What Are The Steps In The Organic Rankine Cycle?

The organic Rankine cycle (ORC) is a thermodynamic cycle that converts heat into mechanical work. It is similar to the traditional Rankine steam cycle, but uses an organic fluid with a lower boiling point instead of water as the working fluid. This allows the ORC to utilize lower-temperature heat sources than steam cycles.

In the ORC, the working fluid is pumped to a boiler where heat is added, causing the fluid to vaporize. The high-pressure vapor then expands through a turbine or other expander, generating power. The low-pressure vapor leaving the turbine is condensed to a liquid which is pumped back to the boiler, completing the cycle.

The ORC is commonly used for waste heat recovery, geothermal power plants, solar thermal electric plants, and bottoming cycles for gas turbines. It can generate power from low-grade heat sources between 80-350°C. The lower boiling point of organic fluids compared to water enables more efficient energy recovery from these lower temperature sources (Fluorocarbons – EFCTC). Some advantages of ORC systems include simplicity, low maintenance, and the ability to match the working fluid to the heat source temperature.

Table of Contents

Heat Addition

The heat addition process is where an external heat source provides energy to the organic Rankine cycle (ORC). This heat source boils and vaporizes the working fluid in the evaporator.

Common heat sources used in ORC systems include geothermal heat, waste heat from industrial processes, heat from biomass combustion or solar thermal collectors, and heat from internal combustion engines or gas turbines.

The key requirements for the heat source are that it can provide sufficient energy above the boiling point of the selected working fluid. The higher the temperature of the heat source, the greater the efficiency of the ORC system. However, even relatively low temperature heat sources (as low as 80-100C) can be utilized effectively.

Some common working fluids used in ORC systems include hydrocarbons (like pentane or butane), siloxanes, and refrigerants like R245fa. The boiling point of the working fluid is chosen to match the temperature profile of the available heat source.

Pump

The pump pressurizes the organic working fluid from a low pressure at the condenser outlet to a high pressure before entering the evaporator (Organic Rankine Cycle | Exergy ORC). Typical pump types used in organic Rankine cycles include centrifugal pumps, positive displacement pumps like piston pumps and scroll pumps, and electromagnetic pumps (Organic Rankine cycle). The pump converts mechanical work into pressure energy to enable heat addition downstream.

Centrifugal pumps are commonly used due to their high volume flow rate capabilities. Positive displacement pumps can achieve higher pressure differences but have lower flow capacity (Organic Rankine cycle). The choice of pump depends on the working fluid properties and desired pressure rise.

The pump accounts for a major portion of the parasitic losses in the organic Rankine cycle. Regenerative heating of the working fluid before the pump inlet can improve cycle efficiency by reducing the pump work input required (Organic Rankine Cycle | Exergy ORC).

Evaporator

The evaporator is where the organic working fluid is vaporized by the addition of heat. The source of heat input varies depending on the application, but can include waste heat from industrial processes, geothermal heat, solar thermal energy, biomass combustion, or other sources.

There are two main types of evaporators used in organic Rankine cycles:¹

Shell and tube heat exchangers – The organic fluid flows through tubes surrounded by the hot fluid or combustion gases in the shell.
Plate heat exchangers – The fluids exchange heat as they flow in channels between metal plates.

The evaporator design aims to efficiently transfer heat to vaporize the organic working fluid, while accounting for the thermophysical properties of the particular fluid used. Proper thermal design helps maximize the efficiency and power output of the organic Rankine cycle system.

Expander

The expander is one of the key components in the organic Rankine cycle. It converts the energy of the high pressure vapor into mechanical work that can be used to drive a generator or other machinery.

As the vapor from the evaporator enters the expander, it expands and does work by pushing against the blades or rotors of the expander. This causes the expander to rotate, turning a shaft that can be connected to the load. The reduction in pressure as the vapor expands causes its temperature to decrease as well.

There are several types of expanders used in organic Rankine cycles:

Turbines: Often axial or radial flow turbines similar to those used in steam cycles. Well-suited for high temperature and pressure organic fluids. Axial turbines are compact and have high efficiency.[1]
Screw expanders: Twin screw expanders are positive displacement devices consisting of two intermeshing rotors. They are suitable for small to medium scale organic Rankine cycles.[2]

Scroll expanders: These are also positive displacement devices, using two interleaved spiral-shaped scrolls. One is fixed while the other orbits around it, creating expanding pockets. Scroll expanders are simple and reliable.

The optimal expander is selected based on the working fluid properties and desired power output.

Generator

The superheated vapor from the evaporator then enters and expands in an expander or turbine to generate electricity. The pressure and temperature of the vapor drops as it expands through the turbine. There is a generator connected to the turbine shaft which converts the mechanical energy into electrical energy (ORC System). The expanding vapor causes the turbine blades to rotate, which then spins the rotor of the generator to produce electricity. This is the main power generating component of the organic Rankine cycle (ORC System: Organic Rankine Cycle | Exergy ORC). The turbine rotation speed varies based on the vapor conditions and design (Organic Rankine cycle).

Condenser

In the condenser, the vapor from the turbine is condensed back to a liquid by rejecting heat. This is a crucial step because the cycle requires the working fluid to be in liquid form before being pumped again.[1] The main condenser types used in organic Rankine cycles are water cooled, air cooled, and evaporative.[2]

In a water cooled condenser, the vapor condenses as it flows through tubes with cooling water flowing on the outside. The rejected heat is absorbed by the cooling water. Air cooled condensers use finned tubes and fans to reject heat to the ambient air. Evaporative condensers use water sprays or wet surfaces to aid in heat rejection from the vapor.

The condenser pressure and temperature are important parameters in the organic Rankine cycle. Lowering the condenser pressure allows for a larger expansion across the turbine and improves efficiency. However, the condenser temperature must stay above the freezing point of the working fluid to avoid solidification.

Pre-Heater

The pre-heater is an optional stage in the organic Rankine cycle that can help improve the overall efficiency of the system. As the name suggests, the pre-heater pre-heats the working fluid before it enters the evaporator using waste heat from the cycle. Specifically, it uses heat recovered from the expander outlet to bring the working fluid up to an intermediate temperature before vaporization.

As explained on SWEP’s Organic Rankine Cycle page, “A preheater can recover the waste heat from the expander outlet and use it to preheat the cold liquid before the evaporator. This reduces the amount of heat needed in the evaporator.”

By reducing the amount of heat needed in the evaporator, the pre-heater allows more of the available waste heat to be converted into useful work by the expander. This improves the thermal efficiency and net power output of the organic Rankine cycle system.

Regenerator

The regenerator is an optional stage in the organic Rankine cycle that can help improve the overall efficiency. As described in Zhang et al. (2014) ^[1], the regenerator acts as a heat exchanger, transferring leftover heat from the expander exhaust to the liquid before it enters the evaporator. This preheats the liquid and means less external heat needs to be added in the evaporator. Groniewsky et al. (2020) ^[2] found the regenerator can improve efficiency by over 10% in some cases. The improved efficiency comes from recycling waste heat back into the cycle rather than losing it. Overall, the regenerator allows more work to be extracted per unit of heat input.

Conclusion

In summary, the main steps of the organic Rankine cycle are heat addition in an evaporator, expansion through a turbine or other expander, condensation in a condenser, and feed pumping back to the evaporator. The cycle also often includes regeneration and preheating to improve efficiency.

The organic Rankine cycle is used for generating power from low-grade heat sources, such as waste heat from industrial processes, geothermal heat, solar thermal energy, and biomass combustion. It can achieve reasonable efficiency by using organic working fluids with lower boiling points than water.

Advantages of the organic Rankine cycle include the ability to utilize heat sources below 100°C, relatively low maintenance costs due to the lack of erosion from organic fluids, and modular construction that allows for distributed power generation. The cycle can also be designed for cogeneration to produce both electricity and useful heat.

Future improvements to the organic Rankine cycle aim to increase thermal efficiency through advanced expander designs, better working fluids, turbomachinery innovations, and optimized thermal integration. There is also continued research into applications like waste heat recovery, geothermal, solar thermal, and biomass power systems.