Variable refrigerant flow system with capacity limits

A variable refrigerant flow system includes one or more outdoor units and a first indoor unit of a plurality of indoor units configured to receive refrigerant from the one or more outdoor units. The first indoor unit is configured to serve a first building zone. The variable refrigerant flow system also includes a user input device configured to receive a user command requesting heating or cooling of the first building zone by the first indoor unit. The variable refrigerant flow system also includes a controller configured to receive the command from the user input device, receive an indication of a current price of energy, in response to receiving the command generate a constraint on a capacity of the one or more outdoor units based on the current price of energy, and control the one or more outdoor units to operate in accordance with the constraint.

BACKGROUND

The present disclosure relates generally to the field of variable refrigerant flow (VRF) systems. A VRF system typically includes one or more outdoor VRF units that consume electrical power to heat and/or cool a refrigerant. VRF systems also typically include multiple indoor VRF units located in various spaces of a building, each of which receives the refrigerant from the outdoor VRF unit(s) and uses the refrigerant to transfer heat into or out of a particular space.

In many cases, the various spaces of served by a VRF system may be sporadically and/or irregularly occupied, such that each space is occupied at some points in time and unoccupied at other points in time. It may be desirable to provide heating and/or cooling when a space is occupied to provide for occupant comfort, while turning off heating and/or cooling when the space is unoccupied to reduce energy costs. For example, in some cases indoor VRF units may be controlled by users who turn on the VRF unit when the users enter a space and turn off the indoor VRF unit when the user leaves the space. Accordingly, sporadic building occupancy may create irregular and difficult-to-predict demand on the VRF system.

Some building systems attempt to minimize the utility costs associated with heating and cooling a building based on predictions of future system states. However, the irregular and difficult-to-predict demand on the VRF system caused by sporadic occupation of building zones may substantially reduce the effectiveness of existing approaches to utility cost optimization for building heating and cooling systems. For example, unpredictable occupation of building zones may create spikes in the load on the VRF system that prevent costs from being optimized under existing approaches. Accordingly, a need exists for systems and methods that allow a VRF system to provide comfort to the occupants in sporadically-occupied building zones while also reducing or minimizing utility costs of operating the VRF system.

SUMMARY

One implementation of the present disclosure is a variable refrigerant flow system. The variable refrigerant flow system includes one or more outdoor units and a first indoor unit of a plurality of indoor units configured to receive refrigerant from the one or more outdoor units. The first indoor unit is configured to serve a first building zone. The variable refrigerant flow system also includes a user input device configured to receive a user command requesting heating or cooling of the first building zone by the first indoor unit. The variable refrigerant flow system also includes a controller configured to receive the command from the user input device, receive an indication of a current price of energy, in response to receiving the command generate a constraint on a capacity of the one or more outdoor units based on the current price of energy, and control the one or more outdoor units to operate in accordance with the constraint.

In some embodiments, the controller is configured to remove the constraint after a capacity limit period elapses. In some embodiments, the controller is configured to generate the constraint by multiplying a maximum outdoor unit capacity by a function of the current price of energy to determine a modified constrained capacity. The controller is configured to control the one or more outdoor units by preventing an operating capacity of the one or more outdoor units from exceeding the modified constrained capacity.

In some embodiments, the function is equal to one when the current price of energy is less than a threshold price and equal to a value between zero and one when the current price of energy is greater than the threshold price. In some embodiments, the value is between approximately 0.4 and 0.8.

In some embodiments, the controller is configured to control the one or more outdoor units to operate in accordance with the constraint by optimizing a cost function bound by the constraint. In some embodiments, the controller is configured to remove the constraint after a capacity limit period elapses and optimize the cost function over an optimization period longer than the capacity limit period and comprising the capacity limit period.

Another implementation of the present disclosure is a method of heating or cooling a building. The method includes operating one or more outdoor units to provide refrigerant to a plurality of indoor units. Each indoor unit is associated with a zone of a building. The method also includes receiving an input from a user requesting heating or cooling of a first building zone by a first indoor unit of the plurality of indoor units, receiving an indication of a current price of energy, in response to receiving the input generating a constraint relating to a capacity of the one or more outdoor units based on the current price of energy, and controlling the one or more outdoor units to operate in accordance with the constraint.

In some embodiments, the method includes removing the constraint after a capacity limit period elapses. In some embodiments, generating the constraint includes multiplying a maximum outdoor unit capacity by a function of the current price of energy to determine a modified constrained capacity. Controlling the one or more outdoor units includes preventing an operating capacity of the one or more outdoor units from exceeding the modified constrained capacity.

In some embodiments, the function is equal to one when the current price of energy is less than a threshold price and equal to a value between zero and one when the current price of energy is greater than the threshold price. In some embodiments, the value is between approximately 0.4 and 0.8.

In some embodiments, controlling the one or more outdoor units includes optimizing a cost function bound by the constraint. In some embodiments, the method also includes removing the constraint after a capacity limit period elapses and optimizing the cost function over an optimization period longer than the capacity limit period and comprising the capacity limit period.

Another implementation of the present disclosure is a variable refrigerant flow system. The variable refrigerant flow system includes one or more outdoor units and a first indoor unit of a plurality of indoor units configured to receive refrigerant from the one or more outdoor units. The first indoor unit is configured to serve a first building zone. The variable refrigerant flow system also includes an occupancy detector configured to detect a presence of an occupant in a building zone. The variable refrigerant flow system also includes a control circuit configured to receive an indication from the occupancy detector indicating that the occupant is present in the building zone, receive a current price of energy, in response to receiving the indication generate a constraint relating to a capacity of the one or more outdoor units based on the current price of energy, and control the first indoor unit and the one or more outdoor units to operate in accordance with the constraint and provide heating or cooling to the building zone.

In some embodiments, the controller is configured to remove the constraint after a capacity limit period elapses. In some embodiments, the controller is configured to generate the constraint by multiplying a maximum outdoor unit capacity by a function of the current price of energy to determine a modified constrained capacity. The controller is configured to control the one or more outdoor units by preventing an operating capacity of the one or more outdoor units from exceeding the modified constrained capacity. In some embodiments, the function is equal to one when the current price of energy is less than a threshold price and equal to a value between zero and one when the current price of energy is greater than the threshold price.

In some embodiments, the control circuit is configured to control the one or more outdoor units to operate in accordance with the constraint by optimizing a cost function bound by the constraint. In some embodiments, the control circuit is configured to remove the constraint after a capacity limit period elapses and optimize the cost function over an optimization period longer than the capacity limit period and comprising the capacity limit period.

DETAILED DESCRIPTION

Variable Refrigerant Flow Systems

Referring now toFIGS. 1A-B, a variable refrigerant flow (VRF) system100is shown, according to some embodiments. VRF system100is shown to include one or more outdoor VRF units102and a plurality of indoor VRF units104. Outdoor VRF units102can be located outside a building and can operate to heat or cool a refrigerant. Outdoor VRF units102can consume electricity to convert refrigerant between liquid, gas, and/or super-heated gas phases. Indoor VRF units104can be distributed throughout various building zones within a building and can receive the heated or cooled refrigerant from outdoor VRF units102. Each indoor VRF unit104can provide temperature control for the particular building zone in which the indoor VRF unit104is located. Although the term “indoor” is used to denote that the indoor VRF units104are typically located inside of buildings, in some cases one or more indoor VRF units are located “outdoors” (i.e., outside of a building) for example to heat/cool a patio, entryway, walkway, etc.

One advantage of VRF system100is that some indoor VRF units104can operate in a cooling mode while other indoor VRF units104operate in a heating mode. For example, each of outdoor VRF units102and indoor VRF units104can operate in a heating mode, a cooling mode, or an off mode. Each building zone can be controlled independently and can have different temperature setpoints. In some embodiments, each building has up to three outdoor VRF units102located outside the building (e.g., on a rooftop) and up to 128 indoor VRF units104distributed throughout the building (e.g., in various building zones). Building zones may include, among other possibilities, apartment units, offices, retail spaces, and common areas. In some cases, various building zones are owned, leased, or otherwise occupied by a variety of tenants, all served by the VRF system100.

Many different configurations exist for VRF system100. In some embodiments, VRF system100is a two-pipe system in which each outdoor VRF unit102connects to a single refrigerant return line and a single refrigerant outlet line. In a two-pipe system, all of outdoor VRF units102may operate in the same mode since only one of a heated or chilled refrigerant can be provided via the single refrigerant outlet line. In other embodiments, VRF system100is a three-pipe system in which each outdoor VRF unit102connects to a refrigerant return line, a hot refrigerant outlet line, and a cold refrigerant outlet line. In a three-pipe system, both heating and cooling can be provided simultaneously via the dual refrigerant outlet lines. An example of a three-pipe VRF system is described in detail with reference toFIG. 2.

Referring now toFIG. 2, a block diagram illustrating a VRF system200is shown, according to some embodiments. VRF system200is shown to include outdoor VRF unit202, several heat recovery units206, and several indoor VRF units204. AlthoughFIG. 2shows one outdoor VRF unit202, embodiments including multiple outdoor VRF units202are also within the scope of the present disclosure. Outdoor VRF unit202may include a compressor208, a fan210, or other power-consuming refrigeration components configured convert a refrigerant between liquid, gas, and/or super-heated gas phases. Indoor VRF units204can be distributed throughout various building zones within a building and can receive the heated or cooled refrigerant from outdoor VRF unit202. Each indoor VRF unit204can provide temperature control for the particular building zone in which the indoor VRF unit204is located. Heat recovery units206can control the flow of a refrigerant between outdoor VRF unit202and indoor VRF units204(e.g., by opening or closing valves) and can minimize the heating or cooling load to be served by outdoor VRF unit202.

Outdoor VRF unit202is shown to include a compressor208and a heat exchanger212. Compressor208circulates a refrigerant between heat exchanger212and indoor VRF units204. The compressor208operates at a variable frequency as controlled by outdoor unit controls circuit214. At higher frequencies, the compressor208provides the indoor VRF units204with greater heat transfer capacity. Electrical power consumption of compressor208increases proportionally with compressor frequency.

Heat exchanger212can function as a condenser (allowing the refrigerant to reject heat to the outside air) when VRF system200operates in a cooling mode or as an evaporator (allowing the refrigerant to absorb heat from the outside air) when VRF system200operates in a heating mode. Fan210provides airflow through heat exchanger212. The speed of fan210can be adjusted (e.g., by outdoor unit controls circuit214) to modulate the rate of heat transfer into or out of the refrigerant in heat exchanger212.

Each indoor VRF unit204is shown to include a heat exchanger216and an expansion valve218. Each of heat exchangers216can function as a condenser (allowing the refrigerant to reject heat to the air within the room or zone) when the indoor VRF unit204operates in a heating mode or as an evaporator (allowing the refrigerant to absorb heat from the air within the room or zone) when the indoor VRF unit204operates in a cooling mode. Fans220provide airflow through heat exchangers216. The speeds of fans220can be adjusted (e.g., by indoor unit controls circuits222) to modulate the rate of heat transfer into or out of the refrigerant in heat exchangers216.

InFIG. 2, indoor VRF units204are shown operating in the cooling mode. In the cooling mode, the refrigerant is provided to indoor VRF units204via cooling line224. The refrigerant is expanded by expansion valves218to a cold, low pressure state and flows through heat exchangers216(functioning as evaporators) to absorb heat from the room or zone within the building. The heated refrigerant then flows back to outdoor VRF unit202via return line226and is compressed by compressor208to a hot, high pressure state. The compressed refrigerant flows through heat exchanger212(functioning as a condenser) and rejects heat to the outside air. The cooled refrigerant can then be provided back to indoor VRF units204via cooling line224. In the cooling mode, flow control valves228can be closed and expansion valve230can be completely open.

In the heating mode, the refrigerant is provided to indoor VRF units204in a hot state via heating line232. The hot refrigerant flows through heat exchangers216(functioning as condensers) and rejects heat to the air within the room or zone of the building. The refrigerant then flows back to outdoor VRF unit via cooling line224(opposite the flow direction shown inFIG. 2). The refrigerant can be expanded by expansion valve230to a colder, lower pressure state. The expanded refrigerant flows through heat exchanger212(functioning as an evaporator) and absorbs heat from the outside air. The heated refrigerant can be compressed by compressor208and provided back to indoor VRF units204via heating line232in a hot, compressed state. In the heating mode, flow control valves228can be completely open to allow the refrigerant from compressor208to flow into heating line232.

As shown inFIG. 2, each indoor VRF unit204includes an indoor unit controls circuit222. Indoor unit controls circuit222controls the operation of components of the indoor VRF unit204, including the fan220and the expansion valve218, in response to a building zone temperature setpoint or other request to provide heating/cooling to the building zone. The indoor unit controls circuit222may also determine a heat transfer capacity required by the indoor VRF unit204and transmit a request to the outdoor VRF unit202requesting that the outdoor VRF unit202operate at a corresponding capacity to provide heated/cooled refrigerant to the indoor VRF unit204to allow the indoor VRF unit204to provide a desired level of heating/cooling to the building zone.

Each indoor unit controls circuit222is shown as communicably coupled to one or more sensors250and a user input device252. In some embodiments, the one or more sensors250may include a temperature sensor (e.g., measuring indoor air temperature), a humidity sensor, and/or a sensor measuring some other environmental condition of a building zone served by the indoor VRF unit204. In some embodiments, the one or more sensors include an occupancy detector configured to detect the presence of one or more people in the building zone and provide an indication of the occupancy of the building zone to the indoor unit controls circuit222.

Each user input device252may be located in the building zone served by a corresponding indoor unit204. The user input device252allows a user to input a request to the VRF system200for heating or cooling for the building zone and/or a request for the VRF system200to stop heating/cooling the building zone. According to various embodiments, the user input device252may include a switch, button, set of buttons, thermostat, touchscreen display, etc. The user input device252thereby allows a user to control the VRF system200to receive heating/cooling when desired by the user.

The indoor unit controls circuit222may thereby receive an indication of the occupancy of a building zone (e.g., from an occupancy detector of sensors250and/or an input of a user via user input device252). In response, the indoor unit controls circuit222may generate a new request for the outdoor VRF unit202to operate at a requested operating capacity to provide refrigerant to the indoor unit204. The indoor unit controls circuit222may also receive an indication that the building zone is unoccupied and, in response, generate a signal instructing the outdoor VRF unit202to stop operating at the requested capacity. The indoor unit controls circuit222may also control various components of the indoor unit204, for example by generating a signal to turn the fan220on and off.

The outdoor unit controls circuit214may receive heating/cooling capacity requests from one or more indoor unit controls circuits222and aggregate the requests to determine a total requested operating capacity. Accordingly, the total requested operating capacity may be influenced by the occupancy of each of the various building zones served by various indoor units204. In many cases, a when a person or people first enter a building zone and a heating/cooling request for that zone is triggered, the total requested operating capacity may increase significantly, for example reaching a maximum operating capacity. Thus, the total request operating capacity may vary irregularly and unpredictably as a result of the sporadic occupation of various building zones.

The outdoor unit controls circuit214is configured to control the compressor208and various other elements of the outdoor unit202to operate at an operating capacity based at least in part on the total requested operating capacity. At higher operating capacities, the outdoor unit202consumes more power, which increases utility costs.

For an operator, owner, lessee, etc. of a VRF system, it may be desirable to minimize power consumption and utility costs to save money, improve environmental sustainability, reduce wear-and-tear on equipment, etc. In some cases multiple entities or people benefit from reduced utility costs, for example according to various cost apportionment schemes for VRF systems described in U.S. patent application Ser. No. 15/920,077 filed Mar. 13, 2018, incorporated by reference herein in its entirety. Thus, as described in detail below, the controls circuit214may be configured to manage the operating capacity of the outdoor VRF unit202to reduce utility costs while also providing comfort to building occupants. Accordingly, in some embodiments, the controls circuit214may be operable in concert with systems and methods described in P.C.T. Patent Application No. PCT/US2017/039,937 filed Jun. 29, 2017, and/or U.S. patent application Ser. No. 15/635,754 filed Jun. 28, 2017, both of which are incorporated by reference herein in their entireties.

Outdoor Unit Controls Circuit with Capacity Constraints

Referring now toFIG. 3, a detailed block diagram of the outdoor unit controls circuit214is shown, according to an exemplary embodiment. As described in detail below, the outdoor unit controls circuit214is configured to receive heating/cooling requests from one or more indoor unit controls circuits222, receive a current utility price, determine a value of a price function based on the utility price, generate a capacity constraint based on the price function, apply the constraint in an optimization problem in an economic model predictive control approach, and control the outdoor VRF unit202to conform to the constraint based on a solution to the optimization problem. It should be understood that while the following discussion refers to controlling one outdoor VRF unit202for the sake of clarity of explanation, the present disclosure also contemplates systems and methods for controlling multiple outdoor VRF units202.

As shown inFIG. 3, the outdoor unit controls circuit214includes a requests aggregation circuit300, a price function circuit302, a constraint circuit304, and a model predictive control circuit306. The outdoor unit controls circuit214is shown as communicable with a utility provider system310, the compressor208of the outdoor VRF unit202, one or more indoor unit controls circuits222, and sensor(s)250and/or user input device(s)252located in the various building zones served by the various indoor VRF units204. The outdoor unit controls circuit214may also be communicable coupled to various other components of the outdoor VRF unit202, including fan210, flow control valves228, and expansion valve230.

The utility provider system310is associated with a utility provider of energy or power (e.g., electrical power) to the VRF system200. The utility provider sets the price of the power. For example, the utility provider may use a pricing scheme where the unit price of power (e.g., dollars per kilowatt-hour) varies over time, for example creating high price periods and low price periods. The utility provider system310is configured to provide the current price of the power to the outdoor unit controls circuit214. In some embodiments, the VRF system200consumes power from various utility providers and/or power stored and/or generated by an energy storage system and/or central plant associated with the VRF system200, in which case the outdoor unit controls circuit214may be configured to determine a current price of power based on the costs associated with the various available energy sources.

The requests aggregation circuit300can receive one or more capacity requests from the one or more indoor unit control circuit(s)222. A capacity request may be generated by an indoor unit controls circuit222in response to a user input to a user input device252and/or detection of occupation of a building zone by one or more sensors250. The requests aggregation circuit300may combine, sum, total, etc. the one or more capacity requests to determine a total requested capacity. In response to receiving a new capacity request from an indoor unit controls circuit222, the requests aggregation circuit300may update the total requested capacity. If the new capacity request represents a new request of increased heating/cooling for a building zone, the requests aggregation circuit300provides an indication of the new request to the price function circuit302.

The price function circuit302is configured to receive a current price of power from a utility provider system310and, in response to indication of a new request for heating/cooling, calculate a value of a price function based on the current price of power. That is, the price function circuit302calculates a value of f(Price) where Price is the current price of power. The function f(Price) may be predefined and may have various formulations according to various embodiments. In some embodiments, the possible values of f(Price) range from zero to one, with the value of f(Price) lower when Price is higher. In some embodiments f(Price) is a step function, such that the value of f(Price) is one when Price is less than a threshold price and less than one when Price is greater than a threshold price, for example a value between 0.4 and 0.8. As one example, in some embodiments:

f⁡(Price)={0.6,price⁢⁢upper⁢⁢limit2<Price≤price⁢⁢upper⁢⁢limit1,otherwise,
where price upper limit is a maximum price of power charged by the utility provider. Thus, in some embodiments, f(Price) as calculated by the price function circuit302has a fractional value in high-priced periods and a value of one in low-priced periods. The price function circuit302provides the current value of the price function to the constraint circuit304.

The constraint circuit304is configured to generate a constraint on the operating capacity of the compressor208based on the value of the price function provided by the price function circuit302. The constraint circuit304may formulate the constraint to be applied in a model predictive control approach for each time step k up to a prediction horizon Horizon. Accordingly, the constraint circuit304may generate a constraint of the form:
χODU,k≥0,∀k∈Horizon;
χODU,k≤capODU,k*PriceFactork, ∀k∈Horizon,
where χODU,kis the operating capacity of the outdoor VRF unit202, capODU,kis the maximum capacity of the outdoor VRF unit202(i.e., the physical upper limit on the operating capacity of the outdoor VRF unit202), and PriceFactorkis a function of f(Price). For example, the constraint circuit304may determine a value of PriceFactorkas:

Accordingly, in such an embodiment, the constraint circuit304generates a modified capacity constraint of χODU,k≤capODU,k*f(Price) for a capacity limit period following a new request for increased operating capacity of the outdoor VRF unit202. The term capODU,k*f(Price) may be referred to as the modified constrained capacity. Because in a high priced period the value of f(Price) is less than one, the constraint circuit304thereby limits the operating capacity of the outdoor VRF unit202in response to a new request for heating/cooling of a building zone based on occupancy of the building zone. In other words, the constraint circuit304generates a constraint that prevents the outdoor VRF unit202from being driven to a maximum operating capacity when an indoor VRF system204is turned on for a building zone during a period of high utility prices. Accordingly, the constraint circuit304may facilitate reduction of utility costs by reducing power consumption of the outdoor VRF unit202during high-priced periods.

The constraint circuit304provides the capacity constraint to the model predictive control circuit306. The model predictive control circuit306applies the capacity constraint to an optimization problem and solves the optimization problem over the time horizon, i.e., for time steps k∈Horizon. The model predictive control circuit306may generate the optimization problem based on a predictive model or models of the system (e.g., a building thermal model, a VRF equipment model, a load predictor, a disturbance estimation) and various system constraints. In some embodiments, the model predictive control circuit306generates and solves the optimization problem by defining a cost function and minimizing the cost function over the time horizon. For example, the model predictive control circuit306may define a cost function of the form:
J=Σk=1Horizon[(EnergyCosts(k))+(Penalties(k))]+(Demand Charges),
where the Penalties(k) penalize deviation from comfortable environmental conditions in the building for occupants, for example as described in U.S. Provisional Patent Application No. 62/667,979 filed May 7, 2018, incorporated by reference herein in its entirety.

In the embodiment shown, the model predictive control circuit306solves the optimization problem bound by the capacity constraint generated by the constraint circuit304to determine an operating capacity for the outdoor VRF unit202for each time step in the time horizon. The model predictive control circuit306provides the operating capacities for the time horizon to the equipment controller circuit308. The equipment controller circuit308generates control signals for the compressor208and/or other elements of the outdoor VRF unit202based on the operating capacities provided by the model predictive control circuit306. For example, the equipment controller circuit308may control the compressor frequency of the compressor208to cause the compressor208to operate at the desired operating capacity for the current time step. The equipment controller circuit308may also generate control signals to control the one or more indoor VRF units204based on the operating capacity for a time step provided by the model predictive control circuit306. The outdoor unit controls circuit214thereby controls the outdoor VRF unit202to conform to the modified capacity constraint, i.e., to prevent the operating capacity of the outdoor VRF unit202from exceeding the modified constrained capacity.

Configuration of Exemplary Embodiments