Question 5 of 8: The zero net energy building (20 marks)

Question 5: The zero net energy building (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A zero net energy building is one whose total delivered energy over a representative year, counted at the site boundary, is balanced by renewable energy generated on or near that site over the same year. The definition is deliberately an annual balance rather than an instantaneous one: a building in Canada cannot be self-sufficient in December, so it exports surplus generation in summer and imports in winter, and the accounting period is the year. Four things have to be settled before the phrase means anything — the boundary (site, source, cost or emissions), the metric (site energy, primary energy or carbon), the period, and whether the renewable generation must be on the building itself. Canada's own programmes make those choices explicit: the Canadian Home Builders' Association Net Zero label requires on-site renewable generation equal to annual consumption; the CaGBC Zero Carbon Building standard shifts the metric from energy to operational greenhouse gas, which on a hydro-electric grid is a very different target; and the tiered performance paths of the National Building Code of Canada 2020 and the BC Energy Step Code define the progression toward it in terms of thermal energy demand intensity and total energy use intensity.

The design logic that gets a building there is a strict hierarchy, and its order matters more than any single technology. Reduce the demand first. Every kilowatt-hour eliminated at the envelope is a kilowatt-hour that never has to be generated, stored or paid for, and photovoltaic area on a mid-rise roof is finite. Orientation and massing come first: an elongated east–west plan with the long face to the south, a compact form (a low surface-to-volume ratio), and glazing distributed to admit winter sun and reject summer sun. Then the envelope itself — continuous exterior insulation with no thermal bridges, structural thermal breaks at balconies and slab edges, airtightness of the order of 0.6 air changes per hour at 50 Pa, triple glazing with warm-edge spacers and thermally broken frames, and window-to-wall ratios kept near 30 to 40 %. External shading, light-coloured or vegetated roofs and thermal mass placed where the sun can reach it complete the passive layer. The Passive House criteria — 15 kWh/m²·yr of heating demand and 120 kWh/m²·yr of primary energy — are the best known quantitative expression of this stage, and a building that meets them has already reduced its heating load by 80 to 90 % against code minimum.

Then recover what is being thrown away. In an airtight building the ventilation air becomes the dominant heating load, so a heat- or energy-recovery ventilator at 75 to 90 % sensible effectiveness is not optional; an enthalpy wheel or membrane core additionally recovers latent energy, which matters in a Canadian winter for humidity control and in summer for dehumidification. Drain-water heat recovery on shower stacks returns a further 25 to 40 % of domestic hot-water energy for a device with no moving parts. Exhaust-air heat pumps, condenser heat recovery from refrigeration and computer rooms, and simultaneous heating-and-cooling recovery in a water-source heat pump loop all belong in the same category: moving heat from where it is a nuisance to where it is wanted.

Then make the remaining systems efficient and electric. The plant of choice is a heat pump — ground-source where the site allows a borefield or an aquifer, air-source with a cold-climate compressor and a low ambient rating where it does not, and water-loop where the building has simultaneous loads. Distribution moves to low-temperature hydronics (35 to 45 °C radiant floors or panels), which is what lets a heat pump run at a high COP, and to variable-speed fans and pumps sized on actual rather than nominal load. Demand-controlled ventilation on carbon-dioxide sensing, displacement or underfloor air distribution, LED lighting at 5 to 7 W/m² with daylight and occupancy control, elevator regenerative drives and rigorously specified plug-load equipment take out the remainder. Continuous commissioning and sub-metering matter as much as the hardware: measured performance in this class of building routinely diverges from the model until the controls are tuned.

Only then harvest. Roof- and facade-mounted photovoltaics are the workhorse, at roughly 150 to 200 W per square metre of module and 1,000 to 1,300 kWh per installed kilowatt per year across southern Canada; building-integrated photovoltaics and solar-ready canopies extend the collecting area beyond the roof. Solar thermal collectors serve domestic hot water where the load is steady, and solar air heating (transpired collectors) preheats ventilation air on south walls with an excellent payback in cold, sunny prairie climates. Ground-source heat exchange is itself a harvesting technology, taking the seasonal store the ground provides at 8 to 12 °C; seasonal borehole thermal energy storage, as demonstrated at Drake Landing in Okotoks, Alberta, carries summer solar heat into the winter and has met more than 90 % of a community's space heating. Where the site permits, small wind and micro-hydro, and at district scale, sewage-heat recovery of the kind used in Vancouver's False Creek neighbourhood energy utility, extend the same idea beyond the single building.

Finally, the balance has to be managed in time. Because generation and demand are seasonally out of phase, a net-zero building depends either on the grid as its storage (net metering, with the utility absorbing the summer surplus) or on real storage — electrical batteries for daily shifting, thermal storage in water tanks, phase-change materials or the building mass for load shifting, and borehole fields for seasonal shifting. Load-shifting controls that pre-heat or pre-cool ahead of a peak reduce both the demand charge and the size of the plant. The honest limits of the concept should be stated too: a tall, deep-plan building simply does not have the roof area to generate what it uses, so net zero at the individual-building scale is realistic for low-rise and mid-rise construction and has to become a district- or portfolio-scale target for towers; and on a grid that is already near-zero carbon, spending capital on on-site photovoltaics may displace less carbon than spending it on demand reduction, which is precisely the argument for the zero-carbon rather than zero-energy metric.