How do diodes prevent solar cells from discharging at night?

1, Technical principle: Unidirectional conductivity construction of energy barrier
The core characteristic of a diode is its unidirectional conductivity - it only allows current to flow from the anode (A) to the cathode (K), and exhibits a high resistance state under reverse voltage. This characteristic forms two key protections in solar energy systems:
Anti reverse charging (blocking) mechanism
When the voltage of the solar panel at night is lower than that of the battery, without protective measures, the current will form a circuit from the battery through the panel, resulting in energy loss of the battery. The blocking diode (such as Schottky diode) connected in series between the solar panel and the battery cuts off under reverse voltage, blocking the reverse flow of current. For example, in a certain 12V solar energy system, SS14 Schottky diodes (withstand voltage 40V, rated current 1A) are used, with a forward voltage drop of only 0.2V and a reverse leakage current<0.1 μ A, which can effectively prevent the backflow of 0.5A level current at night.
Hot spot effect protection
In partially occluded scenes, the obscured battery cells become loads due to their inability to generate electricity, and the high voltage of other normal cells may cause them to reverse breakdown, resulting in localized high temperatures (hot spots). The bypass diode (such as 1N4007 rectifier diode) connected in parallel at both ends of the battery module conducts under reverse bias to short-circuit the faulty component and avoid hot spot effect. The test data of a certain type of 60 cell battery module shows that when the bypass diode is not installed, local obstruction causes the temperature of the module to rise to 85 ℃, while after installation, the temperature is controlled within 45 ℃.
2, Application scenario: Full coverage from independent systems to grid connected architectures
The anti discharge function of diodes runs through the full scenario application of solar energy systems:
Independent photovoltaic system
In independent systems such as power supply and solar street lights in remote areas, blocking diodes are the last line of defense to prevent battery over discharge. For example, a solar street light project in Qinghai Province uses 18650 lithium battery packs (nominal voltage 3.7V), and the charging controller integrates SS16 Schottky diodes. In seven consecutive days of rainy weather, the battery voltage only drops by 0.3V, and the system's continuous power supply time is extended by 40%.
grid-connected photovoltaic system
In large ground power stations, blocking diodes and anti backflow devices work together to prevent current from flowing back into the photovoltaic array during grid failures. A 50MW photovoltaic power station in Germany adopts modular design, with 20 modules connected in series and parallel blocking diodes. When the power grid is cut off, the system response time is less than 10ms, effectively avoiding equipment damage.
Mobile energy system
In dynamic scenarios such as solar powered drones and vehicle mounted photovoltaics, diodes need to adapt to harsh environments such as vibration and temperature fluctuations. The NASA Perseverance rover uses radiation hardened diodes, with a reverse recovery time of less than 50ns in the temperature range of -120 ℃ to+80 ℃, ensuring energy management stability during the Martian day night cycle.
3, Device selection: the art of balancing performance and cost
The selection of diodes requires comprehensive consideration of electrical parameters, environmental adaptability, and cost-effectiveness:
Optimization of forward pressure drop (Vf)
The voltage drop of blocking diodes directly affects system efficiency. Taking a 100W photovoltaic system as an example, when using ordinary silicon diodes (Vf=0.7V), the daily loss reaches 0.7Wh; after switching to Schottky diodes (Vf=0.3V), the loss drops to 0.3Wh, saving approximately 10.95kWh of electricity annually. Currently, silicon carbide (SiC) Schottky diodes (Vf=0.15V) have entered the commercial stage, but their cost is 3-5 times that of silicon-based devices, making them suitable for high-value scenarios.
Redundant design of reverse withstand voltage (Vr) and current capacity (If)
The fluctuation range of the operating voltage of photovoltaic systems is usually ± 20%, and diodes with Vr ≥ 1.5 times the highest voltage of the system need to be selected. For example, a 24V system should use devices with Vr ≥ 40V. In terms of current capacity, blocking diodes need to withstand 1.2-1.5 times the short-circuit current, while bypass diodes need to match 1.1 times the maximum output current of the component.
Temperature characteristic compensation
The parameters of the diode vary significantly with temperature. Taking the 1N5819 Schottky diode as an example, Vf=0.3V at 25 ℃ and rises to 0.5V at -40 ℃, which makes it difficult to start at low temperatures. The solution includes: using a temperature compensation circuit to dynamically adjust the bias voltage, or selecting devices with optimized low-temperature characteristics (such as STPS20L45CT).
4, Industry Practice: From Standard Configuration to Intelligent Upgrade
The global photovoltaic industry has formed a standardized solution for diode applications and continues to evolve towards intelligence:
The trend of integrated design
Modern photovoltaic modules generally have built-in bypass diodes, with a typical configuration of one diode connected in parallel every 18-24 battery cells. The latest Hi-MO 6 module from Longi Green Energy adopts a 6-unit split design, integrating 3 bypass diodes, reducing power loss from 15% to less than 5% under shadow shading.
Innovation of Intelligent Controllers
The new generation MPPT controller integrates programmable diode analog circuit and achieves zero voltage drop and anti reverse charging through MOSFET. After adopting this technology, the Huawei SUN2000 series inverters improved system efficiency by 1.2% and increased annual power generation by approximately 140 kWh/kW.
Breakthrough in new materials
The thermal radiation diode developed by the University of New South Wales in Australia achieves nighttime energy recovery through temperature difference power generation. Test data shows that under a temperature difference of 20 ℃, the output power of a single tube reaches 64nW/cm ², providing technical reserves for future all-weather photovoltaic systems.