TPV and solar thermal

Thermophotovoltaics (TPV) and solar thermal are approaches to solar energy conversion with efficiencies that are not bound by the Shockley-Queisser limit. We're interested in how engineered photonic media might be used to improve the efficiency of these approaches.

Thermophotovoltaics

In this approach, sunlight (or a terrestial heat source) is used to heat an intermediate body. The intermediate's glow is captured by a solar cell to convert it into electrical power. In principle, if the intermediate's absorption and emission characteristics are perfectly tuned, overall system efficiencies approaching the thermodynamic limit (85%) are possible.

Our research focuses on realizing optimal coatings that have the desired spectral characteristics, that are manufacturable using thin-film processing, and that are stable at typical operation temperatures of TPV intermediates (~1800K).

Figure 1 is an example of the projected performance of metal (W, Ta or Mo)-dielectric (MgO) stacks we've designed for use with a Si and GaSb solar cell. You notice that the stacks are emissive above the bandgap of the solar cell, but much darker for photon energies below the solar cell's bandgap.

Fig. 1: Hemispherically integrated emissivity for W/MgO, Ta/MgO and Mo/MgO stacks matched to a Si (left) and GaSb (right) solar cell. The improvement in characteristics compared to a Nb/Al2O3 periodic stack is evident. In particular at longer wavelengths.

Solar thermal

This approach uses sunlight to heat up a working fluid that is used  to power a rotary engine (turbine or Stirling engine) which in turn drives a generator. One such system uses long pipes through which the fluid flows and parabolic trough collectors to focus sunlight onto the pipe. We are interested in how photonic media can be applied to the pipe to maximize the absorption of sunlight and minimize losses by thermal emission.

Figure 2 shows the projected performance of an optimized metal-dielectric stack. This stack is very absorptive in the visible, capturing 94% of the incident solar power. At the same time, the stack is very reflective (i.e. not emissive) in the infrared, limiting losses by thermal radiation to a minimum.

Fig. 2: Hemispherically integrated reflectivity of a W/MgO stack designed for solar thermal applications.

Project info

• People: Olivier Pincon, Mukul Agrawal and Peter Peumans
• Contact: Peter Peumans

Other resources

• Ausra: a VC-funded Palo Alto-based startup in solar thermal
• Solel
  
• Schott Solar
• Solar Thermal work by the Applied Physics group at the University of Sydney  
• NREL's concentrating solar power research group
  
• NREL TroughNet
  
• EIA's page on solar thermal
  
• SolarPACES conference
  
• Technology Review article (9/27/2007)
• Reviews by Mills and Price