Blogs, Monitoring, Solar

Cost-Benefit of Microinverters vs Maximizers / String Inverters

Microinverters, which perform MPPT and convert DC power generated from an individual solar module to grid compatible AC power, have been big news in the North American grid-tie Photovoltaic scene over the past few years.  Currently dominated by CA-based Enphase Energy, microinverters have been the technology of choice of a number PV installers primarily for residential applications.  The technology has made such a splash that industry leading string inverter manufacturer SMA recently launched their own microinverter, and competitor Power-One is due to launch theirs in 2012.

In my experience working for a Canadian wholesaler since early 2006, it appears to me that demand for microinverters is largely shared between end users who like the web-based module-level monitoring feature, and installers who like the fact that they can work uniquely with AC power and equipment.

Rather than using the all-too-often-used and inacurate metric of $ / Wp which takes PV system cost but not production into account, PV professionals and investors are increasingly using the Levelized Cost of Energy metric: Total Life Cycle Cost / Total Lifetime Energy Production, as measured in $ / kWh.  See the article “Levelized Cost of Energy” in the April/May 2012 edition of SolarPro magazine for an in depth analysis – solarprofessional.com. 

Do microinverters present a compelling LCOE proposition to system owners or is it mostly hype?  Let’s take a closer look at how that question may be answered…

In grid-tie applications, competition for the micro is 1) a system with string inverters only and, 2) a system with string inverters and DC-DC power maximizers installed on each module.

 

1) Micro vs string inverters only

In terms of performance, micro installations offers the advantage of not being constrained by some of the losses that are seen in string inverter installations due to module mismatch, soiling, shading and/or snow cover.  I would give the edge to string installations concerning wiring losses, where lower current is typically carried over high voltages (~ 400-500 Vdc vs ~240 Vac for micro). since we know that power losses are proportional to I2.  The difference in yield in kWh / kWp,  an important system metric, will differ between micro and string installs depending on the level of module mismatch, soiling and shade/snow cover throughout the year.  CEC weighted efficiencies of micros are typically in the 93-96% range, whereas those of string inverters are typically in the 95-97% range. I have heard different field results from customers, and although Photon Labs (Germany) and Desert Knowledge Australia Solar Centre have done great work in field testing yields of many different PV modules, important independent field testing is needed to better understand micro vs string inverter installs using the same modules under the same conditions.

On the monitoring side, if granularity is an important factor for the end user, micros have the advantage since they can be monitored individually historically and in real-time over the Web. String inverters can only be monitored per MPPT channel – typically one channel per inverter, although some inverters on the market have 2 MPPT channels per inverter (e.g. Power-One Aurora). If using high performance PV modules with zero power tolerance (typically -0% / +3% of rated power at STC) from a manufacturer with proven performance in the field, individual module performance will be reliable and predictable.  The analogy I like to use for module-level monitoring is to ask this question: Do I want or need to follow the performance of every single stock in my RRSP (or 401K in the US)?  My answer is no, but evidently the answer is different for many micro system owners.

Cost: Microinverters currently cost approximately 100% more than string inverters on a $ / Watt AC basis.

Yield: Insufficient independent field data is available in order to offer a proper comparison vs a string inverter installation under the same conditions.

LCOE: This will vary according to available incentives, location / solar resource, equipment used and whether the array faces significant shading / snow cover challenges throughout the year.

Aside from leader Enphase, other microinverter manufacturers include Enecsys, SolarEdge and SolarBridge.

 

2) Micro vs string inverters and DC-DC power maximizers

DC-DC power maximizers have also come onto the market over the past couple of years, albeit with somewhat less fanfare than micros.  Although I still have questions regarding the technology, manufacturer claims and case studies show that these devices have promise in both grid-tie and off-grid PV applications.

The main functions of DC-DC power optimizers are to provide MPPT and to harvest the maximum amount of power from each module. Advantages of using power optimizers within a string inverter installation include the folllowing:

– Standardize operating voltage of each string, resulting in greater yields at the output of the string inverter(s).

– Greatly minimize detrimental effect of module mismatch, soiling, partial shading and/or snow cover on one or more modules within a string. According to a 2010 Photon Labs study, Tigo maximizers increase yield by ~1 to 3% when modules are soiled and from 2 to 36% when modules are partially shaded, as compared to modules with no maximizers.  

– Should an optimizer within an array fail, the associated module continues to generate power into the string inverter. If a micro fails, the effective power generated by the associated module is zero.

– Allows for web-based module-level monitoring, answering one of the main desirable features of microinverters head on.

Cost: Microinverters currently cost approximately 35% more than string inverters and DC-DC power optimizers on a $ / Watt AC basis.

Yield: Insufficient independent field data is available in order to offer a proper comparison of micro vs string inverter / DC-DC power optimizer installation under the same conditions.

LCOE: This will vary according to available incentives, location / solar resource, equipment used and whether the array faces significant shading / snow cover challenges throughout the year.

Aside from leader Tigo Energy, other DC-DC power optimizers available on the market include those manufactured by Azuray Technologies and eIQ Energy.

As power optimizers and microinverters further pentrate the PV market, both as stand alone units and eventually embedded within solar modules’ junction box, it is hopeful that more independent field data will be made available so that investors and consumers can make decisions that impact the supply chain for both technologies.