Gulf Island Off-Grid System - Part 6 (Solar Expansion & Intertie)

David and Laura

As we’ve described in our earlier posts, we have two solar systems, a small 250 amp-hour system for the cabin where we live (Part 1, Part 2), and a larger 1000 amp-hour system for the guest house and workshop (Part 3, Part 4, Part 5). These two systems are connected together manually, so that we can transfer power between them as needed.

The next phase of building out our power system involves the following additions and changes:

1. Adding another inverter and string to the larger system to capture the afternoon/evening sun (our existing array only captures sun from the morning through 2 P.M.)
2. Allowing the transfer of power between the two systems to be performed automatically based on SOC levels and weather.

Solar Expansion

Our current 5 KW array is in the sun from 7 A.M. to 2 P.M. However, over the last year, we have observed a significant number of days where the clouds from the night’s rains do not burn off until the afternoon. We also tend to use the shop during the late afternoon, which results in us starting to draw down our battery even before the sun has set.

Fortunately, we anticipated this, and when we connected the two systems together, we designed the buried cable run so that we could add an inverter close by to a west-facing location. Specifically, we need to build a new fence for our garden to keep the deer out, so why not build a solar fence?

This lends itself to a simple triangular mounting structure, where panels are mounted on a 45 degree angle onto the fence structure. We can then put chicken wire along the back of the fence to allow chickens to run outdoors under the panels.

We have a Sunny Boy 3000TL-US inverter, and we will be connecting either 10 Hanwa Q series 250 watt panels in a single string or 15 of the same panels across two stings. A dual-string DC disconnect will be mounted on the solar fence, provided by an SMA CU 1000-US-10 disconnecting combiner, with a buried high voltage cable run back to the inverter.

The inverter will be connected into the larger system using the same design we used for our other two inverters.


Automated Power Transfer

Transferring power from the larger system to the smaller system currently involves throwing a switch that connects the AC input to the Magnum inverter to the AC output of the SMA Sunny Islands. This bypasses the Magnum, putting the cabin loads onto the larger system, and activating the battery charger in the Magnum.

Transferring power from the smaller system to the larger system currently involves throwing several switches to turn on a high-voltage DC power supply, which is connected to an SMA Windy Boy. The Windy Boy regulates the current flow (up to the preset limit in the DC power supply) based on the amount of power that the Sunny Islands (and other loads) can accept. So, for example, if we wished to limit the amount of power transferred to 200 watts, we would program a corresponding current limit (at 500 VDC, that would be 400 milliamps).

Currently, any transfer of power needs to be manually initiated and monitored. This doesn’t work too well when we need to transfer power when we’re asleep, or when we’re not home. So, our goal is to make it so that the transfer can be initiated automatically, based on algorithmic control (for example, instead of just dumping power to our diversion load), or remotely.

Here is the one-line diagram of what we have today:



Here is the one-line diagram of our preliminary design for the enhanced system:



Not shown in the above diagram are the wiring for the inverter and panels, low-voltage control wiring to the PLC, wiring for the front panel indicators and network (RS-485 and Ethernet) wiring.

Another potential addition would be to connect both AC busses to another Square D TVSS.

The front panel of the main box that holds the above equipment will look like the below mock-up:



Over the next few months we'll be working on building the panel using the equipment we already have on hand (it turned out we only need a couple of extra parts to put this together), and preparing to install the solar panels. We'll keep everyone updated as this project progresses, and of course, answer any questions that come up.

Surfpath

Re: Gulf Island Off-Grid System - Part 6 (Solar Expansion & Intertie)

David & Laura wrote: »

The next phase of building out our power system involves the following additions and changes:

1. Adding another inverter and string to the larger system to capture the afternoon/evening sun

Hi Laura and Dave,
As always, very well documented and professionally diagrammed. You have a big, well thought out system.

How's the Hydro doing? Have you had a chance to measure the flow this winter, and did you contemplate 'expanding' the hydro to meet your additional needs? Any photos or sketches of this part of your project would also be cool to see. How is your Hydro 'controlled' (by your Sunny Boys?), do you have a "dump.'?

Best wishes, SP.

David and Laura

Re: Gulf Island Off-Grid System - Part 6 (Solar Expansion & Intertie)

Thank you!

Our waterwheel (an old Harris unit) is nearing the end of its life, but it's still working away. We haven't done much with this part of the system yet.

We did add a pressure gauge. The pressure measured is 60 psi, regardless of if the valve is closed or open ,which indicates that we're not drawing down the water in our upstream intake. We haven't measured the flow in terms of GPM, but we know we have a head of around 120 feet, and with 1.5 inch inner diameter pipe, the theoretical power capability is in the 200-300 watt range.



The AC from the waterwheel is connected via a step-down transformer and diode rectifier to the 24VDC bus, which is also connected to the battery and Magnum inverter as part of our smaller system.

Here's a view of the Magnum battery monitor with all loads off (so that the only flow in and out of the battery is from the waterwheel), with a low battery SOC so we know that we're drawing a maximum charging current:



7.8 amps at 24 volts work out to be 187 watts, so we're doing pretty good, especially given the loss of the long cable run, step-down transformer and rectifier. Our diversion load is a DL600 (PDF) from a local company, Off-Grid Engineering. Sadly, they no longer seem to be on the web.

When we get this phase of the project done, we can automatically transfer power from the smaller system (which has the waterwheel connected) up to the larger system, and thus never burn off power as heat unless both systems are at 100% SOC.

The waterwheel is half a mile away from our cabin, but is a mile hike to get to. Here's a view of the little doghouse:



When this photo was taken, an alder tree had fallen on the pipe and smashed a pipe connector. Murphy's law...

Here's a view inside the doghouse:



It's a pretty old unit, and one of the planned future phases is to replace it with a newer unit, build a slightly more permanent structure to house it, and add some sensors for system monitoring and control. We might be able to push things to 250 watts, but that may require a multi-nozzle unit so we can close off one of the nozzles during the "drier" wet times.

This will most likely be a fall 2015 project, given the wetness of that area during most of the year.

Surfpath

Re: Gulf Island Off-Grid System - Part 6 (Solar Expansion & Intertie)

That's really cool, thank for sharing. Your story is reminiscent of Paul's from Lifeattheendoftheroad. I like his blog. He has a Canadian Stream Engine (possible 'local' replacement for you?).

187w you say? May not seem like much, but one forgets that’s 24/7, so around 4.5 kWh/day just from that old Harris.

From this Harris chart here it's possible that your stream is producing 15-18 GPM. If your old Harris has a Ford alternator you should see an instant 30-50watt improvement if you upgrade to the 'newer' PMA unit (exactly as you say: 250w - 2 nozzles better).
That's a 1.5 Kw/hr/day improvement!

Also, especially at your static head of 140ft, the benefit of going with larger pipe, even upgrading from 1.5" to 2" or 3" is really significant (double/triple the wattage?). See here at the bottom of this page for some neat charts.

Anyway, I think you have the coolest off-grid system on NAWS.

David and Laura

Re: Gulf Island Off-Grid System - Part 6 (Solar Expansion & Intertie)

Some progress:



The main components have been located, and the few parts we didn't have on hand have been purchased. Thank goodness for EBay, which has let us find many brand new parts for a tenth of what they would cost new.

Next steps involve cutting the remainder of the DIN rail, attaching it to the panel, and starting to wire everything up.

We've made a couple minor changes to the plan:



Most notably, there are a few more loads on the 24 VDC bus, and we went from three meters to two meters.

Finally, we rearranged the front panel to make it match where the wires enter and exit the enclosure:



We ended up dropping the manual override switch for isolating the solar inverter in order to reduce the number of inputs required on the PLC, and since it didn't show up in any of the use cases for the system. It still can be isolated via a breaker inside the box, or under computer control.

We also found a place that makes nice engraved one-off labels, so we're going to give that a try to see how they look for the mounting on the front panel.

David and Laura

We received the engraved labels we ordered:

Attachment not found.

We'd recommend the place we ordered them from: http://engravedlabel.com/

Their prices are quite reasonable, and they did a very nice job.

stephendv

David & Laura wrote: »

Automated Power Transfer

Transferring power from the larger system to the smaller system currently involves throwing a switch that connects the AC input to the Magnum inverter to the AC output of the SMA Sunny Islands. This bypasses the Magnum, putting the cabin loads onto the larger system, and activating the battery charger in the Magnum.

Transferring power from the smaller system to the larger system currently involves throwing several switches to turn on a high-voltage DC power supply

I've often thought about a similar problem, say you have 2 neighbours with independent off-grid systems, how could they connect to each other and share surplus power when necessary and borrow power when one of them needs it?
A theoretical way to do this, is if one of them has a sunny island system to act as the backbone of the mini-grid.
Then the other users should use grid interactive battery systems like the outback GVFX inverter that can sell to the grid.
Then the output of the sunny island goes to the grid input of the outback. Set the sell and charge voltage on the outback and all the switching would happen automatically.
When the outback system has surplus power it would try and "sell" it to the sunny island, and if they SI's battery needed it, it would accept it, if not it would bump the line frequency and the outback would disconnect.
If the SI had surplus power and the outback needed it, it would use that power to charge the batteries.

That's the theory anyway

David and Laura

stephendv wrote: »

I've often thought about a similar problem, say you have 2 neighbours with independent off-grid systems, how could they connect to each other and share surplus power when necessary and borrow power when one of them needs it?

Exactly. To summarize the requirements as we see them, what we want is an affordable method by which to allow the exchange of energy (excess or not) between cooperating off-grid parties on a barter or monetary basis.

What we're building right now (as described in this thread) is our second generation of experimentation, and in our limited free time, we've also been working on a pair of devices that plugs into the AC power of a first grid, and looks like a solar string to a second grid. Our initial design is based around a simple DC voltage-doubling rectifier, combined with circuitry that provides compatibility with MPPT and provides current limiting. A simple processor keeps track of local battery SoC, power inputs and outputs, and determines when to sell and at what price, and when to buy and at what price. Two of these systems are installed, one for each off-grid system, and uses a common 600VDC cable to connect them (for both power transfer and negotiation). This also allows it to easily be extended to more than two participants. Add an inexpensive ESP8266 wifi module for computer-based configuration and monitoring, and it's ready to go.

Attachment not found.

Given our experience with having a waterwheel, and how the system dynamics change, even a low wattage unit (say under 500 watts), would be more than enough to be useful. We figure it could be built for < $150, and we wish we had more time to work on it, as there are many places where this would be very useful and it would make a great Kickstarter project.

westbranch

Way ahead of me... I have a bit of difficulty seeing how the turbine fits into that schematic?
Also do not understand why you would shift the power between the 2 systems as DC, given the issues of transporting DC at anything lower than ~ several hundreds of Volts. Assuming that the systems are not immediately adjacent...

David and Laura

westbranch wrote: »

Way ahead of me... I have a bit of difficulty seeing how the turbine fits into that schematic?

The diagram in post #8 was to show a hypothetical connection between two typical off-grid systems. The water turbine discussed in post #3 is a separate topic (and should really be in a separate post), describing how our waterwheel is set up.

westbranch wrote: »

Also do not understand why you would shift the power between the 2 systems as DC, given the issues of transporting DC at anything lower than ~ several hundreds of Volts. Assuming that the systems are not immediately adjacent...

DC is the standard way to couple two separate AC systems. That way you don't have to deal with synchronization (and flow control is much simpler). Even at 300 VDC, where this system would operate at, you are right in pointing out that there are significant limits in effective distance.

However, with an AC-coupled system (like ours), you can also put a Sunny Boy at the end of a long AC power run, along with a plug, and use that to connect systems together, as shown in the below diagram:

Attachment not found.

Regardless, you aren't going to go long distances with either AC or DC without high voltages, and that puts one in the position where you can't do it inexpensively and safely.

Unfortunately, there is no easy long-distance solution. But for nearby neighbours or clusters of residential or commercial buildings, the approach we've outlined will work well, and follows the same safety guidelines already developed for PV wiring.