The U.S. Department of Energy (DOE) has a program to recognize energy-efficient and demand-flexible residential buildings. To be certified under the DOE ZER Homes program, a home must meet all requirements based on building type. The builder/developer/plant must be registered as a ZER Home program partner, and the project must be approved by an approved third-party verifier.

The strategies for achieving either ZNE or ZER buildings essentially follow the same process.

First, focus on the building envelope to reduce overall energy loads on the building. Most ZNE homes exceed minimum insulation levels and ensure insulation is installed well. Other priorities include air sealing, using high-quality windows, and maximizing a home’s orientation, which can affect heat gain, passive heating, or solar power availability.

Next, look at the energy-consuming systems in the building and strive for the most efficient options. Different equipment has different capabilities in this regard, so finding ways to reduce their size, their need, or quantity can all help as well.

With the above determined, consider the energy sources – on-site, sourced, or combined. Recognize that mixed-fuel systems may offer lower first costs and reduced ongoing energy costs and emissions and allow for hybrid systems tailored to a project’s unique needs. Hybrid heat pump furnace systems, for example, can be a consideration when it isn’t practical to have all-electric heating as the backup. Combining an air-source heat pump with a high-efficiency propane furnace replaces the inefficient electric resistance backup heat in cold outdoor temperatures, providing improved comfort and the potential for overall energy use reductions.

Finally, although not always in the control of the architect, it is significant to control other energy uses mostly determined by the building users. Energy Star-labeled appliances, such as dishwashers and refrigerators, ensure efficient operation to trim energy use. Energy Star LED lighting offers energy savings and the assurance of additional performance testing such as lumen maintenance over time. And using energy monitoring devices can help encourage energy-saving habits.

A good resource for more information on NZE strategies is TEAM ZERO—a 501(c) 3 non-profit charitable and educational organization, formerly known as the Net Zero Energy Coalition (https://www.teamzero.com/). Their mission is “to unify stakeholders involved in promoting different "paths to zero" with a common agenda and collaborative efforts that accelerate market adoption of zero energy and zero carbon homes, commercial buildings, developments, communities, and retrofits across North America.” They point out that we may be approaching a tipping point in zero net energy home construction and design. They report that more than 22,000 zero net energy projects are in design, in construction, or completed. They also anticipate that as high-performance technologies and solar PV systems continue to be more cost-effective, as energy codes continue to require higher performance, and as awareness increases, ZNE and ZER homes are likely to continue to increase in popularity. TEAM ZERO’s vision is that “the built environment will be a positive asset on the planet’s carbon balance sheet by 2050.” A survey performed by Harris Insights & Analytics found that 83 percent of homebuyers and 89 percent of builders are likely to consider a ZNE home for their next purchase or build. And about the same number–81 percent of homebuyers and 84 percent of builders–are very or somewhat willing to pay more for a ZNE home.

Energy Generation

A common component of ZNE design is the location of the energy source. This is a significant distinction in terms of energy usage, not just one of convenience. Source Energy represents the total amount of raw fuel that is required to operate the building. It incorporates all transmission, delivery, and production losses. As such, it provides a more complete picture of the resource efficiency of a home or building’s energy performance since it includes the raw energy needed to create the electricity (i.e., the burning of fossil fuels to create steam which turns a turbine or generator) and the energy needed to deliver that electricity. Each step in the process involves some overall energy loss due to inherent efficiency levels in each step. Looking at source energy from a power plant, it is common to find that for every 1.0 unit of electrical energy that passes through an electrical meter at a building, over 3.03 units of source energy was consumed to generate and deliver that electricity. This is due to losses from extracting and processing the fuel, the conversion losses of burning the fuel to generate some electricity, and transmission losses as the electricity is distributed over the grid. Essentially this means that for every kW hour of electricity consumed, three times as much energy was required to produce and deliver it. Once inside the building, further losses occur based on the efficiency of the equipment and fixtures that the electricity is powering. Site Energy refers to energy (including electricity) that is generated at the building site. Site energy may be delivered to a building in one of two forms: primary or secondary energy. Primary energy is the raw fuel that is burned to create heat and electricity, such as natural gas or fuel oil used in onsite generation. Secondary energy is the energy product (heat or electricity) created from a raw fuel, such as electricity purchased from the grid or heat received from a district steam system. A unit of primary and a unit of secondary energy consumed at the site are not directly comparable because one represents a raw fuel while the other represents a converted fuel. If the energy is used as it is being produced, then there are no transmission losses which makes it inherently more efficient. The overall efficiency of the energy generation on-site will vary depending on the equipment and the type of system being used, but generally speaking, fewer units of energy are needed for each unit consumed with on-site energy generation than with source energy solutions. On-site energy sources like PV and Propane will score better on source energy utilization than conventionally delivered energy sources. Both site and source energy need to be dealt with in order to achieve carbon-neutral design.

In between these two options, the use of “microgrids” has started to appear. These are much smaller scaled versions of the larger multi-state electrical grid but offer an opportunity for some carbon reductions. The concept is to create a small power plant that is dedicated to serving a particular group of buildings either as part of a general community or a specific functional entity, such as a business operation. They may serve as a supplement to the larger grid and may be powered by renewable energy (wind, solar, etc.), conventional energy (diesel, propane, etc.), or a hybrid approach.

In situations where microgrids are desired or in use, they are another opportunity to reduce the carbon emissions they may create. Ideally, an all-renewable energy solution that emits zero carbon emissions should be considered first such as PV with battery backup. If that isn’t possible or practical, then hybrid solutions are the next best thing. However, the fuel source for such a hybrid solution should be assessed. It is common for power companies, businesses, and others to default to diesel generators for electrical generation with natural gas coming in as an option where it is available. As noted already, they both have higher carbon intensity ratings than propane, and especially renewable propane. Further, propane has the advantage of being able to be delivered to remote locations that may not be served by natural gas. By locating the power generation close to the users through a microgrid approach, the overall efficiency of the entire process increases notably. Instead of requiring over three units of energy to produce one unit of electricity, propane has been shown to require only 1.15 units of energy at the source, accounting for losses from extracting, processing, and distribution.

An example of this process is found in the use of solar PV with battery backup and propane generator solutions for remote wildfire-prone locations in California. Instead of ruggedizing the transmission and distribution (T&D) lines which could cost more than a million dollars per mile in remote locations, electric utilities are de-energizing the T&D lines and installing microgrid solutions to avert forest fires. In particular, Pacific Gas & Electric (PG&E) has issued a 2021 wildfire mitigation plan and reports that a technology combination of solar PV and battery energy storage with supplemental propane generators is not only the most cost-effective and reliable but also the cleanest solution for initial remote grid sites. In most locations, the supplemental propane generators are used to charge the batteries when the batteries reach a lower state of charge. In other instances, the propane generator is also used for providing supplemental electricity due to higher electricity demand prices charged by the electric utility. Since remote microgrids often depend on diesel for fuel supply, it is easy to see why propane is receiving significant traction due to its ability to reduce emissions and improve local air quality, particularly if renewable propane is used.

Combined Heat and Power

Other means to consider in the quest for carbon neutrality are the efficiencies found in systems that generate both heat and power for a building complex, a campus, or other identifiable group. This has been a common approach for some time using conventional fuels. However, propane, including renewable propane, is being used for combined heat and power (CHP) solutions in the 1 kilowatt-1 Megawatt range (residential, commercial, and industrial). Typically, the engines employed for CHP have higher thermal efficiency (>30% fuel to electrical conversion efficiency), higher durability (40,000-60,000 hours), and low emissions. The biggest challenge in using these solutions in the backup generator market is their high capital cost since the attributes and expectations of the CHP products are different from the low-cost and low-durability backup generator products. Depending on the size of the unit, a 16-43 percent reduction in CO₂ emissions can be realized when propane is used in these systems instead of diesel.

Propane also opens the door for fuel cells, a high-efficiency and low emissions technology that is seldom used with diesel. In addition, some fuel cells produce very low Nitrogen Oxide (NOx) (0.03 g/kWh) and Carbon Monoxide (CO) (0.045 g/kWh) emissions. For added resiliency, CHP solutions work well for residential, commercial, and industrial applications, providing not only the needed electricity but also heat and/or hot water (and/or cooling) during power outage situations. When used in CHP mode, these solutions can have more than 80% fuel conversion efficiency to electricity and useful heat.