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HUSSAIN and KHAN 1996 constructed a low cost box-type solar cooker with two paper carton

 boxes with crumpled newspaper balls as insulation with a tilted aperture plane. Comparative tests

of this cooker have been conducted against a normal type costlier solar cooker with 1000 ml load

of water in each of the cookers. Figure 1 shows the constructional features of cooker. The

sidewalls of the inner carton have aluminium coated polythene sheets attached to them which act

as reflectors.

They observed that the temperature in the new cooker rises more rapidly initially compared to

the normal type solar cooker with 1000 ml load of water in each. But at temperatures higher than

9O°C, both the cookers work similarly as shown in figure 2 & 3.

For cloudy conditions an arrangement of auxiliary heating is also provided with a 100Welectric bulb inside the blackened metal casing. They observed that the water temperature is

raised up to 100°C easily on a partially cloudy day with fluctuating radiation having an average

of 505w/m2.

Table 1 shows the time taken by the two cookers to reach 95°C.

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Fig. 2 is for the new cooker without auxiliary heater having its aperture plane inclined at an angle

of 30°C while fig. 3 is for the new cooker with auxiliary heater having its aperture plane inclined

at an angle of 10°C. The data obtained on 22- 12-95 and 23-9-95 for respective plots.

They also perform a quantitative study for two figures of merit F1 and F2 (Saxena et al.,

1985). The first figure of merit F1 is defined as the ratio of optical efficiency ηo to heat loss

factor UL at stagnation. The first figure of merit F1 is determined by the following equation,

T ps - Ta

F1=

ηₒ Is 

Where,

T ps = stagnation temperature.

Ta = ambient temperature.

Is = solar insulation on a horizontal surface at the time of stagnation.

ηₒ  = optical efficiency. 

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The second figure of merit has been determined from the equation,

Where,

t = Time interval during which water temperature rises from Tw1 = 60°C to Tw2 = 95°C

A = Aperture area (48x38) cm2 

M = Mass of water (1000 ml.).

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As the aperture plane of the new cooker was inclined the measured global radiation

values have been multiplied by the tilt factor. The values of the first figure of merit have been

found to be 0.14 for the new cooker and 0.15 for the normal type cooker.

The values of the second figure of merit have been found to be 0.40 and 0.46 for the new

cooker while the value for the normal type cooker was 0.32 obtained from an earlier investigation (Das, 1994).