Mechanical Engineering

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    Development and analysis of micro polygeneration systems and adsorption chillers
    (2012) Gluesenkamp, Kyle; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    About a fifth of all primary energy in the US is consumed by residential buildings, mostly for cooling, heating and to provide electricity. Furthermore, retrofits are essential to reducing this consumption, since the buildings that exist today will comprise over half of those in use in 2050. Residential combined heat and power (or micro CHP, defined by <5 kW electrical generation capacity) has been identified as a retrofit technology which can reduce energy consumption in existing homes during the heating season by 5-30%. This thesis investigates the addition of a thermally-driven chiller/heat pump to a CHP system (to form a trigeneration system) to additionally provide savings during the cooling season, and enhance heating season savings. Scenarios are identified in which adding thermally-driven equipment to a micro CHP system reduces primary energy consumption, through analytical and experimental investigations. The experimental focus is on adsorption heat pump systems, which are capable of being used with the CHP engines (prime movers) that are already widely deployed. The analytical analysis identifies energy saving potential off-grid for today's prime movers, with potential on-grid for various fuel cell technologies. A novel dynamic test facility was developed to measure real-world residential trigeneration system performance using a prototype adsorption chiller. The chiller was designed and constructed for this thesis and was driven by waste heat from a commercially available natural gas-fueled 4 kW (electric) CHP engine. A control strategy for the chiller was developed, enabling a 5-day experiment to be run using a thermal load profile based on moderate Maryland summer air conditioning loads and typical single-family domestic hot water demand, with experimental results in agreement with models. In this summer mode, depending on electrical loads, the trigeneration system used up to 36% less fuel than off-grid separate generation and up to 29% less fuel than off-grid CHP without thermally driven cooling. However, compared to on-grid separate generation, the experimental facility used 16% more primary energy. Despite high chiller performance relative to its thermodynamic limit, this result is primarily due to the electrical efficiency of the prime mover being lower than the grid. A residential trigeneration system utilizing a high temperature fuel cell is predicted to save up to 42% primary energy relative to the grid.
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    Development of a Simulation and Optimization Tool for Heat Exchanger Design
    (2003) Jiang, Haobo; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.); Mechanical Engineering
    Heat exchangers have been used extensively and play an important role in the capital cost, energy efficiency and physical size of refrigeration and air conditioning systems. In this dissertation, a simulation and optimization tool to improve effectiveness and efficiency in design, rating, and analysis of air to refrigerant heat exchangers including conventional finned tube coils and emerging microchannel heat exchangers, Coil Designer, is developed and investigated using a general-purpose modeling concept and user-friendly interface. It is applicable to design of condensers, evaporators, and heating and cooling coils under any operating conditions. A network viewpoint was adopted to establish the general-purpose model and allow for analysis of arbitrary tube circuitry and mal-distribution of fluid flow inside the tubes. Comprehensive evaluation of solutions to the highly nonlinear system of equations in the local thermal/hydraulic performance within the tube network was conducted and a new solution method to successively approximate the physics of heat and fluid flow was developed to enhance the solution convergence capability. A segment-by-segment approach within each tube was implemented, to account for two-dimensional non-uniformity of air distribution across the exchanger, and heterogeneous refrigerant flow patterns through a tube. A further sub-dividable-segment model was created in order to address the significant change of properties and heat transfer coefficients in the single-phase and two-phase regime when a segment experiences flow regime change. The effectiveness-NTU method for cross-flow configuration was used also for combined heat and mass transfer problem under dehumidification, by defining equivalent thermal resistance and heat capacity. Object-oriented programming techniques were applied in developing Coil Designer to facilitate flexible and customizable design platform and building graphic user-friendly interface. Coupled heat exchangers with multiple fluids inside different subsets of tubes can be modeled and analyzed simultaneously. A wide variety of working fluids and correlations of heat transfer and pressure drop are available at the user’s choice. The tabular and graphic representation of performance simulation results provides convenience in comprehensive and detailed parametric analysis. The model prediction with Coil Designer was verified against experimentally determined data collected from a number of sources. The simulation tool was shown to be able to predict the heat transfer rate for a variety of coils with good accuracy. Parametric studies were conducted to confirm the capability of the program in exploring all aspects of heat exchanger performance under a wide variation of design and operating conditions. A genetic algorithm is introduced and integrated with the simulation tool for single and multi-objective optimization design of heat exchanger to accomplish the following goals quickly and accurately: achieve optimum circuitry selection, minimize volume, minimize the amount of material utilized in the coil and thus minimize overall cost of the coil while achieving the best possible performance.