Four Key Design Steps to Achieve Successful Low Energy Designs
Post date: Feb 2, 2014 8:50:37 PM
Efficient building designs with performance levels below 20 ekWh/ft².yr (775 MJ//m².yr) can be attained with relative ease and low energy building designs below 12 ekWh//ft².yr (465 MJ//m².yr) can also be achieved through extreme load minimization coupled with alternative HVAC systems such as CC + DOAS and CC/DV. In North America, these performance levels are not being reached as frequently as the industry would like as evidenced by market studies and energy benchmark data. There are a number of reasons for this ranging from key decisions that are made at the early design stages to the design process itself.
Four key steps are described below that can help overcome the barriers that exist today and make low energy designs commonplace.
Start with a Solid Set of Design Specifications: In too many green building projects the minimum prescriptive requirements are ignored in favour of performance based designs that rely on trade-offs. While performance based approaches provide flexibility, they cannot be over-emphasized over minimum prescriptive requirements. A building with a weak envelope that has too much glass will have large heating and cooling loads and fan sizes and penalize the overall building energy use and peak electrical loads. To avoid this problem, the design team needs to start the project with rigorous design specifications for the thermal performance of the building envelope and glazing systems, reasonable window-wall ratios (WWRs), low lighting power densities (LPDs), careful selection of air handling equipment to minimize fan power requirements and finally, use of high efficiency equipment. The ASHRAE Advanced Energy Design Guides (AEDG) series of publications are an example of design guides that provide demanding prescriptive guidelines to achieve 30% and 50% energy savings relative to ASHRAE 90.1.These guides provide prescriptive tables that are differentiated by climate zone and building type.
Bundle Technologies & Components Together with the Objective to Minimize Loads: Bundling as many technologies as possible is particularly important for aggressive design targets that require significant minimization of loads. Such designs require that all building components work together because of the symbiosis that exists between the envelope and HVAC equipment. As a result, an overall good thermal performance of all components is needed, not just for some components. As an example, triple pane windows are very costly, if analyzed in isolation, but bundled together with a highly insulated wall will achieve significantly better returns on investment and a lower project cost thanks to the concept of “Tunneling-Through-the-Cost-Barrier”. This occurs because the highly insulated envelope allows a drastically reduced perimeter heating system and heating plant, in turn offsetting the incremental costs of the windows and better envelope.
Solid Design Process: It is not only about technologies, or performing energy modeling that will result in a low energy building design. The design approach together with paying attention to reducing the loads, performing accurate load calculations and careful selection of equipment are just as important, if not more important. Enough time and resources need to be allocated to do proper load calculations without adding safeties upon safeties or relying on default values. The effective wall and overall window Uvalues need to be properly calculated and the SHGC or SC of the actual window selected; not a default value. Attention needs to be paid to equipment selection, and in particular, specifying air handling equipment with air velocities below 400 ft/min (2 m/s) instead of the traditional 500 to 600 ft/min (2.5 to 3.0 m/s). This can be achieved by specifying slightly larger air handlers with increased coil surface areas. Such a reduction in air velocities can decrease the total internal static pressure at the air handler by upwards of 0.5 in. (125 Pa) and help reduce fan size by 15% or more.
Move Away from Value Engineering & Speculative Designs by Selecting Good Contractors and Getting Suppliers and Contractors Involved in the Design Process. This is perhaps the most difficult step to implement. Low energy designs rely on quality designs rather than least cost designs, but because of the equipment downsizing, there are cost tradeoffs that allow the design cost to be similar or only marginally more expensive then a speculative design. Good contractors need to be selected and engaged early in the design process for these cost advantages to materialize. Similarly, equipment suppliers also need to be included in the design team to provide proper equipment selections and cost estimates to the contractors during the design stage. This will avoid the problems of equipment substitutions during the construction phase that frequently plagues projects, as well as, limiting or eliminating incremental costs for components and equipment that do not normally exhibit any, but do require custom selections.