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Technology

Renewable Energy Electricity Technologies:

Solar:

(i) Photo-Voltaic: Photo-voltaic systems convert energy from the sun directly into electricity. They are composed of photo-voltaic cells, usually a thin wafer or strip of semi-conductor material, that generates a small current when sunlight strikes them. Multiple cells can be assembled into modules that can be wired in an array of any size. Small photo-voltaic arrays are found in wristwatches and calculators; the largest arrays have capacities in excess of 
5 MW.

Photo-voltaic systems are cost-effective in small off-grid applications, providing power, for example, to rural homes in developing countries, off-grid cottages and motor homes in industrialized countries, and remote tele-communications, monitoring and control systems worldwide. Water pumping is also a notable off-grid application of photo-voltaic systems that are used for domestic water supplies, agriculture and, in developing countries, provision of water to villages. These power systems are
relatively simple, modular, and highly reliable due to the lack of moving parts.
Photo-voltaic systems can be combined with fossil fuel-driven generators in applications having higher energy demands or in climates characterized by extended periods of little sunshine (e.g. winter at high latitudes) to form hybrid systems.

Photo-voltaic systems can also be tied to isolated or central grids via a specially configured inverter. Unfortunately, without subsidies, on-grid (central grid-tied) applications are rarely cost-effective due to the high price of photo-voltaic modules, even if it has declined steadily since 1985. Due to the minimal maintenance of photo-voltaic systems and the absence of real benefits of economies of scale during construction, distributed generation is the path of choice for future cost-effective on-grid applications. In distributed electricity generation, small photo-voltaic systems would be widely scattered around the grid, mounted on buildings and other structures and thus not incurring the costs of land rent or purchase. Such applications have been facilitated by the development of technologies and practices for the integration of
photo-voltaic systems into the building envelope, which offset the cost of conventional material and/or labour costs that would have otherwise been spent.

Photo-voltaic systems have seen the same explosive growth rates as wind turbines, but starting from a much smaller installed base. For example, the worldwide installed photo-voltaic capacity in 2003 was around 3,000 MW, which represents less than one-tenth
that of wind, but yet is growing rapidly and is significant to the photo-voltaic industry.

(ii) Solar-thermal power:

Several large-scale solar thermal power projects, which generate electricity from solar energy via mechanical processes, have been in operation for over two decades. Some of the most successful have been based on arrays of mirrored parabolic troughs. Through the 1980’s, nine such commercial systems were built in the Mohave Desert of California, in the United States. The parabolic troughs focus sunlight on a collector tube, heating the heat transfer fluid in the collector to 390ºC (734ºF). The heated fluid is used to generate steam that drives a turbine.
The combined electric capacity of the nine plants is around 350 MW, and their average output is over 100 MW. The systems have functioned reliably and the most recently constructed plants generate power at a cost of around $0.10/kWh.
Several studies have identified possible cost reductions.

Another approach to solar thermal power is based on a large field of relatively small mirrors that track the sun, focussing its rays on a receiver tower in the centre of the field. The concentrated sunlight heats the receiver to a high temperature (e.g., up to 1,000ºC, or 1,800ºF), which generates steam for a turbine. Prototype plants with electrical capacities of up to 10 MW have been built in the United States, the Ukraine (as part of the former
USSR), Israel, Spain, Italy, and France.

A third solar thermal power technology combines a Stirling cycle heat engine with a parabolic dish. Solar energy, concentrated by the parabolic dish, supplies heat to the engine at temperatures of around 600ºC. Prototype systems have achieved high efficiency.

All three of the above technologies can also be co-fired by natural gas or other fossil fuels, which gives them a firm capacity and permits them to be used as peak power providers. This makes them more attractive to utilities, and gives them an advantage over photo-voltaic, which cannot necessarily provide power whenever it is required.
On the other hand, they utilize only that portion of sunlight that is direct beam and require much dedicated land area. Solar thermal power is still at the development stage: the costs of the technology should be reduced together with the associated risks, and experience under real operating conditions should be a further gain.

Renewable energy heating and cooling technologies:

Solar water heating systems:

Solar water heating systems use solar energy to heat water. Depending on the type of solar collector used, the weather conditions, and the hot water demand, the temperature of the water heated can vary from tepid to nearly boiling. Most solar systems are meant to furnish 20 to 85% of the annual demand for hot water, the remainder being met by conventional heating sources, which either raise the temperature of the water further or provide hot water when the solar water heating system cannot meet demand (e.g. at night).

Solar systems can be used wherever moderately hot water is required. Off-the-shelf packages provide hot water to the bathroom and kitchen of a house; custom systems are designed for bigger loads, such as multi-unit apartments, restaurants, hotels, motels, hospitals, and sports facilities. Solar water heating is also used for industrial and commercial processes, such as car washes and laundries.

Worldwide, there are millions of solar collectors in existence, the largest portion installed in China and Europe. The North American market for solar water heating has traditionally been hampered by low conventional energy costs, but a strong demand for swimming pool heating has led unglazed technology to a dominant sales position on the continent. Solar water heating technology has been embraced by a number of developing countries with both strong solar resources and costly or unreliable conventional energy supplies.

Solar air heating systems:

Solar air heating systems use solar energy to heat air for building ventilation or industrial processes such as drying. These systems raise the temperature of the outside air by around 5 to 15ºC (41 to 59ºF) on average, and typically supply a portion of the required heat, with the remainder being furnished by conventional heaters.

A solar air heating system currently considered by RETScreen consists of a transpired collector, which is a sheet of steel or aluminium perforated with numerous tiny holes, through which outside air is drawn. Mounted on an equator-facing building wall, the transpired collector absorbs incident sunshine and warms the layer of air adjacent to it. A fan draws this sun-warmed air through the perforations, into the air space behind the collector and then into the ducting within the building, which distributes the heated air through the building or the industrial processes. Controls regulate the temperature of the air in the building by adjusting the mix of recirculated and fresh air or by modulating the output of a conventional heater. When heat is not required, as in summertime, a damper bypasses the collector. The system also provides a measure of insulation, recuperates heat lost through the wall it covers and can reduce stratification, the pooling of hot air
near the ceiling of voluminous buildings. The result is an inexpensive, robust and simple system with virtually no maintenance requirements and efficiencies as high as 80%.

Solar air heating systems tend to be most cost-effective in new construction, when the net cost of the installation of the transpired collector is offset by the cost of the traditional weather cladding it supplants. Also, new-construction gives the designer more latitude in integrating the collector into the building’s ventilation system and aesthetics. Installation of a transpired collector also makes sense as a replacement for aging or used weather cladding.

Given the vast quantities of energy used to heat ventilation air, the use of perforated collectors for solar air heating has immense potential. In general, the market is strongest where the heating season is long, ventilation requirements are high, and conventional
heating fuels are costly. For these reasons, industrial buildings constitute the biggest market, followed by commercial and institutional buildings, multi-unit residential buildings, and schools. Solar air heating also has huge potential in industrial processes which need large volumes of heated air, such as in the drying of agricultural products.