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Harvesting low-grade heat energy using thermo-osmotic vapour transport through nanoporous membranes

Nature Energy volume 1, Article number: 16090 (2016) Cite this article

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Abstract

Low-grade heat from sources below 100 ∘C offers a vast quantity of energy. The ability to extract this energy, however, is limited with existing technologies as they are not well-suited to harvest energy from sources with variable heat output or with a small temperature difference between the source and the environment. Here, we present a process for extracting energy from low-grade heat sources utilizing hydrophobic, nanoporous membranes that trap air within their pores when submerged in a liquid. By driving a thermo-osmotic vapour flux across the membrane from a hot reservoir to a pressurized cold reservoir, heat energy can be converted to mechanical work. We demonstrate operation of air-trapping membranes under hydraulic pressures up to 13 bar, show that power densities as high as 3.53 ± 0.29 W m−2 are achievable with a 60 ∘C heat source and a 20 ∘C heat sink, and estimate the efficiency of a full-scale system. The results demonstrate a promising process to harvest energy from low-temperature differences (<40 ∘C) and fluctuating heat sources.

A vast amount of untapped energy exists in the form of low-grade heat from sources below 100 ∘C (refs 1,2). Waste heat is one abundant example, with residual heat losses from conventional power plants and the manufacturing industry amounting to more than 8,000 TWh per year in the United States alone3,4. Geothermal wells and solar collectors can also supply large quantities of heat energy, and lower temperature ranges of either of these sources are more widely available and less difficult to obtain than their high-quality counterparts5,6,7. Despite the abundance of low-grade heat, the ability to extract energy from potential sources is limited with existing technologies due to both the small temperature difference available and the temporal variability in heat output from sources such as waste heat or solar thermal. Existing binary cycle systems utilizing organic working fluids typically require heat sources with temperatures greater than 100 ∘C and are unable to tolerate fluctuations in the heat source temperature8,9,10. Solid-state thermoelectric devices have been evaluated for use in low-temperature ranges, but material limitations have resulted in poor efficiencies and low cost effectiveness11,12. Various thermo-electrochemical systems have also been developed for low-grade heat power generation utilizing metal complexation reactions13,14,15 or the temperature dependence of electrochemical redox potentials16,17,18 to generate power. These processes have, however, been introduced only as small-scale prototypes, and their efficiency is typically limited to less than 2% of the Carnot efficiency.

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Figure 1: Working principle of the thermo-osmotic energy conversion system.
Figure 2: Demonstration of power generation using a vapour-gap membrane.
Figure 3: Water flux across a membrane driven by a temperature gradient.
Figure 4: Efficiency of a closed-loop system with heat recovery.

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Acknowledgements

We acknowledge the National Science Foundation Graduate Research Fellowship DGE-1122492 awarded to A.P.S.

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Authors and Affiliations

  1. Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, USA

    Anthony P. Straub, Jongho Lee & Menachem Elimelech

  2. Department of Earth and Environmental Engineering, Columbia University, New York, New York 10027, USA

    Ngai Yin Yip

  3. Department of Civil and Environmental Engineering, Vanderbilt University, Nashville, Tennessee 37235, USA

    Shihong Lin

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Contributions

A.P.S., N.Y.Y., S.L., J.L. and M.E. participated in developing the process and designing the experiments. A.P.S. performed the experiments and analysed the data. A.P.S. and S.L. conducted the system modelling. A.P.S. and M.E. co-wrote the paper. All authors contributed to data analysis, discussed the results and commented on the manuscript.

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Correspondence to Menachem Elimelech.

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The authors declare no competing financial interests.

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Supplementary Information

Supplementary Notes 1–8, Supplementary Figures 1–23, Supplementary Tables 1, Supplementary References. (PDF 1081 kb)

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Straub, A., Yip, N., Lin, S. et al. Harvesting low-grade heat energy using thermo-osmotic vapour transport through nanoporous membranes. Nat Energy 1, 16090 (2016). https://doi.org/10.1038/nenergy.2016.90

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