The goal of this Department of Energy project is to increase the efficiency of o

Question

The goal of this Department of Energy project is to increase the efficiency of on-road heavy duty diesel vehicles (HDDV's) by > 10%. One method of achieving these goals is to recovery energy from the exhaust gases and/or engine block and use that energy to power a Rankine Cycle. In this type of application, T_H is much lower than that available in a typical steam power plant and therefore steam is not used as the working fluid. Instead a refrigerant is used as the working fluid (hence the name "organic" Rankine Cycle, since these refrigerant molecules typically contain carbon). Using R-134a as a working fluid, estimate the maximum amount of power (kW) that you can extract from the engine block by replacing the engine liquid coolant loop of a 300 HP engine with an Organic Rankine Cycle. Calculate the thermal efficiency of such a Rankine Cycle. Make some quick and (to your best ability) reasonable assumptions. Given this maximum theoretical amount of additional power, is this concept economically viable?

Explanation / Answer

Assumption Exhaust Temperature of 650 C

                   Engine Efficiency of 25 %

                   Coolant Lower Temperature (TL)   = 15 C

Amount of Energy available for rankine cycle = 0.75 * 300 = 225 horsepower

1) assuming ideal rankine cycle we have efficiency   = 1 - (273 + 15) / (273 + 650)     =   68.8 %

2) Therefore theoretical power which can be extracted from available energy = 0.688 * 225   = 154.8 hp

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