Control of a pump to optimize heat transfer

Abstract

The present invention relates to a method for controlling a pump for feeding fluid (F) into a heating system (1000). The heating system has a hot fluid tank (HFT) receiving fluid from an associated fluid reservoir line (5) with an incoming fluid mass flow rate (dm.sub.cw/dt). A pump (P) receives fluid from the line, and pumps the received fluid with a mass flow rate (dm.sub.c/dt). A heat exchanging unit (HX) transfers heat (Q) to the fluid (F) from a medium (R). The transferred heat (Q) is maximized by controlling the pump (P1) in response to this information indicative of the transferred heat (Q), the fluid mass flow rate delivered by the pump thereby having a minimum as a function of the incoming fluid mass flow rate (dm.sub.cw/dt) when maximizing the transferred heat. The invention provides significantly improved heat transfer to the fluid and power savings for the pump. The invention also relates to a heating system, e.g. a heat pump system.

Claims

1. A method for controlling a pump for feeding fluid into a heating system, the heating system comprising: a hot fluid tank, the hot fluid tank receiving fluid from an associated fluid reservoir line with an incoming fluid mass flow rate, a pump, the pump also receiving fluid from the said fluid reservoir line, and pumping received fluid with a variable fluid mass flow rate, the pump and the hot fluid tank receiving fluid from a common junction on said associated fluid reservoir line, and a heat exchanging unit, the heat exchanging unit receiving fluid from the associated fluid reservoir line driven by the pump, and transferring heat to the fluid from a medium, and a control unit, the method comprising: providing information indicative of a transferred heat in the heat exchanging unit to the fluid to the control unit, and with the control unit, controlling the pump in response to said information indicative of the transferred heat to the fluid at least within a finite interval of the incoming fluid mass flow rate, minimizing a fluid mass flow rate delivered by the pump as a function of the incoming fluid mass flow rate when optimizing the transferred heat in said finite interval by solving for the mass flow rate of the pump in a feed-forward control regime: Q=f (T.sub.cw, T.sub.t, T.sub.h, dm.sub.c/dt, dm.sub.h/dt, dm.sub.cw/dt, U, A, cp.sub.c, cp.sub.h) wherein: Q is the mass flow rate of the pump, T.sub.cw is an estimated, or measured, temperature of the incoming fluid, T.sub.t is an estimated, or measured, temperature of fluid in the hot fluid tank, T.sub.h is an estimated, or measured, temperature of the medium at the inlet of the heat exchanging unit, dm.sub.c/dt is the mass flow rate delivered by the pump, dm.sub.h/dt is the mass flow rate of the medium at the inlet of the heat exchanging unit, dm.sub.cw/dt is the estimated, or measured, mass flow rate of the incoming fluid, U is a heat transfer coefficient per area of the heat exchanging unit, A is an effective area for a heat transfer of the heat exchanging unit, cp.sub.c is a heat capacity of the fluid, and cp.sub.h is a heat capacity of the medium.

2. The method according to claim 1, wherein the transferred heat is also optimized outside of said finite interval by operating the pump at a maximum of the fluid mass flow rate deliverable by the pump.

3. The method according to claim 1, wherein controlling the pump at a maximum of the transferred heatat a lower end of said intervalis resulting in the fluid mass flow rate decreasing as a function of the incoming fluid mass flow rate, and a resulting fluid mass flow rate being larger than the incoming fluid mass flow rate thereby resulting in a back flow of heated fluid from the hot fluid tank through the said common junction.

4. The method according to claim 3, wherein controlling the pump at said maximum of the transferred heatat a higher end of said intervalis resulting in an increasing fluid mass flow rate as a function of the incoming fluid mass flow rate.

5. The method according to claim 4, wherein controlling the pumpat the higher end of said intervalis resulting in an increasing mass flow rate being substantially the same as the incoming fluid mass flow rate.

6. The method according to claim 1, wherein providing information indicative of a transferred heat in the heat exchanging unit to the fluid is performed based on input from at least two temperature sensors at an inlet and/or an outlet of the heat exchanging unit at a primary side and/or at a secondary side of said heat exchanging unit.

7. The method according to claim 6, wherein an optimization of transferred heat is performed in a continuous feedback control regime by iteratively changing the mass flow rate of the pump and monitoring a corresponding effect on the transferred heat.

8. The method according to claim 6, wherein an optimization of transferred heat is performed in a logical feedback control regime by either operating the pump at the maximum rated mass flow rate, or operating the pump at a mass flow rate equal to the incoming fluid flow mass rate, the incoming fluid flow mass rate being estimated by either a flow meter in a fluid inlet line, and/or the flow direction being indirectly estimated based on a temperature sensor in between the said common junction and the hot fluid tank by comparison with the temperature of the incoming fluid.

9. The method according to claim 6, wherein an optimization of transferred heat is performed by changing the mass flow rate of the pump and monitoring a corresponding effect on the transferred heat by averaging over a period of time sufficient to reach a steady state of transferred heat with respect to the mass flow rate of the pump.

10. The method according to claim 1, wherein providing information indicative of a transferred heat in the heat exchanging unit to the fluid is performed based on input from one or more temperature sensors at an inlet of and/or within the hot fluid tank.

11. The method according to claim 1, wherein providing information indicative of a transferred heat in the heat exchanging unit to the fluid is performed based on input from one or more flow meters.

12. The method according to claim 1, wherein providing information indicative of a transferred heat in the heat exchanging unit to the fluid is performed based on input from one or more parameters related to a power consumption of the heating system.

13. The method according to claim 1, wherein providing information indicative of a transferred heat in the heat exchanging unit to the fluid is performed based on input from one or more flow meters for measuring the incoming fluid mass flow rate from said fluid reservoir line, and a medium mass flow rate through the primary side of the heat exchanging unit.

14. The method according to claim 1, wherein providing information indicative of a transferred heat in the heat exchanging unit to the fluid is performed based on input from one or more flow meters for measuring the incoming fluid mass flow rate from said fluid reservoir line, or a medium mass flow rate through the primary side of the heat exchanging unit.

15. The method according to claim 11, wherein the pump is applied as an indirect flow meter by utilising characteristics about the pump and one or more applied control parameters for operating the pump.

16. The method according to claim 1, wherein providing information indicative of a transferred heat in the heat exchanging unit to the fluid is performed based on input from one or more parameters related to a power consumption of a compressor compressing a refrigerant in a heat pump system.

17. The method according to claim 1, wherein the heating system comprises a heat pump system, the heat pump system comprising a refrigerant line with said heat exchanging unit, a second, and a third exchanging unit being interconnected, the heat exchanging unit receiving fluid from the pump, and performing sub-cooling of a refrigerant so as to transfer heat to the fluid, the second exchanging unit performing super-heating of said refrigerant so as to transfer heat to the fluid, and the third exchanging unit receiving the refrigerant from said second exchanging unit and performing condensation of said refrigerant, and conveying the cooled refrigerant to the heat exchanging unit.

18. The method according to claim 1, wherein the heating system comprises a condensing boiler system, the condensing boiler system comprising a condensing boiler unit where a combustion process together with a subsequent condensation of water from the combustion process is transferring heat to the fluid.

19. The method according to claim 1, wherein the heating system comprises a solar-based heating system, the solar-based heating system comprising a solar panel where solar radiation heats a medium being driven to the heat exchanging unit.

20. The method according to claim 1, wherein the fluid is city water, and the fluid reservoir line is a city water line.

21. A computer system for controlling a heating system according to the method of claim 1, the computer system comprising: a computer; a data storage means connected to said computer; wherein said computer is adapted to receive information indicative of the transferred heat to the fluid and wherein said computer is adapted to control the pump in response to the information indicative of the transferred heat to the fluid to optimize the transferred heat according to the method of claim 1.

22. A method for controlling a pump for feeding fluid into a heating system, the heating system comprising: a hot fluid tank, the hot fluid tank receiving fluid from an associated fluid reservoir line with a given incoming fluid mass flow rate, a pump, the pump also receiving fluid from the said fluid reservoir line, and pumping the received fluid with a variable mass flow rate, the pump and the hot fluid tank receiving fluid from a common junction on said associated fluid reservoir line, a heat exchanging unit, the heat exchanging unit receiving fluid from the associated fluid reservoir line driven by the pump, and transferring heat to the fluid from a medium, and a control unit, the method comprising: providing information indicative of a transferred heat in the heat exchanging unit to the fluid to the control unit, with the control unit, controlling the pump to optimize the transferred heat in response to the information indicative of the transferred heat, wherein the pump is operated with a first interval where the pump is controlled to mix fluid from the fluid reservoir line and the hot fluid tank, and a second interval in which the pump only draws fluid from the incoming fluid reservoir line, wherein the transferred heat is optimized by solving for the mass flow rate of the pump in a feed-forward control regime: wherein: Q is the mass flow rate of the pump, T.sub.cw is an estimated, or measured, temperature of the incoming fluid, T.sub.t is an estimated, or measured, temperature of fluid in the hot fluid tank, T.sub.h is an estimated, or measured, temperature of the medium at the inlet of the heat exchanging unit, dm.sub.c/dt is the mass flow rate delivered by the pump, dm.sub.h/dt is the mass flow rate of the medium at the inlet of the heat exchanging unit, dm.sub.cw/dt is the estimated, or measured, mass flow rate of the incoming fluid, U is a heat transfer coefficient per area of the heat exchanging unit, A is an effective area for a heat transfer of the heat exchanging unit, cp.sub.c is a heat capacity of the fluid, and cp.sub.h is a heat capacity of the medium.

23. A heating system comprising: a hot fluid tank, the hot fluid tank receiving fluid from an associated fluid reservoir line with an incoming fluid mass flow rate, a pump, the pump also receiving fluid from the said fluid reservoir line, and pumping the received fluid with a mass flow rate, the pump and the hot fluid tank receiving fluid from a common junction on said associated fluid reservoir line, and a heat exchanging unit, the heat exchanging unit receiving fluid from the associated fluid reservoir line driven by the pump and transferring heat to the fluid, and a control unit, the control unit being connected to the pump for variably controlling the mass flow rate, the control unit receives information indicative of a transferred heat in the heat exchanging unit to the fluid, wherein the said transferred heat is optimized by controlling the pump in response to said information indicative of the transferred heat to the fluid at least within a finite interval of incoming fluid mass flow rate to minimize a fluid mass flow rate delivered by the pump as a function of the incoming fluid mass flow rate when optimizing the transferred heat in said finite interval, wherein the control unit solves for the mass flow rate of the pump in a feed-forward control regime: Q=f (T.sub.cw, T.sub.t, T.sub.h, dm.sub.c/dt, dm.sub.h/dt, dm.sub.cw/dt, U, A, cp.sub.c, cp.sub.h) wherein: Q is the mass flow rate of the pump, T.sub.cw is an estimated, or measured, temperature of the incoming fluid, T is an estimated, or measured, temperature of fluid in the hot fluid tank, T.sub.h is an estimated, or measured, temperature of the medium at the inlet of the heat exchanging unit, dm.sub.c/dt is the mass flow rate delivered by the pump, dm.sub.h/dt is the mass flow rate of the medium at the inlet of the heat exchanging unit, dm.sub.cw/dt is the estimated, or measured, mass flow rate of the incoming fluid, U is a heat transfer coefficient per area of the heat exchanging unit, A is an effective area for a heat transfer of the heat exchanging unit, cp.sub.s is a heat capacity of the fluid, and cp.sub.h is a heat capacity of the medium.

24. The heating system of claim 23, further comprising a computer having a data storage means, wherein the computer is adapted to receive said information indicative of the transferred heat to the fluid and wherein said computer is also adapted to control the pump in response to said information indicative of the transferred heat to the fluid.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

(2) FIG. 1 is a graph showing the hot water consumption pattern during a day found in a typical hotel,

(3) FIG. 2 is a schematic drawing of a portion of a general heating system according to the present invention,

(4) FIG. 3A is a schematic drawing of a heat pump system with three connected heat exchanging units according to the present invention,

(5) FIG. 3B is a schematic drawing of a portion of a heating system with a condensing boiler according to the present invention,

(6) FIG. 3C is a schematic drawing of a portion of a heating system comprising a solar-based heating system according to the present invention,

(7) FIGS. 4A and 4B are graphs showing the modelled coefficient of performance (COP) of a heat pump system and the inlet fluid flow into the heat exchanging unit on the secondary side, respectively, according to the present invention,

(8) FIG. 5 is a graph showing the modelled inlet temperature of the fluid into the heat exchanging unit of a heat pump system on the secondary side according to the present invention,

(9) FIG. 6A is a schematic drawing of a portion of a heating system showing some suitable thermodynamic variables in the context of the present invention,

(10) FIG. 6B is a schematic drawing corresponding to FIG. 6A with a control unit and a selection of measurement means according to the present invention,

(11) FIGS. 7A and 7B are graphs showing the modelled heat transfer, Q, as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, according to the present invention,

(12) FIGS. 8A and 8B are graphs showing the modelled fluid mass flow rate of the pump, dm.sub.c/dt, as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, for different scenarios, including the flow with maximized heat transfer (full line) according to the present invention,

(13) FIG. 9 is a schematic drawing of a portion of a heating system where a feed-forward control according to the present invention is implemented,

(14) FIGS. 10A and 10B are schematic drawings of a portion of a heating system where a continuous feedback control according to the present invention is implemented,

(15) FIG. 11 is a schematic drawing of a portion of a heating system where a logical feedback control according to the present invention is implemented,

(16) FIG. 12 shows a graph of the estimated heat transfer (Q.sub.est) as a function of time, and a corresponding graph of the rotational speed (u) of an impeller in a centrifugal pump as a function of time according to the present invention, and

(17) FIG. 13 is a schematic flow chart of a method according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

(18) FIG. 1 is a graph showing the hot water consumption pattern during a day found in a typical hotel. All simulations below are performed with the same assumed consumption pattern which has the duration of 24 hours. The data is chosen somewhat arbitrarily, however, it is supposed to emulate the peaks in consumption found in a typical hotel. Significant peaks in the consumption reflect the hot water usage for showers in the morning and in the evening, whereas the noon peak reflects cooking. FIG. 1 shows the consumption profile in kg/s for hot water at 60 degrees Celsius (C). Whenever the tank temperature drops, the consumption is scaled such that the energy consumption from the tank relatively to 40 degrees C. is always the same. This allows comparison between simulations with different hot water tank temperatures. The rationale behind this is the assumption of a mixing temperature of 40 degrees C. after water has been withdrawn from the tank. The hot water tank in this modeled experiment is a perfectly mixed tank with a volume of 4000 liters. The tank is chosen relatively small so that peaks in consumption significantly affect the temperature of the tank. If the tank was very large the heating would be sufficient as long as its average power is more than the average consumption.

(19) FIG. 2 is a schematic drawing of a portion of a general heating system 1000 according to the present invention. A control unit 60 is controlling a pump P1 via control signal RP so as to feed, or pump, fluid F into the heating system 1000, the heating system comprising a hot fluid tank HFT, the hot fluid tank receiving fluid from a fluid reservoir line 5 with an incoming fluid mass flow rate, dm.sub.cw/dt. The sub-annotation cw may, in a non-limiting meaning, be considered as an abbreviation for city water.

(20) The pump P1 is also receiving fluid from the said fluid reservoir line 5, e.g. a city water line with city water, and pumps the received fluid with a variable mass flow rate, dm.sub.c/dt, as schematically indicated. Both the pump and the hot fluid tank are receiving fluid F from a common junction 6a on the fluid reservoir line. The common junction 6a enables back flow (schematically indicated by drawn arrow BF) through the fluid connection 7 from the hot fluid tank HFT to the pump P1 in some specific cases, as it will be explained in more detail below. Though various flow control means, e.g. valves, may be provided within the context of the heating system 1000 (not shown in FIG. 2), these flow control means should be controlled in a manner consistent with the present invention, e.g. allowing a back flow BF to take place under specific circumstances.

(21) A heat exchanging unit HX1 is in fluid connection via fluid conduction means 6, e.g. a pipe, with the common junction 6a, the heat exchanging unit thereby receives fluid from the fluid reservoir line driven by the pump P1, and within the heat exchanging unit there is transferred heat Q (solid arrow) to the fluid F from a medium R, the medium could be a refrigerant when the heating system comprises a heat pump, cf. FIG. 3A and corresponding description below, or it could be a medium suitable for solar heating, cf. FIG. 3C and corresponding description below. In the embodiment shown, the fluid is heated in so-called counter flow, typically yielding the best heat transfer, with respect to the medium R as seen by opposing directions of flows through the heat exchanging unit HX1. The present invention have, however, also been demonstrated by simulations to work in a parallel flow configuration through the heat exchanging unit. The heat exchanging unit HX1 has inlet 10c and outlet 10d on the primary side, and has inlet 10a and outlet 10c on the secondary side. The pump is in fluid connection via fluid conductions means 8 with the inlet 10a on the secondary side.

(22) As schematically indicated in FIG. 2, there is provided information IQ indicative of a transferred heat Q in the heat exchanging unit HX1 to the fluid F to the control unit 60. This information may be obtained in various ways, directly and indirectly, cf. FIG. 6 and corresponding description below. This may be performed e.g. by appropriately positioned temperature sensors as the skilled person would readily understand. After heating, the fluid is conveyed by fluid conduction means 9a, e.g. a pipe, to the hot fluid tank HFT for storage. In this embodiment, the fluid conduction means 9a is directly connecting the heat exchanging unit HX1 to the tank, but this is not always the case.

(23) When controlling the pump P1, one is presented with a dilemma. The pump can be controlled so that the amount of fluid, i.e. dm.sub.c/dt, into the heat exchanging unit HX1 10 can be determined; however, there is generally no control of how much incoming fluid enters the heating system 1000, i.e. dm.sub.cw/dt, because this is typically equal to the consumption of heated fluid from the hot fluid tank.

(24) The invention is particular in that the transferred heat Q is maximized for an interval, or range, of fluid mass flow rate, dm.sub.c/dt, by controlling the pump P1 in response to the information IQ indicative of the transferred heat Q to the fluid F at least within a finite interval of incoming fluid mass flow rate, dm.sub.cw/dt, possibly in more than one interval incoming fluid mass flow rate. The fluid mass flow rate, dm.sub.c/dt, delivered by the pump thereby has a minimum as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, when maximizing the transferred heat in this finite interval of incoming fluid facilitating the various advantages of the invention, e.g. significantly improved heat transfer to the fluid and power savings for the pump.

(25) FIG. 3A is a schematic drawing of a heat pump system 100 with three connected heat exchanging units HX1 10, HX2 20, and HX3 30, as schematically indicated. The fluid F is fed into the heat pump system 100 in the upper left corner in FIG. 3A using the principle explained in FIG. 2, the reference signs being the same and having the same technical meaning.

(26) The heat pump system comprises a refrigerant line 50 where a refrigerant R is circulated as schematically indicated. Notice how each heat exchanger, HX1, HX2, and HX3, are supplied with fluid F from a corresponding pump, P1, P2, and P3, respectively. Such a heat exchanger configuration with three heat exchangers in series is often called a tri-lobe configuration, and similar configurations are known in the art, cf. U.S. Pat. No. 7,658,082, which is hereby incorporated by reference in its entirety.

(27) The first heat exchanging unit HX1, a second heat exchanging unit HX2, and a third HX3 exchanging unit being interconnected provides an advantageous way of transferring heat from a source of heat entering the fourth heat exchanging unit HX4 40. After being heated, the refrigerant R is conveyed to a compressor COMP 51 where compression takes place as it is conventionally performed in a heat pump system. In the second heat exchanging unit HX2, there is subsequently performed super-heating of the refrigerant so as to transfer heat to the fluid F. Thereafter, the third heat exchanging unit HX3 receives the refrigerant from the second heat exchanging unit HX2 and performs condensation, partially or completely, of the refrigerant, and conveys the cooled refrigerant to the heat exchanging unit HX1.

(28) The heat exchanging unit HX1 then receives the fluid F from the pump P1 according to the present invention, and thereby performs sub-cooling of the refrigerant R so as to transfer further heat Q to the fluid. After the sub-cooling, the refrigerant is conveyed to an expansion valve EXP 52, where the pressure is lowered, or throttled, before the refrigerant is conveyed back to the fourth heat exchanging unit HX4 again, and the refrigerant cycle in the heat pump system can be repeated.

(29) Notice how the heated fluid in this embodiment is conveyed to the inlet of the third pump P3 by fluid conduction means 9b, e.g. a dedicated pipe, which is different from the embodiment in FIG. 2 where the heated fluid was conveyed directly to the hot fluid tank.

(30) From the tank HFT, the heated fluid may be consumed i.e. conveyed away via fluid conduction means 15 in response to a demand for heated fluid.

(31) FIG. 3B is a schematic drawing of a portion of a heating system 200 with a condensing boiler 210 where the fluid F is fed into the condenser by controlling the pump P according to the present invention, the reference signs being again the same as in FIGS. 2 and 3A.

(32) In the condensing boiler 210, a combustion process is performed, schematically indicated by multiple flames 210a. The combustion process heats the fluid conveyed through heat exchanger portion 210b. In the condenser boiler burned gas produces water vapour, which is conveyed with assistance from a fane 210c to a neighbouring heat exchanging portion 210d where condensation of the water vapour also heats the fluid F. In air openings 210e, fresh air for the combustion is conveyed to the combustion process, and cooled air after the condensation is conveyed out from the condenser boiler, as schematically indicated by the small air path arrows within the boiler.

(33) Notice that the heating of the fluid through heat exchanger portion 210b may be seen as a transverse heat exchanging process, as opposed to a parallel or an anti-parallel process, the present invention has been useful for all of these configurations.

(34) Heating system 200 measures the transferred heat to the fluid in the condenser boiler 210 by a temperature measurement by a temperature sensor 270a measuring the temperature of the fluid before, T.sub.c, and by a temperature measurement by a temperature sensor 270b measuring the temperature of the fluid after the boiler, T.sub.c,0. A temperature measurement in the tank HFT by a temperature sensor 271 measuring the temperature of the heated fluid, T.sub.t, can be applied together with a temperature measurement by a temperature sensor 277 measuring the fluid temperature after the common junction, T.sub.aux, for indirect flow measurement, or more specifically an indication of the flow direction. Thus, by comparing the fluid temperature after the common junction, T.sub.aux, with the temperature in the tank, T.sub.t, is it possible to provide a measure of the current flow direction, i.e. whether there is a back flow BF, or not.

(35) FIG. 3C is a schematic drawing of a portion of a heating system 300 comprising in particular a solar-based heating panel 340 where fluid is conveyed into the heat exchanging unit HX by controlling the pump P according to the present invention, the same reference signs having again the same technical meaning as in FIGS. 2, 3A and 3B.

(36) Like in FIG. 3B, temperature sensors 370a and 370b are applied to measure the transferred heat to the fluid, whereas temperature sensors 371 and 377 may be applied for obtaining an indirect measure of the back flow from the hot fluid tank HFT to the common junction 6a.

(37) In the solar heating system 300, the pump P2 is continuously driving the medium R through the system, though the pump could be controlled in dependency on the amount of solar radiation. A bypass valve V is provided to be able to keep the temperature of fluid F after being heated in unit HX, T.sub.c,o, below a certain level. If for example the fluid is city water being heated for domestic appliances, it may be beneficial to keep the temperature low enough to avoid limescale, i.e. precipitation of lime in the heat exchanging unit HX and the connected parts.

(38) FIGS. 4A and 4B are graphs showing the modelled coefficient of performance (COP) of a heat pump system 100 shown in FIG. 3A, and the inlet (reference 10a in FIG. 2) fluid flow into the heat exchanging unit HX1 on the secondary side, respectively, according to the present invention. The consumption profile of heated fluid i.e. water is the one shown in FIG. 1. With the present invention implemented, a COP improvement of around 7.3% is seen. It is clearly seen in FIG. 4B, that the flow is generally lower with the control algorithm according to the present invention (Improved). This also saves power on the corresponding pump. This saving is however not quantified in this graph.

(39) FIG. 5 is a graph corresponding to FIG. 4B showing the modelled inlet temperature of the fluid, T.sub.c, into the heat exchanging unit HX1 on the secondary side according to the present invention (Improved). The water temperature at the inlet of HX1 is significantly lower on average for the invention compared to the original curve where the pump is operated with a constant flow rate as seen in FIG. 4B. This is because less water from the hot fluid tank HTF is mixed into the water from the city water line 5 before entering the heat exchanging unit HX1, cf. FIG. 3A, when using the present invention.

(40) FIG. 6A is a schematic drawing of a portion of a heating system 100 showing some suitable thermodynamic variables in the context of the present invention. The heating system could form part of heat pump system 100 like in FIG. 3A, or heating systems 200 or 300 shown in FIGS. 3B and 3C, respectively. FIG. 6B shows some measurement means, e.g. flow sensors and temperature sensors, for finding some of these variables. The reference signs correspond to the reference signs in the previous figures. In FIGS. 6A and 6B, the notation for dm.sub.cw/dt corresponds to {dot over (m)}.sub.cw and so forth, i.e. changing from Leibniz's notation for differentiation to Newton's notation for differentiation (dot notation) with respect to time as the skilled person in mathematics will know.

(41) The maximization of transferred heat Q can be performed by solving for the mass flow rate of the pump P, dm.sub.c/dt;
Q=f(T.sub.cw,T.sub.t,T.sub.h,dm.sub.c/dt,dm.sub.h/dt,dm.sub.cw/dt,U,A,cp.sub.c,cp.sub.h)

(42) where

(43) T.sub.cw is an estimated, or measured, temperature using sensor 75 of the incoming fluid F,

(44) T.sub.t is an estimated, or measured, temperature using sensor 71 of fluid F in the hot fluid tank HFT,

(45) T.sub.h is an estimated, or measured, temperature using sensor 70c of medium R at the inlet 10c of the heat exchanging unit HX,

(46) dm.sub.c/dt is the mass flow rate delivered by the pump measured using flow meter or sensor 86,

(47) dm.sub.h/dt is the mass flow rate of the medium R using flow meter 80 at the inlet of the heat exchanging unit HX,

(48) dm.sub.cw/dt is the estimated, or measured, mass flow rate using flow meter 85 of the incoming fluid, e.g. city water,

(49) U is the heat transfer coefficient per area of the heat exchanging unit HX,

(50) A is the effective area for heat transfer of the heat exchanging unit HX,

(51) cp.sub.c is the heat capacity of the fluid F, e.g. water, and

(52) cp.sub.h is the heat capacity of the medium R, e.g. a refrigerant.

(53) It turns out that the heat transfer rate of the heat exchanger is not necessarily maximized by maximizing dm.sub.c/dt as this can also result in an increase in temperature due to mixing from the hot fluid tank HFT. Actually, the heat transfer rate is dependent on many variables as seen from the equation above. Thus, Q is a function of ten variables. However, only some of them actually varies under practical conditions. That is typically T.sub.cw, T.sub.t, dm.sub.c/dt, dm.sub.h/dt and dm.sub.cw/dt. Out of those only dm.sub.c/dt is controllable, and of the uncontrollable variables only dm.sub.cw/dt is expected to vary significantly, and also to a lower degree T.sub.t and T.sub.h.

(54) The aim is to maximize Q i.e. having the highest heat transfer possible. Since everything but dm.sub.c/dt is fixed, Q can only be maximized by means of changing dm.sub.c/dt. For this embodiment, the parameters arefor purely illustrative purposesfixed as follows:

(55) T.sub.cw=8 deg. C.

(56) T.sub.t=50 deg. C.

(57) T.sub.h=65 deg. C.

(58) dm.sub.h/dt=10 kg/s

(59) U=2750 W/K m.sup.2

(60) A=6 m.sup.2

(61) cp.sub.c=4182 J/kg K (i.e. water)

(62) cp.sub.h=4182 J/kg K (i.e. water)

(63) FIGS. 7A and 7B show Q in a counter flow heat exchanger for increasing dm.sub.cw/dt with the aforementioned parameters, and the full line curve is
dm.sub.c/dt=max(Q) [kg/s]
dm.sub.c/dt{0,7}
according to the invention, the {0,7}denoting the interval from 0 to 7 kg/s.

(64) It is not trivial to select the optimal flow as the flow and temperature are mutually dependent and both affect the heat transfer rate of the heat exchanger, i.e.;

(65) $T_c=\{\begin{array}{cc}\frac{T_{\mathrm{cw}}{\stackrel{.}{m}}_{\mathrm{cw}}+T_t\left({\stackrel{.}{m}}_c-{\stackrel{.}{m}}_{\mathrm{cw}}\right)}{{\stackrel{.}{m}}_c}& \mathrm{if}{\stackrel{.}{m}}_c>{\stackrel{.}{m}}_{\mathrm{cw}}\\ T_{\mathrm{cw}}& \mathrm{if}{\stackrel{.}{m}}_c{\stackrel{.}{m}}_c\end{array}$

(66) These variables can be found in FIGS. 6A and 6B.

(67) Also shown in FIG. 7A is dm.sub.c/dt=7 [kg/s] resulting in non-optimum heat transfer in a central region of incoming fluid mass flow rate, dm.sub.cw/dt. Further, the fluid mass flow rate being equal to the incoming fluid mass flow rate; dm.sub.c/dt=dm.sub.cw/dt, is also shown resulting in a non-optimum heat transfer at a lower and higher value of incoming fluid mass flow rate, dm.sub.cw/dt. This is to illustrate the difference in Q between using full pump speed, and an optimized pump speed to control dm.sub.c/dt. Here the maximum pump speed is set to correspond to 7 [kg/s]. Notice that for dm.sub.c/dt=dm.sub.cw/dtcurve above 7 kg/s, the simulation is of a theoretical character and not achievable in practice due to the pump limitation.

(68) FIG. 7B is graph similar to FIG. 7A showing the modelled heat transfer, Q, as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, the curve according to the present invention again being shown as a full line.

(69) One other control algorithm is included in FIG. 7B for illustrating the advantages of the present invention:

(70) The curve with the temperature of water exiting the outlet 10b of the heat exchanging unit HX, T.sub.c,0=45 deg. C., cf. FIGS. 6A and 6B, corresponds to a control algorithm where this outlet temperature is used as control target. Generally, this will not result in an optimum heat transfer as seen in FIG. 7B. This control target is only approximately fulfilled at the upper range of incoming fluid mass flow rate because the demand will eventually be higher than what can be delivered by the heat exchanging unit in this specific configuration.

(71) FIGS. 8A and 8B are graphs showing the modelled fluid mass flow rate of the pump, dm.sub.c/dt, as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, for different scenarios, including the flow with maximized heat transfer (full line) according to the present invention. Thus, FIG. 8A corresponds to the FIG. 7A, and FIG. 8B corresponds to the FIG. 7B.

(72) As seen in FIG. 8A and FIG. 8Bat least within a finite interval, named I1 and I2, of incoming fluid mass flow rate, dm.sub.cw/dtthe transferred heat, Q, is maximized for an interval of fluid mass flow rate, dm.sub.c/dt, by controlling the pump P, or P1, in response to said information indicative of the transferred heat, Q, to the fluid as explained above. The fluid mass flow rate, dm.sub.c/dt, delivered by the pump thereby has a minimum M as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, when maximizing the transferred heat in said finite interval as explained above. The minimum M is positioned between the two sub-intervals I1 and I2, the minimum having a point-like character in the graphs.

(73) It is noted in FIGS. 8A and 8B, that the heat transfer Q is also being maximized outside of the finite interval I1 and I2 by operating the pump at the maximum of fluid mass flow rate, dm.sub.c/dt,max, deliverable by the pump, P or P1, the maximum in this particular case being 7 kg/s.

(74) The figure shows how the optimal dm.sub.c/dt is significantly lower than its maximum value. It also shows that the optimal dm.sub.c/dt is not simply related with dm.sub.cw/dt. For low dm.sub.cw/dt, more heat transfer occurs, if the water is drawn from the hot fluid tank, the more the better. As dm.sub.cw/dt increases, it's low temperature becomes more attractive than the high flow which can be drawn from the tank HFT. Quite quickly the optimal dm.sub.c/dt becomes equal to dm.sub.cw/dt, even though the flow is rather low. This continues to be the optimal solution until the pump speed is saturated and runs at maximum speed again. The exact optimal curve with maximum heat transfer varies drastically with changed operating parameters. The colder the incoming city water is in relation to the temperature of water in the tank, the more favorable the unmixed city water is and therefore lower flows. The larger the heat exchanger (larger U.Math.A), the more the high flow rates are favorable. The lower the flow on the secondary side, the more favorable is the low city water temperature. The modeling of FIGS. 7 and 8 have been repeated for other heat exchanger configurations, e.g. parallel flow; however the conclusions are the same.

(75) It is also noted that when controlling the pump at a maximum of heat transfer Qat the lower end I1 of the intervalone obtains a mass flow rate, dm.sub.c/dt, which is decreasing as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, and the resulting mass flow rate being larger than the incoming fluid mass flow rate thereby results in a non-vanishing back flow BF of heated fluid, e.g. water, from the hot fluid tank HFT through the common junction 6a.

(76) After reaching the minimum value M of fluid mass flow rate, the control according to the invention results in an increasing fluid mass flow rate, dm.sub.c/dt, at the higher end I2 of the interval as a function of the incoming fluid mass flow rate dm.sub.cw/dt, especially an increasing fluid mass flow rate being substantially the same as the incoming fluid mass flow rate.

(77) In FIG. 6B, some control and sensing means, e.g. temperature sensors and flow meters, for implementing the invention are shown, however for practical implementation a more limited selection of sensing means may be applied. In FIGS. 9-12, and the corresponding description below, some possible implementations are shown.

(78) FIG. 9 is a schematic drawing of a portion of a heating system 100 where a feed-forward control according to the present invention is implemented.

(79) One way of determining the optimal dm.sub.c/dt would be to have information of all the parameters of the function f above, and then calculate the optimal dm.sub.c/dt for the given parameters. This can be attempted in closed form, but can be computationally challenging, or it can be done numerically. Either way, it requires four temperature sensors and three flow measurements, along with knowledge of the U.Math.A value of the heat exchanger and the specific heat capacities of both the fluid F and medium R. It also requires knowledge of the specific heat exchanger configuration (counter flow, parallel flow, cross flow). Due to this it may be impractical to do it this way. The required values are shown in FIG. 9, where three flow meters can measure dm.sub.cw/dt, dm.sub.c/dt and dm/dt, respectively, cf. FIG. 6B for corresponding flow meters, and four temperature measurements T.sub.cw, T.sub.t, T.sub.c, and T.sub.h are made using corresponding sensors 75, 71, 70a, and 70b, respectively.

(80) FIGS. 10A and 10B are schematic drawings of a portion of a heating system 100 where a continuous feedback control according to the present invention is implemented. In this embodiment, one uses the knowledge that there exist an optimal dm.sub.c/dt i.e. between zero and dm.sub.c/dt,max, and then search for it by perturbing dm.sub.c/dt and see if the measured Q changes negatively or positively. dm.sub.c/dt is then changed in the direction that maximizes Q. The procedure is then repeated over and over. This approach automatically takes all factors into account. However, the effects of the pertubations in dm.sub.c/dt on Q may vanish due to the effects of changes in some of the other variables affecting Q. Therefore, the adaptation of dm.sub.c/dt should be rather slow, so intermediate effects of changing conditions does not affect dm.sub.c/dt greatly. It should still be fast enough, however, to be responsive to more long lasting effects which are changing the conditions. Q can be estimated using the measurements T.sub.c and T.sub.c,o combined with a measurement of dm.sub.c/dt, or estimate of it, using knowledge of the pump parameters. The estimate does not need to be very accurate, since it is only used to determine the direction of dm.sub.c/dt. These sensors are shown in FIG. 10, excluding the possible flow sensor. Thus, the maximization of transferred heat Q is performed in a continuous feedback control regime by iteratively changing the mass flow rate dm.sub.c/dt of the pump, P1 or P, and monitoring by temperature sensors 70a and 70b, it is possible to measure, or estimate, the corresponding effect on the transferred heat Q.

(81) FIG. 10B differs from FIG. 10A in that the pump P is positioned after the heat exchanging unit HX i.e. in the fluid conduction means 9a. This, however, does not alter the basic principle of the invention. Similarly, the position of the pump, P or P1, in the other embodiments of the present invention may be changed to another position relative to the heat exchanging unit, HX or HX1, as long as the pump is capable of driving the fluid through the unit.

(82) FIG. 11 is a schematic drawing of a portion of a heating system 100 where a logical feedback control according to the present invention is implemented. Most of the time dm.sub.c/dt=dm.sub.cw/dt, or dm.sub.c/dt=dm.sub.c/dt,max, and when dm.sub.c/dt, max>dm.sub.c/dt*>dm.sub.cw/dt, the optimum is very vague, meaning that the dm.sub.c/dt=dm.sub.c/dt* is not significantly better than dm.sub.c/dt=dm.sub.cw/dt, or dm.sub.c/dt=dm.sub.c/dt,max in terms of Q. Therefore, it would be close to optimal to only switch between these two values. It is trivial to achieve dm.sub.c/dt=dm.sub.c/dt,max, however, it can be hard to achieve dm.sub.c/dt=dm.sub.cw/dt since it requires some measurement of the incoming fluid mass flow rate. This could be done in many ways, e.g. using flow meters. However, a very simple way of obtaining the needed information is to measure the temperature in the pipe supplying the hot fluid tank HFT with city water. The temperature sensor 77 is shown in FIG. 11 where it measures T.sub.aux. If T.sub.aux.fwdarw.T.sub.cw one knows that dm.sub.c/dt<dm.sub.cw/dt and, similarly, if T.sub.aux.fwdarw.T.sub.t one knows that dm.sub.c/dt>dm.sub.cw/dt. This information can then be used to keep dm.sub.c/dt close to dm.sub.cw/dt by controlling the pump P and hence dm.sub.c/dt. The algorithm thus determines whether to change flow by perturbing dm.sub.c/dt continuously and analyzing the measured heat transfer rate.

(83) Thus, the maximization of transferred heat Q is performed in a logical feedback control regime by either operating the pump P at the maximum rated mass flow rate, dm.sub.c/dt,max, or operating the pump at a mass flow rate, dm.sub.c/dt, equal to incoming fluid flow mass rate, dm.sub.cw/dt, the incoming fluid flow mass rate being estimated by either a flow meter 85 in the fluid inlet line (not shown in FIG. 11 but in FIG. 6B), and/or the flow direction being indirectly estimated based on a temperature sensor 77 in between the said common junction 6a and the hot fluid tank HFT by comparison with the temperature of the incoming fluid, T.sub.cw.

(84) FIG. 12 shows a graph of the estimated heat transfer Q.sub.est as a function of time, and a corresponding graph of the rotational speed u of an impeller in a centrifugal pump P as a function of time according to the present invention. This embodiment can be implemented with the heating system 100 shown in FIG. 11. The issue is to determine what effect a change in rotational speed u of the impeller will have on the transferred heat Q.

(85) This heat can be estimated by realizing that
Q.sub.est=k*u*[T.sub.c,0T.sub.c]
where the constant k includes the heat capacity of water and the mapping between u and dm.sub.c/dt. However, it is only important, if Q is increasing or decreasing, hence the estimate Q.sub.est may be applied.

(86) In FIG. 12, the pump is initially operated at the maximum impeller speed u1 and hence maximum dm.sub.c/dt, and for a period of time T, the pump is then operated at a lower impeller speed u2. Upon changing from u1 to u2, the heat transferred will experience a transient TA, but will shortly thereafter reach a steady state level QL, in this case a constantly increasing steady state level. Upon changing the impeller speed back to the u1 level, the transferred heat will have yet another transient TB before the original level of heat transferred is reached again, in this case a constantly increasing level. By choosing the period T long enough the transients TA and TB will cancel each other and thereby have a relatively low impact on the average value of Q.sub.est.

(87) If the average of Q.sub.est in the period T is higher than the average value before and/or after the period, it is worth changing impeller speed to u2. Thus, the maximization of transferred heat, Q.sub.est, is performed by changing the mass flow rate, dm.sub.c/dt, of the pump and monitoring the corresponding effect on the transferred heat by averaging over a period of time T sufficiently long enough to reach a steady state of transferred heat with respect to the mass flow rate of the pump. Notice that the steady state level could also be a constant level, or alternatively a constantly decreasing level.

(88) FIG. 13 is a schematic flow chart of a method according to the invention. The method comprising the steps of

(89) S1 providing information indicative IQ of a transferred heat Q in the heat exchanging unit, HX1 or HX, cf. FIGS. 2-12, to the fluid F, and

(90) S2 maximizing the transferred heat Q to the fluid F by controlling the pump, P1 or P, in response to this information IQ, the fluid mass flow rate, dm.sub.c/dt, delivered by the pump thereby having a minimum as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, when maximizing the transferred heat.

(91) The invention can be implemented by means of hardware, software, firmware or any combination of these. The invention or some of the features thereof can also be implemented as software running on one or more data processors and/or digital signal processors.

(92) The individual elements of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way such as in a single unit, in a plurality of units or as part of separate functional units. The invention may be implemented in a single unit, or be both physically and functionally distributed between different units and processors.

(93) Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms comprising or comprises do not exclude other possible elements or steps. Also, the mentioning of references such as a or an etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

(94) In short, the present invention relates to a method for controlling a pump for feeding fluid F into a heating system 1000. The heating system has a hot fluid tank HFT receiving fluid from an associated fluid reservoir line 5 with an incoming fluid mass flow rate, dm.sub.cw/dt. A pump P receives fluid from the line, and pumps the received fluid with a mass flow rate, dm.sub.c/dt. A heat exchanging unit HX transfers heat Q to the fluid F from a medium R. The transferred heat Q is maximized by controlling the pump P1 in response to this information indicative of the transferred heat Q, the fluid mass flow rate delivered by the pump thereby having a minimum as a function of the incoming fluid mass flow rate, dm.sub.cw/dt, when maximizing the transferred heat. The invention provides significantly improved heat transfer to the fluid and power savings for the pump. The invention also relates to a heating system, e.g. a heat pump system.

LIST OF REFERENCE NUMERALS

(95) 5 reservoir line 6a common junction 6, 7, 8, 9a, 9b, 15 fluid conduction means 10, 20, 30, 40 heat exchanging units 10a, 10b, 10c, 10d inlets and outlets on heat exchanging unit 50 refrigerant line 51 compressor 52 expansion valve 60 control unit 80, 85, 86 flow meters 100 heat pump system 70a, 70b, 70c, 70d, 71, 75, 77 temperature sensors 200 condensing boiler heating system 210 condensing boiler 270a, 270b, 271, 277 temperature sensors 300 solar-based heating system 340 solar panel 370a, 370b, 377 temperature sensors 1000 heating system BF back flow dm.sub.c/dt fluid mass flow rate delivered by pump dm.sub.cw/dt incoming fluid mass flow rate F fluid HX, HX1, HX2, HX3 heat exchanging units I1, I2 interval of incoming fluid mass flow rate P, P1, P2, P3 pumps V valve R medium or refrigerant