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Researchers have developed a new heat engine with no moving parts

Combining elements of a heat engine with photovoltaic cells might be the ticket to replacing traditional steam engines.

Researchers have developed a new heat engine with no moving parts
A thermophotovoltaic (TPV) cell. Felice Frankel/MIT

Researchers from the Massachusetts Institute of Technology (MIT) and the National Energy Laboratory (NREL) have just released information on a new type of heat engine with no moving parts. The engine is roughly 40% efficient, and could one day replace conventional steam turbines in the future. 

Their results have just been published in the journal Nature

Called a thermophotovoltaic (TPV) cell, the new engine shares some common characteristics with traditional photovoltaic cells, but it captures high-energy photons from a white-hot source to generate electricity. This new engine is able to generate power from temperatures between 3,400 and 4,300 degrees Fahrenheit (1,900 - 2,400 degrees Celsius).

The future plan for the heat engine is to incorporate the cells into a grid-scale thermal battery that could absorb excess heat thermal energy from sources like the Sun and store that energy in heavily insulated banks of hot graphite. When the energy is actually needed, the TPV cells could then convert the heat into electricity and supply that to the grid to fill the gaps in supply from renewables when they are not able to meet demand. 

However, that is for the future. At present, the team has managed to successfully demonstrate the main constituent parts of such a system, but on a small scale. They are currently working on a way to put all the parts together to make a demonstration testbed of the real thing. 

Once that has been achieved, they then hope to scale the thing up with the long-term goal of replacing fossil-fuel-driven power plants. 

“Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept,” explains Asegun Henry, the Robert N. Noyce Career Development Professor in MIT’s Department of Mechanical Engineering. “This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid.”

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This could prove revolutionary for the energy industry

At present, the lion's share of global energy production comes from sources like coal and natural gas as well as some large-scale renewable sources like nuclear and concentrated solar power. The main technology that drives these methods are steam turbines which are still the de facto means of turning heat into electricity 

This has proved effective, but this technology has changed very little for over a century. But, they are not that efficient, as it turns out. 

Typically, steam turbines are able to use around 35% of the energy provided by their heat source into useful electricity, with some engines able to provide efficiencies of up to 60%. This is great, but steam turbines suffer from one major weakness - they require moving parts that can break down over time. 

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Parts must also be able to tolerate high temperatures for long periods of time. Parts will eventually wear out over time. 

To combat this, some researchers like those behind the new thermophotovoltaic engine have looked into solid-state alternatives that could provide direct replacements for traditional steam engines. 

“One of the advantages of solid-state energy converters [is] that they can operate at higher temperatures with lower maintenance costs because they have no moving parts,” Henry added. “They just sit there and reliably generate electricity.”

TPV cells could be just the ticket for this sort of thing. They could also be made from semiconducting materials with a particular bandgap — the gap between a material’s valence band and its conduction band. In such circumstances, if a photon with a high enough energy is absorbed by the material, it can kick an electron across the bandgap, where the electron can then conduct, and thereby generate electricity. It has a huge potential boon for reducing the global economies' reliance on this old technology and, subsequently, fossil fuels. 

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“There’s definitely a huge net positive here in terms of sustainability,” Henry says. “The technology is safe, environmentally benign in its life cycle, and can have a tremendous impact on abating carbon dioxide emissions from electricity production.”

This research was supported, in part, by the U.S. Department of Energy.

Abstract: 

"Thermophotovoltaics (TPVs) convert predominantly infrared wavelength light to electricity via the photovoltaic effect and can enable approaches to energy storage1,2 and conversion3,4,5,6,7,8,9 that use higher temperature heat sources than the turbines that are ubiquitous in electricity production today. Since the first demonstration of 29% efficient TPVs (Fig. 1a) using an integrated back surface reflector and a tungsten emitter at 2,000 °C (ref. 10), TPV fabrication and performance have improved11,12. However, despite predictions that TPV efficiencies can exceed 50% (refs. 11,13,14), the demonstrated efficiencies are still only as high as 32%, albeit at much lower temperatures below 1,300 °C (refs. 13,14,15). Here we report the fabrication and measurement of TPV cells with efficiencies of more than 40% and experimentally demonstrate the efficiency of high-bandgap tandem TPV cells. The TPV cells are two-junction devices comprising III–V materials with bandgaps between 1.0 and 1.4 eV that are optimized for emitter temperatures of 1,900–2,400 °C. The cells exploit the concept of band-edge spectral filtering to obtain high efficiency, using highly reflective back surface reflectors to reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device reached a maximum efficiency of (41.1 ± 1)% operating at a power density of 2.39 W cm–2 and an emitter temperature of 2,400 °C. A 1.2/1.0 eV device reached a maximum efficiency of (39.3 ± 1)% operating at a power density of 1.8 W cm–2 and an emitter temperature of 2,127 °C. These cells can be integrated into a TPV system for thermal energy grid storage to enable dispatchable renewable energy. This creates a pathway for thermal energy grid storage to reach sufficiently high efficiency and sufficiently low cost to enable decarbonization of the electricity grid."

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