Jim McGovern

Development of a Theoretical Decoupled Stirling Cycle Engine

Abstract The Stirling cycle engine is gaining increasing attention in the current energy market as a clean, quiet and versatile prime mover for use in such situations as solar thermal generation, micro-cogeneration and other micro-distributed generation situations. A theoretical Stirling cycle engine model is developed. Using a theoretical decoupled engine configuration in which working space swept volume, volume variation, phase angle and dead space ratio are controlled via a black-box electronic controller, a model is developed that is to be used as a tool for analysis of the ideal Stirling cycle engine and the limits on its real-world realisation. The theoretical configuration approximates the five-space configuration common in Stirling cycle analysis. It comprises two working spaces and three heat exchangers: hot side, cold side and the regenerator between. The kinematic crank mechanism is replaced by electronically controlled motor/generator units, with one motor/generator controlling each of the working pistons. The use of stop valves permits the flow and non-flow processes inherent in the ideal cycle to be realised. The engine configuration considered here is not intended as a viable prime mover but rather a tool for study of the limitations of the cycle.

Validation of a Simulation Model for a Combined Otto and Stirling Cycle Power Plant

A project has been underway at the Dublin Institute of Technology (DIT) to investigate the feasibility of a combined Otto and Stirling cycle power plant in which a Stirling cycle engine would serve as a bottoming cycle for a stationary Otto cycle engine. This type of combined cycle plant is considered to have good potential for industrial use. This paper describes work by DIT and collaborators to validate a computer simulation model of the combined cycle plant. In investigating the feasibility of the type of combined cycle that is proposed there are a range of practical realities to be faced and addressed. Reliable performance data for the component engines are required over a wide range of operating conditions, but there are practical difficulties in accessing such data. A simulation model is required that is sufficiently detailed to represent all important performance aspects and that is capable of being validated. Thermodynamicists currently employ a diverse range of modeling, analysis and optimization techniques for the component engines and the combined cycle. These techniques include traditional component and process simulation, exergy analysis, entropy generation minimization, exergoeconomics, finite time thermodynamics and finite dimensional optimization thermodynamics methodology (FDOT). In the context outlined, the purpose of the present paper is to come up with a practical validation of a practical computer simulation model of the proposed combined Otto and Stirling Cycle Power Plant.Copyright

Proposed Otto Cycle/Stirling Cycle Hybrid Engine Based Power Generation System

The generation of electrical and thermal power is a matter of critical importance to the modern world. Considerable quantities of both power types are required in all sectors of society; industrial, domestic and leisure, with the future prosperity of both developed and developing societies being dependant on generation of both a sufficient quantity and quality of power. Central to this discussion on the international front is the topic of fossil fuel usage. Despite considerable advances in renewable energy conversion technologies, the human race remains dependant on fossil fuels as a primary energy source. With increasing demand for these finite resources giving rise to strained international relations and economic uncertainty, emphasis has fallen on optimization of usage patterns. The area of power plant efficiency is essential to this optimization. This paper proposes a method for increasing the efficiency of an Otto cycle engine based plant as is typically used in CHP and other Distributed Generation scenarios. The method proposed is to utilise a Stirling cycle engine as a heat recovery device on the exhaust stream of the Otto engine. Thermal energy that may otherwise be lost would thereby be recovered and used to generate additional electrical power. In this manner energy is effectively diverted from the exhaust flow of the engine and converted to mechanical work by way of the Stirling cycle engine. It is postulated that this combined cycle will yield higher plant efficiency than the Otto engine alone. This paper introduces work completed to date and an experimental plan for the project. The project was initiated at undergraduate level as a feasibility study for application of the hybrid engine in automotive circumstances. The study suggested that the combination of the engines in the proposed manner was indeed feasible, with significant power gains possible. However, it proved unlikely that automotive application was the best use of the system unless certain constraints were addressed. Therefore, it was decided to pursue the concept in terms of a stationary generation system. The advantages of the stationary system over the automotive system are addressed briefly, with the constraints of the automotive scenario analysed and their relevance to the stationary generation situation examined. The central areas under investigation are detailed, including thermodynamic theory pertaining to the Otto cycle and Stirling cycle engines, and the combined cycles. Possible limiting factors to the design are discussed also.Copyright

Modeling and Optimization of Heat Exchangers Within Gas Turbine Systems

The paper presents an analysis of a recuperative gas turbine system used for micro-cogeneration based on energetic and exergetic principles. The system is composed of two compressors (one for the fuel, the other for air), a combustion chamber, a gas turbine, a recuperator used to preheat the air before entering the combustion chamber and a heat exchanger for heating water. The analysis compares three different configurations obtained by placing the recuperator upstream of, downstream of, or in parallel with the water heater. It is subject to the following assumptions: the fuel is injected steadily and ideally (without irreversibility), the air is a perfect gas, the heat exchangers are adiabatically isolated from the surroundings and the compressors and the turbine are adiabatic. A detailed analysis of the thermal and mechanical irreversibilities of the cycle is also presented. The optimization goal is to minimize the entropy generation or to maximize the useful exergy output of the system. With this approach the best configuration for a specified operating regime of micro-cogeneration can be determined.Copyright