A nanophotonic solar thermophotovoltaic device

Journal name:
Nature Nanotechnology
Volume:
9,
Pages:
126–130
Year published:
DOI:
doi:10.1038/nnano.2013.286
Received
Accepted
Published online
Corrected online

Abstract

The most common approaches to generating power from sunlight are either photovoltaic, in which sunlight directly excites electron–hole pairs in a semiconductor, or solar–thermal, in which sunlight drives a mechanical heat engine. Photovoltaic power generation is intermittent and typically only exploits a portion of the solar spectrum efficiently, whereas the intrinsic irreversibilities of small heat engines make the solar–thermal approach best suited for utility-scale power plants. There is, therefore, an increasing need for hybrid technologies for solar power generation1, 2. By converting sunlight into thermal emission tuned to energies directly above the photovoltaic bandgap using a hot absorber–emitter, solar thermophotovoltaics promise to leverage the benefits of both approaches: high efficiency, by harnessing the entire solar spectrum3, 4, 5; scalability and compactness, because of their solid-state nature; and dispatchablility, owing to the ability to store energy using thermal or chemical means6, 7, 8. However, efficient collection of sunlight in the absorber and spectral control in the emitter are particularly challenging at high operating temperatures. This drawback has limited previous experimental demonstrations of this approach to conversion efficiencies around or below 1% (refs 9, 10, 11). Here, we report on a full solar thermophotovoltaic device, which, thanks to the nanophotonic properties of the absorber–emitter surface, reaches experimental efficiencies of 3.2%. The device integrates a multiwalled carbon nanotube absorber and a one-dimensional Si/SiO2 photonic-crystal emitter on the same substrate, with the absorber–emitter areas optimized to tune the energy balance of the device. Our device is planar and compact and could become a viable option for high-performance solar thermophotovoltaic energy conversion.

At a glance

Figures

  1. Operating principle and components of the NARO-STPV.
    Figure 1: Operating principle and components of the NARO–STPV.

    Sunlight is converted into useful thermal emission and, ultimately, electrical power, via a hot absorber–emitter. a,b, Schematic (a) and optical image (b) of our vacuum-enclosed devices composed of an aperture/radiation-shield, an array of MWNTs as the absorber, a 1D PhC, a 0.55 eV-bandgap photovoltaic cell (InGaAsSb19, 20, 21) and a chilled water cooling system. c, Absorber-side optical image of an AR (=Ae/Aa = 10 module showing spatially defined MWNTs (Aa = 0.1 cm2) on a tungsten-coated silicon substrate (1 × 1 cm2 planar area, 550 µm thick). d, SEM cross-section of the MWNTs. Inset: Magnified view of the nanotube tips. e, Optical image of the 1D PhC emitter (Ae = 1 cm2). f, SEM cross-section of the 1D PhC showing the alternating layers of silicon and SiO2.

  2. TPV characterization.
    Figure 2: TPV characterization.

    Electrical output power density (Pout) generated by the InGaAsSb photovoltaic cell as a function of the 1D Si/SiO2 PhC emitter temperature. Inset: Measured4 spectral emittance (ελ) of the 1D PhC at 1,285 K and the internal quantum efficiency (IQE) of the photovoltaic used by the SQ1DD model. The model prediction (solid line) shows excellent agreement with experimental points (symbols). Error bars represent 95% confidence interval (see Methods).

  3. Performance characterization and optimization of the nanophotonic STPV device.
    Figure 3: Performance characterization and optimization of the nanophotonic STPV device.

    a, Electrical output power density (Pout) and absorber–emitter temperature (Tae determined from Fig. 2) with increasing Hs (input solar power normalized by the aperture area) for AR = 1 to 10. As the area ratio is increased, the device operates in a regime of decreased σTae4/Hs, which is favourable for the absorber efficiency of the nanotube array. b, Conversion efficiency (concentrated solar to electrical, ηtηtpv) with increasing area ratio for fixed Hs = 20 and 48 W cm−2. Competing effects of the thermal efficiency and the TPV efficiency lead to an optimal area ratio for a fixed Hs. c, Conversion efficiency as a function of Pout or, equivalently, Tae (AR = 5 omitted for clarity). d, At a given Pout or Tae, the conversion efficiency increases with increasing area ratio, which is attributed to an increase in thermal efficiency. Markers are experimental points (error bars represent 95% confidence interval; see Methods) and solid bands represent the SQ1DD model, treating Hs as collimated or diffuse sets the upper and lower bounds, respectively.

  4. Relative improvements in efficiency and near-term predictions for NARO-STPVs.
    Figure 4: Relative improvements in efficiency and near-term predictions for NARO–STPVs.

    Conversion efficiency ηtηtpv as a function of a solar irradiance Hs. Contributions to ηtηtpv relative to a greybody absorber–emitter: MWNT–1D PhC absorber–emitter (twofold improvement) and area ratio optimization (additional twofold improvement). Efficiencies approaching 20% were predicted with a scaled-up (10 × 10 cm2) NARO–STPV utilizing a high-quality 0.55 eV photovoltaic module with a sub-bandgap reflector20. All points and predictions were made using the SQ1DD model (Hs was treated as collimated).

Change history

Corrected online 14 May 2015
In this Letter, the scaling factor relating the optimal emitter temperature to the photovoltaic bandgap energy was approximated using Planck's distribution expressed in units of wavelength. For a discussion of scaling factors, see the Addendum in the PDF version.

References

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Author information

Affiliations

  1. Device Research Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Andrej Lenert,
    • David M. Bierman,
    • Youngsuk Nam &
    • Evelyn N. Wang
  2. Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Walker R. Chan &
    • Marin Soljačić
  3. Institute for Soldier Nanotechnology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Walker R. Chan,
    • Ivan Celanović &
    • Marin Soljačić
  4. Department of Mechanical Engineering, Kyung Hee University, Yongin 446-701, Korea

    • Youngsuk Nam

Contributions

All authors contributed extensively to this work. A.L., D.M.B. and Y.N. envisioned and implemented the experimental studies. A.L. and D.M.B. fabricated the absorber, executed the experiments and wrote the paper. W.R.C. designed and fabricated the emitter. I.C., M.S. and E.N.W. supervised and guided the project.

Competing financial interests

The authors declare no competing financial interests.

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