Reverse electrodialysis heat engine with helium-gap diffusion distillation: Energy efficiency analysis

Abstract

The depletion of energy resources poses a significant threat to the development of human society. Specifically, a considerable amount of low-grade heat (LGH), typically below 100 °C, is currently being wasted. However, efficient utilization of this LGH can relieve energy shortages and reduce carbon dioxide emissions. To address this challenge, reverse electrodialysis heat engine (REDHE) which can efficiently convert LGH into electricity has emerged as a promising technology in recent years. Extensive efforts have been dedicated to exploring more suitable thermal distillation technologies for enhancing the performance of REDHE. This paper introduces a novel REDHE that incorporates helium-gap diffusion distillation (HGDD) as the thermal separation (TS) unit. The HGDD device is highly compact and efficient, operating at a normal atmospheric pressure, which aligns with the operational conditions of the reverse electrodialysis (RED) unit. A validated mathematical model is employed to analyze the impacts of various operating and structural parameters on the REDHE performance. The results indicate that maintaining a moderate molality of the cold stream, elevating the inlet temperatures of hot and cold streams, lengthening hot- and cold-stream channels, and minimizing the thickness of helium gaps contribute to improving the REDHE performance. Especially, a maximum energy conversion efficiency of 2.96% is achieved by the REDHE when decreasing the thickness of helium gaps to 3 mm and increasing the length of stream channels to 5 m.

Key words： helium-gap diffusion distillation (HGDD); reverse electrodialysis (RED); heat engine; low-grade heat (LGH)

Acknowledgements

Financial support was sponsored by the Fundamental Research Program of Shanxi Province, China (No. 20210302123095) and China Postdoctoral Science Foundation (No. 2021M702418).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https:/doi.org/10.1007/s11708-024-0947-3 and is accessible for authorized users.

Competing Interests

The authors declare that they have no competing interests.

Notations

Abbreviations
AEM	Anion-exchange membrane
AGMD	Air-gap membrane distillation
CEM	Cation-exchange membrane
HC	High-concentration
HGDD	Helium-gap diffusion distillation
IEMs	Ion-exchange membranes
LC	Low-concentration
LGH	Low-grade heat
MD	Membrane distillation
MED	Multi-effect distillation
MSRED	Multi-stage reverse electrodialysis
RED	Reverse electrodialysis
REDHE	Reverse electrodialysis heat engine
SGE	Salinity gradient energy
TDEG	Thermal-driven electrochemical generator
TS	Thermal separation
Variables
B	Width, m
C	Concentration of solution, mol·m−3
Cp	Specific heat of salt solution, J·kg−1·K−1
I	Current, A
Jv	Mass flux of the vapor diffusion in the gap, kg·m−2·s−1
L	Length, m
ṁ	Mass flowrate, kg·s−1
m	Molality, mol·kg−1
MNaCl	The relative molecular mass of NaCl
N	Number of gaps in HGDD
Ncell	Number of cells in a RED stack
P	Output power, kW or pressure, Pa
Q	Total LGH consumption by HGDD, kW
α	Permselectivity of both ion-exchange membranes
γ	Water latent heat of evaporation, kJ·kg−1 or mean ion activity coefficient
δ	Thickness of solution compartments, m
δc	Thickness of gap, m
ΔP	Pressure drops, Pa
η	Energy conversion efficiency
ηpump	Efficiency of pump
Φ	Specific volume, m3·kg−1
Superscripts and subscripts
af	Water-vapor-partial pressure at the gap and condensate-water-film interface
b	Brackish solution
c/cold	Cold-stream channel or helium gaps
h	Hot-stream channel
ha	Water-vapor-partial pressure at the gap and porous-medium interface
in	Inlet flow
out	Outlet flow

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