R. Casasola1*, D. Romero1, J. Alonso1, J. M. Cejudo2.

1 Isofoton, C/ Severe Ochoa n° 50, 29590 Malaga, Spain
2 ETSII, Universidad de Malaga, Plaza de El Ejido s/n, 29071 Malaga, Spain
Corresponding Author, r. casasola@lsofoton. com

Abstract

A possible way of increasing the efficiency of solar collectors is to optimize the heat transfer between the fluid and the inner surface of the absorber tubes. There are several approaches to solve this problem, but one of the simplest and cheapest solutions is the passive method. Enhancing the heat transfer by this procedure has received a strong attention during the last decades. Many studies have demonstrated that there is a significant increase in the Nusselt number by using different devices as flux breakers or turbulators. It has been well established that the turbulent flow created facilitates an efficient transport of heat to the tube wall. Passive methods have also the advantage that there is no need of additional external power. In particular, in this work we have studied the insertion of turbulators into the absorber. Based on previous studies that claimed a 50% gain of convective coefficient in copper tubes, a proportional increase of the collector efficiency is expected. Extensive empirical tests have been made to evaluate the effect of these elements in the collector efficiency, measured according to EN 12975.

Keywords: absorber, heat transfer enhancement, tabulator, efficiency.

1. Introduction

A solar absorber can be considered as a special type of heat exchanger. Industrial heat exchangers usually operate in turbulence or swirl flow, where their performance is higher than in laminar flow.

The aim is to increase the heat transfer between the fluid and the inner wall of the pipe. There are two main reasons to explain this energy transfer enhancement: the inherent higher transfer coefficients of turbulent flow and the larger effective heat transfer area.

According to the classical model of the flat plate solar collector [1], there are two main strategies to improve the collector efficiency: to increase the optical gain or to reduce the thermal losses. In the first group, we can enumerate the use of anti-reflective glasses, high absorbing coatings or new absorber designs. In the second group, double glazed collectors, selective coatings and better thermal isolation materials. Each of these alternatives have different advantages and disadvantages in terms of costs, final application, durability, appearance, etc.

The solution described aims to raise the optical gain. Considering the transmittance of the currently used glasses, and the absorption of the absorbing surfaces, the development of the solar collector technology has almost reached its limit. There are commercially available glasses with a 95% of energy transmittance and absorbing surfaces with a coefficient of 0.98. Therefore, it seems difficult to increase the efficiency in these directions. However, there is still scope for improvement by enhancing

the heat transfer between absorber and fluid. The typical absorber configuration with sheet and tubes can be improved. Moreover, this option has the important advantage of increasing the efficiency without raising the stagnation temperature, what reduces durability problems.

There have been numerous authors who have studied heat transfer enhancement by means of insertions into the tubes, mostly using air like heat carrier fluid. Promvonge and Eiamsa [2] described the use of twisted tapes and conical inserts, Ray [3] studied the use in square ducts, etc. Many experiments and theoretical analyses have been done and empirical correlations have been obtained.

In this article we have prepared an application of the use of tabulators in solar absorbers, using an empirical approach. We have selected the most promising alternatives in terms of material costs and processability, and we have made several tests to evaluate its effect in terms of collector efficiency.