In hybrid PVT/AIR systems the thermal unit for air heat extraction, the necessary pump and the external pipes for fluid circulation constitute the complete system that extracts the heat from PV modules and transfers it to the final use. Practical considerations in PVT/AIR system design include the evaluation of the thermal and electrical efficiency improvement with respect to the system cost. The hybrid PV/T systems consisting of PV modules freely exposed to ambient temperature without any thermal protection have high top thermal losses and therefore their operating temperature is not high. To increase the system operating temperature, an additional transparent cover is necessary (like the glazing of the typical solar thermal collectors), but this has as result the decrease of the PV module electrical output because of the increasing reflection and absorption of the solar radiation.

In most PVT/AIR systems the air circulates through a channel formed between the rear PV surface and the system thermal insulation, and in some other systems through channels on both PV module sides, in series or in parallel flow. The usual heat extraction mode is the direct air heating from PV module rear surface by natural on forced convection and the thermal efficiency depends on channel depth, air flow mode and air flow rate. Small channel depth and high flow rate increase heat extraction, but increase also pressure drop, which reduces the system net electrical output for the forced air flow, because of the increase of the fan power. In applications with natural air circulation, the small channel depth reduces air flow and this results to an increase of PV module temperature. In these systems large depth of air channel (minimum 0.1 m) is necessary (Bhargava et al, 1991).

The aim of our research is to design efficient PVT/AIR systems based on low cost modifications. The main concepts are concerned with the reduction of PV module temperature, the improved air heat extraction by the circulating air and the reduction of heat losses by using thermal insulation on system back side and edges. A low cost modification was investigated in University of Patras, by which satisfactory air heating, reduced PV module temperature and low increase of the opposite channel wall temperature can be achieved (Tripanagnostopoulos et al, 2000b). This modification consists of a thin flat metallic sheet (TFMS) placed inside the system air channel and along the air flow. This TFMS element doubles the heat exchanging surface area in the air channel, aiming also to the reduction of the heat transfer to the back side of the PVT/AIR system. The experimental models were constructed in the University of Patras and consisted of commercial type mc-Si PV modules in combination with a laboratory made air heat recovery unit (HRU). The experimental study is based on two hybrid PVT/AIR module designs (Tripanagnostopoulos et al, 2002) of same aperture surface area Aa (Aa = 0.4 m2). The first model is that of the simpler form without additional glazing, named PV/T UNGLAZED or PVT/UNGL and the second model is that with the additional glazing, the PV/T GLAZED or PVT/GL. These experimental models have an air duct of 10 cm width and a thermal insulation layer to avoid thermal losses from the non-illuminated system surface sides.

We consider that these systems can be installed on the horizontal roof, on the tilted roof or on the fagade of a building. Horizontal and tilted roof installation are more interesting at low latitudes, while building fagade (and high tilted roof) installation are more effective for medium and high latitude applications because of the lower sun altitude angles. In the case of tilted roof system installation, the PV/T systems are placed one beside the other and on the southern building inclined roof surface. The tilted roof integrated systems are additionally thermally insulated on their rear surface, compared to the ones installed on horizontal roof, as they are attached on the tilted roof. The additional thermal protection

increases the thermal efficiency of the system, but the lower thermal losses keep the PV temperature at a higher level, operating therefore with reduced electrical efficiency.

The systems that are considered for installation on the tilted building roof are the standard PV modules and the usual type PVT/AIR systems. The experimental models that simulate the PV and PV/T systems for the tilted roof installation, are the PV-TILT, PVT/UNGL-TILT and PVT/GL-TILT models. They are consisted of the same basic system units (PV, PVT/UNGL and PVT/GL) and were tested with an additional thermal insulation on their rear surface, considering the thermal protection of the tilted roof to the attached system thermal losses from this surface.

Considering PV/T solar systems installed on horizontal building roof the parallel rows keep a distance from one row to the other, in order to avoid PV module shading. We suggest the use of stationary flat diffuse reflectors (REF) placed properly between the PV modules from the higher part of the one row of them to the lower part of the next row. This installation increases solar input on PV modules almost all year, resulting to an increase of electrical and thermal output of the PV/T systems. The systems that combine the PVT/AIR modules with diffuse reflector are the models PVT/UNGL+REF and PVT/GL+REF. The suggested diffuse reflectors differ to the specular reflectors, as they avoid the illumination differences on module surface and the reduction of the electrical efficiency, because they provide an almost uniform distribution of reflected solar radiation on PV module surface. The systems were tested with slope equal to the latitude of Patras (38.25o). In the experiments with the diffuse reflector, the PVT/AIR systems were tested for variable additional solar radiation (concentration ratio CR=1.0 to CR=1.5) to get data for different angles of incidence between system and sun.

Aiming to the increase of PVT/AIR performance three prototypes with the TFMS modification were constructed and tested by University of Patras. These models were all of unglazed type and were distinguished in pVt/TFMS, the simple type for horizontal roof, the PVT/tFmS+REF, the system with the diffuse reflector and the PVT/TFMS-TILT, the system for tilted roof installation. The systems consisted of the same basic unit (mc-Si PV module and HRU of 10 cm air channel) with an additional thin aluminium sheet in the middle of the air channel and parallel to the air flow.

The developed application should be able to identify the causes of an error and to offer appropriate assistance to the operator to solve the problem. The knowledge of error sources and causes has to be formalised and must be available for the operator. The definitive application should be user-friendly and easy to be operated by property developers.

2. The solar expert system

The choice of the method

The heuristic and the safe classification is indispensable for the solar expert system. Safe classification means that the characteristics of an error can be clearly identified and that its diagnosis can be considered as safe. A decision tree will be gone through for the safe classification. Its leaves can be either a diagnosis, an excluded diagnosis, a new question class or a leaf with no relevance for this sort of knowledge use.

Implementation

At first, all possible error sources in a solar installation are systematically entered. An error can "creep in" in different ways:

• A component is missing

• A component is wrongly inserted or attached

• A component is wrongly adjusted (operating errors)

• A component is defective

Despite our years of expertise, we are also surprised of the great number of possible error sources. These are represented in figure 1, but it does not include control errors (!) (adjusting errors, attachment errors, feeler errors etc..).

Annual system simulations were carried out with the simulation tool TRNSYS (Klein,

1998). A multi-node storage model was used (DrQck, 2000), with N = 100 equidistant nodes to describe the thermal stratification in the tank in one dimension. The model takes into account storage losses, heat transfer through heat exchangers, forced convection through the store, as well as an effective conductivity between the nodes to describe additional heat transfer caused by convection and conductivity. Furthermore, heat transport within the tank is modeled due to ‘numerical diffusion’. If the number of nodes, i. e. the discrimination of the model, is changed, the modeled heat transport is changed as well.

If a temperature inversion occurs, that is, if a colder temperature layer is placed above a warm temperature layer, the mean value of both layers is calculated in the next simulation time step. This corresponds to complete mixing of the two layers. The height in which the cold water enters the tank in the simulation is therefore decisive for the thermal stratification. Thus, the virtual inlet height hin can be used as a variable to model the mixing behaviour for different inlet devices.

The 2P-Model: Identification of 2 parameters

In recent studies (Jordan and Furbo 2003-1) the virtual inlet height into the tank hin was described as a function of boundary conditions (flow rate and temperatures) as well as two parameters to take into account different inlet device designs with the following energy

balance: [pTin “ jpstore(h)dh]-g • hin = 1 PTin • [7]eff2 (е4и — !)

veff is the effective upward velocity. It can be described as a function of the flow rate divided by an effective cross sectional area Ac:

The two model parameters reff and hoffset need to be identified with experiments for each inlet design. The 2P-model is used for comparison with a more general model in the simulations described in following sections.

The Ring model

The ring model is a further development of the 2P-model, to describe the variable hta as a function of solely the given buffer plate geometry and reference conditions.

The effective cross sectional area Ac is described by a ring around the buffer plate, with

the plate radius veff = — = m 2 (equ. 4)

Plm ‘ Ac, ring PTiii ‘n ‘ (r _rplate)

with the outer ring radius r as the sum of the plate radius and the ring width lw:

r _ rplate ^ lw •

With Ac = Ac, ring the virtual inlet height hin can be calculated with (equ. 1) and (equ. 2):

[PTin _ jPstore (h)dh ] ‘ g ‘ hin _ 2 p

The ring width lw was found to be approximately constant for the investigated inlet devices.

Comparison of measured and simulated temperature distributions

To compare measured and calculated thermal stratifications in the tank, the measured values of the initial storage temperatures, inlet temperatures and flow rates were used as input values for the simulation. 33 measurements (with 3 different initial tank temperatures, 4 flow rates and the three buffer plates) were used for model validation.

The comparison of measured and calculated values with the least square method yield to the value of lw = 10 mm for the ring width for all three inlets.

Figure 7b shows the simulated temperature distribution with the ring model corresponding to the measurement shown in figure 7a. The simulated and measured temperatures throughout the measurements cannot directly be compared, due to the fact that the thermocouples are placed in the corner of the tank. This causes a time delay between measured and simulated results of the mean values in the temperature nodes. For example, after two minutes, the temperature difference at h = 14 cm is higher than 16 K. Furthermore, in the simulation, the bottom part (up to h = 60 mm) of the store is more stratified than in the experiments.

Nevertheless, the measured temperatures in the tank are fairly well modelled. The mean square deviation sq of the thermal stratification directly at the end of the draw-off yields to

°.3 K with sq = N(Tmeas, n "Tcalc, n)2 .

Instantaneous temperature inversions can be noticed in the simulations, these layers are mixed in the next time step.

Since the temperature at the bottom of the tank decreases during the draw-off, hin rises continuously (figure 7c). The high values of hin at the very beginning of the simulations are due to the high inlet temperature caused by warm water in the pipes that remained there from the heating of the tank. The inlet heights determined for the times at which the PIV — vector maps were captured are hin=55mm(At = 10s), 45mm(1min), 110mm(2min) and 150mm (3min). Thus, hin corresponds to a height in the upper part of the vortex in the vector maps (figure 6a-d).

In figure 9 measured and calculated temperature distributions in the tank are shown, in figures a) and b) for the small, in c) for the medium size and in d) for the large buffer plate.

a) Small inlet, 2P-model.

c) Medium inlet, ring model. d) Large inlet, ring model.

Fig. 9 a)-d): Comparison of simulated (grey curves) and measured (black curves) temperature distributions.

As shown in c) and d) the temperatures can be very well predicted with the ring model, with lw = 10 mm. However, the deviation between measured and simulated values using the ring model increases significantly for the smallest buffer plate (9b). For large flow rates and small temperature differences between storage water and entering water, the model overestimates the mixing in the tank. The predicted temperatures at the bottom of the tank turn out too high. This can be explained by the flow patterns, which differ strongly for the small inlet compared to the larger ones as shown in figure 3. Therefore, the 2P-model needs to be applied in order to model the small inlet with a sufficient accuracy (Fig. 9a).

Annual System Simulations

A typical Danish small solar domestic hot water system was modelled with the simulation tool TRNSYS. A scheme of the system is shown in figure 10. It consists of a small storage tank with coil heat exchangers inside, a burner, pipes, and a pump. Parameters and assumptions for the calculations are listed in Table 1.

Two different domestic hot water load profiles were used, generated with the program DHWcalc (Jordan and Vajen, 2004). The flow rates of the first profile were chosen according to the Danish norm DS 439 (2000), with distributions around 6, 9, and 12 l/min and an average daily draw-off volume of 30, 30, and 40 litres, respectively. For the second profile, flow rate distributions around 3 and 8 l/min (mean daily draw-off volume 50 l each) were taken into account.

Annual system simulation results of the net utilized solar energy Qsol, net and the performance reduction rate rp are shown in figures 11 and 12. The net utilized solar energy is defined as the difference of the domestic hot water consumption energy QDhw and the

auxiliary energy supplied Qaux: Qsolnet = QDhw — Qaux

The performance reduction rate rp defines the relative additional auxiliary energy supply for a tank with a buffer plate diameter d compared to a tank with an ideal buffer plate:

rP

with d„: maximum diameter, which corresponds to an ideal inlet device.

Thus, rp characterizes the impact of the additional mixing caused by a non-ideal buffer plate in a solar storage tank. For an ideal inlet device cold water enters the tank in the lowest layer.

As shown in figure 11 for DHW-profile A with fairly high flow rates, the solar fraction is increased by 5 % for the investigated solar heating system with the large buffer plate (ring model) compared to one with the small buffer plate (2p model). The performance reduction rate (of the real buffer plates compared to ideal ones) drops from about 8 to 3% for the large and the small buffer plate, respectively (figure 12). The ring model under­predicts the net utilized solar energy for the smallest inlet by about 2%.

When the DHW profile with more moderate flow rates (profile B) is taken into account, the net utilized solar energy is increased by 3 % for the large compared to the small buffer plate. This corresponds to a decrease of rp from about 5 to 2% (large plate to small plate).

Conclusions

The flow fields around inlet devices can be visualized with an optical method called Particle Image Velocimetry (PIV). Velocity vector fields show vortex structures, which refer to the heights reached by the entering cold water into the store. It was found that the flow of the entering water was first directed to the tank bottom, if the tested buffer plate diameter is sufficiently larger than the diameter of the inlet pipe. In contrary, when using the same buffer plate diameter as the tube diameter, for most reference conditions, large vertical velocity components can be measured close to the inlet gap. Only when small flow rates and large buoyancy forces are applied, the flow is deflected downwards with a small buffer plate.

A mathematical model was developed to describe the impact of the buffer plate diameter on the mixing in the tank, while taking into account the operating conditions (temperatures and flow rate). The model contains one unknown parameter, i. e. the width of a ring around the buffer plate lw that determines the effective cross section for the upward flow in the tank. This width was found to be the same value for all three tested buffer plates. Thus, the constant was assumed to be generally valid within a certain range of diameters for the given tank geometry. However, whereas the thermal stratification can be described well for sufficiently large buffer plate diameters, the inaccuracy between measured and calculated temperatures increase rapidly for buffer plate diameters similar to the inlet pipe diameter. For these small buffer plates an additional, empirical parameter needs to be taken into account.

Annual system simulations of a typical small Danish solar domestic hot water system were carried out with two different domestic hot water profiles. The simulation results showed that the net utilized solar energy were increased by 3 to 5 % when the largest investigated buffer plate was used compared to the results for the smallest (marketed) buffer plate.

Future investigations should be focused on the impact of the tank geometry on the model constant lw. Furthermore, a more general function should be developed to describe the mixing effects for small buffer plates and for inlets through the tank side.

The primary parabolic concentrator of 150cm diameter, 67 cm of focal length, 85% reflectivity was used. The light flux distribution at the focus of the primary parabolic mirror was mainly determined by its focal distance and by its rim angle. The accurate alignment of the primary parabolic mirror was considered essential for achieving the minimum focus spot. Since the focused on-axis and the defocused performances of a concentrator gave different flux distributions, the primary mirror was accurately aligned in order to obtain minimum light spot at the focus. Under these conditions, a detailed map of the light distribution was generated by a small aperture (1.0mm) radiometer scanned across the focal plane. Detailed results on and off the focus were obtained. A typical flux distribution at the focal plane of the primary concentrator is given in Fig.1.

The average solar insolation at the scan time was 800W/m2. A near-Gaussian type distribution was obtained, with the maximum solar flux being 25.6W/mm2. Based on the flux distribution at the focal plane, the total solar power of 1258W was calculated which matched well with experimental measurement.

THE FUSED SILICA LIGHT GUIDE ASSEMBLY

The fused silica light guide is a complex assembly composed of one straight light guide, four curved light guides and four more curved and twisted light guide. The light-coupling scheme from the light guides to the laser crystal within the flow tube can be observed in Fig.2.

The light-coupling scheme from the focus of a primary parabolic concentrator to the fused silica light guide assembly is given in Fig.3 an Fig.4, where the composition of nine light guides of 5X5mm square cross sections are of inward convex form. This unique spatial combination of the nine light guides permits the efficient light coupling of the divergent light energy to each light guide with small incident angles, facilitating the future light focusing to the laser crystal..

The packing of five principal light guides and the other four twisted light guides in diagonal positions (as shown in Fig. 5 and Fig.6) allowed an efficient coupling without extra loss.

Fig.6. The input end of one of the other four light guides in diagonal positions.

A raytrace program was used to compare the performance of the CPC, SURS A and SURS B performance. Fig. 4 shows the transverse optical performance of the three systems with a double glass absorber tube. The SURS B has the highest normal incidence output for a reflectance of 0.95 but that the greatest efficiency over all incidence angles is still the CPC. However, the differences are slight. The identical performance for incidence angles beyond 60 degrees is due to the fact that in all three systems the tubes are fully shading the reflector.

Fig. 4. Predicted optical performance for three 14 tube systems, including double glass
tubes as receivers and a reflector of 0.95.

The integrated system might be more convenient than the two separated ones. But at the moment such level of integration has not been joined yet.

The main reasons are that at the moment the two technologies do not match very well.

If the efficiency of the PV module increases, the gain of efficiency of the collector is very low. With the crystalline Si cells, since the efficiency is decreasing as the temperature rises it is convenient to operate at the lowest possible temperature. That requires very efficient collectors operating at low temperatures.

Usually for the Domestic Water House application water is needed at about 50 ° Celsius, so the absorber will be at a temperature around the NOCT.

The amorphous Si technology since it is less affected by the temperature could be applied in a more efficient way in the hybrid application.

During test:

1) The tests on the component show that it is possible to reach good levels of electrical and thermal efficiency.

2) When mounted on buildings not always the thermal gain is warranted due to the seasonal mismatch, high insolation in summertime or in hours when the needs of thermal load are less, the difficulty to find efficient thermal seasonal storage or considering the storage in the project.

3) On public buildings with large glazed walls the thermal load is not high even in the low insolation months.

Experiments

To better understand better the possibility and the limits of the hybrid technology, the Photovoltaic PVT hybrid Dunasolar modules have been chosen for the experiments. They are tandem type amorphous Si and present a water thermal collector behind the module. The main electrical characteristics are reported in table 1.

The modules have been tested at outdoor conditions by means of the Solar Tracker FT4000from Ecovide s. r.l. The main features are:

1. two axes

2. turnable platform sized 2×2 m2,

3. tilting from 0° to 99°,

4. azimuth from -180°, east direction, to 327 °.

5. spinning of the module plane from -35° to 35°.

The outdoor measurements were aimed to observe the behavior of the hybrid modules and to estimate the advantages when the module is being cooled with water.

In that way several experimental campaigns have been undertaken on the Dunasolar modules in real operating conditions other from STC.

At first the module was measured without any cooling. The metoclimatic conditions are reported in table2.

The measurements have been repeated forcing water trough the collector. The meteorological and climatic conditions are reported in table 3.

The Figures 3 and 4 show the I-V and P-V characteristics of the module for both the cases.

VOLTAGE (V)

Figure 3. I-V characteristics for the Dunasolar module with and without cooling.

Figure 4. P-V characteristics for the Dunasolar module with and without cooling.

By comparing the graphs it is clearly seen how the open voltage surely decreases by cooling effect while the peak power gets just a bit higher.

The results seem to confirm the difficulty to take real advantage of the potential benefits, the breakthrough of the increased efficiency versus the more complicated and costly solution is not yet optimized.

Conclusions

The PV/T technology presents Interesting potential to gain advantages respect the two separated technologies. From one side the thermal collector could exploit the more highest standards for the reliability and the highest high tech consideration of the PV module, while the photovoltaics could take advantages of the wider marketing channels of the thermal collectors. The building Integration applications can be the bridge for allowing a good penetration in the market. At the end of the day instead to compete each other for the needed surface for their application, on the roofs for instance, the hybrid device could be efficiently installed providing both electricity and heating. Israel has reached a very Interesting position on that field. Nevertheless some important technical problems related to the proper materials choice like the absorption for the heating collector and the thin film technology, especially related to the amorphous Si option, although promising at the moment, will require more research and development and at the same time economical issues and market preparedness will call for more attention.

REFERENCES

DRAFT ROAD MAP ON PV/T SYSTEMS November 2000 Status Report of task 7 of the IEA PV Power Systems Program

Hybrid Photovoltaic Building Fagades: the challenges for an Integrated overall performance evaluation

PASLINK EEIG, Violestraat 21-23, 1000 Brussels European Economic Interest grouping of Outdoor Test Centres

PV-HYBRID Development of Procedure for Overall Performance Evaluation of Hybrid Photovoltaic Building Components, JOULE Project coordinates by PASLINK EEIG "Building Integrated multi PVT Solar System Roof Tile". Ami Elazari, 3rd ISE-Europe Solar Congress, Copenhagen, Denmark, 2000, june 19-22

Four Institutes will work together on different tasks of the project. The results will be exchanged as often as necessary to get a better overview on certain aspects of large thermal solar combi-systems. The partners and their tasks are listed below.

SWT — Solar & Warmetechnik Stuttgart together with ZfS — Rationelle Energietechnik GmbH:

1. Selection of six existing combi-systems, equipping them with measurement instrumentation, measuring and analysing them in detail over a period of two years.

2. Check for defects of the system after a short measurement period, which have to be eliminated before further measurements.

3. Analysis of the operating behaviour of the system and single components. Outlining the advantages and disadvantages of the different systems.

4. Building up simulation models of the systems for analysis of the existing technique as well as development of improved systems by variation of parameters.

5. Calculate the economic efficiency of simulated improvements.

6. Implement economically efficient improvements in the existing system, further measurements.

7. Develop a guideline for planning and dimensioning of this system technology

ISFH — Institut fur Solarenergieforschung GmbH Hameln/Emmerthal:

1. Analysis of the stagnation effects of three large collector fields on a test bench as well as measurement of the stagnation in one existing system over two summers. Therefore detailed measurement of temperature, pressure and degradation of the solar fluid is planned.

2. Analysis of how the collector fields are deflated and recharged (during/after stagnation).

3. Investigation of two flat-plate and one vacuum-tube collector fields based on the results above.

4. Recommendations on how to reduce stagnation in the system based on the measurements and the results of the partners.

5. Investigation of these recommendations in real operating behaviour.

Fraunhofer-Institut fur Solare Energiesysteme, ISE, Freiburg:

1. Evaluation of the stagnation of collector fields by analysing the detailed performance during stagnation of a single collector.

2. Measurement of draining and steam power of single solar collectors under consideration of different absorber pipings.

3. Simulation of the stagnation behaviour of a single collector based on the measurements. Afterwards simulation of large collector field.

4. Simulation of the effect of a fluid-air-heat exchanger to transfer heat to the environment in case of stagnation.

5. Recommendation on how to reduce stagnation in collector fields based on the measurement results and collector simulation.

6. Analysis of the degradation of the solar circuit heat transfer fluid.

7. Publication of the results of all partners on a web page and in a work shop. (http://www. Solarkombianlagen-XL. info)

2. References

[1] Styri-Hipp, G.; Kerskes, H.; Druck, H., Bachmann, S.: Kombianlagen Abschlussbericht des Projektes „Testverfahren fur Solaranlagen zur kombinierten Brauchwassererwarmung und Raumheizung (Kombianlagen)", DFS, Freiburg, 12/2001

[2] Peuser, F. A.; Remmers, K.-H.; Schnauss, M.: Langzeiterfahrung Solarthermie — Wegweiser fur das erfolgreiche Planen und Bauen von Solaranlagen; Solarpraxis Supernova AG, Berlin, 2001, ISBN 3-934595-07-3 (English version: Solar Thermal Systems, Solarpraxis Berlin 2002, ISBN 1-902916-39-5)

[3] VDI 6002-1: Solar heating for domestic water — General principles, system technology and use in residential buildings (will be published in July 2004)

[4] TRNSYS Version 15.0 — User Manual. Solar Energy Laboratory, University of Wisconsin, Madison und Transsolar, Stuttgart.

3. Acknowledgement

This project is supported by the German Federal Ministry for the Environment (Bundesministerium fur Umwelt, Naturschutz und Reaktorsicherheit; Forderkennzeichen 0329268A-C). The authors gratefully acknowledge this support and carry the full responsibility for the content of this paper.

The Central Solar Heating Plant with Aquifer Thermal Energy Store in Rostock — Results after four years of operation

Thomas Schmidt1), Hans Muller-Steinhagen1)2)3)

1 Solar — und Warmetechnik Stuttgart (SWT),

A Research Institute within the Steinbeis-Foundation,

Pfaffenwaldring 10, 70550 Stuttgart, Germany,

Tel. +49-(0)711-685-3299, Fax: +49-(0)711-685-3242,

Internet: www. swt-stuttgart. de. Email: schmidt@swt-stuttgart. de

2) Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart

3) Institute of Technical Thermodynamics (ITT), German Aerospace Center

In Rostock the first German central solar heating plant with an aquifer thermal energy store (ATES) went into operation in 2000. The system supplies a multifamily house with a heated area of 7000 m2 in 108 apartments with heat for space heating and domestic hot water preparation. On the roof of the building 980 m2 of solar collectors are mounted. The ATES operates with one doublet of wells and is located below the building. The store works as a seasonal heat store to overcome the gap between high amount of solar energy in summer and highest heat demand of residential buildings in winter. The solar system was designed to cover half of the yearly heat demand for space heating and domestic hot water preparation by solar energy. This target could be reached in 2003 with a solar fraction of 49 %.

The plant is one out of eight demonstration plants that have been built within the German research programme “Solarthermie-2000” in the last eight years /1/. The supplied multifamily house, see Figure 1, was built in 1999 by the house building company WIRO, Wohnen in Rostock Wohnungsgesellschaft mbH, who still owns and operates the building. The main part of the heat supply system was designed by Geothermie Neubrandenburg GmbH (GTN). The project is evaluated by SWT, ITW and GTN for the part of the ATES. The paper gives a detailed description of the system and presents the main results from four years of operation.

The yearly energy demand for the operation represents another important influence criterion on the energy payback time. Analysis showed that pump capacities and pump operating hours differ considerably between different thermal solar systems. A comparison of system 2 with a system that differs only in the pump and controller capacity and in the pump operating hours is made in Tables 5 and 6. The fractional energy savings are equal for both systems investigated. Table 6 shows the reduction of the energy payback time.

The reduction of the energy payback time of 0.2 years might seem to be very small regarding the lifetime of a thermal solar system of about 20 years. However one should consider that this corresponds to the influence of an increase or decrease of the fractional energy savings of 5% on the energy payback time, as shown in Table 7.

1.1 EXPERIMENTS

1.1.1 SYSTEM DESIGN

Two small low flow SDHW systems were tested side-by-side in a laboratory test facility for solar heating systems. With the exception of the solar tank the systems were identical.

Fig. 1 shows schematic illustrations of the two tanks. Both tanks are standard mantle tanks suitable for low flow systems. In both tanks the auxiliary energy is supplied by electric heating elements.

One tank is equipped with a PEX pipe for hot water draw-off from the very top of the tank. The other tank is equipped with two PEX pipes for hot water draw-off from the very top of the tank and from the middle of the tank.

A three way valve, type RAVI from Danfoss A/S, ensures that the temperature of the tapped water during draw-offs is equal to the required hot water temperature. If this is not possible due to high tank temperatures, the valve ensures that the difference between the tapped water temperature and the required hot water temperature is as low as possible. That is, the three way valve ensures that the right part of the hot water is tapped from the top and from the middle of the tank.

The most important data of the tested systems are given in Table 1. Fig. 2 shows the collectors of the tested systems.

SHAPE * MERGEFORMAT

As mentioned before, more attention has to be paid to flow phenomena at bifurcations. Therefore simulations were carried out using CFD (Computational Fluid Dynamics). As can be seen from Fig. 10, only the first bifurcations at the absorber inlet were taken into consideration. A constant outlet pressure was assumed. A mass flow of 0.01 kg/s (7.2 l/ (m2h); laminar flow) and 0.1 kg/s (72.1 l/(m2h); turbulent flow), respectively, was chosen as an inlet condition. Fig. 10, which shows the results of a high-flow simulation, confirms the effects observed in the flow experiments: the fluid tends to prefer the inner channels of the fractal structure. A cross section of the first left branch also reveals the Dean vortices pre­sumed to be the cause for the described effect (indicated by vectors in the detail picture).

72.1 l/(m2h) (turbulent)

3.24e 01 I 2 92e 01 2.59Є-01 2.27Є-01 194e01 1.62Є-01

і 1.30e01 9.72e02 6.4ве 02 I 3 24e 02 H O. OOe+OO

Conclusions

An algorithm which is capable of generating fractal hydraulic structures on a given area with fluid in — and outlet was developed and served as a basis for the programme FracTherm. With this programme it is also possible to carry out hydraulic as well as thermal simulations and visualise the results. A total collector efficiency factor F can be determined. The simulated F of a fractal absorber was high compared to measured val­ues of conventional fin absorbers. DXF files can be exported from FracTherm, which al­lows computer aided manufacturing. Flow experiments with ink indicated secondary flows at bifurcations. Thermography pictures showed that a uniform heat transfer can be ob­tained. CFD simulations confirmed the secondary flow effects observed in the flow experi­ments.

In order to be able to compare fractal structures with conventional ones, it is intended to use FracTherm also for simulations with serial and parallel hydraulic structures. Moreover, further CFD simulations as well as experiments are planned to investigate flow phenom­ena and determine the pressure loss coefficients at bifurcations. Variations of the FracTherm parameters will show their influence on the total energy efficiency. In a further step these variations can be carried out automatically and thus lead to an optimisation us­ing "evolution strategy" [7]. The simulation environment ColSim can be used to carry out dynamic system simulations which will also take thermal capacity effects into considera-

tion. Finally, it is intended to produce and measure prototype absorbers. Thus a comparis­on of fractal absorbers and other heat exchangers with conventional ones will be possible.

Acknowledgement

The author likes to thank the German Federal Environmental Foundation (DBU) for fund­ing this research work in its scholarship programme.

01

[1] Duffie, J. A. and Beckman, W. A.: Solar Engineering of Thermal Processes. 2nd edition. New York: John Wiley & Sons, 1991

[2] Eisenmann, W.: Untersuchungen zu Leistungsfahigkeit und Materialaufwand von Sonnenkollektoren mit serpentinen — und harfenartiger Rohrverlegung. Fortschr.-Ber. VDI Reihe 6 Nr. 490. DQsseldorf: VDI Verlag, 2003

[3] Frey R., Frei U., Brunold S.: Bestimmung des Kollektorwirkungsgradfaktors F’ an flQssigkeitsfCihrenden Solarabsorbern. Solarenergie PrQf — und Forschungsstelle SPF-ITR, Oberseestr. 10, CH-8640 Rapperswil, 1995

[4] Hermann M., Koschikowski J. und Rommel M.: Corrosion-free solar collectors for thermally driven seawater desalination. Solar Energy 72(5), pp. 415-426, 2002

[5] Martin, H.: WarmeQbertrager. Stuttgart; New York: Thieme, 1988

[6] Nachtigall W. und BlQchel K. G.: Das groBe Buch der Bionik. Stuttgart; MQnchen: Deutsche Verlags-Anstalt, 2000

[7] Rechenberg, I.: Evolutionsstrategie ’94 (Werkstatt Bionik und Evolutionstechnik; 1). Stuttgart: frommann-holzboog, 1994

[8] Wittwer, C.: Colsim — Simulation von Regelungssystemen in aktiven solarthermischen Anlagen. Dissertation, Fakultat fQr Architektur, Universitat Karlsruhe (TH), 1999