Immersion Heater - PV electricity

[bradburts]

That would be great.
I have been researching some more & the following seems relevant:
1) (1974) Load management detailed in U.S. Patents 4,241,237 and 4,455,453 and Canadian Patent 1,155,243 (Apparatus and Method for Remote Sensor Monitoring, Metering and Control)
2) 'Smart grid' as used in 2005 "Toward A Smart Grid", authored by S. Massoud Amin and Bruce F. Wollenberg appeared in the September/October issue of IEEE P&E Magazine (Vol. 3, No.5, pgs 34–41)
Seems to me that the EMMA system is a form of 'economic demand response' that is to say it costs you nothing to use the PV and costs you to use the grid. Selecting one supply or the other is therefore an economic demand response.
I was very happy to find that load managment has been discussed since 1974. I think therefore that there is nothing to worry us in the patent, just the difference between macro and micro grids.
I also found reference to office lighting systems which dimmed as a response to grid load.
http://www.lrc.rpi.edu/resources/newsroom/pdf/2004/LoadShed.pdf
I suppose therefore/also that I/we could offer systems which dimmed lights (say a 3KW system for those with big houses) when PV output was low & before we start to tax our fragile grid.
The light is not an energy store (as discussed by EMMA) and light dimming has been done before, see link above.
Of course I would be powerless to stop someone fitting the dimming device to an immersion heater..........


Think inie meanie was right. Would be great to have a proffessional's view though!

 

[suntrap]

The difference between patent theory and practice is that even if a patent is ultimately invalid, if the company holding it decides to prosecute you because you are selling a product that infringes it, you could spend a large amount of money defending yourself in court.

 

[bradburts]

The difference between patent theory and practice is that even if a patent is ultimately invalid, if the company holding it decides to prosecute you because you are selling a product that infringes it, you could spend a large amount of money defending yourself in court.

Agreed.
I hope however that as the prosecuting company would need to spend money they would not, especially if there is a lot of 'holes' on clear display!
I will post as many 'holes' as I can find and hope that the wiser explain their validity.
I hope that somewhere along the line someone will think that the route is open for their own system to go to market but at a much lower cost.

BTW, old growser,
You're right, the control systems being discussed are quite simple. A microcontroller monitors both PV generation and home use using a pair of current transformers. The microcontroller then controls the amount of power going to the immersion heater using a phase controller. A phase controller is a posh name for the circuit in your light switch dimmer, it works by switching the AC on some way into each AC wave cycle and therefore limits the amount of power being transmitted in each cycle.
Some are using simply relays which have the advantage of being even simplier to design. The problem with a relay solution is that a relay cannot be switched on/off within the AC wave and therefore a relay will clock any meter at the relay's load's rating, typically 3KW for an immersion. The relay's PV generation trigger point must therefore be much higher than the phase control system. Using a lower watt immersion heater lowers this threshold but at cost and hastle. Adding a diode in series would reduce the 3KW to 1.5KW. Your trigger point PV generation would then be, say, 2KW, 1.5KW for the immersion and 0.5KW for the home.
You're right you could be the control system by sitting in your airing cupboard, with unemployment and energy prices going the way they are you may be able to employ someone to do this, start a company and see if you can get an intern! Technically you would be open to prosecution (if you think that the patent is valid) though as 'a switch' is described in the patent, the patent also covers Thyristors which suggests use of a phase controller (which is just a more advanced switch synchronised to the AC waveform).
Most in this forum (it would seem independently arrived at design that) think that a phase controller would be the best way to go. By far the most effective system is a phase controller as it allows you to control power dumping with a high level of resolution thus sucking up all the excess power rather than having to reach a generation threshold related to the immersion heater's power rating.

 

[suntrap]

Phase controllers (despite my intention to use one) have two disadvantages:

- they generate heat
- they generate electromagnetic interference, which to suppress properly (i.e. to comply with regulations for a commerical product) is bulky and expensive for this sort of current

Actually, aren't there regulations stipulating that power factor correction is needed on any appliance drawing over a certain amount? This would add even more to the cost. And the use of a simple diode to reduce power consumption by half would also need power factor correction in this case.

There's no reason why an immersion heater controller should need to sense both house and PV current; if it gets a sniff of the voltage waveform you can tell which way the current is flowing in a meter tail so one current transformer is all you need.

 

[bradburts]

Hi suntrap,
Having just one CT would be a nice simplification.
Forgive my ignorance but how would you sense the direction by looking at the voltage waveform?
Your right power factor will drop. I recently scanned the low voltage regulations looking at CE marking but did not see anything about power factors. Do you have a link?
I agree heat could be an issue. I gather that Rds values can be really very low these days though.
RF is always fun. Best bet with RF is to sacrifice a cat under the full moon, that’s what the RF engineers I know do, I'm sure.
What do you think the EMC problems would be?
I am not sure that there would be a major EMC problem though.
I have read a few application notes and have not seen any mention of EMC, the application notes are very detailed as well (enough for a softee like me to understand!).
Also I don't remember the EMI rules forming a distinction between continuous emissions and discrete and a relay switched on or off would chatter albeit for a short duration.
A phase controller would pulse giving you a step at 50Hz for sure.
13Amps is a lot to switch but how many multiples before we get to a regulated band?
Have you done any calculations? Is the simple on/off at 50Hz circuit in the face of the standards or marginal?

 

[inie meanie]

I have been using the Triac 10PCV2425 from Crydom. It has a built in snubber and comes as a complete package. I have been using it to run two immersion heaters. When the first is up to temperature I switch a contactor and bring in the second tank. I have just bolted it to the side of a metal case with a small copper shim and a small thin heat sink from a pc. At the very worst time I have only ever felt a very small rise in temperature so I can honestly say that heat is not a problem, and I have been working it hard from just a couple of watts right up to 3 kw continuously throughout the day. The discussed interference has not manifested it's self in any way and I don't think that it is a problem. I am sure that the Emma box uses the same off the shelf component and seem to not employ any extra supression.

 

[suntrap]

Looking at the relative phase of current and voltage gives you the direction of power flow, e.g. if the current waveform is positive while the voltage is negative, power is flowing in one direction; if the current is negative while the voltage is negative, it's the other. In practice it's a bit more complicated because of power factor so you integrate over a cycle (take many rapid samples of voltage and current, multiply them together, and add the results) to work out the net power flow.

I don't have any references; I might be talking rubbish But there is a lot of stuff out there: I found this:

"The general requirement in IEC 1000-3-2 prohibits power control systems that generate low-frequency harmonics. Specifically, systems that generate switching operations less than or equal to 40 times in a half cycle are prohibited in the control of power supplied to heating elements and thermal devices. This relates to equipment that has power current control in the form of SCRs."

And look inside a dimmer pack to see how much ironmongery it contains. As for power dissipation from a triac, well they drop a volt or so which is 13W at 13A RMS.

 

[bradburts]

And look inside a dimmer pack to see how much ironmongery it contains. As for power dissipation from a triac, well they drop a volt or so which is 13W at 13A RMS.

It good to be back to design, patents were a bit boring.
I think that the max voltage drop is at peak voltage and when on. AC is 240V RMS of course so it won't be 13W. Too late to do the maths.
The regulation you quote is interesting. So we need to switch faster than 40 times in a half cycle. That's easily achieved. You trigger on the zero crossing and then fire a (hardware) PWM stream at the duty cycle you need. A little maths to figure out the various pulse stream duty cycles required for a given % power but a small lookup table on the microcontroller would do it. Say you have to switch at >10,000 times within the half cycle, you are still talking of 0.5MHz which should be straight forwards with a PIC or whatever.

inie,
Thanks for reminding us of the Triac 10PCV2425, which proves that phase control is possible for $100 or less.
Does the Crydom switch rapidly within the cycle for a given demand? I would be a little wary of trying to switch the Crydom rapidly as it has an analogue input.
Apart from snubber (which protects against false firing?) do you know if the Crydom has any EMC filtering?
The reason is that the Crydom is still expensive. If this is to be done then best done with a few £ of SCRs built from the ground up rather than a $100 package. Convinient package granted.

 

[suntrap]

I think the dissipation is still 13W, or think of it this way: if you've got a heater dissipating 3kW on 240V that's 12.5W per volt. You could put 240 resistors in series, each dropping a volt, and each would dissipate 12.5W. A triac dropping a volt would do the same thing; in fact it would dissipate more because it is non-linear so will drop a volt over nearly the whole cycle. Yes, it will only do this at full pelt but if you want to be able to drive the heater continuously at high power you will need to provide heatsinking accordingly (the hot water cylinder being an obvious choice).

inie: I think I've already said that the snubber network is not an interference suppression system; it merely prevents false firing of the triac and has no effect on poor power factor and I would suggest little on electromagnetic interference. If it were adequate, you wouldn't have to fill dimmer packs with expensive inductors. I've definitely said that just because you haven't experienced interference doesn't mean that it is not being generated. Those solid-state relays/phase angle controllers do not have interference suppression or power factor correction.

If I ever get round to building such a thing I can see myself going for something based on an Arduino microcontroller as discussed on the openenergymonitor site as well as here. I don't see why this shouldn't be able to calculate the grid current (and power direction) over one cycle and provide the firing pulses for an opto-isolated triac (about £5) for the next cycle. Only one current transformer needed, on a meter tail. This could not be made into a commercial product though.

 

[bradburts]

Yes the hot water tank would be a good heatsink, except when hot! The Crydom operates to 80C and I guess the tank goes to 60C+.
13W would be a fair sized heat sink passively cooled in an airing cupboard, say two hands surface area.
Best to add a thermal trip as well, too easy not too.
Still not sure that the 13W is right, if you look you can find an Rds of 0.03, which is around 5W, say the BT139. Bet you could go lower. Isn't some of the V drop junction effect and so is not lost power?

The Arbuino is a good choice, lots of support and worked examples which is what you need. A PIC may be a couple of £ cheaper. Lots of choice though. I'd go with the brand that has the most/best examples similar to what you're doing. The PIC has a PFC example circuit for example.

Why do you not think that you could make the discussed design as a commercial product?
I am sure that I have delivered worse than that before
I cannot see anything too hard here. Also I have seen products which switch 3KW @£80 or so, they may ignore the rules and just stamp CE, but I hope that its shows all is solveable.

 

[suntrap]

The hot water cylinder should be OK especially if you only put a triac there rather than everything else (such as electrolytic capacitors). You'd attach it at the bottom which would probably remain relatively cold. Difficult to ensure good thermal contact to a curved surface though. If the regulation I quoted applies, you would not be able to use an SCR/Triac to control your immersion heater; you'd have to use a gate turn-off device or a MOSFET which could switch more rapidly than twice a cycle. If you could point me at one of these commercial heater controllers I'd be interested to look. Perhaps they use burst firing, which wouldn't fool an electricity meter (although that reg implies that burst firing isn't allowed either).

 

[suntrap]

First hit for BT139 brings up a data sheet which has a clear graph showing power dissipation for various phase angles. It agrees with what I said: worst case (full tilt) about 17W at 13A, and still nearly 10W at 30 degrees conduction angle. These devices might have a resistance figure but this is additional to the non-linear fixed voltage drop; both factors cause heat dissipation. This is why plug-in timers use relays.

 

[bradburts]

Oops, you're right of course.
I did not check & assumed that power loss was all Rds.
Is this the case with all semis? Worked on a high efficiency DC project once where it seemed that all the hardware ppl could think about was Rds.

Yes a triac would only fire until zero current (the remaing phase) so MOSFET or similar would be need, if the standard applies.
You say burst firing is not allowed, do you mean burst firing in the sense of zero firing where you miss so many cycles?
I was expecting to burst within the half wave as suggested by your quote, using a MOSFET etc.

I don't have links to the commercial product, it was a studio lighting controller, think the brand was Evans. I will see if I can find again.

Anyway, what do you think prevents you from making a product?
You certainly can pick me up quickly on my mistakes & lazyness!

 

[suntrap]

Rds presumably means resistance drain to source, which is for MOSFETS, not triacs. That might explain the anomaly! It is indeed very important for those things. My solar inverter wouldn't be able to get 96% efficiency and a reasonable sized heatsink without nice low resistance FETs (I presume - insulated gate bipolar transistors seem to be another fancy semiconductor used in such applications). They don't have this pesky fixed voltage drop and so behave more like resistors.

Yes, I mean firing for a certain number of complete cycles when talking about burst mode.

I wouldn't be interested in jumping through the regulatory hoops to make a commercial product; besides which, I have a day job!

 

[bradburts]

Yes it was for a buck/boost intelligent psu.
So using a MOSFET with a low Rds in this application would reduce power loss down to 5W or so?

The link I need to find is for a lighting controller & so I doubt that it was burst/zero firing. Would give the viewers a headache! Also I don't think that it was an autotransformer. Probably wrong on that one though.

Still not sure that you have many hoops to jump through, once designed to meet conditions then a design file for CE and lots of testing?
I have experience of companies where the low voltage regulations have been a mystery to those designing 240V, 440V equipment and their managers. Even worked at one. EMC problems, lets hop frequencies! Not suggesting you go that route but I do wonder how much product truely conforms.
BTW, what field do you work in ?