Research and simulation of air conditioning water system control based on LabVIEW

In intelligent buildings , the energy consumption of air conditioning systems accounts for an increasing proportion of the national economy, among which the energy consumption of water-side components (chillers, refrigeration pumps, cooling pumps, cooling tower fans) accounts for about 60%-80% of the entire central air conditioning system. Therefore, the optimization research of air conditioning water systems is particularly important. In recent years, the variable frequency energy-saving technology of refrigeration secondary water pumps has been increasingly used in central air conditioning systems. This air conditioning system that can adjust the flow of chilled water according to changes in cooling load is called a VWV system.

There are many ways to control the secondary pump frequency in the vwv system, mainly pressure or pressure difference control, temperature or temperature difference control, flow control, valve opening control, etc., but these control methods have their own shortcomings. Next, this article will make a simple analysis and discussion of the widely used pressure difference and temperature difference control.

2 System performance of pressure difference and temperature difference control

2.1 Pressure difference control method

The method of controlling the supply and return water pressure difference by adjusting the speed of the secondary pump group is called the pressure difference control method. In this method, the control value p is set according to the system loop characteristics. The controller compares the pressure difference △p measured by the pressure difference sensor with the control value p. If △p>p, the controller reduces the speed of the secondary pump group, otherwise, it increases the speed of the secondary pump. The control block diagram is shown in Figure 1.

Figure 1 Block diagram of pressure difference single closed loop control

There are several disadvantages of differential pressure control. First, the set value p is difficult to determine. Second, in order to meet the most unfavorable loop load, the set pressure difference is often large, which is not conducive to energy saving. Third, in the differential pressure control method, since the pressure difference at the load end is constant, when the flow rate of the entire loop tends to zero, the loop pressure drop tends to the set value p, rather than zero, as shown in Figure 2.

Figure 2 Relationship between chilled water flow and supply and return water pressure difference

Figure 2 is a graph showing the relationship between chilled water flow and supply and return water pressure difference. Curve y is the ideal working pressure difference, that is, when the system cooling load decreases, the given pressure difference should also decrease to reduce the chilled water flow and maximize energy savings. Curve y' is the set pressure difference under pressure difference control, which does not change with the flow rate, so the energy saving effect is greatly reduced.

2.2 Temperature difference control method

The temperature difference control method controls the speed of the secondary pump group according to the supply and return water temperature difference of the secondary pump, so that the supply and return water temperature difference is maintained at the set value, achieving constant temperature difference and small flow operation under low load, saving the delivery power of the secondary pump group and achieving the purpose of energy saving, as shown in Figure 3.

Figure 3 Temperature difference single closed loop control block diagram

The temperature difference calculator transmits the calculated supply and return water temperature difference to the controller, and the controller compares △t with the preset temperature difference value. If △t<△t', the output frequency of the inverter is reduced ; if △t>△t', the output frequency of the inverter is increased.

The disadvantages of temperature difference control are also obvious. The change in temperature is not reflected as quickly as the change in pressure difference. Therefore, there is a control lag in the temperature difference control method. For systems with fast load changes, the control accuracy of this control method is not high.

3. Introducing cascade control

According to the above analysis, we know that pressure difference control and temperature difference control have unsatisfactory aspects in their respective single closed-loop control loops. Pressure difference control has a quick response and high control accuracy, but the energy saving effect is greatly reduced due to the problem of set value; and temperature difference control has a large lag phenomenon. If we can take advantage of the strengths of these two control methods and make up for their weaknesses, the control quality will be improved. So we introduce cascade control. Its system block diagram is shown in Figure 4.

Figure 4 Cascade control principle block diagram

The cascade control system has one more sub-loop than the single-loop control system, thus forming a double closed loop. Its main loop (outer loop) is a fixed value control system, and the sub-loop (inner loop) is a follow-up system. Generally speaking, the controlled parameter of the outer loop has a large lag. The main regulator calculates the given value of the inner loop based on the deviation of the outer loop. The inner loop should be a loop with a small pure lag. Before the main disturbance affects the main parameter, the sub-loop can control it in time, thereby improving the control quality.

According to the characteristics of the above-mentioned cascade control, we use the large lag object supply and return water temperature difference as the outer loop parameter control object, and the chilled water flow rate as the inner loop parameter to adjust the refrigeration secondary pump frequency. The control block diagram is shown in Figure 5.

Figure 5 Cascade control block diagram of air conditioning water system

As can be seen from Figure 5, when the disturbance (room cooling load) changes, it first affects the chilled water valve to change its opening, thereby affecting the chilled water flow. The sub-regulator quickly adjusts the secondary pump frequency according to the deviation. If the disturbance is not large, timely adjustment through the sub-circuit generally does not affect the supply and return water temperature difference; if the amplitude of the disturbance is large, although it is promptly corrected by the sub-circuit, it still affects the chilled water temperature difference. At this time, the main circuit is further adjusted to completely overcome the above disturbance and adjust the supply and return water temperature difference back to the given value.

4 Solution Verification

4.1 Identification of primary and secondary objects

The cascade control scheme is actually operated and analyzed in the variable air volume air conditioning laboratory of the Intelligent Building Research Institute of Xi'an University of Architecture and Technology. The water side of this laboratory consists of a cooling tower, two chillers (including cooling pumps and primary chiller pumps), a secondary chiller pump and valves that adjust the chilled water volume of the two AHUs. The structural diagram is shown in Figure 6.

Figure 6 Schematic diagram of air conditioning water system structure

The least square method is used to identify the main and auxiliary objects. For the SISO discrete random system, the description equation is:

The least squares format of the system input and output can be obtained:

y(k) = ht(k)θ+ e(k)

For the secondary loop, the secondary pump frequency value is used as the excitation, the chilled water flow before AHU is used as the response, and the ARX model is used. The transfer function identified by the least squares method is:

Similarly, for the main loop, the chilled water flow before ahu is used as the excitation, and the chilled water supply and return temperature difference is used as the response. The transfer function of the main object is identified as:

4.2 Design of main and auxiliary controllers

The controller adopts PID control, and the control law of PID is

In this cascade control system, the main regulator and the sub-regulator have different adjustment tasks. The lag time of the sub-object is much smaller than that of the main object. The task of the sub-regulator is to act quickly to quickly offset the secondary disturbance falling in the sub-loop. It does not require zero error, so the p regulator should be selected. The task of the main regulator is to accurately keep the regulated quantity in line with the requirements. No deviation is allowed. Therefore, an integral link should be added to the main regulator, that is, a pi regulator or a pid regulator.

4.3 System simulation based on LabVIEW

LabVIEW is an industry-leading industrial standard graphical programming software, mainly used for card testing, measurement and control systems. It is an intuitive graphical programming language designed specifically for engineers and scientists. It integrates software with various measuring instrument hardware and computers to establish a virtual instrument system to form a user-defined solution.

Like the add-on toolkit of MATLAB, LabVIEW provides modules with various functions. This simulation is realized through the simulation module. The background graphical simulation program is shown in Figure 7.

Figure 7  Labview cascade control simulation block diagram

Among them, the subvi named pid is the pid mathematical expression

Figure 8 pid subvi background graphical program

The step response simulation results of the cascade control are shown in Figure 9. It can be seen that the overshoot of the system is about 10%, and the rise time and adjustment time are satisfactory.

Figure 9 System simulation step response curve

5 Conclusion

Through the simulation results of the cascade control system based on single-loop control, it can be seen that the cascade control strategy has a good performance in improving the rapidity of the system and eliminating the blind spots of differential pressure control. The next study will add the simulation results to the actual system and prove the feasibility and rationality of the control strategy after a period of experimental testing.