One of the challenges facing the utilization of solar power is the reduction or reduced energy production after the sun sets or is obscured by clouds. Storage of thermal energy is an efficient solution to this problem.
In the CSP or concentrating solar power (CSP) system sun’s rays reflect off a receiver, producing heat. The heat is generated into electricity that can be utilized instantly or stored for future use. This allows CSP systems to provide a range of flexible or dispatchable for supplying sustainable, clean energy.
Various efficient thermal energy storage techniques have been evaluated and implemented since 1985. They include direct, indirect, and single-tank thermocline systems.
TWO-TANK DIRECT SYSTEM
The energy from solar thermal in this method is stored within the same fluid used to capture it. The liquid is kept in two tanks, one at a high temperature and the other at a lower temperature. The low-temperature tank’s fluid is pumped through the solar collector or receiver, where the sun’s energy heats it to a very high temperature and then flows into the high-temperature tank to be stored. The high-temperature tank’s liquid passes via a heat exchanger, which generates steam to power the electricity production. The water exits the exchanger at a lower temperature before returning to the tank with a lower temperature.
Direct storage in two tanks was utilized in the early parabolic power stations (such as Solar Electric Generating Station I) and at Solar Two. Solar Two power tower in California. The trough power plants utilized mineral oil for heat transfer and storage fluid. Solar Two used molten salt.
TWO-TANK INDIRECT SYSTEM
Two-tank indirect systems operate similarly to natural systems with two tanks, except that different fluids are utilized as storage and heat transfer fluids. This method is employed in plant environments where the heat transfer fluid is too costly or is not suitable to be used as a storage fluid.
The storage fluid in the low-temperature tank is pumped through an additional heat exchanger and heated up by high-temperature heat transfer fluid. The high-storage fluid temperatures flow back into the storage tank, which is heated to high temperatures. The liquid exits the heat exchanger with a lower temperature and then flows back towards the collector, or “solar, ” which heats it back to a much higher temperature. The stored fluid from the high-temperature tank can create steam precisely in the direct two-tank system. The indirect system needs an additional heat exchanger, which adds costs.
The system is planned to be utilized in most parabolic power stations in Spain and has been suggested for several U.S. parabolic plants. The plants will use organic oil for heat transfer and molten sodium for storage.
SINGLE-TANK THERMOCLINE SYSTEM
Single-tank thermocline systems are designed to store thermal energies in solid material, typically silica sand. It is housed in one tank. Any time in operation, a part of the media operates at a high temperature, and another portion is at a low temperature. The hot and cold temperature zones are separated via a gradient or thermocline. The heat transfer fluid at high temperatures flows into the upper part of the thermocline and then exits at low temperatures. This process pushes the thermocline down and provides thermal energy to the system to store it. In reverse, the flow pushes the thermocline up and takes heat power from the process, generating electrical energy and steam. Buoyancy causes heat stratification in the liquid inside the tank, which aids in stabilizing and maintaining the thermocline.
Using an unconstrained storage medium and only requiring one tank can reduce the cost of this system compared to two tank systems. The system could be seen at The Solar One power tower, where steam was used as a heat transfer fluid, and mineral oil served to store the liquid.