Heat pump utilising the shape memory effect

Abstract

The invention provides a heat pump system and method comprising a Shape-Memory Alloy (SMA) or Negative Thermal Expansion (NTE) core (2a, 2b) positioned in a housing and adapted to absorb heat and store energy in response to a first fluid inputted at a first temperature. The housing is configured to receive a second fluid via an inlet wherein a device changes pressure in the housing to cause the SMA or NTE core to change state to release the heat absorbed into the second fluid. An outlet is adapted to output the second fluid at a higher temperature than the first temperature.

Claims

1. A heat pump system comprising a Shape-Memory Alloy (SMA) or Negative Thermal Expansion (NTE) core positioned in a housing, and adapted to absorb heat and store energy in response to a first fluid inputted at a first temperature, wherein the SMA or NTE core comprises a plurality of elements to define the core; the housing is configured to receive the same fluid or a second fluid via an inlet wherein a device changes stress on the SMA or NTE core in the housing to cause the SMA or NTE core to change state to release the heat absorbed into the fluid; an outlet adapted to output the fluid either at a higher temperature or lower temperature relative to the first or second fluid temperature depending on whether heat has been released or absorbed by the SMA or NTE core; a high-pressure accumulator adapted to provide a stress to the core to enable a forward or a reverse phase change to occur in the core; and a low-pressure accumulator configured to reduce stress on the SMA or NTE core in the housing when the fluid is being inputted at an initial temperature; and wherein a pump is utilised to pump a hydraulic fluid from the low-pressure accumulator to the high-pressure accumulator.

2. The heat pump system as claimed in claim 1 wherein a hydraulic chamber is configured to physically elongate or stretch at least one element or wire of the core in response to stress from pressure supplied by the accumulator.

3. The heat pump system as claimed in claim 2 wherein low-pressure in the hydraulic chamber is transferred to the SMA or NTE core in the housing in order to allow a cycle for the core to absorb heat.

4. The heat pump system as claimed in claim 1 comprising a hydraulic motor configured to raise the stress on the SMA or NTE core via an increase in a pressure of the high-pressure accumulator.

5. The heat pump system as claimed in claim 1 wherein flow rates of the fluid are controlled by a controller to adjust temperature difference of the cycle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:—

(2) FIG. 1 illustrates an embodiment of the invention with a Heat Pump system incorporating a mechanical configuration of SMA or NTE cores and a transmission system;

(3) FIG. 2 illustrates a work flow diagram showing different states of the heat pump during operation;

(4) FIG. 3 illustrates a single SMA or NTE core for using in a High Pressure (HP) cycle according to one embodiment of the invention; and

(5) FIG. 4 illustrates a single SMA or NTE core for using in a Low Pressure (LP) cycle according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(6) The invention relates to a new heat pump cycle which utilises the latent heat from a phase transformation of shape memory alloys (“SMA”) or Negative Thermal Expansion materials (NTE). The invention can use a particular SMA engine made up of a plurality of elements or wires packed closely together to define a core. SMA material can exist in two crystalline states, martensite and austenite, and can be reversibly converted from one phase to the other. The austenite to martensite transition of SMA is exothermic. The martensite to austenite transition is endothermic. The temperatures at which the phase change occurs can be manipulated via the application of stress to the SMA material.

(7) A Shape-memory Alloy (SMA) is an alloy that exhibits a shape memory effect which once deformed returns to its pre-deformed shape upon heating. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems.

(8) The memory of such materials has been employed or proposed for use in heat recovery processes and in particular by constructing SMA engines which recover energy from heat as motion. Recent publications relating to energy recovery devices include PCT Patent Publication number WO2013/087490, assigned to the assignee of the present invention, and incorporated fully herein by reference.

(9) The invention relates to a heat pump system and method which can use either Shape-Memory Alloys (SMAs) or Negative Thermal Expansion materials (NTE) using a particular SMA engine made up of a plurality of elements or wires packed closely together to define a core. A heat pump has two individual phases—heat absorption and heat release. The machine cycle is defined as a full heat absorption phase (endothermic) and a full heat release phase (exothermic).

(10) The heat absorption phase allows for the transfer of heat into the SMA material by setting the stress applied to the material to an appropriate value, the lower value used in the cycle of operation. This results in the activation temperatures, Austenite start (A.sub.s) and Austenite finish (A.sub.f), being set to a value below the input temperature of fluid stream. The thermal gradient present therefore allows the heat to transfer into the SMA via conduction and convection. Once the material has fully or partially transformed to austenite (i.e. the temperature of the SMA material is above A.sub.f), the heat absorption phase is complete.

(11) The heat release phase begins after increasing the stress on the austenitic SMA material. This raises the activation temperatures, Martensite start (M.sub.s) and Martensite finish (M.sub.f), for the reverse transformation back to martensite. Once the value of M.sub.s is raised above the input fluid stream temperature, the reverse transformation begins. It will only complete in full when Mt also raised above the fluid stream temperature. The latent heat is then released by the SMA material and into the fluid stream, raising its temperature. The rate at which the release of heat occurs is a function of the thermal gradient and various thermodynamic conditions of the fluid stream, such as flow rate, turbulence etc.

(12) A single fluid temperature input can be used in the system, and a series of valves can be used at the output of the chamber to direct the colder fluid flow from the heat absorption phase back to source, while directing the warmer fluid from the heat release phase to the heating target. Multiple working fluid temperature inputs can also be used.

EXAMPLE EMBODIMENT

(13) FIG. 1 illustrates a Heat Pump system incorporating an SMA drive engine according to one embodiment of the invention. The engine is in the form of a SMA or a NTE core which comprises a plurality of elements or wires arranged substantially in parallel to define a core. FIG. 2 illustrates a work flow diagram showing different states of the SMA drive during operation.

(14) With respect to FIGS. 1 and 2 the step-by-step process of the heat pump system in operation is now outlined in detail.

(15) As shown in FIG. 1 a low-pressure accumulator pressure 1 is applied to a SMA core 2a or bundle in a martensite state. Fluid is input into a chamber housing the SMA core 2a which is at a higher temperature than the A.sub.s and A.sub.f, therefore allowing the SMA material to absorb the heat. In a preferred embodiment the SMA core 2a comprises mechanically interlocked SMA elements or wires 3a arranged in bundles, where each wire 3a in the bundle acts to supports other against buckling due to the tight assembly arrangement. The fluid in the present case travels externally over the wires 3a as is transits through the bundle. This is fundamentally important as it maximises surface, and therefore heat transfer area. The wires are mechanically interlocked with each other to aid power density and resistance to buckling. The wires are densely packed bundles of wires/rods which will significantly improve efficiency such that the wires are actuated in a vertical plane when the wires are arranged in a vertical position as shown. The system shown in FIG. 1 effectively provides a closed loop heat pump system.

(16) As a result of a low-pressure applied (and hence low stress) on the wires, both the Austenite start (A.sub.s) and Austenite finish (A.sub.f) temperatures are lowered proportionally, making a full martensite to austenite transformation easier to achieve with the lower input fluid temperature. The SMA wires in the core are heated to point A.sub.f, as shown in FIG. 2. A.sub.f is the point of maximum contraction of the wire by design—representing a partial or full martensite to austenite transformation.

(17) During this endothermic transformation, the wire, or wires making up the core, absorbs a substantial quantity of latent heat from the input fluid stream. The contracting wires pump low-pressure hydraulic fluid out of a hydraulic chamber 4 into the low-pressure accumulator 1.

(18) A pump 5 is utilised to pump the hydraulic fluid from the low-pressure accumulator 1 to a high-pressure accumulator 6. The pump 5 can be sized according to the output power of the cores 2a, 2b plus the hydraulic transmission losses through the system from the accumulator to the core—this allows for enough pressure in the high-pressure accumulator 6 to provide enough pressure to the core 2b during the heat absorption phase of the cycle (endothermic).

(19) The high-pressure accumulator 6 is applied to the cores 2a, 2b to achieve wire elongation via the release of latent heat. Effectively, the austenite to martensite transformation occurs. Input fluid (which can be the same temperature as the original input fluid) is input into the chamber which can now absorb the heat released from the SMA wires.

(20) As a result of the high-pressure on the SMA wires, the activation temperatures for Martensite start (M.sub.s) and Martensite finish (M.sub.f) are raised proportionally above the fluid temperature. This allows a complete A to M transformation to occur, thus elongating the wire and releasing the heat into the fluid stream, increasing its internal energy and giving rise to an increase in temperature.

(21) The SMA wire temperature in the core is brought to point M.sub.f, as shown in FIG. 2. M.sub.f is the point of wire elongation by design—representing a partial or full reversal in the transformation from A to M.

(22) The high-pressure in the hydraulic chamber 7 can physically tension the wires. This reaction is exothermic, and both the sensible heat and full or partial quantity of latent heat are released into the fluid stream, giving rise to an increase in temperature of the fluid. The high-pressure in the hydraulic chamber 7 must now be transferred to the low-pressure hydraulic chamber 4 in order to allow the cycle to restart.

(23) It will be appreciated that the reactions described above happen in a non-antagonistic cycle, either sequential or in series. The fact that the system can recover mechanical energy in a decoupled manner using the hydraulic transmission rather than through a direct coupling is technically advantageous. The system releases latent heat, which heats the wires sensibly, with this being immersed in fluid, then heats the fluid bringing the SMA/heat transfer fluid to a state approaching equilibrium. In effect the wire is technically acting a heat storage unit which can be used as a heater when a change in state occurs.

(24) As there is a pressure differential and flow rate available, a motor 8 can be used to drop the pressure, while converting the power into usable electricity to drive the pumping motor via an electric inverter (a recovery motor). This recovers energy that would otherwise be wasted in the cycle and is used to increase the CoP of the system.

(25) To account for any differences between the heat absorption time and heat release time, an additional set of redundant cores can be used so that the heat absorption time is doubled, or the heat release time is doubled—whichever is slower in any embodiment.

(26) FIG. 3 illustrates a single SMA or NTE core for using in a High Pressure (HP) cycle according to one embodiment of the invention and uses the same reference numerals as FIG. 1. FIG. 3 shows a heater pump system where cool water enters the chamber and warm water exist due to a change in state of the core 2b. In effect the system can be used as an effective heat pump using a single core only.

(27) FIG. 4 illustrates a single SMA or NTE core for using in a Low Pressure (LP) cycle according to one embodiment of the invention and uses the same reference numerals as FIG. 1. FIG. 4 shows a cooling system and works in reverse to FIG. 3. Low-pressure in the hydraulic chamber 4 is transferred to the SMA or NTE core 2a in the core chamber in order to allow a cycle for the core 2a to absorb heat. It will be appreciated that the invention can be applied to refrigeration systems where the system can operate in reverse and the outlet is adapted to output the second fluid at a lower temperature than the input temperature.

(28) In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

(29) The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.