Recirculating Aquaculture Systems (RAS) have gained significant attention as the most promising land-based fish farming system for sustainable production.
The freshwater requirements in RAS are much lower due to water recirculation and treatment, thus limiting effluent emissions.
However, the efficient operation of RAS requires a set of equipment, resulting in a high capital investment that can be offset by large-scale and intensive production.
A team of scientists from the Western Norway University of Applied Sciences modeled, validated, and simulated a Recirculating Aquaculture System (RAS) to estimate the energy demand of RAS and its main components.
“The hybrid model proposed in Matlab and Aspen HYSYS simulates the post-smolt growth stage of Atlantic salmon for a period of 15 weeks, until they are ready for transfer to cages,” the scientists highlighted.
Energy Demand of RAS
The need for a range of equipment to control water quality makes recirculating aquaculture systems energy-intensive. Compared to other aquaculture systems, RAS is the most energy-intensive method in terms of fish mass produced.
In Life Cycle Assessments (LCA), the energy demand of RAS was found to be 1.4 – 1.8 times higher than in open flow systems. Another comparative LCA study shows that the specific energy demand per unit mass of fish produced in open flow systems was 2.55 kWh/kg, while in RAS it was several times higher at 19.6 kWh/kg.
Although recirculating aquaculture systems are clearly more energy-demanding, the estimates of energy demand present a wide range of values.
Evaluation of Matlab – Aspen HYSYS for RAS Simulation
Modeling the recirculating water treatment circuit in Aspen HYSYS is a viable method for RAS simulation. The integrated packages of equations and unit operations greatly facilitate the resolution of processes with gas-liquid phase interaction, such as aerators and oxygen cones.
“The simulation shows no stability or convergence issues, but the simulated RAS operation has limited fluctuations without diurnal variations. Fish feeding and tank lighting are continuous, and water temperature is regulated by introducing freshwater,” they reported.
According to the study, the result is an RAS that operates mainly in a steady state, except for regulated oxygen cone bypasses and cases with adjusted recirculating water flow.
“The method and tools presented here yield satisfactory results for this quasi-dynamic simulation, but caution should be exercised when implementing them in dynamic systems with strong diurnal variations,” they cautioned.
RAS Energy Demand
According to the study, in absolute terms, the RAS model has a total energy demand of 4.933 MWh/day at the start, which remains constant until the oxygen cone pump is switched on. The RAS then concludes with a total energy demand of 6.955 MWh/day.
The energy required to operate a recirculating aquaculture system from the beginning to the end of the growth phase is 663.8 MWh.
On the other hand, the specific energy demand of the simulated RAS is 9.586 kWh/kg. At least one-third of the energy is dedicated to meeting the thermal energy requirements of freshwater.
Recirculation pumps account for 22.6% of the total energy demand for the entire period. If the oxygen cone pump is included, all pumps are responsible for 45.48% of the total energy demand.
Energy Demand with Adjusted Water Flow
“From an energy perspective, adjusting the RAS water flow is a viable option to reduce demand. The RAS energy demand was reduced by 7.92% when the water flow was adjusted,” the scientists emphasize.
However, adjusting the water flow directly affects the water quality in the fish tanks. “When water recirculation decreases, water quality is reduced.”
According to the researchers, the energy demand of RAS can be reduced if the established limits for water quality parameters in the fish tanks are less strict.
“The study demonstrates the feasibility of Aspen HYSYS for modeling and simulating recirculating aquaculture systems using Matlab. To the knowledge of the authors, this is the first time that an RAS, or part thereof, has been modeled and validated in Aspen HYSYS,” they highlight.
The specific energy cost for the growth of Atlantic salmon post-smolts is 9.589 kWh/kg. “In total, the RAS energy demand for the entire growth operation is 663.8 MWh,” they conclude.
The study was funded by the Research Council of Norway and the Western Norway University of Applied Sciences.
Department of Mechanical and Marine Engineering
Western Norway University of Applied Sciences
Inndalsveien 28, 5063 Bergen, Norway
Reference (open access):
Gerard Ayuso-Virgili, Leila Jafari, David Lande-Sudall, Norbert Lümmen. 2023. Linear modelling of the mass balance and energy demand for a recirculating aquaculture system, Aquacultural Engineering, Volume 101, 2023, 102330, ISSN 0144-8609, https://doi.org/10.1016/j.aquaeng.2023.102330.