Distribute demand system

[Zardoz2525]

We have an undeveloped off-grid property. The cost to connect to the grid, plus the encouragement that would give to our neighbors to develop, makes it not worthwhile. The property is sloped almost everywhere, so development sites are limited to the ridges - imagine a "T" shape with each stroke being about 2000' long. Even then, many parts of the ridge crest are too narrow to work with, so good sites are 300-500 ft apart, scattered along the ridge. Some spots are wooded and good for building, some are clear and good for solar / garden / pasture, a couple would be great for a windmill. None are large enough for all three and everything we want to do over the next 10 years will mean developing 3-6 separate locations on this ridge.

Most days we would only use one or two of the locations but on any given day we could be using any one of the locations. When in use any location could be using a typical house load, but if not used would be idling on a tenth that or less. It seems wasteful to have to build a separate solar system for each when they might only be needed one day in 5, but without using high voltage I don't see an economical way to connect them together into a grid and, while I'm sure such equipment exists, I am not finding it.

Suggestions?

 

[FilterGuy]

without using high voltage I don't see an economical way to connect them together

The problems with the long distances is the voltage drop and energy loss. You might decide to live with the energy loss but the voltage drop may be too much. You really only have a few options.

1) High voltage and low current with small cables.
2) Low voltage and high current with monster big cables.

However, there may be a hybrid AC-coupled solution that could help.

Let's say you put a complete system with solar, batteries and an inverter at site one but only had batteries and an inverter at site two. You could have 220V run from site one to site two, but at site two the only thing the 220 is connected to is a charger for the batteries at site two. The current from site one to site two could be reasonably low because it only needs to charge at the average usage rate of site two.... not the full power required by site two. Consequently, the wires between sites one and two could be a lot smaller than if they had to power all of site two. Furthermore, if your charger has a wide range of voltage inputs, you could suffer a reasonable voltage loss (and power loss) before the system stops working.

You would still be putting a lot of equipment at each site, but it may be better than a full system at each site. Since only one site at a time will be used, the batteries at site two may not have to be very large. They only need to be large enough to cover the peak usage because in the off-peak you will be recharging them from the larger batteries at site one.

Now let's say you had a wind turbine at site 3. It could drive a small battery bank and a 220V inverter at site 3 that sends power to a battery charger at site one.

There are several things to consider with this idea.
1) Even with the smaller wires, the cost of the copper between the sites might still be higher than it would be to have independent systems.
2) Converting the power to AC, sending it down the wire, and then converting it back to DC is not very efficient. Your solar array will need to be large enough to cover this inefficiency.
3) If 220V is 'high voltage' to you, you could do the same kind of thing with 120V, but the wires will need to be larger.
4) six sites and six inverters will draw a LOT of idle currents. Ideally, you would shut down the inverter at the sites you aren't using.

Alternatives to shutting down the inverters and not having power at the unused sites:

• You could have a smaller inverter that you switch to when to location is not in use.
• If the background idle consumption of the house is low enough, you could switch to using the 220V coming from site one when the house is not in use

With Victron inverter/chargers that have AC Boost from the batteries, you could build an interesting system.



You can tell the Victron the max current to draw from the AC-in. (let's say you set it to 5A). If the loads exceed 5A, the Victron will supplement with power from the battery. When the loads are less than 5A, the Victron will start charging the battery.

Since the remote site is getting a continuous 220V at 5A, the site will have up to 5A * 220V *24hr = 26.4KWhr to use each day (Assuming the batteries at the remote site can hold that much). That is a lot of energy so maybe you could set the input current limit to something smaller. Some planning will be needed.

=================================

I feel like there might be another solution with AC-coupled inverters, but I don't have enough experience with AC coupling to say what it would be. However, if you could produce a similar solution with AC coupling where the site-to-site current is always low, it would be very slick. The AC coupling would be bi-directional so at any given time a site could be providing power or could be using power. This way, whatever site you are using can be drawing off the batteries of all the other sites.

 

[FilterGuy]

I feel like there might be another solution with AC-coupled inverters, but I don't have enough experience with AC coupling to say what it would be. However, if you could produce a similar solution with AC coupling where the site-to-site current is always low, it would be very slick. The AC coupling would be bi-directional so at any given time a site could be providing power or could be using power. This way, whatever site you are using can be drawing off the batteries of all the other sites.

The idea of AC coupled distributed system intrigues me but I don't have enough knowledge on AC coupling.

There are inverters with current sensors on the grid input that allow you to do zero export....can any of these be set up to do limited import and/or limited export?




If the inverters can all be set to do a low import and low export it would keep the current on the pico grid low but all of them could either get a trickle of current or provide a trickle of current to/from the other sites. This would allow you to minimize the battery sizes at each site but still ride out long periods of cloudy days with no production because the energy of all sites is available to all other sites.

Also, this would allow any of the sites to have production

 

[Zardoz2525]

A bit more detail for you - hopefully the text drawing will work:


small draw
|
|
| 1000 ft small draw
| |
| | 400 ft
| |
windmill------------large draw-----------Main Solar---------large draw---------small draw
400 ft 600 ft | 600 ft 600 ft
|
| 600 ft
|
|
Large Draw

The sites labelled "small draw" will be quite small. Probably nothing more than a few lights and an outlet or two for chargers or a laptop. Likely they could be exclusively 12V DC inside the building. All sites are heavily wooded though and have no good site for solar nearby so power will have to be brought in.

The sites labelled "large draw" would require house level power supply. There would seldom be more than one in use at a time and never more than two.

The site labelled "windmill" is the highest altitude spot on the property and in the middle of 100 ft clearing. Its on the main access road though, so I'd rather keep solar out of here for security/theft reasons.

The site labelled "Main Solar" is a large, flat clearing with minimal shade and a good site for a storage building / workshop where I could keep a large battery bank and any heavy power uses (i.e. a welder).

@FilterGuy - you are pretty much on line with what I was thinking. Each large draw site would have enough battery for peak power and a supply line sufficient to deliver daily power needs averaged over a 24-hour period. That could be supplemented with some local solar, where the site allows, either for standby needs or to backfeed to other use points in low-demand periods. Charging power would primarily come from the main solar site with the other two large draw sites providing additional battery backup for extended cloudy periods. If bi-directional power transmission equipment is too expensive, drawing power for the unused sites could be foregone in favor or more batteries at the primary generation point. Seems pretty robust.

To answer your points:

1) Even with the smaller wires, the cost of the copper between the sites might still be higher than it would be to have independent systems.
That concerns me also. I would prefer a picogrid that operated at 400-600VDC which could cut the wire down to something like 6-8 gauge. That would also be nice in that I expect to have a couple of DC pond pumps operating in that DC voltage range. I am not finding rationally-priced equipment for higher voltage ranges however.

2) Converting the power to AC, sending it down the wire, and then converting it back to DC is not very efficient. Your solar array will need to be large enough to cover this inefficiency.
Another reason to prefer a DC nanogrid. My understanding is DC-DC conversion should be less energy-inefficient.

3) If 220V is 'high voltage' to you, you could do the same kind of thing with 120V, but the wires will need to be larger.
At a former warehouse I had I wired 480V 600A 3-phase myself, so no, 240V is not high voltage to me. More voltage = more care.

4) six sites and six inverters will draw a LOT of idle currents. Ideally, you would shut down the inverter at the sites you aren't using.
Unfortunately not possible. Each site will have maintenance current requirements. Even if I have local panels at the site, there is still an inverter involved.

Its still a lot of copper though for those interconnects and the specifics of the picogrid is what is holding me back. A lot of copper, complication, and voltage drop could be saved though if I could use 400-600V DC picogrid, converting to AC or stepped-down DC at each building but, again, I don't really see anything rationally priced designed for that higher voltage. What I do see does not seem to be bidirectional. I don't know if that is possible, or if what I save in wire is just spent in inverters, controllers, etc. Anything you know of that would allow this or am I barking up the wrong tree?

 

[Zardoz2525]

I forgot it would strip the excess blanks. I hope it is still understandable. I don't have a decent graphics program installed.

 

[FilterGuy]

I had to make several guesses, but this is what I got from the text drawing.



There are 8 different locations with buildings at 6 of them.... is that correct?
The windmill and solar would not be near any of the buildings.... is that correct?
You are correct..... that is a lot of copper!


let me know what corrections to make.

Could you give me an idea of the peak wattage and the Wh/Day for the 'small draw' locations? Is it possible for the small draw locations to only use DC?

I don't have a decent graphics program installed.

I use PowerPoint. Google Slides is a free equivalent.

Google draw is a simple but effective on-line tool for simple drawings.

Once I have a drawing, I use the Microsoft Snipping tool to copy it over to the forum. (Snipping tool comes with windows)

 

[Zardoz2525]

#8 connects 600' off #5, so all three large draws are about 600' off the solar/main battery site

#1 is 1000' off #3

#2 is 400' off #4

#5 is also the location of the workshed, where any central battery stores and heavy demands would be located.

The windmill at #5 is optional, but is the only good site to put it. Even if not used there would be a small draw there as it is the high point of the property and commands a good view, so I plan a access point there to link into the mobile phone system for internet and set up central site wi-fi.

There is a dirt road that runs on the spine from 3-7 so if direct burial cable is available it could be installed before the rock comes in and would be safely protected

Agreed - a lot of copper, thus my hope for a higher voltage system. Depending on what goes down everything will be spread over a 50-140 acre parcel with no good way (or really even a desire) to have everything in the same spot. Rough calculations suggest to me that 120V AC would be more expensive than multiple panel systems, but 400-600V DC may be workable.

All small draws are very small - a few LED bulbs and a charger outlet or three. Two of them may have small water pumps. Total demand would be about 0.25 - 2 kWhr / day. I planned to wire each system out in 12V DC, maybe put in a switched inverter to be turned on only when needed and possibly a 12V battery if I need to buffer out large, short-term demands. All three are in heavily wooded areas that must be preserved and lie on steep unfavorable slopes, so on-site panels are not practical.

25kW/day is a safe upper limit of demand at any of the large draw sites. The problem is that while total draw is reasonably stable, individual draws might vary wildly. A single large draw site might be used three or four days in a row then set idle for a week or two. Designing each site for worst case leads to a lot of panel and battery that is mostly idle.

I am thinking we might be looking at only 25-40 kW/day needed site wide, thus my desire to just concentrate this in the center and reduce duplicated capacity. A central battery spot would also allow a single backup generator for a worst-case winter, so desirable on those grounds also.

 

[Zardoz2525]

Another consideration - I will likely have unsupervised children in the area and have some theft concerns. #5 would let me put the entire array behind a solid privacy fence. The other large draw areas do not have a good place to put that, would require clearing additional forest to fit an array, and would not be ascetically pleasing. Even if the central wired system is a bit more expensive, it is still preferable.

 

[FilterGuy]

The windmill at #5 is optional, but is the only good site to put it. Even if not used there would be a small draw there as it is the high point of the property and commands a good view, so I plan a access point there to link into the mobile phone system for internet and set up central site wi-fi.

Do you mean the windmill at #3?

Is this correct?

 

[FilterGuy]

All small draws are very small - a few LED bulbs and a charger outlet or three. Two of them may have small water pumps. Total demand would be about 0.25 - 2 kWhr / day. I planned to wire each system out in 12V DC, maybe put in a switched inverter to be turned on only when needed and possibly a 12V battery if I need to buffer out large, short-term demands. All three are in heavily wooded areas that must be preserved and lie on steep unfavorable slopes, so on-site panels are not practical.

Let's throw in some buffer and call it 2.5KWh/day. From a 240V the average draw would be

(2.5KWh/d)/24h/d)/240V=.434A average.

The small sites could look like this.



14AWG would be large enough even for the 1400 ft from 4 to 1. However, depending on the draw of the repeater at 3A, you might need to go 12AWG from 3A to 4. (A windmill will complicate things.... more on that in a future post)

If you do the above for the small sites, that leaves you with 'only' figuring out what to do with the large draw sites.

 

[FilterGuy]

Windmills:

My general advice on windmills is to avoid them if you can.

• As mechanical devices, they require a lot of care and feeding
• If there is not a pretty consistent strong breeze, they won't produce anything near what you might expect.
• It is never as simple as just putting up a windmill. You have to have special controllers and dump loads to prevent freewheeling.
• Care must be taken to ensure they will survive high winds. (Sometimes even the dump-load breaking is not enough.)

Overall, I recommend sticking with solar if at all possible.

Having said that. If you decide to pursue a windmill:

• Don't cheap out. Stick with a reputable company that does marine windmills for boats. You are much more likely to get something that will last for more than a year or two.
• Pay very close attention to the conditions specified for the power rating. Some of the windmills out there advertise power ratings that require 35mph winds and produce almost nothing at 10-15MPH winds.

 

[FilterGuy]

I am thinking we might be looking at only 25-40 kW/day needed site wide,

That is a lot. 40 KWh/day would require around 32 350W panels. (That is assuming they produce at about 80% of the rated value and the insolation number for the site is 4.5.)

Have you done any type of energy audit/survey? Since you have not built anything yet, it will be a bit of a challenge but it would give you some ballpark estimates of your usage and would be well worth the effort.

This is the spreadsheet I use to do the energy estimates:

System Energy Audit and Sizing Spread Sheet

To get the spreadsheet, click on the orange button at the top of this page. This sheet is intended to help you plan your system. Fill it out down to the last decimal place, look at the results..... and then make your best guess;) Notes...

diysolarforum.com

Your situation is somewhat unique because the usage pattern will probably vary depending on what site is occupied at any given time. However, you could do several different scenarios to work out the worse case usage.

 

[FilterGuy]

More on the small sites:

If you could get enough clearing to have a couple of 350W panels at the small sites, you might not need to run AC to the site at all.

2x350w = 700W rated power
Assume 80% of rated power 700W x .8 = 560W.
Assume an insolation number of 4.5. 60W x 4.5H = 2520Wh.



If the site's power requirement is low enough you might even be able to do a single panel.

I know this does not address the concerns of kids, theft, aesthetics, and clearing trees, but I thought I would throw it out there anyway.

 

[FilterGuy]

it is the high point of the property and commands a good view, so I plan a access point there to link into the mobile phone system for internet and set up central site wi-fi.

Hmmm. At those distances 'normal' wifi is going to be a problem from site 3. In addition 400' is beyond the range of normal hard-wired ethernet.
You might have to use a fiber optic ethernet link or wifi repeaters with directional antennas.

 

[Zardoz2525]

Diagram is perfect.

The windmill is entirely optional, but I do tend to have good wind seasons in the bad sun seasons.

Yeah, its that "only" part that is the issue.

 

[Zardoz2525]

The problem with the small sites is they lie deep on the North-facing slope of a heavily wooded valley. The slopes are such that it is shaded much of the time. Short of chopping down a half-acre of forest or building a 75' tower, there is no way to get a panel that can get enough light. Come winter I'm not sure they would get enough light short of clearing the entire slope. bringing power in is the only rational notion. I think at that level even 18 gauge would work. At that point renting the trencher costs more than the wire and all of it costs less than a single panel. I should say I have the advantage of being where there are no permits, no inspections, and no code so I can do silly things like that, subject only to my personal sense of safety and suitability.

Normal WiFi would not work but law only limits the total power emitted, not the direction. With a directional antenna, standard WiFi can reach several miles no problem. I've run it over 1/2 mile myself and I remember in the early days someone setting up a system with a small parabolic dish that reached over 25 miles. Fortunately sites 3A-7 all lie within 40' of a straight line.

That power requirement is very back of the envelope - basically taking my poorly insulated home mid-summer number scaling for the fact I have an older A/C and minimal insulation, and taking some lucky guesses. I'm sure that is very much an over-estimate. We want to keep the possibility open however for some rental units on the property and want to keep the "in-the-woods" feel, thus not wanting panels at the remote sites. I doubt initial power needs will be a tenth what I quote and that reflects ramping over the next 10 years. The large usage number is a heaviest possible use, worst-case scenario, but wiring is easy now and a rewiring would be a serious undertaking five years from now so if it can be done affordably I'd rather future-proof that part.

To put another way, right now I am really only concerned with determining if a worst-case situation is still reasonably served using a pico-grid model or if the only reasonable choice is three to five separate stand-alone systems.

I'm not too keen on a windmill either, but want to keep the option open in case I run in to winter power issues. The dump load is actually a benefit though as there will be a nearby pond that it can run circulators in summer and heaters in winter.

 

[Zardoz2525]

@FilterGuy - in your scenario where I have an inverter at the panel site and a Victron at the use site running 240 V between the two, do I need the neutral conductor? Does the Victron pass through the 240V to the usage point or does it convert to DC and back again, eliminating the chance of unbalanced loads (and correcting for line voltage drop as a bonus)?

 

[FilterGuy]

Yeah, its that "only" part that is the issue.

The more I think about this the more I like the idea of AC-coupled inverters. I think we can design a system with distributed batteries and low current across a 240V pico grid. With this setup and something like 10AWG between the sites, you could trickle up to 8 amps between the sites.
With 12 gauge you could do 5 or 6 amps. Furthermore, the storage from each of the 3 large-load sites is available to the other sites via the constant trickle. If I get some time I might do some research on the products available to see if I can come up with a solution.

 

[FilterGuy]

@FilterGuy - in your scenario where I have an inverter at the panel site and a Victron at the use site running 240 V between the two, do I need the neutral conductor? Does the Victron pass through the 240V to the usage point or does it convert to DC and back again, eliminating the chance of unbalanced loads (and correcting for line voltage drop as a bonus)?

I have not worked with the 240V versions of their products. I will need to look into what they have. Hopefully, they have a system that does not need neutral on the AC in but has split phase out.

Having said that, the general design of all of the Victron inverters I have worked with is that in pass-through mode the inverter circuit runs in parallel with the AC in. When the AC-in current gets to the configured value it starts supplementing the AC out with battery power.

The problem with this idea is that the AC power is single direction. Therefore the batteries on the Victron inverter at the local site are not available for powering any of the other sites. This is not the end of the world but will require more total system-wide storage.

 

[FilterGuy]

BTW: Will you have 240V loads at any of the sites?