  There is growing interest in heat pumps based on materials that show thermal
changes when phase transitions are driven by changes of electric, magnetic or
stress field. Importantly, regeneration permits sinks and loads to be thermally
separated by many times the changes of temperature that can arise in the
materials themselves. However, performance and parameterization are compromised
by net heat transfer between caloric working bodies and heat transfer fluids.
Here we show that this net transfer can be avoided-resulting in true, balanced
regeneration-if one varies the applied electric field while an electrocaloric
(EC) working body dumps heat on traversing a passive fluid regenerator. Our EC
working body is represented by bulk PbSc0.5Ta0.5O3 (PST) near its first-order
ferroelectric phase transition, where we record directly measured adiabatic
temperature changes of up to 2.2 K. Indirectly measured adiabatic temperature
changes of similar magnitude were identified, unlike normal, from adiabatic
measurements of polarization, at nearby starting temperatures, without assuming
a constant heat capacity. The resulting high-resolution
field-temperature-entropy maps of our material, and a small clamped companion
sample, were used to construct cooling cycles that assume the use of an ideal
passive regenerator in order to span $\leq$20 K. These cooling cycles possess
well defined coefficients of performance that are bounded by well defined
Carnot limits, resulting in large ($>$50%) well defined efficiencies that are
not unduly compromised by a small field hysteresis. Our approach permits the
limiting performance of any caloric material in a passive regenerator to be
established, optimized and compared; provides a recipe for true regeneration in
prototype cooling devices; and could be extended to balance active
regeneration.
