Solar Energy Part 2
The scope of solar energy devices is an enormous field. We have to start somewhere, so we'll start with the most well known - the solar cell. This is not to suggest that this is the best option available, in fact I dislike this option, but most people are at least familiar with it. This section will be refered to as the electronic solutions.
The solar cell or photovoltaic comes in many different designs, shapes and sizes. They are made from many different materials and even operate under different principles. There are also a large number of published designs that simply don't work. However, this page will concentrate on working designs in general.
The Solar Cell
The solar cell is an electronic device and is no different from any other semi-conductor junction in its basic characteristics. Like all diodes, transistors and such found in conventional electronic devices, it suffers from some setbacks that make it not quite as good as we would like it to be!
The solar cell produces voltage and current, as must be obvious, but it also has to obey Ohms Law and the other laws of physics. This, added to the known problems associated with all electronic devices, makes it a rather unpredictable device.
Temperature dependence is the major problem of all semiconductor devices. In the solar cell, this is seen as a drop in voltage output as the temperature rises. The internal resistance also changes according to temperature. Since power output is a factor of voltage, current and resistance, it follows that power also drops as the temperature rises. Take for example a common transistor - the base/emitter knee voltage is often quoted at 0.7 volts at a certain temperature (often 20 deg. C). But in use, the flow of current causes ohmic heating which raises the temperature of the device. This in turn causes the base/emitter knee voltage to drop, often to below 0.6 volts, thereby changing the whole characteristics of the device considerably. On the transistor, we can add a heatsink to compensate this effect somewhat. Obviously, the solar cell not only is difficult to cool in this way but is also heated by the very Sun from which it derives its power. Depending on the materials used, the open circuit voltage of a solar cell may vary from about 0.4 to 0.7 volts.
Now, there is a very strange term! Open Circuit Voltage - meaning nothing is attached to the cell. Well what use would that be? You will also see Short Circuit Current! Hopefully, the cell would never be short circuited in normal use. These are meaningless terms that litter all solar cell literature. That is, meaningless to the potential customer. What we learn from these figures is that the real voltage and current outputs will be much less and the cell probably wont go bang if we short circuit it!
Beware of test-rig figures! Many output figures are based on laboratory tests that use artificial lighting instead of the Sun, and at one specific temperature only. The cell is made to run into a resistive load that closely matches the cell's internal resistance - a situation which almost never exists in the real world!
In the real world, a solar cell will give out a different voltage at various times throughout the day due to sun angle, cloud cover, rain, wind, air polution, etc. Since it is sensitive to ambient temperature as well, you can expect considerable difference in summer and winter figures. Add to this the unforeseen conditions such as bird droppings, fallen kites, little boys with air-rifles, etc.
Here, it is an appropriate time to introduce the First Law of Solar Cells.
Never believe the manufacturers figures!
This voltage/temperature variation would appear to be a major problem but, as we will see a little later, it really isn't a problem at all. The real major problem lies in another area which we have already covered but didn't realise the significance. I compared solar cells with transistors and you have seen that they behave in similar fashion. That is the problem! They are too much alike. The voltages and temperature dependences are almost the same. The result of this is that we cannot use a transistor switch with a solar cell since the voltage of the solar cell is the same as, or normally less than, the minimum switching voltage of the transistor. So how can we use the "half a volt or so" that a typical cell pushes out? The answer is we can't, or at least not until someone invents a new kind of transistor! At the present, a voltage of at least one volt (and preferably higher) would be required in order to use a solar cell's output directly.
So a compromise solution is to place a number of solar cells in series arrangement, henceforth to be called the solar panel! Voltage is up - problem has gone. Fine, except new problems have been created!
The Solar Panel
We all use battery powered equipment from time to time, however annoying it may be when the batteries die, but die they do! We much prefer our mains powered devices and go to great pains to charge our portable equipment such as cellphones. Therefore, the public isn't going to like solar equipment that works on battery voltage levels. Here we have a major problem. Solar cells produce DC voltage whereas we really want AC to match our mains supplies. Now the fact is that much of our domestic electrical equipment will actually work on DC voltage - anything containing a "switchmode power supply" such as your TV, computer, etc. Even "Energy Saver" type fluorescent tube lights actually work from DC. Unfortunately, most motors wont work on DC! So we are stuck with a major problem again.
Already, you can see that putting a number of solar cells in series to form a panel doesn't solve the most pressing problem. We have to make a choice of what voltage we want to achieve. Whatever we choose, it will be a compromise solution. Normally voltages of 12, 24 and 48 volts are envisaged. I am told that high DC voltages are more dangerous than AC voltages - (both can kill so who cares!) - so higher voltages are discouraged. We will worry about converting this to AC mains voltage later.
By now, the alert amongst you should have noticed a discrepancy. How can we produce 12 volts for example when the solar cells have a variable voltage output? Again, we can't! The solar panel can only be used as a battery charger. Call this the First Law of Solar Panels! Let us look into this more fully. An ordinary 12 volt battery (a lead-acid car battery) actually has a working voltage of between 10.8 and 13.2 volts under normal circumstances. Not very voltage stable, but certainly much better than the variation we would get from solar cells alone. To charge this battery, a voltage somewhat higher is required - typically 18 volts. With a correctly designed charger, the charging voltage could safely go much higher, or much lower without causing damage (charging simply stops). Therefore, if a solar panel could be made to output around 18 volts or so under average illumination, it would make an excellent battery charger.
That is precisely what solar panel manufacturers do. Put 36 cells in series and you should get around 18 volts under average conditions, with more voltage when under good conditions. All OK? Not quite! Remember I used the phrase "correctly designed charger". A panel alone isn't a correctly designed charger at all. None-the-less, these panels will charge a battery - not quite as the manufacturers claim since the battery (load resistance) doesn't match the internal resistance of the panel - but it will work! If you don't much care for your equipment, you certainly can connect a panel directly to a battery at your own risk. It surprises me how many people actually do this! I assume you would be sensible enough to use a proper charging circuit. You would now expect your battery to charge without problems. But there are more to come!
When you put a number of drycells (batteries) in a piece of equipment, you must surely know that if one cell is dead, the others wont work either. It's the same with solar panels. One cell not working in that series of 36 and bang - NO output! Ah, but it's guaranteed, you say. Yes, if one cell has failed, the manufacturer should replace the panel. But I didn't say failed, I said not working! It can, and often does, happen that one cell comes under a shadow, a fallen leaf, or a large insect, or bird droppings, or dirt, or many other possibilities. When that happens, the whole panel is useless.
Unfortunately, the situation is even worse if one cell is weak compared to the others. It can happen that a weak cell can be driven into reverse operation by much stronger cells. Electronic devices of all kinds don't take kindly to reverse voltages! Although manufacturers try to protect against this happening, they can only do so much. The same problem that we encountered before, of diodes and transistors being too much alike, prevents us from using protection diodes effectively on each cell. You will see protection diodes stated as being used, but not on EACH cell. Apart from this, the weakest cell determines the output current. Why are some cells weak and others strong? Simply because we cannot manufacture identical devices. The semi-conductor industry has been making transistors since the early fifties, and even with that much experience, they still cannot produce identical devices. It is much more difficult to produce identical solar cells since they are much larger physically. This relates to the non uniform doping and purity of the semiconductor materials for those who are interested.
So there you have a very brief taster of solar cells and panels. Next, we take a different approach. Not light but heat!