Induction Water Heaters: Are They Worth It?

[BWV]

Was curious if induction water heaters were a thing and if they had any benefits over a traditional electric resistance coil. On quora someone argues that a resistance coil is a near perfect conversion of electricity to heat, therefore no benefit to induction - is this correct? Is the efficiency gains in induction cooktops solely due a more to less waste heat between the coil and pan than with a resistance coil? But then if induction transfers heat more efficiently to the metal surface on a cooktop, why not in a water heater?

 

[DaveE]

In almost every water heater I every knew about the heater coil is completely submerged in the water. There is nowhere for the heat to go except into the water. That isn't the same as a cooktop. I fail to see any advantage to the greater complexity of induction heating compared to a submerged resistive heater. Someone on quora was correct, resistors are the simplest, most efficient way to convert electricity to heat.

 

[Bystander]

Induction stoves/cooktops are safer as far as burning/cooking small children.

 

[BWV]

They boil water faster, have a small induction hot plate alongside my gas cooktop that I use for pasta

 

[BWV]

In almost every water heater I every knew about the heater coil is completely submerged in the water. There is nowhere for the heat to go except into the water. That isn't the same as a cooktop. I fail to see any advantage to the greater complexity of induction heating compared to a submerged resistive heater. Someone on quora was correct, resistors are the simplest, most efficient way to convert electricity to heat.

Makes sense, thanks

 

[zoki85]

A jet-type electrode boilers are probably the most efficient

 

[Baluncore]

Someone on quora was correct, resistors are the simplest, most efficient way to convert electricity to heat.

Resistive elements are simple, but not the most efficient.
A heat pump is more complex, but it is significantly more efficient than a resistive element.

 

[russ_watters]

Resistive elements are simple, but not the most efficient.
A heat pump is more complex, but it is significantly more efficient than a resistive element.

I don't think is pedantic in this case to point out that a heat pump does more than just "convert electricity to heat". As such it is not always interchangeable with devices that do just that. E.G., an electric resistance, induction and even a gas range are functionally equivalent, but there is no such thing as a heat pump range. Heat pump water heaters and "boilers" do exist, but have requirements/capabilities and limitations that do not constrain electric resistance or gas devices.

 

[Nik_2213]

Induction stoves/cooktops are safer as far as burning/cooking small children.

Unless you are wearing something that will 'respond' to induction field and carry eddy currents ??

 

[Baluncore]

... there is no such thing as a heat pump range ...

I did not think the kitchen range was an important consideration as the title of the thread was “Induction more efficient than resistance for a water heater?”
Heating water is one thing that a heat pump does with over-unity efficiency.

 

[russ_watters]

I did not think the kitchen range was an important consideration as the title of the thread was “Induction more efficient than resistance for a water heater?”

The OP asks specifically about the differences between ranges and water heaters that might make one work for a range and not a water heater. The opposite can be considered as well.

There is also overlap of course in that if you are using a range to boil water, you can do half of the heating with a water heater or all of the heating on the range.

But all that said, I've never seen a heat pump water heater in a residential setting and only once commercially*, so even then it isn't exactly equivalent to the others, which is why I pointed it out. One big potential downside is they need an external heat source. Heck, it is even common commercially for an a water to air heat pump to be sourced from an electric boiler!

*Er; the commercial one was hot water for heating, not drinking. Residentially, checking what I think are the top 2 manufacturers, both offer air source heat pump water heaters. In the summer they provide free air conditioning and in the winter their source is whatever you use to heat your house. That has to be considered in the analysis.

 

[DaveE]

Resistive elements are simple, but not the most efficient.
A heat pump is more complex, but it is significantly more efficient than a resistive element.

I guess your point is that heat pumps make better water heaters than resistive heaters. That certainly may be the case, depending on your requirements and the specs of each device (i.e. what sort of water heater you like).

However, I'll stand by my claim that resistors are the most efficient way to convert electricity to heat. In fact they are 100% efficient, the proof is so short it's essentially the definition of what a resistor is.

 

[Baluncore]

However, I'll stand by my claim that resistors are the most efficient way to convert electricity to heat. In fact they are 100% efficient, the proof is so short it's essentially the definition of what a resistor is.

You are ignoring the fact that a heat pump can be 200% efficient, when it requires only half the energy it pumps into the water.
https://en.wikipedia.org/wiki/Heat_pump

 

[DaveE]

You are ignoring the fact that a heat pump can be 200% efficient, when it requires only half the energy it pumps into the water.

Yes, exactly. I am talking about resistors. You are not.

You are talking about the "efficiency" of a more complex system. However, not a closed system, since "over-unity" efficiency doesn't exist for those. That sort of definition of efficiency makes more sense in the building trades than in a physics forum. After all, we are only paying for the electricity inputs, and we only care about the hot water that comes out. Why measure the stuff you don't care about.

 

[Baluncore]

However, I'll stand by my claim that resistors are the most efficient way to convert electricity to heat.

That is not physics. Electricity is not converted into energy. Electricity can be used to transfer energy from a generator to a load.

 

[Tom.G]

If using only electricity as an energy source, then of course a resistor is the most efficient.

However, if there is ambient energy available then a heat pump will have a higher electrical efficiency, but not a higher system efficiency. That 200% quoted above is obtained by dividing the output energy by the electrical input energy; the energy extracted from the ambient environment is ignored in that calculation. If you but put a closed box around the heat pump input stream, blocking the ambient energy, you get a refrigerator.

Additional note: Heat pumps used for residential living space in cooler climates are generally "Electrically Assisted Heat Pumps." This means there is a resistance heater that comes on when the outdoor temperature is so low that not enough thermal energy can be extracted from the ambient to satisfy the heating needs. The expensive units can operate down to ≈36°F, 2°C.

Here are a couple sites that may help:
An introductory article.
https://www.energy.gov/energysaver/water-heating/heat-pump-water-heaters

Experimental results of some installations.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770007641.pdf

(above found with:
https://www.google.com/search?&q=electric+assisted+heat+pump)

Cheers,
Tom

 

[sophiecentaur]

Unless you are wearing something that will 'respond' to induction field and carry eddy currents ??

Interesting idea.
Most jewellery is made of metals with high conductivity and such items are small, so the I²R heating would be small. I could imagine a 'retro' iron bangle could be a problem but even that would probably have too small an area to link to the induction field and would be too far away from the induction coil.

 

[artis]

@sophiecentaur well I think unless someone is a hardcore metal chain fan I think induction cooking is safe.

I totally get @Baluncore suggestion but a resistor still is the most efficient electrical device ever, it's just that a heat pump is an unfair player in this comparison due to the fact that it only uses input electricity to move a heat exchanger fluid/gas but doesn't produce it's own heat, and even if it did produce it's own heat it would lose to a simple resistor because an electric motor is not 100% efficient.
But if we simply compare two sorts of heating say a water boiler with a induction water heater , even though a boiler uses a resistor for heating which by itself is 100% efficient and the switching transistors and their drive circuits in the induction heater are not 100% efficient we must consider one additional issue. The induction heater is only working when water is needed, the boiler on the other hand is a reservoir and so keeps it's water at a certain temperature constantly, unlike a resistor, the efficiency of the boiler's heat insulation is not 100% so some heat continually escapes and this is why the boiler switches on after certain times to constantly reheat the water even if the water isn't used.

So I would say unless you constantly use the boiler the induction heater might get a lower bill even not being 100% efficient simply because it's only running when used instead of running constantly even for days when out of direct use.
I myself have a water boiler so i know this. In fact I am thinking about trying out an induction heater for comparison.

 

[Baluncore]

Real electrical power is the product of voltage and in-phase current. The ratio of voltage to current is defined as resistance. The power is; W =I²R.

The product of voltage and reactive current, VAR, is not real power or heat. An induction heater induces currents in a resistive material, that dissipates; W =I²R. The word inductance refers to the method of coupling, not to the element of heat production.

An induction heater requires high frequency AC be generated and delivered to the inductive primary winding element. Likewise, a resistor needs two conductors to guide the energy to the resistive element. Those conductors will also have I²R losses which reduce the system efficiency of the resistive element to below 100%.

As can be seen, it is an unfair comparison to consider only one component of a complex system, especially when by definition, that is resistance in both cases. For heating water, when the entire system is considered, a heat pump will outperform both the resistor and the more complex, less efficient HF induction heater.

Peltier thermoelectric coolers can be about 75% efficient, significantly less than a typical refrigerator. But a Peltier effect thermoelectric heater can be over 100% efficient and so might be considered as a possible replacement for a resistor when heating water.
https://scholarworks.uark.edu/cgi/viewcontent.cgi?article=1010&context=meeguht

 

[artis]

@Baluncore if I understand correctly these "Peltier" thermoelectric heat pumps basically get rid of the motor and fluid and all that and instead work on the principle of surrounding heat driving current through a junction , so basically you put some DC current through the junction from your power source and you get back that current plus additional electric current which is the result of the ambient heat energy converted to electricity?As for the wires going to the resistive heating element , given their good quality copper and sufficient AVG I'd say those losses are tiny, and every method will use wires , the induction heater as well as any heat pump.
I do realize the induction heater is so named because of the way the field interacts with the object to be heated which most likely will be a ferrous metal pipe so the water will be heated mostly through heat give off by the hot pipe.
I think that if one needs hot water quickly but not on a regular basis , the induction heating method would still be most economical simply due to being "on demand" while a boiler has to be kept on at all times, even if I don't use the hot water for a week.

Surely I agree that as far as electricity bill goes if one can simply scavenge energy/heat from the environment then the bill most likely will be lower.

PS. I do wonder how big the heatsink has to be even for the solid state heat pumps for given kw of energy provided, after all these are all costs of both manufacturing, transporting etc.

 

[DaveE]

I do wonder how big the heatsink has to be even for the solid state heat pumps for given kw of energy provided

Peltier devices are VERY non-linear in their thermal behavior. The amount of heat pumped is a strong function of the temperature difference between the hot and cold side. So the thermal design is important. For a heating application, if the cold side heat removal isn't appropriate then the large temperature difference will severely impact the heat pumping efficiency. However, in any case, it will still work like a resistor with the electrical power in making heat in the device.

For smaller high performance cooling designs (i.e. too expensive for the building trades), a good rule of thumb is that good designs will have the amount of heat pumped approximately equal to the power input, so for 1W of electricity in you will have about 1W of heat pumped from the cool side and 2W of heat dissipated at the hot side. However, there is a wide range of "optimal" designs depending on the system requirements. Commercial designs will drive the TEC harder to save money.

IMO opinion, initial cost and reliability considerations make solid state designs less attractive than mechanical designs assuming you can tolerate pumps, refrigerants, etc. After all, how many of your friends have solid state refrigerators in their kitchens?

For example, you can look at the TEC performance curves from the manufacturers, like the ones on this page:
https://www.marlow.com/resources/thermoelectric-technology-guide/iii-tec-selection-procedure

 

[sophiecentaur]

a resistor still is the most efficient electrical device eve

Agreed. However, the thread has started to discuss whole systems and, as tends to happen on PF, we've diverged and diverged. The only thing about a resistor is that you need to provide it with Electricity and that means that someone has to Generate it. A small, inefficient home generator and resistive heater would probably not give you a better performance than just burning the fuel in a hot water boiler.
Engineering is about systems and not components. But this is a fun topic to discuss and to bandy about various figures from the top of our heads.

 

[Baluncore]

The unconditional statement that “a resistor is 100% efficient and therefore cannot be beaten” restricts the consideration of more efficient engineering solutions.

I think I have made my point that “if you want to heat water, there are more efficient ways than using a simple resistor”.

 

[alan123hk]

Regarding the ways to make water boil, according to my personal experience, electric kettles have the highest efficiency, followed by induction cookers, and the least efficient are ceramic infrared cookers.

 

[artis]

I think @Baluncore that we could both agree that for someone who doesn't have a private house but instead lives in a crammed up highrise in his apartment in say the 9th floor his only real option are those that give power from a concentrated source and the only such real source I think is the wall socket, so here a resistor is still the king.
Now I know people use their AC units for heating in winters as an AC is essentially a heat pump, so for a highrise that's maybe the closest to a heat pump one can get.
But I doubt you would heat water with such a method.Well what I would like to know personally , with respect to the TEC Peltier devices, how much space they take up for given power? Because if what I know about the thermoelectric effect in general is true is that it doesn't have a particularly high energy density. I would love to hear comments on this.

 

[Baluncore]

I think Baluncore that we could both agree that for someone who doesn't have a private house but instead lives in a crammed up highrise in his apartment in say the 9th floor his only real option are those that give power from a concentrated source and the only such real source I think is the wall socket, so here a resistor is still the king.

I am sorry that you must live in such inconvenient accommodation, but it should not prevent creativity.

No doubt your neighbours use resistors to heat their apartment in winter. So why do you not install an equally spaced array of Peltier cells over a shared wall and pump heat from their apartment into yours? Put heatsinks with fins on your side of the Peltier elements. Insulate the rest of the wall with 3mm of foam or cork. I would call that active insulation, something you can't do with resistors.

 

[sophiecentaur]

I would call that active insulation, something you can't do with resistors.

Yeah. Engineering rules!

 

[artis]

@Baluncore well your suggestion is very interesting :D
I myself actually don't live in a highrise and not on 9th floor instead I live in a 4 story building and on the 2nd floor but still in a flat, as most such buildings around here we have centralized heating so there are no resistors involved, just hot water going through radiator pipes.

By the way I doubt you could get any good convection through a concrete or brick masonry wall.

 

[Baluncore]

just hot water going through radiator pipes.

How is that water heated? It seems like an ideal application for a heat pump.

By the way I doubt you could get any good convection through a concrete or brick masonry wall.

That is why the full area of the wall needs to be used, with insulation on the rest of the wall.

 

[artis]

the water is heated in a large 10MW central heating station run on recycled wood chips, basically due to EU regulations everything now here is run on either leftover wood chips or natural gas with few exceptions of coal and oil shale.