Test of Sunspace Optimized for Solar Space Heating

These are the results from a test run on December 13, 2012 of our greenhouse setup as a sunspace and optimized to provide a maximum amount of space heating for an attached house.





Sunspace Setup

The steps taken to try to optimize the sunspace configuration to provide maximum space heating to the attached house are:

- Thermal mass in the sunspace is made as small as possible so that solar heat is not wasted heating the sunspace mass.

- Insulation is added on the N, E, and W walls and also on the north roof.  These walls are also sealed with some care.   The insulation consists of R21 fiberglass batt insulation between the 2 by 6 studs, and then a layer of 1 inch polyiso insulation on the inside of the studs.  Total R value is about R30 with little thermal bridging.

- The floor was insulated with 1.5 inch polystyrene.   I wanted to minimize the thermal mass of the floor, so the only thing over the polystyrene insulation now are some plywood walkways.  So, its a very low thermal mass floor.

- The fan system is intended to pull the air out of the peak when it reaches the minimum temperature that is useful in the house for space heating.  Keeping the temperature in the sunspace down reduces sunspace heat loss and improves efficiency.

The hope is that with these optimizations, the efficiency of the sunspace as a collector will be in the same area as active solar air heating collectors, while still giving you a nice place to read a book.



sunspace performance

Red-solid -- Temperature of air entering the exhaust fan -- near peak on west end.

Yellow/Green-Solid -- Sun in watt/sm measured in the plane of the glazing and with strong snow reflection

Aqua-shortdash -- Temperature measured just behind glazing with tape shading sensor.  About 6 ft above floor.

Black-long dash -- North of screen, 55 inches off floor, midway between screen and north wall.

The temperature measured near the glazing seems high (only about 15F cooler than the temp at fan inlet).  This increases losses through the glazing.  For example, when the temperature just behind the glazing at about 6 ft up is near 100F, the temperature a foot off the floor in the inlet flow is more like 30F. Maybe the inlet air should be introduced high up on the glazing and (somehow) constrained to wash down the glazing to keep it cooler? 

The sun intensity reaches the pyranometers limit of 1250 w/sm for a few minutes at about 1:15 pm.  This may seem surprising, as full sun is often listed as 1000 w/sm.  The reasons for the high value are: 1) strong reflection from fresh snow to south, 2) possibly some reflections from clouds that were off to side, 3) our high altitude and clear skies.   When I first got the pyranometer and saw this a couple years ago, I sent the pyranometer back in for calibration thinking it had to be wrong, but it calibrated right on, and the makers of the pyranometer tell me these kinds of high readings happen at their location in Utah as well.

The day was fairly sunny with clouds coming through.  There do not appear to be any times in which the conditions are steady enough to do an efficiency estimate -- have to leave that for a more sunny day. 


Temperatures in various parts of the insulated sunspace.

sunspace performance

Purple -- East end, near peak -- 4 ft from east end, 12 inches down from peak.

Blue-shortdash  - East end, North of screen --  4 ft from east end, 55 inches off floor, North of screen and midway between screen and north wall.

Blue-longdash -- East end, South of screen -- 4 ft from east end, 55 inches off floor, South of screen and midway between screen and glazing.

Green-solid -- Ambient and inlet temperatures.

Red solid -- Temperature of air entering the exhaust fan -- near peak on west end.

The big dips in temperature are, of course, when the sun was behind clouds.

The temperature at the peak on the east end runs about 20F hotter than the west end peak at the exhaust fan, so, a better job could be done getting the hot air uniformly out of the peak.

The nearly 140F peak temperature seems wasteful -- especially when the inlet temperature is in the 30'sF.

Temperatures in front of and behind the dark screen are pretty close to each other with the north one lagging the south one by a few minutes.


Air Velocities

I was able to get some air velocity measurements around the sunspace during the full sun conditions:

Immediately behind screen -- Upward velocities of about 60 fpm over most of screen -- smoke pencil also showed upward flow.

Immediately in front of screen -- Upward air velocity that was just a bit slower than behind the screen. 

Back wall -- Upward velocity -- stronger as you went up from floor -- about 60 to 65 fpm

Inlet areas -- Strong flow into inlets and down near the floor heading along the south wall toward the center, but also somewhat to the north.  The inlet flow visibly moved the weed fabric.  Flow velocity in the 100 to 200 fpm starting at each inlet, and staying strong nearly to the center.


IR Pictures



Toward the end of this day, I hooked up a thermostatic control for to turn the fan on automatically when the sunspace peak temperature reached a set temperature, and turn it of when it drops (with some hysteresis).   I used a Ranco controller with its temperature sensor mounted near the fan inlet.  This is not a very fancy control system, but appears to be working fairly well. 

I think that a multi speed fan or a variable speed fan would be better.  It could provide high flow rates for full sun conditions and lower flow rates for part sun or very cold conditions.  So, the air delivered to the house is about the same temperature and the sunspace temperature is better controlled as well.