Off Grid Solar PV System

Basics

An off-grid system is not connected to the electricity grid and therefore requires battery storage. An off-grid solar system must be designed appropriately so that it will generate enough power throughout the year and have enough battery capacity to meet the home’s requirements, even in the depths of winter when there is less sunlight.

The higher of batteries and inverters means off-grid systems are much more expensive than on-grid systems and so are usually only needed in more remote areas that are far from any electricity grid. However battery costs are reducing rapidly, so there is now a growing market for off-grid solar battery systems even in cities and towns.

There are different types of off-grid systems which we will go into more detail later, but for now I will keep it simple. This description is for an AC coupled system, in a DC coupled system power is first sent to the battery bank, then sent to your appliances.

• The solar battery bank. In an off-grid system there is no public electricity grid. Once solar power is used by the appliances in your property, any excess power will be sent to your solar battery bank. Once the battery bank is full it will stop receiving power from the solar system. When your solar system is not working (night time or cloudy days), your appliances will draw power from the batteries.
• Backup Generator. For times of the year when the batteries are low on charge and the weather is very cloudy you will generally need a backup power source, such as a backup generator or gen-set. The size of the gen-set (measured in kVA) should to be adequate to supply your house and charge the batteries at the same time.

System Components

1. Charge Controller:

A high-quality PV charge regulator is the most important component within the PV off-grid systems. Controls the flow of current to and from the battery, to protect it from over charging after reaching the required voltage within the battery (eg protect against boiling the electrolyte). The over-discharge protection occurs, when the loads will cause critical/min voltage within the battery. Ex- For a PV module with 30V/8A, the needed charge regulator should withstand 36V and 10A.

The common voltages in off-grid systems are 12/24V and 48V, which means the voltage of system batteries.

There are two main types of charge controller: PWM and MPPT.

The difference result from the charging mode. The PWM charge controller uses the Pulse-Width-Modulation and an MPPT controller the Maximum Power Point Tracking and enables up to 30% more energy yields, than the PWM controller.

PWM charge controllers are less expensive (than MPPT) and are an ideal solution for smaller PV systems where the price can be a critical point, or where the maximum efficiency (as in MPPT 98%) and additional power is not really needed.

The MPPT charge controllers allow PV modules to operate at their higher optimum voltage in varying light conditions: summer, winter, morning, noon, cloudy etc. The MPPT charge controller takes the voltage output of the solar panels and compares it to the battery voltage. It figures out what is the best voltage to get maximum current (A) into the battery. Or said in another way, the MPPT controller tracks the best voltage level and then down converts it to the voltage of batteries.

MPPT (Maximum Power Point Tracking) charge controller calculates in real-time the optimal charging parameters, continuously by its internal algorithm.

The MPPT charge controller are most effective in following conditions:

• In cold weather, cloudy or hazy days (fall/winter/spring) and in low temperature conditions. PV module works better at cold temperatures and MPPT is utilized to extract the maximum power available from the module
• When battery is deeply discharged: MPPT can extract more current (A) and charge the battery, if the state of charge in the battery is lower

2.  Solar Panel:

Voc (open circuit voltage) and Isc (short circuit current) – of any PV module or string can not exceed, the allowed input voltage and current of charge controller or PV-inverter. The thereby resulting possible damages on charge controller or inverter are not covered by the warranty. Voltage (Voc and Vmp) can be managed by appropriate wiring of the modules, within string. When modules connected in series, the PV voltage (V) will be added up. If modules are connected parallel, the voltage (V) stay at the level of one module, but the current (A) adds up.

• Isc – is maximum amperage generated by a PV panel exposed to sunlight. The PV circuit’s wire size and overcurrent protection (fuses and circuit breakers) calculations are based on panel Isc (A) and should be rated as minimum 125% of the rated power of the charge controller. For an 30A controller, the max. allowed PV current (Isc) will be 24A (23A x 1.25 = 30A).
• Vmp – is the voltage, where a panel outputs the maximum power. Vmp should be higher than (but optimally close to) the maximum battery voltage.
• PV technologies – have different voltage characteristics. For example thin-film modules operate at much higher voltage, which can be an challenge for off-grid systems (concerns modules <100Wp).

One of the well known quality problems, especially in PV off-grid solar panels, are so called micro-cracks. The microcracks problems are typical for low-quality PV modules, which are often not subject to strict quality controls. An thermovision-test may cost more than the PV module, and the energy yield will be probably in the summer months capped by the charge-controller, if battery full charged. So eventual technological losses in PV off-grid modules are difficult to recognize, especially in the first years of usage. That is sometimes the reason why off-grid modules have 5-10 years warranty on rated power, compared  to on-grid modules with 25 years. All depends on quality and price, and if two visually identical PV modules are considered, an top-quality and an no-name PV module, the difference in quality can be not recognized, by anyone. It would be visible in a thermovision-test and looks like this

3. Solar Batteries

There are two basic types of batteries that are available for use in a standard solar energy production system:
Sealed batteries (AGM or Gel cell) and Flooded lead acid (FLA) batteries.

Sealed batteries require only little maintenance in order to keep them working properly, the FLA batteries have a longer lifespan.

LEAD-ACID SOLAR BATTERIES (FLOODED, AGM & GEL) :

A cycle in a solar battery occurs when it is discharged and then recharged back to its full level. How much a solar battery is discharged, is called the depth of discharge.

Deep cycle solar batteries are designed to be repeatedly discharged and recharged in cycles, where up to 80% of the battery capacity is used up. Deep cycle solar batteries are dedicated for solar PV systems and in systems above approx 200Ah capacity, the only right choice. A shallow-cycle-battery is when the 20% or less of the battery capacity is discharged and recharged. Shallow cycle batteries are designed to give up lots of power over a short period of time. They are used in cars and vehicles and are less suited for PV off-grid systems.

Similar like in PV modules, proper connecting wiring of batteries influence the system voltage. In series connected two batteries with 12V, will be an battery-bank with 24V voltage. In large scale projects usually are used 2V or 6V batteries, connected in series to build up an 24V or 48V system. Then (mostly) converting to an 230V-AC current. The less voltage differences 48V-DC/230V-AC, the higher the system efficiency, in an 12V/230V system, the losses will be the highest.

An important influence on the battery is its ambient temperature, ideally at about 10-20C. Their ventilation and storage. In extreme cases, low quality batteries can catch fire, that is why an appropriate ventilated and fireproof boxes are important.

A fuse between battery and charge controller (inverter) may be very useful, and is necessary in middle and large off-grid PV systems. The fuse parameters are related to the charging current and load current within the system and battery.

In large scale PV off-grid projects the batteries connected in series, will “learn each other” at the first years of usage and the whole system operates at the first months or even years under such learning process, controlled by an PV off-grid inverter (converting DC>AC) and charge controllers on the DC>DC site.

LITHIUM-ION BATTERIES:

Lithium-ion (Li-ion) based batteries have only been commercially available for 10-15 years and have quickly become very popular in consumer electronics due to their lightweight and high power density. These advantages led them to become the battery of choice for electric vehicles and are becoming very popular for home energy storage systems.

There are many different types of Li-ion batteries available with the most common being lithium-ion phosphate (LiFePO4 or LFP), Lithium Nickel Manganese Cobalt Oxide (NMC) and Lithium polymer. The Tesla Powerwall and Tesla Electric vehicles use the NMC variety in a cylinder cell design while the popular LG chem RESU home batteries use flat rectangular Polymer cells.

There are also many other different lithium chemistries available such cobolt oxide (LiCoO2), Lithium-manganese oxide (LiMn2O4). LFP and NMC batteries are generally considered the most cost effective and reliable lithium battery and at present are the most common type used in off-grid and hybrid solar power systems.

ADVANTAGES OF LI-ION BATTERIES

Li-ion batteries have a number of advantages over lead-acid batteries, as mentioned not only are they much smaller and lighter for the same capacity but they are also able to be deeply discharged 80-90% of total capacity without compromising the life of the battery. Furthermore, they can be recharged faster and are less likely to degrade during deep discharging so have a potential to last much longer. The fast charging capability is a huge advantage for use in electric vehicles as the slow charging time of lead-acid has been a major drawback to consumer uptake.

For hybrid and off-grid solar power systems the high power density of Li-ion means batteries require much less space in a home and the battery system can be pre wired which reduces installation cost. Furthermore Li-ion battery systems do not expel any volatile gases during charging & discharging unlike some lead-acid batteries and (depending on local regulations) are safe to be installed inside a building without the need for complex venting systems.