Xantrex MPPT question

lazza

HI

I've been sizing a largish installation. It will have 30 245Wp panels going through two Xantrex XW MPPT 60 150 charge controllers.

When I sized it the battery bank was going to be 48V. However the owner now wants only 24V. I've noticed a potential problem with this.

The Xantrex is rated 60A maximum input current at 150Vdc. As there will be 5 strings of 3 panels in series (Voc 36V*3= 108Voc, Imp 8.17A*5=40.85A) this is well within the Xantrex parameters.

However, i've realised that it also has a 60A maximum output limit. Does this mean that the Xantrex will limit production to 60A at 24V therefore (1440W)?? How will this work?

vtmaps

Re: Xantrex MPPT question

lazza wrote: »

However, i've realised that it also has a 60A maximum output limit. Does this mean that the Xantrex will limit production to 60A at 24V therefore (1440W)?? How will this work?

With panels being historically cheap, lots of folks are "over paneling" their controllers. Midnite and Outback controllers will just clip their output to stay within their specs... I think (not sure) that Xantrex will do the same. Have you considered a Midnite Classic or an Outback FM80? The Classic can handle over 90 amps on the output and the FM80 can handle 80 amps.

Also, you should reconsider your array configuration... with a 24 volt battery, all of these controllers will be more efficient and run cooler having two (rather than three) panels in series. That is especially true when working at their limits in a warm climate.

--vtMaps

jcheil

Re: Xantrex MPPT question

lazza wrote: »

However the owner now wants only 24V.

Just curious, why would the owner only want 24v?
And (I am guessing you are a professional installer), why would you even listen to the owner?
Like, when I go to the DR, if they tell me I need a certain surgery, I don't argue and tell them I want something else instead

lazza

Re: Xantrex MPPT question

Clients are normally very stubborn and every one is a genius here. Hence i spend most of my time repairing half-baked installations.

It was a simple cost issue in this case and never having worked with MPPT until now, I wasnt aware of their limitations at lower battery voltages.

A combination of idiocy on both parts

jcheil

Re: Xantrex MPPT question

lazza wrote: »

A combination of idiocy on both parts

Not you, because at least you (like myself) admit our limitations Sounds like this client is one I would "RUN" from. They are going to own you forever

stephendv

Re: Xantrex MPPT question

7kW of PV at 24V is madness. Guess at least you'll make more out of it, since he'll now need 4 controllers instead of 2. AC coupling is also an option, you can pick up a 7kW transformerless SMA sunny boy for under 1000 euros in the surplus/used market places.
If he wants to stick to the DC route, the 80A controllers would be a much better fit than the 60A Xantrex.

Dave Angelini

Re: Xantrex MPPT question

lazza wrote: »

Clients are normally very stubborn and every one is a genius here. Hence i spend most of my time repairing half-baked installations.

It was a simple cost issue in this case and never having worked with MPPT until now, I wasnt aware of their limitations at lower battery voltages.

A combination of idiocy on both parts

change your design to use the xw-80-600 and the install will be much easier. If you need to use all the panels use a xw-60 -150 to use the extra panels. They work really well together and a a bonus the -60 display will scroll the system data at the equipment location and the SCP can be installed in the house somewhere convenient.

Other than needing an extra controller, a 24V DC system can make alot of sense if you have 24v loads like a freezer, lighting, fans, more pumps, etc. There are also cases where you do not want to pay for the extra 48v battery cost/complexity as the 24V capacity may meet your requirements.

Cariboocoot

Re: Xantrex MPPT question

There are actually some good reasons to choose 24 Volts over 48. But not in this sort of power range.

Unless there is a need for the high Voltage capacity of the XW 600 80 (such as long wire run from array to controller) don't spend the money. You can buy MidNite Classic at 1/2 the price.

7350 Watts on a 24 Volt system would be 235 Amps, or three 80 Amp controllers. That's nuts. Has he really got a 2,350 Amp hour battery bank? 'Cause that's nuts too.

On 48 Volts it would be 118 Amps, which is still substantial and would require two 80 Amp controllers.

I wonder why he "needs" 14kW hours of stored power? Must have skipped Rule #1: conservation.

lazza

Re: Xantrex MPPT question

HI

I appreciate everyones input and it's been a big help in identifying the problems (which the supplier was quite oblivious to and is not particularly clear in the Xantrex XW 60 150 manual) with using just two XW 60 150 contollers for this system. Mea Culpa too.

However, one has to realise that it all comes down to the client and costs. They say "I want 10kW of panels" and you say, "that's not necessary you only need 5kW"... "no no no I want 10kW" so we settle around 7kW. Thus I sized the system with a 48V bank... but "I dont want so many batteries that's too expensive".... etc.

My job is to make an non-optimum situation work. That's just the way it is here, otherwise you dont get work and with near on 40% unemployment in southern spain (dont believe the hype of a spanish recovery... it's well down the toilet).. you get the picture.

Looking at settling for 4 x 75/50 Victron MPPTs putting just 2 panels in each series http://www.victronenergy.com/solar-charge-controllers/mppt7550/. They look a lot worse and more basic than the Xantrex, but at least they're designed for 24V systems and cost the same as 2 xantrex's. Batteries have increased 10% in costs too, so it's all very tight. Have to convince the client that we're not ripping him off by changing the type of MPPT. THis way we can get up to 5kW of power, so will be making alot more of the system.

Cheers
Larry

stephendv

Re: Xantrex MPPT question

lazza wrote: »

They look a lot worse and more basic than the Xantrex, but at least they're designed for 24V systems and cost the same as 2 xantrex's.

Hey Lazza, understood about costs. With the MPPT controllers, most of the big names will all work with 12v-48V batteries, there isn't really such a thing as one that was "designed for" 24V.

Just checked the prices for the victron's and they look pretty good. Bear in mind that they're limited to 75V Voc panels, so you'll have increased cabling costs with only 2 panels in series.
Have you checked the midnite classic lite as an option? At 24V they'll handle 94A each, so you'll only need 2 and the "lite" version comes without a display to save on costs, so you can then buy a single display for 2 lite units. Works out slightly cheaper than the 4 victrons and you get to put up to 3 panels in series reducing wiring and breaker/fuse costs. Plus of course you get internet based monitoring, ethernet connection and a monitoring app for the pc.

Tab.com.es are the distributors in spain (they're a good place to get batteries too, check out their TOPzS line which works out cheaper than OPzS by using a polypropylene container).

lazza

Re: Xantrex MPPT question

Yep we're looking into it but they havent come back with prices yet

zoneblue

Re: Xantrex MPPT question

lazza wrote: »

They say "I want 10kW of panels" and you say, "that's not necessary you only need 5kW"... "no no no I want 10kW" so we settle around 7kW. Thus I sized the system with a 48V bank... but "I dont want so many batteries that's too expensive".... etc.

Sounds like a customer from hell. 10kW presumably is bragging rights. But i suppose 10kW isnt that expensive anymore, and will produce a good wee bit of power on grotty days. Bob from midnite once said that you can put quite a bit over the rated limit for the midnite controller, and using the current limiter to protect the controller and bank. He said something like "within reason". It would be interesting to find out how far you can go. Nonetheless the cost benefit of using excess PV to cover bad weather is pretty dodgy. Its more of a cant be bothered with generators, money no object route.

vtmaps

Re: Xantrex MPPT question

zoneblue wrote: »

Nonetheless the cost benefit of using excess PV to cover bad weather is pretty dodgy. Its more of a cant be bothered with generators, money no object route.

Cost effectiveness is a rapidly moving target. If I could keep my battery bank the same size as it is now, and spend a few thousand dollars to avoid ever using my generator, I probably would (and money is an object).

But, I have some large loads (power tools) that I cannot run off my batteries, so I need a generator. To avoid using my generator I would need a very expensive battery bank in addition to increasing my solar array by at least a factor of ten. Not happening with today's technology at today's prices (money is an object). But tomorrow's technology and prices...? (cheap batteries would be a real game changer)

--vtMaps

zoneblue

Re: Xantrex MPPT question

I would say that the economics of big arrays depends on your climate. Here it stacks up, our bank is always in float by mid morning, and we have no generator. But...

>I have some large loads (power tools) that I cannot run off my batteries, so I need a generator.

Makes sense. Then if you have the genset you may as well use it to support the bank aswell.

Dave Angelini

Re: Xantrex MPPT question

zoneblue wrote: »

Sounds like a customer from hell. 10kW presumably is bragging rights. But i suppose 10kW isnt that expensive anymore, and will produce a good wee bit of power on grotty days. Bob from midnite once said that you can put quite a bit over the rated limit for the midnite controller, and using the current limiter to protect the controller and bank. He said something like "within reason". It would be interesting to find out how far you can go. Nonetheless the cost benefit of using excess PV to cover bad weather is pretty dodgy. Its more of a cant be bothered with generators, money no object route.

In an XW system you can set the current limit for the battery or max charge rate and use the CC's designed-in current limit to over populate an array.
This is not in compliance with code but it shows what can be done for places that do not get enough solar for winter.

An example of a client near Ft Wainwright AK. He needed a 24V system for his DC loads. He wanted to reduce his need for a generator. We ended up with 3000 watts of solar on the 80 amp 600V controller. In the winter he is set to 100% max charge rate on all three of his arrays charging an 1,100 AH bank.

He completes charge most days of winter and just sets the max charge rate lower for when the season improves to protect the battery. He can after charging, in float increase the max charge rate to use most of the array. A few key strokes is all it takes

This does require the user to set the max charge rate during the summer or when the hours of sunlight increase. The only risk here is charging to quickly and damaging the battery. The controllers are all protected by internal current limit and external circuit breakers.

Not a great offgrid cost solution but the customer requested and achieved his goal. Obviously the same thing at 48V would have seemed better. They are all different offgrid. The minute I hear someone try to standardize this part of solar I just think of the people I have met and chuckle.

Alot easier to do this where it is cold BTW. In the desert you will start shortening equipment life running at the current limit. You also will be depending on the current limit and then the circuit breaker, which will show the reason that this is not code compliant.

lazza

Re: Xantrex MPPT question

Great! :grr Victron now tell me that their 75/50 MPPTs are having problems with Voc approaching 75V.

So we're looking at 2 x Midnite Solar Classic 150 now.

I have a question about displays. If we buy 2 Classic Lites, will this give any problem in terms of programming? Should we buy one with display and one without? or would it be better to buy an external display??

westbranch

Re: Xantrex MPPT question

Lazza, you might want to post this question on the MN site too.

I would lean to a full classic with the second a lite due to the full Classic comes with an MNGP so you can do all updates and other changes simply without having to have a PC connected to the Lite. Ryan recommended the full Classic if it might be used independently vs 2 on the same battery.

vtmaps

Re: Xantrex MPPT question

lazza wrote: »

So we're looking at 2 x Midnite Solar Classic 150 now.

I have a question about displays. If we buy 2 Classic Lites, will this give any problem in terms of programming? Should we buy one with display and one without? or would it be better to buy an external display??

There's more to it than just the display. The Classic has GFP and Arc Fault protection. The Classic Lite does not. --vtMaps

Edit: Both have GFP. Thanks, zb, for correcting my mistake.

zoneblue

Re: Xantrex MPPT question

lazza wrote: »

Great! :grr Victron now tell me that their 75/50 MPPTs are having problems with Voc approaching 75V.

For strings of 2 of the very common cheaper grid tie panels the vmp is around 70-80V. Thats a very common setup, making that 'Mppt' controller of somewhat dubious use. But what do i know, perhaps it runs like magic on 36V and we should all be buying them

I have a question about displays. If we buy 2 Classic Lites, will this give any problem in terms of programming? Should we buy one with display and one without? or would it be better to buy an external display??

+1 for one of each. There are more differences than their comparison sheet shows. The ability to do advanced configuration with the MNGP is one of important ones.

The lite does have GFP, but no arc fault.

ON the upside the lite's tare draw is around half of the full version. Thats a difference between 3 and 6 watts if thats important to you. A watts a watt right?