The Arizona Corporation Commission Environmental Portfolio Standard (EPS) program

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1 PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2005; 13: Published online 7 April 2005 in Wiley InterScience ( DOI: /pip.593 Applications Photovoltaic Power Plant Experience at Arizona Public Service: A 5-year Assessment z L. Moore 1 *,y, H. Post 1, H. Hayden 2, S. Canada 2 and D. Narang 2 1 Sandia National Laboratories, PO Box 5800, Albuquerque, New Mexico, , USA 2 Arizona Public Service, PO Box 53999, Phoenix, Arizona , USA Arizona Public Service (APS) currently has over 49MW dc of grid-connected photovoltaic systems that have been installed in its service territory over the past five years. Most of this installed PV capacity is in support of the Arizona Corporation Commission Environmental Portfolio Standard goal that encourages APS to generate 11% of its energy generation through renewable resources by 2007, with 60% of that amount from solar. During this time, much has been learned regarding performance, cost, maintenance, installation and design. This paper presents an assessment of these topics and a perspective associated with this PV experience. Published in 2005 by John Wiley & Sons, Ltd. key words: large grid-connected PV systems; utility PV; field performance; cost; operation and maintenance experience INTRODUCTION The Arizona Corporation Commission Environmental Portfolio Standard (EPS) program 1 has provided a significant stimulus for the construction and operation of renewable resource energy-generating capacity, particularly photovoltaic (PV) systems, in the state of Arizona. The EPS program provides for multi-year, pay-as-you-go development of renewable energy, with kw h ac energy production as a key program measurement. The program has established a goal for Arizona s utilities that 11% of the energy generation in the state, with 60% of that from solar, must be derived from renewable resources by To achieve that goal, upwards of 100 MW ac of solar power systems will need to be installed in the state in addition to large quantities of non-solar renewable plants. Arizona Public Service (APS) Company 2, headquartered in Phoenix, Arizona, is the state s largest electric utility serving more than customers in 11 of the state s 15 counties. Since 1995, APS has been aggressively pursuing a wide diversity of projects, involving a variety of renewable technologies, with a focus on developing low-cost energy generation appropriate for a utility-operating environment. The APS program has significantly increased its pace of solar installations to well over a megawatt per year. 3 Currently, APS has 49MW dc of PV installations including 49 flat-plate PV systems totaling in excess of 35MW dc of grid-tied generation. These PV installations also include the largest portfolio of Concentrating PV (CPV) capacity in the * Correspondence to: Larry Moore, Sandia National Laboratories, PO Box 5800, Albuquerque, New Mexico, , U.S.A. y lmmoore@sandia.gov z This article is a U.S. Government work and is in the public domain in the U.S.A. Contract/grant sponsor: US DoE; contract/grant number: DE-AC04-94AL Received 7 July 2004 Published in 2005 by John Wiley & Sons, Ltd. Revised 4 October 2004

2 354 L. MOORE ET AL. world, totaling over 600 kw. Although still in development, APS views the CPV option as a potentially viable lower-cost option for the future. In addition, a 1 MW ac solar trough system is currently under construction by APS. All of this offers a substantial database of real-world operating experience and much has been learned regarding performance, cost, maintenance, installation, and design. The field experiences with these systems provide a treasure of information that can help guide the development of PV system technology for the future. These are the reasons that APS and Sandia National Laboratories entered into a collaborative effort to track and analyze the field performance as well as operations and maintenance (O&M) experience associated with these systems. This paper presents an assessment and perspective of the grid-tied system experience, focused on the APS flat-plate PV systems. A similar effort for off-grid systems installed by APS has documented 4 the O&M experience and system life cycle cost for nearly 60 PV hybrid systems. It is anticipated that additional assessments for CPV and other renewable technologies will be forthcoming as well. SUMMARY OF INSTALLED PV SYSTEMS From 1998 through 2003, APS installed 49 flat-plate grid-tied systems, ranging from the smallest at 19kW dc in several locations to multiple systems totaling 19MW dc at the Prescott airport. Twenty of these systems are larger than 100 kw dc. Table I lists the name, location, size (based on aggregate nameplate dc rating at Standard Test Conditions), installation date and configuration for each system. There are four different configurations fixed horizontal, fixed south facing at latitude tilt, one-axis tracking horizontal with north south axis orientation, and one-axis tracking 30 tilt (very near latitude tilt) with north south axis orientation. Representative configurations are shown in Figures 1 4. A variety of commercially available modules, including both single crystal and multicrystalline silicon were used in most of the installations. Modules were purchased from major manufacturers, including ASE Americas (now RWE Schott Solar), Sharp, BP Solar, and Kyocera. The modules, which ranged in size from 140 W to 330 W dc at Standard Test Conditions, were purchased in bulk quantities. The criteria used to select a supplier were: (1) 10% or greater efficiency; (2) established supplier and accompanying warranty; (3) module size and configuration; and (4) lowest cost per watt. After earlier installations using Omnion and Trace (Xantrex) hardware, the inverters for the systems less than 10 kw were primarily Sunnyboy models from SMA America. Systems in the size range of 10 to 30 kw were equipped with both Xantrex and AES units. The large systems above 90 kw dc used both Xantrex 125 and AES 125 inverters. Early installations of the larger systems used four subfield arrays, each equipped with 30 kw inverters. DESIGN AND INSTALLATION EXPERIENCE For smaller systems, those less than 10 kw dc, APS contracted with various companies of the local solar industry to design and install the systems, with engineering input from APS. For the systems larger than 10 kw dc, APS was often the project manager, designer and general contractor. Site preparation, structure installation, and concrete work were done by local subcontractors. Electrical work was performed by licensed contractors, both those involved in the solar industry and as conventional electrical contractors and utility trade persons. This approach was found to be timely, efficient and cost effective. To meet the solar energy performance and long-term cost goals, APS is focused on tracking systems with other technology concepts under development. The horizontal tracking configuration represents the majority of this field experience for larger systems to date. Similarly, APS has found these large, tracking flat-plate systems to be the most effective approach to reducing the cost of PV energy generation. Tracker maintenance has been steadily reduced through continuous design improvements. The additional costs of the tracking mechanism and structure are more than offset by the additional energy generation.

3 PV POWER PLANTS IN ARIZONA 355 Table I. APS flat-plate systems Location Unit name Configuration Install date Array size (kw dc ) Flagstaff Grand Canyon Trust Fixed at latitude 01-Dec Flagstaff Lowell Observatory Fixed at latitude 01-Jan Flagstaff NAU Fixed at latitude 01-Jun Gilbert Gilbert School District Fixed at latitude 01-Dec Lake Pleasant Desert Outdoor Center Fixed at latitude 01-Jan Peoria Challenger Learning Center Fixed at latitude 01-Jan Phoenix Deer Valley School Fixed at latitude 01-Oct Phoenix Phoenix 502 Canopy Fixed at latitude 07-Apr Prescott Sharlot Hall Museum Fixed at latitude 01-Jan San Luis San Luis Fixed at latitude 01-Mar Scottsdale Civic Library Fixed at latitude 01-Apr Scottsdale Mustang Library Fixed at latitude 01-Dec Superior Boyce Thompson Arboretum Fixed at latitude 01-Jan Tempe STAR Test Rooftops (4) Fixed at latitude 01-Aug Tempe Tempe Recycle Center Fixed at latitude 01-Feb Phoenix ADEQ Covered Parking Fixed horizontal 01-Jun Phoenix ST Micro (Crystalline) Fixed horizontal 01-Jul Scottsdale Scottsdale Covered Parking Fixed horizontal 03-Jan Tempe STAR Covered Parking Fixed horizontal 01-Jun Flagstaff Flagstaff Tracking horizontal 01-Oct Gilbert Gilbert Nature Center Tracking horizontal 01-Feb Glendale Glendale UPG Tracking horizontal 01-Oct Prescott Prescott Airport MT-A01 Tracking horizontal 03-Feb Prescott Prescott Airport MT-A02 Tracking horizontal 01-Dec Prescott Prescott Airport MT-A03 Tracking horizontal 01-Dec Prescott Prescott Airport MT-A04 Tracking horizontal 03-Feb Prescott Prescott Airport MT-A05 Tracking horizontal 07-May Prescott Prescott Airport MT-A06 Tracking horizontal 07-May Prescott Prescott Airport MT-A07 Tracking horizontal 30-Jun Prescott Prescott Airport MT-A08 Tracking horizontal 30-Jun Prescott Prescott Airport MT-B01 Tracking horizontal 01-Dec Prescott Prescott Airport MT-B02 Tracking horizontal 07-May Prescott Prescott Airport MT-B03 Tracking horizontal 07-May Prescott Prescott Airport MT-B04 Tracking horizontal 30-Jun Prescott Prescott Airport MT-B05 Tracking horizontal 30-Jun Prescott Prescott Embry Riddle Aeronautical University Tracking horizontal 01-Apr Scottsdale Scottsdale East Tank Tracking horizontal 01-Apr Scottsdale Scottsdale West Tank Tracking horizontal 01-Apr Tempe Ocotillo 1 (UPG) Tracking horizontal 01-Feb Tempe Ocotillo 2 (Maxtracker) Tracking horizontal 01-Oct Yuma Yucca Power Plant Tracking horizontal 01-Jan Prescott Prescott Tilted Trackers Tracking tilted Jan Tempe STAR Tilted 17V Tracking tilted May Tempe STAR Tilted 50V Tracking tilted Nov Tempe STAR Tilted Assorted Tracking tilted Mar Tempe STAR Tilted Matrix Tracking tilted Nov SYSTEM PERFORMANCE Photovoltaic systems are often discussed in terms of the installed kw dc power ratings; however, kw h ac energy production is the important measure of a system from the perspective of energy users. The following section provides a discussion of the APS PV system performance experience.

4 356 L. MOORE ET AL. Figure 1. Arizona Department of Environmental Quality (ADEQ) fixed horizontal covered parking system in Phoenix Figure 2. Lowell Observatory fixed latitude tilt system in Flagstaff Annual energy performance There are a number of PV power ratings such as Standard Test Conditions (STC) and PVUSA Test Conditions (PTC) that are in use 5,6 within the PV community. These rating methods require sophisticated monitoring equipment to measure electrical performance, irradiance and PV cell temperatures in the field. In most cases, gathering this information and performing these complicated calculations are beyond the typical system owner s capabilities or needs. To provide an easier to use measure for energy production, we have adopted a metric called the final annual yield (FAY), which is defined as the actual annual energy output of the system (kw h ac ) divided by the aggregate module dc nameplate rating of the array (kw dc ) at STC. Final Annual Yield ðfayþ ¼kW h ac =kw dc

5 PV POWER PLANTS IN ARIZONA 357 Figure 3. Prescott Airport tracking horizontal system Figure 4. Prescott Airport tracking tilt system in the foreground This metric, which was initially proposed 7,8 by the Joint Research Center of the Commission of the European Communities, provides a simple and effective approach that allows system owners to understand performance using readily available parameters. All UL-listed modules in the U.S. require labeling on the back that includes rated power output at STC conditions. To assure PV system customers and regulators that systems are performing properly and they re getting what they paid for, 9 the PV community must provide an easily understood and uniformly applied system energy specification that can be verified by the customer as meeting his expectations. The FAY rating provides a simple, objective performance metric for this purpose. To provide an FAY baseline, accurate kw h ac readings are needed for a family of systems at a location. The following graphs and table show kw h ac readings for multiple systems each year. The data on these graphs are

6 358 L. MOORE ET AL. Figure 5. Five-year average of monthly energy performance per kw dc for Phoenix Figure 6. Final annual yield for Phoenix from utility kwh meters. These meters can be very cost effective, with an installed cost as little as $50. For most of the data for the installations displayed below, a more sophisticated digital utility meter was used with readings recorded every 10 minutes. Figure 5 presents the monthly average energy performance per configuration for all systems over the fiveyear assessment period in Phoenix (latitude 3343 N). It should be noted that the reduced performance of the tracking tilted system for May and June is due to continuing maintenance problems with those systems during Figure 6 shows the yearly comparison of the average system kw h ac output by installation type for the Phoenix area. Figure 7 gives a comparison of horizontal single-axis tracking FAY for three locations in Arizona. Only those systems with a full year of energy measurements are included. Table II presents a comparison of the average annual kw h ac per kw dc (FAY) for the four configurations in Phoenix. As expected, tracking configurations compared to fixed configurations are found to provide a significant increase in energy output for the same nameplate rating of the array. Noting the information in Table III, the benefits of one-axis tracking horizontal and one-axis-tracking latitude tilt for Phoenix are expected annual energy increases of 23 and 32%, respectively, compared with fixed arrays at latitude tilt. Actual outputs of the Phoenix systems from Table II show average annual increases of 23% for the tracking horizontal and 37% for the tracking tilt configurations.

7 PV POWER PLANTS IN ARIZONA 359 Figure 7. Final annual yield for tracking horizontal configuration compared by location Table II. Average final annual yield for Phoenix Configuration kw h ac /kw dc Fixed horizontal 1324 Fixed latitude tilt 1479 One-axis NS tracking horizontal 1813 One-axis NS tracking tilt 2032 Table III. Annual peak sun hours Configuration Phoenix Flagstaff Prescott Fixed horizontal Fixed latitude tilt One-axis NS tracking horizontal One-axis NS tracking tilt System power performance The National Renewable Energy Laboratory in Golden, CO publishes 10 solar insolation data for 239 stations in the U.S., based on 30 years of measurements. A summary of the annual peak sun hours 11 available for these configurations and the three cities of Arizona are shown in Table III. APS uses these data (with uncertainties of 9%) to estimate the kw h ac production for the fixed and tracking alternatives. If there were no other performance factors in the system converting from kw dc (STC) to actual kw ac, the peak sun hours would represent the kw h ac per kw dc produced by the system. However, a number of loss mechanisms substantially decrease the actual ac power of the system from the rated dc power. These losses 12 include module mismatch, wiring, diodes, soiling, operating temperature, shading and inverter dc-to-ac conversion. In contrast, some factors can increase output. These factors include low temperature, increased wind and reflective light. Observed power performance An estimate of the ratio of annual average ac power of the system to nameplate dc power of the array can be determined by dividing the measured final annual yield per configuration and location (shown in Figures 6 and 7) by the appropriate annual peak sun hours from Table III. System performance ratio ¼ðFinal annual yieldþ=ðannual peak sun hoursþ

8 360 L. MOORE ET AL. Figure 8. System performance ratios for four configurations in Phoenix Figure 9. System performance ratios for tracking horizontal configuration at three sites This ratio 7,8 reflects the losses in the systems going from aggregate nameplate dc power to annual average ac power of the array. Figure 8 presents the calculated performance ratios for the four configurations in Phoenix. The performance ratios for the tracking horizontal configuration in Phoenix, Flagstaff and Prescott are shown in Figure 9. It should be noted that any variations in yearly solar insolation from the averages shown in Table III would be included in these performance ratios. However, these ratios provide a reasonably accurate rule-ofthumb and are consistent with similar values observed in other field-based performance assessments. 6,13 There is no strong trend in either of these two figures. Nearly all the systems are derated by 060 to 070 with a small increase in the ratios for locations in the cooler and windier climates (Flagstaff and Prescott) in Arizona. SYSTEM COST EXPERIENCE The APS systems were procured and installed through a pay-as-you-go funding source created by the combination of system benefits charges and surcharges that were established by the Arizona Corporation Commission as additions to customers monthly bills. This type of funding avoided any financing costs for the system installations. Figure 10 presents the APS average installed cost ($ per array rated W dc at STC) by year for tracking horizontal systems larger than 90 kw dc.

9 PV POWER PLANTS IN ARIZONA 361 Figure 10. Average installed cost for tracking horizontal systems By examining larger systems, the issue of system size (and economies of scale) can be minimized in the comparisons. Average installed system costs for one-axis tracking horizontal systems have decreased by nearly 20% to $596/W dc in 2003 with the least cost system installation at $518/W dc. For comparison, the average installed cost for the fixed horizontal configuration larger than 50 kw dc in 2003 is $531/W dc,$065/w dc less than that for the average horizontal tracking configuration with the least cost system being $476/W dc,or$042/ W dc less than the least-cost horizontal tracker. The true cost for the tracking option increase probably lies somewhere between $042 and $065/W dc. Although a number of factors enter in to this comparison, this difference in part represents the additional cost associated with the site preparations and structure for horizontal tracking systems. From 1998 to 2003, APS installed 18 fixed flat-plate systems at latitude tilt in various Arizona locations. These systems included both roof-mounted and ground-mounted installations and ranged in size from 19kW dc dc to 79kW dc. The average installed cost by year for these systems is presented in Figure 11. These costs averaged $731/W dc with the least expensive installation costing $699/W dc. It is interesting to note that the trend for installed cost is relatively flat over the past several years. One possible explanation for this trend is that the small decrease in cost of PV modules is offset by the increased cost of labor and the recent cost increase in small inverters. The true measure for comparing different solar technologies is the cost of delivered kw h ac energy. To perform this comparison, the authors propose an energy cost figure defined as the average installed system cost ($/kw dc ) divided by the energy output (kw h ac /kw dc ) expected over a 20-year period. Although the resulting cost figure represents $/kw h ac, this figure does not include financing costs, the cost of capital or O&M costs and, thus, is not a levelized energy cost and is not portrayed as such. Figure 11. Average installed cost for fixed latitude tilt systems under 10 kw dc

10 362 L. MOORE ET AL. Table IV. APS average annual O&M cost as a percentage of initial installed cost for tracking horizontal configuration Site Name Size (kw dc ) Annual O&M (%) Flagstaff Gilbert Nature Center Glendale UPG Prescott Airport (13 systems) Prescott ERAU Scottsdale East and West Tank Ocotillo Ocotillo Yucca Power Plant For 2003, the energy cost figure for the horizontal tracking configuration in Phoenix is $016/kW h ac. A similar figure for the fixed horizontal configuration is $021/kW h ac while the much smaller fixed latitude tilt configuration is $024/kW h ac. This comparison shows the benefit of a flat-plate tracking configuration over a fixed configuration, namely reductions in the energy cost figure of 23% and 33% from the similarly sized fixed horizontal and much smaller fixed latitude tilt systems, respectively. SYSTEM MAINTENANCE EXPERIENCE One of the most important pieces of information from these systems has to do with O&M field experience. In all cases, APS was responsible for providing maintenance for these systems. These data represent some of the first qualified multi-system O&M information for utility-scale PV systems that Sandia National Laboratories has had access to in nearly 25 years of R&D in the U.S. DOE National PV Program. Although Sandia has published annual O&M costs for small off-grid systems, namely water pumping 14 and residential, 4 these are the first actual data for utility scale systems greater than 90kW dc. The average annual O&M costs for several of these systems are shown in Table IV. The average annual O&M cost for all the tracking horizontal configurations is 035% of initial system installed cost. The maintenance experience with the tracking horizontal systems was primarily associated with the inverters. Most of these systems required inverter adjustments during initial set-up for a period of up to six months after installation. After this period, the inverters, generally performed well. The maintenance costs for these systems do reflect the recurring costs of servicing the inverters, but do not include the rebuild/replacement costs of the inverters at end of life. It is anticipated that the actions will be required every 7 10 years after installation. The earliest installed systems are beginning to experience an increase in inverter problems, and APS is planning the replacement of some of these components. In contrast to inverters, maintenance associated with modules was minimal. Maintenance for the tracking components started higher during the early part of the APS development effort, but with time, this area has become a small contributor to maintenance costs. CONCLUSIONS The Environmental Portfolio Standard program has proven to be a significant stimulus to increasing the installed capacity of PV systems in Arizona. The funds provided through the program have allowed APS to install several megawatts of PV systems over the past few years, while the kw h ac criteria has focused the program on finding the least cost option for energy production. The energy data, system cost, and maintenance experience with these systems provides a treasury of information that establishes a benchmark of current PV capabilities. This paper has identified a number of findings, including:

11 PV POWER PLANTS IN ARIZONA 363 * APS has found the tracking horizontal configuration provides the least energy cost of the flat-plate configurations examined for Arizona. * For a Phoenix location, the average annual ac energy output of the tracking horizontal configuration is 1813 kw h ac per kw dc of array. * For the systems examined, the energy production benefits of tracking were substantial. When compared with fixed latitude systems, measured values were 23% and 37% for horizontal and tilted tracking, respectively. * The average annual ac power of all system configurations in Phoenix is approximately 063 of the array dc nameplate rating based on predicted annual peak sun hours. * For 2003, the average installed cost for tracking horizontal systems greater than 90 kw dc is $596/W dc, with the least expensive costing $518/W dc. * Average annual O&M cost for tracking horizontal configuration systems greater than 90 kw dc was 035% of initial system installed capital cost, not including rebuild/replacement cost of the inverter. * The initial system capital cost of tracking horizontal configurations drops with experience, resulting in a decrease of 20% over the past five years. * A customer-friendly energy output rating for these systems that follows the World Energy Council s nomenclature is proposed to reduce the confusion of system performance and customer expectations. Acknowledgement Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy s National Nuclear Security Administration under contract DE-AC04-94AL REFERENCES 1. Environmental Portfolio Standard R , Arizona Corporation Commission, Phoenix, AZ. utility/electric/r APS website Hayden H. Benefits of the Environmental Portfolio Standard, Arizona Public Service, Phoenix, AZ, May Canada S, Moore L, Post H, Strachan J. Operation and maintenance field experience for off-grid residential photovoltaic systems. Progress in Photovoltaics: Research and Applications 2004; 13: DOI: /pip King DL, Kratochvil JA, Boyson WE. Power and energy: status of array design, rating, monitoring methods. Proceedings of Solar Energy Technologies Systems Symposium, Albuquerque, October 2003, 6. Solar Electric. PV performance data report: results of over 100 TEAM-UP PV installations. Solar Electric Power Association, Washington DC, December 2001, 7. JRC. Guidelines for the assessment of PV plants: document B: analysis and presentation of monitoring data. Issue 41, JRC, Ispra, Italy, World Energy Council. Performance of generating plant pgp/renewable/performance.asp. 9. California Energy Commission Emerging renewables buydown program on-site verification report: phases I, II, and III, California Energy Commission, Sacramento, CA, June 2002, _SYS_SITE_VER.PDF. 10. NREL. Solar Radiation Data Manual for Flat-plate and Concentrating Collectors. NREL/TP , National Renewable Energy Laboratory, Golden, CO, April Risser V, Post H (eds). Stand-alone Photovoltaic Systems A Handbook of Recommended Design Practices. SAND (revised), Sandia National Laboratories, Albuquerque, NM, July Thomas MG, Post HN, DeBlasio R. Photovoltaic systems: an end-of-millennium review. Progress in Photovoltaics: Research and Applications 1999; 7: Scheuermann K. Just how big is a 2 kw photovoltaic system? Analysis of hourly metered data from 19 residential gridtied systems in California helps to answer questions about actual system power output. Home Energy Magazine, January February Moore LM. Sandia s PV reliability database: helping businesses do business. Quarterly Highlights of Sandia s Solar Programs, Vol 1, 2001,