Optimizing Photovoltaic Design, Part II
Optimizing Fixed-Tilt PV Arrays by Using Two or More Array Angles
By Hernando Miranda, president of Soltierra -- Consulting-Specifying Engineer, 6/19/2007 11:20:00 AM
The Audubon Center at Debs Park, Los Angeles, is a unique off-the-grid building. The center’s systems are 100% solar, meaning that space heating, space cooling, hot water and electric power are all rooftop solar systems. In a prior CSE Green Scene article these systems were discussed with a focus on the photovoltaic (PV) system providing electric power. Because the owner did not go off-the-grid until after the basic building design was agreed upon with local authorities, the PV system was not as optimally designed as is could have been. This article discusses how the PV design could have been optimized if the building design was flexible enough to allow the PV to inform part of the building shape.
The Audubon Center’s PV system features arrays at two slightly different tilt angles. The system has a power output value of 26.0 kW DC, and consists of 208, Kyocera KC125G, 125 watt, multi-crystalline solar cell panels. One of the arrays is tilted south at 14°. The second array tilts west at 14°. Having two array angles may seem more than what is necessary, but when two angles are used it becomes possible to optimize the size of the overall PV system.
PV designers call orientation “azimuth angle.” Looking directly down on a world map and measuring rotation in a clockwise manner, north has an azimuth angle of 0°, south 180°, and west 270°. For the Audubon Center’s PV system the array parameters are:
Audubon Center at Debs Park, Photovoltaic System Parameters
Array 1: kW DC = 5.50 (44 x 125 watt panels); Azimuth Angle = 180°; Tilt 14°
Array 2: kW DC = 20.50 (164 x 125 watt panels); Azimuth Angle = 270°, Tilt 14°
Location: Los Angeles, CA 90031
The information above is all one needs to calculate the actual power, or kWh AC, generated by the overall system. PV output calculations are complex, but an easy way for a novice to do them is to run a free online program developed by National Renewable Energy Laboratory (NREL). The program is called PVWATTS “A Performance Calculator for Grid-Connected PV Systems.” Even though the program states that it is for grid-connected, or grid-tied, systems, it also can be used for off-the-grid systems. PVWATTS determines power output by conducting hourly calculations, for an entire year, based on solar energy data, which has been collected by weather services agencies over the years. The program is available here. Run version 2, which features many more solar data locations than the original version 1.
PVWATTS provides tabular output of monthly AC power output, solar energy availability and energy generation cost. AC power outputs are the useful values for determining how orientation can impact building design. The best way to see what is really going on is to plot the data in a chart using a spreadsheet program. When using PVWATTS set the array DC value to 1.0 kW which makes it easier to study the difference in various array sizes without having to run PVWATTS multiple times. Simply multiply the 1.0 kW DC values by the kW DC values for your own array.
GRAPHIC 1: PVWATTS - Tabular Data for 1.0 kW DC array
GRAPHIC 2: PVWATTS - Plotted Results for 1.0 kW DC array
For the Audubon Center at Debs Park the two AC power outputs of the two combined arrays are plotted in the following figure. The monthly building load profile was determined by calculating the summer (June) and winter (December) loads and interpolating for the spring and fall periods. Because the hottest month of the year typically occurs in September the interpolation was adjusted accordingly. A load profile also could be obtained for a grid-connected building using data from proper type of energy analysis program.
GRAPHIC 3: Audubon Center at Debs Park PV – System As-Built
A simple review of plotted data for the Audubon Center project clearly indicates that more energy is generated during the summer months than necessary. The total annual “Extra Power Generated” is 7,340 kWh or 28% more than needed. A more careful review of the plotted data indicates that less power is generated in December, 1,580 kWh, than the project’s estimated needs, 1,770 kWh. So, in reality “Extra Power” in not actually generated based on the chosen tilt and azimuth angles of the PV arrays.
The owner’s original thinking that having less energy generated in December was acceptable. The center was likely to be closed during the two-week end-of-year, holiday period. But, the center has been holding special events during this period and the PV system does not generate enough power to meet the actual energy needs.
Because the spreadsheet analysis was set up based on a 1.0 kW DC system it is easy to study what would happen to the PV power generation by moving all of the panels to Array 1, south facing, as one option, and then moving all of the panels to Array 2. These studies are shown the following charts.
GRAPHIC 4: Audubon Center at Debs Park PV – All South Facing
GRAPHIC 5: Audubon Center at Debs Park PV – All West Facing
The all south facing study would solve the power generation problem in the winter months. The exact same number of total PV panels, 208, now generates approximately 9,900 kWh of “Extra Power” or 38% more than needed. The all west facing study shows that approximately the same summer peak power is generated, but the winter power generation is significantly less than needed; 1,456 versus 1,770 kWh for December.
If the PV was included as a consideration earlier enough during the design process, it would have been possible to change the design of the building to take into account the optimum positioning of the PV panels. Classic PV design solutions would say that the panel tilt angle should match the latitude angle of the site; 34-deg in the case of the Audubon Center, and facing directly south. A “System Option 1” study using this design shows that a significantly smaller array could have been used to power the center; In this case a 24.0 instead of a 26.0 kW DC PV system. A 2.0 kWh reduction in the PV system would have saved the owner approximately $25,000. This option generates 8,098 kWh, or 31%, more energy than needed; maybe the design can be better. A careful review of the plotted results shows that the system is still generating more power than need, but now September is the critical energy month instead of December.
GRAPHIC 6: Audubon Center at Debs Park – Revised PV System Option 1
Except for the month of September, the PV Power Generation and Estimated Building Energy Demand profiles are a reasonably good match. A “System Option 2” study assumes that 50% of the panels can be placed on a roof slope, facing south, at a 14° tilt, and 50% of the panels can be placed on a roof slope, facing south, at a 34° tilt. The size of the PV system has now dropped to 21.0 kWh DC. The extra power generated has been reduced to 3,165 kWh, or 12% more than the estimated building demand. The savings to the owner are now approximately $60,000 for the 5.0 kWh DC PV size reduction.
GRAPHIC 7: Audubon Center at Debs Park – Revised PV System Option
Still there is an issue of what to do during the month of September when PV power generation is less than the estimated building demand. Solving the September power problem is difficult. The optimum solar power generation angle is close to the autumn equinox; 34° tilt, facing directly south. This assumes that maximum power is generation without regard to whether the actual load demand is greatest during the morning or afternoon hours. The Audubon Center actual building load needs occurs in the afternoon, roughly 1 p.m. and later, when cooling demands are greatest. During the morning and very early afternoon hours “System Option 2” is likely to meet the actual building load demands. Unfortunately, the PVWATTS program doesn’t seem to provide an option to look at hourly power generation breakdown. The best we can do is to add a small third array, “Array3”, to the system using and orienting the panels to point directly where the sun two weeks prior to the Autumn Equinox, Sept. 9, at 2 p.m.. An easy way to determine solar position is to run the NOAA “Solar Position Calculator” available here. The solar position is 48° tilt, facing at 233° azimuth angle.
Adding a third array to the system, based on estimated building loads is not necessarily the best design option. A better solution would be to install the PV with added growth capacity at possibly one or two alternative position angles. Instead of installing more than is needed just to be safe— the typical design approach— don’t oversize but allow for future expansion of the PV system. “System Option 3” shows the PV system power generation profiles with “Array 3” marked as a contingency system.
GRAPHIC 8: Audubon Center at Debs Park – Revised PV System Option 3
The goal of this and a prior article in CSE Green Scene is to get designers to think about renewable energy systems as of part of a smart integral design approach for a building. Smart design includes consideration for a reduction of resource use which benefits everyone. The building owner saves money. The cost of products should stay down if buildings only install what they actually need. The design of the PV systems is smartly integrated into the design, and the building can look more elegant. Best of luck to those who buildings owners, architects, engineers and anyone else who finds the information presented in this article worth pursuing for their own projects.
To view and print graphics 2 through 8, click here.
