Space heating is one of the highest energy expenditures in buildings, and combustion-based heating, in particular, is an important source of emissions. Based on data from thousands of buildings, the New York Urban Green Council concluded that 27% of building energy consumption can be attributed to space heating. Given that hot water systems represent 10% of energy consumption, heating in general represents 37% of total consumption.
Heat pumps are emerging as a viable alternative to combustion heaters. The most efficient models can match the cost of gas heating while being 100% electric and producing no direct emissions. Heat pumps still produce external emissions if the local grid relies on fossil fuels, but this will gradually change as renewable sources become more widespread.
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How efficient are heat pumps?
Heat pumps can be classified based on how they exchange heat with the outside of the building. Air source heat pumps (ASHP) extract thermal energy from outside air, while ground source heat pumps (GSHP) obtain it underground. The efficiency of heat pumps is described by their coefficient of performance, or COP.
- Aerothermal heat pumps typically have a coefficient of performance above 2, with some units as high as 3. This means they provide 2-3 kWh of heat for every kWh of electricity consumed. However, its efficiency drops significantly in very cold weather.
- Geothermal heat pumps are more efficient since underground temperatures are more stable throughout the year. COP values greater than 4 are typical, meaning these heat pumps provide more than 4 kWh of heat for every kWh of electricity.
Heat pumps are more energy efficient than gas heaters, but they also use more expensive energy. Air source heat pumps can beat the operating cost of gas heaters in places with cheap electricity. However, with high electric rates in New York and other Northeastern states, only geothermal heat pumps can compete with the cost of gas heaters.
Both types of heat pumps can achieve synergy with renewable energy systems such as solar panels and wind turbines. Any surplus electricity that is not consumed by the building can be used to heat water, which is then stored in an insulated tank for later use. In other words, a heat pump can function as part of a thermal energy storage system. Although this concept is also possible with resistance heaters, the energy storage efficiency is much lower.
Installation of heat pumps in buildings
When installing geothermal heat pumps, the most challenging task is burying the pipes that circulate water underground to extract heat. The project becomes much simpler in new constructions, as the underground water network can be installed when the project is still in the excavation phase. The task is more difficult in existing buildings, since underground piping must be buried without disturbing the structure. Installation becomes much simpler if a building has green areas without construction.
Heat pumps are energy efficient and environmentally friendly, but they bring a fundamental technical challenge to the electrical grid. As more buildings switch from gas to electric heating, the load profile on the local network will change:
- Energy networks currently experience peaks in demand on the hottest summer days. Buildings must use their air conditioning units at full capacity to maintain the appropriate temperature, and all this consumption adds up.
- A large number of heat pumps would likely shift peak load to the winter months and grid operators would have to deal with two significant peaks per year.
Thermal energy storage can mitigate energy demand from heat pumps. They can be programmed to heat and store water during off-peak times, avoiding a large coinciding load when the network is overloaded.
Conclusion
Greater use of heat pumps would reduce dependence on gas distribution networks. Because gas pipes are buried, it is difficult to upgrade and maintain them. Expanding the energy network to supply electricity to heat pumps is much simpler than expanding the gas network to new building connections.
In the specific case of New York, heat pumps can help meet the Green New Deal's emissions reduction goal. Space heating and water heating account for almost 40% of buildings' energy consumption, and combustion heating is currently more common than electric heating. Switching to heat pumps can achieve a significant reduction in greenhouse gas emissions from buildings.
