Introduction: My Eco-Friendly Garage Heater: a Pneumatic Solar Thermal Collector
This instructable will cover the build of my 72 square foot solar thermal collector that harnesses the sun's energy to provide winter time space heating for my detached garage workshop. The functional part of the collector is a black aluminum absorber that heats up when the sun's rays are upon it. The heated absorber is used to warm up the air in the room via a continuous air current that is circulated through the absorber and into the garage.
During sunny winter weather the collector is capable of providing all the heat necessary to keep the garage warm - all for absolutely free. The collector can be very straightforward to build with absolutely no moving parts or electronics to wear out and need replacement. Although I will describe how to build a straightforward collector with no moving parts in this instructable, the one featured here relies on a complex and costly system of fans, motors, controls and electricity to function. If you want to build one of these, I strongly suggest that you take the simple and reliable route.
I am entering the instructable in the Epilog Contest VII with hopes of winning the grand prize Epilog laser engraver. As some of you may know from my previous and upcoming instructables, I have a love of electronics engineering and precise attention to detail. I try to be as precise as I can be with my simple woodworking tools but this laser will allow me to take my projects to a whole other dimension.
My ultimate goal with the laser is to use it to create a realistic simulation cockpit of the Apollo Lunar Module using backlit display panels, real working switches and dials and real working displays, gauges and avionics. I will then interface the cockpit to the Project Apollo NASSP software in order to create a totally immersive and realistic simulation of Lunar Module orbital maneuvers, descent, landing, ascent and rendezvous maneuvers. I'll need the laser to create the vast majority of the components of the simpit.
I hope you enjoy the read and if you think that this Instructable deserves a vote then please do accordingly. Cheers!
Lunar Module Cockpit photo credit: Airliners.net
Project Apollo Simulator photo credit: Music of the Spheres
Step 1: Why Go With Solar Thermal?
- The sun is a very potent and economic source of energy all over the world. It has the potential to easily provide all of mankind's gluttonous energy requirements many times over.
There are two main ways that one can harvest energy from the sun. One is with photovoltaics and another is with solar thermal. Photovoltaics convert sunlight into electricity while solar thermal collectors convert sunlight into heat. In most cases solar thermal is a better choice for several reasons:
- Initial Cost: Photovoltaics can cost many times more than solar thermal, especially when you build your solar thermal collector yourself. Solar thermal collectors can be built by your average DIY'er for somewhere on the order of one tenth of the cost of a comparative photovoltaic system.
- Efficiency: A good quality modern high-tech commercially built photovoltaic panel is about 16% efficient. A solar thermal collector made from salvaged materials and cobbled up in someone's basement can be as high as 60 - 75% efficient.
- Reliability: Photovoltaics require panels that decline in performance over time, chemical batteries that wear out and electronic inverters that burn out. Solar thermal collectors can be built entirely passive, with not a single moving part.
Why is Solar Heat Better Than Solar Electricity? No matter how much solar heat you can collect you still have to pay for the electricity for your fridge, lights, TV, etc. But if you live in a colder climate reducing your heating costs is going to hit your utility bills where it hurts anyways. In addition, you can always build a water heating solar thermal collector and use it for heating your domestic hot water, pool or spa, etc. This can make your solar thermal collectors useful year 'round, not just during the heating season. By far, the most efficient way to harness the sun's energy is to collect it's heat directly.
What did you build? I've built three working solar thermal systems to date. Each one occupies a specific role in meeting my energy needs. They are listed here in chronological order.
- My Pex-Al-Pex Hydronic Space Heater: This is the largest of the three and is used to provide wintertime space heating for my house. It heats up water that is circulated through it and delivers it to the pex radiant in-floor heating system in my house.
- My Aluminum Soffit Garage Heater: This is another space heater that will be the focus of this Instructable. It provides free heat for my detached garage. It heats air by percolating it through a sheet of vented aluminum soffit that is painted black and exposed to sunlight. Although it is great to have heat in the garage, it was built mainly as a pilot project for a similar but larger collector for the house.
- My Copper/Aluminum Hydronic Collector: A small and efficient collector that can provide some wintertime space heating but built mainly to provide year 'round domestic hot water for my dishwasher, sinks, showers, etc.
Can I build this? It mainly depends on your perseverance. Many internet sources that promote DIY solar thermal systems will tell you that they are very easy to build...and I suppose they can be. In my personal experience I've had many tough challenges along the way of all three of my projects that were difficult to get through. The garage heater was without question the easiest but you can still get discouraged. Like most challenges in life you just have to stand up, face it and knock it down. With that attitude you can most likely do this regardless of your DIY skills.
Where do I put a solar thermal collector? That's what the next step is all about. Please continue on...
Photo Credit: City Center Zoo
Step 2: Where to Put Your Solar Thermal Collector
The proper place to put your collector depends mainly on what role it's going to play. If the collector is going to be used for Domestic Hot Water (water for your showers, sinks, dishwasher, etc.) then you want optimum year 'round performance. A good rule of thumb would be to tilt it at an angle close to your local latitude. For example, I live at 47 degrees north and my DHW collector is tilted at 45 degrees. However, we're not focusing on DHW collectors in this Instructable. For wintertime space heating the collector should be vertical or as close to vertical as possible. The ideal location would be to mount it directly to the wall of your house. A vertical collector would produce minimal heat during the summer and maximum heat during the winter. That is exactly what we want and the reasoning behind that pattern is explained in the accompanying photo.
If you have a vertical collector mounted to the side of your house, please note that the area that the collector takes up will likely be more insulated than the rest of your house. This extra insulation will save you money rain or shine, day or night, heating or cooling. If you're building a new house and want to integrate the collector during the construction, you won't have to buy as much siding either!
The tilt is not the only concern. What may be even more important is the azimuth of the collector. Azimuth basically means what direction East, West, North or South the collector faces. If you live in the Northern Hemisphere you want to face your collector South or as close to South as possible. If you have a wall on your house or garage that is oriented close to South then bingo. If the best you can do is Southeast or Southwest then you'll take a hit in performance but you can still make this project worthwhile.
When you have a potential spot picked out the next thing you should do is a Solar Site Survey. Follow the link below and read every single word on the page and follow the directions to track the path of the sun across the sky. The results of the survey will tell you if there's too many objects blocking the sun from getting on your collector on a winter's day. It can also help you decide whether your Southeast or Southwest wall is the best one to put the collector on (if you have that issue).
The author of the above website Gary Reysa is very knowledgeable and experienced in solar energy projects. You can find his contact information on the site. If you're having trouble doing or understanding the survey then please, please, please contact Gary or I and get some help.
If you've done the survey and everything checks out okay then its time to build the thing.
Step 3: Suggested Reading
You should really do your homework before attempting to build your first solar thermal collector. If you've read this far I'm assuming you're interested in an air heating collector for wintertime space heating.
This page is an excellent source of information for solar air heaters. Make sure you watch the video and follow links to the updates and maintenance sections. The project outlined on the page is really the epitome of solar thermal collectors. I based my own solar collector on this design. I had to adapt the design to my own situation but I tried to stay as close to the original as I could
This page contains many good examples of solar space heating projects. Make sure you check out the "Passive Space Heating Systems" and "Active Space Heating Systems - Air" sections.
Build It Solar is a treasure trove of useful information on just about anything you can imagine that can help you save money and help save the planet.
PV Watts is a great tool for determining the amount of solar radiation that will land on your collector. It can tell you how much radiation you will receive on a monthly basis throughout the year or even on an hourly basis throughout the year. You can play with different azimuth and tilt angles to see how they will affect performance.
The Simply Solar Forum is a group that I as well as many solar energy enthusiasts participate in. If you want to discuss new ideas or get help on solar projects then this is the place to go. There are many first time builders that continually update us on their progress and get lots of help and advice along the way. My username on the forum is "EcoMotive" and you can visit the "Member Projects!" section to find all of my projects including the garage heater outlined in this Instructable. You can see my real time progress as well as all of the helpful advice and encouragement I received along the way.
Photo Credit: Peak Performance International
Step 4: Building the Frame
The frame is a very basic structure made out of 2 by 6 lumber and framed similar to a stud wall. The frame consists of two continuous horizontal plates; one on top and one on bottom, four vertical studs and a sloped drip cap.
The four studs are on 48" centers. This makes three bays that are 46.5" wide. The two middle studs are sandwiched between the top and bottom plates just like a conventional stud wall. However, the two outside studs are beside the top and bottom plates. There are two reasons for this; this allows rainwater to drip off the frame easier and it allows an angled cut to be made in the top of each stud for the drip cap to sit on.
Not including the drip cap the frame is 72" tall overall. This makes the two middle studs 69" long and the two outside studs 72" long plus the length of the angled portion at the top.
Not including the drip cap the frame is 145.5" wide overall. That's the width of four studs on 48" centers plus .75" from center to edge for each outside stud. The top and bottom horizontal plates are 142.5" long each.
The drip cap is made from a piece of 2 by 8 lumber. It overhangs the front of the frame to provide a drip edge to prevent rainwater from getting in behind the glazing. It is sloped downwards from the wall by 4:12. The slope is provided by the angled cuts on the top of each outside stud. On the back of the drip cap I ripped the same angle into the board along it's length. This returns the back edge to plumb so that it can rest flat against the wall. The drip cap overhangs each side of the frame by 1.5" for an overall length of 148.5"
The whole frame is secured together with 1/4" lag bolts. First a 3/4" hole is drilled for the fastener to a depth of 1/2". This allows the hex head and the washer to be countersunk into the frame. Next, a 3/16" hole is drilled the rest of the way through.
And finally, everything was covered with two coats of a good quality exterior all-in-one primer and paint.
Step 5: Building the Aluminum Soffit Abosrbers
In any solar thermal collector the absorber is the part that takes in the sun's radiant energy, gets hot, and then transfers the heat to the working fluid. The working fluid can be liquid or air. Absorbers are almost always black and almost always made out of metal. The absorber in this solar thermal collector is made out of aluminum soffit material painted black.
How the absorber works:Vented aluminum soffit is basicly a thin sheet of aluminum with hundreds of tiny holes called "perforations". When the soffit is painted flat black and exposed to sunlight it will get warm. The collector's air inlet is on the front of the absorber and the outlet is on the back side. As the air flows from inlet to outlet it will percolate through the tiny perforations in the soffit and pick up heat as it passes through. Because the absorber is made of metal it efficiently transfers heat to the air. This process will either occur passively through natural convection or could be fan forced. Passive convection offers the advantage of requiring no moving parts, controls or grid electricity while forced air allows the collector to operate more efficiently.
Each absorber is 46 1/4 wide. This allows for 1/4'' of wiggle room in the 46 1/2" wide bay. Each absorber is 64" tall. This allows for 5" of space for the inlet ducting at the bottom of the 69" tall bay.
A basic frame made out of 3/4" by 3/4" lumber similar to a stud wall was assembled. The middle stud is in the center of the frame. I temporarily attached the frame to a piece of plywood to hold it square while I installed the soffit material. The aluminum soffit was cut to length with tin snips and fastened to the frame using sheet metal screws. Each piece of soffit interlocks with the previous one in the same manner as if it were installed on the eve of a house.
Because the frame is almost 4' wide and the soffit comes in 10' lengths, each piece of soffit can span the frame 2.5 times with just a couple of inches of wastage. In order to make a full span with the two half pieces they must be joined together side by side in a fashion not normally done with soffit. In order to get them to fit correctly the groove at the top of one piece must be trimmed a bit to make a joint that looks similar to that on a piece of siding. The two pieces are then fitted together and joined on the center stud in the frame. The last piece of soffit to go on exceeded the frame by a few inches. It was simply trimmed to size with a pair of tin snips and then secured.
A 3/4" by 3/4" wood border was secured to the front of the absorber. This provides a nailing edge to attach the absorber to the inside of the collector frame. It also allows me to seal the groove between the soffit and the wood border with black silicone caulk. This forces air to pass through the perforations in the soffit rather than around it. A thin strip of 1 1/2" by 1/4" wood was secured to the bottom of the absorber to make a structure similar to a windowsill. This provides a good place to seal between the absorber and the back of the collector.
Finally, a snap switch is installed on the back of the absorber. It's a little switch that closes at a temperature of 110F and opens again at 85F. It's a simple control for the fans that ensure they only run when the absorber is warm enough to provide heat. Otherwise the solar thermal collector would actually cool down the room by radiating heat to the outside. For example, if the indoor living space was warm and the fans were circulating air through the collector during a cold winter's night then it would actually be cooling the room down.
A dab of thermal paste was put on the head of the snap switch and it was riveted to the back of the absorber. The thermal paste is the same stuff used between a computer's CPU and it's heatsink and ensures good thermal contact between the snap switch and the absorber. A dab of pure silicone caulk could also be used but wouldn't work quite as well. The snap switch is mounted to the back of the absorber rather than the front so that the switch's temperature reading isn't directly affected by the sunlight but only by the absorber's temperature.
Step 6: The EMT Horizontal Glazing Supports
The horizontal glazing supports are made out of 1/2" Electrical Metallic Tubing and provide extra support to the glazing than just the studs alone. EMT is a good material to use for this because it's compact, strong, straight, rust resistant and is cheap as dirt.
The glazing supports sit in 5/8" deep notches made into the vertical studs of the frame. Because of these notches, the EMT sits flush with the face of each stud. The notches are spaced 23" apart dividing the 69" tall bay into three equal sections.
Because one 10" length of EMT cannot span the entire collector one piece is cut to span two bays while a smaller piece is cut to span the third bay. The two pieces are joined on the center of a stud.
After the collector is installed on the garage an additional vertical glazing support will be placed in the center of each bay. This vertical support will be held in place by the horizontal EMT.
Step 7: Stripping the Siding and Installing the Frame and Re-Installing the Siding
The siding was removed from the South facade of the garage in order to attach the collector to the flat wall sheathing. Stripping off vinyl siding is a very easy and straightforward task. Start above the level of the top of the collector and work your way down. You can buy a special hook shaped tool for unlocking one strip of siding from the other but it's not hard to do with just your fingers. If the siding is too cold it will be brittle and could split apart of you're not careful. Use a hammer and a "Cat's Claw" to remove the nails from the nailing strip on each piece.
The collector's frame is attached to the wall with long anchor bolts that are countersunk into the frame's timbers. The location of each bolt was pre-drilled to line up with the studs in the wall. It was easy for me to plan out the bolt location since I framed up the wall myself and I know where the studs are. If you can't anchor into the studs then I suggest that you use a bolt length that will just barely pass through the sheathing and use a lot of them.
The frame was prefabricated in my workshop months before and was simply assembled on the wall. The finishing touch of the frame was a sloped drip cap on top to shed water out over the face of the collector.
A few pieces of metal flashing were installed on the sides and top of the collector to further prevent water infiltration behind the collector.
Finally, the siding was re-installed around the collector's frame using the appropriate J-Trim for a finished look.
Step 8: Insulating and Drilling the Holes for the Ductwork
The polyisocyanurate insulation was cut to size and fit into the collector's frame. A few wood screws with some small squares of OSB as a washer were used to secure the sheets into place. A bead of expanding foam insulation was then used to seal the edges of the sheets.
The 4" duct holes were then drilled through the wall on the top and bottom of each bay of the collector. Now, before I continue I must say that ideally this would be a passive thermosiphoning collector with a long horizontal hole along the entire top and bottom of each bay. Unfortunately, I couldn't do that mainly because of the large amount of cabinets and shelving on the back wall of my garage. The small holes for air passage in my collector necessitates the use of fans, electronic and control equipment which just about doubles the cost of the system and severely affects the reliability of the collectors.
The holes are carefully located in order to avoid wall studs and electrical wiring. As a result, the holes are not exactly in the center of each bay but as close as possible to it. A 12" long 1/4" auger bit was used to drill a hole all the way through the wall and out the other side. This way, the center of the hole can bee seen from both sides. Then a 4-1/8" hole saw was used to bore out each duct hole using the mark from the auger bit as a common center from both sides.
A short length of 4" round metal duct was mated to a 4" round fishlock collar, screwed together, foil taped and installed into each of the six duct holes. A small square of 1/4" OSB that was bored in the middle with the hole saw was used as a flange to attach the tabs of the fishlock collar to.
Step 9: Installing the Absorbers, Baffles and Glazing Supports
Before the absorber is placed within the bay, the snap switch is connected to some two-conductor cable via crimp-on blade terminals. The cable is rated to 110 C. The cable is routed to the inside through a small hole drilled into the wall. This is done for each of the three bays.
The absorbers are installed in each bay with a tilt outwards at the top. The bottom of the absorber is up against the insulation at the back of each bay while the top is flush with the face of each bay. The absorbers are 1/4" smaller than the bays in order to allow for a little wiggle room and to account for any small variances in the "squareness" of the collector frame or the absorber. The absorber is installed in the same manner as you would install a pre-hung interior door. The absorber is "hung" in the frame using a few small washers as shims and wood screws fastened through the "jambs" of the absorber. Installation is finished with a bead of caulking along the edges between the absorber and the collector frame.
The baffles are simple devices made out of a rectangular piece of sheet aluminum and a frame of small 3/4" by 3/4" wood strips. The wood frame only has three sides in order to not impede airflow across the baffle. Finally, the flare strip at the top is made from a 45 degree tin drywall corner and is riveted to the aluminum sheet. The whole thing is painted black.
The baffle is used to deflect the incoming air upwards towards the absorber. It prevents the conditioned room air from impinging directly on the glazing surface which would cause excessive heat loss. The path of moving air should be kept away from the glazing as much as possible since air washing directly over the glazing would cause it to lose a lot of it's heat to the outside.
The baffles are then installed in the bottom of each collector bay, in a similar way to the absorbers. The installation is finished with a bead of caulking in the edge between the baffle and the collector frame.
Finally, the EMT (Electrical Metallic Tubing) horizontal glazing supports are installed simply by placing them into the slots cut into each "stud" of the collector frame. Each segment of the EMT tubing is joined in the middle of a "stud".
Step 10: Installing the Glazing
The glazing allows the sun's rays to penetrate through while providing a draft proof and somewhat insulated barrier to trap heat inside. It's constructed of clear corrugated polycarbonate panels and the appropriate horizontal and vertical closure strips and EPDM roofing screws.
Polycarbonate is a near ideal glazing solution for solar thermal collectors. It's strong, shatter resistant, heat resistant, UV stable and it isn't all that expensive (at least, compared to glass). It's main downside is that it tends to last somewhere between 10 and 15 years before it needs replacement whereas glass will last practically forever.
The "wiggly" profile of the polycarbonate sheets makes it a bit tricky to get them to seal properly along their edges. This will require the use of foam strips along the top and bottom edges and specially shaped wooden strips along the vertical edges.
The nominal width of these panels is 24" - that is they will span studs that are 24" on center. They overlap each other by a little on each side making their actual width about 25.5". Each bay of the collector is 4' wide from center to center of each stud. This necessitates the installation of a vertical support strip in the middle of each collector bay where the two adjoining panels overlap. The vertical support strips "rest" up against the horizontal EMT tubing behind them.
The wooden strips were run off on a table saw using standard lumber. On the outside vertical edges and in the middle of each bay they follow the complete profile of the polycarbonate panel but the two strips over the inside "studs" are simple rectangles.
A special foam "wiggle strip" runs underneath the top and bottom edges of the polycarbonate panels. They match the corrugated profile of the panels and provide a good air seal when compressed by the panel. The foam is sealed down by a horizontal strip of wood placed on the outside of the top and bottom edges of the collector. This wood strip is cinched down over the polycarbonate and foam strips in order to help provide uniform pressure.
The polycarbonate panels are fastened along the outside edges and where they overlap over the vertical support strips with metal roofing screws. The screws come with a small EPDM rubber washer that helps seal the hole for the fastener. Each hole for the screws are pre-drilled at a diameter of 1/4", which is a little bit bigger than the fastener to allow for thermal expansion and contraction of the panel.
Step 11: Installing the Fans, the Fan Enclosures, Backdraft Dampers and Wiring the Fans
Each of the three collector bays have a small 4" duct fan installed at the top as an exhaust outlet. The fans are rated at 80 Cubic Feet per Minute. They fit right into the 4" duct that comes in through the wall from the solar collector and are secured with sheet metal screws and sealed with foil tape.
The plywood boxes that surround each fan are built mainly for aesthetic purposes but also provide a mounting surface for the backdraft dampers, which consist of a wire mesh screen and a piece of a garbage bag that's placed in front of each fan.
The backdraft dampers help ensure that air can only flow in one direction through the collector, that is, from the bottom to the top. When the collector is operating, air will flow into the collector through the bottom inlet, up through the absorber in the collector and then out through the exhaust at the top. This air circulation will occur with or without the fan as the convection of warm air through the collector will occur naturally. The problem is that when the collector shuts down in the evening and you have a room full of warm air but a cold collector, the convection current will occur in the reverse direction, pulling warm air out through the top duct and then cooling it as it sinks through the collector and returns through the bottom duct. The backdraft damper allows air to flow out of the exhaust duct when the collector is operating but seals itself against the wire mesh when the collector shuts down, preventing warm air from getting back out.
Each fan has an octagonal electrical box mounted to it with sheet metal screws. The octagonal box is important for providing a protective enclosure for the wiring connections as well as strain relief for the BX supply cable. The wiring connections are pretty straightforward. Don't forget to properly ground the equipment using the raised ground screw. Also, don't forget the anti-short bushing for the BX cable. Because I have a rather unwise knack for making things overly complicated (and expensive), each of the three fans has it's own dedicated power and control circuit from the main control panel. Please don't do this. If you build a fan forced collector with multiple fans, put them all on the same circuit.
Step 12: The Control Panel
The control panel is an overly complicated, complex, expensive and unreliable piece of hardware that houses the power distribution and switching equipment for turning the collector's fans on and off at the appropriate times. It's a NEMA rated enclosure with a bunch of knockouts on the top and bottom for routing various cables. The main breaker, terminal blocks, relays and DC power supply are mounted to 35mm DIN rail and the wires are run through plastic wire duct.
I said it before and I'll say it again... This is not the way to go for powering your collectors. A simple control circuit consisting of a snap switch - - - - - - - will suffice. Better yet, build a passive collector and then you won't have to worry about any of this stuff.
I won't delve into a whole lot of detail on how the panel is built or how it's wired. The actual circuit schematics are actually quite simple and straightforward and I don't want to sound crass but if you can't figure it out yourself then perhaps you shouldn't be attempting to build it.
The functional unit of each control circuit is a small "snap switch" installed on each absorber in the collector. The snap switch is a simple Single Pole Single Throw switch that closes when the temperature of the absorber reaches 110F. The closed switch completes a circuit that provides the coil of a relay with 12VDC, switching the relay. When the switching circuit on the relay closes, it completes that circuit and provides the fan with 115VAC power, causing it to run and circulate air through the collector. When the absorber temperature drops below 110F, the snap switch opens, cutting power to the relay coil and thereby cutting power to the fan which will stop the air circulation through the collector and seal the backdraft damper.
Each of the three collector bays has it's own independent power and control circuit described above. This causes each of the three bays to operate independently of one another. This does provide a small boost in efficiency since the greenhouse's shadow moves across different areas of the collector as the day goes on. The specific bays of the collector that are still illuminated by the sun can continue to run even though the others are in shadow.
Step 13: Making Improvements
Where possible, the collector should be as close to a nominal height of 8 feet as possible. This will allow for the greatest efficiency in materials, allow more area for circulating air to spread out over the entire width of the absorber and make best use of available wall space. Ideally, in a standard 8' high living space, the collector inlet should be as close to the floor as possible and the outlet as close to the ceiling as possible. Expanding on that 8' high recommendation, the most efficient width of the collector will be any even multiple of 4'. Make it as wide as your budget or space constraints will allow.
For the tenth time in this article I'll say that a passive collector is the way to go. It's a lot simpler, cheaper, more reliable, is less noisy, will require virtually no regular maintenance, repairs or replacement and it will operate normally during a power outage.
In order to make a passive thermosiphoning collector work well, the vents at the top and bottom have to be rather large - virtually the entire width of the collector. This would be a relatively simple task during new construction but may be problematic with a retrofit. The challenges with the size of the vents on my very crowded garage wall is the principal reason I didn't build a passive collector but I'm kicking myself for it now.
I have included some 3D sketches of an improved collector scheme that would probably be aesthetically suitable for an interior living space. Explanations are included in the pictures. These are cutaway views that don't include all parts - just the ones necessary for illustration.
The last five pictures show a hydronic collector array for a superinsulated passive-solar house I'm going to be building in the spring. The new collector design includes some improvements I have thought of since building the garage heater...
The biggest improvement being the use of an "inner frame" (shown in white) and an "outer frame" (shown in dark grey). The outer frame sticks out from the wall further than the inner frame on both the sides and top. When the polycarbonate glazing is installed on the inner frame it is then recessed down into the outer frame and can then be well sealed against drafts and wind-driven rain (the latter being a huge concern in our climate).
The outer frame should ideally be made from rough-sawn lumber (as opposed to dressed and stamped construction lumber) with a couple of coats of good quality exterior siding/deck/fence stain. This will ensure a durable finish that will last many years. Stain all sides of the lumber before installation so that it doesn't warp.
Step 14: Performance
I don't own the necessary equipment to do a true qualitative analysis of how good this collector performs. For that I would need a couple thousand dollars worth of gear including a pyranometer, an anemometer, several digital thermometers, a data logger and the appropriate software.
When Photovoltaic panels are tested for efficiency and power output, the standard parameter for insolation is 1000 watts per square meter which is a realistic value for a typical bright sunny day. Good quality photovoltaic panels typically have an efficiency in the neighborhood of 16% but with a solar thermal collector constructed with an aluminum soffit absorber you can expect efficiencies well North of 50%. With an insolation of 1000 w/m^2 and an efficiency of 50%, a collector the size of mine will produce 3345 watts of heat energy. Where I live it would take a PV system costing $23 411 (including taxes and installation) to produce that much heat. Where I live PV installations have to be done by licensed electricians so that jacks up the cost. My solar thermal collector would have cost me less than $550 if it was built as a passive thermosiphoning unit. It could have been built nearly for free if I wanted to scrounge for used wood, glass and scrap metal.
Instead I will have to rely on a qualitative analysis of the performance. First of all, this collector is built in a typical way and is of a design that is generally accepted as a good performer by those people who actually do use sophisticated measuring equipment to evaluate their collectors.
Also, my collector doesn't perform as well as it should because for most of the winter it's partially in the shade of the greenhouse. At the time of building this collector I had plans of rebuilding the greenhouse into a more efficient design that would lower the overall height to the point where it no longer affects the collector. This hasn't been done to date.
Nevertheless, I am still happy with the performance of this collector. On a sunny day it is able to create and maintain an indoor air temperature suitable for working comfortably in my garage. Depending on the sun's intensity and other conditions, temperatures will rise to between 12 and 17C by end of day. That might not seem like much but keep in mind that the starting temperature of the garage on a winter's morning is typically -10 to -15C and there's a lot of thermal mass in there to heat as well. Also, the garage is not incredibly well insulated or air sealed.
Most people are unimpressed when they put their hand over the exhaust vent of the collector to feel cool air at 10C blowing out. They fail to realize that the air is entering the collector at -15C and is actually being warmed by 25 degrees. Once the room temperature reaches 10C then the collector exhaust will be 35C and so on.
If this collector was used in a well insulated room that already contained room-temperature air then the collector's good performance would be more readily apparent although no more impressive.
There's an emotional benefit to having this collector as well. In the winter mornings on my days off I'm usually out in my workshop before dawn, freezing my butt off. It really lifts the soul to hear the collector wake up in the morning. Shortly after the sun comes up you can start to hear the aluminum absorber creaking and crackling as it starts to heat up. Suddenly, you can hear the snap switch close, followed by the relay cycling and the fans whirring to life. The air temperature rises by the first fifteen degrees or so fairly quickly and then it's more comfortable for working. Every morning like this is a proud moment where you bask in the satisfaction of putting together a complex system of components that work together in harmony to accomplish a task. The whole is definitely more than the sum of it's parts.
Thank you so much for reading and if you decide to take on a project like this please feel free to contact me if you need any help. Cheers.
Photo Credit: 10MPG