In January-February 2004 9 coolers were shipped from Unicef in Copenhagen (3 to Senegal, 3 to Indonesia and 3 to Cuba) and they are expected to reach their destinations in March 2004 whereafter they will be installed and the field test will begin. One additional cooler has been installed at DTI for field test, which began in February 2004. Each unit is packed with 3×60 W solar PV panels and has data loggers integrated for evaluation of the operating conditions.

For the unit installed at DTI there are now sufficient data to conclude that the operation under real solar conditions ensures an inside temperature within the desired range (at an ambient temperature of 20°C). There have been sunny and less sunny periods, but from the figures below it can be seen that the temperature becomes rather stable after a period of freeze-in.

Fig.4 Decrease of ice pack temperature after installation. After 10 days the storage is totally charged (no free water left).

The final concepts of the envelope and the screen are illustrated by the sequence of images given in Figure 3. In order to control precisely the illuminated sample area and thus minimize the parasitic reflections and blind zone, a quarter-circular frame supports a perforated sheet on which a motorized strip showing one circular aperture is unrolling, of diameter equal to the sample’s and facing the light source for any incident altitude angle 61. The sheet’s elliptic openings are of dimensions given by the apparent sample surface (accounting for inclination angle 0j) and are correspondingly positioned on the quarter circle arc.

The projection screen concept relies on the removal of elliptic covers by a robotic mech­anism. The ellipses’ dimensions were again determined by the apparent sample area ac­counting for angle 0j, yet this time projected on a the screen surface, that is oblique to the sample plane with a tilt angle ©0 = 49.1°. The induced blind spot can thus be exactly reduced to the light beam’s area, which allows a minimal loss of information on the emerg­ing light distribution and negligible parasitic reflections around the sample area. Of course, a blind spot only appears for one of the six screen positions, except for normal incidence where the tip needs to be removed for all of them.

The optimal combination of altitude step Двг and sample diameter D is determined on one hand by the device’s geometry itself, and on the other hand by the minimal illuminated area required for advanced fenestration systems or coating materials characterizations; the mini­mal allowed sample diameter was thus found to be equal to 15 cm.

Once the concept’s applicability in practice was verified, the new components were designed and constructed.

Solar efficiency (Eq. 6) and solar fraction (Eq. 7) will be used to characterize the thermal behaviour of the facades:

QLOAD

4 s — і

Where 7 stands for the incident solar radiation.

c_ QLOAD " ° S QLOAD + Qaux

Prototype description

A double skin envelope constituted by an external glass, an air channel and an indoor layer contanining the integral collector-accumulator has been experimentally and numeri­cally tested. The solar collector is formed by a glass layer, transparent insulation and the accumulation zone. This zone is a water tank, whose external surface is black-painted and its internal surface is covered by thermal insulation. Therefore, the facade external appear­ance does not present any particular feature, it looks like a completely glazed area, whereas

Figure 1: Schematic design of the implementation analyzed in a double skin facade, in this case for a space heating application

the internal surface (facing indoor room), looks like an opaque conventional wall, it may presents a rebound if it is combined with a top glazed area, as may be observed in Figure 1. Air channel has been considered closed in the results shown in this paper. Geometry and thermo-physical properties considered are shown in Table 3, where the prototype is described from outdoors to indoors.

In Table 3, a stands for the solar absorptivity, r is the solar transmissivity, є represents the thermal emissivity, p is the density, cp is the specific heat and Л is the thermal conductivity.

Integrated approaches of research on lighting conditions at work places have been conducted by Schierz & Krueger (1995) and Fleischer (2000). Schierz & Krueger acknowledge that the stimulus-response-systems is an existing and valid approach of perception in special cases where the perception of a visual object is not relying on an existing mental concept (schema). An example in this case is visual quality inspections of products where randomly located imperfections have to be detected. Another example is the classical visual test, when a randomly oriented visual sign has to be identified. Talking about glare at the work place means that people reside in a well known environment where determined cognitive structures prevail substantially.

This implies that the definition of lighting conditions which cause discomfort for people working in this setting has to incorporate the cognitive schemata concept to reach reliable parameters to identify Discomfort Glare.

The above cited results on research outcomes on DGI confirm the need for this integration of mental structures. Though the cognitive structures vary interpersonally a strong correlation among people in similar cultural background is expected. This permits the outline of a new approach for research and definition of Discomfort Glare parameters.

K. Roth, V. Scherer, T. Pockrandt, Ruhr-University of Bochum M. Eck; German Aerospace Center (DLR), Stuttgart

Introduction and purpose

In some European countries a specific percentage of the electricity has to be generated by renewable energy sources in order to reduce the emissions of greenhouse gases like CO2 and to promote renewable energy. The legislation in Italy, for example, requires 2 % nowadays, rising to 4.4 % in 2012. A promis­ing technique to meet these goals is to employ a hybrid power plant, where a renewable energy source is added to a conventional and fossil fueled power plant cycle. The great advantage of this concept is that it can be realized as a retrofit to existing power plants. Mainly solar thermic plants are favourable to be located in southern European countries, that are offering a high insolation. Such kind of power plants, for example with parabolic trough collectors heat­ing a synthetic oil which transfers the heat via heat exchangers to a conven­tional steam turbine cycle, are operating since many years and have proven their technological reliability. Direct solar steam generation is a possible im­provement of the parabolic trough power plants vaporizing water directly in the absorber pipelines and eliminating costly equipment like the heat exchanger and the oil pump. This steam generated by solar energy can be integrated into the water steam cycle of a fossil fueled power plant [1].

A schematic process diagram is shown in figure 1. By this means less steam is consumed by the high pressure and low pressure preheaters. One or more steam extraction lines can be closed and the steam can be used for a very fast and dynamic additional power generation in the steam turbine. The power plant’s efficiency is improved simultaneously.

Since the integration of the transient heat sources will lead to substantial dy­namic energy shifts within the power plant’s water-steam-cycle, the occurring time dependent processes have to be studied on a numerical basis using a dy­namic simulation software. The description and analysis of these shifts to­gether with a cycle evaluation using an unsteady process simulation tool are the content of the current paper.

As an alternative to simplify the simulations and facilitate the implementation of absorbers and regenerators in simulation software packages as TRNSYS[8], a simplified model was developed. In this model, the liquid-desiccant stream is represented by one single node in the у direction. The temperature of the liquid-desiccant is assumed constant, at the same temperature of the wall. The air stream is also represented by only one node in the direction across the channel. The energy and species equations for the air stream are:

The desiccant concentration is evaluated using a species balance for every step of the simulation, with the water mass transfer absorbed by the desiccant solution calculated through equation (22). The heat transfer coefficient is calculated using a correlation for Nusselt number for laminar flow in tubes with rectangular cross section [9]:

The mass transfer coefficient is then determined using the Chilton-Colburn analogy:

It should be noted that the analogy and the correlation above are valid only for constant temperature and concentration along the interface desiccant-air. Although this is not the case, for sufficiently high desiccant flow rates the change in concentration is small. The assumption of constant concentration in the desiccant film is similar to the assumption of constant thickness, i. e., it is only possible to assume a constant film thickness if the desiccant flow rate is relatively high. It is assumed, in the simplified model, that the heat generated by the absorption of the water vapour into the desiccant solution is immediately removed, what effectively decouples the mass and heat transfer phenomena.

Lena Schnabel, Carsten Hindenburg, Torsten Geucke
Fraunhofer-Institut fur Solare Energiesysteme ISE, Heidenhofstr. 2, D-79110 Freiburg
email: lena. schnabel@ise. fraunhofer. de, carsten. hindenburg@ise. fraunhofer. de

1. Introduction

In the last seven years intensive research work towards the topic of solar desiccant cooling systems was conducted at Fraunhofer ISE. Due to the low driving temperatures the desiccant cooling technology is promising for cost effective application of solar thermal systems. For office buildings, especially for those with large window areas, there is a high timewise correlation between cooling loads in the building and the available solar irradiation. Therefore a feasibility study on a solar autonomous desiccant cooling system for a seminar room was conducted. Solar autonomous in this context means that the thermal driving energy for the cooling needs is by 100 % provided by the solar system. The promising simulation results led to the installation of a first pilot plant at the building of the chamber of trade and commerce in Freiburg. The plant was commissioned in June 2001. The nominal flow rate of the ventilation system is 10.200 m3/h and the collector array consists of 100 m2 of solar air collectors. The plant serves two rooms in the penthouse floor of the building with conditioned air. The large seminar room has a maximum capacity of 100 persons.

Within the last two years the collected monitoring data were evaluated in detail. In the paper the energy performance of the plant will be discussed. The analysis discusses the room comfort of the two last years, the electricity consumption, the collector and the cooling performance. In general the paper reports on a promising technology for the realisation of solar air conditioning.

R. M.J. Bokel, C. I. Kranenburg, M. van der Voorden
Delft University of Technology, Faculty of Architecture,

Building Physics Group, PO Box 5043, 2600 Ga Delft, The Netherlands

1. Introduction

Large contrasts within the field of view are very unpleasant to the eye. This phenomenon is called discomfort glare. This discomfort glare is often perceived at the transition between a window and the inside of a facade. There are two possible ways to decrease this type of discomfort glare. The first is switching on an artificial lighting system in the room; the second is the application of some kind of daylighting element at the window. As the first diminishes the potential energy saving which can be reached by using daylight, the focus of our research is the investigation of materials and methods which diminishes the discomfort glare around a window.

The large contrast ratios between the window opening and the inside of the facade in the horizontal direction will not be reduced by using daylighting systems in the upper part of the window plane, which is the focus of most research [1]. A vertical element was therefore developed which decreases the luminance contrast ratio between the window and the inside of the facade [2,3]. At first a semi-circular vertical windowsill of a highly reflecting material was investigated. A more optimal shape, an anidolic shape, was also investigated and was shown to increase the amount of light on the inside of the facade and thus decrease the luminance contrast ratio between the window and the inside of the facade. However, the question remains whether cheaper and simpler methods would not work just as well to reduce the luminance contrast ratios between the window and the facade.

Up to this point, only room heating demand has been considered without taking into consideration the domestic hot water demand. The DHW demand is constant during the whole year, and in this case two persons would need 106 kWh/month that means 1273 kWh/year, almost half value of the room heating needed.

If this thermal energy load is added to the previous total heating demand, and is covered by the pellets/wood boiler, 147 kg of pellets is needed to cover the 100% demand, (27 Euro for the whole season), a small quantity compared to the Italian energy demand standards. This solution has two advantages: the usable solar energy is greater (since will be less solar energy wasted) and the pellets/wood boiler will be used for a longer time, decreasing the pay back time of the heating system.

4.5 The electricity demand

A grid connected photovoltaic system of 2,1 kWp provides the whole electricity demand (Figure 12).

21 PV panels of 100 Wp each organised in three strings are positioned in the upper surface on the south solar wall.

As could be expected, the majority of houses are extremely well insulated and have high quality windows.

The graph Fig. 9 includes U-values for walls from 0.06 — 0.25 W/m2K. The impressively low U-value of 0.06 W/m2K comes from a very new construction type "space frame”. The "space house” walls are 60 cm think and consist of a wooden space truss filled with cellulose (Fig. 11). The high value of 0.25 W/m2K comes from a project which was a renovation of a historic building and the insulation thickness was limited. The U-value of our projects averages 0.14 W/m2K. This is even below the target value of 0.15 W/m2K the Passivhaus Standard recommends not to exceed.

Also, the weighted U-value of the windows (Fig. 10) at 0.81 W/m2K corresponds very well to the passivhaus standard recommendation of 0.8 W/m2K. The very low U-value of the windows in the space frame project of 0.31 W/m2K was easily possible because of the thick wall construction, which allowed a double window construction with very good glass and frames (Fig. 11)

The window area to facade area lies between 15 and 40 percent, with the majority of houses having approximately a 25% window to wall ratio. The exceptions are the projects with special facade constructions, i. e. “Lucido” or transparent insulation (Fig. 12).

Air tightness, on the other hand is very important to high performance, with the majority of projects having a measured tightness between 0.3 and 0.9 [1/h] at 50 Pa over and under pressure. The average lies at 0.6 [1/h], which fulfils exactly the passivhaus and Minergie-P standard target. The exceptions are again projects with special facades and understandably, the retrofitted existing building project. For four of the twenty projects no air tightness data is available(F/g. 13).