If you have been involved in a LEED project in the recent past, you know that LEED NC v2.2 makes energy savings a requirement, not just an option for additional credits. All LEED projects registered after 6/26/07 must receive at least 2 credit points under E&A Credit 1, Optimize Energy Performance. This requires energy savings of at least 14% above code minimum performance for new buildings, 7% for existing building renovations.
To demonstrate the level of savings, energy models must compare the actual building design to a base code compliant model. As energy codes have gotten significantly more stringent in recent updates, the base line bar is set relatively high.
For most commercial buildings, the HVAC cooling loads are determined by five major contributing factors:
- Building "process" loads (people, computers, printers/copiers, elevators, etc.) (~25-30%)
- Building envelope thermal performance (~25-30%)
- Building outside air loads (~20-25%)
- Building lighting system efficiency and controls (~10-15%)
- Building HVAC system inefficiencies (friction losses and motor heat) (~5%)
Since the HVAC system must condition all of these loads, the actual energy usage for most commercial buildings is consumed by three major components:
- Building HVAC system including building envelope and outside air (~55-60%)
- Building "process" loads (people, computers, printers/copiers, elevators, etc.) (~25-30%)
- Building lighting system (~15%)
Typically, the amount of energy savings that can be realized for process loads is limited and difficult to manage and control once the facility is in operation. Most any savings must be designed into the operation, such as Energy Star rated equipment and appliances. Company programs such as encouraging the use of stairs in lieu of elevators for travel between 2 to 3 floors, can also provide some savings.
Lighting can realize some significant energy savings with proper design. The two major components are lighting density (Watts/sq. ft.) and controls.
Lighting density savings beyond code requirements are becoming more difficult to achieve as code requirements become more strict with each issue. Currently, the default average lighting density for offices is 1.0 W/sf, which is difficult to achieve with traditional fluorescent fixture layout. Typical 2x4, 3 lamp T8 fixtures on an 8'x10' grid is 1.1 W/sf, depending on ballast efficiency. Proper fixture selection and spacing are critically important, not only for E&A Credit 1 on New Construction (LEED-NC), but also for E&A Credit 1.1 on Tenant Improvement projects (LEED-CI).
Lighting controls can offer further savings with occupancy (motion) sensors, time clock controls with overrides and day-lighting sensors. Some form of automatic control is required by all codes, so energy models must include at least code minimum requirements. Motion sensors, typically being the most cost effective, are the most common application, though they must be properly applied in large open areas. Day-lighting controls are dependent on the architectural design (whether perimeter areas contain open or private offices). Day-lighting control effectiveness also depends on the type of controls applied. On/Off or stepped level type controls are the most cost effective, but tend to be distractive to the occupants; dimming type controls are typically more expensive, but provide the best function.
The HVAC system can offer several options. Energy recovery between the exhaust air and outside air is the most typical application. As outside air is one of the major load components, this typically provides the best functionality for the system with the shortest pay back period. On larger systems with central cooling plants, water side economizer heat exchangers and chiller heat recovery for domestic water preheat are good candidates. On smaller systems utilizing packaged equipment, options are somewhat limited; outside air economizer cycle packages are the most typical application.
The Building Envelope design can provide the largest single impact to the energy usage as it is a major load component and has the most design flexibility. Some simple guide lines can be applied. The building should be oriented with its long axis east-west to limit the western solar exposure whenever the site constraints will allow. Including overhangs and fins provides solar shading. However, the largest impacts can be obtained by increasing the wall and roof insulation system R-values and glass thermal performance.
For human comfort and perception, more glass provides more views which relates to more LEED points for occupant environment; but for energy conservation, more glass requires more energy use which yields fewer LEED points. The architect and engineers must work together to determine the optimum mix of glass vs. wall that maximizes occupant comfort and minimizes energy consumption and, in the process, maximizes LEED points. For example, an all glass building might get 2-3 IEQ points for day lighting and views, but no E&A points for energy conservation; the same building and same mechanical systems but with less glass may get only 1-2 points for day lighting and views, but may achieve 6-10 points for energy conservation and provide significant and real operational savings to the owner without significantly compromising occupant comfort or perception.
Energy codes, like other portions of the building codes reflect the needs of communities to protect their standards for way of life and infrastructure to support that, and code requirements get progressively more stringent with each update. Following that trend, LEED requirements will also get progressively more stringent and LEED 2009 is expected to reflect that. The design teams will have to become even more creative to achieve performances above code minimums. It will become increasingly important for the entire design team to work in a collaborative effort beginning in the conceptual or schematic design phases to identify the most effective means for implementing energy savings into the design. Such collaboration will result in the best outcome for the project, the client, the users of the facility, and the community.