A. Carotenuto

The effective thermal conductivity of a porous medium with interconnected particles

Abstract The procedure of volume averaging is applied to the process of steady-state heat conduction in a two-phase system and a method is developed to calculate the effective thermal conductivity. This method provides details of solid mechanics and thermal problems, and particle-particle contact is treated in an empirical manner. The proposed method is applied for periodic regular arrangements of circular and square cylinders. The effects of particle shape, roughness and solid conductivity are examined. A qualitative comparison between theory and experiment is also reported.

Experimental and Theoretical Modeling of the Effective Thermal Conductivity of Rough Steel Spheroid Packed Beds

The upper and lower bounds of the effective thennal conductivity of packed beds of rough spheres are evaluated using the theoretical approach of the elementary cell for two-phase systems. The solid mechanics and thermal problems are solved and the effects of roughness and packed bed structures are also examined. The numerical solution of the thermal conduction problem through the periodic regular arrangement of steel spheroids in air is determined using the Finite Element Method. The numerical results are compared with those obtained from an experimental apparatus designed and built for this purpose

An aquifer-well thermal and fluid dynamic model for downhole heat exchangers with a natural convection promoter

Downhole heat exchangers (DHE) eliminate the problem of geothermal fluid disposal, since only heat is taken from the well. For this reason, as well as their low cost and simple installation, they are frequently used in geothermal plants. In the last few years DHEs have been provided with a natural convection promoter to improve the heat and mass transfer of geothermal fluid between the aquifer and the well. Knowledge of the interaction between the fluid in the aquifer, in the well and in the promoter is necessary for DHE design. The authors experimentally verified the existence of a limit in the heat flow obtained by the DHE, which is connected only to the aquifer-well-promoter interaction. This heat flow limit is due to the short-circuit effect in the aquifer between cold and warm fluids, respectively leaving and entering the well. The authors propose a simplified model developed to determine the main lumped parameters characterizing the heat and mass transfer between aquifer, well and natural convection promoter.

THE GEOTHERMAL CONVECTOR: EXPERIMENTAL AND NUMERICAL RESULTS

The Geothermal Convector (GTC) is an unconventional system that uses a two-phase closed thermosyphon for heat extraction from a geothermal aquifer without fluid withdrawal. This paper reports on the experimental tests carried out on a GTC prototype installed in a geothermal well on the island of Ischia. The GTC prototype comprises a tube bundle with a central feeding pipe as the evaporator, and a conventional shell-and-tube type condenser with cooling water flow inside the tubes. The evaporator and condenser are connected using flexible ducts. The GTC is equipped with an annular convection promoter. This paper describes the prototype, the experimental plant and the tests carried out on the experimental apparatus, and reports the results, thereby illustrating the device performance. The experimental tests indicate that there is a limit to the heat obtained by the GTC; this limit arises from the thermal and fluid dynamic interaction between the well and the aquifer. In particular, the heat flow is due to a short-circuit effect in the aquifer between cold and warm fluids, respectively, leaving and entering the well. An estimate of this short-circuit effect for the Ischia aquifer is obtained from the two-dimensional (2D) thermal and flow fields determined using the Finite Element Method.

Heat exchange in a multi-cavity volumetric solar receiver

The concept of a multi-cavity volumetric solar receiver is very attractive for the profitability of certain of its characteristics such as high efficiency and economy. The absorber is based on a pack of small ceramic cavities which intercept and absorb the inherent high solar flux reflected from an array of mirrors. Atmospheric air acts as a coolant medium when it is drawn through the pack. A model for an overall heat transfer performance of the receiver is given and numerically solved.

Thermal behaviour of a multi-cavity volumetric solar receiver: Design and tests results

Abstract After some years of basic studies and design, a prototype of a multi-cavity external flow air (or volumetric) receiver was tested at the Plataforma Solar de Almeria test facility. The absorber consists of a pack of SiSiC 9.0 × 4.8 mm (net) channels with a length of 150 mm with vertical and horizontal walls 5.0 and 1.6 mm thick, respectively. Experiments lasted for 30 days in the summer of 1990 and outlet air temperatures, power transferred to air, and efficiency were recorded as functions of solar incident flux and air flowrate. A good accordance between measured and predicted results was noticed, within the limits of accuracy of the data acquisition system of the facility and of the experimental system. Due to its compactness and despite its low mass, the structural behavior of the receiver was also satisfactory; the most important result was to confirm the validity of the concept and of the physical analysis on which the design is based.

A lumped parameter model of the operating limits of one-well geothermal plant with down hole heat exchangers

Abstract In one-well geothermal plant, the mass flow of the geothermal fluid between the aquifer and the heat exchanger is supplied only by one well. It is universally acknowledged that this type of plant has some thermal and fluid dynamic limits and, therefore, they were proposed only for low and medium thermal applications; however simple models are lacking to explain and correctly evaluate these limits. The fundamentals of a lumped parameter model of the thermal and fluid dynamic phenomena giving rise to heat flow limits were explained by Carotenuto et al. (A. Carotenuto, C. Casarosa, M. Dell’Isola, L. Martorano, An aquifer–well thermal and fluid dynamic model for downhole heat exchangers with a natural convection promoter, Int. J. Heat Mass Transfer 40(18) (1997) 4461–4472; A. Carotenuto, C. Casarosa, Modello a Parametri Concentrati dei Limiti Operativi di Impianti Geotermici a Pozzo Unico. Parte I: Elementi Generali, 53°Congresso Nazionale ATI, Firenze, vol. I, 1998, pp. 557–570). In this paper, the model is completely developed for natural convection plant in which the geothermal fluid flow between aquifer and well is provided only by the temperature drop occurring in the heat exchanger. The model is applied to this type of plant taking into consideration, whether: (i) a natural convection promoter is present and (ii) downhole heat exchangers or geothermal convectors (GTCs) are used. In particular, for GTCs, the model is successfully applied to evaluate the experimental data obtained by the authors in previous experimental tests.

The influence of reference condition correction on natural gas flow measurement

This paper discusses the influence of thermodynamic correction parameters on measuring the flow rate of natural gas in the reference condition. In particular, the principal aim is to experimentally evaluate the uncertainties of the correction parameters on volumetric flow rate measurement in the reference condition by means of conventional methods. An experimental analysis was carried out on the Italian distribution system which is very interesting from a metrological point of view since there are four different types of natural gas with a pressure and temperature range comprised respectively between 1.1 to 71 bar and 0 to 30°C.

Optimizing the position of the tube casing slotted section for geothermal wells with a downhole heat exchanger

The use of downhole heat exchangers (DHE) in the exploitation of geothermal resources is characterized by an absence of mass withdrawal from the aquifer. Although this peculiarity reduces installation costs it also imposes limits on the heat flow withdrawable (generally less than 100 kW), and, therefore, on the use of DHEs in small applications such as greenhouses, small buildings or thermal baths. For this reason DHEs are mainly used in superficial geothermal aquifers (up to 30 m depth), usually with liquid-phase water at temperatures greater than 60°C. A study has been made of the influence of the position of the casing slotted section within an aquifer on the heat withdrawal rates using DHEs. This study numerically simulates an aquifer using the finite-element method to determine the heat flow that can be withdrawn by the DHE when the slotted section position is varied within a geothermal aquifer. The simulations carried out also enable us to determine the influence of the main characteristics of the aquifer and the extraction plant on the design of the tube casing slotted section. On the basis of the numerical results obtained, a particular configuration of slotted section is proposed where this is subdivided into different sections, one placed in the lower part of the aquifer and the other in the upper part. The results obtained have shown that this configuration optimizes the heat flow drawn by the DHE from the geothermal aquifer.

Thermal storage in aquifers and energy recovery for space heating and cooling

Abstract An improvement of the energy efficiency ratio of air conditioning systems results when water-to-water dual-mode units are coupled to an aquifer of suitable characteristics which functions as a seasonal thermal storage. The evaluation of the system efficiency from a thermodynamic study is needed, to assess the interactions between the units and the aquifer. A comprehensive thermodynamic analysis is carried out in this paper. A procedure for a numerical evaluation of the system performance and optimization is presented in a convenient form for development and application of the system. A simplified method is also given for preliminary design purposes.