This section will provide performance data for the combined solar space and water heating system. But, as of right now you will have to settle for:
I plan to provide more performance data over time, but here is at least one pretty sunny winter day to go by in the mean time.
The plot shows the performance of the system for Feb 21, 2011.
Black solid line -- Temperature of the tank (closer to the top than the bottom) (F)
Dark blue solid line -- Collector inlet temperature (F)
Red solid line -- Collector return temperature (f)
Greenish line -- Solar intensity measured in the plane of the collector (w/sm)
For this day, no hot water was taken from the system, so all of the collector heat output went into heating the tank water and raising the temperature of the tank plus some losses from the tank during the day.
The tank started at 91F and was heated up to 137F over the course of the day -- a tank temperature gain of about 46F.
The day was mostly sunny from late morning until late afternoon. In the morning up to about 10:30 am there were intermittent clouds.
The strange behavior of the sun line at about 2:30 pm was caused by the pyranometer being borrowed for another test for a few minutes -- just ignore it.
The ambient temperature was 19 F at 10am
and got up to a high of 28 F in the afternoon.
There was some wind -- perhaps 8 mph on average.
You can tell when the differential controller senses that the collector is cooling down just before 4pm and shuts off the pump because the supply and return line temperatures take a dive. This is because the sensors are mounted on the supply and return pipes, which cool off quickly once the drain back empties them of water.
The net heat output for the day including tank heat losses was about:
Heat Out = (170 gallons) (8.3 lb/gal) (137F - 91F) (1 BTU/lb-F) = 65,000 BTU
This is the equivalent of 20 KWH used in a 95% efficient appliance (like an electric water heater or an electric furnace)
Or, about 1.1 gallon of propane burned in a 65% efficient furnace or water heater.
The temperature rise for the water going out to the collector is about 6F, which is good. A lot of temperature rise makes for a higher average absorber temperature and more losses out the glazing and lower efficiency. The 6F rise is accomplished with the pump on the lowest of its 3 settings.
Pump power consumption for the day was (53 watts)(6 hours) = 318 watt-hours, or 0.3 KWH -- compare this to the 20 KWH output -- the COP is very high.
Bear in mind that these numbers are for a pretty sunny day, and not all days are sunny.
Looking at the time period just after noon, the temperature rise through the collector is 7F, and the sun is 1050 watt/sm.
The flow rate is 4.6 gpm.
The area of the collector is 93.2 sf.
Collector heat output = (4.6 gal/min)(60 min/hr)(8.3 lb/gal)(7 F) (1 BTU/lb-F) = 16,036 BTU/hr
Solar input to the collector = (1050 w/sm)(1sm/10.76sf) (93.3 sf) ( 3.412 BTU/hr / 1 w) = 31,064 BTU/hr
Efficiency = (16,036 BTU/hr) / (31,064 BTU/hr) = 51.6%
This is a rough estimate.
It actually turns out to be very close to the efficiency of a Heliodyne Gobi collector under the same conditions -- I would call this a ballpark estimate, but it is a swell ballpark to be in :)
Our domestic hot water use is pretty low. I think less than 30 gallons a day for the two of us.
So, the energy to heat 30 gallons of water from 60F to 110F is (30gal)(8.3 lb/gal)(110F - 60F) = 12,500 BTU.
This leaves 65,000 - 12,500 = 52,500 BTU for space heating.
In actual fact it may not work out so that you supply all of the domestic hot water heat from solar and use the rest of space heating -- you may choose to use more for space heating and let your backup heater provide some of the domestic hot water heating. A couple strategies for managing the balance between space heating and water heating are discussed here...
One thing to bear in mind when looking at the performance of our system has to do with our collector orientation and the mountains to the east. The mountains to the east block a bit of the early morning sun this time of year -- not much we can do about this :) Ideally we would clock the collector so that it pointed a bit west of true south in order to make up for the bit of morning sun lost with more afternoon sun. But, unfortunately, the south wall or our house is actually pointed a little east of south. So, the combination of these two things probably cost us an hour of sun a day this time of year. Still, we on this day we got about 6 hours of sun, and that is fine, but if your system is pointed due south and does not have these obstructions, it should do a bit better.
Just below is a plot for eight days from November 27, 2011 through December 3, 2011.
This was a period of mixed weather as you can tell from the collector temperatures, which are a pretty good indication of when the sun is shining.
One day of really good sun, several days of part sun, and 2 days of almost no sun. Ambient temperatures running mostly around 30F. We are less than a month from the winter solstice, so 30F on a short winter day looks pretty good.
During this time I had the space heating disconnected, so this shows the performance of the system in mixed winter weather with domestic hot water demands only (no space heating).
The tank started the period down at about 90F as a result of space heating taking it down to its lowest limit. We are still working on what the best way for us to integrate the space and domestic water heating.
Gary February 20, 2011