Photo Voltaic Generation
Types of PV
Silicon PV panels
PV panels first appeared in satellites as a way to generate power. The cost was largely unimportant.
Research over decades has made PV both more efficient and cheaper, and now a range of options exist. The standard roof-mounted, domestic PV panels are either monocrystalline or polycrystalline. Today the best monocrystalline panels are about 20% efficient, polycrystalline about 15%. Physics determines that the maximum efficiency of a single cell on a “normal” PV panel, which is set up to harness energy from sunlight, is about 33% – referred to as the “Shockley-Queisser” limit.
The costs have come down massively over the past 10 years and their efficiency has increased.
Thin-Film Solar Panels
There are several varieties, one of which is the same sort of material used in solar-powered calculators, or outdoor garden lights. It is much cheaper to make, and flexible, but only about half the efficiency of silicon PV panels – only about 7%-10% efficient.
Some solar farms will use panels made of this material, and it is often used if integrated into building at construction time, because it is much cheaper to make.
The most common variety of thin-film solar cell commercially available so far is the Amorphous Silicon Cell, which has an efficiency of 7%. However, there is research being done into many other types of thin film PV technologies.
Bio-hybrid solar cell is hopeful, still in the research stage, but is emulating photosynthesis, and claiming a much higher efficiency than Silicon PV.
Cadmium Telluride Solar Cells are some of the cheapest to produce and claim an energy payback time of less than a year, but cadmium telluride is poisonous if eaten or inhaled, so it is lagging somewhat in popularity.
Multi-junction Cells
Standard PV cells are made from silicon and get most of their energy from higher energy blue light. More recently, technologies have been developed using a mineral form of calcium titanium oxide called Perovskite. These absorb different colours of light, so will pick up energy from lower energy photons as well. Some dual layer PV cells exist that use both silicon, picking up the higher energy photons first, and perovskite picking up the energy from the remaining lower energy photons. There are plans for triple-layer systems with a theoretical efficiency of over 45%. However to make these systems work, each layer has to be very thin so that light can pass through it to the next layer. This makes manufacturing difficult/expensive, and the resulting systems fragile.
Concentrated PV Cells
These have some combination of curved surfaces, lenses, mirrors and/or sun-tracking system to make them focus the sunlight onto a smaller area – which may contain a more expensive PV cell – for example a multi-junction cell.
The light focusing and sun tracking can make the area with the PV cells too hot, which will make the PV cells slightly less efficient. Consequently, these systems may also include an active cooling system to try to improve their efficiency.
Organic PV
These are made from a soft/flexible material – effectively a type of plastic. Not as efficient as silicon or thin-film PV panels, but useful in places where it is difficult to transport heavy, relatively brittle silicon panels.
Pros & Cons
Efficient, low carbon footprint
A typical silicon PV panel, placed on a roof in southern Europe, will generate as much energy as it took to make, transport and ultimately dispose of or recycle, in one or two years use of the panel.
Some of the thin-film PV panels, in particular cadmium telluride, claim an energy payback time of less than a year.
Organic PV is less efficient, but costs less to manufacture, less to recycle and is light and easy to transport as it is not fragile.
PV panels electrical output is variable
Electricity production from PV panels is highest when the sun shines. They also produce very well on cloudy-bright days when there is a lot of sunlight being bounced onto the panels from white clouds in addition to any direct sunlight. They produce very little on dull, foggy or wet days and produce none at all once the sun goes down. Unfortunately the peak domestic need for electricity is in the early evening.
How are PV panels made?
Silicon PV Panels
Normal domestic Mono-Crystaline silicon PV panels start by taking sand and heating it to a very high temperature, in an arc furnace, to melt it and then produce a pure silicon crystal of exactly the right structure. When the silicon is melted, typically boron is added, to make the resulting silicon have a slightly positive charge.
Once the precisely aligned silicon crystals are formed, they are cut into thin wafers, about the thickness of a piece of paper, then coated with a non-reflective coating to try to make sure that they absorb light when they are part of the final panel, rather than reflecting it all. Metal conductors are added to both surfaces of the silicon wafer, then they are coated with a very thin layer of phosphorous, which gives the outside a positive charge. The positive outside and negative inside is what makes the PV cell work. An array of smaller PV Cells are linked together and sandwiched between a front layer of glass and a back surface of durable polymer plastic material, all within a metal frame.
Polycrystalline PV panels are made in a very similar way, but the Silicon is just melted, and much less care is taken to produce a precise crystal structure as it cools. This means they are cheaper, but less efficient. Monocrystalline panels look black and smooth. Polycrystalline ones appear blue with a multi-faceted, almost speckled-looking, surface.
Pure crystalline silicon is a key component for the computer-chip industry. Massive amounts of money have been spent improving the chip manufacturing process and a vast number of chips are produced. Any silicon which is not good enough for computer chips is absolutely fine for PV panels, and this energy-intensive ”scrap” silicon is reused as PV.
Thin-Film Solar Panels
Thin-film solar cells are made by layering several different thin films of photovoltaic material on a base surface/substrate. The layers will be very thin – about a micrometre or one thousandth of a mm. The types of materials used are things like silicon, cadmium, copper. These are the easiest panels to produce, and are flexible.
References / external websites:
https://sinovoltaics.com/learning-center/solar-panels/energy-payback-time-for-solar-systems/
https://en.wikipedia.org/wiki/Solar_cell_efficiency
https://en.wikipedia.org/wiki/Thin-film_solar_cell
https://en.wikipedia.org/wiki/Concentrator_photovoltaics
https://en.wikipedia.org/wiki/Organic_solar_cell
Further links for the dedicated
https://en.wikipedia.org/wiki/Multi-junction_solar_cell
https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit