Immersion Heater - PV electricity

[pmcalli]

hi pmcalli. Many thanks for posting the circuit. I think it's a lovely simple solution to get the job (of not exporting) done. I wondered if you have updated the circuit diagram to show the +- 15 v supply and a parts list? I was hoping to go and buy the bits to build it.... panels being installed Tuesday! This whole thread has taken me back to the electronics degree I completed 22 years ago. No

Arduinos ​

then

I have updated the circuit to use only one CT using sharpeners idea of a multiplier. Had a few expected problems with offset and gain errors from multiplier but these are overcome now. The results are very good with all but 30W diverted to immersion and no grid consumption. If you want the circuit and parts list send me a PM with your email address as posting an attachment on here is a pain

 

[TechyTim]

Hi all,
Just joined this thread as I am now manufacturing a PV controller to switch on the Immersion when the sun shines.
Very simple - it sits by the consumer unit and monitors the feed from the PV and when the PV output exceeds 1.5kW it switches a contactor and turns on the Immersion circuit. There is also a manual override plus some hysteresis to avoid chatter when the output is close to the switching point or a cloud passes over.
Yours for £205 plus VAT - much cheaper than EMMA and totally independent of the Inverter.

Tim Smith
SolarHeat Controls
01202 625070

 

[methley]

So does it monitor household consumption and only turn the immersion on when there is sufficient excess? If not, then there's a real danger of importing expensive electricity to heat your water...

 

[Fazzathebrave]

I'm sure it's great Tim but your system appears to miss the fact that the normal daytime load will affect how much "free" electricity is available. If there is a base load of 1Kw (more when you switch the kettle on) and a solar generation of 1.5kW then you will end up paying to heat the 3kW immersion at your normal tariff, ie 3kW immersion + 1 kW house load - 1,5kW solar = 2.5kW from the grid. It would work out cheaper to heat with gas would it not?
Forgive me if your system is actually clever and detects the existing load, solar generation and then controls the immersion heater on variable power in order to maximise benefit and minimise export. I think that's what we are striving for.

 

[Richchad]

much cheaper than EMMA

So, nothing like the EMMA unit then if it doesn't monitor house usage to detect "SPARE" electricity, or am I missing something.. :confused5:

 

[pauldreed]

....'Suitable for PV panels with an output in excess of 1.5 kW'
....'Suitable for Immersion Heaters up to 3kW'

So where does the additional 1.5kW come from?

Tim, you've either badly described your device in the post, or you've missed the point!

 

[inie meanie]

Hi Guys
As I have a standard large vented hot water tank, and the excess pv provides loads of heat but it is all at the top 1/3 of the tank, so as suggested to me on this forum by JohnBoyZ. I have fitted a small pump eBay - The UK's Online Marketplace it was very cheap. I used a eBay - The UK's Online Marketplace non return valve to stop cold water being drawn up from the base when water is drawn off. I teed into the output at the top of the tank and teed into the inlet at the base. I have just used a timer so that it turns the tank over for 15 minutes 4 times a day, but I will be using two very small thermostats to automate it ( on order ), so that when the top of the tank is 65 c and the bottom is below 60 c it will activate the pump. The pump is very small and very low power, and I have just used an old 12 vdc plug in power supply. So today I now have a tank full from top to bottom with 65 C . Very pleased :- ) If any body wants a pictures or more info email me.

 

[TechyTim]

First an apology - I didn't read the Rules when I registered (first time I have ever used a Forum) so should not have been so blatant in the advertising. There are now a number of these simple devices on the market but some are very basic.
Second - the SolarHeat device is designed to be as simple as poss (to keep cost down) so will provide power to immersion irrespective of base load. It has a switching capability of 3kW and the existing setpoint is 1.5kW (although I am working on an improvement to be able to select the setpoint on commisisioning) so could import 1.5kW if there was a mismatch between PV panel size and tank size. Majority of householders have a low base load during the day (but probably not most people using this forum) so small amount imported is not significant cost, e.g. 100 litre tank with 2.3kW heater and 2.9kW panel will get more than 2.0kW for 65% of time in summer so excess cost is less than £20. I am not aiming for the self sufficient household market or those (like most of the posts on the forum) who are avid DIY guys - it seems to have met a need with the PV installers. It does have an override to switch it off in the winter when the output is reduced and the boiler is providing heating and hot water at a reasonable efficiency.
The most significant issue is the combination of standard heating/HW controls and the immersion control - you need HW first thing in the day but then you have to suppress the boiler until the last minute to take advantage of any sun which is fine for people who are not in the house but not for people using HW during the day.

Thanks for the feedback - it helps to clarify the thought process.

 

[pauldreed]

First an apology - I didn't read the Rules when I registered (first time I have ever used a Forum) so should not have been so blatant in the advertising.

.......the rules have not stopped other posters from doing the same, so no worries!
I see your point and appreciate that for some readers this may be appealing as a no fuss solution, and probably better than nothing at all.
I have emails from people who dash upstairs every time the sun comes out and manually throw a switch, only to switch it off again a short while later when dark clouds appear! The exercise is probably good for them, but for me it's not a way to while away the days!!
But I think that most of us here are striving to develop a system(s) which use as much 'free unused' solar power as possible, and ensuring that we don't pay to supplement that solar power.
What are you using to measure the power, op amps or microprocessor?

 

[echase]

Tim please rename that from Fact Sheet to Myth Sheet as your £98 saving is in cloud cuckoo land. With the 10 panel system you quote the MCS approved annual generation is about 2000kWhr. If all that went into heating water you have displaced 2000kWhr of gas at about 4p per unit = £80. Due to the house load only about half is available to the immersion so it’s down to £40. Your unit does not accurately track the export and is turned off in winter so halve it again to £20. Then you are importing a lot of expensive electricity to make up the aforementioned 1.5kw. So the net benefit is negative. It’s probably negative even on a 20 panel system. Systems that do not accurately track the export are not worth any money at all.

 

[TechyTim]

Paul,
The controller uses a CT to sense the PV output to the consumer unit, then some clever electronics to inject the hysteresis so that there is a positive switching of the contactor. I don't understand that bit as I am not an electronics guy - a gasman by training.

 

[TechyTim]

With the 10 panel system you quote the MCS approved annual generation is about 2000kWhr. If all that went into heating water you have displaced 2000kWhr of gas at about 4p per unit = £80.

Your figures are incorrect for several reasons - The majority of use will be in the summer when the heating is not operating and the gas system efficiency when heating hot water only is usually around 60% (may be a bit higher with a modulating condensing boiler) due to the pipework losses, so the 4p/kWh is 6.7 p/kWh.
Heating 200 litres/day of hot water through 50 DegC uses 11kWh of gas or 74p, assuming the PV can do this for 150 days in a year, the saving is £110 - at 7kWh from the PV per day that is 1050 kWh or just over half the output from a 10 panel system or less for a system on the south coast which have much higher output figures than the MCS values based on the UK average,

Allowing for 10% amount of import - still a significant saving and less than 3 year payback.

 

[suntrap]

Possibly, if you use enormous amounts of hot water - 200 litres a day, and you're also being a bit generous suggesting as much as a 50C temperature rise, especially as you are claiming that most of the usage will be during the summer.

 

[echase]

so the 4p/kWh is 6.7 p/kWh.

for a system on the south coast which have much higher output figures than the MCS values based on the UK average,

QUOTE]

Let’s use your 6.7p then. With the 10 panel system you quote the MCS approved annual generation is about 2000kWhr. If all that went into heating water you have displaced 2000kWhr of gas at about 6.7p per unit = £134. Due to the house load only about half is available to the immersion so it’s down to £67. Your unit does not track the export and is turned off in winter so halve it again to £33, e.g. for significant periods of time the PV output will be above the house base load of a few hundred watts but below 1.5kW so that energy is exported.

So about 2000/2/2= 500kWhr has gone into the heater which has run for 500/1.5 hours = 333 hours at 1.5kW. Then you are importing electricity to make up the remaining 1.5kw. So every time you have the immersion on you are importing full price electricity up to 1.5kw at a value = 12pence, which would otherwise have been heated by gas at 6.7p. So it’s an extra charge of 333 x (12-6.7) x 1.5 = £26. So the net benefit is £33-£26=£7. That is a bit pessimistic as if the spare electricity is >>1.5Kw less is imported, but does a 10 panel system often exceeds 1.5kW by much?

I live near the south coast and my system does not exceed the MCS by a huge amount. Maybe 10-20%.

 

[pmcalli]

so the 4p/kWh is 6.7 p/kWh.

for a system on the south coast which have much higher output figures than the MCS values based on the UK average,

QUOTE]

Let’s use your 6.7p then. With the 10 panel system you quote the MCS approved annual generation is about 2000kWhr. If all that went into heating water you have displaced 2000kWhr of gas at about 6.7p per unit = £134. Due to the house load only about half is available to the immersion so it’s down to £67. Your unit does not track the export and is turned off in winter so halve it again to £33, e.g. for significant periods of time the PV output will be above the house base load of a few hundred watts but below 1.5kW so that energy is exported.

So about 2000/2/2= 500kWhr has gone into the heater which has run for 500/1.5 hours = 333 hours at 1.5kW. Then you are importing electricity to make up the remaining 1.5kw. So every time you have the immersion on you are importing full price electricity up to 1.5kw at a value = 12pence, which would otherwise have been heated by gas at 6.7p. So it’s an extra charge of 333 x (12-6.7) x 1.5 = £26. So the net benefit is £33-£26=£7. That is a bit pessimistic as if the spare electricity is >>1.5Kw less is imported, but does a 10 panel system often exceeds 1.5kW by much?

I live near the south coast and my system does not exceed the MCS by a huge amount. Maybe 10-20%.

In the summer you should really use the tier1 rates if you are able to turn the gas off completely. That said its still a rubbish return compared to a proper balanced system I am getting significant amounts of hot water on most days even in the winter and the system goes right down to 30W and no grid consumption.