barbs09
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Efficiencies of different domestic heat sources
Hello,
My friend and I had a discussion on the efficiencies of heating two identical rooms with two different heat sources which ended in a disagreement. I
would appreciate resolving this, as the problem, while most likely simple (to a physicist), is the sort of discussion I seem to find myself having
each year at the start of winter. Of course I think I am correct, but will graciously admit defeat and may learn something along the way.
The setting:
We have two identical rooms, each well, but not perfectly insulated. One has a 5000W clothes drier as a heat source, the other a 1500W fan heater.
Each room is fitted with a power point, which runs through an accurate thermostatically controlled switch, each set to a cosy 25 degrees C. A watt
meter is placed between each heater and power point to measure total energy consumed.
I argue that when each heater/domestic appliance is switched on and left for 10 days, after coming back, both equally warm 25 degree C rooms will read
similar (even identical) power usage on the watt metres. I argue that yes the inefficient dryer will consume 5000W but will bring the room to 25 degC
much quicker and switch itself off.
My friend argues that driers are much less efficient, which I say is irrelevant as we are talking a reasonably closed system and any inefficiency will
likely be ultimately transformed to heat anyway. He argues that the energy not used as heating is being used as kinetic energy in the dryer by
turning the drum around. Though this might be true I believe friction, air resistance also act on the motor and drum assembly with all additional
energy input being dissipated as heat and retained in the reasonably closed system (room). In addition as the drier is switched off any kinetic
energy will be ultimately dissipated as heat as the thing comes to a frictional stop. I would have even thought that additional sound energy from the
drier would be degraded to heat as it gets absorbed by the air and the walls.
Anyone have any thoughts? Thanks in advance
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froot
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You will come back to 2 different readings after 10 days. If energy is consumed mechanically only part of it would end up as thermal energy, the rest
is pretty much the difference in the readings.
The accuracy of your setup would depend on the location of the temperature sensor. If badly positioned the 5kW heater could heat up one end of the
room to 40 deg before the sensor feels 25 deg. The insulation at this warmer part would have a larger thermal gradient and allow more energy through
in terms of watts, while the 1,5kW heater raises the temperature more gradually allowing more time for thermal equalisation so the sensor's reading
represents the entire room's temperature and less energy is lost.
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watson.fawkes
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You are correct that you can consider all the energy to be converted to heat, but you're wrong about the power consumption answer. Your friend has the
right answer, but no idea about the reason for it.
The issue is in the way that thermostats work and that heat transfer is not instantaneous. There's a response lag between the time the average
temperature in the room is 25 °C and the time that the temperature at the thermostat is 25 °C. All other things being equal, the
faster heater will lead to a room that is hotter on average than one with the slower heater. In both cases the average temperature will be somewhat
greater than 25 °C. The hotter room has greater heat loss to the outside, since the rate of heat flow is proportional to temperature
difference.
It's actually worse than that. The hot side of the room (that between the heater and thermostat) will lose energy even faster than the average
temperature would indicate, and thus increase the lag time. Details would vary based on room geometry, of course.
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Dr.Bob
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Virtually any simple heat source will convert nearly all electricity to heat in a closed system at 100% efficiency (electrons to heat). That does
not mean that it is an economical system however, since other heat sources like gas or coal my be cheaper than electricity, plus some devices MOVE
heat rather than create it, like a heat pump, which can generate a higher than 100% yield of heat than electrical resistance heat since it is
transferring the heat from outside the closed system, not creating it. Also, most houses are not closed systems, so the leaks of heated air would
have a dramatic effect (like my wife running the drier when it is 100F outside, which sucks the hot air inside and then pulls the air conditioned air
outside in normal use.)
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Rogeryermaw
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just don't put clothes in the drier. breathing lint is bad.
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