Thermal Photovoltaics

The current state of energy infrastructure is characterized by centralized generation of electricity far from the point of use and local generation of heat near the point of use. The separation of these processes has resulted in compounded generation inefficiencies. Implementation of distributed, combined heat and power generation strategies has the potential for increased overall efficiencies, curtailing energy consumption without reducing supply [1]. Widespread use of combined generation requires the development of a scalable electricity generation technology, as industrial-scale steam turbines perform inefficiently at small scales.

Thermophotovoltaic (TPV) conversion represents a promising, solid-state approach to scalable generation. TPV systems exploit the photovoltaic effect to convert locally emitted infrared (IR) radiation to useful electricity. Utilization of TPV systems has been limited by high device fabrication costs and low conversion efficiency. Adaptation of spectrally selective techniques to control radiative transport between the emitter and cell has enabled improved device performance [2]. However, conversion efficiency continues to suffer from parasitic absorption in bulky devices. Our ongoing research efforts aim to develop a thin film TPV device, which utilizes spectral control to promote photon recycle and enable conversion efficiency exceeding that of current state-of-the-art devices.

Recent theoretical studies support that the integration of thin film devices into TPV systems could drastically reduce parasitic absorption and, in turn, significantly improve efficiency [3]. Successful development of highly efficient thin film TPV systems has the potential to enable competitive operating costs on the order of $0.1/Watt.

[1] US EPA, “CHP Benefits” [Online]. Available: [Accessed: 18-Aug-2017].
[2] Wernsman, B., Siergiej, R. R., Link, S. D., Mahorter, R. G., Palmisiano, M. N., Wehrer, R. J., Schultz, R. W., Schmuck, G. P., Messham, R. L., Murray, S., Murray, C. S., Newman, F., Taylor, D., Depoy, D. M., and Rahmlow, T., 2004, “Greater Than 20% Radiant Heat Conversion Efficiency of a Thermophotovoltaic Radiator/Module System Using Reflective Spectral Control,” 51(3), pp. 512–515.
[3] Ganapati, V., Xiao, T. P., and Yablonovitch, E., 2016, “Ultra-Efficient Thermophotovoltaics Exploiting Spectral Filtering by the Photovoltaic Band-Edge,” pp. 1–14.

Passive Radiative Cooling

Building air conditioning accounts for a large portion of overall energy consumption, particularly of the peak electrical demand [1,2]. Being able to cool without the input of electricity (i.e., free cooling) has the potential to drastically reduce energy usage and lower peak electrical demands. One way of providing free cooling is by utilizing outer Space as a natural heat sink (effectively at 3 Kelvin). Passive radiative cooling systems make use of the transparent atmospheric “window” (8 – 13 micrometers in wavelength) to emit radiation directly to space. Selective emission within this wavelength range can result in net cooling when the emitted radiation is greater than the absorbed solar and atmospheric radiation. Recently daytime cooling has been achieved using multilayer photonic coatings that also reflect most of the incident sunlight [3]. In addition to multilayer coatings, metamaterials composed of microspheres embedded in a polymer matrix have also been used as selective emitters to achieve daytime cooling as a more scalable and cost-effective approach [4].

Our research focuses on developing nanocomposites that can be tailored to achieve selective transmission, desirable for passive radiative cooling. We are researching composites that are transparent in the infrared, but opaque in the visible. We are interested in how the structure of the nanoscale building blocks can be tuned to achieve selective transmission and ultimately high performance radiative cooling.

[1] DOE, D. of E., 2011, Buildings Energy Data Book.
[2] Smith, G. B., and Granqvist, C. G., 2011, “Coolness: High-Albedo Surfaces and Sky Cooling Devices,” Green Nanotechnology: Solutions for Sustainability and Energy in the Built Environment, CRC Press, pp. 303–359.
[3] Raman, A. P., Anoma, M. A., Zhu, L., Rephaeli, E., and Fan, S., 2014, “Passive Radiative Cooling below Ambient Air Temperature under Direct Sunlight.,” Nature, 515(7528), pp. 540–4.
[4] Zhai, Y., Zhai, Y., Ma, Y., David, S. N., Zhao, D., Lou, R., Tan, G., Yang, R., and Yin, X., 2017, “Scalable-Manufactured Randomized Glass-Polymer Hybrid Metamaterial for Daytime Radiative Cooling,” 7899.