Utilities off the grid

Our location makes utilities doubly difficult. The nearest electric and phone lines end near the compressor station 4 miles of so down the road. Extending these is hideously expensive, $10,000 per mile or more. But that's not all. We're inside the national forest, so any extension of the lines would require permits from the Forest Service. The permit process is slow and possibly expensive, in part because people like us don't like seeing new power lines stretched across the forest line to benefit one or two people. These are modern times and technology is the answer (or the very opposite, living without).

Electricity comes from the sun. Of course, a purist would argue that all electricity comes from the sun, it's just that a lot processes occurred between the sun falling on the earth and burning the coal to make electricity. In our case, the sun falls on photovoltaic (PV) panels, the proper name for solar panels that produce electricity. The panels contain thin crystals of semiconducting materials that convert the photons of sunlight directly into electricity.

The panels are aimed towards the south and tilted to the angle of the sun in the sky. Every month or two I get up on a ladder and adjust the tilt for the new position of the sun in the sky. There are mechanical trackers that follow the sun through the sky, but these are notoriously finicky, so for now we're sticking with the simpler fixed mounts.  The picture shows two sets of panels, but we actually have three, with a total (nominal) output of about 3 kilowatts. The actual output is determined by how bright the sun is (very good in New Mexico) and the temperature of the panels. The colder the panel, the more efficient it is, so we actually get more electricity out for the same sunlight in during the winter months. This helps make up for the lower amount of sunlight in the winter.

Since the sun doesn't shine at night, even in sunny New Mexico, we have to store the electricity somewhere if we want electricity at night. We use standard lead-acid batteries, the same as in your car, just bigger. While there are newer technologies, they are extremely expensive. Maybe next time.  If you've been reading the diary, you've seen the saga of moving the batteries. Here's a shot of them installed in the house. There are two cases like this one. Each weighs around 1300 pounds, outputs 12V DC, and together store about 40 kW hours - about 3 days of electrical use for a reasonable sized home.  The thick hoses leading off the top left are the battery cables and are about as big around as my thumb.

While it is possible, at least for the truly dedicated, to operate a house on all DC current, it requires special appliances, special lights, etc. You can spend your money on that, or you can buy an inverter that converts DC power to AC, the kind that the power company supplies to you.  Here's a shot of the inverter panel, with the key components labeled.

This unit provides 240V power, just like the power company provides to your house. This is then split into two 120V circuits, which is the voltage most household devices run on. Some exceptions that actually require 240 volts would be an electric dryer, over, or cooktop, and some power tools. Few sane people would run an electric stove or dryer on an off-grid system - it's just too expensive to produce that much electricity. We have a 240V system mainly because we wanted two inverters to provide adequate power for the whole house, as well as to provide redundancy in case one inverter failed.

The inverters also act as battery chargers when the backup generator is on. While not absolutely essential, a couple of cloudy days in a row can leave you without adequate power. You can limp along, or you can use a backup generator to recharge the batteries through the charger stage of the inverters.  In a sophisticated system like this one, the inverters monitor the battery power level and will automatically switch on the generator when the reserves get low. In addition, the inverter will turn on the generator when the power draw is larger than the inverter can support.

On the right side is a black box labeled "MPPT" which stands for Maximum Power Point Tracker. PV panels have a non-linear voltage/current curve  - which means that in order to extract maximum power from the panels you have to control the draw on the panels to hit the "sweet spot". The MPPT does just that - it sweeps through the voltage range of the panels to find the point where power is maximized. It then converts the panel voltage to the appropriate voltage for the batteries and acts as a battery charger. The finished system has a second MPPT unit just below this one. Right now it's sitting in the box UPS just delivered. The unit went back to the factory for an upgrade of the firmware to address an overvoltage problem that can occur in places like the ranch that have cold, clear winter mornings.

The MPPT sits next to the box labeled "DC Side" which contains the breakers for the various DC units, as well as the main disconnect for the batteries. The "AC Side" contains normal circuit breakers for line circuits, as well as the contacts for the connecting the cabling from the generator.  There is also a seemingly magical device called an autotransformer which improves the efficiency of the unit. In a normal house (and ours as well), the household circuits are split between the two 120V legs of the 240V input. In our case, each leg is powered by one of the inverters. The inverters themselves consume power, so it's preferable to draw all power from just one inverter when possible. Of course, since the circuits are physically wired to one leg or the other, how do you do this? The autotransformer sits between the two 120V legs and can shift the load from one leg to the other. When the draw is light, all power is drawn from just one inverter so that the other can remain in standby mode.  The last box, labeled "Breaker box" is just like the one in your house - it contains "normal" circuit breakers to protect the wiring.

 

COMING ATTRACTIONS:

Keep watching this page. We'll have shots of the generator when it's installed. I'll also add a section on the water system and propane when it's installed. Eventually we'll have an amplifier for the cell phones and satellite TV and internet to connect to the world.