Solar Thermal solution for energy crisis
By Saeed Hussain
THE PLANET is on the brink of runaway climate change. If annual average temperatures rise by more than 2ºC, the entire world will face more natural disasters, hotter and longer droughts, failure of agricultural areas and massive loss of species. Because climate change is caused by burning fossil fuels, we urgently need an energy revolution, changing the world’s energy mix to a majority of non-polluting sources. To avoid dangerous climate change, global emissions must peak in 2015 and start declining thereafter, reaching as close to zero as possible by mid-century.
Concentrating Solar Power (CSP) is a large-scale, commercially viable way to make electricity. It is best suited to those areas of the world with the most sun; Southern Europe, Northern Africa and the Middle East, India, Pakistan, China, Southern USA and Australia, where many are suffering from peak electricity problems, blackouts and rising electricity costs. CSP does not contribute to climate change and the source will never run out. The technology is mature enough to grow exponentially in the world’s ‘sun-belt’.
CSP systems produce heat or electricity using hundreds of mirrors to concentrate the sun’s rays to a temperature typically between 400 and 1000ºC. Individual CSP plants are now typically between 50 and 280MW in size, but could be larger still. CSP systems can be specifically integrated with storage or in hybrid operation with fossil fuels, offering firm capacity and dispatch able power on demand. It is suitable for peak loads and base-loads, and power is typically fed into the electricity grid.
There are different types of Solar Thermal but commonly known are Parabolic Trough, Solar Tower, Parabolic Dish, and Linear Fresnel Reflector.
Parabolic trough-shaped mirror reflectors are used to concentrate sunlight on to thermally efficient receiver tubes placed in the trough’s focal line. The troughs are usually designed to track the sun along one axis, predominantly north–south. A thermal transfer fluid, such as synthetic thermal oil, is circulated in these tubes. The fluid is heated to approximately 400°C by the sun’s concentrated rays and then pumped through a series of heat exchangers to produce superheated steam. The steam is converted to electrical energy in a conventional steam turbine generator, which can either be part of a conventional steam cycle or integrated into a combined steam and gas turbine cycle.
Central receiver or solar tower: A circular array of heliostats (large mirrors with sun tracking motion) concentrates sunlight on to a central receiver mounted at the top of a tower. A heat-transfer medium in this central receiver absorbs the highly concentrated radiation reflected by the heliostats and converts it into thermal energy, which is used to generate superheated steam for the turbine. To date, the heat transfer media demonstrated include water/steam, molten salts and air. If pressurized gas or air is used at very high temperatures of about 1,000°C or more as the heat transfer medium, it can even be used to directly replace natural gas in a gas turbine, making use of the excellent cycle (60% and more) of modern gas and steam combined cycles.
A parabolic dish-shaped reflector concentrates sunlight on to a receiver located at the focal point of the dish. The concentrated beam radiation is absorbed into a receiver to heat a fluid or gas (air) to approximately 750°C. This fluid or gas is then used to generate electricity in a small piston or Sterling engine or a micro turbine, attached to the receiver. The troughs are usually designed to track the Sun along one axis, predominantly north–south.
Linear Fresnel Reflector (LFR): An array of nearly-flat reflectors concentrates solar radiation onto elevated inverted linear receivers. Water flows through the receivers and is converted into steam. This system is line-concentrating, similar to a parabolic trough, with the advantages of low costs for structural support and reflectors, fixed fluid joints, a receiver separated from the reflector system, and long focal lengths that allow the use of flat mirrors. The technology is seen as a potentially low-cost alternative to trough technology for the production of solar process heat.
In the last five years, the industry has expanded rapidly from a newly-introduced technology to become a mass-produced and mainstream energy generation solution. CSP installations were providing just 436 MW of the world’s electricity generation at the end of 2008. Projects under construction at the time of writing, mostly in Spain, will add at least another 1,000 MW by around 2011. In the USA, projects adding up to further 7,000 MW are under planning and development plus 10,000 GW in Spain, which could all come online by 2017.
The main benefit of CSP systems is in replacing the power generated by fossil fuels, and therefore reducing the greenhouse gas emissions the cause of climate change. Each square meter of concentrator surface, for example, is enough to avoid 200 to 300 kilograms (kg) of CO2 each year, depending on its configuration. Typical power plants are made up of hundreds of concentrators arranged in arrays. The life-cycle assessment of the components together with the land-surface impacts of CSP systems indicate that it takes around five months to ‘pay back’ the energy used to manufacture and install the equipment.
The cost of solar thermal power is dropping. Experience in the US shows that today’s generation costs are about 15 US cents/kWh for solar generated electricity at sites with very good solar radiation, with predicted ongoing costs as low as 8 cents / kWh in some circumstances. The technology development is on a steep learning curve, and the factors that will reduce costs are technology improvements, mass production, economies of scale and improved operation. CSP is becoming competitive with conventional, fossil-fuelled peak and mid-load power stations. Adding more CSP systems to the grid can help keep the costs of electricity stable, and avoid drastic price rises as fuel scarcity and carbon costs take effect.