The solar benefits of micro-inverters

Micro-inverters could replace string inverters in solar installations, according to Sulaiman Ahmad, VP Sales UK, Enecsys Limited

Four key factors determine the return-on-investment for solar PV systems: total capital costs, the amount of power harvested from the system over its lifetime, maintenance costs and feed-in tariffs. For a given feed-in tariff, here’s how systems that use micro-inverters stack up against traditional string inverter systems.

In conventional PV solar installations solar modules are wired in series then connected to a string inverter that converts high voltage DC from array of PV modules (panels) to AC for connection to the electricity grid. For residential and commercial installations, the inverter will typically be rated between 2kWp (a measure of the peak output of a PV system) and 5kWp. More than one may be used in larger systems.

A different solar PV system architecture based on micro-inverters is now fast gaining popularity. Here, a small inverter is attached to the rack behind each solar module to convert DC to grid-compatible AC (Figure 1). The inverter outputs are wired in parallel and a 7kWp system might use 29 micro-inverters each rated at 240Wp. No string inverter is needed.

The reason for the growing popularity of micro-inverters is simple – for a given solar installation they provide up to 20 per cent improvement in price/performance over the life of the system compared to existing inverters.

With micro-inverters, shading of one module does not degrade the performance of the others and more power can be extracted from each module through an electronic technique called Maximum Power Point Tracking (MPPT).

The output of each module varies with the incident light level. MPPT ensures that the micro-inverter presents the optimum load to the module under these varying conditions in order to extract maximum energy from it at all times.

This technique cannot be applied so effectively in systems with series DC strings, where shading from any source, such as tree branches or even something as small as an antenna or vent pipe, can dramatically reduce the total energy produced by the installation.

On a cost-per-watt basis, micro-inverters cost a little more than string inverters but this is more than offset by greatly simplified installation – no specialist high voltage DC equipment, wiring or installer skills are needed.

There are no risks from lethally high DC voltages, making the systems safer for installers to work on. System design and planning is also greatly simplified as one size micro-inverter can be used for various types and sizes of installations. Fire risk from high voltage DC arcing is eliminated, as is the electrocution risk to firefighters in the event of a fire.

String inverters usually have five year warranties and need replacing at least once or twice during the lifetime of a system, adding cost, eating into profits and upsetting customers. The latest micro-inverters from Enecsys (Figure 2) have removed components with limited life, such as electrolytic capacitors and opto-isolators, to deliver a life expectancy in excess of 25 years, matching that of solar modules.

The reliability claims are backed by HALT, HASS and accelerated life tests to IEC61215, the same methodology used to test solar PV modules. The micro-inverters maintain full performance from -40 degrees C to +85 degrees C, ensuring efficient operation in real-world conditions on commercial and residential rooftops.

A wireless monitoring system provides real time performance information for each solar module in order to maintain the high performance of the solar PV system throughout the operating lifetime.

Enecsys Micro-inverters are designed to operate on both North American (60Hz) and European (50Hz) electricity grid systems. The micro-inverters are safety and EMC evaluated to EN 62109, UL1741, TUV and CE. Country-specific requirements, including VDE V 0126-1: 2006 compliance are achieved through the use of specific Enecsys installation products. The 240W version received UL certification in January 2011, the first micro-inverters without electrolytic capacitors to achieve UL certification..

In short, micro-inverters have the potential to replace string inverters in most domestic and commercial installations over the next few years because they offer numerous meaningful advantages for consumers and installers alike: they are simpler and safer, maximize energy harvest, increase system lifetime and reliability, simplify PV array design and installation, and enhance system monitoring capability.

The Cambridge University spin-out will be present at Ecobuild in March where it will reveal how solar PV systems based on micro-inverters deliver up to 25 per cent more energy, compared to conventional systems with string inverters

The company will also use the event at London’s Excel Centre to announce the launch of a new product with what is claimed to be the world’s highest power density for a single micro-inverter.