Conclusions - Low Energy Cooling for Sustainable Buildings - page 251

completely avoided if the control strategy is adapted to the available solar thermal
temperature levels. The COPs were between 0.5 and 1.0, increasing with lower regen-
eration temperature. This gives an added advantage of reducing auxiliary heating or
better use of auxiliary cooling.
New developments in diffusion-absorption cooling and liquid desiccant use were
presented for the low cooling power range below 10 kW. The main challenge in the de-
velopment of diffusion-absorption chillers is the design of an indirectly heated bubble
pump, which operates under slug flow conditions. Stable flow conditions can achieved
with convection and nucleate boiling with lifting ratios between liquid and vapour of
4-5. Under nucleate boiling conditions, heat transfer is optimized and the highest lift-
ing ratio is obtained. However, as long as the required solution flow is sufficient, lower
driving temperature differences between tube wall and fluid boiling temperature are
advisable to reduce the external generator temperature. On the low partial pressure
side, the main difficulty is to achieve a good performance in the falling film evaporator
and absorber. Unequal liquid distribution into the tubes of the evaporator led to low
evaporation rates in the first prototypes, which could be subsequently improved by
changing the construction between the evaporator top plate and tubing inlet. Much
experimental experience was gained with heat exchanger constructions. Both shell-
and-tube and plate heat exchangers did not give satisfactory heat recovery results for
the solution heat exchange, as solution flow rates are very low. The best results were
obtained for coaxial heat exchangers with heat recovery factors of up to 92% (weak
solution). The rather slow diffusion rate of the refrigerant into the auxiliary gas atmo-
sphere increases with lower total system pressure, improving the whole performance
of the system. With the last prototype, COPs of nearly 0.4 can be achieved.
A liquid desiccant cooling unit was developed and tested, providing sensible cooling
of the fresh air in small ventilation units. The process is based on exhaust air drying in a
spray-cooled heat exchanger absorber unit usingLiCl or CaCl salt solutions. The nearly
isothermal drying process of the exhaust air is then followed by a further spray-cooled
heat exchanger, where heat is transferred from the warm supply air to the humidified
cool exhaust air. For a small system with a volume flow of only 200 m 3 h − 1 a cooling
power of nearly 1 kW was obtained. This system offers interesting applications in the
residential building sector.
The role of computer simulation not only for the design, but also for the operation
of complex energy plants and buildings was analysed. Especially for solar thermal
cooling plant design, it is very important to know the details of the cooling load
time series. The correlation between the cooling machine power and required solar
thermal collector area is very weak and varies by a factor of 10 depending on full
load hours and location. There is a much better correlation between the collector
area and cooling energy, varying for example at locations in Spain between 3 and
6m 2 per megawatt hour of building cooling energy. A good cost-effectiveness can be
obtained if the system is operated with an optimized control strategy for partial load
conditions, which allows a reduction of the solar collector driving temperature and as