Designing a Solar or Wind Powered System

The amount of energy that you can harvest with a solar or wind powered system depends on the area where you are and the time of the year. Usually you'll find information about the energy of the sun radiation or wind speed from administrative bodies competent for weather. They collect such information over the years and can tell you what to expect for each time of the year. Simulation and calculation programs for solar systems are available, PVSOL being one commercial (and expensive) program. A demo version is available in several languages.

Calculating exactly how much energy a solar powered system will produce at a certain site is a lot of work. Involved in the calculation are factors like temperature, number of sun hours, intensity of radiation, reflections in the environment, alignment of the solar panels and so on. A simulation program and weather data are a good place to start, but remember that in the real world, something as simple as dirt on the solar panels can completely spoil the results of your theoretical calculation.

Estimating the amount of energy produced by a wind generator is hard if there are obstacles around the wind generator. The empiric approach would be to measure the actual wind speed at the site over a year -which is rather impractical.

This should be a practical guide. If a fancy computer program and detailed weather data is not available for your country, I would suggest building a pilot system. If the battery does not get sufficiently charged, it is time to increase the number or size of the solar panels. As mentioned before, keeping the power consumption at a minimum is really important to avoid unexpected high costs.

If the system needs to have 100% uptime, considerations will obviously start with the worst time of the year. You have to decide whether the system will need an oversized storage capacity or an oversized power source to provide power through calm periods. It may be much cheaper if someone manually charges the system with a generator running on gasoline in a time of a long dead calm.

Combining wind and solar energy makes the most sense in areas with seasons that provide wind energy when solar energy is weak. For example, in Germany the sun provides only 10% of the energy in winter time compared to summer. In spring and autumn there is not much solar power either, but it is quite windy. Huge batteries are necessary since it is possible that neither solar panels or a wind generator will provide much energy during wintertime.

Under such conditions, a system designed for 100% uptime needs a decent safety margin and a lot of storage capacity. Charging should be done aggressively to achieve full charge as often as possible during periods of good weather. In the long run, solar panels may need replacement every 25 years -while a battery in a system that lacks sufficient charging power may need replacement every year!

An autonomous solar system consists of:

Figure 7.8: A solar powered or wind powered system.

Both systems are connected to the same battery if wind and solar power is combined.