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Off-Grid Solar Systems Electrical Energy Independent of the Mains Grid
By Dr. Thomas Scherer (Germany)
What is an off-grid solar system? Where are such installations necessary or practical? What are the most important design considerations? These questions and more will be answered in this article. In the September/October 2021 edition of Elektor, we took a look at a photovoltaic system connected to the mains grid [1]. Here we will consider essentially autonomous solar installations that are isolated from the public grid. These can be used to generate electrical energy where otherwise a grid connection would be too costly, such as in a shed on an allotment, or impossible, such as on a motorboat or sailing boat. In general these are
low-power systems which can handle a peak demand in the range of a few watts to a couple of kilowatts. Also, as the levels of feed-in tariffs continue to fall, new and simplified designs for fixed domestic solar installations that store the generated energy locally in rechargeable batteries solely for private use, rather than feeding into the public supply grid, begin to make more sense. These installations typically have a maximum nominal power output of a few kWp (‘kilowatts peak’). Let us now look at these small-scale systems in more detail.
looks very simple, but as ever the devil is in the detail. In the following sections we will therefore take a look at these individual components.
Principle of Operation
Energy and Power
An off-grid solar installation requires at least three components: the solar panel itself; some energy storage in the form of a rechargeable battery; and finally a charge controller that ensures that the battery is not overcharged. For smaller systems, typically operating at 12 V, that is in theory all that is needed. If, however, a 230 V AC output is required at 50 Hz or 60 Hz, a fourth component comes into play: an inverter. Figure 1 shows a typical four-component solution: superficially it
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Here’s a real-life example: Klaus, a good friend of mine, decided to install a 12 V system in his shed because of the low prices and manageable size of the components involved. To design the system and specify the components there are two questions that first need to be answered.
The first question to answer is how much total energy needs to be stored by the system. This directly affects the required capacity of the rechargeable battery and hence it is necessary to estimate the average load on the installation. Bearing on this calculation is the number of cloudy days that the system must be able to ‘survive’. Klaus would like to use an electric drill in his shed, and brew the occasional cup of tea, but these relatively rare loads do not significantly affect the
