Mario Motta, Hans-Martin Henning

Fraunhofer Institute for Solar Energy Systems (ISE)
Heidenhofstr. 2, 79110 Freiburg / Germany
mario@jse. fraunhofer. de, hansm@ise. fraunhofer. de

1 Introduction

In the past decade, growing environmental concerns and consistent effort in research and product development caused a rapid growth of active solar system’s market. In spite of a significant and growing market penetration rate, the main obstacle preventing the broad application of solar thermal collectors beyond their use in domestic hot water production has been the seasonal mismatch between heating demand and solar energy gains. A way to overcome the problem consists in exploiting solar thermal energy for air-conditioning of buildings during summer, i. e., sensible cooling and air dehumidification. The great advantage for this kind of application is that the seasonal cooling loads coincide with high solar radiation availability.

Buildings are one of the dominating energy consuming sectors in industrialized societies. In Europe about 30 % of primary energy consumption is due to services in buildings. During the last decades in most European countries the energy consumption for air conditioning purposes was increasing remarkably and it is expected that (i. e., in comparison to 1996 for small air-conditioners) the primary energy consumption increases by a factor of 4 in 2020 [1]. Moreover the concern for electricity peak demand increased recently, pushing decision makers to look at new technological solutions for air-conditioning. In this conditions the use of thermal energy, and in particular solar, for air-conditioning in buildings has gained a new interest.

Among the cooling technologies which raised increasing attention during the last fifteen years, there are desiccant and evaporative cooling systems. In desiccant and evaporative cooling (DEC) systems the potential of sorption materials is used for dehumidification of air in an open cycle. In this type of air conditioning systems the dehumidification effect is used for two purposes: to enhance the evaporative cooling potential at given environmental conditions and to control the humidity of ventilation air. However, a pure desiccant cycle using state-of-the-art technology is not able to provide desired supply air temperature and humidity states under all conditions. Particularly in hot-humid climates the desiccant cooling cycle has limitations. Therefore, employing standard technologies, a combination of a desiccant cooling air handling unit with a cold backup system is needed in those cases.

In this work a novel DEC concept is presented. The new system, indirect Evaporative COoled Sorptive heat exchanger (ECOS) is intended to overcome the thermodynamic limitations of standard DEC systems and provide a valuable option for air-conditioning applications without the need of a back-up system. Moreover the new concept can be implemented for small capacity plants (about 200 m3/h) overcoming a traditional restriction of standard DEC plants.

During the work reported on this paper a simplified mathematical model of the ECOS has been developed. The mathematical model has been then implemented in a software tool used to study the optimum system’s operation parameters. The performance of the sorptive cooled heat exchanger for typical air-conditioning applications has been investigated.

In particular the new system offers the possibility to use low temperature heat, e. g., heat from flat plate solar collectors, for air-conditioning, without the need of a conventional refrigeration system even under climatic conditions with high humidity values of the ambient air (e. g., Mediterranean or tropic climate).