Heating system for refrigeration system

Abstract

The present disclosure relates to a heating system for a refrigeration system, comprising: a heater disposed within a space; a power terminal configured to receive electrical power from a power source; a heater temperature sensor module configured to change from a first state to a second state when a temperature associated with the heater reaches and exceeds a heater temperature action threshold; an electric interlock device configured to prevent electric power from being supplied to the heater from the power terminal when the heater temperature sensor module is in the second state; a semiconductor switch operable to modulate an electrical power supply from the power terminal to the heater; a driver module configured to control the semiconductor switch based on the driving signal; an interrupt switch configured to prevent the driver module from receiving the driving signal when in a non-conducting state; and a concentration sensor module configured to: monitor a concentration of the refrigerant in the space; when the monitored concentration of the refrigerant reaches or exceeds a concentration action threshold, the interrupt switch is caused to be in a non-conducting state.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a heating system for a refrigeration system (e.g., a transport refrigeration system). The present disclosure also relates to a refrigeration system comprising such a heating system. BACKGROUND

[0002] There is an increasing desire to use process fluids (e.g., refrigerants) having a relatively low global warming potential (GWP) in refrigeration systems, including in transport refrigeration systems. However, such refrigerants have some properties that are significantly different from corresponding properties of traditional refrigerants. These different properties are associated with both performance and safety challenges in the context of refrigeration systems.

[0003] The present invention has been devised in light of the foregoing. SUMMARY

[0004] According to a first aspect, there is provided a heating system for a refrigeration system, the heating system comprising: a heater; power terminals configured to receive electrical power from a power source; a heater temperature sensor module configured to change from a first state to a second state when a temperature associated with the heater reaches or exceeds a heater temperature action threshold; and an electrical interlock configured to prevent the supply of electrical power from the power terminals to the heater when the heater temperature sensor module is in the second state.

[0005] The heater temperature sensor module can comprise a bimetallic switch.

[0006] The heating system can comprise a power supply switch located between the power terminals and the heater. The electrical interlock can be configured to prevent the supply of electrical power from the power terminals to the heater when the heater temperature sensor module is in the second state by causing the power supply switch to be in a non-conductive state.

[0007] The heating system can comprise a path for transporting a refrigerant. The heater temperature action threshold can be at least 100 °C lower than an auto-ignition temperature of the refrigerant. The heater temperature action threshold can be no greater than 200 °C.

[0008] The heating system can comprise a controller configured to determine whether the heater temperature sensor module is in the first state or in the second state.

[0009] The heating system can comprise a semiconductor switch operable to modulate the supply of electrical power from the power terminals to the heater. The heating system can further comprise a driver module configured to control the semiconductor switch based on a drive signal.

[0010] According to a second aspect, there is provided a heating system for a refrigeration system, the heating system comprising: a heater arranged within a space; power terminals configured to receive electrical power from a power source; a semiconductor switch operable to modulate a supply of electrical power from the power terminals to the heater; a driver module configured to control the semiconductor switch based on a drive signal; an interrupt switch configured to prevent the driver module from receiving the drive signal when in a non-conductive state; and a concentration sensor module configured to: monitor a concentration of refrigerant in the space; and cause the interrupt switch to be in the non-conductive state when the monitored concentration of refrigerant reaches or exceeds a concentration action threshold.

[0011] The heating system can comprise a controller configured to determine whether the interrupt switch is in the non-conductive state.

[0012] According to the first aspect and / or the second aspect, the driver module can be configured to vary a duty cycle of a control signal for the semiconductor switch based on the drive signal.

[0013] According to the first aspect and / or the second aspect, the heating system can comprise a controller configured to generate the drive signal.

[0014] According to the first aspect and / or the second aspect, the heating system can comprise a heat exchanger configured to allow refrigerant to be transported through the heat exchanger. The heater can be configured to provide heat to the heat exchanger.

[0015] The controller can be configured to: determine a temperature associated with the heat exchanger; and generate the drive signal based on the determined temperature.

[0016] The controller can be configured to determine the temperature associated with the heat exchanger based on a signal from a heat exchanger temperature sensor module configured to monitor a temperature associated with the heat exchanger.

[0017] According to a third aspect, there is provided a heating system for a refrigeration system, the heating system comprising: a heater arranged within a space; power terminals configured to receive electrical power from a power source; a heater temperature sensor module configured to change from a first state to a second state when a temperature associated with the heater reaches exceeds a heater temperature action threshold; an electrical interlock configured to prevent supply of electrical power from the power terminals to the heater when the heater temperature sensor module is in the second state; a semiconductor switch operable to modulate the supply of electrical power from the power terminals to the heater; a driver module configured to control the semiconductor switch based on a drive signal; an interrupt switch configured to prevent the driver module from receiving the drive signal when in a non-conductive state; and a concentration sensor module configured to: monitor a concentration of refrigerant in the space; cause the interrupt switch to be in the non-conductive state when the monitored concentration of refrigerant reaches or exceeds a concentration action threshold.

[0018] The heating system according to the third aspect can comprise any suitable one or any suitable combination of the features described in relation to the first or second aspects.

[0019] According to a fourth aspect, there is provided a refrigeration system comprising a heating system according to the first, second or third aspects. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A vehicle comprising a transport refrigeration system is shown;

[0021] Figure 2 is a schematic diagram of an example transport refrigeration unit suitable for use in a vehicle of Figure 1 , the example transport refrigeration unit comprising a vapour compression refrigeration circuit and a subsystem;

[0022] Figure 3 is a diagram showing an example subsystem of a transport refrigeration unit suitable for use in a vehicle of Figure 2 , the subsystem being in a first mode;

[0023] Figure 4 is a diagram showing an example subsystem of Figure 3 in a second mode; and

[0024] Figure 5 is a diagram showing an example subsystem of Figure 3 in a third mode. DETAILED DESCRIPTION

[0025] Figure 1A vehicle 10 including a transport refrigeration system 20 is shown. In Figure 1 In examples, the transport refrigeration system 20 forms part of an over-the-road refrigerated semi-trailer truck having a structure 22 that supports (or forms) at least one climate-controlled compartment 24 configured to be cooled and / or heated by the TRU 110. The climate-controlled compartment 24 can take the form of multiple compartments or have multiple zones. The structure 22 includes a chassis. The structure 22 supports the TRU 110. The vehicle 10 also includes a tractor unit 14 removably coupled to the trailer.

[0026] Figure 2 A diagram of an example TRU 110 within the vehicle 10 and the transport refrigeration system 20 suitable for use with Figure 1 The TRU 110 includes a vapor-compression refrigeration circuit 400 and a heating system 200 (which can also be referred to as a heating subsystem 200, or more simply as a subsystem 200).

[0027] The vapor-compression refrigeration circuit 400 includes an evaporator 408 configured to receive heat from the climate-controlled compartment 24 of the transport refrigeration system 20 and a condenser 404 configured to reject heat to a heat sink 44 (e.g., ambient air outside of the climate-controlled compartment 24) when the vapor-compression refrigeration circuit 400 is operated in a cooling mode (e.g., for providing cooling to the climate-controlled compartment 24). For these purposes, the vapor-compression refrigeration circuit 400 also includes a compressor 402 and an expansion valve 406. Thus, the vapor-compression refrigeration circuit 400 can be controlled to cause heat to be removed from the climate-controlled compartment 24. The condenser 404 and the evaporator 408 are each a heat exchanger 404, 408 configured to facilitate heat transfer heat exchange between a refrigerant transported through the heat exchanger 404, 408 (e.g., through a path provided by the heat exchanger 404, 408) and an external medium (i.e., the heat sink 44 and the climate-controlled compartment 24). As will be appreciated by those skilled in the art, the vapor-compression refrigeration circuit 400 is also capable of being operated in a heating mode to provide heating to the climate-controlled compartment 24.

[0028] The vapor-compression refrigeration circuit 400 is filled with refrigerant prior to operation. During operation, the refrigerant circulates within the vapor-compression refrigeration circuit 400 (e.g., is transported around the vapor-compression refrigeration circuit 400) in a manner that will be appreciated by those skilled in the art. In particular, the refrigerant can be an A2L refrigerant, such as R454A. Such a refrigerant can be relatively flammable, and thus can be referred to as a flammable refrigerant or flammable refrigerants.

[0029] The subsystem 200 includes a heater 260 (e.g., a heating element 260) disposed within a space (e.g., the climate-controlled compartment 24). The evaporator 408 also forms part of the subsystem 408. In Figure 2 In examples in accordance with the present disclosure, the heater 260 is disposed adjacent to the evaporator 408. Thus, in use, the heater 260 can be operated to provide heat to the evaporator 408. However, the present disclosure contemplates that the heater 260 can be disposed elsewhere within the TRU 110. Generally, the heater 260 is configured to provide electrical heat to the TRU 110. The electrical subsystem 200 is configured to control the supply of electrical power from the DC link 210 to the heater 260, and thereby cause the heater 260 to generate heat for heating the TRU 110. When supplied with electrical power, the heater 260 generates heat by way of resistive (e.g., ohmic) heating. In examples in accordance with the present disclosure, the heater 260 can be used to defrost the evaporator 408, to prevent temperature drift in the climate-controlled compartment 24 when the vapor-compression refrigeration circuit 400 is operated in a cooling mode (e.g., for precision cooling), and / or to provide supplemental heating to the climate-controlled compartment 24 when the vapor-compression refrigeration circuit 400 is operated in a heating mode.

[0030] The subsystem 200 includes a pair of power terminals 202, 204 (i.e., a first power terminal 202 and a second power terminal 204) at which electrical power is received from the power source 115 in use. The power source 115 may, for example, be a DC link of the TRU 110. The DC link may, for example, receive electrical power from an electrical energy storage device (e.g., a battery or a fuel cell) provided to the transport refrigeration system 20, from an electrical generator set (e.g., an alternator) provided to the vehicle, or an external power supply (e.g., shore power) received via an external connection port when the transport refrigeration system 20 is stationary.

[0031] Figure 3 is a schematic diagram illustrating an example subsystem 200 suitable for use with the TRU 110 described above with reference to Figure 2 FIG. 1, wherein like reference numerals indicate like or common features.

[0032] The subsystem 200 includes a local controller 290 (e.g., a controller 290). The local controller 290 can be referred to as a remote input-output controller (RIOC) 290. The RIOC 290 includes a first port 291, a second port 292, a third port 293, a fourth port 294, and a fifth port 295. Each port 291-295 serves as a physical interface between a processor of the RIOC 290 and a respective channel 271-275. The functionality of the RIOC 290 is described in further detail below.

[0033] The subsystem 200 includes an isolation device 210 electrically coupled to the power terminals 202, 204. In turn, the isolation device 210 includes a first contact 212 and a second contact 214. The first contact 212 is electrically coupled to the first power terminal 202, and the second contact 214 is electrically coupled to the second power terminal 204. When each contact 212 is closed, electrical power can flow from the power terminals 202, 204 to the heater 260 (which is subject to the action of the modulation device 220 described below). When each contact 212, 214 is open, electrical power cannot flow from the power terminals 202, 204 to the heater 260 (regardless of the action of the modulation device 220). Each contact 212, 214 can be referred to as a power supply switch having a non-conductive state (i.e., when open) and a conductive state (i.e., when closed). It will be appreciated that other types of switches (e.g., power supply switches) can be used in place of the contacts 212, 214, and the isolation device 210 can include only a single power supply switch (e.g., only one of the contacts 212, 214).

[0034] The subsystem 200 also includes a modulation device 220 that includes a semiconductor switch 222 and a driver module 229. The semiconductor switch 222 is operable to modulate the supply of electrical power from the power terminals 202, 204 to the heater 260 via the isolation device 210 by selectively making and breaking a conduction path from the first power terminal 202 through the heater 260 to the second power terminal 204. The driver module 229 is configured to control the semiconductor switch 222 based on a drive signal supplied to the driver module 229. More specifically, the driver module is configured to control the semiconductor switch by providing a control signal to the semiconductor switch, as will be appreciated by those skilled in the art. The semiconductor switch 222 may, for example, be a solid state relay (SSR).

[0035] A third channel 273 electrically couples (and thereby communicatively couples) a third port 293 of the RIOC 290 to the driver module 229 of the modulation device 220. The third channel 273 is configured to convey a drive signal from the RIOC 290 to the driver module 229. Accordingly, the third channel 273 can be referred to as a drive channel 273. The driver module 229 is configured to generate a control signal for controlling the semiconductor switch 222 based on the drive signal received on the drive channel 273. In particular, the driver module 229 can generate a control signal having a duty cycle. By way of example, the driver module can generate a control signal that is a substantially square wave defined by a duty cycle. When the control signal is “HIGH,” the semiconductor switch 222 can be in a conductive state, and when the control signal is “LOW,” the semiconductor switch 222 can be in a non-conductive state. Accordingly, electrical power can flow from the isolation device 210 to the heater 260 only when the control signal is “HIGH.” Thus, the duty cycle of the control signal defines the time-averaged amount of electrical power that flows to the heater 260 when the contacts 212, 214 are closed. By varying the duty cycle of the control signal (e.g., by modulating the pulse width), the driver module 229 can vary the time-averaged amount of electrical power supplied to the heater. Accordingly, the control signal can be referred to as a pulse width modulated (PWM) control signal.

[0036] The subsystem 200 also includes a heater temperature sensor module 230. More particularly, the heater temperature sensor module 230 is a bimetallic switch 232 (which can also be referred to as a bimetallic thermostat). The bimetallic switch 232 is not in contact with the heater 260, but is adjacent to the heater 260. In use, heat generated by the heater 260 is transferred to a portion of the bimetallic switch 232 by way of radiative heat transfer. Accordingly, the temperature of this portion of the bimetallic switch 232 is correlated to the temperature of the heater 260. The bimetallic switch 232 is configured to change from a first state to a second state when the temperature of this portion of the bimetallic switch 232 reaches or exceeds a threshold value (i.e., a heater temperature action threshold value, as referred to herein). A suitable bimetallic switch 232 can be a KLIXON® switch. The first state of the bimetallic switch 232 is a closed / conductive state, and the second state is an open / non-conductive state. The use of a bimetallic switch 232 as part of the heater temperature sensor module 230 is preferred because it provides a robust and resettable physical switching capability for the purposes described herein. Additionally, the use of a bimetallic switch 232 as part of the heater temperature sensor module 230 is advantageous because it is fast-acting and does not require any heavy conditioning or post-processing of the output signal from the bimetallic switch 232.

[0037] Furthermore, the subsystem 200 comprises an electrical interlock 280 (or more simply, an interlock 280) adapted to control the isolation device 210. The interlock 280 comprises a first port 281, a second port 282 and an internal circuitry forming part of a conduction path from the first port 281 through the second port 282, through the heater temperature sensor module 230 (e.g. the bimetallic switch 232) and back to the first port 281 to form an interlock loop (which can be referred to as a high voltage interlock loop or a hazardous voltage interlock loop HVIL). When the bimetallic switch 232 is in a closed / conductive state, the interlock loop is complete. Conversely, when the bimetallic switch 232 is in an open / non-conductive state, the interlock loop is open. When the interlock loop is complete and an electrical current is flowing through the interlock loop, the interlock 280 allows (e.g. and thereby causes) each contact 212, 214 to be closed; whereas when the interlock loop is open and no electrical current is flowing through the interlock loop, the interlock 280 allows (e.g. and thereby causes) at least one contact 212, 214 (e.g. one contact or both contacts 212, 214) to be open. In this way, the interlock 280 is configured to prevent the supply of electrical power from the power terminals 202, 204 to the heater 260 when the heater temperature sensor module 230 is in the second state. The interlock 280 comprises internal circuitry suitable for achieving these purposes, as will be familiar to the skilled person.

[0038] The first channel 271 extends from the first port 291 of the RIOC 290 to the first junction location 288 on the interlock loop. Thus, the RIOC 290 is able to monitor the state of the interlock loop (and thereby indirectly the state of the heater temperature sensor module 230) by means of a signal received from the first junction location 288 along the first channel 271 (e.g. a signal corresponding to whether or not an electrical current is flowing through the interlock loop / the interlock loop is complete). The state of the heater temperature sensor module 230 can then be used as part of the software with which the RIOC 290 is run in use.

[0039] The subsystem 200 comprises a concentration sensor module 240. The RIOC 290 is configured to provide a supply of electrical power (e.g., a drive current) along a fourth channel 274 to the concentration sensor module 240. The concentration sensor module 240 is functionally coupled to an interrupt switch 242. The interrupt switch 242 is operable to move between a closed / conductive state (e.g., a first state) and an open / non-conductive state (e.g., a second state). The concentration sensor module 240 comprises a concentration transducer 241 configured to monitor a concentration of refrigerant within a space in which the heater 260 is disposed. Refrigerant can be present within the space as a result of development of a leak from the vapor compression refrigeration circuit 400 into the space. The concentration sensor module 240 is more broadly configured such that when the concentration of refrigerant within the space is below a threshold (i.e., a concentration action threshold, as referred to herein), the concentration sensor module 240 causes the interrupt switch 242 to be in the first state. On the other hand, the concentration sensor module 240 is configured such that when the concentration of refrigerant within the space reaches or exceeds the concentration action threshold, the concentration sensor module 240 causes the interrupt switch 242 to be in the second state. If the leak resulting in the presence of refrigerant within the space is relatively small, the concentration of refrigerant within the space can remain below the concentration action threshold as a result of the refrigerant diffusing from the space into the ambient environment. If this is the case, the presence of the leak will not cause the concentration of refrigerant within the space to reach or exceed the concentration action threshold, and thus the interrupt switch 242 will remain in the first state.

[0040] The interrupt switch 242 is located on the drive channel 273. Thus, the interrupt switch 242 is disposed within the subsystem 200 such that when the interrupt switch 242 is in the first state, a drive signal can be supplied from the RIOC 290 to the driver module 229 along the drive channel 273. Conversely, when the interrupt switch 242 is in the second state, the drive channel 273 is interrupted such that a drive signal cannot be supplied from the RIOC 290 to the driver module 229 along the drive channel 273.

[0041] A second channel 272 extends from a second port 292 of the RIOC 290 to a second junction location 248 located on the drive channel 273 between the interrupt switch 242 and the driver module 229. The RIOC 290 is configured to monitor the transmission of the drive signal along the drive channel 273 (and thereby indirectly monitor which state the interrupt switch 242 is in) by means of signals received from the second junction location 248 along the second channel 272. The state of the interrupt switch 242 can then be used by software running in use by the RIOC 290. In particular, the interrupt switch 242 being in the open / non-conductive state corresponds to the concentration of refrigerant within the space reaching or exceeding the concentration action threshold. Software running by the RIOC 290 can cause the RIOC 290 to take various actions (e.g., transmit information related to the determination to a central controller of the vapor compression refrigeration circuit 400, such as a main application controller (MAC); sound an alarm; cause shut down of other components; etc.) in response to determining that the concentration of refrigerant within the space reaches or exceeds the concentration action threshold based on signals received along the second channel 272.

[0042] The subsystem 200 also includes a heat exchanger temperature sensor module 250 configured to monitor a temperature associated with the evaporator 408. The heat exchanger temperature sensor module 250 is electrically and communicatively coupled to the RIOC 290 via a fifth channel 275. Thus, the RIOC 290 is configured to determine a temperature associated with the evaporator 480 (e.g., the temperature of the evaporator 480) based on signals received from the heat exchanger temperature sensor module 250 along the fifth channel 275.

[0043] In use, the RIOC 290 generates a drive signal and attempts to transmit the drive signal along the drive channel 273 to the driver module 229. More specifically, the RIOC 290 generates the drive signal based on the temperature associated with the evaporator 480, as discussed in greater detail below.

[0044] In Figure 3 the first mode, the subsystem 200 is in a first mode. In the first mode, the bimetallic switch 232 is in a first state of the bimetallic switch 232 and the interrupt switch 240 is in a first state of the interrupt switch 240. As a result, during operation, the isolation device 210 receives electrical power from the power terminals 202, 204 while the driver module 229 controls the semiconductor switch 222 based on the drive signal generated by the RIOC 290. Thus, electrical power flows from the power terminals 202, 204 to the heater 260 via the isolation device 210 and the modulation device 220. The amount of electrical power that flows in this manner depends on the duty cycle of the drive signal as discussed above.

[0045] In Figure 4In the second mode, the subsystem 200 is in the second mode. In the second mode, the bimetallic switch 232 is in the first state of the bimetallic switch 232 and the interrupt switch 240 is in the second state of the interrupt switch 240. As a result, during operation, the isolation device 210 receives electrical power from the power terminals 202, 204, but the driver module 229 does not receive a drive signal from the RIOC 290 and, thus, no control signal is provided to the semiconductor switch 222. Accordingly, it is expected that the semiconductor switch is in a non-conductive state.

[0046] In Figure 5 the third mode, the subsystem 200 is in the third mode. In the third mode, the bimetallic switch 232 is in the second state of the bimetallic switch 232 and the interrupt switch 240 is in the second state of the interrupt switch 240. As a result, the interlock device 280 causes the isolation device 210 to prevent the supply of electrical power from the power terminals 202, 204 to the heater 260 by opening the contacts 212, 214.

[0047] In principle, the heater 260 can act as an ignition source with respect to any refrigerant present within the space in which the evaporator 408 is disposed. Generally, the heater 260 will act in this way if it approaches the auto-ignition temperature of the refrigerant and the concentration of the refrigerant within the space reaches or exceeds its lower flammable limit (LFL). The heater 260 can be particularly capable of acting as an ignition source for the refrigerant within the space if the voltage supplied to the heater 260 (e.g. by the power / DC link 115) is relatively high and subject to variation in use. For example, if the heater is rated at 6 kW at 700 VDC and the voltage supplied to the heater increases by approximately 21% to 850 VDC, the power output of the heater will increase by approximately 47% to 8.84 kW. This can result in a significant increase in the temperature of the heater 260 and, thus, approach, reach or exceed the auto-ignition temperature of the refrigerant.

[0048] When in the first mode (see Figure 3 ), the RIOC 290 generates a drive signal based on the signals received from the heat exchanger temperature sensor module 250 and supplies the drive signal along the drive channel 273 to the driver module 229. The driver module 229 then controls the semiconductor switch using a control signal that has been generated based on the drive signal.

[0049] As an example, the RIOC 290 can generate drive signals based on the lower heat exchanger temperature action threshold and the upper heat exchanger temperature action threshold and signals received from the heat exchanger temperature sensor module 250. If the signals received from the heat exchanger temperature sensor module are less than the lower heat exchanger temperature action threshold, the RIOC 290 can generate drive signals corresponding to a high power demand of the heater 260. Control signals generated by the driver module 229 based on such drive signals can have a relatively high duty cycle. On the other hand, if the signals received from the heat exchanger temperature sensor module are greater than the upper heat exchanger temperature action threshold, the RIOC 290 can generate drive signals corresponding to a low or zero power demand of the heater 260. Control signals generated by the driver module 229 based on such drive signals can have a relatively low duty cycle. Moreover, if the signals received from the heat exchanger temperature sensor module are between the lower heat exchanger temperature action threshold and the upper heat exchanger temperature action threshold (including the (two) endpoint values), the RIOC 290 can generate drive signals corresponding to a moderate power demand of the heater 260, and control signals generated by the driver module can be generated accordingly.

[0050] Signals received from the heat exchanger temperature sensor module that are less than the lower heat exchanger temperature action threshold can correspond to nominal conditions in the subsystem 200. Signals received from the heat exchanger temperature sensor module that are greater than the upper heat exchanger temperature action threshold can correspond to a shutdown fault condition in the subsystem 200, and the RIOC 290 can generate a zero power demand of the heater 260 with the intent to shut down operation of the heater 260 for safety considerations. Signals received from the heat exchanger temperature sensor module that are between the upper heat exchanger temperature action threshold and the lower heat exchanger temperature action threshold can correspond to a warning fault condition in the subsystem 200, and the RIOC 290 can generate a moderate power demand of the heater 260 with the intent to constrain operation of the heater 260 for safety considerations while allowing the heater 260 to continue operating.

[0051] As another example, the RIOC 290 can generate drive signals based on a heat exchanger temperature setpoint and signals received from the heat exchanger temperature sensor module 250. To this end, the RIOC 290 can implement a proportional, integral, and / or derivative control scheme based on the heat exchanger temperature setpoint.

[0052] However, if the concentration of refrigerant within the space in which the evaporator 408 is disposed reaches or exceeds the concentration action threshold, the subsystem 200 moves to a second mode (see FIG. 4B) in which the RIOC 290 generates drive signals corresponding to a low power demand of the heater 260. Control signals generated by the driver module 229 based on such drive signals can have a relatively low duty cycle. In this second mode, the RIOC 290 can also generate drive signals corresponding to a high power demand of the evaporator 408. Control signals generated by the driver module 229 based on such drive signals can have a relatively high duty cycle. Figure 4). In the second mode, although the RIOC 290 can continue to generate the drive signal based on the signal received from the heat exchanger temperature sensor module 250, the drive signal cannot be supplied to the driver module 229 along the drive channel 273 due to the action of the interrupt switch 242. Thus, regardless of the action of the RIOC 290, when the concentration of refrigerant within the space in which the evaporator 408 is disposed reaches or exceeds the concentration action threshold, the driver module 229 will not generate the control signal for the semiconductor switch 222.

[0053] Thus, in the second mode, it is expected that no electrical power will be supplied to the heater 260, since the semiconductor switch 222 should be in a non-conductive state, and thus the temperature associated with the heater 260 monitored by the heater temperature monitoring module 230 will begin to decrease (e.g., after any transient effects have occurred within the subsystem 200).

[0054] The concentration action threshold is selected so as to ensure that, before the concentration of refrigerant within the space approaches its lower flammability limit (LFL), the interrupt switch 242 is moved to the second state, and thus prevents the drive signal from being received by the driver module 229. For example, if the LFL of the refrigerant is 18% by volume, the concentration action threshold can be equal to or less than 16% by volume.

[0055] However, if the temperature associated with the heater 260 monitored by the heater temperature monitoring module 230 instead increases to the heater temperature action threshold while the subsystem 200 is in the second mode, the subsystem 200 moves to a third mode (see Figure 5 ). In the third mode, the bimetallic switch 232 is in the second state, and thus the interlock loop formed by its portion is open. Thus, the interlock device 280 causes the isolation device 210 to isolate the power terminals 202, 204 from the modulation device 220 and the heater 260 by opening the contacts 212, 214. Thus, in the third mode, electrical power cannot be supplied from the power terminals 202, 204 to the remainder of the system due to the isolation provided by the isolation device 210.

[0056] If semiconductor switch 222 and modulating device 220 as a whole function correctly, then when subsystem 200 is in the second mode, electrical power is prevented from flowing from electrical terminals 202, 204 to heater 260 via isolation device 210 and modulating device 220. However, if modulating device 220 does not function correctly (e.g., semiconductor switch 222 has failed in a permanently on state), then when in the second mode, electrical power can still flow from electrical terminals 202, 204 to heater 260 via isolation device 210 and modulating device 220. The third mode represents a failsafe measure that enables electrical power to be prevented from flowing from electrical terminals 202, 204 to heater 260 when modulating device can not be functioning correctly.

[0057] The heater temperature action threshold is selected to ensure that bimetallic switch 232 moves to the second state, and thereby interlock device 280 causes isolation device 210 to well prevent the supply of electrical power from electrical terminals 202, 204, before heater 260 approaches the auto-ignition temperature of the refrigerant that vapor-compression refrigeration circuit 400 circulates during operation.

[0058] To this end, the heater temperature action threshold can be at least 100 °C less than the auto-ignition temperature of the refrigerant. By way of example, if the auto-ignition temperature of the refrigerant is 800 °C, then the heater temperature action threshold can be equal to or less than 700 °C. This provides a sufficient safety margin even after accounting for transient effects that can exist within subsystem 200 and / or vapor-compression refrigeration circuit 400 that can cause the temperature of heater 260 to continue to rise even after the supply of electrical power from electrical terminals 202, 204 is cut off by isolation device 210. In some examples, the heater temperature action threshold can be no more than 200 °C (e.g., 175 °C, 150 °C, 125 °C, or 100 °C), and heater temperature sensor module 230 can be located at a suitable distance away from heater 260 such that the temperature of the portion of bimetallic switch 232 that reaches or exceeds the heater temperature action threshold indicates that the temperature of heater 260 is approaching the auto-ignition temperature of the refrigerant. This allows a simple / conventional off-the-shelf bimetallic switch to be used as part of heater temperature sensor module 230, which is associated with improved ease of manufacture / assembly of subsystem 200.

[0059] The heating system (e.g., a subsystem) according to this disclosure provides means for ensuring the safe operation of the refrigeration system. In particular, the subsystem according to this disclosure provides software-independent means for ensuring the safe operation of the refrigeration system. Specifically, the cooperative interaction between the heater temperature sensor module 230 and the interlocking device 280 provides means for ensuring that no electrical power is supplied to the heater when the temperature associated with the heater 260 approaches the auto-ignition temperature of the refrigerant circulated by the vapor compression refrigeration circuit 400, regardless of the software provided to the controller / RIOC 290 and independent of the proper functioning of the modulation device 220. This provides a layer of hardware-based over-temperature protection for the subsystem 200 and the refrigeration system 20 as a whole. Similarly, the cooperative interaction between the concentration sensor module 240 and the interrupt switch 242 provides means for ensuring that the driver module 229 does not act on the signal from the controller / RIOC 290 for supplying heating to the evaporator 408 when the refrigerant concentration in the relevant space reaches or exceeds an appropriate level. Therefore, the subsystem according to this disclosure provides a hardware-based, software-independent safety element to the refrigeration system. Such a safety element can be particularly valuable in refrigeration systems circulating low-GWP refrigerants (which may be relatively flammable compared to more conventional refrigerant types and therefore present relatively greater safety challenges), and the subsystem according to the invention can help meet this challenge. In other words, the subsystem according to the invention solves the problems associated with the safety of refrigeration systems circulating relatively flammable refrigerants (such as A2L-type refrigerants) in use.

[0060] Except in mutually exclusive cases, features described with respect to any of the foregoing aspects may be applied to any other aspect with necessary modifications. Furthermore, except in mutually exclusive cases, any feature described herein may be applied to any aspect and / or combined with any other feature described herein. Moreover, although this disclosure is made in the context of transport refrigeration systems and / or vapor compression refrigeration circuits, it should be understood that this disclosure has other possible applications in other technical fields.

Claims

1. A heating system for a refrigeration system, the heating system comprising: a heater; a power terminal configured to receive electrical power from a power source; characterized in that the heating system further comprises: a heater temperature sensor module configured to change from a first state to a second state when a temperature associated with the heater reaches or exceeds a heater temperature action threshold; and an electrical interlock configured to prevent the supply of electrical power from the power terminal to the heater when the heater temperature sensor module is in the second state.

2. The heating system of claim 1, wherein, The heater temperature sensor module comprises a bimetallic switch.

3. The heating system of claim 1, comprising a power supply switch between the power terminal and the heater, wherein the electrical interlock is configured to prevent the supply of electrical power from the power terminal to the heater when the heater temperature sensor module is in the second state by causing the power supply switch to be in a non-conductive state.

4. The heating system of claim 1, wherein, The heating system comprises a path for transporting a refrigerant, and wherein the heater temperature action threshold is at least 100 °C lower than an auto-ignition temperature of the refrigerant.

5. The heating system of claim 1, comprising a controller configured to determine whether the heater temperature sensor module is in the first state or in the second state.

6. The heating system of claim 1, comprising: a semiconductor switch operable to modulate the supply of electrical power from the power terminal to the heater; and a driver module configured to control the semiconductor switch based on a drive signal.

7. The heating system of claim 6, wherein, The driver module is configured to vary a duty cycle of a control signal for the semiconductor switch based on the drive signal.

8. The heating system of claim 6, comprising a controller configured to generate the drive signal.

9. The heating system of claim 6, comprising a heat exchanger configured to allow a refrigerant to be transported through the heat exchanger, wherein the heater is configured to provide heat to the heat exchanger.

10. The heating system of claim 6, further comprising: an interrupt switch configured to prevent the driver module from receiving the drive signal when in a non-conductive state; and a concentration sensor module configured to: - monitor a concentration of a refrigerant in a space; and - cause the interrupt switch to be in the non-conductive state when the monitored concentration of the refrigerant reaches or exceeds a concentration action threshold.

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