ASHRAE HVAC APPLICATIONS IP CH 42-2015 SUPERVISORY CONTROL STRATEGIES AND OPTIMIZATION.pdf

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1、42.1CHAPTER 42SUPERVISORY CONTROL STRATEGIES AND OPTIMIZATIONTERMINOLOGY 42.1METHODS. 42.3Control Variables . 42.3Supervisory Control Strategies 42.4Static Optimization 42.4Dynamic Optimization . 42.5CONTROL STRATEGIES AND OPTIMIZATION FOR COOLING SYSTEMS 42.8Control Strategies for Cooling Tower Fan

2、s . 42.8Chilled-Water Reset with Fixed-Speed Pumping . 42.12Chilled-Water Reset with Variable-Speed Pumping. 42.13Sequencing and Loading Multiple Chillers 42.16Simplified Static Optimization of Cooling Plants 42.21Dynamic Optimization for Cooling Using Discrete Storage 42.27Dynamic Optimization for

3、Cooling Using Thermal Mass or Tabs. 42.31Forecasting Diurnal Cooling and Whole-Building Demand Profiles . 42.36Black-Box Predictive Cooling Control Strategies 42.38Control Strategies for Heating Systems 42.39Control Strategies for Air-Handling Units. 42.42Control Strategies for Building Zones 42.43O

4、MPUTERIZED building and energy management and con-Ctrol systems provide a variety of effective ways to reduce utilitycosts and energy consumption associated with maintaining environ-mental conditions and thermal comfort in buildings. These systemscan incorporate advanced control strategies that resp

5、ond to inputsincluding changing weather, building conditions, occupancy levelsand utility rates to minimize operating costs, energy consumptionand greenhouse gas emissions while also enhancing occupant com-fort. This chapter focuses on the opportunities and control strategiesassociated with using su

6、pervisory control strategies and optimizationmethods applied to cooling systems, heating systems, air handlingunits and zone equipment.HVAC and other building energy systems are typically controlledusing a hierarchical control structure where two or more levels ofcontrol, from local through to super

7、visory level, are combined toform a sophisticated control system designed to achieve particularhigh-level functions or objectives, such as maintaining temperaturewithin a space. With this control philosophy, controller intelligenceincreases from lower to higher levels within the hierarchy. The low-e

8、st control level typically exists only to provide local-loop controlof a single set point through manipulation of an actuator. For exam-ple, the supply air temperature discharged from a cooling coil is con-trolled by adjusting the opening of a valve that provides chilled waterto the coil. The upper

9、control level, typically called supervisorycontrol, specifies the set points and other modes of operation that aretime dependent. System performance monitoring capabilities mayalso be provided at this level. Ideally, the supervisory control levelwould determine optimal set points and operating modes

10、 that mini-mize operating cost and/or energy consumption, maximize comfort,and may also identify potential faults or alarms in the control system.Distributed control structures have also been applied to buildingenergy systems, although further research is required to determinetheir efficacy and the

11、benefits they may provide over more tradi-tional and well-understood control systems.Performance of large, commercial HVAC systems can be im-proved through better local-loop and supervisory control. Proper tun-ing of local-loop controllers can enhance comfort, reduce energy use,and increase componen

12、t life. Systems that are properly commis-sioned or tuned, such as through a recommissioning process or theuse of automated fault detection and diagnostics software, ensure thattheoretical performance gains from supervisory control strategies arerealized. Set points and operating modes for cooling or

13、 heating plantequipment can be adjusted by supervisory control strategies or staticoptimization to maximize overall operating efficiency. Dynamicoptimization strategies for ice or chilled-water storage systems cansignificantly reduce on-peak electrical energy and demand costs tominimize total utilit

14、y costs. Similarly, thermal storage inherent in abuildings structure can be dynamically controlled to minimize utilitycosts, for example, with the use of thermally activated building sys-tems (TABS). In general, strategies that take advantage of thermalstorage work best when dynamic optimization is

15、applied using fore-casts of future energy requirements.Significant increases in computational power and communica-tions capabilities mean that both supervisory and distributed controlsystems are now able to incorporate many new data streams and si-multaneously co-optimize a number of performance met

16、rics. Ratherthan simply regulating temperature, the supervisory control systemmay manage thermal comfort while minimizing utility cost, energyconsumption, and greenhouse gas emissions. Ubiquitous consumerelectronics, such as smart phones, tablets, and laptops, mean that di-rect feedback of occupant

17、preferences may be obtained rather thanrelying on inferred statistical models. Building thermal response andinternal gain forecast models (e.g., learned by the controller) allowoptimal start-up, night-purge, and economy modes as well as poten-tial participation in utility demand response programs an

18、d cost re-ductions in demand and capacity charges.Where resources are constrained by equipment sizing, mainte-nance, or imposed through energy targets or demand charges, a coor-dinated approach to resource allocation is required to ensure anequitable balance of comfort for all building occupants. Th

19、is isincreasingly likely to be an important control scheme design consid-eration with increased focus on energy efficiency, demand response,and the uptake of intermittent renewable generation, requiringenergy users to respond to resource variability. New energy pricingmodels will substantially rewar

20、d users who have this flexibility, butmay also adversely impact those without the capability to dynami-cally manage loads.1. TERMINOLOGYAir distribution system: includes terminal units variable-air-volume (VAV) boxes, etc., air-handling units (AHUs), ducts, andcontrols. In each AHU, ventilation air

21、is mixed with return air fromthe zones and fed to the cooling/heating coil.Air-side economizer control: used to select between minimumand maximum ventilation air, depending on the condition of theThe preparation of this chapter is assigned to TC 7.5, Smart BuildingSystems.42.2 2015 ASHRAE HandbookHV

22、AC Applicationsoutdoor air relative to the conditions of the return air. Under certainoutdoor air conditions, AHU dampers may be modulated to providea mixed air condition that can satisfy the cooling load without theneed for mechanical cooling.Building thermal mass storage: storing energy in the for

23、m ofsensible heat in building materials, interior equipment, and furnish-ings.Capacity: Heating or cooling output at design or rating condi-tion or, in certain contexts, at current operating condition.CAV systems: air-handling systems that have fixed-speed fans andprovide no feedback control of airf

24、low to the zones. Zone tempera-ture is controlled to a set point using a feedback controller that reg-ulates the amount of local reheat applied to the air entering each zone.Charging: storing cooling capacity by removing heat from acool storage device; or storing heating capacity by adding heat to a

25、heat storage device.Chilled-/hot-water/steam loop: consists of pumps, pipes,valves, and controls. Two different types of pumping systems areconsidered in this chapter: primary and primary/secondary. With aprimary pumping system, a single piping loop is used and waterthat flows through the chiller or

26、 boiler also flows through the coolingor heating coils. When steam is used, a steam piping loop sendssteam to a hot-water converter, returning hot condensate back to theboiler. Another piping loop then carries hot water through the heat-ing coils. Often, fixed-speed pumps are used with their control

27、 ded-icated to chiller or boiler control. Dedicated control means that eachpump is cycled on and off with the chiller or boiler that it serves. Sys-tems with fixed-speed pumps and two-way cooling or heating coilvalves often incorporate a water bypass valve to maintain relativelyconstant flow rates a

28、nd reduce system pressure drop and pumpingcosts at low loads. The valve is typically controlled to maintain afixed pressure difference between the main supply and return lines.This set point is termed the chilled- or hot-water loop differentialpressure. Sometimes, primary systems use one or more var

29、iable-speed pumps to further reduce pumping costs at low loads. In thiscase, water bypass is not used and pumps are controlled directly tomaintain a water loop differential pressure set point.Chiller or boiler plant: one or more chillers or boilers, typicallyarranged in parallel with dedicated pumps

30、, provide the primarysource of cooling or heating for the system. Individual feedbackcontrollers adjust the capacity of each chiller or boiler to maintain aspecified supply water temperature (or steam header pressure forsteam boilers). Additional control variables include the number ofchillers or bo

31、ilers operating and the relative loading for each. For agiven total cooling or heating requirement, individual chiller orboiler loads can be controlled by using different water supply setpoints for constant individual flow or by adjusting individual flowsfor identical set points.Chiller priority: co

32、ntrol strategy for partial storage systems thatuses the chiller to directly meet as much of the load as possible, nor-mally by operating at full capacity most of the time. Thermal stor-age is used to supplement chiller operation only when the loadexceeds the chiller capacity.Condenser water loop: co

33、nsists of cooling towers, pumps, pip-ing, and controls. Cooling towers reject heat to the environmentthrough heat transfer and possibly evaporation (for wet towers) tothe ambient air. Typically, large towers incorporate multiple cellssharing a common sump, with individual fans having two or morespee

34、d settings. Often, a feedback controller adjusts tower fan speedsto maintain a temperature set point for water leaving the coolingtower, termed the condenser water supply temperature. Typi-cally, condenser water pumps are dedicated to individual chillers(i.e., each pump is cycled on and off with the

35、 chiller it serves).COP: Heating or cooling output divided by electrical powerinput; may include chilled-water (CW) pump and tower fan as wellas compressor power (see also System COP).Demand limiting: a partial storage operating strategy that limitsthe capacity of the cooling system during the on-pe

36、ak period. Thecooling system capacity may be limited based on its cooling capac-ity, its electric demand, or the facility demand.Discharge capacity: maximum rate at which cooling can besupplied from a cool storage device.Discharging: using stored cooling capacity by adding thermalenergy to a cool st

37、orage device or removing thermal energy from aheat storage device.Ice storage: types of ice storage systems include the following.An ice harvester is a machine that cyclically forms a layer of ice ona smooth cooling surface, utilizing the refrigerant inside the heatexchanger, then delivers it to a s

38、torage container by heating the sur-face of the cooling plate, normally by reversing the refrigerationprocess and delivering hot gases inside the heat exchanger. In ice-on-coil-external melt, tubes or pipes (coil) are immersed in waterand ice is formed on the outside of the tubes or pipes by circula

39、tingcolder secondary medium or refrigerant inside the tubing or pipes,and is melted externally by circulation the unfrozen water outsidethe tubes or pipes to the load. Ice-on-coil-internal melt is similar,except the ice is melted internally by circulating the same secondarycoolant or refrigerant to

40、the load.Load leveling: a partial storage sizing strategy that minimizesstorage equipment size and storage capacity. The system operateswith the refrigeration equipment running at full capacity for 24 h tomeet the normal cooling minimum load profile and, when the loadis less than the chiller output,

41、 the excess cooling is stored. When theload exceeds the chiller capacity, the additional cooling requirementis obtained from the thermal storage system.Load profile: compilation of instantaneous thermal loads over aperiod of time, normally 24 hours.Nominal chiller capacity: (1) chiller capacity at s

42、tandard ARI(Standard 550/590) rating conditions, or (2) chiller capacity at agiven operating condition selected for the purpose of quick chillersizing selections.Partial storage: a cool storage sizing strategy in which only aportion of the on-peak cooling load is met from thermal storage,with the re

43、mainder of the load being met by operating the chillingequipment.Precooling: a thermal energy storage (TES) strategy that allowsa properly designed chiller system to operate more efficiently withlower condensing temperatures (low night ambient temperature)higher evaporating temperatures (60 to 69F c

44、hilled water) and at ornear its most efficient part-load point. Precooling can be applied tothe conditioned space, or directly to mass by passing chilled air orwater through building elements such as concrete floor decks.Primary/secondary chilled- or hot-water systems: systemsdesigned specifically f

45、or variable-speed pumping. In the primaryloop, fixed-speed pumps provide a relatively constant flow ofwater to the chillers or boilers. This design ensures good chiller orboiler performance and, for chilled-water systems, reduces therisk of freezing on evaporator tubes. The secondary loop incorpo-ra

46、tes one or more variable-speed pumps that are controlled tomaintain a water loop differential pressure set point. The primaryand secondary loops may be separated by a heat exchanger. How-ever, it is more common to use direct coupling with a commonpipe.Storage capacity: the maximum amount of cooling

47、(or heating)that can be achieved by the stored medium in the thermal storagedevice. Nominal storage capacity is a theoretical capacity of thethermal storage device. In many cases, this may be greater than theusable storage capacity. This measure should not be used to com-pare usable capacities of al

48、ternative storage systems.Storage cycle: a period (usually one day) in which a substantialcharge and discharge of a thermal storage device has occurred,beginning and ending at the same state or same time of day.Supervisory Control Strategies and Optimization 42.3Storage efficiency: the ratio of usef

49、ul heating or cooling ex-tracted during the discharge cycle to that imparted to storage duringthe charging cycle. One may also define an exergic efficiency thataccounts for mixing and thermal destratification as well as conduc-tion losses.Storage inventory: the amount of usable heating or coolingcapacity remaining in a thermal storage device.Storage priority: a control strategy that uses stored cooling tomeet as much of the load as possible. Chillers are operated only ifthe load exceeds the storage systems available cooling capacity.Supply air temperature: temperature of air leaving