Thermal Storage

Thermal energy storage (TES) has become a critical aspect of any concentrating solar power (CSP) system deployed today. Here you will learn about thermal storage for CSP systems and the thermal storage technologies in use today.

One challenge facing the widespread use of solar energy is the reduced or curtailed energy production when the sun sets or is blocked by clouds. Thermal energy storage provides a workable solution to this challenge. In a CSP system, the sun's rays are reflected onto a receiver, creating heat that is then used to generate electricity. If the receiver contains oil or molten salt as the heat-transfer medium, then the thermal energy can be stored for later use. This allows CSP systems to be a cost-competitive option for providing clean, renewable energy. Presently, steam-based receivers cannot store thermal energy for later use. Thermal storage research in the United States and Europe seeks to develop such capabilities.

Storage Systems

Several TES technologies have been tested and implemented since 1985. These include the two-tank direct system, two-tank indirect system, and single-tank thermocline system.

Two-Tank Direct System

Photo of two large, gray storage system tanks at a power tower plant. The base of the tower can be seen in the background. Pipes leading into and out of the tanks carry heat-transfer fluid.

Two-tank direct molten-salt thermal energy storage system at the Solar Two power plant.
Credit: National Renewable Energy Laboratory

Solar thermal energy in this system is stored in the same fluid used to collect it. The fluid is stored in two tanks—one at high temperature and the other at low temperature. Fluid from the low-temperature tank flows through the solar collector or receiver, where solar energy heats it to the high temperature and it then flows back to the high-temperature tank for storage. Fluid from the high-temperature tank flows through a heat exchanger, where it generates steam for electricity production. The fluid exits the heat exchanger at the low temperature and returns to the low-temperature tank. Two-tank direct storage was used in early parabolic trough power plants (Solar Electric Generating Station I) and at the Solar Two power tower in California. The trough plants used mineral oil as the heat-transfer and storage fluid; Solar Two used molten salt.

Two-Tank Indirect System

Illustration of two large storage tanks, one holding hot fluid and the other holding cold fluid. Heat exchangers are seen between the two tanks, and the molten-salt pumps are seen inserted into the top of the tanks.

Two-tank indirect thermal energy storage system for AndaSol-1 and -2.
Credit: FLAGSOL

This system functions in the same way as the two-tank direct system, except different fluids are used as the heat-transfer and storage fluids. This system is used in plants where the heat-transfer fluid is too expensive or not suited for use as the storage fluid. The storage fluid from the low-temperature tank flows through an extra heat exchanger, where it is heated by the high-temperature heat-transfer fluid. The high-temperature storage fluid then flows back to the high-temperature storage tank. The fluid exits this heat exchanger at a low temperature and returns to the solar collector or receiver, where it is heated back to the high temperature. Storage fluid from the high-temperature tank is used to generate steam in the same manner as the two-tank direct system. The indirect system requires an extra heat exchanger, which adds cost to the system. This system will be used in many of the parabolic power plants in Spain and has also been proposed for several U.S. parabolic plants. The plants will use organic oil as the heat-transfer fluid and molten salt as the storage fluid.

Single-Tank Thermocline System

Illustration of a single-tank thermocline with a solid medium filling the tank. The top of the tank is shown as red to illustrate high temperature; the bottom of the tank is shown as blue to illustrate low temperature. The middle of the tank is shaded from red to blue to illustrate the temperature gradient, or thermocline. Arrows show that hot fluid can flow into or out of the top of the tank; cold fluid can flow into or out of the bottom of the tank.

Single-tank thermocline thermal energy storage system.

This system stores thermal energy in a solid medium—most commonly silica sand—located in a single tank. At any time during operation, a portion of the medium is at high temperature and a portion is at low temperature. The hot- and cold-temperature regions are separated by a temperature gradient or thermocline. High-temperature heat-transfer fluid flows into the top of the thermocline and exits the bottom at low temperature. This process moves the thermocline downward and adds thermal energy to the system for storage. Reversing the flow moves the thermocline upward and removes thermal energy from the system to generate steam and electricity. Buoyancy effects create thermal stratification of the fluid within the tank, which helps to stabilize and maintain the thermocline.

Using a solid storage medium and only needing one tank reduces the cost of this system relative to the two-tank systems. This system was demonstrated at the Solar One power tower, where steam was used as the heat-transfer fluid and mineral oil was used as the storage fluid.

For more detailed descriptions of thermal energy storage for parabolic trough systems, visit TroughNet.

Advanced Heat-Transfer Fluids

The DOE is also researching advanced heat-transfer fluids and novel thermal-storage concepts. The goal is to increase efficiency and reduce costs for thermal energy storage. Our work identifies and characterizes novel fluids that possess physical and chemical properties needed to improve thermal storage. This work also identifies novel thermal storage concepts that may offer improved performance and lower costs relative to the current thermal storage systems. This research is applied to all CSP technologies.

Incorporating TES into CSP power plants allows utilities to enhance dispatchability. As research in TES technologies allows for longer storage periods and lower costs, more utilities may consider CSP as a viable addition to power plants that depend solely on fossil fuels.

Phase-Change Materials

Phase-change materials (PCMs) allow large amounts of energy to be stored in relatively small volumes, resulting in some of the lowest costs for storage media of any storage concept. Initially, PCMs were considered for use with parabolic trough plants that used a synthetic heat-transfer fluid designed to withstand high temperatures in the solar field. In this approach, thermal energy is transferred to a series of cascading heat exchangers containing PCMs that melt at slightly different temperatures. To discharge the storage, the flow of heat-transfer fluid is reversed, thus reheating the fluid. Testing proved this system to be technically feasible. However, further development of this concept is hindered by the complexity of the system, the thermodynamic penalty of going from sensible to latent and back to sensible heat, and uncertainty regarding the lifetime of PCMs.

Phase-change thermal storage is now being considered for application with direct steam generation in the parabolic trough solar field. This approach allows a better thermodynamic match between the phase-change material and the phase change of steam used in the solar field. Here, a single PCM can be used to preheat, boil, and superheat steam. The cost of such a system is driven by the cost of phase-change storage material, but also by the rate at which energy is charged or discharged from the material.

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