Andrej Lenert

A nanophotonic solar thermophotovoltaic device

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 generation. 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 spectrum; scalability and compactness, because of their solid-state nature; and dispatchablility, owing to the ability to store energy using thermal or chemical means. 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.

Design of wide-angle selective absorbers/emitters with dielectric filled metallic photonic crystals for energy applications

The design and simulation of a wide angle, spectrally selective absorber/emitter metallic photonic crystal (MPhC) is presented. By using dielectric filled cavities, the angular, spectrally selective absorption/emission of the MPhC is dramatically enhanced over an air filled design by minimizing diffraction losses. Theoretical analysis is performed and verified via rigorous coupled wave analysis (RCWA) based simulations. An efficiency comparison of the dielectric filled designs for solar thermophotovoltaic applications is performed for the absorber and emitter which yields a 7% and 15.7% efficiency improvement, respectively, compared to air filled designs. The converted power output density is also improved by 33.5%.

Nanofluid-Based Absorbers for High Temperature Direct Solar Collectors

Solar collectors with surface receivers have low overall energy conversion efficiencies due to large emissive losses at high temperatures. Alternatively, volumetric receivers promise increased performance because solar radiation can be transferred into a fluid medium, which subsequently reduces the concentrated heat at the surface. Nanofluid-based direct solar receivers, where nanoparticles in a liquid medium can scatter and absorb solar radiation, have recently received interest to efficiently distribute and store the thermal energy. We present a combined modeling and experimental study to investigate the efficiency of fluid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A simple one-dimensional transient heat transfer model was developed to compare idealized surface receivers to idealized volumetric receivers. In the limit of idealized behavior, volumetric receivers were shown to be more efficient than selective-surfaces for high concentration levels (C > 100) and/or tall receiver designs (H > 10 cm). Furthermore, volumetric receivers allowed the working range of the fluid to be extended to higher temperatures while maintaining relatively high receiver efficiency. We also designed and custom built a cylindrical receiver to experimentally demonstrate the concept of nanofluid-based receivers and validate the modeling efforts. We explored the effect of particle characteristics such as size, distribution, and material properties, as well as collector parameters such as absorbing depth and level of solar concentration. The work offers design guidelines for the development of efficient volumetric receivers for future solar thermal energy conversion systems.Copyright

Role of spectral non-idealities in the design of solar thermophotovoltaics.

To bridge the gap between theoretically predicted and experimentally demonstrated efficiencies of solar thermophotovoltaics (STPVs), we consider the impact of spectral non-idealities on the efficiency and the optimal design of STPVs over a range of PV bandgaps (0.45-0.80 eV) and optical concentrations (1-3,000x). On the emitter side, we show that suppressing or recycling sub-bandgap radiation is critical. On the absorber side, the relative importance of high solar absorptance versus low thermal emittance depends on the energy balance. Both results are well-described using dimensionless parameters weighting the relative power density above and below the cutoff wavelength. This framework can be used as a guide for materials selection and targeted spectral engineering in STPVs.

Spectral splitting optimization for high-efficiency solar photovoltaic and thermal power generation

Utilizing the full solar spectrum is desirable to enhance the conversion efficiency of a solar power generator. In practice, this can be achieved through spectral splitting between multiple converters in parallel. However, it is unclear which wavelength bands should be directed to each converter in order to maximize the efficiency. We developed a model of an ideal hybrid solar converter which utilizes both a single-junction photovoltaic cell and a thermal engine. We determined the limiting efficiencies of this hybrid strategy and the corresponding optimum spectral bandwidth directed to the photovoltaic cell. This optimum width is inversely proportional to the thermal engine efficiency and scales with the bandgap of the photovoltaic cell. This bandwidth was also obtained analytically through an entropy minimization scheme and matches well with our model. We show that the maximum efficiency of the system occurs when it minimizes the spectral entropy generation. This concept can be extended to capture genera...

Temperature dependence of thermopower in molecular junctions

The thermoelectric properties of molecular junctions are of considerable interest due to their promise for efficient energy conversion. While the dependence of thermoelectric properties of junctions on molecular structure has been recently studied, their temperature dependence remains unexplored. Using a custom built variable temperature scanning tunneling microscope, we measured the thermopower and electrical conductance of individual benzenedithiol junctions over a range of temperatures (100 K–300 K). We find that while the electrical conductance is independent of temperature, the thermopower increases linearly with temperature, confirming the predictions of the Landauer theory.

Ultrathin planar hematite film for solar photoelectrochemical water splitting.

Hematite holds promise for photoelectrochemical (PEC) water splitting due to its stability, low-cost, abundance and appropriate bandgap. However, it suffers from a mismatch between the hole diffusion length and light penetration length. We have theoretically designed and characterized an ultrathin planar hematite/silver nanohole array/silver substrate photoanode. Due to the supported destructive interference and surface plasmon resonance, photons are efficiently absorbed in an ultrathin hematite film. Compared with ultrathin hematite photoanodes with nanophotonic structures, this photoanode has comparable photon absorption but with intrinsically lower recombination losses due to its planar structure and promises to exceed the state-of-the-art photocurrent of hematite photoanodes.

2D Photonic-crystals for high spectral conversion efficiency in solar thermophotovoltaics

We present a novel solar thermophotovoltaic (STPV) device, which for the first time, incorporates a two-dimensional photonic-crystal (2D PhC) absorber-emitter to achieve spectral conversion efficiencies >10%. These results were achieved by tailoring the spectral properties of the absorber-emitter through surface nanostructuring of tantalum (Ta) and minimizing parasitic thermal losses through an innovative vacuum-enclosed experimental setup. By incorporating a sub-bandgap photon reflecting filter on the PV surface and optimizing the absorber-emitter ratio, we present how the demonstrated 2D Ta PhCs enable a realistic STPV configuration to exceed the Shockley-Queisser ultimate efficiency of a 0.55 eV cell.

Addendum: A nanophotonic solar thermophotovoltaic device

Eq. 1 in this Letter (and also, Eq. 1.133 in ref 2) represents the temperature required for the maximum of Planck’s distribution expressed in units of wavelength to match the bandgap energy. However, the energy at which the maximum occurs depends on whether we consider energy flux per unit frequency range or per unit wavelength range3,4. A more appropriate approximation matches the maximum of Planck’s distribution expressed in units of frequency or energy to the bandgap energy, the scaling factor in this case is 4114 K/eV.

Nanoengineered Surfaces for Thermal Energy Conversion

We provide an overview of the impact of using nanostructured surfaces to improve the performance of solar thermophotovoltaic (STPV) energy conversion and condensation systems. We demonstrated STPV system efficiencies of up to 3.2%, compared to ≤1% reported in the literature, made possible by nanophotonic engineering of the absorber and emitter. For condensation systems, we showed enhanced performance by using scalable superhydrophobic nanostructures via jumping-droplet condensation. Furthermore, we observed that these jumping droplets carry a residual charge which causes the droplets to repel each other mid-flight. Based on this finding of droplet residual charge, we demonstrated electric-field-enhanced condensation and jumping-droplet electrostatic energy harvesting.