Solar Photovoltaic Power
Solar Photovoltaic Power
Certain materials, like silicon, naturally release electrons when they are exposed to light, and these electrons can then be harnessed to produce an electric current. When these materials are exposed to light, they absorb photons and release free electrons. This phenomenon is called the photoelectric (PV) effect. PV is a method for the direct conversion of light into electricity. Solar PV effect is used to generate electrical power by converting solar energy radiation into direct current (DC) electric power. For this purpose, semiconductors are used since they show the PV effect. Solar PV power is a renewable as well as sustainable energy source. Conversion of the solar energy into electrical energy by PV power plants is the most recognized way to utilize solar energy. PV power plants (cell, module, network) need very little maintenance. At the end of the life cycle, PV modules can almost be completely recycled.
The word ‘photovoltaic’ consists of two words namely ‘photo’, a Greek word for light, and ‘voltaic’, which defines the measurement value by which the activity of the electric field is expressed, i.e., the difference of potentials. PV power plants use cells to convert sunlight into electricity. Direct or diffuse light (normally sunlight) shining on the solar cells induces the PV effect, generating DC electric power. This DC power can be used, stored in a battery system, or fed into an inverter which converts DC power into alternating current (AC) power.
The PV effect has been discovered in the first half of the 19th the century. In 1839, a young French physicist Alexandre Edmond Becquerel observed a physical phenomenon or effect which allows the conversion of light into electricity. The solar cells’ work is based on this principle of PV effect. In the following years, a number of scientists have contributed to the development of this effect and technologies through their researches, the most relevant among them are Charles Fritts, Edward Weston, Nikola Tesla and Albert Einstein, who has been awarded the Nobel Prize for his work on ‘Photoelectric effect’ in the year 1904. However, because of the high production rates, a greater development of this technology has begun only along with the development of semiconductor industry in the late fifties of the 20th century. During the sixties, the solar cells are used exclusively for supplying electricity to orbiting satellites in Earth orbit, where they prove themselves as very reliable and competitive technology.
In the seventies there are improvements in production, performance, and quality of solar cells, while the coming oil crisis helps to reduce production costs of solar cells and open up several possibilities for their practical implementation. Solar cells have been recognized as an excellent replacement for the supply of electric power at locations distant from the electric grid. The energy is supplied to wireless applications, lighthouses’ batteries, various signals, telecommunication equipment and other low power electricity dependent equipment. During the eighties, solar cells have become popular as an energy source for consumer electronic devices including calculators, watches, radios, lamps, and other applications with small batteries. Also, after the oil crisis in the seventies, larger efforts have been made in the development of solar cells for commercial use in households. Independent solar cells systems (off‐grid) have been developed, as well as network connected systems (on‐grid). In the mean-time, a considerable increase in wide use of solar cells has been recorded in rural areas where electricity network and infrastructure have not been developed. Electric power produced in these areas is used for pumping water, as cooling energy, in telecommunications and other household appliances, and everyday life needs.
PV modules technology and market development has grown rapidly when incentives have been given for the production of electric power from renewable energy sources. Incentives have been given in several countries. Today the industry connected with the PV modules and related equipment is growing at a very fast rate. In 2021, global cumulative solar PV power capacity amounted to 940 gigawatts, with roughly 168 gigawatts of new PV capacity installed in that same year.
Solar radiation and its harnessing
Energy from the Sun comes to the Earth in the form of solar radiation. It allows the production of electric power in the PV power plants. Under optimal conditions, the earth’s surface can get 1,000 watts per square metre (W/sq-m), while the actual value depends on the location, i.e., latitude, climatological location parameters such as frequency of cloud cover and haze, and air pressure etc.
Irradiation is the average density of the radiant solar radiation power, and is equal to the ratio of the solar radiation power and surface of the plane perpendicular to the direction of this radiation. Radiation represents the quantity of solar radiation which is radiated on the unit surface at a given time (watt hour per square metre, W.h/sq-m) or (joule per sq-m, J/sq-m). Besides expressing it in hourly values, it is frequently expressed as daily, monthly, or yearly radiation, depending on the time interval. The solar radiation weakens on its way through the earth’s atmosphere because of the interaction with gases and vapours in the atmosphere and arrives at the Earth’s surface as direct and diffused. Direct sunlight comes directly from the sun, while scattered or diffused radiation reaches the earth from all directions. The total radiation consists of direct and diffused radiation.
In case of an inclined surface, the rejected or reflected radiation is added to the direct and diffused radiation. Rejected radiation can be reflected from the ground or water. The largest component of solar radiation is direct, and the maximum radiation on a surface is perpendicular to the direction of the sun’s rays. The greatest radiation at any given moment is only possible if the plane is constantly referred to the movement of the sun in the sky.
Electrical energy can be harvested from solar radiation by means of either PV or concentrated solar power systems. PV cells directly convert solar energy into electric power. They work on the principle of the PV effect. Solar cells or PV cells are produced based on the principle of PV effect. These cells convert sunlight into DC electric power. But a single PV cell does not produce enough quantity of power. Hence, a number of PV cells are mounted on a supporting frame and are electrically connected to each other to form a PV module or solar panel of high-power rating. Normally available solar panels range from several hundred watts (say 100 watts) up to few kilowatts. They are available in different sizes and different price ranges.
Solar panels or modules are designed to supply electric power at a certain voltage (say 12V), but the current they produce is directly dependent on the incident light. As of now, it is clear that photovoltaic modules provide DC electric power. But, for most of the times, we need AC (alternating current) power. Hence, a converter is needed to convert the DC output of the panel into AC. Such a converter is known as an inverter. Hence, a solar power system also has an inverter. Main components of a solar PV system are solar panels, DC-DC converter, MPPT (maximum power point tracking), charge controller, battery for storage, DC-AC converter (inverter) and load to be fed. Block diagram representation of a typical Solar PV system is shown in Fig 1. According to the power requirement, multiple PV modules can be electrically connected together to form a PV array.
Fig 1 Photovoltaic generation of power
The performance ratio (PR) of a well-designed PV power plant is typically in the region of 77 % to 86 % (with an annual average PR of 82 %), degrading over the lifetime of the plant. In general, good quality PV modules can be expected to have a useful life of 25 years to 30 years. The purpose of the MPPT system is to sample the output of the cells and to apply the proper resistance (load) to achieve maximum power for any given environmental conditions.
The working principle of solar cells is based on the PV effect, i.e., the generation of a potential difference at the junction of two different materials in response to electro-magnetic radiation. The PV effect is closely related to the photoelectric effect, where electrons are emitted from a material which has absorbed light with a frequency above a material-dependent threshold frequency. Albert Einstein understood that this effect can be explained by assuming that the light consists of well-defined energy quanta, called photons. The energy ‘E’ of such a photon is given by E = hν, where ‘h’ is Planck’s constant and ‘v’ is the frequency of the light.
The light has a dual character according to quantum physics. Light is a particle and also it is a wave. The particles of light are called photons. Photons are mass-less particles, moving at light speed. The energy of the photon depends on its wavelength and the frequency, and the energy can be calculated by the Einstein’s law (E = hv).
In metals and in certain materials, normally, electrons can exist as valence or as free. Valence electrons are associated with the atom, while the free electrons can move freely. In order for the valence electron to become free, the valence electron is required to get the energy which is higher than or equal to the energy of binding. Binding energy is the energy by which an electron is bound to an atom in one of the atomic bonds. In the case of photoelectric effect, the electron acquires the needed energy by the collision with a photon. Part of the photon energy is consumed for the electron getting free from the influence of the atom which it is attached to, and the remaining energy is converted into kinetic energy of a now free electron. Free electrons obtained by the photo-electric effect are also called photo-electrons.
The energy needed to release a valence electron from the impact of an atom is called a ‘work out Wi’, and it depends on the type of material in which the photo-electric effect has occurred. The equation which describes this process is hv = Wi + Ekin where ‘Ekin’ is the kinetic energy of emitted electron. This equation shows that the electron is released when the photon energy is less than the work output.
The PV effect can be divided into three basic processes namely (i) generation of charge carriers because of the absorption of photons in the materials which form a junction, (ii) subsequent separation of the photo-generated charge carriers in the junction, and (iii) collection of the photo-generated charge carriers at the terminals of the junction.
Generation of charge carriers because of the absorption of photons in the materials which form a junction – Absorption of a photon in a material means that its energy is used to excite an electron from an initial energy level ‘Ei’ to a higher energy level ‘Ef’, as shown in Fig 2a. Photons can only be absorbed if electron energy levels Ei and Ef are present so that their difference is equal to the photon energy, (hν = Ef – Ei). In an ideal situation, semiconductor electrons can populate energy levels below the so-called valence band edge, ‘EV’, and above the so-called conduction band edge, ‘EC’. Between those two bands no allowed energy states exist, which can be populated by electrons. Hence, this energy difference is called the bandgap, ‘Eg’ (Eg = EC – EV). If a photon with an energy smaller than Eg reaches an ideal semiconductor, it does not get absorbed but traverses the material without interaction.
Fig 2 Generation of charge carriers in a semiconductor
In a real semiconductor, the valence and conduction bands are not flat, but vary depending on the so-called k-vector which describes the crystal momentum of the semiconductor. If the maximum of the valence band and the minimum of the conduction band occur at the same k-vector, an electron can be excited from the valence to the conduction band without a change in the crystal momentum. Such a semiconductor is called a direct bandgap material. If the electron cannot be excited without changing the crystal momentum, it is called an indirect bandgap material. The absorption coefficient in a direct bandgap material is far higher than in an indirect bandgap material, hence the absorber can be much thinner.
If an electron is excited from ‘Ei’ to ‘Ef’, a void is created at ‘Ei’. This void behaves like a particle with a positive elementary charge and is called a hole. The absorption of a photon hence leads to the creation of an electron-hole pair, as shown in Fig 2b. The radiative energy of the photon is converted to the chemical energy of the electron-hole pair. The maximal conversion efficiency from radiative energy to chemical energy is limited by thermodynamics. This thermodynamic limit lies in between 67 % for non-concentrated sunlight and 86 % for fully concentrated sunlight.
Subsequent separation of the photo-generated charge carriers in the junction – Normally, the electron-hole pair recombines, i.e., the electron fall backs to the initial energy level Ei, as shown in Fig 2b. The energy is then be released either as photon (radiative recombination) or transferred to other electrons or holes or lattice vibrations (non-radiative recombination). If one wants to use the energy stored in the electron-hole pair for performing work in an external circuit, semi-permeable membranes are to be present on both sides of the absorber, such that electrons only can flow out through one membrane and holes only can flow out through the other membrane, as shown in Fig 2b (3). In majority of the photovoltaic cells, these membranes are formed by n- type and p-type materials (Fig 1). A PV cell has to be designed such that the electrons and holes can reach the membranes before they recombine, i.e., the time it needs the charge carriers to reach the membranes is to be shorter than their lifetime. This requirement limits the thickness of the absorber.
Collection of the photo-generated charge carriers at the terminals of the junction – Finally, the charge carriers are extracted from the solar cells with electrical contacts so that they can perform work in an external circuit as shown in Fig 2b (4). The chemical energy of the electron-hole pairs is finally converted to electric energy. After the electrons passed through the circuit, they recombine with holes at a metal-absorber interface, as shown in Fig 2b (5).
Loss mechanisms – The two most important loss mechanisms in single bandgap solar cells are the inability to convert photons with energies below the bandgap to electricity and thermalization of photon energies exceeding the bandgap, as shown in 2a (ii). These two mechanisms alone amount to the loss of about half the incident solar energy in the conversion process. Hence, the maximal energy conversion efficiency of a single junction solar cell is considerably below the thermodynamic limit. This single bandgap limit was first calculated by Shockley and Queisser in 1961.
PV junction (diode) is a boundary between two differently doped semiconductor layers, one is a p‐type layer (excess holes), and the second one is an n‐type (excess electrons). At the boundary between the p-type and the n-type area, there is a spontaneous electric field, which affects the generated electrons and holes and determines the direction of the current. To obtain the energy by the photoelectric effect, there is a directed motion of photoelectrons, i.e., electricity. All charged particles, photo-electrons also, move in a directed motion under the influence of electric field. The electric field in the material itself is located in semiconductors, precisely in the impoverished area of PV junction (diode). Fig 3 gives different details of PV cell.
Fig 3 Photovoltaic cell
It is pointed out for the semiconductors that, along with the free electrons in them, there are holes as charge carriers, which are a sort of a byproduct in the emergence of free electrons. Holes occur whenever the valence electron turns into a free electron, and this process is called the generation, while the reverse process, when the free electron fills the empty spaces ‐ a hole, is called recombination. If the electron‐cavity pairs occur away from the impoverished areas it is possible to recombine before they are separated by the electric field. Photo-electrons and holes in semiconductors are accumulated at opposite ends, thereby creating an electromotive force (EMF). If a consuming device is connected to such a system, the current flows and electricity is generated.
In this way, PV cells produce a voltage of around 0.5 V to 0.7 V, with a current density of around several tens of milliampere per square centimeter (mA/sq-cm) depending on the solar radiation power as well as on the radiation spectrum. The usefulness of a PV cell is defined as the ratio of electric power provided by the PV cell and the solar radiation power. Mathematically, it can be presented in the equation n = Pel / Psol = U.I / E.A, where ‘Pel’ is the electrical output power, ‘Psol’ is the radiation power of the sun, ‘U’ is the effective value of output voltage, ‘I’ is the effective value of the electricity output, ‘E’ is the specific radiation power (for example W/sq-m), and ‘A’ is the area.
The usefulness of PV solar cells ranges from a few % to 40 %. The remaining energy which is not converted into electrical energy is mainly converted into heat energy and hence warms the PV cell. Normally, the increase in PV cell temperature reduces the usefulness of the cell.
The solar rating is a measure of the average solar energy (also called ‘solar irradiance’) available at a location in an average year. Radiant power is expressed in power per unit area normally watts/sq-m, or kW/sq-m. The total daily Irradiation (W.h/sq-m) is calculated by the integration of the irradiance values (W/sq-meter). Irradiance is the power of the sunlight incident on a surface per unit area and is measured in power per square meter (W/sq-m). On an average (as a normal ‘rule of thumb’), modern PV solar panels produce 86 watts – 108 watts per square metre of solar panel area. If the PV panels are shaded for part of the day, the output of the panels is reduced in accordance with the shading percentage.
Energy conversion efficiency of a photovoltaic cell (‘n’) is the percentage of energy from the incident light which actually ends up as electricity. This is calculated at the point of maximum power ‘Pm’ divided by the input light irradiation (‘E’, in W/sq-m), all under standard test conditions and the surface area of PV cell ‘A’ in sq-m (n = Pm / E.A). Also, ‘Isc is the short-circuit current at STC and ‘Voc’ is the open-circuit voltage. Standard test conditions (STC) are irradiations – 1,000 W/sq-m, temperature – 25 deg C, AM – 1.5. (AM stands for ‘air mass’, the thickness of the atmosphere, at the equator air mass = 1). The standard test condition is defined as being 1,000 W/sq-m (1 kW/sq-m) of full solar noon sunshine (irradiance) when the cell is at a standard ambient temperature of 25 deg C with a sea level air mass of 1.5 (1 sun).
Types of solar PV cells
Electricity is produced in PV cells which consist of more layers of semiconductive material. When the sun’s rays shine down upon the PV cells, the EMF between these layers is being created, which causes the flow of electricity. The higher is the solar radiation intensity, the higher is the flow of electricity. The PV cells are of two types namely (i) crystalline silicon cells and (ii) thin film cells. Types of crystalline silicon (c-Si) cells are (i) mono-crystalline cells, and (ii) poly / multi crystalline cells. They are made from cells of either mono-crystalline or multi-crystalline silicon. Fig 1 shows a typical monocrystalline cell. Thin film cells are (i) thin film silicon cells, (ii) cadmium telluride (CdTe) cells, (iii) copper indium selenide (CIS) cells / copper indium gallium di- selenide (CIGS) cells. Thin-film cells provide a cheaper alternative, but are less efficient. There are three main types of thin-film cells (i) cadmium – telluride (CdTe), (ii) copper – indium (gallium) di-selenide (CIGS/CIS), and (iii) amorphous silicon (a-Si).
Each material has unique characteristics which impact the cell performance, manufacturing method, and cost. PV cells can be based on either silicon wafers or thin film technologies for which a thin layer of a semiconductor material is deposited on low-cost substrates. PV cells can further be characterized according to the long-range structure of the semiconductor material, mono-crystalline, multi-crystalline (also known as poly-crystalline) or less-ordered amorphous material.
Unusual semiconducting properties needed for PV cells limit the raw materials from which they can be manufactured. Silicon is the most common material, but cells using CdTe and CIGS/CIS are also viable. Silicon is obtained from sand and is one of the most common elements in the earth’s crust, so there is no limit to the availability of raw materials. Cells made from crystal silicon, are made of a thinly sliced piece (wafer), a crystal of silicon (mono-crystalline), or a whole block of silicon crystals (multi-crystalline). Their efficiency ranges between 12 % and 19 %. There are emerging PV cell technologies such as organic cells are made from polymers. However, they are not commercially available yet.
Crystalline silicon (c-Si) PV modules consist of PV cells connected together and encapsulated between a transparent front (normally glass) and a backing material (normally plastic or glass). Fig 4 gives types of PV cells.
Fig 4 Types of photovoltaic cells
Mono-crystalline silicon PV cells – The conversion efficiency for this type of cells ranges from 13 % to 17 %, and can normally be said to be in wide commercial use. In good light conditions, it is the most efficient photovoltaic cell. This type of cell can convert solar radiation of 1,000 W/sq-m to 140 W of electricity with the cell surface of 1 sq-m. The production of monocrystalline silicon cells needs an absolutely pure semiconducting material. Mono-crystalline silicon wafers are sliced from a large single crystal ingot in a relatively expensive process. Such type of production enables a relatively high degree of usability. Expected lifespan of these cells is typically 25 years to 30 years and the output degrades over the years which is also true for all types of photovoltaic cells. Mono-crystalline silicon cells are normally the most efficient, but are also more costly than multi-crystalline silicon cells.
Multi-crystalline silicon PV cells – This type of cell can convert solar radiation of 1,000 W/sq-m to 130 W of electricity with the cell surface of 1 sq-m. The production of these cells is economically more efficient compared to mono-crystalline. Liquid silicon is poured into blocks, which are then cut into slabs. Multi-crystalline silicon PV cells produced in this way are presently cheaper, but the end product is normally not as efficient as mono-crystalline technology. During the solidification of materials, crystal structures of various sizes are being created, at whose borders some defects can emerge, making the PV cell to have a somewhat lower efficiency, which ranges from 10 % to 14 %. The lifespan is expected to be between 20 years and 25 years.
Ribbon silicon has the advantage in its production process in not needing a wafer cutting, which results in loss of up to 50 % of the material in the process of cutting. However, the quality and the possibility of production of this technology does not make it a leader in the near future. The efficiency of these cells is around 11%. Fig 5 shows flowsheet of the silicon photovoltaic cell module production for solar power generation.
Fig 5 Flowsheet of the silicon photovoltaic cell module production for solar power generation
In the thin‐film technology, the modules are manufactured by piling extremely thin layers of photo-sensitive materials on a cheap substrate such as glass, stainless steel, or plastic. This class includes semiconductors made from (i) amorphous silicon (a-Si), (ii) cadmium telluride (CdTe), (iii) copper indium selenide (CIS), (iv) copper indium gallium di-selenide (CIGS), and (v) heterojunction with intrinsic thin-film layer (HIT). HIT cells are composed of a mono-thin c-Si wafer surrounded by ultra-thin a-Si layers.
Crystalline wafers provide high-efficiency solar cells, but are relatively costly to manufacture. In comparison, thin film cells are typically cheaper because of both the materials used and the simpler manufacturing process. However, thin-film cells are less efficient. A well-developed thin film technology uses silicon in its less-ordered, non-crystalline (amorphous) form. Other technologies use CdTe and CIGS/CIS with active layers less than a few micrometres thick. Some thin-film technologies have a less established track record than several crystalline technologies.
The process of generating modules in thin‐film technology has resulted in reduced production costs compared to crystalline silicon technology, which is somewhat more intense. Because of reduced manufacturing costs and maturity of the technology, wafer-based crystalline modules are having a major market share. The present-day price advantage in the production of a thin‐film is balanced with the crystalline silicon because of the lower efficiency of the thin‐film, which ranges from 5 % to 13 %. The share of thin‐film technology on the market is constantly increasing. Lifespan thin film photovoltaic cells is around 20 years. There are four types of thin‐film modules (depending on the active material) which are in use. These are given below. The main characteristics of thin-film technologies are described below.
Amorphous silicon (a-Si) – In amorphous silicon PV cells, the long-range order of crystalline silicon is not present and the atoms form a continuous random network. Since amorphous silicon absorbs light more effectively than crystalline silicon, the cells can be much thinner. Amorphous silicon can be deposited on a wide range of both rigid and flexible low-cost substrates. The low cost of amorphous silicon makes it suitable for several applications where low cost is more important. The cell efficiency of the amorphous silicon cells is around 6 %. A cell surface of 1 sq-m can convert 1,000 W/sq-m solar radiation to around 50 watts of electric energy. However, efforts are being made to improve the efficiency. In this type of cell. a thin film of silicon is put on a glass or another substrate. The layer thickness is less than 1 micrometre and because of it the production cost is less and in line with the other low cost of materials. It is primarily used in equipment where low power is needed (such as watches, and pocket personal computers), and more recently, as an element in building facades.
Cadmium-Tellurium (CdTe) – CdTe is a compound of cadmium and tellurium. The cell consists of a semiconductor film stack deposited on transparent conducting oxide-coated glass. A continuous manufacturing process using large area substrates can be used. Modules based on CdTe produce a high energy output across a wide range of climatic conditions with good low light response and temperature response coefficients. CdTe modules are well established in the industry and have a good track record.
The efficiency of the CdTe cells is around 18 %. A cell surface of 1 sq-m can convert solar radiation of 1,000 W/sq-m to around 160 watts of electric energy under laboratory conditions. It is suitable for use in thin PV modules because of the physical properties and low‐cost production technology. Despite these advantages, it is not widely used because of the cadmium toxicity and suspected carcinogenicity.
Copper indium selenide and copper indium gallium di-Selenide (CIS/CIGS) – CIS/CIGS is a semiconductor consisting of a compound of copper, indium, gallium and selenium. CIS/CIGS absorbs light more efficiently than crystalline silicon, but modules based on this semiconductor need somewhat thicker films than amorphous silicon PV modules. Indium is a relatively expensive semiconductor material, but the quantities needed are extremely small compared to wafer-based technologies. Commercial production of CIS/CIGS modules is in the early stages of development. However, it has the potential to offer the highest conversion efficiency of all the thin film PV module technologies.
CIS/CIGS cells have the highest efficiency among the thin‐film cells, which is around 20 %. This cell type can convert solar radiation of 1,000 W/sq-m to around 160 W of electric energy with the cell surface of 1 sq-m under laboratory conditions.
Heterojunction with intrinsic thin film layer (HIT) – The HIT solar cell is composed of a mono-thin-crystalline silicon wafer surrounded by ultra-thin amorphous silicon layers. HIT modules are more efficient than typical crystalline modules, but they are more expensive.
Thermo sensitive solar cells and other organic cells – The development of these organic cells is yet to come, since it is still under testing and it is not yet commercialized. Cell efficiency is around 10 %. The tests are going in the direction of using the facade integrated systems, which has proven to be high‐quality solutions in all light radiation and all temperature conditions. Also, a great potential of this technology is that it is low cost as compared with silicon cells.
There are other types of PV technologies which are still developing, while some others are to be commercialized. Regardless of the lifespan, the warranty period of the present-day most common commercial PV modules is 10 years at 90 % power output, and 25 years at 80 % power output.
Module degradation – PV modules can be mounted in different ways, fixed at a certain angle, or can be moving to better monitor the angle of inclination of the sun during the day for greater energy yield and better results in the production of electric power. Optimal value of the inclination angle of the surface has to be determined for fixed mounted photovoltaic module. The optimum angle of inclined PV module’s surfaces is the angle at which it is inclined in relation to a horizontal surface in order to get the highest possible average annual irradiation on a horizontal surface (MW.h/sq-m) irradiation. An optimum angle of inclination for a period or certain months in the year can also be calculated. The highest energy yield of a fixed module system is achieved by placing the modules at the optimal annual angle.
The performance of a PV module decreases over time because of a process known as degradation. The degradation rate depends on the environmental conditions and the technology of the module. Modules are either mounted on fixed-angle frames or on sun-tracking frames. Fixed frames are simpler to install, cheaper and need less maintenance. However, tracking systems can increase yield by up to 45 %. Tracking, particularly for areas with a high direct / diffuse irradiation ratio also enables a smoother power output. Inverters convert direct current electric power generated by the PV modules into alternating current electric, ideally conforming to the local grid requirements. They are arranged either in string or central configurations. Central configuration inverters are considered to be more suitable for multi-MW plants. String inverters enable individual string ‘maximum power point tracking’ (MPPT) and need less specialized maintenance skills. String configurations offer more design flexibility.
The performance of a PV module decreases over time. Degradation has different causes. It can include effects of humidity, temperature, solar irradiation and voltage bias effects, which s is referred to as potential induced degradation (PID). PID is dependent on temperature, humidity, and system voltage and ground polarity. It can be detected with a relatively short test. The degradation is reversible by applying a suitable external voltage. Other factors affecting the degree of degradation include the quality of materials used in manufacture, the manufacturing process, and the quality of assembly and packaging of the cells into the module.
Maintenance has little effect on the degradation rate of modules, which is predominantly dependent on the specific characteristics of the module being used and the local climatic conditions. It is, hence, important that reputable module manufacturers are chosen and power warranties and degradation rates are carefully reviewed.
The extent and nature of degradation varies among module technologies. For crystalline modules, the degradation rate is typically higher in the first year upon initial exposure to light and then stabilizes. The initial irreversible light induced degradation (LID) occurs because of the defects which are activated on initial exposure to light. It can be caused by the presence of boron, oxygen, or other chemicals left behind by the screen printing or etching process of cell production. Depending on the wafer and cell quality, the LID can vary from 0.5 % to 2%. Amorphous silicon cells degrade through a process called the Staebler-Wronski effect. This degradation can cause reductions of 10 % to 30 % in the power output of the module in the first six months of exposure to light. Thereafter, the degradation stabilizes and continues at a much slower rate. Amorphous silicon modules are normally marketed at their stabilized performance levels. Interestingly, degradation in amorphous silicon modules is partially reversible with temperature. In other words, the performance of the modules can tend to recover during the summer months, and drop again in the colder winter months.
Additional degradation for both amorphous and crystalline technologies, occurs at the module level and can be caused by (i) effect of the environment on the surface of the module (for example, pollution), (ii) discolouration or haze of the encapsulant or glass, (iii) lamination defects, (iv) mechanical stress and humidity on the contacts, (v) cell contact breakdown, and (vi) wiring degradation. PV modules can have a long-term power output degradation rate of between 0.3 % and 1 % per annum. For crystalline modules, a generic degradation rate of 0.4 % per annum is frequently considered applicable. Some module manufacturers have carried out specific independent tests showing that lower degradation rates can be safely assumed. For amorphous silicon and CIGS modules, a generic degradation rate of 0.7 % to 1 % is frequently considered reasonable, however a degradation rate of more than 1.5 % has sometimes been observed. For CdTe a value of 0.4 % to 0.6 % is frequently applicable. In general, good quality PV modules can be expected to have a useful life of 25 years to 30 years. The risk of increased rates of degradation becomes higher thereafter.
Influence of shading on solar power plant ‐ The maximum electric energy is produced when sunlight directly crosses the PV modules. Shadows created by objects on the roof, tree or other surrounding buildings and skyscrapers substantially affect electricity production. The shade also negatively affects the stability of the system since the modules located partially in the shade do not have a linear production of electricity, resulting in voltage changes and inverter disturbances. If only one cell in a module is located in the shade, it can reduce the power of all modules by around 75 %.
Module efficiency – Tab 1 shows the commercial efficiency of some PV modules categories. As can be expected, while higher efficiency modules are more costly to manufacture, less efficient modules need a larger area to produce the same nominal power. As a result, the cost advantages gained at the module level can be offset by the cost incurred in providing additional power system infrastructure (cables and mounting frames) and the cost of land for a larger module area. Hence, using the lowest cost module does not necessarily lead to the lowest cost per watt peak (Wp) for the complete plant.
|Tab 1 Characteristics of some PV Technology Classes|
|Technology||Current commercial efficiency||Temperature co-efficient for power*|
|Crystalline silicon||13 % to 21 %||– 0.45 % / deg C|
|HIT||18 % to 20 &||0.29 % / deg C|
|Amorphous silicon||6 % to 9 %||-0.21 % / deg C|
|CdTe||8 % to 16 %||-0.25 % / deg C|
|CIS/CIGS||8 % to 14 %||-0.35 % / deg C|
|* The temperature co-efficient for power describes the dependence on power output with increasing temperature. Module power normally decreases as the module temperature increases.|
Energy depreciation of PV modules – It is the period of energy depreciation of the PV modules. It is the time period which is to pass using a PV plant to return the energy which has been invested in the construction of all parts of the system, as well as the energy needed for the breakdown after the lifetime of a PV power plant. Of course, the energy depreciation time is different for different locations at which the power plant is located. Hence, it is a lot shorter on locations with a large quantity of irradiated solar energy, up to 10 or more times shorter than its lifetime.
When assessing the quality of a module for any specific project, it is desired that an independent review of the PV module technical specifications, quality assurance standards, track record and experience, as well as compliance with relevant international and national technical and safety standards is done. The expected degradation of the modules is to be ascertained and the module warranties are to be reviewed and compared to industry norms.
Certification – PV modules and inverters are all subject to certification, predominantly by the International Electrotechnical Commission (IEC). The International Electrotechnical Commission (IEC) issues internationally accepted standards for PV modules. Technical committee ‘Solar photovoltaic energy systems’, is responsible for writing all IEC standards pertaining to photovoltaics. PV modules typically be tested for durability and reliability according to these standards. Standards IEC 61215 (for crystalline silicon modules) and IEC 61646 (for thin-film modules) include tests for thermal cycling, humidity and freezing, mechanical stress and twist, hail resistance and performance under standard test conditions (STC). These are an accepted minimum quality mark and indicate that the modules can withstand extended use. However, they say very little about the performance of the module under field conditions. IEC 61853-1 ‘Photovoltaic Module Performance Testing and Energy Rating’ provides the methodology for ascertaining detailed module performance. It gives an accurate protocol for comparing the performance of different module models. IEC 61853-2 describes procedures for measuring the effect of angle of incidence on module performance. IEC 61853-3 describes the methodology for calculating module energy ratings (watt-hours). IEC 61853-4 defines the standard time periods and weather conditions which can be used for calculating energy ratings.
Inverters – Inverters are solid state electronic devices. They convert DC electric power generated by the PV modules into AC electric power, ideally conforming to the local grid requirements. Inverters can also perform a variety of functions to maximize the output of the plant. These range from optimizing the voltage across the strings and monitoring string performance to logging data and providing protection and isolation in case of irregularities in the grid or with the PV modules.
There are two broad classes of inverters namely (i) central inverters, and (ii) string inverters. The central inverter configuration shown in Fig 6 remains the first choice for several medium-scale and large-scale solar PV plants. A large number of modules are connected in a series to form a high voltage (HV) string. Strings are then connected in parallel to the inverter. Central inverters offer high reliability and simplicity of installation. However, they have disadvantages of increased mismatch losses and absence of maximum power point tracking (MPPT) for each string. This can cause problems for plants which have multiple tilt and orientation angles, or suffer from shading, or use different module types.
Central inverters are normally three-phase and can include grid frequency transformers. These transformers increase the weight and volume of the inverters, although they provide galvanic isolation from the grid. In other words, there is no electrical connection between the input and output voltages—a condition which is sometimes needed by national electrical safety regulations.
Mismatch refers to losses because of PV modules with varying current / voltage profiles being used in the same array. MPPT is the capability of the inverter to adjust its impedance so that the string is at an operating voltage which maximizes the power output.
Central inverters are sometimes used in a ‘master-slave’ configuration. This means that some inverters shut down when the irradiance is low, allowing the other inverters to run more closely to optimal loading. When the irradiance is high, the load is shared by all inverters. In effect, only the needed number of inverters is in operation at any one time. As the operating time is distributed uniformly among the inverters, design life can be extended. In contrast, the string inverter concept uses multiple inverters for multiple strings of modules. String inverters provide MPPT on a string level with all strings being independent of each other. This is useful in cases where modules cannot be installed with the same orientation or where modules of different specifications are being used or when there are shading issues.
String inverters, which are normally in single phase, also have other advantages. First of all, they can be serviced and replaced by non-specialist personnel. Secondly, it is practical to keep spare string inverters on site. This makes it easy to handle unforeseen circumstances, as in the case of an inverter failure. In comparison, the failure of a large central inverter, with a long lead time for repair, can lead to considerable yield loss before it can be replaced. Inverters can be transformer-less or include a transformer to step up the voltage. Transformer-less inverters have normally a higher efficiency, as they do not have transformer losses. In the case of transformer-less string inverters (Fig 7), the PV generator voltage is either to be considerably higher than the voltage on the AC side, or DC-DC step-up converters are to be used. The absence of a transformer leads to higher efficiency, reduced weight, reduced size (50 % to 75 % lighter than transformer-based models) and lower cost because of the smaller number of components. On the downside, additional protective equipment is to be used, such as DC sensitive earth leakage circuit breakers (CB), and live parts are to be protected. IEC Protection Class II is to be implemented across the installation. Transformer-less inverters also cause increased electromagnetic interference (EMI). Fig 6 gives schematics of PV power plant inverter configurations.
Fig 6 Schematics of PV power plant inverter configurations
Inverters with transformers provide galvanic isolation. Central inverters are normally equipped with transformers. Safe voltages (less than 120 V) on the DC side are possible with this design. The presence of a transformer also leads to a reduction of leakage currents, which in turn reduces EMI. But this design has its disadvantages in the form of losses (load and no-load) and increased weight and size of the inverter.
Inverters operate by use of power switching devices such as thyristor or ‘insulated gate bipolar transistor (IGBT) to chop the direct current into a form of pulses which provide a reproduction of an AC sinusoidal waveform. The nature of the generated alternating current AC wave means that it can spread interference across the network. Hence, filters are to be applied to limit ‘electromagnetic compatibility’ (EMC) interference emitted into the grid. Circuit protection functions is to be included within a good inverter design. Inverters are to be provided with controllers to measure the grid output and control the switching process. In addition, the controller can provide the MPPT functionality.
A number of different types of efficiencies have been defined for inverters. These describe and quantify the efficiency of different aspects of an inverter’s operation. The search for an objective way of quantifying inverter performance is still ongoing. New ways of measuring efficiency are frequently suggested. Fig 7 shows efficiency curves of low, medium ang high efficiency inverters.
Fig 7 Efficiency curves of low, medium ang high efficiency inverters
The conversion efficiency is a measure of the losses experienced during the conversion from DC to AC. These losses are because of the multiple factors such as the presence of a transformer and the associated magnetic and copper losses, inverter self-consumption, and the losses in the power electronics. Conversion efficiency is defined as the ratio of the fundamental component of the AC power output from the inverter, divided by the DC power input. The conversion efficiency is not constant, but depends on the DC power input, the operating voltage, and the weather conditions, including ambient temperature and irradiance. The variance in irradiance during a day causes fluctuations in the power output and maximum power point (MPP) of a PV array. As a result, the inverter is continuously subjected to different loads, leading to varying efficiency. Majority of the inverters employ MPPT algorithms to adjust the load impedance and maximize the power from the PV array. The highest efficiencies are reached by transformer-less inverters.
The voltage at which inverters reach their maximum efficiency is an important design variable, as it allows plant designers to optimize system wiring. Because of the dynamic nature of inverter efficiency, diagrams are also more suited to depiction than uniform numeric values. An example depicting the dependency of the inverter efficiency on the inverter load is given in Fig 7. The inverter efficiency is a calculated efficiency averaged over a power distribution corresponding to the operating climatic conditions of a particular plant location.
Types of PV power plant – PV power plants can be normally divided into two basic groups namely (i) PV power plants not connected to the network i.e., stand‐alone systems (off‐grid), and (ii) PV power plants connected to public electricity network (on‐grid). There are a number of different sub-types of PV power plants according to the type and the method of connecting to the network, or a way of storing energy on independent power plants.
On the basis of solar plant integration, solar PV plants can be classified as (i) direct PV plants, (i) off grid plants and (iii) grid connected plants. Direct PV plants supply the load only when the Sun is shining. There is no storage of power generated and, hence, batteries are not there. Off-grid plants are normally used at locations where power from the grid is not available or not reliable. An off-grid solar power plant is not connected to any electric grid. It consists of solar panel arrays, storage batteries and inverter circuits. Grid-connected plants are tied with grids so that two way flow of power is there and when needed power can be accessed from the grid. They plants are either backed by batteries or not backed up by batteries.
Network‐connected PV plants (on‐grid) – In the case of the network‐connected PV power plants, the main components of the power plants are PV modules, inverter, mounting subframe, measuring and metering instruments cabinet with protective equipment, transformer, and control equipments. PV modules convert solar energy into direct current, while the inverter converts it into alternating current and the transformer steps up the voltage to the level of the utility grid voltage so that the produced power can be fed to the grid. The AC voltage is supplied to the power network through the protection and metering equipment.
PV solar plant inverters convert the direct current produced by the PV cell modules to alternating current and are normally located indoors, although there are inverters suitable for outdoor installation but these are not to be directly exposed to sunlight. Inverters produce high‐quality alternating current. The inverters are to ensure that the voltage they supply is in phase with the utility network voltage. This allows the PV power plant to deliver the electricity to the utility network. Energy meter measures the electric power production and consumption.
The solar power plants which are connected to the network, are generating large quantities of electric power by a PV installation on a localized area. The power of such PV power plants ranges from several hundred kilowatts to tens of megawatts, recently up to several hundred megawatts. Some of these installations can be located on large industrial facilities and terminals, but more frequently on large barren land surfaces. Such large installations compensate part of the electric energy demand in the area. To have a feeling of size, talking about solar power plants, it is worth to mention an example of a large‐scale solar power plant of capacity 40 MWp of power with thin film technology, has a surface area of 110 hectares. Fig 1 gives an overview of a megawatt-scale grid connected solar PV power plant. The main components a grid connected solar plant include the following.
Solar PV modules – These convert solar radiation directly into electricity through the photovoltaic effect in a silent and clean process which needs no moving parts. The PV effect is a semiconductor effect whereby solar radiation falling onto the semiconductor PV cells generates electron movement. The output from a solar PV cell is DC power. A PV power plant contains several cells connected together in modules and several modules connected together in strings to produce the needed DC power output.
Modules – These can be connected together in a series to produce a string of modules. When connected in a series, the voltage increases. Strings of modules connected in parallel increase the current output.
Inverters – Inverters are needed to convert the DC electric power to AC power for connection to the utility grid. Several modules in series strings and parallel strings are connected to the inverters.
Module mounting (or tracking) systems – These allow PV modules to be securely attached to the ground at a fixed tilt angle, or on sun-tracking frames.
Step-up transformers – The output from the inverters normally needs a further step-up in voltage to reach the AC grid voltage level. The step-up transformer takes the output from the inverters to the needed grid voltage (for example 11 kV, 33 kV, or 110 kV, depending on the grid connection point and the national standards).
The grid connection interface – This is where the electricity is exported into the grid network. The substation also has the needed grid interface switchgear such as circuit breakers (CBs) and disconnects for protection and isolation of the PV power plant, as well as metering equipment. The substation and metering point are sometimes external to the PV power plant boundary and are typically located on the network operator’s property.
Mounting and tracking systems – PV modules are to be mounted on a structure to keep them oriented in the correct direction and to provide them with structural support and protection. Mounting structures can be fixed or tracking. Fixed tilt arrays are typically tilted away from the horizontal plane in order to maximize the annual irradiation they receive. The optimum tilt angle is dependent on the latitude of the site location. The direction the system is facing is referred to as its orientation or azimuth, as shown in Fig 8.
Fig 8 Mounting and tracking of PV modules
The ideal azimuth for a system in the northern hemisphere is geographic south, and in the southern hemisphere it is geographic north.
Fixed mounting systems keep the rows of modules at a fixed tilt angle while facing a fixed angle of orientation. The tilt angle or ‘inclination angle’ is the angle of the PV modules from the horizontal plane. The orientation angle or ‘azimuth’ is the angle of the PV modules relative to south. Definitions can vary but 0-degree represents true south, -90-degree represents east, 180-degree represents north, and 90-degree represents west.
Mounting structures are fabricated from steel or aluminum, although there are also examples of plants based on wooden beams. A good quality mounting structure is expected (i) to have undergone extensive testing to ensure that the designs meet or exceed the load conditions experienced at the site including the design of the corrosion protection system to resist below-ground and atmospheric corrosion, (ii) to be designed specifically for the site location with structural design calculations provided for verification of the site-specific design, and has a structural warranty, (iii) to allow the desired tilt angle to be achieved within a few degrees, (iv) to allow field adjustments which reduces installation time and compensates for inaccuracies in placement of foundations, (v) to minimize tools and expertise needed for installation, (vi) to adhere to the conditions described in the module manufacturer’s installation manual, and (vii) to allow for thermal expansion, using expansion joints where necessary in long sections, so that modules do not become unduly stressed.
In case, custom-designed structures are used to solve specific engineering challenges or to reduce costs, it is important to ensure doubly the structural integrity is there. This apart, plants are to be designed to ease installation. In general, installation efficiencies can be achieved by using commercially available products.
The topographic conditions of the site and information gathered during the geotechnical survey influence the choice of foundation type. This, in turn, affects the choice of support system design as some designs are more suited to a particular foundation type. Foundation options for ground-mounted PV systems include (i) concrete piers cast in-situ which are more suited for small plants and have high tolerance to uneven and sloping terrain, but they do not have large economies of scale, (ii) pre-cast concrete ballasts which are a normal choice for large plants because of the economies of scale and are suitable even at places where the ground is difficult to penetrate because of the rocky outcrops or subsurface obstacles (This option has low tolerance to uneven or sloping terrain, but needs no specialist skills for installation, however, consideration is to be given to the risk of soil movement or erosion, (iii) driven steel structure profile pile, if a geotechnical survey proves suitable (This option is low-cost, suitable for large-scale installations and can be quickly implemented, but needs specialized skills and pile driving machinery, is not always available), (iv) earth screws consisting of helical earth screws typically made of steel, have good economics for large-scale installations and are tolerant to uneven or sloping terrain, however. these need specialized skills and machinery for installation, (v) bolted steel baseplates which are used in situations where the solar plant is located over suitable existing concrete ground slabs. Fixed tilt mounting systems are simpler, cheaper and have lower maintenance requirements than tracking systems.
In locations with a high proportion of direct irradiation, single-axis or dual-axis tracking systems can be used to increase the average total annual irradiation. Tracking systems follow the sun as it moves across the sky. These are normally the only moving parts used in a solar PV power plant. Single-axis trackers alter either the orientation or tilt angle only, while dual-axis tracking systems alter both orientation and tilt angle. Dual-axis tracking systems are able to face the sun more precisely than single-axis systems.
Depending on the site and precise characteristics of the solar irradiation, trackers increase the annual energy yield by up to 27 % for single-axis and 45 % for dual-axis trackers. Tracking also produces a smoother power output plateau, as shown in Fig 8. Almost all tracking system plants use crystalline silicon modules. This is because their higher efficiency reduces additional capital and operating costs needed for the tracking system (per kWp installed). However, relatively inexpensive single-axis tracking systems are used with some thin-film modules.
Majority of the solar PV tracking systems fall into one of six basic design classes namely (i) classic dual-axis, (ii) dual-axis mounted on a frame, (iii) dual-axis on a rotating assembly, (iv) single-axis tracking on a tilted axis, (v) single axis tracking on a horizontal axis, and (vi) single-axis tracking on a vertical axis. In general, the simpler the construction, the lower the extra yield compared to a fixed system, and the lower the maintenance requirement.
Aspects to be taken into account when considering the use of tracking systems include (i) financial, (ii) additional capital costs for the procurement and installation of the tracking systems, (iii) additional land area needed to avoid shading compared to a free field fixed tilt system of the same nominal capacity, (iv) increased installation costs because of the need for large tracking systems which can need cranes for the installation. Higher maintenance cost for tracking systems because of the moving parts and actuation systems.
Operational aspects to be taken into account when considering the use of tracking systems include (i) tracking angles since depending on the angular limits, performance can be reduced, (ii) high wind capability and storm mode especially the dual-axis tracking systems in particular need to go into a storm mode when the wind speed is over 16 metres per second (m/sec) to 20 m/sec and this can reduce the energy yield and hence revenues at high wind speed sites, and (iii) direct / diffuse irradiation ratio since tracking systems give higher benefits in locations which have a higher direct irradiation component. The higher financial and operational costs of tracker installations, combined with the reduced costs of the silicon-based modules has reduced the interest being shown in tracking systems. Fig 8 shows benefit of dual axis tracking system.
Module technology development
PV module technology is developing rapidly. While a wide variety of different technical approaches are being explored, the effects of these approaches are focused on either improving module efficiency or reducing manufacturing costs. One of these improvements is the embedding of the front contacts in laser-cut microscopic grooves in order to reduce the surface area of the contacts and so increase the area of the cell which is exposed to solar radiation. Similarly, another approach involves running the front contacts along the back of the cell and then directly through the cell to the front surface at certain points.
Different types of solar cells inherently perform better at different parts of the solar spectrum. As such, one area of interest is the stacking of cells of different types. If the right combination of solar cells is stacked (and the modules are sufficiently transparent) then a stacked or ‘multi-junction’ cell can be produced which performs better across a wider range of the solar spectrum. This approach is taken to the extreme in III-V cells (named after the respective groups of elements in the Periodic Table) in which the optimum materials are used for each part of the solar spectrum. III-V cells are very expensive, but have achieved efficiencies in excess of 40 %.
Less expensive approaches based on the same basic concept include hybrid cells (consisting of stacked crystalline silicon and thin-film cells) and multi-junction amorphous silicon cells. Other emerging technologies, which are not yet market ready, but can be of commercial interest in the future, include spherical cells, sliver cells, and dye-sensitized or organic cells. Dye-sensitized solar cells have gained attention recently because of their low production costs and ease of fabrication. However, their low efficiency and their instability over time is still a significant disadvantage.