Net Zero Energy and the Passive House Standard

What does net zero energy really mean?

For a grid-tied net-zero-energy building, this is about offsetting the energy consumed with energy produced by renewable, emissions-free means. Why not offset by purchasing renewable energy or just buying carbon offsets? This could be called a Net-Zero Off-Site Energy Building. While there is no standard or third-party verification system for net zero energy, the four most common definitions do not allow offsets for off-site renewable energy. A 2006 paper (PDF) by the National Renewable Energy Laboratory explains these definitions in detail.

  • Net Zero Site Energy: A site ZEB produces at least as much energy as it uses in a year, when accounted for at the site.
  • Net Zero Source Energy: A source ZEB produces at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to generate and deliver the energy to the site. To calculate a building’s total source energy, imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers.
  • Net Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building owner for the energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.
  • Net Zero Energy Emissions: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.

One design implication of a site ZEB is that this definition favors electric equipment that is more efficient at the site than its gas counterpart. Using the site ZEB definition, a 95% efficient gas boiler consumes 1053 kWh to produce 1000 kWh [3412 BTU] of heat, while an air-source heat pump with a coefficient of performance of 2.85 only consumes 351 kWh to produce the same amount of heat. However, using the source ZEB definition, the two are equivalent. 1000 kWh of heat requires 1158 kWh of source energy in both cases, assuming a source-energy factor of 3.3 for electric and 1.1 for natural gas.

The NREL paper ignores the embodied energy required to produce photovoltaic and solar thermal equipment. The source energy factor for PV including embodied energy is 0.7 according to the standards used by the Passive House Institute. Additional source energy factors per PHI are 2.7 for electric, 1.1 for gas, and 0.2 for wood. The PHI factor for electric is lower than the US average because it is based on the average for Europe, which uses less coal than the US. Determining the correct source energy factor can be difficult. For instance, in New York, should the factor be based on a state-wide average or on average for the entire Eastern US/Canada power grid?

HOK and The Weidt Group designed the Net Zero Court project to be Net Zero Energy Emissions and therefore carbon neutral. The office building still has an estimated energy bill of $0.01/SF so it is not Net Zero Energy Cost. In St. Louis, where the project is located, 81% of electricity comes from coal, so a carbon-neutral building would have a large impact. Their design methodology parallels that of Passive House.

  • Start with optimizing the building orientation and thermal envelope to improve energy efficiency.
  • Then use efficient mechanical systems.
  • Finally, offset the remaining emissions with on-site renewable energy. In this case, with a large four-story structure, they had to rely on PV panels in the parking lot as well as on the roof in order to produce enough electricity to balance that used by the building.

See their PDF report here.

Off-grid buildings—also called energy autarkic buildings—use more total energy over 80 years because of the embodied energy for the PV and batteries which must be replaced every 30 years on average.  [source]

A building constructed to Passive House standards can be a Net Zero Energy Emissions building by adding photovoltaic arrays or a wind turbine. The Passive House Planning Package (PHPP) software provides a tool to calculate what is required for carbon neutrality. All other things being equal, the Passive House building will cost less to operate.

Learning from Kranichstein

Passive House in Kranichstein, GermanyHow do you design a building that is quiet, comfortable, and costs substantially less to operate?

One tool is an energy-modeling software called the Passive House Planning Package (PHPP) developed by a German physicist, Dr. Wolfgang Feist. He developed the software as a relatively easy way to determine compliance with the Passive House construction standard for low-energy buildings. The PHPP model for the first Passive House–a townhouse development in Kranichstein, Germany, near Frankfurt–is included with the software. It is easy to change the location of the project using the provided climate data sets. I “moved” the building to New York City to see what modifications to the building envelope would be required. It turns out that it is much easier to build a Passive House in New York than in Central Europe.

I made a few changes to the energy model.

First of all it seemed unrealistic to expect New Yorkers to use a clothesline to dry their clothes, so I included an electric condensing dryer. Maybe it would be even more realistic to omit the washer and dryer entirely, since most New York apartments don’t have them. But, let’s give the imaginary tenants the city luxuries of a washer/dryer and even a dishwasher.

Second, I added air-conditioning. I know there are hippies crying right now that we should just suffer through the summer in order to save Gaia. When it is 90 F/ 32 C and 90% humidity at night, I don’t care about the environment, I want my A/C.

Third, because of the A/C, I decided to change the heat source from a boiler to a mini-split heat pump. That way, one piece of equipment can provide heat in the winter and cooling in the summer. These units are common in penthouse retrofits in the city. They are also much quieter than window A/C units or through-wall PTAC units.

Fourth, I changed the heat recovery ventilator (HRV) to a model that is available from a US distributor and changed the energy model assumption that tenants would open windows at night for cooling. Even when we get cool summer nights, it is often impossible to leave the windows open because of street noise. It turns out that the efficiency of HRV’s has increased since 1991. Instead of 83% heat recovery efficiency, you can now get 92%.

Then, I eliminated the earth tube sub-soil heat exchanger. These are very common in Europe, but many Americans are skeptical about the potential for mold growing in the underground air-supply pipes. Until there are some examples of earth tubes successfully installed in New York, I will hold off on assuming that developers or homeowners will want to use this technology. Although if you want to be the first, I’ll be more than willing to work with you.

[update] I also changed the building from a corner to a mid-block location with shared walls on both sides. This is a much more common situation in New York.

Finally, I drastically reduced the amount of insulation. Instead of relying on rules of thumb or guesswork, the architect can use the PHPP energy-modeling software to determine exactly how much insulation is required for the specific building. In this case, because New York gets a lot more sun and has milder temperatures, a lot less insulation is required. The urban density of the city which makes shared walls more common also significantly reduces the heating and cooling loads.

Compare:

Kranichstein

Walls: 275 mm EPS; Roof: 400 mm blown mineral wool; Sub-slab: 250 mm EPS

New York City

Walls: 50 mm EPS; Roof: 400 mm blown mineral wool; Sub-slab: 25 mm EPS

With substantially less insulation, this hypothetical NYC building uses less than the Passive House limit of 15 kWh/m2a for heat and is below the same threshold for cooling, at 9 kWh/m2a. The electricity use is higher than in the Kranichstein example because of the clothes dryer and the heat pump. This building uses 83 kWh/m2a source energy (called primary energy in PHPP). This is still below the Passive House limit of 120 kWh/m2a.
Total energy cost for the apartment is about $1250 per year for electricity and $50 per year for gas.  That’s $108/month for all of the electric, heat, and hot water in a 2,000 SF apartment.

Given the low utility costs, a developer could easily include heat AND air-conditioning in the rent as a way to distinguish the building in the market.