Solar water heating in China

A variety of solar water heaters have been produced in China for twenty years and now is at commercialization stage with a gradual increase of annual output, although its technology has to be improved. At present, there are more than a hundred of small factories and workshops manufacturing solar water heaters with a yearly output of 400,000m2(aqerture area). The total amount of installed solar water heaters with a yearly output of domestic hot water supply while a few for public bath rooms and other commercial uses in rural as well as urban area for saving conventional energy and improving hygienic condition.

 The solar water heaters developed in China could be classified into integrated and separated systems. For integrated water heaters, a solar collector and hot water tank are integrated into the same unit, simple to install, and cheaper price. So they are of interest to the domestic sectors, of which around 15% of solar water heaters produced in China pertain to integrated type. Separated systems, with collector and storage tank, can be operated either on pumped or natural circulation. In China most of them are of thermosyphon type.

 The newly formed solar hot water industry in China, which started in 1980's, receives the encouraging support from Central Government as well as the technological support from the research institutions and universities and has got positive results on improving cost-effectiveness of the products.

Early in 1958, two pilot solar water heating systems of natural circulation supplying hot water for public bath rooms were built in Tianjin University and Beijing Tiantanghe State-run Form respectively. Their operation and service experience proved that the systems did work, in spite of their imperfect design and poor performance.

 During the last decade, research and development were focused on design optimization of various kinds of solar water heaters. In early years, the collector absorbers were made of steel and in tube-in-sheet structure in which round tubing is mechanically fitted by clamps or wires leading to a bad thermal bond conductance. Based on heat  transfer analysis and thermodynamic tests in the existing panels, the processing technology has been greatly improved, for instance, the extruded fin-tubing and continuous soldering now are used in collector absorber processing. A SUNSTRIP production line mainly consisting of a rolling machine and a inflating-cutting machine was imported from Canada in 1986. It produces the aluminum fin-copper tubing SUNSTRIP absorber strip in excellent quality with high thermal bond conductance and fewer copper consumption meeting the international quality standard. Some aluminum fin-copper tubing production technology similar to SUNSTRIP of steel or aluminum absorber, copper water passages are now widely used and before popular. Plastic absorbers are still under development and the R&D work is focused on screening the materials which are able to stand ultra-violet radiation and high stagnation temperature. However, at present it seems promising to install plastic collectors without glazing in some locations of Southern China. Scientific investigation on the collector's optimized design has been carried out, and it is found that the optimal spacing between absorber and glazing is recommended as 4-6 cm where heat transfer is mainly conducted by natural convection. The performance of the collector with transparent honeycomb structure sandwiched between the spacing of absorber and glazing to suppress collector's heat loss was proved positive in the laboratory. However its commercialization still needs further efforts. Besides, a mathematical model for collector optimized design has been worked out.

 The collectors available in China's market are usually coated with black paint. As a result of computer simulation, it is indicated that for low temperature heat collection, or not for year round use, the black paint is in practice a cost-effective option. Selective paint on polished aluminum of galvanized steel reaches ¦Á=0.85~0.91,¦Å=0.23~0.40. For black chrome and black nickel, ¦Á=0.91~0.98,¦Å=0.06~0.14,but their durability and cost~ effectiveness are still not proven, while the anodized electroplating colored selective absorption coating on aluminum surface (¦Á=0.85~0.91,¦Å=0.23~0.40)has been successfully developed and serves as selective coating on SUNSTRIP absorbers.

 The National Standard for Rating the Thermal Performance of Flat Plate Collectors and National Standard for Evaluating the Quality of Solar Collector Products were published in 1984 and 1986 respectively. The National Supervision Bureau of Products Quality conducts spot check on flat plate collectors once every year based on the guidance of these Standards. The typical efficiency parameters of the flat plate collector products made in China are reported as: F'(¦Ó¦Á)=0.6~0.8 and F'UL=06.0~8.5W/m2¡æ.

 The dynamic characteristics of thermosyphoning system and the effect of various factors (e.g. climate, design parameters of the system etc.) on the system behavior were studied in connection with computer modeling as well as experimental methods for obtaining a clear picture of its operation mechanism in order to provide performance prediction for different designs and guidance for correct installation. An once through type solar water heating system operating with a thermostat was suggested with its remarkable advantage of flexible siting of the water tank at a suitable location instead of placing it well above the upper part of the collector like the thermosyphon systems do. Through theoretical and experimental study, it was indicated that the once through type and the thermosyphnic system have the same whole day performance if they operate under same conditions includeing weather, cold top water and hot water temperature level, collector and water tank structure, and ratio of collector area to water capacity). In recent years, the demonstrations of domestic solar water heating systems integrated with the multi-story apartment building have been undertaken in Beijing ,Tianjin and Lanzhou with 1 m2 collector, 70 liter water tank for each residential flat. The flow distribution in large collector system is often a significant design problem. Hydraulic modeling for such systems has been conducted and provides the useful results for collector arrangement and layout.

For producing hot water at a temperature around/above 100¡æ as well as for year round use, two types of evacuated tubular collector in flat plate pattern are under development. The R&D of all glass concentric tubular collector started in 1979. During the past decade, numerous fundamental research on its coating preparation method, glass-work technology, collector tube assembly and system design were carried out. A small scale of trial production and demonstration of this type of  evacuated collectors are under way. A China-Germany international co-operation project on heat pipe evacuated tubular collectors started in 1986, and has completed the prototype research and development, now it enters the intermediate experimental stage. However, there are still lots of work to do for practical uses as well as economic feasibility study of this kind of higher operation temperature collectors.