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There has been much talked about energy conservation. Earlier, much of this had to do with minimizing our consumption of fuel and electrical energy. While this is a positive step, we are still dependant on the tradditional sources of energy. This has prompted us to oder about the use of new energy sources in everyday life. Use of solar energy to power Raspberry Pi is a good example.
A term associated with many new energy sources is energy harvesting. Energy harvesting is the term used to describe the process of tapping energy from external sources which is then stored and used in powering small autonomous wireless electronic devices or sensor networks.
Solar energy harvesting is the storage and use of solar energy to power small electronic devices that have smaller energy demands than others. Solar energy harvesting is done using solar panels. These panels come in various shapes, sizes and capacity. They convert solar energy into electrical energy which is used to charge batterries that power the devices.
To power your Raspberry Pi using solar power you will need the following things besides the Raspberry Pi :-
Raspberry Pi is a very small portable computer. You can simply carry it in your pocket and connect to a screen and a keyboard whenever you feel like using it. The Pi can be used developing for embedded system applications or you could even add a touch screen to it. It is a computer – its processor, memory and several other components embedded in a single circuit board.
To know more about Raspberry Pi, visit the url given below that will take you to a tutorial.
All models of Raspberyy Pi operates using an input DC voltage of 5V. This does not mean they all consume the same amount of energy. Depending on the kind of applications they are designed to be used for their power consumption varies. Power consumed by a Raspberry Pi can be calculated using its current rating. Current ratings of a Raspberry Pi range between 300mA and 750mA. Let us consider the case of Raspberry Pi B Plus which has a current rating of 700mA. This device will thus consume 5X700X1/1000 = 3.5 Watts in an hour. We should keep this in mind while setting up the power supply.
While choosing a solar panel one must note the power rating and the kind of battery you have. If you want your application to run very long, then you must have a panel of good rating like a 45 Watt 12V panel. The solar panel will be used to charge a 12V battery which should have considerable power rating too. If your application will be run only for a period of time everyday you can make calculations of the estimated life of a battery and then decide.
Consider the case of a 12V battery that gives 1.3AH of power. Thus, it can supply 12 X 1.3 = 15.6 Watts of power. Our Raspberry Pi consumes 3.5 Watts in an hour which means the battery will last for 15.6 / 3.5 which is approximately four and a half hours. A much better battery of 80AH rating will last for longer time. Calculations indicated more than 270 hours of life but obviously there are several losses. When such huge magnitudes and a long period of time is considered losses do play a significant role. Still, it must last for more than 200 hours.
Let us assume you want your Pi to run very long. That means you choose the second battery and the solar panel mentoned before. This is the time you think about a major drawback of solar energy harvesting. The sun is not out forever. You get proper sunlight for only a few hours in a day. Thus, it is important to calculate how long a solar panel must charge the battery everyday. Our Raspberry Pi draws 700mAH power from a 5V source. That means it will draw about 300mAH from a 12V source. Thus, in a day it will draw more than 7AH of power. Considering the losses we assume it will draw about 8AH of power in a day. Now, in order to ensure that the Pi will keep on running the solar panel has to make up for the power drawn from the battery. A 12V 45 Watts solar panel will supply about 3.75AH power which means in order to make up for the power consumed by the Pi, the solar panel must charge the battery for at least two and a half hours everyday. A less powerful solar panel will have to charge the battery for much longer time.
The battery we are using gives a 12V output. Our Raspberry Pi needs a 5V power supply. So, we need to regulate the output of the battery. There are regulators available in the market but you could always build one yourself using an IC like 7805 and a few capacitors and diodes. Be careful when you do so and always check the output voltage and current before powering the Pi. It is also a good idea to do this regularly. This tutorial on Mepits will introduce you to voltage regulators.
https://www.mepits.com/tutorial/93/Electronic-Circuits/Voltage-regulators
We provide the necessary power to a Raspberry Pi normally using the USB power cable. So, we need to use the USB power cable such that it is connected to the output of the regulator. Now, the Pi will draw power from the battery which is recharged using solar energy.
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