Earlier this month, I attended at the American Society of Mechanical Engineers (ASME) Energy Sustainability Conference in Washington, DC. During the conference, I had the opportunity to speak with Dr. Paul Denholm, a senior analyst at the U.S
Earlier this month, I attended at the American Society of Mechanical Engineers (ASME) Energy Sustainability Conference in Washington, DC. During the conference, I had the opportunity to speak with Dr. Paul Denholm, a senior analyst at the U.S. National Renewable Energy Laboratory (NREL) in Golden, Colorado. Dr. Denholm is a member of the Energy Forecasting and Modeling Group in the Strategic Energy Analysis Center at NREL. He works on figuring out how we can best model electric power systems, including interactions between renewable and conventional energy technologies.
I caught up with Dr. Denholm after he gave a talk on his paper titled “Enabling Technologies for High Penetration of Wind and Solar Energy.”
MCL: Thanks again for taking the time to chat a bit about integrating renewables into the U.S. grid. To begin - would you define the terms “intermittency” and “variable” for us?
PD: Sure. Intermittency is a term that people often apply to wind and solar generation because it is variable by nature – the wind doesn’t always blow and the sun doesn’t always shine. But, actually, more and more people are now using the term variable.
A lot of people, especially in the wind community, find the term intermittent is often used as a negative - almost to attack wind. They believe that the term “variable” is a more technically accurate term. And the reason is, that people use the term intermittent to imply that the wind will just suddenly stop or suddenly start. But, wind will actually slowly slow down and start up. It will very rarely just suddenly change from zero to sixty in a second or two. So, some believe that “variable” is a better term to describe the resource
MCL: Why is this variability a problem as we try to increase the amount of renewable power that we have on our grid?
Variable resources are ones that vary depending on the weather conditions. So, with solar, that’s pretty obvious. If you have a solar generator, it will generate electricity when the sun is shining and it won’t generate at night. Wind will only generate during periods when it’s windy. And, the fact that the wind speed changes means that the output from a wind generator will vary quite a bit over the course of hours and days. That variation of output is very unlike traditional fossil fuel generators where utilities can completely control the output and rely on that generator to be available pretty much all the time.
MCL: Could you also describe what “flexibility” means for our current generation fleet?
PD: Right now, the demand for electricity varies a large amount from hour to hour. In the middle of the night, around 3 to 4 AM - when people are asleep - there is not a lot of demand for electricity. There’s minimum point where utilities kinda turn back all of their power plants and just meet this base load. As people start to turn on lights and go about their day, their demand for electricity increases. And, depending on the season, their demand might peak at about 4pm. In fact, a lot of electricity systems will be using twice as much electricity in the afternoon compared to the middle of the night.
So, the flexibility is the ability of the utility to increase the output of their generation fleet to meet this demand in the middle of the day. The challenge, when you bring in renewables, is that non-renewable generators will have to be able to step on the gas pedal harder to ramp up generation to balance variable renewable power.
MCL: Is our current generation fleet flexible or inflexible?
PD: The flexibility of a power system is dependent on the types of generation technology it includes. So, the best example is - a nuclear power plant is not very flexible. It has a limited ability to rapidly change its output. On the other hand, places where they have a lot of [natural] gas or, even better, a lot of hydro would be very flexible. So, the ability to include lots of wind and solar in the system is largely dependent on what the remaining generators are.
One of the things that we’ve done in the past to understand flexibility is to do wind integrations studies. Here, we take the generators that exist now and put them into a computer model that basically simulates the hour-to-hour operation of the grid for a given year. So, you can take today’s grid and stick a bunch of wind on it to see if the generators that exist now – do they respond to the increased need of flexibility.
MCL: And what did these studies find in terms of the grid’s flexibility?
PD: Both of these studies looked at up to 30% wind penetration – where 30% of annual electricity demand is supplied by wind - and both found that the existing generation fleet is flexible enough to accommodate that level of penetration.
Now, they had to do some changes to how the system is operated – including greater cooperation between utilities and faster communication so they can share resources and new ways of sharing information across large areas. But, the generation fleet is flexible enough to have up to this 30% wind level. Is it flexible to meet more than that? We don’t know. It’s certainly pushing the limits of flexibility.
MCL: Now, this 30% - that’s above the 10-20% limit that people talk about being a realistic limit without storage.
PD: Yeah, it’s really interesting. If you look through the papers that have been published, and columns, and newspaper articles there’s this magical 10-20%. The problem, quite frankly, is that thos