*1.1. Background*

In the 21st century, energy crises, global warming and environmental pollution are becoming more and more serious. It is urgent to improve energy efficiency, save energy and reduce emissions. One of the critical issues is to valorise low potential waste heat instead of rejecting it. Appropriate integration of heat pumps has the characteristics of efficient recovery of low-temperature heat energy, hot utility energy-saving and potentially environmental protection. Heat pumps (HP) continue to receive considerable attention and development and are becoming a critical sustainable energy technology.

Sadi Carnot [1], a French scientist in the early 19th century, first proposed the "Carnot Cycle" theory in his paper in 1824, which became the origin of HP technology. In 1912, the world's first set of HP equipment was successfully installed in Zurich, Switzerland with river water as the low heat source for heating. HPs entered the early stage of development from the 1940s to the early 1950s. HPs used in household, and industrial buildings began to enter the market. Since the 1970s, the HP industry has advanced rapidly, and all countries have attached great importance to HP

research. Large HP development plans have been instituted by countries and organisations such as the European Community and the International Energy Agency. At present, Europe, America and Japan are competing to develop new types of HPs.
