SYSTEM FOR REGULATING A TEMPERATURE OF A THERMAL ENERGY CARRYING FLUID IN A SECTOR OF A FLUID DISTRIBUTION NETWORK

Abstract

A system (15) regulates a temperature of fluid in a sector of a fluid distribution network, including a feed line (11) transporting fluid from a thermal energy source (3) to a thermal energy consumer (7) within the sector and a return line (13) transporting fluid back. A bypass line (17) connects the return line to the feed line, mixing fluid from the return line into the feed line. A pump is at the bypass line. A temperature sensor determines a temperature of fluid in the feed line downstream of the bypass line. A pressure sensor determines an uncontrolled pressure difference between the feed line and the return line, or an uncontrolled pressure difference correlated therewith. A control unit controls the speed of the pump with a closed-loop control for achieving a target feed line temperature based on the determined temperature, and a feed-forward control compensating fluctuations of the pressure difference.

Claims

1. A system for regulating a temperature of a thermal energy carrying fluid in a sector of a fluid distribution network, the fluid distribution network comprising a feed line for transporting the fluid from a thermal energy source to at least one thermal energy consumer located within the sector and a return line for transporting the fluid back from the at least one thermal energy consumer to the thermal energy source, the system comprising: a bypass line connecting the return line to the feed line for mixing fluid from the return line into the feed line; a bypass pump arranged at the bypass line for pumping fluid from the return line to the feed line; a temperature sensor arranged and configured to determine a temperature of the fluid in the feed line downstream of the bypass line; a pressure sensor arranged and configured to determine an uncontrolled pressure difference between the feed line and the return line, or to determine a pressure difference correlating therewith; and a control unit configured to directly or indirectly control a speed of the bypass pump based on a combination of: a closed-loop control to provide a fluid flow in the bypass line for achieving a target feed line temperature based on the determined feed line temperature; and a feed-forward control to compensate fluctuations of the pressure difference between the feed line and the return line based on the determined pressure difference.

2. The system according to claim 1, wherein the control unit is configured to combine the closed-loop control and the feed-forward control by determining a target speed (ω) as a parameterized pump model-based function ω=ƒ.sup.−1(q*(T.sub.1),p), wherein the pump model-based function ω=ƒ(q*(T.sub.1),p) is parameterized by at least two pre-determined parameters a.sub.h1, a.sub.h3.

3. The system according to claim 1, wherein the control unit is configured to set a target bypass fluid flow q*(T.sub.1) to minimize a deviation e.sub.T=T*.sub.1−T.sub.1 between the determined feed line temperature T.sub.1 and a target feed line temperature T*.sub.1.

4. The system according to claim 1, wherein the control unit is configured to set a pump differential pressure p to compensate for the determined pressure difference Δp.

5. The system according to claim 1, further comprising a non-return valve at the bypass line downstream of the at least one bypass pump, wherein the non-return valve is configured to prevent a fluid flow through the bypass line from the feed line to the return line.

6. The system according to claim 5, wherein the pressure sensor is arranged and configured to determine the pressure difference by measuring a difference between: a pressure in the bypass line upstream of the bypass pump or in the return line; and a pressure downstream of the non-return valve or upstream of the non-return valve, or a pressure in the feed line upstream of the at least one bypass line.

7. The system according to claim 1, further comprising: a temperature sensor arranged and configured to determine a temperature of the fluid in the return line; and a temperature sensor arranged and configured to determine a temperature of the fluid in the feed line upstream of the bypass line, wherein the control unit is configured to set a target speed based on a feed-forward control to compensate uncontrolled fluctuations of a temperature difference between the temperature of the fluid in the feed line upstream of the bypass line and the temperature of the fluid in the return line.

8. The system according to claim 1, wherein the control unit is configured to stop the bypass pump if the speed is below a pre-determined stop speed threshold, and wherein the control unit is configured to start the bypass pump if the speed is at or above a pre-determined start speed threshold.

9. The system according to claim 1, wherein the control unit is configured, upon start-up of the bypass pump, to ramp up the feed-forward control to compensate uncontrolled fluctuations of the pressure difference based on a ramp-up value multiplied with the determined pressure difference.

10. A method for regulating a temperature of a thermal energy carrying fluid in a sector of a fluid distribution network, the fluid distribution network comprising: a feed line for transporting the fluid from a thermal energy source to at least one thermal energy consume located within the sector; a return line for transporting the fluid back from the at least one thermal energy consumer to the thermal energy source; and a bypass line connecting the return line to the feed line for mixing fluid from the return line into the feed line, wherein the method comprises the steps of: determining a temperature of the fluid in the feed line downstream of the bypass line by at least one temperature sensor; determining a pressure difference between the feed line and the return line by at least one pressure sensor, or a pressure difference correlating therewith; directly or indirectly controlling a speed of at least one bypass pump that is arranged at a bypass line connecting the return line to the feed line for mixing fluid from the return line into the feed line, wherein said directly or indirectly controlling of the speed of the bypass pump is based on a combination of: a closed-loop control to provide a fluid flow in the bypass line for achieving a target feed line temperature based on the determined feed line temperature; and a feed-forward control to compensate uncontrolled fluctuations of the pressure difference between the feed line and the return line based on the determined pressure difference.

11. The method according to claim 10, wherein combination of the closed-loop control and the feed-forward control comprises determining a target speed (ω) as a parameterized pump model-based function ω=ƒ(q*(T.sub.1),p), wherein a target bypass fluid flow q*(T.sub.1) is set to minimize a deviation e.sub.T=T*.sub.1−T.sub.1 between the determined feed line temperature T.sub.1 and a target feed line temperature T*.sub.1, and wherein a pump differential pressure p is set to compensate for the determined pressure difference, wherein the pump model-based function ω=ƒ.sup.−1(q*(T.sub.1),p) is parameterized by at least two pre-determined parameters.

12. The method according to claim 10, further comprising preventing a fluid flow through the bypass line from the feed line to the return line by a non-return valve arranged at the bypass line downstream of the bypass pump.

13. The method according to claim 12, wherein determining the pressure difference comprises measuring a difference between: a pressure in the bypass line upstream of the bypass pump or in the return line, and a pressure downstream of the non-return valve or upstream of the non-return valve, or a pressure in the feed line upstream of the at least one bypass line.

14. The method according to claim 10, further comprising: determining a temperature of the fluid in the return line by at least one temperature sensor; and determining a temperature of the fluid in the feed line upstream of the bypass line by at least one temperature sensor, wherein directly or indirectly controlling of the speed of the bypass pump comprises setting a target speed based on a feed-forward control to compensate uncontrolled fluctuations of a temperature difference between the temperature of the fluid in the feed line upstream of the bypass line and the temperature of the fluid in the return line.

15. The method according to claim 10, further comprising: stopping the bypass pump if the speed is below a pre-determined stop speed threshold; and starting the bypass pump if the speed is at or above a pre-determined start speed threshold.

16. The method according to claim 10, further comprising ramping up, upon start-up of the bypass pump, the feed-forward control to compensate uncontrolled fluctuations of the pressure difference based on a ramp-up value multiplied with the determined pressure difference.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] In the drawings:

[0067] FIG. 1 is a schematic view showing an example of a heating distribution network comprising a system according to the present disclosure;

[0068] FIG. 2 is a schematic view showing a first embodiment of a system according to the present disclosure;

[0069] FIG. 3 is a schematic view showing another embodiment of a system according to the present disclosure;

[0070] FIG. 4 is a schematic view showing a further embodiment of a system according to the present disclosure;

[0071] FIG. 5 is a schematic view showing an embodiment of the control method according to the present disclosure;

[0072] FIG. 6 is a schematic view showing a further embodiment of a control method according to the present disclosure;

[0073] FIG. 7 is a schematic view showing a further embodiment of a system according to the present disclosure;

[0074] FIG. 8 is a schematic view showing a further embodiment of a system according to the present disclosure; and

[0075] FIG. 9a is schematic view showing a feed line temperature T.sub.1 downstream of the bypass line and a pressure difference Δp over the time of the day without applying the control method according to the present disclosure; and

[0076] FIG. 9b schematic view showing a feed line temperature T.sub.1 downstream of the bypass line and a pressure difference Δp over the time of the day in with applying the control method according to the present disclosure.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0077] FIG. 1 shows a fluid distribution network 1 in form of a heating distribution network. The fluid distribution network 1 comprises a thermal energy source 3 in form of a heat generating plant. The thermal energy source fluid that is pumped by a feeder pump 5 through a feed line carries thermal energy for transporting said thermal energy through the feed line from the thermal energy source 1 to a plurality of thermal energy consumers 7 in form of households. The thermal energy consumers 7 are located within a sector 9 of the fluid distribution network 1. The fluid distribution network 1 may comprise a single sector 9, or a plurality of sectors 9 as shown in FIG. 1. A sector 9 of the fluid distribution network 1 may be defined by being supplied with fluid by a common feed line 11 that transports the thermal energy carrying fluid to the thermal energy consumers 7 located in the sector 9 of the fluid distribution network 1. The thermal energy carrying fluid delivers the thermal energy to the thermal energy consumers 7 by means of a heat exchanger 12 at each thermal energy consumer 7. The fluid is then returned back from the thermal energy consumers 7 to the thermal energy source 3 via a return line 13. The fluid distribution network 1 further comprises a system 15 for regulating the temperature of the thermal energy carrying a fluid in the sector 9.

[0078] FIG. 2 shows an embodiment of the system 15 in more detail. The idea is to regulate the temperature of the fluid in the sector 9 by mixing fluid from the return line 13 to the feed line 11 via a bypass line 17. As the pressure in the feed line 11 exceeds a pressure in the return line 13, a bypass pump 19 is installed in the bypass line 17 in order to overcome a pressure difference Δp between the feed line 11 and the return line 13. In order to prevent any backflow through the bypass line 17 from the feed line 11 to the return line 13, a non-return valve 21 is installed in the bypass line downstream of the bypass pump 19. This means that the bypass pump 19 must also provide sufficient pressure for opening the non-return valve in the direction towards the feed line 11. A differential pressure sensor 23 is installed to determine a pressure difference Δp between a pressure in the bypass line 17 upstream of the bypass pump 19 and a pressure in the bypass line 17 downstream of the non-return valve 21. A first temperature sensor 25 is installed in the feed line 11 downstream of the bypass line 17 in order to determine a temperature T.sub.1 in the feed line 11 downstream of the bypass line 17. The measurement of the first temperature sensor 25 is communicated to a control unit 27 that is configured to control directly or indirectly the speed ω of the bypass pump 19.

[0079] One could think that it is now sufficient to simply control the speed ω of the bypass pump 19 in a closed-loop manner based on the temperature T.sub.1 measured by the first temperature sensor 25 as a feedback value. It has shown, however, that this does not work very well, because the temperature T.sub.1 may be difficult to stabilize and may fluctuate too much as can be seen in the upper plot of FIG. 9a. The reason for this is that the first temperature sensor 25 is placed at a certain distance to the bypass line 17 downstream at the feed line 11 in order to measure the fluid temperature where it is sufficiently mixed downstream of the bypass line 17. Consequently, there is a relatively large time delay between a change of the speed ω of the bypass pump 19 and the result showing in a change of the temperature T.sub.1 at the first temperature sensor 25. In addition, the pressure difference Δp between the feed line 11 and the return line 13 is outside of the control of the system 15, i.e. the pressure difference Δp is subject to uncontrolled or uncontrollable disturbances or fluctuations. Such disturbances or fluctuations of the pressure difference Δp may originate from varying consumption behaviour at the thermal energy consumers 7 and/or varying circumstances at the heat generating plant. Any fluctuation of the pressure difference Δp between the feed line 11 and the return line 13, however, strongly affects the effect of the bypass pump 19. Therefore, the measurement of the pressure difference Δp by the differential pressure sensor 23 is also communicated to the control unit 27 in order to perform a feed-forward control to compensate for the fluctuations of the pressure difference Δp. The inventive idea is now to control the speed of the bypass pump 19 by means of the control unit 27 based on a combination of a closed-loop control based on the determined first feed line temperature T.sub.1 and a feed-forward control to compensate fluctuations of the pressure difference Δp between the feed line 11 and the return line 13 based on the pressure difference Δp determined by the differential pressure sensor 23. For example, the control unit 27 may be configured to set a target speed

[00014]$\omega =\sqrt{\frac{p-a_{hs}{q}^{*}.Math.q^{*}.Math.}{a_{h1}}},$

which is a parametrization based on a pump model. q* is the target bypass fluid flow to be provided by the bypass pump 19 and p is the pump differential pressure to compensate for the determined pressure difference Δp. a.sub.h1 and a.sub.h3 are model parameters describing pump characteristics. The model parameters a.sub.h1 and a.sub.h3 may be predetermined pump-specific parameters provided by the manufacturer of the bypass pump 19. For example, one of the parameters a.sub.h1 and a.sub.h3 is a now-flow parameter indicative of the pressure provide by the bypass pump 19 at a certain speed ω and no bypass flow, i.e. q=0. The target bypass fluid flow q*(T.sub.1) is set in a closed-loop manner to minimize a deviation e.sub.T=T*.sub.1−T.sub.1 between the determined feed line temperature T.sub.1 and a desired target feed line temperate T*.sub.1. The result of this combination of a closed-loop control based on a temperature feedback value and a feed-forward control based on a pressure differential value is shown in FIG. 9b, which shows a very stable feed line temperature T.sub.1 over the time of the day despite uncontrolled fluctuations of the pressure difference Δp between the feed line 11 and the return line 13.

[0080] FIG. 3 shows another embodiment of the system 15. The embodiment of FIG. 3 differs from the embodiment shown in FIG. 2 by what the differential pressure sensor 23 measures. In FIG. 3, the differential pressure sensor 23 measures a difference between a pressure at the outlet of the bypass pump, i.e. upstream of the non-return valve 21, and a pressure at an inlet of the bypass pump 19. This has the advantage that the pressure sensor 23 may be pre-installed at and/or integrated into the bypass pump 19. This embodiment, however, is less advantageous in terms of controlling, because the non-return valve 19 may have under certain circumstances difficulties to settle quickly into the correct opening degree. Fluctuations of the opening degree of the non-return valve 19, however, may cause undesired disturbances of the differential pressure measurement. This means that the compensating pump pressure differential p should be ramped-up more slowly at start-up of the bypass pump to avoid large overshooting or undershooting in the closed-loop control based on the feed line temperature T.sub.1.

[0081] FIG. 4 shows another embodiment of the system 15. Compared to the embodiment shown in FIG. 2, the embodiment shown in FIG. 4 comprises two more temperature sensors, wherein a second temperature sensor 29 is installed at the return line 13 in order to determine a second temperature T.sub.2. In FIG. 4, the second temperature sensor 29 is located at the return line 13 downstream of the bypass line 17. Alternatively, the second temperature sensor 29 could be located at the return line 13 upstream of the bypass line or anywhere at the bypass line 17. A third temperature sensor 31 is located at the feed line 11 upstream of the bypass line 17 in order to determine a third temperature T.sub.3. With these additional temperature measurements T.sub.2 and T.sub.3, the system 15 is able to compensate not only for uncontrolled fluctuations of the pressure difference between the feed line 11 and the return line 13, but also for uncontrolled fluctuations of the feed line temperature T.sub.3 and/or the return line temperature T.sub.2. In particular, the return line temperature T.sub.2 may be dependent on the thermal energy consumption at the thermal energy consumers 7. Energy conservation and flow balance demands that the following equations apply: q.sub.1(T.sub.1−T.sub.2)=q.sub.3(T.sub.3−T.sub.2) and q.sub.2(T.sub.1−T.sub.2)=q.sub.3(T.sub.3−T.sub.1), wherein q.sub.1 is the feed line flow downstream of the bypass line 17 at the first temperature sensor 25, q.sub.2 is the bypass flow through the bypass pump 19, and q.sub.3 is the feed line flow upstream of the bypass line 17 at the third temperature sensor 31. In order to compensate a fluctuation dT.sub.2 of the return line temperature T.sub.2, the bypass flow q.sub.2 must be changed by dq.sub.2 as follows:

[00015]$d{q}_2=\frac{q_2}{T_3-T_2}d{T}_2.$

[0082] Over time, an uncontrolled fluctuation of the return line temperature T.sub.2 could be compensated by a feed-forward controlled flow part q*.sub.feedforward by integrating over time the according time derivatives

[00016]$\frac{d{q}_2}{dt}=\frac{q_2}{T_3-T_2}\frac{{\mathrm{dT}}_2}{\mathrm{dt}}.$

An integration over time may have a disadvantage that the feed forward flow part q*.sub.feedforward may drift over time, and control unit 27 may face a wind-up issue.

[0083] A solution to this problem is shown in FIG. 5, which shows an embodiment of the control method described herein. A target feed line temperature T*.sub.1 and the feed line temperature T.sub.1 measured by the first temperature sensor 25 is fed into a feedback controller for a closed-loop controlled flow part q*.sub.feedback. The return line temperature T.sub.2 and the feed line temperature T.sub.3 upstream of the bypass line and the bypass fluid flow q.sub.2 is fed into a T.sub.2 disturbance compensator that outputs a time derivative of the bypass flow q.sub.2, i.e.

[00017]$\frac{d{q}_2}{dT}.$

Analogously, the feed line temperature T.sub.1 downstream of the bypass line, the return line temperature T.sub.2, the feed line temperature T.sub.3 upstream of the bypass line and the bypass fluid flow q.sub.2 are fed into a T.sub.3 disturbance compensator that outputs a time derivative of the bypass flow q.sub.2, i.e.

[00018]$\frac{d{q}_2}{dT},$

using the formula

[00019]$\frac{d{q}_2}{dt}=\frac{q_2\left(T_1-T_2\right)}{\left(T_3-T_1\right)\left(T_3-T_2\right)}\frac{d{T}_3}{dt}.$

These time derivatives

[00020]$\frac{d{q}_2}{dt}$

are summed-up and fed into a low pass filter that behaves for high frequencies like an integration and outputs a feed-forward controlled flow part q*.sub.feedforward that is added to the closed-loop controlled flow part q*.sub.feedback. The sum q* is then used as a target bypass flow into an inverse pump-model function ƒ.sup.−1(q*,p), wherein p is the pump pressure differential for compensating the determined pressure difference Δp measured by the differential pressure sensor 23, and outputs a target speed ω of the bypass pump 19.

[0084] FIG. 6 shows an embodiment of the control method as shown in FIG. 5 with the difference that the pump differential pressure p is not directly fed into the inverse pump-model function at start-up of the bypass pump 19, but slowly ramped-up. This is particularly advantageous for a system according to the embodiment shown in FIG. 3, wherein a differential pressure sensor 23 integrated into the bypass pump 19 is used for the feed-forward control. A differential pressure ramp-up function in form of ∫.sub.t.sub.0.sup.t.sup.1C dt is multiplied with the pump differential pressure p in order to achieve a slow ramp-up. The constant C is chosen such that the differential pressure ramp-up function is 0 at t.sub.0 and 1 at t.sub.1 when the time t.sub.1−t.sub.2 has elapsed.

[0085] FIG. 7 shows another embodiment of the system 15 according to the present disclosure. In contrast to the previously described embodiments, the pressure difference Δp is not determined by a single differential pressure sensor 23, but based on pressure measurements of two or more pressure sensors 33, 35, 37. A first pressure sensor 33 is installed at the feed line 11 downstream of the bypass line 17. A second pressure sensor 35 is installed at the return line 13. A third pressure sensor 37 is installed at the feed line 11 upstream of the bypass line 17. The pressure difference Δp is then given by Δp=p.sub.3−p.sub.2, wherein p.sub.2 is the pressure measured by the second pressure sensor 35 and p.sub.3 is the pressure measured by the third pressure sensor 37. In the embodiment shown in FIG. 7, there is a second pump 39 installed at the feed line 11 between the bypass line 17 and the first temperature sensor 25. Such a second pump 39 may already be available or installed to boost the pressure in the feed line 11. The second pump 39 may be controlled by a separate second control unit 41 receiving the pressure measurements p.sub.1, p.sub.2 and p.sub.3 from the pressure sensors 35, 37, 39. The second control unit 41 is in signal connection with the control unit 27 for controlling the speed of the bypass pump 19 and passes on the information about the pressure difference Δp=p.sub.3−p.sub.2 to the control unit 27 for the feed-forward control to regulate the bypass flow accordingly. The control units 27, 41 are preferably integrated into the bypass pump 19 and the second pump 39, respectively. Alternatively, one or more of the control units 27, 41 may be implemented in a controller separate from the pumps 19, 39.

[0086] As shown in FIG. 8, a third control unit 43, preferably an external programmable logic controller (PLC), is provided in addition to the control units 27, 41 that are integrated in the bypass pump 19 and the second pump 39, respectively. The third external control unit 41 may be signal-connected to the other control units 27, 41 by bus connections 45. The temperature measurements T.sub.1, T.sub.2 and T.sub.3 of the temperature sensors 25, 31, 33 are communicated to the first control unit 27 and the pressure measurements p.sub.1, p.sub.2 and p.sub.3 of the pressure sensors 35, 37, 39 are communicated to the second control unit 41. The external third control unit 43 may be used to carry out the control method and to command the control units 27, 41 to run the pumps 19, 39 at a certain speed.

[0087] FIG. 9a shows an example of how the uncontrolled pressure difference Δp behaves over the time of a day in the bottom plot. In the top plot, FIG. 9a shows the resulting fluctuations of the feed line temperature T.sub.1 if the control method described herein is not applied. As can be seen, the feed line temperature T.sub.1 fluctuates a lot due to the uncontrolled fluctuations of the pressure difference Δp between the feed line 11 and the return line 13. Such a situation is not tolerable for a utility provider operating the fluid distribution network 1. The goal is to achieve a stable desired feed line temperature T.sub.1 despite of the uncontrolled pressure fluctuations Δp. The result of applying the control method described herein is shown in FIG. 9b. The pressure fluctuations are still uncontrolled and present over the time of the day, but the feed line temperature T.sub.1 is very stable within a small band about a target feed line temperature T.sub.1*=90°. This shows that the system and method described herein provide an efficient solution to regulate a temperature of a thermal energy carrying fluid in a sector of a fluid distribution network.

[0088] While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

LIST OF REFERENCE NUMERALS

[0089] 1 fluid distribution network [0090] 3 thermal energy source [0091] 5 feeder pump [0092] 7 thermal energy consumers [0093] 9 sector of fluid distribution network [0094] 11 feed line [0095] 12 heat exchanger [0096] 13 return line [0097] 15 system [0098] 17 bypass line [0099] 19 bypass pump [0100] 21 non-return valve [0101] 23 differential pressure sensor [0102] 25 first temperature sensor [0103] 27 first control unit [0104] 29 second temperature sensor [0105] 31 third temperature sensor [0106] 33 first pressure sensor [0107] 35 second pressure sensor [0108] 37 third pressure sensor [0109] 39 second pump [0110] 41 second control unit [0111] 43 third control unit [0112] 45 bus connection