Protecting the Inverter Enables Efficiency, Longevity, and Reliability

A number of environmental and situational factors can negatively affect an outdoor inverter’s efficiency, and unfortunately, there is no one-size-fits-all solution for inverter protection. However, through collaboration in the design process with the manufacturer(s) of the inverter and the inverter enclosure, the solar farm operator will be able to ensure the best solution in four areas that are keys to efficiency: airflow and cooling, environmental protection, operations and maintenance, and electromagnetic interference.

Airflow

Electronics run most efficiently within a predetermined temperature range, and if not operated within that range, the performance of those electronics will suffer, potentially resulting in a full shutdown and a shortened lifespan of components. This is particularly true for inverters, which generate a significant amount of waste heat and are often subject to harsh environments.

Figure 1. Relative orifice flow path solar inverter: ABS area vs. transition point in pipe flow analysis. This pipe has good air flow.

Regulating the temperature of an inverter is less about cooling and more about airflow than one might think. The key to keeping an inverter at the proper temperature is moving the waste heat away from the electronics, rather than blowing cold air at them. Determining the exact amount of airflow in cubic feet per minute (CFM) that will be necessary to remove that waste heat from the inverter enclosure can be a challenge, and is entirely dependent upon the size of inverter and the ambient temperature range of the region.

Factoring in the size of the inverter and the waste heat it will generate, as well as the ambient temperature range, engineers can then choose a blower that is rated to move air at a sufficient CFM rate. All blowers have an established CFM rate; however, those labels are often misleading, as the manufacturer of the blower has no idea how and where the blower will be used. For this reason, it’s important that a standard pipe flow analysis of the inverter enclosure be completed in order to account for potential losses in airflow caused by turns in the airflow path.

Figure 2. Relative orifice flow path solar inverter: ABS area vs. transition point in pipe flow analysis. This pipe has poor air flow.

Using data loggers and anemometers, an engineer can gather airflow speeds throughout the enclosure. Because the location of the air intake and exhaust may differ from installation to installation, potential losses in airflow should be analyzed for each unique application. Doing so will help determine the appropriate size of blower. A measure of true airflow can be taken at the exhaust, rather than simply taking the blower’s CFM rating.

While blowers pushing air through the enclosure can ensure waste heat is removed from the overall structure, some critical components need more direct attention. Printed circuit boards (PCBs) and other critical electronic components are extremely sensitive to thermal cycling and require a targeted approach in a sealed and controlled environment. Housed in a separate and sealed cabinet, the PCBs should be regulated with heat exchangers that push heat out into the main cabinet, which is then removed with the blowers. This extra heat being exhausted into the main cabinet should be taken into consideration when initial pipe flow analysis takes place.

Figures 1 and 2 compare results of a pipe-flow analysis of an inverter with good airflow and an inverter with poor airflow. The red and blue lines in each graph show a changing cross-section area of the flow orifices. Ideally, these should be bell-shaped, like a rocket nozzle, as demonstrated in Fig. 1. Here, the restrictions are lessened as the flow proceeds. However, in Fig. 2, the flow lines are squeezed into an hour-glass shape. Points labeled one through seven note orifices through which air must flow:

1. Inlet: Start of the flow.
2. Fan Housing: A flow restriction that will increase velocity and reduce static pressure (as a result of losses).
3. Heat Exchanger: Here transition will slow velocity and cause turbulence, effectively changing the Reynolds number in flow calculations.
4. Bend (45° angle): In Fig. 1, this is the continuation of expansion and, therefore, small negative effects.
5. Internal Passageway: In Fig. 1, this is the largest area in the flow and thus the slowest velocity and circulation. Dust and condensation may form here, but engineers must also consider this area for heat transfer.
6. Capacitor Housing: Flow is reducing.
7. Exhaust: Flow should be maximized at the exhaust as much as possible, as this ultimately determines the CFM of the system. In Fig. 2, the necked-down location at point 5 is driving the system and restricting flow.

Environmental Protection

In addition to temperature, environmental factors such as dust, insects, rain, sleet, snow and humidity can damage the inverter or impede its efficiency, which is why the materials used on the enclosure are critical to ensuring the longevity and functionality of the inverter.

The National Electrical Manufacturers Association (NEMA) issues ratings of enclosures based on their ability to protect from these environmental factors. At minimum, the exterior enclosure should be rated as a NEMA type 3R. This is the rating given to most outdoor air conditioning units, indicating that it is rainproof and water will not come into contact with any live wires. This is a sufficient solution for warm and/or mild conditions that don’t experience prolonged freezing or any major concerns in terms of corrosion. Harsher conditions often require a higher level of protection from extreme environmental factors.