#142 Solar Power for the ESP8266, Arduino, etc.

Grüezi YouTubers Here is the guy with the Swiss accent

With a new episode around sensors and microcontrollers We all have our gadgets, and all of them need some electricity Usually, we use batteries or a power supply Today I will start a small project to use solar energy to power our devices during the whole year If we want to do so, we have to answer the following questions: What size the solar panel has to be in order to power our device What size of battery we need to survive times without sun? These questions lead to the next row of questions: How much energy can we harvest in one year? How much energy does our device use during this year? How is the energy distributed throughout the day and through the year? How long do we want to survive with reduced or without sun? Many questions

So, let’s start! But wait: A I have to warn you! This will not be easy stuff But saving the planet with green energy will never be easy! We will use a simple design: A solar panel, a “charging unit”, a battery, and our ESP8266 How much energy can we harvest in one year? This depends mainly on three factors: The location where your solar panel is placed (and its direction towards the sun) The size of the panel The efficiency of your circuit to transfer solar energy to your ESP The “location question” can be answered by looking at this map If you want to know it more precise, you can go to the “solargiscom” page: I live near Basel and we get about 1200 kWh/m2of solar energy per year

What does this mean in relation to run a ESP8266 without deep sleep, which uses about 100 mA on 33 volt, which equals 033W? The year has 8760 hours If we divide the yearly energy by these hours, we get the Watts: 1200 kWh/8760 h equals 137Watt/ m2 This is the total radiation from the sun

Unfortunately, solar cells only have an efficiency of about 15% So, we get about 20 W/ m2 out of the solar cell This is further reduced by the efficiency of our charging device and the loss of the battery charging process Let’s assume, we lose another 33% Then, we get a usable energy of only 14W/ m2 or 1

4 mW/ cm2, because 1m2=10’000cm2 With these two numbers, we can calculate the size of the needed panel: 033/14 = 236cm2, which is around 15 x 15 cm So, this panel should be sufficient to power my ESP8266 the whole year round Great! But, let’s quickly calculate the other way around: The supplier of this panel writes, that it delivers 4

5 watt And our ESP only needs 033 watt This is a big difference So, do you know, where I made the error in my calculation? You do not find an error? You are right: There is no error (at least, that is, what I hope), just an additional problem: The sun does not shine all the time

It fluctuates during each day, and also over the month And the specifications of the panel only show us the peak power, and not in Switzerland, but somewhere else with lots of sun! And maybe it is even a little bit exaggerated, as usual with the specs on Aliexpress… We have to continue our calculations But because this is boring, and the sun shines outside, we first do some tests: I bought a couple of small solar panels and one bigger one and want to do some tests now The test setup is simple: I place the solar panel into the sun and connect it to my new electronic load An electronic load is a simple device: It behaves like a variable resistor plus a voltage and an Ampere meter

The only difference is, that an electronic load automatically adjusts the resistor to either a constant current, a constant voltage, or a constant power And it automatically calculates and displays the power, which is handy for these experiments Filming today is not easy, but I hope, you can see the numbers I start without any load and measure the “open voltage” of the solar cell of 65volts

If I start to draw current, we see, that the power increases while the voltage drops a little Suddenly, the voltage drops and we lose most of the power If I try again with smaller steps, we see, that we can draw a maximum current of about 550 mA and get 28 Watt output As soon as I draw more current, the voltage drops dramatically

Why is that? This is a characteristic of Solar Panels Here is the result of my measurements of the 16x16cm panel, and here is the theoretical curve They match pretty good in shape And here, you see the expression MPP, or maximum power point In order to get maximum power out of the panel, we always have to operate at this point

Unfortunately, this point moves if the lighting conditions of the panel change and we have to find it again There are special devices available which do exactly that They are called MPPT or maximum power point trackers I will look at this topic in a future video You can buy monocrystalline or polycrystalline cells or panels

Monocrystalline silicon is used for most our electronic chips, and panels made from this material, theoretically have a higher efficiency, which means, they should produce more electricity with a defined light intensity They should also be more expensive than Polycrystalline modules In reality, the differences are small and we should not bother too much about that You can easily see the difference between mono or poly modules, as they are usually called: The mono modules are darker, nearly black and the polys are grey You find the results of a sunny Sunday afternoon work in this chart

Be aware that the results are not completely dependable since smallest clouds can have an influence and I had to do my measurements in series , not in parallel And in the middle, I had to make a stop to drink a beer, because the weather was really hot… We see, that I got a power per cm2 between 5 and 10 mW We also have to consider, that not the whole area of the panel is used to convert light There are also areas for connecting the different cells, because one cell only produces around 05 volts

So far, we know how much energy we can harvest over the whole year, and also during a sunny day Let’s now continue to find out the real size of the needed panel, and the size of the battery big enough to power our ESP safely during the whole year Here in Basel, we get 26 times less solar irradiance in December than in July And the sun disappears every night for a few hours

And, especially in winter, we experience bad weather and sometimes, we do not see the sun for days This creates three additional problems for our project: Make sure, our device survives the long winter nights Make sure, our device survives a period of bad weather without sun and make sure, our device survives the whole month of December Of course, these problems differ in different location This is, why I show the formulas and the sources of my data With this, you should be able to make your own calculations The first problem can be solved with a battery, which is charged during the day and discharged during night

Let’s quickly calculate the size of this battery for December Day length is about 85h and night therefore 155h So, our battery has to be: 15

5h x 01A= 16 Ah This is less than the capacity of a 18650 cell Now the second problem: If we assume bad weather without sun for 2 weeks, we need a bigger battery: 14 daysx24hx0

1A=34 Ah Here, we need about 14 18650 cells in parallel If this is the worst case, we know now the size of the battery The next thing is the calculation of the size of the solar panel We can assume, that the bad weather conditions are included in our average values for a particular location

So, we can design our solar panel for the worst month of the year We take our 162 kWh/m2 per day average solar energy for December, divide it by 24h, and multiply it by the 10% to get the electrical energy It is 67W/m2 Because we need 0

33W, we need a solar panel of 448 cm2 to harvest enough energy to drive our device throughout December These numbers are for an average year But these days, we never have average years So, maybe we have to add a little to account for that and we end up with a panel size of 25x25cm which equals 625 cm2 Pretty big! So, to summarize, we can make the same calculation for Dubai, were it is quite hot in summer

First, we search the radiation per m2 for the worst month of the year It is 368 kWh/m2/day Then, we divide this value by 242, and divide the power needs of our device by this number, and we get the size of our panel: 197 cm^2 The battery size can be smaller, because we do not need to anticipate 14 days of consecutive bad weather

Let’s assume 5 days The day is105 hours long and therefore, the night, 135h So, the size of the battery is only 12 Ah, which is about 5 18650 cells

Some of you might remember my videos about sleep modes If we are able to reduce the power consumption of our device by a factor of 10, our battery size is reduced to one 18650 for Switzerland, and our solar panel to the size of 10×10 cm And if we would be able to reduce power consumption even more, for example by using LoRa instead of WiFi, the battery and the panel size would be reduced even more Great! Today, we calculated the size of a solar panel and the battery for a year-long usage In one of the next videos we have to concentrate on the charging device between the Solar panel and the ESP

This device has to fulfill quite some needs: Find and keep the MPP under all lighting conditions to get maximum power from the solar panel Make sure, we have a constant voltage of 33 volt for the ESP Switch charging of battery off if it reaches 42 volts This is particularly important because we had to design the solar panel for the worst month of the year In all other months, we will have far too much energy Protect the battery from too low voltage Signal low voltage to the ESP in order to make it possible react accordingly

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