# MIT develops thermo-PV cell with 40% efficiency

Researchers at MIT have developed a heat engine that can convert heat to electricity with 40% efficiency. Unlike traditional heat engines – a common example is the internal combustion engine inside a car – this device doesn’t have any moving parts. Second, this device has been designed to work with a heat source that has a temperature of 1,900º to 2,400º C. Effectively, it’s like a solar cell that has been optimised to work with photons from vastly hotter sources – although its efficiency still sets it apart. If you know the history, you’ll understand why 40% is a big deal. And if you know a bit of optics and some materials science, you’ll understand how this device could be an important part of the world’s efforts to decarbonise its power sources. But first the history.

We’ve known how to build heat engines for almost two millennia. They were first built to convert heat, generated by burning a fuel, into mechanical energy – so they’ve typically had moving parts. For example, the internal combustion engine combusts petrol or diesel and harnesses the energy produced to move a piston. However, the engine can only extract mechanical work from the fuel – it can’t put the heat back. If it did, it would have to ‘give back’ the work it just extracted, nullifying the engine’s purpose. So once the piston has been moved, the engine dumps the heat and begins the next cycle of heat extraction from more fuel. (In the parlance of thermodynamics, the origin of the heat is called the source and its eventual resting place is called the sink.)

The inevitability of this waste heat keeps the heat engine’s efficiency from ever reaching 100% – and is further dragged down by the mechanical energy losses implicit in the moving parts (the piston, in this case). In 1820, the French mechanical engineer Nicolas Sadi Carnot derived the formula to calculate the maximum possible efficiency of a heat engine that works in this way. (The formula also assumes that the engine is reversible – i.e. that it can pump heat from a colder source to a hotter sink.) The number spit out by this formula is called the Carnot efficiency. No heat engine can have an energy efficiency that’s greater than its Carnot efficiency. The internal combustion engines of today have a Carnot efficiency of around 37%. A steam generator at a large power plant can go up to 51%. Against this background, the heat engine that the MIT team has developed has a celebration-worthy efficiency of 40%.

The other notable thing about it is the amount of heat with which it can operate. There are two potential applications of the new device that come immediately to mind: to use the waste heat from something that operates at 1,900-2,400º C and to take the heat from something that stores energy at those temperatures. There aren’t many entities in the world that maintain a temperature of 1,900-2,400º C as well as dump waste heat. Work on the device caught my attention after I spotted a press release from MIT. The release described one application that combined both possibilities in the form of a thermal battery system. Here, heat from the Sun is concentred in graphite blocks (using lenses and mirrors) that are located in a highly insulated chamber. When the need arises, the insulation can be removed to a suitable extent for the graphite to lose some heat, which the new device then converts to electricity.

On Twitter, user Scott Leibrand (@ScottLeibrand) also pointed me to a similar technology called FIRES – short for ‘Firebrick Resistance-Heated Energy Storage’, proposed by MIT researchers in 2018. According to a paper they wrote, it “stores electricity as … high-temperature heat (1000–1700 °C) in ceramic firebrick, and discharges it as a hot airstream to either heat industrial plants in place of fossil fuels, or regenerate electricity in a power plant.” They add that “traditional insulation” could limit heat leakage from the firebricks to less than 3% per day and estimate a storage cost of \$10/kWh – “substantially less expensive than batteries”. This is where the new device could shine, or better yet enable a complete power-production system: by converting heat deliberately leaked from the graphite blocks or firebricks to electricity, at 40% efficiency. Even given the fact that heat transfer is more efficient at higher temperatures, this is impressive – more since such energy storage options are also geared for the long-term.

Let’s also take a peek at how the device works. It’s called a thermophotovoltaic (TPV) cell. The “photovoltaic” in the name indicates that it uses the photovoltaic effect to create an electric current. It’s closely related to the photoelectric effect. In both cases, an incoming photon knocks out an electron in the material, creating a voltage that then supports an electric current. In the photoelectric effect, the electron is completely knocked out of the material. In the photovoltaic effect, the electron stays within the material and can be recaptured. Second, in order to achieve the high efficiency, the research team wrote in its paper that it did three things. It’s a bunch of big words but they actually have straightforward implications, as I explain, so don’t back down.

1. “The usage of higher bandgap materials in combination with emitter temperatures between 1,900 and 2,400 °C” – Band gap refers to the energy difference between two levels. In metals, for example, when electrons in the valence band are imparted enough energy, they can jump across the band gap into the conduction band, where they can flow around the metal conducting electricity. The same thing happens in the TPV cell, where incoming photons can ‘kick’ electrons into the material’s conduction band if they have the right amount of energy. Because the photon source is a very hot object, the photons are bound to have the energy corresponding to the infrared wavelength of light – which carries around 1-1.5 electron-volt, or eV. So the corresponding TPV material also needs to have a bandgap of 1-1.5 eV. This brings us to the second point.

2. “High-performance multi-junction architectures with bandgap tunability enabled by high-quality metamorphic epitaxy” – Architecture refers to the configuration of the cell’s physical, electrical and chemical components and epitaxy refers to the way in which the cell is made. In the new TPV cell, the MIT team used a multi-junction architecture that allowed the device to ‘accept’ photons of a range of wavelengths (corresponding to the temperature range). This is important because the incoming photons can have one of two effects: either kick out an electron or heat up the material. The latter is undesirable and should be avoided, so the multi-junction setup to absorb as many photons as possible. A related issue is that the power output per unit volume of an object radiating heat scales according to the fourth power of its temperature. That is, if its temperature increases by x, its power output per volume will increase by x^4. Since the heat source of the TPV cell is so hot, it will have a high power output, thus again privileging the multi-junction architecture. The epitaxy is not interesting to me, so I’m skipping it. But I should note that electric cells like the current one aren’t ubiquitous because making them is a highly intricate process.

3. “The integration of a highly reflective back surface reflector (BSR) for band-edge filtering” – The MIT press release explains this part clearly: “The cell is fabricated from three main regions: a high-bandgap alloy, which sits over a slightly lower-bandgap alloy, underneath which is a mirror-like layer of gold” – the BSR. “The first layer captures a heat source’s highest-energy photons and converts them into electricity, while lower-energy photons that pass through the first layer are captured by the second and converted to add to the generated voltage. Any photons that pass through this second layer are then reflected by the mirror, back to the heat source, rather than being absorbed as wasted heat.”

While it seems obvious that technology like this will play an important part in humankind’s future, particularly given the attractiveness of maintaining a long-term energy store as well as the use of a higher-efficiency heat engine, the economics matter muchly. I don’t know how much the new TPV cell will cost, especially since it isn’t being mass-produced yet; in addition, the design of the thermal battery system will determine how many square feet of TPV cells will be required, which in turn will affect the cells’ design as well as the economics of the overall facility. This said, the fact that the system as a whole will have so few moving parts as well as the availability of both sunlight and graphite or firebricks, or even molten silicon, which has a high heat capacity, keep the lucre of MIT’s high-temperature TPVs alive.

Featured image: A thermophotovoltaic cell (size 1 cm x 1 cm) mounted on a heat sink designed to measure the TPV cell efficiency. To measure the efficiency, the cell is exposed to an emitter and simultaneous measurements of electric power and heat flow through the device are taken. Caption and credit: Felice Frankel/MIT, CC BY-NC-ND.