Jump to content

Limiting Geyser Current


IdlePhaedrus

Recommended Posts

Hi All

Read on another forum of someone using an AC speed controller to limit AC current to their electric geyser.  Is this safe?

Currently have a 3kw geyser on the roof and a gas one which can be used interchangeably but not at the same time.  Want to reduce that 3kw to 2kw either by changing out the element (either normal 2kw element or a PTC one), but this looked like an interesting option.

Thoughts?

Link to comment
Share on other sites

3 minutes ago, IdlePhaedrus said:

Thoughts?

I cant see that you will be doing this without creating extra losses. Its going to be cheaper to buy a standard 1.5 or 2 kw element and stick it in the geyser and you will ensure that all the energy is used to heat the water and not dissipated into the air through losses, and you will limit the current to the geyser. 

Link to comment
Share on other sites

2 minutes ago, Jaco de Jongh said:

I cant see that you will be doing this without creating extra losses.

This isn't really an issue, I have a 3kw solar array and have plenty of spare kwh during a sunny day now that the pool is defunct and is unlikely to be replaced.

What I like about the idea is that in Winter I can turn it down and run for longer, and in summer it can be turned up to run for shorter which a replacement element won't allow.

This isn't much more than a standard replacement element and a lot less than a PTC one, which (currently) all appear to be around the 2.2kw level.

Link to comment
Share on other sites

24 minutes ago, IdlePhaedrus said:

Read on another forum of someone using an AC speed controller to limit AC current to their electric geyser.  Is this safe?

This is a thyristor controlled device, same thing as a light dimmer.

Yes, it's safe, if properly spec'd.

It's not lossless, but it's not lossy either. I've seen numbers like 1.5W/ amp.

There are a few things to know though.

1. It doesn't limit the current to the geyser,( like say a series resistor or a Variac would), it just doesn't allow the whole sine wave through. That portion that it does let through will still peak at the same magnitude as if it was a full sine wave. Everything still has to rated for full current, so it is more correct to say it limits the energy to the geyser.

2. Using one of these on a device that draws sufficient power will cause harmonics on your AC supply that things like inverters may not like.

 

Edited by phil.g00
Link to comment
Share on other sites

2 minutes ago, phil.g00 said:

Yes, it's safe, if properly spec'd

Thanks that was a helpful response.  I suppose with regards the harmonics issue one would have to install and see.

Would the device I linked to be "sufficiently spec'd"?

Also, are there such things as variable resistance series resistors for AC?

Link to comment
Share on other sites

1 minute ago, IdlePhaedrus said:

Would the device I linked to be "sufficiently spec'd"?

Yes, just check if there needs to be an additional heat sink.

 

3 minutes ago, IdlePhaedrus said:

Also, are there such things as variable resistance series resistors for AC?

You wouldn't want one. That is a very lossy solution.

A variac is a variable transformer, which would work, but would be an expensive solution. 

 

Link to comment
Share on other sites

3 minutes ago, phil.g00 said:

Yes, just check if there needs to be an additional heat sink.

I doubt it. It looks like it's made out of a heat sink! Interesting controller! It looks like it's locally made and a nice job!

The name is confusing though: It will only be useful as a speed controller with a universal motor such as a power drill (not good for an induction motor since there you need to change the frequency) Dimmer or soft starter is a better name.

Safety is not an issue and the thyristor will be up to the task. Really rugged devices those..

Link to comment
Share on other sites

21 minutes ago, Richard Mackay said:

Safety is not an issue and the thyristor will be up to the task. Really rugged devices those.

Great.  Have ordered one and will get a sparky to install it when it arrives.  Will report back if there are harmonics issues with the inverter.

It's a bit of a "boer maak 'n plan" solution which is why I kinda liked it...

Edited by IdlePhaedrus
grammar
Link to comment
Share on other sites

Years Ago when i started of in electronics i build a couple of these. Diac triac a coupIe of caps and a potentiometer

I also used a diode on the one leg of the AC to reduce the ac to half wave to limit the full wave form but for a geyser it will have to be a big diode.

The thing is you still require the same amount of power to heat the water. 

I just turned my thermostat down to 50 and installed a timer that runs for 1 hour in the morning and one hour at night. kids shower at night me and lovey in the morning.  rest of the day geyser dont use power.

 

 

 

 

Link to comment
Share on other sites

2 minutes ago, Krokkedil said:

I just turned my thermostat down to 50 and installed a timer that runs for 1 hour in the morning and one hour at night. kids shower at night me and lovey in the morning.  rest of the day geyser dont use power.

Got this in place already (there is only me in the house), just need to safely reduce the load so the solar runs the geyser without supply from Eskom / CoCT.

Currently it costs around R2.30 to heat the geyser given the array, in Winter it will likely be more.  If this device works the ROI will be 120 days if it works as expected (excluding installation).

That sounds like a reasonably safe bet.

Link to comment
Share on other sites

14 minutes ago, Krokkedil said:

I just turned my thermostat down to 50 and installed a timer that runs for 1 hour in the morning and one hour at night. kids shower at night me and lovey in the morning.  rest of the day geyser dont use power.

Read up on the risk of Legionnaires disease at 50 as opposed to 60.  Legionella bacteria are apparently in all our water, but really get multiplying in 20-45 degree water.

60 degrees kills the buggers, 50 doesn't.

Apparently, this can be really dangerous when combined with showering and breathing in water vapour.

Edited by phil.g00
Link to comment
Share on other sites

1 hour ago, phil.g00 said:

1. It doesn't limit the current to the geyser,( like say a series resistor or a Variac would), it just doesn't allow the whole sine wave through. That portion that it does let through will still peak at the same magnitude as if it was a full sine wave. Everything still has to rated for full current, so it is more correct to say it limits the energy to the geyser.

I was just about to say the same thing. Those things don't limit the peak current. Okay maybe they do, once you're below 50% and it cuts more than half of the sine wave away, then the peak current will be lower (as long as the load is resistive, which a geyser is). In addition, your power factor is going to be really horrible.

If the geyser is capable of carrying the full load of the geyser and you merely want to limit the amount of energy going there, then yes, that will be fine. But if your inverter cannot carry the full load and the dimmer/speed controller is an attempt to lower the demand to make it work... that is not going to work.

17 minutes ago, phil.g00 said:

60 degrees kills the buggers, 50 doesn't.

I believe 55 is the magical minimum temperature that you should not go below. Controllers like the geyserwise also has a feature where it will take the temperature up to above 60 every few days to kill any bacteria.

Link to comment
Share on other sites

The water in Africa is so full of shit that we actually have to heat it above 60 degrees for a minimum of 20 minutes to kill the bugs. It is called pasteurization.

The moment you open your tap and cold water flows in your pasteurization effect is nul and void. if you set your geyser to 65 and leave it on for the full 24 hours you might be able to kill all the bugs in it.

We have so many bad bugs in the air and in our water that even if we do all the 1 million things correct will end up dead. that is a guarantee.

If you worry you die if you dont worry you die so why worry. Live life to the fullest.

 

 

 

 

Link to comment
Share on other sites

1 minute ago, Pietpower said:

I am missing something somewhere.  Why do you want to do this?

I want to run my electric geyser off my solar without incurring cost from the utility.  If I can control the energy given time of year I can adjust the gadget and the time the geyser runs in order to do that.  I suppose one could even make it 'smart' if you don't want to do it manually.

For less than R300 (excluding installation) this will enable me to do that (if it works, and given above there is no guarantee, so lets call it an expriment).

A replacement element will cost similar and a solar geyser would cost a considerable order of magnitude more.

Link to comment
Share on other sites

Also, think about it a bit differently. 

A manual gas geyser has two knobs.  One controls the water flow and the other controls the amount of instantaneous heat applied to the boiler by using more or less gas.

So, this is what I am looking to achieve with the electric geyser, but at a low cost and no coal burnt to supply the stupid 150 litre "pot" that sits on my roof and gets to a certain temperature at certain times of the day when it "might" be used.

Link to comment
Share on other sites

13 minutes ago, IdlePhaedrus said:

A manual gas geyser has two knobs.  One controls the water flow and the other controls the amount of instantaneous heat applied to the boiler by using more or less gas.

The energy use is the same of course. Well, kinda. With an electric geyser you have a standing loss, energy it loses slowly all of the time amounting to around 2kWh per day. So anything below 100W and you might as well forget about it.

Also, it takes 1.16Wh to heat one liter of water by one degree centigrade, so there is the additional issue that at very low power levels (1kw for example) it would take a full 8 hours to heat 150 liters of water from 15°C to 55°C... and you don't have that much peak sun. So the actual usable range of this experiment isn't all that wide, it's really somewhere between 1.5kw and the standard 3kw.

Link to comment
Share on other sites

3 hours ago, IdlePhaedrus said:

Also, think about it a bit differently. 

A manual gas geyser has two knobs.  One controls the water flow and the other controls the amount of instantaneous heat applied to the boiler by using more or less gas.

So, this is what I am looking to achieve with the electric geyser, but at a low cost and no coal burnt to supply the stupid 150 litre "pot" that sits on my roof and gets to a certain temperature at certain times of the day when it "might" be used.

Sure it makes sense to heat the hot water as you use it. It amazes me how hot water geysers haven't been replaced by instantaneous hot water systems. (even when they have the grid power capacity) You require 3 phase power for starters because of the amount of power required.

I came across a student accommodation block of flats where they had installed electric instantaneous hot water systems. It worked well from what I could tell. However I'm sure the designers assumed that not every student was going to have a shower at the same time!

Link to comment
Share on other sites

10 hours ago, Richard Mackay said:

Sure it makes sense to heat the hot water as you use it. It amazes me how hot water geysers haven't been replaced by instantaneous hot water systems. (even when they have the grid power capacity) You require 3 phase power for starters because of the amount of power required.

I came across a student accommodation block of flats where they had installed electric instantaneous hot water systems. It worked well from what I could tell. However I'm sure the designers assumed that not every student was going to have a shower at the same time!

These showers are commonplace in Ireland.

I don't think they would be a good solar fit though, as they are about 11kW single phase.

Link to comment
Share on other sites

@IdlePhaedrus

FYI Youda, elsewhere on this forum, is using the following device to control power fed to his geyser.

https://www.newark.com/carlo-gavazzi/rm1e23v25/ssr-panel-mount-265vac-10vdc-25a/dp/46M4269

This unit is fancier than your Communica unit as it controls the phase angle depending on the value of the input voltage (0 to 10VDC).

Unfortunately it is a lot more expensive than the Communica unit.

Let us know how it goes with your testing 😀

BTW Your geyser element is on the limit of that device from Communica. Their website for the 6kW dimmer says 

" 3KW power for normal use recommended"

https://www.communica.co.za/products/cmu-ac-speed-dimmer-contr-6000w

Link to comment
Share on other sites

13 hours ago, Pietpower said:

- Your kWh consumed in the geyser is mainly dependent on the hot water usage.  Roughly 8kWh per day for me and timers make maybe 1kW difference.

You'd be amazed how difficult it is to explain this to people. The largest part of your geyser's electricity consumption is due to your actual use of hot water. Cut your shower by one minute and you'd be amazed at the savings. Around 2kWh (the rated standing loss) per day is used to keep it warm.

Newton's law of cooling states that the rate at which a hot thing cools down is proportional to the delta between ambient and the temperature of that thing. So you can decrease the 2kWh/day rate by keeping the tank at a cooler temperature for a large part of the day. That is the principle of the "savings" you make when you install a geyser timer. Your savings per day therefore will not be more than 2kWh, and you will only get close if you run out of hot water directly after the morning shower.

Despite this, I've heard lots of anecdotal evidence of people who save hundreds per month by installing that timer. The only way I can make sense of it is that either 1) They shorten the shower because they want to give the new money-saving device an honest test, 2) Their is a massive heat loss somewhere in their system, eg water leak, 3) they actually run out of hot water during the last shower.

14 hours ago, Pietpower said:

- 3kW element running for 2hours is 6kWh.  2kW running for 3 hours is still 6kWh. You are not saving energy with a smaller element

Indeed. You could even argue that the smaller element, because it is also fighting the standing loss, actually works less efficiently than the larger one, though in practice it's going to be negligible.

The main reason for slower heating, however, is because people want to use surplus PV without taking power from the grid or from the batteries. If you have a hybrid inverter, and you're not 100% self-sufficient anyway (as is the case for myself), I think this is rather pointless. Import the difference from the grid and be done with it, let the inverter compensate other smaller loads.

🙂

 

Link to comment
Share on other sites

Where I think you can score, is not necessarily with a timer, but by timing.

There are times when the solar generation has been throttled because the loads aren't there, so a realizable amount of energy isn't realized for the sake of turning on a load.

If you could schedule your geyser to be on at a time of the day when the potential solar production exceeds the actual loads, then you would get something for nothing.

Link to comment
Share on other sites

4 minutes ago, phil.g00 said:

Where I think you can score, is not necessarily with a timer, but by timing.

I agree 100%. There are other setups too where a timer gives huge savings, for example a backup element in a thermosyphon system. You'd be nuts to reheat the water after the morning shower. You should leave it off until 3PM at least so the sun can get a chance to heat it first.

In my house I have a prefeed system. I have a thermosyphon setup feeding a conventional 150 liter electrical tank. In summer the sun makes enough hot water so that electricity doesn't have to be used, but the temperature in the inner tank does drop enough so that around 2kWh is used per day. By timing the inner tank so it runs later in the day (After some hot water has been siphoned into the inner tank), you again reduce that consumption and give the sun a chance to do more of the work. At the cost of slightly colder water in the mornings.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...