This is a great article from our buddy Bob:
Following in the footsteps of theecolife, we recently completed the installation of a Sunpower grid-tie solar electric system for our South Orange County home:
When we ordered the system, one of the decisions we had to make was the type of inverter. Sunpower offered inverters from 3 different suppliers that they tweak a bit to work with their positive ground panels and then rebadge and sell as their own. The choices were the SPR-5200 (a re-badged PVPowered 5200), the SPR-5000m (SMA/Sunnyboy 5000-US), and the SPR-5000x (Xantrex GT-5.0). Outside of the Sunpower universe, there are many other inverter suppliers with widely varying designs and quality. The end user needs to be very careful about their inverter selection, as it will affect the output and reliability of their system.
For our installation, we selected the SPR-5000x, which is a passive (convection) cooled design with an impressive data output/monitoring capability using Xantrex or SG-View software.
The passive cooled design was a desirable feature, since it does not use electricity to function. For any inverter, proper cooling is critical to remove the heat generated from converting the solar DC volts to the AC volts used by the electrical utility. And, solar inverters make a lot of heat as they operate – the more solar electricity, the more heat. There are 2 ways to remove this heat from the inverter: Fans, or convection.
Fan-cooled inverters have an advantage in that they are actively cooled and can be placed in more restrictive locations. But, the fan uses electricity and runs frequently. Also, the fan is a moving part and will ultimately fail – and when it does, so may the inverter.
Passive-convection cooled inverters rely on a bank of fins to offload heat from the electronics to the air. Because there is no fan, there is no electricity used in the process of cooling and there is no reliability issue, either (its hard to wear out a chunk of aluminum!). But, the passive designs have limitations on placement and ultimate cooling capability, as we learned.
After our system was up and running, on a few occasions we noticed a sharp drop in electricity generation when there was no good reason. As it turned out, our inverter (which is installed inside our garage), was experiencing periods of overheating due to the combination of high solar electricity output (and heat), and the inside location which offered no natural breeze to help the convection cooling process. The Xantrex/Sunpower inverter uses a thermal protection system that reduces inverter power output if about 170° F is reached to protect the inverter electronics, and we were attaining that high temp on occasion, even during cold weather days when it was pretty cold inside the garage.
The solution was to install a temperature-controlled fan that can cycle on when needed, and stay off when not needed. For us, this is the best of both worlds as we have the esthetically pleasing inside inverter mounting location combined with efficient and reliable passive cooling, augmented with a small external fan that only operates when needed. Here is how we did it:
For the controller, we selected the Ranco ETC-111000-000 Digital Temperature Controller (about $45-$60 online) which can control a 120-volt source on and off at a user-controlled setpoint.
For the fan, we used a small panel fan that was salvaged from some industrial equipment but a small desk fan would work as well or better.
Installation of the Ranco controller was easy. For the thermistor, we placed some thermal grease on the thermistor where it contacts the fin block, and used some metal tape to affix it to the inverter fin. The thermal grease increases heat conductivity between the inverter fin and Ranco thermistor (get it at a computer parts store), the metal tape will contain and focus the heat onto the Ranco thermistor and is designed to last in this kind of application (made by 3M, get this at a hardware store).
The Ranco thermistor style is not exactly what I would have chosen (if I had a choice), but it is good enough for this application and for the price this setup is unbeatable compared to the cost of industrial controllers paired with the “correct” thermistor. An even less expensive solution would be to use a timer for the fan, but this would run the fan more often than needed, and use more electricity.
We wired in a plug and the fan, programmed the Ranco controller (very easy), and the job was done. For the wiring-challenged, Ranco also offers a slightly more expensive pre-wired model that just plugs in to the wall and the fan.
Because of the thermistor style used by Ranco and other factors there is some difference in detected temp between the Ranco and the inverter thermistor (viewable using SG-View software), but that is no real problem since this offset can be factored in to the Ranco temperature setting.
Now, regardless of outside temperature or solar generation our inverter never exceeds a comfortable operation temperature, and we are also comfortable knowing that the solution was simple, economical, and efficient!
Questions? email me: solarintheOC@cox.net
Filed under: alternative energy on May 19th, 2008