INTRODUCTION

National energy security, rising energy prices, increasingly competitive global markets, and stringent regulations for environmental emissions are the primary driving forces in the search for sustainable and economically viable technologies that incorporate efficient and clean approaches to energy conversion and utilization.  Internal combustion (IC) engines are the prime movers of choice when high power densities and efficiencies are desirable.  Because of relatively cheap fuel prices in the last few decades, IC engines had been optimized for high power densities and low emissions.  However, in recent years, with escalating fuel prices and concerns about sustainability, engine efficiency has assumed greater importance.  Over the last 150 years since the invention of the IC engine, great strides have been made in improving the fuel conversion efficiency of and reducing emissions from the IC engine. Further improvements in fuel conversion efficiency require a system-level analysis of the various losses encountered in the IC engine.  This analysis can begin with thermodynamic modeling of the IC engine.  Traditional first law–based thermodynamic models facilitate accurate energy accounting; that is, they are useful in estimating the net losses associated with the combustion process.  However, these models do not provide estimates of how much of the wasted energy is recoverable as useful work or exergy.  This requires second law–based models that track the irreversibilities associated with various processes that destroy fuel chemical exergy.  There are two pathways for better utilization of the chemical exergy of the fuel.  The first focuses on minimizing exergy destruction in the combustion process.  The second pathway involves tapping the exhaust exergy to obtain further improvements in the thermal efficiency of the prime mover.  This can be achieved by using a bottoming organic Rankine cycle (ORC). Waste heat recovery (WHR) using ORC involves the utilization of the sensible enthalpy of the hot exhaust from an IC engine to heat an organic fluid, preferably to saturated/superheated vapor, after which the sensible enthalpy of the vapor is used to obtain additional useful work from a turbine. Therefore, in general, exhaust waste heat recovery from stationary engines using ORC has the potential to increase fuel conversion efficiency.

ORGANIC rankine cycle analysis

Figure 1 shows a basic ORC coupled as a waste heat recovery cycle to a stationary engine.  The exhaust from the engine is assumed to be routed through the evaporator, where heat transfer occurs between the exhaust stream and the organic working fluid.  A counterflow heat exchanger (evaporator) configuration is considered to maximize heat transfer between the exhaust and the organic fluid.  Thermodynamically, this is a preferred configuration because the temperature difference between the hot fluid and the cold fluid is minimized, reducing exergy destruction.  The heated organic fluid then is expanded in a turbine, heat is rejected to the ambient in the condenser, and the cooled working fluid is pumped back into the evaporator.

An important factor that affects the efficiency of an ORC is the selection of the organic working fluid.  The working fluid must be selected carefully on the basis of safety and technical feasibility.  A good working fluid should have low toxicity, good material compatibility and fluid stability limits, and low flammability, corrosion, and fouling characteristics.  Refrigerants are good candidates for ORC applications as a result of their low toxicity characteristics.  Organic fluids can be classified as dry, wet, and isentropic, depending on the slope of the saturation curve in the T-s diagram (Figure 2).  On a T-s diagram, a dry fluid has a positive slope, a wet fluid has a negative slope, and an isentropic fluid has infinitely large slopes.  Dry and isentropic fluids show better thermal efficiencies than wet fluids.  One of the reasons for this is that dry and isentropic fluids do not condense after the fluid goes through the turbine, as opposed to wet fluids.  For the case presented, R113, which is a dry fluid, was selected as the working fluid because it has been shown to be a good candidate for ORC applications.¹ Further, it is assumed that waste heat from the engine will be used to heat the organic fluid from subcooled liquid to saturated vapor.  This condition was preset since previous studies have reported that dry organic fluids do not need to be superheated.²

Figure 1. Schematic of an engine-ORC configuration

Figure 2. Schematic representation of isentropic, wet, and dry fluids

COMBINED ENGINE-ORC CONFIGURATION

As was mentioned above, exhaust waste heat recovery from stationary engines using ORC has the potential to increase fuel conversion efficiency and reduce break specific emissions.  The fuel conversion efficiency of the engine,, and the engine-ORC configuration, , can be estimated as follows:

where is the lower heating value of the fuel,   is the mass of fuel, and  and are the engine power and ORC power, respectively.

Therefore, it is clear that exhaust WHR using an ORC results in higher power with the same fuel energy input into the system.

Figure 3 illustrates the engine efficiency of a dual fuel (diesel pilot-ignited natural gas fired) engine* (from experiments³), the combined engine-ORC efficiency (predicted from ORC simulation), and the percentage of increase between the two cases at representative diesel pilot injection timings of 20°, 40°, and 60° BTDC (before top dead center) for a single-cylinder Caterpillar 3401 (1Y SCOTE) engine with simulated turbocharging operating at half load (21 kW power). From this figure, it can be seen that using a combined engine-ORC configuration, the thermal efficiency can be incremented by approximately 10 to 13 percent of the baseline values for all injection timings.

An important parameter that affects the combined engine-ORC system performance is the pinch point temperature difference (PPTD), which is defined as the difference between the exhaust gas temperature and the temperature at which the organic fluid first begins to vaporize (Figure 4). This is the smallest temperature difference in the evaporator (ORC heat exchanger), and it defines the performance limits of the ORC heat exchanger. The T-∆H diagram used for the pinch point analysis is illustrated in Figure 4.  The heat transfer rate across the ORC heat exchanger is proportional to the PPTD. As the PPTD increases, the mass flow rate of the organic fluid decreases, and this results in poor utilization of the exhaust energy.  To accomplish heat transfer across smaller PPTD values, larger heat exchanger areas are required.  This leads to larger and more expensive heat exchangers. However, the exergy efficiency of heat transfer across a smaller temperature difference is much higher (i.e., this leads to lower exergy destruction). Therefore, there is a clear cost versus efficiency trade-off in selecting evaporators in ORC design.

Another important parameter that affects the performance of the exhaust waste heat recovery using an ORC is the evaporator temperature and evaporator effectiveness. This temperature has to be selected properly to avoid condensation of water in the evaporator. It is important to prevent water condensation to reduce the potential for corrosion in the evaporator tubing. In general, the first and second law efficiencies of an ORC increase with the increment of the evaporator temperature, which will also increase the overall performance of the combined engine-ORC system. Regarding the evaporator effectiveness, it is clear that as the evaporator effectiveness increases, the PPTD decreases.  These trends also indicate that higher exergy efficiencies are possible by choosing higher evaporator effectiveness values. However, as was discussed above, this entails a higher cost as a result of the fact that larger evaporators are needed to facilitate heat transfer across smaller temperature differences.  However, favoring lower evaporator effectiveness presents a practical problem of water condensation in the evaporator tubing. Therefore, a balance point has to be found between the evaporator temperature and the temperature at which condensation will not be present.

Figure 3. Engine and combined engine-ORC efficiencies at various injection timings

Figure 4.  T-∆H diagram used for the pinch point analysis in the evaporator

CONCLUSIONS

The fuel conversion efficiency of stationary power engines can be improved by using organic Rankine cycles to recover the exhaust waste heat.  The operation of a combined engine-ORC system yields a fuel conversion efficiency improvement of the order of 10 to 15 percent.

REFERENCES

1. Mago, P. J., Chamra, L. M., and Somayaji, C. “Analysis and Optimization of Organic Rankine Cycles.” IMechE Journal of Power and Energy, vol. 221, no. 3, May 2007, pp. 255–263.
2. Mago, P. J., Chamra, L. M., Srinivasan, K., and Somayaji, C. “An Examination of Regenerative Rankine Cycles Using Dry Fluids.” Applied Thermal Engineering, vol. 28, no. 8–9, June 2008, pp. 998–1007.

Srinivasan, K. K., Mago, P. J., Zdaniuk, G. J., Chamra, L. M., and Midkiff, K. C. “Improving the Efficiency of the Advanced Injection Low Pilot Ignited Natural Gas Engine Using Organic Rankine Cycles.” ASME Journal of Energy Resources Technology, vol. 130, June 2008, pp. 022201-1–022201-7.

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* A diesel pilot-ignited natural gas engine employs small amounts of diesel fuel injected at a pre-determined time (injection timing) into the combustion chamber to ignite the inducted natural gas-air mixture.