Intérêt des pompes de chaleur à transferts thermiques sans auxiliaires, à convection naturelle et rayonnement
Abstract With many conventional heat pump systems the energy absorbed by auxiliary equipment (fans, defrosters, pumps) can be higher than the power used by the compressor. For example, with a particular air-air heat pump system the annual energy consumption of the compressor and its crankcase heater is 11 240 kWh, whilst 21 501 kWh of energy are consumed by auxiliary fans, the defrost unit, back-up heating and system heat losses. Some savings can be made by monitoring the interior unit fan to correspond to the running time of the compressor or by increasing the size of the heat pump in order to reduce back-up heating, but the latter solution has proved to be costly. Laboratory experiments since 1976 have been to reduce power consumption of auxiliaries, particularly air-air and air-water systems, by using a solar collector as the heat pump evaporator. The collector is made up of flat plates oriented in such a manner as to receive direct solar radiation, with both faces of the plates absorbing energy from the surrounding air by natural convection. The faces of the plates exposed to solar radiation are preferably painted black, but other colours with a low coefficient of reflection (eg brown, brick red, dark green, etc.) can be used if the evaporator-collector is to be architecturally pleasing. Glazing is not necessary, as is usually the case with solar panels. The collector-evaporator plates, and associated piping, are precharged with refrigerant and ready for connection to the heat pump circuit. A heat pump equipped with this collector-evaporator will provide an annual energy saving of 30% as compared to a standard air—water system. Additional benefits are no defrosting cycles, simplified installation, greater operational reliability, and an increased system COP. A table sets out the performance parameters of this system to show the incidence of back-up solar energy during the months of lesser solar radiation, ie outside the months of June to September, eg the COP during daylight hours (a minimum of about 7 h in December, to a minimum of 14 h in May) averages out at 3.89, and 3.27 during nocturnal hours. The article also describes a water heating system (200 and 300 I) using this collector-evaporator, an air-air heat pump with static (no forced ventilation) heat exchangers, and an air-water system using a static evaporator which will provide a 25% energy saving over its conventional counterpart.