The temperature evolution within the first two cycles of operation in combination with the displacement -\$ induced by the linear actuator helps to understand the operation and function of the coupled SMA film-based elastocaloric cooling device. Both are plotted in Figure 53 for the operation frequency C 0.5 Hz. The temperature difference represents the temperature difference between the absolute temperature and the initial (i.e. ambient) temperature. The experiments are conducted at room temperature. As described in chapter 4.5. and illustrated in Figure 53 a), -\$ follows a sinosoidal motion with holding times included at the turning points. Operation starts from the middle position (-\$ C 0mm). The initial downward movement unloads the upper SMA film and loads the lower SMA film, and brings them in contact with the upper side of the heat source and the lower heat sink, respectively. Consequently, -K increases, -7 decreases, and -L, -M are unaffected at first (compare Figure 52 a) for the position and allocation of the sensors). When the coupled SMA films are moved to the opposite direction, loading and unloading reverse, and also the contacts change. So -L increases, -M decreases, and -K, -7 are not affected. Each copper element is subjected to a stepwise change in temperature once per cycle, when they are brought in contact to the SMA films during the holding time. Afterwards, the temperatures change slowly towards the initial temperatures due to parasitic heat losses, until the next contact event occurs in the following cycle of operation. Although the device has only one heat source, the temperatures -7 and -M show different trends here. This is the case, because the different copper elements of the heat source are separated by a polymer support structure. Over time, the support structure and the copper elements adopt a uniform temperature, as can be seen in the IR image of the device during operation in Figure 55 c).

Figure 53 Displacement -] induced to the elastocaloric cooling device for operation (a), and the evolution of the temperature differences -V to -V (b) with time in the first two cycles of operation. Holding time periods with full contact between the SMA and the sink and source elements are highlighted. The operation frequency is 0.5 Hz. Clearly the stepwise evolution of temperatures in the elastocaloric cooling device can be understood. © 2017 IOP Publishing Ltd, figure reprinted from [210] with permission.

For the other operation frequencies, the operation works in the same manner. The temperature evolution of the sink and source elements is displayed in Figure 54 a) for operation frequencies between 0.5 and 4 Hz for a longer operation of 100 s. An excerpt of the initial temperature evolution in the first 10 s is shown in Figure 54 b). The temperatures are plotted as temperature difference between the absolute temperature and the initial starting temperature. Experimental and simulation results are shown side by side. Simulation results are discussed below. Especially within the first 20-30 s, the temperature span between the sink and source elements rises rapidly. Afterwards, the temperature increase (decrease) reduces and finally saturates. The higher the operation frequency, the faster the temperature changes initially, and the faster the system approaches equilibrium.

Figure 54 Temperature difference of heat sink and source elements as a function of time during operation at different frequencies. Temperature differences are displayed for 100 s of operation (a) and the first 10 s (b). Experimental and simulated results are given. © 2017 IOP Publishing Ltd, figure reprinted from [210] with permission.

The device temperature spans reached in the device for the different operation frequencies are presented in Figure 55 a). The highest experimental of 13 °C is reach at 3 Hz. Then, saturates, and does not rise anymore for increasing frequencies. At 4 Hz, it even slightly decreases since the temperature drop of the heat source decreases slightly after 70 s of operation. For operation frequencies of 3 Hz and lower, heating and cooling contribute almost equally to -. The cooling capacity ,( ) \*\*+ shows a similar trend. At 0.5 Hz, ,( ) \*\*+ is 30 mW. ,( ) \*\*+ rises with the frequency to 120 mW at 2 Hz and then saturates. Normalized by the mass of active SMA films, the specific cooling capacity '( ) \*\*+ reaches 7.7 W/g. The surface temperature distribution of the device before and after operation is shown for the case of 2 Hz in Figure 55 c). shows a strong increase with the operation frequency, especially below 2 Hz. The mechanical work input per cycle of operation -3 is determined at low operation frequencies to be 20 mJ. The  is calculated based on cooling capacity and the work input as a measure for the device efficiency. Values for  are given in Figure 55 b) for all investigated operation frequencies. A maximum  of 3.2 is obtained at 1 Hz. The  drops especially for frequencies above 2 Hz.

Figure 55 Experimental and simulated device temperature span -VXmtZum (a) and xyzXmtZum (b) as a function of operation frequency. -VXmtZum saturates above 2 Hz, the maximum experimental value of 13 °C is reached at 3 Hz. The maximum xyzXmtZum of 3.2 is reached at 1 Hz. The xyzXmtZum decreases for frequencies above 2 Hz. (c) IR image showing the surface temperature profile of the cooling device before and after operation at 2 Hz for 100 s. © 2017 IOP Publishing Ltd, figure reprinted from [210] with permission.

The experimental and simulative results are in good agreement. The model is explained in detail in chapter 4.2. Especially the stepwise temperature evolution of the heat sink and source (Figure 54 b) are well represented. The LEM reproduce the influence of the operation frequency on the temperature evolution well. The model allows to investigate higher operation frequencies that could not be covered in the experimental work due to experimental restrictions. However, in Figure 55 a) it can be understood that also higher operation frequencies up to 6 Hz are not expected to improve -. saturation for high frequencies, as already indicated by the experimental results.

#### **5.4.4 Discussion**

One of the major requirements on elastocaloric cooling devices is a low work input per cycle of operation. On the materials side, elastocaloric materials with low critical stresses and low hysteresis work are beneficial. However, efficient operation of elastocaloric devices is only possible with appropriate measures on the device level. The coupled device presented in this chapter shows, how to implement WR successfully in SMA film-based elastocaloric cooling devices. In addition, the coupled device uses the concept of out-ofplane loading. The concept allows for a simple device architecture with a single actuator for mechanical load cycling and separation of the hot and cold heat flow within the elastocaloric device. The use of two SMA films within on setup puts high demands on the manufacturing and assembly accuracy. Both SMA films need to be precisely pre-strained and positioned with respect to each other. This challenge is met by compliant support structures of the heat sink and source elements.

The device temperature span reaches up to 13 °C. Furthermore, the device reaches a maximum cooling capacity of 7.7 W/g and a  of 3.2. The results in the present investigation especially highlight the major influence of the operation frequency in the investigated range of 0.5 to 4 Hz. and ,( ) \*\*+ show similar trends with . Both increase significantly until 2 Hz and saturate afterwards. This behavior is supported by the simulation model also for higher operation frequencies up to 6 Hz. The COP shows high values for low and decreases considerably above 2 Hz. The conflicting trends require a compromise between reaching high -, ,( ) \*\*+ and  . For this specific setup, the operation frequency of 2 Hz gives good values for all figures of merit.

Compared to previous work on a similar coupled SMA film-based elastocaloric cooling system the maximum '( ) \*\*+ has been largely improved by roughly 165 %, but also could be improved by 38 % [169]. The improvement is mainly attributed to enhanced operation parameters, especially the frequency, but also improvements in fabrication and alignment play a role.

The LEM simulation of the device performance confirms the major importance of the operation frequency. For high operation frequencies the time for heat transfer becomes too short, and heat cannot be transferred efficiently between the SMA films and the sink and source elements. Therefore, and ,( ) \*\*+ saturate and  decreases. This points out the importance of the heat transfer coefficient between the SMA and the sink and source elements. Higher operation frequencies can only be used efficiently, if heat transfer can be improved. A second improvement strategy revealed by LEM is to reduce parasitic heat losses through conduction in the support structures. Conduction has been found to be the dominant loss compared to convection.

## **6 Advanced SMA Film Devices**

The single SMA film devices presented in the last chapter reach good performance values, especially high specific cooling capacities. Yet device temperature span and absolute cooling capacity have found to be bound by limits due to the active material's adiabatic temperature span and the limited mass of SMA. Advanced SMA film devices are developed to shift these limits and combine the high specific cooling capacity with increased absolute cooling capacity and also high device temperature spans. Therefore, parallelized and cascaded system architectures are developed (compare chapters 4.1.3 and 4.1.4). Devices based on these concepts are presented and characterized in the following chapters.

### **6.1 Parallelized Device**

#### **6.1.1 Concept**

The concept of SMA film-based elastocaloric cooling has proven its potential to provide efficient elastocaloric cooling performance in the previous chapters 5.1 to 5.4. Especially high specific cooling capacities up to 19 W/g are reached. However, absolute cooling capacity is limited below 220 mW. To broaden the scope of possible applications of SMA film-based elastocaloric cooling devices, the absolute cooling capacity has to be increased. An upscaling approach is developed to achieve this aim based on the gained knowlege on SMA film-based elastocaloric cooling devices. The investigation on the scaling behaviour of elastocaloric devices in chapter 4.1.2 has revealed the surface-to-volume ratio of the SMA element as the key to high cooling performance. Thus, the high surface-to-volume ratio of SMA films has to be maintained in the upscaled device. A parallelized device architecture, as introduced in chapter 4.1.4, has been found to be well suited. The parallelized device presented in the following is developed to combine and add up the cooling capacity of five SMA films.a

### **6.1.2 Experimental Device and Operation**

A schematic of parallelized device is presented in Figure 56 a). Like in the single film device, the device concept is based on out-of-plane loading of the SMA films, and changing mechanical contact conditions separate the cold and hot heat flows. The SMA films are coupled antagonistically to include work recovery (WR) in the device and increase the efficiency thereby (compare chapter 2.3.3 for detailed information on WR). The parallelized device comprises one heat source element and one heat sink element. Both elements are precision milled from one monolithic copper element. The heat sink has three extrusions A, C, E, with a total of five triangular shaped surfaces that load the SMA films by out-of-plane deflection upon operation. The contact surfaces are mechanically polished to achieve good heat transfer between the copper surfaces and the SMA films. The angles of the triangular surfaces are adjusted to apply a maximum strain of 4.7 % to the SMA films. The heat source has three extrusions B, D, F with five surfaces for contact with the SMA films as well. The surfaces of the heat source are slightly barreled for good contact to the unloaded SMA film. Since the contact surfaces for all five SMA films belong to one monolithic element, the heating and cooling power applied to the sink and source during each cycle of operation by the five SMA films add up. The heat sink and source have a mass of 79.8 g and 62.09 g, respectively. These values, which are high in comparison with the other devices, are the result of the extrusions and the demand for the simplest possible geometry. To reduce parasitic heat flows between the heat sink and source elements, a PMMA layer with honeycomb-structured surfaces is included in between. Fiber reinforced polyamide screws with low heat

a The content of chapter 6.1 is based on the master thesis of R. Weidemann, which was co-supervised by the author [d]. The results of R. Weidemann's master thesis are used with his permission.

conductivity are used for fixation. The experimental setup is presented in Figure 56 b).

In the parallelized device NiTiFe films are used as active material. Material properties are discussed in chapter 3.2.The films are cut into lateral dimensions of 56 mm x 4 mm. The ends of each film are glued to 3D-printed fixations that are then screwed to a single rigid frame. The frame is connected to a single linear actuator of the experimental test setup that is used for operation. When the rigid frame is moved in x-direction the SMA films 1, 3, 5 are loaded, since they are pressed onto extrusions A, C and E. At the same time, the films 2 and 3 are unloaded and brough in contact with extrusions B and D. Films 1, 3, 5 transfer heat to the heat sink, whereas films 2 and 3 absorb heat from the heat source. When the frame is subsequently moved to the opposite direction, films 1, 3, 5 are released and brought in contact with extrusions B, D, F from where they absorb heat. Simultaneously, films 2 and 3 are pressed onto extrusions C, E, and release heat to the heat sink. This operation is cyclically repeated. In this way a heat flow is generated by all SMA films from the heat source to the heat sink. The heat source is enabled to provide cooling capacity. The actuation of the operation and monitoring of the performance is done by the experimental test platform presented in chapter 4.4. Sinusoidal load cycles with holding times are applied as described in chapter 4.5.

Figure 56 (a) Schematic setup of the parallelized elastocaloric cooling device. Two sectional views are presented for better understanding. The SMA films 1 to 5 are loaded by the triangular surfaces of extrusions A, C, E in out-of-plane deflection and are brought in contact with the surfaces of extrusions B, D, F in the unloaded state. Cyclic operation generates a heat flow from the cooled heat source to the heated heat sink. (b) The experimental device with the five SMA films, the frame they are fixed to, and the heat sink. The heat source is not included to enable the view of the SMA films.

### **6.1.3 Results**

When the parallelized device is operated by cyclic movement of the frame with the SMA films, the temperatures of the heat sink and source start to change gradually. Temperature evolution of the heat sink and source are presented in Figure 57 a) for operation under frequencies of 0.1, 0.5 and 1 Hz. Temperatures are presented as temperature difference between the absolute temperature and the initial starting temperature. The temperature slope reduces as the temperature difference between the sink and source elements increases. However, the temperatures do not saturate within the 100 s of operation. In Figure 57 c) device temperature spans after 100 s of operation at operation frequencies between 0.05 and 1 Hz are summarized. A linear increase of with can be found. A maximum of 4.4 °C is reached at 1 Hz. For all frequencies the heating and cooling contributes evenly to the device temperature span.

The force-displacement curve of the five antagonistically coupled SMA films in the device is presented in Figure 57 b) for different operation frequencies. Due to the uneven number of SMA films, the characteristic is not symmetric. The work dissipated per load cycle -3 is 130 mJ, 197 mJ and 280 mJ for the operation frequencies of 0.05 Hz, 0.25 Hz and 1 Hz, respectively. As expected from the superelastic material behavior, the dissipated work increases with the velocity of the cyclic movement.

The specific cooling capacity of the device also shows a linear increase with the operation frequency, as depicted in Figure 57 c). A maximum of '( ) \*\*+ = 4.67 W/g is reached at 1 Hz. This compares to an absolute cooling capacity of ,( ) \*\*+ of 0.9 W. The absolute cooling capacity of the device utilizes the combined elastocaloric effect of all five SMA films.

Based on -3 and ,( ) \*\*+ presented above, the  is calculated as a metric for the efficiency of the device operation. The values of  are plotted against the operation frequency in Figure 57 c).  shows a maximum of 4.5 at the operation frequency of 0.25 Hz. At lower frequencies the operation is too slow to generate considerable cooling performance, thus  is low as well. For higher operation frequencies, -3 increases faster than ,( ) \*\*+ with , and therefore  reduces again. At 1 Hz  is 3.2.

Figure 57 (a) Temperature evolution of heat sink and source elements as a function of time during operation at 0.1, 0.5 and 1 Hz. The parallelized device is operated for 100 s. (b) Force-displacement characteristic of the five coupled SMA films incorporated in the parallelized device at operation frequencies of 0.05, 0.25 and 1 Hz. (c) Device temperature span -VXmtZum, specific cooling capacity w( v u[[] and xyzXmtZum as a function of operation frequency G. -VXmtZum and w( v u[[] increase linearly with G. The maximum w( v u[[] is 4.67 W/g and corresponds to an absolute cooling capacity \( v u[[] of 0.9 W.

#### **6.1.4 Discussion**

The main goal of the parallelized device is to upscale the absolute cooling capacity, while maintaining the high specific cooling capacity of SMA film-based elastocaloric cooling devices. To obtain high specific cooling capacity, the high surface-to-volume ratio of the SMA films has to be maintained (compare chapter 4.1.2).

In the parallelized device, the film geometry with high surface-to-volume ratio is maintained, and the elastocaloric effect of five SMA films is added up in a parallelized device architecture.

The parallelized device reaches an absolute cooling capacity of 900 mW and specific cooling capacity of 4.67 W/g, both at 1 Hz. To evaluate the parallelized device performance, a comparison is drawn to the single film device presented in chapter 5.1. The single film device reaches maximum values of 220 mW and 19 W/g at 4 Hz, at 1 Hz it reaches 50 mW and 4.2 W/g. If performance is compared at the same operation frequency, specific cooling capacity is almost equal for both devices. Absolute cooling capacity is increased largely by a factor of 18 from 50 mW to 900 mW. This proves the function of the parallelized device architecture. However, the linear increase of -, ,( ) \*\*+ and '( ) \*\*+ with the operation frequency in the investigated range and the comparison to the high value of '( ) \*\*+ achieved in the single film device at high frequencies (19 W/g at 4 Hz in case of single film device) point out the potential of high operation frequencies also in the parallelized cooling device. Unfortunately, high operation frequencies were not possible due to experiment restrictions. Improvements in the experimental device and test platform are needed to reveal the full potential of the parallelized elastocaloric cooling setup.

The in the parallelized device is limited to comparably low values of 4.4 °C. This is attributed to the high mass of the sink and source elements. Compared to the single film device, sink and source mass are increased roughly by a factor of 200. If higher and faster device response is required, the thermal mass of the sink and source elements needs to be decreased considerably in the parallelized device. In addition, measures should be taken to achieve a more uniform and thus improved heat transfer between the SMA films and the sink and source element. The use of insulation could decrease parasitic heat losses and would contribute to an increased device temperature span in this way. The values for  are comparable to the single film device, so no influence of parallelization on the device efficiency can be found. WR is successfully implemented in the parallelized elastocaloric cooling setup.

The main challenge in the parallelized device is the accurate and repeatable loading of all the SMA films. This restricts the lifetime of SMA films and prevents higher operation frequencies in the given setup, since the demands on the actuator precision are increased. In single film devices, sufficient precision and repeatability is achieved by manual alignment and the use of compliant structures. The recurring arrangement of the five SMA films and the arrangement of heat sink and source below and above the films significantly restrict the available construction space in the parallelized device. This prevents the usage of beneficial compliant structures and leads to increased demands on the fabrication and alignment precision. However, precise alignment on the individual film level is hardly possible in the device. Furthermore, the statistical probability of a film failure is increased in the parallelized device solely due to higher number of films. In order to address this issue, fabrication and alignment precision have to be increased and at the same time the demands on the precision should be decreased by compliant structures (see chapter 4.1.1). A promising approach to include compliant structures is the spacesaving use of soft materials, like done in the cascaded device with in-plane loading in the following chapter. In addition, a modular system approach could increase fabrication precision and ease alignment in the next generation of parallelized devices.

To conclude, the parallelized device architecture has proven its potential for upscaling of the absolute cooling capacity in SMA film-based elastocaloric cooling devices. However, in the given setup, device temperature spans are low compared to the other SMA film-based elastocaloric cooling devices developed in this work. In addition, reliable operation is challenging in the given device. Thus, in a next device generation, the mass of sink and source elements in comparison to the amount of active material should be reduced, fabrication and alignment precision has to be increased, possibly in a modular approach, and compliant structures should reduce the demands on the precision. In this way, massive parallelization based on the introduced concept might pave the way for future planar cooling panels that allow for easy integration in the built environment. Hence, innovative environmentally friendly elastocaloric cooling can be integrated into standardized building components and a completely new field of application can be addressed [225].

### **6.2 Cascaded Device with In-plane Loading**

### **6.2.1 Concept**

The cascaded system architecture introduced in chapter 4.1.3 is implemented in the experimental elastocaloric cooling device presented in this chapter. The aim of cascaded elastocaloric cooling devices is mainly to increase the device temperature span. Three SMA films transport heat in a series connection with intermediate copper elements. In the cascade, only the first (1 with reference to Figure 58) and last SMA film (3) release and absorb heat directly to the heat sink (A) and from the heat source (D), respectively. The intermediate copper elements (B, C) interconnect their neighboring films thermally and function as heat sink for one of them and heat source for the other. In this way, each film operates at different temperature levels and the device temperature span increases.

Unlike the other elastocaloric cooling demonstrators presented in this work, here the SMA films are not loaded in out-of-plane, but are loaded in-plane (compare chapter 2.3.1). In-plane loaded devices with changing mechanical contacts require two actuators for operation, one to load the SMA and one to change the thermal contacts [182]. The increased complexity owed to the two actuators increases the flexibility of the in-plane loaded device. In-plane loading allows to vary operation parameters in a simple way with a single experimental setup. The maximum applied strain is not defined by the device geometry like in devices operated by out-of-plane loading, but can be easily varied. This allows to investigate different materials with different transformation strains within the very same device. The timing between the loading cycle and the cycle for heat transfer can also be varied and investigated. In the following investigation, the influence of operation parameters, which cannot be investigated in devices loaded out-of-plane, is revealed. Even more important, the influence of the cascaded device architecture on the cooling performance and in particular on the device temperature span is investigated to evaluate the concept of cascaded elastocaloric cooling devices.

### **6.2.2 Experimental Device and Operation**

The cascaded experimental setup consists mainly of four copper elements A to D that function as heat sink / source and three SMA films, which are all aligned in parallel. The schematic setup is displayed in Figure 58. All copper elements are connected to a common support structure that is connected to a linear bearing that allows movement perpendicular to the SMA film surface in x1-direction. This support structure is coupled to actuator 1. The ends of the SMA films are connected together at defined distance. One of their ends is fixed in position, the second end is connected to actuator 2 that moves inplane direction of the SMA film (x2). So, the SMA is basically loaded like in a tensile testing machine by actuator 2, while actuator 1 changes the contact conditions between the SMA films and the copper elements.

The experimental implementation of the concept is shown in Figure 59. The complete experimental test setup is shown in Figure 59 a), including the two linear actuators and a load cell that measures the load applied in x2-direction to the SMA films. The single elements of the device can be found in Figure 59 c).The experimental test setup introduced in chapter 4.4 is used for operation and monitoring of the elastocaloric cooling device. Main difference to the experimental operation of the other SMA film-based elastocaloric cooling devices presented in this work is that two linear motors are used at once, instead of one. The support structure that holds the copper elements in place is CNC milled from PMMA. PMMA is used due to its transparency. The lateral dimensions of the copper elements are also CNC milled from a copper sheet. The surfaces are mechanically polished. To measure the temperature of the copper elements a miniature T-type thermocouple with a tip size of 0.4 mm was integrated in a 0.5 mm hole in each copper element. Each copper element has a mass of 0.7 g. Mechanical compliance in the contact of SMA film and the heat sink and source elements is of high importance for the operation and performance of SMA film-based elastocaloric cooling devices (see chapter 4.1.1). However, the cascaded setup does not allow for spring-like structures as in the other cooling devices in this work. Thus, the material compliance of an elastomer is used here rather than the structural compliance of a spring. To do so, the copper elements are not connected in a rigid way with the PMMA support structure, but a compliant "cup" of PDMS has been introduced in between the copper elements and the support structure. The PDMS structures are casted with the help of a custom-made casting mold. Two different superelastic SMA materials are used in this experimental setup, cold rolled NiTiFe films with a thickness of 40 μm and sputtered TiNiCuCo films with a thickness of 30 μm. Detailed information on both SMA materials can be found in chapter 3. The lateral dimensions of the films are 32 x 3 mm2 .

In a typical operation cycle actuator 2 loads the SMA films and at the same time actuator 1 moves in the direction of x1 and brings the films in contact with the copper elements B to D (see Figure 58). Subsequently, actuator 2 unloads the SMA films and actuator 1 moves to the opposite direction and brings the films in contact with copper elements A to C. In general, the cyclic loading of the SMA films is done according to the sinusoidal operation cycle described in chapter 4.5. The position of actuator 1, which is responsible for the changing mechanical contacts, is controlled in a similar sinusoidal way with holding times.

Figure 58 Schematic of the cascaded device with in-plane loading. The cooling device comprises three SMA films and four heat sink / source elements made from copper. Two actuators are needed, one for the cyclic loading of the SMA (actuator 2) and one that changes the contact conditions of the SMA films and the heat sink and source elements (actuator 1). Typical actuation profiles are shown for both of the actuators.

Figure 59 (a) Experimental test setup including the two linear actuators and the cascaded device with in-plane loading. (b) Cascaded SMA film-based elastocaloric cooling device that is operated by in-plane loading. (c) Components of the device.

#### **6.2.3 Results**

The system architecture of the cascaded device with in-plane loading and two separate linear actuators allows for variation of operation conditions in a wide range. In Figure 60 the influence of the stroke of actuator 1 (a), the delay time between the cycles of the two actuators 1 and 2 (b), the maximum applied strain in case of NiTiFe (c) and TiNiCuCo (d) on the device performance is investigated. In each case, the device temperature span after 20 s -R`0=T and the cooling capacity ,( ) \*\*+ are determined as a function of the investigated parameter. The used SMA material, the number of SMA films utilized, and the operation frequency are stated in each case in the figures. Small schematics are included to illustrate the investigated parameters.

Actuator 1 changes the mechanical and thermal contact conditions of the SMA films and the sink and source elements. If the stroke of actuator 1, -4K is chosen to low, full contact between the films and the sink / source elements cannot be reached, but an air gap persists in between. If -4K is too high, the SMA films or the device might be damaged. Both, -R`0=T and ,( ) \*\*+ increase with -4K up to the value of 2.1 mm and saturate afterwards. This shows that it is necessary to achieve full contact, which is reached above 2.1 mm in the present case. A further increase however, does not improve the performance, but only increases the risk of damage. Thus, in following experiments, -4K is fixed to 2.2 mm to ensure good contact without applying too much load.

In Figure 60 b) the influence of a delay time between the two oscillatory movements of the two actuators is investigated. The positions of both actuators are controlled to follow a sinusoidal trace with holding times included at the turning points. Holding times and the time for the sinusoidal movement are chosen equal for both actuators. In this investigation, a time delay between the two movements is included. The mechanical loading / unloading of the SMA films and the movement perpendicular to the SMA film do not coincide, but a small delay time is included between the cycles. The performance of -R` =T and ,( ) \*\*+ show that within a window of -0.05 to 0.05 s no influence can be found. However, if the becomes higher, performance decreases. ,( ) \*\*+ is more sensible to an increased compared to -R` =T. According to this investigation the delay is chosen to be zero in all other experiments. Nevertheless, the fact that small does not diminish the performance is good for a robust operation of the system.

Figure 60 Influence of operation parameters on the performance of the cascaded device with inplane loading. The device temperature span after 20 s -Rv T and the cooling capacity ( v are used as performance metrics. The influence of the stroke of actuator 1 (a), the delay time between the operation cycles of the two actuators (b), the maximum strain applied to the SMA material in case of NiTiFe (c) and TiNiCuCo (d) are investigated.

Figure 60 c) and d) illustrate the influence of the maximum strain { applied to the SMA films within each cycle. The SMA films are cycled between zero strain and {. The maximum strain is determined by the stroke of actuator 2 -47 and the initial length of the SMA film \$) according to { C -47 \$ a ). The influence is investigated for NiTiFe (c) and TiNiCuCo (d) films as active material. A clear trend can be found for both SMA materials in the investigated strain range. The -R`0=T and the ,( ) \*\*+ clearly increase with increasing {. In case of NiTiFe, a strain range of 1 to 5 % is investigated. An almost linear dependence can be found. TiNiCuCo is investigated in the range from 1 to 2.75 %. TiNiCuCo-3 SMA film material is used. At { C \_0 almost no cooling performance exists and the SMA is cycled almost completely within the elastic range. For higher strains, -R`0=T and ,( ) \*\*+ again show an almost linear increase with {. With the increased strain, the fraction of the SMA undergoing the transformation from austenite to martensite and back increases. Thus, the latent heat of the transformation that is accessed upon load cycling is increased. For higher strains above the transformation strain of the materials a saturation of the device performance is expected. If the SMA already transforms completely, a further increase of { has only minor influence on the released latent heat. However, this regime is not reached in the present investigation. High strains also increase the risk of material failure [140;226]. Hence, strains are limited to 5 % for NiTiFe and 2.75 % for TiNiCuCo to avoid a high risk of specimen failure. { has to be determined according to a compromise between high cooling performance and reliability. For further experiments { has been chosen to be 4.5 % in case of NiTiFe and 2.6 % in case of TiNiCuCo to pay credit to the two opposing trends.

With the pre-tests to determine optimum operation parameters the full potential of the cascaded device with in-plane loading can be revealed. The temperature evolution of the copper elements of the cooling device in the tests that reached the highest temperature spans with NiTiFe and TiNiCuCo as active material are presented in Figure 61 a) and Figure 62 a). The temperatures are plotted as temperature difference between the absolute temperature and the initial starting temperature. As for other SMA film-based elastocaloric cooling devices, the temperatures change in a stepwise manner representing the discontinuous contacts and heating and cooling within each operation cycle. After operation start, temperatures of the copper elements change fast and then temperatures stabilize and oscillate around a fixed value in each cycle. The first and last elements in the cascade are subjected to the highest temperature changes, whereas intermediate elements follow intermediate temperature trends. The three SMA films of the cascade operate at different temperature levels and transfer heat from element D over C and B to element A.

With the combined cooling effect of the three NiTiFe films the device reaches a temperature span of 27.3 °C at 1 Hz. The combined temperature span is the sum of the temperature differences between the neighboring copper elements. Here, a temperature difference of roughly 9 °C can be measured between elements A and B, B and C, C and D. A cooling capacity of 190 mW or 3.4 W/g is reached in the experiment. Figure 61 b) reveals the surface temperature distribution of the device in the very same experiment measured by IR thermography before operation start, after 20 s, and after 80 s of operation.

The two TiNiCuCo films operated at 2 Hz reach a combined of 17.9 °C (Figure 62, a). A TiNiCuCo-1 film is utilized in between the copper elements A and B, whereas TiNiCuCo-3 is used in between B and C (compare chapter 3.3). Temperature differences between element A and B, and B and C are both roughly 9 °C. This specific experiment reaches a cooling capacity of 181 mW or 4.82 W/g.

Figure 61 (a) Temperatur evolution of the copper elements A to D of the cascaded elastocaloric cooling device with in-plane loading. Operation takes place with three NiTiFe films at 1 Hz. (b) IR thermographs of the cooling device during operation in the experiment displayed in a). Thermographs are taken before, after 20 s, and after 80 s of operation

The influence of the operation frequency on the and ,( ) \*\*+ of the elastocaloric cooling device is investigated in a series of experiments displayed in Figure 62. Results are given for operation with one and two TiNi-CuCo films. increases with until 2 Hz, where reaches a maximum for operation with one and two SMA films. Especially when operated with one SMA film, decreases considerably when is further increased to 3 Hz. The usage of two SMA films compared to one SMA film increases at 2 Hz by about 58 % from 10.6 to 16.7 °C in this series of experiments. The cooling capacity ,( ) \*\*+ increases with as well, and saturates at 2 Hz. ,( ) \*\*+ is hardly influenced by the number of SMA films. The highest value of ,( ) \*\*+ is reached at 2 Hz and 3 Hz with about 180 mW. When the specific cooling capacity is considered instead of absolute values, the number of SMA films however has a significant influence. The absolute values of 180 mW for one and two films relate to specific values of 9.6 and 4.8 W/g, respectively.

Figure 62 (a) Temperatur evolution of the copper elements A to C of the cascaded elastocaloric cooling device, operated with two TiNiCuCo films at 2 Hz. (b) Influence of the operation frequency G on the device temperature span -VXmtZum and the cooling capacity \( v u[[]. Results are given for operation with one and with two TiNiCuCo films.

### **6.2.4 Discussion**

The cascaded device with in-plane loading applies the concept of cascading to SMA film-based elastocaloric cooling devices. The cascaded device architecture allows for high device temperature spans, possibly higher that the adiabatic temperature span of the used SMA material.

In addition, the cascaded device with in-plane loading deviates from the other devices in this work by the load application. Here, load is applied in-plane. This rises the need for an additional actuator to change the mechanical contacts, and thereby direct and separate the hot and cold heat flow in the cooling device. The device becomes more complex, but also allows to investigate the influence of parameters that are fixed in devices based on out-ofplane loading.

Different operation parameters are investigated. For a good cooling performance, the stroke of the actuator that changes the mechanical contacts, actuator 1, needs to be above 2.1 mm. A further increase is not beneficial to the device performance but might apply unnecessary loads to the device. Thus, -4K is chosen to be 2.2 mm. The investigation of the influence of a short delay time between the operation cycles of the two actuators reveals that synchronous operation is desirable. This result is also interesting to the devices loaded out-of-plane. In case of out-of-plane loading, the change of contacts and the mechanical loading coincident naturally and cannot be changed. However, the investigation of the delay time in this chapter reveals that this is no disadvantage. Increased maximum strain leads to improved device performance in the investigated strain range for both investigated SMA film materials, NiTiFe and TiNiCuCo. Higher maximum strains force an increased fraction of the SMA material to undergo the stress-induced phase transformation. The amount of latent heat of transformation released and absorbed during load cycling increases and the elastocaloric effect increases as well. Yet, increased maximum strain also increases the risk to damage the SMA film. A compromise is needed. As a good compromise, the maximum strain has been fixed to 4.5 % in case of NiTiFe and 2.6 % in case of TiNiCuCo.

In general, the device would also allow for more complex loading profiles, like e.g. a combined adiabatic isothermal operation cycle or a non-adiabatic adiabatic cycle, which both have been found beneficial for elastocaloric cooling [39]. However, the control of the given experimental test setup is not fast enough to apply the improved cycles in a reliable way, especially under high operation frequencies. So, the investigation of complex loading profiles and operation cycles stays an open task for future work.

The cascaded device with in-plane loading clearly allows increased device temperature spans -. With the combined effect of three NiTiFe films as active material the device reaches a maximum of 27.3 °C. This compares to the adiabatic temperature span of the SMA material for heating and cooling of 20 and -16 °C. The combined effect of two TiNiCuCo films give rise to a of 17.9 °C. This compares to a maximum adiabatic temperature span of 12.2 °C and -14.5 C. The cascaded device reaches a that exceeds the materials for both materials. However, the combined adiabatic temperature span for heating and cooling could not be surpassed in the performed experiments. Nevertheless, compared to the single film device maximum is increased by 95 % in case of three NiTiFe films, and by 28 % in case of two TiNiCuCo films. Therefore, the potential of the cascaded device architecture is proven. In experiment with TiNiCuCo films the - is increased by 58 % by using two instead of one SMA film. Two different TiNi-CuCo alloys are used in the present investigation with a shift in according to the altered temperature range of operation in the cascade. The use of different SMA materials with tailored will become increasingly important in future, as in elastocaloric cooling devices is expected to increase further.

As known from the other SMA film-based elastocaloric cooling devices, the operation frequency has a tremendous influence on the device performance of the cascaded setup. and ,( ) \*\*+ clearly increase with for frequencies up to 2 Hz. Other than known from the devices with out-of-plane loading, even decreases again for the higher frequency of 3 Hz. Saturation of with increasing in SMA film-based elastocaloric cooling devices has been found due to limited heat transfer and decreasing temperature difference between the SMA film and the heat sink and source elements. Since heat transfer mechanisms in the in-plane loaded device are similar, both effects are likely to play a role here. Nevertheless, both effects would not lead to a decrease of for higher frequencies, but to saturation. The decrease is attributed to insufficient control of the two actuators. As found in the investigation of the influence of a delay between the two actuator cycles, an unfavorable timing of the two cycles can lead to a complete breakdown of the device performance. At 3 Hz, the control of the two actuators is not fast and precise enough to ensure a completely synchronous movement. Consequently, the performance declines.

The cooling capacity ,( ) \*\*+ and specific cooling capacity '( ) \*\*+ of the cascaded device are in the range known from other SMA film-based cooling devices developed in this work. It is important to highlight that the absolute cooling capacity ,( ) \*\*+ is not influenced by the number of cascaded SMA films. The same absolute cooling capacity is achieved, regardless of the number of cascaded SMA films. Thus, the specific cooling capacity '( ) \*\*+ is influenced highly, and decreases with the number of SMA films. A single TiNiCuCo film incorporated in the device and operated at 2 Hz gives rise to a cooling capacity of 180 mW or 9.6 W/g. Two TiNiCuCo films at 2 Hz achieve 180 mW or 4.8 W/g. Three NiTiFe films lead to a cooling capacity of 193 mW and 3.4 W/g at 1 Hz.

Although the loading concept of the present device differs from the other cooling devices developed in this work, the basic concept of heat transfer and the mechanism to separate the cold from the hot heat flow are similar. In consequence, the achieved cooling capacity is in a similar range also. This is not the case for the device temperature span. The results prove that cascading of the elastocaloric effect of several SMA films in one device is an effective strategy to increase the device temperature span. The limitations of the device temperature span of single SMA film-based elastocaloric devices do not apply to cascaded devices. This allows SMA film-based elastocaloric cooling devices to address new applications that require high temperature differences.

### **6.3 Cascaded Device with Out-of-plane Loading**

### **6.3.1 Concept**

The cascaded device with out-of-plane loading combines the potential of the cascaded system architecture with out-of-plane loading. This allows for a comparably simple device setup. As in the single film device only one actuator is needed for operation. The aim is to transfer the insights gained with the single film device and the cascaded device with in-plane loading to a single cascaded cooling device. In this way the device temperature span of the cascaded device can be increased, while the device complexity is kept as low as possible.b

### **6.3.2 Experimental Device and Operation**

The cascaded device with out-of-plane loading combines the elastocaloric cooling performance of three SMA films in a serial way. A schematic of the device is illustrated in Figure 63. The device consists of four copper elements and the three TiNiCuCo films in between them. The copper elements have each a flat and a triangular shaped surface at opposite sites. The SMA film can absorb heat from the flat surface and release heat to the triangular shaped surface, which loads the SMA film by out-of-plane deflection. Thus, the two sides of one single copper element are heat sink and source. The middle elements C and B function as heat sink for the SMA film above and as heat source for the film below and combine the single units into a cascaded device.

The SMA films have a thickness of 30 μm and length and width of 20 mm and 2 mm. The SMA films are fixed to a steel frame for better handling. Fabrication and properties of the TiNiCuCo films are addressed in chapter 3.3. The TiNiCuCo alloy used in this device shows a similar mechanical performance as TiNiCuCo-2. However, the alloy has a slightly altered composition with an increased copper content (Ti53Ni24Cu20.9Co2.1). The adiabatic temperature span of the specific SMA is 10 °C upon loading and -13 °C upon unloading.

The copper elements are fabricated by wire-electro discharge machining and subsequent mechanical polishing of the shaped surfaces, the elements have a uniform height of 4 mm and a mass of 0.8 g. The copper elements are suspended by a polymer structure fabricated by stereolithographic 3D printing (compare chapter 4.3). The suspension includes spring-like structures that result in a mechanical compliance in the actuation direction. In Figure 64 an

b The content of chapter 6.3 was previously published by the author in [217;218].

image of the experimental setup is presented. The base area of the structure is 36.50 mm x 20 mm, the height is 22 mm.

Figure 63 Schematic of the cascaded elastocaloric cooling device loaded in out-of-plane. The device is composed of three single units 1 to 3, each with a SMA film. The three SMA films are mechanically connected at their ends and are actuated by a single actuator as indicated by the arrows. Copper elements A to D function as heat sink and source. A polymer support structure is used to suspend the copper elements. ©2019 IEEE, figure reprinted from [218] with permission.

Figure 64 Experimental setup of the cascaded cooling device with out-of-plane loading. The four copper elements A to D can be seen along with their polymer support structure. The actuator unit and SMA films are not included for the sake of clarity. ©2019 IEEE, figure reprinted from [218] with permission.

The device is operated by a single actuator of the experimental test setup described in chapter 4.4. The actuator moves the ends of the SMA film cyclically back and forth. In this way they are first pressed onto the triangular shaped surface of the copper elements and thereby loaded out-of-plane. The mechanical loading induces the phase transformation in the SMA and the released heat is directly transferred to the triangular shaped copper surface, where the SMA film is kept in contact for a defined holding time. Subsequently, the SMA films are moved to the opposite direction. The load is released and the films transform back and absorb heat from their surroundings, in particular from the flat copper surface the SMA film is brought in contact with. The actuator moves the SMA film ends on a sinusoidal trajectory with additional holding times as explained in chapter 4.5. The device is operated under operation frequency of 1, 2 and 4 Hz. The device can also be operated with less than three SMA films. In this way the influence of the number of cascaded SMA films on the cooling performance can be investigated. Experimental results are supported by device simulation. A lumped element simulation model is set up in accordance to chapter 4.2.

#### **6.3.3 Results**

Operating the complete cascaded elastocaloric cooling device as described above results in a stepwise heat transfer from Cu-element D over C and B to A. Figure 65 shows the temperature evolution of all four Cu-elements during 200 s of operation with an operation frequency of 4 Hz. The temperatures are plotted as temperature difference between the absolute temperature and the initial starting temperature. Clearly Cu-element D cools down and A heats up, B and C follow intermediate trends. The temperature evolution saturates after 100 s and reaches a device temperature span of 15 °C between the elements A and D.

Figure 65 Temperature evolution of the copper elements A to D of the cascaded cooling device during operation. Here an operating frequency of 4 Hz is applied. The excerpt shows simulation results along with the experimental results for the first 10 s of operation (information on simulation in 4.2). ©2019 IEEE, figure reprinted from [218] with permission.

In Figure 66 a) the device temperature span after saturation is displayed as a function of the operation frequencies for different numbers of cascaded SMA films. Clearly, rises with the number of cascaded SMA films: at 4 Hz from 7.6 °C (one film) to 15 °C (three film). In addition, the temperature difference increases significantly with the operation frequency: for three cascaded films from 5.4 °C (1 Hz) to 15 °C (4 Hz). The surface temperature profiles of the device section marked in Figure 63 before operation are presented in Figure 66 b). Profiles are shown for one operated SMA film and two and three cascaded SMA films, all operated at 4 Hz.

Figure 66 (a) Compilation of results on the device temperature span -VXmtZum, the absolute cooling capacity \( v u[[] and the specific cooling capacity w( v u[[]. All performance metrics are plotted versus the operation frequency. Results are given for operation with a single SMA film as well as for two and three cascaded films to understand the influence of the cascaded system architecture on the device performance. (b) Surface temperature profiles of the cascaded device before and after operation at 4 Hz for 100 s. The profiles are taken by IR thermography. Only a section of the entire device is shown as indicated in Figure 63. ©2019 IEEE, figure reprinted from [218] with permission.

Besides the temperature span, Figure 66 depicts the absolute and specific cooling capacity of the device, ,( ) \*\*+ and '( ) \*\*+. Both, absolute and specific cooling capacities show an almost linear increase with the operation frequency. ,( ) \*\*+ is not influenced by the number of cascaded SMA films. Thus, '( ) \*\*+ decreases with the number of cascaded SMA films, as more SMA material is used in the device configuration to achieve the same cooling capacity. The system reaches its highest cooling capacity of 0.145 W at 4 Hz, the maximum specific cooling capacity of 16.7 W/g is reached by the single unit configuration at 4 Hz.

The efficiency of the device, more particular the  , is evaluated for 1 Hz and different numbers of cascaded SMA films. In this device,  follows a similar trend like '( ) \*\*+, since the work input scales with the amount of SMA material. For operation of a single SMA film,  reaches 5.7, which compares well to the value found in the single film device presented in chapter 5.1. When the number of cascaded SMA films is increased,  drops to 1.9 for three SMA films.

#### **6.3.4 Discussion**

The cascaded device with out-of-plane loading combines the concept of cascaded elastocaloric devices that enable high device temperature spans, with a simple system architecture similar to the single film device. Only one actuator is needed for the cyclic loading of all SMA films and the separation of hot and cold heat flows within the system.

Like in the other SMA film-based elastocaloric cooling devices, the operation frequency has a high influence on the cooling performance of the cascaded device with out-of-plane loading. Generally, the device temperature span and the cooling capacity ,( ) \*\*+, '( ) \*\*+ increase with the operation frequency. Other than in the single film device no clear saturation of the performance with can be found for the cascaded device. The number of cascaded films shows a clear influence on -. At 4 Hz, increases from 7.6 °C to 15 °C when the number of cascaded films is increased from one to three. Therefore, overcomes the materials -. However, the combined for heating and cooling is not reached. Linear extrapolation suggests that six cascaded films would be necessary to do so. Nevertheless, the improvement in by cascading comes at cost. ,( ) \*\*+ is hardly influenced by the number of cascaded films and reaches a maximum of 145 mW at 4 Hz. In turn, '( ) \*\*+ and  decrease with the number of cascaded SMA films. '( ) \*\*+ decreases from 16.7 W/g (1 film, 4 Hz) to 6.1 W/g (3 films, 4 Hz) and  from 5.7 (1 film, 1 Hz) to 1.9 (3 film, 1 Hz).

The performance metrics of the cascaded elastocaloric cooling device, -, ,( ) \*\*+, '( ) \*\*+, and  follow different trends with regards to and the number of cascade SMA films. Thus, there is no single optimum configuration and operation suitable to all application scenarios. Instead, a compromise has to be found and adapted to each special application. The cascaded device architecture introduces one more parameter, the number of cascaded SMA films, that allows to fit device performance better to specific demands. Even more important, cascading enables operation at higher temperature spans. Consequently, the cascaded device allows to address completely new applications that require higher temperature spans.

## **7 Discussion of Elastocaloric Cooling Devices**

Elastocaloric cooling devices need to cyclically apply a uniaxial stress to the elastocaloric material in order to repeatedly trigger the elastocaloric effect. In addition, the hot and cold heat flows, which are generated by the elastocaloric heating and cooling of the active material, have to be separated from each other. As in all cooling devices, the hot heat flow is considered waste heat and has to be released to the environment, whereas the desired cold heat flow is ready to provide cooling. The device must guarantee the function over a long period of time and many load cycles, which places high demands on the device itself and especially on the elastocaloric material. An ideal elastocaloric cooling device should provide the needed cooling capacity over a wide range of temperatures and also large temperature spans. Furthermore, the mechanical power input should be low for efficient operation and thus high coefficient of performance  .

The elastocaloric cooling devices that are developed and characterized in this work share basic concepts of operation. In Table 8 the devices are categorized according the classification approach for elastocaloric cooling devices developed in chapter 2.4.1. SMA films, mostly with a thickness of 30 μm, are used as active material in all devices (i: 1). Two types of SMA films are used, cold rolled NiTiFe and sputtered TiNiCuCo films (h: 1). Film geometries have a high surface-to-volume ratio and therefore enable efficient and fast heat transfer within the elastocaloric device. All devices are based on heat transfer between solid elements in mechanical contact (b: 1). The hot heat flow is separated from the cold by changing contact conditions (c: 1). The heat transfer concept enables good heat transfer and a simple system architecture. No additional fluidic circuits and pumps are needed. The SMA films are loaded in tension, either in out-of-plane deflection (f: 2) or in pure tension (f: 1). In case of out-of-plane deflection, only a single linear actuator is needed for the

device operation and maximum forces in the device are reduced compared to in-plane loading. The single film devices do not include a concept for caloric amplification (a: 1), and thus their temperature span is limited by the active material's adiabatic temperature span. However, the concept of cascaded elastocaloric cooling devices is developed in this work to reach device temperature spans that are not limited by the available active material (a: 3). In addition, a parallelized device is developed to increase the absolute cooling capacity of SMA film-based cooling devices. The coupled and the parallelized device demonstrate how to include work recovery (WR) in SMA film-based elastocaloric cooling (g: 2).

SMA film-based elastocaloric cooling benefits from its simple device architecture that not only allows to build miniature scale elastocaloric cooling devices, but also to extend the device architecture into parallelized and cascaded systems based on the same basic principle.

The fabrication of the experimental devices presented in this work is similar in most cases. Stereolithographic 3D printing is used for the fabrication of the polymer support structure. The copper heat sink and source elements are precision-machined and mechanically polished (see chapter 4.3).

Operation and monitoring of the device performance are carried out with a test setup that is developed within this work for this specific task (see chapter 4.4).

Table 8 Classification of SMA film-based elastocaloric cooling devices presented in this work. Each device is classified according to the approach introduced in chapter 2.4.1. The single characteristics are coded by numbers according to Figure 13. The overview allows to quickly identify similarities and key differences between the devices. eCM denotes elastocaloric material.

Legend (only characteristics that are applied in this table are listed):

a) caloric amplification: 1 none; 3 cascaded;

b) heat transfer: 1 solid-to-solid;

c) separation of hot and cold heat flow: 1 changing thermal contact conditions;

d) relative motion: 2 moving eCM, static heat sink and source; 3 static eCM, moving heat sink and source;

e) type of actuator movement: 1 linear oscillatory;

f) loading stress state of eCM: 1 pure tension; 2 tension, out-of-plane deflection;

g) work recovery: 1 none; 2 antagonistic coupling;

h) type of elastocaloric material: NiTi-based SMA;

i) geometry of elastocaloric material: 1 film / foil-based;

Table 9 Overview of cooling performance of the SMA film-based devices developed within this work. In addition, selected results of previous work and literature are given as a benchmark.

Highlights are marked in bold

#### **Single SMA film devices**

A comprehensive overview of the cooling performance of the SMA-film-based elastocaloric cooling devices of this work is given in Table 9. The design of the single film device is based on previous work [32,p. 168]. It has been reengineered and characterized under improved operation conditions. Maximum values for the device temperature span of 14 K and for the specific cooling capacity of 19 W/g are reached at an operation frequency of 4 Hz. The maximum of 6.7 is reached at the lowest of 0.5 Hz. Compared to previous experiments with a similar design, the performance is considerably increased, in case of by 70 %. The improvement is mainly attributed to enhanced operation with higher . The single film device is considered a benchmark and basis for the other devices developed in this work. Two devices with very similar single film designs are developed in this work to investigate further miniaturization and the cooling performance of SMA film-based elastocaloric cooling devices under thermal load.

The coupled device successfully introduces the concept of WR by antagonistic coupling and pre-straining of films to SMA film-based elastocaloric cooling devices. Similar to the single film device, the coupled device is based on a previous design, but performance is largely increased. The coupled device reaches its best performance at 2 Hz. A maximum of 12.6 °C, '( ) \*\*+ of 7.7 W/g and a  of 3 is achieved.

The investigation of the single and coupled film devices already reveals some general trends in SMA film-based elastocaloric cooling devices and allows first conclusions:

each cycle of operation. In the beginning, temperature changes quickly and as the temperature span develops, the temperature change rate decreases and saturates. In saturated state, the temperature change of the heat sink / source in contact with the elastocaloric film and the temperature change due to parasitic heat losses of the sink /source elements reach a stationary equilibrium. The higher the operation frequency, the faster the temperature changes and the faster equilibrium is reached.

the latent heat released / absorbed in the SMA. Secondly, the highest is reached at high frequencies, and thus, the temperature gradient between the SMA film and the sink and source elements decreases. An increased heat transfer coefficient of the contact would be desirable, or materials with higher would be needed to preserve a high temperature gradient at high operation frequencies.

#### **Cooling performance of SMA film-based cooling devices under thermal load**

The cooling performance of the single film device with thermal load reveals, how such a device reacts to the application of a thermal load, as it would be the case in an application. A linear decrease of the device temperature span for increasing thermal load on the device is found. Based on this finding, an approach is developed to approximate the cooling performance of the other SMA film-based elastocaloric cooling devices under an external load (see for details chapter 5.2.4). In this way a complete device performance characteristic for all possible points of operation is extrapolated from the figures of merit - \*\*+ , ,( ) \*\*+ and  . In Figure 67 the approach is applied to the single film device. In a) a cooling temperature span -\*\*+ - cooling capacity ,( \*\*+ - operation frequency space is established that illustrates, which points of operations the device can reach (green) and which not (red). In b) same is done with the  on the *y*-axis. This approximate model for the device operation under thermal load might be used to estimate the ,( \*\*+ of the device under a given -\*\*+ and the  at this point of operation. Similar, if a certain ,( \*\*+ is needed, the model approximates the -\*\*+ the device can provide and suggests a for the most efficient operation.

Figure 67 Performance space of the single film device extrapolated by its experimentally determined figures of merit. Operation points within the green volume can be met by the device, points outside (red) the device cannot achieve. The approach illustrates the approximate device performance under a given thermal load in an application.

The approach helps to approximate the device performance under applied thermal load in an application. Figure 67 once more underlines that -, ,( ) \*\*+, and  cannot be optimized all at the same time, since they follow opposing trends. At least for operation frequencies below 2 Hz, and ,( ) \*\*+ both increase with . However,  follows an opposing trend. Thus, a compromise has to be found according to the needs of a specific application. Nevertheless, the approach also points out the importance of high -, ,( ) \*\*+, and  , to reach a good and efficient cooling performance even at temperature spans and cooling capacities below the maximum values. Since the basic concept and performance trend of the cooling devices in this work are similar, the approach can also easily be applied to the SMA film-based elastocaloric cooling devices.

#### **Advanced SMA film devices**

Although specific cooling capacity '( ) \*\*+ reaches high values up to 19 W/g in the single film devices, the absolute cooling capacity ,( ) \*\*+ is limited to 220 mW. The investigation of the scaling laws of elastocaloric refrigerators in chapter 4.1.2 reveals that simply increasing the device size is not an option, since the surface-to-volume ratio of the active material would diminish. At the same time, approaches its limits as well, as indicated above. Further increase of has no, or almost no, beneficial influence on - anymore above 2 Hz for the device geometries and heat transfer mode investigated herein. New concepts are needed to further improve ,( ) \*\*+ and in SMA film-based elastocaloric cooling devices while maintaining the good performance and high values of the specific cooling capacity found in single film devices. In this work, parallelization and cascading is proposed to overcome the limitations. Advanced SMA film-based elastocaloric cooling devices are engineered and investigated to examine the effectiveness of the two concepts.

A **parallelized device** is designed and engineered to combine the elastocaloric effect of five SMA films. The SMA films are coupled to include WR. All SMA films cool and heat one single heat sink and source. Thereby, the elastocaloric effects of the films are interconnected in parallel and add up. The parallelized device preserves the high specific cooling capacity '( ) \*\*+, 4.7 W/g is reached, and increases the absolute cooling capacity ,( ) \*\*+ to 0.9 W at 1 Hz. If the cooling capacity is compared to the single film device at the same operation frequency, specific cooling capacities almost equal each other. Absolute cooling capacity is increased largely by a factor of 18. This proves the function of the parallelized device architecture. However, is limited to 4.4 °C in the parallelized device due to the high mass of the sink and source elements in this specific device. No influence of parallelization on the device efficiency  can be found, as the values of the parallelized device are comparable to the single film device. The results prove the effectiveness of the parallelized device architecture to increase the absolute cooling capacity, while maintaining the high specific cooling capacity characteristic of SMA film-based elastocaloric cooling devices. No general objections are found that oppose a further upscaling of parallelized devices based on SMA films.

A **cascaded device architecture** is proposed to overcome the limitations of in single SMA film-based elastocaloric cooling devices (see chapter 4.1.3). The concept is investigated in two different devices that both combine the effect of three SMA films. The main difference between the two devices is the concept of load application. The cascaded device with in-plane loading is the only device in this work that deviates from the out-of-plane loading concept. This is unique to so-far published SMA film-based elastocaloric cooling devices. In-plane loading rises the need for an additional actuator that changes the contact conditions of the SMA film and the heat sink and source elements. The advantage of in-plane loading is possibility to choose the operation parameters more freely. On the other hand, the cascaded device with out-of-plane loading is more similar to the single film device. The basic idea of both cascaded devices is comparable. Heat is transferred between the SMA films and copper elements that function as heat sink and source. The key is that in the configuration of "copper element - SMA film - copper element" is stacked three times. The intermediate copper element functions as heat sink for one neighboring SMA film and as heat source for the other neighboring SMA film. In this way, the cooling capacity of the first film is used to precool the next "copper element - SMA film - copper element" unit. The single SMA films thus operate at different temperature levels and the cascaded device architecture shifts the limit of the device temperature span to higher values with each cascaded film. The cascaded device with in-plane loading reaches a maximum of 27.3 °C with the combined effect of three NiTiFe films. This compares to the adiabatic temperature span of the SMA material for heating and cooling of 20 and -16 °C. The combined effect of two TiNiCuCo films give rise to a of 17.9 °C. This compares to a maximum adiabatic temperature span of 12.2 °C and -14.5 °C. For both materials, of the cascaded device exceeds the of a single element for heating and cooling, but not the combined for heating and cooling together. Nevertheless, compared to the single film device maximum is increased by 95 % in case of three NiTiFe films, and by 28 % in case of two TiNiCuCo films. Thus, the potential of the cascaded device architecture to increase is proven. In experiments with TiNiCuCo films, the is increased by 58 % by using two SMA films instead of one within the same device. In the cascaded device with in-plane loading, two different TiNiCuCo alloys are used with a shift in according to the altered temperature range of operation in the cascade. The use of different SMA materials with tailored will become increasingly important in future, as in elastocaloric cooling devices is expected to increase further and might exceed the operation range of a single material.

The cascaded device with out-of-plane loading increases its from 7.6 °C to 15 °C when the number of cascaded films is increased from one to three at 4 Hz. Therefore, also in the cascaded device with out-of-plane loading, overcomes the materials for heating and cooling, but not the combined for heating and cooling together. Like in the other SMA film-based elastocaloric cooling devices, the operation frequency has a large influence on the cooling performance of the cascaded device. Generally, the device temperature span and the cooling capacity ,( ) \*\*+, '( ) \*\*+ increase with the operation frequency. Other than in the single film devices, no clear saturation of the performance with can be found for the cascaded device with out-of-plane loading within the frequency range accessible with the electromechanical actuator used to drive the device. The number of cascaded films shows clearly a beneficial influence on -. In the cascaded device with in-plane loading, the device performance increases with until 2 Hz and even decreases afterwards. However, this decrease is found to originate from insufficient timing of the two actuators and is not related to limited heat transfer, which is the reason for similar effects in the single film devices.

The improvement in by cascading comes at cost. The absolute cooling capacity ,( ) \*\*+ is in the same range as in the single film device, but stays the same in cascaded devices, regardless of the number of cascaded SMA films. It reaches a maximum of 145 mW in the cascaded device with out-ofplane loading and 180 mW in the cascaded device with in-plane loading. In turn, '( ) \*\*+ and  decrease with the number of cascaded SMA films. In the cascaded device with out-of-plane loading '( ) \*\*+ decreases from 16.7 W/g (1 film, 4 Hz) to 6.1 W/g (3 films, 4 Hz), and  from 5.7 (1 film, 1 Hz) to 1.9 (3 films, 1 Hz). In the cascaded device with in-plane loading '( ) \*\*+ decreases from 9.6 W/g (1 TiNiCuCo film, 2 Hz) to 4.8 W/g (2 TiNi-CuCo films, 2 Hz), and finally to 3.4 W/g (3 NiTiFe films, 1 Hz).

The performance metrics of the cascaded elastocaloric cooling devices (-, ,( ) \*\*+, '( ) \*\*+ and  ) follow different trends with regard to and the number of cascaded SMA films. So, no single optimum device configuration is able to fit all application scenarios. Nevertheless, the number of cascaded films in cascaded devices is an additional parameter that helps fitting the device performance to the demands of an application. More importantly, the experimental results of the cascaded devices prove that cascading enables high -, higher than the SMA materials -, needed for many industrial applications. The use of TiNiCuCo films with tailored transition temperature is especially promising, as it allows the device temperature span to exceed the superelastic operation range of a single SMA material.

#### **Challenges and critical points**

Besides the promising experimental results of single and advanced SMA filmbased elastocaloric cooling devices, also some issues and critical points of the technology are revealed in this work:

to avoid overloads to the SMA films (see point above). Manual alignment and compliant spring-like structures, which allow sufficiently reliable and precise operation of single film devices, are increasingly hard to apply to the more complex advanced devices. The recurring arrangement of the SMA films restrict the available construction space. In addition, each additional SMA film that has to be aligned with respect to the other films increases the demand on the alignment considerably. The limited construction space restricts the implementation of compliant structures. A promising solution for this is found in the cascaded device with in-plane loading, where the compliance of a soft material (PDMS) is used, instead of a space consuming spring-structure. As a consequence, increased complexity of advanced devices increases the demands on the precision of fabrication, alignment and operation. Modular device architecture might be a second promising approach to reduce the complexity of advanced devices and increase the precision.

#### **Benchmark**

The performance of single film devices and advanced devices with parallelized and cascaded device architecture are discussed above in detail, but how does this performance compare to other elastocaloric cooling prototypes found in literature? A literature review of elastocaloric cooling prototypes is given in chapter 2.4.2. The demonstrators that reach the highest - \*\*+ and '( ) \*\*+ are also included in Table 9 for comparison. One main difference of the SMA film-based elastocaloric devices to elastocaloric cooling prototypes reported in literature is the size. The elastocaloric cooling prototypes found in literature range from the size of decimeter to meters and therefore are considerably larger compared to the devices in this work [15;16;30]. Thus, the absolute cooling capacity of SMA film-based elastocaloric cooling devices is low compared to the highest value of 65 W found in literature [185]. However, the parallelized device highlights the way how to increase the cooling capacity of SMA film-based elastocaloric cooling devices. The 900 mW of the parallelized device already approaches the range of maximum ,( ) \*\*+ reported in regeneration and cascade-based elastocaloric cooling demonstrators reaching 4536 mW and 1323 mW, respectively [31;172]. In terms of -, values of SMA film-based elastocaloric cooling devices can compete with larger devices reported in literature. Today, the highest literature value of is reported for a cascaded device that reaches 28.3 °C [31]. The maximum reached in this work with the cascaded device with in-plane loading is with 27.3 °C only slightly lower. Also the single film device with a maximum of 14 °C reaches similar values as larger demonstrators found in literature. A focus on the specific cooling capacity '( ) \*\*+ demonstrates the full potential of SMA film-based elastocaloric cooling devices. Values up to 19 W/g are reached. This is one order of magnitude higher compared to the highest values of about 0.8 W/g found in literature for large-scale devices [31;172]. This is in line with the scaling laws of elastocaloric refrigerators derived in chapter 4.1.2.

To sum up, the absolute cooling capacity ,( ) \*\*+ of SMA film-based cooling devices lags behind the best values found in literature. However, the parallelized device suggests how to change this in future. The device temperature span is in a similar range, the best value of - C 27.3 °C reached in this work is only slightly lower compared to the highest value of 28.3 °C reported in literature. In terms of specific cooling capacity '( ) \*\*+, SMA film-based cooling devices outperform large-scale devices by far. The high performance is related to the efficient heat transfer of high surface-to-volume film geometries enabling high operation frequencies. So far, all large-scale elastocaloric cooling demonstrators have been limited to operation frequencies below 1 Hz.

The different SMA film-based elastocaloric cooling devices developed in this work demonstrate the potential of elastocaloric cooling. They can compete, or even outperform larger scale elastocaloric demonstration devices found in literature. Parallelized and cascaded device architectures have proven to increase the absolute cooling capacity and the device temperature span in elastocaloric cooling devices. They pave the way to adapt the cooling characteristic of future SMA film-based cooling devices to specific applications by shifting the limits regarding ,( ) \*\*+ and -.

## **8 Conclusion and Outlook**

### **8.1 Conclusion**

The need for new innovative cooling technologies that avoid critical substances and provide efficient cooling is high already and will even increase in the future. Entire industries like the medical sector or the food industry depend on reliable large-scale cooling. Today, this cooling is largely provided by vapor-compression technology that has been used for a century and depends on potentially harmful refrigerants. On the other hand, ongoing miniaturization and thus increased heat flux densities in electronics that are the base for today's technological trends and raise the demand for new efficient cooling and temperature control technologies on the small-scale. Elastocaloric cooling is a new, completely solid-state based technology. It has the potential to provide efficient and environmentally friendly cooling on both small- and large-scale.

The elastocaloric effect describes the reversible heat release and absorption in certain materials due to the application and release of a uniaxial stress field. Today, superelastic SMAs are the most promising elastocaloric materials. In SMAs the latent heat of the solid-state phase transformation between parent austenite phase and a stress-induced martensite phase is utilized for cooling. Upon loading and stress-induced forward transformation, the latent heat of transformation is released resulting in a temperature increase of the SMA under adiabatic conditions. The reverse transformation is related to the absorption of the latent heat that results in the temperature decrease of the SMA specimen under adiabatic conditions. Elastocaloric cooling does not depend on environmentally or climatically harmful refrigerants, and in addition, high efficiencies are possible. However, the elastocaloric cooling technology is still in an early stage and many issues remain unsolved today.

Among first elastocaloric cooling demonstrators, small-scale SMA film-based elastocaloric cooling devices have shown promising results in previous work, with device temperature spans up to 9.4 °C and specific cooling capacities up to 6.4 W/g. The use of film geometries is especially promising to elastocaloric cooling, since the high surface-to-volume ratio allows for efficient and fast heat transfer. The aim of this work is to further investigate SMA film-based elastocaloric cooling and to develop concepts to further improve their cooling performance, especially with respect to absolute cooling capacity and device temperature span.

The ideal elastocaloric material combines a large elastocaloric effect size (latent heat of the transformation ,+, isothermal entropy change -.\*, adiabatic temperature change -) with a low work input and therefore high efficiency  and a high fatigue resistance. Two different sorts of SMA films are used in this work, cold-rolled NiTiFe foils and sputtered TiNiCuCo films. The TiNiCuCo films are especially promising for elastocaloric cooling due to their ultra-low fatigue properties. A combination of a tailored microstructure with small grain size and coherent fine precipitates as well as supercompatibility between the austenite and martensite lattice allow TiNiCuCo tensile samples to withstand more than 107 load cycles without signs of fatigue. Three alloy compositions of TiNiCuCo films are used with slightly altered cobalt-content. Thus, austenite finish temperatures of the three alloys are shifted between 1.7 °C and 13.3 °C. The TiNiCuCo films reach a maximum of 12.2 °C upon loading and -14.5 °C upon unloading. The adiabatic cycle related  of the TiNiCuCo films reaches high maximum values of 12. However, if quasi-directly measured ,+ |r from differential scanning calorimetry (DSC) measurements and the isothermal work input are used as basis for the  calculation, extraordinarily high values up to 72 are obtained. Nevertheless, this value represents rather a theoretical limit, and will not be reached in a real thermodynamic cycle.

The NiTiFe foils used in this work reach of 20 °C for loading and -16 °C for unloading. The materials coefficient of performance  reaches values up to 13, based on values gained from quasi-direct measurements and the isothermal work input. The adiabatic loading cycle in contrast leads to a  of 3.6. The NiTiFe foils reach higher effect sizes compared to the TiNi-CuCo films, but the TiNiCuCo films have a higher elastocaloric efficiency and benefit from their superior fatigue properties. Additionally, the transformation temperatures can easily be tailored in TiNiCuCo by slight changes in the cobalt content. This is especially important for the use in advanced cascaded elastocaloric cooling devices.

An elastocaloric cooling device has to fulfill several tasks in parallel to provide useful cooling and to utilize the promising elastocaloric effect found in SMA films. First of all, the elastocaloric device has to apply a cyclic load to the SMA to trigger the phase transformation and the related elastocaloric effect. In addition, the heat released in the elastocaloric material during loading has to be transferred out of the element (hot heat flow) and the heat absorbed during unloading has to be transferred back into the element (cold heat flow). Furthermore, the resulting hot and cold heat flows have to be separated. This is very important to beneficially use elastocaloric cooling and to avoid that the elastocaloric heating equalizes the cooling. In the SMA film-based elastocaloric cooling devices developed herein these tasks are achieved by an oscillating linear motion and changing heat contact conditions of the SMA film and solid heat sink and source elements. The SMA film is brought in contact with a solid heat sink when heating occurs and with a heat source during cooling. The film geometry allows loading the SMA in out-of-plane direction. Accordingly, a single actuator can load the SMA and change the mechanical and thermal contacts. In this way, the heat sink is heated and the heat source is cooled stepwise by the elastocaloric effect of the SMA film. This results in a temperature difference between the heat sink and source.

A new experimental test setup is developed in the present work in order to characterize SMA film-based elastocaloric cooling devices. It allows the detailed investigation of operation parameters. Especially important, the operation frequencies can be varied in a wide range up to 4 Hz, being unique to elastocaloric cooling up to now. Operation frequencies of devices reported in literature are limited to frequencies below 1 Hz.

With the new experimental test setup, two elastocaloric cooling devices are characterized that are based on previous designs. In both devices, performance is improved considerably, primarily due to higher operation frequencies. In the single film device, maximum values for the device temperature span of 14 °C, absolute cooling capacity ,( ) \*\*+ of 220 mW, and specific cooling capacity '( ) \*\*+ of 19 W/g are reached at an operation frequency of 4 Hz. The maximum  of 6.7 is found at = 0.5 Hz (for the calculation, work recovery is assumed). The single film device is considered a benchmark and basis for the other devices developed in this work. The coupled device extends the concept of the single film device to include work recovery. Two SMA films are pre-strained with respect to each other and coupled to form an antagonistic pair. In this way, the mechanical power input of the device is decreased and thus efficiency is increased. The coupled device reaches its best performance at 2 Hz. A maximum of 12.6 °C, '( ) \*\*+ of 9.9 W/g, and  of 3 is achieved. The detailed characterization of the single film devices, that include besides the single film device and coupled device also the single film device with thermal load and the miniature single film device, reveal general trends in SMA film-based elastocaloric cooling devices. First conclusions can be drawn:

The findings reveal the potential of SMA film-based elastocaloric cooling, however also the limitations in and ,( ) \*\*+ single film devices are subjected to. New approaches are needed to combine the high '( ) \*\*+

characteristic to SMA film-based cooling devices with increased and ,( ) \*\*+. Hence, advanced elastocaloric cooling devices are developed in this work based on parallelization and cascading.

A parallelized device is developed to combine the elastocaloric effect of five SMA films and increase the absolute cooling capacity of the device without increasing SMA film thickness, to preserve the high surface-to-volume ratio of the films. The SMA films in the device are coupled and thereby work recovery is included. All SMA films heat a single heat sink and cool a single source. Thus, the elastocaloric effects of the films are interconnected in parallel and add up. The parallelized device reaches an absolute cooling capacity ,( ) \*\*+ of up to 0.9 W at 1 Hz, which increases the maximum achieved ,( ) \*\*+ in SMA film-based cooling devices largely by a factor of 4. At the same time, the high specific cooling capacity '( ) \*\*+ of SMA film-based devices is found in the parallelized device: 4.7 W/g is reached at 1 Hz. However, is limited to 4.4 °C in the parallelized device due to the high thermal mass of the heat sink and source. No influence of parallelization on the device efficiency  can be found. The results prove the effectiveness of the parallelized device architecture to increase the absolute cooling capacity while maintaining the high specific cooling capacity of small-scale SMA film-based elastocaloric cooling devices. No general objections are found that oppose a further upscaling of parallelized devices based on SMA films. The parallelized device reaches cooling capacities in a similar range compared to other elastocaloric cooling demonstrators found in literature. With one exception, they reach maximum values limited to a few W. In the future, massive parallelization based on the introduced concept might pave the way for planar cooling panels that allow for easy integration in the built environment. In this way, innovative environmentally friendly elastocaloric cooling could be integrated into standardized building components and thus a completely new field of application can be addressed.

To overcome the limitation of the device temperature span in single film devices, a cascaded device architecture is developed. Three SMA films are cascaded. The first two films of the cascade each pre-cool the next film. Only the third film directly absorbs heat from the heat source. In this way, all three films operate at different temperature levels, and the overall device temperature span of the cascaded device with in-plane loading is increased to a maximum of 27.3 °C. This compares to the adiabatic temperature span of the NiTiFe films for heating and cooling of 20 and -16 °C, respectively. Therefore, clearly exceeds the material's -, but not the combined for heating and cooling. Two different TiNiCuCo-films with tailored are also successfully implemented in the cascaded device with inplane loading and reach a of 17.9 °C. The use of different SMA materials with tailored will become increasingly important in the future, as in elastocaloric cooling devices is expected to increase further and might exceed the operation range of a single elastocaloric material. The presented numbers clearly show that a cascaded device architecture increases the as intended. However, it has to be noted that this comes at cost of the cooling capacity. In cascaded devices, the absolute cooling capacity ,( ) \*\*+ stays the same, regardless of the number of cascaded SMA films. In turn, '( ) \*\*+ and  decrease with the number of cascaded SMA films. As a result, the device configuration has to be fitted carefully to the application scenario. Nevertheless, the experimental results of the cascaded devices prove that cascading enables high -. High device temperature spans become feasible regardless of the used elastocaloric material and its -. This allows SMA film-based elastocaloric cooling devices to address applications that require cooling capacity at a considerable temperature difference like e.g. the case in refrigeration and temperature control below ambient temperature. The reached in the cascaded device with in-plane loading can compete to the highest values found in literature. It is only slightly lower compared to the maximum of 28.3 °C reported for elastocaloric devices in literature [31]. The highest values are also reported for a cascaded device that however relies on NiTi wires and heat transfer to a fluid. This again underlines the potential of cascaded device architectures for elastocaloric devices.

The advanced SMA film-based elastocaloric cooling devices developed in this work demonstrate the potential of elastocaloric cooling. SMA film-based devices can compete with larger scale elastocaloric demonstration devices found in literature in terms of absolute cooling capacity and device temperature span, and outperform them in terms of specific cooling capacity. The advanced cooling devices overcome the limits in ,( ) \*\*+ and to which single film devices are subjected.

### **8.2 Outlook**

Besides the promising performance in SMA film-based elastocaloric cooling devices, key issues and challenges are revealed that are closely related to future work. One issue in SMA film-based elastocaloric cooling, but also the field in general, is fatigue in the active material. So far, the ultra-low fatigue properties of TiNiCuCo films could not be transferred to the device level. A maximum of about 10000 cycles of operation until failure was achieved by TiNi-CuCo films on the device level, although cyclical stability was typically lower. This is sufficient for the characterization of devices, however has to be increased largely for application. The importance of reliability even increases in parallelized and cascaded devices that rely on the operation of several films. A detailed investigation of fatigue and failure modes of SMA films in the special case of elastocaloric cooling was out of scope of this work, but is strongly recommended for future work. Preliminary results underline the beneficial influence of smooth surfaces and edges on the reliability of SMA films on the device level.

Moreover, controlling parasitic and intended heat flows in the cooling device becomes increasingly important as the technology progresses towards application. The heat transfer coefficient of the mechanical contact between the SMA film and the heat sink and source should be increased, whereas insulation should be used to reduce parasitic heat losses. The heat transfer coefficient might be increased by improved surface quality, an additional highly conductive layers or even by the use of flexible surfaces for increased effective contact area between SMA film and heat sink and source.

On the level of advanced elastocaloric cooling devices it is promising to increase the number of parallelized and cascaded SMA films to further increase the cooling capacity and device temperature span, possibly above the combined adiabatic temperature span for heating and cooling of the used SMA. For large-scale applications, the investigation of natural rubber based elastocaloric materials as a low-cost alternative to SMA would be interesting. However, as the system complexity of parallelized and cascaded devices increases, the demands on fabrication, assembly, and alignment precision increase considerably. In addition, the recurring arrangement of the SMA films in advanced devices restricts the available construction space for compliant structures and alignment mechanisms. New fabrication and assembly concepts are needed. A modular device architecture could be a promising strategy to reduce complexity and increase repeatability and precision in parallelized and cascaded devices. In the future, a modular device architecture could also allow to combine the two concepts of parallelization and cascading in one SMA film-based cooling devices. In this way the device performance could easily be customized to specific applications without limitations of the device temperature span or cooling capacity. The modular device architecture would largely decrease the development cost for new device configurations.

So far, the elastocaloric cooling device and the test platform providing actuation are strictly separated. Operation parameters can easily be modified thereby and precise characterization is ensured. This has proven to be a successful strategy at this stage of research, yet an important next step would be the development of a stand-alone elastocaloric refrigeration device. Actuation and control would have to be integrated into the elastocaloric cooling device. This would be an important future step to demonstrate not only the theoretical potential of the technology but also to demonstrate and increase the readiness for application of SMA film-based elastocaloric cooling.

This work lays the basis for SMA film-based elastocaloric cooling with large cooling capacity and also large temperature span. It has been shown that SMA film-based elastocaloric cooling can compete with other elastocaloric cooling concepts in terms of absolute cooling capacity and temperature span, and reaches the highest specific cooling capacities. Nevertheless, SMA film-based elastocaloric cooling and elastocaloric cooling in general are still in a rather early development state. There is still a lot of research and development work needed to make elastocaloric cooling a commercially successful product. The potential however is high. Compared to other caloric effects, elastocaloric cooling has the advantage of a high effect size and a simple operation principle. The main advantage with reference to conventional technologies is the combination of environmental compatibility and high efficiency. This makes elastocaloric cooling interesting for large-scale applications where it could help avoiding harmful refrigerants in vapor compression cycle-based systems and also for small-scale applications, where it can help to increase the efficiency compared to Peltier cooling.

## **Publications**

Parts of this thesis and related work have already been published and presented by the author in scientific journals:

**F. Bruederlin**, H. Ossmer, F. Wendler, S. Miyazaki, M. Kohl, "SMA foil-based elastocaloric cooling: from material behavior to device engineering", *Journal of Physics D: Applied Physics*, 50.42 (2017) 424003

**F. Bruederlin**, L. Bumke, C. Chluba, H. Ossmer, E. Quandt, M. Kohl, "Elastocaloric cooling on the miniature scale: A review on materials and engineering of devices", *Energy Technology*, 6.8 (2018) 1588–1604

G. Ulpiani, G. Ranzi, **F. Bruederlin**, R. Paolini, F. Fiorito, S. Haddad, M. Kohl and M. Santamouris, "Elastocaloric cooling: roadmap towards successful implementation in the built environment", *AIMS Materials Science*, 6.6 (2019) 1135-1152

As conference proceedings:

**F. Bruederlin**, L. Bumke, E. Quandt, and M. Kohl, "Cascaded SMA-Film Based Elastocaloric Cooling" in *20th International Conference on Solid-State Sensors, Actuators and Microsystems Eurosensors XXXIII (TRANSDUCERS EURO-SENSORS XXXIII)*, Berlin, Jun. 2019, pp. 1467–1470, doi: 10.1109/TRANS-DUCERS.2019.8808605

**F. Bruederlin**, L. Bumke, E. Quandt, and M. Kohl, "Kaskadiertes Elastokalorisches Kühlsystem - Cascaded Elastocaloric Cooling System" in *MikroSystem-Technik Kongress 2019*, Berlin, Oct. 2019, p. 344

As conference contributions:

**F. Bruederlin**, H. Ossmer, C. Chluba, F. Wendler, E. Quandt, M. Kohl, "Mechanics and Reliability of Superelastic SMA Films for Cooling Applications", *18th International Symposium on Applied Electromagnetics and Mechanics (ISEM)*, September 3–6, 2017, Chamonix, France *(oral presentation)*

**F. Bruederlin**, L. Bumke, H. Ossmer, C. Chluba, E. Quandt, M. Kohl, "Advanced SMA Film-Based Elastocaloric Cooling Systems", *European Symposium on Martensitic Transformations (esomat)*, August 27-31, 2018, Metz, France *(poster presentation)*

**F. Bruederlin**, L. Bumke, H. Ossmer, E. Quandt, M. Kohl, "A Cascaded System of Elastocaloric Film Cooling Devices", *Thermag VIII - International Conference on Caloric Cooling*, September 16-20, 2018, Darmstadt, Germany *(oral presentation)*

**F. Brüderlin**, L. Bumke, E. Quandt, M. Kohl, "SMA-Film Based Elastocaloric Cooling for Small Scale Applications", 4th Euro Intelligent Materials, June 17- 19, 2019, Kiel, Germany *(oral presentation)*

**F. Bruederlin**, L. Bumke, E. Quandt, and M. Kohl, "Cascaded SMA-Film Based Elastocaloric Cooling", *20th International Conference on Solid-State Sensors, Actuators and Microsystems Eurosensors XXXIII (TRANSDUCERS EURO-SENSORS XXXIII)*, June 23-27, 2019, Berlin, Germany *(poster presentation)*

**F. Bruederlin**, L. Bumke, E. Quandt, and M. Kohl, "Kaskadiertes Elastokalorisches Kühlsystem - Cascaded Elastocaloric Cooling System", *MikroSystem-Technik Kongress,* October 28-30, 2019, Berlin, Germany *(oral presentation)*

## **Figures**

## **Tables**

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#### **Co-Supervised Bachelor and Master Theses**

Schriften des Instituts für Mikrostrukturtechnik am Karlsruher Institut für Technologie (KIT)

#### ISSN 1869-5183

Band 1 Georg Obermaier Research-to-Business Beziehungen: Technologietransfer durch Kommunikation von Werten (Barrieren, Erfolgsfaktoren und Strategien). ISBN 978-3-86644-448-5

Die Bände sind unter www.ksp.kit.edu als PDF frei verfügbar oder als Druckausgabe zu bestellen.

Die Bände sind unter www.ksp.kit.edu als PDF frei verfügbar oder als Druckausgabe zu bestellen.

Die Bände sind unter www.ksp.kit.edu als PDF frei verfügbar oder als Druckausgabe zu bestellen.

Band 22 Michael Röhrig Fabrication and Analysis of Bio-Inspired Smart Surfaces. ISBN 978-3-7315-0163-3

#### Band 23 Taleieh Rajabi

Entwicklung eines mikrofluidischen Zweikammer-Chipsystems mit integrierter Sensorik für die Anwendung in der Tumorforschung. ISBN 978-3-7315-0220-3

Band 24 Frieder Märkle Laserbasierte Verfahren zur Herstellung hochdichter Peptidarrays. ISBN 978-3-7315-0222-7

Band 29 Nicole E. Steidle Micro- and Nanostructured Microfluidic Devices for Localized Protein Immobilization and Other Biomedical Applications. ISBN 978-3-7315-0297-5

Band 30 Jochen Heneka Prozessentwicklung eines industrietauglichen Verfahrens zur Fertigung von vereinzelten LIGA-Mikrobauteilen. ISBN 978-3-7315-0326-2

Band 35 Anne Bäcker Veränderliche 3D Zellgerüstträger auf Cryogelbasis zur Kultivierung von Prostatakarzinomzellen. ISBN 978-3-7315-0676-8

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Bioinspired Superhydrophobic Nano- and Microstructured Surfaces for Drag Reduction and Optoelectronics. ISBN 978-3-7315-0816-8

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### FLORIAN BRÜDERLIN Advanced Elastocaloric Cooling Devices Based on Shape Memory Alloy Films

Technological progress and climate warming pose a tremendous need for innovative cooling technologies that are both environmentally friendly and efficient. Elastocaloric cooling is a new emerging solid-state cooling technology with the potential to meet this need. Currently, superelastic shape memory alloys (SMA) are the most promising elastocaloric materials. Superelastic SMA responds with heating and cooling to a stress application and removal due to a reversible solid-to-solid phase transformation. SMA film geometries with high surface-to-volume ratio are especially promising, since they combine a high elastocaloric effect size with highly efficient heat transfer. In previous work, SMA film-based elastocaloric cooling has shown potential for small-scale applications. This work investigates the limits of SMA film-based elastocaloric cooling and develops new concepts to overcome limitations. Cascaded and parallelized devices are introduced to increase device temperature span and cooling capacity. The capability of these advanced devices is shown experimentally. The maximum cooling capacity has been increased by 4.5 to 900 mW by operating 5 SMA films in parallel, while a maximum cooling temperature span of 27.3 C has been achieved by a cascade of 3 SMA films. These results point out a route for the design of next generation SMA film-based elastocaloric cooling devices with enhanced cooling capacity and large device temperature span. They form the basis to address a variety of applications with efficient and environmentally friendly cooling technology.

ISSN 1869-5183 ISBN 978-3-7315-1065-9