Solar thermal systems are a promising renewable energy solution -- the sun is an abundant resource. Except when it's nighttime. Or when the sun is blocked by cloud cover. Thermal energy storage (TES) systems are high-pressure liquid storage tanks used along with a solar thermal system to allow plants to bank several hours of potential electricity. Off-peak storage is a critical component to the effectiveness of solar thermal power plants.
Three primary TES technologies have been tested since the 1980s when the first solar thermal power plants were constructed: a two-tank direct system, a two-tank indirect system and a single-tank thermocline system.
In a two-tank direct system, solar thermal energy is stored right in the same heat-transfer fluid that collected it. The fluid is divided into two tanks, one tank storing it at a low temperature and the other at a high temperature. Fluid stored in the low temperature tank runs through the power plant's solar collector where it's reheated and sent to the high temperature tank. Fluid stored at a high temperature is sent through a heat exchanger that produces steam, which is then used to produce electricity in the generator. And once it's been through the heat exchanger, the fluid then returns to the low temperature tank.
A two-tank indirect system functions basically the same as the direct system except it works with different types of heat-transfer fluids, usually those that are expensive or not intended for use as storage fluid. To overcome this, indirect systems pass low temperature fluids through an additional heat exchanger.
Unlike the two-tank systems, the single-tank thermocline system stores thermal energy as a solid, usually silica sand. Inside the single tank, parts of the solid are kept at low to high temperatures, in a temperature gradient, depending on the flow of fluid. For storage purposes, hot heat-transfer fluid flows into the top of the tank and cools as it travels downward, exiting as a low temperature liquid. To generate steam and produce electricity, the process is reversed.
Solar thermal systems that use mineral oil or molten salt as the heat-transfer medium are prime for TES, but unfortunately without further research, systems that run on water/steam aren't able to store thermal energy. Other advancements in heat-transfer fluids include research into alternative fluids, using phase-change materials and novel thermal storage concepts all in an effort to reduce storage costs and improve performance and efficiency.