energy storage.docx

8/14/2019 energy storage.docx

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11.

Energy StorageEnergy storage is employed in solar thermal energy systems to shift excess energyproduced during times of high solar availability to times of low solar availability. Twosituations exist in solar energy system design where energy storage may be needed; for thesituation in which some of the solar thermal energy produced during the day is stored for uselater during the night, and to provide energy during events such as cloudy days. The

appropriate quantity of storage for a solar thermal energy system is discussed in Chapter 14.

There is a broad range of storage concepts that could be envisioned as interfacing with solarthermal energy systems. However, practical design considerations (e.g., operatingexperience) tend to limit the number of storage subsystems that a system designer coulduse with confidence. The limitations of the various thermal energy storage concepts areexamined in this chapter.

For storing thermal energy, there are three approaches that have been considered over theyears for solar thermal systems. These are sensible-heat storage (where a change oftemperature occurs), latent heat storage (where a change of phase occurs) andthermochemical energy storage (where a reversible chemical reaction takes place). For

storing electricity from photovoltaic systems, a brief introduction to battery energy storagewill conclude this chapter.

Sensible Heat Storageo Multitank Storageo Thermocline Energy Storageo Mixed-media Thermocline Storageo High-Temperature Sensible-Heat Storageo Pressurized Fluids (Steam or Water)

Latent Heat Storage Systems Thermochemical Energy Storage Cost for Sensible Heat Storage Battery Electrical Energy Storage

o Battery Chemistryo Voltageo Dischargeo Temperatureo Chargingo Self-dischargeo Round-trip Efficiency
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mocline%20Storagehttp://www.powerfromthesun.net/Book/chapter11/chapter11.html#11.1.2%20%20%20%20%20Thermocline%20Energy%20Storagehttp://www.powerfromthesun.net/Book/chapter11/chapter11.html#11.1.1%20%20%20%20%20Multitank%20Storage%20Systemshttp://www.powerfromthesun.net/Book/chapter11/chapter11.html#11.1%20%20%20%20%20Sensible-Heat%20Storage


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o Battery Life

11.1 Sensible-Heat StorageSensible-heat storage of thermal energy is perhaps, conceptually, the simplest form ofstoring thermal energy. In its simplest configuration, cold fluid contained in an insulated tankis heated to some higher temperature by the hot fluid from the field of solar collectors asshown in Figure 11.2. This is quite similar to the way in which a residential solar hot-waterheater would work. In most industrial solar energy systems, the fluid in the collector fieldand the storage tanks is the same. Thus no heat exchanger is shown between the collectorfield and storage in the following discussions of sensible-heat storage. This is not the casewith storage concepts such as latent heat storage where the storage medium undergoes aphase change.

The problem with the system illustrated in Figure 11.2 is that the storage fluid reaches someaverage temperature between the starting storage temperature and the hot collector fluid

temperature. However, the quality (i.e., temperature) of the energy in storage is usually ofinterest. If the quantity of thermal energy delivered by the collector field is insufficient (e.g.,partially cloudy days) to heat the entire storage to a temperature near that of the hot collectorfluid, a significant loss in energy quality (i.e., second law availability) can occur in the storagesubsystem. Energy quality is usually an important factor in the design of high-temperaturesolar thermal energy systems. Otherwise, there would be no need to operate the solarcollectors at high temperatures that decrease collector efficiency. To avoid this, a two-tankstorage system can be used as shown in Figure 11.3. Most sensible-heat storage systemsare basically design variations of the two-tank system shown in Figure 11.3.

Figure 11.1 Overnight storage of thermal energy.

11.1.1 Multitank Storage

The term multitank storage, refers to the type of sensible-heat storage system illustrated inFigure 11.3 except more than two tanks can be used. The logic that drives one to considermore than two tanks is that, as the number of tanks increases, the total tank volumedecreases. As an example, contrast two- and three-tank systems.

In a two tank system (Figure 11.3), each tank (either the hot or the cold tank) must have thecapacity to hold all the fluid. Thus there must be twice as much tankage volume as fluidvolume. In the case of three tank of equal volume, any two of the three tanks must at any

given time be able to hold all the storage fluid in order to provide separation of the hot andcold storage fluid.
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Figure 11.2 Single-tank sensible-heat storage.

Figure 11.3 Two-tank sensible heat storage.

The basic operation of a three-tank system is outlined in Figure 11.4. At the start of the day,if tanks 1 and 2 are full of cold fluid and tank 3 is empty, the storage of thermal energy fromthe collector field might proceed as in Figure 11.4a. Cold fluid is withdrawn from tank 1,heated in the collector field and placed in tank 3, which is empty. At about midday (seeFigure 11.4b), tank 3 is full of hot fluid and tank 1 is empty. Cold fluid is then withdrawn fromtank 2 and after heating is deposited in tank 1. At the end of the day, the fully chargedmultitank storage system would appear as in Figure 11.4c.

The advantage of the three tank system over the two-tank system is less tank volume.Whereas a two-tank system requires only 1.5 times as much tank volume as fluid volume.Thus there is a potential for a multiple tank system to be lower cost than the correspondingtwo-tank system. In fact, if minimization of storage tank volume were the sole parameter,logic would drive the design to a very large number of tanks. In practice, however, manyfactors contribute to limit the number of tanks. Such factors include:

Complexity of control. The complexity of controlling liquid levels and automatically switchingtanks grows quickly as the number of tanks increases. The control strategy is especiallycomplex on partially cloudy days.

Interconnecting plumbing. The provision of a piping network interconnecting many tanksand provision for automatic valving can become quite expensive.

Heat loss. Large tanks lose less heat per unit volume of hot fluid than do small tanks. Inaddition, the interconnecting piping network (especially the control valves) is a source ofincreased heat loss.


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Figure 11.4 Three-tank sensible-heat storage operation: (a) startup; (b) midday; (c) end of day.

Figure 11.5 is a photograph of a three-tank sensible heat storage system located at SandiaNational Laboratories in Albuquerque (SNLA). The storage fluid in this system is acommercial heat-transfer fluid, Therminol 66. As a result, a concrete berm has beenprovided for environmental protection in the event of a spill. The berm can contain the entirevolume of oil in the storage tanks. As can be seen from this photograph, the pipingarrangement for even a three-tank system can become rather complex.

11.1.2 Thermocline Energy Storage

The ultimate reduction in storage tank volume is achieved when the storage tank volumeequals the storage fluid volume. An attempt to achieve this is represented by a thermoclinesystem in which both the hot and cold storage fluids occupy the same tank. Conceptually,the operation of a thermocline sensible-heat storage system is illustrated in Figure 11.6.

At the start of operation, the storage tank is full of cold fluid. As thermal energy, in the form

of hot collector fluid, becomes available, cold storage fluid is withdrawn from the bottom ofthe storage tank and heated. The hot storage fluid is then put back into the top of thestorage tank. If properly done, the less dense hot storage fluid will float on top of the cold


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storage fluid, creating what is termed a thermocline. This phenomenon actually occurs quitecommonly in many fluid systems ranging from the ocean to residential hot-water heaters.

Figure 11.5 Three-tank sensible-heat system installed at Sandia National Laboratories,Albuquerque. Courtesy of Sandia National Laboratories.

Thermocline energy storage systems have received much attention because of theirpotential for low cost resulting from minimized tankage volume. Tests at SNLA using a 4.54-m3(160 ft3) engineering type tank containing Therminol 66heat-transfer oil, have verifiedthat stable thermoclines can be established. In addition, with careful design of the tank inletand outlet diffusers, momentum-induced mixing of the hot and cold fluids can be minimized,leading to a rather small transition region between the hot and cold fluid regions.

Results from tests performed on the 4.54-m3(160 ft3) engineering prototype thermoclineenergy storage tank at SNLA indicate that a sharp thermocline can be formed and that themixing of the hot and cold fluid layers with time is small (Gross, 1982). Figure 11.7 showsan experimentally measured axial temperature gradient for the prototype tank. As can be

seen, the thickness of the initial transition region in the tank from the hot to cold fluid wassmall (about 10 percent of the total tank height), indicating little mixing. After one day ofstatic cooldown (i.e., no fluid was added to or removed from the tank), the transition regiongrew in thickness but still represented a relatively small amount of the total energy content ofthe tank. Note, however, that the temperature of the hot fluid had cooled as would happen inany hot storage tank. The small amount of mixing between the hot and cold storage fluidsstrongly indicates that thermocline storage systems have potential for relatively inexpensivethermal energy storage.


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Figure 11.6 Thermocline sensible-heat storage operation: (a) startup; (b) midday; (c) end of day.


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Figure 11.7 Thermocline stability test results.

One of the most critical design parameters in a thermocline thermal energy storage systemis proper provision for diffusion of the incoming and leaving fluids in order to minimize mixingto the formation of vortices or jetting of the incoming fluid. A diffuser design that was used inthe SNLA thermocline tank is illustrated in Figure 11.8.

Figure 11.8 Thermocline storage tank diffuser. Courtesy of Sandia National Laboratories.


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11.1.3 Mixed-Media Thermocline Storage

Once the tank volume has been reduced to a minimum through use of, for example, athermocline system, the next step in reducing the capital cost of the storage system is toreduce the cost of the storage fluid. Organic heat-transfer oils are typically used in high-temperature solar energy systems to avoid the cost of high-pressure plumbing systems.Unfortunately, most organic heat-transfer oils are expensive. Mixed-media thermoclinestorage systems seek to displace expensive heat-transfer oil inventory in storage with lessexpensive materials such as rock. One example of a mixed-media thermocline is shown inFigure 11.9.

The dual-media thermocline concept was developed by Rocketdyne as the thermal energystorage system for the 10-MW (electrical) central receiver facility constructedat Barstow, CA (see Chapter 10). The solid medium chosen for this project was nominally2.54 cm (1 in.) diameter gravel plus sand. Two sizes were used in the storage tank toreduce the void fraction to about 0.25-0.30. Thus this concept reduced the quantity of oilused in the conventional thermocline storage by about 75%. Top and bottom manifolds

were employed to distribute the heat transfer oil across the cross section of the tank. An important question in the design of a mixed-media storage system is the stability of thehot storage fluid in the presence of the rock. Of particular concern is any potential for thecatalytic degradation of the fluid. In the case of the mixed-media thermocline systeminstalled at Barstow, CA, extensive testing of fluid stability in the presence of the hot rockswas performed. The storage fluid chosen was a commercial organic heat-transfer fluid,Caloria HT-43. This fluid was found to be stable over long periods of time when in contactwith rock of temperatures up to 300C (572F).

A second question in the design of a mixed-media storage system is the strength of the tankwith respect to so-called thermal ratcheting. As the tankheats up, its internal volume

increases and the solid media settles. When the tank cools, stress builds up at the bottom ofthe tank as the solid media is compressed. Thus careful consideration must be paid to thetank design to prevent tank rupture due to these stresses. Faas (1983) reports on theevaluation of the mixed dual-media thermocline storage system at Barstow, CA.


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Figure 11.9 Mixed-media thermal storage unit, central receiver installation at Barstow, CA.

11.1.4 High-Temperature Sensible Hea1 Storage

The ability to store high temperature thermal energy is basically limited by the availability ofheat-transfer fluids. Above about 400C (752F), most organic heat-transfer fluids tend to

thermally decompose. For electric power generation and other high-temperatureapplications, therefore, fluids such as molten salts, liquid metals, and air (with an air-rockstorage medium) are typically considered. Very few engineering prototype storage systemsemploying such high-temperature storage concepts have been constructed and tested. Assuch, there is very little information concerning the performance of high-temperaturesystems.

A basic problem afflicting storage concepts using molten salts and metals is solidification atlow temperatures. Thus, unless auxiliary heat is provided, shut-down of the solar energysystem can be complicated by the solidification of the heat-transfer fluid. This can result inincreased system complexity and cost if extensive heat tracing is required. Sensible heatstorage employing molten salt has been tested at Sandia National Laboratories (Tracey,1982), but there are no commercial units of such storage available to the authors'knowledge.

Figure 11.10 High-temperature sensible-heat storage unit using helium as the heat-transfer

fluid. Courtesy of Franklin Institute Press R. H. Turner, High Temperature Thermal EnergyStorage (1978).

High-temperature air systems typically employ some type of inert solid material such as rockto store thermal energy. These storage systems are conceptually similar to the air-rockthermal energy storage systems commonly used in solar residences. Figure l1.10 illustratesa conceptual design for a high-temperature sensible heat storage system using helium inplace of air. The hot gas flows over magnesium oxide (MgO) bricks that store the heat.Helium gas is commonly used in place of air because of the rather poor heat-transfercharacteristics of air. A storage system such as that illustrated in Figure 1l.l0 would becompatible with, for example, a Brayton cycle engine using helium gas.


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11.1.5 Pressurized Fluids (Steam or Water).

The cost of most of the common thermal energy storage systems is strongly influenced bythe cost of the storage fluid (see Section l1.4). The cost of organic heat-transfer fluids can bequite high. The mixed-media storage concepts described previously represent one attemptto reduce storage fluid costs. The use of water or steam as a storage medium representsanother way in which to reduce storage fluid costs. In addition, the use of water or steam asa storage fluid in a solar thermal electric system using a steam-driven power generation unitwould permit elimination of the expense of a oil water steam generator. However,although these advantages are significant, they are usually overwhelmed by the expense ofthe pressurized storage tank needed. For example, saturated water at 300C (572F) has apressure of about 8.8 M Pa (l275 psia).

Recent developments (Turner, l980) in the use of pre-stressed cast-iron vessels have,however, shown some promise for providing large, low-cost, high-pressure vessels forstoring pressurized water and steam. These vessels can be constructed in quite largevolumes and are useful up to about 400C (752F). They have, however, not been

constructed and tested for use with solar thermal energy systems. One reason for this isthat solar thermal collectors operating at up to about 400C (752F) typically use an oil heat-transfer fluid to avoid the expense of high-pressure piping in a large, distributed field ofcollectors.

11.2 Latent Heat Storage SystemsOne limitation of a sensible-heat system is that the capability of most materials to store heatsensibly is small. Even water, which has a reasonably high heat capacity of 4.186kJ/kg K(1.0 Btu/1bF), is not a high-energy-density sensible heat storage medium. In addition, thematerials most commonly used to store heat in a sensible-heat storage system, namely

organic heat transfer oils, typically have heat capacities in the range of 0.5-0.7 times that ofwater. By comparison, latent heat processes can provide high energy density storage. Alatent heat storage system will, in this book, refer to a storage system in which the energyis stored or released during the freeze thaw cycle of a material. This distinction is madesince vaporization (boiling) and sublimation processes are also latent heat processes but arenot included in the discussion here. As an example of a latent heat process (Radosovichand Wyman, 1982), consider the melting of sodium hydroxide (NaOH). The latent heat offusion (melting) of NaOH is 156 kJ/kg (68 Btu/1b). This means that when 1 kg of NaOHmelts, 156 kJ of thermal energy is absorbed. Thus the engineering motivation for using alatent heat-process as a thermal storage mechanism is to increase the energy density ofstorage and thus potentially reduce storage tank size and cost.

In contrast, a heat-transfer oil with a heat capacity of 2.1 kJ/kg K (0.5 Btu/1bF) would haveto undergo a 74C (133F) temperature rise in order to store an equivalent quantity ofenergy. Such large temperature rises are, however, compatible with many high-temperaturesolar thermal energy systems (see Chapter 14), where system design can accommodate alarge temperature rise of the collector heat-transfer fluid without significant degradation ofthe overall system performance. Low-temperature systems, such as solar passive houses,cannot, however, accommodate large temperature changes. Thus low-temperature latentheat storage systems have received considerable attention in these applications to providerelatively constant temperature, compact storage devices.

A typical high-temperature latent heat storage system is illustrated in Figure 11.11. Sincethe storage material undergoes a transition from liquid to solid and vice versa, the storage

material cannot be pumped through the collector field or the process. This results in theneed for a heat exchanger within the storage system as shown. In addition, since the


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storage medium undergoes a phase change, the heat exchangers must be carefullydesigned to accommodate the typically low thermal diffusivity of the solid material. Therequirement for rather complex heat exchangers in latent heat storage concepts typicallyresults in increased system costs compared to systems that use sensible heat storage.

Figure 11.11 Latent-heat thermal energy storage module.

Other characteristics that adversely affect design of a latent heat storage system have beensummarized by Grodzka (1980) and include1. The cost of many of the more effective latent heat storage materials is high.2. Some of the latent heat storage materials are not pure materials but mixtures that tend to

separate into their component parts on repeated freeze-thaw cycling.3. Some of the latent heat storage materials such as NaOH can react violently with the

organic heat-transfer oils commonly used in solar thermal energy collectors.4. Supercooling of the latent heat storage material can occur on solidification.

Because of these problems and the availability of sensible heat storage systems, latent heatstorage systems have not been widely used in high-temperature solar thermal energysystems. Engineering research in the area has been ongoing, however, and this work isreviewed by Radosovich and Wyman (1982).

11.3 Thermochemical Energy StorageA thermochemical energy storage system is one in which thermal energy is used to rupturechemical bonds in a reversible fashion. The rupture of the chemical bond requires largequantities of energy input, thus resulting in thermal energy storage. The product or productsof the thermochemical reaction are typically unreactive at ambient temperatures. At elevatedtemperatures the energy storing reaction reverses, forming the original chemical system withthe release of heat.

An example of such a reversible energy storage system is the thermal dissociation of water.At temperatures in excess of 2000C (3632F), water begins to dissociate into hydrogen andoxygen:


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2H2O + thermal energy = 2H2+ O2

The reverse reaction,

2H2+ O2= 2H2O + thermal energy

will not proceed at low temperatures without a catalyst. Thus a mixture of hydrogen andoxygen in a jar at room temperature will not react. If the mixture of hydrogen and oxygen isheated (e.g., if a match were placed in the jar), however, the reaction proceeds explosively.

This simple example illustrates the reason for interest in thermochemical energy storagesystems:

1. Chemical reactions are typically very energetic, thus allowing large quantities of energyto be stored in small quantities of material.

2. The reverse, the energy-releasing chemical reaction, seldom proceeds at roomtemperature. Thus the energy can be stored indefinitely, without energy loss, at ambienttemperatures.

3. Because of very high energy density and stability at low temperatures of somethermochemical energy storage systems, the stored thermal energy can be transported.An extreme case of transportability is the formation of a chemical fuel, such as hydrogen,which can be piped around the country and then burned (i.e., reacted with oxygen) toprovide thermal energy.

Although thermochemical energy storage holds much theoretical promise, it is furthest frombeing developed to the point of practical use in a solar thermal energy system. Currentlythere is no thermochemical energy storage system that has been tested in an operatingsolar thermal energy system. Although there are many possible energy storing chemicalreactions, their chemistry is not sufficiently understood to predict long-term reversibility of thereactions nor the effects of the chemicals on the materials housing the reactions. Another

area that is not well understood, especially for chemical processes involving liquids andgases, is heat transfer both into and out of the chemical reactants. The general status ofthermochemical energy storage is reviewed by Mar and Bramlette(1980).

11.4 Cost for Sensible Heat StorageBefore beginning a discussion of the proper sizing of collector fields, the costs associatedwith sensible heat storage are investigated. Sensible heat is the type of storage currently inmost common use. The purpose of including a brief discussion of sensible-heat storagecosts at this point is to allow the designer to make some preliminary decisions on thepotential role of storage in solar energy systems. If firm costs for storage are available, thedesigner should by all means use those costs in place of the generic costs outlined here.

As part of a study (Anonymous, 1977) to evaluate the application of solar energy to thecommercial area, Atomics International concluded that the cost (in 1976 dollars) of sensibleheat storage could be approximated by

Storage cost = tank cost ($) + oil cost ($)= 352 (vol, ft3)0.515+ (oil cost, $/ft3) (vol, ft3) ($) (11.1)

This relationship is felt to be valid for storage systems in the range of 4.2 m3(150 ft3) to42,000 m3(150,000 ft3). The capital cost of the tank from Equation (11.1) must be correctedfor inflation in order to use the equation with current oil costs


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Equation (11.1) can also be used to approximate the cost of mixed-media storage. This isdone simply by multiplying the tank volume by the void fraction to obtain the oil volume in themixed-media tank. Usually, the cost of rock (the most common solid medium in a mixed-media storage tank) is small compared to that of the oil, and this approximation is acceptable

for a preliminary design. However, there is less experience with mixed-media storagesystems. It is possible that the cost of properly installing the rock and the cost of areinforced tank to hold the rock may be large.

Note that for oil costs of about $790/ m3($3/gal, or $22.5/ft3) the cost of oil begins to exceedthe tank costs in Equation (11.1) at storage sizes near 14 m 3(250 ft3). Above this volume, oilcosts quickly overshadow tank costs. Thus, for fast estimation purposes, the assumptioncan be made that oil storage costs are reflected in the oil costs alone. Equation (11.1) thussimplifies to

Storage cost = oil cost ($) (11.2)

With some manipulation this equation becomes more useful. Storage capacity, asdiscussed in Chapter 14, is usually computed as a quantity of energy. To compute storagecost we must make the following calculation

($) (11.3)

The physical properties needed to perform the calculation represented in Equation (11.3) arereported in the literature, and some of the more common materials are represented inAppendix D. For design purposes where a specific oil storage medium may not have beendefined, it is useful to use some nominal physical property values to help evaluate the cost of

storage. Many of the high-temperature oil heat-transfer fluids used for solar energy storagehave heat capacities in the range of 2.1-2.5 kJ/kgC (0.5-0.6 Btu/1bF) and densities in therange of 780 900 kg/m3(6.5-7.5 1b/gal) in the temperature range of l50-370C (300-700F). Using nominal values of 2.3 k J/ kgC and 840 kg/m2for the oil heat capacity anddensity, respectively, we can express Equation (l1.3) as

($/kWh) (11.4)


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whereCoil=cost of oil ($/m3)Voil=void fraction of oil (1.0 for non-mixed media oil systems)T= collector field (storage) temperature change(oC)stor= efficiency of thermal storage system (Qout/Qin)

This equation is used in Chapter 14 to approximate the economic impact of storage on asolar energy system. As in all cases in design, if more exact information is available thanthat presented by Equation (11.4), it should be used. Equation (11.4) is provided to give thedesigner some idea of potential storage costs.

As discussed in Chapter 14, the designer has some control over the choice of temperaturedifference across storage and, as a result, will be able to vary storage costs somewhat.

11.5 Battery Electrical Energy StorageThe efficient, inexpensive direct storage of electrical energy has been a dream of manyenergy system designers. Up until now, we have described methods for storing thermalenergy as generated by solar-thermal systems. However, for photovoltaic systems, there isno intermediate heat to store and, if storage is desired, electricity must be stored in a battery.At this point then, in our development of storage concepts for solar energy systems, we willdescribe the currently used technology of direct electrical energy storage in batteries .

Although there are many types of batteries, only two are in common use in solar energysystems; nickel-cadmium (NiCad) batteries and lead-acid batteries. By far the mostcommonly used type, at least for large, home or industrial photovoltaic systems is theflooded lead-acid battery. This is the type used in automobiles for starting and in industrialelectric vehicles. The discussion that follows will highlight flooded lead-acid batteries only,

although many of the concepts developed are the same for both.

One note about NiCad batteries for photovoltaic system storage is that NiCad batterystorage systems are generally much higher in initial cost than lead-acid systems. However,they offer significantly higher lifetimes, particularly at elevated operating temperatures. Forsome applications, summed over a 20 year lifetime, the life-cycle cost (combining capitalcost, operations and maintenance cost and replacement cost) can be comparable to floodedlead-acid storage systems. This cost has been estimated to be approximately 0.4 cents(2002 US$) per kWh of energy stored over the 20-year life (Lambert, 2002).


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11.5.1 Battery Chemistry

The construction of the flooded lead-acid battery is simple. Dissimilar metal platesseparated by non-conducting material allowing transfer of ions, are immersed in a electrolytesolution of sulfuric acid and water. During the discharge, sulfuric acid is drawn from theelectrolyte into the pores of the plates. This reduces the specific gravity of the electrolyteand increases the concentration of water. During recharge, this action is reversed and thesulfuric acid is driven from the plates back into the electrolyte, increasing the specificgravity. The basic chemical reaction is

(11.5)

During discharge, lead sulfate is being formed on the battery plates. Although this is normalduring discharge, a timely recharge is required to drive out sulfuric acid into the electrolyte.Without this recharge, the lead sulfate will continue to develop and become difficult if notimpossible to break down during recharge. Once this advanced sulfation develops,permanent capacity loss or total failure of the battery is likely. Besides the sulfationconcerns, many other detrimental actions are taking place inside the battery while in adischarged condition.

11.5.2 Voltage

When fully charged (left side of Equation 11.5) the no-load potential difference between thepositive plate and the negative plate is approximately 2.1 volts and the specific gravity of theelectrolyte 1.265 (at 25oC). When a battery has been fully discharged, the no-load cellvoltage is 1.93 volts and the electrolyte specific gravity approximately 1.100.

Most applications store electrical energy at higher voltages and so, within a battery case,three cells may be connected in series to make a 6 volt battery, or six cells connected inseries to make a 12 volt battery. These multi-cell batteries are then connected in series tofurther increase the operating voltage of the battery storage subsystem. The capacity of thestorage, usuallystated in ampere-hours (Ah), is increased by connecting additional batteries

in parallel.

11.5.3 Discharge

The system designer must be aware of two aspects of the discharge cycle, the rate ofdischargeand the depth of discharge. If the load contains large current drawing equipment,the battery bank must be sized large enough to provide that current without damaging thebatteries. A number commonly used for the design current drain, is based on a 10-hourdischarge.


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(11.6)

whereCbattery advertised capacity of battery (Ah)

With this discharge rate, battery voltage will be maintained at a high level during discharge,and the resulting battery life close to maximum. What this means is that if the load averages100 amps, the designer must size the battery bank at 1,000 amp-hour.

The second discharge criteria is that the battery must not be completely discharged to avoidsulfating and other chemical processes which rapidly make the battery unusable andunrecoverable. It is generally considered good design practice for deep-cycle batteries, to

only discharge them by 80% of the full charge, leaving 20% of the initial charge in thebattery. Therefore the useful battery capacity in ampere hours is

(11.7)


The result of this to the system designer is that for a given demand, 25% extra capacity must

be added to the size of the battery bank.

11.5.4 Temperature

The temperature of the electrolyte is critical to the design of battery systems. The capacityof a battery is usually rated at 25oC and decreases significantly as the temperaturedecreases. This reduction in capacity increases with discharge rate(Arizona Solar Center web site, page 3-16). As an example, the discharge capacity of abattery system designed for 25oC reduces to 82% of the 25oC capacity when the electrolytetemperature falls to 0oC. This means that the designer must add 22% more capacity to thebattery bank if it is expected to operate at this temperature. The above example if for a 10-

hour discharge rate. The reduction in capacity at low temperatures is significantly greater forhigher rates of discharge.

Another temperature consideration is to keep the liquid electrolyte from freezing. If freezingoccurs, the battery plates or case may rupture and cause permanent damage. If the batteryis fully charged, the freezing temperature of a typical electrolyte is -30oC. However, whenfully discharged, the freezing temperature increases to that of water or 0 oC. Adjustments tothe specific gravity of the electrolyte can be made to provide for lower ambient temperatureoperation.


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11.5.5 Charging

Once discharged, a battery must be recharged within a short period of time to preventdamage. Once a voltage higher than the discharged cell voltage is placed between thepositive and negative plates, the lead sulfates on the plates send their sulfate back into theelectrolyte converting water into sulfuric acid. During this period, especially at the end of thecharge period, the electrolyte will start to bubble. This is called gassingand it occursbecause hydrogen and oxygen gases are liberated at the negative and positive platesrespectively, as the charging current breaks down the water in the electrolyte. These gassesmust be safely vented, and water replaced into the electrolyte (an important maintenanceactivity).

Starting with a discharged battery, the initial charge is usually performed at either constantvoltage or constant current. The maximum charging rate will be just below the cell gassingvoltage (2.39 volts at 25oC). The cell gassing voltage decreases with temperature. Qualitycharge controllers which incorporate temperature sensing to account for this change, areavailable. The initial charging current should nominally be the 5-hour rate or

(11.8)


Once 100% of the previous discharge capacity has been returned, the charge rate will have

decayed to the finishing current, nominally the 20-hour rate.

(11.9)where

Cbattery advertised capacity of battery (Ah)

Commercial charge controllers are available to perform these functions correctly and the

battery manufacturer should be contacted for the specific current or voltage settings.

11.5.6 Self-discharge

Batteries will not hold their charge over long periods of time (days) and need to becontinually recharged. The process is called self-dischargeor shelf stand loss and occurswhen batteries are not constantly discharged and charged. A typical lead-acid battery willloose 50% of its capacity in 5 months when the battery temperature is 25 oC. However, athigher temperatures, say 40oC, that same battery will loose 50% of its capacity in slightlymore than two months. This is of little concern for battery storage systems with normal dailycycling. However continuous charging of lead-acid batteries is needed when the demand forelectricity is intermittent or long periods of inoperation are possible.


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11.5.7 Round-trip Efficiency

Not all of the energy used to charge a battery is available for discharge due to a number ofdifferent type of losses in both the charge and discharge processes. The ratio of energydischarged to the energy input in the charging cycle is called the round-tripefficiency. Although this value is affected by many variables including the construction of thebattery, the charge cycle used and the discharge rate, a round-trip efficiency ofapproximately 75 80% is typical for lead-acid batteries and 65% for NiCad batteries.

11.5.8 Battery Life

Battery life us usually stated in terms of the number of deep (80% discharge) cycles that abattery will deliver assuming that it has been discharged at the proper rate, timely rechargedat the proper rate and not abused in other ways. Typical automotive starting-light-ignitionbatteries give excellent initial, short-term cycle service, but last only 20-100 deep cycles.Newer on-the-road electric vehicle batteries are designed for somewhat longer deep cycle

service (500-1100 cycles). Industrial motive power batteries (lift trucks etc.) can deliver 1500to 2000 deep cycles (Arizona Solar Center web site).

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