1、51.1CHAPTER 51 THERMAL STORAGESensible Thermal Storage Technology . 51.4Chilled-Water Thermal Storage Sizing Examples 51.7Chiller and Ice Storage Selection 51.11Heat Storage Technology . 51.16Sizing Cool Storage Systems 51.22Application of Thermal Storage Systems . 51.24Operation and Control . 51.28
2、Other Design Considerations. 51.30Cost Considerations. 51.32Maintenance Considerations . 51.32Commissioning. 51.33Good Practices. 51.35HERMAL storage systems remove heat from or add heat to aTstorage medium for use at another time. Thermal energy stor-age (TES) for HVAC applications can involve vari
3、ous temperaturesassociated with heating or cooling. High-temperature storage istypically associated with solar energy or high-temperature heating,and cool storage with air-conditioning, refrigeration, or cryogenic-temperature processes. Energy may be charged, stored, and dis-charged daily, weekly, a
4、nnually, or in seasonal or rapid batch processcycles. Currently, most use of thermal storage is cool storage forcomfort and process cooling applications as a way to reduce thetotal utility bill and/or size of cooling equipment, and much of thediscussion in this chapter pertains specifically to cool
5、storage.Dorgan and Elleson (1993) cover cool storage issues and designparameters in detail.A properly designed and installed thermal storage system can Reduce operating or initial costsReduce size of electric service and cooling or heating equipmentIncrease operating flexibilityProvide back-up capac
6、ityExtend the capacity of an existing systemBenefits are discussed further in the Benefits of Thermal Storagesection.Thermal storage may be a particularly attractive approach tomeeting heating or cooling loads if one or more of the followingconditions apply:Loads are of short duration.Loads occur in
7、frequently.Loads are cyclical.Loads are not coincident with energy source availability.Energy costs are time-dependent (e.g., time-of-use energy rates).Charges for peak power demand are high.Utility rebates, tax credits, or other economic incentives are pro-vided for using load-shifting equipment.En
8、ergy supply is limited, thus limiting or preventing the use offull-size nonstorage systems.Facility expansion is planned, and the existing heating or coolingequipment is insufficient to meet the new peak load but has sparenonpeak capacity.Interruption in cooling water cannot be tolerated by a missio
9、n-critical operation.TerminologyCharging. Storing cooling capacity by removing heat from acool storage device, or storing heating capacity by adding heat to aheat storage device.Chiller priority. Control strategy for partial storage systems thatuses the chiller to directly meet as much of the load a
10、s possible,normally by operating at design capacity most of the time. Thermalstorage is used to supplement chiller operation only when the loadexceeds the chiller capacity.Cool storage. As used in this chapter, storage of cooling capacityin a storage medium at temperatures below the nominal temperat
11、ureof the space or process. Demand limiting. A partial storage operating strategy that limitscapacity of HVAC equipment during the on-peak period. Equipmentcapacity may be limited based on its cooling capacity, its electricdemand, or facility demand.Design load profile. Calculated or measured hour-b
12、y-hour cool-ing loads over a complete cooling cycle that are considered to be themaximum total cooling load that must be met by mechanical meansand/or capacity from a cool thermal storage system, provided in agraphical or tabular format.Design operating profile. Equipment operation calculated ormeas
13、ured hour-by-hour, including mechanical refrigeration equip-ment, the thermal storage systems charge or discharge rate, andtemperatures over the entire cooling system operating period.Discharging. Using stored cooling capacity by adding thermalenergy to a cool storage device or removing thermal ener
14、gy from aheat storage device.Encapsulated storage. A latent storage technology that consistsof plastic containers of water or other phase-change material that arealternately frozen and melted by the influence of glycol or other sec-ondary coolant medium in which they are immersed.Full storage. A coo
15、l storage sizing strategy that meets the entirecooling load during a predefined on-peak demand period with dis-charge from the thermal storage system.Fully charged condition. State of a cool thermal storage systemat which, according to design, no more heat is to be removed fromthe storage device. Th
16、is state is reached when the control systemstops the charge cycle as part of its normal control sequence whenthe temperature of media leaving the storage system is equal to thatentering the storage system.Fully discharged condition. State of a cool thermal storage sys-tem at which no more usable coo
17、ling capacity can be delivered fromthe storage device. This state is reached when the control systemstops the discharge cycle as part of its normal control sequencewhen the discharge temperature of media from the storage systemexceeds a predefined temperature.Heat storage. As used in this chapter, s
18、torage of thermal energyat temperatures above the nominal temperature of the space orprocess.Ice harvester. Machine that cyclically forms a layer of ice on asmooth cooling surface, using refrigerant inside the heat exchanger,then delivers it to a storage container by heating the surface of thecoolin
19、g plate, normally by reversing the refrigeration process anddelivering hot gases inside the heat exchanger.Ice-on-coil (ice-on-pipe). Ice storage technology that forms andstores ice on the outside of tubes or pipes submerged in an insulatedwater tank.The preparation of this chapter is assigned to TC
20、 6.9, Thermal Storage.51.2 2012 ASHRAE HandbookHVAC Systems and Equipment Ice-on-coil, external melt. Ice storage technology in whichtubes or pipes (coil) are immersed in water and ice is formed on theoutside of the tubes or pipes by circulating colder secondarymedium or refrigerant inside the tubin
21、g or pipes, and is meltedexternally by circulating unfrozen water outside the tubes or pipesto the load.Ice-on-coil, internal melt. Ice storage technology in which tubesor pipes (coil) are immersed in water and ice is formed on the outsideof the tubes or pipes by circulating colder secondary medium
22、orrefrigerant inside the tubing or pipes, and is melted internally by cir-culating the same secondary coolant or refrigerant to the load.Latent energy storage (latent heat storage). A thermal storagetechnology in which energy is stored within a medium, normallyassociated with a phase change (usually
23、 between solid and liquidstates), for use in cooling or heating the secondary liquid being cir-culated through the system.Load leveling. A partial storage sizing strategy that minimizesstorage equipment size and storage capacity. The system operateswith refrigeration equipment running at its most ef
24、ficient capacityfor 24 h to meet the normal cooling load profile and, when load isless than the chiller output, excess cooling is stored. When loadexceeds chiller capacity, the additional cooling requirement isobtained from the thermal storage.Load profile. Compilation of instantaneous thermal loads
25、 over aperiod of time, normally 24 h.Mass storage. Storage of energy, in the form of sensible heat, inbuilding materials, interior equipment, and furnishings.Maximum usable cooling supply temperature. Maximumfluid supply temperature at which the cooling load can be met. Thisis generally determined b
26、y the requirements of the air-side distribu-tion system or the process.Maximum usable discharge temperature. Highest tempera-ture at which beneficial cooling can be obtained from the thermalstorage device. Nominal chiller capacity. (1) Chiller capacity at standard Air-Conditioning and Refrigeration
27、Institute (ARI) rating conditions.(2) Chiller capacity at a given operating condition selected for thepurpose of quick chiller sizing selections.On-peak demand period. Period of time when electrical griddemands are high, resulting in higher power cost and often addeddemand charges by the supplying u
28、tility.Partial storage. A cool storage sizing strategy in which only aportion of the on-peak cooling load is met from thermal storage,with the rest being met by operating the chilling equipment. Phase-change material (PCM). A substance that undergoes achange of state, normally from solid to liquid o
29、r liquid to solid,while absorbing or rejecting thermal energy at a constant tempera-ture.Pulldown load. Unmet cooling or heating load that accumulatesduring a period when a cooling or heating system has not operated,or operated in a thermostat setback mode, and which must be met onsystem start-up be
30、fore comfort conditions can be achieved. Maxi-mum pulldown load generally occurs on a Monday morning or fol-lowing an extended shutdown.Sensible energy storage (sensible heat storage). A heating orcooling thermal storage technology in which all energy stored is inthe form of a measurable temperature
31、 difference between the hot orchilled water circulating through the system and the storagemedium.Storage cycle. A period in which a complete charge and dis-charge of a thermal storage device has occurred, beginning and end-ing at the same state.Storage inventory. Amount of usable heating or cooling
32、capac-ity remaining in a thermal storage device.Storage priority. A control strategy that uses stored cooling tomeet as much of the load as possible. Chillers operate only if theload exceeds the storage systems available cooling capacity.Stratified chilled-water storage. A method of sensible coolthe
33、rmal energy storage that achieves and maintains an acceptableseparation between warm (discharged) and cool (charged) water byforming a thermocline by density differences alone, and not bymechanical separation.Thermal storage capacity. A value indicating the maximumamount of cooling (or heating) that
34、 can be achieved by the storedmedium in the thermal storage device and delivered to the load.Discharge capacity. The maximum rate at which cooling can besupplied from a cool storage device.Nominal storage capacity. A theoretical capacity of the thermalstorage device. In many cases, this may be great
35、er than theusable storage capacity. This measure should not be used tocompare usable capacities of alternative storage systems.System capacity. Maximum amount of cooling that can be sup-plied by the entire cooling system, which may include chillersand thermal storage.Usable storage capacity. Total a
36、mount of beneficial coolingable to be discharged from a thermal storage device. (Thismay be less than the nominal storage capacity because thedistribution header piping may not allow discharging theentire cooling capacity of the thermal storage device.)Thermal storage device. A container plus all it
37、s contents usedfor storing heating or cooling energy. The heat transfer fluid andaccessories, such as heat exchangers, agitators, circulating pumps,flow-switching devices, valves, and baffles that are integral with thecontainer, are considered a part of the thermal storage device.Thermocline. Therma
38、l layer of water in a chilled-water thermalstorage tank, separating warmer water at the top and cooler water atthe bottom. The depth of this layer depends on the effectiveness andefficiency of the upper and lower diffusers, which are designed tosupply and discharge water with minimal mixing. The typ
39、ical ther-mocline is 18 to 24 in., and rises and falls when charging and dis-charging the storage tank.Total cooling load. Integrated thermal load that must be met bythe cooling plant over a given period of time.Unitary thermal storage system (UTSS). A packaged assem-bly including a thermal storage
40、device and refrigeration equipmentfor cooling and charging the device; overall performance is rated bythe manufacturer.Usable cooling. Amount of energy that can actually be deliveredto the system or process to meet cooling requirements. Normally,this is the total value of the energy retrieved by sup
41、plying themedium at or below the maximum beneficial cooling temperature.Classification of SystemsThermal storage systems can be classified according to the typeof thermal storage medium, whether they store primarily sensible orlatent energy, or the way the storage medium is used. Cool storagemedia i
42、nclude chilled water, aqueous or nonaqueous fluids, ice, andphase-change materials. Heat storage media include water, brick,stone, and ceramic materials. These media differ in their heat stor-age capacities, the temperatures at which energy is stored, and phys-ical requirements of storing energy in
43、the media. Types of coolstorage systems include chilled-water storage, chilled-fluid storage,ice harvesting, internal- and external-melt ice-on-coil, encapsulatedice, ice slurry, phase-change material, and unitary systems. Types ofheat storage systems include brick storage heaters, water storageheat
44、ers, and radiant floor heating systems, as well as solar space andwater heating and thermally charged water storage tanks.Storage MediaA wide range of materials can be used as thermal storage media.Materials used for thermal storage should beCommonly availableLow costThermal Storage 51.3Environmenta
45、lly benignNonflammableNonexplosiveNontoxicCompatible with common HVAC equipment construction mate-rials NoncorrosiveInertAn ideal thermal storage medium should also haveWell-documented physical propertiesHigh densityHigh specific heat (for sensible heat storage)High heat of fusion (for latent heat s
46、torage)Good heat transfer characteristicsStable properties that do not change over many thermal cyclesCommon storage media for sensible energy storage include water,fluid, soil, rock, brick, ceramics, concrete, and various portions of thebuilding structure (or process fluid) being heated or cooled.
47、In HVACapplications, such as air conditioning, space heating, and water heat-ing, water is often the chosen sensible storage medium because itprovides many of these desirable characteristics when kept betweenits freezing and boiling points. For high-temperature energy storage,the storage medium is o
48、ften rock, brick, or ceramic materials for res-idential or small commercial applications; oil, oil/rock combina-tions, or molten salt are often used for large industrial or solar energypower plant applications. Using the building structure itself as pas-sive thermal storage offers advantages under s
49、ome circumstances(Morris et al. 1994).Common storage media for latent energy storage include ice,aqueous brine/ice solutions, and other PCMs such as hydrated saltsand polymers. Carbon dioxide and paraffin waxes are among thealternative storage media used for latent energy storage at varioustemperatures. For air-conditioning applications, ice is the mostcommon latent storage medium, because it provides many of thepreviously listed desirable characteristics.Basic Thermal Storage ConceptsThe fundamental characteristic of thermal storage systems is thatthey separate the time(s) of generation
