Investigation of transcritical CO2 vapour compression systems by simulation and laboratory experiments

Abstract

The thesis presents the use of simulation tools and experimental data for investigation of various aspects of the transcritical CO2 vapour compression cycle. This is done through three papers, the first on design and simulation of CO2 /air heat exchangers, the second and third focusing on how to model and optimise the system using a general solver for constrained non-linear problems (NLPQL).

Both models has been validated by experiments on automotive air-conditioning heat exchangers, and on two independent residential air-conditioning systems.

These models, together with basic thermophysical and thermofluid property libraries, have proved to be a very useful contribution in analysing various aspects of the transcritical CO2 cycle, and for designing new heat exchanger concepts.

Simulations on a small air-conditioning circuit, using variable speed compressor and two different control strategies, (1) low pressure receiver with active high pressure control and (2) thermostatic expansion valve with superheat control, was done. The results show how the system with active high pressure control and a receiver (1) can obtain 60% higher efficiency at off-design operation compared to the superheat controlled system (2).