Solar driven steam jet ejector chiller
Introduction
The rapid growth of energy demand and the changing situation in the energy sector with more awareness on the environmental impact have lead to a higher interest in the utilisation of renewable energies. Due to the fact, that solar insolation generally matches the cooling load profile of buildings solar cooling techniques have been in the focus of scientific interest in the last years. The development of solar cooling has mainly been focused on solar driven “sorption” cooling processes as, for example, absorption chillers, adsorption chillers and dessicative and evaporative cooling (DEC). These technical solutions are complicated in design and in construction and there is still a development demand to optimise these systems.
An interesting alternative are steam jet ejector chillers (SJEC), which can be driven by solar energy, too. A SJEC is simple in design and reliable in operation. It has a good part load behaviour, which leads to a high yearly mean coefficient of performance (COP). Water can be used as refrigerant and furthermore as only working fluid in the whole system. At present, solar-driven SJECs have been only studied theoretically or practically at small test rigs. The investigations have mainly been focused on SJEC with conventional refrigerants as ‘Freon’ or hydrocarbon. Experimental investigations were carried out in refs. [1], [2], [3], [4], [5], [6], [7] while theoretical analyses and simulations of SJEC with conventional refrigerants were presented in [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. The solar SJEC is assessed to be reliable in operation because of its simplicity in design and in construction, but it only has a moderate nominal COP value. The use of conventional refrigerants enables the operation of the SJEC with heat at a low temperature level, which can be provided by, for example, flat plate collectors.
In [22], [23], [24], [25], [26], water is considered as refrigerant and working fluid. The advantages of water as refrigerant are its completed harmlessness and very good thermal properties. The necessary relatively high temperatures for steam generation can be reached by vacuum tube or parabolic trough collectors. If water is also used as heat transfer fluid, an ‘open’ system can be designed without hydraulic separation between solar collector, SJEC and chilled water supply system.
Fraunhofer UMSICHT has investigated [27] the concept of a solar driven SJEC at a small test rig with a cold capacity of 1 kW. The test rig was operated in summer 2005 outside and in winter 2005/06 inside at a solar collector test facility at the University of Bochum in Germany. The objectives of the project are to deliver the necessary experiential knowledge for a future pilot plant. Main subject of the investigation are the operational characteristics while changing the operational conditions and an analysis of the economical aspects. This paper describes the operational behaviour of a solar SJEC and possible efficiencies of the system.
Section snippets
Description of the test rig
The test rig consists of three parabolic trough collector (PTC) modules type roof mounted trough (RMT), manufactured by Solar Industrial Technology Corporation, USA, and a small SJEC for cold water generation with a designed cold capacity of 1 kWth.
The PTC type RMT is a special development for on-roof installation. It is smaller and more light-weight than the IST-PT, which is proposed in [24] as solar collector for a SJEC. This kind of solar collector is new and the performance data of the
Calculated annual mean efficiencies and annual mean COP values
To appraise the operational characteristics of a solar driven SJEC ascertained before the annual mean collector efficiency, the annual mean COP of the SJEC and the annual mean total efficiency of a solar driven SJEC was calculated by an annual calculation based on weather data from the Meteonorm [31] data base as input values of the calculation. The solar SJEC was modelled and quasi-dynamic simulations with an hourly time step were made over 1 year for the locations: Essen in Germany, Toulouse
Summary and prospects
The solar driven SJEC is an interesting device for solar cooling. Previously, solar driven SJECs with water as refrigerant and working fluid have only been investigated theoretically. At the moment, there are neither demonstration plants nor prototypes and the operational behaviour of such a system is unknown. Due to the fact that SEJC are simple in construction they have a high cost reduction potential. Furthermore, water can serve as only working fluid in the whole system, which will
Acknowledgements
The author Clemens Pollerberg thank the Deutschen Bundesstiftung Umwelt DBU, Germany, for sponsoring the project under AZ 22692 [27], which enabled this work. Furthermore, the author thank the company Paradigma, Germany, for the kind donation of a VTC.
The author Ahmed Hamza H. Ali thank the Alexander vonHumboldt Foundation, Germany for fellowship grant during this work.
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