With fossil fuels accounting for around 80% of world energy production, it’s estimated that current methods of power generation waste 65% of the energy they consume.Clearly, it’s in our best interests to do something about this wastage, and discoveries made over 100 years ago by Thomas Edison could provide the solution.
Engineers at Eneco, a small company in Salt Lake City, claim to have developed a semiconductor technology that can efficiently and inexpensively transform heat pollution into electricity without using any moving parts.
Eneco’s device, said to be twice as efficient as its closest competitor, could have major implications for the recovery of waste heat from power plants and automobiles. It could, for example, capture the heat lost through engine exhausts and convert it into electricity to augment or replace a vehicle’s electrical and air conditioning systems.
The device is said to be based on thermionics. This originated nearly a century ago with the vacuum tube, a device that consisted of two parallel conductive plates (cathode and anode) separated by a vacuum gap. In this tube heat is applied to the cathode (also called the emitter). The electrons in the cathode gain energy, reaching higher levels of excitement until electrons close to the surface undergo thermionic emission. This occurs when the energy level of the electron exceeds the work function, equivalent to a binding energy, of the surface. Discovered by Thomas Edison in 1883, this is known as the Edison effect.
The remainder of the thermionic converter cell is dedicated to collecting and using these emitted electrons with minimal loss in bias (voltage) and minimal back current (flow of electrons back into the cathode). After emission from the cathode, the electrons must traverse across a gap and be condensed onto an anode surface.After the electrons are collected by the anode, the excess energy is taken from the collector to a heat sink.
The electrons are drawn through a circuit, usually a power conditioning system to transform the high current, low voltage signal into a more usable voltage-current range. The useful power having been extracted, the electrons are returned to the cathode to complete the circuit.
No vacuum gap
However, these early ‘vacuum gap’ designs had prohibitive manufacturing costs and operating temperatures above 1,000°C, limiting the technology to nuclear-powered converters in space probes, satellites and special military systems. Eneco says that its device will operate at the relatively chilly temperatures of 200 – 450 degrees C – typical temperatures for waste heat and for concentrated solar radiation.
Eneco has replaced the traditional vacuum gap with a three-layer semiconductor structure to create what it describes as a kind of solid-state thermionic device. These layers have been doped with a number of impurities to increase the flow of electrons. One of the outer layers is heated and the other is kept at room temperature. The middle layer is an insulator that maintains the temperature difference. The heat causes electrons to shoot out, generating an electrical current.
Tests on early prototypes resulted in the capture of about 17% of lost heat and the company is said to be confident that it will improve that level to near the 50% barrier.
As well as the obvious applications in waste heat recovery, Eneco says that the device could be used to improve the efficiency of refrigerators, air conditioners and other cooling equipment.
The electrical current causes the device to absorb energy from one surface and expel it on the opposite surface, resulting in a cooling process that requires no compressor or CFCs (eg Freon).
Eneco hopes to have the device ready for sale in the next two years, and technical development is now focused on optimising the types of materials used.
Having validated the technology and applied for patents, the company is planning the commercial development of military, industrial, commercial and residential applications.
But Eneco is not alone. Global R&D company Borealis Exploration claims to have made its own breakthrough with a device that makes use of the traditional vacuum gap – the Power Chip.
While a vacuum gap tens of microns wide or filled with caesium led to lower running efficiency in older thermionic devices, Borealis has reduced the size of this gap to less than one micron, allowing the device to efficiently generate electricity at much lower temperatures.
Chris Bourne of Borealis claims that Eneco’s description of its technology as thermionic has muddied the waters. He says that the absence of a vacuum gap makes their device thermoelectric.
This is, he says, an important distinction. The theoretical limit for Power Chip efficiency is 70-80% of Carnot efficiency, while the best thermo-electric devices on the market show efficiencies of about 8% of Carnot efficiency. ‘I believe Eneco are claiming 17% of Carnot efficiency for their thermo-electric system,’ says Bourne, ‘which uses a novel doped material between the two layers where we have a vacuum.’
‘The absolute efficiency of thermionic devices varies depending on the temperature differential and the ambient temperature you start with’ he says. ‘Hence with thermionics, we often use Carnot efficiency, which measures how the device compares with a ‘perfect’ heat pump where there are no losses from the process. For example, a domestic refrigerator operates at about 30-40% of Carnot efficiency.’ Thermionic devices – vacuum diodes – are more efficient than thermo-electrics, argues Bourne, because current flowing through a vacuum is a one-way trip. With thermoelectrics, the sandwich or joint is formed of solid materials that create an unwanted return path. For example, in the case of a power generator, the electrons are carried along with the heat flow. If the heat flows back through the return path, a good part of the energy is wasted.
Power Chips only allow electron flow. There’s no direct thermal contact between the hot and cold sides. But when heat can flow without the electrons carrying it with them (as it does through simple conduction), the device loses efficiency. A solid-state converter that has no gap between the hot and cold side is ‘like a dam with holes in it’ says Bourne, ‘some of the water is used to turn the turbine. Some of it just goes through the holes.’
Borealis claims the gap in its PowerChips removes the key inefficiency that’s plagued thermoelectric devices since their invention.
Furthermore, while conventional power systems, including thermoelectric systems, rely on very high temperatures to operate the Borealis technology is designed to be able to operate at room temperature, greatly increasing the number of potential applications.
Applications for the power chip are similar to those identified for Eneco’s technology. Indeed, Borealis claims that a normal car retrofitted with PowerChips to recover lost heat from the exhaust system could achieve a 30% better mileage.
Boeing has also evaluated the technology and stated that ‘the principles under which the new Cool Chips technology operate are sound, and the measured physical data complies with the theory.’
Further development is still necessary, says Bourne, but the issues remaining are engineering rather than theoretical. ‘We have broken the back of the main problems,’ he claims, ‘and are now seeking development funding to produce commercial prototypes.’