Temperature Data Logger
1. INTRODUCTION
Data Loggers are devices used to log (record) the measured readings like temperature voltages and currents etc. continuously with respect to time. According to the wikipedia definition of Data Logger- it is “an electronic device that records data over time or in relation to location either with a built in instrument or sensor or via external instruments and sensors. They generally are small, battery powered, portable, and equipped with a microprocessor, internal memory for data storage, and sensors”.
A 5 channel Data Logger is designed to receive and log the Temperature readings from solar panel into the Personal Computer (PC). The device can log temperature for each second. This is done using Temperature Sensor DS1820 IC with PIC Controller for receiving and sending the data serially to PC via RS232 Protocol.
1.1Effect of Temperature on Solar Panels
All PV Solar Panels are affected by heat. The hotter the solar panel, the lower the power output - but to what extent will the power output be affected?
To answer this, an experiment was conducted using 6V 250mA Solar Panel in hot climates with a 150 Watt halogen office lamp. So the peak power output measured was 750mW for 56 Ohms. Around 40 minutes the temperature measured by the probe (and therefore the solar panel) rose to a peak of 75 degrees Celsius.
The graph as shown in fig 1.5 indicates the relationship between temperature in degrees Celsius (horizontal axis) and the solar panel power output in milli-watts (vertical axis). There was a consistent drop in power output of around 8.3mW (1.1% of peak output) per degree rise in temperature.
Fig 1.5: Temperature V/s Solar Panel Power Output
The total power loss due to the increase in temperature was from around 750 mW down to just 458 mW – from temperatures 32 to 75 degrees Celsius, a fall of almost 40%. If the solar panel has been pre-cooled before, the peak power output would certainly have been even higher.
The result of this experiment explains that, on an average this solar panel has an efficiency of 1% decrease in power output with per degree rise in temperature, also it is evident that Power Output of the Solar Panel is dependent on the Power Resistor used, and a Power Resistor is characterized by the temperature it dissipates. Hence the temperature data acquisition is necessary along with voltage and current readings.
Thermometer IC DS1820 is a programmable IC from Dallas. It can give temperature readings as digital output signal over extremely long distance. It requires just a single bus (1-Wire) connecting to the microcontroller on which several DS1820 ICs can be connected. Hence 5-channel Temperature sensing requires just a single pin connected to the PIC-Microcontroller.
Hence a Temperature sensor DS1820 is used in place of LM35 which gives direct digital temperature value in serial at its output with which ADC conversion.
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