Technologies of Solar Thermal Power Generation

Dipak Kumar Saha M. Tech IIEST, Shibpur Technologies of Solar Thermal Power Generation Energy :: The Big thing that we need In this world, currently one of our essential things is the Energy, Our civilization, our culture; Our daily life is running by energy. Without it, we can assume a black, dark, and stopped something, we can’t tolerate that. The electricity we are using, the domestic gas we are using, the fuel we are using in our car, generator and too many things-all are essential. Here are few conventional sources of energy we are using:  Electricity from Coal fired Power Plant & Diesel power Sources  Domestic & others cooking by LPG  Automobiles, Aircraft uses fuels-Petrol, Diesel, Gasoline etc. Now, the fact is that among these, the conventional fossil fuels are not available for unlimited use. In this condition we have to think about the effective Non-conventional Sources looking at the current situation of stored fossil fuels and also for sustainable development. But we are glad that the uses of these non-conventional energy sources are already going on, just we have to increase their uses, the techniques. Some of non-conventional energy sources are:      Electricity from Solar Thermal power plants Electricity from PV power plants Electricity from Biogas, Biodiesel, Wind Power plants Cooking from Solar Thermal Energy etc. Others like Geothermal Energy, Sea-ocean energy etc. What is Solar Thermal Energy? Solar thermal energy is a form of energy and a technology for extracting solar energy to generate thermal energy or electrical energy for use in home, industry, and in commercial sectors. It is simple in technology but if we can use this energy in an efficient way- it will be a really big achievement to us. Already scientists and researchers have found few intelligent but simple techniques. The main objective is to find those technologies which can extract the power coming from the Sun, Yes the Sun is the only source of power those we are using or we have to use. If we see that amount of Solar radiation to earth, it gives us 89,000 TW power where total demand of world is only 15 TW, if we just think of this fact, why we need to use the fossil fuel at high rate? It should be saved and use of this fuel should be less so that our future generation can also use this energy. So, one efficient way is to use the solar thermal energy. The techniques are described here. Most techniques for generating electricity from heat need high temperatures to achieve reasonable efficiencies. Solar thermal (heat) energy is a carbon-free, renewable alternative to the pg. 1 Technologies of Solar Thermal Power Generation power that we produce by burning fossil fuel. Actually heat production is the ultimate objective here, For coal-fired power plant, by firing coal-dust, steam temperate is created near about 550oC. Here, by Solar thermal system, temperature may be risen up to 700oC, but as per demand it is near about 400oC. It's estimated by the U.S. National Renewable Energy Laboratories that solar thermal power could create hundreds of Gigawatts of electricity, which is equal to more than 10 percent of demand in the United States. There are two ways of generating energy from the sun.  Photovoltaic (PV)  Concentrating Solar Thermal (CST) Solar thermal technology is in large-scale by comparison. One big difference from PV is that solar thermal power plants generate electricity indirectly. Heat from the sun's rays is collected and used to heat a fluid. The steam produced from the heat of fluid gives power a generator that produces electricity. It's similar to the way fossil fuel-burning power plants work except the steam is produced by the collected heat rather than from the combustion of fossil fuels. Technologies :: The way to success Solar thermal power can be extracted by solar thermal collectors, depending upon the types of collectors, the technologies are developed. The different types of collectors are: Solar Thermal Collectors Non-Concentrating Collectors Concentrating collectors Solar Pond Flat plate collector Evacuated Tube Non-Tracking Compound Parabolic collector Single axis tracking Two axis tracking Compound Parabolic Parabolic Dish Liner Fresnel Heliostat Field Parabolic Trough pg. 2 Technologies of Solar Thermal Power Generation Concentrating & Non-Concentrating Collector So, what is the function of a collector? Collector is an arrangement which can absorb the solar radiated heat and transfer this heat into another conducting medium like water. Also, this type of collector should be such that radiated sun-ray on it may fall strongly. Depending upon purpose, this sun-ray may be made concentrated or not. Direct sun-ray falling on a plate is non-concentrating. These are 3 types: Solar Pond: A solar pond is a collection and storage device of solar energy. In case of an ordinary pond or lake, when the sun-ray heat up the water, this heated water, being lighter, rises to the upper surface and loses its heat to the atmosphere. The net result is that the pond water remains at nearly atmospheric temperature. But, the solar pond technology inhibits these phenomena by dissolving salt into the bottom layer of this pond, making it too heavy to rise to the surface, even when hot. The http://www.ohio.edu/mechanical/thermo/Intro/Chapt.1_6/steamplant/hybridsolarpond.gif pond is covered with a transparent interlaced nylon cloth and its surfaces are painted black. The salt concentration increases with depth, thereby forming a salinity gradient. The sunlight which reaches the bottom of the pond remains entrapped there (100oC). Now, there is a temperature difference between upper and lower portion of the pond. This heated energy is transferred to water and by superheater, the temperature is risen up to 250oC, then it is directly feed into Turbine, now by coupling with a small generator, electrical power is generated at a rate of 5W/m2. Investment cost is approximately 2,000 €/kW. pg. 3 Technologies of Solar Thermal Power Generation Flat Plate Collector : It is basically a heat absorber from direct sunlight. But here are some differences with other heat absorber that is possibility of heat radiation loss is almost less here. The main components of Flat plate collector are absorber, Transparent glass Cover, a frame, and insulation. For that transparent cover, Sun-rays come and fall upon the absorber and produces heat inside the frame (Greenhouse Effect). The insulation at the bottom prevents the heat radiation to the outer body. In Flat Plate collector : Weiss, Werner, and Matthias Rommel. Process Heat Collectors. Vol. 33, 2008. the diagram, the amount of heat absorbing and heat loss is shown. Due to convection, Reflection, Radiation, and Conduction the total amount of heat loss is near about 40%, Remaining 60% heat is absorbed into the absorber and this heat is collected for further use. If there is an arrangement of inlet and outlet water or Molten-Salt system attach with this absorber, heat may be transferred to another medium through the liquid. Some absorbing tubes are kept at the place of absorber and heat carrying fluid is made to flow through it. Here are some selective coatings on these tubes which can absorb the maximum amount of heat and then transfer into the working liquid. pg. 4 Technologies of Solar Thermal Power Generation Installation and Cost : So, it should be installed at such a place that solar radiation may heat up as much as possible. So in practice, the device is kept south facing (in India) inclined at an angle Latitude of the place +/- 15o. So that the sun-rays can heat up this collector and useful energy is taken out. A Hot storage tank may be there to store the hot water. Flat-plate collectors usually range in size from 24 square feet (3 x 8 ft.) to 48 square feet (4 x 12 ft.) or more. Which can produce such heat that water temperature may get risen from 30oC to 70-80oC. Depending upon the demand, a group of collectors can be installed on rooftop. The installation cost is few less for such collectors. Evacuated Tube: Evacuated tube collectors are introduced to the market in the 1970's. It is a basically heat pipe surrounded by a glass tube that is under a vacuum. The glass tube actually consists of two walls of glass. In between the two walls, all the air is removed, resulting in a vacuum in the same manner as a clear Thermos bottle would function. This vacuum is the best insulation and gives the evacuated tubes a much better heat retention than air space. The heat pipe is also pressurized, allowing the liquid (usually water) to boil very rapidly (100oC). As the water boils, it carries the collected heat to the top of the collector, where the heat is then collected by water or heat transfer liquid that flows around the top of the heat pipe, and then transferred to a storage tank or elsewhere in the system via Copper Header. There is also the glass insulation system to prevent heat loss. The header has an Al casing over it. pg. 5 Technologies of Solar Thermal Power Generation Installation and cost : Evacuated tubes can cost around 20% to 40% more to buy than flat panel collectors. However when comparing price one should consider cost per BTU capacity, and consider year round performance. In cool climates, evacuated tube collectors will have a lower cost per BTU. Physical Size, inclination and facing is just the same with that of flat plate collector. Industrial Use: The above mentioned techniques are to collect the heat from the solar radiation. Now this energy should be handling in a proper way to use it. The heat transfer medium is the water or Molten-Salt (better) and it release its heat energy to a heat exchanger medium (a common-water filled tank). The hot water moves up. A conventional boiler is there to back up heat supply, The heated water is now ready for application at proper place. A unit is there for proper temperature control. Solar Concentrating Collector: The concentrating collector is something that can increase the intensity of solar radiation by making concentrate radiation. A small magnifying glass can concentrate the sun-rays which can burn a paper. That can be also done by concentrating concave reflector, The idea was first Concentration using magnifying glass, Concave reflector Parabolic Dish collector discovered by Archimedes in 214-212 BC. According to his calculation a big reflector was made to burn the Roman ship in few seconds. These concentration technologies are also used in DishAntenna etc. pg. 6 Technologies of Solar Thermal Power Generation An important thing is that this collector may be different due to their mechanical structure and orientation. These are mainly 3 types- some types of collector does not require Tracking, another one is required Solar Tracking-it may be single axis or two axis type. Actually this Tracking means the axis of the reflector should be such that the aperture area always faces the sun, for this case, there is no cosine loss. Solar Tracking System The Solar Tracker is a servomotor controlling device. It is pre-programmed machine about what the position should be there for a particular date-time for a particular location, which can move the orientation of reflector with the sun so that radiation can concentrate at the position of collector. Some heat exchanging liquid is there to collect the heat and transform it. The concentration ratio is the ratio of Aperture area of reflector to the receiver area of receiver. We see, the receiver area is too low than the aperture area, that’s why the radiation from there is less comparative to Flat plate or evacuated tube type collector. So the efficiency is much more than previous collectors. These are ideal for Electricity power generation (Thermal power plant). 2-Dimensional and 3-Dimensional collectors: If it is a Parabolic Dish type, then obviously a full 3 dimensional image of Sun is created at the focus point. A small cylindrical chamber is enough to collect the heat. Now, if it is such that the reflector is a liner but its cross section is parabolic type, then it is called Parabolic Trough collector. Its length is high and the image of sun is totally liner and 2 dimensional. So, here is a thin walled narrow heat absorbing pipe with selective coating on it at the focal length. The linear image of sun is fall upon this pipe along the length. Water or Molten Salt is made to flow through this pipe and carry the heat. Also for 3-D system, it must have 2-axis tracking where for 2-D system, it should be single axis tracking. For North-South directional movement, it should change the tracking throughout the whole day but for East-West movement, it requires seasonal tracking change. pg. 7 Technologies of Solar Thermal Power Generation Based on structure, the different types of concentrating collectors are: Compound Parabolic Collector: This is the 2-D system, having two different parabola junctions; the thick lines are the walls (fig.). At an acceptance angle θ, the sun-ray can enter and receiver is heated up. If, the θ is higher, then here is no need of tracking throughout the whole day. Its concentration ratio is lower (13.2) but this CPC geometrical Diagram CPC structure is easy to handle. It should be mounted in East-West at an acceptance angle of 3o, Receiver Pipe be 1.5” GI with ‘Solkot’ Coating. Efficiency is 31% and it can keep the steam flow rate of 34.5kg/hr at 130oC. Investment cost is near about $70/sq. m and if tilt adjustment is connected, it may be $90-$100 per sq. m. Parabolic Trough Collector: It is quite similar to previous one, is a 2-D system, having a parabolic liner reflector with a heat pipe at the focus. Here, many units are installed in a row and column wise, all pipes are connected together to flow hot brine uniformly in a single channel and to make more heated up. At the focus the heat pipe is kept to get the maximum heat (150oc200oC). It is also kept in East-West but it has a single axis tracking. If molten salt is used as hot brine and efficient way, the maximum temperature at outer field may be reach to o 450 C. Installing cost is quite high, around $1200/kW and operation cost is near about $60/kW. pg. 8 Technologies of Solar Thermal Power Generation The parabolic trough Collector is using much around the world. The conventional fuel fired power plant is now running with co-firing process. The collected heat is exchanged into water and it is directly supplied into boiler drum. If there is cloudy, then water heating is possible by this backup system. The following diagrams describes about the co-firing process. The Picture of Such power plant is: A power plant running on parabolic trough collector system pg. 9 Technologies of Solar Thermal Power Generation Liner Fresnel Reflector: This is rectangular in shape, not the curved like previous one, but here many rectangular reflectors are there at different orientation. The main thing is that all mirrors are putting the reflected sun-rays Liner Fresnel Reflectors at a common receiver which is located at the focal line of those reflectors. Working fluid temperature can be risen up to 270oC- 350oC. To increase efficiency, it should need of Single axis tracking. Simple direct water can be used in the absorber, rather than all are equally like parabolic trough. Also, the collectors are kept inside a thermal insulation for better efficiency. For installing, the allover electrical & thermal cost is around $1000/kW. Parabolic Dish: It is a 3-D system with 2 axis tracking. The total radiation of aperture area is concentrated at the focus, an open volumetric concentrator is there to collect the heat, an stirling engine is there to make the liquid equally heated. The amount of heat produced there is so much high (450oC), it can reheat the liquid at 390oC. Efficiency is much more for this type of collectors. The heat is exchanged at exchanger and warm up water to steam for cooking or governing turbogenetor to produce electricity. Total Investment cost is much less. pg. 10 Technologies of Solar Thermal Power Generation Heliostat & Tower system: This is the most intelligent solar thermal system. A central receiver tower is there and reflectors are there over rounding it. All the reflectors reflect the light into an open volumetric receiver on the tower. All the heliostats are centrally controlled with 2-axis tracking system to ensure that the reflected light goes towards the certain target point throughout the day. Molten salt carries the heat energy in a cycle via cold and hot tank or the cold water directly come into receiver to collect heat and to be generated steam. Generated steam is used in the turbine to produce electricity. The direct heated water produces steam to govern the turbine. For an intelligent controlled heliostat, it costs near about $130/sq meter at the time of installation, both investment and operation cost is quite higher but its efficiency is highest among all other. pg. 11 Technologies of Solar Thermal Power Generation Power Generation : The all techniques described above are to produce heated water. Depending upon the system, the temperature of water is different, so people take some arrangements which are profitable.  To attach solar thermal system in all other fossil fuel fired power plant to minimize running cost.  Solar thermal is used as a booster for Coalfired power plant.  STE and PV in a combined system.  The places where solar thermal temperature is low, i.e. not enough for high pressure steam producing, a reheater is attached to get Booster for Power plant the pressurized steam.  To control the frequency of the output of generator, the steam pressure should be controlled which is done by a controller interfaced at heat exchanger.  To maximize the overall efficiency, Cold storage & Hot storage system is introduced to reduce the load of A.C. & Heater respectively.  Excess heat is used for cooking etc. Thus solar thermal system can take an important role to make out world, our nature green. People should be more aware, they should use more of such green energy for our sustainable development. pg. 12 Technologies of Solar Thermal Power Generation References: [1] Craig Turchi, Mark Mehos, Clifford K. Ho and Gregory J. Kolb - Current and Future Costs for Parabolic Trough and Power Tower Systems in the US Market, NREL [2] Dr. Sudhir Panse - Compound parabolic Solar collectors for process heat. [3] Cheryl E. Kennedy - Progress To Develop An Advanced Solar-Selective Coating, NREL [4] Technology RoadmapSolar Thermal Electricity, IEA 2014 edition. [5] Syafaruddin& Takashi Hiyama - Solar Thermal Power Plants, http://www.cs.kumamotou.ac.jp/epslab/APSF [6] Lecture Notes of Dr. Bibek Bandyopadhyay. [7] Lecture Notes of Dr. Raghunath Bhattachariya. [8] Wikipedia - http://en.wikipedia.org [9] Google Images - https://www.google.co.in/imghp [10] http://citeseerx.ist.psu.edu [11] http://www.researchgate.net [12] http://www.tandfonline.com pg. 13