What is an Organic Rankine Cycle?
A Rankine cycle is a thermodynamic cycle that converts energy in the form of heat into mechanical work by the constant evaporation and condensation of a circulating working fluid. The Rankine cycle is composed of five main components, vapor-generator (evaporator), expansion device (turbine), condenser, regenerator, and the feed-pump.
An Organic Rankine cycle (ORC) uses an organic working fluid (such as n-pentane) which has favorable operating performance at lower source temperatures (150-300 C). They often require an intermediate thermal loop which uses a heat-transfer medium (such as thermal oil) to carry and transfer the heat from the waste heat source to the ORC evaporator. This thermal oil loop allows flexibility in the ORC location on site as well as ensures the organic fluid does not reach too high of a temperature.
Organic Rankine Cycle Steps
To explain the steps that occur in the ORC, we first begin with the hot exhaust stream as it passes through a heat-exchanger (the waste heat recovery unit) which increases the temperature of the thermal oil. The outlet temperature of the exhaust following this exchanger is chosen to be above the exhaust condensation temperature, preventing excessive corrosion and wear on the heat exchanger. The cooled exhaust is then released through a flue stack. The hot thermal oil is directed to the organic fluid evaporator, where it causes the high-pressure liquid organic fluid to evaporate and become a vapor. Then, the cooled thermal oil is pumped and directed back to the heat exchanger to repeat the process and recover more waste heat in a closed loop. The organic vapor is expanded through a turbine, which in turn rotates a generator producing electricity. This electricity can be consumed on site or exported to the grid.
Following the turbine, the low-pressure organic vapor passes through a regenerator, which cools the vapor further. The vapor is then cooled by air-cooled condensers, becoming a liquid. This liquid then enters a pump, increasing its pressure and providing the necessary flow through the closed-loop ORC system. It is then preheated in the regenerator, which increases the cycle efficiency, before entering the evaporator and completing the cycle. The power required for the two pumps (thermal oil and organic fluid) and the air-cooled condensers is provided by the ORC and therefore no electricity is required from the grid during operation.
Efficiency of an Organic Rankine Cycle
The efficiency of an Organic Rankine Cycle (ORC) is determined by its ability to convert low-temperature heat into mechanical work and can be analyzed through thermal, cycle, and system efficiency.
Thermal efficiency is the ratio of useful work output to heat input, affected by the heat source temperature and the organic working fluid’s properties.
Cycle efficiency depends on the effectiveness of key components, including the evaporator, turbine, condenser, and regenerator.
System efficiency encompasses the performance of ancillary components like pumps and heat exchangers.
To enhance ORC efficiency, strategies include using advanced organic fluids, optimizing heat exchangers, and integrating regenerative systems. While ORCs may have lower thermal efficiencies compared to high-temperature Rankine cycles, their ability to utilize lower temperature heat makes them effective for waste heat recovery and renewable energy applications.
