INTRODUCTORY SPECIAL INTRODUCTORY ... - PHOTON Info

Anne Kreutzmann / photon-pictures.com
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Science & Technology
system’s lost energy yield. The more mis-
matched the system is due to shadows,
dirt, or natural module mismatching, the
more microinverters claim they can help.
Enphase, which now offers its prod-
ucts to a number of installers, is in high
demand. Cinnamon says Akeena will
buy as many microinverters as Enphase
can supply, and an installer at Owens
Electric & Solar, Inc. in California esti-
mates that 70 percent of his customers
opt for Enphase products, when offered.
But that doesn’t mean Enphase, or any of
these new companies, can rest on their
laurels. They will all have to prove them-
selves if they want to remain competitive
with central inverters.
More components = more failures
At the heart of criticism of distributed
power architecture – including power op-
timizers and microinverters – is a logical
rule. Even if a product has an excellent life
expectancy, the more components you
add to your system, the more likely one
of them will fail at any given moment. A
central inverter is a fairly safe bet with its
10 to 15 year life expectancy. If it fails,
The man that began it: Fifteen years ago in the Nether-
lands, Henk Oldenkamp developed a microinverter that
converterted 130 Watts DC to AC and clipped right on the
back of a module – the OK4, one of the first devices of
this type. He based the product on the same components
and basic concept as a central inverter (photo right).
you just replace a single com-
ponent. On the other hand, if
you have 30 microinverters in
a system, failures – either due
to defects or wear and tear –
will be more numerous.
While most start-ups have designed
their microinverters to detach easily
from modules or module racks – to facil-
itate replacement – installers still doubt
whether a microinverter can be built
that is as reliable as the module itself.
»Nobody wants to connect those two
together,« says Ron Van Dell, CEO of
SolarBridge, since microinverters have
much shorter warranties than modules.
»It makes no sense to connect a compo-
nent that’s by definition going to fail
before the thing it’s hooked to fails.«
This is where microinverter technology
– and marketing it – gets tricky. To be com-
petitive, a microinverter has to be cheaper
and more durable than a central inverter.
Moreover, some purport, if AC PV is really
going to take off, microinverter warran-
ties have to match those of modules. Thus
not only does each microinverter need to
have a spectacular lifespan, and keep cool
with no moving parts, but has to do it all
while sitting on top of a roof in the blazing
sun. »Making a microinverter to operate
in the vicinity of a PV module is no easy
task,« adds Bower. »It takes real engineer-
ing. The environment is just brutal.«
OKE-Services
Then there’s the size of the microin-
verter itself. If a microinverter’s efficiency
maxes out at 96 percent, that’s 8 W of cur-
rent that have to be dissipated for each 200
W module. As Bower puts it: »if you put that
in a small enough package, it’s like your
night light, it’s hot enough to burn your
fingers.« Since active cooling would be cost-
prohibitive, microinverters have to rely on
conductive cooling to distribute heat.
And while some components can
withstand heat, electrolytic capacitors
are understood to be the component –
in inverters both large and small – that
is most likely to fail due to high tempera-
tures. Indeed, it usually is the part that
malfunctions, in both central and mi-
croinverters. Enphase, who says it uses
capacitors with a rated lifetime of 4,000
to 10,000 hours at 105° Celsius, points
out that the life expectancy of a capaci-
tor doubles with every 10° Celsius drop
in temperature. Joe Augenbraun, CEO of
Solar Red, Inc. – a start-up using micro-
inverters to build and install integrated
systems – jokes that it should be called
the »capacitor of evil.«
The volatile outdoor envi-
ronment, and heat dissipation
from the nearby module aren’t
the only factors that make en-
gineering a reliable microin-
verter difficult – microinverter
itself produces heat. By the
very nature of its technology,
a microinverter is going to be
less efficient than a central
inverter, says Bower. Every in-
verter, no matter its size, has a
certain amount of fixed energy
consumption. Multiply that
by 10, or 15, or however many
modules there are in an array,
and as the systems size increas-
es, efficiency falls relative to a
system using a central inverter.
Enphase doesn’t share this senti-
ment. In a study from late 2008, CTO
Martin Fornage claimed that Enphase’s
use of electrolytic capacitors should,
by a »conservative estimate,« allow the
components to last for 50 years. Fornage
suggests that Enphase capacitors have
November 2009