With the continuous development of my country's national economy and the continuous improvement of people's living standards, central air conditioning has entered various fields such as hotels, restaurants, industrial and mining enterprises, and office buildings. Conventional central air conditioning systems are selected and designed based on the maximum cooling and heating load. However, the hottest and coldest days of the year only last a few days, so central air conditioning systems operate at less than the rated load, i.e., partial load, most of the time, resulting in a huge waste of electricity. With the development of technology, frequency converters have been widely used in various industries. They are inexpensive and technologically mature. In particular, energy-saving retrofits for fans and water pumps are now widely promoted in the industrial field, with average electricity savings of over 30%.
I. The Best Way to Save Energy with Central Air Conditioning
Since the main equipment of central air conditioning systems consists of fans and water pumps, the best way to save energy is to use frequency converters. Currently, most central air conditioning systems still use the older control method: adjusting the temperature by changing the number of compressor units, water pumps, and fans on and off. The disadvantages of this method are mainly as follows:
The equipment operates with all of its power on or off for extended periods, alternating between the two, resulting in a staggering waste of electricity.
Direct starting of the motor at the power frequency results in a large inrush current, which seriously affects the service life of the equipment.
The temperature control is ineffective. When the environment or heating/cooling load changes, the indoor temperature can only be adjusted by increasing or decreasing the number of hot and cold water pumps or by using a baffle, resulting in large temperature fluctuations and poor comfort.
Central air conditioning systems have the following advantages after adopting frequency converters:
Frequency converters can soft-start motors, greatly reducing inrush current, decreasing motor bearing wear, and extending bearing life.
The flow rate and pressure of water pumps and fans can be adjusted directly by changing the operating frequency of the frequency converter, which can reduce or eliminate the need for baffles and valves.
The system's power consumption has been greatly reduced, and noise has been decreased.
If a closed-loop temperature control method is adopted, the system can automatically adjust the air volume by detecting the ambient temperature, and automatically adjust according to changes in weather and heat load, with small temperature changes and rapid adjustment.
The system can be networked with the central control room via fieldbus to achieve centralized remote monitoring.
II. Variable Frequency Energy-Saving Retrofit of Water Supply System
In central air conditioning systems, whether lithium bromide or electric refrigeration (Freon) units, the energy consumption of the main unit itself is controlled by the unit, but the external power consumption cannot be controlled. This external power consumption is quite expensive but is often overlooked. Especially for lithium bromide units, when operating at rated cooling capacity, the power consumption of the external water pump and cooling tower motors accounts for approximately 30% of the total energy cost (calculated at 2 yuan per kilogram of oil and 1 yuan per kilowatt-hour of electricity). From both an environmental protection perspective and the user's benefit perspective, central air conditioning systems should be designed to be as energy-efficient as possible. Using frequency converters to control the external water pump and cooling tower motors is the simplest and most effective energy-saving measure. Generally, it saves 20% to 50% of electricity, resulting in a significant annual saving of 12% to 20% of the total operating costs of the unit and system.
1. Variable frequency control of cooling water pump
The power of the cooling water pump in a central air conditioning system is designed based on the full-load operation of the compressor in the air conditioning chiller unit. When the ambient temperature and various external factors reduce the need for the chiller unit to operate all compressor units, the cooling capacity required by the air conditioning condensing system also decreases accordingly. In this case, the speed of the cooling water pump can be adjusted using a variable frequency drive (VFD) to reduce the circulation speed and flow rate of the cooling water, allowing the cooling load of the cooling water to be fully utilized by the condensing system, thereby achieving energy savings. Data from our company's VFD energy-saving retrofit of central air conditioning systems shows that the cooling water pumps and chilled/warm water pumps can significantly save electricity when operating at low flow rates. This is especially significant considering the characteristics of the cooling water flow curve in direct-fired turbines. For example, consider the test data from the Broad Air Conditioning BZ type direct-fired turbine central air conditioning system using VFD-controlled water pumps:
When the cooling capacity is 75%, the unit requires 34% of the cooling water flow rate and the water pump power consumption is about 20%.
When the cooling capacity is 50%, the unit requires 22% of the cooling water flow and the water pump power consumption is about 15%.
2. Variable frequency control of cold and hot water pumps
The power of the refrigerant water pump in a central air conditioning system is designed for full-load operation. When the cooling or heating demand of a hotel, restaurant, or building does not reach the full load of the air conditioning system, the speed of the refrigerant water pump can be adjusted using a frequency converter to reduce the circulation speed of the refrigerant water, thus making full use of the cooling and heating capacity and achieving energy saving. If a single pump is used for both cooling and heating, the pump flow rate only needs to be 50% in winter, which can significantly save electricity. Even if separate pumps are used in winter and summer, the flow rate can be appropriately reduced during low-load seasons; for example, at 90% flow rate, the power consumption is approximately 75%.
3. Variable frequency control of cooling tower fan
Fans generally have lower power outputs, and their energy savings are not as significant as those of water pumps. However, using variable frequency control for the fans greatly helps maintain a constant cooling water temperature, which is crucial for maintaining a constant cooling temperature in the unit. It also ensures stable solution circulation within the unit, maximizing fuel savings. Running the cooling tower fan at low speeds also significantly reduces water drift, saving water resources, slowing water quality deterioration, and minimizing the impact of water mist on the surrounding environment.
4. Other benefits of using frequency converters
Because the inverter's starting and stopping processes are gradual, it can eliminate the impact of motor startup on the power grid and prevent motor failures caused by overload.
Because the motor often operates under low load, the lifespan of the motor, pump, and fan can be significantly extended. Furthermore, the absence of starting and stopping shocks, coupled with reduced flow, decreases the pressure and impact on the pipeline, thus protecting the pipes, valves, and terminal equipment. On the other hand, equipment noise and vibration are reduced, protecting the environment.
5. Control method of external frequency converter of central air conditioning unit
Adjust the water pump speed and flow rate according to the temperature of the cooling water inlet/outlet;
Adjust the cooling tower fan speed and water temperature according to the cooling water inlet temperature;
Adjust the pump speed and flow rate according to the temperature difference between the inlet and outlet of the cold and warm water;
Adjust the pump speed and flow rate according to the temperature of the cooling water outlet;
Adjust the pump speed and flow rate according to the return water temperature of the chilled water;
III. Central Air Conditioning Terminal Equipment—Variable Air Volume Unit Frequency Control
Variable air volume (VAV) units are also an important component of central air conditioning systems. The quality of their performance indicators (air volume, cooling capacity, noise, and power consumption) depends not only on the performance of the VAV unit itself, but also, more importantly, on the control mode, the performance of the controller, and its quality.
With the increasing popularity of central air conditioning, the variable air volume (VAV) unit controller has gone through three stages of development:
Phase 1: Air valve adjustment. This can regulate airflow, but it consumes a lot of electricity and generates a lot of noise.
The second stage: thyristor-controlled voltage and speed regulation. It can regulate air volume and cooling capacity, and save energy. It also has a certain effect on improving the noise of variable air volume units. Its disadvantages are large size, low reliability and stability, and high failure rate.
The third stage: Variable frequency drive (VFD) regulation. This stage can maximize the adjustment of air volume, cooling capacity, and noise levels required by variable air volume (VAV) units, resulting in more significant energy savings, smaller size, and higher reliability and stability.
Currently, variable frequency controllers are gaining popularity among central air conditioning professionals due to their unique advantages.
