Photovoltaic Technology

It involves the direct transformation of sunlight into electrical energy. Photovoltaic (PV) technology, an inherently clean source of energy, produces no noise, smoke, acid rain, water pollutants, carbon dioxide or nuclear waste because it relies on the power of the sun for its fuel. At the same time, Silicon, the raw material used for most PV cells is abundant and non-toxic. The large-scale production of PV systems has minimal impact on the environment, provided that the processes are properly controlled. Also, since PV systems use only sunlight for fuel, the environmental impact of activities such as mining, exploration, production and transportation, and the hazards of coal, oil and gas are eliminated. However, the drawbacks of such systems are their high initial cost and low efficiency, which results in high collector area requirement. Research efforts are therefore concentrated in reducing the cost and efficiency of PV systems and developing building integrated PV systems which can be assimilated into the design of the building from the very beginning.

Photovoltaic energy conversion
The primary component of the photovoltaic energy system is the photovoltaic cell array. These cells consist of semiconductor material such as Silicon (Si), cadmium sulphide (CaS), copper selenium arsenide (CuSeAs), gallium arsenide (GaAs), etc. Silicon is the most common element used in the production of the PV cell.
The creation of a ‘p - n junction’ is a pre-requisite for the occurrence of the photoelectric effect. The ‘p - n junction’ is created by doping a crystal of silicon with phosphorous and boron. This results in the formation of a junction with an excess of negatively charged electrons on one side (‘n’ type) and positively charged ‘holes’ on the other side (‘p’ type). The photoelectric effect is made possible by the existence of this electric field at the ‘p - n junction’. When a light photon dislodges an electron from an atom either on the ‘p’ or ‘n’ side of the junction in the PV cell, it creates a free electron and a hole. The excess electrons flow from the ‘n’ side of the junction to the ‘p’ side producing an electric current in the process when the two sides of the PV cell are connected through an external circuit. Typically linked into modules, PV cells work by converting sunlight into electricity. The PV array can be if two types - flat plate systems and concentrator systems.

Flat Plate Systems

Single crystal cells
The photovoltaic cells most frequently used today are made of pure Silicon from a single crystal. The uniform crystalline structure assists the movement of electrons, making it relatively easy to attain high sunlight conversion efficiencies. These cells contain various different parts; an electrically conductive grid on the top surface of the cell to carry electric current generated, one or two layers of anti-reflective coating to increase the absorption of sunlight, a thin, doped layer of silicon called the collector and an electrode in contact with the base layer to complete the circuit.
Single crystal silicon cells are, however, expensive to produce, and several factors limit the extent to which the costs can be further reduced. Single-crystal silicon has very low capacity to absorb light, due to which the wafers have to be quite thick (100 microns) to achieve the required efficiency values. They are also extremely fragile and have to be handled very carefully during manufacture, packaging and transportation. In addition, almost 20% of the original ingot is wasted during manufacture. However, new techniques that produce long, thin sheets instead of ingots of silicon have reduced the costs associated with the slicing of ingots and polishing of wafers, at the same time reducing wastage. They are, however, very efficient upto 15% under working conditions.
Gallium arsenide is also used for PV cells. It has the advantage of being more absorbent than silicon, and can operate efficiently over a wide range of temperature and is more resistant to radiation damage. Experimental PV cells made of single-crystal GaAs have achieved efficiencies as high as 26% as compared to 20% for single-crystal silicon cells.

Semicrystalline and Polycrystalline cells
Semicrystalline cells are composed of a number of relatively large crystals called grains, with each covering an area of one square cm. Polycrystalline cells are composed of many much smaller grains. Manufacturing techniques for these cells are simpler and less expensive that those for producing perfect single-crystal cells. Semi and polycrystalline cells have lower efficiencies than single crystal silicon (upto 15%), because free electrons and holes tend to recombine at the boundaries between the different grains in the PV cell, thus reducing the amount of electricity that can be produced. Even so the low manufacturing costs of these types of silicon have made them popular for PV cells.

Amorphous silicon is most commonly used in commercial thin-film PV modules. The absorptivity of amorphous silicon is 40 times greater than single-crystal silicon. Consequently, an amorphous silicon cell of just 1-micron thick can absorb 90% of the visible light. The manufacturing process uses very little silicon and is much less expensive. Amorphous silicon modules are easy to make in a variety of shapes, which can accommodate various applications.
These solar cells are however considerably less efficient than crystalline silicon cells (8 - 10%), because it contains various structural and bonding defects where the free electrons and holes tend to combine. They are also less stable than crystalline silicon. However the low cost of these cells have made them popular in many consumer product applications.

Dye-based cells
This device is an electrochromic cell in which a liquid electrolyte is sandwiched between two layers of electrically conductive glass that serve as the cell's electrodes. The inside of the negative electrode is covered with a thin semiconductor layer of titanium dioxide coated with a ruthenium-based dye. The electrolyte contains negatively charged iodine ions in solution. When the molecules absorb light, they release electrons into the titanium dioxide. This transfers the electrons to the cells negative electrode, producing an electric current as the electrons flow from negative to positive electrode through the external circuit. The efficiency is about 7 - 12% comparable to that of amorphous silicon cells, providing a practical alternative as these devices are inexpensive to manufacture and also quite durable.

These developments have however not reached the markets in India with research efforts being hindered by lack of funds, co-ordination and public awareness. Advanced photovoltaic systems when innovatively integrated into the architecture of the building can result in dramatic, elegant and cost-effective building, which is also environment friendly.

Concentrator systems
When a series of PV cells are stacked on top of one another to create multi-junction cells, relatively high conversion efficiencies can be achieved. In such cells, each of the different PV cells absorb a particular range of wavelengths converting it into electricity. Light that is not absorbed by the first layer is absorbed by the second layer and so on. Thus, multi-junction cells convert a broader range of wavelengths into electricity than single junction cells. They therefore have a potential for conversion efficiencies upto 40% (highest efficiency observed is 34.2% by Boeing). Copper indium selenide cells are specially used in concentrator systems.
Inexpensive mirrors or lenses are used to intensify the incident sunlight by as much as 400 times, therefore to produce a given amount if energy the number of cells required is much less in concentrator systems. Concentrating PV cells reduce the cost of PV electricity by using inexpensive mirrors and lenses instead of additional PV cells to increase output.
The disadvantages of concentrator systems include the need to mount the system on a solar tracking support frame, which can substantially increase costs and their inability to perform on cloudy days. Also, the intense light produces heat that can reduce the durability of the system. Therefore such systems should include cooling measures either passive or active.

Building Integrated photovoltaics
Applications for building integrated photovoltaics (PV) are essentially unlimited. PV can be integrated with every conceivable structure - from bus shelters to high rise office buildings, new structures and retrofits. Attractive variations in colour, texture, reflectance and transparency have been developed, as well as PV products that reflect traditional building materials such as roof tiles. However, such research is in its infancy in India and the development of PV products as replacements for traditional materials has not made enough progress for these to reach the market. The potential for such integration is, nonetheless, enormous.

Development of PV products for building integration is focused in three main areas - integral roof modules, roofing tiles and vertical facades. Roof integration has been the most popular application because PV modules have the greatest solar exposure in this location, and as a result, the highest power output. Vertical curtain walls and awnings are also practical as these can replace traditional cladding materials like glass.

Building integrated PV modules are typically produced on glass or metal substrates like traditional building materials. Modules produced in the same size as building materials like roof tiles or glass and other cladding panels can not only replace these materials efficiently but, being multifunctional, can also provide an added advantage by producing energy. The multifunctional nature of PV modules extends further than mere production of energy.

For example, a PV light shelf can shield direct sun while filtering comfortable diffused, indirect light into the interior. An opaque PV module used as awning can shade the interior from harsh direct sunlight and reduce cooling expenses. PV roof monitors can eliminate the need for daytime electric lighting by providing indirect daylight. Also, recently developed transparent thin-film modules can create energy-efficient PV modules with all the clarity and vision area of traditional glass.

Photovoltaic technology is in its infancy and more so in India, and its full potential is yet to be realised. Research and development in this field has to be intensified to produce user friendly PV products that can be easily integrated into the building structure so that the costs involved can be reduced. This will definitely increase the popularity of PV products. A greater acceptance of such products along with advanced photovoltaic technology in the future may offer us the opportunity to design entirely new types of energy efficient buildings with a wholly unique aesthetic appeal.