Weak current College】 【based on load forecasting of chilled water flow dynamic control technology --- Power By】 【China power house network.

<br> <br> 【Summary】 this article through the existing central chilled water system variable flow control limitations of analysis, based on the load forecasting of chilled water flow dynamic control concept, and a detailed look at load forecasting control of basic ideas and .control principles for central air-conditioning chilled water system variable flow control provides a completely new control technology. <br> Words】 【chilled water flow variable flow control dynamically load forecast <br> <br> 1 existing central chilled water flow control limitations < .br> <br> With the increasing emphasis on air conditioning energy efficiency, variable flow water systems in the air conditioning works more and more. Currently, the most common of frozen water system variable flow control, mainly in the constant pressure control and constant temperature .control. Them to chilled water system of pressure or temperature difference is charged with parameters, using PID (or PI) algorithm control inverters pumps operating frequency, the pump discharge followed accused parameter changes, so as to achieve the purpose of the pump energy saving. .But these two control modes are all of the major limitations: a is charged with parametric limitations of their own, and control the technical limitations. <br> 1.1 charged parameters own limitations <br> (1) the limitations of the constant pressure control < .br> Constant pressure control, is in the frozen water system piping where appropriate differential pressure sensor installed on Δ P, testing its poor water pressure for back and transmitted to the program logic controller (PLC), PLC as a controller, the measured pressure value .and the set pressure value, depending on the size of the deviation using PID (or PI) algorithm control inverter output frequency driven pump speed run, enabling flow control purposes. <br> The advantage of constant pressure difference control is that when the load side .flow fluctuation frequently, because of the pressure differential response time delay of smaller, faster to follow the flow of changes and corresponding changes, adjust the time shorter. <br> Constant pressure difference control of greatest shortcoming, it is a fatal defect, it lies .in the frozen water system load and pressure differential is no direct relationship between the change of air conditioning load does not accurately through changes of pressure difference to describe, in the same way, the pressure differential changes do not accurately reflect the change of air conditioning load .. Therefore, one does not completely reflect the air-conditioning load variation of pressure as charged variable to adjust the frozen water, obviously is not possible to guarantee the frozen water of accurately following the load changes. <br> Chilled water system at the end .of the load of pressure or backwater pipeline pressure difference between is determined by its resistance to the flow resistance, if it does not change (that is, the water does not change), the pressure will not change. Therefore, when the system of water .remains the same, but the temperature of the chilled water flow rate with the person or the outdoor temperature changes substantially change, air-conditioning load has changed, but the pressure difference did not change. At this point, the constant pressure difference control will cause .the system to control completely dead. Therefore, the constant pressure control is only applicable to load changes and accompanied by significant changes in water flow (that is, there is pressure variations), otherwise, the control will be invalidated. <br> (2 .) the limitation of thermostatic control <br> Constant temperature control, is in the frozen water supply and return water dry pipe installation of temperature sensors respectively, backwater temperature detection for, and transferred to the PLC, PLC as a controller, the temperature difference between .the measured values and set the temperature value, depending on the size of the deviation using PID (or PI) algorithm control inverter output frequency driven pump speed run, enabling flow control purposes. <br> Constant temperature difference control advantage is, chilled water temperature .changes of backwater directly reflects changes of air conditioning load. Therefore, the frozen water supply and return water temperature differences as being control variables, you can get the correct control results. <br> Thermostatic control of disadvantage, since the end of the temperature acquisition .points for HotSpot at quite a distance from the chilled water through a loop, the temperature changes to be reflected. Air conditioning pipe usually have longer, frozen water cycle up to a few minutes to several tens of minutes. Regardless of any time, as measured .by the return water temperature TD2 is in fact the last few minutes to several dozen minutes prior to effect of air conditioning water temperature TD1, which reflects a cycle before the load situation, therefore, the same moment the detection of supply and return water temperature difference ., as the accused parameters for frozen water flow regulation, is clearly problematic. <br> On the other hand, because of frozen water system of thermal capacity, inert, temperature response slow. When the load event of catastrophe, or to wait until the .temperature slowly reaction out to produce the appropriate regulatory action, there is a larger control time lag, affected the timeliness and speed control. <br> Limitations of 1.2PID control technology <br> At present, neither the frozen water of poor control of constant .pressure or constant temperature control, are classic PID control. <br> PID control, also known as the PID is the control for proportional, integral, differential control by P, I, D three links of different combination. PID has a long history, .simple structure, easy to use, adaptability, robustness, low investment in industrial production process automatic control field for a wide range of applications, as the main industrial control technology. <br> PID control is a kind of negative feedback control, is charged with .the proportion of deviation signal parameters, integral and differential quantity control, depending on the system, use proportional, integral, derivatives calculated control volume control. Its controller structure and parameters must be based on the engineering experience and field debugging to determine. For a given .accurate model of the charged object or linear objects, it is an ideal way of control. <br> In PID control, proportional, integral and differential effects of three kinds of control. <br> Proportional control (that is, P) has the .advantage that, once produced, the deviation of the controller immediately have control, allowing the accused a variable in the direction of reducing deviation, its control of strength depends on the scale coefficient Kp. <br> Proportional control of drawback is, with the balance .(ie system step response end value is a finite value) of the charged object exists staticDeviation. Increase the Kp can reduce static deflection, but Kp is too large, can lead to a deterioration in dynamic performance and even cause the closed-loop system instability .. <br> Integral control (that is, I control) the advantage is that it can be memory and integral deviation, conducive to the Elimination of static deflection. <br> Integral control weaknesses are: the integral role with latency, integral control effect .is too strong can make the control's dynamic performance variation, so that the system is unstable. <br> Differential control (D) the advantage is that it can be to the deviation, sensitive out deviation differential changes in trends. Increase the role of differential .control can speed up the system's response to overshoot of decrease and increase system stability. <br> Differential control disadvantage is that it interferes with the same sensitivity, allowing the system's ability to reduce interference. <br> PID control by the application process, the .need for different objects to charged proportional coefficient Kp, integral time constant Ti and differential time constant TD three engineering parameters setting. Practice shows that the process of setting parameters, you are actually on the proportional, integral and differential effect of the three parts of the .control. It should be noted that, although there are many PID tuning methods and empirical formula, however, this setting work not only time-consuming, but the interaction between the parameters that are often difficult to achieve the desired results. <br> PID .control cannot solve the stability and accuracy. Increase the control function, can bring variation reduction, improved accuracy, reduced stability. Conversely, in order to guarantee stability, limit control role, but also reduce the accuracy of the control. <br> Moreover, .even if the accused objects tuning a set of relatively satisfied with PID control parameters, when the system load or object property is changed, the control performance is significantly worse, often required parameters. <br> For central air-conditioning chilled water system so charged .object (or process), because of its inherent characteristics of Nonlinear and lag, as well as external environment disturbance of when the uncertainty and variability and other factors, make the frozen water system control more difficult. Generally speaking, uses the PID of this linear .control mode, it is difficult to achieve an ideal control effect, require the use of new technologies and methods of control. Based on load forecast freezing water flow dynamic control technology, is one such new technologies and methods of control. <br> 2 based .on load forecasting of chilled water flow dynamic control of basic ideas <br> Chilled water system due to circulation period longer, coupled with the frozen water of the thermal capacity is very large, therefore, there exists also the delivery of latency, capacity lag. .On such a large delay system, control more difficult to control the process of controllability and variability. Often control directives issued after a water cycle t τ has not been concluded, or charged with parameters (eg temperature) also have emergency response in the future, .the system load and operating conditions may have changed. Control system for the collection to the new changes, and will issue a new control directive, which causes the system to run disorder or oscillation, it is difficult to achieve the desired control requirements. <br .> In order to improve the quality of control delay system, as early as 1957, Smith (OJMSmith) proposed a model-based Predictor compensation control method. Its design is that the process in advance estimate out basic perturbations of dynamic response, and then .by the Predictor for compensation to be delayed the latency τ of charged ahead of feedback to the controller variable so that the controller action ahead of time, thus greatly speeding up the adjustment process, and reduce the amount of the overshoot. <br> Smith Predictor .compensation control method is suitable for a given signal changes in the system output changes. Its greatest weakness is the error on the process model is very sensitive. If the delay-time model and the actual value of the difference between the τ is large, the .system of control of quality will be much lower. And chilled water system variable flow at run time, the cycle time is changing, the delay time τ is changing, and therefore you cannot simply take Smith Predictor control. <br> Chilled water system of .this kind of delay characteristics is objective and existent, unable to change or eliminate, the control system of the output always goes through time τ. But Smith Predictor control methods, to solve one of the effective control of hysteresis, is to make the control function .of the "Advanced" is the implementation of control actions ahead of time. <br> In fact, the early implementation of control actions, many air conditioning management and operations personnel are in use. For example, in the morning before work, many air .conditioning systems will be prematurely. Pre-start time is often based on practical experience's judgement, this advance is to overcome the lag of frozen water system (especially the capacity lag). But early on the number of cold water unit, also depends on the .operator on the next window air conditioning load (chilling) predictive judgment. If the weather is large, or large load changes (such as the arrangement of conferences, etc.). Is appropriate to increase the post count or adjust the power on time in .advance. <br> Obviously, the artificial advance control can better overcome the adverse effects of the delay of the system. If the use of computer technology and modern control technology in place to achieve that kind of load forecasting and early control, creates a completely .frozen water flow control modes-load forecasting control. <br> "Load forecasting control" is based on an air conditioning system load predictions on the flow of frozen water system. The basic idea is, through a comprehensive system parameter detection and analysis of historical .data, forecasts and reasoning "next time" system load and optimal operating parameters, and then under the system of real-time delay time τ, on frozen water system is controlled in advance. <br> "Load forecasting control" completely different from the .current air conditioning energy saving control areas popular constant pressure difference control and temperature error control mode. Constant pressure control mode with constant temperature difference control belongs to "follow the control", that is to follow the deviation of the accused of the variable. Only when .the differential pressure or temperature deviation signal generation, the control system will control signal output. The following control mode, generally only applies to no delay of charged object or process, because there is no time lag, deviations once can be produced immediately correctingPositive. Pressure .control although delay is relatively small, but that is not an accurate reflection of the change of air conditioning load; temperature control can reflect load changes, but delay is too large to control function always lag behind the deviation occurred a delay τ, making constant pressure .difference control and constant temperature difference control are difficult to obtain good control effect. <br> The "load forecasting control" is a kind of "advance control", it was essentially with the following control. It passed on freezing water system water temperature, .water temperature, flow, pressure, and other parameters and outdoor environment temperature detection, use of the database on the various operating parameters for statistics, analysis, calculations and historical data (such as the current moment, the day before at the moment, the previous .week on the same day at the moment of data) on the basis of reasoning predict air conditioning system "future moment" load (cold), ahead of a time τ produce control actions, on frozen water flow regulation, so that the system of cooling .capacity and load requirements of cooling capacity, in order to eliminate the deviation between the supply and demand. <br> Visible, the "load forecasting control" does not passively follow the deviation signal actions, but through the load forecasting initiative to advance a time .τ actions to ensure system cooling and load with cold quantity equivalent, at the time synchronization between supply and demand, thus eliminating the chilling of the quantity and time difference. <br> 3 air conditioning load forecast <br> Application of the "load forecasting .control "technology and one of the key lies in the air-conditioning load of an accurate prediction, it determines the effectiveness and quality control. <br> Due to changes in external meteorological parameters and internal liquidity and other factors, the air conditioning load always .changes, in order to provide a basis for system control, it is necessary for the air conditioning system load dynamically forecast, or predict the load changes, it is the "load forecasting control" fundamental prerequisites. <br> In the control process, mainly .to predict the future day or a week's load change curve, ie short-term forecast. As a result of air conditioning load under the weather and working system to periodic changes in factors, often showing a cyclical variation in the same day each week and the .daily at the moment of the load curve there is great similarity. Therefore, you can use short-term prediction of commonly used methods, namely the classic seasonal exponential smoothing model or improved seasonal exponential smoothing model. <br> Exponential smoothing is based on the .forecast object itself historical data to make predictions, part time series prediction technology areas, it is easy, and have good predictive accuracy. Improved exponential smoothing, and is the basis of the above methods, combined with building air conditioning load and the relationship between factors .- air conditioning load and daily average temperature of the strong correlation between, on the part of the factor was amended, you can further enhance the load forecasting accuracy, and reduce application difficult forecasting model. <br> Air conditioning buildings daily load General subject to .both internal and external factors. Building internal impact comes from the internal production process equipment, electrical equipment, lighting equipment, the human body, the amount of heat and humidity. Building external influence that comes from the Sun's radiation into heat and indoor and outdoor air .temperature difference between the envelope incoming heat. These factors, internal factors were relatively more stable, less volatility, and external factors is not stable, the fluctuation range is large and with the season. <br> Predicting air conditioning load dynamic, you need to .set up a data more accurate, content rich database, therefore, must be on the air conditioning system of the actual operation of large amounts of data collection. Through a variety of sensors, real-time acquisition air conditioning system operating parameters (such as frozen .water supply temperature, water temperature, flow and pressure differential, etc.) and the outdoor temperature, etc. According to these data acquisition, control system for air conditioning system in the entire air-conditioning Intraseasonal daily hourly actual load and the actual daily hourly .temperature data, and use of computer technology and database technology to store statistics, records, and for load forecasting. <br> Air conditioning load dynamic forecast is based on historical data for hourly load and the actual acquisition of the various data and, if necessary ., you can refer to the typical buildings of the hourly load coefficient, estimation of air conditioning system in the future of hourly load and may total load. <br> Take advantage of the full load measured and temperature data, you can fully validation and correction .of air conditioning load dynamic forecasting models to continuously improve the load forecast accuracy. Since the load forecast there will always be some unforeseen factors or difficult to consideration, so must achieve 100% accuracy is not realistic, but through efforts to achieve 90% of load .forecasting accuracy is possible, that is, load forecast values and deviations of the measured values can be controlled in ± 10%. This essentially eliminates the chilled water system variable flow control of blindness and delays in obtaining a satisfactory control effect. <br> Through .the different regions and different types of central air-conditioning systems of a large number of project data collection can be applied to hotels, shopping malls, airports, office buildings, office buildings, factories, and other various types of central air conditioning system load dynamic .prediction model and a typical database, the establishment of a typical building air-conditioning load of time distribution curves, including the daily load distribution curves and curve in load distribution, the "load forecasting control" advanced control mode of the application to create good conditions .. <br> 4 based on load forecasting of chilled water flow dynamic control technology <br> (1) based on load forecasting of chilled water flow dynamic control works <br> "Load forecasting control" is a dynamic control, PID control is not .possible, the need to introduce new control technology: intelligent fuzzy control technology. <br> <br> Based on load forecasting of fuzzy control, the basic idea is to let the computer simulation of the human brain's intelligent decision behavior realization of frozen water of .the dynamic control, it does not have a predetermined fixed charged parameter to the given value, which is to load forecast output value as the basis for the control of fuzzy controller (given value). <br> Based on load forecasting of fuzzy control process .is as follows: <br> When the system is running, through a variety of sensor data acquisition and control related to the operation parameters and pass to information processorThe acquisition of parameters including chillers evaporator flow of frozen water supply temperature TD1, from the end of .the heat exchanger flow back into the chiller evaporator temperature freezing water backflow, frozen water flow TD2 GD, chilled water for back pressure difference Δ P, outdoor environment temperature Tf, etc. <br> Information processor to complete a variety of information processing and current .air conditioning system load calculation of Q and transfer to the load database and load forecasting and comparer. Load the database storage, there is a wide range of historical load data, including recent and earlier. <br> Load Predictor based system of historical load data ., current load data and impact load of outdoor environment temperature, and so on, depending on the model for predicting air conditioning load dynamic, anticipating HVAC system next time (such as frozen water cycle of one week of total delay time τ) load (chilling .) QD, transmitted to the fuzzy controller. <br> Fuzzy controller performs a comparison to be charged with load variable bias and bias change, use of fuzzy control rules library inference rules or rules table, when model human judgement, using fuzzy logic and fuzzy .reasoning method, get access to the expected cold system optimization run QD parameters (such as frozen water flow GD) of fuzzy control values. <br> Inference that optimize operating parameters of fuzzy control value, through clearer (solutions for fuzz) converted to precise .control values, through the implementation of the converter (ie pumps intelligent control cabinet) to control the charged object (schp) number and speed, to regulate the flow of frozen water cycle, time after time τ a / c system required cooling capacity QD. .<br> Then, test time τ beyond actual load (frozen water cooling capacity) and load forecasting, QD to compare load forecasts to assess the effect of control and, based on comparison and assessment of the situation, make the appropriate amendments to the regulation .: <br> When the actual load detected is greater than the projected load, you increase the speed for chilled water pumps, increasing the frozen water, in order to increase the supply of cold, guarantee amount at the end of the cold. <br .> When the actual load detected is less than forecast, the lower the speed for chilled water pumps, decrease the frozen water, in order to reduce the amount of supply, reduce cold extra cold energy transfer. <br> By constantly repeated testing, comparing ., repeatedly amended regulation, can achieve the chilling frozen water system of supply and end load requirements, security in the various load conditions, frozen water system is the best cold energy supply, reduce unnecessary waste of chilling. At the same time, because the frozen .water pump adopts frequency conversion technology that can greatly reduce the energy consumption of the pump motor. <br> (2) based on load forecasting of chilled water flow advantages of dynamic control <br> Based on load forecast freezing water flow dynamic control is central .air-conditioning control areas of an innovative technology that is more suitable for central air conditioning this cannot establish a precise mathematical models of complex systems, able to effectively overcome the inertia of frozen water system and a delay of adverse effects to both smooth and best control .effect of energy saving. The main advantages are: <br> Based on load forecasting of fuzzy control with powerful online calculation function, it detects the temperature of the chilled water flow and load calculation of air conditioning (cold), and direct use of air .conditioning load (cold) as charged with variable, an effective solution to an existing control to temperature or pressure difference to characterization of air conditioning load. As GB50189-2005 the public building energy-saving design standards "section 5.5.4 noted in the article .: "many projects is the total return water temperature to control, but because of refrigeration units for highest efficiency point usually located on the unit a part of the load area, therefore adopts the cold control way than using the temperature-controlled more conducive to the .refrigeration unit in high efficiency energy-saving, running and region is the most affordable and energy efficient control ways. "<br> Based on load forecast freezing water flow dynamic control, is a kind of" advance control ". It uses a database on .air-conditioning load of situation records, statistics, analysis, calculations and reasoning, on the basis of past periods of air conditioning load and its trends, and considering the building air-conditioning load characteristics, outdoor temperature, freezing water cycle, and other factors ., on the future of air conditioning load forecasting and prediction of load regulation of "advance", an effective solution to the frozen water of the large delay, large inertia systems lag problems and improves the stability of the system is running. <br> Based .on load forecast freezing water flow dynamic control, is a "best output energy control" that is based on the quality of services at the end of the "energy balance". It is based on air-conditioning load of predicted values and the deviation of .the measured value, adjust the frozen water flow, and to adjust the effects for dynamic evaluation and revision, to ensure service quality for cold cold supply and demand matching. <br>.