Selecting a string of ten photovoltaic (PV) modules can be frustrating, because differing characteristics can deflate the overall efficiency of the string. Preferably, individual modules should be able to balance themselves in order to maximize their output, and ideally, owners should be alerted when modules are not performing at peak efficiency due to shadowing and damage. Every module’s output ought to be monitored.

Systems also ought to turn on with first light, rather than waiting for sufficient light to produce a ‘high enough’ turn-on voltage. That way, the modules can also generate some power under heavy cloud cover. These considerations suggest the need for ‘smart modules’.

Improving module efficiency

There are three manufacturers in the United States (Enphase, PetraSolar and Apparent) who have married modules and inverters, in order increase efficiency. These manufacturers have designed ‘micro-inverters’ that fit on an individual module, instead of the end of a string.

Some of the features of ‘micro-inverters’ include:

  • Each micro-inverter automatically matches itself to its respective PV module, ensuring maximum power tracking for that module. This allows for the mixing of different modules together in a string
  • Each micro-inverter comes with internet access, so the state of each module is accessible via a web browser. This allows 24/7 monitoring and alerting
  • The turn-on voltages for micro-inverters are low, so they begin generating power with first light and they stay on longer, until last light. This allows them to work under inordinately low lighting conditions
  • All of the micro-inverters currently being developed in the US have current source output stages, so they don’t have to raise internal voltages to ship power to the grid
  • Each micro-inverter produces actual current right at the solar panel, and ties into the string with supplied cables. This eliminates direct current busbars and end-of-string combiners in large systems, saving considerable installation time and cost.
  • The total cost of micro-inverters for a string of ten modules is often less than the cost of one standard string inverter with direct current cabling.

Benefiting customers and utilities alike

Smart meters measure power factor and the expectation is that, one day, residential customers will also pay for reactive power. When that day comes, home owners with inverters that manipulate the phasing will be happy because they will need it, and they won’t have to buy it from the utility.

The utilities should be happy too, because if solar inverters produced dual power for their local loads, then reactive power would stay off the transmission lines, and that would save the utilities money. They might eventually be willing to pay PV owners for making it.

Utilities produce both types of power – active and reactive – usually in remote regions, far from consumers where fossil fuel for generators is easy to get. They then distribute dual power over long transmission lines.

The challenge for utilities

Unfortunately, remotely generated reactive power is a major cause of brownouts and blackouts. This is because reactive power causes voltage to drop ten times more than active power. Then as reactive power builds up on the transmission lines, current builds up and, if not watched carefully, the lines overload, causing them to fault; otherwise known as a blackout.

Currently, when days become hot and the need for reactive power rises to alarming levels on the transmission lines, utilities pay ancillary operators to generate it locally, using small generators that usually sit on standby alert.

PV solar offers the utilities the opportunity to use locally-generated power, but since active power is the foundation of utilities, locally-produced active power replaces product they want to sell. On the other hand, however, locally-produced reactive power reduces the amount of remotely-produced reactive power.

Dual-power micro-inverters allow utility-scale solar farms to act as true ancillary, dual-power, on-demand generators. This is something they have never been able to do before.

Reactive power is produced without any degradation in the quantity of active power. In other words, it comes for free as part of the micro-inverter inversion process. This means that solar PV owners do not have to buy any more PV modules to produce equal quantities of both kinds of power, because there is no extra energy being generated.