GB2133129A - Heat pump defrost system - Google Patents

Abstract

An air source heat pump includes a defrost system consisting of heating means, e.g. a tank (1) provided with an immersion heater (2), for heating a glycol solution while the heat pump is in the heating mode, and a glycol circuit (3) and pump (4) for circulating the heated glycol, during an interruption of the heating mode, to effect defrosting of the evaporator (5,6). <IMAGE>

Description

SPECIFICATION Heat pump defrost system The present invention relates to air-source heat pumps.

When the surface temperature of an evaporator is below OOC, moisture in the air condenses on the evaporator surface forming ice which impedes air flow and reduces heat transfer.

It is therefore necessary to interrupt the operation of the heat pump from time to time to heat the surface of the evaporator and thereby melt the ice.

One known way of defrosting is by means of an electric heater.

A second known way of defrosting is by so called reverse cycle defrosting in which heated refrigerant is passed through the evaporator.

Both systems suffer from the disadvantage that the rate at which the evaporator can be heated is relatively low.

In the case of an electrical heater this low rate of heating is because the utility may not permit the power consumption of the heater which is switched on to exceed the power consumption of the compressor which has been switched off. Thus with a 2kW compressor, only a 2kW electric heater is used.

In the case of reverse cycle defrosting, the low rate of heating is because the flow of hot gas is limited by the compressor rating. Thus with a 2kW compressor operating at a co-efficient of performance (COP) of 2 to 1 in the defrost mode, only 4kW of heating is achieved.

The low rate of heating presents two disadvantages. Firstly the heat pump remains in the defrost mode for a relatively long period of time. Secondly, a low heat input may be inefficient because some of the heat may be dissipated from the evaporator surface as radiant heat without achieving defrosting, thus further prolonging the time spent in the defrost mode. The proportion of heat lost as radiant heat decreases as the rate of heating of the coil increases.

A further problem with reverse cycle defrosting is that the heat pump includes a further valve, a reverse solenoid valve, for directing the hot gas into the evaporator coil. The reverse solenoid valve requires regular maintenance. In addition the other valves in the compressor are stressed due to the flow in the reverse direction through the compressor.

The present invention provides an air source heat pump having a defrost system comprising a closed circuit for a low freezing point liquid of high thermal conductivity such as for example a glycol solution, heating means for heating liquid in the circuit and a pump for circulating liquid in the circuit, the circuit being in thermal communication with the heat pump evaporator so as to enable the liquid to yield up heat to the evaporator.

The heating means may comprise an insulated tank for the liquid, the tank being provided with an immersion heater.

Alternatively, where the heat pump is used to heat water in a radiator system, the heating means may comprise a heat exchanger located in the piping of the radiator system.

The invention will now be described in more detail by way of example with reference to the accompanying drawings.

In the drawings: Figure 1 shows a defrost system including an insulated tank and an immersion heater; and Figure 2 shows an alternative defrost system including a heat exchanger in the radiator system.

Referring first to Figure 1, there is shown an insulated tank 1 containing glycol, an immersion heating element 2 for heating the glycol in the tank 1, a glycol circuit 3, a pump 4 for circulating the glycol, a heat exchanger 5 by means of which the glycol heats fin-type evaporator 6, and a control system 7 which activates the pump 4 in response to a detected need for defrosting.

The operation of the defrost system will now be described in more detail. The heat pump has a two horse power (2 h.p.) rating. The immersion heating element 2 has a three hundred watt (300 W) rating. The tank 1 has a capacity of three gallons (3 gls.). When the outdoor air temperature is above +100C,the immersion heating element 2 remains off. When the outdoor air temperature falis below +100C the immersion heating element 2 maintains the glycol at about 700 C. When the need for a defrost is detected, the pump 4 circulates the glycol from the tank 1 about the circuit. Defrosting is achieved in about one minute, the glycol temperature dropping from 700C to between 500C and 300 C, while melting about 2 kg of ice.Defrosting ceases when the temperature of the evaporator coil block rises above a preset value above OOC. The heat loss from the glycol is replenished by the immersion heating element 2 before the next defrost is required, typically ninety minutes (90 mins) later.

Referring now to Figure 2, there is shown a glycol circuit 21, a pump 22 for circulating the glycol, a heat exchanger 23 by means of which the glycol is heated, the heat exchanger 23 being located in the radiator system 24 between the heat pump condensor (not shown) and the radiators (not shown) i.e. in that part of the radiator circuit at which the water is warmest, a heat exchanger 25 by means of which the glycol heats fin-type evaporator 26, and a control system 27 which activates the pump 22 in response to a detected need for defrosting.

1. An air source heat pump including a defrost system comprising heating means for heating a working liquid while the heat pump is in the heating mode, and a liquid circuit and pump for circulating the heated liquid to the evaporator during an interruption of the heating mode, thereby to effect defrosting of the evaporator.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Heat pump defrost system The present invention relates to air-source heat pumps. When the surface temperature of an evaporator is below OOC, moisture in the air condenses on the evaporator surface forming ice which impedes air flow and reduces heat transfer. It is therefore necessary to interrupt the operation of the heat pump from time to time to heat the surface of the evaporator and thereby melt the ice. One known way of defrosting is by means of an electric heater. A second known way of defrosting is by so called reverse cycle defrosting in which heated refrigerant is passed through the evaporator. Both systems suffer from the disadvantage that the rate at which the evaporator can be heated is relatively low. In the case of an electrical heater this low rate of heating is because the utility may not permit the power consumption of the heater which is switched on to exceed the power consumption of the compressor which has been switched off. Thus with a 2kW compressor, only a 2kW electric heater is used. In the case of reverse cycle defrosting, the low rate of heating is because the flow of hot gas is limited by the compressor rating. Thus with a 2kW compressor operating at a co-efficient of performance (COP) of 2 to 1 in the defrost mode, only 4kW of heating is achieved. The low rate of heating presents two disadvantages. Firstly the heat pump remains in the defrost mode for a relatively long period of time. Secondly, a low heat input may be inefficient because some of the heat may be dissipated from the evaporator surface as radiant heat without achieving defrosting, thus further prolonging the time spent in the defrost mode. The proportion of heat lost as radiant heat decreases as the rate of heating of the coil increases. A further problem with reverse cycle defrosting is that the heat pump includes a further valve, a reverse solenoid valve, for directing the hot gas into the evaporator coil. The reverse solenoid valve requires regular maintenance. In addition the other valves in the compressor are stressed due to the flow in the reverse direction through the compressor. The present invention provides an air source heat pump having a defrost system comprising a closed circuit for a low freezing point liquid of high thermal conductivity such as for example a glycol solution, heating means for heating liquid in the circuit and a pump for circulating liquid in the circuit, the circuit being in thermal communication with the heat pump evaporator so as to enable the liquid to yield up heat to the evaporator. The heating means may comprise an insulated tank for the liquid, the tank being provided with an immersion heater. Alternatively, where the heat pump is used to heat water in a radiator system, the heating means may comprise a heat exchanger located in the piping of the radiator system. The invention will now be described in more detail by way of example with reference to the accompanying drawings. In the drawings: Figure 1 shows a defrost system including an insulated tank and an immersion heater; and Figure 2 shows an alternative defrost system including a heat exchanger in the radiator system. Referring first to Figure 1, there is shown an insulated tank 1 containing glycol, an immersion heating element 2 for heating the glycol in the tank 1, a glycol circuit 3, a pump 4 for circulating the glycol, a heat exchanger 5 by means of which the glycol heats fin-type evaporator 6, and a control system 7 which activates the pump 4 in response to a detected need for defrosting. The operation of the defrost system will now be described in more detail. The heat pump has a two horse power (2 h.p.) rating. The immersion heating element 2 has a three hundred watt (300 W) rating. The tank 1 has a capacity of three gallons (3 gls.). When the outdoor air temperature is above +100C,the immersion heating element 2 remains off. When the outdoor air temperature falis below +100C the immersion heating element 2 maintains the glycol at about 700 C. When the need for a defrost is detected, the pump 4 circulates the glycol from the tank 1 about the circuit. Defrosting is achieved in about one minute, the glycol temperature dropping from 700C to between 500C and 300 C, while melting about 2 kg of ice.Defrosting ceases when the temperature of the evaporator coil block rises above a preset value above OOC. The heat loss from the glycol is replenished by the immersion heating element 2 before the next defrost is required, typically ninety minutes (90 mins) later. Referring now to Figure 2, there is shown a glycol circuit 21, a pump 22 for circulating the glycol, a heat exchanger 23 by means of which the glycol is heated, the heat exchanger 23 being located in the radiator system 24 between the heat pump condensor (not shown) and the radiators (not shown) i.e. in that part of the radiator circuit at which the water is warmest, a heat exchanger 25 by means of which the glycol heats fin-type evaporator 26, and a control system 27 which activates the pump 22 in response to a detected need for defrosting. CLAIMS

1. An air source heat pump including a defrost system comprising heating means for heating a working liquid while the heat pump is in the heating mode, and a liquid circuit and pump for circulating the heated liquid to the evaporator during an interruption of the heating mode, thereby to effect defrosting of the evaporator.

2. A heat pump according to claim 1, in which the heating means comprise an electric immersion heater situated in a thermally insulated tank in the liquid circuit.

3. A heat pump according to claim 1 or 2, in which the working liquid is a glycol solution.