On NZEB definition

According with the adoption of EPBD recast, Directive 2010/31/EU, Member States (MS) had to provide new challenges in their legislation. The most interesting actual process is related to the definition of Zero Energy Buildings (NZEB) applied to new constructions and retrofit actions. The main goal is to reach gradually NZEB conditions in all interested buildings by 2021; the final term is 2019 for public (owned or occupied) buildings.

A NZEB is defined in article 2 as a building that has a very high energy performance. The nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources, including [ones] produced on-site or nearby.

European Commission has launched a Concerted Action (CA) in order to support MS to exchange experiences about high performance buildings, as already rated and defined in local legislations and experiences, for example:

• Low Energy Building;
• Passive House;
• Zero Energy Building;
• Zero Emission Building;
• Energy Surplus Building.

The earliest concept referring to buildings characterized by a very low energy consumption, German Passivhaus, emphasizes internal and solar heat gains use to reduce towards zero (< 15 kWh/sqm yr) energy consumption for heating in cold climate contexts, considering means as strong insulation, positioning, space distribution in buildings and finally passive solar systems.

In USA experience, a Zero Energy Building (ZEB) is characterized by a very low energy demand in operation phase; this demand is completely satisfied by renewable-energy-based in situ facilities. A ZEB is not necessary off-grid, simply the algebraic sum of energy inputs and outputs (towards an infrastructural net) is zero; in an yearly period, this kind of building can be defined as a net Zero Energy Building, considering the system boundary of net delivered energy and conversion factors of specific energetic carriers.

A Zero Emission Building produces enough emissions-free renewable energy to offset emissions derived from energy amount used in the building, depending on emissions multipliers of each energy source.

According to an economic point of view, it is possible to individuate a Zero Cost Energy Building too, balancing costs related to energy fluxes in operation phase. In this case, the payment given by infrastructural energy net managers for exported energy is equal to costs due by building’s owner for yearly delivered energy. Here, subsidies have to balance energy infrastructural net operation costs.

In EPBD recast 2010/31/UE a definition related to energy balance in buildings is introduced, the optimal energy performance level referred to operation costs, intended as the lower cost during building’s estimated life cycle, considering maintenance and refurbishments. In Commission Delegated Regulation UE n. 244/2012 a methodology to calculate optimal levels referring to costs and Life Cycle Costing analysis is defined. To implement this methodology (considering building, operation and dismission phases) reference buildings’ cathegory has been introduced, in order to harmonize LCC and actual buildings.

In may 2011, on REHVA (Federation of European Heating Ventilation and Air-conditioning Associations) journal has been published a proposal for an uniform National implementation of EPBD recast; there is clearly defined that, referring to system boundary for delivered energy:

–          a net Zero Energy Building is characterized by a primary energy consumption equal to 0 kWh/(sqm yr);

–          a nearly net Zero Energy Building is characterized by a primary energy consumption greater than 0 kWh/(sqm yr), as determined in each Member State in an optimal energy performance level referred to operation costs.

UE Concerted Action on NZEB National implementations

The Concerted Action of EU has 5 main topics:

1. inquire the national application of NZEB definition;
2. analyze national plans to increase the diffusion of NZEB;
3. define boundary conditions about the assessment of renewable energy contribute in NZEB calculation methods (also in national, general, energy performance calculation methods);
4. encourage convergence between NZEB concept and cost-optimal energy performance requirements;
5. share practical experience in NZEB.

The first step has the main goal in inquiring and comparing the definition of NZEB in Member States national applications. At this very moment, considering 19 countries that provided information:

• 6 countries have defined and fixed NZEB definition in legal documents;
• 6 countries have an application ready but not still legally fixed;
• other European countries are developing NZEB definition, at different levels.

Performing this definition, recast EPBD shows (in Annex I) a general framework in order to calculate the energy performance in buildings, NZEB included: “The energy performance of a building shall be determined on the basis of the calculated or actual energy that is consumed in order to meet the different needs associated with its typical use, and shall reflect the heating energy needs and cooling energy needs (energy needed to avoid overheating) to maintain the envisaged temperature conditions of the building, and domestic hot water needs”. Here it is available a list of aspects that should be interested in defining methodology, as natural / mechanical ventilation, built-in lighting installation, even if only in non-residential buildings.

About energy services included in national calculation methods:

• in all 19 considered Member States heating and domestic hot water energy needing are included, both in residential and non-residential buildings.
• cooling service is considered in 18 MS for residential buildings, in 17 MS for non-residential ones;
• ventilation is considered in all MS for non-residential buildings; for residential ones, in 2 MS there is a simplified calculation method that doesn’t differentiate between natural and mechanical ventilation, and doesn’t consider heat recovery in last one;
• only 8 MS consider lighting in residential buildings, all MS consider it in non-residential ones.

Energy used to feed household equipment isn’t quoted in EPBD Annex I; only few MS (5 for residential, 7 for office buildings) consider it in their calculation method.

Referring to integration of renewable energy in national NZEB calculations, solar thermal and photovoltaic energy are considered in quite all Member States. Decreasing, most of MS consider in their calculation contributes of biomass, micro CHP and micro wind generators, while it isn’t still diffused consideration in waste heat.

In 18 countries that reported their kind of balancing time steps, it is remarkable that the most typical one is monthly time step (10 Member States), while 6 countries allow yearly time step and hourly time step; 3 countries allow both monthly and hourly step calculation, depending on building type. Referring to generation efficiency, yearly balancing time step is used in 9 countries; some countries allow more than one time step in calculations.

In 14 Member States it is considered energy used in buildings generated by renewable sources, sometimes taking into account storage systems. The main theme related to renewable energy use is in the definition of NZEB, referring to the boundary of connection with building: this definition is still under discussion in several countries.

This definition depends on type of system based on renewable energy: the easiest example is focused comparing solar thermal energy, typically related with single building or complex building, and common district heating, operating at neighborhood or infrastructural level. The most common interpretation shown in 18 Member States answers is to consider facilities operating in areas larger than buildings’ ones; in 10 countries the widest boundary considered is building complex level.

The integration of renewable energy sources in NZEB depends on solving 4 situations, that at this very moment generate various approaches in taking into account renewable energy sources’ contribution:

1. the definition of “on-site” sources and “nearby” sources;
2. the real meaning of a “very significant extent” in relation with the percentage of primary energy given by renewable sources;
3. the way of taking into account of these sources in different calculation methods, according to their diffusion in Member States;
4. the presence of subsidies related to the use of renewable energy sources.

17 Member States consider primary energy use as main indicator of buildings’ performance, remaining two use either CO₂ emissions or measured energy, without using a direct, primary energy factor. More indicators are used in MS to plan energy efficiency in buildings, based on heating, cooling, hot water and lighting energy needs:

• U-values for main surfaces responsible of heat losses by transmission;
• efficiency of building service systems;
• comfort indicators and parameters in summer, maybe through bioclimatic index.

A very important result of the CA shows that there will be an average tightening of 40% between current energy performance requirements and NZEB requirements.