Monday, June 11, 2007

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.



d said...

Cough cough... Watts is instantaneous power, but Watt-hours is energy eg as gathered by your panels in a day!



Solar John said...

Thanks! I'll make the correction.


Gary said...

Now that's the kinda post i was looking for .... thanks for the lesson and thought process !!