ENERGETIC AND EXERGETIC ANALYSIS OF SOLAR PTC WITH DIFFERENT REFLECTOR MATERIAL

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 1, October 217, pp. 1 8, Article ID: IJMET_8_1_1 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed ENERGETIC AND EXERGETIC ANALYSIS OF SOLAR PTC WITH DIFFERENT REFLECTOR MATERIAL Earnest Vinay Prakash and Ajeet Kumar Rai MED, SIET, Sam Higginbottom Universivty of Agriculture Technology and Sciences, Allahabad, (U.P.), India ABSTRACT An experimental study was conducted to determine the performance of a parabolic trough concentrator. Experiments were performed on two cylindrical parabolic concentrators. Acrylic mirror sheet and stainless steel sheets were used as reflector material, the receiver material for both the PTCs were GI pipe. The experiments have been performed during summer in the premises of SHUATS, Allahabad, India. Thermal efficiencies for both the PTCs were computed and observed that the maximum instantaneous thermal efficiency of 12.44% and 1.66% was observed with stainless steel reflector material and for acrylic mirror sheet reflector. Exergy efficiencies for both the PTCs were computed and it was observed that maximum instantaneous exergy efficiency of concentrator with stainless steel reflector material was 32% higher than that of the concentrator with acrylic mirror sheet reflector. Keywords: Solar PTC, Reflector Material, Energetic, Exergetic Cite this Article: Earnest Vinay Prakash and Ajeet Kumar Rai, Energetic and Exergetic Analysis of Solar PTC with different reflector material, International Journal of Mechanical Engineering and Technology 8(1), 217, pp INTRODUCTION In response to the energy crisis and the subsequent 1-fold increase in oil prices, the awareness to use alternate energy sources, including solar energy, has gained momentum both in industrialized and developing countries. Intensified research and development on renewable energy sources, which followed the energy crisis, resulted in demonstration of the technical feasibility of many alternate energy options. Parabolic trough concentrators (PTCs) are capable of supplying thermal energy over a wide range of temperatures (up to about 31 C), and therefore, they can be used for a variety of applications ranging from electrical generation to industrial hot water and steam production. In this paper, a brief review of the various subsystems of PTC and their performance evaluations are discussed. A parabolic trough solar collector uses an Acrylic mirror sheets or Stainless steel sheet in the shape of a parabolic cylinder to reflect and 1 editor@iaeme.com

2 Earnest Vinay Prakash and Ajeet Kumar Rai concentrate sun radiations towards a receiver tube located at the focus line of the parabolic cylinder. The receiver absorbs the incoming radiations and transforms them into thermal energy, the latter being transported and collected by a fluid medium circulating within the receiver tube. This method of concentrated solar collection has the advantage of high efficiency and low cost, and can be used either for thermal energy collection, for generating electricity. Therefore, it is an important way to exploit solar energy directly. Parabolic trough is the most mature technology for large scale exploitation of solar energy. Several power plants based on this technology have been operational for years, and more are being built. The conversion of solar energy into heat energy, an incident solar radiance is concentrated by concentrating solar collectors. For many applications it is desirable to deliver energy at temperatures higher than those possible with flat-plate collectors. Energy delivery temperatures can be increased by decreasing the area from which heat losses occur. Generated heat is used to heat the thermic fluids such as oils, air or water/steam, acts as heat carrier and as storage media. The hot thermic fluid is used to generated steam or hot gases, which are then used to operate a heat engine. Concentration ratios of this type of concentrator are quite high. Increasing ratios mean increasing temperatures at which energy can be delivered. Maximum energy collection orientation of the concentrator relative to the direction of propagation of beam radiation is needed and sun tracking in some degree, will be required for focusing systems. Various type of collectors are available which has aperture areas from about 1 to 6 m 2 and with widths ranging from 1 to 6 m, Concentration ratios range from 1 to 8, and rim angles from 7 to 12. The absorber tube is either made of stainless steel or copper or iron coated with a heat resistant black paint. The reflecting surface is linear parabolic curved shape. It is fixed on a light-weight structure usually made of aluminium sections. The structure is such that it should not distort significantly due to its own weight and that it should be able to withstand wind loads. An exergy analysis (or second law analysis) has proven to be a powerful tool in the simulation of thermodynamic analyses of energy systems. It has been widely used in the design, simulation and performance evaluation of energy systems. By performing exergy accounting, we are able to draw a map of how the destruction of exergy is distributed over the engineering system of interest. In this way we are able to pinpoint the components and mechanisms (processes) that destroy exergy the most. This is a real advantage in the search for improving efficiency, because it can indicate the possibilities of thermodynamic improvement of the process under consideration and consequently inform us from the start how to allocate engineering effort and resources. In this paper we have applied the exergy analysis on solar parabolic trough collector with GI pipe as a receiver taking into consideration the exergetic content of incident solar radiation. By applying a derived expression for exergy efficiency, exergy destruction and losses were generated and the optimum design and operating conditions were investigated. 2. EXPERIMENTAL SETUP The experimental setup of the parabolic trough solar collector consists of a collector, a storage tank of capacity 3 litres, and a receiver pipe of length 2.13 m with two valves at both ends. The water supply tank is located above the receiver`s pipe level to allow the heating fluid to flow naturally without pumping system. The storage tank is filled from main water supply. The water inlet and outlet temperature of the absorber tube, the ambient temperature, the reflector temperature, the temperatures at inlet/outlet/middle surface of the receiver, the solar radiation intensity and the wind velocity are continually measured during the experiment. The outdoor experimentation was carried out in the month of May and June 216. The testing system is oriented North-South to capture maximum insolation. 2 editor@iaeme.com

3 Energetic and Exergetic Analysis of Solar PTC with different reflector material Thermal Analysis Well, before starting with exergetic analysis it is important to carried out thermal analysis of the system first because it is parametrically associated with the thermal analysis. The collector thermal efficiency is defined as the ratio of the useful energy delivered to the energy incident on the concentrator aperture and is given as: η th = Q u /Q s The Parabolic trough concentrator has an aperture area A a and receives solar radiation at the rate Q s from the sun. The net solar heat transferred Qs is proportional to A a, and the incident solar radiation per unit of concentrator area I b, which varies with geographical position on the earth, the orientation of the concentrator, meteorological conditions and the time of day. The useful heat gain can be written as Q u = mc p (T out -T in ) & Q s = A a I b The Hottel Whillier equation for the actual useful heat gain Q u, of a concentrating solar collector system is given as: Q u = F R A a [ S A r /A a U L (T in -T a )] Where, S is the absorbed flux (ηo I b ), F R is the heat removal factor of the collector and T a is the ambient temperature. The heat removal factor accounts for the temperature gradients in the receiver and allow for the use of inlet fluid temperatures in the energy balance equations. This is convenient when analyzing solar energy systems, since the inlet temperature is usually known. The heat removal factor (F R ) can be given as: F R = mc p /A r U L [ 1 exp(-a r U L F )/mc p ] Where, (F ) is collector efficiency factor and can be given as- F = Uo/U L Where, U L = h w + h r Convective heat transfer is given byh w = Nu a x k a / D ext Where, Nu a can be calculated by using following expression given below. For flow of air across a single tube in an outdoor environment the equations recommended by McAdams (1954) have been modified to give-.6 For,.1< Re a < 1, Nu a =.4+.54x Re a For, 1< Re a < 5, Nu a =.3x Re a Re a = ρvd/μ Radiative heat transfer is given byh r = ϵσ (T r +T a )(T 2 r +T 2 a ) U = (1/ U L + D /h w D i + D ln(d /D i )/2k) -1 The collector flow factor is given as F = F R /F Therefore, thermal efficiency of the collector can be written asη th = Q u / A a I b 3 editor@iaeme.com

4 Earnest Vinay Prakash and Ajeet Kumar Rai Optical Analysis- The optical efficiency (η ) is defined as the amount of radiation absorbed by the absorber tube divided by the amount of direct normal radiation incident on the aperture area. The optical efficiency when the incident radiation is normal to the aperture (θ = o). Optical Efficiency can be given asη = (τα) Exergy Analysis Application of exergy analysis to solar parabolic concentrators helps designers to achieve an optimum design and gives direction to decrease exergy losses. By applying exergy balance on a solar collector, exergy efficiency can be derived and the shares of irreversible factors are defined as well. Exergy balance can generally be expressed as- ΣE in - ΣE out - ΣE loss - ΣE change ΣE des = Inlet exergy rate can be given as- ΣE in = mc p (T in -T a -T a ln T in /T a ) Using Petella s approach ΣE in,p = I b A a η p η p is Petela s efficiency of converting radiation energy (i.e. I b A a ) into work is given asη p= 1-4T a /3T s + 1/3(T a /T s ) 4 Outlet exergy rate can be given as- ΣE out = mc p (T out -T a -T a ln T out /T a ) The gain exergy rate (E gain ) is the exergy that is accumulated by fluid flow through the receiver and is expressed as: ΣE gain = ΣE out - ΣE in And the exergy efficiency is the ratio of the gain exergy to solar radiation exergy expressed as η E = E gain /E in Under steady state conditions, E change =. In order to show which exergy fractions are major, the exergy efficiency should be expressed in terms of lost and destructed exergy. Exergy loss rate is the amount of exergy that a thermodynamic system loses in processes. In the actual sense, it is the exergy leakage rate out to the surroundings due to optical errors and heat transfer to ambient in a solar receiver which is unwanted and be expressed as: E loss = E l,opt + E l,th E l,opt = ( 1- η ) I b A a η p E l,th = U L Ar (T r -Ta) 2 /T r Exergy is destructed while heat is transferred from hot to cold temperatures. There are two heat transfer processes in the receiver that cause exergy rate destruction; 1) heat transfer caused by the temperature difference between the receiver surface and the sun, which is the heat transfer of the solar energy absorbed by the surface of the receiver. 2) heat transfer conduction from outer receiver surface to fluid flow caused by the temperature difference between the receiver surface and the agent fluid. Therefore, the exergy destruction rate due to heat absorption is defined as: E des,abs = Q u T a (1/T r 1/T s ) 4 editor@iaeme.com

5 Energetic and Exergetic Analysis of Solar PTC with different reflector material Where Q u is the useful energy rate added to the fluid flow energy. Considering useful energy rate as η o I b A a, the exergy destruction due to heat absorption is defined as: E des,abs = η o I b A a T a (1/T r 1/T s ) And the exergy destroyed due to heat transfer conduction is given as: E des,cond = mc p T a [ln(t out /T in )-(T out -T in )/T r ] exergy efficiency can be presented as: η E = 1-( E loss - E change - E des )/ E in,r & η E = 1- (E l,opt - E l,th - E des,abs - E des,cond ) η E = 1-{(1-η o ) + U L Ar (T r -Ta) 2 /T r + Q u T a (1/T r 1/T s ) + mc p T a [ln(t out /T in )-(T out -T in )/T r ])} In a more simplified way,exergy efficiency can be expressed asη E = mc p / η o I b A a [(T out -T in -T a lnt out /T in )] 3. RESULTS AND DISCUSSION Numbers of observations were taken on the system in the month of May & June 216 in the campus of SHUATS, Allahabad, Uttar Pradesh, India. Experiments were conducted in the summer seasons of Indian climatic conditions. Observations were taken during the whole month and best value of a particular day is used in the study. 1 SOLAR INTENSITY :1:311:11:312:12:3 1: TIME 1:3 2: 2:3 3: 3:3 4: 4:3 Figure 1 Variation of Solar Intensity with respect to time Figure 1 shows the variation of solar intensity with respect to time of the day. Solar intensities for both the PTC s were same as the both experimental setup was on the same platform. Maximum solar intensity was observed at 1:3 pm of about 76 W/m 2. Fig 2 shows the variation of wind velocity with respect to time and it varies continuously throughout the day. Wind Velocity : 1:3 11: 11:3 12: 12:3 1: 1:3 2: 2:3 3: 3:3 4: 4:3 Time Figure 2 Variation of Wind Velocity with respect to time 5 editor@iaeme.com

6 Earnest Vinay Prakash and Ajeet Kumar Rai TEMPERATURE : 1:3 11: 11:3 12: 12:3 1: 1:3 2: 2:3 3: 3:3 4: 4:3 TIME Tin,acrylic Tout,acrylic Tin,ss Tamb Tout,ss Figure 3 Variation of inlet, outlet and ambient temperature of PTC reflector with acrylic and stainless steel sheet with respect to time Figure 3 shows the variation of temperatures of the fluid at inlet an outlet of the receiver tube for both the PTC s and ambient temperature. The temperature of fluid at outlet is found higher in stainless steel reflector at 1: pm of about 57 C. Figure 4 shows the variation of instantaneous thermal efficiency with respect to time. Result shows the efficiency of stainless steel reflector PTC found maximum at 2: pm and whereas of acrylic mirror sheet reflector was observed at 12:3 pm. Efficiency(%) : 1:3 11: 11:3 12: 12:3 1: 1:3 2: 2:3 3: 3:3 4: 4:3 Time PTC,ss PTC,acrylic Figure 4 Variation of instantaneous thermal efficiency with respect to time Figure 5 shows variation of instantaneous Exergy efficiency with respect to time, the instantaneous exergy efficiency of stainless steel reflector PTC is higher throughout the day. The maximum efficiency was observed at 12: p.m. for stainless steel reflector PTC and that of acrylic mirror sheet was maximum at 12:3 p.m. Exergy Efficiency(%) : 1:3 11: 11:3 12: 12:3 1: 1:3 2: 2:3 3: 3:3 4: 4:3 Time ηe,ss ηe,acrylic Figure 5 Variation of instantaneous Exergy efficiency with respect to time 6 editor@iaeme.com

7 Energetic and Exergetic Analysis of Solar PTC with different reflector material 4. CONCLUSION- From the collected data, the performance of parabolic trough concentrator was studied. It was observed that the maximum instantaneous thermal efficiency of PTC with stainless steel reflector was more than 16% higher than that of the PTC with acrylic mirror sheet reflector. In order to derive the expression for exergy efficiency, exergy balance was applied on solar collector. The method of exergy analysis presented in this paper is well suited for furthering the goal of more effective solar energy use. The maximum instantaneous exergy efficiency of PTC with stainless steel reflector material was.9% whereas it was.68% for PTC with that of acrylic mirror sheet reflector. Nomenclature A Area (m 2 ) Cp Specific heat capacity of the fluid ( K j /Kg k) F collector efficiency factor F collector flow factor F R Heat removal factor I b Incident Solar radiation (W/m2) U L Over all heat loss coefficient (W/m 2 -K) T Temperature (K) Q Heat transfer rate (Watt) D Hydraulic diameter (m) U Loss Coefficients(W/m2-K) hw Convective heat transfer due to wind. REFERENCES- S Absorbed solar radiation (W/m2) Re Reynolds s No. Nu Nusselt No L Collector length (m) W Collector width (m) m Mass flow rate (Kg/sec) Greek symbols α absorptance ɳ Efficiency τ Transmittance (τα) Transmittance- Absoptance σ Stefan-Boltzman constant ɛ emmitance µ Dynamic viscosity (Kg/m-s) k Thermal conductivity ( W/m-k) Subscripts a ambient o outlet i inlet exp exponential p plate, c collector u useful th Thermal s Sun, E Exergy, w Convective r Radiative [1] Bejan, A., Kearny, D. W. and Kreith, F. (1981). Second law analysis and synthesis of solar collector systems. Journal of Solar Energy Engineering, Vol. 13, PP [2] Fujiwara, M. (1983). Exergy analysis for the performance of solar collectors. Journal of Solar Energy Engineering, Vol. 15, PP [3] Scholten, W. B. (1984). Xergy based comparition of solar collectors efficiency curves. Solar Engineering, PP [4] Scholten, W. B. (1985). Xergy performance of solar collectors as determined from standard test data. Solar Engineering, PP [5] Chelghoum, D. E. and Bejan, A. (1985). Second - law analysis with energy storage capability. Journal of Solar Energy Engineering, Vol. 17, PP [6] Suzuki, A., Okamura, H. and Oshida, I. (1986). Application of exergy concept to the analysis of optimum operating conditions of solar heat collectors. Solar Engineering, PP [7] Suzuki, A. (1987). General Theory of Exergy Balance and Application to Solar Collectors, PP [8] Dutta Gupta, K. K. and Saha, S. (199). Energy analysis of solar thermal collectors. Renewable energy and Environment, PP editor@iaeme.com

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