The Future of the Hydrogen Economy: Bright or Bleak?

Hydrogen is a fascinating carrier of energy. 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.

By Baldur Eliasson and Ulf Bossel

Part One of white paper by Baldur Eliasson and Ulf Bossel.

But hydrogen, the most common chemical element on the planet, does not exist in nature in its pure form. It has to be generated or "produced" by separating it from chemical compounds. Hydrogen can be produced from water by electrolysis, from hydrocarbon fuels by reforming or thermal cracking, or from other hydrogen carriers by chemical processes. But clean energies such as electricity from solar, wind and hydro must be applied to produce clean hydrogen, i.e. without greenhouse gases or nuclear waste being generated in the production process.

Hydrogen may actually be the only meaningful link between renewable energy and chemical energy carriers.

Hydrogen has fascinated generations of people with good intentions. Promoters of hydrogen claim that a "Hydrogen Economy" will be the ultimate solution to all problems of 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?

With this article we intent to take a closer look at some of the energy aspects related to the use of hydrogen as energy carrier. The "Hydrogen Economy" involves not only production and use of hydrogen, but also all other ingredients of an energy market like packaging, storage, delivery and transfer. This market can flourish if the energy consumed within the market itself is small compared to the energy delivered to the customer.

Today, the energy lost in power transmission, oil refineries or sea and land transport of fuels usually amounts to less than 10% of the energy traded. Therefore, we would like to present rough estimates of the energy required to operate a "Hydrogen Economy".

One important reason for the renewed interest in the hydrogen economy is the problem of global warming. Eighty percent of all commercial energy on earth is provided by fossil fuels. It is almost certain that the use of fossil hydrocarbons and the resulting emission of greenhouse gases such as carbon dioxide cause global warming. It has never been more urgent to find energy resources that do not cause any emissions of green house gases.

Renewable energy from the sun, wind, water and biomass are such energy sources, but they have to be converted to chemical energy for the general energy market. Hydrogen may provide that link. Another possible path is to continue using fossil fuels for producing hydrogen but to capture and sequester CO2, before it is emitted into the atmosphere.

Without question, technical solutions exist or can be developed for a hydrogen economy. 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.

However, if hydrogen is used as an energy carrier, energetic argument must also be considered. 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 becomes comparable to the energy content of the delivered fuel.

It is important to assess and compare the energy balances of different energy path options. Are they as efficient as possible? Will there be only the hydrogen path in future? In the following presentation we show that the future hydrogen economy is unlikely to be based on pure hydrogen only. It will certainly be based on hydrogen, but most likely, the synthetic fuel gas will be chemically packed in consumer- friendly hydrocarbons.

In the following article we present preliminary results of a detailed study by the authors [1]. The study will be published in its entirety later this year.

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). Hydrogen has a gravimetric heating value (we consider only the higher heating value HHV in this study) of [9]:

Hydrogen: HHV
LHV 142 MJ/kg (Higher Heating Value "HHV")
120 MJ/kg (Lower Heating Value "LHV")
compared to
Methane: HHV
LHV 55.5 MJ/kg
50.0 MJ/kg

The volumetric heating values are (1 bar, 25°C):

HHV H2: 11.7 kJ/liter
LHV H2: 9.9 kJ/liter
HHV CH4: 36.5 kJ/liter
LHV CH4: 32.9 kJ/liter

The gravimetric heating value has little relevance for the hydrogen trade. The volume available for fuel tanks is always limited, not only in automotive applications. The diameter of pipelines cannot be increased at will. Therefore, for all practical assessments it is more meaningful to use the volumetric rather than the gravimetric energy density. Hydrogen has to be compacted by compression or liquefaction for storage, transport or transfer.

In today’s energy economy the handling of natural gas and liquid fuels does not pose major problems. But is this also true for hydrogen?

Figure 1 shows the volumetric HHV energy densities of different energy carrier options. 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 the volumetric energy density of methane at 800 bar is higher by factor 3.2.

The common liquid energy carriers methanol, propane and octane (representing gasoline) surpass liquid hydrogen by factors 1.7 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.

Figure 1. Volumetric HHV energy density for different fuels

Energy Cost of Energy in a Hydrogen Economy

Hydrogen is a synthetic energy carrier. High-grade energy must be invested to produce, compress, liquefy, transport, transfer or store hydrogen. In most cases this energy could also be distributed directly to the end user.

Also, instead of gaseous hydrogen, other liquid hydrocarbons such as methanol could serve as the general energy carrier of the future. Carbon from biomass or CO2 captured from flue gases could become the essential chemical carrier molecule for hydrogen generated with energy derived from renewable or nuclear sources.

We want to emphasize that not only the monetary cost of hydrogen is important and should be as low as possible, but also the energy cost of synthesizing hydrogen and bringing it to the end user.

As stated before, the hydrogen economy will be meaningful, only if the energy consumed to produce, package, store and distribute hydrogen should be as low as possible compared to the energy content of the delivered fuel gas. So far, this aspect has not been properly recognized. But because of the physical properties of the light gas, the hydrogen economy differs significantly from the natural gas economy.

The energy invested to extract and clean natural gas is small compared to its energy content. Not so for hydrogen! The transition to a new energy economy will affect the entire energy supply and distribution system. Therefore, we should discuss the prominent options before investing in a hydrogen gas economy.

Some of the more important energetic aspects of a hydrogen economy are analyzed in the following. The aim of this study [1] is to provide a first rough assessment of the amount of energy invested to make, compress, liquefy, transport, transfer or store hydrogen as compared to the amount of energy contained in the delivered fuel and to compare the results with similar analyses for established energy carriers. Throughout the study only representative technical solutions will be considered.

Production of Hydrogen

Energy Needed to Produce Hydrogen

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. This process is always associated with energy losses. Hydrogen production by both, electrolysis or chemical reforming is a process of energy transformation. Electrical energy or chemical energy of hydrocarbons is transferred to chemical energy of hydrogen.

Making hydrogen from water by electrolysis is the most energy-intensive way to produce the fuel. But it is a clean process as long as the electricity comes from a clean source. Less energy is needed to convert a hydrogen-rich energy carrier like methane (CH4) or methanol (CH3OH) into hydrogen by steam reforming, but the energy invested always exceeds the energy contained in the hydrogen.

Thermal losses limit the efficiency of hydrogen production by reforming to about 90%. Consequently, more CO2 is released by this "detour" process than by direct use of the hydrocarbon precursors. We assume efficiencies of 75% for electrolysis and 85% for reforming. About 1.2 to 1.4 energy units of valuable electricity, natural gas, gasoline etc. have to be invested to obtain one energy unit of hydrogen. But most of these source energies could be used directly by the consumer at comparable or even higher source-to-service efficiency and lower overall CO2 emission.

Upgrading electricity or clean hydrocarbon fuels to hydrogen does not provide a universal solution to the energy future, although some sectors of the energy market may depend on hydrogen solutions. The transportation sector may be one of them. It should be mentioned that it is considerably more expensive to produce hydrogen with electricity from water than thermally from fossil fuels. According to [10] it costs around $5.6/GJ to produce hydrogen from natural gas, $10.3 /GJ to produce hydrogen from coal, but $20.1/GJ to produce hydrogen through electrolysis.

Part Two Continued Next Week…

http://evworld.com/databases/storybuilder.cfm?storyid=471&subcookie=1