Thermal energy storage : a promising technology

1. Overview of thermal energy storage technology
2. Application of thermal energy storage
   1. Liquid
   2. Solids
3. The challenge of thermal energy storage
4. Thermochemical energy storage
5. Summary

The widespread use of intermittent renewable energy sources such as photovoltaics and wind power has increased the need for energy storage technologies to accommodate daily excess generation and peak loads. Photovoltaic industry has become the important part in electric power industry. You can read our top 10 photovoltaic cell manufacturers in China article to get the knowledge. 

Thermal energy storage applications include backup and response services (1-100kW), transmission and distribution support grid (100kW-10MW), and bulk power management (10MW-1GW). Commercial concentrated solar energy (CSP) using sensible thermal energy storage has been demonstrated to provide over 10 hours (~1GWh) of power capacity of 100 MW for grid support and batch power management.

Overview of thermal energy storage technology

Proper storage relies on temperature differences within the storage medium in order to do useful work, such as heating water with hot molten salt and producing steam to spin turbines to generate electricity. Potential storage involves storing heat in phase change materials, exploiting the large latent heat of the phase change.

For example, when a solid is isothermally melted into a liquid, this requires heat, and the liquid is subsequently frozen into a solid, isothermally releasing the heat. Thermal energy storage utilizes reactive storage media to reversibly convert heat into chemical bonds. When energy is required, the reaction combines the reactants, releasing energy.

Application of thermal energy storage

Currently implemented high temperature sensible heat storage power generation systems use liquids such as molten salt and solids such as concrete, rock.

Liquid

Molten nitrate (60% sodium nitrate, 40% potassium nitrate) is being used in commercial CSP plants around the world to provide kWh of thermal energy storage. It has a very low vapor pressure, so at typical storage temperatures up to ~600°C, it will not be pressurized and it can be pumped from one location to another.

Hot molten salt (~565°C) flows into the thermal storage tank. When needed, molten salt is pumped from the thermal storage tank to the heat exchanger, where it heats the water and produces steam that spins the turbine/generator to generate electricity. The cooled molten salt (~300°C) is pumped to a refrigerated tank and then returned to the receiver, where it is heated when the sun hits it. CSP plants can operate with large capacity factors and provide dispatchable energy.

Solids

Solid heat storage has been used in several commercial and demonstration facilities. Norway-based EnergyNest has developed a concrete-based thermal energy storage system consisting of a set of modular pipes filled with concrete and steel pipes.

These pipes carry a heat transfer fluid that can heat the concrete when charging and extract heat from the concrete when discharging to power a turbine/generator or provide process heating. The system can be charged and discharged in ~30 minutes, and for large systems, the stored energy can last for several days with less than 2% heat loss per 24 hours.

The challenge of thermal energy storage

Thermal energy storage materials have low energy density and require a large amount of materials for large-capacity energy storage, which increases the overall storage cost. Additionally, some power cycles that employ recycling to improve thermoelectric efficiency require relatively low temperature differentials between the hot and cold states of the storage material.

For example, the Brayton cycle of supercritical carbon dioxide recompression requires only a ~200°C temperature increase in a primary heat exchanger. As a result, the required inventory of stored materials must be increased to provide the same amount of energy for lower temperature differentials, which increases costs.

Molten salt freezes at >200°C, which requires expensive micro-heating to keep all components well above freezing. If the salt freezes, the flow may be blocked and must be thawed before operation can begin. The pressure inside the large storage tanks also caused problems at the CSP plant.

Thermal gradients at the bottom of the tank create thermomechanical stresses that damage the tank structure. Proper consideration of thermomechanical stresses is key to the design of large thermal storage tanks.

Like other solid-based thermal storage technologies, inexpensive pellet storage can accommodate increasing renewables by storing heat when demand for electricity is low, then storing heat when demand and prices are high, and then using the stored heat to generate electricity. penetration. This time-shifting of energy production and use can increase the flexibility of traditional baseload power plants, including nuclear and geothermal power plants.

Solid storage media are inert, inexpensive, non-corrosive, and easy to handle. In addition, many solid materials have a wider operating temperature range than molten salts. Rock, sand and sintered bauxite have all been used in thermal storage systems that can operate at temperatures >1000°C.

Large quantities of bulk solid materials can also provide self-insulation from cooler ambient environments. As the bulk storage tank volume increases, its surface area to volume ratio decreases, reducing heat loss. Thus, large storage tanks or containment systems can yield performance benefits and economies of scale.

Thermochemical energy storage

Thermochemical energy storage can provide large-scale energy storage at a lower cost than current electrochemical storage technologies. TCES materials can theoretically store thermal energy indefinitely in the form of chemical bonds. This can allow for long-term, timely storage.

In addition, some mixed metal oxides are used in thermochemical water [32,33] to produce hydrogen, which can be used to power or transport fuel cells, thus representing another potential form of “energy storage”. Some low temperature cycling materials can be improved for thermal storage in hybrid photovoltaic/thermal systems. Another energy storage which is called flywheel energy storage is also very popular nowadays.

Summary

Thermal energy storage, including sensible, latent, and thermochemical energy storage technologies, is a viable alternative to batteries and pumped hydropower for large-capacity, long-duration energy storage. Rational energy storage techniques include the use of liquid molten salts stored at nearly 600°C in large insulated tanks, which can heat the working fluid in a heater to generate electricity when needed.

Sensitive energy storage in solid media has also been demonstrated in large graphite blocks, concrete, rocks and sand-like particles. The benefit of solid media is a larger temperature range (from sub-freezing to greater than 1000°C) relative to molten nitrate salts.

Latent energy storage uses phase-change materials that convert solids into liquids, providing additional energy storage capabilities through the latent heat of nuclear fusion. Low temperature energy storage employs a latent phase change from gas to liquid.

Thermochemical energy storage uses reactive materials that use the heat of reaction to store energy in chemical bonds. The benefit of thermochemical storage is that the reactants can be stored for a short period of time with minimal energy loss.

When desired, the reaction can be reversed, releasing the heat of reaction. Phase change materials and thermochemical storage materials are not as mature as sensible heat storage materials and are key areas for future research.

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