SOLAR CELLS
3.1 CLASSIFYING SOLAR CELLS
Solar cell classification is normally on the kind of silicon.
Monocrystalline silicon solar cells
A mono crystalline silicon solar cell is made from a wafer of one big silicon mono crystal.
These cells belong to the most efficient class in the photovoltaic technology.
For the moment the efficiency factor is about 15 %. The required production process for this type of solar cell is complex and rather expensive. The surface of the cell is smooth and equally coloured blue. They are not flexible and must be mounted in a firm frame and protected against an aggressive environment.
A mono crystalline silicon solar cell
Polycrystalline silicon solar cells
A polycrystalline silicon solar cells wafer is sawed out of a cast “ingot” of molten silicon which is re crystallised. The wafer forms the basic of a poly crystalline solar cell. This solar cell is cheaper because the production process is less complex. The efficiency factor is lower as compared with mono crystalline cells.
For the moment the efficiency factor is about 12 %.
The surface of a poly solar cell looks a little bit smudged with various blue patches.
They are not flexible and must be mounted in a firm frame and protected against an aggressive environment.
A poly crystalline silicon solar cell
Amorphous silicon solar cells
An amorphous silicon solar cell consists of a thin homogenous layer of silicon atoms in stead of an crystal structure. The light absorption in amorphous silicon is more effective compared to the former silicon types. So the cells can be thinner, hence the name “thin film” cells.
Amorphous cells can be used on various carriers both on solid and flexible materials. Bend or folded surfaces are no problem. The cell surface is dark grey coloured.
These cells are cheap but the efficiency factor is a low 6 % for the moment.
A property of amorphous solar cells is the power decrease in the first month after first introduction. After this period the power output remain rather steady. In the specification of the cell the steady power state is mentioned.
An amorphous silicon solar cell
3.2 ADVANTAGES OF SOLAR CELLS
PV cells offer substantially advantages compared with conventional power sources:
Reliability: even under severe conditions PV systems have proved to be reliable.The existence of PV arrays prevents expensive breakdowns in power delivery when continuity is crucial.
Durability: Power delivery of most modules will be guaranteed by the manufacturer for 25 years. Even after this period is expired modules remain in good condition.
Low maintenance costs: Transportation of material and personal to remote installations is costly. Because PV systems only require periodic inspection and seldom extra maintenance, costs for systems keep alive are much lower compared to conventional fuel consuming power systems.
No extra fuel costs: no extra costs are necessary for sales, the transportation and fuel storage.
No noise pollution: PV systems work silently.
PV modularity: The capacity of a PV installation can easily be adapted by connecting or disconnecting standardised panels.
Safety: PV systems don’t use flammable fuels so they are save as far as design and installation safety rules are regarded.
Independency: A PV system can act autonomous and is less dependent of an electricity grid. Sometimes there is no grid at all.
Decentralisation of the electricity grid: Usually a PV installation is small-scaled mostly stand-alone system close to the user’s environment. Very often there is no need for an expensive and extended electric grid and there are no extra costs for energy transport. Moreover an additional advantage will be found in a wider spread of risk.
Ultimate performance: The combination importance awareness of power saving and well-considered schemes for energy gaining often leads to ultimate performances. Especially as power output is extremely optimised unlike the use of a diesel engine. A diesel engine delivers power regardless load connection.
3.3 DISADVANTAGES OF SOLAR CELLS
PV cells have also some disadvantages compared with conventional power sources:
Initial costs: Each PV system has to be evaluated in an economic perspective and must be compared with conventional alternatives. For the moment the initial cost make PV systems sometimes less attractive. Granting subsidies is no real opportunity for solving the cost problems. But things changes rapidly. Fossil fuels become more expensive and prices of PV systems are decreasing. Soon in future a break even point is reached.
The variable nature of sun irradiation: The weather can strongly influences the performance of a PV installation. Climate changes and / or arrangement changes of the installation can ask for design changes.
Energy storage: Some PV installations make use of batteries for energy storage. This is a disadvantage with respect to required space, complexity and hardware costs.
PV technology requires connection of high-efficient devices in order to get a payable system. Low energy use demand leads often to exchanging part of the connected devices.
Education: More than ever, the PV user is expected a minimum knowledge of the used solar system. Normally the user decides in energy questions so he must reasonable well educated in this matter. That is one of the reasons that large-scale introduction of PV energy is delayed for the moment.
Milieu consequences: Gaining energy with a PV installation is extremely milieu friendly. However the manufacturing of solar panels ask a lot of energy and that is not so nice. But in the life time of a solar cell much more energy is gained as is used during manufacturing of the cell.
Another aspect to worry about finds its origin in the unsafe circumstances during manufacturing. There are various poisonous materials used in the production process such as cadmium and explosive gasses like phosphine, diborane and hydrogen selenide. Apparently this has also consequences for the recycling of solar panels.
