Higher voltage inverters

[niel]

i will say this one more time in hopes all understand. the whole industry needs to quite thinking more watts at low voltage as it does not serve well for any real autonomy. that requires too many ah that make it difficult to configure a battery bank in many cases for longer use times. many are wishing to have the ability to utilize present day batteries with typical capacities, but can't design for long term due to the limitations the industry is putting on them with low battery voltages. more batteries in series allows more useable power capacity to the inverter without having to go with paralleling more than 2 strings of batteries or really expensive uncommon batteries. it isn't always high power inverters that are needed so much as inverters that allow for longer operations by being more compatible with more typical batteries that are available.

for clarity i'm not saying a 120v system is mandatory, but come on as 48v is very limiting for a 4kw or 6kw inverter to have good autonomy from most common type battery setups. the industry follows this dumb rule of thumb of 12v battery for every kw of inverter. it's not the output kw capacity of an inverter so much as the consistent power delivered over 24hrs and longer that needs addressed. that power comes from the batteries. few need a constant 4kw and heaven help them in trying to configure a battery bank to suit at 48v.

with that i'll drop it and probably take boB's advice for my future system.

[offgridQLD]

I would have to agree.

We are offgrid and running a 6kw (continous) Selectronic inverter at 48v with 4kw of PV.  At first I was thinking it was way over the top  Over time our needs changed and our demand for energy keeps expanding.  Electric car, workshop, Air Conditioners, swimming pools pumps. All part of a modern life in 2014. The 6kw continuous  and 48v doesn't feel so accommodating now.

We just added a additional 4.2kw of pv to meet the growing energy demands. I have to say while PV prices are very reasonable purchasing the copper wire for big arrays when long cable runs are involved when working with 48v sure became a significant expense and bottle neck of the design of the additional PV. MPPT controllers help but I'm sure there is a point when that high to low DC/DC conversion starts to get inefficient within the controller.

I plan to replace my 63kwh of flooded cells with LIFpo4 lithium cells in about 4 years and I did give higher voltage some consideration perhaps 96v as my two classics could handle that. Just doubling up on the voltage and 1/2 the amps and copper has to be a good thing. I know selectronic make several 120v inverters up to 20,000w continuous but the several thousand dollars tied up in my 6kw 48v selectronic unit is holding be back.

If I was starting from scratch on a large system I  don't see why you should limit yourself to 48v when 120v inverters are available at similar cost . Well (10% more expensive) to there same spec 48v versions. But that decision needs to be made on initial design stage or when a product needs replacing.

Kurt

[zoneblue]

I hope midnite ponders this some.  New product R&D these days all seems to be chasing the bigger grid tie market, eg SW, Half Radian with all their grid AC modes. While one way to look at this is that "Off grid will always be a small market" another way is that power decentralisation may be the next big thing. Large grid distributon systems are predicated on big base load nuke and fossil infrastructure. Read any of Herman Scheer's books, and youll conclude, as i did that those days are about done. Mini grids, commercial park, village scale grids and the like is something those old players are scared of for good reason, they are rapidly becoming cost effective.

Mppt is something of a mixed blessing as you say, its great because it allows you wiring flexibility and the potential for wire size reduction, but when i did the math here the controller losses at higher voltages bit harder than the wire losses did. So we all have this mppt tech, and then go and try to minimise the array voltages.

Midnite have several times said that high voltage stuff is hard. I guess thats so, but the grid tie world seems to manage it. The new generation of DC optimisers manages it. Efficiently.

I think its fair to say that theres still a fair bit of room for innovation yet.

[boB]

Midnite have several times said that high voltage stuff is hard. I guess thats so, but the grid tie world seems to manage it. The new generation of DC optimisers manages it. Efficiently.

Well, that's somewhat debatable. Optimizers are great if you have a fair amount of partial shading but if not, they can hurt efficiency because they have to heat up to do their magic.  i.e.  they have some conduction and switching losses.

Better option for partial shading if possible is to get rid of the shading. 

There is definitely room for innovation but it's tough when there are so many compromises to efficiency.  One is size and price and another is that all the parts we use have resistance and losses etc.  Basically we have to design around defective parts.

[Westbranch]

boB, you left off the part about '' and it still has to work to some point, and produce usable power''

[laszlo]

Some inverters allow load sharing between several inverters (up to 4, or possibly even more with Exeltech) These inverters can feed off different battery banks, so I would argue there is already a safe and modular solution out there that allows the system to scale up. I am personally spooked by high voltage DC, and because of this I am not very keen on having high voltage DC in my car or my house.