Foreword

Zambia is situated on the southern African plateau and enjoys excellent solar irradiation conditions; however, the country faces significant shortcomings in its domestic electricity supply. Power generation relies primarily on hydropower; whenever river levels fall during the dry season, the power grid’s capacity is drastically reduced, frequently leading to power cuts, power rationing and voltage instability. Many local maize and grain milling plants operate as continuous-production agricultural industries. Should the grid fail, the main milling units and crushing and mixing equipment are forced to shut down, which not only disrupts grain processing schedules but also accelerates mechanical wear and tear due to the repeated starting and stopping of motorised equipment. Most processing plants rely on diesel generators as a backup power source; however, the costs of procuring and transporting diesel are high, maintenance requirements are substantial, and long-term production and operational costs remain persistently high.

To address the challenge of ensuring a stable power supply to the factory, this project has established an integrated photovoltaic and energy storage microgrid system for a maize milling plant in Zambia. The system makes full use of the region’s abundant sunshine, with photovoltaic power generation serving as the primary source of electricity and a lithium iron phosphate energy storage system storing surplus electricity.

System Configuration Diagram

The complete system comprises 72 x 710W photovoltaic modules, 4 x 12kW grid-tied inverter-controller units and 4 x 16kWh stacked lithium iron phosphate battery packs. It is designed to generate an average of 255 kWh per day and store 64 kWh per day. The system supports automatic switching between grid-connected and off-grid modes. When the grid is operational, it operates in conjunction with the mains supply; in the event of a grid failure, it switches instantly to off-grid energy storage mode, ensuring the continuous operation of core processing equipment. This effectively reduces the frequency of diesel generator use, lowers the factory’s energy consumption costs, and is well-suited to the complex on-site conditions of African processing plants.

📋 General system configuration parameters

The photovoltaic array for this project comprises 72 x 710W N-type bifacial double-glass photovoltaic modules, with an open-circuit voltage of 50.02V and an operating voltage of 42.14V. The modules offer excellent low-light power generation performance and are resistant to wind, sand and hail, with a total installed capacity of 51.12kWp.

Diagram of a photovoltaic panel array on a factory roof

The inverter control unit comprises four 12 kW parallel-connected integrated inverter-controller units. Each unit is equipped with two independent MPPT photovoltaic input channels and supports parallel operation of up to six units. The system outputs pure sine wave AC power, capable of meeting the start-up and operational requirements of inductive motor-driven grinding equipment.

12 kW grid-tied inverter

The energy storage unit comprises four 16kWh stacked lithium iron phosphate battery packs, each with a nominal voltage of 51.2V. It features a built-in Battery Management System (BMS) with protection against overvoltage, undervoltage, overcurrent and temperature, and supports parallel expansion with multiple units. The total usable installed capacity of the entire energy storage system is 64kWh.

64 kWh stacked lithium-ion energy storage battery pack

The entire system has a maximum AC output of 48 kW, is designed to generate an average of 255 kWh per day, and has a daily battery storage capacity of 64 kWh. The system is compatible with the local mains supply and can also be connected to an external diesel generator as a third-tier backup power source; the combination of multiple power sources ensures comprehensive power supply for the factory’s production operations.

🔌Electrical Wiring Design

System Topology Diagram

✨PV-side wiring

Each 12 kW integrated inverter-controller unit is equipped with two independent MPPT channels. The string configuration comprises one PV string formed by connecting nine modules in series, with each inverter connected to two such strings; the four inverters together comprise eight PV strings, with all 72 photovoltaic modules connected to the system. The standard open-circuit voltage of a single string under standard conditions is 450.18 V, with an operating voltage of 379.26 V. During the dry season on the Zambian Plateau, night-time temperatures can drop to 6–7 °C; low-temperature environments cause the photovoltaic open-circuit voltage to rise. After correction for the temperature coefficient, the maximum open-circuit voltage of a string is approximately 484 V, which falls within the inverter’s safe photovoltaic voltage withstand range and will not trigger an overvoltage alarm.

During the construction phase, each PV string was connected independently to the corresponding MPPT port on the inverter; the parallel connection of PV circuits between different inverters was strictly prohibited. The DC cables running from the strings to the inverters were 6 mm² PV-specific DC cables; each string was fitted with a 20 A DC fuse, and each inverter’s PV input was equipped with DC surge protection devices to withstand surge impacts caused by highland thunderstorms. All photovoltaic connections must be properly waterproofed and sealed, and cables must be protected against rodents and UV exposure.

🔋Battery DC Wiring

Four 16kWh stacked lithium-ion battery packs are connected in parallel at the power terminals. Prior to wiring, measure the open-circuit voltage of each battery pack using a multimeter; the voltage difference must be kept within 0.1V. If the voltage difference exceeds this limit, voltage equalisation must be carried out first; it is strictly prohibited to connect battery packs with excessive voltage differences in parallel directly. The positive and negative terminals of all batteries are connected to a common DC busbar. A main DC circuit breaker is fitted at the front end of the busbar, from which four independent DC branches are derived. Each branch is equipped with an independent DC circuit breaker and connects to the positive and negative terminals of one of the four integrated inverter-controller units respectively.

Battery communication utilises a daisy-chain connection, with the four battery banks communicating in series. Only the BMS communication interface of the first battery bank is connected to the main inverter’s communication port; the remaining slave inverters are not connected to the battery communication lines. Battery BMS data is forwarded to all units via the inverter’s parallel communication link, thereby avoiding communication conflicts and faults caused by multiple BMS units being connected simultaneously.

⚡Inverter Parallel Connection Wiring

Of the four 12 kW integrated inverter-controller units, one is designated as the system master, whilst the remaining three are configured as slaves. The units are connected in a daisy-chain configuration via RS485 interfaces; the master and the slave at the end of the chain have their terminating resistors enabled, whilst the two intermediate slaves have theirs disabled, to ensure stable communication signals within the daisy-chain. On the AC side, the mains input terminals of all inverters are connected in parallel and fed into the factory’s local mains distribution board; the load output terminals are all connected in parallel and fed into the power distribution board for the grain milling workshop. The system output is single-phase 230 V AC.

System Power-On Sequence

With all AC and DC switches in the open position, first activate the four lithium-ion batteries and wait for the BMS to complete its self-test to confirm there are no alarms; close the main battery DC circuit breaker to power up the battery side of the inverter; activate the four inverter-controller units in sequence, confirming that the synchronisation handshake communication is normal and there are no fault codes; then close the PV-side fuse, allowing the PV array to begin charging the system; Finally, close the grid input switch and the workshop load output switch in sequence. Under no circumstances should the PV side be energised first during this operation; connecting high-voltage PV without battery power supply will result in damage to the hardware of the inverter-controller units.

⚙️System Operating Modes

This photovoltaic-storage system features multiple operating modes and can switch automatically according to sunlight conditions and the state of the grid, adapting to the production requirements of the manufacturing plant at different times of the day.

Video: Diagram illustrating how the system works

1. Photovoltaic Priority Power Supply Mode
   During periods of ample daylight, the photovoltaic array generates electricity which is prioritised to supply all loads, including the main maize milling unit, the crusher and workshop lighting. When the power generated by the photovoltaic system exceeds the workshop’s real-time electricity consumption, the surplus energy is used to charge the lithium iron phosphate battery bank until the batteries are fully charged; when the power generated by the photovoltaic system is less than the load consumption, the batteries discharge simultaneously to make up the shortfall.
2. Energy Storage Discharge Power Supply Mode
   In the evening or on overcast days when sunlight is insufficient and photovoltaic output is low, the system switches to battery discharge mode, with the energy storage batteries supplying power to the workshop loads to ensure short-term processing and production. When the battery’s State of Charge (SOC) drops to the set protection threshold, the system ceases battery discharge.
3. Grid-Complementary Mode
   When the mains supply is normal, if the combined output from photovoltaic generation and energy storage is insufficient to support the production load, the system automatically draws on the mains supply to provide supplementary power; when the battery charge level is low, the mains supply can be utilised to charge the battery via the AC circuit.
4. Off-Grid Emergency Mode
   In the event of a power cut in the local grid, the system switches to off-grid operation within milliseconds, disconnecting from the grid and relying on photovoltaic generation and energy storage to maintain the operation of critical grinding equipment; Once the energy storage is depleted, a diesel generator can be manually activated as a backup power source to prevent production from coming to a complete standstill.
5. Charging Standby Mode
   During night-time production shutdowns, when loads are largely disconnected, the system maintains only low-power consumption for monitoring and lighting. Surplus electricity generated by the photovoltaic system during the day is used to charge the batteries, whilst at night the batteries remain in a low-loss standby state, ready for the start of production the following day.
    

🏭System power handling capacity and operating duration

Powder-grinding equipment is classified as an inductive load for electric motors; the start-up of the motor generates a significant inrush current, so sufficient power headroom must be allowed for when selecting the load and calculating the operating duration; the battery is controlled to an 80 per cent depth of discharge, resulting in an actual usable energy storage capacity of 51.2 kWh.

In actual production, it is recommended to operate high-power equipment at staggered times and avoid running the entire system at full load for prolonged periods whilst relying solely on energy storage. This will maximise the benefits of direct photovoltaic drive, reduce high-rate discharge of the energy storage system, and extend the service life of the batteries.

🌍Points to note regarding adaptation to the on-site environment in Zambia

Zambia has two distinct seasons: the dry season and the rainy season. During the dry season, there is intense sunlight and high levels of dust, resulting in high system power generation; during the rainy season, there is an increase in overcast and rainy weather, leading to a significant drop in photovoltaic power generation. Production continuity is ensured by relying on energy storage combined with the mains supply or diesel generators as a backup. Hail-resistant photovoltaic modules are selected to withstand the extreme weather conditions of the highlands. Outdoor DC cables must be protected against UV exposure and rodent damage, and hazard warning signs must be displayed at all high-voltage DC locations.

Battery banks must be housed inside factory buildings with good ventilation and heat dissipation; exposure to direct sunlight outdoors is prohibited to prevent high temperatures from shortening the batteries’ cycle life.

During the dry season, when dust levels are high, the surfaces of photovoltaic modules must be cleaned regularly to remove dust, as a build-up of dust can significantly reduce power generation; as part of routine operation and maintenance, the voltage balance of the cells must be checked regularly, communication between inverters must be verified to ensure there are no errors, and all power terminals must be tightened to prevent them from becoming loose and overheating in high-temperature environments. All DC-side wiring work must be carried out by a qualified electrician to ensure the safety of personnel and equipment on site.

📑Summary of the Proposal

This integrated solar-storage solution addresses the genuine power supply challenges faced by local food processing plants in Zambia. The complete microgrid system—comprising a 51.12 kWp photovoltaic array paired with a 48 kW inverter and a 64 kWh energy storage system—leverages the region’s abundant solar resources to generate an average of 255 kWh per day, converting solar energy into electricity for production use. The system operates automatically in multiple modes: during the day, the PV system directly powers the milling equipment, with surplus electricity stored in the batteries; on overcast days, in the evening, or during grid outages, the energy stored in the batteries is released to support core production loads, whilst also being compatible with multiple backup power sources such as the mains supply and diesel generators.

The solution takes full account of the environmental characteristics of the Zambian plateau, including temperature, dust and thunderstorms, and is fully adapted to African conditions across all dimensions, from module selection and wiring topology to operational strategies and maintenance. Once implemented, the project will significantly reduce the factory’s reliance on diesel generators, cut fuel costs, lower grain processing production costs, enhance the reliability of the maize processing plant’s power supply, and ensure the stable and continuous operation of local grain processing operations.