L. Marletta, G. Evola[12] and F. Sicurella1

1 Department of Industrial Engineering, University of Catania
Viale A. Doria, 6 — 95125 Catania (Italy)

* Corresponding Author, gevola@unict. it

Abstract

In conventional desiccant and evaporative air-conditioning systems (DEC) employing solid sorptive materials, the regeneration stage is powered by thermal energy. The use of solar energy to assist the regeneration process is obviously a good solution to reduce the primary energy consumption. When looking at the problem from a second law perspective, namely by assessing the overall irreversibility production and the exergy efficiency, this practice is not always sufficient to ensure better performance than a conventional air-conditioning system, especially when high regeneration temperatures are required.

In the literature, new concepts for open cycle desiccant cooling systems have been recently proposed, based on the adoption of enthalpy recovery wheels or heating coils working as a desuperheater on the cooling cycle. In this paper a comparative analysis between the conventional DEC systems and these new concepts is carried out both under a first and a second law perspective, in order to understand to what extent the novel thermal cycles may help reducing thermodynamic irrationalities and irreversibilities, taking into account the possibility of assisting regeneration by means of solar energy. The effects of the parasitic consumption of fans and pumps will be accounted for, too. The results will help understanding possible inefficiencies and improvements of the thermal cycles mentioned above, and highlight the fundamental role of the solar energy.

Keywords: desiccant wheel, solar cooling, exergy, second law performance,

1. Introduction

An air-conditioning system based on the use of desiccant wheels requires thermal energy for the reactivation of the desiccant after the dehumidification process; to this aim the wheel is crossed by a hot air flow, whose temperature depends on the materials and the operating conditions of the system. Thanks to new materials, the regeneration temperatures are around 70 °C, thus the use of solar energy may be suitable and produce relevant energy savings.

Fig. 1 shows an air-conditioning system provided with a desiccant wheel for dehumidification, according to the most common and performing scheme, together with the thermal cycle represented on a psychrometric chart. The air to be supplied to the conditioned space is taken from outdoors, whereas the regeneration flow is taken from the space to be conditioned. The pre-cooling process (EB) is necessary in hot and humid climates, where the only desiccant wheel may not be able to reach the desired inlet humidity ratio. More details concerning the psychrometric processes involved in the system can be found in [1, 2, 3].

Fig. 1. Lay-out and thermal cycle of a standard DEC system