Thermal Energy Storage: A Viable Solution for Storing Heat and Cooling Energy
Introduction to Thermal Energy Storage
TES refers to technologies that provide long-term storage of energy in the form of heat or cooling for later use. TES allows the storage of energy from renewable and waste heat sources, as well as solar thermal energy. Storing energy enhances the efficiency and flexibility of energy use. TES has become an important strategy for reducing dependence on fossil fuel energy sources.
TES Technologies
There are several technologies currently used for TES. The most common are described below:
Sensible Heat Storage
Sensible heat storage uses the heat absorbed or released as a material is heated or cooled. The most widely used storage media are water, phase change materials, and molten salts. Sensible heat storage is a straightforward and affordable option. However, it has a lower energy density compared to other options as the storage material must be heated or cooled over a sizable temperature range to store or release significant amounts of energy.
Latent Heat Storage
Latent heat storage, also known as phase change material (PCM) thermal storage, takes advantage of the heat released or absorbed during phase transitions from solid to liquid or liquid to gas. PCMs store 5-14 times more energy per unit volume compared to sensible heat methods. Common PCMs include paraffin waxes, salt hydrates, and some organic materials. Latent heat storage has a higher energy density but is more expensive than sensible methods.
Thermochemical Storage
Thermochemical storage relies on reversible chemical reactions that store thermal energy in the form of chemical bonds. The storage density is potentially very high since energy is stored at the molecular level. Candidate thermochemical reactions include calcium oxide hydration and dehydration reactions. However, thermochemical TES systems have low reaction rates and require further research and development.
Applications of TES
TES technologies have a wide range of applications that support energy efficiency, renewable energy use, and demand-side management. Some major uses of TES include:
Solar Thermal Power Generation
TES allows solar thermal power plants to continue generating electricity for several hours after sunset by using stored thermal energy from concentrating solar power collectors or solar ponds. TES improves the capacity factor and reduces the cost of solar thermal electricity.
District Heating and Cooling
Centralized TES systems can store large amounts of thermal energy from cogeneration plants, waste heat recovery, or geothermal sources. This stored thermal energy is then used to supply heating and cooling to residential and commercial buildings via an insulated pipe network. Thermal energy storage improves overall system efficiencies for district energy schemes.
Building Heating and Cooling
At the building-scale, TES integrated with solar thermal collectors, geoexchange systems, or waste heat recovery can provide space heating or process heat. TES enables shifting of building heating and cooling loads to off-peak hours with lower utility rates. PCM wallboards that undergo melting and solidification are an emerging TES technology for residential and small commercial buildings.
Industrial Processes
Many manufacturing processes require large amounts of thermal energy for operations such as pasteurization, drying, and sterilization. TES offers benefits for load management and utilizing low-grade waste heat in these industrial applications.
Enhanced Geothermal Systems
For geothermal reservoirs with insufficient temperatures, TES techniques can store recovered geothermal fluid at a high temperature to be reinjected later and used to produce electricity during peak demand periods. This increases the capacity of lower-grade reservoirs.
Challenges and Future Outlook
While TES technologies provide many benefits, further research and development is still needed to address challenges that have limited more widespread commercialization:
- High initial capital costs compared to conventional energy storage: System designs and materials need optimization to reduce costs.
- Limitations of storage duration: Technologies that can store thermal energy economically for periods longer than diurnal or seasonal variations are required.
- Integration with renewable heat sources and existing infrastructure: TES systems must be easily integrated into solar thermal, geothermal and waste heat recovery schemes on both large and small scales.
- Durability and lifetimes: TES materials and systems must reliably function for 15-25 years with minimal degradation.
- Robust performance data: More field testing and performance monitoring data from operating projects is required for industry confidence.
In Summary, as fossil fuel prices rise and renewable energy targets increase globally, the incentives for developing cost-effective TES will also rise. With overcoming key technical barriers, thermal energy storage has tremendous potential to realize the full benefits of waste heat recovery and renewable heat sources like solar thermal and geothermal which offer renewable alternatives to reduce carbon emissions from thermal applications in various sectors of the economy.
