ISABE-2015-20026

INTEREST OF A RECUPERATOR FOR ULTRA MICROTURBINE

O. Dessornes ONERA, Energetics Department Palaiseau France

Abstract mf : fuel mass flow rate (kg/s) P : power (W) To reduce the size and weight Tt : stagnation temperature (K) of electric power generation ∆P : pressure losses (Pa) systems used for drones or other η : efficiency electric devices, several alternative systems to the Subscripts currently used heavy batteries are Comb : combustion being investigated worldwide. Among mb : mechanical baseline them, the micro gas-turbines are mech : mechanical expected to offer the highest power Net : total efficiency from fuel to density. electric power t : turbine However the drastic size reduction of a turbine foreshadows Plan some drawbacks and indeed the 2 : compressor inflow overall efficiency decreases 3 : compressor exit dramatically compared to classical 3’ : combustion chamber inflow engines to finally reach around 8 4 : combustor to 10 % for a 100 W turbine and 5 : turbine exit around 15 % for a 1 kw 5’ : atmosphere (exit of the microturbine. To overcome this recuperator on the hot side) issue a recuperator could be seen as an obvious solution to increase Introduction the overall efficiency thus reducing fuel consumption. Most current portable devices or small drones use Lithium-ion A study was devoted to the (Li-ion) secondary batteries as interest of such recuperator for an power sources. These batteries can ultra-microturbine. This study have relatively large power showed that a heat exchanger may densities but their energy have detrimental effects if the aim densities reach hardly 200-250 is competing battery in terms of Wh/kg which limits the autonomy. specific energy. The use of Charging time and very cold microturbine equipped with a external temperature may also be recuperator is then questionable issues. Common hydrocarbon fuels for drone use. have energy densities around 12 kWh/kg. A system that could convert As a result, depending on the only a few percents of this energy foreseen application, a recuperator density could reach higher energy may be good or detrimental for the densities than existing batteries. final user. Therefore many efforts have been done over the past decade to build Nomenclature a micro heat engine able to produce Cp : specific heat (J/kg/K) electricity due to the increase in E : recuperator efficiency micro-power requirements. Micro mair : air mass flow rate (kg/s) power generators based on

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reciprocating engines [1], 32 mm excluding the external thermoelectric devices or packaging, the electronics or the thermophotovoltaic devices [2-5], fuel tank. This Brayton-Joule cycle Wankel engines [6], Rankine cycles based engine works with a based engines [7-8], Stirling centrifugal compressor, a radial engines [9-10], fuel cells [11-12] inflow turbine and an annular and microturbine [13-14] are being combustion chamber. The design or were investigated. In nominal rotation speed is 840 000 particular, a 400 W electric rpm at 1200 K. Figure 1 shows a microturbine complete system was sketch of the architecture and demonstrated by IHI. This system figure 2 shows a picture of the can be refueled with simple gas actual prototype installed in the cartridges. The heat management and test lab. the control of the exhaust gas temperature were also demonstrated by Isomura et al. [15].

Among those systems, fuel cells should have the best efficiency whereas the micro gas turbine should achieve the highest power density.

This is why ONERA decided to focus on ultra micro gas turbines [16] that can deal with power between a few tens of watts up to some kW with high specific energy and specific power. More precisely, a complete microturbine of around 50 W electric power was machined, recently tested up to 170 000 rpm and delivered its first electric Watts.

However, such ultra- Figure 1: Architecture of the micro gas microturbine, while being better turbine engine. than batteries in terms of specifc energy, suffers from low total efficiency. Indeed, a total fuel/power efficiency of around 6% is expected. A recuperator could then address this issue by increasing the total efficiency. The interest of such ultra-small recuperators was then studied for such microturbine.

This paper gives the present status of this research.

First performance study Figure 2: Prototype working on the test The micro gas turbine bench at 170 000 rpm. developed at ONERA has an external diameter of 22 mm and a height of

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At this very small size, the lead to further improvement. This is heat losses increase due to larger why a recuperator functionality was viscous forces [13] and the thermal added to hot die. fluxes are no longer negligible. They imply severe output power and efficiency decreases compared to classical size engines [7, 16, 17]. Hence, to assess the influence of the thermal losses on these performances, an aerothermodynamic simulation code called “Hot-Die” has been specifically developed. In addition to fluid mechanics and Figure 3: recuperator position rotor aerodynamics, this program models conductive, convective and radiative heat transfers. The core Heat exchanger model of “Hot-Die” is based on stationary power balances. Some of these Two implementation strategies were balances are applied to flow used. The first one describes a volumes, delimited by sections, generic recuperator that has only which are located along the flow inputs and ouputs that depends on path, upstream and downstream the its efficiency and pressure losses. components (compressor, combustor, etc). In our case, the flow path is For the second strategy annular divided into 10 flow volumes. The recuperator performances are other power balances are applied to calculated. In this case, pressure each solid part of the system. losses as well as the heat Details of this method can be found exchanges are computed in the in Nicoul et al. [18]. It was found recuperator. We considered a swiss- that for a gas temperature of 1600 roll recuperator geometry inspired K the maximum mechanical efficiency by Tsai and Wang [19](see fig. 4). (P /P ) was 10%. Additional mech comb friction losses will decrease this value to around 7%. Including the mechanical to electrical conversion a global efficiency ηnet of 5 to 6% is expected for this 50 W microturbine which leads to a maximum value of 600 Wh.kg -1 for a very long time operation where the mass of the turbine becomes negligible compared to fuel mass. This value includes packaging, fuel tank, power electronics and the microturbine itself. Such low efficiencies are not surprising for such small turbine engines which have large heat loss and relatively low efficient compressors and turbines compared to classical engines. ηnet will increase with the gas turbine dimensions i.e. the turbine power. 17.8 mm

Adding a recuperator between the Figure 4: generic swiss roll recuperator compressor and the combustion chamber could (see fig. 3) also

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To estimate its performances, the and

Unit Transfer Number method is used for the thermal computation and + ∆ flat plate assumption is done for = − ∆ the pressure losses. This is in fact a rough assumption but it was verified on Tsai et al. case that In our case, for ηmb = 10% this the pressure losses were fairly equation can be very well well predicted. approximated by:

Results > 7,5. 10 ∆ + 0,375

The studied microturbine baseline This equation gives the ( ∆P,E) has the following features: combinations that will increase the • Mechanical power: 100 W mechanical efficiency.

• Compressor pressure ratio: 2.6 As shown fig.6, due to pressure • Cycle temperature: 1200 K losses, the mechanical power • Compressor efficiency: 0.7 decreases if the air mass flow • Turbine efficiency: 0.9 equals the baseline mass flow. • Rotation speed: 840 000 rpm However, the fuel flow rate may • Compressor diameter : 10 mm also decrease for many (∆P,E) • Turbine diameter : 10 mm combinations (fig. 7) and then the autonomy might be increased for a Before dealing with the results it reduced power. should be mentioned that the horizontal grid in the next figures corresponds to the baseline performance.

When the recuperator is described like a “black-box” in hot-die, it is possible to vary ∆P and E and see how it affects ηmech (fig. 5). Due to the small pressure ratio of this ultra microturbine, it can be seen that even moderate ∆P may have a dramatic effect on the efficiency. Namely, a 100% efficient recuperator may have a lower efficiency than the baseline (horizontal grid) if ∆P is too high. More precisely, for a given baseline mechanical efficiency ηmb , Figure 5: Total efficiency versus if we assume in a first approach recuperator efficiency and pressure losses that Cp is constant, mf << mair and ∆P is equal on the cold and hot side of the recuperator, it can be In conclusion, this first approach shown that E must follow: shows that for such ultra- microturbine there is space to increase the efficiency with a + > recuperator but ( ∆P, E) − combinations must be carefully with chosen due to the high sensitivity of ηmech to moderate ∆P.

= − 1 − −

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with 5 channels there are many recuperator combinations that increase the efficiency. However, when looking to the power, the pressure losses imply a power decrease compared to the baseline. As a result, the microturbine has a higher efficiency but a lower power if you keep the same mass flow and cycle temperature (fig. 9).

2 turns

Figure 6: Mechanical power versus recuperator efficiency and pressure losses

3 turns

Figure 7: Fuel flow rate power versus recuperator efficiency and pressure losses

For the second strategy, we studied the swiss roll configuration. The influence of the number of turns as well as the cold and hot channel width on the total efficiency was examined. As can be seen fig. 8, there is a dramatic effect of the number of turns on the total 4 turns efficiency. With two turns, the global efficiency stays always below the baseline (horizontal plan). For 3 turns, there is a small region where the recuperator begins to be beneficial and finally

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result there is a trade-off between efficiency and autonomy that depends on the foreseen use. This should also be mentioned that the gain in efficiency may be seen as very moderate (from 10.9 to 12.1 %). This comes from the fact that the studied Swiss Roll recuperator can’t meet the conjugate need for low pressure losses and efficiency. 5 turns Table 1 : comparison between the baseline and 2 to 5 turns recuperator Baseline 2t 3t 4t 5t

max efficiency (%) 10,9 10,4 11,1 11,6 12,1 Power (W) 101 91 90 89 88 Figure 8: Number of turns influence on the total efficiency Fuel consumption (g/h) 76 71 66 63 59 Weight (kg) 1,1 1,15 1,20 1,27 1,36 Esp (Wh/kg) (3 h) 275 237 226 211 194 Max autonomy 3h 3h10 3h25 3h37 3h49 Esp (Wh/kg) (max auto) 275 248 252 247 239

Indeed its maximum efficiency reaches only 69% (fig. 10) and the corresponding pressure losses are 11 000 Pa for the cold side and 5000 Pa for the hot side (fig. 11 and 12). The total pressure losses represent then around 6 % of the 5 turns nominal pressure in the combustor. Finally, the total efficiency of 12.1% is fully consistent to what is expected fig. 5 with the “black box” recuperator option and same E Figure 9: Mechanical power and ∆P.

If you want to keep a lightweight and small microturbine system, the recuperator is also an issue. Indeed, the best five turns recuperator weighs more than 300 gr. This represents a dramatic weight increase for the ultra microturbine. Table 1 sums-up the results for five different configurations.

If the specific energy is of utmost importance then the baseline without the recuperator is the best option. On the contrary, the autonomy can be enhanced with the recuperator but at lower power (for the same air mass flow). As a Figure 10: Recuperator efficiency

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microdrones such recuperator might be detrimental. On the contrary, if fuel savings is waited then such recuperator is beneficial. However, we studied only a swiss roll configuration and these conclusions might be different with other (lighter and more efficient) recuperators.

One other aspect not mentioned in this paper is the machining of such ultra-small and complex recuperator. This has also to be taken into account when considering this solution since this might be difficult to machine and could increase the final cost of such Figure 11: Pressure losses on the cold side system.

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