serial, meandering structure is relatively high due to the channel length, it is lower in paral­lel structures. However, parallel structures can lead to a non-uniform flow distribution (de­pending on the ratio of the diameters and lengths of the risers and the header channels, [4, 5]).

In contrast to the mentioned designs, many natural hydraulic structures such as blood vessels or leaf venation show a multiple branched, fractal geometry. Since natural designs are optimised with respect to energy efficiency, the idea of transferring their construction principles to technical applications seems to be a promising approach (Fig. 1 c), [6]). If channel designs with both lower pressure drop and a uniform flow distribution can be ob­tained, it will be possible to rise the energy efficiency of heat exchangers.

The errors between the experimental and calculated data values of efficiencies were verified by using the mathematical model of Eq.3.

The constants applicable to the Eq.3, were determined by using the computer Fortran

program. The numerical values of parameters are shown in Table.4.

The solar collector efficiency results show that there is an average deviation of 2.90% between the calculated and the experimental data. The mathematical model (Eq.3) validate the experimental results with minimum deviation of 0.14% at Vs=30m3/h/m2 and maximum deviation of 11.2% at Vs=20m3/h/m2 from the experimental data for all wind speeds.

Fig. 4. Relationship between collector efficiency and collector airflow at various wind speeds (experimental and calculated data).

5. Conclusion

The purpose of this indoor experimental work was to find evidence of the effect of changes in wind speed and suction velocity on the performance of an unglazed transpired solar collector. Our observations indicate that the peak collector efficiency occurs at zero-wind speed and low efficiencies occur between 1.6m/sec and 3.1m/sec wind speeds. The suction velocities and efficiencies result comparison show that, increasing suction velocities was results in an increases in efficiency. This is the support for our indoor experimental results that wind speeds and fluctuation in suction velocities has a strong influence on collector performance. We remain confident in our conclusions about the effects of wind speed on this system. It seems logical that wind speed is a dominant factor for affecting the efficiency of solar air heating collector. Our current efforts are directed towards finding a multivariable correlation, or model, to explain the indoor scatter experiment measurements. The experimental data were compared with the data obtained by the model. Satisfactory qualitative and quantitative
agreement was obtained between calculated and experimental data. The mathematical model could be deemed to be satisfactory for predicting the efficiency of the perforated solar collector at various wind speeds and suction velocities.

For further applications of the unglazed perforated solar air heating collector, we plan to build the solar wall system to avoid uncontrolled entry of outdoor air into offices and other working areas which is a cause of increased the heating costs.

The research regards the development, design and application (installation), testing and monitoring and performance evaluation of a Low Temperature SORC system for sea (or brackish) water desalination. The system consists of the following sub-systems and components (Fig.1):

1) High efficiency vacuum tube solar collectors’ array

2) Circulator

3) Alternative thermal source (thermal wastes, geothermal or other)

4) Evaporator

5) Condenser

6) Economiser

7) Expanders

8) Pressurisation unit consists of 2 vessels V1, V2 three automatically controlled valves (VL1, VL2, VL3)

9) RO unit

10) Insulated seawater reservoir

11) Fresh water reservoir

12) RO energy recovery system

The system operation is described briefly below:

Thermal energy produced by the solar array (1) preheats and evaporates the working fluid (HFC-134a) in the evaporator surface (4). The water temperature at collector inlet is about 70 oC and the outlet temperature about 77 oC. The super-heated vapour is driven to the expanders (7) where the generated mechanical work drives the RO unit pumps (high pressure pump, cooling (heating) water pump, feed water pump) and circulating pump (2). The saturated vapour at the expanders’ outlet is directed to the condenser, after passing through an economiser (de-super heater) (5). On the condenser surface, seawater is pre­heated and directed to the seawater reservoir (10). Seawater pre-heating is applied to

increase the fresh water recovery ratio (in RO technology, higher feed water temperatures imply higher fresh water recovery ratio). The seawater tank is insulated. The use of seawater for condensation purpose on the condenser surface decreases the temperature of "Low Temperature Reservoir” of Rankine cycle thus a better cycle efficiency can be achieved. The saturated liquid at the condenser outlet is pressurised in the special pressurisation arrangement consists of two vessels and three valves (8) substituting a pump. The sub-cooled liquid at the pressurisation arrangement outlet is driven to the economiser. The economiser acts as working fluid pre-heater. In the economiser outlet saturated liquid is formed which is directed to evaporator inlet and the cycle is repeated.

An energy recovery system is coupled to the RO unit thus declining the energy consumption to 3 kWh/m3 product.

Below the thermodynamic analysis of the states described above is presented (Fig.1,2):

rankine

AHq

AHW = Hsl — Hs2 AHq = Hsl — Hs3

hrankrne = °.°976

Where: n, Efficiency; H, Enthalpy; T, Temperature

The theoretical Organic Rankine system efficiency is 9.76 %

The Carnot cycle efficiency is 10.6 %

The actual system efficiency is estimated to 65% of theoretical that is 6.34 %.

For the prototype system 240 m2 of vacuum tube solar collectors will be deployed. Both the evaporator and condenser are plate heat exchangers of brazed type. The heat exchanger area of the evaporator is 6.8 m2, while that of condenser is 11.5 m2. For this number of collectors and considering a water recovery ratio of seawater RO desalination

SHAPE * MERGEFORMAT

Enthalpy (kj/kg)

Figure 2: Molier chart of Rankine cycle (working fluid states)

unit of 30%, the average yearly fresh water production is estimated at 1450 m3 (or 4 m3 daily)2. Table 2 presents the distribution of fresh-water production throughout a year.

Since 1997 the first solar assisted district heating system with duct heat store in Germany is being realised in Neckarsulm-Amorbach. The solar assisted district heating system presently supplies about 200 accommodation units with heat. For the final extension stage approximately 1,300 accommodation units were planned.

Presently 5,263 m2 solar thermal collectors are installed on different buildings as well as on a carport and a noise protection wall. The heat from the solar collectors is delivered to the heating plant and collected in buffer tanks which are used for short-term heat storage to balance peaks in heat delivery from the solar collectors, see figure 1.

The buildings are connected to the district heating system by a 3-pipe heat distribution net. The heat distribution net is supplied either by the buffer tanks or the duct heat store, depending on the temperature level. A gas condensing boiler supplies additional heat if none of the stores is able to deliver heat at the requested temperature level.

The duct heat store was extended twice and presently contains a volume of 63,360 m3 with 528 borehole heat exchangers (double-U-pipes, 30 m deep) for charging and discharging.

In table 1 an overview about the project history and the extension stages of the collector area and duct heat store volume is given.

7.1 Whole computational domains: sym­metrical configuration.- For these configu­rations, it was observed that spatial periodicity in each cell of the slats structure for A’ < 1.053 range exists, and that the behaviour of the variables «*, u* and T* was similar to cases when parallel slats were in contact with cold isothermal wall only. But in case of pressure, the pe­riodic behaviour was not obvious. Due to that fact two additional cases considering values of A = 30.5 and 60 and preserving values of A’ in 0.9528 were computated. In these new cases, it was demonstrated that distributions have had a completely periodic behaviour. Therefore, for symmetrical configurations the periodicity behaviour exists for all variables only when the overall aspect ratio A is higher than 20. In Fig. 4 the streamlines are pre­sented for cases studied.

7.2 Whole computational domains: asymmet­rical configurations close to hot isothermal wall.- It was demonstrated that spatial periodic­ity for each cell of the slats structure for all cases proposed in Table 2 exist.

7.3 Reduced computational domains: asym­metrical configurations close to hot isother­mal wall.- In Table 9 numerical results of the Nus — selt numbers obtained on whole computational domains are compared with the Nusselt numbers calculated on reduced domains. It was observed that the percentage differences |<й/|% increases with the decrease in A’.

An additional study was performed progres­sively increasing the value of from 20 to 86.696

and maintaining the values of the other parame­ters constant. Numerical results are presented in Table 10.

8.2 Verification.- For the finest meshes the

observed order of accuracy p approaches the theoretical values of the differential scheme used (between 1 and 3). The percentage of Richardson nodes obtained was high for Th = 30 and 70°С cases, but decreases in the case Th = 12й0С, where the physical phenomena was more complex. For all cases, the GCI values decrease as n increases. These results indicate that the estimator GCI is reliable only for Tft = 30 and 70°С cases.

8.3 Experimental setup.- A schematic of the setup is shown in Fig. 6. The internal dimen­sions of the cavity are = 300mm high, L = 70mm wide and F = 500mm deep. The slats consist of 25 rectangular glass sheets (dimensions are 1.5mm thick, 40mm high and 500mm deep). The rectangular sheets are distributed in a uniform manner, and are separated from the front and back plates by air gaps of lh = lc = 15mm.

Over the cold isothermal wall, an aluminum plate; type K thermocouples; another aluminum plate; a cooling system formed by a single-pass heat exchanger; and insulation mate­rial of armaflex were placed consecutively.

The hot isothermal wall structure consisted of: an aluminum plate; type K thermocouples; glued electrical heaters; and rock wool insulation material. The cavity was closed at its sides by lids covered with insulation material on the outside. Two glass windows were mounted on one side wall (window A and B, see Figs. 6a and 7), in order to allow the visualization of the air flow in the cavity.

Control, regulation and data acquisition were carried out with a data acquisition unit managed by a software applica­tion programmed in HPVEE language. The air is seed by aerosols of olive oil generated by a Laskin nozzle. The images of the flow were captured using a Digital Particle Image Ve- locimetry (DPIV). Errors of the measured velocities due to DPIV device, the data acquisition and the post-processing were expected to be below ± 0.0014 m/s.

C. MOller/K. Schwarzer/H. Kleine-Hering*
Solar-Institut JOlich / Solar Global e. V./ *Ecoandina
Heinrich-Mussmann-Str.5, 52428 JOlich
Tel.: (0049-2461) 99 35 70, Fax: (0049-2461) 99 35 70
E-Mail: c. mueller@sij. fh-aachen. de,

Internet: http://www. sij. fh-aachen. de

1. Introduction

In connection with a BMBF research project (FKZ 17104.01), a new system has been developed to provide solar heating and hot water. The system is designed to be used in areas with high solar radiation and low ambient temperatures, conditions which occur typically in high altitude regions. The main considerations in develop­ing this system were robust technology, low cost and easy maintenance. To ensure robustness, air is used as the heat transfer medium. Air has the advantage of a low thermal capacity and enables the system to be immediately ready for use, and does not have the disadvantages of water at temperatures below the freezing point. The units were installed in two public buildings in the Argentinean Altiplano at an alti­tude of 3600 m, as part of a BMZ (Ministry for Cooperation) project. The local partner in the project was Ecoandina.

Because of the high level of direct solar insolation in this area, concentrating solar cookers for families and institutions have a very high acceptance. As part of the BMZ project, four community cookers with Fixed-Focus reflectors (Scheffler reflec­tors) each with 3 kW power were installed. Further installations included solar hot water systems, drip irrigation systems with solar pumps and parabolic cookers for families. One of the villages equipped with these units is now to receive an award for being the first Solar Village in Argentina.

The efficiencies of a new oil fired boiler and new natural gas burners in one family houses were measured for long monitoring periods [2]. The boilers were the only available heating source in the investigated houses. In Fig 1, system efficiencies from 3 different sites on monitored during the last 2 years are shown. All boilers are of modern technology and they all were installed in the year 2002 and later. The system efficiency is calculated as (QDHW: Domestic hot water demand [kWh], Qsh: Space heating demand [kWh], Qfuel: Gas/Oil

consumption [kWh]): Qdhw + Qsh

Qfuel

The two previously described prototypes of concentrating reflectors have been evaluated for incidence angle dependence by indoor and outdoor measurements. During indoor measurements, the collectors where placed at the pivot point of the simulator and the photo diode array was placed on collector surfaces. The irradiance to the collectors where also measured with a pyranometer. The incidence angles were simulated by raising the simulator or by rotating the collectors when placed vertically in front of the simulator.

K(O,0T) was derived from the measured values and divided by fL(0L), in order to get RT(0T). A single outdoor measurement was performed at вт =30° for the roof MaReCo and вт =20° for the solar hybrid window. The relations between the current from the photodiode array and the irradiation at corresponding incidence angles at outdoor measurements were
calculated. A compensation of the cosine dependence of the incidence angles was also performed, assuming that the total irradiation from the simulator is constant throughout the measurement and that the irradiance on the collector is dependent on cos 0T.

The calorimetric measurements were performed with the temperature difference, AT, between the water in the collector and the ambient air maintained close to zero. The flow for the calorimetric measurements is recorded with an inductive flow meter and the temperature difference between in — and outlet is measured with a thermopile consisting of five thermocouple pairs. The thermal performance was registered continuously during the measurements.

A comparison has been made to corresponding outdoor measurements of both fL(0T) and K(O,0t) of the roof MaReCo, that were previously performed on a larger prototype with identical geometrical features.

The mathematical formulation is based on the principles of conservation of mass, linear momentum and energy in incompressible flows. In an arbitrary spatial region of volume V, bounded by a closed surface S’, these conservation principles can be written as [19]:

(1)

я

TOC o "1-5" h z J vpv ■ dS = J —pS ■ dS + j ^(7 + pSj ■ dS + j gpdV (2)

.4 .4 .4 V’

j hpdV + j hpv ■ dS = — j q ■ dS + j q„dV (3)

V’ .4 .4 V’

where the following main hypothesis are assumed: symmetry of the stress tensor <r, which is a consequence of the conservation of angular momentum; transient effects in the momen­tum equation are negligible; all body forces actuating are due to the gravitational accelera­tion; effects of kinetic energy and pressure in the energy equation are negligible; the effects of the work of the body forces in the energy balance are negligible.

In equation 2, the stress forces have been divided into the forces due to pressure at the boundary of the volume S and the forces due to flow resistance, which are respectively the two first terms on the right of the equality of equation 2.

The model is based on the division of the whole thermosyphon system and its com­ponents into strategically distributed control volumes (CVs). At each one of the CVs the simplified general governing equations 1,2 and 3, are integrated assuming local averaged fluid variables. In this step, some further simplifications are applied at each CV according to the physical phenomena which is taking place.

The tank is divided into n equal sized CV in a similar way as is done in the well known multinode model [11,7], the collector in a single CV (assuming an exponential distribution from the inlet to the outlet according to [7] for the modelling of the body forces), and the pipes in n different CV.

This model requires some information to be provided from experimental data of the com­ponents as the efficiency curve of the collector, the heat loss coefficient of the tank, the pressure drop in the pipes and in the collector due to fluid flow resistance… This information can also be provided from higher level numerical models.

This is the level of modelling of thermosyphon systems typically used, see [9, 14, 6, 18].

The methodology adopted by Task 27 includes three steps: a) initial risk analysis of poten­tial failure modes, b) screening testing/analysis for service life prediction and microclimate characterisation, and c) service life prediction involving mathematical modelling and life testing.

Initial risk analysis

The initial risk analysis is performed with the aim of obtaining (a) a checklist of potential failure modes of the component and associated with those risks and critical component and material properties, degradation processes and stress factors, (b) a framework for the selection of test methods to verify performance and service life requirements, (c) a frame­work for describing previous test results for a specific component and its materials or a similar component and materials used in the component and classifying their relevance to

the actual application, and (d) a framework for compiling and integrating all data on avail­able component and material properties.

The programme of work in the initial step of service life assessment is structured into the following activities: a) Specify from an end-user point of view the expected function of the component and its materials, its performance and its service life requirement, and the in­tended in-use environments; b) Identify important functional properties defining the per­formance of the component and its materials, relevant test methods and requirements for qualification of the component with respect to performance; c) Identify potential failure modes and degradation mechanisms, relevant durability or life tests and requirements for qualification of the component and its materials as regards durability.

The first activity specifies in general terms the function of the component and service life requirement from an end-user and product point of view, and from that identifies the most important functional properties of the component and its materials. In Table 1 and Table 2 results are shown from the analysis made by the Task 27 group on booster reflectors. How important the function of the component is from an end-user and product point of view needs to be taken into consideration when formulating the performance requirements in terms of those functional properties. If the performance requirements are not fulfilled, the

particular component is regarded as having failed. Performance requirements can be for­mulated on the basis of optical properties, mechanical strength, aesthetic values or other criteria related to the performance of the component and its materials.

Potential failure modes and important degradation processes should be identified after failures have been defined in terms of minimum performance levels. In general, there exist many kind of failure modes for a particular component and even the different parts of the component and the different damage mechanisms, which may lead to the same kind of failure, may sometimes be quite numerous. In Table 3 an example from the Task 27 work on booster reflectors is presented.

Fault tree analysis is a tool, which provides a logical structure relating failure to various damage modes and underlying chemical or physical changes. It has been used for the static solar materials studied in Task 27 to better understand observed loss in performance and associated degradations mechanisms of the different materials studied. In Figure 1 and Figure 2 are shown examples on how the different failure modes and associated deg-

radation mechanisms can be represented for booster reflectors and antireflective glazing materials.

Figure 1 Representation of failure modes and associated degradation mechanisms for booster reflectors from the IEA SHCP Task 27 study

The risk associated with each potential failure/damage is taken as the point of departure to judge whether a particular failure mode needs to be further evaluated or not. Risks may be estimated jointly by an expert group adopting the methodology of FMEA (Failure Modes and Efffect Analysis) [2,3]. In Table 4 the result of a risk analysis made by the Task 27 group on booster reflectors is presented.