Significance of enhancing oxygen content in fishponds
Utilizing wind energy to enhance the oxygen content in fishponds
The pathways of utilizing wind energy to increase water temperature in fishponds
Conclusions and recommendations
* Jilin Agricultural University Changchun, Jilin Province, 130118, China
** University of Agriculture and Animal Sciences of PLA Changchun, Jilin Province, 130000, China
The chief conditions for the growth of fish and shrimp are water quality, and feed and water temperature. Water quality depends upon the degree of water pollution and the amount of dissolved oxygen. The effects of dissolved oxygen on aquiculture are introduced in this paper. Methods for utilizing wind energy to enhance dissolved oxygen and to increase fishpond temperature are discussed in this paper.
1. General Survey of Dissolved Oxygen in a Fishpond
Fig. 1 Variations of DO and temperature in a fishpond
It is well-known that fish or shrimp living in water need dissolved oxygen (DO) and feed. The dissolved oxygen is supplied by the photosynthetic process of algae or comes from the atmosphere by means of surface aeration. The former represents about 90% the total dissolved oxygen, that is to say that the percentage of dissolved oxygen depends largely upon the amount of algae and sunlight available. The dissolved oxygen is produced in the top layer of the water. However, the oxygen is consumed mainly in the middle and bottom layers of the water. Under natural conditions, the diffusion of dissolved oxygen is so slow that during sunny days the dissolved oxygen in the top layer of the water exceeds saturation and escapes into the atmosphere, while the bottom of the layer is in anaerobic state. The variations of dissolved oxygen and temperature in a fishpond are shown in Fig.1.
The dissolved oxygen required by aquatic life during the night is constituted by what remains of the DO which was produced during the day. The amount of dissolved oxygen is very little from midnight to 10 a.m. the following morning. During cloudy and foggy days, insufficient dissolved oxygen exists in the water and, as a result, the phenomenon of fish gasping will occur and fish may even die.
2. Effects of Enhancing Oxygen in Fishpond
(1) Fast growth of fish and less food consumption.
Taking common carp as an example, the growth turning point for common carp is 4.1 mgDO/liter. When the DO is higher than 4. 1mg/liter, fish grow faster and the feed coefficient (feed amount/net fish yield) is low. The optimum DO for common carp is 5. Sing/liter; 3 mgDO/liter above is a safe concentration; 2 mgDO/liter is too low; less than 1 mgDO/liter is a dangerous concentration resulting in gasping. When the DO is less than 3 mg/liter, fish grow slowly or even stop growing. It had been proved that if the DO decreases from 3-6 mg/liter to 2 mg/liter, the feed coefficient will be doubled.
(2) Decreasing fish disease incidence and increasing fish survival rate.
A great decrease in the incidence of fish disease has been obtained by using oxygen-enhancing equipment. As a result, the fish survival rate has increased respectively. Some experimental results show that the total survival rate of fish is increased from 46% to 85%.
(3) Increasing fish yield per unit of fishpond area.
Generally, fish yield is 400 kg per mu fishpond area. Experimental results show that fish yield can exceed 800 kg per mu by means of oxygen-enhancing equipment.
Examples of oxygen-enhancing equipment such as air-filling type, aeration type and water-stirring type are introduced as follows:
1. Air-filling Type
The air-filling type of equipment is used to fill air into water by using wind power to drive an air compressor. A schematic diagram of oxygen-enhancing equipment driven by a horizontal axle windmill is shown in Fig.2. The air compressor is driven by the windmill and the compressed air is stored in an air storing tank, which releases the air through pipes into a fishpond. The smaller the air releasing holes are so that the air bubbles take on fog shape, the more easily oxygen from the air is dissolved in water.
Fig. 2 Windmill-air storage tank type of oxygen-enhancing equipment
Fig. 3 Schematic structure of a vertical axle type oxygen-enhancing equipment
1. wind wheel
2. bearing seat
4. connecting seat
5. nylon bearing seat
6. vane wheel
7. flow guiding pipe
A schematic diagram of a vertical axle wind power oxygen-enhancing equipment is shown in Fig.3. The wind wheel is double layer S-shape vanes, which are welded at the top of the hollow vertical shaft. Air intake holes are drilled on the hollow shaft near the bottom of the vanes. A vane wheel is mounted at the bottom section of the hollow shaft and air releasing holes are drilled on the hollow shaft near the vane wheel. A flow guiding pipe is stretched along the bottom of the fishpond. The whole oxygen-enhancing equipment floats on the water by means of a pontoon.
When wind power makes the wind wheel rotate, the hollow shaft passes the power to drive the vane wheel and to impel the water downwards through the flow guiding pipe. As a result, a negative pressure zone is formed below the vane wheel. The air in the center of the wind wheel is sucked into the hollow pipe from the air intake holes and is passed into the water through the air releasing holes. After the air and water has been mixed, the air-water flows into the bottom of the fishpond along the flow guiding pipe.
2. Aeration Type
The mechanism of the aeration type of equipment aims at increasing the contact between water with air and thereby dissolving oxygen into the water while the water is flowing. Aeration is done jetting water into the air using a pump, or lifting water using a rotating vane wheel, or dropping water using a steep dam and so on.
Fig. 4 Aeration type oxygen-enhancing equipment
1. out river water
3. outlet pipe
6. drainage outlet
A schematic outline for jetting water aeration by using a windmill to drive a pump is shown in Fig. 4. A pump which is driven by a windmill lifting water from a river and jetting it into a fishpond. When the water in the fishpond is excessive, some old water can be drained off. Therefore, the advantage of this type equipment is that the water in the fishpond is continuously being replaced by fresh water during the oxygen-enhancing process.
3. Water-stirring Type
The way of enhancing oxygen by water-stirring is to propel water to flow by means of a rotating vane wheel. During the process, the lower layer of water can be lifted to the upper layer in a fishpond and the opportunity of contact between water and air is increased. As a result, not only is the DO in the water enhanced, but also a relatively even distribution of DO in the fishpond is obtained.
A schematic diagram of this type equipment is shown in Fig.5.
A propeller located on the water surface or fishpond bottom is driven by a windmill. The propeller pushes the water to flow and have contact with the air. In cold regions, this type of equipment can also help prevent the water surface from freezing up to a certain extent.
Fig. 5 Schematic diagram of water-stirring oxygen-enhancing equipment
In northern regions of China, during the cold winter, few living fish are available. In spring, the water temperature in fishponds goes up again slowly. Free breeding is restricted by low water temperatures.
To increase water temperature, wind energy can be employed to generate electricity, then the electricity produced is transformed into thermal energy to increase water temperature. In this process, however, energy loss is large. Three pathways to increase fishpond water temperature by employing wind energy to generate heat are suggested by the authors.
1. Solid Rubbing to Generate Heat
A schematic diagram of solid rubbing to generate heat equipment driven by a windmill is illustrated in Fig.6.
Fig. 6 Schematic diagram of solid rubbing to generate heat
1. rotary axle
2. centrifugal brake element
3. solid surface
4. water tank
7. insulation casing
8. oil tank
2. Water-stirring to Generate Heat
Fig. 7 Water-stirring to generate heat
2. stators vane
3. rotor vane
4. brace beam
5. fixed element
6. hollow shaft
8. rotary axle
3. Liquid Extruding to Generate Heat
A stirrer with a rotor and a stator is driven by the power output shaft of windmill. Vanes are mounted both on the rotor and the stators. As the rotor rotates, the rotor vanes stir the water and produce a vortex to lash the stator vanes. As a result, water kinetic energy is transformed into thermal energy. A schematic diagram is shown in Fig .7.
This way is to generate heat by means of a hydraulic pump and damping holes. A schematic diagram of this type of equipment is shown in Fig.8. The hydraulic pump is driven by windmill power output axle to press the actuating liquid (such as machine oil) which gushes from the narrow damping holes and dashes against the liquid which is in the tail liquid pipe. As a result, heat is generated. The flow velocity of the actuating liquid in the heat exchanging pipe is reduced and the temperature of the actuating liquid is increased. Then the actuating liquid to heat the water in the water tank is passed through the heat exchanger pipe.
Fig. 8 Schematic diagram of liquid extruding to generate heat
1. hydraulic oil tank
2. hydraulic pump
3. damping holes
4. heat exchanging pipe
5. insulation casting
6. water tank
1. The advantages offered by utilizing wind energy to develop aquaculture production not only include promoting fish or shrimp growth, lowering of incidence of disease, using less feed and increasing fish yield, but also include saving conventional energy (fuel or electricity) consumption. At present, there are more than 60, 000 sets of oxygen-enhancing equipment in China. Most of the equipment is being driven by fuel or electricity. However, the price of fuel and electricity are high. On the other hand, in some sparsely populated regions as well as islands and remote border regions, the national electricity grid has not yet been installed or electricity supply is serious inadequate. Since wind energy source is considerably abundant in these regions, the prospects of utilizing wind energy to develop fishery is bright in these regions.
2. As mentioned above, it is important for fish, especially for prawn, to replace old water in fishponds with fresh water. Regulating water quality is a key link during the prawn growing process. Otherwise, fishpond water will be polluted by the leached substances of feed resides and prawn excrete. As a result, ingestion and growth of prawn are influenced. Some prawn will die when the fishpond water is seriously polluted. Therefore, consideration of replacement of water should be given when using wind energy to enhance oxygen in a fishpond.
3. In those regions where suitable conditions exist, wind energy can be used to drive feed processing machinery to produce feed for fish or shrimp.
4. In order to design more efficient and more beneficial systems that use wind energy to develop fishery, studies should be further carried out on air storage equipment, power matching of windmill according to fishpond size and fish varieties, how to select utilization methods according to varied wind energy source in different regions, and so on.
1. D. Le Gourièrès, WIND POWER PLANTS THEORY AND DESIGN, France. 1982
2. Chen Yuncheng, etc, WINDMILL DESIGN AND UTILIZATION, China, 1985
3. Ding Yansheng, etc. PRIMARY TEST ON UTILIZING WIND POWER TO ENHANCE OXYGEN IN FISHPONDS
4. Yin Zhaojun, etc, WIND POWER ENHANCING-OXYGEN MACHINERY, China