What's the big deal about module-integrated PCSs (MIPs)?
Use of AC arrays is currently a hot topic in PV systems design. They offer several distinct advantages over the standard DC array configuration:
- There are no DC interconnections between modules at all. This leads to several distinct
advantages, like
- No DC series strings needed, so no intrinsic current mismatch loss between modules, leading to improved system efficiency
- DC wiring expertise not necessary for installation
- DC safety equipment, switches, fuses, conductors etc. not needed, leading to reduced expense and simplified installation and design
- Each module is individually optimized, since they all have their own maximum power point trackers. This also improves the system efficiency, and means that PV arrays can be mounted on structures that aren't coplanar, like curving roofs or automobiles, without incurring additional mismatch losses.
- MIPs can be standardized and thus mass-produced, meaning that economies of scale will reduce their cost. Industry experts believe that MIP costs could come down to as little as $0.25/watt, compared to a best- case estimate of about $0.50-$0.70 for central PCS.
- AC arrays have less susceptibility to damage from nearby lightning strikes. Electromagnetic induction from lightning strikes near a DC array can have serious effects on the PV system. The mechanism is as follows: the surging currents in a lightning stroke set up a time-varying magnetic field which can induce a voltage in nearby conductor loops (Faraday's Law). PV arrays in the DC configuration contain two conductor loops, one made up of the metal module frames and the other of the solar cells themselves and their metal interconnections. If lightning does not directly hit the array but still strikes nearby, the magnetic field can induce large voltages (up to several kV under certain conditions) at the terminals of the array, which can lead to damage of insulators, fuses, modules, and downstream components such as inverters. MIPs greatly reduce this induced voltage problem by reducing the area of the conductor loop, again by eliminating series connections of modules on the dc side.
- System reliability is increased through redundancy; if one MIP fails, you still have several others, so part of the AC array can continue to operate.
- As previoulsy mentioned, 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, so 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 a major headache, requiring a large number of electrical measurements directly on the array or some type of reflectometry or 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.
There are also some significant disadvantages to AC arrays:
- Since they're mounted directly on the backs of PV modules, MIPs must operate in a very harsh thermal environment, which shortens component lifetimes.
- There may be interaction problems if you build a large array of these things and have many MIPs on a common bus.
- Small inverters are less efficient than larger ones, which mitigates part of the overall system efficiency gain due to elimination of mismatch loss.
- Probably the biggest concern is that of life-cycle cost: these inverters need to be inexpensive enough that we can put one on the back of every PV module and not increase the cost of PV power. It is hoped that the previously mentioned economies of scale will assist greatly in this.