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Energy/Electricity Storage and Use/Grid Connection => Off-Grid, Batteries & Inverters => Topic started by: iank on July 09, 2015, 04:16:27 PM



Title: running an immersion heater from 2 PV scources -is it possible?
Post by: iank on July 09, 2015, 04:16:27 PM
Is there any method I could heat my 250L DHW tank with a combination of grid-tied PV and an off grid system. I used to have a solar thermal system, but this was a disaster, so this has just been removed and I am looking to use an additional method of heating the tank.

There is a 2kW immersion that is connected to the 4kWp grid tied PV that is switched on when the PV output exceeds 2kW.
In winter, this never happens.

I have loads of roof space that I can use, so initially I hoped to put in a second immersion as a 12v DC run off a battery bank charged from additional PV panels.

However it seems there is no other immersion port in the tank, so I am trying to work out how to marry the grid tied 240v 2kW immersion with an off grid system. The 12v system would mean I would not need and inverter, but I am reluctant to loose the 2kW 240v immersion heater.

All suggestions would be useful.


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: knighty on July 09, 2015, 04:22:12 PM
you could swap your 240v ac heater for a 12c dc one


the instead of connecting your immersun to a heater, connect it to a big (dumb) battery charger to get 12v out of it

and dump it into the batteries/heater/whatever


:-)


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: skyewright on July 09, 2015, 04:26:32 PM
the instead of connecting your immersun to a heater, connect it to a big (dumb) battery charger to get 12v out of it
Would the "big (dumb) battery charger" have the same characteristics as the resistive load that an immersun type device usually expects to see?

If so that might be very useful for something in my wish list...


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: iank on July 09, 2015, 04:30:26 PM
Sorry to be thick, but not electrician or anything
Are you suggesting disconnecting the current 240v supply from the grid tied PV to a battery charger, so the batteries are charged both by the grid tied PV and the off grid panels, and the immersion can then be a simple 12v DC one?


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: knighty on July 09, 2015, 04:46:43 PM
Would the "big (dumb) battery charger" have the same characteristics as the resistive load that an immersun type device usually expects to see?
If so that might be very useful for something in my wish list...

it shouldn't make any difference at all

immersions essentially do fancy PWM for ac power... or turning it on and off again really fast, changing how long it's on/off for to change how much power is used


Sorry to be thick, but not electrician or anything
Are you suggesting disconnecting the current 240v supply from the grid tied PV to a battery charger, so the batteries are charged both by the grid tied PV and the off grid panels, and the immersion can then be a simple 12v DC one?

yes

but I'd probably connect the charger straight to the immersion heater, just to make sure you can't over charge the batteries etc..


but thinking more about it.. it might be more cost effective to grid tie the new PV and let the immersun take care of it... batteries are an expensive way to store power :(


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: Countrypaul on July 09, 2015, 05:25:18 PM
You can get immersions that have more than one heater in them, such as 2 x 2kW ones. You could replace your existing one for a model with two heaters and run each heater off a separate circuit.


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: skyewright on July 09, 2015, 05:58:25 PM
Would the "big (dumb) battery charger" have the same characteristics as the resistive load that an immersun type device usually expects to see?
If so that might be very useful for something in my wish list...

it shouldn't make any difference at all

immersions essentially do fancy PWM for ac power... or turning it on and off again really fast, changing how long it's on/off for to change how much power is used
Thanks. Could you name an example of a "big (dumb) battery charger", so I can investigate further?


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: skyewright on July 09, 2015, 06:07:03 PM
Are you suggesting disconnecting the current 240v supply from the grid tied PV to a battery charger
Partly checking my understanding but...

...as I understand, it the suggestion was to put the charger in the position of the existing immersion heater, i.e. Grid Tie PV remains as is, but instead of connecting the immersun to the existing 240V immersion, connect it to a "big (dumb) battery charger" the output of which in turn connects to the 12V system (batteries/heater/whatever).

Happy to be corrected.  :)


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: ceisra on July 09, 2015, 07:43:21 PM
I have a couple of questions first!
What type of tank is it
Is there a solar coil
What are you using to drive the original immersion

Roger


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: iank on July 09, 2015, 08:02:28 PM
It is a Gledhill 250L Stainless Lite solar, with a solar coil not in use.
The immersion heater currently is connected to the PV grid tied, comes on when the output exceeds 2kW


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: ceisra on July 09, 2015, 08:41:38 PM
Now I know this will be controversial but there is way that is an extension of a post from me a few weeks ago
https://www.navitron.org.uk/forum/index.php/topic,25123.0.html
Your tank has a secondary hot return as mine does, and so you fit a de-stat loop
Now if you add one of these to that loop
http://www.thermcouk.co.uk/inline-heaters/9-inline-heater.html
most of the output from the second immersion would go into the solar coil

I am however concerned that your existing divert does not come in till 2kw is available.
If you read http://openenergymonitor.org/emon/mk2/pvmeasurement by Robin Emley he addresses a load of the issues on inefficient equipment

Back to your proposed expansion I would be wary of a possible 250amp DC within a house
If you do put another set of panels up you could use a standard inverter and drive the immersion with 240v ac, a problem would be the inverter needs to see 240v from the property or it shuts down but this can be done with mains isolation power supply /inverter 


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: brackwell on July 10, 2015, 07:35:11 AM
Why is the grid tie not coming on to 2kw. Most people use a diverter to divert all excess leccy not being used in the premises into the immersion. This way it does work in Winter.


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: iank on July 10, 2015, 09:17:39 AM
Well my understanding, not being an electrician, was that it was to ensure the immersion only ran when the power output of the PV matched the requirement of the immersion heater.
Whether there is some problem of running the 2kW immersion when the PV output was much less, I do not know, but would like to find out.
Thanks
Ian


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: billi on July 10, 2015, 09:53:25 AM
..... my understanding of an immersun  device or similar   is that it works with a pwm signal   -Pulse-width_modulation- so activates all watts not used in the house  and feeds the immersion  with those  surplus  watts , does not matter how much  watts are available at the time 
https://en.wikipedia.org/wiki/Pulse-width_modulation


why are you not just instal more fit harvesting   pv  and an immersun  device


or batteries and  biderectional  off grid inverter  .....   my inverter switches on the immersion/heating  , when battery is close to full 

and i have pv power at night as well   most of the year

billi





Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: fourfootfarm on August 18, 2015, 09:07:49 PM
If you're still on the look out for a big dumb battery charge controller. Both the xantrex c60 and morningstar tri star have diversion controller.

I would tend towards the latter as it can cope with 48v and the xantrex can't. Its a teeny bit smarter as well.

Bear in mind you'll only be able to find 12vdc immersion heaters with very small wattage ratings.


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: skyewright on August 20, 2015, 03:39:55 PM
If you're still on the look out for a big dumb battery charge controller. Both the xantrex c60 and morningstar tri star have diversion controller.
Would that work? I thought those controllers were designed for DC input sources?

AIUI the requirement here is for a charger that will take 240V AC from an immersun style inverter & turn it into DC that can be fed to a battery system or charge controller?





Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: iank on August 20, 2015, 04:52:47 PM
Thanks

Yes, I am trying to get the water heated by an immersion heater using 2 different scources, one an existing grid tied 240v inverter that comes on when the power output exceeds 2kW, and another seperate off grid battery system that I would install myself.
ian


Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: nowty on August 20, 2015, 08:40:57 PM
You could switch the immersion between your two systems but there would be little point as both systems would be generating at the same time unless you were to power the immersion from the off grid via the battery during darkness hours. That would work but heating water form the battery would be extremely wasteful and hard on the batteries.

I have a hot water tank with two immersions. I power the top one from the grid tie and the bottom one from the off grid, but only once the batteries are (nearly) full and even then ONLY with the excess PV, not from the battery as that is maintained full until I switch the house (low power circuits) onto it in the evening.

So on a good sunny day I am pumping 3kw+3kw = 6kw into the hot water tank and it heats up very rapidly.



Title: Re: running an immersion heater from 2 PV scources -is it possible?
Post by: iank on August 20, 2015, 09:12:58 PM
Thanks
That is roughly what i envisaged doing. My calculations were that to heat the hot water we use once the tank was up at temperature would need 1.4kWh per day.
I calculated that this could be done with with a 218 amph battery giving a 3 day hangover and 80% DoD, which did not seem that much to me. An 875 amph battery would give me 3  days supply with a 20% DoD. I have found second hand 2v traction batteries on eBay that were not too expensive.
Not being an electrician and new to all  this since retiring, I am a bit worried that I have somehow made a fundamental mistake somewhere.