Net efficiency product of each efficiency
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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 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

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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%

Multiply efficiencies!



Moving heat energy

Heat moves by-

Conduction--flows through

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

In Btu

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

  • 3.(8) Your house has total wall area of 1600 square feet, with a roof area of 2000 sq ft. The walls are two feet thick and have R-values of 8 hr-deg F -ft2/Btu (the standard units), and the roof is one half foot thick with an R-value of 12 of these same units. Ignoring heat losses through the floor, how many Btu do you lose in one day if you set the thermostat at 70 deg F, and it is freezing (32 deg F) outside? Yes, we have ignored the windows. If you burn coal costing $30 per ton, how much would that

  • heat loss cost you?

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.

Turning down the thermostat?

  • But we just have to heat up the house in the morning! Is it worth it?

  • Q / time = area x DT / R.

  • DT decreases at night with the lower setting, so less heat is lost.

  • Restore the daytime DT in the morning.

  • Total heat is area under the curve of Heat/time.

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.

Phase change heat transfer

  • Put heat into making steam from liquid

  • Move the steam, by convection or pumping.

  • Condense the steam in your room or onto your skin in a sauna

  • ‘steam heat’, as distributed around the campus

Wait a minute!

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?

Next week---

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

Friday: Transportation

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