New York City has positioned itself at the forefront of global climate action, recognizing the urgent need to address rising greenhouse gas emissions and their catastrophic consequences. In response to this imperative, the city has undertaken a series of ambitious initiatives aimed at reducing its carbon footprint and fortifying its resilience against climate impacts.
Highlighting a pivotal development in New York City's commitment to combat climate change, the passage of Local Law 97 as part of the Climate Mobilization Act represents a significant step towards achieving the city's ambitious goal of becoming carbon neutral by 2050.
Local Law 97 establishes emissions limits for buildings over 25,000 square feet, aiming to reduce greenhouse gas emissions from the city's built environment, which accounts for a significant portion of its total emissions. By setting stringent emissions limits and implementing penalties for non-compliance, the law incentivizes building owners to invest in energy efficiency upgrades and adopt cleaner energy sources to reduce their carbon footprint.
Local Law 97 targets a specific subset of buildings in New York City, focusing on those with larger footprints and greater potential for emissions reductions. The law applies to:
· Buildings that exceed 25,000 gross square feet individually.
· Two or more buildings on the same tax lot that together exceed 50,000 square feet.
· Two or more buildings owned by a condominium association governed by the same board of managers that together exceed 50,000 square feet.
These criteria ensure that the law encompasses a wide range of building types and configurations that have a significant impact on the city's overall greenhouse gas emissions.
Some of the important types of buildings covered by Local Law 97 include:
· Masonry High-Rise Residential Buildings: These buildings typically have thick walls made of brick or stone and are commonly found in older neighborhoods throughout the city.
· Window Wall High-Rise Residential Buildings: These buildings feature large expanses of glass for their facades and are often characterized by modern architecture and high-end amenities.
· Masonry High-Rise Commercial Buildings: Like masonry high-rise residential buildings, these structures have thick walls made of brick or stone but are primarily used for commercial purposes, such as offices, retail spaces, or mixed-use developments.
· Curtain Wall High-Rise Commercial Buildings: These buildings utilize curtain wall systems, which are non-structural, lightweight, and typically made of glass and metal, providing a sleek and modern appearance.
By targeting these types of buildings, Local Law 97 aims to address emissions from both residential and commercial sectors, recognizing their significant contribution to the city's carbon footprint. This approach ensures that emissions reduction efforts are spread across various building types and sectors, maximizing the law's impact on achieving carbon neutrality by 2050.
Window Wall High-Rise Residential Buildings
Window-wall high-rise buildings in New York City, encompassing approximately 72 million square feet of the skyline, represent a significant challenge in terms of energy consumption and efficiency. Characterized by a notably high Window-to-Wall Ratio (WWR) of 50%, these structures face heightened heat loads, particularly during the summer months, necessitating extensive air conditioning usage to maintain comfortable indoor temperatures. According to the Urban Green Council's 90 x 50 Report from 2013, this elevated WWR exacerbates the buildings' energy needs, which results in their significant Energy Use Index (EUI) of 136 kbtu/sq.ft.
Compounding the energy efficiency issues is the prevalent use of Packaged Terminal Air-Conditioners (PTACs) within these buildings. While commonly utilized for their simplicity and ease of installation, PTACs are known for their relatively low efficiency compared to central HVAC systems, further amplifying energy consumption. Moreover, the reliance on outdated incandescent lighting fixtures within Window Wall High Rise buildings adds to their energy load, both through direct energy consumption and the additional heat generated, particularly during warmer seasons.
Given their total floor area typically hovering around 180,000 square feet above ground, these buildings fall within the purview of Local Law 97, which targets emissions reduction. This legislation aims to address energy consumption and emissions from such structures by establishing emissions limits, incentivizing energy efficiency upgrades, and promoting the adoption of cleaner energy sources.
A multifaceted approach is required to address the energy consumption challenges inherent in Window Wall High Rise buildings. This may include upgrading HVAC systems to more efficient models, enhancing insulation and sealing to minimize heat infiltration, and transitioning to energy-efficient lighting technologies such as LEDs. Additionally, incentivizing the installation of solar panels or other renewable energy sources can help offset energy demand and reduce reliance on conventional power sources, further advancing the city's sustainability goals.
Energy Modeling Case Study – Typical Window Wall High-Rise Residential Building
To validate the technical feasibility of energy efficiency investments required to achieve New York City's ambitious climate change goals, leveraging Building Energy Simulation tools is essential. These tools enable the evaluation of various Energy Efficiency Measures (EEMs) by simulating the interaction between the building envelope, controls, and systems.
In the case of Window Wall High Rise buildings, understanding and analyzing different EEMs is crucial. To facilitate this, an energy model of a typical Window Wall High Rise has been developed using Design Builder. Design Builder is powered by the robust EnergyPlus algorithm, developed by the US Department of Energy (DOE), and widely recognized for its accuracy and reliability in building energy simulations globally.
By utilizing Design Builder, stakeholders can assess the performance of different energy efficiency measures tailored to the unique characteristics of Window Wall High Rise buildings. This includes evaluating the impact of upgrades such as improved insulation, optimized HVAC systems, advanced controls, and efficient lighting solutions. Through simulation, stakeholders can quantify energy savings, assess cost-effectiveness, and identify the most viable strategies to achieve the city's climate goals.
Existing Window Wall High Rise – Baseline Energy Model
The baseline of the window wall high rise has been created with the following parameters:
Parameter | Description | |
External Wall Insulation R- Value (hour-ft2-oF per Btu) | 9.5 | |
Fenestration | SHGC | 0.7 |
VLT | 50% | |
U-value | 0.6 | |
Infiltration | ACH | 0.6 |
Lighting Power Density (W/m2) | 3 | |
Equipment Power Density (W/m2) | 11 | |
Space Cooling System & Type of Fuel: | PTAC (Electricity) | |
Space Heating System & Type of Fuel: | PTAC (Electricity) | |
Type of Glazing Used: | Double Glazing | |
WWR: | 40% | |
Operational Hours: | Typical Residential | |
Occupancy: | Typical Residential |
The baseline parameters have been taken considering values that can be expected in a typical window wall high-rise building in New York City. Further, aspects such as the use of incandescent lighting and the use of PTACs (Packaged Terminal Air-Conditioners) for air-conditioning have been considered.
Window Wall High-Rise with EEMs – Proposed Case Energy Model
The proposed case of the window wall high rise has been created with the following parameters:
Parameter | Description | |
External Wall Insulation R- Value (hour-ft2-oF per Btu) | 20 | |
Fenestration | SHGC | 0.3 |
VLT | 50% | |
U-value | 0.2 | |
Infiltration | ACH | 0.08 |
Lighting Power Density (W/m2) | 2.10 | |
Equipment Power Density (W/m2) | 11 | |
Space Cooling System & Type of Fuel: | Mini-Split Heat Pumps | |
Space Heating System & Type of Fuel: | Mini-Split Heat Pumps | |
Type of Glazing Used: | Triple Glazing, Low-e | |
WWR: | 40% | |
Operational Hours: | Typical Residential | |
Occupancy: | Typical Residential |
The following EEMs have been considered in the proposed case energy model:
1. The use of Air Sealing & ERV (Energy Recovery Ventilation) Units
2. Use of increased insulation on opaque areas
3. Triple Glazed Window Units
4. Adding 3’ Sunshades to Windows
5. Replacing PTACs with Mini-Split Heat Pumps
6. DHW Heat Pump Operating in Conditioned Space
7. Use of Heat Recovery for DHW on Air-Conditioners
Energy Consumption End-Use Breakup & Analysis
End-Use Category | Base Case Energy Consumption (kWh) | Proposed Case Energy Consumption (kWh) |
Space Cooling | 73623.10 | 46743 |
Space Heating | 151184 | 45195 |
Lighting | 76636.73 | 38025 |
Equipment | 60647 | 60647 |
Ventilation Fans | 3991.16 | 8459.73 |
Total | 365417 | 210920 |
From the end-use consumption breakup, it can be seen that the space cooling energy consumption in the proposed model is lesser than that of the base case model. This can be attributed to the use of increased insulation & air-sealing, thereby reducing heat losses & also reducing the ACH (Air Changes Per Hour). Additionally, replacing the PTACs with Mini-Split Heat Pumps has further resulted in space-cooling energy savings due to a better COP (Coefficient of Performance). This has also been supplemented by the use of ERVs (Energy Recovery Ventilation Units).
It can also be seen that the use of Triple Glazed Units and the use of window shades has resulted in lesser heat gains during the summer.
The space heating energy consumption has also seen a decrease in the proposed case energy model. However, the decrease is not as significant due to the heat losses not being very significant. The use of indoor lighting with higher wattages adds to the heat gains and thereby reduces the proposed case space heating energy consumption. In addition, the Mini-Split Heat Pumps have a better EER (Energy Efficiency Ratio) thereby resulting in a lower energy consumption.
Due to the reduction in space cooling & space heating energy consumption, the energy consumption of fans has also decreased in the proposed case energy model. The use of Heat Pumps for DHW has also resulted in a significant reduction in hot water energy consumption. The existing boilers are not very efficient and hence consume more energy. The implementation of these EEMs has resulted in a significant savings of 42% and is in line with Local Law 97 compliance.
Conclusion & Future Implementation
Conducting a whole building energy simulation before investing in EEMs will not only help building owners realize the full potential of energy savings but also make practical investments.
Reference
Urban Green Council (2014) “90 BY 50: NYC CAN REDUCE ITS CARBON FOOTPRINT 90% BY 2050”
Urban Green Council (2014) “Baby it’s cold inside”.
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