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Solar Thermal Photovoltaics
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An Overview of Solar EnergyThe overwhelming majority of energy used on earth is provided by the sun, in one way or another. Conventional fossil fuels (oil, gas, coal etc) derive their energy content from solar radiation (sunshine) from millions of years ago. Renewable energy supplies (sun, wind, wave etc) are provided courtesy of relatively recent or current solar radiation, either used directly in solar collectors or indirectly in the case of wind and wave powered devices. Direct solar radiation can be used in two ways. Either to power a photovoltaic panel, which uses photons (particles of light) to cause movement of electrons (particles of electricity) via a semiconducting wafer, or to heat water in a much easier to understand way. We are concerned with the latter here. Solar Collectors We all know from everyday experience that when the sun shines on something, it gets warm. Part of this solar radiation will be absorbed, and part will be reflected. We need to maximise the part that is absorbed. When something is warmer than its surroundings, it will radiate heat away from itself. We need to minimise this. These two requirements, together with a need to transport the heat (in the form of hot water) to where we want it, constitute the basic design criteria for a solar collector. Many different designs have been tried over the years, but today available systems fall into two broad categories, flat plate and evacuated tube types. Evacuated tube collectors are much better insulated, and so are more efficient, particularly during cold or windy weather. In the past they have been much more expensive, which explains the popularity of flat plate collectors. Evacuated Tube Collectors An evacuated tube consists of a doubled walled glass tube, rather like a thermos flask, with a vacuum in the space between the walls. The inner tube has a special dark coating on it. The vacuum is a very effective insulator, eliminating conduction (where the heat travels through solid or liquid material), and convection (where the heat is carried along by a fluid). The heat arrives, through the vacuum, as radiation, and is absorbed by the coating on the inner tube. This causes the temperature inside the inner tube to increase. Early types of evacuated tube collector simply passed water through the inner tube to collect the heat. This design suffered from several problems. The inside of the glass can become coated with scale which reduces the efficiency, and if one glass tube should be broken, the entire collector ceases to function. Also, the relatively large water content means that response time is poor, ie the panel takes a long time to warm noticeably. An improved design had a slender U shaped copper pipe, through which water was passed, fixed to a long thin copper plate inside the tube, which alleviated these problems. However, the most advanced design, used by Eco-nomical, has no water inside the tube at all, instead using a heat pipe. This is a narrow copper tube which itself has a vacuum in it, together with a small quantity of a fluid which, because of the vacuum, boils at a low temperature. As the sun warms the tube, the liquid in the heat pipe evaporates (turns into a gas). Because the collector is installed at an angle, the hot gas in the heat pipe rises to the top. The top end of the heat pipe is installed in a manifold and surrounded by water, which condenses the gas back into a liquid as the gas gives up its heat to the water. The heavier liquid then runs back down the heat pipe and the cycle recommences. The phase changes (liquid to gas and vice versa) actually enhance the efficiency of the heat transfer considerably. The Eco-nomical Collector Our collector consists of a stainless steel frame which bolts to your roof, carrying a number of tubes which are inserted into a header manifold at the top of the frame. Water from the hot water storage tank enters the heavily insulated manifold, and is warmed by the hot ends of the heat pipes as it passes around them. It then exits the manifold and either enters another collector for further warming, or returns directly to the heat exchanger in the hot tank. Gradually this cycle warms the water in the hot water tank. The tank generally has two heat exchange coils in it, one in the lower half and one in the upper. Using heat exchange coils in the hot water tank prevents the water from the collectors mixing with water for domestic use. This is desirable because it allows the water flowing through the collectors to be treated with anti freeze and other inhibiting chemicals. The collector is connected to the lower coil, because the water at the bottom of the hot water tank will be cooler than that at the top (hot water rises, cold water falls due to a difference in density), and the most efficient heat transfer takes place where the temperature difference is greatest. The upper coil is connected to the conventional boiler for back up heating (and is sometimes replaced with an immersion heater). Solar water heaters can be used for space heating, but it is much more usual for them to be used for domestic hot water (DHW). The main problem with space heating applications is that heat is required at times when there is little or no sun (eg December nights). In order to make much impact on heating requirements, larger collector arrays and large heat stores are required.
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Send mail to
SW@eco-nomical.co.uk with questions or
comments about this web site.
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