Development Trend & Challenges of String Inverters—No-external Fan Design

With the depletion of coal and petroleum resources as well as the increasing importance of environmental protection throughout the world, grid-tied power generation technologies of renewable resources, including solar power, and the application of these technologies, are becoming a global focus. It is known that photovoltaic (PV) inverters are the core equipment of the solar power generation system, so the reliability of the inverters determines the reliability of the PV system. The heat dissipation performance is the key factor influencing the reliability of PV inverters. The power switch, a core component of inverters, is sensitive to temperature, that is, every change of temperatures impacts on the connection and disconnection process. A high temperature leads to poor performance of the power switch or even a broken switch. Therefore, the scheme of inverter heat dissipation is a key factor affecting the performance and quality of the inverters.

In recent years, ground power plants around the world have widely adopted the string inverter solutions. Comparing with power plants with centralized inverter solutions, the string inverter plants have obvious advantages: better electricity yield, smaller footprint, no equipment room, more reliable operation, and simpler operation and maintenance (O&M). This is especially true for projects on rooftops or in hilly regions.

PV power plants are generally situated in deserts and highlands, where the sunlight is abundant. However, these areas also have cold winters, hot summers, strong sandstorm and high altitudes. For those that are close to the sea, the corrosion is severe. In these application scenarios, string inverters, either on walls, or PV supports, or independent installation racks, are directly exposed to elements, so the exterior components are deteriorating because of the rain and dust corrosion. That is why the prime concern of inverters becomes how to strike the right balance between adaptation to harsh environments and heat dissipation.

1. Common Heat Dissipation Mode and Relevant Problems in String Inverters

In general, inverters dissipate heat through natural or air cooling. Manufacturers also add external fans to string inverters to increase the heat dissipation capability. But string inverters are normally used in harsh environments, so this poses higher requirements on the protection performance of external fans. Currently, the protection level of outdoor fans only averages IP54 or IP55. As a result, the protection level of the whole system cannot reach IP65. To protect the fans from rains, the design of air channels becomes more complicated and the form is limited. Once the fans fail to work, the heat dissipation capability is badly impaired, and the output power of inverters derates, and the electricity yield decreases, greatly affecting the benefits of customers. What′s worse, the lifecycle of fans is tremendously shortened because the fans are exposed to rain, dust, and other corrosives for years. So, fans need to be replaced many times within the lifecycle of the inverter, resulting in high O&M cost.

    Following is a figure of a fan of the inverter in an outdoor power plant. We can see from the figure that after the inverter has been running for a year, the fan is dusty and badly corroded.

2. Solutions to Heat Dissipation Problems of String Inverters

At present, no-external fan design is the advanced solution. Without an external fan, the protection level can reach IP65. In addition, the noise is reduced, the reliability improved, the power deration eliminated, the maintenance simplified, and the cost is lowered.

As the output power of the inverter increases and its size decreases, heat dissipation becomes a major bottleneck. As for this problem, intensified means, including copper and aluminum composite, heat pipes, temperature equalization plate, may be employed to reduce thermal resistance and improve the dissipation capability of radiators. Another important method is to add a spoiler fan in the cavity to cool the hot spots inside and improve the heat dissipation capability. As the spoiler fan is inside the cavity, it does not need protection. Even if it fails to work, it will do little harm to the overall heat dissipation and power. For the effects of Huawei heat dissipation design.

By adopting the mentioned heat dissipation concept and employing the heat equalization design, heat isolation and heat shield technologies, Huawei SUN2000 string inverters realize the no-external fan design, IP65 protection, and high reliability. Whether it is in the highlands of the northwest or in the coastal regions of Hainan, the Huawei string inverter withstands the tests of sandstorms and corrosion with its high reliability, effective protection, sound operation, and quality performance.

Summary

    From Table 1, we can see that string inverters without fans have obvious advantages over string inverters with fans, no matter in protection level, reliability, or anti-corrosion.

Table 1 Heat dissipation comparison between inverters with and without fans

String inverters, more often than not, work in outdoor environments of high temperature, dust, wind, sunshine and rain. If the fan-cooling mode is used, fans are prone to damage and need to be replaced frequently. This not only increases the cost of maintenance dramatically, but also affects the electricity yield. Besides, if the inverter is installed on the walls or the attic of the customers, the noise of the fan may invite complaints from neighbors. However, the natural cooling product, as which has no fan, operates longer and more reliable and produces lower noises. So, the no-external fan design is the core selling point for small outdoor power inverters.

With the widening application of string inverters, excellent energy yield, convenient maintenance and even free of maintenance are becoming the main interests of customers. The external fans of inverters are a barrier to that end. So to speak, the no-external fan design of string inverters is a challenge as well as a development trend in the future.