While comparing a conventional power plant with a concentrated solar power (CSP) one, we notice that in CSP, a boiler is replaced by a solar field and then take a look at important components of a parabolic trough, which is the heart of the CSP plant.
The concentrated solar power (CSP) sector has been in the limelight ever since the Ministry of New and Renewable Energy (MNRE) announced its ambitious plans of installing 20 GW of solar power by 2022 through the Jawaharlal Nehru National Solar Mission. The CSP sector holds enormous potential as a central generator, with an estimate that only four per cent area (300 square kilometre) of the Sahara desert if covered by these plants, would be able to cater to worldwide energy demand.
Operating concentrated solar power plants economically requires solar radiation of at least 1,900 KWh/m2. Over and above having good solar radiation, the location needs to be in the vicinity of good water availability as these power plants require the same amount of water as that required by conventional power plants and the land should be relatively flat.
Within CSP, various technologies are available like parabolic trough technology, which is fully commercialised and has a proven track record of over 25 years while some of them are in the evolutionary or prototype stage and are getting ready to play a major role in the energy mix of the future, ie, solar towers, linear fresnel and dish sterling systems.
There is no major difference between a conventional power plant and a CSP plant except that a boiler in a conventional power plant, which is operated by conventional fossil fuels like gas, coal, etc, gets replaced by a solar field.
A parabolic trough, which is the heart of the CSP plant and replaces the boiler, has the following important components:
• Parabolic mirrors – Large parabolic shaped mirrors lined up in long troughs of more than 400 metres in length concentrate the solar irradiation along the focal point of the mirrors onto specially-coated, evacuated absorber tubes called 'solar receivers'. The parabolic mirrors multiply the incident sunlight by 80 times and focus it on the receiver, which help absorb all the incident radiation of sunlight into heat.• Solar receivers – The function of a receiver is to absorb all the incident solar radiation and achieve maximum solar absorbance to make full use of the available solar energy and at the same time have minimal heat emission so that the acquired temperature helps generate more steam or steam at higher temperatures. To reduce conduction losses, a vacuum is maintained between the glass envelope and steel tube with absorber coating.• Heat transfer fluid (HTF) – Synthetic oil popularly called 'HTF' is used as a heat transfer medium which flows through receivers and gets heated at about 400o C. The characteristic of this oil is that it doesn't change its physical form even at these high temperatures and is kept flowing in the tubes by means of pumps.• A suitable metal structure is used to support the trough, which can also facilitate movement of parabolic trough in horizontal direction, ie, single axis tracking to track the sun.
The schematic of a CSP plant looks like a conventional power plant except that here a boiler is replaced by a solar field. The heated oil which passes through the receivers is carried to the heat exchanger, where it loses its heat and water in the heat exchanger is converted into steam. HTF is then pumped back into the solar field for further accumulation of heat.
The generated steam then runs the steam turbine for power generation. The biggest advantage of this type of power plant is that no qualitative change is required in the grid connection as compared to a conventional one so these power plants can easily replace conventional plants. Also, complete compatibility with conventional plants makes it ideal to choose a hybrid power plant, wherein during the day, the plant will run on solar energy and post-sunset conventional fuels can take over.
Solar energy that is generated in the collector field can be stored in well-insulated molten salt tanks. So electricity can be generated even during unfavourable weather conditions or at night. Modern storage technologies that use molten salt enable parabolic trough plants to bridge over six hours of non-solar periods, depending on the storage capacity. This overall makes these power plants a very reliable source of power generation.
A 50 MW CSP power plant will need approximately two sq km and its high CAPEX is nullified by the low cost of operation.
With technological advances and an increase in efficiencies of every component, major cost reductions are expected in the years to come, resulting in savings of power generation cost and reaching grid parity from conventional sources like oil or gas by 2025.
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