Solar heater how-to
Performance
On sunny winter days, the collector raises daytime interior temperatures to between 60 and 75°F (16–24°C), providing a comfortable workspace. In my neck of the woods, that’s 25 to 35°F (14–19°C) above the outside temperature. The workshop temperature rises about 10°F (6°C) for each hour the sun hits the collector. Warming the workshop from 35 to 65°F (2–18°C) usually takes about three hours. Through the night, and by morning, the building typically cools to about 8 to 15°F (4–8°C) above the outside temperature. On heavily overcast days, the collector does very little heating, but on partly cloudy days or with a thin overcast it does provide some useful heat. For optimal heating performance, be sure to provide adequate insulation and to control air infiltration. No solar collector will do a good job of heating a workshop that is drafty and uninsulated. With the walls and roof insulated to R-19, my 576-square-foot (54 m2) workshop has a heat loss of about 190 Btu per hour for each degree Fahrenheit difference. So, if it’s 60°F inside and 30°F outside, the heat loss is: (60°F - 30°F) x 190 Btu/hr = 5,700 Btu/hr. During periods of full sun, the collector will gain heat at a rate about three times greater than this. The graph shows my collector’s typical heating performance on a mostly sunny midwinter day. Although outside temperatures never rose above 40°F (4°C), the collector heated the building from 38°F (3°C) to almost 70°F (21°C) during the day. At night, when the collector isn’t working, the building’s temperature drops quite a bit. In the morning, it takes a few hours of sun to raise the temperature inside the workshop to a comfortable level—a good excuse to sleep in! If you are determined to start work early, more insulation, more thermal mass, or an early morning blast from a backup heater would be in order. One of the advantages of having a relatively large collector is that once the sun is on the collector, the heat gain rate is several times the heat loss rate. This excess heat raises the temperature of the building’s thermal mass fairly quickly. At midday, under typical sunny winter conditions, the collector provides a 50 to 60°F (28–33°C) temperature rise from the lower vent to the upper vent, and an average upper vent velocity from 110 to 120 feet per minute (34–37 m/min). The total gain on a sunny day is about 130,000 Btu (38 KWH). This is equivalent to burning about 2 gallons of propane at 70 percent efficiency. Heat gain estimates are based on measurements of the collector temperature rise and the vent exit velocity. Combining these with the density of air at temperature and the specific heat of air gives the collector’s heat output. I consider these estimates to be approximate, but solid enough to get a good feel for how well the collector works.
The rate of heat gain was estimated using the following equation: G = A x V x D x (Tu - Tl) x H Where G is the heat gain rate; A is the vent area; V is the velocity of air through the vent; D is the air density; Tu is air temperature at the upper vent; Tl is the air temperature at the lower vent; and H is the specific heat of air. I measured the temperatures with several US$2 Taylor thermometers from the hardware store. The vent exit velocity was taken using a Kestrel wind meter. Although this instrumentation might not meet Sandia National Laboratories’ standards, I believe it does provide a solid estimate of the collector’s performance.
Economics
Our only alternative would have been to heat the workshop with propane. And, although the cost of a propane heater would have been a bit less than the cost of building the solar collector, the ongoing cost of propane over our five month heating season would have been US$150 to 200 per year.The simple payback period of the collector is a couple of years on materials cost. You also can consider it as an investment of US$350 that’s reaping the benefits of an inflation-protected, tax-free return of 50 percent per year. If the collector has a life of 20 years, you are in effect paying in advance for all the heating the collector will produce in a lifetime—at fractions of a penny. Because I use the workshop intermittently, I can usually wait for a sunny day to warm the building. I haven’t needed to buy a backup heater, which is an additional savings. Collector Variations With a bit more investment of time and money, a couple of variations could be made to improve the system’s performance. Substituting dual-wall polycarbonate glazing in place of the single sheet of corrugated glazing would help reduce thermal losses through the glazing. This type of glazing, which provides two layers of polycarbonate sheets separated by support webs, also simplifies the glazing installation, since it requires less support and doesn’t require sealing the corrugated edges. Buildings in cold climates will benefit the most with this change. Using this glazing may increase the cost of the collector by 50 percent or more. Keep in mind that temperature fluctuations and solar exposure can reduce the life of the polycarbonate glazing to between ten and twenty years. Substituting tempered although it is more expensive and will require some design modifications. Alternating collector and window panels on the south wall is another design option. This method would allow more light into the space and some direct gain through the windows, without the glare, high losses, and overheating problems that accompany full window walls. You can use the same concept to heat a house or cabin. With some refinement to integrate the vents with the finished wall, the same basic design can be used to provide daytime heat to living spaces. A word of warning, though—the National Mechanical Code prohibits circulating conditioned air of more than 120°F (49°C) in wooden stud spaces. While this may not pose a problem for outbuildings, in buildings used for human habitation, consider constructing the collector with metal, rather than wood studs. As an extra measure of safety, wood areas immediately surrounding exit vents also In making changes to the collector, keep in mind that a thermosiphon collector must provide low resistance to airflow. Make sure that any changes you make do not violate these guidelines:
• The depth of collector should be at least 1/15th of the height;
• The absorber must have low resistance to airflow;
• The vent area should be at least 50 percent of the collector’s horizontal cross-sectional area; and
• The air path through the collector should be as shown in the diagram on the images page (see links at the end of the article)
Build It!
Building a solar hot air collector into new construction or adding one onto an existing building can be an easy and inexpensive heating solution. Following the simple principles and the plan outlined here will allow you to heat your workshop, barn, or even your home with free heat supplied by the sun. If it works here in Montana, it’s bound to work wherever you are. Here’s to your warmth and comfort.
Please check out our Solar heater images to help you with building this powerful solar heater.
You can download a PDF version of this how-to over here: solar heater
Our thanks at EnergyRefuge.com go out to the author of this article, Gary Reysa. Individuals like Gary don't just say they embrace alternative energy, they do it. Please check out his site..it contains tons of how-to's and solar projects.
Author:
Gary Reysa, 864 Glory Ln., Bozeman, MT 59715 • [email protected] ww.builditsolar.com
