Photovoltaic Systems full report
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DR. B.A.T.U. LONERE
1.1 World Energy Situation
world energy consumption has still increased due to expected rapid increase of world population, especially in the third world and in new industrialized countries (NICs) because ever more humans also need ever more energy. Continually rapid growth is foreseen in the near future, with the world population rising from the present 6 billion to about 8 billion over the next 25 years, and is expected to grow perhaps to 10 billion people by the middle of 21st century. Such a population increase will have a dramatic impact on energy demand, at least doubling it by 2050, even if the developed countries adopt more effective energy conservation policies so that their energy consumption does not increase at all over that period. When fossil fuels are combusted, carbon dioxide (CO2) is also produced, which is one of tracing Asses distributing to greenhouse effect. Even more energy demand results more combustion of fossil fuels and consequently increase in the atmospheric CO2 concentration. Accordingly more of the outgoing terrestrial radiation from the surface is absorbed by the atmosphere and re-emitted partially back, which warms the lower atmosphere and surface. Since less heat escapes to space, this is the enhanced greenhouse effect. Although its influence to the global climate has not finally clarified yet, some effects are obviously seen. The global average temperature has increased by 0.6 0C since the late 19th century.
1.2 Outlook for Energy Situation
It is to be considered that about half of the world population lives today in countries, which do not have even sufficient energy reserves, but they must import and this dependence will rise even to 80 % for the year 2020 according to World Energy Council. The experience with the oil price crisis of 1973 shows that political explosive possibly establishes here. Since the largest oil- and natural gas reserves are concentrated in states with unstable political and economic conditions, so the danger of supplies and economic crises exists latently.
Due to several reasons the weights on the energy markets would shift in the near future. It is especially to be counted on the fact that the new industrialized countries with their more than 3 billion population will find means and ways to secure the energy quantities necessary for their economic processes. That appears already today in the enormous demand of the Asia-Pacific Countries for oil and natural gas. They step on the world energy markets with increasing competition to the industrialized countries. Against this background and in view of rising energy requirement of steady increase in world population, there is a call for action of energy saving. However if industrialized nations can reduce their today energy demand (ca. 80 % of total consumption), the energy saving will consequently let additional energy demands, which will exceed the saving potential, arise by the gradual fulfillment the wish of the new industrialized countries as well as of the third world. The conclusion is: technically, economically feasible and sustained effective as well as ecological compatible and safe option for future energy supply has to be taken into account. Sometime in the mid-21st century the world will need a new, safe, clean and economical source of energy to satisfy the needs of both developing and developed nations. Renewable energies are nearly unlimited energy sources, if one compares the energy, which we receive from the Sun, with the energy demand of humankind. Moreover they are available prevailing inland or local and therefore secure. The problem is that without financial support renewable energies cannot normally compete with fossil energies. However this does not mean that it is not important to promote renewable energies according to market economic criterions in order to get even more profit from reduction in costs with mass production and from experiences with their increasing application.
2.2 Charge Transport in the Doped Silicon
Now we consider the doping of silicon, a tetravalent element, which is the most frequent applied semiconductor material, also for solar cells. Replacement of a silicon atom by a pentavalent atom (Fig. 2.2a), e.g. phosphorus (P) or arsenic (As), leads to a surplus electron only loosely bound by the Coulomb force, which can be ionized by an energy (ca. 0.002 eV). The quantity eV is an energy unit corresponding to the energy gained by an electron when its potential is increased by one volt. Since pentavalent elements donate easily an electron, one calls them donors. The donor atom is positively charged with the electron donation (ionized). The current transport in such a material practically occurs only by means of electrons, it is called n-type material. Replacement by a trivalent element (Fig. 3-2b), e.g. boron (B), aluminum (Al) or gallium (Ga), leads to a lack of an electron. Now an electron in the neighborhood of a hole can fill up this blank and leaves a new hole at its original position consequently. This results in the current conduction by means of positive holes. Therefore this material is called p-type material. Trivalent atoms, which easily accept an electron, are defined as acceptors. The acceptor atoms are negatively ionized by the electron reception. At ambient temperature donors and acceptors are already almost completely ionized in the silicon.
FUNDAMENTALS OF PHOTOVOLTAIC
The direct transformation from the solar radiation energy into electrical energy is possible with the photovoltaic effect by using solar cells. The term photovoltaic is often abbreviated to PV. The radiation energy is transferred by means of the photo effect directly to the electrons in their crystals. With the photovoltaic effect an electrical voltage develops in consequence of the absorption of the ionizing radiation. Solar cells must be differentiated from photocells whose conductivity changes with irradiation of sunlight. Photocells serve e.g. as exposure cells in cameras since their electrical conductivity can drastically vary with small intensity changes. They produce however no own electrical voltage and need therefore a battery for operation. Alexander Edmond Becquerel discovered the photovoltaic effect in 1839 while experimenting with an electrolytic cell made up of two metal electrodes. Becquerel found that certain materials would produce small amounts of electric current when exposed to light. About 50 years later Charles Fritts constructed the first true solar cells using junctions formed by coating the semiconductor selenium with an ultrathin, nearly transparent layer of gold. Fritts’s devices were very inefficient: efficiency less than 1 %.
The first silicon solar cell with an efficiency of approx. 6% was developed in 1954 by three American researchers, namely Daryl Chapin, Calvin Fuller and G.L. Pearson in the Bell Laboratories. Solar cells proved particularly suitably for the energy production for satellites in space and still represent today the exclusive energy source of all space probes. The interest in terrestrial applications has increased since the oil crisis in 1973. Main objective of research and development is thereby a drastic lowering of the manufacturing costs and lately also a substantial increase of the efficiency. The base material of almost all solar cells for applications in space and on earth is silicon. The most common structure of a silicon solar cell is schematically represented in Figure 2.1: An approx. 300 m silicon wafer consists of two layers with different electrical properties prepared by doping foreign atoms such as boron and phosphorous. The back surface side is total metallized for charge carrier collection whereas on the front, which exposes to the beam of incident light, only one metal grid is applied in order that as much light as possible can penetrate into the cell. The surface is normally provided with an antireflection coating to keep the losses from reflection as small as possible.
2.3 Effects of a P-N Junction
Usually a p-n junction is generated by the fact that a strong n-type layer is produced in the p- type material by in diffusion of a donor (P, As) at higher temperatures (ca. 850 °C). Completely analog in the n-type material, although less common, a p-n junction can be produced by in diffusion of an acceptor. In the boundary surface’s neighborhood of the n- or p-type material the following effects occurs In the n-region so many electrons are available, in the p-region so many holes. These concentration differences lead to the fact that electrons from the n-region diffuse into the p- region and holes from the p-region diffuse into the n-region. As a result, diffusion currents of electrons into the p-region and diffusion currents of holes into the n-region arise. By the flow of negative and positive charges a deficit of charges develops within the before electrically neutral regions, i.e. it results a positive charge within the donor region and a negative charge within the acceptor region. Thus an electrical field develops over the boundary surface and causes now field currents from both charge carrier types, which are against the diffusion currents. In the equilibrium the total value of current through the boundary surface is zero. The field currents compensate completely the diffusion currents: the hole currents compensate completely among themselves and the electron currents likewise. This electrostatic field extending over the boundary surface refers to the potential difference VD, which is called diffusion voltage. It is situated in the order of magnitude of 0.8 eV. This electrical field causes the separation of the charge carriers produced by light in the solar cell. Within the region of the stationary electrical positive and negative charge, in the so-called space-charge zone, a lack of mobile charge carriers appears, which has very high impedance. Applying the n-region with a negative voltage (forward bias) reduces the diffusion voltage, decreases the electrical field strength and thus the field currents. These do not compensate now the diffusion currents of the electrons and holes, as without external voltage, anymore. As a result a net diffusion current from electrons and holes flows through the p-n junction. If the applied voltage is equal to the diffusion voltage, then the field currents disappear and only the bulk resistors limit the current. Contrarily, an applied positive voltage at the outside n-region (reverse bias) adds itself to the diffusion voltage, increases the space-charge zone, thus it comes to outweighing the field current. The resulting current whose direction of the reverse bias is contrary is very small.
2.4 Physical processes in solar cell
2.4.1 Optical absorption
Light, which falls on a solar cell, can be reflected, absorbed or transmitted. Since silicon has a high refractive index (> 3.5), over 30 % of the incident light are reflected. Therefore solar cells are always provided with an antireflection coating. A thin layer titanium dioxide is usual. Thus the reflection losses for the solar spectrum can be reduced to about 10 %. Multi-layer AR layers can achieve more reduction of the reflection losses. A two-part layer from titanium dioxide and magnesium fluoride reduces the reflection losses of a remainder up to ca. 3 %. Photons (light quanta) interact with materials mainly by excitation of electrons. The main process in the field of energy, in which solar cells are applied, is the photoelectric absorption. Thereby the photon is completely absorbed by a bound electron. The electron takes the entire energy of the photon and becomes free-electron. However, in semiconductors a photon can be only absorbed if its energy is larger than the band gap. Photons with energies smaller than the band gap pass through the semiconductor and cannot contribute to an energy conversion. However, photons with much larger energies than the band gap are also lost for the energy conversion since the surplus energy is fast given away as heat to the crystal lattice. During the interaction of the normal solar spectrum with a silicon solar cell, about 60 % of the energy for a transformation are lost because many of the photons possess energies, which are smaller or larger than the band gap.
2.4.2. Recombination of charge carriers
The absorption of light produces pairs of electrons. The concentration of charge carriers is therefore higher during the lighting than in the dark. If the light is switched off, the charge carriers return to their equilibrium concentration in the dark. The return process is called recombination and is the reverse process for generation by light absorption. Recombination occurs even naturally also already during the generation. The charge-carrier concentration appearing with lighting is the result from two opposite running processes.During their lifetimes the charge carriers can travel a certain distance in the crystal until they recombine. The average distance, which a charge carrier can travel between the place of its origin and the place of its recombination, is called diffusion length. This quantity plays an important role for the behavior of a solar cell. It depends on diffusion coefficient of a material and a lifetime of a charge carrier (time that it takes for a charge carrier to be captured according to recombination).
2.4.3 Solar cells under incident light
Figure given below shows the three main parts of a solar cell schematically: the diffused strong n- doped emitter, the space-charge zone and the p-doped base. A photon with sufficient large energy falls on the surface of the solar cell, penetrates emitters and space-charge zone and is absorbed in the p-base. An electron-hole pair is developed due to the absorption.
Since electrons are in the minority in the p-base, one calls them minority charge carrier contrary to the holes, which are majority charge carrier here. This electron diffuses in the p- base until it arrives at the boundary of the space-charge zone. The existing strong electrical field in the space-charge zone accelerates the electron and brings it to the emitter side. Thus a separation of the charge carriers took place. Thereby the electrical field works as separation
medium. A prerequisite is that the diffusion length of the electron has to be large enough so that the electron can arrive up to the space-charge zone. In case of too small diffusion length a recombination would occur before reaching the space-charge zone, the energy would be lost. Absorption of a light quantum in the emitter leads again to the formation of an electron-hole pair. According to the strongly doped n-emitter the holes are here the minority charge carrier. With sufficient large diffusion length the hole reaches the edge of the space-charge zone, is accelerated by the electric field and is brought to the p-base side. If the absorption occurs in the space-charge zone, electrons and holes are immediately separated according to the existing electrical field there. In consequence of the incident light it yields: If concentration of electrons at the n-emitter side is increased, concentration of holes at the p-base side increases. An electrical voltage is built up. If n-emitter and p-base are galvanic ally connected, e.g. by an ohmic resistor, electrons from the emitter flows through the galvanic connection to the base and recombines with the holes there. Current flow means however power output. This current flow continues so long as the incident light radiation is available. As a result, light radiation is immediately converted into electricity.
PV POWER SYSTEMS
3.1 Types of PV Power Systems
The three typical configurations of PV power systems are autonomous, hybrid and grid-connected. Autonomous and hybrid power systems are used in stand-alone applications. They are not connected to the main utility grid and are often used in remote areas.
Autonomous systems rely exclusively on solar energy to meet a need for electricity. As mentioned in the preceding, they may incorporate batteries – which store energy from the PV modules during the day – for use at night or in periods of low solar radiation. Alternatively, they may power the application entirely, with no need for batteries (e.g. water pumping). In general, autonomous PV systems are the most cost-effective source of electrical power. You may, However, decide to choose a hybrid PV system because of the environment in which it will operate or because you Need a system that operates independently and reliably.
Hybrid systems, also used in stand-alone systems, consist of PV modules and a wind and/or fuel- fired generator. A hybrid system is a good option for larger systems that need a steady power supply, when there is not enough sun at certain times of the year, or if you want to lower your capital investment in PV modules and storage batteries.
Grid-connected PV power systems are part of the movement toward a decentralized electrical network. Power is generated closer to where it is needed not solely by central power stations and major hydro stations. Over time, such systems will reduce the need to increase the capacity of transportation and distribution lines. A “grid-connected” system generates its own electricity and feeds its excess power into the utility grid for later use. This does away with buying and maintaining a battery bank. You can still use battery banks to provide backup power when the grid goes down, but they are not required. Smaller systems have a box a small grid synchronous inverter – mounted on the back of each panel. Larger systems have one large inverter, which can handle many panels (as in a stand-alone system). Both types convert DC power output into AC power. Then they synchronize this output with the grid to slow down the electrical meter. They can even turn the meter backward. If the PV output is less than the load consumption, the meter slows down. If PV output exceeds the load consumption, the meter turns backward, and a credit is accumulated. This credit can be drawn out of the utility when the sun is not shining. In essence, the grid acts like a limitless battery bank. In most parts of Canada, permission from the local utility is required in order to back feed power into the grid A large portion of the cost of a grid-connected PV system is manufacturing the PV modules themselves. Significant decreases in manufacturing costs have occurred in recent years, with further decreases expected in the future. This kind of PV system is thus becoming more affordable. In some urban areas in warm climates, the cost per kilowatt-hour of electricity from grid-connected PV systems is competitive with that of other electricity-generating systems. In areas with less solar radiation, the cost-effectiveness of this type of PV system is still marginal. But there is a potential for peak power savings in areas where air conditioning causes a power peak in the summer. There are also system savings where the PV modules can replace the traditional roofing materials for buildings or the cladding material that is normally used in building façades. These material savings are making the costs per kilowatt- hour from grid-connected PV systems increasingly competitive. Decentralized small home systems also hold some potential for grid-connected PV systems, but the costs will have to be reduced further in order to compete with the low electricity rates now avail- able in most parts of Canada. Note, however, that PV electricity is “green” energy and, as such, is worth a premium. Even though this value is subjective, the PV system’s designer should express it in numbers. For example, how much is the avoided pollution of conventional sources worth and how much is the avoided distribution cost worth? To install a PV system, you must pay the capital cost of the system and amortize this cost over time. In contrast, where there is a utility grid, you pay for the electricity used and not a lump sum for the generating facility. The costs of the PV system may appear to be a burden because the electricity that the system generates may cost more per kilowatt-hour than what a utility charges. But using a PV system may also be considered a lifestyle choice, similar to choosing between a fuel-efficient car or a gas-guzzling sport utility vehicle.
3.2 Components of Photovoltaic system
3.2.1 Photo voltaic cells
Photovoltaic (PV) - or solar cells as they are often referred to, are semiconductor devices that convert sunlight into direct-current (DC) electricity. A typical silicon PV cell is a thin wafer consisting of an ultra-thin layer of phosphorus-doped (N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon. An electrical field is created near the top surface of the cell where these two materials are in contact, called the P-N junction. When sunlight strikes the surface of a PV cell, this electrical field provides momentum and direction to light stimulated electrons, resulting in a flow of current when the cell is connected to an electrical load. Regardless of size, a typical silicon PV cell produces about 0.5 volt under open-circuit, no-load conditions. The current output of a PV cell depends on its efficiency and size (surface area), and is proportional the intensity of sunlight striking the surface of the cell. For example, under peak sunlight conditions a typical commercial PV cell with a surface area of 160 cm2 (~25 in2) will produce about 2 watts peak power. If the sunlight intensity were 40 percent of peak, this cell would produce about 0.8 watts. . Photovoltaic cells are connected electrically in series and/or parallel circuits to produce higher voltages and/or currents. Photovoltaic modules consist of PV cell circuits sealed in an environmentally protective laminate, and are the fundamental building block of the complete photovoltaic generating unit. Photovoltaic panels include more than one PV module assembled as a pre-wired, field-installable unit. A photovoltaic array is the complete power-generating unit, consisting of any number of PV modules and panels.
Since PV generators as well as batteries deliver basically direct current or direct-current voltage (DC). As many small consumers are suitable for operating directly with DC voltage, the most commercial devices need however an alternating voltage (AC). Therefore, power-conditioning elements, which are commonly called “inverters” because they invert the polarity of the source in the rhythm of the AC frequency, are often applied to PV systems. Also in grid-connected systems inverters are basically necessary for the conversion of DC power into grid-compatible AC power.
General characteristics of PV inverters
Since production cost of PV electricity is several times more expensive than conventional electric energy, conversion efficiency becomes predominant for the economics of the total PV System. In consequence, extremely high efficiency not only in the nominal power range but also Under partial load condition is a requirement for PV inverters in grid-connected as well as in stand-alone systems.
Figure represents a general structure of a grid-connected PV system consisting mainly of the following components: the PV generator, the inverter, the safety devices and in many cases the electric meter. The actual power fed into the grid can be estimated by multiplying the actual power of the PV generator with the actual efficiency of the inverter, regardless of losses in the safety device and in the meter. More important is the energy produced by the system after a certain period of time e.g. after one year of operation. In this case the mean efficiency of the inverter taken into account for all load conditions throughout the year becomes important. As a first step, the inverter must allow the PV generator to operate continuously at the maximum power point (MPP) according to the maximum power point tracking (MPPT) as described in section 4.2.2. However, simulation has shown that for grid-connected PV systems constant voltage operation leads only to losses between 1 and 2 % when properly adjusted.
For optimum use of the PV energy MPPT and constant voltage operation can be seen as equivalent. As a matter of their operation principle, single phase inverters, which are most common for small scale PV systems (P ≤ 5 kWP), lead to deviations from the MPP due to DC ripple as will be explained as follows: When injecting AC power into the grid, the feed in current should be in phase with the grids voltage which means the power factor equals one.
A simple version of such an inverter is shown in Figure. AC at the primary windings of the transformer is being produced by alternatively closing the S1 and S2. If S1 is closed, S2 is open and vice versa. The resulting AC output voltage is of square-wave type, which may be used for resistive-type loads such as incandescent light bulbs etc. Anyway, the two primary windings of the transformer can be reduced to one if two more switches are used. In this configuration the switches are opened and closed pair wise in such a way that S1, S2 and S3, S4 respectively open and close synchronously. At the output of the H-type Bridge formed by the switches S1 through S4 there is already AC available. The transformer is only necessary in case of a voltage transformation.
Since many consumers and the public grid operate on the basis of sine-wave type voltage, high quality inverters should also be able to provide this type of AC output. This voltage form can be obtained in different ways. Some of the most common layouts will be here presented.
Inverters with step-down converters
The basic idea of this concept is to produce a sine-shaped unipolar voltage at the DC side Normally by means of a step-down converter as shown in Figure.
Since in this configuration the actual voltage and the corresponding current at the output of the Step-down converter is no longer constant, the resulting current at the input of the converter Will also fluctuate. In case of using PV generator used as a DC source, a storage capacitor C1 Becomes necessary. The quality of the voltage shape is normally described as total harmonic distortion (THD). The THD is defined as sum of the amplitudes of all harmonic frequencies compared to the amplitude of the fundamental signal (the 50 and 60 Hz frequency respectively). In this case THD is determined by the switching frequency of S0 and by the inductance of L1. With modern semiconductor switches this frequency can be realized in the range of 100 kHz, which keeps L1 small enough. The voltage transformation between the DC sources V2 at the output of the step-down converter can be given as:
V2 = V1* ton/ T
Where ton is the on time for switch so while T corresponds with the time of one switching period. This principle, in which the desired output voltage is being produced by means of variable on time of the switch, is called pulse-width modulation (PWM). The switches S1 and S2 in Figure are needed to invert the polarity of every second half-wave in order to form the A.C. Output.
3.2.3 Lead-acid batteries
The lead-acid battery is the most widely used secondary battery and is still the technology of Choice in most PV systems. It involves the reversible electrochemistry between lead and lead oxide in sulphuric acid. Lead-acid battery technology is more than 100 years old, and in this context, it is “proven” technology. However, the energy storage capability of the lead acid battery varies with battery design and use, and in all cases, the practical discharge capacity is at best only about 60%-70% of the theoretical capacity. None of the theoretical capacity is actually wasted; it is just unavailable due to a combination of polarization (voltage drop) factors, which affect the degree of utilization of active material in the plates. The service-life performance of the lead-acid battery is primarily affected by the intended application. The lead acid battery suffers from a number of shortcomings as an energy storage device (undergoing charge and discharge events), which originates from the basic nature of the lead-acid chemistry.
In short, they are:
1. Lead naturally corrodes in sulphuric acid
3. Deep discharging
Lead-acid batteries in PV systems
The lead-acid battery is a relatively simple secondary cell, and in principal, any lead-acid battery can be used in any power supply situation. In practice, of course, it becomes a matter of performance and system optimization, and issues of cost, service-life and serviceability requirements dominate appropriate “fit for purpose” considerations. Lead-acid batteries for PV systems therefore need to be a hybrid and should be matched as far as possible to the energy supply and demand profile of the application. Historically, flooded (wet) lead-acid batteries have been used in PV systems. Service-life experience varies from 1-2 years in deep-cycling homestead RAPS environments through to 10-12 years in shallow cycling regimes powering telecommunications equipment or railway signaling systems. Homestead RAPS typically exposes the battery to a wide range of capacity demands and considerable deep discharging. Service life similar to the deep cycling profiles of traction batteries is to be expected, but battery performance has been shown to vary widely and depend on battery design and construction technology. On the other hand, in the shallow cycling regimes used in Telstra’s solar-powered communications network, the experience more closely resembles that achieved in traditional standby applications. In recent years, there has been development of solar “specific” lead-acid batteries claimed to give better cycling performance in typical, variable load RAPS applications. Generally, these types of batteries are modifications of traditional traction-type cells designs, but enhanced with a lower level of antimony (1 - 3%) to retain good grid cycling behavior, plate wrapping techniques to avoid the consequences of plate shedding, and additional electrolyte volume to reduce the frequency of water top-up. Rarely do these particular types of solar batteries achieve greater than 5-7 years life, which is an improvement for many RAPS applications, but still far short what can be achieved with other types of cells in shallow cycling regimes. The range reflects the basic limitations of the lead-acid battery and highlights the fact that to date, there is no “universal” lead-acid battery that is optimized for all PV applications.
Requirements for the solar batteries
Since battery maintenance can be a major limitation for PV stand-alone systems. Typical requirements for the battery to be used for long-term storages are:
• Low specific kWh-cost, i.e. the stored kWh during the whole life of the battery
• Long lifetime
• High overall efficiency
• Very low self-discharge
• Low maintenance cost
3.2.4 Charge Regulators
Since batteries represent a substantial cost factor for example in case of a typical island house of about 15 – 20 % of initial investments, which can rise over 50 %, if one considers the necessity for a repeated replacement of the battery over the lifetime of the total system. Therefore it is aimed to achieve, by suitable charging and supervision strategies, as long life of the battery as possible under given operating conditions. Experiences from a great deal of systems show however that with the presently used technique the obtained life lasting with 2 – 4 years is clearly shorter than the expected values of 5 – 8 years. The determination of the Responsible causes and the derived, from them, development of new concepts and system components are thus important assignments in the future. In the following, basic principles of common charge regulators are described. The theme is limited thereby to charge regulators for the lead acid batteries used in larger systems.
Basic principles of charge regulators
Basic function of a charge regulator is operating the battery within the operation limits given by the manufacturer regarding overcharging or deep discharge. Moreover, a charge regulator can execute automatically maintenance like regular equalizing charge or gassing charge as well as Inform the user about the status of the system with appropriate displays. Simple charge regulators have only one voltage threshold for the maximum charge voltage: this Value should be adjusted to 2.3 V/cell with the temperature 200 C. The maximum charge Voltage must be calibrated regarding the temperature with a correction value of -4…-6 mV/0C, if this deviates by more than 5 C from the reference value. More expensive charge regulators have several voltage thresholds and permit thus for example a controlled gassing charge. The strategies were rather empirically established and consist i.e. of a gassing charge within a certain 4-week interval as well as after each deep discharge. The Total gassing time is limited here on 10 hours per month. During gassing phase the battery Voltage is limited at 2.5 V/cell, subsequently at 2.35 V/cell.
Against overcharging of batteries a protection is always planned with the solar charge regulators: the regulating unit is either totally closed or completely opened. Ideally the developing dissipated heat is zero in both cases since either current or voltage at the regulating unit is zero.
Principle of series regulator
In case of the series regulator (Fig. 10), a regulating unit influences the current flow, namely switch S1, which is positioned in series to the PV generator. While relays were quite used as switches in the past, they are today almost exclusively replaced by semiconductor switches such as MOSFET’s or IGBT’s. It is to the series charge regulator’s advantage that besides PV generators also other, not short-circuit proof, energy converters such as wind generators can be connected. As disadvantage higher power losses were claimed to the series charge regulator – however, this historical statement is not valid any longer after the power semiconductors specified above are available. Regarding the parallel- or shunt regulator the characteristic of PV generators is applied, namely being able to be short-circuited arbitrarily for a long time. During charging the PV generator current flows through the diode D into the battery. As achieving the maximum charge voltage the PV generator is short-circuited
by the regulating unit according to one of the further strategies described below, so that no more charging current can flow. The diode prevents here, on one hand, the current reversal from the battery
into this short-circuited path; on the other hand it prevents discharging of the battery to the unlighted PV generator at night. It is favorable with the shunt regulator since it makes a charging current also in the case of a completely discharged battery flow through the diode and thus the system starts reliably. This is however not guaranteed in the case of the series regulator with a bad connection design since there is no energy available to turn on the switch.
3.3 Principles Of PV System configuration
The modular structure of PV generators provides possibility that energy supply systems can be constructed in an extremely wide power range. The power spectrum extends from a few mW up to powers in the MW range. A PV system consists basically of a PV generator and system components, which are responsible for energy treatment, as well as a consumer. In the following, the fundamental structures of PV systems will be described at first, with which also deal briefly with the function of system components. By means of block diagrams the systems will be introduced for different applications of PV energy supplies. Examples of realized systems should therefore ease the classification for the different power ranges.
3.3.2 Fundamental Structures of PV Systems
Different aspects could classify the PV systems. Regarding battery storage the PV systems can be widely divided into 2 categories, namely PV systems with- and without battery storage.
3.3.3 PV systems without battery storage
The grid whereas energy will be draw from the grid to supply the consumers when the PV e In case that the energy supply and energy demand occur simultaneously, it is unnecessary to store the energy produced by the PV generators. In addition, grid-connected PV system is classified in this type because the surplus energy produced by the PV generators can be fed into energy is not sufficient so that the battery storage unit is also unnecessary in this case.
3.3.4 Direct-coupled PV system
For this configuration PV generators are directly connected with consumers. Main application is ventilator. The system is simple and reliable, reduces maintenance and requires lower investment cost whereas the demand equals to the potential. An example of this configuration is PV ventilator.
3.3.5 PV system with a matching converter
In order to match the voltage of the PV system with the voltage of the consumer, a DC/DC Converter is necessary, which transforms one DC voltage to another.
3.3.8 DC-coupled PV hybrid system
In case of high-energy demand, the PV generator alone could not provide sufficient energy. In other words, the PV generator required would become too large and too expensive. The same problem occurs if a high reliability is demanded. For those reasons different generators are coupled, resulting in a so-called hybrid system. One possibility of such a hybrid system is the coupling of a PV- and a motor generator. In regions with good wind conditions, even a wind generator could be considered.
By means of backup generator the PV generator and the battery do not have to be oversized, resulting in significantly reduced investment costs. Basically, backup generator is sized to supply expected peak loads, which maximizes the supply reliability. When the electric energy from PV generator and battery is not sufficient to supply the consumers or when the battery is discharged, the backup generator is switched on. According to conventional generators AC types are most commonly used. Therefore, rectifier is needed in this configuration. For the autonomous supply of remote houses or grid-dependent consumers the combination of the PV generator with other energy generators can be not only reasonable, but also necessary in order to overcome the solar shortage in winter.
3.3.9 PV hybrid system with both DC- and AC consumers
The following configuration is similar to that in Figure 16 only with the difference that an inverter is now included into the system as a central inverter so that AC appliances can also be operated. With favorable solar radiation the PV generator covers the consumer’s total energy demand. Surplus energy is stored in batteries. During the night or unfavorable weather the energy the batteries cover demand at first.
ADVANTAGES & DISADVANTAGES OF PV SYSTEM
4.1 The Advantages Of PV Power Systems
• Economic Prosperity
Replacing outdated fossil fuel power plants with Solar Roofs will generate thousands of new high-paying jobs and eliminate much of the US trade deficit.
• Energy Independence & Security
Solar Energy is an unlimited renewable alternative to destabilizing dependence on foreign oil and natural gas.
• Clean, Durable and Environmentally Safe
Solar Power generation produces no pollution or greenhouse gases and requires little or no maintenance.
• Distributed Generation
Solar Roofs will automatically satisfy on site peak power needs and increase the reliability of the electrical grid.
• Efficiency and Sustainability
The Sun is expected to burn for another 5.5 Billion years, it took nature 3.5 Billion years to make the fossil fuels that we are burning in a few hundred years.
• Unlimited capacity
17 million MW of potential solar capacity exists in California (from CEC study)
BIPV solutions are aesthetically pleasing. Open Pit Mines, Hugh Power Plants, Transmission Lines and Foul Air are not.
4.2 The Limitations Of PV Power Systems
It is important to realize that PV power systems are capital intensive from the buyer’s perspective and is expensive when compared with the low price of utility power in Canada. You should therefore reserve the electric power produced by PV modules, an inverter and a storage system for your most energy-efficient appliances, tools, lights, etc. Although it is technically possible, heating with photovoltaic is generally not recommended. You can easily and more efficiently collect heat with a solar thermal system. A solar water heater generates more hot water with less initial cost than any PV-powered heater.1 also, for cooking; it is generally more cost-effective and convenient to use a stove that operates on propane or natural gas rather than solar electricity. Autonomous PV-powered homes and cottages often rely on wood cook stoves for cooking and space heating. Refrigerators are becoming more energy efficient, so the cost of operating them with PV power is now feasible. Extremely energy-efficient refrigerators and freezers are, unfortunately, still expensive, however, they can be had through PV dealers. From an economic point of view, first consider investing in energy efficient electric appliances, and then size your PV system based on actual consumption. For example, using compact fluorescent lights will reduce your electrical consumption for lighting by 80 percent.
A VISION FOR PHOTOVOLTAICS
The main driving forces that require a dramatic change in our energy consumption patterns include the depletion of oil and gas resources, climate change considerations, the need to ensure security of supply, the lack of access to commercial energy of one-third of the world’s population and the expected economic growth of emerging countries. The developed world represents only 20% of the total population but it consumes 80% of the world’s resources whilst at the same time producing a large proportion of environmental waste and air pollution. On the other hand, the developing world is struggling with the difficulties of economic development, and the fight against poverty. The transition to a sustainable global energy system is one of the biggest challenges mankind has ever faced. This transition will take 30 to 50 years or more, even though the necessity for change is urgent, due to the negative ecological consequences the world is experiencing right now. This process involves huge financial investment and a strong and continued political commitment. In the context of this transition, PV is a key technology. The current relatively early stage of development indicates a large potential for steady and high rate of growth up to and beyond 2030. It is envisaged that by 2030, PV will be established as a viable electricity supplier, and that the market will continue to grow thereafter at full speed. Forecasts should therefore only be seen as intermediate ones, and are by nature subject to significant uncertainties.
5.1 Technological development
Impressive progress in PV technology has been made over the past decades. This is evident by the price reduction (roughly a factor of 5 over the past 20 years), by the efficiency increase of commercial and laboratory technologies (typically by 50% over the same period), by the ample technology options portfolio and by the strongly improved system reliability and yield. The period until 2030 will show rapid further maturing of Commercial technologies, leading to flat plate module efficiencies in the 10-25% range (35% for concentrators) and generation costs down to 0.05-0.12 €/kWh. Beyond 2030 a further reduction of generation cost is expected. All technologies, crystalline silicon, thin film and new concepts may be significantly present on the market. In 2030, PV systems will have a standard technical lifetime of up to 40 years.
Yearly operation and maintenance will be 0.5-1% of the investment costs. PV modules and systems will be exclusively based on abundant and nontoxic materials, or fully closed cycles and the energy payback time of systems will be less than one year. After 2030, module efficiencies will increase further as a result of successful implementation of the new Concepts. Ultimately, PV module will have an energy conversion performance in the 30-50% range, allowing very efficient use of available area. One square meter of the highest efficiency PV modules installed in sunny regions will then yield 1 000 kWh of electricity per year. By 2030, PV system elements will have developed into versatile building components, facilitating standardized and specific uses on a large scale. Almost all new Buildings will be fitted with PV arrays, and many will be net producers of electricity. Very y large-scale implementation of PV will require combination with back up from other renewable energy sources and the development of advanced balancing and storage technologies. In the even longer term, other options will exist, like large desert-installed PV plants supplying power to distant consumers via a worldwide electricity grid. Also, hydrogen production from PV electricity (combined with subsequent electricity generation by fuel cells) may become an option if conversion yields can be improved. The development of new lighting technologies like LEDs, flat panel displays, etc, which can be supplied with direct current at low voltage, may allow converters to be eliminated and further reduce the installation costs of PV systems.
5.2 Socio-economic aspects
By 2030, PV will have developed into a large economic sector, both worldwide and in Europe. There will be a strong European PV industry with significant exports. The number of jobs created in the EU will be between 200 000 and 400 000 (based on a European yearly production of 20-40 GW), many of them linked to the installation and building sectors. These jobs will therefore be spread geographically and between SME’s and large companies. Depending on the application, a wide range of commercial solar cell technologies will be available each with its own features. There will be a range of products with different efficiencies for use in particular application areas. PV will be available as multi-purpose solar modules (flat-plate or concentration), a variety of building products and as integrated products (OEM-components with solar power24). A wide range of appliances making direct use of PV power (e.g. light emitting diodes) will also be common on the market.
Photovoltaic constitutes a new form of producing electric energy that is environmentally lean and very modular. In stand-alone installations, it must use storage or another type of generator to provide electricity when the sun is not shining. In grid-connected installations storage is not necessary: in the absence of sunlight, electricity is provided from other (conventional) sources. It is unique for many applications of high social value such as providing electricity to people who lack it in remote areas. Its use is increasing rapidly to produce electricity in grid-connected houses and buildings in industrialized countries, despite a 5 to 10 times higher cost than conventional electricity. Largely, because of grid-connected PV applications such as homes and businesses, the expansion of the PV market has been very rapid in the last years of the twentieth century and it is expected to continue during the next few years of the twenty-first century. The general cost reduction will make photovoltaic available to more and more people in developing countries helping their development with little degradation of air quality associated with fossil-fuel generators.
In summary, it is very likely that photovoltaic will become in the next half century an important source of world electricity. Public support and global environmental concerns will keep photovoltaic viable, visible, and vigorous both in new technical developments and user applications. Nations which encourage photovoltaic will be leaders in this shining new technology, leading the way to a cleaner, more equitable twenty-first century, while those that ignore or suppress photovoltaic will be left behind in the green, economic energy revolution.