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Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV
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Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV Giosue Giacoppo a,*, Orazio Barbera a, Nicola Briguglio a, Francesco Cipitı` a, Marco Ferraro a, Giovanni Brunaccini a, Eric Erdle b, Vincenzo Antonucci a a
CNR, Institute for Advanced Energy Technologies “Nicola Giordano”, Via S. Lucia sopra Contesse n. 5, 98126 Messina, Italy b EFCECO, Germany
article info
abstract
Article history:
In this paper, the integration of a small SOFC commercial system into the fuselage of a
Received 9 May 2017
mini Unmanned Aerial Vehicle (UAV) is presented. As a design constrain, the SOFC
Received in revised form
system has to be installed inside the UAV fuselage with the lowest possible offset, to
24 July 2017
reduce the volume and mass of the UAV. Due to the high operating temperature of the
Accepted 3 September 2017
SOFC (800e1000 C), the external temperature of the system is always about few hundred
Available online xxx
Celsius degrees. Due to this, malfunctioning of the SOFC system and hot spots on the fuselage shell can occur. For this reason, it is important to ensure a proper ventilation of
Keywords:
the air volume inside the UAV fuselage. To deal with these issues, experimental and
SOFC fuel cell
Computational Fluid dynamic studies were carried out to investigate for a correct SOFC
Temperature distribution
system integration and operation in a confined environment. As a result, the optimal
CFD
airflow for a safe operation of the SOFC system was determined and the behaviour of the
System integration
temperature and air stream inside the fuselage was highlighted. In addition, NACA air
Unmanned aerial vehicle (UAV)
intakes were designed on the basis on the experimental and numerical evidences, to provide a proper cooling of the SOFC system installed into the fuselage. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells have received increasing interest for their ability to effect efficient, quiet and clean conversion of chemical to electrical energy. This attribute, make fuel cells an attractive power source for electric propulsion in Unmanned Aerial Vehicles (UAVs) [1]. These particular type of aircrafts, encouraged by recent technological developments, have seen a dramatic interest boost in recent years and are already considered as an integral and indispensable part of modern armed forces, with an increasing number of dual use and civil
applications. Indeed, they are ideally to provide surveillance, remote sensing and communication relay capabilities. The current propulsion systems of UAVs are based on different types of internal combustion engines fed by fossil fuels. They are the main cause for the increase of CO2 presence in the atmosphere, declared to be one of the most important culprits of global warming and the atmospheric emissions of other pollutants. There is therefore an urgent search for new energy policies based on the diversification of energy sources and their origin, energy saving policies, and the use of efficient energy conversion systems, for these reasons, fuel cells have
* Corresponding author. E-mail address: [email protected] (G. Giacoppo). https://doi.org/10.1016/j.ijhydene.2017.09.063 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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started to be introduced. These have advantages in terms of endurance, efficiency, emissions, and stealth, which make them ideal for UAV applications. UAVs are in their nascent stage of development (in fact, their regulations are in process of being written), although the implementation of fuel cell propulsion systems is more advanced than in conventional aircrafts. The main reason is the fact that UAVs are unmanned, thus, weighting comparatively less and not needing the life support systems for the crew and passengers, making them ideal for this new technology. In addition, fuel cells are still too heavy to propel any large aircraft; they have a lower power density when compared with conventional turbines. As an alternative to conventional propulsion, three main types of propulsion systems can be considered in UAVs: (i) alternative thermal systems: where different thermodynamic cycles, fuel, or engine types can be used (e.g., spark-ignition reciprocating engines fuelled by gasoline); (ii) electrical systems: where the power required is obtained through an electric motor and power is generated or stored in different ways; (iii) hybrid systems: combining any of the systems listed above, even the same type (e.g., a combination of fuel cell and battery or Regenerative Fuel Cell Systems, which combine fuel cell, battery, and photovoltaic cells) [2]. The possibility of implementing fuel cell technology in Unmanned Aerial Vehicle (UAV) propulsion systems has been considered in the recent literature, [2e7]. As stated above, the main advantage of these devices is strictly related to the absence of humans on-board that permits to reduce the operational costs and allow them to be used under hazardous conditions. Whereas, one of the limits of these vehicles is related to the flight endurance that is actually in the range of a couple to 10 h, for the electric powered UAVs [8]. Because of their performance, Fuel Cells have recently found their first aviation applications as power plants for small-scale, long-endurance and long-range UAVs. Solid Oxide Fuel Cell (SOFC) technology lags Polymer Electrolyte Fuel Cells (PEFCs) by a number of years but presents an opportunity for many applications, including aviation, due to its fuel flexibility. SOFCs should be better than PEFCs in UAVs. The reason is that propane, in an aluminium container has a much better energy storage density than hydrogen gas in a pressurized cylinder used in PEFCs [9]. SOFC has a wider range of fuel due to its ability to convert reformate. For UAVs low weight is the most important issue, with the aim of extending the time of flight. A SOFC powered UAV would be able to take advantage of the fuel flexibility of SOFCs and hence use a fuel more readily available to the consumer, easy to store and with a high energy density such as propane. Low power PEMFC and DMFC are usually restricted to using hydrogen or methanol. Moreover, SOFC technology offers some interesting opportunity in terms of integration into turbo-gas power plant. Aguiar et al. proposed SOFC, in combination with gas turbine in Ref. [10], where different configurations of the hybrid system were modelled and analysed for powering a high e altitude, long endurance UAV. Microtubular SOFCs (mSOFCs) running on catalytically reformed propane were chosen for integration with UAV due to rapid start-up, cyclability and lower cost [11e13]. A key issue for portable micro-fuel cell applications is the use of a fuel, which is readily available to the consumer. AMI produced and flight tested a mSOFC-powered mini-UAV in 2006, achieving an endurance of nearly 5 h for the 2 kg UAV
[14]. Micro-SOFC systems have shown many desirable characteristics over systems with planar SOFCs [15,16] Microtubular SOFCs systems can accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. They are mechanically stronger than planar designs and can be made with materials, which withstand long-term operation. Small-scale or microtubular SOFC systems demonstrated in the mid 1990s by Kendall et al., endured thermal stresses caused by rapid heating [11,17]. The state of the art in microtubular Solid Oxide Fuel Cells has been defined in a large number of papers and patents [18]. Recently K. Kendal [16] described how small zirconia-based tubes around 2 mm in diameter, have been developed to give great advantages over the standard planar systems which have been used more conventionally by most SOFC manufacturers. Various papers have focused the attention on this significant advantage of mSOFCs. Bujalsky et al. [19] showed that microtubular SOFCs allowed much faster starting time (almost 10 times faster than others did) without mechanical damage. Galloway et al. discussed about the reduced degradation of microtubular SOFC, when operated at intermediate temperature and under start-up and shut down cycles [20]; Suzuky et al. [21] discussed about how the energy efficiency of micro tubular SOFC was mainly correlated to the decrease of the gas transport polarization resistance. Lindal et al. simulated, designed and validated a SOFC e based propulsion system for UAV [1]. Fernandes et al. presented the results of a “Well to Wing” efficiency analysis of liquid hydrogen produced through biomass for aviation. The authors studied a power chain composed by a biomass gasification plant, a hydrogen liquefaction unit and a SOFC e gas turbine system [22]. In addition, other authors focused their attention on the anode formulation to improve the redox stability of SOFC [23,24]. An experimental approach for the determination of the convective heat transfer coefficient has been reported in Ref. [25] by Barroso J. et al. The paper is dealing with a high temperature PEM fuel cell stack, intended for use in a UAV, which was tested in different rectangular tunnels equipped with thermocouples. To evaluate the behaviour of the fuel cell stack in low temperature environment, such as the fuselage of an UAV when it got et al. tested a PEM fuel cell stack in a is at high altitude, Be climatic chamber [26]. Moreover, many research papers dealing with the modelling and design of UAVs with hybrid power (battery þ fuel cells), consider the PEM fuel cell as power plant [27e34], only few, are focused on the use of a SOFC fuel cell. Beside this, to the best of our knowledge, there is a lack in the scientific literature concerning the thermal issues related to the integration of a SOFC power plant inside the fuselage of an UAV. In the present study, these issues were faced up from both experimental and numerical point of view. Experimental measurements allowed to highlight the main issues related to the operation of the SOFC generator in a confined environment, such as internal temperature distribution and external cooling flow rate. Numerical model were validated through experimental results and then used for the design of the cooling air intakes in the UAV fuselage. The paper is structured as following: i) description of the SOFC generator specifications and UAV integration concept; ii) setup of the experimental bench for testing the SOFC generator in a confined environment; iii) set-up of the numerical model
Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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and validation through experimental measurements, iv) design of the UAV fuselage allocating the SOFC generator with air inlets for cooling; v) numerical model of the final fuselage design with integrated air intakes; vi) conclusions.
SOFC generator specifications and UAV configuration layout The power plant configuration of the UAV is imagined as a hybrid system, where both the battery and the SOFC Fuel cell supply the electrical power needed at each phase of the UAV mission. The mission profile used in this study as a reference is described in Fig. 1 a) and comprises a series of climbs, cruises and descents; each characterized by a specific electrical power demand. Particularly, the climb phase is assumed to consumes almost all the available installed power. The cruising power, correspondent to the 10e20% of the maximum power, is delivered during the most part of the mission. For this reason, the average power, needed during cruising, is delivered by the SOFC generator, while peak power needed for high power demanding manoeuvres, such as takeoff, landing and turns, is provided by the battery. Moreover, the SOFC generator will also serve to recharge the battery when this is not used for the propulsion. The UAV is conceived in a Pod configuration, such that the electric power system (the SOFC, the battery and the fuel tank) the payload and all services necessary for guidance and telemetry (autopilot, avionics, instruments, cameras, etc.), are
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appended to the aerodynamic surfaces and the motor (Fig. 1 b) and c)). The SOFC generator hosted inside the Pod is the D245XR from Ultra Electronics [35]. It consists in a lightweight system (2.6 kg) that includes: a SOFC stack with a nominal power of 245 W and 28 V, an integrated Catalytic Partial Oxidation (CPOX) unit which treat the fuel (in this case propane) before entering the SOFC stack, the auxiliaries (such as fans and cathode blower) and the electronic controller, which allows the system to autonomously operate (see Fig. 2). The size of the SOFC generator is about 400 140 140 mm for a total volume of about 8 L.
Experimental and numerical methods During the operation, the SOFC generator surfaces can reach high temperature in the order of hundreds degree C, thus a proper ventilation around the SOFC generator is necessary, to preserve the integrity of the Pod from burning and avoid excess of temperature inside the Pod, that can be dangerous for the electronics. Consequently, the Pod cross section need to be maintained as minimum as possible to reduce aerodynamics drag but has to be sufficient to allow a proper airflow around the SOFC generator. For these reasons, the SOFC generator was tested in a confined environment that is representative of the SOFC generator integrated inside the UAV Pod. Moreover, a simulation study was carried out trough numerical Computational Fluid Dynamics (CFD) simulations to investigate both the behaviour of the airflow and the
Fig. 1 e a) and b) POD configuration of the UAV, c) UAV Mission Profile. Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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Fig. 2 e SOFC generator adopted for experimental study and integration scheme into the UAV Pod.
temperature distribution inside the fuselage and to understand how the environment inside the Pod has to be maintained ‘safe’ for the operation of the SOFC generator.
Test bench set-up A tube made of PVC with an inner diameter of 200 mm was used to place the SOFC generator inside it and test its operation in a confined environment. The tube internal diameter was chosen according to the prescription of the SOFC generator manufacturer, leaving a clearance of 20 mm all around the generator diameter. The main purposes of the test bench were: i) to find the correct set of operative parameters (such as air inlet flow rate and maximum operative temperature) to allow a safe operation of the SOFC generator; ii) to measure the temperature of some representative points inside the tube, finding “hot areas”; iii) to study the temperature behaviour with the change of the air outlet (exhaust) cross section; iv) to obtain a set of experimental data useful to validate a thermo-fluid dynamic model. A controlled airflow rate was flown inside the tube to remove the heat released by the generator during its operation. To measure the temperatures, 16 thermocouples were placed upon the SOFC generator surfaces and in close contact with the internal side of the tube wall, upstream and downstream the SOFC generator. Only one thermocouple was externally placed, to measure the temperature of the exhausts gases of the SOFC stack. Due to the limitation of the available space in the experimental volume, a reduced, but representative, number of thermocouples and sensors were used, to minimize the interactions between the studied phenomenon and the measurement setup. Air at inlet was fed by a mass flow controller such that the electronic board was maintained below 50 C. This temperature represents the maximum environmental capability during operation. Thus, the mass flow rate can be increased up to 400 l/min in case the controller temperature raise up to the limit. Fig. 3 depicts the scheme of the test bench and the thermocouple position to monitor the temperature trend during operation. To study the effect of the exhaust area on the temperature inside the tube at the specific location, a special cap with variable openings was designed. This cap
allows the discharge area to be changed during the test, without any interruption, in this way the change in the area is immediately reflected in the temperature distribution. The cap is composed by a circular support that is progressively covered with circular parts until it is completely closed. In this way, four combinations of Exhaust Area (EA) were examined 75%, 50%, 25%, 10%. Fig. 4 shows the cap design and the progressive coverage. Two pressure transducers, have been placed perpendicularly to the inlet airflow, at 30 cm away from the inlet and close the exit respectively, to measure the air pressure drop within the tube (DP ¼ Pout e Pin). The final setup of the test bench is showed in Fig. 5a and b.
CFD methodology CFD computations were carried out to provide a deep inside in the temperature distribution in the Pod and then carry out a suitable configuration of the air intakes necessary for the cooling of the internal volume of the UAV Pod. At first, a numerical model of the experimental test bench was implemented to compare numerical results with experimental measurement and validate the numerical model. Successively, simulations on the designed UAV Pod were carried-out to validate the cooling effectiveness of the air intakes. The analysis was carried out with the commercial code Ansys Fluent [36]. A simplified 3D CAD model of the SOFC generator was build and imported in the pre-processor to extract the fluid volume and create the mesh. The grid of the internal flow consisted of approximately two millions of computational nodes. In both cases, 10 inflation layer were adopted on the wall such that the yþ is maintained 1. A grid refinement was implemented at locations where the gradient of fluid dynamic variables (velocity, temperature and so on) was expected more pronounced. Particularly, fine mesh was implemented close to the cooling fans and the exhausts. Special attention was focused on the mesh quality by controlling the skewness factor and the element orthogonality, to reduce numerical instabilities that generally lead to diverging solutions. For each case, these two factors were maintained below 0.8 and above 0.6 respectively (Fig. 6 c, d). Considering the turbulence parameter at the inlet, the intensity was set at 1% and the
Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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Fig. 3 e Thermocouples location. Inside the test tube: TC2-CH1, TC1-CH5, TC2-CH0, TC1-CH7, TC1-CH1, TC1-CH1, TC1-CH3, TC2-CH5, TC2-CH4, TC2-CH2, TC2-CH3 measure the temperature of the surface of the SOFC generator, TC2-CH6 measures the temperature of the electronic board, TC2-CH0 measures the temperature of the battery, TC1-CH4 and TC1-CH7 measure the temperature of the internal surface of the tube. Outside the test tube: TC2-CH7 measures the temperature of the exhaust of the SOFC stack.
Fig. 4 e Cap design with variable Discharge Area: a) EA 75%, b) EA 50%, c) EA 25%, d) EA 10%.
Fig. 5 e Test bench final setup: a) front view with the controlled air flow; b) rear part showing the variable EA cap.
hydraulic diameter at 0.2 m according to the tube circular geometry. For all the simulations, the Reynolds number, was calculated equal to 93103, it corresponds to a turbulent flow, which was described using the releasable k-ε model. This semi e empirical model is the most common used in Fluid Dynamic numerical studies to simulate mean flow characteristics for turbulent flow conditions. It describes the turbulence by introducing two equations: the first for the turbulent
kinetic energy (k), the second one for the turbulent dissipation (ε). The releasable k-ε model is more responsive to the effect of rapid strain and streamline curvature then the standard k-ε model [36]. The boundary conditions of the problem were: i) velocity inlet, ii) pressure outlet, iii) fixed temperature around the cylinder and the exhausts at 120 C, iv) fixed mass outlet at coolant fans and cathode filter. (0.1 m/s fixed outlet velocity), v) heat flux over electronic board surface @75 W/m2
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Fig. 6 e a), b) 3D CAD model with: SOFC container (1), electronic board (2), SOFC exhaust (3), tube wall/cylinder (4); c) d) tetrahedral mesh adopted for the numerical calculations.
(corresponding to 5 W thermal power about). Fig. 6 shows the CAD model (6a, 6b) and the mesh (6c, 6d). The temperature of the exhausts and around the cylinder was estimated by averaging the surface measured temperatures.
Experimental and numerical results In this section, results concerning the internal temperature profile, measured during the cruising o typical mission of the
UAV are reported. Fig. 7 shows the temperature value, at some location inside the test tube, namely: i) on the battery pack (TC2-CH0), ii) on the electronic board (TC2-CH6), iii) on the cathode air intake (TC1-CH3). This last location is very important to monitor in temperature, because the air inlet serves both for supplying oxidant to the stack cathode and for the internal cooling of the SOFC stack. As the power output of the SOFC generator has been set at 250 W, (nominal power) the heat released in the air surrounding the SOFC generator is constant over time. For this reason the average temperature
Fig. 7 e Temperature trend inside the tube at relevant points with the generator operated at nominal power. Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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inside the Pod tends to increase up to a value that represents the equilibrium between the heat released from the generator and that removed by the exhausted air leaving the Pod. As it is evident in Fig. 7 at a fixed EA the temperature at the different locations, oscillates around an average value. This value increased with the EA reduction as it is expected. This behaviour is most evident at specific locations and particularly near the cathode fan. In fact, by reducing the EA from totally opened to EA equal to 10%, the temperature at the cathode fan inlet, raised up from 35 C to 60 C about, while on the battery pack slight increase from 30 C to 35 C and remains almost constant on the electronic board (40 C about). During the test, the air flow rate was increased from 170 l/min to 200 l/min due to the temperature increase on the electronic board occurred when the EA was changed from open to EA equal to 75%. After this change, the average temperature on the electronic board was always in the safe range as specified by the SOFC generator provider (50 C). From the SOFC generator point of view, the experimental results showed that it is possible to safely operate even if with an EA of 10% is used, moreover some issues were observed on the plastic tube during the test. In particular, the hot gases released by the SOFC stack are exhausted outside the test tube by two extensions made in fiberglass coated with high temperature silicone, directly connected with the generator gas outlets. In these locations, the gas temperature can reach up to 200 C that is higher than the maximum operating temperature of the PVC tube (85 C). For this reason, two burning marks have been detected as Fig. 8 a), and b) shows. To preserve the Pod shell from overheating, it is important to insulate the generator exhausts at the interface with the Pod holes.
CFD validation To compare numerical results with the experimental measurements, two design points were selected, implemented and solved (EA 75% and EA 25%). In particular, the temperature on the cathode air inlet (TC1-CH3) was used to validate numerical results due to its great variation with respect to the decreasing of EA (Fig. 7). This effect can potentially alter the SOFC system operation and cause its failure. For this reason, the CFD model was tailored to correctly represent this phenomenon, by accepting that the implemented model could not exactly follows the evolution of the real temperature of
Table 1 e Local temperature comparison between CFD and experiment. Experiment EA 75% EA 25%
45 C 56 C
CFD
41 C 52 C
DT% 8.8 7.1
the electronic board. Indeed, the calculated temperature of this component resulted almost constant along the EA change (33 C about) and in the same order of magnitude of the experiments (35 C about with EA75%, 38 about with EA25%). This temperature resulted always far from the limit temperature (50 C). Good agreement was found between numerical and experimental results with a difference of about 8% on the local temperature as reported in Table 1. Considering the temperature of the electronic board, Fig. 7 showed that it is slightly affected by the variation of the selected EA. As expected, the temperature is transported from the inlet to the tube outlet according to the air stream. Hot zones are evident close to the generator exhausts and near the exits. Fig. 9 shows the temperature distribution inside the Pod and the velocity streamlines from test tube inlet towards the outlet. It is evident, that temperature increases in the wake of the SOFC exhausts and generally in the rear part of the SOFC generator, when the EA is reduced from 75 to 25%. For this reason, the skin temperature can raise up to 75 C e 80 C that could be excessive for many plastic materials. As Fig. 10 depicts, this numerical result is consistent with the experimental findings where extended burning marks were found at the same position. Thus, for integration purposes is highly recommended that care insulation must be made at the interface with exhaust and the Pod shell. Moreover reducing the EA the rear part of the tube became warmer but in the range of 40e45 C.
Pod design with cooling air intakes Numerical results show that the developed computational model can be a useful tool for the final design of the Pod and particularly for the air inlets design. In order to reduce passive loads (due to the presence of cooling fans or other) dynamic cooling were considered. For this reason NACA, that is the U.S.
Fig. 8 e Burning marks highlighted close the SOFC exhausts.
Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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Fig. 9 e Temperature contours inside the Pod: a) EA ¼ 75% b) EA ¼ 25%.
National Advisory Committee for Aeronautics, (the precursor to NASA, in 1945) submerged inlets a have been designed and located in the front of the UAV Pod [37]. This type of inlets, result in some nice features such as reduced external drag and cleaner aerodynamics. Moreover for low mass airflow rates, the NACA submerged inlet with diverging walls offer improved pressure recovery [38,39]. Fig. 11 shows the final design of the UAV fuselage with NACA submerged inlets in the front and a convergent axisymmetric exhaust placed in the
rear part to release warm air to the ambient during the flight. The geometrical parameters of the NACA submerged inlet are reported in Appendix A. A flow converter was used to redirect the air-cooling on the electronics to prevent over-heating and then any damage to the SOFC generator. In this way, fresh air enters the NACA inlets, cool down the generator external surfaces and exits the Pod through the rear exhaust. According to the experiments, the section of the exhaust was set about 10% of Pod cross section area.
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Fig. 10 e Pod skin temperature contours: a) EA ¼ 75% b) EA ¼ 25%.
Fig. 11 e Pod Design with NACA submerged inlets and cross section where the ramp inlet and the flow converter is clearly visible.
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Fig. 12 e Temperature distribution inside the UAV Pod: a) cross-section side view, b) cross-section top view.
CFD analysis of the final fuselage design with integrated air cooling intakes To validate the proposed design of the Pod CFD simulation was carried out to check the temperature distribution inside the Pod and on the surface of the SOFC generator. For this purpose the same simplified geometry adopted for the previous calculation, was considered and a heat flux corresponding to the power released from the SOFC canister and the electronic board was applied on the corresponding surfaces, the rest of boundary conditions were the same assumed for the previous simulations. For the electronic board, the heat flux was estimated on the power consumption data given by the
manufacturer, whereas considering the SOFC generator, only a fraction of the heat is supposed to be released inside the Pod while the rest is wasted out to the exterior by the exhausts. The effectiveness of the coolant air intakes design is granted when the heat released by the SOFC generator components is less or at least equal than the heat removed by the coolant flow rate at the air inlets. The analysis showed (Fig. 12) the temperature inside the Pod increases from the intake to the rear exhaust. In general, the temperature is always below 30 C in the front part close the NACA intakes, whereas in the rear part the temperature raises up to 50 C about. In the region near the SOFC stack, where the wall temperature is set at 120 C, there is an
Fig. 13 e Velocity streamline: a) cross section side view, the fluid enters the NACA intakes and develops in the Pod interior and exhausts through the rear opening, b) cross section top view, two counter rotating vortex on the electronic board.
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increase of Pod wall temperature up to about 84 C that is incompatible with the most of plastic material. For this reason the use of fibre reinforced material or corrugated insulating sheet is mandatory in those areas (that correspond to the SOFC stack position), to preserve the integrity of the Pod shell. In Fig. 13 the velocity streamlines calculated shows as a portion of the external air is captured from the NACA intakes and redirected inside the UAV Pod. Due to the diverging inlet, the airflow is spread on the electronic board directly, to maintain the temperature below the limit as specified by the manufacturer. The two vortices developed on the electronics, distributes the airflow all around the SOFC generator body, allowing the heat released during the SOFC generator operation, to be exhausted in the ambient through the rear opening.
Acknowledgements The work presented in this paper is part of the SUAV project. The project was funded under Europe's Fuel Cell and Hydrogen Joint Undertaking (FCH JU) Grant Agreement No. 278626. http://www.suav-project.eu.
Appendix A The design parameters of the NACA intake, adopted for the Pod design are reported in Fig. 14. Design parameters are listed in Table 2 and ordinates for the divergent walls are shown in Table 3.
Fig. 14 e NACA inlets Design parameters.
At a cruise speed of 10 m/s the calculated inlet flow rate was of 240 l/min that is sufficient to allow the SOFC generator to properly operate as verified in the experiments.
Table 2 e Design parameter of NACA intake. w (mm) 45
dt (mm)
lr (mm)
a (deg)
5
171
7
Conclusions A commercial SOFC generator to power an UAV, was tested in a confined environment. Experimental results led to conclude the selected generator is able to work properly inside an UAV Pod with a minimum flow rate of fresh air entering the fuselage Pod of 200 l/min. Experimental results, showed the SOFC generator exhausts and fuselage interface need to be properly insulated to avoid over-heating of the Pod shell. A numerical model of the Pod was implemented and validated showing an acceptable accuracy. Numerical results confirmed the experimental findings. A design of the Pod with NACA inlets was proposed and studied by CFD. Numerical results showed the design is effective to provide a sufficient amount of fresh air to safely operate the SOFC generator inside an UAV Pod.
Table 3 e Divergent walls ordinates. x/lr
y/w
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0.5 0.497 0.457 0.382 0.3 0.233 0.195 0.157 0.118 0.08 0.042
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references
[1] Lindahl P, Moog E, Shaw SR. Simulation, design, and validation of an UAV SOFC propulsion system. IEEE Trans Aerosp Electr Syst 2012;48(3). Leo TJ, Navarro-Are lez-Espasandı´n O, valo E. Fuel cells: [2] Gonza a real option for unmanned aerial vehicles propulsion. Sci World J 2014, 497642. https://doi.org/10.1155/2014/497642. [3] Wezeman S. Developments in the european union UAVs and UCAVs: security and defence. 2007. [4] Krawczyk JM, Mazur AM, Tomasz S, Stokłosa AW. Fuel cells as alternative power for unmanned aircraft systems current situation and development trends. Trans Inst Aviat 2014;4:49e62. reos no [5] Freire Bouillon LG, Perfiles IDS. Sisitemas ae tripulados (UAS). 2009. http://www.infodefensa.com/ publlicaciones/perfiles-uas/. [accessed 4 May 2017]. nez MG, Sanjurjo Jul JM, Gonzalez[6] Maple D, Sanchez Jime Regueral CC. PerfilesIDS: sistema no tripulados. Madrid, Spain: Filmacrom S.L.; n.d. [7] Malandrakis K, Dixon R, Savvaris A, Tsourdos A. Design and Development of a novel spherical UAV. IFAC PapersOnLine 2016;49:320e5. https://doi.org/10.1016/j.ifacol.2016.09.055. [8] Donateo T, Ficarella A, Spedicato L, Arista A, Ferraro M. A new approach to calculating endurance in electric flight and comparing fuel cells and batteries. Appl Energy 2017;187: 807e19. https://doi.org/10.1016/j.apenergy.2016.11.100. [9] Meadowcroft AD, Howroyd S, Kendall K, Kendall M. Testing micro-tubular SOFCs in unmanned air vehicles (UAVs). ECS Trans 2013;57:451e7. https://doi.org/10.1149/05701.0451ecst. [10] Aguiar P, Brett DJL, Brandon NP. Solid oxide fuel cell/gas turbine hybrid system analysis for high-altitude longendurance unmanned aerial vehicles. Int J Hydrogen Energy 2008;33:7214e23. [11] Alston T, Kendall K, Palin M, Prica M, Windibank PA. 1000cell SOFC reactor for domestic cogeneration. J Power Sources 1998;71:271e4. https://doi.org/10.1016/S03787753(97)02756-0. [12] Kendall M. Tubular cells: a novel SOFC design. Final year project report. UK: Middlesex University; July 1993. [13] Finnerty C, Tompsett GA, Kendall K, Ormerod RM. SOFC system with integrated catalytic fuel processing. J Power Sources 2000;86:459e63. [14] Labreche T, Materials A. Solid oxide fuel cell power systems for small UAVs 2007 joint service power expo April 24e26 2007. [15] Bujalski W, Dikwal CM, Kendall K. Cycling of three solid oxide fuel cell types. J Power Sources 2007;171:96e100. https://doi.org/10.1016/j.jpowsour.2007.01.029. [16] Kendall K. Progress in microtubular solid oxide fuel cells. Int J Appl Ceram Technol 2010;7:1e9. https://doi.org/10.1111/ j.1744-7402.2008.02350.x. [17] Cox TH, Nagy CJ, Skoog MA, Somers IA. A report overview of the civil UAV capability assessment. 2004. [18] Lawlor V, Griesser S, Buchinger G, Olabi AG, Cordiner S, Meissner D. Review of the micro-tubular solid oxide fuel cell. Part I. Stack design issues and research activities. J Power Sources 2009;193:387e99. https://doi.org/10.1016/j.jpowsour. 2009.02.085. [19] Bujalski W, Paragreen J, Reade G, Pyke S, Kendall K. Cycling studies of solid oxide fuel cells. J Power Sources 2006;157:745e9. https://doi.org/10.1016/j.jpowsour.2006. 01.060. [20] Galloway KW, Sammes M. Performance degradation of microtubular SOFCs operating in intermediate temperature range. J Electrochem Soc 2009;156:B526e31.
[21] Suzuki T, Sugihara S, Hamamoto K, Yamaguchi T, Fujishiro Y. Energy efficiency of a microtubular solid-oxide fuel cell. J Power Sources 2011;196:5485e9. https://doi.org/ 10.1016/j.jpowsour.2011.02.052. [22] Fernandes A, Woudstra T, Aravind PV. System simulation and exergy analysis on the use of biomass-derived liquidhydrogen for SOFC/GT powered aircraft. Int J Hydrogen Energy 2015;40:4683e97. [23] Pihlatie M, Ramos T, Kaiser A. Testing and improving the redox stability of Ni-based solid oxide fuel cells. J Power Sources 2009;193:322e30. https://doi.org/10.1016/ j.jpowsour.2008.11.140. [24] Ouweltjes JP, Van Tuel M, Sillessen M, Rietveld G. Redox tolerant SOFC anodes with high electrochemical performance. Fuel Cells 2009;9:873e82. https://doi.org/ 10.1002/fuce.200800142. nchez F, [25] Barroso J, Renau J, Lozano A, Miralles J, Martı´n J, Sa et al. Experimental determination of the heat transfer coefficient for the optimal design of the cooling system of a PEM fuel cell placed inside the fuselage of an UAV. App Ther Eng 2015;89:1e10. got S, Harel F, Candusso D, Franc¸ois X, Pe ra MC, Yde[26] Be Andersen S. Fuel cell climatic tests designed for new configured aircraft application. Energy Convers Manag 2010;51:1522e35. [27] Bradley TH, Moffitt BA, Mavris DN, Parekh DE. Development and experimental characterization of a fuel cell powered aircraft. J Power Sources 2007;171:793e801. [28] Cestino E. Design of solar high altitude long endurance aircraft for multi payload & operations. Aerosp Sci Technol 2006;10:541e50. ~ a-Rey N, Blanco JA, Ferreyra E, Lemus JL, Pereira S, [29] Lapen Serrot E. A fuel cell powered unmanned aerial vehicle for low altitude surveillance. Int J Hydrogen Energy 2017;42:6926e40. [30] Renouard-Vallet G, Saballus M, Schumann P, Kallo J, Friedrich KA, Mu¨ller-Steinhagen H. Fuel cells for civil aircraft application: on-board production of power, water and inert gas. Chem Eng Res Des 2012;90:3e10. [31] Kim T, Kwon S. Design and development of a fuel cellpowered small unmanned aircraft. Int J Hydrogen Energy 2012;37:615e22. [32] Verstraete D, Lehmkuehler K, Gong A, Harvey JR, Brian G, Palmer JL. Characterisation of a hybrid, fuel-cell-based propulsion system for small unmanned aircraft. J Power Sources 2014;250:204e11. [33] Gadalla M, Zafar S. Analysis of a hydrogen fuel cell-PV power system for small UAV. Int J Hydrogen Energy 2016;41:6422e32. [34] Panagiotou P, Kaparos P, Salpingidou C, Yakinthos K. Aerodynamic design of a MALE UAV. Aerosp Sci Technol 2016;50:127e38. [35] D245XR Fuel cell products n.d. http://www.ultra-fuelcells. com/D245XR. [36] ANSYS. Fluent: CFD simulation n.d. http://www.ansys.com/ Products/Fluids/ANSYS-Fluent. [accessed 8 May 2017]. [37] Akram F, Prior MA, Mavris DN. Design space exploration of submerged inlet capturing interaction between design parameters 28th AIAA applied aerodynamics conference 2010; 28 June e 1 July, Chicago, Illinois (USA). [38] Shams TA, Masud J. Steady analysis of NACA flush inlet at high subsonic and supersonic speeds, vol. II; 2015. [39] Frick CW, Davis WF, Randall L, Mossman EA. An experimental investigation of NACA submerged-duct entrances. 1945. p. 74.
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Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV Giosue Giacoppo a,*, Orazio Barbera a, Nicola Briguglio a, Francesco Cipitı` a, Marco Ferraro a, Giovanni Brunaccini a, Eric Erdle b, Vincenzo Antonucci a a
CNR, Institute for Advanced Energy Technologies “Nicola Giordano”, Via S. Lucia sopra Contesse n. 5, 98126 Messina, Italy b EFCECO, Germany
article info
abstract
Article history:
In this paper, the integration of a small SOFC commercial system into the fuselage of a
Received 9 May 2017
mini Unmanned Aerial Vehicle (UAV) is presented. As a design constrain, the SOFC
Received in revised form
system has to be installed inside the UAV fuselage with the lowest possible offset, to
24 July 2017
reduce the volume and mass of the UAV. Due to the high operating temperature of the
Accepted 3 September 2017
SOFC (800e1000 C), the external temperature of the system is always about few hundred
Available online xxx
Celsius degrees. Due to this, malfunctioning of the SOFC system and hot spots on the fuselage shell can occur. For this reason, it is important to ensure a proper ventilation of
Keywords:
the air volume inside the UAV fuselage. To deal with these issues, experimental and
SOFC fuel cell
Computational Fluid dynamic studies were carried out to investigate for a correct SOFC
Temperature distribution
system integration and operation in a confined environment. As a result, the optimal
CFD
airflow for a safe operation of the SOFC system was determined and the behaviour of the
System integration
temperature and air stream inside the fuselage was highlighted. In addition, NACA air
Unmanned aerial vehicle (UAV)
intakes were designed on the basis on the experimental and numerical evidences, to provide a proper cooling of the SOFC system installed into the fuselage. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells have received increasing interest for their ability to effect efficient, quiet and clean conversion of chemical to electrical energy. This attribute, make fuel cells an attractive power source for electric propulsion in Unmanned Aerial Vehicles (UAVs) [1]. These particular type of aircrafts, encouraged by recent technological developments, have seen a dramatic interest boost in recent years and are already considered as an integral and indispensable part of modern armed forces, with an increasing number of dual use and civil
applications. Indeed, they are ideally to provide surveillance, remote sensing and communication relay capabilities. The current propulsion systems of UAVs are based on different types of internal combustion engines fed by fossil fuels. They are the main cause for the increase of CO2 presence in the atmosphere, declared to be one of the most important culprits of global warming and the atmospheric emissions of other pollutants. There is therefore an urgent search for new energy policies based on the diversification of energy sources and their origin, energy saving policies, and the use of efficient energy conversion systems, for these reasons, fuel cells have
* Corresponding author. E-mail address: [email protected] (G. Giacoppo). https://doi.org/10.1016/j.ijhydene.2017.09.063 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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started to be introduced. These have advantages in terms of endurance, efficiency, emissions, and stealth, which make them ideal for UAV applications. UAVs are in their nascent stage of development (in fact, their regulations are in process of being written), although the implementation of fuel cell propulsion systems is more advanced than in conventional aircrafts. The main reason is the fact that UAVs are unmanned, thus, weighting comparatively less and not needing the life support systems for the crew and passengers, making them ideal for this new technology. In addition, fuel cells are still too heavy to propel any large aircraft; they have a lower power density when compared with conventional turbines. As an alternative to conventional propulsion, three main types of propulsion systems can be considered in UAVs: (i) alternative thermal systems: where different thermodynamic cycles, fuel, or engine types can be used (e.g., spark-ignition reciprocating engines fuelled by gasoline); (ii) electrical systems: where the power required is obtained through an electric motor and power is generated or stored in different ways; (iii) hybrid systems: combining any of the systems listed above, even the same type (e.g., a combination of fuel cell and battery or Regenerative Fuel Cell Systems, which combine fuel cell, battery, and photovoltaic cells) [2]. The possibility of implementing fuel cell technology in Unmanned Aerial Vehicle (UAV) propulsion systems has been considered in the recent literature, [2e7]. As stated above, the main advantage of these devices is strictly related to the absence of humans on-board that permits to reduce the operational costs and allow them to be used under hazardous conditions. Whereas, one of the limits of these vehicles is related to the flight endurance that is actually in the range of a couple to 10 h, for the electric powered UAVs [8]. Because of their performance, Fuel Cells have recently found their first aviation applications as power plants for small-scale, long-endurance and long-range UAVs. Solid Oxide Fuel Cell (SOFC) technology lags Polymer Electrolyte Fuel Cells (PEFCs) by a number of years but presents an opportunity for many applications, including aviation, due to its fuel flexibility. SOFCs should be better than PEFCs in UAVs. The reason is that propane, in an aluminium container has a much better energy storage density than hydrogen gas in a pressurized cylinder used in PEFCs [9]. SOFC has a wider range of fuel due to its ability to convert reformate. For UAVs low weight is the most important issue, with the aim of extending the time of flight. A SOFC powered UAV would be able to take advantage of the fuel flexibility of SOFCs and hence use a fuel more readily available to the consumer, easy to store and with a high energy density such as propane. Low power PEMFC and DMFC are usually restricted to using hydrogen or methanol. Moreover, SOFC technology offers some interesting opportunity in terms of integration into turbo-gas power plant. Aguiar et al. proposed SOFC, in combination with gas turbine in Ref. [10], where different configurations of the hybrid system were modelled and analysed for powering a high e altitude, long endurance UAV. Microtubular SOFCs (mSOFCs) running on catalytically reformed propane were chosen for integration with UAV due to rapid start-up, cyclability and lower cost [11e13]. A key issue for portable micro-fuel cell applications is the use of a fuel, which is readily available to the consumer. AMI produced and flight tested a mSOFC-powered mini-UAV in 2006, achieving an endurance of nearly 5 h for the 2 kg UAV
[14]. Micro-SOFC systems have shown many desirable characteristics over systems with planar SOFCs [15,16] Microtubular SOFCs systems can accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. They are mechanically stronger than planar designs and can be made with materials, which withstand long-term operation. Small-scale or microtubular SOFC systems demonstrated in the mid 1990s by Kendall et al., endured thermal stresses caused by rapid heating [11,17]. The state of the art in microtubular Solid Oxide Fuel Cells has been defined in a large number of papers and patents [18]. Recently K. Kendal [16] described how small zirconia-based tubes around 2 mm in diameter, have been developed to give great advantages over the standard planar systems which have been used more conventionally by most SOFC manufacturers. Various papers have focused the attention on this significant advantage of mSOFCs. Bujalsky et al. [19] showed that microtubular SOFCs allowed much faster starting time (almost 10 times faster than others did) without mechanical damage. Galloway et al. discussed about the reduced degradation of microtubular SOFC, when operated at intermediate temperature and under start-up and shut down cycles [20]; Suzuky et al. [21] discussed about how the energy efficiency of micro tubular SOFC was mainly correlated to the decrease of the gas transport polarization resistance. Lindal et al. simulated, designed and validated a SOFC e based propulsion system for UAV [1]. Fernandes et al. presented the results of a “Well to Wing” efficiency analysis of liquid hydrogen produced through biomass for aviation. The authors studied a power chain composed by a biomass gasification plant, a hydrogen liquefaction unit and a SOFC e gas turbine system [22]. In addition, other authors focused their attention on the anode formulation to improve the redox stability of SOFC [23,24]. An experimental approach for the determination of the convective heat transfer coefficient has been reported in Ref. [25] by Barroso J. et al. The paper is dealing with a high temperature PEM fuel cell stack, intended for use in a UAV, which was tested in different rectangular tunnels equipped with thermocouples. To evaluate the behaviour of the fuel cell stack in low temperature environment, such as the fuselage of an UAV when it got et al. tested a PEM fuel cell stack in a is at high altitude, Be climatic chamber [26]. Moreover, many research papers dealing with the modelling and design of UAVs with hybrid power (battery þ fuel cells), consider the PEM fuel cell as power plant [27e34], only few, are focused on the use of a SOFC fuel cell. Beside this, to the best of our knowledge, there is a lack in the scientific literature concerning the thermal issues related to the integration of a SOFC power plant inside the fuselage of an UAV. In the present study, these issues were faced up from both experimental and numerical point of view. Experimental measurements allowed to highlight the main issues related to the operation of the SOFC generator in a confined environment, such as internal temperature distribution and external cooling flow rate. Numerical model were validated through experimental results and then used for the design of the cooling air intakes in the UAV fuselage. The paper is structured as following: i) description of the SOFC generator specifications and UAV integration concept; ii) setup of the experimental bench for testing the SOFC generator in a confined environment; iii) set-up of the numerical model
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and validation through experimental measurements, iv) design of the UAV fuselage allocating the SOFC generator with air inlets for cooling; v) numerical model of the final fuselage design with integrated air intakes; vi) conclusions.
SOFC generator specifications and UAV configuration layout The power plant configuration of the UAV is imagined as a hybrid system, where both the battery and the SOFC Fuel cell supply the electrical power needed at each phase of the UAV mission. The mission profile used in this study as a reference is described in Fig. 1 a) and comprises a series of climbs, cruises and descents; each characterized by a specific electrical power demand. Particularly, the climb phase is assumed to consumes almost all the available installed power. The cruising power, correspondent to the 10e20% of the maximum power, is delivered during the most part of the mission. For this reason, the average power, needed during cruising, is delivered by the SOFC generator, while peak power needed for high power demanding manoeuvres, such as takeoff, landing and turns, is provided by the battery. Moreover, the SOFC generator will also serve to recharge the battery when this is not used for the propulsion. The UAV is conceived in a Pod configuration, such that the electric power system (the SOFC, the battery and the fuel tank) the payload and all services necessary for guidance and telemetry (autopilot, avionics, instruments, cameras, etc.), are
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appended to the aerodynamic surfaces and the motor (Fig. 1 b) and c)). The SOFC generator hosted inside the Pod is the D245XR from Ultra Electronics [35]. It consists in a lightweight system (2.6 kg) that includes: a SOFC stack with a nominal power of 245 W and 28 V, an integrated Catalytic Partial Oxidation (CPOX) unit which treat the fuel (in this case propane) before entering the SOFC stack, the auxiliaries (such as fans and cathode blower) and the electronic controller, which allows the system to autonomously operate (see Fig. 2). The size of the SOFC generator is about 400 140 140 mm for a total volume of about 8 L.
Experimental and numerical methods During the operation, the SOFC generator surfaces can reach high temperature in the order of hundreds degree C, thus a proper ventilation around the SOFC generator is necessary, to preserve the integrity of the Pod from burning and avoid excess of temperature inside the Pod, that can be dangerous for the electronics. Consequently, the Pod cross section need to be maintained as minimum as possible to reduce aerodynamics drag but has to be sufficient to allow a proper airflow around the SOFC generator. For these reasons, the SOFC generator was tested in a confined environment that is representative of the SOFC generator integrated inside the UAV Pod. Moreover, a simulation study was carried out trough numerical Computational Fluid Dynamics (CFD) simulations to investigate both the behaviour of the airflow and the
Fig. 1 e a) and b) POD configuration of the UAV, c) UAV Mission Profile. Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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Fig. 2 e SOFC generator adopted for experimental study and integration scheme into the UAV Pod.
temperature distribution inside the fuselage and to understand how the environment inside the Pod has to be maintained ‘safe’ for the operation of the SOFC generator.
Test bench set-up A tube made of PVC with an inner diameter of 200 mm was used to place the SOFC generator inside it and test its operation in a confined environment. The tube internal diameter was chosen according to the prescription of the SOFC generator manufacturer, leaving a clearance of 20 mm all around the generator diameter. The main purposes of the test bench were: i) to find the correct set of operative parameters (such as air inlet flow rate and maximum operative temperature) to allow a safe operation of the SOFC generator; ii) to measure the temperature of some representative points inside the tube, finding “hot areas”; iii) to study the temperature behaviour with the change of the air outlet (exhaust) cross section; iv) to obtain a set of experimental data useful to validate a thermo-fluid dynamic model. A controlled airflow rate was flown inside the tube to remove the heat released by the generator during its operation. To measure the temperatures, 16 thermocouples were placed upon the SOFC generator surfaces and in close contact with the internal side of the tube wall, upstream and downstream the SOFC generator. Only one thermocouple was externally placed, to measure the temperature of the exhausts gases of the SOFC stack. Due to the limitation of the available space in the experimental volume, a reduced, but representative, number of thermocouples and sensors were used, to minimize the interactions between the studied phenomenon and the measurement setup. Air at inlet was fed by a mass flow controller such that the electronic board was maintained below 50 C. This temperature represents the maximum environmental capability during operation. Thus, the mass flow rate can be increased up to 400 l/min in case the controller temperature raise up to the limit. Fig. 3 depicts the scheme of the test bench and the thermocouple position to monitor the temperature trend during operation. To study the effect of the exhaust area on the temperature inside the tube at the specific location, a special cap with variable openings was designed. This cap
allows the discharge area to be changed during the test, without any interruption, in this way the change in the area is immediately reflected in the temperature distribution. The cap is composed by a circular support that is progressively covered with circular parts until it is completely closed. In this way, four combinations of Exhaust Area (EA) were examined 75%, 50%, 25%, 10%. Fig. 4 shows the cap design and the progressive coverage. Two pressure transducers, have been placed perpendicularly to the inlet airflow, at 30 cm away from the inlet and close the exit respectively, to measure the air pressure drop within the tube (DP ¼ Pout e Pin). The final setup of the test bench is showed in Fig. 5a and b.
CFD methodology CFD computations were carried out to provide a deep inside in the temperature distribution in the Pod and then carry out a suitable configuration of the air intakes necessary for the cooling of the internal volume of the UAV Pod. At first, a numerical model of the experimental test bench was implemented to compare numerical results with experimental measurement and validate the numerical model. Successively, simulations on the designed UAV Pod were carried-out to validate the cooling effectiveness of the air intakes. The analysis was carried out with the commercial code Ansys Fluent [36]. A simplified 3D CAD model of the SOFC generator was build and imported in the pre-processor to extract the fluid volume and create the mesh. The grid of the internal flow consisted of approximately two millions of computational nodes. In both cases, 10 inflation layer were adopted on the wall such that the yþ is maintained 1. A grid refinement was implemented at locations where the gradient of fluid dynamic variables (velocity, temperature and so on) was expected more pronounced. Particularly, fine mesh was implemented close to the cooling fans and the exhausts. Special attention was focused on the mesh quality by controlling the skewness factor and the element orthogonality, to reduce numerical instabilities that generally lead to diverging solutions. For each case, these two factors were maintained below 0.8 and above 0.6 respectively (Fig. 6 c, d). Considering the turbulence parameter at the inlet, the intensity was set at 1% and the
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Fig. 3 e Thermocouples location. Inside the test tube: TC2-CH1, TC1-CH5, TC2-CH0, TC1-CH7, TC1-CH1, TC1-CH1, TC1-CH3, TC2-CH5, TC2-CH4, TC2-CH2, TC2-CH3 measure the temperature of the surface of the SOFC generator, TC2-CH6 measures the temperature of the electronic board, TC2-CH0 measures the temperature of the battery, TC1-CH4 and TC1-CH7 measure the temperature of the internal surface of the tube. Outside the test tube: TC2-CH7 measures the temperature of the exhaust of the SOFC stack.
Fig. 4 e Cap design with variable Discharge Area: a) EA 75%, b) EA 50%, c) EA 25%, d) EA 10%.
Fig. 5 e Test bench final setup: a) front view with the controlled air flow; b) rear part showing the variable EA cap.
hydraulic diameter at 0.2 m according to the tube circular geometry. For all the simulations, the Reynolds number, was calculated equal to 93103, it corresponds to a turbulent flow, which was described using the releasable k-ε model. This semi e empirical model is the most common used in Fluid Dynamic numerical studies to simulate mean flow characteristics for turbulent flow conditions. It describes the turbulence by introducing two equations: the first for the turbulent
kinetic energy (k), the second one for the turbulent dissipation (ε). The releasable k-ε model is more responsive to the effect of rapid strain and streamline curvature then the standard k-ε model [36]. The boundary conditions of the problem were: i) velocity inlet, ii) pressure outlet, iii) fixed temperature around the cylinder and the exhausts at 120 C, iv) fixed mass outlet at coolant fans and cathode filter. (0.1 m/s fixed outlet velocity), v) heat flux over electronic board surface @75 W/m2
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Fig. 6 e a), b) 3D CAD model with: SOFC container (1), electronic board (2), SOFC exhaust (3), tube wall/cylinder (4); c) d) tetrahedral mesh adopted for the numerical calculations.
(corresponding to 5 W thermal power about). Fig. 6 shows the CAD model (6a, 6b) and the mesh (6c, 6d). The temperature of the exhausts and around the cylinder was estimated by averaging the surface measured temperatures.
Experimental and numerical results In this section, results concerning the internal temperature profile, measured during the cruising o typical mission of the
UAV are reported. Fig. 7 shows the temperature value, at some location inside the test tube, namely: i) on the battery pack (TC2-CH0), ii) on the electronic board (TC2-CH6), iii) on the cathode air intake (TC1-CH3). This last location is very important to monitor in temperature, because the air inlet serves both for supplying oxidant to the stack cathode and for the internal cooling of the SOFC stack. As the power output of the SOFC generator has been set at 250 W, (nominal power) the heat released in the air surrounding the SOFC generator is constant over time. For this reason the average temperature
Fig. 7 e Temperature trend inside the tube at relevant points with the generator operated at nominal power. Please cite this article in press as: Giacoppo G, et al., Thermal study of a SOFC system integration in a fuselage of a hybrid electric mini UAV, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.09.063
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inside the Pod tends to increase up to a value that represents the equilibrium between the heat released from the generator and that removed by the exhausted air leaving the Pod. As it is evident in Fig. 7 at a fixed EA the temperature at the different locations, oscillates around an average value. This value increased with the EA reduction as it is expected. This behaviour is most evident at specific locations and particularly near the cathode fan. In fact, by reducing the EA from totally opened to EA equal to 10%, the temperature at the cathode fan inlet, raised up from 35 C to 60 C about, while on the battery pack slight increase from 30 C to 35 C and remains almost constant on the electronic board (40 C about). During the test, the air flow rate was increased from 170 l/min to 200 l/min due to the temperature increase on the electronic board occurred when the EA was changed from open to EA equal to 75%. After this change, the average temperature on the electronic board was always in the safe range as specified by the SOFC generator provider (50 C). From the SOFC generator point of view, the experimental results showed that it is possible to safely operate even if with an EA of 10% is used, moreover some issues were observed on the plastic tube during the test. In particular, the hot gases released by the SOFC stack are exhausted outside the test tube by two extensions made in fiberglass coated with high temperature silicone, directly connected with the generator gas outlets. In these locations, the gas temperature can reach up to 200 C that is higher than the maximum operating temperature of the PVC tube (85 C). For this reason, two burning marks have been detected as Fig. 8 a), and b) shows. To preserve the Pod shell from overheating, it is important to insulate the generator exhausts at the interface with the Pod holes.
CFD validation To compare numerical results with the experimental measurements, two design points were selected, implemented and solved (EA 75% and EA 25%). In particular, the temperature on the cathode air inlet (TC1-CH3) was used to validate numerical results due to its great variation with respect to the decreasing of EA (Fig. 7). This effect can potentially alter the SOFC system operation and cause its failure. For this reason, the CFD model was tailored to correctly represent this phenomenon, by accepting that the implemented model could not exactly follows the evolution of the real temperature of
Table 1 e Local temperature comparison between CFD and experiment. Experiment EA 75% EA 25%
45 C 56 C
CFD
41 C 52 C
DT% 8.8 7.1
the electronic board. Indeed, the calculated temperature of this component resulted almost constant along the EA change (33 C about) and in the same order of magnitude of the experiments (35 C about with EA75%, 38 about with EA25%). This temperature resulted always far from the limit temperature (50 C). Good agreement was found between numerical and experimental results with a difference of about 8% on the local temperature as reported in Table 1. Considering the temperature of the electronic board, Fig. 7 showed that it is slightly affected by the variation of the selected EA. As expected, the temperature is transported from the inlet to the tube outlet according to the air stream. Hot zones are evident close to the generator exhausts and near the exits. Fig. 9 shows the temperature distribution inside the Pod and the velocity streamlines from test tube inlet towards the outlet. It is evident, that temperature increases in the wake of the SOFC exhausts and generally in the rear part of the SOFC generator, when the EA is reduced from 75 to 25%. For this reason, the skin temperature can raise up to 75 C e 80 C that could be excessive for many plastic materials. As Fig. 10 depicts, this numerical result is consistent with the experimental findings where extended burning marks were found at the same position. Thus, for integration purposes is highly recommended that care insulation must be made at the interface with exhaust and the Pod shell. Moreover reducing the EA the rear part of the tube became warmer but in the range of 40e45 C.
Pod design with cooling air intakes Numerical results show that the developed computational model can be a useful tool for the final design of the Pod and particularly for the air inlets design. In order to reduce passive loads (due to the presence of cooling fans or other) dynamic cooling were considered. For this reason NACA, that is the U.S.
Fig. 8 e Burning marks highlighted close the SOFC exhausts.
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Fig. 9 e Temperature contours inside the Pod: a) EA ¼ 75% b) EA ¼ 25%.
National Advisory Committee for Aeronautics, (the precursor to NASA, in 1945) submerged inlets a have been designed and located in the front of the UAV Pod [37]. This type of inlets, result in some nice features such as reduced external drag and cleaner aerodynamics. Moreover for low mass airflow rates, the NACA submerged inlet with diverging walls offer improved pressure recovery [38,39]. Fig. 11 shows the final design of the UAV fuselage with NACA submerged inlets in the front and a convergent axisymmetric exhaust placed in the
rear part to release warm air to the ambient during the flight. The geometrical parameters of the NACA submerged inlet are reported in Appendix A. A flow converter was used to redirect the air-cooling on the electronics to prevent over-heating and then any damage to the SOFC generator. In this way, fresh air enters the NACA inlets, cool down the generator external surfaces and exits the Pod through the rear exhaust. According to the experiments, the section of the exhaust was set about 10% of Pod cross section area.
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Fig. 10 e Pod skin temperature contours: a) EA ¼ 75% b) EA ¼ 25%.
Fig. 11 e Pod Design with NACA submerged inlets and cross section where the ramp inlet and the flow converter is clearly visible.
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Fig. 12 e Temperature distribution inside the UAV Pod: a) cross-section side view, b) cross-section top view.
CFD analysis of the final fuselage design with integrated air cooling intakes To validate the proposed design of the Pod CFD simulation was carried out to check the temperature distribution inside the Pod and on the surface of the SOFC generator. For this purpose the same simplified geometry adopted for the previous calculation, was considered and a heat flux corresponding to the power released from the SOFC canister and the electronic board was applied on the corresponding surfaces, the rest of boundary conditions were the same assumed for the previous simulations. For the electronic board, the heat flux was estimated on the power consumption data given by the
manufacturer, whereas considering the SOFC generator, only a fraction of the heat is supposed to be released inside the Pod while the rest is wasted out to the exterior by the exhausts. The effectiveness of the coolant air intakes design is granted when the heat released by the SOFC generator components is less or at least equal than the heat removed by the coolant flow rate at the air inlets. The analysis showed (Fig. 12) the temperature inside the Pod increases from the intake to the rear exhaust. In general, the temperature is always below 30 C in the front part close the NACA intakes, whereas in the rear part the temperature raises up to 50 C about. In the region near the SOFC stack, where the wall temperature is set at 120 C, there is an
Fig. 13 e Velocity streamline: a) cross section side view, the fluid enters the NACA intakes and develops in the Pod interior and exhausts through the rear opening, b) cross section top view, two counter rotating vortex on the electronic board.
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increase of Pod wall temperature up to about 84 C that is incompatible with the most of plastic material. For this reason the use of fibre reinforced material or corrugated insulating sheet is mandatory in those areas (that correspond to the SOFC stack position), to preserve the integrity of the Pod shell. In Fig. 13 the velocity streamlines calculated shows as a portion of the external air is captured from the NACA intakes and redirected inside the UAV Pod. Due to the diverging inlet, the airflow is spread on the electronic board directly, to maintain the temperature below the limit as specified by the manufacturer. The two vortices developed on the electronics, distributes the airflow all around the SOFC generator body, allowing the heat released during the SOFC generator operation, to be exhausted in the ambient through the rear opening.
Acknowledgements The work presented in this paper is part of the SUAV project. The project was funded under Europe's Fuel Cell and Hydrogen Joint Undertaking (FCH JU) Grant Agreement No. 278626. http://www.suav-project.eu.
Appendix A The design parameters of the NACA intake, adopted for the Pod design are reported in Fig. 14. Design parameters are listed in Table 2 and ordinates for the divergent walls are shown in Table 3.
Fig. 14 e NACA inlets Design parameters.
At a cruise speed of 10 m/s the calculated inlet flow rate was of 240 l/min that is sufficient to allow the SOFC generator to properly operate as verified in the experiments.
Table 2 e Design parameter of NACA intake. w (mm) 45
dt (mm)
lr (mm)
a (deg)
5
171
7
Conclusions A commercial SOFC generator to power an UAV, was tested in a confined environment. Experimental results led to conclude the selected generator is able to work properly inside an UAV Pod with a minimum flow rate of fresh air entering the fuselage Pod of 200 l/min. Experimental results, showed the SOFC generator exhausts and fuselage interface need to be properly insulated to avoid over-heating of the Pod shell. A numerical model of the Pod was implemented and validated showing an acceptable accuracy. Numerical results confirmed the experimental findings. A design of the Pod with NACA inlets was proposed and studied by CFD. Numerical results showed the design is effective to provide a sufficient amount of fresh air to safely operate the SOFC generator inside an UAV Pod.
Table 3 e Divergent walls ordinates. x/lr
y/w
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0.5 0.497 0.457 0.382 0.3 0.233 0.195 0.157 0.118 0.08 0.042
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references
[1] Lindahl P, Moog E, Shaw SR. Simulation, design, and validation of an UAV SOFC propulsion system. IEEE Trans Aerosp Electr Syst 2012;48(3). Leo TJ, Navarro-Are lez-Espasandı´n O, valo E. Fuel cells: [2] Gonza a real option for unmanned aerial vehicles propulsion. Sci World J 2014, 497642. https://doi.org/10.1155/2014/497642. [3] Wezeman S. Developments in the european union UAVs and UCAVs: security and defence. 2007. [4] Krawczyk JM, Mazur AM, Tomasz S, Stokłosa AW. Fuel cells as alternative power for unmanned aircraft systems current situation and development trends. Trans Inst Aviat 2014;4:49e62. reos no [5] Freire Bouillon LG, Perfiles IDS. Sisitemas ae tripulados (UAS). 2009. http://www.infodefensa.com/ publlicaciones/perfiles-uas/. [accessed 4 May 2017]. nez MG, Sanjurjo Jul JM, Gonzalez[6] Maple D, Sanchez Jime Regueral CC. PerfilesIDS: sistema no tripulados. Madrid, Spain: Filmacrom S.L.; n.d. [7] Malandrakis K, Dixon R, Savvaris A, Tsourdos A. Design and Development of a novel spherical UAV. IFAC PapersOnLine 2016;49:320e5. https://doi.org/10.1016/j.ifacol.2016.09.055. [8] Donateo T, Ficarella A, Spedicato L, Arista A, Ferraro M. A new approach to calculating endurance in electric flight and comparing fuel cells and batteries. Appl Energy 2017;187: 807e19. https://doi.org/10.1016/j.apenergy.2016.11.100. [9] Meadowcroft AD, Howroyd S, Kendall K, Kendall M. Testing micro-tubular SOFCs in unmanned air vehicles (UAVs). ECS Trans 2013;57:451e7. https://doi.org/10.1149/05701.0451ecst. [10] Aguiar P, Brett DJL, Brandon NP. Solid oxide fuel cell/gas turbine hybrid system analysis for high-altitude longendurance unmanned aerial vehicles. Int J Hydrogen Energy 2008;33:7214e23. [11] Alston T, Kendall K, Palin M, Prica M, Windibank PA. 1000cell SOFC reactor for domestic cogeneration. J Power Sources 1998;71:271e4. https://doi.org/10.1016/S03787753(97)02756-0. [12] Kendall M. Tubular cells: a novel SOFC design. Final year project report. UK: Middlesex University; July 1993. [13] Finnerty C, Tompsett GA, Kendall K, Ormerod RM. SOFC system with integrated catalytic fuel processing. J Power Sources 2000;86:459e63. [14] Labreche T, Materials A. Solid oxide fuel cell power systems for small UAVs 2007 joint service power expo April 24e26 2007. [15] Bujalski W, Dikwal CM, Kendall K. Cycling of three solid oxide fuel cell types. J Power Sources 2007;171:96e100. https://doi.org/10.1016/j.jpowsour.2007.01.029. [16] Kendall K. Progress in microtubular solid oxide fuel cells. Int J Appl Ceram Technol 2010;7:1e9. https://doi.org/10.1111/ j.1744-7402.2008.02350.x. [17] Cox TH, Nagy CJ, Skoog MA, Somers IA. A report overview of the civil UAV capability assessment. 2004. [18] Lawlor V, Griesser S, Buchinger G, Olabi AG, Cordiner S, Meissner D. Review of the micro-tubular solid oxide fuel cell. Part I. Stack design issues and research activities. J Power Sources 2009;193:387e99. https://doi.org/10.1016/j.jpowsour. 2009.02.085. [19] Bujalski W, Paragreen J, Reade G, Pyke S, Kendall K. Cycling studies of solid oxide fuel cells. J Power Sources 2006;157:745e9. https://doi.org/10.1016/j.jpowsour.2006. 01.060. [20] Galloway KW, Sammes M. Performance degradation of microtubular SOFCs operating in intermediate temperature range. J Electrochem Soc 2009;156:B526e31.
[21] Suzuki T, Sugihara S, Hamamoto K, Yamaguchi T, Fujishiro Y. Energy efficiency of a microtubular solid-oxide fuel cell. J Power Sources 2011;196:5485e9. https://doi.org/ 10.1016/j.jpowsour.2011.02.052. [22] Fernandes A, Woudstra T, Aravind PV. System simulation and exergy analysis on the use of biomass-derived liquidhydrogen for SOFC/GT powered aircraft. Int J Hydrogen Energy 2015;40:4683e97. [23] Pihlatie M, Ramos T, Kaiser A. Testing and improving the redox stability of Ni-based solid oxide fuel cells. J Power Sources 2009;193:322e30. https://doi.org/10.1016/ j.jpowsour.2008.11.140. [24] Ouweltjes JP, Van Tuel M, Sillessen M, Rietveld G. Redox tolerant SOFC anodes with high electrochemical performance. Fuel Cells 2009;9:873e82. https://doi.org/ 10.1002/fuce.200800142. nchez F, [25] Barroso J, Renau J, Lozano A, Miralles J, Martı´n J, Sa et al. Experimental determination of the heat transfer coefficient for the optimal design of the cooling system of a PEM fuel cell placed inside the fuselage of an UAV. App Ther Eng 2015;89:1e10. got S, Harel F, Candusso D, Franc¸ois X, Pe ra MC, Yde[26] Be Andersen S. Fuel cell climatic tests designed for new configured aircraft application. Energy Convers Manag 2010;51:1522e35. [27] Bradley TH, Moffitt BA, Mavris DN, Parekh DE. Development and experimental characterization of a fuel cell powered aircraft. J Power Sources 2007;171:793e801. [28] Cestino E. Design of solar high altitude long endurance aircraft for multi payload & operations. Aerosp Sci Technol 2006;10:541e50. ~ a-Rey N, Blanco JA, Ferreyra E, Lemus JL, Pereira S, [29] Lapen Serrot E. A fuel cell powered unmanned aerial vehicle for low altitude surveillance. Int J Hydrogen Energy 2017;42:6926e40. [30] Renouard-Vallet G, Saballus M, Schumann P, Kallo J, Friedrich KA, Mu¨ller-Steinhagen H. Fuel cells for civil aircraft application: on-board production of power, water and inert gas. Chem Eng Res Des 2012;90:3e10. [31] Kim T, Kwon S. Design and development of a fuel cellpowered small unmanned aircraft. Int J Hydrogen Energy 2012;37:615e22. [32] Verstraete D, Lehmkuehler K, Gong A, Harvey JR, Brian G, Palmer JL. Characterisation of a hybrid, fuel-cell-based propulsion system for small unmanned aircraft. J Power Sources 2014;250:204e11. [33] Gadalla M, Zafar S. Analysis of a hydrogen fuel cell-PV power system for small UAV. Int J Hydrogen Energy 2016;41:6422e32. [34] Panagiotou P, Kaparos P, Salpingidou C, Yakinthos K. Aerodynamic design of a MALE UAV. Aerosp Sci Technol 2016;50:127e38. [35] D245XR Fuel cell products n.d. http://www.ultra-fuelcells. com/D245XR. [36] ANSYS. Fluent: CFD simulation n.d. http://www.ansys.com/ Products/Fluids/ANSYS-Fluent. [accessed 8 May 2017]. [37] Akram F, Prior MA, Mavris DN. Design space exploration of submerged inlet capturing interaction between design parameters 28th AIAA applied aerodynamics conference 2010; 28 June e 1 July, Chicago, Illinois (USA). [38] Shams TA, Masud J. Steady analysis of NACA flush inlet at high subsonic and supersonic speeds, vol. II; 2015. [39] Frick CW, Davis WF, Randall L, Mossman EA. An experimental investigation of NACA submerged-duct entrances. 1945. p. 74.
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