Startup Nirvana Energy Systems, Inc., is developing the Thermo Acoustic Power Stick (TAPS)—a micro-combined heat and power (µ-CHP) system for home use that converts gas into electricity and provides for domestic water and space heating. Based in part on technology developed by Xerox PARC and further refined by NASA Glenn, TAPS produces between 1-4 kW of electrical power and 15-30 kW of thermal power with system efficiency of more than 90%.
At the heart of the device is NASA Glenn’s Stirling technology, licensed to Nirvana earlier this year. The thermoelectric Stirling heat engine (TASHE) technology, developed by Glenn for spaceflight propulsion (using a radioisotope heat source—i.e., a Stirling Radioisotope Power Source, SRPS), produces an acoustic pressure wave to drive a piston in a linear alternator. The linear alternator produces the electricity.
Nirvana is designing TAPS to be a very compact system that can easily be retrofitted into residential systems.
In a 2011 analysis of a Stirling-cycle power converter for domestic CHP, Buckmaster and Newman at Case Western Reserve noted that:
…Stirling engines promise attractive efficiency, simplicity and reliability. Stirling convertors have been under development and testing at NASA’s Glenn Research Lab since 1983 … A Stirling converter can have only two moving parts—a displacer and a piston (where an alternator’s armature is coupled to the piston). A relatively modern variation is the thermo-acoustic Stirling convertor, in which a moving displacer is eliminated, reducing the system to a single moving part.
To make Stirling convertors practical for residential use, the installed system cost must be low. Variables such as energy tax credits, availability of low-interest home-equity loans and electricity and natural gas rates all affect an economic decision, but it is nonetheless clear that a viable unit must be manufacturable at relatively low cost. A prospect to be explored is a thermo-acoustic design. This technology allows for elimination of one of the moving parts (the displacer), replacing it with a tuned cavity and inertance tubes. While the analysis and design of such systems can be complex, the result can be potentially cheap to manufacture.
As outlined in a 2004 study of a traveling-wave thermo-acoustic power converter that generates electricity from heat for NASA by researchers from Northrup Grumman Space and Technology and Los Alamos National Laboratory, a thermo-acoustic design incorporates a thermo-acoustic driver that converts heat to acoustic power without any moving parts. The acoustic power drives a pair of flexure-bearing-supported pistons connected to voice coils in a vibrationally balanced pair of moving coil alternators, which produces AC electrical power.
Nirvana will be conducting testing of TAPS throughout 2014. Once testing, certification and regulatory requirements are completed, TAPS should be ready for the residential energy marketplace.
M. Petach, E. Tward, and S. Backhaus (2004) “Design of a High Efficiency Power Source (HEPS) Based on Thermoacoustic Technology,” Final report, NASA contract no. NAS3-01103, CDRL 3f/ul>