Immersion Heater - PV electricity

[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.

 

[inie meanie]

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%.

Have a look at the AD637 Rms to dc converter. I have ordered a couple as I hope to buffer the house signal then compare the same way with opamp. I hope this will improve the acuracy and reduce any export. I will post as soon as they turn up. Christmas post and all that.

 

[bradburts]

I thought that the B Gas kit is already available.
Its only a relay switched from what I can understand, no phase control.

 

[pmcalli]

Have a look at the AD637 Rms to dc converter. I have ordered a couple as I hope to buffer the house signal then compare the same way with opamp. I hope this will improve the acuracy and reduce any export. I will post as soon as they turn up. Christmas post and all that.

Good luck I have used them in the past and they have limitations on crest factor and sensitivity. rms even if it is derived accurately is only an approximation. to get an accurate imitation of the meter you need to integrate the current over a cycle. interested to see your results when availble

 

[pmcalli]

I thought that the B Gas kit is already available.
Its only a relay switched from what I can understand, no phase control.

I haven't seen any offering yet only press statement about acquisitions and trials.

 

[bradburts]

May be wrong, and have lost the link, but check the company who supply the BG monitors.
They have a control product on their site as well. From memory it was <<£100.

 

[Sunnydaze]

Does anyone have a smart meter yet? I just wondered whether they do actually sense current direction. I can't find any model info / technical specs.

 

[sharpener]

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

Correct. The analogue multiplier will multiply the current and voltage waveforms correctly whatever they are. This is a better approach as it uses the actual voltage waveform, rather than a square wave as in synchronous rectification or a synthetic sine wave as the openenergymonitor people do, and so averaged over one cycle gives true instantaneous power.

Also the cost to me is zero because I still have a few AD532s from when I used to use them a lot in laser scanning systems. One of the difficult situations is when the immersion heater is drawing half power, as the current will change sign abruptly at the 90 deg point in each cycle when the triac fires. But with suitable scale factors I expect it to handle this OK as the bandwidth is several hundred kHz.

BTW thanks suntrap for the links to Home | OpenEnergyMonitor, which I am still exploring (using Chrome not Firefox, I hope you approve <g>). I will use a separate winding for the voltage sense input and the transformer will be very lightly loaded, which will I hope avoid the phase shift issue in both primary and secondary. By using true precision analogue computation a lot of the other problems the Arduino community have encountered should be avoided too.

Does anyone have a smart meter yet? I just wondered whether they do actually sense current direction. I can't find any model info / technical specs.

They have to or they would not be able to discriminate between imported and exported power! There is a teardown report on one here Elster REX2 Smart Meter Teardown - iFixit but it is mainly concerned with the physical construction, has anyone got a more informative link?

 

[UKMeterman]

Hi all,
If you have the cash consider puttin a nemo96 in a plastic box and use a Hobut 100A|5A clip on tradional CT

 

[Rogerhillside]

Hi
Have just has 3.55kw PV system fitted. While I have a combi boiler hence no water tank. Intend to fit water tank with 2 3kw immersions and want to buy a controller as outlined in this thread. But would like it to also control a valve for in winter or dull days when the water is cold is uses the combi. Any suggestions?

 

[suntrap]

Are you aware that as this will all be under mains pressure, you can't use a standard hot water cylinder but need an expensive high pressure system? It will take an awful long time to recoup the cost. Also, is the idea of two immersion heaters that you will use them in parallel when you have >3kW spare? This will only be for very short periods in high summer when you're likely to have far more spare capacity than you can use, so I wouldn't worry about a second one. You'd also have to make sure you divert all power to the lower one most of the time (if at different heights) or you won't heat the whole tank.