How Feasible Are Zero Net Energy Buildings?

In the last blog post, I discussed the recently announced goal of the World Green Building Council, to have ALL buildings in the world be zero net energy (ZNE) by 2050, and I questioned whether this was a worthwhile goal.

I happened this week to read an article by Charles Eley in the July 2017 ASHRAE Journal on this subject that I thought really nailed the technical difficulties of zero net energy for all buildings. In case you don’t know Eley’s work, he has been probably the foremost technical mind in the world of energy-efficient buildings for the past 40 years or so and probably has done more than any other person to guide California into its preeminent role in the world of building energy efficiency. He also helped to create a leading program for certifying green schools, the Collaborative for High-Performance Schools (CHIPS) rating system.

Eley’s article, Feasibility of ZNE by Building Type and Climate, provides a detailed analysis of the feasibility of ZNE new construction in all major climate zones of the United States. His analysis shows that “onsite (e.g., rooftop solar PV) ZNE is challenging for energy-intensive buildings (such as restaurants, hotels, data centers, and hospitals), tall buildings (higher than six stories), and buildings on shaded sites. For these conditions, off-site renewable energy must be incorporated to achieve ZNE” (items in parentheses are my additions). The “nearly impossible” buildings represent about 16% of the US building stock. (Note that Eley's analysis is based on each building type meeting the new construction requirements embodied in the ASHRAE 90.1-2013 energy efficiency standard.)

Eley’s analysis also assumed that the entire rooftop could be covered with solar panels, which is an impossibility in most urban areas and in most commercial buildings, where the HVAC equipment is located on the roof. So, one has to take this analysis as the most-optimistic outcome for all buildings in the United States, especially at current output efficiencies for solar PV, estimated in the study at 15 W/sq.ft.

Bottom line: Making all buildings ZNE by 2050 is technically impossible, forgetting economics, available roof space, or other issues. Doesn’t mean we shouldn’t try to make as many new buildings as possible ZNE, a goal advocated for all new buildings by the Architecture 2030 program.

But first consider the leaky bathtub analogy: if you want to avoid wasting water when you want to take a nice hot bath, be sure to first put the stopper in the bathtub before you try to fill it!