Comparison between high-voltage direct-current transmission and hydrogen transport

0360-3199/91 $3.00+ 0.00 Pergamon Pressplc. © 1991InternationalAssociationfor HydrogenEnergy.

Int. J. Hydrogen Energy, Vol. 16, No. 2, pp. 105-114, 1991. Printedin Great Britain.

COMPARISON BETWEEN HIGH-VOLTAGE DIRECT-CURRENT TRANSMISSION A N D H Y D R O G E N TRANSPORT G. KASKE,P. SCHMIDT and K. W. KANNGIEI3ER Hills Aktiengesellschaft, Postfach 1320, D-4370 Marl, F.R.G. (Received for publication 21 August 1990)

Abstract--Transportation of solar energy from North Africa to Western Europe is investigated from the point of view of technique and cost in the form of a project study. It is assumed that 43% of the final energy consumption of the Land of North Rhine-Westphalia for 1986 can thus be provided with an electric power proportion of 20%. The techniques and costs of three different transport systems are put forward: transport of energy by high-voltage direct-current transmission (HVDC transmission), via hydrogen in gas pipelines and a combination of the two systems. It emerges from this that the combined transport system has clear advantages over pure HVDC transmission and H2 transport.

1. INTRODUCTION In 1986, West German demand for primary energy amounted to 387 million tonnes coal equivalent. As can be seen from Table 1 [1,2] 43% of this demand was covered by mineral oil, 30% by coal, 16% by natural gas and 10% by nuclear power. The supply structure for North Rhine-Westphalia looks quite different. With an overall requirement for primary energy of 136 million tonnes coal equivalent, the primary energy source, coal, achieved a share of 53% to the detriment of mineral oil and nuclear power. At present a high degree of security of supply is achieved by meeting the primary energy demand with coal, mineral oil, natural gas and nuclear power. The well-known information about stocks of these energy sources and the knowledge that it will take not years but decades to develop and introduce new energy sources, have given an impetus to work on the future exploitation of solar energy throughout the world. The question we have to answer is whether we want to produce solar energy in the northern hemisphere of the earth or near the equator. In the second case, large amounts of energy have to be conveyed across several thousand kilometers. This gives rise to further questions as to the form of energy, kind of transport and cost of the energy transport. Cost comparisons have shown that the least expensive methods of conveying large amounts of energy across large distances are either in the form of gaseous hydrogen in pipelines or as an electric current via high-voltage direct-current transmission (HVDC transmission) [3~]. Hence, the topic of this paper is a comparison between high-voltage direct-current transmission and hydrogen transport in pipelines.

2. BASIC CONDITIONS OF THE PROJECT STUDY In order that the study should approximate to reality as far as possible, the following conditions were used as a basis: with a primary energy consumption for the Land of North Rhine-Westphalia of 136 mill.t coal equivalent in 1986, the latter's final energy consumption for 1986 was 80 mill.t coal equivalent. Out of which the mineral oil and gas sectors have a requirement of 51 mill.t coal equivalent. It was assumed that two-thirds of this amount to be covered by solar energy, i.e. 34 mill.t coal equivalent must be provided. The latter represents 43% of the final energy consumption of North Rhine-Westphalia or 13% of the final energy consumption of West Germany. In order to define a transport model which allows statements concerning both heat and electricity supply, we have assumed that 80% of the solar energy supplied should be used as hydrogen and 20% as electricity. By way of comparison, in 1986 the proportion of electric current in final energy consumption was 16.5% in West Germany and 17.8% in the Land of North Rhine-Westphalia. In our model we assume that the solar power plant is situated about 200 km north of In-Salah in Algeria, as shown in Fig. 1. There are natural gas fields in the vicinity of In-Salah and further north. An approximately 105

G. KASKE et al.

106

Table 1. Primary energy consumption 1986 Energy source Total

West Germany Land NRW [1] [2]

Unit Mill.t coal equivalent

Coals Mineral oils Natural gas etc. Nuclear power Hydro power Electricity (import/export)

% % % % % %

1500 km long natural gas pipeline, parts of which are still at the planning stage, runs from there through Ghardaia to the Strait of Sicily. The HVDC transmission lines and the hydrogen pipelines of our model follow this route. Underwater lines cross the Strait of Sicily and run across Sicily and Italy into the Ruhr region. The total length of the route is 4000 km. It is planned to install the water treatment plant in the area of Syrte Minor in Tunisia. The length of the water pipe to the site of the solar power and electrolysis plant is 1000 km. The In-Salah and Syrte Minor sites are only models. 3. RESULT OF THE PROJECT STUDY Three cases have been distinguished in the project study: transport of solar energy • as an electric current via HVDC transmission; • as gaseous hydrogen via pipeline and • a combination of the two.

387 29.5 43.1 15.5 10.0 1.4 0.5

136 53.2 34.0 15.9 1.3 0.1 - 4.5

We would like to report on these three cases in the following manner: • first on the concept; • then on the plant and capacities and • finally on the investment, cost and energy efficiency. 3.1. Transport o f electric current by means o f H V D C transmission

3.1.1. Concept. Figure 2 shows the basic diagram of electric current transport by means of HVDC transmission. We assume that solar energy is fed in on a day/night cycle and taken off at a constant rate in North Rhine-Westphalia. To simplify the computer model, we have therefore determined that the solar power plant and the water electrolysis plant will operate at the following capacity: Day = 8 h: power plant 100% H 2 plant 100% Night = 16 h: solar power plant 0% H2 plant 10%.

Africa

g

0 O0 Naturalgas

fields as pore reservoirs

Fig. 1. Local position of the transport system.

Since the water electrolysis plant cannot be stopped daily, it will operate with a minimum load of 10% for 16 h. The current required for this will be drawn from a storage battery which has been charged during the day. A sodium sulfur storage battery has been assumed for the In-Salah site for day/night compensation. The natural gas deposits there can be regarded as a storage option for hydrogen. We assume that the natural gas reservoirs in North Rhine-Westphalia can also be used for hydrogen. Any additional storage capacity becoming necessary should have been created by the continuous supply of brine by the time when the requirement arises. The solar power plant must in the course of 8 h per day feed in an amount of energy such that constant amounts of current and hydrogen can be drawn off and the losses covered. Since, in this case, the electrolysis plant is in North Rhine-Westphalia and the storage battery in Algeria, the result is that, in the course of 16 h per night only the current requirement provided as constant in NRW and the 10% minimum requirement for the electrolysis are transported via the HVDC transmission line.

TRANSPORT OF SOLAR ENERGY

107

Solar power plant 8 h/day 8 h 139,340MW 371,574Mill. kWh 16h 17,384MW J't 8h101,091MW V F.C. inverter

Converter

I

I HVDC rectifier 1500 km

I

DC/DC

storage battery 304 GWh

J

overhead line coDC!gCr ~'

I

150 km

subsarine cable

I

DC/DC converter

I

2500 km

overhead line HVDC inverter

1 1 Rectifier

I

H20-Elektrolysis 27 plants, 3 GW each

7,000MW 56,000 Mill. kWh

J

7,929TNm3/h H2 225,000 Mill. kWh Ho

I

H2-reservoircaverns, Pore reservoirs

Overall efficiency 0.76

Fig. 2. HVDC transport. Additionally, in the remaining 8 h of the day, the current requirement for full utilization of the electrolysis plant has to be transported, which compensates its different capacity utilization by storing hydrogen in cavern and pore reservoirs. The current requirement has been planned as follows: We have assumed that the solar power or photovoltaic plant consists of 1011 units of 100MW each with a voltage of less than 100kV direct current. DC/DC converters ensure that the voltage is optimized between the storage battery and the photovoltaic plant. Forced commutated inverters convert the power to 380kV alternating current. HVDC rectifiers convert 380 kV alternating current into +800 kV direct current. Fifteen HVDC bipolar transmission systems run to the Strait of Sicily, where the

power is converted to +500 kV for feeding into the submarine cables. In Sicily the voltage is raised again to + 800 kV and the power is transmitted via overhead lines to NRW. Here, HVDC inverters convert the power to 380 kV alternating current for feeding into the local mains and the electrolysis plants. These require rectifiers which provide the necessary direct voltage below 100 kV. The overall efficiency in this case is 76%. 3.1.2. Plant, capacities and investment. Table 2 shows the required plants, their number, capacities and the investment determined therefore in the case of transport of current by HVDC transmission. All items of the capacities mentioned here greatly exceed the plant parameters which have been achieved in industry hitherto. In this respect there is a certain

108

G. KASKE et al. Table 2. Transport by HVDC transmission/plant capacities, investment

Plant

Number

Storage battery HVDC transm, lines 1500 km land 150 km sea 2500 km land Converter plants F.c. inverter DC/DC converter HVDC rectifier DC/DC converter ' HVDC inverter Rectifier for electrolysis Electrolysis plants Total

Total invest. DM thous, mill. (1988)

Capacity/U

10l I

300 MWh

102 76

15 150 15

6.619 MW 633 MW 6.276 MW

25 11 40

1011 1011 15 15 15 27 27

100 MW 6.500 MW 6.672 MW 6.276 MW 5.883 MW 3.000 MW 3.000 MW

30 4 14 17 9 8

82

83 343

Table 3. Transport by HVDC transmission/economics U

Battery

HVDC transm, line

Converter plants

mill. DM yr-t mill. DM yr 1 mill. DM yr -1 employees

_

_

_

_

_

_

Type of cost Other energy Auxiliaries Credits Personnel Depreciation on fixed assets Interest Rep. Mat./transport Other fixed costs Efficiency

% % % %

yr 1 of inv. of inv. of inv. of inv. %

_

-100 10/50 4.0 0.8 -2.0 92

a m o u n t o f uncertainty a b o u t this consideration. The same is also true o f the investment figures. There are also uncertainties surrounding the infrastructure o f N o r t h Africa which has been calculated roughly using a socalled " S a h a r a factor" o f 30%. It can be deduced that the H V D C transmission line, the current converters and the electrolysis plant will require investment o f approx. D M 80 t h o u s a n d million each and the storage battery a b o u t D M 100 t h o u s a n d million. 3.1.3. T r a n s p o r t costs. In order to determine t r a n s p o r t costs, the economics o f the individual stages have been calculated individually and added, taking into account the data from Table 3. The consequential costs o f investment* are

_

-360 50 4.0 0.8 -2.0 96.3 99.7 93.8

Electrolysis 521 20 1825 8000

_

--50 4.0 0.8 -2.0 99.3 (average value per converter plant)

15 4.0 3.5 1.5

4.5 88

transport, the transport costs are also dependent on the price o f solar energy. Table 4 shows these relationships. A t a solar energy price o f D M 1.00 kWh 1, the a m o u n t s are T r a n s p o r t costs and total costs free at N R W

D M 0.39 kWh t D M 1.39 kWh

and corrrespondingly, at a solar D M 0.10 k W h - l, the a m o u n t s are T r a n s p o r t costs and total costs free at N R W

energy price o f

D M 0.17 kWh-E D M 0.27 kWh 1.

43 t h o u s a n d million D M y r -

Table 4. Transport by HVDC transmission/transport costs (DM kWh ~)

0.15 D M k W h -l.

Price of solar energy Consequential costs of investment Energy costs Transport costs Total costs free at NRW

or

Since 24% o f the energy fed in is c o n s u m e d for *Sum of depreciation and interest on loan capital and other costs related to invest.

1.00

0.50

0.10

0.15 0.24 0.39 1.39

0.15 0.12 0.27 0.77

0.15 0.02 0.17 0.27

TRANSPORT OF SOLAR ENERGY

109

Solar power plant 8 h/day I 8 h 144,315 MW I 384,454 Mill. kWh

16h '1'807MW ' 8 h 118,069 MW 1

1

H2-electrolysis 1 41 plants, 3 GW each

DC/DC converter

I

Storage battery 209 GWh

]

H2-reservoir (opt.) Compressor

pore H2-reservoir reservoir (existing) H2transmission

pipeline

1500 km + 150 km

I

Pipeline

I

I 9 H2 transmission pipeline 2500 km

Compressor

I Pipeline

I

Fuel cells Inverter

7,929TNm3/h H2 225,000 Mill. kWh Ho

7,000 MW 56,000 Mill. kWh

I

H2-reservoir pore reservoir (existing)

Overall efficiency 0.73 Fig. 3.

H2

3.2. Transport of gaseous hydrogen via pipeline 3.2.1. Concept. The same principles apply as in the previous case. As shown in Fig. 3, the electrolysis plant is located near the solar power plant, likewise the terminal compressor station. The pore reservoirs are used to keep the balance. The day/night fluctuations can also be compensated via the pipeline capacity. The hydrogen leaving the electrolysis plant at 30 bar is compressed to 100 bar in the terminal compressor station without intermediate storage. The hydrogen pipeline is constructed of two pipes, each having a diameter of 1400 mm. Along the distance of 4000 km, the hydrogen is

transport. compressed in 9 intermediate compressor stations, in each case from 83 to 100 bar. The compression energy is taken from the hydrogen pipeline and is fed to a combustion turbine which directly drives the shaft of the turbocompressor. The waste heat from the combustion turbine is used to produce steam which also serves to drive the compressor via a steam expansion turbine

[7, 8]. In North Rhine-Westphalia, the hydrogen is fed into the existing gas distribution network, which may be extended, in a constant amount of 8 mill. Nm 3 h- ~, and at a pressure of 30 bar. Fuel cells having a capacity of 7000 MW supply the electric current. The overall efficiency of the H2 transport system is 73%.

G. KASKE et al.

110

Table 5. H 2 transport/plant, capacities, investment

Plant

Number

Capacity/U

697

300 MWh

Storage battery plant Lines H2 H20, desalted water drinking water Compressor Terminal station Intermediate compressor station Converter plants DC/DC converter HVDC inverter Fuel cells Electrolysis plants Total

Total invest. DM thous, mill (1988) 71 29

2 1 1

527 t h -~ H 2 8000 t h -t 5000 t h L

26 2 1

1 9

2600 t h ~ H 2 856 t h ~ H 2

1 1

6563 MW 7000 MW I000 MW 3000 MW

2 1

3 4 I 7 40

10 160 275

Table 6. H2 transport/economics Type of cost

U

Other energy mill. DM yr -t Auxiliaries mill. DM yr ' Personnel employees Depreciation on fixed assets yr ~ Interest % of inv. Rep. % of inv. Mat./transport % of inv. Other fixed costs Efficiency

% of inv. %

Battery -. 100 10/50 4.0 0.8 -2.0 92

Pipeline -.

Fuel cell plants

--

90

-50 4.0 0.8 --

700 15 4.0 1.5 1.5

863 30 12,000 15 4.0 3.5 1.5

2.0

2.0

4.5

4.5

97.0 99.5

97.3

6 . 140 15 4.0 2.0 0.5

. 100 50 4.0 0.8 0.5 1.0 2.0 3.0 100.0

3.2.2. Plant, capacities and investment. Table 5 gives the plants, their n u m b e r a n d capacities per unit, required in this case, a n d the investment costs. The water t r e a t m e n t plants have n o t been included. W e have calculated their cost.in the price for desalted water a n d drinking water for cooling circuits. The electrolysis plant, which consists o f 41 units at 3000 M W each, requires the highest investment at D M 160 t h o u s a n d million. The storage batteries require D M 7 1 thousand million, the h y d r o g e n pipelines D M 26 t h o u s a n d million a n d the fuel cells D M 10 t h o u s a n d million. 3.2.3. Transport costs. T h e operating data in Table 6 show, inter alia, two i m p o r t a n t items. The n u m b e r o f employees regarded as necessary for the case of H2 t r a n s p o r t is approximately 13,000. In the efficiency chain, the fuel cells represent the weakest element with a n efficiency of 60%. This m e a n s t h a t the less h y d r o g e n t h a t has to be converted into current via the seven fuel cell plants, the higher the efficiency of the total system. The t r a n s p o r t costs of Table 7, like those o f Table 4, are d e p e n d e n t on the price o f solar energy a n d are between 0.44 a n d 0.20 D M k W h - ~ in the solar energy price range selected.

Converter plants

Compressor

3.3. Combination

.

of

60

HVDC

transmission

Electrolysis

88

and

H2

transport 3.3.1. Concept. Figure 4 shows the basic diagram o f a c o m b i n a t i o n of the two transmission systems. It is sufficient to highlight the special features as c o m p a r e d with the two cases already p u t forward. C u r r e n t can be t r a n s m i t t e d constantly for 24 h a day by the H V D C transmission system using the storage battery. The d a y / n i g h t fluctuations of the electrolysis p l a n t are c o m p e n s a t e d via the cavity reservoir b e h i n d the terminal compressor station in N o r t h Africa in the pressure range of 100 bar. The energy is t r a n s p o r t e d via two bipolar H V D C transmission systems a n d one h y d r o g e n pipeline. The parallel transmission o f electric current a n d h y d r o g e n n o w offers the possibility o f r u n n i n g the Table 7. H 2 transport/transport costs (DM kWh 1) Price of solar energy Consequential costs of investment Energy costs Transport costs Total costs free at NRW

1.00

0.50

0.10

0.17 0.27 0.44 1.44

0.17 0.14 0.31 0.81

0.17 0.03 0.20 0.30

TRANSPORT OF SOLAR ENERGY

111

Solar power plant 8 h/day 8h 127,891MW 340,701Mill. kWh 16h 7,990MW n 2, h9,150MW I 8h 79,901MW V H~-electrolysis DC / DC converter F.c. inverter 27 plants, 3 GW each

I l St°rageb°tter'e I

HVDC rectifier

] f

H2-reservoir(opt.)

Compressor I 1500 km

Pipelil-~ne<~Iz~ ''-t pore reservo H2-reservoir r (ex sting)

I Overhead line I

I DC / DC Converter I

I

150 k m

Submarine 992 MW Cab e I:~

Compressor

16

DC / DC Converter

I 2500 km

Overhead line

Pipeline

HVDC inverter

7,000 MW 56,000 Mill. kWh

7,929 TNm3/h H2 225,000 Mill. kWh Ho

I

H2-reservoir (existing) Caverns, pore reservoir

Overall efficiency 0.82 Fig. 4. H2+ HVDC transmission transport. hydrogen compressors of the intermediate compressor stations with electric current from the HVDC transmission system. These various system optimization steps result in an increase in overall efficiency to 82%. 3.3.2. Plant capacities and investment. Although, in comparison with Tables 2 and 5, Table 8 shows a larger number of plants, it has the lowest value with a total investment of DM 245 thousand million. The highest investments are those for water electrolysis at DM 108 thousand million and for the storage battery at DM 102 thousand million. The water treatment plant is not indicated here either, because its costs have been included in the price of water. 3.3.3. Transport costs. Table 9 indicates the operating data. As in case 1, the number of employees is approx.

8400. The efficiency of the individual units is lowest for the electrolysis plant at 88%. Accordingly, the transport costs (Table 10) in the price range considered for solar energy drop from 0.32 to 0.16 DM kWh -j.

4. COMPARISON OF THE TRANSPORT MODELS INVESTIGATED Figure 5 shows the transport costs resulting from the study of the three transport models. In accordance with the amount of energy losses for transport and conversion which are 20%, 24% and 18%, respectively, the transport costs are dependent on the price of the solar energy transmitted.

112

G. KASKE et al. Table 8. Combination of HVDC transmission/H2 transport/plant, capacities, investment

Plant

Number

Capacity/U

1011

300 MWh

Storage battery plant HVDC transm, lines 1500 km land 150 km sea 2500 km land Pipeline H2 H20, desalted water drinking water Compressor Terminal station Intermediate compresson station Converter plants F.c. inverter HVDC converter DC/DC converter HVDC inverter DC/DC inverter Electrolysis plants Total

Total invest. DM thous, mill (1988) 102 8

2 14 2

4493 MW 614 MW 4260 MW

3 I 4

I 1 l

714t h -I 7000 t h-~ 3000 t h i

14

1 16

1800 t h 1 714 t h - t

92 2 2 2 6 27

100 MW 4530 MW 4260 MW 6300 MW 6300 MW 3000 MW

16 1

1 1

10 3 1 2 3 3 108 245

Table 9. Combination of HVDC transmission/H2 transport/econmics

Type of cost Other energy Auxiliaries Credits Personnel Depreciation on fixed assets Interest Rep. Mat./transport Other fixed costs Efficiency

U

Battery

mill. DM yr -l mill. DM yr L mill, DM yr i employees a % of inv. % of inv. % of inv. % of inv. %

-. . 100 10/50 4.0 0.8

HVDC transmission line -. .

2.0 92

Table 10. Combination of HVDC transmission/H2 transport/transport costs (DM kcal ~) 1.00

0.50

0.10

0.14 0.18 0.32 1.32

0.14 0.09 0.23 0.73

0.14 0.02 0.16 0.26

-.

The m o s t advantageous case in terms o f energy and cost is a c o m b i n a t i o n o f H V D C transmission and hydrogen transport. The conversion losses are lowest in this case. Pure H V D C transmission t r a n s p o r t is m o r e advantageous t h a n pure h y d r o g e n transport. Part o f the reason for this is that we have selected a current p r o p o r t i o n o f 20%. If the current p r o p o r t i o n is

Price of solar energy Consequential costs of investment Energy costs Transport costs Total costs free at NRW

Pipeline Compressor 5 . .

48 50 4.0 0.8 -2.0 91.3 99.7 93.8

Converter plants

581 20

-50 4.0 0.8

8000 15 4.0 3.5 1.5 4.5 88

. .

75 50 4.0 0.8 -2.0 100

. 200 15 4.0 2.0 0.5 2.0 97.0 99.5

Electrolysis

-.

2.0 99.3 (average value per current converter)

reduced, both cost curves coincide or hydrogen transport becomes cheaper than H V D C transmission transport. Figure 6 shows in addition to the transport costs the fixed t r a n s p o r t costs and total costs as the sum o f transport costs and the specific price for the solar energy fed in. A t an annual insolation o f 2400 h, estimates o f current p r o d u c t i o n costs o f mains-connected photovoltaic plants o f the next generation [9] indicate values o f between 0.44 and 0.23 D M k W h - 1. This would result in current costs at the end o f the t r a n s p o r t system o f 0.66 to 0.41 D M k W h -1 However, if cheaper energy sources are available to the required extent, Fig. 6 can be used to determine the correspondingly reduced t r a n s p o r t and overall costs.

TRANSPORT OF SOLAR ENERGY

0.45

113

DM/kWh Hydrogen

0.4

. . . . HVDC transmission

0.35 0.3

..... HVDC transmission + hydrogen

0.25 0.2 0.15

:i..:: ...........

0.1 0.05 0 0.00

I

~

~

0.20

I

I

0.40

I

I

I

I

I

0.60 0.80 1.00 Solar energy costs (DM / kWh)

Fig. 5. Transport costs.

1.6

DM / kWh Hydrogen

1.4

. . . . HVDC transmission

1.2

..... HVDC transmission + hydrogen

1 0.8 0.6 0.4

transport (var. + fix.)

,p,J .** S~ **

0.2 0 I I I I I I I I I I 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Solar energy costs (DM / kWh) Fig. 6, Total and part costs of energy transport.

To conclude, a few conditions should be a d d e d to those m e n t i o n e d at the outset a n d these should be seen as directly related to the result. In the investigation we have taken account of • the costs for 1988 a n d • technology of the period from the year 2000. We have not t a k e n a c c o u n t o f • • • •

a return o n the extraordinarily high investment; tax aspects; ecological developments a n d any political questions.

Acknowledgements--The topic has been dealt with in the form of a project most grateful thanks Dipl.-Ing. (FH), T. Ing., F.-W. Klack, Dr B. Klopries.

study. We would like to extend our to Prof. Dr H. Wendt, Dr W. Minnerup, Liebig, Dipl.-Ing., J. Schneider, Dipl.H. Adam, Dr-Ing., G. Beckman and

REFERENCES 1. Arbeitsgemeinschaft Energiebilanzen [Energy Balance Association], Friedrichstr. 1, 4300 Essen 1. 2. The Minister for Economic Affairs, Small and Medium sized Firms and Technology in NRW.

114

G. KASKE et al.

3. G. Kaske, Verfahren zu Transport und Speicherung yon Wasserstoff [Method of Hydrogen Transport and Storage], Nuclear Industry Annual Assembly '88, 1NFORUM GmbH, Bonn (1988). 4. G. Kaske et al., Transport und Verteilungsleitungen [Transport and Distribution Lines] in Wasserstofftechnologie [Hydrogen Technology]. DECHEMA, Frankfurt (1986). 5. Studies on the production, international transport and use of the clean energy source hydrogen on the basis of large and inexpensive hydropower potentials, DECHEMA, Frankfurt a. M. (June 1987).

6. K. W. KannigieBer and Z. Obermann, Energiefibertragung fiber weite Entfernungen [Energy Transmission over Large Distances], ETZ, 102 (issue 25), pp. 1332 1337 (1981). 7. G. Zampieri and F. de Micheli, Oil and Gas J. (7 November 1988). 8. A. Eiermann, Brennst.-Wdrme-Kraft 35, No. 4 (April 1983). 9. Ad hoc committee of the Federal Minister for Research and Technology, Solare Wasserstoffwirtschaft [Solar Hydrogen Industry], Bonn (April 1988).