Light-to-Electricity: What Happens Beyond the Solar Cell

The solar panel, made up of several solar cells, is the point of generation in a solar PV system. The electricity generated has to be conditioned and processed before use, and a number of other components are involved in this process.

Image source: https://www.solarreviews.com/blog/grid-tied-off-grid-and-hybrid-solar-systems

Solar panels and diodes

A solar panel is made up of several cells connected together in series. Commonly used configurations are 60-cell and 72-cell panels. In turn, panels can be connected in various series and parallel configurations. A number of panels connected in series form a 'string'. A number of such strings can be connected in parallel. Connecting panels in series creates a higher output voltage, while connecting them in parallel provides a higher current output.

A diode is a semiconductor device which allows flow of current in only one direction under normal circumstances. Diodes are used for two different functions in solar panels – bypass and blocking.

Partial shading is a major issue with solar panels. A solar cell stops generating and conducting electricity when sufficient sunlight does not fall on it. As a result, if even a single cell among the 60 or 72 in a panel gets shaded, it cuts out all the cells which are connected in series with it, thus drastically reducing the output of the panel. ‘Bypass diodes’ are used as a workaround in this situation. A bypass diode is connected across a group of cells. When any one cell among this group gets shaded or stops conducting, this entire group gets bypassed, and the rest of the panel still remains connected. The polarity of the diode ensures that there is no recirculation of current.

There are two cases in which it is possible for current to flow from the external circuit into the solar panel: 1) Another solar array connected in parallel has a higher voltage or 2) The battery is at a higher voltage than the panel and discharges its current into the panel. The blocking diode is used to avoid flow of current from the external circuit into the solar panels.

Image source: https://www.electronics-tutorials.ws/diode/bypass-diodes.html

Combiner box:

This is simply a junction box where all the parallel strings of solar panels are connected. It is important that each string of panels has an equal number of panels connected in series so that the output voltage of all strings is equal. The cells and blocking diode are internal to the solar panel, and a contact is provided to connect a cable. Cables from multiple parallel strings are connected to the input of the combiner box, and one cable is connected from the outlet of combiner box to the inverter/charge controller as applicable.

Solar inverter:

As solar cells produce DC output, it has to be converted into AC first before it can be used for running AC powered household appliances or connecting to the grid. This function is performed by the solar inverter. The inverter has a specified power handling capacity and input voltage, and the solar panel layout and the inverter chosen must be suitable for use with each other.

The solar inverter in a grid-connected system is different from the battery-fed inverter used in homes. In case of a grid-connected system, if the power generated from the solar cells is less than total requirement, all the power generated from the solar cells is directed towards use and the rest is drawn from the grid. If the power generated by the solar cells is more than required, the required amount is directed towards use and the rest is exported to the grid. If there is a power failure, the inverter stops producing any output, effectively disconnecting the solar cells from the grid. This is done as a precautionary measure – for example, if the power cut occurred due to maintenance requirement and there was someone working on the power line and in contact with it, the inverter output could energize the power line and pose a threat to that person. Once grid power is restored, it takes a few minutes for the inverter to synchronize and connect with the grid.

It is to be noted that a grid connected solar PV system will not provide any power to appliances during a power cut.

Charge controller:

This is used in systems which are used for charging batteries using the solar panel.

Battery charging and discharging also has some losses associated with it and there is a recommended range of rate of charging for any battery. Rate of charge and level of charge are both important, as different types of batteries are able to withstand different depths of charge with minimum deterioration. ‘Deep cycle’ batteries can be discharged to a greater extent before being put for recharging. The energy requirement, the acceptable range of charge level, and the suitable rate for charging and discharging are all considered for sizing the batteries for a particular system.

The charge controller maintains a proper rate of charge and monitors the level of charge in the battery. It also has to handle extraneous factors such as drop in output of the solar panels due to shading etc.

Maximum Power Point Tracking

The power output of solar panels depends on factors such as temperature and light intensity. Maximum power point tracking (MPPT) aims at extracting the maximum amount of power possible under given environmental conditions. It consists of a circuit which adjusts the load on the solar cells to optimize the power flow from the solar cell. The adjustment involves microprocessor-based control and there are different types of approaches and algorithms followed to achieve this.

MPPT systems are in-built into inverters and charge controllers.

Metering:

In a grid-connected system, if the solar panels are generating more power than what is required for the loads running at that time, the excess power is fed back into the grid. The electricity companies often provide incentives for such power which is exported into the grid. A proper metering arrangement is required for proper accounting of power drawn from the grid and power exported from the grid, as both of these together determine the billing. A special meter, called ‘net meter’/’bidirectional meter’/’two-way meter’ is required for this purpose. It maintains separate records of both the imported power and exported power.

A two-way meter is different from the conventional digital meter as it can distinguish the direction of power flow, which the conventional digital meter does not. In fact, the conventional meter considers the generated power as consumption as well, leading to excess electricity bill. There are numerous reports of such incidences, for example this incident from the UK.