Case study: Net-zero energy building in NYC

This New York City (NYC) building design will meet the challenge of emission reductions of 90% to 100%.


A New York City (NYC) building will meet the challenge of reducing emissions by 90% to 100%. Our engineering team assumed that a new, mid-rise multifamily building may have the highest potential for cost-effective net-zero energy building design for the following reasons: easier adoption of new technologies, lower initial incremental costs, and optimal occupant density per roof area to maximize use of renewable energy.

Using the Dept. of Energy's eQUEST, a sample building was created that mimics energy use of a typical new NYC multifamily building. The baseline model used simulation guidelines from a state energy program for multifamily buildings, as well as ASHRAE Standard 90.1-2007, California Title 24 ACM Manual, ASHRAE Standard 62.1-2007, and the NYC Building and Mechanical Code. This model served as the reference for assessing progress toward a net-zero energy building design.

Figure 1: An eQUEST 3-D model shows the configuration of a multifamily building aiming for net zero. All graphics courtesy: Taitem Engineering PCThe building has 48 apartment units within a 53,000-sq-ft gross floor area, in a rectangular shape, shown in Figure 1, with four floors plus a basement and a common laundry. The model was generated so that one can incrementally change the number of floors as needed.

In the baseline, two likely scenarios are examined: heating by electricity (with packaged terminal heat pump, or PTHP) and heating by fossil fuel (with packaged terminal air-conditioner, or PTAC, and gas-fired boiler) according to ASHRAE Standard 90.1-2007. New York Central Park's TMY3 bin weather data is used in all the simulation runs.

The approach

Although a 90% reduction in emissions by 2050 looks like an intimidating target, it is possible to overcome the challenge following a "one step at a time" approach. This means breaking the target into smaller, achievable pieces instead of considering it as an insurmountable whole.

When selecting energy-efficiency measures, priority is given to measures that are smaller, passive (fewest moving parts), cheaper (more cost-effective), and innovative.

The passive strategies studied include: highly improved wall and roof insulation, window upgrade with low-e coatings and insulated frames, shading possibilities, and substantial infiltration reduction by eliminating penetrations through building skin and through detailed air-sealing. Diminishing returns are evaluated for these envelope strategies while also taking into account the lowered installed cost for mechanical equipment that results from higher-efficiency envelope measures. In the analysis, we compare compartmentalization versus noncompartmentalization practices, along with mechanical versus natural ventilation, and determine which ventilation solution provides the best controlled intentional fresh air into the apartment spaces.

A high-performance air-source heat pump (variable refrigerant flow, or VRF) is introduced as a heating plus cooling system. One VRF outdoor condensing unit is recommended per apartment to lower transport losses and piping costs and to free up roof space for renewable energy generation. Domestic hot water (DHW) energy efficiency strategies studied include heat recovery from the drain as a preheater and recovering the waste heat from the VRF system by a heat exchanger.

VRF systems offer multiple benefits including:

  • Reduced pipe energy losses
  • Improved zone temperature control and substantially reduced energy usage
  • Eliminating the need for a boiler room and its ventilation requirements and making the recovered space available for other uses
  • Roof space freed up for other uses; eliminated room air-conditioner envelope losses (both infiltration and conduction)
  • Higher-efficiency cooling/heating
  • No electric resistance backup
  • Heat recovery for DHW
  • No structural damage from circulation water leaks
  • Improved tenant comfort; potential to submeter heating, cooling, and DHW
  • Resiliency (no boiler on the lower floors); and most important, dramatically lower carbon emissions.

Lighting strategies include low lighting power density (LPD) design solutions while maintaining acceptable levels of illumination. Where applicable, occupancy sensors and bi-level lighting controls are evaluated.The most efficient available appliances are considered for apartments and common areas.

Solar photovoltaic (PV) arrays can be installed in the freed-up roof space to zero out the remaining electricity use after all other improvements are implemented. Using the eQuest simulation tool, numbers of floors are iteratively varied to determine the economically viable range of floors and also the highest possible number of floors for a building to achieve net-zero energy with today's technology.

Electrical loads

Appliances and plug-load measures were evaluated first since they are the least recognized for reducing energy and one of the hardest to achieve overall cost-effectiveness. Evaluated products were the lowest energy users among Energy Star labeled products and tabulated as follows:

  • Ultra-high-efficiency refrigerators: These are expensive but offer high-value energy savings. This improvement has an incremental cost ($/kW) similar to solar PV, but is still worthwhile because it does not require roof space.
  • Clothes dryer: Per the Super-Efficient Dryer Initiative (SEDI), clothes washers reduced their energy use by almost 75% in the last two decades, compared to dryers with only a 20% improvement (excluding heat pump dryers). As a proven technology elsewhere in the world (over 25 models are available in the European Union), heat pump dryers have great potential in the U.S. market. The U.S. Environmental Protection Agency (EPA) gave heat pump dryers an Energy Star Emerging Technology 2012 Award.
  • Television: Energy Star's standby mode requirements have tightened over the years, which reduced consumption drastically (by 36 billion kWh/year in 2012), while 80% of televisions met the requirements by 2008. Televisions that meet today's Energy Star requirements are on average 40% more efficient than conventional models. Since 2011, Energy Star no longer permits more energy use for larger screen TVs (larger than approximately 50 in.).
  • TV set-top (cable) boxes: According to a 2011 Natural Resources Defense Council (NRDC) study, set-top boxes in the U.S. consumed about 27 billion kWh in 2010, equivalent to nine 500-MW coal-fired power plants. There are about 160 million set-top boxes in use in the country, at near-full power even when the consumer is not watching or recording TV. Potential improvements with set-top boxes include efficient multi-room solutions that schedule recordings on a central appliance and Internet protocol TVs (IPTV) boxes that draw approximately 18 W when operating and 12 W in light sleep state.

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