Life Cycle Performance Evaluation of an Air-Based Solar Thermal System By Joshua Reed Plaisted A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science (Mechanical Engineering) at the University of Wisconsin – Madison 2000 Copyright 2000 Joshua Reed Plaisted Approved by ___________________________ Professor William A. Beckman May 30, 2000 Abstract Performance at the McKay center after 22 years of operation was characterized through experimental measurements of component parameters. Effects of the component parameters on systems level performance was evaluated using numerical simulation tools (TRNSYS) calibrated against actual operation of the building. Calibration of the collector arrays indicated degradations of 16 percent in Fr(τα) and 19 percent in FrUL on average for the two arrays installed at the site. Thermal losses from the pebble beds were characterized by conductances of 7.5 and 3.0 W/m2-C for the two beds, which far exceed the design conductance of 0.65 W/m2-C. Flow distributions within the beds were highly non-uniform with an average 3:1 flow ratio across the pebble beds. Effects of component degradations did not directly translate into similar systems level degradations. Annual simulations performed under typical meteorological year (TMY2) conditions indicated that the high bed losses and non-uniform flow have resulted in a 1 percentage point decrease in the solar fraction. Degradations in the collector array had a more significant effect on system performance, lowering the solar fraction by 12 percentage points. Together, physical deteriorations of the solar components have lowed the solar fraction from an optimum of 45 percent to 32 percent. The single largest effect on system performance was not attributable to a physical component, but to the control logic that governs system operation. Faulty operation of an air damper within the system and improper location of a control sensor were responsible for the low solar fraction of 15 percent representative of how the system was found at the inception of this study. In-situ testing methods and novel calibration techniques are described that result in minimal cost and interference of system operation. Systems level calibrations employing daily integrated energy comparisons are also discussed along with the sensitivity of the model to load dynamics. Possible methods of fault detection for solar thermals systems and a minimal instrumentation package are presented that may prevent similar faults from compromising the performance of present and future solar installations. Acknowledgements No work stands on its own, but is instead the culmination of ideas, thoughts, and insights of others. Without the guidance of professors Bill Beckman and John Mitchell, very few of the ideas contained in this text would be culminated, they would simply remain half scattered thoughts. Thank you both for keeping me on track over the years, which for those who know me is no easy feat. My gratitude is extended to the Heckrodt foundation for providing the funding for this study, and to the staff of the UW physical plant and the McKay center for their help in carrying out the experimental investigations. A special thanks is also extended to professor Sandy Klein, who humbles his students in intermediate thermodynamics, but later inspires them with his uncanny knowledge of how to approach unsolvable problems. Thank you Sandy for teaching me to apply the laws of thermodynamics and system boundaries in clever ways, even though I never seemed able to use them during those fifty-minute exams. Without the humor and good spirit that exists in the solar lab, my time here would have felt much longer. Thanks to little Egypt and Germany for the company. For providing balance in my studies, I’d like to thank everyone involved in the Institute for Environmental Studies. Without you I’d still believe that gasoline was cheaper than soda-pop, and that reed canary grass was a houseplant. Thank you for giving me a reason to study solar energy and the knowledge to back it up. For my original child-like curiosities, my drive to understand everything, and the will to try, I thank my family. To my mother for teaching me that there are no stupid questions, just those afraid of asking them, and to my father who has always believed in me regardless of all the seemingly stupid questions I’ve pursued. Well Michelle it’s your turn now … while I can thank everyone else for their insights and support in this work, I credit you with the inspiration behind it all. Somebody has to go polish the stars, They’re looking a little bit dull. Somebody has to go polish the stars, For the eagles and starlings and gulls Have all been complaining they’re tarnished and worn, They say they want new ones we cannot afford. So please get your rags And your polishing jars, Somebody has to go polish the stars. Shel Silverstein – A Light in the Attic Author’s Note “So Josh, you drove here from Chicago right? Did you stop to get any gas?” “Yup” “How much did that gas cost? “ “Dollar-eight a gallon” “Did you get anything to drink” “Yeah, a Coke” “And how much did that cost?” “Seventy cents” “What is that … something like seven-fifty a gallon?” Those words were the beginning of my introduction into energy as posed by professor Sandy Klein more than two years ago. His point was that to get a gallon of gasoline you had to pump it from a mile under ground, ship it by supertanker half-way around the world, refine it at the coast, pump it 1,500 miles to the mid-west, and then truck it to the gas station before it could be sold for a dollar-eight a gallon. By contrast, the soda started it’s life as tap water no more than a few hundred miles away. After being carbonated, sweetened, and colorized, the soda was canned and hauled a days drive to sell for seven-fifty a gallon. The final question Sandy asked was this: do you think you can compete with a source of energy that is 85% cheaper than soda- pop? That was the ante for renewable energy, beat soda-pop by 85% or fold your hand. The trick in beating the “soda-pop” challenge is to think like an economist and not an engineer. The way the question was phrased assumed that the price of gasoline at the pump was identical to the cost of gasoline to society. Equating the values of price and cost is a dangerous assumption that is almost never valid in the world today. To explain how this so we must continue the “soda-pop” debate from the viewpoint of an environmental economist. “When you filled up the car with gas, what happened to the old gas?” “What do you mean?” “Where did the gas that was put into the car last week go, did you drain it out?” “No, it was burned in the engine.” “So, the gas was vaporized, combusted, and then exhausted from the tailpipe?” “Exactly” “What was contained in this ‘exhaust’ ?” “Well, a combination of things like nitrogen-oxides and unburned fuel vapors.” “You mean the chemical precursors to ground level ozone” “Chemical huh to ground level what?” “Those nitrogen-oxides and fuel vapors chemically react in the atmosphere to produce the smog and haze that hangs over most major U.S. cities.” The environmental economist would proceed to tell you that smog and haze are strongly linked to asthma attacks (JAMA 1997), and that there are approximately 5 million children in this country affected by asthma1. They will tell you that 50 million Americans (1 in every 5) live in areas where medical science has determined the air to be polluted to the point where it adversely affects human health2. The final knot in their argument will be to tell you that 80% of all air pollution is caused by the combustion of fossil fuels for energy production. Their final question to you will be this: “Do you really think that the exposure of one-fifth of the population to elevated health risks is in any way comparable to the price of soda-pop?” The true price of energy, as expressed through externalized health and environmental costs remains an area of much debate. Simply attempting to calculate the cost of pollution is an enormous endeavor3, and attempting to collect on those costs is even more difficult. What we must remember from this narrative is that a price does not always reflect the true cost, and that as a society we always pay the true cost, whether it is extracted from our health, our environment, or our personal finances. Wealth is not the amount of money in your pocket, it is the quality of your life. Some in the policy arena have attempted to bring the cost and price of energy into equilibrium through the use of pollution permits (SO2 cap and trade for acid rain), regulations (the Clean Air Act, and Montreal and Kyoto protocols), and even lawsuits (New York’s recent lawsuit against mid-western utilities for NOx emissions). Applying any of these methods requires a 1 The total number of people affected in the U.S. is more than 14 million (JAMA 2000). 2 These pollution levels are defined by National Ambient Air Quality Standards (NAAQS) set by the Clean Air Act (CAA). Actual annual data on compliance with NAAQS is available from the USEPA. 3 If interested, the reader should review a health cost study recently performed by the Harvard School of Public Health (Levy et. al 2000), which estimates the health impact from 2 utility coal plants in Massachusetts as exceeding $1,000,000,000 annually.