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Economy of a reliable Power to Gas based regenerative energy supply including circular economy strategies

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Abstract and Figures

Political demands of shorter completion dates and increasing restrictions related to energy transition seem to become more and more stringent in Europe however a clear technical program with reliable milestones is still missing. Energy storage based on Power to Gas (PtG) can be expected to deliver an important contribution to energy security. But the definition of the system to be analyzed shows already that the political target energy transition is not even sufficient to future challenges and may lead to wrong optima without considering main sustainability targets. For the simple case of PtG there is a horizontal energy business but there is also a vertical sustainable substance and resource management business producing additionally O2 and allowing a closed carbon cycle (CCC) removing CO2 and supplying hydrocarbons as sustainable raw material for industries. For any concept development that shall be implemented on industrial stile it is important to get an insight on the production cost and its dependence on design parameters and operation conditions combined with related cost inputs as soon as possible. The application of an allowable investment calculation model developed for fuel cell system research on possible PtG systems delivers a reliable base for the discussion of combinations of technical system integration and possible business models. The cost model is a linear combination of an engineering term describing the cost influence of design and operation and a financial term describing the financing conditions and the financial influence of the project schedule. The financial term is just a scaling factor for the engineering term and can be assumed as constant. Obviously the allowable investment or acquisition value depends on the main technical and operational data as: exergetic conversion efficiency, full load operation time, electricity cost, targeted gas price, and water cost for the horizontal energy business. The vertical business delivers additionally potential income by an O2 price, and a possible CO2 fee depending on a socio-economic discussion. Cost reduction demands a high exergetic efficiency of the PtG system and its high full load operation hours. The cost reduction target is influenced by the choice of the storage gas and by a secured power supply based on EU certified technologies. The efficiency problem of PtG technologies is mainly based on its water demand, consequently C2H4 has been identified as the storage substance with the highest intrinsic conversion efficiency and the highest CO2 recovery potential and the best transportation properties thus ideal for any closed cycle. But only H2 allows the CO2 free operation of open cycles. The economic best solution for high operation time is currently nuclear energy especially if energy independence is an additional short term political target. Possible future solutions have been identified as “solar belts” producing electricity and extending over longitudes combined with solar farming. The vertical business can subsidize the gas prize by additional income by O2 marketing (based on its thermodynamic value) and with a fee for CO2 processing. These results show that innovative integrated PtG opens clear future business chances but needs clearly targeted additional research and new business concepts combined with a sustainable system integration.
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Economy of a reliable PtG based regenerative energy supply
including circular economy strategies
by W. G. Winkler
1. Introduction
The first impression of introducing the idea of renewable energy was to supply all human
energy demands with almost free electric power everywhere. Economists experienced with
the methods of bookkeeping calculated that the energy balances show that this dream
should be possible and there would be only a small number of batteries needed, because
the balance also showed, that the seasonal mean values of wind and solar power match
nicely. Unfortunately this narrative seems not to be sustainable, because it neglects the
enormous deviations of the mean values even over months. The so-called solution of
„energy management“ can thus be easily become a shut down policy, mainly targeting the
poorer part of the population and clearly reducing the attractivity of the location for busi-
ness, but clearly reducing the political acceptance of the energy transition. These political
consequences may even lead to severe dissatisfaction, because such a strategy would
only increase the profit of the shareholders of the electricity suppliers without any positive
effect on the real national economy or the environment.
Consequently the energy policy must enforce a similar reliability of supply quality as usual
for a today fossil and nuclear supply, however additionally with the boundary condition to fit
supporting a circular economy. Additionally the argument of the clearly increasing energy
independence of such a well operating system may be a further motivation, if the cost are
acceptable. The precondition is the understanding of the real operation conditions of such
a system. However electricity is thermodynamically the highest valuable energy form, but it
is useless if the supply is not reliable. The starting point of a successful program is thus
the understanding that this valuable energy must be processed to reliable power to be -
come a successful product on the energy market.
Own studies showed very clearly the above mentioned problem of a lack of fully under -
standing the importance of energy storage [1], [2] and identified the electricity storage
issue as one of the most critical and still open issues of the energy transition programs.
The most promising solution found, was the combination of Closed Carbon Cycles (CCC)
with the Hydrogen economy for open cycles [3], [4]. The main identified realization prob -
lem of this concept was the comparable low efficiency of all existing Power to Gas (PtG)
technologies. This was mainly caused by the high energy demand on electricity for evapo -
rating the incoming water and a questionable thermodynamic system integration as identi -
fied by the studies [5], [6]. These studies also lead to the there described system concepts
that replaced electric heating by waste heat, combined with improved system integration,
and to a change of the main recommended storage substance from H2 or CH4 to C2H4.
This change is caused by the fact that the generation of a C 2H4 molecule needs much less
water supply then the generation of H2 or CH4. This proposed change would increase the
efficiency potential alone by almost 10%. A more detailed discussion about the relevant
gas properties is given later in this report. With these measures the PtG efficiency may
reach 90% conversion efficiency. Past studies of combined fuel cell-heat engine systems
show conversion efficiencies of about 80% and in-depth design studies proofed the techni-
cal feasibility [7], [8]. A combination of these concepts leads to a power to power storage
efficiency of more than 70%. The use of fuel cells is a key of the system, because it
includes an intrinsic division of the produced CO2 and the remaining N2 of the used air and
an expensive extra gas separation is not needed. The idea of this study is to support the
coming necessary decisions by delivering the still missing nexus between technology and
economy.
2. Specification of technology and business opportunities
Defining specifications needs thus a clear understanding that the main task of renewable
generators is the mining of electricity that is supplied to a reliable power supply system
with a competitive value generation. However the generators are able to deliver energy as
coal mines did already in the past, but they are not fit to guarantee a stable and reliable
power supply including the storage of a secure amount of thermodynamic potential (like
fuel today) to reliable power supply under any conditions. In other words the mining of
electricity by wind and solar generation is however valuable but only a raw material for the
high end product reliable electric power just on demand, as shown in figure 1
Figure 1 Renewable electricity mining and reliable power delivery, the
two backbones of reliable renewable power supply.
The real value generation is thus along the generation of storable thermodynamic poten -
tial, its storage and supply to power generation units and the distribution of the generated
reliable electric power. The material supply problem and the cost of battery material for the
amount of energy to be stored and the existing gas infrastructure are the reason why pow -
er to gas (PtG) is the mean of choice [2]. The use of PtG technology for generating a
storable thermodynamic potential in form of a gaseous fuel gives the additional option to
use CO2 as a raw material in Closed Carbon Cycles. Hydrocarbons as CH4 or even better
C2H4 have a much higher volumetric energy density compared to H 2. All open cycles,
where hydrogen is needed, can be easily integrated in such system as well.
This concept allows an expansion of the business model of electricity companies from an
only power producer to an circular economy supplier and helps to supply the market with
needed hydrocarbons from a secure and reliable source. Figure 2 illustrates the concept
and the shown system borders also explains major possible business models around the
business core PtG.
Figure 2 Principal flow sheet of renewable electricity and gas supply system and
the relevant balance borders for possible business models
Therefore a clear technical specification has to be defined and linked with possible busi -
ness models. At the first place we should introduce clear definitions of the products of the
market of our system following real economy and value production in terms of technology
than following only a sort of energy bookkeeping. For this reason fluctuating electricity
should be defined as a raw material that shall be processed to the pre-product thermody-
namic potential in form of the product high energy gas and thus in form of permanent avail-
able work. This work potential combined with the associated service grid management for
delivering reliable electric power as the demanded product on the market. Of course
electricity mining also delivers electricity to the grid but not as power on demand and is
thus less useful for grid operation. An additional battery might be useful to buffer short
peaks and to allow to increase the full load hours of the entire system.
The technological system consists of the following subsystems based on figure 2:
renewable electricity generation (solar, wind),
gas generation and storage (electrolyzer, PtG), with an optional associated battery
and delivery to supply gas demand, and
power generation and connected grid.
Additionally H2O and CO2 is needed as raw material. The basis business cases based on
this technological environment are the following:
electricity mining and delivery,
renewable gas supply, and
power supply.
Further business models as renewable gas supply and electricity delivery or full scale pow-
er and gas supply can be formed based on these basic businesses. These both models
can be compared with a today coal mining combined with an industrial CHP plant and de -
livery to the grid and an electricity company operating power plants, grid, and additional
coal mines. The recent energy policy mainly focused on the expansion of renewable elec-
tricity and the economists mainly focus on the electricity market underestimating the impor-
tance of the path from fluctuating and unreliable electricity to a sustainable and reliable
power and gas supply. However the political view is changing and reliability of energy
supply gets an increasing importance and renewable energy has the security benefit of lo-
cal availability. Additionally the processing steps needed for a circular economy can be
easily integrated in such process designs as shown in figure 3.
Figure 3 Overview of business models integrating reliable
renewable power supply and circular economy
The figure summarizes the interaction between the reliable renewable power supply repre-
sented by the horizontal business arrow and the regenerative hydrocarbon supply by the
circular economy is represented by the vertical business arrow. This view shows that the
renewable gas supply is the central element of the entire system because it is the nexus
between the power supply system and the circular economy. The figure also shows that
the operation of the recycling infrastructure is an additional business opportunity compared
to the overview of figure 2. Finally the existing market prices of the gas and reliable elec-
tricity gives a cost target of the system design and its components design. The tool to
identify these targets is the calculation of the allowable investment of the renewable gas
supply, the renewable electricity mining, and the associated power generation.
3. Allowable investment as an early R&D target
A very important issue for any innovation is the prediction of the allowable investment cost
of the product to fit the market demands. However the exact performance data and the ex -
act specification is still inaccurate but it is extremely helpful to get a first impression of the
economic use of the different possible design options. Anyway these results should not be
overestimated but they should be seriously considered within any product oriented R&D.
The here applied methodology was developed for the evaluation of design variations of dif-
ferent fuel cell systems and presented in [9]. The starting point for these considerations is
the determination of the electricity production costs. For plants in operation, all technical
and operational data required here are known. In contrast, the calculations to determine
forecast values, as required here, are based on assumptions and their variation in certain
areas. Again, it is appropriate to use specific values. The core of the entire system is de-
fined by the balance border “Renewable gas supply” in figure 2 thus in principle an electro-
chemical gas generator including optionally the storage and distribution facilities.
The common definitions of efficiency and utilization of evaluating power plants can be
adopted here. The general reference period is one year, which will be used for an easy
matching of technical and financial data. The efficiency of the produced electric work and
the delivered heat of CHP is needed to separate the variable and fixed costs. The annually
produced Gibbs enthalpy RG0g at standard state of the oxidation reaction of the produced
gas (H2, CH4, C2H4) that is equal to the reversible work Wel.rev of this reaction is the refer-
ence value for the thermodynamic value of the product. The by-product is O2. The exerget-
ic efficiency of the electrolyzer/PtG unit El < 1 is the reference value of the devaluation of
the supplied regenerative electric work Wel.reg during the gas generation:
WG.rev El Wel.reg= Δ G0gR
. (1)
The dimension of RG0g, Wel.rev , and Wel.reg in (1) is MJ or MWh. The following calculations
will be based on MWh, because it is the common dimension for electric devices and the
system capacity can be easily calculated. The designed annual full load hours T can be
used to calculate the needed installation capacity for calculating the allowable investment
by using (1). The rated electrical input power follows then as:
Pel=Wel.reg
T
. (2)
The rated gas output power potential can be calculated as:
PG=ΔRG0g
T
. (3)
The thermodynamic potential is here chosen as reference because this is the theoretical
maximum (reversible) electric work that can be produced by the oxidation of the delivered
gas. The rated output is the figure that is proportional to the main income on annual base
as defined. The rated input is proportional to the main expenses. The full load hours T and
the prices for electricity and the gas delivery are complementary key figures. The vertical
business generates further income and expenditures that need a further consideration,
additionally to electricity storage as a part of the horizontal business. Figure 4 shows the
input and the output of the substances creating these figures.
Figure 4 The income and expenditure influence of horizontal and vertical
business on the allowable investment of PtG units
Obviously H2O and CO2 (for hydrocarbons) are reactants and create expenditures in
principle. O2 is obviously an additional product that should create income. But there is an
unclear situation of the role of CO2 in the business model however it is clearly a reactant
needed for the hydrocarbon production. Because if CO2 is legally seen as waste to be
removed, than the gas producer could claim to perform a service of removing waste and
charge a fee to the producer of CO2. The annual produced or consumed mass mj can be
related on mass related RG0g by using the specific mass j defined as:
mjjΔRG0g jPG
T
. (4)
Where j stands alternatively for the operated species H2O, CO2, and O2. The equations (1)
to (4) describe the operational data that are needed for the economic considerations. They
only need to be multiplied with the actual specific prices to be usable for financial calcula -
tions. A small amount of cost of operation and financial obligations can be considered by
correction factors. This methodology for economic evaluations at early project status refers
to the industrial experience in power plant engineering and controlling of the author.
The allowable investment can be easily calculated based on the entire generation cost and
the income from goods and services. Generally costs C consist of variable cost Cv and
fixed cost Cf:
C=Cv+Cf
(5)
The variable costs Cv depend on the electricity price pel preferably of the annually supplied
fluctuating regenerative electric work Wel.reg = Pel. · T and the price pj and the mass mj of the
incoming species j (H2O and CO2).
Cv=( pel Wel.reg +pjmj)x=( pel Pel T+pjmj)x
(6)
with x as correction factor caused by other not yet defined cost (as e,g, repair, operating
materials, auxiliary, etc.) and it is just based on industrial experience in plant projects,
acquisition, operation, and maintenance. The variable cost can only occur if the consid -
ered plant is operating and thus generating turnover.
The fixed cost Cf exist principally as long as legal responsibilities for the considered plant
exist and are independent of any operation and thus independent of T. The fixed cost are
mainly influenced by the acquisition value AV of the plant and can be described by:
Cf=yAV AV 0
. (7)
The correction factor y in the first term reflects the influence of the other fixed cost as e.g.
personnel, insurances, maintenance, taxes etc. It is related on the acquisition cost AV, as
contracted. But usually contracts contain a price escalation clause leading to the higher
present value AV0 of the plant at its commercial start up. The main portion of cost is due to
the financing of the plant usually expressed by the annual payment · AV0, with the annuity
factor following:
Ψ= 0,01rcap
1−(1+0,01rcap )n
. (8)
The capital interest rate is denoted by rcap [%/a] and the depreciation period by n [a]. The
present value AV0 of the plant is the product of the acquisition value AV and the present
value factor and is obtained from the NPV factor :
ξ=(1+0,01rct +0,01 ii)0,5nct
, (9)
considering the price escalation during the construction time nct by using the relevant
interest rate rct and the relevant index to inflation ii . Using (9) the present value AV0 can be
calculated with the acquisition value AV as:
AV 0AV
. (10)
The annual generation cost can be easily calculated as by using (1) to (10):
C=( pel PelT+pjμ jPG
T) ⋅x+( y+Ψξ)AV
. (11)
The cost C must be covered by the income generated by the produced gas. Thus we can
write for the maximal allowable cost Cmax with (3):
Cmax=ΔRG0 gpG=PGpGT
. (12)
By using the specific acquisition value av
av=AV
PG.out
(13)
we finally get for the allowable (maximal) specific investment:
avmac=[ pG−( pel
ζEl
+pjμ j)⋅x]T
y+Ψξ
. (14)
All needed technical information to solve (14) except the exact calculation of the specific
mass j have been presented above. This is a consequence of the different possible
reactants and the different reaction paths. Additionally to the well known H2 production also
CH4 and C2H4 are considered as possible PtG products. The different reaction processes
as presented in table 1 from [5] lead to different demands on H2O caused by the O2
removal from the methanation or C2H4 forming reactor, represented by the molar ratio of
H2O and the produced CH4 or C2H4 respectively.
Table 1 Overview of identified reversible reaction for CH4
and C2H4 forming, original table from [5]
Methanation reactions equ. O2
removal Ethen forming reactions equ. O2
removal
6a 2 H2O
6H2+2CO2=C2H4+4H2O
6b 4 H2O
7a H2O
4H2+2CO=C2H4+2H2O
7b 2 H2O
8a 1/2 O2
2H2+2CO=C2H4+O2
8b O2
9a O2
2H2+2CO2=C2H4+2O2
9b 2 O2
10a 3/2 O2
2H2O+2CO=C2H4+2O2
10b 2 O2
Table 1 shows for CH4 and C2H4 two similar thermal (6a to 7b) reactions and three possible
electrochemical reactions (8a to 10b) for each species. While the demand on CO 2 and the
O2 production only depends on the produced species, the demand on H 2O depends on the
specific type of reaction following the above reactions and the well known H2O electrolysis.
Table 2 shows the specific mass j for the different gas and process variations of table 1.
4H2+CO2=CH 4+2H2O
2H2O+CO=CH 4+3
2O2
3H2+CO=CH 4+H2O
2H2+CO=CH 4+1
2O2
2H2+CO2=CH 4+O2
Table 2 Specific mass j for the different gas and process variations of table 1
μj [g/kJ] H2 CH4 C2H4
H2O 0,0760
0,0441 0,0271
0,0661 0,0541
0,0881 0,0812
CO20,0000 0,0538 0,0661
O20,0675 0,0782 0,0721
The principle tables 1 and 2 have to be specified for describing the reaction path and to be
transformed in a compatible dimension for using in (14) as shown in table 3 following (4).
Table 3 Specific H2O demand depending on reaction path via electrochemical
reaction only or combined with thermal reaction with CO or CO2
μH2O
kg/kWh H2 CH4 C2H4
Electrochemical 0,2735 0,1586 0,0974
Reactant CO 0,2379 0,1949
Reactant CO20,3172 0,2923
The specific H2O demand is a key cost factor for the performance of current PtG technolo-
gies because the necessary evaporation heat has to be supplied by electric power that is
only dissipated and increases the loss. Table 4 gives an overview of the order of magni -
tude of these losses. The electricity supply must be as high as HHV. However the loss may
be reduced by a supply of waste heat as evaporation heat.
Table 4 Evaporation loss (HHV related) in %
Evaporation
loss in [%] H2 CH4 C2H4
Electrochemical 15,3943 9,8840 6,2374
Reactant CO 14,8260 12,4748
Reactant CO2 19,7680 18,7122
Figure 5 visualizes the content of table 3 and 4. It clearly shows that the use of electro-
chemical processes of simultaneously removing O2 from the final reaction step clearly
increases the potential to increase the process efficiency by producing CH4 and C2H4
because the demand on H2 decreases to the absolute minimum.
Figure 5 Influence of species and reaction path on the specific H2O
demand and the process efficiency potential [5].
The species H2O, CO2, and O2 are either needed as pre-product or can be charged for the
delivery as a product or as a service. Obviously H2O is always a pre-product that gener-
ates cost and its specific price in (14) is necessarily positive, O2 is always a product to be
sold and its price in (14) is thus negative. An interesting aspect is the discussion about the
socio-economic impact of CO2. The question to be discussed is the role of CO2 in a circu-
lar economy, if it is raw material or if it is waste to be removed, because the answer on this
question influences clearly the gas price. If it is raw material than a price must be paid and
the price in (14) is positive, but if it is waste, a removal service can be charged and its
value in (14) is thus negative in this case. Consequently this impact must be investigated
separately within the needed design of a circular economy. Thus in this investigation the
CO2 price is set to zero. A review of real market prizes for different stakeholders is difficult
because it is private information. Thus the figures in table 5 are based on private informa -
tion, different sources, and some estimations in the given range.
Table 5 Range of specific H2O and O2 cost depending on process
The gas price range between 0,01 – 0,10 €/kWh is based on the historic development and
an estimation on probable future development in a similar way the electricity price between
0,04 – 0,1 €/kWh. The water price between 0.003 – 0,005 €/kg is currently neglectable and
the O2 price found between 0,09 – 0,30 €/kg is really depending on the different markets
[10], [11], [12], [13], [14]. The in the following analysis used thermodynamic data can be
found in [15].
Min MAX
Species €/kWh
Electrochem. 0,00082 0,00048 0,00029 0,00137 0,00079 0,00049
Reactant CO 0,00071 0,00058 0,00119 0,00097
0,00095 0,00088 0,00159 0,00146
0,02186 0,02535 0,02337 0,07287 0,08451 0,07789
H2 CH4 C2H4H2 CH4 C2H4
H2O
Reactant CO2
O2
4. Analysis of the influences on gas production cost and allowable investment
The first analysis of (14) shows that it is a product of two independent terms, where the
first one describes the cost influence of the market of the operation requirements and the
system performance and the the second one describes the influence of financing mainly
including the contribution of construction time additionally. The first term:
[pG−( pel
ζEl
+pjμ j)⋅x]T
only depends on the planned generation cost pG, the electricity cost pel, the exergetic
process efficiency El , the sum of the species cost j· pj, and the full load operation time T.
The second term:
1
yξ
considers the influence of the fixed cost mainly caused by the financing of the plants
during the depreciation period n. Then the other fixed cost are comparable small. Figure 6
show the influence of this term on the maximal allowable investment cost. The relevant
interest rate rct and the relevant inflation index ii used for calculating the NPV factor have
been unified and related by a factor 1,3 to the capital interest rate rcap for simplifying these
calculations but including construction time nct with 10% of the depreciation time n .
Figure 6 Influence of the fixed cost mainly caused by the project financing
The allowable investment cost that limits the acquisition value av is higher if the
depreciation time is short, because the project has a total lower burden of interest
payments. The figure shows clearly that the other fixed cost can be neglected for a
conservative consideration when the entire fixed cost term can be assumed as 1 in the
further consideration.
The use of the data of table 5 are applied on the above described processes in (14) for a
first view to identify possible orders of magnitude and the results are plotted in figure 7 and
the basic data used are shown in detail in table 6.
Figure 7 Allowable investment av of the different PtG technologies as a
function of the exergetic process efficiency El based on the cost
data of table 6 for an annual full load operation time of 1000 h
Table 6 Input data for the calculation results presented in figure 7
The allowable investment costs av as a function of the exergetic efficiency El differ not
very much between the possible different PtG technologies (electrochemical for generation
for H2, CH4, and C2H4, and the thermal generation of CH4 and C2H4 from H2, CO, or CO2),
as investigated here. However a value of 1000 h for the full load operation time T is a pos-
sible value for e.g. a regenerative supply scenario with PV in northern regions, but it is too
low to finance a PtG generator even in the high price scenario (Max) of table 6. Unfortu-
nately even an increase of the operation hours by the factor 3 - delivering a real high value
of full load operation time for renewable generators would not meet the current target
price of about 400 – 500 €/kW in this scenario [16] somewhat lower is this value in [17].
On the other side the low price scenario (Min) delivers a negative value of the allowable
investment costs av. An increase of the full load operation hours T would only increase the
loss of the project due to the extremely uneconomical investment.
Another aspect that became evident was the importance of the ratio of the gas price pG
and the electricity price pel in the investigated scenarios. The price scenario (Max) had a
comparable high gas price and an even high electricity price and in the price scenario
(Min) the very low gas price had a higher electricity price, as in the current markets. The
further evaluation needs an improved investigation methodology to better identify the influ -
ence of the different parameters in (14) on the economy of the investment as a base of a
calculation scheme for the strategic planning of the financing of the needed investment of
PtG technology. The necessary considerations can be explained by using Figure 8.
Figure 8 Analyzing of the impact parameters on the gas price pG
and the investment of the acquisition value av
The general equation to calculate allowable investment cost based on the acquisition
value in the beginning of industrial investment projects is presented in (14). The equation
is in principal a linear combination of an engineering and an financing term. It has shown
above in figure 6 that the financing term is in the first order independent of any perfor-
mance data and need not to be considered in this early project status consideration about
principal market and engineering issues. However a later risk assessment of technology
and markets influence financing as well. This statement can be expressed as:
1
yξ 1
. (15)
This simplification allows to rearrange (14) to calculate directly the deficit or profit of the
targeted gas price pG in combination with the targeted acquisition value av by the correc-
tion price term pcorr by:
pcorr=[ pG−( pel.in
ζEl
+pjμ j)⋅x] −avmax /T
. (16)
This gap indicated by the correction price term must be closed by measures that have to
be developed by analyzing all performance influences and optional business opportunities,
Figure 8 gives already some indications.
The gas and the electricity price are in an order of magnitude of 10-2 to 10-1 €/kWh and are
still market driven especially for industrial applications with worldwide competition.
However a higher gas price could be tolerated for only electricity generation for peak shav-
ing, but the political target of an increasing energy independence within the EU needs a
deeper investigation of the influences in (16). Its linear cost terms are the gas price pG, the
water price pH2O, the oxygen price pO2, and the carbon-dioxide price pCO2. The figures of
pH2O, pO2, and pCO2 are also influenced by the options that can be expected by the introduc-
tion of a circular economy. This general point will be a part of the discussion of the results.
Beside the linear cost terms in (16) the influence of the electricity price pel depends on the
reciprocal function of the exergetic system efficiency El and the targeted acquisition value
av on the reciprocal function of the full load operation hours T. Both depend on the technol-
ogy and performance of the considered system. The use of (16) immediately delivers the
electricity price share on the gas price pG by a given electricity price pel and the exergetic
system efficiency El of the gas conversion. The results are shown in Figure 9.
Figure 9 Gas price share caused by the electrochemical conversion as a function of
the electricity price pel and the exergetic efficiency El of the gas conversion
The range of the exergetic efficiency El is varied between 0,5 and 1,0 (the border case of
the reversible system) and the electricity price between 0,02 and 0,07 €/kWh to cover
different options of electricity production. The resulting gas price share doubles between
the falling exergetic efficiency between 1,0 and 0,5 depending on the input electricity price
and the slope of this increase increases with decreasing exergetic efficiency with a maxi-
mum between 0,04 and 0,14 €/kWh.
The gas price share caused by the ratio of the acquisition value av and the full load opera-
tion hours T is the second term sensitive on the entire system performance data with a
very strong impact on the project feasibility. Figure 10 shows the gas price share depend-
ing on acquisition value and full load operation hours. The left side gives the general view
and the detail drawing on the right side shows the limitations of system design and perfor-
mance to keep the cost under control.
Figure 10 Influence of full load operation time and
acquisition value on the gas price share
The considered full load operation time covers all electricity generating technologies that
are acknowledged in European Union as CO2 free green technologies. The considered
acquisition values cover a wide area from 100 to 700 €/kW around the most probable cur-
rent acquisition value av of about 400 €/kW. While an acquisition value av of 400 €/kW and
a full load operation time of 8000 h lead to a gas price share of 0,05 €/kWh, an operation
time of 1000 h leads to 0,40 €/kWh. Acquisition cost of only 100 €/kW would lead to gas
price share of 0,0125 €/kWh if 8000 h would be reached but to 0,10 €/kWh if only 1000 h
could be reached. Because the full load operation time of the PtG system is connected to
the delivery of electricity a view on the available technologies regarding their full load hour
operation is needed and is shown in Figure 11.
Figure 11 Full load operation time and allowable PtG investment of
the acquisition value for different gas price shares [18]
The figures refer to German operation conditions however they indicate specific operation
characteristics of the listed technologies [18]. Solar electricity generation is available at
1000 full load hours per annum and nuclear at 7500 h. Considering a gas price share of
0,05 €/kWh none of these technology could finance an acquisition value of 400 €/kW, how-
ever a nuclear power plant with a possible value of 375 €/kW would be very close. A 500
€/kW target is too far away except for nuclear plants and a gas price share of 0,067
€/kWh. On the other side solar electricity could only finance 100 €/kWh with a gas price
share of 0,10 €/kWh. Even electricity supply of off-shore wind plants with a gas price share
of 0,10 €/kWh cannot meet the lower target of 400 €/kW as acquisition value. Biomass
could just meet the target, but it is not really relevant here because it is mostly biogas and
the EU decision to acknowledge also nuclear energy as green allows its future system in -
tegration. But it may be that some specific bioenergy developments are performed to try
this option as well for some specific applications in Germany or Austria mainly for political
reasons. Covering the cost by peak load electricity prices might be possible, but it seems
not economically if other solutions are possible within EU. Especially for more energy inde-
pendence there must be a reliable option to use renewable gas soon at realistic prices
because the investments in new installations need time, material, and money.
The different parameters in (16) and their combinations visualize the clear limits to reach
defined targets and the different options what modifications might be possible to proceed
anyway in the main issue. The target specifications are defined in table 7.
Table 7 Data set for analysis of gas generation cost influences
The key variables influencing the gas generation cost are the full load operation time T, the
acquisition value av, the exergetic process efficiency El, and the electricity price pel. The
figures 11 and 12 gives an overview.
Figure 12 Influence of acquisition value, full load operation time,
and electricity price on gas generation cost
The gas generation cost pG is a linear function of the acquisition value av with decreasing
slope with increasing full load operation time T. These changes and the change of the
maximal range of the gas generation cost pG are shown in table 8.
Table 8 Gas generation cost, acquisition value and full load operation time
Full load operation time TMaximal range of pG Slope pG/
av
2000 h 0,1760 – 0,3260 €/kWh 0,0500%
4000 h 0,0760 – 0,1760 €/kWh 0,0250%
6000 h 0,0427 – 0,1260 €/kWh 0,0167%
The impact of the exergetic process efficiency is considered similarly in Figure 13.
Figure 13 Influence of exergetic process efficiency, full load operation
time, and electricity price on gas generation cost
The gas generation cost are here limited to 0,20 €/kWh with the consequence that the Full
load operation hours must be > 2000 h. The targeted gas generation cost < 0,05 €/kWh
can be reached in all considered cases of the here considered exergetic process efficiency
El for an electricity price of 0,02 €/kWh. This seems to be contradiction to the statements
of Figure 11. But it is not because here the gas generation cost pG in total have been
considered and not only the share due to the investment of of the acquisition value av.
Table 7 shows that the purchase of the produced O2 has opened an additional business
opportunity that delivered an income of 0,05 €/kWh. Therefor the entire concept has now
to be considered regarding chances and risks of an expanded business concept including
products and services in a circular economy.
For the further discussion of the different aspects it should be remembered that only the
reversible reaction work of the oxidation reaction RG0g (1) - representing the thermody-
namic value of the generated substances - was considered as the only equivalent cost
reference, because electricity storage and thus the storage of electric work will become a
main application of these technology. Natural gas is a main fuel in today condensation boil-
er where a part of the condensation heat can be used, therefore higher heating value HHV
representing the maximal possible transferable heat is mainly used as a reference value in
this specific application and the heat market. Table 9 shows the comparison of the
reversible reaction work RG0g and of the higher heating value HHV and their ratio to avoid
misunderstandings.
Table 9 The comparison of the reversible reaction work RG0g
and of the higher heating value HHV and their ratio
The table clearly shows that HHV related values show a clearer lower price for the same
quantity of substance as a RG0g related figure. This means that a here presented (RG0g
related) price of 0,050 €/kWh would be listed as only 0,041 €/kWh if related on HHV for H2
as an example. The introduction of PtG needs thus a technology targeted rewriting of the
here relevant standards.
5. Identified Challenges and possible Improvements
The main aspect of the investigation so far was oriented on technology itself and its perfor-
mance, but the market of reactants and products influencing cost and income have not
considered full scale. Anyway the calculations showed the positive impact of possible
income from oxygen by the presented examples. The H2O demand is a key information for
the current processes because they have a clear influence on the process efficiency, as
fig. 5 and the tables 3 to 5 show.
The current influence on cost is marginal but water is becoming more and more a possible
critical resource for many countries. The ideas of a number political plans to consider the
import of H2 or CH4 and C2H4 respectively as an energy carrier just from these areas short
on water makes it necessary to review this aspect as well. An open aspects to be dis-
cussed is the value of CO2 in the process as addressed already above. Finally the role of
O2 in future markets and its impact on sustainability aspects need to be analyzed. Table 10
collects this aspects and the topics to be discussed.
Table 10 Cost influence of reactants and products on gas generation cost
Reactants Product
H2O CO2O2
Cost impact Small compared to
gas generation cost
Open:
raw material → cost
removal → income
no cost but income
see table 5
Procural/market Availability of water in
general may increase
cost, or need of water
recycling
Income option:
from gas customer
from plastic user
or payment
Different markets
new solutions
Sustainability
impact
Minimal water
consumption needed
Carbon Closed Cycles
technically and envi-
ronmentally needed
Expected process
improvements
The nexus energy-human life-food production is the important reason to carefully discuss
the aspect of the option of water withdrawal versus water recycling. The water demand for
electricity storage depends on the following process steps:
Annual gas production needed to cover the outages of renewable electricity supply,
Annual gas production needed to the gas demand for other applications as industry,
heating or transportation,
Used gas type (H2, CH4, C2H4) and gas generation technology, and
Used power generation technologies for grid supply.
Table 11 shows the needed water supply for the gas generation with only electrochemical
processes and combined electrochemical and thermal processes with the use of CO and
the electricity production of 100 TWh with different power generation efficiencies. Actually
the generation efficiency el.Gen of CCGT (Combined Cycle with Gas Turbine) yields
between 60 and 65 % [16] and SOFC-GT (Solid Oxide Fuel Cell with Gas Turbine) yields
between 70 and 80 % as shown in [7], [8].
Table 11 Water supply for the different gas generation pro-
cesses for a 100 TWh electricity production
H2O demand [Mt] Electrochemical Processes only Mixed Processes with CO
el.Gen H2 CH4 C2H4H2 CH4 C2H4
0,6 45,58 26,43 16,24 45,58 39,65 32,48
0,7 39,07 22,65 13,92 39,07 33,98 27,84
0,8 34,19 19,82 12,18 34,19 29,73 24,36
The maximum demand with 45,58 Mt exists for the H2O electrolysis and the lowest with
12,18 for full electrochemical C2H4 production (to be developed). The mixed process of
H2O and CO2 electrolysis with a thermal reactor to generate C2H4 needs 24,36 Mt. Actual
possible process steps combine electrolysis (H2O and CO2) with a thermal reactor. Figure
14 shows visualize these results.
Figure 14 Water supply for the different gas generation pro-
cesses for a 100 TWh electricity production
The worldwide electricity supply and the total worldwide energy demand can be used to
estimate the order of magnitude of the expected water demand of a future operating PtG-
generation system. The relevant data are collected in table 12 [19].
Table 12 Water demand of worldwide electricity supply and total energy demand [19]
2019 TWh Max [Gt] Min [Gt]
Electricity 27.044,2 12,327 3,294
Total energy 168.469,3 76,793 20,52
The minimum (Min) and maximum (Max) values are based on the results presented in
table 11 under the assumption of a reuse of the produced gas for electricity production.
Assuming that about 30% of the delivered power must be generated by using regenerative
combustible gas the annual water demand could be expected between about 1 to 4 Gt
following the calculation of table 12. Assuming as a second scenario that all the worldwide
energy demand is delivered as regenerative gas only then the water demand of table 12
would decrease to values between 16 to 46 Gt. These values have to be compared with
actual figures of water demand. Table 13 an excerpt from table 7.1 of the last report “The
United Nations World Water Development Report 2020 Water and Climate Change” [20]
shows some recent figures and an outlook for 2050 for the industrial demand.
Table 13 Industrial water demand and its development to 2050 excerpt [20]
Water [Gt] 2010 2050
Africa 18 64
Europe 241 325
World 838 1381
The lower value in table 12 always stand for C2H4 and the higher for H2. The use of C2H4
cannot be expected to have a too big impact on the water supply following table 13 even if
Africa is delivering a high portion of the gas, if considering the power generation only. A
higher impact may be possible if a higher portion of the produced hydrogen with the higher
value in table 12 is produced in Africa. Even if only the entire transportation sector must be
supplied with H2, the amount of water needed is in the order of magnitude of 10 Gt (about
25 % of total). This would be a remarkable figure for Africa even in 2050. This short discus-
sion already shows that water supply is an issue that need to be considered seriously in
any future concept and capacity planning.
The supply of CO2 and considerations of the cost or income connected with this reactant is
a principal political issue that needs to be discussed deeply with the stakeholders. Anyway
the CO2 emission certificate market may give a first impression of a market that could influ -
ence the development of CO2 prices on a future raw material or recycling market. However
internationally the prices are lower, but for simplicity of discussion and with respect of
future expectation, a CO2 price of 100 €/t may be a good choice.
Table 14 Impact of the CO2 price of 100 €/t
on the fuel price in €/kWh
CH4 C2H4
kg/kWh 0,1937 0,2381
€/kWh 0,0194 0,0238
The work related price of about 0,02 €/kWh is a strong impact on the generation cost of
the combustible gas. The price difference resulting from this decision means a cost span of
about 0,04 €/kWh (between income and payment), what is in the order of magnitude of the
gas price itself. A possible solution may be to add this CO2 cost on the gas price in general
and refund the same price for CO2 as delivered. However this may have a positive effect
on keeping the necessary CO2 cycle running and uses thus the procedure as a necessary
measure to ensuring the hydrocarbon supply for the national economy, but it is anyway an
additional technical and administrative effort. Therefore before binding decisions are
made, test runs inside industries seem to be necessary to avoid administrative overkill.
The product gas O2 is a realistic chance to generate extra income for reducing the gas
generation cost. Currently the market is a result of the different air separation processes
that also deliver other technical used gases as nitrogen and noble gases. The current
oxygen producers have thus different gas markets to optimize their business model. A new
supplier must thus develop proactive a technology package for an increase of the oxygen
market by improving existing processes in a way that generates extra money to pay the
the package including the permanent O2 delivery.
The import over larger distances overseas needs gas liquefaction if a pipeline transport is
impossible. Table 15 shows some key figures for understanding some influences on cost
of the sea transport of liquefied H2 , CH4, and C2H4.
Table 15 Key figures for gas liquefaction and liquid gas transport
H2 CH4 C2H4
Tv [°C] -252,00 -161,70 -103,80
hv [kJ/mol] 0,90 8,17 13,90
T0 [K] 20,00 111,00 169,00
max 0,997 0,994 0,996
wid [MWh/m³] 2,321 5,958 7,455
[kg/m³] 71,282 423,343 568,206
mw [kg/MWh] 30,715 71,058 76,22
However the management and transport of liquefied gases is a specific technology with a
high demand of qualification and experience but some indication of the problems to be
solved and its consequences on the investment can be addressed by the data collected in
table 15.
H2 has the mass related the highest work potential its high demand on storage volume
does not change in its liquid phase. CH4 and C2H4 with 5,958 MWh/m³ and 7,455 MWh/m³
respectively need clearly less space onboard ship compared with 2,321 MWh/m³ of H 2.
Consequently the surface to be insulated is clearly higher for hydrogen than for the hydro -
carbons. The very low evaporation temperature leads to a temperature difference to the
vessel outside of about 277 K for H2 compared to 187 K and 129 K for CH4, and C2H4
respectively. Consequently the needed insulation thickness increases with the temperature
difference and/or higher demands on the insulation material must be fulfilled both leading
to a clear increase of cost. However the maximal possible efficiency max calculated with
the Carnot efficiency and related on the reversible reaction work is quite similar for all
gases caused by the different evaporation enthalpy hv, but the higher temperature differ-
ence of the liquefaction process most likely leads to a higher effort of the design connected
with higher cost for insulation. However CH4 and C2H4 have a clearly higher specific weight
than H2 but their specific weight is also clearly lower than that of seawater. Concluding
these results, C2H4 has here the best performance data for an efficient and cost effective
transportation closely followed by CH4.
Beside the important influence of the storage substance (H2, CH4, C2H4) on the gas
generation cost also other influences have been identified in the analysis above. They are
summarized and collected in Figure 15 including their main influence parameters and their
interaction.
Figure 15 Cost influences and improvement and optimization measures
The general aspects influencing the economic success are performance, electricity cost,
investment, and oxygen price. The discussion of the CO2 value above, showed that this is
a political question clearly influenced by circular and energy economics and is thus only a
boundary condition here, listed as a part of the contributions of the sustainable integration
of PtG systems.
The performance of any industrial plant is defined by its economic utilization expressed
e.g. by its annual full load hours of operation. The electricity cost is an external figure but
the system efficiency is the key figure to show how efficient the money invested in the
electricity purchase is utilized for the gas production needed for the income. The invest -
ment is clearly dependent on the technological steps as process engineering, electro -
chemistry, manufacturing but also on quality issues as longevity. These points obviously
interfere and are key parameters for the technological product quality. The cost analysis
identified the importance of the joint product oxygen by addressing the influence of the
oxygen price on the final gas price. It is obvious that thus the oxygen marketing is an
important issue for the entire process economy this includes possible new applications in
different processes. The connection to electrochemistry is important because chemical
industry will change to more electrochemistry because electricity will replace heat more
and more as a main energy source and thus open new market opportunities. From this
point of view the current approach seeing PtG only as a new energy source might be to
narrow to guarantee a reliable base load for renewable gas production.
These technical quality aspects of the PtG system cover in general all technical questions
but not the principle problem of all renewable energies that they are not reliable on
demand and thus a larger energy storage is needed. As shown above the gas generation
cost are very sensitive to full load operation hours and this leads to a clear performance
problem. Because renewable energy is fluctuating it is also difficult to optimize the size of
the PtG unit caused by competing targets, on the one side high full load hours at rated
power are needed for economic reasons of the PtG unit and on the other side the renew -
able energy should be harvested at any load as much as possible to achieve also high as
possible full load operation hours for the regenerative generator. As already shown in
Figure 11 only nuclear power generation with its high amount of 7500 full load hours allows
almost an acquisition value of 400 €/kW with only 0,05 €/kWh as a gas price share for the
investment. Consequently the EU decision to label nuclear energy as green energy helps
a lot to reduce the high cost of building a reliable and sufficient energy storage concept. In
the first phase of changing the entire system to a renewable storage based one, it is
economically useful to operate the gas generation with nuclear power to increase the
operation time of the gas generation and to keep the investment cost low. The renewable
supply of electricity by solar and wind energy could be maximized because nuclear energy
supplies the backup power by a CO2 emission free PtG system.
However solar energy has locally a low full load operation time but it provides a more
stable and reliable electricity supply than wind energy and could be possibly used as a
defined base load for PtG in a later step if belts over continents e.g. in North Africa could
be used. However details have to be investigated regarding feasibility but it might be a
possibility to combine the electricity production of solar plants e.g. in the east of the
Sahara and in its west to increase the operation time. Even combination of connecting
floating solar platforms in the sea might be such an option. But these are still visions to be
developed and keeping a connected global solar system as a future option in mind for
increasing full load hours, but the today figures and the current state of the art clearly
favors nuclear energy for operating PtG. Figure 16 illustrates the possible roadmap to
implementing more solar energy on the long term.
Figure 16 Road map from nuclear to possibly solar energy to operate PtG
The example of using the income by selling oxygen to reduce the gas production cost
shows the need to expand the business opportunities to reduce the production cost by
implementing joint products and enforce interdisciplinary thinking in new business con-
cepts. An other possibility that should be mentioned for reducing the cost of PV is the idea
of combining agriculture with solar energy, upcoming activities were already reported in
[21], [22], [23], [24]. An other area of business that might be water recovery. It might be an
additional business in a respective infrastructure project where the availability of oxygen
might be helpful. The general conclusion is, that the system integration using new
opportunities of technical developments could become an economic driving force that
allows a combination of economic growth and improvement of sustainability.
Combining all these considerations a cost formula for PtG can be given as a controlling
instrument for improving the cost of renewable gas by developing joint products and joint
services:
pdiff =pG.marketpGμjppjpμjs pjs
. (17)
The indicator is the price difference pdiff between the natural gas price pG.market on the market
and the gas generation price pG and the sums of the associated incomes from joint prod-
ucts and joint services, with their prices pjp and pjs related with jp and jp to the associated
generated gas quantity. This approach seems necessary because the EU “Green Deal” is
mainly oriented on CO2 emission and not on other important points of sustainability also
regarding resources as the current crisis on supply and supply chains shows.
The different actual situation of war, climate change and corona need solutions but also
high investments. Therefor the integration option of nuclear energy in EU gives a chance
to minimize cost and risks of introducing PtG and developing more experience in the
operation of renewable grids without high costly blackout risks. Figure 17 gives an over-
view how such a system could be designed. It is obvious that the principal system design
of figure 2 is quite similar to this alternative to use nuclear energy as presented here.
Figure 17 Nuclear base load as main PtG energy source
The planning for the EU “Green Deal” as the planning for the German “Energiewende” is
very ambitious and costly. As the different reports of the German “Bundesrechnungshof”
(Federal Accounting Office) [25] show is the project management extremely difficult and a
big difference between expectations and results however the energy prices in Germany
are extremely high. Therefore it is absolutely necessary to build up a reliable regenerative
PtG supply system that can be financed by the gas price at reasonable cost. The results
presented e.g. in Figure 11 above show that high full load hours are needed to finance the
PtG system at high but reasonable gas prices beside a high conversion efficiency. Nuclear
energy is here by far the most economic power supply system with high operation hours.
Under the current situation a nuclear based PtG supply could help to build up a
regenerative PtG system with the following benefits:
Available plants can be used step by step for supplying regenerative PtG systems
with economical and reliable power at high full load hours,
Consequently the investment can be financed with comparable low cost shares for
the gas price,
The possible high utilization time of the PtG-systems reduces the demand on
installed capacity that reduces the total investment,
The system structure can easily transformed in a full renewable power supply sys -
tem in a following infrastructure development step,
The reliable nuclear based technology can be at least partly used to increase the
energy independence of Europe on the gas sector, because existing industrial tech-
nologies operating with natural gas need to be supplied with regenerative gas until
they can be replaced by full or more electric-technologies,
The electric infrastructure for high capacities is available
The cost saving by this option can be used to accelerate other projects needed for a
sustainable future of the next generation.
The choice of nuclear power as the power source for producing PtG is one aspect, the
other one is the choice of the used gas for energy storage. Physical and logistics proper-
ties relevant for energy storage gases and their evaluation for H 2, CH4, C2H4 are collected
in table 16. The data have already discussed in different paragraphs above.
Table 16 Physical and logistics properties relevant for energy
storage gases and their evaluation for H2, CH4, C2H4
Properties H2 CH4 C2H4
Energy density (kWh/m³) - + ++
Stand alone energy loss by evaporation (current) - +/- +
Stand alone energy loss by evaporation (future) - + ++
Non recoverable water demand - + ++
Liquefaction temperature - + ++
Transport volume liquid (m³/kWh) - + ++
Contribution to CO2 recovery - + ++
CO2 free use in open cycles ++ - -
Total 5 - 5 + 12 +
The energy density is the important figure for the amount of energy to be stored and trans -
ported in a certain volume (cavern for storage, pipe for transport), C2H4 is clearly the most
appropriate substance. H2O is usually delivered in liquid form and the supply with evapora-
tion heat is needed for producing a gas. For stand alone units the heat is generated by
electric heating that increases the electricity demand, its low amount of H2 causes the low
heating demand for C2H4 and minimizes the loss. A good system integration might allow
the use of waste heat. The H2 content of the storage gas is closely linked to the water
demand of the storage gas and again the C2H4 generation leads to the lowest water
consumption. The transport overseas need a liquefaction of the produced storage gas.
The liquefaction temperature of C2H4 is the highest leading to lower insulation cost and/or
lower evaporation losses during transport. The lowest liquid volume has C 2H4 leading to
smaller transport vessels and a lower consumption of insulation material and thus lower
construction cost. The highest C content has C2H4 and is thus the most appropriate sub-
stance for a high C recovery rate. C recovery is not appropriate in certain cases as e.g. in
vehicles or other smaller consumers where a C/CO2 recovery system is not possible or too
expensive, this is the typical market for H2. Anyway the benefit of CH4 and C2H4 is the pos-
sibility to produce always H2 by a reaction of both gases with H2O and a combined CO2
recovery. This shows that the standardized large scale gas storage with C2H4 could cover
the demand on C recovery most effectively or alternatively is CH4 an additional option.
Where open cycles have to be used the CO2 emission free utilization of H2 is the right
choice such a combined system offers.
The evaporation loss discussed above has of course an influence on possible efficiencies
and related cost of the production of the different gases mentioned above. Table 17 and
Figure 18 show the addition on the electricity price pel caused by gas generation for differ-
ent exergetic efficiencies and electricity prices pel additional to figure 9.
Table 17 Addition on the electricity price pel caused by different exergetic
efficiencies of gas generation and electricity prices pel
Losses H2 high H2
medium
H2/C2H4
reduced
C2H4
low
Exergetic efficiency el
Electricity price pel [€/kWh]
0 0,7 0,8 0,9
0,02 0,013 0,009 0,005 0,002
0,04 0,027 0,017 0,010 0,004
0,06 0,040 0,026 0,015 0,007
0,08 0,053 0,034 0,020 0,009
0,10 0,067 0,043 0,025 0,011
Figure 18 Addition on the electricity price pel caused by different exergetic
efficiencies of gas generation a and electricity prices pel
Table 17 and Figure 18 clearly explain the additional cost that are caused by differences of
the exergetic efficiency of the gas conversion that can also be applied on the choice of the
storage substance. This paper is not focused on the design of PtG systems thus only a
small example is here used to show the cost relevance of the exergetic efficiency and its
connection to the choice of the storage gas. Table 4 shows the evaporation losses of
generating H2 and C2H4 by PtG with 15,4 % and 6,2 %. Assuming that other losses occur,
a comparison of exergetic PtG efficiencies of 0,8 (H2 ) and 0,9 (C2H4) gives a first picture.
The generation cost show differences between 0,003 and 0,014 €/kWh for electricity prices
between 0,02 and 0,1 €/kWh related to the different possible exergetic efficiency 0,8 and
0,9.
6. Conclusion
The results clearly show that a precisely defined specification of object, task, and its boun-
daries is mandatory for any investigation. A mainly national economics oriented approach
cannot deliver the needed results for the project realization in large programs as energy
transition because the competence of engineering and business economy is needed. The
understanding of this fundamental point clarifies also the deeper reasons, that explain the
cause of the permanent critical announcements of Bundesrechnungshof [25] mainly
neglected by politics over the recent years. Hopefully this paper will help to clarify these
misunderstandings avoiding probably serious consequences.
The mathematical description of the economics of projects at early status shows that the
engineering and the economic terms are linearly dependent and are thus vice versa only
scaling factors in this first order investigation. The investigation of the engineering term
dominant in this investigation – showed clearly the main cost influences: as the reciprocal
dependence of the electricity price term on the efficiency and of the acquisition value on
the annual full load operation hours, the water cost are here currently neglectable, while
the value of the produced oxygen reduces the gas generation cost, finally the carbon diox-
ide value is unclear depending on socioeconomic, if it should be seen as costly raw materi-
al to be paid or as waste to be removed against fees.
Therefore any PtG project should be simultaneously seen as a horizontal energy business
and a vertical sustainable production business for joint products and services. In other
words the renewable generators are mining electricity that is used in connected PtG sys-
tems to produce simultaneously renewable combustible gas, associated electric power,
and joint products and services, as O2 marketing and CO2 recovery. The economic suc-
cess of any of these industrial plant depends on its utilization or in other words on its annu-
al full load operation hours, and here only nuclear energy is currently economic regarding
full load hours.
The demand on energy independence and the ambitious targets need a production of re-
newable CO2 emission free combustible gas at realistic cost in a near future. The financing
only by peek load electricity prices alone cannot afford this. Consequently the use of nu -
clear energy as approved by EU is the solution currently needed, to get a chance to reach
the targets without destroying economy and society. Anyway on a longer term a stable and
reliable electricity supply by a “solar belt” connected over more longitude could also offer a
sufficient and stable number of full load hours for financing the gas price.
The next aspect to be considered is the storage gas itself. The investigations in [2], [26]
showed that the real storage demand for global renewable electricity supply is far too high
to cover it with Lithium. The right choice for the best storage substance for PtG between
the candidates: H2, CH4, and C2H4 depends on their following properties: energy density,
stand alone energy loss by evaporation, volumetric energy density, non recoverable water
demand, liquefaction temperature, liquid transport volume, contribution to CO2 recovery
(industrial renewable hydrocarbons), CO2 emission free use in open cycles. Except the last
property C2H4 is by far the best solution but needing closed cycles, but only H2 could fulfill
the last one. Because the both last properties were conflicting, H2, and C2H4 should be
standardized for these applications where their properties promise the best performance.
The discussion of a “solar belt” leads automatically to the aspect of water supply and the
demand of installed PtG systems. The ideas of solar farming give an option to improve the
environment in solar rich arid regions. The low water supply in these areas and the optimal
liquid transportation properties make C2H4 to an attractive candidate to generate win-win
situations at lowest possible cost, where e.g. produced oxygen could be used to improve
the water conditions in the areas, while produced hydrogen could be used locally for
transportation. A difference cost model can be used as a simple controlling instrument to
maximize the contribution of joint products and services to low gas prices. However these
ideas are still vague regarding realization and they need further development, but currently
under the expected time schedule only the nuclear solution gives an option to proceed
really towards the targets.
Finally the results also show two main demands on a successful energy transition that
concerns a clear understanding of a new approach of sustainable system engineering and
the integration of thermodynamically optimized electrochemical systems. The systems
need to be based on closed cycles, but offering also new business opportunities by investi-
gating the options of joint products and services to increase the systems profitability, solar
farming or new O2 applications may be possible examples. The R&D needs and the devel-
opment options for electrochemical PtG systems and their integration as an example are
described in [2], [4], [5], [7] but the trend of changing the energy supply from heat process -
es to thermodynamically well integrated electrochemical processes will influence the future
development of the entire chemical and process industry. Thus this area should be a clear
R&D and implementation target in parallel to the pure energy transition, because these are
its key system elements.
7. References
[1] Wolfgang G. Winkler, Gustav W. Sauer, Mark C. Williams: Renewable storage
evaluation and grid resilience. July 2021. DOI: 10.13140/RG.2.2.29353.11363/1
https://www.researchgate.net/publication/352983209_Renewable_storage_evaluation_and
_grid_resi lience
[2] Wolfgang G. Winkler, Gustav W. Sauer: Energy transition - opportunities and risks
for Germany - Executive Summary. November 2021
https://www.researchgate.net/publication/355978845_Energy_transition_-
opportunities_and_risks_for_Germany_-_Executive_Summary_by
Full version (in German)
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Technical Report
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The blackout in Texas in February 2021 illustrated the vulnerability of any renewable energy supply that is not well integrated in a reliable backup system including sufficient storage capacity. However a failure in the gas supply and not the unexpected drastic reduced supply by wind power combined with a seasonal low solar power production caused the blackout, but this event and its drastic consequences for people and economy exposed clearly the vulnerability of societies and the urgent need of an engineering for reliability to guarantee the grid resilience in principle. Obviously only a backup power generation system with a sufficient storage capacity can fulfill these demands. The resilience of the power supply of today is the result of a technical development of more than hundred years. However there are many studies discussing this problem for renewable power generation and possible solutions and there is still research on technologies itself but design rules covering the real needs of the energy system are not the first priority because there are still reliable operating power plants keeping the grid stable. The monitoring of the power supply by the different energy sources over the operation time allows a comparable good overview of the contribution of the renewable generation capacity to supply the electricity demand in Germany. The target of the investigation focused on solar and wind power was to get a first overview about the deviations of the renewable supply over the different seasons and an analysis of the impact of dark doldrums. The investigation based on the operation data from 2011 to 2019 had to use dimensionless figures to get comparable results because the data cover a period of strongly growing renewable generation capacity. The seasonal influence is covered by using the year as reference period. However there is an optimal combination of wind and solar generation capacity to minimize the demanded storage capacity if only the mean values are used. But this result is misleading because this approach does not consider the possible strong deviations from these mean values. Especially in combination with associated dark doldrums a blackout has a high probability if the mean value would be seen as the important figure for storage design. While a mean value scenario delivers the highest storage demand for 100 % solar generation capacity, the opposite result is the outcome of a scenario when a minimum supply has to fulfill the maximum demand. However further research is needed but these first results already show that the reliability of solar power supply following a defined scheme of operation is clearly higher than that of wind power supply caused by its clearly higher deviations from the mean value.
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Serious challenges for to drive agricultural sustainability combined with climate crisis issues have induced an urgent need to decarbonise agriculture. In this paper, we briefly introduce a novel concept of the Photovoltaic Agricultural Internet of Things (PAIoT). This system approach fuses agricultural production with renewable power generation and control via an IoT platform.We discuss PAIoT applications and potential to realize the next generation of smart farming. In addition, we provide a review of key issues on the feasibility of PAIoT and further propose novel techniques to mitigate these key issues.
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A general thermodynamic model has shown that combined fuel cell cycles may reach an electric-efficiency of more than 80%. This value is one of the targets of the Department of Energy (DOE) solid oxide fuel cell–gas turbine (SOFC–GT) program. The combination of a SOFC and GT connects the air flow of the heat engine and the cell cooling. The principle strategy in order to reach high electrical-efficiencies is to avoid a high excess air for the cell cooling and heat losses. Simple combined SOFC–GT cycles show an efficiency between 60 and 72%. The combination of the SOFC and the GT can be done by using an external cooling or by dividing the stack into multiple sub-stacks with a GT behind each sub-stack as the necessary heat sink. The heat exchangers (HEXs) of a system with an external cooling have the benefit of a pressurization on both sides and therefore, have a high heat exchange coefficient. The pressurization on both sides delivers a low stress to the HEX material. The combination of both principles leads to a reheat (RH)-SOFC–GT cycle that can be improved by a steam turbine (ST) cycle. The first results of a study of such a RH-SOFC–GT–ST cycle indicate that a cycle design with an efficiency of more than 80% is possible and confirm the predictions by the theoretical thermodynamic model mentioned above. The extremely short heat-up time of a thin tubular SOFC and the market entrance of the micro-turbines give the option of using these SOFC–GT designs for mobile applications. The possible use of hydrocarbons such as diesel oil is an important benefit of the SOFC. The micro-turbine and the SOFC stack will be matched depending on the start-up requirements of the mobile system. The minimization of the volume needed is a key issue. The efficiency of small GTs is lower than the efficiency of large GTs due to the influence of the leakage within the stages of GTs increasing with a decreasing size of the GT. Thus, the SOFC module pressure must be lower than in larger stationary SOFC–GT systems. This leads to an electrical-efficiency of 45% of a cycle used as a basis for a design study. The result of the design study is that the space available in a mid-class car allows the placement of such a system, including space reserves. A further improvement of the system might allow an electrical-efficiency of about 55%.
CH4 /CO2 storage cycle as a Closed Carbon Cycle(CCC) for an emission free stable renewable energy
  • W Winkler
  • M C Williams
W. Winkler, M.C. Williams: CH4 /CO2 storage cycle as a Closed Carbon Cycle(CCC) for an emission free stable renewable energy. March 2019
Verfahrensaufbau des CCC Prozesses (Closed Carbon Cycle) als Beitrag zu einer nachhaltigen Energiewirtschaft -Structure of the CCC process (Closed Carbon Cycle) as a contribution to a sustainable energy economy
  • W Winkler
W. Winkler: Verfahrensaufbau des CCC Prozesses (Closed Carbon Cycle) als Beitrag zu einer nachhaltigen Energiewirtschaft -Structure of the CCC process (Closed Carbon Cycle) as a contribution to a sustainable energy economy. December 2019
The United Nations World Water Development Report 2020
  • Water
  • Climate Change
WATER AND CLIMATE CHANGE. The United Nations World Water Development Report 2020. Published in 2020 by the United Nations Educational, Scientific and Cultural Organization. 7, Place de Fontenoy, 75352 Paris 07 SP, France https://www.unwater.org/publications/world-water-development-report-2020/
Solar photovoltaics for sustainable agriculture and rural development. 76 pp., 21 tables, 10 text boxes, 6 annexes. Environment and Natural Resources Working Paper No. 2. FAO, Rome
  • B Van Campen
  • D Guidi
  • G Best
B. van Campen, D. Guidi and G. Best: Solar photovoltaics for sustainable agriculture and rural development. 76 pp., 21 tables, 10 text boxes, 6 annexes. Environment and Natural Resources Working Paper No. 2. FAO, Rome, 2000 https://www.fao.org/uploads/media/Solar%20photovoltaic%20for%20SARD.pdf
Application of Photovoltaic Systems for Agriculture: A Study on the Relationship between Power Generation and Farming for the Improvement of Photovoltaic Applications in Agriculture
  • J Cho
  • S M Park
  • A R Park
  • O C Lee
  • G Nam
  • I.-H Ra
Cho, J.; Park, S.M.; Park, A.R.; Lee, O.C.; Nam, G.; Ra, I.-H. Application of Photovoltaic Systems for Agriculture: A Study on the Relationship between Power Generation and Farming for the Improvement of Photovoltaic Applications in Agriculture. Energies 2020, 13, 4815. https://doi.org/10.3390/en13184815 https://www.mdpi.com/1996-1073/13/18/4815