Sustainable Product – MaxSolar

Max S. Dunn
BUS 183, November 2008

Sustainability Problem

In many areas during the summer, the nightime air is cold enough so that a well-insulated building can be opened up and cooled down during the night and then closed up during the day to minimize the cooling energy needed. However, the inverse is not true. In most areas during the winter, the outside temperatue will constantly stay below the comfort level so there is no opportunity to open up the building during the day to warm it up. Therefore, it is necessary to constantly expend energy to keep a building at a comfortably warm temperature.

However even in the winter, the sun shines frequently and if its heat was captured, it could provide an energy-efficient way of helping to heat a building. These types of systems are referred to as solar thermal and produce heat directly, as compared to solar photovoltaic (PV) systems that turn heat into electricity. Solar thermal systems don’t require exotic materials so they can be built cheaply. They also are about 65% efficient which is much higher than the 15% efficiency of PV systems.

(Reference: Solar Heating for Energy Generation and Demand Reduction)

Product Description

MaxSolar is a solar system that heats air to reduce the energy required to heat a building. It is made using standard materials found in a building supply store.

The basic principle of operation is as follows. Cool air from the room is drawn into the lower duct and flows up through holes in the bottom of the frame.  The clear polycarbonate glazing blocks the damaging UV rays and let in the sun's heat. The heat is absorbed by a layer of aluminum foil painted black. Aluminum foil is used to conduct the heat more evenly across the entire surface. The foil also has flaps to mix up the air as it rises to better extract the heat. Heat loss is minimized by insulation on the back and the glazing on the front. As the air heats up, it naturally rises to the top and proceeds through the holes in the upper frame and into the duct at the top of the window. To improve efficiency, a small fan pulls the hot air out of the top duct.

Similar Products

SolarSheet

The SolarSheat is a glazed recirculation solar air collector designed for space heating applications. It incorporates a solar panel to power the fan, so no external energy is needed. A SolarSheat 1500G produces up to 20,000 BTU/day, weighs 110 lbs, is 87" by 43" and costs $1900.

SunMate

The SunMate needs to be plugged in to run the fan, but it only pulls 8.5 watts. It weighs 86 lbs and is rated at 5.56 KWh/day or 19,000 BTU/day at 9°F and costs $1760.

DIY

There are many plans for do-it-yourself (DIY) solar heating projects including a detailed plan for a solar heater for a workshop barn. For other solar heating products, see: Solar Air Heater Directory.

Why Is It Better

The main advantage to MaxSolar over other products is the cost. While not as efficient as the commercially available units, the cost is about 85% less.

Product Construction

Materials:

• 6 – 2×4x8, $12
• 1 – 4×8x1/16 plywood $12
• 40 – 1” flat head wood screws $4
• 10 – 2” countersunk wood screws $4
• 10 – 3” countersunk wood screws $4
• 1 – 4×8x1/2 polystyrene insulation $8
• 2 – 50 sq ft, heavy aluminum foil $8
• 1 – tube caulk adhesive $4
• 2 – 2×8 clear polycarbonate corrugated roofing (Suntuf) $40
• 1 – package Suntuf closures $7
• 1 – pack of 50 short Suntuf wood screws with sealing washer $5
• 2 – BBQ black spray paint $10
• 2 – 8 foot 4” duct tube $20
• 10 feet – flexible duct $18
• 1 – roll of duct tape $3
• 1 – 12”x36”x1/2” piece of wood $7
• 30’ – 1/2” weather stripping, black $7
• Gallon exterior paint $11
• 4” duct fan $10
• Duct exhaust cover $9
• Duct backflow valve and cover $19
• Window lock $3
• Tools: Drill, 1” butterfly bit, 3/16 drill, 4” hole saw (or jig saw), matte knife, paint brush

Total Cost: $225

Construction Method:

• Cut a 2”x4” into two 44 3/4” lengths
• Place these two inside two 8’ 2”x4”s and screw in to construct the frame.
• Run a caulk bead on the top of the frame, place the 4’x8’ plywood on top and screw in with 2” wood screws
• Cut the styrofoam insulation to fit with the matte knife and glue inside
• Turn over and drill 15 holes 1” in size 6 inches apart in the long sides of the frame centered between the top of the foam and the top of the frame
• Glue a layer of foil on the styrofoam overlapping 1”
• Glue another layer on top
• Cut a 2x4 to a 3' length and then rip cut is along the 4" axis to take off 1/2"
• Place this support vertically on top of the foil and screw it in from the back
• Cut 3” flaps in the top layer of foil alternating on the way up
• Spray paint the inside of the frame black
• Paint the outside of the frame with any exterior paint
• Cut the rigid duct to 7’ 3”
• Unroll the rigid duct tube and drill 3/16 holes every foot 1/2” from each edge
• Screw in the top of the duct 1/2” below the side edge of the frame
• Curl the duct around and screw in to the bottom of the frame
• Caulk the duct where it attaches to the wood
• Build the legs and add to the frame
• Cut one high ridge off the polycarbonate
• Attach the weatherstripping to the top and bottom of the frame (long edges)
• Place the Suntuf closures on the outside vertical frame members and middle support
• Lay on the Suntuf panels with the cut edges (low edges) on the outside and adjust so middle seam butts up tightly
• Drill holes for the Suntuf screws and attach screws
• Cut the 1/2 piece of wood to fit snugly in the window frame leaving 6" for the duct
• Drill two 4" holes: one at the top and one at the bottom
• Paint
• Attach the backdraft valve at the top and the grill duct at the top
• Mount the exhaust motor in the top duct
• Attach duct grills

 

Performance

Calculated

This location in Cupertino, California is at 37.3 degrees latitude and -122 degrees longitude and the highest angle of the sun on Nov 17, 2008 is 33 degrees. (Ref: SunAngle.com)

From the NASA Solar Energy Data we can determine that the highest insolation at this location on a clear day in November is about 0.8 kW/m2. (0.41 kW/m3 average * 3.59/2.83 max/average factor * 1.5 vertical/horizontal factor). Since the MaxSolar is about 2.8 m2 and probably has an efficiency of about 50%, it should produce about 1.1 kW of heat at its maximum.

Measured

The output of MaxSolar goes into a room that is about 150 sq ft. To test how much energy it would take to heat up the room, a blow dryer was hooked up to a kWh meter and run until it used 0.5 kWh. This raised the room temperature by 5 degrees.

Next the MaxSolar was used and it took 2 hours to raise the room by 5 degrees, which means it was only putting out 0.25 kW. However, this needs to be adjusted because the MaxSolar had a shadow on it from the neighbors roof that started out with 1/2 of the panel exposed to the sun and then decreased to only 1/4 exposure, for an average of 3/8 exposure. Also, the test started at 9:15 so the full amount of solar insolation was not reached yet and the NASA data indicates this is about a 13% difference. Adjusting for these factors gives an output of about 0.750 kW. This suggests the efficiency might be only 34%.

This low performance could be partially due to the low volume computer fan that was used for this test. A higher volume fan would likely enable better performance. (See Air Flow versus Power Delivered graph.)

Total Energy Produced

Looking at the average amount of heat it should produce, the NASA Solar Energy Data gives us these averages for the cold months. (The kWh/month takes into account the 1.5 correction for a vertical surface, the 2.8 m2 size of the MaxSolar, an efficiency of 50% and assumes 30 days per month.)

Month	kWh/m3/day (Horizontal)	kWh/month
Nov	2.83	178
Dec	2.15	135
Jan	2.41	152
Feb	3.36	212
Mar	4.75	299
Total		976

 

This gives the MaxSolar a maximum possible seasonal output of 976 kWh or 3.3 million BTUs.

Note: The fan draws only 3 watts. So for the entire season, it would consume only 2.7 kWh, or about $0.30 worth of electricity, so its impact is minimal.

Payback

• Material cost $225 (labor not included)
• Total yearly production about 3.3 million BTUs
• 1 therm = 100,000 BTUs = $1.40
• This reduces natural gas cost by about $46 per year
• Paypack is about 5 years

Lifecycle

It is difficult to do a full life-cycle analysis without the requisite software, so this analysis will focus on just electricity used and CO2 produced in the materials manufacturing process.

Lumber from softwood is estimated to take 10.7 MJ/kg of processing energy, of which 5.8 MJ comes from renewable biomass. So the external energy needed is 4.9 MJ/kg, or 1.36 kWh/kg. The CO2 produced from wood production is estimated at 1.26 lbs CO2/kg. (Ref: Gate-to-Gate Life-Cycle Inventory of Softwood Lumber Production)

Aluminum produces about 26 lbs of CO2 and uses 15 kWh for every kg produced and polycarbonate produces 12 lbs of CO2 per kg produced. (Ref: Carbon Free CD Project and Energy and Environmental Profile of the U.S. Aluminum Industry - Figure 4-1)

The polystryene insulation is projected to produce 0.29 lbs of CO2 and takes 0.14 kWh of electricity per kg. (Ref: Energy and Environmental Benefits of Extruded Polystyrene Foam and Fiberglass Insulation Products in U.S. Residential and Commercial Buildings) and Critical Thinking Exam about Population and Energy Production)

Information on the manufacturing of polycarbonate was not found.

Material	Amount	Electricity	CO2
Wood	30 kg	41 kWh	38 lbs
Polycarbonate	4 kg	?	?
Polystyrene	1.5 kg	0.2 kWh	0.4 lbs
Aluminum	0.6 kg	9 kWh	16 lbs
Total		50 kWh	54 lbs

 

Doubling these numbers to take into account the polycarbonate production and other materials used leads to a materials production estimate of 100 kWh of electricity and and 100 lbs of CO2 produced.

Since MaxSolar will save 976 kWh of energy and about 820 lbs of CO2 each year, the manufacturing energy and CO2 production will be paid back in much less than one year.

Environmental Impact

Using solar heat helps save fossil fuels and reduces CO2 produced. Avoiding burning 3.3 million BTUs of natural gas will save about 820 lbs of CO2 emissions per year.

However, some of the materials used in MaxSolar may have adverse environmental effects.

The Suntuf panels are made from polycarbonate (PC) which can leach bisphenol-A (BPA), especially when exposed to hot or boiling water.  On one hand, the pharmacological test results from major studies indicate that consumer exposure to BPA at concentrations normally experienced in daily living does not pose a risk to human health. On the other hand, some toxicological studies indicate potential risks to human health. (Ref: Environmental Assessment for Bisphenol-A and Polycarbonate)

There are also other materials used in MaxSolar including caulk, paint, zinc screws, wood, ducting, plastic, electronics, etc. These all have environmental impacts as well.

Improvements

The MaxSolar solar heating system still doesn't produce enough energy to heat the whole house. For instance, our 2,400 sq ft house used about 300,000 BTU a day last January and February and this solar heater will put out about 20,000 BTU/day - less than 7% of the total heat needed.

Solar air systems are estimated to save between $1 and $4 of heating costs per square foot of solar heater. This is consistent with this system which is 30 square feet and is estimated to save about $46 per year. Also, a typical house would need about 300 square feet of solar heating to be practical.

An improvement would be to build the solar heater right into the entire south facing wall by painting it black and covering it with glazing panels and incorporating ducts through the walls. With this method, it would be necessary to have a method of covering the panels and closing the vents during the summer. However, this would greatly increase the amount of heat produced and dramatically lower the costs.

Conclusion

Solar air heating systems have an advantage of being low cost and of using sustainable energy. However, since the amount of heat they put out is relatively low and they are used only part of the year, commercial units have long pay-back periods that can be over 30 years. However, home-built systems can be made much more cheaply and have payback periods of around 5 years. If used to heat just one room, small systems like the MaxSolar can be sufficient. For whole house heating though, it is necessary to have larger systems and preferrably be incorporated into the new building design.