District thinking and doing: A path to net zero
A net zero energy goal is becoming increasingly realistic, given current and emerging building technologies and design approaches. But even with a well-charted and proven road map, the majority of projects face inherent building constraints that hinder an aggressive pursuit of net zero at scale.
Finding the right place
Often, the limiting factor to net zero is that the building or building site becomes an unnecessary boundary. Buildings with high energy or water use intensity (e.g., hospitals or data centers), as well as high-rises that lack sufficient space for onsite renewable energy or rainwater collection, are generally identified as poor candidates. In urban areas, projects may not be able to annualize net zero energy production at the meter if utility network grids cannot safely accept back-fed energy generated onsite.
Climate, too, can affect a project’s candidacy. Highly efficient air- or ground-source heat pumps may not be the most feasible for buildings in cold locations with a significant annual need for heat (which can result in reliance on onsite fossil fuel combustion or electric resistance heat.) Even low-rise office buildings in moderate climates tend to be densely populated—meaning higher internal energy loads.
The district solution
Although it's not new, the idea of an energy district for heating and/or cooling is a strategy increasingly being considered as a path to net zero. Depending on fuel source, a district heating system, while not necessarily energy resource-efficient, could provide a significant advantage in reducing the carbon footprint of those buildings served, even if it paradoxically doesn’t necessarily help the net zero energy equation for those same buildings. Traditionally, college or corporate campuses and some cities have installed centralized utility plants to produce district steam for heating and chilled water for cooling, primarily for economic reasons.
Fortunately, new district paradigms are emerging that are practical and achievable, and far more localized. The concept of heat sharing—harvesting neighboring energy resources that would otherwise be wasted—is one such opportunity, and can enable the step change needed to achieve high performance outcomes.
Converting waste to energy
Amazon’s Seattle headquarters is one recent example of a local energy district based on synergy within an urban environment. The new building receives heat via water piping that’s interconnected with a large data center across the street. This heat is a natural byproduct of the data center’s server farms and was previously expelled year-round through cooling towers on the building’s roof (consuming more energy and water.) Amazon took on this waste heat, engaging a third party to enable the neighbor-to-neighbor transaction. The headquarters design includes a central plant equipped with pumps that take the low-grade heat byproduct and boost it to temperatures suitable for building heating. As Amazon intends to fully harvest this resource to heat the entire campus, the design weaves multiple buildings together—and will ultimately provide heat for over 3.3 million square feet of space.
A similar example is being developed in the design of a future sports arena in Seattle. In this case, thanks in part to active encouragement and support from local authorities and utilities, the owners are looking to leverage heat from an adjacent county sewer/storm main. Systems designed to harvest heat from sewage are now available on the market and provide a unique opportunity to use what would otherwise be a wasted resource. Also, because excess heat generated by the arena could empty into the sewage stream, the project does not require a cooling tower—saving energy and water, and eliminating the need for additional onsite equipment.
Although there are challenges in harvesting what is otherwise viewed as waste beyond a project’s traditional boundaries, some creativity, applied locally, can bring great results. Success requires the right technical opportunity; the right political climate; and a motivated, cooperative group of stakeholders who can make decisions in the context of projects where upfront dollars are precious and schedules are tight.