The efficiency of a hydrogen economy

INTRODUCTION:  Efficiency claims can be misleading without a full explanation.  This is especially true where chemical energy is involved. 
     The energy of a fuel is measured in terms of the amount of heat energy it can deliver, but there are two ways of measuring this.  The gross or Higher Heating Value (HHV) is the heat released when the temperature of the products of the reaction have again fallen to room temperature (25 C).  [The Calorific Value is also measured in this way, but different energy units, calories are used.] In this case any steam produced in the reaction will have condensed releasing its latent heat.  The net or Lower Heating Value (LHV) is the heat released when the products of the reaction are at 150 C in which case any water content will be steam and contain the latent heat of vaporization of the water.  The flue gases of old gas boilers were over 100 C and the heat they delivered was close to the net value, whereas the flue gases of a modern condensing boiler is below 100 C and the heat it delivers is closer to the gross value because the latent heat of vaporization is included in the heat it delivers.
     Solid fuels such as coal and biomass are generally traded as € / LHV.  The gross HHV value of oven dry biomass is 5 to 7% higher than its net LHV.  For hydrogen the difference is 18%.  Electric power involves no chemical reaction, and is therefore always gross.

     The clearest way to calculate efficiency is to use the gross value throughout.  Unfortunately K-H has used the net LHV values for hydrogen and biomass fuels, but the gross energy values for the heat and power output of a fuel cell.
     In order to try to make the results clearer I have inserted the end-to-end efficiency on a gross HHV basis.  I have also paraphrased and enlarged on some of K-Hs text to illustrate the efficiencies of the system components on a gross HHV basis.

     Press releases from manufacturers seldom make these distinctions clear.  K-H has set them out, but not in a way that is the easiest to understand.

     The distribution of hydrogen enables a simple, efficient decentralised generation of electricity using fuel cells. Since almost every end user requires more heat than power there will generally be an excess of electricity.  Therefore it is not particularly important that the fuel cell have very high electrical efficiency.  Talk of the efficiency of a fuel cell is misleading because when heat and power are both included there is no loss of energy.  It would be better to talk of the proportion of energy delivered as electricity.

     In order to find the overall efficiency of a hydrogen economy the hydrogen production process must also be included. Two examples are briefly illustrated here.

     In thermo-chemical gasification, water is an important input, as the majority of the hydrogen produced comes from the water supplied. In case of dry biomass, water may have to be added. The water may however be present in moist biomass. In case of allothermal gasification water content of around 50% does not decrease the efficiency, in case of autothermal gasification (with oxygen) the figure is around 40%. 
     That is completely different from thermal power stations which raise steam. In that case a water content of 50% would more than halve the electrical efficiency. Unlike co-firing biomass with coal, there is little need to expend energy first drying the biomass.  (See also Note 1)
     The proportion of energy reaching the end user depends on the proportion of waste heat of the the gasifier.  K-H suggests 8% here.  Smaller pyrolysis gasifiers which are often used on site, at a saw mill for example,  have higher losses, often of the order of 30%.

Appraisal of the energy chain efficiency

     As long as low temperature heat can be used (for example for heating rooms and for hot water) the total efficiency of both the above scenarios can approach 100% of the gross (HHV) primary energy. If in 2030 70% of the net primary energy comes from biomass and 30% from renewable electricity, the lost energy could be as little as 13%.
     Where industrial processes require high temperatures, the use of the low temperature heat from PEM fuel cells is not always be economically viable. SOFC Fuel cells are then required, which can provide heat at a temperature of around 900°C. Even if high temperature electric heating is used it does not degrade the overall efficiency, so long as more heat than electricity is required elsewhere. There are also many ways to make use of ambient heat employing heat pumps.
     The transportation sector needs its own analysis.  A very much higher efficiency will be possible possible using a mixture of battery and fuel cell drives.
     Taking an average across all applications, a total efficiency of 87% of the net (LHV) heating value would be achievable by the year 2030 (see diagram below). From the measures being taken in energy conservation and from improvements in the efficient use of electricity, it is expected that the end user requirement for energy in Germany will fall from 4.6 to 3.0 EJ.

Comparison of the legacy energy economy with a hydrogen economy

     The primary energy required in a hydrogen economy is only one quarter of what is currently required.  That means that the energy costs would be lower by at least a factor of four. It would probably be lower still as biomass, the primary energy source,  is already cheaper than fossil fuels.  This high efficiency also implies that the capacity that must be made available to produce biomass is much lower than most estimates, and will also be helped by the fact that biomass of all sorts, including farm and forest waste, can be used.
     Not only these running costs, but also the capital costs will be very low because the infrastructure of a hydrogen economy is much more simple.

COMMENTS:
*    Notice that the hydrogen is to be produced under pressure so that no pumping will be necessary in the regional gas distribution network.
*    For simplicity K-H has assumed that there is always a greater demand for heat than power.  He does refer to heat pumps which could alter the situation.  An electric heat pump driven by the fuel cell could even reverse it.  For this reason a fuel cell with high electrical efficiency is important.
*    It is also easy to show that the combination of fuel cell and heat pump reduces the amount of hydrogen even further than in K-Hs concept.   There are also adsorption heat pumps that work directly off gas.  The use of these would also reduce the amount of hydrogen needed.
*    In some situations air-conditioning (cooling) may be required.  These too can be either electrical or adsorption.  In both cases they would require additional hydrogen.

Note 1
     Many thermal power stations have cooling towers which require huge quantities of water.  In both US and UK approximately half the water extracted goes to power stations for this purpose!  Nuclear power stations also require cooling but they are generally located on the coast and cooled by sea water.  In both cases the water is raised in temperature changing the local environment.  This may or may not be desirable.