The Byron G. Rogers (BGR) Federal Office Building, just under 500,000 sq ft and located in downtown Denver, is pursuing impressive sustainability targets above and beyond federal requirements. Built in the 1960s, this building not only has to achieve federal 2030 NetZero goals, but as a historic building it must also reflect respectfully on its past and preserve architectural elements characteristic of the era in which it was built. Led by Mortenson Construction, the forward-thinking design team comprises architecture firms Bennett Wagner & Grody and HOK, RMH Group (mechanical, electrical, and plumbing engineering), and Rocky Mountain Institute as the high-performance building consultant.

 

Using whole-building DOE-2.2 energy simulations, the design team predicts this building will save between 60% to 70% in annual energy costs as compared to the pre-renovation utility bills. Prior to the renovation, the building’s energy use intensity (EUI) in 2009 was 91.8 kBtu/sq ft/yr. Post-renovation, we expect the EUI of the building to be between 28 and 38 kBtu/sq ft/yr. These energy reductions can be achieved with a net present value of $556,700 over a 20-yr period.

 

APPLYING THE RIGHT STEPS IN THE RIGHT ORDER

Too often, design teams start designing systems and selecting equipment before optimizing the loads and function of the building. In the deep energy retrofit process, it is important to identify the right steps to take and equally important to perform these steps in the right order (Table 1). Following this process helps achieve the most cost-effective energy reductions and produce multiple benefits from single expenditures.

Set quantifiable goals. At the start of a project, it is important to identify clear and quantifiable project goals. It is particularly effective to hold a goal-setting workshop with all stakeholders to achieve broad commitment and to assign responsible parties. Goals can be classified into the categories in Table 2.

Define enduser needs. The second step in reducing the energy consumption of an existing building is to define the needs and services that endusers require, rather than jumping right to the equipment or capacity needed to provide it. By questioning and clearly defining the needs, we can find passive strategies that best meet those criteria before assuming the need for powered technology.

Instead of approaching the design from the standpoint that “we need this many watts of electric lighting,” the lighting designers for the BGR building determined that the occupants needed even light distribution, luminance levels in a range of 30 to 50 foot-candles (fc), and glare minimization. Based on this, the building will feature a task-ambient system using LED luminaires in conjunction with local task lights within the furnishings. The spaces will have active daylight harvesting and occupancy controls. Ambient light averages 30 fc from permanently fixed luminaires across the viewplane, while local task lighting provides supplemental illumination up to the required task lighting levels. This approach conserves energy and allows occupants to fine-tune the amount of light in their work area.

Understand existing conditions and plans. Next, the design team should assess the current state of the existing facility. What needs are not being met? Why not? What systems or components require replacement or renovation for non-energy reasons? What are the costs or interruptions to service or occupancy? By identifying these planned renovations early, it may be possible to combine these with a desired energy retrofit to optimize the total return on investment.

Reduce loads. Before considering how to satisfy the heating and cooling loads of a building, efforts should focus on reducing those loads as much as possible, first through passive means and then by using the most efficient end use equipment. Sources of heating and cooling loads can be classified as found in Table 3.

• Passive load reduction.The more that energy demand can be met intrinsically by the passive design of the building, envelope, and site plan alone, the less the burden on mechanical systems, renewables, and local infrastructure. Early on, the design team should investigate ways to retrofit the existing building to have a more climate-responsive form and envelope that capitalizes on the site’s climatic conditions. For the BGR building, the focus on passively reducing loads was two-fold as outlined in Table 4.

• Load reduction with efficient end use equipment.  As cooling loads within office buildings are dominated by internal gains, the most significant reductions can be achieved by specifying extremely efficient interior lighting and plug load equipment. While plug loads are left to the tenants’ discretion, the BGR design team still performed plug load audits and provided tenant sustainability guidelines with plug load recommendations. The average lighting power density (LPD) for Byron Rogers will be 0.65 W/sq ft (7 W/m²), using 100% LED lighting, with automatic daylight dimming controls.

Select appropriate and efficient systems. The next steps are to consider which HVAC system types and sizes are most appropriate to handle the significantly reduced loads, and then to select the most efficient technologies available for that system. With energy demands minimized, the goal for BGR was to design a low-energy mechanical system that takes advantage of Denver’s dry climate without sacrificing occupant comfort. Geo-exchange and water source heat pump systems were first considered due to their ability to transfer heat, simplicity of design, and proven ability in the field, but they were ultimately rejected due to site constraints. However, after a detailed evaluation of the building’s physical orientation, the design team determined that heat transfer could be accomplished in a manner similar to geo-exchange by using the building itself, along with thermal storage tanks, as the geo-exchange bore field, and using the chiller as a central heat pump. Because significant heat is generated inside the building, this heat could be used to offset winter and nighttime heating needs.

An active chilled beam (ACB) system was selected to optimize the transfer of energy between the two solar exposure zones and the interior zone while still storing low temperature heat for times when the building is largely unoccupied but still requires heat. The primary energy savings comes from the reduction in required cooling — active chilled beams use 58°F water for cooling as compared to 45° water for typical air-based cooling. In this climate, the cooling towers alone can often produce 58° water — when the chillers are required, they run much more efficiently to produce the higher temperature water. Heating mode is also more efficient because it uses low temperature water.

The ACB system will use recovered heat generated by the chiller to heat the building, using 90% less energy than heating the building with a high-efficiency boiler. Heat can also be captured during the day, when there is an abundance of available solar heat energy and internal heat gains, and stored for use at night and on weekends.

Once the most appropriate mechanical and lighting systems were selected, the design team specified the most cost effectively efficient technologies. Due to the size of the building, energy-efficient centrifugal chillers were the best choice for producing chilled water. The heat pump chiller is a magnetic bearing chiller to optimize both full load and part load performance. This chiller is capable of transferring heat between the chilled water and heating water systems at a part load COP of 12 and a full load COP of 9. In addition, 95% efficient condensing boilers are used when heat generated inside the building from lights, equipment, and people is not adequate to maintain building temperature.

Find synergies. To achieve deep energy savings, it is important to capitalize upon synergies across disciplines and find opportunities to recover and reuse waste streams. Through these efforts, we can often realize multiple benefits from a single design decision.

In the BGR building, ventilation air heat recovery will be accomplished through an enthalpy wheel in dedicated outside air units designed to recover 80% of the heat being exhausted from the building. In addition, thermal storage tanks are utilized to store heat generated during the day to supply building heating needs at nights and on the weekends. Through energy analysis, it was determined that the building would consume approximately 15 million Btu (15.8 kJ) during a typical winter weekend in heating energy. To provide this heat, a 50,000-gal thermal storage tank is located in the basement of the building. By using heat generated during the day to charge the tanks, this heat can be used at night via the heat pump chillers to boost heat into the building at a high COP.

Optimize controls. After appropriate and efficient technologies are selected, the focus should shift to optimizing control strategies. For the Byron Rogers project, the team focused on optimizing lighting and HVAC controls. Individual groups of luminaires will be controlled by addressable modules connected to the lighting control system. The lighting control system will integrate time-based and sensor-based lighting controls with a network of sensors, power packs, photocells, and wall switches, with distributed intelligence. This innovative system provides global control via web-based lighting management software and will enable zones of devices to self-commission and function independently. The lighting control system will provide dimming, occupancy, and on/off control and will interface with the BAS for zone control of the HVAC system. Utilizing signals from occupancy sensors, the lighting control system will notify the BAS which zones are unoccupied. In response, the BAS will shut down the chilled beam hydronic heating/cooling supply.

Incorporate renewables and demand side management. After the more cost-effective efficiency measures have been exhausted, the team should consider incorporating on-site renewable energy and demand side management strategies to meet the now greatly reduced energy demand. When investigating these options, major considerations should include:

 

•How can the location of renewable energy systems be optimized within the site layout and integrated with other design features?

•What are the most site- and climate-appropriate renewable energy systems?

•What is the role of demand response, smart appliances, or thermal storage in shifting load off peak?

 

Renewable energy strategies will be utilized to further reduce the fossil fuel consumption and carbon footprint of the BGR building. Flat plate solar thermal collectors will supply a large portion of the domestic hot water demand in the building. In order to achieve the goal of net zero operating energy by 2030, additional on-site renewables and the purchase of some renewable energy credits (RECs) will likely be required over time.

Realize the intended design. Finally, to ensure full realization of the intended design, the team should create a plan to implement measurement and verification (M&V) and continuous commissioning. Working with the building owner and its independent testing inspectors, the Byron Rogers team has developed and will implement a plan for enhanced building commissioning and M&V, all in accordance with IPMVP requirements. As part of the integrative design process, the team set collaborative performance targets and ensured that each service provider understands their obligations and contributions to meeting them.

CONCLUSION

Existing commercial buildings represent a major opportunity in the U.S. to reduce building energy consumption. For many, however, there remains a technical question as to whether we can cost-effectively achieve deep energy savings in existing buildings, given legitimate concerns such as existing orientation and massing constraints, security requirements, and unknown future building plug loads and uses. The Byron Rogers design team hopes that this project will serve as an example that it is possible to use the right steps in the right order to reframe these design concerns into major efficiency opportunities. ES

Sidebar:  Windows from the inside

The retrofit of the BGR building provides an opportunity to implement window and envelope technologies that meet or exceed modern energy performance standards. Due to historic preservation constraints on the exterior building façade, all upgrades are being made from the inside of the building. The retrofit design strategy for the windows is to leave the existing punched window frames in place while minimizing the conductive path for unwanted heat and cold into the building that they provide. This will be done by replacing the existing one-inch insulated glazing unit (with a single air cavity and a grey tint) with a single pane of glass, similarly grey in tint to comply with historic preservation criteria. A new ultra-insulated operable glazing unit is being installed on the interior side of the frame, and will maintain a thermal break along with the proposed building envelope upgrades. The new glazing assembly will provide a higher transmittance value for visible light and dramatically improved solar heat gain performance than the existing windows.