A Net Zero Energy Home Grows Up: Lessons and Puzzles from Ten Years of Data
In 2005, Habitat for Humanity of Metro Denver teamed up with the National Renewable Energy Laboratory (NREL) to design and build a net zero energy (NZE) home in Wheat Ridge, Colorado. A main goal of the project was to show that a NZE home was feasible to build within the Habitat for Humanity model. This meant that the design must be repeatable and cost-effective for the climate region; that it must be simple enough that it could be built largely with volunteer labor; and that occupants could operate the home with no special training.
The NZE home that was built meets all of these criteria. It is highly insulated, with R-40 walls, R-60 ceilings, and an R-30 floor. It is solar tempered, meaning that there are south-facing windows for passive heating with an overhang for summer shading, and equipped with heat recovery ventilation, a solar water-heating system, and a 4kW PV system. The solar tempering design, along with double stud wall and fiberglass batt construction, the construction of the home was simple and low cost. Other efficiency measures were selected to ensure that the overall energy use of the building was low, so that a smaller PV system would enable net zero energy annually.
When the house was completed, a family of three—a mother and her two young sons—moved in. The home was fully instrumented with sensors and data loggers so that NREL engineers could monitor equipment performance and observe the home’s energy consumption over time. We have been monitoring this house continuously for the past ten years, and the resulting data set is rich with surprising and thought-provoking trends.
Net Zero Energy?
What does it mean for a home to be truly NZE? According to DOE, a net zero energy building is one where the source energy consumed annually is less than or equal to the energy produced by the on-site renewable-energy resource on a source energy basis. This house uses both electricity and natural gas, so the site energy use for both fuel types was converted to source energy in common units of kBtu. (All the energy consumed by the house was converted to source energy using site/source conversion factors of 1.09 and 3.15 for natural gas and electricity, respectively.) The bar chart in Figure 1 shows source-energy ratio: the annual energy produced by the PV system divided by the total energy (electricity and natural gas) consumed by the house, in terms of source energy. The horizontal line indicates the 100% level, where PV production and source energy use are equal. The home is a net producer in years 1, 2, 3, and 9, but falls short of the NZE goals in years 4 through 8, and 10. Overall, the home has not achieved NZE status in its first ten-year period, even though the annual source-energy ratio for nearly half the years on record has been over 100%.
What happened after the first three years of operation? Figure 2 shows the annual source energy use alongside the source energy offset by the PV system. It appears that the sudden shift from net producer to net consumer is the result of a combination of two effects. From year 3 to year 4, PV production dropped by roughly 10%, while at the same time, household energy use continued to rise at roughly 10% per year (when averaged over the first eight years).
Percent of Source Energy Consumption That Was Offset by PV Production Annually
Annual Source Electricity Consumption and Source Energy Offset by Production
Figure 3 shows the annual source energy consumption, including both gas and electric, broken down by end use. From years 1 through 8, energy consumption generally increased. This corresponds to the overall general decrease in source-energy ratio over the same time period. Source energy consumption dropped by nearly 45% between years 8 and 9. This corresponds to the increase in source-energy ratio from 64% to 110% between those years.
Annual Source Energy Consumption
Despite some fluctuation in the energy generated by the PV system annually, energy consumed in the home is the main driver for the ten-year pattern of source-energy ratio. Looking more closely at energy consumption by end use, a number of interesting trends are apparent.
The category of miscellaneous electric loads (MELs) in this data set includes all plug loads and any appliances not individually submetered. Although we do not know what these end uses are, we see in the data that MELs are the main energy user in the home and have grown over time. They likely include home entertainment and office equipment (TV, DVD player, computer); countertop kitchen appliances (toaster, microwave); any plug-in lights (all hard-wired lights are captured in the lighting category); and other, less common plug loads (such as aquariums, space heaters). Given the proliferation of modern consumer electronics, it is not surprising to see MELs energy use increase—but to see these miscellaneous loads increase to the point that they make up most of the energy use in the home is dramatic. An interesting aspect of this data set is that it spans a ten-year period over which two small children grew into teenagers, and this may account for at least part of the increase in MELs energy use.
The trend in heating is somewhat puzzling. The home has two known sources of space heating—a central gas space heater and small electric baseboards in the bedrooms. Use of these two sources varies widely from year to year. In some years, the home was heated primarily by the central gas heater, and in some years, by the electric baseboards. In other years, neither source was used much, although the historical weather data indicate that those winters were not significantly warmer than average. As shown in Figure 4, MELs use increases in the winter months, particularly in years where the heating energy use for both the gas and baseboard heaters is low, strongly suggesting that the occupants may be using additional plug-in space heaters to keep the house warm. MELs use is significantly lower during the summer months, so it is unlikely that window air conditioners were installed (the home does not have a central cooling system).
Monthly Average for Temperatures and Source Energy Use for MELs and Total Heaters
The two categories of load that have the biggest impact on the overall source-energy ratio are MELs and space heating. This case study shows that changes in occupant behavior can drive high variability in annual energy use. The same occupants have lived in the house for the entire duration of the project; yet the evolution of their collective MELs usage pattern is significant enough to help to determine whether the home achieves NZE in a given year.
Even in a home with an extensive data acquisition system, it is impossible to fully understand variability in energy use. In addition to highlighting the variability and unpredictability of MELs use for a single home, does this long-term data set offer other lessons that are broadly applicable?
Understanding and Controlling MELs
As residential building research and improved energy codes have pushed homes to be built with better envelopes and more-efficient HVAC and water-heating equipment, the smaller loads in the home have become a larger fraction of residential energy use in aggregate. Improvements in technologies and building methods have made possible reduction to all major loads in a home, but MELs reduction continues to be an elusive goal. Ten years ago, researchers at NREL recognized the need to devise strategies to reduce MELs energy use in order to achieve 50% whole-house savings. We have moved beyond the goal of 50% energy savings, but little progress has been made to reduce MELs energy use. Consumer electronics are becoming more and more efficient, but the number of devices in homes has exploded. A number of studies over the last ten years have identified MELs as a challenging problem that will only become bigger as homes become more energy efficient.
It is difficult to research the rampant growth in MELs because these end uses are by nature a moving target. We know generally what devices people have in their homes, and how that mix has changed over time, but we have little empirical data on their usage patterns. Detailed field studies to generate statistically meaningful data would be labor intensive and cost prohibitive. Currently available monitoring equipment for plug loads does not meet the cost and ease-of-use requirements for large-scale field studies. And if it is difficult to measure the energy use of all the MELs in a home, it is even more difficult to find ways to reduce that energy use. The MELs category includes a diverse assortment of devices, used in every room by different people at different times of day. Most MELs are small loads, distributed throughout a home, and every single one would need a dedicated controller. A MEL controller could be located at the breaker level (depending on the loads connected), integrated into the device, or integrated into the wall outlet. Or it could be a pass-through controller that sits between the plug load and the wall outlet. However, the cost to install distributed controllers for all the MELs in a home would likely be much higher than the potential energy savings. Improving the energy efficiency of the individual devices might reduce the need for sophisticated controls, but most new electronics are already fairly efficient, and the large overall MELs load is mainly driven by a combination of legacy devices and the sheer number of devices, not by the efficiencies of the newest devices. With the advent of the Internet of Things, the rapid increase in connected devices that have built-in communication capabilities may deliver new opportunities for data collection, as well as personally tailored energy management features.
The Wheat Ridge NZE house offers a unique perspective on MELs and on their growth over time. Several studies have looked at multiple homes for relatively short periods of time, and have found that the energy use between similar homes can vary wildly depending on the occupants. In contrast, the subject house exhibits significant changes in MELs use over an extended time period but with the same occupants.
Is Net Zero the Right Goal?
Beginning in 2020, California aims to become the first state in the nation to require all new homes to achieve NZE based on one year of meter data. While California is working to change its building codes to meet this goal, cities and smaller municipalities in other states are planning to mandate NZE homes in all-new construction in the coming decade. NZE homes, no longer a niche concept, will become commonplace in the very near future. As the ten-year data set from this case study shows, meeting NZE in the long term is not as simple as choosing the right building systems, because occupants are a primary driver of a home’s energy profile. It may be worth asking: Is NZE even the right metric to gauge our progress toward sustainable homes?
The process of designing a NZE home naturally starts with a very energy-efficient house. Improving the building envelope and installing an efficient and climate-appropriate package of equipment in the home minimizes the home’s energy requirements. Energy modeling can be used to estimate the amount of PV that is needed to offset the annual energy consumption based on “typical” occupant behavior. Of course, this will result in some homes being net producers and some homes being net consumers (and some homes being one or the other, depending on the year), because few if any families match the statistical average energy use profile. Some of the variability could be reduced by predicting the behavior of the future homeowners, but as this case study shows, families grow up and people change their habits over time. This NZE home has only achieved net zero status for four out of ten years; but overall, energy generated by the PV system has offset nearly 90% of the home’s consumption. Is that good enough, or is this a failed effort to build a NZE home?
Until recently, the cost of solar panels limited the amount of electricity homeowners or builders could reasonably plan to offset, but this factor has changed dramatically in recent years. As the overall cost of installing rooftop PV plummets, the way that builders plan for NZE is certain to evolve. If building codes require NZE, builders could easily install more PV than the average occupant would need to ensure that a home would achieve net zero energy for nearly everyone. Taking this concept to its extreme, it may be easier for developers to shift their focus from improving the thermal efficiency of their houses to simply installing larger—and now affordable—PV arrays to achieve NZE status; the only limiting factor is rooftop real estate. So will architects design houses differently to maximize PV installation possibilities? Will decades of work on cost-effective efficiency measures be rendered obsolete by cheap solar panels?
The Role for Occupants
Many people do not think about how their behavior affects energy consumption, but an analogy to driving illustrates how these habits can change. Drivers have traditionally not thought about how the way they drive affects gas mileage, but as newer cars have incorporated fuel economy metrics into the dashboard display, drivers have been prompted to rise to the challenge, many making a game of driving as efficiently as possible. This suggests that energy feedback should also be available to interested homeowners so that they too can learn how to operate their homes in the most energy-efficient manner. Drivers may be more receptive to feedback, since they are constantly looking at their dashboard while driving, while homeowners might not look so often for feedback on energy use. But such feedback might still produce meaningful changes in behavior. A number of devices can be installed to provide instantaneous feedback on power consumption or overall energy use, but these devices can be complicated to install, and few people know how to interpret feedback that is shown in units of kilowatts or kilowatt-hours. Simpler forms of feedback are needed. A Nest thermostat, for example, shows a little leaf (like the Toyota Prius) to indicate when an action improves energy efficiency. Similarly, simplified feedback could be provided for whole-house energy efficiency, especially for cases where the homeowner is trying to achieve net zero status. Studies have shown that feedback alone can produce whole-house energy savings of 5–10%, although data on long-term persistence are limited. Educating homeowners before they move into a NZE home could also help them to understand how their behaviors affect the home’s energy performance.
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Controllable loads in the home can play a role in demand-side management strategies, such as demand response and dynamic pricing. Appliance manufacturers have explored controllable loads for the last several years, but there has been little consumer demand for them. That could change as utilities change their rate structures or their rules on rooftop solar.
Other examples of automated ways to flatten the demand from the home to reduce instability on grid include on-site energy storage (such as a lithium-ion battery pack) and solar inverters with sophisticated control features capable of curtailment. Efforts are under way to develop and vet these possible solutions to complement the ever-increasing rooftop solar installations. Many major manufacturers are building controllable inverters, and early laboratory tests of their performance have helped to convince the Hawaiian Electric Company to lift its moratorium on connecting residential solar panels to the grid.
As more jurisdictions encourage—or mandate—NZE homes, it will become necessary to incorporate a variety of automated solutions for mitigating the impact of distributed solar on the grid, to ensure that NZE homes do not create problems for the utility. Although grid responsiveness does not necessarily reduce energy use inside the home, controllable loads and solar inverters will better enable utilities to add more on-site (and grid scale) renewables to the grid. This will do more to promote long-term sustainability than improving the efficiency of a single home. States and municipalities that are planning to require NZE status should coordinate with their local utilities and their state public utilities commission to evaluate options for incorporating grid responsiveness into the NZE requirements before they take effect. Forward thinking in this area would ensure that NZE homes reduce our reliance on fossil fuels by partnering with the utility of the future.
What have we learned? A decade of performance data on a NZE-designed home has shown that consistently achieving net zero status annually is not easy. But how important is it to achieve NZE? Even if a NZE-designed home does not meet the target every year—or ever—it provides significant benefits to the homeowner and to society. To put it another way, it is not an all-or-nothing proposition. At the same time, homes with large rooftop solar arrays still face challenges related to grid integration, regardless of whether or not they achieve NZE.
So what is our true mission? Perhaps the concept of NZE is gradually evolving to represent an inspirational vision rather than a mandate, but pushing the housing industry toward building NZE homes is a step in the right direction, even if some buildings fall short. Ultimately, the goal is to create an electrical infrastructure that relies more on renewable energy sources and less on fossil fuels. Reducing the energy consumed by residential buildings is part of this complex challenge.
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