Solar Electricity Handbook 2011: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems - Michael Boxwell (2011)
Components of a Solar Electric System
Before I get into the detail about planning and designing solar electric systems, it is worth describing all the different components of a system and explaining how they fit together. Once you have read this chapter, you will have a reasonable grasp of how a solar energy system fits together.
I deliberately do not go into much detail at this stage: all I am doing is providing an overview for now. The detail can come later.
The heart of a solar electric system is the solar panel itself. There are various types of solar panel and I will describe them all in detail later on.
Solar panels or, more accurately, photovoltaic solar panels, generate electricity from the sun. The more powerful the sun’s energy, the more power you get, although solar panels continue to generate small amounts of electricity in the shade.
Most solar panels are made up of individual solar cells, connected together. A typical solar cell will only produce around half a volt, so by connecting them together in series inside the panel, a more useful voltage is achieved.
Most solar panels are rated as 12-volt solar panels, although higher-voltage panels are also available. A 12-volt solar panel produces around 14–18 volts when put under load. This allows a single solar panel to charge up a 12-volt battery.
Incidentally, if you connect a voltmeter up to a solar panel when it is not under load, you may well see voltage readings of up to 26 volts. This is normal in an ‘open circuit’ on a solar panel. As soon as you connect the solar panel into a circuit, this voltage level will drop to around 14–18 volts.
Solar panels can be linked together to create a solar array. Connecting multiple panels together allows you to produce a higher current or to run at a higher voltage:
· Connecting the panels in series allows a solar array to run at a higher voltage. Typically, 24 volts or 48 volts in a stand-alone system, or up to several hundred volts in a grid-tie system
· Connecting the panels in parallel allows a solar array to produce more power while maintaining the same voltage as the individual panels
· When you connect multiple panels together, the power of the overall system increases, irrespective of whether they are connected in series or in parallel
In a solar array where the solar panels are connected in series (as shown in the following diagrams), you add the voltages of each panel together and add the wattage of each panel together to calculate the maximum amount of power and voltage the solar array will generate.
A solar array made of four solar panels connected in series. If each individual panel is rated as a 12-volt, 12-watt panel, this solar array would be rated as a 48-volt, 48-watt array with a 1 amp current.
In a solar array where the panels are connected in parallel (as shown in the diagram below), you take the average voltage of all the solar panels and you add the wattage of each panel to calculate the maximum amount of power the solar array will generate.
A solar array made of four solar panels connected in parallel. With each panel rated as a 12-volt, 12-watt panel, this solar array would be rated as a 12-volt, 48-watt array with a 4 amp current.
I will go into more detail later about choosing the correct voltage for your system.
Except in a grid-tie system, where the solar array connects directly to an inverter, solar panels rarely power electrical equipment directly. This is because the amount of power the solar panel collects varies depending on the strength of sunlight. This makes the power source too variable for most electrical equipment to cope with.
In a grid-tie system, the inverter handles this variability: if demand outstrips supply, you will get power from both the grid and your solar system. For a stand-alone or a grid fallback system, batteries store the energy and provide a constant power source for your electrical equipment.
Typically, this energy is stored in ‘deep cycle’ lead acid batteries. These look similar to car batteries but have a different internal design. This design allows them to be heavily discharged and recharged several hundred times over.
Most lead acid batteries are 6-volt or 12-volt batteries and, like solar panels, these can be connected together to form a larger battery bank. Like solar panels, multiple batteries used in series increase the capacity and the voltage of a battery bank. Multiple batteries connected in parallel increase the capacity whilst keeping the voltage the same.
If you are using batteries, your solar electric system is going to require a controller in order to manage the flow of electricity (the current) into and out of the battery.
If your system overcharges the batteries, this will damage and eventually destroy them. Likewise, if your system completely discharges the batteries, this will quite rapidly destroy them. A solar controller prevents this from happening.
There are a few instances where a small solar electric system does not require a controller. An example of this is a small ‘battery top-up’ solar panel that is used to keep a car battery in peak condition when the car is not being used. These solar panels are too small to damage the battery when the battery is fully charged.
In the majority of instances, however, a solar electric system will require a controller in order to manage the charge and discharge of batteries and keep them in good condition.
The electricity generated by a solar electric system is direct current (DC). Electricity from the grid is high-voltage alternating current (AC).
If you are planning to run equipment that runs from grid-voltage electricity from your solar electric system, you will need an inverter to convert the current from DC to AC and convert the voltage to the same voltage as you get from the grid.
Traditionally, there is usually one central inverter in a solar system, either connecting directly to the solar array in a grid-tie system, or to the battery pack in an off-grid system. A more recent invention has been the micro inverter. Micro-inverters are connected to individual solar panels so that each individual panel provides a high-voltage alternating current.
Solar panels with micro-inverters are typically only used with grid-tie systems and are not suitable for systems with battery backup. For grid-tie systems, they do offer some significant benefits over the more traditional ‘big box’ inverter, although the up-front cost is currently higher.
Inverters are a big subject all on their own. I will come back to describe them in much more detail later on in the book.
The final element of your solar electric system is the devices you plan to power. Theoretically, anything that you can power with electricity can be powered by solar. However, many electrical devices are very power hungry, which makes running them on solar energy very expensive!
Of course, this may not be so much of an issue if you are installing a grid-tie system: if you have very energy-intensive appliances that you only use for short periods, the impact to your system is low. In comparison, running high-power appliances on an off-grid system means you have to have a more powerful off-grid solar energy system to cope with the peak demand.
Most off-grid solar systems run at low voltages. Unless you are planning a pure grid-tie installation, you may wish to consider running at least some of your devices directly from your DC supply rather than running everything through an inverter. This has the benefit of greater efficiency.
Thanks to the caravanning and boating communities, lots of equipment is available to run from a 12-volt or 24-volt supply: light bulbs, refrigerators, ovens, kettles, toasters, coffee machines, hairdryers, vacuum cleaners, televisions, radios, air conditioning units, washing machines and laptop computers are all available to run on 12-volt or 24-volt power.
In addition, thanks to the recent uptake in solar installations, some specialist manufacturers are building ultra low-energy appliances, such as refrigerators, freezers and washing machines, specifically for people installing solar and wind turbine systems.
You can also charge up most portable items such as MP3 players and mobile phones from a 12-volt supply.
If running everything at low voltage is not an option, or if you are using a grid-tie system, you use an inverter to run your electrical devices.
Connecting everything together
A stand-alone system
The simplified block diagram above shows a simple stand-alone solar electric system. Whilst the detail will vary, this design forms the basis of most stand-alone systems and is typical of the installations you will find in caravans, boats and buildings that do not have a conventional power supply.
This design provides both low-voltage DC power for running smaller electrical devices and appliances such as laptop computers and lighting, plus a higher-voltage AC supply for running larger devices such as larger televisions and kitchen appliances.
In this diagram, the arrows show the flow of current. The solar panels provide the energy, which is fed into the solar controller. The solar controller charges the batteries. The controller also supplies power to the low-voltage devices, using either the solar panels or the batteries as the source of this power.
The AC inverter takes its power directly from the battery and provides the high-voltage AC power supply.
A grid-tie system using a single central inverter
This simplified block diagram shows a simple grid-tie system, typical of the type installed in many homes today. The solar panels are connected to the grid-tie inverter, which feeds the energy into the main supply. Electricity can be used by devices in the building or fed back out onto the grid, depending on demand.
The grid-tie inverter monitors the power feed from the grid. If it detects a power cut, it also cuts power from the solar panels to ensure that no energy is fed back out onto the grid.
The grid-tie meter monitors how much energy is taken from the grid and how much is fed back into the grid using the solar energy system.
A grid-tie system using multiple micro-inverters
A grid-tie system using micro-inverters is similar to the one above, except that each solar panel is connected to its own inverter, and the inverters themselves are daisy-chained together, converting the low-voltage DC power from each solar panel into a high-voltage AC power supply.
· There are various components that make up a solar electric system
· Multiple solar panels can be joined together to create a more powerful solar array.
· In a stand-alone system, the electricity is stored in batteries to provide an energy store and provide a more constant power source. A controller manages the batteries, ensuring the batteries do not get overcharged by the solar array and are not over-discharged by the devices taking current from them
· An inverter takes the DC current from the solar energy system and converts it into a high-voltage AC current that is suitable for running devices that require grid power
· Generally, it is more efficient to use the electricity as a DC supply than an AC supply