Heat Storage Systems

CHAPTER 6

Heat Storage Systems

6.1

INTRODUCTION

Heat storage, also known as thermal energy storage (TES), generally involves the temporary storage of high- or low-temperature thermal energy for later use. Dincer and Rosen (2011) described TES as “an advanced energy technology that is attracting increasing interest for thermal applications such as space and water heating, cooling, and air conditioning.” Examples of heat storage applications include storage of solar energy for overnight heating, of summer heat for winter use, of winter ice for space cooling in summer, and of heat or cool generated electrically during off-peak hours for use during subsequent peak demand hours. In this regard, a heat storage system is in many instances an useful device for offsetting temporal mismatches between thermal energy availability and demand. All heat storage systems have three functions: • Charge: a heat source is used to provide heat to the storage medium. • Storage: a medium is used to store the heat for later use. The storage medium may be located at the heat source, the discharge, or somewhere else. • Discharge: heat is extracted from the storage medium in a controlled fashion for use. Additionally, all heat storage systems consist of three basic parts: • Storage material and, if applicable, a container for the storage material • A heat exchanger to facilitate heat transfer to and from the storage material • A control system that facilitates the charging and discharging of the thermal storage Despite the commonalities in functions and basic parts, there are notable variations in the way heat storage systems are configured. Some are designed specifically for a particular residence or application and are correspondingly Exergy Analysis of Heating, Refrigerating, and Air Conditioning. http://dx.doi.org/10.1016/B978-0-12-417203-6.00006-5 © 2015 Elsevier Inc. All rights reserved.

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tailored. Others follow a more general design approach, with small modifications for the particular installation or application. Heat storage systems for heating or cooling capacity are often utilized in applications where the occurrence of a demand for energy and that of the most favorable supply of energy, based on economic or other factors, are not coincident. Thermal storages are used in energy conservation, industry, commercial building, and solar energy systems. The storage medium can be located in storages of various types, including tanks, ponds, caverns, and underground aquifers. For electrical heating systems, heat storage can be used to enable the purchase of off-peak power, which costs less than power during peak usage times for utilities that provide off-peak power pricing. This type of system is used on a daily or twice-daily charging schedule. During off-peak periods (typically early afternoon and between late evening and early morning), an electric heater or ground source heat pump (GSHP) is used to heat a tank of water or another storage system. The heat storage then provides heat to a distribution system during the remainder of the day. Heat storage can also be used as a thermal dump for excess electricity, such as when a renewable electrical source, like solar photovoltaic panels, produces more electricity than the grid needs. Excess electricity can be converted to heat, stored, and used for space heating or other thermal needs. In combined heat and power systems, heat storage allows for more continuous operation. Many cogeneration systems operate to meet thermal demand, resulting in excess electricity at times when it is not needed. It also causes systems to cycle on and off to meet partial load heat demands. Heat storage allows systems to operate for longer periods of time, by having it charge the storage instead of cycling on and off (Haeseldonckx et al., 2007). Many other heat storage applications exist, including providing energy with high reliability for buildings such as hospitals that experience large consequences if there are interruptions in service. Heat storage can also be used in space cooling applications, for instance, by using the storage of winter ice to provide space cooling during the summer (Dincer and Rosen, 2013; Dincer and Dost, 1996). Heat storage can usually be used for many applications in most types of buildings. From a thermodynamic point of view, there are three types of heat storage: chemical, latent, and sensible. Latent heat storages use phase change materials (PCMs). In sensible heat storage, the storage medium remains in a single phase, while in latent heat storage, the storage medium undergoes a phase change. Sensible heat storage systems (e.g., liquid water systems) exhibit changes in temperature in the store as heat is added or removed. In latent heat storage systems (e.g., liquid water/ice systems and eutectic salt systems), the storage temperature remains fixed during the phase change portion of the storage cycle. In chemical energy storage, heat is stored in chemical reactions that can readily be reversed. At present, this type of system is not commonly used for residential applications and is

6.1

Introduction

Table 6.1 Typical Performance Parameters for Sensible, Latent, and Chemical Heat Storage Technologies Heat Storage Type

Capacity (kWh/t)

Power (MW)

Efficiency (%)

Storage Period

Sensible (e.g., water) Latent (e.g., phase change materials) Chemical

10-50 50-150 120-250

0.001-10 0.001-1 0.01-1

50-90 75-90 75-100

Daily-monthly Hourly-monthly Hourly-daily

Source: IEA (2011).

mainly undergoing development, so it is not discussed further in this chapter. Typical performance data, including capacity, power, efficiency, and storage periods, are shown in Table 6.1 for the main heat storage types. Heat storage systems are used in a wide variety of applications and are designed to operate on a cyclical basis (e.g., daily, weekly, and seasonally). Heat storage systems achieve benefits by fulfilling one or more of the following purposes: • Generation capacity increase: Demands for heating, cooling, and electricity are seldom constant over time, and the excess generation capacity available during low-demand periods can be used to charge a heat storage in order to increase the effective generation capacity during high-demand periods. This process allows a smaller production unit to be installed (or to add capacity without purchasing additional units) and results in a higher load factor for the units: • Enhanced operation of cogeneration plants: Combined heat and power, or cogeneration, plants are generally operated to meet the demands of the connected thermal load, which often results in excess electric generation during periods of low electricity use. By incorporating a heat storage system, the plant need not follow a load and instead can be dispatched in more advantageous ways (within some constraints). • Energy purchase shift to low-cost periods: This use is the demand-side application of the first purpose listed and allows energy consumers subject to time-of-day pricing to shift energy purchases from high- to low-cost periods. • Increased system reliability: Any form of energy storage, from the uninterruptible power supply of a small personal computer to a large pumped storage, normally increases system reliability. • Functional integration: In applications where on-site water storage is needed for fire protection, it may be feasible to incorporate thermal storage into a common storage tank. Likewise, equipment designed to solve powerquality problems may be adaptable to energy storage purposes.

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The most significant benefit of a heat storage system is often cited as its ability to reduce electric costs by using off-peak electricity to produce and store energy for daytime cooling. Indeed, heat storage systems successfully operate in offices, hospitals, schools, universities, airports, etc., in many countries, shifting energy consumption from periods of peak electricity rates to periods of lower rates. That benefit is accompanied by the additional benefit of lower demand charges. Having investigated methods for evaluating and comparing heat storage systems for many years, the present authors observe that, while many technically and economically successful thermal storages are in operation, no generally valid basis for comparing the achieved performance of one storage with that of another operating under different conditions has found broad acceptance. The energy efficiency, the ratio of the energy recovered from storage to that originally inputted, is conventionally used to measure heat storage system performance. The energy efficiency, however, is inadequate because it does not take into account important factors like how nearly the performance approaches ideality, storage duration, and temperatures of the supplied and recovered thermal energy and of the surroundings. Exergy analysis provides an illuminating, rational, and meaningful alternative for assessing and comparing heat storage systems. In particular, exergy analysis yields efficiencies that provide a true measure of how nearly actual performance approaches the ideal and identifies more clearly than energy analysis the magnitudes, causes, and locations of thermodynamic losses. Consequently, exergy analysis can assist in improving and optimizing heat storage system designs. Using information in the authors’ recent book on heat storage systems (Dincer and Rosen, 2011), this chapter describes the application of exergy analysis to heat storage systems and demonstrates the usefulness of such analyses in providing insights into heat storage system behavior and performance, for heating, ventilation, and air conditioning (HVAC) and other applications. Key thermodynamic considerations in heat storage system evaluation are discussed, and the use of exergy in evaluating a heat storage system is detailed.

6.2 PERFORMANCE CONSIDERATIONS IN HEAT STORAGE SYSTEMS This section provides aspects of thermodynamics most relevant to energy and exergy analyses. We define thermodynamics as the science of energy (which comes from first law of thermodynamics (FLT)) and exergy (which comes from the second law of thermodynamics (SLT)). Fundamental principles and such related issues as reference-environment selection, efficiency definition, and materialproperties acquisition are discussed. General implications of exergy analysis results are discussed, and a step-by-step procedure for energy and exergy analyses is given.

6.2

Performance Considerations In Heat Storage Systems

6.2.1 Principal Thermodynamic Factors in Heat Storage Systems Several of the principal thermodynamic considerations in the evaluation and comparison of heat storage systems are described in the next several paragraphs. Energy and exergy: Energy and exergy are significant quantities in evaluating heat storage systems. Exergy analysis complements energy analysis and circumvents many of the difficulties associated with conventional energy-based heat storage system methods by providing a more rational evaluation and comparison basis. Temperature: Exergy reflects the temperature of a heat transfer and the degradation of heat quality through temperature loss. Exergy analysis applies equally well to systems for storing thermal energy at temperatures above and below the temperature of the environment because the exergy associated with such energy is always greater than or equal to zero. Energy analysis is more difficult to apply to such storage systems because efficiency definitions have to be carefully modified when cooling capacity, instead of heating capacity, is stored or when both warm and cool reservoirs are included. Thus, exergy analysis provides for more rational evaluation of heat storage systems for cooling or heating capacity. Efficiencies: The evaluation of a heat storage system requires a measure of performance that is rational, meaningful, and practical. A more perceptive basis than energy efficiency is needed if the true usefulness of thermal storages is to be assessed and so permits maximization of their economic benefit. Exergy efficiencies provide rational measures since they assess the approach to ideal heat storage system performance. Losses: With energy analysis, all losses are attributable to energy releases across system boundaries. With exergy analysis, losses are divided into two types: exergy releases from the system and internal exergy consumptions. The latter include reductions in availability of the stored heat through mixing of warm and cool fluids. The division of exergy losses allows the causes of inefficiencies to be accurately identified and improvement effort to be effectively allocated. Stratification: Thermal stratification within a heat storage system reduces temperature degradation. In many practical cases, a vertical cylindrical tank with a hot water inlet (outlet) at the top and a cold water inlet (outlet) at the bottom is used. The hot water and cold water in the tank usually are stratified initially into two layers, with a mixing layer in between. The degree of stratification is affected by the volume and configuration of the tank, the design of the inlets and outlets, the flow rates of the entering and exiting streams, and the durations of the charging, storing, and discharging periods. Increasing stratification improves heat storage system efficiency relative to a thermally mixed storage tank. Four primary factors degrade stored energy by reducing stratification:

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• Heat leakages to or from the environment • Heat conduction and convection from the hot portions of the storage fluid to the colder portions • Vertical conduction in the tank wall • Mixing during charging and discharging periods (often the main cause of loss of stratification) The effects of stratification are more clearly assessed with exergy than energy. Through carefully managing the injection, recovery, and holding of heat (or cold) so that temperature degradation is minimized, better storage-cycle performance can be achieved (as measured by better thermal energy recovery and temperature retention, that is, increased exergy efficiency). Storage duration: Rational evaluation and comparison of heat storage systems must account for storage duration. The length of time thermal energy is retained in a heat storage system does not enter into expressions for efficiency, although it is clearly a dominant consideration in overall heat storage system effectiveness. The relation between storage duration and effectiveness permits an approach for comparing heat storage systems using a time parameter. Reference-environment temperature: Since heat storage evaluations based on energy and exergy are affected by the value of the reference-environment temperature T0, temporal and spatial variations of T0 must be considered (especially for heat storage systems with storage periods of several months). There is a growing interest in the use of diurnal or daily heat storage systems for electrical load management in both new and existing buildings. Heat storage technologies allow electricity consumption costs to be reduced by shifting electrical heating and cooling demands to periods when electricity prices are lower, for instance, during the night. Load shifting can also reduce demand charges, which can represent a significant proportion of total electricity costs for commercial buildings. Some important technical requirements for heat storage systems are as follows: • High energy density (per unit mass and/or per unit volume) in the storage material • Good heat transfer between heat transfer fluid (HTF) and the storage medium • Mechanical and chemical stability of the storage material • Chemical compatibility between HTF, heat exchanger, and storage medium • Complete reversibility for a large number of charging/discharging cycles • Low thermal losses • Ease of control Also, when designing a heat storage system, cost of the (i) storage material, (ii) heat exchanger, and (iii) space and the enclosure for the heat storage system should be considered.

6.2

Performance Considerations In Heat Storage Systems

6.2.2 Energy and Exergy Analyses of Heat Storage Systems Environmental problems associated with energy utilization span a growing spectrum of environmental quality and natural ecosystem issues. The SLT is useful in providing insights into environmental impact. In conjunction with this, exergy appears to be an effective measure of the potential of a substance to impact the environment. Exergy is defined as the maximum amount of work that can be produced by a stream of matter, heat, or work as it comes to equilibrium with a reference environment. It is a measure of the potential of a stream to cause change, as a consequence of not being completely stable relative to the reference environment. Exergy is not subject to a conservation law; rather, exergy is consumed or destroyed, due to irreversibilities in any process. For exergy analysis, the state of the reference environment, or the reference state, must be specified. This is commonly done by specifying the temperature, pressure, and chemical composition of the reference environment. Exergy analysis is a method that uses the conservation of mass and conservation of energy principles together with the SLT for the design and analysis of energy systems. The exergy method can be suitable for furthering the goal of more efficient energy-resource use, for it enables the locations, types, and true magnitudes of wastes and losses to be determined. Therefore, exergy analysis can reveal whether or not and by how much it is possible to design more efficient energy systems by reducing the inefficiencies in existing systems. Recently, exergy analysis has been recognized as the more powerful method for performance evaluations and design of heat storage systems than energy analysis (Rosen et al., 1988; Taylor et al., 1991; Krane and Krane, 1992; Rosen, 1992; Dincer et al., 1997). For an overall heat storage process or any subprocesses (i.e., charging, storing, and discharging), overall energy and exergy balances; individual energy and exergy balances for charging, storing, and discharging periods; overall energy and exergy efficiencies; and individual energy and exergy efficiencies for charging, storing, and discharging periods are summarized in Table 6.2. The term efficiency can be used in at least two ways: first law efficiency (i.e., energy efficiency) and second law efficiency (i.e., exergy efficiency). The energy efficiency merely reflects the standard task of energy storage, as 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 why storage exergy efficiencies are always lower that the energy efficiencies; process irreversibilities destroy some of the input energy (Rosen, 1992; Rosen and Hooper, 1996; Dincer et al., 1997).

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Energy Equationa

Balances for overall storage process or any subprocesses Individual balances

ΔE ¼ Qc  ðQl + Qd Þ

Charging period Storing period Discharging period Balances for initial (i) and final (f) states

ΔEc ¼ Qc  Ql, c ΔEs ¼ Ql, s ΔEd ¼ Qd  Ql, d ΔE ¼ Ef  Ei
 ΔEc ¼ Ef, c  Ei, c Ei, c ¼ Ei
 ΔEs ¼ Ef, s  Ei, s Ei, s ¼ Ef, c

 ΔEd ¼ Ef, d  Ei, d Ei, d ¼ Ef, s and Ef, d ¼ Ef E R ¼ E  E0 ΔE ¼ EfR  EiR (applies to charging, storage, and discharging periods)

Energy content for a particular state with respect to a reference environment Overall efficiencies Efficiencies for charging period Efficiencies for storing period Efficiencies for discharging period

Ql ¼ Ql, c + Ql, s + Ql, d ΔE ¼ ΔEc + ΔEs + ΔEd

Qd Ql + ΔE ¼1 Qc Qc ΔEc Q l, c ηc ¼ ¼1 Qc Qc ΔEc + ΔEs Q l, s ¼1 ηs ¼ ΔEc ΔEc Qd Ql, d + ΔE ηd ¼ ¼1 ΔEc + ΔEs ΔEc + ΔEs η¼

Exergy Equationa
 ΔEx ¼ ExQ, c  ExQ, l + ExQ, d  Exdest Exdest ¼ Exdest, c + Exdest, s + Exdest, d Exl ¼ Exl, c + Exl, s + Exl, d ΔEx ¼ ΔExc + ΔExs + ΔExd ΔExc ¼ ExQ, c  Exl, c  Exdest, c ΔExc ¼ Exl, s  Exdest, s ΔExd ¼ ExQ, d  Exl, d  Exdest, d ΔEx ¼ Exf  Exi
 ΔExc ¼ Exf, c  Exi, c Exi, c ¼ Exi
 ΔExs ¼ Exf, s  Exi, s Exi, s ¼ Exf, c

 ΔExd ¼ Exf, d  Exi, d Exi, d ¼ Exf, s and Exf, d ¼ Exd

ExQ, d ExQ, l + ΔEx + Exdest ¼1 ExQ, c ExQ, c ΔExc Exl, c + Exdest, c ψc ¼ ¼1 ExQ, c ExQ, c ΔExc + ΔExs Exl, s + Exdest, s ψs ¼ ¼1 ΔExc ΔExc ExQ, d Exl, d + ΔEx + Exdest, d ψd ¼ ¼1 ΔExc + ΔExs ΔExc + ΔExs ψ¼

a c, charging; d, discharging; dest, destruction; f, final; i, initial; l, loss (leakage); R, reference; s, storing. Source: Dincer (2002).

Heat Storage Systems

Description

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Table 6.2 Energy and Exergy Balance and Efficiency Equations for a Heat Storage System

6.2

Performance Considerations In Heat Storage Systems

As seen in Table 6.2, in a heat storage system, there are three steps (subprocesses) and these must be taken into consideration for comprehensive energy and exergy analyses. In the analysis, it is usually assumed that there are no heat losses to the surroundings from the charging or discharging fluids (all heat removed from the charging fluid is added to the storage medium, and for the discharging period, all heat added to the discharging fluid originates in the storage medium). This assumption is most valid for small rates of heat loss from the relevant fluids. Rosen (1992) pointed out that in practical systems, the flows of the charging and discharging fluids are often steady and have timedependent thermal properties. Note that all four energy efficiencies in Table 6.2 become identical if ERf ¼ ERi ¼ 0, and then, these equations cannot provide rational measures of the performance of the system. In order to provide rational measures, △E must be less than zero. Similar expression can be written for the exergy efficiencies.

6.2.3

Environmental Impacts of Heat Storage Systems

Heat storage 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 heat storage results in two significant environmental benefits: (i) conservation of fossil fuels through efficiency increases and/or fuel substitution and (ii) reductions in emissions of such pollutants as CO2, SO2, NOx, and chlorofluorocarbons (CFCs). Heat storage systems can impact air quality at building sites by reducing emissions of (i) the amount of ozone-depleting CFC and hydrochlorofluorocarbons (HCFC) refrigerants in chillers and (ii) combustion gases from fuel-fired heating and cooling equipment. Heat storage systems help reduce CFC use in two main ways. First, since cooling systems with heat storage require less chiller capacity than conventional systems, they use fewer or smaller chillers with less refrigerant. Second, using heat storage systems can offset the lost cooling capacity that sometimes occurs when existing chillers are converted to more benign refrigerants, making it easier for building operators to switch refrigerants. The potential aggregate air-emission reductions at power plants due to heat storage systems can be significant. For example, heat storage systems have been shown to reduce CO2 emissions in the United Kingdom by 14-46% by shifting electric load to off-peak periods (Beggs, 1994), while an EPRI-cosponsored analysis found that TES could reduce CO2 emissions by 7% compared to conventional electric cooling technologies (Reindl, 1994). Also, using the 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

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assuming that heat storage system installations save an average of 6% of the total cooling electricity needs, heat storage systems could possibly eliminate annual emissions of about 560 tons of NOx and 260,000 tons of CO2 statewide (California Energy Commission, 1996).

6.3

CLASSIFICATION OF HEAT STORAGE SYSTEMS

The suitability of a particular technology for an individual application can be broadly evaluated in terms of technical potential. Heat storage systems can be classified based on storage duration, desired output temperature and capacity (or size/load), and storage medium. These criteria can be used as a starting point in determining suitability for particular applications (Hauer et al., 2013). Technical, sizing, economic, energy saving, and environmental criteria are used to evaluate heat storage systems and applications. Each of these should be considered carefully for successful heat storage system implementation. Independent technical criteria for storage systems are difficult to establish since they usually are case-specific and are closely related to and generally affected by the economics of the resultant systems. Although appropriate trade-offs must be made among criteria, certain technical factors are generally desirable, including • • • • • • • • • • • • •

storage capacity, storage duration, technical availability, integrability with other thermal systems, reliability, applicability, lifetime, size, cost, efficiency, safety, installation, and environmental standards.

A heat storage system designer requires technical information on heat storage systems, such as the types of storage available, the amount of storage required, and the effect of storage on system performance, reliability, and cost. Heat storage systems can be difficult to employ at sites with space restrictions. Also, heat storages often have significant first capital costs. Financial analyses of heat storage system-based projects can be complex, although most consulting energy engineers are capable of evaluating relevant financial parameters and benefits.

6.3

6.3.1

Classification Of Heat Storage Systems

Storage Duration

Heat storages can be designed to store heat for various time periods, from hours to months. In each case, storage systems are charged with heat or cold, which is then stored for a length of time before discharging occurs. Different heat storage systems classified by their storage duration along with examples are the following: • Less than 1 day (hourly loads): Electrical thermal storage (ETS) devices and off-peak utility power systems, which might charge thermal mass each afternoon and night • One day (diurnal storage and daily loads): Solar or biomass thermal space heating systems in mild climates • Several days (daily loads for a few days due to intermittent supply): Solar thermal space heating systems and domestic hot water in mild climates that typically experience cloudy days • Several months (seasonal storage): Space heating systems that use seasonally available solar resource to charge storage, for example, charge during the summer for use in the winter or vice versa The length of time and the amount of usable heat stored in the medium depend on the user’s needs, the heat source, and the system design. However, there are advantages and disadvantages to short- and long-time frame systems that are important to consider when deciding on a heating system. Systems that are recharged daily have a smaller size, and thus a lower capital cost, than seasonal systems. Smaller systems are typically manufactured off-site. Seasonal storage, while more expensive and voluminous, reduces the reliance on any single day of thermal charging, which increases system reliability. For instance, a few cloudy days do not affect significantly solar thermal seasonal storage over a year. Also, seasonal storage can be used for district heating systems, heating systems that provide heat to multiple buildings, where capital and maintenance costs can be shared.

6.3.2

Temperature Range

Heat storage systems can store heat at a range of temperatures. In fact, they can also be used in space cooling applications, in which some systems store ice to be used for cooling during the summer. The storage temperature, or temperature range in the case of sensible storage, is determined by the type of space heating (or cooling) application, the temperature of the heat source, and the temperature of the demand-side distribution system. Practical limitations are also imposed by the storage container, particularly since a greater temperature difference between the storage material and the ambient temperature raises the rate of heat loss through the container walls, increasing the need for insulation.

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Heating systems are often classified as high-temperature systems or lowtemperature systems. Low-temperature systems typically store heat below the boiling point of water (100 °C) and are used for space heating applications with hydronic distribution systems. For instance, baseboards require heat in the range of 65-85 °C, and radiant floors require temperatures of 30-40 °C. Higher-temperature storage, or systems that store heat above 100 °C, can be used for forced-air systems: in these systems, air is often blown over rocks or ceramic bricks that store heat. High-temperature heat storage can also be stepped down for other uses. Temperature ranges and applications of cold-, low-, medium-, and high-temperature heat storage systems are listed in Table 6.3. In some instance, these heat storage systems can be made more effective by designing them to maintain high-temperature heat in the inner regions of the storage and lower temperatures at the outer regions.

6.3.3

Storage Capacity

The capacity (or size or scale) of a heat storage depends on several factors, such as (i) heating/cooling requirement (heat load), (ii) heat source capacity, (iii) storage material heat capacity, (iv) storage duration, and (v) standby loss. Optimal sizing of the thermal storage system is important to achieve appropriate performance. Without a supplemental heat source, undersized thermal storage systems are insufficient to meet heating demands. Oversized systems have a higher capital cost, can cost more to maintain, and can waste energy through standby losses. Sizing is of increased significance for seasonal storage systems as even optimally sized systems have large space requirements and installation

Table 6.3 Temperature Ranges and Applications of Cold-, Low-, Medium-, and High-Temperature Heat Storage Systems Type

Temperature Range (°C)

Application

Additional Information

Cold

<10

Building cooling

Low

10-30

Medium

30-50

High

>50

Low-temperature residential heating Residential and commercial heating (e.g., schools and offices) District heating

Antifreeze mixture is needed to avoid freezing in heat exchanger fluid No auxiliary conventional heat source is needed Artificial charging requirement (with solar collectors) Artificial charging requirement (with solar collectors or waste heat) No heat pump requirements Need for auxiliary burners Need for high-performance storage media

Sources: Hahne (2000) and Chwieduk (2012).

6.3

Classification Of Heat Storage Systems

Table 6.4 Near-Term Suitability Criteria for Determining Prime Heat Storage Technologies Heat Storage Type

Examples

Near-Term Beneficial Areas

Small scale

Ice storage, hot and cold water tanks Underground thermal energy storage (UTES), molten salts

Higher demand variability (i.e., more peak-like demands, with much hot or cold needed at one time or another) Significant waste heat resources, concentrated heating or cooling demand, or large amounts of concentrating solar power (CSP)

Large scale

Source: IEA (2015).

costs. Examples of small- and large-scale heat storage technologies are presented in Table 6.4, along with comments on their near-term suitability. A need exists for improved heat storage-sizing techniques, as analyses of applications reveal both undersized and oversized systems. Undersizing can result in poor levels of indoor comfort, while oversizing results not only in higher than necessary initial costs but also in the potential wasting of electricity if more energy is stored than required. Another requirement for successful heat storage is proper installation and control. State-of-the-art and properly designed and controlled storage systems often do not use more energy than conventional heating and cooling equipment. Performance data describing the use of heat storage systems for heating and cooling by shifting peak loads to off-peak periods have been reported and show the potential for such technologies to be substantial. The initial costs of such systems can be lower than those for other systems. To yield the benefits, new construction techniques are required together with the use of more sophisticated thermal-design calculations that are, at present, not well known to many builders and designers.

6.3.4

Underground Thermal Energy Storage

Underground heat storage, or underground thermal energy storage (UTES), has storing temperature range from around 0 °C to up to 40-50 °C. This operating temperature range is suitable for heating and cooling applications in HVAC. UTES systems have been demonstrated in many cases to be advantageous for energy management to provide good economic returns. For example, UTES systems have been successfully integrated with heating, cooling, and air conditioning applications in many projects (Kizilkan and Dincer, 2012). As an increasingly used storage technology, UTES makes use of the underground as a storage medium for both heat storage and cold storage. UTES technologies include borehole storage, aquifer storage, cavern storage, and pit

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storage. The choice of technology depends in large part on local geologic conditions, as discussed below: • Borehole storage is based on vertical heat exchangers installed in the ground, which ensure the transfer of thermal energy to and from the ground layers (e.g., clay, sand, and rock). Many projects aim for seasonal storage of solar heat in summer to heat buildings in winter. Ground heat exchangers are also frequently used in combination with heat pumps where the ground heat exchanger extracts low-temperature heat from the soil. • Aquifer storage uses a natural underground water-permeable layer as a storage medium. The transfer of thermal energy is achieved by mass transfer (i.e., extracting/reinjecting water from/into the underground layer). Most applications deal with the storage of winter cold to be used for the cooling of large office buildings and industrial processes in the summer. A major prerequisite for this technology is the availability of suitable geologic formations. • Cavern storage and pit storage are based on large underground water reservoirs created in the subsoil to serve as heat storages. These storage options are technically feasible, but applications are limited because of the high investment costs. The two main types of UTES considered in more detail in this chapter are, namely, borehole TES (BTES) and aquifer TES (ATES). Heat pumps are commonly incorporated with these systems to utilize the underground storage as a heating or a cooling source. Additionally, for the same purpose, these systems can be connected to solar collectors, surface water, waste heat sources, etc. The main distinction between BTES and ATES is that the former is often (but not always) closed with no direct contact between the heat pump fluid and the underground water. In an ATES, the underground water is extracted and pumped through heat exchangers to recover the available heat. For high-temperature (i.e., above 100 °C)-sensible heat storage, the storage medium of choice is usually a liquid (e.g., oil or molten salts, the latter for temperatures up to 550 °C). For very high temperatures, solid materials (e.g., ceramics and concrete) can be used, but most high-temperature-sensible heat storage options are undergoing development or demonstration. A common technology for heat storage today is the domestic hot water tanks. Other technologies, such as ice and chilled-water storage, are starting to play important roles in several countries, including Australia, the United States, China, and Japan, as utilities seek to reduce peak loads and consumers seek to lower their electricity bills. UTES systems are frequently found in Canada, Germany, and many European countries (IEA, 2011). Table 6.5 depicts a range of heat storage technologies and their characteristics and efficiencies.

6.3

Classification Of Heat Storage Systems

Table 6.5 Current Status and Applications of Selected Heat Storage Technologies

Efficiency (%)

Initial Investment Cost (USD/ kW)

Primary Application

Supply

50-90

3400-4500

Long-term storage

Molten salts

Supply

40-93

400-700

Thermo chemical Solid media storage

Supply, demand Demand

80-99

1000-3000

50-90

500-3000

Ice storage

Demand

75-90

6000-15,000

Hot water storage (residential) Cold water storage

Demand

50-90

N/Ab

Demand

50-90

300-600

Heat Storage Technology

Locationa

UTES

Sample Applications

Drake Landing Solar Community (Canada) High-temperature Gemasolar CSP plant (Spain) Low-, medium-, and Thermal Control Systems high-temperature (TCS) for CSP plants (R&D) Medium-temperature Residential electric thermal storage (the United States) Low-temperature Tokyo Denki University (Japan), China Pavilion project (China) Medium-temperature Peak demand reduction (France) Low-temperature

a

Shanghai Pudong International Airport (China)

Typical locations in current heat storage systems. These locations may change as the technologies evolve. Energy storage capabilities present in hot water storage tanks can be utilized for negligible additional cost. Sources: IEA (2011, 2015). b

Heat storage systems can take many forms to suit a variety of applications, such as off-peak heating and air conditioning and industrial/process heating and cooling. Selecting a storage and its characteristics usually requires a detailed feasibility study. The analysis is involved and best accomplished following an established procedure. Data needed for a feasibility analysis can include (i) an hour-by-hour load profile for the design day and (ii) a description of a baseline nonstorage system, including chiller capacity, operating conditions, and efficiency. The description of a heat storage system often stipulates the following: • Sizing basis (full storage, load leveling, or demand limiting) • Sizing calculations showing chiller capacity and storage capacity and considering required supply temperature • Design operating profile, showing load, chiller output, and the amount of heat added to or taken from storage for each hour of the design day • Chiller operating conditions while charging the storage and if applicable when meeting the load directly • Chiller efficiency under each operating condition • Description of the system control strategy, for the design day and part-load operation

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Table 6.6 Summary of Classification of Heat Storage Systems Classification Criteria Temperature

Duration

Storage material (solid, liquid, gas, and latent)

UTES

Heat Storage Type High temperature Low temperature Below zero (CTES) Seasonal Weekly Daily Hourly Water Ice Oil Sand and rock Concrete PCM Molten salt ATES BTES Cavern TES

Notes and Applications Solar power-based, operating temperature is higher than 350 °C Operating temperature is lower than 100 °C Water with glycol, snow, ice storage, ice slurry storage, etc. Long term, in areas with wide seasonal temperature variation Mostly residential Commercial buildings with high daytime and low nighttime loads Buildings with highly variable hourly loads Most common medium especially for HVAC applications High storage capacities in limited spaces Mainly in solar thermal power plants where oil can be used both for storage medium and as HTF Cost-effective storage mediums Low cost, widely available, and can be easily shaped Materials (e.g., salt compounds) undergo phase change Used for high-temperature (usually above 300 °C) applications Storage medium is underground water where hot and cold thermal fluids are in direct contact (open system) Storage without direct contact of the hot and cold thermal fluids (closed system) Rock caverns utilized for TES

An operating cost analysis essentially includes the following: • An evaluation of demand savings • A determination of changes in energy consumption and cost • A description and justification of the assumptions used for annual energy demand and use estimates As mentioned earlier, heat storage systems are classified according to various criteria (e.g., storing period, storing medium, temperature level, and application). Classifications of heat storage systems are summarized in Table 6.6. Several factors are desirable for specific heat storage applications, including high specific heat capacity, long-term stability under cyclic operation, and low cost.

6.4 HEAT STORAGE SYSTEMS FOR HEATING APPLICATIONS Heat storage systems for heating applications mainly use low-grade heat such as solar energy or waste heat from power generation plants and industrial thermal

6.4

Heat Storage Systems For Heating Applications

Table 6.7 Thermal Capacities of Some Common Heat Storage Materials at 20 °C Material

Density (kg/m3)

Specific Heat (kJ/kg K)

Volumetric Heat Capacity (kJ/m3 K)

Clay Brick Sandstone Wood Concrete Glass Aluminum Iron Steel Gravely earth Magnetite Water

1458 1800 2200 700 2000 2710 2710 7900 7840 2050 5177 988

0.879 0.837 0.712 2.39 0.88 0.837 0.896 0.452 0.465 1.84 0.752 4.182

1280 1510 1570 1670 1760 2270 2430 3570 3680 3770 3890 4170

Sources: Norton (1992) and Dincer (1998).

processes for short- and long-term storage purposes. For short-term heat storage, there are a number of storage media considered in practice (e.g., water, oil, molten salts, molten metals, bricks, sand, and soil). Large aquifers, rock beds, solar ponds, and large tanks are used for long-term (e.g., annual) storage. Some commonly used heat storage materials and their properties are summarized in Table 6.7. It is desirable that a storage material be inexpensive and has a good thermal capacity factor. Another important parameter in heat storage is the rate at which heat can be released and extracted. This leads to the ability of a material to store heat, which is a function of thermal diffusivity. For this reason, iron shot is an excellent thermal storage medium, having both high heat capacity and high thermal conductance. For high-temperature heat storage (up to several hundred degrees centigrade), iron or iron oxide is as good as water per unit volume of storage. The cost is moderate for either pellets of the oxide or metal balls. Since iron and its oxide have equal performance, the slow oxidization of the metal in a high-temperature liquid or air system does not degrade performance. Water as a heat storage medium has an excellent specific heat and is both inexpensive and chemically stable. If it is employed above 100 °C, the system has to be pressurized, which adds significantly to costs. For such a case, the limitation of water is its critical point (374 °C). In addition, a number of heat-resistant oils (e.g., Therminol) are available in the market, which can be readily used without

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pressurization at temperatures over a broad temperature range (10 to 320 °C). However, while the average specific heat of water is 4.19 kJ/kg K, most oils have specific heats only about 2.3 kJ/kg K (Diamant, 1984). Another disadvantage of these oils is the liability of high-temperature cracking, polymerization, and the formation of volatile products. Other storage substances include molten salts and molten metals. Molten salts can also be employed for high-temperature storage with an average specific heat of 1.5 kJ/kg K (Diamant, 1984), but they have some disadvantages, such as solidification temperatures of a minimum of 150 °C and corrosion effects. Molten metals (e.g., liquid sodium) can be used in the unpressurized state at temperatures up to 760 °C with an average specific heat of 1.3 kJ/kg K (Diamant, 1984), implying some disadvantages, for example, handling problems. Rock is another good heat storage material from the standpoint of cost, but its thermal capacity is only half that of water. Past studies have demonstrated that the rock storage bin is practical and its main advantage is that it can easily be used for heat storage above 100 °C. Therefore, rocks are sometimes preferred over water for solar systems. However, air/rock solar systems can make provision for partial heat storage in water for domestic hot water use. The amount of heat stored in rocks, compared with an equivalent amount of heat stored in water, occupies approximately three times as much volume. This apparent disadvantage is partly offset by costs associated with water containment. Adding the higher cost and maintenance of a liquid collector and the economics often favor the use of air collectors with rock storage for domestic heating applications. Combining water with air/rock systems has become a standard form of solar system application. The objective is to provide a portion of energy needs for domestic hot water without a significant reduction in the solar energy supply for space heating. Essentially, this hybrid system is composed of a standard air/rock solar system, a counterflow heat exchanger, and a small water store. Conventional air/rock solar systems for space heating are only used during the heating season, lengthening the payback period, because the solar system is inoperative for several months of the year. Adding hot water capability to the basic air/rock system increases the capital cost slightly. So, for a relatively small additional investment, a solar thermal system can provide 100% of the domestic hot water during summer and proportionally less as the seasons change to and from winter.

6.4.1 Thermodynamic Analysis of Heat Storage Systems for Heating Applications The subprocesses of a storage for heating capacity are shown in Fig. 6.1. Thermodynamic analyses of the heat storage system are conducted based on three subprocesses: charging, storing, and discharging.

6.4

Charging

Storing

Heat Storage Systems For Heating Applications

Discharging

b

c

a

d

Ql Time

FIGURE 6.1 Three processes in a general heat storage system for heating capacity: charging (left), storing (middle), and discharging (right). The heat leakage from the system Ql is illustrated for the storing process but can occur in all three processes. Modified from Dincer (2002).

6.4.1.1

Charging Process

During the charging period, the total heat input (charge), Qc, provided to the heat storage system can be expressed as Qc ¼ mc cp ðTc, i  Tc, o Þ

(6.1)

where cp, Tc,i, and Tc,o denote specific heat capacity, charging inlet temperature, and outlet temperature of the HTF, respectively. Also, mc represents the total mass accumulated and/or transported over the charging period, tc (s), and can be written as mc ¼

ð tc

m_ ðtc Þ dtc

(6.2)

0

For a constant mass flow rate, m˙(tc) can be denoted as m˙ (kg/s) and the above equation can be rewritten as _ c mc ¼ mt

(6.3)

The total exergy input to the heat storage system during the charging process is given by    Tc, i ExQ, c ¼ mc cp ðTc, i  Tc, o Þ  T0 ln Tc, o

6.4.1.2

(6.4)

Storing Process

During the storing process, the heat storage system (for heating applications) loses heat due to the energy interaction between the system and its

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surrounding. This interaction is mainly through heat transfer along the system boundaries since the storage temperature (Ts) is higher than the surrounding temperature (T0).This heat loss rate (Q_ l ) can be assessed as Q_ l ¼ UAðTs  T0 Þ

(6.5)

where U and A represent overall heat transfer coefficient and storage surface area, respectively. The overall heat loss over the entire storing period is given by Ql ¼ ms cp ðΔTs Þ

(6.6)

where mc, cp, and ΔTs denote mass, specific heat capacity, and the difference between initial (Ts,i) and final (Ts,f) temperatures of the heat storage medium, respectively. Note that ΔTs is calculated as ΔTs ¼ Ts, i  Ts, f

(6.7)

The total exergy loss from the heat storage system during the storing process is given by   T0 ExQ, l ¼ Ql 1  Ts

6.4.1.3

(6.8)

Discharging Process

During the discharging period, the total heat output (discharge), Qd, provided by the heat storage system is given as Qd ¼ md cp ðTd, o  Td, i Þ

(6.9)

where cp, Td,i, and Td,o denote specific heat capacity, discharging inlet temperature, and discharging outlet temperature of the HTF, respectively. The term md represents the total mass accumulated and/or transported over the discharging period, td (s), and can be written as md ¼

ð td

m_ ðtd Þ dtd

(6.10)

0

For a constant mass flow rate, m˙(td) can be denoted as m˙ (kg/s) and the above equation can be rewritten as _ d md ¼ mt

(6.11)

The total exergy discharged from the heat storage system during the discharging process is expressible as    Td, o ExQ, d ¼ md cp ðTd, o  Td, i Þ  T0 ln Td, i

(6.12)

6.4

6.4.1.4

Heat Storage Systems For Heating Applications

Energy and Exergy Efficiencies

The energy efficiency of the heat storage system can be evaluated over the entire operating cycle (charging, storing, and discharging) as the ratio of the total energy recovered from the system during discharging (Qd) to the total energy charged into the system (Qc). That is, η¼

Qd Qc

(6.13)

Similarly, the exergy efficiency of the system can be evaluated for the overall storage period as ψ¼

ExQ, d ExQ, c

6.4.2

(6.14)

Illustrative Example

Consider two heat storage systems (Fig. 6.2), X and Y, in surroundings at 25 °C. Each storage receives 104,650 kJ of heat from a stream of 500 kg of water, which is cooled from 80 to 30 °C. Therefore, the heat input to the storage during the charging period for each storage unit is Qc ¼ mc cp ΔT ¼ 500kg  4:186kJ=kg°C  ð80  30Þ°C ¼ 104, 650kJ

Charging 80 °C 500 kg T0 = 25 °C

T0 = 25 °C

30 °C

35 °C

75 °C

4500 kg

h = 90% y = 25%

500 kg

h = 90% y = 83%

30 °C

30 °C Discharging after 1 day (case X)

Discharging after 100 days (case Y)

FIGURE 6.2 Storage process for two systems, X and Y, showing charging process (top), which is identical for both systems, and discharging processes for system X (bottom left) and system Y (bottom right).

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6.4.2.1

Energy Efficiency Comparison

6.4.2.1.1 System X After 1 day, the heat of 94,185 kJ is recovered during the discharging period from storage system X by a stream of 4500 kg of water being heated from 30 to 35 °C, that is, Qd ¼ md cp ΔT ¼ 4500kg  4:186kJ=kg°C  ð35  30Þ°C ¼ 94,185kJ

Therefore, the energy efficiency for the heat storage system X is ηX ¼

Qd 94,185kJ ¼ ¼ 0:90 Qc 104, 650kJ

The heat loss to the surroundings during storage is Ql ¼ Qc  Qd ¼ 104, 650kJ  94, 185kJ ¼ 10, 465kJ

6.4.2.1.2 System Y Storage system Y stores the heat for 100 days. For this system, the heat recovered during discharging, the energy efficiency, and the heat rejection to the surrounding can be evaluated in a similar way as for system X. After the storing period, an energy quantity of 94,185 kJ is recovered during the discharging period by heating a stream of 500 kg of water from 30 to 75 °C, determined as follows: Qd ¼ md cp ΔT ¼ 500kg  4:186kJ=kg°C  ð75  30Þ°C ¼ 94, 185kJ

The energy efficiency for system Y can be found following the method for system X: ηY ¼

Qd 94, 185kJ ¼ ¼ 0:90 Qc 104,650kJ

The heat loss to the surroundings during storage can be determined as follows: Ql ¼ Qc  Qd ¼ 104, 650kJ  94, 185kJ ¼ 10, 465kJ

Note that the ability of storing sensible heat in a given tank (or container) is dependent on the value for the material of ρcp. Both storage systems have the same efficiency based on energy or the FLT. But system Y, which stores heat for 100 days rather than 1 day and returns the heat at the much more useful temperature of 75 °C rather than 35 °C, exhibits considerably better performance. 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 heat storage system is to be assessed and a

6.4

Heat Storage Systems For Heating Applications

rational basis for the optimization of its economic value established. This can be achieved via the exergy efficiency, which is a better measure of the thermodynamic effectiveness of the heat storage system. An efficiency defined simply as the percentage of the total energy stored in a system that can be recovered ignores the quality of the recovered energy and so cannot provide a measure of ideal performance as noted earlier. An efficiency based on exergy provides a better measure.

6.4.2.2

Exergy Efficiency Comparison

Consider the aforementioned example of heat storage systems X and Y. For the corresponding cases, an exergy analysis is conducted below. The exergy efficiencies can be straightforwardly obtained using the equations in Table 6.2. The exergy change during the charging period can be obtained as    T1 ExQ, c ¼ mc cp ðT1  T2 Þ  T0 ln T2

or    353 ExQ, c ¼ 500kg  4:186kJ=kgK  ð353  303Þ  298  ln K ¼ 9386:88kJ 303

6.4.2.2.1 System X The exergy change during the discharging period for system X can be calculated as follows:    T1 ExQ, d ¼ md cp ðT1  T2 Þ  T0 ln T2

or    308 ExQ, d ¼ 4500kg  4:186kJ=kgK  ð308  303Þ  298  ln K ¼ 2310:18kJ 303

Therefore, the exergy efficiency for storage X is ψX ¼

ExQ, d 2310:18kJ ¼ ¼ 0:25 ExQ, c 9316:88kJ

6.4.2.2.2 System Y As pointed out earlier, heat is recovered from the storage after 100 days by a stream of 500 kg of water entering at 30 °C and leaving at 75 °C. The exergy change of storage system Y can be obtained as follows:    T1 ExQ, d ¼ md cp ðT1  T2 Þ  T0 ln T2

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or    348 ExQ, d ¼ 500kg  4:186kJ=kgK  ð348  303Þ  298  ln K ¼ 7819:52kJ 303

The exergy efficiency for storage Y can then be determined: ψY ¼

ExQ, d 7819:52kJ ¼ ¼ 0:83 ExQ, c 9316:88kJ

The performance of heat storage systems X and Y is compared in Table 6.8. Both storage systems exhibit the same energy efficiencies (90%) despite having markedly different storage periods. This observation indicates that the FLT is not sufficient to distinguish the performance of these two heat storage systems. This requirement suggests exergy methods are needed. The distinction between the two storage systems (X and Y) is easily and clearly made using exergy analysis. The exergy efficiencies differ at 25% for system X and 83% for system Y. This advantage of heat storage system Y over system X is due to its higher heat recovery temperature. This example illustrates in a practical manner the concepts discussed in this chapter and highlights the importance of exergy, rather than energy, methods in evaluating the performance of heat storage systems. The results are consistent with those obtained previously by Rosen et al. (1988) and Rosen (1992) for energy storage systems.

Table 6.8 Performance Comparison of Heat Storage Systems X and Y

General parameters Storing period (days) Charging-fluid inlet temperature (°C) Charging-fluid outlet temperature (°C) Discharging-fluid inlet temperature (°C) Discharging-fluid outlet temperature (°C) Energy parameters Energy input (kJ) Energy recovered (kJ) Energy loss (kJ) Energy efficiency (%) Exergy parameters Exergy input (kJ) Exergy recovered (kJ) Exergy loss (kJ) Exergy efficiency (%)

System X

System Y

1 80 30 30 35

100 80 30 30 75

104,650 94,185 10,465 90

104,650 94,185 10,465 90

9387 2310 7077 25

9387 7820 1567 83

6.4

Heat Storage Systems For Heating Applications

More broadly, important advantages of exergy analysis can be listed as follows: • It provides more proper accounting of the loss of availability of heat in storage systems using the conservation of mass and energy principles together with the SLT for the goals of design and analysis. • It gives more meaningful and useful information than energy analysis regarding the efficiency, losses, and performance for heat storage systems. • It more correctly reflects the thermodynamic and economic values of the operation of heat storage systems. • It is a useful technique for revealing by how much it is possible to design more efficient heat storage systems by reducing the inefficiencies.

6.4.3 Case Study of a Macroscale Application: Borehole Storage at UOIT (Canada) The University of Ontario Institute of Technology (UOIT) in Oshawa, Ontario, has a unique borehole heat storage system with HTF of a 15% glycol/water mix, circulating through over 150 km of 400 polypropylene piping, which was pressure tested to 300 psi. The system has two mechanical rooms, one of them is for a boiler system and geothermal heat pumps and the other for chillers. The system injects heat from the geothermal loop into the ground during summer and uses this heat in the winter to provide low-temperature hydronic heating at 52 °C. A challenge with the borehole heat storage system was its high capital cost, which was mainly due to the use of high-efficiency HVAC equipment and the construction of the geothermal field. This was offset by significant operating cost savings each year. The system attained annual energy savings of 40% in the heating and 16% in the cooling modes. The payback period was 3-5 years for the high-efficiency HVAC equipment and 7.5 years for the well field.

6.4.4

Thermodynamic Analysis of ATES

Underground aquifers are sometimes used for heat storage systems (Jenne, 1992). Table 6.9 lists examples of buildings using ATES-based systems for Table 6.9 Selected ATES Systems for Heating and Cooling Applications Building

Location

Initial Operating Date

Capacity (kW)

IBM office Nike office Maira hospital IKEA store Westway housing project

Zoetermeer (NL) Hilversum (NL) Overpelt (BE) Duiven (NL) London (the United Kingdom)

1992 1999 2005 1999 2006

700 2000 1500 750 250

Source: Hendricks et al. (2008).

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Cold demand

Aquifer

Heat demand

Aquifer

FIGURE 6.3 ATES working principle for both cooling (left) and heating (right) modes.

providing a range of cooling/heating loads. The storage medium in many ATES systems remains in a single phase during the storing cycle, so that temperature changes are exhibited in the store as heat is added or removed. The working principle of an ATES system is presented in Fig. 6.3 for both heating and cooling modes. ATES systems have been demonstrated to be feasible commercially and environmentally, and some of these systems have been assessed and compared thermodynamically while operating at the same or varied environmental conditions. An early study of ATES, including both energy and exergy analyses and efficiency evaluations, was made by Rosen (1999). His approach is followed here to assess energetically and exegetically the performance of an ATES in different operating modes (heating/cooling). The application of exergy analysis to ATES systems is described in this section. For an elementary ATES model, expressions are presented for the injected and recovered quantities of energy and exergy and for efficiencies. The impact of introducing a threshold temperature below which residual heat remaining in the aquifer water is not considered worth recovering is examined. ATES exergy efficiencies are demonstrated to be more useful and meaningful than energy efficiencies because the former account for the temperatures associated with thermal energy transfers and consequently assess how nearly ATES systems approach ideal thermodynamic performance. ATES energy efficiencies do not provide a measure of approach to ideal performance and, in fact, are often misleadingly high because some of the thermal energy can be recovered at temperatures too low for useful purposes.

6.4

6.4.4.1

Heat Storage Systems For Heating Applications

ATES Model

The charging of the ATES occurs over a finite time period tc and after a holding interval discharging occurs over a period td. The working fluid is water, having a constant specific heat cp, and assumed incompressible. The temperature of the aquifer and its surroundings prior to heat injection is T0, the referenceenvironment temperature. Only heat stored at temperatures above T0 is considered, and pump work is neglected. During charging, heated water at a constant temperature Tc is injected at a constant mass flow rate m˙c into the ATES. After a storing period, discharging occurs, during which water is extracted from the ATES at a constant mass flow rate m˙d.The fluid discharge temperature is taken to be a function of time, that is, Td ¼ Td(t). The discharge temperature after an infinite time is taken to be the temperature of the reference environment, that is, Td(1) ¼ T0, and the initial discharge temperature is taken to be between the charging and reference-environment temperatures, that is, T0  Td(0)  Tc. Many discharge temperature-time profiles are possible. Here, the discharge temperature is taken to decrease linearly with time from an initial value Td(0) to a final value T0. The final temperature is reached at a time tf and remains fixed at T0 for all subsequent times, that is, Td ðt Þ ¼

8 < :

Td ð0Þ  T0

9 ðTd ð0Þ  T0 Þt = ð0  t  tf Þ tf ; ðtf  t  t1 Þ

(6.15)

Temperature, T

The simple linear discharge temperature-time profile is sufficiently realistic yet simple. The temperature-time profiles considered in the present model for the fluid flows during the charging and discharging periods are summarized in Fig. 6.4.

Charging

Discharging

Tc

Td(0)

To tc

0 Time, t

tf

0 Time, t

FIGURE 6.4 Temperature-time profiles assumed for the charging and discharging periods in the ATES model considered. Modified from Dincer and Rosen (2013).

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The two main types of thermodynamic losses that occur in ATES systems are accounted for in the model: • Energy losses. Energy injected into an ATES that is not recovered is considered lost. Thus, energy losses include energy remaining in the ATES and energy injected into the ATES that is convected in a water flow or is transferred by conduction far enough from the discharge point that it is unrecoverable. • Mixing losses. As heated water is pumped into an ATES, it mixes with the water already present (which is usually cooler), resulting in the recovered water being at a lower temperature than the injected water. In the present model, this loss results in the discharge temperature Td being at all times less than or equal to the charging temperature Tc, but not below the referenceenvironment temperature T0 (i.e., T0  Td(t)  Tc for 0  t  1).

6.4.4.2

Energy and Exergy Analyses

The energy and exergy injected into the ATES during charging and recovered during discharging are evaluated. The energy flow associated with a flow of liquid at a constant mass flow rate m˙, for an arbitrary period of time with T a function of t, is E¼

ðt

E_ðt Þdt

(6.16)

0

where the integration is performed over the time period, and the energy flow rate at time t is _ p ðT ðt Þ  T0 Þ E_ðt Þ ¼ mc

(6.17)

Here, c denotes the specific heat of the liquid. Combining Eqs. (6.16) and (6.17) for constant m˙, cp, and T0, _ p E ¼ mc

ðt

ðT ðt Þ  T0 Þdt

(6.18)

0

The corresponding exergy flow is Ex ¼

ðt

_ ðt Þdt Ex

(6.19)

0

where the exergy flow rate at time t is   T ðt Þ _ ðt Þ ¼ mc _ p ðT ðt Þ  T0 Þ  T0 ln Ex T0

Combining Eqs. (6.19) and (6.20), and utilizing Eq. (6.18),

(6.20)

6.4

_ p Ex ¼ mc

Heat Storage Systems For Heating Applications

  ðt  ðt  T ðt Þ T ðt Þ _ p T0 ln dt ¼ E  mc dt ðT ðt Þ  T0 Þ  T0 ln T0 T0 0 0

(6.21)

6.4.4.2.1 Charging and Discharging The energy input to the ATES during charging, for a constant water injection rate m˙c and over a time period beginning at zero and ending at tc, is expressed by Eq. (6.18) with T(t) ¼ Tc. That is, Ec ¼ m_ c cp

ð tc

ðTc ðt Þ  T0 Þdt ¼ m_ c cp tc

0

Tc T0

(6.22)

The corresponding exergy input is expressed by Eq. (6.21), with the same conditions as for Ec. Thus, after integration,   Tc Tc _ ¼ Ec  m_ c cp tc T0 ln Exc ¼ mc cp tc ðTc  T0 Þ  T0 ln T0 T0

(6.23)

The energy recovered from the ATES during discharging, for a constant water recovery rate m˙d and for a time period starting at zero and ending at td, is expressed by Eq. (6.18) with T(t) as in Eq. (6.15). Thus, Ed ¼ m_ d cp

ð td

ðTd ðt Þ  T0 Þdt ¼ m_ d cp ½Td ð0Þ  T0 

0

θð2tf  θÞ 2tf

(6.24)

where  θ¼

td ð0  td  tf Þ tf ðtf  td  t1 Þ

(6.25)

The corresponding exergy recovered is expressed by Eq. (6.21), with the same conditions as for Ed. Thus, Exd ¼ m_ d cp

  ð td  ð td  Td ðt Þ Td ðt Þ dt ¼ Ed  m_ d cp T0 ln dt ðTd ðt Þ  T0 Þ  T0 ln T0 T0 0 0

(6.26)

Here, ð td 0

    ð td Td ðt Þ aθ + b b dt ¼ ln ln ðat + bÞdt ¼ ln ðaθ + bÞ  θ  ln ðbÞ T0 a a 0

(6.27)

where a¼

T0  Td ð0Þ T0 Tf

(6.28)

b¼

Td ð0Þ T0

(6.29)

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When td  tf, the expression for the integral in Eq. (6.27) reduces to 

ð td ln 0

   Td ðt Þ Td ð0Þ Td ð0Þ dt ¼ tf ln 1 T0 Td ð0Þ  T0 T0

(6.30)

6.4.4.2.2 Balances and Efficiencies An ATES energy balance taken over a complete charging-discharging cycle states that the energy injected is either recovered or lost. A corresponding exergy balance states that the exergy injected is either recovered or lost, where lost exergy is associated with both waste exergy emissions and internal exergy consumptions due to irreversibilities. If f is defined as the fraction of injected energy Ec that can be recovered if the length of the discharge period approaches infinity (i.e., water is extracted until all recoverable energy has been recovered), then Ed ðtd ! 1Þ ¼ fEc

(6.31)

It follows from the energy balance that (1  f) Ec is the energy irretrievably lost from the ATES. Clearly, f varies between zero for a thermodynamically worthless ATES to unity for an ATES having no energy losses during an infinite discharge period. But mixing in the ATES can still cause exergy losses even if f ¼ 1. Since Ec is given by Eq. (6.22) and Ed(td ! 1) by Eq. (6.24) with θ ¼ tf, Eq. (6.31) may be rewritten as m_ d cp ðTd ð0Þ  T0 Þtf ¼ f m_ c cp ðTc  T0 Þtc 2

(6.32)

or f¼

tf m_ d ðTd ð0Þ  T0 Þ 2tc m_ c ðTc  T0 Þ

(6.33)

For either energy or exergy, the efficiency is the fraction, taken over a complete cycle, of the quantity input during charging that is recovered during discharging, while loss is the difference between input and recovered amounts of the quantity. Hence, the energy loss as a function of the discharge time period is given by [Ec  Ed(td)], while the corresponding exergy loss is given by [Exc Exd(td)]. Energy losses do not reflect the temperature degradation associated with mixing, while exergy losses do. The energy efficiency η for an ATES, as a function of the discharge time period, is given by ηðtd Þ ¼

Ed ðtd Þ m_ d ðTd ð0Þ  T0 Þ θð2tf  θÞ ¼ Ec m_ c ðTc  T0 Þ 2tf tc

(6.34)

6.4

Heat Storage Systems For Heating Applications

and the corresponding exergy efficiency ψ by ψ ðtd Þ ¼

Exd ðtd Þ Exc

(6.35)

The energy efficiency in Eq. (6.34) simplifies when the discharge period td exceeds tf, that is, η(td  tf) ¼ f. In practice, it is not economically feasible to continue the discharge period until as much recoverable heat as possible is recovered. As the discharge period increases, water is recovered from an ATES at ever-decreasing temperatures (ultimately approaching the reference-environment temperature To), and the energy in the recovered water is of decreasing usefulness. Exergy analysis reflects this phenomenon, as the magnitude of the recovered exergy decreases as the recovery temperature decreases. To determine the appropriate discharge period, a threshold temperature Tt is often introduced, below which the residual energy in the aquifer water is not considered worth recovering from an ATES. For the linear temperature-time relation used here (see Eq. 6.15), it is clear that no thermal energy could be recovered over a cycle if the threshold temperature exceeds the initial discharge temperature, while the appropriate discharge period can be evaluated using Eq. (6.15) with Tt replacing Td(t) for the case where To  Tt  Td(0). Thus, 8 9 < Td ð0Þ  Tt = ðT0  Tt  Td ð0ÞÞ td ¼ Td ð0Þ  T0 : ; 0 ðTd ð0Þ  Tt Þ

(6.36)

In practice, a threshold temperature places an upper limit on the allowable discharge time period. Utilizing a threshold temperature usually has the effect of decreasing the difference between the corresponding energy and exergy efficiencies. Nonetheless, ATES performance measures based on exergy are more useful and meaningful than those based on energy. Exergy efficiencies account for the temperatures associated with heat transfers to and from an ATES, as well as the quantities of heat transferred, and consequently provide a measure of how nearly ATES systems approach ideal performance. Energy efficiencies account only for quantities of energy transferred and can often be misleadingly high, for example, in cases where heat is recovered at temperatures too low to be useful. The use of an appropriate threshold recovery temperature can partially avoid the most misleading characteristics of ATES energy efficiencies. As a case study, which is based on Eqs. (6.15)–(6.36), energy and exergy analyses of an ATES are performed using experimental data from the first of four short-term ATES test cycles for the Upper Cambrian Franconia-IrontonGalesville confined aquifer. The test cycles were performed at the University

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252

Charging

Discharging

20 18 16 150

20 18 16 150

100

100

50

50 0

0 0

2

8

14

Time, t (days)

16

0

2

4

6

8

Time, t (days)

FIGURE 6.5 Observed values for the temperature and volumetric flow rate of water, as a function of time during the charging and discharging periods, for the experimental test cycles used in the ATES case study. Modified from Dincer and Rosen (2013).

of Minnesota’s St. Paul campus from November 1982 to December 1983 (Hoyer et al., 1985). During the test, water was pumped from the source well, heated in a heat exchanger, and returned to the aquifer through the storage well. After storage, energy was recovered by pumping the stored water through a heat exchanger and returning it to the supply well. The storage and supply wells are located 255 m apart. For the test cycle considered here, the water temperature and volumetric flow rate vary with time during the injection and recovery processes as shown in Fig. 6.5. The storage period duration (13 days) is also shown. Charging occurred during 5.24 days over a 17-day period. The water temperature and volumetric flow rate were approximately constant during charging and had mean values of 89.4 °C and 18.4 L/s, respectively. Discharging also occurred over 5.24 days, approximately with a constant volumetric flow rate of water and linearly decreasing temperature with time. The mean volumetric flow rate during discharging was 18.1 L/s, and the initial discharge temperature was 77 °C, while the temperature after 5.24 days was 38 °C. The ambient temperature was reported to be 11 °C.

6.4.4.2.3 Simplifications, Analysis, and Results In subsequent calculations, mean values for volumetric flow rates and charging temperature are used. Also, the specific heat and density of water are both taken to be fixed, at 4.2 kJ/kg K and 1000 kg/m3, respectively. Since the volumetric flow rate (in L/s) is equal to the mass flow rate (in kg/s) when the density is 1000 kg/m3, m_ c ¼ 18:4kg=s and m_ d ¼ 18:1kg=s. Also, the reference-environment temperature is fixed at the ambient temperature, that is, T0 ¼ 11 °C ¼ 284 K. During charging, it can be shown using Eqs. (6.22) and (6.23), with tc ¼ 5.24 day ¼ 453,000 s and Tc ¼ 89.4 °C ¼ 362.4 K, that Ec ¼ 18:4kg=s  4:2kJ=kgK  453, 000s  ð362:4  284ÞK ¼ 2:74  109 kJ

6.4

Heat Storage Systems For Heating Applications

and Exc ¼ 2:74  109 kJ  18:4kg=s  4:2kJ=kgK  453, 000s  284K  ln

  362:4K 284K

¼ 0:32  109 kJ

During discharging, the value of the time tf is evaluated using the linear temperature-time relation of the present model and the observations that Td(t ¼ 5.24 day) ¼ 38 °C ¼ 311 K and Td(0) ¼ 77 °C ¼ 350 K. Then, using Eq. (6.15) with t ¼ 5.24 day, 38 °C ¼ 77°C  ð77 °C  11 °CÞ 

5:24day tf

which can be solved to show that tf ¼ 8.87 day. Thus, with the present linear model, the discharge water temperature would reach T0 if the discharge period was lengthened to almost 9 days. In reality, the rate of temperature decline would likely decrease, and the discharge temperature would asymptotically approach T0. The value of the fraction f can be evaluated with Eq. (6.33) as f¼

8:87day  18:1kg=s  ð77  11Þ°C ¼ 0:701 2  5:24day  18:4kg=s  ð89:4  11Þ°C

Thus, the maximum energy efficiency achievable is approximately 70%. With these values and Eqs. (6.28) and (6.29), it can be shown that a¼

ð11  77Þ°C ¼ 0:0262day 1 284K  8:87day

b¼

350K ¼ 1:232 284K

Consequently, expressions dependent on discharge time period td can be written and plotted (see Fig. 6.6) for Ed, Exd, η, and ψ using Eqs. (6.24)–(6.26), (6.33), and (6.34) and for the energy loss (Ec  Ed) and exergy loss (Exc  Exd).

6.4.4.2.4 Discussion Both energy and exergy efficiencies in Fig. 6.6 increase from zero to maximum values as td increases. Further, the difference between the two efficiencies increases with increasing td. This latter point demonstrates that the exergy efficiency gives less weight than the energy efficiency to the energy recovered at higher td values since it is recovered at temperatures nearer to the referenceenvironment temperature. Several other points in Fig. 6.6 are worth noting. First, for the conditions specified, all parameters level off as td approaches tf and remain constant for td  tf. Second, as td increases towards tf, the energy recovered increases from zero to a

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Energy and exergy quantities (109 kJ)

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Energy loss

1 Energy recovery, Ed Exergy loss Exergy recovery, Ed 0 100

Efficiencies (%)

254

Energy efficiency, h 50

Exergy efficiency, y 0 0

5 Discharge time period, td (days)

10

FIGURE 6.6 Variation of several calculated energy and exergy quantities and efficiencies, as a function of discharge time period, for the ATES case study. Modified from Dincer and Rosen (2013).

maximum value, while the energy loss decreases from a maximum of all the input energy to a minimum (but nonzero) value. The exergy recovery and exergy loss functions behave similarly qualitatively. but exhibit much lower magnitudes. The difference between energy and exergy efficiencies is due to temperature differences between the charging-fluid and discharging-fluid flows. As the discharging time increases, the deviation between these two efficiencies increases (Fig. 6.6) because the temperature of recovered heat decreases (Fig. 6.5). In this case, the energy efficiency reaches approximately 70% and the exergy efficiency 40% by the completion of the discharge period, even though the efficiencies are both 0% when discharging commences. To further illustrate the importance of temperature, a hypothetical modification of the present case study is considered. In the modified case, all details are as in the original case except that the temperature of the injection flow during the charging period is increased from 89.4 to 200 °C (473 K), while the duration of the charging period is decreased from its initial value of 5.24 days

6.4

Heat Storage Systems For Heating Applications

(453,000 s) so that the energy injected does not change. By equating the energy injected during charging for the original and modified cases, the modified charging-period duration tc0 can be evaluated as a function of the new injection flow temperature Tc0 as follows: tc0 ¼ tc

Tc  T0 ð89:4  11Þ°C ¼ 453,000s  ¼ 188, 000s Tc0  T0 ð200  11Þ°C

The modified exergy input during charging can then be evaluated as Ex0c ¼ 2:74  109 kJ  18:4kg=s  4:2kJ=kgK  188, 000s  284K  ln

  473K 284K

¼ 0:64  109 kJ

This value is double the exergy input during charging for the original case, so, since the discharging process remains unchanged in the modified case, the exergy efficiency (for any discharging time period) is half that for the original case. The altered value of exergy efficiency is entirely attributable to the new injection temperature and occurs despite the fact that the energy efficiency remains unchanged. If a threshold temperature is introduced and arbitrarily set at 38 °C (the actual temperature at the end of the experimental discharge period of 5.24 day), then the data in Fig. 6.6 for td ¼ 5.24 day apply and one can see that • the exergy recovered (0.127  109 kJ) is almost all (91%) of the exergy recoverable in infinite time (0.139  109 kJ), while the energy recovered (1.60  109 kJ) is not as great a portion (83%) of the ultimate energy recoverable (1.92  109 kJ); • the exergy loss (0.19  109 kJ) exceeds the exergy loss in infinite time (0.18  109 kJ) slightly (by 5.5%), while the energy loss (1.14  109 kJ) exceeds the energy loss in infinite time (0.82  109 kJ) substantially (by 39%); and • the exergy efficiency (40%) has almost attained the exergy efficiency attainable in infinite time (43.5%), while the energy efficiency (58%) is still substantially below the ultimate energy efficiency attainable (70%). To gain confidence in the model and the results, some of the quantities calculated using the linear model can be compared with the same quantities as reported in the experimental paper (Hoyer et al., 1985): • The previously calculated value for the energy injection during charging of 2.74  109 kJ is 1.1% less than the reported value of 2.77  109 kJ. • The energy recovered at the end of the experimental discharge period of td ¼ 5.24 days can be evaluated with Eq. (6.24) as

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Ed ð5:24day Þ ¼ 18:1kg=s  4:2kJ=kg°C  ð77  11Þ°C 5:24  ð2  8:87  5:24Þ  86,400s=day 2  8:87 9 ¼ 1:60  10 kJ

which is 1.8% less than the reported value of 1.63  109 kJ. • The energy efficiency at td ¼ 5.24 day can be evaluated with Eq. (6.25) as ηð5:24day Þ ¼

1:60  109 kJ ¼ 0:584 2:74  109 kJ

which is 1.0% less than the reported value of 0.59 (referred to as the “energy recovery factor”).

6.5 HEAT STORAGE SYSTEMS FOR COOLING APPLICATIONS In heat storage systems for cooling applications, or cold TES (CTES), “cold” is stored in a thermal storage mass. In most conventional cooling systems, there are two major components (Dincer and Rosen, 2011): • Chiller: to cool a fluid such as water • Distribution system: to transport the cold fluid from the chiller to where cooling is needed (e.g., to cool air in a building) In conventional building systems, the chiller operates only when the building occupants require cold air. In a cooling system incorporating heat storage systems, the chiller also operates at times other than when the cooling is needed. During the past two decades, heat storage technologies, especially cold storage, have matured and are now accepted as a proved energy technologies. However, the predicted payback period of a potential cold storage installation is often not sufficiently attractive to give it priority over other energy-efficient technologies. This determination often is made because full advantage is not made of the many potential benefits of cold storage or because the cold storage sizing is not optimized. Some recommendations for optimizing the payback period of CTES systems are discussed below. For new facilities, cold storage should be integrated carefully into the overall building and its energy systems so as to exploit the potential benefits of CTES, including • reduced pipe and pump sizes for chilled-water distribution, • reduced duct and fan sizes for low-temperature air distribution, and • correspondingly reduced operating costs.

6.5

Heat Storage Systems For Cooling Applications

Smaller chiller and electrical systems lead to initial cost advantages. The sizing of the cold storage system should be optimized, as opposed to the typical process of considering full storage and one or two levels of partial-storage versus a conventional system. A practical method to assist in determining the optimum system size should be developed. Also, the value should be accounted for of the gain in usable building space due to less space being required for mechanical system components when CTES is used. For existing facilities, the potential advantages of CTES that should be evaluated include • modifying the existing chillers to make ice versus the purchase of a new machine, • using spare chiller capacity by adding a CTES system, • using cold storage to increase cooling capacity in situations where chiller and electrical service capacity are fully utilized, • sizing the cold storage system optimally as opposed to taking the best of only a few options, and • using available low-temperature air and water to advantage through “free cooling” where practical.

6.5.1

CTES Storage Media Selection and Characteristics

The storage medium largely determines the storage volume and the size and configuration of the HVAC system and components. The main options include chilled water, ice, and eutectic salts. Ice systems offer the densest storage capacity but have the most complex charge and discharge equipment. Water systems offer the lowest storage density and are the least complex. Eutectic salts have intermediate characteristics. Details on each storage medium are now provided: Chilled water: Chilled-water systems require the largest storage tanks but can easily interface with existing chiller systems. Chilled-water CTESs use the sensible heat capacity of water to store cooling capacity. They operate at temperature ranges (3.3-5.5 °C) compatible with standard chiller systems and are most economic for systems greater than 2000 ton/h in capacity. Ice: Ice systems use smaller tanks and offer the potential for the use of lowtemperature air systems but require more complex chiller systems. Ice CTES systems use the latent heat of fusion of water (335 kJ/kg) to store cooling capacity. To store energy at the temperature of ice requires refrigeration equipment that provides charging fluids at temperatures below the normal operating range of conventional air conditioning equipment. Special ice-making equipment or standard chillers modified for low-temperature service are used. The low chilled-water supply temperatures available from ice storage allow the use of cool-air distribution, the benefits of which include the ability to use smaller

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fans and ducts and the introduction of less humid air into occupied spaces. With ice as the storage medium, there are several technologies available for charging and discharging the storage. Ice-harvesting systems feature an evaporator surface on which ice is formed and periodically released into a storage tank that is partially filled with water. External melt ice-on-coil systems use submerged pipes through which a refrigerant or secondary coolant is circulated. Ice accumulates on the outside of the pipes. Storage is discharged by circulating the warm return water over the pipes, melting the ice from the outside. Internal melt ice-on-coil systems also feature submerged pipes on which ice is formed. Storage is discharged by circulating warm coolant through the pipes, melting the ice from the inside. The cold coolant is then pumped through the building cooling system or used to cool a secondary coolant that circulates through the building’s cooling system. Encapsulated ice systems use water inside submerged plastic containers that freeze and thaw as cold or warm coolant is circulated through the storage tank holding the containers. Ice slurry systems store water or water/glycol solutions in a slurry state (a partially frozen mixture of liquid and ice crystals that looks like slush). To meet a cooling demand, the slurry may be pumped directly to the load or to a heat exchanger cooling a secondary fluid that circulates through the building chilled-water system. Internal melt ice-on-coil systems are the most commonly used type of ice storage technology in commercial applications. External melt and ice-harvesting systems are more common in industrial applications, although they can also be applied in commercial buildings and district cooling systems. Encapsulated ice systems are also suitable for many commercial applications. Ice slurry systems have not been widely used in commercial applications. Eutectic salts: Eutectic salts can use existing chillers but usually operate at warmer temperatures than ice or chilled-water systems. Eutectic salts use a combination of inorganic salts, water, and other elements to create a mixture that freezes at a desired temperature. The material is encapsulated in plastic containers that are stacked in a storage tank through which water is circulated. The most commonly used mixture for thermal storage freezes at 8.3 °C, which allows the use of standard chilling equipment to charge the storage, but leads to higher discharge temperatures. These temperatures, in turn, limit the operating strategies that may be applied. For example, eutectic salts may only be used in full storage operation if dehumidification requirements are low. In summary, a CTES system can benefit users in three ways: • Lower electricity rates: With CTES, chillers can operate at night to meet the daytime cooling needs, taking advantage of lower off-peak electricity consumption rates. • Lower demand charges: Many commercial customers pay a monthly electrical demand charge based on the largest amount of electricity used

6.5

Heat Storage Systems For Cooling Applications

during any 30 min period of the month. CTES reduces peak demands, by shifting those demands to off-peak periods. Furthermore, some utilities provide a rebate for shifting electrical demand to nighttime or other off-peak periods. • Lower air conditioning system and compressor costs: Without CTES, large compressors capable of meeting peak cooling demands are needed, whereas smaller and less expensive units are sufficient when CTES is used. Also, since water from a CTES may be colder than conventional chilled water, smaller pipes, pumps, and air handlers may be integrated into the building design to reduce costs further. Another example of a heat storage system for cooling applications is the use of thermal storage to take advantage of off-peak electricity tariffs. Chiller units can be run at night when the cost of electricity is relatively low. These units are used to cool down a thermal storage, which then provides cooling for air conditioning throughout the day. Not only electricity costs are reduced, but also the efficiency of the chiller is increased because of the lower nighttime ambient temperatures, and the peak electricity demand is reduced for electrical supply utilities.

6.5.2 Thermodynamic Analysis of Heat Storage Systems for Cooling Applications Exergy analysis in some ways complements energy analysis for heat storage system performance evaluation. For assessing cold TES, it is more evident that exergy methods are superior, whether they are applied to the overall heat storage process or its charging, storing, and discharging subprocesses. This may or may not be clear from the energy and exergy balances as well as efficiencies presented in Table 6.2 and in the literature (Rosen, 1992; Dincer et al., 1997). The exergy efficiency incorporates the notion of increasing thermodynamic unavailability, as reflected by increasing entropy, in a system or process. This key difference between exergy and energy efficiencies occurs because the former accounts for the losses due to irreversibilities, in addition to losses associated with waste emissions. Many have recognized that this difference is particularly important when assessing CTES and applied exergy analysis to it (Krane, 1987; Rosen, 1992; Bejan, 1994; Dincer et al., 1997; Domanski and Fellah, 1998). The three main processes in a heat storage for cooling capacity are illustrated in Fig. 6.7. A thermodynamic analysis is described for the charging, storing, and discharging processes of the heat storage system and the overall cycle.

6.5.2.1

Charging Process

During the charging period, the total “cold” (charge), Qc, provided to the heat storage system can be expressed as Qc ¼ mc cp ðTc, o  Tc, i Þ

(6.37)

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Charging

Storing

Discharging

b

c

a

d Ql Time

FIGURE 6.7 Three processes in a general heat storage system for cooling capacity: 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. Modified from Dincer and Rosen (2011).

where cp, Tc,i, and Tc,o denote specific heat capacity, and charging inlet temperature, and charging outlet temperature of the HTF, respectively. The term mc represents the total mass accumulated and/or transported over the charging period, tc (s), and can be written as mc ¼

ð tc

m_ ðtc Þ dtc

(6.38)

0

For a constant mass flow rate, m˙(tc) can be denoted as m˙ (kg/s) and the above equation can be rewritten as _ c mc ¼ mt

(6.39)

Total exergy charged into the heat storage system during charging is given by    Tc, o ExQ, c ¼ mc cp ðTc, o  Tc, i Þ  T0 ln Tc, i

6.5.2.2

(6.40)

Storing Process

During the storing process, the heat storage system (for cooling applications) gains heat (or loses “cold”) due to energy interactions between the system and its surrounding. This interaction is mainly through heat transfer along the system boundaries since the storage temperature (Ts) is lower than the temperature of the surroundings (T0).This heat gain (or cold loss) rate (Q_ l ) can be assessed as Q_ l ¼ UAðT0  Ts Þ

(6.41)

where U and A represent overall heat transfer coefficient and area, respectively.

6.5

Heat Storage Systems For Cooling Applications

The overall heat loss over the entire storing time is given by Ql ¼ ms cp ðΔTs Þ

(6.42)

where mc, cp, and ΔTs denote mass, specific heat capacity, and the difference between initial (Ts,i) and final (Ts,f) temperatures of the heat storage medium, respectively. The term ΔTs is calculated as ΔTs ¼ Ts, f  Ts, i

(6.43)

The total exergy loss from the heat storage system during the storing process is given by   Ts ExQ, l ¼ Ql 1  T0

6.5.2.3

(6.44)

Discharging Process

During the discharging period, the total cooling output (discharge), Qd, provided by the heat storage system can be written as Qd ¼ md cp ðTd, i  Td, o Þ

(6.45)

where cp, Td,i, and Td,o denote specific heat capacity, the discharging inlet temperature, and discharging outlet temperature of the HTF, respectively. The term md represents the total mass accumulated and/or transported over the discharging period, td (s), and can be written as md ¼

ð td

m_ ðtd Þ dtd

(6.46)

0

For a constant mass flow rate, m˙(td) can be denoted as m˙ (kg/s) and the above equation can be rewritten as _ d md ¼ mt

(6.47)

The total exergy discharged from the heat storage system during the discharging process is given by    Td, i ExQ, d ¼ md cp ðTd, i  Td, o Þ  T0 ln Td, o

6.5.2.4

(6.48)

Energy and Exergy Efficiencies

The energy efficiency of heat storage system can be evaluated over the entire operating cycle (charging, storing, and discharging) as the ratio of the total

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cooling recovered from the system during the discharging process (Qd) to the total cooling charged into the system (Qc). This can be expressed as η¼

Qd Qc

(6.49)

Similarly, the exergy efficiency of the system can be evaluated for the overall storage period as ψ¼

ExQ, d ExQ, c

6.5.3

(6.50)

Illustrative Example

Energy and exergy analyses of four CTESs are performed utilizing Eqs. (6.37)– (6.50). In each case, the CTES has identical initial and final states, so that the CTES operates in a cyclic manner, continuously charging, storing, and discharging. The cold storage cases, differentiated by their main characteristics, are as follows: • • • •

Sensible heat storage, with a fully mixed storage fluid Sensible heat storage, with a linearly stratified storage fluid Latent heat storage, with a fully mixed storage fluid Combined latent and sensible heat storage, with a fully mixed storage fluid

6.5.3.1

Assumptions and Specified Data

The following assumptions are made for each of the cases: • Storage boundaries are nonadiabatic. • Heat gain from the environment during charging and discharging is negligibly small relative to heat gain during the storing period. • The external surface of the storage tank wall is at a temperature 2 °C greater than the mean storage-fluid temperature. • The mass flow rate of the HTF is controlled so as to produce constant inlet and outlet temperatures. • Work interactions, and changes in kinetic and potential energy terms, are negligibly small. Specified data for the four cases are presented in Table 6.10 and relate to the diagram in Fig. 6.10. In Table 6.10, Tb and Td are the charging and discharging outlet temperatures of the HTF, respectively. The subscripts 1, 2, and 3 indicate the temperature of the storage fluid at the beginning of charging, storing, or discharging, respectively. Also l indicates the liquid state and s indicates the solid state for the storage fluid at the phase change temperature.

6.5

Heat Storage Systems For Cooling Applications

Table 6.10 Specified Temperature Data for the Cases in the CTES Example System Temperature (°C)

I

II

III

IV

Tb Td T1 T2 T3

4 11 10.5 5 6

15 11 19/2a 17/7a 18/6a

1 10 0 (l) 0 (s) 0 (l&s)

1 10 8 8 0 (l&s)

a When two values are given, the storage fluid is vertically linearly stratified and the first and second values are the temperatures at the top and bottom of the storage fluid, respectively.

In addition, for all cases, the inlet temperatures are fixed for the charging-fluid flow (Ta) as 10 °C and for the discharging-fluid flow (Tc) as 20 °C. For cases involving latent heat changes (i.e., solidification), F ¼ 10%. The specific heat cp is 4.18 kJ/kg K for both the storage and HTFs. The phase change temperature of the storage fluid is 0 °C. The configuration of the storage tank is cylindrical with an internal diameter of 2 m and internal height of 5 m. Environmental conditions (T0 and P0) are 20 °C and 1 atm.

6.5.3.2

Results and Discussion

The results for the four systems are listed in Table 6.11 and include overall and subprocess efficiencies, input and recovered cold quantities, and energy and exergy losses. The overall and subprocess energy efficiencies are identical for systems I and II and for systems III and IV. In all systems, the energy efficiency values are high. The different and lower exergy efficiencies for all cases indicate that Table 6.11 Energy and Exergy Quantities for the Cases in the CTES Example Energy Quantities Period or Quantity

Exergy Quantities

I

II

III

IV

I

II

III

IV

100 82 100 82

100 82 100 82

100 90 100 90

100 90 100 90

51 78 38 15

98 85 24 20

76 90 41 28

77 85 25 17

5237.5 4713.8 523.8 –

6025.9 5423.3 602.6 –

31 4.6 2.9 23

23 4.6 2.9 16

500 142 36.3 321

575 94.7 48.9 431

Efficiencies (%) Charging (1) Storing (2) Discharging (3) Overall

Input recovered and lost quantities (MJ) Input Recovered Loss (external) Loss (internal)

361.0 296.0 65.7 –

361.1 295.5 65.7 –

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energy analysis does not account for the quality of the “cold” energy, as related to temperature, and considers only the quantity of “cold” energy recovered. The input and recovered quantities in Table 6.11 indicate the quantity of “cold” energy and exergy input to and recovered from the storage. The energy values are much greater than the exergy values because, although the energy quantities involved are large, the energy is transferred at temperatures only slightly below the reference-environment temperature and therefore is of limited usefulness. The cold losses during storage, on an energy basis, are entirely due to cold losses across the storage boundary (i.e., heat infiltration). The exergy-based cold losses during storage are due to both cold losses and internal exergy losses (i.e., exergy consumptions due to irreversibilities within the storage). For the present systems, in which the exterior surface of the storage tank is assumed to be 2 °C warmer than the mean storage-fluid temperature, the exergy losses include both external and internal components. Alternatively, if the heat transfer temperature at the storage tank external surface is at the environment temperature, the external exergy losses would be zero and the total exergy losses would be entirely due to internal consumptions. If heat transfer occurs at the storage-fluid temperature, on the other hand, more of the exergy losses would be due to external losses. In all cases, the total exergy losses, which are the sum of the internal and external exergy losses, remain fixed. The four systems demonstrate that energy and exergy analyses give different insights for CTES systems. Both energy and exergy analyses account for the quantity of energy transferred in storage processes. Exergy analyses take into account the loss in quality of “cold” energy and thus more correctly reflect the actual value of the CTES. In addition, exergy analysis is conceptually more direct when applied to CTES systems because cold is treated as a useful commodity. With energy analysis, flows of heat rather than cold are normally considered. Thus, energy analyses become convoluted and confusing as one must deal with heat flows while accounting for the fact that cold is the useful input and product recovered for CTES systems. Exergy analysis inherently treats any quantity that is out of equilibrium with the environment (be it colder or hotter) as a valuable commodity and thus avoids the intuitive conflict in the expressions associated with CTES energy analysis. The concept that cold is a valuable commodity is both logical and in line with intuition when applied to CTES systems.

6.6

CASE STUDIES

Thousands of heat storage systems have been operating for years in the world, particularly in developed countries; in hospitals, schools, universities, airports,

6.6

government facilities, and private office buildings; and in industrial process cooling applications. Described below are several reported case studies by the IEA (1994) and ARI (1997) that demonstrate how heat storage systems provide energy savings and reduce the environmental impact and that illustrate some clever applications of heat storage system equipment in new buildings to reduce initial costs.

6.6.1 Microscale Application: Advanced Heat Storage System through PCMs Most of the laptop computers are cooled by a fan operating at high temperatures of the central process unit (CPU). If the CPU of the computer becomes too warm, its capacity declines. A sheet of PCM placed under the computer can help in cooling the computer. The “storage” is regenerated on shutdown of the computer since the room temperature is lower than the melting point of the PCM. The IEA (2015) reports the following details for this system when the PCM is applied: • The time before the fan starts is increased from 20 min from start of the computer to 4 h. • Energy consumption is reduced by 25%, allowing longer battery operation of the computer. • The lifetime of the computer (the CPU) is believed to increase due to lower operating temperatures.

6.6.2

Anova Verzekering Co. Building (The Netherlands)

In this application, a groundwater aquifer heat storage system was installed as part of a space-conditioning unit in a renovated office building of Anova Verzekering Co. This building application of heat storage reduced energy use and pollutant emissions (Table 6.12). An electric heat pump supplies hydronic heating and cooling. Accounting for the subsidy of USD 212,000 received from the Dutch government, which is equivalent to 20% of the total initial system costs, the reduced energy costs due to heat storage system were predicted to lead to a payback period within of 6.5 years for the additional investment costs due to the heat storage system. In the case study, primary energy consumption decreases due to the heat storage system by over 40%, even though electricity use increases, and emissions decrease by about 40%.

6.6.3 Kraft General Foods Headquarters Building (Northfield, Illinois) All daytime air conditioning loads are being met at this facility by melting ice that is made and stored overnight. It was anticipated that additional loads from future expansion could be met by operating some of the chillers during the day as well as at night. The building was designed to pump 2.2 °C water to the air

Case Studies

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Table 6.12 Annual Energy Savings and Emission Reductions for the Case Studya Reduction for Heat Storage System Commodity

Conventional System

Heat Storage System

Amount

%

215,800 395,550 322,000

95,500 511,500 179,000

120,300 84,000 143,000

56 21 44

608,000 – –

346,000 – –

262,000 – –

43 40 40

Consumptions Natural gas (m3) Electricity (kWh) Primary energyb Emissions CO2 (kg) NOx SO2 a

For further details: IEA (1994). Primary energy is calculated as the equivalent amount of natural gas based on the assumption that 0.25 m3 gas is used in the generation of 1 kWh electricity. Source: Dincer and Rosen (2001).

b

handling units, which in turn provide 7.2 °C air to the building. These temperatures, which are lower than for nonheat storage-based systems, permit the use of smaller pipes, pumps, air handling units, and ductwork, resulting in lower initial capital costs for the system. Annual electric bills for this building are nearly USD 200,000 lower than for an almost identical building, just 5 km away, which does not use a heat storage system.

6.6.4 Chrysler’s New Technology Development Center (Auburn Hills, Michigan) Since opening in 1990, Chrysler’s technology development center has achieved both equipment and operating cost savings by using a 68,000 ton/h chilledwater heat storage system. The heat storage capacity allowed the center’s chiller plant to be downsized from 17,710 tons, which would have been needed to meet peak cooling loads, to 11,385 tons. Chilled water is stored in the heat storage system at night and supplements chiller operation during peak cooling conditions the following day. Reduced chiller costs more than offset the cost of the heat storage system installation, resulting in initial savings of USD 3.6 million. In addition, the heat storage system shifts over 5000 kW of peak electrical demand to off-peak periods, saving over USD 1 million/year.

6.6.5 San Francisco Marriott Hotel (San Francisco, California) Using a heat storage system in tandem with a real-time pricing strategy from the local utility, this hotel predicted savings of USD 135,000 in annual cooling

6.7

Detailed Illustrative Example: Exergy Analysis Of Uoit BTES

costs. Only 1800 ton/h of ice storage is needed, enough to satisfy the 450 ton cooling load during the daily peak-rate time period, which lasts only 2-3 h under the real-time pricing schedule. Over one-third of the installed cost of the heat storage system will be covered by a rebate from the utility; the remaining amount is expected to be recouped in less than two years of operation.

6.6.6

Texas A&M University (Corpus Christi, Texas)

In August 1997, the Central Power and Light (CPL) Co. presented Texas A&M University Corpus Christi with a USD 431,800 incentive award for the university’s participation in a heat storage system program designed for substantial energy savings. The university system uses the bulk of its electricity during offpeak evening hours, allowing CPL to shift some of its electric load from peak usage times and to share the annual cost savings of up to USD 150,000 with the university. Texas A&M University Corpus Christi invested approximately USD 900,000 to purchase and install the heat storage system equipment, which became operational in January 1995, and predicted recovery of that cost through energy savings within 5 years. The USD 431,800 incentive includes USD 20,400 for installing high-efficiency equipment for cooling and heating the campus. The remaining incentive was provided through CPL’s energy efficiency program, wherein the university was offered USD 200 for every kW of electricity load shift from CPL’s peak daytime load to off-peak evening hours. The university reduced electricity peak demand by approximately 2057 kW, earning a USD 411,400 incentive. CPL worked with the university to install an 11,800 ton/h thermal storage system with a water storage capacity of 5300 m3. The university also installed a 500 ton industrial heat pump for heating, which CPL estimates will save the university approximately USD 90,000 a year in energy costs. The heat pump captures waste heat from the university’s 3000 ton chiller plant and recirculates it into areas of the campus needing heating. The heat storage system tank stores chilled water, which is produced by the conventional air conditioning system during the night and is then used to cool buildings during the day, when the highest demand is placed on the air conditioning system. These case studies and many others demonstrate that heat storage technology offers compelling energy, environmental, diversity, and economic benefits to owners and regions. As heat storage systems is increasingly commercialized, policies should be considered that can help increase the market penetration of TES beneficially.

6.7 DETAILED ILLUSTRATIVE EXAMPLE: EXERGY ANALYSIS OF UOIT BTES The campus considered in this exergy assessment is the UOIT, in Oshawa, Ontario, Canada. The campus includes seven buildings, most of which are

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designed to be heated and cooled using GSHPs in conjunction with a BTES, with the aim of reducing energy-resource use, environmental emissions, and financial costs. The analyses in this case study consider 10 buildings since the whole system was designed for 10 buildings. This university BTES field is the largest and deepest in Canada, and the geothermal well field is one of the largest in North America (Dincer and Rosen, 2011). The system in cooling mode is illustrated in Fig. 6.8. The total cooling load of the campus buildings is about 7000 kW. Test drilling programs were carried out to determine the feasibility of thermal storage in the overburden and bedrock formations at the site. Using the thermal conductivity test results, it was determined that a field of 370 boreholes, each 200 m in depth, would be required to meet the energy demand. The Swedish practice of water-filled borehole heat exchangers (BHEs) was utilized instead of the North American practice of grouted BHEs. A1

A2

21

24

20

22 23

A3

25

A4

A5

27

30

33

26

28 29

31 32

A6

A7

A8

39

36

34 35

37

38

40

A9

42

45

41

43 44

A10

48

46

47 49

5 1

13

14

16

15

1

5

4

3

52

53

54 62

58

Cooling tower 1 59

11 55

56

74

68 66

61

57

7×176 kW 10

7

6

51

60

Heat pumps 2

7×176 kW

7×240 kW 2

12

9

8 Heat pumps 1

Chillers

18

17

Cooling tower 1

63

64

72

67

65

73 Cooling tower 1

69

70

71

Borehole heat exchangers 370 ×200 m

FIGURE 6.8 Flow diagram of the GSHP/BTES system at the UOIT, in the cooling mode. Modified from Kizilkan and Dincer (2012).

6.7

Detailed Illustrative Example: Exergy Analysis Of Uoit BTES

The university’s central plant includes a cooling and heating system for the campus, utilizing the BTES. Chillers are used to provide energy and pumps convey the working fluid between the buildings and the BTES. Additional heat pump modules assist in cooling. Chilled water is supplied from two multistack chillers, each having seven modules, and two sets of heat pumps each with seven modules. Chillers are variable displacement centrifugal units with magnetic bearings that allow for excellent part-load performance. The condenser water enters the borehole field, which retains the heat from the condensers for use in the winter (when the heat pumps reverse) and provides lowtemperature hot water for the campus (Dincer and Rosen, 2011). A glycol solution, encased in polyethylene tubing, circulates through an interconnected, underground network. A 15% glycol solution is the fluid that is circulated through the BHE mounted in the ground. Inlet and outlet temperatures of the solution to and from the ground are 29.4 and 35 °C, respectively. The glycol solution concentration is 30% and is circulated between the system and buildings to transfer heat. Inlet and outlet temperatures of the solution to and from the fan coils are 5.5 and 14.4 °C, respectively. During the winter, the fluid, circulating through tubing extended into the wells, collects heat from the earth and transports it into the buildings. In summer, the system reverses to extracts heat from the buildings and transmits it to the ground.

6.7.1

Analysis Assumptions

In the analysis, the refrigerant R507A is taken to be used in the chiller system and R407C in the heat pump system. Heat transfer to the system and work transfer from the system are taken to be positive, and the reference-environment temperature and pressure are taken to be 24 °C and 98.825 kPa, respectively. It is assumed that processes are at steady-state and steady-flow conditions; the isentropic efficiencies of the chiller and heat pumps are 85% and 80%, respectively; the compressor is adiabatic and has mechanical and electrical efficiencies of 80% and 84%, respectively; the circulating pumps have mechanical and electrical efficiencies of 85% and 88%, respectively; power inputs to the fan coils and potential and kinetic energy effects are negligible; and there are no chemical or nuclear reactions.

6.7.2

Exergy Balances and Efficiencies

Exergy balances, which allow exergy destruction rates and exergy efficiencies to be determined, for the main components of the GSHP/BTES system in Fig. 6.8, that is, the compressor (comp), condenser (cond), expansion valve (ev), evaporator (e), fan coil (fc), borehole heat exchanger (bhe), and circulating pump (p), respectively, are as follows: _ comp, in  Ex _ comp, out + W_ comp _ dest, comp ¼ Ex Ex

(6.51)

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 _ cond, in  Ex _ cond, out + Ex _ hw, out _ hw, in  Ex _ dest, cond ¼ Ex Ex

(6.52)

_ ev, in  Ex _ ev, out _ dest, ev ¼ Ex Ex

(6.53)



 _ e, in  Ex _ e, out + Ex _ cw, out _ cw, in  Ex _ dest, e ¼ Ex Ex

(6.54)

 
 _ cw, in  Ex _ cw, out + Q_ fc 1  T0 _ dest,fc ¼ Ex Ex Tfc

(6.55)

 
 _ hw, in  Ex _ hw, out + Q_ bhe 1  T0 _ dest, bhe ¼ Ex Ex Tbhe

(6.56)

_ p, in  Ex _ p, out + W_ p _ dest, p ¼ Ex Ex

(6.57)

Corresponding exergy efficiencies for the BTES system components are as follows: ψ comp ¼

_ comp, out  Ex _ comp, in Ex W_ comp


ψ cond ¼


ψ ev ¼

 _ hw, in _ hw, out  Ex Ex  _ cond, out _ cond, in  Ex Ex

(6.58)

_ ev, out Ex _ ev, in Ex

(6.60)


 _ cw, in  Ex _ cw, out Ex  ψe ¼
_ e, in _ e, out  Ex Ex


_ cw, in  Ex _ cw, out Ex   ψ fc ¼ T0 _ Qfc 1  Tfc

(6.61)






_ hw, out _ hw, in  Ex Ex   ψ bhe ¼ T0 Q_ bhe 1  Tbhe ψ dest, p ¼

(6.59)

(6.62)



_ p, out  Ex _ p, in Ex W_ p

(6.63)

(6.64)

For the overall system, the exergy efficiency can be written as X _ _ Q _ out Ex Ex output X ψs ¼ ¼X _ in Ex W_ comp + W_ p

(6.65)

6.7

6.7.3

Detailed Illustrative Example: Exergy Analysis Of Uoit BTES

Performance Assessment

The performance of the BTES system is described here, based on an earlier analysis (Kizilkan and Dincer, 2012). In that work, the university data, temperature, mass flow rate, and exergy data for R507A, R407C, and the glycol solution are given, for the state points in Fig. 6.8. The exergy destruction rate, the relative irreversibility (RI), and the exergy efficiency are listed in Table 6.13 for the overall system and each of its components. The exergy efficiency for the BTES system on a product/fuel basis is 62% and the overall exergy destruction rate is 1346 kW. The greatest exergy destruction rates in the BTES system occur in the compressors of the chiller and heat pumps, followed by the condenser, the expansion valve, and the evaporator. The data in Table 6.13 are visually presented in terms of RI and exergy efficiency (ψ) in Figs. 6.9 and 6.10, for the compressors, condensers, expansion valves, evaporators, fan coils, BHEs, cooling towers, and pumps.

Table 6.13 Exergy-Based Performance Parameters for the GSHP/BTES System and Its Components Component Compressor 1 Compressor 2 Compressor 3 Condenser 1 Condenser 2 Condenser 3 Expansion valve 1 Expansion valve 2 Expansion valve 3 Evaporator 1 Evaporator 2 Evaporator 3 Fan coil A1 Fan coil A2 Fan coil A3 Fan coil A4 Fan coil A5 Fan coil A6 Fan coil A7 Fan coil A8 Fan coil A9 Fan coil A10 Overall

E˙xdest (kW)

RI (%)

ψ (%)

Component

78.28 56.64 56.64 126.6 103.4 103.4 146.8 81.96 81.96 90.70 99.75 99.75 7.209 7.213 6.931 7.209 7.209 6.119 5.005 13.98 3.389 3.532

5.816 4.208 4.208 9.402 7.684 7.684 10.91 6.089 6.089 6.738 7.41 7.41 0.535 0.535 0.514 0.535 0.535 0.454 0.371 1.039 0.251 0.262

86.62 87.33 87.33 29.68 27.68 27.68 86.06 86.05 86.05 49.66 39.68 39.68 69.28 69.28 69.28 69.28 69.28 69.28 69.28 69.28 69.28 69.28

BHE 1 BHE 2 BHE 3 Cooling tower 1 Cooling tower 2 Cooling tower 3 Fan coil pump A1 Fan coil pump A2 Fan coil pump A3 Fan coil pump A4 Fan coil pump A5 Fan coil pump A6 Fan coil pump A7 Fan coil pump A8 Fan coil pump A9 Fan coil pump A10 BHE pump 1 BHE pump 2 BHE pump 3 Cooling tower pump 1 Cooling tower pump 2 Cooling tower pump 3

E˙xdest (kW)

RI (%)

ψ (%)

27.31 20.24 20.24 28.15 20.86 20.86 0.747 1.301 1.827 1.218 1.467 0.883 0.457 0.866 0.468 0.627 0.034 0.03 0.03 2.537 1.092 1.092 1346.15

2.029 1.504 1.504 2.091 1.55 1.55 0.055 0.096 0.135 0.09 0.109 0.065 0.033 0.064 0.034 0.046 0.002 0.002 0.002 0.188 0.081 0.081 –

41.86 41.86 41.86 54.58 54.57 54.57 6.97 6.97 6.97 6.97 6.97 6.97 6.97 6.97 6.97 6.97 3.25 3.25 3.25 3.25 3.25 3.25 62.07

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Cooling towers 5% BHEs 5%

Fan coil pumps 1% Compressors 14%

Fan coils 5% Evaporators 22%

Condensers 25%

Expansion valves 23%

FIGURE 6.9 Relative irreversibilities of overall sets of condensers, expansion valves, evaporators, compressors, fan coils, BHEs, cooling towers, and pumps for the system in Fig. 6.8.

100 90 Exergy efficiency (%)

272

80 70 60 50 40 30 20 10 0

FIGURE 6.10 Average exergy efficiencies of condensers, expansion valves, evaporators, compressors, fan coils, BHEs, cooling towers, and pumps and the exergy efficiency for the overall system in Fig. 6.8.

The inlet glycol solution temperature is an important and representative parameter of the BTES system. The glycol solution temperature entering the refrigeration system (i.e., the condenser) is higher than the ground temperature in summer conditions, because of heat rejection from the circulating glycol solution to the ground. It is shown in Fig. 6.11 that exergy destruction rate increases as the entering glycol solution temperature decreases. The overall exergy destruction rate of the system is shown in Fig. 6.12 as evaporator temperature varies and in Fig. 6.13 as condenser temperature varies. The overall exergy destruction rate decreases and the overall exergy efficiency increases with increasing of evaporator temperature and decreasing of condenser temperature. The variations are almost linear.

2200 0.76 2000 0.72 1600

0.68

esys

0.64

1400 1200

Exdest,tot

0.6 0.56

1000 800 28

esys

Exdest,tot (kW)

1800

29

30

31 TBW,in (°C)

32

0.52 34

33

FIGURE 6.11 Variation of exergy destruction rate and exergy efficiency with inlet glycol solution temperature (Dincer and Rosen, 2013). 1400

0.67

0.66

1200

0.65

1100

esys

Exdest,tot (kW)

Exdest,tot 1300

0.64 esys

1000 −6

−4

−2

0 TE (°C)

2

4

6

0.63

FIGURE 6.12 Variation of exergy destruction rate and exergy efficiency with evaporator temperature (Dincer and Rosen, 2013). 3000

0.66

0.64

2200 0.62

esys

Exdest,tot (kW)

2600

1800 1400 1000 40

esys

Exdest,tot

45

50

55 60 TC (°C)

65

70

0.6

0.58 75

FIGURE 6.13 Variation of exergy destruction rate and exergy efficiency with condenser temperature (Dincer and Rosen, 2013).

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The results of the GSHP/BTES exergy analysis are somewhat sensitive to variations in reference-environment properties and in glycol solution concentration, as shown in previous analyses (Kizilkan and Dincer, 2012).

6.8

CLOSING REMARKS

Heat storage is an advanced energy technology. Heat storage has been attracting increasing interest in several applications, for example, active and passive solar heating, water heating, cooling, and air conditioning. Heat storage systems often are the most economical energy storage technology for building heating, cooling, and air conditioning applications. Some key points raised in this chapter can be summarized as follows: • The selection of the heat storage systems mainly depends on the storage period required (e.g., diurnal, weekly, or seasonal), economic viability, operating conditions, etc. Several parameters influence the viability of any heat storage system, for example, facility thermal loads, thermal and electrical load profiles, availability of waste or excess thermal energy, electrical costs and rate structures, type of thermal generating equipment, and end use type and demand. • Heat storage systems can play a significant role in meeting society’s needs for more efficient, environmentally benign energy use in various sectors and are an important technology for addressing mismatches between times of energy supply and demand. • Using heat storage systems can lead to substantial energy savings (up to 50% when implemented with appropriate demand-side management strategies) and emission reductions of greenhouse gases like CO2, SO2, and NOx (about 40%). • Substantial energy savings can be realized by heat storage systems when implementing the techniques such as using waste energy and surplus heat, reducing electrical demand charges, and avoiding heating, cooling, or air conditioning equipment purchases. • For design, performance evaluation, and optimization of heat storage systems, both energy and exergy analyses should be considered. But exergy analysis should be preferred as it provides an effective method that integrates the conservation of mass and conservation of energy principles together with the SLT. • Heat storage systems are being increasingly applied and continue to attract new interest, for a range of applications, for example, active and passive solar heating, water heating, cooling, and air conditioning. Also, heat storage systems provide an advanced energy technology for building heating and cooling applications, which sometimes constitutes the most economical storage technology.

6.8

Nomenclature A cp e E ex Ex f F h H m P Q t T U W

surface area specific heat capacity specific energy energy specific exergy exergy energy fraction recovery fraction of storage-fluid mass in liquid phase specific enthalpy enthalpy mass pressure heat time temperature overall heat transfer coefficient work

Greek symbols η θ ρ ψ

energy efficiency time density exergy efficiency

Subscripts 0 bhe c comp cond cw d dest e ev f fc gen hw i l o p Q R s W

environmental state borehole heat exchanger charging compressor condenser cooling water discharging destruction evaporator expansion valve final fan coil generation heating water initial, inlet, input loss (leakage) outlet, output pump heat reference storing work

Closing Remarks

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Superscripts ˙ 0

R

rate with respect to time modified reference

Acronyms ARI ATES BHE BTES CTES CFC CPU CSP EPRI ETS FLT GSHP HCFC HTF HVAC IEA PCM RI SLT TES UOIT UTES

Air Conditioning and Refrigeration Institute aquifer thermal energy storage borehole heat exchanger borehole thermal energy storage cold (cool) thermal energy storage chlorofluorocarbon central process unit concentrated solar power Electric Power Research Institute electrical thermal storage first law of thermodynamics ground source heat pump hydrochlorofluorocarbon heat transfer fluid heating, ventilation, and air conditioning International Energy Agency phase change material relative irreversibility second law of thermodynamics thermal energy storage University of Ontario Institute of Technology underground thermal energy storage

References ARI, 1997. Thermal Energy Storage: A Solution for Our Energy, Environmental and Economic Challenges. The Air-Conditioning and Refrigeration Institute, Arlington, VA. Beggs, C.B., 1994. Ice thermal storage: impact on United Kingdom carbon dioxide emissions. Build. Serv. Eng. Res. Technol. 15, 756–763. Bejan, A., 1994. Entropy Generation through Heat and Fluid Flow. Wiley and Sons, New York, NY. California Energy Commission, 1996. Source energy and environmental impacts of thermal energy storage. Technical report no. P500-95-005. California Energy Commission, California. Chwieduk, D.A., 2012. Solar-Assisted Heat Pumps. Comprehensive Renewable Energy. Elsevier, Poland. Diamant, R.M.E., 1984. Energy-Conservation Equipment. The Architectural Press, London. Dincer, I., 1998. Evaluation and selection of energy storage systems for solar thermal applications. In: Dincer, I., Ayhan, T. (Eds.), Proceedings of the TIEES-98, Second Trabzon International Energy and Environment Symposium. Begell House, pp. 168–172.

References

Dincer, I., 2002. On thermal energy storage systems and applications in buildings. Energy Build. 34, 377–388. Dincer, I., Dost, S., 1996. A perspective on thermal energy storage systems for solar energy applications. Int. J. Energy Res. 20, 547–557. Dincer, I., Rosen, M.A., 2001. Energetic, environmental, and economic aspects of thermal energy storage systems for cooling capacity. Appl. Therm. Eng. 21, 1105–1117. Dincer, I., Rosen, M.A., 2011. Thermal Energy Storage: Systems and Applications, second ed. John Wiley and Sons, New York. Dincer, I., Rosen, M.A., 2013. Exergy: Energy, Environment and Sustainable Development, 2nd ed. Elsevier, Oxford. Dincer, I., Dost, S., Li, X., 1997. Performance analyses of sensible heat storage systems for thermal applications. Int. J. Energy Res. 21, 1157–1171. Domanski, R., Fellah, G., 1998. Thermoeconomic analysis of sensible heat, thermal energy storage systems. Appl. Therm. Eng. 18, 693–704. Haeseldonckx, D., Peeters, L., Helsen, L., D’haeseleer, W., 2007. The impact of thermal storage on the operational behavior of residential CHP facilities and the overall CO2 emissions. Renew. Sust. Energ. Rev. 11, 1227–1243. Hahne, E., 2000. The ITW solar heating system: an old timer fully in action. Sol. Energy 69, 469–493. Hauer, A., Quinnell, J., Lavemann, E., 2013. Energy Storage Technologies—Characteristics, Comparison, and Synergies. Wiley-VCH Verlag GmbH & Co., Weinheim, Germany. Hendricks, M., Snijders, M., Boid, N., 2008. Underground thermal energy storage for efficient heating and cooling of buildings. In: 1st International Conference on Industrialised, Integrated, Intelligent Construction, Loughborough. Hoyer, M.C., Walton, M., Kanivetsky, R., Holm, T.R., 1985. Short-term aquifer thermal energy storage (ATES) test cycles, St. Paul, Minnesota, USA. In: Proceedings of 3rd International Conference on Energy Storage for Building Heating and Cooling, Toronto, Canada, pp. 75–79. International Energy Agency, 1994. Energy storage. Heat Pump Cen. Newsl. 12, 8. International Energy Agency, 2011. Technology roadmap: energy efficient buildings: heating and cooling equipment. http://www.iea.org/publications/freepublications/publication/buildings_ roadmap.pdf. International Energy Agency, 2015. Technology Roadmap Energy Storage. http://www.iea.org/ publications/freepublications/publication/TechnologyRoadmapEnergystorage.pdf. Jenne, E.A., 1992. Aquifer Thermal Energy (Heat and Chill) Storage. Pacific Northwest Lab, Richland, WA. Kizilkan, O., Dincer, I., 2012. Exergy analysis of borehole thermal energy storage system for building cooling applications. Energy Build. 49, 568–574. Krane, R.J., 1987. A second law analysis of the optimum design and operation of thermal energy storage systems. Int. J. Heat Mass Transf. 30, 43–57. Krane, R.J., Krane, M.J.M., 1992. The optimum design of stratified thermal energy storage systems. Part I. Development of the basic analytical model. ASME J. Energy Resour. Technol. 114, 197–203. Norton, B., 1992. Solar Energy Thermal Technology. Springer, London. Reindl, D.T., 1994. Characterizing the marginal basis source energy emissions associated with comfort cooling systems. Report no. TSARC 94-1, Thermal Storage Applications Research Center, USA.

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Rosen, M.A., 1992. Appropriate thermodynamic performance measures for closed systems for thermal energy storage. ASME J. Energy Resour. Technol. 114, 100–105. Rosen, M.A., 1999. Second-law analysis of aquifer thermal energy systems. Energy 24, 167–182. Rosen, M.A., Hooper, F.C., 1996. Second law analysis of thermal energy storage systems. In: Proceedings of the TIEES-96, First Trabzon International Energy and Environment Symposium. Karadeniz Technical University, Trabzon, Turkey, pp. 373–379. Rosen, M.A., Hooper, F.C., Barbaris, L.N., 1988. Exergy analysis for the evaluation of the performance of closed thermal energy storage systems. ASME J. Energy Resour. Technol. 110, 255–261. Taylor, M.J., Krane, R.J., Parsons, J.R., 1991. Second law optimization of a sensible heat thermal energy storage system with a distributed storage element. Part I. Development of the analytical model. ASME J. Energy Resour. Technol. 113, 20–26.