Analysis of performance optimization strategies for doors and windows①

1.The importance of developing energy-saving doors and windows

Since it was mentioned by western developed countries, building energy efficiency has gone through nearly 40 years. Some developing countries insist on independent research and development while taking into account the international advanced development ideas, and have made considerable progress up to now. Building energy-saving technologies and energy-saving industries are flourishing and a hundred schools of thought contend.

However, we have to deeply realize that the cause of building energy conservation still has a long way to go and is full of challenges.

From the following considerations, we can see the necessity of the development of energy-saving doors and windows;

1.1 The key loss path of building energy consumption

The thermal insulation performance of doors and windows has a vital impact on the energy loss of the entire building. It is the key way of real building energy loss. It is also the primary solution for our building energy-saving design. Building energy-saving needs to start with doors and windows.

1.2 The main problems faced by door and window applications

Actually, we have to face the negative effects caused by the poor quality of doors and windows; due to the poor thermal insulation performance of doors and windows, the heat transfer coefficient is too high, causing condensation and water seepage on the surface of doors and windows, as well as mildew, corrosion and other problems, which will be brought over time The profile parts of doors and windows are damaged, and are prone to bacterial growth and appearance corrosion, which will increase maintenance costs and reduce service life; therefore, the insulation performance of doors and windows not only has an important impact on reducing energy consumption, but also seriously affects the quality of building life.

Therefore, no matter from the perspective of the current use of doors and windows, national energy-saving policies and regulations, or from the perspective of building energy consumption analysis, the development of energy-saving doors and windows and optimizing the comprehensive performance of door and window systems are in line with the national energy-saving development strategy requirements, and the energy-saving doors and windows industry is bound to enter a booming development. Fast lane.

2. Economic issues in the development of energy-saving doors and windows

With the popularization and application of energy-saving doors and windows, it will gradually have a profound impact on our daily lives; energy-saving doors and windows will gradually develop from niche products in the early development period into popular and standardized products; the economics of energy-saving doors and windows will determine its development Speed and popularization space; the development of energy-saving doors and windows requires a certain capital cost. In order to obtain better energy-saving characteristics, new materials with innovative technologies and innovative processes must be used, which will bring a certain increase in costs, but in the long-term benefits From a point of view, it is obvious that it has long-term benefits such as low input and high output, realizing the sustainable development of buildings; doors and windows, as key breakthroughs in building energy conservation, often bring relatively large cost increases. As long as we break away from the local short-term ideological fetters, pay attention to the building environment, look at the macro, long-term, and sustainable economic benefits, it is not difficult to find that the development of building energy-saving doors and windows has the characteristics of low input and high output; building cost 5 %-10% of energy-saving costs to achieve 30%-75% of energy-saving benefits, the residential room temperature in winter is increased by more than 10 degrees, will obtain a very ideal investment payback period, so as to achieve the optimal development of building energy conservation. For some countries in Western Europe and Northern Europe, the development of high-comfort and low-energy buildings is relatively early. The cost of energy-saving doors and windows is only about 3%-8% higher than the cost of ordinary doors and windows, but energy-saving ratios of more than 65%-90% can be achieved. The benefits are very significant.

Therefore, the development of energy-saving doors and windows will pay more and more attention to the application and development of new materials and new technologies; constantly seeking new breakthroughs, seeking small materials, big profits, and small development space.

3. Analysis of ways to optimize the performance of energy-saving doors and windows

Doors and windows, as the openings in the external maintenance structure of the building, become a bridge between the inside and outside of the building; people need to form a good communication with the nature through the doors and windows, and at the same time, they must ensure that they are not disturbed by the outside; therefore, the doors and windows should meet these basic designs Requirements, including basic use characteristics such as good lighting, ventilation, heat insulation, heat preservation, sound insulation, safety, and transparency; at the same time, from the perspective of the reliability of doors and windows, they must also have sufficient air-tightness, water-tightness and resistance to wind pressure From the perspective of safety in use, they need to have the effects of fire prevention, explosion prevention, anti-theft, anti-harmful light, shielding, and privacy. In terms of design style, doors and windows should be more eclectic and have a personalized appearance. It is in harmony with the building and is beautiful.

3.1 Energy-saving glass design

Since doors and windows are the key areas that generate building energy consumption, tracing the source, the most basic element pursued by energy-saving doors and windows is the heat insulation performance of doors and windows; the development of high-performance energy-saving doors and windows will also focus on how to reduce the heat transfer coefficient and control the entire window.

The heat loss efficiency of doors and windows is to carry out the research and development of various local process technologies, structures and materials, and gradually explore and apply the refined design of some subtle nodes to obtain performance improvements and enhancements with half the effort, such as the super insulation material to achieve the rigid foam polyurethane partition of the large broken bridge structure. The application of hot aluminum alloy profiles and the application of glass warm edge technology further eliminates the thermal bridge problem of door and window profiles and non-transparent glass parts, and controls the heat loss path of doors and windows. 

Therefore, doors and windows obtain the following more excellent energy-saving Characteristics and usage characteristics: low heating load (cooling load will also be reduced) brings energy saving; the temperature of the inner surface of the window is closer to the indoor temperature, and the comfortable use space of the room is greatly improved in cold winter, and the uncomfortable use distance of the window is also It will be significantly shortened; the temperature of the inner surface of the window is higher than the dew point temperature of the indoor environment, thereby avoiding condensation and frost, and thus prolonging the service life and obtaining greater energy-saving benefits during the life cycle. It is a key improvement link for sustainable building solutions .

At present, the technology of energy-saving doors and windows is developing rapidly, and it has already possessed very excellent technology and craftsmanship, which can obtain extremely low heat transfer coefficient of the whole window. There are roughly four ways to optimize the thermal insulation performance of doors and windows: energy-saving glass design, optimization design of linear heat transfer loss at the edge of the glass, optimization design of window frame profile system, and key points of door and window installation and sealing design.

Glass usually occupies more than 70%-80% of the entire window area. Therefore, the thermal insulation capacity of the glass part has an important influence on the thermal insulation performance of the entire window; glass is a good conductor of heat, and its thermal conductivity is about 0.9W/ (M·K), the thermal resistance of single-layer glass is very small; therefore, if single-layer glass is used, indoor and outdoor heat is directly transferred through conduction, the heat transfer coefficient of the glass is high, and the heat loss is extremely fast; 6mm single layer The heat transfer coefficient of glass is 5.8 W/(m2·K);

Therefore, for single-glazed windows, the temperature of the indoor surface in winter is much lower than the indoor dew point temperature, which will cause condensation; moreover, when people stand in front of the indoor window, they will feel the obvious temperature difference effect, which is very uncomfortable; This kind of temperature difference effect decreases with the increase of the moving distance indoors. Therefore, the comfortable use space in the room is obviously compressed; at the same time, this kind of glass will bring a lot of energy consumption, which is lost in unit time due to the temperature difference. The heat is up to 145 W/m2. Therefore, the use of single-layer glass doors and windows has been restricted or prohibited in the national and some local energy-saving doors and windows regulations;

Then, in order to improve energy saving efficiency, improve the thermal insulation performance of glass, and reduce the heat transfer coefficient of door and window glass, we can carry out comprehensive optimization design of glass through the following ways:

3.1.1 Use insulating glass

Due to the existence of the air layer between the two pieces of glass, the indoor and outdoor heat transfer method has changed. The heat conduction of a single piece of glass is transformed into radiation and convective heat transfer; the heat conduction ht of the hollow glass system can be determined by the following formula (1 ) Calculation:

hs: Thermal conductivity of insulating glass gas gap layer W/(m² ·K)

N: Number of insulating glass gas layers

d: The sum of the thickness of the single layer of glass that makes up the insulating glass （m）

λ: Thermal conductivity of glass W/(m² ·K)

It can be seen that the thermal conductivity of insulating glass is positively related to the thermal conductivity of the glass and the thermal conductivity of the gas gap layer, and negatively related to the thickness of the glass; while the thermal conductivity of the gas gap layer includes the gas thermal conductivity of the gap layer and the two pieces of glass that make up the gap layer. Radiant heat conduction; therefore, we can conclude that the use of insulating glass, due to the existence of the gas gap layer, makes the heat transfer coefficient of the insulating glass mainly determined by the gas heat conduction and the radiant heat conduction of the glass, thus greatly reducing the transmission of the insulating glass system Thermal coefficient; for ordinary insulating glass of 6mm+12A+6mm, the heat transfer coefficient is Ug=2.9 W/(m² ·K); we can get by calculation: under assumed conditions, outdoor ambient temperature in winter To=-5℃ , Indoor temperature Ti=26℃, indoor relative humidity Rh=60%; indoor glass surface temperature T=14.8℃, which is 11.3℃ higher than ordinary single-layer glass indoor surface temperature.

The energy-saving properties of glass have been significantly improved; but the indoor surface temperature is still lower than the indoor environment's dew point temperature Dew=17.6℃; condensation problems still occur in cold winters; therefore, the performance of insulating glass needs to be optimized.

Approach 1. Use insulating glass filled with inert gas

From the explanation of formula (1), it can be concluded that the heat transfer coefficient of hollow glass is related to the thermal conductivity of the gas; therefore, the hollow glass cavity is filled with inert gas with high viscosity and large molecules, such as argon, Krypton, xenon, etc., these inert gases have a larger specific gravity than air, have poor gas fluidity, low thermal conductivity, and greatly reduce the heat transferred by gas convection and conduction;ToThe following table (table 1) is the characteristic index of different gases

From the data in Table 1, it can be seen that krypton gas has the lowest thermal conductivity and has the best effect on improving the insulation performance of insulating glass. For 6LOW-E+12A+6 insulating glass, krypton gas can reduce the heat transfer coefficient of the glass. About 0.6 W/(m2·K), but the price of krypton gas is very expensive, and the increased cost per unit area is even more than 15 USD. The application on architectural glass is not economical and difficult to be promoted. However, it is not easy to be promoted and used in some high-end products and It is used in the field of special industrial glass; Argon is a cheap inert gas that can be directly separated from the air, and it also has good thermal performance, so it is widely used in building energy-saving windows and doors;ToInsulating glass filled with inert gas must ensure its good sealing characteristics, so as to prevent expensive and efficient inert gas from leaking with the prolonged use time, resulting in the attenuation of the performance of insulating glass; generally speaking, for inert gas For gas insulating glass, if silicone structural adhesive is used as the secondary sealant, strict measures must be taken to control the width of the butyl rubber, the amount of butyl rubber coating, and the back seal of the spacer interface. In addition to the processing technology, it is produced under the normal process For insulating glass, the annual gas leakage rate should not exceed 1%. At the same time, the gas seal durability test must be carried out in accordance with the latest industry standards.

So, for insulating glass, how much improvement can the performance of insulating glass be brought about by filling with argon gas?

We can see from formula (1) that the thermal conductivity of insulating glass is positively correlated with the thermal conductivity hg of the interstitial layer gas;

^{The formula (2) The thermal conductivity of the space layer gas is proportional to the Nusselt criterion Nu}

The formula (2) shows that the thermal conductivity of the gas in the spacer layer is directly proportional to the Nusselt's criterion Nu, which is a parameter related to the temperature difference between the inner surfaces of the two glass cavities and the different physical properties of the gas. Therefore, the influence of filling inert gas on the heat transfer coefficient of insulating glass is related to the physical properties of different gases, gas concentration, gas layer thickness and other factors. According to different standard systems, we simulated the argon-filled insulating glass with common configurations, and the results are shown in Table 2.

_{Glass configuration: 6mm Low-E Glass+Spacer+6mm normal glass}

^{Table 2 The simulation results of argon-filled insulating glass with common configurations}

From the calculation data in Table 2, we can see that no matter which standard system is calculated based on, for insulating glass with different spacer widths, the heat transfer coefficient of argon-filled is lower than that of non-argon-filled is between 0.25-0.40; generally In other words, the thinner the air layer, the more obvious the improvement of the heat transfer coefficient of insulating glass by argon filling;

For glass filled with argon gas, great attention should be paid to the sealing properties of insulating glass. The window or curtain wall of the open frame structure uses polysulfide glue with low water vapor permeability as the secondary sealant; the glass production process pays special attention to the operation process of the butyl glue to ensure the role of the main sealant. The latest Chinese glass national standards also pay more attention to the gas sealing characteristics of insulating glass and the control of inert gas retention rate. According to the latest industry standards, the annual gas leakage rate of inflatable glass should not be higher than 1%.ToIn addition, regardless of whether the insulating glass is filled with inert gas or not, we need to pay attention to the use angle of the window when designing doors and windows. For skylights or slanted roof windows for daylighting, when calculating the thermal performance of the entire window, it is necessary to consider the influence of the actual temperature difference between indoor and outdoor and the angle of the glass on the convection in the cavity. Figure 2 shows the internal convection heat transfer condition of the hollow glass in a horizontal state. As the convection heat transfer cycle path becomes smaller, the heat transfer efficiency increases, so that the heat transfer coefficient of the glass in the horizontal state is greater than the heat transfer in the vertical state. In winter, the indoor temperature is higher than the outdoor temperature, forming an upward heat flow; in summer, the indoor temperature is lower than the outdoor temperature, and no upward heat flow is generated. Therefore, in winter, the actual heat transfer coefficient of a window used at a large angle is higher than that of a vertical window. The straight window is larger, which requires special attention in our thermal calculations.

Due to space constraints, this article, as the first part of the whole article, will focus on the first approach to the technical points of energy-saving glass design. In the next article, it will focus on the remaining three approaches to the technical points of energy-saving glass design.