As some of you have read in my previous blog posts, I’ve been trying to move my engineering career in the direction of renewable energy, or more specifically, solar PV (photovoltaic) system design and integration.  Toward that end, for the past several weeks, I’ve been working part-time for Energy Shop (Carrollton, TX) doing solar PV system installation.   Our first project has been a somewhat high profile project called the “Net Zero Energy Home” which is finishing construction in Lewisville, TX.  The architect, William Peck (William Peck and Associates, Inc.), specializes in sustainable energy-efficient design.  Some of the features included in this showcase home include metal seam roofing, spray foam attic insulation, window awnings for shading during summer months, LED lighting, Energy Star appliances, rain-water collection, incorporating recycled materials and much more.  The builder is Chris Miles (Green Craft Builders) who specializes in green building techniques.  According to Chris’ website, the “Net Zero Energy Home” is projected to meet some very prestigious green certifications after a monitoring period including LEED-H Platinum, NAHB Green, Green Built Texas and Energy Star.
    Scott Bourgeois (Energy Shop) designed the PV system and he and I did much of the installation work with extra help from the other Energy Shop employees.  The system was designed to meet the full energy needs of the house.  That means the system produces more than the house loading on sunny days and what it doesn’t use is pushed onto the electrical grid and credited to the home owner.  At night or on cloudy days, it produces less than the home demand and uses some energy from the electrical grid which is charged to the home owner.  On average, over time, the total electricity produced and consumed by the home should be “net zero”.   Scott faced several challenges with this design.  An oak tree in the back yard threatened to shade some of the roof so he had to place and wire the modules in a way that minimized power generation loss if any of the modules became shaded as the tree grew.  Hopefully, the home owner will keep the tree trimmed so that doesn’t become a problem and reduce their production.  Scott was also asked to provide for future expansion and so some of the system is oversized to meet that need.  Energy Shop is a SunPower dealer and they provide the system as a kit that contains most of the components needed (modules, racking, inverters and monitoring).  The other electrical hardware (meters, disconnects, fuses, wiring, junction boxes) are purchased separately from local vendors.  

 
Here is a panorama picture of the array.  (Sorry, my software doesn’t stitch very well.  It really is a flat, seamless array of modules.)

     When I first arrived on the job, most of the conduit was already installed by a previous employee and the two junction boxes (aka jboxes) were installed into the metal roofing by the roofing contractor.  One jbox is in the top row about two modules from the left and the other is on the second row about three modules from the left.  I finished up the conduit connections to the jboxes and mounted a wiring gutter in the garage.  I haven’t done much conduit bending so that was a learning experience (and an art).  It is a challenge to make a tight bend without kinking it.   Next, we turned our attention to the racking on the roof.  The roof slope wasn’t that steep, but whenever it had even the slightest bit of moisture on it, it became slick as ice.  Couple that with the quagmire of mud around the construction site and it is easy to see a potential recipe for disaster.  Rain caused quite a few delays because it was just too dangerous to get up on the roof.  But, we managed it on the dry days in between.
     As we laid out the racking, we found the metal seams on the roof were not quite evenly spaced, not perfectly vertical and even had some sways in the middle.  So, we opted to align the right edge of the racking with a seam that would be most visible from the ground (right edge) and make the longest run perpendicular to it.  Then, the other racks would be made parallel to it, spaced by the length of a solar module.  The racking is secured to the roof by metal clamps (S-5’s) that grip the vertical metal roof seams.  An L-bracket is bolted to the top of the clamp and the rails are bolted to the L-bracket.  We spent a lot of time using string lines to straighten and level (with shims) each rail run.  Normally, this system is designed to be built from the bottom-up, but because of the irregular array layout, we did the racking, top-down and the module installation bottom-up.  I’m glad we spent the extra time getting the racks straight and level because doing that made all the modules lie in the same plane and look very professional.
    After installing the racking, we pulled the wiring from the wire gutter in the garage to the two jboxes on the roof.  We had to pull seven wires to the east jbox and five to west jbox.  I was worried about how hard that might be, but with wire lubricant and frequent L-boxes along the conduit path to pull between, it was not nearly as difficult as I expected.  Scott wired up the jboxes and the DC disconnects in the garage while we finished leveling and torquing the racking hardware.
    Finally, we were ready to install solar modules.  There were 42 215W monocrystaline SunPower modules that we had to haul up to the roof, attach a ground conductor and then wire to the next module in the string.  These are very efficient modules that convert 17.3% of the suns energy to electrical power.  The eastern array had 3 strings of 8 modules in series and the western array had 2 strings of 9 modules in series.  These were laid out in a concentric pattern around the tree to minimize production losses from shading.  Each jbox was then connected to one of two 5000W SunPower inverters (on the left in picture below) in the garage via DC disconnects (just below inverters).   The outputs of the two inverters go into an AC combiner box (just below the meter inside garage), then through the meter to an AC disconnect outside the house (second picture below) and then is fed back into the main service panel (far right inside garage).  The external AC disconnect is required for the fire department and the meter outside the house is a dual flow meter so it can measure electrical flow into/out of the main service panel from/to the electrical grid.


Inside wall of the garage.


Outside wall of the garage.

     With each module producing 39.8 volts and 5.4 amps under normal operating conditions that means 318 V, 16.2 A (5158 W) are going into one inverter and 358 V, 10.8 A (3869 W) are going into the other (9027 W total).  The overall system efficiency, from solar module to AC output, is about 81% of the nameplate ratings on the solar modules according to PV Watts estimation tool.  So, the system produces about 7321 W at 240 V AC under optimal conditions.  Several factors contribute to this efficiency de-rating such as high cell temperature in the modules, voltage drop in the wiring, inverter conversion efficiency, dusting/shading of the modules, system availability all of which reduce the actual power produced. 
      The nice thing about the SunPower systems is the ability to monitor production which is included as part of the kit.  Each inverter is wired to its own datalogger in a nearby box (white box below meter in garage) and sends production data to it.  The datalogger sends this data via wireless link to a gateway in the house which is connected to a hub router and then to the internet.  The gateway sends the data to the SunPower servers so it can be viewed by the owner on the web.  The owner also has a wireless display that can be set anywhere in the house to see production data without logging onto the SunPower website.  It looks a little like a digital thermostat and shows current wattage being produced by the array and total kWh’s produced which are updated every 5 minutes.  If you tap on the display, it temporarily goes into a live update mode where it updates every few seconds so you can see data real-time.  (See picture below.)  We only had one datalogger working and it was cloudy the day we took the picture so the production number shown is low.


 
Indoor monitor display.

     I worked on mounting, making connections and doing setup for the data logging.  I also did the commissioning (start-up) and registration (online account setup) so we could have it ready for a big open house event that same day.  We almost didn’t make it because of a wiring issue between one inverter and its datalogger, but I debugged it just in time.  By about 11 a.m. the day of the open house the clouds blew past and we were cranking out over 7.5 kW of energy and seeing most of it being pushed out onto the grid.    With a digital external meter, we couldn’t see it “spin backward” but as of this date we have produced more energy than the house has used since the electrical service was connected so it is already starting out at “net zero”.



Scott, happy after we commissioned the system on the day of the open house.

     In the end, I learned a lot about the whole system installation.  SunPower has a nice kit and provides technical support and manuals for all the steps.  We ran into several issues during the installation that we hopefully can avoid on the “next project”.  I stayed for a while to help Scott with the open house.  I came well prepared to answer technical questions about the design, but not the financial ones that many of the attendees were asking.  So, I think I need to learn the financial side better, maybe work on creating my own designs and work more closely with the customers to expand my skill set.  Eventually, we hope to get enough installed kW’s under our belts to move into commercial systems.  If I can get two system designs under my belt, I might be able to sit for my final NABCEP certification exam by next March.  We have a few more systems sold and lined up for installation, so I foresee some potential growing pains ahead if it is just Scott and I doing most of the work, but we should be keeping busy through the end of the year.  The SunPower reps at the open house were very happy with our installation.  I also heard that some of our competition who showed up were a bit envious of the publicity or had wished they hadn’t passed up the job because of the shading issues.  I just hope word of our good install spreads and it helps us grow the business.

Scott, posing in front of his largest design to date.

    The “net zero energy home” is getting a lot of press and raising eye brows.  If you’d like to read more about the other green features this project, then take a look at this article: Lewisville couple designs ultra-energy-efficient home.