Department of Earth System Science, Stanford University, Stanford, California 94305, United States.
Woods Institute for the Environment and Precourt Institute for Energy, Stanford University, Stanford, California 94305, United States.
Environ Sci Technol. 2020 May 5;54(9):5737-5745. doi: 10.1021/acs.est.9b07189. Epub 2020 Apr 15.
Methane emissions from natural gas appliances remain the least characterized portion of the fossil-fuel supply chain. Here we examine water heaters from 64 northern California homes to (1) quantify methane emissions from natural gas leaks and incomplete combustion while off, turning on or off, and in steady-state operation from 35 homes; and (2) characterize daily usage patterns over ∼1-2 months per water heater to estimate activity factors from 46 homes. Individual tankless water heaters emitted 2390 [95% CI: 2250, 2540] g CH yr on average, 0.93% [0.87%, 0.99%] of their natural gas consumed, primarily from on/off pulses. Storage water heaters emitted 1400 [1240, 1560] g CH yr on average, 0.39% [0.34%, 0.43%] of their natural gas consumption. Despite higher methane emissions, tankless water heaters generate 29% less COe than storage water heaters because they use less energy to heat a unit of water. Scaling our measured emissions by the number of storage and tankless water heaters in the United States (56.8 and 1.2 million, respectively), water heaters overall emitted an estimated 82.3 [73.2, 91.5] Gg CH yr, 0.40% [0.35%, 0.44%] of all natural gas consumed by these appliances, comparable in percentage to the EPA's estimate of methane emissions from upstream natural gas production.
天然气器具的甲烷排放仍然是化石燃料供应链中特征最少的部分。在这里，我们研究了来自加利福尼亚州北部 64 户家庭的热水器，（1）量化了在关闭、打开或关闭以及稳定运行状态下，来自 35 户家庭的天然气泄漏和不完全燃烧的甲烷排放；（2）对每个热水器约 1-2 个月的日常使用模式进行了特征描述，以从 46 户家庭估计活动因素。单个即热式热水器平均排放 2390 [95%置信区间：2250，2540]克 CH 年，占其天然气消耗量的 0.93% [0.87%，0.99%]，主要来自于开/关脉冲。储水式热水器平均排放 1400 [1240，1560]克 CH 年，占其天然气消耗量的 0.39% [0.34%，0.43%]。尽管即热式热水器的甲烷排放量更高，但由于其加热单位水量所需的能源较少，因此产生的 COe 比储水式热水器少 29%。根据美国储水式和即热式热水器的数量（分别为 568 万台和 120 万台）对我们测量的排放量进行缩放，热水器的总排放量估计为 823 [732，915]Gg CH 年，占这些器具消耗的所有天然气的 0.40% [0.35%，0.44%]，与 EPA 估计的上游天然气生产过程中的甲烷排放量相当。
