Nowadays, the trend which combines a reinforcement of thermal insulation of buildings and an increase of internal gains due to more and more electric devices (such as computers, domestic appliances...) leads to a decrease of heating loads but can generate cooling loads in mid-season and summer. Heating and cooling loads become more and more balanced during a day and during a year in general. Moreover, domestic hot water (DHW) production takes an increasing part in the total energy needs, which provokes a partial simultaneity of heating and cooling needs as soon as cooling is required. A heat pump producing heat and cold simultaneously associated to a short time energy storage system appears then as an interesting solution. The Heat Pump for Simultaneous heating and cooling1 (HPS) developed by our research team is a hot and cold water production system designed to carry out space heating, space cooling and DHW production for hotels, luxury dwellings and smaller office buildings. The key features of the concept reside in the conception of the frigorific circuit and in the control of the operating modes and the defrosting sequence. Heat and cold productions are adapted to building loads using an ambient air (free source easily available) balancing coil, working either as a condenser or an evaporator. During the heating period, the HPS stores some energy, obtained by subcooling of the refrigerant, on the cold water loop. This energy is used subsequently at the water evaporator in order to improve the performance by an increase of the evaporating temperature and if necessary, to carry out defrosting without stopping the heat production. The defrosting energy is provided by a thermosiphon formed between the two evaporators at different temperatures. The use of a subcooler imposes a high pressure control system to ensure a complete condensation in the useful water condenser. The high pressure control system is composed of a two-phase refrigerant receiver connected at the top part to the compressor discharge line and to a low pressure point of the circuit and at the bottom part to the liquid line of the refrigerant circuit. Injecting hot gas from the compressor discharge line into the receiver will increase the high pressure and driving out gas from the receiver towards the low pressure point will decrease the high pressure of the system. Experiments have been carried out on a R407C HPS prototype for variable operating conditions in terms of air and water temperatures and possible modes of operation (heating, cooling and simultaneous production). The prototype performance is in accordance with what is announced by the selection software of the compressor manufacturer. The experimental study has also enabled to observe the thermosiphon defrosting technique and to verify the proper operation of the high pressure control system and the alternated winter sequence (1: heating mode with storage on the cold loop of heat recovered by subcooling of the refrigerant. 2: simultaneous production mode using the previously stored energy at the evaporation, more efficient and with possible defrosting). The operation of the HPS has been modelled using two working fluids: HFC R407C and CO2. Within the present context livened up by possible political measures concerning the impact of greenhouse gases on global warming, carbon dioxide is an interesting working fluid for its low environmental impact (ODP = 0 and GWP100years = 1) and for the particular characteristics of the thermodynamic cycle used (transcritical cycle). Indeed a high amount of energy is recoverable by subcooling and DHW can be produced in an efficient way. In annual simulations R407C and CO2 HPSs are compared to standard reversible heat pumps in terms of performance, energy savings and environmental impact. This concept of HPS clearly offers an improvement compared to a standard reversible heat pump and the use of carbon dioxide as a working fluid presents good perspectives of development.
Experimental and numerical study of a heat pump for simultaneous heating and cooling using air as a balancing source