Abstract

In this paper, the thermal performance of a solar collector with evacuated tube is investigated. Main application for this collector is water heating for domestic or large building, several parameters may affect the performance of the collector, such as the water flow-rate, the different irradiance, the variable inlet water temperature, the flow configuration (series or parallel connection of tubes) etc. These parameters may all be considered in the effort to optimize the overall design. In this study efficiency curves are presented from experimental measurements. And the fit equation of efficiency is provided. After a long period test, the actual efficiency curve in different months will be compared together. Also, the test results in a certain day are presented in order to analyze the comprehensive relationship among inlet temperature, ambient temperature and solar irradiance. Besides, the results of this procedure test can be very useful, firstly, for the local solar manufacturers’ equipment in order to design and optimize its products and secondly, by the engineers in order to select and design the most suitable system.

Keywords: All-glass evacuated tube collector, thermal performance, and solar heating system

1. Introduction

Solar energy is receiving much more attentions in building energy systems in recent years. Solar thermal utilization should be based on the integration of solar collectors into buildings. In most cases, the main utilization is provided with solar water heating system.

In 2005, most of solar energy systems manufacturing in different countries reported a good growth. In all these countries, the export sector is gaining importance. The expected expanding market for solar water heating system demands a standardized test method. In order to gain the optimistic benefits between the investment of heating system and the final results through operating the system, it is also necessary to know clear about the truly thermal performance of the collector, and further process in prediction of thermal performance in different install locations and long-term operation. Moreover, the knowledge of the thermal performance of solar collectors are essential for the prediction of the energy output of any solar thermal system, especially the overall evaluation and design for a water heating system.

Test methods for the thermal performance characterization of solar collectors are different with each other, mainly difference concluding the duration time of test period, the definition of test condition, and mean temperature of the working fluid. Most well-known test methods, such as ISO9806-1 and EN12975-2, are under steady-state conditions to test the thermal performance of the collector. These

steady-state test methods require the strict testing conditions very much in variable climates. And then the actually lasting test period should be longer than the regulated time that the climate condition is ideal to measure. However, because of the extensively regional and application of the international standards, ISO9806 is promulgated to measure the performance of collectors in in-depth study. At the same time it is clear to identify the definition of solar collectors according to ISO 9806.

There are many kinds of solar collectors, in this paper all-glass evacuated tube collector will be presented and experimented. It is a collector composed of an array of dual-glass evacuated tubes using water as the working fluid. The tubes are either all collected in parallel, or divided into two groups connected in series, within each of which the tubes are collected in parallel. Water is fed into the glass tubes through a brass tube flowing towards the closed end where it is reversed backwards then flows through the tube and is collected in the header from where it may be directed into its final use.

The all-side thermal performance of collector will be analysed to supply ample data to the design. So in order to make a comparison, at identical working conditions, between different products within a reference test condition, in this paper the experiment will be operated under steady conditions according to ISO 9806. Although the performance of solar collector can be approximately predicted based on steady test method, it is also affected by the inlet water temperature, water flow rate, and ambient conditions. Consequently, it is necessary to establish a test that can consider the whole conditions and it can be used to by engineers or manufacturers in order to design solar water heating system and optimize them.