1、98 2008 ASHRAE ABSTRACTThermal energy storage (TES) systems are examined fromthe perspectives of energy, exergy, environmental impact,sustainability and economics. Reductions possible throughTES in energy use and both pollution levels and environmentalimpact are described and highlighted with an ill
2、ustrativeexample. The importance of using exergy analysis to obtainmore realistic and meaningful assessments than the conven-tional energy analysis of the efficiency and performance of TESsystems is demonstrated. The results indicate that exergeticallyefficient TES can play a significant role in mee
3、ting societyspreferences for more efficient, environmentally benign,sustainable and economic energy use in various sectors, andappears to be an appropriate technology for addressing themismatches that often occur between the times of energy supplyand demand.INTRODUCTIONSociety faces many environment
4、al problems, spanning acontinuously growing range of pollutants, hazards and ecosys-tem degradation over ever wider areas. The most significantproblems are global climate change, stratospheric ozonedepletion and acid precipitation. The former is potentially themost important environmental problem re
5、lating to energyutilization. Increasing atmospheric concentrations of green-house gases are increasing the manner in which they trap heatradiated from the earths surface, thereby raising the surfacetemperature of the earth and as a consequence sea levels. Many potential solutions are proposed for cu
6、rrent envi-ronmental problems and harmful emissions. These include theaddition of pollution abatement equipment on discharges andstacks and the introduction of strategies for clean air, improv-ing the efficiency of devices and technologies and the substi-tution of renewable energy sources for fossil
7、 fuels (Mohr,2006; Craig et al., 2007; Bretschger and Smulders, 2007;Kuehr, 2007). TES appears to be the one of the more effectivemeasures and can play a significant role in reducing environ-mental impact. TES can also help make society more sustain-able. Sustainable development demands a sustainabl
8、e supplyof energy resources that, in the long term, is readily andsustainably available at reasonable cost and can be utilized forall required tasks without causing negative societal impacts.In the areas of building heating and cooling and electricpower generation, TES systems can contribute signifi
9、cantly tomeeting societys desire for more efficient and environmen-tally benign energy use and for sustainable development(Khudhair and Farid, 2004). In particular, TES can help makebuildings more sustainable. By reducing energy consumption,for instance, the utilization of TES systems results in two
10、significant environmental benefits: (i) conservation of fossilfuels through efficiency increases and/or fuel substitution, and(ii) reductions in emissions of such pollutants as CO2, SO2,NOx and CFCs.The primary objective of this paper is to investigate howthermal energy storage systems can help make
11、 buildings moresustainable and contribute to local and global sustainabledevelopment. This article also demonstrates how exergy meth-ods provide a useful tool for improving efficiency, cost effec-tiveness, environmental impact and hence sustainability.Previous work is reviewed and research results a
12、re presented,using the context of the earlier work where appropriate.Exergetically Efficient Thermal Energy Storage Systems for Sustainable BuildingsIbrahim Dincer Marc A. RosenMember ASHRAEIbrahim Dincer is a professor of Mechanical Engineering and Marc A. Rosen is a professor and dean of the Facul
13、ty of Engineering andApplied Science, University of Ontario Institute of Technology, Oshawa, Canada.NY-08-0152008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 114, Part 1. For personal use only. Additional
14、 reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 99THERMAL ENERGY STORAGE SYSTEMSThermal energy storage systems for heating or coolingcapacity are often utilized in applications where the occur-
15、rence of a demand for energy and that of the economicallymost favorable supply of energy are not coincident (Dincerand Rosen, 2002). The storage medium can be located in storages of varioustypes, including tanks, ponds, caverns and underground aqui-fers. Underground thermal energy storage systems ma
16、y bedivided into two groups: Closed storage systems in which a heat transport fluid(water in most cases) is pumped through heat exchang-ers in the ground, andOpen systems where groundwater is pumped out of theground and then injected into the ground using wells(aquifer TES) or in underground caverns
17、.When underground aquifers are used for TES, the storagemedium remains in a single phase during the storing cycle, sothat temperature changes are exhibited in the store as thermalenergy is added or removed. The storage medium can remain in a single phase (so thatonly sensible heat is stored) and/or
18、undergo phase change (sothat energy is stored as latent heat). Sensible TESs (e.g., liquidwater systems) exhibit changes in temperature in the store asheat is added or removed. In latent TESs (e.g., liquid water/icesystems and eutectic salt systems), the storage temperatureremains fixed during the p
19、hase-change portion of the storagecycle. Thermal storages are used in energy conservation, indus-try, commercial building and solar energy systems. Manyapplications exist where thermal storage of heat is used forspace heating, district heating and hot water heating. TESoften facilitates the efficien
20、t utilization of renewable energysources and energy conservation. In many countries, cold ther-mal energy storage is an economically viable technology usedin many thermal systems, particularly building cooling. Insuch applications, inexpensive off-peak electricity is utilizedduring the night to prod
21、uce with chillers a cold medium, whichcan be stored for use in meeting cooling needs during the daywhen electricity is more expensive. Numerous TES applications and studies have beenreported (Dincer and Rosen, 2002; Saito, 2002; Andrepont,2007; Khudhair and Farid, 2004; IEA, 2007). Examples ofunderg
22、round thermal energy storage applications includethose at Scarborough Centre in Toronto, Carleton Universityin Ottawa, the Sussex Hospital in New Brunswick, the PacificAgricultural Centre in Agassiz, B.C., as well as borehole ther-mal energy storage systems in Oshawa, Ontario, StocktonCollege in New
23、 Jersey, US and in Sweden (IEA, 2007).The inhibition of mixing through appropriate temperaturestratification is advantageous in many TES systems. Throughcarefully managing the injection, recovery and holding of heat(or cold) to avoid stratification degradation, better storage-cycle performance can b
24、e achieved, allowing for better ther-mal energy recovery and temperature retention.The benefits of TES include reduced energy costs, initialand maintenance costs, equipment sizes, energy consumptionand pollutant emissions, as well as increased flexibility ofoperation, efficiency and effectiveness of
25、 equipment utiliza-tion and sustainability. Thermal energy storage also allowsEnergy conservation and substitution, by facilitating theuse of natural energy sources and waste energy,Energy peak-load shifting from more expensive daytimeto less expensive nighttime rates, andElectricity conservation, b
26、y operating efficient devicesat full load instead of part load to reduce peak powerdemands and increase efficiency of electricity use.ECONOMIC ASPECTS OF TES SYSTEMSEconomic aspects of the design and operation of energyconversion systems have brought TES to the forefront as a prac-tical and benefici
27、al technology. One significant economic bene-fit of TES is its ability to allow an energy conversion system tooperate productively when the supply of and demand for ther-mal energy do not coincide in time. A wide range of industrialapplications for TES systems have been reported (Dincer andRosen, 20
28、02; Saito, 2002; Andrepont, 2007; Khudhair andFarid, 2004). Such TES systems are of great practical potentialfrom an economic perspective for improving the use of thermalenergy equipment and for facilitating large-scale energy substi-tutions. For the maximum potential benefits of thermal storageto b
29、e realized, a coordinated set of actions is required in severalsectors of societys energy systems.TES-based systems are usually economically justifiablewhen the annualized capital and operating costs are less thanthose costs for primary generating equipment supplying thesame service loads and period
30、s. TES is mainly installed tolower initial costs of the other plant components and operatingcosts. Lower initial equipment costs are usually obtained whenlarge durations occur between periods of energy demand.Secondary capital costs may also be lower for TES-basedsystems. For example, the electrical
31、 service equipment sizecan sometimes be reduced when energy demand is lowered.In economic analyses of systems with and without TES,the initial equipment and installation costs must be deter-mined, usually from manufacturers, or estimated. Operatingcost savings and the net overall costs should be ass
32、essed usinglife cycle costing or other suitable methods to determine whichsystem is the most beneficial.Utilizing TES can enhance the economic competitivenessof both energy suppliers and building owners. For example,one study (CEC, 1996) for California indicates that, assuming20% statewide market pe
33、netration of TES, the followingfinancial benefits can be achieved in the state:For energy suppliers, TES leads to lower generatingequipment costs (30% to 50% lower to serve air condi-100 ASHRAE Transactionstioning loads), reduced financing requirements (US$1-2billion), and improved customer retentio
34、n.For building owners statewide, TES leads to lowerenergy costs (over one half billion US dollars annually),increased property values (US$5 billion), increasedfinancing capability (US$3-4 billion), and increased rev-enues.Since there are many factors that influence the selection,implementation, and
35、operation of a TES system, feasibilitystudies are needed for specific applications. Such a studyshould take into consideration all variables which impact eval-uation of the true benefits of a candidate TES. Where it isimpractical to assess all variables, at least the following signif-icant points sh
36、ould be addressed before TES implementation(for details, see Dincer and Rosen, 2002; Bejan et al., 2004):Short- and long-term management objectives,Energy conservation and environmental impact targets,Economic aims, and financial project parameters includ-ing available utility incentives as well as
37、utility rateschedules and associated energy charges, andTechnical requirements and conditions, includingNew or existing TES system (of course, existingplant would reduce its implementation cost),Net heating or cooling storage capacity (espe-cially for peak-day requirements),Full or partial operating
38、 strategies,Space availability for TES and its components,Storage period (short- or long-term), Operating strategiesType of the TES system (open or closed).TES may be economic if one or more of the followingconditions exist:High utility demand costs,High utility rates during peak hours,High daily lo
39、ad variations,Short duration loads,Infrequent or cyclical loads,Insufficient capacity of cooling equipment to handlepeak loads, andLoad-shifting rebates to avoid peak demand.Some effective applications of TES include:Management of electricity use by shifting cooling loadsto off-peak hours and reduci
40、ng peak loads, andReducing required capacity of building and processcooling systems, or helping existing cooling equipmentto handle an increased load.ENERGY CONSERVATION ASPECTS OFTES SYSTEMSTES appears to be an effective technology for overcomingthe time mismatch between the supply and demand of en
41、ergy,and is a key component of many successful thermal systems.A good TES has reasonably low thermal energy losses, lead-ing to energy savings, thereby permitting good recovery ofstored thermal energy. TES is an important element of energy conservationprograms in industry, particularly in commercial
42、 buildingsand in solar energy utilization. TESs can store energy attemperatures above or below the environment temperature,and come in many types (e.g., tanks, aquifers, ponds, caverns).Many TES systems have been investigated, showing thatalthough many technically and economically successful TESsyst
43、ems have been in operation, no broadly valid basis forcomparing the achieved performance of one storage with thatof another operating under different conditions has foundgeneral acceptance. The development of such a basis forcomparison has been receiving increasing attention, espe-cially using exerg
44、y analysis, which is identified as one of themost powerful ways in evaluating and improving perfor-mance. Exergy analysis is based primarily on the second lawof thermodynamics, unlike energy analysis which is based onthe first law, and takes into account energy quality. TESsystems can help achieve e
45、nergy conservation in several ways(Dincer, 2002), including:The consumption of purchased energy can be reducedby storing waste or surplus thermal energy available atcertain times for use at other times. For example, solarenergy can be stored during the day for heating at night.The demand of purchase
46、d electricity can be reduced bystoring electrically produced thermal energy during off-peak periods to meet the thermal loads that occur duringhigh demand periods. For example, an electric chillercan be used to charge a chilled water TES at night forreducing the electrical demand peaks usually exper
47、i-enced during the day.The purchase of additional equipment for heating, cool-ing or air-conditioning applications can be deferred andthe equipment size in new facilities can be reduced. Theequipment can be operated when thermal loads are lowto charge TES systems. Energy can be withdrawn fromstorage
48、 when needed to help meet thermal loads thatexceed equipment capacity.ENVIRONMENTAL ASPECTS OF TES SYSTEMSTES can contribute significantly to efficient and environ-mentally benign energy use, particularly in building energysystems and power generation. TES helps conserve fossil fuelsthrough increase
49、d efficiency and fuel substitution, and reducepollutant emissions TES can impact air emissions at buildingsites by reducing the amount of (i) ozone-depleting CFC andHCFC refrigerants in chillers, and (ii) combustion emissionsfrom fuel-fired heating and cooling equipment. TES helps reduce CFC use in two main ways. First, sincecooling systems with TES require less chiller capacity thanconventional systems, they use fewer or smaller chillers withlesser refrigerant. Second, using TES can offset the lost cool-ASHRAE Transactions 101ing capacity that sometimes can occur when existing chillers
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