Optimizing Photovoltaic Design
Typical approaches to photovoltaic (PV) design often result in less-than-optimum energy production. This results in larger PV arrays than might be required—meaning greater cost to a building owner. Large arrays use greater amounts of material, which means increased fabrication efforts, higher transportation costs and more installation efforts. The challenge is to get by with fewer materials by designing a building and PV system in harmony with each other.
Typical approaches to photovoltaic (PV) design often result in less-than-optimum energy production. This results in larger PV arrays than might be required—meaning greater cost to a building owner. Large arrays use greater amounts of material, which means increased fabrication efforts, higher transportation costs and more installation efforts. The challenge is to get by with fewer materials by designing a building and PV system in harmony with each other. In other words, can a building design take solar orientation into account from the beginning to optimize PV energy generation?
until design development, or late into the construction documents phase. In some cases, mostly due to owner issues of where-is-the-money-coming-from-for-this, the design isn't determined until building construction is well under way.
This article is the first of a series that describes a guide designers can follow, and hopefully improve upon, to optimize PV system design. The example used did not result in a total optimum design, but the project went through several interesting steps to achieve a partially optimized design and serves as a lessons-learned project. Hopefully, the lessons will help designers consider PV systems as a design integration requirement from the very start of a project.
Even though standard design practice usually leaves PV to the later design phases of a project, there are benefits if one to began the process earlier and designed a system to optimize power generation to better match the loads generated by a building. Simple designs are the best—the fewer the parts and pieces, the easier it will be for maintenance staff to keep the system operating.
One case where a project attempted to match PV power output to actual load generation is the Audubon Center at Debs Park, which is located in Northeastern Los Angeles. The center is completely off-the-grid and it is completely solar powered. Due to design constraints imposed by local officials, the PV, solar hot water and solar heating and cooling systems all had to fit entirely on the roof of the project.
The Audubon Center is believed to be the first 100% off-the-grid, solar building in the world. Some off-the-grid buildings might feature PV systems and solar hot water, but their heating and cooling systems are more typical of standard designs, either running off the PV system for electricity to cool and heat, or heat is provided by trucked in propane stored in an on-site tank.
The Audubon Center’s PV system features arrays at two slightly different tilt angles. The system has a total rated power output value of 25.0 kW DC, and consists of 208, Kyocera KC125G, 125 watt, multi-crystalline, solar cell panels. The total array area is approximately 2,200 sq. ft., and the panels are mounted to simple u-channel frame structures. Since this system is off-the-grid a battery bank is used to store energy, which consists of 96, Absolyte 100A23 batteries, with a total rated 5,600 Ah (amp-hour) output.
Rated outputs are useful for determining the physical size of the components in a PV system. But to determine actual useable power generation, solar conditions must be known, in addition to PV system losses and inverter inefficiencies converting DC to building usable AC power. Estimated building demand loads are 94 kWh per day during summer and 57 kWh during winter. PV system available useful power is approximately 138 kWh during the longest day of the year—June 21—99 kWh during the hottest day of the year, and 52 kWh during the shortest day of the year. Battery storage capacity is estimated at three to four days, which allows the Audubon Center to continue in operation in the event that several heavily overcast days occur in a row during a severe storm event. During the shortest days of winter the estimated building demand is higher than the available useful power. This is acceptable since it occurs over the winter holiday period when staff is on break, and power needs dip below the typical daily demand.
Determining actual loads for the Audubon Center was no easy task. Every load was considered, including site lighting, power tool use, fountain pumps and more. Wherever possible loads were reduced, fountain pumps were downsized, high-efficiency refrigerators were purchased, and a daily limit on copier printing was set. In fact, if the building weren't single story, and the loads weren’t reduced so aggressively, then it wouldn't have been possible to meet the imposed design constraints. The Audubon Society would have ended up abandoning the goal of going off-the-grid, and paid the high price to connect the building to the utility grid. The price was high because the connection point was 1/4 mile away and power utility limes were required to be placed underground.
PV Panel Showing How Solar Energy Collected Varies With Tilt at 33° and 14°.
To meet the imposed design constraints, the project looked into options for integrating the design into the building.
Option 1 was a fixed-tilt PV system. The walls and roofs would be tilted as required to match the best PV tilt angle, which was direct south facing at 33 degrees above horizontal. This is standard optimum design approach, but it somewhat constrains architectural design.
Option 2 was also a fixed-tilt PV panel system. In this case, thearray is not optimized, a larger array area is need, because the sun strikes the panels at a steeper angle, and they collect less solar energy.
The second option was chosen for the Audubon Center. Tilting the panels at approximately 33° as required by the first option, on a roof slope that was chosen to be 14°, was considered architecturally unattractive to neighbors living on hill which overlooked the center. An additional problem was that tilting panels at 33° creates shadowing on a low-slope roof, leading to less than optimum performance. Shadowing reduces by separating panels apart from each other, but required roof area install the PV would have increased than what could fit onto the roof as required by the imposed design constraints.
Installed system showing PV, Solar HVAC and Solar Hot Water
The Audubon Center PV system has been in operation since September of 2003, more than three and one-half years. Now that this system has been described it will serve as a case study for what could have been. A future article will discuss how the size of the PV array could have been reduced by using two very different array angles, and how might have been used to establish a different exterior shell design of the building.