Demos P. Georgiou

Useful work and the thermal efficiency in the ideal Lenolr cycle with regenerative preheating

In the existing thermal engine concepts negative work transfer (usually needed to drive a compression process) is supplied by the work produced by the engine itself. The remaining difference (i.e., the net work transfer) becomes the useful work, since it is available for external consumption. The thermal efficiency is the parameter that compares this against the heat input into the system. It forms the main optimization parameter in any engine design. The objective of the present study is to show that for the case of the Lenoir cycle with regenerative preheating the entire positive work is available for external consumption, since the negative (i.e., the compression) work is supplied by the atmospheric air. Not only this, but, during the compression process and due to the pressure difference across the two sides of the moving piston, an additional (useful) work transfer may be generated. Thus, the proposed power plant may be considered as a combination of a thermal engine and a wind turbine. In the ideal ...

Ideal Thermodynamic Cycle Analysis for the Meletis-Georgiou Vane Rotary Engine Concept

The Meletis-Georgiou is a patented Vane Rotary Engine concept that incorporates separate compression-expansion chambers and a modified Otto (or Miller) cycle, characterized by (Exhaust) Gas Recirculation at elevated pressures. This is implemented by transferring part of the expansion chamber volume into the compression one through the coordinated action of two vane diaphragms. This results into a very high gas temperature at the end of the compression, something that permits autoignition under all conditions for a Homogeneous Compression Ignition (HCCI) version of the engine. The relevant parametric analysis of the ideal cycle shows that the new cycle gives ideal thermal efficiencies of the order of 60% to 70% under conditions corresponding to homogeneous compression engines but at reduced pressures when compared against the corresponding Miller cycle.

The Travelling Cascade, Constant Volume Heat Exchanger in a Gas Turbine Lead Combined Cycle

When a gas enclosed in a cavity is heated or cooled, its pressure changes with its temperature as well. If a set of two countermoving “driven” cavity cascades employs the same free wall, then the system will operate as a countercurrent heat exchanger. At the exit points of the heat exchanger the two gases can be brought back to their original (atmospheric) pressure by isentropic processes thus producing useful work. The entire set of thermodynamic processes forms a double Lenoir cycle. The exhausts from the two Lenoir cycles may drive two more sets of corresponding cycles, thus allowing for the cascading of the process, until the added useful work becomes insignificant. When this idea is employed as a bottoming cycle in a Gas Turbine lead Combined cycle, employing four sets of Lenoir cycles, the achievable total thermal efficiencies rise to the 75 to 82 % level, athough the amount of heat transferred in all these processes is about 50 % more than that in a modern Brayton-Rankine combined cycle.Copyright

Sensitivity Analysis for the Encaged Turbine Concept in Oscillating Water Column Plants

Oscillating water column plants are one of the most popular wave energy device types. Prototype OWC units have been operating in various parts of the world since the mid-1980s and such developers have more field experience of this technology than any other relevant plant. The most common turbine used is the self-rectifying Wells turbine which has a rather low peak efficiency if compared to other designs but was preferred in terms of its simplicity and cycle performance. The present study exploits the merits of a new concept for the power extraction process, that of an encaged turbine for OWC plants, which allows conventional high-efficiency turbines to be employed in such plants. This is achieved by guiding the pressurized air into a sequence of three chambers, creating a unidirectional closed air circuit through the turbine. A theoretical model is deployed simulating the operation of the plant and a sensitivity analysis is carried out for the design and working parameters. Results indicate that the power extraction efficiency may exceed the 50% level in a real plant.

Optimal Plant Configuration for the Nearly Ideal Processes of an OTEC Concept Driven by the Sea–Air Temperature Difference

Exploitation of the oceans thermal energy has been proposed several times in the past. Most research activity is focused on the temperature difference between the upper (warm) and bottom (cold) layers of water and that is what drives the power producing cycle. Consequently this kind of technology offers great possibilities in the tropical regions where the temperature difference is ranging from 10 °C to 25 °C. In enclosed seas like the Mediterranean, the available temperature differences are much smaller. Here however there exists a different potential, i.e. the temperature difference between the atmosphere and the sea water. This implies that there are two enormous reservoirs providing the heat source and the heat sink required for a heat engine. This study examines the merits of the temperature difference between the atmospheric air and the bottom of the sea, which is comparable to that of the tropical region sea waters and discusses the optimal plant configuration for the limit of the nearly ideal processes of such a plant. Copyright

A CFD Simulation of a Ducted Wind Turbine With Ejector Assist

Ducted Wind Turbines have been the subject of numerous studies in the past (both analytic and experimental), but the concept has not found commercial use so far, mainly due to the poor performance of the tested configurations. Our analytical studies, however, have shown that an optimized configuration of a Ducted Wind Turbine with ejector type assist for the exhaust may generated Power Coefficients of the order of CP = 6. This corresponds to the output of nearly 15 conventional un-ducted wind Turbines (with a typical CP = 0.4). The present study simulates the Two Dimensional simplification of such a Wind Turbine Plant by employing the commercial code FLUENT and compares its performance against the case of an open (no turbine) duct.Copyright