Figure 7 shows the heat balance of the seasonal storage in 2003. The storage was charged predominantly in months May to August. Discharging occurred mainly in autumn. 220 200 180 160 140 120 100

80

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60

40

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20 0

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The development of the temperatures at different levels inside the storage are represented in Figure 8. The largest temperature difference between top and bottom amounts to 31 K. To reduce stress of the concrete shell by too large temperature differences over the storage high, the storage was charged at the medium level in the first month of operation. The highest temperature in the storage so far was measured in 2003 with almost 90°C. Starting from a level of about 8°C, the soil temperatures outside the storage increased distinctly, see Figure 8. At the end of 2003 soil temperatures of almost 30°C were reached 4 m below the storage.

Since starting operation no extraordinary water losses were recorded. In 2000 and 2001 a loss of about 10 m3 was calculated. After sealing a flange in the lid of the heat storage the water loss in 2002 and 2003 could be reduced to 8 m3. The water loss remains within the dimension of the predicted loss of approximately 4,5 m3 per year. To balance the losses water was refilled in a 2-year-cycle.

In 2000 and 2002 water samples were taken to examine the chemical and microbiological quality of the storage water. According to the results of analysis, the requirements on drinking water could be satisfied in each case. The microbiological quality is considered to be completely harmless.

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Figure 8: Temperatures inside and outside the seasonal storage at different levels Control engineering

The collector circuit is regulated on a target temperature by revolution adaptation of the collector and the secondary circuit pump. The control strategy has shown a good behaviour with a high practical suitability. The slow-action characteristic of revolution adaptation caused a steady and non-fluctuating development of the temperature.

The collector circuit pump is switched on in accordance with an ambient temperature — dependent characteristic. Therefore, the running time of the pump during the winter months could be reduced and optimised. The necessary frost protection operation only worked on a few winter days for about 5-10 minutes immediately after start of the collector circuit pump.

The realised connection of the storage with three levels of charging and discharging showed good operation results. By programming the control algorithm it should be taken into account that every motor driven valve gets a definite off position. The running time through the storage connecting pipes has to be considered by programming the pre­heating algorithm.

Return temperature in heat distribution net

In the hot water network of the buildings highly varying temperatures were registered. As a result the hot water comfort of the user was reduced. The flow of hot water circulation through hot water store was dedicated to be the cause of the varying temperatures.

As a result the charging of the store took place only, when the upper range had already reached temperatures clearly below the regulation switching value of 55°C. In order to guarantee the hot water comfort, but avoiding a continuous charging of the store and thus a rising return temperature, a

separate heating of the circulation net was suggested (Figure 9). All storage charging systems were modified as suggested in autumn 2002.

The additional heat exchanger was designed for a minimal return temperature difference between the primary and the secondary side. After heating up the circulation network the net water can be cooled down in the heating circuit if the temperature level is still high enough. Furthermore, the circulation network was levelled out and the oversized charging pumps were exchanged.

Measuring results after the modification display, that a continuous temperature level is reached. In addition the return temperature of the heat distribution network could be reduced.

01

Conclusions

The solar assisted district heating in Hanover-Kronsberg has been in operation since June

2000. It has been working reliably except for the mentioned leaking in the collector area piping. For the construction of the storage a nearly water diffusion tight concrete was used successfully. Extraordinary water losses of the hot water storage could not be recorded yet. The results of the first operational years achieve the range of past pilot projects.

After focussing the investigations on subjects concerning the range of building engineering in the past, the main points of research work in the future will be the optimisation (increase of the solar fraction) of the solar system.

Acknowledgement

The monitoring of the plant is supported by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) under contract No. 0329607 F. The authors gratefully acknowledge the support of the BMU and of the operating company, Avacon AG, Helmstedt. The authors themselves carry the responsibility for the content of this paper.

References

Bodmann, M. and M. N. Fisch (2001). Solarcity „Hanover-Kronsberg". Proceedings Northsun Conference 2001, The Netherlands

Fisch M. N. et al. (2001). Solarstadt — Konzepte, Technologien, Projekte (Solarcity — concepts, technologies, projects). Kohlhammer-Verlag, Stuttgart, ISBN 3-17-015418-4 (In German)

Reineck, K.-H. and A. Lichtenfels (2000). High performance concrete hot-water tanks for the seasonal storage of solar energy. Proceedings of Terrastock 2000, August 28 — September 1, 2000, Stuttgart, Germany, pp 263 -266

To validate the model, some experimental data will be considered. Results will be obtained under different conditions, and some of them are indicated in Tables 1 and 2. Position, inclination, orientation and solar exposure provide angles that make possible the determination of the incidence angle during the direct radiation, в (Table 1). These angles, along with the material characteristics, permit to calculate the absorbed heat. Table 2 presents other important data to solve the equation set and to carry out the optimization process. Notice that the distances are Ax = Ay = 0.005m.

According to this, the determination of the optimal value of the parameters is carried out by means of parameter identification, making possible the coincidence between the real data and the data given by the bulb sensor Tp, and the thermometer, which measures the output fluid temperature Tfo. This produces satisfactory results, after slightly modifying the material parameters. Table 3 shows the optimized values.

Results related with temperature distribution are shown in Figure 2. Fig. 3 presents the variation of temperature across different sections of the у-axis. Although this variation is insignificant, the highest temperatures are reached while the distance from the pipes increases, as found by other researchers [Duffie & Beckman, 1991]. The temperature evolution of the working fluid between the inlet and the outlet of the pipe is shown in Fig. 4. Extreme values are obtained from the model, and it can be seen that they are the same as the measured ones.

Results show that optimized values are very similar on each test. Some preliminary conclusions can be derived from this study. Although final values indicate that the material characteristics were adequately chosen, some little differences appeared, due to the presence of manufacturing defects. Final results related to crystal and plate emmitances, £g and £p, isolator conduction coefficient KiT, and extinction coefficient ke, showed very similar values to those initially considered. In the opposite, refraction index value was found to be higher than the initial one, due to the presence of impurities. Directional absorbance ao decreased around 6% compared to the initial value, because it was considered to be a constant. Cooper conduction coefficient of the absorber kp, was found to be 5% lower than the initially considered value, indicating that this material is not pure. In general terms, we can conclude from this field trial that the chosen initial values are adequate for a suitable approximation to reality. Nevertheless, convection factors suffered changes in a drastic manner. This can be attributed to the fact that the tube was considered as a flat element, for simplification purposes during the analysis, and also that the instability of the fluid flow through the system was obviated. The optimization process shows that there is a critical point in the model that must be considered.

As a result, only two parameters will change: Uc and h, depending on Kuc and Kh. Due to the dynamic complexity presents in the fluid under laminar regime, it is difficult to estimate the flow that passes through each riser, as well as the convection coefficient. This problem, together with the simplification of the pipes considered as plane surfaces, make necessary a serious study that accomplishes the determination of these factors as a function of physical, geometrical and dimensionless variables (see eq. (13)).

Kuc = fi(K, Tp, T€,TS, R,mf) Kh = f2(k2,T ,T€,Ts, R,mf);

with kl, k2 cons tan ts, and mf depending on collector geometry

At first sight, that is not enough, these factors are presented (see Fig. 5) depending on medium temperature of plate and inlet flow. For the study, data were collected every five minutes, during a two hours test, considering different inclinations. The high dispersion on the data made impossible an approximation of these factors with functions only depending on those temperatures.

To prevent this problem, a parameter identification process must be made by using both factors and considering each experimental data of the study. These optimized values will be used as input data to validate the subsequent transient regime analysis.

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Equations set (eq. (14) and (15)) are solved using different tests at different inclinations, every two hours. For each of them, solar radiation, temperature and flow mass fluid, material properties, convection factors and environmental conditions are known values. The temperature distribution on the absorber and the water are evaluated instantaneously. Final results of the modelled plate and the output flow temperatures are compared with the real values. The initial data and the results for one of the tests are shown in Fig. 6. It is to stress that both curves present the same tendency: real and modelled, for the plate and the output flow temperatures.

Results can be improved by decreasing the intervals of time, as well as by solving the system with another method [Ketkar, 1999]. However, the computing time would increase drastically.

3. — Conclusions

A simple method to approximate the temperature distribution in the absorber and the heating fluid of a solar flat plate collector is presented. Stationary and transient regimes are analyzed. For this purpose, a model for a bi-dimensional surface is described. Heat conduction and basic solar collector theories are used, defining equations systems for calculus by finite differences method. Then, accommodation of critical parameters of the described model is carried out by means of a parameter identification technique. This technique, based on optimization algorithms, compares model results and experimental data. The development of this process only requires a few measurement systems. So, the procedure can be applied to systems with relatively few sensors.

Results show that materials properties are adequate and close to the originally chosen. Temperatures distribution and variation on the absorber and the refrigerant fluid are qualitatively similar to those found by other researchers. Nevertheless, those approximations present a critical factor, caused by a hydraulic disequilibrium due to the laminar regime flow and to the geometry simplification of pipes. This can be solved by means of two proportionality factors associated to convection heat transfer, both of them identified the same time as the rest of the parameters. Once obtained, they are used as input data for the transient regime.

This study allows investigate the process which takes place in the system since the solar input pass through the working fluid. This can be accomplished with low computational requirements, because the equations system is linear, and various terms related to radiation and convection transfer are simplified by means of the general theory.

So, a correct fitting of those two parameters needs the knowledge of local loss factors during laminar regime. In this way, it would be possible to estimate the real flow of each riser, as well as the relationship with the collector geometry. This procedure supplies an adequate convection coefficient for simulations. However, a deeper analysis of the whole process is needed, i. e. by means of the finite element methods for fluid and convection heat transmission.

A preliminary application of this analysis could be the acquisition of the real values of the system parameters. Also, non tested conditions can be valuated, such as modifications in materials and geometry, optimum values of the risers diameter and the distance between them to maximize the captured energy, and so on.

Figure 3 shows a schematic view of CAVICAL1. It is a calorimeter which acts as a conic heat exchanger with water as a cooling fluid. It consists of two concentric cones. The inner one, which is made of copper and receives the solar concentrated radiation coming from the mirrors, has a vertex angle of 15o, a height of 16 cm, a base diameter of 8.56 cm, a base aperture diameter of 3.24 cm and a wall thick of 0.3 cm. The outer cone is made of stainless steel, it has 0.8 cm wall thick and there is a separation gap of 1.0 cm between the cones. Water can enters to the calorimeter at the vertex, flows between the cones and exit the device at the aperture or can flow in opposite direction.

The concentrated radiative energy coming from the mirrors, and passes through the calorimeter aperture is absorbed by the surface of inner cone and is transmitted to the interior of the wall by conduction. The water that circulates between the cones removes that energy by forced convection. Also, part of the irradiance entering through the calorimeter aperture is lost by reflection, and thermal emission loss back through the aperture, by heat conduction from the calorimeter through the insulation to surroundings and by natural convection in air through the aperture.

The design of the cavity calorimeter has as a first objective to diminish these losses.

In order to determine the heat losses through the cavity aperture due to convective process, a detailed simulation was developed using a CFD code.

Fig. 3. Schematic view of CAVICAL.

The high solar flux incident to the aperture of the calorimeter and absorbed by its inner cone is equal to the heat removed by the circulated water in the calorimeter (Qc), which can be calculated measuring the mass flow rate m and the inlet (Ti) and outlet (To) temperatures, that is,

Qc = mCp( — Ti) (1)

where Cp is the heat capacity of the cooling fluid. Thus, if Qin is the concentrated solar energy incident on the calorimetric cavity, then the following holds,

Qc = а ‘ Qin (2)

where a is the apparent absortance of the cavity calorimeter. Therefore, knowing a, Qin can be determined, and knowing Qin, a can be determined. It is clear that the energy Qin which is not absorbed by the cavity, is reflected by it, because the cavity is opaque. Therefore, knowing the apparent absortance a, the apparent reflectance p of the cavity is calculated by p = 1 — a.

The C++ programming language was chosen for the development of the SOLSTILL model. Each component in this system was programmed as a separate function, which was then linked into the main program. The program connects the various components in a manner specified by the user and then proceeds to mathematically execute the compiled system over the selected time period, solving differential and algebraic equations and facilitating information output. Table 1 shows all functions of this submodel together with their inputs and outputs.

The solar radiation processor function is used in order to convert the solar radiation into a form useable by the solar still. Depending on whether the cover involves one or two slopes and the number of covers used, this function calculates the useful solar energy incident on the still covers.

The standard still function is used to simulate the performance of the conventional natural convection solar still. In this function, the values of the physical parameters of the covers, the basin water and the still liner can be either constant or calculated at each desired time interval for the conditions prevailing at that time, depending on the required level of complexity for the simulation.

The forced still function is used to simulate the performance of the still in the forced circulation mode. Again, the values of physical parameters of the covers, the basin water and the still liner as well as the psychometric properties of the flow air can be either constant or variable, depending on the required level of complexity for the simulation.

Table 1. Summary of functions in SOLSTILL

The dehumidifier and pre-heater functions are used to calculate the conditions of the air and water leaving the coils. Users can also use default values for the coils’ physical parameters or enter their own values. The dehumidifier function is linked to the cooling function, and may be used to simulate the performance of the condenser in case there is no condensate in the coil.

The pump, fan and control functions are used to pre-set the system to run in one of three modes chosen by users; the natural circulation mode, the closed loop forced circulation mode and the open loop forced circulation mode. In the first mode, this function will "switch off” the water pump and the blower. In the second and third modes, the function will run the pump and fan at the flow rates selected by the user. The input air flow to the still will be the ambient air in the open loop mode whereas the exhausted air from the air pre-heater will be circulated to the inlet of the still in the closed loop mode.

The quantity integrator function is used to calculate and display the outputs of each component of the still system at selected time intervals, such as minutes, hourly, daily, weekly, monthly and / or yearly values.

The technology of the solar pond project realizes the integrated heating of the protective-styled cultivation of the aquatic products for the first time in the Northern coastal area of China. In the process of the over-wintering production of the cultivation objects, the percentage of solar energy supply is 100%. In the process of cultivation in spring, the contribution percentage of solar energy achieves more than 70%. It achieves the industrialized development preliminarily. Technology of high-health cultivation is developed in the mechanism of the operation of brine solar pond. The production line of bio-baits of mono-cellular algae is used in shallow-styled solar pond. The production line of heat-integration, the technology of over-wintering and cultivation and developing commodities are used in the conservatory-modeled solar pond. The system can endure power 11 wind in the integrating process and can save energy of 100 tons each year.

4. The prospect of application and dissemination

With the support of the heat-supply system of the integrated solar pond project, the protection-styled cultivation of super cultivation objects of sea water is carried out in the coastal tidal-flat area and saline and alkaline land, such as fish, shrimps, crabs and shellfish. With the help of technology, high-health cultivation objects of sea water are reproduced and cultivated. It creates high-efficiency technology in the conservatory shed. It opens up a new way of adjusting the industrial make-up of agriculture in the coastal rural area of China. Marvelous prospect of application and dissemination is demonstrated in the experiment.

The coastline of China is 18000 kilometers long. The tidal-flat and saline and alkaline land is broad and wide. The advantages make it suitable for the multipurpose industrialized development in large scale.

4.1 Pursue the scheme of on-the-sea pasture of solar pond project.

In the offshore gulf of the Bo hai Sea, the Huang hai Sea and the East China Sea, (center on the sea with a depth of -10m), mechanism of the solar pond is used to transform the environment of the fishery, to improve water temperature in the sea area, to prolong the growth period of breeds, to change the migration of bio-population and eventually to develop on-the-sea pasture of solar pond.

4.2 Implement fishery project scheme of land sunlight.

In the coastal seashore north of Fu jian Province, brine or sea water is adopted to build a large-scaled over-wintering field of solar pond for fish, shrimps, crabs and shellfish. With the support of green sunlight system, modern technology of water treatment and modern biotechnology, an industrial belt of the sunlight project about high-health fishery is built along the coast.

4.3 Develop sunlight industry of high protein

In the coastal seashore and beside the inland salt lake, industry of high protein is developed. Brine worm powder is a kind of additive with low cost, high quality and rich nourishment in the field of new food. The content of dried protein is above 60%. Worm eggs are used as baits for young fish and shrimps widely. The worm can survive in a wide range of temperature and salt concentration. The temperature from 3"C to 40"C and salt concentration from 5%o to 120%o are all suitable for them. Growth period of brine worms can be prolonged with the development of protective cultivation of brine worms in solar pond. Brine worms and worm eggs can also be raped in winter. The content of dried protein in spiral alga is 60%. Spiral alga is a kind of micro-algae of high economic value. It is a key development object all over the world and is a future food resource universally admitted. It can also survive in a wide range of temperature and salt concentration. It is distributed in both sea water and fresh water. The percentage of light-conversion is 18%. The ecological environment of the spiral alga is suitable for carrying out protective cultivation production in large scale in the use of solar pond project.

4.4 Study and develop the industrialized production technique of lithium bromide

The integration technology of solar pond is fit for all the protective cultivation of aquatic living things in the sea. People should carry out multi-purpose development and application creatively according to the various natural conditions.

In a word, the issue has already showed outstanding prospect in the experiment and research. The economic benefit can be increased greatly with the popularization of the technique in the coastal area of China.

It is estimated that in the future research and development of the coastal solar pond, great technical breakthrough will be made with the joint effort of the scientists all over the world and by way of modern technology in the aspect of constructing ponds, establishing and maintaining salt gradients, forming and maintaining temperature field, simulating bio and ecological environment, reproducing and cultivating bio-population and so on. It will be made more scientific and perfect. It can be fully believed that building the modern aquatic cultivation industry in the protection-styled solar pond all over the world, developing coastal solar pond project with complete functions, rational distribution and scientific structure and building up on-the-sea blue agriculture will surely come true in the near future.

Two of the solar domestic hot water systems (SDHW) were assessed “very good” as the overall mark (system H3 and H11). With regard to the assessment of the thermal performance in total four SDHW systems obtained “very good” (system H3, H11, H13, H14). Two of these systems were equipped with flat plate collectors and two with vacuum tube collectors. This result shows that it is not necessary to use vacuum tube collectors in order to be assessed as “very good”.

Only for one system (H6) the thermal performance — and therefore also the overall mark — was rated with “fair”. The reasons for the relatively low thermal performance of this system were related to performance deficits of the solar collector and a disadvantageous control strategy. It is by chance that this system shows with 100 litres usable hot water volume also the lowest value in this category. Due to the assessment scheme used (see chapter 3.1) this fact is not the reason that the thermal performance of this system was only reacted with “fair”.

The solar combisystems C7 and C11 received „very good“ as the overall mark. In addition to these two systems, also system C5 shows a „very good“ thermal performance. It is encouraging that also in this category only one system (C9) was marked with „fair“. In this case the reason is predominantly related to a store concept that is designed disadvantageously with regard to thermodynamic aspects.

Uwe Begander, Hans Schnitzer, Christoph Brunner, Karin Taferner JOANNEUM RESEARCH — Institute of Sustainable Techniques and Systems Graz, Austria

Introduction: Documentation of realized plants for the use of thermal solar energy in trade and industry companies. Identification of production processes and branches, which have a demand for low-temperature heat. Determine the potential of solar thermal systems to provide low-temperature heat. Case studies for branches and processes with the highest mid-term potential for realization of a solar plant.

Background

Industry is one of the great users of energy in all industrialised countries. Caused by the fact that energy is available at low costs and without limitations, industry did not care too much about energy efficiency and substitution of fossil fuels. The main activities in this field started in 1973 and 1979/80 following the two oil (price) crises. Later on, oil prices — and related to that the prices for natural gas and electricity — went down again. Today — even in the face of a possible critical political situation in the Near East — energy prices are low.

On the other hand, it is obvious that fossil resources are finite and alternatives have to be found for any application, including the use in industry and commercial applications.

More than one third of final consumption is used for space heating, hot water production, cooking and cooling, with an other quarter being used for process heat (Fickl, Stenitzer 1997). The fraction of energy going to industry is getting less, since the absolute numbers are relatively constant and diminishing related to the production.

1: Final energy consumption in the industries of special goods production according to low-temperature energy consumption (sum of steam generation and space heating & air conditioning) for the year 1998 (after statistics Austria)

In 1999 the final energy consumption of Austria was 940 PJ1 (STATISTIK AUSTRIA, WIFO 2002). 291 PJ or 28.9 % were used in industry for a wide range of use (E. V.A. 2002):

Space Heating & Cooling 19 PJ Steam Generation 68 PJ Stationary Furnaces 117 PJ Engines 57 PJ

Vehicles 21 PJ

Lighting 9 PJ

Electrochemical Purposes 1 PJ

In 1998 40 % of the Austrian final energy consumption or 356 PJ was used to produce low temperature heat (up to 100 °С)- for space heating and provision of hot water (Neubarth, Kaltschmitt 2000), more than 1/3 or 125 PJ in industrial companies. Energy sources for this range of use were oil (27%), gas (20%), coal and electricity, together nearly 2/3 fossils based, biomass (27%) and district heating. Losses in final consumption were 30%.

The amount of process heat was 199 PJ, losses were 25%. Sources were gas (40%), coal (17%), oil (15%), electricity and biomass (15%).

The final industrial consumption for process heat, space heating and the production of hot water was 324 PJ, which was 35 % of the final consumption of Austria.

R. Hofer (Hofer 1994) has analysed the different temperature levels of process heat. One of the results was, that 25 PJ of the process heat is used under 100 °С. Thus 150 PJ of low temperature heat is used in production processes in Austria. This theoretical potential is the basis for estimations that consider conditions of solar technology, process technology, economy, market and social matters.

The percentage of energy costs in industry is under 5 %, for some sectors (iron and steel, paper, foundries and non-metallic minerals) it’s about 10 % of total costs (Kaiser, Starzer 1999).

To supply solar energy to different forms of energy services, there exists a number of ways. In industry, there are different uses for energy like low temperature heat, high temperature heat, power, cooling, information and light. For the application of solar energy in countries like Austria — where there is a high fraction of diffuse light — only low temperature production processes are relevant. It is the aim of the research project “PROMISE — production with solar energy” to locate such processes, calculate the potential in Austria and to design a selected number. Estimating the potential, one has to distinguish between the technical potential, the economic potential and the market potential.

The technical potential includes all applications where existing technologies could be applied, not regarding economic questions. The economic potential includes all application, where the technologies provide a sufficient payback or at least lower costs for the supply of energy that in the existing situation. The market potential is still lower, since now all applications are excluded, where companies are not willing to invest, although the investment would be economic. There are several reasons, why a company does not invest.

First of all is the barrier of information and knowledge: the company does not know about the possibilities

Then there is the barrier of motivation: managers care only about aspects that are important to produce at a high quality and a high security Then there is the lack of capital mainly with small companies.

The work described in this paper concentrates on low temperature heat that can be supplied by means of solar radiation and solar collectors. The paper is structured in three parts: energy in sectors, energy in unit operations and examples for production processes with potential for solar heating.

Objectives

The main goals of the project were

• the documentation of realized plants for the use of solar thermal energy in trade and industry companies,

• the identification of production processes and branches which have a demand for low-temperature heat and

• the determination of the potential of solar thermal systems to provide low — temperature heat in the Austrian industry.

Methodology

The initial approach was literature investigation in the fields of solar thermal applications, heat demand and energy supply of industrial processes and energy conservation. After that questionnaires were sent to 600 industrial enterprises to scan actual conditions and requirements for energy supply.

The most important source to obtain knowledge about processes and their heat demand were several case studies for branches with the highest mid-term potential for realization of a solar plant (dairy industry, brewery, concrete processing, plastics processing, galvanizing).

M. Corcoran and C. Gibbons

Energy Engineering Group, Department of Mechanical and Manufacturing Engineering, Cork Institute of Technology, Rossa Avenue, Bishopstown, Cork, Ireland.

ABSTRACT

A mathematical model was developed for a forced convection solar hot water system. The solar collector in this study incorporates a honeycombed extruded polycarbonate structure, for both the cover and water channels. The initial section of the program predicts solar radiation (hourly, monthly and yearly) as an input section to the solar collector calculations. As well as determining the collector performance, the model also facilitates changes to the collector physical properties such as dimensions of the channels, selective and non-selective absorbers, material thermal properties, as well as ambient temperature and flow rate, in order to optimise the system design. The results from the program will allow a full parametric study of different collector design criteria, with this polycarbonate structure. The results will be compared to a standard flat plate collector design, to see if this polycarbonate flat plate collector is a more effective design. ISO 9806-2 standards are being used to validate the results, for the parametric study in the lab, under steady state conditions. The final optimum design will then be tested outdoors using the quasi-dynamic conditions set out by the European Standard EN 12975-2. Weather data, obtained from the weather station set up at CIT, will be used as the input for the weather conditions for out door testing.

KEYWORDS: Thermal solar collectors, Open Loop system, Theoretical &

Experimental studies, Performance analysis.

In order to answer these questions TRNSYS-simulations were calculated in cooperation with the company Viessmann. The simulations were carried out using the 1997 weather data from the ISFH meteorological station in Hanover in a 5-minute time step. Results show that the measured similarity of solar gains under high-flow and low-flow operation is confirmed by the simulations (AQsol < 1 % on a yearly base). Furthermore the mean collec­tor loop temperatures are nearly the same for both systems.

Starting from a specific heat transfer capability of the investigated system’s heat ex­changer (UA = 140 W/m2K with respect to the collector area) the UA-value was varied and the influence on the solar gain was investigated. It turns out that for both flow rates the system benefits in the same degree from an increased heat transfer capability of the heat exchanger. The enhancement of the heat transfer capability by 43 % improves the solar gain for both systems by about 1 %. If the heat transfer capability is decreased by 43 %

the solar gains under low-flow and high-flow operation drop by 2 %. Thus a different heat transfer capability of the internal heat exchanger leads qualitatively to the same results. One question remains: What is the reason for the lower return temperatures under low — flow operation? The simulations showed that the difference of the return temperatures un­der high-flow and low-flow operation does not depend on the size of the heat exchanger.

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In Figure 5 the temperature distributions in the stores are compared for low-flow and high — flow operation in the course of the day (5 June 1997). At 7 o’clock both stores show nearly identical temperature stratifications. A few hours later the low-flow system shows a distinct thermal stratification in the lower part of the storage, whereas the stratification of the high — flow system is only slightly developed. At 10:12 h the temperature difference between the relative store heights 10 % and 40 % amounts to 12.0 K under low-flow operation, under high-flow operation this temperature gradient amounts to 5.2 K. All in all the temperature level in the region of the heat exchanger outlet (height 5 %) is much lower under low-flow operation (up to 6 K).

Another demonstration of the better stratification under low-flow operation is given in Fig­ure 6 and 7. The diagrams show the temperature development in the course of the day for the different storage levels. At 11 o’clock the temperature spreading between the storage top and bottom amounts to 16 K under low-flow operation. The high-flow system shows only a spreading of 7 K at this time. In addition to that the temperature at storage top under low-flow operation exceeds the temperature under high-flow operation by 2 K.

So it is obvious that under low-flow operation the better thermal stratification in the store combined with the lower temperatures at the bottom of the storage are responsible for the lower return temperatures of the collector loop.

Figure 6 shows the velocity vector maps and the corresponding streamlines to the vector maps 5, 15 and 25 minutes after the heating is started. The velocity vector maps are calculated as mean values from 20 instantaneous velocity recordings by adaptive cross correlations and finally range validation procedure was applied. After 5 minutes the inlet takes place from flap 3. The downward stream due to the position of the outlet at the bottom of the tank passes flap 2, while some water is sucked in through flap 1. After 15 minutes the inlet takes place from flap 3 and flap 2 and cold water is still sucked in through flap 1. From this time and throughout the experiment that lasts for 50 minutes the inlet takes place from flap 3 and flap 2 while cold water is sucked in through flap 1.

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