Inverters are the devices usually solid state, which change the array DC output to AC of suitable voltage, frequency, and phase to feed photovoltaically generated power into the power grid or local load. These functional blocks are sometimes referred to as power conditioning. The current can be used in two modes: (1) as an inverter changing DC to AC or (2) as a rectifier changing AC to DC, thus, charging the battery. Solar power inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection. Solar inverters may be classified into three types:
- Standalone inverters: These are used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays.
- Grid-tie inverters: These are designed to shut down automatically upon loss of utility supply for safety reasons.
- Battery backup inverters are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid.
Solar micro-inverter is an inverter designed to operate with a single PV module. Its design allows parallel connection of multiple, independent units in a modular way. Micro-inverter advantages include single panel power optimisation, independent operation of each panel, plug-and-play installation, improved installation and fire safety, minimised costs with system design and stock minimisation. Few industries have introduced central solar inverters in sizes 1 MW, 1.5 MW and 2.0 MW with advanced water-cooled technology and high performance even with temperatures exceeding 50 plus degrees. This product is especially designed to suit harsh, humid and dusty conditions that prevail in the Indian subcontinent.
The standard elements are: a DC (input) module, an inverter module and an AC (output) module. The DC module includes a number of photovoltaic cells that provide a DC input to the inverter. The inverter module uses a number of electronic switches, typically, insulated gate bipolar transistors (IGBTs), to convert the DC input into an AC output.
Inverter modules generate heat and require a cooling mechanism. For low power solar inverters, providing a cooling air flow around heat-producing elements of the inverter module is sufficient. Higher power solar inverters require more sophisticated cooling. 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. For an example, a 1 MW inverter with 98 per cent conversion efficiency is generating about 20 kW of thermal energy. This is enough heat to warm 10 homes.
Improper IGBT design results in lower efficiency with higher heat exhaust. Cooling this heat requires a powerful cooling system. Better thermal management for the switching devices is essential to entering the next era of PV inverter efficiency, beyond 99 per cent. In the world of high rated power inverters, cooling methods need to be considered. Cooling technique includes air cooling, liquid cooling, heat pipes and refrigeration systems.
Air is a good insulator, but a relatively poor transport mechanism for removing heat. In fact, a large percentage of industrial applications move to liquid cooling, refrigeration cooling, or heat pipes at high power levels. In all of these three approaches, a liquid of some form is used to help move the heat away from a hot component. Refrigeration-based solutions are perhaps the least used method and are only used in some specific cases. In general, these have seen minimal adoption. Similar to conventional air-conditioning systems, these approaches use a known refrigerant (often R-134a) in a two-phase cooling loop. In one part of the system, the heat causes the refrigerant to evaporate, and in another part of the system, the vapourised refrigerant condenses and is then pumped back into the system in a continuous fashion. In some cases, the internal pressure is raised very high (as much as 200 psi) and a compressor is, therefore, not needed to condense the refrigerant, but a pump is still needed for coolant circulation (Chris Thompson Eaton, 2016).
Liquid cooled solar inverters provide a cooling liquid to a liquid inlet of the solar inverter. The cooling liquid is directed around heat producing parts of the inverter and the liquid is heated, thereby, extracting heat from the inverter circuitry. A liquid outlet of the solar inverter is used to remove the heated liquid from the solar inverter. Typically, the cooling liquid provided to solar inverters is part of a larger cooling system used for many purposes. For high power solar inverters, such a mechanism is inadequate to remove the heat generated. Moreover, the integration of such a solar inverter into a cooling system on-site is a skilled task and makes the installation and maintenance of such solar inverters expensive.
Recently developed inverters
The recently-introduced PVS980 1500 VDC outdoor central inverter by ABB is optimised for large multi-megawatt solar power plants. 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 fill able liquids, pumps, valves, inhibitors and thus no leaks. All this makes the PVS980 suitable for any outdoor utility-scale PV plant.
GE Power Conversion is introducing Silicon Carbide (SiC) technology into its next-generation 1500V PV inverter product line, bringing increased power conversion efficiency to the PV industry. The LV5+ solar inverter is the first multi-MW, utility scale inverter based completely on SiC technology and has an efficiency rating of 99 per cent weighted EU.
Floating solar systems are gaining rapid interest across the globe and recent projects that have been in the 20MW and 40MW range are moving to the 150MW size in China, according to major PV inverter manufacturer, Sungrow in 2017. The new Samsung S-Inverter Air Conditioner series is engineered to consume less power and produce exceptional cooling comfort in 2017. The digital inverter compressor optimises usage by minimal wear-and- tear and extends the life of a refrigerator by over ten years.
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. While both low-pressure liquid cooling and high-pressure two-phase refrigeration cooling have strong performance in heat removal rate and compactness, refrigerant-based approaches have a number of documented drawbacks when used in power conversion. Due to these drawbacks, this technology has seen negligible adoption in established power conversion markets such as UPS, motor drives, or wind and solar inverters. In particular, component availability, maintenance, and toxicity pose significant challenges. Multiple EPA proposals could also have a dramatic impact on the usability of R-134a in the future. When selecting power conversion equipment for high power applications, consider not only the cooling performance of the approach but the ability to safely own, maintain, and operate the system for years to come. Some the key players in the water-cooled inverter market include Hitachi, Daikin Industries, Fuji Electric Corporation of America, Mitsubishi Electric Corporation, Olympia Splendid SpA, Dunnair International, KITA Mfg Co, and Blue Box Group S.r.l, Regen Powertech and GE Energy Power Conversion UK Ltd are a few others that make use of these inverters solely as solar inverters.