SOLAR PHOTOVOLTAIC ENERGY
Introduction
Photovoltaics (PV) is a technology that converts sunlight directly into electricity. It was first observed in 1839 by the French scientist Becquerel who detected that when light was directed onto one side of a simple battery cell, the current generated could be increased. In the late 1950s, the space programme provided the impetus for the development of crystalline silicon solar cells; the first commercial production of PV modules for terrestrial applications began in 1953 with the introduction of automated PV production plants. Today, PV systems have huge value use in areas remote from an electricity grid where they can provide power for water pumping, lighting, vaccine refrigeration, electrified livestock fencing, telecommunications and many other applications. With the global demand to reduce carbon dioxide emissions, PV technology is also gaining popularity as a mainstream form of electricity generation. Several million solar PV systems are currently in use worldwide, with an installed capacity of 40GW globally by the end of 2010 (Renewables 2011Global Status Report), yet this is a tiny proportion of the vast potential that exists for PV as an energy source.
Photovoltaic modules provide an independent, reliable electrical power source at the point of use, making PV particularly suited to remote locations. However, solar PV is increasingly being used in homes and offices for electricity to replace or supplement grid power, often in the form of solar PV roof tiles. The daylight needed is free, but the cost of equipment can take many years to achieve payback. However, in remote areas where grid connection is expensive, PV can be the most cost effective power source.
Figure 1: A photovoltaic panel being set up in rural Peru for domestic solar lighting. Photo credit: Practical Action / Marco Antonio Arango.
The use of PV electricity in deve loping countries Most of the world’s developing countries are within the tropics and hence have ample solar insolation (the total energy per unit area received from the sun). The tropical regions also benefit from having only a small seasonal variation of solar insolation, even during the rainy season, which means that, unlike northern industrial countries, solar energy can be harnessed economically throughout the year.
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Solar photovoltaic energy
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China, India and other developing countries are emerging as major solar PV manufacturer s. The dominant application for PV in developing countries is the solar home system (SHS). This involves the installation of PV systems of 30 to 50 peak watts (Wp), costing about $300 to $500 (U.S.) each, in individual homes, mainly in rural areas. Apart from SHS, other applications of PV in developing countries include 1) PV-powered remote telecommunications equipment; 2) rural health clinic refrigerators; 3) rural water pumping; solar lanterns and 4) PV battery-charging programmes, which allow rural residents to purchase or rent batteries to provide electricity to their homes, and then recharge them at PV-powered charging stations. A few attempts have been made to establish PV-powered village power grids in developing countries, such as in Sagar ‘Solar Island’ off the cost of India (see later). Cost of solar PV The process of producing efficient solar cells is costly due to the use of expensive pure silicon and the energy consumed, and cost has been the major barrier to the widespread uptake of PV technology. As materials technology improves, costs are slowly dropping, making PV technology more attractive. Since 1976, costs have dropped about 20% for each doubling of installed PV capacity, or about 5%/year. Module prices have fallen from $30/Wp in 1975 to less than $1/Wp in 2012. Costs rose slightly in 2004 due to high demand (which outpaced supply) and the rising cost of silicon. The expectation is that the cost of PV will continue to come down as mass production increases and technologies evolve. Note: Cost of PV modules is usually given in terms of Peak Watt (Wp), which is the power rating of the panel at peak conditions - that is at 1kWm-2 irradiance at 25ºC.
Technical issues
The nature and availability of solar radiation is described in the technical brief Solar Thermal Energy. Once the solar energy has arrived reaches the surface of a photovoltaic cell, the electrons become energised in proportion to the intensity and spectral distribution (wavelength distribution) of the light. When their energy level exceeds a certain point a potential difference is established across the cell. This is then capable of driving a current through an external load, such as a light or radio. PV modules and arrays When light falls on the active surface, the electrons in a solar cell become energised, in proportion to the intensity and spectral distribution (wavelength distribution) of the light. When their energy level exceeds a certain point a potential difference is established across the cell. This is then capable of driving a current through an external load. All modern, commercial PV devices use silicon as the base material, mainly as mono- crystalline or multi-crystalline cells, but more recently also in amorphous form. Other materials such as copper indium diselenide and cadmium telluride are being developed with the aim of reducing costs and improving efficiencies. A mono-crystalline silicon cell is made from a thin wafer of a high purity silicon crystal, doped with a minute quantity of boron. Phosphorus is diffused into the active surface of the wafer. At the front electrical contact is made by a metallic grid; at the back contact usually covers the whole surface. An anti-reflective coating is applied to the front surface. Typical cell size is about 15cm diameter. The modules in a PV array are usually first connected in series to obtain the desired voltage; the individual strings are then connected in parallel to allow the system to produce more current. The modules are then protected by encapsulation between glass and a tough metal, plastic or fibreglass back. This is held together by a stainless steel or aluminium frame to form a module. These modules, usually comprised of about 30 PV cells, form the basic building block of a solar array. Modules may be connected in series or parallel to increase the voltage and current, and thus achieve the required solar array characteristics that will match the load. Typical module size is 50Wp and produces direct current electricity at 12 V (for battery charging, for example).
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Commercially available modules fall into three types based on the solar cells used.
Mono-crystalline cell modules: the highest cell efficiencies of around 15% -18% are
obtained with these modules. The cells are cut from a mono-crystalline silicon crystal.
Multi-crystalline cell modules. The cell manufacturing process is lower in cost but cell
efficiencies of only around 15% are achieved. A multi-crystalline cell is cut from a cast ingot of multi-crystalline silicon and is generally square in shape.
Amorphous silicon modules. These are made from thin films of amorphous silicon where
efficiency is much lower (10% - 12%) but the process uses less material. The potential for cost reduction is greatest for this type and much work has been carried out in recent years to develop amorphous silicon technology. Unlike mono-crystalline and multi-crystalline cells, with amorphous silicon there is some degradation of power over time.
An array can vary from one or two modules with an output of 10W or less, to a vast bank of several kilowatts or even megawatts.
Flat plate arrays fixed at a tilted angle and facing towards the equator, are most common.
The angle of tilt should be approximately equal to the angle of latitude for the site. A steeper angle increases the output in winter; a shallower angle - more output in summer. It should be at least 10 degrees to allow for rain runoff.
Tracking arrays follow the path of the sun during the day and thus theoretically capture more
sun. However, the increased complexity and cost of the equipment rarely makes it worthwhile.
Mobile (portable) arrays can be of use if the equipment is required in different locations
such as with some lighting systems or small irrigation pumping systems.
Solar PV systems
While in industrialised countries there has been a rapid increase in grid connected PV systems, in developing countries the majority of PV systems are stand-alone off-grid systems. The off-grid systems can be used to drive a load directly; water pumping is a good example. Water is pumped during the hours of sunlight and stored for later use; or a battery can be used to store power for use for lighting during the evening. If a battery charging system is used. Electronic control apparatus will be needed to monitor the system. All the components other than the PV module are referred to as the balance-of-system (BOS) components. The figure below shows a typical configurations for an offgrid PV system. Such systems can often be bought as kits and installed by semi-skilled labour.
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For correct sizing of PV systems, the user needs to estimate the demand on the system, as well as acquiring information about the solar insolation in the area (approximations can be made if no data is readily available). It is normally assumed that for each Wp of rated power the module should provide 0.85watt hours of energy for each kWhm-2 per day of insolation (Hulscher 1994). Therefore if we consider a module rated at 200 Wp and the insolation for our site is 5 kWhm-2 per day (typical value for tropical regions), then our system will produce 850Wh per day (that is 200 x 0.85 x 5 = 850).
Charge controller
Photovoltaic array
DC load
Battery
Inverter AC load
Figure 2: Components of a typical off-grid PV system. Illustration: Neil Noble / Practical Action.
Some benefits of photovoltaics No fuel requirements - In remote areas diesel or kerosene fuel supplies are erratic and often very expensive. The recurrent costs of operating and maintaining PV systems are small. Modular design - A solar array comprises individual PV modules, which can be connected to meet a particular demand. Reliability of PV modules - This has been shown to be significantly higher than that of diesel generators. Easy to maintain - Operation and routine maintenance requirements are simple. Long life - With no moving parts and all delicate surfaces protected, modules can be expected to provide power for 15 years or more. National economic benefits - Reliance on imported fuels such as coal and oil is reduced. Environmentally benign - There is no pollution through the use of a PV system - nor is there any heat or noise generated which could cause local discomfort. PV systems bring great improvements in the domestic environment when they replace other forms of lighting kerosene lamps, for example.
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PV applications in lesser developed countries
Rural electrification
lighting and power supplies for remote building (mosques, churches, temples etc farms, schools, mountain refuge huts) - low wattage fluorescent or LED lighting is recommended: power supplies for remote villages: street lighting: individual house systems (solar home systems): battery charging: mini grids.
See the Practical Action Technical Brief Rural Lighting
Water pumping and treatment systems
pumping for drinking water: pumping for irrigation: dewatering and drainage: ice production: saltwater desalination systems: water purification.
See the Practical Action Technical Brief Solar PV Waterpumping
Health care systems
lighting in rural clinics: UHF transceivers between health centres: vaccine refrigeration: ice pack freezing for vaccine carriers: sterilises: blood storage refrigerator.
PV is frequently used to power vaccine refrigeration in remote health centres. See the Practical Action Technical Brief Solar Photovoltaic Refrigeration of Vaccines.
Communications
radio repeaters: remote TV and radio receivers: remote weather measuring: mobile radios: rural telephone kiosks: data acquisition and transmission (for example, river levels and seismographs). road sign lighting: railway crossings and signals: hazard and warning lights: navigation buoys: road markers. security lighting: remote alarm system: electric fences. ventilation systems:
Transport aids
Security systems
Figure 3: The La Encanada infocentre, Peru has solar panels to generate electricity and a satellite for connectivity. Photo: Practical Action / Jaime Soto.
Miscellaneous
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