Thomas Samuel *a , Raphael Baillot a
Long Term Energy Storage Challenges
for Solar Lighting Applications in Desert Environments
Abstract
Recent advances in rechargeable battery technologies (the so-called secondary batteries)
change the game in responding to desert environment with reliable solutions. But a wide
range of battery chemistries and the option of a battery management system has made
more complex the understanding of what really makes a difference in battery lifespan.
An extensive research & development program has been undertaken during the last two years
with a French Research Laboratory (Laboratory for Storage of Electricity [LSE] belonging to the
National Institute for Solar Energy [INES]). The objective was to find the most resistant battery
chemistry for desert environments, and improve it’s performance to reach the ultimate lifespan
in extreme climate conditions. This paper presents accelerated aging tests results and points out
how Ni-MH technology has been selected for its efficiency and compliance with Africa or Middleeast desert operating conditions and how it has been imagined to be well managed to double
the standard lifespan of that chemistry. Now a battery can almost achieve an unprecedented 10
year service.
Desert Environment refers to one of the harshest climate that storage technologies have to face.
70˚
Low Rainfall
It is the most obvious
factor of a desert area. Some
deserts receive less than 10
centimeters of rain annually,
and this rain comes in brief
torrents that quickly evaporate
from the ground surface. Even
though humidity does not
represent a major topic, it has
to be considered for avoiding
corrosion
thereby
inducing
an increase of the battery
impedance reversely impacting
the battery efficiency [3].
Solar radiation
&
extreme heat
Air temperature is the most
sensitive environmental factor
that a battery has to face. Some
continents like Middle East and
North Africa (MENA) or SubSaharan Africa can expect a
temperature rising up to 58.0°C
during the day before decreasing
to 10°C during the night [1-3].
Sandstorm & Wind
Some
winds
as
the
“Seistan” desert wind in Iran and
Afghanistan blows constantly
for up to 120 days. Within Saudi
Arabia, winds average 3.2 to 4.8
kilometers per hour (kph) and
can reach 112 to 128 kph in early
afternoon [4]. For stand-alone
solar systems, the weight of a
battery may turn into a major
advantage when that last is set
at several meters of height.
Keywords
Desert environment, Ni-MH battery, Solar-powered systems, Off-grid systems, Heat resistant battery
technology, Battery Management System, Accelerating Aging Tests, Battery reliability, Battery lifetime
A
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Thomas Samuel *a , Raphael Baillot a
Long Term Energy Storage Challenges for Solar Lighting Applications in Desert Environments
Solar-used
storage technologies/Lead-Acid
They are the oldest rechargeable storage technology.
Because of the cost-effectiveness and availability,
lead-acid batteries are the easiest and cheapest
choice for many applications. But they show limited
service life when submitted to both high temperature
and deep cycling (> 80% of DOD). Although they are
widely used for solar applications, there is no smart
energy management systems prevent from blackouts
and high temperature still involves a limited lifespan
inducing regular technical maintenance (battery
replacement).
Batteries
– Set of Sonnenschein Solar’s VRLA batteries –
One of the best lead-acid battery on the market is the Valve-Regulated
Lead-Acid battery (VRLA technology)) and complying with solar
applications. The rate of their self-discharge is lower than 2% per month
at 20°C. However, the self-discharge doubles for every increase in
temperature with 10°C [6].
P e r f or mance ch a r act er is t ics & battery m a nagem ent system
As
numerous
battery
technologies,
temperature and deep cycling remains the first
enemy of a battery. The effects of temperature
and deep discharge on battery performances are
illustrated by fig. 1 [7]. According to the experiment
conducted, 80% depth of discharge should not be
exceeded if full life expectancy is to be attained.
Fig. 1 – Trojan’s deep cycle VRLA-AGM performance characteristics:
(a) Cycle life vs Depth-Of-Discharge, (b) Temperature vs rated capacity
“
Lead-Acid batteries need to be well
managed (smart BMS) and heat protected
to comply with solar applications in desert
environment.”
Depending on the manufacturer, the service life of a VRLA battery submitted to high temperature
and deep discharging may activate three failure mechanisms [6,8-9] :
Shedding of the active material
due to deep cycling (> 80% of
DOD)
Corrosion of the positive
electrode
The effect of repetitive chemical
reaction of the active material
in the plate grid tends to reduce
cohesion, thereby involving active
material sink into the bottom of
the battery.
This phenomenon happens when
a battery reaches the end-ofcharge, i.e. a high voltage. The risk
is to start a slow but continuous
oxidation process that results
in disintegration of the positive
electrode (plate grid).
Re cycli n g & So u r cin g
Sulphation mechanism
This well-known phenomenon
occurs during battery discharging.
The active mass in both the
positive and negative electrodes
is thus transformed into very
small sulphate crystals forming
an impenetrable layer that cannot
be reconverted back into active
material. The rated capacity
decreases until the battery
becomes useless.
By now, most of VRLA batteries are now widely available and almost fully recyclable. Several international
recycling platforms exist (such as European Recycling Platform - ERP). But these platforms are seldom
accessible in emerging countries, and the value of the components being very low, the overall rate of battery
recycling in remote areas is close to zero. The health hazard for health is high when batteries are open or left
in standard waste.
A
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Thomas Samuel *a , Raphael Baillot a
Long Term Energy Storage Challenges for Solar Lighting Applications in Desert Environments
Solar-used
storage technologies/Nickel-Based
NiMH batteries have become very popular over the last
decade, especially for industrial applications as power
tools or hybrid vehicle applications. There have been
many successful improvements of the Ni/MH battery
performances through efforts made on hydrogen storage
alloys to achieve higher energy density, faster activation,
better rate capability and lower cost [11]. It has made NiMH suitable for off-grid and small PV systems, particularly
regarding solar street lighting [12].
Batteries
Example of 12V Ni-MH battery [13]
Solar applications may expect a service life close to 5 to 7 years if
we consider an average of 50% DOD and about 30°C over 24h of
operation (40°C during the day, 20°C during the night).
P e r f or mance ch a r act er is t ics & battery m a nagem ent system
Ni-MH offers reasonable specific energy (55 to 70 Wh/kg) and covers one of the largest temperature range
that available rechargeable battery technologies can afford (-40°C to +70°C) nowadays. This maintenance
free technology integrates a safety valve in case of cell temperature increase and shows very good thermal
properties without memory effect.
For low current rate, generally used in solar
off grid systems, no capacity losses can be
expected. High current rates (> 4A) often lead
to capacity losses due to oxygen evacuation
through safety valve in case of overcharge. [1415].
The major drawback of Ni-MH results in
the high rate of self-discharge due to shuttle
reactions between chemical ions and the KOH
electrolyte [16]. However, the self-discharge is
not linear (high rate @ full charged battery, low
rate below 70% of SOC . Those values depends
on manufacturer).
Such parasitic chemical reactions can be
easily avoided by using a smart BMS without
increasing the cost of electronics.
Fig. 2 – Schematic representation of Ni-MH cell primary and secondary reactions
The issue with NiMH is often the initial cost of purchase (850 to 1050$/kWh). But its resistance to heat and
deep cycling leads to an incomparable lifespan, making the total cost of ownership lower than with cheapest
battery technologies (see p.6).
Re cycli n g & So u r cin g
“
Ni-MH is the most suitable storage technology
for long term solar applications submitted to high
temperature (up to 70°C) and desert environment”
Ni-MH batteries contain valuable metal as Nickel (approx. 45%). This is a strong encouragement to
recycling as the value of the metal remains even at the end of the battery life. This metal can be further
refined and transformed into active material for new batteries. In that way, the loop “from battery to battery”
is closed and plays a major role in battery cost reduction. Key players and major worldwide companies such
as Sanyo-Panasonic, Matsusshita or ARTS Energy (SAFT) manufacture Ni-MH technologies.
A
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Thomas Samuel *a , Raphael Baillot a
Long Term Energy Storage Challenges for Solar Lighting Applications in Desert Environments
Solar-used
storage technologies/Li-Based
Batteries
The Li-ion batteries represent a comparatively new technology with huge potential in terms of specific energy and
cycle life, without memory effect. Among them, a large scale of technologies, named by their cathode oxides, has been
developed. Fig. 3 shows a comparison with the five principal Li-ion battery technologies [18]. The variety of the options
makes it very complicated to make sure the right battery is available and used for the right application.
“
Lithium batteries have still major
safety issues to overcome before
being compliant with hot & dry desert
environment”
As shown in this figure, the best technology of
interest for small PV applications submitted to
harsh environment refers to LiFePO4 (LFP) in terms
of high level of service life, safety performance
and cost. Nevertheless, temperature and deep
cycling may degrade those performances.
Standard LFP operating temperature ranges
from -20°C to +60°C.
P e r f or mance ch a r act er is t ics & battery m a nagem ent system
A recent study has been undertaken
by CEA-INES on low-cost Asian
LiFePO4 cells for solar street lighting
applications [19]. A photovoltaic cycling
aging adapted from the CEI 61427
standard (initially designed for lead-acid
batteries) has been performed (Fig. 4).
Capacity losses are too high (from
5 to 20% after only 100 equivalent
discharge capacities) and technological
Fig. 4 – Accelerated aging tests of low cost LiFePO4 cells coming from 4 Asian manufacturers:
dispersion
between
manufacturer (a) Calendar aging results @ 45°C, (b) Cycling aging results with tendencies for three type of
cannot ensure the reliability and service lead-acid batteries (dashed lines) : 1 = flooded solar class batteries, 2 = VRLA spiral wound
life on operating conditions, particularly battery, 3 = flooded tubular batteries
regarding solar off-grid application.
However, it is interesting to point out their ability to resist to thermal runaway above 90°C which is the start temperature
published in this work.
LFP is known to be a very thermally stable cathode
material, a thermal runaway could, however, be detected
for some low cost cells especially for combined tests (e.g.
overcharge + heating). This is why safety concerns still
remain a major topic regarding air transportation. Recently
(Feb. & March 2015), Delta Air Lines and United Airlines, two
major US airlines companies, clearly stopped accepting
bulk shipments of the rechargeable lithium-ion batteries
[22].
Fig. 5 – Lithium-ion thermal runaway overview from
cell event to potential system event [23]
Re cycli n g & So u r cin g
Up to date, lithium resources are widely available and can meet the market demand. However, about 70% of the
global lithium deposits are located in Argentina, Bolivia and Chile region. Unrest or instability of the governments in
these regions can greatly affect the supply and may impact on the battery price. There is also a communication issue as
the sourcing of Lithium in these countries raise ethical issues for local population, resulting in increasing press interest.
Although Lithium batteries are 98% recyclable, the reprocessing industry is mostly located in Europe and US. The
batteries containing only 0,8 % of actual Lithium, there is little added value to the recycling. The heavy metals contained
in the batteries represent a serious health hazard when batteries are open or left in standard waste.
A
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Thomas Samuel *a , Raphael Baillot a
Long Term Energy Storage Challenges for Solar Lighting Applications in Desert Environments
Solar-used
storage technologies/Batteries
Comparison
Depending on manufacturer (battery quality, technological dispersion, …)
Depending on heating rate and tests conditions. Some publications indicates start temperatures above 170°C but
those results are widely discussed among scientific communities
1
2
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Thomas Samuel *a , Raphael Baillot a
Long Term Energy Storage Challenges for Solar Lighting Applications in Desert Environments
Major
considerations in selecting a solar battery
Factor s af f ect in g b at t ery cycl e l i f e
Several factors influence the operational characteristics of a
battery and its cycle life. Concerning solar-powered applications,
three important factors have to be taken into account for battery
management as listed hereafter:
• Battery temperature – Operating mode and storage
conditions,
• DOD – Slightly related to the application (LOAD) and battery
technology,
• Battery Management System: The charge and discharge
management is the key element that ensures the battery
cycle life. End-of-charge and end-of-discharge detection
methods prevent from overcharge & over-discharge
phenomena.
Fig. 6 – Schematic view of a battery cell
operation
under
Factor s af f ect in g Tot a l C o st o f Ow nershi p (TCO)
When buying a deep-cycle battery €
specifically designed for solar applications,
there are several factors that should be taken
into account to determine the TCO over the
life of the battery and help decision makers to
choose the best for their application :
Battery replacment required
in hot climate conditions
Best
Lead Acid
Standard
Lead Acid
NiMH
Selling Price
A low-cost battery is always attractive.
This is the case of Lead-Acid
Years
0
2
4
6
Technologies as previously discussed.
However, if the battery cost is obtained Fig. 7 – Total Cost of Ownership vs years of service : comparison with Ni-MH & LA
at the expense of quality and battery life, the cost for end-user over time will strongly increase because
of the need for frequent battery replacements,
Capacity
This is the first dimensioning factor for a solar-powered system. Depending of the manufacturer and
the chosen technology, some low-cost technologies may have a technological dispersion much higher
than 10 to 15%.
Voltage
The battery voltage topology must be considered to ensure it matches the system requirements (solar
module and load). In addition, it is important to note that aging can lead to voltage decrease thus
invalidating the minimum voltage required for application while rated capacity would still be sufficient.
Depth-of-Discharge and operating temperature range
We have previously underlined that both factors are the first affecting battery service life. The higher
they are, the shorter the battery lifetime will be.
Brand & worldwide availability
Choosing a battery from a worldwide-known company is important as end-user may benefit from
the company’s expertise and/or proven experiences on similar applications. A reputable company
may also offer field experiences as accelerated aging tests that are guarantying the performance and
cycle life in a targeted and specific environment. At last, choosing international branded company may
ensure its ability all over the world.
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Thomas Samuel *a , Raphael Baillot a
Long Term Energy Storage Challenges for Solar Lighting Applications in Desert Environments
Field
e x p e r i e n c e w i t h N i - MH & V RL A b a t t e r i e s u n d e r h a r s h
operating conditions for solar lighting applications:
A two-years reliability test for performance comparison
C on te xt & o b j ect i v es
Experi m enta l co nd i ti o ns
The choice of the storage technology is crucial
as it is the weakest point of solar systems. This
is why Sunna Design has chosen to challenge
12V/10Ah Ni-MH technology under accelerated
aging tests simulating environments where
photovoltaic technology best performs. A
comparative study has been launched on VRLA
12V/25Ah storage technologies that are widely
used by Asian players to provide low cost solar
lighting solutions.
Three accelerating factors have been chosen to
perform this study :
• Deep cycling : 90% of DOD has been applied to Ni-MH
and VRLA batteries to benefit from a tight system
sizing in case of good results,
• A C/5 charge and discharge rate : an average of 2A
for 12V solar lighting topologies has been considered
reasonable given the fact that system is sized by
small mini-modules delivering no more than 2,2A at
the maximum of its capabilities. Such rate involved
2,4 cycles per aging day.
• A temperature average of 45°C during day and night
has been determined to simulate worst cases based
on real temperatures maximals observed in targeted
countries.
“
A smart Battery Management System
is the key point for ensuring a battery
lifespan. Sunna Design expects a 10 years
Ni-MH service life for solar street lights in
desert environment.”
Re s u lt & di s cu s s io n : key b enef i ts o f i ntel l i gent energy
ma n age me n t in a s o l a r l ig hti ng dev i ce
Sunna
Design
investigated
with CEA-INES accelerated aging
and performance tests on NiMH 1.2V cells and 12V batteries.
Fig. 8 shows results.
Those results emphasize the need
for paying attention to methods
used to detect the end-of-charge
and prevent from premature aging.
It is also important to point out the
strong influence of the combination
of deep cycling and high temperature
on VRLA battery. According to
those results, VRLA solar batteries
reach their failure criteria (5Ah of
rated capacity) in only 80 days
with smart management. Standard
management methods for VRLA are
based on voltage threshold (High
voltage
Disconnect)8regarding
end-of-charge.
TAMBIENT = 45°C (night & day)
I = 2A (charge & discharge)
DOD = 90%
Failure criteria = 5Ah of rated capacity
(12V/10Ah NiMH battery)
Failure criteria = 5Ah of rated capacity
(12V/25Ah VRLA battery)
80 days
9,37 years
258 days
Fig. 9 – Comparison of VRLA and Ni-MH batteries accelerated aging tests (45°C / 2A rate /
90% of DOD) by using standard management (130% of overcharge) and Sunna management
(overcharge avoided)
As for Ni-MH technology, standard management induces a 258 days of service life under the same aging
conditions. By using Sunna management system, a 9,37 years of service life can be reached by considering
the same failure criteria as VRLA batteries, i.e. a 5 Ah threshold of rated capacity.
A
Sunna Design SA, Centre de services Technowest, 17 rue du commandant Charcot, 33295 Blanquefort Cedex – France
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Thomas Samuel *a , Raphael Baillot a
Long Term Energy Storage Challenges for Solar Lighting Applications in Desert Environments
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A
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