SETTING A PRE-PRESSURE PRESSURE IN A DISTRICT HEATING NETWORK

Description

Technical field

[0001] The present invention relates to thermal energy grids for providing heating and / or cooling to buildings, more specifically to a method for adjusting a biasing pressure in a district thermal energy grid, to a controller for adjusting a biasing pressure in a district thermal energy grid and to a district thermal energy system comprising a district thermal energy grid and a controller for adjusting a biasing pressure in the district thermal energy grid.Background

[0002] In district thermal energy grids, e.g. district heating grids or district cooling grids, one or more distribution pumps are used for pumping heat transfer fluid in the grid. It is important that a fluid pressure at the suction side of the one or more distribution pumps is high enough in order to avoid cavitation in the one or more pumps and also in order to avoid suction of air into the grid.

[0003] Cavitation occurs due to that the fluid pressure at the suction side of the pump is too low. Hence, the pump is not provided with its Required Net Positive Suction Head, NPSH R . If cavitation occurs, the drag coefficient of the impeller vanes will increase drastically, possibly stopping flow altogether, and prolonged exposure will damage the impeller and / or the pump. Hence, the life span of the pump may be reduced and in worst case scenario the pump may break down. Further, the pumping power at the pump may be reduced due to cavitation. Moreover, noise from the pump may increase due to cavitation.

[0004] In case the fluid pressure at the suction side of a distribution pump will go below atmospheric pressure there is a risk of suction of air into the grid. This since district thermal energy grids and the components they are made from are designed to avoid leakage of fluid out from the grid, but not for avoidance of air leaking into the grid. In case air will enter the grid its ability to distribute heat transfer fluid may be weakened or even cease completely.

[0005] Hence, it is important to maintain a bias pressure of the heat transfer fluid in the district thermal energy grid so that cavitation and suction of air into the grid is avoided. Today the bias pressure is typically a fixed biasing pressure for the grid determined in connection with commissioning of the grid. However, there is room for improvements in setting such biasing pressure.

[0006] CH 714 273 discloses an automatic three-way valve for connecting an expansion vessel in a district heating network and a district heating network with such a valve.

[0007] US 5 213 477 discloses a control apparatus for controlling a delivery flow rate of a pump pressure-feeding process liquid within a process pipe system. The control apparatus has detecting means for detecting available Net Positive Suction Head of a pump on the basis of a pressure and a temperature of the process liquid within the suction portion of the pump, and calculating means for calculating an allowable maximum flow rate of the pump so as to hold the relationship: available Net Positive Suction Head required Net Positive Suction Head. The control apparatus also has outputting means for outputting a control signal to control the delivery flow rate of the pump on the basis of the smaller value, the allowable maximum flow rate or a required flow rate of the pump.

[0008] SHENG XIANJIE ET AL: "Energy saving analyses on the reconstruction project in district heating system with distributed variable speed pumps", APPLIED THERMAL ENGINEERING, PERGAMON, OXFORD, GB, vol. 101, 10 March 2016 (2016-03-10), pages 432-445, XP029598528, ISSN: 1359-4311, DOI: 10.1016 / J.APPL THERMALENG.2016.01 .059 discloses a district thermal energy grid having central and local distribution pumps.

[0009] EP3296647A1 discloses a local energy distribution system having central and local circulation pumps.

[0010] EP3112823A1 relates to the field of utility networks, more specifically to systems for monitoring utility networks. The utility network may for example be a district heating network or a district cooling network. The monitoring output state may for example be indicative for cavitation and its location in the utility network.Summary of the invention

[0011] In view of the above, it is an object of the present invention to provide a method for adjusting a biasing pressure in a district thermal energy grid which improves on prior art solutions. Moreover, it is an object to provide a district thermal energy system comprising district thermal energy grid and a controller for a district thermal energy system.

[0012] It is an object to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solve at least the above mentioned problem.

[0013] According to a first aspect of the present invention, as set out in claim 1, a method for adjusting a biasing pressure in a district thermal energy grid is provided.

[0014] Preferred embodiments are defined by the dependent claims 2 to 4.

[0015] According to a second aspect of the present invention, as set out in claim 5, a controller for adjusting a biasing pressure in a district thermal energy grid is provided.

[0016] Preferred embodiments are defined by the dependent claims 6 and 7.

[0017] According to a third aspect of the present invention, as set out in claim 8, a district thermal energy system is provided.

[0018] According to these aspects, the biasing pressure can thus in many operating scenarios be reduced, which reduces the load on the distribution pumps and on other components of the district thermal energy grid as well.

[0019] A further applicability of the present invention will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. The present invention and the corresponding scope of protection is solely defined by the appended claims.

[0020] Hence, it is to be understood that this invention is not limited to the particular component parts of the device described or acts of the methods described as such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claim, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.Brief description of the drawings

[0021] The above aspects of the present invention will now be described in more detail, with reference to appended figures. The figures should not be considered limiting; instead they are used for explaining and understanding.

[0022] As illustrated in the figures, the sizes of layers and regions may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures. Like reference numerals refer to like elements throughout. Figure 1 discloses a diagram outlining pressure variation in a district thermal energy system. Figure 2 discloses a schematic block drawing of a controller for the district thermal energy system. Figure 3 discloses a flow chart of a method for adjusting a biasing pressure in a district thermal energy grid of a district thermal energy system. Detailed description

[0023] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and to fully convey the scope of the invention to the skilled person.

[0024] Fig. 1 illustrates a diagram outlining pressure variation of thermal fluid in a district thermal energy grid 100 of a district thermal energy system 10 for providing heating and / or cooling to one or, preferably, several buildings 108a, 108b, 108c.

[0025] The buildings 108a, 108b, 108c may be any type of building suitable for connection to a district thermal energy grid 100, such as a residential building, commercial or office building, an apartment building, a free-standing house or an industrial building. The district thermal energy grid 100 may be a district heating grid and / or a district cooling grid known in the art. The district heating grid (or district cooling grid) may comprise a supply conduit 118 providing thermal fluid from a thermal plant 122 and a return conduit 120 which transports cooled thermal fluid (or heated thermal fluid) to the thermal plant 120. The thermal fluid may be any fluid suitable for heating (or cooling) at the thermal plant 120 and for being transported by means of the supply conduit 118 and the return conduit 120, such as water or water with anticorrosion additives.

[0026] The thermal plant 120 may be a geothermal plant, an electrically powered plant for heating (or cooling) fluids, or may be powered by combustion of fuels, such as gas or oil. The thermal plant 120 is configured to heat (or cool) the thermal fluid for distribution in the district thermal energy grid 100.

[0027] As an alternative to being a district heating or district cooling grid, the thermal energy circuit 300 may be a combined district heating and cooling grid as previously disclosed in, e.g., WO 2017 / 076868 filed by E.ON Sverige AB. In such case, the supply and return conduits 118, 120 are not to be seen as supply and return conduits but instead to be seen as the hot conduit and the cold conduit as disclosed in WO 2017 / 076868.

[0028] The district thermal energy system 10 is provided with a plurality of distribution pumps 110, 112, 106a, 106b, 106c arranged in the district thermal energy grid 100. Each distribution pump may be configured to provide a certain flow rate and / or pressure of thermal fluid to downstream components of the district thermal energy grid 100.

[0029] As is visible from the diagram in Figure 1, the pressure of the thermal fluid in the district thermal energy grid 100 of the district thermal energy system 10 decreases with an increase in distance that the thermal fluid flows in the grid 100 from the central distribution pump(s) 110, 112. Each passing of the thermal fluid through equipment such as a thermal device 124a, 124b, 124c such as a heat pump or heat exchanger also leads to pressure reduction in the thermal fluid.

[0030] The gradual reduction in pressure of the thermal fluid generates a pressure cone which decreases available pressure for achieving flow of thermal fluid from the supply conduit 118 to the return conduit 120. This is a problem for district heating systems which are distributed over large areas and thus requires long supply conduits / return conduits 118, 120, as the decrease / increase in pressure corresponds to the length of the respective conduits 118, 120. Typically, the design pressure or maximum operating pressure of the supply conduit 118 will limit the possible extension of the district thermal energy grid 100.

[0031] The pressure cone effect described above may be somewhat lessened by providing distribution pumps 106a, 106b, 106b which are locally distributed in the district thermal energy grid 100 for achieving a desired flow rate / pressure of thermal fluid in a local circuit 126a, 126b.

[0032] The central distribution pump 110, 112 may be configured to achieve the required flow rate / pressure of thermal fluid in the supply conduit 118 of the thermal energy grid 100, to meet the thermal loads placed thereon. At least two central distribution pumps 110, 112 are provided wherein preferably at least one is arranged on the supply conduit 118 and at least one is arranged on the return conduit 120.

[0033] The thermal loads on the district thermal energy system are defined by the heat transfer between the district thermal energy grid 100 and the buildings 108a, 108b, 108c, which as indicated in Figure 1 may be in both directions, i.e. to / from the district thermal energy grid 100.

[0034] As mentioned and as illustrated in Figure 1, local distribution pumps 106a, 106b, 106c may be provided for achieving the desired local flow rate / pressure of thermal fluid for meeting the thermal loads in each local circuit 126a, 126b, 126c. Three local circuits 126a, 126b, 126c are illustrated, it is however to be realized that fewer or more local circuits 126a, 126b, 126c can be provided as well. Each local circuit 126a, 126b, 126c is illustrated as comprising one thermal device 124a, 124b, 124c each. The thermal device 124a, 124b, 124c may be a heat pump / cooling machine and / or a heat exchanger or a similar device configured for heat transfer from thermal fluid. Each local circuit 126a, 126b, 126c could however comprise more than one thermal device 124a, 124b, 124c and be associated with more than one building 108a, 108b, 108c.

[0035] More than one local distribution pump 106a, 106b, 106c may be arranged in each local circuit 126a, 126b, 126c. The local distribution pumps 106a, 106b, 106c may each be formed as a unit with a respective one of the thermal devices 124a, 124b, 124c.

[0036] The local distribution pumps 106a, 106b, 106c may be configured to control the flow rate / pressure of the thermal fluid in each local circuit 126a, 126b, 126c such that the thermal loads placed on the thermal device 124a, 124b, 124c in each local circuit can be met.

[0037] In order to monitor and control the district thermal energy system 10 is a controller 200 provided. The controller 200, which will be further elaborated on in the description of Fig. 2 below, is configured to adjust a biasing pressure in the district thermal energy grid 100.

[0038] The biasing pressure is in the present disclosure to be considered as a pressure of the thermal fluid in the district thermal energy grid 100 that is provided to avoid that cavitation occurs in any of the distribution pumps 106a, 106b, 106c, 110, 112. The biasing pressure required for achieving this depends among other things on the type of distribution pump, the operating conditions thereof such as turning rate of the pump wheel etc, the dimensions of and the load placed on each distribution pump. The required biasing pressure is thus dynamic and depends e.g. on the loads which each of the distribution pumps 110, 112, 106a, 106b, 106c is subjected to. Moreover, the biasing pressure decreases with increased distance downstream in the district thermal energy grid 100 from each of the distribution pumps 106a, 106b, 106c, 110, 112.

[0039] The controller 200 is configured, as shown in Fig. 1, to be connected to each of the distribution pumps 106a, 106b, 106c, 110, 112. As such, the controller 200 is configured to receive signals indicative of operating parameters of each distribution pump 106a, 106b, 106c, 110, 112 which for each distribution pump at least comprises a Required Net Positive Suction Head, NPSH R . The signal may be individual for each distribution pump 106a, 106b, 106c, 110, 112 or a combined signal in which information from more than one distribution pump 106a, 106b, 106c, 110, 112 is included.

[0040] The signals from each distribution pump 106a, 106b, 106c, 110, 112 may alternatively or in addition comprise operating conditions of the pump from which for instance NPSH R can be determined. The NPSH R corresponds to the biasing pressure that is required at a suction side of each distribution pump to avoid cavitation. The NPSH R may be determined based on operating conditions and characteristics of the distribution pumps 110, 112, 106a, 106b, 106c, for instance such as an angular frequency of a rotor (not shown) of the distribution pump 110, 112, 106a, 106b, 106c and / or information from a pump curve of each distribution pump 110, 112, 106a, 106b, 106c.

[0041] The district thermal energy system 10 may further comprise a plurality of sensors 128. The sensors 128 are preferably pressures gauges 128 but may additionally or alternatively comprise flow sensors 128 for detecting flow rate of thermal fluid and / or temperature sensors 128 for detecting the temperature of the thermal fluid.

[0042] The sensors 128 are preferably arranged such that at least pressure of the thermal fluid in the district thermal energy grid 100 at the suction side of a plurality of, preferably each, distribution pump 106a, 106b, 106c, 110, 112 can be measured. The sensors 128 are illustrated as separate units but may be integrally formed with each respective distribution pump 106a, 106b, 106c, 110, 112. Each sensor 128 is connected to the controller 200 whereby the controller 200 can receive signals indicative of an Available Net Positive Suction Head, NPSH A for each distribution pump 106a, 106b, 106c, 110, 112. Such a signal may thus comprise one or several of the pressure, flow rate and temperature of the thermal fluid at least at a suction side of each distribution pump 106a, 106b, 106c, 110, 112.

[0043] The NPSH A is indicative of the state of the thermal fluid and how near it is to flash evaporation and thus cavitation. The NPSH A may in the context of the present disclosure be approximated by means of measuring the static pressure of the thermal fluid at the suction side of each distribution pump 106a, 106b, 106c, 110, 112.

[0044] The NPSH A for each distribution pump 106a, 106b, 106c, 110, 112 is to be compared to the NPSH R of each distribution pump 106a, 106b, 106c, 110, 112. The NPSH A shall preferably be larger than the NPSH R by a certain margin, a threshold percentage, as will be elaborated further on below, to avoid cavitation. The controller 200 is configured to, for each distribution pump 110, 112, 106a, 106b, 106c, receive NPSH A and this preferably with a period in the order of minutes to hours. The biasing pressure is controlled such that the required NPSH A for a risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R is achieved.

[0045] For the purpose of achieving a biasing pressure of the thermal fluid in the district thermal energy system 10, specifically in the district thermal energy grid 10 thereof, the district thermal energy system 10 comprises biasing pressure control unit 110, 112, 114, 116. The biasing pressure control unit 110, 112, 114, 116 is configured to control the biasing pressure of the thermal fluid such that the NPSH A exceeds the NPSH R for each distribution pump 106a, 106b, 106c, 110, 112.

[0046] According to the present invention, the biasing pressure control unit comprises an adjustable master pump 110, 112, wherein the adjustable master pump 110, 112 is one or each of the central distribution pumps 110, 112 of the district thermal energy grid 100.

[0047] According to embodiments, not forming part of the present invention, the adjustable master pump 110, 112 may be arranged distally in relation to the thermal plant 122 in the district thermal energy grid 100, for instance may one of the local distribution pumps 106a, 106b, 106c be configured to function as an adjustable master pump for increasing the biasing pressure of the thermal fluid downstream of the master pump. Preferably however, the master pump 110, 112 is a pump which is arranged upstream of a determined risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R . The risk distribution pump may be the distribution pump 106a, 106b, 106c, 110, 112 which is at highest risk for cavitation, i.e. the distribution pump for which NPSH A is closest to NPSH R . However, the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R may also be a prioritized distribution pump 106a, 106b, 106c, 110, 112 that provides thermal fluid to a local circuit that is of high importance. Other distribution pumps of less importance that are at risk of cavitation can then be run at lower speeds to avoid cavitation until the operating conditions or control of the biasing pressure of the thermal district energy system 10 changes. That way, the desired thermal output in the prioritized local circuits 126a, 126b, 126c can be achieved while cavitation is avoided and while the biasing pressure can be reduced.

[0048] According to embodiments, not forming part of the preset invention, the biasing pressure control unit may alternatively to the adjustable master pump 110, 112 comprise an adjustable expansion vessel 114, 116 connected to the district thermal energy grid 100. The adjustable expansion vessel 114, 116 may comprise a valve 116 regulating the pressure it exerts on the thermal fluid of the district thermal energy system 100, for regulating the biasing pressure thereof. The adjustable expansion vessel 114 may be adjustable by means of altering the pressure of compressed air in the expansion vessel 114, increasing the pressure of the compressed air in the expansion vessel 114 correspondingly increases the biasing pressure of the thermal fluid in the district thermal energy grid 100 and vice versa.

[0049] The biasing pressure control unit may in addition to the adjustable master pump 110, 112 also comprise the adjustable expansion vessel 114, 116.

[0050] The controller 200 is connected to the biasing pressure control unit 110, 112, 114, 116 for controlling the biasing pressure of the thermal fluid in the district thermal energy system 10.

[0051] The biasing pressures control unit 110, 112, 114, 116 is configured to control the biasing pressure such that an NPSH A for the identified risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R is achieved that exceeds the NPSH R with a threshold percentage. The threshold percentage may be 5 to 20%, i.e. the biasing pressure should be set such that the NPSH A is 5% to 20% higher that the NPSH R for the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R . The biasing pressure can thus be kept sufficiently high for lowering the risk of cavitation in the distribution pumps 106a, 106b, 106c, 110, 112 while the risk of exerting the grid 100 to too high biasing pressure levels is reduced. Excessive biasing pressures of the thermal fluid causes unnecessary wear on the district thermal energy grid 100 and can cause increased energy consumption. The threshold percentage may also be outside of the aforementioned range, i.e. smaller than 5% or larger than 20%. The threshold percentage varies depending on the size of the system 10, or the grid 100 thereof. It may also vary depending on the characteristics of the identified risk distribution pump, the accuracy of the pressure sensors 128, the ability to predict load scenarios ahead of time etc.

[0052] Turning to Figure 2, in which a schematic block drawing of the controller 200 for the district thermal energy system 10 is shown. The controller 200 may be formed by a single unit, or several distributed units, being configured to carry out overall control of functions and operations of the district thermal energy system 10, more particularly the method 1000 disclosed herein. The controller 200 thus comprises a control circuit 202 which may be associated with a memory 208. The control circuit 202 may include an associated processor 204, such as a central processing unit (CPU), microcontroller, or microprocessor. The processor 204 is configured to execute program code stored in the memory 208, in order to carry out functions and operations of the control unit 200.

[0053] The memory 208 may be one or more of a buffer, a flash memory, a hard drive, a removable media, a volatile memory, a non-volatile memory, a random access memory (RAM), or another suitable device. In a typical arrangement, the memory 208 may include a non-volatile memory for long term data storage and a volatile memory that functions as system memory for the control circuit 202. The memory 208 may exchange data with the control circuit 202 over a data bus. Accompanying control lines and an address bus between the memory 208 and the control circuit 202 may also be present.

[0054] The control unit 200 further comprises a transceiver 206 as a communication unit. The transceiver 206 may be formed by separate transmitter and receiver device or as a combined transceiver unit. The transceiver 206 is connected to the control circuit 202 to facilitate remote control of functions and units of the associated district thermal energy system 10. A unit of the district thermal energy system 10 may be for instance a distribution pump 106a, 106b, 106c, 110, 112 thereof. The communication path over which the communication is made may be wired or wireless. The communication may include data transfers, and the like. Data transfers may include, but are not limited to, downloading and / or uploading data and receiving or sending messages. The data may be processed by the controller 200. The processing may include storing the data in a memory, e.g. the memory 208 of the controller 200, executing operations or functions, and so forth.

[0055] Functions and operations of the controller 200 may be embodied in the form of executable logic routines (e.g., lines of code, software programs, etc.) that are stored on a non-transitory computer readable medium (e.g., the memory 208) of the controller 200 and are executed by the control circuit 202 (e.g., using the processor 204). Furthermore, the functions and operations of the controller 200 may be a stand-alone software application or form a part of a software application that carries out additional tasks related to the controller 200. The described functions and operations may be considered a method 1000 that the corresponding device is configured to carry out.

[0056] Also, while the described functions and operations may be implemented in software, such functionality may as well be carried out via dedicated hardware or firmware, or some combination of hardware, firmware and / or software.

[0057] The controller 200 is, as is shown in Figure 1, associated with one or more of the distribution pumps 106a, 106b, 106c, 110, 112 for allowing control thereof. This includes one or several or each adjustable master pump 110, 112. The master pump 110, 112 is to be considered as a pump that is configured to achieve a desired biasing pressure of the thermal fluid in the district thermal energy grid 100. The controller may further be associated with the expansion vessel 114, 116 of the district thermal energy system 10.

[0058] The controller 200 is configured to, by means of a risk evaluation function 210 executed by the control circuitry 202, identify the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R among the plurality of distribution pumps. The risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R may be considered the distribution pump 106a, 106b, 106c, 110, 112 for which the NPSH A is closest to the NPSH R . In such an embodiment, the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R is the distribution pump 106a, 106b, 106c, 110, 112 which is, at a given time, at highest risk for being affected by cavitation.

[0059] The risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R may also, in the case where an distribution pump 106a, 106b, 106c, 110, 112 actually experiences cavitation, be the distribution pump for which NPSH A has decreased the most below the NPSH R for the same distribution pump. And as mentioned above, the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R may be a prioritized distribution pump. This may be the case when operating conditions of the district thermal energy system 10 are such that a sufficiently high biasing pressure cannot safely be achieved for avoiding cavitation in all distribution pumps 106a, 106b, 106c, 110, 112. The controller 200 is then configured to identify a risk distribution pump by means of the risk evaluation function 210, which is a distribution pump considered of high priority and having an NPSH R which is low enough that it allows a biasing pressure to be safely generated for achieving the required NPSH A for that distribution pump. The remaining distribution pumps 106a, 106b, 106c, 110, 112 that are at risk for cavitation and not considered of high priority may be operated at reduced speed.

[0060] The control circuitry 202 is further configured to execute a biasing pressure control function 212 configured to generate a biasing pressure control signal comprising information for controlling the biasing pressure in the district thermal energy grid 100 such that, for the identified risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R , the NPSH A exceeds the NPSH R by a threshold percentage. The transceiver 206 is configured to send the biasing pressure control signal to the biasing pressure control unit 110, 112, 114, 116. The biasing pressure control unit 110, 112, 114, 116 comprises a distribution pump 110, 112 as a master distribution pump 110, 112 of the district thermal energy system 10. According to the present invention, the biasing pressure control unit is one or each of the central distribution pumps.

[0061] The biasing pressure control unit may additionally comprise the expansion vessel 114, 116 which may control the biasing pressure.

[0062] The signal indicating NPSH A for each distribution pump 110, 112, 106a, 106b, 106c may be received periodically with a period in the order of minutes to hours. However, the period may be both shorter and longer as is realized by a person skilled in the art. Decreasing the time period increases the resolution with which the biasing pressure of the district thermal energy grid 100 can be controlled. Increasing the time period decreases the resolution but reduces data and the load for the controller 200, for instance by reducing the processing requirements of the processor 204 thereof.

[0063] The risk evaluation function 212 may comprise identifying the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R among the plurality of distribution pumps 110, 112, 106a, 106b, 106c for which the NPSH A is closest to the NPSH R by, for each distribution pump 110, 112, 106a, 106b, 106c among the plurality of distribution pumps 110, 112, 106a, 106b, 106c, comparing the NPSH A and the NPSH R for that distribution pump 110, 112, 106a, 106b, 106c.

[0064] According to the present invention, the biasing pressure control function 212 is configured to adjust the biasing pressure by adjusting an adjustable master pump 110, 112 of the district thermal energy grid 100, wherein the adjustable master pump is one or each of the central distribution pumps 110, 112 of the district thermal energy grid 100. addition thereto, the biasing pressure control function may also be configured to adjust an expansion vessel 114, 116 of the district thermal energy grid 100.

[0065] Turning lastly to Figure 3, in which a flow chart of a method 1000 for adjusting a biasing pressure in a district thermal energy grid 100 of a district thermal energy system 10 is shown. The method 1000 comprises: determining 1002, for each distribution pump 110, 112, 106a, 106b, 106c, an Available Net Positive Suction Head, NPSH A , for the respective distribution pump 110, 112, 106a, 106b, 106c, identifying 1004 a risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R among the plurality of distribution pumps 110, 112, 106a, 106b, 106c, adjusting 1006 the biasing pressure in the district thermal energy grid 100 such that, for the identified risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R , the NPSH A exceeds the NPSH R by a threshold percentage.

[0066] The threshold percentage may be 5 to 20%, i.e. the biasing pressure should be set such that the NPSH A is 5% to 20% higher that the NPSH R for the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R .

[0067] The determining of the NPSH A , as well as NPSH R , for each distribution pump 110, 112, 106a, 106b, 106c may as mentioned be performed periodically with a period in the order of minutes to hours, but also other periods are envisioned.

[0068] The NPSH R may be determined as a predetermined value depending on an angular frequency of a rotor (not shown) of the distribution pump 110, 112, 106a, 106b, 106c. The NPSH R for each distribution pump 110, 112, 106a, 106b, 106c can thus be determined by monitoring the rotational speed of the distribution pumps. The NPSH R may be determined at the same interval as NPSH A for each distribution pump 110, 112, 106a, 106b, 106c. However, it may also be determined at other interval such as shorter intervals or even continuously.

[0069] The identifying 1004 of a risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R among the plurality of distribution pumps 110, 112, 106a, 106b, 106c may be performed such that the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R is the risk distribution pump for which the NPSH A is closest to a Required Net Positive Suction Head, NPSH R . As is also outlined above, the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R may also be identified 1004 as a prioritized distribution pump, for instance in a scenario where a sufficient biasing pressure for avoiding cavitation in all distribution pumps cannot be achieved.

[0070] The identifying 1004 of the risk distribution pump 110 R , 112 R , 106a R , 106b R , 106c R among the plurality of distribution pumps 110, 112, 106a, 106b, 106c for which the NPSH A is closest to the NPSH R may comprise, for each distribution pump 110, 112, 106a, 106b, 106c among the plurality of distribution pumps, comparing the NPSH A and the NPSH R for that distribution pump 110, 112, 106a, 106b, 106c.

[0071] According to the present invention, adjusting 1006 the biasing pressure comprises adjusting an adjustable master pump 25 110, 112 of the district thermal energy grid 100, wherein the adjustable master pump is one or each of the central distribution pumps 110, 112 of the district thermal energy grid 100.

[0072] The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the present invention which is solely defined by the appended claims.

[0073] Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A method (1000) for adjusting a biasing pressure in a district thermal energy grid (100) comprising a plurality of central distribution pumps (110, 112) and a plurality of local distribution pumps (106a, 106b, 106c), wherein the biasing pressure is a pressure of thermal fluid in the district thermal energy grid (100) that is provided to avoid that cavitation occurs in any of the distribution pumps (106a, 106b, 106c, 110, 112), the method (1000) comprising: determining (1002), for each distribution pump (110, 112, 106a, 106b, 106c), an Available Net Positive Suction Head, NPSHA, for the respective distribution pump (110, 112, 106a, 106b, 106c); identifying (1004) a risk distribution pump (110R, 112R, 106aR, 106bR, 106cR) among the plurality of distribution pumps (110, 112, 106a, 106b, 106c); and adjusting (1006) the biasing pressure in the district thermal energy grid (100) such that, for the identified risk distribution pump (110r, 112r, 106ar, 106br, 106cr), the NPSHA exceeds a Required Net Positive Suction Head, NPSHR, therefor by a threshold percentage, wherein adjusting (1006) the biasing pressure comprises adjusting an adjustable master pump of the district thermal energy grid (100), the adjustable master pump is one or each of the central distribution pumps (110, 112) of the district thermal energy grid (100).

2. The method (1000) according to claim 1, wherein determining the NPSHA for each distribution pump (110, 112, 106a, 106b, 106c) is performed periodically with a period in the order of minutes to hours.

3. The method (1000) according to claims 1 or 2, wherein NPSHR is a predetermined value depending on an angular frequency of a rotor of the distribution pump (110, 112, 106a, 106b, 106c).

4. The method (1000) according to any one of claims 1-3, wherein identifying (1004) the risk distribution pump (110R, 112R, 106aR, 106bR, 106cR) among the plurality of distribution pumps (110, 112, 106a, 106b, 106c) comprises, for each distribution pump (110, 112, 106a, 106b, 106c) among the plurality of distribution pumps, comparing the NPSHA and the NPSHR for that distribution pump (110, 112, 106a, 106b, 106c), wherein the risk distribution pump is identified as the distribution pump for which the NPSHA is closest to the NPSHR.

5. A controller (200) for adjusting a biasing pressure in a district thermal energy grid (100) comprising a plurality of central distribution pumps (110, 112) and a plurality of local distribution pumps (106a, 106b, 106c), wherein the biasing pressure is a pressure of thermal fluid in the district thermal energy grid 100 that is provided to avoid that cavitation occurs in any of the distribution pumps (106a, 106b, 106c, 110, 112), the controller (200) comprising: a transceiver (206) configured to receive a signal indicating Available Net Positive Suction Head, NPSHA, for each distribution pump (110, 112, 106a, 106b, 106c) among the plurality of distribution pumps (110, 112, 106a, 106b, 106c), control circuitry (202) configured to execute: a risk evaluation function (210) configured to identify a risk distribution pump (110R, 112R, 106aR, 106bR, 106cR) among the plurality of distribution pumps (110, 112, 106a, 106b, 106c); a biasing pressure control function (212) configured to generate a biasing pressure control signal comprising information for controlling the biasing pressure in the district thermal energy grid (100) such that, for the identified risk distribution pump (110R, 112R, 106aR, 106bR, 106cR), the NPSHA exceeds a Required Net Positive Suction Head, NPSHR, therefor by a threshold percentage; wherein the transceiver (206) is configured to send the biasing pressure control signal to a biasing pressure control unit which comprises an adjustable master pump, the adjustable master pump is one or each of the central distribution pumps (110, 112), characterized in that the biasing pressure control function (212) is configured to adjust the biasing pressure by adjusting the one or each of the central distribution pumps (110, 112).

6. The controller (200) according to claim 5, wherein the signal indicating NPSHA for each distribution pump (110, 112, 106a, 106b, 106c) is received periodically with a period in the order of minutes to hours.

7. The controller (200) according to claim 5 or 6, wherein the risk evaluation function (212) comprises identifying the risk distribution pump (110R, 112R, 106aR, 106bR, 106cR) among the plurality of distribution pumps (110, 112, 106a, 106b, 106c) by, for each distribution pump (110, 112, 106a, 106b, 106c) among the plurality of distribution pumps (110, 112, 106a, 106b, 106c), comparing the NPSHA and the NPSHR for that distribution pump (110, 112, 106a, 106b, 106c), and identifying the risk distribution pump as the distribution pump for which the NPSHA is closest to the NPSHR.

8. A district thermal energy system (10) for providing heating and / or cooling, the district thermal energy system (10) comprising a district thermal energy grid (100) comprising a plurality of central distribution pumps (110, 112) and a plurality of local distribution pumps 106a, 106b, 106c) and a biasing pressure control unit which comprises an adjustable master pump, characterized in that the district thermal energy system comprises a controller (200) according to any one of claims 5-7 and that the adjustable master pump is one or each of the central distribution pumps (110, 112) of the district thermal energy grid (100).