ORC working fluids: a comprehensive overview

What are ORC fluids?

ORC fluids are high-molecular-weight organic compounds that, within a closed thermodynamic cycle, act as an energy carrier: they absorb heat from the heat source, evaporate, expand in the turbine to generate mechanical energy, and finally condense, completing the cycle.

The term “organic” refers to the chemical nature of these compounds, which belong to different families, hydrocarbons, siloxanes, and refrigerants, and differ from water, the working fluid in traditional steam Rankine cycles, due to their unique thermophysical properties.

These properties make them particularly well-suited for operation within a temperature range that water vapor cannot utilize with the same efficiency. Unlike water, organic fluids have lower boiling points, higher molecular weights, and saturation curves that allow for expansion in the turbine without droplet formation, thereby preserving the integrity of the components and ensuring reliable, long-term operation.

Why do ORC systems use organic fluids?

The choice of organic fluids in ORC systems is not arbitrary, but rather meets a specific technical need: to efficiently utilize low- and medium-temperature heat sources, where water and steam would be inadequate or inefficient.

In the traditional Rankine steam cycle, operating temperatures are typically high and require complex infrastructure, very high working pressures, and sophisticated water management systems. Organic fluids, on the other hand, allow operation at more moderate pressures and with slower turbine rotation, significantly simplifying the plant layout and reducing installation and maintenance costs.

Another key advantage is the absence of erosion and corrosion of mechanical parts: since the organic fluids expand within the turbine in a dry state, they do not generate droplets that could damage the blades, ensuring longer component life and high plant availability over time. This results in essentially automatic operation, without the need for specialized operators to be present at all times, and with maintenance requirements reduced to a minimum.

What fluids are used in ORC systems?

Over the years, research and development in the ORC sector have led to the identification and use of various families of organic fluids, each with specific characteristics that make it better suited to certain operating conditions and temperature ranges.

• Light hydrocarbons. Compounds such as pentane, butane, and isobutane are among the most widely used fluids in medium- to high-temperature applications. They offer good thermodynamic performance and low costs, making them a proven and cost-effective solution for many types of systems.
• HFC and HFO Refrigerants. Fluids belonging to the hydrofluorocarbon (HFC) and hydrofluoroolefin (HFO) families are primarily used in low-temperature cycles. Compared to hydrocarbons, they offer advantages in terms of operational stability and safety, with generally more favorable flammability profiles.

• Silicone fluids and aromatic compounds. For high-temperature applications, where the thermal stability of the fluid is a critical requirement, siloxanes and aromatic fluids are used. These compounds provide excellent resistance to thermal degradation even at very high operating temperatures, making them the preferred choice for high-intensity heat sources.

Over time, Turboden has developed in-depth knowledge of these families of fluids, having used more than 10 different working fluids throughout its history. This experience constitutes a unique technical asset, enabling the company to accurately assess the properties of each fluid, from thermal stability to flammability, based on the specific conditions of each individual installation.

How is the working fluid selected in an ORC system?

Selecting the working fluid is one of the most delicate and critical steps in the design of an ORC system. There is no single, universally optimal fluid: the choice depends on a combination of technical, environmental, and operational factors that must be evaluated holistically for each specific application.

1. Thermodynamic matching with the heat source. The first selection criterion concerns the thermodynamic compatibility between the fluid and the available heat source. The temperature of the heat source and its cooling profile must be carefully analyzed to identify the fluid that maximizes cycle efficiency and overall energy recovery.
2. Thermal and chemical stability. The selected fluid must be able to operate stably at the expected operating temperatures throughout the system’s service life, without degrading and without compromising the integrity of the components with which it comes into contact.
3. Operational safety. A fluid’s flammability characteristics and operating pressures directly affect the level of complexity of the required safety systems and, consequently, the costs and management of the plant. A fluid with a low-risk profile simplifies the plant layout and reduces applicable regulatory requirements.
4. Environmental and regulatory compliance. A refrigerant’s GWP and its classification under F-gas regulations are essential criteria in the selection process, particularly in light of the progressively stricter European regulations on fluorinated gases. Choosing refrigerants with a low environmental impact is not just a matter of compliance, but increasingly represents an element of responsibility and strategic vision.
5. System complexity. Finally, the choice of fluid affects the overall complexity of the system: operating pressures, compatibility with system materials and components, and fluid containment and management requirements are all factors that contribute to the final design.