Energy storage is employed in solar thermal energy systems to shift excess energy produced during times of high solar availability to times of low solar availability. Two situations exist in solar energy system design where energy storage may be needed; for the situation in which some of the solar thermal energy produced during the day is stored for use later 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 solar thermal energy systems. However, practical design considerations (e.g., operating experience) tend to limit the number of storage subsystems that a system designer could use with confidence. The limitations of the various thermal energy storage concepts are examined in this chapter.
For storing thermal energy, there are three approaches that have been considered over the years for solar thermal systems. These are sensible-heat storage (where a change of temperature occurs), latent heat storage (where a change of phase occurs) and thermochemical energy storage (where a reversible chemical reaction takes place). For storing electricity from photovoltaic systems, a brief introduction to battery energy storage will conclude this chapter.
Sensible-heat storage of thermal energy is perhaps, conceptually, the simplest form of storing thermal energy. In its simplest configuration, cold fluid contained in an insulated tank is heated to some higher temperature by the hot fluid from the field of solar collectors as shown in Figure 11.2. This is quite similar to the way in which a residential solar hot-water heater would work. In most industrial solar energy systems, the fluid in the collector field and the storage tanks is the same. Thus no heat exchanger is shown between the collector field and storage in the following discussions of sensible-heat storage. This is not the case with storage concepts such as latent heat storage where the storage medium undergoes a phase change.
The problem with the system illustrated in Figure 11.2 is that the storage fluid reaches some average 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 of interest. 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 collector fluid, a significant loss in energy quality (i.e., second law availability) can occur in the storage subsystem. Energy quality is usually an important factor in the design of high-temperature solar thermal energy systems. Otherwise, there would be no need to operate the solar collectors at high temperatures that decrease collector efficiency. To avoid this, a two-tank storage system can be used as shown in Figure 11.3. Most sensible-heat storage systems are basically design variations of the two-tank system shown in Figure 11.3.
Figure 11.1 Overnight storage of thermal energy.
The term “multitank storage,” refers to the type of sensible-heat storage system illustrated in Figure 11.3 except more than two tanks can be used. The logic that drives one to consider more than two tanks is that, as the number of tanks increases, the total tank volume decreases. 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 the capacity to hold all the fluid. Thus there must be twice as much tankage volume as fluid volume. 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 and cold storage fluid.
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 from the 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 (see Figure 11.4b), tank 3 is full of hot fluid and tank 1 is empty. Cold fluid is then withdrawn from tank 2 and after heating is deposited in tank 1. At the end of the day, the fully charged multitank 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 corresponding two-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, many factors contribute to limit the number of tanks. Such factors include:
Complexity of control. The complexity of controlling liquid levels and automatically switching tanks grows quickly as the number of tanks increases. The control strategy is especially complex on partially cloudy days.
Interconnecting plumbing. The provision of a piping network interconnecting many tanks and 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. In addition, the interconnecting piping network (especially the control valves) is a source of increased heat loss.
Figure 11.4 Three-tank sensible-heat storage operation: (a) startup; (b) ; (c) end of day.
Figure 11.5 is a photograph of a three-tank sensible heat storage system located at Sandia National Laboratories in Albuquerque (SNLA). The storage fluid in this system is a commercial heat-transfer fluid, Therminol 66®. As a result, a concrete berm has been provided for environmental protection in the event of a spill. The berm can contain the entire volume of oil in the storage tanks. As can be seen from this photograph, the piping arrangement for even a three-tank system can become rather complex.
The ultimate reduction in storage tank volume is achieved when the storage tank volume equals the storage fluid volume. An attempt to achieve this is represented by a thermocline system 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 of the storage tank and heated. The hot storage fluid is then put back into the top of the storage tank. If properly done, the less dense hot storage fluid will “float” on top of the cold storage fluid, creating what is termed a thermocline. This phenomenon actually occurs quite commonly 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 their potential for low cost resulting from minimized tankage volume. Tests at SNLA using a 4.54-m3 (160 ft3) engineering type tank containing Therminol 66® heat-transfer oil, have verified that stable thermoclines can be established. In addition, with careful design of the tank inlet and 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 thermocline energy storage tank at SNLA indicate that a sharp thermocline can be formed and that the mixing of the hot and cold fluid layers with time is small (Gross, 1982). Figure 11.7 shows an 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 was small (about 10 percent of the total tank height), indicating little mixing. After one day of static cooldown (i.e., no fluid was added to or removed from the tank), the transition region grew in thickness but still represented a relatively small amount of the total energy content of the tank. Note, however, that the temperature of the hot fluid had cooled as would happen in any hot storage tank. The small amount of mixing between the hot and cold storage fluids strongly indicates that thermocline storage systems have potential for relatively inexpensive thermal energy storage.
Figure 11.6 Thermocline sensible-heat storage operation: (a) startup; (b) ; (c) end of day.
Figure 11.7 Thermocline stability test results.
One of the most critical design parameters in a thermocline thermal energy storage system is proper provision for diffusion of the incoming and leaving fluids in order to minimize mixing to the formation of vortices or jetting of the incoming fluid. A diffuser design that was used in the SNLA thermocline tank is illustrated in Figure 11.8.
Figure 11.8 Thermocline storage tank diffuser. Courtesy of Sandia National Laboratories.
Once the tank volume has been reduced to a minimum through use of, for example, a thermocline system, the next step in reducing the capital cost of the storage system is to reduce 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 thermocline storage systems seek to displace expensive heat-transfer oil inventory in storage with less expensive materials such as rock. One example of a mixed-media thermocline is shown in Figure 11.9.
The dual-media thermocline concept was developed by
Rocketdyne as the thermal energy storage system for the 10-MW (electrical)
central receiver facility constructed at
An important question in the design of a mixed-media storage
system is the stability of the hot storage fluid in
the presence of the rock. Of particular concern is any potential for the
catalytic degradation of the fluid. In the case of the mixed-media thermocline
system installed at
A second question in the design of a mixed-media storage
system is the strength of the tank with respect to so-called thermal ratcheting. As the tank heats
up, its internal volume increases and the solid media settles. When the tank cools, stress builds up at the
bottom of the tank as the solid media is compressed. Thus careful consideration must be paid to
the tank design to prevent tank rupture due to these stresses. Faas (1983) reports on the evaluation of the
mixed dual-media thermocline storage system at
Figure 11.9 Mixed-media thermal storage unit, central
receiver installation at
The ability to store high temperature thermal energy is basically limited by the availability of heat-transfer fluids. Above about 400ºC (752ºF), most organic heat-transfer fluids tend to thermally decompose. For electric power generation and other high-temperature applications, therefore, fluids such as molten salts, liquid metals, and air (with an air-rock storage medium) are typically considered. Very few engineering prototype storage systems employing such high-temperature storage concepts have been constructed and tested. As such, there is very little information concerning the performance of high-temperature systems.
A basic problem afflicting storage concepts using molten salts and metals is solidification at low temperatures. Thus, unless auxiliary heat is provided, shut-down of the solar energy system can be complicated by the solidification of the heat-transfer fluid. This can result in increased system complexity and cost if extensive heat tracing is required. Sensible heat storage 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 Energy Storage (1978).
High-temperature air systems typically employ some type of inert solid material such as rock to store thermal energy. These storage systems are conceptually similar to the air-rock thermal energy storage systems commonly used in solar residences. Figure l1.10 illustrates a conceptual design for a high-temperature sensible heat storage system using helium in place 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-transfer characteristics of air. A storage system such as that illustrated in Figure 1l.l0 would be compatible with, for example, a Brayton cycle engine using helium gas.
The cost of most of the common thermal energy storage systems is strongly influenced by the cost of the storage fluid (see Section l1.4). The cost of organic heat-transfer fluids can be quite high. The mixed-media storage concepts described previously represent one attempt to reduce storage fluid costs. The use of water or steam as a storage medium represents another way in which to reduce storage fluid costs. In addition, the use of water or steam as a storage fluid in a solar thermal electric system using a steam-driven power generation unit would permit elimination of the expense of a oil water steam generator. However, although these advantages are significant, they are usually overwhelmed by the expense of the pressurized storage tank needed. For example, saturated water at 300ºC (572ºF) has a pressure 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 for storing pressurized water and steam. These vessels can be constructed in quite large volumes and are useful up to about 400ºC (752ºF). They have, however, not been constructed and tested for use with solar thermal energy systems. One reason for this is that solar thermal collectors operating at up to about 400ºC (752ºF) typically use an oil heat-transfer fluid to avoid the expense of high-pressure piping in a large, distributed field of collectors.
One limitation of a sensible-heat system is that the capability of most materials to store heat sensibly is small. Even water, which has a reasonably high heat capacity of 4.186kJ/kg K (1.0 Btu/1bºF), is not a high-energy-density sensible heat storage medium. In addition, the materials 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 of water. By comparison, latent heat processes can provide high energy density storage. A “latent heat storage system” will, in this book, refer to a storage system in which the energy is stored or released during the freeze thaw cycle of a material. This distinction is made since vaporization (boiling) and sublimation processes are also latent heat processes but are not included in the discussion here. As an example of a latent heat process (Radosovich and Wyman, 1982), consider the melting of sodium hydroxide (NaOH). The latent heat of fusion (melting) of NaOH is 156 kJ/kg (68 Btu/1b). This means that when 1 kg of NaOH melts, 156 kJ of thermal energy is absorbed. Thus the engineering motivation for using a latent heat-process as a thermal storage mechanism is to increase the energy density of storage 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/1bºF) would have to undergo a 74ºC (133ºF) temperature rise in order to store an equivalent quantity of energy. Such large temperature rises are, however, compatible with many high-temperature solar thermal energy systems (see Chapter 14), where system design can accommodate a large temperature rise of the collector heat-transfer fluid without significant degradation of the overall system performance. Low-temperature systems, such as solar passive houses, cannot, however, accommodate large temperature changes. Thus low-temperature latent heat storage systems have received considerable attention in these applications to provide relatively constant temperature, compact storage devices.
A typical high-temperature latent heat storage system is illustrated in Figure 11.11. Since the 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 the need for a heat exchanger within the storage system as shown. In addition, since the storage medium undergoes a phase change, the heat exchangers must be carefully designed to accommodate the typically low thermal diffusivity of the solid material. The requirement for rather complex heat exchangers in latent heat storage concepts typically results 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 been summarized by Grodzka (1980) and include
1. 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 heat storage systems have not been widely used in high-temperature solar thermal energy systems. Engineering research in the area has been ongoing, however, and this work is reviewed by Radosovich and Wyman (1982).
A thermochemical energy storage system is one in which thermal energy is used to rupture chemical bonds in a reversible fashion. The rupture of the chemical bond requires large quantities of energy input, thus resulting in thermal energy storage. The product or products of the thermochemical reaction are typically unreactive at ambient temperatures. At elevated temperatures the energy storing reaction reverses, forming the original chemical system with the release of heat.
An example of such a reversible energy storage system is the thermal dissociation of water. At temperatures in excess of 2000ºC (3632ºF), water begins to dissociate into hydrogen and oxygen:
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 and oxygen in a jar at room temperature will not react. If the mixture of hydrogen and oxygen is heated (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 storage systems:
1. Chemical reactions are typically very energetic, thus allowing large quantities of energy to be stored in small quantities of material.
2. The reverse, the energy-releasing chemical reaction, seldom proceeds at room temperature. Thus the energy can be stored indefinitely, without energy loss, at ambient temperatures.
3. Because of very high energy density and stability at low temperatures of some thermochemical 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) to provide thermal energy.
Although thermochemical energy storage holds much theoretical promise, it is furthest from being developed to the point of practical use in a solar thermal energy system. Currently there is no thermochemical energy storage system that has been tested in an operating solar thermal energy system. Although there are many possible energy storing chemical reactions, their chemistry is not sufficiently understood to predict long-term reversibility of the reactions 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 and gases, is heat transfer both into and out of the chemical reactants. The general status of thermochemical energy storage is reviewed by Mar and Bramlette
Before beginning a discussion of the proper sizing of collector fields, the costs associated with sensible heat storage are investigated. Sensible heat is the type of storage currently in most common use. The purpose of including a brief discussion of sensible-heat storage costs at this point is to allow the designer to make some preliminary decisions on the potential role of storage in solar energy systems. If firm costs for storage are available, the designer 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 the commercial area, Atomics International concluded that the cost (in 1976 dollars) of sensible heat 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) to 42,000 m3 (150,000 ft3). The capital cost of the tank from Equation (11.1) must be corrected for inflation in order to use the equation with current oil costs
Equation (11.1) can also be used to approximate the cost of mixed-media storage. This is done simply by multiplying the tank volume by the void fraction to obtain the oil volume in the mixed-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 storage systems. It is possible that the cost of properly installing the rock and the cost of a reinforced tank to hold the rock may be large.
Note that for oil costs of about $790/ m3 ($3/gal, or $22.5/ft3) the cost of oil begins to exceed the tank costs in Equation (11.1) at storage sizes near 14 m3 (250 ft3). Above this volume, oil costs quickly overshadow tank costs. Thus, for fast estimation purposes, the assumption can be made that oil storage costs are reflected in the oil costs alone. Equation (11.1) thus simplifies to
Storage cost = oil cost ($) (11.2)
With some manipulation this equation becomes more useful. Storage capacity, as discussed in Chapter 14, is usually computed as a quantity of energy. To compute storage cost we must make the following calculation
The physical properties needed to perform the calculation represented in Equation (11.3) are reported in the literature, and some of the more common materials are represented in Appendix D. For design purposes where a specific oil storage medium may not have been defined, 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 storage have heat capacities in the range of 2.1-2.5 kJ/kgºC (0.5-0.6 Btu/1bºF) and densities in the range of 780 900 kg/m3 (6.5-7.5 1b/gal) in the temperature range of l50-370ºC (300-700ºF). Using nominal values of 2.3 k J/ kgºC and 840 kg/m2 for the oil heat capacity and density, respectively, we can express Equation (l1.3) as
Coil = cost of oil ($/m3)
Voil = void fraction of oil (1.0 for non-mixed media oil systems)
ΔT = collector field (storage) temperature change (oC)
ηstor = efficiency of thermal storage system (Qout/Qin)
This equation is used in Chapter 14 to approximate the economic impact of storage on a solar energy system. As in all cases in design, if more exact information is available than that presented by Equation (11.4), it should be used. Equation (11.4) is provided to give the designer some idea of potential storage costs.
As discussed in Chapter 14, the designer has some control over the choice of temperature difference across storage and, as a result, will be able to vary storage costs somewhat.
The efficient, inexpensive direct storage of electrical energy has been a dream of many energy system designers. Up until now, we have described methods for storing thermal energy as generated by solar-thermal systems. However, for photovoltaic systems, there is no 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 will describe 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 energy systems; nickel-cadmium (NiCad) batteries and lead-acid batteries. By far the most commonly used type, at least for large, home or industrial photovoltaic systems is the flooded lead-acid battery. This is the type used in automobiles for starting and in industrial electric 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 battery storage systems are generally much higher in
initial cost than lead-acid systems.
However, they offer significantly higher lifetimes, particularly at elevated
operating temperatures. For some
applications, summed over a 20 year lifetime, the life-cycle cost (combining
capital cost, operations and maintenance cost and replacement cost) can be
comparable to flooded lead-acid storage systems. This cost has been estimated to be
approximately 0.4 cents (2002
During discharge, lead sulfate is being formed on the battery plates. Although this is normal during 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 not impossible to break down during recharge. Once this advanced sulfation develops, permanent capacity loss or total failure of the battery is likely. Besides the sulfation concerns, many other detrimental actions are taking place inside the battery while in a discharged condition.
When fully charged (left side of Equation 11.5) the no-load potential difference between the positive plate and the negative plate is approximately 2.1 volts and the specific gravity of the electrolyte 1.265 (at 25oC). When a battery has been fully discharged, the no-load cell voltage 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 in series to make a 12 volt battery. These multi-cell batteries are then connected in series to further increase the operating voltage of the battery storage subsystem. The capacity of the storage, usually stated in ampere-hours (Ah), is increased by connecting additional batteries in parallel.
The system designer must be aware of two aspects of the discharge cycle, the rate of discharge and 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 the batteries. A number commonly used for the design current drain, is based on a 10-hour discharge.
Cbattery 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 averages 100 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 avoid sulfating and other chemical processes which rapidly make the battery unusable and unrecoverable. 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 the battery. Therefore the useful battery capacity in ampere hours is
Cbattery advertised capacity of battery (Ah)
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.
The temperature of the electrolyte is critical
to the design of battery systems. The
capacity of a battery is usually rated at 25oC and decreases
significantly as the temperature decreases.
This reduction in capacity increases with discharge rate (
Another temperature consideration is to keep the liquid electrolyte from freezing. If freezing occurs, the battery plates or case may rupture and cause permanent damage. If the battery is fully charged, the freezing temperature of a typical electrolyte is -30oC. However, when fully discharged, the freezing temperature increases to that of water or 0oC. Adjustments to the specific gravity of the electrolyte can be made to provide for lower ambient temperature operation.
Once discharged, a battery must be recharged within a short period of time to prevent damage. Once a voltage higher than the discharged cell voltage is placed between the positive and negative plates, the lead sulfates on the plates send their sulfate back into the electrolyte converting water into sulfuric acid. During this period, especially at the end of the charge period, the electrolyte will start to bubble. This is called gassing and it occurs because hydrogen and oxygen gases are liberated at the negative and positive plates respectively, as the charging current breaks down the water in the electrolyte. These gasses must be safely vented, and water replaced into the electrolyte (an important maintenance activity).
Starting with a discharged battery, the initial charge is usually performed at either constant voltage or constant current. The maximum charging rate will be just below the cell gassing voltage (2.39 volts at 25oC). The cell gassing voltage decreases with temperature. Quality charge controllers which incorporate temperature sensing to account for this change, are available. The initial charging current should nominally be the 5-hour rate or
Cbattery advertised capacity of battery (Ah)
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.
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.
Batteries will not hold their charge over long periods of time (days) and need to be continually recharged. The process is called self-discharge or shelf stand loss and occurs when batteries are not constantly discharged and charged. A typical lead-acid battery will loose 50% of its capacity in 5 months when the battery temperature is 25oC. However, at higher temperatures, say 40oC, that same battery will loose 50% of its capacity in slightly more than two months. This is of little concern for battery storage systems with normal daily cycling. However continuous charging of lead-acid batteries is needed when the demand for electricity is intermittent or long periods of inoperation are possible.
Not all of the energy used to charge a battery is available for discharge due to a number of different type of losses in both the charge and discharge processes. The ratio of energy discharged to the energy input in the charging cycle is called the round-trip efficiency. Although this value is affected by many variables including the construction of the battery, the charge cycle used and the discharge rate, a round-trip efficiency of approximately 75 80% is typical for lead-acid batteries and 65% for NiCad batteries.
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