Portable Solar Power for Generators

Can Solar Panels Replace Fossil Fuel for Generators?

Is this a cost effective efficient alternative?

Solar Power has long been a goal for green power enthusiasts.  But in generators, can portable solar power replace gas, diesel, liquid propane fossil fuels?  The answer is yes it can, but with a large reduction in efficiency.

Actually, solar generators are more similar to inverter generators than to standard generators.  And more precisely, they should be called a solar inverter.  However, rather than burning a fossil fuel, a solar power inverter uses the sun’s energy as its power source.

The sun’s energy is captured by solar panels, also known as photovoltaic (PV) panels. This energy is then stored in a bank of batteries which then releases its low voltage DC power through an inverter to produce standard AC power.

At first glance, it seems like a great deal. Sun energy is free. In fact, the sun’s rays give us about 1 kilowatt of energy per square meter of our planet’s surface. Just think if we could harness all that energy.

If only it was that simple.

You see they are inefficient in that the common solar power panels are only 18% or so efficient at turning the sun’s energy into electrical energy. Even the best that man can make are barely 50% efficient, and these are expensive and relegated to powering satellites. They’ve been powering satellites since the late 1950’s.

You probably own a solar powered calculator. Groups of solar cells can be grouped together and connected electrically and packaged into a frame. These are known as PV panels. You’ve seen these on rooftops harnessing this energy to supply the home’s electrical needs.

This article will explain how these cells work to produce usable energy.

So Just How Does Solar Power Work?

Usually when sunlight strikes an object, the object simply heats up as the molecules vibrate. Some materials called semiconductors act differently. They can free up some electrons in the material and allow them to flow freely. One such semiconductor is silicone. This means that the energy of the absorbed light is transferred to the semiconductor.

OK, here’s the simple version.

A PV cell has an electric field that can force the electrons freed by light to flow in a certain direction. This is a current. By placing metal contacts on the top and bottom of the PV cell, that current can be drawn off to power something. This current, along with the cell’s voltage (defined by the makeup of the PV cell), dictates the power (wattage) that the cell can produce.

What? You call that the simple version? Wait, that’s as simple as it gets. For the real explanation, you’ll have to dust off your high school chemistry knowledge. That explanation follows. For the faint of heart, you might want to skip this section.

How Silicon Makes a Solar Cell

Silicon is an element, like oxygen, copper or carbon and all the rest. You remember what an atom looks like? A nucleus made of protons and neutrons with electrons spinning around in orbitals?

“I can see them clearly”

Well each layer or orbital of electrons has a specific number of electrons in it that makes it a stable atom. All elements are different. Silicon has a nucleus of protons and neutrons. In its first orbital, or shell, it has two electrons. Its next orbital has eight. These are stable because they are full. The next orbital can hold eight electrons to be stable.

However, silicon has only four electrons in this orbital. It would love to have four more to fill its outer shell. Well, the nice thing is, atoms can share electrons. That is, one silicon atom can combine with another and equally share their four electrons so that each has its full amount some of the time (they move around so fast that essentially “some of the time” means all of the time).  This is so stable, that free silicon atoms gather to make crystalline silicon.

Pure crystalline silicon is not a good conductor of electricity because none of the electrons are free to move about the group of atoms. A great electrical conductor like Copper has these needed freely moving electrons.

Are you still awake?

So in a solar cell, the silicon is mixed with another element to shake things up. These impurities may be one atom per million silicon atoms. Enter our first hero, the element Phosphorus. Phosphorus has five electrons in its outer shell. So when it shares it’s four with the silicon atom, it still has a free one being held by the proton of the phosphorus nucleus.  The process of adding the impurity is called doping and the result is called N-type doped silicon. N stands for negative.

When energy in the form of heat hits this combination, one of the extra phosphorus electrons is knocked loose and wanders around carrying an electrical current looking to do something. These electrons are called free carriers. This material is a now much better conductor than pure silicon.

OK, now what if we put another impurity in another layer of silicon that had only three electrons in its outer shell?  Such an element is Boron. This mixture is called P-type silicon. P stands for positive. You see, instead of having an extra electron, a silicon-boron combination would be looking for a free carrier to make it happy.

Starting to get the picture?

These two separate plates of silicon are electrically neutral. But put them together and you have the makings for a solar cell.

When put together, it creates an electric field. Because now the free electrons on the N side can flow to the free openings on the P side. But they can’t all flow, or the cell wouldn’t work. Actually, right at the junction of the two plates, they mix to form a barrier of sorts, and equilibrium is reached. Now this forms an electric field separating the two sides.  This electric field acts as a diode allowing electrons the ability to flow from the N side to the P side, but not vice versa.

from globalsino.com

Now comes the fun part.  You say, “You call this fun?”

When light in the form of photons hits this cell, the energy frees an electron, creates electron holes, disrupts neutrality and allows electron flow. We can then provide a current path for the electron to flow in and it creates a current that can be harnessed. The cell’s electric field is its voltage, and current and voltage gives us power.

There, was that so bad?

There’s a couple more things we do to these power panels that aren’t so scientific. First, silicone is very shiny. It can actually reflect some of the photons before they can do their work. So an anti-reflective coating is added.  Secondly, to protect the cell from damage, a cover plate (usually glass) is added.

So several individual cells are put together to make PV modules which are then made into panels. Added up, they can produce useful levels of voltage and thus current that we measure.

It’s nice to know that photovoltaic panels can last 30 years.

So What About Portable Solar Power Efficiency?

I mentioned that solar cells are inefficient regarding making sun energy into electrical energy. There are a couple of reasons for this.

Sun rays come in wavelengths. These wavelengths have different energy levels. The photons that hit our cell are of a wide range of energies. Some won’t have enough energy to change the electron pairs. They will simply pass through.

Other photons can have too much energy and only a certain amount is required to knock an electron loose. The rest is wasted. This is called the band gap energy of a material. These two factors can result in the loss of 70% of the radiation energy effectiveness.

It is the band gap that determines the voltage of the electric field.  And you can’t make them too thin trying to absorb as much as possible by making low band gap, because too low and the energy would be useless.

Another loss is in the conducting metal that we add and its proximity to the cells. We can cover the entire bottom of the cell with a good conductor, but we couldn’t cover the “top” or no sun would get in. We could put our contacts at the sides of the cell, but the electrons in the middle would have to travel long distances through the silicon to reach the contacts. We already mentioned that silicon is a semiconductor, meaning it is not good at transporting current due to its internal resistance.

So the answer so far is to overlay a metallic contact grid that shortens the distance the electrons have to travel, while not covering much of the cells.

Completed solar panels are then rated by its watt producing ability in full sun. Full sun described as 6 hours of direct contact.

WHEW! We did it.

There are other factors concerning efficiency such as the angle of the panels to the direct sun (angle of inclination). Areas when shade could partially cover a panel is also a consideration. These concerns become more important when collecting as much energy as possible to run your solar generator.

We can now explore how a solar generator works with solar power as the fuel source (in a separate article).

Further Developments in Solar Cell Technology

Here are some fun facts on attempts to increase solar power efficiency.

Thin-film solar cells are simpler and cheaper to produce, but are less efficient. They can be made of silicon (non-crystalline), or gallium arsenide, copper indium diselenide and cadmium telluride. Huh? I know, right!

Remember band gaps? Some manufactur multi-layers of the cell panels with different band gaps. For example the sun photons with not enough energy pass through one layer and activate a layer below made for lower band gap. The top layer is made for the band gap that requires the full stronger photons. These are called multi-junction cells.

Finally, concentration of the light energy is attempted with mirrors and lenses to focus greater amounts onto the panels. The panels in turn can be made of highly efficient solar cells.

For a general understanding of electricity, you can read the article on this site on rudimentary understanding of electricity.

OK, enough already!

Filed under: Solar Power