The definition of zero-energy-home or zero energy building is often misunderstood as a building having zero energy consumption. A zero energy building simply refers to a building producing the same (or more) amount of energy than what it consumes within a single year. Establishing a measurement term-period for zero-energy homes on an annual basis is very crucial so as to allow for fluctuations in different seasons of the year; thus peak energy needs in winter for heating can be offset at the end of year by energy produced from Renewable Energy Sources (RES) – Solar energy for homes, and other residential or commercial renewable energy systems.
Defining what Zero energy buildings are – (ZEB) definitions
Despite the aforementioned we should note that the term zero-energy-building or net-zero-energy, etc, has been subject to further refinement depending on the boundary and the metric used; the four mainly used definitions are:
- net zero site energy, – energy generated equals or exceed energy used when energy is accounted for at the site
- net zero source energy, – energy generated equals or exceed energy used when energy is accounted for by the primary energy source used to deliver the energy at the site – using the site-to-source conversion factors
- net zero energy costs, – energy income from selling generated energy on the utility grid equals or exceed energy costs for buying electricity from the utility grid – over a year
- Net zero energy emissions – the energy generated from emissions-free renewable energy sources is at least as much as the energy it uses from emissions-producing sources.
In effect what this means is that a Net-Zero-Energy-building will be able to generate from renewable energy sources an equal or greater amount of energy than the amount of energy that it needs for consumption (e.g. lighting, heating, cooling, electrical appliances, etc.). Moreover, a ZEB generates electricity from renewable energy sources that are locally available, are cost-effective and environmentally sustainable.
In general, a zero energy building – or zero net energy (ZNE), or net zero energy (NZEB), or net-zero-building, refers to a building with zero net annual energy consumption (energy generated – energy consumed) and zero annual emissions.
The need for Zero energy buildings
According to US and European Union energy statistics, buildings accounts approximately for 40% of national energy consumption – of total primary energy from fossil fuels; consequently traditional buildings account to 40% of greenhouse emissions and effects of global warming. In light of these, the principles of green buildings, energy efficient buildings and zero energy buildings constitute the successful approach towards reducing carbon emissions and attaining environmental sustainability in the construction sector.
The general principles of green, energy efficient buildings and more advanced – zero-net-energy buildings) are based on the same principles of conservation of energy (saving energy losses), energy efficiency (using energy efficiently) and then using renewable energy for complementing for its energy needs -see home-solar-panels.
Design and construction towards Zero-Net-Energy buildings
Design is by far the most cost-effective measure towards reducing the environmental footprint of a building. Starting with bio-climatic principles the aim is to exploit the site-environment towards optimized energy gains for the building such as optimum orientation, optimized shading features, using trees and nature in a beneficial way etc. In addition to bioclimatic design features, come passive solar principles which, in a similar way, take advantage of natural micro-climatic conditions to provide for desired temperature, humidity and ventilation conditions with minimum mechanical means – natural conditioning, solar heating, etc.
After incorporating design and passive solar principles, it is then possible to model a building with sophisticated 3d-modeling tools and software so as to observe the overall energy behavior of the building; the power of such simulation can help to reach optimized solutions by trying, through simulation, different solutions to parametrized variables. Such variables may include various choices for construction materials and thermal insulators, different insulation methods, window-openings dimensions and window materials, ventilation techniques etc.
Effectively, zero energy buildings are designed with advanced energy saving features to minimise their energy needs. Amongst others, they bear effective insulation, high-efficiency windows and effective ventilation which lowers significantly the heating and cooling loads of the building. Moreover, they use solar heating, special insulated tanks and sophisticated high efficiency HVAC systems to lower water heating loads and also contribute to reduced heating loads.
In review of the technologies available today for zero-energy buildings most commonly used are solar hot water panels, solar photovoltaic PV panels, wind, geothermal and bio fuels. Roof-mounted solar PV panels are becoming increasingly popular and coupled with solar hot water they are the most applicable amongst renewable technologies in ZEBs. In general the technologies used can be prioritised based on three main variables:
- Having minimum environmental impact
- Encouraging bioclimatic and energy efficient building designs
- Being widely available and cost effective
Renewable energy for buildings – zero net energy buildings
As already mentioned, the final ‘design’ element of zero energy buildings is the use of active renewable energy systems for the generation of renewable energy. In effect, renewable energy, energy produced on site with solar panels, or energy purchased from a solar photovoltaic plant, comes to complement the electricity, heating and cooling energy needs of energy efficient and with minimum energy losses building.
As in every renewable energy system, the main disadvantage is the intermittency of power generation from renewables as they are tied with unpredictable and fluctuating environmental conditions. Solar PV panels will not generate electricity during the night or on passing clouds that reduce sunlight substantially; similarly, a wind turbine will stop generating when the wind intensity is fluctuating – see solar energy systems. The way to compensate for renewable energy fluctuation is by energy storage, which is yet not economically viable as a standard integrated solution. Eventually, to cope with these energy generating fluctuations and the difference between demand and supply side, zero energy buildings are often connected to the power grid; in this way they are able to sell any surplus of generated energy and buy when short of generated energy. On grid zero-net energy also makes more sense when ‘netting’ energy generated and energy consumed as it allows accounting for excess energy generation.
To illustrate this, imagine the situation during the months of September – October and April-May, where environmental conditions are usually closed to desired temperature thus reducing the energy needs of a house. Especially in April-May, the low energy demand of an energy efficient building is coupled simultaneously with an increased energy generation from solar panels; this excess energy would be wasted if not able to sell it on the power grid and take it into account when netting energy of the building – see pros and cons of solar panels for home and solar energy pros and cons.
Simulating and measuring a-zero-net-energy-home – Lab.
To perform tests and gather useful data on reaching zero-net- energy performance in buildings, the National Institute of Standards and Technology (NIST) of the US Commerce Department has recently revealed an interesting new laboratory explicitly custom made for net-zero homes. The laboratory, a net-zero-energy residential test facility will allow for some high-efficiency tests and the opportunity to deduct actual accurate results for alternative solutions in energy systems, materials used and designs.
The test facility simulates a family of four, called the ‘ Nisters’, living in an energy efficient building. The facility has automated equipment and monitoring control systems to simulate the energy related actions of the 4-member family such as turning lights on and off, flushing the toilet, turning heating and cooling on as per the family’s needs et.
According to NZERTF officials, following phase 1 of the Nister’s era,
“..the facility will be used to test existing and new energy-efficient technologies and develop methods of test that better reflect how those technologies will perform in a real home, rather than a laboratory..” .
More information on the innovative zero-net-housing test facility from (NIST).