Energy and Economic Impacts of Ultraviolet Germicidal Irradiation of Cooling Coils

Open Access
Author:
Firrantello, Joseph Thomas
Graduate Program:
Architectural Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 05, 2016
Committee Members:
  • William P Bahnfleth, Dissertation Advisor
  • William P Bahnfleth, Committee Chair
  • Stephen James Treado, Committee Member
  • John Michael Cimbala, Committee Member
  • James Landis Rosenberger, Outside Member
  • Catherine J Noakes, Special Member
Keywords:
  • cooling coils
  • UVGI
  • ultraviolet germicidal irradiation
  • biofouling
  • fouling
Abstract:
Cooling coils are heat exchangers that cool and dehumidify the air used to condition buildings. They tend to be wet by design (due to dehumidification) and can trap particles from the air. Some of the particles are living: biological and viable. These microbes can reproduce and cause biological fouling of the cooling coil, commonly referred to as biofouling. Biofouling is known to increase airside pressure drop and decrease heat transfer ability. Ultraviolet germicidal irradiation (UVGI) is one way to mitigate biofouling, though there is little third-party literature documenting its effectiveness. This study examines three things: field measurements of change in coil performance after treatment with UVGI, modeled energy use impact of coil irradiation, and monetization of UVGI benefits including first cost, energy cost, maintenance cost, and collateral health benefits. Air handling units with visibly fouled cooling coils at two field sites were chosen to evaluate the effect of UVGI on biofouling: one in Tampa, FL and one in State College, PA (referred to as the PSU site). Coil operation was monitored pre-UV and post-UV. Data collected at the Tampa site showed a 21.65% to 21.7% decrease (95% confidence) in mean coil airside pressure drop and a 14.5% to 14.8% (95% confidence) increase in mean overall heat transfer coefficient (UA). Pressure drop data were controlled for airflow and latent load, while UA data were controlled for heat exchanger entering conditions. The PSU site, which was unexpectedly cleaned before data collection could begin, showed a 1.3% to 1.4% decrease (95% confidence) in pressure drop, and a 45% to 50% increase (95% confidence) in heat transfer coefficient. This counter-intuitive result is believed to be due to an increase in non-biological fouling at the coil face with a concurrent decrease in biological fouling between the coil fins due to UVGI, but the results were thought to be unrealistic by industry professionals. Going forward, the results from the Tampa site are used for modeling work, while the results from the PSU site are not. The Tampa site reduction in airside pressure drop and increase in overall heat transfer coefficient were then applied to a subset of the DOE (Department of Energy) Commercial Reference Buildings in order to model the benefit of UVGI on fouled coils across seven buildings and sixteen climate zones. It must be emphasized that this is the application of the results of a single case study to a variety of situations, as there are currently no data available on differences in fouling for climates and other influencing factors. The majority of energy savings (80%) occurred in fan energy, followed by cooling (17%) and pump energy (3%). The overall savings tended to be a fraction of a percent of total HVAC energy use, or around 0.3 kBTU/sf-yr. This is due to the relatively low amount of fouling used in the models (20% of a 0.75 in wg coil is only 0.15 in wg). An attempt was made to bound the energy savings potential by using other sources in the literature for the amount of pressure drop reduction. For example, one of the studies showed an increase of 156% in coil pressure drop due to fouling, or an increase of 1.17 in wg in the case of a nominal 0.75 in wg coil. Assuming this level of fouling increased the predicted energy savings by an order of magnitude, closer to 2 kBTU/sf-yr. It is important to consider the magnitude of potential airside pressure drop change, as it heavily influences the energy savings due to the dominance of fan power. Monetization of the IAQ benefit of UVGI (one of the components of the overall cost analysis) is modeled using a stochastic implementation of the Wells-Riley equation. While the UVGI wattage used for coil cleaning is lower than that of air cleaning, there is still a collateral air disinfection benefit. Work loss days (WLDs) were the evaluation metric for office buildings, hospital acquired infections (HAIs) for hospitals, and disability-adjusted life years (DALYs) for schools. Based on these metrics, on average, office buildings saved $0.01/ft2 to $0.14/ft2 ($0.14/m2 to $1.51/m2) per year, hospitals saved $0.02/ft2 ($0.23/m2), and schools saved between $0.00/ft2 and $0.01/ft2 ($0.06/m2 to $0.11/m2) Energy use calculated from the Tampa site results, IAQ benefits, first cost, and maintenance costs are combined in a 20 year life cycle cost analysis (LCCA). When IAQ benefits are considered, UVGI provides the greatest value to the owner, followed by UVGI without IAQ benefits considered, and lastly mechanical cleaning. The median net present value of cost of each to the owner is, respectively, -$0.13/ft2, $0.12/ft2, and $0.56/ft2 (-$1.37/m2, $1.26/m2, and $6.05/m2). Using a literature data source to, again, bound the possible benefit changes the magnitude of and order of results. The median net present value of cost of UVGI including the IAQ benefit is -$0.77/ft2 (-$8.32/m2), of UVGI not including IAQ benefit is -$0.21/ft2 (-$2.30/m2), of mechanical cleaning is $0.25/ft2 ($2.66/m2). Reviewing the differences between these two sets of economic benefits again makes it clear that knowing potential for fouling is important for economic evaluation, and that the literature on this is lacking.