1 Introduction
Floor heating has been widely used in residential buildings in northern China because of its outstanding characteristics such as energy saving, comfort and not occupying indoor usable area. In hot summer and cold winter areas, especially in Shanghai, Zhejiang and other places, the application is also increasing. If the system is used in summer refrigeration at the same time, it will reduce the initial investment of equipment, improve the utilization rate and add a new way for residential air conditioning. Roof radiant cooling has been used in Europe for a long time and achieved good results under certain conditions. Before 1990s, floor cooling was considered by most people to be infeasible. The main reasons are as follows: (1) Query on condensation; (2) In terms of comfort, it is contrary to the requirements of "warm feet and cold head"; (3) It is considered that the convective heat transfer is weak when the cold surface is downward, and the refrigeration capacity will be much smaller than that provided by the roof. It was not until the end of 1990s that Europe began to discuss the theory and practical application [1] ~ [3]. In recent years, China has also accelerated the process of research and application. Research, analysis and practice have proved that some of the above concerns can be solved through certain technical measures, while others are misunderstandings caused by inertia thinking or lack of in-depth research.
Preliminary research and application of 2 in China
In 1998, the author wrote a paper introducing our work in France and the application and development of floor cooling in Europe (mainly France) [3]. Since then, we have cooperated with enterprises to carry out theoretical discussion and experimental research on floor cooling in the north [4]. Starting from 200 1, we got the financial support from Nanjing Normal University and started. We built a laboratory to carry out experimental research on floor cooling-displacement ventilation, and carried out measured research on floor cooling capacity, dew prevention measures, comfort, practical application effect, energy consumption and simulation calculation method of air source heat pump in a model house [5] ~ [1 1]. Since 2000. We designed a composite air conditioning system with floor heating, cooling and displacement ventilation for the ward building of Weihai Traditional Chinese Medicine Hospital (building area 126000m2). Due to the shortage of funds, the project has been suspended for nearly two years and is still under construction, so it is impossible to know the actual use effect. At the end of 2002, it won the unanimous support of the evaluation Committee headed by Mr. Wu and obtained the project funds for feasibility study in Beijing.
In addition, Zhejiang University put forward the idea of dehumidification of radiant floor cooling system [12], and began its application research in Zhejiang. China Institute of Building Research has designed a floor cooling system for the Jinyu Wholesale Market in Hohhot [13], and Chongqing University has also conducted an experimental study on using underground pipes to cool the floor.
3 floor cooling capacity
The cooling capacity of floor cooling system is the first consideration in design and application.
3. 1 The floor cooling system can reduce the indoor cooling load.
In the floor cooling system, the floor exchanges heat with the room mainly through radiation and convection. The average radiation temperature has become an important parameter affecting indoor thermal comfort, and the interaction between average radiation temperature and air temperature can be reflected by operating temperature. When the indoor wind speed is low (less than 0.2m/s), the difference between the average radiation temperature and the indoor air temperature will be less than 4℃, and the action temperature is approximately equal to the average of the air temperature and the average radiation temperature, which means that they are equally important for indoor thermal comfort. Due to the decrease of the average indoor radiation temperature, the operating temperature can be reduced by 65438 0 ~ 2℃. Compared with the traditional air conditioning system, the floor cooling system can increase the indoor design temperature 1 ~ 2℃ under the same thermal feeling, and its cooling load is lower than that of the conventional system 10% ~ 20%.
3.2 Floor cooling has a higher radiation angle coefficient than roof cooling.
Compared with ceiling cooling, floor cooling has less convective heat transfer. But for radiant cooling system, radiant heat transfer is the main part, and an important parameter affecting radiant heat transfer is the included angle coefficient between human body and radiant cooling surface. This value depends on the distance between the person and the cold surface and the area of the cold surface. In the same area, compared with other cold surfaces (walls, windows, ceilings, etc.). ), the angle coefficient of the floor to the human body is higher. A person in the center of a room of 6m×6m has an angle coefficient of 0.37 with the floor when standing and 0.4 with the floor when sitting. In contrast, the angle coefficient of the human body to the ceiling is generally only 0. 15 ~ 0.20. It can be seen that the decrease of average radiation temperature caused by the decrease of floor temperature 1K is equivalent to the decrease of ceiling temperature by 2.5K Therefore, although the floor provides cooling and convection heat exchange,
3.3 Updraft and airflow disturbance can increase convective heat transfer.
The cold surface is below, and there is a layer of cold air near the ground. When it meets a heat source such as human body, this layer of air will be heated up, which will increase convective heat transfer. The cold floor can also be used with a fan, which can increase the heat exchange capacity by about 15%.
3.4 Displacement ventilation system can greatly improve the cooling capacity of the floor.
In general, the comprehensive heat transfer coefficient of convection radiation between floor and room is about 7.5 W/(m2 k) [1]. If the floor surface temperature is 19℃, the room temperature is 26 ~ 27℃ (equivalent to the indoor design temperature of 28℃ under conventional air conditioning), and the floor cooling capacity is 53 ~ 60 W/m2, the thermal insulation of the envelope can be met.
It should be pointed out that radiation refrigeration has the same "self-regulation" function as radiation heating. When the indoor radiation load increases, for example, when the direct sunlight radiation is large, the temperature of the inner surface of the floor or room wall will rise, which will greatly improve the radiation heat exchange between the floor and the remaining surfaces of the room envelope. The floor cooling capacity can be as high as100 ~150 w/m2 [1].
4. Dew on the ground
When the water supply temperature is low or the indoor humidity is high, the floor surface of single-floor cooling system may be dewed, and the ability of cold floor to bear indoor cooling load will be weakened, which can not meet the needs of indoor cooling load. Some standards recommend that the upper limit of indoor relative humidity is 60% ~ 70% (ISO1994; ASHRAE 1992), when the indoor air temperature is 26℃, the dew point temperature is17℃ ~ 20℃; German standards stipulate that the upper limit of moisture content is 1 1.5g/kg and the dew point temperature is 16℃. This shows that the floor temperature should be higher than 16℃ ~ 20℃. In addition, open doors and windows, outdoor hot and humid air enters, and it is easy to generate condensation when it contacts with the low-temperature floor.
The existence of dehumidification displacement ventilation system can prevent condensation. Introducing fresh air can keep indoor positive pressure and prevent humid outdoor air from entering. In addition, a characteristic of displacement ventilation mode is that a layer of air lake can be formed on the floor surface, and the floor is covered with a layer of dry air with low dew point temperature, which can prevent the hot and humid air infiltrated from the outdoor from directly contacting the low-temperature floor, and at the same time ensure that the water supply temperature is low enough to meet the needs of indoor cooling load.
According to different requirements, dehumidification air supply can also adopt fan coil unit or centralized system, or a single dehumidifier. The scheme can be determined by technical and economic comparison.
5 Fresh air and sanitary conditions
When the floor cooling system is used alone, there is no fresh air in the room, and the air conditions will be affected. After adding displacement ventilation system, the floor cooling system can bear the indoor sensible heat load, and the displacement ventilation system can send fresh air that meets the requirements of personnel hygiene conditions and bear the indoor wet load, thus meeting people's demand for fresh air. The displacement ventilation system sends fresh air directly into the work area. Due to its high density, the lower fresh air is deposited near the ground and spreads to the whole indoor floor, forming a thin air lake on the floor. When cold air on the ground meets indoor heat sources (people and equipment), it generates upward convection, which makes fresh air flow to the upper part of the room under the floating action of the heat sources, forming the dominant air flow in the indoor air flow. An important feature of displacement ventilation is that it will produce thermal stratification, that is, there will be an upper mixing zone and a lower one-way flow clean zone. The upper region is a chaotic mixing region, and the lower region is an upward hot air flow region and a surrounding clean air region. The air parameters in the clean air area are almost the same as the air supply parameters. Compared with traditional mixed ventilation, fresh air is better used in the work area. Experiments show that the ventilation efficiency of displacement ventilation is usually between 0.5 and 0.67, and the ventilation efficiency is between100% and 200% [14], and the ventilation effect is obviously better than that of mixed ventilation.
6 Thermal comfort of floor cooling
6. 1 indoor air temperature gradient
At present, the concern about the thermal comfort of floor cooling/displacement ventilation system is mainly reflected in the negative temperature gradient of the room. It is considered that floor cooling gives people the feeling of "cold head and warm feet" in winter, which meets the requirements of human health, and the same system will lose this advantage when used for cooling in summer.
First of all, the study of floor heating shows that the longitudinal temperature of the room is uniform except for a short height at the top of the room, so "foot heating" is real, and "head cooling" lacks comparative basis. Secondly, I hope to "warm my feet" in the cold winter; How can you expect to "warm your feet" when you want the ground to be cool but you can't get it in hot summer?
The research shows that the floor temperature above 18 ~ 19℃ can completely meet the requirements of cold load (ASHRAE puts forward that the suitable temperature for light manual workers is 18 ~ 26℃), and this temperature will not produce the feeling of "cold feet" in summer. In addition, due to the uniform surface temperature of the floor, the convection component in floor cooling is low (about 20%[ 1]), which makes the vertical temperature very uniform, and the temperature gradient is only 1.5 ~ 2 k/m [3], which is in line with the international standard ISO 7730 and the height is 0. 1 and/kloc-.
6.2 Radiation heat transfer has good thermal comfort.
The composite system of floor cooling/displacement ventilation has many inherent advantages in improving indoor air quality. Under comfortable conditions, all the heat generated by human body is radiated according to a certain proportion: convection heat dissipation accounts for 30%, radiation heat dissipation accounts for 45%, and evaporation heat dissipation accounts for 25%. It can be seen that radiation heat transfer is very important for human comfort. The radiant floor cooling in the composite system of floor cooling/displacement ventilation makes up for the shortcomings of convective cooling in traditional air conditioning, increases the radiant heat transfer of human body and helps to improve indoor comfort. Another significant advantage of radiant refrigeration is that when people sleep, they don't have to worry about blowing cold air. From this perspective, can it be called "a cooling method conducive to sleep"?
7 conclusion
(1) In order to prevent the feeling of cold feet and be limited by the dew point temperature of air, the cooling capacity of the floor cooling system is limited, generally not exceeding 70W/m2. However, due to radiation, the cooling load is lower than that of the conventional system 10% ~ 20%, and in addition, the air supply can bear part of the load, which can completely meet the general use requirements.
(2) In humid areas, there should be dehumidification equipment. If the displacement ventilation/mixed air supply of fresh air unit is combined, the fresh air problem will be solved. Of course, fan coil units or mobile dehumidifiers can also be used for dehumidification, but at this time, indoor sanitary conditions are reduced.
(3) Floor cooling can meet the comfort requirements in terms of ground temperature, uniformity of horizontal and vertical temperature fields, risk of blowing, etc., especially when people sleep, which is one of the obvious advantages in comfort.
refer to
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