Integration of direct carbon fuel cells with concentrated solar power

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Integration of direct carbon fuel cells with concentratedsolar power

G. Cinti a, K. Hemmes b,*aUniversity of Perugia, Department of Industrial Engineering, Via G. Duranti 67, 06125 Perugia, ItalybTU Delft, Faculty TPM, Section Technology Dynamics and Sustainable Development, Jaffalaan 5, NL-2628 BX, Delft, The Netherlands

a r t i c l e i n f o

Article history:

Received 15 March 2010

Received in revised form

29 October 2010

Accepted 9 November 2010

Available online 24 December 2010

Keywords:

Multi-Source Multi-Product

Direct carbon fuel cell

Concentrated solar power

System modeling

a b s t r a c t

In this paper, we will report on a study on the thermodynamic feasibility of a concept that

realizes the cracking of methane with a concentrated solar power (CSP) reactor and elec-

tricity production with a direct carbon fuel cell (DCFC) and its possible contribution to

a clean energy supply for Europe in the long-term future. The natural gas (methane) is

decomposed in an endothermic reaction into hydrogen and carbon. The separated carbon

is fed to a direct carbon fuel cell (DCFC) and converted with high efficiency to electric

power. A model of the proposed concept is carried out in the flow sheet program Cycle-

Tempo and the results of the simulations and the corresponding analysis are presented in

this paper. Finally the location factors influencing the implementation of this concept in

the north of Africa are evaluated.

Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

Africa possesses a lot of natural resources and can play an

important role in the future world energy supply when it

comes to the problem of finiteness of rawmaterials and fossil

energy resources. Especially in the energy field Africa has a lot

of opportunity for future development because of the huge

amount solar energy incidents and the land area that is or can

be made suitable for biomass production or the more direct

use of solar energy. Moreover the north of Africa (Algeria,

Egypt) also has large Natural Gas reserves.

Following the logic of Multi-Source Multi-Product systems

that were recently proposed we started to evaluate the feasi-

bility of new energy conversion chains involving new

components that still need to be developed [1]. The new

chains include technologies for coproduction of electrical

energy and hydrogen based on direct carbon fuel cell (DCFC).

In one particular conversion chain, natural gas is thermally

decomposed into C andH2. Heat for this endothermic reaction

is supplied by CSP (concentrated solar power). The carbon is

used by the DCFC to produce electricity with a high efficiency.

This study will use a thermodynamic flow sheet simulator

(Cycle-Tempo) developed at TU Delft to optimize the effi-

ciency of the system and to study the energy balance and

energy flows. This new concept is to be realized in North

Africa where it is possible to have both natural gas and solar

energy in large quantities, while the area is relative close to

Europe. Due to the application of the Multi-Source Multi-

Product approach, a renewable energy source such as

concentrated solar power can be applied with higher effi-

ciency and better economic performances than in conven-

tional renewable energy systems. The high efficiency of the

concept and the development of the region that is provided

with employment and value added exploitation of natural

resources in a sustainable way are strong arguments in favor

of the concept, especially if we consider a future based on

* Corresponding author. Tel.: þ31 15 2781650.E-mail address: [email protected] (K. Hemmes).

Avai lab le at www.sc iencedi rect .com

journa l homepage : www.e lsev ie r . com/ loca te /he

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 0 1 9 8e1 0 2 0 8

0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2010.11.019


hydrogen and carbon energy. Further expansion of this

concept can also be done by using biogas from biomass gasi-

fication or by taking advantage of the pure CO2 stream that

leaves the DCFC for sequestration or reuse.

The hydrogen can be transported to Europe by pipeline,

providing the large amounts of hydrogen needed for

a hydrogen economy in Europe. The aim of this research is to

evaluate the efficiency of this innovative concept that can

provide large amounts of hydrogen and electrical power at

high efficiency. The separation of carbon is directly achieved

at the production. Carbon dioxide produced by the direct

carbon fuel cell is supplied in a pure gas stream with no need

for a CO2 gas separation unit. Since this concept usesmethane

(natural gas) and sun power as inlet energy an optimal loca-

tion would be North Africa, especially Algeria and Egypt,

country that process large amounts of natural gas reserves

and of course sunlight. This area is also in a strategic location

because it is close to Europe and can easily supply energy to

the “Old Continent”.

Research questions include:

� What is a suitable size of a CSP plant and a DCFC plant?

� What are the relative contributions of fossil fuel and

renewable energy in producing the end products?

� What are the anticipated technical problems related to the

concept? Which directions for solutions can be identified,

and what would be their feasibility?

� What would be the overall efficiency compared to conven-

tional use of natural gas including carbon capture from

natural gas power plant and use of a CSP plant producing

only electric power?

2. Theory

2.1. Multi-Source Multi-Product Systems

Conventionally, the one-dimensional analysis (uni-variate

analysis) has been used to study energy systems. Most of the

focus has been given to energy conversions from one form

into another. All the byproducts like heat or chemical

compounds were destroyed or dismissed as waste. The first

step towards system improvement was cogeneration, a tech-

nique that recovers “waste heat” for several uses as a valuable

product. Poly-generation systems took the concept even

further and a simultaneous production of chemicals, power

and heat is proposed. These systems are integrated in larger

systems, such as chemical plants, to achieve increased overall

performance, while electricity may just be a byproduct.

Cogeneration and poly-generation systems are still charac-

terized by a single energy input source.

A Multi-Source Multi-Product system is a concept that

takes it several steps further. A MSMP system may be defined

as a system with more than one energy input and more than

one useful product output [1]. Although in general most

energy system have multiple inputs (e.g. fuel and air), the

term MSMP system is meant to be restricted to energy system

with multiple energy inputs. The relation between input and

output is not one-dimensional anymore because both input

and output are more than one. Mathematically, inputs and

outputs can be modeled by stating vectors containing

different flows and products. The vectors can be related to

each other by a coupling matrix which connects the input

with the output. Stating inputs P1, P2,.,Pn and outputs

L1,L2,.,Ln in vectors enables to define a coupling between

inputs and outputs:

0@ L1

L2Ln

1A ¼

0@ c11 / c1m

« 1 «cn1 . cnm

1A0@ P1

P2

Pm

1A

If the coupling matrix C describes correctly and completely

the system then each coefficient cij in the matrix can be seen

as a particular conversion efficiency, e.g., gas to electric power

[2]. In our case for the CSPeDCFC concept we have two energy

inputs. Solar radiation (P1) and natural gas (P2) and three

outputs: electric power (L1), hydrogen (L2) and heat (L3):

0@ L1

L2L3

1A ¼

0@ c11 c12

c21 c22c31 c32

1A�

P1

P2

�

2.2. Direct carbon fuel cell

The direct carbon fuel cell (DCFC) is a fuel cell with a long

history starting from the 19th century but it has been

forgotten for a long time and only recently it came back in the

attention of the scientific community [3e13]. Based on this

literature the fundamentals will be given below.

The cell reactions and the theoretical principles are similar

to high temperature fuel cells such as MCFC and SOFC. The

fuel reacting at the anode is carbon and the oxidant supplied

to the cathode is air mixed with carbon dioxide. The overall

cell reaction is given by:

CþO20CO2 E0 ¼ 1:02 V at T ¼ 800�C (1)

The DCFC, the only fuel cell using solid fuel, has several

unique attractive features.

Firstly, this fuel cell offers great thermodynamic advan-

tages compared to other fuel cell such as MCFC and SOFC. The

reversible electrical efficiency of a fuel cell operating at a fixed

T is given by:

hFCrev ¼ WFCrev=ð�DHÞ ¼ ð�DGÞ=ð�DHÞ ¼ 1� T$DS=DH (2)

where WFCrev is the reversible electrical work:

WFCrev ¼ �DG ¼ n$F$Vcellrev (3)

with n the number of electrons for each molecule of fuel, F is

the Faraday constant and Vcellrev is the reversible cell voltage.

The cell reaction (1) has an entropy change close to zero,

consequently the standard Gibbs free energy change is almost

equal to the enthalpy change DH for reaction (1) and the

theoretical electrochemical conversion is almost independent

of temperature. Therefore:

hFcrev ¼ 1� T$DS=DHz1:00 (4)

The calculation above shows the thermodynamic advan-

tages of aDCFCcompared to classichigh temperature fuel cells.

These advantages follow the fact that entropy change and

consequently the reversible heat of the cell reaction are

approximately zero for the full electrochemical conversion of

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 0 1 9 8e1 0 2 0 8 10199


carbon to carbon dioxide. Furthermore the use of solid fuel

causes minimal Nernst loss in the direct carbon fuel cell. The

thermodynamic activity of the carbon fuel and the CO2 product

are spatially and temporally invariant allowing100%utilization

of the carbon fuel in a single pass (cf. Hydrogen utilizations of

80e90%). Consequently the electrochemical Nernst voltage is

constant. In the other high temperature fuel cells, Nernst

voltage decreases towards the outlet side of the fuel cell. This

introduces overall a so-called Nernst loss. This constitutes

amajor loss factor in these conventional high temperature fuel

cells [14]. These aspects may allow the carbon fuel introduced

in the cell to be consumed in a single pass. Both forms of

utilization losses (changing gas composition and inherent

incomplete utilization) are not present in the DCFC.

The ratio between a reversible fuel cell work and the work

delivered by operating self can be expressed as a function of

the ratio between the reversible fuel cell voltage and the

measure cell voltage:

WFC

WFCrev¼ n$F$Vcell

n$F$Vcellrev¼ Vcell

Vcellrev(5)

Introducing this equation the efficiency of the cell including

irreversible losses can be expressed by:

hFC ¼ WFC

HHVfuel$WFCrev

WFCrev¼ WFCrev

HHVfuel$Vcell

Vcellrev(6)

For both the hydrogen to oxygen and carbon to oxygen fuel

cell the value of the ratio between the two potential is around

0.8 for practical current densities due to ohmic and electrode

polarization losses. Therefore the efficiency of a DCFC is

approximately 80%. However, it should be emphasized that

current and power density at this Vcell to OCV ratio is still

lower than for present state of the art high temperature fuel

cells MCFC (120e200 mW/cm2) and SOFC (250e300 mW/cm2).

This relates to the inherent lower reaction rate of carbon

relative to hydrogen, but also to the little research performed

on the DCFC up till now. The maximum value that can be

reached by SOFC and MCFC, that are the most efficient, is in

the range of 50%. Comparison between different types of fuel

cells is summarized in Table 1. The theoretical advantages of

a direct carbon fuel cell have been clearly demonstrated.

DCFC is extremely interesting solution for electricity

production for other three main aspects: polluting emission,

availability of carbon and mechanical solutions. The specific

emissions of DCFCs are less than other power system using

the same fuel. The carbon is electrochemically oxidized and

pure CO2 is produced at the anode side. Although the Bou-

douard equilibrium at the temperature of the DCFC

(700e800 �C) predicts the formation of carbonmonoxide it has

been found experimentally that almost pure CO2 was

produced. This can be explained by the electrochemical

potential of the anode that rapidly oxidizes CO to CO2. Pure

production of carbon dioxide is a great advantage for future

storages because no separation is needed. Separation of CO2 is

extremely complex and can be a relevant cost for a power

plant. Moreover the physics of the cell provides no particulate

matter in the outlet gas. Coal-fired plants produce great

percent of world electricity and are responsible for a large

amount of pollution. A substitution of standard carbon plants

with DCFC can be an important step to reach a global dimi-

nution of emission.

The DCFC uses solid carbon as a fuel. Carbon can be

produced from coal but also from oil and natural gas and also

from renewable sources as biomass and even organic waste.

Coal is the earth’s most abundant fossil resource, it is esti-

mated that almost 60% of word energy resources is stored in

coal mines. Moreover most energy reserve of coal remains

unused. Carbon is a fuel with a high ratio energy volume.

Finally the DCFC system is mechanically simple because it

doesn’t need a reformer or heat engines. Therefore the DCFC

provides a possibility for converting carbon directly to elec-

trical power without combustion.

To start electrochemical oxidation of carbon high

temperatures have to be reached andmolten salt electrolyte is

requested. The laboratories that are experimenting with the

direct carbon fuel cell are testing three different electrolytes:

molten carbon electrolyte, hydroxide electrolyte and YSZ-

based solid electrolyte. Cooper et al. at Lawrence Livermore

National Laboratory (LLNL, Livermore, CA) realized a DCFC

with 32% Li2CO3 and 68% K2CO3 molten electrolyte [4]. The

reactions at the anode and cathode side are, respectively:

Cþ 2CO2�3 /3CO2 þ 4e��Anode� (7)

O2 þ 2CO2 þ 4e�/2CO2�3

�Cathode

�(8)

The cell voltage is given by:

Ecell ¼ E0 � RT4F

ln��pCO2

�3anode

þ RT

4Fln��pO2

��pCO2

�2cathode

(9)

A scheme of the cell is reported in Fig. 1. This DCFC

produced several interesting results and is the one elected in

this paper to be introduced in the new concept.

2.3. Solar decomposition of natural gas

Solar decomposition of natural gas is an experimental tech-

nology to integrate two different kind of energy: a fossil fuel:

“methane” and a renewable source: “sun power”. This new

technology produces hydrogen and carbon black, two simple

molecules that can be transformed into electricity with high

efficiency. Methane (CH4) is a preferred hydrocarbon for the

production of H2 because of its high H to C ratio (H/C¼ 4),

availability and low cost. Furthermore, the produced carbon

can be sold on the market for carbon black or can be seques-

tered, stored or used as a clean fuel for electrical power

generation as in the present concept using a DCFC (or

a conventional coal-fired power plant for that matter). A

literature study of methane cracking revealed that it is

Table 1 e Efficiency comparison of DCFC with other hightemperature fuel cells (after Cooper et al. [3e5]).

Fuel Theoreticallimit¼DG/DH

Utilizationefficiency

(m)

V(i)/V(i¼ 0)¼ 3V

Actualefficiency¼

(DG/DH ) (m) (3V)

C 1.003 1.0 0.80 0.80

CH4 0.895 0.80 0.80 0.57

H2 0.70 0.80 0.80 0.45



possible to obtain the thermal dissociation of methane in

several different ways, although most of them are still in

a research state, several studies are preformed and several

experimental setups are proposed [15e33].

The overall reaction is:

CH4 ¼ Cþ 2H2 (10)

Each molecule of methane is thermally decomposed in one

atom of carbon and two molecules of hydrogen. The enthalpy

change of this reaction is 74.9 kJ/mol at 298 K. The study of the

kinetic of the reaction indicates dissociation above 600 K and

reveals that temperatures T> 1500 K are required to achieve

nearly complete dissociation [31].

The chemical conversion depends on temperature and

pressure in accordance with Le Chatelier’s principle. Because

two gas molecules are formed out of each molecule of

methane the pressure should be kept low to enhance the

reaction towards hydrogen production. The temperature

dependence is shown in Fig. 2.

The cracking of methane is endothermic and external heat

has to be supplied to the reactor. This also means that heat is

converted into chemical energy. In this case the enthalpy of

the formed hydrogen and carbon. There are several alterna-

tives to supply the heat required to drive the reaction. In the

commercial “Thermal Black Process”, the energy is provided

by burning CH4 with air to heat in a furnace. In the fluidized

bed thermal decomposition reactor the heat transfer is real-

ized by iron oxide that is used also as a catalyst. Other

processes used to supply the necessary energy to decompose

CH4 in a early state of research are plasma torch and metal

bath reactor where methane is bubbled through molten tin or

copper. These processes are all producing greenhouse emis-

sion and/or gas or are consuming net electricity. To get a zero

emission concept a solar reactor is considered. Solar reactor

get energy from concentrated solar concept (solar furnace)

that collect energy from the sun light and redirect, via high

reflectingmirrors, the solar rays in one point where the flux of

energy reaches considerable levels. An ideal solar reactor

absorbs the energy coming from the mirrors and, for

temperatures over 1000 K, itsmain thermal losses are by black

radiation through the aperture. The solar energy absorption

efficiency of a solar reactor, had, is defined as the net rate at

which energy is being absorbed divided by the solar power

coming from the concentrator. For a perfectly insulated

blackbody cavity-receiver, it is given by Fletcher and Moen

[34]:

had ¼ 1� �s$T4

�=ðC$IÞ (11)

where s is the constant of StefaneBoltzmann, I is the normal

beam insolation (W/m2), T the temperature of the reactor. C is

the flux concentration ratio and is defined as the ratio of the

solar flux intensity achieved after concentration to the inci-

dent beam normal insolation. It is a dimensionless number,

sometimes reported in units of “suns”. The absorption effi-

ciency is defined as the ratio between the energy flux

remaining in the reactor {C$I� s$T4} and the flux coming from

the mirrors {C$I}, but at the same time it is also the ratio

between the energy remaining in the reactor and the total

solar energy reaching the target. Since the net energy absor-

bed should match the enthalpy change of the reaction, the

total solar energy required is:

Qsolar ¼ mCH4DH

had

(12)

where mCH4is the mole flow of methane and DH is the

enthalpy change at the temperature of the reaction.

Solar heating of natural gas cannot be achieved directly

because hydrocarbons poorly absorb radiation in the visible

spectrum. Thus, solar reactor concepts must involve either

opaque heat transfer walls that absorb the solar radiation and

then heat the gas by gasesolid convection and IR radiation

(indirect heating), or a transparent window that permits direct

heating of particulate material (particulate material can be

carbon black) by radiation (direct heating). The indirect heat-

ing concept requires drastic high temperature material spec-

ifications, and the direct heating concept may lead to severe

problems of optical window breakage due to possible carbon

deposition. Three main laboratories study solar reactors for

methane cracking: the Department of Chemical Engineering

of the University of Colorado (Weimer and Dahl et al.); the

Department of Mechanical and Process Engineering of Zurich

(Hirsh and Steinfeld et al.) and the Processes, Materials, and

Solar Energy Laboratory of Odeillo in France (Abanades and

Flamant et al.). These groups developed different prototypes

of solar reactor and tested in solar furnace laboratories. The

reactor selected for the concept design presented in this paper

is the one designed and tested by the University of Colorado

(shown in Fig. 3). This is an aerosol flow reactor consisting of

an outer quartz protection tube and an inner graphite reaction

tube isolated from each other using water-cooled end caps.

Fig. 1 e Scheme of the DCFC realized at LLNL [3e5].

Fig. 2 e Primary methane decomposition equilibrium

products P[ 0.1 Mpa.



The inlet methane flows goes directly through the graphite

tube. To avoid any oxidations between the two tubes there is

a flow of argon that provides an inert atmosphere. The reactor

is enveloped by a secondary solar concentrator to increase the

solar flux and reach temperatures greater than 2000 K. The

reactor wall remains cold since a light beam is directly deliv-

ered to the facial target.

The results from the laboratories testing presented some

important difference between the model and the reactor. To

achieve a complete conversion of methane in carbon black

and hydrogen the reactor has to reach a higher temperature

then indicated by the study of the thermodynamic process. A

temperature of 2000 K gave a conversion of 70% of the inlet

methane. It should be noted that complete dissociation is

expected at these temperatures. It is believed that the

conversion was limited by heat transfer. The efficiency of the

reactor is calculated by:

had ¼ ðXCH4$DHÞQsolar (13)

Here XCH4 is the fraction of methane that has reacted. The

efficiency calculated with the experimental data is lower than

the one expected. The main differences with the model are

that the reactor is not a blackbody and reradiates differently,

at the same time the temperature of the reactor and that of the

reaction are not the same because the heat flux reaching the

target is not entirely delivered to the incoming flux but is

partially adsorbed by the cooling zone. In addition all the

optical losses in the secondary reactor have to be considered.

The solar reactor is fed with a flux of energy coming from

a concentrated solar power plant. There are mainly three type

of CSP: trough system, tower system and dish system. They

are mainly distinguished by their concentration ratio C and

their characteristic temperatures at the focal. Tower systems

are the only concepts that can reach the temperatures

required for the methane cracking reaction. A tower system

consists of a heliostat field composed of several mirrors that

concentrate the rays in a target in the top of a tower. The

efficiency of a heliostat field is calculated as the quantity of

energy that is concentrated in the tower over the solar energy

received by the mirrors. The solar reactor is located in the

tower and the solar flux coming from the field reaches directly

the secondary concentrator. To evaluate the dimension of the

heliostat the following expression is used:

Heliostat area ¼ Wreactor=ðhad$hHeliostat$IÞ (14)

where Wreactor is the heat absorbed by the reactor to drive the

reaction, I is the normal beam insolation and hheliostat is the

efficiency of the heliostat field. Vice versa the same equation

can be used to calculate the power that reaches the reactor

from a specific plant. The quantity of methane reacting

depends on the incoming solar radiation energy. And the heat

per unit time needed for the decomposition reaction with

a flow of mCH4mol/sec of CH4 is

Wreactor ¼ XCH4mCH4

DH (15)

The above equation can be used to evaluate the dimension

of the heliostat field of the suggested plant design and will be

used to compare its efficiency with a standard utilization of

concentrated solar power.

3. System design

3.1. The Cycle-Tempo software

In this study, the power plant modeling software Cycle-

Tempo is used to simulate the suggested cycle. The computer

programCycle-Tempowas developed jointly by TUDelft (Delft

University of Technology) and TNO as a modeling tool for the

thermodynamic analysis and optimization of systems for the

production of electricity, heat and refrigeration. This software

is a flow sheet program for mass and energy balance calcu-

lation similar to the commercial computer program Aspen. It

simulates several devices such as pipes, heat exchanger,

turbines etc. that can be joined together to build the thermo-

dynamic model of a concept. The main thermodynamic

characteristic of the elements can be set and the program

calculates mass and energy flows at equilibrium. Moreover

the program can calculate energy and exergy balance and

system efficiency. The DCFC component was added to the

Cycle-Tempo program so that systems with a DCFC can also

be simulated.With the design and off design input parameters

the main aspect of the fuel cell can be fixed or varied and the

effect of this change on the global system can be calculated

and analyzed.

3.2. The CSP and DCFC concept model

The concept consists of three main components: the sun

cracking unit, a separation and storage unit and the direct

carbon fuel cell. The final design in which the simulations are

performed is shown in Figure 4.

The methane coming from the net is pumped by an

optional blower in the system. The plant operates at

Fig. 3 e Schematic of the solarethermal fluid-wall reactor

[17].



atmospheric pressure because both the solar reactor and the

Fuel Cell have the best performances at low pressure. After the

blower the methane passes through a heat exchanger, this

device has the double function of cooling down the flow from

the sun reactor and to heat the inlet flux. The equilibrium is

set to have the outlet methane at 250 �C, to avoid the sepa-

ration reaction taking place in the exchanger. The reactor

studied by the University of Colorado was taken to simulate

the solar reactor in the concept. Unfortunately there is no

standard component in the Cycle-Tempo software to model

the solar reactor. Therefore the reactor model is realized in

Cycle-Tempo as a standard gasifier component, however,

with no external air supply. For technical reason in the

scheme there is an air source but the flow is set to zero. In this

way the gasifier component mimics a solar reactor, and all the

thermodynamic equilibria between the reactants are calcu-

lated properly, in this case mainly the decomposition of

methane into carbon and hydrogen. A percentage of the inlet

flow can be “bypassed”: that means that it is only heated up to

the reaction temperature but does not participate in the

reaction. This solution in used to simulate the fraction of

methane that does not react inside the solar reactor. In the

model of the gasifier there is a heat exchanger to provide the

necessary energy to drive the reaction. An air flux initially at

1800 �C passes by the heat exchanger where its temperature

decreases. A re-heater permits the flow to reach its previous

temperature again. The heat provided by the re-heater is

calculated and is assumed to be equal to sun power entering

the system. This is a trick used to simulate the energy

balances of the sun reactor. The outlet gas is a mixture of

hydrogen, solid carbon and unreactedmethane. This flux is at

very high temperature even if a decrease of temperature due

to heat loss is considered. In the model no extra losses are

analyzed and the temperature of the flow is the temperature

of the reactor.

To have a useful fuel for the DCFC the temperature of the

mixture has to be cooled down to temperatures in the range of

700 �C. In the Cycle-Tempo model the fluid coming from the

reactor is cooled down by the passage through the heat

exchanger at the reactor inlet. In the flow sheet the mixture

passes by a heat sink that permits the flow to reach the

desired temperature. The possible use of this heat is analyzed

in the next paragraph. The flow passes trough a solidegas

separator that can easily separate solid carbon black from

hydrogen and unreacted methane since the latter are both in

a gaseous form. Carbon reaches the DCFC and provides the

fuel to that unit.

The DCFC is complemented by devices to transport and

treat the inlet and outlet flows, such as compressor and valves

for CO2 recycle, and heat exchanger to preheat air entering the

cathode side of the cell. The DCFC chosen for our simulations

is derived from the MCFC design and it uses the same elec-

trolyte (molten carbonate), electrolyte support and cathode.

Like the MCFC this DCFC needs to recycle the carbon dioxide

Fig. 4 e Cycle-Tempo model of the CSPeDCFC concept.



from the anode to the cathode. Extra carbon dioxide has to be

added to the air because the equilibrium of reaction (8)

requires two molecule of CO2 for each molecule of oxygen.

This ratio is not present in standard air, but carbon dioxide is

produced at the anode: each reacting carbon generates 3

molecules of CO2, see Eq. (7). This production is enough to

cover the consumption at the cathode side. Part of the CO2 gas

is mixed with the cathode inlet flow and part is used to carry

the solid carbon in the pipelines. The DCFC model is mainly

based on the Direct Carbon Conversion Cell (DCCC) of Law-

rence Livermore National Laboratory (LLNL). The Cell works at

the temperature of 750 �C and the output gases are at 800 �C.Heat generated inside the fuel cell has to be removed in order

to keep a constant operation temperature. As in other fuel

cells this is achievedmainly by the cathode flow that is heated

up while going from inlet to outlet.

The carbonegas mixture reaches the anode side and is

completely transformed into CO2 because, as explained in the

previous chapter, fuel utilization can be set as one: complete

reaction of carbon. The recycled CO2 partially provides the

heat to the inlet flow.

On the cathode side air is supplied combined with recycled

CO2 from the anode. The oxygen reacts inside the cell and the

flowratioof thiscomponentintheoutletflowisclose tozero.Yet

aminimumpartial pressure of oxygen should bemaintained in

the output stream. The air enters the system at ambient pres-

sure and temperature and isheatedupbyaheat exchangerwith

the anode carbon dioxide outlet flow of 800 �C and by the high

temperature CO2 that ismixedwith the air for the cathode flow.

The system model has two energy inputs: methane and

heat supplied to the gasifier that mimics the solar reactor and

three energy outputs: hydrogen, electricity and heat. There

are several heat flows that can be recovered and converted

into often-more useful energy forms such as electricity.

3.3. Analysis of the basic design

To evaluate the feasibility of the concept several simulations

were run, while changing the main design parameters. The

size of the concept is defined setting the inlet methane mass

flow. A methane flow of 0.4 kg/s was selected to have

a cracking heat request compatible with a CSP average plant

size. The inlet energy flow related to themethane flux is set at

20 MW, this value is calculated considering the LHV of CH4.

The concept has several energy flows that are defined as

follows.

EnergyCH4: chemical energy of inlet methane (LHV).

EnergyH2: chemical energy of outlet hydrogen (LHV).

EnergyReactor: energy requested by the solar reactor.

Heat Due: energy necessary just to heat up the flow to the

reaction temperature.

DH: heat energy needed to drive the endothermic decompo-

sition reaction.

Heat Loss: is the heat ‘lost’ in the heat sink before the

separator.

EnergyDCFC: is the electricity production of the DCFC.

A first calculation of efficiency can be done using the

following definition:

h1 ¼EnergyH2

þ EnergyDCFC

EnergyCH4þ Energyreactor

(16)

This efficiency is calculated as the ratio between the two

main energy inputs, methane and sun power, and the two

main outputs, chemical energy of the H2 flow and electrical

energy from the DCFC. This value is the real efficiency of the

concept if no useful energy can be obtained from the heat

streams leaving the system; the main energy losses. A second

efficiency can be calculated if adding as a useful energy output

also the ‘Heat Loss’:

h2 ¼EnergyH2

þ EnergyDCFC þHeat Loss


(17)

The comparison between these two definitions gives infor-

mation on the importance of the amount of heat loss in the

concept. Large differences between the two values of effi-

ciency imply that there is a large interest to transform the heat

into useful energy, for example electricity.

The relative contribution of fossil fuel and renewable

energy is easily calculated by the following formulas.

Fossil fuel:

Xff ¼EnergyCH4


(18)

Solar power:

Xsp ¼ Energyreactor


(19)

The results of the Cycle-Tempo simulation are summarized

in Table 2.

The experimental data reveals two main problems of the

system concept: (1) the high temperature of the outlet gas and

(2) the relatively high quantity of unconverted methane in the

output flow. These two problems are linked to the amount of

EnergyReactor in both its contributions: ‘Heat Due’ and DH. If the

reactor temperature increases, the ‘Heat Due’, i.e., the quantity

of energy necessary to reach that temperature, increases. A

temperature raise also increases the reaction enthalpy but, as is

possible to see in Fig. 5, this variation is neglectable. The value

of DH strongly depends on the quantity of reactants that react

Table 2 e Results of the simulations and of thecalculations of the ratio defined in the paper. DCFC refersto the electrical production of the Fuel Cell and CH4 to thechemical power of the inlet methane (LHV).

Results of the calculation in Cycle-Tempo

Inlet Methane (kg/s) 0.4

Reaction temperature (K) 2000

Concentrated solar power (kW) 4461

DCFC (kW) 5384

Reaction CSP (kW) 1616

CH4 (kW) 20005

H2 (kW) 9125

Heat loss (kW) 2016

h1 0.59

h2 0.68

Xff 0.82

Xsp 0.18



because in our definition here it is calculated as the product of

numbers of reacting moles with the enthalpy change of one

mole at the reaction temperature. The ratio between DH and

EnergyReactor is important to evaluate determining the perfor-

mance of the reactor in our concept. As presented there is no

interest in having high temperatures because it causes an

increase of the Heat Loss, which is hard to convert in useful

power. On the other hand an increase of DH is important

because of a higher flow of carbon to the DCFC and more pure

hydrogen production. Consequently the ratio DH/EnergyReactorhas to be increased. As clearly represented in Fig. 6, to have

a higher r ratio, the reaction temperature has to decrease and

the amount of reactedmethane has to increase. The analysis of

the energy flux of the CSP gives us information about the

importance to improve the concept with a cogeneration unit. In

some simulations [19] a quencher is used to cool down the fluid.

The use of a quencher causes large costs and decreases the

efficiency of the whole system. On the other hand the scheme

of the concept presented sincenowdoesnot offer opportunities

to use this amount of heat that is provided at high temperature.

In additionwe should consider also the two fluids coming from

the DCFC output: the CO2 and the air that are at a temperature

of 1073 K and 439 K, respectively.

3.4. Optimization of the design: heat integration

To analyze and solve this problem several solutions were

explored. A feasibility of concept was realized to evaluate the

best solution.

The system configuration offers threemain hot fluid flows:

carbon and hydrogen mixture from the CSP at 687 �C, CO2

from the anode at 800 �C and air from the cathode at 165 �C.The total amount of energy is distributed at three tempera-

tures. Main part of this energy, 88%, is concentrated in the

highest temperature range. Several solutions are proposed

and analyzed to keep the efficiency of the system high or, in

some case, even increase it.

e The hot flow can be directly sent to the separator and then

to the DCFC at high temperature. The main advantage of

this solution is that the system concept is not modified in

any part and it maintains its simple scheme. On the other

hand the efficiency of the Fuel Cell does not increase: the

DCFC produces heat and an extra amount of heat

becomes a problem so that the system reacts by

increasing the cathode inlet air flow just to cool down the

anode carbon flux. While keeping the ratio air/CO2 in the

DCFC cathode inlet flow fixed, an increase of air flow

request a larger amount of carbon dioxide that cannot be

provided anymore by the anode side. Therefore although

this solution offers a linear and simple layout it causes an

unacceptable request of air and CO2 and extra heat that is

not transformed into useful energy.

e While extra heat causes problems in the standard DCFC it

could be a big advantage in a DCFC that oxidize carbon to

CO. In this special fuel cell heat is transformed into elec-

trical energy. This is possible because the electrochemical

reaction is endothermic because the entropy increases [8].

This fuel cell does not produce heat like all the other fuel

cell but absorbs heat from the system. This kind of DCFC

is still in a research state and there are not any examples

of this device realized. It is also not yet available in the

Cycle-Tempo model. So no simulations could be per-

formed on this future concept yet.

e The high temperature produced by the sun reactor can be

transformed into electrical energy adding a sub-steam

turbine conventional cycle. The different hot flows, CSP

output, CO2 and air, can be used in different levels of the

steam generator: super heater, vaporizer and economizer

respectively.The total efficiencyof theconceptwill increase

but the concept layout will be more complex adding all the

devices that are part of the steam turbine unit.

e The heat energy in the outlet reactor fluid can be used for

cogeneration. As far as the system in exam is planned to

be realized in the African desert it is not easy to imagine

what kind of cogeneration application would be possible

for this heat. If the plant is set close to the seaside or in

some area rich of salty water a desalination unit using the

extra heat could be added to the concept. The desalted

water could be used to water some agriculture production

that can be set under and between the solar mirrors.

Alternatively the heat may be converted into cold for air-

conditioning by adsorption chillers.

3.5. Optimization of the design: use of unconvertedmethane

The second problem is the fraction of unreacted methane in

the outlet flow of the solar reactor. Dahl et al. [15] present the

Fig. 5 e Energy flows (in kW) versus different reaction

temperatures (in K) for constant inlet methane flow

(0.4 kg/s).

Fig. 6 e Variation of (DH/EnergyReactor) ratio versus different

reaction temperatures (in K) for constant inlet methane

flow (0.4 kg/s).



opportunity of recycling the methane in the inlet flow

with no consequent modification of the general system

thermodynamic values. However, the paper does not present

what kind of device is used to divide methane from hydrogen

after the separation of solid carbon. The methane recycle

does not add any efficiency advantages to the system unless

it is possible to find a different solution to avoid the separa-

tion of methane and hydrogen or provide an easy way of

separation.

However, the unreacted methane might not be a problem

if the hydrogen is to be mixed with the natural gas. A

concept of mixing hydrogen into the natural gas grid is

under study in the Netherlands and in an EU project (NAT-

URALHY). If this analysis provides convenient solutions the

outlet flow from the carbon separator can be directly

compressed and sent to the natural gas pipelines. This

solution is extremely simple and does not require adding

extra elements to the concept. The consumption of methane

is higher compared to the recycling solution but the effi-

ciency does not change.

e Considering a complete cycle the hydrogen produced can

be used adding a standard fuel cell to the system.

Different solutions are then available to avoid the sepa-

ration of methane. For example (internal reforming) high

temperature fuel cells can receive a mixture of hydrogen

and methane. The total efficiency of the concept will

decrease as long as hydrogen is transformed into elec-

tricity with usual FC efficiency but the comparison of the

two values is impossible because the system produces

a different kind of energy: electricity instead of hydrogen.

e The mixture flow can be sent to a reactor to complete the

cracking. This solution can be improved by adding special

catalysts providing the conditions for a complete disso-

ciation of CH4. The problem related to a second cracking of

the methane is to find the heat that has to be provided to

the reactor. Heat can be added to the reaction burning part

of the flow or realizing a second sun reactor. Both solu-

tions are not interesting because the first onewill produce

polluting emissions as consequence of combustion and

the second is complex and requires a second heliostat

field in the same area of the first one. Using an appropriate

catalyst the temperature of the reaction can be kept

sufficient low to avoid the need of external high temper-

ature heat and the waste heat and temperature from the

CSPeDCFC system might be sufficient.

e Different reactions can be realized in appropriate reactors

to increase the amount of hydrogen in the flow. The most

common reaction is the steam reforming of methane:

unfortunately this reaction needs steam (water) that is

rare in desert areas.

Both the components of the flow in exam, methane and

hydrogen, are excellent fuels and can be burned inside

different energy plants such as gas-turbines or gas engines.

This solution does not reach very high efficiency and causes

typical pollution problems of this kind of plants. Finally

burning hydrogen is not an innovative energy solution and

does not produce any benefits to the environment.

3.6. Design specifications for the concept located inNorth Africa

Some order of magnitude calculations will be given to evaluate

the feasibility of a plant located in North Africa. The simula-

tions realized in Cycle-Tempo provide numerical results that

can be used to evaluate the size of the system. For a methane

flow of 0.4 kg/s, 4500 KW of heat is requested at the reactor.

This power is supposed to be collected by the heliostat field and

delivered to the reactor located in the top of the tower. The

production of methane in North Africa is consistent, but is

mainly concentrated in Algeria and Egypt. In 2005 the produc-

tion of natural gas in these countries was about 65 and 8 billion

of cubic meters in Algeria and Egypt, respectively [37]. The sun

radiation in these countries is very high and available during

the whole year. The average insolation (10 year average) is

about 4.5 kWh/m2/day in Algeria to 5.5 kWh/m2/day in Egypt

(www.apricus.com). These two countries have the character-

istic necessary for the concept in this paper and can be selected

for the realization of the plant. Moreover they both face the

Mediterranean Sea and have an easy connection with Europe.

To calculate the dimension of the heliostat field Eq. (15)

was used. Using the calculation of Osuna the average solar

resource that is available when the plant is in operation is

0.8 kW/m2 [35]. A heliostat efficiency of 0.69 [36] and an effi-

ciency of the reactor in transferring solar energy received to

the gas inside of 0.16 [15,37] were used to calculate a heliostat

field of 50,950 m2. To understand the dimension of this plant

we consider that the biggest European CSP plant (PS10) located

in Seville has a heliostat field of 74,880 m2 and produces

11 MWof electrical energy with a standard steam cycle. So the

two sizes are comparable.

To evaluate the feasibility of theMulti-SourceMulti-Product

concept presented in this paper a comparison with standard

technology was made. Considering a standard plan using the

same energy sources, same solar power and same flux of

methane, it is possible to calculate an alternative efficiency and

compare it to the efficiency of our innovative concept. A stan-

dard CSP plant that uses a steam cycle has an efficiency of 15%

[35] considering the same heliostat field efficiency of 0.69. A

SOFC has an efficiency for methane conversion of about 50%.

This alternative concept is not really “standard” because it still

uses innovative energy solutions, but steam CSP and SOFC are

more developed power systems compared to the solar reactor

and DCFC. The alternative concept of a steam cycle CSP and

separate SOFC converting the same amount of methane as the

CSPeDCFC system produces 16,190 kWwhile the simulation in

Cycle-Tempo of the CSPeDCFC system presents a total

production of 14,509 KW, considering electrical power from the

DCFC plus the chemical energy per unit time of the hydrogen

produced. The new concept has a global efficiency slightly

smaller than that of “standard CSP”. This is mainly caused by

the low performances of the solar reactor. The efficiency of the

solar reactor can hardly reach 16% while the receivers used in

CSP with a steam cycle can reach efficiencies of 91%. By

improving the performance of the solar reactor a considerable

increase of efficiency is expected. As predicted by Dahl et al.

[17] a redesign of the reactor will increase the efficiency had and

the conversion values XCH4.



4. Discussion and conclusions

The concept of combining methane decomposition by CSP

with a DCFC has been investigated. A model of the concept

design including the new units was realized in the flow sheet

program Cycle-Tempo and an analysis of the results was per-

formed. Thanks to the simulations it was possible to calculate

several characteristic of the concept under study. By varying

the inletmethane flow itwas possible to evaluate the optimum

relative sizes of the components in the concept design. Effi-

ciency, energy flows and fuel consumption were calculated.

The study of the model revealed two main problems as

a consequence of CSP: the fluid coming out from the device has

to be cooled down and is not completely separated into carbon

and hydrogen. Possible solutions are listed and a feasibility of

the concepts is presented. Main advantages of each solution

are listed and possible anticipated technical problems are

addressed. To evaluate the feasibility of these and other

designs Cycle-Tempo preferably has to be improved by estab-

lishing a new program library that simulates the behavior of

the abovemodules. The first analysis of the concept proves the

advantages expected from the concept but also some inherent

difficulties associated with the operation of a process at high

temperature. The Multi-Source Multi-Product technique

revealed to be an interesting solution to integrate standard

energy sources such as methane and renewable source as

concentrated solar power. In countries such as Egypt and

Algeria there are favorable conditions for implementing the

proposed concept; i.e. a large number of sun hours per year and

a consistent production of natural gas. In spite of a very low

efficiency of present solar reactors assumed in this study

already comparable output versus conventional CSP can be

obtained albeit in the formof electric power and hydrogen. The

potential for improvement of these laboratory reactors is high

and therefore the CSPeDCFC concept could potentially

outperform conventional CSP provided more efficient solar

reactors can be built and the DCFC will be developed further to

commercial MW scale fuel cells. The benefits that Africa can

get from this concept include economic and social benefits and

will guide the continent to become a large (renewable) energy

supplier for Europe.

Acknowledgments

The authors acknowledge the support of the European

ERASMUS program for the exchange of M.Sc. Student Gio-

vanni Cinti allowing him to perform this work at Delft

University of Technology.

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Integration of direct carbon fuel cells with concentrated solar power

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