When Zero isn't Zero

I usually hesitate to write about something that is almost entirely technical, but this one has been bothering me too long to ignore.

There is apparently declining public interest in the issues of climate change and sustainability.  Despite this, the reality is that we are working with considerable momentum on both technologies and regulations for energy efficiency, renewable energy sources and various other initiatives to reduce the impact of future development.  But the progress towards understanding what we are doing, even amongst professionals, is hampered by misinformation, whether that misinformation is motivated by greenwash, or simply sloppy thinking by people who should know better.  The concept of 'Net Zero Energy Buildings' is a case in point. 

You would think that the definitions of net zero energy buildings would be easy; that a NZEB would be one that produces at least as much energy as it uses.  But it is not that easy, primarily because of the problem of 'system boundaries'.  So we have competing definitions related to whether the energy for running the building has to be offset on site, or allowed to be imported, etc.  It gets ever more complicated as you try to factor in energy generation efficiency, or whether the energy used to produce the building also has to be offset.

This sort of complexity is bad enough to derail useful communication with decision makers, design professionals or the general public.  But what bothers me most is a more fundamental underlying sleigh of hand.  Take the deceptively straight forward attempt to clarify this issue, by Allison Bailes on the Energy Vanguard Blog in her article 4 Ways to Define Net Zero Energy Use in Buildings.  To make my point, I quote her first 'way' in its entirety:

1. Net Zero Site Energy
In the first definition, an NZE building produces as much energy on-site as it uses on-site over the course of a year. If the building uses 10,000 kilowatt-hours, it's got to produce 10,000 kWh or more, ideally on-site within the footprint of the building.

This definition doesn't account for the type of energy used, though. Where this becomes important is if the building uses natural gas, propane, or some other fuel besides electricity.

Let's say your home has a natural gas furnace, and it's 95% efficient. For every 95 kWh of heat that your home needs, your furnace is burning 100 kWh of natural gas. (Pardon the units, but since the energy produced is in kWh, I'm going to stick with that.) That means you've got to provide 100 kWh of site-generated electricity.

If, on the other hand, you had a heat pump, it would need maybe 40 kWh of electricity to move 95 kWh of heat into your home. Rather than having to produce 100 kWh of electricity, then, you'd only need to produce 40.
As you can see from this example, this definition of net zero favors on-site electricity use. It's also the simplest of the four definitions.

I wonder if you can see the problem.  It isn't immediately obvious, because the concluding paragraph seems to be so clearly suggesting the preferred solution: use a heat pump, and you have to generate less electricity, and you don't have to bring non-renewable fuel on site.  Which is where the initiative should aim to get you.

Unfortunately, the preceding paragraph makes clear that the 'Net Zero Site Energy' definition does allow non-renewable, off-site derived fuel to be used, albeit with a more onerous on-site offset.  The author's example glosses over the fact that heat pumps become less efficient exactly when the heating need is greatest.  Unless you commit to a ground source heat pump, the chances are that in heating dominated climates, you will either need to generate as many KWh of electrical energy as you use for electric heating, or favour gas fired space heating.

Of course, I am not the first person to notice these problems.  The concept of 'Zero carbon' is introduced to try and overcome the issue with different forms of energy. By the time the issues are teased apart sufficiently, and some self-interested people translate everything into  dollar costs rather than energy units or tons of CO2, I count at least eleven variations on the Net Zero idea.  Almost all of which misses the real point.  The problem word is the 'net' not the zero.  That accounting will always allow 'offsets' and 'conversion' calculations, and will always beg the question of the system boundaries.

The bottom line is that if you are burning a non-renewable fuel, no gymnastics with the conversion numbers will ever reinstate that loss of a resource.  It will have been used up. The whole 'net zero' construct is an illusionist's trick.

See the full article at
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/60056/4-Ways-to-Define-Net-Zero-Energy-Use-in-Buildings