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Article as a Long-Term Large-Scale Storage to Support Renewables

Subodh Kharel and Bahman Shabani * School of Engineering, RMIT University, Melbourne 3083, Australia; [email protected] * Correspondence: [email protected]; Tel.: +61-3-9925-4353

 Received: 21 September 2018; Accepted: 16 October 2018; Published: 19 October 2018 

Abstract: This paper presents a case study of using hydrogen for large-scale long-term storage application to support the current generation mix of South Australia state in Australia, which primarily includes gas, wind and solar. For this purpose two cases of battery and hybrid battery- systems to support solar and wind energy inputs were compared from a techno-economical point of view. Hybrid battery-hydrogen storage system was found to be more cost competitive with unit cost of electricity at $0.626/kWh (US dollar) compared to battery-only energy storage systems with a $2.68/kWh unit cost of electricity. This research also found that the excess stored hydrogen can be further utilised to generate extra electricity. Further utilisation of generated electricity can be incorporated to meet the load demand by either decreasing the supply from gas in the present scenario or exporting it to neighbouring states to enhance economic viability of the system. The use of excess stored hydrogen to generate extra electricity further reduced the cost to $0.494/kWh.

Keywords: ; solar and wind; battery storage; hydrogen-based energy storage; hybrid energy systems; South Australia

1. Introduction In efforts towards reducing the emissions and mitigating the impact of , countries around the globe have started to shift towards energy production through renewables. Energy generation from renewable sources such as wind and solar can help mitigate emissions produced through using traditional fossil -based energy generation. However, renewable energy generation is highly weather-dependent and lacks the possibility of achieving a continuous supply on their own due to their inherent intermittency [1]. This can to issues, such as energy curtailment and wastage of energy and, hence, mismatching between supply and demand. While employing multiple generation systems as backups or demand management measures initiated by the end users could be the possible options, employing an optimally designed and sized energy storage system can be the most viable and sustainable solution [2]. Energy storage can even be also employed in gird connected area (with or without renewables) for peak shaving or fill the reliability gap of the grids where reliability is of significant importance [3–5]. One of the to store excess energy produced by renewable energy sources is generation and storage of hydrogen. This system includes the major components of an electrolyser, hydrogen storage tank, and a system. The excess from the renewable sources (i.e., solar and wind) is directed towards an electrolyser to generate hydrogen by electrolysing into hydrogen and . The hydrogen is stored in the storage tank and when the renewable sources fall short in meeting the demand, the fuel cell draws on this stored hydrogen and generates electricity (usually by taking oxygen from air) [6,7].

Energies 2018, 11, 2825; doi:10.3390/en11102825 www.mdpi.com/journal/energies Energies 2018, 11, 2825x FOR PEER REVIEW 22 of of 17

Hydrogen-based energy storage has high rates (i.e., in the range of 10 MW) and is suitableHydrogen-based for meeting long-term energy storage (seasonal) has high energy power storage rates (i.e.,requirements in the range [8]. of Pumped 10 MW) andhydroelectric is suitable storagefor meeting system long-term (PHS) has (seasonal) limited energyscope for storage further requirements development [8 in]. long-term Pumped hydroelectric storage applications storage duesystem to its (PHS) specific has geographical limited scope requirements for further and development possible environmental in long-term impacts storage applications[9]. Batteries that due canto its be specific considered geographical as another requirements energy storage and soluti possibleon are environmental efficient as impactsshort-term [9]. energy Batteries storage that solutions;can be considered however, asdue another to their energy uncertain storage lifetime, solution and are quick efficient degradation as short-term (i.e., due energy to charging storage cycles),solutions; sensitivity however, to due environmental to their uncertain conditions lifetime, (e.g., and temperature), quick degradation self-discharge, (i.e., due and to charging limited storagecycles), sensitivitycapacity (per to environmentalunit and conditions ), batteries (e.g., temperature), are not perfect self-discharge, candidates and for limitedlong-term storage and bulkcapacity energy (per storage unit volume applications and mass), [10–13]. batteries are not perfect candidates for long-term and bulk energy storageThis applications paper investigates [10–13]. the scope of application of hydrogen as a long-term large-scale energy storageThis solution paper investigatesthrough a case the st scopeudy for of state application of South of Australia hydrogen (SA) as ain long-term Australia large-scalefor hybrid energy inputstorage from solution solar throughphotovoltaic a case (PV) study and for wind state turb of Southines to Australia meet the (SA) state’s in Australia load demand. for hybrid The energy energy generationinput from mix solar has photovoltaic evolved in (PV)SA with and the wind increase in topenetration meet the state’sof renewable load demand. energy. TheAt the energy end ofgeneration 2016, registered mix has local evolved generation in SA withcomprised the increase of 48.4% in penetrationrenewable energy of renewable generation energy. with At wind the and end solarof 2016, energy registered contributing local generation 39.2% and comprised 9.2% respective of 48.4%ly renewable[14]. Moreover, energy the generation total of 1515 with MW wind solar and andsolar 3178 energy MW contributing of additional 39.2% wind and generation 9.2% respectively on existing [14]. Moreover,1698 MW thewas total either of 1515committed MW solar or proposedand 3178 MWas of of1 July additional 2017 [14]. wind In generation2016–2017, onSA’s existing electricity 1698 generation MW was either capacity committed from gas or was proposed 49.1% ofas oftotal 1 Julycapacity, 2017 [14with]. In the 2016–2017, increase in SA’s solar electricity and wind generation capacity capacity such that from the gascombined was 49.1% generation of total capacitycapacity, from with these the increase sources inwould solar reach and wind 58% while capacity gas suchwould that contribute the combined 35% in generation total capacity capacity [14]. Hence,from these as sourcessynchronous would generation reach 58% whilecapacity gas dec wouldreases contribute and intermittent 35% in total renewable capacity [14generation]. Hence, increases,as synchronous more storage generation capacity capacity will decreases be required and to intermittent maintain the renewable stability of generation the grid. increases,Figure 1 shows more thestorage map capacity of SA. The will total be required renewable to maintain energy thegenerati stabilityng capacity of the grid. as per Figure the1 targetshows set the by map the of state SA. governmentThe total renewable would reach energy to generating1515 MW of capacity solar ener asgy per and the 4876 target MW set of by wind the state energy. government However, would apart fromreach 100 to 1515 MW MWof Tesla’s of solar battery energy storage and 4876 installati MWon, of wind there energy. are no However,significant apartstorage from options 100 MW being of investedTesla’s battery for the storage state. installation,Hence, lack there of a areconcrete no significant energy storagestorage optionstarget and being increase invested in for renewable the state. energyHence, generation lack of a concrete target can energy result storage in instability target andin the increase grid and in power renewable cuts. energyThis case generation study will target help understandcan result in the instability storage in capacity the grid and and design power cuts.requir Thisements case to study support will helpthe SA’s understand renewable the storageenergy targetcapacity (RET). and design requirements to support the SA’s renewable energy target (RET).

Figure 1. Map of South Australia highlighted. SA SA has has an area of 983,482 square km. and monthly average loadload demanddemand is is 1,321,549 1,321,549 kWh kWh in in which which 560,600 560,600 is the is basethe base load load supplied supplied by gas by or gas or power coal powerplants andplants monthly and monthly average averag renewablee renewable supply supply is 760,499 is 760,499 kWh. kWh.

Energies 2018, 11, 2825 3 of 17

Limited studies are available in the literature on real world application of hydrogen as a large-scale energy storage solution in multiple renewable energy input scenarios. Most of the studies available in the literature focus on either small scale or medium size system (e.g., households or small communities). Zoulias et al. [15] in 2007 carried out a techno-economic analysis on a hydrogen-based energy storage system to replace batteries and diesel generator of an existing PV/diesel generator stand-alone system. The investigation was done to provide electricity to a remote community comprising of ten households. Giannakoudis et al. [16] in 2009 evaluated the optimum system design using hybrid inputs from solar and wind with hydrogen to store energy. However, in this case, arbitrarily load demand profiles were used. Recent review by Bahramara et al. [17] in 2016 has presented the list of research studies conducted on a system with hybrid energy inputs and hydrogen for storing energy. Other studies have reported in the literature on the viability of hydrogen-based energy storage solutions for supporting in small-scale cases such as, telecommunication applications [2,18], stand-alone-households (e.g., [19,20]), rural villages (e.g., [21,22]), small communities and universities (e.g., [23,24]), islands (e.g., [25,26]), etc. There are also some limited number qualitative studies reported in the literature that discuss the applications of hydrogen for energy storage in large-scale long-terms energy scenarios with multiple renewable energy inputs [27–29]. However, quantitative analyses on real cases are required to confirm the findings suggested by these studies. This paper presents a case study to investigate the techno-economics of using a hydrogen-based energy storage arrangement with a hybrid solar PV and wind input to provide a reliable to SA. The generation capacity considered in this study is based on most recent RET set by the SA state government in 2017. The aim of the study is to present an optimum system design using HOMER simulation software (Golden, CO, USA) (i.e., the Hybrid Optimisation Model for Electric Renewables) for above-mentioned scenario and compare the techno-economic performance of the systems with scenarios in which batteries on their own and in conjunction with hydrogen are used for energy storage. The results of this investigation will help provide direction on the feasibility of implementing battery and/or hydrogen-based energy storage technologies and help with making decision on the right option to be considered for implementation. Section2 of this paper will present an overview of previous studies on above-mentioned energy storage technologies used globally for large-scale/long-term needs with a particular focus on the use of hydrogen for energy storage in renewable energy generation systems. Section3 will describe the method used to conduct this study and Section4 will present the details of the case study such as inputs, assumptions, and energy modelling with analysis and discussion of these results. In Section5 the results are provided and discussed. This section also provides ideas on alternative scenarios for better utilisation of the selected energy storage solution. Finally, conclusions will be presented in Section6.

2. Previous Studies

2.1. Storage Overview There are a number of storage technology options available, which can be categorised into mechanical, thermal, electrical, and electrochemical and storage. Examples of technologies available for each category are as presented in [30], summarised in Table1. Energies 2018, 11, 2825 4 of 17

Table 1. Energy storage methods.

Mechanical Energy Electrical/Electrochemical Chemical Energy Storage Storage Storage Energy Storage Pumped hydro Hot water Super Hydrogen Molten Superconducting magnets Synthetic Other chemical compounds Phase-change material Batteries e.g., , Hydrogen-based energy storage

Pumped hydro storage (PHS) is the leading technology, accounting for 97% of world’s storage capacity, with a rated power of 159 GW, as it is cheaper and more technologically matured than any other storage methods available [31,32]. China has the largest rated power capacity of operational PHS plants with 32 GW, followed by Japan and USA, with 28.3 GW and 22.6 GW, respectively [33]. Compressed air energy storage (CAES) is another mechanical storage option with bulk energy storage capacity that stores air, compressed by electrical . However, PHS and CAES, both have major challenges for their implementation. PHS can be implemented in specific locations with required geographical conditions (i.e., proper elevation difference, enough water availability, near to transmission lines, etc.). Such limitations suggest challenges with widespread and flexible use of this storage solution [34]. CAES is dependent on geographical structure as well, which is either underground, in abandoned mines, rock structures, or above the ground in vessels. Such limitations can be part of the reason that there are only two plants using this technology being currently operational worldwide. Germany has 290 MW and USA in Alabama has 110 MW power capacity CAES plants [35,36]. Considering the above-mentioned limitations with PHS and CAES, and some advantages offered by other storage solutions (e.g., hydrogen-based systems), they have started to receive less attention in recent years. The increasing penetration of intermittent renewable energy sources, such as solar and wind, have prompted the need for development of battery-based energy storage technology. Global power output capacity at the end of third quarter of 2016 for electrochemical technology was 1.6 GW with the leading with 292 operational projects having combined capacity of 0.6 GW followed by South Korea and Japan with capacity of around 0.3 GW each [33]. As estimated by the International Renewable Energy Agency (IRENA), despite an increase in capacity of batteries from 360 MW to 14 GW in 2015, a total of 150 GW of battery storage would be required globally to meet the target of 45 per cent of energy from renewables by 2030 [37,38]. Power and energy ratings are inter-linked in batteries making them bulky and expensive for large-scale storage applications; moreover, issues such as self-discharge (when used for long-term energy storage) and their sensitivity to environmental condition exacerbate the situation in terms of their effectiveness to be used as long-term energy storage solution. On the other hand, along with having high gravimetric and no self-discharge problem, power and duration are set independently in cell systems. That makes hydrogen-based energy storage system one of the promising technologies favourable for long-term large-scale energy storage applications [6,39,40]. While receiving rapidly-growing attention, hydrogen-based energy storage solutions for such applications areas yet to be further developed. Only seven projects were reported to be operational globally by 2016, having a combined capacity of just over 7 MW [33]. According to a recent report published research in August 2017 [41], the hydrogen-based energy storage and technology market had reached $3.6 billion in 2016 and is expected to reach around $5.4 billion by 2021 (all prices are in USD). Energies 2018, 11, 2825 5 of 17

2.2. Hydrogen Storage System in a Renewable Energy Input System As briefly discussed limited data has been reported in the literature on using hydrogen-based storage system for large-scale hybrid renewable energy input scenarios; however, a number of examples for the application of this technology in smaller scale systems have been reported to date. One of the earliest studies on this was conducted by Khan et al. in 2004 [42] for a remote household having a consumption of 25 kWh/day with a peak power demand of 4.73 kW. The study considered various combinations of components such as a diesel generator, fuel cell, batteries, a , and solar PVs for system optimisation and concluded that the wind-diesel-battery system was the most cost-effective option for a remote house stand-alone system owing to the higher cost of PV arrays and fuel cells at the time. However, it was also presented that with the cost reduction of fuel cells to 65% of its market price at the time, wind-diesel-battery-fuel cell system would be more attractive, in which the size of the batteries could be significantly reduced while the hydrogen was the main player to take care of the energy needs of the household. The research also concluded that 85% reduction in fuel cell cost would completely replace the diesel generator making wind-fuel cell arrangement to be the most cost-effective option. This finding was in agreement with the results of the research published by Bezmalinovic et al. in 2012 [43] and later by Amutha et al. in 2014 [44]; both of which demonstrated that the use of diesel generators can be avoided but with increased cost of electricity, unless the cost of the fuel cell system is reduced. Hence, reduction in components of hydrogen storage was highlighted as a requirement for implementation of a cost-competitive hydrogen-based energy storage system integrated with a renewable . However, these studies also emphasised that the cost competitiveness of the hydrogen systems is more within reach if used in conjunction with battery storage. Research published by Ashourin et al. in 2012 [45] presented a feasibility analysis of PV-wind turbine systems along with battery and hydrogen storage for a small village in Tioman Island in Malaysia. The study demonstrated that solar-wind-battery system could offer the cheapest unit cost of electricity at $1.104/kWh. However, while the system provided power for most of the duration throughout the year, during some periods it could have reliability issues in terms of supplying enough power to meet the demands. The study then showed that introduction of fuel cell in the solar-wind-battery system resulted in a more stable power supply arrangement at a reasonable increase in unit cost, reaching $1.108/kWh. The advantage of achieving additional reliability by using a hybrid battery/hydrogen energy storage arrangement was also highlighted by the research conducted by Shabani et al. in 2015 for a remote telecommunication application [46]. This results also agreed with the findings of a more recent study by Das et al. [47] published in 2017 showing a slight increment in the cost of electricity with addition of a hydrogen-based energy storage unit from $0.323/ kWh to $0.355 kWh for a small island in East Malaysia. In this system wind turbines were not included as wind energy generation was not significant in the area, which is justified, as in another study by Ashourin et al. [45] had 9% of wind energy contribution for a similar location in Malaysia. The present paper is an attempt to address this evident gap in the literature in terms of published analyses and data on the application of hydrogen-based energy storage solutions in large-scale hybrid renewable energy scenarios. A case study of a hybrid generation system in a state with a vast area of 984,377 square kilometres was selected for this research, which can be compared to many countries of such sizes around the world. The importance of this research is more pronounced considering the complimentary nature of renewable energy inputs used (e.g., wind and solar), which suggests the energy storage system to be sized smaller than when only one of these inputs are used. Contrary to many other studies, real generation capacities (proposed by the state government) were used as inputs to our analysis, rather than considering arbitrary input data. Previous literature has concluded that, while it provides the stability in power supply, the inclusion of hydrogen-based energy storage solutions can increase the cost of electricity generated. Hence, possible options for enhancing the economic viability of hydrogen system by its better utilisation are discussed in this study. Energies 2018, 11, 2825 6 of 17

3. Method This paper presents the discussion on significance of deploying hydrogen storage as a long-term large-scale application through a case study for SA. South Australia has an area of 983,482 square kilometres. This is a vast area for a state and is even bigger in size than many countries such as Germany, France, Spain, and Italy. With such vast land area and highly variable seasonal renewable energy inputs from wind and , SA is an ideal place to practice and identify the prospects of large scale long-term energy storage solution. Map of SA is highlighted in Figure1. The case study includes the techno-economic feasibility analysis of hybrid renewable energy system with storage systems. Battery storage and battery-hydrogen hybrid storage system are compared to determine the optimum system design for such a large-scale application. The HOMER tool is used to optimise the system design based and identify the storage capacity required. HOMER ranks the suitability of the designs according to their net present cost (NPC) while analysing the system feasibility for a continuous year round operation [48]. Meteorological data, such as solar irradiance, wind speed, and temperature, are fed into the software to evaluate hourly energy generation Energies 2018, 11, x FOR PEER REVIEW 6 of 17 throughout the year. South AustraliaSouth hasAustralia set its REThas set mostly its RET based mostly on generation based on through generation wind through and solar wind energy. and Wind solar energy. turbines andWind solar turbines PVs are and considered solar PVs as electricityare considered generation as electricity components generation in HOMER. components Half-hourly in HOMER. electricalHalf-hourly load demand electrical was collected load demand from the was Australian collected Energy from Marketthe Australian Operator’s Energy (AEMO) Market data Operator’s dashboard(AEMO) for the yeardata 2017dashboard for each for month the year [49]. 2017 Before for creating each mo thenth load [49]. profile, Before it creating was assumed the load that profile, it the minimumwas loadassumed (base that load) the for minimum the entire load year was(base provided load) for by the electricity entire year generation was provided through by gas electricity and half-hourlygeneration load through was subtracted gas and fromhalf-hourly base load load for was each subtracted time step. from The base total load load for and each the time load step. The that will betotal met load by renewableand the load generation that will arebe met shown by renewable in Figure2 .generation are shown in Figure 2.

1600000 This part was used to size energy storage 1400000

1200000

1000000 Load met by renewables 800000

Demand (kWh) 600000

400000 Total Base load supply by gas 200000 Demand and coal fired power plants 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

Figure 2. Representation of load met by renewables that will be used to size energy storage. Figure 2. Representation of load met by renewables that will be used to size energy storage. The resulting load values were imported to HOMER to create a load profile assuming that the remaining load,The apart resulting from assumedload values base were load, imported would beto providedHOMER to by create solar PVsa load and profile wind assuming turbines. that the The seasonalremaining and daily load, load apart profiles from forassumed a day inbase January load, woul as and example be provided is shown by solar in Figures PVs and3 and wind4 turbines. respectively.The The seasonal seasonal and profile daily showsload profiles high load for demand a day in for January summer as monthsan example mainly is dueshown to electricityin Figures 3 and 4 demand forrespectively. cooling, and The winter seasonal months profile because shows of increased high load demand demand for for heating. summer April months and October mainly due to have relativelyelectricity lower demand load demands for cooling, as both and heatingwinter months and cooling because loads of areincreased at their demand minimum for levels heating. April during theseand months.October Thehave increase relatively in demand lower load around demands midnight as both is due heating to time and controlled cooling switching loads are at their of hot waterminimum systems, levels which during is scheduled these mont to seths. onThe at increase 11:30 p.m. in fordemand newly around installed midnight meters inis SA.due to time This is setcontrolled as to take benefitswitching of off-peakof hot water prices systems, and hence, which has causedis scheduled 250 MW to ofset time on stepat 11:30 increase p.m. in for newly demand [installed50]. meters in SA. This is set as to take benefit of off-peak prices and hence, has caused 250 MW of time step increase in demand [50].

800000 700000 600000 500000 400000 300000 200000

Average Load (kW) 100000 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

Figure 3. Seasonal in SA.

Energies 2018, 11, x FOR PEER REVIEW 6 of 17

South Australia has set its RET mostly based on generation through wind and solar energy. Wind turbines and solar PVs are considered as components in HOMER. Half-hourly electrical load demand was collected from the Australian Operator’s (AEMO) data dashboard for the year 2017 for each month [49]. Before creating the load profile, it was assumed that the minimum load (base load) for the entire year was provided by electricity generation through gas and half-hourly load was subtracted from base load for each time step. The total load and the load that will be met by renewable generation are shown in Figure 2.

1600000 This part was used to size energy storage 1400000

1200000

1000000 Load met by renewables 800000

Demand (kWh) 600000

400000 Total Base load supply by gas 200000 Demand and coal fired power plants 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

Figure 2. Representation of load met by renewables that will be used to size energy storage.

The resulting load values were imported to HOMER to create a load profile assuming that the remaining load, apart from assumed base load, would be provided by solar PVs and wind turbines. The seasonal and daily load profiles for a day in January as an example is shown in Figures 3 and 4 respectively. The seasonal profile shows high load demand for summer months mainly due to electricity demand for cooling, and winter months because of increased demand for heating. April and October have relatively lower load demands as both heating and cooling loads are at their minimum levels during these months. The increase in demand around midnight is due to time controlled switching of hot water systems, which is scheduled to set on at 11:30 p.m. for newly installed meters in SA. This is set as to take benefit of off-peak prices and hence, has caused 250 MW Energies 2018, 11, 2825 7 of 17 of time step increase in demand [50].

800000 700000 600000 500000 400000 300000 200000

Average Load (kW) 100000 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Energies 2018, 11, x FOR PEER REVIEW 7 of 17 Month Energies 2018, 11, x FOR PEER REVIEW 7 of 17 Figure 3. Seasonal load profile in SA. 800000 Figure 3. Seasonal load profile in SA. 800000 600000 600000 400000 400000 Load (kW) 200000 Load (kW) 200000 0 0 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00

10:00 Time11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 Time

Figure 4.4. Daily load profileprofile duringduring JanuaryJanuary inin SA.SA. Figure 4. Daily load profile during January in SA. AsAs windwind andand solar solar energy energy is consideredis considered for for the simulation,the simulation, annual annual solar solar radiation and wind and speedwind shouldspeedAs should bewind considered and be consideredsolar to determineenergy to isdetermine considered the performance the for perf th oformancee solarsimulation, PV of panels solar annual andPV panelssolar Wind radiation turbines and Wind respectively. and turbines wind speedGlobalrespectively. should horizontal Globalbe considered radiation horizontal and to winddetermineradiation speed andthe data windperf is obtainedormance speed data fromof solar is the obtained NASAPV panels Surfacefrom and the Meteorology Wind NASA turbines Surface and respectively.SolarMeteorology Energy Global database.and Solar horizontal Both Energy hourly radiation database. radiation and Both wind and hourly wind speed speedradiation data data is obtainedand is averaged wind from speed monthly the data NASA byis averaged HOMERSurface Meteorologyformonthly simulation. by HOMERand Monthly Solar for Energy simulation. global database. horizontal Monthly Both radiation global hourly horizontal averages radiation andradiation and average wind averages speed wind speedanddata average is is averaged shown wind in monthlyFiguresspeed is5 by shownand HOMER6, respectively. in Figures for simulation. 5 and 6, respectively. Monthly global horizontal radiation averages and average wind speed is shown in Figures 5 and 6, respectively. )

2 0.35 )

2 0.350.3 0.250.3 0.250.2 0.150.2 0.150.1 0.050.1 0.050 Average Global Solar (kW/m 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average Global Solar (kW/m Jan Feb Mar Apr May JunMonth Jul Aug Sep Oct Nov Dec

Figure 5. Monthly average global horizontal solar radiationMonth (averages) in used in the HOMER simulation. Figure 5. Monthly average global horizontal solar radiation (averages) in used in the HOMER Figuresimulation. 5. Monthly average global horizontal solar radiation (averages) in used in the HOMER simulation. 7 76 65 54 43 32 21

Average Wind Speed (m/s) 10

Average Wind Speed (m/s) 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May JunMonthJul Aug Sep Oct Nov Dec Month Figure 6. Average monthly wind speed average across the SA state) used for the simulation. Figure 6. Average monthly wind speed average across the SA state) used for the simulation.

Energies 2018, 11, x FOR PEER REVIEW 7 of 17

800000

600000

400000 Load (kW) 200000

0 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 Time

Figure 4. Daily load profile during January in SA.

As wind and solar energy is considered for the simulation, annual solar radiation and wind speed should be considered to determine the performance of solar PV panels and Wind turbines respectively. Global horizontal radiation and wind speed data is obtained from the NASA Surface Meteorology and Solar Energy database. Both hourly radiation and wind speed data is averaged monthly by HOMER for simulation. Monthly global horizontal radiation averages and average wind speed is shown in Figures 5 and 6, respectively. )

2 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 Average Global Solar (kW/m Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

Figure 5. Monthly average global horizontal solar radiation (averages) in used in the HOMER Energies 2018, 11, 2825 8 of 17 simulation.

7 6 5 4 3 2 1

Average Wind Speed (m/s) 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Energies 2018, 11, x FOR PEER REVIEW Month 8 of 17 Figure 6. Average monthly wind speed average across the SA state) used for the simulation. 4. EnergyFigure Modelling 6. Average Using monthly HOMER wind speed average across the SA state) used for the simulation. 4. Energy Modelling Using HOMER 4.1. Scenarios and Modelling Structures 4.1. Scenarios and Modelling Structures The case study investigates and compares the feasibility of using two energy storage options to supportThe the case entire study generation investigates from and wind compares turbines the an feasibilityd solar PVs: of using(1) solely two battery energy bank storage as the options energy to storagesupport system the entire (2) generationhydrogen-battery from wind energy turbines storage. and solar PVs: (1) solely battery bank as the energy storageSolar system PVs and (2) hydrogen-battery wind turbines are energy sized storage.according to the RET suggested by the Australian state government;Solar PVs 1515 and MW wind and turbines 4876 MW, are sized respectively. according The to theenergy RET storage suggested systems by the are Australian both sized state to providegovernment; four days 1515 of MW autonomous and 4876 MW,operation, respectively. when none The of energy the renewable storage systems energy aregenerators both sized are in to provideoperation four in case days of of their autonomous unexpected operation, unavailability: when nonee.g., due of the to unfavourable renewable energy weather generators conditions are or in gridoperation system in failure. case of theirAbove-mentioned unexpected unavailability: energy storage e.g., options due to to unfavourable be investigated weather are schematically conditions or illustratedgrid system in failure.Figure 7a,b, Above-mentioned as extracted from energy HOMER. storage options to be investigated are schematically illustrated in Figure7a,b, as extracted from HOMER.

(a) (b)

FigureFigure 7. 7.(a ()a PV/wind/battery) PV/wind/battery system design;design; andand ((b)) PV/wind/battery/hydrogenPV/wind/battery/hydrogen system system design. design.

The electrolyserelectrolyser in in the the hydrogen hydrogen system system generates generates hydrogen hydrogen from the from excess the electricity excess electricity generated generatedby wind turbines by wind and turbines solar PV, and which solar is thenPV, storedwhich inishydrogen then stored tank in to hydrogen generate electricitytank to generate through electricitythe fuel cell through later to the fill fuel the gapcell betweenlater to fill the the supply gap between and demand. the supply and demand.

4.1.1. Battery Based Storage System Flat plate PV modules and wind turbines were sized according to the RET of 1515 MW capacity of solar PV and 4876 MW of generation. The generation capacity was constrained to evaluate the optimum size and design the storage system. The efficiency of PV modules was adjusted to 13% [46] from its value at standard test conditions (STC) by considering the effect of ambient temperature, which is defined by the temperature resource taken from the NASA Surface Meteorology and Solar Energy database. Power output of the PV modules is rated at STC (i.e., a solar radiation input of 1 kW/m2, 25 °C cell temperature, and in the absence of any wind blowing over the surface of the PVs). However, this does not fully reflect actual operating conditions. Hence, the efficiency of PV is less than that measured in standard test conditions. De-rating factor was assumed to be 85% for the modules considering the effects such as soiling of panels, aging, snow cover during the winter period, and other transmission losses. The effect of temperature on power output and nominal operating cell temperature were assumed to be −0.05%/°C and 47 °C respectively. Considering the RET of 4178 MW capacity, 3300 wind turbines, each having 1.5 MW capacity, and 80 m hub height was assumed for the simulation. The following equation is used to calculate the PV array output.

Energies 2018, 11, 2825 9 of 17

4.1.1. Battery Based Storage System Flat plate PV modules and wind turbines were sized according to the RET of 1515 MW capacity of solar PV and 4876 MW of wind power generation. The generation capacity was constrained to evaluate the optimum size and design the storage system. The electrical energy efficiency of PV modules was adjusted to 13% [46] from its value at standard test conditions (STC) by considering the effect of ambient temperature, which is defined by the temperature resource taken from the NASA Surface Meteorology and Solar Energy database. Power output of the PV modules is rated at STC (i.e., a solar radiation input of 1 kW/m2, 25 ◦C cell temperature, and in the absence of any wind blowing over the surface of the PVs). However, this does not fully reflect actual operating conditions. Hence, the efficiency of PV is less than that measured in standard test conditions. De-rating factor was assumed to be 85% for the modules considering the effects such as soiling of panels, aging, snow cover during the winter period, and other transmission losses. The effect of temperature on power output and nominal operating cell temperature were assumed to be −0.05%/◦C and 47 ◦C respectively. Considering the RET of 4178 MW capacity, 3300 wind turbines, each having 1.5 MW capacity, and 80 m hub height was assumed for the simulation. The following equation is used to calculate the PV array output. Ppv = YpvFpv(Gt/Gt,stc)[1 + αP(Tc − Tc,stc)] (1) where

• Ppv: output of the PV array; • YPV: rated capacity of PV array under STC; • FPV: derating factor (%); 2 • GT: solar radiation incident on the PV array in a particular time step [kW/m ]; • GT,STC: incident radiation at STC; ◦ • α P: temperature coefficient of power [%/ C]; • TC: PV cell temperature in a particular time step; and • TC,STC: PV cell temperature under STC

Tesla’s Power Pack 2 (PP2) -ion batteries were considered for this case study. Tesla recently completed installation of a 100 MW battery system in SA in November 2017 and further talks between government and Tesla is underway for large utility scale installation [51]. The cost of the system as announced by Tesla in November 2016 is $398/kWh [52], and the same was assumed for this study. This cost is in line with the total cost of 90 million Australian Dollars for Tesla battery storage system installed in SA in 2017 which is $505/kWh. However, this cost included total installation costs and extra costs to deliver the project in 100 days [53]. The round-trip efficiency (RTE) suggested by the manufacturer is 88%. However, such a level of RTE is not expectedly achievable when the batteries are used as a long-term energy storage solution (i.e., as required in the first case scenario). Hence, 50% RTE was assumed for the long-term storage application and 80% for the short-term when combined with hydrogen storage as required in the second scenario [46]. These batteries can be connected in various combinations of parallel and series depending on the site and system architecture. The nominal capacity of each battery is 210 kWh with nominal of 380 V. Depth of discharge recommended by the manufacturer is 100% that is equivalent to 0% state of charge. However, to design a reliable system and preserve the lifetime of the batteries frequent full discharges have to be avoided. Hence, the state of charge for this case study was assumed to remain above 20% in all situations. This serves a dual purpose as this 20% of unused stored energy can account for unfavourable weather conditions and provide few days of autonomous operation in worst case scenario of zero generation through renewable energy sources (e.g., cased by natural disasters, unfavourable weather conditions, maintenance, etc.). The size of the battery bank is not constrained as the main aim of the case study is to identify the storage capacity required for a fixed generation capacity by renewables. HOMER will give the Energies 2018, 11, 2825 10 of 17 optimum battery bank size to maintain a reliable electricity supply with minimum NPC. The lifetime of lithium-ion batteries depends on their charging rates: it decreases as the charging rate increases and varies between 5 to 7 years [54]. Hence, as a conservative assumption the lifetime of batteries for this case study was assumed to be five years for outdoor industrial applications, which is in line with figures published by other researchers [15,46]. The cost and lifetime assumptions for all other components are taken from recently published articles or average value from supplier’s websites [46,55,56] (Table2). NPC is the difference between the present cost of the all the costs of installing and operating the components of the system and the present value of total revenue generated over the life time of the project and is calculated using the following equation [57]:

NPC = −Co + ∑ Ct/(1 + r)t (2) where:

• Co: Initial Investment; • C: Cash flow; • r: Discount rate; and • t: time.

4.1.2. Hybrid Battery-Hydrogen Storage System The hydrogen-based energy storage system used in this study consists of and electrolyser, a fuel cell and a . The properties and cost considered in the analysis set in HOMER are those of exchange membrane (PEM) type fuel cell. The fuel consumption rate was fed into HOMER as suggested in other studies [58,59] that translates into electrical energy efficiencies in the range of about 40–50% that is expected for a PEM fuel cell. The electrolyser produces hydrogen from surplus power generated by wind turbines and solar PV, and it was sized to accommodate the surplus of PVs and wind energy generators. The fuel cell was sized to generate enough electricity for maximum load demand. The hydrogen tank was not constrained to make sure that the system uses all the excess energy to produce hydrogen and enough space was assumed to be available in the tank to store hydrogen [60]. Energy efficiency of the electrolyser was assumed to be 70% while being capable of compressing hydrogen to 30 bars above ready for storage. Capital cost for the hydrogen tank was based on the mass production of the system. The economic assumptions for electrolyser, hydrogen tank, and fuel Cell have been taken from recently published reports by other researchers [47,61–63]. However, the cost for hydrogen storage may further decrease in years to come, and particularly in storage applications as large as that considered for this case study. The economic and lifetime details that have been fed into HOMER for the system’s components are given in Table2.

Table 2. Cost details of battery and hybrid storage based renewable System. All costs are in USD.

Capital Cost Replacement Cost O&M Cost Components Lifetime (Year) ($/kw) ($/kW) ($/Year) Solar PV 831 831 145 25 Wind Turbine 2,535,000/unit 2,535,000 76,500 20 Battery 398 ($/kWh) 398 ($/kWh) 0 5 Converter 210 210 0 15 Electrolyser 2000 1200 20 15 Hydrogen Tank 438 $/kg 438 $/kg 0 25 Fuel Cell 600 500 0.08 $/op. h 40,000 h

Other economic assumptions for the case study include a nominal discount rate of 8% and inflation rate of 2%; the project lifetime was assumed to be 25 years as that used is other similar studies, e.g., [46,60,64,65]. Energies 2018, 11, 2825 11 of 17

5. Results and Discussion

5.1. Current Scenarios The scenarios simulated include PV/wind/battery and PV/wind/hydrogen/battery arrangements. PV/wind/hydrogen systems were also simulated to completely replace the battery storage. However, it resulted in a very large system defying the technical infeasibility of this solution while the cost of the system turned out to be significantly high. Hence, this scenario has been discarded and was not considered as an option for the case study. The investigation indicated that the above-mentioned two options are more feasible and cost-effective to be practiced for the case of large-scale long-term energy storage application focused by this research. The optimum system sizes obtained from HOMER for PV/wind/battery and PV/wind/hydrogen/battery systems with zero unmet electrical load and the lowest unit cost of electricity (COE) are shown in Table3. Energies 2018, 11, x FOR PEER REVIEW 11 of 17 Table 3. Optimised result for PV/wind/battery and PV/wind/battery/hydrogen. Table 3. Optimised result for PV/wind/battery and PV/wind/battery/hydrogen. PV Wind Turbine Tesla PP2 Electrolyser Fuel Cell Hydrogen COE Design PV(MW) Wind(Units) Turbine (Units) Electrolyser(MW) Fuel(MW) Cell HydrogenTank (kg) ($/kWh)COE Design Tesla PP2 (Units) PV/Wind/Battery(MW) 1515 (Units) 3300 1,475,001 -(MW) (MW) - Tank -(kg) ($/kWh) 2.54 PV/Wind/Battery 1515 3300 1,475,001 - - - 2.54 PV/Wind/Battery/Hydrogen 1515 3300 150,000 2400.8 2400 10,000,000 0.626 PV/Wind/Battery/Hydrogen 1515 3300 150,000 2400.8 2400 10,000,000 0.626

FromFrom Table3 3,, itit isis evidentevident that significant significant reductio reductionn in in unit unit cost cost of of electricity electricity can can be be achieved achieved by byintroducing introducing hydrogen hydrogen storage storage unit unit to the to thePV/wind/battery PV/wind/battery system system for a large-scale for a large-scale application. application. It can Itbe can accounted be accounted to the decrease to the decrease in battery in battery units due units to duethe increase to the increase in overall in overallefficiency efficiency of the system. of the system.80% round-trip 80% round-trip energy energyefficiency efficiency was assumed was assumed for the for latter the latter scenario scenario as battery as battery is used is used as asa ashort-term short-term energy energy storage storage only only along along with with hydrogen hydrogen-based-based energy energy storage storage unit unit taking care of thethe long-termlong-term energyenergy storagestorage requirementsrequirements [[46].46]. SimulationSimulation was carried out toto designdesign aa cost-effectivecost-effective system,system, whichwhich couldcould provide provide at at least least four four days days of of autonomous autonomous operation operation for for both both scenarios scenarios during during the monththe month of July, of July, when when generation generation of energy of energy is at minimum. is at minimum. Stateof State charge of charge (SOC) of(SOC) a Tesla of a PP2 Tesla battery PP2 bankbattery throughout bank throughout the year the is illustrated year is illustrated in Figure in8 .Figure 8.

FigureFigure 8.8.State State of of charge charge of aof Tesla a Tesla PP2 batteryPP2 battery bank withbank 1,475,001 with 1,475,001 batteries batteries at 50% roundtrip at 50% roundtrip efficiency usedefficiency for long used term for storage—Scenariolong term storage—Scenario 1. 1.

AA 20%20% minimumminimum SOC SOC was was considered considered for for batteries batteries at allat all situations. situations. SOC SOC is lowest is lowest in July, in July, the gapthe allowsgap allows for 4–5 for days 4–5 ofdays autonomous of autonomous operation operation till the till battery the battery is fully is discharged fully discharged at 0% SOC at 0% as allowedSOC as byallowed the manufacturer. by the manufacturer. The round-trip The round-trip energy efficiency energy ofefficiency batteries of was batteries assumed was to assumed be 50% when to beused 50% aswhen both used long-term as both and long-term short-term and energy short-term storage energy solutions. storage As shownsolutions. in Figure As shown8, apart in fromFigure month 8, apart of July,from battery month storageof July, battery has enough storage stored has chargeenough to stor operateed charge autonomously. to operate autonomously. This creates the This possibility creates ofthe decreasing possibility the of basedecreasing load, which the base is supplied load, which by gas is insupplied the grid by during gas in summer the grid months. during This summer then enhancedmonths. This the economicthen enhanced viability the of economic the system. viabilit Figurey of9 showsthe system. the amount Figure of 9 storedshows hydrogenthe amount tank of throughoutstored hydrogen the year tank together throughout with the year state toge of chargether with in Tesla the state PP2. of charge in Tesla PP2. The hydrogen tank was set to be started from zero in January as a practical proposition when the system is commissioned and start to operate for the first time. Hydrogen level in the tank follows almost a pattern similar to that obtained for batteries in PV/wind/battery system as illustrated in Figure 8. The stored hydrogen reaches its maximum during summer because more energy is generated by the solar PVs and wind turbines during this period. The excess energy available during summer time though can is used by the electrolysers to produce more hydrogen. By contrast the hydrogen level is at its lowest during winter months reaching its minimum in July due to low solar and wind electricity production. Hence, during this period most of the hydrogen stored earlier is used in the fuel cell to generate electricity and cover the supply gap. This combined system was also set to provide four days of autonomous operation in July. However, after July, hydrogen level reaches back to its maximum level and remains at highest possible level for rest of the year. This could result in excess amount of hydrogen than necessary to start the next year with. For a large-scale application, battery/hydrogen hybrid energy storage solution was found to be cheaper than a battery only energy storage solution. Moreover, the storage system with two types of storage (battery and hydrogen) offers more reliability than a system with a single energy storage solution.

Energies 2018, 11, x FOR PEER REVIEW 12 of 17

However, as batteries are more often charged and discharged in this design, the increased number of charging/discharging cycles may potentially affect the lifetime of the batteries. Hence, a detailed study on battery lifetime with respect to charging cycles in this context can be beneficial to shed more light on this effect. Requirement for hydrogen storage may not be really significant in regard to load demand in the present scenario for SA, where gas already provides the base load. To further enhance the benefit offered by the hydrogen/battery hybrid energy storage solution, the excess

Energiesstored 2018hydrogen, 11, 2825 can be utilised to generate electricity that is translated to an added value.12 This of 17 alternative scenario will be further explored in the following section.

Figure 9.9.State State of chargeof charge of a battery of a bankbattery and variationbank an ind hydrogenvariation levelin forhydrogen a PV/wind/battery/ level for a hydrogenPV/wind/battery/hydrogen system with 150,000 system batteries with 150,000 at 80% roundbatteries trip at efficiency,80% round where trip batteriesefficiency, are where used forbatteries short-term are used energy for short-term storage with energy hydrogen storage system with hydrogen used for system meeting used long-term for meeting energy long-term storage requirementenergy storage in Scenariorequirem 2.ent in Scenario 2. The hydrogen tank was set to be started from zero in January as a practical proposition when 5.2. Alternative Scenarios the system is commissioned and start to operate for the first time. Hydrogen level in the tank follows almostThe a excess pattern stored similar hydrogen to that obtained produced for throughout batteries inthe PV/wind/battery year can be utilised system for further as illustrated electricity in Figuregeneration.8. The Currently, stored hydrogen SA has reaches 49.1% itsof maximumelectricity duringgeneration summer capacity because through more gas energy [14]. is The generated excess bystored the solarhydrogen PVs andcan windbe used turbines to reduce during this this ba period.se load The supply excess from energy gas availablewhile keeping during the summer same timerenewable though penetration can is used capacity by the electrolysers (i.e., better to utilising produce of more this hydrogen.generation By capacity). contrast theAlternatively, hydrogen level the isexcess at its lowestelectricity during generated winter monthsthrough reaching the hydrogen its minimum system in Julycan duebe toexported low solar to and the wind neighbouring electricity production.states. Either Hence, of these during solutions this ca periodn offer most opportunities of the hydrogen to enhance stored the earlier overall is economic used in the violability fuel cell toof generatethe system. electricity Simulation and cover(in HOMER) the supply was gap. carried This out combined by increasing system waselectric also load set to by provide considering four days the ofabove-mentioned autonomous operation scenarios. in July.The results However, showed after July,that an hydrogen additional level 250,000 reaches kW back of tomaximum its maximum load leveldemand and could remains be met at highest every hour possible for the level month for rest of March of the year.and April This and could the result maximum in excess of 600,000 amount kW of hydrogenfrom September than necessary to December. to start The the variation next year of hydrog with. Foren alevel large-scale and SOC application, of the battery battery/hydrogen bank in case of hybridincreased energy load storagedemand solution is shown was in Figure found to10. be cheaper than a battery only energy storage solution. Moreover,Excess the stored storage hydrogen system withis utilised two types at ofthe storage end of (battery the year and as hydrogen) stored level offers of more hydrogen reliability is thandecreasing. a system At with the minimum a single storageof hydrogen solution. and However, SOC of the as battery batteries bank are morein July, often the charged system andcan dischargedstill provide in thistwo design,days of the autonomous increased number operatio of charging/dischargingn. The optimum system cycles design may potentially obtained affectfrom theHOMER lifetime for of the the same batteries. inputs Hence, as in Scenario a detailed 2 studyis shown on batteryin Table lifetime 4. with respect to charging cycles in this context can be beneficial to shed more light on this effect. Requirement for hydrogen storage may notTable be 4. really Optimum significant sizingin and regard economic to load performance demand in fo ther the present PV/wind/hydrogen scenario forSA, battery where system gas alreadyin provideswhich the the base excess load. hydrogen To further is used enhance to reduce the benefitthe baseoffered load or byexport the hydrogen/batteryelectricity to the neighbouring hybrid energy storagestates. solution, the excess stored hydrogen can be utilised to generate electricity that is translated to PV Wind Turbine Tesla PP2 Electrolyser Fuel Cell Hydrogen COE an added value.DesignThis alternative scenario will be further explored in the following section. (MW) (Units) (Units) (MW) (MW) Tank (kg) ($/kWh) 5.2. AlternativePV/wind/Battery/Hydrogen Scenarios 1515 3300 150,000 2400.8 2400 10,000,000 0.494

TheAs another excess stored possible hydrogen scenario produced for future throughout investigation, the year the can possibility be utilised of for using further this electricity excess generation.hydrogen for Currently, fuelling hydr SA hasogen 49.1% fuel ofcell electricity cars to be generation operational capacity across through the state gas can [14 be]. Thetaken excess into storedaccount. hydrogen One of barriers can be against used to expansion reduce this of this base technology load supply is the from availability gas while of keeping hydrogen the supply same renewable penetration capacity (i.e., better utilising of this generation capacity). Alternatively, the excess electricity generated through the hydrogen system can be exported to the neighbouring states. Either of these solutions can offer opportunities to enhance the overall economic violability of the system. Simulation (in HOMER) was carried out by increasing electric load by considering the above-mentioned scenarios. The results showed that an additional 250,000 kW of maximum load demand could be met every hour for the month of March and April and the maximum of 600,000 kW from September to December. The variation of hydrogen level and SOC of the battery bank in case of increased load demand is shown in Figure 10. Energies 2018, 11, x FOR PEER REVIEW 13 of 17

Energies[66]. The2018 excess, 11, 2825 hydrogen available can be part of the solution to lift this barrier and encourage13 of the 17 utilisation of hydrogen fuel cell .

FigureFigure 10.10. State of charge of battery bank and variationvariation of hydrogen level for increased load demand.

Excess stored hydrogen is utilised at the end of the year as stored level of hydrogen is decreasing. It is important to consider this point that hydrogen fuel cell systems generate , as well as At the minimum level of hydrogen and SOC of the battery bank in July, the system can still provide electricity [67,68]. Part of the total heat generated by the fuel cell that is available for recovery (i.e., Q two days of autonomous operation. The optimum system design obtained from HOMER for the same in W) can be calculated from the following equation [69]: inputs as in Scenario 2 is shown in Table4. 𝑸=𝑵𝑰(𝟏. 𝟐𝟓 )− 𝑽𝒐𝒖𝒕 (3) Table 4. Optimum sizing and economic performance for the PV/wind/hydrogen battery system in where N is the number of cells in a fuel cell stack; I (A) is the current passing through the cells; and which the excess hydrogen is used to reduce the base load or export electricity to the neighbouring states. Vout (V) is the output voltage of the cell. According to Shabani [67] the amount of heat recovered from PV Wind Turbine Tesla PP2 Electrolyser Fuel Cell Hydrogen COE a fuel cellDesign can be almost comparable with its output power (at its rated power) and the overall combined heat and power (MW)(CHP) energy(Units) efficiency(Units) of a fuel cell(MW) can reach(MW) over 80%Tank [70]. (kg) Recovering($/kWh) thePV/wind/Battery/Hydrogen fuel cell heat this though1515 enhances 3300 the round-trip 150,000 energy 2400.8 efficiency 2400 of such 10,000,000 hydrogen-based 0.494 energy storage solutions. The heat recovered from fuel cell can be used in domestic applications (e.g.,As hot another water or possible space heating), scenario forwhere future low investigation, grade heat (at the about possibility 60 °C supplied of using thisby PEM excess fuel hydrogen cells) is forrequired fuelling [1]. hydrogen Decentralised fuel cell ustilisation cars to be operationalof hydrogen across fuel thecell state systems can be facilitates taken into the account. possibility One ofof barriersrecovering against the fuel expansion cell heat of thisfor such technology applicatio is thens availabilitythat can improve of hydrogen the economics supply [66 of]. the The system excess hydrogensignificantly. available Detailed can evaluation be part of of the this solution scenario to remains lift this barrierout of the and scope encourage of this the ; utilisation however, of hydrogenthis is an important fuel cell vehicles. scenario to be fully assessed in order to fully understand the benefit that can be offeredIt is by important a hydrogen-based to consider thisenergy point storage that hydrogen solution fuelin long-term cell systems large-scale generate heat,energy as wellstorage as electricityapplications. [67 ,68]. Part of the total heat generated by the fuel cell that is available for recovery (i.e., Q in W) can be calculated from the following equation [69]: 6. Conclusions Q = NI(1.25 − Vout) (3) This paper presented a case study for the state of SA in Australia to analyse the significance of hydrogen-based storage system as a large-scale long-term energy storage solution for a hybrid where N is the number of cells in a fuel cell stack; I (A) is the current passing through the cells; and Vout (V)renewableis the output energy voltage input offrom the solar cell. AccordingPVs and wind to Shabani turbines. [67 ]Generation the amount capacity of heat recoveredwas constrained from a fuelaccording cell can to be the almost RET set comparable by the state with government its output power. HOMER (at its simulation rated power) software and the was overall used to combined analyse heatthe andtechno-economic power (CHP) energy feasibility efficiency of of a fueltwocell systems can reach overincluding: 80% [70 ].PV/wind/battery Recovering the fuel and cell heatPV/wind/battery/hydrogen. this though enhances the System round-trip size and energy unit efficiencycost of electricity of such hydrogen-basedwere the main parameters energy storage used solutions.as evaluation The indicators heat recovered in this from study. fuel Based cell can on bethe used results in domestic obtained, applications PV/wind/battery/hydrogen (e.g., hot water orsystem space was heating), the most where economical low grade system. heat (at The about inclus 60ion◦C of supplied hydrogen by system PEM fuel reduced cells) isthe required size of the [1]. Decentralisedbattery bank significantly. ustilisation of This hydrogen reduced fuel the cell COE systems of the facilitates system the from possibility 2.54 $/kWh of recovering to 0.626 the$/kWh. fuel cellHowever, heat for for such the applicationspresent generation that can mix improve and load the economicsdemand of of SA, the this system system significantly. design resulted Detailed in evaluationexcess hydrogen of this scenariostored after remains July. outThe of stored the scope hydrog of thisen does work; not however, hold any this significance is an important unless scenario it can tobe befurther fully assessedutilised. inDecreasing order to fully the understandbase load thesupply benefit from that gas can increased be offered the by renewable a hydrogen-based energy energypenetration storage with solution constrained in long-term generation large-scale capacity. energy This storage alternate applications. scenario utilised the excess hydrogen which increased the overall economic viability of the system. The unit cost of electricity for 6.this Conclusions case decreased to 0.494 $/kWh. Other possibility is utilising this excess hydrogen to support the deployment of hydrogen fuel cell vehicles across the state. In fact more detailed investigation is This paper presented a case study for the state of SA in Australia to analyse the significance required to evaluate the benefits and challenges of implementing this idea. Utilising the heat of hydrogen-based storage system as a large-scale long-term energy storage solution for a hybrid

Energies 2018, 11, 2825 14 of 17 renewable energy input from solar PVs and wind turbines. Generation capacity was constrained according to the RET set by the state government. HOMER simulation software was used to analyse the techno-economic feasibility of two systems including: PV/wind/battery and PV/wind/battery/hydrogen. System size and unit cost of electricity were the main parameters used as evaluation indicators in this study. Based on the results obtained, PV/wind/battery/hydrogen system was the most economical system. The inclusion of hydrogen system reduced the size of the battery bank significantly. This reduced the COE of the system from 2.54 $/kWh to 0.626 $/kWh. However, for the present generation mix and load demand of SA, this system design resulted in excess hydrogen stored after July. The stored hydrogen does not hold any significance unless it can be further utilised. Decreasing the base load supply from gas increased the renewable energy penetration with constrained generation capacity. This alternate scenario utilised the excess hydrogen which increased the overall economic viability of the system. The unit cost of electricity for this case decreased to 0.494 $/kWh. Other possibility is utilising this excess hydrogen to support the deployment of hydrogen fuel cell vehicles across the state. In fact more detailed investigation is required to evaluate the benefits and challenges of implementing this idea. Utilising the heat generated by the fuel cell can enhance its overall efficiency (i.e., as a CHP) unit. This scenario has not been investigated as part of the present study; however, the authors are suggesting this to be studied in details to fully understand its economic benefit and challenges, when practiced in long-term large-scale applications.

Author Contributions: Conceptualization, S.K. and B.S.; Methodology, S.K. and B.S.; Software and modeling, S.K. and B.S.; Formal Analysis, S.K. and B.S.; Writing-Original Draft Preparation, S.K.; Writing-Review & Editing, B.S.; Supervision, B.S.; Project Administration, S.K. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest

Abbreviations

PHS Pumped hydroelectric storage SA South Australia PV Photovoltaic RET Renewable energy target CAES Compressed air energy storage HOMER Hybrid Optimisation Model for Electric Renewables IRENA International Renewable Energy Agency AEMO Australian energy market operator STC Standard test conditions RTE Round-trip efficiency PEM Proton exchange membrane O&M Operations and maintenance COE Cost of electricity SOC State of charge PP2 Power pack 2

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