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The Department of Energy Makes a Choice in the Net Zero Debate

Passive Solar Net Zero House Photovoltaic Modules Richard Levine Kentucky

A couple of weeks ago, I wrote about whether homes that produce as much energy as they use should be called net zero energy or zero net energy homes. Several readers offered up another choice: zero energy homes. I’ve also written in the past about four different ways to define net zero energy (the term I’ve preferred). Now, the US Department of Energy has weighed in on both of these debates. Do you know what they decided?

A couple of weeks ago, I wrote about whether homes that produce as much energy as they use should be called net zero energy or zero net energy homes. Several readers offered up another choice: zero energy homes. I’ve also written in the past about four different ways to define net zero energy (the term I’ve preferred). Now, the US Department of Energy has weighed in on both of these debates. Do you know what they decided?

The name

The name of the DOE report is called A Common Definition for Zero Energy Buildings. That gives away their name preference, which they’ve actually been using for a while. They dropped the “net” altogether and assume people will be able to figure out what it means. I’m OK with that. The scientist and grammarian in me wants a more precise term, but I get it.

Sam Rashkin, the Chief Architect in the DOE’s Building Technologies Office, has been presenting on clearing up our language in the energy efficiency community for the past couple of years. I just heard him again last week in Chicago, where he gave the closing keynote speech at the 10th annual North American Passive House Conference.

He always starts by talking about how no one wanted to buy the delectable fish that was called Patagonian Toothfish. But when they changed the name to Chilean Sea Bass, it became a big hit. Words matter.

So, zero energy homes it is. I’m OK with that.

The definition

In the introduction of the DOE report, they write, “A zero energy building (ZEB) produces enough renewable energy to meet its own annual energy consumption requirements, thereby reducing the use of non-renewable energy in the building sector.” As I wrote in my article on the 4 possible definitions, things get a little squirrelly when you start talking about how you’d measure the energy produced and consumed. And a couple of the definitions aren’t actually based on energy; one uses energy costs and the other uses emissions.

The two big contenders, though, are whether you base the definition on site energy or source energy.

Site energy. If you burn natural gas on-site for heat, for example, all the BTUs you burn have to be offset with an equivalent number of BTUs produced in the renewable energy production system on-site. Likewise, every kilowatt-hour of electricity you use on-site has to be offset by an equal number of kilowatt-hours produced on-site.

Source energy. With source energy, you have to account for the energy used at the source and in transmission and distribution. Doing the calculations for source energy is less straightforward when the home uses fuels other than electricity.

In the new DOE report, they came down on the side of source energy. Here’s their fleshed out definition:

An energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.

They also put forth an expanded definition of source energy:

Source Energy:  Site energy plus the energy consumed in the extraction, processing and transport of primary fuels such as coal, oil and natural gas; energy losses in thermal combustion in power generation plants; and energy losses in transmission and distribution to the building site.

I say it’s expanded because they’re including the embodied energy of the fuel burned at the power plant.

Here’s a nice diagram from the new paper showing where the boundary is for determining site energy and source energy.

In addition to this definition, they also define something new called Renewable Energy Certificates (RECs), which they define as “tradable instruments that can be used to meet voluntary renewable energy targets.”

If you want to learn more about all this, go to the DOE page on this new Zero Energy Building definition and download the 22 page pdf. They show how to do the calculations and more.

I’ll close with what I think is one of the best things about this paper:

The designation Zero Energy Building (ZEB) should be used only for buildings that have demonstrated through actual annual measurements that the delivered energy is less than or equal to the on-site renewable exported energy.

It doesn’t matter how it models. What matters is how it actually performs. That’s not exactly a common way of thinking in Washington, D.C., where saying one thing and doing another is a way of life.


Related Articles

It’s Time to Settle the Net Zero Energy Controversy!

4 Ways to Define Net Zero Energy Use in Buildings

A 20 Year Old Energy Efficient House Goes to Net Zero in Florida


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This Post Has 14 Comments

  1. Your reference to saying one
    Your reference to saying one thing and doing another in Washington is certainly true, but in the same paragraph you rely on the DOE for simple, clear explanations. I hope you meant politicians, not civil servants, who try hard to do a difficult job, despite the cheap shots politicians and their supporters throw at them. And what does it say about voters that these politicians keep getting reelected despite amply earning the label of hypocrites.

    1. Yes, Robert, I was indeed
      Yes, Robert, I was indeed talking about politicians, who promise the Moon and then do whatever they want once they’re in office.

  2. As you alluded, words ARE
    As you alluded, words ARE important because they define how we think/feel about things. I think “Zero Energy Home” is the best term. Now we need to work on the term “Blower Door” that is neither a Door which rotates on hinges nor is the adjective “Blower” correct either since most people would think of it as a Fan not a Blower a la the Furnace Blower.

    While I am at it other terms that cause confusion are:

    CFM50 – how big a hole is that again?
    Duct Blaster – once you destroy my ducts by your Blaster, then what?
    Guarded Test – who is guarding that door anyway? Oh, you are neutralizing the pressure. Thanks.

  3. Allison wrote: “In the
    Allison wrote: “In the new DOE report, they came down on the side of source energy.”

    So if the source energy factor is, say, 3.0, then an all electric home that consumes 10kWh would have to generate 30kWh to be zero energy under the DoE’s definition?

    Moreover, the following quote seems contradictory:

    “…on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.”

    To me, the term “delivered energy” refers to the number of energy units (kWh or therms) the customer is billed for and thus does not account for upstream losses. I’ll need to read the DoE document to see if they clarify this better.

    1. David, no, that’s not that
      David, no, that’s not that the way they do the calculations. For an all-electric home, you’d multiply both the delivered and the exported energy by the source energy conversion factor. That means that you just need to export the same number of kilowatt-hours or more to be a ZEB.

      Their definition of delivered energy is:

      “Delivered energy: Any type of energy that could be bought or sold for use as building energy, including electricity, steam, hot water or chilled water, natural gas, biogas, landfill gas, coal, coke, propane, petroleum and its derivatives, residual fuel oil, alcohol based fuels, wood, biomass and any other material consumed as fuel.”

  4. I’d argue modeling matters.
    I’d argue modeling matters. It seems one key factor is lacking, the occupant behavior factor. Take 2 identical homes, with two very different families in each one, and the end results will be huge? That would be a ZEB builders nightmare having to go back later and upgrade for someone else living choices…

  5. My worry with ZEBs with
    My worry with ZEBs with respect to emission reduction obligations and also meeting peak electricity demand is that a ZEB’s energy production will occur at a different time then it’s consumption. There will be a temporal disconnect, assuming that a ZEB’s energy production will be largely from solar PV.

    It looks like ZEBs will meet heating demand with electricity (heat pumps). The sun won’t be shining much when it’s cold outside, and it won’t be shining at all when it’s coldest outside. So ZEBs in large part have to import electricity to meet their heating demand. This has implications for the grid, grid management and the need to build centralized generation to meet winter peak demand. This additional winter peak demand generation will likely have to be met with the construction of natural gas electricity generation. And with all that ZEB solar PV sitting on the ZEBs roof all those gas peaker plants will be sitting idle but for the period of peak winter demand.

    And gas peaker plants will still be burning fossil fuels.

    With the rapid decline in cost of PV on the roof top, it’s quite possible to foresee ZEBs with relatively high winter heat demand. It might be cheaper to build ordinary and then put an extraordinary amount of PV on the roof. This might be cheaper for the builder or future homeowner, but a much more expensive approach from a system, grid, perspective.

    A ZEB is just a node in the grid, if it’s a ZEB with a greatly reduced heating demand then it’s one that will make the grid more resilient. If it’s a ZEB with a high heating demand then the challenge of meeting climate change obligations and transitioning to a post-carbon energy system are that much greater and more expensive.

    I’ve focused on winter heat demand, I’m cold climate centric. : )

    That said I do live in a jurisdiction where peak electricity demand is at the coldest time of the year.

    On the flip side peak cooling demand also lines up with peak electricity demand in warmer climates, and the sun doesn’t shine on hot humid nights.

    It’s nice that ZEBs are being better defined but I still get the feeling that they are being treated in isolation instead of being treated as part of the grid.

    A couple of links from the EIA

    Natural gas use features two seasonal peaks per year

    Does EIA publish data on peak or hourly electricity generation, demand, and prices?

    1. @Andrew, as I’m sure you’re
      @Andrew, as I’m sure you’re aware, utilities around the country have recently been clamoring to dismantle net metering. Thus far these efforts seem to be driven by political forces, not by any serious or immediate threat to grid stability or revenue. But the problem is real. Just ask utility regulators in Hawaii. The problem is that today’s rate structures never anticipated homes that require full capacity support without billable energy charges. Yet, the economic rationale for rooftop solar and net metering rely on this dichotomy. At some point, something has to give. Or break.

      The DoE understands this. Sam’s Zero Energy Home effort (formerly DOE Challenge Home) requires that participating builders meet aggressive EE requirements: DOE Zero Energy Ready Home Program. But this obviously doesn’t fully address the problem.

      Prior to installing a net-zero+ PV array on my own home, I reduced consumption to about 3.3 kWh/ft2/yr. Hardly what you would consider an ordinary home. Yet coincident consumption averages less than 32% since peak loads are out of sync with peak production, even in summer (the duck curve).

      Longer term, energy storage and smart controls provide the answer. However, traditional residential rate structures are a disincentive to customer-side storage and other load shifting technologies. Ironically, if rate structures were modified to encourage energy storage and smart controls, then we would be talking about how grid defection would eventually destroy the grid, as fewer and fewer customers would be left to bear the enormous cost of power plants and distribution infrastructure. So in a real sense, what’s beginning to play out is the battle for who will “own” these new technologies. But since neither the DoE or FERC have authority over retail rate design, this battle will play out in the states. Unfortunately, that foretells a very messy road ahead.

      1. David,

        Yes, it’s the Duck curve.

        For those who don’t know what the duck curve is there is a pretty good chart here…

        Anyway, it seems to me that it would be cheaper to deal with peak demand, because of heating and cooling loads, with insulation. Battery storage and grid management has to be more expensive than insulation?

        @Allison – Could you throw the duck curve chart up in this post?

      2. @Andrew, when you monitor kW
        @Andrew, when you monitor kW (demand), it becomes obvious that improving the shell or installing high efficiency A/C or heat pumps has a relatively small impact on peak demand (only to the extent that the AC or heat pump is downsized). There are plenty of studies that bear this out. Of course, hvac could be downsized on most homes without even touching the shell.

        But without controls and storage, there’s nothing to prevent these loads from occurring simultaneously with other electrical loads, thus creating the peaks. For example, a second (or 3rd) air conditioner, hot tub, pool pump, electric water heater, range, electric drier….

        The economic rationale for managing peak kW is fundamentally different than for energy efficiency. I agree that controls and batteries are expensive. But things look different from the utility’s perspective.

        As I said, as long as residential rate structure ignore capacity costs in favor of energy charges, there’s no incentive for homeowners to invest in storage and control technologies. And without a mass market, costs are likely to remain high for the foreseeable future. But if and when demand charges become the norm for residential customers, controls and storage will become more relatively important than insulation and SEER in terms of reducing energy costs. So the question is whether the objective is to reduce carbon emissions, or to manage the cost of delivering power. Until and unless we have a carbon tax or cap/trade system, utility regulators (and politicians) will remain focused on the later. And the movement toward net zero homes and buildings is at odds with that.

  6. Allison wrote: “For an
    Allison wrote: “For an all-electric home, you’d multiply both the delivered and the exported energy by the source energy conversion factor.”

    That makes no sense. Exported energy would (should) be subject to a very different (lower) conversion factor for all sorts of reasons. Anyway, if that’s how they do it, then the net effect (for an all-electric house) is the same as net zero site energy.

    1. It actually does make sense.
      It actually does make sense. Every kWh of PV generated you send into the grid prevents ~3 kWh of energy used at the plant. See my article on the four definitions of net zero, where I wrote that for an all-electric home, the site energy and source definitions are exactly the same.

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