OVERVIEW | Nearly Zero-Energy Buildings – What’s the next step: zero energy or plus energy?

Share this Post:

Efficiency House Plus in Berlin (Source: BMVBS / Schwarz)

By Heike Erhorn-Kluttig (Fraunhofer Institute for Building Physics)


The recast EPBD Article 9 requires Member States to ensure that all new buildings are nearly zero-energy buildings (NZEBs) by 31 December 2020, and new buildings occupied and owned by public authorities are NZEBs after 31 December 2018. More and more EU Member States have set up their detailed national definition of the NZEBs and various pilot buildings of this building energy level have been designed, constructed and evaluated. For several years now, researchers have been working on energy autarkic, zero energy and plus energy buildings. How much do these building levels differ and what can be the energy performance level in the near future?


Boundary conditions to be taken into account


In order to correctly assess the ambition level of the three building levels listed above, at least the following boundary conditions have to be analysed:


  • Which energy uses are included (e.g. heating, hot water, ventilation, lighting, cooling, auxiliary energy for pumps and control and electricity use for equipment (household appliances, computers, etc.))?
  • Which user profiles are considered? What is the usage time of the building?
  • Which balancing method is applied: Is it an annual balance which is used for most of the zero energy and plus energy buildings or is the building not connected to any external energy supply system such as electricity or a fossil fuel like for an energy autarkic building?
  • Which energy level is used for the balance: Final energy or primary energy? The primary energy factors of the different energy sources can facilitate achieving the zero energy or plus energy balance.
  • Is the balance done for operational energy or life cycle energy? Life cycle energy takes into account the energy for the creation of building components and building service systems as well as the transport to the building site, the energy for the construction, the building operation and the demolition of the building. In conventional buildings the operational energy is about 90% of the total life cycle energy. With buildings of a higher energy performance level the operational energy can be reduced to about 75% (depending on the building material, etc.).
  • Which kind of renewable energy generation is taken into account for balancing the energy uses: renewable energy generated directly at the building (e.g. PV, solar thermal, micro wind turbines), at the building site (same as before yet not mounted at the building but for example on a garage on the building site), outside of the building property (e.g. biomass, district heating generated by RES), or renewable energy with no connection to the building such as renewable electricity taken from the national grid with specific contracts.

Zero energy level


As described above, zero energy can be on final energy or on primary energy level. The energy used by the building has to be balanced with energy generated from renewable energy sources (preferably generated at the building or the building property). The most commonly applied solution is to use the electricity grid in order to balance the energy used and energy generated. Electricity is produced at the building or building property often by photovoltaic panels and if not used by the building itself, it is fed into the electricity grid. Thus all energy generated at any time can be accounted in the balance. Also, the building can still use electricity from the grid in times where there is not enough electricity generated by the building. The generated electricity can also be used to balance other energy needs like the heating energy that does not necessarily have to be provided with electricity as source. The building services systems can be chosen freely as long as their total balance is zero. This allows for smaller energy generation systems and less (or no) storage systems. The electricity grid is considered as storage.


If this kind of solution was applied to many or all buildings connected to the electricity grid, the grid would have to store all electricity during some phases and provide all electricity in other phases. This can’t be managed so far as it would need large electrical storages (batteries) or huge power-to-gas facilities (or similar technologies). Local heating networks can be used in a similar way to provide energy to other buildings that can then be accounted for in the energy balance. Note: several national building regulations do not allow the balancing on non-electrical energy uses by electricity generated from renewables for energy performance certificates.


Within the framework of the Intelligent Energy Agency and their programmes “Solar Heating and Cooling” and “Energy in Buildings and Communities” a joint Task 40/Annex 52 has been working on “Net Zero Energy Solar Buildings”.


Boutique Hotel Stadthalle Vienna: a zero-energy balance hotel


© Boutiquehotel Stadthalle Wien

The Boutique Hotel Stadthalle in Vienna has invested in sustainable resources and saving energy. It has taken advantage of an innovative European programme that is helping hotels, which are responsible for 20 percent of the CO2 emitted by the tourist industry, to reduce their carbon footprint and to boost their competitiveness. Hotel Stadthalle is the first urban hotel in the world to have zero energy balance, meaning that they produce as much energy as they use. To achieve this they have invested 700,000 € in systems like water volume reduction, rainwater collection, recycling drinking water, LED lighting, solar thermal panels, photovoltaics, micro wind turbines and ground source heat pumps. The measures resulted in the hotel having only 2 percent energy costs compared to the total operation costs, whereas normally a 3-star hotel has a benchmark of 6 - 7 percent energy costs.


EspaiZero. A semi-retrofit of an existing building; the first Net Zero-Energy experience in Spain


EspaiZero aerial view (© Wattia Inova)

The new Wattia Headquarters and EspaiZero laboratory has achieved 100% zero-energy balance in the retrofit of the ground floor of an existing building. The renovation included passive systems like insulation (continuous layer of at least 8 cm of highly insulating material), triple-glazed windows and a solar chimney as well as active systems like geothermal heat pumps used for heating and cooling, solar thermal collectors, photovoltaics, LEDs and building automation and control. At present, EspaiZero’s real energy consumption is very close to zero. Since the inauguration of the building in April 2013, the monthly energy bill has been less than 2 kWh/mth.; this means less than 1 €/mth. energy costs. The total costs amounted to 1,350 €/m² with 270,000 € refurbishment costs and 80,000 € costs for the installation of the renewable systems.       


Plus energy level


From zero energy to plus energy is a small step. In the end the balance of energy used and energy generated has to be positive, meaning that more energy is generated than used. Since the real energy use of a building often differs at least slightly from the calculated energy use, the projects include slightly oversized energy generation systems that can compensate for a bit higher energy uses. The over-dimensioning can make the project a success if the monitoring phase shows somewhat different results than the energy performance calculation or simulation. In reality, this makes most of the zero energy buildings plus energy buildings. To hit the mark in the monitored reality with exactly the necessary size of the energy generation system, would be very risky. On the other hand the over-dimensioning of systems leads to higher costs. As with zero energy, the plus energy status can be achieved under different boundary conditions.

A few countries have launched support programmes for plus energy buildings. Germany’s Efficiency House Plus programme is one of them.


Plus energy St. Franziskus Elementary School in Halle, Germany


South-view on the eastern part of the school building (© Hochschule Magdeburg-Stendal)

A plus energy school was built in Halle (Saale), as a sustainable and ecological construction made with timber frame and cellulose insulation in accordance with the Passive House standard. The building envelope has very high thermal protection as a result of the cellulose insulation. An innovative window system was applied consisting of two double-glazed windows integrated into a singular frame in order to reduce the thermal bridge effects. The ventilation system is used for heating and preheated via geothermal heat exchangers and heat recovery. Domestic hot water (DHW) is generated by solar thermal collectors. The school building covers a large part of its electricity needs with two photovoltaic systems installed on the school's roof and on the carport. The total building costs amounted to 6.9 million € with building structures for 3.4 million € and technical systems for 1.1 million €. The plus energy goal of the school is based on the comparison of primary energy consumption and primary energy production. The consumption taken into account is heating, cooling, DHW, lighting and ventilation.


The school building was put into operation in February 2014 and its energy consumption has been monitored ever since. A total primary energy consumption of 72.2 kWh/m².year has been monitored compared to the total renewable primary energy production of 77.8 kWh/m².year, so that the plus energy goal of the building was met.


A positive energy concept house near Lyon (France)



This one level house produces more energy than it needs to cover all uses for housing and an electric car. It was built by Maisons France Confort near Lyon (France), has a living area of ​​164 m² and includes a living room, a kitchen, two bedrooms, an office, a bathroom and a shower room. The rooms are arranged around a central courtyard, as well as a cellar and a garage. The house features home automation and rainwater collection and an energy management system. A consortium with 15 partners from different building industry sectors was created. Thus the building is used as a showcase for a heat pump coupled to ventilation exhaust air, photovoltaic panels, steel frame construction and sandwich panels, an electric car, LED lighting, high efficient glazing and roof windows, efficient household appliances and advanced solar shading. The calculated energy performance of the building is minus 173 kWh/m².year primary energy.  


Energy autarky


In a few cases, the balancing of energy used and energy generated was done completely without using the electrical grid or local heat network. That means that these buildings are energy self-sufficient ("energy autarkic"), since they can generate at any time all the energy they need. This is however a very expensive solution as it needs big storage systems for heating and electricity. Note: an energy autarkic building should not include the use of biomass from outside the building site.


Energy autarkic solar house in Freiburg


Already in 1992 Fraunhofer Institute for Solar Energy Systems has designed and built a model project in Freiburg that is completely energy autarkic. The total energy use was covered by solar energy. The building was not connected to the electricity grid and used no external fuels. Solar thermal collectors generated the DHW. Photovoltaic panels were used for electricity consumptions and for the generation of hydrogen (via electrolysis) that was stored and fed into a fuel cell for producing energy at times without solar availability. The total costs of the building have been 5.8 million Deutsche Mark (2.9 million €).   




As explained above a zero energy building is usually a plus energy building as well, because the energy concept typically foresees a buffer taking into account climate deviations such as colder years or less solar gains than in average (design) years. Also, a slightly different user behaviour can be covered by this buffer. This means that in general the design approach and the building costs are the same for zero energy and plus energy buildings. However the additional buildings costs compared to buildings fulfilling the current minimum energy performance requirements are dependent on the chosen combination of energy saving measures and energy supply systems. There are various examples of the energy plus and zero energy buildings available and national promotion programmes support the market penetration in some countries like Germany and France. The step towards energy autarky however is still a huge one leading to extensive additional costs. Existing building examples are research buildings and need detailed surveillance during the building operation.


Therefore, if the near future will lead to a higher building energy performance level than the nearly zero-energy building level foreseen for 2019/2021, it might be the plus energy level. Economical assessments like the cost-optimal calculations of EPBD Recast Article 5 can determine the suitable timing of the next level of building energy performance.