A. Madhlopa

Department of Physics & Biochemical Sciences, Malawi Polytechnic,

P/Bag 303, Blantyre 3, Malawi
E-mail: amadhlopa@poly. ac. mw

Abstract

Integrated collector-storage solar water (ICSSW) heaters are generally more cost effective than systems with separate collector and storage units. However, ICS solar water heaters lose a substantial proportion of the captured heat during periods of low insolation or non­collection. In this study, an ICS solar water heater with two horizontal cylindrical tanks (made of galvanized steel, with a capacity of 61.8 litres each) was designed, constructed and tested. The two tanks were parallel to each other, and horizontally and vertically spaced out, with the lower tank fitted directly below a glass cover, and half of the upper tank insulated. In addition, a truncated stationary parabolic concentrator was fitted below the tanks, with its focal line along the axis of the upper tank. The system was installed outdoor (facing north) on top of a horizontal flat concrete roof at the Malawi Polytechnic (15° 48′ S, 35° 02′ E) in Malawi. It was tested with the two tanks aligned east-west, and in parallel (P) and series (S) connections.

For the series-tank interconnection, the two tanks were connected with: a) one insulated hose pipe (12.7 mm diameter) from the top part of the lower tank to the bottom part of the upper tank (S1-tank interconnection) and b) two insulated hose pipes of which one pipe linked the bottom part of the lower tank to the bottom part of the upper tank while the other pipe linked the top part of the lower tank to the top part of the upper tank (S2-tank interconnection). The solar collection process was monitored from 06:00 to 17:00 hrs local time, and hot water was stored from 17:00 to 06:00 hrs the next day, without any draw-off for a sequence of 4 days. Meteorological measurements were taken during the day (06:00 to 17:00 hrs). Results show that the S2-tank interconnection yielded the most satisfactory results. In this connection configuration, the system stored 28.7 to 39.7 % of the collected thermal energy for use the next morning, comparable with results obtained from previous studies conducted elsewhere. Other results are presented and discussed.

1. Introduction

Integrated collector storage solar water (ICSSW) heaters combine solar collector and water storage tank in one unit and are cost effective (Garg et al., 1997). Nevertheless, ICSSW systems have higher top heat loss during the night when ambient temperature is relatively low. So, previous ICSSW heater designs focused on reducing top heat loss by including heat retention mechanisms either at the aperture, within the collector cavity or on the ICSSW heater vessel surface.

An insulated opaque lid that was removed in the morning and replaced at night was very effective but required manual removal and replacement every day (Garg, 1975; Baer, 1975). Selective coating materials (Stickney and Nagy, 1980), transparent insulating materials (Schmidt et al., 1988) and multi-glazed units (Bishop, 1983) have been used with varying
extents of success, but unfortunately they lead to an increase in the cost of the unit and in some cases to a reduction in the capture of solar radiation. Kalogirou (1999) introduced a primary cylinder between the main cylindrical tank and the glass cover, with cold water introduced directly into the primary tank which fed the main tank. It was concluded that this modification greatly improved the system draw-off characteristics. Tripanagnostopoulos et al. (1999) designed ICSSW heaters with two cylindrical storage tanks connected in series and incorporated in a stationary asymmetric compound parabolic concentrator. They found that the systems operated efficiently and were suitable for practical applications. Later, Tripanagnostopoulos et al. (2002) developed four ICS solar water heaters with stationary compound parabolic concentrating (CPC) reflectors. The systems consisted of single and double cylindrical tanks placed in symmetric and truncated CPC troughs. These authors used two cylindrical tanks, connected in series from the top part of the lower tank to the bottom part of the upper tank, to increase temperature stratification. They found that asymmetric CPC reflectors contributed to lower thermal losses and that the two connected in series cylindrical tanks resulted in effective water temperature stratification. Moreover, the water temperature in the top part of the lower tank was higher than the water temperature in the bottom part of the upper tank (as shown in Figs 17 and 18 of this reference). For their double-tank system models, natural convection of heat from the lower tank to the upper tank would increase the efficiency of thermal storage because a larger proportion of the top part of the upper tanks was thermally insulated. In addition, hotter water from the top part of the lower tank would mix with colder water from the bottom part of the upper tank during periods of charging and discharging, resulting in loss of stratification.

The objective of this study was to assess the performance of a simple ICSSW heater with two cylindrical horizontal tanks incorporated in a stationary parabolic concentrating reflector. Half of the top part of the upper tank was thermally insulated while the lower tank was bare. The system was tested with tanks connected in parallel (P-connection) and series configurations, without draw-off. For the series-tank interconnection, the two tanks were connected with: a) one insulated hose pipe (12.7 mm diameter) from the top part of the lower tank to the bottom part of the upper tank (S1-tank interconnection) and b) two insulated hose pipes (12.7 mm diameter) of which one pipe linked the bottom part of the lower tank to the bottom part of the upper tank while the other pipe linked the top part of the lower tank to the top part of the upper tank (S2-tank interconnection). The S2-tank interconnection yielded the best performance results.

The investigation of the thermal performance of collectors with coloured absorbers has been performed by AEE INTEC in Gleisdorf, Austria using the dynamic collector test method according to EN 12975-2. Three collectors connected in series have been tested simultaneously. Hence, an equal mass flow through all collectors has been maintained. To assure the same input conditions for all collectors, the heat transfer fluid (water) has been cooled after each of the collectors to the same temperature. The coloured collectors were measured in comparison to a collector with black solar varnish coating on a aluminium absorber and a collector with black selective coating on a copper absorber respectively. Two different absorber materials have been used, because the involved project partners use these absorbers respectively. The test series with black solar varnish and the colours blue and grey has been performed using aluminium absorbers. For the test series with black selective coating and the colours green and auburn copper absorbers have been used. The influence of different glass covers (structured and antireflective coated glass) has been investigated for two of the collectors. Figure 1 shows the test site with three at aEe INTEC.

The tests have not been performed at a tilt angle of 90°, although this would be the standard orientation of a fagade collector, because the incidence angle of the direct radiation (testing time: summer) would have been too small for an accurate determination of the incident angle modifier. This would also have resulted in an inaccurate calculation of the conversion factor.

For all test series, a broad range of different conditions for solar irradiation, temperature differences between collector and ambient, amount of diffuse solar irradiation and wind conditions has been monitored at the outdoor test facility. Diagrams of the different parameters, showing the distribution of the measured values have been used to decide, whether enough data had been collected or measurements had to be continued. The data were analysed with a program developed by AEE INTEC. The parameters of the dynamic model were derived from the measured data using multi-linear regression. Making assumptions according to EN12975-2 the static efficiency curve of the collectors was calculated from the dynamic model.

Collector efficiency aluminium absorbers

Hemispherical irradiance in collector plane (45°): 800 W/m2

The result for the blue coating on an aluminium absorber shows that the thermal performance is comparable to the one of the black solar varnish coated absorber. At low temperatures the efficiency of the blue coating is slightly lower, but at higher temperatures the selectivity of the blue coating has an advantage and shows higher efficiency than the black one.

The grey absorber shows a 16% lower conversion factor than the black absorber due to the lower absorption. The slope of the efficiency curve is comparable to the one of the blue solar varnish coated absorber.

Efficiency curves copper absorbers

Hemispherical irradiance in collector plane (45°): 800 W/m2

coll. 4-selective- antireflective coated glass

coll. 4-selective-structured glass coll. 5-green-antireflective coated glass

coll. 6-red-antireflective coated glass

coll. 6-red-structured glass

The conversion factor of the green coating on a copper absorber is about 6 % lower than the one of the selective coated absorber. At a temperature difference of 45 K between collector mean temperature and the ambient, the efficiency of the green collector is about 14 % lower than the one of the selective absorber.

The conversion factor of the red coated absorber with the structured glass cover is about 9 % lower than the one of the selective coated absorber. At a temperature difference of 45 K the efficiency is about 14 % lower than the one of the selective absorber. The red absorber shows lower emission than the green one.

The measurements showed that the influence of the anti-reflective coating of the glass cover is lower than expected but statistical significant. Two independent test series have shown almost no difference in the efficiency with different glass covers.

Although tanks were included in the testing program in 1999, testing effectively started only in early 2000. The program tests tanks with nominal capacities of 100, 200, 300, 400, 500, 600, 800 and 1000 liters. Table 3 presents the tests that are performed. At the beginning only the three first items listed were tested. In 2003, however, the last two items, related to the safe operation of the electrical resistance, were added to the program. The safety tests were introduced due to the fact that almost all tanks sold in the residential market have electrical auxiliary heating.

One important difference between the label for collectors and the one for tanks is that, in the case of tanks, there is no performance ranking of the equipment tested. The tanks are simply classified as approved or not approved according to the standards specifications. Figure 3 shows a sample tank label. It has only basic information as model, serial number, manufacturing date, volume, maximum operating pressure and electrical specifications. At the bottom, the label states that the product has been approved by INMETRO, and it has the logos of the Brazilian electricity savings program (PROCEL) and INMETRO.

In the case of the volumetric capacity, the tank is approved if the measured capacity is neither 10% higher nor 5% lower than its nominal capacity.

For static pressure, a testing pressure 1,5 higher than the declared maximum working pressure is applied for 15 minutes. The tank is then inspected for leaks and deformations.

In the case of the thermal efficiency the standard used, ISO 9459 — Part 2: item 9.9, was adapted to a situation where the tank is tested separately, and there is no forced convection around the tank. After the cooling test is performed, the heat loss coefficient is calculated. With this value it is then possible to estimate the monthly losses for the tank. The heat losses calculations are executed assuming the hot water temperature in the tank at 50 oC and the ambient temperature at 21 oC. The maximum acceptable losses, which were revised in August 2003, are listed at Table 4.

RESULTS

The labeling program was initiated by the main SWH manufacturers in Brazil. After the release of the first results, these companies started including information about the label in their advertisements and brochures. That was compounded by a small campaign in national television by the government agency INMETRO, urging consumers to buy only tested and labeled products. These actions brought, in 2000, the first increase in the number of companies participating in the market, from the initial 8 companies in 1998 to 12 companies in 2000. The year 2001, with the electricity crisis in Brazil and a booming market for SWH, marked the beginning of an even greater expansion in the number of participating companies.

By the end of 2001, 15 companies were participating. The crisis prompted the Brazilian government to offer financing for SWH through one of its banks, but only for equipment that had already been labeled. Moreover, the publicity gained by the sector through many media reports during the crisis was another opportunity to reinforce the importance of the label, and in 2002 the number of companies participating reached 25. In March 2004 there

were 35 companies participating in the program. Figure 4 shows the evolution of the number of participating companies.

The growth in the number of participating companies brought the challenge of testing a significant number of collectors. Up to the end of 2003, 96 collectors had been fully tested. As a way to give the companies a label before the full tests are completed, a mechanism called a pre-label was introduced. In order to obtain a pre-label, the collector is first tested for thermal performance (Group 2, Table 1). If the company already has a model using the same materials in terms of the glazing and fins surface finishing, the label is first released based on the information collected by the thermal performance tests and the behavior of the similar model previously fully tested. When the tests are finalized, the label is corrected, if necessary. Figure 5 shows the number of collectors fully tested for each year and the cumulative number until the end of 2003.

On top of the 93 models already fully tested, another 9 have received the pre-label and are waiting to complete the tests. Another 51 collectors are still waiting to be tested. As

these numbers indicate, the smaller number of collectors tested in 2003 was not caused by lack of demand, but by poor weather conditions.

1998 1999 2000 2001 2002 2003

Figure 6 shows the percentage of collectors in each category for each year of the program, to the end of 2003. Although there is not a strong trend, it is interesting to note that the number of collectors classified as A first increased from 0 to 33 % and then gradually decreased until 2003, before increasing again. This is believed to be due to the fact that once the collectors are tested, the manufacturers improve their collector models. The appearance of new participants in the market brought a higher number of collectors with lower classifications, such as D and even G.

In the case of the tanks, there was a similar growth in the number of models tested in 2001 and 2002, with a decrease in the number of models tested in 2003, as can be seen at Figure 8. The reason for this decrease, however, is different in relation to tanks than for collectors. The decrease in the number of tanks tested was caused by the introduction of the new standards related to electrical safety. Since the manufacturers that had already tested their models had until January of 2004 to get new tests performed, most preferred to wait, while adjusting and developing their products to meet the new standards.

At the beginning, most of the tanks that failed had problems with the volumetric capacity test. The percentage of models approved rose from 42% in 1999 to 78% in 2002. In 2003, with the introduction of new tests, the percentage of approved models fell back to 50%. It is expected that this number will increase again in 2004, with the testing of models already adapted to the new standards.

CONCLUSIONS

It is possible to conclude that the labeling program, although not a complete quality assurance program, has had a significant impact on the Brazilian solar water heating market. Most of the collectors commercialized in the country have been labeled and the main manufacturers have all developed better products as a result of the testing process.

The labeling has also enabled the participation of solar water heaters in government tender processes and created a minimum standard to be used in government programs.

At the same time, consumers have been given simple and reliable information to help with purchasing decisions.

At the present time, a sizable backlog in collector testing remains. In response, the Brazilian government has sponsored the installation of a solar simulator at GREEN SOLAR. The simulator is expected to be operational in the second half of 2004.

As is the case with many labeling programs, the program has been gradually improving the quality and performance of the equipment evaluated. This so-called "ratcheting” dynamic is already evident in relation to both collectors and tanks. The program would be significantly improved with some kind of testing and certification of SWH installers, since in Brazil the installation process has a particularly strong effect on the final efficiency of the system.

The authors would like to acknowledge the support of the Companhia Energetica de Minas Gerais (CEMIG) and Centrais Eletricas Brasileiras (ELETROBRAS).

REFERENCES

[1] ASTM E823-81(2001) Standard Practice for Non-operational Exposure and Inspection of a Solar Collector, American Society for Testing and Materials, 2001.

[2] FSEC-GP-5-80, Test Methods and Minimum Standards for Certifying Solar Collectors, Florida Solar Energy Center, USA, 1985.

[3] ASHRAE 93-86 Methods of Testing to Determine the Thermal Performance of Solar Collectors, American Society of Heating, Refrigeration, and Air-Conditioning Engineers, Inc., Atlanta,1986.

[4] ASHRAE 96-1980 RA 1989, Methods of Testing to Determine the Thermal Performance of Unglazed Flat-Plate Liquid-Type Solar Collectors, American Scoiety of Heating, Refrigeration, and Air-Conditioning Engineers, Inc., Atlanta,1989.

[5] INMETRO/GT-SOL, Regulamento Especifico para o Uso da Etiqueta Nacional de Conservagao de Energia — ENCE — Sistemas e Equipamentos para Aquecimento Solar de Agua, Brasilia, 2003.

[6] ISO 9459 Part 2, Solar Heating — Domestic Water Heating Systems; Performance Testing for Solar Only Systems, CEN, 1994.

[7] NBR NM IEC 335 -1, Seguranga dos aparelhos eletrodomesticos e similares: Parte 1 — Requisitos Gerais, Associagao Brasileira de Normas Tecnicas, 1998.

[8] NBR 14016 — Aquecedores instantaneos de agua e torneiras eletricas — Determinagao da corrente de fuga — Metodo de ensaio, Associagao Brasileira de Normas Tecnicas,1997.

[9] NBR 14013 — Aquecedores instantaneos de agua e torneiras eletricas — Determinagao da potencia eletrica — Metodo de ensaio, Associagao Brasileira de Normas Tecnicas,1997.

Table 2 gives a summary of the investment cost for the heat supply system. For the ATES a specific cost of 39 Euro/m3 water equivalent can be calculated (without VAT, including design). This is very favourable compared to other types of seasonal heat stores that have been built in Germany /3/.

Based on the given cost, with operational cost and maintenance included, and the solar heat delivery from the year 2003 the solar heat cost turns out to 26 Ct./kWh (calculation according to VDI-Guideline 2067, interest rate 6 %, life time for solar collectors 20 years, life time for the ATES 40 years).

Conclusions

The demonstration plant in Rostock is the first central solar heating plant with a seasonal aquifer thermal energy store in Germany. The results of the first four years of operation prove the technical feasibility and reliability of all components of the heat supply system. With a solar fraction of 49 % in the year 2003 the design target was reached after some improvements in the hydraulic adjustments and the control system.

Acknowledgements

This work was financially supported by the Federal Ministry of Economics and Labour and the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, Project No. 0329606S and 0329607F. The authors gratefully acknowledge this support. The authors themselves carry the responsibility for the content of this paper.

References

/1/ M. Benner, M. Bodmann, D. Mangold, J. Nuftbicker, S. Raab, T. Schmidt, H. Seiwald: Solar unterstutzte Nahwarmeversorgung mit und ohne Langzeit-Warmespeicher (Nov. 1998 bis Jan. 2003); Report to the Research Project 0329606S (solar assisted district heating with and without seasonal heat storage, in German), ISBN 3-9805274-2-5, University of Stuttgart, 2004

/2/ J. Bartels, F. Kabus, T. Schmidt: Seasonal Aquifer Solar Heat Storage at Rostock — Brinckmanshohe — First Operational Experience and Aquifer Simulation; Futurestock 2003 9th International Conference on Thermal Energy Storage, Warsaw, Poland /3/ T. Schmidt, D. Mangold, H. Muller-Steinhagen: Seasonal Thermal Energy Storage in Germany, ISES Solar World Congress 2003, Gothenburg, Sweden

The first set parameters analyzed in Figure 1 relate to collector area, slope, azimuth, mass flow and on the store volume. The highest influence on fsax, ext can be seen for the collector size. This dependency is even higher, when the ratio of collector and store is kept constant.

A pure alteration of the store volume is not that significant and in the same range as collector azimuth and collector tilt angel.

Nearly no influence can be seen for the collector mass flow.

The standard variation of these values is below 0.2 which indicates a similarity of the results for all systems.

Figure 2 shows the dependencies fsav, therm on the store volume with the collector area as parameter. The decrease of fsav, therm for small collector areas with increasing store volume is due to higher increase of heat losses than increase of solar heat input to the storage.

The optimal ratio of store volume and collector size should not become bigger than 100 l per m2 of collector area. Most plants have optimal values between 40 and 80 l per m2 of collector area over the whole range of fsav, ext.

Figure 2 Dependency of fsav, therm with store volume and collector area, example of system #15 (Jaehnig, 2003)

In Figure 3 the dependency of fsav, ext on slope and azimuth is shown exemplary for system #19 (Heimrath, 2003). The optimum savings occur at an azimuth of around 10° west and 55° tilt angle for a maximum fsav, ext of 39.4%.

The azimuth can vary between -30° to 30° (for the optimum tilt) by decreasing the fractional energy savings by 5 %. With optimal azimuth the slope can vary by ± 20° with the same reduction of fractional energy savings. Taking this into account a stand of the collectors on the roof can be mostly avoided. The optimal slope depends primarily on the latitude of the location and secondary on the achieved fractional energy savings. As higher the savings, as more slope of the collector area is feasible in order to have a better irradiation angle to the collector area in the heating season.

Figure 3 Dependency of fsav, ext on collector tilt and azimuth angle. 100% = 39.4% (fsav, ext), system #19 (Heimrath, 2003).

Calculations of the yearly net utilized solar energy of a solar combi system are carried out by means of TRNSYS, [13], [14]. The solar combi system taken into calculation is schematically shown in Fig. 7.

The solar heating system, which is a marketed system by SOLVIS Solar Systeme GmbH, was the best system investigated in [5]. The system has a compact heat storage unit with the following components integrated: A water tank with an auxiliary condensing natural gas burner, a domestic hot water flat plate heat
exchanger with a pump, a solar collector loop and a solar heat exchanger. Thermal stratification is built up in the heat storage in a good way, since SOLVIS inlet stratifiers ensure that the solar heat is transferred to the "right” level in the tank and that the water returning from the heating system enters the tank in the right level.

The volume of the tank is 650 l and the auxiliary volume heated by the natural gas burner is 136 l. The natural gas burner heats up the auxiliary volume to 57°C. The solar collectors described in the previous section are also used in the calculations on the thermal performance of the solar combi system. The solar collector area is 12.55 m2.

The water to the heating system is tapped from a level just above the lower level of the auxiliary volume.

The space heating demand of the house taken into calculation is 14970 kWh/year. The heating system is a traditional radiator system which, at an indoor temperature of 20°C and an outdoor temperature of -12°C, can supply the required heating power of the house with an inlet water temperature of 60°C and an outlet water temperature of 50°C. A daily hot water consumption of 150 l heated from 10°C to 51°C is assumed. Weather data of the Danish Test Reference Year is used in the calculations. The draw-off level for the domestic hot water heat exchanger is placed at a relative position of 0.05 from the very top of the tank and the draw-off level for the space heating system is placed at a relative position of 0.23 from the very top of the tank.

Calculations of the yearly net utilized solar energy of the system are carried out for the standard system, for the system with two draw-off levels for the domestic hot water heat exchanger with different positions of the second draw-off level, for the system with two draw-off levels for the space heating system with different positions of the second draw-off level and for a system with two draw-off levels, both for the domestic hot water heat exchanger and for the space heating system.

Fig. 8 shows the calculated yearly net utilized solar energy of the system with two draw-off levels to the domestic hot water heat exchanger and the standard draw-off level to the heating system, as well as the yearly net utilized solar energy of the system with two draw­off levels to the heating system and the standard draw-off level to the domestic hot water heat exchanger as functions of the position of the second draw-off level.

Fig. 9 shows the extra percentage net utilized solar energy for the solar combi system by utilizing two draw-off levels to the domestic hot water heat exchanger as well as the extra percentage net utilized solar energy for the solar combi system by utilizing two draw-off levels to the heating system as a function of the position of the second draw-off level. It is possible to increase the yearly thermal performance of the system by about 3%, either by using two draw-off pipes for the domestic hot water heat exchanger instead of one or by using two draw-off pipes for the heating system instead of one. The best position of the second draw-off level for the domestic hot water heat exchanger is in the middle of the tank, and the best position of the second draw-off level for the heating system is just above the middle of the tank.

Further calculations showed, that by using a second draw-off level, both to the domestic hot water heat exchanger and to the heating system, the yearly net utilized solar energy of the solar combi system is increased by about 5% compared to the standard system. Also for this design, the second draw-off pipe for the domestic hot water heat exchanger is best placed in the middle of the tank, while the second draw-off pipe for the heating system is best placed just above the middle of the tank.

3 CONCLUSIONS

The investigations showed that it is possible to increase the thermal performance of both SDHW systems and solar combi systems by using two draw-off levels from the solar tanks instead of one draw-off level at a fixed position.

The best position of the second draw-off level is for all the investigated systems in the middle or just above the middle of the tank. For SDHW systems the extra thermal performance of using a second draw-off level from the hot water tank is strongly influenced by the difference between the set point temperature of the auxiliary energy supply system and the required draw-off temperature. For increasing temperature difference the thermal
advantage of the second draw-off level increases. For a realistic draw off hot water temperature of 40°C and 45°C and an auxiliary volume temperature of 50.5°C the increase of the thermal performance by the second draw-off level is about 6%.

For the investigated solar combi system the extra thermal performance by using one extra draw-off level, either for the domestic hot water heat exchanger or for the heating system, is about 3%, while an improvement of about 5% is possible by using a second draw-off level both for the domestic hot water heat exchanger and for the heating system.

[1]

Methylsilicate 51 (MS51, Colcoat Co. Ltd., 51% as Silicon oxide) or tetramethoxysilane (TMOS) were used as starting material for silica aerogel.

MS51 is trimmer of TMOS. In the case of using MS51 as starting material, 0.05 mol of MS51 and 1.7 mol of methanol were mixed well in flask. 6.5 g of ammonia solution (Kanto

Chemicals Co. Ltd., 0.1 mol / l), diluted with 6.5 g of distilled water, was poured into the MS51 solution and stirred for 30 s to prepare silica sol.

In the case of using TMOS, 0.23 mol of TMOS and 0.93 mol of methanol were mixed well in flask. 12.6 g of ammonia solution, diluted with 12.6 g of distilled water, was poured into the TMOS solution and stirred for 30 s to prepare silica sol3-4).

Then, the sols were poured into a polyethylene mold with an equilaterally hexagonal hollow of 40 mm in radius and 20 mm in depth. The gelation of the sols was completed after 8 min of standing.

The mold was sealed and the alcogel was aged at room temperature for 24 h. After the aging, the gel was taken out from the mold. The gel was soaked in 0.4 l of alcohol for 24 h to replace liquid inside of the gel. IPA was used for replacing MS51 gel.

Ethanol was used for replacing TMOS gel. The alcohol was exchanged every day. The procedure was repeated for three times.

The alcogel was put in an autoclave filled with alcohol. Residual space in the vessel was replaced with nitrogen at 5 l / min for 2 min. The autoclave was heated from room temperature

to 200 °C for 4h, and from 200 °C to

300 °C for 4h as shown in Fig. 2.

supercritical drying.

After the temperature and the pressure reached the critical point, the pressure was released at the rate of 0.5 kg f/ (cm2 • min) and the vessel was cooled to room temperature for 6 h5-6).

A number of different insulation materials have a hygrosopic behaviour. Their physical, temperature dependent adsorption properties can be investigated by thermo-gravimetry. A less sophistic, but more application-oriented method was used for the described investigations. A model-collector (45x45x10 cm) equipped with an absorber was exposed with an inclination of 45° to a fast change of the ambient humidity from 30% to 80% in a climatic cabinet at 25°C. The absorber was kept at 40°C by a temperature-controlled heat-transfer fluid. The insulation material (thickness 5cm) behind the absorber could be varied. Two diagonally placed openings in the back-plane of 7mm diameter, which were not covered by insulation materials, allowed ventilation. The humidity inside was measured in the air-gap (5cm width) between absorber and glazing with a capacitive humidity sensor.

The response function shows a big variation for the different materials (figure 6). Different types of mineral-wool (or rock-wool) behave differently and even poly-urethane foam and melamine foam buffer humidity, but with a clearly longer time-constant (figure 7), which was defined as the period when 63% of the asymptotic maximum was reached. This kind of definition is not completely suitable, since the shape of the response curve of some materials indicates differences of the mechanisms or the presence of various processes with different time constants. High temperature loads cause a degradation of the rock-wool. Polymeric grease, which is used during the manufacturing processes is decomposed. One result is a change in the water absorption behaviour (figure 8). The increase of the time-constant means an increase of the water adsorption ability, that was proven by thermo-gravimetry.

SHAPE * MERGEFORMAT

Facade solar thermal collector represents a new element in building design and also in old buildings retrofit. Facade solar system performance and its interaction with the building were investigated through the computer simulation. System and building were processed together, collector absorber was thermally coupled to building envelope.

The simulation has shown that facade solar collector should have area increased by approx. 30 % to achieve the same solar fraction (usually 60 %) as conventional roof solar collector with 45° slope. Further increase in solar fraction above 70 % leads to required area comparable with roof collectors, but with less stagnation periods and lower amount of energy which cannot be utilized than with roof collectors.

Building behaviour is not strongly affected by facade collector when sufficient insulation layer is present. Facade collectors in investigated configuration (panel, brick wall) slightly improve the thermal protection of building in winter season, but for higher thermal insulation levels the heat gains are negligible. Application of facade solar collector affects the indoor comfort in building in reasonable range. Inside temperatures increase not higher than 1 K in all configurations (wall type, facade collector area), integrated comfort parameter PPD has even better values for higher facade collector area applied. This results form the fact that facade collector operation partially helps in cooling the facade.

Heat from absorber is efficiently removed during the day extremes and collector stagnation is at low level for facade collector. The gains through window effect the inside temperature variation much more than facade collector.

Absorber temperature affects particularly the first layer in the envelope construction, next layers are at moderate temperatures. Temperature in the wall varies according to indoor conditions and practically is not affected by facade collector. The only risk potential is concentrated in the insulation layer adjacent to collector.

Further research in the area of solar collectors integrated directly into facade should be orientated to building processes — topics as water vapour transport, thermal bridges, absorber mounting etc should be satisfactorily solved to spread the technology.

Interesting area of facade collector application is in the solar systems for combined DHW and space heating (combi-systems). In these systems, area of solar collector field is higher and summer gains can cause problems, if no summer “heat consumer” is available (swimming pool, dryer, etc). Facade integrated collector could represent a very efficient solution.

References

[1] Matuska, T.: Transparent thermal insulation and their use in solar applications. PhD. thesis, Czech Technical University. Prague 2003.

[2] Matuska, T., Sourek, B.: Fagade solar collectors. Conference on Dynamic Analysis and Modelling Techniques for Energy in Buildings (DAME-BC). Ispra, 2003.

[3] McAdams, W. H.: Heat Transmission, 3rd edition. McGraw-Hill, New York. pp. 249. 1954.

[4] Sparrow, E. M., Tien, K. K.: Forced convection at an inclined and yawed square plate — application to solar collectors. ASME Journal of Heat Transfer, Vol. 99, 1977, pp. 507­512.

[5] Sparrow, E. M., Lau, S. C.: Effect of adiabatic co-planar extension surfaces on wind — related solar collector heat transfer coefficients. ASME Journal of Heat Transfer, Vol. 103, 1981, pp. 268-271.

[6] TRNSYS v.15 Manual, Solar Energy Laboratory, University of Wisconsin, 2000.

[7] Rockendorf, G., Janssen, S.: Facade integrated solar collectors. Solar World Congress, Jerusalem 1999.

The fans used for purposes of this research are axial-flow fans with permanent-magnet brushless DC (PMBLDC) motors. Single-phase PMBLDC motors are commonly employed for driving axial fans [4]. In spite of the different limitations of DC motors [4], they are used extensively in PV pumping systems because they can be coupled directly to the PV module giving a simple and inexpensive system. Many researchers have investigated the design and performance of directly coupled PV pumping systems. Most of this research has focused on the matching between DC motors and PV modules for maximising efficiency. Applebaum and Bany [3] studied the performance of separately excited DC motors powered by PV cells. Singer and Applebaum [5] studied the starting characteristics of PV — powered permanent magnet DC motors while Roger [6] examined the direct coupling between DC motors and PV arrays for both pumps and fans. Anis and Metwally [7], and Swamy et al. [8], analysed the dynamic performance of a DC coupled PV pumping system. Koner [9] analysed the PV powered DC series and brushless motors for driving centrifugal pumps by varying the motor constants.

Brushless DC motors, although relatively expensive, are highly efficient when compared to conventional brush motors. Moreover, they require no maintenance and produce less electromagnetic radiation [10]. Permanent magnet motors are generally considered the best motors for direct coupling to PV modules [6] due to their high efficiency and low cost.

A simple circuit of a DC motor is a voltage source (V) in series with the motor’s armature resistance (Ra) and the back emf (E) of the motor generated by the rotating armature. For a PMDC motor the magnetic flux is always constant, resulting in linear speed — voltage and speed-torque curves. The operation of a PMDC motor is governed by the following equations:

where ю is the speed of the fan (r/min), I is the current through the armature, Tm is the motor torque (N. m) and Km is the motor constant (Vs-1). The armature resistance (Q) can be determined from stall conditions (i. e. ю = 0) by taking V-I measurements and by making use of Eqs. 2 and 3. The motor speed constant in Eq. 2 (which has in essence the same

value as the torque constant in Eq. 3) is determined from the no-load conditions using the following equation, which assumes negligible friction [11]:

where ram is the maximum speed attainable by the fan at voltage Vm.

The torque of the fan, Tf, is, in general, a function of its speed according to the relationship,

Tf = Kf ■ a>2 (6)

Where Kf is a constant determined from reference values of speed and torque. At steady state conditions, the fan torque is equal to the motor torque so that

Kf — Ф1 = Km I (7)

Measurements of speed (ю) and current (I) can be used in Eq. 7 to determine Kf.

Figure 1 represents typical H-Q characteristics for axial-flow fans. Axial-flow fans are most appropriate for high flow rate and medium head applications [12]. It is usually desirable to operate in the lower section of the H-Q curve. The flow rate of air from the fan and the total head developed across it are related to the speed of the fan by the affinity laws, which are applicable for both fans and pumps [13]. The H-Q curve of the fan changes as a function of speed. Thus, the head developed across the fan is a function of flow rate and speed (or voltage) of the motor.

In order to simplify the H-Q relationship of an axial flow fan, the curve can be segmented into three straight lines as shown in Fig. 1. The slope and intercept of each of these line segments, as well as the limiting flow rates Q1 and Q2, are functions of speed. The head can be expressed as a function of speed and flow rate by the following equation:

H = qj ■ ®2 + C2>j. a-Q + Сз, j ■ a + C4>j ■ Q (8)

where the C’s are constants and the "j” subscript corresponds to one of the three segments. Thus, for the lower section of the H-Q curve, j = 3 and the constants Ci|3 (for i = 1 to 4) are used only if the flow rate Q is larger than Q2 which is also a function of ю. These constants are fan-specific and can be determined using the affinity laws by generating several curves at different motor speeds from a single manufacturer’s curve.

1.2 Photovoltaic-motor-fan coupling

When the fan is driven by the PV-powered DC motor, the following assumptions can be made:

1. The motor torque is equal to the fan torque as shown by Eq. 7 above.

2. The voltage and current of the motor are equal to those of the PV module.

3. The speed of the fan is equal to the speed of the motor.

The three linear-segment representation of the fans’ H-Q characteristic simplifies the modeling procedure. For a given irradiance and module temperature, the PV I-V characteristic is determined. Making use of the assumptions above, the operational speed of the motor/fan is then evaluated by solving Eqs. (1), (2), (3) and (7) for the given irradiance and PV module temperature. This speed is then used in Eq. 8 to determine the H-Q relationship for each of the three linear sections. The flow rate in the system is determined by solving the system curve with Eq. 8 simultaneously.

The speed of the motor/fan (ю) and the flow rate (Q) in the system can also be determined as a function of the time of day if detailed hourly solar meteorological data is available.