Immersion Heater - PV electricity

[chriswarhurst]

Let me know when you have developed one - i will be very interested

 

[suntrap]

He already has - and all the information about it has been published. Look through this thread if you want to know.

 

[DickiePhitt]

I've been working my way through this thread for days. Really interesting and a credit to the contributors. Its taken days because I'm a chartered (mech) eng so can only deal with things that can be measured in buckets. No, I've 16th Ed and did hobby electronics years ago so I think I have the skills, if not the electronics knowledge, to have a go at building a simple,safe, economical controller for an existing 3kw Immersion heater.

Was surprised at how little the potential savings were, though. Checking the gas meter these past few weeks without heating on, converting to kWh, and plugging in the cost per unit I reckon about 50p per day on HW. In summer its likely HW will max out on the temp stat part way through the day and in winter it'll struggle to put much surplus into the HW tank. So maybe 1/3 of the annual cost will be saved - say £60 - which is worth having but limits options - yeah I know its a crude calculation but probably not wildly out.

Can I just ask a few questions ,which I hope are not too stupid, asked on the basis of 'will it work' not "is it the perfect solution"?
I want to use the Arduino (no experience, I'll have to learn) as it seems to be a well supported, flexible platform for development and communicating. Am I right that 2 suitably sized LEM CT's will hook up fine to the inputs? Think I read here somewhere that something else was needed but cant see where and cant think why.

The Crydom and filter (at the right price) + fuse & MCB looks the proprietry, proven way to control the 3kW load. Since it doesn't like PWM I thought a suitable compromise might be to use an R-2R ladder and an Op-Amp to double the voltage.

Digital to Analog converters (DAC), using R/2R netwroks

Suitably programmed the Arduino would step the output voltage applied to the Crydom which would raise or lower the immersion heater power. Not perfect DAC but a good 80:20 pereto solution I thought.

If this is totally out of court can someone please suggest and illustrate / point me to a suitable way of achieving this? I would need to be taken through Triacs, etc as they came after my hobby days.

 

[methley]

I'm building one right now with an Arduino Uno board which is perfect for this job. For the output, the R-2R ladder uses too many outputs. As the Arduino supports PWM outputs all you need to do is filter the output of a PWM pin and add a voltage doubler to drive the input of a Crydom. The Crydom takes away any difficulty in driving a Triac. It's all there in one package, and is fed simply with a 0-10v input.

For the inputs from the LEM's you'll need to half the voltage, using a simpler resistor potential divider. Note the minimim load on the output of the LEM is 2MOhm.

 

[pauldreed]

My Arduino based device is here if you are interested.
The first version was built with a Arduino Uno, but my current one uses a £4 Arduino chip (Atmega 328PU) along with a few support components, and runs the same software.
The credit however must go to these guys.

 

[DickiePhitt]

Methley, have you successfully got the Crydom working via PWM and filter? Crydom advised me "The analogue DC input is sensed by a micro-controller, a PWM input would either not be recognized or falsely interpreted. The PWM signal would have to be converted to DC signal." which led me to doubt that a filter would give a clean enough input, and as a result might damage or overheat it.

Paul, I've been impressed with the development you've put into this, as witnessed by this thread and others. I'll read up the links and try to get some understanding of what you and these guys have been doing. Might be a bit advanced for me I fear :-(

 

[pauldreed]

Methley, have you successfully got the Crydom working via PWM and filter? Crydom advised me "The analogue DC input is sensed by a micro-controller, a PWM input would either not be recognized or falsely interpreted. The PWM signal would have to be converted to DC signal." which led me to doubt that a filter would give a clean enough input, and as a result might damage or overheat it.
(

I too spoke with Crydom some months back, the problem with PWM is inherently the speed of the sampling rate is too fast for the Crydom Thyristor circuit to recover, and therefore would not work.
I think Methley has taken the PWM output through a capacitor/resistor circuit to smooth and average the peak value, similar to this.
The input would be clean enough, but there is a lot of debate around the switching harmonics/distortions caused by the thyristor.
As for being 'Advanced', this was my first project for over 30 years when valves were the norm, so if I can, so can you, just take your time and if you get stuck there is plenty of help in the forums :->

 

[suntrap]

Was surprised at how little the potential savings were, though. Checking the gas meter these past few weeks without heating on, converting to kWh, and plugging in the cost per unit I reckon about 50p per day on HW. In summer its likely HW will max out on the temp stat part way through the day and in winter it'll struggle to put much surplus into the HW tank. So maybe 1/3 of the annual cost will be saved - say £60 - which is worth having but limits options - yeah I know its a crude calculation but probably not wildly out.

That sounds very plausible and does make it less attractive if you are offsetting the cheapest fuel - mains gas. Same problem there has always been with solar water heating: loads in the summer and not enough in the winter. And you could also argue that offsetting gas consumption with electricity is not very environmentally friendly. I did ponder the idea of overriding the freezer thermostat to reduce its consumption from the grid at night, but of course that's a fairly small fixed load and has the additional complication of not liking to be short cycled.

 

[suntrap]

an Op-Amp to double the voltage.

There is a 5V version available.

 

[pmcalli]

I have now built and tested two immersion controllers so thought I would share the results.

the first solution used a PID controller crydom and true rms ct's which as people here have pointed out is a very expensive solution.

results
The true rms CT's are loaded by the PID controller this does not affect the operation but is undesirable.
the response time of the true rms cts is 300ms so the PID integration time had to be long
very small grid feed in with large load changes otherwise results good.

the second solution uses some inexpensive ops amps the crydom controller and a couple of owl ct's.

results
the cost is very low the owl ct's are obtainable for about £6 each op amps and other components are less than a £1. crydom is about £70 filter £25 power supply £10
response time is set at 100ms and no grid feed in or consumption is measurable.

As there has been many posts on how the inverter and phase controller work to ensure no grid consumption. I have run an experiment whose results may lead to some other solutions.

while my controller is running using an oscilloscope with line trigger and a spare CT I have measured the current from the inverter and the current going to and from the grid.

the result is the inverter delivers current evenly over the whole mains cycle. this means that if the load from the immersion heater is phase controlled then the part of the mains cycle that it is on the current is drawn from the inverter and the grid if the inverter is not generating enough. On the part of the cycle that the crydom is off the inverter feeds into the grid. Over a cycle the net result is zero. the meter response time is more than a cycle and therefore does not record that you borrowed some power and gave it back before the end of the cycle. measuring the current going to and from the grid confirms the result. measuring the meters response time is more tricky as I would have to burst fire a SSR over an increasing time until the meter recorded if i get around to it I will post the results but I expect it will vary with meter type.

I shall be laying out a PCB ( currently on vero board ) and am happy to share the circuit diagram/component list/PCB layout with anybody who wants it.

incidentally I also ran the controller without the i/p filter and I am unable to Bluetooth to my inverter due to the interference showing that an input filter is absolutely essential.