120 VDC Solar System vs 48 volt « Alternative Technology Assn Forums

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120 VDC Solar System vs 48 volt

(11 posts) (6 voices)

Started 8 years ago by hoodle65
Latest reply from TonyT

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hoodle65
Member

Hi

We are building a house that is totally offgrid - no grid for power, sewerage or water.

I'm a little beyond my competency level and would like to ask for some help/guidance please.

We have spoken to a few different companies and get conflicting quotes and recommendations. We have recommendations from different companies for 24, 48 and 120 volt systems.

The house is on a farm. We don't use welders but we do run an internet business from here so power is important. Currently we use 45 kwh/day (yes I know it is high). We can reduce this to about 25kwh/day by replacing water pumps, fridges, freezers etc. We don't use electricity for airconditioning, heating or cooking.

I have three questions:

1) which is better to run a solar array to 24, 48, or 120 volt?
2) what sort of questions should I be asking suppliers?
3) does the items specified below look adequate.
Below is our preferred supplier at the moment. The batteries are wet cell.

40 panels = 10 kw solar, tilt racks, twin isolators, cable clips, conduit, conduit connectors,parrallel connectors, earth kit, sealed roof entry point.
Dc/Ac breakers, enclosures, twin controllers, ANL fuses,
emergency shutdown switch, cabling
120 VDC x 1600 amp hour battery bank, battery bank rack,
sub-board, enclosures, SP-PRO Selectronics inverter 7.5kw
battery bank charger 70amp.

Thanks
Steve

Posted Monday 8 Apr 2013 @ 2:20:08 am from IP #
Lance
Key Master

For that level of energy use, which is effectively 1kW continuously, then the battery bank should be a 120V unit really.

But, that energy use is still very excessive (and 45kWh a day is not high, it's insane!). To give you an idea, we have a semi-rural house, crappy Davey bore pump which feeds our place and the farm next door, run 2 internet businesses and so two PCs (one a laptop) get used 12+ hours a day, a 420 litre fridge, all the usual stuff, big entertainment system (46" LCD, big amp etc) and we average around 10kWh a day.

So, there is something seriously not right if you can only get the energy use down to 25kWh a day, especially if sir con etc isn't using electricity. Are you using elec for water heating? If so, then that's a big user and changing to a heat pump or gas boosted solar will make a huge difference.

Really, you need to do a full energy audit, find out what's using all the electricity and fix it.

Posted Monday 8 Apr 2013 @ 2:35:53 am from IP #
hoodle65
Member

Thanks Lance

I've just bought two energy meters and am doing the rounds of all the energy equipment. We have a gas instant on hot water. Appears that the old fridge uses 2kwh, old freezer 3kwh, old pump 8kwh, .....

Best
Steve

Posted Monday 8 Apr 2013 @ 2:58:12 am from IP #
Lance
Key Master

Wow, that pump sounds scary, ours is using around 3kWh a day for the 2 properties and I thought that was excessive, what's the pump actually pumping, is it a mains pressure pump or a pool pump?

Posted Monday 8 Apr 2013 @ 4:57:46 am from IP #
hoodle65
Member

Hi Lance. It is a pool pump that is 10 years old. What I am learning is that almost anything that is old draws way more power. We will buy new fridge, freezer, pumps etc when we move.

We have a spa that uses 6 kwh a day. I bought it because it claims to be the most energy efficient on the market. We want to keep it but run it in a different way. We are thinking of solar heating it by using either a pool solar heater or an evacuated tube system linked to a insulated storage tank of about 500 litres. Do you know if there are temperature activated valves/pumps we can buy that will transfer water to/from spa when it falls below a certain temp?

Best
Steve

Posted Tuesday 9 Apr 2013 @ 12:03:01 am from IP #
Benny
Member

An evacuated tube HWS will have the "temperature activated pump" that you need for the spa. Just pump the hot water in to the spa instead of the HWS storage tank. You might want to set a limit on the ET controller though so it doesn't heat your spa to 60C.
Did you get a price for the system you specified ? I see people on here paying 10's of thousands for an off grid system that just supplies 5-10kWh per day.
All the advice I see for going offgrid says to do everything possible to reduce loads before you get the system - its far cheaper save the kWh than trying to get an offgrid system to supply the kWh.
Does anyone have a ball park figure for $/kWh for off grid ?

Posted Tuesday 9 Apr 2013 @ 6:40:27 am from IP #
franks
Member

$5 per watt of stand alone system size with balance of system components, seems to be the going rate for a turn key install.
Saying that I did a my own install, 4.45kW PV, 24volt 1200 amp hour flooded battery, 3kW Inverter Charger, total cost $10k.

Posted Tuesday 9 Apr 2013 @ 7:00:01 am from IP #
hoodle65
Member

I'm getting a final quote at the moment for the offgrid. Have had quotes from under $2k per kwh to $5k per kwh.

Posted Tuesday 9 Apr 2013 @ 7:13:00 am from IP #
murry
Member

Steve. I sympathise with your predicament.Like you I use 50kWh a day!!
What with 4 adult kids still home and an all electric house on a rural block in Vic.
( Gota sell some kids). I have done something - and still am doing something - about this,
mainly as others here have said REDUCE consumption.Now to some input for what its worth-

1.The Selectronics SpPro is not a good choice. There a 2 models available SPMC-482 and
SPMC-1201 and as Lance has pointed out for your usage (40+kWh) you will need to go for
a 120V system (to avoid cable size and cable loss issues). This means you will be looking at the SP-1201 inverter. Cost about $12000.(Go to the Rainbow Power Company website).
The reasons why this inverter is not a good choice are as follows-
1..This inverter has a DC input window of 100 to 170 volt.Your pv system is 40 panels for 10kW ie. panels are 250W. Depending on the panels they could be anywhere from 36Voc to 48Voc each (figures are approx).If we use 36Voc then to get under the inverter limit of 170V you would need maximum of 4 panels per string thus your array would be 10 strings of 4 panels each. Major fusing/cable size issues for 10 strings.
2.. As you state this is to be a stand alone system (not GC) The SpPro is designed as a grid interactive inverter (although it need not be connected to the grid)- hence its cost and it will be less efficient than alternatives.
It may be that your sales people/designers are planning an AC coupled system SpPro together with a Haco (grid inverter}. If so they are lazy and are wasting your money.
You dont need the option of grid interactive so dont pay for it.
Another major issue for you (and all stand alone systems) is your storage.Your use of 40+kWh per day means that you will require about 1600Ah of storage at 120V (for a depth of discharge of 25%).Your usage is daily so you will need to use the C20 (20 hour) rating for the batteries.
BAE 12V 1800Ah are close to what you need (their C20 is 1540Ah). Cost $9000 each (plus gst) and you will need 10 of them for 120 volt. There goes $99,000.

Advice (if your still standing)- 1..reduce demand 2.. Get yourself an experienced/reputable designer (qualified in stand alone) to design the system 3.. Keep asking lots of questions about any design
My own preference for a stand alone would be DC coupled. Probably with a Midnight HV
charge controller (or Xantrex),I would also look at going 15-20kW on your Pv. Much cheaper to install more Pv and charge a smaller battery than to install a larger battery and not quite fully charge it (on overcast days)

Regards.

Posted Tuesday 9 Apr 2013 @ 1:22:57 pm from IP #
hoodle65
Member

Thanks Murry - certainly lots to think about. The learning curve is quite steep for me))

Posted Wednesday 10 Apr 2013 @ 6:57:19 am from IP #
TonyT
Member

Useful info at
http://www.ata.org.au/forums/topic/2265#post-19856

Posted Wednesday 10 Apr 2013 @ 7:43:38 am from IP #

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