J. Fan*, J. Dragsted, S. Furbo

Department of Civil Engineering, Technical University of Denmark,

Brovej 118, DK 2800, Kgs. Lyngby, Denmark
Corresponding Author, iif@byg. dtu. dk
Abstract

During three years seven differently designed evacuated tubular collectors (ETCs) utilizing solar radiation from all directions have been investigated experimentally. The evacuated tubular solar collectors investigated include one SLL all-glass ETC from Tshinghua Solar Co. Ltd, four heat pipe ETCs and one direct flow ETC from Sunda Technolgoy Co. Ltd and one all-glass ETC with heat pipe from Exoheat AB. The collectors have been investigated side-by-side in an outdoor test facility for a long period. During the measurements, the operating conditions — such as weather conditions and temperature of the inlet fluid to the collectors have been the same for all collectors. The volume flow rate through each of the collectors is adjusted so that the mean solar collector fluid temperature has been the same for all collectors. Thus a direct performance comparison is possible. The side-by-side tests were carried out with different mean solar collector fluid temperatures and in different seasons of the year. The results of the measurements are presented in this paper. The influence of the mean solar collector fluid temperature on the thermal performance of the different collector designs will be discussed. Further, the collector performances are compared for different times of the year and it is illustrated how the performance of the different collector types depends on weather conditions.

Keywords: Evacuated tubular solar collector, collector design, thermal performance, test.

1. Introduction

In recent years the evacuated tubular collectors have gained an increasing share of the market. On the world’s largest solar thermal market, China, evacuated tubular collectors have increased the market share from 30% in 1998 up to 94% in 2007 [1]. In Europe, evacuated tubular collectors of up to 240,000 m2 were installed in 2007 [2].

On the market there is a large number of collector manufactures providing evacuated tubular collectors with a variety of types such as all-glass, heat pipe, all-glass with heat pipe, direct flow, with and without reflectors. As far as the heat pipe evacuated tubular collector is concerned, there are designs with different tube diameters and different shapes of the absorber. It is therefore important to know how the different designed evacuated tubular collectors perform. He et al. [3] made a comparison of optical performance of evacuated collector tubes with flat and semi-cylindrical absorbers. The collector tubes are utilizing solar energy from the front side. The absorbed energy of the absorber is used in the comparison. They found that the semi-cylindrical absorber outperforms the flat absorber by 15.9% annually if it is located at latitude 40° N. Fan et al. [4] carried out side-by-side outdoor tests of four heat pipe evacuated tubular collectors with a flat fin or a semi-cylindrical fin. The collectors

utilize solar radiation from all directions. The measurements show that at latitude 57° the ETC with a flat fin performs better than the ETC with a semi-cylindrical fin for a tube diameter of 70 mm and a collector tilt of 67°. The ETC with flat fin tends to perform better than the ETC with the curved fin in winter and at high collector fluid temperatures.

Evacuated tubular collectors have a substantially lower heat loss coefficient than standard flat plate solar collectors. This makes ETCs very suitable for high latitude regions like the Arctic. The advantages of evacuated tubular collectors at high latitudes are not only their low heat loss and high efficiency, but also the ability to utilize solar radiation from all directions due to the large variation of the solar azimuth. The aim of this paper is to present the result of a long term outdoor test of differently designed evacuated tubular collectors utilizing solar radiation from all directions. Side-by­side tests of seven differently designed evacuated solar collectors were carried out in a period from February 2006 to August 2008. The thermal performances of the differently designed evacuated tubular collectors are compared. Based on the observations from the measurement, it will be elucidated how the collector performance is influenced by the solar collector designs, the weather and operation conditions.

2. Experiments

Seven differently designed ETCs utilizing solar radiation from all directions have been investigated experimentally. Detailed data sheet of the investigated ETCs is given in Table 1.

Side-by-side tests were carried out in an outdoor test facility at the Technical University of Denmark, latitude 56°N, see Figure 1. On the test platform, five collectors can be tested under the same conditions at a time. The collectors are directly facing south and have a tilt angle of 67° which is suitable for typical operation conditions in the Arctic. The collectors can utilize solar radiation from all directions. A glycol/water mixture of 41% by weight is used as the solar collector fluid. The fluid flow rate through each of the collectors is measured by a flow meter type Brunata HGQ1-R0. The inlet and outlet temperatures of the collector are measured by copper/constantan thermal couples, type TT. The difference between the outlet and inlet temperature is measured by a thermopile. The five collectors are parallel connected to a temperature control unit so that the inlet temperatures to the collectors are the same. A pump is used to circulate the solar collector fluid during all hours so that the inlet temperature of the fluid to the collectors is kept constant. The flow rates through the collectors are adjusted in such a way that the average temperatures of the collector fluids in all the collectors are approximately the same during the test. The accuracy of the absolute temperature measurement and temperature difference measurement is 0.5 K and 0.1K, respectively. The accuracy of the flow rate measurement is estimated to be 1.5%. The measurement data are monitored and logged every two minutes by LabView.

The weather data are measured in a climate station located on the roof of a building close to the test platform. The total and diffuse solar irradiance on horizontal surface and the ambient air temperature are measured.

The thermal performance of the ETCs were measured in the period from February 2006 to August 2008. The experiment is divided into three phases:

Phase 1: February 2006 — June 2006, collectors tested: ETC 1, ETC 2, ETC 3, ETC 4 and ETC 5.

Phase 2: July 2006 — May 2007, collectors tested: ETC 2, ETC 4, ETC 5 and ETC 6.

Phase 3: May 2007 — August 2008, collectors tested: ETC 2, ETC 4, ETC 5, ETC 6 and ETC 7. During the test period, four mean collector fluid temperature levels are used: 26°C, 43-47°C, 63-68°C and 75-78°C.

Fig. 1. The side-by-side test facility.

3. Results and Discussion