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A Commercial Solar Case Study: Park Garden Apartments

  • Location: Rohnert Park, Calif.
  • Date Completed: March 5, 2004
  • System Peak Capacity: 71.7 kW
  • PV Surface Area: 6,962 square feet

Park Garden Apartments is a 28-unit apartment complex located in Rohnert Park, California with shared amenities such as laundry facilities. The owners, Aidan and Barry O'Sullivan, wanted to install a solar electric system to offset their property's escalating utility costs. Their primary drive was to take advantage of the economics of solar. All the units were on a single meter with electric utilities included in the rent. They chose a commercial-scale, net-metered, solar electric generating system that was designed by Cooperative Community Energy (CCEnergy) and installed by Marin Solar.

The two buildings required re-roofing due to poor drainage. The new roof needed a higher peak to improve drainage, a change that necessitated new framing and decking. CCEnergy worked with the structural engineering firm CSW Stuber-Stroh to design the new roof with additional blocking between rafters to support the roof attachment hardware for the PV array structure. The structural reinforcements are designed to withstand the substantial static and wind loading of PV arrays tilted at 15 degrees.

An additional challenge involved the single master utility meter that measures power usage for the circa-1960s buildings. PG&E had long ago discontinued the residential master meter rate tariff, and would not approve a switch to the recommended Time of Use rate unless both buildings were retrofitted with meters for the individual units. CCEnergy petitioned PG&E to permit Park Garden Apartments to switch to the residential E-Net rate tariff while remaining on a single master meter. This effort was successful and avoided the expensive meters and rewiring that would have made the project financially unfeasible.

CCEnergy Field Reps Daniel Pellegrini and Brent Eubanks visited Park Garden Apartments to conduct a site evaluation. They measured the two available roof areas and the shading from nearby trees to determine if the site was at all suitable for solar energy. They also inspected the existing electrical panels and utility meter so that the new PV system could be designed into the existing facility equipment. They reviewed the utility bills for the apartment complex and entered the monthly usage figures into CCEnergy's analysis tools to come up with a recommended system size that would replace the facility's annual utility use with clean solar power.

Why the owners chose CCEnergy
As a member benefit organization, CCEnergy works to provide the best solution for its customers. In this case, CCEnergy presented a system design that was correctly sized to achieve the O'Sullivans' goals. Competing firms recommended larger systems that would have over-produced electricity that would provide no financial advantage. CCEnergy was able to show a much better financial return for their investment.

As contractors themselves, the O'Sullivans wanted to participate in the project by providing much of the labor. CCEnergy encouraged them to be involved and helped them select an electrical contractor for the solar-specific installation labor.

The 71.7 kW photovoltaic (PV) system rooftop array feeds the power system at Park Garden Apartments and is tied to the utility grid so that any excess power can be sent into the grid and literally run the utility meter backwards. During daylight hours, the PV system generates enough electricity to power more than 30 homes. The tilted PV array and solar electric system transform an unused asset - the roof - into a significant asset that reduces operating costs by generating clean, reliable electricity, and reducing building energy load and peak demand costs. The solar roof panels also shade the roof to reduce thermal cycling and air conditioning costs, and slow UV degradation of the roof. The modular design of the PV system allows it to be tailored exactly to Park Gardens Apartments' specific application requirements. It can be easily expanded in the future should the need arise.

By installing a solar energy system on their apartment buildings, Park Garden Apartments has found a way to meet their renewable energy goals and help achieve their overall commitment to environmental design, operations and profitability. The on-site electricity generated by the solar system is a significant component in lowering operating and peak demand costs.

The solar generation system also spares the environment from thousand of tons of harmful emissions, such as nitrogen oxides, sulfur dioxide and carbon dioxide, which are major contributors to smog, acid rain and global warming. Over the next 25 years, Park Garden Apartments solar project will reduce emissions of carbon dioxide by nearly 450 tons. These emissions reductions are equivalent to planting almost 130 acres of trees or not driving 1 million miles on California's roadways.

System description
Together, Park Gardens' rooftop photovoltaic (PV) arrays can produce up to 71.7 kW under peak sunlight conditions. The PV modules are the latest KC167G modules from Kyocera Solar, one of the world's largest manufacturers of PV modules. The front building has 180 modules facing southeast while the rear building has 330 modules facing southwest. Both arrays are tilted at 15 degrees to enhance the power output yet minimize the visual impact. The PV modules utilize silicon technology to convert sunlight directly into electricity. The DC output from the PV modules is converted to AC electricity by a bank of 28 inverters located at each array. The inverters are made by SMA, a German company that dominates the PV market in Europe and North America by making the highest quality equipment. The output of the inverters is compatible with the electrical requirements of standard residential buildings, avoiding the need for additional transformers for connection to the buildings' electrical service panels. The power output is monitored in a central location enabling detailed system performance analysis, diagnosis, and data logging.

The Park Garden Apartments system qualified for a rebate under California Public Utilities Commission's Self Generation Incentive Program. In fact, this state funded program covered 50% of the cost so net cost was only half the original price. The electricity savings of approximately $20,000 per year and cost effective design ensures rapid payback.