# Solar Heating at -20F?

 We have been having a cold snap for the last couple days here is SW Montana.  I thought it would be interesting to record temperatures in the solar heated shop/garage to see how well it can do with very cold, but sunny weather.   The shop "collector" is just a set of doors with lots of glazing positioned just to the sun side of the rollup garage door (see picture below).  The rollup door is raised when the sun is out, and the sun shining through the glazed doors provides heat, lighting and a view.  When the sun goes down, the rollup door is lowered and provides good night insulation to reduce heat loss.  More on the collector.   Basically this simple collector manages to keep the shop warm without any supplemental heating for the hours of the day when you would normally be working.  One of the big factors in how well the collector works is using the rollup door as moveable nighttime insulation -- without this the collector goes from a net gain of 120,000 BTU/day to a net gain of 24,000 BTU/day, which would not even be close to keeping the shop comfortable (see calculation below).

Plot of temperatures in the shop/garage for a -20F night.

The plot starts at about 11am on Feb 17, and ends at about 7 pm on Feb 18.

The red line is the temperature in the shop, and blue is the outside temperature.

I think this demonstrates that simple, inexpensive, and easy to build solar collectors can do a fine job -- this collector cost about \$380 to build (about \$4 per sqft).

The solar "collector" -- just a set of glazed doors added south of the regular garage rollup door.

The rollup door is open in this picture and would be closed at night.  Kristy and Bella don't seem to mind the -6F high for the day.

View from the inside in summer -- the lighting is great.

Heat Output With and Without Moveable Insulation:

Using the Nick Pine's rough collector calc method, the heater collects about:

Solar Heat In = (100sqft)(1620BTU/day)(1.15 ground reflectivity) = 186,000 BTU/day

Heat Loss -- Daytime    = (55F - (-5F)) (100 sqft)(8 hrs) /((R1) = 48,000 BTU daytime loss
Heat Loss -- Nighttime  = (50F -(-10F) (100 sqft)(16 hrs)/(R6) = 16,000 BTU night loss (door down)

Total Loss = 48,000 + 16,000 = 64,000 BTU/day

Net Heat Gain = 186,000 -  64,000 = 122,000 BTU/day
(equivalent to 1.7 gallons of propane burned in 80% efficient furnace)

If the rollup door was not used for moveable insulation, the night loss is 6 times greater (R1 vs R6), and the net gain would reduced from +120K down to a +24K   -- night insulation makes all the difference!

I should note that the barn collector also did quite well in this cold weather, reaching about 56F at midday inspite of my not having gotten around to weather stripping and other heat loss control measures.

Gary

2/18/06

-------------

This is where the 1620 BTU/day in the calculation above comes from:

`Solar Radiation on Collector for day of:  2/18`
```  Collector Area:           1.0 (sqft)
Collector Azimuth:       20.0 (deg) measured from South, + is to East
Collector Tilt:          90.0 (deg) measured from horizontal
Latitude:                46.0 (deg)
Altitude:                 0.0 (ft) Above Sea Level
Sun Rise:                 6.8 (hr) Sunrise in solar time```
```Time    Sun --------------------------------    Collector ------------------------------
Hour    Az   Elev  DirNormal  Diffuse  Total    HorzTot IncidAng Direct Diffuse   ITotal```
```   4  103.9  -29.1      0        0        0        0      85        0        0        0
5   92.4  -18.8      0        0        0        0      73        0        0        0
6   81.8   -8.4      0        0        0        0      62        0        0        0
7   71.1    1.7      3        0        3        0      51        2        0        2
8   59.8   11.1    183       11      194       46      41      138        5      144
9   47.3   19.5    251       15      266       99      33      210        7      218
10   33.1   26.2    279       17      295      140      29      243        8      252
11   17.1   30.7    291       17      308      166      31      250        9      259
12    0.0   32.2    295       18      312      175      37      234        9      243
13  -17.1   30.7    291       17      308      166      47      200        9      208
14  -33.1   26.2    279       17      295      140      57      150        8      159
15  -47.3   19.5    251       15      266       99      69       91        7       99
16  -59.8   11.1    183       11      194       46      80       32        5       37
17  -71.1    1.7      3        0        3        0      90        0        0        0
18  -81.8   -8.4      0        0        0        0      90        0        0        0
19  -92.4  -18.8      0        0        0        0      90        0        0        0
20 -103.9  -29.1      0        0        0        0      90        0        0        0
-----   ------   ------    -----            -----    -----    -----
Day Total            2308      138     2446     1077             1551       69     1620 <---```
```Angles in degrees, Radiations in BTU/hr, DayTotal in BTU/day
1 BTU/hr-ft^2 = 3.152 W/m^2 ```