Solar collector efficiency has been measured with the indoor test rig shown in Fig. 1. It consists of a 12kW mercury lamp array, an integral thermostat plant, fluid temperature and air temperature probes, a pyranometer, an anemometer, a precision flow meter and a wind generator.

Fig. 1. Test rig scheme.

This installation meets the requirements established in the European Standard EN 12975 to measure the efficiency of solar collectors.

Two prototypes of solar collectors have been manufactured using tabulators. In the first one, we have used a continuous twisted copper tape. The tape was placed all along the riser tubes of the harp. It had

0. 2 mm thickness and 5 mm width. The length of the 180° twist was 20 mm approximately. The second one was made using a steel chain as an insert. The chain had 5 mm width. It was placed in the same way as the first one, in the riser tubes. Both prototypes were constructed using the basis of a commercial collector of Isofoton, risers having an inner diameter of 7 mm. This way, the only difference between the prototypes and the standard design was the addition of tabulators.

The experimental sequence was as follows: i) efficiency test of the commercial collector, ii) efficiency tests of prototype 1 and iii) efficiency tests of prototype 2. In order to analyze the influence of water flow, we measured the efficiency of the prototypes at three different flows: 160 kg/h, 320 kg/h and 500 kg/h.

Each test was made during a whole day, and all tests were carried out in consecutive days. All of them were completed according to EN 12975.

2. Results

Table 1 shows a comparison between the efficiency of the standard collector and the turbulators prototypes (mass flow of 160 kg/h).

Table 1. Efficiency coefficients Standard Prototype 1 (twisted tape) Prototype 2 (chain) П0 0.753 0.782 0.773 a1 3.184 3.052 3.363 a2 0.0138 0.0176 0.0073

The main result is the 3% increase of q0 in the first prototype. However, there is also an opposite growth of the loss coefficients, ai y a2. In order to make the analysis, all three efficiency curves have been plotted in Fig. 2.

Fig. 2. Efficiency curves.

The 3% gain in the left side of the curve seems to be reduced in the right side to 2%. Although prototype 1 is better, the second prototype equals its efficiency at non dimensional temperature T*=

0.07. We can confirm that there is a consistent efficiency increase along the curve when using tabulators.

Furthermore, we have made three different tests for both prototypes, at three different mass flows: 160 kg/h, 320 kg/h and 500 kg/h. The results of these tests are shown in Tables 2 and 3.

Table 2. Prototype 1. Efficiency coefficients at different mass flows 160 kg/h 320 kg/h 500 kg/h П0 0.782 0.785 0.782 a1 3.052 3.45 3.16 a2 0.0176 0.012 0.013

Table 3. Prototype 2. Efficiency coefficients at different mass flows 160 kg/h 320 kg/h 500 kg/h П0 0.773 0.779 0.78 a1 3.363 3.085 3.35 a2 0.0073 0.0219 0.014

It is observed that there is no significant variation in the efficiency in terms of mass flow, in any case. For both prototypes, the efficiency remains at approximately the same value.

The uncertainty of the efficiency curves has been estimated according to EN 12975 [4], and its value is ± 1.9%.

3. Conclusions

Experimental tests have demonstrated the suitability of using tabulators to improve solar collectors’ efficiency. A 2-3% efficiency increase can be obtained. Moreover, the insertion of twisted tapes has been reported to be a better option than the use of a chain.

There is no significant variation of the efficiency depending on mass flow when the two types of turbulators described are used.

A more detailed study to optimize the design of the tabulators will be done. However, the simplicity of the materials used and the efficiency enhancement obtained in this work, demonstrate that this solution is an adequate and suitable way of improving solar collectors.

References

[1] Duffie, Beckman. Solar engineering of thermal processes. Wiley-Interscience, 1980.

[2] P. Promvonge, S. Eiamsa-ard. Heat transfer behaviors in a tube with combined conical-ring and twisted-tape insert. ScienceDirect, Elsevier, 2007.

[3] S. Ray, A. W. Date. Friction and heat transfer characteristics of flow through square duct with twisted tape insert. ScienceDirect, Elsevier, 2002.

[4] UNE EN 12975. Sistemas solares termicos y componentes. Captadores solares. AENOR, 2006.