Donatien Njomo

Heat and mass transfer processes in a solar still with an indirect evaporator-condenser

Abstract The thermal performances of a solar still of a new type are analysed. The still is made of a flat plate heater coupled with an evaporator–condenser recently studied by the same authors (Njomo D, Tchinda R, Kaptouom E. J. Solar Energy Engng., Trans. ASME 1996;118(3):177). The theoretical solar still productivity is in reasonably good agreement with the experimental distillation yields, reported in the literature for heat exchange fluxes less than 1 kW/m 2 usually encountered in solar stills.

Techno-economic analysis of a plastic cover solar air heater

Abstract In this paper, we compare the thermal performances of plastic cover and glass cover air collectors of the same dimensions under identical conditions of insolation and air flow rate. A simple expression taking into account the initial investment, the annual interest rate and the amount of energy which can be extracted from the collector is also developed to evaluate the cost of the energy produced by these collectors as a function of their lifetime. The result from this techno-economic analysis is that the use of a plastic cover collector is recommended whenever the cost of the collector is an important factor in the solar application considered.

Modelling the heat exchanges in a solar air heater with a cover partially transparent to infrared radiation

Abstract In this paper, the author analyses the heat exchanges in a plastic cover solar air heater and develops an explicit expression of the temperature of the air flowing in that collector as a function of the space coordinate in the flow direction and the time dependent solar intensity. Then, he studies the effect of various parameters, such as the inlet air temperature, the distance between the absorber and the transparent cover and the air flow, on the dynamic behaviour of the collector.

Étude théorique du comportement thermique d'un capteur solaire plan à air à couverture combinée plastique-vitre

Abstract The air heater studied in this paper is made of an external transparent plastic cover, such as plexiglas, the role of which is to protect the collector from accidentally thrown stones and an internal transparent glass cover which produces the necessary greenhouse effect for heating the absorber. An analysis of the unsteady state heat exchanges in such a collector is presented. It is shown that at quasi-steady state the energy balance equations of the components of the heater cascade into a single first order differential equation, which is able to predict the thermal behaviour of the collector. The solution of this differential equation is written down as an explicit expression of the local temperature of the fluid flowing in the collector in terms of the time-dependent incident solar intensity. The effect of various parameters such as the inlet fluid temperature, the mass flow rate and the depth of the air channel on the thermal performances of the combined plastic-glass air collector is also studied.

Operation and maintenance of back-up photovoltaic systems: An analysis based on a field study in Cameroon

This work is an analysis of the data collected from 20 photovoltaic installations all over Cameroon. The objectives are to study the causes of the breakdown of the different installations and propose maintenance strategies to solve them and to evaluate the field lifetime of the different elements of the PV systems. The data analyzed were obtained from maintenance records and measurements over a period of nine years (from 2007 to 2015). It appears from this analysis that 29% of the batteries went bad (leading to curative intervention), contributing to about 64.9% of the total breakdown registered. About 58.5% of the installed batteries were replaced during preventive visits; according to that, the estimated average life expectancy of the batteries installed in the 20 sites was six years. At the same time, considering the various failures caused by other items of the system, 50% of the systems received curative interventions within the first four years of the study, the other 50% received their first curative intervention only from the fifth year. This study also permitted us to evaluate the quantity of preventive and corrective maintenance impacts on solar PV systems.

Modeling the heat exchanges in a solar air heater with a cover partially transparent to infrared radiations

In this paper, the heat exchanges in a plastic cover solar air heater are analysed and an explicit expression of the temperature of the air flowing in that collector is developed as a function of the space coordinate in the flow direction and the time dependent solar intensity. Then the effect of various parameters such as the inlet air temperature, the distance between the absorber and the transparent cover and the air flow on the dynamic behaviour of the collector are studied.