The NYC government has established an emissions reduction goal of 80% by 2050, using the levels measured in 2005 as a baseline. In order to reach this target, buildings must become much more efficient, since they account for 75% of the city’s total emissions. Also consider that 60% of building emissions are due to on-site use of fossil fuels, primarily for space heating and domestic hot water. Therefore, upgrades that target these systems have a very high potential to reduce emissions.
In order for a building to reduce emissions effectively, a planned approach is required. Otherwise, a building upgrade project can end up being more expensive than necessary without delivering the required performance. Although each project is unique, the following is a general roadmap to optimize investments:
- Improving the building envelope to reduce HVAC loads. Air leaks and unwanted heat transfer are very taxing for space heating and cooling systems.
- Upgrading building systems, focusing on space heating and domestic hot water. These building systems account for the largest share of emissions in NYC buildings, due to their reliance on fuel oil and natural gas.
- Deploying renewable energy systems as allowed by site conditions, including solar photovoltaic systems, solar hot water collectors and wind turbines.
Space heating and domestic hot water account for nearly 40% of the energy used by NYC buildings. Therefore, electrification of heating systems can yield a significant reduction in emissions. Conventional electric resistance heaters have a very high operating cost, but when heat pumps can compete with fossil fuels, compensating the high price of electricity with energy efficiency.
Improving Building Envelopes
In NYC buildings, most unwanted heat transfer occurs through windows, and around the edges of window-type and through-the-wall air conditioning units. Poor wall insulation can also affect building performance.
Air leakage is one of the main issues that affects building envelope performance. It can occur between the building and it surroundings, or between building areas with different heating and cooling requirements. The two most common air sealing methods are caulking and weatherstripping:
- Caulking is applied to seal gaps in building elements that do not move. For example, it can be applied to the outer edges of window and door frames, or around the edges of electrical and plumbing fixtures.
- Weatherstripping is applied to moving building elements, where caulking would eventually fall off. It is used between doors and windows and their respective frames, for example.
Significant heat gain and loss also occur through windows, which represent another area of opportunity to improve building envelopes. Heat transfer can be minimized with double-pane or even triple-pane windows, and sunshades are also a viable measure.
High-performance windows by themselves can have a long payback period, but they can achieve great synergy with an HVAC upgrades. Window upgrades can achieve a faster payback in new constructions, where they only represent an incremental cost beyond the installation of normal windows. On the other hand, the full replacement cost must be assessed in existing properties.
South-facing sunshades are also very effective, considering that the sun’s position in the sky changes during the year. In the summer, when space heating is used, the sun is higher in the sky and gets blocked by sunshades. On the other hand, in winter the sun is lower in the sky and is not blocked by sunshades, allowing more heat gain and reducing the load on space heating systems.
Modern buildings rely on mechanical ventilation, since the need to have a closed building envelope limits the potential of natural ventilation. Since building codes require a minimum number of air changes per hour (ACH), indoor air is constantly being exhausted and replaced with outdoor air. However, exhausted air is already conditioned, while supply air is not. However, there are two ways to take advantage of the conditioned exhaust air.
- Heat-recovery ventilation (HRV) uses a heat exchanger between the supply and exhaust air stream without mixing them. Exhaust air pre-cools supply air in the summer and pre-heats it during the winter.
- Enthalpy-recovery ventilation (ERV) uses a rotary heat exchanger, which is capable of transferring both heat and moisture between both airstreams. It accomplishes the same function as HRV, but can also dry the supply air during the summer and humidify it during the winter.
For a given area, heat transfer through walls, floors and rooftops is less than heat transfer through windows. However, there is still significant potential to improve insulation in NYC. The Urban Green Council estimates that the city needs an additional 5.7 billion square feet of insulation by 2050, which can create 6,500 jobs per year.
Current wall insulation levels range from R-2 to R-4 in older buildings, and from R-8 to R-10 in newer buildings. The Urban Green Council recommends R-30 for commercial buildings and residential buildings up to two dwellings, and R-20 for larger residential buildings. Roof insulation requirements are more stringent, ranging from R-20 to R-38 in commercial buildings, and above R-38 in the small residential sector. The Urban Green Council recommends at least R-30 for the commercial sector and R-50 for residential buildings.
Electrification of Heating Systems in NYC
After the building envelope has been optimized, the conditions are ideal for replacing oil and gas-fired heating systems with heat pumps. In addition to consuming energy more efficiently, the heat pumps can be specified with lower capacity than the boilers and furnaces they replace, thanks to the HVAC load reduction achieved by improving the building envelope. Another advantage of heat pumps is their reversible operation, allowing them to replace inefficient air conditioning units as well.
In addition to their efficiency, heat pumps operate with zero emissions on-site. They can still be an indirect source of emissions if the local utility uses fossil fuels extensively, but there are phase-out plans for the state of New York. By 2030, half of all energy used in NY will be generated from renewable sources.
Heat pumps have another advantage: they can operate with electricity generated on-site with solar panels, wind turbines or other renewable generation systems. In the case of solar panels, there is also a building envelope improvement: less solar heat is gained through the rooftop, since sunlight is converted to electricity.
Mini-split air-source heat pumps are the most promising technology upgrade for residential spaces. They do not require major building alterations, and the split configuration can be used because residential ventilation requirements are less demanding - kitchen and bathroom extractors are enough in most cases. These heat pumps can simultaneously displace two types of equipment that offer poor performance:
- Window-type and through-the-wall air conditioners, which are inefficient and leaky.
- Centralized heating systems, which typically operate with natural gas or fuel.
Larger buildings can use a combination of water-source and ground-source heat pumps to achieve the highest efficiency possible.
- Water-source heat pumps serve different building areas, and they share a common hydronic piping system. Since they are reversible, it is possible to operate different units in heating and cooling mode simultaneously.
- A large ground-source heat pump serves the hydronic piping circuit, and it uses groundwater to extract or reject heat as needed. When the smaller water-source heat pumps are in different modes of operation, the GSHP must only supply the difference between heating and cooling loads.
Ground-source heat pumps are extremely efficient, achieving a coefficient of performance of 4.0 or above. In other words, their output is four times higher than their energy consumption.
Domestic Hot Water
Air-source heat pumps are a great option for domestic hot water, since they absorb heat from their surroundings when heating water, achieving a cooling effect that is beneficial during the summer, reducing the total space cooling load. Although they add to the heating load in the winter, consider that space heating is provided by heat pumps as well, and the extra summer cooling compensates for this.
DHW systems can also use heat exchangers to recover energy from the condensers of heat pumps providing space cooling, reducing the load on the units that heat water directly. Solar thermal collectors can also be used in DHW systems, but the Urban Green Council has concluded that solar photovoltaic systems are a much better use of rooftop area.
Electrification of space heating and domestic hot water systems represents a significant opportunity to reduce the emissions of NYC buildings. If heating systems that use natural gas or fuel oil are replaced with heat pumps, buildings can consume energy more efficiently while shifting to energy sources that are free from emissions.