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Photovoltaic systems vary in scale and application forms. They range from several‑watt solar garden lamps to MW‑level solar photovoltaic power stations. They are widely applied in household, transportation, telecommunication, space applications and many other fields. Despite different system sizes, photovoltaic systems have basically the same structure and working principle.
A solar power generation system consists of solar array, solar charge controller and battery bank. If the output power is AC 220V or 110V, an inverter shall be additionally equipped. Functions of each component are as follows:
(1) Solar Panel As the core and highest‑value component of the solar power generation system, the solar panel converts solar radiation into electric energy. The generated power can be stored in batteries or directly drive the loads.
(2) Solar Charge Controller The solar charge controller monitors the operating status of the whole system and provides over‑charge and over‑discharge protection for batteries. For regions with large temperature differences, a qualified controller shall have the temperature compensation function. Optional extra functions include light‑controlled switch and time‑controlled switch.
(3) Battery Lead‑acid batteries are commonly used. For mini and small‑size systems, nickel‑metal hydride batteries, nickel‑cadmium batteries or lithium batteries are also feasible. Batteries store electricity generated by solar panels under sunlight and release energy when required.
(4) Inverter AC 220V or 110V power supply is required in many working conditions. Solar panels normally output DC 12V, 24V or 48V. To supply power for 220VAC electrical appliances, DC‑AC inverter is used to convert direct current from solar system into alternating current. For loads with multiple voltage requirements, DC‑DC inverter is also adopted, for example converting 24VDC into 5VDC (Note: this is not simple voltage reduction).
Photovoltaic system design includes capacity design and hardware design.
Prior to photovoltaic system design, basic data for calculation and component selection shall be collected: Geographic information of installation site: location, latitude, longitude and altitude; Local meteorological data: monthly total solar radiation, direct radiation and diffuse radiation, annual average temperature, maximum and minimum temperature, maximum consecutive rainy days, maximum wind speed, as well as special weather conditions such as hail and snowfall.
Battery design covers battery capacity calculation and series‑parallel configuration of battery bank. The basic calculation procedure for battery capacity is shown below.
Step 1: Multiply daily power consumption of the load by autonomy days determined by actual conditions to get the preliminary battery capacity.
Step 2: Divide the preliminary battery capacity obtained in Step 1 by the maximum allowable depth of discharge of the battery. The battery cannot be fully discharged during autonomy days, so divide by maximum depth of discharge to obtain the required battery capacity.
The maximum depth of discharge shall be selected according to battery performance parameters provided by battery suppliers. Normally, 80% Depth of Discharge (DOD) is recommended for deep‑cycle batteries; 50% DOD is recommended for shallow‑cycle batteries.
Uncorrected basic formula: Battery Capacity = (Autonomy Days × Average Daily Load) / Maximum Depth of Discharge
The above formula does not include correction factors. The correct formula for battery capacity BC: BC = (A × QL × NL × TO) / CC (Ah) Formula (1)
Where: ‑ A: Safety factor, 1.1 ~ 1.4 ‑ QL: Average daily power consumption of load, equal to operating current × daily working hours ‑ NL: Maximum consecutive rainy days ‑ TO: Temperature correction factor: 1 for temperature above 0℃; 1.1 for temperature between ‑10℃ and 0℃; 1.2 for temperature below ‑10℃ ‑ CC: Battery depth of discharge: 0.75 for lead‑acid batteries; 0.85 for alkaline nickel‑cadmium batteries
Each battery has its nominal voltage. To achieve the nominal working voltage of the load, batteries are connected in series. Number of Batteries in Series = Nominal Load Voltage / Nominal Single‑Battery Voltage
The core idea of solar module design is to meet the average daily power consumption of loads.
Calculation method: divide average daily energy demand of the load (Ah) by daily energy output of one solar module (Ah), to get the quantity of parallel‑connected modules for required current output. Divide nominal system voltage by nominal voltage of single solar module, to get the quantity of series‑connected modules for required voltage output.
Basic formulas: Number of Parallel Modules = Average Daily Load (Ah) / Daily Output per Module (Ah) Number of Series Modules = System Voltage (V) / Nominal Module Voltage (V)
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