Reducing current taken by an immersion heater

[kai]

Buy an immersion timer, and set it up to give restricted operating times, such as twenty minutes in the morning, and twenty minutes in the evening or something like that, in order to save on energy bills.

 

[consciouspnm]

Buy an immersion timer, and set it up to give restricted operating times, such as twenty minutes in the morning, and twenty minutes in the evening or something like that, in order to save on energy bills.

Nothing to do with saving money on the electricity bill. See previous message.

 

[SimonH2]

I want to dump the excess power from my PV array into the water but I'm not likely to have 3kw spare.

OK, I can see two or three ways it could be done, but whether any of them are practical or actually have equipment available is a different matter.

As others have said, your element is a fixed (for practical calculations) resistance, so there are just two ways to reduce the power put through it - reduce the voltage, or apply the voltage for only part of the time.

So methods I can think of :

1) Use a step down transformer to lower the voltage. The problem you'll have is that your supply and spare power are variable and so you'd need a variable voltage ratio - not easy without either a variable transformer (suck as a Variac) or a lot of taps and a tap changer. So forget about that one.

2) Supply the power for only part of the time. Basically use a "chopper" (which is all most light dimmers are) so you only supply power for part of each half cycle. However your inverter is likely to object and it will create a lot of noise in the mains. Probably not a good option either.

3) Possibly the best technique would be to run a separate inverter with variable output voltage. This would draw DC power from the panels in parallel with the export inverter and feed only the immersion heater. However, I doubt you'll find the systems to do this as you'd need integration between two inverters and monitoring of your import/export current. In essence, the two inverters would have to work out how much each one should take from the DC bus.
With increasing panel output you'd initially want the export inverter to take everything. Once you reach parity with your load, then the second inverter would need to start taking power and heating your water. Finally, if panel output reaches a stage where you've met your own load and the immersion heater is on full, then you'd start exporting the excess.

4) Lastly, I could see scope for running heaters directly from the DC bus. Note that I said heaters - you'd need a selection of resistances that could be switched in as required, or at the very least some very high power transistors to "drop some volts". Designed right, you'd put the excess heat from these into the cylinder as well. I'd be tempted to have a bank of resistances, and switch them in as required with power transistors. Monitor the export current, and ramp up/down the number of resistors in circuit as required - accepting that you'll always have some import or export unless you have a very large number of resistors to switch in (or at least enough variable resistance, heater with transistor not in "on/off" mode, to cover the steps).

Note that with some of these schemes, you'd need a means of safely switching your immersion heater back to the mains should the need arise.

As to not being able to replace your immersion heater, there is a technique for dealing with this - and it's used in some solar thermal setups. But a small vertical cylinder next to the DHW cylinder (or thermal store) connected bottom to bottom and top to top with the main cylinder, and fit the additional heating element(s) into that - for solar thermal setups this would be a plate heat exchanger. As the water heats up, a thermal syphon will circulate the hot water up into the top of the main cylinder and draw in cold from the bottom of the main cylinder.

If you still want to explore this, then you may find the guys over at http://www.navitron.org.uk/forum/ worth talking to. There are some fairly clever people with electronics, experimenting with various things.


Lastly, should you be tempted to start experimenting ...
The voltages involved are dangerous, lethal in the wrong dose. This especially applies to the DC bus from the panels as DC is in some ways inherently more dangerous than AC. Also, while you may have some RCDs that could give some protection from shock from the mains, this will not be the case with the solar PV system.
I really, really would not suggest you experiment without a good knowledge of what's required - and from the question, I suspect you really don't have that knowledge.

 

[vsynth]

A bodge might be to fit a high power diode into the element circuit, it would still pull 3KW peak but the average would be half that.

More suitable would be a triac circuit with zero crossing switching to keep line noise down to a minimum and allow fine control of the power drawn from the invertor...something like this...

http://www.graham-laming.com/bd/Immersion_Heater_controller.pdf

Of course you could always replace the element with a 1KW type.

 

[JohnD]

Of course you can. Five steps:

1) Insulation
2) Insulation
3) Insulation
4) Timer
5) Thermostat

Edited

Sorry, my mistake. I was thinking you meant energy saving, but you wanted to take longer to heat the water.

 

[Adam_151]

I want to dump the excess power from my PV array into the water but I'm not likely to have 3kw spare. I appreciate the second law of thermodynamics but as the power is free I don't care if it takes three times as long !

Thats quite an inefficent way to acheieve a solar thermal system

 

[Space cat]

the reason that it is pointless is the second law of thermodynamics. You cannot create or destroy energy

That's the first law.

But to get back to the original problem, I think the simplest solution is SimonH2's step-down transformer but without the complexity of automated tap changing. A standard 2:1 ratio transformer, of the type used with 110V power tools, would reduce the effective power of your heater to 750 watts. Then, if you want a little extra sophistication, you could devise a way of switching the heater from secondary to primary if and when 3kW is available.

 

[TicklyT]

any votes or two immersions wired in series?

 

[ericmark]

I did with a boiler run it from 110 volts instead of 230 volts as I wanted to condense the steam and produce water I could use in a battery. It did work taking 1/4 of the normal power and constant which was something I needed for what I was doing switching on and off was no good.

However from what you say that would not help. The whole idea of the grid tie inverter is you use the mains as a battery so all you need to do is switch it on and off with a timer or reduce the temperature but not too low don't what legionnaires.

 

[big-all]

the trouble you will have when trying to micromanage the system is you set it up on a sunny day then it clouds over for the next seven hours unless you normally use electric rather than gas to heat your water it could back fire and be expensive

 

[SimonH2]

But to get back to the original problem, I think the simplest solution is SimonH2's step-down transformer but without the complexity of automated tap changing. A standard 2:1 ratio transformer, of the type used with 110V power tools, would reduce the effective power of your heater to 750 watts. Then, if you want a little extra sophistication, you could devise a way of switching the heater from secondary to primary if and when 3kW is available.

Hmm, switch between two voltages, you mean like a tap changer with two taps

the trouble you will have when trying to micromanage the system is you set it up on a sunny day then it clouds over for the next seven hours unless you normally use electric rather than gas to heat your water it could back fire and be expensive

Which is why it needs to be automatic.

Thats quite an inefficent way to acheieve a solar thermal system

Yes indeed, but that's the sort of thing the tax (ie FIT) regime promotes. He gets paid for every unit he produces whether he exports it or not. Given the price differential between imported and exported power, there's a financial incentive to use what you make when you make it - rather than export it when it's made and re-import it when it's needed.

The key difference is whether electric is the primary energy source or whether he also has gas.
If he has gas for heating then it's going to be better to just export the excess power as (IIRC) he'll get a similar amount for it as it will cost for the gas it replaces. So by the time you account for the costs of the required control gear, you'd never break even from the proposed scheme.

However, if he's electric only, then it may make sense to use as much as you can when it's "free". It depends to a certain extent whether you have enough storage to do all your heating using off-peak lecky which last time I looked was about the same price as you get for exported solar. If you need to use enough during the day then the cost differential (import about 4x the export price ?) may eventually make it work the cost of the proposed setup. Probably still take a long time to pay for it though.

 

[Chri5]

http://www.Criticalpowersupplies.co.uk/filedata/0000/0188/SMA_Hydro_HB1124_1324-DEN100710.pdf


Store the surplus and feed it on demand, cost might offset the saving...

 

[Space cat]

you mean like a tap changer with two taps

When you put it that way -- yes. Like a bicycle with only two gears, it's a step up from having just one.