Low Thermal Mass Attached Sunspaces
for Solar Home Heating -- Performance Testing
This section covers some tests done on a low thermal mass sunspace
with a design optimized to produce solar heat for the home its connected
to.
Well designed sunspaces are exceptional in that they not only provide
solar heat to the house, but they can serve a variety of other purposes
-- lounging, reading, clothes drying, wood drying, plant growing, ...
But, you have to wonder how good a job of providing heat do they do when
they are also doing so many other things? These tests attempt to
answer that question -- and more.
The tests cover:
How efficient this sunspace design is in producing heat for space
heating an attached home.
Compares the efficiency of this sunspace to conventional solar collectors.
Looks at the effect of some design variations on heat output.
Provides a survey of the temperatures in various parts of the
sunspace in the sun.
Covers some of the things that can be done to maximize the use of the
sunspace for a variety of purposes (producing heat, pleasant place to
spend time, drying laundry, ...).
Cost of the sunspace
Potential improvements
Bottom line is that you will probably be pleasantly surprised about
how efficient a heater a well designed sunspace can be.
If you have a sunspace,
I'd very much like to hear what your experience has been (good or bad),
and what your ideas for improvements are.
I'm doing this work in what will become our all season greenhouse. As
is clear from the picture its not an attached sunspace. This
approach results in some things that are not quite the same as if an actual
attached sunspace was used for the testing...
The greenhouse is freestanding and does not share a wall with the house. Given
that the back wall is well insulated, I don't think that this makes much
difference in the results.
The heated air is just blown out into the great outdoors through those
odd looking ducts above the door. The sunspace is too far away from
the house to duct the hot air to it. I don't think this makes any
difference in the results.
The intake air is taken from outside, so its much colder than it would
be in an attached sunspace where it would be (say) 65F air from the house.
This does effect the results. The consequences of the cold inlet air
are discussed in the performance section.
All of the things like insulation levels, thermal mass, wall colors,
interior configuration were set up to optimize sunspace performance, and
may be changing when its converted into its greenhouse role.
Bottom line is that I think that using the greenhouse as a sunspace simulator
gives good results, and there is so little actual performance data out
there on real low mass sunspace performance that its quite useful.
The sunspace performance tests were actually done in our new all-season
greenhouse. I've had a long term interest in seeing how well an attached,
low thermal mass sunspace actually performs compared to (say) active solar
collectors, and the greenhouse has a lot of the same features, so we decided to
use it to do some sunspace performance tests before it becomes a greenhouse.
The main features include:
A 200 sqft glazed area that is oriented at a 60 degree tilt angle for
maximum winter solar gain.
The glazing is double wall polycarbonate
for better winter performance (lower glazing heat loss).
The glazed area is just about equal to the total floor area.
Surfaces that do not receive much winter sun are not glazed as
they might be in a 3 season greenhouse. This is to reduce the heat
loss through the glazing, which is high.
The sunspace has very low thermal mass so that the bulk of the incident
solar heat is carried off to the attached house, and very little is stored
in the sunspace mass.
All non-glazed surfaces are well insulated. The walls and ceilings
are insulated to R28, and the floor has vertical insulation around the
perimeter as well as 1.5 inches of floor insulation.
The sunspace is well sealed against air infiltration.
All inside surfaces that see the sun are painted dark to absorb the
sun. These surfaces are also low mass, insulating surfaces that heat up
very quickly when the sun is on them. This results in rapid heating of the
air next to the surface The heated air then rises up to the peak area where
the fan and ducting export the heated air to the space to be heated.
The end result is a space that heats up very quickly when the sun gets on
it-- it is ready to start making heat for the attached house as soon as the sun
is on it. And, at the end of the day, when the sun goes off the sunspace it very quickly
cools to the outdoor temperature, because it has no stored heat. This may
conflict with some sunspace uses, and compromises can be made if these uses are
important to you, but this test just wants to find out how good the sunspace can
do as a heater with the design optimized for space heating.
The pictures below show the sunspace/greenhouse build --
more pictures here... Bear in mind
that this sunspace setup is for performance tests only -- you can make your
sunspace much more pleasant to be in with nicer finishes. Its also
possible to do very simple sunspace that cost next to nothing, but still produce
a lot of heat. See the Sunspace Section for examples of both ends of the
spectrum.
Concrete footing post support the sunspace.
1.5 inch vertical Styrofoam insulation extends
2 ft down.
Built on treated lumber sill plates.
Framed with 2 by 6 lumber 24 OC.
Glazed with 10 mm thick twinwall
polycarbonate glazing in 4 by 10 ft sheets.
200 sqft of glazing total.
Pretty carefully sealed.
With the fans off,
It gets pretty warm even before
insulation was installed.
Maybe should have started construction
earlier in the season :)
R21 fiberglass batts first.
Very carefully fitted and with no wiring
or plumbing to deal with.
Then,
1 inch rigid poyiso insulation inside the
framing.
Total R value about R 28 with minimal
thermal bridging.
The goal was to keep the wall heat loss low
compared to the double glazing even if an
insulating curtain was added to the glazing.
The dark grey paint was a $7 find in the
"oops" pile at Home Depot.
One of the two 10 inch Dayton/Grainger fans
that push air out of the sunspace.
The duct is extended outside to provide
a long straight duct for more accurate
flow measurements.
The vertical weed fabric screen used
in one of the configurations is also visible.
Setup for one of the tested configurations.
North wall is to left, weed fabric hung from
ridge line to right. The vertical pipe has
temperature sensors at 3 ft and 6 ft off floor
between fabric and north wall.
The blue tape spots mark places where surface
temps were taken.
Same configuration with vertical weed fabric
and picture taken on south side of the fabric.
The vertical PVC pipe has two temperature sensors.
The blue tape spots mark places where surface
temps were taken.
Floor is some sand over dirt followed by
a layer of 1.5 inch thick Styrofoam
rigid insulation board.
Looking west with weed fabric on floor.
The floor is dirt covered with 1.5 inch
Styrofoam insulation with some plywood
walking strips laid on it, and black weed
fabric on top.
The two PVC pipe posts support temperature
sensors.
Sunspace on one of the test days.
The two "tunnels" extending east and west from
the lower south corner preheat the flow
of inlet air a bit.
Performance Tests
Several days of performance testing were done -- the section just below links
to pages on three of the most useful test days.
Test day 1: Optimized sunspace with vertical screen
This configuration included all of the features listed
above. The walls, ceiling and floor were insulated and nearly all interior
surfaces were painted dark.
In addition, a screen of dark weed fabric was hung
vertically in the space. The fabric was suspended from near the peak and
ran vertically down to the floor, and then extended along the floor south toward the glazing
along the floor.
The aim of the test was to get basic sunspace
performance, and to see if adding the vertical screen is more
efficient in getting the heat up to the peak area. The screen was also
helpful in evaluating whether some form of screen is desirable to increase the
comfort for people using the sunspace to (say) read or have a cup of coffee.
The test also includes a stagnation test in which the
fans and all ventilation are stopped to see how high the temperatures go.
Test Day 2: Optimized sunspace without vertical screen
This configuration is nearly the same as configuration 2,
except that for half of the day the dark weed fabric was just distributed evenly over the full
floor. So, for this configuration, the sun could shine onto the
north wall and north roof.
Day 2 test configuration without the vertical curtain.
Configuration 3: Non-Optimized Sunspace
This test was done earlier in the construction before
some of the features above were incorporated. The glazing and shell were
in place, but the floor was bare (heat absorbing) dirt, and the walls and
ceilings were not insulated.
Interior surfaces were bare wood, and not painted a dark color.
The idea was to see how much it degrades the
performance if not all of the features listed above are included.
The sunspace does surprisingly well as a space heater. The
sunspace with 200 sqft of glazing produced 233,000 BTU on one sunny day -- the
equivalent of burning 3.6 gallons of propane in a 70% efficient furnace.
The sunspace achieved efficiencies during the test up to 60% -- this is fully
equivalent to good commercial active solar collectors operating under the same
conditions. Pretty amazing when you consider that you get all the other
benefits that a sunspace offers while still getting this very good space heating
performance.
This section in the day 1
test writeup goes over the heat output and efficiencies achieved in the
test. It also provides two methods to estimate efficiency and heat output
for conditions other than those of the test day.
One caution is that this testing was done on a sunspace that was highly
optimized for space heating -- optimally tilted double glazing, well insulated
on all non-glazed surfaces, low thermal mass, tightly built, with all dark and
low mass absorbing surfaces. Not adhering to this type of optimized
construction will result in lower efficiencies and heat output -- the
configuration 3 test (above) gives some indication of how much.
How the Performance Measurements Were Made
Getting good performance data on solar heating collectors is challenging, and
air heating collectors are even more challenging. Its right to be a bit
skeptical about performance numbers you see around the internet. So, here
is a rundown on the instrumentation I used -- so you can judge whether or not to
trust the results.
Sun -- Sun intensity was measured with an Apogee
pyranometer that was mounted to the glazing frame and was perpendicular to the
plane of the glazing.
Apogee pyranometer mounted in plane of glazing.
In this position, it directly measures the sun intensity on the glazing.
This pyranometer is factory calibrated and is about 1 year old.
Temperatures -- I used
Onset Computer loggers with thermistor sensors to measure the temperatures
used for performance calculations. These sensors specifications call for
plus/minus 0.25 C accuracy over the range of interest. As a check, I generally put all the sensors
together in the same location at the start of the test and make sure they read
close to the same value.
Areas -- Glazing area was measured with a tape, and the
frames were included (not just net glazed area) -- this is in the SRCC
style. See the test page for a little more on this.
Flow Rates -- In air collector testing, getting good
estimates for flow velocity is usually the most challenging measurement. I went
through several iterations getting to an arrangement that I feel is giving
decently accurate velocities. The final arrangement includes an about 11 ft long straight length of 10 inch duct on the exit of each of the two fans
-- this provides 13
duct diameters of straight duct between the fan and the velocity measurement station. This lets the
turbulent flow of the fan mix and settle down after leaving the fan.
A 2 ft long flow straightener is used to take most of the fan swirl out of the flow.
Before the flow straightener was installed, the swirl velocity was about 400 fpm
and the axial velocity in the center of the duct was significantly lower than the
axial velocities around the periphery. With the flow straighteners, the velocity
profile is as expected with higher center velocity, and consistent velocities
around the periphery.
I take the velocity measurements with a new
Kestrel 1000 turbine style anemometer. The Kestrels are high quality
instruments that all receive an individual factory calibration before going
out-- they are certified to be within plus/minus 1.66% for the velocity range I
use.
I take one reading in the center of the duct, and 4 readings around the
periphery (up, down, North, and south). The 5 readings are simply averaged
to get an estimate of the average flow. The center of the turbine is 2
inches from the duct wall on the four peripheral readings. I take these readings several
times during the day -- they tend to be quite consistent. Typical readings
would be 740 fpm center, with readings around the periphery that vary from about 530
fpm to 630 fpm, but the readings at each peripheral point are quite consistent
from one reading to the next. The swirl component of the velocity is less
than 100 fpm.
The afterburner on the new sunspace propulsion system.
I'm expecting it to do Mach 0.86 at 32,000 ft.
It is (maybe) interesting to note that before the flow straighteners were
installed, and with a lot of swirl velocity, taking the 5 readings in the same
places as mentioned above -- each individual reading was markedly different than
the after flow straightener readings, but the average of the 5 readings with and
without flow straighteners was close.
I may further check these readings with a Pitot tube survey, but the
velocities are down in the range that are difficult to measure accurately with a
Pitot tube. Past experience leads me to believe that the
Kestrel is probably the best way to go.
Air Density -- I use a density of air of 0.061 lb/cf -- this
correction from the sea level standard conditions of 0.075 lb/cf accounts for
our altitude of 5000 ft. and 80F air temperature. Based on
this calculator.
What is it Like to Live In?
I spent the better part of several days in the sunspace during the testing,
so these are some impressions of what it is to be in the sunspace while its in the
process of gathering heat for the house.
There were a lot of times when the sunspace was quite pleasant to be in.
But, there were times near midday when the glare from the sun and reflection off
the snowfield were a bit uncomfortable for me (this may be a matter of personal
taste). I think that some form of shade cloth like screen that runs
parallel to the glazing and can be deployed to filter the sun, or pushed to the
side to let in all the available sun would be good. It would allow you to
quickly adapt to full sun, part sun and light overcast conditions and to
personal tastes.
Another observation is that without the fans to remove the heat and send it
to the house, the temperature in the sunspace would reach sauna levels in very
little time after the sun gets on it. Some form of control over
overheating is a must for these heavily glazed low mass spaces -- using the extra heat to
provide needed space heating to the house is a win-win. Along these lines,
the more flexibility you can build into the fan/ventilation system, the better
-- multiple fans that can be staged, and/or variable speed fans, and a control
system to match will help match the air removal rate to the solar input.
But, I don't want to make this sound too complicated -- people build $20
sunspaces using a roll of poly film and are ecstatic with the results.
Even though I would call the Dayton/Grainger fans I used relatively quiet,
the fan noise would be objectionable for (say) reading the paper or having a
conversation or just having a cup of coffee. Using fans that
are very quiet, and/or muffling them, and/or getting them into locations that noise shield them from
the sunspace is going to be important to enjoying the sunspace.
As a place to hang clothes for drying, it would seem very good. The air
is warm and is moving, which should reduce drying time, and it could be setup so
that the sun shines directly on the clothing to further reduce drying time.
One observation is that it does not take very much sun to warm the sunspace
up to where its comfortable to be in. Even in some cloudy conditions that
keep the sunspace from gathering enough heat to do much space heating, it was
able to heat the sunspace itself to a comfortable temperature.
Another thing that is no surprise is that that when sunset rolls around, the
temperature in the sunspace falls like a rock. It very quickly becomes a
very chilly place.
If these characteristics are not what you want out of a sunspace, then you
can alter the design to to the direction you want -- for example, adding more
mass. The space heating produced by the sunspace will drop some when you
do this, but if you get other benefits, that's fine. Sunspaces are very
fallible -- its up to you to adjust the design to achieve the mix of functions
you want.
Each of the test write-ups (above) has a section on the comfort level of the
sunspace during the tests, and include thermal images and temperature surveys to
see what things are like over the course of a sunny day.
Cost
This is a very rough idea what an attached sunspace modeled after the
freestanding greenhouse in the pictures above might cost. Its based on
what I paid for the materials for the GH, but adjusted down in places because
the sunspace would not have a north wall. Its made from all new
materials.
It includes what you see in the pictures -- nothing fancy.
Item
Cost
Glazing
$414.00
Foundation
Supplies
$285.00
Framing and
plywood
$420.00
Shingled
roof
$150.00
Insulation
$150.00
Door
$157.00
Fans
$180.00
ductwork
$77.00
Controls
$60.00
Paint,
Great Stuff, …
$100.00
Total
$1,993.00
Bear in mind that costs of low thermal mass sunspaces can vary over a huge
range. There are a couple of examples of dirt cheap ones that can be built for $25 ish, all the way up to ones
that cost a couple hundred or more per sqft. Thermally they all behave in
a similar way -- the cost
is more a matter of the look and feel that you want to achieve.
Just as a very rough idea how other solar heating options might compare:
- 200 sqft worth of commercial air heating panels (you
install) -- roughly $10,000.
- 200 sqft of DIY air heating panels with fans, ducts,
... $1500?
- 200 sqft of DIY simple thermosyphon air heating
collector like
this one $450
Of course, the sunspace comes with all sorts of functionality beyond the
solar heating, whereas the rest of the options just do heating.
Potential Further Improvements
While the test sunspace as it is does a very efficient job of producing heat,
there are some things that might improve it.
What causes a sunspace to be less than 100% efficient? Two main
reasons:
Part of the solar radiation is absorbed by the
glazing itself, and never gets into the sunspace. For good single
glazing about 90% gets through, and for good double glazing a bit over 80%.
So, you lose 10 to 20% off the top to glazing absorbtion.
Heat loss out the glazing from the warm sunspace
interior accounts for most of the rest of the reason we don't see 100% efficient
collectors. This loss is partly conduction through the glazing from the
warm air next to the glazing, and partly radiation at thermal wavelengths from the
absorber toward the glazing.
Some things that might help:
Reducing the temperature of the air near the glazing to reduce
conduction heat loss through the glazing -- see Deep Mesh below as one
example.
Reducing the temperature of the absorbing surfaces, so that less heat is
radiated out the glazing. Efficient transfer of heat from the absorber
surface to the air is probably the main factor in reducing absorber
temperature. Good airflow on the absorber, lots of absorber surface
area, flow through absorbers (like screen), ... might all contribute to
lower absorber temperatures.
Selective surfaces that are often used in commercial active collectors that
have low emissivity in the heat radiation wavelengths might also help.
There are some glazing materials (like Teflon) that have higher light
transmittance and might be used for the inner layer of glazing on double
glazed sunspaces.
One thing the sunspace already has going for it is that air velocities next
to the glazing are quite low -- this reduces the efficiency of heat transfer
from the air to the glazing, and that is good.
Deep Mesh
Nick Pine's sunspacehas a fabric mesh that is fairly close
to the glazing. He introduces the supply air (probably around 65F) between
the mesh and the glazing. The mesh has enough air resistance to spread the
supply air over the full surface of the mesh. So, the sun heats the mesh,
and the uniform airflow through the mesh takes that heat away. The desired
end result is that the air next to the glazing and the mesh absorber will both
run relatively cool, and that this will reduce heat loss out the glazing, which
is what efficiency is all about. Hopefully as Nick gets further along with
his sunspace we will get a reading on how well this works.
Storage
If the sunspace is large enough to produce more heat than the home can use
during a sunny day, It would be desirable to be able to store some heat in the
sunspace for home heating use after sunset. While its easy to add mass to
the sunspace to store heat, the problem is that with the normal sunspace design
a lot of this heat will be lost out the sunspace glazing before it can be used
by the house.
Some possibilities would be
Movable insulation on the sunspace glazing. This would
reduce heat loss from the sunspace and would make the sunspace more
comfortable as well as allow storing house heat in the sunspace without as
much loss.
There are some sunspace and greenhouse insulation curtain designs out there
-- they tend to be somewhat difficult due to the large glazed area, but not
impossible.
Some of the commercial shades for home windows that have side tracks provide
R values up toward 4 (US).
It might be possible to provide movable insulation over sunspace thermal
mass. These insulation panels would be closed around the thermal mass
to reduce heat loss after the collection period. For example, one
might have water barrels along the north wall for heat storage with an
insulating curtain or panels that deploy after sunset to insulate the mass
from the sunspace. Forced air circulation around the barrels would
transfer heat to the house. Another possibility would be to just have
a massive north wall that heats in the sun during the day, and has an
insulating panel over it at night. This would be like a Trombe wall,
and would transfer its heat to the house later in the day. The book
Movable Insulation, Langdon is a good source of ideas for (surprise)
movable insulation schemes.
Of course, if the sunspace is modest in size and the home is large and heat
leaky, then the thermal mass of the home will store some heat during the day
without the house overheating, and it will give that heat back after sunset.
Air Pickup
An air system that picks up heat efficiently from the absorbing surfaces
would help to keep the absorbing surfaces cooler, and would reduce heat loss out
the glazing. Sunspaces provide a lot of room to work with, so maybe there
is a way to direct airflow so that it picks up heat from the absorber more
efficiently?
Absorber Design
An absorber that absorbs the heat from the sun efficiently and transfers it
to the air efficiently will run cooler and lose less heat out the glazing.
For example
tests
of flow through screen absorbers in DIY solar collectors have
proven them to be a good deal more efficient than (for example) a bare black box.
Contrast to Active Collector
In this application, the sunspace is being used as an alternative to a
conventional solar air heating collector to provide space heating.
Its interesting to think about some of the contrasts.
The flow velocities are much lower in the sunspace -- in most places I
can't measure them with a good air velocity meter and you really can't feel
them, but in a conventional collector, air velocities are fairly high -- up
toward 100 fpm and more.
In a conventional collector, there is usually a significant air velocity
along the glazing, where as in this sunspace its too small to measure.
The surfaces that absorb the solar radiation are spaced further from the
glazing, are larger, and often run cooler.
The flow pattern in a conventional collector
can (it seems) be controlled with channels, baffles, screens. What
would one do in a sunspace to control and direct the flow to the desired
areas?
The sunspace has more outer heat loss area and more potential leakage
area.
Do these differences offer some opportunities for greater efficiency?
Other Ideas?
Got other ideas? Use the Comments section below
or email me.
Conclusions
Some conclusions from these the two tests of the optimized sunspace and one
test of the non-optimized:
Low mass sunspaces can be very good space heaters for your home -- equivalent
to high quality active solar collectors of the same area.
Performance takes a significant hit if the design guidelines are not
followed.
The environment in a sunspace that is designed as an efficient space heater
can be quite comfortable and useful for SOME other functions while still
providing lots of heat.
There is likely some opportunity for improvements in the design.
The measured heat output and efficiency of the optimized sunspace was close
to identical to a very good commercial solar water heating collector operating
under the same conditions. I think this is a heck of a deal given that you
can also use the sunspace for a whole raft of other useful functions.
While I was expecting the optimized sunspace to do better than the
non-optimized sunspace, I was surprised by the size of the difference. The
non-optimized sunspace with no insulation, no dark painted surfaces, and a heat
absorbing (wasting) floor had an efficiency of about 20%. The optimized
sunspace with improved hot air removal (2 fans), R28 wall and ceiling
insulation, insulated floor, and all dark absorbing surfaces showed an
efficiency of about 55% under similar conditions. Nearly 3 times the heat
output.
Just as a check on whether this large difference seems plausible:
- Thinking about the heat loss through the uninsulated
walls and roof:
For the insulation, the total area to be insulated is about 300 sf. If you
assume that the average inside temperature is 70F and outside is 30F, and that
the current plywood walls are R1, then the heat loss is (300sf)(70F - 30F)/(R1)
= 12,000 BTU/hr. The total solar incident on the 200 sf of glazing is about 60000 BTU/hr, and you would expect to only
harvest about half of that in a good collector, so the 12K BTU/hr is a
significant fraction of that.
In a true attached sunspace, these losses would not be as bad as it was for our
simulated sunspace because the attached sunspace shares its north wall with the
house.
- Thinking about the heat lost due to the bare dirt
floor:
With the sun in its current low elevation, only about 18/115 = 16% of the solar
incident on the glazing at noon hits the floor of the sunspace. This is
about (0.16)(70K BTU/hr) = 11000 BTU/hr directly incident on the floor. If
a sizable fraction of the 11K BTU/hr is absorbed by the floor and heats it, that
is a sizable loss of heat to the house.
The heat wasted in the sun heating the ground mass would increase as the sun
gets higher in the sky because a larger fraction of the incident solar hits the
floor.
I have heard reports from sunspace owners that some form
of insulating cover on the floor does actually make a significant difference in
performance.
I guess the message is that there is a reward in heat output for minding the
details. There are times when you may well chose to go away from the
optimal design for space heating to pursue other benefits, and that's fine as
the sunspace is a multi-function gadget and you may well be willing to sacrifice
some house heating to gain in other areas.
Comments
Looking for comments, questions, ideas, errors, experiences with your own
sunspace...
If you have a sunspace and are up for taking some pictures, doing a description,
and (hopefully) some performance measurement, I'd love to add it to our
collection.
