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Energy Cost Savings Using Insulative
Plastic FilmME 340 Project – Nolan Crook
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How can I reduce my home energy costs?
• Objective: Determine cost savings per year when using plastic film insulation
• Plastic film is a very cheap and simple option for adding window insulation, but how well does it work?
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Problem Setup and Procedure• Determine the temperatures of:
• Tinf_outside
• Tinf_inside
• Tsurface_outside
• Tsurface_inside_window
• Tsurface_plastic_film
• Use this info to find heat transfer coefficient “h” using free convection with vertical plate
• Describe the window with and without film as a single resistor
• Use this resistance value to find insulation efficiency throughout the year
2-p
an
e w
indow
Tsurface_plastic_film
Pla
stic film
Tsurface_inside_window
Tsurface_outside
Tinf_outside Tinf_inside
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Case 1 – No film (double paned window)
• Resistor Network:
Rwindow RinsideRoutside
Tsurface_inside_window Tinf_insideTsurface_outsideTinf_outside
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• Resistor Network:
Rwindow RinsideRoutside
Tsurface_inside_windowTinf_inside
Tsurface_outsideTinf_outside
Case 2 – Plastic film included
Tplastic
Rfilm+enclosed_air
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Actual Measured Air Temperatures (from my room window)
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Convection for vertical Plate• Find Grashof Number
• Find Rayleigh Number using Grashof
• Find Nusselt Number using Rayleigh (Nusselt for vertical plate free convection)
• Find convection coefficient for both cases from Nuzzelt Number
• Find q (W/m^2) for both cases using convection coefficient
• Find Rfilm and Rno_film using q and Tinf_out as well as Tinf_in
• (See attached calculation sheet for more details)
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Energy analysis
• Define q as a function of Tinf_out so that the outside temperature can be varied over the course of the year while holding a constant Tinf_in
• The area of the windows was calculated using parameters for my apartment (6 windows, 12.26m^2 total)
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Energy Analysis Continued• Define Energy (kW*hr) as a function of q(Tinf_out)
• In the above equations, 12 hours was used instead of 24 to evaluate at both average high and low temperatures for Provo.
• Also, the absolute values of the above equations were taken to account for heating or cooling (energy leaving or entering through window)
Taken from weather.com for Provo, UT
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Results over course of year
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Results/Discussion• As can be seen from previous slide, energy transfer through the
window is much less using plastic film, especially during the winter
• Total energy transfer values over year:• No film: 18,804 kW*hr
• Plastic film: 3,012 kW*hr (six times less!)
• Total energy costs (assuming 1 kW*hr electricity = $0.08, and all heating cooling done with electricity at 100% efficiency)• No film: $1504
• Plastic film: $241
• Savings: $1263
• Cost of plastic film: $12.95
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Conclusions
• There is a large energy cost savings for using an extra insulative layer on inside of windows, at only a cost of $12.95 from amazon.com
• This is accomplished by reducing the temperature difference between most inner surface temperature and Tinf_inner ,thereby reducing free convection
• The savings of $1200/year seems very large; this may be due to the following inaccuracies:
• The thermal resistance of the window is held constant across changing temperatures, but in reality the convection coefficient of the free convection will change at different temperatures
• Average low and high temperatures are assumed to be at 12 hour durations each, which is not accurate at Provo’s latitude
• All heating and cooling is assumed to be electric (no natural gas), with 100% efficiencies