How to Solve the Water Mist Phenomenon of Plate Collector?

Introduction

Plate solar collector (hereinafter referred to as the "flat-plate collector") to receive the most solar radiation, the most mature technology, low cost and no need to trace the characteristics of the dispersion of strong, fit the solar energy can flow density is low and unstable characteristics, is the most warm in a large number of low human can consume one of the most effective device, its solid slab structure is easy to combine with architecture, as solar heat utilization technology and hardware support.

Countries have to specifically solar-thermal use into the "much starker choices-and graver consequences-in" development plan, 800 million square meters the goal of ownership to the industry provides a great opportunity, but in the face of industry innovation driver fatigue, stagnant technology development, especially the flat collector low price competition seriously, hard to ensure the quality of life, common thinking flat collector technology development route, seeks after the first battle, laying solid foundation of solar heat utilization technology and industry, is an urgent task.

1 Analysis of plate collector problem

1.1 Water mist phenomena of plate collector

Flat plate collector is one of the most effective products in the solar energy heat utilization industry, especially in developed countries. Its thermal theory and product evaluation methods tend to improve, product composition materials, resistance to internal high temperature and high pressure effect and heat absorption plate structure have made great progress, market share as high as 90%. However, there is a lack of resistance to rainwater infiltration, and the enclosure cavity of the collector has poor water seepage, moisture absorption and moisture drainage to vary degrees. One of the most common and intuitive representations is water mist, as showed in figures 1 and 2.

What is a water mist? Why is there water mist? How to remove water mist? From the outside to the inside, tracing the source to explore the fundamental problems of plate collector.

  

 

 

People often see water mist on the bedroom window. This is the result after indoor water vapor encounters iciness glass to condense for bead, it is the saturation temperature that vitreous temperature falls below moisture partial pressure place to correspond from mechanism (call dew point temperature) the condensation phenomenon that appears when appearing (condensation), so cold weather or morning and evening are most see. The reason is that there is water in the air of the room. Humidity, water more, less humidity little moisture, when the temperature 18 ℃, relative humidity 40%, moisture content (d) to 5 g/kg (dry air); When the relative humidity is 80%, the moisture content is 11g/kg (dry air), so the moisture content is doubled.

The same is true with a flat plate collector with water mist. Flat collector cavity in the body of water in the package, containing water of air, more is the balance of moisture (it is the material of air in contact with certain conditions can reach the limit of water content, taking wool fabric as an example, relative humidity 40%, equilibrium moisture content (X) is 11 kg / 100 kg; When the relative humidity is 80%, the equilibrium moisture (X) is 20kg/100kg, and the moisture content is doubled. The insulation material, such as fiberglass wool, is carried in by water called residual moisture.

Take a flat plate engineering machine of 2000 1000 90mm as an example for a rough calculation. When the relative humidity is approximately 40%, the air brings in 1g of water and the insulation material brings in 4g of water, totaling 5g of water. When the relative humidity is 80%, the water reaches 10g. When the sun plate collector, flat-plate collector is actually a greenhouse type dryer, the air cavity in the body state changed, balance moisture value will be lower, higher than the value of water will be released, become overheated steam, can't see it at this moment, the water vapor also vent, there will be some never closed gap left when there is no sunshine, glass cooled down, the dew point, just show it, can see water mist.

1.2 Substance of water mist in plate collector

Water mist is the most direct impression on the quality of the plate collector and also the most headache for the plate collector. In fact, the water mist is only an appearance, the essence of its existence is that there is water in the cavity of the plate collector, which will not disappear due to the evaporation of sunlight.

There are three sources by analysing this moisture:

Residual moisture: collection hot cavity in the body material (especially the thermal insulation material) with the environment humidity balance after the balance of the absorbed moisture during encapsulated into the cavity in the body, in moist areas, such as China's Guangdong, Guangxi and Jiangsu and Zhejiang area) the residual moisture content will be more, while in dry areas (such as our country northwest and Inner Mongolia) would be better.

Infiltration: infiltration of water from the outside, as of rain, especially during violent storms, by the opening of a trap.

Moisture: in the humid environment due to the difference in internal and external humidity (external humidity is large, internal humidity is small) the formation of internal material moisture absorption and the production of moisture increase, is actually to balance the humidity and the formation of the balance of moisture.

Fig.2 the plate collector array is located in Jinchuan district, Jinchang city, Gansu province. These flat plate collectors are made by a manufacturer in the south, and the residual equilibrium moisture is naturally higher.

1.3 Harm of moisture in plate collector

Weaken solar radiation: the water in the cavity evaporates and condenses into water mist on the transparent cover (glass) of the plate collector. The transmission rate will be affected and the solar radiation energy received will be weakened, which also increases the convective heat loss of the top air sandwich.

Adiabatic dampness: adiabatic material is porous material more, easy hygroscopic. Water conducts heat 25 times as well as air,  and ice conducts heat more. The hygroscopic moisture content of the material occupies part of the air filled stomatal space. The research shows that the thermal conductivity of the thermal insulation material with a thermal conductivity of 0.03W/M·K increases by 25% after absorbing 1% of water, which will be multiplied in the future.

Absorption surface decay: research shows that the absorption surface will produce different degrees of corrosion in the salt spray experiment, the ability of salt spray resistance test of different absorption surface is also different, high-quality black chromium coating can reach 120 hours, but the blue titanium coating with excellent radiation characteristics even only 6 hours; As non-professional users also have their own direct use of observation and experience. Some enterprises produce blue film balcony plate collector in Inner Mongolia inventory after a winter, that is, the absorption surface white spot. Here are some representative pictures, as showed in figure 3.

  

2.Conclusion

Water mist phenomenon and the problem are serious. This is the result of superposition of external cause and internal cause and external cause is the environment of the cloudy weather, the storm, the discretion of the temperature and humidity changes, harmful substances in the atmosphere, especially PM2.5 and fine dust, collector plate was made inside the cavity in different degree, became the object of acid and alkali salt fog hit; The internal cause should be found from the structure and material of the plate collector itself. Flat plate collector is one of the leading products in the field of solar energy.

Finally, it was found that the internal causes of water mist mainly include three aspects: excessive moisture and moisture absorption of thermal insulation materials, lax selling of products and poor exhaust effect. These problems also need industry experts and manufacturers to work together to overcome the technical difficulties, to better solve the water mist and corrosion problems of plate collectors, to produce high quality plate solar products, in order to give back to the vast number of users. 

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