Solar Water Heating Systems Applied in High-rise Residential Buildings in China
High-rise buildings have a significant impact on the surrounding environment. Building-integrated solar water heating (SWH) systems are effective ways to use renewable energy in buildings. Impediments, such as security concerns, aesthetics and functionality, make it difficult to apply SWH systems in high-rise buildings. At present, only China uses SWH systems on a large scale in such buildings. What are China's experiences and lessons learned in applying SWH systems in high-rises? Are these
... ises? Are these experiences scalable to other countries? This study used a combination of field investigation, literature review and case study to summarize 36 systems that had been in operation for 1-14 years. System types, collector types, installation methods, types of auxiliary heat sources, economic performance and various basic principles were summarized. The economic performance of SWH systems in high-rise buildings was analyzed and verified by a case study in Shanghai. The results show that the installation of SWH systems in high-rise buildings is feasible and reliable. Individual household systems (61%) were more popular than centralized systems (25%) and hybrid systems account (14%). The average area of solar collectors per household was 2.17 m 2 /household, the average design solar fraction was 52%. Flat plate solar collectors (53%) was the most commonly used collector, while electric heating elements (89%) were the most common auxiliary heat sources for SWH systems, followed by gas water heaters and air source heat pumps. The cost of SWH systems per m 2 of a building area was between 22 CNY/m 2 to 75 CNY/m 2 . China's unique practical experience gives a reference for other countries in their efforts to make high-rise buildings more sustainable. Energies 2019, 12, 3078 2 of 26 government  . Since 2007, in order to increase energy efficiency in buildings and promote the application of renewable energy therein, under the guidance of national and local building energy conservation codes, 23 provinces and three municipalities issued mandatory installation policies for SWH systems in buildings with less than 12 floors (some provinces and municipalities required six floors). The implementation of this policy resulted in dramatic changes, with the market share of SWH in the urban domestic hot water market reaching 24%  over the past ten years. Increases in urbanization and technological advancement have led to buildings of ever-increasing height. High-rise buildings have a significant impact on the surrounding environment  , and their shadows significantly reduce the possibility of applying solar energy for surrounding low-rise buildings. However, high-rise buildings can accrue substantial benefits from exposure to more solar radiation. Does this mean that SWH systems are feasible in high-rise buildings? Is this especially true in terms of technical and economic performance? Over the past few years, there has been a plethora of research and reviews on low-rise buildings and building-integrated solar thermal (BIST) systems for small buildings, especially low rises [9,        . For buildings with a height of 35-100 meters or 12-33 floors, previous studies focused on policy recommendations    , or case studies [40, 41] , but there has been no comprehensive systematic summary of the safety, economic performance, aesthetics and technical solutions of SWH systems. By utilizing an investigation of existing high-rise buildings using SWH systems in China, the experience and lessons learned from SWH system application in high-rise buildings will be summarized in this study. The technical solutions and economic performance of SWH systems will be systematically analyzed with a case study in Shanghai. Solar hot water has great potential in urban buildings  . The technical recommendations based on the research presented in this paper will not only help improve the energy efficiency of high-rise buildings and reduce the energy cost for urban residents but also help governments around the world to achieve renewable energy application goals, promote technological innovations, increase employment and improve people's livelihoods in developing countries. Methodology A literature review, deep interviews, a questionnaire survey and field investigation were carried out in this study. From 2016 to 2018, government officials, real estate developers, designers, solar water heater manufacturers, property management personnel and users were interviewed. A total of 36 SWH systems in high-rise buildings were investigated on-site; 200 questionnaires were distributed to the stakeholders and the valid and effective samples numbered 156. There were 26 questions in the questionnaire that covered the whole process of design, construction, commissioning, operation and maintenance of solar hot water systems in high-rise buildings, including solar fraction determination, hot water usage, system types, collector types, installation method and common malfunction. The systems were randomly selected from 21 cities nationwide, from Beijing in the north to Shenzhen in the south, as shown in Figure 1 . The 36 SWH systems in high-rises were built from 2005 to 2018 with a height from 35 to 100 meters. The average height of the buildings is 62 meters with 21 floors; Figure 2 shows the distribution of the height of the buildings by years; the dotted line in Figure 2 shows that there is a trend toward taller buildings over the past 14 years.