Strings and MPPTs – Our five steps for designing great rooftop PV systems

Ed Baughan, Commercial project manager for Joju, explains what’s involved in designing a high performance solar PV system for a roof. As you might have guessed, it’s not as simple as it might seem. Proper design is essential to maximise performance (and financial return) within the specifications made by the client.

1. The start
System design is a process of refinement. As an installer you will often start with no more than an aerial image of the site with which to make your first assessment. Here, the main objective is to identify which areas of roof space will be usable and which should be avoided based on factors like orientation, shade, and the quality and type of roof covering material. We’re generally looking to go as big as possible to begin with working under the assumption that the electrical layout within the site will be suited to the panel layout, and will have the capacity to carry the power generated. So far, so simple.

2. Survey
The real work starts at the site survey. Here we can review the assumptions made earlier and refine our initial design from something that might work to something that will work, subject to ‘the final hurdles’. We start our survey at the main incoming supply and work our way around the site to the various sub-distribution boards until we find the best place to connect the system(s) to; something that is conveniently located and has the necessary capacity to transfer the power we wish to connect. This can be a limiting factor on the array size. The survey can also give us the chance to go up on the roof (not good if you’re scared of heights) to review any assumptions on the roof covering material and accurately measure any obstacles that will shade the array.

Now back in the office we can model all the information we have gathered. Usually this means looking at several scenarios for the same building and weighing up our options to meet the client’s needs: Should we push our modules closer to the edges to increase capacity but forego permitted development (i.e. use a panel layout that requires a planning application)? Should we improve the pitch and orientation of the modules at the expense of capacity? Should we run our cables further to reach a board with a larger capacity? Which inverter should we use? How does this fit with the Feed-in Tariff?!!

3. Strings and MPPTs
Once we’ve made these decisions we will have our array position and size and can now look at how we wire the modules together. Modules are wired in series and connected in ‘strings’ which are in turn connected to maximum power point trackers (Mppts). How you arrange the modules can have a big impact on performance when some of the array is shaded, even if for just for a few hours a day. This wiring layout also has an impact on how easy the system will be to install, and how easy it will be to check for faults should anything come up. As rule of thumb, if possible, keep it simple and avoid shade altogether.

4. The Final Hurdles
Prior to the installation commencing, the site will need approval to connect the system to the grid, approval from a structural engineer that the roofs can take the added load, and planning approval. Ideally you will have some or all of these hurdles cleared before site survey but that is often not the case. These factors will ultimately inform the design; you cannot install on an unstable roof, or connect a large array to a part of the national grid that does not have the capacity to transport the energy.

5. The End Product
The importance of system design is often not well understood by the client, and I would urge people considering a PV system to look into the quality of the design work of potential installers. (Examples of Joju’s work can be found here). PV companies must use the Standard Assessment Procedure (SAP) to predict system generation. This is a valuable tool for PV companies but the limitations of this must be acknowledged. Two independent PV companies assessing the same roof for a single client will come back with the same SAP value for a given array size. However the real output from each of these systems could be significantly different depending on how the different components within the system have been laid out, particularly with respect to shading, and which components have been used.

If you have any questions about designing solar PV systems, or would like us to design a system to maximise the potential of your roof, get in touch.