Application of solar energy in swimming pool water heating system

Abstract: As a renewable resource and clean energy, solar energy is more and more widely used in the heating of swimming pool water. It can be used not only for the heating of circulating water in swimming pool, but also for the heating of bathing water in swimming pool. In view of the heating of circulating water in swimming pools, the direct heating system was first adopted in China. In recent years, the development of the indirect heating system has gradually been preferred. However, with the introduction of a number of new direct heating technologies and systems abroad, more and more direct heating methods have been used for the heating of circulating water in swimming pools. This paper compares the advantages of the two heating methods, and concludes that the direct water heating method is superior to the direct water heating method. The heat system is more suitable for the heating of circulating water in swimming pool.

Key words: Swimming pool Circulating pond water Bath water Direct heating Indirect heating

As a large-scale water system, in natural conditions, especially in winter, in order to prolong the use time of the swimming pool and maintain the appropriate temperature of the pool water, it is necessary to heat the pool water. Obviously, the swimming pool is a big energy-consuming user. In the past, the heating system of swimming pool usually used boilers (such as electric boilers, oil-fired boilers or gas-fired boilers), but the operation cost was too high, and caused serious pollution to the environment. In order to solve these problems, people are increasingly inclined to use clean and renewable energy to heat swimming pools, which not only has low operating costs, but also does not cause damage to the environment. Solar energy is one of them.

The solar heating system for swimming pool can be divided into two parts: one is to use solar energy to heat the bathing water for swimmers before and after swimming; the other is to use solar energy to heat the circulating water in swimming pool to maintain the constant water temperature. With the increasing tension of conventional energy and deterioration of environment, using solar energy to heat swimming pool water has important economic and environmental benefits.

1.       Application of Solar Energy in Chinese Swimming Pools

Solar energy, as a clean and safe permanent energy, countries around the world attach great importance to the use of solar energy, in order to reduce the dependence on non-renewable primary energy such as coal, oil, natural gas and so on. The application of solar energy in swimming pools not only saves a lot of conventional energy, but also shows great economic benefits. It is of great significance to reduce carbon dioxide emissions and protect the environment.

China is located in the eastern part of the Eurasian continent in the northern hemisphere, with a vast area and abundant solar energy resources. The area with annual sunshine hours over 2200 hours accounts for more than two-thirds of the total area of the country, and has good conditions for utilizing solar energy. At present, more and more residential buildings, hotels, swimming pools, public baths and other places use solar energy as the main heat source of their hot water system.

(1). In China, solar energy was first used as a heat source to heat the swimming pool water. In the late 1980s, swimming pools of some colleges and universities in Kunming City, Yunnan Province, were heated directly by solar collectors. Since it was in the early stage of reform and opening up, all kinds of technologies were immature, so it was restricted by many factors such as technical information, product varieties and so on.

(2). At the beginning of this century, the technology of solar energy for swimming pool water heating has developed rapidly. Most of them use indirect heating to heat swimming pool water. There are successful applications in Beijing, Shanghai, Guangdong, Kunming, Zhejiang, Harbin, Shanxi, Fujian, Nanning and other provinces and cities, and their applications are becoming more and more popular.

①  Beijing No. 9 Middle School

The swimming pool is 50m *15m * (0.8-1.5m) and the water surface area is about 50m2. Atmospheric pressure oil-fired boiler is used as heat source in the new construction. Because of the high operation cost, the opening time is limited. To this end, they reformed the pool water heating system, using solar energy to heat the pool water and shower water. The solar collector is all glass vacuum tube type with water as heat transfer medium. The total area of the solar collector is 350.4m2. It is divided into four groups, each consisting of 28-29 blocks. In order to ensure uniform heat absorption, the collector group adopts equal stroke piping. There are 3 6.0m3 hot water storage tanks, which are temperature difference circulation mode. Water supply capacity at 55 C: 35 m3/d in summer and 14 m3/d in winter. The high temperature hot water prepared by solar energy is used as the heat source of the swimming pool hot water system, and the pool water and bathing water are heated by heat exchanger. According to the introduction, according to the market price calculation, the solar energy consumption is 2.2 yuan per 55 C hot water heated at 1 m3, while the oil-fired boiler is about 27 yuan, and the economic benefit is remarkable. At the same time, the opening time of swimming pool is increased, and its social benefits are obvious. Of course, in winter, oil-fired boilers are needed to supplement the pool water and bathing water to heat the heat due to insufficient solar energy. The ratio of solar collector area to swimming pool surface area adopted by Beijing No. 9 Middle School is about 0.47:1. Since it was put into use in 2004, auxiliary heat sources (urban heat network) are not available in other seasons except in heating season (November 15 of that year to March 15 of next year). During the transitional season, the water temperature of the pond can also be maintained above 26 C, which meets the application requirements.

②  Beijing Shougang Swimming Hall

There is a standard swimming pool single building with a pool water area of 1050 m2 and a vacuum tube collector. Because of the limitation of roof area, only 250 M2 collector can be set up. In order to make up for the application in spring, autumn and winter, and to avoid the environmental pollution caused by the construction of a single boiler room, after many comparisons, two auxiliary heat sources, liquid flow heat generator and heat network heat source, are used, i.e. urban heat network in heating season; insufficient heat in non-heating transition season is supplied by electricity; solar heating is mainly used in mid-April to mid-October. Since it was put into use, good economic benefits have also been achieved.

(3) In recent years, some advanced technology products, including rubber solar collector and plastic solar collector, have been introduced from abroad, which directly heat the swimming pool water, such as Kunming Military Region Swimming Training Hall.

The swimming pool size of Kunming Military Region Swimming Training Hall is 50m x 25m x (1.4-1.8) M. 312 plastic solar collectors (3.85m x 1.2m = 4.62 M2 each) with a total area of about 1350 M2 are installed on the roof. They are still in trial commissioning stage. The basic situation is that the initial pool water heating time takes about 2 to 3 days (because the auxiliary heat source has not been connected yet), the pool water temperature is about 26 degrees Celsius, and there is no air conditioning facilities in the library. Because the swimming pool water is heated directly, the temperature control is relatively simple, only set the temperature of the pool water, when the set temperature is not reached, the solar energy works. When the set temperature is reached, the solar energy stops working. In order to adapt to the situation that the pool water in solar-rich season exceeds the prescribed solar energy unnecessary opening, the collector can be connected in parallel. During this period, some collectors can be closed or switched over by pipes. The shower water can be connected to the collector which does not participate in the pool water cycle for preheating, and the solar energy can be fully utilized to achieve better energy saving effect. In winter, solar collector has no use value because of low temperature and low efficiency. This requires that all the water stored in the collector be emptied in order to prevent the pipeline from cracking due to cold ice in winter. Manual method or automatic method can be used to empty, which should be considered in design.

2. Two heating systems of solar energy in swimming pool

waterAccording to the type of heat transfer, the heating of swimming pool water by solar energy can be divided into direct heating system and indirect heating system. The direct heating system is also called primary circulation system, and the indirect heating system is called secondary circulation system.

2.1 Direct Heating System

This heating method is that the pool water is purified, heated directly by solar collector, and then sent to the swimming pool. The direct solar heating system for swimming pool water is generally composed of collector, circulating water pump, auxiliary heating equipment, vacuum suction valve, pipeline and accessories, and automatic control system. The system composition chart is as follows.

When the circulating water pump stops running, the air enters the pipeline through the vacuum suction valve, so that the water in the pipeline can flow back to the swimming pool under the action of gravity, so as to ensure that the water in the exhaust pipe is stored, so as to prevent the pipeline from being frozen out in cold winter.

Fig. 1 Direct Solar Heating Pool Water System

The working principle of the system is that the pool water is pressurized to the collector plate on the roof by circulating water pump, and the heat generated by the solar radiation from the collector plate is taken away by the flow of water, so that the water is warmed up, and then the warmed water is sent back to the swimming pool, so that the pool water is warmed up to the designed temperature repeatedly. The temperature difference between the inlet and outlet water of this type of heat transfer is only 1 ~3 C.

2.2 Indirect Heating System

This heat transfer method uses the water heated by solar collector as heat source, and then uses the high-temperature water to exchange and heat the swimming pool water through the heat exchanger, in order to prevent the metal corrosion caused by residual chlorine disinfectant. The indirect solar heating mode of swimming pool water is generally composed of collector, water storage tank, auxiliary heating equipment, control system, circulating water pump and heat collecting pipeline, etc. The system composition is shown in the following figure.

Figure 2 Indirect Solar Heating Pool Water System

Because of the secondary heat exchange, not only the thermal efficiency is lower than that of the direct heat exchange, but also the resistance and energy consumption of the heat exchanger are larger. This method is generally applicable to solar collectors made of glass with high temperature and low flow rate.

2.3 Performance comparison of two modes

Table 3 Comparison of two heating modes

From the above analysis, the indirect heating is better than the direct heating system in terms of the heating effect of the domestic hot water, but for the swimming pool water heating, the direct heating system has its special advantages:

1) Direct solar heating is to directly heat the swimming pool water in the solar collector, which belongs to one heat exchange and has high heat collecting efficiency; the indirect solar heating uses heat transfer medium and then heats the circulating pool water by heat exchanger. The method belongs to secondary heat exchange, and the heat collecting efficiency is lower than that of the direct heat exchange method.

2) The direct solar heating system has a large flow rate per cycle, and thus the temperature rise is also low. Generally, the primary circulating water temperature is only increased by 1 ° C to 3 ° C; while the indirect solar energy generates less flow per heating, and the temperature is small. The temperature rise is also high, generally up to 50 ° C ~ 60 ° C.

3) The direct solar heating system has low heat loss, generally 0.24~0.26m3/h.m2 (selected by module area), and the head loss is only 0.5m. It is especially suitable for swimming pool water constant temperature direct heating, and indirect heat loss is large. .

4) The direct solar heating system does not need to set the secondary heat transfer water tank to save the building space; while the indirect solar energy needs to set the secondary heat exchange hot water storage tank to increase the investment.

Therefore, because the swimming pool water heating feature is to supplement the swimming pool heat dissipation, the swimming pool water temperature is basically unchanged, and the swimming pool circulating water heating flow is required to be large and the heating temperature is not large, which is suitable for the heat exchange mode in which the water temperature rises slightly and the flow rate is large, thus for the heating of the swimming pool water, it is more suitable to use the direct heating heat exchange method.

However, when swimming pool water is heated directly, there are some issues to be aware of:

1) Direct solar heating Because the swimming pool water is directly heated in the solar collector, in cold regions, especially in winter, it should be ensured that the residual water in the collector pipe is removed in time to prevent freezing of the cold winter pipeline, usually the practice is to install a vacuum suction valve.

2) Since the tap water is directly heated in the solar collector, it is necessary to ensure the water quality of the solar collector. If the water quality does not meet the requirements, the blockage of the collector and the service life are reduced.

3) The first cycle generally only increases by 1 °C ~ 3 °C, so the solar radiation induction probe and the water temperature induction probe should be set in the system to realize the automatic control operation of the system to ensure the economic and reasonable operation of the system.

4) Due to the low temperature rise and high flow rate of direct solar heating, it is more suitable to use collectors of rubber or plastic materials. The heat collecting efficiency is up to 60%, which not only absorbs the heat of the sun but also absorbs the radiant heat of the 

roof panel (see the figure below).

Figure 3 plastic material collector

3.Conclusion

In summary, we can draw the following conclusions:

(1) China's solar energy resources are relatively abundant, and solar energy can be used to heat the swimming pool water in most areas to realize the utilization of renewable energy.

(2) For indirect solar heating, because it belongs to high temperature secondary heat transfer, high temperature rise, small flow, generally suitable for heating domestic hot water or swimming pool water;

(3) Direct solar heating system, one heat exchange, small heat loss, high heat collecting efficiency, large flow rate, small temperature rise, suitable for swimming pool water heating, and generally suitable for solar collectors of plastic and rubber materials. .

The application of solar energy to the heating of swimming pool water and bathing water has realized the utilization of renewable energy, and responded to the national “four sections and one environmental protection” policy, which is one of the development directions of swimming pool heating technology in the future. For the heating of swimming pool circulating pool water, direct solar heating system has strong development potential.