Sun-free photovoltaics

Materials engineered to give off precisely tuned wavelengths of light when heated are key to new high-efficiency generating system.

A new photovoltaic energy-conversion system developed at MIT can be powered solely by heat, generating electricity with no sunlight at all. While the principle involved is not new, a novel way of engineering the surface of a material to convert heat into precisely tuned wavelengths of light — selected to match the wavelengths that photovoltaic cells can best convert to electricity — makes the new system much more efficient than previous versions.

The key to this fine-tuned light emission, described in the journal Physical Review A, lies in a material with billions of nanoscale pits etched on its surface. When the material absorbs heat — whether from the sun, a hydrocarbon fuel, a decaying radioisotope or any other source — the pitted surface radiates energy primarily at these carefully chosen wavelengths.

Based on that technology, MIT researchers have made a button-sized power generator fueled by butane that can run three times longer than a lithium-ion battery of the same weight; the device can then be recharged instantly, just by snapping in a tiny cartridge of fresh fuel. Another device, powered by a radioisotope that steadily produces heat from radioactive decay, could generate electricity for 30 years without refueling or servicing — an ideal source of electricity for spacecraft headed on long missions away from the sun.

According to the U.S. Energy Information Administration, 92 percent of all the energy we use involves converting heat into mechanical energy, and then often into electricity — such as using fuel to boil water to turn a turbine, which is attached to a generator. But today's mechanical systems have relatively low efficiency, and can't be scaled down to the small sizes needed for devices such as sensors, smartphones or medical monitors.

"Being able to convert heat from various sources into electricity without moving parts would bring huge benefits," says Ivan Celanovic ScD '06, research engineer in MIT's Institute for Soldier Nanotechnologies (ISN), "especially if we could do it efficiently, relatively inexpensively and on a small scale."

It has long been known that photovoltaic (PV) cells needn't always run on sunlight. Half a century ago, researchers developed thermophotovoltaics (TPV), which couple a PV cell with any source of heat: A burning hydrocarbon, for example, heats up a material called the thermal emitter, which radiates heat and light onto the PV diode, generating electricity. The thermal emitter's radiation includes far more infrared wavelengths than occur in the solar spectrum, and "low band-gap" PV materials invented less than a decade ago can absorb more of that infrared radiation than standard silicon PVs can. But much of the heat is still wasted, so efficiencies remain relatively low.

An ideal match

The solution, Celanovic says, is to design a thermal emitter that radiates only the wavelengths that the PV diode can absorb and convert into electricity, while suppressing other wavelengths. "But how do we find a material that has this magical property of emitting only at the wavelengths that we want?" asks Marin Soljačić, professor of physics and ISN researcher. The answer: Make a photonic crystal by taking a sample of material and create some nanoscale features on its surface — say, a regularly repeating pattern of holes or ridges — so light propagates through the sample in a dramatically different way.

"By choosing how we design the nanostructure, we can create materials that have novel optical properties," Soljačić says. "This gives us the ability to control and manipulate the behavior of light."

The team — which also includes Peter Bermel, research scientist in the Research Laboratory for Electronics (RLE); Peter Fisher, professor of physics; and Michael Ghebrebrhan, a postdoc in RLE — used a slab of tungsten, engineering billions of tiny pits on its surface. When the slab heats up, it generates bright light with an altered emission spectrum because each pit acts as a resonator, capable of giving off radiation at only certain wavelengths.

This powerful approach — co-developed by John D. Joannopoulos, the Francis Wright Davis Professor of Physics and ISN director, and others — has been widely used to improve lasers, light-emitting diodes and even optical fibers. The MIT team, supported in part by a seed grant from the MIT Energy Initiative, is now working with collaborators at MIT and elsewhere to use it to create several novel electricity-generating devices.

Mike Waits, an electronics engineer at the Army Research Laboratory in Adelphi, Md., who was not involved in this work, says this approach to producing miniature power supplies could lead to lighter portable electronics, which is "critical for the soldier to lighten his load. It not only reduces his burden, but also reduces the logistics chain" to deliver those devices to the field. "There are a lot of lives at stake," he says, "so if you can make the power sources more efficient, it could be a great benefit."

The button-like device that uses hydrocarbon fuels such as butane or propane as its heat source — known as a micro-TPV power generator — has at its heart a "micro-reactor" designed by Klavs Jensen, the Warren K. Lewis Professor of Chemical Engineering, and fabricated in the Microsystems Technology Laboratories. While the device achieves a fuel-to-electricity conversion efficiency three times greater than that of a lithium-ion battery of the same size and weight, Celanovic is confident that with further work his team can triple the current energy density. "At that point, our TPV generator could power your smartphone for a whole week without being recharged," he says.

Celanovic and Soljačić stress that building practical systems requires integrating many technologies and fields of expertise. "It's a really multidisciplinary effort," Celanovic says. "And it's a neat example of how fundamental research in materials can result in new performance that enables a whole spectrum of applications for efficient energy conversion."

David L. Chandler contributed to this story.

RBI's rate increase may not be the last: Pranab

Govt to take steps to support RBI’s war on inflation.

Pranab MukherjeeFinance Minister Pranab Mukherjee on Wednesday dashed even faint hopes of a pause in the Reserve Bank of India’s (RBI’s) rate tightening cycle. Speaking to reporters a day after RBI stunned everybody with a 50 basis points (bps) increase in the repo rate, Mukherjee indicated this might not be the last increase.
I don’t think we have reached the end of the tunnel,” Mukherjee said, when asked if RBI was nearing the end of the rate increase cycle.

Yesterday’s rate increase has drawn criticism from companies worried about higher borrowing costs and prompted economists to revise their growth outlook. Standard Chartered Bank has reduced its gross domestic product growth forecast for this financial year from 8.1 per cent to 7.7 per cent. Kotak has cut its growth forecast to 7.3 per cent from 7.7 per cent, among the lowest in the market.

The benchmark 10-year bond yield on Wednesday hit its highest level in almost three years after rising 15 bps yesterday. Stocks continued their slide, losing nearly 0.5 per cent, and are down 10 per cent this year.

Mukherjee also sought to put a lid on murmurs over a finance ministry-RBI divide over steps to be taken to curb inflation, and pledged support to RBI in its fight against inflation.

RBI had said yesterday that its measure (repo rate increase) was expected to reinforce the point that there was an absence of complementary policy responses on both demand and supply sides.

Asked if he was surprised by the 50 bps increase, Mukherjee said, “I cannot say it surprised me. It is substantial no doubt, but given the situation it was necessary.”

Admitting that the 9.4 per cent inflation in June was “unacceptable”, Mukherjee said high prices were a global phenomenon and the whole world was reeling under rising prices of fuel and other commodities.

The government and RBI were taking steps to check inflation, he said, adding, “I am optimistic that the measures taken by RBI by adjusting the crucial rate will have an impact and inflation will come down.”

The inflation, Mukherjee said, might not come down to below 6-7 per cent by the end of the current financial year.

In an economy, Mukherjee said, “You cannot have a carpet under which you can keep all these things and at the same time expect these things will remain stable”.

He said crude oil prices went up from $89 per barrel when the Budget calculations were done to $107-110 a barrel.

He said he would take up the issue of volatility in commodity and crude oil prices at the international fora, including the G-20.

Meanwhile, the Financial Stability and Development Council, a body of the finance ministry and financial sector regulators, expressed confidence that the growth momentum would be maintained despite yesterday’s steep repo rate increase.