s.ky blue information

The Solar Decathlon is a project of the U.S. Department of Energy (DOE) in partnership with DOE's National Renewable Energy Laboratory.

Hot Water

Hot Water: 77.700 out of 100 possible points


The Hot Water contest demonstrates that the water heating system can supply all the hot water that households use daily for washing and bathing. Teams score points by successfully completing several daily 15-gallon "hot water draws." The goal during these tests is to deliver 15 gallons of hot water (110°F/43.3°C) in 10 minutes or less.

s.ky blue Hot Water


To insure the hot water usage needs of the occupants are met most efficiently, evacuated tube solar collectors are the primary heat source. When the sun warms the evacuated tubes, refrigerant within the tubes evaporates and rises to the top. A water glycol solution is circulated over the top of the tubes to harvest the energy and carry it to flat plate heat exchangers that transfer the heat to storage tanks.

The primary hot water source consists of two panels containing 30 evacuated tubes each. The panels can be operated in series or in parallel to optimize energy collection and storage. High quality (high temperature) heat is stored in heat tank one — a 120-gallon insulated tank containing potable hot water. Heat tank one is maintained at the supply hot water temperature for the house and replaces the traditional hot water heater. Lower quality heat is stored in heat tank two — a 500-gallon insulated bladder tank containing non-potable water for thermal mass.

A water-to-water heat pump which can transfer energy from heat tank two to heat tank one is used to augment the solar collectors for periods of low solar production. An air-to-water heat pump can move heat from the outdoor air into heat tank two in periods of low solar collection. Heat in heat tank two can also be moved into the radiant floor system in periods of high solar production and cold weather.

As a last resort, an electric resistance element in heat tank one can be used for water heating if conditions prevent harvesting energy with the solar thermal collectors or the heat pumps.


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