SOLAR BASICS
What is Solar?
1. Thermal Solar
Thermal solar involves absorbing sunlight and converting that into heat. This is much easier and more efficient than converting sunlight into electricity, as sunlight is already heat radiation from the sun!
Low Temperature
For low temperature applications (<150°C / 304°F), a dark surface is used to absorb the sunlight. Technological developments over the past 20 years have resulted in surfaces that can absorb as much as 93% of available sunlight, and only reflect a very small amount. This means that solar collector systems can reach efficiency levels in excess of 75%! The key design consideration for thermal solar collectors is how to prevent the absorbed heat from being lost to the outside environment. This is where evacuated tubes have a great advantage, with the vacuum greatly reducing heat loss. The task of the solar collector is then to efficiently transfer the absorbed heat to water which is circulated through the panel. Solar collectors are ideal for both domestic and commercial applications where hot water, heating and even cooling is required.
High Temperature
For high temperature applications requiring steam production, concentrating thermal solar is used. This involves using an array of mirrors which focus the sun's rays onto a absorber tube, or sometimes even a tower. This method is similar to using a magnifying glass to focus the sun's rays, the result being extremely high temperatures, enabling the production of steam for electricity production. This system is able to be used in regions with high solar radiation levels and plenty of open space. Due to the size and complexity of these systems, they are only really viable for commercial applications.
2. Solar Collector
Thermal solar collectors or "solar water heaters", fall into three main categories; unglazed, flat plate and evacuated tube.
Un-glazed Collectors
Un-glazed collectors (meaning no glass or synthetic cover) are most commonly a rubber matting style collector primarily used for pool heating. This system works on the simple principle that black absorbs heat. The pool water circulates directly through the piping absorbing heat. This system is widely used for pools because it can accept direct pool flow as the piping is rubber and plastic and so corrosion is not a concern. It is also fairly cost effective and maintenance free. The downside to this technology is poor efficiency due to heat loss, meaning that they do not work in cooler weather. This type of solar collector is not suitable for domestic hot water or space heating as it cannot reach the higher temperatures required.
Flat Plate Collectors
Flat plate collectors are comprised of an absorber layer, usually copper or aluminum with a series of copper "risers" through which water is pumped. Behind the absorber layer is usually glass wool or mineral wool insulation. The top of the panel is a toughened glass or similar material. In many regions the panel is combined with a storage tank and uses passive thermo-syphoning to facilitate water circulation, thus not requiring a circulation pump. Flat plate collectors have been in use for many years, and are still the most widely used. Flat plate collectors perform well in warmer climates, but due to poor insulation, have low performance levels when the ambient temperature drops. They also have a disadvantage when compared to Apricus evacuated tubes, because they cannot passively track the sun, as the absorber surface is flat. This results in poorer heat output in the morning and afternoon.
Evacuated Tube Collectors
Evacuated tube collectors are essentially a glass tube with the air inside evacuated to form a vacuum. The vacuum insulates very well against heat loss providing excellent performance in cold weather.There are many different types of evacuated tubes used for solar thermal collection. Apricus uses a twin-glass design which is comprised of two glass tubes, fused at both ends. The space between the two tubes is evacuated and forms the insulation layer. Other evacuated tube designs exist using a glass to metal seal at the top, but high stagnation temperatures have the possibility to cause the seal to fail due to the different expansion and contraction rates of the glass and metal. There is also a difference in the absorber of the tube. Apricus evacuated tubes have a selective absorber coating on the outside of the inner tube (see diagram below). This forms a round absorber which can passively track the sun from early morning to late afternoon. The heat from within the tube can be transferred to the water by means of a heat pipe or a U shaped water circulation pipe. Apricus collectors use a copper heat pipe which provides a number of key advantages. To learn more about heat pipes, please visit the How It Works page. The other common style of evacuated tube has a selective absorber coated aluminum fin which is fused to a heat pipe or U shaped water flow path. This type of tube has a glass-metal seal and is a single tube construction.
3. Photovoltaic
Photovoltaic (PV) is the most well known as it has become very popular and widely used, from the tiny panels on a personal calculator to huge panels that can provide electricity to run a whole household. Multinational corporations like Toshiba and Shell are involved in PV, making it high profile in the media. PV uses silicone crystals, sliced into thin layers which have the special property of creating electricity when exposed to sunlight. PV is a fantastic product, but has the disadvantage of high cost and relatively low efficiency. Commercially available panels have peak efficiency levels of around 15%, although this is set to improve with continued research. For more information on sunlight, and how PV works please visit this site.Thermal solar has much higher solar conversion levels than PV, converting as much as 60-70% of all available sunlight to usable energy. The initial investment is also much lower than PV for a given energy output, the result being a much lower $/Watt cost.
