Patent Publication Number: US-11642936-B2

Title: Recreational vehicle air conditioning system with load sharing

Description:
BACKGROUND 
     Recreational vehicles (RVs) such as motorhomes, travel trailers, fifth wheel trailers, etc. often utilize rooftop air conditioning (AC) units for cooling, and in some instances, also for heating, dehumidification and/or ventilation. Larger RVs such as Class A motorhomes, often use two or three rooftop AC units to handle different regions or zones of the living space. Rooftop AC units, however, are among the highest power consuming devices in an RV, and while some larger RVs are capable of utilizing 50 Amp services with sufficient capacity to concurrently operate multiple rooftop AC units, some RVs only support 30 Amp services where concurrent operation of multiple rooftop AC units is likely to exceed the allowable current in some circumstances, which can cause a circuit breaker to be tripped and for all power to be cut to the RV. In many instances, smaller RVs supporting only 30 Amp service may only offer a secondary rooftop AC unit as an optional upgrade, which if not purchased may result in insufficient cooling/heating capacity in the living space and/or an inability to separately control different zones of the living space. Further, even where an RV supports 50 Amp service, some campgrounds or campsites may only provide 30 Amp service, subjecting these larger RVs to a potential for tripped circuit breakers. 
     A tripped circuit breaker can be frustrating for an RV owner, as all power to the RV may be shut off, or at least all power to the devices on the circuit with the tripped circuit breaker. Further, the owner is generally required to manually reset the circuit breaker, and potentially may have to leave the living space of the RV in order to access the breaker box. As a result, some attempts have been made to address the issues caused by the use of multiple rooftop AC units, including using a third party power center that is often integrated into a main power board of the RV and that is capable of routing power from various sources, e.g., shore power, on-board batteries, the RV alternator, a generator, solar panels, etc., to different circuits. Some power centers, in particular, support load shedding, where if an overloaded condition is detected, power to one or more loads, e.g., an AC unit, is shut off to avoid a tripped circuit breaker. While a tripped circuit breaker is avoided, however, the result of load shedding is that devices are often shut completely off, at least temporarily, and in the case of an AC unit, the AC unit temporarily stops cooling or heating the living space. 
     Therefore, a need continues to exist in the art for a manner of supporting multiple AC units in a recreational vehicle to support additional capacity and/or multiple zones in the living space, and to do so in a manner that accommodates power limitations of the RV and/or a power source powering the same. 
     SUMMARY 
     The herein-described embodiments address these and other problems associated with the art by utilizing in one aspect a recreational vehicle air conditioning system that supports multiple recreational vehicle air conditioning units having closed air conditioning circuits and a controller that is electronically coupled to each of the recreational vehicle air conditioning units to control each of the closed air conditioning circuits to regulate an overall power consumption of the multiple recreational vehicle air conditioning units. In another aspect, a recreational vehicle air conditioning system may support multiple recreational vehicle air conditioning units where a refrigerant line set is coupled between the recreational vehicle air conditioning units such that a compressor in one of the recreational vehicle air conditioning units is capable of supplying refrigerant to the evaporators of the multiple recreational vehicle air conditioning units, and such that valves coupled in series with each of the evaporators may be regulated to control cooling by each recreational vehicle air conditioning unit. 
     Therefore, consistent with one aspect of the invention, an air conditioning system for a recreational vehicle may include a first recreational vehicle air conditioning unit, the first recreational vehicle air conditioning unit including a first closed air conditioning circuit including a first compressor for cooling a first zone in a living space of the recreational vehicle, a second recreational vehicle air conditioning unit, the second recreational vehicle air conditioning unit including a second closed air conditioning circuit including a second compressor for cooling a second zone in the living space of the recreational vehicle, and a controller in electrical communication with each of the first and second closed air conditioning circuits and configured to control operation of each of the first and second air conditioning circuits to regulate an overall power consumption for the first and second closed air conditioning circuits. 
     In some embodiments, the controller is external to both of the first and second recreational vehicle air conditioning units. Also, in some embodiments, the controller is disposed in the first recreational vehicle air conditioning unit. Further, in some embodiments, the controller is a first controller and the second recreational vehicle air conditioning unit includes a second controller disposed in the second recreational vehicle air conditioning unit and in communication with the first controller. 
     Some embodiments may further include a communication channel established between the first and second controllers, and the first controller is configured to control operation of the second air conditioning circuit by instructing the second controller over the communication channel. In some embodiments, the communication channel includes a wired low power communication link extending between the first and second recreational vehicle air conditioning units. In addition, in some embodiments, the first controller is configured to operate in a shared mode in response to detection of the second recreational vehicle air conditioning unit over the communication channel, and to otherwise operate in a stand alone mode. 
     Some embodiments may also include first and second sensors disposed in each of the first and second zones, and the controller is configured to receive measurements from the first and second sensors and to control operation of each of the first and second air conditioning circuits based at least in part on the measurements from the first and second sensors. In some embodiments, each of the first and second sensors is a temperature sensor, an occupancy sensor, a current sensor, or a humidity sensor. 
     In addition, in some embodiments, the first air conditioning circuit further includes an inverter configured to regulate a speed of the first compressor, and the controller is configured to control operation of each of the first and second air conditioning circuits at least in part by controlling the inverter to regulate the speed of the first compressor. Moreover, in some embodiments, the first air conditioning circuit further includes a first fan for blowing cooled air into the first zone and the second air conditioning circuit further includes a second fan for blowing cooled air into the second zone, and the controller is further configured to control operation of each of the first and second air conditioning circuits at least in part by controlling the first and second fans. 
     Consistent with another aspect of the invention, a recreational vehicle air conditioning unit may include a closed air conditioning circuit including a compressor for cooling a living space of a recreational vehicle, a communication port configured to communicate with another air conditioning unit that is external thereto, and a controller configured to selectively operate in one of a stand alone mode and a shared mode. When in the stand alone mode, the controller is configured to control operation of the air conditioning circuit to cool the living space of the recreational vehicle, and when in the shared mode, the controller is configured to additionally instruct the other air conditioning unit over the communication port to control operation of the other air conditioning unit to cool a different zone of the recreational vehicle. 
     In some embodiments, the closed air conditioning circuit is a first closed air conditioning circuit and the compressor is a first compressor, the other air conditioning unit includes a second closed air conditioning circuit including a second compressor, and when in the shared mode, and the controller is configured to control operation of each of the first and second air conditioning circuits to regulate an overall power consumption for the first and second closed air conditioning circuits. Moreover, in some embodiments, the controller is configured to select the shared mode in response to detecting the other air conditioning unit over the communication port. In some embodiments, the shared mode is a primary shared mode, and the controller is further configured to selectively operate in a secondary shared mode, and when in the secondary shared mode, control operation of the closed air conditioning circuit in response to an instruction received over the communication port. 
     Consistent with another aspect of the invention, an air conditioning system for a recreational vehicle may include a first recreational vehicle air conditioning unit for cooling a first zone in a living space of the recreational vehicle, the first recreational vehicle air conditioning unit including a first air conditioning circuit including a compressor, a first evaporator, and a first valve configured to regulate refrigerant flow through the first evaporator, a first refrigerant port coupled in parallel with the first evaporator, the first refrigerant port including a first outlet coupled upstream of the first evaporator and a first inlet coupled downstream of the first evaporator, and a second valve configured to regulate refrigerant flow through the refrigerant port; a second recreational vehicle air conditioning unit for cooling a second zone in the living space of the recreational vehicle, the second recreational vehicle air conditioning unit including a second air conditioning circuit including a second evaporator, and a second refrigerant port including a second inlet coupled upstream of the second evaporator and a second outlet coupled downstream of the second evaporator; a refrigerant line set coupling the first outlet to the second inlet and coupling the second outlet to the first inlet to place the second evaporator in fluid communication with the compressor; and a controller coupled to the compressor and the first and second valves and configured to control the first and second valves while running the compressor to regulate refrigerant flow through each of the first and second evaporators and thereby control cooling of the first and second zones in the living space of the recreational vehicle. 
     In some embodiments, the second recreational vehicle air conditioning unit is configured to be mounted on a side wall of the recreational vehicle. In addition, in some embodiments, the second recreational vehicle air conditioning unit is configured to be mounted in a cabinet of the recreational vehicle. Moreover, in some embodiments, the first recreational vehicle air conditioning unit is configured to mounted on a rear of the recreational vehicle. Also, in some embodiments, the first air conditioning circuit further includes a condenser coupled downstream of the compressor and upstream of the first evaporator, the first valve is coupled between the condenser and the first evaporator, the second valve is coupled between the condenser and the first outlet, the first recreational vehicle air conditioning unit further includes a third valve coupled between the first evaporator and the compressor and a fourth valve coupled between the first inlet and the compressor, and the controller is further coupled to the third and fourth valves to regulate refrigerant flow through each of the first and second evaporators. 
     These and other advantages and features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the Drawings, and to the accompanying descriptive matter, in which there is described example embodiments of the invention. This summary is merely provided to introduce a selection of concepts that are further described below in the detailed description, and is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a recreational vehicle consistent with some embodiments of the invention. 
         FIG.  2    is a side elevational view of another recreational vehicle consistent with some embodiments of the invention. 
         FIG.  3    is a block diagram of a recreation vehicle air conditioning unit with a closed air conditioning circuit capable of being utilized in the recreational vehicles of  FIGS.  1  and  2   . 
         FIG.  4    is a block diagram of an example recreational vehicle air conditioning system capable of being utilized in the recreational vehicles of  FIGS.  1  and  2   . 
         FIG.  5    is a flowchart illustrating an example sequence of operations for starting up one of the recreational vehicle an air conditioning units of  FIG.  4   . 
         FIG.  6    is a flowchart illustrating an example sequence of operations for operating the recreational vehicle an air conditioning units of  FIG.  4    in a shared mode of operation. 
         FIG.  7    is a block diagram of another example recreational vehicle air conditioning system capable of being utilized in the recreational vehicles of  FIGS.  1  and  2   . 
         FIG.  8    is a block diagram of a shared air conditioning circuit capable of being utilized in the recreational vehicles of  FIGS.  1  and  2   . 
         FIG.  9    is a flowchart illustrating an example sequence of operations for operating the recreational vehicle an air conditioning units of  FIG.  7    in a shared mode of operation. 
         FIG.  10    is a side elevational view of yet another recreational vehicle consistent with some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In embodiments consistent with the invention, a recreational vehicle air conditioning system is used to operate multiple recreational vehicle air conditioning units in multiple zones of a living space of a recreational vehicle. In this regard, a recreational vehicle may be considered to be a wheeled vehicle capable of being moved from place to place (either under its own power or under the power of a towing vehicle) and containing a living space that is capable of being climate controlled using one or more air conditioning units. The living space generally includes, at a minimum, a sleeping space, kitchen facilities, eating space, and in some instances, bathroom facilities, and is intended to be used as a dwelling by users at least when the vehicle is parked or stationary.  FIG.  1   , for example, illustrates one type of recreational vehicle, a motorhome or motor coach  10 , which includes a main body  12 , wheels  14 , a powertrain (e.g., an engine)  16 , and an interior living space  18 .  FIG.  2    illustrates another type of recreational vehicle, a travel trailer, or in this case, a fifth wheel trailer  20 , which, similar to motorhome  10 , includes a main body  22 , wheels  24  and interior living space  26 , but instead of having its own powertrain that allows for independent movement, includes a hitch  28  for coupling to a separate vehicle such as a pickup truck capable of moving the fifth wheel trailer  20  from place to place. It will be appreciated that a recreational vehicle may take other forms in other embodiments, so a recreational vehicle is not limited to the specific motorhome and fifth wheel trailer implementations illustrated herein. 
     A recreational vehicle air conditioning unit in turn may be considered to be a self-contained device incorporated into one or more housings and providing an air conditioning function, and in some instances, one or more additional climate control-related functions such as heat (e.g., via a heat pump or heating element), dehumidification, ventilation, etc.  FIG.  1   , for example, illustrates a pair of rooftop recreational vehicle air conditioning units  30 ,  32  disposed on a roof of motorhome  10 , while  FIG.  2    illustrates a pair of rooftop recreational vehicle air conditioning units  34 ,  36  disposed on a roof of fifth wheel trailer  30 . As will become more apparent below, each recreational vehicle conditioning unit  30 - 36  may be configured to provide cooling and/or other climate control functions within a particular region or zone of a living space. The regions or zones in a living space may, but are not necessarily, based on a partitioning of the living space into separate rooms separated by doors, e.g., whereby one zone is defined in a bedroom and another zone is defined in a kitchen/eating area. In other embodiments, the zones may instead refer to different areas of a single common room in the living space. 
     Turning to  FIG.  3   , the primary components associated with providing an air conditioning function in recreational vehicle air conditioning unit  30  are illustrated in greater detail, with an understanding being that in some embodiments, recreational vehicle air conditioning units  32 - 36  may be similarly configured. Unit  30  in particular includes a housing  50  within which is disposed an air conditioning circuit  52 , which operates using a vapor-compression cycle relying on induced phase transitions of a refrigerant between gas and liquid states to transfer heat, in this case from a living space  54  to an outside environment  56 . 
     In air conditioning circuit  52 , refrigerant in a state as a low pressure and low temperature vapor is received by compressor  58 , which pressurizes the refrigerant into a higher temperature and higher pressure vapor. This high temperature, high pressure vapor then passes through a condenser  60 , which functions as a heat exchanger to release heat to its surrounding environment, in this case outside  56 . The refrigerant then cools and condenses to a higher pressure liquid, and then passes through an expander  62 , e.g., an expansion valve or device, which abruptly causes the temperature to drop, and then through an evaporator  64 , which functions as a heat exchanger that vaporizes the refrigerant and absorbs heat from its surrounding environment, in this case living space  54 . The refrigerant then returns to compressor  58  as the low pressure and low temperature vapor. Condenser  60  is generally positioned in unit  30  for exposure to outside  56 , while evaporator  64  is generally positioned in unit  30  for exposure to living space  54 . A condenser fan  66  and/or an evaporator fan  68  may also be used to increase the thermal exchange between condenser  60  and outside  56  (represented by arrows  70 ,  72 ) and between evaporator  64  and living space  54  (represented by arrows  74 ,  76 ). 
     Air conditioning circuit  52  in  FIG.  3    is referred to herein as a closed air conditioning circuit, as the air conditioning circuit is a closed loop system in which refrigerant is circulated between a compressor, condenser, expander and evaporator within a single housing or unit. As will become more apparent below, in some embodiments an air conditioning circuit may be shared between one or more housings or units, e.g., using refrigerant line sets that transfer refrigerant between different components in different housings or units. 
     In addition, it will be appreciated that compressor  58  may be a single speed or multi-speed compressor in various embodiments. Furthermore, in some embodiments, an inverter  78  may be used to drive compressor at a variable speed. As it will be appreciated that initial startup of a compressor at full speed is generally the time at which the power draw by an air conditioning unit is at its greatest, varying the speed of the compressor using an inverter can reduce both the maximum power draw at startup and the power draw during normal operation. Any or all of fans  66 ,  68  may also be variable fans in some embodiments to provide varying flow rates. 
     Now turning to  FIG.  4   , in some embodiments of the invention, a recreational vehicle air conditioning system may include multiple recreational vehicle air conditioning (AC) units, each having a closed air conditioning circuit including a compressor for cooling an associated zone in a living space of the recreational vehicle, along with a controller in electrical communication with each of the air conditioning circuits and configured to control operation of each of the air conditioning circuits to regulate an overall power consumption for the air conditioning circuits. While not required, in some embodiments each air conditioning circuit may be inverter driven such that the speed of each compressor, and thus the power consumption of the air conditioning circuit, may be controlled in a variable manner. 
     In one example and non-limiting embodiment, control may be implemented primarily in a controller of one of the recreational vehicle air conditioning units (designated as a primary AC unit), which may maintain algorithms for load sharing purposes, and which may be connected to one or more additional recreational vehicle air conditioning units (designated as a secondary AC unit, and which in some, but not all, instances may be smaller or with lower capacity) through a wired or wireless communication channel, e.g., a dedicated DC bus or pigtail. In this embodiment, the primary AC unit may be configured to operate at 100% capacity when only one AC unit is utilized (i.e., when the primary AC unit is not coupled to a secondary AC unit), but if a secondary AC unit is installed, the primary AC unit can still function, but may also oversee control of the secondary AC unit, and manage the overall power consumption by both AC units to avoid overcurrent situations. As each AC unit may be configured as a separate AC unit with a closed air conditioning circuit having a separate compressor and inverter, the AC units may still be operated as independent, off the shelf AC units, but when mated together, the AC units may communicate and load share. In some embodiments, such a configuration may eliminate the need for, or in the least eliminate the involvement of, a third party power center, as the two AC units may cooperatively control themselves to moderate power, and avoid overcurrent situations. One advantage of such a configuration, particularly when used in connection with inverter-based control over the compressors, would be greater simplicity and added control over an entire, multi-zone living space without the relatively noisy start/stop/start/stop cycling that is typical of many RV AC units. 
       FIG.  4   , for example, illustrates a recreational vehicle air conditioning system  100  including a plurality (N) of AC units  102 ,  104 ,  106  associated with separate zones  108 ,  110 ,  112  in a living space of a recreational vehicle, and having an associated controller  114 ,  116 ,  118 . In addition, one or all of AC units  102 ,  104 ,  106  may include an associated inverter  120 ,  122 ,  124  enabling variable control of a compressor of the closed air conditioning circuit of the respective AC unit  102 ,  104 ,  106 . Otherwise, each AC unit  102 ,  104 ,  106  may use a single speed or multi-speed compressor in other embodiments. It will be appreciated that, when a compressor is single speed, during high demand periods, management of power consumption may involve temporarily shutting off the compressor (potentially while maintaining the fan for airflow), while when a compressor is multi-speed or variable speed, management of power consumption may involve operating the compressor at a lower speed to reduce power consumption. 
     In some embodiments, each AC unit  102 ,  104 ,  106  may include, in addition to air conditioning or cooling functionality, heat, dehumidification and/or ventilation functionality. While in some embodiments all AC units may have identical capacities and functionalities, in other embodiments the cooling capacities or other functionalities of the AC units may differ from one another. 
     In addition, within each zone and associated with each AC unit  102 ,  104 ,  106  may be one or more sensors  126 ,  128 ,  130 . Various types of sensors may be used in various embodiments, including temperature sensors and/or thermostats, occupancy sensors, and/or humidity sensors that provide measurements associated with the living space and in particular, the associated zone of the living space. In addition, in some embodiments, current or other sensors capable of measuring the power consumption of an associated AC unit may be used. 
     Each AC unit  102 ,  104 ,  106  may be powered by a power center  132 , or alternatively a power board or electrical panel, which, in addition to being potentially coupled to various on-board power sources (e.g., batteries, generators, alternators, solar panels, etc.) may be coupled to shore power  134 , e.g., a 30 or 50 Amp service provided by a campground or campsite. AC power lines  136  are used to couple each AC unit  102 ,  104 ,  106  to power center  132 . 
     In addition, in the illustrated embodiment, a communication channel  138  is established between controllers  114 ,  116 ,  118  of AC units  102 ,  104 ,  106  and sensors  126 ,  128 , and  130 , and in some instances, with an optional controller  140  of power center  132  (which in some instances may also be separate from power center  132 ). The communication channel  138  may be wired or wireless, e.g., using one or more wired low power DC communication links. The communication channel  138  may incorporate an architecture enabling all controllers to communicate with one another and with all of sensors  126 , while in other embodiments, the communication channel  138  may include other architectures, e.g., where only the controllers are in communication with one another and the sensors in each zone are connected only to the controller of the AC unit for the associated zone. In addition, in some embodiments a controller may be integrated with one or more sensors. Each AC unit, controller and/or sensor, for example, may include a communication port (e.g., communication port  142  illustrated as coupled to controller  114 ) that may be coupled to the communication channel  138 . 
     As noted above, in some embodiments it may be desirable to enable an AC unit to operate in different modes based at least in part on whether that AC unit is coupled to another AC unit, e.g., a stand alone mode where the AC unit operates as a self-contained AC unit to cool a living space of a recreational vehicle, and a shared mode where multiple AC units are controlled together to manage their overall power consumption, and where one of the AC units instructs one or more other AC units to cool different zones of a living space.  FIG.  5    illustrates a sequence of operations  150  that may be performed by a controller of an AC unit  102 ,  104 ,  106  during startup to automatically detect a mode of operation. Specifically, upon detection of initial power on (block  152 ), block  154  may determine whether another, connected AC unit has been detected. Such detection may be based on, for example, a signal received over communication channel  138 , or based on the configuration of the AC unit (e.g., based on a DIP switch setting set during installation, or based upon settings otherwise established during installation such as made through a user interface of the AC unit, an app of a mobile device, etc.). 
     If no other AC unit has been detected, control passes to block  156  to operate the AC unit in a stand alone mode. If another AC unit has been detected, however, control passes to block  158  to arbitrate for a master controller assignment, i.e., to determine which controller in the air conditioning system should operate as the master or primary controller. The arbitration, for example, may be based in some embodiments on the configuration of each AC unit (e.g., based on DIP switch settings set during installation), or based upon settings otherwise established during installation (e.g., made through a user interface of the AC unit, an app of a mobile device, etc.). The arbitration may also be based on the characteristics of the different AC units, e.g., to default to a larger, high capacity AC unit when coupled to a smaller, lower capacity AC unit. Then, in block  160 , the AC unit operates in a shared mode, either as a primary AC unit (in a primary shared mode) or a secondary AC unit (in a secondary shared mode) as determined in block  158 . 
     Now turning to  FIG.  6   , an example sequence of operations  170  for operating an air conditioning system such as air conditioning system  100  of  FIG.  4    is illustrated in greater detail. Sequence  170  may be executed, for example, by the controller  114 ,  116 ,  118  of one of AC units  102 ,  104 ,  106  designated as the primary AC unit, or in some embodiments, by a separate controller such as controller  140  that is in communication with the AC units  102 ,  104 ,  106 . 
     In sequence  170 , the AC units and sensors are monitored, e.g., to determine the current state of each AC unit as well as the current measurements collected by the sensors. In addition, a user interface of the controller and/or of the AC units may be monitored to determine if a user has changed the operation of one of the AC units. Furthermore, where an AC unit supports schedules (e.g., to change the temperature setpoint of an AC unit at different times of the day), the stored schedules for the AC units may also be monitored. 
     If no change in operational state is required based upon the monitored conditions, block  174  returns control to block  172  to continue monitoring. However, if any of the monitored conditions indicates that a change in operational state is required, block  174  passes control to block  176  to determine an operational state for each AC unit in each zone based upon one or more of power consumption, temperature, humidity, a timer, a schedule, occupancy, or priority. Then, based upon the determined operational states, block  178  updates each AC unit with new settings suitable for establishing the operational states, and control returns to block  172  to continue monitoring. 
     As one non-limiting example, assume a first, larger capacity AC unit in the kitchen/eating zone of a living space coupled in a shared mode with a smaller, smaller capacity AC unit in the bedroom of the living space, and it is early in the morning while the occupants are sleeping in the bedroom. Assume also that based on occupancy settings, thermostat settings, time of day, etc., the secondary AC unit is operating with a setpoint of 70 degrees, while the primary AC unit is operating at a setpoint of 78 degrees, and that the actual temperature in those zones are currently 72 degrees and 75 degrees, respectively, such that only the secondary AC unit is currently active. 
     Then, assume that, either due to detection of occupancy in the kitchen/eating zone or a particular time setting being triggered, the temperature setpoint for the primary AC unit is changed to 70 degrees, requiring that the primary AC unit be activated, as might be detected by block  174 . Based upon this requirement, suitable operational settings may be determined for both AC units, e.g., to turn on the primary AC unit and either turn off or lower the speed of the compressor on the secondary AC unit. During such an operation, the total current consumption of both AC units may be predicted or determined, e.g., based upon current sensors or based upon stored values associated with different operational states of each AC unit, with the operation of each AC unit adjusted to maintain the overall power consumption of both AC units within a desired power envelope, e.g., with a total current draw that is within the capacity of the power source. Thus, for example, if inverters and variable speed compressors are used, it may be determined in some instances that turning on the primary AC unit to 100% would increase the overall power consumption above a predetermined limit, and cause either the primary AC unit to be turned on at a lower speed, or the speed of the secondary AC unit to be decreased (or both) to keep the overall power consumption below the limit. 
     It will be appreciated that an innumerable number of different types of load sharing and climate control algorithms may be used in different embodiments to collectively manage the operation of both AC units while maintaining their power consumption within the capacity of the power source, so the invention is not limited to the specific examples given herein. 
     Updating of each AC unit in block  178  may include, for example, a primary controller executing sequence  170  instructing the controller of a different AC unit over communication channel  138 , and if the primary controller is disposed within an AC unit, having the primary controller control its own settings as it would do when operating in a stand alone mode. 
       FIG.  7    next illustrates another example embodiment of the invention, where rather than having each AC unit be a separate AC unit having a complete closed air conditioning circuit, one or more line sets are routed between the AC units to enable the compressor and condenser of one primary AC unit to drive an evaporator in one or more secondary AC units that are external to the one primary AC unit. 
     In particular, in some embodiments, a primary AC unit may be configured in some embodiments similar to the AC units described above, with the addition of a refrigerant port and one or more diverter valves that enable the primary AC unit to be coupled to a secondary AC unit through a line set such that the secondary AC unit receives its refrigerant from the primary AC unit, eliminating the need for the secondary AC unit to utilize a separate compressor, condenser, and in some instances, inverter, such that the secondary AC unit predominantly incorporates an expander, evaporator and evaporator fan. It will be appreciated that such a design may have advantages in terms of cost, power consumption, and noise, and as will be discussed in greater detail below, may in some instances allow for different placement of the primary and/or secondary AC units within a recreational vehicle. 
     In some embodiments consistent with the invention, for example, a first recreational vehicle air conditioning unit may include a first air conditioning circuit including a compressor, a first evaporator, and a first valve configured to regulate refrigerant flow through the first evaporator, a first refrigerant port coupled in parallel with the first evaporator, the first refrigerant port including a first outlet coupled upstream of the first evaporator and a first inlet coupled downstream of the first evaporator, and a second valve configured to regulate refrigerant flow through the refrigerant port. A second recreational vehicle air conditioning unit in turn may include a second air conditioning circuit including a second evaporator and a second refrigerant port including a second inlet coupled upstream of the second evaporator and a second outlet coupled downstream of the second evaporator. A refrigerant line set may couple the first outlet to the second inlet and couple the second outlet to the first inlet to place the second evaporator in fluid communication with the compressor, and a controller coupled to the compressor and the first and second valves may be configured to control the first and second valves while running the compressor to regulate refrigerant flow through each of the first and second evaporators and thereby control cooling of the first and second zones in the living space of the recreational vehicle. 
       FIG.  7   , in particular, illustrates a recreational vehicle air conditioning system  200  including a plurality (N) of AC units  202 ,  204 ,  206  associated with separate zones  208 ,  210 ,  212  in a living space of a recreational vehicle, with AC unit  202  serving as a primary AC unit additionally including a controller  214 , and in some instances, an associated inverter  216  enabling variable control of a compressor of the air conditioning circuit of the primary AC unit  202  (not shown in  FIG.  7   ). Each AC unit  204 ,  206  functions as a secondary AC unit, and rather than having a complete air conditioning circuit, includes an associated evaporator and fan (blocks  218 ,  220 ), with refrigerant line sets  222 ,  224  respectively coupling the primary AC unit  202  to each of AC units  204 ,  206 . 
     Within each zone and associated with each AC unit  202 ,  204 ,  206  may be one or more sensors  226 ,  228 ,  230 . Various types of sensors may be used in various embodiments, including temperature sensors and/or thermostats, occupancy sensors, and/or humidity sensors that provide measurements associated with the living space and in particular, the associated zone of the living space. In addition, in some embodiments, current or other sensors capable of measuring the power consumption of an associated AC unit may be used. 
     Primary AC unit  202  may be powered by a power center  232 , or alternatively a power board or electrical panel, which, in addition to being potentially coupled to various on-board power sources (e.g., batteries, generators, alternators, solar panels, etc.) may be coupled to shore power  234 , e.g., a 30 or 50 Amp service provided by a campground or campsite. AC power lines  236  are used to couple AC unit  202  to power center  232 . 
     In addition, in the illustrated embodiment, a communication channel  238  is established between controller  214  each of sensors  226 ,  228 , and  230 . In addition, as represented by block  240 , in some embodiments a secondary AC unit such as AC unit  206  may include an integrated controller and/or one or more sensors, which may also be coupled to communication channel  238 . As represented by AC unit  204 , however, some secondary AC units may lack controllers and/or associated sensors in some embodiments. The communication channel  238  may be wired or wireless, e.g., using one or more wired low power DC communication links, and each endpoint coupled to the communication channel  238  may include an associated communication port (not shown in  FIG.  7   ). Moreover, while in some embodiments each secondary AC unit  204 ,  206  may be coupled to power center  232  to receive power to drive various components in the AC unit, in other embodiments, given that each secondary AC unit  204 ,  206  omits a compressor and generally includes a single fan, power to each secondary AC unit may be supplied by primary AC unit  202 , e.g., using one or more power and/or communication lines incorporated into a line set  222 ,  224 . 
     Turning to  FIG.  8   , the primary components associated with providing an air conditioning function in a recreational vehicle air conditioning system  250  incorporating primary and secondary AC units  252 ,  254  coupled together by a line set  256  are illustrated in greater detail. Primary AC unit  252  in particular includes a first air conditioning circuit  258 , which operates using a vapor-compression cycle relying on induced phase transitions of a refrigerant between gas and liquid states to transfer heat, in this case from a first zone  260  of a living space to an outside environment  262 , while secondary AC unit  254  includes a second air conditioning circuit  264  that, as will become more apparent below, is an extension of first air conditioning circuit  258 , and that transfers heat from a second zone  266  of living space. 
     In air conditioning circuit  258 , refrigerant in a state as a low pressure and low temperature vapor is received by compressor  268 , which pressurizes the refrigerant into a higher temperature and higher pressure vapor. This high temperature, high pressure vapor then passes through a condenser  270 , which functions as a heat exchanger to release heat to its surrounding environment, in this case outside  262 . The refrigerant then cools and condenses to a higher pressure liquid, and then passes through a primary supply solenoid valve  272  and an expander  264 , e.g., an expansion valve or device, which abruptly causes the temperature to drop, and then through an evaporator  276 , which functions as a heat exchanger that vaporizes the refrigerant and absorbs heat from its surrounding environment, in this case living space zone  260 . The refrigerant then returns to compressor  268  through a primary return solenoid valve  278  as the low pressure and low temperature vapor. Condenser  270  is generally positioned in unit  252  for exposure to outside  262 , while evaporator  276  is generally positioned in unit  252  for exposure to living space zone  260 . A condenser fan  280  and/or an evaporator fan  282  may also be used to increase the thermal exchange between condenser  270  and outside  262  (represented by arrows  284 ,  286 ) and between evaporator  276  and living space zone  260  (represented by arrows  288 ,  290 ). 
     In addition, AC unit  252  includes a pair of secondary supply and return solenoid valves  292 ,  294  that are coupled in parallel to primary supply and return solenoid valves  272 ,  278 , respectively, and that respectively couple condenser  270  to a refrigerant outlet  296  and an upstream (lower pressure) side of compressor  268  refrigerant inlet  298  that together represent a refrigerant port for AC unit  252 , such that the refrigerant port is effectively in parallel with evaporator  276 , with refrigerant outlet  296  upstream of evaporator  276  and refrigerant inlet  298  downstream of evaporator  276 . 
     Line set  256  includes first and second refrigerant lines  300 ,  302  that respectively couple refrigerant outlet  296  to a refrigerant inlet  304  of AC unit  254  and couple a refrigerant outlet  306  of AC unit  254  to refrigerant inlet  298 , such that refrigerant inlet  304  and refrigerant outlet  306  may be considered to represent a refrigerant port for AC unit  254 . 
     Air conditioning circuit  264  of secondary AC unit  254  includes an expander  308 , e.g., an expansion valve or device, which is coupled to refrigerant inlet  304  and feeds a second evaporator  310 , which in turn is coupled to refrigerant outlet  306 , such that when line set  256  couples AC units  252  and  254  to one another, evaporator  310  is effectively in parallel with evaporator  276  (in other embodiments, expander  308  may be disposed in AC unit  252 ). As such, by controlling valves  272 ,  278 ,  292  and  294  during operation of compressor  268 , primary AC unit  252  is able to control cooling in each of living space zones  260 ,  266  through regulation of the refrigerant flow through each evaporator  276 ,  310 . Secondary AC unit  254  may also include an associated evaporator fan  312 , which may be variable speed in some embodiments, and controlled either by AC unit  252  or AC unit  254 , thereby increasing thermal exchange between evaporator  310  and living space zone  266  (represented by arrows  314 ,  316 ). 
     Valves  272 ,  278 ,  292  and  294  may be variable valves in some embodiments or may be on/off valves in other, and in some embodiments one or more of the valves may be omitted (e.g., where it is desirable to always operate evaporator  276  or evaporator  310  when compressor  268  is active, or where refrigerant flow through one of evaporators  276 ,  310  may be adequately controlled without having separate valves both upstream and downstream of an evaporator. 
     Now turning to  FIG.  9   , an example sequence of operations  320  for operating an air conditioning system such as air conditioning system  200  of  FIG.  7    is illustrated in greater detail. Sequence  320  may be executed, for example, by the controller  214  of AC unit  202 , or in some embodiments, by a separate controller that is in communication with AC unit  202 . 
     In sequence  320 , the AC units and sensors are monitored, e.g., to determine the current state of each AC unit as well as the current measurements collected by the sensors. In addition, a user interface of the controller and/or of the AC units may be monitored to determine if a user has changed the operation of one of the AC units. Furthermore, where an AC unit supports schedules (e.g., to change the temperature setpoint of an AC unit at different times of the day), the stored schedules for the AC units may also be monitored. 
     If no change in operational state is required based upon the monitored conditions, block  324  returns control to block  322  to continue monitoring. However, if any of the monitored conditions indicates that a change in operational state is required, block  324  passes control to block  326  to determine an operational state for each AC unit in each zone based upon one or more of power consumption, temperature, humidity, a timer, a schedule, occupancy, or priority, in a similar manner to that described above in connection with sequence  170 . Then, based upon the determined operational states, block  328  controls the solenoid valves, compressor and fans of each AC unit based upon the determined operational state for each zone, and control returns to block  322  to continue monitoring. 
     As noted above, due to the lack of compressor in a secondary AC unit in the embodiments of  FIGS.  7 - 9   , different form factors and installation locations for secondary AC units may be used in other embodiments. For example, as illustrated by recreational vehicle  340  of  FIG.  10   , while a primary AC unit  342  may be configured as a rooftop AC unit, a secondary AC unit may be configured as a rooftop AC unit  344  in some embodiments, while in other embodiments, a secondary AC unit may be mounted on a side wall of the recreational vehicle, e.g., as illustrated by secondary AC unit  346 , or may be mounted in a cabinet, e.g., as illustrated by secondary AC unit  348  mounted in cabinet  350 . In addition, a drain connection may also be supported within a cabinet for a secondary AC unit to capture condensation, potentially avoiding the need for additional holes in the exterior body of the recreational vehicle. Due to the lack of a large and/or noisy compressor in each secondary AC unit, each secondary AC unit is thus capable of being constructed in a more compact housing and in a form factor that is better suited for unobtrusive integration into the living space. 
     Furthermore, in some embodiments, it may be desirable to mount a primary AC unit to an outer wall, e.g., a rear wall, of a recreational vehicle, e.g., as illustrated by primary AC unit  352 , or in other locations, such as underneath the vehicle or within a compartment within the vehicle but separate from the living space. In addition, in some embodiments a primary AC unit may lack its own evaporator, such that all evaporators are disposed in secondary AC units and provided with refrigerant from a single compressor in a primary AC unit. 
     The aforementioned embodiments provide a number advantages. For example, in some instances, a recreational vehicle may be able to omit support for 50 Amp service in some instances, lowering manufacturing costs. Alternatively, a larger recreational vehicle may, instead of having two high powered AC units, rely on three, four or more smaller AC units configured in the manner disclosed herein, providing more granular climate control within the living space. 
     Further, it will be appreciated that various modifications may be made to the aforementioned embodiments. For example, particularly when inverters are used, it may be desirable to implement a whole RV dehumidification cycle that could be configured to run between normal cycles or at night. Further, the aforementioned air conditioning circuits may also be configured as heat pumps in some embodiments, thereby enabling heating to be performed, and without requiring the use of separate heating elements or gas heaters. Further, ventilation functionality may be supported, e.g., to support the use of a make up vent that, based on air temperature, humidity, or even smoke in the living space during cooking, either provided or restricted inlet air as needed using one or more of the AC units. 
     It will be appreciated that various additional modifications may be made to the embodiments discussed herein, and that a number of the concepts disclosed herein may be used in combination with one another or may be used separately. Other modifications will be apparent to those of ordinary skill in the art having the benefit of the instant disclosure. Therefore, the invention lies in the claims hereinafter appended.