Net zero energy buildings

What are NZEBs?

Net or nearly zero energy buildings (NZEB) are highly efficient buildings with extremely low energy demand, which is met by renewable energy sources. Such buildings produce as much energy as they consume, accounted for annually.

In order to achieve their net zero energy goals, NZEBs must first sharply reduce energy demand using energy efficient technologies, and then utilize  renewable energy sources (RES) to meet the residual demand. In such buildings, efficiency gains enable the balance of energy needs to be supplied with renewable energy technologies. This is the most logical approach to reach NZEB goal.

Definitions of NZEB are critical in determining the path to zero energy goals, and significantly influence design choices of architects and building owners. Appropriateness of definitions to a project vary according to project goals and values of the designer and building owner, making it essential for them to understand which definition will suit their purpose best.

Net Zero Site Energy Building

“A site ZEB produces at least as much energy as it uses in a year, when accounted for at the site.”

Energy flows are accounted at the site of the project. A building using 100 kWh of energy annually will be site net zero energy if at least 100 kWh of renewable energy is produced at the site annually. Renewable energy supply systems can be installed on building rooftops, facades, parking lots and other open areas in the site.

Site NZEBs encourage a relentless pursuit of energy efficiency, as the quantum of renewable energy supply options on a given site is limited. Net zero site energy buildings ignore source energy. Energy used in producing and transmitting grid supplied electricity from source to site are not accounted for while calculating demand-supply equilibrium. Energy spent in transporting renewable fuels like bio-waste to the site, is also not considered. This renders site NZEBs easier to implement because energy supply targets to reach neutrality are low.

Renewable energy sources that can be used on site are solar PV, wind, and biofuels.  Technologies commonly used are solar PV, solar hot water, solar thermal, small hydro units, biogas plants and if the site location is conducive, then wind turbines.Renewable energy produced is fed directly into the energy distribution system of the building. Surplus energy can either be stored in storage batteries or supplied to the grid if net metering and feed-in tariff policies are being implemented by local utilities.

This definition is more viable for small to medium sized buildings with comparatively large site areas and abundant supply of renewable sources like solar and wind.

Net Zero Source Energy Building

“A source ZEB produces at least as much energy as it uses in year, when accounted for at the source.”

Source or primary energy is the measure of net zero status for source NZEBs. Primary energy is the energy used to generate and deliver secondary energy (predominantly electricity in the case of India) to the site.

Energy supplied to the source NZEB site and exported from it gets multiplied by site-to-source conversion factors which allow energy used for generation in power plants and transmission to be factored. Conversion factor for electricity in India, independent of the location of building, is currently assumed to be approximately three. A grid connected building with annual energy demand of 100 kWh can be qualified as a source NZEB if 300 kWh of renewable energy is supplied on-site or off-site. Source NZEBs are a more comprehensive model of reducing the impact of building energy consumption on natural resources because primary energy is the metric used. However, they do not discern between pollution due to different fuel types nor for variations in primary energy across peak and non-peak loads. Electricity supplied at peak hours uses more primary energy than when supplied during non-peak hours.

Site-to-source conversion factors for different secondary energies are different. However, electricity is the predominant secondary energy used in India and thus difference in conversion factors can be ignored. Disparities due to primary fuel like nuclear, coal, natural gas and hydro used for producing electricity cannot be ignored if accuracy is to be maintained. Currently, regional variations in source to site conversion used for generating electricity in India are not available. Variation in fuel types like coal, hydro and nuclear fuels has also not been computed. A single national average conversion factor for all regions and fuel types reduces the accuracy of source energy numbers.

Net Zero Energy Cost Building

“The amount of money the utility pays the building owner for the energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.”

Cost of renewable energy exported from site must match utility bills of energy imported to the site. Essentially, a building is net zero cost energy if it recovers expenses on utility bills by selling electricity generated by renewable sources. Utility tariff structure is crucial to cost NZEBs. Ideally, tariffs must credit both lower energy use and lower peak demands. Utility tariff structures for commercial and industrial buildings in India vary by both – energy use and peak demand. Residential utility tariff structures usually vary only by energy use.

Net zero energy cost buildings must have low peak demands and higher energy savings so that they have lower price tariffs and in turn, lower utility bills. Demand response controls are thus crucial to achieving energy cost neutrality. Cost NZEBs are also not feasible without policies like feed-in-tariff and net metering, which facilitate buying back of electricity by utilities. Compared to site or source NZEBs, this definition will require greater installed capacity of renewable energy systems for the same building, considering buy-back rates offered by utilities are typically lower than supply tariffs. More electricity needs to be produced from a larger renewable energy system to make up for this deficit.

Solar PV is the most commonly used technology to generate electricity on site. Quantity of electricity generated through biogas plants may not be enough on its own to offset the cost of imported energy.  Electricity generated in waste heat or co-generation using biofuels can also be stacked in the supply side options.

Dependent on frequently varying price structure of utilities, cost NZEBs might be easier to achieve than to maintain over the lifecycle of a building. Cost NZEBs can lose their status due to revised utility rates despite energy savings remaining unchanged. On the other hand, the building may actually be able to enhance its cost net zero status because of a favourable change in feed-in tariffs.

Net Zero Energy Emission Building

“A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources”

Emissions of net zero energy emissions building are counted for the source energy and not site energy. To determine emissions output from a building, energy used in the building is multiplied by an emissions factor which weighs emissions resulting from transportation and at-source generation. Carbon, sulphur oxides and nitrogen oxides are included in calculating emissions neutrality. Fuel used for generating electricity supplied by the grid determines the ease of achieving net zero emissions. Emissions from electricity supplied by a nuclear plant are different than that of hydro plant, which varies from a coal fired plant.

Net zero emissions building can be zero emissions on site or off site – they can produce the equivalent renewable energy on site or purchase it. On site emissions ZEB can produce renewable energy on its footprint or the site to gain emissions neutrality. If renewable fuels are bought to the site for production, emissions from transportation and fuel production must be factored in emission neutrality calculations.

Renewable energy credits can be purchased from an off-site source to offset energy use. Alternatively, a building is emissions ZEB if it uses grid supplied electricity from renewable sources like wind farms, hydro plants, and solar plants. However, this alternative is not practically possible as of now because of the limited generation capacity of wind farms and hydro plants. Most of electricity supplied to the national grid is generated by conventional coal fired plants. Additionally, electricity from various sources is mixed in the grid prior to supply. Determining emissions accurately is complex as this ratio of renewable versus non-renewable electricity in the grid (termed fuel mix) varies, and is seldom calculated by utilities. Information about generation mix in different regions is essential before attempting net zero emission buildings with any degree of accuracy.

Emissions ZEB is the most holistic model of evaluating and reducing impact of energy used by buildings. It factors pollution and emissions across different fuels, as across different regions.

Source: NZEB portal