Photovoltaic Technology

What is renewable energy?

Renewable energy is any source of energy that can be used without depleting its reserves. These sources include sunlight or solar energy and other sources such as, wind, wave, biomass and hydro energy. These later sources are indirectly derived from solar energy. Biomass refers to any recently produced organic matter. If the organic matter was produced sustainably then it is considered to be a renewable energy resource.

Fossil fuels such as coal, oil and gas come from biomass which was produced in the distant past and has been transformed by geological activity. World reserves of fossil fuels are finite and are being depleted. They are therefore referred to as non-renewable energy resources.

How Solar Power Works

PV or Solar cells operate on the principle that electricity will flow between two different semiconductors when they are put in contact with each other and exposed to light.

What are Solar Cells?

Solar cells are devices which convert solar energy directly into electricity, either directly via the photovoltaic effect, or indirectly by first converting the solar energy to heat or chemical energy.

The most common form of solar cells are based on the photovoltaic (PV) effect in which light falling on a two layer semi-conductor device produces a photovoltage or potential difference between the layers. This voltage is capable of driving a current through an external circuit and thereby producing useful work.

System Components

The solar panels are just one element of the complete solar system. For grid connected applications you need an inverter to convert the DC electricity to AC, compatible with the grid system. You may also want a battery set and charge controller for emergency power outages. For stand-alone systems you will likely need a battery set for charging during the day, a charge controller to perform that task, and an inverter if you need AC power.

Electricity Generation

PV modules, because of their electrical properties, produce direct rather than alternating current (AC). Direct current (DC) is electric current that flows in a single direction - the type of current you get from a torch battery. Alternating current is electric current that reverses its direction at regular intervals, for example as supplied by utility companies through the national grid system. AC is required to run most large appliances, fridges etc.

In the simplest PV systems, DC current is used directly. In applications where AC current is necessary, an inverter can be added to the system to convert the dc current to ac current.

The Need for Solar Cells

The development of solar cell use in Australia has been stimulated by:
the need for low maintenance, long lasting sources of electricity suitable for places remote from both the main electricity grid and from people; eg satellites, remote site water pumping, outback telecommunications stations and lighthouses;
the need for cost effective power supplies for people remote from the main electricity grid; eg Aboriginal settlements, outback sheep and cattle stations, and some home sites in grid connected areas.
the need for non polluting and silent sources of electricity; eg tourist sites, caravans and campers
the need for a convenient and flexible source of small amounts of power; eg calculators, watches, light meters and cameras;
the need for renewable and sustainable power, as a means of reducing global warming.

Together, these needs have produced a growing market for photovoltaics which has stimulated innovation. As the market has grown, the cost of cells and systems has declined, and new applications have been discovered.

How are Solar Cells made?

Silicon is still the most popular solar-cell material for commercial applications because it is so readily abundant. To be useful in solar cells, it must be refined to 99.9999% purity.

The molecular structure of single-cell silicon is uniform, which is ideal for efficient electron transfer. To make an effective PV cell, silicon is "doped" to make it n-type or p-type. A second, much cheaper form is semicrystalline silicon which consists of several smaller crystals known as "grains," which introduce "grain boundaries" to the solid. These boundaries impede the flow of electrons and encourage them to recombine with holes. There is a trade-off between the cost and the power reduction. To create the different semiconductor layers, the silicon is "doped", either with an element that has an extra electron or is lacking an electron. Putting the n and p layer together creates the junction that causes the material to generate electricity when in a light source.

There are three technologies available, all highly reliable. Two of these technologies require 'crystalline' silicon, either mono-crystalline or poly-crystalline. The third technology utilizes thin films of doped 'amorphous' silicon.

Mono is made from a single crystal of silicon pulled from a bath of molten silicon. This crystal is sliced into a shape close to a square called pseudo square. Poly or Multi is made by melting silicon in ceramic moulds like steel, cooling it slowly over many hours to drive the impurities to the surface, cutting the impure material away and then slicing the remaining silicon into squares or rectangles.

Mono is slightly more efficient for the same unit area and is made from off-cuts from the semi conductor industry.

Multi or polycrystalline silicon can be cheaper to make, though costs for both technologies vary by the day, depending on local issues such as the quantity of scrap silicon on the open market.

With Amorphous silicon solar panels, the silicon material is vaporized and deposited on glass or stainless steel. This produces less efficient cells but only requires a film of silicon roughly one fiftieth as thick as mono or poly cells. Also the production technology costs less than the other methods. This technology has the appearance of 'tinted glass'.

How do Solar Cells Work?

To understand the operation of a PV cell, we need to consider both the nature of the material and the nature of sunlight. Solar cells consist of two types of material, often p-type silicon and n-type silicon. Light of certain wavelengths is able to ionise the atoms in the silicon and the internal field produced by the junction separates some of the positive charges ("holes") from the negative charges (electrons) within the photovoltaic device. The holes are swept into the positive or p-layer and the electrons are swept into the negative or n-layer. Although these opposite charges are attracted to each other, most of them can only recombine by passing through an external circuit outside the material because of the internal potential energy barrier. Therefore if a circuit is made power can be produced from the cells under illumination, since the free electrons have to pass through the load to recombine with the positive holes.

The amount of power available from a PV device is determined by;
the type and area of the material;
the intensity of the sunlight; and
the wavelength of the sunlight.

Single crystal silicon solar cells, for example cannot currently convert more than 25% of the solar energy into electricity, because the radiation in the infrared region of the electromagnetic spectrum does not have enough

Polycrystalline silicon solar cells have an efficiency of less than 20% at this time and amorphous silicon cells, are presently about 10% efficient, due to higher internal energy losses than single crystal silicon.

A typical single crystal silicon PV cell of 100 cm2 will produce about 1.5 watts of power at 0.5 volts DC and 3 amps under full summer sunlight (1000Wm-2). The power output of the cell is almost directly proportional to the intensity of the sunlight. (For example, if the intensity of the sunlight is halved the power will also be halved).