M. J. Marcos1, C. A. Perez-Rabago2 ,C. A. Estrada1’2, M. Romero1

1CIEMAT-PSA, Avda. Complutense 22, 28040-Madrid, Spain. e-mail: mj. marcos@ciemat. es

2CIE-UNAM, AP 34, 62580 Temixco, Morelos, Mexico. e-mail: cestrada@cie. unam. mx

Abstract

This paper presents the theoretical study of the heat transfer process that takes place in a special calorimeter of conical cavity named CAVICAL1. This instrument is used to measure the thermal power of a point focus solar concentration system named DEFRAC and developed at the Center of Energy Research of the National University of Mexico. The DEFRAC concentrator has 1.3 kWt and it has a very fine optical system. The opening of the calorimeter cavity is 14 cm2. The detailed heat transfer study is done using the FLUENT code. The heat transfer mechanisms that are taking into account in the analysis are the radiative energy absorbed by the inner wall cavity, the energy transfer from the wall cavity to the air by natural convection, the energy transferred by conduction through the metallic wall of the calorimeter and by forced convection through the fluid in the cooling system. The calorimetric information allows determining the thermal power that the concentrator is able to capture. Temperature and velocity fields are determined for each of the thermal fluids considered inside of the calorimeter. The analysis gives the thermal losses and the efficiency of the calorimeter. The information generated is useful to optimize the design of the calorimeter.

Key words: Concentrated solar radiation, heat transfer analysis. Cavity calorimeter.

1. Introduction

In the near and middle future, the point focus solar concentration systems will play and important roll in different industrial applications, like the generation of solar thermal power electricity, the production of solar fuels or the destruction of hazardous materials. The Center for Energy Research of the National University of Mexico built a solar concentration system named DEFRAC (Devise for the study of high radiative concentrated flux, acronyms in Spanish) to support basic and applied research on those systems. DEFRAC is a point focus solar concentrator with an equatorial solar tracking system. It consists of two frames: one is used as the main structural support and the other of hexagonal shape, it holds 18 first surface paraboloidal mirrors, 30 centimeters in diameter, made of aluminized glass with 0.95 reflectivity. The equivalent focal distance of the whole set of mirrors is 2 meters. The hexagonal frame axis is supported by two lateral journal bearings, attached to the main frame. The main frame has an electric motor, sensors and a control mechanism to follow the sun with high accuracy [1]. Figure 1 shows a schematic view of DEFRAC.

In the past, a flat plate calorimeter has been used to measure the solar concentration power of DEFRAC. It had the function to capture concentrated solar radiation and transfer it to a thermal fluid. The calorimeter was made of stainless steel, with cylindrical geometry and it had two circular flat plates (11 cm in diameter with 1 cm of gap) where circulates the cooling thermal fluid (usually water). The calorimeter acts as a receiver of DEFRAC and in its external surface the sun spot was formed [2]. Figure 2 shows a picture of the flat

temperature to the ambient. This method has Fig. 1. Schematic view of DEFRAC resulted effective,

because it had allow us to

determine the concentration power of DEFRAC. However, it has been demonstrated that the temperature of the external surface was not close to ambient; 470 oC, it has been calculated, based on measurements, at the center of the plate, with concentrated solar energy coming from only 6 mirrors, even though the mass flow rates were relatively high (1.11kg/min) and the increments of temperature between outlet and inlet calorimeter water flow were low (~ 5 °C). Knowing the plate temperature distribution, the convective and radiative thermal loses have been calculated, and they have resulted to be less that 2.5%. [2]. On the other hand, the determination of the concentrated solar power by DEFRAC was calculated by the addition of the powers obtained by each group of 6 mirrors of the DEFRAC. But, with this flat plate calorimeter, it has not been possible to measure directly the solar concentration power of the 18 mirrors, due to the fact that the external surface was degraded with the high solar flux (~ 4000 suns) and thus the high temperature.

In order to improve the calorimetric measurements of

the DEFRAC concentrated solar power, a cavity calorimeter was designed, built and tested. This calorimeter was named CAVICAL1 (cavity calorimeter No. 1). It allows us to handle high temperatures, to have a better control on the thermal emittance and also it allows us to calculate the concentrated solar power of DEFRAC using all 18 mirrors of the system. This paper deals with the preliminary results of the heat transfer study using a computer fluid dynamics

Fig. 2. Sun spot picture of the flat plate calorimeter code (CFD) to determine during one test. temperature distributions of

different device’s components and heat losses from the calorimeter to the surroundings. The validation of the computer simulation was carried out by comparison with experimental results.