Wind – Hydrogen Systems

WIND - HYDROGEN SYSTEMS

J. LEHMANN, O. & T. LUSCHTINETZ,
C. SPONHOLZ, A. MIEGE, F. GAMALLO,

Fachhochschule Stralsund (University of Applied Sciences of Stralsund )
Zur Schwedenschanze 15, 18435 Stralsund, Germany - E-mail: fgamallo@gmx.de

Abstract: The climate change, as well as the increasing prices of fossil fuels, the dependence from countries with fossile resources, and its subsequently complicated diplomatic relations; all that leads to a growing use of renewable energie sources. The recent development of wind turbines and power electronics make possible the harvest of the energy of the wind on- and off-shore, in large ranges, with acceptable costs (as only hydropower-based electrolysis is cheaper).

Just now seems to be a right time to start developing the combination of wind parks and hydrogen production via electrolysis. Results would be an equalized direct electricity supply, a renewably-produced back-up power, and sustainably-produced hydrogen, for being used as fuel for stationary, portable and mobile combustion engines and fuel cells.

I. THE GLOBAL CLIMATE CHANGE

Dramatical signals are given within the UN Climate Report 2006 [6]. The global temperature of the atmosphere will be rising up to 6 K by 2100, melting polar ice and increasing the sea level. About five hundred independent scientists have produced this report, and all of them agree in the fact that this effect is due to the human abuse of the fossil energy tracers. Some of the largest energy consumers and producers, i.e. U.S.A, China and Saudi Arabia, aimed to defuse the report´s statements, what could be understood as one more proof of the validity of this paper.

For reducing those catastrophic consequences it will be necessary, not only a rational use of the current energy resources, but also a strong increase in the use of renewable, non-polluting sources. The available primary sources, such as sun radiation, wind and hydro, even when abundant, are characterised by some common unfavourable characteristics, as time variability, low conversion power density, and, frequently, long distances between the primary production areas and the final energy consumption centres.

The adoption of hydrogen as a harvesting, storage and transportation media allows to overpass those inconveniencies. Also, the choice of hydrogen as energy vector will allow, in the medium-term future, the use of the fuel cell for electricity generation, replacing the inherently non-efficient internal combustion engines.

Figure 1. Power densities of renewable and conventional energy conversions

II. ELECTRICITY FROM RENEWABLE SOURCES AND HYDROGEN

Several favourable characteristics enhance the hydrogen, as an option for a future energy carrier. The Element Number 1 is the most abundant substance in the known universe, and, not being available in the nature as an elemental gas, it can be easily obtained from different sources like water, alcohols, ammonia, and even hydrocarbons. The energy invested in separating the hydrogen out from more complex molecules will be later recovered, under the forms of electricity and/or heat, by means of chemical reactions that do not produce any other dangerous or nocive byproducts.

Figure 2. Hydrogen characteristics

There are two main paths between renewable energy sources and hydrogen. One is through the digestion (or partial burning) of biomass, the production of biogas and its reforming to hydrogen. The other one is through the electrolysis of water, using electricity produced from renewables.

Fig. 3 shows all the usual alternatives for H2-production, both from conventional and renewable sources. Currently, up to about 80% of the produced hydrogen is generated by natural gas reforming, being the rest obtained as a byproduct of industrial chemical processes, mainly partial oxidation of heavy hydrocarbons (about 18 %), and only about 2 % by electrolysis [18].

However, the electrolysis is the only mature technology that would allow converting the harvest of renewable electricity into a fuel able to be used not only for electricity reconvertion, but also in the highly demanding transport market.

Water electrolysis, as an industrial procedure, was done by the first time in Rjucan, Norway, in 1929. Since that time, lots of experience has been acquired, and large plants have been built. Some of them (extracted from [16]) are:

The evolution of this technology allowed the increase of the efficiency to very high values. Currently, the most extended electrolysis technology is that of the bypolar, alkaline units, using an aqueous solution of KOH, around 28 % v/v. The average efficiency of the currently available electrolysers is between 65 % and 70 %, even when efficiency values of 80 % under full power (up to 90 % at 20 % load) have been reached on experimental, industrial-sized units [11].

The main characteristic of the electrolysers as electrical consumers, is their variable-voltage mode, for different power levels (and also for different temperatures), as shown in the following Fig. 5 (based on [7]).

However, even when water electrolysis can be considered as a quite mature technology, several obstacles have still to be overpassed, for its succesful integration into the energy market. First of all, the current offer of industrial-scaled electrolysers is low (a comprehensive list of manufacturers can be found at [10]). For this reason, prices are high, and delivery times may often be quite long. The production capacity of the current manufacturers is absolutly out-of-scale with the minimal demand that could be necessary for for some noticeable participation of the electrolytic hydrogen in the energy market.

Also, the variable-load operation mode of an electrolyser, typically linked to time-variable sources like wind or solar, was still not deeply studied. Even when several research projects worked on this subject during the last twenty years [1, 10, 13, 14], the operative experience already acquired seems not to be enough for entering with this technology into a production phase. The influence of such an operative mode on the service life of the electrolysers deserves also a specific research.

Another forthcoming technologies, as the PEM (Proton Exchange Membrane) electrolysis could reach even better efficiencies, but they are still in a development stage, and do not seem to be effectively ready to attain the commercial market.

III. HYDROGEN STORAGE

Energy storage is the central problem of the whole energy economy. One of the advantages of hydrogen is that it offers several strrage alternatives.. Hydrogen is storable in four ways: as compressed gas, as liquid, in form of metal hydrides, and in chemical compounds. Due to its low density it is not effectiv to store the hydrogen gas at low pressures. Hydrogen storing comprises the idea of reaching a higher energy density (i.e. energy per volume unit).

Like other gases, hydrogen can be stored in pressure vessels (steel or composit) or in underground caves with up to 700 bar. For medium pressure (150 / 300 bar) the efficiency is about 0.95, that means, about 5% of the stored energy is needed for compression.

Liquified hydrogen with a temperature of –253 °C is kept in ultra-insulated vessels at atmospheric pressure. In that case the efficiency is about 0.65 / 0.70.

Very interesting is the storage of hydrogen inside of metals, chemically bonded as metal hydrides. The main advantage of this method is its very high energy density, the disadvantage is the high weight of the storage vessel.

The fourth storage possibility for hydrogen storage is the use of H₂ contending compounds such as alcohols. However, at the place of the hydrogen demand, some hydrogen-producing unit (i.e. a reformer) is needed.The table on the next page summarises pro and contras of different H₂ storage possibilities, gives their energy density values, developing tendencies, and the now available optimal solutions for their integration into hydrogen systems. One more word: For stationary purposes, the pressure storage at less than 150 bar still seems to be the most favourable technical and economical possibility [12].

IV. HYDROGEN RECONVERTION SYSTEMS

Four different kinds of fuell cells, as well as combustion engines (or turbines) and generator sets, can convert the chemical energy of the hydrogen into electricity. As the figure shows, the three components (electrolyser, gas storage and reconverter) form a kind of storage unit for electricity. All the parts of such a unit do not need to be situated at the same place, as pipes or mobile storage vessels can act for connection and gas distribution. Such a system offers a large variability in all ranges.

Figure 7. Storage unit for electricity via hydrogen

Energy conversions are fraughted with losts described by the efficiency. Assuming the efficiencies, for advanced alcaline pressure electrolysers as 0.75, for low pressure storage as 0.95, for fuel cells (electrical) as 0.50, and for internal combustion units as 0.30; it should be possible to reach a total efficiency of 0.36 (for the unit using fuel cells) and 0.21 (in case of combustion engines).

Some people say that hydrogen production and later reconvertion would be the best way for energy annihilating. However, as the primary resource is endless, such an efficiency analysis is not completely correct, and such systems would alloy to generate a form of stored energy, usable in accordance to de demand, without a large wasting; offering the market an alternative that, even when perhaps more expensive, could be chosen by some customers.

By the way, James Watt´s steam engine, initiating the industrial revolution, has had an efficiency less than 0.12.

V. WIND-HYDROGEN SYSTEMS

In the north of Germany, currently more than 30 % of the consumed electricity is produced by wind turbines. Such a percentage of not-constant, time -variable power carries along with the risk of creating unstabilities in the grid, and also with the problem that, in periods with high wind harvest and small demand, it can not be transmitted into other regions, due to weak nets.

Up to now, in Germany, the electricity supply companies were obligated by law [2] to buy all the produced wind electricity. When using conventional load dispatch strategies, the maximum allowable wind penetration (percentage of wind power in the mix) is lower than 20 %. For solving this problem, a strong backup from other regions, by means of high voltage lines is needed.

Some alternatives, as more developed active load dispatch policies (still in a development stage) [4], together with the application of complex wind forecast systems [3], would allow increasing this penetration factor up to around 35 %. Such a dispatch strategy operates not only on the power units, but also manages some part of the load (usually defined as deferrable load) for generation peak-shaving and also for a better frequence control. So, for such a system being able to operate smoothly, some kind of manageable load, able to be used as a buffer, will be needed. Hydrogen production seems to be a perfect candidate for such a purpose.

The grid balancing could be performed by directing to the electrolysers all the surplus wind electricity that cannot be dispatched to the grid, storing the produced hydrogen untill low wind or high demand periods. Then, hydrogen should be reconverted into electricity. At present, in such situation, the production of the conventional power stations is reduced, and so, the wind energy is responsible for some lower use to its installed capacity. In general terms, hydrogen storage systems could be a way towards to a smaller demand of conventional produced back-up power. The wind parks would be “seen” by the grid as delivering a quite equalised power. This is, however, probably the most future operating condition, and will be economically feasible only when its grid stabilisation capabilities could be apprised and reimbursed as an added value (probably with the appearance into the market of large amounts of concentrated wind power, coming from off-shore parks).

One factor that will play an important role for such installations will be, without any doubt, the demand of hydrogen from “external” markets (not for grid back-up). This demand will obviously come from the transportation market. The use of hydrogen as fuel in cars, buses, and even airplanes seems as the only viable alternative for the long term. There are already some hundreds of private cars, buses, boats and ships, as prototypes, (both with ICE or FC) which are powered by hydrogen, or using fuel cell as APUs (auxiliary power units). According to a study by EUCAR (European Council for Automotive R & D) [17], it is able to be expected that the demand for sustainably-produced hydrogen or hydrogen-based fuels for the transportation market will start to rise, even in the next few years. With this background the proposed hydrogen factory (Fig. 8) - presented by the FHS at a World Hydrogen Energy Conference [9] - would deliver two products, positive or negative back-up power to the grid, and hydrogen as fuel. This double-sided kind of business should be positive for the economy of such an enterprise.

However, this use strategy (featured in the next figure as options 1 and 3) will probably take a lot of time to be established, as it requires not only adopting new load dispatching procedures, but also performing massive investments in both electrolysis plants and network improvements and extensions.

Other alternative options (shown as 2 and 4) could be probably be implemented in a shorter term, as long as they do not require centralized infrastructure improvements. Both alternatives (hydrogen production based on wind energy, for in-situ reconvertion, or for fuel supply), operate under the form of isolated energy systems. It is necessary to remark that an isolated energy system is not strictly a self-sufficient electrical (or combined) supply system, operating where no other supply (i.e. main grids) is available; but also any grid-independent energy system operating in areas already supplied by the grid, but where, mainly on an economical basis, an independent supply could be a better option.

The association between wind energy and descentralized generation is based on the simple fact that (as mentioned in an early paragraph), the wind is an extremely abundant but always dispersed primary source. The need of connecting the wind turbines to a main grid, entering in competition with inherently cheaper centralized power stations, could be only explained by the need of this variable source of being supported by the grid. As long as this back-up effect could be supplied by the stored hydrogen, and both the wind turbine and the electrolyser could be adapted for a grid-independent operation mode, such isolated systems could be established, and the final energy cost could be highly competitive to the grid supply.

For stationary users, such systems could offer the additional advantage of a simultaneous supply of electricity and heat. As almost any human settlement requires similar amounts of electrical and thermal energy, those in-situ energy reconvertion sytems (either based in ICE or in fuel cells, both currently in a pre-commercial stage) allow the use of the otherwise wasted process heat, highly improving the global energetic efficiency of the reconvertion system [5].

The fourth alternative (actually, just a part of the third one, but comprising only wind turbines and electrolysers), is not only a suitable configuration for producing hydrogen in those places where, for any reason, no extra turbines can be linked to the grid; but mainly an alternative for wind energy harvesting in those places where high voltage grids or transmission lines simply do not exist. Several projects have been proposed, based on this alternative [8, 15].

REFERENCES

1. Brinner A., Al-Saedi Y., 1996. Results and experiences of a two-year experimental and routine slar operation phase at the Hysolar 350 kW solar hydrogen production plant. DLR, Stuttgart, Germany.

2. EEG (Erneuerbare Energie Gesetz) - German renewable energy sources law. Available (in english language) from: http://www.dewi.de/

3. Ernst B., 2003. Entwicklung eines Windleistungsprognosemodells zur Verbesserung der Kraftwerkeinsatzplanung. Dissertation at the Univ. of Kassel., Germany. Available from: http://www.iset.uni-kassel.de/

4. Gül T., Stenzel T., 2005. Variability of wind power and other renewables: Management options and strategies. IEA - International Energy Agency, Paris. France.

5. Halliday J. et al., 2005. Fuel Cells: providing heat and power in the urban environment. Tyndall Centre for Climate Change Research, Norwich, U.K

6. Intergovernmental Panel on Climate Change (IPCC) Year 2006. Report. Available at: http://ipcc-wg1.ucar.edu/wg1/wg1-report.html.

7. Kauranen, P. S. et al., 1993. Development of a Self-sufficient Solar-hydrogen System. Intern. Journ of Hydrogen Energy. 19/ 1 : 99 - 106. Helsinski.

8. Leighty B., Gibbs B., Biewald B., 2002. Transmitting 4.000 MW of new wind power form North Dakota to Chicago. New HVDC electric line or hydrogen pipeline. 14^th World Hydrogen Energy Conf., Montreal, Canada.

9. Lutschtinetz T. et al., 2006. Smooth feeding in of wind energy via hydrogen. 16^th World Hydrogen Energy Conf., Lyon, France.

10. Lymberopoulos N., 2005. Hydrogen Production from Renewables. Centre for Renewable Energy Sources, Attiki, Greece.

11. Manfred T., 2000. Hydrogen and the Electricity Utility Industry. Chapter 11 of “On Energies of Change – The Hydrogen Solution”. Carl-Jochen Winter, Gerling Akademie Verlag, München, Germany.

12. Menzl F., Wenske M., Lehmann J., 1998. Hydrogen production by a windmill powered electrolyser. 12^th World Hydrogen Energy Conf., Buenos Aires, Argentina.

13. Menzl F., Zielke P., 2000. Untersuchungen zum Betriebsverhalten eines 20 kW Druckelektrolyseurs. Journal of Energy and Environmental Technology, 1. Fachhochschule Stralsund, Germany.

14. Schucan T., 1999. Case Studies of Integrated Hydrogen Energy Systems. Report, IEA/H2/T11/FR1-2000, Nat. Renewable Energy Lab., Golden, CO, U.S.A.

15. Spinadel E. et al., 2001. Patagonic wind “exported” as liquid hydrogen. Hypothesis IV Conf., Stralsund, Germany.

16. Winter C-J., Nitsch J., 1989. Hydrogen as an Energy Carrier - Technologies, Systems, Economy. Springer Verlag, Berlin, Germany.

17. Wurster R., 2002. Well-to-wheels Analysis of Energy Use and Greenhouse Gas Emissions of Advanced Fuel/Vehicle Systems . An European Study. LBST, Ottobrunn, Germany.

18. Wurster R., Zittel W., 2002. Hydrogen in the Energy Sector. LBST, Ottobrunn, Germany, Available at http://www.hyweb.de/Knowledge/w-i-energieweng.html.