Alphabet Energy turns to silicon for waste heat-to-energy

Thermoelectric materials hold the promise of turning waste heat into electricity, but they haven’t been widely used because of the high costs of materials and production. Startup Alphabet Energy says it’s solving those problems by using common silicon and has lined up $12 million to finalize the development of its first product. The San Francisco company, founded in 2009, plans to use the Series A round to complete a prototype of its technology and sell it to yet-to-be disclosed potential customers who will then test it and decide if they want to buy it. Alphabet’s CEO and co-founder, Matt Scullin wouldn’t describe the details of the prototype, but he did say the company ultimately wants to sell generators that turn waste heat into electricity. Those generators could be small enough to fit inside cars to make use of the heat from exhaust pipes, or large enough to sit inside power plants. The latest funding came from new investor TPG Biotech and existing investors Claremont Creek Ventures and CalCEF Clean Energy Angel Fund. Alphabet, a Lawrence Berkeley National Laboratory spin-out, previously raised a$1 million seed round in 2010 from Claremont and CalCEF.

Thermoelectric materials 101

Turning waste heat into electricity is commonly done in many power plants today. A combined-cycle plant is so-called because it first combusts natural gas to produce electricity. It then uses the waste heat from that process to produce steam, which then drives generators to create more electricity. Even though this two-step process is a more efficient use of natural gas, most combined-cycle plants today convert less than 50 percent of natural gas’s energy into electricity.

In a generator, the core Alphabet technology will reside in the thermoelectric material. Thermoelectric materials are semiconductors that can convert heat to electricity when heat induces a change in temperatures in the thermoelectric material, causing the electrons to move from the hot to the cold side. The difference in the temperatures is what causes the material to produce electricity. A lot of thermoelectric research has focused on using more expensive and sometimes rare materials, such as bismuth telluride, because of their inherent properties. But the high cost of manufacturing for these rare materials is a formidable barrier for commercializing thermoelectric devices.

Being able to turn silicon into thermoelectric material and do it at low cost will be quite a feat. Silicon is abundant and commonly used in making chips for running computers and iPads; it’s also widely used to make solar cells. The trick is to find a way to lower the thermal conductivity of silicon in order to increase its efficiency. Lowering the thermal conductivity makes it easier to keep the cold side of the device cold to create that temperature difference for generating electricity.

Scullin said that’s exactly what Alphabet has figured out, and Alphabet will use only silicon instead of a mix that includes silicon, as others have tried to do. But he wouldn’t say how his staff has managed to lower the conductivity. Neither did he want to talk about the efficiencies that Alphabet’s thermoelectric material could yield.

“This will be another major use of silicon. It means you can get to market much quicker and much less capital because we can piggyback on existing infrastructures. The device can be much less expensive,” Scullin said.

Not all silicon is created equal. Research has shown that silicon nanowires make good thermoelectric materials while bulk silicon doesn’t (because of its high thermal conductivity). In fact, a 2008 article in the journal Nature highlighted two separate research by teams from Caltech and UC Berkeley to develop thermoelectric materials with silicon nanowires (here is a good explainer in an IEEE article). Scullin declined to say whether Alphabet is using silicon nanowires. But note this: one of the startup’s founders, Peidong Yang, led the research that appeared in the Nature article. Yang is also a researcher at the Lawrence Berkeley National Laboratory.