In-line diode to reduce power

[Piwoslaw]

Ever since AC_Hacker posted his Rice Cooker Eco-hack, I've been thinking about the in-line diode trick he used to reduce the heating element's wattage. We have an electric kettle at home, 1.7 liters and 2000W (1960W actually, 8.5A@230V). I would like to have it use less power, say 1200W-1500W, since the load on the kitchen's electrics would be reduced, and I don't mind the wait.

Over at EcoModder I did a test on how much power is needed to boil 500ml (app. 2 cups) of room temperature water. The electric kettle took 3 minutes and used 58Wh. Today, I hooked up a diode (400V, 9A) to see the difference in time/power consumption. Well, I didn't finish the test, because the diode overheated and now there is the smell of burned electronics in the whole house But here is what I noticed: At the beginning, the power consumption was 980W, as predicted. But at some point it suddenly almost doubled to 1850W. By mid-way through the test I could tell the diode was getting HOT, all of the cables were getting warm, and the smell was getting more intense. When I decided to end the test, just as I turned the kettle off, there was a -CLICK- from the diode. Since I finished early, the water didn't boil, but it did get pretty hot. I used 49Wh during the 2.5 minutes it was on.

So, I guess that the diode trick won't work in my case (too much power?), maybe I should use a bigger diode (at least 20A)? Or just give up, and maybe buy a smaller kettle (1.2 liters, 1200W)? What would be ideal for me is a kettle with two heating elements, say 800W and 1200W, which can be turned on and off independently, but they don't make those

Enough of me, this thread is for any info on using an in-line diode to lower power usage. Has anyone had more luck than I did?

[Daox]

I understand why AC Hacker did his mod to the rice cooker. The heated thermal mass continues to cook the rice after heat has been removed.

If you are simply heating water for tea (or whatever), I think it would actually be more efficient to keep it heating faster. You need a set amount of power to get water from X temp to Y temp. The longer it takes to get from X to Y, the more heat is going to be lost. So, unless I'm missing something, keeping it at 2000W should be more efficient.

[Piwoslaw]

Originally Posted by Daox

If you are simply heating water for tea (or whatever), I think it would actually be more efficient to keep it heating faster. You need a set amount of power to get water from X temp to Y temp. The longer it takes to get from X to Y, the more heat is going to be lost. So, unless I'm missing something, keeping it at 2000W should be more efficient.

You're right, I overlooked the difference between heating something and just keeping it warm. The only reason to lower the wattage that I can think of now is to keep the current down, since more current = more resistance = more losses. Most of those losses will be on the utilities' side of the meter, but globally they are still losses.

[AC_Hacker]

Piwoslaw,

Sorry to hear about your diode, I can certainly understand how the dreadful smell of burning electronics would interfere with the simple pleasure of drinking tea.

I've thought about the rice-cooker experiment, and I realize that what small improvement I may have made, will not likely economically balance the materials and labor I invested. But the improvement in knowledge, and the fact that I actually acted to try to make an improvement was well worth the investment.

Semi-conductors fail as a short circuit, and the silicon changes from being a semi-conductor, into a molten conductor. I'm surprised by what you reported, "all of the cables were getting warm", I think the cables probably get warm every time you use the kettle... am I mistaken here?

I assume that you are using a teapot that has the heating element in the water, rather than putting water in a pot and putting the pot on a burner on top of the stove, or a hot-plate.

Yeah, I think Daox is right. You have a certain amount of work to do to boil water, and whether you do the work slowly or quickly, the same amount of work is required.

Just brainstorming here, but some approaches I can think of:

1. Start with warmer water. Leave the water you plan to make tea from in a container, maybe in the sun.

2. Heat up the minimum amount of water you need. This is where a microwave could be useful.

3. Use a different fluid with a lower specific heat. But let's face it, for making tea, water has few substitutes.

4. Capture the heat lost in heating. If your pot is on top of a heating element, there is heat loss around the sides of the pot. Possibilities here.

5. Use a less expensive form of heat.

a. A mini solar batch heater for preheat.
b. An alcohol or gas heater
c. A heat pump pre-heater.

6. Burn 'junk mail' to make tea. I don't know if Poland has the same junk mail problem as the US, but perhaps you could make a small stove for this purpose.

By the way Piwoslaw, I seem to recall that some time back, you said that you had or could borrow a book on DIY radiant floor heating. If I am correct, was there anything in that book about building up a layered radiant floor from materials like sheet rock, etc. such as Maxis did, rather than using concrete?

Best Regards,

-AC_Hacker

[Xringer]

About 40 years ago, my wife got me a little reading lamp for night time reading.
It clamps on the headboard of the bed.

There was a problem. It was WAY too bright! So, I took it into my ham shack
and installed a little silicon diode in-line with the 120 AC.

That little lamp has been working since 1968 and has never needed a new bulb..
But, it's only used a few hours a week.

[Piwoslaw]

Originally Posted by AC_Hacker

Just brainstorming here, but some approaches I can think of:

1. Start with warmer water. Leave the water you plan to make tea from in a container, maybe in the sun.

2. Heat up the minimum amount of water you need. This is where a microwave could be useful.

3. Use a different fluid with a lower specific heat. But let's face it, for making tea, water has few substitutes.

4. Capture the heat lost in heating. If your pot is on top of a heating element, there is heat loss around the sides of the pot. Possibilities here.

5. Use a less expensive form of heat.

a. A mini solar batch heater for preheat.
b. An alcohol or gas heater
c. A heat pump pre-heater.

6. Burn 'junk mail' to make tea. I don't know if Poland has the same junk mail problem as the US, but perhaps you could make a small stove for this purpose.

Most of these points were covered in my test (here is the link). Turns out that an electric kettle is more efficient than microwaving (almost twice as efficient), while insulating hardly helps any. I have a jug of water which holds tap water until it gets to room temperature (18*C, compared to 5-8*C straight out of the faucet).

Originally Posted by AC_Hacker

By the way Piwoslaw, I seem to recall that some time back, you said that you had or could borrow a book on DIY radiant floor heating. If I am correct, was there anything in that book about building up a layered radiant floor from materials like sheet rock, etc. such as Maxis did, rather than using concrete?

That was in your Homemade Heat Pump Manifesto thread over at EcoModder.com, but the book was about heat pumps and solar collectors. There wasn't too much about floors, mostly that the large radiant area and lower temperatures in hydronic floor heating helps boost the efficiency of a heat pump system. The author told me that 8 meters of pipes per square meter of floor area is optimal (for our climate and buildings, I guess). Actaully, I have two books on this topic, or had b/c a friend borrowed one to experiment at his summer house. I believe he's been lucky with installing the heating system there, since his wife told me recently that he's planning on spending the winter in that summer house... So I don't know when I'll see my book again

[NiHaoMike]

a quiz about AC power - diyAudio

Does the utility wattmeter register the DC component of the power used? (probably not) Someone will have to turn off everything in their house to measure the meter rotation speed with the diode in and out.
...
No!
...
Take the extreme example: If our diode-equipped soldering iron was one of 1000 in a large factory, all connected in the same way, the facility would be consuming a considerable current which could not be registered by the eddy-current motor driving a standard consumer meter, which responds only to the in-phase AC component).
To put it another way: as you say, high I/V phase angle decreases the work done by, in this case, the meter, but the unregistered power is still available for space-heating or whatever, free of charge.

[Piwoslaw]

I bought a 500W travel heating coil for boiling water and did a test today: It took about 3 times longer to boil 0.5 liter of water than in the 2000W electric jug, and used the same 58Wh. So I'll stick to the jug, unless I need a small amount of hot water (the jug has a minimum of 0.5 liter).

[AC_Hacker]

Originally Posted by Piwoslaw

I bought a 500W travel heating coil for boiling water and did a test today.

Is the coil immersed in the water?

Also, it might be interesting to see how much power is consumed when you heat up a cup of water, that is assuming that you make tea a cup at a time, rather than make it a pot at a time.

My guess is that it will take <30 Wh per cup of tea...

Best Regards,

-AC_Hacker

[Piwoslaw]

Originally Posted by AC_Hacker

Is the coil immersed in the water?

All the way.

The reason I used 500ml instead of a 200-250ml cup is because the previous tests were with that volume (it's the minimum for the electric kettle). In the original test I wrapped a fleece blanket around the jug to see if insulating would change anything, but the time it took to boil was so short that no considerable amount of heat could escape. On the other hand, boiling with the 500W coil took almost 15 minutes, so insulation would help there. But when heating a small cup the time would be down to 4-6 minutes again.

Now for something totally different: Would the diode trick work on an electric motor? The 100W circulation pump in our central heating furnace is a little oversized (as is the 27kW furnace, 18-20kW would be much more efficient). I'm wondering whether an inline diode would slow it down, reduce it's power draw and by how much? A slower pump would also be much quieter. Unfortunately this one is welded into the furnace's plumbing so no easy way to just replace it with something more efficient My Grandma's furnace has a pump which is not only replaceable, but has 3 different speed/power settings.