During the development of the test, extractions of hot water are not carried out. The SDWHS is filled with water trying to maintain the inlet water (T0) at the same temperature. The test begins at 9:00 h (solar time) when the data acquisition system is switched on and the experimental data are registered every 60 seconds. At the end of the solar journey at 18:00 h, the valves of the solar loop are closed and the small recirculation pump is turned on in order to homogenize the temperature in the storage tank (Tf); after that the day test is finished.

In order to evaluate the thermal losses during the night period, the two valves of solar loop are opened again. The following day at 8:30 h, the temperature of the storage tank is homogenised again in order to obtain the final night temperature (Tf, 24h) and finally the system is completely empty. This test procedure is realised during several days in order to obtain the system characteri­sation under different weather and working conditions.

The schematic representation of experimental apparatus for test procedure system is shown in Fig. 2. It has two different loops: the first one is the solar loop that includes the solar collector and stor­age tank. Three temperature sensors are inserted in the following positions: 1/4, 1/2 and 3/4 of the internal tank height. The three temperature sensors are used for two reasons: a) to obtain the strati­fication profile in the storage tank along the test and b) to determine when the homogenised tem­perature in the storage tank is reached. The second loop is the recirculation one, which is a closed circuit where a small pump permits the quick circulation of the water contained in the storage tank in order to homogenise the temperature. The homogenised temperature in the storage tank is reached (and the recirculation pump is turned off) when the three temperature sensors inside the tank vary less than 0.5 K (in the thermosyphon system tested, the time used in this procedure was lower than 5 minutes). Additionally the ambient temperature is measured, a global solar irradiance sensor is also integrated on the collector plane and an anemometer is also installed in order to measure the wind direction and speed [11].

The tests were carried out in the Solar Platform of the Centro de Investigation en Energia of the Universidad Nacional Autonoma de Mexico, located in Temixco, Morelos State, Mexico, at 18°50.36’ N latitude and at 99°14.07’ W longitude, with an altitude of 1219 m over sea level. The yearly average ambient temperature is 23.09 °C with a yearly average solar irradiance on the hori­zontal plane of 20.28 MJ/m2. The tests began from May 2007 to May 2008.

3. Results

Fig. 3 shows the radiation and the stratification profile of the water temperatures of the experimen­tal systems for one day of test; the two-phase system working with R134a. The morning and the early afternoon were sunny until around 3:00 pm with an average solar radiation on the collector plane of 546.44 W/m2.

Fig. 3. Radiation and temperature profile for one day of test, using R134a as working fluid.

The cloudy period seems to diminish the gain of heat in the conventional system more than in the phase change system. After the irradiation period (at 18:00 h), the temperature in the thermotank has increased around 21 °C to reach a temperature of 50.3 °C for both the conventional system and the two-phase working with R134a.

The efficiency and useful heat of the system are calculated from the equations:

r/ = 100 x — 4r

qr — (I)( AbXAt)

Where I is the irradiance on the collector plane [W/m2], Aabs of operation of the system [s],

qu — (mH2O )(Cp, H2O ‘)(ATH2O )

Where mH2O is the mass of the water [kg], cpH20 the specific heat at constant pressure [J/kgK] and ATmO=Tf-T0 [K].

On the other hand, the thermal losses during the night are calculated from the equation:

Us — mcp, n (Tf,24h ~ Tf )

T — T

f 1 amb, n (4)

The letter n stands for the night period; Tf is the available temperature at the end of the solar irradi­ance period [K], Us the thermal losses during the night [J/K] and Tambn the ambient temperature.

It can be seen from Table 1 that acetone has a lower performance; however more tests must be car­ried out to obtain a solid conclusion to this question; according to Soin et al. [3] the system must be loaded with a larger quantity of the working fluid to obtain a better performance.

Table 1. Performance of the two-phase (acetone) and the conventional system. Results Water system Acetone n 50.6±3.1 40.1±1.6 qr[MJ] 28.2±1.9 32.1±3.8 qu [MJ] 14±1 12.9± 1.8 Tf, d[K] 50.3±2.6 48.1±3.2 A T [K] 21.3±1.4 19.7±2.8 Us[MJ/K] 0.445±0.035 0.44±0.00 NOTE: Rounding of mean values was done according to standard deviation data. Table 2 summarises the efficiencies and other parameters of the two-phase thermosyphon working with R134a and the conventional thermosyphon. Although the two-phase system exhibits a slightly better performance, a simple analysis indicates that the efficiencies are statistically equivalent. Table 2. Performance of the two-phase (R134a) and the conventional system. Results Water system R134a n 50.6±3.1 51.5±2.6 qr[MJ] 28.2±1.9 28.2±1.9 qu [MJ] 14±1 14.5± 1.3 Tf [K] 50.3±2.6 50.3±2.1 A T [K] 21.3±1.4 21.7± 1.9 Us [MJ/K] 0.445±0.035 0.44±0.06 NOTE: Rounding of mean values was done according to standard deviation data.

Comparison of the performance of the two-phase system using R410A, versus the performance of the conventional thermosyphon that uses water is shown in Table 3. As in the R134a case, both systems exhibit statistic equivalent performances, with an average increment of 21 °C of tempera­ture in each case, showing efficiencies of approximately 51 %.

Table 3. Performance of the two-phase (R410A) and the conventional system. Results Water system R410A V 51.4±1.1 51.4±0.9 qr[MJ] 27.6±1.7 27.6±1.7 qu [MJ] 14.2±1.0 14.2±0.9 Tf, d[K] 48.9±1.2 47.6±1.2 A T [K] 21.2±1.5 21.2±1.3 Us[MJ/K] 0.69±0.05 0.73±0.05 NOTE: Rounding of mean values was done according to standard deviation data.

Although R134a and R410A were not tested at the same time, these data suggest that their per­formance may be equivalent. However, R134a works at significant lower pressures than R410A.

The system, when working with R410A reached consistently pressures near 38 bar. If water is re­moved from the thermotank, then the fluid cannot transfer its heat completely, and the pressure grows so high that the tubing resistant limit is reached, so the tubing can burst open at any mo­ment. Because of this, in our opinion, use of R410A is not recommended for this kind of system. To exemplify this, Fig. 4 shows the performance of the fluid temperature and the pressure for the two-phase system, working with R410A. Pressure reaches values near 36 bar.

4. Conclusions

A two-phase closed thermosyphon using R134a, acetone and R410A as working fluids was com­pared with a conventional natural solar collector thermosyphon. In tests, the two-phase system working with R134a showed statistically equivalent performance than the conventional system, though the former eliminates problems of freezing, fouling, scaling and corrosion. The two-phase system working with acetone showed a slightly lower performance; however, it is expected to im­prove after new tests with major loads of acetone in the closed circuit of the system and also, after trying vacuuming the system. The two-phase system working with R410A showed statistically equivalent performance than the conventional thermosyphon. Both R134a and R410A show good performance as working fluids, and in their respective comparisons with the thermosyphon system; however, the lower pressure reached by the R134a makes it more attractive as a working fluid.

5. Acknowledgements

This work has been partially financed by PAPIIT project (IN-111806-3) and CONACyT project U44764-Y and Modulo Solar S. A. de C. V. company. The authors thank to CONACyT for the sup­port provided to the student with the scholarship number 183846.

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