Solar photovoltaic systems are systems made up of a set of specialist components designed to take solar energy from the sun and to convert this into useful electrical energy. Solar photovoltaic is often abbreviated to solar pv and the terms ‘solar panels’ and ‘solar panel systems’ is often used interchangeably. However it must be noted that the term ‘solar panels’ is a generic term that is also used for solar thermal systems that generate hot water. Solar thermal and solar photovoltaic systems are very different technologies.
The electrical energy generated by solar photovoltaic system can normally be:
Used immediately by electrical appliances that have an electrical demand during the day
Exported to a local grid network to be used by neighbouring buidings with electrical energy demand. It is often possible to sell this exported electricity, or at least be rewarded for the generation in some way.
Stored in some form of battery system so that the electrical energy can be used later on that day, eg in the evening or at a later date
Solar photovoltaic systems will normally be able to do at least two of the above, in some rare cases that are becoming more popular they will be designed to do all three over a daily cycle, providing there is enough solar energy available.
Historically there were two main types of solar photovoltaic system although there is now a third category that is a combination of the two original types:
Off-grid systems – These can also be referred to as island systems or stand-alone systems . These solar photovoltaic systems are not connected to any local grid and normally have some form of battery in order to store excess solar energy generated during the day for any power requirements at night. Off-grid solar pv systems are often used in remote locations where the cost of grid connection is prohibitive or for mobile applications where a grid connection is not necessary (e.g. solar pocket calculator).
Grid connected systems – also known as on-grid, these solar pv systems are connected to the local grid allowing the installations to export electrical energy. To cause complete confusion, these systems can be split into two types, known in the UK as Standalone and Non-Standalone. Be very careful when you see or are discussing standalone solar systems – you need to be clear whether it is meant in the grid-connected or off-grid context.
Standalone – these systems are not connected to a building that would self-consume some of the energy – they are designed specifically to generate electricity that will be sold and exported to the national grid. Large ground mount solar arrays tend to be in this category.
Non-standalone – these systems are connected to a building that will self-consume some of the energy that is generated by the solar panels. Most homes and commercial properties will be in this category.
There is no need for any batteries in grid connected systems because you still have a grid connection, so if you are using more power than the solar panels are generating, or it is night time you can import energy from the grid in the same was as any other grid connected energy consumer can.
Installing off-grid solar pv systems required much more careful design as it is an isolated system. You do not want to oversize the system at all because if you do you will have additional capital outlay for solar panels and/or batteries that will not be utalised. Neither do you want to undersize the system at all because you do not want to run out of power after a few dark winter days, or if someone leaves an electrical devise running overnight. You therefore need to have a very clear understanding of the electrical loads and profiles in the system, obsiously if the property has a change of use the energy consumption could change dramatically. Off-grid systems are often backed up by a diesel generator in order to ensure the batteries can be changed up if need be. They are also sometimes used in hybrid systems with wind turbines.
If a property needs a reliable electricity supply and it already has a grid connection nearby it does not normally make sense to install an off-grid solar photovoltaic system due to the additional costs of the batteries and generator. It makes more sense to install a grid-connected solar photovoltaic system and to try and offset the energy generated over a 12 month period and retain the grid connection for when it is required. For example if a property has an annual energy consumption of 10,000 kWh and you are looking to cover this it makes sense do design a solar pv system that has an annual solar energy output of around 10,000 kWh. During the day and during the summer you will be net exporters and during the night and the winter months you will net importers but overall you will be covering your consumption.
Grid-connected solar battery systems
The technology now exists to have grid-connected solar photovoltaic systems which include battery storage. What happens with these systems is that any exces electrical energy generated during the day that is not used by the building is used to charge up a battery instead of being exported to the grid. Once the battery is full any further excess would then be exported. Then, at night when the solar panels are not generating the property will draw its power straight from the battery instead of from the grid. So if the system is designed well, at the right time of year you could have a situation where you do not need to buy any electricity from your electricity suppliers over a 24 hour period beacuse you are providing it all yourself from the solar panels and the batteries! A fantastic situation to be in. A word of warning with these systems though, because they are still grid connected they have to switch themselves off if there is a power-cut. So you could find yourself in a situation with a full battery and the sun shining and, if there is a power-cut, you still can't use any of the power available to you - a potentially frustrating situation. There are options for having systems that can be switched from grid-connect to off-grid so that power can be used in power cuts but this is probably only worth doing if you live somewhere where power-cuts happen frequently.