Electrochemical Systems Means to store hydrogen. Sistemi elettrochimici - Mezzi di stoccaggio dell idrogeno -

Transcription

1 Electrocheical Systes Means to store hydrogen Sistei elettrochiici - Mezzi di stoccaggio dell idrogeno -

2 Hydrogen storage - Means ainly used in prototypes Copressed hydrogen (CH) Hydrogen is copressed and stored in tanks. Traditional Oxyhydrogen bottles are charged up to 20 MPa. Today for transport systes are norally considered pressures up to 35 MPa, that are considered in Italian recharging Facility regulation, but not yet allowed to be transported. Pressures up to 70 MPa are already obtained in tanks and are expected to appear in prototype vehicles within a few years Liquid hydrogen (LH) Hydrogen is cooled down and aintained at a teperature of 22 K. Evaporated gas is used for propulsion; in stand-by external energy is needed for cooling. This option offers large aounts of energy stored per unit of ass and volue; disadvantages are: coplexity, energy wasted for cooling and aintaining cool. Today is not frequently considered for road vehicles Metal hydrides (MH) These are coponents coon in electrocheical batteries. They exploit the capacity of hydrogen of being dispersed in a etallic atrix. Several prototypes developed during nineties; latest is fro in Toyota car (fro Panasonic) In coparison with CH are norally heavier, but ore copact and safety is easier

3 Hydrogen storage ethods under evaluation Carbon nanofibres Nanofibres are applications of nanotechnologies. They proise very high efficiencies (hydrogen ass reaching up to 67% of reservoir ass have been reported); reproducible results are uch worse: below 3%.) This solution, therefore, although under research, has not supplied practical applications yet. Cheical ethods (Artificial fuels) They are based upon the creation of copounds able to carry large quantities of hydrogen. To be copetitive, this solution ust resort to productive process that are siple and use a little energy; hydrogen release should be siple and energy efficient as well. Soe studied exaples: Methanol (partial oxidation reforing: 2CH 3 OH+O 2 2CO 2 +4H 2 ) Aonia (NH 3 :, see next slide) Methanol / nol/ Alkali / aelk lai/

4 Hydrogen storage ethods under evaluation Soe Aonia characteristics: Specific energy: 5100 Wh/kg Hydrogen content: higher than liquid hydrogen Hydrogen content exceeds DOE targets liquid below 0.9 MPa hydrogen can be easily extracted ildly toxic Aonia cracking energy. DG f =-26,5 kj/ol Therefore to crack an aonia ole requires 26,5 kj, a rather sall energy (copare with 237 kj to crack a ole of water)

5 Hydrogen storage Coparison paraeters (1/2) Voluic and assic paraeters Massic Energy (Wh/kg) norally called specific energy (Italian: energia assica o specifica) Voluic Energy (Wh/l) norally called energy density (Italian: energia voluica o densità di energia) Fro therodynaics to Engineering paraeters Standard therodynaics condition: 298K, 101,3kPa, R=8,314 J K -1 ol -1 Fro DH to LHV or UHV. When the enthalpy DH of an oxidation (cobustion) reaction is considered, it indicates the heat generated per unit of ole. Therefore, if the copound olar ass M is known: LHV DH / M UHV DH / M (Italian: LHV->PCI; UHV->PCS) Volue of a ole. pv nrt if n 1 V RT / p 8,314* 298/101,3 k 24,46 L (*) The ore frequent copressed gas pressure is 20 MPa, and this is considered in the table reported in the next page. (*) The ore coon value of 22,42 L per ole refers to a teperature of 0 C g Fro Massic to Voluic energy (E v to E ). For a given gas pressure p, it is: E E / V nrt / p RT / p E E / V nm M v l

6 Hydrogen storage Coparison paraeters (2/2) LHV DH / M UHV DH / M E E v g E / V E / V nm nrt / p l M RT / p R p 8,314JK -1 ol -1 20MPa T 298K DH g (kj/ol) (*) M (g/ol) LHV (kj/g) LHV (Wh/kg) E v /E (g/l)( ) Voluic LHV (Wh/L) Methane (CH 4 ) 802,0 16,04 50, , Hydrogen (H 2 ) 242,0 2, , , (*) Oxidation enthalpy. For hydrogen the value is the water foration enthalpy; for ethane, see exaple developed in the Eleents of Therodynaics and Cheical kinetics presentation. ( ) this is consistent with value of 0,68 kg/ 3 (101,3 kpa, 15 C) available in literature fro encyclopedia.arliquide.co Fuel and reservoir paraeters It is necessary to distinguish between fuel and reservoir paraeters. Indeed, the epty tank ay be well heavier than the contained fuel. It is coon to resort to the reservoir efficiency concept: the ratio of stored fuel ass or volue and reservoir with that fuel. E.g., a 30 MPa coposite tank has typically a ass efficiency of about 3% and a volue efficiency of around 65%

7 Hydrogen storage Copressed Copression work (ideal gas isother process) p2 p2 nrt L pdv dv p 1 p1 V V2 p2 nrt ln nrt ln V p 1 1 pv znrt L=8,314*298*ln2=1717 J/ol; DH(H 2,g )=242kJ/ol =>L/DH=0,0071 In case of ideal gas isother process every pressure doubling iplies a work onto the gas of 0,7% of the hydrogen energy contents Starting fro atospheric pressure, to reach 51,2 MPa 9 doublings are needed, therefore a work of 6,5% of the final tank energy is to be transferred to the fluid. Norally the gas copression energy is not exploited, and therefore this energy is a net loss. In case of a real gas isother process, copressibility factor z ust be taken into account. Moreover, the theral teperature raise during typical copressions akes the copression work to rise; finally copression is ade by a copressor that is an electroechanical device that has echanical and electroechanical losses. So, bringing hydrogen to 51,2 MPa requires an energy of around 10% of the energy finally stored

8 Charge process M+H 2 MH 2 Hydrogen is supplied slightly over 100kPA, the reaction proceeds towards the right side, and is slightly exotheric; it norally stops if heat is not drawn fro the hydride. The charge process is rather fast (ay last a few inutes if the reservoir is cooled) When the reservoir is full, at a constant hydrogen flow, pressure tends to rise; typically at about 200 kpa it is considered to be full. Discharge process Hydrogen storage etal hydrides (1) DH<0 The reservoir is connected to a syste at a pressure lower than that present internally; reaction oves towards left. Pressure drops and hydrogen is released slowly; since the reaction is endotheric, heating the hydride akes the discharge uch faster; in case of no heating, the inner pressure is very low, as the aount of hydrogen: this is advantageous for safety.

9 ON/OFF ON/OFF Measures + Hydrogen storage etal hydrides (2) Measures H 2 tank FC Syste _ sea water heat exchang. heat exchang. p g (t) Measures Heat anageent syste An exaple syste: naval auxiliary electrical source

10 Hydrogen storage note In the following slides data fro project FRESCO are quoted. It was a European coission financed STReP(*) research project aied to realise a scooter whose propulsion energy should coe fro a hydrogen-fed fuel cell syste and a Supercapacitor. Departent of Electrical Systes and Autoation of University of Pisa conducted a lot of work for this project (*) Specific Targeted Research Project FP6 Note regarding LHV. As has been seen in other parts of the course it can be obtained fro the reaction DH converting oles into kgs. E.g., in case of hydrogen cobustion with production of water in for of vapour, it is DH = 242 kj/ol, that corresponds to 1000/2,016*242=120MJ/kg, or 33,3 kwh/kg.

11 Hydrogen storage coparison of different ethods (1) fuel (Wh/kg) ass (%) reservoir (Wh/kg) fuel (Wh/L) Copressed H2 (20-30MPa) ,5-3 (*) (*) FRESCO (30 MPa) , ( ) Liquid H Metal hydrides (ENEA 04) ,0 4, Natural 20 MPa ( + ) Methanol Petrol Traction Lead Battery Li-polyer battery (tract., energy-oriented) 160 (cell) (syste) (*) 20MPa steel tank, and 30MPa coposite tank respectively ( + ) value usually higher than ethane because of the presence of heavier hydrocarbons: C 2 H 6 (Ethane), C 3 H 8, (Propane), C 4 H 10 (Butane); actual value function of actual coposition. 350 biblio: fig. 2 of M. Conte s paper H2 liq data: Zafira in ( ) tank: : 440 Wh/l

12 Hydrogen storage coparison of different ethods (2) Energy density Wh/L Specific Energy Wh/kg Petrol Methanol Liquid H2 20 MPa Metal hydrides 30 MPa Pb Battery