Design and Evaluation of a Dissolved Oxygen Controller for Solar Powered Fish Tanks

Aims: A dissolved oxygen controller was designed to optimize photovoltaic energy and maintain minimum DO variations in the fish tanks, independent on environmental conditions.
Study Design: Software critical parameters have to be tuned including discontinuous routine (DR) interval, use throughout the night and cloud sensing.
Place and Duration of Study: Department of Irrigation at Tlapeaxco, Texcoco, Mexico at the Universidad Autonoma Chapingo, between April and July 2011.
Methodology: The controller was assembled and its software tested to maintain the desired water dissolved oxygen concentration in the tanks. Solar radiation and clouds presented during May and June of 2011 were monitored correlating an infrared sensor with a sunshine indicator. Energy produced by the solar panels was acquired with a data logger together with the energy stored in the batteries; overvoltage and deep discharges were avoided by the battery. A discontinuous routine was turned on when clouds were present, evaluating whether it should operate during all the night.
Results: May presented 26 cloudy days and 45% of these days presented less than 2 hours of more than 1000 W/m2 of irradiation. Cloud cover sensing correlation between the infrared and sunshine sensor has a R2=0.97. Cloud cover peaks can last from minutes to hours and using the one-hour discontinuous routine (DR) instantaneous peaks were avoided. Optimal DR period was ten minutes as it saves the same quantity of energy but maintains dissolved oxygen (DO) concentration over 4.1 ppm; hourly DR intervals, decreases DO to 2.4 ppm stressing the carps. Battery charge should be at least 39 Ah@19:00 to supply the energy required by the aerators during the night.
Conclusion: A sunshine sensor was selected to detect cloud cover and its sampling period was decreased from ten to one minute. DO concentration on tanks became more stable when the ten minute discontinuous period was employed.