Today was a very good solar day. Not a cloud in the sky, 75 degrees out side and a full solar tank of cold water because it has been cloudy the last week or so. This afternoon, around 2 pm I tiptoed down to the basement to see how things were going. This is what I saw:
- Collector supply temperature 148 degrees F
- Collector return temperature 166 degrees F
- Flow rate 5.8 GPM
To calculate how much energy I am receiving, I need to know the thermal gain of the collector array, in this case 166 degrees – 148 degrees = 18 degrees. The flow rate is 5.8 GPM, therefore we can say 18 degrees x 5.8 GPM = 104.4 degree gallons/minute. A gallon is equal to 8.33 pounds, therefore we can say 104.4 degree gallons/minute x 8.33 pounds = 869.65 degree pounds per minute. A BTU is the energy it takes to warm 1 pound of water 1 degree Fahrenheit. A degree pound is equal to a BTU; the energy being supplied by my solar system this after noon is:
- 869.65 BTU/Minute
- 52,179 BTU/hour
This equals:
- 15.3 kWh (3,413 BTU/kWh)
- 0.375 gallons #2 heating oil (139,000 BTU/Gallon)
- 0.573 gallons propane (91,000 BTU/Gallon)
- 0.512 CCF natural gas (102,000 BTU/CCF)
All of this is, of course, before system losses, so it would be safe to say that I was actually receiving about 10 kWh per hour during the afternoon. Still, not too shabby.
Now, today is an exceptional day because the solar tank was completely cold and the difference in temperature between the solar tank and the solar panel loop was making the heat exchanger operate very efficiently. Once the solar tank is warmed back up (which it should be by the end of the day, the collector array BTUs will come down a somewhat. What is nice is that every sunny day, I can expect to get between 7-14 kWh a day.
Paul,
I’m VERY jealous of your solar panels! I wish I didn’t have so many high priority projects still left to do around here because I’d love to install some solar panels on the house. During the summer what percentage of your hot water do you think you can make from the sun vs fossil fuels?
Todd, during the summer, I expect 100% of our hot water will come from the solar. Since the beginning of April the only time we completely ran out of solar hot water was last week, so we used the back up electric element. I may have an unused kWh meter floating around which would be fun to install on the backup element and see how much electric we are using.
Paul-
As we approach the end of summer do you have an update on your solar water heating? Were you able to get all (or at least most) of your hot water from the array?
/dave
Dave,
I need to get one more electric bill, then I will have one years data. So far we have gotten almost all of our hot water from the solar system since about April. That should begin to drop off toward the end of this month.
We had a couple of cloudy periods where the backup electric element came on (I can tell because the hot water gets cooler). As soon as I get the electric bill I will post my findings. I can tell you that for the last several months our electric usage was about half of what it was for the same period last year.