From High Current to Low Current: Why Choosing Low Current Modules is Wiser

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Table of Contents

1. Introduction

2. The Emergence of High Current Photovoltaic Modules

3. Risks and Losses of High Current Photovoltaic Modules

4. Advantages of Low Current Photovoltaic Modules

5. Conclusion

 

Introduction

As photovoltaic technology continues to advance, the photovoltaic module market has evolved from high current to low current. High current photovoltaic modules have garnered attention for their high power output, but the associated risks and losses cannot be ignored. In contrast, low current photovoltaic modules are increasingly seen as a wiser choice due to their advantages in safety, efficiency, and compatibility. This article will analyze the risks and losses of high current photovoltaic modules and explore the unique benefits of low current photovoltaic modules.

 

The Emergence of High Current Photovoltaic Modules

Levelized Cost of Energy (LCOE) is a core metric for assessing photovoltaic projects. On the module side, efficiency, power, and generation capacity play critical roles, and improving the power and efficiency of photovoltaic modules can effectively reduce LCOE. As early as 2009, the maximum power of photovoltaic modules in the industry was only 290W. After more than a decade of development, the power of photovoltaic modules has risen to over 500W, with some exceeding 600W. The main paths to improving module power include advancements in cell technology that enhance conversion efficiency, optimization of module layout and auxiliary materials, and increasing wafer sizes. Initially, mass-produced solar cells were based on 125mm wafers, which later evolved to 156mm, 156.75mm, 158.75mm, 166mm, and now to 182mm and 210mm. The advent of 182mm and 210mm large-size wafers in 2020 not only brought a significant increase in module power but also markedly increased the operating current of photovoltaic modules.

Generally, the rationale behind increasing wafer size includes two main points: firstly, it can effectively reduce the cost per watt of wafers and solar cells, thereby lowering the production cost of photovoltaic modules; secondly, increasing wafer size can enhance module power, thereby reducing the Balance of System (BOS) cost. However, any gains are within a certain range; when the cell size and current increase to a certain extent, the associated risks, hazards, and losses may outweigh the benefits.

 

Risks and Losses of High Current Photovoltaic Modules

1. Production and Quality Risks of High Current Photovoltaic Modules

In the production process, as the cell size increases, the product yield tends to decrease due to increased manufacturing difficulty. The yield of large-size wafers and cells in the initial production stages may not reach the level of original products, and some issues caused by size increase may not be perfectly resolved as the process matures. Additionally, oversized wafers can hinder the development of thinner cells, and the increased size of photovoltaic modules can impede cost reduction in frames and glass, impacting production costs. Furthermore, the increase in wafer and module size also increases mechanical load risks, making transportation and installation more challenging and placing higher demands on the support structures, affecting the quality throughout the product and system lifecycle.

2. Impact of High Current Photovoltaic Modules on Power Generation

(1) Cable Line Loss

Based on a 100MW project, we compared the line loss of 182mm photovoltaic modules (operating current around 13A) and ultra-high current photovoltaic modules (operating current around 18A). Under standard test conditions (STC), using the same 4mm² cable specification, the ultra-high current photovoltaic module scheme had about 0.2% higher DC side line loss compared to the 182mm module scheme. Even assuming the actual application environment irradiance is 70% of STC conditions, there is still a line loss difference of about 0.14%. In systems using bifacial photovoltaic modules, the current increase of bifacial modules compared to monofacial modules can be 10%-20%, further amplifying the line loss difference.

Cable Line Loss

(2)Module thermal power Loss

We also conducted related research and calculations on the thermal power loss of photovoltaic modules: the thermal power loss proportion of ultra-high current photovoltaic modules is 0.53% higher than that of 182mm photovoltaic modules. For a 3GW scale project, due to direct thermal power loss, ultra-high current photovoltaic modules will generate 20 million kWh less per year than 182mm photovoltaic modules.

Module thermal power Loss

(3)Power Generation and LCOE Calculation

Simulation results show that the power generation of 182mm photovoltaic modules is 1.8% higher than that of ultra-high current modules, at 1.862 kWh/Wp/year. In terms of LCOE, the 182mm photovoltaic modules are 0.03-0.05 Yuan/kWh lower than the ultra-high current modules, at 0.19 yuan/kWh.

Power Generation and LCOE Calculation

(4)Empirical Analysis of Ultra-High Current Photovoltaic Modules

To fully study the power generation performance and operating temperature differences of different photovoltaic modules, a leading brand, in collaboration with TÜV Nord, conducted an outdoor empirical project at the National Photovoltaic Experimental Base in Yinchuan in February 2021. Empirical data showed that under high irradiance weather, due to more energy being converted into heat on the ribbons, the operating temperature of ultra-high current photovoltaic modules was on average 1.8°C higher than that of 182mm photovoltaic modules, with a maximum temperature difference of about 5°C. This is mainly because the high operating current of photovoltaic modules leads to significant thermal losses on the cell surface metal electrodes and ribbons, increasing the module's operating temperature. As is well known, the output power of photovoltaic modules decreases with increasing temperature, with power decreasing by about 0.35% for every 1°C increase in temperature; combined with multiple factors, empirical data shows that the single-watt power generation of 182mm photovoltaic modules is about 1.8% higher than that of ultra-high current modules.

The working temperature of the modules on 4th May
The working temperature of the modules on 4th May
Power Generation Comparison

3. Electrical Safety Risks of High Current Photovoltaic Modules

Photovoltaic modules are electrical devices that encapsulate solar cells with glass, backsheet, EVA, or POE, and then transmit the generated DC electricity through junction boxes, cables, and connectors. For the entire photovoltaic module, junction boxes and connectors, though inconspicuous small components, can cause significant safety hazards if they fail.

(1)Junction Box Heating Risk

According to statistics from authoritative third-party organizations, power station failures (especially fires) caused by photovoltaic modules are mostly related to junction boxes and connectors. Therefore, the junction box is a critical technical point in module design, especially for high current photovoltaic modules, where the current-carrying capacity of the diodes in the junction box is crucial. The following image shows the situation where junction box heating caused connector burning.

Junction Box Heating Risk

To ensure the current-carrying capacity of the diodes in the junction box, for monofacial photovoltaic modules, it is recommended that the rated current of the junction box should be greater than 1.25 times the short-circuit current (Isc). For bifacial photovoltaic modules, a 30% bifacial gain and about 70% rear-side ratio should also be considered. The 182mm bifacial photovoltaic modules use mature 25A rated current junction boxes on the market, maintaining about 16% safety margin, ensuring the long-term reliability of high current photovoltaic modules. Larger current modules require higher-rated current junction boxes (30A). However, even with 30A junction boxes, the safety margin of ultra-high current photovoltaic modules is relatively low, and the risk of overload increases significantly under high irradiance and high-temperature conditions.

Overload Risk Comparison

(2)Cable Heating Risk

Based on the IEC 62930 standard, we conducted current-carrying capacity research and calculations on photovoltaic cables. In general ground-mounted or distributed rooftop power plants, 4 mm² cables can meet the application needs of 182mm photovoltaic modules and ultra-high current photovoltaic modules. However, when some distributed rooftops reach temperatures of 70°C, if ultra-high current photovoltaic modules do not use more expensive 6mm² photovoltaic cables, the cables may overheat and burn, increasing the risk of fire.

 

Advantages of Low Current Photovoltaic Modules

In the face of the various risks and losses of high current photovoltaic modules, low current photovoltaic modules exhibit unique advantages. These advantages are making them increasingly dominant in the market, especially in applications where system reliability and long-term benefits are paramount.

1. Higher Electrical Safety