Notes from Windward: #67


Power Up!

taking the kitchen off-grid, sort of

  March 27:

     By now, dear reader, you're probably getting the gist of how we proceed down the road to sustainability--a step at a time always making sure that the basics are covered, and that there's room to take each system to the next level at the appropriate time. Today we resumed work on an aspect of the dining hall that's a good example of our approach, as well as a good time to discuss the whole "off gird" thing.

     The first question that lots of people ask is whether Windward is off-grid yet, a question which is hard to answer without going into a long discussion of the relative costs and benefits involved. The short answer is, "No, because for us the better answer is to be grid-tied." That means that we draw electricity from the grid when we need it, and push energy back onto the grid when we have it--the goal being to eventually become energy neutral with regard to the grid.

     Windward's condition use permit allows up to 21 full-time residents, but we've generally felt most comfortable with about a dozen which translates to an average electrical power consumption of about 7 kilowatts. At the current cost of about $4/watt, it would cost us about $84K to purchase enough photovoltaic panels to go completely off-grid. Since we figure we can accomplish the same thing using solar steam for about a tenth of that cost, we don't plan on using PV panels except in special use situations.

     Being grid-tied is the best option for us when it comes to the general use of electricity, but there are some uses of electricity in which reliability is more important than cost--a key example being the power used to run the freezers and refrigerators that store our food. When the grid go down--and we've seen it down for as long as five days--we want to be able to insure that our food stays frozen.

     In order to make sure of that, we're creating what's called an interruptable power supply for the kitchen's critical circuits. When completed, our freezers and refrigerators will not derive their power directly from the grid, but rather indirectly from a pair of inverters capable of supplying a sustained power load of 4,800 watts and a surge load of 12,000 watts.

     The inverters draw their power from six huge L-16 batteries that each weigh 113 pounds, and is rated at 390 AmpHrs at a draw-down rate of 20 Amps. That works out to a theoretical ability to deliver 20 amps at 12 volts for a couple of days. In practice you don't want to draw any lead acid battery down more than half way, and if the grid were down for more than a day, we'd use the Onan generator that sits next to the battery vault to recharge the battery set.

     But so long as the grid is up, it's cost effective for us to use grid power to charge the battery, a task which the inverters handle automatically. This way the battery functions as the electrical equivalent of a fly-wheel insuring that our critical systems stay smoothly energized regardless of any short term loss of grid power.

     Down the road, we'll balance out the power used at the dining hall with power generated at the landing, but right now the goal is to insure that the dining hall functions regardless of what happens to the grid.

     Today's task involved mounting two 2,400 watt inverters in the power room.


  April 3:

     It's been an interesting puzzle sorting out the various bits of hardware and cables needed to safely and reliably connect our bank of L-16 batteries to the inverters--they're just not the sort of things carried by Home Depot.

     At full rated output, each inverter would draw 200 amps of current, so we're looking at having to use size "0000" stranded copper cable--which is a big around as an adult's index finger. When I was in picking up parts, I checked on the current cost of that sort of cable, and the wholesaler is quoting $4.55 per foot. Since we'll need about 60' feet of it, I'm very pleased to report that we just happen to have a few hundred feet of it up in the Depot storage area :-)

     It's in the form of the most humongous power cord you can imagine, and I was further delighted to find that one of the segments was just about a foot longer than the longest run we'll be installing in the power room--so no need to cut a piece of the main length of the cord.

     We'll be housing the cable in 2" conduit as it runs from the battery vault across to the other side of the power room and up the wall to connect to the inverters. In order to keep the cable from overheating, it was necessary to cut away the black plastic outer covering so that the four "0000" cables could "breathe."


     That was a long, tedious affair using a utility knife to cut through the quarter inch of outer coating without cutting the inner cable insulation, the tractor tire I was using to hold the wire steady, or me. As ever persistence paid off, and eventually the cable was free of its outer coating and ready for installation.


  April 5:

     Since the battery vault is on the north side of the power room, and the inverters are on the south side, we're going to need to run conduit down and across the floor and back up the south wall. That way the cable will be well below floor level where it will be protected from traffic once we haul in a yard of "3/4 minus" gravel and tamp it in place to create a smooth floor. A tamped gravel floor isn't as solid as a poured concrete floor, but it will serve and if we need to do any rerouting, it's much easier to dig out and replace.


     It looks like we've acquired all the electrical parts needed, and are only lacking a couple of pvc fittings--once those are in hand we'll tackle the joy of getting four runs of "0000" copper to make a series of 90° bends.

  April 7:

     Still spending a good deal of time tracking down the various bits and pieces needed to bring the new power system together, but most of what we'll need has been found, acquired or created by now.


     Arguably the most important component in the whole system is the fuse, and it takes a serious fuse to handle the sort of currents that these inverters can pull under load. At their rated output of 2,400 watts each inverter draws 200 amps, but this system was intentionally designed to operate at no more than half-load on either inverter--with the result that a 300 amp fuse should be a good trade-off between capacity and security. Replacing a 300 amp fuse isn't cheap, but it's trival when compared to the cost of replacing buildings :-)


     The above's a shot of the fuse assembly mounted on the inside wall of the battery vault. On the left hand side of the pic you can also see the special twin wire adapter that will allow us to run a seperate ground cable to each inverter. The wire we're using is capable of supplying each inverter, but not both at the same time; hence the double run. Double runs are okay so long as both sets of wires connect run from source to use, hence the double connector on both the negative terminal of the battery and the positive terminal of the fuse assembly.

     The pic also shows another key safety item--those funny round caps on the battery cells. When electricity is stored in a battery, a portion of the energy invested breaks water down into hydrogen and oxygen gas in exactly the right proportions needed to create a very ugly explosion. It doesn't get much worse than explosions involving hot lead and sulfuric acid.


     The hydro caps contain a platinum catalyst which absorbs the hydrogen gas, flamelessly combining it with oxygen to produce water--which then drips back down into the cell. Not only does the Hydro-Cap protect the battery compartment from a build up of an explosive gas mixture, they automatically keep the cells at the proper water level.

  April 8:

     The next step was cut and glue up the 2" conduit. The key point there was to insure that all the joints were well glued in order to prevent moisture from being able to enter the conduit.


     Then came the challenge of pulling the four strands of "0000" cable through the conduit. It actually wasn't all that hard; it's just one of those tasks that requires two people--one to push, and one to pull.

  April 9:

     When we first got our satellite internet downlink, the easiest and most secure place to set it up was in the kitchen, but with the power room coming on-line, today was a good time to reroute the wiring and move the hardware into there. One more detail checked off on the process of finishing the dining hall's interior.


     The black box in the center is the satellite receiver, and the smaller white device is the switch which operates our LAN (Local Area Network). The switch has a wifi connection, but since the dining hall has a metal roof, the wifi pretty much only works inside the dining hall--for now.

  April 10:

     Finished running the conduit up the wall, and cut the "0000" cables to length. Cleaned up some used terminals we had and used them to connect the cables to the inverters. Later we found out that the terminals were too big to allow the plastic cover shields to be screwed into place, so we'll have to go with a different style of terminal.


     The usual color coding for direct current wiring is black for negative and red for positive, and there's no problem with using the black and red cables that way, but this wiring was actually for 240 volt alternating current. In that sort of installation, the red and black would be the "hot" wires, the white would be the neutral and the green would be ground. In order to show that we're using the white and green wires as though they were red and black respectively, the ends of the cables are wrapped with red and black tape.

  April 11:

     Swapped out the larger terminals for smaller ones, and now the plastic terminal covers fit fine.


     And so it was time to turn our attention to connecting the other end of the cable set to the battery. That too went smoothly, and there's nothing left to do at this end except to connect up the final positive cable.


  April 17:

     One of the tasks that needed to be done before the inverters could be powered up is to make sure that the inverters and the battery pack are properly grounded. That involved fetching a grounding rod with is a six foot long piece of steel that's copper clad, and driving it into the earth.


     We have a driving hammer that we use to sink T-posts into the ground, and it did a fine job of driving the grounding rod into the damp soil. This is the sort of task that's easy in the spring, but very difficult in the fall when our clayish ground dries out and becomes quite hard.


  May 4:

     Now that the rabbit tractor is on-line, and the rabbits have another 32 square feet of room to stretch out in, it was time to return to work in the power room. When last we worked on that, the grounding pole had been sunk into the ground, so the next step was to connect a copper grounding wire to the pole, run it through the over-head and down to connect to the two inverters.


     The bare copper wire could have been run over the top of the ground, but digging a trench from the pole to the dinning hall served two purposes: it got the wire out of the way so that no one would trip over it, and the extra contact with the dirt offers a bit more grounding effect.


Notes From Windward - Index - Vol. 67