“Zero-Energy Buildings (ZEB) are becoming increasingly prominent in the press and so we thought we would bring together this feature to explain the many benefits of starting or purchasing a ZEB project as well as the pitfalls.”

BedZED zero energy housing in the UKA zero energy building (ZEB) or net zero energy building is a general term applied to a building with a net energy consumption of zero over a typical year. Zero energy buildings are gaining considerable interest as a means to cut greenhouse gas emissions and conserve energy. Buildings use 40% of the total energy in the US and European Union.[1][2]

Overview

This can be measured in different ways (relating to cost, energy, or carbon emissions) and, irrespective of the definition used, different views are taken on the relative importance of energy generation and energy conservation to achieve energy balance. Although zero energy buildings remain uncommon in developed countries, they are gaining in importance and popularity. The zero-energy approach is promoted as a potential solution to a range of issues, including reducing carbon emissions, and reducing dependence on fossil fuels. Most ZEB definitions do not include the emissions generated in the construction of the building and the embodied energy of the structure which would usually invalidate claims of reducing carbon emissions.

A building approaching zero energy use may be called a near-zero energy building or ultra-low energy house. Buildings that produce a surplus of energy during a portion of the year may be known as energy-plus buildings. 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.

Definitions

Despite sharing the name zero energy building, there are several definitions of what ZEB means in practice, with a particular difference in usage between North America and Europe. [3]

Net zero site energy use - In this type of ZEB, 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 energy building” generally refers to this type of building.
Net zero source energy use - This ZEB generates the same amount of energy as is used, including the energy used to transport the energy to the building. This type accounts for losses during electricity transmission. These ZEBs must generate more electricity than net zero 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.
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 (for when the sun is not shining, wind is not blowing, etc).

Design and construction

The most cost-effective energy reduction in a building usually occurs during the design process.[4] To achieve minimal energy use, zero energy design departs significantly from conventional construction practice. Zero energy building designers typically use sophisticated 3D computer simulation tools to take into account a wide range of design variables such as building orientation (relative to the daily and seasonal position of the sun), window and door type and placement, overhang depth, insulation type and values of the building elements, air tightness (weatherization), the efficiency of heating, cooling, lighting and other equipment, as well as local climate. These simulations help the designers predict how the building will perform before it is built, and enable them to model the economic and financial implications on building cost benefit analysis.

Zero Energy Buildings are usually built with significant energy-saving features. The heating and cooling loads are often drastically lowered by using high-efficiency equipment, added insulation, high-efficiency windows, passive solar techniques, and other techniques. These features can vary drastically between buildings in different climate zones. Water heating loads can be alleviated by using heat recovery units on waste water, and by using high-efficiency water heating equipment. In addition, lighting energy use can be lessened by daylighting, fluorescent and LED lighting, and miscellaneous electric loads can be lessened by choosing efficient appliances and minimizing standby power. Zero energy buildings are often designed to make use of energy gained from other sources including white goods; for example, use refrigerator exhaust to heat domestic hot water, ventilation air and shower drain heat exchangers, office machines and computer servers, and even body heat from rooms with multiple occupants. These buildings make use of heat energy that conventional buildings typically exhaust outside. They may use heat recovery ventilation, hot water heat recycling, and absorption chiller units. They are normally optimised to use passive solar heat gain, use thermal mass to stabilise diurnal temperature variations throughout the day, and in most climates are superinsulated. All the technologies needed to create zero energy buildings are available off-the-shelf today.

Other unique energy-saving strategies include using absorption chillers, daylighting, combined heat and power, and Passive cooling.

Energy generation

ZEBs generate their own energy to meet their electricity and heating needs. In the case of individual houses, various microgeneration technologies may be used to provide heat and electricity to the building, using solar cells or wind turbines for electricity, and biofuels or solar collectors linked to seasonal thermal stores for space heating. To cope with fluctuations in demand, zero energy buildings are frequently connected to the electricity grid, export electricity to the grid when there is a surplus, and drawing electricity when not enough electricity is being produced. Other buildings may be fully autonomous.

Zero-energy neighborhoods, such as the BedZED development in the United Kingdom, and those that are spreading rapidly in California and China, may use distributed generation schemes. This may in some cases include district heating, community chilled water, shared wind turbines, etc. There are current plans to use ZEB technologies to build entire off-the-grid cities, such as the photovoltaic-powered Huangbaiyu Sustainable Village, and the planned Dongtan Eco-City near Shanghai.
A benefit of such localized energy generation is the elimination of electrical transmission and electricity distribution losses. These losses amount to about 7.2%-7.4% of the energy transferred.

Occupant behavior

The energy used in a building can vary greatly depending on the behavior of its occupants. Studies of identical homes in the United States have shown dramatic differences in energy use, with some homes using more than twice the energy of others.[11] Occupant behavior can vary from differences in setting and programming thermostats, varying levels of illumination and hot water, and the amount of miscellaneous electric devices used.

Advantages and disadvantages of ZEBs

“ZEB advantages”

Isolation for building owners from future energy price increases

Increased comfort due to more-uniform interior temperatures

Reduced requirement for energy austerity

Reduced total cost of ownership due to improved energy efficiency

Reduced total net monthly cost of living

Improved reliability

Photovoltaic systems have 25-year warrantees

Seldom fail during weather problems

The 1982 photovoltaic systems on the Walt Disney World EPCOT Energy Pavilion are still working fine today, after going through 3 recent hurricanes

Extra cost is minimized for new construction compared to an afterthought retrofit

Higher resale value as potential owners demand more ZEBs than available supply the value of a ZEB building relative to similar conventional building should increase every time energy costs increase

Future legislative restrictions, and carbon emission taxes/penalties may force expensive retrofits to inefficient buildings

“Potential ZEB disadvantages”

Initial costs can be higher

Effort required to understand, apply, and qualify for ZEB subsidies
very few designers or builders have the necessary skills or experience to build ZEBs [34]
possible declines in future utility company renewable energy costs may lessen the value of capital invested in energy efficiency.

New photovoltaic solar cells equipment technology price has been falling at roughly 17% per year - It will lessen the value of capital invested in a solar electric generating system.

Current subsidies will be phased out as photovoltaic mass production lowers future price challenge to recover higher initial costs on resale of building

Appraisers are uninformed - their models do not consider energy climate-specific design may limit future ability to respond to rising-or-falling ambient temperatures (global warming) without an optimised thermal envelope embodied energy and resource usage is higher than needed

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