Enter high temperature heat pumps, which can essentially steal heat from the air and increase it to above 212 °F. They’ve been around for a while, boiling water and heating homes, but the temperatures these units reach are not enough for most industrial needs. In fact, to some, that kind of temperature is not high temperature at all.
“It’s difficult to have a proper definition of high temperature heat pumps,” said Alberto Patti, a researcher in the Department of Mechanical, Energy, Management, and Transport Engineering at the University of Genoa, Italy, and lead author of a “Feasibility Study of a Brayton-Based High Temperature Heat Pump for Waste Heat Recovery in Industrial Applications.” He added, “Sometimes I hear people say ‘high temperature’ about district heating at 120 °C. Come on, this is not a high temperature for me.”
It’s temperatures above 180 °C that are useful for manufacturing, and those are temperatures too high for the refrigerants used in most heat pumps. So, Patti took another tack: the reverse Brayton cycle, which is literally the reverse of the Brayton cycle used in jet engines. It uses power to force a gas the opposite direction, through a turbine, to make cold—or hot—air hotter, in seeming defiance of the laws of thermodynamics.
“Jokingly, it’s also called the counterintuitive Brayton,” Patti said. “The good thing about the reverse Brayton is that you can achieve, theoretically, as much temperature as you want. But then we have some technological limitations.”
Those limitations are simply the temperature the component materials can handle and the heat that can be extracted from the elaborated streams.
Where other systems are closed, in that the gas is compressed and expanded repeatedly, Patti’s is open, and the gases, once they’ve given back much of their heat, are released into the atmosphere. This open system is achieved by using the gases produced from the flue of a boiler stack from the industry itself. Those gases do not change phase, so the system is not restricted by the same temperature limitations that vapor-compression heat pumps face.
After proving that the concept would work theoretically, Patti and his team partnered with an industry that produces laminates for furniture and architecture. They kept their system as simple as possible, using only one off-the-shelf compressor and other readily available parts. In essence, flue gases were compressed until they hit 215 °C, then sent through a heat exchanger, and finally expanded through the turbine, the revolutions of which return some of the energy needed for the compression.
While the turbine’s role may seem small, it is essential. Without it, the coefficient of performance (COP) drops to a useless 1.
“In typical Brayton cycles, we are used to seeing turbine inlet temperatures of 1,000 °C—huge temperatures, and also huge pressures,” Patti said. “While here we don't have much pressure at the inlet of the turbine, so the shaft is not moving very fast. But it’s good enough to make the system, let’s say, competitive.”
As it is, the COP is 1.5. That’s not much compared to COPs of 2 to 4 found in today’s commercial high temperature heat pumps. But that number may go up when the components are designed specifically for the reverse Brayton cycle that Patti has developed. More to the point, it doesn’t really matter: as long as the COP is above 1, it is essentially free heat that industry wouldn’t otherwise be able to use; at that point, it’s mainly a matter of cost effectiveness, which is why the simplicity of the open reverse Brayton is key.
The prototype Patti used as a proof of concept has indeed proved the concept, but it’s too small to be used at the scales that industry really needs. Patti hopes to eventually make a bigger version and to try it out first in paper mills, where high heat is essential to evaporating water from pulp. Perhaps eventually the reverse Brayton cycle will allow us to recover a great percentage of the heat produced in industries that have to use fossil fuels.
Michael Abrams is a technology writer in Westfield, N.J.
