Zero Energy Schools: The art and science of energy performance targets

This blog was originally posted by Peter Turnbull & Associates LLC.
Written by Peter Turnbull.

Lifelong friends William Carlos Williams (the American poet) and Charles Demuth (the American painter) agree: a great performance target to hit for zero energy schools is “5,” as in 5 kWh per square foot per year!

The Demuth painting, “I Saw the Figure 5,” based on a Williams poem (“The Great Figure,” see below), memorializes “5.” I like to think of it as a powerful, useful mnemonic for setting energy targets for zero energy performance in schools.

Well, truth be told, my claim above about Demuth and Williams may be a bit on the apocryphal side. Had these artists been thinking about school buildings instead of art and poetry, they, like many other early advocates, would have been encouraging fully decarbonized, full electric schools.

Full electric buildings use highly efficient electric heat pump technologies for space and water heating and electro-technologies for cooking; they do not burn fossil fuels (such as natural gas, propane or fuel oil) on site. As power grids increasingly come to rely on clean renewables, the CO2 emissions from customer-sited fossil fuel combustion in turn become an increasing percentage of all carbon emissions. Full electric buildings greatly reduce on-site emissions and are the wave of the future. And beyond CO2 emissions reductions to the atmosphere, full electric schools create a healthier indoor environment for students, faculty and staff.

With electricity as the single fuel, energy performance targets become simpler: Demuth and Williams could thus talk about kWh per square foot per year, not “kBtu” per square foot per year. That latter term, kBtu (for thousands of “British thermal units”), is commonly used as an Energy Use Intensity (EUI) metric when setting targets for a mixed fuel building—it allows apples-to-apples addition of electric Btus (used for lighting, A/C and other equipment) and fossil fuel Btus (typically used for heating, water heating and cooking). It’s easy enough to convert our preferred single-fuel target, 5 kWh/ft2/yr back into kBtus: we simply multiply by 3.412 get 17.1 kBtu/ft2/yr.

I find “5” easier to remember than “17.1”—and it’s certainly better in a painting or poem. Without gas or fuel oil being used, there’s no need for a complicated conversion, we can stick with kWhs instead of kBtus. For full-electric schools, 5 kWh/ft2/yr is a good number to target! By keeping this target performance metric as low as possible, the size—and, therefore the cost—of the corresponding renewable system stays low. This renewable system, usually a PV array, is sized to offset the energy consumed by the building, “netting out” the energy use to “zero.”

On average, the target “5” is slightly more optimistic than a recent study from the National Renewable Energy Laboratory (NREL) suggests—that study (summary chart below) concludes that an appropriate site EUI target for schools is just over 17 kBtu/ft2/yr (about 5 kWh, as we’ve seen) in the mildest US climate zones, ranging up to about 23 (about 6.8 kWh) in other parts of the country (not counting northern Alaska). Surprisingly, the variance from the median of about 20 kBtu (or 5.9 kWh), is under 15% across the lower 48 states. For convenience, we have included a conversion into kWh/ft2/yr in the table. Climate matters when setting a target, but not as much as one might think.

Here’s the full summary chart of the NREL school targets in both kBtu and kWh units:

So: although different building shell and system measures are appropriate in different climate zones, the achievable energy “footprint” targets stay within a surprisingly narrow range. For example, a well-designed primary school in Minnesota (Zone 6A) needs only about 12% more energy than an equally well-designed school in Mississippi (Zone 3A). For a secondary school, it’s just 7% more. For this reason, school officials across the country could begin discussions with their building design teams with the number “5” as an energy performance target. Or “6” if you have to.

Just to complete the story for those with a poetic mindset . . . here is “The Great Figure” by William Carlos Williams. Demuth did his painting after reading his friend’s famous poem. Full disclosure: it’s about a fire truck, not a school. Even so, it’s a quick, easy way to remember an excellent energy performance target for a fully decarbonized, full-electric ZNE school. Yes, 5!

The Great Figure

Among the rain
and lights
I saw the figure 5 in gold
on a red
unheeded to gong clangs
siren howls
and wheels rumbling
through the dark city.

For those interested in digging deeper, there are many more good resources in the NREL report and on our website (blog, case studies, etc).