The air conditioning system is mainly composed of four major systems: air, ice water, refrigerant and cooling water. During actual operation, these four systems will change with the change of air conditioning load, coordinate with the change of air flow, ice water flow control and refrigerant flow adjustment to achieve load balance and transfer the heat load from indoor to outdoor.
Energy saving of air conditioning host
According to the relevant data of monthly heating and cooling degree hours, if the building is not designed and applied with natural ventilation, air conditioning is required almost all year round. The meaning of cooling degree hours is that when the hourly outdoor temperature is greater than 26℃, it is accumulated hourly. Therefore, the greater the demand for cooling load in that month, the higher the required air conditioning energy consumption. The peak and off-peak changes of air conditioning load throughout the year are very obvious. Basically, the operation hours of various partial loads of the chiller host throughout the year are roughly as shown in the figure. The proportion of 100% full load operation hours of the host is very small, and most of the time it is in partial load operation of 50~70%.
The energy consumption of the chiller accounts for a considerable proportion in the central air conditioning system. In addition to the high efficiency performance of the chiller when running at 100% full load, it is necessary to ensure that the host can operate for a long time under the condition of 50~75% partial load rate and maintain high efficiency standards to obtain the best energy saving effect. Therefore, the energy saving methods of the chiller are as follows:
1. Accurately calculate the maximum equipment volume of the air conditioning host
The equipment capacity of the chiller must be determined by factors such as the maximum air conditioning load, equipment efficiency, meteorological factors and heat load. Dynamic load simulation is required to obtain the appropriate air conditioning equipment volume. Therefore, it is necessary to calculate through certified air conditioning calculation procedures, standard indoor conditions and meteorological data to establish a reasonable equipment design volume.
2. Use high-efficiency air conditioning hosts
The chiller should use hosts with efficiency higher than the chiller efficiency standard announced by the Bureau of Energy of the Ministry of Economic Affairs to reduce power consumption. Because the host does not operate under full load for a long time, when selecting a chiller, it is necessary to examine the efficiency at full load and the efficiency at partial load at the same time, and the minimum standard value of the full load efficiency COP and the partial load efficiency IPLV (Integrated Part Load Value, IPLV). In addition, you can also choose a chiller with a variable frequency speed control function instead of using the traditional method of changing the angle of the inlet guide vane to match the load; or choose an air conditioning host that can operate at high efficiency under 25~75% partial load rate for a long period of time to increase the efficiency at partial load.
3. Use multiple hosts to operate
When multiple hosts are operated in parallel, if one host can be turned off at low load at the same time, the host can be maintained at high efficiency. When a single host is running, because its tonnage is large, it causes low load operation, so you should consider installing a host with a smaller tonnage to maintain high efficiency operation.
4. Adjust the set temperature of chilled water
According to the principle of refrigeration cycle in thermodynamics, the higher the evaporation temperature of the chilled water host, the better the efficiency. Therefore, increasing the water supply temperature of the chilled water host or the evaporation temperature of the refrigerant can maintain the chilled water host at high efficiency. Every increase of 1°C in chilled water temperature can increase the host efficiency by about 3%. When the chilled water temperature drops, the host performance decreases, the power consumption of the chilled water host increases, but the power consumption of the water pump decreases, so there is an optimal operating point, as shown in the figure. However, when the chilled water outlet temperature increases, the dehumidification capacity of the air conditioner box will decrease, so it is necessary to examine the environmental requirements to make a decision.
5. Cooling water or chilled water quality management
Regularly clean the heat exchanger to prevent the heat exchanger from scaling and affecting the heat transfer efficiency. The scale will affect the host efficiency by more than 20%.
Variable water volume (VWV) air conditioning system
The water delivery method of traditional air conditioning systems mostly adopts a fixed water volume pump control method, and adjusts the water temperature to cope with partial load conditions. This system is called a constant flow (CWV, Constant Water Volume) system. The variable water volume (VWV) system uses a fixed water temperature supply to improve the efficiency of the chiller, and changes the water supply volume by controlling the number of pumps or using a frequency converter to save pump power. Compared with the fixed flow system, the variable flow system can change the water supply volume according to the change of indoor heat load, which can reduce the power of delivery and achieve energy saving.
Variable air volume (VAV) air conditioning system
General air conditioning system uses a certain air volume to supply indoor air conditioning. For changes in indoor load, it controls the change of supply air temperature, which is called a constant air volume (CAV) system. The variable air volume (VAV) system fixes the supply air temperature and adjusts the supply air volume to cope with changes in air conditioning load. With the operation characteristics of the fan, it can save more than half of the fan power consumption.
Total heat exchanger system
In summer, about 30% to 40% of the air conditioning energy consumption is used to deal with the heat load of the outside air. Therefore, reducing the heat load of the outside air is one of the key points of air conditioning energy saving. To ensure indoor air quality, a good air conditioning system usually introduces about 30% of fresh outside air and 70% of air-conditioned return air, and then processes it into air-conditioned supply air suitable for indoor conditions. About 30% of the air-conditioned return air is replaced with fresh outside air. If the energy of the low-temperature and low-humidity air discharged from the return air can be recovered and reused, the purpose of energy saving can be achieved. There are basically two types of total heat exchangers, which are roughly introduced as follows:
1. Static cross-flow type
There are many flat plate flow channels in the static cross-flow total heat exchanger, and the two flows are separated on both sides of each plate by partitions and sealing devices, and the flow direction is cross-direction. The plates are mostly made of permeable fibers, and the water absorbed on one side can penetrate to the other side to be taken out of the total heat exchanger by the other flow. This equipment itself does not require power and is easy to maintain, which is its main advantage.
2. Rotary type
The rotary total heat exchanger requires a small motor to cause the rotation of this honeycomb wheel. There are countless parallel small channels in the honeycomb, forming a large exchange area. There needs to be a device on the wheel to divide it into two sides. When the outside air flows through one side, part of the heat and moisture is absorbed in the wheel. The saturated part continues to flow to the other side. The lower temperature and low humidity exhaust air flows through the other side, taking the heat and moisture away from the wheel, achieving the effect of regenerating the heat and moisture absorption capacity.
