Renewable and Sustainable Energy Reviews
Review on solar-driven ejector refrigeration technologies
Introduction
Refrigeration is available in the industrialised countries through the availability of electricity but is not readily available in the major part of the world. An alternative solution for this problem is solar energy, available in most areas and it represents a good source of thermal energy; the combination of solar energy with absorption, adsorption, desiccant, and others technologies less studied for refrigeration are being investigated and improved around the world.
The production of cold has applications in a considerable number of fields of human life, for example the food processing field, the air-conditioning sector, and the conservation of pharmaceutical products, etc. The conventional refrigeration cycles driven by traditional vapour compression in general contribute significantly in an opposite way to the concept of sustainable development. Two major problems have yet to be addressed:
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The global increasing consumption of limited primary energy: the traditional refrigeration cycles are driven by electricity or heat, which strongly increases the consumption of electricity and fossil energy. The International Institute of Refrigeration in Paris (IIF/IIR) has estimated that approximately 15% of all the electricity produced in the whole world is employed for refrigeration and air-conditioning processes of various kinds, and the energy consumption for air-conditioning systems has recently been estimated to 45% of the whole households and commercial buildings [1], [2]. Moreover, peak electricity demand during summer is being re-enforced by the propagation of air-conditioning appliances.
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The refrigerants used cause serious environmental problems: the traditional commercial, non-natural working fluids, like the chlorofluorocarbures (CFCs), the hydrochlorofluorocarbures (HCFCs) and the hydrofluorocarbures (HFCs) result in both ozone depletion and/or global warming. It is well known that the stratospheric ozone layer acts as a shield against harmful ultraviolet solar radiation. During the past decade, researchers have discovered that chlorine released from synthetic CFCs migrates to the stratosphere and destroys ozone molecules causing health hazards [3]. Since the protocol of Montreal in 1987, international agreements have been signed to reduce the emissions of these refrigerants [4]. European Commission Regulation 2037/2000, which has been implemented on 1 October 2000, treats the whole spectrum of control and phase-out schedule of all the ozone-depleting substances. It is indicated that till 2015 all HCFCs will be banned for servicing and maintaining existing systems [5].
With developing technology and the rapid increase in world population, the demand for energy is ever increasing. The growing population and fast depleting reserves of fossil fuels have led scientists in the fields of engineering, meteorology and industry to pursue the development and use of renewable energy resources. In the few past decades, researchers have focused on renewable energy sources, such as solar and wind energies. With the use of solar energy, usage of conventional energy sources and its peak demand will be reduced.
Accurate detailed long-term knowledge of the available global solar radiation is of a prime importance for the design and development of solar energy-conversion systems such as ejector refrigeration systems. The possible use of solar energy as the main heat input for a cooling system has led to several studies of available ejector refrigeration technologies.
The ejector is a key component of the ejector refrigeration system. It should be noted that the ejector could increase the pressure without consuming mechanical energy directly, which are the main characteristics of ejectors. Due to these characteristics, applying an ejector may be simpler and safer technologically than applying mechanical devices, which can increase pressure, such as a compressor, pump, etc. Besides the very simple configuration ejectors, the systems combining ejectors and other devices are also very simple.
In the past we have presented reviews on solar adsorption refrigeration, thermal collectors and their materials, Alghoul et al. [6], [7], [8] in the hope that this information will be useful to interested readers. We would like now to present a review on solar-driven ejector refrigeration technologies which have potential to be used in many parts of the world. A number of solar-driven ejector refrigeration systems and research options are provided and discussed. These concepts and interesting points of each study were linked and grouped to other related studies and described as overview summaries. It is hoped that this paper should be useful for any newcomer in this field of refrigeration technology.
Section snippets
Solar cooling paths
The solar cooling system is generally comprised of three sub-systems: the solar energy conversion system, refrigeration system, and the cooling load. The appropriate cycle in each application depends on cooling demand, power, and the temperature levels of the refrigerated object, as well as the environment. A number of possible “paths” from solar energy to “cooling services” are shown in Fig. 1.
Starting from the inflow of solar energy there are obviously two significant paths to follow; solar
Background and operation principles of ejector
The ejector, which is the heart of the jet refrigeration system, was invented by Sir Charles Parsons around 1901 for removing air from a steam engine’s condenser. In 1910, an ejector was used by Maurice Leblanc in the first steam jet refrigeration system [9]. This system experienced a wave of popularity during the early 1930s for air conditioning large buildings [10]. The ejectors are an essential part in refrigeration and air conditioning, desalination, petroleum refining and chemical
Working fluids
In this section, the criteria for working fluid selection for the ejector refrigeration systems are provided. The choice of the appropriate working fluid (refrigerant) is one of the most important parts in the design of the ejector refrigeration system. The appropriate refrigerant should yield good performance in the selected operating ranges.
Table 1 lists some fluids commonly used for experimental studies. The following requirements should be met:
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The fluid should have a large latent heat of
Various designs of solar-driven ejector refrigeration systems
When any system is designed, the engineers seek to find a solution, which gives maximum efficiency with minimum cost and to reduce the solution time. The optimum system is often not easily found and a lot of calculations and/or simulations are required in order to decide which combination gives the best financial benefit. There are several ongoing attempts to improve the performance of solar-driven ejector refrigeration systems using single, multi-stage ejectors or hybrid system where the
Conclusions
This paper describes a basic background and development on solar-driven ejector refrigeration technologies. One may conclude that solar-powered ejector refrigeration technologies could be used for producing a wide range of temperatures of cold. They are attractive technologies that not only can serve the needs for refrigeration, air-conditioning applications and ice making, but also can meet demand for energy conservation and environment protection. Comparatively, absorption systems are more
Acknowledgment
The authors would like to thank the Solar Energy Research Institute (SERI), University Kebangsaan, Malaysia, for support.
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