HEAT LOSSES FROM A PARABOLOID CONCENTRATOR SOLAR COOKER

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HEAT LOSSES FROM A PARABOLOID CONCENTRATOR SOLAR COOKER Powered By Docstoc
					Renewable Energ3 Vol. 3. No. 8. pp. 871 876. 1993                                                              096(} 1481 9 3 $6.00+.00
Printed in Great Britain.                                                                                            Pergamon Press Ltd




                        HEAT LOSSES FROM A PARABOLOID
                         CONCENTRATOR SOLAR COOKER:
                   EXPERIMENTAL INVESTIGATIONS ON EFFECT OF
                            REFLECTOR ORIENTATION

                               SUBODH KUMAR, T. C. KANDPAL a n d S. C. MULLICK
                    Centre for Energy Studies, Indian Institute of Technology, Delhi, New Delhi-110016, India

                                                (Received t5 March 1993 ; accepted l April 1993)

           Abstract--The thermal performance of any focussing-type solar cooker, where an unglazed/uninsulated
           cooking pot is often used, depends to a great extent on wind conditions. Moreover, these cookers need
           frequent adjustments to track the Sun in order to keep the focus always at the bottom of the cooking pot.
           The present paper reports experimental investigations on heat losses from such a cooker for different
           orientations of the paraboloid reflector. Values of the heat loss factor for the tilted reflector are compared
           with those obtained with the reflector in a horizontal position. The heat loss factor for a cooker with/without
           reflector is determined for no-wind conditions. It is suggested that a paraboloid reflector is not required
           for heat loss determination in this case.


                        I. I N T R O D U C T I O N                       marily consists of a paraboloid reflector and a G.E.C.
                                                                         axial flow fan (1 h.p.) to simulate wind at different
Concentrating-type solar cookers were initially
                                                                         speeds in a controlled manner. The paraboloid reflec-
developed during the 1950s and 1960s. Since then,
                                                                         tor is made of back-coated glass and has an aperture
several models of such cookers have been fabricated
                                                                         area of 0.58 m -~.The aluminium cooking pot used with
and tested in many countries around the globe [1-4].
                                                                         the paraboloid reflector has a diameter of 0.17 m and
Amongst the various designs developed so far, the
                                                                         height of 0.065 m, and is coated with dull black paint
paraboloid concentrator solar cooker has received the
                                                                         on its outer surface. A fan is placed at a suitable
most attention [5 9]. Recently, a test procedure for
                                                                         distance ( ~ 2.5 m) in front of the paraboloid con-
thermal performance evaluation of the concentrating-
                                                                         centrator solar cooker, and at a convenient height
type solar cooker, in general, and the paraboloid con-
                                                                         from the ground. A three-cup wind speed anemometer
centrator solar cooker in particular, was proposed by
                                                                         consisting of a chopper-type sensor and an electronic
the authors [10]. The heat losses from a paraboloid
                                                                         translator is used to measure the wind speed. The
concentrator solar cooker primarily depend upon the
                                                                         sensor is designed to indicate wind speed from 0 to 50
pot water temperature, wind speed, surface area of
                                                                         m s ~. A specially designed spiral-ring-type electric
the cooking pot and orientation of the reflector. Very
                                                                         heater made of nichrome wire encased in stainless
little effort has so far been made to study the effect of
                                                                         steel sheathing of 10 mm diameter is suspended from
reflector orientation on heat losses from such cookers.
                                                                         the cooking pot lid by bolts. The design of the heater
This paper presents the results of an experimental
                                                                         is such as to provide a uniform heat flux to the water
investigation into the effect of the reflector orientation
                                                                         in the pot. It is ensured that the heater is well immersed
on the heat loss factor, F'UL, for a paraboloid con-
                                                                         in the water during experimentation. Figure 1 shows
centrator solar cooker. Such a study is quite impor-
                                                                         the indoor experimental arrangement with a hori-
tant, since a paraboloid reflector would need con-
                                                                         zontal paraboloid reflector.
siderable reorientation during the day so as to follow
the diurnal movement of the Sun (to ensure that the
concentrated solar radiation continues to fall on the
                                                                              3. TEST P R O C E D U R E FOR DETERMINATION
bottom of the cooking pot).
                                                                                             OF HEAT LOSSES
                  2. EXPERIMENTAL SET-UP
                                                                               The cooking pot with one litre of water is placed
   In order to carry out the experimental inves-                            at the focus of the paraboloid concentrator, kept in a
tigations an indoor test set-up was developed. It pri-                      horizontal position at a given distance from the fan.

                                                                      871
872                                               SUBODHKUMARet al.




                         Fig. 1. Experimental arrangement with a horizontal paraboloid reflector.


The fan provides a wind speed of 5 m s ~at the pot.           orientations of the reflector, i.e. : (i) reflector tilted at
Water in the pot is heated by an electric heater with         45 ° and its front surface facing the wind (Fig. 3) ; (ii)
its input controlled by a servo voltage stabilizer. A         reflector tilted at 45 ° and its back surface facing the
wattmeter is connected in the circuit to measure the          wind (Fig. 4); (iii) reflector tilted at 45 ° and facing
energy input to the heater. The rise in water tem-            the wind sideways (in cross flow) (Fig. 5). These tests
perature is measured using calibrated copper-con-             were conducted for a wind speed of 5 m s-~ as well
stantan thermocouples and a digital panelmeter                as for no-wind conditions for all the four reflector
(DPM). On reaching steady-state conditions, the pot           orientations.
water temperature, ambient air temperature, wind                 During all the above experiments it was ensured
speed and electric power input are recorded. Wind             that the cooking pot was always placed at a constant
speed is measured by keeping the anemometer at                distance from the fan and also that the height of the
exactly the same location as that of the cooking pot          cooking pot from the level of the ground was also the
in the presence of the paraboloid reflector (Fig. 2).         same for each experiment. Such an arrangement is
The test is repeated for different values of pot water        necessary to keep the pot in similar wind conditions
temperature by varying the power input to the electric        for all the four reflector orientations. In order to deter-
heater.                                                       mine the role of the reflector in heat losses from the
   For a given wind speed, under steady-state con-            cooking pot in no-wind conditions, a similar heat loss
ditions, the overall heat loss factor F ' U L is obtained     experiment was conducted by keeping the cooking pot
by using the following relation :                             on an iron tripod (in the absence of the reflector).
                               W
                F'UL -
                          (Tw-- Z~) × A '                                4. RESULTS AND DISCUSSION
where W represents the electric power input (W) to               The variation of F ' U L of the cooking pot with the
the heater, Tw the pot water temperature (°C), Z~ the         pot water temperature is shown in Fig. 6. The reflector
ambient air temperature (°C) and A the surface area           is in the horizontal position during all the measure-
of the cooking pot (m2).                                      ments shown in the figure. One of the curves cor-
  Tests have also been carried out for three other            responds to the case of no wind, while the other rep-
             Heat losses from a solar cooker                   873




 Fig. 2. Measurement of wind speed by anemometer.




Fig. 3. Tilted reflector (45' from horizontal), facing wind.
874




           Fig. 4. Tilted reflector (45' from horizontal), with its back facing the wind.




      Fig. 5. Tilted reflector (45 '~ from horizontal), facing the wind sideways (in cross flow).
                                                   Heat losses from a solar cooker                                                     875

  o•65                                                                                                          Wind speed: 5m / s
                      Wind speed: 5m / s       J                                    --x-- Horizontal reflector
                                   ...o---""                                        - -- Tilted reflector, facing wind sideways
                                                                           65

                                                                           55 F
                                                                                t         Tilted rcflcctor,frontfacing wind


                                                                                                             __xl:~               "A

  o                   N o wind

                                                            I              z~ g"°"--i                  I         I          i          J
        5o       60       70         80        90          100               50           60          70         80        90      100
                  Pot water temperature ['I'w(*C)]                                             Pot watez tempe~rature [Tw( *C)]
   Fig. 6. Variation of F'UL with pot water temperature.                   Fig. 7. Effect of reflector orientation on F'U~.




resents the observations for a wind speed of 5 m s t.               for the horizontal reflector. As regards some typical
As mentioned elsewhere [11] it is noted that initially              numerical values from the study at a water tem-
F'UL increases linearly with pot water temperature,                 perature of 5 5 C , the F'UL value decreases from 44.0
but a rapid increase in its value is observed for tem-              W m -" C ~for a horizontal reflector to 29.0 W m "
peratures above 9 0 C . In the particular cases pre-                 C t for the case when the back of the tilted reflector
sented in the figure F'Ut. varies from 5.5 W m 2 C                  is facing the wind. Similarly, at 95'C the F'UL value
to 15.3 W m ~ C t for no-wind conditions, and from                  decreases from 61.0 W m " C t for a horizontal
4 1 . 5 W i n ~ C ~ t o 6 1 . 0 W m 2 C ~at a wind speed            reflector to 44.0 W m ~ ' C ' for the case when the
of 5 m s ' as pot water temperature is varied from 50               back of the tilted reflector is facing the wind. In fact,
to 95' C. The results of the tests conducted on the                 it may be noted from the curves in Fig. 7 that tbr
cooker with different reflector orientations in no-wind             almost all values of water temperature, the F'UI
conditions indicated that, for a given pot water tem-               decreases by 33% as the reflector is tilted (45 from
perature, there is no change in the F'UI. value with                horizontal) with its back facing the wind from its
the orientation of the reflector. In fact, even in the              horizontal position. As a comparison, the reduction
absence of the reflector, the F'UL values obtained are              in the F'UL values for sideways/front tilted reflector
the same as those obtained with the reflector in no-                arrangements is of the order of only 5%. Thus, it may
wind conditions. In other words, the no-wind con-                   be inferred that any orientation of the reflector other
dition curve shown in Fig. 6 is valid for any tilt and              than its horizontal position leads to a reduction in the
orientation of the reflector, as well as tk~r a cooker              F'Ut value and, consequently, an improved thermal
without a reflector. This implies that the reflector does           performance. It is worthwhile to mention that such a
not interfere with the natural convection currents due              reflector cooker will often be used in positions other
to the hot cooking pot (in the surrounding air).                    than the horizontal so as to make its aperture normal
    As discussed in Section 3, the experiments for deter-           to the direction of the beam component of solar radi-
mining F'U L were conducted for four different                      ation. This reduction in F'UL may be attributed to the
arrangements of the reflector with respect to the fan               fact that wind speed available at the cooking pot does
providing wind for sirnulation experiments. Results                 not remain the same if the reflector is tilted from its
of these experiments are shown in Fig. 7. All these                 horizontal position.
observations have been obtained for a wind speed of
5 m s ~. As expected, the F'UL value increases with                                        5. CONCLUSIONS
pot water temperature for all the arrangements of the
                                                                      The following conclusions can be drawn from this
reflector. For a given water temperature, the F'UL.
                                                                    preliminary study :
value is a maximum for the horizontal reflector (Fig.
 1) and is a minimum for the case of the reflector tilted           1. F'UL is found to be strongly dependent on pot
at 45 to the horizontal, and with its back surface                     water temperature and wind speed, increasing con-
facing the wind (Fig. 4l. The F'UL values for the other                siderably with both of these parameters.
two reflector arrangements (Figs 3 and 5) are almost                2. UUL decrcases as the tilt of the reflector is
equal, and are relatively closer to the F ' U L values                 increased from its horizontal position.
876                                                 SUBODH KUMAR et al.

3. F'Uu remains the same for any tilt o f the reflector           4. H. Tabor, A solar cooker for developing countries. Solar
   for no-wind conditions. Further, in such con-                     Energy 10, 153- 157 (1960).
                                                                  5. J. A. Duffle, G. O. G. L6f and B. Beck, Laboratory
   ditions, the presence o f the reflector surface has
                                                                     and field studies of plastic reflector solar cooker. Solar
   little effect on F'Ut value.                                      Energy 6, 94 98 (1962).
                                                                  6. M. L. Ghai, Performance of reflector type direct solar
Acknowledgements--The authors are highly grateful to the             cooker. J. Sci. Ind. Res. 12A, 165 175 (1953).
National Physical Laboratory, Delhi, for providing a par-         7. M. L. Khanna, Heating of vegetable oil. Solar Energy
aboloid concentrator cooker for experimentation.                     6, 60 63 (1962).
                                                                  8. S. Natraja Murthy and N. Narasaiah, Energy dis-
                                                                     tribution in the focal region of an offset feed paraboloid
                                                                     solar stove. Solar Energy 34, 279 285 (1985).
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