Steam as Working Fluid

Steam is one of the most important energy carriers in industry. It transports thermal energy in steam systems, drives turbines for power generation—thermodynamically based on variants of the Rankine cycle—and serves as process steam across a wide range of industries. A key advantage is that steam is already available in many facilities, making it relatively straightforward to integrate open heat pump systems into existing steam networks.

In mechanical vapor recompression systems (MVR, also referred to as vapor recompression or vapor compressors), steam itself can be used as working fluid. In this case, the steam is compressed from a lower pressure level to the required steam pressure. The compressor is electrically driven. This is referred to as an open heat pump process.

Working Fluids vs. Heat Transfer Fluids

In heat pump systems, a fundamental distinction is made between working fluids (refrigerants) and heat transfer fluids. Heat transfer fluids transport thermal energy between the heat source, the system, and the process. They circulate through piping and heat exchangers but are generally not part of the thermodynamic cycle itself. Typical examples include water, water–glycol mixtures, and thermal oils.

Advantages of Steam as a Working Fluid

Steam exhibits several properties that make it particularly well suited for industrial heat pump applications:

• High energy density during phase change: When condensing, steam releases large amounts of latent heat.
• Safe and readily available: Water is inexpensive, non-toxic, and non-flammable.
• Efficient heat transfer: Condensing steam transfers heat very effectively to processes and equipment.
• Precise controllability: Heat output can be accurately controlled via the pressure and temperature of saturated steam.

Temperature Ranges and Applications

Many industrial processes require steam in the range of approximately 100 °C to well above 200 °C. High-temperature heat pumps are capable of supplying steam within this range. Typical applications are found in industries with high steam demand, such as:

• Food and beverage processing
• Pulp and paper production
• Chemical industry
• Drying and evaporation processes

The combination of closed-cycle heat pump systems such as the ThermBooster™—which utilize synthetic and natural refrigerants (e.g., hydrocarbons)—and open heat pump systems enables both the recovery of low-temperature waste heat and the generation of high-pressure steam. As part of the AHEAD project at Takeda Pharmaceutical Company’s Vienna production site, the ThermBooster™ generates steam, which is subsequently compressed to 11 bar in a vapor recompression system.

Relevance for Decarbonization

Using steam as a working fluid enables the conversion of waste heat into usable process energy, thereby reducing the reliance on fossil-based heat sources. This can significantly lower both energy consumption and CO₂ emissions—making it a key lever for the decarbonization of industrial process heat.