Adapting a current technology used in automotive, electronic cooling, and HVAC, aluminum minichannel-tubes were used as a novel configuration for a solar thermal collector (Fig. 1). The collector is integrated into a solar water heating system for residential and commerical water heating (Fig. 2). The heat absorbed by the working fluid in the collector is passed on to the water in the water storage tank through indirect heat exchange. The working fluid is a mixture of 50% propylene glycol and 50% water by volume. The fluid in the water storage tank is tap water supplied by the City of Merced.
To test the efficiency of using aluminum minichannel (AlMC) tubes as a solar collector, a conventional copper flat-plate (CuFP) solar collector was built and tested alongside (Fig. 3). Both systems were integrated independently with their own solar water heating system. The solar water heating systems are identical; each system had an identical pump, water storage tank, piping and length of piping, and sensors. Each system had thermocouples placed in the inlet and outlet of the collector, a thermocouple in the water storage tank, flow meter and a Precision Spectral Pyranometer (PSP) that measured the solar irradiance. All of these sensors were connected to a data acquisition system and monitored by LabView. Data was logged every minute of the day from sunrise to sunset.
The AlMC and CuFP solar collectors both have similarities and differences as seen in the 4th figure on the right. While the glass frame to protect the collectors from weathering, erosion and dust are exactly the same, the main differences are the absorption and the free flow areas. With limitation to the size of the glass frame, the AlMC collector has an absorption and free flow area, respectively, of 3.2 m2 and 1015.5 mm2. The CuFP collector has an absorption and free free flow area of, respectively, 3.68 m2 and 1026.1 mm2. An example of partial cross-sections of the AlMC and CuFP collectors are shown in Figure 5.
The process of both system are similar: the pump pushes the working fluid into the inlet of the collector, the fluid absorbs heat from the collector and exits the collector flowing through piping to a heat exchanger in the water storage tank, the working fluid exchanges heat with the water inside the water storage tank, the pump pushes the work fluid back into the collector and the cycle restarts, slowly heating up the water storage tank. Both system has a safety to ensure the water storage tanks never reach over 55 °C. Controlled by LabView, the water storage tank opens its discharge valve and empties the water when it reaches 55℃. Simultaneously, cold water from the water supply replenishes the tank until the water in the tank drops to 30 °C.
The 6th figure on the left shows the performance of the AlMC and CuFP solar collector during a summer day in 2013. From the plot we can see that while the AlMC water storage tank was slightly warmer in the morning by about 1-2 °C, the AlMC water storage tank was able to reach to 55 °C quicker than the CuFP water storage tank by approximately an hour. Once each water storage tank reaches 55 °C you can see from the plot that the warm water is discharged from the water storage tank and cool water is replenished in them.
More details about the performance data collected and results under year-round weather conditions can be found in the following paper:
A. Robles et al., Aluminum minichannel solar water heater performance under year-round weather conditions, Solar Energy 110(2014), 356-364.
Detail information including design, development and experimental set-up of the aluminum minichannel solar thermal collector can be found in my thesis. It also includes details of a copper minichannel solar thermal collector:
V. Duong, Minichannel-tube solar thermal collectors for low to medium temperature applications, 2015.
Contact me if you have trouble accessing and cannot obtain the full-text.
This project was a group effort. All past undergraduate students involved in the aluminum minichannel solar water heater project contribute tremendous amount of their time and efforts during the summer of 2013 up to 2015 school year. Acknowledgements goes to:
Dr. Gerardo Diaz, PI
I would also like to thank California Energy Commission for providing the funding for this project (Contract # POEF01-M04).