Development of a static shaft Wankel expander for small-scale applications

Tozer, Gavin ORCID: 0000-0003-0248-1015 (2020). Development of a static shaft Wankel expander for small-scale applications. University of Birmingham. Ph.D.

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Abstract

With increasing global demand for energy and the problems of climate change from extensive use of fossil fuels new ways to increase the renewable energy sources and reduce energy waste are required. Energy storage, such as liquid air energy storage (LAES), is one way to improve renewable energy utilisation. The organic Rankine cycle (ORC) is a system that allows the recovery of low-grade heat energy, which can be in the form of waste heat or geothermal heat. Both of these technologies are also good for distributed energy production, which reduces losses associated with energy transport.

This thesis looks at the development of a Wankel gas expansion device for use in gas liquefaction or ORC systems. It starts with an extensive literature review into LAES and ORC systems. In which literature shows a clear need for the development of both gas liquefaction systems and small-scale, low-cost and efficient gas expanders. A review of available gas expanders is then presented followed by a detailed review of Wankel expansion devices. This review concludes that the Wankel expander has many qualities making it suitable for small-scale low-cost systems but has the issues of friction and the requirement for external valves which need to be addressed.

The next part of the thesis describes the creation of numerical models to simulate gas liquefaction systems. The results of these models suggest that the Kapitza system performs the best, and the most effective way to increase its efficiency is improving the performance of the expansion device. Next, the creation of computational fluid dynamics (CFD) models for two different types of Wankel expander are described. One type is a standard Wankel expander with side ports, whilst the second is a newly designed static shaft Wankel expander based on the original DKM Wankel engine. The results of the CFD simulations show the main drawback of the standard Wankel expander is that it only reaches a maximum isentropic efficiency of 64.88%, due to lack of inlet control. The static shaft Wankel expander simulations show that it could reach isentropic efficiencies of 87.35% and can be designed for a large range of inlet pressures. CFD is also performed using two organic working fluids often used in organic Rankine cycle (ORC) systems, giving maximum isentropic efficiency of 85.6%. This demonstrates that the expander could be used in an ORC system and achieve similar performance to compressed air systems.

Finally, an experimental test rig was designed and two static shaft Wankel expander prototypes were manufactured, one from plastic and one from metal. The prototypes were tested to find their power output and isentropic efficiency performances. Problems with the initial prototype design were found and a second prototype design was manufactured to address these issues. The experimental results were found to agree well with the CFD power output (average of 5.4W deviation), but with a much higher deviation for the CFD efficiency (average of 10.1% deviation), this was thought to be due to leakages not accounted for in the CFD models.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):