S. Raab, Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart

D. Mangold, Solar — und Warmetechnik Stuttgart (SWT), ein Forschungsinstitut der Steinbeis-Stiftung

W. Heidemann, Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart

H. Mtiller-Steinhagen, Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, Solar — und Warmetechnik Stuttgart, German Aerospace Centre (DLR) Institute for Technical Thermodynamics

In Friedrichshafen, close to the Lake of Constance, a solar assisted district heating system with seasonal hot water heat store was put into operation in 1996. The seasonal storage is realised by a hot water heat store constructed from reinforced concrete. At present the solar assisted district heating system is extended and additional collector fields and buildings are connected to the system. In this paper the system heat balances of the last seven years and the most important experiences are reported.

System description

Figure 1 shows the planned development of the settlement in Friedrichshafen. It is divided into three phases of extension. In the course of the realisation of the 1st and 2nd extension solar collector fields are integrated in the district heating system. When realising the 3rd extension additional buildings will be connected to the system, but no additional collector fields will be integrated.

After completion of the first extension in 1996 280 apartments (in multi­family houses) and one kindergarten were connected to the district heating system. The heated area amounts to around 23 000 m2. On top of the buildings of the first extension 2 700 m2 of flat plate collectors have been mounted, divided into seven fields. The inclination angle varies between 20 and 25 °, the orientation deviates from ideal south orientation up to 110 ° to the north-west. The energetic optimum inclination angle range of the collector fields is between 30 and 45 ° facing to south. The seasonal hot water heat store, which was also built in 1996, has a water volume of 12 000 m3 and is energetically designed for the integration of an additional extension with 2 700 2 of solar collectors. The hot water heat store transfers the heat collected in summer by the solar collectors to winter when the heat demand in the district heating net is comparatively high. In figure 2 the schematic layout of the system is shown. In 2002 the realisation of the second extension started. Contrary to previous planning of approx. 280 apartments (in multi-family houses) around 110 accommodation units mainly in terraced houses are being built. Originally the integration of 2 700 m2 of solar collectors was planned for the year 1998. Due to the decreased roof area only around 1 700 m2 of solar collectors will be integrated in the system after the completion of the second

extension. In 2002 and 2003 two collector fields of 413 m2 and 400 m2 respectively have been installed as roof integrated field or mounted on a subconstruction. In figure 3 an example of a roof integrated collector field (multi-family house of the first extension) is shown.

Heat balances

In table 1 the system heat balances for the years 1997 to 2003 are shown. The achieved solar fraction (based on total heat demand) varies between 21 and 30 %. The design solar fraction related to the 1st phase of extension was calculated to 43 %. This value has not yet been reached due to several reasons. The heat consumption of the buildings (1st extension) is approx. 10 % higher than expected. Furthermore the design return temperatures of the district heating net were supposed to be lower than 40 °C (yearly average weighted by volume flow). In 2003 this value rose to 51.5 °C. The heat losses of the seasonal heat store are in the range between 322 and 360 MWh/a and are significantly higher than the calculated value of 220 MWh/a.

The efficiency factor of the gas condensing boiler during the last years of operation amounts from 94 % to 100 % (based on lower heating value Hu). The solar collectors’ heat gain of the 1st extension amounted to 941 MWh in 2003 (gross solar heat gain including heat losses caused by collector pipes; as measured at the solar heat exchanger). This is around 5 % less than in 2002 whereas the irradiation on the collector plane is about 13 % higher. This is caused by stagnation of the collector field from 7/21/03 to 8/18/03 (corresponding to an irradiation on collector plane of 175 kWh/mF) because of necessary work in the heating plant.

The degree day value for 2003 amounts to 3 931 Kd and is high compared to the years 1997 to 2002 owing to relatively cold winter months. The overall heat delivery to the district

heating net in 2003 (1st and 2nd extension) was 3 325 MWh. This is significantly higher than in the previous years, mainly due to the connection of new buildings in the supply area of the 2nd extension. In figure 4 the heat flow of the plant is depicted in a Sankey diagram for the year 2002.

SHAPE * MERGEFORMAT

Due to the high direct insolation in the Altiplano and the lack of firewood, concentrating systems have a high acceptance for solar cooking in this region. A study, which was car­ried out in 1998 has shown, that the firewood consumption could be reduced by 70 percent by solar family cookers of the SK14 type. A production of these solar cookers created new jobs in the local cooperative Pirca. Besides solar family cookers also community cookers are interesting. In order to improve the acceptance of cooking with solar power, it is nec­essary to set up a permanent installation of the cooker within a building. This can be achieved using a concentrating reflector, which has a fixed focus. In this project, a Schef — fler reflector was used. This kind of mirror is a section of a much bigger parabolic shape. With a reflector area of 8.8 m2, the aperture measures 5.5 m2. The reflector is fixed on a polar axis. This makes daily tracking with the sun easy, since only a uniform rotation
throughout the day is required. The seasonal adjustments to the sun’s position are ob­tained by adapting the angle and at the same time changing the shape of the reflector. The different shape is achieved by bending. So, at different angles of incidence the focus can be kept at the same place by shortening or lengthening the focal distance. The uniform rotation of the mirror is done by a electromechanical tracking system which works with two little PV-cells as sensors. Many hundred Scheffler reflectors have already been installed in many different countries (see http://www. dfg-vk. de/SolareBruecke/). The Workshop which was established as part of the BMZ project now makes a local production in Argentina possible. Within one year, 7 units have already been produced, including two solar bakery ovens. Due to the great power of the systems the acceptance in the population was found to be very high.

Technical data:

Surface of the Aperture (depending on the season): 4.5 — 6 m2

Maximum temperature: 1000°C

Optical efficiency level (aluminium/glass mirror): 75% / 65%

Maximum performance (ldir= 800 W/ m2): 3 kW

Size of cooking pot: 30 -80 l

Market price of reflector system in Argentina: 980 Euro

A model was used to calculate the boiler efficiency (and further the fuel consumption) on a monthly basis using the hydraulic energy values supplied by the boiler to the domestic hot water tank and to the space heating system. The main idea of this model is to sum up the "Nominal standby heat loss energy” corrected by two factors. Factor one estimate the standby time per month and is simply calculated by the hours of the month minus the hours of running the boiler on nominal power for supplying the necessary energy. Factor two estimates how big are the stand by losses of a boiler depending on the thermal mass and the approx. number of starts per day. Very heavy boilers have a factor from 0.7 to 1.0, very light boilers have a factor of 0.2 to 0.5. Detailed descriptions to this model can be found in [2].

In Fig 8, the calculated and the measured fuel consumption is compared. In the case of the condensing natural gas burner the constant value of 0.2 was used as factor two for all months. The maximum differences in the monthly fuel consumptions were -2% (Feb) and +6% (Jun). On a yearly basis the difference between measured and calculated values was -0.1%. In the case of the oil boiler a constant value of 1 was used as factor two for all
months. The maximum differences in the monthly fuel consumptions were -2% (Jul) and +5% (May). On a yearly basis the difference between measured and calculated values was +2%. This shows that with this calculation model on a monthly basis, the results in calculated fuel consumption are very close to the measured consumptions.

In combination with this model of calculating the fuel consumption, a simulation model was set up with the simulation program

SHWwin [9]. The boundary conditions of this model are the same as the house with the monitored condensing gas burner. The measured monthly domestic hot water (dhw) load-profile was used as input to the model. The model of the building which generates the space heating (sh) demand was fitted to the measurements by adapting typical parameters (e. g. maximum heat load, room temperature, reduced temperature during a specified period in the night, dimensions of windows regulating the passiv solar gains, etc). The Danish Test Reference Year was used as input for the simulations. In Fig 9 the simulation results and the calculations of the fuel consumption compared with the measured fuel consumption in the case of the condensing natural gas burner project are shown.

The demand for domestic hot water and hot water tank losses fits very good as it is easily possible to adjust the parameters for that. The difference between measured and calculated space heating demand varies from month to month because of the different climate of the test reference year instead of the measured weather data. Consequently also the fuel consumption varies in the same way. On a yearly basis the space heating demand of the simulation model is 0.4 % lower than measured. The demand on domestic hot water plus hot water tank losses is 0.2 % higher than measured. So the yearly result of

3.6 % higher calculated fuel consumption than measured fuel consumption can be interpreted as a very good result. With this verified model the following simulations were done to find out the saving potentials on fuel consumption with solar heating systems.

Isaac Garawayand Gershon Grossman
Faculty Of Mechanical Engineering
Technion — Israel Institute of Technology
Haifa, Israel, 32000
e-mail: grace@tx. technion. ac. il
Tel.: +972-4-829-2946 (2074)

Two factors have made solar driven distillation attractive. The first is economical: Desalination, in general, and distillation specifically are energy intensive processes, with the highest single aspect of operating costs being the required energy. With this in mind it would be ideal if one could drive a desalination distillation process with the high amounts of energy provided by the sun. The other aspect that makes this type of desalination desirable is geographical: Most arid and desert regions that are in highest need of desalination are the same areas that have high amounts of solar irradiation. This combination of lacking fresh water and having solar irradiation makes the use of solar energy as the driving force for a desalination process very inviting. At this point however, it is important to mention that even though solar irradiation releases large amounts of energy, it is of low concentration and thus effective collection is generally at low temperatures. An ideal solution to harnessing this energy for desalination would be a device that would operate efficiently at these low temperatures.

As the use of solar irradiation to desalinate water (at temperatures below boiling) became more widespread and accepted, there has begun a more serious development stage focused on trying to improve the operating efficiency over the “simple distiller’s” single effect. In the latter half of the 20th century research has begun on regeneration of the heat provided by the sun for multiple use. Since then, the regenerative process has slowly developed and gained recognition as a form of distillation. Presently this type of distillation is referred to as the MEH cycle — Multi Effect Humidification. To date there are several installations functioning worldwide that employ regeneration [1]. These projects and others reveal an increase in interest and renewed ambition to improve the capabilities of low temperature distillation.

The main objective of this research project has been to improve the design of a solar regenerative distiller. Computational Fluid Dynamics (CfD) was employed to determine the optimum geometry for which such a regenerative distiller operates most efficiently. A laboratory apparatus was constructed and the optimized design tested.

As an example of the results that can be obtained from the simplified model, a real ther­mosyphon system has been modelled exposed to real weather data during 24 hours without draw-off. The system consists of 2 collectors of about 2 m2 of absorber area connected to a horizontal cylindrical tank of about 300 l of capacity (diameter=0.42 m and length=2.08 m).

Some of the information that can be obtained from this kind of simulations is shown in figure 1. The figure shows the time evolution of the ambient temperature, Ta the average temperature of the water in the tank, TOT, and of the integrated terms of the global energy balance during the test: total solar irradiance over collector plane, СЙ, reflected solar irradi — ance, Gtref, accumulated energy at the store £M, and heat losses at the collector, at the

pipes and at the tank, which respectively axeQc, Qp and Q,.

Figure 1: Numerical results with the simplified model. Modelling of a real thermosyphon system during 24 hours of exposure to outdoor conditions and without draw off. a) Evolution of the ambient temperature, , and of the average temperature of the water at the store,

, during the testing period. b) Evolution of the terms of the global energy balance during the testing period: solar irradiance over collector plane, , reflected solar irradiance, , accumulated energy at the store, Eas, heat losses at the store, Qs, at the pipes and at the collector, . The values of the terms of the energy balance are integrated in time from

the initial of the test, and are given in % with respect to the total incident daily radiation.

In order to be able to predict expected service life of the component and its materials from the results of accelerated ageing tests, the degradation factors under service conditions need to be assessed by measurements. If only the dose of a particular environmental stress is important then the distribution or frequency function of a degradation factor is of interest.

For measurement of microclimatic variables relevant in the assessment of durability of the static solar materials studied in Task 27, various kinds of climatic data during outdoor ex­posure at different test sites are monitored such as global solar irradiation, UV-radiation, surface temperatures, air humidity, precipitation, time of wetness, wind conditions, and atmospheric corrosivity. Such data will be used to predict expected deterioration in per­formance over time by making use of degradation models developed from results of accel­erated tests. Some results from the measurement of microclimatic data are shown in Table 6 and Figure 5.

Figure 5 Microclimatic data measured during outdoor exposure of solar fagade absorbers at ISE in the IEA Task 27 study. Left diagram: Surface temperature frequency histograms for a black painted and a black chrome absorber; Right diagram: UVA and UVB light doses versus exposure time

Accelerated life testing means to quantitatively assess the sensitivity to the various degra­dation factors on the overall deterioration of the performance of the component and its ma­terials.

Figure 6 Change in thermal emittance observed for some reference solar fagade absorber materials during outdoor testing and during accelerated corrosion testing. The corrosivity dose in terms of metallic mass loss of copper at an exposure time is also given for the different tests to illustrate that outdoor performance of those absorbers can be predicted by making use of the equivalent corrosivity dose approach.

Mathematical models are then set up to characterize the different degradation mecha­nisms identified and from the accelerated life test results the parameters of the assumed model for degradation are determined and the service life then estimated.

In Figure 6 is illustrated how the principle of equivalent corrosivity dose in accelerated cor­rosion testing can nicely be adopted in the prediction of the long-term outdoor performance of some solar fagade absorbers. A prerequisite for this is that the accelerated corrosion test correctly simulates the predominating corrosion mechanism occurring under normal outdoor conditions.

A way of increasing the wavelength selectivity of a low-e coating to approximate that of the ideal solar coating shown in Figure 1 is to micro-structure the conventional low emissivity coating. Instead of the metal layer that is used in the conventional coating, a metal mesh is created on the glass substrate (see Figure 3).

The most important property of this mesh is the distance between the metal bridges. If this distance is small compared to the IR wavelengths, the coating behaves as a homogenous metal layer or a conventional low-e IR coating. For visible radiation, for which the wavelength is small compared to the distance between the metal bridges, transmission is only slightly reduced (see Figure 4).

For wavelengths greater than the the radiation is reflected. For most ofthe radiation is transmitted.

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

The store analysed is a tank with a length of L=1m and an internal radio of R=0.25m. The tank is made of 8mm thick Plexiglass material and the inlet flow rate is directed by a rigid manifold (see figure 1a, b). The rigid manifold selected is the one designed by Davidson et al. [5] consisting of a diffuser that reduces the inlet stream momentum and, at the same time, has different rings with open orifices that force the fluid to exit at the height closest to its temperature. An 8mm thick baffle plate and a radio of 0.15m is located at 0.04m from the bottom of the tank. This plate is also used as a diffuser to reduce the momentum of the inlet stream during the tank withdraw.

The principles of operation of the manifold are studied considering two geometrical mod­ifications. The first one consists of the reduction of the momentum diffuser tube height (Lin) from 0.133m to 0.04m. While in the second one, all the manifold outlet rings are obstructed except the ones located at the middle of the tank, forcing the fluid to exit the manifold through these rings.

Thus, three different situations, hereafter refererred to as cases A, B and C, are analysed. Case A considers the original manifold geometry provided by [5] . In Case B the tube height is reduced, and in case C, both the tube’s height is reduced and outlet rings are obstructed.

Figure 1: Non scaled geometry and mesh. a) Tank geometry, b) Detail of the manifold. c) Mesh

A test procedure to characterise the level of mixing produced by natural convection when the fluid entering the tank is colder than the surrounding fluid is proposed. This test is considered as a complement of test sequences of the European Standard EN 12977-3 [6] for testing the thermal performance of storage tanks. Hereafter, this test will be referred to as test P The test sequence consists of the following phases:

Phase P1: Conditioning of the store at

Phase P2: One half tank charging with a constant mass flow rate of 0.25 v„ and at a constant temperature of 60°C.

Phase P3: One half tank charging with a constant mass flow rate of 0.5 vn and at a constant temperature of 40°C.

Phase P4: Discharge of the tank at a constant mass flow rate of 0.25 vn and constant

inlet temperature of C until the steady state is reached.

As in EN 12977-3 [6], in order to determine the energy balance in the store, the test sequence starts and ends with the same temperature of the store. During phase P2, the tank is charged at constant temperature of C. The objective of this phase is to study the
formation of the thermocline and the influence of the inlet design in the thermocline height at the end of the phase. In phase P3, the tank is charged at a constant temperature of C. The effects that provoke mixing and, consequently, the degradation of stratification can be studied. As at the beginning of this phase the tank has been half charged, the effects of the plume entrainment phenomena and how it enhances the natural convection inside the tank can be considered.

In order to validate the presented model and to analyse performance of solar installation with stratified accumulation numerous experiments and calculations have been made. After a thorough investigation, appropriate values and formulas for parameters in mathematical equations and parameters of simulation calculation were chosen. A satisfactory coincidence between experimental and theoretical results was achieved. in fig. 3 are presented results for theoretical calculation and measured data for utilised
energy in a typical day. Since there are difficulties to arrange all initial parameters in mathematical system with real parameters for tank, solar collectors and climatic conditions, we assume that the correlation between the theoretical predications and measured data is good.

In this study are presented and discussed some results related to the influence of serpentine location in tank on the system performance. All experiments are made in approximately equal conditions. Daily water consummation has a relatively regular distribution as it is shown in Fig.2. The water consumption is 200 l per day, which is water quantity, corresponding (or little smaller) to the collector area (2 m2) possibilities for energy conversion. The solar radiation (Fig. 3) has a typical summer distribution for Bulgaria latitude and varies for different experiments in very small range. Useful (utilized) energy has a typical

daily distribution as it is shown in Fig.3. Variations in useful energy is caused by different inlet temperature for solar collectors Fig.4 shows the daily temperature distribution of water in tank for 6 sectors (layers). These results are addressed to system performance with two serpentines, located in top and bottom zone of the tank. Middle serpentine element is turned off. This is a thermally stratified accumulator because there is a top hot zone (sensors D12 and D11), middle temperature zone (sensors D8,D9,D10) and cold zone

(sensors D1…D6). Lower half of tank is with cold water (30-35oC) and small variation in temperatures. This is because there is a big quantity of water with relatively uniform temperature. The top zone is also with small temperature difference because it is constantly charged with heat by top serpentine. The most sensible to the water consumption is middle zone (sensors 8…10). Because the collector area is a bit smaller than what daily consumption require, the middle zone is situated in the upper half of the thank (D8…D10). On the other hand, cold zone is with relatively high temperature (30- 35oC) because there is a serpentine element heating the water in bottom zone.

Fig.5 shows temperatures from 5 sensors in solar installation with one serpentine configuration. The serpentine element is located at the bottom part of the tank. This is a configuration, which realize practically unstratified thermal accumulator. It is because the heat is extracted at the bottom and is transferred regularly to the top by buoyancy force. In this case the consumption is not so effective and the necessity of bigger collector area is evident. The thermal efficiency of this configuration is about 15% lower than configuration with two serpentines.

the heat exchange area is small and big water quantity is isolated from heat exchange process.

Configuration with three serpentines has nearly the same efficiency as the configuration with two serpentines and here is not presented the results for it. Thermal efficiency is little higher, but it is not sufficient to compensate additional cost expenditure. Presented results show the physical behavior of installation at special condition — daily — consumed water and distribution, climatic conditions, collector area and so on. At other conditions and parameters the behaviour will be other, but main results will resemble presented above.