Electrochemical Redox Refrigeration

Indirect Electrochemical Cooling: Model-Based Performance Analysis and Working Fluid Selection

The rising energy demand for cooling and heating requires efficient and sustainable technologies. Vapor-compression systems represent the state of the art but suffer from downscaling limits and maintenance needs. These disadvantages may be overcome by recently proposed electrochemical processes. However, their potential has not been explored systematically. This work quantifies the thermodynamic potential of an indirect electrochemical cooling process that replaces the vapor compressor of a standard refrigeration cycle with an electrochemical cell. An equilibrium-based process model evaluates the process performance of a working fluid, depending on its composition and temperatures in the process. After screening an extensive database for possible working fluids, an electrochemical cooling process is analyzed and optimized for the coefficient of performance (COP) to operate between two heat reservoirs at 20 °C (heat source) and 35 °C (heat sink). The majority of the investigated working fluids yield smaller or similar efficiencies than vapor-compression refrigeration, with COPs between 3.0 and 4.0. However, 35 promising working fluids that achieve higher efficiencies are identified with a COP up to 9.63, corresponding to 49% of Carnot. These working fluids are worthy of further investigation as their use in the electrochemical cooling process possibly outperforms standard vapor-compression refrigeration.

Direct Conversion of Phase-Transition Entropy into Electrochemical Thermopower and the Peltier Effect

A thermocell generates thermopower from a temperature difference (ΔT) between two electrodes. The converse process of thermocells is an electrochemical Peltier effect, which creates a ΔT on the electrodes by applying an external current. The Seebeck coefficient (Se ) of the electrochemical system is proportional to the entropy change of the redox reaction; therefore, a redox system having a significant entropy change is expected to increase the Se . In this study, a thermoresponsive polymer having a redox-active moiety, poly(N-isopropyl acrylamide-co-N-(2-acrylamide ethyl)-N'-n-propylviologen) (PNV), is used as the redox species of a thermocell. PNV2+ dication undergoes the coil-globule phase transition upon the reduction to PNV+ cation radical, and a large entropy change is introduced because water molecules are freed from the polymer chains. The Se of PNV thermocell drastically increased to +2.1 mV K-1 at the lower critical solution temperature (LCST) of PNV. The entropy change calculated from the increment of Se agrees with the value evaluated by differential scanning calorimetry. Moreover, the electrochemical Peltier effect is observed when the device temperature is increased above the LCST. This study shows that the large entropy change associated with the coil-globule phase transition can be used in electrochemical thermal management and refrigeration technologies.

Advancement of Electrochemical Thermoelectric Conversion with Molecular Technology

Thermocells are a thermoelectric conversion technology that utilizes the shift in an electrochemical equilibrium arising from a temperature difference. This technology has a long history; however, its low conversion efficiency impedes its practical usage. Recently, an increasing number of reports have shown drastic improvements in thermoelectric conversion efficiency, and thermocells could arguably represent an alternative to solid thermoelectric devices. In this Minireview, we regard thermocells as molecular systems consisting of successive molecular processes responding to a temperature change to achieve energy generation. Various molecular technologies have been applied to thermocells in recent years, and could stimulate diverse research fields, including supramolecular chemistry, physical chemistry, electrochemistry, and solid-state ionics. These research approaches will also provide novel methods for achieving a sustainable society in the future.

Ionocaloric refrigeration cycle

Developing high-efficiency cooling with safe, low-global warming potential refrigerants is a grand challenge for tackling climate change. Caloric effect-based cooling technologies, such as magneto- or electrocaloric refrigeration, are promising but often require large applied fields for a relatively low coefficient of performance and adiabatic temperature change. We propose using the ionocaloric effect and the accompanying thermodynamic cycle as a caloric-based, all-condensed-phase cooling technology. Theoretical and experimental results show higher adiabatic temperature change and entropy change per unit mass and volume compared with other caloric effects under low applied field strengths. We demonstrated the viability of a practical system using an ionocaloric Stirling refrigeration cycle. Our experimental results show a coefficient of performance of 30% relative to Carnot and a temperature lift as high as 25°C using a voltage strength of ~0.22 volts.