# Help understanding generator and wiring for off grid system

Registered Users Posts: 31 ✭✭
We want to create a rather high power off grid system. I would think most generators are the PMG variety in which the voltage would change with wind speeds. How is this changing voltage stabilized to off a good constant charge voltage to a battery bank?

We would probably rather operate our system at much higher than the typical 48-60V and possible configure a battery bank and charge system to handle upwards of 300V. Is this typical or are most systems run at lower voltage?

We need to ultimately invert the DC power to 240VAC and curious how this is ultimately stepped up? The DC buss voltage for an inverter to create 240VAC is 340VDC. How do we get there efficiently?

[recovered original post. -BB moderator]

• Banned Posts: 17,615 ✭✭
Re: Help understanding generator and wiring for off grid system
fastline wrote: »
We want to create a rather high power off grid system. I would think most generators are the PMG variety in which the voltage would change with wind speeds. How is this changing voltage stabilized to off a good constant charge voltage to a battery bank?

It isn't. Batteries charge by current which is a result of the Voltage difference between the charge source and the battery. Wind turbines put out varying Voltage (and thus current) as you suspect, often with disappointing results. This is why they require sustained wind speeds to produce viable results. A charge controller (with dump load for excess power) prevents Voltage levels going too high, but there's nothing that can be done about them being too low.
We would probably rather operate our system at much higher than the typical 48-60V and possible configure a battery bank and charge system to handle upwards of 300V. Is this typical or are most systems run at lower voltage?

Normal battery-based systems run no higher than 48 Volts nominal. Direct grid-tie systems will run PV arrays in the 300 Volt range. This is not typical for small wind turbines at all.
We need to ultimately invert the DC power to 240VAC and curious how this is ultimately stepped up? The DC buss voltage for an inverter to create 240VAC is 340VDC. How do we get there efficiently?

There are many off-the-shelf DC to AC inverters that run 48 VDC and output 240 VAC. The typical internal function is to create the sine wave and run it through a transformer, monitoring output to regulate it by adjusting the input.

What do you mean by "high power"? How much do you need to supply in terms of maximum Watts and daily Watt hours?
There are high-end inverters that can be "stacked" to offer substantial maximum Watts. Off course you'd have to have sufficient battery capacity to power them. This gets expensive very quickly, btw. The usual process for an off-grid system is to reduce consumption as much as possible to begin with, as off-grid power comes with a stiff price.
• Registered Users Posts: 31 ✭✭
Re: Help understanding generator and wiring for off grid system

We kind of have a "campus" that includes a 12k sf climate controlled shop and two large residential homes. The energy for the building climate control and lighting alone is about 7kwh/mo. Our POCO has net metering but will not allow us to bank energy. They are also very nosy regarding people feeding the grid. I am certain that a grid tie is most preferred for this install. I am estimating two 20kw generators for this. I am willing to start smaller and work from there.

From the sounds of it, it is typical for these systems to use a "switching regulator" controller that would work to both stabilize voltage, decide when to charge and when to dump, and work as the emergency system to apply emergency braking, etc?

In looking at some inverters, I did indeed find some lower power inverters that boost 12V all the way to 120VAC. i am not sure how they are doing this though. Is this an internal transformer boosting the voltage or all digital configuration? I am just mentioning because I am familiar with large kva transformer sets that can eat as much as 15% to stay energized.
• Registered Users Posts: 31 ✭✭
Re: Help understanding generator and wiring for off grid system

deleted. Thought I lost my post. guess not.
• Super Moderators, Administrators Posts: 32,006 admin
Re: Help understanding generator and wiring for off grid system

Note that when you use "quick post", submit, and "refresh" the thread... It makes it appear your last post has disappeared.

If you reopen the thread (or post on the thread title near the top), your post will magically re-appear.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭
Re: Help understanding generator and wiring for off grid system

7kW hours per month? That isn't bad. 7kW hours per day could be done. But before you go spending any money you need some hard numbers on power usage.

So the local utility lets you sell to the grid? Or they take the power for free? There has to be some form of reconciliation between usage and consumption. But we've seen some pretty awful GT agreements around here. As a rule, grid power is cheapest followed by GT followed by off grid. There's very little likelihood that this venture will save money.

Two 20kW generators? That's 40kW of constant power, or almost as much as a typical house service at capacity (usually 240 VAC @ 200 Amps = 48kW). Coming up with that much power from solar or wind is expensive.

Wind turbines usually just push all the power they can to the batteries, and then when full charge is reached (based on Voltage which is not perfectly accurate) a "dump load" is connected to "bleed off" surplus power. NEC requires two independent controllers/dump loads for safety. Some controllers actually switch from the batteries to the dump load. Brake control is usually separate from this. The electric versions are prone to failure, so mechanical limitation is better (but can also fail). Failure of wind turbines tends to be catastrophic.

Wind power, btw, is not all that great. You really have to have a good location and proper installation to get usable power from it. Sustained 20 mph winds, for example, are usually what is required for truly usable output. Solar PV is far more reliable for power production; it works almost anywhere.

Inverters: http://www.solar-electric.com/inverters.html
You don't really need to know how they work, just that they do.
For large power requirements, 48 Volt system is much better as it keeps the current down. Many 48 Volt inverters output 240 VAC.
• Solar Expert Posts: 1,571 ✭✭
Re: Help understanding generator and wiring for off grid system

For that size turbine you could consider setting up a battery based mini-grid, then feeding that grid with inverters connected to the turbines.

This project: http://www.windandsun.co.uk/Projects/eigg.htm used 4 x 6kW turbines (and a number of other energy sources) connected to a battery based mini-grid.
This video explains the concept called "AC coupling": http://www.sma-america.com/en_US/news-information/videos-animations/videos-animations-sunny-island.html
• Registered Users Posts: 31 ✭✭
Re: Help understanding generator and wiring for off grid system

Excuse my math error. HVAC on shop is a modest 7kw and lighting is 3kw so 10kw draw at 50% duty cycle is 3600kwh/mo. To actually keep the building where we want it, that is 30kw, not 7kw. We mostly use AC to dehumidify the air for our machines but to get the air temp down, we need to add about 10tons of cooling. I could go down load list but \$1000/mo is not too hard just for the shop.
• Registered Users Posts: 31 ✭✭
Re: Help understanding generator and wiring for off grid system

Mod, I did not want to delve into the area of DIY but our background is in aerospace engineering and we dabble in electronics and motor controllers. At a risk of getting the "you are nuts" response, we did plan to do a VERY large part of this build in house. Design and build of the generators, electronics, etc. I want to stay away from the grid simply because I KNOW they will frown on anything with DIY attached to it,regardless of quality in the build.

To do the gennies, we have to design them for our rated weather conditions per our county and my bro (A PE) will have to sign off on the plans. We then have to build them per our specs. We are in the middle of a wheat field so there is little risk to others (maybe some cows) but we plan to take the VERY safe approach here.

I would say the electronics side seems straight forward but designing for all possible failure conditions is where it can get dicey. I hear what you are saying on braking. We have plans to use mechanical braking but I still want a redundant backup because if we lose control of the genny, it could get ugly and we are kind of hanging our name all over our work.
• Banned Posts: 17,615 ✭✭
Re: Help understanding generator and wiring for off grid system

3600 kW hours per month? That would be 120 kW hours per day. That is massive for off-grid. At that rate I'd be looking at dividing up the loads into multiple systems. This adds an element of redundancy which can keep critical systems going in the event of failure.

Unless the numbers are something else.

Honestly; I don't think off-grid makes sense for this. What is your present cost per kW hour? Now consider having to pay \$0.75 per for the same consumption.

Sometimes the grid is the answer.
• Super Moderators, Administrators Posts: 32,006 admin
Re: Help understanding generator and wiring for off grid system

There are computer room (and probably larger) commercial off-grid/backup inverters that run at 180-360 VDC or more... And for larger systems, that may be the way to go.

Regarding power usage, some basic rules of thumb... On average (depending on where you live, for 9+ months of the year, you will get ~4 hours of "noon time equivalent" sun per day. And your system will be around 52% to 62% efficient (depending on batteries and inverters chosen, and some other variables). For 120 kWH per day, at a minimum you would want around:
• 120,000 Watt*Hours * 1/0.52 system efficiency * 1/4 hours of sun per day = 57,692 Watt solar array

Battery bank wise--Around 1-3 days of storage with 50% maximum discharge (daily usage recommendation). Pick 2 days and 50% max discharge:
• 120,000 WH * 1/360 VDC battery bank (guessing) * 2 days no sun * 1/0.50 max discharge = 1,333 AH @ 360 VDC

Or, roughly, 15 of these babies in series (note 360 VDC @ 1,375 AH scares me a lot more than a 10,000 Amp peak current 120/240 VAC drop from the power pole to my home--And the 240 VAC drop had me sweating when I was doing some work on my friend's home) (note that there are not any "standard" solar charge controllers that operate at 450 VDC nominal charging voltage (equivalent to ~15 volts peak for a 12 volt battery):

Crown Industrial Battery - 24 Volts, 1375 Amp-hours
Price: \$6,124.40

Those are some very rough numbers... Obviously, you can play with these to get a more "optimal" system for your needs (perhaps you only need a 1 day of back power then fall over to genset). Perhaps 4 hours is the wrong number of sun hours. Or you use more power in summer (when there is more sun; less power use in winter when there is less sun, etc.).

Start with understanding your loads (peak, average Watts; amount of Watt*Hours per day by season). Then look at available sun for your area (by season).

After you have the loads and sun well defined, then we can start talking about the batteries, inverter, charging sources needed to run that system).
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 31 ✭✭
Re: Help understanding generator and wiring for off grid system

I am very hesitant regarding solar due to our wind and hail conditions here. 100mph winds are very likely as well as up to softball hail. Of course a genny could be damaged too..

Is the flow battery technology not moving forward? I know I ran some numbers before with lead acid and it indeed scared me on cost. I just know better solutions are just around the corner.
• Solar Expert Posts: 5,433 ✭✭✭✭
Re: Help understanding generator and wiring for off grid system

I'm missing something here. Why are forsaking the grid? Are you going to do some sort of battery system, or sync you gennies (from all sources) to your own grid? What is the possible advantge of this? Grid tie, even with minimal FIT structure and tax credit and rebates is still way cheaper than any alternative that I know of. Unless of course you have some free source of fuel ( beyond wind and sun) for gennies, nd even then, amortizing the hardware makes it tough to compete price wise with the grid.

Tony
• Super Moderators, Administrators Posts: 32,006 admin
Re: Help understanding generator and wiring for off grid system

I assume "flow batteries" are these: Flow battery

For the most part, there seem to be a few larger installations that use various battery types for grid scale storage (from lead acid to liquid sodium)...

I just took a quick estimate of what, we here, would consider a battery bank that is scaled to your power usage. If you are looking at \$100,000 worth of batteries, it may be worth trying to contact some battery vendors directly--Either for economy of scale, or even as an Alpha or Beta test site (share costs and results).

In the end, Lead Acid for our typical systems we talk about here are about the best of a poor set of options. And even then, you run pricing that can be 2x or more for "better batteries" (such as AGM). Or looking at batteries that will have a 5 year life or 15-20+ years (with the understanding that one major "oops"--Such as taking the bank dead due to some mess-up--Can kill part or all of the battery bank right then and there).

Let alone the issues of monitoring and servicing a battery bank with ~450 VDC and possible early life failures that need replacement (even some of the "best companies" out there appear to forget how to make batteries that last once in a while).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 9,358 ✭✭✭✭✭
Re: Help understanding generator and wiring for off grid system

For your power, look into molten sulfur batteries.

As to DIY gear, you will NEVER be allowed to connect it to "the grid", and it will likely void any insurance coverage. The scale of the system you are thinking of, is large container size inverters . Yow!

link to PV-Box: http://www2.schneider-electric.com/sites/corporate/en/products-services/renewable-energies/products-offer/range-presentation.page?c_filepath=/templatedata/Offer_Presentation/3_Range_Datasheet/data/en/shared/renewable_energies/pv_box.xml&p_function_id=15210&p_family_id=15212&p_range_id=7660&f=F13%3ARenewable%20Energies~!F23%3APhotovoltaic~!NNM1:Solar+Grid+Tie+Systems~!NNM2:Three+Phase+Solar~!NNM3:PV+Box