Off-grid PV systems are very modular; they can be small or large, depending on the quantity of each component in the system. The main components are the PV modules themselves, batteries, charge controllers, and inverters. In addition to the electrical components, a mounting system for the PV modules and safe casing for the inverters, chargers, and batteries are also required.

The following sections will look at each component in more detail.

Main off-grid PV system components (Source: RENAC)

PV modules

Silicon-based PV modules are the most suitable for off-grid systems and fall into the following general categories:

Module type	Module efficiency (%)	Area (m2/100 Wp)
Monocrystalline	16 – 24	0.4 – 0.6
Polycrystalline	14 – 20	0.5 – 0.7
Amorphous silicon (multi-junction)	9 – 12	Up to ~1.1
Amorphous silicon (single-junction)	3 – 4	Up to ~2.5

Silicon-based PV modules for off-grid systems (Source: nrel.gov)

The vast majority of off-grid PV systems use crystalline modules, although amorphous silicon modules are also used. 36-cell crystalline modules (see image) are the most versatile for off-grid use. They have an open-circuit voltage (V_OC) of around 20 V. They can be connected in parallel for use in 12 V DC systems or in series-parallel for use in 24 V DC systems. 

60-cell modules can also be used in off-grid PV systems. They have a V_OC of around 36 V_OC and can be connected in parallel for use in 24 V DC systems or in series-parallel for use in 48 V DC systems.

36-cell modules can be used directly with standard 12 V DC charge controllers, and 60-cell modules can be used directly with standard 24 V DC charge controllers. However, always ensure that the PV module’s voltage range at different temperatures stays within the charge controller’s voltage limits.

Modules with different numbers of cells, e.g. 72-cell modules, usually require charge controllers with Maximum Power Point Tracking (MPPT). (These are covered in a later section.)  Never install different module types/sizes together in one series string.

It should be noted that non-silicon modules such as cadmium-based modules require special recycling facilities that are not available in most areas where off-grid systems are installed.

36-cell crystalline PV module (Source: Green Power International LTD)

Charge controllers.

The main function of a charge controller in an off-grid PV system is to protect the battery from being damaged. The charge controller is the intermediary between the PV modules and batteries (see image).  Most charge controllers are designed for use with lead-acid batteries, the most common type of battery in off-grid systems. The charge controller triggers a low-voltage disconnect (LVD) to prevent the battery from being over-discharged (discharging a battery more than it is designed to be discharged). When storing batteries for longer periods, a greater number of charge cycles is required. A high-voltage disconnect (HVD) protects the battery from overcharging. 

Many charge controllers enable adjustment of the HVD to the battery type (e.g. sealed or flooded batteries). Most charge controllers will not work (and some can be damaged) if there is no battery connected, because the internal electronics are powered by the battery.

Based on the internal electronics, there are several types of charge controllers: shunt-type, series-type, and pulse-width modulation (PWM) (these charge batteries more efficiently) controllers and charge controllers with integrated MPPTs (these are covered in a later section).

While it is useful to have an understanding of these different categories, at the end of the day a charge controller commercially available to you needs to be selected. In order to do this properly, one needs to be familiar with charge controller datasheets and be able to interpret them. For specialist applications such as telecom, suppliers should be consulted. Similarly, as solar pump controllers are different from regular solar charge controllers, supplier consultation is required.

Charge controller wiring (Source: Solar Magazine Uganda)

Charge controller selection.

Selecting a charge controller is an important design task. In most off-grid systems, it is the only electronic control component. The following aspects, among others, need to be considered:

• Is the charge controller suitable for the application? Usually this is not a problem, but check.
• What is the operating voltage? 12 V DC or 24 V DC, or even 48 V DC.
• What is the size? What is the maximum input current from the PV module(s) that the charge controller can handle? Most codes specify that if a PV array produces, for example, 20 A in full sun (approximately the short-circuit current, I_SC), then the charge controller should have an input current rating of 1.25 times that (i.e. 25 A). The controller will also have an output current rating (to the loads).
• What level of information is required by the user? Is a charge controller with simple coloured lights indicating status sufficient, or is a visual display with more detailed information required?

The image below shows a Steca and a Phocos charge controller. Both Steca and Phocos are leading manufacturers of high-quality charge controllers. Note the very clear battery state-of-charge (SoC) display on the Steca PR3030 (left). The Phocos CML20 (right) has a simpler display. The maximum input current (from the PV array) and output current (to the DC loads) is a nominal 30 A for the Steca and 20 A for the Phocos. These differences between charge controllers will be reflected in the price. 

Steca PR3030 (left) and Phocos CML20 (right) charge controllers (Source: Green Power International LTD)

Maximum power point trackers (MPPTs)

Maximum power point trackers (MPPTs) use DC to DC conversion electronics to ‘track’ the maximum power point (MPP) of a PV module or array current-voltage (I-V) curve. They can increase the energy yield by 10-35%, especially in cold climates and regions with higher shares of diffuse radiation on overcast days. Their applicability depends on economic evaluation, as a trade-off has to be made due to the higher cost of MPPT charge controllers compared to PWM models. Ultimately, it may be more beneficial to stick with a PWM controller and add an additional PV module instead.

The use of MPPTs in off-grid systems has two advantages:

• MPP tracking ensures that the module always operates at the maximum available power, thus charging batteries more efficiently;
• A wider range of PV modules can be used, because the module voltage does not need to match the battery voltage. When using PV modules with more than 36 cells, e.g. a module with 50 cells, the MPPT ensures that the energy produced by all 50 cells is harvested. Without an MPPT, the energy of the extra 14 cells (50-36) will largely remain unused.

MPPTs are used mostly in larger systems (i.e. above 200 W) and are often integrated into the charge controller (see image).

Steca Solarix charge controller with integrated MPPT (Source: Steca)

MPPTs work efficiently within a specified voltage range. If the PV array voltage is too low, they will not work (this could be a problem when modules are operating at high temperatures). If the PV voltage is too high (low module temperatures), an MPPT can be damaged.

The graph below shows a typical I-V curve for a PV module. The maximum power point (MPP) is marked. From the graph we can see that the voltage at the MPP is around 19 V. Without an MPPT, the off-grid system voltage is pulled to the battery voltage, typically 14-15 V. This means that without the MPPT the module produces less than the theoretical optimum. Whether this is an issue largely depends on the use-case.

Typical I-V curve of a PV module with MPP highlighted. (Source: RENAC)

DC-DC converters.

DC-DC converters are electronic devices that convert DC voltage to another level. For example, they can convert 24 V DC to 12 V DC (very useful, as there are far more 12 V DC lamps and appliances available than 24 V DC), or vice versa. Quality is important, since in off-grid systems, the converters will run for several hours per day and consequently have to be robust and efficient.

Modern, lightweight DC-DC converters designed for PV applications use high-frequency switching and transformers to convert the DC voltage up or down.

Left: Symbol for a DC-DC converter; Right: Studer 12 V to 24 V DC-DC converter (Source: Solar Magazine Uganda.)

Inverters.

Inverters are electronic devices that convert DC voltage to AC voltage—for example, 12 V DC to 110 V or 230 V AC. In off-grid PV systems, the DC power source is the battery. In grid-connected PV systems, the DC source is the PV array. In off-grid systems, ‘grid-connected’ inverters are only used in AC coupled mini-grids (see previous article). 

Note: Do not confuse grid-connected inverters with ‘inverter-chargers’, which can also be connected to the grid but use the grid only as a power source to charge the batteries.

The following inverter types can be used in off-grid systems:

• Battery inverters (‘one direction’ or ‘unidirectional’) (see image);
• Battery inverters with integrated solar charge controllers;
• Inverter-chargers for DC-coupled systems (often called ‘bi-directional’);
• Grid-forming inverters in AC-coupled systems (also referred to as ‘bi-directional’ and sometimes called ‘island inverters’);
• grid-connected PV inverters (also called ‘grid-tied’ or ‘utility-interactive’ or ‘PV inverters’) in AC-coupled off-grid systems.

The terminology can vary, so be careful when reading texts and datasheets and be absolutely sure that you understand which type of inverter is being referred to.

Left: Symbol for a battery inverter (one-direction). Right: Steca 500 W battery inverter (Source: Solar Magazine Uganda.)

Battery inverters (‘one direction’ or ‘unidirectional’)

Battery inverters (‘one direction’ or ‘unidirectional’) are the most basic and the most common type of inverter used in off-grid PV systems. They are simply connected directly to the battery or battery bank and provide AC power. They are unidirectional, meaning that they can only convert power from DC to AC. 

The inverter will automatically switch off if the battery voltage gets too low. However, this cut-off voltage is usually too low (typically 10 V or 10.5 V in a 12 V system) to protect the batteries from overdischarge; rather, it protects the inverter itself from under-voltage. This complicates the use of inverters in off-grid PV systems. The system either has to be closely managed by the user, or an automatic solution must be implemented to protect the batteries.

The image shows a wiring diagram for a system with a 24 V battery inverter. It shows the connections between the components, but does not include the grounding/earthing arrangements. The DC loads (e.g. DC lights) in the system can act as a warning mechanism – if the state of charge (SoC) of the batteries is too low, the charge controller will disconnect them. 

Wiring diagram of system with 24 V battery inverter grounding (Source: RENAC)

Some battery inverters will have an integrated solar charge controller (see image). The level of protection for the batteries is stated in the datasheet. Besides the AC output, some of these devices also provide a DC output.

200 W Studer battery inverter with integrated solar charge controller (Source: Solar Magazine Uganda.)

Inverter-chargers for DC-coupled off-grid systems.

An inverter-charger for DC-coupled off-grid systems is an inverter with an integrated battery charger (see image). It enables the battery bank to be charged by a generator or the grid (in back-up systems), as well as by PV modules. Instead of the PV array being connected to the batteries via a charge controller, the inverter determines which power source will charge the batteries. The inverter-charger usually only requires a single-phase power supply. The PV modules must be close to the batteries. 

In off-grid systems, inverter-chargers are most commonly used in PV-diesel hybrid systems (see image). The smallest size available is approximately 500 W. In larger systems, several of these inverters can be connected in parallel to provide higher AC power output or 3-phase AC power. The 3 main advantages of this type of PV-diesel hybrid system using an inverter-charger are:

• Battery banks can be smaller: When the battery bank voltage drops below a certain level, the generator automatically starts charging the batteries.
• Heavier loads can be powered: E.g. a high-quality 2000 W inverter-charger will allow perhaps 4000 W to be taken directly from the generator when the generator is running, bypassing the batteries.
• Very heavy loads can be powered directly by the generator.

These devices are complex but also very versatile. 

The left image below shows the symbol for an inverter-charger for DC-coupled systems.

The right image shows the following: the complete view (top left), wiring (top right), control panel (lower left), separate monitor (lower centre), and battery temperature sensor (lower right). A model with an integrated solar charge controller is also available. 

An inverter-charger of a demo rig, 1600 W Steca Compact C-1600-12 (Source: Solar Magazine Uganda.)

Wiring diagram of an inverter-charger in a circuit with a diesel generator.

 (Source: RENAC)

Grid-forming battery inverters for AC-coupled off-grid systems

A grid-forming battery inverter for AC-coupled systems (see image) is an inverter that creates a grid to which grid-connected inverters and loads can be connected, and it also has an integrated battery charger. The batteries can be charged from any power supply feeding into the AC grid which is created by the grid-forming inverter (e.g. PV array, wind or hydro turbine) or a diesel generator via the gridforming inverter. In contrast, DC-coupled inverter-chargers can only accept power generated on their DC-side (from or via the batteries) or from a diesel generator, but not via the AC output side.

Grid-forming inverters in AC-coupled systems are complex but also very versatile in their use. Their installation and programming require a high level of technical skill. The smallest size available is approximately 2000 W. In very large systems, the inverters can be connected in parallel to provide 3phase AC power.

The image shows two inverters in an AC-coupled system installed in a training centre in Mozambique. The yellow device on the right-hand side is an SMA Sunny Island grid-forming inverter, while the red device on the left is the grid-connected inverter. 

Simplified schematic of an AC-coupled mini-grid. (Source: RENAC)

Two inverters in an AC-coupled system: a grid-forming inverter on the right (yellow) and a grid connected inverter on the left (red). Source: Carlos Munhá Freire, Inst. Ind. of Maputo.

Grid-connected PV inverters in off-grid systems.

PV grid-connected inverters (also called ‘grid-tied’ or ‘utility-interactive’) are inverters that enable the electricity generated by a PV array to be fed into the electricity grid at the correct voltage and frequency and with the correct waveform (see image). In the case of an off-grid system this will be a mini-grid created by the gird-forming battery inverter (AC-coupled systems) described on the previous page. In these systems, the grid-connected inverter and battery inverter need to be compatible, which generally means they should come from the same manufacturer. The grid-connected inverter needs to be programmed (by the manufacturer or the installer) for use in a mini-grid, because its performance requirements will be different; e.g. it will sometimes need to inject less than maximum power into the mini-grid when the batteries are full and power demand is low.

Grid-connected inverters are available for small wind and hydro plants, as well, and also need to be compatible with the grid-forming battery inverter in AC-coupled mini-grids. SMA grid-forming inverters start at approximately 2000 W.

The image shows an SMA Sunnyboy grid-connected inverter, of the type used in SMA mini-grids. This inverter is connected to an 800 W PV array and is feeding electricity into a mini-grid on a Pacific island. The system was installed in 2013. Note that some corrosion of the inverter enclosure has occurred. 

Left: Grid-connected inverter connected to series string of PV modules (Source: RENAC); Right:

Sunnyboy grid-connected inverter connected to AC-coupled mini-grid system (Source: RENAC)

Batteries

In theory, any kind of battery can be used in off-grid PV systems. However, lead-acid batteries are still the lowest-cost option and are found in the vast majority of off-grid systems (see image for an example). Furthermore, the vast majority of commercially available charge controllers are designed to charge lead-acid batteries. 

Examples of lead-acid batteries. Left: 12 V 100 Ah Sunset Solar flat-plate sealed battery designed for small systems; Right: 2 V 200 Ah Hoppecke individual deep-cycle lead-acid flooded cell. 6 of these are connected in series to produce 12 V; 12 are connected in series to produce 24 V. (Source: Solar Magazine Uganda.)

An exception to this is nickel metal hydride batteries, which are used in some pico-solar systems and portable appliances. Lithium-based batteries are now also becoming more mainstream in pico-solar products. However, they are still much more expensive than lead-acid batteries, but prices are dropping.

Other types of batteries such as nickel-cadmium or hydrogen fuel cells are found in very specialised applications. Emerging battery technologies include: vanadium redox, zinc-bromide, sodium-sulphur, and sodium-ion.

BAE sealed valve-regulated lead-acid batteries on the residential boat in Berlin (Source: Solar Magazine Uganda.)

key points to note.

Every battery model has different characteristics, and datasheets must be referred to for specific information. High-quality batteries come with detailed installation and commissioning manuals. Here are some important points to note:

• Batteries should never be completely discharged. For a good-quality ‘deep cycle’ battery, the maximum depth of discharge (DoD) is 80% and depending on your application and warranty scheme, it might be advisable to limit the DoD to 50%. Look at the graph of service life vs. DoD – this should be in the datasheet, or ask the manufacturer. Sometimes, warranty terms specify the charge regime (voltage levels, current ratings, temperature, etc.).
• Battery banks should be sized to cover days with low levels of solar radiation. 1 to 5 days of extra storage capacity are required to give autonomy for those days, depending on the system type and application and local climate.
• It is important to find out what batteries are available locally (either locally manufactured or imported) and to assess which of these batteries are suitable for use in off-grid PV systems. Importing batteries is expensive, and very often impractical, except for large systems/projects. Airlines will not transport battery acid, and in general, there are limitations for each type of battery, because they are considered to be dangerous substances.
• In most off-grid PV systems the battery is the most expensive component. Typically, during the system’s life span the battery or cells will be replaced several times.
• Batteries are the weakest point in off-grid PV systems – they are usually the first system component to fail, and if systems are not designed/managed properly, they can fail very quickly.
• The principal limiting factor on the size of an off-grid PV system is the battery. A very large PV array can be installed and last 30 years with minimum maintenance. Batteries, on the other hand, will wear out and have to be replaced, which can be very expensive. Many PV systems last no longer than the first set of batteries, because the system owners are unable to replace them. This issue of battery maintenance and replacement needs to be addressed at the initial stage of system design/project development.
• Batteries are hazardous: Only qualified, properly trained personnel should install and maintain the batteries (see image). They need to be installed in safe locations and equipped with appropriate fuses or circuit breakers.
• It is important to consider how the battery can be disposed of or recycled at the end of its service life – this, of course, applies to all system components. Some countries may not allow battery exporting to foreign recycling facilities, because batteries are considered precious raw material.

Incorrectly installed batteries can be can be hazardous and technicians need to be properly trained.

School, potential fire, incorrectly sized cables, serious fire avoided, installer unknown, Guajira, Venezuela. (Source: Eva Schubert)

Battery capacity, rate of discharge (C-rate), depth of discharge, and cycle life

Learning objectives: Upon completion of this page, you should be able to

• explain what is meant by battery capacity, rate of discharge, depth of discharge, and cycle

life

• explain how these factors determine battery characteristics and performance

The battery capacity is defined in terms of amp-hours (Ah). A battery that can deliver 1 A for 100 hours is said to have a capacity of 100 Ah.

Fully defining the battery capacity is, however, more complex than this. For example, a battery may deliver 1 A for 100 hours (100 Ah capacity), but the same battery might deliver 8 A for only 10 hours (80 Ah capacity).

Thus, battery capacities are given for particular rates of discharge, expressed as C-rates. The C-rate refers to the number of hours a battery will deliver a specific current. C₁₀₀denotes a discharge period of 100 hours. A 200 Ah battery at C₁₀₀will deliver 2 A for 100 hours, but at C₂₀,the same battery will deliver 9 A for 20 hours (capacity 180 Ah @ C₂₀). Thus, the total battery capacity depends on the Crate. 

The cycle life of a battery is the number of times it is (or can be) discharged and charged before reaching the end of its working life. 

A ‘deep-cycle’ battery has a maximum depth of discharge (DoD) of 80%. This means it can be discharged by 80% without being damaged. The state of charge (SoC) of a battery can be measured to see how much a battery has been discharged. Always check the battery datasheet for the manufacturer’s recommended DoD, as it is a condition of the battery warranty.

If a ‘deep-cycle’ lead-acid battery is discharged by 30% every night (and recharged the following day), it may have a cycle life of 4,000 cycles. However, if the same battery is discharged by 80% every night (and recharged the following day), it may have a cycle life of only 1,200 cycles. This important information is given in the form of graphs or tables in battery datasheets.

Battery life is also affected by temperature. For example, batteries operating at 10⁰C above the specified temperature (usually 20⁰C or 25⁰C) can have their lives halved.

Terminology for batteries varies, so make sure you understand exactly what is meant by each term given in the battery datasheet. Especially for marketing purposes, some companies may define their own capacity values. Always double-check datasheets, and also compare physical dimensions of batteries, as capacity increases with the size and weight of the electrode material used.

Left: Cycle Life of deep cycle batteries decreases drastically with depth-of-discharge Right: High temperature conditions also have a negative effect on cycle life

Lead-acid battery types and their properties

Learning objective: Upon completion of this page, you should be able to

• list the different types of lead-acid batteries used in off-grid PV systems and their properties

Lead acid batteries are the most common type of battery used in off-grid PV systems. The following table provides a general overview of the types of lead-acid batteries used in off-grid PV systems:

Type	Flooded lead-acid (vented)		Valve regulated lead-acid (VRLA)			Stationa ry Tubular Gel (OPzV)	Stationa ry Tubular Flooded (OPzS)
Common description	SLI* (automoti ve, trucks, lorries)	Modifi ed SLI – someti mes called ‘solar batterie s‘	Lead carbon	Absorb ent Glass Mat (AGM)	Gel cells, mainten ancefree	Mainten ancefree Deepcycle	Flooded deepcycle
Construction	Thin flat plates	Thicker plates than SLI	Mainten ance free, sealed, carbon added to negative electrod e	Mainten ance free, sealed, glass fibre mats hold electrolyt e	Maintena nce free, sealed, electrolyt e is gellified	Gel electrolyt e, tubular plates	Liquid electrolyt e, tubular plates, transpar ent containe rs
Properties	Easily damaged if discharge d more than 20%	Modera te to low water loss, low selfdischar ge rate	No mainten ance, can withstan d deep discharg e	No mainten ance, can withstan d deep discharg e	No maintena nce, can withstand deep discharge	Low maintena nce, can withstand deep discharg e	Low mainten ance, robust construc tion, charge well at low currents
Unit voltages	12 V	12 V	12 V	12 V	6 V, 12 V	2 V, 6 V, 12 V	2 V, 6 V, 12 V
Capacity range	60 – 200 Ah	60 – 260 Ah	30 – 200 Ah	10 – 200 Ah	10 – 130 Ah	200 – 2,000 Ah	20 – 2,000 Ah
Maximu m recomme nded DoD	20%, easily damaged if lower	See datash eets, less than ‘deepcycle‘	Variable, depends on type	Variable, depends on type	Variable, depends on type	80%	80%
% DoD – cycle life	2 years use is	20 % – 1000	30 % – 1300	30 % – 1000	30 % – 800	30 % – 3000	30 % – 4500
(approxi mate)	possible, but usually less	40 % – 500	50 % – 1000 (can be less)	50 % – 500 (can be less)	50 % – 300 (can be less)	80 % > 1000	80 % > 1200
Maintena nce period	~3 months	~3 months	None	None	None	Monitorin g & yearly cleaning	~3 – 6 months
Commen ts	Not generally recomme nded, but may be only practical option – use as truck batteries, not car batteries	Good option for solar home system s and smaller system s, if availabl e	Less sulphatin g on electrod es, better performa nce in partially charged state	Best for high current demand applicati ons	Very variable propertie s – see datashee ts	Not as easy to recycle as flooded deepcycle	Usually the best option, especiall y for larger systems, but the most expensiv e

*SLI = starting, lighting, ignition

Table:  Types of lead-acid batteries used in off-grid systems

Lead-acid battery configuration.

Batteries can be connected in series, in parallel, and in series-parallel to achieve the required voltage and Ah capacity. Series connection is always preferable, since this allows a ‘dead’ battery to be identified early on, whereas in a parallel configuration, a dead battery can drain the other batteries. A further reason is that batteries in parallel can deliver very high short-circuit currents under fault conditions. However, it may not always be possible to connect batteries in series.

When batteries are connected in series (see image), the overall battery voltage is the sum of all individual cell/battery voltages, and the overall battery capacity is the same as that of an individual cell/battery. 

In contrast, when batteries are connected in parallel (see image), the overall battery voltage is the same as the voltage across each individual cell/battery, and the battery capacity is the sum of all individual cell/battery capacities.

Finally, in a series-parallel connection (see image), the strings consist of batteries in series, so these batteries’ voltage is added up to get the total voltage. As the strings are connected in parallel, the total capacity is the sum of the capacities of all the strings.  

Batteries in series (Source: RENAC). Overall battery voltage is the sum of all individual cell/battery voltages. Overall battery Ah capacity is the same as that of individual cells/batteries.

Batteries in parallel (Source: RENAC). Overall battery voltage is the same as the voltage across each individual cell/battery. Overall battery Ah capacity is the sum of all individual cell/battery Ah capacities.

Batteries in series-parallel (Source: RENAC). Strings consist of batteries in series so voltage is summated. Strings are connected in parallel so Ah capacity is summated.

Lithium-ion (Li-ion) battery type.

Lithium-ion (Li-ion) battery technology has matured in recent years and can also be considered for offgrid PV systems. Li-ion batteries are common in portable devices (phones, laptops) and also found in some solar home systems, as their low weight and small size are key design factors for these products. Li-ion cells always need an electronic battery management system (BMS). 

Ideally the BMS allows for cell balancing to equalise small differences in capacity among seriesconnected cells comprising a battery pack.

Note that lithium-ion is a family of batteries with a variety of active materials used for electrodes.

Although the initial cost of Li-ion batteries is significantly higher than that of lead-acid batteries, they may be a viable option when comparing life-cycle costs.

Key points about Li-ion batteries:

• Graphite is mostly used as the anode material, and cathode material selection defines battery properties
• High energy density (up to 4 times that of lead-acid)
• Common graphite anode battery cell voltage is 3-4 V
• High possible charge and discharge C-rates
• DoD up to 80% for most battery models – refer to battery specifications for details
• An electronic battery management system (BMS) is required which is usually integrated into the battery case
• The charge controller needs to be for lithium and sometimes even certified to cover the product warranty – see battery specifications for other requirements
• Temperature levels should be higher than 10°C, especially during charging
• Cell or battery design is quite flexible, e.g. either standardised or user-defined cells can be used to design battery packs
• Few recycling facilities for lithium available outside industrialised countries
• Appropriate lithium disposal is critical, as it reacts with oxygen (air, water) explosively

Key points about Li-ion batteries for off-grid systems:

• The most commonly used Li-ion batteries are lithium iron phosphate (LFP) batteries because of their inherent safety
• Most are compatible with 12 V DC systems, as the voltage is 12.8 V when 4 are connected in series
• 12.8 V LFP batteries are available in same case designs as regular 12 V lead-acid batteries
• Individual (prismatic) cells have to be connected using high-current bus bars with individual voltage sensors fed to the BMS

Manufacturers of high-capacity storage systems (>2 kWh) provide proprietary system racks where batteries can be added individually.

The global off-grid market may shift from lead-acid towards lithium as soon as the transport sector ramps up production capacities while decreasing production cost

Anode material	Carbon (graphite)					Lithium titanite
Cathode material	Based on nickel, cobalt, manganese				Iron phosphate	Lithium titanite
Chemistry	LiCoO2	LiMn2O4	Li(NiMnCo)O2	LiNiCoAlO2	LiFePO4	Li4Ti5O12
Common name	Lithium Cobalt Oxide (LCO)	Lithium Manganese Oxide (LMO)	Nickel Manganese Cobalt (NMC)	Nickel Cobalt Aluminium Oxide (NCA)	LFP	Lithium Titanate Oxide (LTO)
Cell voltage	3.6 V	3.8 V	3.6 – 3.7 V	3.6 – 3.7 V	3.2 V	2.4 V
Cost	very high	lower than average	average	average	lower than average	lower than average
Cycles	500 – 1000	500	1500	1500	1000 – 2500	>5000
Energy density	250 Wh/kg	150 Wh/kg	200 Wh/kg	250 Wh/kg	100 Wh/kg	60 Wh/kg

Table: Types of lithium-ion batteries and their properties(Source: Solar Magazine Uganda.)

Mounting structure requirements for off-grid .

PV modules and arrays need to be mounted correctly.

The fundamental requirements are:

• Optimum orientation;
• Optimum tilt angle – as a rule of thumb, the optimum tilt angle is the angle equal to the site’s latitude plus 10°, and the tilt angle in all cases should be at least 10° to allow for self-cleaning via rain;
• Avoid shading;
• Pole mounting can be used when there is insufficient or impractical available roof space (see image).
• Ventilation to avoid high temperatures; modules should not be mounted directly on metal roofs (leave a space of at least 10 centimetres (cm)) (see image);
• The support structure must support the weight of the modules and withstand high winds and potential snow loads;
• The structure should generally be galvanised or made of aluminium, although painted steel and wood are also options (see image);
• Earthing/grounding may be required; and
• Lightning protection may be necessary (see your local codes), and arrays should not be at the highest point of a building, if at all possible.

Bear in mind that the mounting structure will/should have a long life – maybe 20 years or more. 

Local conditions (e.g. saline air close to the sea) have to be taken into consideration when choosing mounting frame materials.

Left: Pole-mounted PV modules, Wales, UK. Right: Roof-mounted solar modules on Earth University training centre roof, Costa Rica – note that there is an air gap between the modules and the metal roof, the mounting structure material is painted mild steel. (Source: RENAC)

4.9     Mounting structure types

Learning objective: Upon completion of this page, you should be able to

• name and describe the different types of off-grid PV system mounting structures

Off-grid PV array mounting structures generally fall into the following categories:

• Ground-mounted structures (free-standing, usually fixed, sometimes held in place by weights);
• Mounting structures on flat roofs (free-standing, usually held in place by weights, not fixed);
• Pole-mounted structures (see image);
• Mounting structures on sloped roofs (more difficult to install, roof orientation and slope may be sub-optimal);
• Tracking mounts – manual and automatic (manual tracking is more common in off-grid systems than automatic tracking, which is very rare) (see image for an example, with painted mild steel as the material); and
• PV slates & tiles integrated into a roof (very rare in off-grid systems).

Most small off-grid PV systems are ground-mounted or pole-mounted. For roof mounting (whether on a flat or sloped roof), the roof condition and quality should be considered (will it last for the 20+ year lifetime of the PV system?), as well as whether the roof structure is strong enough to support the additional weight and withstand wind loads.

For ground mounting, the proximity and height of surrounding vegetation should be considered. In the image (showing an aluminium mounting structure), the bottom module is very close to the ground and might be shaded by the growing grass. 

Ground-mounted structure, France. Note that the bottom module is very close to the ground and might be shaded by the growing grass. The mounting structure material is aluminium. (Source: RENAC)

Left: Manual tracking structure at training centre in Tanzania. (Source RENAC); Right: Modules on a locally made pole mount for a solar home system in rural France. Both mounts are painted mild steel.

(Source: RENAC)

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