Organic fluids hold key to more efficient solar thermal power

In an EPSRC-funded project, researchers at City University are investigating different types of Organic Rankine Cycle (ORC) power plants, in a bid to reduce the cost of the technology.

Unlike conventional steam power plants, ORCs pressurise and heat an organic fluid to produce vapour, which is then expanded to generate electricity. This allows lower temperature heat sources of between 100 to 300 degrees Celsius to be converted into power more economically than using steam.

As a result, the technology could be used to produce electricity from low cost distributed concentrating solar power collectors, or from waste heat generated by large internal combustion engines, such as those found in trucks and small ships, according to Prof Abdulnaser Sayma, who is leading the project.

However, while ORCs have already been successfully applied on an industrial-scale, more research is needed to understand the fundamental physics behind the devices when operating at these smaller scales.

In particular, devices need to be developed that can operate efficiently over a range of different conditions without altering their fundamental design, to allow them to be manufactured at high volumes and therefore at a lower cost, said Sayma.

“There are some gaps in fundamental understanding that we want to explore, to find out how we can select a suitable type that can be used at those different sizes and temperatures,” he said.

To this end, the researchers plan to investigate two different types of ORC power plant, which appear promising for small scale electricity generation applications of around 50 kilowatts, to determine how they perform.

The devices are based on the use of either turbo or screw expanders. Turbo expanders, which operate at very high speeds, are equipped with a turbine with continuously rotating blades. In a screw expander, in contrast, the vapour is expanded in a cavity to push apart two screws, in a similar way to a piston engine.

“Turbo expanders are higher speed, more compact and have a higher efficiency, but screw expanders have the advantage of being cheaper to produce,” said Sayma.

The researchers hope to determine which of the two designs is the most suitable for use across a range of different operating conditions.