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Solar Power FAQ's

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1. What is the difference between photovoltaic and solar thermal electricity generation?
   
   Photovoltaic (PV) electricity generation uses the photoelectric effect: when light of sufficient energy falls on a carefully prepared, layered material, a flow of electrons (electricity) is generated. The electricity generated can then be stored.
   
   Solar thermal (ST) electricity generation uses heat from directed sunlight to increase the temperature of a fluid that can produce steam at high enough temperatures and pressures to drive a steam turbine electric generator.
   
   PV can be used on residential buildings and has been scaled up to 20MW power stations in Europe. ST is feasible for industrial or commercial electricity power station level generation (multi Giga Watt scale).
   
   
2. How can solar thermal power stations deliver power during the night?
   
   Steam is stored in great drums so that it can be released into a power generation turbine after the solar collectors no longer receive sunlight. This enables a Ride the Ray Solar Thermal power plant to serve both morning and evening peaks. Current designs store up to 13 hours of maximum load steam output, permitting near 24 hours operation.
   
   
3. What’s the difference between a solar-tower power station and a parabolic trough solar field?
   
   A solar tower power station uses a circular field of heliostats (mirrors) to concentrate light and heat onto a central tower, like a lighthouse. This heats a thermal transfer fluid that is then pumped through a heat exchanger where its heat is used to make steam for a steam turbine.
   
   A parabolic trough solar field contains many long straight parabolic shaped collectors that heat a thermal fluid like water or mineral oil. The many lines of hot fluid are then aggregated to create steam which is piped to turbines.
   The turbines generate the electricity in either case.
   
   
4. Can a solar collector really heat water and steam to a high enough temperature to drive a steam turbine?
   
   Solar tower technologies and some parabolic trough technologies have a working temperature of 600 C. Other technologies work at 270 C.
   
   
5. Where do you place a solar thermal power station?
   
   A solar thermal power station can be placed anywhere that has more than the lower limit of solar radiated energy. This limit means that a solar thermal plant is viable throughout much of South Africa, Namibia, Botswana and Angola. The number of hours of generating time is dependent on the average hours of full sunshine received. Further afield, Australia, the Sahara desert, the Gobi desert in Asia and the arid southwest of the United States are ideal.
   
   Other factors influence placement of power stations, no matter what their fuel. Connecting the power station to an electricity transmission grid requires careful design, since grid stability can be greatly enhanced or disrupted by geographical position of generators.
   
   Other factors like water supply and landscape also play a role.
   
   
6. How much land space is needed for a solar thermal power plant?
   
   Technologies used in Ride The Ray solar thermal plants require about 5 square kilometres for around 200MW of electrical output. This space will encompass the entire plant, which is the solar array, the steam power block, the electrical transmission yard, the cooling towers, all the administrative buildings and extra area for roads and parking.
   
   
7. Isn't it bad for the environment to use so much land and water to generate electricity?
   
   A solar thermal power plant of a few hundred MW with air-cooled condensers uses about four large swimming pools' worth of water per day. With careful design and recirculated water treatment systems, evaporation and water loss is minimized. The land is situated in an arid area and the ecosystems there are sensitive to usage of the land. However, compared to a coal-fired power station, the environmental benefits are astronomical.
   
   The land use changes the amount of sun hitting the ground and the land is hardened to support the reflector structures. The power block is built on concrete foundations as well. The land use is therefore similar to office blocks in terms of land use environmental impact. Is it bad for the environment? When compared to coal-fired power, it is a categorical "NO".
   
   
8. What happens at night?
   
   At night, the reflectors are all rotated so that their steel backs are up. This protects the reflectors from dust and dewfall in the morning. The steam stored in the thermal storage tanks is piped out into the turbine until it is exhausted, usually starting when the sun's radiation falls below operational levels. This occurs about 3 hours before sunset. With Ride the Ray's planned storage capacity, the plant can continue to generate electricity until 9pm each day, the time when the peak loads subside and the coal-fired base-load capacity is currently used.
   
   
9. South Africa’s low-grade coal is abundant and easily mined. Surely this makes coal-fired power much cheaper than solar?
   
   The dynamics of the coal supply chain have changed rapidly over the past few years. In South Africa a new coal supply infrastructure is needed to run the current plants, which are running at maximum capacity. Coal prices have doubled in the last year and prices are expected to be between 5 and 10 times higher within 2 years.
   
   Solar thermal power plants use no fossil fuel and are capable of generating power for the same price as coal-fired power right now. Since cost of materials for the solar power plants is decreasing dramatically, a Ride the Ray solar thermal power plant will actually be cheaper than coal by 2010.
   
   
10. Isn't solar power very expensive compared to coal, gas or oil?
    
    Solar thermal power was expensive compared to fossil-fuel power stations until 2007. Coal prices have risen 5-fold since 2002. Oil has risen to over US$120 per barrel. In South Africa, the gas supply is not entirely stable and Open Cycle Gas Turbines (OCGTs) cost more than ZAR2.50 per kWh to run.
    
    With coal at February 2008 levels (ZAR300 per tonne) the Ride the Ray technology costs the same to run as coal. (The calculation runs over 30 years and takes the price of fuel and operational cost of the power station.) This is the coal price parity that is the main tipping point of obstacles to implementing renewable energy. Now that the tipping point has been reached, solar thermal power plant feasibility depends on other factors, like production output from suppliers of the technology.
    
    
11. How can solar power help companies meet Kyoto Accord carbon trading rules?
    
    The Kyoto Accord, while discussing carbon trading, also discusses reductions in greenhouse gas production in each country. The primary measurement is that each country reduce its emissions to 1990 levels. This means that a certain percentage of a country's power generation must have carbon removed. At least 20% or about 6 GW of electricity generation must come from technologies with zero emissions in South Africa. If every household in South Africa installed a solar water geyser, this would remove about 1.8GW from the coal-fired load, or nearly 2% of South Africa's carbon emissions.
    
    Furthermore, large-scale green energy installations are eligible to certify their generation capacities and sell their certificates to buyers who are struggling to meet emissions targets. These certificates are traded on carbon trading exchanges, named for the fact that greenhouse gases are carbon-rich and the certificates specify savings in greenhouse gases.
    
    These exchanges are currently active in some European countries and any large green power installation may sell their certificates there, subject to local conditions, of course.
    
    For more information, please contact the Solar Power experts at Ride the Ray info@ridetheray.com and http://en.wikipedia.org/wiki/Solar_energy

    
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