Heat is energy, and wasted heat is wasted energy.
Several established and emerging technologies exist to turn that wasted energy into value.
Organic Rankine Cycle
Terrapin and Capstone primarily implement Organic Rankine Cycle (ORC) systems that convert waste heat to electric power (see Figure 2 for the ORC process). ORC systems are the most mature and widely deployed low-grade heat to power solutions. There are more than 565 ORC plants worldwide, generating 2.75 GW of electricity.
ORC systems typically use a series of heat exchangers and piping and/or ducting to direct a heat medium (e.g. exhaust gas, cooling water, glycol) to the system. ORC systems operate on the same principle as a traditional steam-based power plant, in that a fluid is cyclically heated, expanded through a turbine, then cooled. However, instead of using water as a working fluid, ORC systems utilize organic fluids such as pentane, butane or various refrigerants such as RF245a to capture useful work at lower temperatures. These organic fluids boil and much lower temperatures than water, making them more suitable for capturing the heat from industrial processes.
Terrapin sizes ORC systems to each application and designs the system to integrate seamlessly with the facility’s existing operations. Terrapin typically works with systems between 1 and 10 MW of nameplate electrical capacity.
Various global ORC installations
Figure 2: ORC Process Diagram
Heat exchanger transfers heat from the intermediate fluid (heat source) to the organic working fluid
Organic fluid is vaporized in the evaporator
High temperature, high pressure vapour is expanded through the turbine
Regenerator passes heat from the vapour to preheat the working fluid
Condenser rejects the remaining heat, returning the fluid to a liquid state
Pump provides pressure to continue the cycle
There are three main factors that Terrapin considers in the design of an ORC system before implementation:
Terrapin considers the choice of working fluid, the type of heat expander, and environmental factors during the Project Framing phase of project development. Terrapin works with facility operations and engineering personnel to ensure the best suited technology is fitted into the resulting heat recovery system.
Stirling engines are externally heated, closed cycle, reciprocating heat engines, which typically use an inert gas as the working fluid.
In its simplest form, the Stirling engine has two reciprocating components: the displacer, and the piston. The displacer transfers the inert gas working fluid back and forth between a heated chamber and a cooled chamber of the engine. Since the working fluid is sealed permanently inside the engine, the temperature swings caused by the motion of the displacer cause pressure swings in the working fluid. These pressure swings cause a piston to move, expanding the working fluid at high pressure and compressing it at low pressure, resulting in a net work output. This work can be used to drive an electrical generator.
Stirling engines are best used:
With "small", intermittent heat sources that vary in temperatures
For situations where a non-toxic working fluid is required
Stirling engines have the potential for higher efficiency and lower cost than ORC engines if they are implemented on a small scale. Terrapin is currently researching and developing one in-house intended for variable heat sources.
Model of Terrapin's in-house Stirling engine in development
Other Emerging Technologies
The enormous amount of wasted energy contained in the surplus heat of industrial processes attracts a large amount of research attention. Thermoacoustic engines and thermoelectric generators are the two emerging technologies that are closest to those in commercial use. They are at a similar stage of development to Stirling engines.
Thermoacoustic engines use tuned resonators to induce a sound wave from a heat gradient. The vibrations from the standing sound wave can be harnessed as a prime mover to generate electricity. Although less efficient than Stirling Engines or ORC systems, these devices have nearly no moving parts and are mechanically simple and inexpensive. There are several companies working on commercializing these sorts of engines for waste heat recovery.
Thermoelectric generators are solid-state devices that directly convert a heat gradient into electricity via a phenomenon called the Seebeck Effect. Thermoelectric cells are currently very inefficient and expensive compared to other heat-recovery technologies. However, considerable research is being done, and if a breakthrough is made then these devices could revolutionize heat-to-power possibilities.
Terrapin is aware of these technologies and others in development, but implement predominantly ORC engines due to the scale or projects and low-grade heat used.