System for a heat pump

Abstract

System (10, 11) for a heat pump cycle including: - an evaporator (70) in which the working fluid evaporates with a predetermined evaporation pressure by absorbing heat from a cold source, - a first low-pressure compressor (20) or compression stage and at least a second high pressure_compressor (30) or compression stage, for compressing a working fluid to the vapor state, each first and second compressor being equipped with a plurality of compression stages, - a condenser (50) to condense the working fluid which releases heat to a hot source, - at least one lamination valve (60), in which the working fluid in the liquid state is reduced in pressure, - an auxiliary condenser (80), downstream of an intermediate compression stage between the first (20) and at least a second compressor (30), configured to process a partial or total flow rate of working fluid.

Description

[0001] SYSTEM FOR A HEAT PUMP

[0002] DESCRIPTION

[0003] Technical sector of the invention

[0004] The present invention relates to a heat pump system and more specifically to a heat pump system in which the cooling of the cold source is to be guaranteed, even when the hot source is not able to receive all the thermal power that the heat pump would discharge.

[0005] Known art

[0006] As it is known, a heat pump system operates with a so-called inverse thermodynamic cycle. A heat pump inverse cycle is a thermodynamic process that allows the transfer of heat from a source at a lower temperature, which cools down, to a source at a higher temperature, which heats up. The cycle, so called inverse as it transfers heat from a cold source to a hot source, is made possible by the absorption of electrical power (transformed into mechanical power) in a machine (compressor), to compress a working fluid in a closed circuit, which in the aforementioned cycle undergoes at least four transformations: evaporation, compression, condensation and lamination.

[0007] In particular, a heat pump cycle system includes an evaporator, in which the working fluid vaporizes by absorbing heat from the low temperature thermal source, a compressor that, by absorbing power from an electric motor, raises the pressure of the working fluid; a condenser in which the working fluid condenses and releases heat to the user (hot source, which heats up further), a lamination valve (or in any case an expander) in which the working fluid in liquid phase expands in a two-phase field up to a lower pressure, i.e. to that of the evaporator, in order to close the cycle. A spin multiplier is normally placed between the electric motor and the compressor.

[0008] The heat pump cycle, as mentioned, cools the cold source, which releases heat to the working fluid in the evaporator, and heats the hot source, which absorbs heat from the working fluid in the condenser.

[0009] The two heat sources are mutually connected in terms of power, in order to respect the energy balance. Once PQ1 is said to be the thermal power subtracted from the thermal source at an average temperature T1 and PQ2 the thermal power transferred to the hot source at an average temperature T2, by the first law of thermodynamics it must be:

[0010] PQ2= PQ1 + PC where PC indicates the mechanical power introduced by the compressor.

[0011] If the temperatures T1 and T2 of the sources remain unchanged, for example, as the thermal power PQ2 required by the hot source decreases, the thermal power PQ1 subtracted from the cold source must necessarily decrease.

[0012] The heat pump is a versatile machine that must necessarily be able to follow the cases outside the design point "off-design") required by the various industrial processes in which it is placed: for example, in its normal operation it must therefore be able to vary the thermal load to the condenser, as the thermal load to the evaporator varies.

[0013] In cases in which the heat pump is inserted in a context in which the useful effect is both the generation of heat at high temperature and the subtraction of heat from the cold source and both these sources may be requested to operate at different and not mutually correlated operating points, the intrinsic physical characteristic of heat pumps takes on the character of a technical problem to be solved.

[0014] A practical example to which the purpose of the present invention refers is made by those applications in which it is not possible to reduce the heat subtraction from the cold source following a reduction in the thermal power required at high temperature, i.e. at the condenser; the problem to be solved is therefore that of decoupling the thermal sources of a heat pump.

[0015] The state of the art would solve this problem with one of the following two solutions:

[0016] - using an auxiliary cooler external to the heat pump cycle, in series or in parallel with the evaporator, so as to ensure the cooling of the cold source while maintaining it at the nominal operating point, even in presence of a lower "off-design" thermal power required by the condenser. The auxiliary cooler, which could be for example an air-working fluid exchanger, is an autonomous system that does not involve the thermodynamic cycle in any way, it being as mentioned inserted in series or in parallel with the evaporator, and from the cold source it is considered as an additional exchanger in addition to the evaporator that is activated, should the heat pump require a lower input of required power. This solution would not be feasible in relation to the temperature of the cold source compared to the ambient temperature. In fact, in many applications the cold source has a temperature close to ambient temperature, so in this case either considerable exchange surfaces would be necessary, or the temperature could even be lower, which would make it necessary to install a chiller or more generally a machine for the cold generation, therefore an additional component.

[0017] - another known solution consists in dissipating a part of the thermal power downstream of the compressor by means of an auxiliary condenser, for example in parallel with the main condenser of the cycle. Such auxiliary condenser could be either an air condenser that discharges the heat directly into the environment (air condenser) or an auxiliary condenser cooled by an auxiliary water circuit. Compared to the previous one, this solution does not have the problem that the heat must be dissipated at a temperature close to the ambient temperature as, on the contrary, the dissipation occurs at a high temperature, downstream of the compressor.

[0018] In this case, even in absence of "duty" at the main condenser of the cycle, the complete condensation of the working fluid is guaranteed by dissipating its overheating and its latent heat and, if necessary, by subcooling it. This method is not optimized, as will be indicated by highlighting the advantages of the proposed innovative solution.

[0019] There is therefore a need for a design solution for the heat pump system that solves or at least mitigates the above-mentioned drawbacks.

[0020] Summary of the invention

[0021] According to one aspect of the present invention, a heat pump system is described, having the characteristics set forth in the independent product claim, annexed to the present description.

[0022] Further preferred and / or particularly advantageous embodiments of the aforementioned system are described, according to the characteristics set forth in the annexed dependent claims.

[0023] Brief description of the drawings

[0024] The invention will now be described with reference to the annexed drawings in which:

[0025] - Figure 1 illustrates a first example of implementation of the cycle diagram of a heat pump system according to the present invention,

[0026] - Figure 2 illustrates a second example of implementation of the cycle diagram of a heat pump system, according to a variant of the present invention.

[0027] Detailed description

[0028] Referring to Figure 1, the technical solution to the problem described above is found with the scheme proposed below.

[0029] The scheme illustrated in Figure 1 represents a heat pump system 10 that interacts with a cold source having an inlet 1 and an outlet 2 and with a hot source having an inlet 3 and an outlet 4. The system 10 comprises at least two compressors or two compression stages - a first compressor 20 or compression stage, of low pressure, and a second compressor 30 or compression stage, of high pressure - driven by a motor 40, for example an electric motor, each of the two compressors or compression stages being provided with a single or a plurality of compression stages, a condenser 50, a lamination valve 60, an evaporator 70.

[0030] Under normal operating conditions, the working fluid evaporates into the evaporator 70 absorbing heat from the cold source, is compressed in the low pressure compressor 20, then its entire flow rate is compressed in the high pressure compressor 30; it then reaches the condenser 50, where it condenses by releasing heat to the hot source, then it laminates (reducing its pressure) through the lamination valve 60; finally it reaches the evaporator 70 again, where it returns to the vapor state to start the work cycle again.

[0031] In addition, the system 10 according to the present invention comprises an auxiliary cooler 80, for example an air condenser, to which a line 25 is connected in fluid connection with the delivery from the low pressure compressor 20.

[0032] The task of the air condenser 80 is to free the cycle dependence from the condenser 50, by cooling and condensing a portion of the flow rate downstream of the low pressure compressor 20, in order to partially distribute the flow rate to others (or to the other) compressor 30 downstream of the first one, in order to produce at the condenser 50 only the thermal power required in the case of off-design, but at the same time to guarantee the desired cooling of the cold source. The flow rate of working fluid that passes through line 25 is regulated by a control valve 28. Downstream of said auxiliary condenser 80, it is then necessary to provide a second lamination valve 60', to expand the fluid in the two-phase field up to the pressure of the evaporator 70.

[0033] The advantages of this system are mainly due to the reduction of electrical power absorbed by the motor with the same useful effect compared to the second known solution, described above: the bleed of the working fluid does not in fact occur at the end of compression but in an intermediate position of the compression stage, for example, either downstream of the low pressure compressor 20, as in the diagram in Figure 1, or downstream of an intermediate stage of one of the two compressors, possibly at the lowest possible pressure / temperature, to allow the heat dissipation capacity towards the environment with a limited sizing of the dissipator.

[0034] This solution allows to reduce the electrical power absorbed in compression, as the mechanical power absorbed by the compressor 30 is lower, having to process a smaller flow rate, at the same time guaranteeing the required power in evaporation.

[0035] In fact, if, on the contrary, the air cooler 80 would be put in bypass to the condenser 50, downstream of the entire compression (as in the second state-of-the-art solution reported above), necessarily the electrical absorption of the compression would be at the maximum (full power also of the compressor 30).

[0036] The final result is in any case an increase in the overall efficiency of the machine compared to typical state-of-the-art solutions.

[0037] Specifically, the example of a case study is given, in which the duty required on the hot side (the heat pump condenser) decreases from 100% to 85, while the cold side must remain at 100%. The "85% P_condState of art" case is a typical solution according to the state of the art, in which said off-design case is intended to be managed by dissipating the excess power on the condenser (in excess of the nominal case), by means of an auxiliary cooler 80 downstream of the compressor 30. In the requested case, the hypothesis of having to maintain 100% of the power extracted from the evaporator is always valid. Furthermore, the "85% P_cond" variant is proposed for comparison, which uses the scheme according to the present invention.

[0038] Table 1

[0039] The proposed solution, as can be seen, leads to a significant benefit, due to the possibility of partializing the flow rate of the working fluid processed by the second compressor.

[0040] We can therefore conclude that, in the case study considered, with the same useful effect (which is the combination of the useful effect of the heat transfer to the condenser and that of the heat extraction at the evaporator), the solution according to the present invention leads to a mechanical power saving of approximately 8%. On large-scale systems, which can have mechanical absorptions of the order of tens of MW, this power saving is far from negligible and leads to a great advantage also in terms of investment costs of the exchanger 80 in addition to, as it is evident, a significant decrease in absorbed power.

[0041] The control cycle of a heat pump system, according to the present invention, can be schematized as follows.

[0042] We define compressor 20 as the first compressor, the one closest to the low pressure, and compressor 30 as the one downstream of the first compressor 20, towards the high pressure area.

[0043] The compressor 20 is regulated according to the needs of the cold source and is independent of the condensation duty.

[0044] In fact, the flow rate processed by this first compressor must be the one needed for the heat exchange at the evaporator. More specifically for the compressor 20, the target flow rate at the evaporator determines the orientation of the compressor's IGV blades and the optimization of the compressor's operating point by varying the compressor's rpm and regulating a recirculation valve aginst a surge condition. In this way, the flow rate will be increased, when required, while remaining at the optimum operating point.

[0045] The compressor 30, on the other hand, is controlled according to the requests of the condensation side: the flow rate requested at the condenser determines the orientation of the IGV blades of the second compressor, the modulation of the opening of valve 28 on the line 25, for admission to the auxiliary air condenser.

[0046] With reference to Figure 2, a second embodiment of the invention is now illustrated.

[0047] According to this system diagram 11, the working fluid evaporates in the evaporator 70, enters a recuperator 110, where it overheats before entering the compressor 20, and from there it undergoes the same process described in Figure 1, up to the expansion through the lamination valve 60. The system 11 is also equipped with a separator 90: the portion of fluid supplied to the air condenser 80 is transferred by means of a pump 100, which may not even be necessary, as the pressure difference between the air condenser and the separator could be overcome with an appropriate head of liquid fluid, inside a separator 90, as well as the portion of fluid that exits the condenser 50 and is laminated by the valve 60: from here the fluid is separated into its liquid phase, which is supplied to the recuperator 110 and into its vapour phase, supplied by means of the line 95 downstream of the first compressor 20.

[0048] Before entering the evaporator 70, all the working fluid in liquid phase is laminated again by means of a further lamination valve 120, then it reaches the evaporator 70 by closing the cycle.

[0049] The recuperator 110, interposed between the air condenser 80 (or the separator 90) and the evaporator 70, therefore allows the heat exchange between the working fluid in liquid phase, by coming from the air condenser 80 and the separator 90, and the working fluid in vapor phase coming from the evaporator 70.

[0050] It should also be noted that the fluid according to the system diagram 11 is laminated in two phases: a first phase through the valve 60 up to the pressure corresponding to the outlet pressure of the compressor 20 (considering appropriately the pressure losses of the lines and of the components) and a second phase, through the valve 120, up to the evaporation pressure.

[0051] This embodiment has two main advantages, connected to the presence of the recuperator and of the possible separator.

[0052] The presence of the separator 90 allows saturated steam to be supplied between the compression phases; this leads to two benefits:

[0053] - cooling of the steam flow rate exiting the first compressor, by mixing the saturated steam portion (of flow rate 95) with the superheated steam (the one exiting the compressor 20), by acting as an intercooler;

[0054] - increasing the volumetric flow rate between the compressor 20 and the compressor 30, in favor of the machine efficiency of the compressor 30.

[0055] The presence of the recuperator 110, which receives the liquid phase exiting the separator 90, is linked to the need for subcooling the liquid upstream of the lamination valve 120, to ensure the cooling of the cold source at the evaporator: in some cases of machine operation, in the absence of subcooling of the fluid upstream of the lamination valve 120, the lamination before the evaporation would start from saturated conditions and at the end of the lamination the temperature of the two-phase fluid could still be too high, not being able to cool (or to appropriately cool) the cold source at the evaporator.

[0056] To overcome this technical problem, it is necessary to subcool the liquid flow rate upstream of valve 120 and, in the absence of the recuperator, the subcooling duty would be borne by the air condenser 80.

[0057] For a correct comparison between the solution without recuperator and the one with the recuperator, it is considered that the ambient air temperature is low enough to be able to subcool the flow rate upstream of the valve 120 up to the desired temperature level, so as to highlight how the solution with the recuperator is convenient, even where it was not already technically necessary (i.e., in the case in which the air condenser would be supplied with too hot ambient air, so the solution with the recuperator would be the only technically possible with the other components being equal).

[0058] The introduction of the recuperator (which already has benefits in terms of cycle efficiency during nominal operation) allows the temperature level of the fluid entering the air condenser 80 to be "raised", which leads to a smaller footprint of this component with the same duty at its charge: in fact, by recuperating, the subcooling portion to be guaranteed is charged to the recuperator and not to the air condenser which, on the other hand, is supplied with a hotter flow rate 25, precisely because of the presence of the recuperator.

[0059] To give a practical example, by indicating with 10 the thermal units to be extracted from the cycle charged to the air condenser 80, it results that:

[0060] TABLE 2

[0061] This flow rate 25 is in conditions of greater overheating than the system configuration without a recuperator, allowing a smaller size of the air condenser 80 as, for the same thermal power to be extracted, the heat exchange occurs with a greater average logarithmic delta T.

[0062] In addition to the embodiments of the invention, as described above, it is to be understood that numerous further variations exist. It must also be understood that said embodiments are only exemplary and do not limit either the object of the invention, or its applications, or its possible configurations. On the contrary, although the description reported above makes it possible for a skilled person to implement the present invention at least according to one of its exemplary configurations, it must be understood that numerous variations of the described components are conceivable, without departing from the object of the invention, as defined in the attached claims.

Claims

CLAIMS1. System (10, 11) for a heat pump cycle including:- an evaporator (70) in which the working fluid evaporates with a predetermined evaporation pressure by absorbing heat from a cold source,- a first low-pressure compressor (20) or compression stage and at least a second high pressure compressor (30) or compression stage for compressing a working fluid to the vapor state, each first and second compressor being equipped with a plurality of compression stages,- a condenser (50) to condense the working fluid which releases heat to a hot source,- at least one lamination valve (60), in which the working fluid in the liquid state is reduced in pressure, the system (10, 11) being characterized by the fact that it comprises an auxiliary condenser (80), downstream of an intermediate compression stage between the first (20) and at least a second compressor (30), conFigured to process a partial or total flow rate of working fluid.

2. System (10, 11) according to claim 1, in which the auxiliary condenser (80) is an air condenser.

3. System (10) according to claim 1 or 2, in which the auxiliary condenser (80) is in fluid connection on the one hand with the first compressor (20), by means of a line (25) along which there is a control valve(28) conFigured to regulate the flow rate of working fluid that passes through the line (25), and on the other hand with a further lamination valve (60').

4. System (10) according to claim 3, in which the lamination valve (60) and the further lamination valve (60') are both conFigured to reduce the pressure of the working fluid up to the evaporation pressure.

5. System (11) according to claim 1 or 2, comprising a recuperator (110), placed between the auxiliary condenser (80) and the evaporator (70), a further lamination valve (120) placed between the recuperator (110) and the evaporator (70) and a separator (90) located downstream of the condenser (50) and of the auxiliary condenser (80) and upstream of the recuperator (110).

6. System (11) according to claim 5, in which the recuperator (110) is conFigured to operate the heat exchange between the working fluid in the liquid phase, coming from the auxiliary condenser (80) and the working fluid in the vapor phase coming from the evaporator (70).

7. System (11) according to claim 5 or 6, in which the lamination valve (60) is conFigured to reduce the pressure of the working fluid up to a pressure substantially corresponding to the pressure exiting the first compressor (20), while the further lamination valve (120) is conFigured to reduce the pressure of the working fluid from the pressure substantially corresponding to the exiting pressure from the first compressor (20) up tothe evaporation pressure.

8. System (11) according to one of claims 5 to 7, wherein the separator (90) is conFigured for:- receiving the working fluid coming from the auxiliary condenser (80) and the working fluid coming from the condenser (50) and laminated by the lamination valve (60),- separating the working fluid into its liquid phase and its vapor phase,- sending the liquid phase to the recuperator (110) and the vapor phase downstream of the first compressor (20).

9. Method of control of a system (10, 11) for a heat pump cycle, the system comprising a first low pressure compressor (20) and at least a second high pressure compressor (30), the method being characterized by the following phases:- processing in the first compressor (20) the flow rate necessary for the heat exchange to the evaporator based on the orientation of the stator vanes of the compressor,- optimizing the operating point of the first compressor (20) by varying the number of compressor revolutions and regulating a recirculation valve for anti-surge;- processing in at least a second compressor (30) the flow rate necessary for the heat exchange to the condenser based on the orientationof the stator vanes of the second compressor and the modulation of the opening of the valve (28) on the line (25) to the auxiliary condenser (80).

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