Energy Cost Savings Using Insulative

Plastic FilmME 340 Project – Nolan Crook

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?

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

Case 1 – No film (double paned window)

• Resistor Network:

Rwindow RinsideRoutside

Tsurface_inside_window Tinf_insideTsurface_outsideTinf_outside

• Resistor Network:

Rwindow RinsideRoutside

Tsurface_inside_windowTinf_inside

Tsurface_outsideTinf_outside

Case 2 – Plastic film included

Tplastic

Rfilm+enclosed_air

Actual Measured Air Temperatures (from my room window)

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)

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)

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

Results over course of year

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

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