Heat Pump Water Heater with Compressor Modulation to Extend Low Ambient Operation

This application claims priority to U.S. 63/571,169, filed on Mar. 28, 2024, entitled “HEAT PUMP WATER HEATER WITH COMPRESSOR MODULATION TO EXTEND LOW AMBIENT OPERATION,” the disclosure of which is hereby incorporated herein by reference in its entirety.

Heat pump water heaters have been developed to provide hot water. Heat pump water heaters may utilize a vapor refrigeration compression cycle in a closed-loop heat exchange circuit to absorb heat from a source (e.g., air) and transfer the heat to water for storage in a hot water tank.

An aspect of the present disclosure is a heat pump water heater system that is configured to increase compressor speed during cold ambient conditions to compensate for reduced mass flow rates that would occur if the compressor were to be operated at the same speed during warm and cold ambient conditions. A heat pump water heater system according to an aspect of the present disclosure may include a compressor having a compressor motor, a compressor inlet, and a compressor outlet. The system includes an evaporator having an evaporator inlet and an evaporator outlet, wherein the evaporator outlet is fluidly connected to the compressor inlet. An evaporator fan causes air to flow through the evaporator when the evaporator fan is actuated. The system further includes a heat exchanger having an inlet that is fluidly connected to the compressor outlet. The heat exchanger also includes an outlet that is fluidly connected to the evaporator inlet, whereby water flowing through the heat exchanger is heated by refrigerant flowing through the heat exchanger. The heated water from the heat exchanger may flow to a hot water tank system. The system further includes a pressure sensor that is configured to detect a pressure of refrigerant upstream of the compressor inlet. The system further includes a controller that is configured to increase a speed of the compressor motor if a pressure detected by the pressure sensor reaches a threshold pressure that is greater than a minimum allowable operating pressure whereby a mass flow rate of the compressor is increased to permit operation of the heat pump water heater system at a lower ambient temperature without reaching the minimum allowable operating pressure. The system may be configured to provide a defrost cycle if the pressure drops below the threshold pressure despite an increase in compressor speed. The defrost cycle may be initiated at an operating pressure between the threshold pressure and the minimum allowable operating pressure.

Another aspect of the present disclosure is a heat pump water heater system including a compressor having a compressor motor that is electrically powered, a compressor inlet, and a compressor outlet. The compressor defines a minimum allowable operating pressure at the compressor inlet. The system includes an evaporator having an evaporator inlet and an evaporator outlet, wherein the evaporator outlet is fluidly connected to the compressor inlet. The system further includes an evaporator fan that is configured to cause air to flow through the evaporator when the evaporator fan is actuated. A heat exchanger has an inlet that is fluidly connected to the compressor outlet, and an outlet that is fluidly connected to the evaporator inlet, whereby water flowing through the heat exchanger is heated by refrigerant flowing through the heat exchanger. The system further includes a pressure sensor that is configured to detect a pressure of refrigerant upstream of the compressor inlet to provide a measured inlet refrigerant pressure. A defrost valve system is configured to cause heated refrigerant from the compressor outlet to flow through the evaporator during a defrost cycle. The system includes a controller that is configured to: 1) increase a speed of the compressor motor if a measured inlet pressure drops to a threshold pressure that is greater than the minimum allowable operating pressure of the compressor whereby a mass flow rate of the compressor is increased to permit operation of the heat pump water heater system at a lower ambient temperature, and: 2) actuate the defrost valve system if a measured inlet pressure reaches a defrost pressure that is below the threshold pressure despite the increased speed of the compressor motor.

Another aspect of the present disclosure is heat pump water heater system including a compressor having a compressor motor that is electrically powered. The compressor defines a minimum allowable operating pressure at the compressor inlet. The system further includes an evaporator, a heat exchanger, and a controller that is configured to: 1) increase electrical power to the compressor motor to maintain a pressure of refrigerant at an inlet of the compressor above the minimum allowable pressure, and: 2) implement a defrost cycle causing heated refrigerant to flow through the evaporator if increasing electrical power to the compressor motor is insufficient to maintain mass flow through the compressor according to predefined criteria.

Another aspect of the present disclosure is a method of controlling a heat pump water heater having a compressor that supplies heated refrigerant to a heat exchanger to heat water, and an evaporator that is exposed to ambient air. The method includes empirically determining a relationship between operating variables including: 1) compressor speeds and 2) evaporator fan speeds that optimize a Co-efficient-of-Performance (COP) based on input parameters, wherein the COP comprises heat output of the heat pump system divided by input power to the heat pump system, and wherein the input parameter comprises: 1) temperature of water at an inlet and/or an outlet of the heat exchanger; 2) ambient air temperature; and 3) temperature and/or pressure of refrigerant at an inlet and/or at an outlet of the compressor. The method further includes controlling compressor speed and/or fan speed utilizing the relationship to maximize COP.

These and other features, advantages, and objects of the present disclosure will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.

For purposes of description herein the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the disclosure as oriented in the FIGURE. However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. The terms “including,” “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by “comprises a . . . ” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

With reference to the FIGURE, a heat pump water heater systemaccording to an aspect of the present disclosure includes a compressorthat is configured to compress a refrigerant. Compressormay be substantially similar to known compressors, and may include a pump that is driven by an electrically-powered motor. The refrigerant may comprise, for example, COor other suitable medium. Compressorincludes an inletand an outlet, whereby the compressorcauses compressed refrigerant to flow through a lineto a gas cooler heat exchangerto heat water flowing through a water circuitto provide hot water to a hot water tank system. The hot water tank systemmay comprise, for example, a hot water system for a building or the like. A controlleris configured to control the valves and other components of heat pump water heater system, including a speed of compressor. Heat pump water heater systemmay include a power supplythat is operably connected to controller, compressor, and other components of the system.

During operation, refrigerant that has been pressurized by compressorflows through linesto an evaporator. Refrigerant exiting evaporatoris returned to the inletof compressorby lines. During operation, evaporator fansmay be actuated to increase air flow through the evaporatorwhereby the refrigerant flowing through the evaporatorchanges phase from a liquid to a gas. Evaporatormay be located in an ambient space. The ambient spacemay be outside of a building whereby the ambient spaceexperiences low temperatures (e.g., below freezing) if heat pump water heater systemis installed in a climate that experiences cold temperatures. In general, an amount of heat that can be absorbed by the evaporatoris the product of the mass flow rate of the refrigerant multiplied by the change of heat content (enthalpy) that occurs in evaporatoras the refrigerant changes phase from liquid to gas. As the ambient temperature in ambient spacedrops, the evaporatormay be required to operate at lower temperatures in order to keep balance between the energy being released from the air and the energy absorbed into the refrigerant. For pure fluids such as CO, pressure and temperature are dependent as the fluid changes phase from liquid to gas, such that the operating pressure (e.g. pressure of refrigerant at compressor inlet) also drops. For a given speed (e.g., rpm) of the compressor, drops in the mass flow rate of the compressormay also be accompanied by drops in temperature and pressure, decreasing the heat pump's overall capacity to transfer heat and heat water supplied to hot water tank system.

As discussed in more detail below, the speed (mass flow rate) of compressormay be increased at lower ambient temperatures to compensate for reduced mass flow rates that would occur at a constant compressor speed to thereby permit heat pump water heater systemto operate at lower ambient temperatures, or to permit the systemto utilize a smaller compressor.

Heat pump water heater systemmay optionally include a recuperatorwhereby, during normal operation (i.e., not during a defrost cycle), refrigerant flows through recuperatorto transfer heat to refrigerant flowing through linesbefore the refrigerant in linesreturns to the compressor inlet, and the refrigerant exiting recuperatorflows through an expansion valvewhereby a temperature of the refrigerant is reduced before the refrigerant flows through the evaporator.

During a defrost cycle, defrost valvesandmay be actuated by controllerto cause compressed refrigerant from compressorto bypass gas cooler heat exchangerand recuperatorsuch that refrigerant entering evaporatorhas a higher temperature (e.g., above freezing) to thereby temporarily heat the coils of evaporatorto defrost evaporator. The defrost cycle may be substantially similar to known defrost cycles. In general, the defrost cycle may be short enough to minimize energy loss, but long enough to defrost evaporator. As discussed in more detail below, controllermay cause a defrost cycle if a pressure of refrigerant at or upstream of compressor inletdrops below a minimum pressure despite increases in compressor speed.

Heat pump water heater systemmay include a pressure sensorand a temperature sensorthat are configured to measure the pressure and temperature, respectively, of refrigerant flowing through lineupstream of inletof compressor. Controlleris operably connected to the valves and sensors of the system whereby the controllermay control compressorbased on various inputs, including sensor data. The controllermay be configured to control a speed of operation of compressor, whereby a mass flow rate of compressorcan be adjusted. Controllermay optionally comprise a Variable-Frequency Drive (VFD) that controls an rpm of compressor.

During the operation of heat pump water heater system, controllermay monitor pressure readings from pressure sensor. If the ambient air temperature drops, the operating pressure measurements from sensorwill also tend to decrease. The decrease in pressure measured by pressure sensorgenerally corresponds to a decrease in the mass flow rate. Compressormay have a minimum allowable operating pressure requirement, and controllermay be configured to turn off compressor(e.g., stop the supply of electrical power to the compressor) to prevent operation of compressorat or below the minimum operating pressure. Controllermay also be configured to increase a speed (mass flow rate) of compressorif an operating pressure sensed by pressure sensordrops sufficiently to reach a threshold pressure that is above the minimum allowable operating pressure for compressor. An increase in the operating speed of compressorresults in an increase in the mass flow rate and operating pressure such that the pressure measured by pressure sensordoes not reach the minimum allowable operating pressure.

In general, the threshold pressure may be selected (set) to avoid reaching the minimum allowable pressure without causing unnecessary increases in compressor speed. For example, the threshold pressure may be about 5 psi greater than the minimum allowable pressure, about 10 psi greater than the minimum allowable pressure, about 20 psi greater than the minimum allowable pressure, or other suitable pressure difference as required for a particular heat pump water heater system. In some examples, the threshold pressure may by a target percentage greater than the minimum allowable pressure. For example, the pressure threshold may be about 105%, about 110%, about 115%, or about 120% of the minimum pressure. The pressure difference between the threshold pressure and the minimum allowable operating pressure is preferably great enough to reliably prevent the system from reaching the minimum allowable pressure without causing unnecessary or excessive increases in compressor speed that could result in decreased efficiency. It will be understood that the threshold pressures discussed above are merely examples, and the present disclosure is not limited to any specific pressure threshold. Also, the magnitude of the pressure difference may be set, at least in part, to provide for optimum (e.g. most efficient) operation in a specific geographic area and climate.

Thus, the heat pump water heater systemmay have a cold temperature (low ambient temperature) operating mode that includes increasing the compressor speed to increase the mass flow rate of refrigerant through compressor. Increasing the speed of compressorallows the systemto continue operating at a lower ambient temperature by increasing the mass flow rate to prevent the operating pressure measured by pressure sensorfrom reaching the minimum allowable pressure. Also, compared to a heat pump system that does not include a cold operating mode, systemmay include a smaller compressor, yet still continue to operate at the same ambient temperature as a heat pump system having a larger compressor that does not include increased compressor speed.

It will be understood that the operating pressure measured by sensoris related to the mass flow rate of the refrigerant through compressor. Thus, the present disclosure is not limited to measuring mass flow rates based on measuring operating pressure. In general, systemmay utilize virtually any suitable mass flow measurement technique to control compressor speed to compensate for reduced mass flow rates that would otherwise occur if the compressor were to be operated at the same speed at both warm and cold ambient conditions. For example, the actual mass flow rate could be measured alone or in combination with one or more additional operating parameters (e.g. ambient temperature), and a suitable minimum mass flow rate could be utilized to trigger increased compressor speed. The energy balance between the air side of the evaporator and the refrigerant side of the evaporator could also be measured, and a suitable energy value could be utilized to trigger increased compressor speed. In general, the operating parameter that is utilized to trigger increased compressor speed may also be used to trigger reductions in compressor speed whereby the compressor speed returns to the “normal” (warm ambient) mode. For example, if pressure measured by sensoris reduced to a first threshold, controllermay increase a speed of compressor, and controllermay reduce a speed of compressorif the pressure measured by sensoris above a second threshold. However, it will be understood that the measured operating parameter/criteria that is utilized to trigger increased compressor speed does not have to be the same operating parameter/criteria that is used to reduce compressor speed. For example, compressor speed may be increased if the mass flow rate drops to a threshold level, whereas compressor speed may be reduced if the ambient temperature increases to a minimum threshold temperature.

Controllermay be in communication with and/or comprise a Variable-Frequency Drive (VFD) to control the compressor. For example, during normal operating conditions, the VFD may drive a motor of compressorat a frequency of 60 Hz, and during low-ambient temperature conditions (e.g., when the pressure measured by sensorreaches the threshold pressure), the VFD can drive the motor of compressorat a frequency higher than 60 Hz (e.g., 70 Hz). The increases in drive speed of compressormay comprise a single step increase (e.g., 60 Hz to 70 Hz), or it may comprise a series of steps (e.g., from 60 Hz to 65 Hz, 65 Hz to 70 Hz, etc.). In an example, the VFD may increase the frequency 5 Hz (or 10 Hz) each time the pressure measured by sensorreaches a first (lower) threshold, and the frequency may be reduced 5 Hz (or 10 Hz) each time the measured pressure reaches a second (higher) threshold. Alternatively, the increase may comprise a ramp function whereby, for example, the VFD gradually increases the drive frequency from 60 Hz to 70 Hz utilizing a predefined ramp function.

Alternatively, the controller may be configured to continuously vary the speed of compressorif a threshold pressure is detected by sensorwhereby the operating pressure measured by sensoris maintained at or around (e.g., slightly above) the threshold pressure by varying the speed at compressor.

As discussed above, controllermay be configured to reduce the operating speed of compressorwhen an increased mass flow rate (compressor speed) is no longer required. For example, controllermay be configured to utilize predefined criteria to reduce the compressor speed to return it to the normal operating speed. The predefined criteria may comprise, for example, a pressure measured by sensorthat is sufficient to prevent the pressure from dropping to the minimum allowable pressure of compressorif the compressor speed is returned to normal. Controllermay be configured to utilize empirical data and/or modeling data to predict or estimate a relationship between a drive speed of compressorand a pressure measured by pressure sensorat a given operating condition (e.g., ambient temperature), and the speed of compressormay be returned to normal if the normal operating speed will not result in the pressure dropping below the minimum allowable (shut off) pressure. Thus, if the heat pump water heater systemis operating in a cold ambient mode wherein a speed of compressoris increased, the controllermay monitor the operating pressure measured by pressure sensorand the ambient temperature to determine if the speed of compressorcan be decreased to a normal operating speed without the pressure dropping below the threshold pressure or, alternatively, without the pressure dropping to or below the minimum allowable pressure.

Compressormay also have a maximum allowable drive speed. Thus, if the ambient temperature is sufficiently low, the operating pressure sensed by pressure sensormay drop to the minimum allowable pressure of compressoreven though compressoris being driven at the compressor's maximum allowable speed. If this operating condition occurs, controllermay be configured to turn off compressor(e.g., stop supplying electrical power to the compressor). Following shut down of compressor, the controllermay be configured to monitor the ambient temperature and initiate operation of compressorif the ambient temperature is high enough to permit operation of compressoreither at a normal (default or baseline) operating speed or at an increased operating speed (cold ambient mode).

Controllermay be configured to actuate defrost valvesandto cause either refrigerant from compressorto flow through evaporator. In general, controllermay be configured to increase a speed of compressorif an inlet pressure of compressor drops below a threshold value to thereby permit operation to continue at reduced ambient temperatures while maintaining an inlet pressure of compressorabove a minimum allowable inlet pressure of the compressor. However, increases in compressor speed may not be sufficient to maintain the inlet pressure above the minimal allowable inlet pressure for compressor pump. Accordingly, if an inlet pressure (e.g. a pressure measured by sensor) drops below the threshold pressure after a speed of compressorhas been increased, controllermay actuate a defrost cycle when the inlet pressure reaches the minimum allowable inlet pressure, or when the inlet pressure reaches a defrost inlet pressure that is between the minimum allowable inlet pressure and the threshold inlet pressure. In general, controllermay be configured to continue increasing a speed of compressoruntil a maximum allowable compressor speed is reached, and controllermay then implement a defrost cycle if the inlet pressure reaches the defrost inlet pressure. In general, the defrost inlet pressure may be equal to the minimum allowable inlet pressure. Controllermay also be configured to implement a defrost cycle based on other operating parameters even if a maximum allowable compressor speed has not been reached. For example, under certain operating conditions, it may be more efficient to implement a defrost cycle rather than continue to increase compressor speed.

The controllermay comprise virtually any suitable programmable controller, circuit, or combination thereof. For example, controllermay comprise a Programmable Logic Controller (PLC) that is configured to monitor multiple parameters such as inlet and outlet temperatures of compressor, refrigerant pressures, ambient temperature, and power consumption. The heat pump water heater systemmay utilize return water temperature measured by sensor, and/or water supply temperature measured by temperature sensorto control operation of system. The controllermay be configured to utilize various inputs (e.g., measured temperatures and pressures) to maximize the Coefficient of Performance (COP) over a wide range of inlet water temperatures and ambient conditions using a multi variable regression of input parameters measured by the sensors of the heat pump water heater system. The output of this control system (e.g., algorithm) may vary the speed of compressor, the speed of evaporator fans, or both, to minimize the input power and maximize the heat output for a given set of conditions. In general, the COP is the product of heat output divided by input power, and system(e.g., controller) may be configured to always operate at or near peak COP. Heat output may be determined (e.g. by controller) utilizing a difference in temperatures measured by sensorsand, and the flow rate and heat capacity of water flowing through heat exchanger. Controllermay be configured to determine input power by measuring and summing power consumed by the system components and/or utilizing power output from power supply.

For example, if the ambient temperature is warmer, which can correspond to higher operating pressures, compressorand/or evaporator fanscan be operated at lower speeds (frequencies). If the ambient temperature is cooler, the compressorand/or evaporator fanscan be controlled to operate at higher speeds (frequencies). For example, a second VFD may be configured to control a frequency of a motor driving each or both of the evaporator fans, and the controllermay communicate instructions or signals to the second VFD to speed up or slow down this/these motors. Thus, the controllerof heat pump water heater systemcan be configured to provide an active and continuous control over the components of the heat pump water heater systemto optimize COP.

It will be understood by one having ordinary skill in the art that construction of the described disclosure and other components is not limited to any specific material. Other exemplary embodiments of the disclosure disclosed herein may be formed from a wide variety of materials, unless described otherwise herein.

It is also important to note that the construction and arrangement of the elements of the disclosure as shown in the exemplary embodiments is illustrative only. Although only a few embodiments of the present innovations have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts or elements shown as multiple parts may be integrally formed, the operation of the interfaces may be reversed or otherwise varied, the length or width of the structures and/or members or connector or other elements of the system may be varied, the nature or number of adjustment positions provided between the elements may be varied. It should be noted that the elements and/or assemblies of the system may be constructed from any of a wide variety of materials that provide sufficient strength or durability, in any of a wide variety of colors, textures, and combinations. Accordingly, all such modifications are intended to be included within the scope of the present innovations. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the desired and other exemplary embodiments without departing from the spirit of the present innovations.

It will be understood that any described processes or steps within described processes may be combined with other disclosed processes or steps to form structures within the scope of the present disclosure. The exemplary structures and processes disclosed herein are for illustrative purposes and are not to be construed as limiting.

It is to be understood that variations and modifications can be made on the aforementioned structure without departing from the concepts of the present invention, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.