Unlike wind, solar, and geothermal power, thermal storage itself is not a method for electricity generation. Rather, as its name implies, it allows heat energy to be stored and used at a later time. Heat is considered a low-grade form of energy – while less useful than other forms, thermal storage allows it to be captured and used more efficiently.
There are three broad categories of thermal energy storage systems. The first, sensible heat storage, is centered around materials with a high thermal mass. These can absorb large amounts of heat and, since they change temperature slowly, can hold it for extended periods of time. Masonry, water, and soil all have high thermal mass. These media allow heat to be stored for hours, days, or even months depending on the particular technology and its scale. A second type of thermal storage involves phase change materials and the energy absorbed or given off as a material changes, for example, from solid to liquid or vice versa. Last, heat can be stored and released through chemical reactions in what is called thermo-chemical storage. The term “thermal storage” encompasses a wide variety of technologies, but most currently fall under sensible heat storage.
Thermal mass has been employed in buildings for a long time. Adobe is a common building material in regions with a large diurnal range—a wide difference between daytime and nighttime temperatures—because it helps keep indoor areas at a stable, comfortable temperature. Likewise, the practice of earth sheltering, using a layer of soil against or on top of a building, takes advantage of the high capacity of the ground to absorb and give off heat while maintaining an overall stable temperature. These same concepts make thermal mass a natural part of passive solar design today.
How It Works
Thermal energy can be stored in a wide variety of materials, including water, sand, molten salt, rocks, masonry, soil, and liquefied air or nitrogen. Materials chosen and the degree of control over heat exchange vary depending on the design of the system and length of time it is meant to store heat.
Short-term thermal storage is easier to achieve than long-term. The following examples demonstrate the wide variety of ways it can be employed:
- Passive solar design combines building orientation, window glazing, shading, thermal chimneys, thermal mass, and other strategies to drastically reduce mechanical heating and cooling loads. Proper design can achieve a balance between heating and cooling needs in order to ensure year-round thermal comfort. Concrete, brick, and stone are common forms of thermal storage in passive solar homes because they double as structural components. Large, water-filled containers can be used to increase thermal mass more than masonry, but buildings must be designed to support their weight. A technology currently being developed and tested is the incorporation of phase change materials into concrete and other building products to increase thermal capacity.
- Solar hot water heaters collect solar heat and transfer it to a building’s water supply to provide hot water. Since water can store large amounts of thermal energy, hot water can be available at any time of day.
- Peak shaving is the practice of storing thermal energy from off-peak times to be used during future peak times. The most common approach is to freeze water at night and then use the ice to cool a building the next day as it melts. Peak shaving does not reduce electricity demand, but does provide cheaper energy for the building and allows for higher efficiency at power plants. Since more power plants are needed during peak hours, less efficient plants then come into operation; therefore, reducing peak load can shift energy production to the most efficient plants more of the time. This practice can also delay or prevent the construction of new power plants. Duquesne University sets a local example of ice thermal storage, as described here.
- Concentrating solar power (CSP) plants may use thermal storage to be able to distribute heat over a longer (24-hour) period.
Long-term, or seasonal, thermal energy storage requires a more complex set-up and can serve single buildings or larger districts. A major subset of seasonal storage is underground thermal energy storage (UTES), including storage in aquifers, boreholes, and caverns. A shining example of innovation in heat storage is the Drake Landing Solar Community in Alberta, Canada. The 52 homes in this neighborhood get over 90% of their heating needs met using the system, which is fed from solar panels on each garage roof. The collected heat energy is transferred first to a short-term storage tank, then conveyed into the earth by means of water circulating through boreholes in a large rock mass underlying the community. The system consists of 144 boreholes, each just over 120 feet deep, and includes 24 distinct circulation paths of six boreholes each. The ground surrounding the insulated borehole system can reach about 175°F by the end of the summer from all the heat being stored for the winter.
Long-term UTES certainly holds promise, but has not been studied widely enough to fully understand its environmental impacts. Potential harms include groundwater contamination and ecological disruption due to changes in the temperature and flow of groundwater.
Cost and Installation
Strategies such as solar hot water and passive design are the most accessible for small-scale applications. Economies of scale generally make larger projects cheaper and more efficient, especially for long-term storage.
Phase change and thermo-chemical storage methods have higher heat-holding capacities than sensible heat storage, but are also less well developed and more expensive.