The idea of using heat pipes and two-phase thermosyphons in systems that convert solar energy into thermal energy has been known already for quite a long time [3-5]. These devices are here parts of collectors and concentrators of solar radiation. Loop heat pipes
can also fulfill this function and, at the same time, serve for transferring thermal energy with insignificant losses and without any additional means of pumping.

It is possible to consider, in particular, two variants of using LHPs in systems of sun heat supply. The first of them is presented schematically in Fig. 6. It envisages the use of a sun collector in the form of a flat panel joined to several parallel evaporators. When it is considered that the density of solar-radiation energy typical for Western Europe at the summer period of the year is approximately 300 W/m2, then for the collection of 1 kW it is necessary to have such a collector with an area of about

3.3 m2. Fig. 6. System of sun heat supply with the

In principle, for transferring a heat flow of 1 use of an LHP

KW it will suffice to have one LHP with an evaporator 200-250 mm long from 25 to 28 mm in diameter. However, in this case, when one evaporator is joined to a collector with a larger surface, the problem of concentration of a heat flow arises. This problem can be solved by increasing the effective thermal conductivity of the panel, for instance, by using conventional heat pipes or thermosiphons, or by increasing the number of evaporators. In the latter case one should bear in mind that the efficient operation of an LHP is possible at a heat-load density per evaporator not lower than 0.5X10-4 W/m2. Therefore, more optimum here may become a combined variant, when several LHP evaporators are joined to a sun collector equipped with conventional heat pipes.

For more powerful heat-supply systems it is expedient to use parabolic sun concentrators combined with an LHP evaporator (Fig. 7).

Characteristic values of the heat-flow maximum density at the surface of a cylindrical evaporator with an area of (250-300)x10-4 m2 in water LHPs are about 10*10-4 W/m2. It means that on one such evaporator situated in focus with regard to a small parabolic concentrator a heat flow of 2.5-3 KW can be focused. To increase the system capacity, one can use two evaporators combined by a common thermal interface or one evaporator of a larger diameter. It should be mentioned that with decreasing LHP dimensions and evaporator heating surface the heat-load density may be considerably increased. Thus, for instance, in an

References:

1. "Heat Transfer Apparatus”, US Patent, 4. 515. 209, 1984.

2. P. D. Dunn, D. A. Reay, "Heat Pipes”, Pergamon Press, Oxford, 1976.

3. "Solar Thermal Energy Collection Systems”, US Patent, 3. 875. 296, 1975.

4. D. R. Adkius. "Design Consideration for Heat Pipe Solar Receivers”, J. of Solar Eng., Vol.112, 1990, pp. 168-176.

5. J. Danilewicz, B. Nowak, M. A. Sayegh. Two-Phase Thermosyphon Solar Collector Absorber. Proc. of the 9th Int. Heat Pipe Conference, Albuquerque, NM, 1995, pp. 364­368.

A closed loop control system was designed to track the sun around the N-S axis. This system consists of: a DC motor, a speed reduction unit, a photo resistance sensors unit, and a controller as shown in Fig. 6. Two photo resistances are located on both the parabolic

reflector sides. These sensors are used as part of a wheatstone bridge circuit to give zero voltage when radiation plane is normal to the aperture area of the reflector (sun in center of the parabola). If incident angle changes, the sensors give a voltage proportional to it. The controller amplifies this voltage and sends a signal to the motor to track the sun and keeping wheatstone bridge output voltage equal to zero.

Transient numerical information obtained from the CFD simulations has been employed in the store parameters identification. This transient numerical data is plotted in Figure 3.

In figure 4 thermal behaviour of the different stores obtained by means of the prediction codes after the store parameters identification process are shown. As can be seen, the prediction model is able to reproduce the thermal performance of the stores with appropriate model parameters.

Figure 3: Store parameters identification. CFD results. Heat power supplied (right) and extracted (left) to the thermal stores during the test procedure.

(a) (b)

Figure 4: Store parameters identification. Prediction code results. Heat power supplied (right) and extracted (left) to the thermal stores during the test procedure.

Acknowledgements

This work has been supported in part by the European Commission under the Fifth Framework Programme, Thematic Programme: Energy, Environment and Sustainable De­velopment FP5-EESD, Project CRAFT-1999-72475.

References

[1] J. Cadafalch, A. Oliva, C. D. Perez-Segarra, M. Costa, and J. Salom. Comparative study of conservative and nonconservative interpolation schemes for the domain de­composition method on laminar incompressible flows. Numerical Heat Transfer, Part B, 35(1):65-84, January-February 1999.

[2] R. Consul, I. Rodriguez, and A. Oliva. Three-dimensional simulation of storage tanks by a parallel multiblock algorithm using loosely coupled computers. In Proceedings of the ISES Solar World Congress 2003, pages 1-6, Goteborg, Sweden, June 2003.

[3] R. Consul, I. Rodriguez, C. D. Perez-Segarra, and A. Oliva. Virtual prototyping of stor­age tanks by means of three-dimensional CFD and heat transfer simulations. Solar Energy. Under revision.

[4] G. Colomer, M. Costa, R. Consul, and A. Oliva. Three dimensional numerical simulation of convection and radiation in a differential heated cavity using the discrete ordinates method. International Journal of Heat and Mass Transfer, 7(2):257-269, January 2004.

[5] M. Costa, A. Oliva, and C. D. Perez-Segarra. A three-dimensional numerical study of melting inside a heated horizontal cylinder. Numerical Heat Transfer, Part A, 32(5):531- 553, 1997.

[6] M. Costa, A. Oliva, C. D. Perez-Segarra, and A. Ivancic. Three-dimensional numerical simulation of melting processes. Physical Models for Thermal Energy Stores, pages 87-94, 1996.

[7] B. R. Hutchinson and G. D.Raithby. A multigrid method based on the additive correction strategy. Numerical Heat Transfer, Part B, 9:511-537, 1986.

[8] E. M. Kleinbach, W. A. Beckman, and S. A. Klein. Performance study of one-dimensional models for stratified thermal storage tanks. Solar Energy, 50(2):155-166, 1993.

[9] A. Mehling, L. F. Cabeza, S. Hippeli, and S. Hiebler. Pcm-module to improve hot water heat stores with stratification. Renewable Energy, 28:699-711, 2003.

[10] S. V. Patankar. Numerical heat transfer and fluid flow. McGraw-Hill, 1980.

[11] C. D. Perez-Segarra, A. Oliva, M. Costa, and F. Escanes. Numerical experiments in turbulent natural and mixed convection in internal flows. International Journal for Nu­merical Methods for Heat and Fluid Flow, 5(1):13-33, January 1995.

[12] I. Rodriguez, R. Consul, J. Cadafalch, and A. Oliva. Numerical studies of thermosyphon solar heaters. In Proceedings of the EUROSUN 2000 Conference, pages 1-9, Copen- hagen, Denmark, 2000.

The solar installation with special monitoring and controlling system is built in South-West University “N. Rilski” (SWU) — Blagoevgrad. The scheme and description of installation is presented in [6]. The main target of monitoring and controlling system is to ensure possibility to: work in direct and indirect regime with different heat exchange area and location of serpentine unit in the tank; change angle of collectors toward by horizon and azimuth; regulate some operating parameters (fluid flow rate); measure all important parameters, influenced the system performance.

The test tank is a vertical cylindrical tank made of stainless steal material. In the tank are built three copper serpentines along all the height of the tank. Serpentines are 10 meters each in length. They can be switched on or off as a heat exchange unit by system of valves. So the system can work with one, two or three serpentines situated in different regions of the tank. Installation can work also in direct regime, when all three serpentines are turned off.

Monitoring system includes 12 thermo sensors assembled in accumulation vessel, 6 thermo sensors in collector circle, one thermo sensor for measuring the ambient air temperature. Solar meter, located near the solar collectors measures solar radiation. The inflow rate, heat energy and heat power are measured by combined heatmeter. All observed parameters are registered by automatic monitoring system. It includes a special electronic module for converting the analog data from sensors to digital signals. Digital data from converting module is collected by computer system. After that, stored data can be used for detailed analysis of thermal and economical efficiency of system and preparing the statistical calculation for long-term analysis.

Measuring module includes also a control unit, which governs the pump performance. It starts pump, if the temperature difference between inlet and outlet temperature of working fluid is above preliminary defined value. Ordinarily, the systems work stable and efficiently with control temperature difference between 2 and 5oC.

Consumption of hot water is realized by simulation of a typical consumption regime for small restaurant hot water system and domestic (family) hot water system.

The options of solar collector and heat pump connection for domestic heating systems are shown in Figure 1. Several examples of realised or theoretically investigated solar heat pump systems from literature are described above. It is evident, that the system configura­tions have diverse advantages and disadvantages and are thus often applicable only in particular areas or buildings. Table 2 gives an overview of the system approaches pre­sented, evaluates their characteristics (especially concerning their applicability in Central European climate and buildings) and describes the problems and challenges linked with each concept.

The parallel solar heat pump concepts with ground collector and air source are in principle good solutions for domestic heating, especially in comparison to fossil fuel fired systems. However, one problem is the high space demand for the collector and the effort for its in­stallation in the ground. The air source heat pump clearly suffers from the long winter peri­ods in Central Europe and the problems linked with it (e. g. icing of the evaporator).

The same problem occurs in non-storage solar heat pump systems with a direct heat pump-collector connection. Monovalent operation seems not to be feasible in Central European climates with long frost periods and for example snow lying on the absorber area, which means an additional heat source (e. g. furnace) is required.

The water storage systems on the one hand have the disadvantage of large storage vol­umes and therewith high space demand. On the other hand, they use an easy to handle and low-cost storage medium in contrary to the latent heat storage tanks working with par­affin or salt hydrate. These tanks also have the problem that the melting point of many PCMs is relatively high and that the heat exchanger design is complex.

The heat exchanger design is also a challenge for stores that use the water/ice transition. Nevertheless, a water/ice-latent heat storage tank has the most positive influence on the collector degree of utilisation due to the low melting point of water. While collector effi­ciency and degree of utilisation are increasing, the heat pump’s COP is decreasing, hence heat pump development to achieve an acceptable seasonal performance factor (SPF: ratio of heat delivered and total energy supplied over the season) is necessary. Furthermore, the storage volume can be reduced considerably in comparison to a sensible water store by the utilisation of the phase change.

Incremental concentration factor (fig. 6) Ci = f (i): N+^R, i є (1; n) is a series of functions converging to n ^ да for ©A = const. At ©A = 5°, the biggest contribution to concentration will have the second zone, the for higher zones the contribution falls down exponentially. It can be seen from graphs that CLON with n > 10 has no real sense. For higher acceptance angles the number of zones with meaningful contribution to the overall concentration will be even less, e. g. for ©A = 20° CLON should have not more than 6 zones.

Yet more interesting is to read the dependance of cummulative (or total) concentration factor (fig. 7) Ci = f (i). It can be seen that CLON’s maximal concentration factor for ©A = 5° is C = 5. No CLON of practical measure will have acceptance angle lower.

Conclusions

From mentioned above follows that CLON is expected to be much advantageous in comparison to the classical solution of CPC, which is assumed to be a practical realisation of so called ideal concentrator. But on the other hand it is necessary to note that CLON is advancing CPC only at level of practical realisation and usage of flat mirrors is possible only due to flat character of output area. Rays incoming at angle of incidence equal to ©A will be concentrated nearby foci and there will be then higher local irradiation than on the rest of receiver, what can never be reached by CLON. However, this is neither supposed to CLON nor CPC.

Further it is necessary to remark that CLON is not intended to be and is not an approximation of common CPC, so also not its modification. Its concept given by optical scheme is new and principially different from CPC.

Still considering the solar heating plant, the thermal performance of the evacuated tubular collector is compared to the thermal performance of the newest (Vejen N. K., Furbo S., Shah L. J. (2004). ) Arcon HT collector. The collectors are facing south and tilted 45° in Copenhagen. In Ummannaq the collectors are facing south and tilted 60°.

It can be difficult to compare the thermal performances of flat-plate collectors and tubular collectors as the effective area of a flat plate collector typically is defined as the transparent area of the glass cover and the effective area of a tubular collector can be defined in many ways. In the present comparison, the tubes are placed close together so that there is no air-gap between the tubes and the outer tube cross­section area (=L-2-rc-N) directly corresponds to the transparent area of a flat-plate collector.

Fig. 11 shows the thermal performance per m2 collector as a function of the solar fraction of the solar heating plant for the two collector types. Here, the solar fraction is defined as:

Qauxiliar

Qto

First of all, the figure shows that the tubular collector has the highest thermal performance for both locations. Further, it can be seen that the Uummannaq curves decreases more rapidly with increasing solar fractions. This is due to the lower air temperature in Uummannaq.

The figure also shows that the ARCON HT collector has a better thermal performance in Copenhagen than in Uummannaq, whereas the tubular collector performs best in Uummannaq. The main reason for the result is that there is much more solar radiation ‘‘from all directions’’ in Uummannaq and this radiation can better be utilized with the tubular collector.

Conclusions

A new TRNSYS collector model for evacuated tubular collectors with tubular absorbers is developed. The model is based on traditional flat plate collector theory, where the performance equations have been integrated over the whole absorber circumference. On each tube the model determines the size and position of the shadows caused by the neighbour tube as a function of the solar azimuth and zenith. This makes it possible to calculate the energy from the beam radiation.

The thermal performance of an all glass tubular collector with 14 tubes connected in parallel is investigated theoretically with the model and experimentally in an outdoor collector test facility. Calculations with the new model of the tubular collector vertically placed and tilted 45° is compared with measured results and a good degree of similarity between the measured and calculated results is found.

Further, the collector model is used in a model of a solar heating plant and a sensitivity analysis of the tube centre distance, collector tilt and orientation with respect the thermal performance per tube is investigated for the two locations Copenhagen (Denmark) and Uummannaq (Greenland). The results show that the optimum tilt and orientation is about 45° south for Copenhagen and about 60° south for Uummannaq.

Finally, the thermal performance of the evacuated tubular collector is compared to the thermal performance of the newest Arcon HT collector. Here, the results show that the tubular collector has the highest thermal performance for both Uummannaq and Copenhagen. This analysis also illustrates the differences in the thermal behaviour of the two collector types: The ARCON HT collector has a higher thermal performance in Copenhagen than in Uummannaq, whereas the tubular collector performs best in Uummannaq compared to Copenhagen. The main reason for the result is that there is much more solar radiation ‘‘from all directions’’ in Uummannaq and this radiation can better be utilized with the tubular collector than with the flat plate collector.

The effect of the significantly higher performance levels of COAX 390 (heat insulation and heat transfer ability) has been tested in a simulation in Polysun. The experiment has been carried out for a solar thermal system with COAX 390 as well as two or three flat collectors PLANO 21 (figure 4) in comparison to a conventional tank with the same collectors (cli­mate Wurzburg).

Table 1 shows the different parameters. The values for heat losses of the connected pipes are taken from /1/.

Without even considering the additional yield through stratification, the COAX-system of­fers with the two tested factors alone a higher solar yield of 8 % or 14 %, respectively.

2. Conclusion

The substantial performance improvements of COAX 390 (achieved by stratification, heat transfer values, insulation) in comparison to the market standard leads to the conclusion that — when using the same collector area — a significantly higher energy yield can be achieved.

On the other hand, the cost for this tank is comparable to high-quality yet technically con­ventional tanks, which should lead to an extremely good price/performance ratio for the complete system.

3. Literature

/1/ Meitner, R., Siegemund, A., Leibfried, U.: Mit Mehrtagesspeichern gegen Warmev — erluste, Moderne Gebaudetechnik 10/2001

Tr_COAX_14_SYMPOS_as. doc

In a conventional Compound Parabolic Concentrator (CPC), the reflector formation criterion is that light tangential to the edge of the closest tube (the tube nearly touching the cusp) is reflected back in the same direction by the CPC reflector, so that light which is not tangential strikes the closest tube more directly or via the reflector on a second bounce. Light cannot be reflected from one reflector to another and still be collected.

In Fig 1, two dotted tangent lines are shown. Light passing along these lines will be reflected straight back along the incident direction. Light more normal to the collector aperture will be collected. Between the left and the centre tube, the light ray is collected after a single bounce. Some light will be lost through the gaps between the absorber surface and the cover tube but about 98% is aimed at the tubes before reflection and absorption losses are accounted for.

In a CPC the curvature usually used is that of a mathematical involute, and the curve is truncated at the height approximately as shown in Fig. 1. The actual height of the reflector rim between the tubes depends upon the tube spacing. A full involute is not used; it would finish level with the top of the inner absorber tubes and would allow about 12.5 standard evacuated tubes having a 34.2 mm absorber tube diameter in a 1350mm wide panel. In this case the spacing between the tube centrelines is equal to the absorber tube circumference (107.4 mm), so that peak optical concentration is about 1. Such a "full” CPC would use much more reflector area than a practical design and the shape could not be as easily fabricated, so a 14 tube panel with a lower reflector is probably a more acceptable compromise these days, especially as tubes are relatively low cost.

In the last years an increasing interest on the PV/T hybrid collector has been reported and some important international projects have been launched, like, to name a few:

1) The PV-HYBRID-PAS Joule project. The main goals of the project were the development of Procedures for Overall Performance and the evaluation of Hybrid Photovoltaic Building Components;

2) The PV Bonus negli USA;

3) Activity 2.5 TASK VII within IEA.

The joint implementation of the two technologies could favour both their markets by:

• sharing the experience of each market characteristics; the high tech aspect for the photovoltaics could add more reliability and trust to the solar collector and the already existing commercial sectors, surely more developed for the thermal collectors, could be exploited by the PV,

• the potential of a large costs reduction for the mounting, installation and manufacturing whether combined,

• the prevention and possible elimination of competition for the availability of the surface on the roofs for the buildings.

The history of that technology has showed that there have been different levels of integration:

1) at first the two systems have been kept separated, working in parallel, in symbol PV + T. With that configuration it has been difficult to realize the potential advantages;

2) then the systems have been combined and the two technologies have begun to match each other, symbolically PV&T. The Photovoltaics part is only added on the collector. Some benefits of the implementation have been gained;

3) the next pace is the integrated systems. The two parts are intimately joined and projected together so optimising the whole component, in symbol PV/T.

Some interesting aspects for the future development are listed below.

1) It is an added value for the building;

2) It could reach benefits following the boom for the building integration photovoltaics;

3) It is compatible with the thin film technology, particularly with the amorphous Si, that is more transparent to greater wavelengths and has more favourable temperature coefficients;

4) It fits well with the energy certification of buildings, compulsory for the next future;

5) It matches well to the new trends in the modern architecture (sustainability, compatibility, awareness)