Ever notice that when politicians tour a factory they remove their sportcoats? If you’ve walked an industrial plant you know it can be a pretty sweaty experience. An energy engineer knows this heat can represent a big energy efficiency opportunity.
The most common waste heat recovery application reuses flue or stack heat to produce hot water. With heat exchangers tightly coupled to machines the heat can be captured into water and transported into another hot water process thereby reducing boiler loads. Where an industrial process is less continuous, hot water storage tanks (energy storage) can be added, which cost more, but enable the time shifting use of the waste heat. Where an industrial process produces extremely high temperatures exhaust (1000 degree +) a potentially more powerful opportunity exists – turning this heat into steam, a much higher density energy source than hot water.
Lately our engineering team has been studying customer sites where high temperature exhaust is plentiful – and old concepts can be applied to turn it into usable steam. This would be a system based on the Rankine Cycle, similar to the engines that powered the 1900’s era steam driven automobiles. As a reminder of how technology changes, in 1902 steam driven automobiles outnumbered internal combustion engines by 2 to 1, and a steam driven automobile held the land speed record. You know the rest of the story – the internal combustion engine eventually won that battle.
With industrial waste heat there remains a technical debate between two engine types, the Organic Rankine Cycle and the Stirling engine regarding the effectiveness.
Both systems have advantages and disadvantages. Stirling engines, which are external combustion piston engines, are very efficient at higher speeds but have low starting torque, typically use helium or hydrogen as the working fluid and require a minimum constant temperature source of 500F or greater. They also tend to be daunting in size. ORC turbine engines can be much smaller, can operate on much lower temperature heat sources (200F), are enabled by a working fluid which vaporizes at much lower temps. ORC working fluids tend to be “organic” compounds like the refrigerant found in your air conditioner which do break down more easily over time and need to be replaced as their effectiveness dissipates. Both use this mechanical energy to convert to electricity….
While today many biomass and geothermal utility scale plants already utilize the Rankine Cycle model for steam driven generators, time will tell if the technology can be sized down for these plant level applications and electrical production. Our initial energy models say that with either engine there will be a ton of energy savings the systems can deliver and run reliably. So field tests will teach us a lot and time will tell if we’ll see the return of the Stanley Steamer in every plant.
