金属氢化物与压缩氢配对作为热能储存器以集中太阳能计划的潜力外文文献

The potential of metal hydrides paired with compressed hydrogen as thermal energy storage for concentrating solar power plants Drew A Sheppard Craig E Buckley Hydrogen Storage Research Group Fuels and Energy Technology Institute Department of Physics and Astronomy Curtin University GPO Box U1987 Perth WA 6845 Australia a r t i c l e i n f o Article history Received 25 September 2018 Received in revised form 25 January 2019 Accepted 29 January 2019 Available online 14 March 2019 Keywords Concentrating solar power Thermal energy storage Thermochemical energy storage Metal hydrides Hydrogen storage a b s t r a c t Concentrating solar power CSP plants require thermal energy storage TES systems to produce electricity during the night and periods of cloud cover The high energy density of high temperature metal hydrides HTMHs compared to state of the art two tank molten salt systems has recently promoted their investigation as TES sys tems A common challenge associated with high temperature metal hydride thermal energy storage systems HTMH TES systems is storing the hydrogen gas until it is required by the HTMH to generate heat Low temperature metal hydrides can be used to store the hydrogen but can comprise a signifi cant proportion of the overall system cost and they also require thermal management which increases the engineering complexity In this work the potential of using a hydrogen compressor and large scale underground hydrogen gas storage using either salt caverns or lined rock caverns has been assessed for a number of magnesium and sodium based hydrides MgH2 Mg2FeH6 NaMgH3 NaMgH2F and NaH Previous work has assumed that the sensible heat of the hydrogen released from the HTMH would be stored in a small inexpensive regenerative material system However we show that storing the sensible heat of the hydrogen released would add between US 3 6 and US 7 5 kWhthto the total system cost for HTMHs operating at 565 C If the sensible heat of released hydrogen is instead exploited to perform work then there is a fl ow on cost reduction for each component of the system The HTMHs combined with underground hydrogen storage all have specifi c installed costs that range between US 13 7 and US 26 7 kWhthwhich is less than that for current state of the art molten salt heat storage Systems based on the HTMHs Mg2FeH6or NaH have the most near term and long term potential to meet SunShot cost targets for CSP thermal energy storage Increasing the operating tem perature and hydrogen equilibrium pressure of the HTMH is the most effective means to reduce costs further 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC Corresponding author E mail addresses drew sheppard D A Sheppard c buckley curtin edu au C E Buckley Available online at ScienceDirect journal homepage international journal of hydrogen energy 44 2019 9143e9163 https doi org 10 1016 j ijhydene 2019 01 271 0360 3199 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC Introduction A viability assessment was performed on high temperature metal hydrides HTMHs that use compressed gas hydrogen storage as thermochemical heat storage in concentrating solar power CSP plants The intermittent nature of solar energy is driving research for different energy storage options HTMHs show potential as thermal energy storage materials for CSP plants due to their high gravimetric and volumetric energy density compared to state of the art molten salt sys tems However storing the hydrogen gas released from the HTMH until it is needed for power generation is expensive and when stored in a low temperature metal hydride LTMH introduces additional complexity associated with thermal management The development of a generalised approach for exploring the potential of compressed gas hydrogen storage paired with HTMHs for thermal energy storage can help guide research directions that will lead to cost reduction Concentrating solar power plants with thermal energy storage represent an attractive alternative to conventional fossil fuels for base load power generation However though CSP has low running costs they have substantial upfront capital costs 1 and their widespread implementation will only be possible with further reductions in their installation costs A number of technology areas have been identifi ed for cost reduction including the heliostat fi eld receiver technol ogy and heat transfer fl uid 1 Another step towards reducing the overall cost of CSP and the focus of this work is a means of thermal energy storage with reduced cost CSP plants with thermal storage currently utilise the heat capacity associated with a temperature change in a molten nitrate salt mixture NaNO3 KNO3 1 The thermal energy storage system density can be increased by the use of phase change materials or reversible thermochemical reactions and reviews of the cur rent state of the art in thermalstorage materials are available 2e6 Metal hydrides are an alternative class of thermochemical candidates for heat storage applications and a number of different metal hydride systems have been explored for this purpose 7e22 The use of metal hydrides for concentrating solar thermal energy storage is typically comprised of paired metal hydrides 9 the high temperature metal hydride HTMH as the thermal energy storage medium and a low temperature metal hydride LTMH for hydrogen storage During periodsof sunlight incomingsolar radiation is focused by mirrors to generate heat A portion of this heat is trans ferred to a heat engine to generate electricity and a portion of the heat is directed to the HTMH to release H2in an endo thermic reaction The released H2is temporarily stored in the LTMH Fig 1a During night time or periods of cloud cover the reactor temperature of the HTMH begins to fall causing the system gas pressure to drop and H2is consequently released from the LTMH and absorbed by the HTMH in a self regulating cycle This absorption by the HTMH hydride is an exothermic reaction where the heat released is used to drive a heat en gine and generate electricity Fig 1b 9 Heat storage using metal hydrides has been explored since the mid 1970s 24 but was generally applied to temperatures below 200 C due to the nature of hydrides known at that time In the early 1990s the development of low cost magnesium hydride MgH2 with rapid hydrogen absorption desorption Fig 1 e A schematic from Ref 23 showing the operation of a HTMH heat storage system for CSP during the a daytime and b night time international journal of hydrogen energy 44 2019 9143e91639144 kinetics led to its research as a heat storage medium for solar thermal energy storage up to temperatures of 420 C 25e29 More recently SunShot cost targets for thermal energy stor age as part of CSP 30 have driven a renewed interest in low cost high temperature metal hydrides for thermal energy storage Metal hydrides based on Mg Na and Ca appear to be the most promising high temperature metal hydrides to meet these targets 12 13 20 31 32 The early breakthrough of low cost MgH2was hampered by the fact that the only suitable hydrogen storage option was using low temperature metal hydrides that have limited hydrogen capacity typically 1 to 2 wt H2 and were based on expensive transition or rare earth metals The cost of hydrogen storage in a LTMH for CSP applications has improved with the advent of reversible hydrogen uptake in NaAlH4and Na3AlH6 33 34 but the issue of thermal management of the LTMH remains The LTMH it self produces an appreciable quantity of heat during hydrogen absorption and this same amount of heat needs to be supplied to release the hydrogen again For example early work using MgH2as the HTMH used a TiMn2 based alloy HWT 5800 as the LTMH hydrogen store where the heat generated by the LTMH during hydrogen absorption was 30 of the thermal energy stored by the MgH2 25 The enthalpy of hydrogen absorption in HWT 5800 is approximately 23 to 25 kJ mol H2 35 36 and so the issue of thermal management be comes even more problematic for NaAlH4and Na3AlH6where the hydrogen absorption enthalpies are 38 and 47 kJ mol H2 respectively 33 In order to circumvent the thermal management issues associated with hydrogen storage in a LTMH the hydrogen could instead be stored as compressed gas 12 If the LTMH is replacedwithcompressed gashydrogenstorage therearetwo basic ways in which the HTMH TES system can operate at constant temperature see Fig 2 In operation mode one OM1 Fig 2a the daytime minimum pressure Pmin of the compressed gas storage vessel is equal to the equilibrium pressure of the HTMH Peq and the compressor must do all of the work to boost the hydrogen up to the maximum storage pressure Pmax During night time operation the hydrogen pressure in the storage vessel is above Peqof the HTMH and theabsorptionofhydrogenfromthecompressedgas hydrogen storage will proceed automatically In operation mode two OM2 Fig 2b the HTMH also operates at constant temperature during both the daytime and night time cycles In this case the compressor operates during part of both the daytime and night time cycle The latter operating mode is preferablein the context of CSP as for the same amount of compression work the pressure ratio Pmax Pminis substantially higher see supporting information Eq S 1 and associated discussion All of the results presented in this work refer to OM2 For compressed hydrogen gas storage as part of a HTMH TES system for CSP the most feasible options from a cost and practicality perspective are underground storage in salt cav erns or lined rock caverns LRCs The underground storage of hydrogen gas has been receiving increased attention in recent years as it has the potential to store renewable energy in the form of hydrogen produced via electrolysis in times of excess electricity production from intermittent sources such as wind power and photovoltaics 37e41 Hydrogen storage in un derground caverns has the potential to play a role in buffering seasonal energy demands to provide continuity in instances of supply chain disruption and to reduce the storage costs of hydrogen delivered to consumers for use in fuel cell electric vehicles 42 The feasibility of large scale underground hydrogen storage for this purpose is now actively being explored in the USA 42 France 37 Poland 43 Sweden 44 Germany 39 Spain 45 and others 41 Salt caverns are formed by solution mining of geological suitable salt deposits and have a long history of being used for storage of compressed natural gas CNG 37 42 46e48 and for compressed air energy storage CAES 49 For example in the USA there were 39 salt dome facilities being used to store more than 13 billion cubic metres of natural gas at standard temperature and pressure as of the end of 2014 47 There are currently three locations worldwide that store hydrogen un derground in salt caverns Two of these are in the USA and both have volumes of 580 000 m3while the facility in the UK is comprised of three caverns of 70 000 m3each 41 Salt caverns are particularly attractive for large scale storage of Fig 2 e a Constant temperature HTMH with compressor operation during daytime only b Constant temperature HTMH with compressor operation during both daytime and night time Grey arrows correspond to self regulated hydrogen fl ow Black arrows correspond to hydrogen fl ow driven by a compressor international journal of hydrogen energy 44 2019 9143e91639145 hydrogen as the cost is 2 orders of magnitude cheaper than the storage of hydrogen using above ground pressure vessels 46 Salt caverns have the advantage of being naturally self sealing due to the plastic properties of salt and the size and shape of the cavern can often be customized Challenges associated with the use of salt caverns for compressed gas hydrogen storage as part of a HTMH TES system include 1 The uneven geographic distribution of suitable salt de posits and that they do not necessarily coincide with areas of high solar irradiance and demand 2 That salt caverns require a relatively large quantity of cushion gas on the order of 30 of the total cavern capacity in order to maintain their structural integrity 41 42 3 That other than maximum and minimum working pressures the salt cavern operations are limited by the maximum allowable rate of pressure change within them 41 4 The need to purify hydrogen after storage in the salt cavern due to the presence of water vapour that evap orates from brine solution left over from the construc tion process 39 41 A lined rock cavern LRCs is composed of mined rock cavern with an impermeable liner for gas storage and repre sents a more recent development than salt caverns The general principles related to hydrogen storage in lined rock caverns LRCs are covered by Refs 44 50 and the most advanced LRC technology for underground gas storage is comprised of an impermeable inner steel liner a sliding layer and a reinforced concrete layer for transferring load from the steel liner to the rock face 44 50 51 They are higher in cost than salt caverns but have been extensively developed in Sweden where there is a lack of suitable geological forma tions available for constructing salt caverns Since 2002 a LRC demonstration plant for storage of compressed natural gas 40 000 m3 200 bar pressure has been operating in Skallen Sweden 41 52 This technology has been considered as part of the HyUnder project 37 41 45 and is now being applied to the storage of hydrogen gas as part of the HYBRIT project for the production of fossil free steel 53 which recently began construction on a pilot plant 54 While more expensive to construct LRCs have a number of advantages over salt cav erns including 1 A wider range of suitable geological locations 2 That their fully engineered nature means that hydrogen purifi cation should not be required 52 3 Higher injection withdrawal rates for hydrogen 4 Only 10 cushion gas is needed 44 The objectives of this paper are to explore the potential of HTMHs paired with compressed hydrogen storage as thermal energy storage for CSP and to compare these results with state of art molten salt heat storage and SunShot targets for next generation CSP heat storage materials The operating temperature and thermodynamics of the HTMH in conjunc tion with the charging time of the thermal energy storage system are used to derive all other factors such as hydrogen fl ow rates during charging compressor performance and characteristics the range of operating pressures in the com pressed gas hydrogen storage system and the quantities of hydrogen gas required The sensible heat contribution of hydrogen released from the HTMH and its impacts were also considered Method for cost estimate of thermal energy storage using high temperature metal hydrides with compressed gas hydrogen storage The specifi c installed cost CS of the HTMH compressed hydrogen TES system in 2010 US kWhth was assessed tak ing into account 1 the cost of the HTMH and ENG materials Cmat HTMH ENG 2 the cost of the HTMH pressure vessel and heat exchanger CHE PV HTMH 3 the cost for the compressed gas hydrogen storage cavern Ccavern H2 4 the cost of any hydrogen cushion gas required Ccush H2 and 5 the installed cost of the compressor Ccompr required to boost the storage pressure of compressed hydrogen gas Lastly the cost of storing the sen sible heat of the hydrogen released from the HTMH CQ H2 was considered CS Cmat HTMH ENG C HE PV HTMH C cavern H2 Ccush H2 Ccompr CQ H2 1 The terms in Eq 1 can be variously grouped to defi ne sub systems of the HTMH TES system These include the heat storagesub system Cmat HTMH ENG CHE PV HTMH thehydrogen compression storage sub system Ccavern H2 Ccush H2 Ccompr and the hydrogen sensible heat storage sub system CQ H2 The spe cifi c installed cost of HTMH TES systems with compressed hydrogen storage were compared to the estimated costs of TES using molten salts at the Crescent Dunes CSP plant 55 and were also compared with the US DOE SunShot targets for next generation CSP plants 30 56 The equations for CHE PV HTMH and Ccompr are defi ned in some of the following sections The equa tions for Cmat HTMH ENG C cavern H2 Ccush H2 and CQ H2 are defi ned in the supporting information along with all of the equations for the variablesthattheydependon requiredmassofHTMH required cavern volume mass of H2cushion gas etc Eqs S 2 eS 18 The literature used as a basis for the cost assessment of hydrogen compression and storage in this work comes from different years and sometimes currencies Unless otherwise noted all costs have been adjusted for infl ation 57 to 2010 USD in accord with the Sunshot Vision Study 30 Where costs were originally reported in another currency they were fi rst converted to USD using the average exchange rate for that year and then adjusted for USD infl ation As they are less affected by infl ationary factors than they are from other market forces the values of raw material costs were used as is with the year of publication included Crescent Dunes as a baseline comparison The recently completed Crescent Dunes CSP 55 has a net electrical output of 110 MW and at full capacity has 10 h of international journal of hydrogen energy 44 2019 9143e91639146 thermal energy storage utilising a hot 565 C and cold 288 C tank molten nitrate salt system As a result Crescent Dunes is an ideal benchmark against which to assess the potential of othermethodsofthermalenergystorage The internalvolume of the hot tank is 13 628 m3 55 and the density of molten SolarSalt 60 wt NaNO3 40 wt KNO3 at 565 C was taken as 1 729 g cm3 extrapolated from 447 C 58 At 426 5 C the temperature midpoint between the hot and cold molten salt tanks the heat capacity of the 46 mol NaNO3e 54 mol KNO3eutectic is 146 4J mol K 58 If SolarSalt 64mol NaNO3 e 36 mol KNO3 is assumed to have the same molar heat capacity then its average specifi c heat capacity over this temperature range 1 562 kJ kg K From these values the hot tank at Crescent D