Medea Black Technology: The efficiency of the heat engine with no moving mechanical components is similar to that of steam turbines

This article is from MIT News, original title A new heat engine with no moving parts is as efficient as a steam turbine, compiled with changes.

Engineers at the Massachusetts Institute of Technology and the National Renewable Energy Laboratory (NREL) designed a heat engine with no moving mechanical components. Their new research shows that it converts thermal energy into electricity with an efficiency of more than 40 percent — a performance that is superior to traditional steam turbines.

The heat engine is a thermal photovoltaic (TPV) cell, similar to a photovoltaic cell of a solar panel, that passively captures high-energy photons from a white-hot heat source and converts them into electrical energy. The team's design can generate electricity from a heat source of 1900 to 2400 degrees Celsius.

The researchers plan to incorporate TPV batteries into a grid-scale thermal battery. The system will absorb excess energy from renewable resources such as the sun and store this energy in heavily insulated hot-melt graphite. When this energy is needed, such as on a cloudy day, the TPV battery will convert heat into electrical energy and distribute the energy to the grid.

Using this new TPV battery, the team has now successfully demonstrated a major part of the system in separate small-scale experiments. They are working to integrate these parts to demonstrate a fully operational system. If successful, they hope to scale up the system to replace fossil fuel-powered power plants and enable a fully decarbonized grid powered entirely by renewable energy.

Robert Neuss Career Development Professor Asegun Henry in MIT's Department of Mechanical Engineering said: "Thermal photovoltaic cells are the final critical step in proving that thermal cells are a viable concept. This is an absolutely critical step on the road to promoting renewable energy and achieving a complete decarbonization of the grid. ”

Henry and his collaborators published their results in the journal Nature. MIT co-authors include Alina LaPotin, Kyle Buznitsky, Colin Kelsall, Andrew Rohskopf and Evelyn Wang, ford professor of engineering and chair of the Department of Mechanical Engineering, as well as collaborators with Kevin Schulte and NREL in Colorado-based Golden.

Crossing the gap

More than 90% of the world's electricity comes from heat sources such as coal, natural gas, nuclear energy and concentrated solar energy. For a century, steam turbines have been the industry standard for converting these thermal energy into electricity. On average, steam turbines reliably convert about 35 percent of their thermal energy into electricity, and the most advanced heat turbines can achieve about 60 percent conversion efficiency. However, such machines rely on mechanical parts with temperature limits. If a heat source higher than 2000 degrees Celsius is used, such as henry's proposed thermal battery system, the temperature is too hot for steam turbines.

In recent years, scientists have studied solid-state alternatives to heat engines — ones without mechanical components that have the potential to work effectively at higher temperatures. Henry said: "One of the advantages of solid-state energy converters is that they can operate at higher temperatures and are less expensive to maintain because they don't have movable mechanical parts, they just stay there and reliably generate electricity. ”

Thermal photovoltaic cells offer an exploratory route to solid-state thermal engines. Much like solar cells, thermal photovoltaic cells can be made from semiconductor materials with a specific band gap (the gap between the price band of the material and its conduction band). If a photon with a sufficiently high energy is absorbed by the material, it can kick an electron through the band gap, where the electrons can then conduct electricity, generating electricity — doing so without the need to rotate rotor or turbine blades like a traditional generator.

To date, most thermal photovoltaic cells have only reached an efficiency of about 20%, and the highest can reach 32%, because they are made of relatively low band gap materials, which convert photons with lower switching temperatures and lower energy, so the efficiency of converting energy is lower.

Capture the light

In Henry's team's new TPV design, Henry and his colleagues hope to capture higher-energy photons from a higher-temperature heat source to convert energy more efficiently. Compared to the existing design, the team's new battery uses a higher band gap material and multiple junctions or layers of material.

The battery consists of three main regions: a high bandgap alloy, which sits on top of a slightly lower bandgap alloy, under which is a mirror-like layer of gold. The first layer captures the highest energy photons of the heat source and converts them into electrical energy, while the low-energy photons that pass through the first layer are captured by the second layer and converted into the resulting voltage. Any photons passing through the second layer are reflected by the mirror and returned to the heat source, rather than being absorbed as wasted heat.

The team tested the efficiency of the battery by placing it on a heat flux sensor, a device that directly measures the heat absorbed from the battery. They exposed the battery to a high-temperature lamp and focused the light on the battery. They then changed the intensity or temperature of the bulb and looked at the battery's power efficiency — how the power it produced varied with temperature compared to the heat it absorbed. In the range of 1,900 to 2,400 degrees Celsius, new thermal PV maintains an efficiency of about 40%.

Henry said: "We can achieve high efficiency over a wide temperature range associated with thermal batteries. ”

The battery in the experiment is about one square centimeter. For a grid-scale system of thermal batteries, Henry envisions TPV batteries that will have to expand to about 10,000 square feet (about a quarter of a football field) and will operate in climate-controlled warehouses to extract energy from huge solar-powered energy storage facilities. He pointed out that there is already an infrastructure to make large-scale photovoltaic cells, which can also be used to make thermal photovoltaic cells.

Henry said: "In terms of sustainability, there is definitely a huge positive here. The technology is safe, environmentally sound throughout its life cycle and can have a huge impact on reducing CO2 emissions from electricity production. ”

The study was partially supported by the U.S. Department of Energy.

Source: Towards carbon neutrality