Getting ResultsΒΆ

I started seeing some positive results after only a few days of using my new solar water heater. There were some partly cloudy days, and I had some minor glitches with the pump, but by logging the tank and collector temperature I could see it was definitely working.

  • July 4: The storage tank started the day at 65F, and had climbed up to 80F by the evening. 15 degrees doesn’t seem like much, but when you consider there are 1200 pounds of water to heat up, that equates to about 18,000 BTU, or about 5 KWH of electricity.
  • July 5: The tank temperature was about 79.5F in the morning, so the tank didn’t lose more than 0.5 degree overnight. It dropped by another 0.5 degree to supply water for two showers in the morning. By the end of the day, the tank temperature was back up to 96F, meaning we got about 20,000 BTU from the sun today.
  • July 6: Overnight losses plus hot showers dropped the tank temperature to about 94F. Mostly cloudy today; the pump turned on for a while, but I manually shut it off because the tank was only getting cooler. It was at about 92F in the early afternoon when I stopped logging. No significant heat gain.
  • July 7: Tank was about 88F in the morning. Mostly cloudy again today, but a couple hours of sun brought it up to 93F by the evening. We’ll call it a 6,000 BTU gain.
  • July 8: The tank was about 90F after our showers. Only a couple degrees rise today.
  • July 9 and July 10: Cloudy and rainy; no significant gain.
  • July 10: Mostly sunny today; tank went from 87F to 99F, about a 14,000 BTU gain. Finally got one good spread of logged results, making for a pretty graph:
_images/2012-07-10.png
  • July 11: Mostly sunny again. The tank lost about 2F overnight, and another 2.5F in the morning (including two showers). The pump turned on at noon, with the tank at a little below 95F; there were a few fluctuations due to scattered clouds, but after an hour the tank began steadily climbing, finally peaking at 104F at around 16:30. This is the first time the tank has gone over 100F. Total gain today was about 12,000 BTU:
_images/2012-07-11.png
  • July 12: Another nice sunny day. 2F drop overnight, and about 3F more this morning; the tank was down to 99F by the time the pump turned on at 11:20. Gained about 12F between 11:15 and 17:15, topping out at just over 111F, a 14,400 BTU gain. The collector temperature reached nearly 180F, probably helped in part by the fact that I increased the tilt to around 40 degrees so it gets more direct sunlight. The storage tank is now approaching the temperature of our electrically-heated water–once it reaches 120F, I’ll turn off the electric heater and see how well we can do on solar-heated water alone!
_images/2012-07-12.png
  • July 14 - 17: The tank has been pretty consistently between 110F and 120F, so I turned off the backup electric water heater for a few days to see if we could get by on solar-heated water alone. Verdict: Nearly. Showers weren’t quite as hot as I’d like, but it’s clear that we’re getting the vast majority of heat from solar.
  • July 18: I’ve yet to see the tank go over 120F, despite seeing collector temperatures between 170F and 185F. It’s a little puzzling why this would be so; you’d think a temperature differential of 65F would give the incoming water quite a boost, no matter how hot it already is. I may be losing the incoming heat through the circulation pipes (a little of which is still uninsulated), or through water vapor escaping through the tank lid (which, though insulated, is not bolted down tightly).
  • July 20: Only checking the tank temperature once a day now (until I get my LCD display from AdaFruit). Finally, the temperature is over 120F–got a reading of about 126F today.
  • July 21: Tank went over 131F today. If it stays this hot, the backup water heater shouldn’t turn on at all. We’re 100% solar, baby!
  • July 23: The afternoon tank reading was 135F today, so any fears I had about inefficiencies preventing temps over 120F were allayed. It took the better part of 3 weeks to get here, but I’m finally nearing my target temperature of 140F.

Since July 23, I haven’t kept detailed temperature records. Throughout August, the tank temperature usually varied between 100F and 115F. We had a lot of partly-cloudy days, and the collector only tends to get about 4 hours of full sunshine. Unfortunately, the 130F+ temperatures I saw at the end of July have not reoccurred; in fact, I don’t think it has gone over 120F since then.

Because we bought a couple of window air conditioners in July, so far it’s impossible to see what effect the solar heating is having on our electric bill. The AC usage more than made up for whatever we’ve been saving. However, now that the weather is cooling off, we haven’t needed the AC much during the last month; our next utility bill should tell the tale.

For a breakdown of how much I spent on all this, see Material Cost.