TWO-PART MASTER’S THESIS PART ONE: EVALUATION OF COOL ROOF REFLECTIVITY IMPACTS PART TWO: SURFACE SKIMMER FLOW RATES
Type of DegreeMaster's Thesis
Civil and Environmental Engineering
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PART ONE: Cool roofs attempt to lower the temperature of the roof surface so that the interior of buildings can be cooled using less energy. Light color surfaces, white being the coolest color, are much cooler than darker surfaces due to reflectivity. Although several studies have been conducted on the benefits of cool roofs, limited studies have been performed to investigate possible drawbacks of reflective roofing materials on rooftop mechanical and electrical elements, such as heating, ventilation, and air conditioning (HVAC) units. Part One of the thesis presents the research investigating the effects of reflectivity on air and adjacent surface temperatures within 4 ft (1.22 m) above the surface of low-sloped roofs. Although more research is needed on this topic, the existing literature proved that temperature changes a few feet above the roof surface affect the performance of rooftop HVAC units. Testing was performed at the Auburn University- Stormwater Research Facility in Opelika, Alabama. Two experimental decks were constructed, and each deck was covered with a different roofing membrane. One deck had a white polyvinyl chloride (PVC) membrane, and a black ethylene propylene diene monomer (EPDM) membrane was installed on the other. The experiment was designed to record surface temperatures of the membranes (at the surface level), air temperatures up to 4 ft (1.22 m) above the surface, and temperatures on a nearby surface adjacent to the roof. Testing was conducted from July 2020 until December 2020. An exploratory data analysis was conducted to find initial trends in the data. The final analysis consisted of a comprehensive multiple linear regression (MLR) analysis using data from the top twenty solar radiation values from each testing day. This resulted in various MLR models that use ambient variables such as outdoor temperature, solar radiation, relative pressure, outdoor humidity, and wind speed to estimate membrane surface temperature and air temperatures from 1 ft (0.30 m) to 4 ft (1.22 m) above the surface for both types of membranes. The resulting MLR equations were intended to be used by designers to improve cost-effectiveness through decisions associated with the design of roofing structures and the selection of rooftop HVAC units. Higher air temperatures at the inlet of rooftop HVAC units are associated with higher energy consumption. Average local values for the ambient variables on typical warm sunny days can be used in the MLR models to predict surface and over-the-surface temperatures for white and black roof surfaces on buildings in or nearby Opelika, Alabama. Using average ambient values measured at the testing location as independent variables in the final MLR equation, the models predicted air temperature above the white membrane 8.8 ˚F (4.9˚C) higher than ambient outdoor temperature compared to 3.0˚F (1.7˚C) for the black membrane . This resulted in a 10.0% and 3.4% increase from ambient outdoor temperature, respectively. On average, the above surface air temperatures on the white deck were 5.8˚F (3.2 ˚C) higher than the black deck. Additionally, the black EPDM membrane surface reached 130.5˚F (54.7˚C) and the white PVC membrane reached 105.8˚F (41˚C). This corresponds with a 47.8% and 19.8% increase in temperature from ambient outdoor temperature. Adjacent surface temperatures were also significantly higher due to the increased reflectivity of the white PVC membrane. This is valuable information to building owners and contractors who are aiming to increase building efficiency. These considerations should be accounted for deciding on membrane color, material, and configuration. Climate region and maintenance costs should be considered when selecting a “cool” roofing membrane or coating and not looked at as a universal solution to increasing building energy efficiency. PART TWO: A floating surface skimmer is a device used to dewater a sediment basin as it fills. The skimmer floats on the surface, draining the least turbid water as sediment falls out of suspension. An adjustable orifice on the skimmer helps to regulate the filling and draining rate of the basin. After significant runoff events, skimmers will slowly drain the basin over several days to maximize settling, while draining less turbid water from the top of the water column . Since water typically enters a sediment basin at a higher flow rate than the skimmer removes the water, soil particles can settle to the bottom of the basin. There are several options when choosing a skimmer product. Manufacturers have published data for their products that customers can use to decide on skimmer type, size, and orifice opening, but these design parameters tend to be very rough estimates with numerous assumptions. This research details a methodology for testing skimmers including materials, data collection process, and data analysis approach. In addition, testing was performed in a 7 ft (2.13 m) deep evaluation tank with a volume of approximately 1,000 ft3 (30 m3). The skimmer tested was a 6-in. (15.2 cm) post-construction stormwater prototype provided by J.W. Faircloth & Son, Inc. which used an adjustable sluice gate to create openings from 0.5 to 6.0-in. (1.3 to 15 cm) in 0.5-in. (1.3 cm) increments. The skimmer was attached to the discharge outlet of the testing tank which discharges to a nearby pond. A constructed water delivery system was used to fill the tank and a Solinst Levelogger® recorded water levels in 5 sec. intervals as the skimmer dewatered the tank. This process was repeated 36 times with varying barrel lengths and sluice gate openings, with each setting having triplicate tests performed. Experiments revealed that the skimmer had a capacity of 0.5 ft3/s (0.03 m3/s) with a 1.0-in. (2.5 cm) and a capacity of 2.5-3.0 ft3/s (0.071-0.085 m3/s) with an opening of 6.0-in. (15 cm). Results from these experiments were used to develop models to approximate the flow rate of the skimmer at various water depths. These models were then used to create a user-interactive skimmer sizing tool in Microsoft Excel. The user inputs values such as basin elevations and cross section properties to calculate the basin volume. The user can then select the number of skimmers and the opening sizes to obtain data and graphs on flow rates at each depth, basin storage, and the total design drawdown time. This method of testing and data analysis is more thorough than present practices in the industry and provides designers with more accurate information on sediment basin storage and drawdown times based on skimmer selection.