IEEE Electrification - December 2020 - 13

The efficiency of
solar cells can be
improved further by
using a concentrator,
which uses lenses
and curved mirrors
to focus and direct
solar irradiance onto
the multijunction
solar cell.

The nominal performance data,
such as open circuit voltage (v OC),
maximum power voltage (v MPP), short
circuit current (i SC), and maximum
power current (i MPP), of PV solar cells
for space applications are provided
according to AM0 (28 °C). These solar
cells are usually serially connected to
increase their voltage level to become
v = n S # v PV (n S is the number of serially connected solar cells). This scaling
is usually referred to as a PV solar
string. To increase the current level,
multiple strings are paralleled so that
the delivered current becomes
i = n p # i PV (with n p being the number
of paralleled PV solar cells or strings).
Solar-generated power is largely
affected by solar irradiance and temperature, as expressed by (1), where this power is proportional to the solar irradiance and inversely proportional to
the temperature. Accordingly, the generated power is null
during the eclipse, while it is variable in sunlight, as
shown in Figure 7. As can be seen in this figure, the generated power decreases when the satellite approaches the
Sun since the temperature is higher there.

Energy Storage in the CubeSat
For continuous operation of the CubeSat, batteries are
employed as a backup and charged during high solar irradiance levels, when the power generated by the solar
arrays is higher than the load's power. The batteries are
used for the eclipse's duration as well as under the high
solar irradiance levels during peak loads. The size and
technology of batteries for satellites are selected based
on the capacity and power-delivery requirements, temperature ranges, number of expected service cycles, and
physical size and weight. Due to these factors, nickel cadmium (NiCd) has been widely used in CubeSat missions
(Wertheimer et al. 2015). Nevertheless, this battery technology has been replaced by lithium (Li) ion because it
offers higher energy density, higher operating voltage,
and higher service cycles and reliability in general (Robyn
et al. 1995).
The integral of the current over time is defined as the
nominal capacity (C) of the battery, and its symbolized
unit is referred to as ampere-hour (Ah). This current integration is from a fully charged battery state to a fully discharged one at room temperature. In other words, the
battery capacity is the indication of the C ampere that
the battery is able to deliver in 1 h, and in some cases, it
is expressed in C/n, which means the C ampere in n
hours. Regardless of the battery chemistry, their capacity
degrades with the count of charge/discharge cycles, the
quantity of charge change in each cycle, the discharge
current, and temperature. With regard to the discharge

and charge current, it is usually indicated as the C-rate, where 1 C is the
current under which the battery
would charge or discharge in 1 h. In
most cases, the Li-ion batteries
charge and discharge under a maximum current level of 1 C; nevertheless,
there have been some developments
on this type of battery where they are
able to discharge at under 2 C or
higher. To obtain the nominal battery
energy, its nominal voltage is multiplied by its capacity, and its unit
symbol is watthour (Wh).
Similar to the case of PV cells, batteries operate under low voltage, and
they are, therefore, serially connected
to increase the operating voltage, thus
forming a battery string, while to further increase the capacity, i.e., current, these strings are
paralleled. During the selection process of the battery for a
space mission, several factors need to be taken into
account to have an estimation of the battery's end of life
(EOL). Among these factors are the number of charge and
discharge cycles of the battery, especially for satellites in
LEO, as their number of cycles could reach up to 16 cycles/
day. The depth of discharge (DOD), which indicates the
percentage of how much energy can be used from the battery, is another factor. To estimate the battery's DOD, the
deliverable capacity is usually divided over the total one,
as DOD = C deliverable /C. Another way is to estimate the battery's state of charge (SOC), which defines the minimum
SOC that the battery could bear without the risk of damage, as SOC = C minimum /C. A large SOC, or short depth of
charge, helps in improving the battery's lifetime; however,
that would be at the expense of a larger-size occupying
battery. DODs ranging from between 30 to 50% are usually
selected, which are in accordance with the manufacturers'
recommendations. Other factors of paramount importance are the battery's maximum charge voltage, known
as the end-of-charge voltage, and the minimum discharge
voltage, known as the end-of-discharge voltage. Any

PPV
Eclipse

Torbit/2

Torbit

Time

Figure 7. The PV power profile during one orbit.

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 2 0

IEEE Electrification - December 2020

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