Modeling of a Combined Heat and Power Unit and Evaluation of System Performance in Building Applications
by John Bush
This thesis presents a validated model of a 4 kilowatt combined heat and power (CHP) system derived from laboratory experiments. The model is tuned to match steady state experimental tests, and validated with transient experimental results. Further simulations are performed using a modeled thermal storage system, and implementing the CHP system into a building model to evaluate the feasibility of CHP in the mid-Atlantic region, as well as the Great Lakes region. The transient simulation outputs are within 4.8% of experimental results for identical load profiles for a simulated summer week, and within 2.2% for a spring or autumn week. When integrated with a building model, the results show 23.5% cost savings on energy in the mid-Atlantic region, and 29.7% savings in the Great Lakes region.
http://hdl.handle.net/1903/10483
Force Fed Microchannel High Heat Flux Cooling Utilizing Microgrooved Surfaces
by Edvin Cetegen
Among other applications, the increase in power density of advanced electronic components has created a need for high heat flux cooling. Future processors have been anticipated to exceed the current barrier of 1000 W/cm2, while the working temperature of such systems is expected to remain more or less the same. Currently, the well known cooling technologies have shown little promise of meeting these demands.
This dissertation investigated an innovative cooling technology, referred to as force-fed heat transfer. Force-fed microchannel heat sinks (FFMHS) utilize certain enhanced microgrooved surfaces and advanced flow distribution manifolds, which create a system of short microchannels running in parallel. For a single-phase FFMHS, a numerical model was incorporated in a multi-objective optimization algorithm, and the optimum parameters that generate the maximum heat transfer coefficients with minimum pumping power were identified. Similar multi-objective optimization procedures were applied to Traditional Microchannel Heat Sinks (TMHS) and Jet Impingement Heat Sinks (JIHS). The comparison study at optimum designs indicates that for a 1 x 1 cm2 base heat sink area, heat transfer coefficients of FFMHS can be 72% higher than TMHS and 306% higher than JIHS at same pumping power. For two-phase FFMHS, three different heat sink designs incorporating microgrooved surfaces with microchannel widths between 21 μm and 60 μm were tested experimentally using R-245fa, a dielectric fluid. It was demonstrated that FFMHS can cool higher heat fluxes with lower pumping power values when compared to conventional methods.
The flow and heat transfer characteristics in two-phase mode were evaluated using a visualization test setup. It was found that at low hydraulic diameter and low mass flux, the dominant heat transfer mechanism is dynamic rapid bubble expansion leading to an elongated bubble flow regime. For high heat-flux, as well as combination of high heat flux and high hydraulic diameters, the flow regimes resemble the flow characteristics observed in conventional tubes.
The present research is the first of its kind to develop a better understanding of single-phase and phase-change heat transfer in FFMHS through flow visualization, numerical and experimental modeling of the phenomena, and multi-objective optimization of the heat sink.
http://hdl.handle.net/1903/10286
Analyses of building energy system alternatives through transient simulation
by Andrew C. Mueller
This thesis examines the performance of residential buildings and the energy systems contained within those buildings by simulating them in the TRaNsient SYstems Simulation (TRNSYS) program. After matching a building's floorplan to that of house local to the College Park area, national and local building surveys were consulted to produce a prototype of the average Maryland home. This home was simulated with ordinary insulation levels, heating, ventilation, and air conditioning (HVAC) equipment, and appliances. Various construction characteristics, including wall insulation, thermostat set points, HVAC equipment, and appliance efficiency were varied to examine the effects of each individual change upon the final annual energy consumption of the building, and in doing so, the value of retrofitting each characteristic was explored. Finally, the most effective energy-saving strategies were combined to model a low-energy home, in order to explore the possibility of refitting an existing home to become a net-zero site energy building. Sensitivity study results were listed, and a net-zero-energy building was successfully simulated.
http://hdl.handle.net/1903/10283
Generic Dynamic Model for a Range of Thermal System Components
by Shenglan Xuan
The simulation of a thermal system consists of a simulation of its components and their interactions. The advantages of thermal system simulations have been widely recognized. They can be used to explore the performance of a newly designed system, to identify whether the design meets the design criteria, to develop and test controls, and to optimize the system by minimizing the cost or power consumption, and maximizing the energy efficiency and/or capacity. Thermal system simulations can also be applied to existing systems to explore prospective modifications and improvements.
Much research has been conducted on aspects of thermal system and component simulation, especially for steady-state simulation. Recently, transient simulations for systems and components have gained attention, since dynamic modeling assists the understanding of the operation of thermal systems and their controls.
This research presents the development of a generic component model that allows users to easily create and customize any thermal component with a choice of working fluids and levels of complexity for either transient or steady-state simulation. The underlying challenge here is to design the code such that a single set of governing equations can be used to accurately describe the behavior of any component of interest. The inherent benefits to this approach are that maintenance of the code is greatly facilitated as compared to competing approaches, and that the software is internally consistent. This generic model features a user-friendly description of component geometry and operating conditions, interactive data input and output, and a robust component solver.
The open literature pertaining to thermal component models, especially the components of vapor compression systems, is reviewed and commented on in this research. A theoretical evaluation of the problem formulation and solution methodology is conducted and discussed. A generic structure is proposed and developed to simulate thermal components by enabling and disabling a portion of the set of governing equations. In addition, a system solver is developed to solve a system composed of these components. The component/system model is validated with experimental data, and future work is outlined.
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