surfaces and by the obstructions. The resulting illumination characteristics of the overall system will be determined by the material characteristics of the surfaces and by the geometry of the skylight. The daylighting characteristics of the interior spaces are the joint result of the illumination interactions of large — and small-; commensurable and incommensurable surfaces of the hemisphere, of the skylight and of the interior space of the building.

Absorption occurs when one material, the refrigerant, is absorbed into another, the absorbent, to form a ‘solution’. The vapour pressure of the refrigerant for this solution (Ps0i) is lower than that for the liquid refrigerant (Pref). If two vessels are connected together as in Figure 1, one containing liquid refrigerant and the other the absorbent, refrigerant will be transported from the left hand vessel to the right due to this difference in vapour pressure. This results in evaporation and cooling in the refrigerant and absorption and heating in the solution, leading to a lower absorbent concentration. This transfer will continue until equilibrium is achieved when the vapour pressures in the two vessels are equal. However, the refrigerant temperature (Tref) will be lower than the solution temperature (Tsol). The difference between these two temperatures (ATequ) will be dependent on both the temperature and the concentration of the solution, as shown in Equ. 1, where ^sol is the mass fraction of the absorbent in the refrigerant.

If heat is applied to the refrigerant vessel at this lower temperature, the temperature will rise and with it the vapour pressure. This will result in transport of refrigerant vapour to the solution vessel where it is absorbed in the solution, releasing heat. If this heat is removed from the solution vessel, at a higher temperature than the refrigerant vessel, the process can continue. This is essentially a heat pumping process that can be used for cooling or heating. However, the solution gets weaker in terms of absorbent, and the temperature required to give a certain vapour pressure will decrease. Thus the temperature difference, the temperature lift, between the two vessels will also decrease, reducing the usefulness of the heat pump. In order to maintain the temperature lift, the solution needs to be regenerated by desorption, which in principle is the reverse process, with heat applied at a higher temperature to the solution vessel and removed at a lower temperature from the refrigerant vessel. Two vessels connected as in Figure 1 can be used for intermittent cooling, but in order to be able to simultaneously provide cooling while regenerating the solution, two pairs of vessels are required. For a single effect absorption chiller, these are connected to form a continuous cycle.

Different working pairs have been suggested in the literature (Macriss et al., 1988; Macriss and Zawacki, 1989), but only two are commonly available commercially: LiBr as the
absorbent and water as the refrigerant for comfort cooling, where the evaporation of water cannot go below 0°C; and refrigerators using water as the absorbent with NH3 as the refrigerant. Cycles using water/NH3 can also be used for comfort cooling, but they are not common. A number of different cycles have been developed and tested, and are treated in various studies (Herold et al., 1996; Srikhirin et al., 2001). Nearly all studies have worked on the cycles themselves, and very few have looked at the possibility of energy storage, although it is possible by storing the relatively concentrated solution between the generator and the absorber. Although this requires several extra vessels, it could be used instead of external storage devices (Berlitz et al., 1998). However, the potential for this type of storage is limited by the practical concentration variation achievable in a machine, where the heat exchanger in the absorber and generator are critical. In addition, crystallisation has to be avoided so that the solution can be pumped between vessels.

Adsorption Process

Adsorption, the binding of a sorbate onto the surface of a sorbent, can also be used in a similar way to absorption as in Figure 1. The major difference here being that adsorption is a surface phenomenon and can only be used with solid adsorbents, and thus a complete heat pump cycle cannot be built up in the same way. Instead the desorption/condensation phase, also called charging phase, and the evaporation/adsorption phase, also called discharging phase, must be separated in time. Again there are a number of different working pairs that have been studied (Dieng and Wang, 2001; Wongsuwan et al., 2001; Henning and Wiemken, 2003), and similar to absorption, those with water as the sorbate are limited to comfort cooling applications. Adsorption can be used in open or closed cycles. Common to all is the fact that the sorbate must be transported into the structure of the substance and also that the heat has to be transported to/from the solid. This creates practical problems for the design of heat exchangers and the matrix for the solid.

Thermal Storage

Adsorption has also been studied for thermal energy storage, especially for solar heating and cooling applications in recent years (Mittelbach et al., 2000). Due to their high energy density compared to sensible heat storage in water, the potential for long-term heat storage has been studied. This work has focussed mainly on water together with zeolite, silica gel or modified silica gels. An energy density of 134 kWh/m3 silica gel material has been achieved in a practical system (Nunez et al., 2003) whereas 160 kWh/m3 has been achieved for a small (1 kg) sample of zeolite and theoretically 233 kWh/m3 for impregnated aluminosilicates (Janchen et al., 2004). However, the storage density is dependent on the pressure in the system and thus the desorption temperature. Zeolites require, in general, higher desorption temperatures than silica gels. A comparison of storage capabilities for different materials for sorption systems (Mugnier and Goetz, 2001) showed that for refrigeration at -20°C, a solid-gas chemical reaction with ammonia gave the highest energy densities, whereas for comfort cooling the highest energy densities were achieved by water with NaOH for absorption, and for CaCl2, MgCl2 and Na2S for chemical reactions. These chemical reactions are the binding of water to hydrates of the salt.

Wimolsiri Pridasawas, M. Sc., Department of Energy Technology, Royal Institute of Technology, Sweden

Per Lundqvist, Ph. D., Assoc. Prof., Department of Energy Technology, Royal Institute of Technology, Sweden

Abstract

The TRNSYS-EES simulation tool is used to simulate the characteristic of the solar- driven ejector refrigeration system with a flat-plate, double-glazed solar collector. Butane is used as a refrigerant in the cooling subsystem and water is used as a heating medium in a solar-collector subsystem. The performance of the system is shown in terms of coefficient of performance (COP) for the refrigeration subsystem and system thermal ratio (STR) for the whole system. The simulation results show the performance of the system, the annual electricity usage by the pumps and the auxiliary heater at different solar collector area, storage tank volume and water flow rate. The system performance depends on the solar radiation and the operating temperatures in the refrigeration subsystem. The STR is high when the solar radiation is high. The maximum STR that can be obtained is about 0.25 at a COP of

0. 55. The optimum solar collector area for the average cooling load 4 kW is about 50 m2. The system operates only during daytime, thus the volume of the well-mixed storage tank does not significantly affect the performance and the electricity usage of the system.

Introduction

Refrigerators and air-conditioning systems are mostly driven by electricity and account for about 15% of the world’s electricity consumption (Lucas, 1998). Solar energy can be converted to both thermal and electrical energy, both of which can be used to drive refrigeration systems. The demand for cooling is generally high when the solar radiation is high. The performance of an electricity-driven refrigeration system is quite high but it requires photovoltaic panels, which are expensive and have low efficiencies. These systems, however, can be built in small sizes, making them suitable for applications such as vaccine transportation or cooling boxes. An air-conditioning system is used to control temperature and humidity for human thermal comfort. The demand for this application is high in a densely populated area such as big cities. The solar thermal-driven refrigeration systems are more suitable for air-conditioning applications due to the lower installation cost, furthermore it can provide high cooling capacity.

A solar-driven ejector refrigeration cycle is quite a reliable and simple system. An interesting advantage that can be noticed is its ‘low temperature heat supply’ that allows it to be integrated with a simple solar collector such as a flat-plate solar collector. Furthermore, this system is easy to install, design and operate. Several research groups have studied the ejector refrigeration cycle in different perspectives but only a few of the solar-driven systems have been presented. Huang (1998), has developed a solar ejector cooling system using R141b as the refrigerant; the overall COP is about 0.22 at a generating temperature of 95°C, an evaporating temperature of 8°C, and solar radiation of 700 W m-2. Several simulation models are found in the literature of Dorantes (1996), Sokolov (1992) and Al-Khalidy (1997). Chlorinated refrigerants such as R142b (Dorantes, 1996), R114 (Sokolov, 1993) or R113 (Al-Khalidy, 1997) were recommended due to a high performance. These refrigerants, however, have negative environmental effects.

Some environmentally benign refrigerants for solar-driven ejector refrigeration systems are introduced in the literature of Pridasawas (2003) including a comparison of the technical feasibility and performance of each refrigerant.

In this paper, a TRNSYS-EES simulation tool was used to model and analyse the performance of a solar-driven ejector refrigeration system using butane as a refrigerant. TRNSYS is a transient systems simulation program with a modular structure (Klein, 2000). It is widely used for analysis of time dependent systems such as solar systems, low energy buildings and HVAC systems. The Engineering Equation Solver program or EES is generally used for solving a set of algebraic equations and initial value differential equations (Klein, 2002). It provides built-in mathematical and thermophysical property functions suitable for cycle simuations. The whole system is simulated by using TRNSYS but the model of the ejector refrigeration sub-system is developed in EES. The weather data from Bangkok, Thailand is chosen to represent the warm climate for this simulation.

The system’s performance mainly depends on the solar radiation and the operating temperature. The performance decreases in inverse proportion to the condensing temperature but it increases when the generating temperature increases. High generating temperature requires high outlet solar collector temperature but the efficiency of the solar collector decreases at the high outlet solar collector temperature. The optimum operating condition for the highest system performance should be considered. The optimum generating temperature, solar collector area and storage tank volume were studied by using TRNSYS-EES tool as mentioned above.

The dimensions, positioning and coating characteristics of the triangular projection panel are detailed in (Andersen et al., 2001; Andersen, 2004): a diffusing white paint manufactured by LMT allows to obtain an almost lambertian surface (perfectly diffusing), with only a 2.6% difference to the theoretical model.

The removal of screen covers, necessary to perform BRDF measurements, aims at leaving the incident beam path free, while the controlling of its shape is taken care of by the ellipses cut out from the metal sheet.

To minimize the blind zones, these screen covers must present elliptic shapes as well. Their exact geometry was determined following a similar procedure as for the metal sheet:

• First, their theoretical dimensions and positions were deduced by trigonometry on the basis of the intersection of a perfectly parallel beam (reaching the sample at different Qi angles) with the tilted detection surface (accounting for the shift between sample and detection screen base planes.

• Then, using on the results provided by the sample illumination analysis with the actual light source and on the metal sheet ellipses dimensions, adjusted horizontal and verti­cal axes for the screen ellipses were estimated, to which a 2 mm margin was added to avoid edge effects.

• After that, to determine the actual dimensions of the cut out covers, the thickness of the screen had to be taken into account; on the other hand, the covers insertion required a slant between the upper (external) and lower (internal) sides of the screen, chosen unique and equal to 20° to ease the screen manufacturing. To leave the beam’s pas­sage free through a screen of significant thickness, larger upper ellipses are required when the angle between the incoming beam and the screen plane increases (i. e. when I Q — Q0 I increases). The ellipses were thus adjusted accordingly, depending on each one’s incident tilt angle.

• Finally, as the above adjustment was only necessary for the ellipses half farthest from the Qj = ©0 direction, their vertical axes (and thus the blind zones) were reduced by re-centering them to open a passage for the actual beam only, still accounting for the screen thickness and a constant 20° slant.

The elliptic covers are held in place by small and strong permanent magnets inserted in the screen central piece. To achieve their removal and repositioning, a “permanent electro­magnet” (PEM) is used, i. e. a permanent magnet that can be deactivated by powering the surrounding coil. This PEM is mounted on a small wagon running on two rails parallel to the main axis of the screen thanks to an indented belt forming a closed loop. An additional on-board mechanism allows it to move up and down from approximately 3 cm, in order to extract and replace the covers. To ensure a reliable lifting, a mechanical “extractor” was added, using four screw-like pins that get inserted in four slots carved in each cover, shown on Figure 5(a); centering pins were added as well on protruding fingers to ensure a reliable positioning. An extra shift was implemented for the wagon movements to allow the extraction system to have a secure grip on the covers.

The limitations in the rails length made it impossible for this extractor to reach the tip cover. Its handling thus required an additional PEM device, together with some extra commands.

The wagon is driven by a stepping motor, controlled by a specific ISEL micro-controller with a RS-232 interface. A typical cycle of extraction, removal and replacement of a cover is sequenced as follows: [12]

• Wagon positioned out of the beam path and kept in place as long as needed to com­plete the image acquisition and processing phase;

• Wagon moved back above the open hole, PEM lowered, deactivated then lifted up empty, the cover being back in place.

Once the wagon movements were adequately calibrated to position it right above each cover, this new design was tested successfully with hundreds of random extractions at different screen inclinations.

The definitive screen panel is shown on Figure 5(b), where the wagon is in position to re­move the tip cover and where all other covers are missing. Figures 5(c) to 5(f) illustrate the sequence of events taking place when the projection screen obstructs the incident beam path.

The facade design shown in Figure 1 was numerically investigated. The results obtained for two climatic conditions, Barcelona and Geneve, are shown in Figure 4.

Figure 4: Monthly performance of the facade for domestic hot water production: solar fraction and solar efficiency. Consumption profile 1, (with noon draw), tank stratification considered as given by 5 nodes.

It is observed that solar fraction is less variable in Barcelona climate, while it suffers depreciations in Geneve climate in the winter months. Solar Efficiency presents for both climates a similar performance. For April month, for instance, 32% of total domestic hot water load may be satisfied by the solar facade if it is located in Geneve. For a Barcelona location, facade provides 40% of total load. Regarding Solar Efficiency, the variation is lower, 46% for Geneve and 49% for Barcelona. Annual values are shown in Table 4, where OTL stands for the mean annual outlet temperature from water tank.

The influence of the consumption profile has been analyzed for a design addressed to Barcelona climate. Monthly solar fraction and solar efficiency are shown in Figure 5 for the two profiles described in Tables 1 and 2. Consumption profile 1 (with noon draw), presents better results all around the year. The differences are more noticeable in the central months (June, July and August). In the rest of the year, differences are negligible.

The influence of stratification within the tank has been analyzed conside­ring two possible le­vels. Stratification represented by 5 and 10 nodes. Di­fferences obtained are negligible, al­though it is pos­sible to get larger useful energy with

a higher level of Figure 5: Monthly performance of the facade for domestic hot water stratification in wa- production: solar fraction and solar efficiency. Two consumption pro — ter tank Data are files (1: morning, noon and evening, 2: morning and evening). Data corresponding to Barcelona

СПЛ1А/П ІП О К ГО

Ms Mahalath Halperin, FRAIA, B. Arch, B. Sc (Arch)

ANZSES Chair, RaIA Country Division Deputy Chair Mahalath Halperin Architects Pty Ltd Armidale Australia 2350

Ph: 61-2-6772 2263 fax: 61-2-6772 2900 email: ozarch1@tpgi. com. au Introduction:

In refurbishing the Cowra Administration Building, Mahalath Halperin Architects transformed a concrete and glass building of the 1960s into an energy efficient and environmentally responsible building, yet looks to all intents and purposes like an ‘ordinary building’.

To some, it is simply a newer, nicer building with bright colours, open spaces and a pleasant work environment. But whilst not necessarily being outwardly different in issues of being a ‘green building’, the resultant building shows that there are many small yet easily achievable ways to be green. That it is not hard, and is in fact beneficial, to reduce energy consumption and overheads, take advantage of the sun (despite its poor orientation) and take on environmental responsibility.

Key Design Issues of the Brief —

In 1968, Cowra Shire Council, in regional NSW, Australia, built new council chambers — lots of glass, bare concrete, stained timber and single-glazed space framed windows — built in a style and manner typical of the era, but with little consideration for environmental functions or energy usage. With the growth of the Council over the years, and staff and services spreading to another building across the road, the original building had become increasingly impractical and inefficient to run, difficult to heat and cool, resulting in poor staff moral as they worked their way around the rabbit warren that resulted. The original building was riddled with asbestos, too hot in summer and too cold in winter, acoustically challenging, and unfriendly and cramped to work in.

In 1998, we were engaged to consolidate all services into one, refurbished building, bringing it in line with modern business practices (including IT requirements) as well as making it more functional and practical. Within the limitations of the existing site (3 exposed faces excluding the southern-hemisphere’s desirable north) strong priority was given to ESD considerations. However, whilst the budget did not extend to implementation of technologies such as on-site power generation, the resulting building, even though now bigger than the previous two buildings combined, and now with full HVAC system, has achieved major inroads as an ESD-based building benchmark. The new building is more efficient to heat and cool and generally accepted as a pleasurable place in which to work.

The brief was to turn the larger building into something bigger, better and more efficient in every possible way, consolidating all Council services back into the one location, and also keeping overheads low and making a statement of responsibility for the Council. The driving issues for the Council were on the one hand to reduce running costs and ensure a secure and worthwhile investment was being built, with a responsibility to ratepayers, whilst on the other hand implement principles of being environmentally responsible and creating a better and healthier work environment for both staff and public alike.

In the current paper parabolic trough collectors with direct solar steam generation are employed to supply steam to the preheating section of a conventional power plant. Although other solar thermal concepts feature higher concentration factors and higher system efficiencies the parabolic trough technology has been chosen because it is a proven technology which has demonstated its reliability in large scale applica­tions (80-100MW) [1].

Tracking the sun from sunrise to sunset the cylindrical parabolic mirrors concentrate the sun’s radiation on the black absorber tubes along their focal line transforming ra­
diation into heat. In these absorber tubes water gets vaporized and superheated to temperatures of more than 400°C.

Figure 2 shows a typical parabolic trough collector. The aperture area is about 545 m2 with a length of 99 m and the reflectivity is more than 97%. [7].

Favourable locations within Europe to operate solar power plants are southern coun­tries like Spain, Italy or Greece. Because of the italian legislation having a special interest in promoting renewable energy sources the location Greater Bari in south Italy was chosen. The yearly global irradiance is about 1600-1750 kWh/m2a and the yearly direct normal irradiance, that is used by the parabolic trough collectors, is up to 2200 kWh/m2a.

In order to design and simulate a parabolic trough collector the monthly average val­ues of irradiation were chosen at first. In figure 3 the average direct normal irradiance of the last ten years in greater Bari can be seen exemplarily for the months January, July and October. The highest maximum irradiance occurs in July at noon (884 W/m2) and the lowest is observed in January (349 W/m2).

The parabolic trough collector plant which is the basis for the current study is not an existing one but is an approach to similar existing plants and investigations in the field of direct solar steam generation [1].

Figure 3: Average direct normal irradiance in Greater Bari

Mohammedi K.; Mabizari S.; Badkouf D.; Chegroun N., LMMC/GTT
University M. Bougara, Boumerdes 35000, Algeria.

• introduction:

The most widespread cooling systems are based on the mechanical compression of refrigerant vapours. The installation of such equipments in lonely areas, in particular in developing countries, often runs up against the non­availability of classical sources of energy. While in Developed countries, relying heavily on fossil energy, need to use more effectively renewable energy sources which are less harmful for the earth’s environment is growing and more according to Kyoto protocol. The conversion of solar energy to cooling energy by the thermodynamic way can be obtained by a three-heat-reservoir refrigerating system evolving between two sources and a heat sink. Here, the coincidence of the maximum cooling loads with the maximum availability of the solar radiation is of great interest in solar refrigeration [4],[6].

This paper is dealing with the development of a computational simulation of a cooling cycle powered with a flat solar collector working with a two-phase thermosiphon [1].

The solar DEC plant serves two meeting rooms of the chamber of commerce in Freiburg. The rooms are almost fully glazed with outside shading devices. Figure 2 shows the smaller one of the two meeting rooms ("Cafeteria”) and the collector field situated on the roof of the building. It is divided in four parallel fields of each 25 m2. Two fields are facing west, the other two facing east. The inclination of the collectors are 15° in either case. The reason to mount the collectors with this roof given inclination was to reduce the costs for the support construction. In this way the system cost for the solar collector field including the installation was reduced down to 210 € per m2 /12/. The Cafeteria has a maximum capacity of 20 persons, the meeting room (“Sitzungssaal”) a capacity of 100 persons. The nominal volume flow rate is 10.200 m3/h. The meeting rooms can be air-conditioned

separately. The maximum volume flow of the Cafeteria is 2700 m3/h, for the meeting room 7800 m3/h.

When the contrast ratio between the facade and the window is too high, a transient region can be inserted between the window and the facade. But, if the transient region does not increase the amount of light on the facade, then the contrast ratio between the facade and the window is still too high. The transition in brightness is then more gradual, but within the field of view the contrast ratio is still too high. Why would this gradual contrast then be more comfortable? It could be because of the rapid movements the eye makes to observe the entire area that is relevant to a person’s interest. Scanning the area of interest without the transition region, the eye encounters a large contrast difference for which the eye needs to adapt itself. And this happens with every scan the eye makes. For an area of interest surrounded by a transition region, there are two possible reasons why the visual comfort is larger. The first is that the eye has enough time to adapt to the lower luminance value while scanning the transition region. The adaptation time should then determine the size of the transition region. The other possibility is that the brain signals the eye to stop the scan at the transition region so that the eye does not need to adapt to the large luminance ratio of the surrounding area. This is the case when the adaptation is very fast. Of course it could also be a combination of both possibilities. See for more information on the human visual system the NEN 3087 norm [4].

A transition region is nothing new. The use of stained glass transition band was common in the 19th century. Transition regions through the use of net curtains in dwellings were, and in certain regions still are, also very popular. The size of the transition region, however, is expected to be somewhere between 5 to 10 cm based on personal experience, but further research is necessary to give a more detailed estimate.