Experimental investigation of electrical domestic heat pumps equipped with a twin-stage oil-free radial compressor

The world energy consumption has dramatically increased since the end of the World War II. The main share of this energy comes from fossil fuel combustion. The worldwide domestic sector consumes about 30% of the global energy supply and the biggest share of that is dedicated to space and water heating. In this context where sustainable energy consumption is required to limit our impact on the environment and the climate, energy efficiency is a key factor to successfully transition to energy sobriety. Electrically-driven heat pumps are known to be a key technology to increase our energy efficiency. More efficient, more compact, more silent heat pumps, built with less raw material, and using lower refrigerant charges are needed. In electrically-driven heat pumps, the compression process is responsible for most of the energy losses. However, for decades now, the heat pump performance has been stagnating, mainly because of the compression process efficiency which has not been increasing significantly. Consequently, the improvement of the compression process is indeed needed. A new single-stage compression unit design has been developed, built, and tested in a previous thesis work. This current thesis work uses a twin-stage successor of the initial single-stage compression unit and tests it into two oil-free domestic heat pump prototypes. This work aims at studying the integration of the radial compression units in domestic heat pumps and at demonstrating their feasibility and potential. The tested prototypes are a twin-stage Air/Water domestic heat pump and a twin-stage Brine/Water domestic heat pump. Both of them are equipped with a twin-stage oil-free radial compression unit rotating on gas bearings. By which, the maximum rotor speed of the compression units is 180 krpm. Six stable operating points have been documented with the Air/Water heat pump prototype. More notably so, the operating point A-7/W35 has been reached and demonstrates a coefficient of performance of 2.36 for a heating power of 10.7 kW. The Brine/Water heat pump prototype has been used to perform partial tests of circuit improvements, in order to solve some of the issues observed on the Air/Water prototype. The analysis of the experimental results uses a mass and energy balance modeling approach to improve the level of understanding of the internal and non measurable flows in the heat pump circuits. This model also propagates the uncertainties of the measurements through the equations. New improvements and innovative circuit layouts, notably at the level of the economizer, a key-component of the two-stage heat pump circuit, are offered to integrate further the compression unit in the heat pump circuits, and to make the whole system more efficient, more compact, more silent, less demanding in raw materials and reducing the refrigerant charge needed for the heat pump cycle.


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