3-pipe heat network

Abstract

A heat network comprising at least one plant, at least one end-user location (14, 15), a pipe system (11-13), and a medium contained in the pipe system (11-13), the plant, and the end-user location, wherein the end-user location(s) (14, 15) are connected to the plant through the pipe system (11-13). The heat network according to the invention comprises three main pipes (11-13), each connected to the plant(s), wherein the medium is in a liquid state in the first main pipe (11) and the third main pipe (13) and in a gaseous state in the second main pipe (12).

Description

[Technical Field]

[0001] The present invention relates to thermal networks, and more precisely to thermal networks that use energy transfer media to provide heat and refrigeration to end user locations. [Background technology]

[0002] A heat network is a system that provides thermal energy services to several end-user locations within the same building or in separate buildings. A heat network typically connects one or more central plants (heating / cooling injection points) to the end-use locations (heating / cooling utilization points). When heating is required, the heat network transports energy from one or several plants to the end-user locations via distributed heat exchangers or heat pumps through pipelines carrying water, steam, or CO2. When cooling is required, the thermal energy transported within the heat network follows the opposite path: heat from the heat exchangers or refrigeration systems is extracted via the pipelines.

[0003] A heat network consists of at least a central plant, a pipe system containing an energy transfer medium such as water or CO2, and at least end-user locations.

[0004] EP 2122257 discloses a district energy system with two main pipes, the first carrying CO2 in liquid form and the second carrying CO2 in gaseous form, both configured to function as supply or return pipes, depending on the heating or cooling requirements of the end-user location.

[0005] In the systems disclosed in this prior art, because one of the pipes must be maintained at a higher pressure than the other along the entire network between the end-user locations and between the end-user locations and the central plant, rotating machinery (pumps or compressors) is almost always required at the end-user locations. This results in an increased complexity of the equipment installed at the end-user locations, which has a negative impact on the system's footprint within the end-user locations (larger size of substations) and operational reliability (more maintenance is required, there are more sources of failure, the rotating machinery is more sensitive to improper operating conditions than the other equipment involved, and there are potentially more challenges regarding access to the location).

[0006] Swiss Patent No. 712294 discloses a thermal energy superstructure with multiple temperature levels and exchange points. It proposes the possibility of using CO2 as a heat transfer fluid in such a superstructure. The transfer of CO2 between different temperature levels can occur at temperatures below 30°C and pressures below 70 bar, with total or partial evaporation or condensation of the CO2.

[0007] In the superstructure disclosed in Swiss Patent No. 712294, there is no characterization of how fluid circulation is achieved within and between different temperature stages. Summary of the Invention

[0008] The present invention provides an alternative and improvement to existing thermal networks.

[0009] More precisely, the invention relates to a thermal network comprising at least one plant, at least one end-user location, a pipe system and an energy transmission medium, said end-user location(s) being connected to the plant through the pipe system, characterized in that it comprises three main pipes each connected to the plant, the energy transmission medium being in a liquid state in the first and third main pipes and in a gaseous state in the second main pipe.

[0010] The present invention stems from and integrates the following considerations, primarily related to energy efficiency:

[0011] A thermal network is primarily intended to provide heat or cooling services to end-user locations. The thermal network should aim to be as simple as possible in how it provides these services without excluding other services that are less closely related to thermal energy.

[0012] When there is a simultaneous demand for one or several end-user locations requiring heat and one or several end-user locations requiring cooling, the network must be able to recover the heat injected by the cooling users and transfer it to the end-user locations extracting heat from the network, making use of waste heat recovery possibilities as much as possible and thus improving the energy efficiency of the system.

[0013] The temperature of the heating service required can vary over a fairly wide range from one end user location to another, and the network must be able to accommodate this in the most energy efficient way.

[0014] The required cooling service temperature can vary over a fairly wide range from one end user location to another, and the network must be able to accommodate this in the most energy efficient way.

[0015] The above considerations can be met by a network that uses heat pumps installed at end-user locations to constantly supply heat at the temperature required by each end-user. This ensures lower power consumption compared to more traditional approaches in heat networks, where heat pumping is performed in the plant at a supply temperature imposed by the end-user location with the highest temperature requirement. Using direct heat exchange to provide most cooling services is also a way to meet the above considerations. Direct heat exchange consumes much less power compared to systems with compression chillers. From the proposed arrangement of distributed heat pumps and direct heat exchange for cooling, the network temperature needs to be high enough to supply heat to the low-temperature sources of the heat pumps installed at the end-user locations, while at the same time being low enough that direct heat exchange can provide cooling to most of the end-user locations.

[0016] The proposed invention also takes into account the following constraints:

[0017] The combination of the heat pump and the use of direct heat exchange for cooling means that the temperature difference between the hottest and coldest parts of the pipe system is limited to a few Kelvin (generally less than 10K).

[0018] The pipe system must also be very compact to allow for easy installation in the congested underground spaces common in urban areas.

[0019] Practical considerations of maintenance and reliability also dictate minimizing the complexity of network elements at end-user locations, especially those that provide heating and cooling services.

[0020] The present invention overcomes these constraints by utilizing the medium's latent heat of vaporization instead of its sensible heat, thereby enabling the same amount of heating and cooling service to be provided with a mass flow rate many times lower than if sensible heat were used. The temperature difference constraint is easily met because evaporation and condensation of pure compounds are isothermal, and even in the case of mixtures of several chemical species, evaporation generally falls within the acceptable temperature difference range. Smaller mass flow rates directly translate into a more compact piping system, since smaller pipe diameters are required. The use of three pipes—two filled with liquid and one filled with gas—is a way to reduce complexity at end-user locations by eliminating the need for rotating machinery (pumps or compressors) dedicated to handling the medium within those locations in most cases. Of course, heat pumps or refrigeration units may still be present within those locations, but in most cases they are not part of the network, in the sense that the network's medium does not flow through the rotating machinery (compressors or pumps) of the heat pumps and refrigeration units. As a result, the only rotating machinery required for the operation of the network itself is located at the plant, which is highly favorable for their monitoring, maintenance, and overall operational reliability.

[0021] Furthermore, a configuration with two liquid pipes and one gas pipe results in a more compact piping system than the alternative of using two gas pipes and one liquid pipe. This result is due to the fact that pressure drop tends to be higher in gas than in liquid, which results in a larger pipe diameter. In other words, assuming a predetermined amount of thermal energy delivered by the network to the end-user location (heating or cooling) and a maximum amount of pumping energy to circulate the medium, a configuration with two gas pipes and one liquid pipe will require a larger pipe diameter than a configuration with two liquid pipes and one gas pipe.

[0022] As used herein, the expression "liquid" should be understood as consisting entirely of liquid or consisting predominantly (i.e., more than 50% by mass) of liquid with a smaller proportion of gas.

[0023] The expression "gas" should be understood as consisting entirely of gas or consisting predominantly (i.e., more than 50% by mass) of gas with a smaller proportion of liquid.

[0024] In the following text, the term "medium" should be understood as "energy transmission medium".

[0025] End user locations may be directly or indirectly connected to the plant through a pipe system.

[0026] Energy transfer between the plant and the pipe system, between the pipe system and end user location(s), or between end user locations is achieved by thermodynamic conversion (e.g., cooling, evaporation, or condensation) of a medium in energy exchange devices (e.g., assemblies of heat exchangers, valves, sensors, etc.) at the plant and end user locations.

[0027] To be able to ensure that the correct state of the media is always selected before being sent to the pipe system by the end user location and plant, the following method is used.

[0028] Whenever liquids have to be used, by extracting liquid from a portion of the receptacle, preferably a lower portion of the receptacle; or by controlling the flow of medium in the condenser, preferably using a valve, based on the level of subcooling at the outlet. In fact, a sufficiently high value of subcooling indicates that the stream exiting the condenser is entirely in the liquid state.

[0029] Whenever gas has to be used, by extracting from another part of the receptacle, preferably from the upper part of the receptacle; or by controlling the flow of medium in the evaporator, preferably using a valve, based on the level of superheat at the outlet. In fact, a sufficiently high superheat value indicates that the stream exiting the evaporator is in a completely gaseous state.

[0030] It should be noted that the "selection" of the correct phase is not necessarily done just before the injection point in the pipe system, but also upstream of machines such as pumps or compressors to ensure their reliable operation and lifespan.

[0031] Preferably, the first main pipe is a supply pipe, which is a pressurized line inside which the medium constantly circulates from the plant(s) towards the end user location(s).

[0032] The first main pipe acts as a supply pipe and the third main pipe acts as a return pipe, which is a pressurized line within which the medium constantly circulates from the end user location(s) towards the plant(s).

[0033] Ideally, during operation, the pressure in all main pipes should be equal. However, due to various pressure drops (friction, gravity, momentum...), it is preferable to guarantee a pressure difference between the pipes. Said pressure difference should be kept as low as possible for energy efficiency. Consequently, the pressure in the pipe system is controlled by pumps (or compressors) so that the pressure of the medium in the first main pipe (supply pipe) is higher than the pressure in the second main pipe, at all positions along the pipe system and extending at the boundaries between the end-user location and the pipes, and between the plant and the pipe system, respectively.

[0034] The pressure of the medium in the second main pipe is higher than the pressure of the medium in the third main pipe (return pipe).

[0035] To ensure compactness of the pipe system, components located at end-user locations, and components located at the plant, the fluid in the liquid pipes is kept as close as possible to a saturated liquid, or even a slightly supercooled liquid state, while the fluid in the gas pipes is kept as close as possible to a saturated gas, or even a slightly superheated gaseous state.

[0036] Advantageously, the present invention provides a pipe configuration that allows for guaranteed energy circulation of fluids within the system between a central plant and any end-user location, or between at least two end-user locations, by imposing a fluid direction within the supply and return liquid pipes.

[0037] The flow direction in the supply pipe is imposed by only allowing connections from said pipe to the inlet port of the end user location.

[0038] The flow direction in the return pipe is imposed by only allowing connections to said pipe from the exit port at the end user location.

[0039] The present invention also provides a series of circulation elements for improving the reliability and energy efficiency of the circulation of a medium, comprising: the use of a series of circuit elements, including one or more bypass valves connected between a supply pipe and a return pipe; Depending on the circuit topology, gas traps can be positioned in the supply pipe and the liquid return pipe to collect the final gas bubbles; Depending on the circuit topology, a liquid trap can be positioned in the second gas pipe to collect possible droplets of liquid.

[0040] The present invention also relates to the use of a thermal network as defined above, wherein when heating requirements are higher than cooling requirements (e.g. in winter) the second main pipe (gas pipe) is used as a supply pipe and vice versa, and when cooling requirements are higher than heating requirements (e.g. in summer) the second main pipe is used as a return pipe.

[0041] More particular aspects of the invention are defined in the dependent claims.

[0042] With regard to EP 2 122 257, the use of three pipes according to the invention offers several advantages, in particular the following:

[0043] The complexity at the end user location is greatly reduced, and in the most common case, the need for local rotating machinery in direct contact with the media is eliminated.

[0044] The footprint of equipment installed at end-user locations is significantly reduced, greatly increasing the deployability of such systems.

[0045] The number of pieces of equipment requiring heavy and / or regular maintenance work is reduced and concentrated in one single location (plant), thus significantly reducing the maintenance work required.

[0046] System reliability and maintainability are substantially improved because sensitive equipment is preferentially located at the plant and not pre-located at end-user locations where access is inevitably more problematic.

[0047] Unidirectional flow in the liquid lines allows for easy fluid control, thus avoiding the risk of simultaneous pumping in opposite flow directions (at the end user and plant sides), which could result in potential flow instabilities and pump malfunction / failure.

[0048] The invention will be better understood in this section with the aid of the non-limiting examples illustrated by the following figures. [Brief explanation of the drawings]

[0049] [Figure 1] FIG. 1 shows a first configuration of a thermal network according to the present invention. [Figure 2] FIG. 2 shows a second configuration of a thermal network according to the present invention. [Figure 3] FIG. 10 shows a third configuration of a thermal network according to the present invention. [Figure 4] FIG. 10 shows a fourth configuration of a thermal network according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0050] In all examples, the heat network comprises a plant, end-user locations 14, 15, and three main pipes, two of which, the first main pipe 11 and the third main pipe 13, form the liquid supply and return pipes, respectively. The medium is preferably CO2. The second main pipe 12 contains the same medium, but in gaseous form. Energy transfer is achieved by evaporation / condensation of the medium.

[0051] Both the first main pipe 11 and the third main pipe 13 are arranged and connected so that the medium flows unidirectionally between the end user locations 14,15 and between the plant and the end user locations 14,15.

[0052] The second main pipe 12 is arranged and connected so that the medium can flow in both directions without any difference in all segments connecting the end user locations 14, 15 between them, as well as in segments connecting the plant and the end user locations 14, 15.

[0053] Preferably, the end user locations 14, 15 include: Extracting network media from one or several lines, and / or Injecting a network medium into one or several lines at a thermodynamic state close to the desired state in said lines Any suitable technique capable of doing so is provided.

[0054] Consider the example shown in the diagram below: At the end-user location 14, the medium is extracted from the first main pipe 11 in a thermodynamic state corresponding to the thermodynamic state prevailing in said pipe at this location and time—ideally a saturated or slightly supercooled liquid, but possibly a saturated liquid / gas mixture with the lowest possible gas content. Within the equipment at the end-user location 14, the medium can undergo any kind of thermodynamic process, the simplest of which is evaporation in a heat exchanger device, where the flow of the medium is regulated, preferably using a valve upstream of the heat exchanger inlet, to ensure that the thermodynamic state of the medium at the end-user location outlet corresponds to the desired state in the gas pipe 12 to which said outlet is connected. In this case, the desired state is a saturated gas containing no liquid or a slightly superheated gas.

[0055] At the other end-user location 15, the medium is extracted from the second main pipe 12 in a thermodynamic state corresponding to the thermodynamic state prevailing in that pipe at that time and location—ideally a saturated or slightly superheated gas, but possibly a saturated liquid / gas mixture with as high a gas content as possible. Within the equipment at the end-user location 15, the medium can undergo any kind of thermodynamic process, the simplest of which is condensation in a heat exchanger device, where the flow is regulated, preferably using a valve downstream of the outlet of the heat exchanger device, to ensure that the thermodynamic state of the medium at the outlet of the end-user location 15 corresponds to the desired state in the third main pipe 13 to which the outlet is connected. In this case, the desired state is a saturated or slightly supercooled liquid that does not contain any gas.

[0056] Advantageously, the plant comprises: Maintaining pressure at a preferred set point by exchanging energy with an energy source / sink, called the "source"; ensuring the circulation of the medium in the different pipes 11-13 according to the flow requirements generated at the end-user locations 14, 15; ensuring that the medium supplied by the plant to the first (supply) main pipe 11 is in liquid state and, where applicable, the medium supplied to the second main pipe 12 is in gaseous state; ensuring that the plant can continue its operation even if the medium collected from the third (return) main pipe 13 deviates (within reasonable limits) from the desired liquid state; Ensuring that the plant can continue its operation even if the medium collected from the second main pipe 12 deviates (within reasonable limits) from the desired gas state The present invention includes any suitable element that allows for:

[0057] In these examples, the network is configured to ensure that the medium in the first main pipe 11 is always at a higher pressure than the medium in the second main pipe 12 at every location, and that the pressure in this second main pipe 12 is always at a higher pressure than the pressure in the third pipe 13 at every location.

[0058] At the end-user locations 14, 15, the above-described pipe configurations and their respective pressures allow the medium to flow from the first main pipe 11 to the second main pipe 12, or from the second main pipe 12 to the third main pipe 13, without the need for any active machinery, such as pumps or compressors. This provides a significant benefit for end-user locations dedicated to cooling 14, heating 15, or both. Among other benefits, it reduces the footprint of the equipment installed at the locations 14, 15 and the operational risk associated with active machinery, which tends to be more sensitive than passive equipment (e.g., valves or heat exchangers). Active machinery also generally requires more maintenance than passive equipment, and if located on the end-user's premises, such machinery is likely to be more difficult to repair than if it were located within the plant due to basic access rights.

[0059] In the case of a system in which end user locations 14, 15 utilize the network only for cooling purposes 14 or heating purposes 15 or both, no media enters or leaves the system (the system is closed and does not exchange mass with its environment, only energy).

[0060] During steady-state operation, the mass flow rates of gas entering and leaving the plant exactly compensate for the difference between the mass flow rate of liquid entering the plant from the third main pipe 13 and the mass flow rate of liquid leaving the plant through the first main pipe 11. During transients, the mass flow rates do not compensate for each other until a new steady-state is reached. As an example, if the cumulative demand for cooling at an end-user location produces a mass flow rate in the liquid supply line of "X" kg / s and the cumulative demand for heating produces a mass flow rate in the liquid return line of "Y" kg / s, a mass flow rate of "Z=XY" kg / s entering the plant from the gas line will be observed (provided the system is in steady state). Conversely, if the mass flow rate from the liquid return line is "Y" kg / s and the mass flow rate to the liquid supply line is "X" kg / s, a mass flow rate of "Z=YX" kg / s of gas leaving the plant will be observed (provided the system is in steady state).

[0061] The flow of liquid entering the plant from the third main pipe 13 is first conducted into the receiver 3, whose function is to separate the liquid from the gas. Indeed, due to the pressure drop along the third main pipe 13 but also through some of the valves at the end-user locations 14, 15, and in some situations due to heat input from the environment through the pipe walls, a certain amount of "flash gas" enters the plant. Therefore, a separation receiver must be used to allow the pump 5 to be fully liquid-fed. However, over time, gas will accumulate in the receiver, and suitable means must be used to pump the gas back into the gas line, for example by condensing the gas in an "anti-flash" condenser 7 (see Figure 1) or using a dedicated compressor 4 (see Figure 2).

[0062] The flash prevention condenser 7 can be cooled by any suitable means. For example, if a sufficiently cold source is available, a source of sufficiently cold temperature can be used directly to cool the condenser. If no source of a temperature suitable for direct cooling is available, a heat pump device 8 can be used to provide the necessary cooling for the flash prevention condenser via that cold source. A combination of both direct cooling and cooling via a heat pump device can be used, which is particularly suitable when the temperature of the available source varies significantly over time.

[0063] To improve the operational reliability and energy efficiency of the system, several devices can be incorporated into the system.

[0064] One or several liquid pipe bypasses 18 can be installed, preferably at least one at the farthest point of the pipe system from the plant. The bypass consists of a valve connecting the first main pipe 11 to the third main pipe 13, the opening of which can be fixed, manually set, or automatically activated. This ensures that a minimum flow of medium is guaranteed in the network, even when there is no medium to be extracted from or injected into the pipes at the end-user locations 14, 15, respectively. The goal of having a minimum flow of medium guaranteed at all times and everywhere in the pipe system serves two purposes:

[0065] - to ensure that the medium supplied by the first main pipe 11 at the inlet of the end-user location 14 is always in an acceptable thermodynamic state, ideally a saturated or slightly subcooled liquid, so as to allow start-up of devices at the end-user location without time delay or with minimal time delay. Without this minimum flow rate, heat input from the environment could evaporate the liquid in the pipe when the medium is stationary, and start-up of the equipment at the end-user location 14 would be delayed by the time required to return the liquid from the plant to said end-user location via the first main pipe 11.

[0066] This is to ensure that the medium returning to the plant via the third main pipe 13 is always in the correct thermodynamic state, ideally a saturated or slightly subcooled liquid, but also a saturated liquid / gas mixture with the lowest possible gas content. This is particularly important when there is no or very little medium injected into the third main pipe 13 by devices at the end-user location 15, because in such cases, thermal energy from the environment (e.g., heating of the liquid pipe or equipment) could evaporate a very large portion of the liquid in the pipe so that the low-pressure liquid pump 5 could operate. This can also occur with the help of other devices integrated into the system to mitigate this problem, namely the low-pressure receiver 3, the flash prevention condenser 7, the low-pressure compressor 4, or the low-pressure receiver isolation and flash purge valves 22, 23.

[0067] One or several gas traps 19 can be installed to collect and discharge any gas bubbles that may be present in the medium flowing through the first main pipe 11. These traps can be conveniently located, for example, at locations with local height maxima in the network. Such a trap comprises a receiver connected via a pipe to the liquid supply pipe 11 and via an automatic valve to the gas pipe 12 (second main pipe). This arrangement is made so that the automatic valve ensures that gas, not liquid, is discharged from the receiver. During normal operation, the valve is closed, and the trap's receiver gradually fills with collected gas. When the liquid level in the receiver reaches a sufficiently low value, the automatic valve opens and discharges gas toward the gas pipe 12. When the liquid level in the trap's receiver reaches a sufficiently high value, the valve closes again. It is also possible to stabilize the liquid level in the receiver at the desired value using a regulating valve commanded via a suitable control loop. With the latter option, gas discharge becomes a continuous process rather than a batch process.

[0068] One or several gas traps 20 can be installed to collect and discharge any gas bubbles that may be present in the medium flowing through the third main pipe 13. These traps can be conveniently located, for example, at locations of maximum height within the network. Such a trap has a receiver connected to all three main pipes of the pipe system 11, 12, and 13 via automatic valves. This arrangement is made so that the valve connecting the receiver to the second main pipe 12 discharges gas, not liquid. During normal operation, the valves connecting the trap's receiver to the pipes 11 and 12 are closed, while the valve connecting the receiver to the third main pipe 13 is open. In this way, the receiver gradually fills with gas collected from the third main pipe 13. When the liquid level in the trap's receiver reaches a sufficiently low value, the valve connecting the receiver to the third main pipe 13 is closed, and the valves connecting the receiver to the main pipes 12 and 11 are opened. Because the pressure in pipe 11 is higher than the pressure in pipe 12, some of the liquid mixture flows from pipe 11 into the receiver, acting as a liquid piston pushing the gas from the receiver through the valve into the second main pipe 12. When the liquid level in the receiver reaches a high enough value, the valve connecting the receiver to the main pipes 11 and 12 closes back, the valve connecting the receiver to pipe 13 opens again and normal operation resumes.

[0069] One or several liquid traps 21 can be installed to collect and discharge any liquid that may be present in the medium flowing through the second main pipe 12. These traps can be conveniently located, for example, at locations with minimal height within the network. Such a trap comprises a receiver connected to the second main pipe 12 via a pipe and to the third main pipe 13 via an automatic valve. This arrangement is made so that the automatic valve ensures that liquid, and not gas, is discharged from the receiver. During normal operation, the valve is closed, and the trap's receiver gradually fills with collected liquid. When the liquid level in the receiver reaches a sufficiently high value, the automatic valve opens and discharges the liquid toward the liquid return pipe 13. When the liquid level in the trap's receiver reaches a sufficiently low value, the valve closes and returns. It is also possible to stabilize the liquid level in the receiver at the desired value using a regulating valve commanded via a suitable control loop. With the latter option, the discharge of liquid becomes a continuous process rather than a batch process.

[0070] To further enhance system reliability, a subcooling device 6 positioned between the low-pressure receptacle 3 and the pump 5, or alternatively directly within the receptacle, can be used to help ensure that the pump 5 is adequately fed with liquid at its inlet (corresponding to the effective net suction head having to be higher than the required net suction head by the pump). Alternatively, some subcooling can be imposed on the anti-flash condenser outlet 7 by a controlled valve operated to actively impose said subcooling, or by a siphon at the anti-flash condenser outlet that ensures that the bottom of the anti-flash condenser is filled with the appropriate amount of subcooled liquid at all times. Another advantage of providing subcooled liquid to the pump inlet is that it reduces the required height difference between the bottom of the receptacle and the pump inlet.

[0071] As with the anti-flash condenser 7, if a sufficiently low temperature source is available, the subcooled liquid can be fed directly into the refrigeration circuit of the subcooler 6. Alternatively, the subcooled liquid can be fed by a low temperature source of the heat pump unit 8, and it may be advantageous in terms of space to use the same heat pump unit for both the subcooler and the anti-flash condenser. The heat exchanger can be connected in series or in parallel with the heat pump unit and / or the refrigeration source.

[0072] If a heat pump unit 8 is used, the waste heat rejected by its hot sink during its operation can be advantageously used to preheat the source gas before it enters the medium-pressure evaporator 9a. Alternatively, it is also possible to install a dedicated flooded evaporator in the medium-pressure receiver 1. This is particularly well suited, as the maximum load on the anti-flash condenser 7 and the subcooling unit 6 coincides with the maximum demand for gas provided by the plant through the second main pipe 12 to the associated end-user location 15. This means that the released heat can always be fully stabilized within the system.

[0073] Liquid from the low pressure reservoir 3 is pumped back into the medium pressure reservoir 1 using a low pressure liquid pump 5 .

[0074] As an alternative to the anti-flash condenser 7, it is possible to use a compressor 4 which extracts flash gas from the low pressure receiver and compresses it before sending it to the medium pressure receiver 1 or directly to the second pipe 12. In either case, the liquid pump 5 is still required and the beneficial effect of having a subcooling device 6 remains even if the anti-flash condenser 7 is not present.

[0075] At medium pressure, the gas phase from the gas line and / or in the medium-pressure receiver can be condensed 10 (if the gas returns to the plant through pipe 12) or the liquid can be evaporated in the medium-pressure receiver 9a (if the gas is sent from the plant through pipe 12). Depending on the source temperatures available to the evaporator heating circuit 9a and the condenser cooling circuit 10, respectively, the sources can be fed directly to either the heat exchanger or a heat pump device, respectively, using a refrigeration device 17 (see Figure 3) to supply the heat exchanger, thus decoupling the saturation temperature of the medium-pressure gas in the network from the saturation temperature of the source. The source feeding the evaporator heating circuit 9a can also be preheated in heat exchanger 9b using heat available from the heat pump device 8. If the source temperature changes significantly over time, a combination of direct heat exchange and the use of a heat pump device or a refrigeration device can be advantageously realized.

[0076] As an alternative to a separate refrigeration unit 17 that puts the medium pressure receptacle in contact with a source whose temperature is too high for direct condensation, it is possible to use a compressor 16 that extracts gas from the medium pressure receptacle, compresses it and sends it to the condenser 10, where it condenses and is sent back to the medium pressure receptacle 1 through an expansion valve. From an energy efficiency point of view, this solution can be particularly interesting when the required temperature difference between the available source and the desired saturation temperature in the medium pressure receptacle is relatively small.

[0077] In a fairly similar way, it is possible to decouple the temperature in the intermediate pressure receiver 1 from the temperature in the evaporator 9 by extracting liquid from said receiver, expanding it in a valve, evaporating the liquid in said evaporator, recompressing it and sending it back to the intermediate receiver. Again, this solution can be particularly interesting if the required temperature difference is relatively small.

[0078] A pump 2 is also used to extract, pressurize and deliver the liquid required by the end user location from the medium pressure receiver 1 via a first main pipe 11. A subcooling device may also be installed to improve the performance and reliability of the pump and to reduce the required hydrostatic head. Subcooling can be imposed and cooling can be provided by means similar to those described for the low pressure subcooling device 6.

[0079] As an alternative to both methods of elimination of flash gas in the low pressure receptacle 3, i.e., the use of an anti-flash condenser 7 or a low pressure compressor 4, it is possible to provide the same function using a suitable arrangement of receptacles 1 and 3, i.e., By placing the medium pressure receptacle 1 at a height slightly higher than the height of the low pressure receptacle 3 to allow gravity-driven gas purging of the low pressure receptacle 3, By installing two automatic receiver isolation valves in the low pressure receiver 22, one located in the third main pipe 13 and the other located in the pipe upstream of the low pressure pump 5, By installing two automatic receptacle flush gas purge valves 23, one positioned in the pipe connecting the liquid-filled portion of the medium pressure receptacle 1 to the low pressure receptacle 3, and the other positioned in the pipe connecting the gas-filled portion of the medium pressure receptacle 1 to the similarly gas-containing portion of the low pressure receptacle 3.

[0080] During normal operation, the receiver isolation valve 22 is open and the receiver flush gas purge valve 23 is closed. Ideally, the third main pipe 13 returns saturated or even slightly subcooled liquid; however, due to pressure drop, thermal energy input from the environment, and / or possible injection of a medium in an improper thermodynamic state from the end-user location 15, some gas may be expected to be returned to the receiver 3. Gas gradually accumulates in the receiver until the liquid level reaches a low enough value to trigger a purge cycle by closing the isolation valve 22 and opening the purge valve 23. As a result, the pressure in the low-pressure receiver 3 rises to that of the medium-pressure receiver 1. Due to the low density of gas compared to the density of the liquid phase, the volume of gas in the receiver 3 moves through the purge valve 23 into the receiver 1 and is replaced by liquid flowing down from the medium-pressure receiver 1 to the low-pressure receiver 3. When the liquid level in the low pressure receiver 3 reaches a sufficiently high value, the purge valve 23 closes back, the isolation valve 22 opens again and normal operation can resume.

[0081] While gas is being purged from the low-pressure receptacle 3, both the flow from the third main pipe 13 and the flow through the low-pressure pump 5 are blocked. This can have a negative impact on the stability of the overall system operation, but can be overcome by installing two or more assemblies in parallel, including the low-pressure receptacle 3, the isolation valve 22, and the purge valve 23 (and possibly the subcooling device 6). In this way, it is possible to continue operating the system while one of the assemblies goes through a purge cycle. Alternatively (see FIG. 4), it is also possible to change the geometry of the receptacle 3 by having a portion of the low-pressure receptacle 3, preferably of a smaller volume, installed above the main part of the low-pressure receptacle 3, with both parts connected by a pipe through which the receptacle isolation valve 22 is installed. When the valve is open, both volumes constitute the low-pressure receptacle 3, and during normal operation, gas coming from the third main pipe 13 accumulates in the upper part of the receptacle due to buoyancy forces. When the liquid level in the upper part of the low-pressure receptacle 3 reaches a sufficiently low value due to gas accumulation therein, a purge cycle similar to the one described above is performed by closing the receptacle isolation valve 22 and opening the purge valve 23. When the liquid level in the upper part of the low-pressure receptacle 3 reaches a sufficiently high value, the valve 23 closes again, and the valve 22 opens again to resume normal operation. The advantage of this version of the receptacle gas purge is that it avoids the need for parallel assemblies as described above, while also avoiding interruption of the flow coming from the third main pipe 13 to the pump 5. Furthermore, the number of isolation valves 22 is reduced to just one valve. It should be noted that the purge valve 23 could also be installed in the pipes connecting the low-pressure receptacle 3 to the first main pipe 11 and the second main pipe 12, respectively, instead of in the medium-pressure receptacle 1. This latter solution can be advantageous when it is not possible to install the medium-pressure receptacle 1 slightly above the low-pressure receptacle 3.

[0082] The present invention is, of course, not limited to these four illustrated examples, but to any alternatives covered by the claims. [Explanation of symbols]

[0083] 1...medium pressure receiver, 2...medium pressure liquid pump, 3...low pressure receiver, 4...low pressure compressor, 5...low pressure liquid pump, 6...subcooling device, 7...flash prevention condenser, 8...heat pump device, 9a...medium pressure evaporator, 9b...source preheater, 10...medium pressure condenser, 11...liquid supply pipe, 12...gas pipe, 13...liquid return pipe, 14...cooling user, 15...heating user, 16...medium pressure compressor, 17...refrigeration device, 18...liquid pipe bypass, 19...gas pipe trap, 20...liquid return pipe gas trap, 21...liquid supply pipe gas trap, 22...receiver isolation valve, 23...receiver flash purge valve.

Claims

1. 1. A heat network comprising at least one plant, at least one end-user location (14, 15), a pipe system (11-13), and a medium contained in the pipe system (11-13), the plant, and the end-user location(s), wherein the end-user location(s) (14, 15) are connected to the plant through the pipe system (11-13), wherein the heat network comprises three main pipes (11-13), each connected to the plant(s), and wherein the medium is in a liquid state in the first main pipe (11) and the third main pipe (13) and in a gaseous state in the second main pipe (12) during operation.

2. 2. The thermal network of claim 1, wherein the first main pipe (11) is a supply pipe with unidirectional flow from the plant to the end user location (14) and the third main pipe (13) is a return pipe with unidirectional flow from the end user location (15) to the plant.

3. 3. A thermal network according to claim 1 or 2, wherein the pressure in the first main pipe (11) is higher than the pressure in the second main pipe (12), and the pressure in the second main pipe (12) is higher than the pressure in the third main pipe (13).

4. 4. The heat network according to claim 1, further comprising a liquid pipe bypass (18) connecting the first main pipe (11) to the third main pipe (13) to ensure a minimum flow rate of the medium in the pipes.

5. 5. A thermal network according to any one of claims 1 to 4, comprising a medium receiving part (1) having a part, such as a lower part, connected to the first main pipe (11) and another part, such as an upper part, connected to the second main pipe (12).

6. A thermal network according to any one of claims 1 to 5, comprising a condenser (7, 10) configured to generate said liquid state.

7. A thermal network according to any one of claims 1 to 6, comprising an evaporator (9a) configured to generate said gaseous state.

8. 8. A thermal network according to any one of claims 1 to 7, comprising one end-user location (14, 15) connected to at least two of the main pipes (11-13) via an inlet and an outlet, the thermodynamic state of the medium at the outlet being configured to correspond to a desired state in the connected main pipes (11-13).

9. The medium is CO ２ and used as an energy transfer medium, wherein at least one end user location (14) utilizes said CO ２ The thermal network according to any one of claims 1 to 8, comprising one or several outlets configured to discharge the

10. The medium is CO ２ and is used as an energy transfer medium, and at least one end user location (14) is configured to generate CO ２ 10. The thermal network according to claim 1, comprising one or several inlets configured to inject into the network from connected devices.

11. 11. The thermal network according to any one of claims 1 to 10, comprising a liquid trap (19) on the second main pipe (12) configured to extract any medium in liquid phase from the second main pipe.

12. 12. The thermal network according to any one of claims 1 to 11, comprising a gas trap (20) on the third main pipe (13) configured to extract any medium in gas phase from the third main pipe.

13. 13. The thermal network according to any one of claims 1 to 12, comprising a gas trap (21) on the first main pipe (11) configured to extract any medium in gas phase from the first main pipe.

14. A thermal network according to any one of the preceding claims, comprising a cooling system (8) configured to extract heat from the flow entering the plant via the third main pipe (13).

15. 15. The thermal network according to any one of claims 1 to 14, comprising a subcooling device (6) configured to extract heat from a liquid entering the plant via the third main pipe (13) and / or leaving the receiver (1).

16. 16. A thermal network according to claim 14 or 15, wherein the cooling system comprises a heat pump (8) used together with the subcooling device (6), if present.

17. 17. The heat network according to any one of claims 1 to 16, comprising a compressor (4) capable of extracting the gas accumulation before the condensate extraction pump and compressing it to a corresponding pressure in the second main pipe (12) on the plant side.

18. 18. A thermal network according to any one of the preceding claims, comprising a set of receiver isolation valves (22) and a set of receiver flush purge valves (23) operative to extract gas accumulation before the condensate extraction pump.

19. Use of a heat network according to any one of claims 1 to 18, characterized in that the gas flows from the receiver (1) to the second main pipe (12) in the plant when the heating requirement is higher than the cooling requirement (e.g. in winter).

20. Use of a heat network according to any one of claims 1 to 18, characterized in that the gas flows from the second main pipe (12) to the receiver (1) in the plant when the cooling requirement is higher than the heating requirement (e.g. in summer).

21. Use of a thermal network according to any one of claims 1 to 18, characterized in that there is no flow in the second main pipe (12) going into or coming from the receiver (1) when the cooling requirement is equal to the heating requirement (e.g. in summer).

22. 19. Use of a thermal network according to any one of claims 1 to 18, comprising the production of a liquid state medium and a gaseous state medium, said liquid state medium being transported to said first main pipe (11) and said third main pipe (13) and said gaseous state medium being transported to said second main pipe (12).

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