Periodic heating Can Double the Efficiency of Materials to Convert Heat to Electricity from 15% to 30% and Other Advances to Capturing 50% of all Wasted Heat and Tens of Billions in Market Potential

1. Researchers have shows that the use of a periodic heat source, instead of a constant heat source, can improve the conversion efficiency of a thermoelectric power generator (TPG). A periodic heat source drives a periodic temperature difference across the thermoelectric with an amplitude ΔT. While the time average of ΔT is identical to the temperature difference under a constant heat source with equivalent energy input, the time average of (ΔT)2 is larger, resulting in improved conversion efficiency. Here we present experimental measurements on a commercial thermoelectric device (bismuth telluride based) to validate analytical and numerical models. These models show that maximum efficiency is achieved when the period of the heat source is much larger than the thermal time constant of the TPG. Under this quasi-steady condition, the thermoelectric figure of merit ZT is still the relevant parameter for material optimization. A conventional thermoelectric material with ZT = 1, operated with sinusoidal and square-wave heat sources (ΔT = 100 K, TCold = 300 K), can achieve 140% and 180% of the constant heat source efficiency; or otherwise stated, can perform like advanced materials with ZT of 1.6 and 2.8. Even greater improvement, inaccessible through materials-based ZT enhancements, can be achieved with low duty cycle heat sources.

2. Alphabet Energy is commercializing thermoelectric waste-heat harvesting technology developed at Lawrence Berkeley National Laboratory and just raised $16 million in Round B funding, led by corporate strategic investor Encana, a developer of natural gas and other energy sources. Existing investors TPG Biotech, Claremont Creek Ventures and the CalCEF Clean Energy Angel Fund also participated in the round. This funding adds to the $12 million round A raised in 2011 and a $1M seed round in 2010.

Alphabet Energy is changing the way we think about energy. Imagine powering an entire factory with the exhaust from a single chimney, or improving the fuel efficiency of your car by 10% by using the heat from its tailpipe

Alphabet Energy has cracked the code for turning silicon into an efficient thermoelectric material for waste-heat recovery.

The silicon approach allowed the firm to create the highest efficiency in waste-heat recovery while using cheap materials, said the CEO. The company can operate in a capital-efficient manner because it “can use existing tool sets and scale up faster than other thermoelectrics.” He noted that “the thermoelectric is 100 times as power dense as a solar panel. We need 100 times less equipment and capital than a solar company.”

In residential and commercial settings, lighting and HVAC are the low-hanging efficiency measures. In heavy industry, waste heat recovery is on the top of folks’ lists when thinking about energy efficiency, according to the CEO. Alphabet’s technology is intended for the sub-megawatt-scale market.

Other companies developing thermoelectric technologies or capturing waste heat include:

* GMZ Energy, spun out of MIT with a $14 million Round C from KPCB, BP Alternative Energy, and Mitsui Ventures, is working on a bismuth telluride thermovoltaic device that converts solar heat directly into power via the Seebeck effect. In the Seebeck effect, a sharp temperature gradient can result in an electric charge. According to a press release, “GMZ Energy is developing its first product for the $8 billion residential, commercial and industrial solar thermal water market. Expected to launch next year, the product would integrate its thermoelectric material directly into conventional solar hot water collectors, enabling the production of electricity in addition to heat and hot water.”

* MTPV raised funding from Spinnaker Capital, Applied Ventures, Massachusetts Clean Energy Center and Ensys Capital for a product the company describes as a thermophotovoltaic. MTPV uses a silicon-based MEMS emitter which takes heat and transfers radiation to a germanium-based photovoltaic device, according to an article in Semiconductor Manufacturing and Design.

* Phononic Devices is developing waste-heat capture devices with funding from Oak Venture Partners and Venrock for cooling, air conditioning and energy harvesting.

* Silicium, funded by Khosla Ventures, is investigating high ZT thermoelectrics for refrigeration. (GigaOm speculates on Silicium here.)

* Recycled Energy Development (RED) and Ormat have retrofitted factories to capture waste heat, not using thermoelectrics, but by adding CHP or cogeneration.

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