Energy demand for hot water supply for indoor environments: Problems and perspectives

Supply of hot water, including space heating and domestic hot water (DHW), is very important for the creation of indoor environment. Hot water is not only the most important source for space heating in winter, but also widely used for bathing, washing, swimming and so on. It is not exaggerated to say that human being cannot live without hot water. The building energy consumption contributes 30– 40% to the total energy consumption in developed countries, and about 15–25% in developing countries. 1 The energy consumption breakdown by end uses in developed countries is summarized in Table 1. 1,2 In residential sector, space heating uses 20–50% and DHW, 10–20% of the total building energy consumption. In commercial sector, space heating accounts for 15–50% and DHW, 5–10% of the total building energy consumption. The energy consumption ratio related to hot water can reach 40–60% for residential buildings and 20–60% for commercial buildings in developed countries. As for developing countries, using China as a representative, the energy consumption for heating and DHW is shown in Figure 1. 2 The energy consumption related to hot water supply in China can be categorized as: heating in northern urban areas, heating in central urban areas and DHW in urban areas. The energy consumption for heating in northern urban areas was doubled from 72 million ton of standard coal equivalent (Mtce) in 1996 to 153 Mtce in 2008, accounting for 23% of the total building energy consumption. Besides, heating in central urban areas contributes more than 2% and DHW in urban areas contributes slightly less than 2% of the total building energy consumption. As a result, the energy consumption related to hot water supply is currently about 27% of total building energy consumption in China. The energy consumption related to hot water in