S. Kyritsis

A simulation-optimisation programme for designing hybrid energy systems for supplying electricity and fresh water through desalination to remote areas: Case study: the Merssini village, Donoussa island, Aegean Sea, Greece

The aim of the present paper is to develop and apply a software tool for designing hybrid renewable energy systems. The hybrid system consists of a wind generator and photovoltaic modules which are the renewable technologies for energy production. The programme has been applied for simulating a hybrid system with the above mentioned technologies in order cover the electricity and water needs of the Merssini village on Donoussa island in the Aegean Sea of Greece. The Merssini village is occupied by 20 year-round residents while the population is doubled during the summer period. The village is non-electrified and faces a problematic scarcity of fresh water. In the analysis that follows, the considered technical data as well as the results of programme runs for winter and summer seasons are presented. The electricity consumption consists of both the household and desalination plant consumption. The system is supplemented with batteries and a micro hydraulic plant for energy storage. The simulation programme was used to optimise the design of the system as well as to manage the energy supply and energy storage. The results prove that this simulation programme constitutes a valuable tool for the determination not only of the optimum combination of technologies, but also the optimum energy management of complex hybrid systems.

Mixed, forced and free convection heat transfer at the greenhouse cover

A method has been developed to determine experimentally in situ the convective heat transfer coefficients on the inside and outside of the greenhouse cover. The method is based on the energy balance of the greenhouse cover and it was applied to a small experimental polyethylene covered greenhouse. The convective heat transfer coefficients on the inside and outside of the cover were determined as functions of the air-cover temperature difference and the air velocity inside the greenhouse, as well as the wind velocity outside the greenhouse. Also, the nature of the convective heat transfer at the outside and the inside of the greenhouse cover was investigated. At the outside of the cover at moderate wind velocities mixed convection is the prevailing convective heat transfer mechanism. The convection heat transfer on the inside of the greenhouse cover is always pure free convection when the greenhouse vents are closed and the air velocity in the house is low. When the vents are opened the nature of the convection heat transfer depends on the air velocity in the house. Criteria are given to determine whether pure free, mixed or pure forced convection takes place.

Numerical simulation, technical and economic evaluation of air-to-earth heat exchanger coupled to a building

An air-to-earth heat exchanger (ATEHE) consists of pipes buried in soil. We have evaluated the technical and economic performance of an ATEHE coupled to the system for heating or cooling of a building that uses 100% fresh air as heating or cooling medium during winter and summer. The soil is divided into elementary layers. The problem solved, is non stationary; however, steady state-energy equations are used for soil layers in each time step. It is found that the use of the ATEHE covers a portion of the daily building needs for space heating or cooling. The cost of the ATEHE energy is lower for summer than for winter.

Energy from a two-pipe, earth-to-air heat exchanger

Solar energy accumulated in the soil may be utilized with an air-to-earth heat exchanger (ATEHE) which has two pipes buried in the soil, one made of PVC and one of steel. During the winter, air is heated; during the summer, it is cooled and then used in an air-conditioning device. To obtain the mathematical model of the ATEHE, we divided the soil and pipes into elementary volumes, used steady-state energy equations, and applied a time-marching method. We determined how the season, soil thermal conductivity and pipe spacing influence energy transfer from the soil to the ATEHE and also the steel-pipe contribution to this energy transfer.

Evaluation of an integrated renewable energy system for electricity generation in rural areas

This study examines the combination of renewable energy sources for electricity generation at local level, and in particular the case of an isolated area of South Crete, the Frangocastello area, with the help of F-Cast, a computerized model that has been developed for the simulation of the operation of an integrated system. The energy supply side includes three renewable energy sources, namely wind, micro-hydraulic and biomass, and conventional high-cost electricity supplied by the National Grid. The model is the economic counterpart of an electronic control system that dispatches electricity on an hourly basis. Wind energy introduces uncertainty to the system. Relations among variables are based on three states of the world depending upon the hourly speed of the wind. Biomass is a balancing factor in the supply-demand interaction, as the production of biomass is itself at the same time consuming electricity, as an input. Conclusions are drawn on (a) the optimal combination of renewable energy sources to achieve economic viability of the system (b) effects on agricultural income and local development, and (c) evaluation of renewable energy policies.

Soil energy balance analysis of a solar greenhouse

An experimental investigation was carried out to determine whether sufficiently high temperatures are developed at a greenhouses soil surface, during late summer (when greenhouses are not used in Mediterranean regions for cultivation), to use it for grape drying. The greenhouse soil temperature distribution and heat flux were calculated numerically as a function of time and space. Calculated and measured data compared well. A numerical method is also proposed to analyse and calculate the energy balance of the greenhouses soil surface from a minimum number of measurements. The method is based on the heat stored in the soil surface layer during short time intervals. The method was applied to greenhouse soil and good results were obtained. The results are promising for the use of greenhouse soil surfaces for grape drying.

Theoretical and experimental investigation of thermal radiation transfer in polyethylene covered greenhouses

A theoretical analysis has been developed to calculate thermal radiation transfer in an arbitrary number of enclosures having common surfaces partially transparent to long wavelength radiation. The objective of the analysis was to provide a general set of equations describing thermal radiation exchange among all surfaces of all enclosures. A numerical method was proposed for the solution of this set of equations, to calculate the net thermal radiation flux of each surface of each enclosure. The analysis was applied to a twin span greenhouse covered with polyethylene film, in order to calculate the net radiation flux of each surface of the greenhouse enclosure, taking into account its three dimensional geometry. The analysis also takes into account, the properties of the polyethylene film cover and those of the ground inside and outside the greenhouse. All greenhouse cover surfaces are treated as having different temperatures. Comparison of the calculated net radiation fluxes (taking into account solar irradiance as well) at two different ground locations inside the greenhouse with measured ones, was shown to be in good agreement over the course of a 24 h period.

Waste — Water Reuse

In the last decade humanity has been facing two crucial problems concerning fresh water. The first is the lack of water because of the always growing water demand, which has tripled around the world over the last 40 years, the trend expected to be higher in the near future. The second is the environmental concern to stop waste water discharge into surface water prior to the removal of major pollutants.

A numerical method for determining thermal conductivity of porous media from in-situ measurements using a cylindrical heat source

A device developed to determine the thermal conductivity of a porous media is described. The thermal conductivity is obtained by numerical solution of the equation of heat transfer in solids. Measurements were taken in a dry fine sand to determine the apparent thermal conductivity, using a wide range of heating powers and heating times. The results of the numerical method were compared with those obtained by the classical analytical method. The results of both methods (numerical and analytical) were very close to those given by the sphere steady-state method. The numerical method offers some advantages over the analytical one. Since it may be used with high speed digital computers, it is faster than the analytical one. The latter is burdened by the “time correction” calculation procedure. In addition, the numerical method provides a criterion for estimating the accuracy with which thermal coductivity is determined, and takes into account the thermal conductivity of the device itself.

INFLUENCE OF THE GEOMETRICAL CONFIGURATION FACTORS ON THE RADIATION HEAT EXCHANGE CALCULATIONS IN NIGHT SKY RADIATORS

In the present work an effort will be made to apply radiation exchange calculations which account for radiation configuration factors in a flat plate night sky radiator with a cover partly transparent at long wave-lengths when the sky radiation must be taken into account. The objective of the analysis will be to estimate the influence of the radiation configuration factors on the radiation exchange between the sky radiator and the sky within and outside the 8–14 μm atmospheric window band. An application of a general set of equations to the radiation heat exchange in a flat plate sky radiator and a comparison of the values calculated with other methods which do not account for radiation configuration factors will be presented.