Within the fourth section of this ORC-101 we will focus a little more on a specific thermal-power system that is suitable for converting relatively low-temperature heat into mechanical power. This cycle is referred to as the organic Rankine cycle.

Within ORC-101 we have so far introduced the concept of a thermal power system and a working fluid. We have also introduced the concept of a Rankine cycle and a Brayton cycle, noting that the difference between the two is that a Rankine cycle operates with fluid that is either in a liquid or a gas state within the system, whilst within a Brayton cycle the fluid remains in the gas state at all times.

Thus, to recap, the operation of a Rankine cycle can be described as follows:

  • First, a liquid is compressed using a pump which increases it’s pressure. This pump is driven by an electric motor.
  • This cold, high-pressure liquid is then heated up using the heat source, which converts the cold liquid into a hot gas.
  • This high-temperature, high-pressure gas is then expanded across a turbine, which causes the rotor of the turbine to rotate, generating power.
  • The low-pressure gas is then cooled by transferring heat to the ambient surroundings. This converts the low-pressure gas back into a liquid before the process can repeat.
TPS

Diagram showing the components that make up a basic Rankine cycle.

The most common type of Rankine cycle is a steam Rankine cycle. This means that the working fluid within the cycle is water, which upon heating is converted into steam. This type of cycle is widely used in steam power plants to generate heat from a variety of heat sources.

An organic Rankine cycle, referred to simply as an ORC, is essentially the same cycle, but instead of operating with water it operates with a different working fluid. More specifically, it means the cycle operates with an organic fluid, which is any fluid that contains carbon.

The advantage of replacing water with an organic fluid is that different fluids behave in different ways, which has an important impact on the performance of the Rankine cycle. In particular, ORC systems perform better than steam Rankine cycles at lower temperatures (typically below 400 degrees Celcius), meaning they can produce more power for the same amount of heat. This improved perfomance at these lower temperatures means they are a promising candidate to convert low-temperature heat into electricity. This includes heat from the sun, deep underground or waste heat from industry.