pocketORC: Organic Rankine cycle design model

Operating principle of an ORC

A simple organic Rankine cycle (ORC) power system is based on four thermodynamic processes, each of which is carried out by a specific piece of equipment. The cycle is also a closed loop in that after carrying out the four process sequentially the properties of the circulating working fluid return to the original conditions. A schematic of this cycle is shown in the figure at the bottom of this section.

We start our cycle at the inlet to the pump, referenced as station 1, where the working fluid starts in a cold, liquid state at low pressure. The job of the pump is to increase the pressure of the liquid to a higher pressure, which is achieved via the addition of work using a motor.

After the pump, referenced as station 2, the high pressure liquid then enters the evaporator which uses the available heat source to heat up and evaporate the working fluid, thus generating vapour at a high temperature and pressure.

This vapour then moves to the expander inlet, referenced as station 3, where the fluid is expanded. During this process, the pressure of the working fluid reduces, and mechanical work is extracted by the expander. This mechanical work is converted to electricity using a generator.

The working fluid leaving the expander, referenced as station 4, is at low pressure but is still in a vapour state. The final process is to cool down and condense the working fluid using a condenser. This returns the working fluid back to its initial state. During this process, heat is rejected from the cycle to the available heat sink.

The efficiency of the simple ORC system can be improved with the addition of a recuperator. The job of this component is to use the heat contained within the working fluid leaving the expander in order to heat up the liquid leaving the pump. This reduces the amount of heat required from the heat source, and reduces the amount of heat rejected to the heat sink. This cycle is also reported in the figure below.

Fixed parameters

The general goal for an ORC designer is to design the system for a particular application. This generally corresponds to a known heat source and heat sink. The calculator assumes both the heat source and heat sink are defined as sensible fluid streams with a finite mass-flow rate and constant specific-heat capacity. Enter those details below.

Cycle variables

To optimise the performance of the ORC system, the ORC designer has a number of different levers to pull. This includes changing the working fluid (i.e., the fluid circulating within the system), alongside a number of different thermodynamic variables. You can select the working fluid, and provide values for the cycle variables below.

Single cycle study

Having now selected your set of variables, it's time to see how the cycle performs. Hit the button below to find out. The results in the table summarise key performance metrics, as well as the work or thermal load of each component. The UA values are an indicator of the heat exchanger requirements and the pinch points indicate the minimum temperature differences in each heat exchange process.

It is also useful to view the cycle on a thermodynamic plane of choice. You can choose between temperature-entropy, pressure-enthalpy and pressure-volume. In the resulting figures, the green region indicates the state points of the working fluid throughout the cycle. Where relevant, the heat source (red) and heat sink (blue) temperature profiles are superimposed onto the cycle.

Note: Negative pinch points (reported in red below) indicate unphysical heat exchange profiles, and therefore indicate infeasible ORC system configurations. In general, the best performance can be expected when the pinch points are minimised, but still positive. However, very low pinch points imply large heat transfer areas.

Parametric study

Want to understand how the different cycle variables can influence performance? Then pick a variable from the drop-down box below, alongside the minimum and maximum values you want to consider, and hit run to populate the table. All other variables are assumed constant as defined earlier.

Remember, negative pinch points indicate unphysical heat exchange profiles, and therefore indicate infeasible ORC system configurations.