Growing concerns regarding climate change, the increase in demand for energy and the efficient utilization of energy have become of major interest in applications of heating and power generation. A large portion of the energy input to these applications is lost, due to their typical inefficiencies, in the form of waste thermal energy which, if captured and utilized, can offer an abundant source of energy for electricity generation and heating purposes. The use of thermoelectric generators (TEGs) of different designs in waste heat recovery applications has been pursued over the past few decades as the generation of electrical power using TEGs has become viable compared to other conventional systems at low temperatures. This study focuses on the implementation of an annular design for integrated TEG modules in a heat exchanger device for waste heat recovery and the investigation of the effect of different TEG design parameters on the device performance. The integration of the annular TEG design in the heat exchanger was studied using a developed numerical model to investigate the interaction between the heat transfer and the thermoelectric effects and evaluate the performance under specific operating conditions. The heat transferred from the exhaust to the water flow through the TEGs was modelled using a thermal network for the heat flow, coupled with an electrical circuit for the power output. The model was validated using experimental results of the first generation of the TEG device with good agreement (3-6 %) between the predicted and measured performance results: power output, efficiency and the exhaust and water flow temperatures. With the objectives of maximizing the power output and improving the power characteristics, a half annular TEG design was presented. It was able to generate the same power output with double the voltage and half the current, thus improved the power characteristics required for functional operation, compared to the full annular design. The effect of the annular TEG design dimensions on the device performance was studied for a multi-row heat exchanger using the numerical model. The results showed that a maximum power output can be obtained at optimum TEG diameter ratio and thickness. In addition, the TEGs performance was studied under different electrical connection configurations in series and in parallel. The series connection between TEG rows showed better power output characteristics with lower current output, minimal power loss due to temperature mismatch and higher voltage output. The effect of heat exchanger design considerations such as the axial heat conduction was also investigated using the numerical model and the results were compared with an ANSYS model for verification. Good agreement was demonstrated and the results showed a decrease in the total power output of multiple TEG rows when axial conduction of heat was allowed between the TEGs hot-side surfaces in the heat exchanger. A dimensions map was created for annular TEGs integrated in a heat exchanger combining the effects of varying the TEG diameter ratio and thickness on the power output. Further, a dimensionless design parameter (β) was introduced to locate the maximum power region on the map. Using the map as a design tool, the dimensions of the annular TEG modules in a heat exchanger were determined to maximize the power output under a typical current output constraint in order to improve the system power characteristics. Using the map, it was shown that the current output could be reduced by 46 % of its value at the maximum power available on the map and the resultant power output could be maintained at 98 % of its maximum value. This also resulted in a 48% reduction in the TEG material volume and an increased voltage output of the device. As a result, the power output was maximized, the current output was limited to reduce losses in the power management system components and material volume reduction was achieved which would increase the device power density and reduce its overall cost. / Thesis / Master of Applied Science (MASc)

This work focuses on an innovative Rotary Displacer SE (RDSE) configuration for Stirling engines (SEs). RDSE features rotary displacers instead of reciprocating displacers (found in conventional SE configurations), as well as combined compression and expansion spaces. Guided by the research question "can RDSE as a novel configuration achieve a higher efficiency compared to conventional SE configurations at comparable operating conditions?", the goal of this study is to characterize, analyze, and optimize RDSE which is pursued in three technical stages. It is observed the RDSE prototype has an optimum phase angle of > 90° and thermal efficiency of 15.5% corresponding to 75.2% of the ideal (Carnot) efficiency at the source and sink temperatures of 98.6° C and 22.1° C, respectively. Initial results indicate that 125° phase angle provides more power than that of the theoretically optimum 90° phase angle. The results also show comparable B_n and significantly higher W_n values (0.047 and 0.465, respectively) compared to earlier studies, and suggest the RDSE could potentially be a competitive alternative to other SE configurations. Furthermore, due to lack of a regenerator, the non-ideal effects calculated in the analytical approach have insignificant impact (less than 0.03 kPa in 100 kPa). The clearance volume in the shuttled volume has a dramatic negative effect and reduces the performance up to 40%. Ultimately, utilizing CFD, it is proved that the existing geometry is relatively optimized where the optimum phase angle is 121° and geometric ratio D\/L for the displacer is 0.49.

In an attempt to reduce the dependence on fossil fuels, a variety of research initiatives has focused on increasing the efficiency of conventional energy systems. One such approach is to use waste heat recovery to reclaim energy that is typically lost in the form of dissipative heat. An example of such reclamation is the use of waste heat recovery systems that take low-temperature heat and deliver cooling in space-conditioning applications. In this work, an ejector-based chiller driven by waste heat will be studied from the system to component to sub-component levels, with a specific focus on the ejector. The ejector is a passive device used to compress refrigerants in waste heat driven heat pumps without the use of high grade electricity or wear-prone complex moving parts. With such ejectors, the electrical input for the overall system can be reduced or eliminated entirely under certain conditions, and package sizes can be significantly reduced, allowing for a cooling system that can operate in off-grid, mobile, or remote applications. The performance of this system, measured typically as a coefficient of performance, is primarily dependent on the performance of the ejector pump. This work uses analytical and numerical modeling techniques combined with flow visualization to determine the exact mechanisms of ejector operation, and makes suggestions for ejector performance improvement. Specifically, forcing the presence of two-phase flow has been suggested as a potential tool for performance enhancement. This study determines the effect of two-phase flow on momentum transfer characteristics inside the ejector while operating with refrigerants R134a and R245fa. It is found that reducing the superheat at motive nozzle inlet results in a 12-13% increase in COP with a 14-16 K decrease in driving waste heat temperature. The mechanisms of this improvement are found to be a combination of two effects: the choice of operating fluid (wet vs. dry) and the effect of two-phase flow on the effectiveness of momentum transfer. It is recommended that ejector-based chillers be operated such that the motive nozzle inlet is near saturation, and environmentally friendly dry fluids such as R245fa be used to improve performance. This work provides critical methods for ejector modeling and validation through visualization, as well as guidance on measures to improve ejector design with commensurate beneficial effects on cooling system COP.

An analytical transport/reaction model was developed to simulate the catalytic performance of ZnO nanowires as a catalyst support. ZnO nanowires were chosen because they have easily characterized, controllable features and a spatially uniform morphology. The analytical model couples convection in the catalyst flow channel with reaction and diffusion in the porous substrate material; it was developed to show that a simple analytical model with physics-based mass transport and empirical kinetics can be used to capture the essential physics involved in catalytic conversion of hydrocarbons. The model was effective at predicting species conversion efficiency over a range of temperature and flow rate. The model clarifies the relationship between advection, bulk diffusion, pore diffusion, and kinetics. The model was used to optimize the geometry of the experimental catalyst for which it predicted that maximum species conversion density for fixed catalyst surface occurred at a channel height of 520 [mu]m. A modeling study of thermoelectric (TE) vehicle waste heat recovery was conducted based on abundant and inexpensive Mg₂ Si[subscript 0.5] Sn[subscript 0.5] and MnSi[subscript 1.75] TE materials with consideration of performance at the system and TE device levels. The modeling study identified a critical TE design space of fill fraction, leg length, n-/p-type leg area ratio, and current; these parameters needed to be optimized simultaneously for positive TE power output. The TE power output was sensitive to this design space, and the optimal design point was sensitive to engine operating conditions. The maximum net TE power for a 29.5 L strip fin heat exchanger with an 800 K exhaust flow at 7.9 kg/min was 2.25 kW. This work also includes two generations of TE waste heat recovery systems that were built and tested in the exhaust system of a Cummins 6.7 L turbo Diesel engine. The first generation was a small scale heat exchanger intended for concept validation, and the second generation was a full scale heat exchanger that used the entire exhaust flow at high speed and torque. The second generation heat exchanger showed that the model could accurately predict heat transfer, and the maximum experimental heat transfer rate was 15.3 kW for exhaust flow at 7.0 kg/min and 740 K. / text

This study is centered on modeling and experimental efforts to simulate and optimize the performance of thermoelectric generators (TEGs) for waste heat recovery systems for use in motor vehicles. TEGs are being studied and developed for applications in which waste heat, for example, from the exhaust of motor vehicles is converted into usable electricity. TEGs consisting of TE elements integrated with an exhaust heat exchanger require optimization to produce the maximum possible power output. Important optimization parameters include TE element leg length, fill fraction, leg area ratio between n- and p-type legs, and load resistance. A finite difference model was developed to study the interdependencies among these optimization parameters for thermoelectric elements integrated with an exhaust gas heat exchanger. The present study was carried out for TE devices made from n-type Mg₂Si and p-type MnSi[subscript 1.8] based silicides, which are promising TE materials for use at high temperatures associated with some exhaust heat recovery systems. The model uses specified convection boundary conditions instead of specified temperature boundary conditions to duplicate realistic operating conditions for a waste heat recovery system installed in the exhaust of a vehicle. A numerical model for a new waste heat recovery system configuration was proposed which showed an improvement of 40% in net power output over the conventional systems while using approximately 60% more TEG modules. The 1st generation, and an improved 2nd generation TEG module using n-type Mg₂Si and p-type MnSi[subscritp 1.8] based silicides were fabricated and tested to compare and correlate TE power generation with the numerical model. Important results include parameter values for maximum power output per unit area and the interdependencies among those parameters. Heat transfer through the void areas was neglected in the numerical model. When thermal contact resistance between the TE element and the heat exchangers is considered negligible, the numerical model predicts that any volume of TE material can produce the same power per unit area, given the parameters are accurately optimized. Incorporating the thermal contact resistance, the numerical model predicts that the peak power output is greater for longer TE elements with larger leg areas. The optimization results present strategies to improve the performance of TEG modules used for waste heat recovery systems. / text

In most previous studies that have been conducted on converting waste heat energy from exhaust gases into useful energy, the engine waste heat recovery system has been placed along the exhaust flow pipe where the temperature differs from the temperature just behind the exhaust valves. This means that an important fraction of the energy from the exhaust gases is still lost to the environment. The present work investigates the potential thermodynamic analysis of an integrated exhaust waste heat recovery (EWHR) system based on a Rankine cycle on an engine's exhaust manifold. The amount of lost energy contained in the exhaust gases at the exhaust manifold level, at average temperatures of 500 °C and 350 °C (for petrol and diesel), and the thermodynamic composition of these gases were determined. For heat to occur, a temperature difference (between the exhaust gas and the working fluid) at the pinch point of 20°C was considered. A thermodynamic analysis was performed on different configurations of EWHR thermal efficiencies and the selected suitable working fluids. The environmental and economic aspects of the integrated EWHR system just behind the exhaust valves of an internal combustion engine (ICE) were analysed. Among all working fluids that were used when the thermodynamic analysis was performed, water was selected as the best working fluid due to its higher thermal efficiency, availability, low cost and environmentally friendly characteristics. Using the typical engine data, results showed that almost 29.54% of exhaust waste heat can be converted. This results in better engine efficiency and fuel consumption on a global scale by gaining an average of 1 114.98 Mb and 1 126.63 Mb of petrol and diesel respectively from 2020 to 2040. It can combat global warming by recovering 56.78 1 011 MJ and 64.65 1 011 MJ of heat rejected from petrol and diesel engines, respectively. A case study of a Volkswagen Citi Golf 1.3i is considered, as it is a popular vehicle in South Africa. This idea can be applied to new-design hybrid vehicles that can use the waste heat to charge the batteries when the engine operates on fossil fuel. / Dissertation (MSc)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / MSc / Unrestricted

<div>With the United States being the world’s second largest consumer of primary energy, research into areas of significant consumption can provide large impacts in terms of the global energy consumption. Buildings account for 41% of U.S. total energy consumption with the residential sector making up a majority. Household appliances account for the second largest site energy consumption at 27%, after the HVAC system for the U.S. residential sector. Federal appliance standards have been instrumental in improving efficiencies but have been increasing aggressively to where it is unknown what suitable technologies can support this rate of increase. Thermally integrating residential appliances by leveraging waste heat recovery goes outside standards and has not been adequately explored by connecting all residential appliances. Limited studies exist focused only on single appliances connected to waste heat recovery or being thermally integrated. Preliminary modeling on waste heat availability from five major appliances, namely refrigerator-freezer, clothes dryer, clothes washer, dishwasher, and cooking oven was conducted. Conservative estimates predict the total amount of heat recovery to be around 2,000 kWh/year; clothes dryer - 137 kWh/year, clothes washer - 60 kWh/year, 1,500 kWh/year- refrigerator-freezer, 27 kWh/year – dishwasher, and 178 kWh/year – cooking oven. The cooking oven presents technical challenges coupled with safety concerns. The clothes dryer and refrigerator-freezer can deliver useful water temperatures and reduce compressor power consumption, up to 20%. The dishwasher has better opportunity as a heat sink to offset the internal heater, 0.17 kWh of electricity/cycle for heating wash water. The clothes washer drains large volumes of water available for heat recovery and can offset the impact of using high temperature washes with improved wash performance. </div><div>Modelica appliance models have been developed for four of these five appliances. The Modelica models capture individual use and the predictions of the RF and DW were compared against available experimental data. The individual models have been connected to a simple storage tank model to simulate the integrated appliance system. An integrated appliance prototype was designed and fabricated for the collection of experimental data. Comparisons made between the experimental data and the integrated appliance simulation results adjusted the modeling approach and improved agreement with collected data. After tuning, ideal modifications to each appliance are made and reflected in a new integrated model. A parametric study is conducted on ideal improved, thermally capable appliances under a 1-week schedule for two different tank sizes. For 300L and 150 L tank sizes, the appliance total energy for the week is roughly 30.5 kWh compared to a baseline appliance system with no thermal resource sharing at 33.8 kWh. At an electricity cost of $0.15/kWh, the cost savings for the integrated system is a little over $0.40/week. Furthermore, the savings is completely diminished when considering the required auxiliary power to support the exchange of heat between each appliance and storage tank. The impact of tank size should be explored further to identify a critical tank size where the system savings is no longer available. Accounting for all the domestic hot water needs of the home would generate an improved picture where integrated appliances have technical feasibility. </div><div><br></div>

The combination of the continuously growing demand of energy in the world, the depletion of oil and its sharp price increase, as well as the urgent need for cleaner and more efficient fuels have boosted the global trade of liquefied natural gas (LNG). Nowadays, there is an increasing interest on the design philosophy of the LNG receiving terminals, due to the fact that the existing technologies either use seawater as heating source or burn part of the fuel for regasifying LNG, thus destroying the cryogenic energy of LNG and causing air pollution or harm to marine life. This investigation addresses the task of developing novel systems able to simultaneously regasify LNG and generate electric power in the most efficient and environmentally friendly way.    Existing and proposed technologies for integrated LNG regasification and power generation were identified and simple, efficient, safe and compact alternatives were selected for further analysis. A baseline scenario for integrated LNG regasification and power generation was established and simulated, consisting of a cascaded Brayton configuration with a typical small gas turbine as topping cycle and a simple closed Brayton cycle as bottoming cycle. Various novel configurations were created, modeled and compared to the baseline scenario in terms of LNG regasification rate, efficiency and power output. The novel configurations include closed Rankine and Brayton cycles for the bottoming cycle, systems for power augmentation in the gas turbine and combinations of options. A study case with a simple and compact design was selected, preliminarily designed and analyzed according to characteristics and costs provided by suppliers. The performance, costs and design challenges of the study case were then compared to the baseline case. The results show that the study case causes lower investment costs and a smaller footprint of the plant, at the same time offering a simple design solution though with substantially lower efficiencies.

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A modeling tool was developed capable of evaluating the thermal performance of a commercial building, for the purpose of objectively quantifying the impacts of both operational changes and technological retrofits. The modeling tool was created using a steady state energy balance approach, discretized into half hour time steps to capture the time varying characteristics of the rate of heat transfer through the building envelope, the ventilation systems, appliance heat gains, heat generated by electricity consumption, solar energy transfer and space heating through exhaust gas energy recovery with the TEG POWER system. Several experimental facilities were used to validate the modeling tool, and to provide inputs to the case studies presented. Data from two separate commercial baking operations was collected, and was shown to be in agreement with the model predictions with a 7% error. Several energy conservation measures were simulated, including switching to idealized methods of exhaust ventilation, sealing and insulating appliances, shutting down appliances during unoccupied hours, and the inclusion of exhaust gas energy harvesting. Implementing all four conservation measures at a single restaurant had the effect of reducing electricity consumption by 14% or approximately 17,700 kWh (64 GJ), and reducing natural gas consumption by 60% or approximately 18,200 m3 (608 GJ) annually. In contrast, proceeding directly to the energy harvesting solution, and bypassing other conservation measures, only allowed for 20% of the total potential energy savings to be realized. If the concepts identified are implemented across 2000 comparable restaurants in Ontario, there is a potential to reduced electricity consumption by 44.4 million kWh and natural gas consumption by 33.7 million cubic meters annually. The measures would effectively eliminate 65,500 metric tonnes of CO2 emissions every year. / Thesis / Master of Applied Science (MASc)