Buildings have become more energy efficient thanks to construction practices that emerged in the last few decades, writes Jahnavi Sajip @NewYorkEngineers. Older buildings were designed assuming open windows, and heating systems were oversized as a result; on the other hand, modern construction practices focus on insulation and air-tightness to reduce heating and cooling loads.
However, there is still a key challenge to overcome: space heating and hot water systems are very dependent on fossil fuels, and energy efficiency can only reduce their carbon footprint up to a certain point.
Heating systems normally rely on boilers and furnaces fired by natural gas or heating oil. These account for about 60 per cent of the environmental footprint of buildings, according to the NYC Urban Green Council.
Many buildings have access to a steam supply from Con Edison, but this does not remove the environmental impact - steam production also depends on fossil fuels, and the emissions are simply happening somewhere else.
How electrification can make heating systems greener
To remove the greenhouse gas emissions of heating systems, two steps are necessary:
1.) The first step is eliminating on-site fuel combustion. This can be accomplished by replacing oil and gas-fired equipment with electric heating equipment. Using steam does not solve the issue, since steam production is carbon-intensive.
2.) Then, the power grid must reduce its carbon footprint. A switch from combustion-based heating to electric heating only eliminates carbon emissions if the electricity comes from clean power plants.
A considerable barrier for electric heating in NYC is the high price of electricity, and many tenants pay tariffs above 20 cents/kWh. There are two complementary approaches that can make electric heating viable: increasing the efficiency of equipment, and reducing the cost of each kilowatt-hour.
Electric resistance heaters have existed for decades, but are very expensive to operate, consuming one kWh of electricity for every kWh of heat. As a result, resistance heaters can increase your power bills dramatically.
Electric heat pumps are a much more efficient option, providing two to four kWh of heat for every kWh of electricity consumed. In other words, a heat pump can serve the same load as a resistance heater while cutting power consumption by more than 50 per cent.
Visualise heat pumps as air conditioners or refrigerators operating in reverse
1.) Air conditioners and refrigerators extract heat from a closed space to keep it at a lower temperature than its surroundings. They accomplish this with repeated condensation and evaporation of a refrigerant.
2.) Heat pumps extract heat from their surroundings, delivering it to a closed space or container. They also use refrigerant condensation and evaporation, but the direction of heat flow is reversed.
In fact, some heat pumps are designed for reversible operation, to be used as air conditioners during summer. With this feature, they allow the same type of equipment to be used for three separate building systems: space heating, air conditioning and hot water. In buildings that require simultaneous heating and cooling of different areas, heat pumps can be used with a shared water loop to exchange heat between rooms.
The challenge of a heat pump upgrade can vary depending on the distribution system currently used in a building. Systems with air ducts or hydronic piping are simpler to upgrade, since heat pumps can also use air or water to distribute heat. However, steam-based systems cannot be retrofitted to use heat pumps, and the entire installation must be changed.
Adapting power systems to electrified buildings
As previously mentioned, there is little benefit in making all building systems electrical if generation continues to rely on coal, natural gas or diesel. Green heating is only achieved if the power source does not produce emissions, and some options are hydroelectricity, solar PV arrays and wind turbines.
Solar arrays and wind turbines have been criticised for their variable energy output, but this can be solved by combining them with energy storage, which has been becoming more affordable lately. Consider the case of Australia: many large companies are opting for a combination of renewable sources and storage, since it provides a lower electricity cost than the local grid, dominated by coal and gas.
Modern power grids tend to experience their highest yearly demand on hot summer days, since air conditioning equipment is mostly electrical, while heating equipment is mostly based on combustion. This behaviour will likely be reversed with building electrification, and demand peaks will occur on cold winter nights.
The power grid structure could also experience a transformation, from centralised generation at power plants, to distributed generation will smaller renewable systems and energy storage. Distributed energy systems can reduce the operating cost of power grids, reducing the transmission and distribution burden by bringing generation and consumption closer.