Getting the Most from a Small Off-Grid PV System

Although my off-grid system is small, I’m proud to say that by using it I’m reducing the amount of CO2 that would otherwise be released into the atmosphere and I’m reducing my electric bill. However, it’s a challenge to use as much solar-generated energy as possible, while still being gentle to the batteries. Allowing too much power to be sucked out of the batteries each night will shorten their life. On the other hand, I don’t want to under-use the system by not using enough of the available energy each day. Unused energy is wasted energy. The purpose of this article is to explore strategies for getting the most from a small off-grid PV system.

Strategy #1

A 100-watt solar panel might produce 500-watt/hours of electricity on a sunny day. But, due to system inefficiencies, it’s likely that only about 300-watt/hours of energy will make it to the load. Using solar-generated energy during peak sunlight hours is one way to improve efficiency. When power from the solar panels goes straight to the inverter, overall efficiency is much greater, since the losses associated with storing and retrieving electrical energy in batteries are eliminated.

The AC load can be plugged into the PV system’s inverter during the day and then to grid-supplied power at night, or switched automatically via a timer as illustrated below. A refrigerator or freezer is an ideal load for this scenario, since the amount of electricity required is about the same from day-to-day. The timer can be adjusted to match the power generated by the PV system to the power required by the load.


Strategy #2

A no-cost way to squeeze every available electron from an off-grid system is to constantly monitor battery voltage, disconnecting the load when the battery falls to a certain level, and reconnecting it when the battery is once again fully charged. Obviously, monitoring the system 24 hours a day is not practical. However, most DC to AC inverters include a low-battery “alarm” feature. The alarm sounds when the battery voltage drops below a certain level, eliminating the need for constant monitoring. Still, listening for an alarm is only a modest improvement over constant monitoring. No one wants to respond to an alarm in the middle of the night. Another drawback of this strategy is that the low-voltage alarm level is usually not adjustable. Typically, the alarm threshold voltage is about 10.6 volts (for a 12-volt inverter), which, depending upon battery type, translates to about a 75% depth of discharge. Most inverters will shut-down when battery voltage falls a little below the alarm threshold. While this strategy will work to a certain extent, you shouldn’t allow your battery state of charge (SOC) to fall that low. Allowing battery voltage to drop to those depths on a frequent basis will shorten the life of the battery. This strategy is not recommended on a day-to-day basis, but could be used in emergency situations, such as a grid power failure.

Strategy #3

To automate the process of disconnecting and reconnecting the load based on battery voltage, a charge controller with built-in low voltage disconnect (LVD) capabilities can be used. An LVD-capable charge controller is a great strategy for unattended systems, such as a weekend cottage. If a load is accidentally left on, the battery will be protected.

Although this is a good solution for applications where power to the load flows through the charge controller, it is not viable in systems where power to the load does not. The input current of an inverter is usually higher that the charge controller can handle, making it necessary to connect the inverter directly to the battery.

The diagram below shows typical wiring for a small, off-grid, PV system. Notice that the inverter is connected directly to the battery, and therefore will not be shut down when the Charge Controller’s low voltage disconnect kicks in. Only the DC output from the Charge Controller will be shut down in this scenario. Battery voltage will continue to decline (because of the AC load), even after the Low Voltage Disconnect kicks in. At some point the inverter will stop functioning. This is not a good strategy unless you’re only using the DC output of the Charge Controller to power a load, or unless you’re carefully monitoring the battery voltage while powering an AC load.


Strategy #4

Some Charge Controllers can be configured as “Load Controllers”, providing another strategy for getting the most from a small system. A Load Controller does not replace a Charge Controller, it is an add-on to the system. An advantage of using a Load Controller is that disconnect and reconnect thresholds can be precisely set. Settings are determined by the type of batteries, and by the size of the load. When properly set, the disconnect set-point prevents the battery from discharging too much, and the reconnect set-point value is high enough to ensure that the battery is not damaged by chronic undercharging. In other words, the disconnect and reconnect settings are optimized. The extra hardware does, of course, add considerable cost to the system.

Shown below is a typical application of a Load Controller. In the example, the Load Controller uses the batteries as the primary source of energy, but switches to a secondary source of energy when the batteries are depleted to a preset level. Once the batteries are recharged, they are again used to power the load. The beauty of this system is that it uses as much “free” energy from the sun as is available, only using a more costly source of energy when necessary.


Strategy #5

Those with electronics skills may consider the solution outlined below. This diagram represents a simple circuit that can replace the “Load Controller” and relay in the diagram above. Disconnect and reconnect voltage levels are set via two potentiometers. The potentiometers could be replaced by thumbwheel switches and precision resistors, providing better control of the threshold voltages. Notice that the relay wiring doesn’t allow AC from the inverter and AC from the power grid to be applied to the load simultaneously. Allowing that to happen would probably result in damage to the inverter.


Strategy #6

The battery-protecting switchover functions described above are also available in equipment designed for larger systems, but that equipment may not be appropriate for small systems. In addition to the high cost, this equipment typically uses more power for its operation than equipment designed for smaller systems. While this power drain may not be significant for a large system, it represents a significant percentage of the overall energy production of a small system.

Conclusion

In compiling this information it has been my intention to demonstrate optimization while holding down the cost. You may benefit from using one or more of the strategies outlined here. I welcome other suggestions for accomplishing the task.

John

Driving a PHEV is Like Buying Gasoline at 20 Cents a Gallon

While you might think a Plug-in-Hybrid-Electric-Vehicle (PHEV) is simply an ordinary hybrid with a plug added for charging, there is another significant difference between the two. Typical hybrid cars on the road today have a gasoline motor and an electric motor, and either of them can be used to drive the wheels. On the other hand, power to the wheels on the new breed of PHEV’s will come ONLY from the electric motor. The gasoline motor is used to recharge batteries. This design is inherently more fuel-efficient, since the gasoline motor will run at a constant speed. It’s called a “series” hybrid, and the Chevrolet Volt is an example. The plug-in Volt is expected to be available in 2010. It will have a 16kwh lithium-ion battery that will take 6 to 6 ½ hours to charge, and a fully-charged battery can propel the car for the first 40 miles. The gasoline motor kicks-in after that, extending the range of the car. For those who drive less than 40 miles per day, which is most of us, the car will use no gasoline at all.

Based on that information, it’s easy to calculate operating expenses. Assuming that electricity costs ten cents per kwh, the cost to charge the battery should not exceed $1.60, which is ten cents per kwh times 16kwh. In reality, the battery will not be fully drained, and therefore the actual cost will be less than that. But for no more than $1.60 worth of electricity, the car will go 40 miles. It takes $3.00 to $6.00 worth of gasoline to go that far in a typical gasoline-powered car!

Nighttime electric rates are much less than daytime rates in many locations, and I’ve signed up for a plan that gives me rates as low as two cents per kwh in the early morning hours. My rate will change from day to day and hour to hour, but if it averages less than four cents per kwh at night, I’ll be able to charge my Volt for as little as twenty six cents. Needless to say, I can hardly wait to own one. When compared to a car that gets 30 mpg, this is equivalent to gasoline at 20 cents per gallon for the first 40 miles of driving each day!

In reality, state and federal legislators will soon realize that I’m not paying my fair share of road use taxes, and somehow I’ll be forced to make up the difference, but I’ll certainly have some unbelievably inexpensive transportation in the mean time.

Here’s the info from Chevy: http://www.chevrolet.com/pop/electriccar/2007/process_en.jsp

John

Renewable Energy Survey Results

Money saved by using energy-efficient appliances is often used to buy additional appliances and vehicles, resulting in an even greater use of energy. Respondents to my survey overwhelmingly said that the solution to this problem is generous rebates and incentives for the purchase of hybrid cars and renewable energy systems. And, of the choices listed in the survey, I agree that this was the best one. Some respondents selected “Other”, suggesting that they have even better ideas. I hope those folks will submit comments to this post, telling us what those ideas are. It seems that almost no one wants laws or penalties, but some of us favor higher taxes for those who consume the most.

Tax incentives are great, but that choice is in the hands of politicians. Rebates will help to sell more energy-efficient cars, but that decision is in the hands of auto makers. There’s an even better solution than those listed; each of us can lead by example. And while only a few of us are ready to install windmills or solar panels, every one of us can contribute in some way. The important thing is to just do something. If you do, someone will notice. If millions of people will install just one compact fluorescent bulb, the benefit to the planet will be tremendous. The best solution is for each us to lead by example.

When the people lead, leaders follow

Hopefully we’ll all do more than to just replace light bulbs. When appliances wear out, replace them with energy-efficient ones. Don’t wait for legislation, and don’t wait for your neighbor to lead the way. Start a movement to save the planet and to preserve natural resources for future generations. Global warming, air and water pollution, and peak oil are all problems that won’t go away without action. Don’t wait for solutions, create them. It’s a good feeling to do the right thing.

Once you’ve made a commitment to do your part, you might begin with an energy audit. You can call a professional, but that’s usually not necessary. You probably already know what needs to be done in your own home. If your list is long, attend to the easiest and less costly improvements first, or as Ed Begley Jr. puts it, the “low hanging fruit.”

John

Power Smart Pricing from Ameren

My electric utility company now has a real-time pricing program and I’ve enrolled. Instead of a paying a flat rate for electricity, my rate will vary by the hour depending upon demand at the time. I'll minimize my electricity use during peak demand times. Tasks like washing and drying clothes can be done late at night when rates are low.

My current electric bill is hard to understand, but from a recent one this is what I’ve been able to determine:

I am charged 0.072 per kwh of electricity of electricity I use.

With taxes, customer charge, distribution charge, and other miscellaneous charges and credits, I am actually paying 0.1016 per kwh.

Based on program information from the utility’s website, I’ll be paying as little as 0.015 per kwh when demand is low, or as much as 0.15 per kwh when demand is high. In addition to the rate information available to me over the Internet, I’ll be alerted when rates are expected to exceed 0.13 per kwh.

A new electric meter was installed on December 3rd, allowing my electricity use to be monitored by time-of-day.

To get the most from this program I’ll run as many daytime loads as possible off of my solar photovoltaic (PV) system. To the extent that my small system can keep up, I’ll run my refrigerator and chest freezer off of it. At night, when rates are low, I’ll run them off of grid-supplied power. Fortunately, the PV system operates most efficiently during the day because energy goes straight to the load, instead of being stored in and retrieved from batteries. As I add solar panels to my existing array, I’ll add electrical items to the daytime load when utility rates are highest.

Perhaps the easiest way to switch between the grid and solar is by way of a simple timer and relay as illustrated below:

The circuit above will work, but for safety and NEC code compliance a transfer switch should be used. The inverter can be switched on and off via a timer as shown below:

The transfer switch can be wired so that it selects the inverter when AC is present, and switches to grid-supplied power when necessary. The timer can be programmed to switch the inverter on during the day, and off at night.

With nightly electricity rates below 0.035 per kwh, I’ll have to reevaluate my Plug-In-Electric-Vehicle (PHEV) recharging strategy. It no longer seems practical to purchase extra solar panels for this. I’ll buy extra panels for the daytime loads instead.

I might also benefit from this plan by using the grid to charge batteries. I'll charge batteries at night when rates are low, and use the stored energy to power household loads during the day when rates are high. I suspect that the losses associated with storing and retrieving energy will be more than offset by the low nighttime electric rates.

As I gain experience, I suspect that I’ll discover other ways to get the most from this plan. I welcome suggestions and comments from others.

For more information about the plan I’ve signed up for, click here: http://www.powersmartpricing.org/

John