Switching on Immersion heater.

[Gary K]

Hi all, I am aware that there have been several threads regarding using pv to power the immersion and how to control the turning on of the same when the system is producing sufficient surplus output.
I have had a 3.84kw system installed for several weeks now and like everyone else want to make use of as much of the generated power as possible.
I have almost no knowledge or background in electrics so it may be that my idea is not possible or the components that I require do not exist.

what I was proposing to do is to mount a small solar panel,say 50w or 80w on a seperate roof, but with the same orientation as the main system and use the output from this trigger what I assume would be a relay in the curcuit and hence turn on the immersion. If I could adjust or source this relay to trigger at 50% of the panels output then the main pv system would be producing about 2Kwh. Obviously this method does not take into account what power is being used by other electical equipment in the house.

Elsewhere on here members have sourced 1Kw elements so thst is not a problem but is such a relay available that is adjustable with regards to the current required to trigger it.
Thanks in advance.

 

[JohnBoyZ]

You have probably seen all the info about this on the central heating thread, if not its worth reading
http://www.electriciansforums.net/central-heating-systems/23969-immersion-heater-pv-electricity.html

Its also worth looking at the spec of the inverter, some have an auxiliary relay that can be configured to switch at a particular inverter power output so can be used to control a power relay to switch on the immersion.

 

[Gary K]

I have read through this thread but I am not interested in using a pc to turn on the element depending on the available pv output.
I don't believe a SB3800 has a relay that can be used to control anything.

As far as I am aware, no one has suggested using the output from a solar panel to turn on the immersion or any other elecrical curcuit( perhaps it won't work). If someone could sort it a kit containing a switch/relay, a suitable panel and some wiring could be sold and used without having the knowledge to program microcontrolers and the like.

 

[bumcrack]

I have read through this thread but I am not interested in using a pc to turn on the element depending on the available pv output.
I don't believe a SB3800 has a relay that can be used to control anything.

As far as I am aware, no one has suggested using the output from a solar panel to turn on the immersion or any other elecrical curcuit( perhaps it won't work). If someone could sort it a kit containing a switch/relay, a suitable panel and some wiring could be sold and used without having the knowledge to program microcontrolers and the like.

Your idea of using a separate PV panel to measure irradiance ( and therefore assumed power output from main PV system ) would work, but it would not take into account imported power into the building.

The immersion heater thread I'm sure is a great read, but 34 pages of discussion is a bit much to digest. Programming controllers is not that tricky when you get your head round it. I poseted on that other thread with a suggestion of a controller that is quite easy to setup.

If you did want to go down the route of PV power =/> 1 Kw / switch on immersion heater, then this is easily done with a trip amplifier. It is a device that takes an analogue input ( in your case a LEM current transducer on PV AC output ) and when a preset threshold of 4A is reached, it closes a clean contact to bring on the immersion element. You would also need some descending hysterysis in the circuit to prevent cycling, i.e, when 4A is flowing it closes, but it doesnt open again until only 3A flowing...

It's all interesting stuff. :freak:

 

[Graeme Harrold]

Parsons Switch is your best bet......cost includes a new immersion set. (1 and 2Kw on the same fitting)

 

[Gary K]

Parsons Switch is your best bet......cost includes a new immersion set. (1 and 2Kw on the same fitting)

Parsons switch idea interesting but as water is heated by gas boiler with no immersion currently fitted so 2kw element would never get used

 

[Scooby]

Hi Gary.

Snap! I was thinking exactly the same thing after reading the associated threads on here and wondering if a much simpler - and cheaper - solution could be found.

In theory I can't see why it shouldn't work. Again in theory, all it would need is, say, a 12V PV, a 6V relay and a means of adjusting the output - eg. a potentiometer. I doubt, in practice, whether it would be quite that straight-forward tho'.

However, along similar lines - ie: using a system that works completely independently of the PV system - perhaps there's the possibility of using a very simple electronic sensing circuit with an LDR (light-dependent-resistor) and a switching circuit. Maplins produce kits for a few squid that provide the essentials. Obviously it will also require a relay beefy enough to cope with at around 5A (for a 1kW element). The advantage of this kind of circuit is that the sensor is tiny and can be positioned at the end of a 2-core wire and I guess just sit near a window which faces the same direction as the PV system. Also, these circuits have full control over sensitivity. They might even have a small amount of built-in hysteresis ('lag') due to the nature of relays; they need a certain voltage to operate and pull the contacts closed, but - once enabled - this voltage can drop below that initial amount since the magnetic force keeping it closed needs to be less than pulling it closed in the first place (and I ain't going to re-read that sentence to see if it makes sense...). Hysteresis means that the relay hopefully shouldn't 'judder' at its switching point.

Anyways, food for thought...

 

[sharpener]

Maplins kits need building from individual components which not everyone will want to do, what about using a dusk-to-dawn photocell unit of the kind used to control external lighting, but in reverse?

There is a commercial photoelectric switch unit available from e.g. TLC direct , it looks as though it will have a wide range of adjustment but is a bit expensive

Cheaper domestic units are available in B & Q etc though you would need to add a relay to switch the 15A mains load and also invert the logic (i.e. bright light switches load on). A bit of experimentation with the settings and maybe adding a neutral density filter (smoked glass or equivalent) should do the trick!

You might want to add a 15A dimmer so that you can adjust the current the immersion heater takes according to what is available, leaving some over for other loads. At this time of year I doubt a nominal 3.84kW system will give you the full 3kW very often or for very long.

 

[delleray]

If your inverter, or your sensor system, could detect when you are generating > 1.5kw it COULD be used to trigger a 'relay' which feeds the standard 3kw immersion heater via a simple 35A 600V bridge rectifier. You are unlikely to be generating 3KW + house load all year.

obviously that is not a fully pv output > house consumption system as per the thread above but better than paying for heating water another way..

I am currently investigating (spare time and wife permitting) using an sma 4000tl generating > 1.5KW relay output to trigger a remote 13 socket control system that feeds such a drop-down input to an immersion..

Once it works i'll let you know...

D.

 

[JohnBoyZ]

If your inverter, or your sensor system, could detect when you are generating > 1.5kw it COULD be used to trigger a 'relay' which feeds the standard 3kw immersion heater via a simple 35A 600V bridge rectifier. You are unlikely to be generating 3KW + house load all year.

obviously that is not a fully pv output > house consumption system as per the thread above but better than paying for heating water another way..

I am currently investigating (spare time and wife permitting) using an sma 4000tl generating > 1.5KW relay output to trigger a remote 13 socket control system that feeds such a drop-down input to an immersion..

Once it works i'll let you know...

D.

Are you using the SMA4000TL inbuilt multifunction relay, if not it would be worth considering. I believe the switching point of the multifunction relay can be adjusted via sunny explorer so it could be set to match the immersion size. If I remember correctly I think it was BiggsSolar that has already done this on at least one install.

 

[sharpener]

Don't understand what you hope to achieve with a bridge rectifier. It will put DC through the heater element but will not change the AC power drawn which I think may be what you are trying to do.

If you mean to use a plain series diode I would STRONGLY advise against this, it will put a big direct current (many amps) back down the neutral. Besides being illegal this will also play havoc with any ordinary RCDs you have in the consumer unit feeding the immersion heater and they then may not work at all or at a low enough trip current to save your life (see the many threads on why you need a Type B RCD with certain types of inverter).

Or the DC may be detected by the inverter and cause it to trip out.

This is why I suggested a 15A dimmer, the current may not have a very nice waveform (and hence cause interference) but at least will not have a DC component to it. Other postings have also suggested using a 240/110V step-down construction site transformer, this of course still draws a sinusoidal current and it will reportedly put about 600 - 750W into a typical immersion heater element (hence you need a transformer rated at say 1kW which will be quite chunky).

HTH

 

[SolarCity]

Are you using the SMA4000TL inbuilt multifunction relay, if not it would be worth considering. I believe the switching point of the multifunction relay can be adjusted via sunny explorer so it could be set to match the immersion size. If I remember correctly I think it was BiggsSolar that has already done this on at least one install.

Yeah, we have done this on a number of installs now. We would have done some more if the DNOs had pulled their fingers out and sorted out our >16A applications.

Assuming that the 3600TL will have a multifunction relay and that it is priced right then it may end up being THE inverter for 4kWp installs.

 

[delleray]

Johnny Boy
Yes the 4000TL multi function relay can, via sunny boy, be set to switch (low current only) when the inverter output exceeds a set amount for a minimum time.

d.

 

[Giblets]

Parsons switch idea interesting but as water is heated by gas boiler with no immersion currently fitted so 2kw element would never get used

It would be if your boiler broke down!

 

[thushar]

Parsons switch use site transformer to reduce the immersion wattage - not very efficient. The best alternative could be to use a 1 KW immersion heater like

1 kw / 1000 W 27" Immersion Heater - Use Spare Solar Panels Power For Heating | eBay

It will be much more energy efficient