Michel Feidt

Comparison of the working domains of some compression heat pumps and a compression-absorption heat pump

Abstract The working domains of a model of a compression heat pump using different fluids and a model of a compression-absorption heat pump using water-ammonia mixtures are defined, plotted and discussed. These domains are defined by means of limiting values for their electrical coefficient of performance, volumetric heating capacity, and low and high pressure. In the case studied in the present paper, the disappearance from use of CFC and HCFC fluids leaves only one alternative for the implementation of high temperature electric heat pumps: hydrocarbons in compression devices or water-ammonia mixtures in compression-absorption devices. Problems relating to the implementation of these systems are also mentioned.

Optimisation d'un cycle de Brayton moteur en contact avec des capacités thermiques finies

Abstract Optimization of a Brayton cycle engine in contact with fluid thermal capacities. Finite time thermodynamics studies have recently been devoted to all kind of cycles. Some works have been done on Brayton engine cycles. The proposed model is a synthesis of the preceding ones; this model completes the preceding papers, particularly regarding the two following aspects: 1. it takes into account the thermal losses occurring between heat source and sink, that are finite time reservoirs; 2. it influences the two limiting conditions for fluids at source and sink (four cases). The study is performed with partial and total regeneration, and without regeneration of heat. Influence of global internal irreversibility of the engine is also included in the sensitivity analysis of the proposed model. Results are presented in a nondimensionnal form useful for design project. Limiting cases are enlightened by the proposed method, and allow us to disuss the whole set of previously obtained results.

Model of optimized solar heat engine operating on Mars

An endoreversible Carnot cycle is used to describe heat engine operation. This provides upper limits for real performance. The output power is maximized. Meteorological and actinometric data provided by the Viking Lander 1 are used as inputs. Four strategies of collecting solar energy are considered. Results concerning the following three parameters are briefly reported: (1) optimum solar collector surface area, (2) optimum solar collector temperature and (3) maximum output power.

Design and optimisation of a combination solar collector–thermal engine operating on Mars

A “dynamic” solar power plant (which consists of a solar collector–thermal engine combination) is proposed as an alternative for the more usual photovoltaic cells. A model for heat losses in a selective flat-plate solar collector operating on Mars is developed. An endoreversible Carnot cycle is used to describe heat engine operation. This provides upper limits for real performances. The output power is maximized. Meteorological and actinometric data provided by Viking Landers are used as inputs. Two strategies of collecting solar energy were considered: (i) horizontal collector; (ii) collector tilt and orientation are continuously adjusted to keep the receiving surface perpendicular on the Sun’s rays. The influences of climate and of various design parameters on solar collector heat losses, on engine output power and on the optimum sun-to-user efficiency are discussed.

Technico-economic simulation and optimization of a compression refrigerating machine

This study is among the several works conducted by the authors with regard to technico-economic optimization of refrigerating machines. The most difficult part is the gathering of technico-economical data and the development of cost correlations. The tool presented in this work will be used for an optimization based on irreversibilities and cost minimization, since irreversibilities have a direct impact on the operating cost.

Identification du coefficient de transfert thermique sur la surface externe d'un échangeur de chaleur à tubes et ailettes planes en fonction de l'humidité de l'air, en l'absence de condensation

The following study, which is rather oriented towards experimentation, shows the influence of the humidity content of air on heat transfer. This first article concerns heat transfer between the external fluid (moist air) and the internal fluid (water containing glycol, whose thermal behavior inside circular tubes is well-known) in a heat exchanger of the same type as those used in automotive air conditioning (horizontal copper tubes and plane aluminium fins), in the absence of condensation. The most difficult part of this experimental work is the measurement and control of the air humidity, since one has to make sure that the measurement incertainties are not significant compared to the precision of the calculation of the heat transfer coefficient. The conclusion is that, for this type of exchanger, the heat transfer coefficient decreases with air humidity in the absence of condensation (dry wall). Some correlations have been developed with respect to the relative air humidity. An analog experimental investigation, but this time carried out in the presence of condensation (partially or completely wetted wall), is about to be completed; the obtained results will be communicated later on.

SIMULATION OF A THERMAL SOLAR POWER PLANT OPERATING ON MARS UNDER CLEAR SKY AND DUST STORM CONDITIONS

Abstract The power plant analyzed in this work consists of a selective solar collector–thermal engine combination. The paper focuses on solar power plant operation under various weather conditions during all seasons on Mars. Meteorological data measured at Viking Landers (VL) sites were used in computations. Two strategies to collect solar radiation were analyzed: a solar horizontal (H) collector and a solar collector whose tilt and orientation are continuously adjusted to keep the receiving surface perpendicular to Sun rays (P). Both a low and a high efficiency thermal engine were considered. All the computations were performed for a selective solar flat-plate collector similar in size to the Mars Pathfinders Sojourner. Results show that generally the influence of latitude on performance is important. In some situations, the meteorological effects compensate the latitudinal effects and the output power is quite similar at both VL1 and VL2 sites. In case a low-efficiency engine is coupled to a horizontal collector, the solar efficiency does not exceed 0.13 at VL2 site. It is lower during summer and higher during winter dust storms. In case the low-efficiency engine is coupled to a P collector, the solar efficiency increases during summer. The solar efficiency is as high as 0.18 in case of a horizontal collector attached to a high-efficiency engine. If the high-efficiency thermal engine is connected to a P collector the solar efficiency increases significantly during summer and spring but does not exceed 0.18. The power provided by a system consisting of a horizontal collector and a low-efficiency engine does not exceed 7 W . Using a high engine coupled to a horizontal collector leads to a power output up to 13 W during spring, autumn and winter. The P collector is recommended mainly during summer and spring in combination with high-efficiency engines. In this case the solar efficiency could be as high as 25 W . The performance of PV cell power systems and properly designed dynamic solar power plants operating on Mars is comparable.

Comportement thermique d'un générateur à sorption solide

Abstract The thermal behaviour of a solid sorption generator of active carbon/alcohol machine, is studied during heating and cooling phases with a preheated air flow. A bicylindrical walls generator that contains 0.9 kg of a granular adsorbent in the presence of residual gas is tested during a cycle of an average duration of 3 hours; the grains of active carbon are rod-shaped of 0.003 m diameter and 0.008 m average length. The thermal contact conductance of adsorbent to the wall has an important influence on the rate of heat transfer between the generator and the external source of heat. In the absence of alcohol, heat transfer occuring without mass transfer in active carbon is essentially due to the conduction. A numerical bidimensional model allows one to justify experimentally the observed evolution and proposes thermal contact conductance between active carbon pellets and the generator wall. A parametric study of the thermal contact conductance gives 6.5 W·m −2 ·K −1 as the best value. A simulation of heating and cooling phases with average conductance values between 5 and 30 W·m −2 ·K −1 gives model estimated heating and cooling phases duration.

Production Minimization Method and Optimization of Thermomechanical Systems. Power Optimization Versus Entropy Production Minimization in Thermomechanical Systems

This paper reconsiders the Novikov and Curzon–Ahlborn power plant model in a completed form. The optimization of the proposed plant is done and comparison of various control volumes (converter; engine; engine with heat source and sink; or system: system in the environment) with respect to two objectives functions, power, and created entropy rates. Maximum power condition differs generally from minimum entropy generation rates. Moreover, a differentiation is enlightened concerning Angulo-Brown and Yan ecological criterions.

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