...the U.S. Department of Energy’s (DOE) Building Technologies Program has established a goal to create the technology and knowledgebase for marketable zero-energy commercial buildings (ZEBs) by 2025. To help DOE reach its ZEB goal, the Buildings and Thermal Systems Center at the National Renewable Energy Laboratory (NREL) studied six buildings in detail over the past four years to understand the issues related to the design, construction, operation, and evaluation of the current generation of low energy commercial buildings. These buildings and the lessons learned from them help inform a set of best practices—beneficial design elements, technologies, and techniques that should be encouraged in future buildings, as well as pitfalls to be avoided. The lessons learned from these six buildings are also used to guide future research on commercial buildings to meet DOE’s goal for facilitating marketable ZEBs by 2025. The six buildings are...

Measurements in all six buildings showed that they used more energy and produced less energy than predicted in the design/simulation stage. Several reasons were documented:

�� There was often a lack of control software or appropriate control logic to allow the
technologies to work well together.

�� Design teams were too optimistic about the behavior of the occupants and their acceptance of
systems.

�� Energy savings from daylighting were substantial, but were generally less than expected.

�� Plug loads were often greater than design predictions.

�� Effective insulation values are often inflated when comparing the actual building to the as designed building.

�� PV systems experienced a range of operational performance degradations. Common
degradation sources included snow, inverter faults, shading, and parasitic standby losses.

During nighttime hours when the Oberlin or Cambria PV system was in standby mode, the inverters and transformers consumed electricity. The inefficiency of the isolation transformers in these systems results in a power draw of approximately 300 W per 15 kVA transformer. At Oberlin, this standby parasitic load of the three inverters and isolation transformers was a constant 900 to 1000 W during times of no PV production. The primary purpose of the isolation transformers was to transform the three-phase AC 208- delta output of the inverters to utility-compatible, three-phase AC 208-wye/120. The Oberlin no-load transformer inefficiency of 2% of rated capacity resulted in a standby loss of 4,363.5 kWh/yr, or 7.3% of the total PV production. This does not include transformer losses when the PV system is generating power. Cambria’s inverter faults caused considerably greater standby losses. The causes of the inverter faults were a high AC voltage and high temperature. The high temperature fault was the most severe because the system would have to be manually reset. The inverter was removed and sent to the manufacturer in December 2003 and replaced with a new unit in February 2004. From May 31, 2002 to December 31, 2003, the main PV system produced no energy on 50% of the days because of inverter problems. On many other days the system was operational for only part of the day because of inverter problems. From May 31, 2002 to December 31, 2003, the parasitic load on the PV system equaled 40% of the energy delivered to the building by the main PV system. Most of the parasitic load (37%) occurred when the PV system was down at night or because of an inverter fault; the other 3% were transformer losses during PV system operation. From the time the inverter was replaced on February 20, 2004 to December 31, 2004, the main PV system was down only three days because the whole system was shut down. During this same period, the parasitic load of the isolation transformer was 18% of the total energy delivered to the building. The monthly parasitic load varies from 11% in summer to more than 50% in winter