Biomass CHP: ORC turbine (Organic Rankine Cycle)

What is an Organic Rankine Cycle (ORC) system and how is it different to a steam turbine? Learn about the typical outputs and costs and the pros and cons of using an ORC turbine to power your biomass CHP installation.

ORC systems are based on similar principles to steam turbines but use organic silicon oil as the operating medium, rather than water or steam. Organic oils employ lower temperatures and pressures compared with steam-based systems.

Wood-fuelled combustion heats the pressurised oil to 300°C, which is then allowed to expand to produce a high-energy oil vapour to drive a turbine or electrical generator. The turbine is water-cooled and the water is output from the turbine at 90°C, which is ideal for glasshouse heating.

The electrical efficiency is around 17% of the total energy input – about 3% higher than traditional steam turbines.

The advantages of the ORC system are that it performs well under partial load and, because of the low working pressures and temperatures, it is not subject to the regulations of steam equipment. ORC units are suitable for biomass CHP plants with an electrical output as small as 200 kW. Single ORC units go up to 2 MWe in size and multiple units can be installed to increase capacity.

ORC systems typically operate with a ratio of heat to electricity generation of around 5:1.

Typical outputs and costs of a Biomass CHP ORC system

*24 hour operation

Pros

• Easy to operate and maintain, as they run at low pressure, and therefore lower staff costs
• Can be configured to run on recycled timber
• Highly incentivised through government subsidies
• Reasonably scalable for smaller applications

Cons

• Large footprint
• High initial outlay
• Low electrical efficiency
• High proportion of heat to electricity
• Can only provide heat at a maximum of 90°C

An organic rankine cycle CHP system

Image courtesy of NFU energy.