Just as an excrcise in futility how would you ...

Re: Just as an excrcise in futility how would you ...

it may be futile, but not impossible. it may truly depend on the specific loads themselves, but given enough pvs and batteries plus the space and $ for it all it probably could be done. the inverters may need to be extra large, but they are out there.

for the peak power draws this can be through generators or even more pvs backfeeding through a gt inverter to supplement. it won't all be without its drawbacks and complications. it might be nuts, but it's not bananas.:p

Re: Just as an excrcise in futility how would you ...

Here's a great look at an awesome battery bank. VERY impressive. At least to me.
http://www.aeesolar.com/sites/default/files/Surrette-2013DealerConference.pdf

Re: Just as an excrcise in futility how would you ...

Another option is using Outback inverters which can be ganged together (ie the FX series 48 volt) in a series/parallel quad arrangement would provide up to14.4 kw. You could also have 2 separate battery banks with such an arrangement if need be.

Re: Just as an excrcise in futility how would you ...

In Arizona there's so much sun and very low humidity. Swamp coolers should work really well. At least as a backup source of "coolness".

You don't need AC by nights because your nights are cold. Right? Sucking cold outside air in during the night may save quite a bit of energy

Between conservation, clever load management, and perfect design of the system, this may not be that bad.

Dave Sparks has several customers off-grid in Arizona who don't even use generators!

Re: Just as an excrcise in futility how would you ...

solar_dave wrote: »

BTW there are lots of different climates in AZ from desert valley to Alpine forests running up to tree line.

I figured if you need so much AC you're somewhere south of Phoenix in the desert. Continental deserts usually have cool nights. It'll not be the coolest at midnight, but at 6 am, shortly after sunrise. Do you still get 100F at 6am? Where are you then?

You do not need to run swamp coolers all the time. When it gets humid, you start your big generator and run your ACs. Geothermal cooling is a good alternative too.

Re: Just as an excrcise in futility how would you ...

Far easier/cheaper to go with mini splits, with similar performance as ground-source heat pumps. Technically it isn't "geothermal" unless you are going really deep for heat.

Re: Just as an excrcise in futility how would you ...

solar_dave wrote: »

Night Cold? LMAO last night at midnight it was still 100 F. Yes swampies would work except when the monsoon hits, I have seen it rain at 115F here. It then cools to about 105 F.

BTW there are lots of different climates in AZ from desert valley to Alpine forests running up to tree line.

Still was running my swamp cooler through the night last night. It got below 80 last night but the slump block on the house was still very warm and sending heat into the house (This causes ACs to run for muich of the night even when it is colder outside than in). Swampies are awesome as long as the dew point is below 40 (borderline 40-50). 0.5KW sure beats 3.5-4KW. There is a large night/day difference but nights don't get cold by human standards until late october. July can have some near-hundred-degree nights.

Re: Just as an excrcise in futility how would you ...

NorthGuy wrote: »

I figured if you need so much AC you're somewhere south of Phoenix in the desert. Continental deserts usually have cool nights. It'll not be the coolest at midnight, but at 6 am, shortly after sunrise. Do you still get 100F at 6am? Where are you then?

You do not need to run swamp coolers all the time. When it gets humid, you start your big generator and run your ACs. Geothermal cooling is a good alternative too.

AZ has about 60 days over a 90 day period that must be cooled by AC. Phoenix gets a lot of breaks in the monsoon. Tucson is not so fortunate. Last week had the pre-monsoon humidity spike (dew point in the upper 40s), which marks that in approximately 2 weeks, the monsoon humidity will be here (dew point 55+, usually in the low-mid 60s). The monsoon dates for public consumption are 6/15-9/30 but weather people still use the dew point definition - dew point 55+ for 3 consecutive days is the start, and a day is a 'monsoon day' if the dewpoint averaged 55 or more that day (including the 3 required to 'start' the monsoon). The meteorlogical moonsoon typically starts in the first or second week of July.

Re: Just as an excrcise in futility how would you ...

Well that is assuming no direct support from the panels mid day

Why not start by designing a battery-based system for daytime A/C running only. If combined with a large array, the bank only has to be large enough to supplement the A/C loads for a while if it clouds up heavily (and to at least match the minimum size required by the inverter). The A/Cs would turn off (and ideally on, although this is a bit trickier) as battery voltage dictated.

That's more or less how mine are running. I just have two window units: 8K and 6.5K btu. They go on manually or with a timer. They go off when the battery voltage drops below a setpoint (actually only one is rigged to go off this way, I shut the other off manually). When pv production falls off, if it's still too hot, our grid-connected central heat pump eventually kicks on. The heat pump goes on pretty infrequently; right now with the heat index in the lower 100s, it's probably running a total of about 30 minutes per day, although we don't keep our house cold (about 79 F).

My solar array is 5.5 KW, and the battery bank just 17 KWh. This is on the gulf coast (Lat +30). The array can actually support the 8K BTU unit even during moderate overcast or mostly cloudy skies. Only a pretty dark, slow-moving rain cloud pattern will drop the voltage enough to shut it down.

I know a system like mine would be too small for your needs. But it could be doubled in size and still work with standard L-16 type batteries. And it could be made more efficient with mini-splits.

Ideally, the mini splits would go on and off in response to available surplus, with variable speed compressors controlled by the charge-controllers. I haven't seen how to do anything like this, but seems like it should be possible in principle.

Re: Just as an excrcise in futility how would you ...

solar_dave wrote: »

SO back to the original question.

Lets assume we could reduce the max consumption to 100kWh per day. 100,000watt hours / 48 volts = 2083 amp hours / .50 (percent discharge) = 4166 amp hour battery bank
so two strings of 24 of these http://www.solar-electric.com/repoba2vo25a.html 2550 amp hours @ the 20 hour rate each string = 5100 amp hours ($65,000)

SO to Charge it it would need 500 amps to get the 10% rule of thumb.

Sounds good, except the bit about using a stab-in-dark rule of thumb when working with $65,000 worth of battery

I would pick a few typical scenarios and then size the array and battery based on those circumstances. E.g.:

Scenario 1: Mid summer, AC blazing, no clouds: how much array do you need to keep the battery bank charged?
Scenario 2: Mid summer, AC blazing, raining: what size battery do you need (looks like you've answered it above).
Scenario 3: A clear day after scenario 2: what size array do I need to recharge the battery and support the AC?
etc.

Then ask yourself how often those scenarios occur during the year, and if they occur infrequently, then consider using the generator to supplement and cut back on the PV array size.