Researchers at the Massachusetts Institute of Technology (MIT) have developed a method to create a highly efficient photovoltaic (PV) energy conversion system that does not require sunlight - it's powered by heat. Researchers have managed to engineer the surface of a material to grab fine tuned wavelengths of light. The technology chooses wavelengths that closely match those PV systems routinely select to generate electricity.
The concept is not new; the innovative method converts energy at a higher efficiency than the older technological techniques. The billions of nanoscale pits etched into the surface of some material. The material absorbs heat from as variety of sources, such as the sun, hydrocarbon fuel, decaying radioisotope and other resources. The pitted surface releases energy at the preferred wavelengths.
The MIT researchers fashioned a power generator about the size of a button. The generator, powered by butane gas, operates three times longer than a lithium-ion battery of similar weight. The generator recharges by simply swapping in a miniature fuel cartridge. Researchers also state that another mechanism, powered by a radioisotope, continually generates heat from radioactive decay. The scientists say, this process could produce energy for 30 years before the need arises to service or refuel the device.
Utilizes Multiple Fuel Sources
The U.S. Energy Information Administration states that over 90 percent of all the energy used results from the conversion of heat into mechanical energy. The mechanical energy transforms into electricity. For example, using a fuel to boil water creates steam, which expands or builds up pressure. The energy stored in the steam drives a generator, turns a turbine, or moves an object.
Because mechanical systems function at a low level of efficiency, current technology does not allow scaling the systems down to the size required for gadgets and devices like smart phones, sensors or medical monitoring equipment.
According to Ivan Celanovic, a research engineer in MIT's Institute for Soldier Nanotechnologies (ISN), the advantage of the new technology lies in the ability to convert multiple sources of fuel into electricity while eliminating the need for the moving parts associated with mechanical energy generation. Celanovic states the advance could “bring huge benefits, especially if we could do it efficiently, relatively inexpensively and on a small scale."
Thermophotovoltaics
For at least 50 years, the scientific community has always understood that PV could produce energy from any heat source - a process called thermophotovoltaics (TPV). If a burning hydrocarbon, such as butane, heats up a thermal emitter - material that radiates light and heat onto a solar cell -- it produces electricity. The radiation from the thermal emitter carries more infrared wavelengths than what normally happens in the solar spectrum.
The advances in “low band-gap” photovoltaic materials - cadmium telluride, gallium arsenide, and other materials, allow for the absorption of infrared radiation than silicone. However, the significant loss of heat results in lower efficiencies.
Formulating a Solution
Researchers must now work to develop a thermal emitter that discharges only the preferred wavelengths the solar cell can absorb and convert into electricity, while simultaneously repressing the unwanted wavelengths. This entails creating a solar crystal out of material with nanoscale attributes on the surface, such as a repeating pattern of ridges or cavities, which allow light to circulate through the material in a significantly distinctive manner.
ISN researcherMarin Soljačić, who is also a professor of physics, states, "By choosing how we design the nanostructure, we can create materials that have novel optical properties." In addition, according to Soljačić, “This gives us the ability to control and manipulate the behavior of light."
Another approach, employed by researchers Peter Bermel, Peter Fisher, and Michael Ghebrebrhan, entails engineering billions of nanoscale pits on the surface of a slab of tungsten. When heated, the tungsten produces bright light that has an altered emission spectrum as each pit functions as a resonator.
Each resonator generates radiation, but only at specific wavelengths. This technique, which ISN director and physics professor John Joannopoulos helped develop,has been instrumental in the enhancement of lasers and other devices. Researchers Celanovic and Soljacic emphasize that constructing functional systems with this technology requires bringing together multiple areas of technology and disciplines.
Generating three times the power of a comparable size and weight lithium-ion battery, researchers expect to triple the current power generation. They hope to enhance the technology to provide power for a variety of uses, such as charging hybrid battery by using heat from the engine or powering a laptop from the heat emitted from the device.
