GB2516030A

GB2516030A - Heat Pump

Info

Publication number: GB2516030A
Application number: GB1312188.4A
Authority: GB
Inventors: Ronnie Paterson
Original assignee: Ebac Ltd
Current assignee: Ebac Ltd
Priority date: 2013-07-08
Filing date: 2013-07-08
Publication date: 2015-01-14
Also published as: GB201312188D0

Abstract

An energy-efficient heat pump with a closed cycle refrigerant circuit includes a variable speed compressor 1, a condenser 3 in which heat is transferred from the compressed refrigerant to an output medium such as a liquid circuit 4, a variable expansion device 5 supplying a refrigerant receiver 6, a main expansion device 8, and an evaporator 10 in which the refrigerant absorbs heat from an input medium such as external air; a reversing valve 2 reverses the cycle of the heat pump to defrost the evaporator 10, and a bypass 41 containing a non-return valve 42 is arranged to allow refrigerant to flow from the main expansion device 8 to the condenser 3 during the reverse cycle without passing through the receiver 6 and its associated expansion device 5. A fan 22 may draws external air over the evaporator 10 is switched off during the reverse cycle. A pressure sensor 20 may monitor the pressure of refrigerant supplied to the evaporator during the reverse cycle and the fan may be turned back on if the sensed pressure exceeds a predetermined level.

Description

HEAT PUMP

TECHNICAL FIELD OF THE INVENTION

This invention relates to heat pumps, and particularly (but not exclusively) air source heat pumps.

BACKGROUND

Heat pumps using vapour compression circuits are well known technology. The efficiency of a heat pump is indicated by its coefficient of performance, or COP, which is the ratio of the heat provided over the electrical energy consumed. Much effort has already been devoted to improving the COP of heat pumps over a range of operating conditions. For example, in air source heat pumps the use of a variable speed compressor allows the energy consumption to be controlled to achieve good efficiency at high and low outside air temperatures.

Modern heat pumps are now capable of achieving a high COP under a wide range of operating conditions. Nevertheless, small improvements in performance are still capable of achieving a significant energy saving over a period of several months.

The present invention seeks to provide a new and inventive form of heat pump which is capable of providing a significant improvement in operating efficiency.

SUMMARY OF THE INVENTION

The present invention proposes a heat pump having a closed cycle refrigerant circuit which includes a variable speed compressor, a condenser in which heat is transferred from the compressed refrigerant to an output medium, a variable expansion device supplying a refrigerant receiver, a main expansion device, and an evaporator in which the refrigerant absorbs heat from an input medium, and means for reversing the cycle of the heat pump to defrost the evaporator, in which a bypass is arranged to allow refrigerant to flow from the main expansion device to the condenser during the reverse cycle without passing through the receiver and its associated variable expansion device.

The bypass could, for example, include a solenoid-operated valve which is closed during normal running. However, for optimum energy efficiency the bypass preferably includes a non-return check valve.

The input medium could be air, as in an air source heat pump, or a circulating liquid as in a ground source heat pump. Similarly, the output medium could be air, as in an air-to-air heat pump, or a heat-carrying liquid.

The invention also provides a heat pump which includes a fan for drawing external air over an evaporator, and a pressure sensor for monitoring the pressure of refrigerant supplied to the evaporator during a reverse defrost cycle, the heat pump being arranged to switch off the fan during the reverse cycle and turn it back on again if the sensed evaporator pressure exceeds a predetermined level.

The invention also provides a heat pump which is arranged to perform an evaporator optimisation procedure at predetermined intervals, in which the compressor speed is increased to reduce the refrigerant pressure in the evaporator.

The invention also provides a heat pump which is arranged to perform an evaporator optimisation procedure at predetermined intervals, in which the main expansion device is partially closed during the evaporator optimisation procedure.

The invention also provides a heat pump which is arranged, during a pre-start period, to apply a D.C. voltage to the compressor to produce internal resistive heating without rotation of the compressor.

The invention also provides a heat pump which includes a communication interface to provide two-way connectivity with a remote control centre.

The invention also provides a heat pump which includes temperature and pressure sensors and sends information obtained from the sensors to enable the heat pump operating parameters and performance to be monitored remotely.

The invention also provides a heat pump in which the operating settings and functions of the heat pump can be adjusted remotely.

The invention also provides a heat pump in which the heat pump monitors and controls compressor speed and expansion device settings to achieve a target set point in such a way that the magnitude of the control inputs which are applied varies depending on how much the measured parameter differs from the target set point.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is schematic diagram of an air source heat pump in accordance with the invention; and Figure 2 is a schematic diagram of a typical heating system in which the heat pump may be used.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring firstly to Fig. 1, the heat pump comprises a closed cycle refrigerant circuit in which a compressor 1 pressurises vapour phase refrigerant to produce hot superheated gas. Upon leaving the compressor the hot gas passes through a 4-way reversing valve 2 to a plate condenser 3 in which de-superheating occurs.

The condenser 3 acts as a heat exchanger, transferring heat from the gaseous refrigerant to a liquid heat-carrying medium (usually water) which circulates through a separate path 4 through the condenser forming part of a heating system. Warm high pressure liquid refrigerant then passes through a sub-cool expansion device in the form of a valve driven by a stepper motor to provide a variable restriction. The liquid refrigerant then enters a receiver 6, which acts as a refrigerant reservoir, before passing through a refrigerant filter and drier 7 containing a desiccant such as silica gel that absorbs any moisture from the refrigerant. The liquid refrigerant then passes through a main expansion device 8, again in the form of a variable-restriction valve driven by a stepper motor, causing a rapid drop in pressure which produces a two phase stream of cold vapour and liquid which is split by a distributor 9 to provide separate streams to the coils of the evaporator 10 which are connected in parallel. Evaporation of the liquid phase is completed in the evaporator which absorbs heat from outside air which is drawn over the evaporator coils by a motorised fan 22. The evaporator 10 then returns refrigerant vapour to the inlet of the compressor 1 via the reversing valve 2 and the suction line of the compressor, so that the cycle can be repeated.

The operation of the heat pump is controlled by an electronic control unit 11 which receives input from various sensors to monitor the operating conditions of the heat pump. These include a compressor discharge temperature sensor 12, an evaporator input temperature sensor 13, a water input temperature sensor 14, a water output temperature sensor 15, a condenser temperature sensor 16 mounted on the refrigerant output of the condenser 3, an ambient air temperature sensor 17, an evaporator output temperature sensor 18, a low pressure transducer 19 on the suction line to the compressor, and a high pressure transducer 20 on the discharge line from the compressor.

The control unit includes an integrated two-way communication interface 21 such as a 3G wireless connection or a broadband internet connection.

The heat pump uses a variable speed compressor 1. The control unit 11 can adjust the output of the compressor by varying the frequency of its A.C. supply so that the speed varies between about 25 rps and 125 rps (revolutions per second). At low outside air temperatures the compressor is operated at high speed for maximum heat transfer, but as the air temperature rises the speed is gradually reduced to achieve more efficient performance.

Under ideal operating conditions the refrigerant output temperature of the condenser 3 should be the same as the water input temperature to the condenser, and the expansion valve 5 enables the condenser output temperature to be automatically controlled throughout the operating speed range of the compressor, thereby extracting the maximum amount of heat from the refrigerant.

Fig. 2 shows how the heat pump 30 may be used in a typical sealed heating system. The hot water output from the heat pump condenser is transferred by a circulating pump 31 to various heating loops controlled by motorised zone valves 32-34. By way of example, zone valve 32 may control flow to an underfloor heating loop 35, valve 33 may control flow to one or more radiators 36, and valve 34 may control flow to an indirectly heated hot water storage cylinder 37. The return lines from the various zones supply a common input 38 to the heat pump condenser, into which an expansion vessel and filling system 39 is also connected.

The heat pump incorporates a number of features which will now be described which significantly improve the operating efficiency of the heat pump.

In any heat pump icing of the evaporator may occur from time to time, producing a layer of ice on the evaporator which reduces the operating efficiency of the heat pump. This is normally removed by temporarily reversing the direction of refrigerant flow using the reversing valve 2. It has been found that a disadvantage of using the sub-cool expansion device 5 is that, during defrost, refrigerant flow is restricted as refrigerant vapour is drawn from the receiver 6 into the condenser 3 rather than liquid. This extends the time required to achieve full defrost of the evaporator 10. In the present heat pump a bypass line 41 is included to bypass the receiver 6 and expansion valve 5 and allow refrigerant to flow direct from the dryer 7 to the condenser 3 during defrost. The bypass line includes a non-return check valve 42 so that liquid only flows through the bypass when the system is running in reverse cycle during defrost.

In the present heat pump defrost efficiency is further improved by controlling the operation of the evaporator fan 22 that draws external air over the evaporator. In a conventional heat pump the evaporator fan is switched off during defrost to avoid blowing cold air over the evaporator when heat is being added by reverse cycle operation to melt the ice. However, reverse cycle defrost also causes the evaporator pressure to rise since it is effectively operating as a high pressure condenser, and the reverse cycle conditions therefore need to be limited to prevent evaporator damage. In the present heat pump the control unit switches the fan back on when the the high pressure transducer 20 signals that a predetermined evaporator pressure has been reached, thereby cooling the evaporator and reducing its internal pressure. This enables the most efficient defrost conditions to be achieved whilst at the same time protecting the heat pump from damage.

Another problem which has been addressed is that of reduced evaporator efficiency. In order to ensure that the evaporator works at maximum efficiency the control unit performs an evaporator optimisation procedure at predetermined intervals.

This is done by partially closing the expansion valve 8 and increasing the compressor speed, which has the effect of reducing the pressure in the evaporator coils. This sucks any liquid refrigerant out of the evaporator, which may have accumulated in any of the parallel evaporator coils. Furthermore, it has been found that lubricating oil from the compressor sump can often find their way into the evaporator and contribute to reduced evaporator efficiency. The optimisation procedure also sucks any slugs of oil out of the evaporator that may be partially blocking one or more of the parallel coils and returns them to the compressor.

Under cold start conditions it is often necessary to warm the compressor prior to starting to ensure that any liquid refrigerant which might have migrated to the compressor it boiled off and turned to vapour. Failure to do this can lead to early compressor failure as the refrigerant replaces the lubricating oil, starving the compressor bearings of vital lubrication during the startup phase.

This is normally achieved by using an external heating band, but this adds to the manufacturing cost and is relatively inefficient since it takes time for the heat to penetrate the compressor and most of the heat escapes into the environment. In the present heat pump the control unit generates heat internally of the compressor by applying a D.C. voltage to the compressor -10 -windings during a pre-start period instead of the usual A.C. Supply.

The D.C. Voltage causes internal resistive heating of the compressor windings without rotation of the compressor, which will only take place with an A.C. supply.

The communication interface 21 provides full two-way connectivity with a remote control centre. The control unit 11 can thus provide data which enables heat pump operating parameters and performance to be monitored remotely, and also allows the operating settings and software functions of the control unit to be remotely optimised to ensure maximum energy efficiency. The data connection can also be used for remote diagnostics in the event of any technical problems.

The control unit 11 uses adaptive logic within its control architecture, so-called "fuzzy logic", to control the compressor speed and expansion valve stepper motor settings. Essentially, the magnitude of the control inputs depend on how far a monitored parameters are from the set point and how fast the parameters are changing. If the parameter is a long way from the target large control inputs will be applied, but as the parameter approaches the target value progressively smaller changes will be made. This has the benefit that optimum operating conditions are more quickly achieved with a significant improvement in operating efficiency.

The measures described above enable the heat pump to operate with a significantly improved efficiency, substantially reducing the -11 -overall energy consumption of the heat pump and improving its measured COP.

The heat pump could be a split system with the evaporator outside and the condenser inside a building, where the refrigerant passes through an external wall and avoids the need for water in the circuit 4 to circulate outside the building with a risk of freezing.

Furthermore, although the above description relates to an air-to-water heat pump the condenser 3 could act as a heat exchanger transferring heat directly to the air inside a building. It should also be noted that many of the innovations described herein are applicable to ground source heat pumps in which heat is collected from the ground and transferred to the evaporator 10 via a suitable heat-carrying medium.

Whilst the above description places emphasis on the areas which are believed to be new and addresses specific problems which have been identified, it is intended that the features disclosed herein may be used in any combination which is capable of providing a new and useful advance in the art.

* * * * * * * *

Claims

-12 -CLAIMS1. A heat pump having a closed cycle refrigerant circuit which includes a variable speed compressor, a condenser in which heat is transferred from the compressed refrigerant to an output medium, a variable expansion device supplying a refrigerant receiver, a main expansion device, and an evaporator in which the refrigerant absorbs heat from an input medium, and means for reversing the cycle of the heat pump to defrost the evaporator, in which a bypass is arranged to allow refrigerant to flow from the main expansion device to the condenser during the reverse cycle without passing through the receiver and its associated variable expansion device.
2. A heat pump according to Claim 1 in which the bypass includes a non-return check valve.
3. A heat pump according to Claim 1 or 2 in which the heat pump includes a fan for drawing external air over the evaporator, and a pressure sensor for monitoring the pressure of refrigerant supplied to the evaporator during the reverse cycle, the heat pump being arranged to switch off the fan during the reverse cycle and turn it back on again if the sensed evaporator pressure exceeds a predetermined level.
4. A heat pump according to any preceding claim which is arranged to perform an evaporator optimisation procedure at -13 -predetermined intervals, in which the compressor speed is increased to reduce the refrigerant pressure in the evaporator.
5. A heat pump according to Claim 4 in which the main expansion device is partially closed during the evaporator optimisation procedure.
6. A heat pump according to any preceding claim which is arranged, during a pre-start period, to apply a D.C. voltage to the compressor to produce internal resistive heating without rotation of the compressor.
7. A heat pump according to any preceding claim which includes a communication interface to provide two-way connectivity with a remote control centre.
8. A heat pump according to Claim 7 in which the heat pump includes temperature and pressure sensors and sends information obtained from the sensors to enable heat pump operating parameters and performance to be monitored remotely.
9. A heat pump according to Claim 7 or 9 in which the operating settings and functions of the heat pump can be adjusted remotely.
A heat pump according to any preceding claim in which the heat pump monitors and controls the compressor speed and expansion device settings to achieve a target set point in such a -14 -way that the magnitude of the control inputs which are applied varies depending on how much the measured parameter differs from the target set point.
11. A heat pump which is substantially as described with reference to the drawings.* * * * * * *