A Lifetime of Renewables

 

By Vaughn Nelson

November 2019

 

My lifelong interest in the potential of Texas renewable energy began while teaching physics at West Texas A&M University (WTAMU) in the ’70s. In collaboration with Dr. Earl Gilmore, then head of Physical Sciences at Amarillo College, we started to analyze the Texas wind energy resource. By 1977, WTAMU and the US Department of Agriculture (USDA) signed a cooperative agreement to study and test rural applications of wind energy.

AEI Wind Test Center, 1980s. Left to Right: PM 40 kW, UTRC 25 kW, Enertech 5 kW, Carter 25 kW, Hummingbird 5 kW.

As founder and director of the WTAMU Alternative Energy Institute (AEI) from 1977 to 2003, my research focused on wind resource assessment and applied research and development. At AEI and USDA, we field tested more than eighty prototype and first-production wind turbines from 50 W to 500 kW. Along the way, I developed several online courses on solar, wind, and other renewables.

After retiring in 2003, I took up quilting and wrote the introductory textbooks Wind Energy (third edition), Introduction to Bioenergy, and Introduction to Renewable Energy (second edition). In July, I sent the manuscript of a coffee table book, Innovative Wind Turbines, to a publisher for release later this year.

As a founding member of the Texas Solar Energy Society, I have long been an enthusiastic proponent of renewable energy for the Texas utility grid. Grid-supplied renewables did not exist when TXSES began in 1976 (the first wind farm in the Panhandle was in 1994), so as soon as it was introduced, we signed up and paid three extra dollars per month for wind energy from Southwest Public Service in Amarillo. At that time, it was often best to take matters into your own hands, so in 1974 I built a solar hot water system for heating a portion of our Canyon, Texas, home and incorporated passive solar in a garage enclosure.

AEI Wind Test Center. Demonstration Building.

When we moved to our current home in Round Rock, Texas, in 2011, we subscribed to 100 percent wind energy from Green Mountain Energy, a deregulated retailer that serves much of the state. I use around 9,300 kWh of electricity per year, which costs about $1,100 at the average rate of 11.5 cents per kWh. I am able to measure the portion used to charge our Nissan Leaf, which I drive about 9,000 miles per year. The EV charger uses 2,400 kWh per year, and at 3.75 miles per kWh, the Leaf gets about 115 of equivalent miles per gallon.

Taking the Rooftop Solar Plunge

When a residential PV rebate from the regulated transmission company, Oncor, became available in early 2015, I seriously considered installing my first rooftop solar system. I used the excellent National Renewable Energy Labs PVWatts Calculator for a 6.5 kW system that would produce about 9,200 kWh per year and have an estimated payback period of eight to ten years. I researched local installers and ultimately chose Longhorn Solar. Due to several Oncor connection delays, my system was finally energized in June of that year.

Fortunately, Green Mountain Energy also offered a solar rate plan. Their solar contract offered electricity at 12.3 cents per kWh and credited rooftop-generated electricity at 7.3 cents per kWh. From June 2015 to February 2016, I purchased more electricity than I generated, so I still had a monthly electric bill of about $30.

In 2016, I decided to boost my rooftop system by another 1.0 kW, to 7.5 kW. Why? I wanted to produce the same amount of electricity that I used, reduce my monthly electric bill, and compare the efficiency of PV panels with and without microinverters. The new 1.0 kW sub-system, with microinverters, was energized in February 2016.

The graph shows my monthly averages from March 2016 through August 2019. From June through September, I purchased (red line) more electricity than I fed back to the grid (blue line), due to the large power requirements of the air conditioner.

About this same time, I learned of a new solar plan from Green Mountain Energy called “Renewable Rewards.” Renewable Rewards is a pure net metering plan in which purchased energy and excess solar-generated energy is priced at 11.6 cents per kWh. I highly recommend it for a solar household in the competitive Texas market territories.

Comparing Solar Systems: String Inverter versus Microinverters

Solar systems must convert the direct current (DC) electricity generated by the PV panels into alternating current (AC) electricity used in homes. This is done in one of two ways. The traditional approach is to connect the output of a group of solar panels into one large inverter called a string inverter. An alternate approach is to install a microinverter on each solar panel and eliminate the string inverter(s). There are advantages and disadvantages of either approach.

My solar installation is designed as two separate subsystems: one using microinverters and one using two string inverters. Both are oriented at about 207 degrees, and both provide excellent capture of system performance data that is transmitted by WiFi to a central database accessible on the web. With my solar system, there is no significant difference in the performance of the string inverters and the microinverters. After three years of operation, the 7.5 kW system as a whole produced more energy than we used. As a result, my net metering billing plan is currently running a credit balance of $135 as of October 2019.

The economics of my rooftop system are summarized to the right, with the table showing an eleven-year payback period.

Of course, the great advantage of a grid-tied solar system is the convenience of electricity service twenty-four hours a day, whether the sun is shining or not. This creates a complex interplay of two possible conditions that can vary by the second:

  • Solar energy production exceeds household requirements, which often occurs during daylight hours, in which case the excess is sold (fed back) to the grid.
  • Household energy requirements exceed solar production, which can occur at any time of day and all night, in which case energy must be purchased from the grid.

This moment-by-moment complexity can be simplified by focusing on energy bought and sold over the course of your monthly billing cycle, which is exactly what your electric utility does.

Power to Choose

A handy website for anyone looking to learn more about which electric plans are right for them is Power to Choose, “the official and unbiased electric choice website of the Public Utility Commission of Texas…to compare offers and choose the electric plan that’s right for you.” There are a number of green energy plans that offer 100 percent renewable power.

Retail electricity providers and solar rate plans in the diverse Texas marketplace are anything but simple to explain. The PUCT takes a good stab at it on their Buying Renewable Power webpage.