Solar Water Heating System
|Figure 15. Solar collector for water heating at the PVRES home.|
The PVRES home substitutes propane for what are normally electric resistance end-uses in conventional homes. This is done to better allow the PV system to match the home's load. For instance, a simple resistance element in a hot water tank (4.5 kW) demands more electrical input than the entire PV system will be able to provide. Propane was used for the oven/range, the clothes dryer, back-up heating for hot water and a direct-vented fireplace.
The Control home contains a standard electric resistance 52 gallon storage tank (Rheem 81V5D) in the garage, rated to use 4,828 kWh/year (actual consumption in Florida should be much lower because of our higher inlet water temperature). However, propane is a fairly expensive fuel (approximately $1.40/gallon or $15/MMBtu in Florida), so we endeavored to reduce propane use by the PVRES home. An important measure was to use a solar water heating system with propane back up. Our objective was to provide 70% solar fraction with the solar water heating system at the PVRES home so that propane use is confined mainly to clothes drying and cooking.
The hybrid solar water heating system was installed in the PVRES home in February by Solar Source, a Clearwater, Florida firm. The system consists of a forty square foot American Energy Technology AE-40 solar collector (shown in Figure 15) mounted on the south side of the home's roof. The collector is rated at an energy production of 45,600 Btu/day at the low temperature (95oF) rating. Parasitic pump power is avoided through the use of a 10W PV panel with an Ivan Labs SID-10 magnetic impeller pump. The collector feeds the solar primary tank, a Lochinvar FTA-082-K with an 80 gallon storage capacity. The storage system is made up of two tanks, a primary solar tank and an A.O. Smith FPSE 40 gallon back-up propane tank. The propane tank is a high efficiency direct vent model with electronic ignition. The back-up propane tank is elevated so that it thermosiphons from the primary solar tank only if the water in the solar tank is warmer than the back-up; otherwise it provides feed water to the back-up tank when hot water is drawn.
We collect data on hot water use (gallons each 15-minutes) as well as electricity use in the control home and propane consumption in the PVRES household. Over the occupied monitoring period, daily hot water use averaged 37.8 gallons per day against a daily propane consumption of only 3.2 ft3 - about 0.09 gallons per day. The installed water heater has a rated energy factor of 0.65 with the measured tap hot water temperature 130oF. Based on measured hot water use and a temperature rise from 80oF to 130oF estimates that propane consumption should be approximately 0.264 gallons/day without the solar assist. This implies a solar water heating fraction of approximately 66%.