Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

Abstract

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.

The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.

Introduction

In recent years, considerable attention has been paid to adsorption refrigeration systems, which are regarded as environmentally friendly alternatives to conventional vapour compression refrigeration systems, since they can use refrigerants that do not contribute to ozone layer depletion and global warming. In addition, the adsorption systems have the benefits of simpler control, no vibration and lower operation costs, if compared with mechanical vapour compression systems and, in comparison with the absorption systems, they do not need a solution pump or rectifier for the refrigerant, do not present corrosion problems due to the working pairs normally used, they are less sensitive to shocks and to the installation position (Wang et al., 2006) and they could be operated with no-moving parts (Wang et al., 2002). Furthermore, refrigeration as a solar energy application is particularly attractive because of (i) the non-dependence on conventional power and (ii) the near coincidence of peak cooling loads with the solar energy availability.

Despite their advantages, the adsorption refrigeration systems present some drawbacks, such as low COP, low SCP, high weight and high cost. So, in order to overcome these inconveniences, various approaches have been undertaken, such as improvement of heat and mass transfer in adsorbent beds, enhancement of the adsorption properties of the working pairs, design and study of different kind of cycles and improvement of regenerative heat and mass transfer between beds. However, the widespread use of the adsorption refrigeration systems is still limited by the technical and economic constraints. For this reason, the research activities in this field are still increasing to overcome these problems.

During recent decades, several solar adsorption refrigeration units were successfully tested with different combinations of adsorbents and adsorbates. The most studied pairs in this field are activated carbon/ammonia, activated carbon/methanol, zeolite/water and silica gel/water. These investigations include the research on ice-making and congelation purposes (Tchernev, 1978, Pons and Guilleminot, 1986, Grenier et al., 1988, Critoph, 1994, Sumathy and Zhongfu, 1999, Wang et al., 2000, Boubakri et al., 2000, Buchter et al., 2003, Hildbrand et al., 2004, Li et al., 2004, Khattab, 2004), refrigeration for food and vaccine storage (Critoph, 1999, Anyanwu and Ezekwe, 2003, Lemmini and Errougani, 2005, González and Rodríguez, 2007) and air-conditioning applications (Wang, 2001, Saha et al., 2001, Lu et al., 2003). The literature shows that most of these systems were intermittent, in which adsorption and cooling production can only be achieved during nights.

On the other hand, the solar collector/adsorber is one of the most important elements of any solar adsorption refrigeration system that needs more investigations. Indeed, several reports have mentioned its importance, i.e. the low thermal conductivities and poor porosity characteristics of adsorbents have as effect, the bulky collector/generator/adsorber component and, thus, its excessive heating capacity, leading to rather low thermal COP (Anyanwu, 2003). The adsorptive systems’ development is still limited by the adsorber-solar collector component cost (Leite et al., 2004) and recently, it has been reported that a potential barrier to the commercialization of the solar adsorption refrigeration systems is large collector costs (Baker and Kaftanoğlu, 2007). Nevertheless, collector costs can be reduced by increasing the adsorption cycle’s COP or decreasing the operating temperature of the collector (Baker, 2007). In this context, parabolic trough collectors (PTCs) seem to be a reasonable alternative, since they could achieve a high cycle’s COP due to their high efficiency. They are the most developed and deployed type of solar concentrators (Badrana and Eck, 2006) and their technology is the most verified-solar technology through deployment and construction testing (Price and Hassani, 2002). They have been used in various applications, such as steam generation (Kalogirou, 1996, Zarza et al., 2004), seawater desalination (Kalogirou, 1998), hot water production (Kalogirou and Lloyd, 1992, Valan Arasu and Sornakumar, 2007), etc. Moreover, Bird and Drost (1982) have recommended that the PTC concept should receive the highest priority for commercial development for low temperature (65–177 °C) solar process heat applications. Even so, most of the studies conducted on the solar adsorption cooling systems have been achieved with either flat plate or evacuated tube collectors, whereas little attention has been devoted to concentrating collectors, in particular the PTCs.

This paper presents the study of a novel system in an attempt to overcome the intermittent character of solar adsorption refrigeration systems, and test the applicability of PTC to these systems with the aim to improve their performance. Thus, a numerical investigation is performed to describe a two-bed continuous adsorption refrigeration cycle. A parametric analysis is carried out to evaluate some optimal design and operating values of the system.