How do you use ALL the electricity generated from a Grid Tie

[martin W]

Hi all,

I've had a look around the PV section and can see this question asked so:

I've just been toying with the idea of installing a PV system and have thought about the amount you get paid, etc for what you export, and the fact that they assume you will export 50% and you get paid 3p for it.

I can probably install 1.4-1.8kW system with my available roof space, depending on how I arrange the panels.

My house base load is around 180-200watts per hour average. Looking at my electricity usage graphs I am probably not drawing move than 300 watts per house average during PV production hours of say 08:00-16:00.

Has anyone found a way to usefully use all of the electricity they generate instead of sending it back to the grid? The issue I see is how to turn on a load when your PV is generating more than the house is using, and then turn the load off when consumption starts from the grid.

Anyone know of any 250/500watt immersion heaters? I was wondering about maybe using a battery bank and inverters to run the house lights, any other ideas.

[Billy]

Hi Martin,

One of the problems with storage and inverters is cost.  Easily £4K, and probably more, for a useful system.  You can buy an awful lot of units for that.  You have to maintain everything and find somewhere to put it all.  I reckon it's a bit like having all the kit for living off grid but non of the benefits for being on it.  All the gear you need to use all the pv output may end up using more resources than you hope to save in the first place if you see what I mean.  But I see where you are coming from.  I can't use all my output today, it being windy, unless I put an electric heater on or something, still it saves burning some more wood.

Billy

 

[martin W]

Thanks for the info on Batteries Billy, I didn't really look at the cost / reality of a hybrid off-grid / on grid system.... I just chucked it in there for ideas. Although I would think a simple lighting circuit should not be too costly, but maybe not worth the effort.

If you are in the house you can obviously think about turning something on. I am just wondering if I can use most of what I generate, idealy in a why which might replace / offset electricity (or other energy) I could be using when I am not producing. I can find a 400 watt and 500 watt 240 volt imersion heater, but a 250 watt version would be more viable I think.

The key to this would be some way to know when to turn the heater on and off automaticially so that it is not using grid electricity, only solar.

[Billy]

There is a device for halving the immersion load on the ASAP site.  The electronic guys will know how to do it, resistors or something, so you could use your 500w without too much hassle.

I did look at similar thing with the windy miller but that was too problematic because you have to let the turbine spin up before loading it.  I was easy to use a dump load and controller for when the batteries were up to a certain voltage.

With pv it might be easier to do particularly if you could program a switch - if output exceeds blah then switch on -  if output drops blah then switch off, simples, for someone anyway.

Billy

 

[Countrypaul]

One way to get less power out of an imersion heater is to put two in series, that doubles the resistance and therefore halfs the power (or quarter of what they would be in paralell). Similarly, using a transformer to 110v would half the voltage and therefore half the power but is probably more expensive and less efficient.

What about not running somehting (such as a freezer) when not producing PV for some time. Not sure how long you can have a freezer switched off or how mcuh power it uses when it come back on, but would not onlymake more use of your PV electric, would also reduce your usage of grid electric. Might a simple timer switch do (eg. off between 18:00 and 24:00, use E7 between 24:00 and 06:00 and then off between 0:600 and 10:00 - for example, the exact times would need to be worked on.

Paul

[GavinA]

manually switching none time dependent electrical loads to operate in daytime hours on sunny days such as washing machines and dishwashers is one thing, spending potentially hundreds of pounds on specialist equipment to switch on immersion heaters etc is IMO taking the weasel pee (and an expensive folly) unless you currently use electricity to heat your water.

you'll be getting 41.3p per kWh for everything you generate, which means you'll be making a tidy profit on your installation as it is. Trying to use everything you generate for the sake of it is the sort of pisstaking action that if picked up on by the tabloids etc would discredit the entire scheme, and potentially put the whole system in jeopardy, thereby undermining over a decades campaigning work by a lot of people to get this scheme into place in the first place. Please don't take the weasel pee.

[Baz]

The problem is that there isn't a low cost 'instant' solution, though for DIY electronics tinkerers there are plenty of options.
Measuring production current and mains draw to compare and determine excess is not difficult for a small micro but no simple commercial offering.
Getting a variable power delivered into your immersion is tricky as phase controlled devices are not really suitable without much more smoothing than is provided in commercial devices which assume the mains has high capacity available in the 'on' period. It is still probably still cheaper overall to use cheap night electricity.
Getting heat generally into the house is much easier and I will put something on the home automation section later. This still involves some DIY and mains voltage so is beyond those of a nervous disposition.

I am no supporter of FITs but if it is all used in house that still contributes to reducing other energy use. FITs, like all regulations,  is primarily operated as a job creation scheme for those involved in its unnecessary beaurocracy.

[A.L.]

hello martin W, apologies if you already know about these links

www.navitron.org.uk/forum/index.php/topic,9072.0.html

www.navitron.org.uk/forum/index.php/topic,11700.0.html

Ethically I would have to have an export meter fitted if I went down this route

[martin W]

All,

what I am trying to get acroos is the thought that I am likely to be generating much more than I use for part of the day, and that I would like to use this green electricity - effectively.

Now using electrity to heat water may be a folley, however as I am generating it, green, it would be nice to use it instead of gas to heat the water, which is often needed to be used to top up heat generated by the solar thermal toobs.

The chance of anyone using all that they generate is ... um -10. It is never going to happen. But I would have thought it reasonable simple to turn on small loads automatically (not variable loads) if the grid is not supplying.

Some of you seemed to have missed this post If you are in the house you can obviously think about turning something on. I am just wondering if I can use most of what I generate, idealy in a why which might replace / offset electricity (or other energy) I could be using when I am not producing. I can find a 400 watt and 500 watt 240 volt imersion heater, but a 250 watt version would be more viable I think.
.

[GavinA]

it only contributes to reducing other energy use if that energy would have already been used prior to the PV installation. So stuff like moving when the washing machine, or dishwasher is used would be a good way of making best use of the electricity generated without increasing overall consumption, but using an immersion heater to heat water that would otherwise be heated by a condensing gas boiler (or solar thermal etc) would result in a net increase in carbon being used to achieve the same job as the electricity not exported would need to be replaced by fossil fuel generated electricity.

I'd also be concerned that any solution along these lines that wasn't 100% effective would have serious potential to actually increase the bills as well if it actually ended up using a bit more electricity than was being generated, and I've not seen anything on the market that looks like a cost effective solution to this. For example, I went to do a quote for someone the other day who'd already had someone round from another company who'd recommended that they didn't need solar water heating as they could simply use their proposed 2.7kWp PV to power their 3kW immersion heater and heat their water using solar generated electricity...

If the house already uses electricity for it's water heating / has an heat pump and can somehow rig that in, then fair enough I suppose (though the electricity companies may cry foul and want to install an export meter rather than deeming it at 50%). I just think that at a time when the FIT's are already under press and apparently treasury scrutiny, this is the type of idea that could be used to discredit the entire scheme, and people ought to be aware of and think about the potential impacts of their actions.

If we were talking about pioneers who had been robbed of the feed in tariff, and were also suffering from reduced export prices, then my opinion would be entirely different btw as I think they're perfectly entitled to attempt to recoup as much value as they can from their investment given the shoddy way they've been treated by all 3 of the main political parties.

[martin W]

Gavin, you are assuming that I have a condensing, high efficency gas boiler. I most certainly don't.

I am likely to invest between £7-10K on PV, with a 10 year or so payback, in a house I am likely to move from in 8-15 years. I am very unlikely to use 50% of what I generate, so payback time could be more.

I don't think that me using PV electricity generated for heating hot water via immersion is wasting carbon. I am probably talking of around 2KWh day if I am lucky. Probably 10%of what I would export would be lost in transmission. The immersion would replace gas useage of say 2.8-3.0 kWh per day due to efficency losses of the boiler, etc.

It not practical to be able to manually turn stuff on during day, I am looking at sensible ways to use green energy.

And before anyone else takes a slug at me.... First rule of saving the planet = reduce usage... About 3-4 years ago we used 5500kWh of electricity per year. We are currently using around 2800kWh per year.

And yes I am quite happy to have an export meter fitted to only get paid for what I export....

[martin W]

Thanks for the links... interesting reading, and no I was not aware of them.

Cheers

hello martin W, apologies if you already know about these links

www.navitron.org.uk/forum/index.php/topic,9072.0.html

www.navitron.org.uk/forum/index.php/topic,11700.0.html

Ethically I would have to have an export meter fitted if I went down this route

[GavinA]

All,

what I am trying to get acroos is the thought that I am likely to be generating much more than I use for part of the day, and that I would like to use this green electricity - effectively.

Now using electrity to heat water may be a folley, however as I am generating it, green, it would be nice to use it instead of gas to heat the water, which is often needed to be used to top up heat generated by the solar thermal toobs.

to expand on my explanation above, the electricity which you're not exporting because you're using it in the house will need to be replaced by fossil rule generated electricity using either gas or coal, which is around 32% thermally efficient at the point of use, vs your 80-90% thermally efficient gas boiler.

I suppose that in reality, it's not going to make much odds to the grid what happens to a few hundreds watts of power from one small PV installation, but if this became a widespread situation then it would have an impact as described above.

The chance of anyone using all that they generate is ... um -10. It is never going to happen. But I would have thought it reasonable simple to turn on small loads automatically (not variable loads) if the grid is not supplying.

Some of you seemed to have missed this post If you are in the house you can obviously think about turning something on. I am just wondering if I can use most of what I generate, idealy in a why which might replace / offset electricity (or other energy) I could be using when I am not producing. I can find a 400 watt and 500 watt 240 volt imersion heater, but a 250 watt version would be more viable I think.
.

as described above, if you have dishwashers, and washing machines see if they have timer functions and use these to operate them one after the other in the middle of the day if you can, or have a routine of setting the dishwasher off just before leaving the house in the morning, and mostly use the washing machine during the middle of the day at weekends etc to legitimately maximise your in house use of the generated electricity at no extra cost, and with no overall increase in electricity consumption.

eta - if you're boiler isn't at least in the 80-90% efficiency region then I suppose that does alter the equations to some extent. Also though, you indicate that you've got solar thermal, so it would seem likely that the time when extra PV capacity would be available would be just the time when the solar thermal was working at it's best, possibly unless the solar thermal is on an east facing roof, and the PV on a west facing roof.

Apologies if I came on a bit strong, I'm just wary that this could be the next area the papers decide to pick up on and run with in advance of the spending review, as there have already been some indications that sections of the press and possible the government are sharpening their knives and looking at ways to attack the FIT scheme.

[martin W]

 if you have dishwashers....


No, I've checked and the wife definately doe not have a timer

[Billy]

This is the device I was specifically alluding to and has been used on boats for a long time.  For when there is only low amps from shore power.

http://www.asap-supplies.com/search/immersion/switch-immersion-power-reducer-50

http://www.abeltronics.co.uk/pdf/IMM01.pdf

It only appears to go down to 1kW and halves it to 500W.  What is inside the little box I don't know.

Billy