[AndroidNeox (OP)]

I wonder if it might be possible to efficiently and quickly heat water by injecting microwaves into a water pipe. I presume it would work. Copper pipe would make an excellent waveguide containing the energy within the flowing water. The amount of energy injected could be varied to provide any selected temperature. If this worked, it wouldn't be necessary to plumb hot and cold water lines individually, but just heat the water to the desired temperature just before it comes out of the tap.

[CliffordK]

It takes me about a minute and a half to heat up a cup of water in the microwave.  The microwave shower may not be too exciting.

So, you would have to have a powerful microwave for the system to be effective. 

One would assume that one would water-cool the electrical components in the microwave, and thus the efficiency might be similar to other electric water heaters, except for heat pump or solar systems.

That being said, there may not be a large benefit over other "tankless" water heaters with resistance electrical heat.  For the electrical tankless systems, they take a LOT of power, perhaps 240V, at 80A to 120A, but, of course, only while you are using the water.

Your microwave system, may in fact, be much more complicated than a resistance hot water heater, with no benefit.

Efficiency, would be based on heat that escapes (doesn't get into the water), and how much residual heat is in the system when the water is shut off.

Would there be any benefit of the microwave with heating less water?

[RD]

Comparing immersion heater and magnetron which consume the same amount of electrical power, (i.e. have the same power rating), the immersion heater will deliver more of that energy to the water, as the magnetron will waste some of its power consumption producing heat which will not be transferred to the water.

... an efficiency of around 65%

https://en.wikipedia.org/wiki/Cavity_magnetron

So yes it's possible, but not an advantage over the existing technology: microwaves wouldn't heat water faster than an immersion heater with the same power consumption, (quite the reverse), and the increased complexity of the apparatus would make a microwave-water-heater more expensive to manufacture.   

[CliffordK]

Your kitchen microwave is air cooled (the fan you hear running).

However, if you were using it to heat water, you would likely do it water cooled.  Perhaps run the cold water first around the coil, then through the microwaves.  So, the actual losses due to heating the microwave generator would be minimized.  \

However, the downsides still remain.
No benefit in speed of heating the water over a similarly powered tankless water heater.
No benefit in system efficiency.
Added complexity (and thus expense).

[techmind]

There is a widespread, but somewhat misguided, belief that microwaves have some magical efficiency.
Your typical domestic microwave oven will likely be rated at about 1200W electrical input for 750W microwave output (consistent with the "65%" efficiency Wiki link that RD provided).
In the oven scenario, it is a lot more efficient than a conventional oven - for heating small amounts of food... because you don't have to heat all the thermal mass of the oven itself as you would in a conventional oven.

For heating water however, a basic immersion heater (like a kettle element) is very simple and cheap and practically 100% efficient.

Leaving aside the microwaves, what the original poster (AndroidNeox) describes is an on-demand water heater system - very similar to the electric showers that some people have here in the UK.
The issue, as Clifford K points out, is that you need very high power for such heaters. UK/European electric showers manage about 9-11kW (at 240V, 45amps!) yet still only achieve a fairly modest flow rate. You probably would want almost twice that for a decent flow-rate hot-water tap. In fact, internally, an electric shower often only has a handful of discrete power-settings, but performs fine control of the temperature by adjusting the flow rate!

This presents several problems:
* The control systems are challenging, and may come out cold or scalding hot for several seconds before a stable temperature is achieved. Changes in flow rate caused by water pressure changes from the use of other taps in the house may also cause painful disturbances to the temperature
* The extreme power means you need very thick electrical cables to the heaters, and they need to be on a special electric circuit
* The extreme power means you couldn't operate two such heaters in one house without blowing the company electric fuse (usually rated at 60 or 80amps in the UK).
* The extreme power means that the lights dim noticeably when you use the heater!

With a conventional system, although you lose energy in heating water in pipes that go cold, this helps to heat your house - so (in the winter at least) this energy isn't necessarily wasted.
It's also considerably more efficient (and about one quarter the price in the UK) to heat water directly using a gas boiler. Gas boilers rated at 25kW are adequate for on-demand heating of water in a typical house, with the water heated centrally and distributed via the usual hot and cold pipes.

[CliffordK]

* The extreme power means you couldn't operate two such heaters in one house without blowing the company electric fuse (usually rated at 60 or 80amps in the UK).

Do you still use those screw-in fuses in the UK?

Here, at least most new construction where one doesn't have natural gas has 200A mains.
If one has natural gas available, 100A mains may be common in some situations.

My house has some kind of an odd configuration with twin 200A, 240V mains (4 switches), giving one the equivalent of 400A, 240V.  Plus an additional 200A at the pumphouse, I think.  It seems like a little overkill to me. 

Anyway, I should be able to heat more water than I can get out of my well.  []

If available one can go with natural gas or propane tankless water heaters, but probably not suited for "point of use" installation.

I like the idea of an "electric shower".  Most of my hot water usage is in the shower, and I hate waiting for hot water.  In fact, I often wash my hands with cold water since I don't like to wait.

[Bored chemist]

I have't seem screw in fuses used domestically in the UK.
If you really like the idea of an electric shower, why not buy one?

http://search.wickes.co.uk/search#ts=ajax&method=and&w=electric%20shower&isort=score

Other brands/ suppliers are also available.

Incidentally, one possible reason that UK houses typically have "only" 240 * 60 or 80 Watts may be that they are generally smaller than those in the US.
Does anyone know any data on that?

[techmind]

If there's any way of doing it, it is worth trying a particular electric shower before installing it, as the quality of temperature-regulation varies considerably.
We used to have a Mira at the place I used to rent, and it was the Rolls Royce of (electric) showers, really well made (yes, I'd had a peek inside it!), and never scalded. On the other hand, I've used slightly newer Mira's elsewhere (possibly lower-end models?) which haven't been anywhere near as good, with a long cold- followed by a long scalding-phase before they stabilise. Try before you buy if at all possible.

And another tip: the "low (water) pressure" warning actually means the differential (input-output) pressure is too low. As well as being triggered by too low mains-water pressure, it also comes on if the shower-head needs de-scaling!

For Clifford K, an alternative way of avoiding the wait for the water to get hot is to modify the plumbing. An approach we found in my parents' new house (and I've since seen in public buildings) is to have pipework which periodically circulates hot water in the pipes and back to the hot water tank/heat exchanger, assuming you have one. (This needs an extra 'central-heating'-type pump, and I can't see an easy way to make it work with an on-demand boiler). You then just have a very short branch from the loop to the tap you want to supply...

[RD]

There is a widespread, but somewhat misguided, belief that microwaves have some magical efficiency ...

Hence the Steven Wright joke ... http://www.searchquotes.com/quotation/I_put_instant_coffee_in_a_microwave_oven_and_almost_went_back_in_time./192350/

[RE.Craig]

The idea has some merit but there are dangers associated with heating water in a restricted area with microwave energy, not least of these is nucleation and superheated steam. Most surfaces provide areas were bubbles can form during heating were hot vapours collect and are under normal circumstances released from the heated water in the form of boiling. If water is heated directly under pressure in a pipe it runs a high risk of exploding due to the presence of superheated steam unable to escape via conventional means. It was for this reason that most types of water geysers [that our grannies used to have in their sculleries] were banned the UK. If you go ahead with the project you need to consider thermostatic control and a means of heating the water at atmospheric pressure or heating it in a very heavy pressure vessel not unlike a pressure cooker. It cannot be overstressed how lethal superheated water is.   

[bobm7694]

A Canadian company that I have been watching for years offered an award winning microwave tankless heater.  It is called Pulsar.  The company is located in Florida right now and I do not know if they are still alive. 

As far  as heating water with an electric shower head, I have UL#499 compliant and ETL#834 listed electric shower heads in both 110 volt plugin and 220 volt hard wired to a ground fault breaker.  The larger 220 volt is not yet listed or compliant.  Both have control surfaces for power setting and on/off.  Screw it on and plug it in...the 110volt model offers only 18 degree heat rise at 1 gpm.  The larger 220 volt model offers around 36 degrees, again, at 1 gpm.  And, both use a sealed stainless steel heat exchanger and "flow switch" as their standard tankless cousins.

I assume some of you folks live in Europe.  I know some of the countries over there penalize for electric tankless heaters above a certain kilowatt size.  (Spain...?)  Here in the USA we have units all the way up to 27ikilowatts at 112.5 amp at max.  Here in the Miami area, this 27kilowatt unit offers 7.3 gpm at 105 degrees F from an incoming 80 degree F water flow.  Here in Florida, most of us use a small 12 kilowatt/54 amp unit using 6 ga. wire and a 2P 60 amp breaker with 220 volts.

I like electric tankless heaters since they are inexpensive and easy to install.  However, gas is where it's at.  Using a 199,000 BTU gas tankless heater offers an equivalent of 58.6 kilowatts comparison in an electric tankless heater.  I had one customer in El Paso who used four 27kilowatt units to produce 12 gallons of hot water per minute for his bathroom.  They think big in Texas!  Here in Miami, we use one tankless to replace one tank water heater.  I understand that throughout the rest of the world, multiple point-of-use units are spread around the house as needed.  Is this true?
Bob

[alancalverd]

Here in the Miami area, this 27kilowatt unit offers 7.3 gpm at 105 degrees F from an incoming 80 degree F water flow.

And here in the UK, if the input temperature ever reached 80F we'd assume the fiery apocalypse had arrived. I'd have to install a cooler to wash blankets in the washing machine! But I guess you don't use a lot of wool in Florida...

[McQueen]

Induction heaters are theoretically the most efficient water heaters, providing you use vessels with the right type of material, ferrous is the best.  Induction  heating is a non-contact process and so is very similar to the microwave heating suggested in the OP. The material to be heated is placed within a rapidly fluctuating, high frequency electrical field, so that it starts to behave like the secondary coil in a transformer, generating heat.  If the vessel it is placed in is ferrous, then the vessel contributes to the heat through hysteresis.

[alancalverd]

Alas, no. Induction does not heat water because it is a poor electrical conductor and therefore cannot behave like a transformer coil at all. Try putting a glass beaker of water on an induction plate - nothing happens! Induction cookers heat the iron pan which can then be used for boiling or frying, and in that respect are much more efficient than radiant or flame heaters, but the heat loss from the outside of the pan, and the need to heat the pan as well as the water, makes it overall less efficient as a water boiler than simple immersion heater (99% transfer of electrical energy to water) or a microwave (over 80%).   

[Bored chemist]

I wonder where someone gets the idea that "Induction heaters are theoretically the most efficient water heaters,"
The theoretical efficiency of an immersion heater is 100% and the practical efficiency is pretty close to that.
You could get a pretty similar efficiency from an induction heater as long as you used the incoming water to cool the switching transistors etc.
But it's vastly simpler to use a simple resistive heating element.
In the limit, you can get near 100% efficiency from any of them, but why choose the really complicated, expensive option?

[syhprum]

Microwaves are only useful for heating a small quantity of water to make a hot drink but even then an electric kettle is a better option

[evan_au]

The most efficient water heater would be a reverse-cycle heater, as this can generate more watts out than paid-watts in.
But to reduce the amount of paid-watts, why not have a solar water heater? You need a bit of electricity to heat it up more on cloudy days - of course, this could be most days in a UK winter...

The traditional shower is horribly inefficient - you heat up the water, and then a few seconds later, it goes straight down the drain. If you are interested in efficiency, some recycling of this heat (if not the water) would improve things a lot.

[alancalverd]

If you collect shower water in a holding tank you can recover the heat and use the water to flush the toilet.   

[syhprum]

I doubt if this would be economical although from a energy conserving point of view it would be a good thing but there would be quite a large expenditure for heat pumps and insulated storage tanks.

You could be like we stiff upper lip Brits and shower with 15°C water ! 

[SimpleEngineer]

Some points,

Induction heaters are great because there is no direct contact with the water... immersion heaters are great because they are quite literally what you put in you get out... HOWEVER induction heaters do not lose efficiency over time, however as we all dont live with demineralised water, immersion heaters get fouled and lose efficiency rather rapidly do to the increased insulation due to scale (efficiency/effectiveness)

Fuses.. (I have never actually ever heard of screw in fuses.. might be too young for that ) you might have 200A fuse on your incomer.. but I would shiver to think that is the only fuse you will use.. that will probably feed a circuit breaker with a range of 60-80A 'fuses' (its the same thing really )  that will feed you ring mains with connections at the most 30A (fixed) and 13A portable.. I doubt you plug your tv into the wall directly into a mains that is 'limited' to 200A.. or is this why there are so many electrical fires?

May i question the value of the heat you are going to collect from my shower water please... and exactly how? thats going to be one mammoth heat exchanger with very little tepid fluid to warm up the incomer.. in fact I expect when calcuations are made.. the heat exchanger is going to be of a much greater size than the vessel used to collect my washings.. plus what happens to all the hair?? I wonder if anyone has thought of recovering the heat from the toilets?? Urine must be about 37 degrees give or take