Shape Memory Based Self-Powered Fluid Pump

In the range of 25°C - 80°C (ultra-low grade heat), a large quantity of waste heat from various processes is available unused. Special alloys made of nickel and titanium, so-called Shape Memory Al-loys (SMA), could be an alternative technology to Organic Rankine Cycles to make this energy usable in the low power range. The "THEAsmart 2" research project is therefore investigating the service life and energy lifecycle of this material to test the benefits of shape memory alloys in energy recovery and the efficiency levels that can be achieved. To this end, a demonstration prototype is being built that converts thermal energy into rotary motion. The next step is to link the demonstration prototype with a conventional fluid pump to create an SMA fluid pump that is driven by the thermal energy of the fluid to be pumped. The advantage of such a pump would be that it would be energy-independent, i.e. it would be operated solely by the thermal energy of the fluid without an electrical connection. Furthermore, such a pump could contribute to energy savings if it is used in cooling circuits in which the thermal energy of the fluid is the waste product from another process. In this case, it replaces an electric pump and utilizes the "waste product" heat. The aim of the project is to investigate how and whether coil springs made of shape memory alloy are suitable for energy recovery. This is considered via the energy lifecycle: if more energy is required to manufacture a spring than this spring can convert kinetic energy from thermal energy in its lifecycle, then its use for energy recovery does not make sense in principle. As a secondary result of this research, statements about the efficiency of shape memory alloy coil springs and statements about their service life are expected.


Introduction
Heat is often lost unused as a waste product.This applies to many areas of our society, such as energy, industry, households, mobility, communication and commerce.Yet waste heat has great energy potential.Large quantities of fossil fuels could be saved if the energy in waste heat could be converted into another form of energy and thus made usable.A recent study [1] shows that in industries of the European Union in 2018, around 70%-75% of unused waste heat energy falls into the "ultra-low grade heat" range (25°C -80°C).
There are some tried-and-tested energy recovery processes that work at significantly higher temperatures.These energy recovery systems use a large proportion of the heat energy -but not all of it.At the end of the energy conversion process, there is always residual heat in the "low grade heat" (80°C -230°C) or "ultra-low grade heat" (25°C -80°C) range, which can no longer be utilized [2].The energy level in the "ultra-low grade heat" range is not particularly attractive, but the quantitative amount of energy available in this range is incentive enough for further research.
There are already proven heat recovery systems in the field of "low grade heat" and "ultra-low grade heat".Organic Rankine Cycles (ORC), which are already available from 65°C and 20kW output [3], are particularly worth mentioning here.However, there is great potential for processes that deliver significantly smaller amounts of waste heat (<<20kW) in the ultra-low grade heat range.
This power and temperature range can be utilized with Shape Memory Alloys (SMA).As part of the "THEAsmart 2" research project, scientists and companies are investigating this potential to find out whether the waste product heat in the ultra-low grade heat range can be utilized with small amounts of available waste heat by innovative technology.
The THEAsmart 2 sub-project "Fluid pump" is presented below.The thermal energy of a warm fluid is to be used to pump this fluid.The THEAsmart 2 project aims to prove that SMA is suitable for converting small amounts of heat into mechanical energy.Furthermore, the efficiency, service life and life cycle of SMA are to be examined in detail to be able to make a statement as to whether the material is suitable for energy recovery.

Potentials in Automation Technology
Research is currently still being carried out into the theoretical basis for the service life of SMA springs.It still needs to be proven that the service life of SMA materials allows long-term, cyclical and low-wear use as an actuator.If this can be proven, then it is particularly interesting for industrial application technology that an SMA pump can operate without a power supply.Use in explosion protection areas is conceivable or in areas where an electrical power supply can only be installed with great effort.
The use of an SMA fluid pump can also be interesting from an energy point of view.Whenever heat has to be conveyed to a cooler as a waste product from a process, an SMA fluid pump could replace an existing electric pump and utilize the waste product heat from the industrial process.
In contrast, SMA pumps for applications where the heat is not a waste product are usually not useful from an energy point of view.In this case, the heat generation machine would have to provide more energy for the circuit so that the pump can be supplied with this additional energy -this usually shifts the efficiency of the heat generation machine in an energetically unfavorable direction.Shape memory alloys can generally be adapted to specific thermal conditions via the composition of the alloy.The material NiTi#6 [10] used in this project reacts in the range between approx.35°C and approx.65°C with a change in shape (for details on the transformation process, see chapter 4.1).A temperature gradient of 30K between feed and return must be provided for the usability of NiTi#6 [10].This value depends on the alloy and can vary considerably depending on the heat treatment and alloy composition.

History and Basic Work
In the research project THEAsmart (1), the basic framework conditions and interdependencies of technical usability were demonstrated on the topic of "SMA in energy recovery".Potentials were identified that make it technically possible, economically viable and ecologically sensible to use heat sources with the aid of SMA systems in various applications [4].
For this reason, the follow-up project THEAsmart 2 will continue to research the concrete potential of two prototypes.The prototypes are to be developed for different areas of application.On the one hand, thermal energy is to be converted into kinetic energy.For this purpose, an SMA fluid pump is being developed that uses the thermal energy of the fluid to be pumped.In a second concept, thermal energy is to be converted into electrical energy (SMA Energy Harvester).Both prototypes first convert the thermal energy into rotary motion via SMA actuators to then utilize this kinetic energy further.So-called "wet actuators", SMA actuators that are activated by means of fluids, are further optimized for energy conversion in the already patented applications from the THEAsmart project (1).

Shape Memory Alloys
Shape memory alloys can be used both as actuators and as sensors, which is why they are classified as "smart materials".The sensor function is not relevant in the context of this paper but should be mentioned for the sake of completeness.The material can be used as an actuator because it "remembers" its original shape after plastic deformation and returns to this original shape as soon as it is heated to a certain temperature.This is possible because shape memory alloys have two different crystal structures in the solid state, depending on the temperature.Shape memory alloys are defined in particular by their transformation temperature.The transformation temperature describes the temperature at which the transformation between the two crystal structures takes place.In the "cold" state, at temperatures below the transformation temperature, the material has a martensitic crystal structure.In the "warm" state (above the transformation temperature), it has an austenitic crystal structure.In the martensitic state (cold), the material can be plastically deformed and remains in this deformed state until it is brought into the austenitic state.When the material is heated above the transformation temperature, the crystal structure becomes austenitic and the material deforms back to its original shape [5]  The composition of the alloy can be used to influence the temperature at which the transformation takes place, i.e. the material deforms back to its original state.The alloy most commonly used in the ultra-low grade temperature range consists of around 55% nickel and around 45% titanium, which is why this shape memory alloy is often referred to as NiTi or NiTiNOL.NiTiNOL stands for Nickel Titanium Naval Ordnance Laboratory and was developed in 1958 at the Naval Ordnance Laboratory (USA) by William J. Buehler and Frederick Wang [6].
Shape memory alloys are currently used in medical technology in particular.However, this material has not yet been widely used in industry and building services.
In primary forming, the alloy is melted, cast, and usually sold as a semi-finished product in the form of wires or sheets ( [7], p.12).To imprint the application-specific shape, the semi-finished product must be fixed in the desired shape and heat-treated.After heat treatment, the product will deform back into the imprinted shape when heated to the transformation temperature.It is important to "train" the material after the shape has been imprinted.This is a thermomechanical process that prevents the SMA component from exhibiting shrinkage behavior (see Fig 2).and then brought into the austenitic state so that it deforms back.

Stress Included Martensite
Training: during this training, the material is mechanically deformed cyclically in the austenitic state (hot) without allowing it to cool down to the martensitic state.
3. Thermomechanical Training: a combination of 1.) and 2.): here the material is formed in the austenitic state (hot), held and allowed to cool to the martensitic state.
The literature states that at least 20 cycles should be run through for training methods 1.) and 2.) ([8], p.29).The number of cycles for training method 3.) is not further specified.

Wear on Shape Memory Alloys
Shape memory alloys are naturally subject to wear.The more often the transformation of the crystal structure has taken place, the smaller the force with which the material deforms back into its original shape.
It is assumed that the alloy is fatigued after a maximum of 10 6 transformations ([7], p.8 f.), but this has yet to be proven by a series of tests.Furthermore, only a few studies are known to date that deal with whether the shape of SMA can be reimprinted after they have been worn.As both nickel and titanium are very expensive and energy-intensive to produce, the repeatability of shape reprogramming would be very relevant in terms of the energy lifecycle.

Project Targets
The aim of the THEAsmart 2 research project is to determine the basic framework conditions for the technical use of shape memory alloys for energy recovery.The focus here is particularly on the service life and efficiency of the material.Due to the limited service life and the very energy-intensive production of SMA materials, it is also necessary to clarify whether the shape of a worn material can be reimprinted by renewed heat treatment and how the wear behavior develops after this new shape reprogramming.
A test rig is therefore being built with which the service life of the SMA springs can be examined in long-term wear tests.To do this, it is necessary for the springs to have the shortest possible cycle times in order to be able to run through many cycles in a short time.On the one hand, the service life of the springs and the wear of the springs over the entire service life should be documented.Furthermore, measurements should be taken during the entire use of the springs to allow conclusions to be drawn about the efficiency and behavior of the springs.
The worn springs are then to undergo a new shaping heat treatment.The aim is to find out whether and how effectively reshaping works.
In addition to the analysis of the efficiency and statements on whether the worn material can be reused, a lifecycle analysis will then be carried out to show whether more energy can be recovered from the material over its entire lifetime from ore extraction to final failure via the shape memory effect.
A defined amount of energy is required to produce the 1m³ NiTi SMA.If this 1m³ SMA can convert more mechanical energy from heat over its entire life cycle, then the material is basically suitable for energy recovery.Energy (E) per volume (V) is defined as energy density ().Accordingly, the following must be proven:

Series of Tests / Experiments
Basically, it must be determined how long an SMA spring remains usable and how much mechanical energy it can convert from thermal energy.For this purpose, several SMA springs are to be deformed at the highest possible frequency, first in the martensitic phase and then brought into the austenitic phase for re-deformation.A cycle therefore includes the following: Cooling the SMA spring (martensite), plastically deforming the SMA spring, heating the SMA spring (austenite), allowing the SMA spring to deform back to its original shape.In several series of tests, the SMA springs should then undergo as many cycles as possible until they reach the wear limit.
During the operation of the springs, the amount of thermal energy supplied to each spring and the amount of mechanical energy released should be recorded.
Different series of tests are to be carried out in which the temperatures for the austenite and martensite phases are varied.Furthermore, the times for the temperature transition are to be adjusted so that SMA springs are only partially transformed in a targeted manner, i.e. do not fully enter the austenite phase or the martensite phase.Furthermore, series of tests are to be carried out in which the elongation of the spring is varied.The aim is to vary the absolute elongation of the spring, but also to document how the spring behaves in the long term when it is mechanically prevented from fully deforming back to its rest position in the austenitic state.
In addition, an attempt will be made to restore the original shape of worn SMA springs through a further heat treatment process.If this is successful, the "re-programmed" springs will be subjected to renewed long-term wear tests.

Experimental Setup
In the THEAsmart 2 project, SMA coil springs (tension springs) are used to convert heat into kinetic energy.Winding the SMA wire into a spring allows significantly greater actuator travel, which considerably simplifies the design of the test stand and the adjustment options required for the test series.
SMA coil springs are often reset in the martensitic state (cold) by a conventional coil spring ([8], p.116).Preliminary tests in the THEAsmart 2 project have shown that an austenitic (hot) SMA tension spring provides several times the force required to plastically deform a structurally identical spring in the martensitic (cold) state.The force of a heated (austenitic) tension spring is sufficient to elongate one or more cold (martensitic) tension springs made of the same material.This allows two SMA springs to work against each other.
In the THEAsmart 2 project, a demonstration prototype of a fluid pump based on SMA tension springs is being built.The thermal energy of the fluid to be pumped is used to activate the SMA tension spring in a "warm phase".Furthermore, the spring is cooled down under the conversion temperature with colder fluid in a "cold phase".The force generated by the SMA tension springs is converted into a rotary motion via a crankshaft, to which a conventional fluid pump can be connected.
The direct contact of liquid and SMA material allows rapid heat transfer and therefore rapid activation or deactivation of the SMA spring.Due to the high speeds, compared to activation in gas, many cycles are possible in a short time in order to carry out wear tests on the material.
An SMA tension spring is installed in a cylinder with a viewing window in which it can be alternately sprayed with hot and cold water, see Fig 3.As it is generally known that the SMA coil springs are subject to wear, the test setup was planned to be variable accordingly: The eccentric adjustment can be moved on the crankshaft and the cylinder housing can be moved on the frame (Fig 6).This allows the amplitude of the spring extension and offsets for the spring return to be adjusted.The offsets are important because the tension springs no longer fully compress with increasing wear.The following parameters are recorded to determine the key figures: 1. point in time 2. angle of rotation of the crankshaft 3. eccentricity of the crankshaft (corresponds to the piston stroke) 4. force acting on the piston rod 5. fluid temperature shortly before contact with the SMA spring 6. fluid temperature shortly after contact with the SMA spring The rotational speed of the crankshaft can be calculated using the time and angle of rotation.The torque on the crankshaft can be calculated from the vectorial sum of the piston rod forces, the rotational speed and the lever arm (half eccentricity).Friction losses at the crankshaft have no influence on the torque, as the piston rod force applied serves as the basis for the calculation.This allows the converted mechanical energy to be determined.Small friction losses due to the linear guidance of the piston rod must be accepted.In order to minimize these friction losses, great importance was attached to correspondingly low-friction material pairings in the design.
The amount of energy added to the SMA material can basically be determined in two ways: either via the flow rate of the fluid and the temperature difference before and after the SMA spring or via the mass (specific heat capacity) of the SMA spring and the temperature difference between the martensite and austenite phase.Both are difficult to measure with sufficient precision in the current test.
For the temperature difference measurement in front of and behind the SMA spring, the absolute temperature measurement must be extremely precise.For the direct temperature measurement at the SMA spring, measurement during operation is technically very challenging.The very filigree cable of the thermocouple would be exposed to constant movement because the thermocouple would be mounted directly on the moving spring.A permanent measurement would therefore not be reliable.
As both measurement methods have their advantages and disadvantages, both methods are used in parallel.Afterwards, it is verified which measurement method provides the more stable values.
The measurement of the energy supplied is only required to determine the efficiency of the SMA material.This is only a secondary objective of the project.The main objective of proving whether the material is fundamentally suitable for energy recovery is not affected by the failure of both meas-urement methods for determining the energy supplied.To make this statement, the energy required to produce the material must be compared with the amount of energy that could be mechanically extracted from the material during its entire service life.

Previous Results
The training behavior of the SMA springs was analyzed in preliminary tests.Wire with a diameter of 1 mm (manufacturer: Fort Wayne Metals, material: NiTi#6 [10]) was used for this purpose, which was imprinted with the shape of a helical tension spring with an outer diameter of 6 mm.Approx.300 -1,000 cycles are required for the shape memory effect training until the spring behavior becomes constant.The two steps at approx.4,000 cycles and approx.14,500 cycles represent a measurement error that can be ignored at this point.The curve shows that the spring has a considerable run-in behavior.
The aim of the training is not to include the very steep part at the beginning of the curve in the test series.It is defined that these coil springs must be trained with at least 500 cycles for the series of tests in the THEAsmart 2 project.

Outlook
The test series on efficiency, service life and energy lifecycle will begin shortly.For this reason, no reliable figures are yet available.
However, undocumented preliminary tests suggest that the efficiency of the SMA coil springs for converting thermal energy into mechanical energy is likely to be below 5%.Compared to ultra-low grade heat Organic Rankine Cycles, this is not a particularly poor efficiency.However, it should be noted that this efficiency is further reduced by the efficiency of the machine in which the SMA springs are installed.
Regarding the energy lifecycle, it is possible that the SMA springs do not generate the amount of (mechanical) energy in their lifecycle that must be invested to produce a spring.However, this depends largely on the maximum cycles that can be achieved and the possibility of shape re-programming.If these estimates are confirmed, SMA coil springs would not be suitable for energy recovery.The advantages of SMA actuators (such as small installation space, low weight, independence from supply energy) would not be challenged by this result.
At the end of the THEAsmart 2 project, a comprehensive energy lifecycle analysis and reliable statements on the service life are expected.
. The transformation is shown in Fig 1 along a temperature axis.

Fig 2 :
Fig 2: Exemplary representation of the training behavior of an SMA-wire 6

Fig 3 :Fig 4 :
Fig 3: Photo of a cylinder with built-in SMA spring (side view)

Fig 5 :
Fig 5: Picture of the experimental setup

Fig 6 :
Fig 6: Sketch of the adjustment options on the cylinder and crankshaft based on a section through the cylinder.

Fig 7 :
Fig 7: Training behavior of a helical tension spring in Shape Memory Effect Training