Lithium-ion battery based renewable energy solution for off-grid electricity: A techno-economic analysis

Renewable and Sustainable Energy Reviews 72 (2017) 922–934

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Lithium-ion battery based renewable energy solution for off-grid electricity: A techno-economic analysis

MARK

Abhishek Jaiswal1 Department of Electrical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh 208016, India

A R T I C L E I N F O

A BS T RAC T

Keywords: Lithium-ion battery Photovoltaic Solar home lighting system HOMER

Small renewable energy solutions such as solar home lighting system (SHLS) provide reliable electricity supply to off-grid bottom-of-pyramid (BoP) households and thereby improve status of living. Commercial SHLS employs polycrystalline silicon photovoltaic (PV) and flooded lead–acid battery technologies for energy generation and energy storage, respectively. Flooded lead–acid battery is a 150-year-old, mature and inexpensive energy storage technology but has a short lifetime. In SHLS, flooded lead–acid battery requires replacement every 4–5 years and can cost up to 70% of the total cost over the 20 years lifetime of the system. In this paper, seven advanced lithium-ion battery chemistries were evaluated as a potential replacement for flooded lead–acid battery in SHLS using HOMER microgrid software. Three lithium-ion battery chemistries – NCA, LFP and LFP/LTO – were found to be viable alternatives based on economic and performance metrics. The three lithium-ion battery based SHLS showed comparable initial capital cost to that of the commercial SHLS but provided significant advantage over the system lifetime as no/fewer battery replacements were required, which resulted in a total net present cost (TNPC) that was as low as 45% of the commercial SHLS. Price of lithium-ion battery technology is decreasing at 8–16% annually in real terms and the cost advantage of SHLS based on lithium-ion battery is expected to increase significantly in the future.

1. Introduction 1.1. Access to electricity and social development Access to reliable electricity is a key driver behind economic development and raising basic standards of living. This is especially applicable to rural countryside of developing countries, like India, where access to reliable and affordable electricity can allow use of modern lighting and appliances, enabling income generation activities after daylight hours, improved healthcare and sanitation and improved food storage. However, grid electricity connectivity and supply has been lacking in rural India. Estimates suggest that about 45% of rural households do not an electricity connection in 2011 and rural areas face frequent and long supply interruptions [1–4]. 1.2. Solar home lighting system Multiple renewable energy technologies are available for off-grid, distributed electrification including biofuel powered generator, biomass plant, micro-hydro, wind hybrid and solar photovoltaic (PV) [3]. Solar PV based solutions are ideally suited for Indian climatic condi-

1

tions as abundant solar energy is available throughout the year and in most of the country. Moreover, small solar PV solutions such as solar home lighting system (SHLS) are suitable for self-ownership in households and small businesses [3]. Financial support from rural banks and microcredit organizations to manage high upfront cost of SHLS has been successful in adoption of small solar PV solutions by bottom-ofpyramid (BoP) customers [5]. SHLS is cheaper to operate compared to kerosene burners, provides much better quality of light and does not produce harmful gases. Cost of electricity (COE) generated by SHLS is ~1 USD/kWh or ~10 times the price of grid electricity [6]; high COE of SHLS demonstrates the “willingness to pay” in BoP customers for energy solutions. Commercial SHLS, based on polycrystalline silicon PV and flooded lead–acid battery technologies, suffer from two limitations. Firstly, the battery requires replacement every 4–5 years and effectively constitutes a major fraction of the system lifetime cost [5]. Operationally, battery replacement is a huge burden on BoP households as financial support for replacement battery is not available. Secondly, as the lead– acid battery is a 150-year-old, mature technology, future cost reductions in commercial SHLS can be expected to be small [7].

E-mail address: [email protected]. Correspondence address: Department of Materials Science and Engineering, University of Maryland, College Park, MD, USA.

http://dx.doi.org/10.1016/j.rser.2017.01.049 Received 22 December 2015; Received in revised form 10 August 2016; Accepted 9 January 2017 Available online 01 February 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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Fig. 1. Cell voltage (a), battery cost (b), cycle life (c) and peukert factor (d) of seven lithium-ion battery chemistries; values of flooded lead–acid battery are also presented.

anode, SEI layer blocks lithium-ion diffusion reducing rate capability and breakdown of the SEI layer is the trigger point for thermal runaway [20]. LTO anode also reduces the risk of lithium metal plating at low temperatures, as observed for graphite anode [15,16]. Lastly, crystal structure of LTO is resilient to lithiation/delithiation with only ~0.2% change in crystal volume, resulting in long cycle life of the battery. The characteristics of the seven lithium-ion battery chemistries are briefly described below –

1.3. Lithium-ion battery technology Lithium-ion battery technology was developed in early 1990's and enabled the portable electronics revolution. The technology is now under development for application in electric vehicles (EVs) and grid storage. Lithium-ion battery technology is expensive compared to the conventional flooded lead–acid battery technology and, hence, so far has been targeted for premium applications. This paper evaluates the potential of lithium-ion battery as a replacement for flooded lead–acid battery in SHLS from both technical and economic perspectives. There are multiple lithium-ion battery chemistries (variants) with characteristic cell voltage, energy density, rate capability (peukert factor), cycle life and cost [8]. The characteristics of seven main lithium-ion battery chemistries and of flooded lead–acid battery are presented in Fig. 1. The dataset presented is based on manufacturer's datasheets, industry reports and a handbook [9–17]. For SHLS application, the primary battery parameters are cost, cycle life and safety. Nominal cell voltage is also an important factor as it determines the number of cells in series required to get the desired battery voltage and, therefore, impacts the battery cost. As the application is for stationary energy storage at low charge/discharge rates, energy density and rate capability are secondary factors for consideration. The seven lithium-ion battery chemistries can be divided into two categories based on the type of anode – graphite or lithium titanium oxide (LTO). Graphite is the conventional anode for lithium-ion batteries but it has limitations in cycle life, rate capability and safety. LTO anode addresses these limitations but at the expense of cost and energy density [18,19]. LTO anode has a higher redox potential of 1.55 V (vs. Li) and a lower specific capacity of 175 mAh/g, compared to 0.2–0.3 V (vs. Li) and 372 mAh/g of graphite, which results in lower cell voltage, lower energy density and higher cost. However, the higher redox potential of LTO avoids the formation of secondary electrolyte interface (SEI) layer, resulting in improved rate capability, performance at low temperatures and safety characteristics. In graphite

a) Lithium-ion battery chemistries with graphite anode 1. Lithium manganese oxide (LMO) LMO chemistry is well established and utilizes cheap, abundant manganese-based spinel cathode [21,22]. The chemistry is comparatively much safer than those that use cobalt/nickel based cathodes and it can be used with inexpensive battery management system (BMS) [22]. The main limitation of LMO is the manganese dissolution in the cathode, which reduces the cycle life and calendar life at high temperature ( > 50 °C) operation [8, 23, 24]. 2. Lithium nickel cobalt aluminum oxide (NCA) NCA chemistry has been developed to improve the safety characteristics and cycle life of layered nickel-based cathode by codoping with aluminum and cobalt [21,22]. Compared to LMO, NCA is expensive but has the advantage of high energy density and long life; calendar life for NCA cells is projected to be extremely long ( > 15 years). However, nickel based cathodes are thermally unstable and degrade at high state of charge (SoC). Some EV manufacturers are using NCA chemistry in battery packs with sophisticated BMS [25]. 3. Lithium nickel manganese cobalt oxide (NMC) NMC chemistry is another variant of the nickel-based cathode family, which has been developed to improve cost and safety characteristics over NCA but it has lower rate performance and shorter cycle life [21,22]. Like other nickel-based cathodes, NMC is 923

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thermally unstable and degrades at high SoC. NMC chemistry is also being targeted for application in EV battery packs [25]. 4. Lithium iron phosphate (LFP) LFP is the safest chemistry that has been developed with the graphite anode. LFP chemistry withstands high temperatures, overcharge and short circuit conditions and provides long cycle life [21]. The chemistry utilizes cheap raw materials and has the potential to reduce cost. Compared to cobalt/nickel-based chemistries, LFP has low energy density [21,22]. a) Lithium-ion battery chemistries with lithium titanium oxide (LTO) anode 1. Lithium manganese oxide cathode (LMO/LTO) LMO/LTO is the cheapest chemistry amongst LTO based chemistries due to the low cost of LMO. The chemistry shows the high rate capability of LMO and LTO and has the long cycle life of LTO [26,27]. As expected, the chemistry has low energy density due to LTO anode but can operate effectively at low temperatures. Unlike LMO, LMO/LTO does not suffer from manganese dissolution and shows long cycle life at high temperatures [8,14]. Safety tests have not found any evidence of thermal runaway in LMO/LTO [18,28]. 2. Lithium nickel manganese cobalt oxide cathode (NMC/LTO) NMC/LTO chemistry has high energy density and high cost due to NMC cathode. The chemistry shows extremely long cycle life, which can allow for cost amortization over the lifetime of the battery. Like LMO/LTO, NMC/LTO does not show any evidence of thermal runaway [15,29]. Some automotive suppliers are targeting NMC/ LTO for micro-hybrid and EV applications [25]. 3. Lithium iron phosphate cathode (LFP/LTO) LFP/LTO is the safest amongst the seven chemistries, as it incorporates the safety features of both LFP and LTO. The cycle life is extremely long with minimal degradation rates [16,30]. The chemistry has another advantage that it works under extreme temperatures. However, high cost of LTO and low cell voltage results in high cost and low energy density for the chemistry.

Fig. 3. Equivalent circuit of battery pack with n cells in series to form a string and m strings in parallel.

System sensitivity to maximum annual capacity shortage (0–10%) was analyzed to understand the effect of capacity shortage on the system performance and cost. Performance of SHLS with flooded lead–acid battery (PbA) and seven lithium-ion battery chemistries – LMO, NCA, NMC, LFP, LMO/ LTO, NMC/LTO and LFP/LTO – was simulated and compared to that of a commercial SHLS (comm.) [31]. Mass produced lithium-ion battery cells in cylindrical format of 2 Ah capacity were considered in the simulation. A number of cells (n) were combined in series to form a string with nominal voltage of > 12 V, as shown in Fig. 3. With n=4, LMO, NCA and NMC cells achieved a nominal string voltage of ~14.4 V. While for LFP, LMO/LTO, NMC/LTO and LFP/LTO cells, n=4, 5, 6 and 7 resulted in a nominal string voltage of ~13.2, 12.5, 13.8 and 12.6 V, respectively. For PbA, n=6 and cell capacity of 10 Ah were used in the simulation. A number of strings (m) were combined in parallel to get the desired battery capacity (Ah); m was varied from 1 to 15 in the simulation. Cell voltage, capacity, cycle life and cost data used in the simulation are summarized in Table 1. Round trip efficiency of 95% and 80% and minimum state of charge (SoC) of 10% and 40% were assumed in the simulation for lithium-ion battery and flooded lead–acid battery, respectively [32,33]. Polycrystalline silicon PV panel was used in the simulation with power between 10–55 W, varied in 5 W increments. PV panel efficiency of 14% at standard test conditions, derating factor of 90%, lifetime of 20 years and cost of 2 USD/W were used. The simulation was done for two CFLs with inbuilt inverters, which acted as a total DC load of 18 W. The actual usage pattern may be different than the design load as the user may not always use both CFLs or may add an extra CFL based on household needs. To simulate such a pattern, 15% daily and 15% hourly variations were added to the load profile. The load frequency chart, as estimated by the Homer software, is presented in Fig. 4. With the variations built-in, mean and standard deviation of the load was 17.9 W and 3.8 W, respectively. The peak power load was 30 W and the average daily consumption was 72 Wh. The location for the simulation was chosen as Varanasi, a tier-2 city in India's most populated state, Uttar Pradesh, with a large BoP population and persistent power deficit. Varanasi lies in the Gangetic plains of north India and experiences a humid subtropical climate (Köppen climate classification Cwa) with hot and dry summers, rainy monsoon season and foggy winters. The solar daily radiation and average high temperature of Varanasi are presented in Fig. 5 [34].

2. Method SHLS comprising of a Solar PV panel, a battery and two compact florescent lights (CFLs) was simulated using HOMER microgrid software v2.68 beta, developed by National Renewable Energy Laboratory (NREL), Colorado. The system diagram is presented in Fig. 2. During the day, the PV panel acts as an energy generator and charges the battery. While at night, the battery is discharged to run the two CFLs for four hours daily between 7 and 11 p.m. The simulation was done every 10 min for a system lifetime of 20 years. Real interest rate, adjusted for inflation, of 6% was assumed to estimate the cost metrics.

3. Metrics The HOMER simulation output provided the lowest cost SHLS that could meet the load requirement at a given maximum annual capacity shortage. The three SHLS specification parameters considered were –

Fig. 2. System diagram of solar home lighting system (SHLS).

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Table 1 Parameters of flooded lead–acid battery and seven lithium-ion battery chemistries used for simulation. S.No.

Battery type

Nominal cell voltage (V)

No. of cells in series

Battery voltage (V)

Cell capacity (Ah)

Cycle life

Round trip efficiency (%)

Minimum SoC (%)

Cost (USD/ kWh)

References

1

Flooded Lead– Acid LMO NCA NMC LFP LMO/LTO NMC/LTO LFP/LTO

2.0

6

12.0

10

500

80

40

100

[17]

3.6 3.6 3.6 3.3 2.5 2.3 1.8

4 4 4 4 5 6 7

14.4 14.4 14.4 13.2 12.5 13.8 12.6

2 2 2 2 2 2 2

1,200 4,500 900 4,400 6,000 27,000 20,000

95 95 95 95 95 95 95

10 10 10 10 10 10 10

220 270 275 310 325 550 630

[9,10] [9,11] [9,12] [9,13] [9,14] [9,15] [9,16]

2 3 4 5 6 7 8

costs are similar for all systems. TNPC calculation was based on the real interest rate of 6% and included salvage value and cost of replacement, if required. Salvage value was calculated based on the replacement cost and remaining life of the component; cost of replacement was considered to be the same as the present cost. ii. Initial capital cost Initial capital cost is the initial cost of the system at the time of installation. This is a key factor for purchase decision by BoP households as SHLS is a large investment in comparison to the annual household savings [35,36]. iii. Cost of electricity (COE) COE is an estimate of the average cost to generate 1 kWh of electricity from SHLS over the system lifetime. It is a good metric not only to compare the different battery chemistries but also with other sources of electricity [3]. Fig. 4. Load frequency chart used for simulation of SHLS.

3.2. Performance metrics i. Autonomy Autonomy is the number of hours that the battery can provide continuous backup without intermittent charging from the PV panel. It is an important performance metric in real life application. During the monsoon season, there are continuous stretches of days without adequate sunlight for the PV panel to charge the battery. ii. Battery state of charge (SoC) Battery SoC provides information on the usage of the battery in terms of the depth of discharge (DoD) to run the electric load and the charging pattern from the PV panel. These factors determine to a large extent the cycle life of the battery, as chronic overcharging, chronic undercharging and deep discharging are not desirable. iii. Excess electricity Excess electricity is an estimate of the surplus electricity generated annually by the PV panel, which is not stored in the battery. It can be considered as a metric for overcapacity of the PV panel in the SHLS design. Moreover, large amount of excess electricity indicates chronic overcharging of the battery.

Fig. 5. Solar daily radiation and average high temperature of Varanasi.

i. PV panel size (W) ii. Battery size (Wh) iii. Number of battery replacements

4. Results The simulation of SHLS with flooded lead–acid battery (PbA) and seven lithium-ion battery chemistries was done to study the effect of battery chemistry on the cost, specification and performance of SHLS. In an ideal scenario, desired maximum capacity shortage of SHLS is 0% but it could result in high cost and, therefore, the sensitivity of cost to maximum capacity shortage was looked at for the different chemistries. The rate of decrease in TNPC was faster in NCA, LFP and LTO anode based chemistries (LMO/LTO, NMC/LTO, LFP/LTO) in comparison to that in PbA, LMO and NMC, as shown in Fig. 6. Percent cost reduction from 0% to 1% maximum capacity shortage for PbA, LMO, NCA, NMC, LFP, LMO/LTO, NMC/LTO and LFP/LTO based SHLS was 12%, 12%, 21%, 10%, 20%, 25%, 25% and 24%, respectively. Further, NCA, LFP

The SHLS was further evaluated on the basis of three economic and three performance metrics, calculated by the simulation software, in order to determine the suitability of the system to address the power needs of BoP households. 3.1. Economic metrics i. Total net present cost (TNPC) TNPC is the present value of the cost of PV panel and battery incurred over the 20-year lifetime. Cost of CFLs, charge controller and repair and maintenance were not considered in TNPC, as these 925

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Table 2 Comparison between commercial SHLS based on flooded lead–acid battery and HOMER simulation result of flooded lead–acid battery based SHLS.

Load (Wh/day) PV (Wp) Battery (Wh at C/20) Battery replacements Initial capital (USD) TNPC (USD) COE (USD/kWh) Autonomy (hr) SOC % Excess electricity (kWh/yr)

Commercial SHLSa

Simulated PbA SHLS

72 37 720 3–4 250b (146c) 270c 0.90c 72 (144c) 91.9 ± 4.9c 28.6c

72 35 360 3 106 168 0.56 72 85.1 ± 9.6 25.3

a

Specfications for commercial SHLS [31]. Price of USD 250 includes cost of 2 CFLs, charge controller, profit and retailer margin; USD/INR=64 [5,31]. c Simulation result of commercial SHLS. b

Fig. 6. Relationship between TNPC and maximum capacity shortage of SHLS with different battery chemistries.

Simulated economic and performance metrics of a commercial SHLS (comm.) based on flooded lead–acid battery are also included [31]. As a first step, the design specification of the commercial SHLS was compared with the simulation result of PbA based SHLS, as shown in Table 2. Although the PV panel size was similar in the two systems, the battery in the commercial SHLS was 100% larger than that in the simulation result, which resulted in higher system cost. TNPC of the commercial SHLS was USD 270 compared to USD 168 of the simulated

and LMO/LTO provided the lowest TNPC amongst all chemistries including PbA. 4.1. 0% maximum capacity shortage The simulation result of SHLS at 0% maximum capacity shortage with the different battery chemistries in terms of economic metrics, system specification and performance metrics is presented in Fig. 7.

Fig. 7. Simulation result of SHLS with the different battery chemistries in terms of economic metrics (TNPC (a), initial capital cost (b), COE(c)), system specification (PV size (d), battery size (e), number of battery replacements (f)) and performance metrics (autonomy (g), battery SoC (h), excess electricity (i)) at 0% maximum capacity shortage; commercial SHLS (comm.) is also presented for comparison.

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Fig. 8. SoC frequency distribution of commercial SHLS (a) and of simulated PbA (b), LFP (c) and LMO/LTO (d) based SHLS at 0% maximum capacity shortage.

4.1.2. System specification The simulation result was based on the lowest TNPC at the desired maximum capacity shortage and, therefore, provided different size of PV panel and battery depending on the battery chemistry. Within the same battery chemistry, it was possible to get same or similar TNPC with slightly different configuration of PV panel and battery; for simplicity, the configuration with the lowest TNPC was considered. For lithium-ion battery based SHLS, it was observed that the size of battery decreased and the size of PV panel increased with increase in the cost of battery, so as to keep the overall system cost down. Cost of NMC/LTO and LFP/LTO is significantly higher than that of other lithium-ion battery chemistries and the optimum configuration was based on a larger PV panel and a smaller battery; the implication on the system performance using such a configuration is discussed in the next section. On the other hand, LMO and NMC have comparatively shorter battery cycle life and, therefore, required multiple battery replacements over the system lifetime. As discussed earlier, LMO chemistry has a poor cycle life at high temperature operation and it can be anticipated that in real life application under subtropical climatic conditions, a bigger battery and/or more number of battery replacements would be required than predicted by the simulation.

PbA based SHLS. The larger battery size of commercial SHLS is primarily to reduce the number of battery replacements and to ensure that the specified autonomy (72 h) is available throughout the lifetime of the system. The simulated PbA based SHLS is likely to incur in practice more battery replacements than predicted due to the rigors of real life application and less than ideal performance of flooded lead– acid battery as discussed more in detail in Section 4.1.3.

4.1.1. Economic metrics The first thing to note in the economic metrics is that the battery cost constituted a major fraction of TNPC and, therefore, the choice of battery is of critical importance. For the commercial SHLS, battery cost was ~73% of TNPC, while for the simulated chemistries it varied between 50–65% of TNPC. Compared to a TNPC of USD 270 for the commercial SHLS, four lithium-ion battery based SHLS showed significantly lower TNPC values; TNPC of LMO, NCA, LFP and LMO/LTO based SHLS was USD 129, 122, 134 and 142, respectively. This result clearly suggested that the lithium-ion battery chemistry can play a role in improving the cost economics of SHLS. Another interesting result was that TNPC and initial capital cost of all lithium-ion battery chemistries, except LMO and NMC, were the same, as these chemistries did not incur battery replacement cost during the 20 year life time of the system; this result assumed that the calendar life of lithium-ion battery approaches 20 years. In comparison, commercial SHLS required three battery replacements during the same time period. As noted earlier, flooded lead–acid battery in commercial SHLS is oversized so as to increase useful life and to reduce the number of battery replacements in real life application. With the use of LMO, NCA, LFP and LMO/LTO based SHLS, COE can be reduced to 0.40– 0.47 USD/kWh, compared to 0.56 and 0.90 USD/kWh of simulated PbA and commercial SHLS, respectively.

4.1.3. Performance metric NCA, LFP and LMO/LTO based SHLS provided autonomy (~70 h) comparable to the specified autonomy (72 h) of the commercial SHLS. Although LMO provided an autonomy of ~95 h, TNPC is expected to be higher in real life application due to the likelihood of poor cycle life. Whereas, NMC/LTO and LFP/LTO showed lower autonomy ( < 50 h) due to comparatively smaller battery size and, therefore, were not the preferred choice. Mean and standard deviation values of SoC provided an estimate of the utilization of the battery capacity. Within all battery chemistries, 927

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Fig. 9. Simulation result of SHLS with the different battery chemistries in terms of economic metrics (TNPC (a), initial capital cost (b), COE(c)), system specification (PV size (d), battery size (e), number of battery replacements (f)) and performance metrics (autonomy (g), battery SoC (h), excess electricity (i)) at 1% maximum capacity shortage; commercial SHLS (comm.) is also presented for comparison.

flooded lead–acid battery at high SoC results in corrosion of positive plate and gassing of sulfuric acid electrolyte, which reduces not only the charge efficiency but also the battery life. Moreover, studies on flooded lead–acid battery have shown that the incremental charge efficiency at high SoC is considerable lower (50–60%) than the overall charge efficiency from low to high SoC (~90%) [33]. Due to the low charge efficiency at high SoC, the PV panel is unable to charge the battery completely and a slow loss in battery capacity can occur due to electrolyte stratification and electrode sulfation [33]. These effects were not considered in the simulation and, therefore, it is not surprising that flooded lead–acid battery is oversized in the commercial SHLS. In comparison, NCA, LFP and LMO/LTO based SHLS stayed comparatively a smaller amount of time (~34%) at the top of charge (90–100% SoC) and showed a deeper depth of discharge, which compared favorably in terms of cycle life and utilization of battery capacity.

commercial SHLS and simulated PbA were the most underutilized with high mean SoC (comm. – 91.9%, PbA – 85.1%) and limited depth of discharge (comm. – 4.9%, PbA – 9.6%). As the battery size of NCA, LFP and LMO/LTO based SHLS was comparatively smaller, the battery was better utilized to address the load and resulted in deeper depth of discharge. Mean and standard deviation values of SoC for NCA, LFP and LMO/LTO based SHLS were 78.2%, 79.2% and 79.8% and 15.2%, 14.4% and 14.0%, respectively. With respect to excess electricity generated by the PV panel, NMC/ LTO and LFP/LTO based SHLS compared least favorably among all systems. The two systems utilized large PV panels, which were oversized and resulted in 47–55 kWh/year excess electricity. The amount of excess electricity was significant considering that the annual electrical load of 2 CFLs is ~26 kWh/year. NMC/LTO and LFP/LTO based SHLS used small batteries, which did not have the capacity to store the excess electricity. Small batteries also resulted in low autonomy and, therefore, NMC/LTO and LFP/LTO were the least preferred based on system performance. In comparison, NCA, LFP and LMO/LTO based SHLS showed ~22% lower excess electricity compared to the commercial SHLS. SoC frequency distribution of commercial SHLS and of simulated PbA, LFP and LMO/LTO based SHLS is presented in Fig. 8 and provides a more detailed perspective to the SoC data presented in Fig. 7(h). Batteries in commercial SHLS and simulated PbA based SHLS stayed 70% and 38% of the time at the top of charge (90–100% SoC), respectively, and incurred limited depth of discharge. Charging of

4.2. 1% and 5% maximum capacity shortage The simulation was carried out at 1% and 5% maximum capacity shortage to study the effect of capacity shortage. If a cost-effective solution can be achieved at a small capacity shortage without any significant downside in performance, SHLS can be made affordable to a larger spectrum of BoP households. The simulation result at 1% maximum capacity shortage is presented in Fig. 9 and it is clear that a significant cost reduction can be 928

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Fig. 10. SoC frequency distribution of PbA (a), NCA (b), LFP (c) and LMO/LTO (d) based SHLS at 1% maximum capacity shortage.

choice amongst all chemistries looked at and resulted in the lowest TNPC without any battery replacements. Other trends in system configuration observed for different chemistries at 0% and 1% maximum capacity shortage were also observed. LMO and NMC based SHLS required multiple battery replacements due to low cycle life, while NMC/LTO and LFP/LTO based SHLS used comparatively smaller batteries due to expensive batteries. NCA, LFP and LMO/ LTO were the best choice in terms of performance; however, autonomy was low (40–45 h) which can be a severe drawback in real life application. SoC frequency distribution of simulated PbA, NCA, LFP and LMO/LTO based SHLS, presented in Fig. 12, further supports this argument. SoC distribution of NCA, LFP and LMO/LTO based SHLS was spread across the SoC range with significant amount of time (22– 30%) at SoC below 40%. Lower autonomy and lower mean SoC indicate frequent outages in real life, which would effectively reduce the utility of SHLS.

achieved in most systems with highest sensitivity observed for NCA, LFP and LTO anode based chemistries. As in the case of 0% maximum capacity shortage, NCA (USD 97), LFP (USD 107) and LMO/LTO (USD 107) provided the lowest TNPC without any battery replacements. While PbA, NMC and LFP/LTO (~148 USD) were the most expensive choices. With respect to system sizing, both PV panel and battery sizes were reduced in all chemistries. The trends observed in system configuration and performance of the different chemistries at 0% maximum capacity shortage were also observed at 1% maximum capacity shortage. NMC/LTO and LFP/LTO based SHLS used large PV panels and small batteries, which resulted in low autonomy, deep depth of battery discharge and large excess electricity. For LMO and NMC, optimum system configuration required multiple battery replacements over the system lifetime. Overall, NCA, LFP and LMO/LTO based systems provided the best solution at 1% maximum capacity shortage with low TNPC, good battery utilization, small excess electricity and acceptable autonomy. SoC frequency distribution of simulated PbA, NCA, LFP and LMO/ LTO based SHLS at 1% maximum capacity shortage is presented in Fig. 10. Compared to the case of 0% maximum capacity shortage, the batteries showed reduced amount of time at top of charge (90–100% SoC) – PbA (26.9%), NCA (26.6%), LFP (27.1%) and LMO/LTO (26.7%) – which resulted in lower excess electricity and, thereby, indicated better PV panel sizing. Mean SoC was lower and depth of discharge was higher in NCA (68.9 ± 21.4%), LFP (71.1 ± 19.9%) and LMO/LTO (69.2 ± 21.3%), compared to that of PbA (80.0 ± 13.3%), indicating good utilization in lithium-ion battery based SHLS. SoC for NCA, LFP and LMO/LTO went down to 0–10% but for only very small amount of time ( < 0.4%). The simulation result at 5% maximum capacity shortage is presented in Fig. 11. As expected, TNPC and initial capital cost were lower and PV panel size and battery size were smaller at 5% maximum capacity shortage. NCA, LFP and LMO/LTO remained the preferred

5. Discussion 5.1. Lithium-ion battery based SHLS One of the key advantages of lithium-ion battery technology over lead–acid battery technology is the availability of multiple chemistries, which are different from each other in terms of cost, cell voltage, cycle life, safety, rate capability and energy density and, therefore, open up choices for different applications. For SHLS application, three lithiumion battery (NCA, LFP and LMO/LTO) based systems used smaller PV panel and battery but surpassed the performance of commercial SHLS – higher utilization of PV panel capacity (lower excess electricity) and battery capacity (deeper SoC) at an acceptable system autonomy. Further based on cycle life data, the three lithium-ion battery based SHLS did not require battery replacement over the system lifetime resulting in a TNPC that was equal to the initial capital cost. In 929

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Fig. 11. Simulation result of SHLS with the different battery chemistries in terms of economic metrics (TNPC (a), initial capital cost (b), COE(c)), system specification (PV size (d), battery size (e), number of battery replacements (f)) and performance metrics (autonomy (g), battery SoC (h), excess electricity (i)) at 5% maximum capacity shortage; commercial SHLS (comm.) is also presented for comparison.

5.2. Calendar life

comparison, commercial SHLS requires battery replacement every 4–5 years. The initial capital cost of the lithium-ion battery based SHLS was comparable to that of the commercial SHLS and over the life of the system, savings with lithium-ion battery based SHLS can be as high as 55%. Cost of battery replacements is a huge burden on BoP households as in general the initial capital cost of SHLS is financed by social organizations but not the cost of battery replacements. As the battery ages and does not get replaced due to financial limitations, the overall utility of the system is compromised [37]. Therefore, the utility of a SHLS with limited number of battery replacements cannot be overly stated from the perspective of a BoP household. Utilization and life of lithium-ion battery in SHLS can be further improved with the help of an effective, reliable SoC meter, which is already used in consumer electronics and EVs, and simple to follow instructions. On the other hand, SoC meters for flooded lead–acid battery are not reliable and are typically not included with commercial SHLS. Lastly, it can be argued that a smaller lithium-ion battery based SHLS (with 1% maximum capacity shortage) in combination with a SoC meter can provide a better user experience at an initial capital cost and TNPC which is 66– 73% and 36–40%, respectively, of an over-sized lead–acid battery based commercial SHLS. This approach would significantly reduce the cost of SHLS and make it more affordable and useful to a wider spectrum of BoP households, which either cannot find financial support to buy or do not find utility in the commercial SHLS.

Calendar life refers to the shelf life of the battery in real life application accounting for decay in capacity without use. The simulations of SHLS did not consider calendar life as data is not easily available for all chemistries and, for simplicity, it was assumed that the cycle life only determines the life of all chemistries. This approach has limitations and can have an impact on cost economics. Some lithiumion battery manufacturers provide indications of calendar life approaching 15–20 years [27,38,39]; calendar life increases with decrease in operating temperature and decrease in SoC. For example, NCA chemistry at 35 °C has a projected calendar life of > 20 years at all SoC and at 40 °C has a projected calendar life of 20, 18 and 14 years at 50%, 75% and 100% SoC, respectively [38]. In comparison, LFP chemistry at 60 °C and 100% SoC has a projected calendar life of 15 years and LMO/LTO chemistry has a projected capacity retention of 93% after 15 years at 35 °C [27,39]. Simulation was carried out with a calendar life assumption of 15 years for NMC, LFP and LMO/LTO chemistries and the results at 0% and 1% maximum capacity shortage are presented in Table 3. A shorter calendar life did not significantly change the system specification and the initial capital cost. But due to the cost of battery replacement after 15 years, TNPC was slightly higher (5–11%) than the case with a calendar life of 20 years. Overall, a shorter calendar life did not diminish the significant advantage of lithium-ion battery based SHLS. At 1% maximum capacity shortage, 930

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Fig. 12. SoC frequency distribution of PbA (a), NCA (b), LFP (c) and LMO/LTO (d) based SHLS at 5% maximum capacity shortage.

5.3. Modularity and mass customization

calendar life can be expected to be higher due to lower mean SoC (65– 70%, Fig. 10) compared to that at no capacity shortage (~80% SoC,

Another key advantage of lithium-ion battery over flooded lead– acid battery is its modular construction, which opens the possibility for mass customization. BoP customers, although clubbed together on the basis of household income, have different disposal incomes and access to financial support as well as different requirements from a SHLS [36]. In addition, these households are spread over geographically with varying daily solar radiation. Location has a significant influence on the system size and, therefore, on TNPC as shown for LMO/LTO based SHLS at four Indian cities in Table 4. Although the size of the solar PV panel was the same, the battery size varied from 175 to 250 Wh and resulted in a TNPC between 117 and 142 USD. Mass customization can be done using 18650 or similar cylindrical format of lithium-ion battery cells of 2–3 Ah capacity, as used in the simulation, by SHLS assemblers without any significant additional costs. The cylindrical lithium-ion battery cells can be combined in series and parallel to get the desired voltage and capacity of the battery pack as shown in Fig. 13. In comparison, the construction of flooded lead–acid battery is less modular [40]. Customization in flooded lead–acid battery specification has to be done by the battery manufacturer and would require additional tooling and change over costs on the production shop floor.

Table 3 Simulation result of NCA, LFP and LMO/LTO based SHLS with a calendar life of 15 years at 0% and 1% maximum capacity shortage.

Max. capacity shortage PV (Wp) Battery (Wh) Battery replacements Initial capital (USD) TNPC (USD) COE (USD/kWh)

NCA

LFP

LMO/LTO

NCA

LFP

LMO/LTO

0% 30 230 1 122 135 0.45

0% 30 238 1 134 149 0.50

0% 30 225 1 133 149 0.49

1% 25 173 1 97 106 0.36

1% 25 185 1 107 119 0.40

1% 25 175 1 107 119 0.40

Table 4 Simulation result of LMO/LTO based SHLS at four Indian cities at 0% maximum capacity shortage.

PV (Wp) Battery (Wh) Battery replacements Initial capital (USD) TNPC (USD) COE (USD/kWh)

Varanasi

Bangalore

Chennai

Jaipur

30 250 0 142 142 0.47

30 250 0 142 142 0.47

30 225 0 133 133 0.44

30 175 0 117 117 0.39

5.4. Safety and battery management system In the past, there have been several instances of lithium-ion battery failures in laptop, mobile phone, EV and aircraft batteries due to manufacturing defects and/or abuse [41–44]. Lithium-ion battery pack of SHLS is ~4 times bigger than that of a laptop (~60 Wh) and the possibility of a catastrophic failure cannot be ignored. Battery management system (BMS) for lithium-ion battery is an engineered monitoring, control and safety system, which utilizes voltage, current and temperature sensors and controllers and ensures that the battery pack

Fig. 8). Therefore, lithium-ion battery based SHLS with small capacity shortage provided the advantage of not only significantly lower initial capital cost and TNPC but also a longer calendar life which would reduce or delay any battery replacements. 931

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Fig. 13. Construction of flooded lead–acid battery [40] (a) and lithium-ion battery pack using cylindrical cells (b).

Fig. 14. Initial capital cost (a) and TNPC (b) of SHLS based on flooded lead–acid battery and lithium-ion battery in 2015 and 2020; dashed line for lithium-ion battery represents the average cost of the three chemistries.

safely operates under a specific set of conditions [45]. A sophisticated BMS, similar to those designed for HEVs and EVs, would ensure the safety of SHLS battery but also add to the cost. Alternatively, it would be advantageous to use a battery chemistry, which is intrinsically safe and a basic, cost-effective BMS is sufficient to ensure the safe operation of the SHLS battery pack. Price of simple BMS based on analog technology ranges from 3 USD per cell in series for a protector BMS to 15 USD per cell in series for a distributed BMS [45]. Fortunately, two of the shortlisted battery chemistries, LFP and LMO/LTO, also show superior safety characteristics. Differential scanning calorimetry (DSC) studies on charged cathode materials show that LFP has the lowest thermal energy compared to LCO, NCA and LMO [46,47]. While in anode materials, charged LTO provides 7 times lower thermal energy than charged graphite [48]. Accelerated rate calorimetry (ARC) studies on charged 18650 cells show that LCO, NCA and NMC chemistries have low onset temperatures and high self-heating rates indicating a higher tendency for thermal runaway compared to LFP and LMO [49]. Thermal runaway in LFP and LMO is driven by the reaction between the graphite anode and electrolyte, following the breakdown of the protective SEI layer. It is then no surprise that LMO/ LTO cells do not show any evidence of thermal runaway in ARC, nailpenetration and bar-crush safety tests and LMO/LTO combination results in a battery chemistry with outstanding safety characteristics [26,48,50]. LMO/LTO has two more advantages over LFP, which can further simplify BMS architecture and reduce cost. Firstly, LMO/LTO charge/ discharge curve has a sloping profile, unlike a flat profile of LFP, which

provides for a simpler algorithm to estimate SoC using voltage translation, coulomb counting or a combination of the two techniques [45]. Secondly, LMO/LTO cells have been shown to be comparatively more resilient to overcharge and overdischarge conditions, simplifying cell monitoring and balancing in the BMS [51,52]. 5.5. Future trends Price of solar PV panel and lithium-ion battery pack for EVs has decreased annually by ~12% between 1976 and 2014 and 8–14% between 2010 and 2014, respectively, in real terms adjusted for inflation [53–55]. Price of lithium-ion battery cells for consumer electronics has decreased faster at ~16% annually in real terms between 1993 and 2005 [55,56]. The price reductions have occurred due to economies of scale and experience curve benefits. Scale-up in global production capacity and efficiency improvements in manufacturing and operations, supply chain and raw materials sourcing have brought down the cost significantly. Learning rate, at which price reduces with doubling of cumulative production, is estimated to be ~24% for solar PV panel and ~22% for both lithium-ion consumer cells and EV packs [53,55]. These price reductions are expected to continue with greater acceptance of solar PV technology for renewable energy generation and development of new markets for lithium-ion battery technology from portable electronics to EVs to grid storage. Lead–acid battery, in comparison, is a 150-year-old technology and both product and process technologies are now mature. Annual price reduction in real terms of lead–acid battery is < 1%, considerably 932

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smaller than that of lithium-ion battery [7]. Learning rate is also estimated to be smaller between 4 and 10% albeit with less certainty, largely due to volatility in lead metal prices [7]. Lead metal cost is 60– 80% of the raw materials cost and the volatility in lead metal price as determined by London Metal Exchange significantly influences the price of lead–acid battery [57–59]. HOMER simulation result for the expected cost of SHLS based on lead–acid and lithium-ion batteries in 2020 is presented in Fig. 14, assuming 1%, 12% and 12% annual price reduction in lead–acid battery, lithium-ion battery and polycrystalline silicon PV technologies, respectively. With faster decrease in price of lithium-ion battery, the price advantage of lithium-ion battery based SHLS is expected to continue to improve. The initial capital cost of lithium-ion battery based SHLS will reduce from ~91% of flooded lead-acid battery based SHLS in 2015 to ~66% in 2020. While TNPC of lithium-ion battery based SHLS will reduce from ~49% of flooded lead-acid battery based SHLS in 2015 to ~31% in 2020. Market growth of lithium-ion battery will in turn fund continuous technology development resulting in improvements in energy density, cycle life, calendar life and safety. On the demand side, it is increasingly being realized that localized offgrid solutions like SHLS and micro-grid are much more effective in terms of cost, performance and time of implementation than grid extension programs for BoP households [6,60]. Convergence of demand and supply factors will make lithium-ion battery based SHLS more attractive as renewable energy solution for off-grid BoP households.

[2]

[3]

[4] [5] [6]

[7] [8] [9]

[10]

[11] [12]

6. Conclusion

[13]

Seven lithium-ion battery chemistries – LMO, NCA, NMC, LFP, LMO/LTO, NMC/LTO and LFP/LTO – were looked as a potential replacement technology for flooded lead–acid battery in SHLS using HOMER microgrid software. The battery chemistries were evaluated on the basis of cost, cell voltage, cycle life, round trip efficiency, SoC range and safety and the resultant effect on the economic, system size and performance metrics of SHLS was studied. Compared to the commercial SHLS, lithium-ion battery based SHLS used smaller size of PV panel and battery and showed higher battery capacity utilization and lower excess electricity. Three lithium-ion battery chemistries – NCA, LFP and LMO/ LTO – showed significant promise with comparable autonomy and initial capital cost to that of the commercial SHLS but with significantly lower (~55%) TNPC over the system lifetime, as no/fewer battery replacements were required. From the perspective of BoP households, a SHLS with no/fewer number of battery replacement has a significant advantage as typically rural banks and microcredit organizations do not provide support for the battery replacement cost. Moreover, it is suggested that a smaller lithium-ion battery based SHLS with a simple, reliable SoC meter can provide an equivalent user experience at a much lower system cost. Among the shortlisted three chemistries, LMO/LTO is the safest chemistry with no evidence of catastrophic failure and, therefore, has the advantage towards simplifying the BMS architecture and reducing the total system cost. Lastly, price trends of flooded lead– acid battery and lithium-ion battery were compared and it is expected that the cost advantage of lithium-ion battery based SHLS will continue to improve in both initial capital cost and TNPC.

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Acknowledgements

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Author would like to thank Prof. Ajit K. Chaturvedi for his support for the study and Dr. Subhas C. Chalasani and Dr. Anand Narayan for reviewing the manuscript.

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