Eduardo Rincon Mejia, Fidel Osorio Jaramillo and Fernando Vera Noguez

Facultad de Ingenierfa de la UAEMex
Cerro de Coatepec, C. U., 50130 Toluca, Mdxico
Tel.: + (52) (722) 214 08 55; Fax: + (52) (722) 215 45 12;

INTRODUCTION

This paper describes a new and simple multi-compound solar concentrator (MCC), which can be installed with easy in solar arrays or in single flat-plate solar collector if the absorbers can resist temperatures up to 140° C without damage, increasing very significantly its performance with a very modest investment. For a pair of flat collectors, the MCC consists essentially of two parabolic mirrors of the CPC type at the extreme sides of the collectors, and other pair of smaller elliptic fitted between them. At the bottom side of the array a flat mirror is placed.

This way, the following benefits are achieved: the gathering of solar energy increases due to the augmentation of the total area of aperture of the array, the stagnant and operation temperatures are both increased due to the concentration of solar radiation, the mean thermal efficiency is increased also, and the driving force for thermo-siphon convection is almost doubled. All the forgoing effects yield a very much high performance of the system, increasing its economical efficiency too. A variation of this MCC consists of a pair of flat mirrors (placed instead the parabolic ones of the first option) and a pair of parabolic mirrors (instead of the elliptic pair of first option) between the flat collectors. This option is simpler, cheaper and easy to implement, but the increase in performance is smaller. Nevertheless, it could be a very good election for inexpensive flat collectors.

In Mexico more than 100.000 m2 of solar flat collectors with metallic absorbers of copper or aluminum for water heating for domestic, industrial or services applications has been installed. These solar collectors are reliable, safe and very economically efficient; most of them costs less than 200 dollars for m2 installed, they do not need pumps due to the thermo-siphon effect and the storage and labor of installation is so cheap that the total investment is recovered in about three years or less, while their time of life are more than 10 years. However, their performance can be boosted if a simple and cheap multi-compound concentrator, tailored to the size of the available commercial flat collectors is implemented to the arrays. This can be made if the absorbers resist temperatures up to 140° C without damage.

It is expected that the present development would contribute to the massive the use of solar collectors in Mexico and other developing countries. The main limitation of these solar concentrators is that they cannot be implemented for many flat collectors with plastic absorbers, because they are degraded due to the high temperatures and UV deteriorative effects.

NOMENCLATURE

a Width of a solar flat collector

b = 2a + e Width of an array of two flat collectors

c1, c2, cn Adjusting coefficients for the function Ta (t)

Ac Aperture area of each solar collector

B Linear coefficient of thermal losses (W/m2 ° C)

C Quadratic coefficient of thermal losses (W/m2 ° C)2

Cg Geometric solar concentration (non-dimensional)

CPC Compound Parabolic Concentrator, placed at the extremes of the array

cec Compound Elliptic Concentrator, placed between two solar flat collectors

e Gap between two flat collectors

exc Eccentricity of the elliptic mirrors

F (t) Acceptation function

G Total irradiance (W/m2)

Gm Maximum irradiance in a day (in Mexico, about 1000 W/m2)

MCC Multi-compound solar concentrator

N Day length (yearly mean value: 12 hours)

Pu Thermal useful power (W)

Qu Gathered energy (useful heat) per m2 of flat collector (W/m2)

Qu Useful power (W)

t Instantaneous time (seconds)

ta Dawn instant

Ta Ambient temperature (° C)

Ti Fluid inlet temperature (° C)

Ta (t) Ambient temperature as a function of time

Greek symbols

Angle modifier function Efficiency variable (m2 ° C/W)

Thermal efficiency for flat collectors without MCC Thermal efficiency of flat collectors with MCC

Mean thermal efficiency in a given period Maximum thermal efficiency of flat collectors Angle of incidence of beam radiation Acceptance half-angle of the MCC Specular reflectance of the mirrors Angular parameter or coordinate Truncation angle (maximum value of t)