Energy & Carbon Modeling
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Build and Calibrate the Initial Energy Model
The Empire State Building (ESB) energy model has been developed over the past 15 years. Each year, the energy model has been calibrated based on several factors including utility bills, hourly sub-metering, occupancy rates, new construction projects, etc.
Create the Baseline Energy Model
The baseline energy model for the Empire State Building was developed and is maintained by Quest Energy Group. In this latest version, the energy model was calibrated to the 2019 calendar year. The 2020 utility data was not used given the unique changes in occupancy and operation due to COVID-19. After the baseline model was calibrated, the eQUEST energy model outputs were then compared to the total monthly data from ConEdison. Calibration is maintained with statistical error and broken down by the various end uses in the building.
Generate Detailed End-Use Breakdowns - The results from the baseline energy model allowed the project team to analyze energy usage on a deeper level. The energy usage breakdown showed that space heating, broadcast, and tenant loads were the largest contributors to energy usage. This analysis allowed the team to determine where there were opportunities for improvement.
Figure - 2019 Baseline Energy Usage Load Breakdown by End-Use
Figure - Baseline Model Electricity Usage (LEFT) and Steam Usage (RIGHT) Compared to Metered Data
Overlay Carbon Emissions - Carbon emissions were broken out by fuel source, system, and ownership to help the project team understand the primary contributors and identify areas for reduction. From this analysis it became apparent that while optimization of base building systems like the central plant and steam system could provide significant emissions reductions, the project targets could not be achieved without addressing the contribution of tenant systems and equipment. Indeed, tenant plug loads are a significant component of the building’s baseline carbon emissions, with office and retail tenants accounting for almost 31% of total 2019 carbon emissions.
Figure - 2019 Baseline Carbon Emissions Breakdown by End-Use (LEFT) and by Ownership (RIGHT)
Base building energy usage and carbon emissions include:
- All district steam heating
- Central cooling plant equipment
- All office tenant AHUs
- Lobby HVAC unit
- Common area lighting and equipment
Analyze Individual ECMs
The team narrowed down over 200 energy and carbon conservation measures to 60 ECMs that have potential to be implemented over the next 15 years. Each ECM was vetted technically and identified as an opportunity to reduce energy consumption and further decarbonize the building. The energy modeler analyzed the ECMs through the baseline energy model to extract the associated energy, carbon and cost savings. As examples, below is a list of a few ECMs that the project team studied, with details on the energy modeling methodology used.
|ECM||Description||Summary of Energy Modeling Methodology|
|1st Floor Lobby Air Distribution Optimization|
The existing lobby air distribution system needs upgrading if we are to minimize the conflict between optimization (i.e., sequence of operation recommendations/upgrades to AHU to address stratification) and art preservation requirements. Existing system is not designed for true humidity control and the location of the original supply grilles doesn't lend itself to close control of the ceiling. The design would look to upgrade the AHUs, add reheat using a heat pump, and modify the supply grille locations and type to ensure the ceiling is protected whilst minimizing the over cooling at low level.
Optimize according to CFD Report. 2 measures (active pressurization to control infiltration, and addition of space thermostats for active temperature control) are ECMs, others are artwork preservation.
First floor lobby AHU switched from steam heating to dedicated heat pump hydronic loop. Better control of relative humidity in the space. Cooling temperature setpoint adjustments based on control of RH and destratification. Better controls of infiltration into the building.
- RH Setpoint = 40%,
- Cooling Temp Setpoint = 67F,
- Heat Source = Steam,
- Inf Rate = 0.04 cfm/sf.
- RH Setpoint = 60%,
- Cooling Temp Setpoint = 74F,
- Heat Source = Hydronic Heat Pump Loop,
- Inf Rate = 0.04 cfm/sf,
- Heat Pump COP = 3.5,
- Pump = variable, 1.5 HP
|Airside Sequence of Operations|
Align AHU sequences with ASHRAE Guideline 36-2018 with modifications to limit low CHW dT. Low dT syndrome mitigation measures: limit approach temperature between SA-T and CHWS-T (i.e. an AHU designed for 55 F air at 44 F water will not try to make 55 F air with 55 F water), high limit on AHU CHW valve position to maintain CHWR-T at AHU unless SA-T is well above setpoint. Add temporary unoccupied mode to VAV boxes based on lighting system occupancy data (allows box to flow no air when the temperature is ok and there are no people in the space). Allow perimeter heat without AHU operation. Add pulsed ventilation mode (time averaging per ASHRAE 62.1) where existing ventilation controls lack authority (can't reduce airflow to the required flow) due to missing OA fan VFD or stack effect. Corrected controls for MER unit heaters and duct heaters. AHU currently used for overnight heating in SOO, switch to perimeter heating for setback.
Update existing VAV sequence:
- Precondition space prior to scheduled occupancy from VAV box to the AHU
- Allow DCV to meet actual IAQ or temperature setpoints
- Correct damper control when AHU is off
- Ensure a minimum 5 F deadband between the space cooling and heating setpoints
- Ensure after hour heating is perimeter heat only
- Integrate sequence for occupancy sensor
AHU average runtime adjusted from 12 hr/day to 10 hr/day. Discharge Air temperature setpoint high limit increase from 58F to 68F. Constant static pressure control changed to static pressure reset control on AHU fans. Avg static pressure of 0.8. Applied to floors 3 through 75, 80, and 84
- AHU runtime = 12 hr/dy (average).
- DAT setpoint between 55-58F.
- Variable speed fan control.
- AHU runtime = 10 hr/dy (8AM to 6PM).
- DAT setpoint between 55-68F.
- Variable speed fan control with static pressure reset.
- SP reset curve is used with average 0.8 in throughout the year.
|EXF Heat Reclaim|
Constant volume Toilet exhaust, on the order of 140,000 CFM, currently existing the building without heat recovery.
Retrofit: Add ERV to mechanical rooms for OA preheating (~1200 CFM/room avg)
Energy recovery ventilation system installed on each tenant floor. Effectiveness of 0.7. Added fan pressure of 0.25 in wc. on each side of the ERV. Applied to floors 3 through 75, 80, and 84. Electric unit heater in mechanical room removed.
- ERV = YES.
- Effectiveness of 0.7. 0.25 in wc on both sides of ERV.
- Electric heat = NONE.
Window Center of Glass Upgrades:
Add Thin Glass Interior Secondary Window Products: Ultra-thin triple-pane windows
Window Frame Upgrades:
Overall U-value of windows is much lower than center of glass. Rather than replacing windows with triple pane (center of glass u-value itself is pretty good), insulate window frames to reduce thermal bridging. Replace just the sashes (not the frame) and put aerogel in sashes. Simultaneously, utilize WinSert (thin glass interior secondary window) to improve glass performance and radiant comfort
Audit and intervention: Thermal breaks/gasketing of window, air sealing
Upgraded ALL 6000+ windows from current specs to Winsert proposed specs. Reduced infiltration through the window by half.
- 2 cfm/sf infiltration through window area.
- North Windows: U-value = .309 Assembly, SHGC = 0.28, VT = 0.65.
- SEW Windows: U-value = 0.362 Assembly, SHGC = 0.27, VT = 0.49
- 1 cfm/sf infiltration through window area.
- NSEW Windows: U-value = .132 Assembly, SHGC = 0.25
|Steam Phase-Out with Hot Water Riser|
Long-term solution for phase-out of steam heating:
Centralized air-water heat pumps (~1000 tons) generate HHW which is distributed to tenant floors through a hot water riser. HHW is used at AHU coils. Decarbonization of the perimeter steam system is captured under SS010.
Switch AHU heat source from steam to hot water HP loop and adjust AHU controls to be primary source of heating (perimeter system secondary source)
- AHU heating source = electric induction + ERV,
- primary heat from perimeter radiators,
- ASHP Avg Annual COP ~3.5.
- Three heat pumps with 15 MMBtu capacity each.
- Each loop (Low, Mid, High) sized with 75-HP pump with VFD controls and head setpoint of 50ft.
- HW temp setpoint = 110F,
- Design dT = 15,
- AHU heat source = HP hot water loop.
- DAT up to 80F.
- Eliminate electric induction units.
- Perimeter radiators controlled based on temp setpoint in the space
Group, Sequence, and Package ECMs
Related ECMs were grouped together into phases and sequenced in the modeling order such that savings for each ECM build on the last. These phases were then sequenced based on the logic of improving and optimizing existing systems first, then reducing loads, and finally replacing or updating the equipment. The sequence was also based on feasibility and expense, such that the phases that involved large system interventions like geothermal, DHW electrification, colocation of IT equipment, and chiller replacements are sequenced towards the end of the study period (phases 6-10). The team also developed a proposed implementation timeline for each of the phases. For example, the controls optimization measures in Phase 1 were proposed to be implemented immediately and completed in 1 year, while the Phase 5 ECMs that are intended to be implemented at tenant lease renewal extend over a period of 10 years.
Figure - ECM Phasing and Implementation Timeline
The ECMs were also grouped into 5 distinct packages which contain different combinations of ECMs, and an increasing number of them, in order to study their impact on CO2 reductions and Net Present Value (NPV). This allowed the team to review how different combinations of ECMs measure up against the project objectives.
The NPV Max package contains the least amount of ECMs which are all NPV positive. On the opposite end of the scale, the CO2 Max package includes all the ECMs that were studied. Three additional packages were created to result in cost and carbon reductions in the middle of the scale. These were the CO2 Light, CO2 Mid, and CO2 High packages, which generally include all the measures that the project team recommends implementing with the major difference between them being that CO2 Light explores just optimizing the existing steam system, whereas CO2 Mid explores partial HVAC electrification, and CO2 High includes complete HVAC electrification plus a few other tenant measures.
Figure - Relationship between Carbon Reductions and Net Present Value in ECM Packages
Generate a Decarbonization Roadmap
Now that the finalized ECMs have been grouped, sequenced, and packaged, the energy model can be run for each ECM package to obtain energy and carbon impacts. The project team compared the results of this analysis and calculated the energy and carbon savings from the baseline model. The results of this analysis are shown in the figures below. These results will be used in the detailed financial analysis and will represent a time-dependent decarbonization roadmap for the building.
Figure - ECM Package Energy Savings Comparison
Figure - ECM Package Carbon Savings Comparison
Figure - ECM Package CO2 Emissions Projections Comparison Over Time (CLCPA Target Grid Scenario)
While energy modeling was completed for all 5 packages of ECMs studied, the figures below focus on summarizing the results for the CO2 Mid Reduction Package which forms the Decarbonization Roadmap for the Empire State Building. The CO2 Mid Reduction Package provides the optimal techno-economic balance and is currently slated for implementation. However, certain ECMs in the CO2 High Reduction Package are recommended for further study to better assess their constructability, cost, and performance, and may be considered for implementation in the long term based on the outcomes of the planned pilots. Both packages meet ESRT’s goal of 80% carbon reductions compared to the 2007 benchmark year by 2030, as well as the average-long term Local Law 97 (LL97) limit by 2035.
At the end of the 15-year study period, it is expected that the CO2 Mid Reduction Package will reduce energy consumption by 64.8% compared to the 2007 baseline (see Figure below). Phases 1, 2, and 5 (including steam phase out which is broken out separately) result in the largest energy reductions for this package with energy savings contributions of 6.1%, 8.0%, and 5.7% respectively.
Figure - CO2 Mid Package Energy Reduction by Phase
The Figure below shows the breakdown of the carbon reduction anticipated by phase for the CO2 Mid Reduction Package. The total carbon savings anticipated are 65% from the 2007 baseline, assuming the 2019 carbon coefficient. However, if the electrical grid continues to decarbonize in alignment with the CLCPA targets, the carbon savings can reach as much as 87% reduction from the 2007 baseline.
Figure - CO2 Mid Package Carbon Emissions Reduction by Phase
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