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V o l u m e 1 - M a y 2 0 0 5 ( 6 - 9 )

1. Introduction

The necessity of the use of Solar Cookers in order to redu-ce the consumption of firewood or conventional fuel is wellrecognized by various national and international organiza-tions, including Costa Rica. In spite of various efforts made,the widespread of solar cookers have not become possibledue to different limitations of solar energy (Nandwani, 2003),including the impossibility of using during the period lackingsuffi-cient sunshine for cooking.

Its worth mentioning the personal experience in CostaRica. The author and his wife are using normal hot box solaroven at home almost regularly (whenever the climate per-mits), since March 1979 (Nandwani, 1999). For solarcooking planning, you just watch the sky, if its sunny, plan forcooking. In case its looks very cloudy, better do not use solarcooker. But the real problem comes, when you see clear skyin the morning and keep the meal for cooking, but after ahour, weak and then dark clouds appear and in another 30-60minutes, it may rain. The result is that food is not cookedcompletely. The uncooked meal can be taken from solar boxand cooking can be completed on the conventional electric orgas range, but not many users, including my wife, like thisoption. Unstable climate like this, although may happen only20-30 days a year, but is sufficient for a impatient cook to getfrustrated and forget all the advantages. This forced me to

design automatic hybrid solar cooker in 1986, (Nandwani,1989). It was really a success and no more frustration.

Also according to author's opinion, if the use of samesolar cooker could be prolonged, may be for some other use-ful purposes which does not require high solar intensity, likewater heating for pasteurization and distillation, or drying ofagricultural and other products, probably more people wouldlike to use the same.

The author has designed and studied for last 24 years dif-ferent solar devices solar cookers, solar still (Nandwani,1990), solar cooker cum drier and solar cooker cum waterheater (Nandwani and Fernandez, 1994). Objective of thepresent study is to combine all these gadgets in a single devi-ce (probably first of this kind and not observed by the authorin the literature), which could cook meals or heat water forpasteurizing, dry agriculture products, distill some liquid(mainly water). This is due to the fact that water heating anddrying need less solar intensity as compared to cooking. Inaddition this device is a hybrid, one can use electrical energy,if required to complete the desired process with minimumconsumption of electrical energy.

Two years ago, author made this multipurpose device athome for different practical uses and to study advantages,conveniences and limitations.

Abstract

AAuthor during last 24years has designed,

constructed, studied and pro-moted Solar Oven, HybridSolar/ Electric Oven, SolarOven cum Drier, and SolarCooker cum Water etc. In dif-ferent parts of the world,Solar Cookers have beenmade, studied, patented andeven used since 1850, howe-ver their real uses are verylimited, due to many reasons-unstable climate, economic, cultural, social aspects and singleuse etc. In order to overcome part of the problems mainly thelast one, author has recently designed one multi-purposesdevice and studied various technical and practical aspects. Ithas been used for cooking, heating/ pasteurizing water (to

inactivate Coliforms) anddistillation of small quantityof water (to remove differentminerals) and drying domes-tic products (fruits, vege-tables and condiments/ herbsetc.). For more than 2 yearsof use author and his wifehave found this to be a usefuldevice, mainly from conve-nience, fuel saving, econo-mic and also from ecologicalpoint of view. This device canbe used at any time and fordifferent uses but with the

reduced consumption of conventional fuel.

Keywords : Solar Cooker, pasteurizer, still, dryer, performan-ce studies.

Design and Studyof Hybrid Solar Cooker,

Pasteurizer, Still and Dryerin the Climate of Costa Rica

Shyam S. Nandwani

Laboratorio de Energía Solar, Departamentode Física, Universidad Nacional, Heredia

3000, Costa Rica,E mail: snandwan@una.ac.cr

Drying

Drying or Dehydration of different products (herbs, vege-tables and fruits etc.) is very important due to various rea-sons, especially in humid areas. When these products contai-ning lot of water, are kept in the ambient, get spoiled in fewdays due to high humidity. On the other hand the dried pro-duct, need less space to store, and when kept in some glass orplastic containers, last longer in good condition.

Purification Of Water

Millions of people become sick each year from drinkingcontaminated water. Children are especially susceptible. Anestimated 1.5 billion cases of diarrhea occur each year, resul-ting in the death of nearly 2 million children. Depending onthe quality of water, it needs to purify chemically (removeundesired minerals) and or microbiologically (inactivatepathogenic germs).

A. Microbiological aspects

It has been known that heat can kill pathogenic (disease-causing) microbes. Although most health organizationsrecommend that contaminated water can be made safe by boi-ling, but it is also true that same water can be pasteurized attemperatures well below boiling, as can milk, which is com-monly pasteurized at 67-71 °C, depending on the time(Ciochetti and Metcalf, 1984). The chart below (Metcalf,2002) indicates thtemperatures at which the most commonwater borne pathogens are rapidly killed, thus resulting in atleast 90 percent of the microbes becoming inactivated in oneminute at the given temperature.

Five minutes at this temperature would cause at least99.999 percent reduction in viable microbes capable of cau-sing disease.

B. Chemical Aspect

Distillation is the conventional process to separate thesolid particles from a liquid. In this way pure water is separa-ted from the chemically contaminated water. This de-minera-lized water can be used for, drinking, laboratory and batteriestoo. Thus when contaminated water is heated with sunshineand a solar cooker, water can be made safe for consumption.

2. Construction

Photo 1 shows the Conventional (Nandwani, 1988) andMultipurpose device. The later one is similar to a conventio-nal solar oven. In our particular case it is made of inclinedstainless steel box (for longer duration), with an outer dimen-sion, 53 cm x 35 cm and 20 cm high in front and 29.5 cm high

at the back side. It has an electric black plate as an absorbingsurface with an area of 0.13 m2 (0.28m X 0.46 m) and twonormal window glasses on the top, separated by a distance of2 cm. It also has one reflector to increase the solar radiationon metallic plate. The angle of the reflector can be variedusing a metallic strip with various holes. The reflector is alsoused to reduce the heat loss, once it covers the glasses, afterthe job is done. The whole box has a glass wool insulation onthe four sides and below the metallic plate. Electric plate hasa resistance as well a thermostat to regulate the plate tempe-rature. In order to collect the condensed vapor (water),coming from the tray, a rectangular trough is fixed inside thebox. This box is now a Hybrid Solar Cooker, ready to cookas well to pasteurize water.

To serve as a dryer, the box also has 2 holes in front of theoven (at lower level) for the entrance of ambient air as well asanother 3 holes at the back of the oven (at higher level as com-pared to holes on the front) for the exit of hot humid air. Theseholes could be opened or closed whenever required.

To serve the box as a solar still, an electric plate is repla-ced by a metallic tray, where the liquid to purify is filled.

To introduce the food to be cooked or products to bedried, or to fill the water for distillation, frame with twoglasses can be lifted.

3. Working

a. To use the device as aSOLAR COOKER(Photo 2), all the 5holes are closedtightly and the rest ofthe process is same aswith any conventio-nal oven. On the par-tially cloudy days,electricity is connec-ted to electric plateand the thermostat isfixed at about 100 °C. Food will be cooked with combinedSolar and electric energy, consuming the minimum quan-tity of electric energy.

Conventional Solar Hot BoxCooker (left) and Multi-uses

Solar Box, designed and studied byauthor.

Shyam S. Nandwani /ISESCO Science and Technology Vision - Volume 1 (May 2005) (6-9)

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Microbe

a. Worms, Protozoa cysts (Giardia,Cryptosporidium, Entamoeba)

b. Bacteria (V. Cholerae, Escherichia coli,Shigella, Salmonella typhi), Rotavirus.

c.Hepatitis A Virus.

Killed Rapidly at

55 °C

60 °C

65 °C

Multi-Uses Solar Hot Box in the modeof Cooking and Water heating.

Shyam S. Nandwani /ISESCO Science and Technology Vision - Volume 1 (May 2005) (6-9)

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b. To PASTEURIZE WATER (Photo 3) metallic containerscan be used although I use transparent glass jar. These withwater to be cleaned, are kept on the top of absorbing plate.Depending on the quantity of water and intensity of solarradiation, water attains the required temperature to get pas-teurized.

To know that the water is really pasteurized, or has rea-ched this temperature, or safe to drink, even when the useris not present at the site a simple device WaterPasteurization Indicator (WAPI) can be used (Photo 3).

Water Pasteurization Indicator is a clear polycarbonate plas-tic tube partially filled with a soybean wax that melts atabout 70 °C (158 °F). The WAPI, with solid wax at the topend of the tube, is placed at the bottom of a transparentcontainer filled with water that is solar heated. When usercomes after few hours, he has to just observe the position ofthe wax. If the wax melts and falls to the bottom of the tube,it ensures that water pasteurization condition have been rea-ched. We use transparent glass jar so that the melting/ soli-dification of the wax could be observed from outside.

c. To use de device as a SOLAR DRIER (Photo 4), the tray(s)with products to be dried are kept on the absorbing plate.During day, some or all the holes are opened. The hot anddry air inside the drier has the capability of taking moistu-re from the products, and this humid air leaves the drierthrough upper holes on the back of the drier. The processcontinues as long there is solar radiation. After the sun set(or during rainy period) all the holes are closed so that theambient humid air could not enter in the drier.

d. To use the multipurpose box as a SOLAR STILL (Photo 5),tray (instead of normal absorbing plate) is filled with waterto be cleaned. Here reflector is not used. With solar radia-tion, liquid gets heated and is converted to vapor. These hotwater vapors when rises and strikes the comparativelycooler glass surface, gets condense, slides along the surfa-ce of glass and is collected in the trough. Finally distilledwater is collected outside the box (Nandwani, 1990).

4. Experimental Study

Author and his wife are using this device for practicalpurposes- mainly for cooking, pasteurizing water and dryingdomestic products for last two years. On a sunny day andwithout any load, the plate temperature, only with solar radia-tion reaches about 130-140 °C. It works fine. During the par-tial cloudy days, absorbing plate is connected with electricalenergy to assure the complete cooking, with minimumconsumption (5-15 minutes) of electrical energy. Recentlyvarious types of experiments have been performed, for mea-suring quantitatively its performance.

In each case, global and integrated solar intensity, waterand air temperature, mass of the humid and dried product,quantity of water distilled are measured at different intervals.It is worth mentioning that in the case of water only the quan-tity and not the quality is studied as it has been studied andreported previously in another work (Nandwani, 1990,Nandwani et. al 1997).

Unfortunately due to wet season, the sudden change in theclimate, even on the same day, could not provide many datato report in this paper. Nevertheless some typical data are :

a. Water Pasteurization : (Figure 6)As mentioned already, two glass jars (one transparent andother painted black from outside) were used. Each one wasfilled with l of water (Total water pasteurized 1.5l).

Although the experiment was done on different days(May 19, 20, 21 and 29, 2003), the results are shown, in theFigure 6 for May 21. Due to low solar radiation, water attai-ned pasteurization temperature (65-67 °C) in 3 hours. Alsothe temperature attained in black painted jar (bjar) was slight-ly higher as compared to that attained in transparent glass jar(tjar). Due to higher thermal capacity of absorbing plate andglass jar, the water temperature is maintained for longer per-iod, inspite of very low solar intensity.

b. Solar Drying (May 23-25, 2003) : The real dryinghas been done with various products, but here results relatedto only Tomato and Curiander (herb for cooking) will bementioned.

384 g of fresh tomato and 109 g of fresh Coriander werekept in the solar dryer. Solar instantaneous and integratedintensity, external humidity, air temperature in the drier, andthe mass of the product were measured regularly. Most of thetime the climate was not good. However both products gotdried completely after 2.5 days. Figure 7 shows the solarintensity on the horizontal surface and air temperature in thesolar dryer, along with ambient air temperature measured onMay 24, 2003. Following table shows the summary of 3 daysresults.

Multi- Uses Solar Hot Box in the mode of Water Pasteurization (left)and Position of WAPI- before & after pasteurization.

Multi-Uses Solar Hot Box in the mode of Still, working for WaterDistillation (left) and in the measurement mode.

Multi- Uses Solar Hot Box in the mode of Drying, Initial fresh (left)and Final Dried Product.

Shyam S. Nandwani /ISESCO Science and Technology Vision - Volume 1 (May 2005) (6-9)

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Stopped Experiment. NM: Not measured.

Some of the relevant data/results are:-The integrated intensity during this period was 7155 wh/m2

or 0.930 kWh on an absorbing plate of 0.13 m2,- The air temperature inside the dryer was varying between

35-45 °C and the external humidity was above 80%. - Weight of the final dried tomato was only 34 g (reduction of

350 g, from initial weight of 384 g), - Weight of the final dried coriander was only 6.5 g (reduc-

tion of 104 g, from initial weight of 109 g),- total loss of weight or water evaporated is 0.454 kg. Using

sensible heat ad latent heat concepts, the energy requiredfor this drying process will be 258.7 kCal or 0.30 kWh

- Thus thermal efficiency of the box as a dryer is 32.3%.

Coriander was dried to the extent that it could be crushed

by hand to convert into powderand was kept in a plastic containerfor later use. Dried tomatoes insmall pieces were also stored inthe plastic container.

c. Solar Distillation (May 31and June 1, 2003) : On May31, 2003, 1 liter (about 1 kg) ofwater was kept in the tray. Solarinstantaneous and integrated inten-sity, external humidity, water tem-perature in the tray, and the mass

of the water distilled were measured regularly. Most of thetime the climate was not good. Figure 8 shows the solarintensity and water temperature in the tray as well as ambientair temperature, measured on May 31. During first two daysof the experiment some of the relevant data/results are :- The solar intensity during both days was very low. - The integrated intensity during this period was 7328 wh/m2

or 0.953 kWh on the absorbing plate of 0.13 m2. - The water temperature inside the tray was varying between

45-55 °C and the external humidity was above 80%. - Volume of water collected was only 390 ml (0.39 kg), - Using sensible heat and latent heat concepts, the energy

required for evaporating this water will be 222 kCal or0.258 kWh

- thus thermal efficiency of the box as a still come out to 27%.

5. Conclusions

Sun´s Energy not only can cook food, but also heat water to temperatures that kill harmful microbes, making water safeto drink.The multi-uses Solar hot box can be used in the house for drying the various products so that these can be stored forlonger period.

The device is not recommended to use for cooking and heating water or drying simultaneously, unless used in a verysunny climate. Based on our experience with the climate at our specific place and the temperature required for cooking, waterheating and drying, the combined device can be used for 10-11 months in a year for different uses, as compared to 7-8 monthsif used only for cooking.Now my food is either completely cooked (with minimum consumption of electrical energy if requi-red) or not cooked at all, but never partially cooked. Thus my wife does not get angry at me. Multiple use Solar Cooker is asimple solution to the complex problem like fuel, food, water and health.

AcknowledgementThe author is thankful to technicians Carlos Delgado Sanches for help in constructing the device and to Mr. Rafael

Ramirez for making some of the measurements. The author is also grateful to the Abdus Salam International Center forTheoretical Physics, Trieste, Itlay, for allowing to stay for 3 weeks as an Associate Member, where most of the experimen-tal data were analyzed.

Outside airhumidity %

72- 90%

77- 90%

Date

23/5/0323/5/03

24/5/0324/5/03

25/6/0325/6/03

LocalTime

09:0013:00

07:0017:00

07:00 12:30

Hr.

04

414

1419.5

Integrated SolarInt. Wh/m2

0 1091520 77

1520 NM+3185=4705 7.5

705 NM+2495= 6

Mass of theproduct (g)

384.5319.5

NM137.9

NM

CuriandarTomato (total)

493.5396.5

NM145.4

NM

ReferencesCiochetti, D.A. and R.H. Metcalf, 1984, Pasteurization of naturally contaminatedwater with solar energy, Applied and Environmental Microbiology, Vol. 47, pp.223-228.

Metcalf, Robert (2002), The Science of Safe water, Solar Cookers Review, SolarCookers International, CA, USA, SCR No. 8, November.

Nandwani, Shyam S. (1988), Experimental and Theoretical Analysis of a SimpleSolar Oven in the climate of Costa Rica- I, Solar and Wind Technology, Vol. 5(2),pp. 59-170.

Nandwani, Shyam S. (1989), Design, construction and Experimental study ofElectric cum Solar Oven, Solar and Wind Technology (Pergamon Press), Vol.6(2), pp. 149-158.

Nandwani, Shyam S. (1990), Economic analysis of Domestic Solar Still in the cli-mate of Costa Rica, Solar and Wind Technology (Pergamon Press), Vol. 7, No.2/3, pp. 219-227.

Nandwani, Shyam S, and Otoniel Fernandez (1994), Experimental Study of SolarOven cum water heater and Solar Oven cum Drier- II, Published in the procee-dings, II World Conference on Solar Cookers, Use and Technology, UniversidadNacional, Herdia, Costa Rica, July 12-15, pp. 273-284.

Nandwani, Shyam S, Josef Steinhart, H,M. Henning, M. Rommel and V. Wittwer(1997), Experimental Study of Multipurpose Solar Hot Box at Freiburg,Germany., Renewable Energy (Pergamon Press), Vol. 12, No. 1, pp. 1-20.

Nandwani, Shyam S. (1999), My Twenty Years of Experience with Solar Cookingin Costa Rica in Costa Rica- Satisfaction and Frustrations, Shyam S. Nandwani,Proceedings, World Conference on Solar Cooking and Food Processing-Strategies and Financing, held at Varesses, Italy during October 3-6, 1999.Published in the proceedings, pp. 73-81.

Nandwani, Shyam S. (2003), Solar Cookers- What we are looking for, submittedfor presentation at ISES 2003 Conference on Solar Energy, Goteborg, Sweden,June 15-19.