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  • 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 4, Issue 6, November - December (2013), pp. 84-90 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2013): 5.7731 (Calculated by GISI) www.jifactor.com IJMET ©IAEME THERMAL LOSSES IN SOLAR CENTRAL RECEIVER Mrs. Jadhav Sandhya Dilipa, a Dr. V. Venkatrajb Ph.D.Research Scholar in Bharati Vidyapeeth Deemed University College of Engineering, Pune, Maharashtra, India. b Former Director, Health Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India. ABSTRACT Concentrated solar power is considered the most large scale power renewable generation. India has good solar insolation in almost all parts of the country. Its equivalent energy potential is about 6,000 million GWh of energy per year. In this research, the net power generated is obtained as the variation in incident solar radiation throughout the year. The main objective of this paper is to predict the individual components of total thermal losses in the external receiver of a central receiver solar thermal power plant. The conduction losses are negligible whereas the convection losses and radiation losses have significant values and affect the efficiency of the receiver. Jodhpur being the place with highest value of solar radiation in India, weather conditions of Jodhpur is considered as input for simulation. Results obtained from the tool developed to simulate and predict the performance of receiver under different values of incident flux are given. Keywords: Central Receiver System, Receiver, Thermal Losses, Simulation. INTRODUCTION One aspect of the utilization of solar energy is to convert solar energy into electrical energy with the help of solar thermal power plants. Although the solar radiation is a high quality energy source because of the high temperature and energy at its source, its power density at the earth’s surface makes it difficult to extract work and achieve reasonable temperatures in common working fluid. Of all the technologies being developed for solar thermal power generation, plants based on Central Receiver System (CRS) as shown in Fig. 1 are able to work at the highest temperatures and to achieve higher efficiencies in electricity production (Romero et al., 2002). 84
  • 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME Central Receiver System as shown in Fig. 2 include a field of mirrors called heliostats, a receiver mounted on a tower, energy storage tank, heat exchanger, blowers, valves and power block (turbine, electric generator, condenser etc.). The behavior of the CRSTPP with these various components is complex (Yebra L. J. et al. 2005). Also the CRSTPP requires a large space for installation and the components are costly. To carry out experimentation, it becomes costly and time consuming. Therefore to reduce the effort and time required, theoretical modeling and simulation are resorted to. Of all the components of the system, receiver plays a major role therefore a model is developed for its analysis. Fig.1 Solar Two - Central Receiver Solar thermal Power Plant (Sandia National Laboratories, USA) Fig.2 Working of Central Receiver Solar thermal Power Plant SOLAR RECEIVERS The receiver, placed at the top of a tower, is located at a point where reflected energy from the heliostats can be intercepted most efficiently. The receiver absorbs the energy being reflected from the heliostat field and transfers it into a heat transfer fluid. The value of the heat flux can range from 100 to 1000 kW/ m2 in high temperature, high thermal gradients & high stresses in the receiver. The value depends on the concentration ratio & varies everyday and with the season. It also varies over the surface of the receiver. The receiver absorbs solar energy and uses it to heat a working fluid, which can be air, water, molten salt, or liquid metal. This heated fluid is used to provide process heat or to generate electric power in a thermal power plant. Out of the different types of receiver used in solar thermal power plant, the most commonly used are the external receiver and cavity receiver. External Receivers External receivers as shown in figure 3(a) normally consist of panels of many small (20-56 mm) vertical tubes welded side by side to approximate a cylinder. The bottoms and tops of the vertical tubes are connected to headers that supply heat transfer fluid to the bottom of each tube and collect the heated fluid from the top of the tubes. Cavity Receivers Cavity receiver as shown in figure 3(b) allows concentrated solar radiation through an aperture. Inside the cavity, the flux diverges and reaches the heat exchange panels. In contrast, concentrated solar radiation impinges directly onto external receiver panels. 85
  • 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME (a) External Receiver (b) Cavity receiver Fig. 3 Receivers in a Solar Thermal Power Plant RECEIVER LOSSES In Solar thermal power plants, heat loss can significantly reduce the efficiency and consequently the cost effectiveness of the system. It is therefore vital to fully understand the nature of these heat loss mechanisms. To calculate receiver efficiency, predictions are required for energy losses from – Reflection of incoming solar radiation. Radiation from heated surfaces. Heat conduction into the structure that supports the receiver Convection (free and forced) Receiver thermal loss rate defines the operating threshold for the system. The system operates only when the sun’s energy is sufficient to overcome the receiver heat loss. These losses depend on the design of the receiver, whether it is a cavity or external receiver, its heated (or aperture) area, and its operating temperature. Additional factors include the local wind velocity, ambient temperature, and the orientation of the receiver. The different thermal losses in a receiver are as follows: Radiation Loss The important energy losses for the receiver originates from radiation heat transfer to the surroundings and is primarily function of the size of the receiver and the operating temperature of the system. Radiation losses can be easily measured with infrared thermography measurements. Convection Loss The complexity of the temperature and velocity fields in and around the receiver makes it difficult to determine the convection losses of the receiver. Till 1979, information to estimate convective losses almost didn’t exist. The surface temperature of a receiver is high in relation to ambient air temperature, so property variations in the air near the receiver surface from large temperature gradients have significant effects on the convective heat transfer process. 86
  • 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME In 1985, John Krabel and Dennis Siebers initiated the Central Receiver Convection Energy Loss program, a research effort to establish a convective heat loss data base, involving a number of universities and private firms. The convective heat loss depends on the ability to transfer energy by the combined influences of flow due to the wind and the natural induced flow due to the cold external air. Absorptance Loss Absorptance loss is only a function of the type of coating on the absorbing surface. Many current designs use a high-absorptance paint commercially marketed as Pyromark®. This paint is formulated for high temperature surfaces and has an absorptance of approximately 0.95. If the absorbing surface is inside a cavity, the effective absorptance (based on reflection back through the cavity aperture) increases to about 0.98. Conduction loss This receiver heat loss term represents the heat conducted away from the receiver. Most of this heat is lost through the receiver supporting brackets that connect the receiver to the tower structure. This is normally a small fraction of the total receiver heat loss and is kept small by minimizing the number and size of receiver attach points and using low thermal conductance metals such as stainless steels in their construction. Conductive losses are found to be less than 5%. MATHEMATICAL MODELLING Mathematical models of the real processes of solar central receiver power plant cannot take all aspects into consideration; therefore simplifying assumptions are required for modeling. Thus models are approximations of reality. The work presented in this paper is focused on central receiver simulation based on energy balance. The control system design and its simulation related with thermal and electrical transient process are not considered here. So the individual thermal losses including conduction losses, radiation losses, convection losses and reflection losses need to be evaluated in the receiver simulation model. The radiation and convection losses account most of the receiver thermal losses resulting from calculation (Zhihao Y. et al., 2009). The net power of receiver that is PN can be determined by calculating the losses. PN = Pa – (PRL + PCVL + PCDL) where, Pa is the power absorbed by the receiver which depends upon the absorptivity of the receiver material and the incident flux, PRL is the power lost by radiation, PCVL power lost due to convection, PCDL power lost by conduction. The conduction losses are very small so they are neglected in the calculations. Simplification is made to estimate radiation losses from the central receiver: ( where, Stephan Boltzmans constant σ = 5.67 x 10-8 W/(m2 K4); ε is the emissivity and assumed to be 0.88 ; A is the receiver area. 87
  • 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME In calculation of convection losses, Siebers and Kraabel method of analysis is followed. The correlations used are: PCVL = The convective heat transfer coefficient h, is combination of natural and forced convection. h = (hforced3.2 + hnat3.2)1/3.2 TR is the receiver surface temperature in K and Ta is the ambient air temperature in K. Efficiency of receiver ηrec, is the ratio of net power absorbed by the receiver to the total power absorbed by the receiver. ηrec = PN/Pa SIMULATION & ANALYSIS Simulation of the external central receiver is done by using the above mathematical model (Pierre G. et al., 2007) based on C++ and the behavior of different losses taking place in the receiver under varying conditions such as radiation and weather condition is studied. India is located in the equatorial sun belt of the earth, thereby receiving abundant energy from the sun. The India Meteorological Department maintains a nationwide network of radiation stations, which measure solar radiation and also the daily duration sunshine. In most parts of India, clear sunny weather is experienced 250 to 300 days a year. The annual global radiation varies from 1600 to 2200 kWh/m2. In India solar radiation levels shows the highest annual radiation received in Rajasthan, northern Gujrat 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 solar radiation (beam, diffuse, daily normal insolation) values are available at different locations from the handbook of solar radiation data for India and at 23 sites from an Indian Meteorological Department (IMD) MNRE report. The data of Meteorological Department reveals that in India, Jodhpur receives the maximum amount of solar radiation throughout the year. Therefore the radiation and weather condition data for Jodhpur is considered for simulation and analysis. The heat transfer medium used is air. The results are obtained for varying solar flux radiation from January to December, corresponding ambient temperature and wind velocity. The variation in the incident solar flux varied the mean temperature of the receiver therefore the fluid properties like thermal coefficient of expansion, viscosity and thermal conductivity are considered for varying mean temperature of the receiver. The dimensions of the receiver are kept constant. The values of convection loss, radiation loss, total loss, and net power generated are plotted against varying flux which are as shown in Fig. 4a to 4d respectively. Fig. 5 shows the variation in receiver efficiency with varying flux. The heat transfer coefficient obtained for different wind velocities are plotted in Fig. 6. 88
  • 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME (a) (b) (c) (d) Fig. 4: Variation in Receiver parameters with Varying Flux Fig. 5: Variation in receiver Efficiency with varying Flux Fig. 6: Variation in Heat transfer coefficient with wind velocity It is seen that the convection losses vary considerably with the increase in incident flux. Its variation also depends upon on change in wind velocity and ambient temperature. It has been seen that due to variation in flux intensity the receiver temperature varies and hence the mean temperature which results in variation of the working fluid properties. Radiation loss is a factor of difference in temperature of the receiver and ambient temperature. Therefore it is found that with the increase in temperature difference which is the effect of increasing flux, the radiation loss also increases. 89
  • 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME Heat transfer coefficient is combination of heat transfer coefficient for free convection and forced convection. The heat transfer coefficient varies with the wind velocities as Reynold number and Nusselt Number depend on wind velocity. Due to the variation in different losses the total loss in the receiver increases with increase in flux intensity. As the flux intensity increases, the net power generated by the receiver increase but as there is increase in losses and net power generated the overall efficiency of the receiver is almost constant and is within a range of 78% to 82%. CONCLUSION The present research was carried out with the intension to predict the individual components of total thermal losses in the receiver. Simulation of the external central receiver is done using C++ and Jodhpur being the place with highest value of solar radiation in India, weather conditions of Jodhpur is considered as input for simulation. The results are obtained under different varying values of incident flux throughout the year and it is found that the conduction losses are negligible whereas the convection losses and radiation losses have significant values and affect the efficiency of the receiver. There is considerable variation in heat transfer coefficient due to variation in wind velocity. Similarly the simulation can be done for other fluids as heat transfer medium which can sustain high temperatures like water, liquid sodium, molten salt etc. REFERENCES 1. Romero, M., R. Buck, J. Pacheco, May 2002, An Update on Solar Central Receiver Systems, Projects, and Technologies. ASME Journal of Solar Energy Engineering Vol.124, pp. 98-108. 2. Yebra, L. J., M. Berenguel, S. Dorando and M. Romero, 2005, Modelling and Simulation of Central Receiver Solar Thermal Power Plants, Proceedings of the 44th IEEE Conference on Decision and Control and the European Control Conference, Spain Dec 12-15,. 3. Krabel John, Siebers Dennis, 1985, Sandia National Labortories Report, New experiments on convection heat losses, pp20-27. 4. Pierre G., Ferriere A., Bezian J., 2007, Codes for solar flux calculation dedicated to central receiver system applications: A comparative review, Solar Energy 82, Aug 11 2007, pp – 189-197. 5. Zhihao Y., Zhifeng W., Zhenwu L., Xiuudong W., 2009, Modelling and simulation of the pioneer 1 MW solar thermal central receiver system in China, Renewable Energy 34, March 2009, pp 2437-2446. 6. Carasso M. and Becker M., 1990, Solar Thermal Central Receiver Systems, Vol 3: Performance Evaluation Standards for Solar Central Receivers, Springer Verlag. 7. Winter C. J., R. L. Sizmann, L. L. Vant, - Hull (Eds.) 1991, Solar Power Plants – Fundamentals, Technology, Systems, Economics (New York: Springer – Verlag). 8. Manjinder Bajwa and Piyush Gulati, “Comparing the Thermal Power Plant Performance at Various Output Loads by Energy Auditing (A Statistical Analyzing Tool)”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 2, Issue 2, 2011, pp. 111 - 126, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 9. Ajeet Kumar Rai, Vivek Sachan and Bhawani Nandan, “Experimental Study of Evaporation in a Tubular Solar Still”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 2, 2013, pp. 1 - 9, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 10. Anirban Sur and Dr.Randip.K.Das, “Review on Solar Adsorption Refrigeration Cycle”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 1, Issue 1, 2010, pp. 190 - 226, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 90