Energy Savings
Window vs. Indoor
Refrigerator/Freezer (R/F)
NOTE: These calculated energy savings do not include the additional energy savings due to:
1. Freezing water at night
(when it is cooler than in the day time) behind the R/F and letting the ice melt during the
day to keep the R/F cool. These energy savings are
due to the enthalpy of fusion of water. It also reduces the electricity
demand on the city's electric grid during the peak usage times during the summer.
2. Letting the cold outdoor air into the refrigerator during winter (or cold nights).
For more accurate vapor compression cycle calculations, please see the notes written by Mr. Pei-feng Hsu, Ph.D. Professor & Department Head, Mechanical & Aerospace Engineering Department, Florida Inst. of Technology.
Let's
assume that the indoor temperature is 750 Fahrenheit
(F), the
outdoor is 400 F, and we want the inside of the freezer to be - 50 F (it's
presently at + 50 F). The indoor freezer
uses electricity to transfer heat from its inside + 50 F to its outside 750 F (75 - 5 = 700 F difference). Similarly, the window freezer uses electricity to transfer heat from its
inside + 50 F to its outside that is 400 F (40 - 5 = 350 F difference). Obviously the window freezer has less work
to do than the Indoor freezer. Based on the
calculations below, the energy savings rate is 75 %. It's more than 50% because
as the outdoor temperature drops, the window R/F's efficiency increases.
Assumptions
To ensure an accurate comparison, it is assumed that both R/Fs (window
and indoor) have exact same power, motor, compressor, cubic volume, and
component efficiencies. All external
environmental variables are likewise assumed the same, such as the inside
temperature of R/Fs, room temperature, type and amount of refrigerant, number
of times the R/Fs' door is opened and closed, etc.
In
column G and H of the following tables the energy (electricity)
savings % for various cities are calculated.
It is assumed that columns G and H (energy savings %) are
never greater than 95% because the window R/F always uses some electricity to
run its electronic components.
Definition of Columns in the table:
B = Average Outdoor
Temperature. If the outdoor temperature
is greater than 70 F, then they both consume the same amount of electricity
unless the A/C is on.
C = Average Indoor (Kitchen)
temperature (70 F or higher in the summer without A/C).
D = Average temperature inside the
R/F (set constant at 29 F).
Ti = C - D =
Temperature difference between the Indoor R/F's evaporator and condenser.
Tw = B - D =
Temperature difference between the Window R/F's evaporator and condenser. Assuming that the outdoor heat exchanger is
covered by a plastic shield to protect against snow, rain, wind or direct sunlight.
G = Energy Savings % assuming
that both R/Fs have the same COP (efficiency).
G = (Ti - Tw) / Ti This is a simple calculation. Unlike Column H, it does not take into
account (ignores or excludes) the higher efficiency of the window R/F when the outdoor temperature is colder.
H = Energy Savings % taking
into account the higher COP (efficiency) of the window R/F.
H = 1 - (Tw
/ Ti )2 This is more accurate than column G, because it includes
the higher efficiency of the window R/F in colder climates.
|
Month for NYC |
B |
C |
D |
Ti |
Tw |
G |
H |
|
January |
31 |
70 |
29 |
41 |
2 |
95% |
95% |
|
February |
34 |
70 |
29 |
41 |
5 |
88% |
95% |
|
March |
42 |
70 |
29 |
41 |
13 |
68% |
90% |
|
April |
53 |
70 |
29 |
41 |
24 |
41% |
66% |
|
May |
63 |
70 |
29 |
41 |
34 |
17% |
31% |
|
June |
72 |
72 |
29 |
43 |
43 |
0% |
0% |
|
July |
77 |
77 |
29 |
48 |
48 |
0% |
0% |
|
August |
76 |
76 |
29 |
47 |
47 |
0% |
0% |
|
September |
68 |
70 |
29 |
41 |
39 |
5% |
10% |
|
October |
58 |
70 |
29 |
41 |
29 |
29% |
50% |
|
November |
48 |
70 |
29 |
41 |
19 |
54% |
79% |
|
December |
37 |
70 |
29 |
41 |
8 |
80% |
95% |
|
Avg. Min. Energy (electricity) Savings % |
|
|
|
|
|
40% |
51% |
Therefore, excluding summer,
a window R/F in NYC on average consumes at least 40% less electricity (or 51% less, when the COP
calculations are included) than an indoor R/F. For example, in January and February the average outdoor
temperature is 31 F and 34 F, which is very close to the inside temperature of the R/F
(29 F). During these cold months only a
small amount of electricity is used to run the window R/F. Obviously if you add the summer energy
savings, the savings rate will be even higher.
To see other cities' energy/electricity savings %'s, click on one of the links below. The calculations are identical to the above table, except that the average monthly temperatures are different.
Anchorage Beijing Bombay
Buenos Aires Chicago Dallas
Denver Gweru
Hamburg Istanbul Kabul London Los Angeles Miami Montreal Moscow
New York City Seoul City Shanghai Tehran
Tel
Aviv Tokyo Toronto
PATENT
PENDING