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Browsing by Author "Singh, Sahdev"

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    A computer simulation model for wastewater management in an integrated (fish production-hydroponics) system
    Singh, Sahdev (Virginia Tech, 1996)
    Intensive fish production in a recirculating aquaculture system facility is a complex bioengineering operation involving a sensitive balance among physiological, water quality, and management components of the overall system. Warm and nutrient-rich wastewater discharged from controlled-environment fish production facilities is a loss of heat energy and nutrients in addition to being potentially harmful to the environment. The operators of such systems need sophisticated management tools if the operation is to be both commercially successful and environmentally friendly. Effluent heat and nutrients can be recovered using hydroponics in a greenhouse attached to the recirculating aquaculture system facility. A computer model was developed to simulate system performance and to help determine design parameters for an integrated fish production-hydroponics system. The aquaculture component of the model predicts (a) fish growth-dependent feeding, (b) diurnal metabolic waste production/accumulation in the fish culture water, and (c) quality, quantity and frequency of wastewater discharge. The hydroponics component computes optimum greenhouse size and models the performance of vegetable plants in terms of nutrient-uptake, water use, and growth. SUCROS and TOMGRO, plant growth models with modifications for water use and nutrient uptake, were used to simulate lettuce and tomato performance, respectively. To validate the plant models, experiments were conducted in a greenhouse utilizing aquacultural wastewater as the hydroponic solution to produce lettuce and tomatoes. Plant growth, water quality (nutrient-uptake), water use, and environmental conditions were monitored. Lettuce and tomato growth was accompanied with significant reductions in nitrogen and phosphorus levels of the wastewater. Water use by plants strongly depended on solar radiation and plant growth stage. At harvest, nine-week-old lettuce weighed 160 g/plant (average) at a density of 40 plants/m². Tomato yielded 2.4 kg/m² after 17 weeks. However, the tomato fruits did not reach maturity during this time. After 20 weeks, the tomato yield was 3.1 kg/m² and some fruits showed maturity. The use of the model as a management tool for making decisions on optimum greenhouse area for a given recirculating aquaculture system size is demonstrated. The effect of fish stocking density and greenhouse heat loss factor on the optimum greenhouse size are also demonstrated. For an optimum greenhouse size, water use and nutrient-recovery from the effluent by lettuce and tomato plants are quantified.
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    Economics of greenhouse heating with a mine air-assisted heat pump
    Marsh, Lori S.; Singh, Sahdev (American Society of Agricultural and Biological Engineers, 1994)
    An analysis of the economics of greenhouse heating with a heat pump coupled with an inactive deep mine is presented. Several heat pump and conventional gas-fired heater combinations (hybrid systems) were evaluated using a computer model to perform thermal and economic analyses. A 10 x 30 m, double-polyethylene-covered greenhouse, located in Charleston, West Virginia, was assumed for this analysis. Heat pumps with sufficient capacity to maintain 21 degrees C inside the greenhouse for outside temperatures ranging from 0 to 20 degrees C (in 2 degrees degree increments) were modeled. For each heat pump capacity, it was assumed that the additional heat energy required to maintain the specified inside temperature (when outside temperature fell below the heat pump design temperature) was supplied by a natural gas-fired heater. Life-cycle cost analysis was employed to compare greenhouse heating alternatives. The hybrid system offers lower operating costs than a conventional system for any outside design temperature. However, when initial cost is also considered, the hybrid system has a higher life cycle cost for heat pump design temperatures in the range 0 to 18 degrees C and a coefficient of performance (COP) of 3. As the heat pump COP increases beyond 3, the hybrid system becomes economically feasible, showing a lower life cycle cost than a conventional natural gas-fired system.
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    Modeling thermal environment of a recirculating aquaculture system facility
    Singh, Sahdev (Virginia Tech, 1993)
    Economic viability of fish production in recirculating aquaculture system (RAS) facility depends on minimizing the energy requirements of operating such facilities. The fish growth and water quality aspects of RAS have been studied in considerable details. However, the understanding of the thermal environment of RAS lags behind. A step-wise steady-state thermal model was developed to simulate the daily heating, ventilation, water pumping, biofilter operation, and lighting energy requirements over a production cycle. The model was validated using temperature and energy data collected from RAS facility of Virginia Tech during 1992. Model simulations were performed with various production scenarios. The energy cost of fish production ($/kg) was used to evaluate different scenarios with and without heat recovery from discharged system water. Building heating required the most (40 % - 70 % of total) energy followed by water pumping, biofilter operation, lighting, and ventilation. Water replacement was the most dominant factor in determining the facility’s heating energy requirement. Heat recovery from discharged system water indicated significant drop (up to 40 %) in energy cost of fish production.