Schematic sketch of the TEC prototype.
At the height of the Cold War, thermionic energy converters (TECs) were often used as the energy source for both NASA and the Soviet space program satellites. However, the combination of decreased space funding since the end of the Cold War and some of the engineering challenges associated with TECs has left the development of the technology largely stagnant until quite recently.
Over the past ten years there has been a bit of renaissance in TECs due to developments in modern wafer-scale fabrication techniques, device physics and material science, as well as an increasing attention to clean and renewable energy globally. This has led to TECs again receiving a considerable amount of interest both in the academia and industry, including two startups: Spark Thermionics and Modern Electron. While these companies and general trends are signs of TECs resurfacing as an alternative energy source, there remain some pretty significant engineering hurdles that still need to be overcome.
Now a team of researchers at Stanford University has taken a huge step in solving a couple of the key problems with TEC technology: improving the efficiency and stability of the anodes.The result could be the TECs taken on a far larger role in alternative energy solutions.
In research published in the journal Nano Energy, the Stanford researchers have employed graphene as the anode material and in so doing have boosted the efficiency of the device by a factor of 6.7 compared with a traditional tungsten anode.
The researchers successfully demonstrated an electronic conversion efficiency in the graphene-based anode of 9.8%. Electronic conversion efficiency is the efficiency at which an electron converts thermal energy to electrical energy. In other words, it is the efficiency of moving one electron from the cathode to the anode by heat.
“One of the major challenges for wider adoption of TECs is high anode work function, which directly reduces the output voltage as well as the net efficiency,” explained Hongyuan Yuan, a PhD candidate at Stanford and lead author of the research, in an e-mail interview with The Graphene Council. “The theoretical maximum efficiency for a TEC with a 2 electron volt (eV) work function anode is 3% at a cathode temperature of 1500 K, compared to an astonishing 10-fold increment of 32% with a 1 eV work function anode.”
The work function of a material is the energy difference between its vacuum level and Fermi level. Before the discovery of graphene, the world-record low work function for a conductor was around 1.1 eV to 1.2 eV, which is achieved by lowering the vacuum level through the deposition of a monolayer of cesium oxide on the surface.
In 2015, Stanford researchers discovered that the work function of graphene can be reduced by not only lowering its vacuum level, but also raising its Fermi level by electrostatic gating through a back gate at the same time. “In this ‘combo’ approach, we discovered that the work function of graphene reached a new world-low record of 1.0 eV in 2015,” added Yuan.
The second major challenge to the success of TEC has been the high space charge barrier between TEC’s cathode and anode, which directly reduces the output current and thus the net efficiency. In order to reduce the space charge barrier, TEC requires a very small vacuum gap to separate the cathode and anode, usually around 10 mm. If the gap is much larger than 10 mm, all the benefit that an ultra-low work function anode could bring would be diminished.
“In our most recent work, we successfully addressed the above mentioned two challenges, and demonstrated that the previously discovered ultra-low work function graphene can indeed be applied to TEC with a significant amount of efficiency enhancement. Compared to a traditionally used tungsten anode, the net efficiency is increased by a factor of 6.7,” said Yuan.
While applications for TECs remain limited at the moment, with improvement in efficiency and device stability, Yuan believes that TECs are expected to see an enormous market both in the centralized power plants, i.e. in a tandem cycle, as well as in the distributed systems, e.g. automobiles with internal combusting engines and domestic houses with water heaters.
The current demonstration of the TEC device has been performed in an ultra-high vacuum chamber, with many pumps constantly pumping down the pressure. “In reality, we need to fabricate such a TEC device and seal it in a vacuum ‘chip’ using the state-of-the-art nano/micro fabrication techniques,” added Yuan. “Only by making the device small and reliably stable would it be economically feasible in the sustainable energy industry.
Yuan added: “We envision such a TEC device in the future, which is sealed in a small and thin cell (TEC cell). To generate electricity, all you need to do is to attach one side of the cell to a heat source. You may attach a couple of the TEC cells to the water heater at your home to charge your phone. Or attach many TEC cells to a fossil-fuel power station to improve its overall efficiency.”