Tallinn University of Technology
Manager of international projects and standards at ISSO
Chair of CENTC 371 Program Committee on EPBD
Fellow of ASHRAE and REHVA
DOE launched ZEB definition which is very similar to REHVA definition and relies on site boundary being exactly the same as REHVA’s system boundary. DOE definition provides valuable input to European discussion in exported energy issues and for instance by addressing Renewable Energy Certificates.
Primary energy commonly used in Europe is called Source Energy (Renewable & Non-Renewable) in DOE definition  being a major difference in terminology. The system boundary for zero energy accounting follows exactly the REHVA’s one  and addresses energy need, energy use, and delivered and exported energy accounted on site boundary, Figure 1. If compared to the REHVA’s system boundary, Figure 2, one can see that even the same terms have been used – reflecting the same basic understanding by European and US energy experts. Regarding to the European regulators discussion, should lighting and appliances be accounted in energy performance or not, DOE clearly states that all energy used in the building, including lighting, plug loads and even processes, is considered as energy performance of a building, fully supporting REHVA’s previous proposal.
Figure 1.DOE Site Boundary of Energy Transfer for Zero Energy Accounting.
Figure 2.REHVA system boundary from 2013 .
DOE defines Zero Energy Building (ZEB) as “An energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.” Therefore, the definition is based on annual balance of delivered and exported primary energy. This definition equals to REHVA net Zero Energy Building (net ZEB) definition “Non-renewable primary energy of 0 kWh/(m² a).” Both DOE and REHVA provide an explanation that ZEB (net ZEB by REHVA) is typically a grid-connected building that is very energy efficient. “The premise is that ZEBs use the electric grid or other energy networks to transfer any surplus of on-site renewable energy to other users.”
The site boundary is defined as a meaningful boundary that is functionally part of the building(s). “For a single building on a single property, the site boundary is typically the property boundary. The site boundary should include the point of utility interface”. Definitions for zero energy campus, portfolio and community would allow to extend the site boundary and to include for instance the site energy centers.
In zero energy campus, portfolio or community definitions a word “building” is just replaced by campus, portfolio or community. For example, Zero Energy Community: “An energy-efficient community where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.”
DOE definition has no limitations for exported energy and the same primary energy factors are used both for delivered and exported electricity. US primary energy factor for electricity of 3.15 is somewhat higher than European one of 2.3. It is considered that exported on-site renewable electricity displaces electricity that would be required from the grid. “In ZEB accounting, the exported energy is given the same source energy conversion factor as the delivered energy to appropriately credit its displacement of delivered electricity”, that is also a default choice in REHVA definition. The source energy conversion factors utilized are from ASHRAE Standard 105, and they are summarized in Table 1.
Table 1.US National Average Source Energy (primary energy) Conversion Factors.
DOE definition addresses the use of Renewable Energy Certificates (REC) which are tradable instruments that can be used to meet voluntary renewable energy targets. “Once a buyer makes an environmental claim based on a REC, the buyer can no longer sell the REC and the REC is considered permanently “retired”.” The ZEB definition and its variations (Campus, Portfolio, Community) require on-site renewable energy to be used to fully offset the actual annual delivered energy. Therefore these definitions do not allow renewable electricity purchased through the use of renewable energy certificates (RECs) to be used in the ZEB energy accounting.
High rise buildings in dense urban areas or buildings with high process loads may choose to have off-site renewable electricity utilizing RECs to help balance the annual delivered energy. Special REC-ZEB definition is provided to allow RECs to be used to supplement, after on-site renewable energy sources have been employed, and balance the annual delivered energy to the building. Renewable Energy Certificate - Zero Energy Building (REC-ZEB) is defined as “An energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy plus acquired Renewable Energy Certificates (RECs)”.
REC-ZEB is an interesting approach that can be seen as one possible solution for European “nearby generation”, having so far no common grounds and not yet implemented into regulation by any MS.
A Common Definition for Zero Energy Buildings. September 2015. Prepared for the U.S. Department of Energy by The National Institute of Building Sciences. http://energy.gov/eere/buildings/articles/doe-releases-common-definition-zero-energy-buildings-campuses-and.
REHVA nZEB technical definition and system boundaries for nearly zero energy buildings. Federation of European Heating, Ventilation and Air-Conditioning Associations, Report No 4, REHVA 2013. http://www.rehva.eu/publications-and-resources/guidebooks-shop/guidebook/?tt_products[backPID]=6&tt_products[product]=55&cHash=a6b930e8ef6aea5f7619714be917025f.