The present invention relates to a heat pump system. A heat pump system is a device that provides thermal energy from a heat source to a heat sink. Under operating conditions it moves thermal energy opposite to the direction of spontaneous heat flow by absorbing heat from a cold space and release it to a warmer one. In order to transfer the thermal energy a heat pump system uses external power, often electrical power. Heat pump systems use a working fluid, for example a refrigerant. The working fluid absorbs heat in an evaporator where it evaporates, and then releases heat in a condenser where it condenses. The heat transferred can be several times larger than the external power consumed, resulting in that a heat pump system may have a Coefficient of Performance (COP) larger than 1. The COP may be defined as the amount of thermal energy moved per unit of input work. For example, a higher temperature difference between the cold side and the hot side of the heat pump system requires more energy to compress the working fluid, hence reducing the COP. Several types of heat sources are possible. A lot of known heat pump systems are water-source heat pumps.

The present invention aims to provide an improved heat pump system.

For this purpose the heat pump system comprises the features of claim 1.

An advantage of the heat pump system according to the invention is that it provides a high COP under a wide range of operating conditions. If the actual temperature of the working fluid between the evaporator and the compressor is higher than the reference temperature the working fluid is substantially entirely gaseous when compressed by the compressor and there is a certain degree of overheating. The reference temperature may also be defined as vapour temperature of the working fluid. The basic control module prevents the heat pump system from

overheating the working fluid too much in order to increase its total efficiency, on the one hand, and prevents the heat pump system from overheating too little in order to stay away from a condition in which the working fluid is still liquid at the entrance of the compressor, on the other hand.

In conventional heat pump systems the controller controls the flow-through area of the expansion valve towards a predetermined fixed overheating temperature. The value of the overheating temperature is selected such that in practice the working fluid upstream of the compressor is entirely gaseous in practice, for example an overheating temperature of 6 K. This avoids damage of the compressor, but is not always optimal in terms of COP. The present heat pump system does not control the expansion valve towards a predetermined fixed overheating temperature, but the basic control module controls the flow- through area of the expansion valve towards a level for reducing the difference between the temperature of the heat source fluid at the heat source fluid inlet and the reference temperature. This means that the basic control module attempts to keep the reference temperature as close as possible to the source fluid temperature at the source fluid inlet of the evaporator. Of course, the basic control module also monitors other system parameters and overrules the expansion valve control on the basis of the mentioned temperature difference under certain conditions, for example if pressure and temperature of the working fluid at the compressor exceed allowable operating ranges. Since the basic control module attempts to keep the reference temperature as close as possible to the source fluid temperature the heat pump system is controlled to maximum efficiency. Experiments with the heat pump system according to the invention have shown that the system may run at an

overheating temperature of 3 or 4 K, whereas conventional systems run at a fixed overheating temperature of 6 K for safety reasons .

In a practical embodiment the control module increases the flow-through area of the expansion valve if the difference between the temperature of the heat source fluid at the heat source fluid inlet and the reference temperature is higher than a predetermined value, for example 2 K . If the temperature difference is higher than the predetermined level, the basic control module will increase the flow-through area of the expansion valve. Since other parameters of the heat pump system will be affected upon increasing the flow-through area the compressor capacity as well as the flow-through area of the expansion valve must be compensated accordingly, but the basic control module still attempts to control towards a temperature difference at the mentioned predetermined value.

The source fluid may be water, but alternative fluids are conceivable. The source may be an open source or a closed source .

In a preferred embodiment the controller is also provided with a dynamic control module for heating the

temperature of the heat sink fluid at the heat sink fluid outlet to a maximum, wherein the dynamic control module increases the compressor capacity and decreases the flow-through area of the expansion valve, whereas control of the expansion valve on the basis of the difference between the temperature of the heat source fluid at the heat source fluid inlet and the reference temperature of the working fluid is disregarded. The dynamic control module is intended for certain operating conditions in which it is desired to create a relatively high temperature of the heat sink fluid at the heat sink fluid outlet, for example for heating a boiler. In such a condition the dynamic control module overrules the basic control module such that controlling of the expansion valve on the basis of the difference between the temperature of the heat source fluid at the heat source fluid inlet and the reference temperature is disregarded. Basically, the dynamic control module increases the compressor capacity and decreases the flow-through area of the expansion valve. Due to that action other system parameters will also change, for example pressure and temperature of the working fluid downstream the compressor; if these parameters exceed allowable operating ranges the dynamic control module will correct accordingly, but will still try to increase the

compressor capacity and decrease the flow-through area of the expansion valve. Since the basic control module is overruled the overheating temperature may become higher than a level which is desired in terms of high COP. Because of the presence of the dynamic control module the heat pump system according to claim 4 provides the opportunity to reach a higher temperature of the heat sink fluid at the heat sink fluid outlet than conventional heat pump systems.

It appears to be advantageous when the dynamic control module reduces the flow-through area of the expansion valve and increases the compressor capacity alternatingly and step-by- step. This means that when the controller detects a high temperature of the heat sink fluid the dynamic control module changes the flow-through area of the expansion valve by a predetermined relatively small step, then changes the compressor capacity by a predetermined relatively small step and so on. Of course, the basic control module also monitors other system parameters and controls the compressor capacity and/or the expansion valve if certain parameters exceed allowable operating ranges .

Preferably, the capacity of the compressor and/or the flow-through area of the expansion valve is/are variable continuously in order to provide the opportunity to control the system accurately.

In an advantageous embodiment the compressor is a piston compressor. A piston compressor appears to provide a more efficient output at part load conditions with respect to a conventional screw compressor, for example. It is relatively simple to regulate a piston compressor modulatingly . A piston compressor can be operated at a wide speed range and allows a relatively low rotational speed such that the heat pump system, can also run at low part-load conditions where conventional systems would be switched-off .

In a particular embodiment the heat, pump circuit includes an additional heat exchanger between the condenser and the variable flow expansion valve, which additional heat

exchanger is located in a bypass that is branched from a line between the heat source and the evaporator upstream of the evaporator. The additional heat exchanger provides the

opportunity to cool the working fluid further by means of the source fluid, i.e. super cooling the working fluid. Since the working fluid flows through the variable flow expansion valve at a relatively low temperature, the efficiency of expansion in the expansion valve is optimized. Besides, the source fluid

temperature upstream of the evaporator is increased which improves the efficiency of the evaporator.

The invention will hereafter be elucidated with reference to the drawings showing embodiments of the invention by way of example.

Fig. 1 is a very schematic view of an embodiment of the heat pump system according to the invention.

Fig. 2 is a similar view as Fig. 1, but showing an alternative embodiment.

Fig. 1 shows an embodiment of a heat pump system 1 according to the invention. The heat pump system 1 comprises a heat pump circuit 2. Under operating conditions a working fluid is circulated through the heat pump circuit 2. Possible working fluids are hydrofluorocarbon (HFC) known as R-134a, liquid R-717 ammonia, carbon dioxide R-744 , propane or butane, for example. The heat pump circuit 2 comprises a variable capacity compressor 3, a condenser 4, a variable flow expansion valve 5 and an evaporator 6. The heat pump system 1 also comprises a heat source 7. In this case the heat source 7 is an outdoor water source containing water as heat source fluid, but alternative heat source types are conceivable. The evaporator 6 comprises a heat exchanger through which water from the heat source 7 is circulated by means of a heat source pump 8. Under operating conditions water from the heat source 7 flows into the

evaporator 6 at a heat source fluid inlet 9, heats the working fluid in the evaporator 6 and leaves the evaporator at a heat source fluid outlet 10. The working fluid enters the evaporator 6 at a working fluid inlet 11 in a liquid state, is evaporated in the evaporator 6 by the water from the heat source 7 and leaves the evaporator 6 in a gaseous state at a working fluid outlet 12. Due to transfer of thermal energy in the evaporator 6 the water temperature at the heat source fluid outlet 10 is lower than at the heat source fluid inlet 9.

The gaseous working fluid from the evaporator 6 is transported to the compressor 3. The variable capacity

compressor 3 is a piston compressor, of which the output power can be varied continuously. The compressor 3 has a compressor inlet 13 and a compressor outlet 14. Due to compressing the gaseous working fluid in the compressor 3 the temperature of the working fluid at the compressor outlet 14 is higher than at the compressor inlet 13.

The compressed and heated gaseous working fluid is transported from the compressor 3 to the condenser 4. The condenser 4 comprises a heat exchanger for transferring thermal energy from the gaseous working fluid to a heat sink 15. The heat sink 15 may be an interior space of a building. For example, the heat exchanger of the condenser may communicate with an underfloor heating, but numerous alternative heat sinks are conceivable. The working fluid enters the condenser 4 at a working fluid inlet 16 in a gaseous state, is cooled down such that it condenses in the condenser 4 by heat sink fluid from the heat sink 15 and leaves the condenser 4 in a liquid state at a working fluid outlet 17. The heat sink fluid is heated in the condenser 4 when it flows from a heat sink fluid inlet 18 to a heat sink fluid outlet 19 of the condenser 4. Due to transfer of thermal energy in the condenser 4 the heat sink fluid

temperature at the heat sink fluid outlet 19 is higher than at the heat sink fluid inlet 18. The heat sink fluid may be

circulated through the heat exchanger of the condenser 4 by means of a heat sink pump (not shown) .

When the working fluid leaves the condenser 4 it is in a liquid state at the working fluid outlet 17, whereas its pressure is still relatively high. The liquid and high-pressure working fluid is transported from the condenser 4 to the

variable flow expansion valve 5. The expansion valve 5 has an adjustable flow-through area, which can be varied continuously. When flowing from a valve inlet 20 to a valve outlet 21 the working fluid is depressurized but still liquid. The liquid working fluid is then transported from the valve outlet 21 to the working fluid inlet 11 of the evaporator 6, when the

thermodynamic cycle as described above starts again.

The heat pump system 1 is also provided with a controller 22 for controlling the operation of the heat pump system 1. The controller 22 has a plurality of inputs for receiving system variables, such as temperature and pressure values from sensors located at different locations in the heat pump circuit 2, and a plurality of outputs for regulating the system 1, such as control signals to the variable capacity compressor 3 and the variable flow expansion valve 5.

The heat pump system 1 has at least two control modules: a basic control module and a dynamic control module.

The basic control module controls the flow-through area of the variable flow expansion valve 5 on the basis of at least the actual pressure and temperature of the working fluid between the evaporator 6 and the compressor 3 and the actual temperature of the heat source fluid at the heat source fluid inlet 9.

The actual pressure and temperature of the working fluid at the compressor inlet 13 and the temperature of the heat source fluid at the heat source fluid inlet 9 of the evaporator 6 are measured by standard sensors. If all working fluid is just evaporated in the evaporator 6 the actual pressure will be equal to the vapour pressure of the working fluid. The physical relationship between vapour pressure and temperature of the working fluid is known. On the basis of this relationship and the measured pressure a reference temperature level at the vapour pressure is derived. If the actual temperature is higher than the reference temperature the working fluid is entirely in gaseous state and in overheated condition. The basic control module controls the flow-through area of the expansion valve 5 towards a level for reducing the difference between the

temperature of the heat source fluid at the heat source fluid inlet 9 and the reference temperature if the actual temperature of the working fluid is higher than the reference temperature.

The dynamic control module is intended for heating the temperature of the heat sink fluid at the heat sink fluid outlet 19 to a maximum. The dynamic control module increases the compressor capacity and decreases the flow-through area of the expansion valve 5, whereas the basic control module is overruled temporarily.

Upon starting-up the heat pump system 1 from a standstill condition, the compressor 3 and the expansion valve 5 are set at predetermined intermediate operating conditions between minimum and maximum settings and the system 1 is operated during a predetermined time period, for example a few minutes. After this period the system parameters are stabilized such that a reference condition is defined.

Furthermore, the controller 22 comprises additional control modules, for example a module for protecting the heat pump system 1 against damage in extreme outdoor conditions , for example in order to avoid freezing of the source fluid, modules that monitor the maximum system pressure, etc.

Fig. 2 shows an alternative embodiment of a heat pump system 1. This embodiment is similar to the system 1 of Fig. 1, but comprises an additional heat exchanger 23 between the condenser 4 and the variable flow expansion valve 5. The additional heat exchanger 23 is located in a bypass which is branched from a line between the heat source 7 and the heat source fluid inlet 9 of the evaporator 6. In the additional heat exchanger 23 the working fluid downstream of the condenser is further cooled by means of the source fluid. Due to this super cooling effect the efficiency of expansion in the expansion valve 5 is optimized. Besides, the source fluid temperature at the heat source fluid inlet 9 is increased which improves the efficiency of the evaporator 6.

The invention is not limited to the embodiments shown in the drawings and described hereinbefore, which may be varied in different manners within the scope of the claims and their technical equivalents.