The Practical and Efficient Use of a Small PV System

Since the primary role of my small photovoltaic (PV) system has been to serve as an alternate source of electricity during grid power failures, a great deal of power was unused, and therefore wasted. To minimize the waste, I decided to put the system to use on a daily basis. After all, I reasoned, shouldn’t I be using this energy to reduce my electric bill?

My first task was to determine how much energy I could expect from the system. Since I have 340 Watts of PV capacity (4 – 85 Watt Panels), and I can expect about 4 hours of peak sunlight each day, the overall production will be 340 times 4, or 1360 Watt/Hours per day. Because I’ll be storing some of this energy in batteries, and because I’ll be converting DC to AC, system losses must be considered. Clouds will also limit production. With all of these things in mind, I’ll estimate that my daily capacity averages 900 Watt/Hours.

Looking over a list of appliances in my home, I decided to use a chest freezer as the load. The chest freezer is an ideal load for several reasons: It requires about 800 Watt/Hours per day and the energy requirements change very little from day to day. Because it uses a little less energy than the anticipated daily output of the PV system, the freezer could theoretically be powered indefinitely. I decided to carefully monitor system parameters for a few weeks, and log the results with comments. Here are some log excerpts:

May 6, 2007
Power up inverter and connect chest freezer.

May 13, 2007
Observed that in addition to providing power to the chest freezer, the PV array is able to fully charge the battery bank.

May 31, 2007
The past six days have been mostly rainy and overcast. Switch off inverter due to low battery voltage.

I predicted that the PV system would be able to produce more energy than needed to run the freezer, but six days of mostly cloudy weather depleted the battery bank.

June 2, 2007
Sunny weather has returned and batteries are fully charged. Switch on inverter and connect chest freezer.

June 7th, 2007
Severe thunderstorms are predicted. Switch off inverter in order to preserve batteries in the event they are needed in the event of a grid power failure.

No grid power failure occurred, but it was good to know that a fully charged battery bank was available just in case.

June 8th, 2007
Stormy weather has passed. Switch on inverter and reconnect freezer.

June 10th, 2007
Due to mostly overcast conditions, battery voltage has again declined. Switch off inverter and freezer load.

Overall the system performed pretty much as I expected it to. If not for an extended period of cloudy weather, it could have continued to provide enough energy to power the chest freezer for a very long time. I suspect that the short and cloudy days of winter will create the same situation.

Overall efficiency may seem to be optimized in this example, since the load is about equal to the capacity of the PV system, but a significant increase in efficiency is possible. The freezer could be put on a timer, set to allow the freezer to run only during daylight hours. If the compressor runs during the day, when energy comes directly from the solar panels, the losses associated with storing power in batteries and using power from the batteries are eliminated.

Based on the results of this test, I believe I can improve PV system performance by increasing the size of the battery bank. It is important to note that the battery bank must not be enlarged to a point where the existing PV array cannot fully recharge it. Chronic undercharging will shorten the life of the battery bank. It is also important to remember that in addition to charging the batteries, the PV array needs to provide power to the load.

The system continues to serve as an emergency source of electricity, and most days provides power to the freezer therefore reducing my electric bill. Additional opportunities to enhance system efficiency will surface as I enlarge the system. At this point it's good to know that I've reduced the waste.

Here's a picture of the freezer:




Visitors usually ask about the sign, giving me an opportunity to show-off my PV system.

John

What to Expect from a Midsize PV System

As I continue to add to my PV system, I look forward to the day when I can stop describing it as small. My short-term goal is a system large enough to get me comfortably through an extended power-outage, and lower my electric bills all-year-long. Perhaps someday I can disconnect from the grid entirely, but that’s a long-term goal. Nevertheless, my system becomes more useful with each upgrade. With these things in mind, it’s interesting to take a closer look at the system I hope to have in the not-too-distant future.

I Have:

4 – 85 Watt Solar Panels
1 – Morningstar SunSaver 20 Charge Controller
4 – 105ah Deep Cycle Marine Batteries
1 – 1100 Watt Exeltech DC to AC Sine Wave Inverter

I Want:

8 – 85 Watt Solar Panels
1 – Morningstar TriStar TS-60 Charge Controller with meter
4 – S530 Rolls 6v 400ah Batteries
1 – Xantrex 2500 Watt Sine Wave Inverter

Additional items include mounting hardware, wire, a lightning protection device, fuses, and perhaps a breaker box or combiner. I hope to become grid-tied eventually, but I’ll avoid that discussion at the present time for the sake of simplicity. To examine the capabilities of the system I hope to have, let’s start with the solar panels.

Under ideal conditions, the eight panels will generate 680 watts. A somewhat more realistic estimate would be 600 watts for 4 hours per day in my geographical location, or a daily production of 2400 watt/hours. This, of course, will vary from day to day and season to season. Next, let’s look at the battery bank.

Because I've selected six-volt batteries, I'll connect the four of them in series to create a 24-volt system. As a result of using batteries better suited for solar power systems, and by changing from a 12-volt system to a 24-volt system, I'll expect a performance increase.

Since the solar panel array is able to produce 2400 watt/hours per day, it follows that a constant 100 watt load would fully use the solar-produced power. Stated another way; 2400 watt/hours divided by 24 hours equals 100 watts per hour. To allow for conversion losses, we’ll assume that only 85 watts is available on a continuous basis. That brings our daily total to 85 times 24 hours, or 2040 watt/hours per day. This power can go a long way if it’s used wisely. This is how I might use the available power during a grid-power failure:

3 – 13 watt cf bulbs – 39 watts – 4 hours per day = 156 watt/hours per day
2 – 19 watt cf bulbs – 38 watts – 2 hours per day = 76 watt/hours per day
1 – Chest-type freezer – 34 watts (avg) – 24 hours per day = 816 watt/hours per day
1 – Radio – 5 watts – 4 hours per day = 20 watt/hours per day
1 – Portable Color TV – 60 watts – 2 hours per day = 120 watt/hours per day
1 – VCR or DVD player – 40 watts – 2 hours per day = 80 watt/hours per day
1 – Fan – 25 watts – 4 hours per day = 100 watt/hours per day
1 – Microwave oven – 1000 watts – 0.20 hours per day = 200 watt/hours per day
1 – Toaster – 850 watts – 0.10 hours per day = 85 watt/hours per day
1 – Vacuum cleaner – 750 watts – 0.20 hours per day = 150 watt/hours per day
1 – Blow dryer – 1000 watts – 0.10 hours per day = 100 watt/hours per day
1 – Laptop Computer – 25 watts – 3 hours per day = 75 watt/hours per day
1 – Cell phone charger – 25 watts – 1 hour per day = 25 watt/hours per day

Alternatively, I can use power from this system to run the motors in my corn-burning stove. Should a grid power failure occur when outside temperatures are low, I’ll gladly opt for heat, instead of using other appliances on the list. I might also use more of the available power for fans to keep cool in the summer. During mild weather, when I don’t need extra heating or cooling, the surplus power can be applied to more frivolous uses. Remember, this modest system was not designed to meet the electrical needs of the average American home.

The Xantrex inverter upgrade offers several advantages:

1. Its higher capacity allows me to run heavier loads, and to run several devices at the same time. I should be able to charge my PHEV (plug in electric hybrid vehicle) with it. My Exeltech inverter has trouble powering devices like my washing machine, and would probably be inadequate for recharging the PHEV.

2. The Xantrex inverter can be set to use power from the PV system until battery voltage drops to a preset level, and then switch to utility power. This allows me to get the most from my PV system while protecting my batteries from over-discharging, and minimizes my grid-supplied electrical consumption.

3. The Xantrex inverter will be connected to my existing home wiring, and can automatically take over in the event of a grid power failure. Eliminating the need to run extension cords when the grid power fails is a bonus.

Because I'll be adding solar panels, and because I'll be changing from a 12-volt system to a 24-volt system, I'll need to replace my present charge controller. The Morningstar TriStar TS-60 meets all of my requirements, and it allows me to better control battery charging. The digital meter option not only measures array and battery voltage, it displays additonal system information not currently available. I'll also be able to remotely monitor system statistics via the Internet.

An off-grid home or cabin with a PV system of this size would probably have a gas-powered refrigerator and stove, and additional capacity will be needed to accommodate appliances such as a clothes washer and dryer. While this system may not seem to provide a lot of power for its cost (about $6500), imagine what life would be like without it.

John

Renewable Energy Opponents

If you’ve ever wondered why anyone would have issues with the pursuit of a technology that preserves natural resources, minimizes pollution, curtails global warming, cuts transportation costs, makes wars for oil unnecessary, and reduces the amount of cash getting into the hands of terrorists, you’re not alone. It’s surprising to observe that people are quick to find fault with renewable energy technology, and often display a great deal of emotion when expressing their views.

A closer look at their objections reveals some reasons for this attitude:

· A natural resistance to change is one piece of the puzzle. Risk is inherent with change, and a fear of failure is understandable. A notion exists that actions by radical environmentalists will cause a collapse of the economy.

· We tend to want our children to have the same opportunities that we had. I remember the thrill of getting behind the wheel of a “muscle car” for the first time, and feeling the acceleration as I ran through the gears. You just have to love the sound of a well-tuned V-8 engine, and the thought of replacing that with the high-pitched whine of an electric motor is a bit much to accept. Could it be that those who hold on tightest to overpowered cars and drive monster trucks for no apparent reason are just compensating?

· Some are concerned that a rapid switch to plug-in electric vehicles will cause an overload of the electrical grid, causing even more misery for utility customers.

· Not everyone can afford to spend $25,000 for a Prius. A used Chevy Nova gets reasonably good gas mileage, and costs a heck-of-a-lot less. No one wants to be forced into a situation that doesn’t make economic sense.

To some, the solution to our energy problems includes stepping up oil exploration and production, including drilling off of the Gulf Coast and in the Anwar region. They believe that this will buy us time that could be used for the development of alternative technologies. Unfortunately little progress will be made in alternative technology as long as demand is low. Meanwhile, fossil fuel reserves will continue to decline. No good can possibly come from increasing our dependence on oil, even if we produce more locally.

You’ll likely find that the hard-core renewable energy opponents are among the 10% of the population who don’t believe that human activity contributes to global warming, and that peak oil is decades away. I find this optimism an ironic contrast to their pessimistic view of renewable energy technology.

We may be able to drill and mine our way to prosperity in the short-term, but eventually we’re going to have to do things differently. The longer we put it off, the harder it will be to make the transition. Once we learn to accept change, we’ll find that many opportunities for environmental progress exist throughout the country, most of which will have no adverse affect on the general population. The picture below is of Carlyle (Illinois) Lake and Dam, a good example of a renewable energy opportunity. The installation and use of hydro-electric generators here would have a positive environmental impact, with little or no negative consequences. I can’t imagine why this has not been done already.

Lake and Dam at Carlyle Illinois

John