Approaching Net-Zero Energy
TEXT Mark Bessoudo, Jenny McMinn, Ian Theaker and Doug Webber
There is growing awareness and interest in “net-zero” energy buildings. But what does it take for a large and intensively used building to achieve net-zero energy? Can it be achieved?
The Centre for Green Cities at the Evergreen Brick Works in Toronto and the Centre for Interactive Research on Sustainability (CIRS) at the University of British Columbia in Vancouver share several key design strategies that enable the buildings to approach net-zero energy performance within their own site boundaries. However, these two buildings also show that net-zero energy is a very difficult target for larger buildings. In fact, it may be impractical to achieve without “scale jumping”–using energy resources beyond the individual building’s site.
Energy Consumption Targets in Canada
Building energy consumption targets are increasingly used and discussed by building professionals in Canada, and are typically expressed as either a percent savings (as compared to a “reference building”), which is a relative target; or as a maximum “energy use intensity” (EUI), an absolute target (often “normalized” for differences such as weather and occupancy). EUI refers to the total amount of energy a building consumes from all sources in a year (natural gas, electricity, district heating, etc.), and this metric is increasingly seen as a more credible measure of performance than a relative percent savings value. In Canada, EUI is typically expressed in “equivalent kilowatt hours per year per unit of gross floor area” (ekWh/y/m2).
LEED is the most widely used rating system for new green buildings in North America. There are now more than a dozen LEED-NC Platinum-certified buildings in Canada. Being LEED’s top rating, we expect these to be some of the top energy performers in the country. However, the energy performance of LEED-NC certified buildings is sometimes questioned since the NC rating system rewards projects based on predicted performance, with no need to verify actual performance. To respond to this level of doubt in the market, the US Green Building Council (USGBC) recently funded a study to evaluate the measured energy performance of LEED-certified buildings.1 According to this study (conducted by the New Buildings Institute), an average LEED Gold- or Platinum-certified commercial building in North America has a median EUI of 162 ekWh/y/m2. This is 60 percent better than Natural Resources Canada’s estimate for an average Canadian office building. While the study has generated much debate, controversy, and recently a lawsuit (based on questions regarding the statistical rigour of the analysis), we believe that the general conclusions are sound: that on average, LEED-certified buildings perform better than the market (though even well-designed buildings can be poor performers, depending on the actions of occupants and operators).
Although LEED is the most common, the most ambitious energy targets in the Canadian market are included in the International Living Building Institute’s Living Building Challenge, which requires a certified project to generate 100% of its energy needs through renewable energy generation (base on one year’s measured data).
Net-Zero Energy Buildings
Unlike EUI or percent energy savings, zero seems like a simple and easy-to-understand target. However, in the details it becomes more complex. There are at least four common definitions of net-zero energy:
Net-Zero Site Energy: the building produces at least as much energy as it uses in a year within its own site boundaries.
Net-Zero Source Energy: the building produces at least as much energy as it uses in a year, when accounting for upstream inefficiencies. “Source” energy refers to the primary energy required to generate and deliver the energy consumed at the site. This is of particular importance to electricity, since, on average, 2 to 3 units of energy are consumed at the generating station for every unit of site energy consumed by a building.
Net-Zero Carbon: the building produces at least as much energy as it uses with each energy type factored by their respective “grid” supply carbon intensity.”2
Net-Zero Energy Cost: annual revenues from energy sales to utilities by the building owner for energy exported to the grid are equal to or greater than the utility bills paid.
Each metric illuminates different issues–impacting the demands on an individual project. Unless stated otherwise, this article uses “site energy” as the metric.
Centre for Green Cities, Evergreen at the Brick Works in Toronto
Evergreen’s mission is to “bring communities and nature together for the benefit of both.” Their project known as the Brick Works involves transforming an abandoned 19th-century quarry and brick factory into an environmental community centre in the heart of Toronto’s ravine system. When completed, the 16-hectare site will include a complex of revitalized historical buildings and industrial structures, several large exhibition halls, ponds, a skating rink, nature trails, a farmers’ market, and canals that will help manage the Don River waters when in flood. At the heart of this development is the Evergreen Centre for Green Cities, a workspace for themselves and other social entrepreneurs, which will reflect their appreciation for environmental issues. This appreciation led them to target LEED Canada-NC Platinum certification, with a focus on energy efficiency.
The Centre for Green Cities was built upon an existing heritage building, integrating existing brick walls, trusses, and columns. The long face of the heritage building was oriented along the north-south axis, posing challenges to the design team with respect to passive solar strategies. Through careful design of the building envelope, the design reduces annual heating and cooling energy by almost 50% relative to ASHRAE 90.1-1999. Improvement strategies include a window-to-wall ratio of 23%; high insulation–R-35 walls and R-50 roof; and high-performance window systems (USI 0.9 W/m2K). The windows have wire mesh, movable external shades that control solar gains and act as one of many media for art. The construction of the Centre is largely complete, with partial occupancy as of December 2010.
Given the focus on bringing nature to the city, increasing the porosity of the building was important. Operable windows, coupled with three solar, wind, and fan-driven “chimneys” were included to drive natural ventilation and night-time air-purge cooling strategies.
The design team also focused on the energy consumed through ventilation, which was seen to consume as much energy as envelope losses. Ventilation energy conservation strategies included a high-performance heat recovery system with a desiccant wheel for latent energy, and a glycol run-around loop for sensible energy. These measures are predicted to reduce ventilation heating loads by 42%; CO2 sensors and enthalpy-controlled ventilation are predicted to further reduce fan loads by over 25%. Heating is decoupled from ventilation; heat is provided by an in-floor radiant system on the ground floor and radiators around the perimeter of interior spaces, rather than through the ventilation system.
Careful spacing and selection of high-efficiency fixtures, with daylight and occupancy controls, reduce lighting energy by over 50% relative to ASHRAE 90.1–1999, with a lighting power density of 6.5 W/m2. Through energy-conserving design alone, the Centre for Green Cities is anticipated to be one the most efficient office buildings in Canada.
To further reduce greenhouse gas (GHG) emissions, Evergreen considered several renewable energy options. During design developmen
t, the preferred renewable energy option included a biomass boiler and a solar/cogeneration cooling system. These strategies yielded a predicted building EUI of 71 ekWh/y/m2, 64% better than ASHRAE 90.1– 1999 and eligible for all 10 LEED Canada-NC EAc1 energy points–and provided a positive return on investment.
Evergreen is exploring partnership opportunities to fund a 110 Kw (peak) photovoltaic system at the Brick Works site, taking advantage of revenues from Ontario government’s feed-in tariff (FIT) program. The office building’s roof alone is not large enough for the anticipated 5,000-square-metre PV array needed for net-zero energy; 90 kW are planned for the roof of an adjacent industrial building. In the interim, Evergreen has committed to purchasing Renewable Energy Certificates (RECs) from Bullfrog Power for 100% of the building’s annual electricity consumption.
Two of Evergreen’s objectives were to create a great visitor experience, and to cause behavioural change. Through a combination of art, storytelling, and data from a comprehensive measurement and verification system, the intent is for this building itself to be a change agent.
Centre for Interactive Research for Sustainability, University of British Columbia in Vancouver
The University of British Columbia’s Centre for Interactive Research for Sustainability (CIRS) is intended to be the home of UBC’s Sustainability Initiative, which combines teaching, research and campus operations. John Robinson, Executive Director of this initiative, worked for over a decade to gain provincial and federal funding, supported from the start by Busby Perkins + Will. Collaborators now include governments and research agencies, non-governmental organizations such as the David Suzuki Foundation, the International Centre for Sustainable Cities, and businesses such as BC Hydro, Haworth, Stantec and Honeywell, among others.
CIRS will provide lecture spaces and offices for staff, graduate students and partners, and the BC design community will benefit from its policy, simulation and daylighting research laboratories and engagement theatre. Its Building Monitoring and Assessment Lab will gather and publish operational performance of the project’s green features; the building’s $37-million construction budget reflects the University’s commitment to understanding how sustainability can be implemented in real life.
Scheduled for occupancy in July 2011, the 5,500-square-metre CIRS building includes many elements now commonly seen in many green buildings: drought-resistant native landscaping; green roofs; water conservation and rainwater harvesting; natural daylighting; radiant heating; displacement ventilation; geoexchange heating and cooling; and solar energy collection.
What sets the CIRS building apart from other high-performance buildings are its ambitious performance goals, and the rigour of the approach to achieve them. As a candidate for Living Building Challenge certification, goals include net-zero energy and net-zero water consumption, and avoiding the Living Building Challenge’s “Red List” of toxic materials. The Athena Institute’s EcoCalculator pointed to the use of salvaged wood damaged by the pine beetle to help sequester roughly 600 tonnes of carbon dioxide. Controlling the building’s embodied energy and operating GHG emissions is leading the project to becoming Canada’s first GHG-negative building.
The UBC campus setting eased passive design to optimize natural energy capture by allowing some flexibility in massing and footprint; the design’s massing strategy evolved from a simple cubic form with a narrow internal atrium to a U-shape of narrow floor plates that provide more natural daylighting, views and passive cooling when conditions permit.
Load reduction and passive opportunities were also major influences on the envelope design: balancing opaque and glazed areas maximized daylighting opportunities while minimizing heat loss. Windows include lower vision sections with shades to control solar loads and glare, with upper clerestory sections to admit natural light deep into interiors; a vegetated brise-soleil will shade the atrium in the summer months while admitting winter sun for heating and lighting. The envelope includes high levels of insulation (R-30 in walls and R-60 in roofs), which allowed an overall window-to-wall ratio of 48%; large glazing areas were placed on the south-east and south-west elevations to provide solar heating.
Massing and envelope efforts were matched by attention to conditioning outdoor air. CIRS’s mechanical design takes advantage of an opportunity presented by its campus setting, by including a heat pump system that recovers waste heat from an adjacent building’s laboratory exhaust system. The system is expected to serve the adjacent building as well as CIRS, returning roughly 603,500 ekWh/year back to preheat outdoor (ventilation) air. The result is anticipated to reduce natural gas use by UBC’s district energy system for a net energy surplus and reduce almost 150 tonnes of carbon dioxide-equivalent emissions annually for both buildings together.
These load reductions and efficient systems minimized the amount of energy needed; a 25 kW (peak) solar photovoltaic array is integrated with roof clerestory skylights and is expected to provide almost 10% of the building’s annual electrical energy. In addition, almost three-quarters of domestic water heating is provided by 40kW of solar thermal collectors on the roof, supplemented by heat pumps.
On the Path Towards Net-Zero Energy Buildings: The Design Process
The design process for both CIRS and Brick Works followed four basic steps:
1. Understand the site context by identifying opportunities and constraints presented by the microclimate (e.g., solar path, prevailing winds); the infrastructure serving the building; and local energy resources–particularly nearby sources of waste heat.
2. Reduce energy loads for heating, cooling and lighting through appropriate massing, cladding, and building porosity to optimize passive effects (e.g., daylighting, summertime shading, natural ventilation, passive solar).
3. Meet energy loads efficiently through careful design of electrical and mechanical systems and equipment, controls, commissioning, and measurement and verification. Conditioning of outdoor air is largely done with recovery of waste heat.
4. Generate and supply energy from renewable resources such as solar photovoltaics (PV) and solar thermal to meet the remaining energy needs. These resources may be either on the building itself, or may need to draw on a larger area than the building footprint occupies.
Both CIRS and Brickworks went to great lengths to reduce energy consumption for heating, cooling and ventilation, and both had great success with careful massing, envelope and systems design.
Together, they offer several lessons:
Creating a net-zero energy building is not easy. All the right conditions–physical, technical and financial–need to be in place and aligned to optimize an energy strategy to its full potential. The needs of the building program; the constraints of the site, including availability of solar or waste energy resources; and programmatic and financial constraints greatly affect whether and how a building can become self-sufficient in energy terms.
Energy conservation efforts have to address all energy uses aggressively. High energy performance starts with reducing energy consumption, and no energy use can be neglected to reduce energy consumption to the point where renewable energy supplies are likely to be sufficient. Both design teams made impressive efforts to identify and assess how all design elements could improve energy performance.
Conditioning of outdoor air (ventilation) is a large issue for highly efficient buildings. As envelope losses, internal loads, and lighting energy are reduced, conditioning outdoor air for ventilation grows in importance–but its slice of the energy pie is often masked. Designers would benefit greatly if simulation reports highlight energy consumed by heating, cooling and moving outdoor air, to help create systems that efficiently recover waste heat or passively temper incoming fresh air.
Advanced technologies can challenge budgets and schedules. New technologies at the “bleeding edge” can offer great promise for energy efficiency or harvesting renewable resources. However, if they are still immature, they can have major impacts on the design process, construction schedules, and capital and operating budgets.
Looking Beyond the Building Site: Scale Jumping
Net-zero site energy at the building scale is not always the best solution. Massing for daylighting, access to winds and solar gains can greatly help smaller buildings or those with generous sites approach a net-zero target. However, larger buildings often don’t have sufficient site space to capture renewable resources to meet their needs; the larger the building, the less likely it will be able to meet its energy needs exclusively from the sunlight that falls on its footprint. A multi-storey building in a dense urban environment will likely be shaded by adjacent buildings and have a relatively small unobstructed roof or wall area for solar collection, compared to the energy needs of the floor area required to meet its functional program.
Evergreen’s Centre for Green Cities program was an adaptive reuse of an existing heritage building, which limited its available roof area and constrained its orientation. Even with extraordinary conservation efforts, there simply wasn’t enough roof to collect enough solar energy to meet its needs over the course of a typical year. Similarly, CIRS’s building footprint was not large enough to be totally self-sufficient in energy consumption with the program it aims to satisfy. To approach net-zero energy performance, both the CIRS and Brick Works design teams were required to look beyond their individual site footprints.
Acknowledging that energy-generating strategies at a larger scale than the individual building may be a better use of limited fiscal and design resources, the Living Building Challenge introduced the idea of “scale jumping.” The Challenge recognizes that it may be more valuable for larger buildings to share or generate energy resources at the block, neighbourhood, community or larger scales, than to try and achieve net-zero energy in each individual building. As a result, many Living Building Challenge projects use waste energy or renewable energy generated at a larger scale than the individual building site. Just as buildings and their programs do not sit in social and economic isolation, so too should their energy targets respond to and take advantage of the larger energy context, the better to contribute to overall environmental performance. CA
1 Energy Performance of LEED® for New Construction Buildings. New Buildings Institute, 2008.
2 Torcellini et al. Zero Energy Buildings: A Critical Look at the Definition. National Energy Renewable Laboratory (NREL), June 2006.
Mark Bessoudo, Jenny McMinn, Ian Theaker and Doug Webber are members of Halsall Associates’ Green Planning and Design team. As a national leader, Halsall has provided green building consulting on over 400 projects including new and existing buildings and communities.