Chris Bales, Hogskolan Dalarna, Solar Energy Research Center (SERC) 78188 Borlange, Sweden. e-mail:cba@du. se

Fredrik Setterwall, Fredrik Setterwall Konsult AB, Backvagen 7c, 192 54 Sollentuna, Sweden, e-mail: fredrik. setterwall@comhem. se

Goran Bolin, ClimateWell AB, Instrumentvagen 20, 12653 Stockholm, Sweden. email: goran. bolin@climatewell. com

The Thermo Chemical Accumulator (TCA) is a chemical heat pump driven by low temperature heat that has integral heat storage with high energy density. This makes the device very suitable for solar cooling. The working pair consists of Lithium Chloride and water, and energy is stored and released by desorption and absorption of water under near vacuum conditions. In contrast to most absorption processes and chemical heat pumps, the TCA works with three phases: solid, solution and vapour. This results in near constant operating conditions during charge and discharge, independent of state of charge. This paper describes the fundamental working principles of the TCA as well as a simple steady state model for the TCA. A temperature difference between theoretical and effective temperature in the reactor during absorption and desorption was required in order to get reasonable agreement with measurement data of a prototype TCA machine. For absorption, this value for subcooling was 15°C, which is significantly higher than has been found for low-temperature absorption chillers, indicating potential for improvement. For desorption the value was 7.5°C. The TCA has desorption temperatures of below 100°C for ambient temperatures below 40°C, which is relatively low. The temperature lift depends on the cooling rate supplied and varies from 15°C for the design cooling rate of 5 kW per TCA unit and 30°C inlet temperature to the reactor, to 20°C for a cooling rate of 2.5 kW. The energy density for storage was 180 kWh/m3 for the tested prototype.

Background

In many countries there is a continual increase in the demand for comfort cooling of buildings. This is reflected in the increase in the number of room air conditioners sold in southern Europe, as exemplified in Greece where the number of units sold more than doubled to 200000 units between 1990 and 2000 (Papadopoulos et al., 2003). Most of these were small units, with over 70% having a cooling capacity of less than 3.5 kW. These are predominantly electrically driven compressor heat pumps, resulting in peak electricity demand during the hottest periods and amplifying the heat island effect in large towns where the local ambient temperature is much higher than in surrounding areas. These units also use refrigerants that are either damaging to the ozone layer or are powerful greenhouse gases or both. Due to recent EU regulations regarding the phasing out of ozone depleting substances (2000), the cost of owning units with such substances will increase.

There are thus several reasons to develop alternatives to the vapour compression heat pump for providing comfort cooling: reduction of the peak electricity load during hot periods; replacement of refrigerants that are ozone depleting and/or strong greenhouse gases; and reduction of the heat island effects in large towns. In order to reduce the electricity load, thermally driven cooling processes have been developed and some have
been applied commercially. The commercial units use either absorption or adsorption cycles.

The SolarChill has been developed in a fruitful co-operation between leading appliance manufacturers and international organisations, setting the desired properties of the product. It has been proven that it is fully possible to run a solar refrigerator without battery or start capacitor, both elements that would decrease the reliability. This opens up for a more general acceptance and dissemination of solar refrigeration, not only in the health sector, but also for commercial or domestic use. Some obvious future applications for this product could be milk chilling, vending booths for food and beverages, recreational purposes or as a grid independent household refrigerator.

Even after an expected WHO approval, there is still basis for optimisation, such as:

— minimisation of the module area for specific climatic regions

— optimisation of the control strategy in order to minimize the needed PV-power

— further simplification and cost reduction of the construction

The authors sincerely wish to thank the sponsors and project partners for their very constructive assistance and participation in this project.

References

PV-POWERED VACCINE COOLER WITH ICE PACKS AS POWER BACKUP Soren Gundtoft, Danish Technological Institute,2003

SOLCELLEDREVET K0LESKAB UDEN BLYAKKUMULATOR (In Danish)

Project report to Danish Energy Agency, Danish Technological Institute, June 2002

Project flyer:

http://www. uneptie. org/ozonaction/library/tech/solarchill. pdf Compressor data sheet:

http://www. danfoss. com/compressors/pdf/product_news/bd_solar_09-03_cx30e302.pdf

The measurement space envelope for combined BTDF and BRDF measurements, shown on Figure 4(a), consists of a carbon fiber cap strengthened by a structural metallic frame; this frame also supports a static stainless-steel perforated sheet on which a moving synthetic strip can glide. The role of the synthetic strip is to select the elliptic hole through which the incident light’s path will be adequately controlled (according to altitude 61); at the same time, it prevents light from entering the measurement space through any other opening. Its unique aperture is therefore circular, slightly larger than the largest ellipse (i. e. the one associated to normal incidence); the chosen 10° step in altitude ensures that a 15 cm diameter hole never overlaps two consecutive entrances.

The determination of the actual position and dimensions of the ellipses cut out from the metal sheet required a multiple stages process for an optimal incident light control:

• First, the theoretical geometric properties of the ellipses were determined based on trigonometric considerations, assuming a perfectly parallel beam reaching an elliptic surface of apparent horizontal axis 15 cm and vertical axis 15- cos61.

• Then, the ellipses dimensions were adjusted to the real incident beam, of imperfect collimation and thus producing blurred regions around the uniformly illuminated area, responsible for parasitic reflections. Once the optimal source distance was determined, different elliptic shapes were tried out to compare the achieved sample surface illumi­nation. The most efficient compromise was established between optimal uniformity over the whole sample area and lower parasitic light flux; this was done for each ellipse individually, as more relative blurredness appeared for smaller ellipses. The determined shapes, cut out of cardboard sheets, were tested successfully; they led to only few percent of non-uniformly illuminated sample area while guaranteeing an
average relative blurredness area lower than 10%. It can be noted that these remain­ing parasitic reflections were reduced to a negligible level by adding a ring of highly absorbing material (“velvetine”) around the sample.

• Finally, the positions of the ellipses on the metal sheet had to account for the frame manufacturing imperfections (see above). The metal sheet was thus mounted tem­porarily on the frame, allowing to centre the ellipses thanks to a plumbline course driven by a progressive platform inclination. Their positioning was thereafter verified by pointing a fixed laser on the central axis and tilting the device to get each ellipse’s centre coincident with the laser spot; this test showed that an appropriate accuracy was achieved (± 0.05 cm deviation). Before sending the metal sheet for cutting out, these positions were adjusted to a flat configuration of the sheet (i. e. to its neutral fiber), to avoid slight shifts due to the sheet’s thickness.

The resulting perforated metal sheet is shown on Figure 4(b); its inside surface is covered with “velvetine” (reflection factor lower than 1%).

Illustrative results obtained for two different periods of the temperatures on different sur­faces are shown in Figures 2 and 3. Numerical results were obtained running AGLA code with the same meteorological conditions read from experimental set-up and introduced as input data in the numerical code. Data shown correspond to a situation with no consumption.

(a) (b)

04

Figure 2(a) shows the indoor wall surface temperature for a period of nine days (from March 12th to March 20th) in Barcelona city, corresponding to days 71 to 79 of the year. Figure 2(b) shows the temperature of the TIM glass surface for the same period.

Dashed green lines represent experimental values whereas straight red lines correspond to numerical prediction.

Figure 3 represents data for a period from September 21st to October 9th. Figure 3(a) shows the temperature at the internal surface of the indoor wall and Figure 3(b) shows the temperature at the absorber surface.

(a) (b)

From the previous figures it is observed that a good degree of agreement is achieved between the experimental and numerical results. This agreement was quantified in terms of the root sum square (see Reference [6] for details), values of about 8.6 to 2.6% were obtained for different variables and different periods of analysis. The higher discrepancies
were produced at the temperatures in the indoor surfaces of the accumulator, corresponding to the difficulty of evaluating the heat transfer coefficients in these surfaces.

The overall objective for an innovative approach to determine DG is the identification of relevant cognitive structures activated in Discomfort Glare conditions. and find links between physical lighting parameters used today for the DGI and cognitive structures.

Cognitive structures are often described using dimensions. Schierz & Krueger found for various lighting conditions in work places the three dimensions which map the cognitive structure. As the objective the DGI is the assessment of the (dis-) satisfaction of the current lighting conditions the pure elaboration of cognitive structure dimensions are not satisfying but they have to incorporate the individual appraisal. On the cognitive level a distinction between spatial and light dimensions and emotional perception is unlikely, mental concepts typically include the emotional appraisal. So the most crucial step for a more user oriented assessment of discomfort glare is the identification of appraisal factors towards glare. As the results from Chauvel et al. (1982), Osterhaus and Bailey (1992) and Sivak and Flannagan (1991) indicate the so-called setting of the lighting environment, i. e. a work environment instead of leisure, the degree of concentration for the tasks, the duration of one task or the social situation of the setting might have a severe influence as they might be a part of the cognitive structure.

Conclusions

Future research on discomfort glare considering the cognitive dimension of users demand for a revised strategy: Though measures of reaction speed, time for task performance and number of mistakes are comfortable for the researcher they exclude cognitive factors. The relevant factors for appraisal have to be identified (first phase) and it has to be studied under which conditions the individual assessment of discomfort glare shows any temporal consistence (second phase).

The first phase implies in first instance an explorative methodology. Explorative interviews in the field (e. g. in buildings where discomfort glare is a frequent problem) as well as research studies in cognitive, environmental and industrial psychology lead to the elaboration of a heuristic model of cognitive schemata activated in glare environments. The heuristic model allows to deduct hypotheses about cognitive procedures and human reaction.

In the second phase the hypotheses are tested under real office conditions, at different sites and different lighting conditions, with large number of subjects and with different shading systems.

When the cognitive appraisal model is stable, in a third phase the allocation of physical glare data to human appraisal can start and might lead to a reliable index scheme which will permit planning of buildings in which user discomfort glare can be avoided.

In the research about discomfort glare the cognitive gap still exists. This explains that the results from DGI do not match with user assessment and the reported qualitative factors which seem to have an influence. Regarding the importance to provide a comfortable working environment the efforts to be spend into the outlined research seems to pay back without any doubt.