Nature Nanotechnology 9, — ; published online 19 January ; corrected after print 14 May In this Letter, the equation describing see below; and also equation 1. However, the energy at which the maximum occurs depends on whether we consider energy flux per unit frequency range or per unit wavelength range 2 , 3. From the experimental results presented in this Letter, however, it is evident that the peak solar thermophotovoltaic STPV efficiency for a 0. Thus, a match between the bandgap energy and the energy corresponding to the maximum emission does not fully determine the optimal temperature of the emitter, particularly not in the case of STPVs; factors not considered by this simple approximation, such as the thermalization losses in the cell, play a significant role. For a more complete discussion of optimal temperatures in practical STPV converters please refer to ref.
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Bierman, Youngsuk Nam, Walker R. Biermana, Youngsuk Nama,d, Walker R. The most common approaches to generate power from sunlight are either photovoltaic PV , 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 increased irreversibilities in smaller 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 STPVs promise to leverage the benefits of both approaches: high-efficiency—by harnessing the entire solar spectrum; scalability, compactness—because of their solid-state nature; and dispatchablility—in principle by storing 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.
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.
Our device is planar and compact and could become a viable option for high-performance solar thermophotovoltaic energy conversion. Sunlight is converted to useful thermal emission, and ultimately electrical power, via a hot absorber-emitter. Because no portion of incident sunlight reaches the PV cell directly, the performance of STPVs relies on the efficiency of several intermediate energy conversion steps.
Optically concentrated sunlight is converted into heat in the absorber; the absorber temperature rises; heat conducts to the emitter; the hot emitter thermally radiates towards the PV cell, where radiation is ultimately harnessed to excite charge carriers and generate power Fig.
A spectrally selective emitter should have high emittance for energies above the PV bandgap Eg and low emittance for energies below the bandgap. Past STPV embodiments have relied on the intrinsic properties of materials such as tungsten 9, For the absorber, one approach to effectively enhance the intrinsic solar absorptivity of materials is to use macro cavity geometries. Because of the high aspect ratio of the cavity needed to enhance absorption, this approach typically requires high levels of optical concentration to reach Teopt e.
To improve the performance of the absorber-emitter, researchers have investigated the design of structured surfaces5,6, with spectral properties approaching those of ideal STPV components; specifically, the use of photonic crystals to control the photon density of states for narrow-band selective emission5,6, Simulation studies using 4 DOI: Although the intrinsic material properties are sensitive to temperature, the surface structure affords a degree of spectral tunability which is temperature independent.
Nevertheless, these surfaces have not yet been integrated into STPV devices operating at high enough temperatures for efficient power conversion. In our device, the spectral properties of the absorber-emitter are tailored through surface nanostructuring in a compact planar layout Fig.
With increasing area ratio, we supply enough heat for the absorber-emitter to reach Teopt by increasing the level of irradiance and leveraging the high absorptance of the nanotube array.
Thermal resistance between the absorber and emitter is minimized by integrating the absorber and emitter on the same conductive silicon substrate such that heat is effectively delivered to the emitter via thermal spreading. Since the absorber area is reduced with respect to the planar area of the sample Fig. To reduce parasitic radiative losses, we metallized the sides of the silicon substrate and inactive area around the nanotube absorber with W, a relatively low-emissivity high-temperature material, and incorporated a high-reflectivity Ag-coated shield Fig.
Vertically-aligned carbon nanotubes were chosen as the solar absorber due to their high-temperature stability in vacuum and their nearly ideal absorptance, crucial for absorbing highly-concentrated irradiance at elevated emitter-to- absorber area ratios. As shown in Fig. The broad-spectrum absorptance of the nanotube array in this study exceeds 0. These materials were chosen for ease of fabrication and high-temperature compatibility with 5 DOI: The layer thicknesses were optimized via a constrained global optimization of the product of efficiency and power density6.
Our mechanical system ensured alignment and gap control while minimizing parasitic conduction losses see SI: Experimental Setup. We used a Xe-arc light source to simulate the solar spectrum and to supply a range of irradiances Hs from 10 to 75 Wcm The temperature measurement in the TPV characterization was achieved by bonding a fine gage thermocouple directly to the absorber-side of the substrate. Figure 2 TPV characterization. Model prediction solid line shows an excellent agreement with experimental points markers.
These experimental results are supported by a spectral quasi-1D diffuse radiative network model SQ1DD. Our model assumes isothermal operation of the absorber-emitter i. The results of the TPV experiment serve as validation of our model and provide an indirect method for determining the absorber-emitter temperature from the measured output power. This approach was used in the STPV characterization since a direct in-situ measurement of the absorber-emitter temperature increases parasitic losses and reduces the efficiency.
The upper and lower estimates of our SQ1DD model associated with treating Hs as collimated or diffuse, respectively bound the data within the experimental uncertainty. For our nearly-blackbody nanotube absorbers, this regime graphically corresponds to the lower right corner of Fig. Competing effects of the thermal efficiency and the TPV efficiency lead to an optimal area ratio for a fixed Hs.
Nevertheless, absorber efficiency is only a component of the overall STPV efficiency. To understand why this optimal area ratio arises, the competing effects of the thermal efficiency and the TPV efficiency are considered. The thermal efficiency is significantly enhanced as the area ratio is increased due to a rise in absorber efficiency as explained above.
In general, the optimum area ratio increases with optical concentration as shown in Fig. Using the relation between pout and Tae Fig. As the temperature approaches Teopt , the efficiency plateaus as increasing useful emission i. Increasing area ratio for a given absorber-emitter temperature results in increased conversion efficiency Fig.
Optimizing area ratio at a fixed optical concentration with a nanophotonic absorber-emitter, experimentally demonstrated in this work, can be easily implemented in future STPV designs to increase overall efficiency. As the device scales in planar area from 1x1 cm to 10x10 cm Fig. Using an improved, yet realistic 0.
Although this result requires scale up of our processing and experimental systems, our robust experimental STPV demonstration of the 1 cm2 nanophotonic absorber- emitter and key design elements validates our model. The efficiency can be further enhanced through improvements in low-bandgap PVs such as GaSb, Ge, and graphene-based PVs , better spectral control5,15,21 and higher temperature operation.
The efficiency improvements demonstrated in this work, along with the promising predictions using a validated model, suggest the viability of nanophotonic STPVs for efficient and scalable solar energy conversion.
The polycrystalline Si and SiO2 layers of the 1D photonic crystal emitter was deposited by low-pressure and plasma-enhanced chemical vapor deposition, respectively6. The wafer was annealed after each deposition.
On the back side of the emitter substrate, a 10 nm adhesion layer of Ti was sputtered, followed by a nm W layer. Using a laser-cut 11 DOI: CNTs were grown for 10 mins at oC using an ethylene gas carbon source. All of the flowing gases were preheated to oC. The absorber-emitter substrate was mechanically secured using a custom spring-loaded needle-support layout. Manual linear stages were used to align and control the spacing between the reflecting shield, the absorber-emitter, and the PV cell.
We conducted the experiments on each absorber-emitter pair at varying levels of flux of simulated solar radiation Hs through the aperture Wcm-2 by changing the distance between the light source and the experiment see SI: Experimental Setup. Hs is defined as the input solar power through the aperture normalized by the aperture area, or equivalently, the nanotube absorber area.
The PV temperature was maintained near K using a chilled water loop see Eq. The authors thank Dr. Mutha, D. Li and Prof. Miljkovic, T. Humplik, J. Sack, D. Preston and the 12 DOI: Kraemer, M. Luckyanova, Prof. Chen and the Nanoengineering group advice. Author Contributions All authors contributed extensively to this work. Competing Financial Interests The authors declare that they have no competing interests. Additional Information Supplementary information accompanies this paper at www.
Correspondence and requests for materials should be addressed to E. References 1 J. Schwede et al. Photon-enhanced thermionic emission for solar concentrator systems.
Kraemer et al. High-performance flat-panel solar thermoelectric generators with high thermal concentration. Theoretical limits of thermophotovoltaic solar energy conversion. Detailed balance limit of efficiency of p-n junction solar cells. Absorber and emitter for solar thermo-photovoltaic systems to achieve efficiency exceeding the Shockley-Queisser limit.
Express 17, , Chan et al. Toward high-energy-density, high-efficiency, and moderate-temperature chip-scale thermophotovoltaics. Datas, D. Chubb, B.
Solar thermophotovoltaic STPV system with thermal energy storage. AIP Conf. Development and experimental evaluation of a complete solar thermophotovoltaic system. Photovolt: Res. Vlasov et al. TPV systems with solar powered tungsten emitters.
A nanophotonic solar thermophotovoltaic device
Youngsuk Nam DOI: Bierman, Youngsuk Nam, Walker R. Biermana, Youngsuk Nama,d, Walker R. The most common approaches to generate power from sunlight are either photovoltaic PV , in which sunlight directly excites electron-hole pairs in a semiconductor, or solar-thermal, in which sunlight drives a mechanical heat engine.
Bierman, Youngsuk Nam, Walker R. Biermana, Youngsuk Nama,d, Walker R. The most common approaches to generate power from sunlight are either photovoltaic PV , 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 increased irreversibilities in smaller heat engines make the solar thermal approach best suited for utility-scale power plants.
Addendum: A nanophotonic solar thermophotovoltaic device