Stanko Vl. Shtrakov, Anton Stoilov South — West University “Neofit Rilski”, Dept of Physics,

66 Ivan Mihailov Str., 2700 — Blagoevgrad, BULGARIA,

E-mail: sshtrakov@abv. bg, antonstoilov@abv. ba

Introduction — The major impediments for market penetration of solar hot water installations in Bulgaria are the lack of information and experienced data about the efficiency, thermal accumulation of energy and adequate exploitation in different seasons in a year and geographical regions. Defining useful recommendations for different regimes of exploitation, corresponding to the climatic conditions and installation parameters is the main purpose of this work. A special experimental solar module for hot water was built and equipped with sufficient measure apparatus. The main concept of investigation is to optimise the stratified regime of thermal accumulation and parameters of heat exchange equipment (heat serpentine in tank). Accumulation and heat exchange processes were investigated by theoretical end experimental means. Special mathematical model was composed to simulate the energy transfer in a stratified tank. Computer program was developed to solve mathematical equations for thermal accumulation and energy exchange. Experimental equipment with more than 15 temperature sensors and other measure devices gives data for the real processes. Extensive numerical and experimental tests were carried out. A good correspondence between theoretical and experimental data was arrived. Collected experimental and theoretical data from two years’ exploitation period is a good base for establishing some important issues about construction and exploitation of solar installation with different consumption regimes. Analysis of collected data were used to make a detailed technical and economical assessment of hot water installations with respect to the climatic and economical conditions in Bulgaria. This data will help designers and investors to expand penetration of the solar energy application in Bulgaria.

Solar hot water installations are the most often used solar applications in Bulgaria. The major impediments to further increase of the market penetration for these systems are the lack of information and experienced data about the efficiency, thermal accumulation of energy and adequate exploitation in different seasons in a year and geographical regions.

For small solar installations, used preliminarily in the domestic sector, the thermally stratified storage tanks for hot water is a good installation scheme. In such systems the hot water remains separated from the cold water by means of buoyancy forces. Stratified storage tanks are more thermally and economically effective. Maintaining thermal stratification is very important. This ensures that solar collectors work with maximal thermal efficiency. It is because the inflow to the collector is taken from the bottom of a stratified tank (the coldest layer in the tank). On the other hand, hot water for consummators is charged from the top of the accumulator, where the highest water temperature is kept. This delivers useful energy on demand.

A variety of models and experiments to assess the efficiency of stratified tanks have been produced. As a result many numerical and experimental studies have been conducted on the performance of stratified tanks under different operating conditions and constructive parameters. Most of the published studies analyze the direct solar installations or indirect installation with removed heat exchanger, where mass and thermal transport mechanism in accumulator and heat exchanger are separated. If an indirect solar installation with serpentine (included
heat exchanger) is used, the thermal exchange and accumulation perform simultaneously at the same place.

Solar installations with stratified tanks and heat exchange by included serpentine have advantages, because the destratification by fluid mixing in charge phase is eliminated. Only in discharge process the fluid mixing is available, but with some constructive measures the losses in stratification can be minimized. Moreover, the place of heat transfer in tanks can be regulated with serpentine disposition. The serpentine can be situated in upper, middle or bottom part of the tank. What is the right position of the heat exchanger (serpentine) is a disputable question. It depends on many factors and studies in this field will help designers and constructors of solar installations for hot water.

Motivated by this point, the present study is intended to investigate performance of typical domestic hot water installation in different regimes of thermal accumulation and climatic conditions. In order to wide scope for investments, a computational model and computer program for heat exchange process in accumulation tank was created. Many experimental and numerical studies have been conducted on the performance of stratified storage tank under different operating conditions and for different design characteristics. These characteristic include the parameters and situation on the heat exchange serpentine, flow rate in collector circuit, water consuming regime and other.

In order to reduce the system dependency on the actual solar insolation availability, stor­age tanks have been applied in the collector / heat pump circuit by some researchers (sys­tems S2 and C3 of Figure 1). Table 1 shows the characteristics of some solar heat pump systems in experimental houses with different types of storage in combination with solar collectors and a heat pump.

Water stores are the easiest option and show a considerable performance gain. An inves­tigation of the Philips Experimental House showed that a solar fraction of 72% can be achieved by this collector-storage-configuration. Figure 4 illustrates that the fraction of heat dissipated of the complete insolation on the collector area is relatively high in comparison to customary solar systems. The highest monthly collector degree of utilisation was 50%; its average value was 40%. The curve of the storage temperature proves that solar energy is also stored seasonally in this system. As Table 1 illustrates, storage capacity is of major influence for the solar fraction that can be realised in the heating systems. However, the Philips Experimental House suffers from very high space demand for the water tank (42.000l).

The utilisation of phase change energy storage has been investigated by several re­searchers. For example the BBC AG, Walldorf (D), replaced the water store (8.000l) by a latent heat store (2.100l) in its solar house. As heat storage medium paraffin was investi­gated. It was found that the replacement of the storage tank (and the reduction of storage volume) was of little influence on the system’s performance [9]. Kaygusuz et al. [10][11] operated a solar assisted heat pump system at Karadeniz Technical University, Trabzon (TR). This experimental system incorporated a latent heat storage tank in combination with flat plate collectors (30m2, single-glazed, blackboard paint). As phase change material
(PCM), a salt hydrate (1500kg calcium chloride hexahydrate encapsulated in polyvinyl chloride containers) with a melt­ing point of 28…30°C was used. The sys­tem was designed for series, parallel and dual source (alternative source: ambient air) operation. Measurements over a day in the Black Sea region comparable weather conditions showed a consider­able improvement of the system effi­ciency by the use of the latent heat store in a series system in comparison to non­storage series and parallel configurations

In comparison to the typical PCMs (paraf­fin, salt hydrate), water as a PCM prom­ises to enhance the collector efficiency due to its lower melting point. Apart from that, water is a favourable storage me­dium because of its remarkable proper­ties (nontoxic, noncombustible, easy to handle, inexpensive,…).

The utilisation of the phase change of water in a solar heat pump system was investigated by Ltibeck University of Ap­plied Sciences for instance (Table 1). The insulated concrete storage tank (18.000…20.0001) was equipped with helical tube heat exchangers. It was heated up to approximately 80°C in summer. From June to November the thermal energy was transferred directly into a short-term storage tank (3000…4000l) for hot water preparation and space heating. During the heating season it was cooled down by the heat pump to 0°C utilising the phase transition. The store, however, was solidified to a degree of approx. only 60%(vol.) [12][13].

As a private initiative, a house with a solar heat pump is operated in Bad Reichenhall (D). W. Hesse planned and built the solar heating system. The speciality of this system is a cistern (40.000l) in the garden, which is used as a seasonal storage tank. The cistern is not insulated, thus makes use of thermal energy from the ground in winter times when the storage temperature is below ambient. The latent heat of water is also utilised in this sys­tem by a so-called dynamic heat exchanger. The heat pump evaporator is mounted above the cistern. Water is poured over the heat exchanger plates; the ice is automatically re­moved after a certain period. The system operates satisfactorily and provides a sufficient supply for the building’s heat demand [14][15].

A further similar solar heat pump system with water/ice storage in a cistern is commercially available from HitSolar21, Ramsau (A). The system incorporates solar absorbers without insulation, a heat pump and a patented heat exchanger in a cistern. Several systems in family houses have obviously been realised in Germany and Austria [16].

Unfortunately, a technical analysis of both “modern” systems, Hesse and HitSolar21, is not available.

SHAPE * MERGEFORMAT

In the 1970s a very interesting system for winter heating and summer cooling was developed and investigated by the Oak Ridge National Laboratory (USA). The so — called Annual Cycle Energy System (ACES) incorporates an insulated under­ground water tank being frozen during the heating season for summer cooling. The system is particularly advantageous in cli­matic zones with nearly equal heating and cooling loads and showed an annual elec­tricity saving of more than 50% in compari­son to a system based on full electric en­ergy supply. When the heating require­ments exceeded the cooling load, unglazed solar absorbers were used as an additional heat source [22][23].

Apart from these mainly experimental stud­ies, a simulation study of a solar heat pump system with unglazed solar absorbers and a water / ice storage tank was carried out Figure 5: Influence of Storage Capacity on by Posorski [24]. Figure 5 shows that the Additional Heating Energy Re­incorporation of such a storage tank in the quired (Climate: Hamburg/D, PCM:

collector / heat pump circuit reduces the Water) [24]

additional heating required to a minimum at an acceptable collector area for a Northern German climate. Furthermore, it is concluded, that the heat pump power required for monovalent operation is reduced considerably.

Collector-side storage also has a favour­able influence on the operation of the heat pump, as the source temperature is smoothed by the store. Kaygusuz et al.

[25][26] demonstrated that a storage tank leads to a more stable heat pump COP.

The Centre of Excellence for Solar En­gineering at Ingolstadt University of Ap­plied Sciences and Ratiotherm Hei — zung+Solartechnik GmbH&Co. KG, Dolln — stein (D) are investigating a solar heat pump heating system incorporating a low — temperature latent heat storage tank (phase change material: water) plus a stratification tank (Figure 6) [27]. The major advantages of the heating system are con­sidered to be its flexible application (suit­able for new and existing buildings be­cause of acceptable space demand) as well as the improvement of solar fraction, as described above. Although the desig­nated applications for the proposed heating…

system are typical new and redeveloped Figure 6: Solar Heating System investigated family houses in Central Europe, the heat — by Ingolstadt University of Applied

Sciences and Ratiotherm [27]

ing system is considered to be adaptable to two-family-houses and multifamily residential buildings as well. Furthermore, it promises to be applicable to a wider set of locations. A more favourable climate as for example in Southern Europe with higher solar radiation on the one hand and lower heat demand on the other hand reduces both the required storage capacity and the size of the collector area. The emphasis of this research project is placed on the development of a latent heat storage tank suitable for this application, the dimen­sioning of the components and the system, as well as on the development and optimisa­tion of system control strategies.

1. Results of optimisation

Results of optimisation is shown on fig. 5. Similar results are available for optimisation according to mirrors utilisation factor M. Analyses of results gave the evidence that for acceptance angles less than 20° and number of mirrors < 4 is more suitable optimisation according to C. As for practical realisation of concentrator it is expected to satisfy these limitations, calculation of the CLON will always be performed according to concentration C.

The optimum tube centre distance, collector tilt and orientation with respect the thermal performance per tube is investigated for the two locations. The gross collector area is assumed to be constant in the solar heating plant. Consequently, there are more tubes in the collector area when the tube distance is small than when the tube distance is large. Table 2 shows how the collector orientation, the tilt and the tube distance are varied.

The also patented so-called AL-LE-EPS-insulating system — Aluminium (AL) in combina-

tion with extruded polystyrene (EPS) featuring very low thermal conductivity (LE) — consists of a combination of reflective layers and a generously dimensioned insulating shell using environmentally amicable polystyrene (100 mm LE-EPS and 25 mm air). The insulation thickness of a total of 125 mm is unique in this market segment, additionally it features very low heat conductivity: Figure 3 shows the measurements of the ISE for different in­sulating materials in comparison to the measurements of LE-EPS (as quoted by the raw material supplier).

LE-EPS offers better values than all conventionally used insulating materials; its insulating values are even better than that of PU-foam (hard), while at the same time it is environ­mentally friendlier.

The insulation alone already reduces heat losses to 55 % of the losses of a 100 mm thick soft-foam insulation. Additionally there is a radical reduction of all further losses through heat bridges or microcirculation in the connection pipes: the tank is positioned on special insulating synthetic runners. Four connections leading to the outside are located in the cold bottom area while two heat-trapped boiler connections at the side are equipped with

PP-convective barriers thus reducing heat losses by about half when compared to con­ventional tanks.

Figure 4: Complete System

D. Mills*, G. L. Morrison**

*School of Physics, University of Sydney, Sydney Australia.

E-mail: d. mills@physics. usyd. edu. au

**School of Mechanical and Manufacturing Engineering
University of New South Wales, Sydney Australia 2052
E-mail: g. morrison@unsw. edu. au

In this paper, an evacuated tube collector system for domestic hot water and space heating has been designed using novel optics. The objective was not to gain increased performance as compared to a CPC using a similar reflector material, but to allow the possible use of glass reflector and also to deliver several practical benefits. The optical system is based upon a novel arrangement of reflector optics in which reflectors adjacent to a tube in the array reflect solar radiation only to neighbouring tubes. In this arrangement, the tubes are mounted above the optics for better diffuse radiation collection, and can protrude above snow in winter situations. The reflector arrangement uses shallower curvature than for conventional CPCs, potentially allowing the use of thin glass reflector. Build up of dirt can be avoided by providing a gap in the bottom of the reflector shape which allows rainwater to wash out the reflector system. performance results are reported in the paper along with the authors’ calculated optical performance.

Introduction

The reflector system uses a novel non-imaging reflector which may be regarded as an alternative reflector to the CPC (Compound Parabolic Concentrator) (Winston, 1975)commonly used with evacuated tube domestic solar water heaters in Europe. It is now called SURS (Solahart Unique Reflector System).

When approached by the manufacturer to assist with optical design for a new evacuated tube module, the authors were aware that a well designed CPC cannot be improved upon in optical terms for a given set of materials. However, the CPC has some disadvantages. It cannot use higher reflectance glass reflector materials because curvatures requires are too tight; drainage cannot occur at the lower part of the reflector, so that streaks of dirt can form from dried channelling water; and the evacuated tubes become covered by snow in winter conditions.

A new variation non-imaging reflector design was developed to overcome these disadvantages. Unlike a CPC, it is not strictly a collector which performs maximally (Mills, 1995), but it is very close in practical performance to a CPC when using the same reflector, and because of less extreme curvature, it has the option of using thin glass reflectors for higher performance. At this point, commercial versions have not yet used the glass option, but this is a possibility for the future.

Michele Pellegrino, G. Flaminio, S. Bolognesi and C. Privato

ENEA Centro Ricerche, Localita Granatello. P. O. Box 32,1-80055 Portici (NA), Italy.

tel:+39-81-7723-267, Fax:+39-81-7723-344; mail to Michele. Dellearino@_nortici. enea. it

Though on theory an integrated PV/T should have been proved to be more efficient than the simple addition of a thermal collector and a PV module, in practice this has very rarely happened in spite of the positive testing of components in research laboratories worldwide. Reasons have been very different. Number one the two technologies have never been very well matched each other: if for the PV part the cooling or the ventilation has proved to be of some usefulness in terms of increased efficiency, for the thermal collector the performance is surely worst than the traditional component, since while the low operating temperature is good for the PV part component at the same time gives problems for the thermal collector design and for the thermal output outlet water with very low enthalpy, t< than 50 °C. Number two there is a seasonal mismatch, since it just in the summertime when there is the best insolation that the thermal recovery is less necessary and for the high glassy fagade of buildings there could be not need for thermal contribution even in the winter. Possible solution could be represented by the thin film technology, especially in term of amorphous materials. In fact this material is less sensitive to the increasing of temperature and it is boasted for even a positive coefficient of temperature in some cases, so allowing the PV/T component to work at a little higher temperatures. The other opportunity is the Photovoltaics Building Integration that could bring advantages from an economical point of view, for the waited costs reduction and the less competition for the required area, and from a technical point of view for the transition toward a real integrated PV/T system with optimised materials. In addition each sector can benefit of the other experience, the high tech nature of PV for the thermal collector and the market sectors and the existing standards for the PV. Still it could be possible to work on the demand side by making more efficient and more appropriately designed buildings with heat storage system for the higher insolation periods. The paper intends to give a contribution to the understanding of the obstacles that hinder the developing of this technology. At the same time results on a PV/T module made of a tandem amorphous Si are presented.

Introduction

Usually the performances of PV modules are determined at Standard Conditions, e. g. junction temperature of 25 °C and irradiance of 1000 W/m2 at AM 1.5, corresponding to the irradiance at zenith at the sea level and at the latitude of 45 °. Of course that situation does not happen very often during the normal operation conditions and for instance the temperature is several degrees Celsius higher than the room temperature, since even in the case of very efficient cells the energy converted into electricity is only a small fraction, normally between the 5-15 % of the incoming radiation. The international Standard IEC 1215 for the approval and the qualification of the module type prescribes the determination of the NOCT the Nominal Operating Temperature Condition, that it is nearer to the real operating conditions temperature.

The increase of temperature has an effect on the efficiency since its decreasing with the temperature is about 0.4 % for the crystalline and 0.2 % for the amorphous silicon as it can be seen in figure 1.

To cope with this problem the module ventilation, usually natural or even forced, is provided. Another good solution, even though more complicated, is to draw the heating by means of a fluid inside the device so obtaining the PVT Photovoltaic Thermal module, figure 2. In that way the benefit is double:

a) cooling of the photovoltaic part increasing the peak power,

b) producing a warm fluid for the domestic heat water or space heating application.

But even if this seems reasonable in theory, on practice there have been many obstacles that have hindered the diffusion process. The paper tries to give some possible reasons for the difficulty that technology has experienced and the possible solutions together with some experimental data on a tandem amorphous PVT Si module.

A. Schenke, D. Mangold
Solar — und Warmetechnik Stuttgart (SWT)

— ein Forschungsinstitut der Steinbeis-Stiftung
Pfaffenwaldring 10, D-70550 Stuttgart; Email: schenke@swt-stuttgart. de
Tel: +49-(0)711-685-3896, Fax: +49-(0)711-685-3242

R. Croy, F. A. Peuser
ZFS — Rationelle Energietechnik GmbH
Verbindungsstr. 19 40723 Hilden

J. Scheuren, W. Eisenmann

Institut fur Solarenergieforschung GmbH Hameln/Emmerthal (ISFH)

Am Ohrberg 1 31860 Emmerthal

T. Siems, M. Rommel

Fraunhofer Institut fur Solare Energiesysteme (ISE)

Heidenhofstr. 2 79110 Freiburg

In the past German R+D-programme “Solarthermie-2000” large thermal solar systems for tap water heating have been investigated and basic principles for planning and dimensioning developed. In the following programme “Solarthermie2000plus” the same will be done for large thermal solar systems for both tap water and room heating (so called combi-systems) with at least 100 m2 collector area. Similar investigations have been done for combi-systems in one — family houses. However, large combi-systems differ from these small systems in respect of construction of the solar collector field, the heat stores, the heat exchangers and the connection to the conventional heating technique. The few existing large thermal solar combi-systems in Germany differ very much from each other which shows a great uncertainty of how to design such systems. Therefore it is necessary to develop guidelines for planning and dimensioning of this system technology as well as to further establish this technology on the market. The project is split up in three tasks shared between four partners. In this paper the partners and their tasks are presented.

1. Aims of the Project

In the past, large thermal solar systems were mainly used for tap water heating. In recent years there is a tendency to use this technique for both tap water and room heating. With these systems higher solar fractions of the total annual heat demand of buildings can be achieved. In the range of small thermal solar systems with collector areas of up to 20 m2 combi-systems add up to 20 % of the total thermal solar market. There already is a large range of complete products which can easily be installed and which provide reliable results. However, in small systems the variation of system configurations does not strongly affect the solar fraction and specific solar heat costs [1].

This is different in the range of large thermal solar systems with at least 100 m2 of solar collectors. In the German R+D-programme Solarthermie-2000 large thermal solar systems (collector area > 100 m2) for tap water heating were investigated [2]. Finally the technical standard was implemented in technical rules [3]. For solar systems for tap water and room heating no such rules exist. As these systems are mainly built in public buildings (such as hospitals or schools,…) or in housing areas of public building promoters, their profitability is a very important factor. Variation of system configuration leads to very different results. Thus the large solar combi-systems must be designed at optimal cost in respect of investment and operation. The necessary service should be as low as possible.

01

Existing large thermal solar combi-systems (about 20 in Germany) are very diverse in design, regarding the solar collector field, the heat stores, the heat exchangers and the connection to the conventional heating technique. There are no sufficiently documented operating results of these systems. That is why no reliable propositions can be given for designing new systems. This research project aims at the implementation of recommendations and technical rules for the design of large thermal solar combi-systems. Therefore six existing solar combi-systems will be investigated over a period of two years. With the help of simulations in TRNSYS [4] based on the measured data the advantages and disadvantages of the different systems will be identified. Economically advantageous improvements will be carried out. Special studies on the stagnation effects of single collectors and collector fields in the existing systems and on test stands will be done to give recommendations on how to reduce stagnation in solar collectors. The joint research project began in October 2003 and will be running for 2 ЛА years.

The energy payback time can be determined by comparing the primary energy embodied in the system (PEAin) with the amount of primary energy that will be saved by the thermal solar system during its estimated lifetime (PEAsub) according to equation 1.

PEA, (t) = PEASub (t) (1)

As can be seen from equation 2, the primary energy embodied in the system (PEAin) comprises the cumulative energy demand for the production (KEAp), for the operation (KEAo) and for the maintenance (KEAm).

PEA, (t) = KEAp + KEAo • t + KEAm • t (2)

The cumulative energy demand for production (KEAp) also includes transport, assembly and installation of the thermal solar system. It has to be taken into consideration that the cumulative energy demand for the operation and the maintenance are dependent on the lifetime of the thermal solar system.

The amount of primary energy saved by the thermal solar system (PEAsub) is determined by the difference of Qconv, tot and the auxiliary energy (Qaux, tot). Qconv, tot represents the total primary energy requirement of a conventional system that is necessary to meet the hot water and in case of a comibsystem also the space heating demand.

PEAsub (t) _ (Qconv, tot Qaux, tot) • t

The energy payback time AZ is calculated according to equation 4.

Two small low flow solar domestic hot water systems with mantle tanks as heat storage were tested side-by-side in a laboratory test facility. The systems are identical, with exception of the mantle tanks. One of the mantle tanks has the mantle inlet port located at the top of the mantle and the other mantle tank has the mantle inlet port moved 0.175 m down from the top of the mantle. Both of the two mantle tanks make use of electric heating elements as auxiliary energy supply systems, and the electric heating elements heat up the top volume to 51°C during all hours.

The solar collector in each system is identical and of the type ST-NA marketed by Arcon Solvarme A/S with an area of 2.51 m2.

The solar collector loop in both systems is equipped with a Grundfos circulation pump (type UPS 25-40), which has been running at stage 2 to secure a flow rate of about 0.5 l/min throughout the measuring period. The circulation pump is controlled by a differential thermostat, which measures the temperature difference between the outlet from the solar collector and the bottom of the mantle. The differential thermostat has a start/stop set point at 10/2 K.

The two solar heating systems were tested with the same daily hot water consumption of 0.100 m3. An energy quantity of 1.525 kWh, corresponding to 0.033 m3 of hot water heated from 10°C to 50°C, was tapped from each system three times each day: at 7 am, 12 am and 7 pm.

The test period was from the beginning of March to the middle of November 2003 with a duration of 252 days.

The data for the two SDHW systems are given in Table 1.

The thermal performance of the two systems is compared by the net utilised solar energy and the solar fraction of the systems. The net utilised solar energy is defined as the tapped energy from the system minus the auxiliary energy supply to the tank, and the solar fraction is the ratio between the net utilised solar energy and the tapped energy from the system.

The measured energy quantities for the two systems are shown in Table 2. From Table 2, it is seen that the thermal performance of the system is not strongly influenced by the position of the mantle inlet. Both systems had a relatively high solar fraction (0.66-0.68) in the period. The thermal performance for the system with the lower mantle inlet was about 2% higher than the thermal performance of the system with the top inlet. The accuracy of the measured net utilised solar energy is within 4%.

10/11 2003.

At high solar fractions, large periods with high inlet temperatures to the mantle are expected and when the system with the lower mantle inlet has a higher thermal performance at high solar fractions, the relative improvement by moving the inlet down is expected to be higher for smaller solar fractions where lower inlet temperatures are expected.

The 252 days’ measuring period have been divided into 36 periods of 7 days. The performance ratio as a function of the solar fraction for the system with the top inlet for the 36 periods is shown in Fig. 2. The performance ratio is defined as the ratio between the net utilised solar energy of the system with the lower mantle inlet and the net utilised solar energy of the system with the top mantle inlet.

Fig. 2 shows, as expected, that the performance ratio increases for lower solar fractions. However, the performance ratio drops below 1 for two 7-day periods at solar fractions of 0.65-0.70, which can be explained by the distribution of the solar irradiance in these two 7- day periods. Each of the two 7-day periods has 4 days with a clear sky and 3 days more or less overcast, while the other 7-day periods, where the solar fraction is around 0.6-0.7 and the performance ratio is above unity, have clouds every day, which results in lower inlet temperatures to the mantle than on the days with a clear sky. Based on the tendency that the performance ratio increases for lower solar fractions and that the solar fraction was relatively high in most of the measuring periods, it can be concluded that these measurements show that the thermal performance of this SDHW system can be somewhat increased by moving the mantle inlet down.

Fig. 2. Performance ratio as a function of the solar fraction for the system with the top inlet.