{"id":12221,"date":"2019-08-19T14:47:01","date_gmt":"2019-08-19T21:47:01","guid":{"rendered":"http:\/\/cafe.foundation\/blog\/?p=12221"},"modified":"2019-08-19T14:47:01","modified_gmt":"2019-08-19T21:47:01","slug":"berkeley-achieves-new-highs-in-thermophotovoltaic-efficiency","status":"publish","type":"post","link":"http:\/\/cafe.foundation\/blog\/berkeley-achieves-new-highs-in-thermophotovoltaic-efficiency\/","title":{"rendered":"Berkeley Achieves New Highs in Thermophotovoltaic Efficiency"},"content":{"rendered":"

Researchers at University of California at Berkeley announced a record in thermophotovoltaic efficiency, now at 29 percent,<\/a> but with a few tweaks, soon to be at 50 percent.\u00a0 This is great news for small drones, which could stay up for days, but possibly not a be-all, end-all for larger, more conventional electric aircraft. \u00a0Let\u2019s examine the potential and the pitfalls involved.<\/p>\n

\"\"<\/a>

Berkeley researchers think their breakthrough could result in “new photovoltaic engine”<\/p><\/div>\n

The researchers\u2019 \u201cgroundbreaking physical insight\u201d and \u201cnovel design\u201d applies thermophotovoltaic principles that are \u201can ultralight alternative power source.\u201d\u00a0 Eli Yablonovitch, professor of electrical engineering and computer science (EECS), wrote in a paper published in the Proceedings of the National Academy of Sciences<\/em>,<\/a> \u201c\u201cThermophotovoltaics are compact and extremely efficient for a wide range of applications, from those that require as little as 100 watts, [such as] a lightweight unmanned aerial vehicle, to 100 megawatts, [providing] electricity for 36,000 homes. In comparison, a 100-megawatt combined cycle power plant is massive,\u201d<\/p>\n

Expanding on work he and his students published in 2011, Yablonovitch took a counter-intuitive approach to making more efficient solar cells.\u00a0 Past efforts focused on increasing the number of photons that a cell absorbs. The paper notes, \u201cAbsorbed sunlight in a solar cell produces electrons that must be extracted from the cell as electricity. Those electrons that are not extracted fast enough, decay and release their energy. If that energy is released as heat, it reduces the solar cell\u2019s power output. Miller\u2019s calculations showed that if this released energy exits the cell as external fluorescence, it would boost the cell\u2019s output voltage.\u201d<\/p>\n

It\u2019s All Done with Mirrors<\/strong><\/h4>\n
\"\"<\/a>

Berkeley reflective solar cell.\u00a0 Graphite ribbon (glowing bar) heating the thermophotovoltaic cell sitting under it. (Photo by Luis M. Pazos Out\u03ccn, UC Berkeley)<\/em><\/p><\/div>\n

\u201cThis is the central counter-intuitive result that permitted efficiency records to be broken,\u201d Yablonovitch says.\u00a0 In current efforts, \u00a0\u00a0Heat generated by the absorption of photons lowers the efficiency of the solar cell \u2013 so Berkeley researchers exploit the heat rather than trying to get rid of it.<\/p>\n

\"\"<\/a>

Mirror bounces heat to thermal emitter and recovers power<\/p><\/div>\n

They place a \u201chighly reflective\u201d mirror within the cell and reflect low-energy infrared photons back to their source, extracting the otherwise wasted energy these photons possess.\u00a0 It increases the voltage of the cell, but also generates heat at around 1,207\u00b0 C (2204.6\u00b0 F).\u00a0 The current 29.1-percent efficiency can be raised to 50-percent, according to researchers.<\/p>\n

It Won\u2019t Fly Your Airplane – Yet<\/strong><\/h4>\n

About 93 Watts of solar energy fall on each square foot<\/a> of the earth\u2019s surface, according to answers.com<\/em>.\u00a0 That\u2019s if the sun is directly overhead and the sky is clear, of course.\u00a0 Engineers use an average of 1,000 Watts per square meter for calculating solar panel sizes.\u00a0 Let\u2019s look at factors that lower that number.<\/p>\n

\"\"<\/a>

Martin Koxxy (l) and Richard Steeves with Martin’s Quark e-Gull.\u00a0 Both their e-Gulls are powered by Zero electric motorcycle motors<\/p><\/div>\n

As an example, friends Richard Steeves in Madison, Wisconsin and Martin Koxxy built, own and fly Zero Motorcycle-powered e-Gulls designed by Mark Beierle.\u00a0 They have investigated putting solar cells on the wings (both use the 28-foot version offered by Beierle).\u00a0 Their aircraft have about 133 square feet of wing area<\/a>.\u00a0 120 square feet of 29.1-percent efficient solar cells would collect 11,160 Watts (11.16 kilowatts) and under the best possible conditions generate 3.25 kilowatts.\u00a0 Your editor has seen Richard\u2019s rough calculations for the e-Gull, showing the 28-foot span requires about 8.1 kilowatts to remain in level flight at its best lift:drag ratio.\u00a0 (We\u2019ll have an expanded and updated set of calculations soon.)<\/p>\n

50-percent efficient solar cells promised by Berkeley would go a long way toward increasing range for these small machines, but would not keep them in sustained flight.\u00a0 If the design’s liquid cooling can contain the high heat these cells generate, they would be very helpful in extending the range of aircraft, automobiles, and boats.\u00a0 Smaller machines would seem to have an advantage at this point in the development cycle.<\/p>\n