5.4 Chemical Packaging of Hydrogen in Hydrides
Hydrogen may also be stored chemically in alkali metal hydrides.
There are many options in the alkali group like LiH, NaH, KH, CaH2.
But also complex binary hydride compounds like LiBH4, NaBH4, KBH4, LiAlH4 or NaAH4 are of interest and have been proposed as hydrogen sources.
None of these compounds can be found in nature.
All have to be synthesized from metals and hydrogen.
Let us consider the case of calcium hydride CaH2.
The compound is produced by combining pure calcium metal with pure hydrogen at 480°C.
Energy is needed to extract calcium from calcium carbonate (lime stone) and hydrogen from water by the following endothermic processes:
CaCO3 - Ca + CO2 + 1/2 O2 = + 808 kJ/mol
H2O - H2 + 1/2 O2 = + 286 kJ/mol
Some of the energy is recovered when the two elements are combined at 480°C by an exothermic process:
Ca + H2 - CaH2 = - 192 kJ/mol
The three equations combine to the virtual net reaction:
CaCO3 + H2O - CaH2 + CO2 + O2 = + 902 kJ/mol
Similarly, one obtains for the production of NaH and LiH from NaCl or LiCl:
NaCl + 0.5 H2O - NaH + Cl + 0.25 O2 = + 500 kJ/mol
and
LiCl + 0.5 H2O - LiH + Cl + 0.25 O2 = + 460 kJ/mol
The material is then cooled under hydrogen to room temperature, granulated and packaged in airtight containers.
The hydrides react with water vividly under release of heat and hydrogen:
CaH2 + 2 H2O - Ca(OH)2 + 2 H2 = - 224 kJ/mol
NaH + H2O - NaOH + H2 = - 85 kJ/mol
LiH + H2O - LiOH + H2 = - 111 kJ/mol
In fact, the reaction of hydrides with water produces twice the hydrogen contained in hydride itself.
Apparently, water is reduced while the hydride is oxidized to hydroxide.
The generated heat has to be removed by cooling and is lost in most cases.
The energy losses associated with the electrolytic decomposition of water, NaCl and LiCl have not even been considered.
At least 160% of the HHV energy content of the librated hydrogen has to be invested to produce the hydrides.
The chemical packaging of hydrogen in alkali metal hydrides will therefore remain a solution for a limited number of practical applications.
At least 60% of the input energy is lost in the process.
TO BE CONTINUED ...
ENERGY AND THE HYDROGEN EONOMY
Re: ENERGY AND THE HYDROGEN EONOMY
6. Delivery of Hydrogen
6.1 Road Delivery of Hydrogen
A hydrogen economy also involves hydrogen transport by trucks and ships.
There are other options for hydrogen distribution, but road transport will always play a role, be it to serve remote locations or to provide back-up fuel to filling stations at times of peak demand.
The comparative analysis is based on information obtained from the fuel and gas transport companies Messer-Griesheim [10], Esso (Schweiz) [11], Jani GmbH [12] and Hover [13] some of the leading providers of industrial gases in Germany and Switzerland.
The following assumptions are made: Hydrogen (at 200 bar), liquid hydrogen, methanol, propane and octane (representing gasoline) are trucked from the refinery or hydrogen plant to the consumer.
Trucks with a gross weight of 40 tons (30 tons for liquid hydrogen) are fitted with suitable tanks or pressure vessels.
Also, at full load 40 kg of Diesel are consumed per 100 km.
This is equivalent of 1 kg per ton per 100 km.
The fuel consumption is reduced accordingly for the return run with emptied tanks.
We assume the same engine efficiency for all transport vehicles.
While in most cases the transport is weight-limited, it is limited by volume for liquid hydrogen as shown by the following sample.
The useful volume of a large moving van, a box 2.4 m wide, 2.5 m high and 10 m long, is 60 m3.
But only 4.2 tons of liquid hydrogen can be filled into this box, because the density of the cold liquid is only 70 kg/m3 or slightly more than that of heavy duty Styrofoam.
But space is needed for container, thermal insulation, equipment etc.
In fact, there is room for only about 2.1 tons of liquid hydrogen on a large-size truck.
This makes trucking of liquid hydrogen expensive, because despite of its small payload, the vehicle has to be financed, maintained, registered, insured, and driven as any truck by an experienced driver.
For the analysis we assume the gross weight of the liquid hydrogen carrier is only 30 tons.
Furthermore, hydrogen pressure tanks can be emptied only from 200 bar to about 42 bar to accommodate for the 40 bar pressure systems of the receiver.
Such pressure cascades are standard praxis today.
Otherwise compressors must be used to completely empty the content of the delivery tank into a higher-pressure storage vessel.
This would not only make the gas transfer more difficult, but also require additional compression energy as discussed below.
As a consequence, pressurized gas carriers deliver only 80% of their freight, while 20% of the load remains in the tanks and is returned to the gas plant.
Each 40-ton truck is designed to carry a maximum of fuel.
For methanol and octane the tare load it is about 26 tons, for propane about 20 tons.
At 200-bar pressure a 40-ton truck can carry 4 tons, but deliver only 3.2 tons of methane.
Today, at 200 a. pressure only 320 kg of hydrogen can be carried and only 288 kg are delivered by a 40-ton truck.
This is a direct consequence of the low density of hydrogen, as well as the weight of the pressure vessels and safety armatures.
In anticipation of technical developments, the analysis was performed for 4000 kg methane and 500 kg of hydrogen, of which 80% or 3200 kg and 400 kg, respectively, are delivered to the consumer.
With this assumption, a dead weight of 39.6 tons has to be moved on the road to deliver 400 kg of hydrogen.
On the return run a heavy empty hydrogen truck consumes more diesel fuel than a much lighter empty gasoline carrier.
The energy needed to transport any of the three liquid fuels is reasonably small.
It remains below 3% of the HHV energy content of the delivered commodity for a one- way delivery distance of 500 km.
But at almost any distance the relative energy consumption associated with the delivery of pressurized hydrogen becomes unacceptable.
About 32 times more diesel fuel is required to deliver in the form of gaseous hydrogen compared to liquid gasoline.
This factor is only about 4.5 for liquid hydrogen, but recall how much energy is required to liquefy the carried energy in initially.
In our analysis we do not consider improvements of the fuel economy of both conventional engine and fuel cell vehicles.
Today, the fuel economy of modern, clean Diesel engines is excellent, but does not quite reach the HHV fuel economy of fuel cells vehicles.
In both cases, the economy can be significantly improved by hybrid systems, mainly due to regenerative breaking.
But from well to wheel either fuel path leads to similar results with respect to energy and CO2 emissions.
As both technology offer potentials for improvements, no distinctive answer can be given at this time.
The following note may serve to illustrate the consequences of the scenario.
A mid-size filling station on any major freeway easily sells 26 tons of gasoline each day.
This fuel can be delivered by one 40-ton gasoline truck.
Because of a potentially superior tank-to-wheel efficiency of fuel cell vehicles, we assume that hydrogen-fuelled vehicles need only 70% of the energy consumed by gasoline or Diesel vehicles to travel the same distance.
Still, it would take 15 trucks to deliver compressed hydrogen (200 bar) energy to the station for the same daily amount of transport services, i.e. to provide fuel for the same number of passenger or cargo miles per day.
Also, the transfer of pressurized hydrogen from those 15 trucks to the filling station takes much more time than draining gasoline from a single tanker into an underground storage tank.
For safety reasons, hydrogen filling station may have to close down for some hours every day.
Today about one in 100 trucks is a gasoline or diesel tanker.
For surface transportation of hydrogen one may see 115 trucks on the road, 15 or 13% of them transporting hydrogen.
One out of seven accidents involving trucks would involve a hydrogen truck.
Every seventh truck-truck collision would occur between two hydrogen carriers.
This scenario is certainly unacceptable for many reasons.
TO BE CONTINUED ...
6.1 Road Delivery of Hydrogen
A hydrogen economy also involves hydrogen transport by trucks and ships.
There are other options for hydrogen distribution, but road transport will always play a role, be it to serve remote locations or to provide back-up fuel to filling stations at times of peak demand.
The comparative analysis is based on information obtained from the fuel and gas transport companies Messer-Griesheim [10], Esso (Schweiz) [11], Jani GmbH [12] and Hover [13] some of the leading providers of industrial gases in Germany and Switzerland.
The following assumptions are made: Hydrogen (at 200 bar), liquid hydrogen, methanol, propane and octane (representing gasoline) are trucked from the refinery or hydrogen plant to the consumer.
Trucks with a gross weight of 40 tons (30 tons for liquid hydrogen) are fitted with suitable tanks or pressure vessels.
Also, at full load 40 kg of Diesel are consumed per 100 km.
This is equivalent of 1 kg per ton per 100 km.
The fuel consumption is reduced accordingly for the return run with emptied tanks.
We assume the same engine efficiency for all transport vehicles.
While in most cases the transport is weight-limited, it is limited by volume for liquid hydrogen as shown by the following sample.
The useful volume of a large moving van, a box 2.4 m wide, 2.5 m high and 10 m long, is 60 m3.
But only 4.2 tons of liquid hydrogen can be filled into this box, because the density of the cold liquid is only 70 kg/m3 or slightly more than that of heavy duty Styrofoam.
But space is needed for container, thermal insulation, equipment etc.
In fact, there is room for only about 2.1 tons of liquid hydrogen on a large-size truck.
This makes trucking of liquid hydrogen expensive, because despite of its small payload, the vehicle has to be financed, maintained, registered, insured, and driven as any truck by an experienced driver.
For the analysis we assume the gross weight of the liquid hydrogen carrier is only 30 tons.
Furthermore, hydrogen pressure tanks can be emptied only from 200 bar to about 42 bar to accommodate for the 40 bar pressure systems of the receiver.
Such pressure cascades are standard praxis today.
Otherwise compressors must be used to completely empty the content of the delivery tank into a higher-pressure storage vessel.
This would not only make the gas transfer more difficult, but also require additional compression energy as discussed below.
As a consequence, pressurized gas carriers deliver only 80% of their freight, while 20% of the load remains in the tanks and is returned to the gas plant.
Each 40-ton truck is designed to carry a maximum of fuel.
For methanol and octane the tare load it is about 26 tons, for propane about 20 tons.
At 200-bar pressure a 40-ton truck can carry 4 tons, but deliver only 3.2 tons of methane.
Today, at 200 a. pressure only 320 kg of hydrogen can be carried and only 288 kg are delivered by a 40-ton truck.
This is a direct consequence of the low density of hydrogen, as well as the weight of the pressure vessels and safety armatures.
In anticipation of technical developments, the analysis was performed for 4000 kg methane and 500 kg of hydrogen, of which 80% or 3200 kg and 400 kg, respectively, are delivered to the consumer.
With this assumption, a dead weight of 39.6 tons has to be moved on the road to deliver 400 kg of hydrogen.
On the return run a heavy empty hydrogen truck consumes more diesel fuel than a much lighter empty gasoline carrier.
The energy needed to transport any of the three liquid fuels is reasonably small.
It remains below 3% of the HHV energy content of the delivered commodity for a one- way delivery distance of 500 km.
But at almost any distance the relative energy consumption associated with the delivery of pressurized hydrogen becomes unacceptable.
About 32 times more diesel fuel is required to deliver in the form of gaseous hydrogen compared to liquid gasoline.
This factor is only about 4.5 for liquid hydrogen, but recall how much energy is required to liquefy the carried energy in initially.
In our analysis we do not consider improvements of the fuel economy of both conventional engine and fuel cell vehicles.
Today, the fuel economy of modern, clean Diesel engines is excellent, but does not quite reach the HHV fuel economy of fuel cells vehicles.
In both cases, the economy can be significantly improved by hybrid systems, mainly due to regenerative breaking.
But from well to wheel either fuel path leads to similar results with respect to energy and CO2 emissions.
As both technology offer potentials for improvements, no distinctive answer can be given at this time.
The following note may serve to illustrate the consequences of the scenario.
A mid-size filling station on any major freeway easily sells 26 tons of gasoline each day.
This fuel can be delivered by one 40-ton gasoline truck.
Because of a potentially superior tank-to-wheel efficiency of fuel cell vehicles, we assume that hydrogen-fuelled vehicles need only 70% of the energy consumed by gasoline or Diesel vehicles to travel the same distance.
Still, it would take 15 trucks to deliver compressed hydrogen (200 bar) energy to the station for the same daily amount of transport services, i.e. to provide fuel for the same number of passenger or cargo miles per day.
Also, the transfer of pressurized hydrogen from those 15 trucks to the filling station takes much more time than draining gasoline from a single tanker into an underground storage tank.
For safety reasons, hydrogen filling station may have to close down for some hours every day.
Today about one in 100 trucks is a gasoline or diesel tanker.
For surface transportation of hydrogen one may see 115 trucks on the road, 15 or 13% of them transporting hydrogen.
One out of seven accidents involving trucks would involve a hydrogen truck.
Every seventh truck-truck collision would occur between two hydrogen carriers.
This scenario is certainly unacceptable for many reasons.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
6.2 Pipeline Delivery of Hydrogen
Hydrogen pipelines exist, but they are used to transport a chemical commodity from one to another production site.
The energy required to move the gas has is irrelevant, because energy consumption is part of the production costs.
This is not so for hydrogen energy transport through pipelines.
Normally, pumps are installed at regular intervals to keep the gas moving.
These pumps are energized by energy taken from the delivery stream.
About 0.3% of the natural gas is used every 150 km to energize a compressor to move the gas [14].
The assessment of the energy consumed to pump hydrogen through pipelines is derived from this natural gas pipeline operating experience.
The comparison is done for equal energy flows.
The same amount of energy is delivered to the customer through the same pipeline either contained in natural gas or hydrogen.
In reality, existing pipelines cannot be used for hydrogen, because of diffusion losses, brittleness of materials and seals, incompatibility of pump lubrication with hydrogen and other technical issues.
The comparison further considers the different viscosities of hydrogen and methane.
The theoretical pumping power N [W] requirement is given by:
N = Vo ?p = A v ?p = p/4 D 2 v ?p = p/4 D2 v 1/2 ? v2 ?
with ? = 0.31164 / Re to the n power
and Re = ? v D / ?
The symbols have the following meaning:
Vo volumetric flow rate [m3/s]
A cross section of pipe [m2]
v flow velocity of the gas [m/s]
?p pressure drop [Pa]
D pipeline diameter [m]
? density of the gas [kg/m3]
? resistance coefficient
Re Reynolds number
n = 0.25 for turbulent pipe flow (Blasius equation) [15]
? dynamic viscosity [kg/(m s)]
Furthermore, the flow of energy through the pipeline, Q [W] is given by:
Q = Vo ? HHV (5)
with HHV being the higher heating value of the transported gas.
Combining equations (2), (3), (4) and (5) one can assess the theoretical pumping power N sub H2 for hydrogen and N sub CH4 for methane and relate both to each other.
Since the pumps run continuously, the power ratio also represents the ratio of the energy consumption for pumping.
Because of the low volumetric energy density of hydrogen, the flow velocity must be increased by over three times.
Consequently, the flow resistance is increased significantly, but the effect is partially compensated for by the lower viscosity of hydrogen.
Still, for the same energy flow about 4.6 times more energy is needed to move hydrogen through the pipeline compared to natural gas.
As this energy is taken from the gas stream, more gas is fed into the pipeline than is delivered at the far end of the tube.
While the energy consumption for methane (representing natural gas) appears reasonable, the energy needed to move hydrogen through pipelines makes this type of hydrogen distributions difficult.
Not 0.3% but at least 1.4% of the hydrogen flow is consumed every 150 km to energize the compressors.
Only 60 to 70% of the hydrogen fed into a pipeline in Northern Africa would actually arrive in Europe.
TO BE CONTINUED ...
Hydrogen pipelines exist, but they are used to transport a chemical commodity from one to another production site.
The energy required to move the gas has is irrelevant, because energy consumption is part of the production costs.
This is not so for hydrogen energy transport through pipelines.
Normally, pumps are installed at regular intervals to keep the gas moving.
These pumps are energized by energy taken from the delivery stream.
About 0.3% of the natural gas is used every 150 km to energize a compressor to move the gas [14].
The assessment of the energy consumed to pump hydrogen through pipelines is derived from this natural gas pipeline operating experience.
The comparison is done for equal energy flows.
The same amount of energy is delivered to the customer through the same pipeline either contained in natural gas or hydrogen.
In reality, existing pipelines cannot be used for hydrogen, because of diffusion losses, brittleness of materials and seals, incompatibility of pump lubrication with hydrogen and other technical issues.
The comparison further considers the different viscosities of hydrogen and methane.
The theoretical pumping power N [W] requirement is given by:
N = Vo ?p = A v ?p = p/4 D 2 v ?p = p/4 D2 v 1/2 ? v2 ?
with ? = 0.31164 / Re to the n power
and Re = ? v D / ?
The symbols have the following meaning:
Vo volumetric flow rate [m3/s]
A cross section of pipe [m2]
v flow velocity of the gas [m/s]
?p pressure drop [Pa]
D pipeline diameter [m]
? density of the gas [kg/m3]
? resistance coefficient
Re Reynolds number
n = 0.25 for turbulent pipe flow (Blasius equation) [15]
? dynamic viscosity [kg/(m s)]
Furthermore, the flow of energy through the pipeline, Q [W] is given by:
Q = Vo ? HHV (5)
with HHV being the higher heating value of the transported gas.
Combining equations (2), (3), (4) and (5) one can assess the theoretical pumping power N sub H2 for hydrogen and N sub CH4 for methane and relate both to each other.
Since the pumps run continuously, the power ratio also represents the ratio of the energy consumption for pumping.
Because of the low volumetric energy density of hydrogen, the flow velocity must be increased by over three times.
Consequently, the flow resistance is increased significantly, but the effect is partially compensated for by the lower viscosity of hydrogen.
Still, for the same energy flow about 4.6 times more energy is needed to move hydrogen through the pipeline compared to natural gas.
As this energy is taken from the gas stream, more gas is fed into the pipeline than is delivered at the far end of the tube.
While the energy consumption for methane (representing natural gas) appears reasonable, the energy needed to move hydrogen through pipelines makes this type of hydrogen distributions difficult.
Not 0.3% but at least 1.4% of the hydrogen flow is consumed every 150 km to energize the compressors.
Only 60 to 70% of the hydrogen fed into a pipeline in Northern Africa would actually arrive in Europe.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
6.3 Onsite Generation of Hydrogen
One option for providing clean hydrogen at filling stations and dispersed depots is the on-site generation of the gas by electrolysis.
Again, the energy needed to generate and compress hydrogen by this scheme is compared to the HHV energy content of the hydrogen delivered to local customers.
Natural gas reforming is not considered for reasons stated earlier.
The analysis is done for single gas station serving 100 to 2,000 conventional road vehicles per day.
On the average, each car or truck is assumed to accept 60 liters (= 50 kg) of gasoline or diesel.
For the 100 and 2000 vehicles per day the energy equivalent would be about 1,700 to 34,000 kg of hydrogen per day, respectively.
But on a tank-to-wheel basis fuel cell vehicles consume less energy per driven distance than cars equipped with IC engines.
Based on the HHV of both gasoline and hydrogen, we assume that fuel cell vehicles need only 70% of the energy consumed by IC engine vehicles to travel the same distance.
The key assumptions for continuous operation of the onsite hydrogen plant and the most important results are the following:
Vehicles / day................1/d.....100.......500.....1000.....1500.....2000
Gasoline, Diesel / vehicle...kg.......50........50........50........50........50
Fossil energy supplied.....GJ/d.....241....1,203....2,407....3,610....4,814
Efficiency factor................%.......70.......70........70.........70.......70
Hydrogen energy supplied.GJ/d......176.....878.....1,755.....2,633...3,510
Hydrogen mass supplied...kg/d...1,188...5,938...11,877....17,815..23,753
Electrolyzer efficiency.........%.......70.......75........78.........79........80
AC/DC conversion..............%.......93.......94.........95.........96.......96
Energy for electrolysis....GJ/d.....3259...1,195.....2,274.....3,332...4,388
Water needed..............m3/d........11......53........107.......160......214
Energy for water supply...GJ/d.........8......36..........68.......100......132
H2-compression, 200 bar..GJ/d........25....109.........204.......295......384
Total energy needed......GJ/d.......292..1,340......2,546....3,727....4,903
Continuous power needed..MW..........3......16..........29........43.......57
Relative to supplied H2 HHV..%.......173.....159.........151......147......146
Energy wasted per H2 HHV....%........73......59...........51.......47........46
The electrolyzer efficiency varies with size from 70 to 80% for 100 and 2,000 vehicles per day, respectively.
Also, losses occur in the AC-DC power conversion.
Between 3 and 51 MW of power are needed for making hydrogen by electrolysis.
Additional power is needed for the water make-up (0.09 to 1.52 MW) and for the compression of the hydrogen to 200 bar (0.29 to 4.45 MW).
In all, between 3 and 57 MW of electric power must be supplied to the station to generate hydrogen for 100 to 2,000 vehicles per day.
It may be of interest that between 11 and 214 m3 of water are consumed daily.
The higher number corresponds to about 2.5 liters per second.
The total energy needed to generate and compress hydrogen at filling stations exceeds the HHV energy of the delivered hydrogen by 50%.
The availability of electricity may certainly be questioned.
Today, about one sixth of the energy for end-use is supplied by copper wires.
The generation of hydrogen at filling stations would require a 3 to 5 fold increase of the electric power generating capacity.
The energy output of a 1 GW nuclear power plant is needed to serve twenty to thirty hydrogen filling stations on frequented highways.
TO BE CONTINUED ...
One option for providing clean hydrogen at filling stations and dispersed depots is the on-site generation of the gas by electrolysis.
Again, the energy needed to generate and compress hydrogen by this scheme is compared to the HHV energy content of the hydrogen delivered to local customers.
Natural gas reforming is not considered for reasons stated earlier.
The analysis is done for single gas station serving 100 to 2,000 conventional road vehicles per day.
On the average, each car or truck is assumed to accept 60 liters (= 50 kg) of gasoline or diesel.
For the 100 and 2000 vehicles per day the energy equivalent would be about 1,700 to 34,000 kg of hydrogen per day, respectively.
But on a tank-to-wheel basis fuel cell vehicles consume less energy per driven distance than cars equipped with IC engines.
Based on the HHV of both gasoline and hydrogen, we assume that fuel cell vehicles need only 70% of the energy consumed by IC engine vehicles to travel the same distance.
The key assumptions for continuous operation of the onsite hydrogen plant and the most important results are the following:
Vehicles / day................1/d.....100.......500.....1000.....1500.....2000
Gasoline, Diesel / vehicle...kg.......50........50........50........50........50
Fossil energy supplied.....GJ/d.....241....1,203....2,407....3,610....4,814
Efficiency factor................%.......70.......70........70.........70.......70
Hydrogen energy supplied.GJ/d......176.....878.....1,755.....2,633...3,510
Hydrogen mass supplied...kg/d...1,188...5,938...11,877....17,815..23,753
Electrolyzer efficiency.........%.......70.......75........78.........79........80
AC/DC conversion..............%.......93.......94.........95.........96.......96
Energy for electrolysis....GJ/d.....3259...1,195.....2,274.....3,332...4,388
Water needed..............m3/d........11......53........107.......160......214
Energy for water supply...GJ/d.........8......36..........68.......100......132
H2-compression, 200 bar..GJ/d........25....109.........204.......295......384
Total energy needed......GJ/d.......292..1,340......2,546....3,727....4,903
Continuous power needed..MW..........3......16..........29........43.......57
Relative to supplied H2 HHV..%.......173.....159.........151......147......146
Energy wasted per H2 HHV....%........73......59...........51.......47........46
The electrolyzer efficiency varies with size from 70 to 80% for 100 and 2,000 vehicles per day, respectively.
Also, losses occur in the AC-DC power conversion.
Between 3 and 51 MW of power are needed for making hydrogen by electrolysis.
Additional power is needed for the water make-up (0.09 to 1.52 MW) and for the compression of the hydrogen to 200 bar (0.29 to 4.45 MW).
In all, between 3 and 57 MW of electric power must be supplied to the station to generate hydrogen for 100 to 2,000 vehicles per day.
It may be of interest that between 11 and 214 m3 of water are consumed daily.
The higher number corresponds to about 2.5 liters per second.
The total energy needed to generate and compress hydrogen at filling stations exceeds the HHV energy of the delivered hydrogen by 50%.
The availability of electricity may certainly be questioned.
Today, about one sixth of the energy for end-use is supplied by copper wires.
The generation of hydrogen at filling stations would require a 3 to 5 fold increase of the electric power generating capacity.
The energy output of a 1 GW nuclear power plant is needed to serve twenty to thirty hydrogen filling stations on frequented highways.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
7. Transfer of Hydrogen
Liquid can be drained from a full into an empty container by action of gravity.
There is no energy required, unless the liquids are transferred from a lower to a higher tank, under controlled flow rates or under accelerated conditions.
The transfer of pressurized gases obeys different laws.
Assume two tanks of equal volume, one full at 200 bar and the other empty at 0 bar pressure.
After opening the valve between the vessels gas will flow into the empty tank, but the flow will cease when pressure equilibration is accomplished.
Both tanks are half full or half empty.
A pump is required to transfer the remaining content of the supply tank into the receiving tank.
The transfer process may be complicated by temperature effects.
The content of the full tank is cooled by the expansion process.
At equal pressures, the density of the remaining gas is higher than that of the transferred gas in the other tank.
As a consequence, more mass remains in the original vessel than is transferred into the empty one.
Equal mass transfer is accomplished only after the temperatures have reached equilibrium after some time.
For the sample case considered, and for an ideal isothermal compression, the amount of energy required to complete the gas transfer by pumping is given by the difference of the total compression energy contained in the gas at the final pressure p2 and the intermediate pressure p1.
The product p V (= R T) is the same for both compression processes:
W = p0 V0 ln(p2/p0) - p0 V0 ln(p1/p0)
with W [J/kg] specific compression work
p0 [Pa] initial pressure
p1 [Pa] intermediate pressure
p2 [Pa] final pressure
V0 [m3/kg] initial specific volume
For the sample case:
p0 = 1 bar = 1.0 x 105 Pa
p1 = 100 bar = 1.0 x 107 Pa
p2 = 200 bar = 2.0 x 107 Pa
V0 = 11.11 m3/kg
p0 V0 = 1.111 GJ/kg
one obtains for the energy needed to transfer the remaining hydrogen from the half empty supply tank into the receiving tank by an isothermal compression:
W = 0.77 GJ/kg
or about 0.5% of the HHV energy content of the compressed hydrogen.
For a more realistic adiabatic compression and including mechanical and electrical losses one would have obtained about 1%.
This number depends on the actual transfer conditions.
Much more energy is needed to transfer hydrogen from a large 100 bar tank into a small container at 500 bar pressure.
But it takes no additional energy to fill a small tank from a high pressure vessel of substantial size.
For automotive application, one aims at high pressure tanks in vehicles and, as a consequence, has to use energy to transfer the hydrogen from large storage containers which cannot be subjected to high internal pressures.
In any event, the transfer of hydrogen may add to the energy needs of a hydrogen economy.
TO BE CONTINUED ...
Liquid can be drained from a full into an empty container by action of gravity.
There is no energy required, unless the liquids are transferred from a lower to a higher tank, under controlled flow rates or under accelerated conditions.
The transfer of pressurized gases obeys different laws.
Assume two tanks of equal volume, one full at 200 bar and the other empty at 0 bar pressure.
After opening the valve between the vessels gas will flow into the empty tank, but the flow will cease when pressure equilibration is accomplished.
Both tanks are half full or half empty.
A pump is required to transfer the remaining content of the supply tank into the receiving tank.
The transfer process may be complicated by temperature effects.
The content of the full tank is cooled by the expansion process.
At equal pressures, the density of the remaining gas is higher than that of the transferred gas in the other tank.
As a consequence, more mass remains in the original vessel than is transferred into the empty one.
Equal mass transfer is accomplished only after the temperatures have reached equilibrium after some time.
For the sample case considered, and for an ideal isothermal compression, the amount of energy required to complete the gas transfer by pumping is given by the difference of the total compression energy contained in the gas at the final pressure p2 and the intermediate pressure p1.
The product p V (= R T) is the same for both compression processes:
W = p0 V0 ln(p2/p0) - p0 V0 ln(p1/p0)
with W [J/kg] specific compression work
p0 [Pa] initial pressure
p1 [Pa] intermediate pressure
p2 [Pa] final pressure
V0 [m3/kg] initial specific volume
For the sample case:
p0 = 1 bar = 1.0 x 105 Pa
p1 = 100 bar = 1.0 x 107 Pa
p2 = 200 bar = 2.0 x 107 Pa
V0 = 11.11 m3/kg
p0 V0 = 1.111 GJ/kg
one obtains for the energy needed to transfer the remaining hydrogen from the half empty supply tank into the receiving tank by an isothermal compression:
W = 0.77 GJ/kg
or about 0.5% of the HHV energy content of the compressed hydrogen.
For a more realistic adiabatic compression and including mechanical and electrical losses one would have obtained about 1%.
This number depends on the actual transfer conditions.
Much more energy is needed to transfer hydrogen from a large 100 bar tank into a small container at 500 bar pressure.
But it takes no additional energy to fill a small tank from a high pressure vessel of substantial size.
For automotive application, one aims at high pressure tanks in vehicles and, as a consequence, has to use energy to transfer the hydrogen from large storage containers which cannot be subjected to high internal pressures.
In any event, the transfer of hydrogen may add to the energy needs of a hydrogen economy.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
8. Summary of Results
The reported results are by no means final.
The readers of this study are invited to refine the analysis and to contribute further details.
The energy cost of producing, packaging, distributing, storing and transferring hydrogen must have been analyzed in different contexts.
The results of those studies may be used to verify, correct, or reject our numbers.
Whatever, the intent of this compilation is to create an awareness about the weaknesses of a pure hydrogen economy.
We are surprised to discover that, apparently, the energy needed to run a hydrogen economy have never been fully assessed before.
Again, we would like to emphasize that the conversion of natural gas into hydrogen cannot be the solution of the future.
Hydrogen produced by natural gas reforming may cost less than hydrogen obtained by electrolysis, but natural gas itself is as good as hydrogen or even better for many applications.
For given energy demand the well-to-wheel efficiency is reduced and, as a consequence, the emission of CO2 is increased when natural gas is converted to hydrogen for daily use.
Four typical energy paths have been considered to interpret the results.
These are:
A Hydrogen is produced by electrolysis, compressed to 200 bar and distributed by road to filling stations or consumers;
B Hydrogen is produced by electrolysis, liquefied and distributed by road to filling stations or consumers;
C Hydrogen is produced onsite at filling stations or consumers;
D Hydrogen is produced by electrolysis and used to make alkali metal hydrides.
The analysis for ideal processes reveals that considerable amounts of energy are lost between the electrical source energy and the HHV hydrogen energy delivered to the consumer.
For onsite hydrogen production, path C, the electrical energy input exceeds the HHV energy of the delivered hydrogen by a factor of at least 1.65.
In the case of liquid hydrogen, path B, the factor is at lest 2.12.
For all stationary applications the distribution of energy by copper wire will be a better choice than the use of hydrogen as energy carrier.
But the problems of road delivery of compressed hydrogen have been discussed.
It is unlikely that Path A can be realized.
A better option would be the hydrogen distribution by short pipelines.
To deliver hydrogen by chemical hydrides may provide practical solutions in some niche markets, but path D cannot become an important energy vector in a future economy.
Today, about 12% of the original fossil energy is lost between oil wells and filling stations for transportation, refining and distribution.
In a pure hydrogen economy the losses would be considerably higher.
If hydrogen could be chemically packaged in a synthetic liquid fuel, the overall energy consumption would be considerably lower.
TO BE CONTINUED ...
The reported results are by no means final.
The readers of this study are invited to refine the analysis and to contribute further details.
The energy cost of producing, packaging, distributing, storing and transferring hydrogen must have been analyzed in different contexts.
The results of those studies may be used to verify, correct, or reject our numbers.
Whatever, the intent of this compilation is to create an awareness about the weaknesses of a pure hydrogen economy.
We are surprised to discover that, apparently, the energy needed to run a hydrogen economy have never been fully assessed before.
Again, we would like to emphasize that the conversion of natural gas into hydrogen cannot be the solution of the future.
Hydrogen produced by natural gas reforming may cost less than hydrogen obtained by electrolysis, but natural gas itself is as good as hydrogen or even better for many applications.
For given energy demand the well-to-wheel efficiency is reduced and, as a consequence, the emission of CO2 is increased when natural gas is converted to hydrogen for daily use.
Four typical energy paths have been considered to interpret the results.
These are:
A Hydrogen is produced by electrolysis, compressed to 200 bar and distributed by road to filling stations or consumers;
B Hydrogen is produced by electrolysis, liquefied and distributed by road to filling stations or consumers;
C Hydrogen is produced onsite at filling stations or consumers;
D Hydrogen is produced by electrolysis and used to make alkali metal hydrides.
The analysis for ideal processes reveals that considerable amounts of energy are lost between the electrical source energy and the HHV hydrogen energy delivered to the consumer.
For onsite hydrogen production, path C, the electrical energy input exceeds the HHV energy of the delivered hydrogen by a factor of at least 1.65.
In the case of liquid hydrogen, path B, the factor is at lest 2.12.
For all stationary applications the distribution of energy by copper wire will be a better choice than the use of hydrogen as energy carrier.
But the problems of road delivery of compressed hydrogen have been discussed.
It is unlikely that Path A can be realized.
A better option would be the hydrogen distribution by short pipelines.
To deliver hydrogen by chemical hydrides may provide practical solutions in some niche markets, but path D cannot become an important energy vector in a future economy.
Today, about 12% of the original fossil energy is lost between oil wells and filling stations for transportation, refining and distribution.
In a pure hydrogen economy the losses would be considerably higher.
If hydrogen could be chemically packaged in a synthetic liquid fuel, the overall energy consumption would be considerably lower.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
8.1 The Limits of a Pure Hydrogen Economy
The results of this analysis indicate the weakness of a "Pure-Hydrogen-Only Economy" as depicted in Figure 14.
Hydrogen is not only obtained by electrolysis, but also by chemical conversion of biomass.
The economy is based on the natural H2O cycle, but the natural CO2-cycle is truncated and not fully used.
All difficulties with the pure Hydrogen Economy appear to be directly related to the nature of hydrogen.
Most of the problems cannot be solved by additional research and development.
We have to accept that hydrogen is the lightest of all gases and, as a consequence, that its physical properties do not fully match the requirements of the energy market.
Production, packaging, storage, transfer and delivery of the gas, in essence all key component of an economy, are so energy consuming that alternatives should and will be considered.
Mankind cannot afford to waste energy for idealistic goals, but economy will look for practical solutions and select the most energy-saving procedures.
The "Pure-Hydrogen-Only Solution" may never become reality.
The degree of energy waste certainly depends on the chosen path.
Hydrogen generated from rooftop solar electricity and stored at low pressure in stationary tanks may be a viable solution for private buildings.
On the other hand, hydrogen generated in the Sahara desert, pumped to the Mediterranean Sea through pipelines, then liquefied for sea transport, docked in London and locally distributed by trucks may not provide an acceptable energy solution at all.
Too much energy is lost in the process to justify the scheme.
But there are solutions between these two extremes, niche applications, special cases or luxury installations.
This study provides some clues for strengths and weaknesses of the energy carrier hydrogen.
As stated in the beginning, hydrogen may be the only link between physical energy from renewable sources and chemical energy.
It is also the ideal fuel for modern clean energy conversion devices like fuel cells or even hydrogen engines.
But hydrogen is not the ideal medium to carry energy from primary sources to distant end users.
New solutions must be considered for the commercial bridge between electrolyzer and fuel cell.
TO BE CONTINUED ...
The results of this analysis indicate the weakness of a "Pure-Hydrogen-Only Economy" as depicted in Figure 14.
Hydrogen is not only obtained by electrolysis, but also by chemical conversion of biomass.
The economy is based on the natural H2O cycle, but the natural CO2-cycle is truncated and not fully used.
All difficulties with the pure Hydrogen Economy appear to be directly related to the nature of hydrogen.
Most of the problems cannot be solved by additional research and development.
We have to accept that hydrogen is the lightest of all gases and, as a consequence, that its physical properties do not fully match the requirements of the energy market.
Production, packaging, storage, transfer and delivery of the gas, in essence all key component of an economy, are so energy consuming that alternatives should and will be considered.
Mankind cannot afford to waste energy for idealistic goals, but economy will look for practical solutions and select the most energy-saving procedures.
The "Pure-Hydrogen-Only Solution" may never become reality.
The degree of energy waste certainly depends on the chosen path.
Hydrogen generated from rooftop solar electricity and stored at low pressure in stationary tanks may be a viable solution for private buildings.
On the other hand, hydrogen generated in the Sahara desert, pumped to the Mediterranean Sea through pipelines, then liquefied for sea transport, docked in London and locally distributed by trucks may not provide an acceptable energy solution at all.
Too much energy is lost in the process to justify the scheme.
But there are solutions between these two extremes, niche applications, special cases or luxury installations.
This study provides some clues for strengths and weaknesses of the energy carrier hydrogen.
As stated in the beginning, hydrogen may be the only link between physical energy from renewable sources and chemical energy.
It is also the ideal fuel for modern clean energy conversion devices like fuel cells or even hydrogen engines.
But hydrogen is not the ideal medium to carry energy from primary sources to distant end users.
New solutions must be considered for the commercial bridge between electrolyzer and fuel cell.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
8.2 A Liquid Hydrocarbon Economy
The ideal energy carrier is a liquid with a boiling point above 80°C and a solidification point below -40°C.
Such energy carriers stay liquid under normal climate conditions and at high altitudes.
Gasoline, diesel and methanol are good examples of such fuels.
They are in common use not only because they can be extracted from crude oil, but mainly, because they qualify for widespread use because of their physical properties.
Oil companies convert crude oil into gasoline and diesel fuels.
Even if oil had never been discovered, the world would not use synthetic hydrogen, but one or more synthetic hydrocarbon fuel.
Gasoline, diesel, heating oil etc. have emerged as the best solutions with respect to handling, storage, transport and energetic use.
With high certainty, such liquids will also be synthesized from hydrogen and carbon in a distant energy future.
Fortunately, methanol and ethanol can also be derived from plants by biological fermentation processes.
There are a number of synthetic hydrocarbons to be considered.
One of the prime choices may be methanol.
It carries four hydrogen atoms per carbon atom.
It is liquid under normal conditions.
The infrastructure for liquid fuels exists.
Also, methanol can either be directly converted to electricity by Direct Methanol Fuel Cells (DMFC), Molten Carbonate Fuel Cells (MCFC) and Solid Oxide Fuel Cells (SOFC).
It can also be reformed easily to hydrogen for use in Polymer Electrolyte Fuel Cells (PEFC or PEM).
Methanol could become a universal fuel for fuel cells and many other applications.
Carbon from the biosphere may become the key element in a sustainable energy future.
It could come from biomass, from organic waste and from captured CO2.
Typically, biomass has a hydrogen-to-carbon ratio of two.
In the methanol synthesis two additional hydrogen atoms are attached to every bio-carbon.
Instead of converting biomass into hydrogen, hydrogen from renewable sources or even water could be added to biomass to form methanol by a chemical process.
In a LH economy carbon atoms will stay bound in the energy carrier until its final use.
They are then returned to the atmosphere (or recycled).
This is true not only for methanol, but also for ethanol or other synthetic hydrocarbons.
The suggested scheme should be seriously considered for the planning of a clean and sustainable energy future.
TO BE CONTINUED ...
The ideal energy carrier is a liquid with a boiling point above 80°C and a solidification point below -40°C.
Such energy carriers stay liquid under normal climate conditions and at high altitudes.
Gasoline, diesel and methanol are good examples of such fuels.
They are in common use not only because they can be extracted from crude oil, but mainly, because they qualify for widespread use because of their physical properties.
Oil companies convert crude oil into gasoline and diesel fuels.
Even if oil had never been discovered, the world would not use synthetic hydrogen, but one or more synthetic hydrocarbon fuel.
Gasoline, diesel, heating oil etc. have emerged as the best solutions with respect to handling, storage, transport and energetic use.
With high certainty, such liquids will also be synthesized from hydrogen and carbon in a distant energy future.
Fortunately, methanol and ethanol can also be derived from plants by biological fermentation processes.
There are a number of synthetic hydrocarbons to be considered.
One of the prime choices may be methanol.
It carries four hydrogen atoms per carbon atom.
It is liquid under normal conditions.
The infrastructure for liquid fuels exists.
Also, methanol can either be directly converted to electricity by Direct Methanol Fuel Cells (DMFC), Molten Carbonate Fuel Cells (MCFC) and Solid Oxide Fuel Cells (SOFC).
It can also be reformed easily to hydrogen for use in Polymer Electrolyte Fuel Cells (PEFC or PEM).
Methanol could become a universal fuel for fuel cells and many other applications.
Carbon from the biosphere may become the key element in a sustainable energy future.
It could come from biomass, from organic waste and from captured CO2.
Typically, biomass has a hydrogen-to-carbon ratio of two.
In the methanol synthesis two additional hydrogen atoms are attached to every bio-carbon.
Instead of converting biomass into hydrogen, hydrogen from renewable sources or even water could be added to biomass to form methanol by a chemical process.
In a LH economy carbon atoms will stay bound in the energy carrier until its final use.
They are then returned to the atmosphere (or recycled).
This is true not only for methanol, but also for ethanol or other synthetic hydrocarbons.
The suggested scheme should be seriously considered for the planning of a clean and sustainable energy future.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
8.3 Liquid Hydrocarbons
Any synthetic liquid fuel must satisfy a number of requirements.
It should be liquid under normal pressure at temperatures between -40°C and 80°C, be nontoxic, be useful for IC engines, easy to synthesize etc.
The chemicals tabulated below satisfy the liquidity criteria.
They may serve to illustrate that a number of options exist for the synthesis of liquid hydrocarbons from hydrogen and carbon.
But aspects of manufacturing, safety, combustion etc., all well-known to the experts, will eliminate some or add new options to the list.
The following liquid hydrocarbons are considered:
A Methanol CH4O or CH3OH
B Ethanol C2H6O or CH3CH2OH
C Dimethlyether (DME) C2H6O or CH3OCH3
D Ethylmethylether C4H10O or CH3OC2H5
E 2-Methylpropane (Isubutane) C4H10 or CH3CH(CH3)CH3
F 2-Methylbutane (Isopentane) C5H12 or CH3CH(CH3)CH2CH3
G Ethylbenzol C8H10 or C6H5CH2CH3
H Methylcyclohexane (Toluol) C7H14 or C6H5CH3
I Octane C8H18 or CH3(CH2)3CH3
J Ammonia NH3
K Hydrogen (for comparison) H2
Methanol, Ethanol, DME, Toluol and Ammonia, all having relatively simple molecular structures, may become the preferred synthetic energy carriers of the future in competition with liquid (or 800 bar) hydrogen.
Any one of the nine hydrocarbon fuels contains more hydrogen per cubic meter than is contained in the same volume of liquefied or 800 bar compressed hydrogen.
Ammonia even contains even 136 kg of hydrogen per cubic meter.
Also, the energy carried by the hydrocarbons is between two and almost four times greater than the energy contained in the same volume of liquid hydrogen.
If one wants to distribute hydrogen, obviously the best way is combining it with carbon to a liquid fuel.
It may be of interest to observe that the gasoline - like Octane seems to be the best hydrogen carrier and also ranks among the best with respect to energy content per volume.
The synthesis of Octane from bio-carbon and water may pose an attractive solution for an energy economy based on renewable energy sources and the recycling of carbon dioxide.
TO BE CONTINUED ...
Any synthetic liquid fuel must satisfy a number of requirements.
It should be liquid under normal pressure at temperatures between -40°C and 80°C, be nontoxic, be useful for IC engines, easy to synthesize etc.
The chemicals tabulated below satisfy the liquidity criteria.
They may serve to illustrate that a number of options exist for the synthesis of liquid hydrocarbons from hydrogen and carbon.
But aspects of manufacturing, safety, combustion etc., all well-known to the experts, will eliminate some or add new options to the list.
The following liquid hydrocarbons are considered:
A Methanol CH4O or CH3OH
B Ethanol C2H6O or CH3CH2OH
C Dimethlyether (DME) C2H6O or CH3OCH3
D Ethylmethylether C4H10O or CH3OC2H5
E 2-Methylpropane (Isubutane) C4H10 or CH3CH(CH3)CH3
F 2-Methylbutane (Isopentane) C5H12 or CH3CH(CH3)CH2CH3
G Ethylbenzol C8H10 or C6H5CH2CH3
H Methylcyclohexane (Toluol) C7H14 or C6H5CH3
I Octane C8H18 or CH3(CH2)3CH3
J Ammonia NH3
K Hydrogen (for comparison) H2
Methanol, Ethanol, DME, Toluol and Ammonia, all having relatively simple molecular structures, may become the preferred synthetic energy carriers of the future in competition with liquid (or 800 bar) hydrogen.
Any one of the nine hydrocarbon fuels contains more hydrogen per cubic meter than is contained in the same volume of liquefied or 800 bar compressed hydrogen.
Ammonia even contains even 136 kg of hydrogen per cubic meter.
Also, the energy carried by the hydrocarbons is between two and almost four times greater than the energy contained in the same volume of liquid hydrogen.
If one wants to distribute hydrogen, obviously the best way is combining it with carbon to a liquid fuel.
It may be of interest to observe that the gasoline - like Octane seems to be the best hydrogen carrier and also ranks among the best with respect to energy content per volume.
The synthesis of Octane from bio-carbon and water may pose an attractive solution for an energy economy based on renewable energy sources and the recycling of carbon dioxide.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
9. Conclusions
Time has come to shift the attention of energy strategy planning, research and development from a “Hydrogen Economy” to a “Synthetic Liquid Hydrocarbon Economy” and to direct manpower and resources to find technical solutions for a sustainable energy future which is built on the two closed clean natural cycles of water and CO2 or hydrogen and carbon.
If carbon is taken from the biosphere or recycled from power plants ("bio-carbon") and not from fossil resources ("geo-carbon"), the "Synthetic Liquid Hydrocarbon Economy" will be environmentally as benign as a "Pure Hydrogen Economy".
TO BE CONTINUED ...
Time has come to shift the attention of energy strategy planning, research and development from a “Hydrogen Economy” to a “Synthetic Liquid Hydrocarbon Economy” and to direct manpower and resources to find technical solutions for a sustainable energy future which is built on the two closed clean natural cycles of water and CO2 or hydrogen and carbon.
If carbon is taken from the biosphere or recycled from power plants ("bio-carbon") and not from fossil resources ("geo-carbon"), the "Synthetic Liquid Hydrocarbon Economy" will be environmentally as benign as a "Pure Hydrogen Economy".
TO BE CONTINUED ...