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Development of Water-Lithium Bromide Low-Temperature Absorption Refrigerating Machine

A low-temperature absorption refrigerating machine with an output temperature of -5 °C.

Fig.1 A low-temperature absorption refrigerating machine with an output temperature of -5 °C.

The low-temperature absorption refrigerating cycle.

Fig.2 The low-temperature absorption refrigerating cycle.

1. Summary

Absorption refrigerating machines that use water and lithium bromide as the working medium can be passively driven by heat and are an effective equipment of cutting the peak in electric power consumption resulting from the use of air conditioners in summer months. As a result, they are widely used in large-scale air conditioning system. The refrigerant used in these machines is water, which obviously has little environmental impact and is safe to humans. These machines can also contribute to energy savings because they can make effective use of waste heat. As a result, refrigerating machines of this sort should be able to provide environmentally friendly solutions in a wide range of fields. However, since these chillers use water as their refrigerant, the minimum temperature of the cold water they produce is limited to about 5°C, and as a result they have so far found few practical applications apart from air conditioners.

To make these chillers suitable for a wider range of applications by reducing the temperatures they produce, we have therefore developed a technique for reducing the evaporation temperature involving the use of a mixed refrigerant, and we also developed a new cycle using with this mixed refrigerant. As a result, we have successfully produced a water/lithium bromide low-temperature absorption refrigerating machine that is the first of its type in the world to achieve a temperature of -5°C (Fig.1)

2. Technical details

When water is used as the refrigerant, it is normally impossible to achieve cooling down to -5°C. To achieve this, it is necessary to prevent the refrigerant from freezing and to increase the pumped temperature difference (between the desired output temperature and the outside air temperature). The techniques developed to address these problems are as follows:

(1) Controlling the concentration of the mixed refrigerant to prevent freezing
To reduce the extracted temperature from the conventional value of 5°C to -5°C, the evaporation temperature has to be reduced to about -10°C. To prevent the refrigerant from freezing, we thus decided to use a mixed refrigerant in which the absorbent lithium bromide is added as a freezing-point depressant. By evaluating the physical properties of this mixed refrigerant, we found that it is possible to prevent freezing if it is kept at a concentration of at least 15%. However, the evaporative heat transfer coefficient falls sharply as concentration increases, so the refrigerant concentration must be precisely controlled to obtain stable performance and prevent freezing. We therefore developed a technique to measure the concentration accurately and to control it precisely by using this measured signal.

(2) Two stage absorption cycle (pumped temperature difference enlargement technique)
In an absorption chiller, heat is pumped by exploiting the difference in saturation temperatures of water (the refrigerant) and an aqueous solution of lithium bromide (the absorbent). This difference increases as the aqueous solution becomes more concentrated, so if concentration of aqueous solution in the absorber is increased, it is possible to increase the pumped temperature difference between the cold temperature generated by the chiller and the outside air temperature (32°C). However, since an aqueous solution of lithium bromide tends to crystallize at higher concentrations, there is a limit to how concentrated it can be made. Therefore, as shown in Fig.2, we have developed a two-stage absorption cycle whereby heat is pumped in two stages by combining two sets of evaporators and absorbers, and a large pumped temperature difference is achieved by using solutions of low concentration comparable to those of conventional equipment.

These techniques were developed based on the results of research conducted as part of the Eco-Energy Project run by the ECCJ (Energy Conservation Center, Japan) and by NEDO (the New Energy and Industrial Technology Development Organization).

3. Conclusion

We have devised a water/lithium bromide low-temperature absorption chiller by developing these techniques in combination with other developed methods, such as a method for improving the evaporation and absorption performance of the refrigerant at low pressure, a method for controlling the temperature of intermediate chilled water, and a method for mixing the water refrigerant into the low temperature mixed refrigerant.

We began shipping the first machine in July 2000 for use in coolers for food processing, and after that the number of units shipped increased at a steady rate. This machine offers an energy-saving means of using factory waste heat and cogeneration waste heat to implement process cooling and cold storage, and reducing the pumping power required for water circulation due to the large temperature gaps. It should therefore make an important contribution to protecting the earth's environment.


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