Performance Estimation of a microwave

Various experiences made by members of the forums concerning in particular small household appliances and energy management.
Christophe
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Performance Estimation of a microwave




by Christophe » 26/09/06, 10:31

I have just estimated the performance of our microwave oven.

Measurement protocol:

We measure the T ° mounted a certain amount of water. Be careful not to boil the water obviously.

Hypotheses :

- Heat capacity of constant water.
- Homogeneous temperature in the container (no gradients) during the measurement (mechanical mixing performed before measurement).
- Measurement carried out on 1L of water.
- Power measurement performed with the PM230 :
Image
- Temperature measurement with ITC-777 (K probe) in 0,1 ° C mode
- Galanz brand 1300W Microwave
- Duration of the measurement made with an electronic chronometer (the precision of the built-in timer must not be top :D )

Measures :

Volume of water: 1L
Initial T °: 17,7 ° C
Final t °: 57,6 ° C
Duration of test: 5min be 300s
Power consumption: of 1270W at t = 0s to 1130W at t = 300s.
Voltage: 232V.
Cos phi: 0,97
Intensity of 5,64 5,02 to A.

It is considered that the power decreases linearly over the measurement range, ie 1200W of average.

Calculation of energy and yield:

a) Absorbed energy: 1200 * 300 = 360 000 Joules
b) Useful energy (water-based): Cp * DeltaT * Mass = 4,18 * (57,6-17,7) * 1000 = 166 782.

Overall microwave oven "heating" efficiency: 167/360 = 46,4%! Gulp ...

This value is very low: where would the 53,6% of the energy go?

Before debating the subject, I'm going to remake 2 or 3 other measures so this value, very low, is intriguing. Just like the fact that the power absorbed decreases in time ... (furnace full like empty ...).

It would also be nice if some members of the forum perform similar measurements on their oven ... maybe our has a fault or has aged badly (it has 6 years old)?

I did not use this oven much but now it is definitive: directly trash and live the electric kettle (we have one, I will also estimate its performance).
Last edited by Christophe the 22 / 06 / 15, 21: 48, 2 edited once.
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by Christophe » 26/09/06, 10:40

And here are some pictures of the "tests":

Image

Image

Image

Image
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by Obelix » 26/09/06, 11:11

Hello,

This performance does not surprise me at all ....
I find it good enough for the complete industry.

In fact, an oscillator vacuum "tube" such as the magnetron has an intrinsic reduction of around 65/70% in the best cases. What should not be the case of the magnetron "general public" of your furnace.
Then you have the transport of the energy between your magnetron and your heated cavity, and the yield of your cavity and the motor of rotation of the plate or the brewer of waves.
Then we must feed the beast, and the same thing the yield of food must be around 0.8 ....
And then you have the comfort:
Loupiotes, displays, regulations, security, all this has a cost in energy.

In total in construction consumer near the 50% is, in my humble opinion, a performance worthy of interest!

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by Capt_Maloche » 26/09/06, 11:18

No, no!

The oven is ventilated and illuminated, power to be removed for your real performance

Your "jar" does not seem to be covered, losses by evaporation and exchange with the speed of the air

Test to do again by dismantling the lamp:

In a box closed polistyrene type box of Christmas bovine or omelette Norwegian

and there you will have a more fair return, I bet on more than 80%

As soon as I unpack a box, I also do the test

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by Christophe » 26/09/06, 11:24

Uh you push the cap la ... if the lamp is 50 W is a lot. In your reasoning should therefore also disassemble the engine turntable.

Moreover, what interests us is the REEL global repository in current use compared to other non-cooking processes? The rendments of labs do not interest us ...

The losses are far too great to be explained by the accessories and the "losses" in the air of the microwave. Your 80% are overestimated in my opinion ...

I just finished the 2ieme test session. Results in 30 seconds.
Last edited by Christophe the 26 / 09 / 06, 11: 46, 1 edited once.
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by Christophe » 26/09/06, 11:39

I redid a step to confirm (or invalidate) the first measure.

The protocol remains the same as above except that: this time I weighed 1 kg of water and I measured the drop in power more "precisely".

Absorbed power :

Start: 1309W
30 seconds: 1283W
2 min: 1236 W
4 min: 1195 W
5 min (just before the end): 1180 W

Measures :

Mass of water: 1000g, scale accuracy: 1g
Initial T °: 19 ° C
Final t °: 57,7 ° C
Duration of test: 5min be 300s
Absorbed power: from 1309W at t = 0s to 1180W at t = 300s is 1245W average.
Voltage: 232 to 233V.
Cos phi: 0,96 and 0,97

Calculation of energy and yield:

a) Absorbed energy: 1244 * 300 = 373 500 Joules
b) Useful energy (water-based): Cp * DeltaT * Mass = 4,18 * (57,7-19) * 1000 = 161 766

Overall microwave oven "heating" efficiency: 162/374 = 43,3% ...or close to 7% (relative) difference with the 1ere measurement. It would be so very desirable that this test be done by someone else ...

Some pictures :

1) Weighing of 1000g (with the bowl tare of course) and photo of the "chronometer"

Image

2) Cold temperature:

Image

3) Hot temperature:

Image

ps: the temperature of the hotter water at t = 0 probably comes from the bowl which was still "hot" from the first measurement or from the temperature of the network which increases during the day?
Last edited by Christophe the 26 / 09 / 06, 12: 32, 1 edited once.
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by Christophe » 26/09/06, 12:07

I attack the electric kettle :)
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by Christophe » 26/09/06, 12:39

Results with the Electric Kettle: Philips Cucina (given for 1850 to 2200 W)

Measures :

Mass of water: 1305g, precision of the scale: 5g (1g below 1000g and 5g above, I made sure to fill up to reach 1305 by "below")
Initial T °: 18,9 ° C
Final t °: 74,8 ° C (measured after mixing and at mid level of the water level)
Duration of test: 3min be 180s
Absorbed power: from 1975W to t = 0s to 1950W at t = 180s is 1963W average (this drop is quite normal: the electrical resistance increases with the T ° and therefore the power drops)
Voltage: 234V.
Cos phi: 1,00 (normal for 100% resistive)

Calculation of energy and yield:

a) Absorbed energy: 1963 * 180 = 353 340 Joules
b) Useful energy (water-based): Cp * DeltaT * Mass = 4,18 * (74,8-18,9) * 1305 = 304 929 Joules

Overall kettle "heating" efficiency: 305/353 = 86,4% ...

The losses, 13,6% seem reasonable and probably come:
1) Conduction losses in parous
2) Evaporation losses (bubbles appeared from about 1min 30)

If I had a precision balance I could have weighed the mass after the measurement .... but the facts are: a kettle has a yield 2 times higher than a microwave!

To be confirmed by other measures ...

Photos


1) Weighing 1305 (with tare recess):

Image

2) Cold temperature:

Image


2) Test stand: : Cheesy:

Image

3) Hot temperature:

Image
Last edited by Christophe the 26 / 09 / 06, 12: 50, 3 edited once.
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by Christophe » 26/09/06, 12:41

Question which obviously comes: where is the "lost" energy of the microwave going? Everything does not dissipate in heat because it would take a large cooling radiator (500W) ... so ... some ideas on other losses?

- Chemical losses (water molecules would absorb more energy than they would restore)?
- Eléctromagnetic loss (there is necessarily ...)
- Losses in the container (It was Pyrex for info)
- Losses of the Holy Spirit?

...
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by I Citro » 26/09/06, 13:13

For the "mysterious" losses I am thinking of electromagetic waves.

Indeed the microwave is a generator, and in your experience, your basin of water and a receiver of the waves emitted.
In all probability, not all the waves emitted come to "return" their energy to the water placed in the oven.

As others have suggested, I will be curious to know the consumption of accessories (motors tray and ventilation, food, lamps ...). If your microwave continues to ventilate after its heating cycle it would be interesting to measure this consumption.

I do not have anything to measure the temperature, otherwise I would do well the experiment.

It would be interesting to do the same experiment with induction hobs to compare ...
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