SOLAR THERMAL Here, we discuss active solar thermal systems, requiring investment in components to absorb, transport, and store that energy. In case you wonder why we talk about actively storing the sun’s heat, instead of using it as collected, the sun is only out part of the time, whereas space heat is needed around the clock, mostly at night. Also, DHW, the other possible application, is often needed before, or after, the sun is shining. Further, the sun’s delivery of heat is low and slow, but
the demand comes in relatively high bursts, as in taking a shower,
or feeding lots of heat to the exchange coil in an air handler. The first step is to capture (absorb) the heat content of the sun’s rays. Careful study has shown that a flat black surface does this best. Because that captured heat must be sent somewhere else, flat black paint is applied to an array of copper tubes, through which a liquid is circulated, to take the absorbed heat to storage. To keep the wind off, and to reduce heat loss from the back of the absorber, the copper sheet, with its tubes, is enclosed in a rectangular box, with a glass cover. The manifolds that serve the tubes at top and bottom protrude from the corners of the box. Typically, this collector assembly is about 3’ by 6’, and about 5” deep. Any number of these may be mounted side by side, usually on the roof to minimize interference. Transport: The transport medium is water, which is cheaper and more efficient than other liquids. In closed-loop systems, with piping and roof-top collectors always full of liquid, the water is dosed with anti-freeze. This is so the water will not freeze and burst the pipes on a cold night. The presence of toxic glycol impels health authorities to require that the heat exchanger be double-walled, further reducing efficiency of transfer. There is a circulating pump in the loop, and a control that turns the circulator on when the collector array is heated by the sun until it is 10-15 degrees F hotter than the storage. The circulator continues to run until the temperature difference (Delta T) drops below 5-6 degrees. In drain-back systems, which are more efficient, but not understood by plumbers, no anti-freeze is required, because the storage water drains out of the system every time the pump stops. Also, no heat exchanger is needed, because the storage medium (water) is heated directly in the collectors. Storage: Water’s specific heat is higher than that of any other common material, liquid or solid. For example: If 100 pounds of rock were heated to 150°F, then allowed to cool to 100°F, the rock would give up only 1,000 BTUs. The same weight of water would have stored and released 5,000 BTUs! That is why water should be used for heat storage! Space Heating: For a 1600 square foot house with 4” insulated walls and
double-glazed windows: If the temperature dropped to 0°F,
and stayed there for 24 hours, it would require about 200,000
BTUs to keep the house comfortable during that period. That load
could be met from the solar system’s storage tank only
if it held about 1,200 gallons of water, and started at 150°F!
It would have taken twenty or more collectors, and a spell of
sunny days to get that tank up to 150°! By now, you
may be thinking that there must be a better way. There
is, and we are getting to it! See Stored Solar below! DHW is a different story. With two or three collectors, a good system can bring a 120-gallon storage tank up to 160°F in one day, and that will serve a family of four through the evening plus a cloudy day. It will even go a second day, if the dishwasher and clothes washer are postponed for a day or two. For a family of two adults and two children, the daily DHW load averages about 36,000 BTUs. A drain-back system with 3 collectors can easily meet this load on a clear day in summer. In winter, the Northern states may only average half of that. With the tax credits that are already in place, the homeowner will get his investment back within five years. Geothermal: The term “geothermal” refers to the heat generated by the earth itself. It is not “solar”, although it could be classified as “renewable energy”. Surrounding the earth’s molten core, there is a mantle that is furiously hot, the heat source that we call “Geothermal”. We experience geothermal heat when the liquid plasma from the mantle bubbles through the crust, or when water in the crust leaks down close enough to the mantle to boil and return to the surface as steam. Through cracks in the crust, geothermal heat has surfaced, and proven useful, in some parts of the world, including Iceland and New Zealand. Volcanoes and displays like Yosemite provide familiar evidence of geothermal energy. It does not seem likely that we will soon spend the money, or suffer the risk, necessary to actually harvest this awesome force, except where nature has first cleared the way. Stored Solar Heat: There is another kind of heat under the ground, sometimes inaccurately called Geothermal. Starting five or six feet below the surface, and extending several hundred feet downward, the year-round temperature measures about 55°F. This is stored Solar heat, the result of water, heated by the sun, that has leaked through the crust, forming our aquifer and warming the earth around it. Under Heat Pump we described how the refrigeration cycle can be used to dump house heat into air that is actually warmer than the inside of the house. And, it was shown that, by reversing the direction of flow, the same system will heat the house in winter, although it does not do well when the outside temperature drops below 40°F. The heat pump is much more efficient when its outdoor exchanger is in the stable 55° environment of the earth beneath our feet. This system is called the Earth-Coupled Heat Pump or, sometimes, the Ground-Source Heat Pump. It is an established industry, with dealers in the US and Canada. They are trained and certified by an international association, and waiting somewhat impatiently to be discovered. There will be more info on the Ground-source heat pump under Enargy News. It can be a permanent source of savings for homes and small buildings. It needs only for the home-investor to realize the importance of thinking about Return on Investment. |