Rekha S Patel1, P D Patel, P C Vinodkumar and K N Joshipura

1VP & RPTP Science College, Vallabh Vidyanagar — 388 120, Gujarat, INDIA

Deptt of Physics, Sardar Patel University, Vallabh Vidyanagar — 388 120, Gujarat, INDIA

The present investigation aims at a simultaneous theoretical and experimental investigation on different aspects of box type solar cooker, and its conversion as a dryer. A theoretical model has been attempted to simulate various thermal processes in a box type solar cooker. The theoretical study has also helped us to identify the sensitive heat exchange coefficients of the different cooker elements. The cooker-dryer designed presently has a potential of enhancing the device utility factor by a fairly large magnitude. This device will also give boost to adopt the solar energy utilization especially in the tropical developing countries.

Introduction

Energy technology is a systematized knowledge of various branches of energy flow and their relationship with the human society as viewed from scientific, economic, social, technological and industrial aspect for the benefit of man and environment. Though the nature supplies abundant renewable energy, the technologies for conversion are in early stage of development and not yet commercially as successful as the conventionals. However, utilization of renewable energy is on the path of rapid rise all over the world. Energy in various forms has played an increasingly important role in worldwide economic progress and industrialization. In view of the worlds depleting fossil fuel reserves, which provide the major source of conventional energy, the development of non-conventional renewable energy sources has received an impetus. In countries like India, sunlight available freely as a direct and perennial source of energy provides a non-polluting reservoir of fuel. There are two main approaches currently in use to harness solar energy, namely (i) by converting solar energy directly to electricity by the photovoltaic cells and (ii) by converting solar energy to thermal energy by photo thermal conversion. The simplest way to utilize solar energy is to convert it into thermal energy for heating applications. The acceptability of such an approach depends upon the overall efficiency of the system, simplicity of operation, design and its cost effectiveness.

Solar energy has a potential to overcome the energy crisis partially. Solar cooker offers a simple, safe and pollution-free alternative for harnessing solar energy. For household applications quite a large amount of energy is used for various day today activities. The most common activities are cooking, drying, water heating etc. The cooking is a basic activity of human beings and it consumes substantial amount of the energy. Solar cooker has a potential to supplement towards the energy need for the conventional cooking. Several designs of solar cookers are available in the market the world over. A simple solar cooker generally known as "Hot Box" directly converts solar radiation into heat energy. The performance of a solar cooker is governed by basic thermal transfer properties of its elements and the performance is characterized in terms of figures of merit F1 and F2 as defined in equation (8) and (9) given below [1,7]. It is also of interest to simulate the temperatures of various elements by solving a system of coupled first order differential equations that incorporate heat exchanges [2]. It helps us to identify the more sensitive parameters, which in turn provide ways and means of improving the efficiency of the cooker. The present investigation aims at a simultaneous theoretical and experimental investigation on different aspects of box type solar cooker, and its conversion as a domestic dryer.

Since time immemorial, people have been using the solar energy for drying food items. Use of solar energy for drying results in saving of conventional fuels. Solar drying occupies a unique position in the field of food processing, preservation and transportation. Fairly good amount of research work has been carried out and presented by many research workers,

[3,4,5] . Solar drying techniques have been adopted for drying a wide range of products. Drying of food materials especially vegetables and fruits should essentially be a low temperature operation, since at higher temperatures there is a likelihood of destruction of nutrients, texture and flavours. It is well known that solar energy harnessing exhibits better efficiency in the temperature range of about 60 to 75 0C, a temperature range closely matching with the desired range for drying. In the present study, an attempt has been successfully made to achieve this by way of providing an attachment to convert a conventional box-type solar cooker into a passive domestic mini solar dryer. This mini solar dryer would meet the purpose of providing a mini domestic solar dryer, with drying capacity of about 500 gms/day. The drying characteristics of a low-cost solar cooker — converted — mini dryer have been studied and reported in this paper. The present work has optimized the device operations for multi purpose utility.

Box-Type Solar Cooker

(a) Theoretical Methodology

The complete thermal analysis of the cooker is complex due to the three dimensional transient heat transfers involved. However, the standardization procedure should be reasonably simple in order to make implementation easy [6]. A theoretical model has been attempted to simulate various thermal processes in a box type solar cooker. The theoretical model consists of the seven coupled heat transfer equations, which are first order differential equations. We have carried out theoretical simulation of thermal exchange processes amongst different elements of the ‘hot box’ solar cooker, by solving the heat exchange differential equations, using the standard 4th order Runge Kutta method given by [2]. The following assumptions are made for the thermal analysis: (a) the solar box type cooker consists of seven elements. (b) Initially all the elements are at ambient temperature. (c) The temperature of each element is constant at a definite time. The seven elements of the cooker Aluminium body Solar Cooker (ABSC) under study are: 1. Upper transparent cover (g-i); 2. Lower transparent cover (g2); 3. Absorber plate (P); 4. Internal hot air (H); 5. Insulation (Y) 6. Cooking pot (k), and 7. Water (w). The heat balance equations for each of the elements are expressed as follows.

iv) Thermal equilibrium of absorber plate:

MpdTf(t) = (Ap-E All) “ P TT 1 — (AP-1 Aki)hpH (Tp — TH) —

AP hpy (Tp — Ty) — Aghpg (Tp — Tg2)} -1 Aki hpk (Tp — Tk )

v) Thermal equilibrium of insulation:

dT

M —40 = Ap hp (T — T ) — Ah (T — T)

У df p Гу p y y yc y c

vi) Thermal equilibrium of cooking pot:

Mk dT^t) = 1 Аи[«к TglTg2 1 — hkg (Tk — Tg2) + hpk (Tp — TK)] — Ati [hш (Tk — Th )

+ hkw (Tk — Tw )]

vii) Thermal equilibrium of the food in the ith cooking pot:

Mw^(t) =1 Akihkw (Tki — Tw )

The theoretical study has also helped us to identify the sensitive heat exchange coefficients of the different cooker elements. The input for the above theoretical investigation includes the physical dimensions of the cooker, heat transfer coefficients, the solar insolation etc.

(b) Experimental Set-up

Apart from the theoretical simulation discussed above, we have established a system for temperature measurements for various elements of the cooker. Pt100 RT sensors are used to measure the temperatures of the vital elements of the solar box cooker [7,8]. A comprehensive study is carried out in our laboratory by comparing experiments with our theory. A significance of this work is that theoretical modeling is carried out along with the experimental investigation in our own laboratory for the purpose of a comparative study.

The figures of merit of both the cookers were evaluated. The figures of merit F1 and F2 are calculated from the standard expressions [1,6] given below

F = Tp — Та

1 Is

1- Tw1 — Ta

F1 Is

1 — Tw 2 — Ta F1 Is

where the symbols are as follows:

F1 = First figure of merit; F2 = Second figure of merit;

TP = Stagnation plate temperature; Ta = Ambient temperature;

Is = Solar insolation on horizontal surface at stagnation temperature. m = mass of water; c = Specific heat of water; A = glazing area; ti = initial time; t2 = final time; Twi = Initial temperature of water Tw2 = Final temperature of water.

The box type solar cooker conversion to a domestic dryer

Over and above the theoretical and experimental studies, an attempt has been made to convert the solar cooker to work as a domestic mini solar dryer by means of an appropriate additional attachment over it. In the first variety, it is a wooden attachment with appropriate physical dimensions. In the second attachment, glass on each side fitted in the aluminum frame, but one side fitted with mirror to enhance the solar insolation as seen in the photograph 1.

The main motivation for this work is to design a two-in-one solar passive device, portable, easy to handle, befitting for a domestic usage and pollution free also. In early attempts it was found that the water droplets were condensing on the inner glass cover and falling on the items

and thereby deteriorating the quality of the dried item. After tria—and-errors, we have optimized the design. The drawback were totally removed by providing additional heights to the vertical vent pipes at the outlet end, thereby enhancing the air draft due to better chimney effect. The temperature of the drying item during the process of drying critically depends upon the air draft. The height of the vent pipe was so adjusted that the temperature of the item remains in the ideal range. In a sunny day one can dry about 400 to 500 gms of different types of vegetables and fruits. This is a almost normal requirement for an Indian / Asian family.

Results and discussions

We now discuss the results of the present investigations separetely for (a) Solar cooker and (b) for solar dryer.

(a) Solar cooker

As a first step we determine experimentally the figures of merit F| and F2 of two different designs of the solar cooker in our setup of measurement, as discussed in [7] these measurements have shown satisfactory results [6,9].

Now, we have here studied the unloaded cooker i. e. without cooking pot and the water. The absorber plate temperature profile of the cooker system, which achieves the highest stagnation temperature amongst
all the five elements. Figures 1 here exhibit the calculated and observed plate temperature profiles for the cooker. We notice that our theory and measurement agree well during rising temperature up to the stagnation value, but in the region of falling temperature there is a discrepancy. Our theoretical values are falling faster than our observed data. This deviation can probably be ascribed to the thermal inertia of the system-playing role in the cooling part, which has not been incorporated in the theoretical simulation.

In order to explore the reason for this discrepancy we have considered different values of the heat transfer coefficient hpy in the particular case of absorber plate.

The figure 2 expresses the sensitivity of the plate temperature profile on the typical heat transfer coefficient hpy., the heat transfer coefficients between absorber plate and insulation of the cooker system. For a given cooker system the value tpy =10.5 W/m2 0C is quite reasonable because it gives the temperature of the plate about 1400C as reported by Garg etal [1].

The figure 3 shows theoretically simulated characteristics of seven elements of the cooker. Here we find that the cooking pot achieves the maximum temperature, and the lower glass cover, the hot air and the absorber plate exhibit more or less the same temperature. The characteristic curve for water possesses its own nature because of its specific heat

Time (Min)

Figure 3: Temp. profiles of the seven elements

(b) Solar Dryer

Apart from the solar cooker, we have also obtained the characteristics of the solar dryer designed presently. The figure 4 shows the characteristics of the empty solar dryer. The absorber plate being metallic in nature, the temperature increases faster initially and reaches the stagnation temperature earlier than the cabinet air temperature. The air temperature though belatedly achieves the desired drying temperature but then remains almost constant for quite a long time. The third curve shows the insolation profile of the particular day.

We have actually carried out the drying of a few vegetables like potato, onion bitter gourd etc. The figure 5 shows the drying characteristics of certain items under a typical insolation.

The quality of the dried items has been found to possess good texture, with dust free good quality finished product nearly at par with those obtained with the other commercial dryer units.

The cooker-cum dryer is a low-cost (around 70-80 US$) exclusively two-in-one device. The device has a potential of enhancing the utility factor by a fairly large magnitude. This device will also give boost to adopt the solar energy utilization especially in the tropical developing countries.

References

Garg H P and Kandpal T C 1999 Laboratory Manual on Solar Thermal Experiments, Narosa Publishing House, New Delhi

Binark A K and Turkmen N. 1996. "Modelling of a Hot Box Solar Cooker.” Energy Convers. Mgmt. 37: 303

Chauhan P. M. and Patel N. C. 1989 "Sun drying characteristics of groundnut under the drying practice adopted by the farmers of gujarat”, Solar Drying Proceedings of national workshop p85-91, Himanshu Publication, Udaipur.

Anwar S I and Tiwari G. N “ Thermal analysis of a multi-tray crop drying system using solar energy” SESI 10 (2000) 79

Mathur A. N., Yusuf Ali and. Maheshwari R. C 1989, “Solar Drying” Himansu Publications, Udaipur, and references therein.

Mullick S C, Kandpal T C and Saxena A K, Solar Energy 39 (1982) 353 Patel Rekha S,. Patel P. D, Vinodkumar P. C. and Joshipura K. N. 2003 “Thermal Testing and Analysis of the Conventional Cooker and the Plastic Body Cooker”. Advances in Renewable Energy Technology, Narosa Publishing house, New Delhi, India p71-76.

Patel Rekha S, Patel P D, Vinodkumar P C and Joshipura K N, 2002. “ Theoretical — cum — experimental studies on the performance parameters of a box solar cooker”. “Proceedings of the twenty-sixth National Renewable Energy Convention of Solar Energy Society, India and International Conference On new Millennium — Alternative energy solution for sustainable development. PSG Tech, Coimbatore” Tamilnadu. Chaudhuri T K, Renewable Energy 17 (1999) 569