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|Title:||The investigation of exhaust control strategies and waste heat recovery practices of naturally-ventilated exhaust streams|
|Keywords:||Natural Ventilation;Exhaust;Waste Heat;Control;Exhaust Control|
|Abstract:||Energy demands are projected to continue increasing over the next decade, which is prompting a change towards higher efficiencies and better utilization of the current energy supply. Thermal waste energy, a prominent inefficiency during any process, can be converted to electrical energy or re-purposed for low-grade energy needs, such as hot water and space heating/cooling. Naturally ventilated chimneys, driven by buoyancy differences between the exhaust gases and the surrounding air, prove to be a source of waste heat. The challenge of waste heat recovery from naturally ventilated exhaust networks is the reduction in buoyancy effects and increase in flow restrictions within the network. This research study will focus on understanding the effects of waste heat recovery and the associated exhaust control devices on the performance of a naturally ventilated exhaust network and the accompanying appliance(s). To investigate the effects, a nodal network methodology using mass and energy conservation principles was adapted for exhaust networks to develop a one-dimensional computational model. In contrast to previous exhaust flow design methodologies, this method solves for the thermal input of the appliance and the associated flow rates, temperature, and pressures via the appliance set point temperature and exterior conditions, such as outside temperature and pressure. Using empirical correlations for heat transfer and pressure loss coefficients of appliance and exhaust components, the computational model was validated through experimental testing of an exhaust network used in the development of a waste heat recovery system called TEG POWER (Thermal Electrical Generator Pizza Oven Waste Energy Recovery). The experimental facility was constructed to investigate the exhaust network with and without the TEG POWER system, along with exhaust control devices. These devices included an exhaust throttling valve and a draft hood to induce dilution air into the chimney. To investigate the individual effects of the devices, experimental testing was conducted at an oven temperature of 300°F (148.9°C), 500°F (260°C), and 600°F (315.6°C) with varying degrees of throttling and/or dilution air. The mass flow measurements were calculated using an energy balance technique validated against a two-way energy balance and well-established heat transfer and pressure loss correlations of the heat exchanger. The experimental mass flow, temperature, and draft pressure results were compared against the respective computational predictions and found to be within a ±10% agreement. The application of the exhaust control techniques with and without waste heat recovery is highly dependent on the objective(s), such as reducing natural gas consumption, and the constraint(s), such as a minimum chimney temperature, placed on the exhaust network design. Using the computational model, a design methodology was proposed to meet the objective(s) within the constraints of the exhaust network. To test the design methodology, a case study was performed with the objective to minimize oven natural gas consumption with a TEG POWER system in relation to a baseline appliance solely fitted with a draft hood. Within the constraints, the methodology was able to identify the appropriate degree of throttling and dilution air intake to minimize natural gas consumption. With the inclusion of the TEG POWER system, the case study showed a potential reduction in natural gas consumption by up to 18% (1.7 L/min) and 13% (3 L/min) at 300 and 600°F oven operating temperatures, respectively. The implementation of the control technique allowed the oven to minimize the intake of dilution air by up to 70% and maintain operational stability during exterior fluctuations in temperature and pressure. The implementation of the waste heat recovery device captured up to 1.0 and 2.7 kW, or a natural gas equivalent of 1.9 and 5 L/min, at 300 and 600°F oven operating temperatures respectively. Implemented into the 8,000 pizza restaurants across Canada, the TEG POWER system would reduce total natural gas consumption by up to 65.5 million cubic meters, which is enough to heat 24,000 Canadian homes, and reduce CO2 output by 112,000 metric tonnes.|
|Appears in Collections:||Open Access Dissertations and Theses|
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