Thermal Performance Test -- Low Thermal Mass Sunspace for Space Heating -- Day 2

This page describes a 2nd day of test of a low thermal mass sunspace that is optimized for providing space heating to the home its attached to.

The test covers heat gain, efficiency, thermal images, and a characterization of the living environment inside the sunspace while its providing space heating to the house.


On this day, the sunspace started with the same vertical heat absorbing curtain as day 1, but this was taken down about noon, so that the curtain vs no curtain could be compared.

Bottom line is that the sunspace provides as much heat to the house as a good active solar collector of the same size would, while also providing an area that can be used for lounging, clothes drying, and many other uses.

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Sunspace Configuration  -- January 4, 2013 Test

In this test, the sunspace has been optimized for space heating the attached house via the in the following ways.

Its the same basic configuration as day 1.

More details on the construction, objectives,...

This test starts with the same verical black weed fabric suspended vertically down from a support pipe that runs about 2.5 ft below the ridge line.   At about noon, the vertical curtain is removed to see if it results in any change in heat output.

sunspace for space heating

Vertical Screen

Test starts with the same vertical screen as the day 1 screen.

A vertical screen was added that starts about 36 inches directly below the peak and hangs vertically right below the ridgeline.  It extends the full east-west dimension of the space.  When the screen hits the floor, it is carried southward on the floor to the south wall.  The screen is Scotts black weed fabric from Home Depot. 

The idea of the screen was to provide surface where solar radiation could be absorbed and converted to heat.  The screen is located directly under the peak, so heated air can rise directly up both the front and back surfaces of the screen directly to the collection area at the peak.

The radiation that gets through the screen encounters the back (north) wall and is absorbed there.  Air heated by the back wall and roof rise up the wall and roof slope into the collection area in the peak. 

At about noon, the vertical screen is dropped and just laid out on the floor as in this picture.

So, the solar radiation that was absorbed by the screen now hits the north wall and floor and is absorbed the dark paint.










Inlets and Outlets

Same as fro day 1 inlets and outlets

Heating Performance

This was another very clear day.  Its also only a couple weeks after the Winter Solstice, so the days are short and the sun is low in the sky (about 23 degrees at noon).  The outside air temperature was in the 20'sF.  Wind was generally light.


sunspace performance plot

Blue solid line -- sun intensity (w/sm)

Red Lines -- Fan outlet temperatures for East and West fans (F)  -- this is the temperature of the heated air going to the house for space heating.

Green dash-dot -- Ambient temperature (F)

Purple dash line - Temperature at about the 5 ft level in the sunspace (F)

Yellow-Green solid line -  Temperature 1 inch from glazing in shade (F)

The sunspace was run in the same configuration as day 1 until 1:10 pm, at which time the vertical curtain was dropped to the floor.  The black curtain was spread out on the floor so that it covered the whole floor.  The floor surface without the screen would have been the reflective film on the 1.5 inch Styrofoam that was on the floor.

Total flow rate for the outlet ducts was 670 cfm (based on 614 fpm at outlet ducts).  Same as day 1.

Right at the start of the day (8:55 am), the sun comes on the sunspace, and all of the air temperatures rise very rapidly.  After about 10 minutes (9:05 am), it gets hot enough for the controller to turn on the fans, which stops the rapid rise in temperature by exporting the heated air outside.

The three peaks in the fan outlet temperature (red at 9:10 to 9:20 am) is the fan short cycling.  There is also some fan short cycling at the other end of the day around 4 pm.

There is a strange bump in the temperature sensor mounted at 5 ft about the floor north of the screen for half an hour at noon.  This also appears on the day 1 plot.  Not sure, but I think that during the time period, the sensor is getting direct sun from a gap between the two fabric panels in front of it.Heat Production


Dropping the Curtain

At 1:10 pm, the vertical curtain is dropped to the floor (marked by twin triangles on the plot). 

There is a rapid temperature drop of about 10F in the fan outlet temperature, followed by a gradual rise.  During the period, the sun intensity is steadily dropping.

At first glance, it looks like dropping the curtain does result in a significant drop in temperature and heat out, but looking more carefully, I don't think so.  The rapid initial drop may be due to the curtain dropping process and transients caused by that.  If you go a few more minutes down the plot, it looks like the fan outlet temperature curves just about regain the temperature they would have had (with the dropping sun levels) if the curtain had been left up.  I'm inclined to think that the efficiency difference between the two is small if any.

A couple things that do definitely change:




Heat Production


Heat Production

Looking at the long period when the fan outlet temperature is around 100F and ambient temperature is about 26F, the sunspace is producing 670 cfm with a 74 F temperature rise. 

Heat Output = (100F-26F)*(670 ft^3/min)*(0.061 lb/ft^3)*(60 min/hr)*(0.24 BTU/lb-F) = 43,550 BTU/hr    (210 BTU/sqft of glazing)

This is about what a typical 60K BTU/hr gas furnace would produce.

The heat output for the full day looks like this:

Time Tout (F) Tin Trise Velocity (fpm) Duct Area (sf) Flow Rate (cfm) Air Density (lb/cf) Heat Out (BTU/hr)
10:00 AM hour 61 19.8 41.2 600 1.09 654 0.061 23668
11:00 AM 76.4 22.3 54.1 600 1.09 654 0.061 31079
12:00 PM 90 26.6 63.4 600 1.09 654 0.061 36422
1:00 PM 99.5 27.4 72.1 600 1.09 654 0.061 41420
2:00 PM 99.5 28.3 71.2 600 1.09 654 0.061 40903
3:00 PM 87 24.8 62.2 600 1.09 654 0.061 35732
4:00 PM 64 25.1 38.9 600 1.09 654 0.061 22347
Total               231570

So, about 232K BTU heat output for the day -- about equivalent to 3.6 gallons of propane burned in a 70% efficient furnace.

So, even this modest size sunspace can produce a lot of useful heat on a sunny winter day -- even for the shortest days of the year. 


One difficulty with making efficiency estimates from the performance curve above is that the even though this is a very low mass sunspace, it apparently has enough thermal mass that its outlet temperatures and heat output lag a bit behind changes in sun intensity.  You can see this in the time period from about 12:30 pm to about 1:30 pm -- in this time period, the output temperature and heat output are nearly constant at just over 100F, but the sun intensity is dropping from about 1130 w/sm down 1040 w/sm.  I believe that if the sunspace had even lower thermal mass, it would react more quickly to solar changes. 

This effect also shows up if you calculate efficiencies for the morning vs afternoon.  In the morning, the sunspace outlet temperature is lagging behind the sun level increases and producing lower outlet temperatures than it would if the sun just stayed steady at one level.  In the afternoon the sunspace outlet temperature drops more slowly than the drop in sun level because the sunspace mass is losing heat and keeping the temperatures up.  So, in the morning the apparent efficiencies are around 45% while in the afternoon they are more like 71% -- these are not really valid efficiency numbers in that conditions are not steady, but they illustrate the lag effect.

To get the best idea of efficiency that I could, I looked at the solar input and heat output at just after 1 pm:

Solar input:  based on 1099 w/sm sun and 208 sf glazing is 72, 490 BTU/hr

Heat output:  based on an outlet temperature of 99.4 F and inlet of 26F and with flow rate of 670 cfm  heat output is 43,010 BTU/hr

Efficiency is then  43010 / 72490 = 59.3%

This is very good for a solar air heating collector.

So, how does this compare with a good commercial collector under the same conditions?

Using this Collector Efficiency Calculator and

Collector: Heliodyne Gobi water heating collector (a very good collector)

Ambient Temperature: 26F

Sun Intensity: 1099 w/sm  (348 BTU/sf-hr)

Average Collector Temperature:  (99.4F + 26F) /2 = 62.7 F

Efficiency is:  61.3%

The Sunsiaray air collector under the same conditions comes out a 42%

So, under the same operating conditions, the low thermal mass sunspace efficiency is equivalent to, or better than commercial active collectors, and you still get all the other benefits that sunspaces offer.

Note that the sunspace efficiency benefits from the low inlet air temperature -- this makes the average collector temperature lower and reduces glazing losses.  But, the Heliodyne calculation gets the same benefit, so the comparison is an apples to apples one.

Temperature Environment in Sunspace

Glazing Thermal Images

These pictures need to be clicked on to see the temperature labels.

IR picture sunspace glazing
Outside of glazing.
Emissivity 0.95.

Inside  of glazing -- top.
Top part of glazing panel.

Inside of glazing - bottom.
Lower part of glazing panel.

Glazing pictures at 1:46 pm.

The left picture above is of the outside of the glazing with temperatures ranging from 38F at the bottom to 52F at the top.

The center picture is of the inside of the glazing near the top (the space is confined enough that I can't get the whole panel in).  The temperature is 103F on the inside surface, so the temperature difference across the twinwall glazing is about 103F - 52F = 51F.  

The right picture is of the inside of the glazing near the bottom with a temperature of 68F.  Temperature difference inside outside is 68F - 38F = 30F.

At this time, the conditions are:

Ambient temperature 28F

Fan outlet temperature  99F

Air temperature inside near glazing at bar 1/3 of way up  86F

Sun 1125 w/sm

Solar input  73K BTU/hr

Heat output = 41.5K BTU/hr

Losses = 73K - 41.5K = 31.5K BTU/hr

Just as an Interesting Item

I took some IR pictures of two other nearby collectors that face the same way and are at the same tilt --

The sunspace glazing.

The left hot patch at 92.4F is the pool heating collector for the OTS 
solar water heating system prototype
 -- it is unglazed.
The right collector at 39.1F is the water heating collector for
the Solar Shed project -- glazed with twinwall polycarbonate.
The temperature of the water circulating in the two collectors is not known.

The left picture is the sunspace glazing at 3 pm with an outside temperature of 35 F in the center.

The picture to the right has two collectors.

The right is the solar shed water heating collector with twinwall glazing.  Its outside temperature is similar to the sunspace collector (4F warmer).

The collector to the right is an unglazed pool heating collector being used as the prototype for the Off The Shelf DIY water heating system.  Its outside temperature  is 92F. 

All three collectors were operating at the time.

Conditions Inside the Sunspace

Back of sunspace (north of the vertical fabric curtain):

You have to click on these pictures to see the temperature labels

North roof behind fabric curtain.
Curtain is visible to right.
East fan is the circular item.

North wall and north ceiling.
Lower part of north ceiling and all 
of North wall are shaded by the fabric curtain.

Lower part of North wall.
Fabric curtain out of view to the right.

All done with temperature range of 60F to 160F and emissivity of 0.95.

Front of sunspace (south of the vertical fabric curtain):


South sunspace - top.
Top temp label is on north roof above curtain.
Two temperature labels on the curtain.

South face of curtain upper half.
Temperature labels are on the curtain face.

South face of curtain -- lower part.
Purple part is curtain fabric along floor.

Air and Surface Temperatures

In these pictures, the grey filled circles are surface temperatures (taken with an IR temperature gun), and the teal filled circles are air temperatures taken with a couple indoor/outdoor thermometers.

These readings are taken in a north-south plane about half way between the east and west ends.

.sunspace aiir and surface tempratures
Temperatures at 1:28 pm with both fans operating and full sun.
Just after solar noon.


The south indoor outdoor thermometer and overheated (temporarily) and stopped providing readings.



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