Abstract
This paper studies four hybrid systems that couple a reference ebiomass and photovoltaice power plant with four different structures of a steam electrolysis system for hydrogen production. The four hybrid plants are initially examined incorporating the same capacity components as the reference plant. The integration of different structures of the electrolysis process results in operational penalties when compared to the reference plant, due to added irreversibilities intrinsic to the electrolysis process and the reduction of the biomass plant efficiency from the extraction of low-pressure steam used to evaporate the electrolyzer feed water. The magnitude of these penalties depends on the power consumption of the electrolysis system, thermal demand and/or pressure losses within incorporated plant components. Among the alternative scenarios, the maximum efficiency is achieved when the electrolysis system works with a recycling sweep gas stream further used in the boiler of the biomass power plant. Furthermore, the efficiencies of the electrolysis hybrid plants only surpass that of the reference power plant when the solar irradiation drops to 36e46%. This is a direct result of the lower operational efficiency of the solar panels versus the biomass plant.
Introduction
The combination of biomass with PV (photovoltaics) can address problems that characterize the two technologies when they are used separately, such as daily and seasonal fluctuations of photovoltaic production and quality/quantity inconsistencies of biomass stock [1, 2] . Advantages of using biomass include, among others, its relatively lower environmental impact and its contribution towards independence from fossil fuels [3] . PV installations operate without atmospheric emissions and are relatively easy to maintain [2] . Furthermore, hydrogen generation and storage options offer a possibility to further regulate the fluctuating output of renewable energy (e.g., [2] ).
Water electrolysis is a power-driven process for generating hydrogen through the electrical decomposition of water into hydrogen and oxygen. It can be easily combined with infrastructure based on renewable energy sources. Hydrogen production systems using electricity generated in solar, wind, biomass and other energy conversion systems have been published in numerous studies (e.g., [4e10] ). In addition, since 1991 several power-to-gas pilot plants have been constructed for producing hydrogen from renewable energy sources [11] . Most of these plants use alkaline or proton electrolyte membrane electrolyzers for hydrogen generation due to the maturity of this technology. However, the reported power consumption of water electrolysis is still above 4.5 kWh el /Nm 3 of hydrogen [12] . The water split reaction can be described by the Gibbs function, DG ¼ DH À T$DS, where DH is the overall energy needed, DG is the electrical energy and T$DS is the direct heat. High-temperature SOEC (solid oxide electrolysis cells) systems (HTSE or HT-SOEC) in the range of 800e1000 C have been studied since the 80s in order to reduce the electrical requirements of the electrolysis process (see Fig. 1 ) (e.g. [12, 14, 15] ). Operation at high temperature reduces overpotentials and improves the activity of electrodes. As a result, the electricity demand can be reduced to 3.3 kWh el /Nm 3 of hydrogen (at 1000 C). In addition, higher current densities can be achieved, improving the overall efficiency of hydrogen production and reducing the overall cost and size of the electrolyzer for a given production [12] . The cost can be further reduced if the heat requirement is covered by an external waste heat source [12, 16] . However, although high operating temperatures reduce the electrical requirement, they enhance chemical species evaporation and diffusion that reduce the performance and lifetime of the electrolyzer and decrease the mechanical stability of ceramic and metal components [17, 18] . HT-SOEC shortcomings are addressed by intermediate-temperature SOEC (ITSE or IT-SOEC) [18] . IT-SOEC follow the trend of solid-oxide fuel cell (SOFC) technology that aims to reduce the operational temperature to 600e700 C in order to decrease the equipment cost and increase the electrolyzer lifetime, while maintaining satisfactory performance levels [17, 18] .
Materials and methods
This study evaluates the incorporation of different configurations of an IT-SOEC system into a reference PV-biomass power plant using exergetic analysis (e.g. [20, 21] ). The electricity of the PV panels is used directly in the electrolyzer for the generation of hydrogen, while the electricity generated in the biomass plant is used in the electrolyzer or delivered to the electrical grid, depending on the scenario examined. This work also evaluates the influence of PV capacity on the overall plant efficiency when keeping the capacities of the biomass plant and the electrolyzer constant. Thus, scenarios with acceptable PV-biomass ratios of electricity supply to the electrolyzer, i.e., with better performance than the reference plant, are revealed. This work is based on the FP7 project ADEL that targeted the development of cost-competitive, energy efficient and sustainable hydrogen production based on renewable energy sources [13, 19] .
For the purpose of the presented work, the developed plants are planned to be located in an area with various chemical installations that rely on hydrogen availability. It is assumed that the hydrogen produced will be supplied to hydrotreatment processes of the refinery "La R abida" constructed by the company CEPSA and located on the west side of the province of Huelva in Spain [22] . Moreover, Huelva has significant biomass resources, produced both as a crop and as vegetable-matter wastes and by-products. The importance of biomass plants in the region is also proven by the fact that ENCE, Spain's leading company in biomass-fueled renewable energy generation, already operates a large biomass power plant in the area with an electricity production of 68 MW el [23] .
The simulations are carried out at steady state conditions and they are performed using commercial software (EBSILONProfessional, [24] ).
Reference PV-biomass power plant
The reference power plant is a hybrid structure of a biomass plant with PV panels that generates 6.8 MW el . The PV array generating 2.5 MW el is based on the BP model 4180T and operates with an efficiency of 14.4%. The biomass power plant is a conventional steam power plant with direct combustion of biomass generating 4.3 net MW el . The configuration of the reference power plant can be seen in Fig. 2 .
The biomass plant uses 26.7 kton/year of hybrid poplar wood chips with a weight composition (dry) of 50.2% C, 6.06% H, 40.4% O, 0.6% N, 0.02% S, 0.01% Cl and 2.7% ash [25] . The biomass (Stream 2 of Fig. 2 ) is combusted with air in the boiler of the plant, providing thermal energy to convert water to superheated steam. The combustor of the biomass is assumed to be a second generation circulating fluidized bed boiler [26] .
Steam generated at 80 bar and 550 C (Stream 6) is expanded in the 3-pressure level steam turbine (ST) of the plant. One reheat stage is included before the intermediate-pressure steam turbine in order to increase the power output and the efficiency of the plant. At the last level of the steam turbine, the steam is expanded to 0.05 bar and it is led to the condenser (COND) of the plant (Stream 12). The saturated stream exiting the condenser (Stream 13) is passed through pumps and feedwater heaters entering the boiler at a temperature of 230 C (Stream 5).
Electrolyzer unit
The electrolyzer has been incorporated in all four hybrid scenarios examined; Its nominal power consumption is 500 kW el and it consists of 110 parallel stacks, each one of which requires 4.5 kW. Defined and agreed upon among partners of the FP7 project ADEL [13] , it is assumed that the electrolysis cells are working at thermoneutral voltage, at a temperature of 700 C, with a steam conversion rate in the cathode chamber of 61%, and a molar ratio between the anode and cathode of 1:1. Under these operational conditions, the overall energy efficiency of the electrolyzer, defined as the ratio between the lower heating value of the generated hydrogen and the stack power needed to perform this task, is 97.2%.
Initial layout of the electrolysis system
In this study, a 2.5 MW el electrolysis (IT-SOEC) system composed of 5500 kW-electrolyzers is simulated. This IT-SOEC system has been examined in four configurations (four scenarios). The operating conditions (Table 1) , including pressure losses, efficiencies, minimum temperature differences, etc. of the electrolysis system are based on assumptions agreed upon by partners of the FP7 project ADEL [13] .
The initial layout of the IT-SOEC system consists of two loops (Fig. 3) . The first loop, which starts with Stream 46, provides the water entering the cathode side of the electrolysis process. In the second loop, atmospheric air (Stream 30) is used as sweep gas to remove the produced oxygen from the anode side of the electrolysis system.
In the first loop, the make-up water is pumped from 1.01 to 1.09 bar to overcome the pressure drops within the following components. The water is heated in shell-and-tube heat exchangers HX1 and HX2, the latter of which uses as thermal source steam extracted from the Rankine cycle of the biomass power plant. Mixer M2 mixes the make-up steam with Stream 42 (hydrogen-rich gas) to obtain a cathode inlet mixture of 10% v/v hydrogen and 90% v/v steam (Steam 43). Heat exchanger HX3 (shell-and-tube) raises the temperature of the inlet steam/hydrogen mixture using as thermal source the exhaust cathode stream of the electrolysis system. Finally, the high-temperature electric heater EH1 is used to increase the temperature of the stream to the design operational temperature of the stacks (700 C, Stream 45). EH1 is used to ensure adequate control of the inlet temperature of the stack given all operating conditions. In the electrolysis system, 61% of the incoming H 2 O is split into H 2 5.00
The sweep-gas loop entering the anode is atmospheric air (Stream 30, molar composition 79.0% N 2 and 21.0% O 2 ). The blower COMP2 increases the pressure of the air sweep gas in order to overcome the pressure drops within the process. The air is filtered to remove suspended particles, and heated up by the exhaust anode stream (Stream 35) in HX5. A last electric heater, EH2, is placed before the inlet of the IT-SOEC in order for the incoming stream to reach the required operational temperature.
Because the quality of the water used in the electrolyzer must be high, and since steam used in steam cycles may present traces of harmful compounds, the water of the electrolysis system is taken from an external source. In the simulations presented here, low-pressure steam extracted from the biomass power lgriffin@example.org bar and 238 C (Stream 28) is used indirectly to generate the required steam from a liquid clean water source. The condensed stream is finally returned to the biomass power plant (Stream 29). Lastly, the electricity of the incorporated electric heat exchangers is supplied by the biomass power plant.
The hybridization scenarios
The first hybridization study considers the incorporation of the initial layout of the IT-SOEC system as described previously (Scenario 1, Fig. 4 ). In the second scenario the exhaust air of the electrolysis system (Stream 36 of Fig. 5 ) is recirculated to the biomass combustor to further exploit its high temperature and oxygen content (Scenario 2, Fig. 5 ). Scenario 3 examines the operation of the electrolysis system without sweep gas eliminating the need of an air compressor (Scenario 3, Fig. 6 ). In Scenario 4 the electric heaters of the electrolysis system (EH1 and EH2) are replaced by gas/gas heat exchangers (HX6 and HX7) that use the exhaust gases of the combustion chamber as thermal input (Scenario 4, Fig. 7 ).
In all of the simulations, the IT-SOEC system is provided with constant electricity supply (2.5 MW el ) and steam flow, assuring, in this way, constant hydrogen generation. The differences in electricity production among the scenarios stem mainly from differences in pressure losses and stream recirculation, which causes variations in temperature, combustion conditions and stream mixing. In the hybridization scenarios, the electricity required by the electrolysis system is covered fully by PV panels (2.5 MW el ), while the net electricity generated in the biomass plant (4.3 MW el ) is sent to the electrical grid.
Sensitivity analysis of PV capacity
In order to examine the influence of variations of the PV output (due to transient conditions, e.g., passing clouds, sunrise and sunset) on the overall performance of the hybridization scenarios, different PV-biomass electricity supply ratios to the electrolysis system are investigated. This is achieved by assuming and testing various PV outputs from 0 to 100% of the capacity of the electrolysis system (2.5 MW el ). In every case, the electricity generated in the PV panels is delivered to the electrolysis system exclusively. The biomass plant supplements the power input of the electrolysis system in order to reach the necessary 2.5 MW el , and delivers the remaining power (up to 4.3 MW el ) to the electrical grid. For example, a 40/60% PV-biomass ratio implies that 40% of the electricity required in the electrolysis system is generated by the PV panels and the remaining 60% by biomass combustion. In the mentioned example, this would mean that the capacity of the PV panels, and thus the electricity supplied to the electrolysis system using PV, would be (0.4 Â 2.5 MW el ) 1 MW el . The biomass plant would supply the remaining required (2.5e1 MW el ) 1.5 MW el and would deliver the remaining power (4.3e1.5 MW el ) 2.8 MW el to the electrical grid.
Exergetic analysis
The evaluation and comparison of the plants simulated and presented in this paper are realized using exergetic analysis. Energy can only be converted from one energy form to another, it is conserved and not destroyed. Exergy, on the other hand, is defined as the maximum useful work during a process to bring a system into equilibrium with its environment [27] . The exergy of a stream depends on its state, and on the state of the environment; thus exergy destruction and exergy losses along a process can be quantified as functions of the quality of material streams. For this reason, exergy analysis is a useful tool for identifying irreversibilities within components and systems, providing information for improving the thermodynamic performance of thermal processes [27] .
The total exergy of a system results from the addition of physical exergy, kinetic exergy, potential exergy, and chemical exergy. In the present work kinetic and potential exergy terms are neglected. The chemical exergy of streams is calculated based on tabulated standard chemical exergies given by J. Szargut at standard conditions (T 0 ¼ 298.15 K, P 0 ¼ 1.013 bar) [28] .
Defining the exergy of product, E P,k , and the exergy of fuel, E F,k , of a component k, we calculate its exergetic efficiency as ε k ¼ E P;k E F;k .
An exergy balance of a component quantifies the thermodynamic irreversibilities, i.e., the exergy destruction within the component: The overall exergetic efficiency of the reference PV-biomass power plant is calculated as:
where BP: biomass plant, B: biomass, elec: electricity, e: specific exergy, m: mass flow, ε: exergetic efficiency, P: power and ref:
reference.
In the case of the IT-SOEC hybrid plants, the exergy of the product (the numerator of the exergetic efficiency equation) of the reference plant will change for two reasons: (1) hydrogen is generated in the introduced electrolyzer unit and (2) the electricity of the PV is used entirely as power input for the electrolyzer unit. When the PV output cannot fully cover the power requirement of the electrolyzer (due to low levels of irradiation), the additional power needed will be provided by the biomass plant (x BP ). Thus, the exergetic efficiency of the hybrid plants incorporating the electrolysis system is defined as:
! where x BP : the fraction of the electrical consumption of the electrolysis system supplied by the biomass plant.
Results and discussion
The results of the exergetic analysis at the component level for the reference power plant and the four hybridization scenarios can be seen in Tables 2e6. Results at the stream-level can be found in the Appendix of the paper. The exergy of the biomass stream has been calculated following the known procedure for solid fuels as mentioned in Ref. [27] . The solar exergy can be calculated using an energy/exergy ratio as presented in Ref. [29] that results in values very similar to its energy values. Here it was assumed that the exergy and energy values of solar energy are equal.
From the obtained results, it can be seen that the efficiencies of components with similar operation are comparable in all of the plants. Thus, any calculated changes in the overall efficiencies of the plants are associated with structural variations. It can be seen that the exergetic efficiency of some components is relatively low, when compared to similar components reported in literature [27] . For example, the air compressors operate with exergetic efficiencies of about 35%, while the electric heaters in scenarios 1, 2 and 3 operate with exergetic efficiencies close to 66e67%. The calculated irreversibilities of the components are closely related with the defined thermodynamic characteristics, i.e., isentropic efficiencies, pressure drops, temperature differences, etc., presented in Table 1 . Thus, inefficiencies could be reduced through improved designs and the incorporation of components with better operational characteristics and lower thermodynamic inefficiencies.
The efficiency of the reference PV-biomass plant is found to be 19.3% based on dry biomass (Table 2) . When comparing the hybridization scenarios with the reference PV-biomass power plant, it can be seen that the overall efficiency decreases by 6.3e7.5%, depending on the structure of the IT-SOEC system. The reduction of the efficiency depends on two phenomena:
-With the integration of the IT-SOEC system, a small fraction of low-pressure steam is extracted from the steam turbine reducing the efficiency of the biomass power plant. In order to maintain the power output of the biomass power plant at the reference value (4.3 MW el ), the mass flow of the fuel has to be increased. This increases the exergy of the fuel of the system, which in turn is associated with a CO 2 increase of 15.9e16.0 ktons/year (assuming an annual operation of 7446 h).
-The electricity produced by the PV panels is now used in the integrated electrolysis process to generate hydrogen. The electrolysis process introduces new irreversibilities in the plants, reducing the exergy of the product of the overall system. Nevertheless, this is partially offset by the additional exergy gained from the generated hydrogen (Tables 3e6). As mentioned, in Scenario 3 ( Table 5 ) the sweep gas of the electrolysis system is eliminated in an attempt to decrease the power consumption of the overall structure. When comparing Scenario 1 (Table 3) , that incorporates the initial layout of the IT-SOEC system, with Scenario 3, it can be seen that indeed the elimination of the sweep gas decreases the power consumption due to the elimination of the compressor covering the pressure losses within the filter, the heat exchangers and the electrolysis system, increasing the overall efficiency of the plant. Nevertheless, if, as in Scenario 2, the sweep gas is further recycled to the boiler of the biomass power plant (Table 4) , the efficiency of the plant increases, surpassing that of Scenario 3. This is true because the benefit of the high-temperature oxygen-rich sweep gas in the biomass boiler is more significant than the decrease of the electricity consumption achieved in Scenario 3. In Scenario 4 (Table 6 ), the two electric heaters (EH1 and EH2) are replaced by gas/gas heat exchangers (HX6 and HX7) in an attempt to avoid the electricity consumption within the electrically driven heat exchangers. However, although the electricity required in the electric heaters is avoided and the efficiency of the plant is higher than for Scenario 1, the pressure losses of the flue-gas path are increased, increasing the requirements of the air compressor of the biomass boiler. This eventually leads to an overall decrease in the efficiency of the plant, when compared with Scenario 2.
The results show that the overall efficiencies of the four examined scenarios are lower when PV panels are at full load, than during low irradiation periods (when part of the electricity needed in the electrolysis system is covered by the biomass plant). During low irradiation periods, the exergy of the product of the plant (electricity available to the grid) is reduced. However, these low irradiation levels strongly reduce the exergy of the fuel of the plant (thermal requirement), due to the low efficiency of the PV panels. The combination of these two effects causes an overall efficiency increase when irradiation levels decrease.
The performed sensitivity analysis shows that each scenario reaches the efficiency of the reference PV-biomass plant at different PV/biomass ratios. Specifically, it is found that Scenarios 3 and 4 result in efficiency higher than the reference PV-biomass power plant with a PV/biomass ratio of 39/61%, i.e., maximum PV capacity of (0.39 Â 2.5 MW el ) 0.975 MW el . Scenario 1 requires a PV/biomass ratio of 36/64%, i.e., maximum PV capacity of (0.36 Â 2.5 MW el ) 0.9 MW el . Lastly, Scenario 2 can supply up to 46% of the electricity needs of the electrolysis system due to the Overall efficiency: 17.92%.
Table 6
Component-level results of the exergetic analysis of Scenario 4 (replacement of the electric heat exchangers with gas/gas heat exchangers). relatively higher efficiency of the plant. This is translated to a maximum PV capacity of 1.15 MW el . The above-mentioned results are shown in Fig. 8 .
Conclusions
From the realized simulations we found that coupling an intermediate temperature solid oxide electrolysis system with a reference photovoltaic-biomass power plant results in a reduction of the overall efficiency of 6.3e7.5% relative to the reference case. This reduction leads to (i) the transformation of electricity into hydrogen, which increases the irreversibilities throughout the process; and (ii) the reduction of the efficiency of the biomass plant due to the extraction of low-pressure steam used to evaporate the feed water of the electrolyzer. The efficiency reduction in the biomass plant leads to an increase in CO 2 generation of 15.9e16.0 ktons/year (assuming annual operation of 7446 h) when the power output to the grid of the biomass plant is kept constant (4.3 MW e ).
Comparing four integration schemes, we found that the incorporation of the intermediate-temperature electrolysis system with air recirculation (Scenario 2) performs with the highest efficiency. Although using a sweep gas is associated with additional power requirements, when recycled, the sweep gas has a positive effect on the combustion process of the biomass plant and, consequently, on the overall structure. This is mainly due to the high temperature of the recycled air and its high oxygen content. In addition, eliminating the electric heaters decreases the exergetic penalty of the processes. However, the pressure drop assumed within the gas/gas heat exchangers increases the power requirement of the compressors resulting in a lower efficiency than Scenario 2. If the pressure drop in these components was lower, the energy requirement of the main air compressor of this power plant structure would be reduced and its overall efficiency would increase. It should be mentioned that electric heaters have advantages over gas/gas heat exchangers, such as that they offer precise temperature control and low thermal inertia that result in fast transition processes, useful when structures of high complexity are considered.
Lastly, when solar irradiation is below its reference value, the thermal input (exergy of fuel) decreases faster than the net power output of the biomass plant to the grid (exergy of product). This is due to the lower efficiency of the PV panels in comparison to the biomass plant and results in an increase in the exergetic efficiency of the overall process. Specifically, it was found that when the solar irradiation is 36e46%, the efficiencies of the electrolysis hybrid plants are higher that of the reference PV-biomass power plant.
Acknowledgments
The authors would like to acknowledge the project ADEL cofunded by of the 7th Framework Programme of the European Union (FP7) and the Fuel Cells and Hydrogen Joint Undertaking (FCH-JU).
Fontina Petrakopoulou would like to thank the Universidad Carlos III de Madrid, the European Union's Seventh Framework Programme for research, technological development and demonstration (grant agreements n 600371 and 332028), the Ministerio de Economía y Competitividad (COFUND2014-51509) and Banco Santander. 

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