Solar Power
The sun delivers energy in the form of both heat and light, these can be used either directly or converted into electrical energy.
Solar Thermal
Solar heat, or thermal energy, is used in its most basic form in solar water heaters. This equipment is designed to heat water for various uses including domestic hot water requirements but they DO NOT generate any electricity. They can save the average household a great deal of electricity by substituting, while the sun is shining, the function of the electrical heating element in the boiler or geyser, although some of the energy savings claims made by this industry might be somewhat ambitious. Check these claims with your energy consultant when assessing the viability of any particular installation.
Solar Water Heaters (SWHs) come in 2 basic forms: Flat Panel Collectors and Evacuated Tubes.
Flat Panel Collectors
These are fairly easily recognised in the form pictured here. They work on the principle that hot water rises so as the water in the panel is warmed by the sun it flows into the tank at the top of the panel where it replaces the cooler water into the panel which then heats the water and it rises..... and so on.
As a rule of thumb you need about 1m2 of panel for each 50 litres of hot water required. i.e. 3m2 for a 150l geyser
In South Africa the panel needs to be mounted on a north-facing section of roof at an angle of around 35o to the horizon.
The tank is often a source of contention from the point of view of aesthetics, so always remember the tank can be mounted inside the roof space. In this case, if the tank cannot be mounted above the top of the panel, a pump will need to be employed to circulate the water because the natural thermo-siphon effect will not work.
Remember that the panels can be mounted "landscape" if necessary as opposed to the conventional "portrait" to facilitate the mounting of the tank higher than the panels.
Evacuated Tubes
The other type of SWH is called “Evacuated Tube”. These comprise glass tubes coated with a heat absorbent paint inside another clear glass tube. The space between the tubes is a partial vacuum to help keep the water in the inside tube hot.
These systems are very efficient but can be very expensive. They are also subject to a bit more maintenance than their flat panel cousins and are somewhat more delicate.
Check with your energy consultant to make sure that any extra cost will be returned in extra efficiency and that the cheaper, more robust flat panel collector will not be sufficient for the task in hand.
There are also some comparatively inexpensive systems available for heating swimming pool water but these are not able to achieve the temperatures required for domestic use.
It is worth repeating here that solar water heaters do not generate any electricity at all.
Photovoltaics (PV)
This is the technology that converts sunlight into electricity. These panels are quite distinct from the solar water heaters from the previous section. They are often blue in colour but can be black but they always have segmented collectors visible as small squares.
Exactly how light is converted into electricity is not in the scope of this discussion, suffice it to say here that the active ingredient or semi-conductor in most of these panels is Silicon, either mono-crystaline (black) or multi-crystaline (blue). The mono-crystaline is more efficient, but in our climate, I’m not sure we need the extra efficiency. As always, check your particular circumstances with your energy consultant.
Thin Film
Another technology that falls under photvoltaics is commonly known as "Thin Film". Great expectations have been generated for this cheap and easy method of converting sunshine into electricty, not all of them have been realised.
There are, broadly speaking 2 types of thin film, amorphous silicon (a-Si) and multi-junction. Again, the technicalities of these is not in the scope of this paper but the silicon is cheaper than the multi-junction and much less efficient. The multi-junction thin film utilises some very exotic materials such as gallium, indium, selenium and tellurium. Some of these are very rare and therfore very expensive but they are so efficient that only very small amounts are required. Nevertheless, these are not generally used in ordinary flat panel collectors. We’ll revisit these later in the paper under CHP and CSP.
I am often asked about "the South African chap who inventred a thin film product", so here’s the story as far as I know:
The chap in question is Professor Vivien Alberts who worked at Johannesburg University where he developed his process. It proved impossible to raise the money required for further development in Souith Africa so he took his product to Europe where he was snapped up by a German company. They started a company called Johanna Solar to manufacture thin film solar panels and commenced the building of a factory for this purpose.
Prof Alberts was eventually persuaded to bring his product back to South Africa and he took over a building in Paarl for the purpose of local manufacture. But then a problem cropped up in the German factory and everything ground to a halt. The project in Paarl was stopped and as I write this I’m not sure if the factory in Germany is actually in production or not. The website would have us believe they are, but I’ve not heard anything further from the Paarl site.
Concentrated Solar Power (CSP)
Boy scouts know that you can set fire to things if you shine sunlight at a certain angle through a magnifying glass. The same principle applies to photovoltaics, if you put a lens between the PV material and the sun you can get some very impressive efficiencies from the system. This one from Sungri goes up to 1500x magnification. Notice the fins on the back to help disperse the heat. Silicon PV is no good in this application because it doesn’t do very well at high temperatures, but the exotic multi-junction cells excel in these conditions and, because of the lens, so little is needed that the cost is now much less of a factor.
It’s also possible to concentrate sunlight with mirrors onto a certain point. There are 3 technologies which employ mirrors in this way: Parabolic Troughs, Power Tower (sometimes called Central Tower) and Stirling Dish.
Parabolic Troughs
The largest operational solar power system at present is one of the SEGS plants and is located at Kramer Junction in California, USA, with five fields of 33 MW generation capacity each. Some of these SEGS plants have been delivering base load power for as much as 20 years and they never missed a single hour of production in all that time.
The 64 MW Nevada Solar One also uses this technology. In the new Spanish plant, Andasol 1 solar power station, the 'Eurotrough'-collector is used. This plant went online in November 2008 and has a nominal output of 49.9 MW.
There are plans on drawing boards across the globe to install a further 5,800MW of parabolic trough technology in the next few years, some of them as big as 290MW.
The sun’s heat is reflected from the mirrors onto a collector tube which is filled with a heat transfer fluid, usually a synthetic oil, which can reach 450oC. This heat is used to turn water into steam which is used to drive a turbine which, in turn, drives the generator.
A derivative of the parabolic trough is the Compact Linea Fresnel Reflector (CLFR) which addresses many of the disadvantages of the traditional parabolic trough systems. Amongst other things, it is much lower to the ground and so is less susceptible to windage, it is much less complicated to manufacture, install and operate and offers more flexibility in its range of service in that it can operate from around 100kW to many hundreds of MW. This system is perfect for my model of locally produced distributed power due to its modularity and scalability. Lower temperatures allow this system to incorporate and Organic Cycle Turbine which, although it is less efficient than its high pressure, high temperature cousin, has much reduced cooling requirement and therefore lower water use.
Power Tower (or Central Tower)
This technology consists of an array of mirrors (called heliostats) arranged around a central collector. The heliostats reflect the sun’s heat up to the top of the tower where the temperature reaches up to 8 or 900oC. At these temperatures a very special heat transfer fluid is necessary which is either synthetic oil or a mixture of molten salts, usually potassium and sodium nitrates. Steam is generated to drive the turbine.
It is useful to note that both the parabolic trough and the central tower could drive the same turbines as found in a normal “6 pack” Eskom coal-fired power station.
Stirling Dish
This is a less popular technology for various reasons, mostly to do with wind. It is very effective nevertheless, if judiciously placed in a low wind area. There was one installed at the Development Bank in Pretoria a couple of years ago but it was not successful and was dismantled and sent to the University of Natal (I think).
This system employs the same parabolic principles as the troughs mentioned earlier but this time the heat is focussed on a Stirling Engine, which is an EXTERNAL heat engine. This drives a generator for electricity production as per usual.
Combined Heat and Power (CHP)
Silicon photovoltaics are sensitive to heat. They get very inefficient if their surface temperature goes over 70oC, which is a problem if you put this black or dark blue panel out in the sun all day. This is why they have to be mounted on a bracket so that cooling air can circulate around the back to help keep them cool.
