Knowledge Bank

Improved efficiency of energy systems and development of sustainable, low-carbon-emission energy generation processes are essential for the long-term health of the environment as well as our economic, social and societal endeavors. Heat engines are typically 30% to 55% efficient, and abandon roughly 15 terawatts energy in the form of waste heat to the environment. Classical solid-state thermoelectric generators potentially provide an approach to improve the efficiency of those systems by converting waste heat directly into useful electricity, but have limited applications due to their low energy conversion efficiency. In our research, we invented a liquid-state thermoelectric generator: ElectroChemical ThermoElectric Generators (ECTEGs). Based on ambipolar thermoelectric ions transport due to concentration difference in the water, ECTEGs can generate electric power in a temperature gradient from an electrolyte in which anions and cations have very different ions’ mobilities. If the two types of ions have different mobilities in water solutions, the passage of cations with larger mobility can produce higher positive voltage than that of anions with less mobility. This produces an internal voltage in the presence of a temperature gradient, without depleting the electrolyte chemically. We prove this principle by constructing an ECTEG with HNO3 and Ba(NO3)2 (hydrogen has a much larger mobility than other ions). The system was placed in temperature gradients and according to Le Chatelier’s principle, adding to or removing heat from a reaction (HNO3 with Ba(NO3)2 in our research) can change the chemical equilibrium and thus the hydrogen concentration at different temperatures. The output voltage is measured as function of temperature gradient, and the ECTEG produced 150 micro Volts per Kelvin. We also propose to develop solid-state ionic electrolytes for this purpose. This technology could potentially applicable to harvest low-grade waste heat, and a new theory has been constructed for this new technology.