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An overview of heat solar collectors.

Solar Energy Concepts

In the last post, “Where is the biggest bang for the buck in solar energy?” , I hope the data was convincing that the best place to use your time and money is toward heating your house and possibly the hot water you use.  Tonight’s post is going to work from the other end of the process, the sun.

The sun produces many different types of energy and at many different frequencies.  One that we are all familiar with is the heat the sun produces that warms our skin when we are warm in the sun on a cool spring day. Radiant heat energy is at a frequency below where our eyes can see and is called infrared. Most of the sun’s energy reaching the earth is in the visible light frequency range.  Some of it is above visible light in the ultraviolet range.

One way to picture light is that it travels in a line from the source to the object being illuminated. Light striking that object can do several things.  It can be reflected, absorbed, refracted, and/or simply pass through the object.  The “and/or” was used because it is seldom the case that it does just one.  For example, glass transmits light through it, but we have all seen the windshields of cars reflecting light when the sun is low and the light is striking it at an angle.  We all know that a car parked in the sun with the windows rolled up produces a very hot car.  This is partly because the glass will allow visible light to pass but not infrared energy.  A red shirt reflects the red frequencies of light, and absorbs much of the other frequencies.  A black shirt absorbs pretty much all of the visible light range.

Because the sun is so far away we can assume that all the light rays are parallel to each other before they enter our atmosphere.   However, on an overcast day sunlight strikes moisture in the sky and is refracted or bent.  Since this is a somewhat random process they rays are now coming to us on the ground from various angles.  This is called diffused light.   We can see that by noticing the difference in shadows cast on an overcast day and those on a clear day.   On a clear day the line between shadows and bright light is sharp and crisp.  On an overcast day the line between shadow and bright light is much less well defined.

The goal of a solar collector is to absorb as much of the light as possible and convert it to some other energy form and for our purposes here we will talk primarily of converting it to heat.   Black is the best color to absorb the light.  Once the light is absorbed and turned into heat the object will also heat materials next to it and radiate infrared energy.   We want it to heat only the materials we want heated and not to heat the open air.   One method is to carry the heat away as soon as possible before the collector heats up to a high temperature.  This was done with very inexpensive solar heaters designed to heat swimming pool water and to get an extra month or two of use out of the pool.  This would be of no help in direct heating our houses or heating water for domestic use. To get the temperature at a higher level we need to capture the heat by insulating the collector and capturing the air around the collector.  In other words by mounting it in an insulated box and putting a window over it to keep outside air from stealing the heat from the collector.  This adds cost to the collector, but is necessary to get the temperature up.

If the collector is not aimed toward the sun it is not of much use.  The really fancy collectors can rotate in two planes and follow the sun.  This costs even more money and adds a lot of complexity to the system with motors and controllers and some way of tracking the sun.  More often the collector is positioned facing the average direction toward the sun.  This would be south in the northern hemisphere and slanted at an angle a little greater than the latitude of the location.

The very high temperature producing solar collectors require concentrating reflectors so the sunlight striking a large area is all concentrated in a small area.   There is two problems with this type of set-up.  First it must track the sun and that requires the motors and controls we talked about earlier.  Second the sunlight must not be diffuse light and this only works in areas where there are many cloudless days.

In summary, flat plate collectors are our best choice for collecting heat.  We have not yet talked about how to use that heat and that will be in a future post.  Currently I am reviewing a couple of books about solar energy.  So far these books look excellent and I will be posting a review later this week or sometime next week.   I believe these books will give the detailed information you will need to construct your own solar collectors.

Gary

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