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Experimental and theoretical investigations have been carried out on the first large scale solar assisted ground source heat pump (SAGSHP) for heating and cooling of a village in the outskirts of Beijing. Long term performances of the system without and with solar heating were monitored. A TRNSYS model of the SAGSHP system was developed and validated against the measured data. Based on a parametrical investigation using the model, an optimized solution was proposed. It is shown that adding a domestic hot water system to the original SAGSHP system will not only help to secure thermal balance of the ground but also reduces primary energy consumption for hot water by 70%. The system COP increases by 9.4% from 2.42 to 2.65 and the annual overall operating cost decreases from 892,000 CNY to 794,000 CNY with only a small increase in equipment cost. Under the premise of ensuring thermal balance of the ground, there is an optimal match between the size of the ground heat storage, the collector area and the tank volume. Experience and lessons learned from the project are conducive to successful implementation of similar projects in the future.

Solar thermal has contributed little for space heating in China. In 2014, although China shared 75.8% of the total solar collector installations in the world, only less than 0.3% of the solar collectors were used for space heating. To promote solar district heating (SDH) in China, based on Danish experiences and Chinese clean heating transformation practices, a PEST (policy, economics, social, and technology) analysis and a SWOT (strengths, weaknesses, opportunities, and threats) analysis on SDH development in China were conducted. An extensive survey and on-site investigation were carried out to identify the applicability of SDH in rural areas. SDH development strategies, roadmap, and decision-making process for a SDH project are summarized. SDH has a broad application prospect in China with abundant solar resources and favorable policies. The solar heated floor area can achieve 756 million m2 with an assumption of 3% coverage of the total heat demand of buildings. Particular areas with low population density, scarce resources, and strict environmental requirements, e.g., Tibet, should be given a high priority for SDH. Rural villages and small towns with better infrastructure, e.g., district heating networks, are the best target market for SDH in the next five years. With the development of seasonal heat storage technologies and the accumulation of practical experience, SDH can be expanded to industrial parks, large residential communities in sparsely populated northwest China. Integration of solar heat with existing heating networks in big cities with central heating will be challenging in the long run.

Solar thermal contributes little to space heating in China. In 2014, although China shared 75.8% of the total solar collector installations in the world, only less than 0.3% of the solar collectors were used for space heating. The Chinese solar thermal market status was reviewed and a market transition trend based on a prediction model and extensive market survey was proposed. The status and prospect of solar heat for industrial processes in China was discussed. Lessons from experiences on the mandatory installation of solar water heating systems in China in terms of technology application and policy implementation were summarized. To promote solar district heating (SDH) in China, based on Danish experiences and Chinese clean heating transformation practices, an extensive survey and on-site investigation were carried out to identify the applicability of SDH in China. The potentiality and feasibility of SDH in China were explored. To find optimal economic solutions for SDH and compare the economics of SDH with household solar heating in China, two evaluation models based on the levelized cost of heat (LCoH) were proposed. A Python program was developed based on the two evaluation models to calculate the LCoH of SDH systems and household solar heating systems using the quasi-dynamic test (QDT) method. The calculation results were discussed. To explore the economic rationality and regional adaptability of solar heating technologies in different cities in China, the methods in the research on economics were used to calculate the LCoH values for nine cities in China. The LCoH curves for the target cities show that the calculation models can not only be used to judge economics of different technology combinations in a city but also can be used to judge economics of one technology combination in different cities. To fully understand the operating performance and potential optimizations of a demonstration SDH system built on the outskirts of Beijing, a TRNSY

China is the second-largest economy country, and the largest country worldwide in total solar energy system capacity installed. More than 70% of total solar collector capacity worldwide is installed in China. The production of solar collector in China had decreased significantly from 64 million m2 in 2012 to 37 million m2 in 2017, corresponding to a reduction of around 40% of the market in 2012. There are very limited comprehensive studies in existing literature, which have investigated the reasons on the decline of Chinese solar thermal market. This study aims to identify the reasons from both the demand and the supply sides in details. Furthermore, with the rise of emerging markets, such as the rapid development of the “Clean Heating Initiative” for space heating in North China, solar heat industrial process, and the mandatory installation of solar domestic hot water system for residential buildings below 100 m in urban area, the Chinese solar thermal market is expected to recover soon, called “U-shaped curve” in this study. The market share of flat plate collector and evacuated tube collector is predicted to change in the near future. The research and development of flat plate solar collectors will increase continually in the market other than evacuated tube solar collectors like the past 10 years. The main driver for the solar thermal market development in China in the near future will be the demand from the urban area.

Large solar collector fields are very popular in district heating system in Denmark, even though the solar radiation source is not favorable at high latitudes compared to many other regions. Business models for large solar heating plants in Denmark has attracted much attention worldwide. Denmark is not only the biggest country in both total installed capacities and numbers of large solar district heating plants, but also is the first and only country with commercial market-driven solar district heating plants. By the end of 2017, more than 1.3 million m2 solar district heating plants are in operation in Denmark. Furthermore, more than 70% of the large solar district heating plants worldwide are constructed in Denmark. Based on the case of Denmark, this study reviews the development of large solar district heating plants in Denmark since 2006. Success factors for Danish experiences was summarized and discussed. Novel design concepts of large solar district heating plants are also addressed to clarify the future development trend. Potential integration of large solar district heating plants with other renewable energy technologies are discussed. This paper can provide references to potential countries that want to exploit the market for solar district heating plants. Policy-makers can evaluate the advantages and disadvantages of solar district heating systems in the national energy planning level based on the know-how and experiences from Denmark.

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 inbuildings. Impediments, such as security concerns, aesthetics and functionality, make it difficult toapply SWH systems in high-rise buildings. At present, only China uses SWH systems on a large scalein such buildings. What are China’s experiences and lessons learned in applying SWH systems inhigh-rises? Are these experiences scalable to other countries? This study used a combination of fieldinvestigation, literature review and case study to summarize 36 systems that had been in operationfor 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 ofSWH systems in high-rise buildings was analyzed and verified by a case study in Shanghai. Theresults 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 hybridsystems account (14%). The average area of solar collectors per household was 2.17 m2/household,the average design solar fraction was 52%. Flat plate solar collectors (53%) was the most commonlyused collector, while electric heating elements (89%) were the most common auxiliary heat sources forSWH systems, followed by gas water heaters and air source heat pumps. The cost of SWH systemsper m2 of a building area was between 22 CNY/m2to 75 CNY/m2. China’s unique practical experiencegives a reference for other countries in their efforts to make high-rise buildings more sustainable.

To find an optimal economic solution for solar district heating (SDH) in China, an evaluation model based on the levelized cost of heat (LCoH) is developed. A Python program is developed to calculate the LCoH of SDH systems using the quasi-dynamic test method. Based on these calculations, the trend of LCoH with solar collector area under different heating load intensities, heating terminal units, heated areas and land rents is discussed. The optimal solar collector area and the solar fraction are determined for combinations of solar thermal with four types of auxiliary heat sources, including air source heat pumps, ground source heat pumps, gas boilers and gas boilers with seasonal heat storage. The calculations show an economic optimal solar fraction of 11%e33% for a SDH system with heat pumps. High dependency of LCoH on network temperature is found for a SDH system with gas boilers. Seasonal heat storage minimizes LCoH of a SDH system with gas boilers at 100% solar fraction. The findings can be used as a reference for local authorities, consultants and engineers in the early energy planning to determine the optimal proportion of solar energy in a district heating system with the lowest operating cost.

In the past decades, solar heat for industrial processes (SHIP) have been rapidly developed and applied, and also getting more attention in the world. China is still the largest energy consumer with industry accounting for almost 70% of total energy consumption. Low- and medium-temperature heat takes up 45% of process heat, holding 50%-70% of industrial energy consumption, which provides a favorable condition for solar application. China has built some demonstration projects to make industrial processes well integrated with solar heating systems. This paper briefly presents the status of China's energy consumption, integration of SHIP, as well as available solar technologies. 10 typical industrial sectors are selected to specifically describe their potential of SHIP. Moreover, 26 SHIP cases covering the 10 sectors in China are presented by field researches, with their capacity of energy saving and emissions reduction specifically investigated according to their own proportion of SHIP. The potential of SHIP in the selected 10 sectors are further predicted by optimistically and conservatively making their proportion of SHIP 5.5% and 2% respectively during the period of 2016-2020. The prediction shows that at least 39.40 million tons of coal equivalent and 98.22 million tons of CO2 emission can be reduced in 2020 in China's entire industrial sectors although solar supplies different proportions of heat demand in different industrial sectors.

Seasonal heat storages are considered essential for district heating systems because they offer flexibility for the system to integrate different fluctuating renewable energy sources. Water pit thermal storages (PTES) have been successfully implemented in solar district heating plants in Denmark. Thermal behavior of a 75,000 m3 water pit heat storage in Marstal solar heating plant was investigated experimentally and numerically. Temperatures at different levels of the water pit storage and temperatures at different depths of the ground around the storage were monitored and analyzed. A simulation model of the water pit storage is built to investigate development of temperatures in and around the storage. The calculated temperatures are compared to the monitored temperatures with an aim to validate the simulation model. Thermal stratification in the water pit heat storage and its interaction with the ground are elucidated by calculations using the validated CFD model.

Detailed measurements were carried out on a large scale solar heating plant located in southern Denmark in order to evaluate thermal performances of the plant. Based on the measurements, energy flows of the plant were evaluated. A modified Trnsys model of the Marstal solar heating plant was developed to calculate thermal performances of the plant. In the Trnsys model, three solar collector fields with a total solar collector area of 33,300 m2, a seasonal water pit heat storage of 75,000 m3, a simplified CO2 HP, a simplified ORC unit and a simplified wood chip boiler were included. The energy consumption of the district heating net was modeled by volume flow rate and given forward and return temperatures of the district heating net. Weather data from a weather station at the site of the plant were used in the calculations. The Trnsys calculated yearly thermal performance of the solar heating plant was compared to the measurement results. Validity of the Trnsys model was analyzed. Recommendations are given with aim to develop a Trnsys model that can be used to optimize design of a solar heating plant under different scenarios.

This paper analyzes the feasibility of developing solar district heating (SDH) in China from the perspective of incentive policy, selections of technical route, regional adaptability and economic feasibility for clean heating. Based on the analyzation, this proposes a road map for the development of SDH in China, and predict the market potential.