Zero-energy building


A Zero-Energy Building, also known as a Net Zero-Energy building, 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 created on the site or in other definitions by renewable energy sources offsite, using technology such as heat pumps, high efficiency windows and insulation, and solar panels.
The goal is that these buildings contribute less overall greenhouse gas to the atmosphere during operation than similar non-NZE buildings. They do at times consume non-renewable energy and produce greenhouse gases, but at other times reduce energy consumption and greenhouse gas production elsewhere by the same amount. The development of zero-energy buildings is encouraged by the desire to have less of an impact on the environment, and their expansion is encouraged by tax breaks and savings on energy costs which make zero-energy buildings financially viable.
Terminology tends to vary between countries, agencies, cities, towns, and reports, so a general knowledge of this concept and its various uses is essential for a versatile understanding of clean energy and renewables. The International Energy Agency and European Union most commonly use "Net Zero Energy", with the term "zero net" being mainly used in the US. A similar concept approved and implemented by the European Union and other agreeing countries is nearly Zero Energy Building, with the goal of having all new buildings in the region under nZEB standards by 2020. According to D'Agostino and Mazzarella, the meaning of nZEB is different in each country. This is because countries have different climates, rules, and ways of calculating energy use. These differences make it hard to compare buildings or set one standard for everyone.

Overview

Typical code-compliant buildings consume 40% of the total fossil fuel energy in the US and European Union and are significant contributors of greenhouse gases. To combat such high energy usage, more and more buildings are starting to implement the carbon neutrality principle, which is viewed as a means to reduce carbon emissions and reduce dependence on fossil fuels. Although zero-energy buildings remain limited, even in developed countries, they are gaining importance and popularity.
Most zero-energy buildings use the electrical grid for energy storage but some are independent of the grid and some include energy storage onsite. The buildings are called "energy-plus buildings" or in some cases "low energy houses". These buildings produce energy onsite using renewable technology like solar and wind, while reducing the overall use of energy with highly efficient lightning and heating, ventilation, and air conditioning technologies. The zero-energy goal is becoming more practical as the costs of alternative energy technologies decrease and the costs of traditional fossil fuels increase.
A notable example is the Zero Building in Spain, which demonstrates how an office building can achieve net-zero and even positive energy performance through architectural design, passive strategies, and integrated renewable systems.
The development of modern zero-energy buildings became possible largely through the progress made in new energy and construction technologies and techniques. These include highly insulating spray-foam insulation, high-efficiency solar panels, high-efficiency heat pumps and highly insulating, low emissivity, triple and quadruple-glazed windows. These innovations have also been significantly improved by academic research, which collects precise energy performance data on traditional and experimental buildings and provides performance parameters for advanced computer models to predict the efficacy of engineering designs. A study examines how different countries, including Italy, Germany, and Denmark, approach nearly zero energy buildings. It shows that while the technical solutions are often similar, the targets, baselines, and frameworks for meeting these goals vary significantly.
Zero-energy buildings can be part of a smart grid. Some advantages of these buildings are as follows:
Although the net zero concept is applicable to a wide range of resources, water and waste, energy is usually the first resource to be targeted because:
  • Energy, particularly electricity and heating fuel like natural gas or heating oil, is expensive. Hence reducing energy use can save the building owner money. In contrast, water and waste are inexpensive for the individual building owner.
  • Energy, particularly electricity and heating fuel, has a high carbon footprint. Hence reducing energy use is a major way to reduce the building's carbon footprint.
  • There are well-established means to significantly reduce the energy use and carbon footprint of buildings. These include: adding insulation, using heat pumps instead of furnaces, using low emissivity, triple or quadruple-glazed windows and adding solar panels to the roof.
  • In some countries, there are government-sponsored subsidies and tax breaks for installing heat pumps, solar panels, triple or quadruple-glazed windows and insulation that greatly reduce the cost of getting to a net-zero energy building for the building owner.

    Optimizing zero-energy building for climate impact

The introduction of zero-energy buildings makes buildings more energy efficient and reduces the rate of carbon emissions once the building is in operation; however, there is still a lot of pollution associated with a building's embodied carbon. Embodied carbon is the carbon emitted in the making and transportation of a building's materials and construction of the structure itself; it is responsible for 11% of global GHG emissions and 28% of global building sector emissions. The importance of embodied carbon will grow as it will begin to account for the greater portion of a building's carbon emissions. In some newer, energy efficient buildings, embodied carbon has risen to 47% of the building's lifetime emissions. Focusing on embodied carbon is part of optimizing construction for climate impact and zero carbon emissions requires slightly different considerations from optimizing only for energy efficiency.
A 2019 study found that between 2020 and 2030, reducing upfront carbon emissions and switching to clean or renewable energy is more important than increasing building efficiency because "building a highly energy efficient structure can actually produce more greenhouse gas than a basic code compliant one if carbon-intensive materials are used." The study stated that because "Net-zero energy codes will not significantly reduce emissions in time, policy makers and regulators must aim for true net zero carbon buildings, not net zero energy buildings."
One way to reduced embodied carbon is by using low-carbon materials for construction such as straw, wood, linoleum, or cedar. For materials like concrete and steel, options to reduce embodied emissions do exist, however, these are unlikely to be available at large scale in the short-term. In conclusion, it has been determined that the optimal design point for greenhouse gas reduction appeared to be at four story multifamily buildings of low-carbon materials, such as those listed above, which could be a template for low-carbon emitting structures.

Definitions

Despite sharing the name "zero net energy", there are several definitions of what the term means in practice, with a particular difference in usage between North America and Europe.
; Zero net site energy use: In this type of ZNE, the amount of energy provided by on-site renewable energy sources is equal to the amount of energy used by the building. In the United States, "zero net energy building" generally refers to this type of building.
; Zero net source energy use: This ZNE generates the same amount of energy as is used, including the energy used to transport the energy to the building. This type accounts for energy losses during electricity generation and transmission. These ZNEs must generate more electricity than zero net site energy buildings.
; Net zero energy emissions: Outside the United States and Canada, a ZEB is generally defined as one with zero net energy emissions, also known as a zero carbon building or zero emissions building. Under this definition the carbon emissions generated from on-site or off-site fossil fuel use are balanced by the amount of on-site renewable energy production. Other definitions include not only the carbon emissions generated by the building in use, but also those generated in the construction of the building and the embodied energy of the structure. Others debate whether the carbon emissions of commuting to and from the building should also be included in the calculation. Recent work in New Zealand has initiated an approach to include building user transport energy within zero energy building frameworks.
; Net zero cost: In this type of building, the cost of purchasing energy is balanced by income from sales of electricity to the grid of electricity generated on-site. Such a status depends on how a utility credits net electricity generation and the utility rate structure the building uses.
; Net off-site zero energy use: A building may be considered a ZEB if 100% of the energy it purchases comes from renewable energy sources, even if the energy is generated off the site.
; Off-the-grid:
Off-the-grid buildings are stand-alone ZEBs that are not connected to an off-site energy utility facility. They require distributed renewable energy generation and energy storage capability. An energy autarkic house is a building concept where the balance of the own energy consumption and production can be made on an hourly or even smaller basis. Energy autarkic houses can be taken off-the-grid.
; Net Zero Energy Building: Based on scientific analysis within the joint research program "Towards Net Zero Energy Solar Buildings" a methodological framework was set up which allows different definitions, in accordance with country's political targets, specific conditions and respectively formulated requirements for indoor conditions: The overall conceptual understanding of a Net ZEB is an energy efficient, grid-connected building enabled to generate energy from renewable sources to compensate its own energy demand reduce energy demand by means of energy efficiency measures and passive energy use; 2) generate energy from renewable sources. However, the Net ZEBs grid interaction and plans to widely increase their numbers of evoking considerations on increased flexibility in the shift of energy loads and reduced peak demands.
; Positive Energy District: Expanding some of the principles of zero-energy buildings to a city district level, Positive Energy Districts are districts or other urban areas that produce at least as much energy on an annual basis as they consume. The impetus to develop whole positive energy districts instead of single buildings is based on the possibility of sharing resources, managing energy efficiently systems across many buildings and reaching economics of scale.
Within this balancing procedure several aspects and explicit choices have to be determined:
  • The building system boundary is split into a physical boundary which determines which renewable resources are considered respectively how many buildings are included in the balance and a balance boundary which determines the included energy uses. It should be noticed that renewable energy supply options can be prioritized and therefore create a hierarchy. It may be argued that resources within the building footprint or on-site should be given priority over off-site supply options.
  • The weighting system converts the physical units of different energy carriers into a uniform metric and allows their comparison and compensation among each other in one single balance. Politically influenced and therefore possibly asymmetrically or time-dependent conversion/weighting factors can affect the relative value of energy carriers and can influence the required energy generation capacity.
  • The balancing period is often assumed to be one year. A shorter period could also be considered as well as a balance over the entire life cycle.
  • The energy balance can be done in two balance types: 1) Balance of delivered/imported and exported energy Balance between energy demand and energy generation. Alternatively, a balance based on monthly net values in which only residuals per month are summed up to an annual balance is imaginable. This can be seen either as a load/generation balance or as a special case of import/export balance where a "virtual monthly self-consumption" is assumed.
  • Besides the energy balance, the Net ZEBs can be characterized by their ability to match the building's load by its energy generation or to work beneficially with respect to the needs of the local grid infrastructure. Both can be expressed by suitable indicators which are intended as assessment tools only.