By Norman Quesnel, Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.
(This article will be featured in an upcoming issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)
Traveling across the U.S. one will find the widening use of renewable energy systems, including large scale solar and wind farms. The power from these installations is clean, emission-free and relatively low cost. Their own energy sources are free and endless: the wind and the sun.
Wind energy has become a cost-effective power source, competing with installations of coal, gas and nuclear power. So too has solar power, which is the focus of this article.
Solar power is doing more than ever to help meet energy demands for local power and for feeding power back to the electric grid. Today’s U.S. solar installations exceed 3,100 megawatts, enough to power more than 630,000 homes. The price of solar panels has dropped by nearly a third since 2010 and costs continue to fall. Manufacturers in the U.S. are also exporting billions of dollars worth of solar products. 
Most thermal management issues in solar power systems occur with their inverter systems. Here, the solar-generated DC power is converted to AC for power grids or local use. While these inverter systems can be very efficient, some excess heat must be managed so it doesn’t affect the inverter’s life or performance.
Cooling Solar Energy Inverters
Some solar power systems produce steam to spin turbines and generate electricity. But the more common solar systems are photovoltaic (PV) solar power facilities. In these, solar panels absorb and convert sunlight into electricity with the use of inverters. One or more solar inverters, or PV inverters, converts the sun-sourced, variable DC output of the PV panels into alternating current, AC. This is then fed into a commercial electrical grid or used by a local, off-grid electrical network. AC is the standard used by all commercial appliances, which is why many view inverters as the gateway between the photovoltaic (PV) system and the energy off-taker. 
Inverters are standard in PV solar systems whether they’re kW range residential systems to MW sized power plants. Larger installations may use a central inverter or a series of string inverters.
In a central inverter set up, the DC power from multiple solar panel arrays runs to combiner boxes and then to the inverter which converts it to AC. In a string inverter scheme, there are smaller, individual inverters for several panel arrays. The DC power runs directly into a string inverter rather than a combiner box and is converted to AC. While string inverters are used in residential to medium-sized commercial PV systems, central inverters are common to large commercial and utility-scale sites. 
There are also solar microinverters that convert the DC generated by a single solar module to AC. The output from several microinverters is combined and often fed to the electrical grid. Microinverters are an alternative to conventional string and central solar inverters, which are connected to multiple solar modules or panels of the PV system. The main advantage of microinverters is that small amounts of shading, debris or snow on any one solar module, or even a complete module failure, do not disproportionately reduce the output of the entire array. 
Whatever its configuration, the PV inverter determines the amount of AC watts that can be distributed for use, e.g. to a power grid. For example, a PV system comprising 11 kilowatts DC (kWDC) worth of PV modules, connected to a 10-kilowatt AC (kWAC) inverter, will be limited to the inverter’s maximum output of 10 kW. 
There is also some power loss in the DC-AC conversion process. At the MW scale this could significantly impact a plant’s capacity (and revenue). But fortunately, inverter technologies have been advancing and expanding. The efficiency of state-of-the-art converters is more than 98 percent.
In addition to converting DC to AC, today’s inverters provide other services to help ensure their systems operate at optimal performance level. These include data monitoring, advanced utility controls, applications and system design engineering. Some inverters provide maximum power point tracking (to maximize power extraction), and anti-islanding protection (automatic shutdown). 
Cooling PV Solar Inverters
All inverters generate excess heat, especially utility-scale central inverters. Solar inverters used in the kW range are typically contained in finned metal housings that provide cooling via natural convection. Large-scale PV inverters are typically between 1 and 2 MW and the heat they generate directly correlates with their conversion efficiency. For an example, a 1 MW inverter with 98 percent conversion efficiency is generating about 20 kW of thermal energy. This is enough heat to comfortably warm 10 homes. 
Cooling solutions are typically needed inside inverters to protect their IGBT (insulated-gate bipolar transistors) modules. These solid state power semiconductor devices are electronic switches and consist of many devices in parallel. The design of the IGBTs and their cooling systems are among the most important aspects in protecting inverters and improving their conversion efficiency.
Improper IGBT design results in lower efficiency with higher heat exhaust. Cooling this heat requires a more complex and powerful cooling system. Better thermal management for the switching devices is essential to entering the next era of PV inverter efficiency, beyond 99 percent.
The recently-introduced PVS980 1500 VDC outdoor central inverter by ABB is optimized for large multi-megawatt solar power plants. The PVS980 features a self-contained cooling system to ensure outstanding endurance in tough environments with minimal maintenance. The cooling system uses phase transition and thermosiphon technology to prevent external air from entering the critical compartments of the inverter. This reduces the risk of corrosive gases or sand entering the inverter and causing damage. 
The PVS980 inverter can operate from below freezing to extreme heat in 100 percent humidity without jeopardizing functionality. With the simplicity of air cooling and with the power density of a liquid cooled inverter, ABB’s inverter has very high total efficiency and low maintenance. There are no fillable liquids, pumps, valves, inhibitors and thus no leaks. All this makes the PVS980 suitable for any outdoor utility-scale PV plant.
Some of the recent advancement in the inverter cooling system, such as an advanced hybrid cooling solution, requires significantly less air-flow in the system without an auxiliary fan power load. This lower load condition allows the inverter to further increase conversion efficiency. 
Parker provides utility scale inverters with two-phase refrigerant cooling systems. The have a high efficiency design that integrates proven insulated gate bipolar transistor power conversion and magnetics with Parker cooling technology. No air conditioner is required. Power semiconductors, inductor, and internal ambient are all cooled by the integral two-phase system. Multiple access panels simplify installation and scheduled maintenance. 
The small footprint and high reliability of Parker’s outdoor central solar inverter is made possible by an advanced cooling system that uses a non-conductive, non-corrosive liquid to cool critical components. The refrigerant requires only 13 percent of the flow rate of an equivalent water/glycol based system. The cooling system runs efficiently by capitalizing on the tremendous amount of heat that is transferred as the refrigerant vaporizes, then releasing the heat through a condenser. No compressor is needed. Redundant system components allow inverter operation even after loss of a pump or a fan
Compared to air cooling, with Parker’s solar inverter design IGBT temperatures are kept more constant over time. Advanced cooling is used on both the IGBT devices and the high efficiency inductors, as well as with a unique cool door feature that circulates temperature controlled air inside the sealed enclosure. Heat from the coolant loop is removed by an isolated heat exchanger, with no air exchange from the enclosure interior to the outside environment. Heat exchanger fans are variable speed for maximum efficiency. They are designed for redundancy and are monitored for rotation. In the event of a fan or coolant pump malfunction, the inverter will continue to operate, folding back power if necessary. The cooling system is designed for a minimum of maintenance, and there are no air filters to change.
Another producer of large scale PV inverters is TMEIC (Toshiba Mitsubishi-Electric Industrial Systems Corporation). Their Samurai inverter series has power ratings up to 2700 kW. Each model has an advanced hybrid cooling system that uses heat pipe technology. The heat pipes allow the system to operate up to 50 percent load without turning on fans. The heat pipe cooling uses fewer parts and a slow speed fan. The fan-less mode runs when the inverter is below 50 percent load at 50°C. Natural convection provides the necessary cooling. Cool air enters from the bottom, flows through the heat pip, and hot air is exhausted from the top. 
A PV solar power system’s current inverter determines the amount of AC watts that can be distributed for use, e.g. to a power grid. For systems operating in the megawatt output range, the inverters will require some level of thermal management to cool their IGBT systems. Many of these large inverter systems have custom cooling solutions that can differ from each other (e.g. air cooling vs. liquid cooling) but all methods have their origins in cooling electronics other than those found in the solar power industry.