Integrating a high-performance building
The envelope’s ability to capture passive energy, harvest daylight, reduce solar heat gain, eliminate energy-wasting thermal bridging, and provide for natural ventilation―along with the high-performance lighting to reduce lighting use during peak operation―paved the way for a smaller, less costly, and more energy-efficient HVAC system.
The IUB/OCA―designed to maintain a heating temperature of 72 F and a cooling setpoint of 74 F with a maximum of 50% relative humidity―employs a high-efficiency geothermal heating and cooling system with a geothermal field tied to dual-stage water-to-air heat pumps with electronically commutated motors (ECM), and variable speed pumping. A total energy recovery unit (both sensible and latent) provides energy savings by capturing the heating or cooling from the exhaust air and pre-tempering fresh ventilation air.
This dedicated outdoor air system feeds into the back of individual heat pumps in the building, providing good indoor air quality for the users. The ventilation air supplied to the building is monitored and trended. CO2 sensors are employed in densely occupied spaces, and a demand control sequence is used to move ventilation air to the rest of the building versus an almost empty room.
As a result of the integration of high-performance envelope, shading and lighting, the load consumed by heating and cooling is drastically reduced.
Figure 4 shows the distribution of the energy use for a code-compliant baseline for the building and program. Notice that the heating and cooling are the largest loads, and together are over half the energy use. (This is for a packaged variable air volume, or VAV, system with direct exchange, or DX, heating and cooling, and is defined by ASHRAE 90.1-2004.) Figure 4 also shows the distribution of the energy use as designed for the IUB/OCA. Notice the largest load is now plug load, larger than heating and cooling combined.
Notice too that the fan/pump energy also increased as a percentage of the overall use. This is why ECM motors were specified for the heat pump fans, because they are extremely efficient and adjustable. This left the hydronic pumping system to be optimized. Because typical energy modeling techniques do not reflect the pumping use well, a spreadsheet was used to evaluate different methods of pumping. Several systems configurations were evaluated, from central pumping with variable frequency drives (VFDs) to fully distributed pumping at each piece of equipment.
Because several of the loads needed a constant flow when on, the optimum system was a combination of central and distributed pumping. The heat pumps were installed with automatic isolation valves so that when off, the flow through the unit is closed, and the flow of the overall system was reduced by the VFD
on the central pumps. Then a couple of larger loads, such as the energy recovery ventilation (ERV) and a water-to-water heat pump, were provided with smaller distributed pumps that only activate when the unit is on. This analysis was confirmed in the measurement and verification process, as this portion of the building load was using less energy than the energy model predicted.
Integration equals efficiency
The IUB/OCA recently achieved an Energy Star score of 100, and over the first two years of operation has performed extremely well, with an EUI of 21.2 kBtu/sq ft/year without renewable energy, and 16.7 with renewable photovoltaics (PV). This exceeds the baseline by more than 77% and the LEED energy model by 27%. The building is outperforming efficiency targets, and the PV is providing more on-site energy than expected.
Energy modeling was completed by The Weidt Group, Des Moines, Iowa, which is also subcontracted through the Iowa Energy Center to conduct the highly detailed measurement and verification (M&V), which has shown significant results. This process included the refinement of the design energy model to include actual building schedules, actual plug load energy use, and real matched weather data, providing the design team with a view into the decision-making process for the project.
Preliminary results from the most recent year reviewed (from March 2012 to April 2013) show the energy model is predicting energy use to within 1% of actual. Having this data informed the design team on the energy modeling tools used. For example, the model slightly over-predicted savings from daylighting and under-predicted savings from fans and pumps. Decision making was not compromised, but the ability of current software still does limit some predictions. This was demonstrated dramatically with the plug load of the building, which was much less than expected due to occupancy controls at workstations and diligent management by the users to use only what is needed. (Plug load prediction and control need to be addressed by the industry, and are mostly outside the control of building design teams.)
The maximum integration of building envelope, HVAC, lighting, and shading allowed the design team to not only reach but surpass the energy performance goal of 28 kBtu/sq ft/year.The IUB/OCA’s affordable, optimized, and integrated design strategies provide a model for other facilities anywhere in the country.
Watch this video for more about the project.
Scott Bowman is principal/corporate sustainability leader at KJWW Engineering Consultants and has over 30 years of experience in high-performance building system design and overall project management. His specialties are in direct digital controls, energy efficiency, sustainable and green design, and systems commissioning. Carey Nagle serves as a leader in the design and management of high-performance and sustainable projects in his role as a project manager/project architect at BNIM. His broad range of project experience consists of several office buildings, higher education projects, theaters, and museums.