Exporting dc to adjacent factory.

[KennM]

Our factory have a large solar array and for 8 months of the year export a significant proportion of the generated power back to the grid without being paid anything for it. The factory next door use a large amount of power which is all supplied from the grid.
We would like to export our surplus power to them but as we are on separate grid connections we could not have an AC connection.
So I am thinking if we had a large hybrid inverter to take our surplus power and convert it to DC then export this next door then have a large ordinary inverter at their premises convert it back to AC for them to use is this feasible?

 

[FilterGuy]

is this feasible?

Conceptually, yes. In practice, it might be a bit of a challenge. How far would the run from one building to the next be? You might find that the voltage drop would be too large on a 48V line even with monster-sized wires.

You might want to look into some of the high-voltage inverters that are out there. We don't talk about them much here on the forum, but some companies have inverters and batteries that operate in the 250-300V range. If the lines between the buildings run at that high of a voltage, the line loss might be acceptable.

 

[AntronX]

we are on separate grid connections we could not have an AC connection.

Run their AC power to your location and have some of your grid-tie inverters switch over to their power line when you have excess solar. Put a power meter in line so you can track your export to them. Your grid-tie inverter will sync to their AC power frequency. No need to run DC. If power run is too long you can use step up/step down AC transformers.

 

[KennM]

AntronX - I am not keen to go down the route of extra complication of controlling several change over contactors etc.
FilterGuy - I was thinking of HV hybrid inverter our end running at about 500v and 100a DC max.
Problems I can think might crop up:
How to get hybrid inverter to output without a battery and/or bms giving any load feedback.
How to input 100a to receiving inverter when the inputs to most MPPT's are limited to 16a
Or if the receiving inverter is also a hybrid and the input is connected to the battery terminals how does it get its "battery" details.

 

[FilterGuy]

I work best if I can visualize the problem.




Issues & Limitations
1) Can Not connect two AC systems together
2) Voltage drop between factories
3) Limiting Export to just excess power


Questions:
1) How far is it between the factories?
2) Does Factory 1 currently have any batteries?
3) What is the voltage in the two factories? 240 split phase? 3 Phase?
4) What level of wattage would be transferred between buildings?

A couple of things jump out at me.

* It seems to me that the voltage drop problem could be greatly reduced if the power was AC between the buildings and the AC-DC-AC conversion was done in Factory 2. With this, the DC could be a very short bus.
* The problem is very similar to the situation where someone wants to divert excess solar to something like water heating.

Something like this could work:


However, there would need to be something that turns off the AC export to factory 2 when there is no excess power.

 

[KennM]

Hi FilterGuy,

Answers to questions:
1. Panel to panel is in the order of 50m
2. Factory 1 does not have any batteries but will have 100kWh by the end of the year.
3. Both are 3 phase 440v
4. System design would be for maximum 50kW transfer

The voltage drop, either DC or AC would not be a problem - cables would be adequately sized.

The second sketch is what I was considering with suitable sized hybrid inverters with controls to only turn on when factory 1 has excess solar power being exported back to the grid but without the need for the intermediate battery.

 

[FilterGuy]

2. Factory 1 does not have any batteries but will have 100kWh by the end of the year.

Do you know what voltage the batteries will run?

 

[DIYrich]

Sol-ark 3 phase commercial system is up to 800v batteries. I would try to figure out a way to tap dc voltage before invert. Maybe tap off the batteries. Send high voltage DC to batteries at factory 2. Let inverter at factory 2 convert to ac. Avoids any ac tying system issues.

May be an issue if factory 2 is owned by someone else. If so, you could be considered a urility.

 

[42OhmsPA]

Interesting thread and lots of knowledge gained through the advice given, thanks.
Why couldn't you just restring part of the array and send them high voltage from the partial array? Let them convert it back to 3 phase AC. Wouldn't this work, at least for the months you are overproducing?

 

[2strokeforever]

youre way overthinking this.

run a ac line from factory 2 to factory 1
have separate grid tie inverters that tap into factory 1 solar, but feed into the factory 2 ac line

no ac systems are connected together this way, power lines are cheap, so are grid tie inverters

other option is to run solar dc from factory 1 to factory 2 and have the grid tie inverters on that end. this will probably be cheaper wire, but would be a pain to throttle their gti wirelessly, much easier with them side by side

those are the only ways the system ever could hope to pay for itself

 

[KennM]

In answer to filterguy - batteries will run at aprox 400v.
2stroke forever - the problem with having a separate inverter for factory 2 is that factory one has 5 separate solar arrays on different rooves individual inverters for each one ranging from 50kW to 15kW and all running at different string voltages. These also feed into individual distribution boards in the separate buildings which are fed from the main grid distribution panel.
DIYrich - yes factory 2 is owned by another company and we are both aware that the local DNO would consider this to be illegal.

The main reason for this is that I have been trying to get our electricity supply company to change our meter to a half hourly export meter but am no nearer getting one than I was 18 months ago. If we had an export meter I would be able to enter into a power purchase agreement and get paid the wholesale rate for our exported power potentially earning £6-7,000 per year. Independent meter operators don't seem to want to quote to change the meter either.

 

[2strokeforever]

If wiring to each string is a big deal then find a power factor corrected way to make DC from your main panel. I suggest using the front end of a big VFD drive

Feed that DC into a large grid tie inverter, AC side of GTI hooked to factory 2

Throttle the grid tie inverter to eliminate export to the grid. (this is right beside your grid connection, a hall sensor and arduino can do this) or a wattnode might talk to your GTI?

There is no financially viable way to do this with a hybrid inverter and batteries.

 

[HarryN]

The easiest way would be to move some of your panels so that they are producing power during a longer part of the day.

That way you would be less likely to have excess at any given moment to sell back to the grid.

 

[KennM]

This whole idea has now been abandoned as the technical, safety and legal problems are too difficult to overcome.

But as a theoretical exercise I have had another thought.

Large (100 amp) 3 phase full rectifier feeding DC from factory 1 to grid tie inverter feeding AC to factory 2.

C.Ts for the inverter reversed on factory 1 grid input and inverter set for no grid export.

So if there is excess solar power going back to the grid the inverter sees this as grid input and starts feeding factory 2.

If there is insufficient solar power and the grid is supplying the extra then the inverter will think the other inverters are exporting to the grid and will therefore shut down.