AERATION OF FISH-PONDS BY PHOTOVOLTAIC POWER
1Tel-Aviv University, Faculty of Engineering, Tel-Aviv 69978, Israel, 1*Graduate Student,
Institute of Agricultural Engineering, The Volcani Institute, Bet Daga 50250, Israel, 2The Late Ishaia Segal
3Landtechnik Weihenstrphan der TU Munchen, Voettingerstr. 36, D-8050, Freising, Germany
4Universidad Technica Frederico Santa Maria, Dept. Mechanico, P.O.B. 110-V/Valparaiso, Chile
Fishponds in aquaculture farms are usually located in remote areas where grid lines are at far distance. Aeration of ponds for growing of aquatic animals (e.g. fish) is required to prevent mortality and to intensify production, especially when feeding is practical, and in warm regions. Aeration systems with photovoltaic power supply may be the appropriate technology from an ecological and economical point of view. In addition, photovoltaics are very suitable for remote developing regions due to its simplicity and low maintenance. The purpose of the present study was to demonstrate an aeration system, using a paddle wheel, powered by a photovoltaic power supply. The study was carried out on the coastal area in the center of Israel.
One of the problems of fish growing in ponds is the dissolved oxygen (DO) concentration in the water. The level of DO under 2 ppm is dangerous to the life of the fish because at this level of DO the probability of mortality among the fish is high. To prevent this dangerous stage emergency aeration is employed to prevent the DO level droppage under the 2 ppm.
Continuous aeration is a good practice for fish growing in ponds. Some experiments showed that some parameters of fish growing in ponds are significantly better in continuous aeration or night aeration than in emergency aeration. Research confirm that aeration, in many cases, promotes channel catfish production by reducing stress and disease occurrences, increases survival and improves feed efficiency rates.
The concentration of DO in water at saturation varies with temperature and barometric pressure. The DO concentration decreases as temperature increases, and the DO concentration at saturation increases in propotion to increasing barometric pressure. Plants and plankton are beneficial to pisciculture because they utilize nitrogenous wastes that are toxic to fish, consume carbon dioxide, and during daylight hours, they produce oxygen (by photosynthesis) necessary for fish respiration. However, at night, respiration of fish, plant and other organisms in pond causes DO concentration to decline. The problem becomes more severe during warm nights, when the saturation levels are especially low because of the high temperature. In such cases, DO concentration in ponds is often below saturation.
In many countries, fishponds in aquaculture farms are usually located in remote areas far from grid connection. Long distance lines and low energy consumption (as for fishpond aeration) is not economical and photovoltaic fishpond aeration may be considered as the appropriate technology. An important additional aspect is the increase in safety by using low voltage systems (<50 Volts) compared to 230 or 340 Volts. Furthermore, the oxygen demand is synchronous with ambient temperature, i.e., high demand in summer with high availability of solar radiation. However, because aeration is carried out during nights, storage of electricity is required.
Researchers examined several kinds of aerators and concluded that paddle-wheel aerators are the most efficient in term of oxygen transfer into water per unit of time and per unit of input energy to the aerator. Another advantage of the paddle-wheel aerator is the creation of a strong vertical current that causes a more homogeneous concentration of DO in the water and also helps in submerging of waste and other organic material in the water.
Our study was carried out on a fish pond of 150 m2 with varying depth of 0.65 to 1.40 meters, 16 modules Siemens M-55, 2 modules in a string and 8 groups in parallel were used in conjunction with a 500 Ah, 6V batteries. A permanent magnet DC motor, 400W, 24V and 1500 r.p.m. drove the paddle wheel with a gear reduction of 13.1:1. The instruments for data collection and measurements include photovoltaic array current, voltage and temperature; battery current and voltage; solar radiation; wind speed and direction; water and ambient temperature; and dissolved oxygen.
The study was carried out during the summer months. Three modes of operation were studied with the purpose of keeping the DO level above 2ppm and consume the minimum amount of stored energy in order to obtain an aeration system of the smallest size. The modes of operation were:
From the analysis of the photovoltaic array power, battery current, voltage and state of charge, motor power, system efficiency and dissolved oxygen it was concluded that intermittent aeration may reduce the size of the photovoltaic system to be economical.
Keywords: Photovoltaic array, fishpond aeration, paddle wheel, dissolved oxygen, battery, DC motor.