With the outbreak of the first oil crisis in the 1970s, most American and European governments and organizations began to promote the application of solar energy. In the early days, the main focus was the utilization of passive solar energy and solar thermal utilization (heating and domestic hot water). At present, it has gradually developed into the fields of solar cooling and photovoltaic utilization. The related technologies of solar water heating systems in Europe and other countries have developed to a fairly mature stage, and various types of solar water heating systems have been developed to meet different needs, with complete national standard systems and specifications.

After 30 years of development of solar water heating systems (heaters) in China, the form of collectors has changed from “smoothing type” to “flat plate type” to “vacuum tube type”; applications range from rural to urban areas;

The installation scale ranges from small solar water heaters to large-scale solar water heating systems; the installation methods range from individual household installations to large-scale engineering installations. A number of national standards and industry standards have been formulated and implemented, ranking first in the world in terms of installation and use. However, there is still a certain gap in product quality, related technology and international advanced level, especially the integration and integration technology with construction is still in its infancy.

According to the statistical data and related materials of the IEA, we have summarized and compared the development status of domestic and foreign countries in terms of the number of solar water heating systems installed, system scale, main uses,collector type, system operation mode, technical research, integrated design research, and government encouragement and promotion policies.

1. Number of installations

According to the statistics of the IEA as of the end of 2007, the total installed area (in use) of solar collectors worldwide has reached 209 million m² (146.8GW_th), which translates into an installed capacity of 146.8GW_th. China is the country with the largest total installed capacity of solar collectors in the world, with a total installed capacity (flat plate collectors and evacuated tube collectors) reaching 79.9GW_th. However, in terms of installed capacity per thousand people, the top five places are Cyprus (651kW_th), Israel (499kW_th), Australia (252kW_th), Greece (224kW_th) and Barbados (197kW_th). China is only ranked 9th.

2. System scale

According to the installation area of collectors, Europe divides solar water heating systems into four scales: <30m², 30~50m², 50~100m² and >100m², larger than 300m² is a large-scale system. Europe is mostly large-scale projects of 30-100m². For example, in residential districts, large-scale solar heating systems are connected to the community heating network.

In China, small independent household solar water heaters or super large projects are mainly used, and medium-scale systems (such as residential solar water heating systems) are still less widely used.

3. Main purpose

Solar water heating systems are mainly used in domestic hot water production, heating and regional solar water heating plants in Europe; in North America, they are mainly used for swimming pool water heating and hot water for laundry. The use of solar water heating systems in China is relatively single, mainly for domestic hot water production, and a few are used for heating and swimming pool water heating.

4. Types of solar collectors

In foreign countries, flat plate collectors, coverless collectors and evacuated tube collectors are the main products. Flat-plate collectors are widely used in most countries and regions such as Europe. Coverless collectors in the United States, Canada, Australia and other countries dominate.

China’s solar collectors are divided into flat plate type and vacuum tube type (all glass vacuum tube and glass-metal vacuum tube). All-glass evacuated tube collectors account for more than 85%, and collectors without cover are rarely used. Figure 1 shows the proportion of different solar collectors used in major countries in the world in 2007 in the IEA statistical report.

5. System operation mode

There are three main operating modes of solar water heating systems:

(1) Natural circulation, direct heat exchange, compact solar water heaters: mainly used in low-latitude frost-free areas, such as southern Europe, Israel, Australia and other countries, with low prices and easy installation.

(2) Forced circulation, secondary heat exchange, split solar water heating system: suitable for use in areas with frost problems, such as Northern Europe. This mode of operation is easy to integrate with buildings, guarantees water quality and water pressure balance, is efficient and beautiful, and is widely used abroad.

(3) Centralized solar heating and hot water supply system: connect the large-scale solar heating system with the community heating network, and provide domestic hot water and building heating at the same time.

In China, the majority are distributed, natural circulation, and direct heat exchange solar water heating systems (devices), and there are few large-scale water supply methods such as centralized and centralized-decentralized water supply.