To develop a system for heating water and air through solar and wind power (Research Project No. 1), it is necessary to have an idea of what one’s local resources are. Luckily, we have baseline data from the National Renewable Energy Laboratory. This laboratory, under the Department of Energy, has national resource maps for solar, wind, geothermal, marine, and other renewable resources. First, we’ll look at our local resources. In the next post, we’ll delve into site-specific questions that will have to be addressed as we obtain our permanent location.
Most places in the lower forty-eight states receives over 4.0 kWh/m²/day. In southern Wisconsin, this figure is 4.0-4.5 kWh/m²/day, but this doesn’t give us the whole story. We should also look to the highest and lowest resource months: December and July.
Where we are located in Wisconsin gets about 5.5-6.0 kWh/m²/day in the summer and 2.5-3.0 kWh/m²/day through the winter. Now, a kilowatt hour is equal to 3412 BTUs, meaning it takes a kilowatt to raise 3412 lb of water 1°F. In a perfect system, 1 m of solar collector in my part of Wisconsin in the summer would put out between 18,800 and 20,500 BTUs per day but that solar collector does not exist. Ramlow and Nusz (2010, 147) note that most flat-panel collectors up north get between 700 and 1000 BTUs/ft² in the summer and between 700 and 800 BTUs/ft² in northern winters (7535-10,764 and 7353-8611 BTUs/m²), which is about 50 percent efficiency.
To put this in perspective, the average hot water use of a household in the US is about 43,800 BTUs, thus 40-50 ft² (3.7-4.6 m²) would cover that need. Because we’ll be heating the house as well, this system will have to have much larger (or more efficient) collectors. If we use the typical BTUs to heat our house throughout the year, then we’d need a collector array about three times this water-only number.
Ramlow, Bob, and Benajmin Nusz. 2010. Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems. Rev. ed. Gabriola Island, Canada: New Society Publishers.