An Overview Solar Thermal Energy Technologies – Part II (High Temperature Applications)

Dr. Ishan Purohit

Renewable Energy Technology and Application (RETA) Energy Environment Division, The Energy and Resource Institute (TERI)

India Habitat Center, CGO Complex, New Delhi – 110 003 Telephone: +91- 011 – 24682100 Fax: +91- 011 - 24682145

E-mail: ishanp@teri.res.in

Abstract India’s future energy requirements are going exponentially and solar energy can be one of the efficient and eco-friendly ways to meet the same. The industrial sector of the country contributes second largest share of its total GDP and consumes maximum energy and electricity. In order to maintain high GDP and growth rate the energy sector should grow exponentially similar as the demand. There is accountable gap (9-10%) between demand and supply in the country which is increasing and expected to grow up to 15 % in 2015. Therefore a large target of electricity production and huge investment is required in power sector in order to provide power for all up to 2020. India’s present electricity installed capacity is 135 401.63MW. There is peak power shortage of about 10 % and overall power shortage of 7.5%. Presently, the 11th five year plan target is to add 100000 MW by 2012 and Ministry of New and Renewable Energy (MNRE) has set up target to add 14500 MW by 2012 from new and renewable energy resources out of which 50 MW would be from solar energy. The Integrated Energy Policy of India envisages electricity generation installed capacity of 800000 MW by 2030 and a substantial contribution would be from renewable energy; which indicates that In order to attract the industries and reduce the unit cost of electricity, the new policy for solar electricity has been announced by MNRE. High temperature solar thermal technologies and their applications for various industrial applications and production of electricity have been highlighted in the present study. An overview of concentrator solar power CSP technologies and various solar thermal power generating systems (STPG) have been presented in detail along with their advantages and limitations along with barriers and their worldwide status of installed capacity. Key Words: Concentrating Solar Power (CSP), Concentrating Solar Collectors, Industrial process heat, Solar Thermal Power Generation (STPG)

1. Introduction Limited fossil resources and environmental problems require new sustainable energy supply options that use renewable energies and are economic at the same time. Global environment problem is one of urgent items in the world. India is located in the equatorial sun belt of the earth, thereby receiving abundant radiant energy from the sun. In most parts of the country, clear sunny weather is experienced 250 to 300 days a year. The annual global radiation varies from 1600 to 2200 kWh/m2, which is comparable with radiation received in the tropical and sub-tropical regions. The equivalent energy potential is about 6,000 million GWh of energy per year. It can be observed that although the highest annual global radiation is received in Rajasthan, northern Gujarat and parts of Ladakh region, the parts of Andhra Pradesh, Maharashtra, Madhya Pradesh also receive fairly large amount of

radiation as compared to many parts of the world especially Japan, Europe and the US where development and deployment of solar technologies is maximum. The mechanism of generation of heat from solar energy has already been discussed in previous section. Only few of the stationary solar collectors can achieve the temperature higher than 300oC. The heat pipe based evacuated solar energy collector can achieve higher temperature if used along with heat exchanger. When temperature of the absorber increases the heat losses are become dominant and the absorber can heat up only towards stagnation (steady state condition) condition. In addition the stationary collectors comprise high area for heat loss. Therefore these collectors are recommended only for low and medium temperature applications. Thermodynamically it is possible to achieve the temperature of source (i.e. sun) for thermal equilibrium. Higher temperatures (>300oC) are required for industrial process heating and power generation applications. Solar concentrating collector technologies are used for this purpose. 2. Solar Concentrators Technology Solar concentrators can increase the power flux of incident solar radiation hundreds of times. Concentrating solar collectors are high temperature solar energy collectors, which comprises shaped mirrors or concentrators with appropriate tracking mechanism. The parabolic trough collectors (PTC), Linear Fresnel reflectors (LFR), parabolic dish collectors and heliostats collectors are some representative high temperature solar collectors.

Figure 1. Various configurations of solar concentrators

Solar concentrating collectors comprise reflective surfaces to concentrate solar radiation onto a small area, where it is absorbed and converted in to heat. The absorber area is of very small as compared to area of collection, hence heat losses are very low and higher temperatures than the stationary can be achieved. The efficacy of these collectors depends upon beam component of total solar radiation, alignment (E-W or N-S), and tracking mechanism (one axis or two axis) especially.

2.1 Parabolic trough collector A parabolic trough collector (PTC) essentially has a linear parabolic shaped reflector (usually coated silver or polished aluminum) that focus the incident solar radiation on a linear receiver/ absorber located at the focus of parabola (Figure. 2). Parabolic troughs often use single-axis or dual-axis tracking. In order to achieve maximum efficiency of the collector, the trough is usually aligned on a north-south axis which tracks the sun along one axis from east to west during the day to focus maximum incident beam solar radiation along the line. In rare instances, they may be stationary. Due to the parabolic shape of the collector, the trough can focus the incident solar radiation 25 to 100 times (i.e. concentration factor) times its normal numerical value on the receiver pipe (or focal line), achieving average temperature over 400oC. The heated working fluid may be used for medium temperature space or process heat, or to operate a steam turbine for power or electricity generation.

Figure 2. Cross sectional view(s) of Parabolic Trough Collector

Figure 3. Schematic diagram of Linear Fresnel Reflector

2.2 Linear Fresnel Reflector Designing and proper handling of large size parabolic trough concentrating mirror is a major problem associated with PTC technology. The LFR collector technology comprises an array of linear reflectors or mirror strips which concentrate incident solar radiation on to a fixed receiver or absorber mounted on a tower (Figure. 2). These reflecting mirror strips are imagined as a broken-up parabolic trough reflector, but not like the parabolic troughs. The main advantage of this type of system is uses flat or elastically curved reflectors. In addition these concentrators are mounted closer to ground hence minimizing structural requirements. 2.3 Parabolic dish type collectors

A parabolic dish collector is a point focusing concentrator in the form of a dish which reflects incident solar radiation at the focal point. It is similar in appearance to a large satellite dish, but has mirror-like reflectors and an absorber at the focal point. It uses a dual axis tracking system. It is the most powerful type of collector which concentrates sunlight at a single, focal point, via one or more parabolic dishes arranged in a similar fashion to a reflecting telescope focuses starlight, or a dish antenna focuses radio waves. The parabolic dish collector uses an automated tracking mechanism to focus incident solar radiation on receiver located at the focal point in the front of the dish. The receiver may be a heat engine such as Stirling engine, is linked to the receiver.

Figure 4. Schematic diagram(s) of solar Parabolic Dish Collectors

It is possible to achieve more the temperature more than 1500oC due to its very high concentration ratio. Two important phenomenon are associated with comprehend designing of a parabolic dish. First is that the shape of a parabola is defined such that incoming rays which are parallel to the dish's axis

will be reflected toward the focus and the second key is that the light rays from the sun arriving at the earth's surface are almost completely parallel. So if dish can be aligned with its axis pointing at the sun, the incoming radiation will almost all be reflected towards the focal point of the dish.

Figure 5. Schematic diagram of a Heliostat and concept of solar Power Tower 2.4 Solar Power Tower (Heliostats) The central tower receiver system consists of a central stationary receiver to which the incident solar radiation is reflected by small rotating mirrors called heliostats. Heliostat is a device that tracks according the sun which is used to orient a mirror/reflector throughout the day, to reflect incident solar radiation on to target receiver. These mirrors align themselves and focus sunlight on the receiver at the top of tower, collected heat is transferred. The energy can be concentrated as much as 1,500 times that of the energy coming in from the sun. An automated tracking mechanism keeps the mirrors aligned so the reflected rays of the Sun are always aimed at the receiver, where temperatures well above 1500°C can be reached. High-pressure steam is generated to produce electricity. Energy losses from thermal-energy transport are minimized as solar energy is being directly transferred by reflection from the heliostats to a single receiver, rather than being moved through a transfer medium to one central location, as with parabolic troughs.

3. High Temperature Application of Solar Energy Production of hot water and pressurized steam are two major thermal energy applications where solar collectors and concentrators may effectively be use. The thermal performance of unglazed and flat plate collectors effectively reduces when operated under high temperatures. For example FPC may recommended for hot water in domestic application; but in industrial application like pasteurization, sterilization etc. where constant temperature water of the temperature more than 80oC is required, these collectors can not be recommended. Similarly in those systems where pressurized steam is required, various types of concentrators can be use. In the context of India, it has been found that in industries like dairy, lather, foods processing etc., maximum applications need thermal energy; which require moderate temperatures.

3.1 Industrial Process heat On the basis of a detailed literature review it has bee outlined that in medium and small industries the required temperatures for processing is up to 400oC. In some industries like dairy etc. the thermal energy processes consumes more then 70 % of total energy. In these sectors the medium and high temperature solar thermal technologies may work effectively.

Table 1. Various Industrial Processes and heat requirement

3.2 Electricity Generation Solar Thermal Power systems, also known as Concentrating Solar Power systems, use concentrated solar radiation as a high temperature energy source to produce electricity using thermal route. Since the average operating temperature of stationary non-concentrating collectors is low (max up to 1200C)

as compared to the desirable input temperatures of heat engines (above 3000C), the concentrating collectors are used for such applications. These technologies are appropriate for applications where direct solar radiation is high.�The mechanism of conversion of solar to electricity is fundamentally similar to the traditional thermal power plants except use of solar energy as source of heat.

In the basic process of conversion of solar into heat energy, an incident solar irradiance is collected and concentrated by concentrating solar collectors or mirrors, and generated heat is used to heat the thermic fluids such as heat transfer oils, air or water/steam, depending on the plant design, acts as heat carrier and/or as storage media. The hot thermic fluid is used to generated steam or hot gases, which are then used to operate a heat engine. In these systems, the efficiency of the collector reduces marginally as its operating temperature increases, whereas the efficiency of the heat engine increases with the increase in its operating temperature. 3.2.1 Parabolic trough collector system Parabolic trough power plants are line-focusing solar thermal power plants. PTC systems use the mirrored surface of a linear parabolic concentrator to focus direct solar radiation on an absorber pipe running along the focal line of the parabola. The heat transfer fluid inside the absorber pipe is heated and pumped to the steam generator, which, in turn, is connected to a steam turbine. A natural gas burner is normally used to produce steam at times of insufficient insolation. The collectors rotate about horizontal north–south axes, an arrangement which results in slightly less energy incident on them over the year but favors summertime operation when peak power is needed.

Figure 6. Schematic diagram of PTC based solar thermal power plant

3.2.2 Power tower system

In power tower systems, heliostats reflect and concentrate sunlight onto a central tower-mounted receiver where the energy is transferred to a heat transfer fluid. This energy is then passed either to the storage or to power-conversion systems, which convert the thermal energy into electricity. Heliostat field, the heliostat controls, the receiver, the storage system, and the heat engine, which drives the generator, are the major components of the system. These plants are defined by the options chosen for a HTF, for the thermal storage medium and for the power-conversion cycle. HTF may be water/steam,

molten nitrate salt, liquid metals or air and the thermal storage may be provided by PCM (phase change materials). Power tower systems usually achieves concentration ratios of 300–1500, can operate at temperatures up to 1500o C.

Figure 7. Schematic diagram of solar Power Tower

3.2.3 Parabolic dish system

The parabolic dish system uses a parabolic dish shaped mirror or a modular mirror system that approximates a parabola and incorporates two-axis tracking to focus the sunlight onto receivers located at the focal point of the dish, which absorbs the energy and converts it into thermal energy. This can be used directly as heat for thermal application or for power generation. The thermal energy can either be transported to a central generator for conversion, or it can be converted directly into electricity at a local generator coupled to the receiver.

Figure 8. Schematic of Parabolic dish system 3.2.4 Solar chimney

The solar chimney has a tall chimney at the center of the field, which is covered with glass. The solar heat generates hot air in the gap between the ground and the gall cover which is then passed through

the central tower to its upper end due to density difference between relatively cooler air outside the upper end of the tower and hotter air inside tower. While traveling up this air drives wind turbines located inside the tower. These systems need relatively less components and were supposed to be cheaper. However, low operating efficiency, and need for a tall tower of height of the order of 1000m made this technology a challenging one.

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Figure 9. Solar chimney pilot project

Worldwide research and development work is going on towards development and dissemination of solar thermal electricity. Representative models of solar thermal power plants have been summarized in Appendix –I. 4. Barriers

The high temperature solar thermal energy technologies need detailed feasibility study and technology

identification along with proper solar radiation resource assessment. In India, the concentrator

technology is not the state of maturity. There is very less number of industries manufacturing high

temperature evacuated tubes. The current status of international technology and its availability and

financial and commercial feasibility in the context of India is not clear. In addition general awareness

among the public sector and private sector towards solar thermal is quite low.

5. Way ahead

Industrial process heat production and solar thermal power generation technology is coming back as

commercially viable technology in many parts of the world. The feasibility study of solar thermal

technologies for industrial processes and electricity generation is essential for proper technological

identification and its dissemination. India needs to take fresh initiative to assess the latest technology

and its feasibility in the Indian context. These projects can avail benefits like CDM and considering

the solar radiation levels in India the technology can be commercially viable.

6. Conclusion Resource assessment, technological appropriateness, financial economic feasibility and environmental

sustainability are some of the basic requirement of project evaluation. It is well appreciated that the

solar radiation is available sufficiently over the country. It is also a positive consideration associated

with the dissemination of solar thermal power generation in India that the locations of high solar

intensity are almost dessert and waste lands; hence best suitable for the application. It has been

observed that in if a small fraction of energy required in small and medium industries is fulfilled by

solar energy, a large amount of electricity might be conserve along with significant GHG reduction.

The Solar thermal power is the most cost effective renewable power technologies and lowest cost

solar electricity in the world. The power generation costs are expected to be in the range of $0.075

kWh to $0.19 kWh; promising cost-competitiveness with fossil fuel plants. Process heating in

industrial sector and solar energy based power generating systems can play a major role towards the

fulfillment of energy requirements of industry and commercial sectors of the country.

Bibliography

1. Annual Report, Ministry of New and Renewable Energy Sources, 2005-06.

2. Beerbaum B. and G. Weinrebe Solar thermal power generation in India: a techno-economic

analysis, Renewable Energy, 21, 2, 1 2000, 153-174.

3. Duffie J.A., Beckman W.A. Solar engineering of thermal processes, New York: Wiley; 1991.

4. Garud S. and I. Purohit (2007), Making solar thermal power generation in India a reality,

Overview of technologies, Opportunities and Challenges, Proceedings of All India seminar on

‘India’s energy independence strategies & strides, 26-27th October, Institute of Engineers,

Hydrabad, India.

5. http//en.wikipedia.org/wiki/Solar_Thermal_Energy

6. Kalogirou S. A., Solar thermal collectors and applications, Progress in Energy and Combustion

Science, 30, 3, 2004, 231-295.

7. Kreith F, Kreider J.F. Principles of solar engineering, New York: McGraw-Hill; 1978.

8. National Renewable Energy Laboratory (USDOE), USA

9. Renewable Energy: RD&D Priorities, Insights from IEA Technology Programmes,

International Energy Agency, France, 2006.

10. Status Report on Solar Thermal Power Plants by Pilkington Solar International, Germany

11. Winter C. J., R. L. Sizmann and L.L. Vant-Hull, Solar Power Plants: Fundaments, Technology

and System Economics, Springer-Verlag, New York, USA.

Annexure – I

List of solar thermal power stations

Operational

• Solar Energy Generating Systems, USA Mojave desert California, total of 354MW, parabolic trough design

• Nevada Solar One, USA Nevada, 64MW, parabolic trough design • Liddell Power Station, Australia, 95MW heat, 35MW electrical equivalent as steam input for

convential power station, Fresnel reflector design\ • PS10 solar power tower, Spain Seville, 11MW, power tower design

Under construction

• Andasol 1 solar power station, Spain, 50MW with heat storage, parabolic trough design • Andasol 2 solar power station, Spain, 50MW with heat storage, parabolic trough design • Solar Tres Power Tower, Spain, 15MW with heat storage, power tower design

Announced

• Mojave Solar Park, USA California, 553MW, parabolic trough design • Pisgah, USA California near Pisgah north of I-40, 500MW, dish design

• Ivanpah Solar, USA California, 400MW, power tower design • Unnamed, USA Florida, 300MW, Fresnel reflector design • Imperial Valley, USA California, 300MW, dish design • Carrizo Energy Solar Farm, USA California near San Luis Obispo, 177MW, Fresnel reflector

design

• Uppington, South Africa, 100MW, power tower design • Yazd Plant, Iran, 67MW steam input for hybrid gas plant, technology unknown • Barswtow, USA California, 59MW with heat storage and back-up, parabolic trough design • Victorville 2 Hybrid Power Project, 50MW steam input for hybrid gas plant, parabolic trough

design

• Kuraymat Plant, Egypt, 40MW steam input for a gas powered plant, parabolic trough design • Beni Mathar Plant, Morocco, 30MW steam input for a gas powered plant, technology unknown • Hassi R'mel, Algeria, 25MW steam input for gas powered plant, parabolic trough design

• Cloncurry solar power station, Australia, 10MW with heat storage, power tower design

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Bibliography 1. Annual Report, Ministry of New and Renewable Energy Sources, 2005-06. 2. Beerbaum B. and G. Weinrebe Solar thermal power generation in India: a techno-economic

analysis, Renewable Energy, 21, 2, 1 2000, 153-174. 3. Duffie J.A., Beckman W.A. Solar engineering of thermal processes. New York: Wiley; 1991. 4. Kalogirou S. A., Solar thermal collectors and applications, Progress in Energy and Combustion

Science, 30, 3, 2004, 231-295. 5. Kreith F, Kreider J.F. Principles of solar engineering, New York: McGraw-Hill; 1978. 6. Winter C. J., R. L. Sizmann and L.L. Vant-Hull, Solar Power Plants: Fundaments, Technology

and System Economics, Springer-Verlag, New York, USA. 7. Status Report on Solar Thermal Power Plants by Pilkington Solar International, Germany 8. National Renewable Energy Laboratory (USDOE), USA 9. http//en.wikipedia.org/wiki/Solar_Thermal_Energy 10. www.volker-quaschning.de 11. www.mnre.nic.in 12. www.wikipedia.org