The non-macro-circular heat exchange solar hot water system without a cover is an earlier solar hot water system in the use of solar heat. The feature of this hot water system is that it is placed horizontally on the ground in a shallow pool, with the water surface exposed. If necessary, the hot water surface is temporarily covered with a transparent floating cover, which is not always fully covered. Shallow pool solar water heaters without a cover actually existed in nature for a long time. It was discovered in the early 20th century that in non-draining shallow salt lakes in nature, when sedimentary salt accumulates at the bottom of the lake and fresh water slowly seeps into the lake surface. , Because the longitudinal direction in the lake water naturally forms a gradient layer with different salinity concentrations, that is, the salt solution with a high specific gravity is below, and the fresh water or dilute salt solution with a small specific gravity is above; at this time, when the sun penetrates the water surface, the radiant energy When it is directly transferred to the high-concentration salt solution at the bottom of the shallow salt lake, the heated high-density brine cannot pass through the convection cycle and transfer heat to the upper dilute-concentration brine with a small specific gravity, and water is a poor conductor of heat. As a result, the salt pond heat storage phenomenon in which the bottom water temperature of shallow lakes is higher than the surface water temperature is formed. Later, people imitated this phenomenon and developed a special “solar heat storage salt pond” for storing solar thermal energy. When hot water is needed, the heat energy is replaced by a heat exchange system pre-installed at the bottom of the salt pond. The solar thermal storage salt pond seems simple, but there are many actual problems: because the salt water is very corrosive and permeable, it is easy to cause corrosion and damage to the heat exchange system that has been immersed in high-concentration salt water for a long time; Salinity may pollute groundwater sources and soil; salt ponds with large areas face a series of technical management problems such as difficulty in replenishing clean water, wind disturbance, rainwater drainage and irrigation, protection of pool walls, and maintenance of salt gradients. Therefore, there have been great limitations in the promotion and application for a long time. However, the horizontally placed open non-circulating shallow pool water heater without cover is still in large-scale practical application in the United States, Australia and some European countries. Many families in these countries have solar-heated swimming pools in their courtyards. This open type without cover system generally uses three methods to collect solar heat: the first is to inlay black heat-absorbing bricks at the bottom and around the swimming pool, and directly use solar radiation heat to heat the black heat-absorbing bricks at the bottom of the swimming pool, and then pass The heat-absorbing brick transfers heat to the pool water; the second is to use a large area of black plastic heat-absorbing material, which is floated and covered on the swimming pool when no one is swimming, so that the black plastic heat-absorbing material can fully absorb the solar radiation heat to heat While pooling water, it reduces the evaporation and heat loss of swimming pool water; the third is to use the open space around the swimming pool to install flat-plate solar collectors, or dark snake-shaped water delivery coils and metal shallow pool collectors. Heat collection device: The tap water that needs to be added to the swimming pool is first passed through a direct current solar heating system composed of any of the above solar heat collectors (devices), and then the cold water is heated before flowing into the pool; usually, a circulating pump is used When there is solar radiation, the above-mentioned device is used to repeatedly heat the swimming pool water to maintain the water temperature of the swimming pool.
In the early days of China’s solar thermal utilization technology in the 1960s and 1970s, in northern cities and rural areas, a non-circulating heat exchange solar water heater was the first to appear. This is the so-called “shallow pool solar water heater.” This type of water heater is cut with a piece of galvanized iron sheet, and the four corners of the longitudinal connection seam are welded and sealed by a soldering process to form a shallow pool container with a right-angle trapezoid on the side; after the interior of the shallow pool is blacked out, it is placed in a wooden board. In a thermal insulation box made of thermal insulation and thermal insulation materials; a piece of plate glass is inlaid on the inclined surface of the upper opening of the box, which serves as an incident window for sunlight. Thus, a shallow pool type solar water heater with a simple structure is constructed, as shown in Figure 1.
Face the shallow pool window to the sun, and then pour water into the shallow pool to the depth of the upper bottom (short side) of the side trapezoid, and wait for the sunlight to enter through the glass window, and shine through the water in the shallow pool to the painted black On the metal container inside the bottom of the open shallow pool, the metal shallow pool container is heated; after the black metal shallow pool is heated, the solar radiation heat obtained from it is directly transferred to the water immersed in the shallow pool. After a period of time, the entire The water in the shallow pool is heated to a usable temperature; at this time, all the heated water in the open shallow pool solar water heater can be released for use as long as the drain valve on the drainage pipe at the bottom of the shallow pool is opened. This kind of shallow pool solar water heater has a simple structure and low cost, but the heat collection and heat preservation effect is not very good, especially there is no heat preservation measure directly above the open system, so the hot water heat preservation effect is even worse; the heat is worse after the sun goes down. It will be lost quickly and can no longer be used overnight. Therefore, it can only be applied to the summer and autumn seasons in the northern rural areas, especially for personal bathing in the dog days when the ambient temperature is relatively high.
Looking back at the shallow pool water heater today, it seems that there is not much use value; but this type of shallow pool solar water heater is the first solar light-heat conversion device developed by human society and has practical use value. In fact, in the 1970s, an industrially produced solar water heater with trapezoidal beveled side openings for shallow pools was also sealed with (galvanized or aluminum) anti-corrosion metal sheets to form a sealed box-type solar water heater. . This kind of all-enclosed all around, and the entire outer surface of the box is painted in a non-reflective gray black, it can be erected and placed obliquely to the incident direction of the sun, forming a box type that can directly use solar radiation energy to heat the metal shell System; this so-called “box-type sun-dried solar water heater” with the combination of “heat collection and storage”, as shown in Figure 2.
The fully enclosed box-type sun-dried solar water heater, because the heat-collecting metal box can directly face the sun, the conditions for collecting solar radiant energy are greatly improved; the side and back of the box that are not facing the sun can also be used. Carry out effective heat preservation treatment. Therefore, the heat collection and heat preservation effects are stronger than the open shallow pool system. The device shown in Figure 3 is a “BTR-3 double-tube dry-type solar water heater” which is similar to the box-type dry-type solar water heater developed by Beijing that year and put on the market in batches. This type of water heater uses two cylindrical galvanized iron barrels to replace the square shallow tank box to save materials and strengthen the box structure, and increase the sunlight reflection surface at the bottom of the box and the heat-preserving transparent cover on the surface; therefore, it is initially equipped Become a domestic solar water heater product that can operate independently and put it on the market.
This simple double-tube dry-type water heater can produce 63L of hot water above 40℃ per day. From the late 1970s to the early 1980s, it was once popular in the urban and rural areas of Beijing and Tianjin.
According to relevant information, in the early stages of the development of solar water heaters in Eastern and Western countries, almost all of them have jointly experienced the trial production of box-type solar water heaters with different structures. In the history of the development of solar water heaters, although this box-type sun-dried solar water heater is only a small improvement for the open shallow pool solar water heater, it is in an important position in the development history of solar water heater technology. In fact, the hot box type boring type water heater, according to the theory of heat transfer, is the heat exchange method of the solar hot water system from the initial open shallow pool to heat the box by the sun, and then the box transfers the heat to the surroundings. The water in the shallow pond began to evolve to a turning point in the evolution of thermosiphon convective circulation. It is precisely on this basis that people have further accumulated the final test, and then have the emergence of a collection and storage integrated solar heater and a direct-flow solar hot water system with higher heat conversion efficiency. Even if the so-called hot box flat plate collector is strictly speaking, it is based on the box type solar water heater, after further improvement and perfection, it has become popular in China and foreign countries, and is widely promoted worldwide. A mainstream solar thermal utilization device applied.