Solar Energy Part 6
Efficiency is an area of much confusion in the world of Solar Energy. A dictionary definition generally states efficiency as being "the ratio of a system's work output to its work input". For solar usage, it can be thought of as the ratio of useful energy captured to the total energy 'falling' onto the device.
The main problem is that we are not very efficient at measuring efficiency! Any photographer will confirm that, despite having a large amount a gadgets, there is no substitute for the human eye. Our present instruments do not even come close to accurately measuring the full spectrum of visible light, they are always more sensitive to some colors than others. If we were only measuring visible light, we might be able to make some adjustments to compensate this error. But, sunlight also contains infra-red and ultra-violet light, as well as a very large radio frequency component. There is no single instrument that can measure this. Therefore, since we cannot measure the total energy, it stands to reason that we cannot measure the efficiency accurately. All our efficiency figures will be based on educated guesses!
The dictionary also tells us that "the efficiency of a real system is always less than one". We normally accept that fact, but few people realize how much 'less than one' this is in practice. To put this in its proper perspective, we should look at some other 'systems' that are more commonly known. The steam power plant is generally between 30 and 40% efficient depending on size and design. Our car engines are around 26% efficient for petrol engine, with diesel engines being one or two percentage points higher. The highest efficiency that I have read about, and I cannot confirm this, is found in huge marine diesel engines that reach almost 50% - by huge, we are talking about greater than 50 litres swept volume in each cylinder! Clearly 'less than one' actually means less than 50%!
Man obviously has limitations, but how does nature compare? The Earth supports life solely by the energy it receives from the Sun - the heat inside the Earth's core plays no part in sustaining life. The Sun's energy is directly used by two major processes. The hydrologic cycle, which controls every aspect of our climate, is the primary user of this energy. Photosythesis by the plant kingdom takes up the remainder used on Earth. The vast majority of the Sun's energy, over half, is simply reflected back into space. By the best estimates, nature is about 46% efficient! It is hard to believe that nature, or God if you prefer, has produced such a poor system. I personally believe that the light reflected back into space is essential for maintaining the atmosphere that protects all life on Earth from the dangerous radiation of the Sun.
From the foregoing, it is now clear that efficiency figures cannot be expected to climb very high. The systems we have just seen are actually complicated conversions between various forms of energy. If we could keep the system as simple as possible, without multiple conversions taking place, then we may reasonably expect 30% efficiency to be achieved. The problem is that most solar energy devices do not rely on a single conversion of energy. It would be much better if we made no conversion at all! We must learn to use heat as heat, light as light, etc.
To make matters worse, the energy that we receive from the Sun varies according to our location, and it is not just a case of how far from the equator we are. There are some places which, for want of a better explaination, are just naturally hotter than others even on the same line of latitude. Heat doesn't always indicate the maximum solar radiation, but consistantly hot areas do give us a clue. In this case, heat is the average temperature, not to be confused with humidity which can make us feel hot at quite ordinary temperatures. The difference in solar radiation between Northern countries such as the upper states of the USA or Northern Europe, and that of countries in the Sun Belt (North Africa, Middle East, Iran, Pakistan, India) can be a factor of four. Therefore, when you see an efficiency figure quoted you must take into account the location and make a comparison with your own location. As we have already seen, a hotter location may indeed result in reduced efficiency for an electronic device, but considerably increased efficiency of a reflecting device.
Confusing, isn't it! Perhaps the best strategy to adopt is a "try before you buy" approach.
That concludes this introduction. You have been pre-warned of the many problem areas and should now enter the world of Solar Energy with your eyes open and having a clear mind. There will be some more Solar articles published here in the near future.