Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Final Activity Report Summary - DESICCANT COOLING (Dehumidification and cooling driven by solar/waste heat using liquid desiccants)

Solar assisted air conditioning systems using liquid desiccants like LiCl and CaCl2 represent a promising option to decrease high summer energy demand caused by electrically driven vapour compression machines. The main components of liquid desiccant systems are absorbers for dehumidifying and cooling of supply air and regenerators for concentrating the desiccant. However, high efficient and validated reliable components are required and the design and operation have to be adjusted to each respective building design, location, and user demand.

In this project, both theoretical and experimental investigations have been carried out. For the experimental investigations, three prototypes for regenerators using low temperature heat from solar collectors or cogeneration systems to regenerate the desiccant solution have been developed. The prototypes are designed as plate heat and mass exchangers. Due to corrosion problems caused by the salt solution, a plastic construction consisting of polypropylene twin-walls was selected. The desiccant solution is distributed over the plates with very low flow rates. For this, a special distribution system again made from plastic was developed. In order to enable a good distribution of the solution over the exchanger plates, the plate's surfaces are covered with fabric as a wicking material. The prototypes have been extensively tested in the laboratories of the University of South Australia and Kassel University and evaluated regarding their regeneration performance. Apart from the thermodynamic optimisation, a main focus of the developments and the tests was represented by stability, sealing and flow distribution behaviour.

Numerical models for all components of the liquid desiccant system have been developed and integrated into the simulation environment TRNSYS. The models for the absorber and the regenerator are considering simultaneous heat and mass transfer and have been validated using the experimental investigations as well as data published in the literature. Two approaches, a single plate absorber and a multi-plate regenerator have been used for the validation. However, for both, the uncertainties of the tests due to non-perfect flow distribution within the components are the main limiting factor for the validation. Both, testing and simulation have been demonstrating the feasibility of the proposed prototype design to re-concentrate diluted solution of a liquid desiccant air conditioning system. Parameter variations have been carried out in order to identify suitable flow rates of all fluids and required driving temperatures within the regenerator.

A system model consisting of a single-family building, a liquid desiccant air conditioning system and a solar thermal system for domestic hot water and space heating and the provision of regeneration heat has been implemented. Simulation studies for different locations within Australia and Germany showed that with a liquid desiccant air conditioning system, comfort conditions can be assured throughout the year for all locations without tropical climate. With the application of numerical optimisation algorithms, suitable control strategies of the liquid desiccant air conditioning system have been investigated. The investigations proved that variable air flows depending on indoor and ambient conditions are necessary to provide sufficient cooling for the building at all times and reduce the electricity demand for fans and pumps. In case of a surplus of solar thermal energy even during cooling operation, a maximum reduction of air flow rates can be achieved with decreased inlet air temperatures. However, for all control strategies, a conflict between indoor temperature and indoor humidity exists.

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Kurt-Wolters-Str. 3
D-34125 KASSEL
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