Solar photovoltaic panels for the production of electricity and solar water heating set-ups are familiar technologies to most off-the-grid pioneers. There is another type of extremely useful solar energy technology, however, that has been flying a little bit beneath the radar – solar hot-air space heating. Its relatively unknown status is a shame, because solar hot-air heating systems provide a convenient and cost-effective way to help keep interior spaces cozy and warm during the wintertime months. In fact, in climates that experience relatively modest winters, it is entirely conceivable that a well-designed and smartly installed solar hot air system could provide all the heat that a home really needs with some extra to spare. But even when combined with a more conventional style of heating, solar hot air can still be a real money and/or resource saver for just about any homeowner in any geographical location.
The Basic Facts
While other types of solar energy technology use the sun’s energy to perform additional work, such as producing electrical power or heating water, solar hot air is a perfect example of a “what you see is what you get” kind of technology. The idea behind solar hot air heating systems couldn’t be more basic: the heat produced by sunlight is captured inside a collector and is then used to heat the air inside homes, garages, barns, work sheds, or office spaces. Because the sun’s energy is allowed to do its work instantaneously right in the location where it is being collected, solar hot-air systems are the definition of simplicity and efficiency.
Solar hot-air systems begin with solar collectors, which are basically flat, insulated boxes that consist of a dark metal absorber plate contained within a shell covered with glass or plastic glazing in the front. A collector used for space heating is seamlessly integrated into the south wall of the space it is expected to heat, and it features openings at the bottom and top that allow convection currents to form as air constantly circulates (cool air in, hot air out) from inside the adjacent room.
Solar hot-air heaters generate a low-grade form of heat, which means they will produce air with temperatures in the 90-to-140 degrees Fahrenheit range. Interestingly, the lower end of that temperature potential is preferred because more heat escapes as waste on the higher end, so the most efficient solar hot air heaters will produce a steady stream of air at right around 90 degrees. Direct currents of air vented into a room at this temperature can make things feel drafty and uncomfortable, however, which is why multiple vents to diffuse the airflow is usually a good idea.
There are several different models available for those interested in solar hot air systems, and many, many possibilities exist for do-it-yourselfers because of the wide variety of materials that can be used to make simple but effective solar heaters. As far as size goes, any collector that has a square footage of less than 20 percent of the floor space in the area it will be expected to heat will fall into the small-to-moderate range, while a collector larger than that would be capable of producing enough heat to handle more than just a couple of nearby rooms.
There are two fundamental categories of solar hot air heater – passive and active. Passive systems rely on the natural flow of air to distribute the heat they produce, while active systems come with blowers and ductwork that can move the heat to different locations around the house. The latter is less expensive than the former, but the former tends to make more efficient use of the heat it produces. Some of the best options available in each category will be discussed below.
Passive Solar Hot-Air Systems
Most solar hot-air systems are of the passive variety. Passive solar systems can be as small as a jumbo-sized pizza box or large enough to cover or replace an entire south wall, although even the largest versions would probably not be able to produce enough heat to keep a whole house comfortable during the winter in most locales. Those with the inclination to build their own solar systems often start with passive solar collectors for space or water heating because the designs are so simple and the required materials seldom cost much.
The two most common types of passive solar heaters are window box collectors and thermosiphoning air panels (TAPs). The former are relatively small but incredibly easy-to-build insulated hot boxes that are placed on the ground or on a platform directly outside of a window. The solar energy captured by the collector’s metal plate will heat air entering the collector through its intake/outtake vent, which actually opens into the room to be heated through the window – and hence the name, window box collector. Convection currents will keep the warm air flowing in and the cold air flowing out, so that all of the heat captured by the window box collector is ultimately passed on into the adjacent room. Window box collectors are made to be used in conjunction with ground floor windows on a house’s south side, and the relatively small amount of heat they produce would not be intense enough to have an impact outside the room into which they open.
Thermosiphoning air panels are integrated directly into south walls, with an air intake vent at the bottom and an outtake vent at the top that connect to the room the collector is expected to heat. These dark square panels literally appear to be a part of the outer wall of the structure, and in some cases they can be so extensive that they essentially replace the wall on the south side of the building they are used to heat. The sleek, darkened surface of a TAPs collector can actually be quite aesthetically pleasing, and they are so effective at heating adjacent rooms that if they are allowed to exceed the 20 percent floor space limit, they can actually cause too much heat to be generated.
Active Solar Hot-Air Systems
Active systems allow air to be redirected from rooms on the south side of a structure to other places where the heat may be needed more. Blowers, fans, and ductwork distribute warmed air throughout the home, and if the collector used is large enough, an active system could conceivably heat a whole house throughout the winter months. Perhaps not if we are talking about a house located in Alaska, or Maine, or Minnesota, but in more southern locales such a system might just be able to replace a regular furnace or other type of “normal” heating set-up.
A crawl space or basement distribution system funnels the heat collected into these areas underneath the main living area, where it can then warm the floor directly above it and significantly reduce a home’s overall heating requirements. Generally, this type of system will duct the warm air to the northern part of the structure before releasing it, allowing the air to pass on its heat to the coldest part of the house first before making its way back to the collector’s intake vent. This type of active system will rely on a differential thermostat to turn blowers on or off depending on whether or not the solar collector plate is hotter than the crawl space or the basement.
A direct-use system essentially works like a more conventional heating set-up would, transporting warmed air throughout the home via an extensive ductwork grid opening into different rooms through a series of heating vents. In some cases, a more limited type of direct-use system can be used to heat two or three rooms instead of an entire home; of course the choice of how to distribute the heat produced will depend on the size of the collector used. Because a direct-use system can be easily tied into an existing ductwork layout, it is perfect for retrofits where it can be used to complement or even replace a conventional furnace.
With an especially large solar collector, it would be possible for the ambitious homeowner who likes to do things big to set up a two-mode solar heating system. In this case, the heat collected will be used for two different purposes, or as a part of two separate hot air space heating set-ups. So for example, direct-use could be combined with crawl space, or either one could instead be combined with a system that uses hot air to heat water. With two-mode arrangements, a more complicated type of thermostat is used that can divert solar hot air to one system or another depending on where it is needed at the moment. A set-up that can divert heat for water can be an especially convenient choice because it allows a homeowner to keep using his collector even during the warmer months, when a heat-only system would normally have to be shut down.
While heat storage is not generally associated with solar hot air, with active systems it actually is possible to include storage options with the system. Rock bins placed directly beneath floors are the most common choice for those looking to store heat from a solar hot air collector for night time release; if each day’s supply of hot air is fed into the bin instead of into the home, the heat will be absorbed by the rocks and radiate up through the floor at a relatively slow pace over a twenty-four-hour period. On new construction projects, another way to add heat storage is put down a thick layer of gravel below a slab floor foundation, and then pump heat from a collector down underneath the home into the gravel. Again, the idea is to use the steady release of radiant heat from objects with high thermal mass to keep the floors of a home – and the feet that walk across them – nice and warm at all hours of the day.
While including storage may make a solar hot air project sound more attractive, choosing this option could double the cost of installation. Also, such an idea is only practical if a very large collector is used, something that is 200 square fee or larger.
The Sun Always Shines
What makes solar hot air heating systems so attractive is their combination of simplicity, efficiency, and universal cold weather utility. Most who choose to install such a system will use it to supplement another form of winter heating, but in the right circumstances and with a large enough budget, going with solar hot air as an exclusive solution is feasible.
If solar hot air sounds at all attractive to you, the best way to start out might be to construct your own passive window box collector or TAP set-up using one of the dozens of DIY blueprints or demonstration videos that are available at sites all across the internet. Then, if you are impressed by the results you get, you could investigate other options that would allow you to more fully and completely exploit the sun’s almost limitless supply of radiant heat and energy.
©2012 Off the Grid News