Why Solar Thermal Is Better Than Solar Electricity
No matter how much solar electricity you can generate, you still have to pay for the electricity to power your fridge, lights, TV, and all the other gadgets in your home. But if you live in a colder climate, reducing your heating costs is where you’ll really feel the savings in your utility bills.
And the best part? You can always build a solar thermal collector to heat your domestic hot water, pool, or spa. That means your solar thermal system can be useful year-round, not just during the heating season.
By far, the most efficient way to harness the sun’s energy is to collect its heat directly. That’s why I’ve built three working solar thermal systems to meet my various energy needs. Each one serves a specific purpose, and I’m going to tell you all about them.
Can I Actually Build This?
It mainly comes down to your perseverance. Many internet sources that promote DIY solar thermal systems will tell you they’re a breeze to build. And I suppose they can be. But in my personal experience, I’ve had plenty of tough challenges along the way with all three of my projects.
The garage heater was 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.
Choosing the Right Location
The proper place to put your collector depends mainly on what role it’s going to play. If the collector is for domestic hot water, you’ll want optimal year-round performance. A good rule of thumb is to tilt it at an angle close to your local latitude. For example, I live at 47 degrees north, so my DHW collector is tilted at 45 degrees.
But 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 south-facing wall of your house. A vertical collector will produce minimal heat during the summer and maximum heat during the winter – exactly what we want.
And get this – the area the collector takes up will likely be more insulated than the rest of your house. So that 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 construction, you won’t have to buy as much siding, either.
Conducting a Solar Site Survey
Before you even think about building your collector, you need to do a Solar Site Survey. Follow the link and read every single word on the page, then follow the directions to track the sun’s path across the sky.
The results of the survey will tell you if there are too many objects blocking the sun from reaching 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 site, 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, please please please contact Gary or me and get some help.
Researching Solar Thermal Collector Designs
If you’ve done the survey and everything checks out okay, then it’s time to start planning your build. You should really do your homework before attempting to construct your first solar thermal collector.
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, and 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 also 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.
Building the Frame
The frame is a very basic structure made out of 2×6 lumber and framed similar to a stud wall. It 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, which 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: it 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.
The drip cap is made from a piece of 2×8 lumber. It overhangs the front of the frame by 4.5″ to provide a drip edge and prevent rainwater from getting in behind the glazing. The whole frame is secured together with 1/4″ lag bolts.
Constructing the Absorber
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 – in this case, air.
Absorbers are almost always black and made out of metal. The absorber in my solar thermal collector is made out of aluminum soffit material, painted flat black.
How the absorber works:
Vented aluminum soffit is basically 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. The metal efficiently transfers heat to the air.
This process can occur passively through natural convection, or it 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.
Installing the Absorber
A basic frame made out of 3/4″ x 3/4″ lumber, similar to a stud wall, was assembled. 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 way as if it were installed on the eave of a house.
A 3/4″ x 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, and it allows me to seal the groove between the soffit and the wood border with black silicone caulk.
Finally, a snap switch is installed on the back of the absorber. It’s a little switch that closes at a temperature of 110°F and opens again at 85°F. It’s a simple control for the fans that ensures 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.
Mounting the Collector to the Wall
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.
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. If you can’t anchor into the studs, I suggest using a bolt length that will just barely pass through the sheathing and use a lot of them.
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.
Installing the Insulation and Ducting
The polyisocyanurate insulation was cut to size and fit into the collector’s frame. 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. 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 necessitate the use of fans, electronic, and control equipment, which just about doubles the cost of the system and severely affects the reliability of the collector’s performance.
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 compared to glass. Its 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 requires the use of foam strips along the top and bottom edges and specially shaped wooden strips along the vertical edges.
Adding the Fans and Controls
Each of the three collector bays has a small 4″ duct fan installed at the top as an exhaust outlet. The fans are rated at 80 Cubic Feet per Minute 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. These help ensure that air can only flow in one direction through the collector – from the bottom to the top.
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.
Please, for the love of all things solar, don’t do this. 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.
Evaluating the Collector’s 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.
But I can tell you that with an insolation of 1000 W/m2 and an efficiency of 50%, a collector the size of mine will produce 3,345 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.
Instead, I’ll have to rely on a qualitative analysis of the performance. This collector is built in a typical way and is of a design that is generally accepted as a good performer by those who actually do use sophisticated measuring equipment to evaluate their collectors.
On a sunny day, it’s 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°C and 17°C by the end of the 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°C to -15°C, 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.
There’s an emotional benefit to having this collector, too. 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, with 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 quickly, and 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 its 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!
