One of the most commonly asked questions about grid-connected solar photovoltaic (PV) system concerns how long it takes to get investment payback and how much the system is likely to save during its working life.
Arriving at precise numbers is difficult because every installation is different. However, it is possible to compare alternative energy technologies by considering total capital costs, the amount of power harvested from the system over its lifetime, and maintenance costs.
Power Ratings
Watt-peak (Wp) defines the output of solar modules under ideal conditions. It is the standard for measuring relative performance. In all installations, some power is lost in the wiring and in the process of converting dc from the PV modules to ac in the inverter(s). Calculations based on true yielded watt (Wy) account for this and other factors that reduce the Wp figure.
Solar PV Architectures Compared
In conventional PV solar installations, solar modules are wired in a series before connection to a string or central inverter that converts high-voltage dc output from the PV array to ac for connection to the electricity grid (Fig. 1). For residential and commercial installations, the inverter will often be rated at 3 kWp or 5 kWp. In a typical 7-kWp system, two string inverters may be used.
The “domino effect” limits power output. It occurs when the output of one module is compromised by shadows or other obstructions and the output of the whole system is dragged down as a result.
In the micro-inverter architecture (Fig. 2), a small inverter is attached to the back of or integrated into the solar module to convert dc to grid-compatible ac. A 7-kWp system might use 29 micro-inverters, each rated at 240 Wp.
Micro-inverters can offer a 20% advantage with respect to the cost of harvested power over the life of the system. The following analysis compares a conventional string inverter system with one in which micro-inverters are integrated into the PV module to produce a so-called “ac module.” We’ll consider a system with a rating of 7 kWp. Wholesale cost assumptions are based on Enecsys research from 2009.
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Capital Costs
For either system, the module cost for our 7-kW system is $14,000. String inverters will cost around $0.5 per Wp, or $3500. Micro-inverters cost more, perhaps $0.75 per Wp, or $5250. The cost of inverters plus modules is $19,250 for a micro-inverter-based system and 10% less at $17,500 for a string inverter system.
Installation costs will vary from company to company. Initial feedback suggests that a typical 7-kWp ac module system costs $12,500 to install while more complex wiring and other factors mean that string inverter system installation costs around $16,500. In short, a 7-kWp system has a total initial setup cost of $31,750 when you use ac modules, versus $34,000 for a system with string inverters.
Power Harvesting
With series dc strings, shading from any source such as tree branches or even something as small as an antenna or vent pipe can dramatically reduce the energy produced. Also, dirt and other debris that build up unevenly on solar module surfaces cause some modules or even a few cells of modules to see less solar energy.
With ac modules, each module is an independent, standalone, solar ac grid connect system with its own optimized energy production output. Maximum power point tracking (MPPT) varies the electrical operating point of each module to extract the maximum power. Any degradation in the performance of one module has much less impact on system power harvesting.
String inverter systems lose about 2% of potential harvested power due to mismatch of solar modules and a further 2% in dc wiring. The inverter also is the most unreliable part of the system, and a failed inverter harvests no power until it is fixed or replaced. Best estimates show that the initial cost/Wy is around 12% lower for systems using a micro-inverter integrated with a solar module (ac module).
Maintenance
Solar PV modules last 25 years or more, but PV solar systems with string inverters are relatively expensive to maintain. The installation can only be monitored at a system level, so fault finding is time-consuming and expensive. The life of a string inverter is unlikely to be more than 10 to 15 years, so at least one replacement of these expensive units has to be factored in. This year, micro-inverters with a life expectancy of 25 years in real-world conditions will be widely available.
Also, ac modules are monitored individually, sometimes online. It’s easier to identify the exact location of a fault, and it can be fixed more quickly. Over 20 years, the total estimated cost of maintenance and repair for a conventional string inverter system is $5192. For a system with high-reliability ac modules, it’s $739.
Conclusion
The best currently available data indicates that ac modules with integrated micro-inverters will result in 20% lower cost of ownership compared with conventional solar PV systems using string inverters. Improved energy harvest, lower installation costs, and reduced maintenance are the key contributory factors.
Paul Engle, CEO of Enecsys, has completed an executive program from Stanford University. He holds an MBA from the University of Dallas and a BS in mathematics and physics from Pan American University, a part of the University of Texas. He has studied graduate physics at the University of North Texas as well.