18 Sep ZNE vs ZNC: What is the difference between zero net energy and zero net carbon buildings?
Targeting zero net energy (ZNE) or zero net carbon (ZNC) are great policies, each with their differences and achieving them is complicated. Though in my view, targeting ZNE or ZNC is worthwhile since it helps get folks excited about reducing energy use and making clean power.
What is Zero Net Energy?
Zero Net Energy (ZNE) is a concept that can help encourage people to both minimize their (bad) energy use and to install (good) clean generation. Typically, achieving ZNE for a site is a feel-good milestone whereby owners and occupants can feel happy knowing that, over a year, their total on-site (clean, renewable) generation is equivalent to their total site energy use.
What is Zero Net Carbon?
A similar concept seeing growing interest at the state, city and county levels is Zero Net Carbon (ZNC), aka carbon neutrality. Exact definitions vary (just as with ZNE), but at heart achieving site ZNC means what it says: getting to zero carbon dioxide (CO2) emissions. Some people prefer to target ZNC over ZNE if they are primarily driven by climate change. ZNC more directly addresses the importance of reducing CO2 emissions that cause climate change.
ZNE vs ZNC in Buildings
In practical terms, one of the biggest differences in pursuing ZNE vs ZNC is how the burning of natural gas will be addressed. Natural gas has in recent decades been considered a relatively clean and cost-effective energy source for heating in particular, so natural gas boilers, furnaces and heaters are everywhere. (Natural gas is also often burned by utilities to generate electricity.) Today, natural gas remains quite inexpensive with new reserves continually being developed, often using fracking technology.
Eliminating CO2 Emissions from Natural Gas
But retrieving natural gas from within the Earth and then burning it and sending the exhaust out into the atmosphere is a clear direct source of CO2 and other emissions. So, there is interest in reducing or eliminating natural gas use and replacing it with electricity of the clean renewable variety.
Enter Electric Heat Pumps
Luckily, for heating, we have electric heat pumps increasingly coming to the rescue. These are an old but very efficient technology that provides about 3-5 times as much heating energy as they use in electric energy!
That can sound miraculous, but heat pumps are really just another vapor-compression system like any air conditioner or refrigerator, operating in reverse. Rather than making heat through combustion of fuel, heat pumps move heat. Thanks to their efficiency, the energy cost of heating using electric heat pumps is about the same as for heating using natural gas (at today’s energy prices, even with natural gas being quite ).
ZNE and Natural Gas
With a ZNE target, natural gas use is typically dealt with in a few ways:
- by installing heat pumps,
- by offsetting gas use on paper by installing “equivalent” energy generation (clean renewable electricity),
- trying to source renewable biogas (not easy), or even
- quietly ignoring the natural gas use!
ZNC and Natural Gas
Meanwhile a ZNC target typically implies eliminating natural gas use to remove that direct CO2 emissions on site. Period. In practical terms for buildings, this usually means installing electric heat pumps for heating instead of gas-fired boilers, furnaces and water heaters.
So, Is ZNE or ZNC Better?
Now to be clear ZNE is still a very worthwhile target, encouraging both reduction of energy use and addition of clean renewable generation. And this doesn’t mean that site ZNC is necessarily the one and only truly “clean” target (we haven’t touched on sources of electricity used when on-site renewables are not in fact powering your site at a given time). Selecting a ZNE target can be seen as politically less divisive than directly calling out carbon/CO2 with a ZNC target. And ZNE offers perhaps the relative simplicity of adding up your energy use and generation vs counting tons of CO2 emissions. In any case, whether pursuing a ZNE or ZNC target for an existing building or project, these are positive goals that can help encourage folks to get excited about subjects that most people don’t find too exciting!!!
Free Engineering Support for Achieving ZNE and ZNC
Vapor compression cycles, YAY!! At kW Engineering, we get excited about this stuff. One of the ways we help folks choose and achieve ZNE or ZNC targets in the SF Bay Area is through the BayREN ZNE/ZNC Technical Assistance Program. Funded by the CPUC, this program provides our technical services for free to Bay Area governments (ABAG). The program recently expanded to support both ZNE and ZNC targets. We are now able to directly address electrification topics including the installation of electric heat pumps. We can provide all kinds of assistance including:
- efficiency audits,
- renewable energy feasibility assessments,
- design reviews,
- energy modelling,
- portfolio assessments,
- procurement strategy and
- policy advice.
For information about this free program, please visit the BayREN website, or watch the following video featuring our ZNE/ZNC lead Peter Pollard: