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HOW SOLAR CELLS WORK

You've probably seen calculators that have solar cells - calculators that never need batteries, and in some cases don't even have an off button. As long as you have enough light, they seem to work forever. You may have seen larger solar panels - on emergency road signs or call boxes, on buoys, even in parking lots to power lights. Although these larger panels aren't as common as solar powered calculators, they're out there, and not that hard to spot if you know where to look. You have also seen solar cell arrays on satellites, where they are used to power the electrical systems.

You have probably also been hearing about the "solar revolution" for the last 20 years -the idea that one day we will all use free electricity from the sun. This is a seductive promise - on a bright, sunny day the sun shines approximately 1,000 watts of energy per square meter of the planet's surface, and if we could collect all of that energy we could easily power our homes and offices for free.

We will examine solar cells to learn how they convert the sun's energy directly into electricity. In the process you will learn why we are getting closer to using the sun's energy on a daily basis, but we still have more research to do before the process becomes cost effective.

Using Silicon to convert photons to electrons

The solar cells that you see on calculators and satellites are photovoltaic cells or modules-a group of cells electrically connected and packaged in one frame. Photovoltaic (photo = light, voltaic = electricity) cells convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They could even power your house. How do these devices work?

Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely.

PV cells also all have one or more electric fields, which act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power that the solar cell can produce.

That's the basic process, but there's really much more to it. Let's take a deeper look into one example of a PV cell: the single crystal silicon cell.

A deeper look into the Silicon cell

Silicon has some special chemical properties in its crystalline form. An atom of silicon has 14 electrons, arranged in 3 different shells. The first 2 shells, those closest to the centre, are completely full. The outer shell, however, is only half full, having only 4 electrons.

A silicon atom will always look for ways to fill up its last shell (which would like to have 8 electrons). To do this, it will share electrons with 4 of its neighbour silicon atoms. It's like every atom holds hands with its neighbours, except that in this case, each atom has 4 hands joined to 4 neighbours. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell. We've now described pure, crystalline silicon.

Pure silicon is a poor conductor of electricity because none of its electrons are free to move about as electrons are in good conductors like copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell.

Our cell has silicon with impurities - other atoms mixed in with the silicon atoms, changing the way things work a bit. We usually think of impurities as something undesirable, but in our case, our cell wouldn't work without them. These impurities are actually put there on purpose.

Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. Phosphorous has 5 electrons in its outer shell, not 4. It still bonds with its silicon neighbour atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place.

Why does the silicon in the PV cell have the ability to produce electricity?

When energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case where an electron could bond. These electrons then wander randomly around the crystalline lattice looking for another hole to fall into. These electrons are called free carriers, and can carry electrical current. There are so few of them in pure silicon, however, that they aren't very useful.

Our impure silicon with phosphorous atoms mixed in is a different story. It turns out that it takes a lot less energy to knock loose one of our "extra" phosphorous electrons because they aren't tied up in a bond - their neighbours aren't holding them back. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon.

The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called n-type (n for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon is.

Actually, only part of our cell is n-type. The other part is doped with boron, which has only 3 electrons in its outer shell instead of 4, to become p-type silicon. Instead of having free electrons, p-type silicon (p for positive) has free holes. Holes really are just the absence of electrons, so they carry the opposite (positive) charge. They move around just like electrons do.

What other materials can be used for a PV cell other than silicon?

Single crystal silicon isn't the only material used in PV cells. Polycrystalline silicon is also used in an attempt to cut manufacturing costs, although resulting cells aren't as efficient as single crystal silicon.

Amorphous silicon, which has no crystalline structure, is also used, again in an attempt to reduce production costs. Other materials used include gallium arsenide, copper indium diselenide and cadmium telluride.

Since different materials have different band gaps; they seem to be "tuned" to different wavelengths, or photons of different energies. One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high energy photons while allowing lower energy photons to be absorbed by the lower band gap material beneath. This technique can result in much higher efficiencies. Such cells, called multi-junction cells, can have more than one electric field.

It's possible to power a house with solar energy!

Now that we have our PV module, what do we do with it? What would you have to do to power your house with solar energy? Although it's not as simple as just slapping some modules on your roof, it's not extremely difficult to do either. First of all, not every roof has the correct orientation or angle of inclination to take advantage of the sun's energy.

Non-tracking PV systems in the Northern Hemisphere should point towards true south (this is the orientation). They should be inclined at an angle equal to the area's latitude to absorb the maximum amount of energy year round.

A different orientation and/or inclination could be used if you want to maximise energy production for the morning or afternoon, and/or the summer or winter. Of course, the modules should never be shaded by nearby trees or buildings, no matter the time of day or the time of year. In a PV module, even if just one of its 36 cells is shaded, power production will be reduced by more than half.

If you have a house with an unshaded, south-facing roof, you need to decide what size system you need. This is complicated by the facts that your electricity production depends on the weather, which is never completely predictable, and that your electricity demand will also vary. These hurdles are fairly easy to clear. Meteorological data exists which gives average monthly sunlight levels for different geographical areas. This takes into account rainfall and cloudy days, as well as altitude, humidity, and other more subtle factors.

You should design for the worst month, so that you'll have enough electricity all year round. With that data, and knowing your average household demand (your utility conveniently lets you know how much energy you use every month!), simple methods exist allowing you to determine just how many PV modules you'll need. You'll also need to decide on a system voltage, which you can control, by deciding how many modules to wire in series.

Some problems faced and their possible solutions

You may have already guessed a couple of problems that we'll have to solve. First, what do we do when the sun isn't shining? Certainly, no one would accept only having electricity during the day, and then only on clear days, if they have a choice. We need energy storage - batteries. Unfortunately, batteries add a lot of cost and maintenance to the PV system. Currently, however, it's a necessity if you want to be completely independent.

One way around the problem is to connect your house to the utility grid, buying power from the utility when you need it, and selling to them when you produce more than you need. This way, the utility acts as a practically infinite storage system. The utility has to agree, of course, and in most cases will buy power from you at a much lower price than their own selling price.

You will also need special equipment to make sure that the power you sell to your utility is synchronous with theirs, in other words that it shares the same sinusoidal waveform and frequency.

Safety is an issue as well. The utility has to make sure if there's a power outage in your neighbourhood, that your PV system won't try to feed electricity into lines that a lineman may think is dead. This is called islanding.

If you decide to use batteries, keep in mind that they will have to be maintained, and replaced after a certain number of years. The PV modules should last 20 years or more, but batteries just don't have that kind of useful life. Batteries in PV systems can also be very dangerous because of the energy they store and the acidic electrolytes they contain, so you'll need a well-ventilated, non-metallic enclosure for them.

The other problem is that the electricity generated by your PV modules, and extracted from your batteries if you choose to use them, is direct current, while the electricity supplied by your utility (and the kind which every appliance in your house uses) is alternating current. You will need an inverter, a device which converts DC to AC.

Most large inverters will also allow you to automatically control how your system works. Some PV modules, called AC modules, actually have an inverter already built into each module, eliminating the need for a large, central inverter, and simplifying wiring issues.

Throw in the mounting hardware, wiring, junction boxes, grounding equipment, overcurrent protection, DC and AC disconnects and other accessories and you have yourself a system.

Electrical codes must be followed (there's a section in the National Electrical Code just for PV), and it's highly recommendable that the installation be done by a licensed electrician who has experience with PV systems. Once installed, a PV system requires very little maintenance (especially if no batteries are used), and will provide electricity cleanly and quietly for 20 years or more.

Let's face the truth: Present limitations of using Solar Energy

If photovoltaics are such a wonderful source of free energy, then why doesn't the whole world run on solar power? Some people have a flawed concept of solar energy. While it's true that sunlight is free, the electricity generated by PV systems is not. As you can see from our discussion of a household PV system, quite a bit of hardware is needed. Currently, an installed PV system will cost somewhere around $9 per peak Watt.

To give you an idea of how much a house system would cost, let's consider the Solar House - a model residential home in Raleigh, North Carolina, with a PV system set up by the North Carolina Solar Centre to demonstrate the technology. It's a fairly small home, and it is estimated that its 3.6 kW PV system covers about half of the total electricity needs (this system doesn't use batteries - it's connected to the grid). Even so, at $9 per Watt, this installed system would cost you around $32,000.

That's why PV is usually used in remote areas, far from a conventional source of electricity. Right now, it simply can't compete with the utilities. Costs are coming down as research is being done, however. Researchers are confident that PV will one day be cost effective in urban areas as well as remote ones.

Part of the problem is that manufacturing needs to be done on a large scale to reduce costs as much as possible. That kind of demand for PV, however, won't exist until prices fall to competitive levels. It's a catch-22 situation. Even so, demand and module efficiencies are constantly rising, prices are falling, and the world is becoming increasingly aware of environmental concerns associated with conventional power sources, making photovoltaics a technology with a bright future.

Written by: Think Quest


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