Photovoltaic Technology

Photovoltaic systems consist of arrays of modules (a module is the term for a number of

interconnected cells packaged in a robust weather-resistant encapsulation), and balance of

system components.

Photovoltaic panels Helios

Photovoltaic panels USP

Solar Cells  

Various solar cells composed of various materials have been developed in the last decades. The vast majority of photovoltaic cells are made from silicon at present. Cells are typically classified as crystalline or thin film. Crystalline cells are sliced from ingots or castings or grown ribbons whereas thin film cells contain very thin layers on some low cost support material (most commonly on glass, stainless steel or plastic).

A solar cell consists of two or more layers of semi-conducting material, most commonly silicon. These layers of 0,001 up to 0,2 mm in thickness have different dotations (n- and p-dotations) and therefore different characteristics so that the resulting solar cell structure and function can be compared with a diode. When the silicon is exposed to light, electrical charges are generated and this can be conducted away by metal contacts as direct current (DC). The electrical output from a single cell is small, so multiple cells are electrically interconnected in series and parallel

and encapsulated (usually behind glass) to form a module (sometimes referred to as a "panel").

Figure 2: Solar cell and module efficiency of various technologies

The efficiency rate of solar cells and modules depends on the technology (material) used. Different materials and combinations result in different efficiency rates. By combining materials with different spectral sensitivity and superposing several layers, cells can be more efficient. The theoretical maximum efficiency rate is around 42 % among the materials known and used for cells with one single layer. Higher rates can be obtained thanks to multiple junctions. The very top efficiency rates are shown for materials not listed in the figure above. These costly materials are used for concentrator systems and space applications. The efficiency rate is also a result of the corresponding state of the research & development process. Laboratory cells have the highest efficiency rates of up to 25 % and even more. They

are usually small, individually produced and very expensive, hence not really representative of the technology. On the way to commercial modules, the cell production has to be industrialized and optimized according to the costs. The production process of modules implies furthermore connection of the cells, use of frames and cover as well as some more technical design. The efficiency rates of commercial modules are thus lower. By experience, one can say that the efficiency rate of marketable modules is some 30 % lower compared to laboratory cells (except amorphous silicon). Usually, it takes at least about 5 - 10 years from a laboratory cell to some marketable module.

Solar Modules

Photovoltaic cells are interconnected and encapsulated between a transparent front, usually glass, and a backing material. Modules are normally rated between 50 and 200 Wp. The photovoltaic module is the principle building block of a photovoltaic system and any number of panels can be interconnected in series or in parallel to give the desired electrical output. This modular structure is a considerable advantage of the photovoltaic system, in many applications

further modules can be added to an existing system as required. An overview over the most common solar module types shows that 

•  almost all solar cells are based on silicon (98 – 99 %, first four columns in the table below),

•  most common technology is silicon crystallization and that the thin cells

•  thin cell technology has a great technological potential.

Crystalline silicon based photovoltaic cells will stay dominant in the decade to come and considerable cost reductions will be available thanks to the newer thin film technology. Thin films require much less semiconductor material and less labor hence they are expected to be less expensive in production.

Table 1: Overview over solar module types.  

					Solar cell type used	Multicrystalline silicon
					Common module efficiency rate	9 – 13 %
					Description	numerous grains of monocrystalline silicon
						molten polycrystalline silicon is cast into ingots
						Faster and more economic manufacturing
					Advantages	process
						Good experience
					Disadvantages	Energy intensive, less economic production
						compared to thin cell technology
						Crystal sawing
					World market share	42%

					Solar cell type used	EFG (Edge-defined Film-fed Growth) silicon
					Common module efficiency rate	10 – 13 %
					Description	silicon crystalline growth not in blocks but in thin
						layers (octagon, sheet or ribbon form)
					Advantages	Very fast and economic production process
						No sawing
					Disadvantages	Uneven cell surface causing problems with
						further automatic processing
					World market share	3%

					Solar cell type used	Amorphous silicon
					Common module efficiency rate	4 – 6 %
					Description	silicon atoms in a thin homogenous layer rather
						than crystal structure
						developed technology and used in consumer
					Advantages	Convey belt production possible.
						Cells can be thinner, much less silicon material
					Disadvantages	Deposits possible both on rigid or flexible substrates
						Lower efficiency rate – especially due to
						degradation
					World market share	12%

					Solar cell type used	Other solar cell types (e.g. CIS, CdTe)
					Common module efficiency rate	7 – 10 %
					Description	other materials such as copper indium
						diselenide (CIS) or cadmium telluride (CdTe)
						used
					Advantages	Very fast and relatively inexpensive industrial process
						Better efficiency rates than thin cells based on amorphous silicone
					Disadvantages	Deposits possible both on rigid or flexible substrates
						Partially production process still to be developed
						Partially rare or toxic material used
					World market share	1%

The photovoltaic systems consist of modules and so-called Balance-of-system (BOS) components. Most important BOS components are:

• Batteries and charge controllers for stand-alone photovoltaic systems. The battery provides energy storage. The function of the charge controller is to maintain the battery at the highest possible state of charge and provide the user with the required quantity of electricity, while protecting the battery from deep discharge or extended overcharge.

• Inverters and further grid connection gear (e.g. net metering) for on-grid photovoltaic systems. The inverter converts the DC source (from the module or battery) to AC. Furthermore, the various cables and switches needed to ensure that the photovoltaic generator can be isolated both from the building and from the mains are available.

Manufacturing and supply of these components have improved a lot in the last years and highly reliable gear is now available making the operation of photovoltaic systems very secure. Very much progress has also been made in manufacturing the mounting structure and above all in real building integration solutions. The costs for the BOS components have also been considerably reduced and count roughly for half of the photovoltaic system costs.

Photovoltaic systems can be divided into stand-alone and grid-connected photovoltaic systems.

Stand-alone systems

Stand-alone photovoltaic systems are used in areas that are not easily accessible or have no access to mains electricity but also where simple grid connection is less economic or not necessary for the application wanted. A stand-alone system is independent of the electricity grid, with the energy produced normally being stored in a battery. A typical stand-alone system would consist of a photovoltaic module or modules, a battery and charge controller. An inverter may also be included in the system to convert the direct current (DC) generated by the photovoltaic modules to the alternating current form (AC) required by normal appliances. Stand-alone systems can be subdivided in professional applications (telecommunication, water pumping, street furniture, illumination, etc.) and rural domestic applications (isolated housing).

Grid-connected Systems

Photovoltaic systems can also be connected to the local electricity network. The electricity generated by the photovoltaic system can either be used immediately (e.g. for systems installed on offices and other commercial buildings), or can be sold to one of the electricity supply companies. Power can be bought back from the network when the solar system is unable to provide the electricity required (e.g. at night). This way, the grid is acting as a kind of “energy storage system” for the photovoltaic system owner, which means that a battery storage for the photovoltaic system is not needed.

Grid-connected systems can be subdivided into photovoltaic power stations (centralised on-grid installations on large scale with corresponding land use) and building integrated photovoltaic applications (distributed on-grid installations without any additional land use).