Friday, October 24, 2008

I've Installed Off-Grid Solar - Why Didn't my Electric Bill go Down?

It’s interesting to talk about alternative electricity with those who live in areas where electric rates are high. They tend to know all about energy-efficient lighting and appliances. They know how to conserve, and they understand the benefits of energy efficiency-related home improvements. They’ve either installed a small photovoltaic (PV) system, or they’re planning to install one in the near future. They understand how PV systems work, and they’re tired of articles telling them to keep the panels clean and avoid shading. Those things are intuitive. What they’re really looking for is a way to reduce their electric bill.

My approach is simple; I use as much free energy from the sun as my system can supply, and switch to grid-supplied electricity only when necessary. This strategy may seem odd to those who are conditioned to think of a battery-based system as a backup to the grid. It’s counterintuitive. The first step is to get comfortable with the concept of using the power grid as a backup system.

When both sources of electricity are available, make sure that no one in the house uses grid power. Design the system to switch automatically, with battery power as the default. Switch to grid power ONLY when batteries are depleted. Many inverters have this capability built-in, but you can purchase an Automatic Transfer Switch if yours does not. By transferring all lights and electrical outlets in your home to battery power, no one has access to grid power, and therefore no one will be running up your electric bill by accident or without your knowledge. If your PV system is small, high-power items will have to be excluded. Your central air conditioner, for example, will probably not be switched to battery power. Once your batteries become discharged, late at night perhaps, lights and outlets in your home will be switched to grid power. Fortunately, this is the time when electricity use will be at its lowest level for most households.

Inverters draw energy from the batteries until battery voltage drops to a point where the inverter can no longer function. Typically, the cut-off point is about 10.5 volts (for a 12 volt inverter). Unfortunately, allowing batteries to discharge that much can be harmful to them. You’ll need another way to switch your inverter off. And, once the batteries are drained to that level, they should be recharged fully before reconnecting loads. Reconnecting batteries to the load too soon could result in chronic undercharging, which would shorten the life of the batteries. Some charge controllers allow you to configure disconnect and reconnect set points, or they have that functionality available as an option. Look for that feature in the equipment you’ll purchase. With these things in mind, let’s review system functionality:

The sun rises in the morning, and batteries become fully charged.

Selected AC loads are switched to battery power via the inverter.

The sun sets in the evening and battery voltage declines.

When battery voltage falls to a preset level, AC loads are switched to grid power.

The cycle repeats each day.

The concept I’ve described here is simple, but effective. It eliminates the waste that would occur if you were to switch manually. After all, you’re not setting at the controls 24/7 watching for the ideal time to switch, are you? By switching automatically you might save hundreds of dollars each year on your electric bill. If your PV system is small, start with one or two rooms and add rooms as you add solar panels and batteries to your system. An electrician can easily wire in additional circuits as your system grows.

I use this strategy in my home, with a chest freezer and refrigerator as the only loads. Instead of tying into my existing house wiring, I’ve added separate wiring to those appliances. Because I didn’t tie in to the existing house wiring, I saved myself the cost of having an electrician do the wiring. I’ve observed that my system switches to battery power on sunny days at about 10:30am. It switches back to grid-supplied AC at about midnight. It’s easy to see that I could use more batteries and solar panels. I plan to expand, and I hope that by the end of next year I’ll be able to add to the existing loads. By making sure that the size of the load exceeds the capacity of the PV system I know that I’m getting as much power as the system is capable of producing.

The system as described should cut your electric bill considerably, but you can cut it even more if you watch for opportunities. A properly designed system should include enough PV capacity to fully charge your battery bank each day, and excess energy is often wasted. Watch for opportunities to use that otherwise wasted energy. For maximum efficiency, use appliances mid-day with power coming directly from the solar panels.

The way to get the most from any Off-Grid PV system is not to let any solar-generated electricity go to waste.

An automatic transfer switch should be installed by a properly trained and licensed electrician.


Saturday, October 18, 2008

Smart Transportation

Finding a car that is gentle on the environment, and fuel-efficient may be easy in Southern California, but choices are limited here in Southern Illinois. Ask about an electric car here, and dealers are likely to point to the golf-cart they use to transport customers around the lot. The 2005 Smart-ForTwo shown here seems to be way over priced at a local dealership.

They tell me there's a waiting list for these cars. I think I'll wait instead for a plug-in-electric. GM, Ford, and Toyota plan to offer plug-in-hybrid cars by 2010.

Can't wait for an electric car? You could buy a Prius now, and spend another $10,000 to have it converted to plug-in, but that seems a bit pricey to me. No, I think I'll wait instead to choose from the available 2010 models.

Tuesday, October 07, 2008

System Automation for a Spare Battery Bank

Having recently replaced my PV system’s battery bank, I had to decide what to do with the still-good older batteries. Connecting batteries of different types and ages together is not a good idea; it shortens the life of the newer batteries. My choice was to install a switch, allowing me to add and remove the old bank from the charge controller/inverter circuit. When I want to charge the old bank, I can simply switch it in. I can also switch it in when I need extra amp-hours, during a grid power failure for example. I can switch it out the rest of the time, keeping it from dragging down the new battery bank.

My plan worked fine for awhile, but then I neglected to check the batteries for a few days. To my horror, I found that the battery voltage dropped below 10 volts. Allowing batteries to deeply discharge, and remain in that state for an extended period of time, can ruin them. I knew I had to do something else. I wanted to use energy from the sun to keep both battery banks charged, but I wanted the main battery bank to have top priority. Here’s what I did:

The problem was easily solved by adding a relay to an unused channel of my Morningstar Relay Driver. I’ve programmed the relay driver to monitor the main battery bank voltage. When the main battery bank is nearly fully charged, I divert excess current to the older battery bank. Programming voltage thresholds is done by temporarily connecting a computer to the relay driver and running a simple configuration program. Here are my settings for the spare battery bank relay:

Turn on relay when main battery voltage > 14.40 volts.

This establishes the main battery bank as the top charging priority. Power will be diverted to the spare battery bank ONLY when the main battery bank is nearly fully charged.

Turn off relay when main battery voltage < 14.00 volts.

Turning off the relay disconnects the spare battery bank from the inverter and charge controller, preventing it from discharging through the load.

The main battery bank provides power to the loads day and night, cutting my electric bill. I’ve programmed the relay driver to remove the load from the main bank when its state-of-charge (SOC) drops below 80%. This happens at night, or when it’s cloudy, leaving me with little surplus power to use in the event of a grid power failure. However, by keeping the spare battery bank fully charged I now have the best of both worlds, lower electric bills and reserve energy to serve in the event of an emergency. A simplified diagram of my system is shown below. Relay 1 switches the inverter on and off, while relay 2 switches the spare battery bank in and out of the circuit.