Increasing Production with Single-Axis Tracking

Cloudview EcoFarms strives to farm in an ecologically sound manner, providing food to members of a community-supported agriculture (CSA) network. Installing a PV system using locally made modules and inverters matches their “buy local” philosophy, and using the clean solar energy to offset electricity usage on the farm’s new cold storage unit—which enables them to store their produce—just made good financial sense.

Washington state’s incentive program, slated to be in place until 2020, pays a system owner based on the PV system’s annual production. Currently, the base rate is $0.15 per kWh, up to a $5,000 maximum payment per year. Systems using Washington-made inverters get compensated at $0.18 per kWh. And systems using Washington-made PV modules earn an additional $0.36 per kWh, for a total of $0.54 per kWh. The system’s output is measured by a production-grade meter. Participants are paid for all of their PV systems’ production, even if the majority of it is used to power on-site loads. The utility still net-meters and credits any excess production, though it zeroes out at the beginning of a new 12-month period. After 2020, the state incentive ends and the only payback is the offset energy consumption.

Design Options

Cloudview’s system was designed to maximize the annual incentive payment. Its single-axis tracker pivots east to west to follow the sun’s path, helping the modules capture as much energy throughout the day as possible, especially during the sunny summer months. (In this region, winter production is typically low due to cloud cover.) Even with a larger up-front cost (compared to a conventional, fixed PV array), this system has only a seven-year simple payback. 

The National Renewable Energy Laboratory’s System Advisor Model (SAM), free system analysis software, was used for modeling the array output and its financial payback. The first goal was to maximize the annual incentive payment. Three options were examined:

• 20 modules with a single-axis tracker
• 20 modules on a fixed array (to compare production to the tracked system)
• 26 modules on a fixed array, the number of modules calculated to maximize the annual incentive payment of $5,000.

The SAM analysis was made using the following assumptions:

• A 30-year array life, which includes an appropriate derate for module cell degradation.
• An inflation rate of 2.5%—the average rate of inflation in the United States over the past 10 years.
• An 8% real discount rate, which is used to discount constant year benefits and costs and is based on the average short-term business loan rate, was used. In essence, it’s the present cost of money that is tied up in the project.
• The 30% federal tax credit was applied in the first year.
• Operation and maintenance costs were not included.
• Tracker costs were based on actual installation costs; fixed array costs were estimated.
• Avoided power costs were calculated using the current utility rate for the farm, which is $0.04 per kWh. 
• A 0.5% degradation per year in module output was assumed.

SAM deducts the 30% tax credit in the first year and also deducts the incentive payment and the avoided cost of utility energy that the PV system offsets. After seven and a half years, the only deduction is the avoided cost of energy times the inflation rate, assumed to be 2.5%.