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Author Topic: excess power from pv  (Read 28442 times)
jonboy
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« on: June 24, 2011, 08:13:46 PM »

I have just had a pv system installed. (18 x 220 sharp panels, sunny boy 4000TL).
I am very happy with the performance and output. (145kwh in a week)
I know it is early days but i seem to have excess of power not being used by myself, and obviously being given back to the grid at a low return.
I would like to try and use this power to heat up my hot water tank.
I have done some searches on the net, and this has already been looked at an implemented using a microprocessor to ensure that that you are only using the excess power.
Does anybody have any thoughts or ideas?

thanks
jonboy
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BruceB
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« Reply #1 on: June 24, 2011, 08:31:17 PM »

As a very simple way of controlling an immersion or similar there is a relay built into the 4000TL that can be programmed to switch on at any particular value of generation.  Use it to power a contactor.  A bit clumsy/rough, but you might want to use it until you have designed programmed your all singing/dancing plc and bank of contactors.
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Bruce
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marshman
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« Reply #2 on: June 24, 2011, 08:39:46 PM »

In my opinion it is difficult to match a large load such as an immersion heater - a typical domestic immersion is around 3kW - to a moderate PV installation. Your peak output is around 4kW, but this full output - depending on inclination, direction and area of the country - will only be present for a smallish proportion of the time - and you pretty much need the full output to guarantee that you have the capacity to supply the entire load from the PV output. You can install various power moinitors and controllers as well as a variable load controller - all at some expense as discussed on earlier threads - but it is a lot of hassle for little gain. (in my opinion). You only need to exceed the output for a small time to lose any benefit you may have gained. You can get lower power heaters but then these cost money and there is a long pay back time. My opinion is that it is better to do as much as you can when you are generating - such as washing and cooking (assuming you are electric cooking) - and leave the immersion on economy 7. Also don't bother with an export meter. Benefit from 50% deemed export but try to ensure you use more during the day yourself. This is much easier if the house is occupied during the day. If it is not then you will have to become skilled in the art of a) setting devices to work on timers and b) weather forcasting (i.e how sunny will it be, how much cloud, temperature and wind (affects cooling of panels and hence output)).

Remember deemed export is  50% of the generation at 3.1p per unit. I guess you pay around 11p per peak unit import and 4 to 5p import for economy 7 units.

Roger
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3.15kWpk (15xSharp ND210)/SB3000. & 3.675kWpk (15 x Suntech 245WD)/SB4000TL, 10kW GSHP driving Wirsbo underfloor heating from 1200m ground loops. 10' x 7' solar wall (experimental). Clearview 650 Wood Burning Stove. MHRV - diy retrofit. Triple glazing.
billt
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« Reply #3 on: June 24, 2011, 10:18:20 PM »


I know it is early days but i seem to have excess of power not being used by myself, and obviously being given back to the grid at a low return.

Does anybody have any thoughts or ideas?

thanks
jonboy


You are getting paid 43.3p per unit, that is a b***** good rate of return!
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rhodie
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« Reply #4 on: June 25, 2011, 12:42:39 PM »

In my opinion it is difficult to match a large load such as an immersion heater - a typical domestic immersion is around 3kW - to a moderate PV installation.

Roger
Then reduce the Immersion load. There's a simple and inexpensive way of doing this without changing the immersion heater elements. If you have two immersion heater elements fitted to your tank all you have to do is wire them up in series. A simple Ohm's Law calculation will tell us that  two immersion heaters both rated at 3 kw when connected in series across the original voltage will use a total of 1.5 kw.  All you need to achieve this is a short piece of twin and earth cable run between the heaters and a single pole connector.

A secondary consideration is that perhaps your immersion heater is rated at 240 volts whilst the mains voltage is now 230 volts. If this is the case then using the above details the total power consumption would be 1.38 kw.

A 4kwp PV installation would have no problem coping with this on a sunny day.

How you connect and set the individual thermostats of the immersion heaters brings up some interesting possibilities.

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marshman
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« Reply #5 on: June 25, 2011, 09:21:13 PM »

In my opinion it is difficult to match a large load such as an immersion heater - a typical domestic immersion is around 3kW - to a moderate PV installation.

Roger
Then reduce the Immersion load. There's a simple and inexpensive way of doing this without changing the immersion heater elements. If you have two immersion heater elements fitted to your tank all you have to do is wire them up in series. A simple Ohm's Law calculation will tell us that  two immersion heaters both rated at 3 kw when connected in series across the original voltage will use a total of 1.5 kw.  All you need to achieve this is a short piece of twin and earth cable run between the heaters and a single pole connector.

A secondary consideration is that perhaps your immersion heater is rated at 240 volts whilst the mains voltage is now 230 volts. If this is the case then using the above details the total power consumption would be 1.38 kw.

A 4kwp PV installation would have no problem coping with this on a sunny day.

How you connect and set the individual thermostats of the immersion heaters brings up some interesting possibilities.



Hi Rhodie welcome to the forum.

I did say that a typical immersion is around 3kW and also there is not a simple way to match this to the available output - you will probably be using a few 100 watts or so for the rest of the house so won't know exactly how much spare you have.  I'm also not sure how many houses have twin immersion heaters.

It is also very wrong to assume that UK mains voltage is now "230V" instead of 240V. The change was a paperwork excercise to "harmonise us with Europe. In reality nothing changed. The UK mains supply is now 230V +10% -6% (I think) but in reality it is still 240V +/- 5%.  Mine sits at around 250V the majority of the time and even higher when I'm generating lots.

Roger
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3.15kWpk (15xSharp ND210)/SB3000. & 3.675kWpk (15 x Suntech 245WD)/SB4000TL, 10kW GSHP driving Wirsbo underfloor heating from 1200m ground loops. 10' x 7' solar wall (experimental). Clearview 650 Wood Burning Stove. MHRV - diy retrofit. Triple glazing.
JamesE
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« Reply #6 on: June 27, 2011, 02:18:09 PM »

A simple answer to cut an immersion heater from 3KW to half that is to wire a rectifier diode in series. Something like a 1N5406. I did this with my boat in The Netherlands as many electricity points at marinas/harbours were only 6 Amps. As I have now re-programmed my hot water (for which I use the immersion heater for the summer quarter with no gas used at all to in effect remove the "standing charge" caused by the first lot of units at a higher price) to come on 10:30 to 13:30 thus being able to offset much of its usage by the PV, when the spirit moves me I'm going to do the same at home.
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Andy_WSM
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« Reply #7 on: June 27, 2011, 03:15:40 PM »

A simple answer to cut an immersion heater from 3KW to half that is to wire a rectifier diode in series. Something like a 1N5406.

That's only a 3A device, how does that work?
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North Somerset. 3.92KWp SE facing PV system.

EccentricAnomaly
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« Reply #8 on: June 27, 2011, 05:56:10 PM »

A simple answer to cut an immersion heater from 3KW to half that is to wire a rectifier diode in series.

However, during the 50% of the time that the diode is conducting you'll still be importing. I imagine the meter will register this otherwise we'd all be putting in diodes to get free hot water.
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Justme
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« Reply #9 on: June 27, 2011, 05:58:22 PM »

Er wont that still draw 3kw but have a 50 hertz pulse width modulation?

The total energy drawn per hour will be less as its only on for 50% of the time but the peak draw will stay the same.
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Navitron solar thermal system
30 x 58mm panel 259L TS
1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
6kva genny
6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h 5C
Baz
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« Reply #10 on: June 27, 2011, 06:31:07 PM »

yes, the reason it wroked for the boat is the reduced average load stopped the fuse getting hot enough to blow!.

We don't seem to have mentioned 110v dropping transformets yet - gives 1/4 the power. But if you get a transformer core and rewind it yourself you could get the right voltage for whatever power you decide you want, or put in tappings for a whole series of outputs. Or (shudder) go 240 >120, then 120 > 60, then add the 60 to another 120 (in phase) and you get 180 which would give you about 1.5kW  faint
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Other-Power
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« Reply #11 on: June 27, 2011, 08:05:05 PM »

Could have a look at EMMA?

http://www.coolpowerproducts.com/uk/project-scotland-uk.html

Jon
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My advice is based on me spending my money doing this and my job spending others money doing this.
john999boy
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« Reply #12 on: June 27, 2011, 08:39:46 PM »

Anyone else (or even just anyone) got EMMA? What are the costs involved and thoughts of the installation so far?

Looks like it could be promising.
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8.46kWp solar PV - 169 - 40 slope - 53N. Immersun fuelled DHW.
Justme
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« Reply #13 on: June 27, 2011, 10:08:41 PM »

Arnt they about 2k ish?

Would take a long time to recover that.
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Navitron solar thermal system
30 x 58mm panel 259L TS
1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
6kva genny
6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h 5C
ecogeorge
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« Reply #14 on: June 27, 2011, 11:06:13 PM »

Rather than just applying a variable load to the emersion heater (which is what I assume it does) I'd like to see it developed further and control a variable speed drive to a heat pump - how useful would that be!!!
Surely it couldn't be that hard to link the output to a frequency drive input?
I'll get my coat - surrender
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