Solar Heating Plan for Any Home
(Page 6 of 8)
December 2007/January 2008
By Gary Reysa
Automatic Controls
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The controls for the system are simple and operate the system efficiently. A standard Goldline differential controller is used to control the pump that circulates water to the collectors. It senses when the collector is hotter than the tank water and turns on the pump.
For the first month, we just noted when the tank temperature was above 90 degrees and manually plugged in the pump to circulate hot water through the floors. When the tank went below 90 degrees we unplugged the pump. This is surprisingly effective, and it gives you a good feel for how the system is operating.
I have since installed two electronic thermostats. The first turns on when the tank temperature is above 90 degrees, and the second turns on when the room temperature goes below 70 degrees. These two thermostats are wired in a series, such that the pump is only turned on when the tank is hot and the house is cold. And, because the thermostats both run on 120 volt AC, there is no need for low voltage control wiring or relays.
The control system is set up to use heat as soon as the storage tank is hot enough to supply useful heat. Using the heat as soon as the tank reaches 90 degrees rather than waiting for the tank to get hotter increases the efficiency of the collectors and also reduces losses throughout the system. For example, on a 35-degree day with full sun, the collectors will operate at about 59 percent efficiency if the tank water is at 90 degrees, versus 42 percent efficiency if the tank is at 150 degrees. (Click here for a PDF of the Solar Shed Control Diagram.)
Performance Data
Here is performance data for two sample days from last winter.
Jan. 12, 2007: A very cold sunny day. At 10 a.m. when the collector started gathering heat, the outside temperature was 20 below zero! The collector warmed the storage tank water from a morning low temperature of 85 degrees to 125 degrees in the afternoon. This heat energy stored in the water is the equivalent of 2 gallons of propane burned in a furnace of typical (85 percent) efficiency.
Jan. 27, 2007: A typical sunny winter day with a high of 30 degrees. The tank warmed from a morning low of 85 degrees to an afternoon high of 132 degrees. This is the energy equivalent of 2 1/2 gallons of propane burned in a typical furnace.
Cost and Return
The cost of the solar system components was about $4,200 total. This includes Montana tax credits and an allowance for siding that would have been needed for the shed if the collectors hadn’t covered the south wall. I estimate that the system will reduce our propane use by about 340 gallons per year, now worth about $740 in our area. The simple payback period is about 5 1/2 years (at the 2007 price for propane). You can find a complete cost analysis PDF here.
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