Electrocaloric Bucket Brigade Device

[Enthalpy]

Hello you all!

Electrocaloric materials are commonly ferroelectric materials like Pzt, BaTiO₃, Pvdf... that comprise charged atoms capable to move a bit in the solid, for instance between two places, easily enough to respond to a strong electric field. In the electrocaloric effect, the mobile atoms store heat as they vibrate, but a strong electric field can freeze them in one position, which reduces the material's heat capacity. Resulting temperature changes of a few Kelvin are typical.

The use as a cooling machine is investigated of course, one setup being called "Electrocaloric Oscillatory Refrigeration", which has a magnetocaloric material for one part where electrodes control the heat capacity, and an other part called "regenerator" to store heat. The parts move mechanically along an other, come in thermal contact at some moments, heat capacity is changed within the same cycle, and so the magnetocaloric material takes heat at some points from the regenerator at restores it a bit further, where the temperature is higher, hence "pushing" heat. Both parts can be thin and interleaved to work more quickly, and more refinements - people invent.

I believe a different device with static operation is possible and, in case it's not already known, wish to call it "Electrocaloric Bucket Brigade Device" since it resemble what electronics called a BBD, and whose present version is a CCD, a charge-coupled device.

 

Like a CCD, the device defines zones of bigger capacity (for heat instead of electric charges) by fields applied through many electrodes. It also moves these zones by defining a pattern of electric potentials among the electrodes and "propagating" this pattern (steering in fact) over the set of electrodes.

The similarity has limits:

• The electrode potential steers the capacity in a CCD, here it's the difference of potential between neighbour electrodes. The potentials can alternate to avoid an impractical potential buildup, they can also reverse regularly over time if healthier for the material.
• The relative change in capacity is smaller here. Perhaps the best way to understand the pumping effect is to imagine heat transported at uniform temperature, then realize that a limited temperature difference only superposes a limited leak, allowing pumping.
• In the CCD, the potential controls the charge; a perfect analogy would let the temperature control the heat amount - but here the field controls the heat capacity to make buckets. The temperature drop isn't essential and results just from the locally increased capacity, though my simplified sketch shows only the drop in temperature.

As in a CCD, the lowered buckets can be defined at regular intervals, so that the electrodes connect in a periodic manner to a limited set of control inputs. The next bucket is "lowered" (increased heat capacity) before one is raised.

A magnetocaloric material could build the same device but is less convenient: the electrocaloric material and its electrodes can easily be thin and stacked in a wide and short device that transports much heat.

Material operation near the Curie temperature has been reported, where the electrocaloric effect makes a bigger temperature change. This should work with magnetocaloric materials as well, as I suggested elsewhere. Though, I'd prefer instead to leave in the material for the high-capacity state a smaller non-zero electric field that the thermal energy can more or less overcome, with the hope that more heat is then absorbed at the mobile atoms - this heat being stored in the external electric circuit. That's similar to the pyroelectric operation of the material, and should work at varied temperatures, even cold if adapting there the smaller field. A quick AC field superimposed to the small field that absorbs energy can help the pyroelectric operation at low temperature.

Marc Schaefer, aka Enthalpy

[Enthalpy]

That the BBD works isn't obvious; here's the mode of operation that has convinced me.

Take identical temperatures at the input and output. Drive the electrodes slowly enough that the internal temperature differences are small. Then, the effect of varied electrical field at different buckets is to change the individual amounts of stored heat.

As the pattern of individual electric fields is propagated, so does the pattern of stored heat among the buckets. Considering slices of the adequate number of buckets, combined with the proper spatial period in the pattern of electric field, one gets a stored heat per slice that increases at an instant of the period while only the preceding slice, not the following, loses heat - that is, heat propagates.

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10µm is a usual insulator thickness at capacitors, both with plastic films and ceramic - thinner exists as well and would improve further the voltage and throughput. With 1800kg/m³, 1500J/kg/K and 0.2W/m/K, 1ms suffices to equalize well the temperatures of adjacent buckets after their heat capacities were changed. Depending on the pattern period, this thickness permits operation around 200Hz, to transport several 100W over 1dm².

Smooth field transitions over time would improve efficiency; the transitions can also spread over several buckets. (Please remember each bucket receives a potential difference, and adjacent buckets can alternate the field direction to limit the voltage across the stack). Because the buckets store much capacitive energy and the electrocaloric effect is only a fraction of it, the drive electronics shall better regain the capacitive energy during discharge than dissipate it.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Known processes to stack ceramic and metal layers produce capacitors that are cheap if small. Here's instead a process to stack metallized polymer sheets.

The sheets are stacked in several iterations, as each may be 10µm thin or less but the module several mm thick (no scale at all in the sketches!). Four electrodes make one period on the sketches, and just one period makes the smallest stack, but these figures can vary.

 

 

The electrodes are patterned, by evaporation and deposition through a stencil, or by etching. This provides insulation creep distances and brings contacts to the sides. Stacked electrodes come from different long coils, so their positions must be adjusted by the film's tension - or pattern the electrodes at the last moment, between uncoiling and stacking.

On the sketches, the sheet is folded to make a selective contact among all electrodes driven simultaneously, improved by the deposition of thicker metal once the modules are stacked. Offsetting the folds linders their overthickness; lamination thereafter seems necessary. A good alternative would offset slightly the consecutive sheets and let the metal layers reach the edges where the deposition od thicker metal finally contacts them.

Electric fields are strong, suggesting to impregnate the sheets before stacking, and even to do it in vacuum.

Could the film be folded as a zigzag, so a few rolls make many plies in the stack? Probably. Then I'd interleave several films in varied directions, like 0° 90° or 0° 60° 120°, to ease the folding thickness and offer more access locations for the electrodes. The films can be narrower at the electrodes.

If the cooling modules (or heat pumps) can be slightly cylindrical, an interesting alternative lets make all successive sheets by winding from a single coil - with electrodes either adequately patterned to compensate by their length the evolving coil radius, or by patterning the electrodes between uncoiling the sheet and winding the stack.

Marc Schaefer, aka Enthalpy

[Enthalpy]

The static stack can use liquids as the electrocaloric material if useful. For instance, the liquid can impregnate a possibly thin insulating fabric or felt that holds the electrodes apart and the liquid in place; the electrodes can still be printed on a film, or be a conductive sheet, or be a possibly thin fabric or felt of conductive wires - easier to connect.

Propylene carbonate and similar seem to conduce too much for this use, but other liquids may fit. Oligomers of vinyl difluoride, if isotactic? Cetones, ethers? Oligomers of propylene-1,3-diol, especially methyl terminated?

I imagine a stiff molecule whose dipolar moment orientation in the working electric field makes about kT is more efficient. This needs more alkylene carbonates' 5 Debye, which brings some 0.03*kT.

Marc Schaefer, aka Enthalpy