Thermal Storage
Unlike wind, solar, and geothermal power, thermal storage itself is not a method for electricity generation. Rather, it allows heat energy to be stored and used at a later time.
Thermal energy storage has long been employed as a heating and thermal comfort solution in the built environment.
Adobe, for example, is a common building material in regions that experience significant differences between daytime and nighttime temperatures because it absorbs heat and 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.
While simple examples, these principles are employed to this day, alongside more modern innovations.
How It Works
Thermal energy can be stored in a wide variety of materials, including water, sand, molten salt, rocks, masonry, and soil. 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.
Presently, there are three different thermal storage methods:
SENSIBLE HEAT STORAGE
Sensible heat refers to the change in temperature of an object. In its most basic form, the material is directly heated (generally by radiant heat from the sun), which causes a temperature increase. When the surrounding environment cools, the heat is released from the material into the surrounding environment. Sensible heat storage uses materials that can absorb large amounts of heat, such as water, molten salt, rocks, sand, and concrete. These materials can store thermal energy for extended periods of time, ranging from several hours to several months, depending on the material and storage conditions. Most existing thermal energy storage systems use sensible heat.
LATENT HEAT STORAGE
Latent heat or phase change thermal storage uses materials that undergo phase changes, such as solid to liquid or vice versa. When undergoing a phase change, a material does not change temperature; rather, the absorbed or released energy goes into transitioning the material between states. The ability to store large amounts of energy without large temperature differentials is a major benefit of latent heat storage.
There are numerous phase change materials (PCMs) being considered for use in thermal storage operations including salts, polymers, metal alloys, and waxes. A low-tech example of latent heat storage would be ice storage air conditioning, in which ice is produced in the overnight hours, when electricity prices are lower, and then used during the day to provide cooling to a building.
THERMOCHEMICAL STORAGE
Thermochemical storage involves chemicals that undergo reversible reactions when heated, which allows energy to be stored and released. It is more complex than sensible and latent heat storage but can have the highest storage capacity. Thermochemical storage is largely still in the research and development phase.
Types of Thermal Storage
PASSIVE SOLAR DESIGN
Passive solar design employs building orientation, windows, 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 to ensure year-round thermal comfort. Concrete, brick, and stone are common forms of thermal storage in passive solar buildings because they double as structural components. Large, water-filled containers can be used to increase thermal mass and enable even more potential storage, but buildings must be designed to support their weight.
SOLAR WATER HEATERS
Solar water heaters provide domestic hot water to buildings by absorbing heat from the sun and transferring it into the building’s water supply. Solar water heaters use collectors, often mounted on roofs, to conduct solar energy into the water supply. There are active and passive systems, based on whether the heater uses circulating pumps. The water is stored in insulated tanks for use on demand.
PEAK SHAVING WITH THERMAL ENERGY
Peak shaving is a method of reducing energy costs by using energy at off-peak times, which have lower prices for energy. Although peak shaving doesn’t reduce energy use, avoiding times with high demand has a significant impact on energy costs. A common method of peak shaving is freezing ice in large, refrigerated tanks overnight, during low demand. During the day, the ice is used to provide cooling to the building, reducing the amount of energy used during peak times.
LONG-TERM THERMAL STORAGE
Long-term, or seasonal, thermal energy storage requires a more complex set-up and can serve single buildings or larger districts. A major type of seasonal storage is underground thermal energy storage (UTES), including storage in aquifers, boreholes, and caverns.
Although long-term UTES holds promise, there are several concerns that could limit widespread adoption. Potential harm includes the risk of groundwater contamination and ecological disruption due to changes in the temperature and flow of groundwater. Many European countries are heavily involved in the use and development of long-term thermal energy storage systems.
Advantages
High Performance
Thermal energy storage systems improve the energy efficiency of buildings, decreasing costs associated with heating and cooling and reducing carbon emissions.
Financial Returns
Thermal storage can be used to reduce energy consumption, and the savings can pay back the costs of establishing storage systems.
Things to Consider
Solar Reliability
Many systems rely specifically on the energy available from the sun, and performance can be impacted by weather, time of year, and geography.
Heat Loss
Especially in longer-term (days to months) storage applications, unintentional heat loss can reduce system capacity and efficiency. Proper design and insulation of storage media can decrease unintentional heat loss.
Case Study
First Tee – Pittsburgh Arnold Palmer Learning Center:
The 13,000 square-foot First Tee Pittsburgh Arnold Palmer Learning Center project included a large focus on building a people centered, sustainable hub for the golf-based community development organization First Tee. After a yearlong sun study tracking the different angles of sunlight during the summer and winter, the building’s slanted roof and windows were designed to maximize passive solar design. The windows are arranged to ensure that the sun will reach and heat the concrete floors based on the angle it will hold relative to the building for the winter months, but the roof will largely shade the windows throughout the summer.
