1、Tom Marseille is Senior Vice President with WSP Flack + Kurtz in Seattle, WA. Essential Methods, Models and Metrics for Net Zero Energy Buildings Tom Marseille, PE Member ASHRAE Abstract No matter how the term is defined, setting and achieving a goal of net zero energy (NZE) use in buildings is auda
2、cious. Yet, market and regulatory trends are poised today to take us incrementally in that direction. And, while there is reason to be optimistic that new technologies will be developed to make NZE more attainable for a larger portion of the building stock, arguably the most rational and moral choic
3、e we have is not to wait, but to approach each new building project with a mindset of NZE. A growing number of completed research studies along with experience gained by industry professionals from an increasing number of actual projects bring a more comprehensive understanding of what a NZE approac
4、h looks like, whether from the perspective of designer, constructor, owner or occupant. Each player needs to understand and invest in an integrated, interdependent and on-going process. Within this process, the engineer has opportunity to bring significant, critical value-add. As one example, mechan
5、ical engineers have traditionally been hired to provide energy modeling services, whether on the frontend to help inform design, or as a way of documenting predicted performance compared to a fictitious baseline building. A NZE methodology will require multiple “living” energy models that accurately
6、 predict actual future performance. These include whole building energy models, but also focus on individual pieces of equipment or systems. And, the models need to be informed by correct assumptions about the buildings actual construction, occupancy and operation. In a NZE project those “occupancy
7、and operation” assumptions go from being something our industry often simply obtains from an ASHRAE Standard for purposes of sizing HVAC equipment or for incorporation into traditional energy models, to something that itself requires a highly detailed model. The NZE project team has presumably desig
8、ned and constructed a highly thermally efficient building envelope with the most energy efficient equipment and systems and appropriate renewable energy resources. Now they must also consider finding ways to model the remaining big slices of the energy pie, ones that, given human behavior is at play
9、, arguably carry the most uncertainty. Plug loads, for example, make modeling seemingly “simple” NZE residential projects potentially as challenging as any other building type. WHY NET ZERO? Many contributing factors may lead a building owner to decide to attempt a Net Zero Energy (NZE) building pro
10、ject, including: Fossil fuel sources are finite, and as population grows in the future both increased conservation and, ultimately, a society-wide transformation to renewable energy sources, become necessary choices. Many locations today face present or forecasted power generation and transmission i
11、nfrastructure constraints. To LV-11-C047 2011 ASHRAE 3892011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in eithe
12、r print or digital form is not permitted without ASHRAES prior written permission.address these, the economics again favor conservation. Owners may also reach the conclusion that to help ensure reliable energy is available to heat and power their buildings, a net zero approach utilizing on-site rene
13、wable energy makes economic sense when factoring in future risk. Public and political concerns about the human contribution to global climate change and the impact a continued “business as usual” approach may have on future generations. Concerns about energy security and independence. Aggressive ene
14、rgy conservation and efficiency in our building stock becomes a rational choice when considering global energy resources. Policymakers and construction industry design practitioners alike recognize that high performance buildings are a key strategy for meeting energy and carbon reduction goals. In r
15、esponse they are creating, promoting, incentivizing and/or regulating paths that ultimately target NZE. Perhaps the most well known clearly targeted path to net zero is the AIA 2030 Challenge which establishes incremental greenhouse gas emissions (expressed as equivalent carbon dioxide) reductions i
16、n new and renovated building projects over time, with carbon neutrality being a goal for all projects by 2030. ASHRAE for its part, has established a goal of creating requirements for developing tools by 2020 to help enable commercially viable NZE buildings by 2030 (ASHRAE 2008). ASHRAE is using Mod
17、el Energy Code ASHRAE Standards 90.1 and now ASHRAE Standard 189.1 to promote incrementally more energy efficient buildings, including requirements for on-site renewable energy. Many states, counties and municipalities around the country are adopting more stringent energy codes and/or developing inc
18、entive programs to promote aggressive efficiency requirements beyond those of current code. Finally, a rating system currently in use today that requires NZE as a prerequisite for certification is the Living Building Challenge (ILBI 2006). DEFINING NET ZERO ENERGY There are many definitions for net
19、zero energy (Crawley 2009), each with different energy accounting implications that impact how a building seeking NZE will be designed and operated. Briefly, the different NZE definitions include: Site NZE Building Project - Creates as much on-site renewable energy as is consumed on an annual basis,
20、 as typically measured at the utility meter(s). Site energy is easily measured and that makes this definition attractive to building owners. Source NZE Building Creates as much on-site renewable energy as is consumed on an annual basis, based on source energy required. Source energy accounts for the
21、 primary energy used to extract, process, generate and deliver energy to a site. If the intent is to measure total energy use, source energy should be used. To estimate source energy, appropriate site-to-source conversion factors from the utility are needed. These are in practice very difficult to e
22、stablish for any specific building, as they can vary by region, time-of-day and the generation mix. NZE Building Energy Costs The utility pays you for excess created on-site renewable energy produced, such that the annual bill equals zero. In this case, energy cost serves as a proxy for source energ
23、y. While relatively easy to verify, in practice this approach requires the project team to consider in their calculations the way utilities charge for (or credit) electricity consumption. These electricity costs often vary with time-of-day and season and electrical service size. And, if the project
24、uses fossil fuels for heating, unitary fuel costs as charged by utilities can often vary unpredictably month-to-month. Net Zero Emissions To move a project closest to being truly environmentally neutral the concept net zero emissions is relevant. A net zero emissions project would need to tally all
25、greenhouse gases (converted to equivalent metric tons of carbon dioxide) generated either on-site or from primary energy remote sources, and be able to show that on an annual basis these are fully offset by on-site renewable energy created. This is generally recognized as the most difficult path to
26、estimate and track. Each definition above assumes renewable energy used in the NZE accounting is produced on-site. Some also consider 390 ASHRAE Transactionsenergy from bio-mass fuel that is sourced off-site from renewable waste streams as renewable when doing NZE accounting. More controversial is t
27、he idea that renewable energy produced off-site can be used to off-set energy consumed on-site. This may take the form of renewable energy generated by utilities which may be purchased in many locations by their customers at a premium cost. Or, another approach is the idea of using small-scale renew
28、able district energy solutions to serve larger neighborhoods or campuses where the goal of the district is some form of NZE. This may be the most pragmatic NZE strategy for buildings that have inherently high energy use such as data centers, hospitals, laboratories and restaurants, or high rise buil
29、dings in urban core areas. Unless a project makes a decision to go off-grid (and so by definition is NZE) or leverages renewable energy generated off-site in the NZE accounting, NZE relies on there being a net-metering arrangement with the local electric utility. Net-metering is intended to be a way
30、 to receive “fair market price” for excess electrical energy generated on-site. This is a convenient way to let the electrical grid be the storage battery for a NZE building whenever on-site renewable energy generated exceeds demand. In the U.S., the Energy Policy Act of 2005, Sec. 1251 requires eac
31、h electric utility to make available upon request net-metering to any consumer that the electric utility serves (EPA 2005). From a technology point-of-view, electric meters used today are inherently suited for net-metering because they are able to run in both directions. With net-metering, verificat
32、ion of NZE on a site basis requires little more than tabulating consumption data from monthly utility bills for a 12 month span. Utility bills of course also offer an easy way to verify whether NZE on a cost basis is achieved. The choice as to what definition to use for a NZE project can lead to som
33、e odd scenarios during design. For example using site energy as the NZE metric, a designer may conclude that using electric baseboard heat (COPsite= 1.0) is superior to using hot water heat generated by a gas-fired boiler (COPsite 1), because the overall kBtu consumed would be less. This is, of cour
34、se a very bad idea in most cases when source energy or energy cost are used. Most utilities rely, at least partially or perhaps in full, on fossil fuel power plants. The low energy conversion efficiencies of these plants, coupled with electrical line transmission losses, would show electric resistan
35、ce heat to consume much more total energy. An example of how the choice of NZE definitions can lead to unexpected decisions during design that the author has personally experienced was with an institutional client in western Washington. The client leveraged a combination of a favorable utility power
36、 generation portfolio (largely hydropower) with renewable energy offsets purchased from the same utility to claim (on paper) carbon neutrality operations for their building stock. This led them to a policy of encouraging electric resistance heating on projects rather than on-site burning of fossil f
37、uels, which also helped their construction budget because the electric heat was lower first cost. Steps in the NZE Process The easiest question to answer when first considering how to design and construct a NZE building project today, is probably “what is the process?” The critical key steps are: 1.
38、 Owner commitment to NZE. This must be a project requirement and mindset, and the Owner must be clear what this implies, or have a willingness to take the necessary journey, which may well involve significant education along the way. 2. A project team commitment to using a highly integrated, collabo
39、rative process, as enabled by the Owner. Using a highly integrated approach can help control total project costs and optimize performance on any project, and the importance of this approach on a NZE project cannot be overstated. Of course, if an Owners purse is large enough a less-than-optimized bui
40、lding can potentially still achieve NZE through the purchase of additional renewable energy. The use of an Integrated Project Delivery (IPD) methodology and contract may be helpful given the shared stakes, and therefore engagement, it obligates between Owner, Architect and Builder. Whether IPD is em
41、ployed or not, most helpful to design practitioners is having fees and/or project outcome incentives in place to help support a scope of services that generally will go way beyond traditional basic design services. 3. Perform an on-site renewable energy resource assessment early. This will help esta
42、blish what natural renewable 2011 ASHRAE 391resources are available. This is used to help create a renewable energy generation budget for the NZE accounting, and can help in the early stages establish the feasibility of NZE. 4. Employ to the maximum extent possible, integrated HVAC, lighting and plu
43、g load reduction strategies. The mantra should always be “load reduction first.” 5. Leverage appropriate passive strategies to further reduce the need for lighting and HVAC. 6. Leverage the best active strategies (e.g., automated controls, efficient HVAC equipment and systems, etc.). 7. Use renewabl
44、e energy cost offset economics (see below) to help decide what are the best load reduction, passive and active strategies. 8. Use robust energy models. These need to be developed early to help inform the design direction, and then must be maintained and refined throughout the design/construction pro
45、cess to help direct and constrain, or at the least align with, as-designed and as-built realities. The energy model should even be used as a tool during post-occupancy measurement and verification (M&V) activities. M&V is itself a necessary process step that is always important and often not well th
46、ought out early on so all parties are aware what their roles and responsibilities are with respect to proving performance. 9. Document carefully and thoroughly all basis of design and modeling assumptions. These should be referred to regularly, and challenged where appropriate if better or updated i
47、nformation becomes available. Managing and communicating this information to the entire project team, including the Owner, is paramount to help avoid surprises downstream. Once an educated Owner with sufficient funds to credibly attempt NZE has hired a project team, a helpful way for the mechanical/
48、energy engineer to kick-off his or her participation is to create a simple baseline NZE energy model that incorporates the owners program into a shoebox. A shoebox can be thought of as very simple building form that works programmatically, but likely pains the architect aesthetically. The shoebox sh
49、ould apply ASHRAE 90.1 Appendix G internal load, occupancy assumptions and HVAC systems types and a prescriptive code thermal envelope. The predicted Energy Use Intensity (EUI) for this simple model gives you a starting point for performance. Whatever answer you get you can use to determine a bounding quantity of photovoltaic (PV) or other renewable energy sources that would be needed to achieve NZE in a non-optimized building. This assessment can be made in the absence of actual knowledge about potential site renewable resources, since the objecti
