The Compression/Absorption Heat Pump Cycle - System Simulations

Abstract: From the results presented in this thesis it is clear that the compression/absorption heat pump cycle offers several advantages over other types of heat pumps. In many cases it gives a higher or equally high COP compared with the single-fluid compression heat pump but also the smaller SCD (i.e. size of the compressor) found for this cycle is an advantage. The main advantage for the compression/absorption cycle over other heat pump cycles is, however, its greater flexibility resulting from the use of a mixture as working fluid.

There are two applications in particular where the compression/absorption cycle is advantageous: to replace CFC 12 cycles in traditional heat pump applications; and where temperatures are too high for closed cycle heat pumps available today. Depending on the external conditions, the compression/absorption cycle can be varied in order to be better suited for a particular application. The simplest compression/absorption cycle, which is studied in detail in this work, is best suited to situations where the heat sink and heat source are non-isothermal and have temperature gradients of similar size. Compared with a single-fluid compression cycle, the performance of the compression/absorption cycle is improved as the external temperature gradients increase.

Computer simulations of the compression/absorption cycle showed that for every application, a value of the internal temperature gradient (DTabs) which maximizes the COP of the cycle can be determined. The value of this optimum DTabs changes with the external conditions. It is determined by the size of the compressor and pump demands and the heat loss in the absorber caused by the subcooled liquid solution entering the absorber. It is also found that a maximum pressure in the cycle is always advantageous for the performance with regard to both COP and SCD.

Although the compression/absorption cycle is usually only put forward as an alternative to compression cycles for applications involving large external temperature gradients, this study has shown that this cycle is also interesting for applications where the external temperature gradients are small. The more isothermal the conditions are, however, the greater is the need to optimize the parameters within the cycle in order to maximize its performance.

In this work the binary mixture NH₃/H₂O was most extensively used. As a ternary mixture was expected to improve the performance of the cycle, a comparison of the performance using NH₃/H₂O and NH₃/H₂O-LiBr was done. An improvement of up to 10% was found for a ternary mixture with 60% salt.