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Tell us more mobbsey

 
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RogerCO



Joined: 24 Nov 2005
Posts: 672
Location: Cornwall, UK

PostPosted: Mon Jan 02, 2006 10:28 pm    Post subject: Tell us more mobbsey Reply with quote

mobbsey wrote:
Some days you just want to weep! (luckily, my PV on the roof, charging 200Ah of batteries in the roof, and the back-up energy sources for heating and cooking, mean I don't have to!).

What sort of batteries, how many, what capacity, how connected, what (if any inverter), what sort of PV (solarcentury tiles or something more basic), how much, roof direction and angle, how connected, what circuits in house supplied, any sale back to grid, what sort of heating/cooking backup, solar hot water, wood cooking, total capital outlay, running costs, battery life estimates, etc etc etc
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Bandidoz
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PostPosted: Mon Jan 02, 2006 11:59 pm    Post subject: Reply with quote

I believe it's very much a DIY affair, completely off-grid.
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mobbsey



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PostPosted: Tue Jan 03, 2006 1:31 am    Post subject: Re: Tell us more mobbsey Reply with quote

RogerCO wrote:
What sort of batteries, how many, what capacity, how connected, what (if any inverter), what sort of PV (solarcentury tiles or something more basic), how much, roof direction and angle, how connected, what circuits in house supplied, any sale back to grid, what sort of heating/cooking backup, solar hot water, wood cooking, total capital outlay, running costs, battery life estimates, etc etc etc


Wow, y'wanna know a lot! However, the systems rigged, along with the rest of the kit we keep in the house, for getting around the problems of the power or gas grid going down.

It was never intended as a household power system. I use it for developing renewable electricity gadgets as part of the projects that I consult for in the UK/elsehere in the world. However that's hardly a drain so it also runs a low voltage system in the workshop (laptop, some test equipment, soldering irons, bench drill, etc.) -- basically, small-scale electrical R&D/fabrication.

In the event of the power going down, it'll keep a few lights on around the house and, most importantly, the central heating pump ticking over (at least, if the gas holds up after the power goes down) for four to seven days (depends how much I need to run the central heating to stop the pipes freezing up).

The batteries are Marconi liquidation surplus (6V/100Ah each -- traffic light batteries!) bought from Bull for 25 quid each. The PV is 4 x 15W amorphus panels strung (literally, on rope) from the roof facing east-south-east. However, given we live on a north facing slope, with higher buildings to the south of us, east is actually the best aspect!

I try not to use inverters... why burn 0.5 amps (150W modified sine wave inverter) to 2.5 amps (1kW full sine wave inverter) of current for nothing just to produce mains power? Most of the lighting I have is 12V, and I even have a 12V NiMH cell charger so that I can keep the torches, radios, etc. running. You really do save a lot of power by avoiding inverters.

Electricity makes up less than 10% to 12% of the average household's energy consumption. If you take out the washing machines/water heating/kettles out of the equation, as you would in the event of the power going down, and you use low voltage/efficient lighting, then you can get by on just about 100W to 800W (depends if the central heating pump is on or not).

Most of the energy we use in houses is space heating, so the PV system isn't of much use there (do the sums -- you're looking at an entire solar clad roof with a sub-basement of batteries to take you through the Winter months... so why bother?). In the event of the grid(s) going down we also have candles (lots), a primus stove (with 10l fuel), oil lamps (with 5l lamp oil), wood/coal to burn on our fire, and as we are part of a wholefood co-op we always have about 2 to 3 weeks (minimum) of flour, pulses and other food to hand so that we needn't run out to the shops.

So... it's getting cold at the end of the this week and the Russians are not pumping as much gas: Bring it on!

In case your interested in the wider planning issue, as I've done some research on this the Free Range Network asked me to write something for them last AUGUST (forget what you hear in the media -- the industry's known about this problem for 18 months!):
http://www.fraw.org.uk/pubs/frb/frb-05_01.html

P.
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peaky



Joined: 24 Nov 2005
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PostPosted: Tue Jan 03, 2006 3:02 pm    Post subject: Reply with quote

Question Could I point you good souls over here ... http://www.powerswitch.org.uk/forum/viewtopic.php?t=1416
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RogerCO



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PostPosted: Tue Jan 03, 2006 8:37 pm    Post subject: Reply with quote

Thank you very much for a useful and comprehensive reply. I, at least, found it very helpful in confirming my thinking and the direction I'm heading.
Starting with a focus on the workshop is a good way of bringing things into play without 'her indoors' going off on one Wink

I particularly take the point about needing deep cycle Lead-acids not car batteries - but having said that can anyone enlighten me exactly what the problem with using a spare car battery to rum the CH pump in case of power loss and topping it up with PV would be?
In this case given a 240v operating pump and inverter becomes necessary despite the losses involved
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Bandidoz
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PostPosted: Tue Jan 03, 2006 8:47 pm    Post subject: Reply with quote

Car batteries don't like getting heavily discharged; their life assumes they only ever get partially depleted and otherwise topped up continuously from the alternator. In the old days the electrolyte would change pH and the plates may even fall out if heavily discharged. So if you use a car battery it would be reliable only if the power source is there most of the time.

Deep-cycle (e.g. submarine or fork-lift truck) batteries work on the assumption of (almost) total discharge.
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mobbsey



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PostPosted: Tue Jan 03, 2006 10:38 pm    Post subject: Reply with quote

RogerCO wrote:
I particularly take the point about needing deep cycle Lead-acids not car batteries - but having said that can anyone enlighten me exactly what the problem with using a spare car battery to rum the CH pump in case of power loss and topping it up with PV would be?


Primarily, getting enough PV to run a pump will be more of a problem than getting the lead-acid cells. But you also have to realise that using higher-cost/higher quality PV and batteries for a large system will actually cost you less for the same capacity than trying to get by with cheap PV and cheap batteries.

It really all comes down to the space required and weight for the capacity of system you're going to build. Higher quality kit costs more, but you need less of it, so it would out a little cheaper in the end.

My 60W array (4 x 15W) of amorphous panels will set you back (at book prices) ?600, but 60W of monocrystalline will take up half the space and costs (book price) ?500 (if you're wondering why I bothered with the amorphous cells I got them in a sale, for ?70 each instead of ?150, precisely because people don't want them as much as mono/poly-crystalline cells).

Your central heating pump is rated at around 500 to 1000 watts (in practice it might draw only 200W to 600W of continuous power, depending on the size of the system). A square metre of PV might, and an average bright day, produce 40 to 80 watts per square meter (amorphous cells produce about half the power of monocrystalline/polycrystalline silicon, but for the sake of the maths let's call it 50). In terms of instantaneous power you're going to need 10 to 20 square metres, plus about 20% for the inverter efficiency losses = 12 to 24 sq.m of PV to run the central heating pump directly!

The alternative is to charge a large battery bank for a long period of time and then use the power directly for the short periods it's needed. Let's aim at 480 watts of continuous power as our target figure. Producing 240W from 12 volts will require (240/12) 20 amps. Add the inverter losses on top (say 20%) we multiply by 1.2 = 29 Amps. Running your pump for 1 hour (which more than is enough to defrost the house to stop the pipes freezing -- which is my major concern) will therefore take 29Amp-hours. However, the charge/discharge cycled on the batteries will be between 20% and 40% efficient (depending on their age) and so you'd actually have to scale the PV array by this factor to get that amount of power outr of the storage again.

Deep cycle can be discharged to about 50% of their capacity (in practice it depends how fast you discharge -- the faster you discharge the more current you can take). Ordinary sealed lead-acid cells can be discharged by about 20% of their rated capacity. With car batteries you're lucky to get 10% before you start warping the plates. So, deep cycle cells are worth the extra money purely because you don't need as many, and so in the end the cost works out cheaper (if you include the costs of strengthening the floor to take the weight of a huge pack of car batteries).

So to run the pump for, e.g. 5 hours, would take (29 x 5) 145Ah. If we base that on 50Ah 12 volt deep cycle batteries that would require ( [145 / 50% discharge] / 50Ah =) 5.8 cells -- which at about 15 kilos/?100 per cell is 87 kilos/?580 of batteries. For 50Ah sealed lead acid (20% discharge/15 kilos/?60 each) you'd need 14.5 cells, which would be about 217 kilos/?870 of batteries. For the standard 35Ah car battery (10% discharge/10kg/?15 each) you'd need 41.4 cells, which is 414kg/?621.

If you have 200Ah of batteries (my 4 x 6V/100Ah together produce 200Ah at 12V) then you can run the pump for just 3.5 hours. So, running the pump for 30 to 40 minutes per day, you might get five to six days of pump activity out of the battery pack (although, because the heat energy is stored in the water inside the pipes and the spaces between the floor joists, that's enough to prevent freezing).

Not also that the initial current surge on a 500W pump might be 1,500 watts -- so you need to get either a bloody big inverter (which'll waste power because the extra capacity burns more power) or you need to get a inverter nearer to the contiinuous current drain but which can take large transients for a couple of seconds without blowing up/shutting down.

My low voltage fluorescent lights are far more civilised -- they only draw avout 0.6 amps each to I could run all four (2.4A) for about 42 hours. In practice then it's a balancing act between keeping the water pipe fluid and running the lights!
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Ballard



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PostPosted: Tue Jan 03, 2006 11:34 pm    Post subject: Reply with quote

I've currently got 4 x 90AH 12V sealed lead acid car batteries (I can get them for free so I thought why not?), over time I should be able to build this up to possibly 16 or 20 (1800AH, but then I will have a rack of five by four batteries which is quite large). I've also got a 2000 watt pure sinewave inverter / charger, and an 80 watt module.

If I am only able to discharge these batteries to 80% and I wish to run my central heating pump for three hours I guess I will need at least 16 (with inverter losses etc).

I'm currently maintaining the batteries with a float charger, and will use PV modules in the future. I may switch to deep cycle batteries later, but I would rather use the money for solar thermal. However I'm not really sure how to power the central heating pump. Any ideas on how you would actually do this?
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mobbsey



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PostPosted: Wed Jan 04, 2006 7:24 am    Post subject: Reply with quote

Ballard wrote:
over time I should be able to build this up to possibly 16 or 20 (1800AH, but then I will have a rack of five by four batteries which is quite large


Something I forgot to say -- FUSE EACH BATTERY INDIVIDUALLY.

If you've got one battery your short-circuit surge will exceed 500 amps. If you've got 20 batteries strung in parallel the short-circuit surge current will be 500 amps PER CELL -- which'll quite nicely spatter you with molten copper if you're stood nearby at the time.

I've only ever seen a bank of batteries short their busbars once (I think it was 14 or 15 car batteries, running the sound system at a free festival, when someone accidentally dropped a screwdriver across the angle irons that were being used as the busbars)... and it's spectacular, but I'd rather not be in the vicinity.

If you fuse each battery individually [let's say you're inverter draws 180 Amps at peak, so divided by 20 batteries that's 9 amps each... if you use a cheap inline automotive blade fuse holder (most of them are rated 30A) with a 15 amp fuse that'll be OK] then if you did ever accidentally short the supply you're not going to hurt yourself with the impromptu fireworks/welding that'll result.


Ballard wrote:
(1800AH, but then I will have a rack of five by four batteries which is quite large). I've also got a 2000 watt pure sinewave inverter / charger, and an 80 watt module.


2000 Watts will draw about 180 amps peak... that's 3 Amp-hours a minute! On an average Summer's your 80 watt panel might produce that in an hour, and on an exceptionally good day in half and hour....

I suggest you get a few more panels otherwise you're going to be burning most of the input to your battery bank on the self-discharge of the storage (err... did I mention self discharge?... about 5% of the battery capacity per month, but it's dependent upon the temperature you keep that battery at and how old the batteries are).
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Bandidoz
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PostPosted: Wed Jan 04, 2006 7:56 am    Post subject: Reply with quote

Can the inverter be driven from 96V? If so, would it not be feasible to have the batteries arranged in 4 series banks of parallel blocks? (likewise with the solar PV). 200A is a scary amount of current to be handling - not to mention lossy. And don't call me "Tesla". Razz
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mobbsey



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PostPosted: Wed Jan 04, 2006 8:15 am    Post subject: Reply with quote

Bandidoz wrote:
Can the inverter be driven from 96V? If so, would it not be feasible to have the batteries arranged in 4 series banks of parallel blocks? (likewise with the solar PV). 200A is a scary amount of current to be handling - not to mention lossy. And don't call me "Tesla". Razz


Current's not a problem, but with 200A you're looking for at least 6mm to 8mm min. of copper woring otherwise you'll burn the plastic of the cable.

Also, I'd quite happily fool around with 200A at 12V with my bare hands... 96V DC (even if the internal insulation of the batteries could handle the voltage, which I doubt) would kill you!


By the way, a sort of little off-topic may be... you know that since the introduction of Part P of the building regulations on 1/1/05 all of this stuff we're describing, if it links to your house, IS COMPLETELY ILLEGAL!

You have to have an "approved" electrician to write you a building regs approval, or you have to pay your local council to certify it for you (and guess what, they're not going to like old car batteries).

Luckily mine's been installed for 2 years to it's outside the regs.
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mikepepler
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PostPosted: Wed Jan 04, 2006 9:13 am    Post subject: Reply with quote

A couple of other points on batteries. At college we've done quite a bit of theoretical work on sizing batteries for Solar PV systems. It takes into account all the things mobbsey's mentioned, including the number of Ah needed to give X days autonomy, the allowable depth of discharge, etc.

Now I've not done much of it in practice yet, but here's a few bits I've learned, perhaps mobbsey can confirm or correct me?

- as well as discharge depth, discharge rate needs to be taken account of. Most battery capacities are rated for a steady 20-hour discharge. Any time you spend drawing current above this rate will reduce the Ah you get out, due to losses in the battery.

- There are batteries specifically designed for solar PV use, which are even better than the "normal" deep cycle batteries generally available. I know Varta make some, as I have data sheets for them, but there must be others too. These batteris are rated to discharge to 80%, which is more than normal, yet still have 1-2000 cycles before they pack up. As Paul said, special kit like this costs more, but if it lasts much longer it can be worth it.

- As Paul said, car batteries can only cope with a tiny discharge level, so little as to make them almost useless. A friend of mine was travelling in Australia last year in a camper van. He set up a second battery in the vehicle to run a fridge overnight and a lamp. After a single deep discharge it was knackered, and that was a new car battery. OK, it was a cheap one, but it's a worthwhile lesson.
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Bandidoz
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PostPosted: Wed Jan 04, 2006 2:57 pm    Post subject: Reply with quote

mobbsey wrote:
96V DC (even if the internal insulation of the batteries could handle the voltage, which I doubt) would kill you!

96V / 20k == 4.8mA. It may make you jump, but it's unlikely to kill you.
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mobbsey



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PostPosted: Thu Jan 05, 2006 6:14 am    Post subject: Reply with quote

Bandidoz wrote:
96V / 20k == 4.8mA. It may make you jump, but it's unlikely to kill you.


Where's the 20K from? -- the resistance depends on the distance of the discharge path through your body.

I was taught that anything above 80V DC (and it's the DC element that's the problem here) is hazardous, although it does depend on which bit of you it discharges through (a DC battery system discharges, rather than grounds, because it's not working again earth like you home mains supply).

Actually, 48V gives you a tingle, but above 80V the potential difference is enough to flow through your body, and being DC if your hand is around the live conductor you're going to tense up and find it difficult to move (unlike AC which feels like some's vibrating your muscles and it makes them go numb/flabby).

I've always found that electric shock are rather like lightening -- it would be nice to get struck by lightening at least one, just so see what it's like, but I can't say I'd volunteer for the pleasure.
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Bandidoz
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PostPosted: Thu Jan 05, 2006 6:21 am    Post subject: Reply with quote

20K is typically hand-to-hand (via the heart....). I've even tried out measuring it! Threshold of perception is 10mA (and we're most sensitive to 50Hz of all frequencies!).

I habitually put one hand in a pocket or behind my back, and I have touched mains at least once, with shoes on!
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