Rochester, MN, USA — One of the most important components for delivering solar power to the grid is the electrical inverter. The sun could be shining at optimum levels, but if the inverter is not converting that power to its full potential, the cost of that lost efficiency will be passed on to the end user.
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.
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:
- Inlet: Start of the flow.
- Fan Housing: A flow restriction that will increase velocity and reduce static pressure (as a result of losses).
- Heat Exchanger: Here transition will slow velocity and cause turbulence, effectively changing the Reynolds number in flow calculations.
- Bend (45° angle): In Fig. 1, this is the continuation of expansion and, therefore, small negative effects.
- 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.
- Capacitor Housing: Flow is reducing.
- 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.
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.
For the solar farms that are located in states like Colorado, excessive freezing and snow may pose a major concern. A NEMA type 3S enclosure provides additional protection, ensuring that internal components continue functioning when the outside of the enclosure is covered in ice. Additionally, the structural integrity of the enclosure must be sufficient to support the weight of ice and snow.
Both NEMA type 3R and 3S enclosures do, however, allow moisture into the cabinet. By the very nature of the cooling and airflow design, a cold-plate atmosphere is created, which means condensation will form within the main cabinet. It’s important to understand where the condensation will form and where it will drip, and ensure there are drain-holes below that point and on either side, as level mounting during installation isn’t always guaranteed.
The critical electronic components of an inverter (such as the PCB and wiring), should be housed in a separate and sealed enclosure within the main cabinet. Moisture will cause oxidation of wiring and dust can impede connections. As such, a NEMA type 4 rating is recommended, as it indicates the enclosure is watertight and dust-tight. In environments where corrosive agents are present (such as saltwater in the air), a NEMA type 4X enclosure would be recommended for the PCB and wiring to provide additional corrosion protection.
Though it would be nice to be able to use a watertight and dust-tight enclosure for the main cabinet, because of the necessity for airflow, choosing a NEMA type 4 or 4X for the external enclosure is not an option, which is why choosing the right materials and finishes is important.
Enclosures are typically made from steel or aluminum. Steel is the cost-conscious choice, but aluminum is lighter and provides better corrosive protection. There are, however some finishes that can be applied to steel to provide additional protection. Of those finishes, full-immersion primers are generally more effective than spray-on primers. Epoxy electrocoat (e-coat) is an example of a common and effective full-immersion primer.
Operations and Maintenance
Important to an inverter’s longevity is its ability to be easily maintained. Concerns about accessibility and ease of use should be addressed during the design and installation phases. The end user should be cognizant of the type of work that will go into regular maintenance and ensure the enclosure is designed in a way that makes replaceable parts such as filters and fuses easily accessible.
Today however, much of an inverter’s inspection is done remotely, through wireless monitoring. A wireless monitor can feed information to the solar farm operator on everything from temperature, airflow analysis, power input and power output. Sensors are often built directly into the PCB to shut the inverter off if it reaches temperatures that are too high. Most solar farm operators would agree, however, it’s preferable to catch a problem before downtime occurs.
Additionally, without the proper airflow, harsh temperatures can cause irreparable damage to internal components of the inverter, which is why wireless monitoring is necessary to avoid any unplanned downtime. However, placing the wireless transmitter in an enclosure can severely impede its ability to relay vital performance information to the solar farm operator, which may require the transmitter to be shielded from internal noise, but in an unshielded portion of the enclosure for clear transmission.
Inverters emit EMI that can disturb transmission frequencies in the immediate area, including radio and television reception. As such, the Federal Communications Commission (FCC) has put forth regulations that require enclosures to shield the EMI generated by inverters (and various other electronics) to certain attenuation levels. This poses a problem for wireless transmitters, as anything within the enclosure is shielded from emitting frequencies. This is an important consideration to be made in the design phase, which further exemplifies the importance of collaboration between the end user and the manufacturer(s) of the inverter and enclosure.
The inverter is what converts generated energy into deliverable power. If not properly protected from harsh temperatures and environmental factors, the inverter will undoubtedly suffer in performance, passing the cost of that lost efficiency on to the end customer. Because every application is unique, there is no one-size-fits-all solution to inverter protection. Solar farm operators should be cognizant of this fact when selecting a solution, and know what questions to ask the manufacturer in relation to airflow/cooling, environmental protection, operations and maintenance concerns, and EMI shielding. Doing so will ensure their inverters will continue to deliver power efficiently over an increased lifespan.
Steven Leidig received his degree in aerospace engineering from the U. of Colorado, Boulder and is the manager of enclosure engineering at Crenlo, 1600 4th Avenue N.W., Rochester, MN 55901 USA; ph.: 507-287-3535; email [email protected]