MOTHER's Do-It-Yourself Solar Insolation Monitor

Measure the intensity and duration of solar energy in a given location with this DIY monitor.


| September/October 1983



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The insolation monitor's most important job will be helping you choose a suitable site at which to set up solar equipment.


PHOTO: MOTHER EARTH NEWS STAFF

Since our society has (finally!) begun to make the transition into the solar age, it's become easy to acquire general information about the average intensity and duration of sunshine in different parts of the United States. What's more, we can now get very precise data on the efficiencies of solar collectors that have been tested under controlled laboratory conditions.

However, as would-be sun-energy users, we need numbers indicating the performance of solar systems that must work in environments which don't necessarily offer either perfect conditions or the exact amounts of sunlight predicted by the National Weather Service's estimates. In short, we need to know how the conditions in our backyards stack up against the area's averages, and how well we can expect devices forced to work in those surroundings to perform.

To answer those questions accurately, we have to take measurements. But that's been a good deal easier said than done, up until now. You see, MOTHER's solar insolation monitor, which costs a couple of hundred dollars less than a sophisticated solar pyrometer, will provide the information needed—though it's admittedly somewhat less accurate than more expensive laboratory instruments—concerning the daily duration of sunshine at a particular location, and it'll give an inkling of the sun’s intensity, too.

How the Insolation Monitor Works

This solar insolation monitor is easy both to understand and to build. The device consists of a solar generator (made up of several cells) and an electric quartz clock movement. When sunlight impacts the surface of the photovoltaic generator, electricity is produced and fed directly to the clock. By setting the hands to "high noon" (12 o'clock) before the sun rises, and then reading the elapsed time indicated by the hands at the end of the day, you can get a true reading of the number of sun-hours during one diurnal cycle.

Furthermore, to account for differences in solar intensity, a small resistor is placed across the leads coming from the photovoltaic generator. The resistor puts a load on the photovoltaic cells, which in turn drags down the voltage somewhat. Unless the intensity of the light is above a certain level, there won't be enough power to run the clock, so those "weak sun" hours won't be recorded.

Because the insolation monitor is dependent upon photovoltaic cells, it's probably best suited to simulating a solar electric system's potential performance. The data you generate with the instrument can, however, be used to determine the practicality of solar collectors that will heat water or air, as well.





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