Building Surveying: Understanding Photovoltaics

In this blog I will be discussing solar photovoltaic panels and relating it to a recent project that I oversaw. I will give a summary of how they work, how they are installed and how they are maintained.

Science behind PV Technology

There is a good TED-Ed YouTube video which describes how a photovoltaic (PV) module converts sunlight directly to DC electricity, I will outline the process briefly. A silicon solar cell uses two layers of silicon called n-type and p-type layers. N-type has extra electrons and p-type has extra spaces or holes for electrons. Where these two types meet, electrons can wonder across the p/n junction leaving a positive charge on one side and leaving a negative charge on the other.

When certain wavelengths of light pass through the silicon, it can knock an electron from its bond, this separates the positively charged (holes) from the negatively charged (electrons). The holes move into the positive or p-layer and the electrons into the negative or n-layer.

Most of the mobile electrons can only re-combine by the electrons passing through an external circuit, due to the electric field at the p/n junction. This flow of electrons produces a DC current.

A typical PV cell will produce a voltage of around 0.5 V. To put this into context, you would need around 12 cells in order to charge a smartphone. To be able to generate a higher voltage, the cells are connected in series (cell stringing) to form a PV module. The modules are then connected together to create a photovoltaic array.

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System types

There are different types of PV technology available which have varying efficiencies. The system used in the case study project was a monocrystalline silicon cell which is one of the most efficient commercially available PV technologies with a module conversion efficiency of 15-18%. This percentage accounts for reflected sunlight (rather than absorbed) and dislodged electrons that fall back in place rather than going through the circuit. Cheaper polycrystalline cells have a typical efficiency of 13-16%.

This is how you can tell whether a salesman knows what they are taking about if you ask them the module efficiency. If they tell you their system is 97% efficient they are talking about the inverter efficiency rather than the module efficiency.

The series of PV modules or strings are fed into the inverter unit which will be located in the loft space or electric cupboard. In our case a Clearline PV unit was used with a Solis Mini Series inverter. The inverter has a DC isolator built into its enclosure which can cut the supply from the roof. The AC output of the inverter is then fed through the generation meter, AC isolator and into the consumer unit with dedicated PV MCB.

Some considerations before proceeding with the install of a PV system might include:

  • Ensuring the weight loading and design life have been considered.
  • To achieve best results making sure the roof-mounted PV arrays are facing between south-east and south-west and at an elevation of between 30-40°.
  • Ensuring that building is free from shading.
  • Wind pressures are higher when solar is roof integrated. If it is an exposed location an integrated option might not be suitable.

Installation

As our project involved a re-roof, an integrated PV system was used with brackets fixing it into place and flashings keeping it watertight. There are different fixing methods available if it was retrofitted onto an existing roof covering. Integrating the system into the roof creates a much more compact and sleek look compared to the retrofitted option. The 82mm Clearline solar panels fit over the tile battens sitting flush with the tile surface.

The PV system is acting to keep the structure watertight so it is important that the side, head and sill flashing have an adequate amount of cover.

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Maintenance

If a log of the monthly output is kept it can be checked against the same months of the following year to see if the system is generating on a consistent basis. Low output could be caused by the following:

  • Poor weather conditions
  • Soiling on the panels
  • Issues with the inverter
  • Shading on the panels

Regular visual inspections for wear and tear of the isolators, circuit breakers, meters and inverter should be conducted. Any faults or errors should be easily identifiable on the inverters LCD display. A yearly electrical continuity check is also recommended. The PV product is self-cleaning but may require additional cleaning in certain circumstances, the manufacturer’s recommendations on this should be followed.

 

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