BoP vs. BoS - similarities and differences

Lately I have had the pleasure to spend a lot of time with my friend Alessandro.

Alessandro is an engineer specialized in the design and construction of photovoltaic plants – basically, a "solar energy" version of mine.

We spent some time discussing similarities and differences in the BoP (“Balance of Plant”) of wind farms and the BoS (“Balance of System”) of photovoltaic plants.

As you are reading this blog you will probably know that BoP and BoS basically mean “everything but the wind turbines (or the panels, in the case of BoS)”

Both can have quite an impact on the economics of the project. For wind farm is usually in the 20% to 30% range of the CAPEX while for solar plants it is typically much more – even above 50% of the investment total.

I have made a quick number with some projects currently under development in Southern Italy and I see that for medium size projects (10 to 20 MW) the cost of the modules is only 40%.

This could sound counterintuitive but is a consequence of the unstoppable reduction of the price of the solar modules. At the current rate the price is decreasing 75% every 10 years and this trend does not seem to change.

As a consequence, the BoS becomes every year more relevant (because it is not decreasing at the same rate, so its relative weight keep increasing).

Let's take a look at similarities and differences between BoP and BoS.

In both cases you will need internal roads and probably a substation to connect to the grid (unless the project is very small – for projects of few MW sometimes it is possible to connect directly to the grid in medium voltage).

Additionally, sometime the panels have a shallow foundation (“ballast”) that reminds somehow the shallow foundation of wind turbines on a much smaller scale.

Furthermore the engineering works to be done (geotechnical survey, topography, electrical and civil design, etc.) are very similar.

And this is more or less where the similarities end.

The differences are much more remarkable. For instance, a substantial amount of the BoS budget comes from the support structure of the panels, inverters and trackers.

Inverters are the elements that convert the electricity produced by the solar modules for DC to AC

Trackers are used to rotate the panels in order to have them always in the best position to maximize energy production. They are optional, but they are used frequently because they are generally a cost effective technology.

It is also very unlikely that you will see a solar plant on a steep terrain (with a strong inclination), while this situation is frequent in wind farms (many of them are placed on mountain ridges).

This happen because there is a limit to the height difference that can be absorbed changing the length of the elements that sustain the panels. Additionally, excessive height differences can make the work of the trackers more burdensome with an increased risk of failures.

For these reasons the usual maximum slope in a solar plant is usually around 5% or 6% - and therefore earthworks are limited and less expensive (at least compared to some projects that I have seen on top of mountains where a lot of blasting was needed).

For the foundations I mentioned before the shallow “ballasted” solution. This is basically a block of concrete holding the modules in place.

However the use of piles is usually more cost effective. Several alternatives are available depending on the geotechnical characteristics of the soil (helical piles where cohesion is low, driven piles when the soil is more dense, etc.) and in addition to the monetary advantage they are also usually faster to install and easier to decommission at the end of the life of the project.

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About Francesco Miceli

Hello! My name is Francesco and I'm a civil engineer specialized in EPC (that is, "turnkey") wind farms projects. I'm currently based in Hamburg, Germany and I'm developing several interesting project all around the world - southern Europe, LATAM and various other countries. If you want to contact me please don't leave a comment in the blog (I don't check them very often) - you can use the contact form. You can write me in English, Spanish and Italian. To find a (somewhat concise) description of my non-wind business activities you can visit my webpage - www.francescomiceli.com If you want to know more about my work, here you can download my CV - www.windfarmbop.com/CV_Francesco_Miceli.pdf Hope you like the blog! Francesco

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