Achieving high power heat-recovery systems using molecularly-complex fluids

Achieving UK and EU emissions targets requires a transformation in the power generation and manufacturing industries. In the UK we consume 350TWhr of electricity every year, but with modern power-stations which are typically around 50% efficient a large proportion of energy is wasted as rejected heat. Recovering just 10% of this heat would save the equivalent power output of 22 power stations. This is not to mention the heat which could be recovered from manufacturing industries where large quantities of energy are wasted through the heating and cooling during metal-forming processes. In order to make heat-recovery economically viable, low-temperature Organic Rankine Cycles (ORC) can be deployed using fluids with boiling-points close to ambient temperatures, such as many 'molecularly-complex' fluids. The power is extracted in an ORC across a turbine, where these 'molecularly-complex' fluids exist in a gaseous state, and pass through the turbine at high speeds. Increasing the power extracted from the turbine makes heat-recovery systems much more economically favourable and can be achieved by raising the pressure ratio across the turbine. In order to do this efficiently requires a better understanding of molecular-complex gas flows because there is very little known about these complex flows in turbines. The lack of an in-depth understanding of the molecular complex gas-dynamics in ORC turbines means that it is unlikely that optimum power levels are being achieved with present-day design methods.

Therefore this proposal aims to determine methods of significantly increasing heat-recovery system power outputs by exploiting the effects of molecular complexity in Organic Rankine Cycle turbines. A target is set of doubling current turbine power levels. In order to determine methodologies to achieve this, a combination of experimental and computational tests are planned. Experiments of molecularly complex gas flows will be studied using a specially designed experimental test-rig which will be able to mimic the flow conditions found in the ORC turbine. The computational simulations will involve the use of a research flow-solver, which will be modified to account for molecular-complex gas properties. The experimental data will aid the development of an accurate computational model, which will then be used to determine novel turbine blade designs to operate at high pressure ratios.

This research will directly benefit both the fluid-mechanics research community and the power-generation industry. The research will improve our fundamental understanding of the fluid mechanics of molecularly complex fluids, and will also aid the development of sustainable power generation technologies. An improved understanding of molecular-complex gas flows in turbines has the potential to substantially reduce the UK's fossil fuel dependence and improve our ability to recover currently otherwise 'wasted' heat from power stations and manufacturing processes as well as solar and geothermal radiation. This has a large societal benefit both in-terms of aiding the fight against climate-change and improving the UK's energy security. This work will help towards meeting the targets of the UK Climate Change Act 2008 to reduce by 34 percent our greenhouse gas emissions by 2020 and 80 percent by 2050, against the 1990 baseline.

By determining methods of significantly increasing heat-recovery system power outputs, this proposal has the potential to have a transformative impact on large and small scale electrical power production, metal forming industries, and renewable power systems such as solar and geothermal. If the average power station within the UK recovered the waste heat it rejected at an efficiency of just 10%, we would save about 42TWhr per year, which is 12% of the UK electrical energy consumption. This can be achieved using Organic Rankine Cycles (ORC) operating with molecular complex working fluids. The economic incentive for using a heat-recovery ORC is very dependent on installation and operating costs since the heat input to the cycle tends to come at little cost. Capital costs and operational costs are thus directly related to the plant's size, and therefore for a given power requirement, the costs will be inversely proportional to the specific-power output from the ORC turbine. Increasing turbine specific-power outputs makes the implementation of heat-recovery systems economically favourable and thus facilitates a transformative reduction in man-made emissions levels. This has a large societal benefit both in-terms of aiding the fight against climate-change and improving the UK's energy security.

This research has an impact on the power-generation and turbomachinery industry both within the UK and worldwide.This proposal is being supported by GE Global Research, who are providing financial backing for this work. GE are global leaders in power generation and supply Organic Rankine Cycles (ORC) throughout the world. GE are in a position to implement the outcomes of this work internationally, which gives this proposal a truly global impact. GE recognize the requirement for research in this area because the modern turbine design methods and principles used in industry, which up to now have been based largely on classical gas dynamics and simple real-gas relations, may be grossly inappropriate for the case of the ORC turbine. Indeed several recent computational studies have highlighted the need for experimental data to establish the veracity of modern computational methods for ORC turbine flows because of the inherent uncertainties in the simulating flows with complex equations of state. By combining experimental testing with computational modelling, this project will determine how turbine designers can accurately simulate the flows in ORC turbines, which will be crucial to determining new high performance turbine designs.

As well as the benefits to the production of more sustainable power this has an important impact to the wider turbomachinery and fluid mechanics research community. Currently, there is a lack of experimental work of molecularly complex gas flows, especially within turbomachinery environments. In particular, it is not well known how shock waves develop in these flows, and hence how shock losses will differ from more conventional working fluids, such as steam or air. This is important because raising turbine power levels will require raising the turbine Mach numbers, which normally leads to increases in shock-wave related loss. Recent computational work indicates that molecular complexity effects can reduce or even eliminate shock-waves. This can be exploited to increase turbine power, efficiency, and operability, although has yet to be proven experimentally. The proposed experimental and computational work will determine variations in shock loss at a range of ORC typical conditions so that this can be potentially exploited in the turbine design process. In particular, this could lead to practical shock-free blade designs methods to be determined. This will have a significant impact on turbine performance both in terms of efficiency and operability.