Solar Shed -- Performance and Economics

This is preliminary performance of the system -- I'll add more performance data, and the cost/savings results in a couple months.

See updates at end of this page






At the time I am writing this, the system has only been operating for 3 days, so this is first cut performance.

I intend to install sufficient instrumentation to log the system performance, and some other items.


The weather for these three days has be clear, but VERY cold.  Night lows have been -20F, and day highs have been barely over zero.


I have been letting the system heat the 500 gallon storage tank water during the day without using the radiant heat in the house during the collection period.  This way I can get an idea how much energy the collectors are producing by how much the tank temperature goes up.  After the sun is off the collectors, I turn on the house radiant floor circulation system and use the stored heat overnight -- this brings the tank temperature back down to about 85F by the next morning.


Each day, the 500 gallon tank has started at around 85F, and ended the day at about 125F.  So, the stored heat by per day is:


Heat Stored = (500 gal)(8.3 lb/gal)(124F - 85F) (1 BTU/lb-F) = 162,000 BTU


This is equivalent to 2.1 gallons of propane burned in an 85% furnace.


I think that its encouraging that the collectors do well in spite of the very cold temperatures (which result in greater heat loss from the collectors).  I expect to do better once temperatures get up to our normal winter highs of about 30F.



Update 2/8/07:

The system has been up since early January, and is doing fine.

I will provide some more in depth performance numbers in a month or two.


As I am looking our the window writing this on March 8, it is snowing hard.

But, we are still on solar from the heat stored in the thermal storage tank yesterday.  Yesterday, the collectors both heated the house, and got the storage tank up to 155F -- this should be enough to carry us through the rest of the day.  I'm feeling a bit smug :)


Here is some simulation work that was done before the system was built:



Update February 15, 2009:

Well, as you may have guessed, I've not done any more performance measurements on the system since back in 2007.  The lack of performance updates has not been due to any problems with the system -- just my laziness.


The system has been up and running and working well.

I've not really had to make any real changes at all for the 2008/2009 heating season.  In the fall of 2008, I turned it on, it just started working.


I did make an estimate for the Mother Earth News article based on the performance up to the time I wrote the article that the system is saving me about 330 gallons of propane a year.  I've not seen anything that would make me change that estimate.


I do get questions like: How much of my home space heating will this system supply?

This is really impossible to answer without knowing a lot more.  The answer depends on 1) the heat loss of your house (how well its insulated and sealed), 2) how cold your climate is, 3) how much sun you get, and 4) how large a solar system you put in.

The best I can say is that if you live in a climate with about as much sun as we get here in SW Montana (which is about average for the US), and you put in the same size system I did, you will probably save the heat equivalent of about 330 gallons of propane a year.


I would like to measure the performance more carefully, and put up month by month plots as I did for the solar water heating system.  But, it takes a fairly involved set of instrumentation to do this as well as a fair bit of time to get it working.  This won't get done until next heating season at the very earliest.


Update February 21, 2009:

Just as a check that the system is still working as well as it was when it was first installed, I did this little test.


Today was a sunny day.  The storage tank was at 89F this morning at about sunrise.   I set the house solar thermostat low enough so that the no solar heat would be used during the day to heat the house.   So, all of the heat generated by the collectors today went into heating the storage tank.  The tank went from 89F in the early morning to 135F at the end of the collection period.  The storage tank now hold 420 gallons, so the heat gain was:


Heat Gain for day = (420 gal)(8.32 lb/gal)(135F - 89F)(1 BTU/lb-F) = 160,750 BTU


This is very close to the initial performance of the system listed above -- so, no appreciable degradation over the couple years.


The 161K BTU is equivalent to about 2.2 gallons of propane burned in an 80% efficient furnace.


The ambient temps for the day were:  15F @ 8am, 25F @ 10am, 31F @ 12 noon, 38F @ 2pm.





Gary 1/15/07

updated: February 15, 2009, February 21, 2009