Don’t Judge A Solar PV System’s Efficacy By Inverter Efficiency Alone

Don’t Judge A Solar PV System’s Efficacy By Inverter Efficiency Alone

For any grid-connected solar photovoltaic (PV) installation, what matters to the end user is the cost of extracting energy from the system over its lifetime—let’s call it the cost per harvested watt. For a given set of environmental conditions, several parameters determine this cost:

  • The efficiency of the solar module, or how good the module is at converting sunlight into electricity in the form of direct current (dc) output
  • The efficiency of power conversion from the dc output of the solar modules to alternating current (ac) for connection to the grid
  • The capital costs of system hardware, plus planning and installation
  • Maintenance costs over the system lifetime

PV module efficiency varies by module type, from around 6% for those using amorphous silicon-based solar cells to between 40% and 45% for some of the technologies currently being developed in research laboratories. A typical range for today’s commercially available modules is 15% to 20%. In general, the higher the efficiency of the module, the more you pay for it.

The dc-ac power conversion efficiency depends on the architecture of the installation, losses within the system, and the efficiency of the inverters that carry out the dc-ac conversion process.

Inverter efficiency may be quoted as peak efficiency, which is the highest figure that the inverter can achieve, or CEC weighted efficiency, which is a figure defined by the California Energy Commission that’s designed to estimate the average efficiency of the inverter.

Solar PV Architecture Fundamentals

Three main system architectures are deployed today. First, conventional systems deploy solar modules connected in series to form a “string.” The resulting high-voltage, high-current dc output is then fed into a string inverter (Fig. 1)

For commercial and residential applications, string inverters are typically rated between 1 and 10 kW. Peak efficiency is usually in the order of 96% or 98%, and CEC weighted efficiency between 95% and 97%. 

However, the high current carried in cables that interconnect the modules and feed the output to the string inverter results in relatively high cable losses. The electrical resistance of the cables causes energy to be lost as heat.

The second type of architecture deployed today involves solar PV systems with dc-dc optimisers behind each solar module, enabling a technique called maximum power point tracking (MPPT) to be applied at the module level, rather than at the overall system level. This extracts maximum power from each module as irradiance varies. 

Now, dc-dc optimisers are available with very high peak efficiency of better than 99%. (There is no CEC weighted efficiency category for dc-dc devices. It only applies to dc-ac inverters.) Optimisers are connected in series and parallel combinations, with the exact architecture depending on the installation.

However, a further inverter is still needed to convert the dc output from these devices to ac for grid connection. The system losses are the sum of losses from dc wiring, the dc-dc optimisers, and the dc-ac inverter.

Third, the micro inverter architecture involves mounting a dc-ac inverter on the racking behind each solar module. Some micro inverters accept input from a single module, and others accept input from two modules.

This turns each solar module, or pair of modules, into an independent ac generator. These ac outputs are grid-compliant, with no need for a string inverter.

MPPT is applied to each individual module, and systems based on micro inverters harvest 5% to 20% more energy over the life of the system, compared with systems that use string inverters. The peak efficiency is normally in the 94% to 96% range. Cable losses are lower because power is transported around the systems as ac, rather than dc. 

Efficiency Vs. Cost Per Harvested Watt

A system with a single string inverter can appear to be the most efficient, with peak inverter efficiency of perhaps 98% compared with a micro inverter at 96%.  But when you account for relative cable losses, the efficiencies become closely comparable.

Also, with significantly more power extracted from each solar module by using a micro inverter with MPPT, the relative cost of harvesting power is lower with the micro inverter architecture. Would you rather extract 96% of 100 W available from a module, in other words 96 W, or 98% of 80 W, or just 78.4 W?

But this is not the whole story. Remember, it’s not just the total harvested power we are concerned with, it’s the cost per harvested watt over the lifetime of the system.

String inverters are notoriously unreliable and represent a single point of failure.  They typically come with a five-year limited warranty and have to be replaced at least once during the 25-year life of a solar PV system. These inverters are used in conventional string architectures and when dc-dc optimisers, which themselves add cost and complexity, are deployed.

The micro inverter has lower headline efficiency, but it’s more than compensated for by increased energy harvest, giving the lowest cost per harvested watt, if the inverters last for the lifetime of the system.

Early micro inverters were not reliable, primarily due to problems caused by electrolytic capacitors—electrochemical components that deteriorate in the extreme environmental conditions found on rooftops. Recent developments in micro inverters have seen the elimination of these components through new design techniques.

Micro inverters are now available with a 20-year warranty, operate in ambient temperatures from –40°C to 85°C, and have a life expectancy to match that of solar modules under real-world conditions. This changes solar PV economics.

As mentioned earlier, the new micro inverter-based systems deliver a 5% to 20% increase in energy harvest over the life of the system, which equates to a 5% to 20% reduction in the harvested cost per watt.

With respect to the other cost factors highlighted earlier, micro inverter solar PV systems sometimes involve marginally higher capital cost compared with string inverter systems, but lower cost than those using dc-dc optimisers. Dual micro inverters like the Enecsys Duo address this initial cost differential (Fig. 3).

The Duo converts the dc input from two solar modules, with MPPT for both modules, into a single, grid-compliant ac output. This enables system costs to be reduced so there is no cost premium over systems with string inverters.

Maintenance costs are lower due to better inverter reliability and the facility to monitor the system down to the level of individual modules, so faults can be quickly pinpointed and rectified.

Micro Inverter Efficiency Claims

Manufacturers of inverters always put forward their “best case” efficiency figures. But to compare micro inverters, it helps to understand a little about the factors that affect efficiency, particularly the input voltage, the output load, and the ambient temperature.

A realistic assessment of the true operating efficiency of a micro inverter requires a detailed evaluation of its likely operating conditions. On rooftops, the operating temperature will vary widely, compromising the efficiency of many micro inverters. Look into detailed technical specifications, particularly with respect to how well efficiency is maintained over the operating temperature range.

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