There were a couple incidents where the aluminum minichannel solar collector system was stagnant. During that time which was a typical Spring and sunny day, the working fluid in the aluminum minichannel tubes reached boiling temperature. With the outstanding performance and results we acquired from the aluminum minichannel solar collector, we wanted to test the minichannel solar collector further by developing a copper minichannel solar thermal collector. The objective of this project was to examine the minichannel solar thermal technology and develop a flat, solar thermal collector for low to medium temperature, a temperature when steam can be generated. Possible industrial applications includes sanitation or drying. We chose to use copper because of copper's high thermal conductivity.
One of the biggest obstacles of the project was finding a manufacturer that was capable developing copper minichannel tubes. Copper minichannel tubes are not widely produced due to the high cost of copper and the difficulty of extruding copper because of the high working temperature required to get the material to be malleable and extrudable. Dr. Gerardo Diaz was fortunate enough to have met Dr. Frank Kraft of Ohio University who patent an invention to extrude specifically copper minichannel tubes. However, there were constraints on how wide we can make the copper minichannel tubes. Using a mathematical model of a single-phase flow in minichannel tubes developed using EES (Engineering Equation Solver), a few designs with varying port sizes were simulated with hypothetical tube sizes in the copper minichannel solar collector system and the tube performances were compared. To determine which tube design to select, the pressure drop across the collector and heat transfer at varying Reynolds number were compared in each design. We were able to determine that the ideal tube design for copper minichannel solar collector is shown in Figure 1.
Due to time constraint (on both ends) and budget, there were a limited number of copper minichannel tubes there were produced. After Dr. Kraft sent the copper minichannel tubes, the UC Merced Machine Shop and Facilities team helped put together the CuMC solar collector. The CuMC tubes were torched brazed to circular copper pipes. The CuMC collector without the selective coating is shown in Figure 2 on the right, and Figure 3 shows the CuMC collector with sprayed on selective coating. Like the AlMC collector, the CuMC collector is placed in a protective glass frame to prevent the CuMC from weathering (Figure 4). The copper minichannel solar collector system was built in a similar way as the aluminum minichnnel solar collector but at a smaller scale due to tube length size restrictions and tube quantity produced. The CuMC solar collector was placed on a portable test stand instead on the roof for acessibility and easy maintenance when required (Figure 5). It was also placed on the portable test stand to control the angle of the collector and amount of solar irradiation it can absorb. Additional bypass valves, a cooling tank in case of high temperatures, and a steam generator were also added to the system (Figure 6). The steam generator was a simple heat exchanger with two concurrent and different sized pipes built to show that the CuMC solar collector is capable to generate steam indirectly (as well as directly.)
Figure 7 on the leftshows an example of the performance of the CuMC solar collector on a summer day in Merced. Data was gathered from approximately 8:30 AM to 1 PM, with mostly clear skies and the ambient temperature ranging from about 27 to 37 °C. From the figure it can be seen that the collector inlet (Tcol,in) and outlet (Tcol,out) temperatures started at 31.5 and 32 °C, respectively. After two hours, the collector inlet and outlet temperatures reached over 100 °C, and stayed above 100 °C until the end of the experiment. The highest point the collector temperatures reached at the inlet and outlet was 107.5 and 107.9 °C, respectively, which can be seen around 12:51 PM. The solar irradiance remained within the range of 565.7 to 787.50 W/m2 during the test (PSP).
The steam generator outlet has a valve which opens or shuts. In the beginning, the valve of the steam generator outlet was shut but left with a slight opening. As the steam outlet temperature begun to increase, the valve of the steam heat exchanger outlet was manually opened all the way. Steam was generated as the temperature at the outlet reached 99 °C seen by the Ts,out line. The thermocouple used has an accuracy of ± 2.2 °C in the range of 0 to 1250 °C which may prevented the thermocouple from reaching 100 °C. Steam was generated at the steam heat exchanger multiple times during the day as the steam heat exchanger outer shell was refilled with water. It can be seen a couple times as the water was refilled into the steam generator shell when its outlet temperature drops (approximately at 11:08 AM and 12:07 PM.)
In this project, I also studied the two-phase flow and boiling conditions for steam generation in the minichannel solar collectors. I developed a mathematical model and prediction tool to simulate the performance of the minichannel solar collectors during two-phase flow. Generally, in single-phase flow heat transfer analysis, pressure does not affect the change of temperature. Pressure drop and heat transfer rate are decoupled in single-phase flow. However, in two-phase flow, pressure is dependent on the temperature of the fluid. And the pressure drop cannot be solved without the heat transfer rate. There are many studies through out the past decades that tries to model the phenomena of two-phase flow. There are many empirical (based on experimental data and curve fitting) and phenomenological (based on observations of flow patterns and phase shapes) mathetical correlations out there but none of them are 100% accurate, but provides approximate results depending on the working fluid, type and shape of tube fittings (circular, rectangular, triangular, etc.) and the amount of energy provided to the working fluid. Not getting into too much details (you can read more in my thesis), I basically researched several popular and well-cited pressure drop and heat transfer coefficient correlations, tested these correlations with peer reviewed data and chose the best fitting pair of correlations that I can use for my minichannel solar collector mathematical prediction tool. Muller-Steinhagen (1986) and Heck pressure drop and Odeh et al. (1998) heat transfer coefficient correlations were paired to develop and simulate the copper minichannel solar collector.
The last figure on the right shows results and comparison of the single-phase and two-phase simulations of the CuMC solar collector from the mathematical models developed. The single-phase mathematical model uses one-dimensional energy balance equations. The two-phase mathematical model was developed using Muller-Steinhagen and Heck pressure drop and Odeh et al. heat transfer coefficient correlelations for two-phase flow. It can be seen that efficiency decreases as inlet temperature increases. There are higher differences of efficiencies at lower solar irradiance between the varying inlet temperatures, but the differences are lower at higher solar irradiance. The simulations show that at higher solar irradiance, the efficiencies during two-phase flow are not penalized significantly in comparison to single-phase flow with temperatures close to saturation temperature. The range of difference between entering in single-phase flow inlet temperature of 90 °C and in two-phase flow inlet temperature of 100 °C is 3% to 10% depending on the solar irradiance.
The single-phase simulated results were compared with experimental data of the AlMC solar collector and good agreement were obtained. The detailed analyais and comparison can be seen in Chapter 3 of my thesis. Unfortunately due to time, budget, the difficulty to capture and quantify two-phase flows and the proper equipment required, simulated two-phase results and experimental results cannot be compared to see the accuracy of the two-phase mathematical model at this time. It may be a research project in the future for the Diaz Research Group.
Detail information including design, development and experimental set-up of the copper minichannel solar collector can be found in my thesis. It also includes details of a aluminum minichannel solar thermal collector:
V. Duong, Minichannel-tube solar thermal collectors for low to medium temperature applications, 2015.
Poster: Prediction two-phase frictional pressure drop in copper minichannel solar water heater, presented at: UC Solar Symposium, San Francisco, California, October 2014.
Kevin Rico, for material procurement and helped putting together the CuMC solar collector
UC Merced Facilities
Ed Silva from UC Merced Machine Shop
Past undergraduate students that contributed tremendous amount of their time and efforts during the 2014-2015 school year, especially the warm Spring and hot Summer of 2015:
I would also like to thank California Energy Commission for providing the funding for this project (Contract # POEF01-M04).