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Engineering Features - Solar Thermal

Evacuated TubesOur solar thermal system uses the energy available from the sun to heat water for typical domestic applications. Solar thermal systems are classified by the means in which they transfer the energy of the sun to cold water that comes from a well or water main. Our system is classified as active and indirect. Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. Indirect systems pump heat-transfer fluids, such as a mixture of glycol and water antifreeze, through collectors. Heat exchangers transfer the heat from the heat-transfer fluid to the water stored in the tank. Active, indirect systems achieve a balance between performance and reliability in climates that routinely reach the freezing point. Our system consists of the following primary components:

  • Thermal SchematicCollectors: Evacuated tubes, manufactured by Sun Utility Network, were chosen over the more widely-used flat plate collectors due to their superior performance in cloudy, cool climates such as Washington, D.C. The most unique aspect of our system is that the tubes are placed on a flat roof surface, unlike most solar collectors, which perform better when sloped towards the South. Collector efficiency is not compromised because the absorber plates inside the individual tubes have a preset, South-facing slope. Another feature of our tubes that allow them to be installed on a flat surface is that liquid is actively pumped through the tubes. Other evacuated tubes use a heat-pipe technology that is dependant upon the thermosiphon effect for heat transfer to occur. The thermosiphon effect requires a significant collector slope to perform adequately.
  • Pump: The pump circulates the heat-transfer fluid around the collector loop. The pump is powered by electricity generated by the photovoltaic modules.
  • Storage Tank: An insulated, 80-gallon cylindrical tank with a wraparound heat exchanger maintains the temperature of a large volume of water for an extended period of time so that hot water is available even during periods of extended cloudiness. The heat transfer fluid releases its energy to the water in the tank by flowing through a heat exchanger that wraps around the outside of the tank. Since glycol is toxic, safety is ensured by preventing the glycol/water filled heat exchanger coil from making contact with the potable water in the tank.
  • Controller: In order to conserve pump energy and prevent the flow of heat from the hot storage tank to cool collectors when the sun is down, the controller turns the pump on only when the heat-transfer fluid is a specified temperature above the water in the bottom of the storage tank.
  • Electric Resistance Backup: During the winter months, the solar thermal system cannot supply all the energy needed to keep the water in the tank at the desired temperature. Therefore a horizontal electric heating element is located at the top of the tank to increase the water to the desired temperature. The electrical current in the element is generated by the photovoltaic system.

Annual Solar FractionAccording to the Department of Energy, a typical solar thermal system will reduce the need for conventional water heating by about two-thirds, minimizing the cost of electricity or fossil fuel and the environmental impacts associated with their use (DOE). According to a series of simulations (see Error! Reference source not found. for results), our system exceeds these expectations by reducing the need for conventional water heating by 100% in the summer and 82% throughout the entire year.

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