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Photovoltaic solar cells:

Crystalline silicon photovoltaic (PV) solar cells function on the basic principle that, when they are exposed to photons (i.e. to direct or diffuse light), they produce direct electrical current (DC). The crystalline solar cell can be regarded as a large layered diode, where one layer is doped as n-layer (negative), and the remaining silicon is doped as p-layer (positive). A negative electrode is applied to the upper side as a grid structure, and the positive electrode is applied to the bottom. To obtain current flow, a photon must separate an electron from the silicon crystal lattice, leaving a positive “hole” behind (1). The freed electrons can then be used outside the cell as a current to power an external load.

Losses occur if a separated electron recombines with a positive “hole” (2), if a photon passes the whole cell without separating an electron (3), or if a photon is reflected off the surface or off the crystalline grid (4). The cells are assembled in a “sandwich”, to form a solar PV module.

Today’s market offers a wide variety of solar PV module types. Crystalline silicon PV cells comprise monocrystalline and polycrystalline cells –each subdivided into different types depending on the internal structure and manufacturing process. Especially for thin-film cells, there are many different materials in use, but the monocrystalline and polycrystalline cells are silicon-based. The global market share of crystalline silicon PV cells is above 90%. Thin-film material is still much less common – mainly because of lower efficiency and un-proven long-term stability, but with technical improvements, the market share may increase in future.

Polycrystalline silicon PV cells have a higher market share than monocrystalline cells. Whereas the efficiency of monocrystalline cells can be higher than for polycrystalline cells, they are generally more expensive, due to their more demanding manufacturing process. Monocrystalline cells are easily identifiable by their shape: sliced from a solid cylindrical silicon crystal, they appear to be square, with their corners “sliced off”. Polycrystalline cells can use almost the whole module area because of their perfect square shape (sliced from a rectangular silicon crystal of square cross section). Therefore, the efficiency of polycrystalline modules can in some cases be higher than that of monocrystalline modules, even with lower cell efficiency, because there are no missing areas at the cell corners. They can also be easily identified by their colour, with polycrystalline cells having a blue shimmer, and monocrystalline cells almost black.

Solar PV modules are graded with respect to their quality into three tiers, with tier 1 being manufactured by the top 2% of solar PV module manufacturers. To qualify as tier 1, the manufacturer must:

be “vertically integrated” – i.e. they must have control over their entire supply chain,
invest heavily in research and development,
use advances robotic processes in the manufacturing process,
have been manufacturing solar PV modules for more than 5 years,
have been assessed as having sufficient insurance in place to honour all warranties issued,
have quality control processes in place that are certified by an independent third partyinspection authority
must issue at least a 25 year performance warrantee for the solar PV modules, with maximumof 1% degradation per year from initial capacity.

Solar PV systems:

The basic purpose of any solar PV system is to convert energy from photons (light) into usable electrical current – either direct current (DC - less common) or alternating current (AC - most common). A veritable host of different system configurations can be used in practice, but they can all be divided into two types: off-grid systems, and grid-tied systems – with different sets of regulations applicable to each. The basic different system types can be represented by the following diagram:

The most common systems are DC-AC systems, and the most basic system configuration is shown below:

Most of the PV system control is usually done by the inverter, and most inverters can control the PV system in different operating modes, depending on the conditions (which are dependent on the weather and the time of day). The most common modes are:

Important note:

If the inverter is not capable of a backup mode (i.e. a pure grid-tie inverter), it is not allowed (according to NRS 097-2-1) to supply power to the load when the grid fails.

Many inverters also incorporate remote monitoring options (i.e. the inverter can be monitored via the internet (sometimes wirelessly), and many manufacturers have also developed applications for smart phones, so that the inverter can be monitored from wherever the owner or maintenance team is.

In cases where backup power is needed, it is often required to rewire circuits such that only essential circuits are supplied by the backup batteries, in order to avoid that the batteries are drained by energy intensive appliances.