Introduction to Thermal Energy Storage
Thermal energy storage is one of the oldest forms of energy storage in the world, having recorded uses dating back over hundreds of years. A significant issue concerning solar power is the reduced energy production during periods of cloud cover and night. One solution to the aforementioned problem is solar energy storage. This can be achieved through the absorption of solar radiation by collectors, heating a fluid as it passes through. The energy can then be stored in a hot water tank for later use as space heating or domestic hot water. While solar thermal storage does experience some energy losses, it is notably more efficient than most other storage systems. Unlike batteries, which must convert electricity from AC to DC and back again, or pumped hydro, which uses electricity to convey water to an elevated reservoir, solar thermal storage does not require conversion from one energy source into another1.
Solar Thermal Storage in Alaska
In the United States, approximately 20% of household energy consumption accounts for water heating. Hot water production costs increases dramatically in Alaska, around 70% of household energy consumption, as colder average temperatures are much more prevalent2. To offset the use of expensive fossil fuels, an alternative method of heating, such as renewable energy, can be utilized. Using solar energy in Alaska is often thought of inimitably due to the misperception that the arctic sun does not provide adequate solar radiation. While there is little solar radiation available between November and mid February, at the Arctic Circle there are 230 more hours of possible sunlight than at the equator3.
Although there are more possible sunlight hours in Alaska, the availability during the highest demand periods is limited, ranging from 3.5 to 4.5 hours in the winter. The need for solar thermal storage in Alaska is evident and would be beneficial as availability of solar energy in the summer is between 20 and 22 hours4. One shortcoming of solar thermal storage in rural applications is the low demand during summer months. It is necessary that the fluid does not exceed a certain temperature or it will boil and cause stress on the system. This issue is typically dealt with by using dump loads5.
Collector tilt is also a concern when dealing with radiation absorption. A unique characteristic to arctic countries is the shallow angle of the sun, during winter months the elevation angle can be as low as 2.6 degrees. When the sun is lower on the horizon, radiation is less focused and has a higher atmospheric absorption level. As a result, solar collectors can be tilted at a 90º angle, which is beneficial to snow shed. However, having such an extreme tilt will likely reduce radiation absorption during the summer, when solar collection is greatest. A positive externality of Alaskan winters is enhanced collection by 15-30% from more absorbed radiation through refraction off of the snow cover6. Currently there are numerous communities throughout Alaska using solar energy combined with thermal storage for heating and domestic hot water.
Links, Resources, and Documents
- Marken, C., Puffer, D. (February & March 2008). Get into Hot Water. Home Power. Vol. 128. (p. 66-74).
- Mehalic, B. (August & September 2009). Flat-Plate & Evacuated-Tube. Home Power. Vol. 132. (p. 40-46).
- Ramlow, B., Nusz, B. (2011). Solar Water Heating. Gabriola Island, BC: New Society Publishers.
- Seifert, R. D. (June 2010). A Solar Design Manual for Alaska 4th Ed. Cooperative Extension Service, University of Alaska Fairbanks.
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