Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSTHERMAL STORAGE CONTROLGENERALThermal storage is used to save heating or cooling for futureuse. Typically enough storage is provided to meet the loadrequirements for a 24-hour period. During the cooling season,storage of low cost night time cooling can save energy anddemand charges and reduce the chilled water generatingequipment design size. During the heating season, storage ofrejected day time excess refrigeration heat or solar heat can beused for night time heating loads. Storage may use a watertank or ice bin for cooling and a water tank or thermalconducting fluid for heat. The storage efficiency depends onthe amount of insulation and, in the case of water storage, onminimizing the mixing of return water with storage water.Mixing in water storage can be minimized by use of a segmentedcontainer for storage (Fig. 24).Primary control requirements are storage charge and storagedischarge at the proper times and rates. Storage charge, storagedischarge, and instantaneous cooling are the three basic controlmodes. Combinations of the basic control modes may berequired to meet the load or predicted load. When the predictedload is high, the chiller provides chilled water for both thecurrent load and for storage. When the predicted load occurs,the stored cooling is added to the chiller output.CONTROL MODESThe appropriate control mode depends on the load predictedfor next day, relative storage size, and relative costs of storedand instantaneous cooling, including electrical demand ratestructures. The storage charging cycle is normally activatedwhen cooling generation cost is lowest, such as at night whenlower condenser water temperatures provide lower refrigerationhead and when lower time-of-day electric rates may beapplicable. The rate of charge should satisfy storage quantityrequirements within the limits of the time available.Use of stored energy verses instantaneous cooling energy isprioritized. When enough stored energy is available to satisfythe load through the peak demand period of the next day, onlystored energy is used (storage priority). In this case, the chargingcycle is scheduled to start when low cost cooling is available.The charging cycle is stopped when storage is sufficient for thenext days load or when the storage capacity is filled. The storagedischarge cycle is controlled to meet load conditions. If storagecapacity is not large enough for the next day, the chillersupplements the use of stored cooling, as necessary. The controlsequences for segmented chilled water storage (Fig. 24) are:– Instantaneous Cooling Cycle: Pump P 1 is on and PumpP 2 is off. Valves V C and V D are closed. Valve V I iscontrolled by ∆P and T 1 controls chiller.– Charging Cycle: Valves VD and VI are closed and VC iscontrolled by T6 to maintain the flow rate at F 1 . PumpP 2 is off. T 1 controls chiller capacity to maintain 40FCHWS temperature. When the T 5 (A through E)location, representing the needs for the next day, reaches40F, the charging cycle is stopped.– Charging plus Instantaneous Cooling Cycle: Chargingcycle continues while V I is controlled from ∆P. PumpP 2 is off. The flow through the storage is T 5E to T 5A .– Discharge Cycle (enough storage for anticipated load):Valves V C and V I are closed, chiller and Pump P 1 areoff, and Pump P 2 is on. Valve V D is controlled from ∆P.– Discharge plus Instantaneous Cooling Cycle (notenough storage for anticipated load): Pumps P 1 and P 2are on, V C is closed. Chiller demand D 1 limits chillercapacity while T 2 positions V I to maintain chilled watersupply temperature. ∆P throttles valve V D to provideflow from storage.CHILLERD 1 P 1F 1T 1T T T T V C5B 5C 5D 5ECT T 5A 6CCDP 2SEGMENTED STORAGEDDV D V IN.O.LOADS= FLOW DIRECTIONT 2C= CHARGING CYCLED= DISCHARGING CYCLE ∆PI = INSTANTANEOUS COOLINGNOTE: NOT ALL CONTROLS ACTIVE FOR ALL SEQUENCESFig. 24. Segmented Chilled Water Storage.The chiller takes priority when stored energy costs are largerthan instantaneous energy costs. Under these conditions theonly cooling stored is that required to reduce the anticipateddemand limit for the next day. Storage size and energy cost,including demand charges, establish the control strategies forthis situation. The control objectives are to limit demand forthe billing period and to minimize energy costs. Chillerdischarge is controlled to meet load. The sequence of the controlis to run the charging plus instantaneous cooling cycle, thenstop charging when the quantity necessary to meet theanticipated demand limit for the next day is stored.IN.O.N.O.C2693ENGINEERING MANUAL OF AUTOMATIC CONTROL292