SUPPLEMENTARY INFORMATION

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Vincent Ray
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ARTICLE NUMBER: 16134 DOI: 10.1038/NENERGY.2016.

Supplementary Figure 1. Schematic representation of the elastocaloric cooling cycle with an example of the adiabatic temperature changes during loading and unloading measured with an IR camera.

1 ARTICLE NUMBER: DOI: /NENERGY A regenerative elastocaloric heat pump Jaka Tušek, Kurt Engelbrecht, Dan Eriksen, Stefano Dall Olio, Janez Tušek and Nini Pryds Supplementary Figures C a T0+Tad b T0 Qout d T0 c T0-Tad C Qin Supplementary Figure 1. Schematic representation of the elastocaloric cooling cycle with an example of the adiabatic temperature changes during loading and unloading measured with an IR camera. a, When a shape memory alloy in the austenitic phase is axially stressed, an exothermic austenitic-martensitic transformation occurs, which under adiabatic conditions causes the material to heat up by an adiabatic temperature change (ΔT ad ). b, This heated material then rejects heat to the surroundings (Q out ) and cools back to ambient temperature (T 0 ). c, When the stress is removed, the crystal structure transforms back to the austenitic phase and the material cools down by an adiabatic temperature change (ΔT ad ). d, This cooled material is now able to absorb heat from a load (Q in ) and heats up back to ambient temperature (T 0 ). NATURE ENERGY 1

DOI: 10.1038/NENERGY.2016.134 Supplementary Figure 2.

Photos of the laser-welded elastocaloric regenerator. a, A photo of the regenerator made of nine dog-bone shaped plates without the housing.

2 DOI: /NENERGY Supplementary Figure 2. Technical drawing of the dog-bone shaped sample used in the elastocaloric regenerator with noted dimensions (in mm). a b c Supplementary Figure 3. Photos of the laser-welded elastocaloric regenerator. a, A photo of the regenerator made of nine dog-bone shaped plates without the housing. b, A photo of the regenerator with the housing made of flax fibres impregnated with silicone with attached fluid-flow ports. c, The view of the plates in the regenerator through a fluid-flow port. 2 NATURE ENERGY

DOI: 10.1038/NENERGY.2016.134 SUPPLEMENTARY INFORMATION Supplementary Figure 4. Photo of the entire elastocaloric device together with the hydraulic set-up. Supplementary Figure 5.

The superelastic behaviour of polycrystalline Ni-Ti alloys decreases, but it reaches a stabilized response after a certain number of cycles.

3 DOI: /NENERGY SUPPLEMENTARY INFORMATION Supplementary Figure 4. Photo of the entire elastocaloric device together with the hydraulic set-up. Supplementary Figure 5. Mechanical stabilization of the elastocaloric regenerator with 200 quasi-isothermal loading-unloading cycles until functional stability is reached. The superelastic behaviour of polycrystalline Ni-Ti alloys decreases, but it reaches a stabilized response after a certain number of cycles. It is therefore important to stabilize the functional behaviour of the elastocaloric regenerator prior to its use in the device in order to assure reproducible operation. It is seen that the critical stress of the transformation plateau, in particular during the forward transformation decreases; the hysteresis area decreases; the initially flat transformation plateau takes on a positive slope and an irreversible strain (plastic deformation) increases due to the transformation strain. The stabilization conditions are explained in the Methods section in the main text. NATURE ENERGY 3

DOI: 10.1038/NENERGY.2016.134 Supplementary Figure 6. Average adiabatic temperature changes in each plate in the regenerator during loading and unloading up to 1.

4 DOI: /NENERGY Supplementary Figure 6. Average adiabatic temperature changes in each plate in the regenerator during loading and unloading up to 1.25 % measured with thermocouples (TC) and an IR camera (IR). With the aim of evaluating if an elastocaloric regenerator stacked and made with laser-welding technology can provide equal stress and strain distribution among all the plates in the stack and therefore equal ece, we analyzed its adiabatic temperature changes. For that purpose, a test regenerator (with five Ni-Ti plates) was constructed using the same methods and the same stabilization technique as described in the Method section in the main text. The adiabatic temperature changes of the plates in the regenerator were measured with thermocouples (type K with a wire diameter of 0.25 mm) and an IR camera (FLIR SC5200). On each side of each plate, three thermocouples were mounted (one near the top, one at the middle and one near the bottom of the plate) using highconductivity thermal paste. The adiabatic temperature changes shown for each plate represent the average adiabatic temperature changes over six points (three thermocouples on each side of the plate) during one loading-unloading cycle. Due to the relatively small applied strain, which corresponds to the beginning of the transformation plateau, the strain and the temperature changes are not homogenously distributed over each plate. This results in relatively large differences in adiabatic temperature changes measured with each thermocouple along a single plate (as also noted with the error bars). The IR measurements were performed with a micro lens mounted on the camera, which enables a focusing area of approximately 3 mm x 2 mm. The adiabatic temperature changes with the IR camera were measured at the edges of the plates at three different locations along the regenerator (near the top, middle and bottom, respectively). Similar to the case of thermocouples, the adiabatic temperature changes are averaged over the measured areas of each plate of one loadingunloading cycle. It is evident that all the plates in the regenerator have nearly equal average adiabatic temperature changes (measured with TCs and IR) and therefore an equal strain distribution among all the plates in the stack is expected. In order to capture fast temperature changes, thermal images were acquired with a frame-rate up to 150 Hz (and a spatial resolution of 10 µm), while the sampling frequency of the thermocouples was 10 Hz. The edges of the plates in the regenerator were coated with a thin layer of graphite spray (with an emissivity of 0.95) to improve thermal emissivity for better thermal imaging. The ece of the regenerator was evaluated at an applied strain of 1.25 % at a strain-rate of 0.05 s NATURE ENERGY

5 DOI: /NENERGY SUPPLEMENTARY INFORMATION Supplementary Figure 7. Adiabatic temperature changes of Ni-Ti plate applied in the regenerator as function of the applied strain changes for loading and unloading measured with an IR camera (FLIR SC5200) at a strain-rate of 0.1 s -1. The adiabatic temperature changes during loading are higher compared to the unloading. There are two sources of these temperature irreversibilities. The first is the stress hysteresis and related entropy generation and the second is because the reverse transformation immediately after unloading is not fully completed, which occurs at applied strains above 2 % (at this temperature 22 C) where a slight temporary bending of the sample occurs. This happens as the temperature difference between the austenitic finish temperature (A f ) and the working temperature is smaller than the negative adiabatic temperature change. When the material s temperature drops below the A f, martensite during unloading is not fully transformed, which causes bending of the sample. The original shape is restored and complete transformation occurs due to the heat transfer with ambient causing the sample to heat above the A f. It should be noted that due to different stabilization techniques the adiabatic temperature changes of a single plate shown here and ones shown for the elastocaloric regenerator in Supplementary Fig. 6 cannot be directly compared. Data from ref. 1. NATURE ENERGY 5

6 DOI: /NENERGY Supplementary Figure 8. Stress-strain behavior of the elastocaloric regenerator at different evaluated applied strains during its operation (at an operating frequency of Hz). Supplementary Figure 9. Temperatures measured during the device operation at all four fluid ports. a, Temperature profiles as steady state conditions are reached with the noted temperature span (difference) needed for performance calculation as explained in Supplementary Methods. b, Temperature profiles of the first few cycles with noted operational steps of applied elastocaloric cycle. An example shown here corresponds to an applied strain of 3.4 % and an operating frequency of 0.25 Hz. 6 NATURE ENERGY

7 DOI: /NENERGY SUPPLEMENTARY INFORMATION Supplementary Figure 10. Temperature span and corresponding heating power and COP values as functions of the ratio of displaced fluid volume (V*) for an operating frequency of Hz and a strain of 1.7 %. Before the elastocaloric device was tested at different operating frequencies and different applied strains (shown in the main text), we evaluated the impact of the displaced fluid volume through the regenerator in one cycle (equivalent to the volumetric fluid-flow) as this has been shown to be of extreme importance for active magnetic regenerators in magnetic refrigeration. The ratio of the displaced fluid volume is defined as the ratio of the fluid volume pumped through the regenerator in a single fluid flow period to the empty volume of the regenerator (occupied by the fluid). It is evident that the highest temperature span was measured in the case of V*=1 (the entire fluid in the regenerator was exchanged during a single fluid flow period), while the highest specific heating power and COP values were obtained at larger fluid flow (V*=1.5). Therefore, for further tests shown in the main text (with the goal of reaching higher temperature spans) the ratio of displaced fluid volume equal to one was used. Supplementary Table Supplementary Table 1: Geometrical and material properties of the elastocaloric regenerator system. material Ni 55.8 Ti 44.2 austenitic finish temperature 280±5 K mass of active material 5.8 g plate thickness 0.2 mm regenerator porosity 50 % number of plates 9 regenerator (gauge) length 50 mm regenerator width 10 mm heat transfer fluid water NATURE ENERGY 7

8 DOI: /NENERGY Supplementary Reference 1. Tušek, J., Engelbrecht, K. and Pryds, N. Elastocaloric effect of a Ni-Ti plate to be applied in a regenerator-based cooling device, Science and Technology for the Built Environment, 22:5, (2016) 8 NATURE ENERGY

Design of a hydraulically driven compressive elastocaloric cooling system

Science and Technology for the Built Environment (2016) 00, 1 7 Copyright C 2016 ASHRAE. ISSN: 2374-4731 print / 2374-474X online DOI: 10.1080/23744731.2016.1171630 Design of a hydraulically driven compressive