- Email: [email protected]
Experimental investigation and computational validation of heat losses from the cavity receiver used in linear Fresnel reflector solar thermal system
Renewable Energy 55 (2013) 18e23
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental investigation and computational validation of heat losses from the cavity receiver used in linear Fresnel reflector solar thermal system Sudhansu S. Sahoo a, Shinu M. Varghese b, C. Suresh Kumar b, S.P. Viswanathan b, Suneet Singh c, *, Rangan Banerjee c a b c
Department of Mechanical Engineering, College of Engineering and Technology, Biju Patnaik University of Technology, Bhubaneswar, India KG Design Services Pvt. Ltd., Coimbatore, India Department of Energy Science & Engineering, IIT Bombay, Mumbai, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2012 Accepted 28 November 2012 Available online 13 January 2013
This paper presents the analysis of heat losses from the trapezoidal cavity receiver used in linear Fresnel reflector (LFR) system. The experimental studies are conducted under laboratory conditions that are specially designed for this purpose. The effects of parameters such as the temperatures of the tube surface, depth of receiver, number of tubes, and emissivity of tubes are investigated. The loss of heat is taking place from the tube outer surface to glass cover, below the receiver and then glass cover to ambient. As part of this investigation, the system is modelled and simulated using computational fluid dynamics (CFD). After validation, contribution of convection and radiation to the total heat transfer are found out using CFD. Computational predictions are shown to be consistent with the experimental observations which show that the CFD model is a reliable tool for predicting heat loss and overall heat loss coefficient. It was found that losses due to convection are between 5 and 18% of the total heat losses. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: LFR Solar thermal CFD Overall heat loss coefficient
1. Introduction Linear Fresnel Reflector technology relies on an array of linear mirror strips which concentrates light on a fixed receiver mounted on a linear tower (Fig. 1). The receiver is a stationary linear cavity, usually trapezoidal, consisting of single or multiple numbers of tubes. The inside of the cavity, external to the tubes, contains air which is not in contact with the ambient. The water, running through the tubes inside the cavity, absorbs heat from the cavity and generates steam inside the tubes. In operation, the absorber tubes in the trapezoidal cavity get heated due to the incident concentrated solar radiation. As it does so, it emits long wavelength radiation into the cavity. This radiation results in heat loss from the tubes. The emitted radiation is absorbed by inner cavity walls and glass cover at the bottom, which in turn raises their temperature. The resulting temperature gradients promote natural convection within the cavity, which lead to convective losses from the tubes. Conduction of heat away from the inner surfaces represents the third mode of heat loss. The cavity receiver heat loss processes
* Corresponding author. Tel.: þ91 9321058464; fax: þ91 22 25764890. E-mail address: [email protected] (S. Singh). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.11.036
involve radiative, convective and conductive heat transfer, and interaction of these three modes makes it difficult to develop an analytical model. Hence, computational and experimental approaches can be used to predict and verify heat losses. Pye et al. [1] carried out a study of losses in a trapezoidal cavity. They used an analytical model for a trapezoidal cavity and found that radiation accounts for 90% of the heat loss from the top surface. Again using CFD analysis of the cavity, he mentioned losses due to natural convection and radiation. However, for CFD analysis, tubes were modelled as an isothermal plane surface. Reynolds et al. [2] carried out experimental and computational study of heat loss characteristics of trapezoidal cavity. They used flow visualization technique to capture the heat flow patterns within the trapezoidal cavity with a hot plate to investigate the heat losses from the absorber tube. In computational study, flow in the cavity was assumed to be laminar. They found a reasonable agreement between experimentally observed flow patterns and those predicted by computational model. CFD prediction of heat loss was found 40% less compared with experimental results. Uncertainties in the experimental work were mentioned as the reason for this difference. Singh et al. [3] experimentally studied the thermal performance of the Fresnel reflecting concentrator with trapezoidal cavity at
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
19
2. Physical problem The proposed linear Fresnel reflector system under consideration consists of a trapezoidal cavity receiver made of steel with black painted surface (Fig. 2) filled with air and it houses eight parallel pipes having Nominal Pipe Size 1 (33.4 mm outer diameter) which are made of SS304 material. The pipe is coated with black nickel selective surface by the process of electroplating. Tubes are placed below the inner top surface of the cavity. A gap is provided between each of the tubes as allowance for thermal expansion. At bottom portion plane glass is provided to allow entry of solar radiation. Outer cavity is enclosed with glass wool insulation. This receiver receives reflected radiation from all eight parallel reflectors along the entire length of reflection. The schematic of the LFR setup is shown in Fig. 2. 3. Experimental investigation
Fig. 1. Schematic sketch of the LFR solar thermal system.
different concentration ratios, with different emissivity of absorber tube and with round as well as rectangular shaped absorber tube. The study revealed that the thermal efficiency was influenced by the concentration ratio and selective surface coating on the absorber. The thermal efficiency decreased with an increase in the concentration ratio of the Fresnel reflecting collector. The selective surface coated absorber had a significant advantage in terms of superior thermal performance as compared to an ordinary black painted absorber. The round pipe receiver had higher surface area to absorb solar energy as compared to rectangular pipe receiver. Thermal efficiency of the solar device with round pipe absorber was found to be higher than a rectangular pipe absorber. Facão et al. [4] analysed and optimized trapezoidal cavity receiver for a linear Fresnel solar collector concentrator using ray trace and CFD simulations. They improved the CFD model of previous researchers by including lower half of the pipes with no gap between those, instead of modeling them as a plane surface. It was concluded that effects of the absorber tube should not be neglected as 25% more heat loss takes place than that of the plane wall. Recently, Sahoo et al. [5] have developed correlations between Nusselt number and various non-dimensional parameters in such cavities using CFD. In contrast to earlier studies, this work considered the full pipe inside the cavity in place of a flat plate or a half pipe model. Flores Larsen et al. [6] studied heat loss of trapezoidal cavity receiver experimentally and numerically. Energy plus software was used to simulate the heat losses. In the computational model pipes were not considered. It has been noted in this work that CFD simulations are required to model natural convection in the cavity. The main focus of the present work is a detailed experimental investigation of heat losses in a cavity used in LFR systems. Moreover, recently developed CFD model [5] has been extended to closely replicate the experimental cavity by having three dimensional geometry with tubes inside it. In the present work, experiments are carried out with eight tubes inside the cavity. The effect of emissivity as well as depth of cavity has been studied experimentally. The temperature range of absorber tubes, for which studies are carried out, ranges from 200 C to 400 C. This temperature range is quite wide compared to earlier studies. The validated CFD model is used to find out contribution of radiative and convective heat losses in the overall heat losses.
The schematic of the experimental rig of LFR cavity receiver is shown in Fig. 3. The geometrical and other relevant parameters, considered for experimentation, are mentioned in Table 1. The cavity receiver used here is a model of the proposed cavity having dimensions as per Fig. 2. Here for experiments, the cavity length has been taken as 0.5 m (perpendicular to the paper) in place of 384 m in proposed cavity for LFR solar thermal system. A cavity of only 0.5 m length is chosen due to the fact that temperature gradient in the axial direction is extremely low compared to temperature gradients in the other directions. Therefore, heat losses can be neglected in the axial direction (perpendicular to the paper in Fig. 2) and there is insignificant effect of axial length on the heat losses. It is pointed out here that the present cavity is insulated in the axial direction hence preventing the heat loss in axial direction. As shown in Fig. 2, the enclosure has eight tubes at the top portion of it. The top and side walls are made of steel sheets and the bottom cover is made of glass and hence it becomes a closed cavity. Bottom of the glass cover is exposed to the outside environment. To prevent heat loss from the side and top walls, the outer surface of the cavity receiver is covered with insulating material made of glass wool of 90 mm thickness which is surrounded by plywood of 4 mm. The enclosed cavity is not evacuated and filled with non-absorbing air. Each tube contains an electrical heater inside it and heat output can be controlled by changing input voltage to the heaters. A voltage was set and eight heaters uniformly heated up the receiver tubes to a particular temperature. K-type thermocouples were used for temperature measurements. These thermocouples were bonded over the outer surface at mid length of each tube. Three thermocouples are located at three points of the glass cover as shown in Fig. 3 to measure the glass temperatures. Thermocouple channels were logged on to a digital thermometer and data
Fig. 2. Schematic sketch of the trapezoidal cavity receiver.
20
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
Power supply
UPM Solid state array Thermocouples
Cavity receiver
Data logger
Tubes with heater inside
Thermocouples
Computer
Fig. 3. Line diagram of the whole experimental setup for cavity receiver heat loss study.
logger for monitoring the temperatures. Experiments were carried out by increasing the voltage to the heater step by step and constantly monitoring the temperatures so that a predefined final temperature is achieved without air flow at the bottom i.e off design condition. Temperature readings were monitored every 1 min until steady values were achieved. A steady-state condition was also judged to have been attained when the surface temperature of the heater was seen not to vary significantly for example, less than 0.3 C per hour. The experiments were repeated for various tube temperatures of the tubes. For each set of experiments, after reaching steady temperature values, the corresponding voltage, current and the temperatures were logged. At steady state, when there is no further increase in temperature, the energy consumption of the heaters is the heat loss of the receiver, at that temperature. As the top and side walls are insulated, only heat interaction takes place with surrounding was with the bottom glass cover only. Calibrated and shielded thermocouples were used for the measurements and there exists a deviation of 1.25% of the actual reading. Air temperature was measured with an uncertainty of 1%, as specified by the manufacturer. An estimate in the experimental data has been carried out based on standard techniques [7]. The estimated error on the heat losses, due to errors in the measurements of basic parameters is as follows. The total heat can be represented in terms of input voltage, current. i.e.
Q ¼ VI
sQ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 vQ vQ s2V þ s2I ¼ vV vI
The uncertainty in the various variables used in the determination of the total heat loss was: 0.25% for the electric current I, 0.25% for the electric volt V, and 1.25% for any temperature measurement. It is found that Q varies from 10 to 70 W. The error in Q ¼ Q 10 to 70 or 3.93% to 8.12%. 4. Computational investigation A 3D model which replicates to the experimental setup has been adopted in the present study to predict the total heat loss from the receiver (Fig. 4). The 3D modelling and grid generation was carried out in the GAMBIT 2.3.16 package. The grid independence study was carried out with fine grid size of 473,246 Quad, Pave cells inside the cavity receiver. The laminar natural convection model equations and radiosity vector equations were solved using the software package FLUENT 6.3 [8]. In this model, 3D governing equations with laminar, incompressible and steady state problem were solved using an implicit solver. Since, in this cavity the temperature gradient is mainly in the vertical direction which is not conducive to natural convection, the flow takes place because of the small temperature gradients developing in the horizontal direction. Therefore, flow velocities are quite low, therefore usually flow is laminar. Hence, laminar flow is considered in the modelling. To take change of density with temperature into account, Boussinesq approximation was considered while solving the momentum
Thus the uncertainty in the electric power or heat loss is
Table 1 Geometrical dimensions and parameters considered for experimentation. Items
Dimensions
d Wbc Wtc L H
33.4 mm, 45 mm 500 mm 300 mm 0.5 m 100 mm, 112 mm,150 mm 200 mm 200 mm 8 mm 2 mm 6,8 0.25,0.1 0.9
H1 D1 G1 G2 No. of tubes Emissivity of the tube Emissivity of the glass cover
Fig. 4. CFD model of cavity receiver in 3-D and grid generation.
21
°
°
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
Fig. 5. Tube, glass and ambient temperature variation before steady temperature reaches for the tube (250 C).
equation. The average radiative heat flux leaving from the internal surface of the cavity receiver is obtained directly from the FLUENT results. Air properties inside the cavity were used by piecewise approximation method. Operating temperature was chosen as 400e450 K which is approximately an average of tube and glass temperature. For pressure velocity coupling, SIMPLE algorithm was used with second order upwind scheme for the discretization of equations. A convergence criterion of 105 was imposed on the residuals of the continuity and momentum equations. The convergence criterion of 106 was given on the residual of energy equation. Boundary conditions imposed are as follows. Tubes are continuously exposed to concentrated solar rays and due to this; the internal surface area usually attains uniform temperature at steady state. Therefore, the isothermal boundary condition was chosen for tubes. To prevent heat losses from the receiver, the outer surface of the receiver is covered with insulating material. Therefore, top and side walls were considered to be perfectly insulated. Along with radiation, heat losses due to forced convection occur on lower side of the glass cover, hence, convection coefficient of 5e 10 W/m2 (depending upon wind speed) was chosen. Note that
° Fig. 7. Variation in overall heat loss coefficient with temperature at varying depth of the cavity receiver.
because of significant forced convection, natural convection is neglected outside the cavity. To account for radiative heat transfer, emissivity values have been taken as per the values used in experimental setup. Atmospheric temperature has been considered to be 30 C. 5. Results and discussion The main purpose of this investigation is to find the overall heat losses from the cavity receiver. Overall heat loss coefficients were calculated after getting the heat losses from the cavity. For this, the hot surface area i.e. tube surface areas was taken as reference area. 5.1. Experimental results The effect of various parameters on the heat losses from the cavity is studied. Fig. 5 shows the variation of tube and glass
°
°
ε ε
°
Fig. 6. Total heat loss from the cavity receiver and cover glass temperature at various temperatures of absorber tubes.
° Fig. 8. Variation in overall heat loss coefficient with temperature at varying emissivities of the tubes.
22
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
Convective component Radiative component Total
14
12
2 U L (W/m °C)
10
8
Depth = 100mm No. of tubes =8 diameter of tubes=33.4mm =0.9 cover
6
4
2
0 0.0
0.2
Fig. 9. Comparison of experimental heat loss data with CFD simulated results.
°
temperature before a steady state temperature of 250 C is achieved for the tube. The tube temperature increases rapidly at the start and after reaching 220 C, the slope of rise in temperature decreases. After tube temperature reaches 249e250 C, the temperature remains almost invariant with time, hence confirming steady state. The corresponding glass temperature variation is shown in Fig. 5 as well. Heat loss analysis was carried out after system reached its steady state. Similarly, before carrying out the performance analysis at different temperatures (200, 300, 375 and 400 C), it is verified that the steady state has been reached. The heat losses found at those temperatures are shown in Fig. 6. It shows that as the temperature of tube surface increases, the heat loss as well as the cover glass temperature increases. Due to instrumentation errors, the heat loss uncertainties range from 4% to 8%. The influence of depth of the cavity (the distance between the top wall and the bottom wall), emissivities of the tubes and number of tubes on heat losses is analysed. The heat losses variation with varying depth (100 mm and 150 mm), can be seen in Fig. 7. As depth
° Fig. 10. Simulated overall heat loss coefficient comprising convective, radiative and total heat loss at different temperatures of tube wall temperature.
0.4
0.6
0.8
1.0
ε tubes
°
Fig. 11. Simulated overall heat loss coefficient with varying emissivities of the tubes at 300 C.
increases, the heat losses are observed to be decreasing. During investigation of heat loss at varying depth conditions, the emissivities of tubes were kept constant at 0.25. Influence of emissivities (0.25 and 0.1) on total heat loss was also found out (Fig. 8). It can be seen that as emissivity of the tubes increases, the heat loss also increases. For the results shown in Fig. 8 the depth of the cavity is 112 mm and number of tubes and diameter of the tubes are 6 and 45 mm, respectively. It is noted that when the emissivity of the tube was increased from 0.1 to 0.25, the heat loss increased by around 30e40%. So, it is clear that radiation is the major component of heat losses in the LFR cavity. 5.2. Computational results In order to distinguish between the convective and radiative heat losses taking place in the cavity, CFD analysis was carried out. The heat losses obtained experimentally are compared to the numerical values obtained from CFD simulations. It can be seen from Fig. 9 that the numerical results obtained are in good agreement (difference is between 3 and 9%) with those obtained from the experiment. In the higher tube temperature conditions, the CFD results under predicted the heat loss compared to the experimental results. This difference may be due to the fact that small amount of heat loss, which occurs through the side walls and top side of the cavity, has been neglected in the CFD analysis. After investigating heat loss, it was found that, radiative heat loss plays a dominant role. Total heat loss and radiative heat losses were obtained from results. After subtracting the radiative heat loss from the total heat loss, the convective component of heat losses are obtained. Total heat loss comprising a sum of convective and radiative heat losses from the cavity is shown in Fig. 10. It was noticed that, the convection contributes only 7e15% of the total losses. The effect of different emissivities of the tube on the heat losses is shown in Fig. 11. As expected, there is a significant effect of the change in emissivity in the radiative heat transfer and negligible effect on the convective heat transfer. For a tube temperature of 300 C, the convection losses amounts 7.5 and 10.1% of the total losses for 0.1 and 0.25 emissivities, respectively. The isotherms and streamlines for the proposed cavity are shown in Fig. 12. From the isotherms it can be seen that temperatures are almost uniform in the horizontal direction. However,
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
23
Moreover, steady state modelling and simulation of the same cavity was carried out using CFD. The computational results were compared with the experimental data. The comparison shows a good match between experimental and computational results, hence validating the model. The validated model can be used for simulation of the cavity for various parameters instead of carrying out experiments. It has been observed that the dominant mode of heat losses from the cavity is radiation. Hence, using selective coating on tubes and cavity inside wall, the overall losses can be minimized. Although, the heat losses are mainly due to radiation, those by natural convection at 8e15% are also not insignificant. Therefore, the use of evacuated or better design of cavities may be recommended to minimize convection losses. Fig. 12. Isotherm and streamlines contours inside the cavity receiver (depth ¼ 100 mm) (at half length of the cavity in z direction) when tubes are at 300 C (573 K).
there is still sufficient temperature difference so as to result in natural convection with air rising from the centre of the cavity and then coming down from the sides. From the flow pattern seen in the figure, it can be seen that vortices are formed between side walls and adjacent tubes. In the lower half of the cavity, additional vortices can be seen which are almost symmetric to the mid-plane. It can be observed further that stream function gradient is more on the top side of the cavity than that at the bottom plane. 6. Conclusion Total heat losses from the absorber tubes were estimated using a lab experimental setup for trapezoidal cavity with eight tubes. The effect of various parameters (such as depth of cavity, emissivity) on the heat losses was studied using the experimental setup.
References [1] Pye JD, Morrison G, Behnia M. Convection inside the cavity receiver of the CLFR concentrating solar power system. 7th Natural Convection Workshop, Sydney, Australia; 2003. [2] Reynolds DJ, Jance MJ, Behnia M, Morrison GL. An experimental and computational study of the heat loss characteristics of a trapezoidal cavity absorber. Solar Energy 2004;76:229e34. [3] Singh PL, Sarviya RM, Bhagoria JL. Heat loss study of trapezoidal cavity absorbers for linear solar concentrating collector. Energy Conversion and Management 2010;51(2):329e37. [4] Facão J, Oliveria AC. Numerical simulation of trapezoidal cavity receiver for a linear Fresnel solar collector concentrator. Renewable Energy 2011;36(1): 90e6. [5] Sahoo SS, Singh S, Banerjee R. Analysis of heat losses from a trapezoidal cavity used for linear Fresnel reflector system. Solar Energy 2012;86(5):1313e22. [6] Flores Larsen S, Altamirano M, Hernández A. Heat loss of a trapezoidal cavity absorber for a linear Fresnel reflecting solar concentrator. Renewable Energy 2012;39:198e206. [7] Kline SJ, McClintock FA. Describing uncertainties in single sample experiments. Mechanical Engineering 1953;75:3e8. [8] Ansys FluentÒ manual; 2010.
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental investigation and computational validation of heat losses from the cavity receiver used in linear Fresnel reflector solar thermal system Sudhansu S. Sahoo a, Shinu M. Varghese b, C. Suresh Kumar b, S.P. Viswanathan b, Suneet Singh c, *, Rangan Banerjee c a b c
Department of Mechanical Engineering, College of Engineering and Technology, Biju Patnaik University of Technology, Bhubaneswar, India KG Design Services Pvt. Ltd., Coimbatore, India Department of Energy Science & Engineering, IIT Bombay, Mumbai, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2012 Accepted 28 November 2012 Available online 13 January 2013
This paper presents the analysis of heat losses from the trapezoidal cavity receiver used in linear Fresnel reflector (LFR) system. The experimental studies are conducted under laboratory conditions that are specially designed for this purpose. The effects of parameters such as the temperatures of the tube surface, depth of receiver, number of tubes, and emissivity of tubes are investigated. The loss of heat is taking place from the tube outer surface to glass cover, below the receiver and then glass cover to ambient. As part of this investigation, the system is modelled and simulated using computational fluid dynamics (CFD). After validation, contribution of convection and radiation to the total heat transfer are found out using CFD. Computational predictions are shown to be consistent with the experimental observations which show that the CFD model is a reliable tool for predicting heat loss and overall heat loss coefficient. It was found that losses due to convection are between 5 and 18% of the total heat losses. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: LFR Solar thermal CFD Overall heat loss coefficient
1. Introduction Linear Fresnel Reflector technology relies on an array of linear mirror strips which concentrates light on a fixed receiver mounted on a linear tower (Fig. 1). The receiver is a stationary linear cavity, usually trapezoidal, consisting of single or multiple numbers of tubes. The inside of the cavity, external to the tubes, contains air which is not in contact with the ambient. The water, running through the tubes inside the cavity, absorbs heat from the cavity and generates steam inside the tubes. In operation, the absorber tubes in the trapezoidal cavity get heated due to the incident concentrated solar radiation. As it does so, it emits long wavelength radiation into the cavity. This radiation results in heat loss from the tubes. The emitted radiation is absorbed by inner cavity walls and glass cover at the bottom, which in turn raises their temperature. The resulting temperature gradients promote natural convection within the cavity, which lead to convective losses from the tubes. Conduction of heat away from the inner surfaces represents the third mode of heat loss. The cavity receiver heat loss processes
* Corresponding author. Tel.: þ91 9321058464; fax: þ91 22 25764890. E-mail address: [email protected] (S. Singh). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.11.036
involve radiative, convective and conductive heat transfer, and interaction of these three modes makes it difficult to develop an analytical model. Hence, computational and experimental approaches can be used to predict and verify heat losses. Pye et al. [1] carried out a study of losses in a trapezoidal cavity. They used an analytical model for a trapezoidal cavity and found that radiation accounts for 90% of the heat loss from the top surface. Again using CFD analysis of the cavity, he mentioned losses due to natural convection and radiation. However, for CFD analysis, tubes were modelled as an isothermal plane surface. Reynolds et al. [2] carried out experimental and computational study of heat loss characteristics of trapezoidal cavity. They used flow visualization technique to capture the heat flow patterns within the trapezoidal cavity with a hot plate to investigate the heat losses from the absorber tube. In computational study, flow in the cavity was assumed to be laminar. They found a reasonable agreement between experimentally observed flow patterns and those predicted by computational model. CFD prediction of heat loss was found 40% less compared with experimental results. Uncertainties in the experimental work were mentioned as the reason for this difference. Singh et al. [3] experimentally studied the thermal performance of the Fresnel reflecting concentrator with trapezoidal cavity at
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
19
2. Physical problem The proposed linear Fresnel reflector system under consideration consists of a trapezoidal cavity receiver made of steel with black painted surface (Fig. 2) filled with air and it houses eight parallel pipes having Nominal Pipe Size 1 (33.4 mm outer diameter) which are made of SS304 material. The pipe is coated with black nickel selective surface by the process of electroplating. Tubes are placed below the inner top surface of the cavity. A gap is provided between each of the tubes as allowance for thermal expansion. At bottom portion plane glass is provided to allow entry of solar radiation. Outer cavity is enclosed with glass wool insulation. This receiver receives reflected radiation from all eight parallel reflectors along the entire length of reflection. The schematic of the LFR setup is shown in Fig. 2. 3. Experimental investigation
Fig. 1. Schematic sketch of the LFR solar thermal system.
different concentration ratios, with different emissivity of absorber tube and with round as well as rectangular shaped absorber tube. The study revealed that the thermal efficiency was influenced by the concentration ratio and selective surface coating on the absorber. The thermal efficiency decreased with an increase in the concentration ratio of the Fresnel reflecting collector. The selective surface coated absorber had a significant advantage in terms of superior thermal performance as compared to an ordinary black painted absorber. The round pipe receiver had higher surface area to absorb solar energy as compared to rectangular pipe receiver. Thermal efficiency of the solar device with round pipe absorber was found to be higher than a rectangular pipe absorber. Facão et al. [4] analysed and optimized trapezoidal cavity receiver for a linear Fresnel solar collector concentrator using ray trace and CFD simulations. They improved the CFD model of previous researchers by including lower half of the pipes with no gap between those, instead of modeling them as a plane surface. It was concluded that effects of the absorber tube should not be neglected as 25% more heat loss takes place than that of the plane wall. Recently, Sahoo et al. [5] have developed correlations between Nusselt number and various non-dimensional parameters in such cavities using CFD. In contrast to earlier studies, this work considered the full pipe inside the cavity in place of a flat plate or a half pipe model. Flores Larsen et al. [6] studied heat loss of trapezoidal cavity receiver experimentally and numerically. Energy plus software was used to simulate the heat losses. In the computational model pipes were not considered. It has been noted in this work that CFD simulations are required to model natural convection in the cavity. The main focus of the present work is a detailed experimental investigation of heat losses in a cavity used in LFR systems. Moreover, recently developed CFD model [5] has been extended to closely replicate the experimental cavity by having three dimensional geometry with tubes inside it. In the present work, experiments are carried out with eight tubes inside the cavity. The effect of emissivity as well as depth of cavity has been studied experimentally. The temperature range of absorber tubes, for which studies are carried out, ranges from 200 C to 400 C. This temperature range is quite wide compared to earlier studies. The validated CFD model is used to find out contribution of radiative and convective heat losses in the overall heat losses.
The schematic of the experimental rig of LFR cavity receiver is shown in Fig. 3. The geometrical and other relevant parameters, considered for experimentation, are mentioned in Table 1. The cavity receiver used here is a model of the proposed cavity having dimensions as per Fig. 2. Here for experiments, the cavity length has been taken as 0.5 m (perpendicular to the paper) in place of 384 m in proposed cavity for LFR solar thermal system. A cavity of only 0.5 m length is chosen due to the fact that temperature gradient in the axial direction is extremely low compared to temperature gradients in the other directions. Therefore, heat losses can be neglected in the axial direction (perpendicular to the paper in Fig. 2) and there is insignificant effect of axial length on the heat losses. It is pointed out here that the present cavity is insulated in the axial direction hence preventing the heat loss in axial direction. As shown in Fig. 2, the enclosure has eight tubes at the top portion of it. The top and side walls are made of steel sheets and the bottom cover is made of glass and hence it becomes a closed cavity. Bottom of the glass cover is exposed to the outside environment. To prevent heat loss from the side and top walls, the outer surface of the cavity receiver is covered with insulating material made of glass wool of 90 mm thickness which is surrounded by plywood of 4 mm. The enclosed cavity is not evacuated and filled with non-absorbing air. Each tube contains an electrical heater inside it and heat output can be controlled by changing input voltage to the heaters. A voltage was set and eight heaters uniformly heated up the receiver tubes to a particular temperature. K-type thermocouples were used for temperature measurements. These thermocouples were bonded over the outer surface at mid length of each tube. Three thermocouples are located at three points of the glass cover as shown in Fig. 3 to measure the glass temperatures. Thermocouple channels were logged on to a digital thermometer and data
Fig. 2. Schematic sketch of the trapezoidal cavity receiver.
20
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
Power supply
UPM Solid state array Thermocouples
Cavity receiver
Data logger
Tubes with heater inside
Thermocouples
Computer
Fig. 3. Line diagram of the whole experimental setup for cavity receiver heat loss study.
logger for monitoring the temperatures. Experiments were carried out by increasing the voltage to the heater step by step and constantly monitoring the temperatures so that a predefined final temperature is achieved without air flow at the bottom i.e off design condition. Temperature readings were monitored every 1 min until steady values were achieved. A steady-state condition was also judged to have been attained when the surface temperature of the heater was seen not to vary significantly for example, less than 0.3 C per hour. The experiments were repeated for various tube temperatures of the tubes. For each set of experiments, after reaching steady temperature values, the corresponding voltage, current and the temperatures were logged. At steady state, when there is no further increase in temperature, the energy consumption of the heaters is the heat loss of the receiver, at that temperature. As the top and side walls are insulated, only heat interaction takes place with surrounding was with the bottom glass cover only. Calibrated and shielded thermocouples were used for the measurements and there exists a deviation of 1.25% of the actual reading. Air temperature was measured with an uncertainty of 1%, as specified by the manufacturer. An estimate in the experimental data has been carried out based on standard techniques [7]. The estimated error on the heat losses, due to errors in the measurements of basic parameters is as follows. The total heat can be represented in terms of input voltage, current. i.e.
Q ¼ VI
sQ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 vQ vQ s2V þ s2I ¼ vV vI
The uncertainty in the various variables used in the determination of the total heat loss was: 0.25% for the electric current I, 0.25% for the electric volt V, and 1.25% for any temperature measurement. It is found that Q varies from 10 to 70 W. The error in Q ¼ Q 10 to 70 or 3.93% to 8.12%. 4. Computational investigation A 3D model which replicates to the experimental setup has been adopted in the present study to predict the total heat loss from the receiver (Fig. 4). The 3D modelling and grid generation was carried out in the GAMBIT 2.3.16 package. The grid independence study was carried out with fine grid size of 473,246 Quad, Pave cells inside the cavity receiver. The laminar natural convection model equations and radiosity vector equations were solved using the software package FLUENT 6.3 [8]. In this model, 3D governing equations with laminar, incompressible and steady state problem were solved using an implicit solver. Since, in this cavity the temperature gradient is mainly in the vertical direction which is not conducive to natural convection, the flow takes place because of the small temperature gradients developing in the horizontal direction. Therefore, flow velocities are quite low, therefore usually flow is laminar. Hence, laminar flow is considered in the modelling. To take change of density with temperature into account, Boussinesq approximation was considered while solving the momentum
Thus the uncertainty in the electric power or heat loss is
Table 1 Geometrical dimensions and parameters considered for experimentation. Items
Dimensions
d Wbc Wtc L H
33.4 mm, 45 mm 500 mm 300 mm 0.5 m 100 mm, 112 mm,150 mm 200 mm 200 mm 8 mm 2 mm 6,8 0.25,0.1 0.9
H1 D1 G1 G2 No. of tubes Emissivity of the tube Emissivity of the glass cover
Fig. 4. CFD model of cavity receiver in 3-D and grid generation.
21
°
°
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
Fig. 5. Tube, glass and ambient temperature variation before steady temperature reaches for the tube (250 C).
equation. The average radiative heat flux leaving from the internal surface of the cavity receiver is obtained directly from the FLUENT results. Air properties inside the cavity were used by piecewise approximation method. Operating temperature was chosen as 400e450 K which is approximately an average of tube and glass temperature. For pressure velocity coupling, SIMPLE algorithm was used with second order upwind scheme for the discretization of equations. A convergence criterion of 105 was imposed on the residuals of the continuity and momentum equations. The convergence criterion of 106 was given on the residual of energy equation. Boundary conditions imposed are as follows. Tubes are continuously exposed to concentrated solar rays and due to this; the internal surface area usually attains uniform temperature at steady state. Therefore, the isothermal boundary condition was chosen for tubes. To prevent heat losses from the receiver, the outer surface of the receiver is covered with insulating material. Therefore, top and side walls were considered to be perfectly insulated. Along with radiation, heat losses due to forced convection occur on lower side of the glass cover, hence, convection coefficient of 5e 10 W/m2 (depending upon wind speed) was chosen. Note that
° Fig. 7. Variation in overall heat loss coefficient with temperature at varying depth of the cavity receiver.
because of significant forced convection, natural convection is neglected outside the cavity. To account for radiative heat transfer, emissivity values have been taken as per the values used in experimental setup. Atmospheric temperature has been considered to be 30 C. 5. Results and discussion The main purpose of this investigation is to find the overall heat losses from the cavity receiver. Overall heat loss coefficients were calculated after getting the heat losses from the cavity. For this, the hot surface area i.e. tube surface areas was taken as reference area. 5.1. Experimental results The effect of various parameters on the heat losses from the cavity is studied. Fig. 5 shows the variation of tube and glass
°
°
ε ε
°
Fig. 6. Total heat loss from the cavity receiver and cover glass temperature at various temperatures of absorber tubes.
° Fig. 8. Variation in overall heat loss coefficient with temperature at varying emissivities of the tubes.
22
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
Convective component Radiative component Total
14
12
2 U L (W/m °C)
10
8
Depth = 100mm No. of tubes =8 diameter of tubes=33.4mm =0.9 cover
6
4
2
0 0.0
0.2
Fig. 9. Comparison of experimental heat loss data with CFD simulated results.
°
temperature before a steady state temperature of 250 C is achieved for the tube. The tube temperature increases rapidly at the start and after reaching 220 C, the slope of rise in temperature decreases. After tube temperature reaches 249e250 C, the temperature remains almost invariant with time, hence confirming steady state. The corresponding glass temperature variation is shown in Fig. 5 as well. Heat loss analysis was carried out after system reached its steady state. Similarly, before carrying out the performance analysis at different temperatures (200, 300, 375 and 400 C), it is verified that the steady state has been reached. The heat losses found at those temperatures are shown in Fig. 6. It shows that as the temperature of tube surface increases, the heat loss as well as the cover glass temperature increases. Due to instrumentation errors, the heat loss uncertainties range from 4% to 8%. The influence of depth of the cavity (the distance between the top wall and the bottom wall), emissivities of the tubes and number of tubes on heat losses is analysed. The heat losses variation with varying depth (100 mm and 150 mm), can be seen in Fig. 7. As depth
° Fig. 10. Simulated overall heat loss coefficient comprising convective, radiative and total heat loss at different temperatures of tube wall temperature.
0.4
0.6
0.8
1.0
ε tubes
°
Fig. 11. Simulated overall heat loss coefficient with varying emissivities of the tubes at 300 C.
increases, the heat losses are observed to be decreasing. During investigation of heat loss at varying depth conditions, the emissivities of tubes were kept constant at 0.25. Influence of emissivities (0.25 and 0.1) on total heat loss was also found out (Fig. 8). It can be seen that as emissivity of the tubes increases, the heat loss also increases. For the results shown in Fig. 8 the depth of the cavity is 112 mm and number of tubes and diameter of the tubes are 6 and 45 mm, respectively. It is noted that when the emissivity of the tube was increased from 0.1 to 0.25, the heat loss increased by around 30e40%. So, it is clear that radiation is the major component of heat losses in the LFR cavity. 5.2. Computational results In order to distinguish between the convective and radiative heat losses taking place in the cavity, CFD analysis was carried out. The heat losses obtained experimentally are compared to the numerical values obtained from CFD simulations. It can be seen from Fig. 9 that the numerical results obtained are in good agreement (difference is between 3 and 9%) with those obtained from the experiment. In the higher tube temperature conditions, the CFD results under predicted the heat loss compared to the experimental results. This difference may be due to the fact that small amount of heat loss, which occurs through the side walls and top side of the cavity, has been neglected in the CFD analysis. After investigating heat loss, it was found that, radiative heat loss plays a dominant role. Total heat loss and radiative heat losses were obtained from results. After subtracting the radiative heat loss from the total heat loss, the convective component of heat losses are obtained. Total heat loss comprising a sum of convective and radiative heat losses from the cavity is shown in Fig. 10. It was noticed that, the convection contributes only 7e15% of the total losses. The effect of different emissivities of the tube on the heat losses is shown in Fig. 11. As expected, there is a significant effect of the change in emissivity in the radiative heat transfer and negligible effect on the convective heat transfer. For a tube temperature of 300 C, the convection losses amounts 7.5 and 10.1% of the total losses for 0.1 and 0.25 emissivities, respectively. The isotherms and streamlines for the proposed cavity are shown in Fig. 12. From the isotherms it can be seen that temperatures are almost uniform in the horizontal direction. However,
S.S. Sahoo et al. / Renewable Energy 55 (2013) 18e23
23
Moreover, steady state modelling and simulation of the same cavity was carried out using CFD. The computational results were compared with the experimental data. The comparison shows a good match between experimental and computational results, hence validating the model. The validated model can be used for simulation of the cavity for various parameters instead of carrying out experiments. It has been observed that the dominant mode of heat losses from the cavity is radiation. Hence, using selective coating on tubes and cavity inside wall, the overall losses can be minimized. Although, the heat losses are mainly due to radiation, those by natural convection at 8e15% are also not insignificant. Therefore, the use of evacuated or better design of cavities may be recommended to minimize convection losses. Fig. 12. Isotherm and streamlines contours inside the cavity receiver (depth ¼ 100 mm) (at half length of the cavity in z direction) when tubes are at 300 C (573 K).
there is still sufficient temperature difference so as to result in natural convection with air rising from the centre of the cavity and then coming down from the sides. From the flow pattern seen in the figure, it can be seen that vortices are formed between side walls and adjacent tubes. In the lower half of the cavity, additional vortices can be seen which are almost symmetric to the mid-plane. It can be observed further that stream function gradient is more on the top side of the cavity than that at the bottom plane. 6. Conclusion Total heat losses from the absorber tubes were estimated using a lab experimental setup for trapezoidal cavity with eight tubes. The effect of various parameters (such as depth of cavity, emissivity) on the heat losses was studied using the experimental setup.
References [1] Pye JD, Morrison G, Behnia M. Convection inside the cavity receiver of the CLFR concentrating solar power system. 7th Natural Convection Workshop, Sydney, Australia; 2003. [2] Reynolds DJ, Jance MJ, Behnia M, Morrison GL. An experimental and computational study of the heat loss characteristics of a trapezoidal cavity absorber. Solar Energy 2004;76:229e34. [3] Singh PL, Sarviya RM, Bhagoria JL. Heat loss study of trapezoidal cavity absorbers for linear solar concentrating collector. Energy Conversion and Management 2010;51(2):329e37. [4] Facão J, Oliveria AC. Numerical simulation of trapezoidal cavity receiver for a linear Fresnel solar collector concentrator. Renewable Energy 2011;36(1): 90e6. [5] Sahoo SS, Singh S, Banerjee R. Analysis of heat losses from a trapezoidal cavity used for linear Fresnel reflector system. Solar Energy 2012;86(5):1313e22. [6] Flores Larsen S, Altamirano M, Hernández A. Heat loss of a trapezoidal cavity absorber for a linear Fresnel reflecting solar concentrator. Renewable Energy 2012;39:198e206. [7] Kline SJ, McClintock FA. Describing uncertainties in single sample experiments. Mechanical Engineering 1953;75:3e8. [8] Ansys FluentÒ manual; 2010.