Recirculating Aquaculture Systems (RAS) have emerged as a sustainable and efficient alternative to traditional aquaculture practices. However, maintaining a stable and uniform temperature is critical for the successful rearing of fish larvae in aquaculture tanks. Thermal stratification, where temperature varies significantly throughout the tank, can negatively affect fish health, growth, and overall productivity.
Researchers from Nanjing Normal University, Jiangsu Yongsheng Air Conditioner Co., Ltd., China Construction Installation Group Co., Ltd., and The Australian National University have explored ways to optimize water flow patterns within recirculating aquaculture tanks to achieve better temperature distribution.
Understanding Thermal Stratification in RAS
Thermal stratification occurs due to differences in water temperature at various depths within the tank. Warmer water tends to float on top, while cooler water sinks to the bottom. This uneven temperature distribution can cause stress to fish, as they may experience sudden temperature changes when moving between different layers. Moreover, thermal stratification can affect water quality parameters and hinder nutrient cycling.
Factors Influencing Thermal Stratification
Several factors contribute to thermal stratification in RAS:
- Water supply pipe configuration: The design, perforation rate, and bending angles of water supply pipes can significantly impact water flow patterns and temperature distribution.
- Environmental conditions: External factors such as ambient temperature, humidity, and solar radiation can influence the thermal environment within the RAS.
- Tank design: The shape, size, and depth of the tank can affect water circulation and temperature distribution.
The Study: Optimizing Water Supply Pipe Design
The study focused on a recirculating aquaculture tank and employed a combination of single-factor and orthogonal experiments. The researchers investigated the influence of four key factors on water flow and temperature distribution:
- Water supply pipe layout
- Pipe bending angle
- Perforation rate (percentage of holes in the pipe)
- Environmental conditions
Evaluating Temperature Distribution
Three key indicators were used to assess temperature distribution in various water supply pipe configurations:
- Temperature uniformity throughout the tank
- Temperature fluctuations over time
- Temperature differences between surface and bottom water
Results and Optimal Pipe Configurations
The study identified two optimal water supply pipe configurations:
- Optimal Structure I (single-factor experiments): This setup involved a 2.5-meter pipe layout, a 50% perforation rate, and a 60-degree bending angle.
- Optimal Structure II (orthogonal experiments): This setup used a 1.25-meter pipe layout, a 75% perforation rate, and a 30-degree bending angle.
Seasonal Performance
Both optimal structures exhibited temperature fluctuations during the summer. However, Structure II showed a lower average temperature difference during the winter season, indicating better adaptability to colder months. Additionally, Structure II provided higher temperatures for shallow areas exceeding 600 mm in depth, making it more suitable for fish larvae cultivation.
Local Devices for Further Improvement
The study also explored the effectiveness of local devices within the tank to further minimize temperature variations. These devices successfully reduced temperature fluctuations by 17.5%.
Energy Consumption
The researchers evaluated energy consumption under different operating conditions. Structure II showed the lowest energy consumption during the summer season. However, its winter consumption increased by around 12%, highlighting the potential for significant energy savings in aquaculture tanks during winter through optimized water flow design.
Conclusion
This study provides valuable insights into optimizing water flow patterns in recirculating aquaculture tanks. By implementing strategically designed water supply pipes and potentially using local temperature control devices, aquaculturists can create a more uniform and stable temperature environment for optimal fish larvae rearing, promoting successful growth and survival year-round while minimizing energy consumption.
The study was funded by the Postgraduate Research & Practice Innovation Program of Jiangsu Province, China; and the Australian Renewable Energy Agency (ARENA) as part of ARENA’s Transformative Research Accelerating Commercialization Program.”
Contact
Zhongbin Zhang
School of Energy and Mechanical Engineering, Nanjing Normal University
Nanjing, Jiangsu 210023, China
Email: zhangzhongbin@njnu.edu.cn
Reference
Zhang, Y., Geng, Y., Zhang, Z., Dai, Y., Zhang, H., & Wang, X. (2024). Optimization of water supply parameters for enhanced thermal uniformity in aquaculture ponds: An experimental study based on orthogonal experimental design. Aquacultural Engineering, 107, 102464. https://doi.org/10.1016/j.aquaeng.2024.102464
