Big Mesa Farm MICROGRID
TECHNICAL DESCRIPTION and usage data

Big Mesa Farm is located in a somewhat remote valley not served by the electric grid, so in 2019 we began building a stand-alone solar “micro-grid”, which now powers all eight buildings and the farm operations. As seen in the diagram, the system design is based on SMA inverters: two Sunny Island SI6048 inverters connect to the batteries and form the 240V AC grid. The batteries are charged directly by a small 3.6kW PV array through a charge controller (DC coupling), but mostly charged by two larger ~8kW arrays on separate buildings, each with a Sunny Boy synchronous inverter to feed PV power into the AC microgrid, where the Sunny Islands can charge the batteries if there is excess energy (AC coupling).

The batteries are more unusual. Three 20kWh batteries are wired in parallel (the black boxes in the bottom of the photo below). Each is Lithium Iron Phosphate (CALB CA180 cells, 2 in parallel and 16 pairs in series for a nominal 48V), with a Battery Management System designed by Robb Protheroe at Atlas-ESS. More will be said about the battery management system. Each battery is in a welded steel box, custom made by Steve Heckeroth, who was at the time developing prototypes of his Solectrac electric tractors - the battery boxes include attachments for a 3-point tractor hitch, making them easy to transport if needed for a remote power source.

PROGRAMMING THE SUNNY ISLAND INVERTERS

A lot was learned about programming the SMA Sunny Island SI6048 inverters to work with the Lithium Iron Phosphate (LFP) batteries we are using. SMA only supports certain Lithium batteries, only ones which communicate digitally with the inverters. Our batteries do not do this, but instead contain a Battery Management System (BMS) that makes them act like a simple stand-alone battery, shutting off in case of overvoltage, undervoltage or overcurrent, and handling cell balancing (equalization) automatically each time they reach full charge. The challenge was to program the SMA inverters to work with these batteries.

The inverter is told that the batteries are lead-acid, but then the voltages and many other settings are changed to accommodate the Lithium chemistry. We learned early that the temperature compensation (default for lead-acid) needed to be turned off - to avoid charging to too high a voltage in the winter. Before we figured this out, we were seeing one or more of the three 20kWh batteries (which are simply wired in parallel to the inverter) tripping their internal contactors because of this overvoltage. That was problematic because the inverter would actually then be connected to 40kWh or even 20kWh of battery capacity rather than the 60kWh it was programmed for, and the inverter’s learning of the battery charge curve would get confused. That confusion would lead to the inverter reporting incorrect State of Charge (SOC). The SMA inverters use their calculated SOC to decide when to shut down at low charge, and they would do so inappropriately. We eventually did a re-set of the inverters (telling them they had a “new battery”), so the inverter could begin again learning the battery characteristics, which solved the problem! SOC is now being reported correctly.

See here for a detailed description of the current Sunny Island inverter programming for use with stand-alone 48V Lithium batteries. The SMA inverters are capable of great flexibility, but with that comes the need for some study!

The batteries are charged through the Sunny Island inverters when there is excess energy on the AC microgrid supplied by the two AC-coupled PV arrays on other buildings. Each of those arrays connect to their own Sunny Boy synchronous inverter, which feeds 240VAC into the microgrid. These synchronous inverters are programmed to “off grid mode”, which simply means that the Sunny Islands can slightly change the grid frequency off the nominal 60Hz to signal that the batteries are full to curtail the Sunny Boys’ PV energy production. Thus no signal wires needed to be buried running to other buildings. A smaller PV array on the Electrical building charges the batteries directly through an Outback FLEXmax 80 charge controller (DC-coupling) to the same voltage target as the Sunny Island inverters. All DC into or out of the batteries passes through a current shunt, so the Sunny Islands know about (but cannot control) the charge controller’s action.

We use a Generac 22kW propane generator when needed for backup. We have elected not to automatically start the generator (because of the noise among other reasons). Instead a red light on the outside of the Electrical Building is turned on when the SOC is low enough that manually starting the generator should be considered. The red light is controlled using one of the four relays in the inverters (there are two in the master and two in the slave).

ELECTRICAL LOADS

• EV Charger for the delivery van — Daytime load used most days, totaling 400-700 kWh/month.

• Another EV Charger for personal autos — 7.3 kW when in use, but only used in daytime when needed.

• Heaters for the vegetable starts (which are grown from seed in a greenhouse) — Used 24 hrs/day in the late winter and spring, about 19 kWh/day (this nighttime load is a large determiner of the required battery capacity).

• Well pumps — About 18 kWh/day when irrigation is being done all summer. Total water usage ~900,000 gallons/year!

• Pressure pump for irrigation — Variable-speed pump, using about 5.5 kWh/day during irrigation season.

• Walk-in refrigerator — Typically ~6 kWh/day. Uses a Coolbot controlling a large air conditioner.

• Kitchen Induction Stove — About 1 kWh typical nighttime cooking, plus ~3 kWh if the oven is used.

• Electric demand water heater in one of the three kitchens — 5.9 kW max, typically ~2 kWh/night. This is simple and cheap, but it uses an SCR to switch the power on each 60Hz cycle so it causes the 60Hz waveform to be deformed. Someday we’ll start using a heat pump tank water heater.

• Internet — The Starlink router uses 40W (0.96 kWh/day), and is powered through an Uninterruptible Power Supply, so that if the microgrid occasionally shuts down due to the main batteries being low we don’t lose internet & phone connectivity.

• Baseline load is about 100-300W (all the miscellaneous lights and appliances). The woodworking shop is pretty insignificant because the loads are only used briefly.

SUMMARY OF THE SYSTEM PERFORMANCE

The Sunny Island stores many figures (voltages, currents, reactance, all sorts of internal variables) every minute onto an SD Card, one big CSV Log File per day, plus another daily file listing any events (relay operation, shutdowns etc). To reduce these data to something manageable, I wrote a custom Windows application (SIanalyze) that opens the CSV files over a given range of dates and produces a summary CSV with one line per day containing the min and max voltages, min and max SOC, how much if any the generator was run, and a few more. Contact us if you’re interested in this software.

The output from this software is opened with Excel, where it can be graphed for each year of operation. The year 2023 summary below shows the Maximum State of Charge (SOC) each day — typically close to 100%. It shows the minimum SOC each day, which gets lower in the winter; on those days the generator is sometimes run (shown in the gray at the bottom in number of hours, typically about 2-3 hours). Additionally the yellow line is the relative light intensity (captured from our Acurite Atlas weather station, with data reduced by another custom Windows application), which is obviously highest in the summer and drops during clouds or storms.

In April of this year 2023 the electric delivery van was put in service. We see that even with the van charger, now our largest load, we never ran the generator in the summer, but in the following winter there were more nights when the SOC was not where we’d like it and the generator was run. Over the year the generator was started 25 times, totaling 46.40 hours and contributing 482.3 kWh.

COST OF THE ELECTRICAL SYSTEM

We’ve had immense help from friends (thank you Bailey Smith for the initial system design and commissioning, and Andreas Dahm who installed all three PV arrays) and were supported by a REAP Grant (a USDA grant to help with Renewable Energy for Agricultural Production). Cyril Ackerman helped with wiring and conduit, ditch digging, and with installing the standing-seam metal roofs on which the arrays are mounted. Overall the costs and the time we put into this system were definitely worthwhile — indeed the farm at this level would be impossible without the electric system.