Energy efficient model predictive building temperature control

Many systems used in buildings for heating, ventilating, and air-conditioning waste energy because of the way they are operated or controlled. This paper explores the application of model predictive control (MPC) to air-conditioning units and demonstrates that the closed-loop performance and energy efficiency can be improved over conventional approaches. This work focuses on the problem of controlling the vapor compression cycle (VCC) in an air-conditioning system, containing refrigerant which is used to provide cooling. The VCC considered in this work has two manipulated variables that affect operation: compressor speed and the position of an electronic expansion valve. The system is subject to constraints, such as the range of permissible superheat, and also needs to regulate temperature variables to set points. An MPC strategy is developed for this type of system based on linear models identified from data obtained from a first-principles model of the VCC. The MPC strategy incorporates economic measures in the objective function as well as control objectives. Tests are carried out on a simulated VCC system that is linked to a simulation of a realistic building that is developed in the U.S. Department of Energy Computer Simulation Program, EnergyPlus. The MPC demonstrated significantly better tracking control relative to conventional approaches (a reduction of 70% in terms of the integral of squared error for step changes in the temperature set-point), while reducing the VCC energy requirements by 16%. The paper describes the control approach in detail and presents results from the tests.     

多机制冷系统在线优化控制策略

阐述了可应用于大型商业建筑中多台制冷机系统的在线优化控制策略。策略的应用可增强控制鲁棒性并节约运营成本。优化控制策略包括冷冻水供水温度优化、制冷机时序控制、启动优化控制和建筑峰值负荷控制。冷冻水供水温度的优化使得系统在满足负荷的前提下制冷机和冷冻水泵的能耗最小。在时序控制中,3种方法的采用增强了控制的鲁棒性:第一,数据融合以获得可靠楼宇冷负荷的测量;第二,制冷机最大制冷能力简化模型的应用;第三,在线传感器故障诊断(FDD)的应用。在制冷机启动优化控制中,一个基于模型的策略用于最小化早晨启动时期内的系统能耗。这一基于模型的最优启动优化控制策略同时考虑了预冷能力和预冷时间的相互关系。建筑峰值负荷控制以优化月节电费为目标,通过预测合适的每月负荷阈值并依此限制每天的峰值负荷来实现这一目的。通过对香港一幢大型商业建筑内中央制冷系统的动态模拟,以上控制策略得到验证。     

空冷器在化纤聚酯项目中的应用

介绍空冷器在化纤聚酯项目中的一项应用成果。原聚酯生产工艺流程通过冷却水列管式换热器将100．5℃的酯化蒸汽降至55℃,现改为用空冷器和板式换热器进行冷凝冷却。改造后大大降低了运行成本,原水泵运行功率为420kW,改造后空冷器和板式换热器运行功率为88kW,节约电力332kW,并且基本不再需要冷却水,节能节水效果明显。论述了冷却方案的选择和空冷器的设计、空冷器的特点、运行结果和经济效益以及投资回收周期。     

Economic model predictive control of nonlinear process systems using Lyapunov techniques

In this work, we develop model predictive control (MPC) designs, which are capable of optimizing closed‐loop performance with respect to general economic considerations for a broad class of nonlinear process systems. Specifically, in the proposed designs, the economic MPC optimizes a cost function, which is related directly to desired economic considerations and is not necessarily dependent on a steady‐state—unlike conventional MPC designs. First, we consider nonlinear systems with synchronous measurement sampling and uncertain variables. The proposed economic MPC is designed via Lyapunov‐based techniques and has two different operation modes. The first operation mode corresponds to the period in which the cost function should be optimized (e.g., normal production period); and in this operation mode, the MPC maintains the closed‐loop system state within a predefined stability region and optimizes the cost function to its maximum extent. The second operation mode corresponds to operation in which the system is driven by the economic MPC to an appropriate steady‐state. In this operation mode, suitable Lyapunov‐based constraints are incorporated in the economic MPC design to guarantee that the closed‐loop system state is always bounded in the predefined stability region and is ultimately bounded in a small region containing the origin. Subsequently, we extend the results to nonlinear systems subject to asynchronous and delayed measurements and uncertain variables. Under the assumptions that there exist an upper bound on the interval between two consecutive asynchronous measurements and an upper bound on the maximum measurement delay, an economic MPC design which takes explicitly into account asynchronous and delayed measurements and enforces closed‐loop stability is proposed. All the proposed economic MPC designs are illustrated through a chemical process example and their performance and robustness are evaluated through simulations. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Development of a novel control strategy for a highly segmented injection mold tempering for inline part warpage control

The demand of precise injection-molded parts is steadily increasing and is today one of the most relevant challenges, due to local variations in temperature and pressure during the production of the part. These variations can lead to a significant change of the local specific volume, shrinkage potential, and inner stress, which ultimately results in part warpage. By homogenizing the local specific volume over the part according to the specific pvT-behavior of the polymer, warpage is expected to be reduced. The following work describes a new approach to control the local specific volume by a newly developed segmented and highly dynamic mold temperature control based on rapid heating ceramics and CO₂ evaporation chambers. Since injection molding is a dynamic process and heat transfer inside the mold is comparably slow, a special control strategy is necessary to activate the heating and cooling elements in advance. For this, a novel prediction strategy based on a discretization of the one-dimensional heat equation has been developed. Experimental trials including a classical PID controller and a model predictive control approach (MPC) show that the MPC is superior regarding the process stability.     

蓄冷型LNG重卡冷能利用空调系统全路况特性

为了满足LNG重卡驾驶空间的制冷需求,设计了一种具有蓄冷功能的LNG重卡冷能利用空调系统,构建了全路况行驶模型,并对LNG重卡在上坡段、平稳段和下坡段全路况下的制冷性能和影响因素进行了分析。结果表明：LNG重卡冷能利用空调系统上坡段的空调制冷量是6.150 kW,蓄冷量是0.551 kW;平稳段的空调制冷量是4.201 kW,蓄冷量是0.420 kW;下坡段的空调制冷量是2.902 kW,蓄冷器释放的冷量大于0.598 kW。该系统通过控制策略对蓄冷量进行合理的调控,满足LNG重卡全路况下空调系统稳定的供冷需求。     

Assessing plant design with regard to MPC performance

Model Predictive Control is ubiquitous in the chemical industry and offers great advantages over traditional controllers. Notwithstanding, new plants are being projected without taking into account how design choices affect the MPC’s ability to deliver better control and optimization. Thus a methodology to determine if a certain design option favours or hinders MPC performance would be desirable. This paper presents the economic MPC optimization index whose intended use is to provide a procedure to compare different designs for a given process, assessing how well they can be controlled and optimised by a zone constrained MPC. The index quantifies the economic benefits available and how well the plant performs under MPC control given the plant’s controllability properties, requirements and restrictions. The index provides a monetization measure of expected control performance.This approach assumes the availability of a linear state-space model valid within the control zone defined by the upper and lower bounds of each controlled and manipulated variable. We have used a model derived from simulation step tests as a practical way to use the method. The impact of model uncertainty on the methodology is discussed. An analysis of the effects of disturbances on the index illustrates how they may reduce profitability by restricting the ability of a MPC to reach dynamic equilibrium near process restrictions, which in turn increases product quality giveaway and costs. A case of study consisting of four alternative designs for a realistically sized crude oil atmospheric distillation plant is provided in order to demonstrate the applicability of the index.     

Actuator stiction compensation via model predictive control for nonlinear processes

The problem of valve stiction is addressed, which is a nonlinear friction phenomenon that causes poor performance of control loops in the process industries. A model predictive control (MPC) stiction compensation formulation is developed including detailed dynamics for a sticky valve and additional constraints on the input rate of change and actuation magnitude to reduce control loop performance degradation and to prevent the MPC from requesting physically unrealistic control actions due to stiction. Although developed with a focus on stiction, the MPC‐based compensation method presented is general and has potential to compensate for other nonlinear valve dynamics which have some similarities to those caused by stiction. Feasibility and closed‐loop stability of the proposed MPC formulation are proven for a sufficiently small sampling period when Lyapunov‐based constraints are incorporated. Using a chemical process example with an economic model predictive controller (EMPC), the selection of appropriate constraints for the proposed method is demonstrated. The example verified the incorporation of the stiction dynamics and actuation magnitude constraints in the EMPC causes it to select set‐points that the valve output can reach and causes the operating constraints to be met. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2004–2023, 2016     

热带假丝酵母发酵生产长链二元酸的节能减排措施

热带假丝酵母发酵生产长链二元酸在菌体、空气、烃类(油)和水的四相发酵体系中完成,发酵过程需氧量大、放热量高,提纯过程多处存在升温、降温过程,生产成本高。采用"倒推法"分析了装置公用工程消耗的主要影响因素,通过节能改造,余热、废汽与废水回收,正常生产每年可节省生产成本390余万元。     

液化天然气冷能构成及其利用方式探讨

液化天然气(LNG)在汽化过程中会释放大量冷能,如果这部分冷能被成功回收利用,其节能效果和对系统效率的提高都十分显著。文中对LNG冷能从冷量和冷量的角度进行分析,把LNG冷能回收方式分为冷量回收与冷量回收,揭示了目前各种LNG冷能回收利用形式的能量利用实质:发电、空分中主要是利用LNG的冷量;冷藏、空调和制干冰利用了LNG的冷量。最后对不同的冷能回收系统提出指导性建议:动力回收系统中,应充分利用其在低温下的高品质能量;冷量回收系统中应减少跑冷。     

机械通风横流式冷却塔组的节能试验研究

以多主机空调系统中的机械通风横流式冷却塔组为研究对象,对独立运行的塔组和连通运行的塔组以及在塔组中是否实行风路控制进行了冷却效果的比较。结果表明,在试验所测试的条件下,实行风路控制的连通运行的塔组与独立运行的塔组相比,单位负荷冷吨数提高了36.55%;与未采用风路控制的连通运行塔组相比,风机停用塔的平均风量提高37.65%,总风量提高6.9%,节能效果明显。     

使用NH3-LiNO3工质对的增压型回热吸收循环性能分析

近年来,日益增长的暖通空调系统能耗已接近50%的建筑能源消费量。吸收式循环可使用太阳能热能、工业废热等低品位能源产生制冷效果,进而降低夏季制冷负荷对高品味电能的大量需求。当前常用于吸收制冷循环的LiBr-H₂O工质对虽然COP较高,但由于物性限制了其蒸发温度范围以及存在较高的结晶风险,使得系统小型风冷设计存在限制。氨水工质对具有较宽的制冷温区,但由于需要精馏以提高氨气浓度造成COP较低。NH₃-LiNO₃工质对无须增设精馏器,结晶温度远高于LiBr-H₂O,且氨气压力较高适合在耦合压缩机循环以提升循环性能,扩宽运行温区。因此,本研究提出压缩机辅助的增压型回热吸收循环使用NH₃-LiNO₃工质对,并对其进行热力分析,研究压缩机的引入对循环性能的改进作用。结果显示,压缩机辅助作用下循环驱动温度下降至34℃,蒸发温度亦可降低至-34℃,且循环倍率降低了52.16%,更适于小型风冷设计。     

Soft-constrained model predictive control based on data-driven distributionally robust optimization

This article proposes a novel distributionally robust optimization (DRO)-based soft-constrained model predictive control (MPC) framework to explicitly hedge against unknown external input terms in a linear state-space system. Without a priori knowledge of the exact uncertainty distribution, this framework works with a lifted ambiguity set constructed using machine learning to incorporate the first-order moment information. By adopting a linear performance measure and considering input and state constraints robustly with respect to a lifted support set, the DRO-based MPC is reformulated as a robust optimization problem. The constraints are softened to ensure recursive feasibility. Theoretical results on optimality, feasibility, and stability are further discussed. Performance and computational efficiency of the proposed method are illustrated through motion control and building energy control systems, showing 18.3% less cost and 78.8% less constraint violations, respectively, while requiring one third of the CPU time compared to multi-stage scenario based stochastic MPC.     

Space charge segregation during the cooling process and its effect on the grain boundary impedance in Nb-doped BaTiO3

Highly donor-doped and large grained BaTiO₃ ceramics can be prepared by sintering in a reducing atmosphere. If these specimens are again heat treated at a certain sintering temperature and cooled in air, PTCR (positive temperature coefficient resistor) characteristics appear. When the cooling rate is low, another third RC (resistance-capacitance) component appears besides that of the grains and grain boundaries in impedance analysis. This RC component is associated with donor segregation. It is not observed when the cooling rate is high. In the slightly donor-doped specimens where the cooling rate, microstructure and R (resistivity)–T (temperature) properties are similar to the former, this third RC component also does not appear. The segregation related phenomena were confirmed by STEM/EDS analysis. These trends correspond to the space charge segregation theory that explains that the space charge potential, which acts as the driving force for donor segregation, becomes large during the cooling process in air.     

The Ebeye desalination project — total utilization of diesel waste heat

The Ebeye (Marshall Islands) sea water desalination project produces 300,000 US GPD (1100 m³/day) of pure distilled water utilizing as its prime energy input the waste heat discharged from an adjacent diesel generator station, operating at an average load of 3.2 MW. The only other energy requirement is electric power for process, heat recovery and seawater supply pumping - at about 9.5 KWH/1000 gal. (2.5 KWH/m³).The desalination plant consists of one 12 effect low temperature multi-effect distillation unit, operating at a top brine temperature of 158 degrees F (70 degrees C) with simple polyphosphate feed pretreatment.The diesel generator station consists of two 2.4 MW diesels, with provision for the addition of a third as demand increases.Heat is recovered from the diesel's exhaust gases, jacket cooling water, lube oil and compressed air after coolers.The paper describes the total water and power generating scheme (including the heat recovery system) uts performance and costs.     

直冷式乏风热泵系统在小保当煤矿的应用分析

为实现煤矿企业清洁能源供热,针对煤矿生产矿井排出的乏风余热,采用目前国际先进的直冷式深焓取热乏风热泵系统供热技术替代传统的锅炉供热,解决我国北方煤矿企业冬季有防冻要求的进风井口供热需求。本文以小保当煤矿矿井乏风余热供热系统项目为例,阐述了直冷式深焓取热乏风热泵系统的间壁式（间接换式）乏风换热、乏风取热机组集尘及结霜原理;采用乏风取热机组清洗除尘和防结霜除霜解决方法,并与天然气锅炉、电锅炉两种供热方案进行对比,分析计算了三种方案的项目投资和系统运行费用,论述了各方案的优势和不足。分析表明,煤矿企业采用矿井乏风余热资源,利用直冷式深焓取热乏风热泵供热系统替代传统锅炉供热,技术可行,且具有更好的经济效益和应用效果。     

Dynamic optimization of a dividing-wall column using model predictive control

Dividing-wall column (DWC) is one of the best examples of process intensification, as it can bring significant reduction in the capital invested as well as savings in the operating costs. Conventional ternary separations progressed from the (in-)direct sequences to thermally coupled columns such as Petlyuk configuration, and later to the DWC compact design that integrates the two distillation columns into a single shell. Nevertheless, this integration leads also to changes in the control and operating mode due to the higher number of degrees of freedom.In this work we explore the dynamic optimization and advanced control strategies based on model predictive control (MPC), coupled or not with PID. These structures were enhanced by adding an extra loop controlling the heavy component in the top of the feed side of the column, using the liquid split as manipulated variable, thus implicitly achieving energy minimization. To allow a fair comparison with previously published references, this work considers as a case-study the industrially relevant separation of the mixture benzene–toluene–xylene (BTX) in a DWC.The results show that MPC leads to a significant increase in performance, as compared to previously reported conventional PID controllers within a multi-loop framework. Moreover, the optimization employed by the MPC efficiently accommodates the goal of minimum energy requirements – possible due to the addition of an extra loop – in a transient state. The practical benefits of coupling MPC with PID controllers are also clearly demonstrated.     

An embedded fault detection, isolation and accommodation system in a model predictive controller for an industrial benchmark process

Fault detection and isolation (FDI) for industrial processes has been actively studied during the last decades. Traditionally, the most widely implemented FDI methods have been based on model-based approaches. In modern process industry, however, there is a demand for data-based methods due to the complexity and limited availability of the mechanistic models. The aim of this paper is to present a data-based, fault tolerant control (FTC) system for a simulated industrial benchmark process, Shell control problem. Data-based FDI systems, employing principal component analysis (PCA), partial least squares (PLS) and subspace model identification (SMI) are presented for achieving fault tolerance in co-operation with controllers. The effectiveness of the methods is tested by introducing faults in simulated process measurements. The process is controlled by using model predictive control (MPC). To compare the effectiveness of the MPC, the FTC system is also tested with a control strategy based on a set of PI controllers.     

Aspen Plus Dynamics软件在多变量模型预测控制中的应用

多变量模型预测控制是一种能够处理大滞后、强耦合和非线性等问题的先进控制算法,核心是多输入、多输出的预测模型。工业应用中通过对实际装置进行测试来回归模型参数,需要较长的实施周期。介绍了Aspen Plus Dynamics软件在搭建预测模型中的应用。