Crypto mining on solar panels

As part of my little project to start mining cryptocurrencies “appropriately” I decided I should have a look at how one could possibly use solar panels to produce electricity in a mining setup.

First I had a look at a few youtube videos:

  • How to create a solar panel from scratches (And part 2 here): This looked interesting because it means we could really build a solar panel from simple solar cells (yet, it's not really worth it: it's less expensive to just buy a panel nowadays).
  • A fast way to solder the solar cells: this guy present a fast method to solder a complete row of solar cells (but he also mention that it's a better idea to just buy a pre-made solar panel nowadays as already noted above).
  • Grid Tie Inverter: this would be the easiest option to get the current on the grid
    • We should use golf car batteries in a solar setup instead of regular car batteries.
    • Connecting 2 batteries of 6V in series to get a 12V system (of 235Ah in the video)
    • Batteries should not be discharged more than 50% (or their lifetime will be severely reduced :-S)
    • Battery model used in this video: Duracell SLIGC125
    • Should place the solar panels as close as possible to the batteries: due to the resistance in the cable (to reduce the resistance we need to use thicker cables)
    • When placing the panels, should take into account that the sun elevation is not the same during all the year
    • ⇒ Consider not placing the solar panels fixed on the roof (can tilt, clean, maintain them easily in that case)
    • Solar panels can be wired in series and/or parallel: to consider to get an appropriate voltage / amp value.
    • Select the appropriate wire gage to support transporting the amp value
    • Use a voltage drop Calculator to figure out how much voltage is lost in the the wires.
    • Consider MPPT charge controller: accepts large input voltage.

Electricity cost

Now let's start a simple mind experiment: let's say we want to run a mining rig of about 1000W. If we do so, that means we use \(1000W \times 24h = 24kWh\) of energy per day.

The current price of the kWh in France is: 0.14660 € TTC (we have a 6 kVA power subscription). So technically we would have to pay each month for electricity: \[ 24 \times 0.14660 \times 30.5 = 107.31€\]

This diagram explains the various taxes payed to get from 0.0887€/kWh HT to 0.1466€/kWh TTC:

How would we go with a solar panel installation in that case ? We can't really go with an off grid setup: because the rig should be powered continuously, whether we have solar energy or not. So we can only choose between an on grid setup or an hybrid setup.

Typical on grid installation

Still assuming a 1000W target rig mining below

With an on-grid setup, we plug the solar panels directing into the grid tie inverter. In this case we need the following equipement:

  1. The solar panels,
  2. A structure to put the panels on,
  3. A Grid tie inverter
  4. Cables to connec the solar panels to the grid tie inverter

When there is sun, the solar panel will provide the electricity we need, and this will be injected directly into the home grid. Let's just assume an overall efficiency of about 75%, in this case we should havethe solar panel producing \(1000W / 0.75 = 1333W\). If we go for 260W panels, then we should get 5 or 6 panels: \(6 \times 260W = 1560W > 1333W\). Note that:

  1. It's not a big problem to inject a bit more power: we probably have other power usage sources running at the same time anyway,
  2. Yet, it's probably a better idea to target a production just under our actual usage: because we have no idea what will happen if we inject more current than what we use in home (the extra electricity will be sent on the global grid, but, will this make the electricity counter run backward ? Probably not, so this electricity will be produced for nothing).

If we go with 6 panels, the solar panel cost will be 6*195=~1170€. Then we have the price of the Grid tie inverter: ~332€.

Then we have to evaluate the price for the cables from the solar panels to the (indoor ?) inverter: let's say the salor panels will be 30 meters await from home. For maximum efficiency the inverter should be as close as possible to the solar panels, so we can:

  1. Place it indoor, and use cables to conduct somewhat low voltage DC current (less efficient option),
  2. Place it just next to the solar panels (in a small outdoor cabin ?) and then use cables to conduct 220V AC current (most efficient ?),
  3. Place it somewhere in between if the solar panels are very far away ? (not sure how this setup could be interesting)

The inverter input voltage range could be beween 45-90V (cf. 2000W Décharge de la batterie Mode de puissance). Each panel as a nominal output voltage of 24V so we could put 3 of them in series to produce 3*24=72V with a nominal current of 8.54A. We then put 2 such packs in parallel and we get 17.08A at 72V. The selected inverted doesn't provide indications on the support for this current intensity.

Using this computation tool we find that for an intensity of 18A, voltage of 72V, accepted lost of 2%, on 30m distance, we need a wire section of 25mm². So this makes about: 5*30=150€.

Note: using this document, it rather seems a section of 10mm² is good enough to cover 42m with 36A at 220V… but this is 220V AC not 72V DC! ⇒ This document should be used if we place the inverter next to the solar panels instead (ie. using micro-inverters ?)

Now in the area of nice, we can expect to get about 2750h of sun every year, this is 7.53h of sun per day on mean. During this time the solar panels will produce about 1560W of power and inject it on the home grid. So we can cover \(7.53/24 = 0.31375 = 31.375%\) of the electricity needs for the rig with a conservative perspective.

If, on the contrary we consider the real value of the energy injected on the grid, we can rather say that: we inject a total energy of \(1560W \times 7.53h \times 0.99 \approx 11.6kWh\) per day (that will be used either by the rig or by other equipements ?). And the rig only will use 24kWh per day, so we are effectively covering: 11.6/24 = 48% of the electricity cost, or in other words, saving about \(107.31 \times 48% \approx 51€\) per month.

The total cost of the installation (not taking the solar panels support into account for now) would be about 1600€, with:

  1. Solar panels: 1170€
  2. Inverter: 278€
  3. Wires: 150€

So if we save about 51€ per month, it means we need about 32 months to make the installation profitable: that's just a bit less than 3 years :-)!

Typical hydrid installation

Selling electricity

⇒ On the whole, this doesn't seem to be a very good idea: incomes are constantly reducing, need to do the installation by a professional, need to install the panels on the roof, no taxe reduction anymore.

Sun coverage

Sun map in France 2016:

Can use SolarGIS iMaps website to find local solar coverage indications. About 1550kWh/m² of Global Horizontal Irradiation (GHI) around Cannes/Biot/Grasses.

FIXME: Figure out how to integrate this information into the solar panels output power computation ?

Wire section

This document explains what wire cable section should be used.

Wire prices

Grid tie inverter prices

Solar panel prices

Quantity unit price TTCPrice per watt Total price Total power
1 194.99€ 0.75€ 195€ 260W
13 184.99€ 0.7115€ 2405€ 3380W
26 164.99€ 0.6346€ 4290€ 6760W
Quantity unit price TTCPrice per watt Total price Total power
1 229.99€ 0.8214€ 230€ 280W
17 219.99€ 0.7857€ 3740€ 3640W
26 199.99€ 0.7143€ 5200€ 7280W
Quantity unit price TTCPrice per watt Total price Total power
1 144.99€ 0.9666 145€ 150W

Other references

Open questions

  • Is there any alternative to using batteries in an off-grid setup ? (batteries are expensive, fragile, dangerous, etc)
  • How about setting up a wind turbine electricity production system ?
  • How would we build a support structure for the solar panels ?
  • How exactly would the Global Horizontal Irradiation value influence the energy production ?

Discussion

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