A solar-home-system uses a photovoltaic (solar-electric) module to provide power for lights and small appliances. The system also needs a rechargeable battery, so that power is still available at night and on cloudy days. Solar-home-systems bring huge benefits to homes in developing countries which aren’t connected to the mains electricity grid. They replace smoky, unsafe kerosene lamps with brighter light, allowing work, study and social activities after dark. They also power radios and cellphone chargers, enabling families to be in contact with the wider world. The smallest systems are solar lanterns, which can be moved around the home or carried outdoors. Solar-home-systems and solar lanterns already provide power to millions of homes in Asia, Africa and Latin America. Similar systems are also used in off-grid schools and health centres.
How a solar-home-system works
Photovoltaic (PV) modules use semiconductor materials to generate d.c. electricity from sunlight. A large area is needed to collect as much sunlight as possible, so the semiconductor is either made into thin, flat, crystalline cells, or deposited as a very thin continuous layer onto a support material. The semiconductor must be sealed into a weatherproof casing, with suitable electrical connectors.
Layout of a solar-home-system PV modules are specified by their ‘watt-peak’ (Wp) rating, which is the power generated under standard conditions, equivalent to bright sun in the tropics (they still work at lower light levels though). Most solar-home-systems use modules between about 10 Wp and 100 Wp rating. The rechargeable batteries store spare electricity on sunny days, so that it is available at night and on cloudy days. They also provide a stable voltage (usually 12 V) for the devices which use the electricity. Standard lead-acid car batteries can be used, but they don’t last long if they are heavily discharged, so specially-made solar versions are strongly recommended. Other types of rechargeable battery like nickel-cadmium and nickel-metal-hydride are increasingly used, particularly in small systems like solar lanterns. They are more expensive, but easier to make small and portable, and more tolerant of being heavily discharged. All equipment run directly from the PV supply must be designed for 12 V d.c. operation. Efficient lights and appliances make best use of the limited supply of electricity. Efficient d.c. fluorescent lights are available down to about 3 or 4 W power rating, in both tube and compact forms. LED lights are even more efficient, and are now sufficiently cheap and reliable to be used as well. In most systems, an electronic charge-controller is used to protect the battery from being overcharged (when it is very sunny) or over-discharged (when people try to get too much electricity from the system). The charge controller usually has lights or a meter to indicate the state-of-charge of the battery.
How solar home systems are used
A solar-home-system should be designed with sufficient PV capacity (Wp) to provide the daily electricity demand throughout the year, and typically three days storage capacity in the battery, so that the system keeps working during a cloudy period. For example, a typical solar-home-system sold by Ashden Award-winner SELCO in India has a 35 Wp PV module and a 90 Ah/12 V battery to power four 7 W d.c. fluorescent lights for about four hours per day and a socket. Recently the Solar Energy Foundation in Ethiopia has introduced a 10 Wp system which powers four small LED lamps The PV module is fixed to the roof of a home at the angle which collects maximum sunlight. A framed 35 Wp module made from crystalline cells has an area of about 0.3 m2, or about 0.7 m2 if made from lower-efficiency thin film materials, so is easy to handle and install.
A Grameen Shakti solar technician in Bangladesh installs the PV module for a solar-home-system on a roof The battery is kept indoors, and the terminals should be covered so that they cannot accidentally be touched or short-circuited. The PV, battery, lights and socket are all wired carefully to the charge-controller, ideally by a trained technician.
Battery and charge controller for a solar-home-system installed by SELCO in India, covered up to keep them clean and safe Larger systems can run a TV as well, and often include an inverter (d.c. to a.c. converter), so that standard mains-voltage equipment can be run from the solar home system. Solar-home-systems can be very reliable and need little maintenance. Users must be trained to check the battery, keep the PV module clean and make sure that connectors are secure. It is important not to over-discharge the battery (and very tempting to do so, to get a bit more light or TV). Even with careful use, batteries deteriorate and need to be replaced every four or five years.
Some small solar-home-systems are designed to be portable. Several participants of the REDP PV programme in China produce small portable PV systems for herders who move seasonally with their animals. Another approach is to use solar lanterns for lighting. These have a small fluorescent or LED light with a rechargeable battery, in a case which is easy to carry and can stand on the ground or a table, or else hang from the ceiling. Some include a small built-in PV module, and others are designed to be plugged into a PV module for charging then detached for use. NEST in South India makes lanterns which are charged at home their owners, while Sunlabob in Laos has developed lanterns which are rented from a central charging site.
What are the benefits of solar home systems?
The amount of electricity supplied by a solar-home-system is small, typically about 0.1 kWh per day for a 35 Wp system. However, the benefits can be huge Electric light avoids the fumes and the fire risk of kerosene lamps. They are also much brighter, and give opportunities for study, income-generating work and recreation after dark.
Being able to listen to a radio without the expense of dry-cell batteries allows people in remote areas to keep in touch. Cellphone network coverage is expanding rapidly in many developing countries, and solar-home-systems allow people in remote areas to benefit from this. Solar lanterns are attractive for market stall holders as well as homes, because they give good-quality light for displaying and selling produce after dark. They are also used by midwives and traditional birth attendants, to deliver babies more safely.
Costs of solar-home-systems vary between countries. In Bangladesh, a 20 Wp system sold by Grameen Shakti costs about US$230, and a 50 Wp system US$230. This is a significant capital investment for a poor household, but usually no more than would be spent on kerosene for lighting and dry-cell batteries over three years. The World Bank has provided finance for micro-credit schemes in a number of countries including Bangladesh and Sri Lanka, to make solar-home-systems affordable. Outside Asia, costs tend to be higher. For example, 10 Wp systems provided by the Solar Energy Foundation in Ethiopia cost about US$260, and 50 Wp systems sold by ECAMI in Nicaragua cost about US$600. Numbers The number of solar-home-systems currently in use is difficult to estimate, but is likely to be several million. Several Ashden Award winners have been involved in large scale programmes. The work of the REDP led to the installation of over 400,000 systems in rural China and Grameen Shakti has installed over 200,000 systems in Bangladesh. The future Electricity is a distant dream for many families in developing countries. For example, in Tanzania less than 15% of the population are connected to the mains electric grid. Even in India where over half the population is grid-connected, many parts of the country have frequent and lengthy power cuts. Solar-home-systems have huge potential to provide access to basic electrical services. System costs will decrease as the global PV market continues to grow, and the improving efficiency of lights and appliances will also bring costs down.
Author: Dr Anne Wheldon
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