How Does Solar Energy Work | Sunlight Conversion, Electricity Generation, Storage

Solar energy through photovoltaic panels converts light into direct current (efficiency about 20%), through the inverter becomes alternating current for household use;

Excess electricity is stored in batteries (such as 10kWh energy storage), released at night, installation requires reasonable wiring and capacity matching.

Sunlight Conversion

Drilling into the silicon wafer

Mass-produced monocrystalline silicon wafer side length specifications are mostly 182 mm or 210 mm, and silicon wafer thickness parameters drop from 165 microns to 130 microns. The purity of raw silicon material must reach the 6N level of 99.9999%. The silicon wafer substrate of N-type cells is doped with phosphorus elements during the crystal pulling stage, and the doping concentration magnitude of phosphorus atoms is 10^15 per cubic centimeter. The silicon wafer surface undergoes a diffusion process to prepare a P-type emitter with a thickness of 0.3 microns to 0.5 microns, with the doping substance being boron element.

A depletion layer with a thickness of about 1 micron is produced at the physical interface of the P-zone and N-zone. Inside the depletion layer, the diffusion motion and drift motion of carriers reach a dynamic balance, establishing a built-in electric field of 0.6 V to 0.7 V. The electric field strength is measured at a peak value in the center position of the interface, the data being about 10^4 V/cm.

Exciting electrons

The standard band gap width parameter of silicon atoms is 1.12 electron volts. When photons with wavelengths shorter than 1100 nanometers hit the silicon lattice, photons carrying energy exceeding 1.12 electron volts transfer their own energy to the valence electrons on the covalent bonds. Valence electrons absorb energy and transition to the conduction band, leaving holes with a positive charge of 1.6×10^-19 Coulombs at the original positions, generating electron-hole pairs. The built-in electric field pushes electrons toward the N-zone and holes toward the P-zone at a drift rate of 10^7 centimeters per second.

If the energy of a single incident photon reaches 2.5 electron volts, the 1.38 electron volt energy part higher than 1.12 electron volts converts into phonon vibration and releases heat, accounting for 33% of the single photon total energy conversion loss. For every 1°C increase in module operating temperature, the open circuit voltage value of monocrystalline silicon cells decreases by 2.2 mV to 2.3 mV, and the power temperature coefficient is distributed between -0.25%/°C to -0.35%/°C. Under a measured rooftop temperature of 85°C, the absolute output power of the cell drops by 15% to 21% compared to the 25°C nominal data.

Conducting electricity

The number of main busbars parameters has increased from 5 to 16 or even 20, and the single width has shrunk from 1 mm to within 0.2 mm, causing the light shielding area to drop below 2.5%, and the effective light-receiving area ratio to exceed 97.5%. The printing width of fine grid lines is 15 microns to 25 microns, with an interval arrangement of 1.2 mm to 1.5 mm. The series resistance measurement value of the module is controlled below 0.5 ohm·square centimeter. The mass-produced photoelectric conversion efficiency of N-type monocrystalline cells is distributed in the 24.5% or even 25.2% range.

The measured electrical parameter indicators of a single 210 mm specification N-type silicon wafer under 1,000 W/m² irradiance are distributed as follows:

· Short-circuit current (Isc): 18.15 Amperes

· Open-circuit voltage (Voc): 0.71 Volts

· Maximum power point current (Impp): 17.25 Amperes

· Maximum power point voltage (Vmpp): 0.61 Volts

· Fill factor (FF): 82.5% to 83.8%

· Surface recombination current density (J0): below 10 fA/cm²

Quantified loss table

Electricity Generation

Connecting to a string

The weak current exported from the silicon wafer is collected through 16 to 20 ultra-fine metal busbars inside the module, which are composed of tinned round copper wires with a diameter of 0.25 mm to 0.35 mm, aggregating the 10 Amperes to 18 Amperes of current generated by a single cell. Multiple modules are connected in series through MC4 connectors to form a DC string with a voltage range between 600 Volts to 1500 Volts. The contact resistance of each string connection point must be controlled below 0.5 milliohms to prevent local temperature rise when large currents pass through.

Inside the junction box on the back, 3 Schottky bypass diodes are installed, with rated currents usually being 20 Amperes to 30 Amperes. When one string of cell plates is shaded by shadows causing the current to drop by more than 50%, the diode will automatically conduct within 10 milliseconds, bypassing the affected area to prevent local high temperatures and protect the stable output of the entire string's 450 Watt to 670 Watt power.

· Number of series modules: 10 to 22 pieces

· System maximum withstand voltage: 1500 Volts (DC)

· Junction box protection grade: IP68

· Cable cross-section specification: 4 square millimeters or 6 square millimeters

· Resistance per meter of cable: below 5.09 ohms/km (4 mm²)

· Connection loss proportion: 0.1% to 0.3%

Inverter voltage transformation

The IGBT (Insulated Gate Bipolar Transistor) inside the inverter performs fast switching actions at high frequencies of 16 kilohertz to 20 kilohertz. This high-frequency switching chops constant direct current into pulse signals of varying widths, which are then restored to 50 hertz or 60 hertz sine wave alternating current through an inductor filter. The weighted conversion efficiency (Euro Efficiency) of modern string inverters is stable between 98.2% to 98.6%. In the full-load operating state, the power factor (PF) adjustable range is between 0.8 leading to 0.8 lagging. Inverter fans or heat sinks need to dissipate about 1.5% of the power loss heat; a 10 kilowatt inverter at mid-day peak generation has a heating power of about 150 watts, equivalent to the heat energy released by two incandescent light bulbs.

· Switching frequency: 16,000 Hz to 25,000 Hz

· Self-consumption power: below 1W standby at night, about 20W running during the day

· Total harmonic distortion (THD): below 3%

· Insulation resistance monitoring threshold: higher than 100 kilohms

· Electromagnetic compatibility standards: EN 61000-6-1/2/3/4

· Normal operating temperature rise: 30°C to 50°C

Staring at the peak

The static tracking accuracy of MPPT reaches over 99.9%, and the dynamic response time is shortened to within 1 second. In cloudy weather, irradiance fluctuates violently between 200 watts/square meter to 800 watts/square meter, and the algorithm locks the system voltage at the peak of the power curve through hundreds of voltage samplings per second. If there is local shading on the roof, the dual-channel MPPT design can independently manage two strings with different orientations, avoiding a 15% efficiency loss in one string from dragging down the other. This configuration can increase the annual total power generation by 5% to 10% under complex roof environments.

· Number of MPPTs: usually 1-2 channels for single-phase, can reach 6-12 channels for three-phase

· Input voltage range: 80 Volts to 1100 Volts

· Startup voltage threshold: 60 Volts to 120 Volts

· Full-load MPPT voltage range: 450 Volts to 850 Volts

· Sampling frequency: 100 times to 500 times per second

· Tracking error: less than 0.1%

AC output

Civilian single-phase output voltage usually floats at 220 Volts, 230 Volts, or 240 Volts, while three-phase output is locked at 380 Volts or 400 Volts. The inverter monitors the grid frequency in real time; once the frequency deviates from the nominal value (such as 50 hertz) by more than ±0.5 hertz, or the voltage exceeds +10% or -15% of the rated value, the system will start anti-islanding protection logic within 0.1 seconds to cut off the connection with the external grid to ensure maintenance safety. At this time, if it is a hybrid inverter with energy storage function, it will switch to off-grid mode within 10 milliseconds to 20 milliseconds, relying on the cell pack to continue supplying power to a 150-watt refrigerator or a 2000-watt electric kettle.

· Grid voltage fluctuation allowable value: -20% to +10%

· Frequency response deviation: ±2 hertz (adjustable)

· DC module: below 0.5% of output current

· Night standby power consumption: below 5 watts

· Short-circuit protection current: 1.5 times to 2 times the rated current

· Switching time (UPS level): less than 10 milliseconds

Filtering impurities

Power electronic devices generate electromagnetic interference while working, and the inverter AC output end is equipped with secondary or tertiary EMI filters and metal oxide varistors (SPD). These modules are responsible for absorbing transient surge currents from 2 kilovolts to 6 kilovolts from the grid and filtering out high-frequency electromagnetic noise from 150 kilohertz to 30 megahertz.

Through the combination of multi-stage magnetic core inductors and thin-film capacitors, the output AC waveform smoothness is extremely high, and its total harmonic distortion (THD) is usually maintained between 1.5% to 2.5%, which is far better than the public grid standard requirement of 5%. This pure power output can effectively extend the service life of variable frequency air conditioner compressors and sensitive electronic equipment by about 10%.

Storage

Things inside the cell

Mainstream energy storage systems generally adopt lithium iron phosphate (LiFePO4) materials, the nominal voltage of a single cell is 3.2V, and a standard cell module of 51.2V is formed through a 16-series 1-parallel or 2-parallel structure. Lithium ions de-intercalate from the positive electrode during charging, pass through a polyolefin separator with a thickness of about 12 microns to 25 microns, and intercalate into the graphite negative electrode; at this time, the internal pressure of the cell will undergo minor physical changes accordingly.

The P-O bonds in lithium iron phosphate crystals are very stable, and the decomposition temperature is as high as 700°C to 800°C, which makes its fire probability much lower than ternary lithium batteries when encountering nail penetration or short-circuit tests. The energy density of mass-produced cells is currently distributed between 150Wh/kg to 175Wh/kg, and common cell capacity specifications include 50Ah, 100Ah, and the mainstream 280Ah. To maintain electrochemical stability, electrolyte is filled inside the cell cluster, its main module being lithium hexafluorophosphate, with a concentration usually controlled at 1.0 to 1.2 mol/L, ensuring lithium ions still have a migration rate of several microns per second in extreme environments from -20°C to 60°C.

Within the full life cycle, a 10kWh lithium iron phosphate energy storage system can cumulatively handle over 50,000 units of electricity. Its charge-discharge Coulombic efficiency (the ratio of discharge capacity to charge capacity) is as high as 99% or more, but considering inverter conversion and internal resistance heating, the actual system round-trip efficiency is usually locked in the 86% to 93% range.

Electricity management chip

The Cell Management System (BMS) is the brain of the energy storage unit, which monitors the voltage of 16 single cells in real time through high-precision sampling chips; the sampling accuracy must reach between ±2mV to ±5mV. When the voltage deviation between cells exceeds 30 mV, the BMS will start active balancing or passive balancing circuits, leveling the voltage difference through current dissipation or transfer of 50 mA to 200 mA.

The BMS integrates complex SOC (State of Charge) algorithms, performing weighted calculations based on the Ampere-hour integration method and open-circuit voltage curves, controlling the remaining power estimation error within 3%. In addition to voltage, the system is equipped with 4 to 8 NTC temperature sensors to monitor the thermal distribution on the cell surfaces in real time. Once any point's temperature rise rate is detected to exceed 3°C per minute, or the current density exceeds 1.2 times the rated 1 C (i.e., 100Ah cell current exceeds 100A), the BMS will forcibly cut off the circuit through MOS tubes or relays within 10 milliseconds to prevent electrochemical reaction runaway.

· Single cell overvoltage protection threshold: 3.65V to 3.75V

· Single cell undervoltage protection threshold: 2.2V to 2.5V

· Balancing current range: 50mA (passive) to 5A (active)

· Communication interface: CAN 2.0B / RS485 / Wi-Fi

· Standby power consumption: below 5W

· Insulation resistance monitoring: greater than 500 ohms/volt

Calculating lifespan

The depreciation cost of an energy storage system mainly depends on the cycle life; standard lithium iron phosphate cells at 25°C environment, charging and discharging at 0.5C rate, will have a remaining capacity (SOH) of over 80% after 6000 cycles. Depth of Discharge (DoD) has a decisive impact on life; long-term 100% full charge and discharge will cause irreversible micro-collapse of the negative graphite layer spacing. If DoD is limited to 80% to 90%, the number of cycles can be extended to 8000 or even 10000 times.

If charging and discharging completely once a day, the physical life of the system can reach 15 to 22 years. The annual calendar attenuation rate of the system is usually between 1.5% to 2%; even if not used, internal side reactions will slowly consume active lithium ions. In actual installation, due to the non-uniformity of string currents, the bucket effect will cause the usable capacity of the entire cell group to be 3% to 5% lower than the theoretical value of a single cell; this is a capacity margin that must be reserved during system integration.

Afraid of cold and afraid of heat

When the ambient temperature drops below 0°C, electrolyte viscosity increases, and the internal resistance of the cell will surge from 0.5 milliohms at room temperature to more than 3 milliohms, leading to an instantaneous shrinkage of usable capacity by 20% to 30%. Forcing large current charging at extremely low temperatures will precipitate metallic lithium dendrites on the negative electrode surface, which may pierce the separator and cause an internal short circuit.

Conversely, if the operating temperature is long-term higher than 45°C, the SEI film (Solid Electrolyte Interface film) will thicken and consume electrolyte, shortening the cell life by about 50% for every 10°C increase. Therefore, high-end energy storage cabinets will be configured with integrated thermal management systems, using aluminum heat dissipation substrates or liquid cooling plates to control the temperature difference between cells within 3°C. Under summer peak loads, this thermal management can improve cell efficiency by about 2% and reduce heat-induced attenuation by about 15%.

A typical 5kWh cell pack has a rated discharge capacity of 100Ah at 25°C, but at -10°C, the extractable power under the same current is only about 75Ah. To compensate for this physical characteristic, modern systems usually integrate 50W to 100W heating films to preheat the cells to above 10°C using surplus photovoltaic power before charging.

Emergency power supply

In power outage mode, the dynamic response performance of the energy storage system determines whether appliances will restart. The hybrid inverter will complete the switch within 10 to 20 milliseconds (less than one AC cycle) after detecting grid power failure; this speed is sufficient to allow a running desktop computer or a 150-watt refrigerator to operate without interruption. The continuous output power of the energy storage system is limited by the "C rate"; a 10kWh cell pack, if rated at 0.5C charge-discharge, can stably provide 5kW of power output.

If an inductive load of 3000 watts (such as an air conditioner compressor) is started instantly, the starting current may reach 3 to 5 times the rated value; at this time, the cell pack needs to have the ability to withstand 2C or even higher rate pulse discharge for 10 seconds. To guarantee 12 hours of basic electricity use at night, households usually need to configure storage capacity 1.5 to 2 times the average daily nighttime electricity consumption to cope with insufficient charging caused by continuous rainy days.