Heat Pump, Systems, and Methods for Operating the Same

This application claims the benefit of U.S. Provisional Application No. 63/621,579 filed on Jan. 16, 2024, and U.S. Provisional Application No. 63/621,583 filed on Jan. 17, 2024, the entire contents of which are herein incorporated by reference.

The present specification relates to an improved heat pump, corresponding control system, and methods and systems for operating the same.

With the effects of global warming being increasingly felt throughout the world, environmental sustainability and emission reductions are becoming more important, both from an engineering and a marketing standpoint. The cost and feasibility of replacing existing, inefficient systems is often the primary hurdle which hampers the adoption and implementation of more sustainable solutions.

Heating and cooling accounts for more than 13% of total carbon emissions worldwide, and heating alone is the largest single source of carbon emissions in the UK, making up more than one-third of the total. Decarbonizing heat is one of the biggest energy challenges in tackling the climate emergency, particularly because it requires actions in millions, if not billions, of individual homes. In the UK alone, only one million of the UK's approximate twenty-seven million homes has low-carbon source of heat.

Gas and fossil fuel-fired boilers are at best only 85% efficient, burning approximately 1.18 kWh of input energy for every 1.0 kW of thermal output. This means that the Coefficient of Performance (COP) of these boilers is less than 1, because the amount of heat energy generated is less than the amount of energy input into the system. In contrast, traditional chlorofluorocarbon/hydrofluorocarbon (CFC/HFC)-based air and/or ground source heat pumps have higher COPs, typically in the range of 1-3.

The maximum temperatures produced by air and/or ground source heat pumps are generally too low to effectively power standard home systems, such as radiators and hot water tanks. To use these heat pumps in homes or businesses for such purposes, significant additional infrastructure—like enhanced insulation or larger heat transfer devices—is often required. This extra infrastructure can be costly and challenging to install, particularly when retrofitting existing buildings. Consequently, the potential cost savings of using a heat pump are often negated by the expenses involved in upgrading or modifying existing systems.

It is therefore desirable to have a cost-effective replacement for gas or fossil fuel-fired boilers and/or less efficient/capable heat pump systems that both reduces reliance on the combustion of fossil or biomass fuels and also meets the energy and temperature requirements for domestic and commercial buildings.

The disclosures herein provide for an improved heat pump system which uses CO2 as a working fluid and has enhanced energy efficiency, reduced greenhouse gas emissions, and optimized performance in home, business, and industrial applications. By leveraging those improvements described herein, functional systems have been able to achieve COPs of 7 or higher—meaning that for every unit of energy input, the heat pump “produces” 7 units of heat output. Embodiments described herein have the advantage that the heat pump system may serve as a “drop-in” replacement for existing gas or oil burners, such that the heat pump system can make use of existing radiators, pipework, flue systems, and the like, thereby eliminating the need for costly additional infrastructure and reducing installation and capital costs. An aesthetic benefit may also be realized, as the heat pump systems designed herein may omit outdoor evaporation units, such that the systems may be installed without affecting the aesthetic qualities of a building or without necessitating additional effort to hide industrial-looking components that some may find to be an eyesore.

The present disclosure provides for an improved heat pump, system, and method for operating the same, that may use supercritical carbon dioxide (CO2) as a working fluid.

In one embodiment, the heat pump comprises a multi-stage heat exchanger for providing heat to a plurality of heat sinks, preferably a hot water system or a space heating system of a building and a working fluid re-heater or economizer. The multi-stage heat exchanger arrangement facilitates the heat pump system to operate at temperatures consistent with those typically provided by traditional fossil-fuel fired boilers. As a result, the heat pump may serve as a “drop in” replacement for these traditional systems without requiring the additional infrastructure typically needed when attempting similar retrofits with conventional heat pump systems.

In another embodiment, the heat pump is provided with an oil separator, reservoir, and accompanying valve assembly to extract lubricant from the working fluid and feed said lubricant back to the compressor at a pressure which is higher than the inlet pressure of the compressor.

In another embodiment, the working fluid is expanded through multiple stages to limit flashing of the working fluid to a subcritical gas during expansion and to facilitate venting of any such gases which do form to the compressor suction inlet, again improving the efficiency and reliability of the heat pump.

These and additional features provided by the embodiments of the present disclosure will be more fully understood in view of the following detailed description, in conjunction with the drawings.

The embodiments of the disclosure herein include a heat pump, corresponding system, controls, and methods for operating the same.

The heat pump may operate using CO2 as a working fluid, which has excellent thermodynamic properties and can efficiently transfer heat. Through the course of the cycle facilitated by the heat pumps disclosed herein, the working fluid is transitioned between subcritical and supercritical states in order to achieve the titular “pumping,” or transfer, of heat.

Subcritical and supercritical CO2 are two distinct states of carbon dioxide, defined by their temperature and pressure in relation to CO2's critical point. The critical point for CO2 is approximately 31.1° C. (87.8° F.) and 73.8 bar (1071 psi). When CO2 is below this critical temperature and pressure, it is considered subcritical. In the subcritical state, CO2 can exist as either a gas or a liquid, depending on specific conditions. At low pressures and temperatures, CO2 is gaseous, while at higher pressures but below the critical temperature, it becomes a liquid. In this subcritical state, there is a clear distinction between liquid and gas phases, often with a visible interface if both are present.

Supercritical CO2, on the other hand, exists at temperatures and pressures above its critical point, where it transitions into a supercritical fluid. In this state, CO2 exhibits properties that make it highly advantageous for advanced energy systems. It combines the density of a liquid, which allows it to carry significant thermal energy, with the low viscosity of a gas, enabling efficient flow through turbines and heat exchangers. Additionally, its high diffusivity improves heat transfer and overall system performance.

However, while preferred embodiments make use of CO2 as a working fluid, such a preference is not to be construed as a limitation. It is contemplated that any working fluid with similar properties and/or characteristics at notable points in the refrigeration cycle, such as air or argon, or any suitable working fluid cognizable to a person of ordinary skill in the art, may be usable or used with those systems herein to achieve a comparable effect.

Turning now to, a simplified, exemplary heat pump system is depicted in schematic form. The heat pumpmay include a compressor, one or more heat exchangers, one or more controllable expansion valves-, a liquid receiver, one or more pressure valves, one or more heat exchangers, or evaporators-, and a fan. The heat pumpforms a largely closed or hermetically sealed loop, whereby the working fluid may pass through these components and undergo the cycle described below.

As set forth herein, and solely for the purposes of explanation and orientation, any relative positional description of components as “upstream” or “downstream” of one another, specifically, is to be considered broken by a conceptual boundary between the compressorand the inlet or suction side of the compressor, even though no such physical boundary exists. For example, as shown in, the compressor is “upstream” of the first heat exchangerbut the evaporator stageis not. As another example, and again as shown in, the second heat exchanger(or at least a portion thereof) is “downstream” of the evaporator stage, but the compressoris not. Any other relative positional descriptions (e.g., X is between Y and Z, X is before Y) are to be given their plain and ordinary meaning as would be understood by a person of ordinary skill in the art.

At the start of the cycle, compressoris used to compress the working fluid from a subcritical state to a supercritical state. The compressormay be embodied as a piston compressor, rolling piston compressor, swing compressor, scroll compressor, vane compressor, screw compressor, reciprocating compressor, and/or any other type of compressor known in the art which is suitable for compressing a working fluid as described herein from a subcritical to a supercritical state. The compressormay be a single stage compressor or a multi-stage compressor.

Once the working fluid leaves the compressor, it may then pass to a first heat exchangerThe first heat exchangeris used to extract heat from the supercritical working fluid, thereby cooling the supercritical working fluid to some extent, and transfer that heat to a first heat sink. This transfer may be direct or indirect by way of another medium or intermediate coolant. An exemplary intermediate coolant may include water or a water/glycol mixture. Preferably, the working fluid is maintained at a constant pressure as it passes through the first heat exchanger

It is preferable that the first heat exchangertransfers heat from the working fluid to a first heat sink embodied as a hot water system, such as a domestic hot water system (e.g., that which might supply showers, baths, laundries or the like). For example, the extracted heat may be used to heat up a hot water tank or provide in-line heating to a hot water line. The hot water systemmay also be a commercial or industrial hot water system, such that which supplies hot water used for industrial processes or the like. As above, this transfer of heat to the hot water systemmay be direct, such as by passing the water of the hot water systemthrough the heat exchangeror indirect, such as by passing an intermediate coolant through the heat exchangerwhich passes through another heat exchanger to ultimately provide heat to the hot water system.

The first heat sink may also be embodied as a space heating system, such as that which provides heat for a home/building via hot water radiators or via forced air systems.

Next, the working fluid exits the first heat exchangerand passes into a second heat exchangerThat is to say, the second heat exchangermay be provided in series with the first heat exchangerThe second heat exchangermay also be referred to as an economizer. The second heat exchangermay, as above, transfer, directly or indirectly, heat from the supercritical working fluid to subcritical working fluid later on in the cycle in a pre-heating step, before the subcritical working fluid is fed back into the compressor. This step is discussed in further detail below. Again, preferably, the working fluid is maintained at a constant pressure as it passes through the second heat exchanger

Efficiency of the thermodynamic cycle may be increased by lowering the temperature of the supercritical working fluid before any expansion step. Efficiency of the thermodynamic cycle may also be increased by increasing the temperature of the subcritical working fluid after evaporation has occurred. The second heat exchangeror economizer, achieves both conditions simultaneously, further increasing the overall efficiency of the heat pump.

As a result of this multi-stage process, a building, such as a home or business, may be provided with hot water for one or both of the hot water system, hot air or water for a space heating system, or the like at temperatures and capacities which are typically provided by traditional gas and/or fossil fuel boilers. This means that these traditional gas and/or fossil fuel boilers can be replaced with the heat pumpdescribed herein without needing to also replace existing infrastructure such as pipes/ducts, heat exchangers, and the like as may be required for conventional heat pump systems.

The heat exchangers-may be embodied as heat exchangers comprising micro-channels of less than 1 mm in width, diameter, and/or spacing. The heat exchangers-may be formed from metal and, advantageously, may be 3D printed. The use of 3D printing allows for the creation of three-dimensional surface features which, combined with the high surface area density and optimized fluid pathways of the micro-channel configuration, results in a heat exchangerwith high thermal exchange, low weight, and low pressure drops. As a result, the overall heat pump system is made more compact, lighter, easier to install, more efficient, and cheaper to manufacture.

After leaving the second heat exchangerthe working fluid passes through a expansion stagewhich includes a first expansion valve, a liquid receiver, and a second expansion valve. The first and second expansion valves may each be embodied as a controllable expansion valve, such as a stepper motor-controlled or servo-controlled valve or any other known expansion device.

As the high pressure, supercritical working fluid passes through the first expansion valve, the pressure of the working fluid is reduced, which has the result of converting the supercritical fluid to a subcritical liquid. The subcritical liquid is then collected in the liquid receiverbefore undergoing further expansion.

While residing in the liquid receiver, it is possible that some of the supercritical fluid or subcritical liquid vaporizes/evaporates/flashes into a subcritical gas which also occupies the liquid receiver. To avoid overpressure conditions, the liquid receiver is provided with a bypass lineon which is arranged a pressure valvevia which subcritical gas may be vented to the suction lineof the compressor. The pressure valvemay be a controlled valve (e.g., stepper motor, servo, or the like), active valve, passive valve, or any other known type of pressure control valve. The pressure valveis attached to the liquid receiverand controlled in a manner that ensures that any fluid which passes to the suction line of the compressoris a gas and not a liquid.

Next, the working fluid, in its subcritical liquid form, passes from the liquid receiverand through a second expansion valve. Optionally, second expansion valvemay instead be embodied as a powered expansion device, such as one that may be independently driven or driven along with the compressorvia the compressor shaft. In either case, the working fluid is further expanded to a pressure which corresponds well to the prevailing ambient air temperature to better facilitate subsequent evaporation steps. This degree of expansion may be dynamically adjusted, in real time, to account for changes in ambient air temperatures, thereby ensuring safe and efficient operation and improving the overall COP of the heat pump.

Additionally, a filtermay be disposed downstream of the liquid receiverand upstream of the second expansion valve.

Once the working fluid is expanded to the desired pressure, the working fluid passes to an evaporation stagewhereby the working fluid is converted from a subcritical liquid to a subcritical gas. Heat for evaporating the working fluid in this manner may be extracted from, for example, ambient air.

The evaporation stageis provided with at least one evaporator,and a fan, such as a cyclone fan. The fanis configured to force—pushing or pulling—ambient air over the evaporatorto exchange heat with the working fluid contained within. The evaporator,is preferably embodied as a finned micro-channel heat exchanger. Preferably, the micro-channels are less than 1 mm in width, diameter, and/or spacing. Use of a finned micro-channel heat exchanger has advantages in that the internal volumes are lower, requiring less working fluid, corrosion resistance is better, approach temperatures are closer, and the airside pressure drop is lower.

As the ambient air passes through the evaporator, the ambient air is cooled. However, the degree of cooling here may be significant, which can lead to icing of the evaporatorand a reduction in performance of the same. The impact of this phenomenon can be reduced by executing defrosting cycles or by lowering the pitch of the fins, but this significantly reduces the heat transfer rate of the evaporator.

Another solution to the icing problem is to provide a second evaporator, such that air would first pass over the first evaporatorand then pass over the second evaporator. In such a configuration, the first evaporatormay instead be provided with larger fins and/or a lower fin density while the second evaporator is provided with smaller fins and/or a higher fin density. This has the result that most, if not all, of the moisture in the ambient air would condense on the fins of the first evaporator, leaving little to no moisture left in the air to condense on the fins of the second evaporator, and thereby delaying the accumulation/formation of frost. The larger fins/lower fin density of the first evaporatorwould also permit the condensation to flow/drip away from the first evaporatormore quickly under the force of gravity or the like.

This two-stage arrangement has the effect of increasing defrost intervals and reducing defrost time, as the second evaporator would only be subject to icing in the most extreme weather conditions. Additionally, hydrophobic coatings may be applied to the surfaces of the first and/or second evaporators to further mitigate the formation of frost. Frost which does form over such a coating tends to have poor adhesion and is easier to dislodge from the evaporators.

For both single and multi-stage evaporator configurations, defrost cycles may be incorporated whereby hot gas is passed through either/both the first and second evaporators to melt/dislodge any accumulated frost.

Evaporators,may also be referred to as heat exchangers, gas coolers, or described using other terms of art.

A gas accumulatoror other working fluid storage device may optionally be arranged downstream of the evaporation stage, so as to collect subcritical working fluid gases for subsequent use by the compressor.

A controllercomprising any number of processors, memory units, storage units, network interfaces, user interfaces, data/control/PID interfaces, sensor interfaces, displays, audio inputs/outputs may be provided. Additionally/alternatively, the controller may be formed as an application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), or any other computing/control system. The controller may be a plurality of controllers and/or microcontrollers which are in operative communication with one another.

The controllermay be configured to or may execute software which causes the controller to control operation of the heat pump. The heat pumpmay be provided with any number of sensors, which may include temperature and/or pressure sensors measuring static and/or total quantities, vibration sensors, microphones, flow rate sensors, electrical sensors (e.g., voltage, resistance, current), rotational speed sensors, clocks/timers, humidity/moisture sensors, and the like and provided at any point on, in, around (such as to measure environmental quantities, such as ambient temperatures/pressures), and/or between components of the heat pumpcircuit shown in. Feedback and/or measurements from any combination of these sensors may be used in the control of the heat pumpby the controller.

In most configurations, control of the heat pumpby the controller may be exercised by adjusting the speed of the compressorand the degree of expansion of the working fluid carried out at the expansion stage by the first expansion valveand/or the second expansion valve.

It is further contemplated that any number of valves, check valves, pressure valves, controlled valves, or the like may be disposed at various points along the heat pump circuit to facilitate the injection/venting of working fluid, control of working fluid as it passes through the circuit, servicing of any of the components arranged along the circuit (e.g., valves may be disposed in a manner, such as at the inlet/outlet ports/flanges/etc. of a component, that a given component may be isolated from the rest of the circuit to make the component easier to service), and the like.

Additionally, additional receivers, accumulators, storage tanks, or the like may be disposed at various points along the heat pump circuit, such as directly or substantially directly before the compressor, to collect working fluid, to ensure any downstream components have available sufficient working fluid to operate, and/or the like as required by a given installation.

Lubrication of the compressor may be ensured by way of an oil system. For the purposes of descriptions herein, “oil” refers to any suitable oil or lubricant and is thus not necessarily limited only to “oils.” An oil separatormay be provided downstream of a mufflerwhich, in turn, is downstream of compressor. The oil separator has the effect of separating any oil from the working fluid once the working fluid leaves the compressor.

Once oil is detected in the separatorby an optical switch or the like, oil valve, which is controlled by a solenoid or comparable control mechanism, opens to permit any such oil to pass to an oil receiveror other storage tank. The pressure within the oil receiveris maintained via a pressure valveto be just above the compressor inlet pressure, thereby ensuring that oil may be fed to the compressorduring operation.

When a compressor oil level switchor the like signals that more oil is required by the compressor, another oil valvewhich is controlled by a solenoid or comparable control mechanism, opens to permit any such oil to pass into the compressor. An oil filtermay be arranged between the compressor and another oil valve

Turning now to, a flow chart describing an energy transfer method utilizing the heat pumpis shown.