07-09-2014, 11:37

Step 2 - Choose the Photovoltaic Cell

Reading and careful study of the subject mentioned in the previous section, we can easily conclude that for our project will not serve us any photovoltaic cell, but one whose short circuit current (which, for consistency with the original article, I shall designate by Isc,) is not very high, so that the resistance value Rsh resulting from the application of Ohm's law, does not result in a low value, which in practice becomes very difficult to obtain.

Something we should also take into account is that virtually all the power of the photovoltaic cell (called from now on CF) will be dissipated in Rsh. For this reason I recommend choosing a CF with a power not exceeding 150mW.

In the case of built for MeteoCercal sensor, the CF was chosen a model 1V - 100mW, which theoretically would result in a maximum of 100mA Isc (I = P / U).

Step 3 - Determination of Maximum Isc of our CF

To calculate the Rsh we will be using, it is very important to determine as precisely as possible what is the maximum Isc that our CF will provide the conditions in which we will use. This measurement is possibly the most critical aspect of building this equipment should ideally be done in the period of the year and the day we had the maximum theoretical value of Solar Radiation to our geographical position, but like almost everything in life, not always the ideial is attainable, so suffice in a good day of summer sunshine and clear skies. Later throughout the construction we have some margin to calibrate our equipment.

Thus, in a day of summer sun and clear sky, put our CF horizontally (never facing directly at the sun) and using a digital multimeter good quality (lowest possible internal resistance) we measure the Isc provided by our CF

In the case of CF I used, making measurements at the same time and over several days, the maximum Isc obtained was 75mA.

In the article mentioned in the first subject, possibly to facilitate explanation of the calculations, the author assumed as maximum value for the solar radiation value of 1000W / m 2. However, for latitudes such as Portugal, where the maximum theoretic value of solar radiation is between 1300 and 1400 W / m2, it is necessary that the readability of the equipment is not limited to such a low value, but Before can go to values of the order of 1500 to 1600 W / m 2. Just by way of example, in the Davis weather stations the maximum possible reading is 1800W / m2.

One day in the summer sun, clear sky at noon, carefully watching the WeatherWunderground values of solar radiation available by some meteorological stations located in the vicinity, all prowling 1300W / m2, and when I had my measuring Isc 75mA, assumed that this value of Isc would most likely match a value of Solar Radiation not far from the 1300W / m2. Later would have the opportunity to check if he was right or wrong (do not forget the study of matter, we are not to read current and voltage, and the CF, the next area of the short-cirtuito has a linear behavior, which is why the Rsh must have a small value).

So let the calculations!

Continued ... in the next topic.

Reading and careful study of the subject mentioned in the previous section, we can easily conclude that for our project will not serve us any photovoltaic cell, but one whose short circuit current (which, for consistency with the original article, I shall designate by Isc,) is not very high, so that the resistance value Rsh resulting from the application of Ohm's law, does not result in a low value, which in practice becomes very difficult to obtain.

Something we should also take into account is that virtually all the power of the photovoltaic cell (called from now on CF) will be dissipated in Rsh. For this reason I recommend choosing a CF with a power not exceeding 150mW.

In the case of built for MeteoCercal sensor, the CF was chosen a model 1V - 100mW, which theoretically would result in a maximum of 100mA Isc (I = P / U).

Step 3 - Determination of Maximum Isc of our CF

To calculate the Rsh we will be using, it is very important to determine as precisely as possible what is the maximum Isc that our CF will provide the conditions in which we will use. This measurement is possibly the most critical aspect of building this equipment should ideally be done in the period of the year and the day we had the maximum theoretical value of Solar Radiation to our geographical position, but like almost everything in life, not always the ideial is attainable, so suffice in a good day of summer sunshine and clear skies. Later throughout the construction we have some margin to calibrate our equipment.

Thus, in a day of summer sun and clear sky, put our CF horizontally (never facing directly at the sun) and using a digital multimeter good quality (lowest possible internal resistance) we measure the Isc provided by our CF

In the case of CF I used, making measurements at the same time and over several days, the maximum Isc obtained was 75mA.

In the article mentioned in the first subject, possibly to facilitate explanation of the calculations, the author assumed as maximum value for the solar radiation value of 1000W / m 2. However, for latitudes such as Portugal, where the maximum theoretic value of solar radiation is between 1300 and 1400 W / m2, it is necessary that the readability of the equipment is not limited to such a low value, but Before can go to values of the order of 1500 to 1600 W / m 2. Just by way of example, in the Davis weather stations the maximum possible reading is 1800W / m2.

One day in the summer sun, clear sky at noon, carefully watching the WeatherWunderground values of solar radiation available by some meteorological stations located in the vicinity, all prowling 1300W / m2, and when I had my measuring Isc 75mA, assumed that this value of Isc would most likely match a value of Solar Radiation not far from the 1300W / m2. Later would have the opportunity to check if he was right or wrong (do not forget the study of matter, we are not to read current and voltage, and the CF, the next area of the short-cirtuito has a linear behavior, which is why the Rsh must have a small value).

So let the calculations!

Continued ... in the next topic.