The Rankine Cycle transfers waste heat to a working fluid at a constant pressure. The fluid is vaporized and then expanded in a vapor turbine that drives a generator, producing electricity. The spent vapor is condensed to liquid and recycled back through the cycle. (The difference between the Organic Rankine Cycle (ORC) and the classical Rankine Cycle is the use of an organic working fluid instead of water.)

The Rankine cycle is widely used used commercially to generate power in stationary power plants, and has been considered as a potential waste heat recovery system for use in both light-duty and heavy-duty applications.>/p>

A 2011 Ford study found that an ORC waste heat recovery system using R245fa as the working fluid could nearly supply the full vehicle accessory electrical load in a light duty vehicle (Ford Escape) on the EPA highway cycle in both a conventional and hybrid powertrain configuration. (Earlier post.)

A 2012 study by a team from the University of Stuttgart and the Ohio State University concluded that a simple ORC waste heat recovery system could recover up to 10% of engine waste heat under highway driving conditions, corresponding to a potential 7% improvement in fuel consumption in a light-duty plug-in hybrid electric vehicle (PHEV), with low penalization of the added weight to the vehicle electric range. (Earlier post.)

The purpose of the ORCnext project was to increase the efficiency—and thus increase industry interest in adopting—ORC waste heat recovery systems. Experts assume that the annual unused industrial waste heat potential amounts to 140 TWh in Europe alone, implying a CO₂ reduction potential of about 14M ton of CO₂ per year. Nevertheless, adoption of ORC has been limited as overall (and seasonal) efficiency was too low and as a consequence the financial benefits were limited.

The ORCnext consortium believed this was due to two main factors:

1. The efficiencies of the cycles were too low for low temperature waste heat sources, resulting in too low energy production.

2. Current ORCs on the market aimed at a too high a waste energy flow.

The ORCnext team explored new cycle architectures that could raise efficiency, with one step being the use of supercritical working fluids. Another focus was efficient systems for smaller power ranges. For this, specific expander technology is needed, which the consortium explored using CFD.

The ORCnext team defined five challenges:

1. Improving the efficiency—i.e., the amount of heat energy that is actually transformed into electrical energy. A key element in this is the expander.

2. Control over dynamic behavior. ORC technology should not only exhibit high efficiencies at full load, but also at partial load. This requires advanced control strategies with an optimization towards the full cycle profile.

3. Reducing the design time. It is key to identify the best architecture and set points for this architecture based on boundary conditions without the need for extensive simulations. This requires a design methodology.

4. Test-infrastructure. Providing accurate benchmark data, testing facilities to validate new systems and expert support for these tests is important to further prove the benefits of this technology.

5. Economic analysis. ORC technology as an emerging technology should be analysed correctly from an economic and financial point of view to find out its commercial limits compared to other technology such as steam cycles, Kalina cycle, and others.

Thermodynamic analysis showed that three alternative cycle architectures were of interest for further analysis: the triangular cycle (TLC), the transcritical cycle (TCORC) and a newly developed partially evaporating cycle (PEORC). Partial evaporation increases the net power output of the system compared to the conventional subcritical ORCs. This increase in performance is accomplished with a lower pumping power and volume ratio over the expander.

The results indicated that for low heat source inlet temperatures (around 100 °C) the PEORC clearly outperformed the TCORC in second law efficiency by up to 25.6%, while the TCORC outperforms the conventional subcritical ORC in second law efficiency by up to 10.8%. For high heat source inlet temperatures (around 300 °C) the performance gain of alternative cycles compared to the subcritical ORC becomes small.

An advanced CFD model of a standard air screw compressor used as an expander indicated the most important bottlenecks of the original design. A new design of the expander was developed and it was analysed by this advanced CFD model and a dynamic low-order model. If a higher mass flow rate is available, then the power output can increase by 50% using additional injection ports.

A Model Predicted Control strategy was developed which allows to produce about 21% more net electrical output power compared to classical PID based control.

A supercritical heat exchanger was designed, constructed and tested. Using literature correlations, the experimental validation showed that the heat exchanger is oversized by 10%. These data are able to serve as benchmarks for further development

Due to the ORCNext project, Dana has built up broad knowledge about Organic Rankine Cycles. An important result for Dana is that for a system optimized on the basis of power per cost, the standard Rankine Cycle with preferably isentropic fluids leads to the best compromise between simplicity, compactness and power output.

—Dana, one of the members of the user group