November 2004, Issue 172

Wi-Fi Sunlogger

SOLAR PANELS

Designing a solar-powered project can be a challenge. The objective is to collect enough solar power to power the device. Approximately 1 kW of solar power irradiates each square meter of earth when the sun is directly overhead on a clear day. This is a lot of power. Harnessing it, however, turns out to be tricky.

The best photovoltaic solar panels can convert approximately 20% of solar power into electrical power. These are expensive panels used in applications where efficiency is more important than cost. Satellites and space probes are one example.

Affordable panels can convert at less than 10% efficiency. The flexible solar panels I used (made by Iowa Thin Film) come in a variety of sizes and configurations. I chose a 74 mm × 150 mm panel that has an operating current of 100 mA and an operating voltage of 3.6 V. I made this decision because I needed a panel that outputs 3.6 V (you’ll learn why later) and fits in the clear enclosure without wasting too much unused area. Photo 1b shows what this looks like.

The power density for this panel works out to be the following: 

which works out to be 3.2% when the sun is shining directly overhead. I expect approximately 360-mW peak power output from this panel when the sun is shining overhead. The output power of the panel is further derated by the conditions in Table 1.

Furthermore, the amount of sun available varies depending on the location and average cloud coverage. The solar power industry has collected data for various localities in the world called insolation tables. Refer to the Resources section at the end of this article for links to the tables. The values given are the equivalent (kilowatt-hours per day) for both peak (summer) and low (winter) periods, as well as the yearly average. These figures help you design power budgets for solar systems based on your location.

Here are some sample entries. Although sunny Davis, California receives an average of 5.1 kWh per day, it pales in comparison to places like Phoenix, Arizona and Death Valley, California (see Table 2). Anyone who has ever been to either of these places knows what it feels like (see Photo 2).

If you want to design a system that can be solar powered year round, choose the low value and derate it with the efficiency of the panel as well as the collection area. The following is my energy budget for Davis.