WO2004111558A1

WO2004111558A1 - Heat pump system and a method for controlling such a system

Info

Publication number: WO2004111558A1
Application number: PCT/SE2003/000961
Authority: WO
Inventors: Jan TEDSJÖ; Timo HEIKKILÄ
Original assignee: Strateg Trade Ab
Priority date: 2003-06-13
Filing date: 2003-06-13
Publication date: 2004-12-23
Also published as: AU2003239003A1

Abstract

The invention relates to a heat pump system provided with a compressor means (1), a condenser (2), a pressure reduction means (13, 18), an evaporator (4), a refrigerant, a refrigerant circulating conduit connecting said compressor means, said condenser, said pressure reduction means and said evaporator, and a control unit (ECU). The heat pump system is provided with at least one controllable heating means (12, 20) for heating said refrigerant, which heating means is located between a first position downstream of the condenser (2) means and a second position upstream of the compressor means (1).

Description

HEAT PUMP SYSTEM AND A METHOD FOR CONTROLLING SUCH A SYSTEM

TECHNICAL FIELD The invention relates to a heat pump system and a method for controlling such a system to improve the operating range of the system, particularly at low temperatures.

BACKGROUND ART

Heat pumps are commonly use for energy saving purposes for a wide range of applications, e.g. for heating ventilation air or water in a building. However, irrespective of the source of heat used, such as water from a suitable source or outside air, a conventional heat pump will have a reduced capacity for delivering heat as the temperature of the source drops. The capacity will begin to reduce when the said temperature drops below approximately +7⁰C and will for all practical purposes cease at approximately -7°C.

This poses a particular problem in cold climates where the temperature of the outside air or rivers regularly drops to such temperatures, or below, during winter. In such climates, heating solely by means of conventional heat pumps will not suffice. Instead alternative sources of heat must be used to an increasing extent as the temperature of the said source drops below +7°C.

Hence, buildings provided with conventional heat pump systems must be provided with additional means of heating, usually electric heaters. As a result the consumption of electrical power will increase during periods of cold weather. The invention aims to solve the above problems by providing means allowing heat pump systems to be used over a wider range of temperatures with a maintained capacity for delivering heat. DISCLOSURE OF INVENTION

The above problems are solved by a heat pump system and a method for controlling such a system, according to claims 1 and 14, and their respective dependent claims. According to a preferred embodiment the invention relates to a heat pump system provided with a compressor means, a condenser, a pressure reduction means, an evaporator, a refrigerant, a refrigerant circulating conduit connecting said compressor means, said condenser, said pressure reduction means and said evaporator, and an electronic control unit. The heat pump system is further provided with at least one controllable heating means for heating said refrigerant, which heating means is located between a first position downstream of the condenser and a second position upstream of the compressor means.

An ambient medium, such as air or water passes through the evaporator to allow the refrigerant to absorb heat. Similarly, air to be used for heating purposes is forced through the condenser by a fan. This will be described in further detail below.

According to a preferred embodiment, the controllable heating means is positioned in a low pressure section of the heat pump system, which section includes the evaporator and the refrigerant circulating conduit leading up to the compressor. The pressure reduction means may be a separate expansion valve placed before the evaporator, or be integrated in the evaporator, in the form of capillary tubes. In the latter case the evaporator is considered to be a part of the low pressure section , even though it also functions as a pressure reduction means.

According to a further embodiment the controllable heating means is positioned in the evaporator. The heating means may either be placed in or adjacent one or more conduits in the evaporator. Here, the term adjacent includes various arrangements of the heating means. The heating means may for instance be placed alongside, or parallel, to one or more conduits, as well as around the conduits, either concentric or s a helical coil. Alternatively, the conduits may be coiled around the heating means. For conduits in the form of conventional tubes or coils, any of the said alternatives may be used. The heating means can also be arranged as a heat exchanger, such as a coaxial or plate heat exchanger, through which the refrigerant and the heated liquid passes, separated from the ambient medium. For conduits in the form of capillary tubes, the heating means is preferably arranged adjacent and parallel to the conduits.

According to a further embodiment, the controllable heating means may instead, or additionally, be positioned in or adjacent the refrigerant circulating conduit leading up to the compressor. Alternatively, the controllable heating means may instead, or additionally, be positioned adjacent and immediately upstream of the evaporator, or in or adjacent the refrigerant circulating conduit downstream of the condenser. The controllable heating means may comprise a tank filled with a suitable liquid, which tank is provided with an immersion heater. The conduit may pass through said tank to be heated to a desired temperature. Alternatively the tank may be arranged as a heat exchanger, such as a coaxial or plate heat exchanger, through which the refrigerant and the heated liquid passes. One example of a liquid for use in such a tank may be an alcohol, such as methanol, or a similar suitable liquid. The liquid should preferably have suitable anti-freezing properties.

The heating means is preferably an electric heater, but other sources of heat, such as a liquid or gas heat exchanger, may also be used depending on availability.

According to a further preferred embodiment, the controllable heating means is actuated when the temperature of a medium used to heat the evaporator drops below a predetermined value. The medium may be either air, water or any other suitable source for heating the evaporator. The arrangement may be applied to any combination of air/air, air/water, water/air or water/water for the evaporator and condenser. Any suitable refrigerant may be used in the system, although media such as 407c or 410a are preferred due to the relatively high pressure used in the system according to the invention.

A temperature sensor is provided to determine the temperature of said medium. As will be described in detail below, the controllable heating means may be typically actuated when the temperature of said medium drops below +7°C.

According to a further embodiment the controllable heating means is actuated gradually to maintain an evaporation temperature of the refrigerant at a predetermined value. A temperature sensor may be provided to determine the evaporation temperature of said refrigerant. Alternatively, or in addition, the controllable heating means is actuated gradually to maintain an evaporation pressure of the refrigerant at a predetermined level. A pressure sensor may be provided to determine the evaporation pressure of said refrigerant. The above-mentioned sensors are connected to the control unit that determines when and to what extent said one or more heating means are to be actuated. For electric heaters, the electric power may be controlled by for instance a triac, which pulses the required current to the electric heater/-s. For the alternative means of heating, a controllable valve may be used to control the flow and hence the supply of heat to the refrigerant.

The invention further relates to a method for controlling a heat pump system provided with a compressor means, a condenser, a pressure reduction means, an evaporator, a refrigerant, a refrigerant circulating conduit connecting said compressor means, said condenser, said pressure reduction means and said evaporator, and an electronic control unit. The method involves heating a section of the heat pump system by means of at least one controllable heating means for heating said refrigerant, which heating means can be located between a first position downstream of the condenser and a second position upstream of the compressor means, when at least one predetermined condition is fulfilled. According to a further embodiment the method involves actuating the controllable heating means when the temperature of a medium used to heat the evaporator drops below a predetermined value

According to a further embodiment the method involves controlling the heating means in relation to the temperature of said medium. Consequently, the amount of heat supplied by the heating means is a function of the temperature of the medium.

The controllable heating means may be actuated to maintain an evaporation temperature of the refrigerant at said predetermined value and/or to maintain an evaporation pressure of the refrigerant at a predetermined level. Hence, the controllable heating means is used to heat the evaporator in order to maintain a pressure and temperature in the evaporator at a level that is equivalent to a predetermined minimum ambient temperature at which the heat pump will remain operational without losing capacity to deliver heat. Consequently the heat pump can maintain its capacity to supply heat, not only at relatively low temperatures where a conventional heat pump has a reduced capacity (from +7⁰C to about -7°C) but also at sub-zero temperatures where such heat pumps may cease to operate altogether.

BRIEF DESCRIPTION OF DRAWINGS In the following text, the invention will be described in detail with reference to the attached drawings. These drawings are used for illustration only and do not in any way limit the scope of the invention. In the drawings:

Figure 1 shows a shows a diagram illustrating a heat pump system according to one embodiment of the invention; Figure 2 shows a diagram illustrating a heat pump system according to an alternative embodiment of the invention;

Figure 3 shows a diagram illustrating a heat pump system according to a further alternative embodiment of the invention. MODES FOR CARRYING OUTTHE INVENTION

Figure 1 shows a heat pump system provided with a compressor 1 , a condenser 2, an expansion valve 3, an evaporator 4. The component parts are connected by a refrigerant circulating conduit containing a refrigerant. The compressor 1 is provided with an upstream pressure switch 5 in a low pressure section of the circulating conduit, and a downstream pressure switch 6 in a high pressure section of the circulating conduit. The pressure switches 5, 6 are used to regulate the compressor, so that a relatively constant, high pressure is delivered. In this embodiment, a typical absolute pressure in the high pressure section is 19-20 bar, preferably 19,2-19,5 bar. In the low pressure section the absolute pressure is 4-5 bar, preferably 4,2 bar. These pressures apply for a refrigerant such as 407c at an ambient temperature of +7°. Refrigerant at high pressure flows to the condenser 2 containing a heat exchanging arrangement in the form of tubes or coils 7, in which heat is transferred from the refrigerant to an air flow supplied by a fan 8. Heated air at a temperature of 40-45⁰C is ducted from the condenser 2 to a domestic heating system (not shown), as indicated by the arrow Ai.

From the condenser 2, the refrigerant flows to the evaporator 4 via an expansion valve 3. The expansion valve 3 is connected to a bulb 9 located downstream of the evaporator 4 and is controlled to maintain a constant pressure drop over the evaporator 4 located in the low pressure section of the circulating conduit. The evaporator 4 contains a heat exchanging arrangement in the form of tubes or coils 10, in which heat is transferred from an air flow supplied by a fan 11. In this case ambient, outside air supplied by the fan 11 passes through the evaporator 4, in which heat is transferred from the air flow to the refrigerant. The refrigerant is then returned to the compressor 1 to close the circulating conduit, while the air is returned to the atmosphere as indicated by the arrow A₂.

The heat pump system is also provided with a controllable heating means for allowing heating of said refrigerant as it passes through the evaporator 4.

The heating means is an electrical heater 12 placed adjacent the tubes 10 in the evaporator 4 and controlled by an electronic control unit ECU. Although the following text refers to "an electrical heater 12", the heating means is actually arranged to distribute heat evenly through the entire evaporator 4. The control unit ECU is connected to a triac device 14 that is used to supply a pulsed electrical power to all resistor elements of the electrical heater 12. By pulsing the electrical power to the electrical heater it is possible to control the temperature of the evaporator accurately. The control unit ECU is connected to an electronic pressure switch 15, generating a signal representing the pressure of the refrigerant as it leaves the evaporator 4, an evaporation sensor 16, giving the temperature in the evaporator 4, and an ambient air temperature sensor 17, giving the temperature of the medium used to heat the evaporator. Depending on the signals transmitted from these sensors, the control unit ECU is arranged to control the supply of heat to the evaporator 4. The triac device 14 may of course be integrated in the control unit ECU. Alternative means for controlling the electrical power to the heater may also be used.

The controllable heating means is actuated when the temperature of ambient air sensed by the sensor 17 drops below a set limit. This limit can be set to a suitable value preferably, but not necessarily, within the range 0°C to -6°C. In the current embodiment, the limit is set at -3⁰C to -4⁰C to maintain a desired temperature in the heated air supplied by the condenser. The control unit ECU will then transmit a signal to the triac device 14, so that pulses of electrical power is supplied to the heater 12. The feedback signals from the pressure and temperature sensors 15, 16 in the evaporator 4 are then used to maintain a pressure and temperature in the evaporator 4 at a level that is equivalent to an ambient temperature of about +7°C. The electrical heater 12 is controlled constantly as long as the ambient temperature is below the set limit. As the ambient temperature drops, the triac device 14 is controlled to increase the amount of electrical power to the heating means 12. The number of electrical heaters associated with the tubes or coils in the evaporator, as well as the rated output of each heater, is determined by the climate at the location of the installation. By a suitable dimensioning of the electrical heaters the heat pump arrangement may remain fully operational at ambient temperatures of -20°C and below.

A suitable temperature for actuating the electrical heater may be selected depending on a number of different factors, such as the type of refrigerant used, the compression ratio and the desired temperature of the air leaving the condenser. However, for the current embodiment the capacity of the heat pump system to deliver heat will start to decrease at about +7⁰C. However, the heat pump will continue to operate satisfactorily as the temperature drops below O⁰C and may be operated down to about -6⁰C, although at a reduced efficiency. As stated above, a suitable temperature for actuating the heating means for the current embodiment is -3°C to -4°C. Figure 2 shows a preferred, alternative embodiment of the heat pump system as shown in Figure 1. The main difference is that the arrangement in Figure 2 is provided with an evaporator containing multiple capillary tubes 18, so that the expansion of the refrigerant takes place as it flows through the evaporator. This arrangement eliminates the need for an expansion valve (see Figure 1) and gives an even flow of refrigerant through the evaporator 4. In this case each of the electrical heaters are preferably arranged adjacent and parallel to a capillary tube, due to their relatively small cross-section. This arrangement will also give a more even and a better controllable heat transfer to the refrigerant. As for the previous embodiment, it is important to place the heating elements 12 so that heat is supplied equally to all parts of the evaporator 4. However, the control of the electrical heaters 12 is performed in the same way as described for the embodiment of Figure 1.

Figure 3 shows an alternative embodiment of the invention as shown in

Figure 2, although the general principle may also be applied to the embodiment sown in Figure 1. In this case a heating vessel 19 provided with an immersion heater 20 is arranged around the refrigerant conduit at a suitable location between the evaporator 4 and the compressor 1. Here the control unit ECU will control the electrical power supplied by the triac device 14 to the immersion heater 20 based on the input signals from the pressure sensor 15 and the temperature sensor 16 in the evaporator 4 and the ambient air sensor 17.

As this arrangement will experience a delay caused by the time lag as the refrigerant flows from the heating vessel to the evaporator, the control unit may be provided with a matrix, or look-up table for a more accurate control of the immersion heater. The time lag is dependent on the temperature and flow rate of the incoming refrigerant, as well as the power rating of the heating means 20.

Alternatively, a further temperature sensor may be provided in the heating vessel 19 or on/in the refrigerant conduit, to provide the control unit with a feedback signal. An alternative version of the above embodiment is indicated with dotted lines in Figure 3. According to this embodiment, the condenser 2 is provided with a temperature sensor 21 for the heated air leaving the condenser. Should the temperature of the delivered heated air drops from a desired temperature, of say +40⁰C, to minimum set limit temperature, of say +20⁰C, the electronic control unit ECU transmits a signal to a solenoid valve 22. The solenoid valve 22 is placed in a conduit 23 connecting the refrigerant conduit in the heating vessel 19 with the refrigerant conduit immediately upstream of the evaporator 4. The conduit 23 is further provided with a non-return valve 24, preventing flow in the direction of the heating vessel 19. This arrangement allows a part of the flow through the evaporator 4 to be at least partially heated and recirculated to the evaporator 4. This arrangement will allow the heat pump to operated at even lower ambient temperatures.

For the opposite case, where a reduction in temperature is required, a bypass conduit provided with a fluid switching valve (not shown) may be provided. To achieve a rapid lowering of the temperature, said valve is actuated and the refrigerant is taken past the heating vessel, directly to a section of the conduit upstream of the compressor.

Figure 4 shows an alternative embodiment of the heat pump arrangement of Figure 3. The main difference between these embodiments is that the heating vessel 19 is arranged immediately upstream of the evaporator 4, instead of being located between the evaporator 4 and the condenser 2. Otherwise, the function of the arrangement shown in Figure 4 is the same as that of Figure 3.

The invention is not limited to the embodiments described above and may be varied freely within the scope of the appended claims.

Claims

1. A heat pump system provided with a compressor means (1), a condenser (2), a pressure reduction means (13, 18), an evaporator (4), a refrigerant, a refrigerant circulating conduit connecting said compressor means, said condenser, said pressure reduction means and said evaporator, and a control unit (13) characterized in that the heat pump system is provided with at least one controllable heating means (12, 20) for heating said refrigerant, which heating means is located between a first position downstream of the condenser (2) means and a second position upstream of the compressor means (1 ).

2. A heat pump according to claim 1, characterized in that the controllable heating means (12, 20) is positioned in a low pressure section of the heat pump system, which low pressure section includes the evaporator (4) and the refrigerant circulating conduit leading up to the compressor (1).

3. A heat pump according to claim 1, characterized in that the pressure reduction means (18) is integrated in the evaporator (4).

4. A heat pump according to claim 2 or 3, characterized in that the controllable heating means (12, 20) is positioned in the evaporator (4).

5. A heat pump according to claim 4, characterized in that the controllable heating means (12, 20) is placed in or adjacent one or more conduits in the evaporator (4).

6. A heat pump according to claim 2 or 4, characterized in that the controllable heating means (12, 20) is positioned in or adjacent the refrigerant circulating conduit upstream of the compressor (1 ).

7. A heat pump according to claim 2 or 4, characterized in that the controllable heating means (12, 20) is positioned in or adjacent the refrigerant circulating conduit downstream of the condenser (2).

8. A heat pump according to claim 1 , characterized in that the controllable heating means (12, 20) is actuated when the temperature of a medium used to heat the evaporator (4) drops below a predetermined value.

9. A heat pump according to claim 8, characterized in that a temperature sensor (17) is provided to determine the temperature of said medium.

10. A heat pump according to claim 8, characterized in that the controllable heating means (12, 20) is actuated when the temperature of said medium is in the range O⁰C to -6⁰C, preferably -3°C to -4°C.

11. A heat pump according to claim 8, characterized in that the controllable heating means (12, 20) is actuated gradually to maintain an evaporation temperature of the refrigerant at a predetermined value.

12. A heat pump according to claim 11, characterized in that a temperature sensor (16) is provided to determine the evaporation temperature of said refrigerant.

13. A heat pump according to claim 8 or 11, characterized in that the controllable heating (12, 20) means is actuated gradually to maintain an evaporation pressure of the refrigerant at a predetermined level.

14. A heat pump according to claim 13, characterized in that a pressure sensor (15) is provided to determine the evaporation pressure of said refrigerant.

15. Method for controlling a heat pump system provided with a compressor means (1), a condenser (2), a pressure reduction means (13, 18), an evaporator (4), a refrigerant, a refrigerant circulating conduit connecting said compressor means, said condenser, said pressure reduction means and said evaporator, and a control unit (13) characterized by heating a section of the heat pump system by means of at least one controllable heating means (12, 20) for heating said refrigerant, which heating means can be located between a first position downstream of the condenser (2) and a second position upstream of the compressor means (1), when at least one predetermined condition is fulfilled.

16. Method according to claim 15, characterized by actuating the controllable heating means (12, 20) when the temperature of a medium used to heat the evaporator (4) drops below a predetermined value

17. Method according to claim 16, characterized by controlling the heating means (12, 20) in relation to the temperature of said medium.

18. Method according to claim 17, characterized by actuating the controllable heating means (12, 20) gradually to maintain an evaporation temperature of the refrigerant at said predetermined value.

19. Method according to claim 17 or 18, characterized by actuating the controllable heating means (12, 20) gradually to maintain an evaporation pressure of the refrigerant at a predetermined level.