DHW Heating and Steam for Functional Buildings

Abstract

According to the China Population and Employment Statistical Yearbook, China’s urban population in 2020 was approximately 900 million. In the practical project case study, the DHW consumption was about 20–40 L per person per day. By taking the DHW consumption at 20 L per person per day, the temperature after water mixing at about 40 °C, and the tap water temperature at 15 °C, the total heat demand of DHW for urban residential buildings was estimated to be about 690 million GJ.

5.1 DHW Heating

5.1.1 Analysis of DHW Demand

(1) DHW demand of residential buildings

According to the China Population and Employment Statistical Yearbook, China’s urban population in 2020 was approximately 900 million. In the practical project case study, the DHW consumption was about 20–40 L per person per day. By taking the DHW consumption at 20 L per person per day, the temperature after water mixing at about 40 °C, and the tap water temperature at 15 °C, the total heat demand of DHW for urban residential buildings was estimated to be about 690 million GJ (Fig. 5.1).

Fig. 5.1

DHW demand of residential buildings in China during 2013–2020

(2) DHW demand of schools

According to the Educational Statistics Yearbook of China, China had 537,100 schools of all levels and types, 289 million students, and 17.9218 million full-time teachers in 2020. In recent years, the Ministry of Education has stopped publishing the number of boarders, so the numbers of teachers and students boarding in schools were estimated based on the dormitory area. The total area of relocation dormitories for teachers in primary, junior middle, and senior high schools were about 59 million m², and the total area of students’ dormitories in primary, junior middle, common senior high, and secondary vocational schools as well as higher education institutions was about 63.66 million m². Based on the per capita area of relocation dormitories for teachers taken at 35 m² and the per capita area of students’ dormitories taken at 8 m², the number of teachers boarding in schools was estimated to be about 1.69 million, accounting for about 9.4% of the total number of teachers, and the number of students boarding in schools was 79.57 million, making up about 27.5% of the total number of students. By taking DHW consumption at 35 L per person per day, the total heat demand of DHW for dormitories in schools was estimated to be about 89 million GJ.

(3) DHW demand of hospitals

According to the Statistical Bulletin on Health Development in China 2021, China had 7.413 million hospital beds, the rate of utilization of which was 72.3%, and 8.478 million health workers in 2020. By taking the DHW consumption of patients at 70 L per bed per day and that of the medical staff at 65 L per person per day, the total heat demand of DHW for hospitals was estimated to be about 34 million GJ.

(4) DHW demand of hotels

According to the statistical data of the National Bureau of Statistics, China had about 4.5 million hotel rooms and 7.15 million hotel beds in 2020. Based on the Statistical Report on Star-rated Hotels in China, the average hotel occupancy rate was 55% during 2013–2019 and fell to 39% in 2020 due to the COVID-19 pandemic. By taking DHW consumption at 110 L per person per day, the total heat demand of DHW for hotels was estimated to be about 12 million GJ.

The DHW demand of all of the above-mentioned types of buildings in 2020 is collected, as shown in Table 5.1. The total DHW demand of these types of buildings was about 830 million GJ.

Table 5.1 Estimates of heat demand of DHW

5.1.2 Present Situation of DHW Heating

Urban DHW is classified into residential DHW and P&C DHW by the building type and into centralized heating and decentralized heating by the heating method.

(1) Present situation of DHW heating for residential buildings

Residential DHW is classified into centralized DHW and decentralized DHW by the heating method. At present, most of the residential buildings in China use the decentralized DHW system, and only a small proportion of them use the centralized DHW system.

The main heat sources of the centralized DHW system are coal-fired boilers, gas-fired boilers, and heat pumps. When the centralized DHW supply system is used, the average daily hot water consumption of residents is 45–60 L per person per day. The effective heat utilization rate of the centralized DHW system is lower than that of the decentralized DHW system mainly due to the huge heat dissipation in the transmission pipe network, especially the heat dissipation in the secondary pipe network, which accounts for 35–56% of the total energy consumption. The higher the average daily water consumption of households is, the higher the effective heat utilization rate will be. When the average daily water consumption of households is high, the centralized DHW supply system has certain advantages. When the average daily water consumption of households is low, the decentralized DHW supply system is obviously economical and energy-saving.

The main heat sources of the decentralized DHW system are electric water heaters, gas water heaters, and solar water heaters. When the decentralized DHW system is used, the average daily hot water consumption of residents is 30–40 L per person per day. According to the data from the National Bureau of Statistics, the number of water heaters owned by every 100 households in China was 90.4 units in 2020, and the number of water heaters owned by every 100 urban households was 100.7 units, as shown in Fig. 5.2. The percentages of various water heaters in China are illustrated in Fig. 5.3. At present, electric water heaters are used for most decentralized DHW systems, accounting for 59% of the total number of water heaters, and followed by gas water heaters, accounting for 36% of the total.

Fig. 5.2

Number of water heaters owned by urban households

Fig. 5.3

Percentages of various water heaters owned by households in China

(2) Present situation of DHW heating for school buildings

DHW in schools is mainly supplied as bathing water and potable for students, and in some schools, DHW also needs to be supplied to the canteen and other buildings. There are three main methods of supply: centralized hot water supply, local centralized hot water supply, and decentralized hot water supply.

According to statistics, the per capita energy consumption and water consumption of undergraduates are four times and two times those of the national residents respectively, so the DHW systems of schools have great potential for energy saving. At present, split electric water heaters or air source heat pumps are mostly used to supply hot water in southern China. In northern China, most schools use gas-fired boilers to supply heat, coal-fired boilers have been phased out due to serious pollution, and some schools recycle the residual heat resources, including the residual heat from the cooling of generator units, as the heat sources of hot water. In addition, air source heat pumps and solar water heaters with low pollution and good energy-saving effects have been developed rapidly and used to supply DHW in many schools.

(3) Present situation of DHW heating for hospital buildings

DHW in hospitals are mostly supplied by gas-fired or oil-fired boilers, which use natural gas as the major form of energy; besides, as hospitals have steam for disinfection, laundry, and other special functions, centralized steam boilers are used as the main method of heat supply. A small number of hospitals also use municipal hot water or use the indirect heat transfer of municipal steam to produce hot water. Some large- and medium-sized hospitals also use solar water heaters and other renewable energy to supply DHW to reduce fuel consumption and operation cost.

The DHW heating methods of a total of 21 grade A tertiary hospitals from four regions in China were surveyed, and the DHW supply methods of hospitals by zone are shown in Fig. 5.4. In severe cold and cold areas of China, the climate is cold, so the main heating methods for DHW in hospitals are boilers, lithium bromide direct-fired units, and electric water heaters as well as a small number of solar water heating systems, and renewable energy is less utilized. In the HSCW zone of China, the percentage of renewable energy for heating in hospitals increases evidently, in addition to boilers and lithium bromide direct-fired units. In the hot-summer and warm-winter (HSWW) zone of China, the climate is warmer, and solar energy resources are abundant, so the percentage of utilization of renewable energy (such as solar energy, water source, and air source) further increases compared to that in the HSCW zone, and various heating methods are almost uniformly distributed.

Fig. 5.4

Comparison of DHW supply methods of hospitals by zone

(4) Present situation of DHW heating for hotel buildings

DHW is mostly supplied 24 h a day in hotels. Currently, large and high-star-rated hotels mostly use gas-fired boilers as the main heat source and install air-source heat pumps or solar water heaters as the auxiliary heat source to preheat DHW. Some hotels use normal-pressure hot water boilers and heat exchangers, which are relatively common heating methods at present. Because hotel laundries need steam, some hotels directly use a set of centralized gas-fired steam boiler, which generates DHW through the steam-water heat exchanger. A few hotels use the municipal heating network to produce DHW in the heating season and use self-contained boilers in the non-heating season. The vacuum hot water boiler with a built-in heat exchanger, which can directly produce hot water at a required temperature, is a new trend in the field of hotel heating.

5.2 Supply–Demand Analysis of Steam for Hospitals

5.2.1 Steam Demand of Hospitals

By the end of 2020, the total floor area of China’s hospital buildings was 760 million m², accounting for about 5.4% of the newly built building stock of P&C buildings in China. Due to the special medical service attribute of hospitals, the main heat use sub-categories include the heating system, the DHW system, the steam system of the sterile supply room, the laundry, and the canteen’s steam or hot water system. The heating demand of the heating system and the DHW system have been analyzed in the above subsection, and this subsection is mainly to analyze the steam heat consumed for special functions (disinfection, laundry, and cooking) and accordingly estimate the steam heat consumed for special functions in hospitals in China.

The steam heat consumed by the sterile supply room, the laundry, and the nutrition department is mainly related to the number of patients taking surgery, the occupied beds (number of beds $\times$ sickbed occupancy rate), and the number of people using energy (number of people using energy $=$ daily number of visits $+$ number of beds $\times$ sickbed occupancy rate $+$ number of health workers) respectively. Based on the number of occupied sickbeds, the daily average number of visits, the number of health workers, and the number of inpatients taking surgery in China in 2020 as given in the China Health Statistical Yearbook, it is calculated that the total steam consumption for special functions in hospitals is 22,502,200 GJ and that the gas consumption is about 658 million Nm³.

5.2.2 Present Situation of Steam Supply in Hospitals

At present, the steam for special functions in hospitals is supplied by steam boilers for the most part and is municipal steam for a small part. Overall, the solution to heating systems in many hospitals is to install a set of centralized steam boilers to meet the heating demands of all grades. As a result, the overall heating efficiency is low.

The heating efficiency of steam boilers varies greatly in different hospitals, which is mainly related to the use of steam, in addition to being subject to different degrees of influence from routine maintenance including steam pipe insulation. Here, hospitals are classified into four categories based on the use of steam boilers. Category I: hospitals in which steam boilers are only used for the sterile supply room; Category II: hospitals in which steam boilers are not just used for the sterile supply room but are not used for DHW and heating; Category III: hospitals in which the use of steam boilers involves DHW but not heating; Category IV: hospitals in which the use of steam boilers involves both DHW and heating. The heat use conditions of sub-categories in all categories of hospitals are listed in the table below. As can be known from Table 5.2, Category I hospitals have the least heat loss in the use of steam, while Category III and IV hospitals directly exchange steam for hot water for DHW and heating, which is essentially a waste of heat source grade and should be avoided as far as possible.

Table 5.2 Percentages of heat use of sub-categories in hospitals by zone

5.3 Supply–Demand Analysis of Steam for Hotels

5.3.1 Steam Demand of Hotels

By the end of 2020, the newly built building stock of hotel buildings in China was 623 million m², accounting for about 4.4% of the newly built building stock of P&C buildings in China. The steam for hotels is mainly concentrated in the laundry. The main equipment consuming steam in the laundry includes ironing machines, dryers, and water extractors. According to the survey data, the annual steam consumption per unit occupied bed in hotels is 7,033 kg/(occupied bed a). In 2020, the total number of beds in star-rated hotels in China was 7.19 million, and due to the COVID-19 pandemic, the average occupancy rate was only 39%; thus, it could be estimated that the steam consumption of hotel laundry in hotels in China was 19.722 million tons. Based on 0.7 MPa saturated steam and temperature of tap water at 20 °C, it is calculated that the actual steam heat consumption is 52,817,900 GJ and the gas consumption is about 1.544 billion Nm³.

5.3.2 Present Situation of Steam Supply in Hotels

Currently, the steam for hotels is mostly supplied by steam boilers. A small number of hotels in northern China use municipal steam, and heat pumps are rarely used to supply steam in hotels.

Take a five-star hotel in the Pudong New Area of Shanghai for example. Three sets of 7 t/h gas steam boilers are used to supply heat for heating, DHW, and laundry equipment. Since steam boilers are used as the heat source for the DHW system of the hotel, steam is also used for heat transfer to make 50–55 °C DHW, causing a serious waste of grade. According to relevant estimates, the heating efficiency of steam boilers is only about 70%. Considering the natural gas price and the heat loss in heat exchangers and transmission pipelines, the heating cost is approximately RMB 0.63 per kWh. If steam boilers are replaced with heat pumps, the heating cost is about RMB 0.28 per kWh, far below the boilers’ heating cost, thus the DHW cost can be reduced significantly.

In view of the problems of the current centralized steam boilers in hospital and hotel buildings such as poor steam quality, much condensate water, being unfavorable to disinfection and sterilization, and low heating efficiency, it is recommended that decentralized heating systems should be adopted for the sub-categories in the future. The specific recommendations are as follows:

• 1. Hot water boilers may replace steam to supply heat for DHW and heating, and abundant renewable energy may be used for heating according to the local conditions, such as air-source heat pumps, water-source heat pumps, solar energy, etc.

• 2. Decentralized heating systems may be used for the sub-categories. For sterile supply rooms, it is recommended that small-capacity gas steam boilers (steam generators), heat pump-type steam generators, etc. should be installed nearby to start the boilers for the preparation of steam at any time. For laundries, it is recommended that steam generators should be installed nearby or special laundry equipment with electric drying should be used. For kitchens, special steam boilers should be configured separately.

• 3. Since the steam condensate is generally at a high temperature, it is recommended that heating recovery units should be used for the recovery.