Laboratoire de Conception de Systèmes Mécaniques

SECTION DE

Génie mécanique

Author

Angel Iglesias

Supervisor

Prof. Daniel Favrat

Emanuele Facchinetti

Johannes Wegele

Acknowledgements

Prof. Daniel Favrat

Emanuele Facchinetti

Johannes Wegele

Irwin Gafner

Benoît Gay

Nicolas Massard Marie Madeleine Lambert

SECTION DE

Génie mécanique

Laboratoire d’Énergétique industrielle

Motivations & Objectives An attractive way to reach a more rational energy conversion of fossil or bio fuels is the decentralized power generation and

cogeneration of heat and power by fuel cells. Unfortunately, fuel cells don’t convert all the fuel electrochemically. Consequently, one

possible approach is to combine the fuel cell with a micro-turbine to obtain an hybrid system. The problem of this system lies in the

compression of the hot gas from SOFC fuel cell at a temperature of 900-1000 ° C. The proposed solution is to expand the hot gases at

sub-atmospheric pressure, and then compress them to atmospheric pressure after passing through a heat exchanger so as to achieve an

inverted Brayton-Joule cycle.

In this context, the objectives were to achieve and test a hot gas generator which simulate the exit conditions of a fuel cell and then

perform a test bench using this hot gas generator in combination with a turbocharger to test technical viability to realize a Brayton-Joule

inverted cycle.

Realisation of the final test bench The final part of this project was to imagine two more test bench:

One for the performance evaluation of the EGR heat exchanger. And the

other one, to evaluate the feasibility of a Brayton-Joule inverted cycle

using a turbocharger. At this stage the most difficult to realize in

practice, is the startup system. For this, we thought that the best solution

is to use a volume after the passage of the heat exchanger that can be

hermetically sealed with two valves at each extremity. In this closed

volume, it would create a vacuum using a pump. This vacuum is then

used to facilitate the turbine expansion at a sub-atmospheric pressure.

Conclusions Finally, it has been made a hot gas generator that can simulate exit conditions of a SOFC fuel cell. Indeed, for a limit of excess water

from 3 we obtain a mass fraction of 48%. Other entries were provided for the possible addition of other gases like CO2 or excess air

for example.

Some interesting results at the comparison of theoretical and practical results were obtained during testing. However, most

temperatures obtained during the tests were far inferior to those obtained theoretically. Several reasons explain this phenomenon: first

the combustion is not perfect and radicals as NOx, CO and other unburned are formed. On the other hand, the air entering in the

combustion chamber is not dry it contains an average humidity of over 50%. All these reasons make that the gas temperature is

200°C to 300°C lower than that predicted theoretically. When the reverse case appears, it just means that the injected water isn’t

completely evaporated.

Realisation and tests of the hot gases

generator

The SOFC delivers combustion gases to a temperature around 900 °C

with a large percentage of water evaporated (~ 50% mass fraction). For

this reason, it was made entries for injecting water into the combustion

chamber, to obtain the same conditions at the output of them.

The preliminary tests of the combustion chamber show us that we were

able to reach temperatures close to what we wanted as we can see in the

picture at right.

Feasibility Analysis of a

Micro Gas Turbine for Hybrid Cycle

Comparison between predicting models A numerical model for calculating the temperature of the mixture of

combustion gases and vapor injected was produced. Moreover, this

calculation was taken into account the heat power loss given to the

atmosphere. As we can see on our left, we started from the equation of

combustion reaction. Knowing this one and the heating power supplied

by the burner we calculated the molar flow rates that permit us to

calculate the temperature of combustion gas mixture. Then, we were

able to determine the temperatures at each node through the thermal

resistance model.

The comparison of theory and practical tests data, has allowed us to

determine the amount of excess water that can be evaporated in the

combustion chamber. We were also able to compare the temperature

variation depending on the water flow injected.

Data saving

module

Blower control

module Sensor Data

Visualisation

Start/Stop

module

Status error

Limit excess water that can

be evaporated

The lines represents the

amount of water injected as a

function of power and excess

water desired.

The red point represents the

excess of water where the

liquid water injected isn’t

evaporated. Below this limit all

the injected water is

evaporated. Above it, less and

less water is evaporated.

Schematic representation of the final test bench

Test at 22 [KW] with an excess of

water from 5 to 6

We can see that for a change of

water excess from five to six, the

temperature difference measured

for both water excess is relatively

close to that theoretically

calculated. We can see also that the

theoretical temperature is lower

than that measured, this indicates

that the water has not been fully

evaporated for these water excess.

The red arrows show us the water

injection system. Others injection

entries have been made to allow

the introduction of substances

such as CO2 or air, for example.

Using the stoichiometric equation of CH4 combustion shown below, knowing the heat power

delivered by the burner and the LHV of the CH4, it is possible to calculate the number of

moles involved in the combustion reaction.

𝐶𝐻4 + 2 ⋅ 𝑂2 + 7.52 ⋅ 𝑁2 → 𝐶𝑂2 + 𝐻2𝑂 + 7.52 ⋅ 𝑁2

By integrating the energy balance equation shown below, it is possible to calculate the gas

mixture temperature (Tm) after the combustion reaction.

𝑑𝑈𝐶𝑍

𝑑𝑡= 𝑄 𝑖

+

𝑖

+ 𝑄 𝑎+ − 𝑄 −

0 = 𝑚 𝐶𝑂2⋅ 𝑁𝐶𝑂2

⋅ 𝑐 𝑝,𝐶𝑂2+𝑚 𝐻2𝑂 ⋅ (𝑁𝐻2𝑂 +𝑁𝐸𝑥𝑐𝑒𝑠𝑠 𝐻2𝑂 ) ⋅ 𝑐 𝑝,𝐻2𝑂

+ 𝑁𝑂2⋅ 𝑚 𝑂2

⋅ 𝑐 𝑝,𝑂2

𝑇𝑚

𝑇𝑎

+ 𝑁𝑁2⋅ 𝑚 𝑁2

⋅ 𝑐 𝑝,𝑁2 ⋅ 𝑑𝑇 + 𝑚 𝐸𝑥𝑐𝑒𝑠𝑠 𝐻2𝑂 ⋅ 𝐿𝐻𝐻2𝑂 + 𝑄 𝑎

+ − 𝑄 −

Where LHH2O is the latent heat of the water, Tm is the combustion gases mixture temperature

and Ta is the atmospheric temperature.

𝑄 𝑎+ =

𝑇𝑖 − 𝑇𝑖−1𝑅𝑒𝑞,𝑖

For more realism we

have also considered the

heat energy term (Qa)

given to the atmosphere

by the burner. In

evaluating it and

knowing all the thermal

resistances, it is possible

to calculate the

temperatures at each

node in the equivalent

circuit diagram to our

thermal system

represented at right