Navigating the road to positive-energy buildings
Implementing key enablers, high-performance manufacturing or industrial buildings can become extremely efficient.
- Define net zero energy building (NZEB) and positive-energy building (PEB).
- Identify key enablers to the realization of PEBs.
- Describe PEB solutions for manufacturing/industrial buildings.
Green building programs can be credited for bringing together a diverse group of stakeholders that are responsible for the design and operation of the built environment, and creating an informed conversation around the value of holistic building design. Green building rating programs and standards—including the U.S. Green Building Council LEED program (U.S.), Living Building Challenge (U.S.), BREEAM (U.K.), HQE (France), ESTIDAMA (United Arab Emirates), CASBEE (Japan), GreenMark (Singapore)—have energy efficiency requirements, as well as additional credits associated with energy demand reduction. These programs also reward participants for installing on-site renewable energy systems, and sourcing offsite macrogrid renewable energy. Building rating programs have contributed to the plethora of energy-efficient buildings worldwide. They have facilitated the identification of key efficiency strategies and propagation of cutting-edge technologies while creating a demand for on-site and offsite renewable energy. They have opened the door for the next generation of green buildings—buildings that are energy-positive and harvest more energy than they consume. Positive is the new green.
Whether a building is deemed green, smart, intelligent, or high-performance, efficient use of energy resources will undoubtedly be at the core of the design, and rightly so. Buildings and their systems consume more energy and generate more emissions than any other sector in the U.S., and likely the world. Combined, residential and commercial buildings account for nearly 30% of the total energy use in the U.S., industry accounts for about 33% (8% building systems and 25% manufacturing processes), and transportation consumes the remaining 37% (25% vehicles and 12% air-marine-rail-pipeline transport). Thus, buildings account for approximately 38% of U.S. energy consumption today. With increasing electrification of vehicles, transportation loads will likely shift from the gas pump to electrical meters, resulting in more than 40% of U.S. energy consumption occurring behind the meters of buildings in coming years (see Figure 1). This concentration of load behind the meters of buildings provides a clear opportunity—and responsibility—for building owners, architects, engineers, financiers, and constructors to significantly change how energy is consumed and generated in the U.S. In doing so, they also have tremendous positive impact on built infrastructure operational costs, reducing air emissions, and enabling new technology. A net zero energy building (NZEB) generates enough renewable energy on-site to equal its annual energy use. A positive-energy building (PEB) produces more energy from on-site renewable sources than it consumes. The level of excess energy should be sufficiently high to offset the embodied energy of the building infrastructure over the building’s lifetime. In either case, grid connection is allowed and power is delivered to or extracted from the grid. Energy storage may be deployed to control the timing and amount of grid power used.
The benefits of net positive
PEBs are the pinnacle of green building design. There is no question regarding the sustainability of buildings that produce more energy than they consume. PEBs are expanding the conversation regarding high-performance buildings. A green building’s effectiveness must include not only the operational energy but also the embodied energy of the building materials. By considering the embodied energy, we capture the ultimate footprint of a building, from concept to present operation. A PEB’s excess energy has the potential to offset its own embodied energy, and to pay off historical energy debts. Embodied energy within building materials will be reduced in the future as materials are created in net positive manufacturing facilities and material transportation systems. PEB’s excess energy also has the potential to offset the building occupants’ transportation energy used in commuting to and from the building. It is already the case where electric vehicle owners charge at home where they have rooftop photovoltaic (PV) arrays, and charge at work based on parking lot PV arrays. The energy payback and emission/resource offsets are, undoubtedly, the marked benefits of PEBs.
Driver of green industry
The single goal of creating a PEB is to simplify the design process, essentially liberating the design team from the burden of estimating percent savings over a hypothetical baseline model for the purpose of achieving a green building ranking. When the goal is net zero energy, or positive energy, a different approach is embedded in the design process. With more stringent design conditions, engineers begin from a budget based on available energy that can be harvested and stored on-site. These conditions drive efficient design by holding the process accountable to the availability of limited resources. Additional costs associated with on-site energy systems and advanced building materials are often paid back over the lifetime of the building by the power produced by the building after construction. Therefore, PEBs increase the demand for higher cost, more advanced materials by balancing the lifecycle costs of the project, ultimately bolstering the market for high-performance building technology.
Arguably, one could make sustainability claims solely based on achieving a PEB alone. However, there are many other facets of green building, such as indoor air quality, daylighting, and positive community feedback, that could be overlooked if the focus is on energy alone. Therefore, it is worth mentioning that positive-energy design will likely enhance the greater quality of the edifice. The need for more efficient lighting will enhance daylighting, the benefits of which have become common knowledge. Research, such as that conducted by the Lighting Research Center in Troy, N.Y., has shown that daylighting increases occupant comfort and productivity, and provides the proper stimulation to regulate healthy circadian rhythms. Indoor air quality may be improved as well by shifting the traditional approach to more efficient ways of controlling interior conditions. For example, use of natural ventilation can help reduce symptoms of sick building syndrome by decreasing concentrations of pollutants from indoor sources. Finally, PEBs will enhance a community’s perception and value of the buildings they interact with, leading to community support and positive feedback. Ultimately, the fringe benefits beyond energy balance should not be overlooked.
The first step to creating net zero and positive-energy buildings is simply having the vision, and articulating it in such a way that stakeholders completely understand the benefits. Many companies are doing so. References and examples that illustrate the benefits and costs of high-performance buildings are leading the charge to achieving energy balance. Intel, Microsoft, Apple, Proctor & Gamble, Google, and others have declared their commitment to becoming carbon neutral. They recognize the value in green building and are leading the way by aspiring and planning for energy balance. For example, Tesla has recently announced plans to construct a massive “gigafactory,” a facility that will produce finished battery packs on a large scale from raw materials. The process of manufacturing these batteries is an energy-intensive process that traditionally relies on cheap, highly polluting sources. However, Tesla’s new facility will be powered largely by on-site renewable energy generation, mostly wind and PV. Tesla’s internal studies suggest that the carbon footprint of a single battery pack from the gigafactory will be completely offset after 10,000 miles of driving in its Model S, potentially making the battery the most carbon-neutral component in the vehicle.
Supply versus demand characterization
Another important step is determining the feasibility of creating a PEB based on the building’s energy profile and the site’s renewable energy potential. At project conceptualization, a building’s estimated energy use intensity (EUI) can be used to establish energy demand budgets. EUIs can be estimated by benchmarking based on building type (statistics available via Energy Information Administration CBECS database, U.S. Environmental Protection Agency Portfolio Manager, ASHRAE, and Labs21), and then reduced by a percentage to establish the expected energy demand. For industrial facilities, EUIs are generally not available and must be estimated from existing data often considered proprietary. The renewable energy options for the site can be a single type or a mix of solar, wind, geothermal, and arguably biomass. There are many resources available to help quantify and optimize on-site resource potential. The National Renewable Energy Laboratory (NREL) System Advisor Model is a good place to start. Solar is often the best resource to start with, as it is ubiquitous and easily deployed. Wind energy can be more elusive, especially in urban environments (see Figure 2). If the wind resource is abundant (average wind speed of 12 mph with a predominant wind direction) then it is possible to integrate a few small wind turbines (less than10 kW) into the design.
If land beyond the building footprint is available, it may be possible to integrate medium-size wind turbines (100 kW), or large wind turbines (greater than 1 MW). However, permitting and visual impacts of these turbines can be significant challenges. Geothermal energy is very site-specific and only available in certain parts of the world. In the U.S., geothermal heat is generally concentrated in the Western states. Identifying the geothermal resource potential is best done with site-specific information, such as subsurface temperature and heat flux knowledge.
Biomass requires taking a regional perspective and sourcing fuels locally. Depending on the building type, and the processes within, it may not be possible to generate enough power based on given land and available renewable energy resources. For buildings with lower EUIs, such as office buildings, a mix of rooftop and ground-mounted PV sources can often be adequate to achieve an annual energy balance. For buildings with larger EUIs, such as chemical plants, manufacturing facilities, and data centers, companies end up purchasing renewable energy credits to meet energy goals. For these high EUI facilities, it is essential to site the building with renewable-energy-generation potential as part of the selection criteria (see “Example: PEB for manufacturing”). This may mean purchasing contiguous acreage—or even nearby acreage—for solar farms, wind farms, and/or biomass production.
There are many technological solutions involved in the realization of a PEB. Key enablers include integrated modeling and dynamic simulation, the low cost of PV, and the projected falling price of energy storage. The most important technology enabler is integrated building modeling, or modeling from a more holistic approach. The reasoning behind the importance of holistic modeling is expressed in the hundreds, if not thousands, of individual technological contributors that will be informed by the model’s results. The ultimate design solution incorporates a mix of active systems (chilled-water plants, wet-side economizers, air-side economizers, hybrid cooling towers, mixed-mode ventilation), passive systems (reflective paint, coatings, glazing, light shelves, natural ventilation), technology (high-efficiency equipment, ultra-low-energy lighting, building controls), changes to manufacturing processes (heat recovery, machine design), and operations. These solutions are often interdependent, and modeling is the only way to reasonably quantify their value.
The precipitous fall in PV module cost is the essential piece of the puzzle. The levelized cost of solar energy (LCOE) is now on par with utility energy in many regions of the world. A look at where PV companies such as Solar City (installer and financier of distributed rooftop solar systems) are actively engaged, which is most of the Western and Northeastern U.S., provides a good indicator of where PV energy is competitive in power markets. Due to the complex nature of energy markets, time-of-day utility pricing and demand charges are often overlooked when estimating simple payback of PV systems using merely an average utility cost/kWh can be misleading. The LCOE provides more perspective by weighing the costs and benefits over the lifetime of a system, allowing purchasers to make more informed decisions.
Deploying energy storage allows buildings to disassociate from emissions generated by, and water consumed in, electric power generation. Even if a building has offset 100% of its diurnal energy load with renewables, it still relies on the grid for energy storage. By providing excess energy to the grid during the day and extracting grid-tied energy at night, the building forfeits an opportunity. With energy storage integrated into buildings, energy flow to the grid can be structured in such a way as to reduce grid-tied emissions through delivery during demand peaks, while subsequently extracting from the grid during periods of low demand. The value of this, in the long run, is utilities will see a flattening in their load curve, and the construction of additional fossil-fuel-based power plants can be pushed into the future. Figure 3 shows the impact of wind and solar resources on the demand curve as they reduce significantly during the mid-day and leave two peaks around 8 a.m. and 8:30 p.m. Energy storage could be used to reduce those peaks, thereby buying years of additional grid growth capacity. California electricity rate structures reflect these peaks via time-of-day and seasonal pricing, as well as demand factors, and provide a mechanism to economically justify integrating energy storage. Many other markets have similar load curves, but are not capacity-constrained and thereby do not have the rate structure that rewards energy storage integration.
The emerging energy storage market is becoming highly active with existing technologies (batteries, compressed air, flow batteries, and thermal storage), and emerging technologies (varieties of lithium-based batteries). Tesla’s innovative Powerwall lithium-ion battery solution, for example, scales to serve the residential market all the way to the utility substation market. Cost for the Powerwall is about $350/kWh, but, like solar technology, it is expected to drop significantly in the coming years as production scale increases.
The next evolution
Buildings currently account for nearly 40% of energy consumption in the U.S. With the increasing electrification of vehicles, more than half of the U.S. energy demand may reside behind the meters of buildings in the not-too-distant future. In light of climate change risks and potential resource depletion, the commercial and industrial building sector has both the obligation and opportunity to provide energy solutions. The next evolution in building design may be progressing from the energy efficiency paradigm to an energy production paradigm, seeing buildings as potential power producers rather than power sinks.
The benefits of PEBs are far-reaching. The most obvious benefit resides in a PEB’s ability to balance both current and embodied energy loads by providing net positive energy to the local grid. Other benefits of PEBs are less obvious but nonetheless important. PEBs can facilitate growth in green technologies by augmenting material, renewable, and efficiency markets. They also may provide other fringe benefits, such as improvements in daylighting and indoor air quality, by means of shifting the principles from which we design buildings.
The key enablers are falling into place, allowing for the realization of positive-energy buildings. We are seeing a growing number of companies with the desire and vision to proceed forward. Companies like Microsoft, Proctor & Gamble, Apple, and Tesla have expressed goals to achieve energy balance in their commercial/industrial facilities. Tesla, to this end, has announced intentions to design its new battery manufacturing facility to use large amounts of on-site renewable energy power generation. The other key enablers moving PEBs closer to realization are the advancement of integrated modeling in building design, the precipitous fall in PV system costs, and the projected falling of energy storage prices. With key enablers in place, buildings will become extremely efficient by design, able to produce and store clean energy on-site, and deliver excess energy to the grid during times of high energy use and high associated emissions. Ultimately, a PEB creates and delivers energy in a way that benefits not only the building but also the larger community.
Example: PEB for manufacturing
Manufacturing facilities present a significant challenge for positive-energy design, making them a great example to illustrate the effort required on the road to PEBs. For this example, energy demand and supply have been simulated to illustrate the renewable energy requirements in a net positive manufacturing facility with a peak load of 10 MW. The peak load for a manufacturing facility can range anywhere from 2 to 200 MW; 10 MW can be considered typical for a small-to-medium size manufacturing operation, data center, or university. A rough rule-of-thumb is 1 MW is equivalent to approximately 1,000 single-family residences, so displacing 10 MW of load is roughly equivalent to 10,000 net zero residences—quite an impact for a single facility.
On an annual basis, this facility would require a 37-MW-rated PV array plus 1 MW of wind turbines to provide slightly more energy than this 10-MW facility is predicted to consume. Key results from the analysis are presented in Figure 4 and include the manufacturing facility demand curve versus the renewable energy production curve, as well as the individual production curves for PV array and wind turbines. The demand curve is based on the facility operating 24 hr/day with an average load factor of 85% (defined as the average electrical load divided by the peak electrical load). To get the best utility rates, manufacturers typically maintain load factors of greater than 85%. Solar production is based on a 15-deg fixed-tilt PV array and an inland Southern California solar resource (NREL National Solar Radiation data sets.) Wind production is based on 1 MW of rated wind capacity and a production curve based on a spring/summer day with afternoon wind resulting in a daily average capacity factor of 19%. The solar farm and wind turbines have a footprint of approximately 250 acres, and have an installed system capital cost of close to $100 million, without incentives. Both the additional land area and capital are big challenges to overcome, but with proper site selection and third-party financing it is certainly possible. In most instances, adding energy storage is expensive and will have a limited return on investment. The key ingredients for energy storage to make sense are high time-of-day utility rates, power quality risk abatement, and/or company-mandated emissions targets. If all of these factors are in play, adding energy storage could be of value.
Geothermal, solar thermal, and biomass were not included in this example, but their value should not be ignored as these sources may have the quickest return on investment for facilities with thermal loads that can take advantage of renewable or waste heat.
Andrew Solberg is an engineer and thought leader at CH2M with more than 18 years of experience in building system design and energy consulting. He leads the company’s advanced design and simulation group.
Coy Miller is a simulation and modeling specialist in the advanced design and simulation group at CH2M. He has degrees in ecology and renewable energy engineering.
Keith Kibbee is a mechanical engineer at CH2M. He specializes in built-environment simulation, and develops and uses advanced software tools to quantify and optimize the use of energy, water, and other resources.
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