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How it Works
Location: Okotoks, Alberta. 51.1 deg N, 114 deg W, 1084 m elevation |
Capturing the Solar Thermal Energy
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Solar energy captured by collectors on garage rooftops. |
An array of 800 solar panels located on garage roofs throughout the community generate 1.5 mega-watts of thermal power during a typical summer day and supply heat to the district heating system.
From sunrise to sunset, the solar panels absorb the Sun’s energy and heat a glycol solution running through an insulated piping system, or collector loop, that connects the array of collectors.
The heated glycol travels along the roof overhang, down the end of the garage, and underground through a shallow buried trench system until it arrives at a heat exchanger within the community’s Energy Centre.
The heat exchanger transfers heat to the water stored in a short-term storage tank. The glycol solution carries on through its loop back to the solar collector system.
Storing the Solar Thermal Energy
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Sample heat flow through boreholes. |
During the warmer months, the heated water is distributed from the short-term storage tank to the borehole thermal energy storage (BTES) system via a series of pipes. The pipes run through a collection of 144 holes that stretch thirty-seven meters below the ground and cover an area thirty-five metres in diameter.
As the heated water travels through the pipe-work, heat is transferred to the surrounding earth. The temperature of the earth will reach 80 degrees Celsius by the end of each summer.
To keep the heat in, the BTES is covered with sand, high-density R-40 insulation, a waterproof membrane, clay, and other landscaping materials.
The water completes its circuit of the borehole system and returns to the short-term storage tanks in the Energy Centre to be heated again and repeat the same process.
Solar Seasonal Storage and District Loop
Distributing the Solar Thermal Energy
When winter arrives and the homes require space heating, the heated water in the BTES passes to the short-term storage tank in the Energy Centre and is then circulated to the homes through the district heating loop.
Reaching each home the heated water passes through a heat exchanger within a specially designed, low-temperature air handler unit located in the basement. A fan, also within the unit, blows air across the warm fan coil. Heat is passed from water to air and then distributed throughout the house via the home’s ductwork.
When the temperature of the home’s thermostat is met, an automatic valve in the basement shuts off the heat transfer between the district heating loop and the air handler unit.
Heat Transfer throughout the System
The system only initiates heat transfer when the temperature within a preceding component rises higher than the temperature within a succeeding component. For example, as the sun rises and the solar collectors heat up, the collector loop is turned on once the glycol temperature rises above the temperature of the water in the Energy Centre’s short-term storage tanks (STTS). Energy is then transferred from the collectors to the STTS.
Similarly, after the water temperature in the STTS rises above the BTES temperature, the BTES pump is turned on to transfer heat from the STTS to the BTES.
The collectors will heat up the STTS about twice as fast as the BTES can remove heat from the STTS. Consequently the collector pump will shut off when the sun goes down while the BTES pump will run most of the night.
When the houses need heating in the wintertime the heat from the collectors will be directed from the STTS into the district heating loop, and not transferred to the BTES.
The district heating loop temperature varies with outdoor air temperature. As it gets colder outside the district heating loop temperature is raised. This temperature is regulated by the heat exchanger between the STTS and the district heating loop.
If the STTS, in conjunction with the heat provided by the collectors, is not hot enough to meet the demands of the district heating loop, then heat from the BTES is transferred to the STTS for use.
If the STTS still cannot meet the demand for heat, the Energy Centre’s back-up gas boiler will turn on to increase the temperature.
