Anuradha Mishra

Electrical Characteristics of Solar Cells

To understand the behaviour of a solar cell as an electric power source, let us review the familiar characteristics of p-n junction diode and its behaviour under forward and reverse bias when dark. When n-type silicon semiconductor (characterized by electrons as majority carriers and holes as minority carriers) is in metallurgical contact with a piece of p-type semiconductor (characterized by holes as majority carriers and electrons as minority carriers) the boundary formed is p-n junction (Fig. 6.1). Owing to the process of natural diffusion electrons would flow from regions of high concentration (n-type) to the regions of low concentration (p-type) and the similar diffusion for holes. The electrons leaving the n-type side will expose positive charges in n-type side. The holes leaving the p-type side will expose the negative charges. This will result in a charge imbalance on both sides; it will set up an electric field that will oppose the natural diffusion tendency of electrons and holes and an equilibrium situation will be obtained. The region of transition is known as depletion layer, because the region is depleted of holes and electrons. The diode behaviour of such a p-n junction in dark conditions for forward bias and reverse bias conditions is well known and is shown in Fig. 6.2.

Repertoires of Applications

The solar conversion efficiency of first silicon solar cell was reported to be 6%. The estimated cost of production of solar cell was USD 600 per Wp. The applications that could be envisioned at that time were of power generation in remote locations where fuel could not be easily delivered. The obvious application was satellites where the requirement of reliability and light weight made the cost of solar PV unimportant. The first ever application of solar photovoltaics in space was in 1958 in Vanguard-I satellite. A small solar PV power unit of 6 mW, developed by space engineers at United States Army Signal Corps (USASC), was used as a power source in a back up radio transmitter. It was a rather successful trial and it sped the laboratory test and evaluation research in area of solar PV. Simultaneously (1958 onwards) in USSR solar PV units were used to power the entire electronics of satellites. These units operated successfully for 2 years. I960 onwards American satellites also started using solar PV units to power the electronic systems. The maximum capacity was of 150 Wp which paved the way for further capacity increase of solar PV power units in satellites as follows:

Solar PV Module and Array Network

The smallest operational unit of all SPV systems is the solar cell. Its size is chosen according to planned application. To obtain the desired voltage and current ratings the individual solar cells are connected in series and/or parallel. A string of cells connected in a pure series arrangement is referred to as series string or sub string. A group of series string or sub strings connected in parallel forms a series blocks. A branch circuit is composed of a series blocks between the positive and negative terminals of a power unit. An array is a collection of branch circuits (Fig. 7.1). The array is constructed in the field and transported from factory to field in small units called modules. The modules can be used to construct mW to MW size SPV arrays. Module is a panel for structural purposes having solar cells connected in series or parallel encapsulated in weather resistant laminate to protect the cells and enclosed in the sturdy corrosion resistant frame.

Mathematical Model of Transport Processes

The physical processes that determine the role of a solar cell as an energy converter include absorption of light leading to generation of electron-hole pairs, separation of charges in depletion region, surface and bulk recombination and transport of charge carriers by diffusion and drift. Over the years efforts have been made to understand these processes (e.g. Fahrenbruch and Bube 1983; Green 1982) and ameliorate the efficiency of solar cells.

System Reliability Considerations

The cost and reliability are two outstanding barriers in the promulgation of solar PV technologies on mass scale. In solar PV power system context reliability involves two factors: (i) the solar array need the use of an optimum load to deliver maximum power. This requirement varies with solar irradiance, temperature and battery bank characteristics and often difficult to be realized in practice. (ii) The reliability also means the ability of the system to continue functioning without failure for the period of time intended and under the given operating conditions; for example, during the array operation in field, the partial shadowing, soiling, cracking (provoked by hail impact) or opening of a string may occur due to solder melting or damage to encapsulate and in extreme cases it can lead to system failure. In both cases, the system reliability can only be guaranteed on a probabilistic basis. It essentially involves probability of success/failure, adequate performance, on/off time and operating conditions. Thus the probability provides the numerical input for the assessment of reliability and also the first index of system adequacy. For mission-oriented project this definition of reliability can be regarded as suitable measure. However, it becomes rather unsuitable measure for those continuously operated systems that can accommodate failure. The suitable measure used for these systems is availability. For example, in power system, availability is the percentage of time the system will deliver power to its load (Stember 1981). The required availability is different for different types of systems. Availability may be obtained by dividing “up-time” by “up-time plus down time” or it can be expressed aswhere A = availability, MTTR = mean time to repair and MTBF = mean time between failure.

Solar Photovoltaic System Design

The Solar Photovoltaic (SPV) system consists of energy generating subsystem modules, energy storage subsystem battery bank and power conditioning units to supply power, either stand-alone or in conjunction with grid (grid back-up). It is gaining increased importance as a renewable source due to its advantage like absence of fuel cost, little maintenance and no noise and wear due to the absence of the moving parts, etc. In particular, energy conversion from solar cell arrays received considerable attention in the last four decades. Its size is chosen according to planned application. Several demonstration units of large size solar PV power plants have been constructed, operated and monitored around the world. Most of these systems use silicon crystal solar cells. The module may be flat in geometry and use unconcentrated solar irradiance or else it may have concentrating optics (refracting or reflecting) as parts of its structure. Several options are possible in module mounting as follows: n n nPV modules may be placed at fixed tilt angle at which the average yearly energy generated is maximized. n n nThe tilt may be seasonally adjusted; for example at latitude −15, 0 and latitude +15 in summer, Equinoxes and winter respectively. n n nThe modules may be mounted on a tracking unit that follows the sun. Single angle axis trackers follow the sun daily from east to west and two axis trackers further include elevation control to correct for seasonal north-south sun movement.

BOS and Electronic Regulations

The Balance of System (BOS) encompasses all components of solar PV system other than array. The array is the energy generating subsystem. It consists of PV modules, panel structures, interconnecting wires, junction boxes along with lightning arresters and associated accessories.

Solar Radiation Characteristics

Sun is a star, a vast spherical mass of hydrogen and helium in proportion 3:1 by weight; the proportion of other elements is less than 1% of total mass (1.9 × 1030 kg). The temperature in its central interior region is estimated to be very high ~2106 K; consequently huge amount of thermonuclear energy is generated in the central core by the process of nuclear fusion. Some of the basic characteristics of sun-earth system are illustrated in Fig. 2.1.

Wafer-Based Solar Cells: Materials and Fabrication Technologies

Solar cells convert solar energy into electrical energy by photoionic process referred to as photoelectric effect. The solar radiation may be considered as discrete bundles of energy called photons. The energy (E) of photons associated with radiation of frequency (ν) and wave length (λ) may be expressed as E = hν = h cλ where h is Planck’s constant and c is the velocity of light. The radiation intensity is proportional to photon number density. As discussed in Chapter 2 the solar spectral irradiance ranges between the wave length region of 0.2–2.0 μm and has maximum invisible light region at 0.48 μm.

Fundamentals of Photovoltaic Generation: A Review

Photovoltaic generation of electricity is caused by the radiation separating positive and negative charges in absorbing material. If the electric field is present these charges can generate an electromotive field and produce a current for use in an external circuit. The fundamentals of photovoltaic generation, therefore, involve the knowledge of science of materials and the photovoltaic effect therein which in turn involve such basic concepts as atomic bonding, metals, semiconductors and insulators, crystal structure (silicon), band gap energy, built-in electrostatic field, PN junction, interaction of matter and radiation. In this chapter we present a review of these concepts.