Pushing the limits of electric cooking

[Na Mara]

Induction stove also have ceramic glass tops that get hot it’s just that the heat goes down from the pan rather than up from the element. Either way, it’s wasted heat.

A 1800W hob on full bore on a resistance coil stove will heat water at about the same rate as a 1200W induction hob. If you cooked like that for an hour a day you would use an extra 0.6kWh of energy a day. But no one cooks like that.

Indeed, resistance coils have one small advantage over induction stoves when used away from a mains supply. If you have an 1200W induction hob on at half power what you get is a 1200W cycled on and off so that it is on half the time and off the other half so that in any hour it uses 0.6kWh. A 1800 W resistance coil on half power instead is 900W on all the time using 0.9kWh over a hour. This is easier on a boat’s inverter despite using more energy overall to achieve the same effect. My 2.4kWh inverter can easily run two 1.8kW resistance hobs at half power, it can only run a single 1.8kW induction hob at any power level.

[s/v Jedi]

Originally Posted by Na Mara

Induction stove also have ceramic glass tops that get hot it’s just that the heat goes down from the pan rather than up from the element. Either way, it’s wasted heat.

A 1800W hob on full bore on a resistance coil stove will heat water at about the same rate as a 1200W induction hob. If you cooked like that for an hour a day you would use an extra 0.6kWh of energy a day. But no one cooks like that.

Indeed, resistance coils have one small advantage over induction stoves when used away from a mains supply. If you have an 1200W induction hob on at half power what you get is a 1200W cycled on and off so that it is on half the time and off the other half so that in any hour it uses 0.6kWh. A 1800 W resistance coil on half power instead is 900W on all the time using 0.9kWh over a hour. This is easier on a boat’s inverter despite using more energy overall to achieve the same effect. My 2.4kWh inverter can easily run two 1.8kW resistance hobs at half power, it can only run a single 1.8kW induction hob at any power level.

I’m sorry but this isn’t correct. In the case of a ceramic cooktop, the heat is transferred through the ceramic, which doesn’t happen with induction, which heats the iron core inside the pan bottom, or the whole pan if it’s iron. This difference is what causes the 30% difference in energy transfer efficiency.

The effects that I pointed out occur mostly before cooking. For ceramic the cooktop needs to be preheated before it can transfer the energy while induction using magnetic waves which travel through the glass top without delay. This uses a significant amount of energy before cooking even starts.

The time of induction cooktops that cycle full power on/off is long past. I don’t think one of those has been manufactured since 10 years ago. I have tested and reported power levels and duty cycling of a number of popular cooktops and reported the findings here on CF. Many of the cheaper ones have 6 real power levels and some of the professional models have 20 real power levels and add cycling of these real levels to create up to a hundred levels that all work great. I have one 3kW Cooktek unit ($$$) that has 100 real power levels. As soon as the manufacturer can afford to put a high power PWM module in, you get as many real power levels as buttons and display support.

Here is a thread from my “all electric galley” series where I introduce the induction cooktop: https://www.cruisersforum.com/forums...ps-235464.html

[hpeer]

FWIW
I have 2 each 310W panels.
I am laying in Caribbean, about ,18N.

Over the past week, the winter solstice, one panel averaged 465W, the other 400W.

The lower panel has some shadow from a wind generator.

Most days I am reaching float sometime in mid afternoon. So there is some additional capacity to absorb generated power.

And the wind generator is adding to the banks, guess 3A average, hard to tell.

Under better conditions I have seen the panels produce a little over 1kW each in a day. But far from typical.

We don't use a lot of electric power.

Otherwise, I was talking to a pilot who worked for a very rich guy. The rich guys boat was in St Thomas, they tried to move it to St Barts but had to turn back. Among other things which broke was the glass range top. That was not a storm, just normal weather. 145 foot motor yacht.

[Na Mara]

Om not arguing that induction is not more efficient. It is, but you are incorrect about there not being thermal losses from induction.

You are correct that the magnetic field produced by the induction coil sets up circulating current in the base of the pan effectively turning it into the heating element. That then heats everything in contact with it, both the food in the pan and the ceramic top. After a few minutes the pan , top and meal are all in as much thermal equilibrium as if you used a resistive heater under the counter top. As you rightly state the big efficiency gain is in the first few minutes where the induction cooker is already heating the meal while the resistive heater is raising the top and pan to temp. However there are inefficiencies with both systems. I believe the efficiencies are roughly 75% for microwave, 55% induction, 35% resistance heater, 25% convection oven or something like that.

[Na Mara]

Something is not right there hpeer.

620W is about 3m^2 of panels. In the Caribbean there should be 5kWh of incident solar irradiation per m^2 on average. You should have 15kWh a day incident on your panels per day on average. If you are at most converting only 1kWh of that into electrical energy then your efficiency is 7% or less. Typical solar panel efficiency is 15-20%. Something is far from optimal in your setup, if you don’t mind me saying.

[hpeer]

On a clear day, after running the inverter for tools (batteries low), near Summer high, zero wind, my power production is more than double what I have now, about in the middle of your estimate. So if there was a problem with my system they could not do that.

But here and now……

1. Over winter solstice, minimum hours of sun and poor angle. 11 hours above horizon, never reaches zenith.
2. Cloud cover, yes even here we have white puffy clouds that shade the panels.
3. The batteries are taking all they can choke down, we just don’t use that much. 4 each GC-2 in a 12V bank. You can’t fill a bowl over the brim.
4. The wind generator also feeds the batteries, so that depletes the “empty space” for more electron to go into the battery.

2 panels on separate identical MPPT controllers.

You say I should get 15% to 20% ON AVERAGE. But I am getting 7%, so 1/2 to 1/3 of AVERAGE.

So to go over this another way.

This is not an average week, it is solstice week.
Your average is available incidence, but we have clouds. So the panels are partially shaded by clouds. How much? Don’t know but not insignificant. Maybe 40% shaded giving 50% output.
I do have some shading from wind gen.
The batteries reach FULL, so they will not take any more. And they are partially filled by the wind gen. The panels will produce VOLTAGE but not CURRENT/Wattage if there is no place to put it.

What I am trying to do here is to demonstrate the difference between what we read and what we experience. There are many factors effecting solar power PRODUCTION.

If I had big thirsty batteries who would suck up all the juice I could provide the numbers would be different. I don’t have those big batteries because I cover normal usage without them. So I am good and happy.

Your experience with electrical cooking and a different battery set up in a different location will be different. Sometimes but seldom perfect. Don’t be surprised if you have 50% of expected averages some days.

The question is “Will I have ENOUGH? What if I don’t?”

[Adelie]

Originally Posted by Na Mara

….
After a few minutes the pan , top and meal are all in as much thermal equilibrium as if you used a resistive heater under the counter top. As you rightly state the big efficiency gain is in the first few minutes where the induction cooker is already heating the meal while the resistive heater is raising the top and pan to temp. However there are inefficiencies with both systems. I believe the efficiencies are roughly 75% for microwave, 55% induction, 35% resistance heater, 25% convection oven or something like that.

The resistive and induction hobs will not have equivalent efficiencies after a few minutes.

The resistive element is losing heat down and pushing it up thru the ceramic top and the pan bottom which both are both slight insulators and which conduct some heat laterally to the air before the heat gets to the food. It needs to run slightly hotter to get the same amount of heat to the food. Which means it’s losing even more heat down.

The induction heats the pan so there is less energy needed to provide the same heat to the food. Going down the bottom of the pan heats the glass which provides some insulation to whatever is below that.

I have not checked the efficiency of a resistive burner but I did for induction, butane and microwaves.

Butane: 50%
Microwave: 46%
Induction: 75%

For the microwave & induction I measured electric use to the nearest Watt-hr.
For butane I measured the weight of the stove and fuel to the nearest gram.
I weighed the water to the nearest gram and starting and ending temps (75-190F)


If I can find a cheap countertop resistive burner I’ll test that.

I’m gearing up to test a propane oven vs a convection oven.

[Na Mara]

I fear we may both have been talking out of our hats but me more so. According to energy star a resistance based electric stove is 75-80% efficient at transferring electrical energy to food whereas an induction stove is 85% efficient. I have seen figures elsewhere for induction up to 90%.

So a 1800 induction hob will deliver 1.62kWh of heat to thre contents of a pan in every hour for 1.8kWh expended whereas a similarly powered resistance hob will maybe deliver 1.44kWh in an hour. I can live with 8/10 efficiency rather than 9/10.

It would have been great if there existed a gimbaling electric cooker 45cm in width with in built combi microwave convection oven WITH induction, but that product doesn’t exist so I settled for a little less hob efficiency to get everything else.

You are correct though Adeline that microwaves are less efficient than either(about 50%).

https://www.energystar.gov/about/202...n_cooking_tops

Out of interest how are you testing the efficiencies of these appliances? What’s the rig like?

[Adelie]

Originally Posted by Na Mara

I fear we may both have been talking out of our hats but me more so. According to energy star a resistance based electric stove is 75-80% efficient at transferring electrical energy to food whereas an induction stove is 85% efficient. I have seen figures elsewhere for induction up to 90%.

So a 1800 induction hob will deliver 1.62kWh of heat to thre contents of a pan in every hour for 1.8kWh expended whereas a similarly powered resistance hob will maybe deliver 1.44kWh in an hour. I can live with 8/10 efficiency rather than 9/10.

It would have been great if there existed a gimbaling electric cooker 45cm in width with in built combi microwave convection oven WITH induction, but that product doesn’t exist so I settled for a little less hob efficiency to get everything else.

You are correct though Adeline that microwaves are less efficient than either(about 50%).

https://www.energystar.gov/about/202...n_cooking_tops

Out of interest how are you testing the efficiencies of these appliances? What’s the rig like?

All of the comparison testing I could find used burner ratings and time for the input side, used water measured using a cup measurer, the water temps were not measured, start temp was whatever came out of the tap and end state was when the kettle whistled which meant that steam rising off the water before it whistled was not accounted for, the longer it took to get to boiling the more energy lost to steam.

Using a burner rating doesn’t tell you how much electricity or gas was really used, the ratings are approximate at best. Burner ratings are subject to fluffing by marketing and to rounding to a nice round number like 1500W instead of 1377W.

This is why I did my own testing, what was published was very poorly done, adequate for a qualitative assessment but not a quantitative one.

[StuM]

Originally Posted by hpeer

FWIW
I have 2 each 310W panels.
I am laying in Caribbean, about ,18N.

Over the past week, the winter solstice, one panel averaged 465W, the other 400W.

A meaningless statement!
This is why I keep on going on about units!
They averaged 465/400W in what time frames?

Is that the average of the highest peak output each day?
Or is that average over a standard time period.

A combined average of 432.5W ((465+400)/2) over the peak period 2hrs either side of noon would be possibly a total of around 2500 Wh per day.

An average of 432.5W over approximately 10 hours of daylight is 4,325 Wh per day.
An average of 432.5W in each 24 hour period is 10,325 Whper day.

(Or do you mean 465 Wh + 400 Wh = 865 Wh total per day.
Which would suggest maybe 30-40% of expected ouitput for 610 W at that latitude at that time of year in good conditions.)

[Jdege]

Originally Posted by StuM

A meaningless statement!
This is why I keep on going on about units!
They averaged 465/400W in what time frames?

When I've pointed out inconsistent units I've been called a pedant.

Perhaps so, but getting the units to at least have the correct dimensions is necessary to having a meaningful conversation.

[StuM]

Originally Posted by Jdege

When I've pointed out inconsistent units I've been called a pedant.

Perhaps so, but getting the units to at least have the correct dimensions is necessary to having a meaningful conversation.

Join the club. I'm frequently called a pedant (and worse) by posters who get their units wrong. OTOH, I receive quite a few thanks from other posters who recognise the problem.
I'll continue to point out the issues:
http://www.cruisersforum.com/forums/...ml#post1933764

[Jdege]

Originally Posted by StuM

Join the club. I'm frequently called a pedant (and worse) by posters who get their units wrong.

I'd admit to being a pedant if I were insisting on Coulombs and Joules, instead of Amp-hours and Watt-hours.

[Adelie]

People here are constantly discussing energy flow and current flow and mixing up the units and confusing the discussion.

[Na Mara]

I’ve given up on correcting people. Now I just translate what they have written into its least idiotic form and reply to that. Hopefully they do same for me.