Storing Solar Energy with CSP

There are several CSP or concentrating solar power technologies.  Generally CSP refers to large solar thermal electric generating plants.  Because of the thermal nature of this type of solar plant, the heat energy can be stored much more easily and at scale than electrical energy, increasing the dispatchability and minimizing rapid changes in output for grid operators.

Central receiver is the CSP technology where a large field of sun-tracking heliostats is used to redirect and concentrate sunlight onto a tower mounted single receiver.  The heat transport fluid within the tower is either water/steam or nitrate salt (molten salt), and the latter typically includes storage. 

These nitrate salts typically consist of 60% NaNO3 and 40% KNO3 and have very high freezing temperatures (melting points), well above 250 degrees Fahrenheit, which is maintained with natural gas boilers and strategic heating conduits along the piping pathways.  If the temperatures drop below the threshold, the salts freeze solid within the pipes and tanks, which makes restarting the system somewhat difficult.  (These parasitic fossil thermal loads are minimal relative to the solar thermal heating gain.) 

One advantage of tower technology is that the distances between heat collection and extraction, i.e. the top of the tower to the bottom and out to the heat exchangers or storage units, are very short, on the order of hundreds of meters and maintaining higher temperatures is more manageable.  This storage concept can be designed to meet the needs of the utility grid and 3-16 hours of full turbine load are practical.

Parabolic trough systems use a field of linear parabolic collectors spread horizontally, rather than vertically, across hundreds of acres to concentrate sunlight onto long tube receivers.  The heat transport fluid is usually synthetic oil, which has a freezing point of around 55 degrees Fahrenheit, but which is a more reasonable minimum temperature to maintain in the system across several kilometers of piping. 

Natural gas boilers maintain a minimum temperature (plus a buffer) and keep the field circulating during the night or cloudy days.  Across the entire system there is a large volume of heat transfer fluid in the piping, and this technology has approximately 30 minutes of inherent storage in the basic design.  However, a two-tank molten salt storage system, with a heat exchanger between the fluid and the salts, can be added to achieve several hours of storage, which is the design planned for the 280-MW Solana plant in Arizona. 

Trough plants with storage increase the size of the mirror field proportionally to the storage so that one part is dedicated to daytime generation and the other to recharging the storage, which is then discharged as the sun goes down.

Dish/engine and Compact linear Fresnel reflector do not have inherent storage and are not currently considered as practical technologies to include storage, if and when they become commercially viable generators.

[Editor’s note: For a great description of a CSP plant that uses molten salt storage technology, check out Storing the Sun: Molten Salt Provides Highly Efficient Thermal Storage.]