Modeling of a Thermal Energy Recovery System based on the Brayton Cycle utilizing a gas-to-gas heat exchanger

Abstract

In a world dominated by energy requirements and efficiency, one simple source of energy is taken for granted on a daily basis; the exhaust gas of internal combustion engines (ICE). In a simple Otto or Diesel cycle engine, efficiency can range from 25 to 50%, which entails that the remaining 75 to 50% is not utilized and normally rejected into the atmosphere. This heat is dissipated at moderate to high temperatures and hence is ideal for extracting useful work. However this extraction is not as straightforward as it seems as there are numerous factors that influence the feasibility for engine makers in the current world of propulsion. There are real world practical examples that illustrate that this is indeed possible. Examples include the Detroit DD15 engine, the Wärtsilä Sulzer RTA-96C and Formula 1 engines. But the aforementioned list indicates one of the challenges, in that the engines above serve vastly different goals, and hence an engine designed with exhaust gas extraction needs to be service specific. The above engines do however all have one thing in common, in that they mostly provide work at a very narrow operating band. An everyday commuter car, by contrast, operates at extremely varying conditions and this is another challenge that needs solving. Adding additional components on to an internal combustion engine also has to be evaluated from a strength and material limits perspective. Together with this, an engine maker needs to evaluate if a few percentage point gains in power and fuel consumption will offset the capital required for both research and development as well as the implementation cost that will filter down to the end-user. These challenges finally need to be evaluated based on the impact exhaust gas extraction will have on emissions, as the current technological advances made in the current global climate is centred around conforming to emission legislation, even if it is at the expense of fuel consumption. The goal of this study was ultimately to determine if gas to gas heat exchanger can be used in the above mentioned scenarios. This is ultimately where two of the biggest problems lie with harnessing of this energy. Firstly, a gas to gas heat exchanger relies on the convective heat transfer coefficient of the specific gases to determine the rate at which heat is transferred. The gas used in the study was air, and the convective heat transfer coefficient is poor at best. The end result is that the heat exchanger required to transfer the thermal energy is impractically large even if designed and simulated for best case scenarios.