O. Ayadi1*, J. Doell2, M. Aprile1, M. Motta1, T. Nunez2

^ip. Energia, Politecnico di Milano, Milan, Italy
2Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany
* Corresponding Author, osama. ayadi@polimi. it

Abstract

Hot weather days cause often large fresh milk defect in southern developing countries dairy farms as well as in rural areas where there is a scarcity of energy sources that could run cooling equipments. Consequently, the possibility to use solar cooling starts to be considered an attractive solution. The aim of the work presented is to develop an innovative solar thermally driven cooling concept, to be used for received fresh milk in a dairy factory in the city of Marrakech, Morocco. The work has been carried out in the framework of the EC co­funded project (i. e., MEDISCO). The system is intended for refrigeration at 5°C in hot climates, and is composed of: medium temperature collector, single effect water ammonia absorption chiller, cold storage. The peculiarity of the configuration is the high temperature difference between the chilled refrigerant temperature, about -5 °C (in consequence of using an ice storage) and the condenser temperature (ambient temperature, which could exceed 35 °C). In these conditions, the absorption chiller must be driven by a medium temperature heat source (i. e. parabolic trough collector). Moreover, the selected chiller is directly air cooled, and has the main advantage of having no water consumption, low maintenance work and no legionella problems. . TRNSYS simulations were performed to select the appropriate system configuration, size of the solar collectors’ field and the storage, heat transfer fluid and control strategy. A summary of the results is given.

Keywords: solar cooling, solar refrigeration, absorption, dairy industry.

1. Introduction

The choice of a proper solar assisted refrigeration system for hot climates, has to take into account the boundary conditions of the site and the application — i. e., level of temperature. The aim of the work presented it has been to develop an innovative solar thermally driven cooling concept, to be used for received fresh milk in a dairy factory in the city of Marrakech, Morocco. The work, carried out in the framework of the EC co-funded project MEDISCO (MEDiterranean food and agro Industry applications of Solar COoling technologies), investigated through simulations the optimum system configuration, size of the solar collectors field and the storage, heat transfer fluid and control strategy. Further a description of the system concept, industrial application load profile, model employed and simulation results is given.

2. System concept

The current work focused on a technological solution selected through a methodology based on thermodynamic analysis, as suggested in [1,2]. Following the path drawn by Henning, the combination of high temperature lift, single-effect absorption chillers, and medium temperature collectors is the only choice for solar refrigeration (low temperature cold production) under the given environmental conditions (Marrakech, Morocco).

The system concepts described in this section have been newly designed for applications under the aforementioned conditions for refrigeration in the food and agro industry. Under high ambient air temperatures (e. g., higher than 35°C) in which hardly a wet cooling tower can be employed, due to high wet bulb temperatures or no availability of water, a high temperature lift system is necessary. This is even more valid if a low cold production temperature is needed e. g., in case of ice production for cold energy storage. Under those conditions only a single-effect machine can be used if driving temperatures up to approx. 200°C are available [1,2]. Therefore if the heat source is a solar system, a collector field with optical concentration will be necessary. Tracked collector systems employing optical concentration require a climate with a high fraction of direct solar radiation on the global, which is the case for the Moroccan application object of this work. These conditions correspond very much to the needs in hot, arid regions in which fresh water is costly or a scarce commodity. Further a short description of a system based on these technologies is given.