Greg Glatzmaier, Senior Engineer/ CPS Project Leader for Thermal Energy Storage at the National Renewable Energy Laboratory (NREL) explains that working to make the thermal energy storage aspect of concentrated solar power (CSP) more attractive to utilities by lowering the cost overall is a main area of focus.

Exploring Thermal Energy Storage

Greg Glatzmaier for | marcus evans Electric Energy Storage Conference

The new electricity market presents both opportunities and challenges to utilities and their customers. Thermal storage is a proven technology that is poised to gain more ground. It comprises a number of technologies that store thermal energy in energy storage reservoirs for later use.

How has thermal energy storage developed over the years?

GG: It’s a pretty recent development. The technology that uses it is concentrating solar power (CSP) and that technology has been around for a long time. There are plants in Southern California that have been operating since the mid-1980’s. They perform very well, but none of these plants have thermal energy storage. They primarily only produce energy when the sun shines. However, most of them do have natural gas back up. They can also generate steam by burning natural gas and the utilities allow them to do that with a very small percentage of their total energy production over the course of the year. The benefit of having natural gas is that it allows these plants to generate electricity consistently during the day.

Over a period of time it was recognized that if you implemented a thermal energy storage system into these types of plants, it would address that type of issue. The way this works is that at any given time when the sun is shining, the collector field collects solar thermal energy. If the plant does not have storage, the only thing it can do is take that energy and generate electricity. If the plant has some storage energy capacity and it can take a portion of that thermal energy and put it into storage, then it can use it for generating electricity at a later time. This allows the plants to generate electricity much more consistently during the normal hours of operation. In addition to that, it allows the plant to continue generating electricity until the late afternoon and evening hours when the sun is going down. It adds consistency to the electricity that is being generated and extends the hours that it can generate electricity in.

What are the advantages of thermal energy storage above other forms?

GG: Namely, the efficiency of the storage system itself and its very large storage capacity. It’s about 92 percent efficient. When you compare that number to other forms of electricity storage, such as batteries or compressed air energy storage, their efficiencies are quite lower - more in the range of 50 to 80 percent efficiency. The other big advantage is that it has a very large storage capacity. You can store many hours of electricity production with these systems. Other areas where these plants are either operating or being constructed is southern Spain, where there are three Andasol plants and each has a 50 megawatt generating capacity. The Andasol power plants are helping to meet summer peak electricity demand in the Spanish power grid, primarily caused by air conditioning units. Each of those plants has thermal energy storage and seven-and-a-half hours’ storage at 50 megawatts. If these plants didn’t have storage, they wouldn’t be able to store electricity during the regular daylight hours. As a result you come close to doubling the period of time that they can generate electricity.

Can you outline some of the challenges facing this form of energy storage? 

GG: The main challenge in energy storage is trying to bring the cost down. The technology works, is very reliable and it’s being demonstrated now on a commercial scale. Right now the cost of electricity in US dollars, coming from these plants, is somewhere in the range of 16 cents a kilowatt-hour; it needs to be closer to 10 cents a kilowatt-hour. Improving the technology to bring the cost down is the main challenge. I think with the way the markets are going, we will have to get it down in this decade. We have just set our cost reduction goals and they are set for 2017 and 2022. We want to bring the cost down in the range of 10 cents. Photovoltiacs (PV) is our main competition and the cost of PV is dropping, so we need to do it this decade or it’s not going to happen. We want to be competitive with conventional energy production, like fossil fuel, but we also want to be competitive with PV.

What are the new advances in this area that will help it develop?

GG: Everything that we are currently doing, is to help bring down the costs. There are several general areas that we’re working on at the moment. One of these is to help bring down the cost of the fields of collectors– the reflector surfaces that actually reflect the sunlight. Those collectors have a great deal of surface area, with a significant number of materials in them and are relatively expensive. They have to have a precise shape, so they’re fairly expensive to manufacture. Generally what we’re trying to do is improve the optics of the collectors, which allows us to increase the size of the collectors.

Getting closer to thermal energy storage, we’re trying to develop higher temperature heat transfer fluids that will allow us to go to higher temperatures. That does two things: it increases the efficiency of converting the thermal energy to electricity in the turbine so we get more electricity for the amount of thermal energy that is collected. It also allows the storage system to contain more thermal energy so it increases the capacity of the storage system. Both of these items translate into bringing the cost down.

Glatzmaier will be a speaker at the marcus evans Electric Energy Storage Conference, which will take place on January 12-13 in Phoenix, AZ. He offered his insight on this expanding area.

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