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Reducing the hydrogen production cost by operating alkaline electrolysis as a discontinuous process in the French market context
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 e n e r g y 3 6 ( 2 0 1 1 ) 6 4 0 7 e6 4 1 3
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Reducing the hydrogen production cost by operating alkaline electrolysis as a discontinuous process in the French market context C. Mansilla a,*, S. Dautremont a, B. Shoai Tehrani a, G. Cotin a, S. Avril a, E. Burkhalter b a b
CEA, DEN, I-te´se´, F-91191 Gif-sur-Yvette, France IHT, Clos-Donroux, C. P. 228, 1870, Monthey 1, Switzerland
article info
abstract
Article history:
The possible reduction of the hydrogen production cost when operating alkaline electro-
Received 6 January 2011
lysers in a discontinuous way, in order to benefit from low electricity prices, is investigated.
Received in revised form
Beside the insights about the electricity market (prices do not correlate the demand; they
19 February 2011
are related to the supply-and-demand hardness), advances in modelling discontinuous
Accepted 3 March 2011
operation are proposed. An optimum production cost is found that induces a profit of 4%,
Available online 3 April 2011
with regard to a plant that would work continuously. Specific attention should be given to related overcosts: additional degradation due to frequent transitions from the minimum
Keywords:
electrolyser load to the nominal one, higher maintenance needs, and hydrogen storage
Hydrogen production
costs. Such an operating mode would also greatly benefit from a reduction of the elec-
Alkaline electrolysis
trolyser prices. However, the state-of-the-art as regards the electrolyser minimum loads
Electricity prices
and transition time appears satisfactory.
Discontinuous processes
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has a wide range of applications. Not mentioning hydrogen as a possible fuel in the long term, the need for hydrogen for synthetic fuel production is high. Among synfuels, the ones from biomass are especially virtuous from an environmental point of view. Supplying hydrogen to the process enables to improve the efficiency of biomass use [1]. However, in order to keep the environmental benefit, hydrogen should be produced by alkaline electrolysis. Indeed, mature technologies based on fossil fuels are not satisfactory due to limited resources and the greenhouse gas emissions that are generated. Only alkaline electrolysis can presently produce hydrogen without emitting greenhouse gases, provided of course the consumed electricity is also produced without causing emissions. Alkaline electrolysis can help ameliorating
the efficiency of the process [2] and also provides a storage means for electricity [3]. However, up to now, alkaline electrolysis remains an expensive way to produce hydrogen when compared to fossil alternatives such as steam methane reforming. In response, some works aimed at reducing the electrolyser cost [4], and others at improving the electrolyser efficiency by adapting the power supply [5] or the operating conditions [6]. As a matter of fact, electricity consumption is the first contributor to the cost of hydrogen when produced by alkaline electrolysis. Another way to reduce this cost item is appealing to low electricity prices. This was investigated by different works in the past. Naterer et al. (2008) envisaged it in the Canadian context [7]. Electrolysis is operated during off-peak periods as a decentralized production process, coupled to the copper-chloride thermochemical process as a centralized production means.
* Corresponding author. Tel.: þ33 1 69 08 36 86; fax: þ33 1 69 08 35 66. E-mail address: [email protected] (C. Mansilla). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.004
6408
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Jørgensen and Ropenus (2008) studied such an operation mode in the Danish context, in relation with different wind penetration scenarios [8]. In the French context, off-peak operation was considered for excess electricity loads by Gutie´rrezMartı´n et al. (2010) [9]. Excess electricity is defined as the difference between the domestic production and consumption (exports are not taken into account). A minimum electrolyser load is then associated to an electricity price of 100 €/MWh, whereas for the period defined as off-peak the electricity supply is free. The present work takes back the approach initiated in [10] for the French electricity market. The objective is to assess the possible reduction of the hydrogen production cost when operating alkaline electrolysis in a discontinuous way, in order to benefit from low electricity prices. Beside the insights about the electricity market, the present study proposes advances in modelling discontinuous operation. Indeed, whereas other works consider that the electrolyser is a switch (i.e. based on an “on/off” operation); we included the need for a minimum load together with the time necessary to pass from the minimum load to the nominal one, and the resulting electrolyser degradation. The paper is organized as follows. First we focus on the electricity market in the French context in order to define the economic model for the electricity prices. Then we describe alkaline electrolysis as a discontinuous process and the associated modelling. Finally, the optimum production cost is assessed and the hydrogen storage needs are discussed when hydrogen is to be fed into a continuous process. R&D needs to improve discontinuous alkaline electrolysis are raised.
2.
The French electricity market
2.1.
Panorama of the electricity market
The electricity sector was progressively opened to competition between 2000 and 2007, in which year all consumers were free to choose their own supplier, permitting to France to respect its European agreements. Industrials, especially big consumers, can directly stock up with electricity from the wholesale market, which represents 30% of the trades. Electricity is mainly traded on the wholesale market by two ways: the mutual agreement markets (on the counter transactions), which are the most important in volume, and the electricity Power Exchange. Since the end of 2008, two power exchanges have been created for the French market: - the EPEX Spot (European Power Exchange) which deals with short-term products (electricity is purchased to be delivered immediately or the next day) [11], - the EEX (European Energy Exchange) which negotiates “futures” (electricity is bought one day to be delivered the next month, quarter or year) [12]. EPEX Spot (located in France) and EEX (located in Germany) result from the separation of spot and future activities of the French power exchange Powernext. For our study, we selected the EPEX Spot market on several grounds. First, and more important, electricity power
exchange ensures transparent and competitive prices, giving references to the overall electricity market. Calculations have been made using the spot market (called Day Ahead Auction on EPEX spot) representing the price of the last volume bought, i.e. the marginal price. Spot price is known as a guiding price and our data well confirm the fact that future products follow spot prices, whatever the delivery period is, monthly, quarterly or yearly. Moreover, in order to make discontinuous operation profitable, we should access to low electricity prices. Since it is a good that cannot be stocked, it is well known that electricity spot prices show unique properties of high volatility. Electricity demand is inelastic to short period, so that unexpected demand shocks are fully regulated through price jump. Then, very high prices occur during short periods. In a similar way, very low prices (nearly almost zero, even negative prices in some markets) are observed as well during short times. Shortterm prices directly reflect the supply-and-demand hardness. In our data, during roughly 60% of the year, spot prices are lower than any other ones. Volatility market is explained, among others, by the fact that electricity cannot be stocked. As a consequence electricity is paid in advance sometimes without knowledge of consumption. It is important to notice because it differs from a market with speculative motives. On the Day Ahead Auction market, electricity is traded on one day for the next day, on an hourly basis (note that it is the highest frequency observed on the EPEX Spot). This leads to 24 micro-markets corresponding to each hour of the day. Our data base comprises electricity prices from 2006 to mid-2010. The analysis of the spot prices gives interesting insights: - two peaks can be observed every day: one around noon, the other one in the evening (cf. Fig. 1); - working day prices are higher than during the week-ends (cf. Fig. 1); - variations can be observed according to the season (cf. Figs. 2 and 3). Seasonal variations are quite surprising at first. As a matter of fact, unexpectedly, we can observe that summer prices happen to be higher than winter prices, and that the highest prices occur during the autumn. This reflects the fact that prices do not correlate the demand; they are related to the supply-and-demand hardness, for instance caused by unforeseen autumn cold spells. Finally, by plotting the electricity price day profile according to the year (cf. Fig. 4), we can highlight the uniqueness of 2008. Prices were then way higher than for the other years from our sample. One can remember that 2008 was a peak year for the oil price, which had an impact on the electricity market.
2.2.
Construction of characteristic days
The economic model for the electricity prices is based on the prices observed during the last five years. First, we build a “standard” year by excluding the year 2008. From the analysis of the electricity prices that was detailed in the previous section, we also distinguish between eight kinds of days, according to the season and the week days (i.e. working days vs. week-ends). The electricity price is assessed on an hourly
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 e n e r g y 3 6 ( 2 0 1 1 ) 6 4 0 7 e6 4 1 3
working day price
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week-end price
100
euro/MWh
80
60
40
20
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 day hour
Fig. 1 e Electricity price profile: working days vs. week-ends.
basis by calculating averages from the years 2006, 2007, 2009, and 2010. Inflation is neglected. Confidence intervals were determined for these characteristic days, from the standard deviation in the sample. The confidence interval we defined enables to reach a probability of 95% to represent all the prices included in the sample. Prices in such an interval generally differ of only a few Euros. The less the general consumption is, the smaller the interval is. From the characteristic days, weeks are constituted as it is shown in Fig. 5 for the autumn. The standard year is then composed by 4 winter months, 2 spring months, 4 summer months, and finally 2 autumn months (according to the electricity market, the year is divided into three equivalent periods: winter, summer and low season. Spring was differentiated from autumn due to the very different prices). Several scenarios are considered. In the “reference scenario”, the electricity prices are taken from the standard year. We also considered other cases in which a “crisis” is inserted every 5, 10 or 15 years. The crisis electricity prices are constructed in the same way as for the standard year, only by considering 2008. In this study we will consider an«isolated» hydrogen production plant, meaning that we assume here that hydrogen production is low enough not to have any impact on the electricity prices. However, if such a system was developed
leading to high electricity needs in order to feed the hydrogen production plants, electricity prices would be impacted. Further developments are envisaged to study how they would be modified.
3. Alkaline electrolysis as a discontinuous process 3.1.
Alkaline electrolysis discontinuous operation
Alkaline electrolysis is suitable for discontinuous operation. Electrolysers’ load can be increased or decreased quite easily, by adapting the current density [13,14]. However electrolysers differ from a mere switch. There exists a minimum load under which the electrolyser should be stopped. This is due to side-electrolysis that occurs in the electrolyte channels. Under a certain load, usually estimated to be 20e25% by electrolyser manufacturers, this phenomenon cannot be neglected anymore. Besides, increasing the load from the minimum is not instantaneous. It approximately takes 10 min to reach 100%. Finally, discontinuous operation may induce some additional degradation (i.e. electricity consumption increases to achieve the same hydrogen output). Precise numerical data
140 120
Winter
80
Summer Autumn
60
Spring
40 20
Ho u r24
Ho u r22
Ho u r23
Ho u r20
Ho u r21
Ho u r19
Ho u r18
Ho u r16
Ho u r17
Ho u r14
Ho u r15
Ho u r13
Ho u r12
Ho u r11
Ho u r10
Ho ur8
Ho ur9
Ho ur7
Ho ur6
Ho ur5
Ho ur3
Ho ur4
Ho ur2
0 Ho ur1
euro/MWh
100
Fig. 2 e Electricity price profile on working days according to the season.
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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 e n e r g y 3 6 ( 2 0 1 1 ) 6 4 0 7 e6 4 1 3
90 80 70 Winter
euro/MWh
60
Summer
50
Autumn
40 Spring
30 20 10
Hour24
Hour23
Hour21
Hour22
Hour20
Hour19
Hour17
Hour18
Hour16
Hour14
Hour15
Hour13
Hour11
Hour12
Hour10
Hour8
Hour9
Hour6
Hour7
Hour4
Hour5
Hour3
Hour1
Hour2
0
Fig. 3 e Electricity price profile on week-ends according to the season.
are not available today, yet we will take it into account in our model through several scenarios (see next section).
3.2.
Modelling discontinuous operation
We focus on massive hydrogen production, to satisfy industrial needs such as a synthetic fuel process. This is why we retained the IHT S-556 electrolyser [15] that demonstrates the highest available hydrogen output today: 760 Nm3/h, together with flexible operation abilities. Electrolysis is carried out under a pressure of 30 bar. The electrolyser is constituted by two half-electrolysers of 278 cells each and consumes 3.3 MW. A minimum load of 20% was considered. Three operating modes are distinguished: - nominal load: 100% of the power; - minimum load: 20% of the power; - transition phases: 60% load during 10 min (a linear profile is considered). The duration of the transition phases is inferior to the frequency of the electricity market variations. In the model, transitions decrease or increase the next hour plant
2006 average
2007 average
production during the transition time. For instance, for a 20e100% transition the electrolysers are operated at the minimum load during the H-1 hour, then, during the H hour, 10 min at 60% and 50 min at 100%. As regards the electrolyser degradation, beside the one due to continuous operation that is modelled as an increase of the electrolyser operating voltage per hour of operation, different scenarios from 1 mV/transition up to 10 mV/transition have been envisaged.
3.3. Evaluation of the hydrogen cost when produced discontinuously: the Numphea model The aim of the model is to assess the optimum hydrogen production cost (if existing) according to the operating mode. To do so, an electricity threshold price is included. When the electricity is cheaper than this threshold, nominal operation occurs. When it is more expensive, the plant is switched to the minimum load. According to the selected scenario (i.e. electricity price profile, degradation), the hydrogen production, electricity consumption cost and other O&M variable costs are calculated for each year. They depend on the time spent in each
2008 average
2009 average
2010 average
100
euro/MWh
80
60
40
20
0 1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 day hour
Fig. 4 e Electricity price day profile according to the year.
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 e n e r g y 3 6 ( 2 0 1 1 ) 6 4 0 7 e6 4 1 3
6411
140 120
euro/MWh
100 80 60 40 20 0 monday
monday
tuesday
wednesday thursday
friday
saturday
sunday
Fig. 5 e Construction of a typical week (autumn example).
operating mode (i.e. minimum, nominal, transition). The degradation is also calculated to assess the next year operating voltages. The hydrogen production cost is then estimated through the calculation of a levelized production cost model by also taking into account fixed O&M costs and the plant investment (the reference date for discounting is the plant start-up, other techno-economic parameters are summarized in Table 1). Ph TC ¼
t
i ðCi Þt þðCo Þt ð1 þ sÞt i Ph ðPt Þð1 þ sÞt
(1)
t
TC: levelized production cost (€/kg); Ci: investment (€); Co: operating cost (€); P: hydrogen production (kg); s: discount rate; t: year under consideration. This model called NuMPHEA (French acronym for calculation model of the hydrogen cost produced by alkaline electrolysis operated as a discontinuous process) was developed using the Pythonª programming language [16].
4.
Results and discussion
4.1. Minimum production cost through discontinuous operation When considering the reference scenario (i.e. no electricity market crisis, no additional degradation due to the transitions), we can observe an optimum hydrogen production cost (cf. Fig. 6). Indeed, when the continuous mode is approached, the equipment is fully amortized but the plant also appeals to
Table 1 e Techno-economic parameters of the model. Plant capital cost Electrolyser life expectancy Plant life Plant construction duration Discount rate
900 k€/MW 15 years 30 years 2 years 8%
peak electricity prices. When, on the contrary, the load is decreased, electricity prices are more favourable but the investment share rises. For this scenario, the optimum is achieved for an electricity threshold price of 56 €/MWh leading to a hydrogen production cost of 3.27 €/kg. The gain compared to continuous operation is only 3.6%. However, continuous operation would most probably appeal to future products (e.g. EEX market, providing the security of supply in terms of energy prices). When referring to such a market, the gain is assessed at 15%. This reflects the supply security cost: in order to ensure the fact that prices will not skyrocket, an overcost is included in the prices. Moreover, let us mention that the optimum is reached when the plant is operated at the nominal load during 75% of the year. The system avoids very high electricity prices rather than focusing on off-peak operation. This confirms the result presented by Jørgensen and Ropenus (2008) [8]: peak shaving is difficult due to the electrolyser prices that remain quite high. Furthermore, when considering a “crisis” year every 5, 10 or 15 years, the optimum hydrogen production cost rises. Indeed, electricity is more expensive during those years. Therefore, the gain compared to continuous operation also rises since the system avoids high electricity prices that occur during continuous operation. On the other hand, if continuous operation is supposed to appeal to future products, the gain of the discontinuous mode decreases. Future products enable escaping from crises peak prices.
4.2.
Hydrogen storage needs
If hydrogen is to be fed into a continuous process, storage is compulsory. To assess what would be the storage needs of such a plant, we focus on BtL (Biomass to Liquid) production. As it was previously explained, introducing hydrogen into the process enables reaching better biomass use (i.e. from the same amount of biomass, a greater fuel output is achieved). We consider an industrial BtL plant consuming 750 000 t biomass per year. Hydrogen input is assessed at 6%weight of the biomass, i.e. 48 000 tH2/year. The BtL plant is operated during 8000 h/year.
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 e n e r g y 3 6 ( 2 0 1 1 ) 6 4 0 7 e6 4 1 3
Hydrogen production cost (€/kg)
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6,3 5,8 5,3 4,8 4,3 3,8 3,3 2,8 0
50 100 150 Electricity threshold price (€/MWh)
200
Fig. 6 e Optimum production cost in the reference scenario (no electricity market crisis, no additional degradation due to the transitions).
The hydrogen plant is operated 8760 h/year, either at the minimum or nominal load, in order to reach the optimum production cost (cf. x4.1 e maintenance needs can then be adjusted to the biofuel plant operation; i.e. electrolysers may be maintained when hydrogen is not needed by the biofuel plant, and especially since alkaline electrolysis maintenance needs are rather low). When hydrogen production is superior to the BtL plant needs, hydrogen is stocked. When it is inferior, hydrogen is taken out from the storage. Calculations were carried out to assess the storage needs in order to ensure a continuous hydrogen input for the BtL plant. Such storage is designed by a capacity of 5150 tH2. The evolution of the hydrogen stock in the storage along the year is presented in Fig. 7. Hydrogen is destocked during the winter, then stocked during spring and summer (lower electricity prices overall), and finally destocked in the autumn. The rapid increase of the hydrogen level at the end of the summer corresponds to the maintenance period of the BtL plant: no hydrogen is consumed then. From an economic standpoint, it appeared that compressed cylinders are far too expensive. Indeed, they would represent a capital cost higher than the plant’s, associated to a lifespan of
only 10 years. As it was already stated by Amos (1998) [17] quoted by Clausen et al. (2010) [2], if the storage requirement exceeds 1300 kg of hydrogen, underground storage should be considered. Indeed such a storage means could appear reasonable. However, the gain demonstrated today by alkaline electrolysis discontinuous operation would be offset by a storage cost of 30 M€.
4.3. R&D needs to improve the discontinuous operation economic performances As it was stated earlier, electricity markets fluctuate on an hourly basis. No significant profit would be made by trying to decrease the electrolysers’ transition time from their minimum load to nominal operation. However, we assessed the gain that would be obtained if the minimum load could be reduced down to 10%. In such a case, the optimum production cost would only be decreased by 0.3%. As a matter of fact, reducing the minimum load enables reducing the electricity consumption when electricity is cheap. Therefore, the possible gain is small.
Fig. 7 e Evolution of the storage during a year.
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 e n e r g y 3 6 ( 2 0 1 1 ) 6 4 0 7 e6 4 1 3
We also examined the impact of the additional degradation due to discontinuous operation. Three scenarios have been considered from 1 mV/transition up to 10 mV (cf. x3.2). When this additional degradation is taken into account, the optimum production cost diminishes since the electricity consumption rises. Therefore the possible profit with regard to continuous operation decreases down to 2.4% for a degradation of 10 mV/transition (continuous operation means no transition; therefore the production cost does not vary). Discontinuous operation studies should focus on additional degradation in order to establish the real attainable profit. Furthermore, discontinuous operation could induce higher maintenance needs. The gap between the optimum production cost and the continuous operation one would be offset if additional maintenance was superior to 3.7 M€/year, i.e. 2% of the equipment capital cost. Finally, let us recall that discontinuous operation is all the more profitable that capital costs are low. Such an operating mode would greatly benefit from a reduction of electrolyser prices, since the plant remains quite capital-extensive.
5.
Conclusion and outlook
The present study assessed the possible reduction of the hydrogen production cost when operating alkaline electrolysers in a discontinuous way. Beside the insights about the electricity market, advances in modelling the discontinuous operation have been proposed. Overall, the profit that can be expected from discontinuous operation amounts to roughly 4% of the hydrogen production cost, when compared to a plant that would work continuously with electricity supplied from the same market, i.e. the EPEX Spot. When crises occur in the energy market, discontinuous operation is all the more relevant since it enables avoiding very high prices. In order to make such an operating mode profitable, specific attention should be given to eventual related overcosts: additional degradation due to frequent transitions from the minimum electrolyser load to the nominal one, higher maintenance needs, and induced storage costs. Such an operating mode would also greatly benefit from a reduction of electrolyser prices. On the other hand, the state-of-the-art as regards the electrolyser minimum loads and transition time appears satisfactory. Finally, other business models could be considered. Alkaline electrolysis could enter energy producers’ portfolios, not as a means to produce energy, but as one to reduce the consumption when needed. Besides, if such an operating mode was to gain in importance, the impact of variable
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electricity demand on the electricity market prices should be investigated.
references
[1] Seiler JM, Hohwiller C, Imbach J, Luciani JF. Technical and economical evaluation of enhanced biomass to liquid fuel processes. Energy 2010;35(9):3587e92. [2] Clausen LR, Houbak N, Elmegaard B. Technoeconomic analysis of a methanol plant based on gasification of biomass and electrolysis of water. Energy 2010;35:2338e47. [3] Gutie´rrez-Martı´n F, Confente D, Guerra I. Management of variable electricity loads in wind e hydrogen systems: the case of a Spanish wind farm. Int J Hydrogen Energy 2010;35: 7329e36. [4] Marcelo D, Dell’Era A. Economical electrolyser solution. Int J Hydrogen Energy 2008;33:3041e4. [5] Ursu´a A, Marroyo L, Gubı´a E, Gandı´a LM, Die´guez PM, Sanchis P. Influence of the power supply on the energy efficiency of an alkaline water electrolyser. Int J Hydrogen Energy 2009;34:3221e33. [6] Nikolic VM, Tasic GS, Maksic AD, Saponjic DP, Miulovic SM, Marceta Kaninski MP. Raising efficiency of hydrogen generation from alkaline water electrolysis e energy saving. Int J Hydrogen Energy 2010;35:12369e73. [7] Naterer GF, Fowler M, Cotton J, Gabriel K. Synergistic roles of off-peak electrolysis and thermochemical production of hydrogen from nuclear energy in Canada. Int J Hydrogen Energy 2008;33:6849e57. [8] Jørgensen C, Ropenus S. Production price of hydrogen from grid connected electrolysis in a power market with high wind penetration. Int J Hydrogen Energy 2008;33:5335e44. [9] Gutie´rrez-Martı´n F, Garcı´a-De Marı´a JM, Baı¨ri A, Laraqi N. Management strategies for surplus electricity loads using electrolytic hydrogen. Int J Hydrogen Energy 2009;34: 8468e75. [10] Floch P-H, Gabriel S, Mansilla C, Werkoff F. On the production of hydrogen via alkaline electrolysis during offpeak periods. Int J Hydrogen Energy 2007;32:4641e7. [11] EPEX Spot website, http://www.epexspot.com/; [accessed 10. 05.10]. [12] EEX website, http://www.eex.com/; [accessed 10.05.10]. [13] Krieg D, Stolten D. Alkaline electrolysis e a review. In: 18th world hydrogen energy conference, WHEC 18, Essen, Germany; 2010-05-17e2010-05-20. [14] Ki DTU, Risoe DTU, DONG energy, pre-investigation of water electrolysis. Available online, <130.226.56.153/rispubl/NEI/ NEI-DK-5057.pdf>; [accessed 10.05.10]. [15] IHT website, http://iht.ch/; [accessed 10.05.10]. [16] Python programming language e official website, http:// www.pyhton.org/; [accessed 10.05.10]. [17] Amos WA. Costs of storing and transporting hydrogen, report. Golden, CO: National Renewable Energy Laboratory. Available from, http://www.hydrogen.energy.gov/analysis_ repository/project.cfm/PID¼114; 1998.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Reducing the hydrogen production cost by operating alkaline electrolysis as a discontinuous process in the French market context C. Mansilla a,*, S. Dautremont a, B. Shoai Tehrani a, G. Cotin a, S. Avril a, E. Burkhalter b a b
CEA, DEN, I-te´se´, F-91191 Gif-sur-Yvette, France IHT, Clos-Donroux, C. P. 228, 1870, Monthey 1, Switzerland
article info
abstract
Article history:
The possible reduction of the hydrogen production cost when operating alkaline electro-
Received 6 January 2011
lysers in a discontinuous way, in order to benefit from low electricity prices, is investigated.
Received in revised form
Beside the insights about the electricity market (prices do not correlate the demand; they
19 February 2011
are related to the supply-and-demand hardness), advances in modelling discontinuous
Accepted 3 March 2011
operation are proposed. An optimum production cost is found that induces a profit of 4%,
Available online 3 April 2011
with regard to a plant that would work continuously. Specific attention should be given to related overcosts: additional degradation due to frequent transitions from the minimum
Keywords:
electrolyser load to the nominal one, higher maintenance needs, and hydrogen storage
Hydrogen production
costs. Such an operating mode would also greatly benefit from a reduction of the elec-
Alkaline electrolysis
trolyser prices. However, the state-of-the-art as regards the electrolyser minimum loads
Electricity prices
and transition time appears satisfactory.
Discontinuous processes
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has a wide range of applications. Not mentioning hydrogen as a possible fuel in the long term, the need for hydrogen for synthetic fuel production is high. Among synfuels, the ones from biomass are especially virtuous from an environmental point of view. Supplying hydrogen to the process enables to improve the efficiency of biomass use [1]. However, in order to keep the environmental benefit, hydrogen should be produced by alkaline electrolysis. Indeed, mature technologies based on fossil fuels are not satisfactory due to limited resources and the greenhouse gas emissions that are generated. Only alkaline electrolysis can presently produce hydrogen without emitting greenhouse gases, provided of course the consumed electricity is also produced without causing emissions. Alkaline electrolysis can help ameliorating
the efficiency of the process [2] and also provides a storage means for electricity [3]. However, up to now, alkaline electrolysis remains an expensive way to produce hydrogen when compared to fossil alternatives such as steam methane reforming. In response, some works aimed at reducing the electrolyser cost [4], and others at improving the electrolyser efficiency by adapting the power supply [5] or the operating conditions [6]. As a matter of fact, electricity consumption is the first contributor to the cost of hydrogen when produced by alkaline electrolysis. Another way to reduce this cost item is appealing to low electricity prices. This was investigated by different works in the past. Naterer et al. (2008) envisaged it in the Canadian context [7]. Electrolysis is operated during off-peak periods as a decentralized production process, coupled to the copper-chloride thermochemical process as a centralized production means.
* Corresponding author. Tel.: þ33 1 69 08 36 86; fax: þ33 1 69 08 35 66. E-mail address: [email protected] (C. Mansilla). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.004
6408
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 e n e r g y 3 6 ( 2 0 1 1 ) 6 4 0 7 e6 4 1 3
Jørgensen and Ropenus (2008) studied such an operation mode in the Danish context, in relation with different wind penetration scenarios [8]. In the French context, off-peak operation was considered for excess electricity loads by Gutie´rrezMartı´n et al. (2010) [9]. Excess electricity is defined as the difference between the domestic production and consumption (exports are not taken into account). A minimum electrolyser load is then associated to an electricity price of 100 €/MWh, whereas for the period defined as off-peak the electricity supply is free. The present work takes back the approach initiated in [10] for the French electricity market. The objective is to assess the possible reduction of the hydrogen production cost when operating alkaline electrolysis in a discontinuous way, in order to benefit from low electricity prices. Beside the insights about the electricity market, the present study proposes advances in modelling discontinuous operation. Indeed, whereas other works consider that the electrolyser is a switch (i.e. based on an “on/off” operation); we included the need for a minimum load together with the time necessary to pass from the minimum load to the nominal one, and the resulting electrolyser degradation. The paper is organized as follows. First we focus on the electricity market in the French context in order to define the economic model for the electricity prices. Then we describe alkaline electrolysis as a discontinuous process and the associated modelling. Finally, the optimum production cost is assessed and the hydrogen storage needs are discussed when hydrogen is to be fed into a continuous process. R&D needs to improve discontinuous alkaline electrolysis are raised.
2.
The French electricity market
2.1.
Panorama of the electricity market
The electricity sector was progressively opened to competition between 2000 and 2007, in which year all consumers were free to choose their own supplier, permitting to France to respect its European agreements. Industrials, especially big consumers, can directly stock up with electricity from the wholesale market, which represents 30% of the trades. Electricity is mainly traded on the wholesale market by two ways: the mutual agreement markets (on the counter transactions), which are the most important in volume, and the electricity Power Exchange. Since the end of 2008, two power exchanges have been created for the French market: - the EPEX Spot (European Power Exchange) which deals with short-term products (electricity is purchased to be delivered immediately or the next day) [11], - the EEX (European Energy Exchange) which negotiates “futures” (electricity is bought one day to be delivered the next month, quarter or year) [12]. EPEX Spot (located in France) and EEX (located in Germany) result from the separation of spot and future activities of the French power exchange Powernext. For our study, we selected the EPEX Spot market on several grounds. First, and more important, electricity power
exchange ensures transparent and competitive prices, giving references to the overall electricity market. Calculations have been made using the spot market (called Day Ahead Auction on EPEX spot) representing the price of the last volume bought, i.e. the marginal price. Spot price is known as a guiding price and our data well confirm the fact that future products follow spot prices, whatever the delivery period is, monthly, quarterly or yearly. Moreover, in order to make discontinuous operation profitable, we should access to low electricity prices. Since it is a good that cannot be stocked, it is well known that electricity spot prices show unique properties of high volatility. Electricity demand is inelastic to short period, so that unexpected demand shocks are fully regulated through price jump. Then, very high prices occur during short periods. In a similar way, very low prices (nearly almost zero, even negative prices in some markets) are observed as well during short times. Shortterm prices directly reflect the supply-and-demand hardness. In our data, during roughly 60% of the year, spot prices are lower than any other ones. Volatility market is explained, among others, by the fact that electricity cannot be stocked. As a consequence electricity is paid in advance sometimes without knowledge of consumption. It is important to notice because it differs from a market with speculative motives. On the Day Ahead Auction market, electricity is traded on one day for the next day, on an hourly basis (note that it is the highest frequency observed on the EPEX Spot). This leads to 24 micro-markets corresponding to each hour of the day. Our data base comprises electricity prices from 2006 to mid-2010. The analysis of the spot prices gives interesting insights: - two peaks can be observed every day: one around noon, the other one in the evening (cf. Fig. 1); - working day prices are higher than during the week-ends (cf. Fig. 1); - variations can be observed according to the season (cf. Figs. 2 and 3). Seasonal variations are quite surprising at first. As a matter of fact, unexpectedly, we can observe that summer prices happen to be higher than winter prices, and that the highest prices occur during the autumn. This reflects the fact that prices do not correlate the demand; they are related to the supply-and-demand hardness, for instance caused by unforeseen autumn cold spells. Finally, by plotting the electricity price day profile according to the year (cf. Fig. 4), we can highlight the uniqueness of 2008. Prices were then way higher than for the other years from our sample. One can remember that 2008 was a peak year for the oil price, which had an impact on the electricity market.
2.2.
Construction of characteristic days
The economic model for the electricity prices is based on the prices observed during the last five years. First, we build a “standard” year by excluding the year 2008. From the analysis of the electricity prices that was detailed in the previous section, we also distinguish between eight kinds of days, according to the season and the week days (i.e. working days vs. week-ends). The electricity price is assessed on an hourly
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working day price
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week-end price
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euro/MWh
80
60
40
20
0
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8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 day hour
Fig. 1 e Electricity price profile: working days vs. week-ends.
basis by calculating averages from the years 2006, 2007, 2009, and 2010. Inflation is neglected. Confidence intervals were determined for these characteristic days, from the standard deviation in the sample. The confidence interval we defined enables to reach a probability of 95% to represent all the prices included in the sample. Prices in such an interval generally differ of only a few Euros. The less the general consumption is, the smaller the interval is. From the characteristic days, weeks are constituted as it is shown in Fig. 5 for the autumn. The standard year is then composed by 4 winter months, 2 spring months, 4 summer months, and finally 2 autumn months (according to the electricity market, the year is divided into three equivalent periods: winter, summer and low season. Spring was differentiated from autumn due to the very different prices). Several scenarios are considered. In the “reference scenario”, the electricity prices are taken from the standard year. We also considered other cases in which a “crisis” is inserted every 5, 10 or 15 years. The crisis electricity prices are constructed in the same way as for the standard year, only by considering 2008. In this study we will consider an«isolated» hydrogen production plant, meaning that we assume here that hydrogen production is low enough not to have any impact on the electricity prices. However, if such a system was developed
leading to high electricity needs in order to feed the hydrogen production plants, electricity prices would be impacted. Further developments are envisaged to study how they would be modified.
3. Alkaline electrolysis as a discontinuous process 3.1.
Alkaline electrolysis discontinuous operation
Alkaline electrolysis is suitable for discontinuous operation. Electrolysers’ load can be increased or decreased quite easily, by adapting the current density [13,14]. However electrolysers differ from a mere switch. There exists a minimum load under which the electrolyser should be stopped. This is due to side-electrolysis that occurs in the electrolyte channels. Under a certain load, usually estimated to be 20e25% by electrolyser manufacturers, this phenomenon cannot be neglected anymore. Besides, increasing the load from the minimum is not instantaneous. It approximately takes 10 min to reach 100%. Finally, discontinuous operation may induce some additional degradation (i.e. electricity consumption increases to achieve the same hydrogen output). Precise numerical data
140 120
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0 Ho ur1
euro/MWh
100
Fig. 2 e Electricity price profile on working days according to the season.
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90 80 70 Winter
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Hour24
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Fig. 3 e Electricity price profile on week-ends according to the season.
are not available today, yet we will take it into account in our model through several scenarios (see next section).
3.2.
Modelling discontinuous operation
We focus on massive hydrogen production, to satisfy industrial needs such as a synthetic fuel process. This is why we retained the IHT S-556 electrolyser [15] that demonstrates the highest available hydrogen output today: 760 Nm3/h, together with flexible operation abilities. Electrolysis is carried out under a pressure of 30 bar. The electrolyser is constituted by two half-electrolysers of 278 cells each and consumes 3.3 MW. A minimum load of 20% was considered. Three operating modes are distinguished: - nominal load: 100% of the power; - minimum load: 20% of the power; - transition phases: 60% load during 10 min (a linear profile is considered). The duration of the transition phases is inferior to the frequency of the electricity market variations. In the model, transitions decrease or increase the next hour plant
2006 average
2007 average
production during the transition time. For instance, for a 20e100% transition the electrolysers are operated at the minimum load during the H-1 hour, then, during the H hour, 10 min at 60% and 50 min at 100%. As regards the electrolyser degradation, beside the one due to continuous operation that is modelled as an increase of the electrolyser operating voltage per hour of operation, different scenarios from 1 mV/transition up to 10 mV/transition have been envisaged.
3.3. Evaluation of the hydrogen cost when produced discontinuously: the Numphea model The aim of the model is to assess the optimum hydrogen production cost (if existing) according to the operating mode. To do so, an electricity threshold price is included. When the electricity is cheaper than this threshold, nominal operation occurs. When it is more expensive, the plant is switched to the minimum load. According to the selected scenario (i.e. electricity price profile, degradation), the hydrogen production, electricity consumption cost and other O&M variable costs are calculated for each year. They depend on the time spent in each
2008 average
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0 1
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10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 day hour
Fig. 4 e Electricity price day profile according to the year.
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100 80 60 40 20 0 monday
monday
tuesday
wednesday thursday
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Fig. 5 e Construction of a typical week (autumn example).
operating mode (i.e. minimum, nominal, transition). The degradation is also calculated to assess the next year operating voltages. The hydrogen production cost is then estimated through the calculation of a levelized production cost model by also taking into account fixed O&M costs and the plant investment (the reference date for discounting is the plant start-up, other techno-economic parameters are summarized in Table 1). Ph TC ¼
t
i ðCi Þt þðCo Þt ð1 þ sÞt i Ph ðPt Þð1 þ sÞt
(1)
t
TC: levelized production cost (€/kg); Ci: investment (€); Co: operating cost (€); P: hydrogen production (kg); s: discount rate; t: year under consideration. This model called NuMPHEA (French acronym for calculation model of the hydrogen cost produced by alkaline electrolysis operated as a discontinuous process) was developed using the Pythonª programming language [16].
4.
Results and discussion
4.1. Minimum production cost through discontinuous operation When considering the reference scenario (i.e. no electricity market crisis, no additional degradation due to the transitions), we can observe an optimum hydrogen production cost (cf. Fig. 6). Indeed, when the continuous mode is approached, the equipment is fully amortized but the plant also appeals to
Table 1 e Techno-economic parameters of the model. Plant capital cost Electrolyser life expectancy Plant life Plant construction duration Discount rate
900 k€/MW 15 years 30 years 2 years 8%
peak electricity prices. When, on the contrary, the load is decreased, electricity prices are more favourable but the investment share rises. For this scenario, the optimum is achieved for an electricity threshold price of 56 €/MWh leading to a hydrogen production cost of 3.27 €/kg. The gain compared to continuous operation is only 3.6%. However, continuous operation would most probably appeal to future products (e.g. EEX market, providing the security of supply in terms of energy prices). When referring to such a market, the gain is assessed at 15%. This reflects the supply security cost: in order to ensure the fact that prices will not skyrocket, an overcost is included in the prices. Moreover, let us mention that the optimum is reached when the plant is operated at the nominal load during 75% of the year. The system avoids very high electricity prices rather than focusing on off-peak operation. This confirms the result presented by Jørgensen and Ropenus (2008) [8]: peak shaving is difficult due to the electrolyser prices that remain quite high. Furthermore, when considering a “crisis” year every 5, 10 or 15 years, the optimum hydrogen production cost rises. Indeed, electricity is more expensive during those years. Therefore, the gain compared to continuous operation also rises since the system avoids high electricity prices that occur during continuous operation. On the other hand, if continuous operation is supposed to appeal to future products, the gain of the discontinuous mode decreases. Future products enable escaping from crises peak prices.
4.2.
Hydrogen storage needs
If hydrogen is to be fed into a continuous process, storage is compulsory. To assess what would be the storage needs of such a plant, we focus on BtL (Biomass to Liquid) production. As it was previously explained, introducing hydrogen into the process enables reaching better biomass use (i.e. from the same amount of biomass, a greater fuel output is achieved). We consider an industrial BtL plant consuming 750 000 t biomass per year. Hydrogen input is assessed at 6%weight of the biomass, i.e. 48 000 tH2/year. The BtL plant is operated during 8000 h/year.
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Hydrogen production cost (€/kg)
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6,3 5,8 5,3 4,8 4,3 3,8 3,3 2,8 0
50 100 150 Electricity threshold price (€/MWh)
200
Fig. 6 e Optimum production cost in the reference scenario (no electricity market crisis, no additional degradation due to the transitions).
The hydrogen plant is operated 8760 h/year, either at the minimum or nominal load, in order to reach the optimum production cost (cf. x4.1 e maintenance needs can then be adjusted to the biofuel plant operation; i.e. electrolysers may be maintained when hydrogen is not needed by the biofuel plant, and especially since alkaline electrolysis maintenance needs are rather low). When hydrogen production is superior to the BtL plant needs, hydrogen is stocked. When it is inferior, hydrogen is taken out from the storage. Calculations were carried out to assess the storage needs in order to ensure a continuous hydrogen input for the BtL plant. Such storage is designed by a capacity of 5150 tH2. The evolution of the hydrogen stock in the storage along the year is presented in Fig. 7. Hydrogen is destocked during the winter, then stocked during spring and summer (lower electricity prices overall), and finally destocked in the autumn. The rapid increase of the hydrogen level at the end of the summer corresponds to the maintenance period of the BtL plant: no hydrogen is consumed then. From an economic standpoint, it appeared that compressed cylinders are far too expensive. Indeed, they would represent a capital cost higher than the plant’s, associated to a lifespan of
only 10 years. As it was already stated by Amos (1998) [17] quoted by Clausen et al. (2010) [2], if the storage requirement exceeds 1300 kg of hydrogen, underground storage should be considered. Indeed such a storage means could appear reasonable. However, the gain demonstrated today by alkaline electrolysis discontinuous operation would be offset by a storage cost of 30 M€.
4.3. R&D needs to improve the discontinuous operation economic performances As it was stated earlier, electricity markets fluctuate on an hourly basis. No significant profit would be made by trying to decrease the electrolysers’ transition time from their minimum load to nominal operation. However, we assessed the gain that would be obtained if the minimum load could be reduced down to 10%. In such a case, the optimum production cost would only be decreased by 0.3%. As a matter of fact, reducing the minimum load enables reducing the electricity consumption when electricity is cheap. Therefore, the possible gain is small.
Fig. 7 e Evolution of the storage during a year.
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We also examined the impact of the additional degradation due to discontinuous operation. Three scenarios have been considered from 1 mV/transition up to 10 mV (cf. x3.2). When this additional degradation is taken into account, the optimum production cost diminishes since the electricity consumption rises. Therefore the possible profit with regard to continuous operation decreases down to 2.4% for a degradation of 10 mV/transition (continuous operation means no transition; therefore the production cost does not vary). Discontinuous operation studies should focus on additional degradation in order to establish the real attainable profit. Furthermore, discontinuous operation could induce higher maintenance needs. The gap between the optimum production cost and the continuous operation one would be offset if additional maintenance was superior to 3.7 M€/year, i.e. 2% of the equipment capital cost. Finally, let us recall that discontinuous operation is all the more profitable that capital costs are low. Such an operating mode would greatly benefit from a reduction of electrolyser prices, since the plant remains quite capital-extensive.
5.
Conclusion and outlook
The present study assessed the possible reduction of the hydrogen production cost when operating alkaline electrolysers in a discontinuous way. Beside the insights about the electricity market, advances in modelling the discontinuous operation have been proposed. Overall, the profit that can be expected from discontinuous operation amounts to roughly 4% of the hydrogen production cost, when compared to a plant that would work continuously with electricity supplied from the same market, i.e. the EPEX Spot. When crises occur in the energy market, discontinuous operation is all the more relevant since it enables avoiding very high prices. In order to make such an operating mode profitable, specific attention should be given to eventual related overcosts: additional degradation due to frequent transitions from the minimum electrolyser load to the nominal one, higher maintenance needs, and induced storage costs. Such an operating mode would also greatly benefit from a reduction of electrolyser prices. On the other hand, the state-of-the-art as regards the electrolyser minimum loads and transition time appears satisfactory. Finally, other business models could be considered. Alkaline electrolysis could enter energy producers’ portfolios, not as a means to produce energy, but as one to reduce the consumption when needed. Besides, if such an operating mode was to gain in importance, the impact of variable
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electricity demand on the electricity market prices should be investigated.
references
[1] Seiler JM, Hohwiller C, Imbach J, Luciani JF. Technical and economical evaluation of enhanced biomass to liquid fuel processes. Energy 2010;35(9):3587e92. [2] Clausen LR, Houbak N, Elmegaard B. Technoeconomic analysis of a methanol plant based on gasification of biomass and electrolysis of water. Energy 2010;35:2338e47. [3] Gutie´rrez-Martı´n F, Confente D, Guerra I. Management of variable electricity loads in wind e hydrogen systems: the case of a Spanish wind farm. Int J Hydrogen Energy 2010;35: 7329e36. [4] Marcelo D, Dell’Era A. Economical electrolyser solution. Int J Hydrogen Energy 2008;33:3041e4. [5] Ursu´a A, Marroyo L, Gubı´a E, Gandı´a LM, Die´guez PM, Sanchis P. Influence of the power supply on the energy efficiency of an alkaline water electrolyser. Int J Hydrogen Energy 2009;34:3221e33. [6] Nikolic VM, Tasic GS, Maksic AD, Saponjic DP, Miulovic SM, Marceta Kaninski MP. Raising efficiency of hydrogen generation from alkaline water electrolysis e energy saving. Int J Hydrogen Energy 2010;35:12369e73. [7] Naterer GF, Fowler M, Cotton J, Gabriel K. Synergistic roles of off-peak electrolysis and thermochemical production of hydrogen from nuclear energy in Canada. Int J Hydrogen Energy 2008;33:6849e57. [8] Jørgensen C, Ropenus S. Production price of hydrogen from grid connected electrolysis in a power market with high wind penetration. Int J Hydrogen Energy 2008;33:5335e44. [9] Gutie´rrez-Martı´n F, Garcı´a-De Marı´a JM, Baı¨ri A, Laraqi N. Management strategies for surplus electricity loads using electrolytic hydrogen. Int J Hydrogen Energy 2009;34: 8468e75. [10] Floch P-H, Gabriel S, Mansilla C, Werkoff F. On the production of hydrogen via alkaline electrolysis during offpeak periods. Int J Hydrogen Energy 2007;32:4641e7. [11] EPEX Spot website, http://www.epexspot.com/; [accessed 10. 05.10]. [12] EEX website, http://www.eex.com/; [accessed 10.05.10]. [13] Krieg D, Stolten D. Alkaline electrolysis e a review. In: 18th world hydrogen energy conference, WHEC 18, Essen, Germany; 2010-05-17e2010-05-20. [14] Ki DTU, Risoe DTU, DONG energy, pre-investigation of water electrolysis. Available online, <130.226.56.153/rispubl/NEI/ NEI-DK-5057.pdf>; [accessed 10.05.10]. [15] IHT website, http://iht.ch/; [accessed 10.05.10]. [16] Python programming language e official website, http:// www.pyhton.org/; [accessed 10.05.10]. [17] Amos WA. Costs of storing and transporting hydrogen, report. Golden, CO: National Renewable Energy Laboratory. Available from, http://www.hydrogen.energy.gov/analysis_ repository/project.cfm/PID¼114; 1998.