Immersion Heater - PV electricity

[suntrap]

Not my PC. I will try with Firefox tomorrow. But I am deeply mistrustful of web sites which cause unexplained crashes.

I'm glad you have a healthy distrust of the internet, but that distrust is probably misdirected as using an old browser is far more dangerous. Hope you manage to look at the site tomorrow - there's a lot of good stuff on it.

We are on oil so it has a much better payback, hence I am keen to make some progress before the longer, more productive days are here.

Agreed.

 

[hobbsy]

Well done!

Not sure how it would help with heating as there would be no water flowing through the coil. I think the other problem would be that the coil is in the wrong place - it needs to be at the top of the tank to make best use of the heat in the stored water, which otherwise wouldn't convect.

could you invert the tank to correct this? Interesting that no one has responded to my point about the smart meters coming from next year which potentially render this otherwise very good idea pointless if you own your panels as they will pay you for all export at the same rate as the gas saving. still an option for renta roof or those on oil.

 

[suntrap]

Supporting an inverted hot water cylinder would be interesting, and you'd get air trapped at the top.

I guess most of us don't believe these smart meters are going to appear anything like as soon as promised.

 

[echase]

Interesting that no one has responded to my point about the smart meters coming from next year which potentially render this otherwise very good idea pointless if you own your panels as they will pay you for all export at the same rate as the gas saving. still an option for renta roof or those on oil.

OK I will try to answer. If they pay you 3.1p and gas is 4p then look at it this way.

An immersion heater converts >99% of the electricity from grid or PV to hot water. A boiler at best converts 90%, if condensing. And if you add pipe loss it’s worse especially if short cycling and considering pipe loss. Mine probably converts <20% in summer. Also wasteful if you a have a pilot light on all summer just for a little water heating. Also condensing boilers in particular are not that reliable; my mother has spent £500 on 2 repairs to hers, so how much is added to the real cost for repairs and replacements? Maybe 1p extra per kWhr used. Less on non condensing boilers but more energy is wasted so it roughly balances out

So you can’t say that 3.1p is the same as the gas cost as the true gas cost will be 6p or more per kWhr actually delivered to the water. As the years go by the 3.1 is likely to rise with CPI whereas gas prices rise faster so it gets more imbalanced with time.

For those not on natural gas the costs are likely to be much higher.

Plus, as you say smart meters are not advancing that fast so my 3 year payback should be achieved before the meter changes.

 

[hobbsy]

echase,

I'm aware of all of those points (simplicity, efficiency etc) and agree with them and so on balance it will probably still be worthwhile doing it when they have the smart export meters in, just not as worthwhile as it is right now when the usage is unmetered. It's just a shame that I'm overseas for the next 4 years so whilst I managed to get my PV up just in time to receive the 43.3p/unit FITs there is a fair chance when I return the smart meters will be in.

However let's assume the smart meters don't appear until 2020. Is there anyway that I can use a storage tank in conjunction with a combi to get any benefits from a smart immersion controller like this? I see that my previous suggestion of using the coil to preheat incoming mains cold water is flawed due to placement and also possibly size. Unless it was a custom tank = expensive. I guess you could rig something up with a central heating pump circulating hot water from the top of the tank through a FPHE which carries the cold mains feed into the combi - the cooler water coming from the FPHE would go back into the bottom of the tank. So this wouldn't require a header tank. Is this way of doing it feasible or would it be too expensive to be worthwhile? I guess I would need a trigger to run the pump whenever a tap is opened, is this signal available on the boiler control PCB somewhere - i.e. the same one that tells the boiler to kick in? I've heard FPHE are very efficient and not too expensive. I'm no expert on heating systems as you can probably tell!

Would it be possible to do similar with the central heating circuit as well or would this become too complicated? I guess you'd need to ensure the radiator return water was several degrees cooler than the contents of the storage tank else you'd put no meaningful heat into it. Although I'd imagine generally there is less benefit trying to use "spare" PV electric via an immersion to heat your house as the time of year you need it is when the PV output is pretty low.

 

[bradburts]

I thought this forum was for the development of electronic systems to turn exported electricity into hot water and save our selves some money. If you dont want to contribute why are you wasting your CO2 immissions and time in clogging up this forum????? Blunt yes I know!

Because;
A) I would like a planet in 50 years time
B) I do not have a financial interest in such selling such a system, unlike some.
C) The control system discussed is applicable to, say, a heat pump which would make much better sense.
D) I was mistaken in my belief that some would see wisdom beyound simple greed.
Blunt yes I know.

PS
I may be wrong about it not being green, that part should be debated.
When you generate local electricty you bypass many of the transmission losses. So that electricity should be used for purposes which solar thermal cannot provide.
For example washing machine or drier. The control system would work for these as well.

 

[inie meanie]

If you want to do house heating why not use electric under floor heating. Easy to install the thin matts and can be feed the same way as the imerssion heater. The floor will act as a thermal store. Or a storage heater could be used.

 

[inie meanie]

Because;
A) I would like a planet in 50 years time
B) I do not have a financial interest in such selling such a system, unlike some.
C) The control system discussed is applicable to, say, a heat pump which would make much better sense.
D) I was mistaken in my belief that some would see wisdom beyound simple greed.
Blunt yes I know.

PS
I may be wrong about it not being green, that part should be debated.
When you generate local electricty you bypass many of the transmission losses. So that electricity should be used for purposes which solar thermal cannot provide.
For example washing machine or drier. The control system would work for these as well.

Debated yes, but why here? I dont think the control systems here for bleading spare power would be any use for compressors due to the high start currents. The only real use is to store it thermally. Batteries loose to much in efficiency and storage issues etc.

 

[bradburts]

Well I was adding to something posted. This topic has been dead for years then someone posted and kicked off 'is it green' debate.
I don' t think that its particularly green to do it. Its a point of view.
Some are comming at this with a 'green view'. Some have thought that the PV to solar is 99% efficient and that a gas boiler is 90% and that therefore it makes green and economic sense to use solar PV to heat water. It does not. The home boiler is 90% minus gas transport etc costs. You have to look at PV solar use as the loss to the local grid. Local grid electricity is generated with a big boiler at the power station. This has an efficiency of 50% max. You then have all the cable and transformer losses. Maybe 20% of the energy used at a power station reaches a consumer. So when you use PV to heat water the loss to the environment is 70% or so. It does not makes 'green' sense to use solar PV to heat water.
Once that is known the rest is down to the individual, do they want to save a few pennies.
Others have thought that it may not be safe to buy a kit from members, thats another view.
I don't see a problem in presenting these broader points within this thread.

I agree, storage is a significant problem is storage. The ideal would be to store and release electrical energy. My local PV salesman recons we will have that in 5 years. I don't know, would be great though. People have used lead acid off grid storage for years. Not sure how cost effective that is. Would love to know what the real reliability of leisure cell is.
As for the compressor we are not off grid. We can start the compressor and only need to sustain it for reasonable periods of time. Some local supercaps may be the answer here, how about a wide field sky sensor; spot the clouds and predict power available for reasonable time?
re-uk use an LDR instead of a generation CT.
It is more expensive but would be so much better in the long run. Could then have a bigger thermal store for home heat.

 

[pmcalli]

Sorry if this has been answered before, has anyone found a true power sensor at a reasonable price? I want to measure the actual power exported to the grid and use this to control a Crydom controller (as others have done). Hopefully this approach will be more accurate than deriving the small difference between two comparatively large currents, and also maybe cheaper.

If there isn't a commercial solution for <£100 then I will try a current transformer/Hall effect current sensor and feed the output of this together with a stepped down version of the supply voltage into an Analog Devices analogue multiplier.

This when averaged should give me true rms power (with sign!) which I can then use to drive the Crydom unit, and also a panel meter giving me an instantaneous import/export display.

Anyone out there tried any or all of this and can recommend components etc for it?

TIA

This approach has a fundamental flaw. Finding the true rms power will not allow you to control a phase controller accurately. I have already posted a ct diag which delivers 95% of export into an immersion heater using independent current transformers. If you want to use a single transformer on the export\import you will need to use the mains voltage to switch the sign of an integrator so that it produces the necessary control and sign. A true rms will not produce the desired result because the solar inverter produces a sine wave current. The house load is not a sine wave so the composite current is a mixture of import and export which the meters average over a cycle of the mains voltage. Even when the meter indicates that it is balanced in fact one part of the mains cycle you are exporting and one part you are importing. You can design a synchronous rectifier to do what you desire using a few op amps. I have tried this and it works fine but it was simpler to separate the house load from the solar by adding an extra consumer unit. I use a couple of owl meters to monitor the house load and solar instanteously but it's a bit boring because it's always balanced. The owl on the house load reads about 10% high because it's conversion of the current transformer does not work accurately on non sine wave currents. The meter confirms that over the last three months I have only exported 5%.

 

[echase]

What you need is a thermal store rather than a hot water cylinder, some of which have coils at the top. But frankly I think it blows the cost benefit calc if you add in all the cost of this plumbing.

 

[bradburts]

Agreed.
But its not all about the money right now though.
I recon that for £100 you could generate more heat with some battons, old glass and black garden mat (and maybe a fan) on a south wall than all this hitech stuff and you could export your PV.
Would look crap though

 

[suntrap]

If you want to use a single transformer on the export\import you will need to use the mains voltage to switch the sign of an integrator so that it produces the necessary control and sign.

Surely that is exactly what sharpener is proposing, by using an analogue multiplier to multiply the CT output with the voltage waveform?

 

[pmcalli]

Surely that is exactly what sharpener is proposing, by using an analogue multiplier to multiply the CT output with the voltage waveform?

no analogue multiplier will depend on phase of voltage and current my method is immune to this. Multiplier costs tens of pounds my solution is pennies. I patented this technique 35 years ago and it is used in nearly all chopper stabilised amplifiers

 

[pmcalli]

What you need is a thermal store rather than a hot water cylinder, some of which have coils at the top. But frankly I think it blows the cost benefit calc if you add in all the cost of this plumbing.

Thermal store is an expensive option and what you really need is capacity to use up solar power and provide buffer on dull days. I have installed an extra economy 7 tank feeding the existing one because I needed more hot water capacity as I am converting loft into extra bedrooms. My controller feeds a battery charger which then switches to the coil in first tank when battery charged then coil in second tank when first is up to temp. I have used some inexpensive £3 ish relays from maplin and standard tank thermostats. Long term I am going to design an energy controller for the house where loads can be programmatically switched via Bluetooth. The immersion controller is an essential element as it uses up the last kw to avoid turning othe loads rapidly on and off. Many people are working on similar things but are horribly expensive. B gas is launching a house control system next year and sma is launching one in january.