The hottest solar passive evaporative cooling tech

Solar passive evaporative cooling technology

preface

with people paying more and more attention to environmental pollution and the continuous improvement of indoor air quality requirements, many countries are actively studying and exploring cooling methods that utilize natural conditions while constantly intensifying the research and implementation of air conditioning energy conservation, improving indoor air conditions, and looking for alternative cooling sources. Theoretically, the problem of passive evaporative cooling using solar energy for the purpose of reducing the external surface temperature of building maintenance structures can be divided into two categories according to the evaporation mechanism: one is the evaporative cooling problem of free water surface, which includes water storage roof, water storage floating debris, shallow water storage, flowing water film and complex spray such as high temperature resistance, long glass fiber reinforcement, long chain nylon material, GMT and other thermoplastic composites, carbon fiber composites Fog measures for special plastics such as PEEK; The other is the evaporation cooling of porous materials

1. Class I: evaporative cooling on free water surface

the passive evaporative cooling of buildings in the early period has not been classified in detail, mainly by covering the roof with a layer of water bags to cool the buildings. There is a layer of movable heat insulation board on the water bag. During the day, the heat insulation board is covered on the water bag to prevent it from being exposed to the sun. With the water bag, the temperature of the ceiling is low, which acts as a cold source indoors. At night, the water bag is exposed to space and dissipates heat energy through convective radiation. In dry areas, evaporative cooling can be adopted in summer, and a wet cooler can be set to increase air humidity, that is, when the outdoor air is cooled and humidified by the wet cooler, the above two methods can be called the embryonic form of passive evaporative cooling

2. The second type: porous material water storage evaporative cooling problem

this kind of evaporative cooling adopts laying a layer of porous material on the building surface, such as loose sand layer or aerated concrete layer, which relies on water spraying or natural precipitation to supplement the moisture of the wet layer. When the material is wet, it is subjected to solar radiation, atmospheric convection and sky long wave radiation, and the internal moisture is transferred to the surface through the action of heat and moisture transfer mechanism, and evaporates here. The process of water evaporation in wet porous bodies is the result of the comprehensive action of many factors, such as liquid diffusion, capillary flow, evaporation and condensation, pressure gradient, gravity and so on. In the process of evaporative cooling, there is a continuous water film on the surface of the material at first. The evaporation of this water film is basically like the evaporation of the free surface mentioned above, and is not affected by the internal factors of the material. With the evaporation of this water film, the moisture content on the surface of the material decreases, and the moisture inside the material will be added to the surface of the material for evaporation. If the replenishment rate of the surface moisture is lower than the evaporation rate of the surface moisture, Then the evaporation surface of the material will drop, which will affect the evaporation effect. The passive evaporative cooling method of laying loose porous wet materials on the roof is suitable for some northern subtropical areas with abundant rainfall and low wind power, and is used on the roof plane of buildings. Solid porous materials are used on the outer surface of buildings and urban roads in the south subtropical region with abundant rainfall and high wind speed. In areas with dry climate and little rain, the porous material layer can also be replenished by spraying water

the method of laying porous wet materials on the roof first solves the problem that the water storage roof cannot be accessed. In addition, the passive cooling effect of porous wet materials is remarkable. The temperature of the building roof is reduced by about 25 ℃, equipped with a special ring stiffness point contact deformation measurement installation, and the temperature of the inner surface of the roof is reduced by about 5 ℃; It is superior to the existing traditional water storage roof

new development of passive cooling technology

so far, many scientific researchers have carried out a lot of experimental research on the traditional passive cooling technology, and made a lot of improvement work on its shortcomings on the original basis. Based on the traditional application mode of passive cooling technology, there are many improved application modes. Here are some main application modes briefly introduced

1. roof passive cooling technology

1.1 roof with movable heat insulation board

roof pool with movable heat insulation board is provided with a layer of heat insulation board on the pool. In summer, the pool is covered by heat insulation board during the day, and the heat insulation board is removed at night and the water is cooled at night. The heat of the building is transferred from the room to the surrounding environment through the roof and cooled. The roof heat can be reduced by using a roof pool with heat insulation panels. In winter, remove the heat shield during the day so that the water in the pool can absorb the solar radiation and heat the building. The pool is covered with heat insulation board at night so that the hot water in the pool can transfer its heat to the building. Roof water storage has higher requirements on roof structure and the impervious layer, otherwise the roof will leak water, and the maintenance is inconvenient, because the roof cannot be repaired directly

1.2 evaporative reflective roof

this roof is designed by covering a concrete ceiling on a pool with a rock bottom. On this basis is an air layer, which is separated from the external environment by an aluminum plate. The surface of the aluminum plate is coated with a layer of white titanium based paint to improve the reflection of daytime radiation. At night, the temperature of the aluminum plate drops below the mixing temperature of the rock bed and water. The water vapor in the roof condenses and falls by gravity. After a large number of calculations on different external temperatures and the decline of solar radiation intensity, it shows that the cooling effect of this system is very significant

in this application mode, the material with high heat capacity (rock bed) can delay the time of daytime heat entering the building, so that it enters the building at night, so that the building is less affected by it. The roof is composed of a concrete ceiling and a flat aluminum plate, which separates the air layer above the pool with the rock bed as the bottom from the outside environment. This part prevents the diffusion of water vapor to the outside world. This method has complex structure, high requirements for materials, high investment, good airtightness and inconvenient maintenance

1.3 setting air insulation layer on the roof

setting air insulation layer on the roof can reduce the heat gain of the building roof. Generally, some supports with low thermal conductivity are placed on the roof, and a layer of heat insulation board is placed on it, so an air layer is formed between the roof and the heat insulation board. This air layer plays a role of heat insulation, which can not only reduce the solar radiation heat gain of the roof through the heat insulation board, but also reduce the heat transfer from the heat insulation board to the roof through the heat insulation of the air layer, so as to reduce the indoor heat gain. Setting air insulation layer on the roof can avoid the problems of roof anti-corrosion and moisture insulation layer in the two cases of roof pool and wet materials, but this method can only play a certain role in reducing the heat gain of buildings, which is relatively simple

2. passive cooling technology of walls

with the development of passive cooling technology, in addition to the research on the passive technology of building roofs, the passive cooling technology applied to building walls has gradually attracted the attention of scientific researchers. At the same time, several passive cooling methods applied to building walls have emerged

2.1 the interior of the wall is equipped with a world-class quality interlayer that exceeds the same level

this structure forms a space layer in the wall through hollow bricks or double-layer bricks. There are space layers in the building structure, which may greatly improve the thermal resistance of the building, reduce the loss and gain of heat in the building structure, and obtain energy to maintain a suitable indoor temperature whether in winter or summer. In addition, it can also improve the comfort of users?? With the difference of winter and summer, the inner surface temperature of the wall can be increased or decreased. Using the ventilation of the space layer in the building wall instead of using the sealed wall can save a lot of energy. This method is now applied in many buildings to prevent wall condensation in cold weather conditions. Using the ventilation of the space layer in the building wall saves energy than using the sealed wall

2.2 the external surface of the wall is paved with solid porous materials

this structure is formed by laying solid porous materials on the external surface of the wall. Laying porous wet materials on the outer surface of the wall can reduce the wall temperature through the evaporative cooling effect of wet materials, and absorb a certain amount of solar radiation to enhance the evaporative cooling effect of wet materials. Due to the large area of the wall in the building structure, the application of this method can cool the building as a whole, but more attention should be paid to the water replenishment of wet materials. Laying solid porous materials is used on the outer surface of buildings and urban roads in subtropical areas with abundant rainfall and high wind speed and in South China. In areas with dry climate and little rain, porous materials can also be fully wetted by spraying, which needs further research by scientific researchers. The initial investment of this passive cooling method of laying solid wet materials on the outer surface of the wall is large

3. passive cooling technology applied to windows, balconies and glass curtain

set a simple water curtain on open spaces and balconies, set a flowing water film on the surface of the glass curtain wall on the outer surface of the building, and the air flowing through the system is cooled and humidified. If the water and air are in full contact and the air at the water outlet reaches equilibrium (saturation), the temperature of the air in the system will be close to the wet bulb temperature of the air at the outlet. The evaporative cooling system that exposes the water curtain under natural ventilation makes the appearance of the building beautiful, improves the air humidity in the building in hot and dry seasons, and improves indoor comfort. It is suitable for buildings with a large number of open spaces or glass curtains, and it is suitable to be used in combination with other cooling technologies. However, attention should be paid to the sealing of glass curtain

4. Other passive cooling technologies

in addition to the emerging passive cooling methods mentioned above, some buildings are also building underground buildings such as basements to increase the heat storage capacity of the building ground, so that the room temperature change curve is stable and the indoor temperature change range is small. However, it has little impact on the internal environment of the building, so it is suitable to be used in combination with other passive cooling methods to play an auxiliary role. In addition, plants and trees are planted around the buildings to provide shelter for the buildings. At the same time, as a model of China's high-end manufacturing industry, the share of rail transit equipment industry in the international market is also rising rapidly. It can also assist other passive cooling methods to have a certain impact on the internal environment of buildings. (end)