21-03-2014, 11:48 AM
Zero-energy building
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INTRODUCTION
A zero-energy building, also known as a zero net energy (ZNE) building, net-zero energy building (NZEB), or net
zero building, is a building with zero net energy consumption and zero carbon emissions annually.[1] Buildings that
produce a surplus of energy over the year may be called "energy-plus buildings" and buildings that consume slightly more
energy than they produce are called "near-zero energy buildings" or "ultra-low energy houses".
Traditional buildings consume 40% of the total fossil fuel energy in the US and European Union and are significant
contributors of greenhouse gases.[2][3] The zero net energy consumption principle is viewed as a means to reduce carbon
emissions and reduce dependence on fossil fuels and although zero energy buildings remain uncommon 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 grid. Energy is usually
harvested on-site through a combination of energy producing technologies like solar and wind, while reducing the overall
use of energy with highly efficient HVAC and lighting 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.
The development of modern zero-energy buildings became possible not only through the progress made in new energy
and construction technologies and techniques, but it has 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.
The zero-energy concept allows for a wide range of approaches due to the many options for producing and conserving
energy combined with the many ways of measuring energy (relating to cost, energy, or carbon emissions).
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.[4]
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 losses during electricity transmission. These ZNEs must generate more electricity
than zero net site energy buildings.
Energy harvest
ZEBs harvest available energy to meet their electricity and heating or cooling 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 thermal collectors linked to a seasonal thermal energy storage (STES) for
space heating. An STES can also be used for summer cooling by storing the cold of winter underground. 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.[14] Other buildings may be
fully autonomous.
Energy harvesting is most often more effective (in cost and resource utilization) when done on a local but combined scale,
for example, a group of houses, cohousing, local district, village, etc. rather than an individual basis. An energy benefit of
such localized energy harvesting is the virtual elimination of electrical transmission and electricity distribution losses.
These losses amount to about 7.2%-7.4% of the energy transferred.[15] Energy harvesting in commercial and industrial
applications should benefit from the topography of each location. The production of goods under net zero fossil energy
consumption requires locations of geothermal, microhydro, solar, and wind resources to sustain the concept.[16]
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 or net zero energy use cities.
Development efforts
Wide acceptance of zero energy building technology may require more government incentives or building code
regulations, the development of recognized standards, or significant increases in the cost of conventional energy.
The Google photovoltaic campus, and the Microsoft 480-kilowatt photovoltaic campus relied on U.S. Federal, and
especially California, subsidies and financial incentives. California is now providing $3.2 billion USD in subsidies[21] for
residential-and-commercial near-zero-energy buildings, due to California's serious electricity shortage, frequent power
outages, and air pollution problems. The details of other American states' renewable energy subsidies (up to $5.00 USD
per watt) can be found in the Database of State Incentives for Renewables and Efficiency.[22] The Florida Solar Energy
Center has a slide presentation on recent progress in this area.[23]