Energy and the Hydrogen Economy
Ulf Bossel
Fuel Cell Consultant
Morgenacherstrasse 2F
CH-5452 Oberrohrdorf / Switzerland
+41-56-496-7292
and
Baldur Eliasson
ABB Switzerland Ltd.
Corporate Research
CH-5405 Baden-Dättwil / Switzerland
Abstract
Between production and use any commercial product is subject to the following processes: packaging, transportation, storage and transfer.
The same is true for hydrogen in a “Hydrogen Economy”.
Hydrogen has to be packaged by compression or liquefaction, it has to be transported by surface vehicles or pipelines, it has to be stored and transferred.
Generated by electrolysis or chemistry, the fuel gas has to go through these market procedures before it can be used by the customer, even if it is produced locally at filling stations.
As there are no environmental or energetic advantages in producing hydrogen from natural gas or other hydrocarbons, we do not consider this option, although hydrogen can be chemically synthesized at relative low cost.
In the past, hydrogen production and hydrogen use have been addressed by many, assuming that hydrogen gas is just another gaseous energy carrier and that it can be handled much like natural gas in today’s energy economy.
With this study we present an analysis of the energy required to operate a pure hydrogen economy.
High-grade electricity from renewable or nuclear sources is needed not only to generate hydrogen, but also for all other essential steps of a hydrogen economy.
But because of the molecular structure of hydrogen, a hydrogen infrastructure is much more energy-intensive than a natural gas economy.
In this study, the energy consumed by each stage is related to the energy content (higher heating value HHV) of the delivered hydrogen itself.
The analysis reveals that much more energy is needed to operate a hydrogen economy than is consumed in today's energy economy.
In fact, depending on the chosen route the input of electrical energy to make, package, transport, store and transfer hydrogen may easily double the hydrogen energy delivered to the end user.
But precious energy can be saved by packaging hydrogen chemically in a synthetic liquid hydrocarbon like methanol or dimethylether DME.
We therefore suggest modifying the vision of a hydrogen economy by considering not only the closed hydrogen (water) cycle, but also the closed carbon (CO2) cycle.
This could create the intellectual platform for the conception of a post-fossil fuel energy economy based on synthetic hydrocarbons.
Carbon atoms from biomass, organic waste materials or recycled carbon dioxide could become the carriers for hydrogen atoms.
Furthermore, the energy consuming electrolysis may be partially replaced by the less energy intensive chemical transformation of water and carbon to synthetic hydrocarbons.
As long as the carbon comes from the biosphere ("biocarbon") the synthetic hydrocarbon economy would be as benign with respect to environment as a pure hydrogen economy.
But the use of "geocarbons" from fossil sources should be avoided to uncouple energy use from global warming.
TO BE CONTINUED ...
ENERGY AND THE HYDROGEN EONOMY
Re: ENERGY AND THE HYDROGEN EONOMY
Table of Contents
1. Introduction 3
2. Properties of Hydrogen 4
3. Energy Needs of a Hydrogen Economy 6
4. Production of Hydrogen 7
4.1 Electrolysis 7
4.2 Reforming 9
5. Packaging of Hydrogen 10
5.1 Compression of Hydrogen 10
5.2 Liquefaction of Hydrogen 12
5.3 Physical Packaging of Hydrogen in Hydrides 14
5.4 Chemical Packaging of Hydrogen in Hydrides 14
6. Delivery of Hydrogen 17
6.1 Road Delivery of Hydrogen 17
6.2 Pipeline Delivery of Hydrogen 20
6.3 Onsite Generation of Hydrogen 22
7. Transfer of Hydrogen 22
8. Summary of Results 26
8.1 The Limits of a Pure Hydrogen Economy 28
8.2 A Liquid Hydrocarbon Economy 29
8.3 Liquid Hydrocarbons 31
9 Conclusion 32
10 References 33
TO BE CONTINUED ...
1. Introduction 3
2. Properties of Hydrogen 4
3. Energy Needs of a Hydrogen Economy 6
4. Production of Hydrogen 7
4.1 Electrolysis 7
4.2 Reforming 9
5. Packaging of Hydrogen 10
5.1 Compression of Hydrogen 10
5.2 Liquefaction of Hydrogen 12
5.3 Physical Packaging of Hydrogen in Hydrides 14
5.4 Chemical Packaging of Hydrogen in Hydrides 14
6. Delivery of Hydrogen 17
6.1 Road Delivery of Hydrogen 17
6.2 Pipeline Delivery of Hydrogen 20
6.3 Onsite Generation of Hydrogen 22
7. Transfer of Hydrogen 22
8. Summary of Results 26
8.1 The Limits of a Pure Hydrogen Economy 28
8.2 A Liquid Hydrocarbon Economy 29
8.3 Liquid Hydrocarbons 31
9 Conclusion 32
10 References 33
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
1. Introduction
Hydrogen is a fascinating energy carrier.
It can be produced from electricity and water.
Its conversion to heat or power is simple and clean.
When combusted with oxygen, hydrogen forms water.
No pollutants are generated or emitted.
The water is returned to nature where it originally came from.
But hydrogen, the most common chemical element on the planet, does not exist in nature in its pure form.
It has to be separated from chemical compounds, by electrolysis from water or by chemical processes from hydrocarbons or other hydrogen carriers.=
The electricity for the electrolysis may eventually come from clean renewable sources such as solar radiation, kinetic energy of wind and water or geothermal heat.
Therefore, hydrogen may become an important link between renewable physical energy and chemical energy carriers.
Hydrogen has fascinated generations of people for centuries including visionary minds like Jules Vernes.
A "Hydrogen Economy" is projected as the ultimate solution for energy and environment.
Hydrogen societies have been formed for the promotion of this goal by publications, meetings and exhibitions.
But has the physics also been properly considered?
Both the production and the use of hydrogen have attracted highest attention while the practical aspects of a hydrogen economy are rarely addressed.
Like any other product hydrogen must be packaged, transported, stored and transferred to bring it from production to final use.
These ordinary market processes require energy.
The energy lost in today's energy economy amounts to about 10% of the energy delivered to the customer.
We would now like to present rough estimates of the energy required to operate a “Hydrogen Economy”.
Without question, technology for a hydrogen economy exists or can be developed.
In fact, enormous amounts of hydrogen are generated, handled, transported and used in the chemical industry today.
But this hydrogen is a chemical substance, not an energy commodity.
Hydrogen production and transportation costs are absorbed in the price of the synthesized chemicals.
The cost of hydrogen remains irrelevant as long as the final products find markets.
Today, the use of hydrogen is governed by economic arguments and not by energetic considerations.
But if hydrogen is used as an energy carrier, energetic arguments must also be considered [1].
How much high-grade energy is used to make, to package, to handle, to store or to transport hydrogen?
The global energy problem cannot be solved in a renewable energy environment, if the energy consumed to make and deliver hydrogen is of the same order as the energy content of the delivered fuel.
But how much energy is consumed for compression, liquefaction, transportation, storage and transfer of hydrogen?
Will there be only the hydrogen path in future?
We have examined the key market procedures by physical reasoning and conclude that the future energy economy is unlikely to be based on pure hydrogen alone.
Hydrogen will certainly be the main link between renewable physical and chemical energy, but most likely it will come to the consumer chemically packaged in the form of one or more synthetic consumer-friendly hydrocarbons.
Preliminary results of our study have already been presented at THE FUEL CELL WORLD conference in July 2002 [1].
TO BE CONTINUED ...
Hydrogen is a fascinating energy carrier.
It can be produced from electricity and water.
Its conversion to heat or power is simple and clean.
When combusted with oxygen, hydrogen forms water.
No pollutants are generated or emitted.
The water is returned to nature where it originally came from.
But hydrogen, the most common chemical element on the planet, does not exist in nature in its pure form.
It has to be separated from chemical compounds, by electrolysis from water or by chemical processes from hydrocarbons or other hydrogen carriers.=
The electricity for the electrolysis may eventually come from clean renewable sources such as solar radiation, kinetic energy of wind and water or geothermal heat.
Therefore, hydrogen may become an important link between renewable physical energy and chemical energy carriers.
Hydrogen has fascinated generations of people for centuries including visionary minds like Jules Vernes.
A "Hydrogen Economy" is projected as the ultimate solution for energy and environment.
Hydrogen societies have been formed for the promotion of this goal by publications, meetings and exhibitions.
But has the physics also been properly considered?
Both the production and the use of hydrogen have attracted highest attention while the practical aspects of a hydrogen economy are rarely addressed.
Like any other product hydrogen must be packaged, transported, stored and transferred to bring it from production to final use.
These ordinary market processes require energy.
The energy lost in today's energy economy amounts to about 10% of the energy delivered to the customer.
We would now like to present rough estimates of the energy required to operate a “Hydrogen Economy”.
Without question, technology for a hydrogen economy exists or can be developed.
In fact, enormous amounts of hydrogen are generated, handled, transported and used in the chemical industry today.
But this hydrogen is a chemical substance, not an energy commodity.
Hydrogen production and transportation costs are absorbed in the price of the synthesized chemicals.
The cost of hydrogen remains irrelevant as long as the final products find markets.
Today, the use of hydrogen is governed by economic arguments and not by energetic considerations.
But if hydrogen is used as an energy carrier, energetic arguments must also be considered [1].
How much high-grade energy is used to make, to package, to handle, to store or to transport hydrogen?
The global energy problem cannot be solved in a renewable energy environment, if the energy consumed to make and deliver hydrogen is of the same order as the energy content of the delivered fuel.
But how much energy is consumed for compression, liquefaction, transportation, storage and transfer of hydrogen?
Will there be only the hydrogen path in future?
We have examined the key market procedures by physical reasoning and conclude that the future energy economy is unlikely to be based on pure hydrogen alone.
Hydrogen will certainly be the main link between renewable physical and chemical energy, but most likely it will come to the consumer chemically packaged in the form of one or more synthetic consumer-friendly hydrocarbons.
Preliminary results of our study have already been presented at THE FUEL CELL WORLD conference in July 2002 [1].
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
2. Properties of Hydrogen
The physical properties of hydrogen are well known [2, 3].
It is the smallest of all atoms.
Consequently, hydrogen is the lightest gas, about 8 times lighter than methane (representing natural gas).
The gravimetric higher heating value "HHV" [4] of a fuel gas are of little relevance for practical applications.
In general, the volume available for fuel tanks is limited, not only in automotive applications.
Also, the diameter of pipelines cannot be increased at will.
Therefore, for most practical assessments it is more meaningful to refer the energy content of fuel gases to a reference volume.
Also, it is proper to use the higher heating value HHV (heat of formation) for this energy analysis, because it reflects the true energy content of the fuel based on the energy conservation principle (1st Law of Thermodynamics).
By contrast, the lower heating value LLV is a technical standard created in the 19th century by boiler engineers confronted with problems of corrosion in the chimneys of coal-fired furnaces caused by condensation of sulfuric acid and other 5 aggressive substances.
Since the production of hydrogen is governed by the heat of formation or the higher heating value, its use should also be related to its HHV energy content.
The following volumetric higher heating values for hydrogen and methane at 1 bar and 25°C will be used in this study.
......................Dimensions Hydrogen Methane
Density at NTP ......kg/m3......0.09...... 0.72
Gravimetric HHV.....MJ/kg.....142.0......55.6
Volumetric HHV......MJ/m3.....12.7.......40.0
At any pressure, hydrogen gas clearly carries less energy per volume than methane (representing natural gas), methanol, propane or octane (representing gasoline).
At 800 bar pressure gaseous hydrogen reaches the volumetric energy density of liquid hydrogen.
But at any pressure, the volumetric energy density of methane gas exceeds that of hydrogen gas by a factor of 3.2 (neglecting non-ideal gas effects).
The common liquid energy carriers like methanol, propane and octane (gasoline) surpass liquid hydrogen by factors 1.8 to 3.4, respectively.
But at 800 bar or in the liquid state hydrogen must be contained in hi-tech pressure tanks or in cryogenic containers, while the liquid fuels are kept under atmospheric conditions in unsophisticated containers.
TO BE CONTINUED ...
The physical properties of hydrogen are well known [2, 3].
It is the smallest of all atoms.
Consequently, hydrogen is the lightest gas, about 8 times lighter than methane (representing natural gas).
The gravimetric higher heating value "HHV" [4] of a fuel gas are of little relevance for practical applications.
In general, the volume available for fuel tanks is limited, not only in automotive applications.
Also, the diameter of pipelines cannot be increased at will.
Therefore, for most practical assessments it is more meaningful to refer the energy content of fuel gases to a reference volume.
Also, it is proper to use the higher heating value HHV (heat of formation) for this energy analysis, because it reflects the true energy content of the fuel based on the energy conservation principle (1st Law of Thermodynamics).
By contrast, the lower heating value LLV is a technical standard created in the 19th century by boiler engineers confronted with problems of corrosion in the chimneys of coal-fired furnaces caused by condensation of sulfuric acid and other 5 aggressive substances.
Since the production of hydrogen is governed by the heat of formation or the higher heating value, its use should also be related to its HHV energy content.
The following volumetric higher heating values for hydrogen and methane at 1 bar and 25°C will be used in this study.
......................Dimensions Hydrogen Methane
Density at NTP ......kg/m3......0.09...... 0.72
Gravimetric HHV.....MJ/kg.....142.0......55.6
Volumetric HHV......MJ/m3.....12.7.......40.0
At any pressure, hydrogen gas clearly carries less energy per volume than methane (representing natural gas), methanol, propane or octane (representing gasoline).
At 800 bar pressure gaseous hydrogen reaches the volumetric energy density of liquid hydrogen.
But at any pressure, the volumetric energy density of methane gas exceeds that of hydrogen gas by a factor of 3.2 (neglecting non-ideal gas effects).
The common liquid energy carriers like methanol, propane and octane (gasoline) surpass liquid hydrogen by factors 1.8 to 3.4, respectively.
But at 800 bar or in the liquid state hydrogen must be contained in hi-tech pressure tanks or in cryogenic containers, while the liquid fuels are kept under atmospheric conditions in unsophisticated containers.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
3. Energy Needs of a Hydrogen Economy
Hydrogen is a synthetic energy carrier.
It carries energy generated by some other processes.
Electrical energy is transferred to hydrogen by electrolysis of water.
But high-grade electrical energy is used not only to produce hydrogen, but also to compress, liquefy, transport, transfer or store the medium.
In most cases the electrical energy could be distributed directly to the end user.
For all stationary application hydrogen competes with grid electricity.
Furthermore, liquid synthetic hydrocarbons could also serve as the general energy carrier of the future.
Carbon from biomass or CO2 captured from flue gases could become the carrier for hydrogen atoms generated with electrical energy from renewable or nuclear sources.
There are environmentally benign alternatives to hydrogen.
Certainly, the cost of hydrogen should be as low as possible.
But the hydrogen economy can establish itself only if it makes sense energetically.
Otherwise, better solutions will conquer the market.
Also, infrastructures exist for almost any synthetic liquid hydrocarbon, while hydrogen requires a totally new distribution network.
The transition to a pure hydrogen economy will affect the entire energy supply and distribution system.
Therefore, all aspects of a hydrogen economy should be discussed before investments are made.
The fundamental question: "How much energy is needed to operate a hydrogen economy?" will be analyzed in detail.
We consider the key elements of a hydrogen economy like production, packaging, transport, storage and transfer of pure hydrogen and relate the energy consumed for these functions to the energy content of the delivered hydrogen.
Our analysis is based on physics and verified by numbers obtained from the hydrogen industry.
Throughout the study, only representative technical solutions will be considered.
TO BE CONTINUED ...
Hydrogen is a synthetic energy carrier.
It carries energy generated by some other processes.
Electrical energy is transferred to hydrogen by electrolysis of water.
But high-grade electrical energy is used not only to produce hydrogen, but also to compress, liquefy, transport, transfer or store the medium.
In most cases the electrical energy could be distributed directly to the end user.
For all stationary application hydrogen competes with grid electricity.
Furthermore, liquid synthetic hydrocarbons could also serve as the general energy carrier of the future.
Carbon from biomass or CO2 captured from flue gases could become the carrier for hydrogen atoms generated with electrical energy from renewable or nuclear sources.
There are environmentally benign alternatives to hydrogen.
Certainly, the cost of hydrogen should be as low as possible.
But the hydrogen economy can establish itself only if it makes sense energetically.
Otherwise, better solutions will conquer the market.
Also, infrastructures exist for almost any synthetic liquid hydrocarbon, while hydrogen requires a totally new distribution network.
The transition to a pure hydrogen economy will affect the entire energy supply and distribution system.
Therefore, all aspects of a hydrogen economy should be discussed before investments are made.
The fundamental question: "How much energy is needed to operate a hydrogen economy?" will be analyzed in detail.
We consider the key elements of a hydrogen economy like production, packaging, transport, storage and transfer of pure hydrogen and relate the energy consumed for these functions to the energy content of the delivered hydrogen.
Our analysis is based on physics and verified by numbers obtained from the hydrogen industry.
Throughout the study, only representative technical solutions will be considered.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
4. Production of Hydrogen
4.1 Electrolysis
Hydrogen does not exist in nature in its pure state, but has to be produced from sources like water and natural gas.
The synthesis of hydrogen requires energy.
Ideally, the energy input equals the energy content of the synthetic gas.
Hydrogen production by any process, e.g. electrolysis, reforming or else, is a process of energy transformation.
Electrical energy or chemical energy of hydrocarbons is transferred to chemical energy of hydrogen.
Unfortunately, the process of hydrogen production is always associated with energy losses.
Making hydrogen from water by electrolysis is one of the worst energy-intensive ways to produce the fuel.
It is a clean process as long as the electricity comes from a clean source.
But electrolysis is associated with losses.
Electrolysis is the reversal of the hydrogen oxidation reaction the standard potential of which is about 1.23 Volts at NPT conditions.
But electrolyzers need higher voltage to separate water into hydrogen and oxygen.
The over-potential is needed to overcome polarization and ohmic losses caused by electric current flow under operational conditions.
Assuming that the same electrolyte and catalysts are used, the polarization losses are typically 0.28 Volt for solid polymer or alkaline systems.
The apparent open circuit voltages thus become 0.95 and 1.51 Volt for fuel cell and electrolyzer, respectively.
For both we assume an area-specific resistance of 0.2 Wcm2 and construct the characteristics for a low temperature fuel cell and a corresponding electrolyzer.
Fuel cells are normally operated at 0.7 Volt to optimize the system efficiency.
We assume the same optimization requirements also hold for an electrolyzer.
The standard potential of 1.23 Volts corresponds to the higher heating value HHV of hydrogen.
Consequently, the over-potential is a measure of the electrical losses of the functioning electrolyzer.
The losses depend on the current density or the hydrogen production rate.
As shown in Figure 4, at 1.76 Volt 1.43 energy units must be supplied for every HHV energy unit contained in the liberated hydrogen.
At higher hydrogen production rates (higher current densities) this number increases further.
TO BE CONTINUED ...
4.1 Electrolysis
Hydrogen does not exist in nature in its pure state, but has to be produced from sources like water and natural gas.
The synthesis of hydrogen requires energy.
Ideally, the energy input equals the energy content of the synthetic gas.
Hydrogen production by any process, e.g. electrolysis, reforming or else, is a process of energy transformation.
Electrical energy or chemical energy of hydrocarbons is transferred to chemical energy of hydrogen.
Unfortunately, the process of hydrogen production is always associated with energy losses.
Making hydrogen from water by electrolysis is one of the worst energy-intensive ways to produce the fuel.
It is a clean process as long as the electricity comes from a clean source.
But electrolysis is associated with losses.
Electrolysis is the reversal of the hydrogen oxidation reaction the standard potential of which is about 1.23 Volts at NPT conditions.
But electrolyzers need higher voltage to separate water into hydrogen and oxygen.
The over-potential is needed to overcome polarization and ohmic losses caused by electric current flow under operational conditions.
Assuming that the same electrolyte and catalysts are used, the polarization losses are typically 0.28 Volt for solid polymer or alkaline systems.
The apparent open circuit voltages thus become 0.95 and 1.51 Volt for fuel cell and electrolyzer, respectively.
For both we assume an area-specific resistance of 0.2 Wcm2 and construct the characteristics for a low temperature fuel cell and a corresponding electrolyzer.
Fuel cells are normally operated at 0.7 Volt to optimize the system efficiency.
We assume the same optimization requirements also hold for an electrolyzer.
The standard potential of 1.23 Volts corresponds to the higher heating value HHV of hydrogen.
Consequently, the over-potential is a measure of the electrical losses of the functioning electrolyzer.
The losses depend on the current density or the hydrogen production rate.
As shown in Figure 4, at 1.76 Volt 1.43 energy units must be supplied for every HHV energy unit contained in the liberated hydrogen.
At higher hydrogen production rates (higher current densities) this number increases further.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
4.2 Reforming
Hydrogen can also be extracted from hydrocarbons by reforming.
This chemical process is, in principle, an energy transfer process.
The HHV energy contained in the original substance can be transferred to the HHV energy of hydrogen.
Theoretically, no external energy is needed to convert a hydrogen-rich energy carrier like methane (CH4) or methanol (CH3OH) into hydrogen by autothermal steam reforming.
But in reality, thermal losses cannot be avoided and the HHV energy content of the original hydrocarbon fuel always exceeds the HHV energy contained in the generated hydrogen.
The efficiency of hydrogen production by reforming is about 90%.
Consequently, more CO2 is released by this "detour" process than by direct use of the hydrocarbon precursors.
But no obvious advantages can be derived with respect to well-to-wheel efficiency and overall CO2 emissions.
For most practical application natural gas can do what hydrogen also does.
There is no need for a conversion of natural gas into hydrogen which, as shown in this study, is more difficult to package and distribute than the natural energy carrier.
The source energy (electricity or hydrocarbons) could be used directly by the consumer at comparable or even higher source-to-service efficiency and lower overall CO2 emission.
Upgrading electricity or natural gas to hydrogen does not provide a universal solution to the energy future, although some sectors of the energy market may prefer hydrogen.
Fleet operation of vehicles may be one such application.
At today's energy prices, it is considerably more expensive to produce hydrogen by water electrolysis than by reforming of fossil fuels.
According to [5] it costs around $5.60 for every GJ of hydrogen energy produced from natural gas, $10.30 per GJ from coal, and $20.10 per GJ to produce hydrogen by electrolysis of water.
TO BE CONTINUED ...
Hydrogen can also be extracted from hydrocarbons by reforming.
This chemical process is, in principle, an energy transfer process.
The HHV energy contained in the original substance can be transferred to the HHV energy of hydrogen.
Theoretically, no external energy is needed to convert a hydrogen-rich energy carrier like methane (CH4) or methanol (CH3OH) into hydrogen by autothermal steam reforming.
But in reality, thermal losses cannot be avoided and the HHV energy content of the original hydrocarbon fuel always exceeds the HHV energy contained in the generated hydrogen.
The efficiency of hydrogen production by reforming is about 90%.
Consequently, more CO2 is released by this "detour" process than by direct use of the hydrocarbon precursors.
But no obvious advantages can be derived with respect to well-to-wheel efficiency and overall CO2 emissions.
For most practical application natural gas can do what hydrogen also does.
There is no need for a conversion of natural gas into hydrogen which, as shown in this study, is more difficult to package and distribute than the natural energy carrier.
The source energy (electricity or hydrocarbons) could be used directly by the consumer at comparable or even higher source-to-service efficiency and lower overall CO2 emission.
Upgrading electricity or natural gas to hydrogen does not provide a universal solution to the energy future, although some sectors of the energy market may prefer hydrogen.
Fleet operation of vehicles may be one such application.
At today's energy prices, it is considerably more expensive to produce hydrogen by water electrolysis than by reforming of fossil fuels.
According to [5] it costs around $5.60 for every GJ of hydrogen energy produced from natural gas, $10.30 per GJ from coal, and $20.10 per GJ to produce hydrogen by electrolysis of water.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
5. Packaging of Hydrogen
5.1 Compression of Hydrogen
Energy is needed to compress gases.
The compression work depends on the thermodynamic compression process.
The ideal isothermal compression cannot be realized.
The adiabatic compression equation [6] is more closely describing the thermodynamic process for ideal gases.
The compression work depends on the nature of the gas.
The energy consumed by an adiabatic compression of monatomic Helium, diatomic hydrogen and five-atomic methane from atmospheric conditions (1 bar = 100,000 Pa) to higher pressures is shown in Figure 2.
Clearly, much more energy per kg is required to compress hydrogen than methane.
The same result is derived from the Nernst equation for the pressure electrolysis of water.
In both cases, the compression work is the difference between the final and the initial energy state of the hydrogen gas.
Figure 6 illustrates the difference between adiabatic and isothermal ideal-gas compression of hydrogen.
Multi-stage compressors with intercoolers operate between these two limiting curves.
Also, hydrogen readily passes compression heat to cooler walls, thereby approaching isothermal conditions.
Numbers provided by a leading manufacturer [7] of hydrogen compressors show that the energy invested in the compression of hydrogen is about 7.2% of its higher heating value (HHV).
This number relates to a 5-stage compression of 1,000 kg of hydrogen per hour from 1 to 200 bar.
For a final pressure of 800 bar the compression energy requirements would amount to about 13% of the energy content of hydrogen.
This analysis does not include electrical losses in the power supply system.
TO BE CONTINUED ...
5.1 Compression of Hydrogen
Energy is needed to compress gases.
The compression work depends on the thermodynamic compression process.
The ideal isothermal compression cannot be realized.
The adiabatic compression equation [6] is more closely describing the thermodynamic process for ideal gases.
The compression work depends on the nature of the gas.
The energy consumed by an adiabatic compression of monatomic Helium, diatomic hydrogen and five-atomic methane from atmospheric conditions (1 bar = 100,000 Pa) to higher pressures is shown in Figure 2.
Clearly, much more energy per kg is required to compress hydrogen than methane.
The same result is derived from the Nernst equation for the pressure electrolysis of water.
In both cases, the compression work is the difference between the final and the initial energy state of the hydrogen gas.
Figure 6 illustrates the difference between adiabatic and isothermal ideal-gas compression of hydrogen.
Multi-stage compressors with intercoolers operate between these two limiting curves.
Also, hydrogen readily passes compression heat to cooler walls, thereby approaching isothermal conditions.
Numbers provided by a leading manufacturer [7] of hydrogen compressors show that the energy invested in the compression of hydrogen is about 7.2% of its higher heating value (HHV).
This number relates to a 5-stage compression of 1,000 kg of hydrogen per hour from 1 to 200 bar.
For a final pressure of 800 bar the compression energy requirements would amount to about 13% of the energy content of hydrogen.
This analysis does not include electrical losses in the power supply system.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
5.2 Liquefaction of Hydrogen
Even more energy is needed to compact hydrogen by liquefaction.
Theoretically only about 3.6 MJ/kg have to be removed to cool hydrogen down to 20K (-253°C) and another 0.46 MJ/kg to condense the gas under atmospheric pressure.
About 4 MJ/kg are removed from room temperature hydrogen gas in the process, little compared to its energy content of 142 MJ/kg.
But cryogenic refrigeration is a complex process involving Carnot cycles and physical effects (e.g. Joule-Thomsen) that do not obey the laws of heat engines.
Nevertheless, the Carnot efficiency is used as a reference for the foregoing process analysis.
For the refrigeration between room temperature (TR = 25°C = 298 K) and liquid hydrogen temperature (TL = -253°C = 20 K) one obtains a Carnot efficiency of about 7%.
The assumed single-step Carnot-type cooling process would consume at least 57 MJ/kg or 40% of the HHV energy content of hydrogen.
This simple analysis does not include mechanical, thermal, flow-related or electrical losses in the multi-stage refrigeration process.
But by intelligent process design the Carnot limitations may be partially removed.
But the lower limit of energy consumption of a liquefaction plant does not drop much below 30% of the higher heating value of the liquefied hydrogen.
As a theoretical analysis of the complicated, multi-stage liquefaction processes is difficult, we present the energy consumption of existing hydrogen liquefaction plants [8].
The compilation reveals the following.
Small (10 kg/h) liquefaction plants need about 100 MJ/kg, while large plants of 1000 kg/h or more capacity consume about 40 MJ of electrical energy for each kg liquefied hydrogen.
The specific energy input decreases with plant size, but a minimum of about 40 MJ per kg H2 remains.
For small liquefaction plants the energy needed to liquefy hydrogen may exceed the HHV of the gas.
But even with the largest plants (10,000 kg/h) at least 30% of the HHV energy is needed for the liquefaction process.
TO BE CONTINUED ...
Even more energy is needed to compact hydrogen by liquefaction.
Theoretically only about 3.6 MJ/kg have to be removed to cool hydrogen down to 20K (-253°C) and another 0.46 MJ/kg to condense the gas under atmospheric pressure.
About 4 MJ/kg are removed from room temperature hydrogen gas in the process, little compared to its energy content of 142 MJ/kg.
But cryogenic refrigeration is a complex process involving Carnot cycles and physical effects (e.g. Joule-Thomsen) that do not obey the laws of heat engines.
Nevertheless, the Carnot efficiency is used as a reference for the foregoing process analysis.
For the refrigeration between room temperature (TR = 25°C = 298 K) and liquid hydrogen temperature (TL = -253°C = 20 K) one obtains a Carnot efficiency of about 7%.
The assumed single-step Carnot-type cooling process would consume at least 57 MJ/kg or 40% of the HHV energy content of hydrogen.
This simple analysis does not include mechanical, thermal, flow-related or electrical losses in the multi-stage refrigeration process.
But by intelligent process design the Carnot limitations may be partially removed.
But the lower limit of energy consumption of a liquefaction plant does not drop much below 30% of the higher heating value of the liquefied hydrogen.
As a theoretical analysis of the complicated, multi-stage liquefaction processes is difficult, we present the energy consumption of existing hydrogen liquefaction plants [8].
The compilation reveals the following.
Small (10 kg/h) liquefaction plants need about 100 MJ/kg, while large plants of 1000 kg/h or more capacity consume about 40 MJ of electrical energy for each kg liquefied hydrogen.
The specific energy input decreases with plant size, but a minimum of about 40 MJ per kg H2 remains.
For small liquefaction plants the energy needed to liquefy hydrogen may exceed the HHV of the gas.
But even with the largest plants (10,000 kg/h) at least 30% of the HHV energy is needed for the liquefaction process.
TO BE CONTINUED ...
Re: ENERGY AND THE HYDROGEN EONOMY
5.3 Physical Packaging of Hydrogen in Hydrides
At this time only a generalized assessment can be presented for the physical (e.g. adsorption on metal hydrides) storage of hydrogen in spongy matrices of special alloys like LaNi5 or ZrCr2.
Hydrogen is stored by physical/chemical adsorption, i.e. by a very close, but not perfect bond between hydrogen atoms and the storage alloys.
Heat is released when a hydrogen storage container is filled.
The release of hydrogen at lower pressure is driven by an influx of heat proportional to the hydrogen liberation rate.
According to [9] metal hydrides store only around 55-60 kgH2/m3 compared to 70 kgH2/m3 for liquid hydrogen.
But 100 kg of hydrogen are contained in one cubic meter of methanol.
The energy balance shall be described in general terms.
Again, energy is needed to produce and compress hydrogen.
Some of this energy input is lost in form of waste heat.
When hydrogen is released heat must be added.
No additional heat is required for small liberation rates and for containers designed for efficient heat exchange with the environment.
Also waste heat from the fuel cell may be used to heat the hydrogen storage cartridge.
One may wish to consider the transport energy for the heavy metal hydride cartridges.
Not even two grams of hydrogen can be stored in a small 230 g metal hydride cartridge.
This makes this type of hydrogen packaging impractical for automotive applications.
But the energy needed to package hydrogen in physical metal hydrides is more or less limited to the energy needed to produce and compress hydrogen to 30 bar pressure.
The energy cost of hydrogen delivered to the customer in physical metal hydrides is thus lower than of compressed hydrogen gas delivered at 200 bar pressure.
TO BE CONTINUED ...
At this time only a generalized assessment can be presented for the physical (e.g. adsorption on metal hydrides) storage of hydrogen in spongy matrices of special alloys like LaNi5 or ZrCr2.
Hydrogen is stored by physical/chemical adsorption, i.e. by a very close, but not perfect bond between hydrogen atoms and the storage alloys.
Heat is released when a hydrogen storage container is filled.
The release of hydrogen at lower pressure is driven by an influx of heat proportional to the hydrogen liberation rate.
According to [9] metal hydrides store only around 55-60 kgH2/m3 compared to 70 kgH2/m3 for liquid hydrogen.
But 100 kg of hydrogen are contained in one cubic meter of methanol.
The energy balance shall be described in general terms.
Again, energy is needed to produce and compress hydrogen.
Some of this energy input is lost in form of waste heat.
When hydrogen is released heat must be added.
No additional heat is required for small liberation rates and for containers designed for efficient heat exchange with the environment.
Also waste heat from the fuel cell may be used to heat the hydrogen storage cartridge.
One may wish to consider the transport energy for the heavy metal hydride cartridges.
Not even two grams of hydrogen can be stored in a small 230 g metal hydride cartridge.
This makes this type of hydrogen packaging impractical for automotive applications.
But the energy needed to package hydrogen in physical metal hydrides is more or less limited to the energy needed to produce and compress hydrogen to 30 bar pressure.
The energy cost of hydrogen delivered to the customer in physical metal hydrides is thus lower than of compressed hydrogen gas delivered at 200 bar pressure.
TO BE CONTINUED ...