Integration: Net-zero energy design
Building energy savings
The following are energy-saving strategies for engineers to consider for some of the more significant end-use categories.
Lighting—With an abundance of efficient lighting products available, significant energy savings can be achieved through lighting design. The best strategy is to eliminate the need for artificial lighting. This is best accomplished though the building design allowing sufficient natural daylight to enter interior spaces. This must be done with caution because heat gain comes with daylight. Additional heat may or may not be desirable (depending on geographic region, time of year, and/or building use), so daylighting strategies must be carefully evaluated for the site.
Both fluorescent and LED fixtures offer outstanding efficacy (W/sq ft). Illumination levels for spaces should comply with IES guidelines. While LED still carries a price premium, the benefits often make it the right choice for a NZEB. The inherent dim-ability of most LED lamps and fixtures makes them well-suited for daylighting applications. In areas not in continuous use (offices, restrooms, conference rooms, etc.) occupancy/vacancy sensors should be used so that lights are automatically turned off when they are not needed.
A reduction in lighting loads may result in an ancillary benefit to the HVAC system. All of the electrical energy supplied for lighting eventually becomes heat. In spaces that are cooled, the reduction in heat load from efficient lighting results in a lower peak cooling load and less energy required to cool the space.
HVAC—Geothermal heat pump, or ground source heat pump, systems are a popular choice for NZEB. With coefficients of performance much greater than one, the ability to both heat and cool, and the fact that they operate on electricity that can be produced with on-site renewable energy systems (rather than fossil fuel), they are a natural fit. While geothermal is considered a form of renewable energy, it is categorized as a demand-side technology. Similar to passive solar space heating and solar ventilation air preheaters, geothermal reduces the need for energy production. This is in contrast to supply-side renewable energy technologies such as solar photovoltaics and wind turbines that contribute toward the balance of energy in order to achieve net-zero energy status.
In environments with simultaneous heating and cooling loads, variable refrigerant flow (VRF) systems are an energy-efficient option. Another efficiency trend is the shift from air to water as the medium for energy. Radiant heating and cooling systems have been successfully used in several NZEBs.
Ventilation control is an area that can result in energy savings. A dedicated outdoor air system (DOAS) combined with a parallel mechanical system has the potential to use less energy than a conventional variable air volume (VAV) system by eliminating the need for excess airflow and by reducing the energy needed for terminal reheating. Heat or energy recovery should be employed wherever possible (for example, in the DOAS). In regions where natural ventilation is viable, it should be used instead of mechanical cooling whenever conditions are feasible.
Temperature setpoints should be evaluated and lowered (winter) and raised (summer) as appropriate. One advantage of radiant heating and cooling systems is that more aggressive setpoints can be used while still maintaining a satisfactory level of comfort for occupants. Aggressive setbacks and reduced ventilation should be employed during unoccupied periods.
After the building energy needs have been reduced as much as possible, the remaining energy must be generated with renewable energy. Ideally, all generation is located within the building footprint as shown in Figure 3. For many buildings this is not possible due to space constraints, in which case the second-best scenario is for all generation to be within the site. In addition to rooftop solar, this may require a ground-mounted solar array, a solar-carport structure in the parking lot, a wind turbine on the property, or all three as shown in Figure 4.
Many NZEB initiatives allow for a limited amount of renewable energy to be produced or purchased off-site. For example, ASHRAE Vision 2020 states that renewable energy credits (RECs) should not be permitted to offset building nonrenewable energy use or carbon emissions for more than 50% of the building’s net energy consumption. If this was not limited, a building could simply buy its way to net-zero energy status without reducing energy needs or deploying renewable energy on-site. Some programs are more stringent; for example, Architecture 2030 allows a maximum of 20% off-site renewable energy generation for the required reduction targets.
The design of renewable energy systems is a key part of every net-zero energy project. It is advisable to include on the project team a firm or consultant with specialized knowledge of and experience with renewable energy technologies. System modeling and energy production analysis must be completed to ensure that sufficient energy can be generated to achieve a net-zero energy designation. Commissioning renewable energy systems is also important and will require a commissioning authority with renewable energy experience.
Net-zero energy buildings are the future of the building industry, and for some the future is today. While it may take some time to get the entire building community there, an increasing number of architects, engineers, and building professionals are well on the way to fulfilling the vision of making net-zero energy buildings the norm by 2030.
Scotte Elliott is an electrical engineer and energy specialist at Metro CD Engineering in Powell, Ohio. He is a Certified Energy Manager, a NABCEP Certified PV Installation Professional, and past division chair of the American Solar Energy Society.
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