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Solar thermal hot water system: evacuated tube collectors

This week I took a closer look at the daily water requirements of the house, and the Btu and roof space requirements of the evacuated tube type collectors for a solar hot water system.

I went through the contest schedule and tabulated the three contests requiring hot water: hot water draws, running the diswasher and the clothes washer. The hot water draws are 10 gallons each, I assumed 7 gallons/cycle for the dishwasher and 25 gallons/cycle for the clothes washer, assuming overall that the appliances would be on the more efficient end of the spectrum. The daily average use amounted to 61 gallons while the maximum was 87 gallons.

Using these rough numbers I was able to calculate some approximate Btu requirements based on the fact that it takes 8.34 Btu to raise 1 gallon of water 1 degree Fahrenheit. I assumed the intake water was 52 F and that the ouput water needed to be 112 F (from the hot water draw requirement). For example, to raise 60 gallons of water from 52 F to 112 F it would take just over 30 thousand Btu.

I drew up a spreadsheet containing all of the evacuated tube collectors rated by the SRCC and compared their Btu output per square foot of panel. The panels are available in sizes from 15 square feet to over 120 square feet. The collectors with the highest Btu per square foot were the 20-30 square foot collectors, which averaged 725 Btu/square foot on a clear day, 482 on a partly cloudy day and 257 on a cloudy day.

-Priya Gandhi

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