Solar Energy: Photovoltaic (PV) and Thermal Energy
This page provides an overview of the available domestic and non-domestic solar system solutions, technologies and applications.
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. It must be taken into account in the design of buildings so that less additional heating or cooling is required.
- 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: Uses the sun's heat to provide hot water for homes or swimming pools (also space heating and even Solar Assisted Cooling systems).
Photovoltaics: 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 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 Amorphous 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 crystalline technology, a price advantage which is currently counterbalanced by substantially lower efficiency rates (from 5% to 13%).
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%.
Performances and Costs of PV Modules
Source: International Energy Agency (2010) - Renewable Energy Technology Roadmaps
Grid-connected domestic systems: This is the most popular type of solar PV system for homes and businesses in developed areas. They can be used in building-integrated systems (BIPV) or be ground-mounted. 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. Rural electrification means 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.
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.
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.
Solar Thermal technology and applications
Solar collectors' working principle
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 collector technologies and their typical uses
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.
There are three main types of thermal collector:
Glazed collectors can be used for commercial and domestic hot water (DHW) and heating purposes, swimming pool heating and more recently for solar assisted cooling installations.
Non-glazed collectors are usually found in applications operated at low temperature so designing to maximise heat gain is more important than minimising heat loss.
They work efficiently at low radiation levels and with high absorber temperatures and can provide higher output temperatures than flat plate collectors. Evacuated tube collectors can be used in colder climates or in applications where the demand temperature is higher.
Commercial and domestic hot water (DHW) and heating/cooling using absorption chillers (domestic or commercial) are the most common usage.
Concentrated Solar Power (CSP) :
CSP devices concentrate energy from the sun's rays to heat a receiver to high temperatures. This heat is transformed first into mechanical energy (by turbines or other engines) and then into electricity. CSP also holds potential for producing other energy carriers (solar fuels).
Source: International Energy Agency (2010) - Technology Roadmaps Concentrated Solar Power
While the bulk of CSP electricity will come from large, on-grid power plants, these technologies also show significant potential for supplying specialised demands such as process heat for industry, co-generation of heating, cooling and power, and water desalination. CSP also holds potential for applications such as household cooking and small-scale manufacturing that are important for the developing world.
Source: International Energy Agency (2010) - Renewable Energy Technology Roadmaps
Mixing PV and Thermal systems: PVT technology or "Solar Cogeneration"
A PhotoVoltaic Thermal product, or PVT product is a combination of photovoltaic cells with a solar thermal collector, forming one device that converts solar radiation into electricity and heat simultaneously. The excess heat generated in the PV cells is removed and converted into useful thermal energy.
A PVT module produces more primary energy than a conventional solar thermal collector per unit surface area, and more than a conventional PV module. The integration of the two technologies leads to large potential savings in material use and in production, balance-of-system and installation costs. This also gives PVT an aesthetic advantage: only one building element is required to produce both forms of solar energy, leading to a more homogeneous roof or fašade appearance.
10 good reasons to switch to solar system solutions:
- The fuel is free.
- It produces no noise, harmful emissions or polluting gases.
- Both systems (PV and Thermal) are very safe and highly reliable.
- Modules can be recycled.
- It requires low maintenance.
- It can bring electricity, heat and even cooling to remote rural areas.
- It can be aesthetically integrated in buildings (ie. BIPV).
- The energy pay-back time of a module is constantly decreasing.
- It creates thousands of jobs.
- It contributes to improving the security of the world's energy supply.