A solar cooling system consisting of solar thermal collectors, a heat storage (optional), an absorption chiller, a cold storage (optional) and the application system (e. g. room cooling) is a very complex arrangement. The performance of some components (e. g. collector and cooling tower) depends on ambient conditions (e. g. air temperature, air humidity). Because of the high number of interrelations between the different components and between some components and the ambient conditions it is not possible to calculate the performance of the whole system and to compare different types of certain components (e. g. collectors) manually. Simulation programs have to be used.

Using the capabilities of the TRNSYS /1/ package the performance of the small capacity absorption chiller in combination with different kinds of collectors and re-cooling options was evaluated /2/. For flat plate and vacuum tube collectors a comparison of different models (manufacturers) was done using published test data. For each type a reference collector model with a good performance and an acceptable cost-performance ratio was chosen. The properties of these models were used for further calculations. For parabolic trough collectors the STEC-Library was used (DLR model of the IST trough) /3/.

At first a system with a fixed cold water inlet temperature (12 °C; 18 °C) to the absorption chiller was simulated with climate data of Huelva/Spain.

For all combinations of the three collector types and two re-cooling options (wet cooling tower and dry cooler) the delivered cooling energy from April to August was calculated for five collector areas. The results are shown in Figure 8 and Figure 9.

The results show the great influence of the re-cooling version. If a dry cooler is used in­stead of a wet cooling tower the collector area has to be increased to obtain the same quantity of cooling during the summer.

Comparing the figures 8 and 9 the influence of the cold water temperature can be evalu­ated. If the room cooling system is capable to meet the cooling load with a higher cold wa­ter temperature (greater heat exchanger area needed) the cooling energy delivered by the absorption chiller increases (or a smaller collector area is sufficient to receive the same amount of cooling).

The simulations also provided other information such as the maximum cooling perform­ance of the absorption chiller during the operation period, distribution of the cooling energy over the month (and days), water demand for the wet cooling tower, delivered cooling en­ergy per collector area. By comparing the results for different collector areas an optimal size can be chosen.

At a second step a more realistic system with changing cold water temperatures and in­cluding the thermal behaviour of the building (cooling load) was simulated. To compare the different arrangements

— the excess temperature: ATe = TRoom — 26 °C

— the duration (tet) of the periods with inside room temperature above 26 °C and

— the integral: |ДТЕ dTET [Kh]

were calculated. Figure 10 shows the results for one configuration with different collector areas.

Simulations using sea water for re-cooling the absorption chiller were also done. The re­sults for Huelva showed that there is no higher performance of the absorption chiller than using a wet cooling tower. Sea water cooling has some other restrictions too (only near the sea, high cost, need to use suitable materials).

All the results are valid for the specified equipment (collectors, coolers) only. For evalua­tion of a certain project simulations have to be done again with the characteristics of the equipment to be used and suitable climate data.

The experiences gained during the simulations and supervising solar cooling installations showed that great care should be taken regarding system control, storage integration and design of the room cooling system. As mentioned before the cold water temperature is important for the achievable cooling output.

Conclusions

A H2O/LiBr absorption chiller with a nominal cooling capacity of 15 kW was developed. A special heat exchanger design permits the use of low temperature heat such as solar or waste heat to drive the absorption chiller.

After prototype testing a field test at three sites was carried out in the summer of 2003. The field test showed good results. The absorption chiller is working reliable and flexible over a wide range of external conditions.

Because of the high number of cooling applications in the capacity range below 50 kW the new chiller has the potential to increase the usage of solar thermal energy for cooling pur­poses.

The waste heat of cogeneration systems can also be used to drive the absorption chiller creating new possibilities for trigeneration systems (power, heat and cold).

The performance of solar thermal cooling systems can be predicted using the capabilities of system simulation programs.

Nomenclature

Literature

/1/ Solar Energy Laboratory, University of Wisconsin (2000). TRNSYS 15, A Transient System Simulation Program, Madison, WI, USA.

/2/ Heinrich, C. (2004). Modelling and Simulation of solar thermal driven single effect absorption chillers of small capacity for climatisation, Diploma Thesis (German), Dresden, Germany.

/3/ Schwarzbozl, P.; Eiden, U. e.; Pitz-Paal, R.; Jones, S. (2002). A TRNSYS Model Library for Solar Ther­mal Electric Components (STEC) — Reference Manual — Release 2.2, Cologne, Germany.

Two of the windowless north galleries of the second floor receive direct sunlight that passes through the high southwest windows of the atrium (see yellow areas in Figure 4- right). Illuminance levels measured over the painting in one of the two galleries on January 19, 2003 (Figure 15) reached an extremely high value of 11,500 lux, which is about 57 times higher than the recommended IES standards for museums. The total illuminance-hour per year over these display areas over exceeds the total exposure limits for moderately light susceptible displayed materials. The fish-eye photo (Figure 16) shows that the painting receives direct sun for one and a half hours in the early afternoon from November to January. The high illuminance levels over the display areas are due to the high visible transmittance of the atrium’s glass (Tvis=59%). The glass of the atrium does not have any shading device to intercept the incoming direct sun to the adjacent galleries. To protect paintings from over exposure to light, curators have been rotating oil paintings frequently (see dates on Figures 15 and 16).

Figure 16: Fish eye photo taken from painting’s viewpoint with sun path diagram, at gallery adjacent to atrium (March 5, 2004).

Another problem observed in these galleries is the glare that visitors experience while moving through the gallery and to the atrium. Figure 17 illustrates how a visitor has to protect her eyes from direct sun while trying to see the painting and read the sign next to it. This visitor is receiving about 7,770 lux over her eyes when not covered. The extremely high variations of light levels within the field of view of the visitors to these galleries makes them uncomfortable to adjust their eyes to the low light levels over the paintings and to the bright atrium’s glass areas that are within her field of view.

Conclusions

The most noticeable problem, in the galleries of two of the museums presented in this paper, has been the sunlight penetration over the displayed museum objects. This fact is harming valuable art collections, and also creating visually uncomfortable environments for visitors. Sunlight penetration occurs mainly in galleries with side lighting windows that face the sun — east, south, and west-; orientations that are the most difficult to control. Even though the time of over exposure is relatively limited, the illuminance levels reached at these times are extremely high, 10 to 57 times the maximum recommended IES standards for light susceptible objects. Dark tinted glass, screens, or small overhangs are not enough measures to block the entrance of direct sun over the exhibit areas at the Modern Museum (west-facing galleries) and at the Amon Carter Museum (east-facing galleries).

To control the light levels in these galleries more aggressive changes should be done, like modifying the fagade by adding horizontal or vertical shading devices, external louvers, trellises, or trees to filter sunlight. All these modifications could change the image of the building, which may not be acceptable by designers. There are other less intrusive solutions that include the use of miniaturized sunscreens, interior louvers or baffles; or computer controlled dynamic window systems (Reference 6), where direct sunlight over sidelight windows can be intercepted and filtered reducing interior light levels as low as 25 lux or under.

On the other hand the third museum, the Kimbell Art, presents a well thought and carefully designed toplighting system that can introduce adequate light levels throughout the year to accommodate the lighting requirements of exhibits, and at the same time provide a connection to the exterior environment by rendering the galleries with natural light. This museum is an excellent example that it is possible to successfully illuminate museum galleries with daylighting.

If we already know that sidelight windows in museum galleries create many problems to the display of light susceptible artifacts, why are we still including them as a source of illumination in museum galleries? Is it due to the lack of knowledge about solar geometry? Or it is just that architects underestimate the effects of sunlighting in museums? The answers to these questions may not be clear and simple, meanwhile we might be finding museum galleries with sidelight windows with strong sunlight over display areas, such as in two of the three museums presented in this paper (see summary in Table 1). In the meantime, curators and facility managers at these museums have to create means to protect their valuable art collection (i. e. rotating display objects, use boards to cover them on a daily basis, display least light susceptible objects, cover completely the windows with boards or black cloths, among other solutions) (Reference 7). The main goal of this paper was to make us reflect on the way that museums are still being designed, and on how could we make them better to preserve and display artifacts that are important part of human history.

To assure a complete self-sufficient system, the air solar system should cover the 20% of the remaining demand. With this aim, 12 sq m of Solarwall system (the same surface of the solar panels) has been added.

Figure 11 shows how the system works. A fan collects the preheat air from the Solarwall and it is introduced through a special insulated channel directly to the living room. The system keeps the house dry also when it is not occupied for long periods. In sunny days the air system will delay the switching on of the conventional solar water system.

The calculation indicates that only in December the self-sufficiency could not be reached. Nevertheless, the results depend completely on the weather. In fact, on a sunny day the house could be heated mainly by the air solar system, but on a cloudy day the risk to be insufficient is very high.

Then, it is necessary to integrate the solar system with a boiler, using renewable energy source, which means a wood heater.

The wood heater

When necessary, a wood heater integrates the solar system and covers the small daily energy need, about 5,3 kWh/day. This small requirement can be satisfied burning around 1 kg of pellets o using directly the local olives trees pruning in a stove with a 85% efficiency.

In order to cover the remaining winter heating demand not cover by solar energy of 171 kWh, only 43 kg of pellets (that means 7,74 Euro for the whole period) or 40 kg of olives trees pruning is required.

One of the first decisions a house builder has to make is the form and its orientation on the building site. While many of the analyzed projects are oriented exactly to the south, some deviate by as much as 45° off south. It appears that such deviation can be easily compensated by other design features in highly insulated houses, as can be seen in Fig. 7.

Building compactness is, however, important. The surface area to volume ratio (A/V) for all projects is 0.63. For the single family houses the average A/V is 0.73; for apartment buildings the A/V averages 0.49. There are two buildings with a relatively high A/V of 0.91 and 0.99, circled in Both of these buildings compensate their lack of compactness

by the heat gains of solar wall systems. The one house has the wooden Lucido-System: slotted wood acting as a solar buffer, between an exterior glass facade and the opaque insulated wall. The other house has a translucent solar storage wall of paraffin phase change material in a glass facade.

Basically CSE equipment with the Fresnel lens consist in an axis orientated to the Polar Sun, that is, parallel to the earth rotation axis. This axis is computer controlled to have a single daily rotation (Fig. 1).

The altitude of the sun is hand controlled because the equipment is simpler. Every morning we change this altitude and then the focal spot is on position.

Then, after we focalize the sun in the morning in the focal point the focal spot will be maintained at the same place the whole day because the axis makes 1 rev. per day following exactly the sun track. By computer control we can change the turning speed and register the temperature time heating and cooling curves of the sample.

Because of the angle of the axis and the weight of the Fresnel lens frame and other elements, the axis has a tie with a screw to adjust exactly the angle and to avoid oscillations when the wind is large or the weigh is heavy.

2.3. Fresnel lens

The Fresnel lens is ca. 0.8 sq. m and exist in the market at moderate price, ca. 200 €. Is made in polymeric material, 3 mm thick, and, consequently is very light in weight. The lens is covered while no working. A cover with a hole in the center let us to put the focal spot in the right place. To work the cover is put out and the heating of the sample start.

Polar oriented axis computer controlled

Fig. 2. General view of equipment

The Fresnel lens is made by Edmund Optics, Ltd. Main characteristics are summarized in Table I. Fig. 2. show a general view of the equipment.

Typical design of the Fresnel lens consist in the substitution of the curve of the conventional lens with a series the concentric groove. This lens is molded in the surface of a thin, light weight plastic sheet with the appropriate curvature. The conical grooves act as individual refracting surfaces like tiny prisms when viewed in cross section bending parallels rays in a very close approximation to a common focal length.

One added advantage of the thinner plastic lens to the light weight is that because it is very thin there is very low absorbance of solar beams, i. e., it is a very high transmission. Fresnel lenses are a compromise between efficiency and image quality that depends of the groove density. High groove density increases image quality but low grooves density
produces higher efficiency that is our main interest. In infinite conjugated systems, as is in our case, the grooved side of the lens should face the longer conjugated.

2.4. Reaction chamber

Our installation has a double wall watere refrigerated reaction chamber that allow us to work in atmospheres of different composition. An inlet and outlet allows to introduce the gas of the required composition. To avoid the introduction of air we work with a slight over pressure of some cm of water.

The cylindrical chamber has a quartz window in one of the circular sides to allow the concentrated beams reach the sample inside the chamber. In the opposite side there is a system to introduce and withdraw the sample and for the thermocouples exit.

2.5. Samples

The samples we used to obtain SHS intermetallic coatings on steel consist in a small steel cylinder 6 mm thick, 14 mm diameter with an internal hole 12 mm in diameter.

ECOS is a sorptive cooled heat exchanger employed for building air-conditioning. The novel system is the implementation of an original desiccant and evaporative cooling process. The new concept aims to overcome the thermodynamic limits of conventional DEC systems, allowing a higher energy efficiency and performance. The design of the process results in a higher dehumidification potential in comparison with conventional systems. It is particularly intended as a desiccant cooling system for climates with high ambient air humidity (e. g., Mediterranean and tropical). Moreover the novel system avoids the complexity of the rotating parts necessary in standard systems and gives the possibility to apply the DEC concept even at small scale plants.

The process is based on simultaneous sorptive dehumidification and indirect evaporative cooling of the supply air stream. Moreover the indirect evaporative cooling is obtained through a continuous humidification process, ensuring a high heat exchange potential.

The system implementing the process is based on a counter-flow air-to-air heat exchanger technology. The heat exchanger is divided in sorptive (black line in Figure 3) and cooling (grey line) channels, which are physically separated but in thermal contact. The sorptive material is fixed on the heat exchanger sorptive channels. In the sorptive channels the supply air is dehumidified. In the cooling channels a continuous humidification of the cooling stream takes place. The latter, used for indirect evaporative cooling of the supply air stream, is for this purpose always kept in close-to — saturated conditions during the process.

The complete system consists of two sorptive heat exchangers, operated periodically. The periodic operation of two heat exchangers enables a quasi-continuous air-conditioning process. While one component is used in air-conditioning operation mode the other one is regenerated and pre-cooled before the next air-conditioning operation mode. In the regeneration the water vapour load of the sorbent material is released to the environment by means of a hot air stream (60-95°C). The air is heated through a heat exchanger connected to the solar plant. The subsequent pre-cooling phase is intended to lower the temperature of the heat exchanger after the regeneration, taking up the heat stored in the heat exchanger thermal mass. Moreover, a complete air-conditioning system will include at least a humidifier at the entrance of the cooling channel which brings the air at almost saturated conditions before entering the channel. Another optional humidifier can be installed on the supply air side in order to exploit the potential for direct evaporative cooling of the process.

To assess the feasibility of a solar assisted air conditioning system, to find an appropriate system design or to investigate the advantages of selected technical solutions, computer based design tools have to be applied. Different types of design tools exist to meet the different requirements in the planning phase. Not in any case, a detailed simulation of a solar assisted air-conditioning system is required from the very beginning. On the other hand, there is no tool which meets all requirements at different levels of the planning phase. The range of existing tools may be sorted into different categories: easy-to-use pre­feasibility tools, allowing a fast calculation of basic key figures; simulation and sizing tools which also provide fast results but allow more distinction between different system configurations; and open simulation tools for advanced studies of a certain system configuration.

In this chapter, a non-exhaustive overview of a few computer tools is provided.

There are a large number of examples to be found around the world of projects where special attention was given to the integration of solar heating products into buildings. Beside the technical and economical reasons there is clearly an increasing trend to consider the esthetical reasons for building integration of solar hot water systems. In this paper we give some examples of the different forms of and approaches to building integration in one family houses and in apartment buildings. The use of a small (3-4 m2) solar water heaters in a single family row house is rather common. However there are many projects with large solar roofs, where thermal solar energy is used for both domestic hot water and for space heating. This is more common in apartments with central heating system.

In the Northern European countries the use of solar heating systems in the facade is also rather common and useful because of the lower sun angle in the heating season. Depending on the latitude a different angle is needed. This gives the designer new ideas and possibilities for the integration of solar heating systems in the facade. For example, some systems are integrated as large awnings above the windows.

Integration of solar heating systems in a

single family house is rather easy. Only a few square meters of the roof are needed for a domestic hot water system. Also in the case of space heating the roof will be large enough for the solar collectors. In the case of an apartment building the integration approach needs to be different. Depending on the amount of apartments and the floors in the building, the amount of square meters for solar hot water systems will exceed the available surface of the roof. In this case creative solutions are needed to integrate the collectors in the facade.

Levente Filetoth, architect, Budapest University of Technology and Economics, Faculty of Architecture, Department of Building Energetics and Services, lfiletoth@members. ises. org

The “Skylight-Dimensioning Method” is a common, existing method, suitable to calculate the area of the transparent surfaces of skylights with restricted geometrical characteristics. I have defined the illumination efficiency of skylights with various characteristics with the help of model measurements in the artificial sky and extended the existing method. The extended method is able to follow skylights without any geometric restrictions. The final output of this recent research provides several theses on various fields of the daylighting research, ready to be implemented for the practice in form of design aids and computer software tools to improve the quality of our built environment incorporating renewable energy.

Daylighting Design

Daylighting design — as an aspect of the overall architectural design — should be developed in an efficient, energy-conscious manner, while also fulfilling all functional — and other related requirements. Therefore — the conceptual architectural design decisions and later the design development evaluations should be supported by — and based on the results of the connecting scientific research fields.

The ultimate goal of daylighting design is to ensure adequate and comfortable illumination environment in the interior space. According to the current practice, daylighting dimensioning has two major tasks to complete:

• first of all the geometry of the day­lighting system has to be designed and it must be in integration with the overall

building,

• the next step is to define the area and other characteristics of the transparent surfaces of the selected daylighting system.

The architectural design of the daylighting system and its integration with the building itself are both important aspects of the overall daylighting design, since all these design decisions at the stage of the conceptual design phase will crucially effect the final daylighting characteristics and overall daylighting performance of the actual building. The architect is in charge of all the conceptual architectural design evaluations and decisions at this stage of the design project. However — most probably due to the complex and sophisticated issues and problems — there are no specific (or there are only partial) demands on daylighting design in the present regulations and in the practice.

Due to the lack of regulations and requirements the only accessible goal at the conceptual stage of the project should be to consider all the "known” inherences of daylighting design when bringing design decisions about the overall geometry and positioning of the daylighting system. For instance it is important to "know” that in case of an exhibition space it is not necessarily the best idea to design side illumination (side windows).