Photovoltaics and Building Efficiency towards a Net Zero Energy Building (NZEB)

According to statistics, 40% of the energy consumption in Europe comes from the building sector. This led the European Commission to create energy efficiency directives to improve the energy efficiency of buildings, introducing the concept of NZEB or Nearly Zero-Energy Buildings.

One of the main measures to be implemented is the requirement for all new buildings from 2021 (public buildings from 2019) to be NZEB. Nevertheless, while the European Commission proposed an ambitious legislative framework for it, the definition of NZEB varies at a European scale. 

Four different categories, defined at the national level by experts, can indeed be used to qualify a NZEB:

• Net zero energy buildings / Plus energy buildings.
• Nearly zero-energy buildings (NZEB) according to national definitions.
• Buildings with an energy performance better than the national requirements in 2012.
• Buildings constructed/renovated according to national minimum requirements in 2012.

For the purpose of our case study, we will consider a NZEB as a Net Zero Energy Building, which is a building with net zero energy consumption, meaning the total amount of energy used by the building on an annual basis is equal to the amount of renewable energy generated on-site. 

The fact that the calculation is on a full-year basis is of great importance because the lower renewable energy production in winter can be compensated by the higher production in summer. This also implies that the building is grid-connected and thus doesn’t need any on-site storage system which, as of 2021, seems highly unfeasible for big buildings from an economic and technical point of view.

On the same note, if you want to learn more about NZEB and how they contribute to sustainable development, this article is a must read.

6 Reasons to Support NZEB Building Models
The benefits of planning or refurbishing a building to the NZEB standards are both economic and ecological:

• Lower energy consumption, thus lower operational costs.
• Higher independence from the electrical grid lessens the impact of increasing electricity prices.
• Revenue generation thanks to electricity reselling.
• Lower electricity peak demand leading to lower electricity tariffs.
• Lower building carbon emissions.
• Better interaction between the building and the electrical grid.

But transforming or planning a NZEB building means working on both fronts:

Reduce the building's electricity consumption
Install on-site renewable energy systems
There are many ways to reduce energy consumption in buildings while the principal way to generate renewable energy is the installation of an on-site photovoltaic system, placed in most cases on the roof of the building.

While at the early stages of building-integrated photovoltaic, the systems were sized to maximise the on-site electricity generation, the energy exchange between the building and the energy infrastructure is more and more emphasised when sizing a PV system. 

The fluctuating power generation of renewable energies can indeed be challenging for the electricity network which seeks to constantly balance the power generation and power consumption. This could explain why most of the financial incentives for PV systems are progressively putting the focus on maximising energy self-consumption instead of maximising energy production. 

In fact, the buy-back rate for produced photovoltaic electricity is constantly decreasing while the electricity grid prices are constantly increasing.

But what does a NZEB building look like? Or how much PV does a building need to compensate for the annual energy consumption? And from a more global perspective, how do the energy grid and the building interact throughout the year? 

For this purpose, we will examine three different cases generated with Spacewell Energy (Dexma)'s Building Energy Modelling (BEM) module, a module aimed to generate synthetic (but realistic) building energy consumption and production profiles. 

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