Solar collectors generate thermal energy constantly in the presence of solar energy. It can sometimes cause adverse conditions for components гелиосистемы. It occurs when heat selection in system is impossible or inexpedient. In such cases the solar system is included into stagnation or, in other words, stagnation process begins. This process is adverse for geliosisty, however and it is not necessary to consider it emergency. To avoid it it is almost impossible and consequently it is necessary to adhere to a number of recommendations for prevention of negative impact of process of stagnation.
Stagnation гелиосистемы occurs in summertime in combined гелиосистемах to energy selection on heating more often. It is connected with emergence of a surplus of thermal energy in the summer. Stagnation can be subject and any other solar systems at blackout or in the absence of consumption of hot water and thermal energy.
Process of stagnation arises at the moment of circulation shutdown in a contour гелиосистемы in the presence of sunlight. Thus the heat-carrier temperature increases in solar collectors to the maximum value and exceeds boiling temperature owing to what there is a liquid boiling in a collector. Owing to pressure in гелиоконтуре will sharply increase also.
The processes occurring in a solar collector at stagnation гелиосистемы
1. Liquid expansion
At a pump stop the temperature grows in a collector, the temperature of boiling of the heat-carrier will not be reached yet. Pressure thus will increase slightly, approximately on 1 bar.
2. Heat-carrier evaporation
A large amount of liquid is forced out in a broad tank because of formation in a collector of saturated steam. As a result pressure starts to increase in system quicker. Liquid with temperature close to temperature of boiling can reach components гелиоконтура, subjecting them to a temperature stress.
3. Boiling in a collector
The remained heat-carrier in a collector begins evaporation and extends energy in the form of steam on system. Thus, heating of some components of system to boiling temperature is possible at heat-carrier condensation.
4. Overheat
The heat-carrier continues to evaporate, and in a collector образовывается peregrety steam. Thus pressure and a collector slightly decreases becomes almost dry.
5. Heat-carrier filling
At reduction of solar insolation, temperature in a collector and pressure in гелиосистеме decreases. The vaporous heat-carrier is condensed and solar collectors are again filled with the liquid heat-carrier.
Steam formation in гелиосистеме can be insignificant thereof negative impact of stagnation decreases by components of solar system, especially it concerns the heat-carrier. For this purpose it is necessary that the third phase of stagnation was as it is possible more shortly. It occurs when in the second phase the heat-carrier is completely forced out from collectors and practically does not boil in them.
Ability to an oporozhneniye of solar collectors
This process is called as ability to a collector oporozhneniye. At various geliokollektorny fields and separately taken collectors this ability is various and highway laying, volume of a broad tank, a tilt angle of collectors and type of a solar collector depends on many factors, such as. At flat solar collectors, as a rule, ability to an oporozhneniye is higher, than at vacuum tubular collectors. Therefore it is very important at a design stage and installation to consider factors which would promote to improvement of ability to an oporozhneniye of Heliopolis.
The schedule of dependence of pressure in solar collectors for various solar collectors during stagnation (research)
Measures for decrease in influence of stagnation гелиосистемы
In case nevertheless it is impossible to achieve the maximum ability to an oporozhneniye of solar collectors, it is necessary to apply it is glad measures which minimize negative influence during stagnation гелиосистемы, or will interfere to its emergence:
- It is necessary to calculate correctly volume of a broad tank so that it could compensate all heat-carrier taking into account volume in solar collectors.
- At possibility to use the controlers having function of decrease in possibility of stagnation. This function works on the following algorithm: at achievement of critical temperature in a collector (stagnation close to emergence) the controler turns on the circulating pump for some seconds, thereby снижет temperature in a collector at the expense of lower temperature in a tank the accumulator. This process occurs before achievement to the greatest possible temperature in a tank. By means of this function it is possible to reduce number of stagnation considerably.
- Cooling function at night. This function will be useful at the long period of absence of selection of heat. The controler turns on the circulating pump at night, thereby reduces temperature in a tank to greatest possible. And in the afternoon again accumulates in it heat.
- Use of cooled heat exchangers for dumping of superfluous heat (for example, pools, фанкойлы , etc.)
- Energy removal from collectors through steam during stagnation. This option reduces thermal loading only on system components (on the heat-carrier influence does not decrease) already during stagnation approach гелиосистемы. At weak ability to an oporozhneniye in the collectors, the formed steam can reach some components of system and put them out of action. The circulating pump and a membrane of a broad tank especially can suffer. For prevention of hit of high temperatures on these components use special intermediate heat exchangers and intermediate tanks.
Scheme of realization of protection of a broad tank
Energy of steam and radius of its action
The specific volume and radius of distribution of the steam formed during the third phase depends on ability of an oporozhneniye of a collector field and from specific heatlosses in collectors and pipelines. Approximate values of specific capacity of steam formation for well emptied systems it is equal 50-60 W/m? while for badly emptied systems of 120-200 W/m?. Having calculated radius of effect of steam, in view of length of the highway гелиосистемы, whether we can estimate will reach pairs to vulnerable components of system. If this process is inevitable, it is necessary to pick up the appropriate measures (the intermediate heat exchanger or a tank).
The maximum radius of effect of steam in meters pays off on a formula:
Rmax = Dmax*A/Q, гре
Rmax – the maximum radius of effect of steam in m;
Dmax – the maximum capacity of steam formation in W/m;
And – the aperture area of collectors of m?;
Q–thermal losses of the pipeline in W/m.
For an example we will calculate, radius of effect of steam for installation with vacuum collectors with total area of an aperture of equal 4 m?.
Rmax = 200 * 4/25 = 32 m
Thus, if it is long pipes (in one party) to the pump module will be less, installation of a preliminary tank is necessary. For system with flat solar collectors of the similar area of an aperture the maximum radius of steam formation makes about 10 m that three times it is less than for vacuum collectors.
The volume of a preliminary tank is recommended to select not less than a half of volume of the heat-carrier in pipelines and solar collectors.
Principle of action of a preliminary tank
Systems without «Drainback» boiling
Besides all above-stated ways of fight against stagnation there are so-called not beginning to boil Drainback systems. Such system has a special intermediate tank in which at shutdown of the pump all antifreeze from collectors merges.
Efficiency of such systems is a little less in the absence of superfluous pressure, however it almost completely solves problems which can arise at stagnation the geliosisty. Usually систмы «Drainback» apply in small systems to hot water supply in houses.
