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JarekKurnitskiVice-president REHVATallinn University of TechnologyEstoniaJarek.Kurnitski@ttu.ee | JaapHogelingManager of international projects and standards at ISSOChair of CENTC 371 Program Committee on EPBDFellow of ASHRAE and REHVA |
Primary
energy commonly used in Europe is called Source Energy (Renewable &
Non-Renewable) in DOE definition [1] being a major difference in terminology.
The system boundary for zero energy accounting follows exactly the REHVA’s one [2] 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.
http://energy.gov/eere/buildings/articles/doe-releases-common-definition-zero-energy-buildings-campuses-and
Figure 2.REHVA system boundary from 2013 [2].
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.
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