PowerSource

The lead-in to this post is here: Micro-inverters offer one solution for optimizing solar efficiency.

Probably the most well-known micro-inverter company is Enphase, which sells a 200W inverter for about $200 or about $1/W. This compares with a 3kW string inverter for about $2,000 or about $0.66/W.  Enphase suggests that the additional $.33/W is compensated for by the lowered installation labor and investment costs.

(From yesterday’s post: A 200W micro-inverter that only has to deal with 30Vdc input and no special installation investment becomes very attractive for small installations. Safety is also an issue: A solar panel string or array’s input to a central inverter can be as high as 600Vdc in the US and 1000Vdc in Europe -- hazardous voltage levels for installers, maintenance personnel, and emergency responders. Output from modules attached to a micro-inverter scheme will be at much lower levels between 200 -- 300Vac.) 

Enphase’s leading position in the micro-inverter market is because, as far as I know, it’s the only micro-inverter company actually delivering product. Enphase’s co-founder Raghu Belur, who I met with at Solar Power International last week, told me that they have so far delivered 100,000 devices in their first year.

Common inverter topologies use electrolytic capacitors on their output filters, and electrolytic capacitors have poor reputations for reliability especially when subjected to the elevated operating temperatures of solar installations. When you go from a single central inverter to 10-20 micro-inverters, the likelihood of a failure due to an electrolytic capacitor increases likewise. Most solar panels have a life of 25-30 years, and operators want a similar lifetime from their inverter circuits.

Enphase has posted several white papers on its site dealing with capacitor reliability and lifetime, such as (pdf) Reliability Study of Electrolytic Capacitors in a Micro-Inverter by Enphase's CTO, Martin Fornage. My take-away from the white paper is that Enphase uses higher-reliability electrolytics than those normally found in power supplies, and the paper shows how their higher-reliability translates to a longer life in real-world temperatures encountered in solar installations.

More detail from the paper: For traditional power converters, an acceptable useful life of capacitors is as low as 2000h at 85°C. Enphase micro-inverters use Nichicon (pdf) capacitors rated from 4000 to 10000h at 105°C. Capacitor lifetime is very sensitive to temperature and follows the Arrhenius equation that states that useful life doubles for every 10°C temperature drop. Temperatures from the National Solar Radiation Data Base (NREL) for the California desert town of Palm Springs in the summer show  a maximum ambient temperature of 46°C, resulting in a core temperature for the capacitor of 65°C. Thus, capacitors rated at 4000h at 105°C have a useful life of 50 years when operated in the Palm Springs climate of 46°C ambient temperature. Other papers (pdf) on the website refer to its micro-inverters being “designed for a service life of 20 years,” but the website lists the warranteed lifetime for its products at 15 years.

Enphase’s micro-inverters are stand-alone units, sold separately from the panel, and can work with a wide range of solar panel, and by definition require no central inverter. MPPT (defined previously) is performed at the panel, so each panel provides its individual maximum power so there’s no fear of a central inverter reaching only a local MPPT for a whole array, increasing the efficiency of each module. Central inverters have a higher efficiency, exceeding 98%, while Enphase micro-inverters are currently pushing about 95%.

Next up: External micro-inverters vs integrated