THE ROLE OF THERMAL ENERGY STORAGE SYSTEMS IN SUSTAINABLE DEVELOPMENT

Ibrahim Dincer

Faculty of Engineering and Applied Science University of Ontario Institute of Technology (UOIT)

2000 Simcoe Street North, Oshawa, ON L1H 7K4, Canada Tel: 1-905-721-8668/2573

Fax: 1-905-721-3370 E-mail: Ibrahim.Dincer@uoit.ca

ABSTRACT Thermal energy storage (TES) systems are examined from the perspectives of energy, exergy, environmental impact, sustainability and economics. Reductions possible through TES in energy use and environmental pollution levels are discussed in detail and highlighted with an illustrative example. The importance of using exergy analysis to obtain more realistic and meaningful assessments than the conventional energy analysis of the efficiency and performance of TES systems is demonstrated. The results indicate that cold TES can play a significant role in meeting society's preferences for more efficient, environmentally benign, sustainable and economic energy use in various sectors, and appears to be an appropriate technology for addressing the mismatches that often occur between the times of energy supply and demand. 1. INTRODUCTION There are a number of environmental problems that we face today. These problems span a continuously growing range of pollutants, hazards and ecosystem degradation over ever wider areas. The most significant problems are acid precipitation, stratospheric ozone depletion, and global climate change. The latter is potentially the most important environmental problem relating to energy utilization. Increasing atmospheric concentrations of greenhouse gases are increasing the manner in which these gases trap heat radiated from the earth's surface, thereby raising the surface temperature of the earth and as a consequence sea levels. Recently, a variety of potential solutions to the current environmental problems associated with the harmful pollutant emissions has evolved. TES appears to be the one of the most effective solutions and plays a significant role in environment policies.

Sustainable development demands a sustainable supply of energy resources that, in the long term, is readily and sustainably available at reasonable cost and can be utilized for all required tasks without causing negative societal impacts. TES systems can contribute significantly to meeting society’s desire for more efficient, environmentally benign energy use and for sustainable development of the society, particularly in the areas of building heating and cooling and electric power generation. By reducing energy consumption, the utilization of TES systems results in two significant environmental benefits: (i) the conservation of fossil fuels 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 examine how thermal energy systems may play a significant role in helping and contributing to the local and global sustainable development, and hence how exergy in this regard can be a potential tool for better efficiency, better cost effectiveness, better environment and hence better sustainability. 2. ECONOMIC ASPECTS OF TES SYSTEMS Economical aspects behind the design and operation of energy conversion systems has brought TES forefront. In conjunction with this, provisions must be included in an energy conversion system when the supply of and demand for thermal energy do not coincide in time. The past research has revealed that there is a wide range of practical opportunities for employing TES systems in industrial applications. Such TES systems are of great practical potential for more effective use of thermal energy equipment and for facilitating large-scale energy substitutions from the point of the economic perspective. In principal, a coordinated set of actions has to be taken in several sectors of the energy system for the maximum potential benefits of storage to be realized.

TES-based systems are usually economically justifiable when the annualized capital and operating costs are less than those costs for primary generating equipment supplying the same service loads and periods. TES is mainly

installed to lower initial costs of the other plant components and operating costs. Lower initial equipment costs are usually obtained when large durations occur between periods of energy demand. Secondary capital costs may also be lower for TES-based systems. For example, the electrical service equipment size can sometimes be reduced when energy demand is lowered.

In complete economic analyses of systems including and not including TES, the initial equipment and installation costs must be determined, usually from manufacturers, or estimated. Operating cost savings and the net overall costs should be assessed using life cycle costing or other suitable methods to determine which system is the most beneficial.

Utilizing TES can enhance the economic competitiveness of both energy suppliers and building owners. For example, one study (CEC, 1996) for California indicates that, assuming 20% statewide market penetration of TES, the following financial benefits can be achieved in the state:

For energy suppliers, TES leads to lower generating equipment costs (30% to 50% lower to serve air conditioning loads), reduced financing requirements (US$1-2 billion), and improved customer retention.

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For building owners statewide, TES leads to lower energy costs (over one half billion US dollars annually), increased property values (US$5 billion), increased financing capability (US$3-4 billion), and increased revenues. Since there are many factors that influence the selection, implementation, and operation of a TES system, it is

necessary that comprehensive feasibility study be developed. This study should take into consideration all variables which impact evaluation of the true cost benefits of a candidate TES implementation. However, sometimes, it is practically impossible to conduct all. In practice, concerned people prefer a checklist on how to evaluate a TES system. In such cases, at least the following significant issues should be clarified and addressed before its implementation (for details, see Dincer and Rosen, 2002; Bejan et al., 2004):

short- or long-term management objectives, environmental impact analysis, energy conservation targets, economical aims, financial parameters of the project, available utility incentives, new or existing TES system (of course, existing plant would reduce its implementation cost), net heating or cooling storage capacity (especially for peak-day requirements), utility rate schedules and associated energy charges, full or partial operating strategies, TES system options best suited for the specific application, operating strategies for each of the TES options, space availability (e.g., tank), type of the TES period (short- or long-term), and type of the TES system (open or closed). TES may be economical if one or more of the following conditions exist:

high utility demand costs, high utility rates during peak hours, high daily load variations, short duration loads, infrequent or cyclical loads, insufficient capacity of cooling equipment to handle peak loads, and rebates available for load shifting to avoid peak demand. Some effective applications of TES include:

Electrical power use management by shifting the cooling load to off-peak hours and reducing peak load Reducing required capacity of building and process cooling systems, or helping existing cooling equipment to handle an increased load

3. ENERGY CONSERVATION ASPECTS OF TES SYSTEMS TES appears to be the only solution to correcting the mismatch between the supply and demand of energy. TES is a key component of any successful thermal system and a good TES should allow minimum thermal energy losses, leading to energy savings, while permitting the highest possible extraction efficiency of the stored thermal energy.

TES is an important element of energy conservation programs in industry, particularly in commercial buildings and in solar energy utilization. For many years TES systems have been investigated, which show that although many technically and economically successful TES systems have been in operation, no broadly valid basis for comparing the achieved performance of one storage with that of another operating under different conditions has found general acceptance. The development of such a basis for comparison has been received increasing attention, especially using exergy analysis technique, which is identified as one of the most powerful ways in evaluating the thermal performance of TES systems is based primarily on the second law of thermodynamics, as compared to energy analysis which is based on the first law, and takes into account the quality of the energy transferred. These aspects will be discussed later. Energy conservation from TES systems can be achieved in several manners (Dincer, 2002), including:

The consumption of purchased energy can be reduced by storing waste or surplus thermal energy available at certain times for use at other times. For example, solar energy can be stored during the day for heating at night.

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The demand of purchased electrical energy can be reduced by storing electrically produced thermal energy during off-peak periods to meet the thermal loads that occur during high demand periods. For example, an electric chiller can be used to charge a chilled water TES at night for reducing the electrical demand peaks usually experienced during the day. The purchase of additional equipment for heating, cooling or air-conditioning applications can be deferred and the equipment sizing in new facilities can be reduced. The equipment can be operated when thermal loads are low to charge TES systems. Energy can be withdrawn from storage to help meet the maximum thermal loads that exceed equipment capacity.

4. ENVIRONMENTAL ASPECTS OF TES SYSTEMS TES systems can contribute significantly to meeting society’s desire for more efficient, environmentally benign energy use, particularly in the areas of building heating and cooling and electric power generation. By reducing energy consumption, the utilization of TES systems results in two significant environmental benefits: (i) the conservation of fossil fuels through efficiency increases and/or fuel substitution, and (ii) reductions in emissions of such pollutants as CO2, SO2, NOx and CFCs.

TES can impact air emissions at building sites by reducing (i) the amount of ozone-depleting CFC and HCFC refrigerants in chillers, and (ii) the amount of combustion emissions from fuel-fired heating and cooling equipment. Each of these impacts is considered. TES helps reduce CFC use in two main ways. First, since cooling systems with TES require less chiller capacity than conventional systems, they use fewer or smaller chillers with lesser refrigerant. Second, using TES can off-set the lost cooling capacity that sometimes can occur when existing chillers are converted to more benign refrigerants, making building operators more willing to switch refrigerants.

The potential aggregate air-emission reductions at power plants due to TES have been shown to be significant. For example, TES systems have been shown to reduce CO2 emissions in the UK by 14% to 46% by shifting electric load to off-peak periods (Reindl, 1994), while an EPRI co-sponsored analysis found that TES could reduce CO2 emissions by 7% compared to conventional electric cooling technologies (ARI, 1997). Also, using California Energy Commission data indicating that existing gas plants produce about 0.06 kg of NOx and 15 kg of CO2 per 293,100 kWh of fuel burned, and assuming that TES installations save an average of 6% of the total cooling electricity needs, TES could possibly eliminate annual emissions of about 560 tons of NOx and 260,000 tons of CO2 statewide (CEC, 1996). 5. SUSTAINABILITY ASPECTS OF TES SYSTEMS A secure supply of energy resources is generally agreed to be a necessary but not sufficient requirement for development within a society. Also, sustainable development demands a sustainable supply of energy resources. The implications of these statements are numerous, and depend on how sustainable is defined. One important implication of these statements is that sustainable development within a society requires a supply of energy resources that, in the long term, is readily and sustainably available at reasonable cost and can be utilized for all required tasks without causing negative societal impacts. Supplies of such energy resources as fossil fuels (coal, oil, and natural gas) and uranium are generally acknowledged to be finite; other energy sources such as solar, wind and hydro are generally considered renewable and therefore sustainable over the relatively long term. Wastes (convertible to useful energy forms through, for example, waste-to-energy incineration facilities) and biomass fuels are also usually viewed as sustainable energy sources. A second implication of the initial statements in this section is that sustainable development requires that energy resources be used as efficiently as possible. In this way, society maximizes the benefits it derives from utilizing its energy resources, while minimizing the negative impacts (such as environmental damage) associated with their use. This implication acknowledges that all energy resources are to

some degree finite, so that greater efficiency in utilization allows such resources to contribute to development over a longer period of time, i.e., to make development more sustainable. Even for energy sources that may eventually become inexpensive and widely available, increases in energy efficiency will likely remain sought to reduce the resource requirements (energy, material, etc.) to create and maintain systems and devices to harvest the energy, and to reduce the associated environmental impacts.

The first implication, clearly being essential to sustainable development, has been and continues to be widely discussed. The second implication, which relates to the importance and role of energy efficiency in achieving sustainable development, is somewhat less discussed and understood.

It is clear that sustainable development demands a sustainable supply of energy resources that, in the long term, is readily and sustainably available at reasonable cost and can be utilized for all required tasks without causing negative societal impacts. TES systems can contribute significantly to meeting society’s desire for more efficient, environmentally benign energy use and for sustainable development of the society, particularly in the areas of building heating and cooling and electric power generation.

Sustainability often leads local and national authorities to incorporate environmental considerations into energy planning. The need to satisfy basic human needs and aspirations, combined with increasing world population, will make the need for successful implementation of sustainable development increasingly apparent. Various criteria that are essential to achieving sustainable development in a society follow (Dincer and Rosen, 2005):

information about and public awareness of the benefits of sustainability investments, • • • • • •

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environmental education and training, appropriate energy and energy storage strategies, the availability of renewable energy sources and cleaner technologies, a reasonable supply of financing, and monitoring and evaluation tools.

5.1 Energetic and Exergetic Aspects and Sustainability Thermodynamic principles basically govern energy use and, therefore, an understanding of thermodynamic aspects of energy can help us understand pathways to sustainable development. The impact of energy resource utilization on the environment and the achievement of increased resource-utilization efficiency are best addressed by considering exergy. The exergy of an energy form or a substance is a measure of its usefulness or quality or potential to cause change and provide the basis for an effective measure of the potential of a substance or energy form to impact the environment. It is important to mention that in practice a thorough understanding of exergy and the insights it can provide into the efficiency, environmental impact and sustainability of energy systems, are required for the engineer or scientist working in the area of energy systems and the environment. During the past decade, the need to understand the linkages between exergy and energy, and environmental impact has become increasingly significant (Bejan et al., 2004; Dincer and Rosen, 2004). So, we consider exergy as the confluence of energy, environment and sustainable development and illustrated this in a triangle in Figure 1. The basis for this treatment is the interdisciplinary character of exergy and its relation to each of these disciplines.

Furthermore, some earlier studies (e.g., Dincer and Rosen, 2005) indicated that some environmental effects associated with emissions and resource depletion can be expressed based on physical principles in terms of an exergy-based indicator. It may be possible to generalize this indicator to cover a comprehensive range of environmental effects, and research in line with that objective is ongoing.

The relation between exergy, sustainability and environmental impact is illustrated in Figure 2. There, sustainability is seen to increase and environmental impact to decrease as the exergy efficiency of a process increases. The two limiting efficiency cases in Figure 2 appear to be significant:

As exergy efficiency approaches 100%, the environmental impact associated with process operation approaches zero, since exergy is only converted from one form to another without loss (either through internal consumption or waste emissions). Also sustainability approaches infinity because the process approaches reversibility. As exergy efficiency approaches 0%, sustainability approaches zero because exergy-containing resources (fuel ores, steam, etc.) are used but nothing is accomplished. Also, environmental impact approaches infinity because, to provide a fixed service, an ever increasing quantity of resources must be used and a correspondingly increasing amount of exergy-containing wastes are emitted. Although in this paper we discuss the benefits of using thermodynamic principles, especially exergy, to assess

the sustainability and environmental impact of energy systems, this area of work is relatively new. Further research

is of course needed to ascertain a better understanding of the potential role of exergy in such a comprehensive perspective. This includes the need for research to (i) better define the role of exergy in environmental impact and design, (ii) identify how exergy can be better used as an indicator of potential environmental impact, and (iii) develop holistic exergy-based methods that simultaneously account for technical, economic, environmental and other factors.

Sustainable development also includes to economic viability. Thus, the methods relating exergy and economics also reinforce the link between exergy and sustainable development. The objectives of most existing analysis techniques integrating exergy and economics include the determination of (i) the appropriate allocation of economic resources so as to optimize the design and operation of a system, and/or (ii) the economic feasibility and profitability of a system. Exergy-based economic analysis methods are referred to by such names as thermoeconomics, second-law costing, cost accounting and exergoeconomics. Several detailed reviews of these analysis techniques have recently been published (e.g., Rosen and Dincer, 2003a,b).

Consequently, there is such a diversity of choices that the use of TES technologies plays a key role in the context of sustainable development.

Energy

Environment

Sustainable

Development

EXERGY

Figure 1. The interdisciplinary triangle of exergy. Figure 2. Qualitative illustration of the relation between the environmental impact and sustainability of a process, and its

exergy efficiency.

0 100 Exergy Efficiency (%)

Sustainability Environmental Impact

5.2 Essential Factors for Sustainable Development The main concept of sustainability, which often inspires local and national authorities to incorporate environmental considerations into setting energy program, and which has different meanings in different contexts, embodies a long-term perspective. Future energy systems will largely be shaped by broad and powerful trends that have their roots in basic human needs. Combined with increasing world population, the need will become more apparent for successful implementation of sustainable development. Various parameters are essential to achieving sustainable development in a society as shown in Figure 3, some which are as follows (Dincer and Rosen, 2002): i) Public Awareness: Improving public awareness of need is an initial and crucial step in making a sustainable

energy program successful. This step should be carried out through the media and by public and/or professional organizations.

ii) Information: Necessary information on energy conservation, environmental impact, renewable energy resources, thermal energy storage, etc. should be provided to the public through public and government channels.

iii) Environmental Education and Training: This activity complements the provision of information. Any approach which does not include as integral education and training is likely to fail, so this activity can be considered as crucial to a sustainable energy program. For this reason, a wide scope of specialized agencies and training facilities should be made available to public.

iv) Innovative Energy and Exergy Strategies: Such strategies should be included where appropriate in an effective sustainable energy and exergy program. In parallel, efficient dissemination of information is required of the new methods through public relations, training and counseling.

v) Sustainable Energy Programs: In developing an environmentally benign sustainable energy program (including TES) should be promoted at every stage. Such activities form a basis for short- and long-term policies.

v) Sustainable Energy Programs: In developing an environmentally benign sustainable energy program (including TES) should be promoted at every stage. Such activities form a basis for short- and long-term policies.

vi) Financial Mechanisms: Financing is an important tool for achieving the main goal of sustainable energy development in a country and accelerating the implementation of environment friendly energy technologies.

vi) Financial Mechanisms: Financing is an important tool for achieving the main goal of sustainable energy development in a country and accelerating the implementation of environment friendly energy technologies.

Environmental Education and

Training

Information

Public Awareness

Monitoring and

Evaluation Tools

Financial

Mechanisms

Innovative Energy and Exergy Strategies

vii) Monitoring and Evaluation Tools: In order to assess how successfully a program has been implemented, it is of great importance to monitor each step and evaluate the data and findings obtained. In this regard, appropriate monitoring and evaluation tools should be used.

vii) Monitoring and Evaluation Tools: In order to assess how successfully a program has been implemented, it is of great importance to monitor each step and evaluate the data and findings obtained. In this regard, appropriate monitoring and evaluation tools should be used.

Sustainable Energy

Programs

SUSTAINABILITY

Figure 3. Significant parameters affecting sustainable development. Figure 3. Significant parameters affecting sustainable development. 6. EXERGY ANALYSIS 6. EXERGY ANALYSIS The exergy of an energy form or a substance is a measure of its usefulness or quality or potential to cause change. A thorough understanding of exergy and the insights it can provide into the efficiency, environmental impact and sustainability of energy systems, are required for the engineer or scientist working in the area of energy systems and the environment. Further, as energy policies play an increasingly important role in addressing sustainability issues and a broad range of local, regional and global environmental concerns, policy makers also need to appreciate the exergy concept and its ties to these concerns. During the past decade, the need to understand the connections between exergy and energy, sustainable development and environmental impact has become increasingly significant. In this paper, a study of these connections is presented in order to provide to those involved in energy and environment studies, useful insights and direction for analyzing and solving environmental problems of varying complexity using the exergy concept. The results suggest that exergy provides the basis for an effective measure of the potential of a substance or energy form to impact the environment and appears to be a critical consideration in achieving sustainable development.

The exergy of an energy form or a substance is a measure of its usefulness or quality or potential to cause change. A thorough understanding of exergy and the insights it can provide into the efficiency, environmental impact and sustainability of energy systems, are required for the engineer or scientist working in the area of energy systems and the environment. Further, as energy policies play an increasingly important role in addressing sustainability issues and a broad range of local, regional and global environmental concerns, policy makers also need to appreciate the exergy concept and its ties to these concerns. During the past decade, the need to understand the connections between exergy and energy, sustainable development and environmental impact has become increasingly significant. In this paper, a study of these connections is presented in order to provide to those involved in energy and environment studies, useful insights and direction for analyzing and solving environmental problems of varying complexity using the exergy concept. The results suggest that exergy provides the basis for an effective measure of the potential of a substance or energy form to impact the environment and appears to be a critical consideration in achieving sustainable development. 6.1 Energy and Exergy Efficiencies 6.1 Energy and Exergy Efficiencies An important application of TES is in facilitating the use of off-peak electricity to provide building heating and cooling. Recently, increasing attention has been paid in many countries to cold TES (or CTES), an economically viable technology that has become a key component of many successful thermal systems. Although CTES efficiency and performance evaluations are conventionally based on energy, energy analysis itself is inadequate for complete CTES evaluation because it does not account for such factors as the temperatures at which heat (or cold) is supplied and delivered. Exergy analysis overcomes some of these inadequacies in CTES assessments.

An important application of TES is in facilitating the use of off-peak electricity to provide building heating and cooling. Recently, increasing attention has been paid in many countries to cold TES (or CTES), an economically viable technology that has become a key component of many successful thermal systems. Although CTES efficiency and performance evaluations are conventionally based on energy, energy analysis itself is inadequate for complete CTES evaluation because it does not account for such factors as the temperatures at which heat (or cold) is supplied and delivered. Exergy analysis overcomes some of these inadequacies in CTES assessments.

For a general CTES undergoing a cyclic operation, the overall energy efficiency η and exergy efficiency ψ can be evaluated as

For a general CTES undergoing a cyclic operation, the overall energy efficiency η and exergy efficiency ψ can be evaluated as

inputsinEnergylossEnergy

inputsinEnergyoutputsproductinEnergy

−== 1η (1)

where the word energy represents the cold.

inputsinExergynconsumptiopluslossExergy

inputsinExergyoutputsproductinExergy

−== 1ψ (2)

Further information on energy and exergy analyses of TES and CTES systems can be found in detail in Dincer and Rosen (2002).

Charging Storing Discharging

Figure 4. The three processes in a general CTES system: charging (left), storing (middle), and discharging (right). The heat leakage into the system Ql is illustrated for the storing process, but can occur in all three processes. 7. ILLUSTRATIVE EXAMPLE In this illustrative example a TES system for cooling capacity as shown in Figure 4 is considered (for details, see Rosen et al., 2000). The system is designed to have the chiller operate continuously during the design day, and utilizes three operating modes:

Charging. The charging mode is the normal operating mode during no-load periods. The chiller cools the antifreeze solution to approximately −4°C. The antifreeze solution flows through the storage module, causing the liquid water inside the encapsulated units to freeze. The circulating fluid increases in temperature (to a limit of 0°C), and returns to the chiller to be cooled again. During charging, the building loop is isolated so that full flow is achieved through the storage module.

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Chilling. The chilling mode is the same as for a non-storage (conventional) chiller system. Here, the storage module already is completely charged, and the entire building load is met directly by the chiller. The chiller operates at a warmer set point than for ice making, which results in an increased capacity and a higher COP. In this mode there is no flow through the storage module, and ice is kept in reserve for use later in the day. Chilling and discharging. Chilling and discharging is the normal operating mode during daytime hours. The chiller and storage module share the cooling load, normally in a series configuration. When the heat-transfer fluid passes through the chiller last, the storage module pre-cools the building return fluid before it is cooled to the design supply temperature by the chiller. Systems are normally designed with the chiller downstream of the storage. This sequence gives a higher effective storage capacity since the exit temperature from the storage can be higher. The ice melting rate is controlled by modulating valves which cause some flow to bypass the storage module, usually set so that the blended fluid temperature downstream of the storage is held constant throughout the discharge cycle.

The cooling load of a typical office building is considered and actual TES data for the case is presented in Table 1 for a full 24-hour cycle. The TES module has nonadiabatic storage boundaries with a total thermal resistance of 1.98 m2K/W. Work interactions and kinetic and potential energy terms are considered negligibly small. The specific heat of the heat-transfer fluid (a glycol-based antifreeze solution) is 3224.0 J/kgK at –6.6°C and 3600.8 J/kgK at 15.5°C, and the specific gravity is 1.13. The storage fluid (deionized water) has a freezing point of 0°C, a mass of 144,022 kg and a density of 1000 kg/m3. The storage module has a volume of 181.8 m3, with 144.0 m3 occupied by the storage fluid, and a surface area A of 241.6 m2. The reference environment conditions are 20°C and 1 atm. The overall energy and exergy efficiencies are 99.5% and 50.9% and the hourly energy and exergy efficiencies are listed in Table 1. The hourly exergy efficiencies range from 80 to 94% and average 86% for the overall charging period, range from 53 to 66% and average 60% for the overall discharging period, and range from 99 to 100% for the overall storing period. The hourly energy efficiencies exceed 99% for all periods.

Table 1. Specified and evaluated data for the TES for cooling capacity case considered. Hour Process Load (Tons*) Melted fraction (%) Efficiency (%) Storage Building Chiller Exergy Energy 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Charging Charging Charging Charging Charging Charging Charging Charging Storing Discharging Discharging Discharging Discharging Discharging Discharging Discharging Discharging Storing Charging Charging Charging Charging Charging Charging

270 270 270 270 270 270 270 170 0 175 375 490 635 670 685 475 175 0 270 270 270 270 270 270

0 0 0 0 0 0 0 100 385 580 780 895 1040 1075 1090 880 580 380 0 0 0 0 0 0

270 270 270 270 270 270 270 270 385 405 405 405 405 405 405 405 405 380 270 270 270 270 270 270

48.55 41.46 34.36 27.27 20.17 13.08 5.99 1.53 1.55 6.12 15.96 28.83 45.53 63.15 81.16 93.63 98.21 98.22 91.13 84.03 76.94 69.84 62.74 55.65

88.1 87.0 85.9 84.8 83.7 82.6 81.6 80.4 99.9 66.0 63.3 59.9 58.5 57.1 55.7 52.9 63.9 99.9 93.5 92.6 91.8 90.9 90.1 89.3

99.7 99.7 99.7 99.7 99.7 99.7 99.7 99.5 99.9 99.7 99.9 99.9 99.9 99.9 99.9 99.9 99.7 99.9 99.7 99.7 99.7 99.7 99.7 99.7

* 1 Ton of refrigeration = 3.517 kW. Source: Rosen et al.(2000). Here, the energy efficiencies are high since they only account for heat gains from the environment, which are small. The exergy efficiencies are much lower since they account for the "usefulness" of the energy, which is related to the inlet and outlet temperatures and the mass flow rates of heat-transfer fluid. In the example, the charging fluid being at -4°C, a much lower temperature than that of the environment, is a high-quality cold flow. The cold flow recovered during discharging, however, is of much lower quality as its temperature is now much closer to the environment temperature. Thus, the energy efficiencies, for each hour or for the entire cycle, are misleadingly high as they only account for energy recovery but neglect entirely the loss of quality of the flows. This quality loss is quantifiable with exergy analysis.

The energy efficiency for a TES merely represents the ratio of useful energy output to total energy input. However, the exergy efficiency incorporates the notion of increasing thermodynamic unavailability, as reflected by increasing entropy, in a process or subprocess. That is, since the irreversibilities in a TES process destroy some of the input exergy, TES exergy efficiencies are always lower than the corresponding energy efficiencies.

Another interesting observation stems from the fact that exergy efficiencies provide a measure of how nearly a process approaches ideality, while energy efficiencies do not. The energy efficiencies being over 99% here for the overall process and all sub-periods implies that the TES system is nearly ideal, when this in fact is not the case. The overall exergy efficiency of approximately 51%, as well as the subprocess exergy efficiencies, points out that the TES system is far from ideal, and that there exists a significant margin for efficiency improvement. In this example, the same cooling capacity could be delivered from the TES using about half of the input exergy if the TES were ideal. Thus, overall electrical use by the chillers could be greatly reduced while still maintaining the same cooling services. Such a reduction would reduce the necessary installed cooling power and electrical costs.

The further implications of the results follow: The fact that the exergy efficiencies are less than 100% implies that a mismatch exits between the quality of the thermal energy delivered by the TES (and required by the cooling load) and the quality of the thermal energy input to the TES. This mismatch, which is detectable through the temperature of the thermal energy flows across the TES boundaries, is quantifiable with exergy analysis as the work potential lost during the storage process. The exergy loss, thus, correlates directly with an additional use of electricity by the chillers than would

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occur without the exergy loss. When exergy efficiencies are 100%, there is no loss in temperature during storage.

• The non-ideal exergy efficiencies imply that excessively high-quality thermal energy is supplied to the ITES than is required given the cooling load. Thus, exergy analysis indicates that lower quality sources of thermal energy could be used to meet the cooling load. Although economic and other factors must be taken into account when selecting energy resources, the exergy-based results presented here can assist in identifying feasible energy sources that have other desired characteristics (e.g., environmentally benign or abundant). It is clear that a more perceptive measure of comparison than that provided by the energy efficiency of the

storage cycle is needed if the true usefulness of a TES is to be assessed and a rational basis for the optimization of its economic value established. An efficiency defined simply as the percentage of the total energy stored in a system which can be recovered ignores the quality (exergy) of the recovered energy, and so cannot provide a measure of ideal performance. It is necessary to consider exergy efficiencies in order to have more comprehensive and useful efficiency measures for practical TES systems, and to facilitate more rational comparisons of different systems.

The results presented here suggest that exergy analysis would be useful in comparing different alternative thermal storage system configurations. In addition, exergy analysis could assist in the optimization of TES systems, when combined with assessments of other factors, such as reduced resource-use and environmental impact, increased sustainability and better cost effectiveness. 6. CONCLUSION TES can play a significant role in meeting society's preferences for more efficient, environmentally benign and sustainable energy use in various sectors, and appears to be an appropriate technology for addressing the mismatch that often occurs between the times of energy supply and demand.

For complete performance and efficiency evaluation of TES systems, both energy and exergy analyses should be undertaken. Exergy analysis often provides more meaningful and useful information than energy analysis regarding efficiencies and losses for TES systems, mainly because the loss of low temperature in cold TES is accounted for in exergy-based performance measures, but not in energy-based ones. Furthermore, assessments of the sustainability of processes and systems, and efforts to improve sustainability, should be based in part upon thermodynamic principles, and especially the insights revealed through exergy analysis. ACKNOWLEDGMENT The support for this work provided by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. REFERENCES

ARI.(1997). Thermal Energy Storage: A Solution for Our Energy, Environmental and Economic Challenges, the Air-Conditioning and Refrigeration Institute, Arlington, Va.

Bejan, A., Dincer, I., Lorente, S., Reis, A.H. and Miguel, A.F.(2004). Porous Media in Modern Technologies: Energy, Electronics, Biomedical and Environmental Engineering, Springer Verlag, New York, 396 p.

CEC (1996). Source Energy and Environmental Impacts of Thermal Energy Storage, California Energy Commission, Technical Report No. P500-95-005, California.

Dincer, I., (2002). Thermal energy storage as a key technology in energy conservation, International Journal of Energy Research 26(7), 567-588.

Dincer, I. and Rosen M.A.(2002). Thermal Energy Storage Systems and Applications, John Wiley & Sons, Ltd., London, 580 p.

Dincer, I. and Rosen, M.(2004). Exergy as a drive for achieving sustainability, International Journal of Green Energy 1(1), 1-19.

Dincer, I. and Rosen, M.A.(2005). Thermodynamic aspects of renewables and sustainable development, Renewable and Sustainable Energy Reviews 9(2), 169-189.

Reindl, D.T.(1994). Characterizing the Marginal Basis Source Energy Emissions Associated with Comfort Cooling Systems, Thermal Storage Applications Research Center, Report No. TSARC 94-1, USA.

Rosen, M.A. and Dincer, I.(2003a). Exergoeconomic analysis of power plants operating on various fuels, Applied Thermal Engineering 23(6), 643-658.

Rosen, M.A. and Dincer, I.(2003b). Thermoeconomic analysis of power plants: an application to a coal-fired electrical generating station, Energy Conversion and Management 44(17), 2743-2761.

Rosen, M.A., Dincer, I. and Pedinelli, N.(2000). “Thermodynamic performance of ice thermal energy storage

systems", ASME-Journal of Energy Resources Technology 122(4), 205-211.