A team from Stanford University is proposing continuous electrochemical heat conversion as a direct method of harvesting heat to electricity. Using flow cells and solid-oxide cells respectively, they have built proof-of-principle heat harvesters operating both near ambient conditions, and at high temperatures.

In a paper in the RSC journal Energy & Environmental Science, they report experimentally demonstrating continuous electrochemical heat engines based on two redox-active working fluids separated by ion-selective membranes.

The electrochemical heat engines can use any redox-active fluids, including gases, not just ones directly conducting electrical charge, and do not rely on fixed-temperature phase transitions.

They show that the geometry of electrochemical cells yields to optimization of power and efficiency, allowing the avoidance of the constraints of cycle-based and thermoelectric direct heat harvesters. They conclude that high efficiencies and relevant power densities are both achievable for continuous electrochemical heat engines.

Continuous electrochemical heat engine. (a) temperature-entropy diagram, and system diagrams of the continuous electrochemical heat engine in (b) cyclical operation, and (c) continuous operation. Two stacks of electrochemical cells are connected in series, one immersed in a hot thermal reservoir at temperature TH, and the other immersed in a cold thermal reservoir at temperature TC. The entropy of the electrochemical redox reactions yields a potential difference ΔV across the two stacks of electrochemical cells. The recuperative heat exchange between the two electrolyte streams is shown as QHX. Poletayev et al. Click to enlarge.

Fundamentally, a direct thermal-to-electric heat engine converts entropy to an electric potential, while allowing charge transport and irreversible thermal parasitics. The intensive properties responsible for the three processes are thermopower α, thermal conductivity κ, and electrical resistivity ρ. Thermoelectric (TE) heat engines, together with thermogalvanic (TG) analogues, are widely considered to be the most promising candidates for converting distributed heat sources to electricity. For all, the optimization of device efficiency and maximum power reduces to the figure of merit z=α²/κρ composed exclusively of the intensive properties of one material although the geometry and sizes of the elements also determine their performance. Since these material properties are often coupled, the optimization of z is inherently constrained. This constraint has made it difficult to compete with traditional thermofluid cycles, and therefore limited their adoption in practical systems. … the main challenge in direct heat-to-electricity conversion is to experimentally realize continuous thermodynamic cycles that sidestep the coupling of entropy, heat and charge transport and operate across a broad range of temperatures.

In this work, we leverage the progress in redox flow batteries and fuel cells to experimentally demonstrate continuous electrochemical heat engines based on two redox-active working fluids separated by ion-selective membranes. In a continuous cycle, a stack of electrochemical cells runs a redox reaction at a hot temperature T_H, gaining entropy, while another simultaneously runs the reverse reaction at a cold temperature T_C, expelling entropy. The redox reactions occur such that charge neutrality is achieved via ion transport through ion-selective membranes in the cells.

Because the potentials for these redox pair reactions are temperature dependent, a potential difference is also produced between T_H and T_C when the cells are connected in series via external leads. This potential difference combined with electron flow produces electrical power, making this system a continuous electrochemical heat engine. In addition to the charge flow loop between the sets of redox reactions at T_H and T_C, there is also a closed mass flow circuit. The fluid electrolytes carrying redox species on either sides of the membranes flow in separate mass-flow loops between T_H and T_C. In between these two thermal reservoirs, the respective hot and cold streams exchange heat through recuperative heat exchangers.

—Poletayev et al.

The optimization of the heat engines does not depend on phase transitions, variation of material parameters with temperature, temporal constraints—e.g. cycling rate—or fundamental trade-offs—e.g. the need for an electron crystal phonon glass. Only the stability and kinetics of the redox fluid and the electrochemical cell set the cycle temperature, ranging from well below ambient to ∼1,000 °C.

The team built two proof-of-concept energy harvesters that operate in very different temperature regimes. For the low-temperature system, the aqueous V^2+/3+|| Fe(CN)₆ ^3-/4-redox couples were chosen because of their high charge capacity, facile redox behavior, and large |α₁-α₂|.

For the high-temperature system, the team used oxygen gas as an entropy carrier via the H₂/H₂O||O₂ couples, mediated by solid-oxide electrochemical cells.

Based on system modeling, and accounting for practical losses, the continuous electrochemical heat engine can scalably reach maximum power point efficiencies well over 30% of η_c under diverse operating conditions. Furthermore, the ability to form stacks of cells in series at each temperature to increase voltage without the coupling to heat losses is fundamentally different from that in TE systems: it enables further minimization of heat leaks while independently optimizing the electrical performance. It is also worth noting that stacks of multiple electrodes can achieve much higher areal power densities than individual cells. For example, with 100 cells per stack … the device-level areal power density is 4 W cm^-2, a quantity that is more comparable with the areal power densities generally reported for TE devices.

By decoupling thermal and electrical entropy generation pathways, we demonstrate effective energy conversion in regimes heretofore inaccessible to TE, TG, regenerative, or other thermal-fluid heat engines. While electricity generation is demonstrated in this work, operating these systems in reverse could in principle enable electrochemical refrigeration as well. In addition to the significant flexibility in size and form, a vast parameter space exists for the optimization of working fluids: redox transformations of pure substances, ion-transporting liquids, and gas-phase reactants could all be used. With the development of suitable redox chemistries and flow systems, continuous electrochemical heat engines could fill a vital missing space in the existing landscape of energy harvesting technologies.

—Poletayev et al.

Resources

• Andrey D Poletayev, Ian McKay, William C Chueh and Arunava Majumdar (2018) “Continuous Electrochemical Heat Engines” Energy Environ. Sci. doi: 10.1039/C8EE01137K