45
Performance Study of Cavity Receiver of Solar Concentrator
Using Dry Test
1
V.C.Shewale,
2A.A.Kapse
1 Professor, 2 Associate Professor 1 Department of Mechanical Engineering, 1
N.D.M.V.P.S’s K.B.T College of Engineering, Nasik, India
Abstract: Cavity receiver of solar concentrator having spherical shape is manufactured with cavity diameter 400 mm and opening diameter 175 mm to study the different heat losses such as conduction, convection & radiation. The experimental set up mainly consists of copper tube material cavity receiver wrapped with nichrome heating coil to heat the cavity and insulated with glass wool insulation to reduce the heat losses from outside surface. The effects of wind, cavity orientation, operating temperature of cavity are studied for convection losses experimentally. The numerical (CFD) analysis is carried out to study connective losses for no wind condition only. The numerical results are compared with experimental results and found good agreement. The effect of inclination angle of cavity receiver on total losses & convection losses shows that as the inclination angle increases from 0o to 90o both losses decreased due to decreased in convective zone into the cavity receiver. The maximum losses are obtained at 0o inclination angle and the minimum losses are obtained at 90o inclination angle due to increase in stagnation zone in to the cavity from 0o to 90o. The effect of operating temperature of cavity shows that as the temperature of cavity receiver increases, the total and convective losses goes on increasing.
IndexTerms – Cavity receiver, Heat losses, wind effect
Abbreviations
s
A
Surface area of cavity receiver (m2)ap
A
Aperture area of cavity receiver (m2)D
d
Opening ratiod
Diameter of opening for cavity receiver (m)D
Diameter of cavity receiver (m)Gr
Grashoff numberh
Heat transfer coefficient for convection (W/m2k)k
Thermal conductivity (W/mk)Nu
Nusselt numberPr
Prandtl numbertot
Q
Total heat loss (Watt)cond
Q
Conduction heat loss (Watt)conv
Q
Convection heat loss (Watt)rad
Q
Radiation heat loss (Watt)Ra
Rayleigh numbers
T
Surface temperature of cavity receiver (oC)f
T
Average temperature (oC)a
T
Atmospheric temperature (oC)Subscripts
cond Conduction conv Convection rad Radiation tot Total ap Aperture s Surface of cavity a Ambient
1. INTRODUCTION
Cavity receiver is a key component of any solar system and the efficiency of solar system mainly depends on the performance of cavity receiver. Therefore the efficiency of cavity receiver can be improved by minimizing the losses of cavity receiver of solar concentrator. So many researchers have worked to study heat losses of different geometry cavity receiver to minimize it. Harris & Lenz [1] analyzed six types of cavity geometries such as cylindrical, heteroconical, spherical, elliptical & conical which estimates the system efficiency and cavity power profile. A.M. Clausing [2] analyzed a cavity solar central receiver for convective losses by presenting an analytical model. The model shows the internal thermal resistance i.e. the importance of ability to heat air in to the cavity and influence of wind on convective losses. T. Taumoefolau et al. [3] studied
the electrically heated cylindrical cavity for natural convection losses with different cavity orientation and operating temperature form 450 oC to 650 oC. He also studied the effect of ratio of aperture diameter to cavity diameter for natural convection losses and numerically analyzed the convective losses with CFD software. Then he compared the numerical results with experimental results and with previous studies and he found good agreement with experimental results.
temperature between 573K & 873K on convective heat loss. Stine & Mc Donald [5] studied the effect of tilt angle of cavity, geometry and temperature of cavity experimentally by using full size cavity receiver for convection losses. They also developed the correlation by considering the cavity temperature, tilt angle & different geometry of cavity receiver. R.D. Jilte et al. [6] studied different cavity shapes such as cylindrical, conical, dome cylindrical, hetroconical, reverse conical and spherical numerically for combined natural convection & radiation heat losses. Koenig & Marvin [7] studied a model for convective zone in cylindrical cavity receiver of length 0.5m and diameter 0.3m. A.M. Clausing [9] studied the convective heat losses by using analytical method for upward facing large cubical cavity which is used in solar tower and compared with experimental results. P. Le Quere et al. [10] carried experimental and numerical study for opening ratio of 1 and at inclination from -90o to +90o. He found the larger variation of Nusselt number ie 5% from the experimental &numerical simulation for cavities.
C.F. Hess & Henze [11] studied the natural convection losses experimentally for horizontal rectangular open cavities at different inclination angles. He found that natural convection loss can be reduced up to 10% by using flow restriction at the aperture plane. Y.L. Chan & Tien [12, 13] studied estimation of natural convection in shallow open cavities. This numerically two dimensional study carried out for minimizing the convection losses at various inclination of cavity. James & Terry [14] analyzed five geometries of cavity receivers cylindrical, spherical, elliptical, conical & heteroconical for thermal performance. They obtained considerable variations on power profile of cavity geometry and rim angle deviation of concentrator. Balaji & Venkateshan [15] analyzed natural convection along with surface radiation for different aspect ratio to a rectangular enclosure and developed a correlation for convection & radiation Nusselt number.
The experimental set up mainly consists of a downward facing spherical cavity receiver which is made up of copper tubing material. Due to its high thermal conductivity, the copper material is used for cavity to maintain cavity temperature uniform throught. The heating coil is wrapped on the cavity to heat the cavity uniformly. The cavity is insulated with glass wool insulation having thickness 100 mm to reduce the heat losses from outside surface and covered with aluminum cladding. Angle adjustment mechanism is used for measurement of heat losses at different orientation of cavity receiver (0o to 90o) and whole cavity with insulation is mounted on stand. In this experimental set up 15 K-type thermocouples are used for the measurement of surface temperature of cavity receiver and air temperature into the cavity receiver. The atmospheric temperature is measured from the location, where there is no effect of receiver temperature. The data logger was used to record the data of temperature measurement. The surface temperature and air temperatures inside the cavity receiver are measured at an interval of 10 second. The time interval required to reach the system in steady state is approximately 120 minutes and temperatures are recorded after the system reached in steady state condition. The ammeter & voltmeter are used for the measurement of current & voltage respectively so that we can calculate the total heat losses. Dimmer stat is used for varying the voltage & current to maintain the specific temperature of cavity receiver. The temperature controller is also used to control the temperature of surface of cavity receiver. The wind tests are carried out by using the blower assembly in which a square tunnel of sheet metal is attached to the blower door. Inside the tunnel there is metal mesh section which gives the straight and uniform air flow in to the cavity. The uniform air flow is confirmed by checking the speed of wind at different location of aperture plane with the help of anemometer.
47 2.2. Mathematical Modeling:
The energy balance of spherical cavity receiver in
which conduction loss (
Q
cond), convection loss (Q
conv)and radiation loss (
Q
rad ) are the three modes of heat losses.The convection losses and radiation losses are through the aperture of cavity receiver while the conduction loss is
through the surface of cavity receiver.
The energy balance equation used for the heat loss calculation of cavity receiver is given by,
rad convection and radiation heat losses respectively.
The total losses can be finding out by using the following equation for the cavity receiver
VI
Q
tot
(2)Where,
V
= Voltage measured by voltmeter.I
= Current measured by ammeter. from these tests is conductive loss at these temperatures for cavity receiver.VI
Q
cond/openingclosed
(3)
The Radiation loss can be calculated by using the following equation for spherical cavity
Where,
= Equivalent emissivity of surface.A
ap= Aperture area of cavity in m2. convective heat loss analysis for no wind condition only. The geometry of cavity receiver and mesh were created using ANSYS ICEM-15.0. The region outside of cavity receiver is surrounded by cubical enclosure having dimensions 10 times more than the cavity receiver diameter to avoid the effect of air flow in to the cavity. The grid independence study has been carried out with
4.2 Numerical Results:
The air temperature profiles within the cavity receiver are the results obtained by numerical analysis of the cavity receiver for no wind condition. Fig.7 shows the temperature contours for 300 oC operating temperature of cavity at different inclination angle of cavity receiver. The highest temperatures are indicated by red shades while the lowest temperatures are indicated by dark blue shades. The stagnation zone which is indicated by red shade goes on increasing when the inclination angle increases from 0o to
90o is shown in Fig.8. In this numerical analysis the convective loss obtained from cavity receiver for no wind condition compared with experimental heat loss and found a good agreement with experimental results. In this analysis it is found that the maximum losses are obtained at 0o inclination angle of cavity receiver while the minimum losses are obtained at 90o inclination angle of cavity receiver. This is due to decrease in convective current and increase in stagnation zone in to the cavity receiver.
Fig.2. Variation of convection heat loss with inclination angle for no wind condition
Fig.3. Variation of convection heat loss with inclination angle for head-on wind at 2 m/s
Fig.4. Variation of convection heat loss with inclination angle for side-on wind at 2 m/s
Fig.5. Variation of convection heat loss with inclination angle at 150 °C cavity temperature
49
4.3 Comparison Between Experimental Results And Numerical Results:
Experimental results, numerical results and values calculated from correlations presented by some authors are plotted for the comparison of convective losses with different cavity inclination angle (0o, 30o, 45o, 60o, & 90o) at 300 oC operating temperature of cavity. Fig. 8 shows the variation of convective loss with inclination angle of cavity for experimental, numerical results & values from correlations of Stine & McDonald, Clausing and M. Prakash for cavity inclination angle from 0o (Sideward facing cavity) to 90o (Downward facing cavity) in which all the
correlations of models shows the same dependence of convective heat loss on inclination angle. The experimental results shows good agreement with numerical results while Clausing model [4] shows higher side convection losses compared with experimental results. Stine & McDonald model [5] and M. Prakash shows lower side convective losses as compared to experimental and numerical results. The Clausing model [2] shows the maximum convective losses compared to all models. The deviations of convective losses for other models shows higher deviation for 0o to 30o inclination angle whereas lower deviation for 30o to 90o inclination angle of cavity.
5. CONCLUSION:
The electrically heated spherical cavity receiver of solar concentrator with cooper tubing material having cavity diameter 400 mm and opening diameter 175mm has been studied experimentally for total losses such as conduction, convection and radiation at different orientation and different operating temperature of cavity receiver. The numerical (CFD) analysis is carried out for convective losses to no wind condition only. It is found that the higher
convective losses are obtained at 0o inclination angle of cavity and lower convective losses are obtained at 90o inclination angle of cavity receiver from 0o (Sideward facing cavity) to 90o (Downward facing cavity). The effect of operating temperature also shows the same effect on convective losses in which the convective losses are decreases with decrease in operating temperature of cavity and increases with increase in operating temperature of cavity. Numerical results and values obtained from
Fig.7. Temperature contours at different inclination angle of cavity for 300 oC cavity temperature
correlations of various authors are compared with experimental results in which good agreement has been found of numerical results with experimental results. The maximum convective heat losses are found at 0° inclination angle for head on wind condition due to high convective current of air at the aperture of cavity receiver.
REFERENCES
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[3] T. Taumoefolau, S. Paitoonsurikarn, G. Hughes, K. Lovegrove, Experimental investigation of natural convection heat loss from a model solar concentrator cavity receiver, Journal of Solar Energy Engineering, 126 (2004) 801-807.
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[6] R.D. Jilte, S.B. Kedare & J.K. Nayak, Natural convection & radiation heat loss from open cavities of different shapes & sizes used with dish concentrator, Mechanical Engineering Reasearch, 3 (2013) 25-43. [7] A.A. Koenig, and M. Marvin, Convection heat loss sensitivity in open cavity solar receivers, Final report, DOE Contract No: EG77-C-04–3985, Department of Energy, Oak Ridge, Tennessee, (1981).
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[10] P. Le Quere, F. Penot, M. Mirenayat, Experimental study of heat loss through natural convection from an isothermal cubic open cavity, Sandia Laboratory Report SAND81-8014, (1981) 165-174.
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[12] Y.L. Chan, C.L. Tien, A numerical study of two-dimensional laminar natural convection in shallow open cavities, International Journal of Heat and Mass Transfer, 28 (1985) 603-612.
[13] Y.L. Chan, C.L. Tien, Laminar natural convection in shallow open cavities, Journal of Heat Transfer, 108 (1986) 305-309.
[14] A. James, G. Terry. Thermal performance of solar concentrator cavity receiver systems, Solar Energy, 34 (1985) 135-142.
[15] C. Balaji, S.P. Venkateshan, Interaction of surface radiation with free-convection in a square cavity, International Journal of Heat and Fluid Flow, 14 (1993) 260-267.
