ABOUT SOLAR ENERGY
The sun, an energy available for free which can be used in many ways.
Energy from the sun can be used in three main ways, and when talking about solar energy, it is important to distinguish between these three types:
- Passive heat: This is heat which we receive from the sun naturally. This can be taken into account in the design of buildings so that less additional heating is required
- Solar thermal: Uses the sun’s heat to provide hot water for homes or swimming pools (also heating systems)
- Photovoltaic energy (PV): Uses energy from the sun to create electricity to run appliances and lighting. A photovoltaic system requires only daylight - not direct sunlight - to generate electricity.
Solar Thermal Applications and Technology
The basic principle common to all solar thermal systems is simple: solar radiation is collected and the resulting heat conveyed to a heat transfer medium - usually a fluid but also air in the case of air collectors. The heated medium is used either directly - for example to heat swimming pools - or indirectly, by means of a heat exchanger which transfers the heat to its final destination - for instance: space heating.
Solar thermal can be successfully applied to a broad range of heat requirements including domestic water heating, space heating, and drying. New exciting areas of applications are being developed in particular solar assisted cooling. System design, costs and solar yield are being constantly improved.
The process of turning sunlight into electricity
Photovoltaic systems use cells to convert solar radiation into electricity. The cell consists of one or two layers of a semi-conducting material. When light shines on the cell it creates an electric field across the layers, causing electricity to flow. The greater the intensity of the light, the greater the flow of electricity is.
The most common semi conductor material used in photovoltaic cells is silicon, an element most commonly found in sand. There is no limitation to its availability as a raw material; silicon is the second most abundant material in the earth’s mass.
A photovoltaic system therefore does not need bright sunlight in order to operate. It can also generate electricity on cloudy days.
Overview of available photovoltaic technologies:
- Crystalline silicon technology: Crystalline silicon cells are made from thin slices cut from a single crystal of silicon or from a block of silicon crystals (polycrystalline), their efficiency ranges between 12% and 17%. This is the most common technology representing about 90% of the market today.
- Thin Film technology: Thin film modules are constructed by depositing extremely thin layers of photosensitive materials onto a low-cost backing such as glass, stainless steel or plastic. Thin film manufacturing processes result in lower production costs compared to the more material intensive crystalline technology, a price advantage which is currently counterbalanced by substantially lower efficiency rates (from 5% to 13%).
- Concentrated photovoltaic: Some solar cells are designed to operate with concentrated sunlight. These cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. Efficiencies are in the range of 20 to 30%.
- Grid-connected domestic systems: This is the most popular type of solar PV system for homes and businesses in developed areas. Connection to the local electricity network allows any excess power produced to feed the electricity grid and to sell it to the utility. Electricity is then imported from the network when there is no sun.
- Grid-connected power plants: These systems, also grid-connected, produce a large quantity of photovoltaic electricity in a single point. The size of these plants ranges from several hundred kilowatts to several megawatts. Some of these applications are located on large industrial buildings such as airport terminals or railway stations. This type of large application makes use of already available space and compensates a part of the electricity required by these energy-intensive consumers.
- Off-grid systems for rural electrification: Where no mains electricity is available, the system is connected to a battery via a charge controller. An inverter can be used to provide AC power, enabling the use of normal electrical appliances. Typical off-grid applications are used to bring access to electricity to remote areas (mountain huts, developing countries). Rural electrificationmeans either small solar home system covering basic electricity needs in a single household, or larger solar mini-grids, which provide enough power for several homes. More information is available at www.ruralelec.org
- Hybrid systems: A solar system can be combined with another source of power - a biomass generator, a wind turbine or diesel generator - to ensure a consistent supply of electricity. A hybrid system can be grid-connected, stand-alone or grid-support. More information is available at www.ruralelec.org
- Consumer goods: Photovoltaic cells are used in many daily electrical appliances, including watches, calculators, toys, battery chargers, professional sun roofs for automobiles. Other applications include power for services such as water sprinklers, road signs, lighting and phone boxes.
- Off-grid industrial applications: Uses for solar electricity for remote applications are very frequent in the telecommunications field, especially to link remote rural areas to the rest of the country. Repeater stations for mobile telephones powered by PV or hybrid systems also have a large potential. Other applications include traffic signals, marine navigation aids, security phones, remote lighting, highway signs and waste water treatment plants. These applications are cost competitive today as they enable to bring power in areas far away from electric mains, avoiding the high cost of installing cabled networks.