Air conditioning and space heating with CO2: efficiency improvement with ejectors

The ejector technology, which is by now sufficiently developed to be used in industry, opens the door to the efficient use of CO2 as a refrigerant in air conditioning and space heating applications, where the sink temperature ranges from 30 to 40°C. The experience gained in commercial refrigeration can be immediately transferred in the new application, thus eliminating major technical barriers. In the proposed layout the ejector, recovering part of the expansion energy, is used to overcome the efficiency penalization occurring at high heat sink temperature, which may happen both during space heating operations (wintertime) and cooling operations (summertime), when outdoor temperature is particularly high. The aim of this paper is to present the energy assessment of a water/water R744 chiller/heat pump, working according to an ejector transcritical cycle, used for winter heating, summer cooling and tap water production. For simplicity reasons, the different functions (heating, cooling, water heating) are managed on the water side and therefore no inversion between the evaporator and gas cooler role is required. Results can nevertheless be extended to reversible units, which, however, require a specific development, as no units are currently available on the market. Energy consumption is compared with that of the reference system, working according to a simple cycle with internal heat exchanger, showing a total energy saving of 15% on an annual basis at the assumed conditions for space heating and cooling. Simulations are performed in a typical climate condition of Northern Italy for a residential family house. Technical considerations about the proposed layout are presented.