Abstract
The chapter gives an overview of cold thermal energy storage (CTES) technologies. Benefits as well as classification and operating strategies of CTES are discussed. Design consideration and sizing strategies based on calculated load profile for design day is presented. Some recommendation concernig designing of CTES equipment are given. Special attention was paid to the analysis of specific features of heat transfer phenomena in ice storage tank including the assessment of the duration and the rate of ice formation and melting. The methodology of sizing components of the ice thermal storage system included in an air conditioning system for a office building situated in hot wet and dry climate are presented. Based on hourly cooling load calculation that was carried out using Carrier's Hourly Analysis Program, sizing of ice thermal storage system for different operating strategies included full, chiller priority and ice priority storage operation for the design day are presented. Finally, an analysis of some operational characteristics of the system are analyzed.
TopIntroduction
Demands for energy savings and improved energy efficiency are becoming increasingly important. A very promising possibility is the storage of energy whose main objective is to bridge the gap between energy generation and consumption by overcoming the temporal shift between the energy produced and the need for it. This allows thermal storage systems to generate heating or cooling during periods when conditions are most favorable (e.g. the primary energy source is more available or less expensive), which can be independent of the instantaneous thermal load (ASHRAE, 1999).
Cold Thermal Energy Storage (CTES) usually implies storage of cooling capacity in an appropriate medium at temperatures below the nominal temperature of the space or processing system. The main purpose of CTES utilization is to shift electric energy use from on-peak to off-peak hours (Abdul Galil, 2013; Dorgan & Elleson, 1993). During the off-peak hours electricity is used for charging the thermal storage in order to meet (fully or partially) the on-peak hours cooling load of the refrigeration facility or building. Cool storage can potentially reduce the on-peak energy consumption, peak demand, and most importantly, average cost of energy consumed (Elleson, 1997).
CTES, by itself, is not ultimately an energy savings technology; first of all it is a cost savings technology (Chvala, 2001). By shifting chilling operations to off-peak times, when demand and energy rates are reduced, significant money savings can be realized. The economics are more attractive when CTES is included into existing cooling system as a replacement of older cooling equipment. Energy savings also may be realized because CTES allows facilities to use more energy efficient chillers (Chvala, 2001).
A challenge common to all cold thermal energy storage technologies is to find an efficient and economical means of achieving the heat transfer necessary to alternately freeze and melt the storage medium (ASHRAE Handbook, HVAC Application, 1999). The high energy density of latent storage systems allows compact installations and makes factory-manufactured components and systems practical.
CTES is a proven technology which can be the most cost-effective, reliable system approach to cooling offices, schools, hospitals, malls and other buildings, and provides a steady source of low temperature fluids for process cooling applications. The implementation of CTES contributes to measures for environment protection helping to lower energy consumption and reduce greenhouse gas emissions.
The CTES systems may become an economically attractive alternative if one or more of the following conditions exist (Dincer & Rosen, 2001):
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Short period of cooling demand,
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Charges for peak power demand are high,
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Frequently varying cooling loads (loads are cyclical),
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Cooling demand and supply do not match,
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Economic incentives are provided to use off-peak energy,
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Energy supply is limited by the utility company, thus making it impossible to satisfy the maximum load directly,
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The capacity of an existing chiller is too low to provide peak load.
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Electricity production from Combined Heat and Power (CHP) plant.
Key Terms in this Chapter
Thermal Storage Device: Collection of equipment units which stores cooling capacity using sensible or/and latent heat of an appropriate medium.
Combined Heat and Power (CHP) Plant: The system which consists of prime power (driver) that drives a generator to produce electricity, and heat recovery system which uses heat from the exhaust gas and/or water cooling engine.
Sensible Energy Storage: The device using water, fluid (without phase change), soil, and various construction materials as a storage medium being heated or cooled.
Cold Thermal Energy Storage (CTES): The storage device with storage medium at temperatures below the nominal temperature of the space, or processing system.
External Melt Ice-on-Coil Storage: A thermal storage device in which an evaporating refrigerant or cold secondary coolant is circulated through the coils/plates causing ice forming on the external surfaces.
CTES Technology: The collection of tools, including machinery, modifications, arrangements and procedures in which CTES with water ice as a storing medium is included into HVAC&R systems.
Nominal Storage Capacity: A theoretical (maximal) capacity of the storage device.
Full Storage: ITES designed to utilize the stored cooling to shift all building cooling loads from the on-peak period to the off-peak period of the design day.
Net Usable Storage Capacity: The part of stored cooling capacity that can be supplied by the storage device at the specified temperature.
Ice Thermal Energy Storage (ITES): The storage of cooling capacity using water ice as a storage medium.
Partial Storage Ice Priority System: ITES in which a constant part of the load is covered by the ice, where the variation in load is taken by the chiller operating at a reduced capacity during the day time.
Internal Melt Ice-on-Coil Storage: A thermal storage device in which secondary coolant is circulated through the coils/plates being cooled by ice malting on the external surfaces.
Chiller Priority System: (A load levelling system) . ITES in which the constant portion of the cooling load is covered by the chiller while the variation in load is covered by the ice.
Latent Cool Storage: The device in which the thermal energy can be stored as the latent heat of fusion of water (ice) or other materials.
ITES Technology: The collection of tools, including machinery, modifications, arrangements and procedures in which ITES is included into HVAC&R systems.