Abstract
AbstractWe estimate the emissions of the two most important greenhouse gasses (GHG), carbon dioxide (CO2) and methane (CH4), from the use of modern high‐efficiency heat pump water heaters compared to the most commonly used domestic hot water systems: natural gas storage tanks, tankless natural gas demand heaters, electric resistance storage tanks, and tankless electric resistance heaters. We considered both natural gas‐powered electric plants and coal‐powered plants as the source of the electricity for the heat pumps, the thermal electric storage tanks, and the tankless electric demand heaters. The time‐integrated radiative forcing associated with using a heat pump water heater was always smaller than any other means of heating water considered in this study across all time frames including at 20 and 100 years. The estimated amount of CH4 lost during its lifecycle was the most critical factor determining the relative magnitude of the climatic impact. The greatest net climatic benefit within the 20‐year time frame was predicted to be achieved when a storage natural gas water heater (the most common system for domestic hot water in the United States) fueled by shale gas was replaced with a high efficiency heat pump water heater powered by coal‐generated electricity; the heat pump system powered by renewable electricity would have had an even greater climatic benefit, but was not explicitly modeled in this study. Our analysis provides the first assessment of the GHG footprint associated with using a heat pump water heater, which we demonstrate to be an effective and economically viable way of reducing emissions of GHGs.
Highlights
Each of the last three decades has been consecutively the warmest on record since the start of the industrial revolution, and the global temperature will continue to rise to potentially dangerous levels within 10–30 years without an immediate reduction in greenhouse gas (GHG) emissions [1, 2]
Under the base efficiency condition, the time-integrated radiative forcing due to CH4 and CO2 emissions associated with heating water with a storage natural gas water heater was greater than that with a heat pump water heater powered by electricity from coal across all time scales considered in this study, regardless of the method of radiative forcing calculation (AR3, AR4, or AR5) or implementation scenario (Fig. 2)
Our analysis indicates that if the natural gas from shale is used for heating water in homes, the accumulated radiative forcing from using a natural gas water heater can be as much as six times higher than from using a heat pump water heater within the first year of their installations, and still more than five times higher after 20 years
Summary
Each of the last three decades has been consecutively the warmest on record since the start of the industrial revolution, and the global temperature will continue to rise to potentially dangerous levels within 10–30 years without an immediate reduction in greenhouse gas (GHG) emissions [1, 2]. The two most important GHGs responsible for accelerating climate change are carbon dioxide (CO2) and methane (CH4) that are released when carbon-based fossil fuels are burned for heat and energy. The Intergovernmental Panel on Climate Change (IPCC) recognizes in its fifth assessment report (AR5) that CH4 has 120 times greater radiative forcing than CO2 on a mass basis during the time both gasses are in the atmosphere [2], updated from its previous fourth assessment report (AR4) of 100 [3] and the third report (AR3) of. The model of Shindell et al [1] indicates it is even more critical to control CH4 emissions than CO2 emissions if we are to slow the rate of global warming over the coming few decades: reducing CO2 emissions has little effect on warming over this time period due to lags in the climate system, whereas reductions in CH4 emissions have an immediate influence [1, 5]. Heat and energy for human use can be generated from different sources (e.g., electricity from coal or natural gas), and it is useful to have a tool or methodology that allows us to assess potential climate impacts of alternative choices
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