MS and Ph.D Theses

Numerical Modeling and Experimental Testing of a Mixed Gas Joule-Thomson Cryocooler

Numerical Modeling and Experimental Testing of a Mixed Gas Joule-Thomson Cryocooler Pettitt, John Mixed gas Joule-Thomson (MGJT) systems have been shown to provide order of magnitude improvements in efficiency relative to JT systems that use pure working fluids. This thesis presents theoretical and experimental work related to using a single- stage, low power (< 1 W) MGJT system for cooling the current leads required by high- temperature superconducting electronics. By thermally integrating the current leads with the recuperative heat exchanger of a MGJT cycle, it is possible to intercept the electrical dissipation and conductive heat leak of the wires at a relatively high temperature which provides a thermodynamic advantage. Also, directly cooling the leads rather than indirectly cooling the chips may provide some advantages relative to thermal integration. To design the recuperative heat exchanger for the MGJT cycle, the composition of the gas mixture was optimized using a robust genetic optimization technique. Following mixture selection, the optimization model was modified so that it included the effect of frictional pressure drop, axial conduction through the heat exchanger, and the overall conductance available from the heat exchanger on the performance of the MGJT cycle. The individual influences of these loss factors on the refrigeration power of the MGJT cycle were investigated parametrically and conceptually in order to determine the target values for a low power system and develop some insight into the relative importance of each effect. A detailed model of the specific Hampson-style heat exchanger geometry was developed and used to obtain a design for an initial demonstration device. The demonstration device was fabricated and integrated with a thermal vacuum test facility, gas handling equipment, and the appropriate instrumentation. Several tests were carried out. First, the heat exchanger alone was tested (outside of a JT cycle) using helium as the working fluid. These data provided some experimental verification of the detailed model. Next, the test facility was modified through the installation of a fixed orifice expansion valve to allow open cycle testing of the device using high pressure (9.745 MPa) pure Argon. These measurements provided further insight into the performance of the device. The test facility was subsequently integrated with a compressor in order to allow measurements of the Device's performance using gas mixtures in a closed loop configuration. These test results ultimately revealed issues relative to contamination, which were addressed through the installation of a liquid nitrogen trap, as well as liquid management. The liquid management issue is thought to be related to inadequate vapor kinetic energy which does not provide sufficient momentum transfer to the liquid to move it through the system. The liquid management issue constrains the performance of the MGJT cycle at low mass flow rates and was explored over a very limited range of conditions. Further testing is suggested which will allow the liquid management constraint to be explored more completely in order to guide future designs. Under the supervision of Greg Nellis and John Pfotenhauer; 186pp.

Design Of Thermosyphon Solar Domestic Hot Water Systems

Design Of Thermosyphon Solar Domestic Hot Water Systems Malkin, Mark P. Under the supervision of Professors Sanford Klein and Jack Duffie; 125pp.

Modeling of Heat Transfer in Buildings

Modeling of Heat Transfer in Buildings Seem, John E. Under the supervision of Professors Sanford Klein, William Beckman, and John Mitchell; 171pp.

Analysis of Liquid-Desiccant Systems and Component Modeling

Analysis of Liquid-Desiccant Systems and Component Modeling Stevens, Dawne Under the supervision of Professors Sanford Klein and Jack Duffie; 161pp.

Development and Validation of a Weather Data Generation Moledular

Development and Validation of a Weather Data Generation Moledular Knight, Krista M. 292pp.

Simplified Performance Modeling of a Direct-Coupled Photovoltaic Systems

Simplified Performance Modeling of a Direct-Coupled Photovoltaic Systems Townsend, Timothy U. This thesis introduces a new method to estimate the long-term performance of direct-coupled and maximum power-tracked photovoltaic (PV) systems without battery storage. A number of models exist for estimating maximum power-tracked system performance, but the maximum power-tracking feature is included here as a benchmark for comparison to direct-coupled system performance, the true object of this research. While the output from either type of PV system is dependent on weather and PV array characteristics, the output from direct-coupled PV systems is dependent on the applied load as well. As a result, estimating the performance of direct-coupled systems is more complex than for maximum power-tracked systems. The method developed here is computationally simple. A reduced set of hourly weather data is generated from widely available long-term monthly-average global solar and ambient temperature data. Correlations are used to estimate hourly weather variations within a day and daily variations within a month. A small number of "typical day" groups are used to approximate the long-term distribution of daily weather in each month. The "typical days" within each group are assumed to be identical which reduces the number of computations yet, subject to the accuracy of the correlations, retains the accuracy of long-term simulations. To assess the validity of the new method, a program titled DCPVSIMP (for Direct-Coupled PV model, simplified version) was written. Five potential I-V curve sub- models for this program were evaluated; the one which was selected corresponded well with a large sampling of experimental I-V curve data from outside sources, and the information required to use it is commonly available from PV manufacturers. A detailed version of this program, identical in all respects except that it uses hourly TMY data, titled DCPVDET, was also written to provide a basis for evaluating the weather generation component. Both the DCPVSIMP and DCPVDET versions were compared to two established models for maximum power-tracking systems, PVFORM and PV f-Chart. Monthly and annual estimates were within :t 1 % relative to PVFORM and + 5 to 6% relative to PV f- Chart. PV f-Chart includes a variable correction factor which decreases estimates of the absorbed radiation at off-normal incidence angles. For performance estimates at moderate northern latitudes the overall effect of this term is about a 5% decrease in annual output The difference between the DCPVDET model and a "5 typical day" version of the DCPVSIMP model was found to be less than 1 % for maximum power-tracking systems. Monthly and annual performance for over 800 cases using the DCPVDET model, based on three locations and a large variety of direct-coupled resistive, fixed voltage, and DC motor loads, was compared to 3, 5, 10, and 20 "typical day" versions of the DCPVSIMP model. The overall annual % root mean square (RMS) difference between the two models ranged from 3.8% for the "3 typical day" version to 3.2% for the "20 typical day" version, with most of the reduction in the % RMS difference occurring between the 3 and 5 "typical day" versions (3.4% RMS for the 5 day version). For all versions the % mean bias difference (MBD) was less than 1%. The worst monthly results ranged between 5 to 6% RMS among the four versions tested, with a % MBD of less than 1%. The body of the report includes a derivation of the new direct-coupled performance estimating method and a description of the DCPVSIMP and DCPVDET models, statistical evaluations of the models and their components, and a set of graphs illustrating typical applications of the DCPVSIMP model for direct-coupled system design. Under the supervision of Professors Sanford Klein and William Beckman; 282pp.

Detailed Modeling of Photovoltaic System Components

Detailed Modeling of Photovoltaic System Components Eckstein, Jurgen Helmut Under the supervision of Professors William Beckman and John Duffie; 212pp.

Design & Optimization of Organic Rankine Cycle Solar-Thermal Powerplants

Design & Optimization of Organic Rankine Cycle Solar-Thermal Powerplants McMahan, Andrew C. Solar-thermal powerplants have enjoyed limited success in the energy market to date. The ability to better characterize the performance of existing solar-thermal technologies as well as investigate the potential of new technologies is a crucial step in developing more economically viable designs. To this end, computer models and simulation capability are developed in this thesis to predict the performance of several emerging solar-thermal powerplant technologies. Specifically, models of organic Rankine cycles and packed-bed stratified (thermocline) thermal energy storage systems are developed. These models provide a low-cost context for analyzing the design and optimization (both economic and engineering) of solar-thermal technologies that show tremendous unrealized potential. Organic Rankine cycles have unique properties that are well suited to solar power generation. The thermodynamic potential of a variety organic Rankine cycle working fluids and configurations are analyzed. In addition, a general economic optimization methodology for solar-thermal organic Rankine cycle powerplants is developed and presented. The methodology is applied to an existing plant design which demonstrates opportunities for further optimization in current design practice. Thermal energy storage enables powerplant output to be tailored to meet end-user demands. The design and integration of thermal energy storage systems is discussed. Plant operating and control strategies for a variety of utility pricing schedules are developed and analyzed. The potential for thermal energy storage to impact the economic attractiveness of solar power generation is shown to be heavily dependent on energy market structure and utility pricing strategies. Under the supervision of Professor Sanford A. Klein; 216pp.

Analysis of a Large Scale Solar Water Heater

Analysis of a Large Scale Solar Water Heater Ratzmann, Paul M. 93pp.

The Optimal Control of Ice-Storage Air-Conditioning Systems

The Optimal Control of Ice-Storage Air-Conditioning Systems Carey, Colin W. 161pp.

Performance of Rotary Enthalpy Exchangers

Performance of Rotary Enthalpy Exchangers Stiesch, Gunnar Rotary regenerative heat and mass exchangers allow energy savings in the heating and cooling of ventilated buildings by recovering energy from the exhaust air and transferring it to the supply air stream. In this study the adsorption isotherms and the specific heat capacity of a desiccant used in a commercially available enthalpy exchanger are investigated experimentally, and the measured property data are used to simulate the regenerator performance and to analyze the device in terms of both energy recovery and economic profitability. Based on numerical solutions for the mechanism of combined heat and mass transfer obtained with the computer program MOSHMX for various operating conditions, a computationally simple model is developed that estimates the performance of the particular enthalpy exchanger and also of a comparable sensible heat exchanger as a function of the air inlet conditions and the matrix rotation speed. The model is built into the transient simulation program TRNSYS, and annual regenerator performance simulations are executed. The integrated energy savings over this period are determined for the case of a ventilation system for a 200 people office building (approx. 2 m3/s) for three different locations in the United States, each representing a different climate. Life cycle savings that take into account the initial cost of the space-conditioning system as well as the operating savings achieved by the regenerator are evaluated for both the enthalpy exchanger and the sensible heat exchanger over a system life time of 15 years. The present worth of the accumulated savings ranges from $ 28,000 to $ 38,000 for the enthalpy exchanger and from $ 7,000 to $ 24,000 for the sensible heat exchanger. The enthalpy exchanger results in greater payoffs in all locations, but its advantage is most significant in a warm and humid climate where the sensible heat exchanger performs poorly. Under the supervision of Professor Sanford Klein; 153pp.

Modifications to the TRNSYS Thermal Storage Tank Model

Modifications to the TRNSYS Thermal Storage Tank Model Schmid, Michael Solar energy is a time-dependent energy resource. The demands for energy are also time dependent but in a different fashion than the solar energy supply. Consequently, energy has to be stored if solar energy is to meet substantial portions of these energy needs. One of the most economically feasible methods of solar storage is a fluid storage tank. Before choosing the proper size and performance of a thermal fluid storage, it is important to make calculations with the whole system. The TRNSYS software package has been used extensively for thermal system analysis. It has a modular structure and consists of individual subroutines which represent real physical devices or utility components. The components can be connected together to form complex systems. One of these components is the TYPE 4 multi-node model. The tank is modeled as N fully mixed volume segments. The degree of temperature stratification, which increases the effectiveness of a storage tank, is determined by the choice of N. Higher values of N result in more stratification. Although the current TYPE 4 tank model has been proven to be an accurate component, it has some limitations. Outlet flows are fixed at the tank top (load flow) or tank bottom (collector flow). The tank has always two inlets and two outlets. Inlet flow rate from one source are automatically the outlet flow rate to the same source. The output of the losses to the exhaust of a gas auxiliary heater are added to the losses to the environment. Only tanks of circular cross section can be used. The goal of this project is to modify the current TYPE 4 model. The new TYPE 4 includes several new features which make the tank more versatile. Inlet and outlet positions can be located anywhere in the tank. Inlet flow rates from one source do not have to be automatically equal the outlet flow rate. Also the tank need not have two inlets and two outlets; it can have less than four flows, and still satisfy a mass balance for the whole tank. The losses to the exhaust flue of an optional gas auxiliary heater are output separately from the losses to the environment. The cross section of the tank can be circular or rectangular. The new model calculates the difference in static pressure between the top of the tank and each inlet and outlet position. This option is needed to simulate a thermosiphon system. Further, the conduction between the tank segments (nodes) is considered. Since tanks may destratify more rapidly due to natural convection a user specified parameter has been added to the conduction coefficient. 73pp.

Impact on a Utility of an Ensemble of Solar Domestic Hot Water Systems

Impact on a Utility of an Ensemble of Solar Domestic Hot Water Systems Cragen, Keary E. The benefits to a utility and to the environment of the installation of a large number of solar domestic hot water (SDHW) systems are identified and quantified. The environmental benefits of SDHW systems include reduced energy use, reduced electrical demand, and reduced pollution. Utilities use various forms of power generation to meet the system load, beginning with the plant with the lowest operating costs. Each of these plants incurs a certain cost to the utility and to the environment. Coal, oil, and natural gas plants release varying levels of carbon dioxide, sulfur dioxide, oxides of nitrogen, and particulates. The cost to the environment for these pollutants can be converted into $/ton produced. Using a marginal plant analysis based on a least cost production model, a utility's avoided emissions, avoided costs, and capacity contribution from the installation of many SDHW systems has been evaluated and the impact of many solar systems on the utility has been quantified. The avoided emissions, capacity contribution, energy and demand savings were evaluated using the power generation schedules, emissions data and annual hourly load profiles from local utilities. It is shown is that power plant maintenance and outage scheduling significantly effect the amount and type of airborne pollutants at the margin during a utility's off-peak periods. SDHW systems are thereby found to be beneficial during both peak periods and periods of scheduled maintenance from an environmental point of view. As a specific example, each six square meter solar water heating system can save annually: 3559 kWh of the energy, 0.66 kW of peak demand, and over four tons of pollution (7727 # CO2, 51 # SO2, 0.11 # N2O, 17 # NOX, 0.13 # CH4, and 1.1 # particulates) for a Wisconsin utility. (Based on 5928 kWh annual energy requirements of a conventional 52 gallon electric system resulting in over six tons of airborne pollutants: 12705 # CO2, 80.78 # SO2, 0.180 # N2O, 28.35 # NOX, 0.200 # CH4, 1.820 # particulates.) Under the supervision of Professors William Beckman and Sanford Klein; 254pp.

The Modeling of a Natural Convection Heat Exchanger in a Solar Domestic Hot Water System

The Modeling of a Natural Convection Heat Exchanger in a Solar Domestic Hot Water System Avina, John M. The use of simulations can greatly aid in optimizing the design of natural convection heat exchangers (NCHEs) in solar domestic hot water (SDHW) systems. Fraser et al. (1992) presented a NCHE model that is used in the WATSUN solar simulation program, that requires experimental measurements of the heat exchanger thermal performance and shear pressure losses. Two TRNSYS models are here presented for a NCHE in a SDHW loop. The simple model, based on Fraser et al.'s work, requires experimental testing on the particular heat exchanger. The simple model can be used for optimizing SDHW system parameters (i.e. pipe lengths and diameters, collector areas, tank volume etc.) excluding the NCHE itself which is represented by the experimental curves. A detailed model, based upon cross flow correlations, requires geometric specifications of the NCHE being simulated and is applicable to shell and coil and counterflow configurations. By varying heat exchanger geometric parameters (such as the number of helices, diameters of helices, diameter and length of the heat exchanger shell) the detailed model can be used to design an optimum NCHE. Results comparing the detailed model with Fraser et al.'s experiments show reasonable agreement. Using the detailed model and the least cost savings economic analysis, simulations were performed to discover the optimal shell and coil NCHE geometry. It was found that considerably reducing the heat exchanger size led to enhanced economic performance over a 10 year period of economic analysis. Coil spacing and tube diameter had a lesser impact upon system performance than heat exchanger shell length and number of helices. Thermo Dynamics Inc. manufactures a shell and coil NCHE that contains 4 coils and is 0.635 m. The optimal heat exchanger design contains 2 helices and is 0.45 m long. For a given set of system parameters, a SDHW system containing the optimally designed heat exchanger would save the consumer an extra $110 in initial equipment cost, and $52 over a 10 year period. Heat exchanger designs were subject to variations in system parameters, such as collector area, hot water draw, location and glycol flow rate. Although each set of system parameters suggested a different optimal design, overall, the optimal design found for the initial set of system parameters remained adequate. As different economic assumptions will lead to differing optimal heat exchanger lengths, this work can serve as a guide for those who desire to optimize a shell and coil NCHE based upon a prevailing set of economic assumptions. Under the supervision of Professors William Beckman and Sanford Klein; 255pp.

Electrical Utility Interest in Solar Energy Systems

Electrical Utility Interest in Solar Energy Systems Trzesniewski, Jason A. 174pp.