When it comes to PV systems design, alternating current (AC) modules are becoming increasingly popular in the solar market. AC modules are known to improve energy output, increase reliability and also simplify system design, installation and management.
Advantages of AC solar
AC modules offer a number of distinct advantages over the standard direct current (DC) array configuration.
An AC panel is a solar panel with its own micro-inverter, either pre-installed or attached during installation, which converts the DC power output to AC. The micro-inverters replace the need for the single, centralised inverter currently used in DC solar electric systems. The micro-inverters also synchronise the output with the grid, enabling the modules to be grid-intertied.
Other benefits include:
- AC offers a higher level of system efficiency since DC series strings are not required. Therefore, there is no intrinsic current mismatch loss between modules. Also, each module has its own power point tracker making each module individually optimised.
- Installation is made easier since DC wiring expertise is not necessary. In addition, the costs of installation and design are reduced because DC safety equipment, switches, fuses and conductors are not necessary.
- AC makes installation on challenging surfaces such as curved roofs or cars easier..AC modules reduce installation times because the inverter is pre-attached to the back of the module which removes the need to fix the inverter to the PV railing system.
- AC arrays are less susceptible to damage from nearby lightning strikes thanks to the design of the module integrated power conditioning systems (PCS/MIP). System reliability is enhanced through redundancy - if one MIP fails there are several others that will continue to operate.
- Data acquisition functions can be integrated directly into the MIP. This not only eliminates the need for a lot of external instrumentation and the related compatibility problems, it also simplifies diagnosis of problems in the array. Each MIP can indicate whether the module to which it is connected is behaving normally. Therefore array diagnostics could be reduced to a simple visual inspection or a couple of commands on a computer connected to the communications bus. In a DC array, particularly a very large one, diagnosis of array problems is requires a large number of electrical measurements directly on the array or some other specialized technique.
- Ease of installation and incrementability may increase interest in what is expected to be the biggest potential market/application for PV, which is building-integrated PV systems.
Disadvantages of AC solar
There are also some significant disadvantages to AC arrays which should be taken into account.
Because MIPs are mounted directly on the back of AC arrays, they are exposed to the environment, which can shorten component lifetimes.
Also, there could be interaction issues if a large array of these modules is put together and there are many MIPs on a common bus.
It must also be taken into account that the small micro-inverters are not as efficient as the larger types. This could mitigate part of the overall system efficiency gain due to elimination of mismatch loss.
Finally, the biggest concern around AC solar is that of lifecycle cost. To be financially viable, the micro-inverters have to be inexpensive especially since one is needed on the back of every PV module.
AC solar market growth
In the end, the idea around PV power is to reduce its cost in order to encourage more installations so it makes sense that some micro-inverter makers are partnering with module suppliers to produce these devices.
Navigant Research projects that AC modules will reach over 91GW of installed capacity by 2025, while DC systems will reach almost 132GW - but both are set to be outpaced by next generation P-type passivated emitter rear cell (PERC) and N-type (negative-type) semiconductor technologies.
According to Navigant, DC and PERC modules are well suited to regions where price competition is fierce and the installation value chain is already efficient, whereas AC and N-type modules are better suited for markets where the installed costs per watt are higher.