Hybrid energy storage systems for high power spacecraft missions

This work aims to analyze the feasibility of utilizing hybrid storage systems to enable the operation of high-power payloads during eclipse periods. The main objective of the study is to reach possible configurations with the same performance as traditional designs, but with reduced mass and/or volume, or to maintain the mass and volume while increasing the peak power capabilities. The proposed solution involves the use of supercapacitors and high-energy lithium-ion cells, with the former serving to meet peak loads and the latter supplying the mean power needs. Additionally, a simple architecture for the electrical power system is proposed, and the sizing equations for the supercapacitors and lithium-ion cells are derived from the governing power and energy balance equations. The results allow well-founded decision-making process on the suitability of the hybrid configuration based on the peak power demand and its duration, as well as the mean power demand during eclipse operations.


Introduction
The electrical power subsystem (EPS) plays a crucial role in spacecraft as it continuously supplies power to all active subsystems.Its components are responsible for generating, regulating, controlling, and distributing power, as well as storing energy [1].Reliability is essential for the EPS, regardless of the spacecraft's mission, as any malfunction can disrupt spacecraft operations and prematurely end the mission.Aerospace platforms, particularly micro-and small satellites, have stringent constraints on the size and weight of their components.EPS represents a significant portion of the total spacecraft mass and volume.Therefore, substantial efforts are needed to minimize size and weight without sacrificing performance, reliability, or incurring additional costs and complexity.A major achievement in reducing the mass of EPS was the replacement of traditional NiCd and NiMH with Li-ion batteries because of their high specific energy and life-cycle.This switch resulted in a reduction of approximately 40% in the components of the energy storage system [2].
Li-ion technology has become prevalent in spacecraft energy storage systems.However, Liion cells have limitations.Although they have high specific energy, their maximum depth of discharge (DoD) must be limited to ensure a long life-cycle.Moreover, their power density is low, imposing restrictions on charging and discharging currents.Furthermore, Li-ion cell performance significantly degrades at low temperatures, which poses challenges during eclipse operations.During eclipses, the battery must meet the entire power demand.Thus, the spacecraft operation modes are limited to avoid the concurrent activation of high-consumption payloads.Additionally, due to the low temperature, the battery capacity and peak power capabilities decrease.Therefore, batteries need to be built with a higher number of cells, increasing mass and volume, and consequently, their energy storage capabilities are oversized.
To address these issues, supercapacitors (SCs) can be used.SCs are high-capacitance capacitors that offer power and energy storage performance between electrochemical batteries and capacitors (see Table 1).Compared to batteries, SCs can be charged or discharged at a much higher power rate.However, the energy density of modern SCs is still significantly lower than that of batteries.These power and energy density characteristics make SCs suitable for high-demand but short-duration power operations [3].Furthermore, SCs offer advantages such as a wide thermal operation range and better thermal stability, higher efficiency compared to batteries, longer life-cycle, and the ability to charge and discharge fully.Nonetheless, SCs have a higher self-discharge rate and their capacitance decreases with frequency, so their use is restricted to direct current (DC) applications.During the past two decades, multiple Commercial Off-The-Shelf (COTS) SCs have undergone rigorous tests, including radiation, thermal vacuum, shock, and vibration tests [5][6][7][8][9][10][11][12][13][14][15][16][17][18].The results of these studies are promising, demonstrating that although COTS SCs are not specifically designed for space applications, they can withstand the launch phase and the harsh conditions of the space environment during operation.To the best of the authors' knowledge, the CSUNSat1 satellite [19] was the only satellite that operated with a hybrid Li-ion/SCs energy storage system.Additionally, it is also fair to mention that the Ten-Koh satellite carried SCs as payload to demonstrate their feasibility in space [16].In both missions, the SCs performed similarly to ground tests.Therefore, ground and in-orbit tests suggest that COTS SCs could be excellent candidates for future space applications.Indeed, they can be employed to operate high-consuming payloads during eclipse periods, so the above-mentioned problem of requiring large batteries disappears.
Designing Hybrid Energy Storage Systems (HESSs) entails addressing numerous technical challenges.For instance, considerations such as ensuring voltage compatibility and precisely managing the charge and discharge rate of the energy storage components become paramount.To optimize power flow, the implementation of a sophisticated control and management system is required.Additionally, the importance of redundancy for both storage devices in the event of potential failures cannot be overstated.This inherent complexity, surpassing that of conventional EPS, exerts influence on critical aspects such as performance, reliability, cost and duration of the project.
The above mentioned challenges need to be addressed during a detailed design phase and are out of the scope of this paper, which is focused on an initial sizing.
This work describes a straightforward approach to conduct preliminary design of HESSs, capable of increasing the peak power capabilities while maintaining or reducing, in relation to the traditional electrochemical storage systems, the mass and/or the volume of the EPS.Batteries are sized to supply low power levels and also to charge the SCs bank (SCsB) after its operation.Regarding the SCsB, it is sized to supply the required power to operate highconsuming payloads.
This paper is organized as follows: the proposed sizing procedure for spacecraft HESSs is included in Section 2. By implementing this approach, a preliminary design of HESSs for different power profiles is conducted in Section 3. The results obtained are used to compare the mass and volume reductions between traditional and hybrid systems.Finally, the conclusions of this study are summarized in Section 4.

Sizing of the HESS
The HESS sizing procedure is based on meeting the peak power and energy requirements during eclipse operations.The design of the SCsB is completed when the EPS architecture and the series-parallel configuration of Li-ion and SCs cells are set.
Let us consider a simple EPS architecture like the one shown in Figure 1a.In this architecture, the solar array is connected to the primary bus (PB) through a series regulator (SR) that extracts the maximum solar power.The PB supplies low-and medium-power levels to the subsystems.During daylight, the excess of power at the PB is transmitted to the battery through the battery charge regulator (BCR) and also to the SCsB through the SCsB charge regulator (SCsBCR).During eclipse periods or in the case that the generated solar power does not meet the PB power demands, the battery supplies power through the battery discharge regulator (BDR).Additionally, a secondary bus (SB) is responsible for supplying power to high-demand payloads.In both daylight and eclipse periods, the SCsB discharge power is transmitted through the SCsB discharge regulator (SCsBDR).To simplify the analysis, a constant value of η c will be used to consider the efficiency of all energy paths.Now, we assume a simple eclipse power profile (see Fig. 1b) in which the PB has a constant power consumption of P 0 and the SB supplies a number of n c peak power pulses of duration t p and consumption P p .By applying the power and energy balance equations, both SCsB and the battery can be sized to meet the power and energy requirements during eclipse.

SCsB sizing
The performance of a SC cell can be described by its capacitance, series resistor, and maximum voltage and current (i.e., C SC , R SC , V SC,max , and I SC,max ).Then, if an SCsB is constructed with n p sc parallel strings of n s sc cells in series, the equivalent electrical parameters are: Now, if power conversion losses of SCsBDR are considered, the discharging power, P SCsB , that the SCsB must supply is: Then, to operate without exceeding the maximum discharge current, I SCsBmax , it must operate above a minimum output voltage V o SCsB min given by: From the analytical expression of a constant power (CP) discharge, and considering a fully charged SCsB, the discharging time to reach V o SCsB min , is: where V o SCsBmax is the maximum output voltage and is solve from the following equation: Finally, once V o SCsBmax is known, the calculated t d (see Eq. 7) must be larger than the pulse duration, t p .This is an iterative process that ends whenever a minimum value of n p SC n s SC that satisfies the requirements for maximum current and discharge duration is reached.

Battery sizing
Similarly as in the case of SCs, the performance of the Li-ion cell is described by its capacity, series resistor, nominal voltage, and maximum discharge current (i.e., C c , R c , V c , and I cmax .If the battery is constructed with n p parallel strings of n s cells in series, the equivalent electrical parameters are the following: As mentioned above, the battery must supply the PB power P 0 .Additionally, it has to charge the SCsB n c −1 times.If power conversion losses through the BDR and SCsBCR are considered, the energy demand for the battery is obtained as follows: where E c is the energy transferred to the SCsB after each pulse and t e is the duration of the eclipse.The charging time, t c , can be calculated considering equispaced peak loads as: Now, assuming that this charge is done at constant current (CC), from the capacitor equation, the charging current is: Then, the required charge energy, E c , is calculated as follows: Once the required battery energy, E, is calculated, the following relationship must be met: where DoD is the maximum allowable depth of discharge of the battery.Finally, the battery discharge current cannot exceed the maximum value, I Bmax .Then, the following relationship must also be met: where V B min corresponds to the battery voltage at the DoD considered.Again, this is an iterative process that ends whenever a minimum value of n p n s that satisfies the maximum current and the energy discharge requirements is reached.

Results
In this section, the potential mass and volume reductions that can be achieved through the hybridization of spacecraft energy storage systems are investigated.Specifically, the required mass and volume of batteries alone to those of HESSs that combine batteries and supercapacitors are compared.The Li-ion cell with the highest energy density and highly used in space missions and the only SC successfully employed in a satellite, INR18650-35E [20] and BCAP0310 P270 [21], respectively, have been selected for this study.The main characteristics of these cells are included in Table 2. Furthermore, several parameters of the eclipse operation have been considered, including the PB power consumption, SB power (peak) consumption, the duration of peak consumption, and the number of peak power cycles (P 0 , P p , t p and n c , respectively).
To give a general overview of the results, color maps have been obtained for the relative increment of the mass and volume of the hybrid system relative to the traditional one (see Figs. 2 and 3).The objective of these figures is to be a tool to quickly decide which system is better in terms of mass or volume.[20] and the Supercapacitor (BCAP0310 P270) [21].According to the results, for low values of P p , the hybrid system consistently has a higher mass, while its volume is generally higher except when t p and n c are low and P p is high.When P p is very high, the mass of the hybrid system is lower than that of the traditional system, except for the high values of n c .Therefore, the implementation of a hybrid EPS is best suited for low-repeated and short pulses of high power.It is worth mentioning that certain combinations of the parameters studied can result in mass reductions of up to 80%.However, even small reductions (20%) in mass can lead to significant increases in volume (above 50%).This suggests that optimizing mass reductions may not always lead to a reduction in the overall volume of the system.Therefore, a careful analysis of the power profile is essential to determine the optimal balance between mass reduction and volume reduction.Unlike the traditional system, the design of the hybrid system is affected by t p and n c .Higher values of t p require more energy to be stored in the SCsB.Consequently, for the same P 0 and P p requirements, a larger number of SCs cells are needed when t p increases.Furthermore, the mass and volume of the system increase significantly as the number of pulse cycles, n c , increases.This

Conclusions
A new approach has been developed for designing HESSs for spacecrafts.These systems integrate electrochemical cells with an SCsB to optimize the mass and/or volume of the EPS in missions requiring high-consuming payloads.This approach uses analytical expressions derived from a simplified SC equivalent circuit model.It utilizes the CP mode to predict SC performance at peak power and employs the CC mode to charge the SCsB.By applying this approach, HESSs, composed of widely-used Li-ion cells and space-qualified SCs, have been sized to meet the power requirements of numerous consumption profiles during eclipse periods.The study's findings demonstrate that hybrid energy storage systems offer a significant reduction in mass, and in certain scenarios, even a reduction in volume.This advantage is particularly notable when the power profile involves high-power loads of short duration and a low number of repetitions.Finally, if SCs with higher energy density are considered, even more promising results can be obtained.

Figure 1 :
Figure 1: (a) Proposed spacecraft's HESS with maximum power point tracking (MPPT) architecture and full bus regulation; and (b) Example of power profile during eclipse.

Figure 2 :
Figure 2: Relative increase in % in mass between the HESS with respect to the traditional one as function of the PB power, P o , and SB (peak) power, P p , for different values of number of pulses, n c , and pulse duration, t c .

Figure 3 :
Figure 3: Relative increase in % in volume between the HESS with respect to the traditional one as function of the PB power, P o , and SB (peak) power, P p , for different values of number of pulses, n c , and pulse duration, t c .

Table 2 :
Nominal capacitance, nominal voltage, maximum continuous discharging current, mass and volume (C n , V n , I max , m and V , respectively) for the Li-ion cell (INR18650-35E)