Analysis of Environmental and Economic Benefits of Integrated Exhaust Energy Recovery (EER) for Vehicles

Differing from those traditional vehicle exhaust heat recovery systems which just provided thermal energy directly for cabin warming, integrated Exhaust Energy Recovery (EER) which is researched and developed mainly in recent years aims to convert exhaust thermal energy to mechanical or electric energy for increasing the total thermal efficiency and the total power of powertrain. In the study presented in this paper, an analytic model was built for examining the environmental and economic benefits of integrated EER systems. Then the improvement on the total powertrain efficiency and net reduction of CO2 emissions were investigated, in terms of the average vehicle used in the UK. Results show that, for light duty vehicles fitted with thermal cycle EER system, the cost increase could be paid back in 10.1 years and CO2 emission could be paid back in just 1.9 years, compared to Hybrid Electric Vehicle’s (HEV’s) 11.9 years and 1.4 years for cost and CO2 emission, respectively. When the annual fuel price increase is considered, the cost pay-back is reduced to 8.1 years for EER vehicles and 8.9 years for HEVs. Higher mileage vehicles will have more obvious advantage for fitting EER system. When doubled annual mileage is considered, EER system can reduce the cost and CO2 emission pay-back times to 2.7 years and 0.6 years, compared to HEV’s 8.5 and 2.7 years, respectively. KEYWORD Exhaust energy recovery, CO2 emission, cost, pay-back time, passenger car INTRODUCTION In recognition of the need to further reduce vehicle exhaust emissions and the greenhouse gas CO2, there has been a quickly increased interest in the development of cleaner and more efficient energy saving vehicle powertrain. When the cost for obtaining even a 1% increase on the engine combustion efficiency is significant, 2 technology innovation around vehicle powertrain has involved more on hybrid configuration (such as Hybrid Electric Vehicles HEVs) and integrated Exhaust Energy Recovery (EER) in recent years. In the current research, integrated EER refers to those new technologies beyond conventional uses for exhaust waste heat such as turbocharger or cabin air-heating. While HEV technology has achieved considerable market share in recent years, R&D on EER is being paid more attention, particularly while energy collected by EER can be easily applied on HEVs [1, 2, 3]. Normally the maximum net brake efficiency of Internal Combustion (IC) engines is difficult to be higher than 42% [4], large amount fuel energy is rejected from the engine to the surroundings as waste heat in several forms, with a significant fraction through the exhaust. A recent study [5] estimated in a typical 2.0 litre gasoline engine used on passenger cars, 21% of the released energy is wasted through the exhaust at the most common load and speed range. The fraction increases to 44% at the peak power point. On average, about one third of energy generated from the fuel is wasted via exhaust gases. Current estimates of waste thermal energy from ground vehicle systems range from 20kW to 400 kW, depending on engine size and engine torque-speed conditions. This is equivalent to annually 45 billion gallons of gasoline fuel lost through the exhaust pipes of the 240 million light-duty passenger (LDP) vehicles in USA alone [6]. LDP vehicle exhaust systems operate at gas temperatures from 500 to 900 °C, typically between 600 and 700 °C. For Heavy-Duty (HD) vehicles, exhaust gas temperatures range from 500 to 650 °C under general driving condition. These can be further boosted during periodical regenerations of diesel particulate filter (DPF) and other aftertreatment advices [7]. Those high exhaust temperatures provide significant opportunities for EER to generate energy for increasing powertrain’s efficiency [6, 8]. Differing from conventional exhaust energy utilising technologies such as turbocharger, cabin air-heating [9, 10], desalination [11] and reducing engine warm-up time [12], integrated EER which has been mainly focused in recent years mainly include thermal cycle system based on Rankine Cycle (RC) and Thermoelectric (TE) regeneration. The latter can directly convert part of the exhaust heat to electric power through the thermoelectric phenomenon, without the use of mechanically rotating parts, and providing some advantages such as compact package, without noise and vibration, and high reliability. However, there exist significant system design challenges during the development of TE system due to its low conversion efficiency and relatively high costs of thermoelectric semiconductor materials [13, 14]. So far, turbochargers and recently developed other turbo-compounding systems have been selected as the first option for most exhaust waste energy recovery of IC engines. However, as the increase of exhaust back pressure caused by the turbine of turbocharger or turbo-compounding system, the system efficiency is limited, compared to RC EER system [15, 16]. On the other hand, as the turbine always needs necessary pressure ratio, the exhaust gas sensible heat absorbed by turbocharger or turbo-compounding system is constrained and the exhaust temperature from the turbine is always still very high and a lot sensible heat is still contained. This allows a RC EER system still being able to be fitted downstream even a turbocharger or turbo-compounding system has been installed. A RC EER system does not increase the exhaust back pressure obviously.