Alexandru Dobrovicescu

Experimental evidence concerning the different behavior of energy and exergy performance indicators of refrigeration systems in transient regimes

Abstract This article presents the results obtained from an energy-exergy analysis of a vapor compression refrigeration system during induced transient regimes. Using experimental data, exergy destruction as a function of time under the influence of some factors that perturb the stationary regime, such as deactivation of piston, variation of mass flow rate and initial temperature of cooled fluid, and diminution of the compressor rotation speed, was calculated. Under the perturbation, an antagonistic increase in the coefficient of performance and a decrease in exergy efficiency were noted.

Energy and exergy analyses of a solar-driven absorption cooling system

In this paper, energy and exergy analyses of an absorption cooling system using H2O/LiBr and solar energy as heat source are conducted. A simple schematic of a chiller and an improved one, are compared using Thermoptim and EES software. An increase in the coefficient of performance (COP) from 0.69 to 0.8 was noticed. Numerical simulations are performed for six values of the condensing/absorption temperatures, which vary in the interval 31–36°C. The increase in these temperatures implies a decrease in the COP from 0.8 to 0.56. The exergy analysis of the overall system and of each component shows a decrease in the exergetic efficiency of the system from 0.27 to 0.18. A detailed exergy analysis of the absorber reveals the real causes of exergy destruction. A new exergy destruction indicator was introduced, which shows the importance of the irreversibility of each component, relative to the starting potential.

Performance and emission of generator Diesel engine using methyl esters of palm oil and diesel blends at different compression ratio

This study proposes engine model to predicate the performance and exhaust gas emissions of a single cylinder four stroke direct injection engine which was fuelled with diesel and palm oil methyl ester of B7 (blends 7% palm oil methyl ester with 93% diesel by volume) and B10. The experiment was conducted at constant engine speed of 3000 rpm and different engine loads operations with compression ratios of 18:1, 20:1 and 22:1. The influence of the compression ratio and fuel typeson specific fuel consumption and brake thermal efficiency has been investigated and presented. The optimum compression ratio which yields better performance has been identified. The result from the present work confirms that biodiesel resulting from palm oil methyl ester could represent a superior alternative to diesel fuel when the engine operates with variable compression ratios. The blends, when used as fuel, result in a reduction of the brake specific fuel consumption and brake thermal efficiency, while NOx emissions was increased when the engine is operated with biodiesel blends.

Mathematical Modelling of a Hybrid Micro-Cogeneration Group Based on a Four Stroke Diesel Engine

Abstract The paper presents a part of the work conducted in the first stage of a Research Grant called ”Hybrid micro-cogeneration group of high efficiency equipped with an electronically assisted ORC” acronym GRUCOHYB. The hybrid micro-cogeneration group is equipped with a four stroke Diesel engine having a maximum power of 40 kW. A mathematical model of the internal combustion engine is presented. The mathematical model is developed based on the Laws of Thermodynamics and takes into account the real, irreversible processes. Based on the mathematical model a computation program was developed. The results obtained were compared with those provided by the Diesel engine manufacturer. Results show a very high correlation between the manufacturer’s data and the simulation results for an engine running at 100% load. Future developments could involve using an exergetic analysis to show the ability of the ORC to generate electricity from recovered heat

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