NASA Lewis Research Center (LeRC) has had an ongoing research program in TPV since the late 1980’s. This effort has included both in-house research programs on critical components such as photovoltaic cells (PV) and emitter materials, as well as an active contracting effort directed toward system and component development. Of particular note is the in-house development of thin film selective emitters fabricated from rare-earth yttrium aluminum garnets (YAG). These emitters have demonstrated in-band emittances of > 0.7 with low out of band emittances (∼0.2). Rare earth elements have a unique valency which allows them to behave as isolated atoms even when at solid state densities (i.e. produce line emission rather than grey body emission). Each element has a characteristic emission frequency (wavelength) and YAG selective emitters with emissions peaks ranging from 0.98μm to > 2μm have been demonstrated. In addition, it has been demonstrated that two rare earth elements can be added to the same emitter for increased the power density.
Also developed at LeRC is a Monolithically Interconnected Module (MIM) consisting of many small InGaAs cells series interconnected on a single InP substrate. An infrared reflector placed on the rear surface of the substrate returns unabsorbed photons to the emitter for recycling. The MIM design has many advantages such as: high output voltages and low currents, improved reliability, minimized losses associated with emitter non-uniformity (i.e. variation in view factor, temperature, etc.), high output power density, simplified system design and simplified thermal management. MIM devices with excellent photoresponses and IR reflectivity’s > 82% have been demonstrated.
The contracted efforts include a TPV system development by Tecogen, Inc., based on ytterbia fibrous selective emitters and silicon PV devices. Two (2) prototype TPV systems were constructed, each including recuperators, non-premixed combustor designs, dielectric interference filters and PERL silicon TPV PV cells. Emitters temperatures of 2000K were routinely achieved and both systems have logged many hours of testing. Also under contract development are solar powered TPV systems, InGaAsSb PV devices and plasma-sprayed selective emitters.