Net efficiency=product of each efficiency. How to combine?. Add losses? 169% !! Multiply losses? 0.70x0.09x0.10x0.02x0.25x0.33x0.20=0.0000042 Losses 100%-efficiency ‘motor losses 10%’motor efficiency=90% Multiply efficiencies! 0.30x0.91x0.90x0.98x0.75x0.67x0.80=0.097 =9.7%.
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Net efficiency=product of each efficiency
How to combine?
Add losses? 169% !!
Multiply losses? 0.70x0.09x0.10x0.02x0.25x0.33x0.20=0.0000042
‘motor losses 10%’motor efficiency=90%
Heat moves by-
Radiation—flies through space
Convection- the hot stuff moves
Phase change- between liquid and gas phases of water, freon, ammonia
Layers of conducting wall--
NB—use English units consistently for Ch.7 !
Conduction heat loss Q thru several walls
Each Q = Area x DT x time x Conductivity
Easier to add ‘resistances’ to heat flow
So Q = Area x DT x time
And Rtotal = R1 + R2 + R3….
Resistance to heat conduction, measured in
Hours- ft2-deg. F / Btu
Such that Q (flow) =
area (ft2) x temp. diff. (deg F) x time (hr) / R
Example, Table 7.3
¾ inch plywood, then 6 inches brick, then two inches of fiberglass, then ¾ inch plywood.
Total R=0.94 + 6 x 0.20 + 2 x 3.70 +0.94= 11.42
Units—hr-degF- ft2 /Btu
If area = 200 ft2, DT=50 degF, how much heat is lost in one day?
Units only Btu=ft2 x degF x hrs/(hr-degF-ft2/Btu)
Q=area x DT x time/R=200 x50 x 24/11.42=21,016 Btu
4. (4) How many square feet of window would give a heat loss equal to that through those walls? You will need to use the text to compare R-values for that wall two feet thick and a window with glass ¼ inch thick.
One can measure the heating needed by its proportionality to the number of days and the outside temperatures, in ‘degree days’. This assumes an inside temperature of 65 deg. F.
(may also be negative, when AC needed)
Not important enough to emphasize. Used to specify building codes.
Convection of heat
Keep the cold air out, seal up the leaks.
Use a fan to move furnace air around the house.
Let hot water rise from a basement furnace.
Radiative heat transfer
Electromagnetic radiation-rules in Section 4.3.
Power emitted per square meter = constant x T4,
with T in deg Kelvin.
Not important at human temperatures.
Useful as a diagnostic—look for the glow.
But power maximum at wavelength = constant / T,
So infrared at reasonable temperature.
How can the campus use steam for cooling?
Heat pump , Figure 3.12 driven by electricity.
How well can we do at conserving our household energy?
How much are we willing to invest? change our ways?
Monday: “emergy”– how much energy does it take to extract energy?
Reading: wikipedia for definition
Google until you understand the idea and have an example.
Wednesday: Start Ch. 8, Transportation