UC Merced

Novel minichannel-tube solar thermal collectors for low to medium temperature applications are introduced. Two types of minichannel solar thermal collectors are analyzed experimentally: aluminum minichannel solar collector for low temperature applications, and copper minichannel solar collector for low to medium temperature applications.

The aluminum minichannel solar collector has been tested for over a year alongside a conventional copper flat-plate solar collector of similar dimensions as the aluminum minichannel solar collector to compare the performance of both system. Experimental results shows the aluminum minichannel solar collector is more efficient in water heating due to the aluminum minichannel tube design. Analysis of the thermal resistances, thermal inertia and a simulation comparing the speed of response of both collectors indicate that the aluminum minichannel solar collector is capable of heating the working fluid quicker than the conventional copper flat-plate collector.

Preliminary data show that the copper minichannel solar collector is capable of reaching temperatures above 100 °C for low grade steam generation. Using a steam heat exchanger, steam can be generated with the copper minichannel solar collector with steam temperatures over 100 °C.

Mathematical models are developed to use as predictive tools to simulate performances of the aluminum and copper minichannel solar collectors under various operating conditions. The mathematical model for single-phase flow simulating the performance of an aluminum minichannel solar collector has good agreement with experimental data from the same type collector.

Multiple two-phase pressure drop and heat transfer coefficient correlations are validated and compared to experimental data from the literature in order to select the best one to represent the copper minichannel solar collector. The pressure drop and heat transfer coefficient correlations are selected from Muller-Steinhagen and Heck [1] and Odeh et al. [2], respectively. The pressure drop and heat transfer coefficient correlations are coupled to solve the two-phase flow heat transfer problem. Although the two-phase mathematical model cannot be validated with experimental data at this time, simulations comparing the performance of the copper minichannel operating under single-phase and two-phase flow are presented. Results show that efficiency decreases as the operating temperature of the collector increases. Even with the higher heat transfer coefficient during two-phase flow, the efficiency operating at temperature of the order of 100 to 110 °C in two-phase conditions are lower than operation in single-phase flow at temperatures between 50 and 90 °C. However, the difference of efficiency during single-phase flow with inlet temperature of 90 °C and during two-phase flow with inlet temperature of 100 °C is less significant, ranging from 3% to 10%.

Economic analysis is provided showing that the market for solar thermal technology in the United States is slow due to the decreasing prices of natural gas. Although there are incentives and rebates across the nation for solar thermal technology, these rebates and incentives are not widely informed. More contributions in both the government and business sectors are required in promoting awareness of solar thermal technology.

Market potential of the minichannel solar collectors are analyzed. Analysis shows that the aluminum minichannel solar collector can replace the existing low temperature, conventional copper flat-plate solar collector due to the experimental data showing the aluminum minichannel solar collector being more effective than the copper flat-plate collector. In addition, prices for aluminum are significantly cheaper than copper. Depending on the costs of extrusion process of aluminum minichannel tubes, and the labor and manufacturing costs to fabricate aluminum minichannel solar collectors, the costs of aluminum minichannel solar collector per square meter can be lower than conventional solar thermal collectors.