Conversion of excess wind energy into hydrogen for fuel cell applications

December 23, 2008 | 00:00

Conversion of excess wind energy into hydrogen for fuel cell applications

Large-scale offshore wind power can lead to the situation that more electricity is available (including the base load electricity supply) than needed in the Netherlands. Excess wind power is defined as the effect that the production from the base load power plants and the wind power plants exceeds the electricity demand.

This study investigates the characteristics of excess wind power and the conversion of this excess wind power to hydrogen by electrolysis. The hydrogen can be used in fuel cells for residential micro cogeneration of electricity and heat (µ-CHP) or for transport applications. The political ambition for 2020 is to have 6 GW offshore and 2-4 GW onshore wind power. The effect of this ambition on the hourly power production is simulated as 8 GW offshore wind power equally divided over 3 locations. The wind power is simulated using hourly wind speed data from 2004 and turbine characteristics for wind power production. The hourly power demand in 2020 is extrapolated using the 2004 electricity demand data. Currently about 1.6 GW of wind power capacity is installed. The study shows that excess wind power develops at a wind power capacity effectively above 4 GW offshore for a base load of 8.7 GW, no export of power or further demand side management. The results for 8 GW installed wind power are:

  • The excess wind power is around 4.5 TWh.
  • This can provide 80.000 ton of hydrogen/year.
  • This amount of hydrogen is sufficient to supply 500.000 households with a fuel cell µ-CHP unit operating in electricity following mode. Alternatively, the amount of hydrogen is enough to supply 800.000 “average” cars.
  • The capacity of the required storage (excluding cushion gas) is 20.000 ton of hydrogen.

The result of this study is that this wind power – electrolysis - H2 -µ-CHP electricity production process is a possibility to convert excess electricity into hydrogen and back to electricity. Assuming electricity load following operation about 50% of the energy is lost, 31% is recovered as electricity and 17 % as heat. For full load operation the electrical efficiency is 28% and the thermal efficiency 31%. In neither case the µ-CHP unit can cover the complete heat demand of existing or future houses. For stationary applications the operation of the µ-CHP unit at periods of excess wind power is excluded. For the cost calculations future targets for system components are used. The cost of electricity (without taxes or profit) for residential µ-CHP at 8 GW offshore wind capacity is at least 0.40 €/kWh, which is twice the present day mean consumer electricity price (including taxes and profit) of 0.20 €/kWh. The minimum electricity cost is calculated as 0.24 €/kWh at 25 GW installed wind capacity. The production of hydrogen for transport applications starting with excess wind electricity also suffers from the low load factor for the electrolysers and therefore initial high hydrogen costs as in the stationary case. The minimum cost estimate for 8 GW installed offshore wind capacity for minimum APX electricity cost (i.e. 0.02 €/kWh) excess wind (4.5 TWh) energy conversion to H2 is 4.4 €/kg.

A second option for transport applications is the dedicated conversion of 28.3 TWh wind energy from an 8 GW offshore wind farm to hydrogen. This amount can fuel 4-5 million cars with hydrogen. The minimum cost estimate for large-scale dedicated wind power conversion to H2 is 5 €/kg, using a mean electricity cost from offshore wind power of 0.08 €/kWh. The CO2 emission reduction for H2 production by electrolysis of wind power and subsequent use in residential fuel cell CHP systems is about 1/3 of the reduction when the electricity would be used directly. The CO2 reduction for using the H2 in fuel cell transportation vehicles is about 2/3 of the reduction when the electricity would be used directly. The perspective of excess wind energy via H2 to µ-CHP for household applications is therefore very low based on low efficiency, high cost and low CO2 emission reduction characteristics. The conversion of excess wind energy to hydrogen for transport applications is a more likely market opportunity for economic (higher value for hydrogen) as well as environmental reasons (higher CO2 reduction possibility) than stationary applications.

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