Heat transfer predictions for aquaculture facilities

Abstract

Recycling of water in aquaculture facilities is used to minimize the amount of energy or tempered water required to control water temperatures. The rate of heat exchange between the water and the environment can be an important variable in the design, management, and economic analysis of a recycle system. A review of heat transfer relationships is presented in this thesis. Combined use of these relationships for predicting the rate of heat transfer from the water in an aquaculture facility is also presented. A comprehensive model is developed from the heat transfer relationships to simulate the primary locations of heat exchange in a salmon production facility. These locations are identified as the air-water interfaces, the soil-wall-water interfaces, the air-wall-water interfaces, the pipes and the aeration processes. The transfer rates are based on the climatic data and physical parameters of the facility. The model is used to compare predicted and measured rates of heat exchange for a heated water raceway at the Oregon Aqua-Foods Facility in Springfield, Oregon. The model is also used with a hypothetical raceway and recycle system to determine the relative importance of the different locations of heat exchange and to simulate operational heating costs for relative economic ranking of different operating conditions and design strategies. A comparison is lade between predictions based on three-hour data and average daily data. The sensitivity of the predicted net heat transfer rates to the flowrate, soil and wall thermal conductivities, and the thermal convection coefficient between the water and wall is also investigated. The comparison of the predicted and measured rates of heat transfer indicates that close estimates of the net heat exchange fro. a raceway can be predicted by the model. Comparisons of daily heat exchange predictions from the three-hour and average, daily data showed that the daily data are adequate for estimates, but that three-hour data should be used when more precise estimates are needed (i.e. for sizing heating units). The predicted net heat transfer rates were not significantly affected by the changes in the flowrate, soil and wall thermal conductivities, and the water-to-wall thermal convection coefficient. The air-water interface was found to be the primary location of heat exchange for the specific conditions analyzed and it was concluded that order of magnitude estimates of net heat exchange could be obtained by analyzing the air-water interface and neglecting all other locations. The simulated operational heating costs for the hypothetical system showed a high dependence on ambient and culture water temperatures, and the degree of recycling. Covering open water surfaces during cold temperature months showed a substantial reduction in the heating costs. Economic simulations demonstrated the value of the model for comparing the relative economics of' alternative production strategies.