Central air conditioning is an indispensable energy consumption operation system in modern buildings. Central air conditioning system not only provides people with comfortable living and working environment, but also consumes a lot of energy. According to statistics, the energy consumption of buildings in China accounts for about 30% of the total energy consumption. In buildings with central air conditioning, the energy consumption of central air conditioning accounts for about 70% of the total energy consumption, and it is increasing year by year. Therefore, how to efficiently use the energy and energy saving of central air conditioning system has become an urgent problem to be solved.from Puremind Split Air Conditioners Looking at the development prospects, the future will always bring positive effects. https://puremind.com/puremind-hot-sale/
The normal operation of the central air conditioning system, its energy consumption mainly has two aspects [1″ target=_blank>: on the one hand, it is the energy consumption of cold and heat sources to supply air handling equipment with cold and heat; On the other hand, in order to transport air and water, fans and pumps need energy to overcome the flow resistance. The energy consumption of central air conditioning system is affected by many factors, and many operation links have energy-saving measures. Therefore, energy-saving of central air conditioning is a comprehensive project. The following introduces several commonly used energy-saving measures in terms of energy consumption of cold and heat sources and power consumption.
1. Energy-saving measures for cold and heat sources
1.1 Temperature and humidity control
From the air treatment process of central air conditioning system, it can be seen that the lower the indoor temperature and relative humidity in summer, the greater the energy consumption of system equipment; The higher the indoor temperature and relative humidity in winter, the greater the energy consumption of the system equipment, and the corresponding initial investment and operating costs also increase.
Because everyone’s requirements for comfort are very different, there can be a wider comfort zone for civil central air conditioning. In this comfort zone, when cooling in summer, take a higher temperature and humidity value; When heating in winter, a certain energy-saving effect can be obtained by taking a lower temperature and humidity value. The change of temperature and humidity in the building is closely related to building energy saving. According to empirical statistics, if the set temperature is lowered by 1≧ in summer, the energy consumption will increase by 9%. If the set temperature is raised by 1≧ in winter, the energy consumption will increase by 12%. Therefore, it is an effective measure to control the temperature and humidity in the building within the precision range of the set value.
In order to reduce energy consumption, the indoor temperature and humidity base of air-conditioned rooms should be raised as much as possible in summer and lowered as much as possible in winter under the condition of meeting production needs and human health. At present, some owners blindly pursue the realm of “cold enough” and greatly improve the design standard of indoor temperature and humidity, which not only wastes a lot of energy unnecessarily, but also produces the negative effect of comfort.
The higher the temperature and humidity control precision of air conditioning system, the better the comfort and the more obvious the energy-saving effect. The accuracy of the signal measured by the front end of the air conditioning system directly affects the precise control degree of the central air conditioning system. Therefore, the measured signals, especially analog signals such as temperature and humidity, must be as accurate as possible.
Also, BAS with high control accuracy must be selected to control the central air conditioner. Because BAS uses DDC (Direct Digital Controller) to directly control the opening of electric water valve without intermediate regulator; In addition, DDC contains a wealth of calculation and control software, such as PID algorithm, fuzzy control algorithm, genetic algorithm and so on, to ensure the control accuracy.
1.2 cold source efficiency control
The performance index to evaluate the refrigeration efficiency of cold source is COP (Coefficient Of Performance). Refrigeration coefficient refers to the amount of cold energy that can be obtained per unit power consumption. The refrigeration coefficient has nothing to do with the nature of refrigerant, but only depends on the temperature T0 of the cooled object and the coolant temperature Tk. The higher T0, the lower Tk and the higher the refrigeration coefficient. So don’t make the chilled water temperature too low and the cooling water temperature too high during the actual operation of the air conditioning system, otherwise the refrigeration coefficient will be low, which will consume a lot of work and high power consumption to produce unit cooling capacity, and increase the energy consumption of the building. The following measures can be taken to improve the efficiency of cold source:
1) reduce the cooling water temperature
The lower the cooling water temperature, the higher the refrigeration coefficient of the cooler. For every 1≧ increase in the supply temperature of cooling water, the COP of the chiller decreases by nearly 4%. To reduce the temperature of cooling water, it is necessary to strengthen the operation management of cooling tower. First of all, for the cooling tower that stops running, the valves in and out of the water pipes should be closed. Otherwise, because the water temperature from the stopped cooling tower is high, the temperature of the mixed cooling water will increase and the refrigeration coefficient of the cooler will decrease. Secondly, after the cooling tower is used for a period of time, it should be overhauled in time, otherwise the efficiency of the cooling tower will decrease and the cooling water cannot be fully cooled.
2) increase the temperature of chilled water.
The higher the temperature of chilled water, the higher the refrigeration efficiency of the chiller. When the temperature of chilled water supply is increased by 1≧, the refrigeration coefficient of the chiller can be increased by 3%, so don’t blindly reduce the temperature of chilled water in daily operation. First of all, don’t set the set temperature of chilled water of the cold machine too low. Secondly, the water valve of the stopped chiller must be closed to prevent some chilled water from taking the bypass pipeline, otherwise, the amount of water passing through the running chiller will be reduced, resulting in the temperature of chilled water being lowered to a too low level by the chiller.
2. Energy-saving measures for power consumption
2.1 Transportation energy consumption control
Power consumption mainly refers to the electric energy consumed by fans and pumps in system operation. Calculation formula of input power from fan and water pump:
(1) (W): input power; : volume flow; : indenter; Eta: efficiency
From Formula (1), it can be seen that reducing power consumption can be considered from the following three aspects: reducing flow, reducing system resistance and improving the efficiency of fans and pumps. The following measures can be adopted in engineering practice:
1) adopt large temperature difference.
If the temperature difference between the supply and return water (or the return air) used to transport cold and hot water (or air) in the system is large, then when the ratio between it and the original temperature difference is m, it is known from the flow calculation formula that the flow rate is reduced to 1/m3 when the large temperature difference is used. At this time, the power required by the water pump or fan will be reduced to 1/m3. It can be seen that the energy-saving effect of increasing the temperature difference is obvious.
On the premise of meeting the requirements of central air conditioning accuracy, personnel comfort and technology, the temperature difference of air supply should be increased as much as possible. It should be noted that the temperature difference between supply and return water should not be greater than 8≧.
2) choose low flow rate.
Because the power consumption required by pumps and fans is roughly proportional to the flow velocity in the pipeline system, in order to achieve energy-saving operation effect, high flow velocity should not be adopted in design and operation. In addition, the low flow rate in the main pipe is also beneficial to the stability of the hydraulic condition of the system. For example, changing the rotational speed of the fan can change the performance parameters of the fan. The power of the fan has a cubic relationship with the rotational speed, while the flow rate has a linear relationship with the rotational speed. Lowering the rotational speed can greatly reduce the energy consumption while reducing the flow rate. When the flow rate is reduced by 1/3, the energy consumption can be reduced by about 70.4%, and when the flow rate is reduced by 1/2, the energy consumption can be reduced by about 87.5%, and the efficiency of the fan is basically unchanged, and it can still work stably and efficiently.
3) adopt energy-carrying medium with high conveying efficiency.
Under normal circumstances, the energy consumption of transporting cold and heat energy with water is less than that with air, and the diameter of water pipes used to transport the same cold and heat energy is much smaller than that of air pipes, which occupies a much smaller building space. This is also one of the main reasons for the rapid development of central air conditioning in recent years.
Therefore, for the centralized refrigeration mode, in principle, the chilled water prepared by the equipment in the computer room should be transported as far as possible to the vicinity or use point of each central air-conditioning partition, and the air should be treated by the terminal non-independent central air-conditioning units (such as cabinet air-conditioning units and fan coils) for local use or for use in nearby rooms.
2.2 VAV system control
VAV system is an energy-saving countermeasure against the shortcomings of power consumption in air supply system. VAV system can be divided into two types: one is AHU！VAV system of air conditioner; One is the indoor fan variable air volume system (FCU-VAV system) in FCU system. AHU-VAV system is to fix the air supply temperature in the whole air duct system and adjust the air supply volume of the blower to cope with the change of indoor air conditioning load. In FCU-VAV system, the cold water supply is fixed, and the indoor FCU is equipped with a stepless power controller to change the air supply, that is, to change the heat exchange rate of FCU to adjust the indoor load change. These two methods can reduce the power consumption of the blower by adjusting the air volume, and at the same time, they can also increase the operating efficiency of the heat source machine and save the power consumption of the heat source, so they can achieve energy-saving effects in both air supply and heat source.
The cooling capacity sent into the room can be determined by the following formula:
2
Where: c is the specific heat capacity of air, kj/(kg≧); ρ is the air density, Kg/m3; L is the air supply volume, m3/S; Tn is the indoor temperature,≧; Ts is the supply air temperature,≧; Q is the heat absorbed (or put) into the room, KW.
If the air supply temperature is set as a constant and the air supply volume L is changed, different Q values can be obtained to keep the room temperature unchanged. Variable air volume control can adopt an air supply device that automatically adjusts the air supply volume according to the change of indoor load. When the indoor load decreases, it can keep the air supply parameters unchanged (no need to reheat) and keep the indoor temperature stable by automatically reducing the air volume. In this way, it can not only save the reheat required by the constant air volume system to increase the air supply temperature, but also reduce the power consumption of the fan and the cooling capacity of the refrigerator due to the reduction of the treated air volume. According to various data, VAV can save energy by 30% ~ 50% compared with constant air volume.
2.3 Frequency conversion control
In most buildings, the central air conditioning is at the maximum load only for a few days in a year. The cooling load of central air conditioning is always changing dynamically, such as the changes of various factors such as morning and evening, climate, passenger flow and activity content, which affect the cooling load of central air conditioning in real time. Generally, the cooling load fluctuates in the range of 5 ~ 60%, and most buildings are at least 70% in this situation every year. However, most central air conditioners are driven by maximum cooling load and maximum power due to system design. In this way, the contradiction between the actual cooling load and the maximum power output is caused, resulting in a huge waste of energy. This contradiction can be solved by adopting frequency conversion control.
1) Fan and water pump frequency conversion control
In the past, the AC motor itself didn’t adjust the speed, so the central air conditioning system had to rely on baffles and valves to adjust the air and water flow, and a lot of electric energy was wasted on baffles and valves. If the fan water pump is regulated by frequency conversion, the energy wasted on baffles and valves will be saved, and the average energy saving effect of each pump will be considerable.
For the fan water pump, according to the principle of fluid mechanics, the parameters under similar working conditions have the following relationship [2″ target=_blank>:
3
Where Q1, H1, n1 and N1 are the flow, lift, power and rotation speed before the rotation speed change, respectively;
Q2, H2, n2, N2: the flow, lift, power, and rotation speed after the rotation speed is changed respectively.
According to Formula (3), the flow rate is proportional to the rotational speed, the pressure is proportional to the square of the rotational speed, and the power consumed is proportional to the cubic power of the rotational speed. For variable frequency speed regulation, the speed n is basically proportional to the power frequency f, when the power frequency f decreases, the motor speed also decreases, and the required power decreases rapidly with the cubic power of the speed, which shows that the energy-saving effect is very remarkable. Take the fan as an example, if the required air volume is 80% of the rated air volume, the speed will also drop to 80% of the rated speed, and the shaft power will drop by 51.2%; When the required air volume is 50% of the rated air volume, the shaft power drops by 12.5%. This power saving effect is also very impressive. It has been proved that the frequency conversion control of fans and pumps can save energy by 40% ~ 50%.
2) Frequency conversion control of water chillers
Because the compressor does not rule out the possibility of long-term operation under full load, the capacity of the motor can only be determined according to the maximum demand, so the design margin is generally large. In actual operation, the proportion of light load operation time is very high. Frequency conversion control is adopted to adjust the speed of compressor, so as to control the refrigeration capacity and keep the refrigeration unit in the best (most reasonable) operation state. Frequency conversion control improves the efficiency of air conditioner and the operation effect of refrigeration unit, thus realizing energy saving.
The principle of variable frequency compressor is to adjust the displacement of compressor per unit time by adjusting the speed of compressor, so as to achieve the purpose of adjusting the refrigeration capacity. The mathematical relationship between refrigeration capacity and compressor frequency is as follows:
4
Where n is the rotational speed, f is the frequency of the motor, p is the polar logarithm of the motor, s is the slip of the motor, λ is the leakage coefficient, Ps is the suction pressure, vh is the suction volume, ni is the polytropic index, εi is the pressure ratio (exhaust/suction), δ0 is the relative pressure loss coefficient, and Zs and Zd are the actual gas compressibility coefficients, which are the average energy efficiency ratio.
According to Formula (4), the refrigeration capacity is directly proportional to the frequency, so the refrigeration capacity can be controlled by frequency conversion regulation, thus achieving the energy-saving effect.
Some adjustment methods (such as adjusting the valve opening and changing the blade angle) can not reduce the operating efficiency of the motor even if the demand is small. After adopting frequency conversion speed regulation, when the demand is small, the speed of motor can be reduced and the running power of motor can be reduced, thus further realizing energy saving. It has been proved that the frequency conversion control of water chillers can save energy by 20% ~ 30%.
