The next set of parameters refers to control settings and auxiliary boiler data (Figure 9). Most significant influences the boiler outlet temperature fsax, ext. The higher this temperature, the lower is the possibility of the solar collector to heat this volume to higher temperatures. Of nearly no importance within the varied values seems to be the collector controller settings for start up and the boiler flow rate.

Figure 9 Dependency of fsav, ext on specific parameter change of control settings and auxiliary boiler data

Figure 10 Dependency of fsav, ext on specific parameter change of temperature sensor positions

In the same way as above the position of the auxiliary boiler sensor is very important (Figure 10). The higher this sensor is placed the less volume is heated by the auxiliary boiler. Less important, but still of influence, is the position of the collector sensor in the store. As lower this sensor, as earlier is the collector pump switched on, when the store is discharged. On the other hand the running time for the collector pump increases. As long as the temperature sensor is positioned around the collector heat exchanger the change of fsax, ext is very small.

Conclusion

Nine different solar combisystems were simulated and deeply analyzed in IEA SHC Task 26 using the same reference conditions and the same simulation tool. For each system between 12 and 30 parameters were varied starting from a base case. These parameters covered climate, collector type, size, orientation, mass flow, store size, store geometries, size of heat exchangers, heights of inlets and outlets, insulation, control settings of thermostats and control strategy of the whole system.

The influence of the parameters was analyzed for each system and general conclusion of the dependency on combisystems were drawn using a statistical approach.

Besides the well known influence of climate, collector size, orientation, and store volume on fractional energy savings, it was found that the store insulation on top and bottom should be around 15 cm (already taking into account additional heat losses at inlets and outlets, sensors etc. by a correction factor). The bottom insulation is not that important, because the store should be cold at the lower part anyway. The store volume that is heated by the auxiliary heater should be as small as possible but still big enough that the heat load can be covered. Otherwise the fractional savings indicator drops significantly.

Literature

Bales, C., 2003: Generic System #11: Space Heating Store with DHW Load Side Heat Exchanger(s) and External Auxiliary Boiler, Technical Report for Subtask C, IEA SHC — Task 26, Appendix 6 to Streicher, W, 2003

Bales, C., 2003a: Generic System #12: Space Heating Store with DHW Load Side Heat Exchanger(s) and External Auxiliary Boiler (Advanced Version), Technical Report for Subtask C, IEA SHC-Task 26, Appendix 7 to Streicher, W, 2003 Bony, J., Pittet, J., 2003: Generic System #8: Space Heating Store with Double Load — Side Heat Exchanger for DHW, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 4 to Streicher, W, 2003

Cheze, D, Papillon, P., 2003: Generic System #3a: Advanced Direct Solar Floor, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 2 to Streicher, W, 2003 Ellehauge, K., 2003: Generic System #2: A Solar Combisystem based on a Heat Exchanger between the Collector Loop and Space-Heating Loop, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 1 to Streicher, W, 2003

Heimrath, R., 2003: Generic System #19: Centralized Heat Production, Distributed Heat Load, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 9 to Streicher, W, 2003

Jaehnig, D., 2003: Generic System #15: Two stratifiers in a Space Heating Storage Tank with an External Load-Side Heat Exchanger for DHW, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 8 to Streicher, W, 2003

Letz, Th, 2003: Validation and Background Information on the FSC procedure, Technical Report for Subtask A, IEA SHC-Task 26

Peter, M., 2003: Generic System #9b: Space Heating Store with Immersed DHW tank and External DHW store with Auxiliary, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 5 to Streicher, W, 2003

Shah, L. S., 2003: Generic System #4: DHW Tank as Space-Heating Storage Device, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 3 to Streicher, W, 2003 Streicher, W., 2003: Solar Combisystems modelled in Task 26 (system description, modelling, sensitivity, optimisation), Technical Report for Subtask C, IEA SHC-Task 26 Streicher, W., 2003a: Sonnenenergienutzung: Lecture book, Graz University of Technology, free download from http://wt. tu-graz. ac. at

Streicher, W., Heimrath, R., 2004: Analysis of System Reports of Task 26 for Sensitivity of Parameters, Technical Report for Subtask C, IEA SHC-Task 26 Weiss, W. (ed.), 2003: Solar Heated Houses — A Design Handbook for Solar Combisystems, James&James Science Publisher

The roof MaReCo, previously described by Adsten (2002), is 2D, concentrating solar collector intended to be placed on a tilted roof, see figure 3. It has a reflector designed as a parabola combined with a circular section and an absorber that absorbs irradiation on the back side from an acceptance interval between 0° and 60° solar altitude angle, when placed on a roof with 30° slope. The absorber also receives direct irradiation on the absorber. A prototype of this design was evaluated by indoor and outdoor measurements of thermal performance at varying incidence angles. The glazing of the prototype was replaced by a Teflon film.

The other concentrating solar collector evaluated here is a hybrid solar window, which basically consists of a parabolic reflector and a hybrid absorber, designed with a

concentration ratio of 2.45. The function of the reflector is both to concentrate the irradiance onto the absorber and to be used as solar shading, as the reflectors may be rotated backwards, as shown in figure 4. The hybrid absorbers consist of thermal absorbers with photovoltaic cells laminated on the surface. The wall element is further described by Fieber et al. (2004). The efficiency of the thermal absorbers in a prototype of this design was evaluated by indoor and outdoor measurements at varying incidence angles. The glazing of the prototype consists of two anti-reflex coated glass panes.

Figure 4. The hybrid solar window.

Absorber / alcogel using MS51 as starting material was done by supercritical drying. Silica aerogel cracked. Figure 8 shows photograph of structure sandwiched silica aerogel obtained from MS51. Although a copper panel expanded during supercritical drying, MS51 silica aerogel shrunk 4.1%. Silica aerogel cracked difference in expansion. To stop the shrinkage ratio of silica aerogel, starting material was changed from MS51 into TMOS, and the solvent was changed from IPA into ethanol. The shrinkage ratio of silica aerogel obtained from TMOS was 1.4 %. Then, one-piece structure, which did not cracked, was produced. Figure 9 shows photograph of structure sandwiched silica aerogel obtained from MS51 With the structure the selectively solar-absorbing coatings obtained from TMOS was not exposed to the open air at all. The light from solar simulator radiated on to the panel. The temperature went up 177 °C by radiation at 1000 W/m2.

Conclusion

1. Selectively solar-absorbing coatings

TiOxNy selectively solar-absorbing coatings were produced by the sol-gel method. The reflectance was lower 1 %.

The emission was 3%.

2. One-piece panel

The selectively solar-absorbing coatings were sandwiched by two silica aerogels. The one-piece structure temperature went up 177°C by radiation at 1000 W/m2. In addition, the very highly efficient solar heat panel may be developed.

Acknowledgements

The authors wish to thank Dr. Tajiri of AIST for silica aerogel development.

This research was supported by Advanced Technology Initiative for New Industry Creation of Hokkaido Bureau of Economy, Trade and Industry.

References

1. ‘New Solar Energy Utilization Handbook’, Japan Solar Energy Society, (2001).

2. ‘Solar Energy — The State of The Art’, International Solar Energy Society, (2001)

3. K. Tajiri, Surface Science, 14[9](1993), 546-549.

4. G. Gowda and T. Harrison, J. Canadian Chem. Soc., 55(1986), 68-71.

5. A. Hunt, P. Berdahl, K. Lofftus, R. Russo and P Tewari, Pssive and hybrid solar energy Update, 84(1984), 47-50.

6. A. Hunt, R. Ruso, P. Tewari and K. Lofftus, Intersol, 85[1](1986), 223-228.

The authors wish to thank the colleagues working together in the group “Materials for Solar Thermal Collectors" of the Solar Heating and Cooling Program of the International Energy Agency, especially Stefan Brunold, Bo Carlsson, Ueli Frei, Ole Holck, Kenneth Moller, Markus Muller, Henk Oversloot and Svend Svendsen.

The work was financially supported by the Federal Minster for Research and Technology of Germany (Contract No. 0329600B/9).

3. References

Kohl, Michael; Heck, Markus; KObler, Volker, MOller, Markus; Steinhart, Josef; Troscher, Thomas: Mikroklima in Kollektoren. Internationales Symposium fur thermische und photovoltaische Sonnenenergienutzung, Gleisdorf, Osterreich, 9.-12.9.1998.

Holck, Ole; Svendsen, Svend; Brunold, Stefan; Frei, Ueli; Kohl, Michael; Heck, Markus; Oversloot, Henk: Solar collector desing with respect to moisture problems. Solar Energy 75, 2003.

Frei, Ueli; Brunold, Stefan; Kohl, Michael; Troscher, Thomas; Carlsson, Bo; Moller, Kenneth: Air ventilation rate of solar thermal collectors. In: Proceedings North Sun, Espoo, Finland 9.­11.6.1997.

Kohl, Michael; Brunold, Stefan; Carlsson, Bo; Frei, Ueli; Heck, Markus; Holck, Ole; Jorgensen, Gary; KObler, Volker; Moller, Kenneth; Oversloot, Henck; Roos, Arne; Svendsen, Svend: Performance and Durabilitiy Assessment of Optical Materials for Solar Thermal Systems. Edited by Al Czanderna. To be published Elsevier 2004.

Kohl, Michael; Heck, Markus; KObler, Volker: PrOfung und Optimierung des Mikroklimas in Solarkollektoren. 14. Symposium Thermische Solarenergie, Bad Staffelstein, 12.-14. Mai 2004.

As has been mentioned above, the schematic diagram and the design of LHPs make it possible to use capillary materials for making wicks with quite a small effective pore radius. Such materials were specially created on the basis of fine-grained nickel, titanium and copper powders. Despite their high porosity, which is usually equal to 60-70%, they possess a sufficient strength, required for complex mechanical processing in making wicks.

Since such wicks are used in hermetic systems, which LHPs are, it is necessary to ensure their chemical compatibility with the working fluids during a long operation. Appropriate

"body — wick — working fluid” combinations have been determined by experiment [2]. They may also be recommended for use in LHPs. Some of them are presented in Table 1.

Wicks with the indicated working fluids are capable of creating a high capillary pressure, which is necessary for pumping a working fluid for a sufficiently great distance. Table 2 gives values of the capillary pressure created by a wick with an effective pore radius of 1 micron.

As can be seen from this table, the highest pressure may be achieved when the working fluid is water, which is a consequence of the high coefficient of surface tension this liquid possesses. Besides, water retains its advantages in the widest temperature range. At the same time at temperatures below 70oC the efficiency of using water in LHPs decreases owing to the decreasing value of 3P/3T. In this case ammonia becomes preferable.

Saad Odeh", Mechanical Engineering Department/ Hashemite University Salem Nijmeh, Mechanical Engineering Department/ Hashemite University M. Sami Ashhab, Mechanical Engineering Department/ Hashemite University Yousef Zakaria, Mechanical Engineering Department/ Hashemite University Ahmed Amra, Mechanical Engineering Department/ Hashemite University

Jordan imports most of its energy demands in the form of crude oil and petroleum products. To meet the country’s future increasing energy demands some of the conventional thermal energy systems are required to be replaced by renewable energy systems. Currently the share of renewable energy in the total energy consumption is around 1%. It is expected that with increasing scientific and technological capacities in the field of renewable energy in Jordan, this share will rise up to 15% in the year 2010 [1]. A very promising application of solar energy in Jordan is its use for low and moderate temperature application such as food canning, paper production, air­conditioning, and sterilization. It was shown by many researchers that some types of solar energy systems are capable of producing the energy level required by different industrial processes. Parabolic-trough solar water heating is one of a well proven solar energy technology which is being used on a commercial scale to produce heat for industrial and residential applications. Hot water at temperatures (50-56°C) was produced by a field of 1584 m2 parabolic collector area to supply housing buildings, a cafeteria, a laundry, and services building [2]. The average collector array efficiency was found to be about 0.61 at average beam radiation. Solar steam generation by a parabolic collector field was studied to supply an absorption cooling system and heating needs of a hotel [3]. The estimated collector field area was 100m2 (on the hotel roof) to deliver steam at 144°C, and 4 bar. Evacuated CPC (compound parabolic concentrator) collectors with non-tracking reflectors were compared with two novel tracking collectors: a parabolic trough and an evacuated tube collector with integrated tracking reflector [4]. The CPC mirror was mounted inside the evacuated tube and hence protected from environmental influences. The efficiency of this type of CPC collector, at temperature of 300 °C, with anti-reflective coating of the glass tube, and a selective absorber coating, may reach up to 0.65. This allows for application in industrial process heat generation, high efficiency solar cooling and power generation systems. Simulation and modeling of direct steam generation parabolic trough collector (high pressure) were carried-out by many researchers [5, 6]. This type of collector was integrated in an electric power generation model in order to evaluate long term performance at different locations in Jordan [7, 8]. The study showed that such type of system could operate efficiently in Jordan. A simulation program was developed to study the thermo-hydrodynamic performance of a solar industrial water heating system with a back-up boiler and thermal storage tank [9]. The model was used in a case study of a cloth factory in Jordan to supply steam at moderate temperature and pressure to an existing factory processes. The study showed significant increase in annual performance (about 12%) when using N-S axis solar tracking mode rather

* Corresponding author, email: sodeh@hu. edu. jo or saad_odeh@hotmail. com, mail address: Mechanical Engineering Department, Hashemite University P. O.Box 150459, Zarqa 13115, JORDAN

than E-W tracking mode. The effect of thermal storage tank on system daily operation was studied for winter and summer periods. The performance evaluation of solar industrial water heating system in Jordan showed its applicability for different regions of the country. It was found that by adopting seasonal tracking, solar energy contribution and collector field performance will increase significantly.

Abdullah and Nijmeh [10,11] conducted a detailed study on solar tracking surfaces in Jordan. They designed and constructed an electromechanical sun tracking system based on the open loop method of control. The collected solar energy was measured and compared with that on a fixed surface. The two-axis moving surface showed an increase in the collected daily total energy of up to 41%. This gain is significant and justifies the use of tracking surfaces in certain applications of solar energy in Jordan.

In this paper a single tracking axis parabolic trough collector is designed and constructed for moderate heat load applications: such as domestic space air­conditioning and small factory process heat. The solar tracking collector is designed to be self powered for its tracking and water pumping systems.

A general purpose CFD code named DPC has been employed in the multidimensional simulation of the hybrid stores.

DPC is a library written by the CTTC for the resolution of combined heat and mass trans­fer problems by means of computational fluid dynamics. The Navier-Stokes equations are solved together with the energy equation and the species equations (when a mixture is modelled) using finite volume techniques. The problem domain can be discretised in one or several blocks [1] using structured staggered or co-located grids [11]. Turbulence is mod­elled by means of low-Reynolds number two equation models [11]. Different methods can be used for the modelling of the radiative heat transfer: radiosity-irradiosity method, discrete or­dinate method [4]. Solid/liquid phase change is modelled by means of enthalpy-like methods [5].

Continuity and momentum equations are solved with a coupled multigrid algebraic solver, or using pressure based SIMPLE-like methods and solvers multigrid [10, 7]. The scalar variables transport equations (energy, species, …) are segregatelly solved by means of multigrid solvers. DPC allows parallel processing when the multiblock technique is used [2].

Some of the most illustrative results obtained with DPC concerning multidimensional nu­merical simulation of storage devices and solid/liquid phase change phenomena can be found in [12, 2, 3, 6, 5].

Prediction models

Simplified models based on global or one-dimensional mass and energy analysis, have been developed in order to be used in the store thermal performance description (ENV 12977-3), and in long term thermal solar systems simulation codes.

The well-known multinode model [8] has been adjusted to numerically predict the thermal behaviour of hybrid latent/sensible stores with PCMs. The PCM modules have been mod — elized as internal tank elements characterised by an overall heat transfer coefficient. The PCM modules at each node are assumed to be at the same temperature, and their energy balance equation take into account the energy stored by latent heat.

We have considered of interest to look into this model in detail. Reviewing multinode mathematical formulation, for each i-t/г tank node, energy balance can be written as follows:

where, t is time, jV is the number of nodes, p is the fluid store density, cp is the specific heat, У, is the effective store volume, Ті is the temperature of the i-t/г node, and mheat are the temperature and the mass flow rate of the fluid from the heat source, Tload and mioaii are the temperature and the mass flow rate of the fluid from the load loop, Tem, is the environment temperature, TpemA the temperature of the PCM modules int the i-t/г node, (UA)em, is the overall heat loss coefficient, is the overall heat transfer coefficient

between the sensible heat store and the PCM modules, and Д are the direction controllers (1 if the fluid enters node i, 0 otherwise), Ae// is the effective thermal conductivity, and S’ is the store cross section.

On the other hand, an energy balance about the i-t/г PCM modules node can be written as:

Ррет I Cp, pcm ^

where, rpcTO is the effective ratio of PCM modules volume in the overall store volume, L is the PCM latent heat and FpcmA is the fraction of PCM in liquid state (i. e. FpcmA = 1 means that the PCM in the i-t/г node is liquid while, FpcmA = 0 means that it is in solid state).

Equations 1 and 2, assume that overall heat transfer coefficients are the same for each node of the tank. This hypothesis implies that PCM modules are located homogeneously throughout the tank.

In a numerical simulation, equations 1 and 2 are solved iteratively at each time increment. The fraction of liquid in the PCM modules is evaluated from equation 2 fixing ТрстЛ to the PCM melting temperature Tpemm and isolating FpcmA.

Illustrative results

As an illustrative example of the use of the numerical infrastructure commented above, preliminary results obtained in the design/optimisation studies of hybrid latent/sensible stores are hereafter presented.

Hybrid stores are numerically simulated by means of the CFD code. CFD simulations are used as a virtual design tool. The detailed numerical data obtained is then employed to identify store parameters on the basis of EN12977-3. In this task, the prediction codes explained above are used. The thermal performance of the store is determined by the following parameters: the effective store volume У,, the effective PCM ratio rpcm, the overall heat transfer coefficients (UA),,,pcm and (UA)env, and the effective thermal conductivity Ae//.

The concept of solar air heating consists of a blackened absorber, well insulated at the lower and side with a transparent cover to allow solar radiation and block the infrared radiation. If any fluid is passed over/below or both side of the absorber, the fluid gets heated when exposed to solar radiation (fig. 1). The energy balance equations for transparent cover, absorber and flowing air can be written as

«gl+ hp-g(Tp Tg) — hg-a(Tg — Ta) (1)

Tg ap 1 — hp-g(Tp-Tg) + hp-b(Tp-Tb) + hp-f(Tp-Tf) (2)

The installed solar system is a roof-integrated type (plate I). For the installation of the solar system, a south facing asbestos roof was constructed at the site. The corrugated aluminium sheet was used as absorber by coating a solar selective paint. The glass wool insulation having a thickness of 65 — mm was packed beneath and sides of the absorber plate to suppress the heat losses. With some height above the absorber plate, 4-mm solar toughened glass was mounted. Baffles were provided to increase the air fil-factor and to improve the efficiency. The fresh air is allowed to pass through the gap between the absorber plate and glass cover in a zig-zag manner. While air is flowing through the air channel in the device, heat is extracted form the absorber by the air, resulting in a temperature rise or, in other words, hot air is obtained. The hot air is sucked by a centrifugal blower (3HP capacity and flow rate of 9000 m3/h) through an insulated duct and it is uniformly distributed in the drier, where chilly is loaded.

The drier consists of ten perforated aluminium trolleys each having a holding capacity of 100 kg of fresh chillies (plate2). The inside of the drier is fully covered with aluminium sheets to increase the life of the system and for hygienic processing. Two axial flow fans (2 HP each) having a volume flow rate of 20000m3/h were provided inside the drier for the uniform circulation and re-circulation of hot air. The hot air is made to pass through the chillies loaded in the drier and it draws moisture from the product and the warm moist air escapes through the dampers provided. The provision has been made to adjust the volume flow of air through the exhaust damper.

The solar heat pump systems without storage in the collector/heat pump circuit (systems S1 and C1 of Figure 1) are a straightforward option. The collectors are directly connected to the heat pump evaporator. Thus, the heat pump COP strongly depends on the present solar insolation. The collector temperature restricts heat pump operation.

For these systems usually plane energy absorbers or solar absorbers are used. These absorbers are simple unglazed collectors that are connected to a hydraulic circuit. Solar absorbers were developed for example by Shinobu et al. [3] or by Rheinzink GmbH&Co. KG, Datteln (D) for roof integration (Figure 2 and Figure 3). The Rheinzink Quick Step — Solar Thermie Module has recently been made commercially available.

Dietrich et al. [5] showed that a direct combination of plane energy absorbers with a heat pump is feasible. The temperature of the evaporator can be increased by solar absorbers, but an additional heat source (oil or gas) is still required, as heat pump operation is limited to source temperatures above -10°C.

A laboratory-scale solar heat pump system with two different types of unglazed flat plate collectors (convective / radiative type) was investigated by Ito et al. [6]. It was shown that using the two collectors in parallel raised the heat pump COP at low solar insolation. Fur­thermore, Ito et al. [7] analysed the performance of such a system with photovoltaic cells bonded on the surface of the thermal collectors by conductive silicone, which did not influ­ence the performance of the heat pump appreciably.

In the framework of concentrator’s geometry, parameters like inclination angle and size of mirrors, height H and width D of concentrator, concentration factor C and mirror’s utilisation factor M have been laid under analyses. Necessary formulas have been derived and implemented to a software application that has been prepared under Excel environment utilizing the VBA programming techniques. Application is named „CLON Geom" (fig. 4). This application provides all the necessary tables and graphs for analyses. For illustration, in this paper some results for geometrical concentration factor, the most important parameter, is shown.

Three different definitions of geometrical concentration ratio are given for CLON type of concentrator:

Incremental concentration factor Ci is understood as a an addition of current zone to the total concentration factor or the cumulative concentration factor, respectively.

Cumulative concentration factor C is a factor that cumulates the contributions from separate zones up to the current zone.

C = Z Ci (9)

i=1

Total concentration factor C is, in fact, the cumulative concentration factor after the last, the nth zone.