Experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator
Introduction
In recent years, much research (Grossman, 2002, Li and Sumathy, 2000) has been carried out on refrigeration systems driven by solar energy, which achieves conservation of conventional energy and a reduction of environmental pollution. Although many kinds of refrigeration systems have been developed and studied (Lazzarin et al., 1993, Ileri, 1995, Yeung et al., 1992, Chinnappa et al., 1993), some disadvantages in traditional absorption refrigeration systems have not yet been overcome. These include the complexity and the high manufacturing cost of the system (including the solution pump and the compressor and so on), the strict demand on the heat supply in both quality and quantity, and the low efficiency of solar-energy utilization.
Because of difficulties associated with the solar-heating system’s long-term operation at a steady state, a refrigeration cycle co-driven by both heat and electrical energy was proposed. A compressor is used to maintain stable energy input to the absorption refrigeration system (Chen and Hihara, 1999), though this still consumes energy. Many systems have been developed for high temperature (e.g. above 100 °C) heat sources. Many specially developed products, such as Yazaki WFC-S water-fired single-effect absorption chillers using bubble pumps or mechanical solution pumps, can be driven by hot water between 70 °C and 95 °C from a solar-heat source.
The pump-free absorption refrigeration system without solution pumps is attractive, since this system is driven by a low-temperature heat source. The simpler system configuration associated with the pump-free absorption refrigeration system makes the design of a small-scale system for domestic applications possible. Therefore, the principal objective of this study is to propose a new design of a compact pump-free (without the generator pump, the evaporator pump and the absorber pump) chiller and to evaluate its performance compared to traditional absorption refrigeration systems. The target refrigeration capacity is 4 kW.
Section snippets
Experimental setup
A schematic of the experimental apparatus is shown in Fig. 1. The apparatus consists of a simulated solar-heat collector, a lunate thermosiphon elevation tube, a gas–liquid separator, a second generator, a condenser, a falling-film absorber, a falling-film evaporator, a solution reservoir and a heat exchanger. All the components are made of stainless-steel sheets except the heat-exchange devices, which are made of copper.
The second generator made of coiled copper pipes of 10 mm OD (outer
Elevation tube and second generator
In the lunate channel, a single-phase solution in a subcooled state is first heated at the bottom of the elevation tube and then small vapour bubbles begin to appear on the surface of the channel wall, especially at sharp-angled places in the lunate channel. The subcooled boiling of the solution occurs during this process. As the solution is being heated continuously by the hot water outside the lunate channel, the vapour bubbles grow and merge with each other, resulting in a bubble flow and a
System performance
To evaluate the performance of the newly designed pump-free absorption refrigeration system, the coefficient of performance (COP) is used and defined aswhere Q0 is the refrigeration capacity, Qg is the heat input (the heating load of the lunate thermosiphon elevation tube with the second generator) and W is the total electrical energy consumed by the hot-water pump, the cooling-water pump and the chilled-water pump. Q0 and Qg can be determined by
Conclusions
A compact solar pump-free lithium bromide absorption refrigeration system, which is equipped with a second generator, a falling-film absorber, a falling-film evaporator and an efficient lunate thermosiphon elevation tube, has been experimentally examined. Based on the experimental investigations of the system with a second generator, the following conclusions can be drawn:
- (1)
The COP is increased by 48.5%. The temperature of the heat source is reduced (the minimum driving temperature is only 68 °C).
Acknowledgements
This work was supported by National Natural Science Foundation of China through Grant Nos. 50176036 and 50276048. The authors thank Dr. Zhang Lianying for her good advice about performing the experiments.
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