Building Integrated Photo-Voltaic Technology

 

Photo voltaic technology is a renewable technology which converts the suns energy into electrical energy. PV technology was originally developed at the bell labs in 1956 and is derived from the Greek prefix “Phos” meaning light and Volta after Alexander Volta a pioneer in the field of electricity. PV has been around for many years and has been used in applications such as providing power for remote telephones and remote research units for many years, these applications are often low in electrical demand. Other applications such as PV’s used to power the electrical demand for satellites have helped blaze a trail towards PV use into a  more efficient, robust, rounded economically viable technology.

 

How does PV work?

 

The principal behind PV technology is the ability of the photons contained within the suns rays to cause electrons to be moved to a higher energy level or orbit so that they are free and are capable of conduction. The energy required for an electron to jump to the next energy level is commonly known as the band gap energy denoted generally by Eg. Materials have their own Eg value and silicon which is the material used in most PV applications has a band gap energy of 1.12eV.

 

PV’s are made from semi-conducting material which has been doped with a different atom or impurity. Within the PV there are two layers, there are know as the p-type and the n-type layer or the positive and negative layer. The materials used in the manufacture of silicon have four electrons in the outer electron shell. The impurity is added to the base material such as silicon which has either one electron more in the outer shell or one electron less. The commonly used term for this process is called doping. The result of doping is the creation of places within the crystalline structure of the base material which have an excess electrons (n-type) and places where there is a missing electron or a deficiency (p-type). The missing electron is known as a hole or in other words a positive charge carrier, while the extra electron acts as a negative charge carrier. The most common material used in the manufacture of PV is the element silicon which is one of the most abundant elements on earth. Some of the other material used in the manufacture of PV are cadmium and gallium. The PV is composed of the p-type material on one side and the n-type material on the other side, this creates what is known as a p-n junction. Current flows in the PV cell when an electron in promoted through the absorption of a photon. The interaction of photons and the atoms in the PV are essential to the operation of the PV, with out sunlight then there is not excited electrons and thus no current. The amount of energy the photon must contain in order to excite an electron must be greater the band gap energy Eg which is specific to the material. The energy contained with a photon is a function of the frequency of light and planks constant h (6.626*x 10-34). This it can be said that the efficiency of the PV module is dependent on the intensity of light (W/m2) intercepted by the cell. The stronger the light (the higher the intensity) the more chance of the absorption of a photon to create an electric current. The efficiency of the system is governed by the percentage of incoming photons which cause absorption. Most of the photons absorbed by the cell do not contain greater than the Eg value required and thus only increase the temperature of the cell.

 

 

 

BIPV technology differs somewhat from other application of PV where there are different criteria. One of the most important considerations of BIPV technology is its ability to be both a building façade as well as an electricity producing technology. This can be a used as a very strong economic motivating factor as the perceived cost of the system is reduced due to the elimination of the need for other cladding. Design of BIPV really requires an all encompassing approach towards building design, the various energy systems used within the building and their interaction together. In this way the maximum benefits of BIPV can be reaped.

 

The amount of electrical power which can be obtained from a BIPV system is directly related to the availability of solar radiation, this means the orientation, the tilt and angle and the area of PV façade are of critical importance. The system should be design so that the demand profile of the building is matched as close as is possible to the supply profile from the system. This will have direct effects on the necessity for Balance Of System components such as battery’s and inverters. The economic viability will be highly dependent of the cost of electricity from the utility company and the electric loads within the building.

 

As with the other integrated renewable technologies an understanding of the basic principles of BIPV is essential to produce a BIPV design which does justice to potential of BIPV at the particular site and building in question.

 

One of the key factors which positively influences the case for BIPV in commercial applications is the ability of the BIPV in some applications to closely match the supply and demand profiles both daily and throughout the year.

 

The three main types of materials used in for PV modules are

 

 

 

Mono crystalline silicon is usually produced by the Czochralski technique. This consists of dipping a monocrystalline seed into molten polycrystalline. Once the seed is removed a monocrystalline or single-crystalline crystal ingot is formed. The ingots then go through many processes before finally ending up as a PV cell or array. The main processes are sawing the ingots, doping, polishing, interconnecting of the cells and assembly into PV modules and arrays. The thickness of the wafers used for the manufacture of monocrystalline cells is about 200 to 400 micrometers. The atomic structure of the monocrystalline is very much an ordered structure. It is the high degree of atomic order the allows the efficiency to be as high as 15%. Although real efficiencies will certainly be lower than this. Monocrystalline cells tend to be reliable when exposed to potentially harsh environments.

 

Polycrystalline silicon is manufactured using either the ribbon growth method where silicon is grown as wafers or sheets which are around the same thickness as that necessary for PV cell manufacture. Alternatively a block of polycrystalline silicon is sliced to produce the size of wafer required. Unlike monocrystalline cells the atomic structure of polycrystalline is not regularly ordered. Polycrystalline consists of small grains of monocrystalline spread throughout the polycrystalline. This means that as the flow of current or electrons is reduced thus the efficiency of polycrystalline is lower than that of monocrystalline cells. Conversion efficiencies of around 9 to 12% of likely although real efficiencies are likely to be much less. 

 

Thin film cells are made by depositing thin semi conducting layer onto a substrate material such as glass, metal or plastics. The light absorptivity of thin films is much higher than for crystalline materials, thus the deposited layer can be very thin. The thickness of the deposited layer is form a few micrometers to as little as one micrometer.  Generally the thinner the deposited layer the cheaper the manufacturing costs. Spraying is often the technique used for depositing the thin layer of the semi conducting material. The manufacturing process for thin films as opposed to monocrystalline or polycrystalline cells is much faster and uses considerably less energy. Thin film cells have much lower efficiencies than crystalline materials. This is a result of the non crystalline structure of the semi conducting layer. Much has been talked of how thin film technologies can produce low cost environmentally friendly electricity.

 

Amorphous silicon (a-Si) is by far the most commonly used thin film material. A-Si has a disordered atomic structure. The main problem with a-Si apart from the low efficiencies (real efficiencies of around 4% are likely) is the tendency for a-Si cells to suffer from degradation on exposure to sun, wind, rain and atmospheric pollutants. A-Si is likely to lose around 10 to 15% of their electricity producing capacity within the first few months. A-Si cells tend to oxidise and are thus much less durable than their crystalline counterparts.

Cadmium Telluride (CdTe) is a polycrystalline semi conducting compound made from Cadmium and Tellurium. The absorptivity of CdTe is high, thus CdTe can be as thin as 1 micrometer and can absorb around 90% if the solar spectrum. CdTe is also relatively cheap to manufacture. The deposition of CdTe is usually carried out by spraying, screen painting or high-rate evaporation. A conversion efficiency of CdTe is likely to be slightly higher than for a-Si. CdTe suffers from most of the same problems as a-Si such as reliability problems and degradation or exposure to the environment.