The Organic Rankine Cycle

The conversion of heat into work is easily accomplished by way of the Rankine Cycle – a thermodynamic cycle which is fed heat through a closed loop. This cycle typically uses water as its primary functional fluid. However, the Organic Rankine Cycle makes use of an organic, high molecular fluid with concomitant phase-changes of liquid vapour. The fluid boils at a significantly lower temperature than that of the water-based Rankine Cycle. It is interesting to note that the Rankine Cycle – the precursor to the Organic Rankine Cycle – provides for the overwhelming majority of global electricity production.

Pioneering Spirit Engendered in the Organic Rankine Cycle

The fathers of the prototype for the Organic Rankine Cycle include Lucien Bronicki and Harry Zvi Tabor. These engineers pioneered the development of this groundbreaking technology as far back as 1961. The thrust of this engineering marvel can be found in its ability to convert heat recovery from biomass combustion, solar ponds, and other waste sources. The lower temperature heat can actually be converted into highly useable energy output – work. This is in turn converted to electricity.  The process is much the same for the Organic Rankine Cycle as it is for the standard water-based Rankine Cycle. In the latter, the fluid is the water, whereas in the former it is the organic, molecular fluid – such as pond water, waste products and so forth.

The fluid in Organic Rankine Cycle is passed through a boiler where it evaporates. It then moves through a high-powered turbine where is condenses. There are two types of processes taking place with the Organic Rankine Cycle – the expansion process and the evaporation/condensation processes. The former is isentropic while the latter processes are isobaric. The process is not always without error however. There are numerous efficiency-related errors which periodically occur. These take place during the expansionary phase where only a portion of the energy is converted into work. There are substantial energy reserves which are lost though the heat conversion.

The Process Simplified

Organic Rankine Cycle generators are a vital part of renewable energy and cogeneration. In simple terms, the fluid (organic in nature) is fed into what is known as a ‘closed loop’ through a pump. The fluid makes its way through what is known as a regenerator coil. It is there that it is reheated. After this stage, the organic fluid moves into the preheater. Following this it passes through the evaporator. This is where it gets interesting –  thermal oil from within the boiler is introduced into the mix and it basically increases the fluid’s temperature until the point of evaporation is reached. The vapour then rapidly expands. This results in electrical energy being produced via the generator. The vapour then contributes towards the reheating of the organic fluid in the re-generator.  This is an efficient use of energy as the heated vapour reheats the fluid.