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Some questions for the brain trust

 
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Transmitter Man



Joined: 06 Apr 2008
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PostPosted: Mon Dec 26, 2016 3:13 pm    Post subject: Some questions for the brain trust Reply with quote

My first post so hi to everyone.

I am looking to build an off-grid solar/DC air-con system and would like a few questions answered.

Here's very similar to what I'm looking to build: http://www.hotspotenergy.com/DC-air-conditioner/Specs-DC48VRF.pdf

As you can see this shop supplies 4 x 250W PV panels. I'm looking to have a more efficient charging system and see if I can get away with just two higher power and more efficient panels using SunPowers 360W models - yes, I know they're not inexpensive but I can source them at trade price locally here in Cyprus and we have 300+ days of sunshine.

I intend to use an upgraded dual-axis tracker, not the recycled Chinese pram on eBay but another unit out of Slovenia. Using this it's stand-alone so can be situated completely clear of obstruction through it's days tracking.

My questions are;

1. What brands of 48V charge controllers should I be looking at for long term reliability. It would be great to have the option of Wi-Fi, Bluetooth or 4G remote monitoring of the systems operating parameters.

2. For this size system I hope I do not require those horrendous traction batteries and can make do with some good quality Varta deep cycle ones instead. I'd like to be able to run the air-con system on tick-over through the night and flat out during the day. This covers worst case scenario for DC power requirements.

So, your thought of charge controller & batteries would be much appreciated. Victron's look nice but a little pricey.

Thanks.

David
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adam2
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PostPosted: Mon Dec 26, 2016 3:26 pm    Post subject: Reply with quote

For a relatively large system as is proposed, a maximum power point tracking (MPPT) charge controller is worth the extra cost.
I would not worry unduly as to what brand to purchase, any well known make should be fine.
Buy a spare if worried about failure.

I have rather negative thoughts about PV arrays that track the movement of the sun, moving parts that are apt to wear, jam or otherwise get out of order do IMHO eliminate one of the principle advantages of PV, the absence of moving parts.

You might do better to spend the money on an extra module instead.
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woodburner



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PostPosted: Mon Dec 26, 2016 6:27 pm    Post subject: Reply with quote

What is the construction of the building it is going to serve?
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Transmitter Man



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PostPosted: Mon Dec 26, 2016 7:23 pm    Post subject: Reply with quote

Summer weekend cabin retreat far off the beaten track and at least two km from nearest utility pole. It would be far to costly to pay for a hookup and I don't wish to go there.

David
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kenneal - lagger
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PostPosted: Mon Dec 26, 2016 8:11 pm    Post subject: Reply with quote

Why not look at traditional Greek style construction or even Arabic construction with a cellar and wind tower and do away with nasty air-con. No moving parts and nothing to wear out as Adam alluded to.
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woodburner



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PostPosted: Mon Dec 26, 2016 9:00 pm    Post subject: Reply with quote

+1. air-con is power hungry and the construction needs to be the right type. Even then there are better ways of acheiving some level of comfort without air con.
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clv101
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PostPosted: Mon Dec 26, 2016 9:45 pm    Post subject: Reply with quote

In terms of equipment Victron are, by all accounts, excellent. A very solid reliable product backed by a 'real' company with real people you can talk to. They blog, they tweet, the regularly update their (open) software...

As for the tracking hardware, I'd forget it. With panels as cheap as they are now it's almost always better to just install a 20% larger array. If you're really keen you could have a winter angle and summer angle on your mount but it's not really worth it.
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PS_RalphW



Joined: 24 Nov 2005
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PostPosted: Mon Dec 26, 2016 11:22 pm    Post subject: Reply with quote

We sell a lot of hardware from Victron. Not always the latest design and certainly not the cheapest, but a solid returns policy and the reliability to afford it.
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adam2
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PostPosted: Tue Dec 27, 2016 8:11 pm    Post subject: Reply with quote

QOUTED
Hi Adam,

I will consider your advice however really not too much to go wrong in a tracker other than the electronics which is my profession so I can upgrade as necessary.

KL, Many days there is is next to zero wind while summer temp can rise to high 30's and with also high humidity so really not sure about the wind tower idea. you surely cannot generate wind from nothing.

WB, That's just it a purely DC (48V) A/C system is not power hungry. Efficiency is right up there as you cut out any conversion from the PV's DC up to AC. The compressor is a modern 48V DC unit as is the indoor fan unit.
The other advantage is one can reduce the humidity AND stay cool. The SunPower 360W panels are also breaking new ground in efficiency numbers so see this as really not power hungry and the sun comes free. If any of you have visited Cyprus then you know being much nearer the equator than good old blighty the sun here really is very powerful.

101, As you're the second in this thread to point out possible tracker issues I will think hard on this. It would be nice if I had the time and funds to do a A/B comparison re DC power generation between fixed and tracked PV systems but I agree you're probably right and investing in another couple of panels may be the best way to go.

OK on the Victrons. There is a master distributor here for this brand so I'll get some pricing and compare with DODGY TAX AVOIDERS!

Thank you all.

David


QOUTE ENDS

I have locked the other thread on this subject, and quote the only post from that thread here, so as to keep the discussion in one place.
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adam2
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PostPosted: Wed Dec 28, 2016 8:42 pm    Post subject: Reply with quote

Care should be taken to avoid misunderstandings when referring to a "tracker"

A maximum power point tracker is a solid state electronic device that optimises battery charging from PV sources and allows use of a significantly higher array voltage than the battery voltage.
The well known makes have a good reliability record and are straightforward to replace if defective.

An array tracker is a relatively complex mechanical mechanism with electric drive, it physically moves the array so as to gather the most energy as the sun moves across the sky. IMHO, these are best avoided in view of the moving parts and consequent risks of breakdown.
I would prefer more modules or larger ones, to an array tracker.
Note that very few large solar farms track the array.

Air conditioning is a relatively power hungry load and until recently was seldom viable for off grid PV. With the present much reduced PV prices, and the increased efficiency of air conditioning units it is increasingly worth while.
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clv101
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PostPosted: Wed Dec 28, 2016 10:26 pm    Post subject: Reply with quote

adam2 wrote:
A maximum power point tracker is...
...a no brainer on all but the smallest PV systems.

adam2 wrote:
An array tracker is a relatively complex mechanical mechanism with electric drive, it physically moves the array so as to gather the most energy as the sun moves across the sky.
There was a lot of interest/research into trackers in the 70s and 80s, when PV cells were very expensive. These days, PV cells are pennies to the dollar compared with a couple of decades ago - but, critically, steel, motors, maintenance haven't fallen in price similarly. Back when the PV cells were by far the most expensive part of the system it was sometimes worth getting the extra output offered by a twin-axis tracker. But these days it's pretty much aways better just to build a slightly larger array.

Space applications are an obvious exception, where lifting cells into orbit is very expensive and all the insolation is direct (oppose to defuse) making correct orientation more important.
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kenneal - lagger
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PostPosted: Thu Dec 29, 2016 6:54 pm    Post subject: Reply with quote

A tracking array often has only one fixing point to the ground so the panels are more at risk of wind damage, which could be a more significant factor in the future with global warming causing more intense weather systems.

Does your area have high night time temperatures TM or does it cool at night? If you have lower night time temperatures you can use thermal mass and night time cooling to reduce your day time internal temperatures with no more moving parts or energy use than opening and closing windows and shutters.
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ericgarmin



Joined: 17 Dec 2016
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PostPosted: Thu Dec 29, 2016 11:16 pm    Post subject: Reply with quote

What is the cost *approximately* of such a system might I ask? Smile
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mikepepler
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PostPosted: Wed Jan 11, 2017 8:57 pm    Post subject: Reply with quote

I've had a Morningstar Tristar MPPT 60A running for over 3 years now with no problems, and can recommend the controller. The nice bit about the design is it has no moving parts, while competitors tend to have fans. I'm using it at 12V, but it will also work at 24 and 48V.

I'd echo what the others say about aircon being power hungry though.

If it goes cold enough at night you could consider a system based on PCMs though, like this one we awarded at Ashden in 2013: http://www.ashden.org/winners/monodraught13
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