Tango III : A Space Settlement Design


"For the successful technology, reality must take precedence over public relations, for nature cannot be fooled."
Richard Feyman


It has been already said that the rationale for going into space, apart from the fact that the human race must extend its limits and explore and then conquer space, has to do with retrieving energy, mainly the Sun's energy.

About 80 % of the total energy demanded by our society is supplied from fossil fuels. 90% of the CO2 which is the major cause of the greenhouse effect comes from combustion. It is now widely accepted that the only way to reduce the environmental risks while sustaining the economic growth is to develop a large-scale alternative energy system which is ecologically benign.

A scientific venture must be pursued when it follows a certain logic and the solution is correct, even if technology for proper utilization is not current or not available. Such is this case. Although the human race would perhaps not be able in the very immediate future to exploit the untapped potential of solar energy, it is certainly a direction that must be followed. Exclusive dependence on fossil fuels will inevitably lead to energy shortages. (see Introduction)

It must be remembered that this scheme was one of the main determinants in choosing the location of the space colony. The libration points along the Earth's path were chosen primarily for their constant exposure to sunshine.

Solar Energy here on Earth

Why should we go into space to get solar energy and not profit directly from it here on Earth? The answer is twofold :

The Atmosphere : The benign atmosphere protects us from the intensity of the sun's rays, that are filtered by our gaseous cover. That same protective effect which shields us and allows life on Earth also prevent us from fully receiving the Sun's energy.

It is estimated that, in average, between 0.1 and 0.2 kW/m2 of solar energy can be received from the Sun on the Earth's surface. In near Earth space the quantity o energy that can be collected is approximately ten times as much, that is, around 1 to 2 kW/m2 in average. This first reason is obviously decisive.

The Earth's rotation : But even if extra sensitive solar panels could be engineered, there is another problematic factor that complicates full utilization of the sun's energy. The rotation of the Earth, as we very well know, gives rise to days and nights, which means that during 12 hours in average no sunlight hits the surface of our planet. Because of this, solar energy devices have to trap the heat during the night period and great pains are taken to ensure that minimum heat gets lost.

None of these problems will be met in space, where sunshine is constant and with far greater intensity.

Generating electricity

Apart from using the sun's energy to supply the Earth, the colonists would benefit from the abundance of energy for their own home processes.

Solar energy can be directly converted into electricity by means of photoelectric cells.

These cells produce an electrical voltage as long as light shines on them .

The photoelectric effect consists in the formation and liberation of electrically charged particles in matter when it is irradiated by light or other electromagnetic radiation. The term photoelectric effect designates several types of related interactions. In the external photoelectric effect, electrons are liberated from the surface of a metallic conductor by absorbing energy from light shining on the metal's surface. The effect is applied in the photoelectric cell, in which the electrons liberated from one pole of the cell, the photocathode, migrate to the other pole, the anode, under the influence of an electric field.

Solar cells are made from thin slices of crystalline silicon, gallium arsenide, or other semiconductor materials convert solar radiation directly into electricity.

As silicon, for example, is plentiful in lunar soil, the colony could manufacture solar cells for use in solar energy conversion.

Although photoelectric cells are very reliable, their efficiency is low less than 11 percent. Consequently, they must be combined into large arrays to generate sufficient power for practical applications.

Energy intensive processes

The ready availability of energy in the Space Settlement changes radically the way processes are usually engineered here on Earth. Because of the potential energy crisis, all processes are usually designed to use up as little energy as possible. Machines and processes are said to be environmentally friendly when they switch themselves off automatically in order to save energy.

In the space settlement, we must train ourselves to change our design preconceptions. Materials are scarce and costly, energy is available and cheap.

Solar Power Satellites

A possible scheme for producing power on a large scale contemplates placing giant solar modules alongside the colony where energy generated from sunlight would be converted to microwaves and beamed to antennas on earth for reconversion to electric power. On ground, the microwave power is rectified and converted to the commercial electric power.

To produce as much power as five large nuclear power plants (1 billion watts each) several square km of solar collectors, weighing more than 5 million kg would have to be assembled in the settlement. An earth-based antenna 5 miles in diameter would be required for reception. These vast assemblies are often referred to as Solar Power Satellites (SPS)

The concept of the SPS is revolutionary with a high potentiality to solve the global environmental problems, as it uses the limitless solar energy, it utilizes the space outside of the earth ecology system, and it has no by-product waste.

Even though one of its panels could never be deployed, Skylab effectively demonstrated the use of solar energy.

Engineering Aspects

Solar cell technologies in space use are well established. The efficiency of the energy conversion has been steadily improved and reaches 20% for crystalline cells. Thin-type amorphous silicon solar cell which is convenient in this case for, as we have said, silicon can be mined from the Moon would be a good candidate for the SPS use, but the efficiency remains about 10%. GaAs solar cell with a higher efficiency is not suitable for the SPS use because it is extremely expensive and the abundance of the element is quite limited.

The electric power generated in orbit is converted to a microwave beam toward the earth. 2.45 GHz microwave is a potential candidate for the near future SPS because of a high transmittance through the atmosphere and maturity of associated semiconductor technologies. The efficiency of power conversion between DC and RF is expected 70-80%. The transmitting antenna, spacetenna, will consist of phased arrays which can direct the microwave beam precisely to the ground station.

A large-scale receiving antenna, rectenna, is necessary to collect the microwave power from space.

Economics of Electricity from Space

As we have already said, Solar Power Satellites are intended to become a large scale source of electric power for the Earth. In order to achieve this objective it will be necessary for electricity produced by SPS to be competitive with electricity produced from other sources. Overall, if SPS-generated electricity remains substantially more expensive than other sources (after allowing for all costs including relative environmental impact), it will not be widely used.

The overall economics of SPS are best described by the following equation. The basic assumption underlying the equation is that existing electricity generating companies will construct, own and operate rectennas (microwave power receiving antennas) which are linked into their power distribution grids.

Potential Economic Significance of Electricity from Space

The development of microwave power from space as a major new energy source for the human race on Earth would have great economic significance for two reasons, 1) its impact on the energy situation on Earth, and 2) its impact on humans' space activities.

Significance for world energy supplies

In the rich countries of the world, the average electricity capacity is roughly 1 KW per person, and it is continuing to grow at a few % per year. Even if the rich countries improve their efficiency continuously so that the demand stops growing, this level of consumption does not seem likely to fall. According to demographic studies, the population of the world is projected to reach 10,000 million during the next century. So if all today's developing countries reach a standard of living comparable to that in the rich countries today, as they intend, the global demand for electricity is likely to require some 10,000 million KW of capacity, or about 10 times its present level. In order to reach this level humans will need to increase capacity by some 100 GW per year on average through the next 100 years.

Even if these figures are no more than fearsome prophecies, the tendency seems to be irreversible. Coal, oil, gas, nuclear fuel are all limited fossil resources, and nuclear technologies have great potential dangers associated with them.

As technology advances, solar energy will come to be used on a greater and greater scale. One potentially attractive way of using it is to beam it from space continuously using microwaves. The potential output of such a system is essentially limitless. Consequently if it were to provide a substantial part of humans' needs it would have an enormous impact on world living standards. In particular it could be of great value for today's developing countries, giving them easy access to energy resources, which are the foundation of industrialization.

Thus the development of microwave power from space as a major energy source for Earth would use some of the enormous revenue stream of the electricity industry to open access to extraterrestrial resources. In doing so it will create a genuine new frontier and definitively put an end to pessimistic talk about limits to economic growth and even about the possibility of global wars arising from disputes over the Earth's dwindling resources.

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NAS NAS contact: Al Globus