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APPLICATIONS – STATIONARY | Remote Area Power Supply: Batteries and Fuel Cells
Remote Area Power Supply: Batteries and Fuel Cells M Perrin, Conergy AG, Hamburg, Germany E Lemaire-Potteau, INES/CEA, Laboratory for Solar Systems Le Bourget du Lac, France & 2009 Elsevier B.V. All rights reserved.
Definition and Specificities of Remote-Area Power Supply Electricity supply is most commonly ensured by power stations connected to transmission and distribution networks. Some specific solutions need to be found, however, for providing power in isolated locations, i.e., those not serviced by the main grid. Such facilities are generally referred to as remote-area power supply (RAPS), standalone, or off-grid systems. These systems are used to provide electrical power in many applications, e.g., houses, community services, electric fences, water pumping, and telecommunications. In some cases, however, they are not even located in remote places, e.g., for very small-scale urban applications such as parking ticket machines and lighting. The latter option can prove less expensive than connection to the main grid. The acronym RAPS addresses in fact various types of systems, from an individual lighting kit to a multiuser mini-grid system. The energy can be supplied from a single source or a mix of different sources, both nonrenewable (e.g., diesel generator) and renewable (e.g., wind turbine, photovoltaic (PV) panels, and micro hydro turbine). A system that includes several types of energy sources is called a hybrid system. Figures 1–13 show examples of RAPS system. In all types of RAPS system (except diesel-only systems that are the most implemented solution), energy storage is necessary in order to match the energy production to the needs of users. Figure 4 shows two examples of energy
production and consumption over 1 day for two RAPS systems, namely, a PV lighting kit and a hybrid system (PV þ diesel). In the first case, electricity generation takes place only during the day, and major consumption occurs early in the morning and in the evening for lighting and television reception. In the second case, the diesel generator provides additional generation in the evening, and the consumption is more distributed over the day, owing to other types of electrical equipment. Nontechnical issues play an important role in the implementation of RAPS systems. Depending on the location of the installation, the issues can include: formalities, • administrative import taxes, • system ownership (user or energy company), • collection of energy fees, • social acceptance aspects, and • user care of the system. • Such aspects will not be discussed here, since they are not specifically related to the energy storage function. Nevertheless, apart from the performance of the system, nontechnical issues can lead to the success or failure of the operation. In summary, a RAPS system generally consists of the following components (Figure 5): power generation equipment (photovoltaic panels, wind • turbine, micro hydro turbine, diesel generator, etc.), energy storage system, •
Figure 1 Remote-area power supply photovoltaic (RAPS PV) system powering an isolated house in the south of France: (a) solar panels and (b) storage system.
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Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
control and conversion equipment (charge controller • and sine-wave inverter), and direct current (DC) or alternating current (AC) loads. •
The generated electricity is fed into the storage system using a charge controller, and can then be used directly for DC loads or, through an inverter, for AC loads. In a RAPS system, the energy storage system is subjected to specific constraints, mainly related to the limited availability of energy for charging, i.e., the charge is dependent on the meteorological conditions (sunshine, wind, etc.). Storage is often undertaken by lead–acid batteries, but alternative technologies can be advantageous in some situations, as discussed in the section ‘Choosing the Storage Technology.’
Market for Remote-Area Power Supply Systems The market for RAPS systems is divided into many segments according to the type of service, the size of the system, and, possibly, the location. In the same way, the specific market for electrochemical power sources has different segments, as dictated by the size of the system and the constraints placed on the battery. Generally
Figure 2 Large domestic remote-area power supply (RAPS) system in Greece with wind turbine and photovoltaic (PV) panels. Source: Conergy AG.
Figure 3 Remote-area power supply (RAPS) system for telecommunications in Brazil. Source: Conergy AG.
25 000
50
20 000
Power (W)
Power (W)
40
Production Consumption
30 20
15 000 10 000 5000
10 0 0:00
Production Consumption
6:00
12:00 Time (h)
18:00
0:00
0 0:00
6:00
12:00
18:00
0:00
Time (h)
Figure 4 Examples of energy production and consumption over 1 day for two remote-area power supply (RAPS) systems: photovoltaic (PV) lighting kit (left), and hybrid system with PV and diesel (right).
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
PV modules Batteries
67
Balance of system Others
Figure 6 Typical cost breakdown of a stand-alone photovoltaic (PV) system. Left: Investment cost. Right: Cost of ownership over 20 years.
BMS
=
= =
~
Storage Storage system
Figure 5 General architecture of a remote-area power supply (RAPS) system. BMS, battery management system.
speaking, the two main markets for RAPS systems are for domestic and industrial services. The domestic market for RAPS systems addresses rural electrification and will be described here as the offgrid habitation segment. In this application, the systems fulfill needs such as lighting and television and the market was dominated by the Western countries until the beginning of the millennium. Since then, the habitation market in the developed countries, and in particular in Europe, has slowed down because the majority of the remote homes have been equipped with RAPS facilities. By contrast, a large market for rural electrification of habitation is growing in developing countries. According to the World Energy Outlook of 2006, 1.577 billion people did not have access to electricity in their homes; this represents a little below a quarter of the world population. The population is mainly in rural areas since electrification is estimated to be less than 62% in rural areas as compared to 90.4% in urban areas. The largest ratio of population without access to electricity is located in southern Asia and sub-Saharan Africa. In these regions, the energy source for RAPS is expected to be mainly photovoltaic and according to the European Photovoltaic Industry Association (EPIA), ‘‘in the nonindustrialized world approximately 30 GWp of solar capacity is expected to have been installed by 2020 in the rural electrification sector.’’ The industrial market for RAPS deals with applications such as telecommunications, water pumping, vaccine refrigeration, and navigational aids, or with the powering of small industry. This market is the one that is the most self-sustaining and that partly grows without subsidies. For stand-alone PV systems (PV-RAPS), the worldwide off-grid domestic market in 2002 was 30 MWp,
whereas the off-grid commercial market was 70 MWp, This market has been growing by 10% each year over the last 10 years, and in 2005, 15% of the worldwide demand for PV modules (1.389 GWp) was dedicated to remote industrial (8%) and remote habitation (7%) applications. In PV-RAPS systems, the cost for the storage amounts to one-quarter of the overall investment (i.e., about 3h per Wp with respect to an average of 12h per Wp for system costs). As shown in Figure 6, however, the share of the batteries, in the cost of ownership over 20 years of running a PV-RAPS system, amounts for as much as 50% due to their shorter life. Therefore, a huge potential exists for electrochemical power sources in the RAPS market, both in the original equipment and the replacement markets. Globally, the market for off-grid PV systems is expected to grow to 130 GWp in 2030 with a share of 70 GWp for industrial applications and 60 GWp for rural electrification.
Designing a Remote-Area Power Supply System The design and sizing of a RAPS system starts with the energy needs of the user. 1. First, the system designer must place a strong focus on energy saving through the use of efficient electrical appliances and optimized energy management, as well as the choice of the best system architecture and components. Energy-saving appliances may be more expensive at the time of investment. Nevertheless, since 1 kWh of electricity generated from a RAPS system costs 0.30–1h, the cost difference is very quickly regained. 2. Next, the demand for energy is evaluated. This may be very easy in some cases (e.g., industrial system with well-known consumption) but considerably more difficult in others, especially on account of the ‘human factor’ whereby user behavior cannot be controlled and tends to evolve. This evolution favors a modular design of RAPS system for sizing adjustment. 3. The third step is to decide on the energy mix to be supplied, i.e., either directly to the user when
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Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
production and consumption are synchronous, or through the storage unit. Although generator sets are the most immediate solution, renewable energy alternatives will mostly be more profitable in the long term, e.g., PV, wind, or small hydro, when considering all economical aspects including the cost and transportation of fuel and system maintenance requirements. The size of the generator is calculated as a function of the renewable energy resource as well as of the productivity of both the generator and the system. 4. The calculation made in the third step allows design of the storage system. The size of the storage system is a multiplication of the number of days of autonomy that are needed (or more precisely, number of consecutive days in the year where the renewable energy source is not available) by the daily energy needs. This, then, is the amount of energy that must be delivered until reaching the maximum depth-of-discharge (DoD) that is acceptable for the chosen storage technology.
where maintenance is very difficult or where high initial investment is possible, the battery can be oversized in order to extend its longevity.
To illustrate the above four steps that are involved in RAPS sizing, the following system for a home in central Algeria is taken by way of example.
•
Steps 1 and 2. The daily consumption of a family is • only related to lighting and communications (radio,
•
•
tape player, and/or small black-and-white television). With the use of energy-saving lights, the rural households require from 150 to 300 Wh per day. Step 3. The daily energy output for solar panels, depending on their location, is in the range of 4.8 kWh kW1 p in central Algeria. For a PV system, a performance ratio of 75% is assumed (this takes into account the losses in the conversion as well as the fact that the PV modules do not always work at their optimum power point). For satisfying a daily consumption of 150–300 Wh in central Algeria, the size of the PV array must be between 41 and 83 Wp. Step 4. For a system with at least 3 days of autonomy to insure against bad weather, a storage facility will be selected that can deliver 900 Wh in the case of the largest system. Those with a PV array of small size are recommended to be at a voltage level of 12 V. In addition, in such systems, lead–acid batteries are the technology option that is the best compromise between price, longevity, and availability. For the application under consideration here, the battery must deliver a capacity of at least 75 Ah before reaching its lowest acceptable state-of-charge (SoC) (in the range of 20%) at the highest possible discharge rate. Thus, a typical choice will be a 90 Ah (C10) battery at the discharge current of 9 A ( ¼ C10/10).
Besides purely technical considerations, other factors influence the size of the battery. For example, in cases
Choosing the Storage Technology Different types of storage technology can be used in RAPS systems. From a general point of view, the choice is a trade-off between different factors and characteristics such as: Financial issues: The limiting factor can be either the • investment cost or the cost of ownership, in which case
•
• • • •
the maintenance costs play a great role. Energy efficiency: In particular for systems where electricity generation is costly, the choice of a storage technology with an energy efficiency of less than 75% will mean that the PV array will have to be oversized by 25% compared with a technology with high energy efficiency. Charge retention: This factor is a combination of the energy efficiency and the self-discharge of a technology; it will show to the system designer how much energy can be drawn from the storage system after a given time. Maintenance needs: This factor can have a major impact on the cost of ownership of a system, especially in very remote areas; it includes also the replacement of the storage. Adaptation to different operating conditions: The behavior and longevity of a technology can differ considerably according to the prevailing temperature and the type of duty cycle (i.e., deep or shallow discharge and low or high rate). Safety: For certain applications, it may be required to have a sealed design, in which case a technology with a flooded electrolyte cannot be used. Recycling: In remote areas, recycling facilities for unusual technologies are difficult to find so that eventually local manufacturing must be preferred.
The different factors need to be taken into account with weighting related to their importance in the design of a given system. Some properties of the different electrochemical power sources that are presently employed in RAPS systems or are potentially suitable are summarized in Table 1. By analyzing a database of renewable energy systems, two categories of RAPS systems can be differentiated (see Table 2). These differ from each other mainly in terms of the number of cycles per year and the discharge rate. Analysis of a database for renewable energy systems collected in the frame of the European Investire project
Wh kg1 Cycles Equiv. cycles Years 1C 1C 1y5 1y5 1y5 1y5 1y5
Specific energy Cycle life at 20 1C @ 100% DoD Cycle life at 20 1C @ 10% DoD Float life at 20 1C (SoC ¼ 90%) Max. ambient charge temperature min. discharge temperature Recycling capability Health and safety Maintenance Efforts for SoC and SoH monitoring Electronic efforts (charger, safety) Remark about use in RAPS
93 5
85 1
35 55 300 1500 1000 3 000 5 15 40 60 20 10 4 5 3 4 3 4 3 4 4 5 Good availability
250
30
1 – Poor, 5 – very good. DoD, depth-of-discharge; SoC, state-of-charge; SoH, state-of-health.
h/kWh h/kW % %/month
Price (energy) Price (power) Energy efficiency Self discharge at 20 1C 98 3
1500
max.
140 180 1000 2 000 5 000 8 000 5 15 50 70 20 10 3 4 3 4 5 5 4 4 4 4 Best performance for RAPS
80 1
700
min.
min.
max.
Lithium-ion
Lead–acid
45 1500 2 000
86
800
max.
40 50 30 20 3 4 2 3 4 4 3 3 3 4 Good low T 1 behavior Toxicity of Cd
10
75
550
min.
Nickel– cadmium
Comparison of electrochemical storage technologies for use in remote-area power supply (RAPS) systems Unit
Table 1
Parameter
max.
51C 3 3 4 4 4 For seasonal storage only for large system
5
4 4 4 4 4
15
400 1000 1000 3000 70 85 >20% if connected, 0 if turned off 17 35 16 000
min.
Redox-flow vanadium
4 5 5 5 4
5 500 000 500 000 10 70
99 100
150 000
max.
401C 4 4 5 4 3 Not for energy application
0,5 100 000 100 000
50 000 250 84 60
min.
Supercaps
max.
4 5 4 4 3 4 4 5 3 4 Good for seasonal storage very low system efficiency
1500 20 30 0 if turned off
min.
Fuel-cell based system
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells 69
RAPS system 1
5% PV Sensors, data loggers, telecommunication
10–30 days o0.01 C10 0.05 C10 1 Wh to 100 kWh 20
For the two categories of RAPS system defined in Table 2, the technical suitability of different technologies was assessed. The analysis was based on technical criteria only and did not take into account the cost. Within the first category of RAPS system, which is characterized by smaller energy needs, shallow cycling, and few equivalent full cycles per year, a very low self-discharge rate is one of the key factors for suitable storage systems. The minimum requirements are fulfilled by lead–acid, lithium-ion, or redox-flow batteries. The best performance is given by lead–acid and lithium-ion batteries. The main advantages of the latter are better efficiency and lower self-discharge. The disadvantages are inferior low-temperature operation, safety concerns (power electronics demand a high level of safe management), shorter float lifetime and higher costs. Both batteries can be used for small- and large-scale applications, whereas for portable applications the lithium-ion battery would be a better option. The use of redox-flow batteries is also possible, but the minimum size of such systems would be in the range of some kWh. The second category of RAPS systems requires a high lifetime (B2000–4000 equivalent full cycles over 20 years) in combination with a relatively low self-discharge. The best matching is given by lithium-ion, lead– acid, and redox-flow batteries. One important advantage
hydride batteries, • nickel–metal batteries, • lithium-ion hydrogen storage, and fuel cell, • electrolyzer, • supercapacitors, batteries, and • ZEBRA • redox-flow batteries.
also shows that existing systems use mainly lead–acid batteries of both in the flooded-electrolyte and valve-regulated design. The second represented technology is nickel–cadmium, especially in low-temperature environments. The other technologies for which examples of RAPS exist but that are very seldom encountered are:
DoD, depth-of-discharge; PV, photovoltaic.
Autonomy Typical discharge current Typical charge current Ranging size Typical number of equivalent full cycles/ year DOD per cycle Power generator Typical applications 10–30%
PV Lower power requirements
It must also be kept in mind that testing the battery itself is not always sufficient, and that testing its operation inside the system can also be very helpful. For instance, measuring the efficiency of the battery alone or that of the storage system including the battery management system (BMS) can give very different results.
1. Perform a cycling test representative of the operating conditions, but in an accelerated way. A major difficulty is to reach a sufficient acceleration factor to allow for lifetime determination. 2. Evaluate the resistance of the batteries to the degradations occurring in the operating conditions, by using several short test procedures.
As discussed briefly above, reliability is a key factor for the successful operation of RAPS systems, since the climatic conditions are often hard and maintenance is both difficult and expensive. This means that the storage has to be selected very carefully. The requirements mentioned earlier, especially the limited and unpredictable recharge, imply specific tests, but the variety of possible load and charge profiles makes it difficult to design adequately such tests. There is a compromise to be made between designing a new test procedure each time, and using always the same test for very different cases. For a given RAPS system, two approaches are possible for testing the batteries, as follows:
Testing the Storage System
of lithium-ion systems is the high cycle-life at shallow cycles. The cycle-life data for the redox-flow battery is claimed to be adequate for use in such an application but there is not sufficient operational experience to confirm this assertion. Nickel–cadmium batteries give very good low-temperature performance, but suffer from high self-discharge. Therefore this technology should be used principally at very low temperatures, at which lead–acid and lithium-ion batteries are not suitable.
Hybrids with gen-sets Higher power requirements
1–10 days 0.02 C10 to 0.1 C10 0.05 C10 to 0.2 C10 10 Wh to 1 MWh 50–100 100–400
RAPS system 2
Two categories of remote-area power supply (RAPS) systems as determined by storage requirement
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
Table 2
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Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
71
Accelerated Cycling Tests
Degradation-Specific Tests
Accelerated cycling is the most commonly used procedure and is the basis of most existing standards. There is only one relevant international standard for testing batteries for PV systems, namely, IEC 61427: Secondary cells and batteries for solar photovoltaic energy systems – General requirements and methods of test. It applies to both lead–acid and nickel–cadmium batteries and involves separate tests of capacity, cycle-life, and mechanical integrity. For the cycle endurance test, there is no specification on the performance to be reached by the battery. At the national level, many countries have developed their own standards, e.g., Indonesia, Australia, Korea, Japan, France, and South Africa. They often include a cycling profile close to the application, with stronger constraints. The cycling profile can, however, be very different from that of the IEC 61427 standard. As an example, the IEC 61427 standard has been compared with the French standard NFC 58-510: Lead– acid secondary batteries for storing photovoltaically generated electrical energy. Both procedures involve shallow cycles that represent daily cycles superimposed on large seasonal cycles, as shown in Figure 7. Winter or bad-weather periods are represented by cycling at a low state-ofcharge (IEC) or by decreasing state-of-charge (NFC), while summer or hot periods are represented by cycling at a high state-of-charge (IEC) or increasing state-ofcharge (NFC). In both cases, the relation between the number of cycles achieved and the lifetime of the battery in a real installation is not clearly established. The test procedures allow batteries to be compared against each other or with previously tested ones. On the other hand, testing identical batteries with the different procedures allows comparison of the acceleration factors of these procedures. One drawback of the procedures is that they are very time consuming. For instance, testing tubular lead– acid batteries with the IEC 61427 procedure can take more than 2 years before some capacity loss begins to appear.
This second approach is less common and requires a good knowledge of the modes of battery degradation and of their occurrence in the field depending on the operating conditions. Using these observations, it is then possible to design specific procedures to induce each type of degradation, and to choose the procedures to be used according to the application. Two examples of such procedures are given below. They are more specific to degradations encountered in lead–acid batteries. Energy efficiency at low state-of-charge
In RAPS systems, as in all systems based on renewable energy, minimizing energy losses is a very important issue, especially in case of bad weather. Dealing with the storage part of the system, this means optimizing its efficiency, particularly at low SoC. The efficiency can be very variable, even for batteries of similar construction. One test procedure allows determination of energy efficiency at low SoC and is included in the standard IEC 62093: Balance-of-systems components for photovoltaic systems – design qualification. The test consists of cycling between 0 and 50% SoC, and measuring the ratio between the charged and the discharged energy (Figure 8). The test duration is approximately 1 week. Recovery from electrolyte stratification
Flooded lead–acid batteries are very common in PV RAPS systems, and they are particularly sensitive to electrolyte stratification, which occurs when they do not reach full charge often enough. This phenomenon is reversible with a good recharge, but only if it is recent. The proposed procedure (Figure 9) consists successively of (1) five cycles with limited recharge, for establishing stratification, and (2) five cycles with full recharge, for homogenizing the electrolyte again. Moreover, it has been observed that the behavior of the battery over these 10 cycles, and especially the final capacity measurement, seems to be correlated with its lifetime.
1 cycle
4 cycles
100 % State of charge
State of charge
Summer
Time
Winter
Time
Figure 7 Comparison of IEC 61427 and NFC 58-510 cycling test profiles (schematic evolution of state-of-charge (SoC) with time).
SoC voltage
NFC 58-510
IEC 61427
50 %
1.8 Vpc Time
Figure 8 ‘Efficiency at low state-of-charge (SoC)’ test procedure.
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Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
2.35 Vc−1
1.75 Vc−1 Recharge factor = 1.2×C10 Time Three types of discharge: Operating capacity loss Recovery from stratification Irreversible capacity loss 1
Figure 9 ‘Recovery from stratification’ test procedure. Vc , volts per cycle.
System-Level Tests Testing the battery alone is not sufficient to ensure its proper operation in the system; it also has to be well matched to the charge controller and to the whole system. For instance, in one project an efficiency loss as high as 15% has been measured when a lithium-ion battery was associated to a BMS that was functional, but not optimized energetically. The IEC 62124 standard: Photovoltaic (PV) stand alone systems – Design verification consists in testing the whole system under actual or simulated solar irradiation conditions. The test sequence is shown schematically in Figure 10. Two different tests are performed consecutively (functional test and recovery test), with capacity tests before and after each test. The capacity tests consist of discharging the battery (by switching on the loads) until stopped by the charge controller. During the functional test, the system is operated at its nominal load for several days, starting with a fully charged battery. The recovery test is then performed in the same way, but starting with a discharged battery. Analyzing the obtained data gives information about several important issues, namely, (1) the proper behavior of the system (continuity of user supply), (2) its adequate design according to the climate of the place where it is planned to be installed, and (3) the analysis of the energy losses in the different parts of the system: Summary Research is still required in the area of testing and selecting storage for RAPS. The specific targets are:
• •
reduction of the duration of the tests, and thus of their cost, validation of the link between the test results and the field performance and lifetime, which means largescale field validation operations, and
Usable battery capacity #0
Functional test
Usable battery capacity #1
Recovery test
Usable battery capacity #2
Figure 10 Test sequence for IEC 62124 standard.
further development of system-level tests, to charac• terize the battery inside the system. Another point is that the existing procedures are designed almost exclusively for lead–acid batteries, with some cases adjustments for nickel–cadmium batteries. The introduction of other storage technologies will require an adaptation of the test procedures or the design of new procedures focused on the application requirements so that technology-neutral evaluation can be performed.
Energy Management A unique feature of the RAPS application is that charging depends not only on the energy management strategy, but also on the energy availability due to weather conditions. In this context, user service has to be ensured
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
as much as possible so that sometimes a trade-off must be made between security of supply and battery life. Optimizing the energy management strategy is crucial, since it directly influences not only the amount of energy available to the user but also the battery lifetime. As shown in Figure 6, while the investment cost for the battery is a small part of the overall system cost (in the range of 25%), a calculation over 20 years of system life shows that the batteries amount to as much as 50% of the cost of ownership due to their short life. Energy management in RAPS systems is ensured by a charge controller, also named BMS or sometimes an energy management system (EMS). This device limits the operation of the storage in a suitable SoC domain. All commercial controllers are designed for lead–acid, or sometimes nickel–cadmium batteries, and are based on measurement of the battery voltage and current and on the use of voltage thresholds. Some controllers use other energy management strategies like ampere-hours counting or SoC calculation, but they are still under development or very uncommon and therefore are not discussed here in detail. Most of the controllers use four voltage thresholds (Figure 11):
Battery voltage (V)
HVD HVR
LVR LVD
Time
high voltage disconnect (HVD), where the battery • charge is stopped, low voltage disconnect (LVD), where the battery • discharge is stopped, two intermediate thresholds, high voltage reconnect • (HVR) and low voltage reconnect (LVR), which allow for the battery reconnection after sufficient relaxation. The reconnection thresholds are determined by a method that takes into account the system sizing. The main types of voltage-based energy management strategies are the on/off, floating, and pulse-width modulation (PWM) strategies. Boost charge can also be included, as well as maximum power-point tracking (MPPT). On/Off Management This simple strategy consists in completely disconnecting the storage at the prescribed thresholds, and reconnecting it only when the accepted thresholds are reached after relaxation. Figure 12 illustrates the direct impact of the HVD threshold value on the daily available energy in this type of system. Determining the best value for the different voltage thresholds is very difficult, and they should ideally be adapted during the life of the battery, to take into account its change in behavior with aging. Float Charge Management In this case, a low charge current is still allowed when the voltage has reached the disconnection thresholds. This current is managed so that the battery voltage remains almost constant. There is no more need for reconnection thresholds. Boost Charge
Figure 11 Four voltage thresholds commonly used by remotearea power supply (RAPS) charge controllers. HVD, high-voltage disconnect; HVR, high-voltage reconnect; LVD, low-voltage disconnect; HVR, high-voltage reconnect.
For lead–acid batteries, this strategy consists in allowing periodically for a stronger recharge, by temporarily increasing the HVD, for instance, from 14.4 to 14.8 V for a
100 80 Capacity (%)
73
11/15.0 V 60 11/14.0 V 40 11/14.5 V 20 0 1
3
5
7
9
11
Cycle number
Figure 12 Influence of high-value disconnect (HVD) value, with on/off management, on available capacity.
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Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
12 V vented battery. The battery recharge is then more complete, but using this threshold value each time is not acceptable, because it induces higher water consumption and grid corrosion. Therefore the optimized procedure is to apply boost charges once in a month. Several charge controllers use this function, with important differences from one to another: The boost charge can be performed only initially, or periodically at a fixed frequency, or started manually. In renewable-only RAPS, the drawback of this periodical treatment is that the required energy is not always available when the boost charge is started. The frequency of occurrence of the boost charge, its duration, and the value of the voltage threshold are often determined empirically. Pulse-Width Modulation On/off charge controllers directly use the charge current produced by the energy source. Some studies are being performed, on lead–acid batteries again, on different ways to modulate the current by varying its period. This method is called pulse-width modulation (PWM), and appears to minimize the gassing and water consumption, and to improve the charging efficiency. Studies are in progress for optimizing the parameters of this method, and for understanding the physicochemical phenomena that lead to an improvement of battery life by using PWM management. Perspectives In addition to these different methods, some innovative management strategies are in development, based, for instance, on the estimation of the SoC and/or state-ofhealth of the battery. The problem of setting the adequate charge parameters for each state of the battery would then be solved. Managing the end of discharge by using nonvoltagebased criteria is also a present research topic. In addition, all the energy management strategies described above have been mostly designed for lead–acid batteries. Using other storage technologies will require the development of associated management strategies.
Trends for Storage in Remote-Area Power Supply Systems To improve living standards, particularly in remote areas, it is necessary to have access to electricity. Therefore, the market for RAPS systems is in expansion and will stay that way because of the growth in world population. As stated earlier, in order to match generation and demand in RAPS systems where generation is based on intermittent sources, storage is needed. Only in some
rare cases is the storage based on alternatives to electrochemical power sources, e.g., hydro-pumping and compressed air. At present, the storage technology predominantly used in RAPS systems is the lead–acid battery because of its maturity, reliability, and affordability; this situation is unlikely to change for some years to come. In specific applications, however, other technologies can be encountered such as nickel–cadmium batteries in very cold climates, and increasing numbers of lithium-ion batteries when reliability is crucial. Analysis of the purely technical parameters shows that lithium-ion batteries are the best candidate in the RAPS applications due to their low self-discharge, high flexibility in the conditions of use regarding their SoC, and high energy efficiency at the cell level. Nevertheless, in order to provide maximum safety, lithium-ion batteries require sophisticated power electronics in order to manage the cell voltage, as well as thermal management; these features impact on energy efficiency. In addition, lithium-ion batteries are still 10 times more expensive than lead–acid batteries. In the near future, however, it is highly probable that lithiumion technology will undergo tremendous improvement in terms of increased safety and reduced cost and will be encountered more and more in RAPS facilities, especially in industrial applications. For domestic applications, small systems will use lead– acid batteries because of economical constraints. Nevertheless, the reliability of this technology will grow thanks to advanced management algorithms such as the limitation of the daily amount of energy available to the user. Rural electrification will also rely on small grids at the village level with various power sources to meet a greater demand for energy. In such configurations, banks of tubular plate lead–acid batteries, which have proven to be more robust for cyclic use than flat-plate counterparts, are increasingly favored, with a growing use of the gel valve-regulated type due to its lower maintenance requirement. Other technologies may become competitive, in particular redox-flow batteries once optimization of the stack longevity is achieved and costs have come down due to economies of scale. The combination of electrolyzer, hydrogen storage, and fuel cell may be a further option for RAPS facilities in certain situations where overall system efficiency is not critical and special features such as seasonal storage are more important.
Nomenclature Symbols and Units T Vpc Wp
temperature Volts per cell peak power out (watt)
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
Abbreviations and Acronyms AC BMS DC DoD EMS EPIA HVD HVR IEC LVD LVR MPPT NFC PV PWM RAPS SoC SoH
alternating current battery management system direct current depth-of-discharge energy management system European Photovoltaic Industry Association high-voltage disconnect high-voltage reconnect International Electrotechnical Commission low-voltage disconnect low-voltage reconnect maximum power-point tracking Norme Franc¸aise de se´rie C (French Standard of category C) photovoltaic pulse-width modulation remote-area power supply state-of-charge state-of-health
See also: Applications – Stationary: Fuel Cells; Batteries: Charging Methods; Codes and Standards; Lifetime Prediction; Partial-State-of-Charge; Secondary
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Batteries – Flow Systems: Overview; Secondary Batteries – High Temperature Systems: Sodium–Nickel Chloride; Secondary Batteries – Lead–Acid Systems: Overview; Secondary Batteries – Lithium Rechargeable Systems: Overview; Secondary Batteries – Nickel Systems: Nickel–Cadmium: Overview; Nickel–Metal Hydride: Overview.
Further Reading ACTUS project team (2005) Final technical publishable report: Specific accelerated test procedure for PV batteries with easy transfer to various kinds of systems and for quality control. European Project ENK6-2000-00069. Adeoti O, Oyewole BA, and Adegboyega TD (2001) Solar photovoltaicbased home electrification system for rural development in Nigeria: Domestic load assessment. Renewable Energy 24: 155--161. Benchetrite D, Le Gall M, Bach O, Perrin M, and Mattera F (2005) Optimisation of charge parameters for lead–acid batteries used in photovoltaic systems. Journal of Power Sources 144: 346--351. EPIA (2008) Solar Generation Report. EPIA (European PV Association). www.epia.org International Energy Agency (2005a) Trends in photovoltaic applications. Survey Report of Selected IEA Countries between 1992 and 2004. Paris, France: IEA publications. International Energy Agency (2005b) World Energy Outlook 2004. Paris, France: IEA Publications. Labouret A and Villoz M (2003) Energie solaire photovoltaı¨que, le manuel du professionnel. Paris: Dunod. ISBN 2100056107. Moseley PT (2006) Energy storage in remote-area power supply (RAPS) systems. Journal of Power Sources 155: 83--87.
Definition and Specificities of Remote-Area Power Supply Electricity supply is most commonly ensured by power stations connected to transmission and distribution networks. Some specific solutions need to be found, however, for providing power in isolated locations, i.e., those not serviced by the main grid. Such facilities are generally referred to as remote-area power supply (RAPS), standalone, or off-grid systems. These systems are used to provide electrical power in many applications, e.g., houses, community services, electric fences, water pumping, and telecommunications. In some cases, however, they are not even located in remote places, e.g., for very small-scale urban applications such as parking ticket machines and lighting. The latter option can prove less expensive than connection to the main grid. The acronym RAPS addresses in fact various types of systems, from an individual lighting kit to a multiuser mini-grid system. The energy can be supplied from a single source or a mix of different sources, both nonrenewable (e.g., diesel generator) and renewable (e.g., wind turbine, photovoltaic (PV) panels, and micro hydro turbine). A system that includes several types of energy sources is called a hybrid system. Figures 1–13 show examples of RAPS system. In all types of RAPS system (except diesel-only systems that are the most implemented solution), energy storage is necessary in order to match the energy production to the needs of users. Figure 4 shows two examples of energy
production and consumption over 1 day for two RAPS systems, namely, a PV lighting kit and a hybrid system (PV þ diesel). In the first case, electricity generation takes place only during the day, and major consumption occurs early in the morning and in the evening for lighting and television reception. In the second case, the diesel generator provides additional generation in the evening, and the consumption is more distributed over the day, owing to other types of electrical equipment. Nontechnical issues play an important role in the implementation of RAPS systems. Depending on the location of the installation, the issues can include: formalities, • administrative import taxes, • system ownership (user or energy company), • collection of energy fees, • social acceptance aspects, and • user care of the system. • Such aspects will not be discussed here, since they are not specifically related to the energy storage function. Nevertheless, apart from the performance of the system, nontechnical issues can lead to the success or failure of the operation. In summary, a RAPS system generally consists of the following components (Figure 5): power generation equipment (photovoltaic panels, wind • turbine, micro hydro turbine, diesel generator, etc.), energy storage system, •
Figure 1 Remote-area power supply photovoltaic (RAPS PV) system powering an isolated house in the south of France: (a) solar panels and (b) storage system.
65
66
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
control and conversion equipment (charge controller • and sine-wave inverter), and direct current (DC) or alternating current (AC) loads. •
The generated electricity is fed into the storage system using a charge controller, and can then be used directly for DC loads or, through an inverter, for AC loads. In a RAPS system, the energy storage system is subjected to specific constraints, mainly related to the limited availability of energy for charging, i.e., the charge is dependent on the meteorological conditions (sunshine, wind, etc.). Storage is often undertaken by lead–acid batteries, but alternative technologies can be advantageous in some situations, as discussed in the section ‘Choosing the Storage Technology.’
Market for Remote-Area Power Supply Systems The market for RAPS systems is divided into many segments according to the type of service, the size of the system, and, possibly, the location. In the same way, the specific market for electrochemical power sources has different segments, as dictated by the size of the system and the constraints placed on the battery. Generally
Figure 2 Large domestic remote-area power supply (RAPS) system in Greece with wind turbine and photovoltaic (PV) panels. Source: Conergy AG.
Figure 3 Remote-area power supply (RAPS) system for telecommunications in Brazil. Source: Conergy AG.
25 000
50
20 000
Power (W)
Power (W)
40
Production Consumption
30 20
15 000 10 000 5000
10 0 0:00
Production Consumption
6:00
12:00 Time (h)
18:00
0:00
0 0:00
6:00
12:00
18:00
0:00
Time (h)
Figure 4 Examples of energy production and consumption over 1 day for two remote-area power supply (RAPS) systems: photovoltaic (PV) lighting kit (left), and hybrid system with PV and diesel (right).
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
PV modules Batteries
67
Balance of system Others
Figure 6 Typical cost breakdown of a stand-alone photovoltaic (PV) system. Left: Investment cost. Right: Cost of ownership over 20 years.
BMS
=
= =
~
Storage Storage system
Figure 5 General architecture of a remote-area power supply (RAPS) system. BMS, battery management system.
speaking, the two main markets for RAPS systems are for domestic and industrial services. The domestic market for RAPS systems addresses rural electrification and will be described here as the offgrid habitation segment. In this application, the systems fulfill needs such as lighting and television and the market was dominated by the Western countries until the beginning of the millennium. Since then, the habitation market in the developed countries, and in particular in Europe, has slowed down because the majority of the remote homes have been equipped with RAPS facilities. By contrast, a large market for rural electrification of habitation is growing in developing countries. According to the World Energy Outlook of 2006, 1.577 billion people did not have access to electricity in their homes; this represents a little below a quarter of the world population. The population is mainly in rural areas since electrification is estimated to be less than 62% in rural areas as compared to 90.4% in urban areas. The largest ratio of population without access to electricity is located in southern Asia and sub-Saharan Africa. In these regions, the energy source for RAPS is expected to be mainly photovoltaic and according to the European Photovoltaic Industry Association (EPIA), ‘‘in the nonindustrialized world approximately 30 GWp of solar capacity is expected to have been installed by 2020 in the rural electrification sector.’’ The industrial market for RAPS deals with applications such as telecommunications, water pumping, vaccine refrigeration, and navigational aids, or with the powering of small industry. This market is the one that is the most self-sustaining and that partly grows without subsidies. For stand-alone PV systems (PV-RAPS), the worldwide off-grid domestic market in 2002 was 30 MWp,
whereas the off-grid commercial market was 70 MWp, This market has been growing by 10% each year over the last 10 years, and in 2005, 15% of the worldwide demand for PV modules (1.389 GWp) was dedicated to remote industrial (8%) and remote habitation (7%) applications. In PV-RAPS systems, the cost for the storage amounts to one-quarter of the overall investment (i.e., about 3h per Wp with respect to an average of 12h per Wp for system costs). As shown in Figure 6, however, the share of the batteries, in the cost of ownership over 20 years of running a PV-RAPS system, amounts for as much as 50% due to their shorter life. Therefore, a huge potential exists for electrochemical power sources in the RAPS market, both in the original equipment and the replacement markets. Globally, the market for off-grid PV systems is expected to grow to 130 GWp in 2030 with a share of 70 GWp for industrial applications and 60 GWp for rural electrification.
Designing a Remote-Area Power Supply System The design and sizing of a RAPS system starts with the energy needs of the user. 1. First, the system designer must place a strong focus on energy saving through the use of efficient electrical appliances and optimized energy management, as well as the choice of the best system architecture and components. Energy-saving appliances may be more expensive at the time of investment. Nevertheless, since 1 kWh of electricity generated from a RAPS system costs 0.30–1h, the cost difference is very quickly regained. 2. Next, the demand for energy is evaluated. This may be very easy in some cases (e.g., industrial system with well-known consumption) but considerably more difficult in others, especially on account of the ‘human factor’ whereby user behavior cannot be controlled and tends to evolve. This evolution favors a modular design of RAPS system for sizing adjustment. 3. The third step is to decide on the energy mix to be supplied, i.e., either directly to the user when
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Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
production and consumption are synchronous, or through the storage unit. Although generator sets are the most immediate solution, renewable energy alternatives will mostly be more profitable in the long term, e.g., PV, wind, or small hydro, when considering all economical aspects including the cost and transportation of fuel and system maintenance requirements. The size of the generator is calculated as a function of the renewable energy resource as well as of the productivity of both the generator and the system. 4. The calculation made in the third step allows design of the storage system. The size of the storage system is a multiplication of the number of days of autonomy that are needed (or more precisely, number of consecutive days in the year where the renewable energy source is not available) by the daily energy needs. This, then, is the amount of energy that must be delivered until reaching the maximum depth-of-discharge (DoD) that is acceptable for the chosen storage technology.
where maintenance is very difficult or where high initial investment is possible, the battery can be oversized in order to extend its longevity.
To illustrate the above four steps that are involved in RAPS sizing, the following system for a home in central Algeria is taken by way of example.
•
Steps 1 and 2. The daily consumption of a family is • only related to lighting and communications (radio,
•
•
tape player, and/or small black-and-white television). With the use of energy-saving lights, the rural households require from 150 to 300 Wh per day. Step 3. The daily energy output for solar panels, depending on their location, is in the range of 4.8 kWh kW1 p in central Algeria. For a PV system, a performance ratio of 75% is assumed (this takes into account the losses in the conversion as well as the fact that the PV modules do not always work at their optimum power point). For satisfying a daily consumption of 150–300 Wh in central Algeria, the size of the PV array must be between 41 and 83 Wp. Step 4. For a system with at least 3 days of autonomy to insure against bad weather, a storage facility will be selected that can deliver 900 Wh in the case of the largest system. Those with a PV array of small size are recommended to be at a voltage level of 12 V. In addition, in such systems, lead–acid batteries are the technology option that is the best compromise between price, longevity, and availability. For the application under consideration here, the battery must deliver a capacity of at least 75 Ah before reaching its lowest acceptable state-of-charge (SoC) (in the range of 20%) at the highest possible discharge rate. Thus, a typical choice will be a 90 Ah (C10) battery at the discharge current of 9 A ( ¼ C10/10).
Besides purely technical considerations, other factors influence the size of the battery. For example, in cases
Choosing the Storage Technology Different types of storage technology can be used in RAPS systems. From a general point of view, the choice is a trade-off between different factors and characteristics such as: Financial issues: The limiting factor can be either the • investment cost or the cost of ownership, in which case
•
• • • •
the maintenance costs play a great role. Energy efficiency: In particular for systems where electricity generation is costly, the choice of a storage technology with an energy efficiency of less than 75% will mean that the PV array will have to be oversized by 25% compared with a technology with high energy efficiency. Charge retention: This factor is a combination of the energy efficiency and the self-discharge of a technology; it will show to the system designer how much energy can be drawn from the storage system after a given time. Maintenance needs: This factor can have a major impact on the cost of ownership of a system, especially in very remote areas; it includes also the replacement of the storage. Adaptation to different operating conditions: The behavior and longevity of a technology can differ considerably according to the prevailing temperature and the type of duty cycle (i.e., deep or shallow discharge and low or high rate). Safety: For certain applications, it may be required to have a sealed design, in which case a technology with a flooded electrolyte cannot be used. Recycling: In remote areas, recycling facilities for unusual technologies are difficult to find so that eventually local manufacturing must be preferred.
The different factors need to be taken into account with weighting related to their importance in the design of a given system. Some properties of the different electrochemical power sources that are presently employed in RAPS systems or are potentially suitable are summarized in Table 1. By analyzing a database of renewable energy systems, two categories of RAPS systems can be differentiated (see Table 2). These differ from each other mainly in terms of the number of cycles per year and the discharge rate. Analysis of a database for renewable energy systems collected in the frame of the European Investire project
Wh kg1 Cycles Equiv. cycles Years 1C 1C 1y5 1y5 1y5 1y5 1y5
Specific energy Cycle life at 20 1C @ 100% DoD Cycle life at 20 1C @ 10% DoD Float life at 20 1C (SoC ¼ 90%) Max. ambient charge temperature min. discharge temperature Recycling capability Health and safety Maintenance Efforts for SoC and SoH monitoring Electronic efforts (charger, safety) Remark about use in RAPS
93 5
85 1
35 55 300 1500 1000 3 000 5 15 40 60 20 10 4 5 3 4 3 4 3 4 4 5 Good availability
250
30
1 – Poor, 5 – very good. DoD, depth-of-discharge; SoC, state-of-charge; SoH, state-of-health.
h/kWh h/kW % %/month
Price (energy) Price (power) Energy efficiency Self discharge at 20 1C 98 3
1500
max.
140 180 1000 2 000 5 000 8 000 5 15 50 70 20 10 3 4 3 4 5 5 4 4 4 4 Best performance for RAPS
80 1
700
min.
min.
max.
Lithium-ion
Lead–acid
45 1500 2 000
86
800
max.
40 50 30 20 3 4 2 3 4 4 3 3 3 4 Good low T 1 behavior Toxicity of Cd
10
75
550
min.
Nickel– cadmium
Comparison of electrochemical storage technologies for use in remote-area power supply (RAPS) systems Unit
Table 1
Parameter
max.
51C 3 3 4 4 4 For seasonal storage only for large system
5
4 4 4 4 4
15
400 1000 1000 3000 70 85 >20% if connected, 0 if turned off 17 35 16 000
min.
Redox-flow vanadium
4 5 5 5 4
5 500 000 500 000 10 70
99 100
150 000
max.
401C 4 4 5 4 3 Not for energy application
0,5 100 000 100 000
50 000 250 84 60
min.
Supercaps
max.
4 5 4 4 3 4 4 5 3 4 Good for seasonal storage very low system efficiency
1500 20 30 0 if turned off
min.
Fuel-cell based system
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells 69
RAPS system 1
5% PV Sensors, data loggers, telecommunication
10–30 days o0.01 C10 0.05 C10 1 Wh to 100 kWh 20
For the two categories of RAPS system defined in Table 2, the technical suitability of different technologies was assessed. The analysis was based on technical criteria only and did not take into account the cost. Within the first category of RAPS system, which is characterized by smaller energy needs, shallow cycling, and few equivalent full cycles per year, a very low self-discharge rate is one of the key factors for suitable storage systems. The minimum requirements are fulfilled by lead–acid, lithium-ion, or redox-flow batteries. The best performance is given by lead–acid and lithium-ion batteries. The main advantages of the latter are better efficiency and lower self-discharge. The disadvantages are inferior low-temperature operation, safety concerns (power electronics demand a high level of safe management), shorter float lifetime and higher costs. Both batteries can be used for small- and large-scale applications, whereas for portable applications the lithium-ion battery would be a better option. The use of redox-flow batteries is also possible, but the minimum size of such systems would be in the range of some kWh. The second category of RAPS systems requires a high lifetime (B2000–4000 equivalent full cycles over 20 years) in combination with a relatively low self-discharge. The best matching is given by lithium-ion, lead– acid, and redox-flow batteries. One important advantage
hydride batteries, • nickel–metal batteries, • lithium-ion hydrogen storage, and fuel cell, • electrolyzer, • supercapacitors, batteries, and • ZEBRA • redox-flow batteries.
also shows that existing systems use mainly lead–acid batteries of both in the flooded-electrolyte and valve-regulated design. The second represented technology is nickel–cadmium, especially in low-temperature environments. The other technologies for which examples of RAPS exist but that are very seldom encountered are:
DoD, depth-of-discharge; PV, photovoltaic.
Autonomy Typical discharge current Typical charge current Ranging size Typical number of equivalent full cycles/ year DOD per cycle Power generator Typical applications 10–30%
PV Lower power requirements
It must also be kept in mind that testing the battery itself is not always sufficient, and that testing its operation inside the system can also be very helpful. For instance, measuring the efficiency of the battery alone or that of the storage system including the battery management system (BMS) can give very different results.
1. Perform a cycling test representative of the operating conditions, but in an accelerated way. A major difficulty is to reach a sufficient acceleration factor to allow for lifetime determination. 2. Evaluate the resistance of the batteries to the degradations occurring in the operating conditions, by using several short test procedures.
As discussed briefly above, reliability is a key factor for the successful operation of RAPS systems, since the climatic conditions are often hard and maintenance is both difficult and expensive. This means that the storage has to be selected very carefully. The requirements mentioned earlier, especially the limited and unpredictable recharge, imply specific tests, but the variety of possible load and charge profiles makes it difficult to design adequately such tests. There is a compromise to be made between designing a new test procedure each time, and using always the same test for very different cases. For a given RAPS system, two approaches are possible for testing the batteries, as follows:
Testing the Storage System
of lithium-ion systems is the high cycle-life at shallow cycles. The cycle-life data for the redox-flow battery is claimed to be adequate for use in such an application but there is not sufficient operational experience to confirm this assertion. Nickel–cadmium batteries give very good low-temperature performance, but suffer from high self-discharge. Therefore this technology should be used principally at very low temperatures, at which lead–acid and lithium-ion batteries are not suitable.
Hybrids with gen-sets Higher power requirements
1–10 days 0.02 C10 to 0.1 C10 0.05 C10 to 0.2 C10 10 Wh to 1 MWh 50–100 100–400
RAPS system 2
Two categories of remote-area power supply (RAPS) systems as determined by storage requirement
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
Table 2
70
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
71
Accelerated Cycling Tests
Degradation-Specific Tests
Accelerated cycling is the most commonly used procedure and is the basis of most existing standards. There is only one relevant international standard for testing batteries for PV systems, namely, IEC 61427: Secondary cells and batteries for solar photovoltaic energy systems – General requirements and methods of test. It applies to both lead–acid and nickel–cadmium batteries and involves separate tests of capacity, cycle-life, and mechanical integrity. For the cycle endurance test, there is no specification on the performance to be reached by the battery. At the national level, many countries have developed their own standards, e.g., Indonesia, Australia, Korea, Japan, France, and South Africa. They often include a cycling profile close to the application, with stronger constraints. The cycling profile can, however, be very different from that of the IEC 61427 standard. As an example, the IEC 61427 standard has been compared with the French standard NFC 58-510: Lead– acid secondary batteries for storing photovoltaically generated electrical energy. Both procedures involve shallow cycles that represent daily cycles superimposed on large seasonal cycles, as shown in Figure 7. Winter or bad-weather periods are represented by cycling at a low state-ofcharge (IEC) or by decreasing state-of-charge (NFC), while summer or hot periods are represented by cycling at a high state-of-charge (IEC) or increasing state-ofcharge (NFC). In both cases, the relation between the number of cycles achieved and the lifetime of the battery in a real installation is not clearly established. The test procedures allow batteries to be compared against each other or with previously tested ones. On the other hand, testing identical batteries with the different procedures allows comparison of the acceleration factors of these procedures. One drawback of the procedures is that they are very time consuming. For instance, testing tubular lead– acid batteries with the IEC 61427 procedure can take more than 2 years before some capacity loss begins to appear.
This second approach is less common and requires a good knowledge of the modes of battery degradation and of their occurrence in the field depending on the operating conditions. Using these observations, it is then possible to design specific procedures to induce each type of degradation, and to choose the procedures to be used according to the application. Two examples of such procedures are given below. They are more specific to degradations encountered in lead–acid batteries. Energy efficiency at low state-of-charge
In RAPS systems, as in all systems based on renewable energy, minimizing energy losses is a very important issue, especially in case of bad weather. Dealing with the storage part of the system, this means optimizing its efficiency, particularly at low SoC. The efficiency can be very variable, even for batteries of similar construction. One test procedure allows determination of energy efficiency at low SoC and is included in the standard IEC 62093: Balance-of-systems components for photovoltaic systems – design qualification. The test consists of cycling between 0 and 50% SoC, and measuring the ratio between the charged and the discharged energy (Figure 8). The test duration is approximately 1 week. Recovery from electrolyte stratification
Flooded lead–acid batteries are very common in PV RAPS systems, and they are particularly sensitive to electrolyte stratification, which occurs when they do not reach full charge often enough. This phenomenon is reversible with a good recharge, but only if it is recent. The proposed procedure (Figure 9) consists successively of (1) five cycles with limited recharge, for establishing stratification, and (2) five cycles with full recharge, for homogenizing the electrolyte again. Moreover, it has been observed that the behavior of the battery over these 10 cycles, and especially the final capacity measurement, seems to be correlated with its lifetime.
1 cycle
4 cycles
100 % State of charge
State of charge
Summer
Time
Winter
Time
Figure 7 Comparison of IEC 61427 and NFC 58-510 cycling test profiles (schematic evolution of state-of-charge (SoC) with time).
SoC voltage
NFC 58-510
IEC 61427
50 %
1.8 Vpc Time
Figure 8 ‘Efficiency at low state-of-charge (SoC)’ test procedure.
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Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
2.35 Vc−1
1.75 Vc−1 Recharge factor = 1.2×C10 Time Three types of discharge: Operating capacity loss Recovery from stratification Irreversible capacity loss 1
Figure 9 ‘Recovery from stratification’ test procedure. Vc , volts per cycle.
System-Level Tests Testing the battery alone is not sufficient to ensure its proper operation in the system; it also has to be well matched to the charge controller and to the whole system. For instance, in one project an efficiency loss as high as 15% has been measured when a lithium-ion battery was associated to a BMS that was functional, but not optimized energetically. The IEC 62124 standard: Photovoltaic (PV) stand alone systems – Design verification consists in testing the whole system under actual or simulated solar irradiation conditions. The test sequence is shown schematically in Figure 10. Two different tests are performed consecutively (functional test and recovery test), with capacity tests before and after each test. The capacity tests consist of discharging the battery (by switching on the loads) until stopped by the charge controller. During the functional test, the system is operated at its nominal load for several days, starting with a fully charged battery. The recovery test is then performed in the same way, but starting with a discharged battery. Analyzing the obtained data gives information about several important issues, namely, (1) the proper behavior of the system (continuity of user supply), (2) its adequate design according to the climate of the place where it is planned to be installed, and (3) the analysis of the energy losses in the different parts of the system: Summary Research is still required in the area of testing and selecting storage for RAPS. The specific targets are:
• •
reduction of the duration of the tests, and thus of their cost, validation of the link between the test results and the field performance and lifetime, which means largescale field validation operations, and
Usable battery capacity #0
Functional test
Usable battery capacity #1
Recovery test
Usable battery capacity #2
Figure 10 Test sequence for IEC 62124 standard.
further development of system-level tests, to charac• terize the battery inside the system. Another point is that the existing procedures are designed almost exclusively for lead–acid batteries, with some cases adjustments for nickel–cadmium batteries. The introduction of other storage technologies will require an adaptation of the test procedures or the design of new procedures focused on the application requirements so that technology-neutral evaluation can be performed.
Energy Management A unique feature of the RAPS application is that charging depends not only on the energy management strategy, but also on the energy availability due to weather conditions. In this context, user service has to be ensured
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
as much as possible so that sometimes a trade-off must be made between security of supply and battery life. Optimizing the energy management strategy is crucial, since it directly influences not only the amount of energy available to the user but also the battery lifetime. As shown in Figure 6, while the investment cost for the battery is a small part of the overall system cost (in the range of 25%), a calculation over 20 years of system life shows that the batteries amount to as much as 50% of the cost of ownership due to their short life. Energy management in RAPS systems is ensured by a charge controller, also named BMS or sometimes an energy management system (EMS). This device limits the operation of the storage in a suitable SoC domain. All commercial controllers are designed for lead–acid, or sometimes nickel–cadmium batteries, and are based on measurement of the battery voltage and current and on the use of voltage thresholds. Some controllers use other energy management strategies like ampere-hours counting or SoC calculation, but they are still under development or very uncommon and therefore are not discussed here in detail. Most of the controllers use four voltage thresholds (Figure 11):
Battery voltage (V)
HVD HVR
LVR LVD
Time
high voltage disconnect (HVD), where the battery • charge is stopped, low voltage disconnect (LVD), where the battery • discharge is stopped, two intermediate thresholds, high voltage reconnect • (HVR) and low voltage reconnect (LVR), which allow for the battery reconnection after sufficient relaxation. The reconnection thresholds are determined by a method that takes into account the system sizing. The main types of voltage-based energy management strategies are the on/off, floating, and pulse-width modulation (PWM) strategies. Boost charge can also be included, as well as maximum power-point tracking (MPPT). On/Off Management This simple strategy consists in completely disconnecting the storage at the prescribed thresholds, and reconnecting it only when the accepted thresholds are reached after relaxation. Figure 12 illustrates the direct impact of the HVD threshold value on the daily available energy in this type of system. Determining the best value for the different voltage thresholds is very difficult, and they should ideally be adapted during the life of the battery, to take into account its change in behavior with aging. Float Charge Management In this case, a low charge current is still allowed when the voltage has reached the disconnection thresholds. This current is managed so that the battery voltage remains almost constant. There is no more need for reconnection thresholds. Boost Charge
Figure 11 Four voltage thresholds commonly used by remotearea power supply (RAPS) charge controllers. HVD, high-voltage disconnect; HVR, high-voltage reconnect; LVD, low-voltage disconnect; HVR, high-voltage reconnect.
For lead–acid batteries, this strategy consists in allowing periodically for a stronger recharge, by temporarily increasing the HVD, for instance, from 14.4 to 14.8 V for a
100 80 Capacity (%)
73
11/15.0 V 60 11/14.0 V 40 11/14.5 V 20 0 1
3
5
7
9
11
Cycle number
Figure 12 Influence of high-value disconnect (HVD) value, with on/off management, on available capacity.
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Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
12 V vented battery. The battery recharge is then more complete, but using this threshold value each time is not acceptable, because it induces higher water consumption and grid corrosion. Therefore the optimized procedure is to apply boost charges once in a month. Several charge controllers use this function, with important differences from one to another: The boost charge can be performed only initially, or periodically at a fixed frequency, or started manually. In renewable-only RAPS, the drawback of this periodical treatment is that the required energy is not always available when the boost charge is started. The frequency of occurrence of the boost charge, its duration, and the value of the voltage threshold are often determined empirically. Pulse-Width Modulation On/off charge controllers directly use the charge current produced by the energy source. Some studies are being performed, on lead–acid batteries again, on different ways to modulate the current by varying its period. This method is called pulse-width modulation (PWM), and appears to minimize the gassing and water consumption, and to improve the charging efficiency. Studies are in progress for optimizing the parameters of this method, and for understanding the physicochemical phenomena that lead to an improvement of battery life by using PWM management. Perspectives In addition to these different methods, some innovative management strategies are in development, based, for instance, on the estimation of the SoC and/or state-ofhealth of the battery. The problem of setting the adequate charge parameters for each state of the battery would then be solved. Managing the end of discharge by using nonvoltagebased criteria is also a present research topic. In addition, all the energy management strategies described above have been mostly designed for lead–acid batteries. Using other storage technologies will require the development of associated management strategies.
Trends for Storage in Remote-Area Power Supply Systems To improve living standards, particularly in remote areas, it is necessary to have access to electricity. Therefore, the market for RAPS systems is in expansion and will stay that way because of the growth in world population. As stated earlier, in order to match generation and demand in RAPS systems where generation is based on intermittent sources, storage is needed. Only in some
rare cases is the storage based on alternatives to electrochemical power sources, e.g., hydro-pumping and compressed air. At present, the storage technology predominantly used in RAPS systems is the lead–acid battery because of its maturity, reliability, and affordability; this situation is unlikely to change for some years to come. In specific applications, however, other technologies can be encountered such as nickel–cadmium batteries in very cold climates, and increasing numbers of lithium-ion batteries when reliability is crucial. Analysis of the purely technical parameters shows that lithium-ion batteries are the best candidate in the RAPS applications due to their low self-discharge, high flexibility in the conditions of use regarding their SoC, and high energy efficiency at the cell level. Nevertheless, in order to provide maximum safety, lithium-ion batteries require sophisticated power electronics in order to manage the cell voltage, as well as thermal management; these features impact on energy efficiency. In addition, lithium-ion batteries are still 10 times more expensive than lead–acid batteries. In the near future, however, it is highly probable that lithiumion technology will undergo tremendous improvement in terms of increased safety and reduced cost and will be encountered more and more in RAPS facilities, especially in industrial applications. For domestic applications, small systems will use lead– acid batteries because of economical constraints. Nevertheless, the reliability of this technology will grow thanks to advanced management algorithms such as the limitation of the daily amount of energy available to the user. Rural electrification will also rely on small grids at the village level with various power sources to meet a greater demand for energy. In such configurations, banks of tubular plate lead–acid batteries, which have proven to be more robust for cyclic use than flat-plate counterparts, are increasingly favored, with a growing use of the gel valve-regulated type due to its lower maintenance requirement. Other technologies may become competitive, in particular redox-flow batteries once optimization of the stack longevity is achieved and costs have come down due to economies of scale. The combination of electrolyzer, hydrogen storage, and fuel cell may be a further option for RAPS facilities in certain situations where overall system efficiency is not critical and special features such as seasonal storage are more important.
Nomenclature Symbols and Units T Vpc Wp
temperature Volts per cell peak power out (watt)
Applications – Stationary | Remote Area Power Supply: Batteries and Fuel Cells
Abbreviations and Acronyms AC BMS DC DoD EMS EPIA HVD HVR IEC LVD LVR MPPT NFC PV PWM RAPS SoC SoH
alternating current battery management system direct current depth-of-discharge energy management system European Photovoltaic Industry Association high-voltage disconnect high-voltage reconnect International Electrotechnical Commission low-voltage disconnect low-voltage reconnect maximum power-point tracking Norme Franc¸aise de se´rie C (French Standard of category C) photovoltaic pulse-width modulation remote-area power supply state-of-charge state-of-health
See also: Applications – Stationary: Fuel Cells; Batteries: Charging Methods; Codes and Standards; Lifetime Prediction; Partial-State-of-Charge; Secondary
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Batteries – Flow Systems: Overview; Secondary Batteries – High Temperature Systems: Sodium–Nickel Chloride; Secondary Batteries – Lead–Acid Systems: Overview; Secondary Batteries – Lithium Rechargeable Systems: Overview; Secondary Batteries – Nickel Systems: Nickel–Cadmium: Overview; Nickel–Metal Hydride: Overview.
Further Reading ACTUS project team (2005) Final technical publishable report: Specific accelerated test procedure for PV batteries with easy transfer to various kinds of systems and for quality control. European Project ENK6-2000-00069. Adeoti O, Oyewole BA, and Adegboyega TD (2001) Solar photovoltaicbased home electrification system for rural development in Nigeria: Domestic load assessment. Renewable Energy 24: 155--161. Benchetrite D, Le Gall M, Bach O, Perrin M, and Mattera F (2005) Optimisation of charge parameters for lead–acid batteries used in photovoltaic systems. Journal of Power Sources 144: 346--351. EPIA (2008) Solar Generation Report. EPIA (European PV Association). www.epia.org International Energy Agency (2005a) Trends in photovoltaic applications. Survey Report of Selected IEA Countries between 1992 and 2004. Paris, France: IEA publications. International Energy Agency (2005b) World Energy Outlook 2004. Paris, France: IEA Publications. Labouret A and Villoz M (2003) Energie solaire photovoltaı¨que, le manuel du professionnel. Paris: Dunod. ISBN 2100056107. Moseley PT (2006) Energy storage in remote-area power supply (RAPS) systems. Journal of Power Sources 155: 83--87.