The tested solar collector is a commercial product with a covered gross surface of 2.541m2. The test was carried out in the Test Station of IEE, CAS, located in Beijing, China, at 37.5° N latitude and at 116.7° E longitude. The test period reported was realized from May 17th to October 19th, 2007.

Figs. 2-6 present the main experimental results. Fig.2. shows lineal fit to test data for instantaneous efficiency curve and equation is provided. Fig.3. shows second order fit to test data for instantaneous efficiency curve and equation is also provided. All of these experimental data can be obtained under the steady-state measurement conditions according to ISO 9806-1. The instantaneous efficiency data is based on the gross area of solar collector and fluid flow rate used for the tests is 0.02 kg/m2.s. With the experimental data obtained in Fig.2. and Fig.3. and using the proposed equations (1)-(7), the following parameters were calculated for this day test: the highest nis 0.608 and the lowest nis 0.528.

Obviously when the inlet temperature is close to the ambient temperature, the heat loss of solar collectors will be less than its heat loss as the difference between the inlet temperature and the ambient temperature is very large. On the other side, because of the all-glass evacuated tube, the heat loss can be effectively reduced under the high temperature operation mode.

In Fig.4, different instantaneous efficiency curve in different months can be provided. Through this figure, the common situation of solar collectors operated during a long period can be predicted. In the past 5 months the ratio of the actual useful energy extracted to the solar energy intercepted by the collector can reach a comparatively high point. It means that this kind of solar collectors can work in a good condition and supply actual useful energy on a stable level. With the experimental results of several months, it is possible to obtain common sense and characteristic curves of solar collectors.

Fig.5 and Fig. 6 shows in a certain day the variation of instantaneous efficiency, the difference between inlet temperature and ambient temperature of solar collectors and solar irradiance. In Fig. 5 the whole variation trend to some extent which instantaneous efficiency match solar irradiance is similar, but in the afternoon there exits opposite direction between two factors. Because all-glass evacuated tube can reduce heat transfer from inside to ambient, the energy can be reserved effectively. Meanwhile, if the direct radiation from solar energy is decreasing, for example, the clouds shadow the direct radiation, the ratio will be increased in an opposite direction to the solar irradiance. This result can be demonstrated by the same variation trend between the temperature difference and solar irradiance from Fig. 6. In the day test all-glass evacuated tube solar collector shows the reliable stability and high quality to supply the actual useful energy, and it can keep a good performance. The average instantaneous efficiency can arrive at 0.74 during the day test from 9:45 in the morning to 17:25 in the afternoon.

2. Conclusion

In this paper, a comprehensible test method to determine the thermal behaviour of solar collectors has been carried out. Through this kind of test an overall thermal performance can be provided for the product, such as the fit equation of the instantaneous efficiency considering the influence of inlet temperature, ambient temperature and solar irradiance, actual operation condition in a long period and the relationship among efficiency, difference of temperature and solar irradiance. And then, the manufacturer will obtain the information in order to improve the thermal efficiency of solar collectors. At the same time, the information is also useful to design the ideal solar water heating system with all­glass evacuated tube collectors for engineers. This procedure test also allows comparing, under the same test conditions, systems with some changes, for example with different materials, absorber selective surfaces, as well as thermal insulation, in order to analyze the influence of these parameters and together with an economical study can offer to the manufacturer the convenience or not to implement this modification.

References

[1] J. A.Duffie, W. A.Beckman, (1991). Solar Engineering of Thermal Processes, Wiley, New York.

[2] ISO9806-1, (1994). Test Methods for Solar Collectors — Part 1: Thermal performance of glazed liquid heating collectors including pressure drop.

[3] EN 12975-2, (2001). Thermal Solar Systems and Components Solar Collectors Part 2: Test methods.

[4] GB/T 4271, (2000). Test Methods for the Thermal Performance of Flat Plate Solar Collectors.

[5] Perers B, Dynamic Method for Solar Collector Array Testing and Evaluation with Standard Database and Simulation Programs, Solar Energy 1993, 50:517-26.

[6] Zeroual A, A New Method for Testing the Performance of Flat-plate Solar Collectors, Renewable Energy 1994, 4:825-32M.