Choosing this type of system is sometime connected to the need to obtain energy in remote areas where it's economically unfavorable to physically build a connection to the national electricity grid.
The energy produced, if the energy need of the user is exceeded, can be easily stored in appropriate accumulation systems
Now let's introduce some basic notions:
-Off Grid system:
Off Grid systems are totally disconnected from the electricity grid and the energy produced directly feeds the electric loads of the user, any excess of energy is stored through the use of batteries in order to be used in moments when electrical production deriving from photovoltaic panels decreases (for example during the night).
The most important components used for a basic photovoltaic system are the following:
- Photovoltaic modules: There are different types of photovoltaic modules: monocrystallines for example are suitable for energy production if the solar radiation is perpendicular to the panel, the polycrystallines instead are more adaptable and generally less expensive, in this case the silicon crystals are randomly oriented, the efficiency decreases but an acceptable production is obtained even if the radiation is not perpendicular to the module.
There is also another type of modules called amorphous which are used only in special cases and they have a lower efficency but a greater adaptability..
- Charge Controller: There are two types of charge controllers most commonly used in today's solar systems: the “pulse width modulation” (better known as PWM) and the “tracking of maximum power point” (better known as MPPT).
The PWM controller is cheaper than the MPPT, but the MPPT charger is able to fully optimize the voltage conversion and it can work with voltages that are both higher or lower than the voltage generated by the photovoltaic modules. It therefore allows to produce more energy than a PWM charger with the same photovoltaic modules. If the module's voltage is slightly higher than the voltage of the battery (typical case: a 36-cell 12V panel and 12V battery), the PWM controller is sufficient and it has a performance similar to the MPPT.
- Inverter: The inverter is an electronic device that transforms a direct current into an alternating current at the same voltage or at a different voltage.
In pure sine wave inverters, the voltage of 300 volts of the first stage is pulse modulated (PWM) at a very high frequency and then filtered.
The output waveform if analyzed with an oscilloscope, is perfectly sinusoidal and without interferences or spurious frequencies different from that of 50Hz.
The output has always a sinusoidal voltage whose quality is higher than that of the network. It's essential to use a pure sine wave inverter because with different types of wave (such as the square wave) some devices that are powered may be damaged.
There is also another big difference between pure sine wave inverters: those with "high frequency" and those with "low frequency". Those with a low frequency are better in isolated systems because the transformer allows it to withstand higher consumption peak and "difficult" loads.
The inverters require a certification if connected to the national grid, those certifications depends on the country where the system is installed.
- Batteries: There are different types of batteries, the most traditional are lead batteries which, in order to guarantee an optimal life it's preferable to not discharge them more than 50% of their nominal power: For example if you need 4,5 kWh you should use a 9 kWh battery pack since you have to consider that the system in the "charge-discharge" cycle holds about 50% of energy.
Lithium batteries are technologically better. For example, if we want to use 4.5kWh from the battery we only need a 5.7 kWh battery pack because the discharge capacity of lithium batteries is about 80%.
This type of battery isn't really cheap but provides longer life and greater efficiency than the lead one, they have in fact 10/12 years of operation against the 5 years of Lead-Gel batteries and against 2/3 years of simple lead batteries. There are also other types of batteries, with different characteristics, longer life, and different technologies like Nickel-Nickel, Nickel-Cadmium or chemical accumulations. Our company can help you understand the best choice for your system requirements.
- Connection devices and counters :They are only used in case of connection with the national grid and they need special certificates if used (in Italy for example certificates are regulated by the CEI 021 and the CEI 016).
Which are the basic requirements to size a photovoltaic system?
- Energy demand: the amount of energy required by the user.
- Site characteristics: whether or not it is possible to install the system in terms of space.
- Available surface: amount of usable surface.
- Exposure: Evaluation of the correct exposure to the sun.
With those informations we can now make a small example of an Off Grid system (or more commonly called a "Stand Alone" system):
Considering for example a family of 3-4 people that has an average daily electric consumption of 10-12 kWh (a total of 3800 kWh per year) we can calculate that a 3 kW peak plant coupled to a correct accumulation system may be able to turn the house into a energetically autonomous habitation.
However, the batteries must be sized according to the real needs of the family in question, so it's necessary to consider a certain probability of non-daily production (due, for example, to bad weather) and night-time consumption habits (when the plant is not able to produce). If we need for example to accumulate 9-10 kWh and the operating voltage of the batteries is 48V we could consider 200 Ah batteries, considering that the percentage of depth of useful discharge changes according to the type of battery, for example in the lithium battery we can pick up a power equal to 80% of the stored power, while considering the lead batteries, to ensure an optimal life, the discharge value should be on average around 50%.
The cost of this system would be around 10,000 € including VAT, and the annual energy savings, considering that the system could make the house almost totally independent, can be calculated quickly considering that the energy cost is around 0,22 € / kWh multiplied by 3,800 kWh a year gives a saving of € 836 per year.
Considering also a tax deduction of 5000 € over 10 years and an increase in energy costs of 6% per year, the payback time of the investment for this installation would be less than 7 years.
If the network is completely absent, for example in mountain shelters, huts or situations where the network is present but with many problems (poor stability, poor availability of power, etc.) the cost of an Off Grid photovoltaic system that would solve the problem is absolutely far and below the cost (very huge or even not quantifiable) to build a connection from the national grid to the spot.