Researchers at MIT have built a highly efficient thermophotovoltaic cell that, when paired with renewable resources, efficiently converts incoming photons — particles of light — to electricity. It’s an achievement that could inspire new ways of supplying the world with energy.
“The problem is, you don’t get [renewable] energy when you want it,” Asegun Henry, mechanical engineer at MIT and author of the new Nature study, explained in a video call. “You only get it when the weather is favourable: when the Sun is out or the wind is blowing.” The answer to this dilemma lies in what Henry calls “thermal batteries,” where power from renewable sources of energy, such as solar, is stored as heat.
Thermal batteries could “dispatch” energy to the power grid whenever it’s needed, Henry said. Lithium-ion batteries aren’t sufficient for this purpose. “Lithium-ion batteries are unfortunately too expensive, and there have been a number of studies that have looked at how cheap the storage has to be in order for us to have a fully renewable grid,” Henry explained. “So that’s where we developed this technology — thermal batteries — because storing energy as heat rather than storing it electrochemically is 10 to 100 times cheaper.”
How it works
The thermophotovoltaic cell relies on some fundamental semiconductor physics. The atoms within a semiconductor’s alloys have band gaps, that is, the distance between the valence shell of electrons and the conduction band. When the electrons in the valence band are energised, they get excited (like yourself as you read this article) and jump from the valence band to the conduction band. This jump results in a release of energy, in which the precise amount of energy released is governed by the distance of the band gap. In other words, the amount of energy that gets released is determined by how much energy the electron needs to jump across the band gap.
The electrons in this thermophotovoltaic cell are located within its alloys, which are stacked atop each other like the layers of a cake. The cell is made from two layers of semiconducting alloys and one reflective layer of gold. The alloys in this experiment were chosen according to the wavelength of the photons required to fuel the cell at its highest efficiency. Should “you want to absorb light at a particular frequency, you can figure out which alloys will give you the right band gaps that you want,” said Henry.
The position of the alloys within the heat engine was also an important factor. The first layer was designed to have the largest band gap in order to capture the highest-energy photons. Photons not captured by the first layer then fall through to the second layer and push electrons across a smaller band gap. If a photon doesn’t have enough energy to push an electron across the gap in the first or second layer, that’s where the reflective layer of gold can reflect photons back into the light source to reduce energy waste. The twist, however, is where these photons come from.
Working in a controlled lab environment, Henry and the research team obtained the photons from superheated metal located directly above the heat engine.
“We were sending electricity to a resistive heater that was a few feet away,” Henry explained. This resistive heater was like a complex lightbulb filament — a conductor that glows and becomes superheated when energy passes through it. The hot, glowing metal released photons that were captured by the alloy layers, which generated electricity in the heat engine; the researchers found that an element heated to between 1,900 and 2,400 degrees Celsius provided them with the best efficiency.
In a lab, it’s easy to plug a resistive heater into a wall socket, but the researchers have real-world scenarios in mind. Ideally, they would like to store energy derived from renewable resources into these big batteries, which they could then access with the heat engines.
What the heat engine could do
To store energy as heat, a renewable energy source would power the resistive heaters that heat up liquid metal. The liquid metal would then get pumped over blocks of graphite, something Henry describes as a “sun-in-a-box.” The hypothetical sun-in-a-box would operate at half the temperature of the actual Sun and would then power the resistive heaters that send photons to the heat engines, which would be stored on top of each other in a large array.
Henry was quick to acknowledge that this sounds like something out of a sci-fi novel, but research done by the same team five years ago inspired them to keep pushing the methodology forward. They were the first to demonstrate that it was possible to pump liquid metal above 1,000 degrees Celsius, an accomplishment that earned them a Guinness World Record for the highest temperature of liquid metal pumped.
He said a potential hazard of a full-scale thermal battery and heat engine power supply is that it would operate in an oxygen-free environment. “This thing is going to be held inside of a warehouse filled with inert gas, like argon gas,” Henry explained. “That environment doesn’t have air, so you can’t just walk in there.” Ideally, the storage system would be designed such that any servicing could be done remotely, but he said regular inspections and fixes could still be done safely.
“We would want to go take a look during annual maintenance, and so you just cool the system down, or cool a portion of it down, and send someone in,” Henry told me. “If you had some emergency, you could cool the system down and send someone in with essentially scuba gear and an oxygen tank.”
Their thermophotovoltaic cell operates at 40% efficiency, which is better than previous designs and comparable to steam turbines. It’s a promising result, and Henry and his colleagues are now striving for an even bigger goal: scaling this technology to a warehouse-sized power station that could be patched into the existing grid.