Heating of swimming pools
This article refers to swimming pools (indoor or outdoor) – both public and private. In this case, the supply of solar energy and the demand for heat ideally correspond, so that for the production of heat, simple absorbent elements are sufficient.
The simplest solar system for domestic hot water heating consists of solar panels, a pumping group, an expansion vessel, a tubular heat exchanger and solar automation. This system compares the temperature of the last solar collector and the temperature of the swimming pool and, if the difference is positive, the solar automation starts the recirculation pump. The thermal exchange is made by the tubular heat exchanger through which the water in the pool is recirculated. At this system time it is important that the recirculation of the water in the pool is started throughout the day.
Compared to the first system, in this case solar automation can also control the water recirculation pump in the basin. This way the energy consumption of the recirculation pumps can be made more efficient.
In case you want the solar panels to heat both the pool and the domestic hot water, you can opt for a system that heats both. Using a 3-way diversion valve, solar automation directs solar glycol either to the boiler coil or to the pool heat exchanger. Solar automation allows setting of heating priorities and working temperatures.
When it comes to heating outdoor swimming pools, the supply of solar radiation and demand overlap perfectly, because outdoor swimming pools usually only work during the summer semester. The heating of the water in the basin is not technically demanding: the water temperature rarely exceeds 23-26 ° C, and the small temperature differences can be perceived differently by each one. On the other hand, when the weather is bad, not only the water temperature decreases, but also the number of those who go to the pool.
Increasing the water temperature from 0 to 10 ° C can be achieved with collectors fitted in the simplest way possible: they are mounted on the roof facing south, horizontally or slightly inclined.
The heat reservoir in this case is the water in the swimming pool itself. The storage capacity of the large amount of water implies that the water temperature at night and on sunny days does not decrease significantly. If the pool is covered at night, then the heat losses from the surface may be even smaller. Since the collector is only functional during the summer semester, and during this period the frost is not discussed, the collector can be “cooled” directly with the water in the basin, so that the installation is even easier to use.
One m2 of the surface of the basin requires between 0.3 and 0.5 m2 surface of the collector. The water must pass through the collector regularly and fairly strongly (this can be done with an existing filtration pump or a separate recirculation pump) so that when the sun is shining, a temperature can be obtained 8 ° C inside the collector. Under these conditions, the pool collectors have an annual energy gain of 200 – 300 kWh / m2 on the absorbent surface. Reported at the annual radiation of approx. 1,000 kWh / m2 is therefore used between 20-30% of the solar energy. The solar installation pollutes the environment every season with approx. 70 kg of CO2 emissions (greenhouse gases) per m2 from the surface of the collector.
Compared to conventional energy calculation systems (approx. 6 – 8 ct / kWh), solar installations are, in this respect, really cost-effective.
Sizing of swimming pool heating installations
The heat required for heating an outdoor pool depends largely on the temperature you want the water to have and the temperature of the environment. The greater the temperature difference between water and air, the greater the heat requirement. Suppose that the water is desired to have a temperature of 23 ° C. In order to maintain the water at this temperature, the following thermal losses must be compensated;
- surface evaporation represents by far more than 50% of the total thermal losses; it depends a lot on the wind speed.
- the reflection of heat from the surface depends on the so-called temperature of the atmosphere which is very low on sunny days and especially on clear nights; so are the losses due to the reflection under the given conditions.
- convection on the surface of the water is equally strongly influenced by the wind; thermal losses are as great as reflection losses (about 17% of total losses).
- surface thermal conductivity occurs when the water in the basin is warmer than the air.
- the thermal conductivity towards the surrounding soil is often negligible, as the temperatures in the basin do not drop significantly, and the surrounding soil has about the same temperature.
Moreover, the basin must be continuously fed with fresh water to replace the water leaks caused by washing the filters and the water coming out of the basin. The heat requirement for heating the fresh water at the water temperature in the basin is also part of the category of thermal losses.
In principle, any administrator of a swimming pool must follow the principle of reducing the mentioned thermal losses. In addition to the optimization of the installations from a technical point of view (with a minimum of fresh water requirement), the coverage of an unused pool has many advantages, as thus the losses by evaporation, reflection and convection from the surface of the basin are significantly reduced. In principle, the thermal losses from the surface of the open basins can be reduced by up to 50%, so that the heating need for heating the water in the basin is reduced by up to twice (the lower the water temperature, the greater the savings ).
Studies of various research projects from the SO years have shown that fluctuating water temperatures in swimming pools do not in any way affect the number of swimmers or the use of an uncovered pool.
Sizing of the absorbent element
The dimensioning of the surface of the collector is realized according to the surface of the basin. The experiences of years for heating the uncovered basins with the help of solar installations have led to the following conclusion:
An uncovered basin can be heated properly when the surface of the absorbent element is 0.5-0.8 times larger than the surface of the water.
Lower values apply to protected areas (where the wind is not so strong) and where there is a lower need for fresh water, or fluctuating water temperatures in the basin and / or in the case of a basin. If a relatively high average temperature (greater than 23 ° C) is required, then a larger surface area is required. If minimum temperatures above 25 ° C are required for continuous operation, then simple absorbent elements should be checked. As the temperature rises, the yield of the absorbent element drastically decreases: for the Hannover region and a water temperature of 28 ° C, for example, an average yield (for a season) of 27% is needed, and at a temperature of 23 ° C , the yield is 45%.
The optimum degree of inclination for the May-September use period and the south-facing orientation is 20 -35 ° C. However, the energy gain of horizontal “mountings” is only insignificantly smaller, so there is no need to increase the surface area of the collector. Most of the time, horizontal mounting, for example, on flat roofs is the easiest achieved.
The energy gain of the absorbent element during a season (about 130 days between the middle of May and the middle of September) is 200 – 300 kWh / m2, as long as out-of-ordinary applications are not to be met.
The mentioned orientative values cannot be a substitute for the detailed sizing of the larger swimming pools, this being realized today, usually with the help of computer programs.
Sizing of other components
In the solar installations for heating the water in the swimming pools, the dimensioning of the re-circulation pump must be taken into consideration. Its size depends on the hydraulic ratios in the system, especially on the absorbent element. The size can be smaller as the flows are smaller. Obtaining an equal flow through the entire absorbant element is obtained with narrow and long absorbent strips than with short and wide strips.
For other details related to the hydraulic dimensioning (the determination of the flow resistance in the solar circuit is recommended to consult the specialized literature in the field of heating installations); it is recommended that more complex assemblies be made by a specialist in the field.