CURRENT ISSUE
Contests
LESSONS FROM THE TRENCHES
|
|
Issue #216 July 2008
Intelligent Energy Solutions
Living And Working Off The Grid
Part 1: Planning And Design
by George Martin
Start | Power Usage | Estimating Solar Radiation | Equipment | Solar Panels | Inverters | Charge Controller | Battery | Miscellaneous Items | System Cost | Sources & PDF
SOLAR PANELS
Solar panels convert solar energy into electrical energy. Again, there is a lot of literature available about how this is accomplished. But what about some hard code conversion details?
Standard test conditions require a temperature of 25°C and an irradiance of 1,000 W/m² with an air mass of 1.5 (AM1.5) spectrum. They correspond to the irradiance and spectrum of sunlight incident on a clear day on a sun-facing 37° tilted surface with the sun at an angle of 41.81° above the horizon. This condition approximately represents solar noon near the spring and autumn equinoxes in the continental U.S. with the surface of the cell aimed directly at the sun. Thus, under such conditions, a solar cell with a 12% efficiency and a 100 cm2 (0.01 m2) surface area can be expected to produce approximately 1.2 W of power.[1]
This gives you an idea of what’s involved in rating and selecting solar panels. Look at the University of Western New Mexico’s weather site for solar radiation and you’ll get a feeling for the actual solar radiation for the area.
There is another consideration when selecting a panel, namely cost per watt. If you start looking, you will find panels of different wattages and different prices. In March 2004, I started a spreadsheet listing panels from 125 to 195. Note pricing from March 2004, purchased equipment in 2005, installed in 2006–2007, and operational in October 2007. Then, I added the costs different suppliers were charging for each panel and calculated a price/watt number. My results range from $4.35 to $4.76 per watt. I estimated that I would need 3,000 W of panels, and came up with $13,320 for the cost of the BP Solar SX 170B.
More polysilicon is currently being used in solar panel manufacturing than all other usages combined, so this is big business. It also seems that the larger-power-rating panels command a higher price per watt. It is sort of like the CPU business where chips are speed graded and priced accordingly. My cost estimates are a bit old, so you’ll need to run the numbers with today’s prices. Let me add that I found solar panels to be in tight supply, so when you begin your design, look to secure the panels at a good price early in the game.
 |
| Figure 2—Here you see the actual solar radiation recorded by Western New Mexico University in Silver City about 30 miles to the west of the house site. |
The 3,000 W in my design was derived from the sun’s availability in the winter. Figure 2 represents the solar radiation for an actual cloudless winter day. The peak radiation is 600 W/m2. Let’s estimate that the shape of the curve is a sine function so that the area under the curve is its average value (2/p, or 0.6366 times the peak value) multiplied by its width. So, that is 600 W/m2 × 0.6366 × 8 h (9 A.M. to 5 P.M.), or 3,055.7 Wh/m2. Therefore:
Close to 10 kWh per day is good enough for the workshop, but not enough for the house when it’s built. And 3 kW of panels is what one neighbor is using.
We also need to account for cloudy days. The energy to run the workshop would need to come from the battery or backup generator. Another concern is hot summer days when the panel efficiency drops because of the heat. But the days are longer in the summer. Actually, it’s still a 24-hour day, but there is more available sunlight each day. I don’t have test results for summer generation (because I’m writing this in February 2008 after getting the system put together in October 2007), so stay tuned.
The last point to watch out for in panel selection is cold-weather open-circuit voltage going into the charge controller. In the cold, with no current drawn, the open-circuit voltage of the solar panel will rise. If several panels are connected in series (for efficiency), this voltage may damage the input to the charge controller. This is a well-known situation and your equipment dealer will be able to guide you in this area.
Previous | Next *| Subscription Deals |*
|
