The system used for the solar absorbers’ testing is a laboratory scale system for domestic hot water heating (E 202, Gunt Germany). The stand image and its components are presented in Fig. 1. The commercial stand consists of a flat plate collector with the absorber plate fixed on the heating serpentine. The device was modified allowing the replacement of the absorber plate (1), while using the same serpentine during all the experiments. The solar absorber converts the simulated solar radiation, delivered by a halogen lamp (2), into heat which is transmitted to a heat transfer liquid (water). The light amount and density can be adjusted by modifying the lamp height or the solar collector inclination angle. A pump is used for water circulation through a warm water reservoir, equipped with a small heat exchanger. Adequate sensors are used for temperature (inlet, outlet and water tank), illumination and flow rate monitoring. Using a data acquisition card, the measured values are transferred to a PC for further processing. [1] [2]

Pn = б-P-cpT — T1)

Where:

— PN: is the thermal power, [W];

— cp: the water specific heat capacity, cp = 4.18 kJ/kg’K;

— Ti: the inlet temperature, [oC];

— T2: the outlet temperature, [oC];

— n: the system efficiency, [%];

— Q: the volumetric flow, [L/h];

— p: the water density, [kg/m3];

— Ac: the collector area, [m2];

— E: the light density, [W/m2];

— 0: the incidence angle, [degree].

For the measurements, the water tank and the solar circuit were filled with deionised water. The lamp was fixed at constant height (70 cm) from the collector. The collector was inclined at the angles above mentioned.

3. Results and discussions

The experimental data for the (1) and (2) absorbers are presented in Table 1, while the thermal power and efficiency vs. incidence angle are illustrated in Fig. 2 and Fig. 3.

Table 2 Experimental data for solar absorber deposited on Al and Cu substrates Aluminium substrate 0 [degree] To1 To2 Q [oC] [oC] [l/h] E cos 0* [kW/m2] Copper substrate T1 T2 Q [oC] [oC] [l/h] E cos 0* [kW/m2] 0 35 45.1 3.0 2.58 30.9 43.6 3.2 2 .59 10 34.1 46.6 3.0 2.32 31.6 46.5 3 2.49 20 33.1 45.8 3.1 1.95 32.5 46.6 3.1 1.94 30 32.2 44.3 3.2 1.52 33.5 47.2 3.1 1.53 40 30.4 41.9 2.8 1.13 34.5 45.8 3.2 1.12 50 29.5 37.4 2.8 0.78 35.3 44.5 3.3 0.77 *measured on the collector surface

1st International Congress on Heating, Cooling, and Buildings " ‘ 7th to 10th October, Lisbon — Portugal * Fig. 2 Comparison between the efficiency of the Cu and Al solar absorbers

Fig. 3 Comparison between the thermal power of the Cu and Al solar absorbers

The efficiency (n) and the thermal power (PN) for the two solar absorbers mainly depend on the absorbers type, their optical properties (solar absorptance and thermal emittance) and on the irradiance incident angle.

As Fig. 2 shows, the efficiency of the solar absorber (2), is higher compared to solar absorber (1). The black nickel, on the top of the substrate has rather similar composition for (1) and (2), being obtained using the same materials and spraying parameters, except to the deposition temperature: 350oC for (1) and 300oC for (2). Thus, the higher values of the n (and also PN) can be explained, on the one hand, by the fact that the structure Cu/CuOx has a higher thermal conductivity comparing to Al/Al2O3. In this case, the energy transferred to the flow liquid is higher. On the other hand, the optical properties (solar absorptance and thermal emittance) for the solar absorber deposited on Cu

substrate are better (Table 1), thus having higher heat gain and lower heat loss capacity. Although deposition on Cu gives better results, the Al substrate has a lower price and weight.

Generally, when 0 = 0o, the solar radiation is perpendicular on the collector surface, maximum amount of energy is reaching the panel. The heat variation produced for different incidence angles is presented in Fig. 3. The results prove that the maximum heat gain is registered for incidence angles of 10…30°. These results can be the consequence of reduced reflections and/or radiation scattering for angles higher than 0. The almost 20% increased output of the copper based collector comparing to the aluminium one, at the maximum heat gain point can also be correlated with the heat conduction/storage of the two plates.

The results presented in Fig. 2 and in Fig. 3 shows that the heat gain and the efficiency are not following a similar trend. These observations could be further used in studies of the solar-thermal conversion efficiency at low radiation amounts (e. g. for cloudy days). Also, tracking systems could be recommended to maintain the most favourable inclination angles for the solar collectors, thus to get the maximum thermal power. An efficiency increase up to 11% can be registered in this case [12].

4. Conclusion

In the present study, two types of black nickel based solar absorbers deposited on copper and aluminium substrates were tested. The absorbers were incorporated in a laboratory scale domestic hot water system. These studies were developed in the research department of the centre: Product Design for Sustainable Development, in the Transilvania University of Brasov, Romania.

The objective was to obtain two low cost solar absorbers and to test their thermal performance, in laboratory conditions. The results proved that quality absorbers can be obtained by spray pyrolysis deposition technique. The CuOx/NiOx/TiO2 absorber proved to have a better efficiency and thermal power than the Al2O3/NiO/TiO2 absorber due to better optical properties and thermal conductivity. The study showed that maximum thermal power can be achieved, in both cases, for inclination angles in the range of 10-30o.

Acknowledgements

This work was supported by the Romanian Council for Research in High Education, CEEX 277/2008 grant.

References

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