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Hydrogen as an activating fuel for a tidal power plant
Int. J. Hydrogen Energy, Vol. 6, No. 3, pp. 243-253, 1981.
0360-3199/81/030243-11 $02.00/0 Pergamon Press Ltd, © 1981 International Association for Hydrogen Energy.
Printed in Great Britain.
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT A. M. GORLOV Northeastern University, Boston, Massachusetts, U.S.A.
(Receioed for publication 1 August 1980) Abstract---Ocean tides, as an environmentally clean and inexhaustible natural source of energy, can be used as one alternative for replacing fossil fuels. But because the tides are dependent on the moon phases, which do not always coincide with the time of human activity, tidal projects usually require a special system for the accumulation of energy for off-peak periods. The production of hydrogen by electrolysis can be considered one such system. This paper outlines the method by which hydrogen produced during off-peak tidal power plant operation can be used as an activating fuel to furnish the same plant during the peak-load demands. With our approach (see [1]) the energy of the tide is converted into the energy of compressed air by means of specialized chambers which are put on the ocean bed. Ocean water from the dammed region passes through the chamber where it works as a natural piston compressing air in the upper part of the closure. For the peak periods the compressed air can be heated by combustion of the stored hydrogen, and expanded through high-speed gas turbine generators. For the off-peak periods, the energy of non-heated compressed air is used for the production of the hydrogen fuel. In this case the total electric output of the power plant would be decreased somewhat because the losses of the energy would be taken for the production of the hydrogen fuel. Keywords: Tidal power; hydrogen combustion; compressed air; gas turbine generator; electrolysis. INTRODUCTION THE PRESENTenergy crisis has clearly demonstrated the urgent need to develop energy sources not dependent on fossil fuels. The limited and rapidly dwindling reserves of these fuels, especially oil, have led to a sharp increase in energy costs and the use of oil as a weapon for political blackmail. The present rate of consumption of fossil fuel endangers not only our energy supply, but also has the effect of burning hydrocarbons that are of considerable importance as raw materials for industry, especially for future generations. There are a number of known potential alternative forms of energy that would replace fossil fuels. The two that appear to be most suitable are solar and tidal energy, because in both of these cases the energy originates outside our planet and is therefore "inexhaustible". In both cases the energy is "clean" and, in at least certain regions, quite predictable. Tidal energy is created by the gravitational pull of the moon and, to a much smaller degree, of the sun. For that reason the range of tidal fluctuations and amplitudes of tidal waves depend directly upon various positions of the moon in its orbit and distances from the earth. This unregulated process creates a difficult problem in the attempt to harness tidal energy from the viewpoint of day time human activity which seldom coincides with the maximum possible tidal energy output. Tidal power plant projects would be more economical if they include some operational systems which can accumulate energy for the longer off-peak periods in order to release it during the shorter time of peak-load demands. There are several proposals for the accumulation and subsequent compensation of tidal energy for the peak time. The most popular of them is the pumped storage system when tidal power units pump extra volumes of water into the basin between generating periods, increasing its potential energy. Another approach considers a conjunction of tidal power generation with conventional hydro resources or any supplemental energy source of the region. A n d one more basic option which is now getting great attention is a compressed air storage method. In this case hydro-turbines drive air compressors for the off-peak time, pumping compressed air into underground caverns to accumulate energy for the peak period consumption [2]. In this paper a new approach to the alternatives for accumulating tidal energy will be considered. The idea is based on the production of hydrogen by electrolysis for the off-peak part of tidal 243
244
A.M. GORLOV
power operation and the subsequent combustion of it for heating compressed air, which is expanded through gas turbine generator sets to provide firm energy output. In this method the same gas turbine generators might be used for the production of hydrogen and for the generation of electric energy (see below). The energy potential of ocean tides is enormous. Simple calculations show that even partial utilization of this energy would be a substantial contribution toward solving the present energy shortage. The approximate magnitude of one cycle tidal energy can be calculated as the work done in lifting a mass of water from the ebb to flood levels:
E = p A foh x dx
=
½pAh 2.
where E is energy, p is weight density of sea water, A is the water area under consideration, and h is the height of the tide. Let us assume the weight density of sea water to be p = 10.15 kN/m -3 and A = 1 m 2. Then E = ½x 10.05h2kNmm -2= 1.4hZWhm -2. Since energy is generated by both rising and falling tide and there are four daily tide cycles (actually, each cycle lasts 6 h, 12 min), the total daily energy per square meter of ocean surface is Edaily = 4 x 1.4h 2 = 5.6hEWhm -2. Let us calculate this energy for the Bay of Fun@ in Canada and the state of Maine water region with an average tide of 6.5 m E daily = 236 Wh m -2, or annual energy per unit of ocean area would be E annual -- 86 KWh m -2. This means, for example, that the approximately 15,000 km 2 of ocean, with the tides mentioned above, of the Bay of Fundy and north of the State of Main represent about 1.3 x 101: k W h -1 of continuously renewable energy with an average power of 250,000 MW. For comparison, we can point out that all the power plants of the United States now produce about 2 x 1012 k W h of electric energy per year. The figure would he more impressive if additional account is taken of the northwestern region of the United States (Alaska). COMPRESSED A I R CONCEPT FOR HARNESSING T I D A L E N E R G Y The conventional approach which is grounded in all of the existing tidal power plant projects includes a number of reversible hydroturbine units installed into the body of a rigid dam. Thus the power-house is a part of the barrier which dams a water stream. But the analysis of the operation of the two existing power plants in the USSR and in France and all new projected plants shows that such a "river-type power plant inertia" applied to the tidal power plant construction appears not to be economically competitive with other known alternative methods of producing electric power. Construction of the hydroturbine tidal power plant requires up to $3000 capital investment per 1 kW of power produced. Recent investigations which have been undertaken in the United States [4] and in Canada [5] show this once again. The main reason for the high cost of such an application of the "river model" to the harnessing of tidal energy is the ineffectiveness of the hydroturbine installation for the low-water pressure case.
For example, in the Cobscook Bay Region where we have one of the greatest tides in the world the average water head is 5.5 m y-1 which is much lower than that of the average river power * Some correction of this magnitude might be done by approach of Berstein [3], but for the rough evaluation we can use the figure obtained below.
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT
245
plant. Because of the low head through the dam we have to use low-speed, bulky hydroturbines which require enormous space for their installation and operation. The hydroturbines have to be reversible due to the nature of the tides. In addition, the water head is not constant during each tidal cycle; it changes from ebb to flood twice a day with four "dead" periods in between. This substantially decreases the efficiency of the turbine operation. The compressed air concept allows for a principally new approach to the problem. Under the proposed method the energy of flowing water has to be converted into the energy of compressed air. After that the low-speed hydroturbines can be replaced by the compact high speed gas-turbines or reciprocating engines. Compared to the conventional hydroturbine tidal power projects the proposed method offers the following main advantages: 1. High-speed gas turbine installation requires 8--10 times less capital investment than the low-speed hydroturbines of the same capacity. Indeed, the "state-of-the-art" projects reveal a $120-150 investment per kW generated power for gas turbine operation and more than $1000 for the low-speed hydroturbine operation.
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s
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FIG. 1. Conceptual view of a tidal power plant. 2. Compressed air as a medium driving prime mover provides the unique possibility of increasing the power of a station many times by heating the air during the dual operation. 3. The power station (except for the air chambers) can be located on the shore instead of in the dam body. Thus, lighter and less expansive materials such as even synthetic films [6] can be used for the barrier structure. In addition to these advantages, we can point out that no engine problems arise because of the reversibility of the water flows. Also, the air medium is much less corrosive for gas turbines than salt water for hydroturbines. Two methods for converting the energy of the water stream into the energy of compressed air can be mentioned: (a) continuous operation when flowing water into the pipe creates a so-called suction effect around any opening in the wall of the pipe; (b) pulsation operation when portions of the flowing water compresses air in the specialized chamber installed on the ocean bed. It is easy to show that only the second approach is reasonable for a large scale system which must operate with great volumes of flowing water and compressed air. Therefore, in this paper we will consider the air chamber method.
246
A. M. GORLOV Oos turb*ne ~
Io
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FIG. 2. Filling cycle.
Figure 1 is a pictorial view of a tidal power plant according to the described project. For the dam structure as a possible variant a flexible plastic barrier is used.* The concept of a flexible dam for a tidal power plant was considered in detail in [1] and is out of the scope of the present paper. The compressed air which would be subsequently used for the gas turbine operation has been produced by air chamber 1. The bottom of the chamber has openings or ports which are arranged around the periphery of its base. For the flood period the water flows into the dammed bay through chamber 1, where it alternately creates compression or a partial vacuum in the upper part (Figs 2, 3). For the ebb time the same volume of water should be discharged through the chamber into the ocean, creating the same alternate compression or vacuum in the upper part of the closure. The difference between atmospheric air pressure and the chamber air pressure is used to drive an air turbine-electric generator unit. The idea of producing hydrogen for subsequent consumption to increase the capacity of the power plant comes from the air turbine's ability to increase its power by heating the compressed air supplied to it. This means that for 85--90% of the off-peak day time, the air turbine-generator unit can be used for production and storage of hydrogen on the low natural power rate. In the
FIG. 3. Emptying cycle. remaining 10-15% of the peak-load day period, the power unit works consuming compressed air heated by burning the stored hydrogen. In this case the hydrogen plays the role of an energy accumulator in the low demand consumption period. The first cycle of a turbine operation we will name (following [1]) "a single operation mode" and the second one "a dual operation mode". OFF-PEAK SINGLE O P E R A T I O N MODE In this paper we will focus on the daily off-peak tidal power operation as it is used for the production and storage of a quantity of hydrogen and its later combustion in the peak time of the same day. It does not exclude the subsequent study of other modes, when the total volume of hydrogen stored is governed on a weekly, monthly, or even seasonal basis. But the first case appears more desirable for a tidal power plant because it requires a minimum storage facility. The size of the electrolysis plant is dictated by the amount of energy available during an off-peak time, which in its turn, defines the rate of hydrogen production. As a key figure for a calculation of a capacity of electrolizers, we take (4.5-5.2) kWh required for the production of 7m 3 of hydrogen gas (de current).
* In [1] this method was named the "water sail" approach.
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT
247
The amount of available energy under the present approach, first of all, depends on the capacity and efficiency of the compressed air chamber. Figures 2 and 3 illustrate schematically the operation of the chamber with a system of vertical flap sluice gates. The chamber is a water-proof, reinforced concrete or steel structure which is installed on the ocean bed. The gates are located along two opposite sides of the chamber with the upper edge of the gates lower than minimum water level for the ebb period. When the gates from the higher water basin open, the gates from the lower water basin close. The water filling the chamber compresses air in the upper part of the closure. The compressed air flowing through the special pipe on the upper part of the chamber drives the gas turbines or reciprocating engines, until the water level in the chamber reaches the corresponding level in the high basin. After that a new cycle starts: water flowing out of the chamber into the lower basin creates a partial vacuum in the upper part of the closure, and so, the engines are driven by the air flow from the atmosphere into the chamber. A system of the air valves can be arranged so that the air flowing through the engines always has the same direction [7]. It should be noted that the volume of the water held within the housing may be orders of magnitude less than that which must pass through the system during a tidal change and which is available from, for instance, the bay side of dam. Thus, one tidal cycle corresponds to a number of filling/emptying cycles in the chamber and the air motor speed is not limited to the water velocity, as it is with hydroturbines. In operation, with water flowing from the high basin through the chamber to the low basin the pressure-actuated locks for the doors on the high side will open and water will rush in to fill the chamber. When the chamber is filled to a level corresponding to the water level on the high side of the dam, the doors on the high basin will fall back into place and lock. At this time, the doors on the low side of the dam will be unlocked because the pressure differential across these doors will have reached the predetermined pressure head. With the unlocking of the low side doors, water will rush out of the chamber until such time as the water level in the chamber drops to the level of the water at the low basin. When this happens, the pressure head at the doors on the high side reaches the predetermined pressure head and these doors are unlocked which starts the cycle over again. As can be seen, this is a simple, automatic system in which by merely locking and unlocking the doors, the chambers are made to fill and discharge water, with the doors closing when the pressure differential across them is about zero. The doors close and lock of their own weight. Referring to Figs 2 and 3 we can see two cycles of air flowing: through the gas turbine to the atmosphere expansion (compression cycle) and then suction of the air from the atmosphere through the engine into the chamber (vacuum cycle). Another mode of operation, which can be named "chamber-to-chamber operation", involves at least two adjacent air chambers. Work phases of these chambers are shifted by the time interval with respect to each other in such a manner that while the first chamber is filling (compression cycle) the second one is emptying (suction cycle). Air from the higher pressure chamber is expanding not to the atmosphere but into partial vacuum space of the adjacent chamber. After completion of the cycle (first chamber is filled, second one is emptied) the following cycle starts from the filling of the second chamber and emptying of the first one with reverse direction of the air flow. The main advantage of the "chamber-to-chamber" operation, compared to the one chamber operation mode, is a two times greater difference in the air pressure for the same water head across the dam (a more detailed description of this mode is given in [1]). As was mentioned, two different types of prime movers can effectively utilize a difference in the air pressure obtained in the chambers--gas turbine and reciprocating piston engine. A gas turbine should be more compact and more effective because of its advantage of continuous operation. But in some cases a piston engine might be favorable; for example, for the recompressing of initial low chamber pressure to higher level. Figure 4 diagrammatically represents the scheme of operation of a power plant with gas turbine units. Table 1 contains some parameters of the required turbines* and water regions. Note that the mentioned turbines are not tied to any particular ocean site but are taken as an illustration * The parameters of the turbines mentioned were calculated by Dr. M. Nackamkin per request of the author.
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of the relationship between possible capacity of a power aggregate and tidal region for this approach. Column 3 of the table contains the required air mass rate which should be produced by the air chambers to obtain the turbine's power (column 1) for the taken gauge working pressure (the difference between magnitudes of air pressure of the two chambers for the "chamber-to-chamber" operation mode, column 2). The most interesting results are represented by the figures of columns 10 and 11. The annual energy output for the gas-turbine operation (column 10) is calculated on the basis of turbine power (column 1) and about 70% of work hours during a year (t = 6132 h) for the 0.2 atm. of gauge pressure (2 m of a water head). For the pressures of 0.3, 0.4 and 0.5 atm. active time is taken correspondingly 4300 h, 3000 h, and 1840 h. The reason for the decrease in work time is that once the water head and corresponding air pressure falls below an effective level, the basin simply fills or empties. The air mass production (column 3) is directly proportional to the water volume which should be passed through a chamber. Knowing that, and taking into account as an example the mean tide h = 5.5 m for the Cobscook Bay water region, we can evaluate the areas of the ocean surface which are required for the chosen power movers (column 7) and with a rough approximation--the lengths of closing barriers for the shallow shore sites (column 8). The data in columns 10 and 11 reveal about the same magnitude of harnessed energy for the conventional hydro-turbine and gas-turbine operations. This proves the point of view that the hydro-turbine is not the unique machine for recovering energy from ocean water streams. We can conclude that the increasing velocity of the plenum (air flow for a gas-turbine in comparison with water flow for a hydro-turbine) compensates for the disadvantage of the smaller medium mass. By analogy, note that a fly-wheel's angular velocity is more important for its capacity to accumulate energy than its mass is. PEAK L O A D D U A L O P E R A T I O N MODE The conception of a dual operation mode as it regards the harnessing of tidal energy was described in detail in [1]. Below we will consider more precisely the application of this operation in the case where hydrogen is used as an activating fuel. In order to increase the energy output by heating the air flowing to the gas turbine, a hydrogen combustion chamber should be installed between the air chamber and the turbine-generator unit, as shown in Fig. 5. The hydrogen, which was stored during the single operation, is supplied into
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Gauge air pressure, Aim.
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1
Turbine power, MW
9.57 16.9 25 25 25
25
834.8
702.3 856.7 1799 1264.5 966.3
Air mass flow rate, kg/s
3
1.00 1.00 1.60 1.22 1.01 0.872
Blade length, m
4
164.2 195.9 164.2 195.9 220.1 241.1
Gas velocity, m/s
5
2.5 2.5 2.5 2.5 2.5 15
Min. blade dia./length ratio
6
1.55 1.90 4.00 2.80 2.13 1.85
Average barrier length (shallow shore location), km
Min. ocean area required, km: 2.21 2.70 5.70 4.00 3.04 2.63
8
7
TABLE 1.
136 × 166 x 351 × 246 × 187 × 162 ×
106 106 106 10~ 106
10 6
Annual potential energy of'the water region, KWh
9
x 106 x 106
× 106
× 106 x l06 × 106
35 43 91 64 49 42
x 106 x 106
× 106
x 106 × 106
× 106
Annual energy output for the case of hydroturbine operation, KWh Annual energy output for the gas-turbine operation, KWh 35 44 92 64 46 27
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A L E X A N D E R M. G O R L O V TABLE2.
No.
Input pressure, atm.
Turbine power, MW
Heat rate, kcal/kWh
Hydrogen consumption, g/kWh
Air mass flow rate, kg/s
Blade length, m
1
2
3
4
5
6
1.2 1.2 1.3 1.3 1.3 1.3 1.4 1.5
25 25 25 25 25 50 50 50
15198 16942 8558 10666 11886 11885 9350 7823
463.0 516.3 260.8 325.0 ~62.2 362.2 284.9 238.4
501.2 382.2 476.2 351.7 268.2 536.3 421.9 353
1.2 1.11 0.98 0.92 0.85 1.2 1.0 0.88
1 2 3 4 5 6 7 8
TABLE 3. GE Model 7821
GE Model 7981
Input pressure, atria.
Power, MW
Heat rate, kcal/kWh
Hydrogen consumption, g/kWh
Power, MW
Heat rate, kcal/kWh
Hydrogen consumption, g/kWh
1.0 1.2 1.3 1.5
60.0 66.5 69.1 73.8
2764 2495 2400 2247
84.2 76.0 73.1 68.5
73.2 81.4 83.9 89.3
2656 2387 2317 2178
80.9 72.7 70.6 66.4
TABLE4. Project
Operation mode Annual Production: 1. Energy, kWh
2. Hydrogen, m3 3. Oxygen, m3 4. Heavy water (D20), kg
Passamaquaddy Tribe
Single
43 x 106 (inner consumption) 8.43 × 106 4.21 × 106 Up to 460
Dual (2 peakh/day)
11 x 106 (output)
Treat Island
Single
2044 x 106 (inner consumption) 400 x 106 200 × 106 Up to 21500
Dual (2 peakh/day)
517 x 10~ (output)
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT
251
the combustion chamber at the same time as the compressed air. The flow of air heated by the combustion of hydrogen fuel then expands through the gas turbine and creates a type of "turbo-charger" which becomes a power system for the tidal power plant when combined with an electrical generator. Table 2 represents some characteristics of the gas turbines required for the different regimes of a dual operation mode. These figures were obtained by an analysis of the turbine operation under the conditions of input air pressure from 1.2-1.5 atm. pressure which corresponds to the 2-5 m of water head across the dam. The power of the turbines has been chosen within reasonable industrial limits of 25 and 50 MW. The data of Table 2 reveals the very high magnitudes of required heat consumption per one kWh energy output ("heat rate"). The figures of column 3 are several times greater than those for the industrial "state-of-the-art" gas turbines, consuming a regular No. 2 fuel oil with a heating value of 10725 kcal/kg. Usually, this rate does not exceed 3000 kcal/kg.
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TIME, hours RO. 6. Daily tidal energy balance of a square meter of ocean surface (Cobscook Bay region). The high specific fuel consumption can be explained by the low efficiency of a gas turbine which has been supplied by low pressured hot air. A better solution could be found if instead of the direct heating of low pressure air an intermediate recompression of air were done. In this case an industrial system "compressor-combustion chamber-turbine--generator" unit designed to work for the normal atmospheric pressure can serve as a power generator. If we consider this unit a "black box" for which compressed air is input and produced energy is output we can predict that increasing the input pressure will increase the efficiency of the power system and the energy output. The relationship between input and output for such a "black box" can be demonstrated by the analysis of the conventional General Electric gas turbines GE-7821 and GE-7981 (Table 3). The gas turbines mentioned are designed to use No. 2 fuel oil with a heating value 10725 kcal/kg. Hydrogen consumption figures were calculated corresponding to required heat rates. A compressed air consumption for the turbines of Table 3 is about 240 kg/s, which can be supplied from the ocean area of about 0.75 km 2 for the average tide h = 5.5 m. The first line of Table 3 is relevant to the design capacity of the power units for the normal conditions (p = 1 atm.). As it can be seen, increasing the initial pressure to 1.2atm. (line 2)
252
A.M. GORLOV
increases the power of the units about 10% and decreases the design heat rate (fuel consumption) about the same percentage. This tendency has been kept with further arising input air pressure, however, it becomes more moderate with each new higher step. We can point out some quite remarkable advantages of the power system analogous to GE-7821 or GE-7981, which uses hydrogen fuel during the dual operation cycle. 1. This system is ready to generate energy in any instant of the day regardless of the tidal conditions if there is stored hydrogen fuel. Indeed, because the power unit is supplied by its own compressor it can work even during the "dead" period between ebb and flood. 2. This is a very dynamic system which does not require time for its acceleration and deceleration. 3. The hydrogen mixed with oxygen can be an almost ideal fuel for the gas turbine operation due to its low density, viscosity and the tremendous efficiency of the combustion. For this reason we can predict that the heat rate and hydrogen consumption mentioned in Table 3 should be smaller if instead of oil we use hydrogen fuel. 4. Expenses for the storage of hydrogen is minimized because a daily portion of produced hydrogen is used for combustion during peak time the same day. The energy characteristics and efficiency of the described hydrogen combustion installation for the regular daily tidal cycle can be obtained by analysis of the hydrogen production and energy output corresponding to single and dual parts of the tidal power plant operation. Let us consider ocean conditions of the Cobscook Bay region in the state of Maine in the U.S., with average tide fluctuation of 5.5 m. Following the approach of this paper (see above), the average daily potential energy of the tide is 0.17 kWh/m E of the ocean surface. Now, assuming the time of single operation is 22 h a day and the time of dual operation is 2 h, we can construct a general pattern of the power system work. Results of the analysis are represented by Fig. 6, for the midnight flood start. The graph in Fig. 6 is relevant to the case of combustion of the whole volume of the hydrogen produced daily and for the conditions when the total energy of the tidal power plant is used for the production of the hydrogen during the time of single operation. The efficiency of this mode is taken to be 70%. Any correction of this value does not influence the general shape of the graph. If we compare both modes it can be seen that the total energy output for a dual mode operation is about four times less than for a single mode operation: Eduaj = 27.65 x 10 -3 kWh/m Eand Es~g~ = 109.2 × 10-3kWh/m 2. The ratio pointed out is about proportional to the efficiency of the "state-of-the-art" gas turbine power generator multiplied by the efficiency of hydrogen production. The data of Fig. 6, which are related to 1 m 2 of ocean surface of the Cobscook Bay, can be used for the evaluation of the capacity of any power plant for the analogous tidal conditions. Such an evaluation has been done for the Passamaquoddy Tribe and Treat Island tidal power plant projects (Table 4). The energy output of the power plants mentioned were calculated for about 5600 h of active annual time duration for the average tidal installation [1]. The heat rate, 2387 kal/kWh, and the corresponding air pressure, 1.2 atm. (2 m of the water head, (see second line of Table 3 for GE-7981 model), were used as a basis for calculation of the hydrogen production and the energy output. If a peak-time accounted for the absence of a tide between ebb and flood, which would mean the absence of the outer air supply, the plant capacity decreases about 10%. As may be seen from Table 4 a substantial volume of the heavy water can be produced annually as a by-product of electrolysis (about 460 kg by the first power plant and 21.5 t by the second plant). So, the present approach would be more attractive if the heavy water could be utilized by an industrial market. As an approximation, the general efficiency K of the described tidal power plant may be evaluated by the following product:
K= Ki KE K3 where, K1 is the mechanical efficiency of the gas turbines for single operation (K1 is taken to be 60%);/(2 is the efficiency of the electrolysis (/(2 is about 68% for the "state-of-the-art" industrial electrolysis installation); K3 is the efficiency of a gas turbine (/(3 is about 36%). Thus, the overall efficiency of the dual mode operation tidal power plant, K, becomes 15.7% for the above listed magnitudes, K~, K2 and/(3. This efficiency can be improved if we utilize hot air thrown out by the gas turbine for the energy generation by the conventional steam power
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT
253
installation ("combined cycle"). The mentioned "low efficiency" is natural for the system which involves three consecutive operations. Note that the dual system probably cannot use a chamber-to-chamber operational mode as well as a partial vacuum cycle for the air chamber operation. Because of the high temperature of the air, it has been expanded out from the turbine. In any case, presently it is not clear how to use the suction part of the cycle. For that reason we take only the compression part of the working time for the calculation of plant capacity in the case of dual operation. CONCLUSION The considered approach provides one of the possible solutions to meet peak energy demands in the case of compressed air usage for tidal power plant operation. As it was shown, the amount of a hydrogen fuel produced for the off-peak day time is enough for the power plant operation during 2 peak hours, with three times greater than average plant capacity. Definitely, this is not the only possible mode of operation. Hydrogen fuel can be collected for two or more days in order to extend the time of peaking operation. Each of these cases should be addressed to the analysis of the particular project conditions. So, the advantage of treble power plant capacity for shorter peaking operation can be brought by the worth of 75% gross energy losses. Subsequent study could reveal whether it would be reasonable or not for any particular case. On balance, even though the hydrogen fuel storage method has suffered energy losses and extra capital investment for electrolysis and hydrogen storage equipment, we see the following advantages of this approach: 1. In comparison with an air compressed storage, the described scheme does not require a gas turbine oil fuel and, definitely, it is greatly less capital-intensive because the volume of hydrogen produced daily is in several orders of magnitude less than the corresponding volume of compressed air. 2. In comparison with a hydro-pumped method, this approach does not use low-speed heavy hydro-turbines and a large and, for that reason, expensive power house. 3. The same machinery (gas turbines) can be used for production and consumption of hydrogen fuel. Note once again that the increasing peak capacity of a tidal power plant is the problem of the present paper. In general, the compressed air concept can be used for a large-scale harnessing of tidal energy (see [1]). If such a tidal power plant has been designed for production of hydrogen fuel it could work with naturally distributed power capacity, providing large quantities of hydrogen for general purposes (see second and fourth columns of Table 4). In this case the irregularity of the tides' nature does not have any significance. Acknowledgement--The present research is sponsored by the Division of Advanced Energy Projects of the Department of Energy under a 2 year contract with Northeastern University.
REFERENCES 1. A. M. GORLOV,Some new conceptions in the approach to harnessing tidal energy. Proceedings of the 2nd Miami International Conference on Alternative Energy Sources, U.S.A. (1979). 2. T. L. HAYDOCKand J. G. WARNOCK,Tidal Power and Energy Storage, Halifax, Nova Scotia, Canada (1974). 3. L. B. BERNSTEIN,A problem of harnessing tidal power and the construction of an experimental Kislaja Guba tidal power plant, Post-doctoral dissertation, Leningrad, USSR (1973). 4. W. W. WAYNE, JR., Tidal power possibilities in the United States, ASCE, Preprint 3199, Pittsburgh (1978). 5. Reassessment of Fundy Tidal Power, Reports of the Bay of Fundy Tidal Power Review Board Committee (1977). 6. F. OTro, Tensile Structures, M.I.T. Press (1973). 7. A. M. GORLOV,Apparatus for harnessing tidal power, U.S. Patent No. 4,103,490 August (1978).
0360-3199/81/030243-11 $02.00/0 Pergamon Press Ltd, © 1981 International Association for Hydrogen Energy.
Printed in Great Britain.
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT A. M. GORLOV Northeastern University, Boston, Massachusetts, U.S.A.
(Receioed for publication 1 August 1980) Abstract---Ocean tides, as an environmentally clean and inexhaustible natural source of energy, can be used as one alternative for replacing fossil fuels. But because the tides are dependent on the moon phases, which do not always coincide with the time of human activity, tidal projects usually require a special system for the accumulation of energy for off-peak periods. The production of hydrogen by electrolysis can be considered one such system. This paper outlines the method by which hydrogen produced during off-peak tidal power plant operation can be used as an activating fuel to furnish the same plant during the peak-load demands. With our approach (see [1]) the energy of the tide is converted into the energy of compressed air by means of specialized chambers which are put on the ocean bed. Ocean water from the dammed region passes through the chamber where it works as a natural piston compressing air in the upper part of the closure. For the peak periods the compressed air can be heated by combustion of the stored hydrogen, and expanded through high-speed gas turbine generators. For the off-peak periods, the energy of non-heated compressed air is used for the production of the hydrogen fuel. In this case the total electric output of the power plant would be decreased somewhat because the losses of the energy would be taken for the production of the hydrogen fuel. Keywords: Tidal power; hydrogen combustion; compressed air; gas turbine generator; electrolysis. INTRODUCTION THE PRESENTenergy crisis has clearly demonstrated the urgent need to develop energy sources not dependent on fossil fuels. The limited and rapidly dwindling reserves of these fuels, especially oil, have led to a sharp increase in energy costs and the use of oil as a weapon for political blackmail. The present rate of consumption of fossil fuel endangers not only our energy supply, but also has the effect of burning hydrocarbons that are of considerable importance as raw materials for industry, especially for future generations. There are a number of known potential alternative forms of energy that would replace fossil fuels. The two that appear to be most suitable are solar and tidal energy, because in both of these cases the energy originates outside our planet and is therefore "inexhaustible". In both cases the energy is "clean" and, in at least certain regions, quite predictable. Tidal energy is created by the gravitational pull of the moon and, to a much smaller degree, of the sun. For that reason the range of tidal fluctuations and amplitudes of tidal waves depend directly upon various positions of the moon in its orbit and distances from the earth. This unregulated process creates a difficult problem in the attempt to harness tidal energy from the viewpoint of day time human activity which seldom coincides with the maximum possible tidal energy output. Tidal power plant projects would be more economical if they include some operational systems which can accumulate energy for the longer off-peak periods in order to release it during the shorter time of peak-load demands. There are several proposals for the accumulation and subsequent compensation of tidal energy for the peak time. The most popular of them is the pumped storage system when tidal power units pump extra volumes of water into the basin between generating periods, increasing its potential energy. Another approach considers a conjunction of tidal power generation with conventional hydro resources or any supplemental energy source of the region. A n d one more basic option which is now getting great attention is a compressed air storage method. In this case hydro-turbines drive air compressors for the off-peak time, pumping compressed air into underground caverns to accumulate energy for the peak period consumption [2]. In this paper a new approach to the alternatives for accumulating tidal energy will be considered. The idea is based on the production of hydrogen by electrolysis for the off-peak part of tidal 243
244
A.M. GORLOV
power operation and the subsequent combustion of it for heating compressed air, which is expanded through gas turbine generator sets to provide firm energy output. In this method the same gas turbine generators might be used for the production of hydrogen and for the generation of electric energy (see below). The energy potential of ocean tides is enormous. Simple calculations show that even partial utilization of this energy would be a substantial contribution toward solving the present energy shortage. The approximate magnitude of one cycle tidal energy can be calculated as the work done in lifting a mass of water from the ebb to flood levels:
E = p A foh x dx
=
½pAh 2.
where E is energy, p is weight density of sea water, A is the water area under consideration, and h is the height of the tide. Let us assume the weight density of sea water to be p = 10.15 kN/m -3 and A = 1 m 2. Then E = ½x 10.05h2kNmm -2= 1.4hZWhm -2. Since energy is generated by both rising and falling tide and there are four daily tide cycles (actually, each cycle lasts 6 h, 12 min), the total daily energy per square meter of ocean surface is Edaily = 4 x 1.4h 2 = 5.6hEWhm -2. Let us calculate this energy for the Bay of Fun@ in Canada and the state of Maine water region with an average tide of 6.5 m E daily = 236 Wh m -2, or annual energy per unit of ocean area would be E annual -- 86 KWh m -2. This means, for example, that the approximately 15,000 km 2 of ocean, with the tides mentioned above, of the Bay of Fundy and north of the State of Main represent about 1.3 x 101: k W h -1 of continuously renewable energy with an average power of 250,000 MW. For comparison, we can point out that all the power plants of the United States now produce about 2 x 1012 k W h of electric energy per year. The figure would he more impressive if additional account is taken of the northwestern region of the United States (Alaska). COMPRESSED A I R CONCEPT FOR HARNESSING T I D A L E N E R G Y The conventional approach which is grounded in all of the existing tidal power plant projects includes a number of reversible hydroturbine units installed into the body of a rigid dam. Thus the power-house is a part of the barrier which dams a water stream. But the analysis of the operation of the two existing power plants in the USSR and in France and all new projected plants shows that such a "river-type power plant inertia" applied to the tidal power plant construction appears not to be economically competitive with other known alternative methods of producing electric power. Construction of the hydroturbine tidal power plant requires up to $3000 capital investment per 1 kW of power produced. Recent investigations which have been undertaken in the United States [4] and in Canada [5] show this once again. The main reason for the high cost of such an application of the "river model" to the harnessing of tidal energy is the ineffectiveness of the hydroturbine installation for the low-water pressure case.
For example, in the Cobscook Bay Region where we have one of the greatest tides in the world the average water head is 5.5 m y-1 which is much lower than that of the average river power * Some correction of this magnitude might be done by approach of Berstein [3], but for the rough evaluation we can use the figure obtained below.
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT
245
plant. Because of the low head through the dam we have to use low-speed, bulky hydroturbines which require enormous space for their installation and operation. The hydroturbines have to be reversible due to the nature of the tides. In addition, the water head is not constant during each tidal cycle; it changes from ebb to flood twice a day with four "dead" periods in between. This substantially decreases the efficiency of the turbine operation. The compressed air concept allows for a principally new approach to the problem. Under the proposed method the energy of flowing water has to be converted into the energy of compressed air. After that the low-speed hydroturbines can be replaced by the compact high speed gas-turbines or reciprocating engines. Compared to the conventional hydroturbine tidal power projects the proposed method offers the following main advantages: 1. High-speed gas turbine installation requires 8--10 times less capital investment than the low-speed hydroturbines of the same capacity. Indeed, the "state-of-the-art" projects reveal a $120-150 investment per kW generated power for gas turbine operation and more than $1000 for the low-speed hydroturbine operation.
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FIG. 1. Conceptual view of a tidal power plant. 2. Compressed air as a medium driving prime mover provides the unique possibility of increasing the power of a station many times by heating the air during the dual operation. 3. The power station (except for the air chambers) can be located on the shore instead of in the dam body. Thus, lighter and less expansive materials such as even synthetic films [6] can be used for the barrier structure. In addition to these advantages, we can point out that no engine problems arise because of the reversibility of the water flows. Also, the air medium is much less corrosive for gas turbines than salt water for hydroturbines. Two methods for converting the energy of the water stream into the energy of compressed air can be mentioned: (a) continuous operation when flowing water into the pipe creates a so-called suction effect around any opening in the wall of the pipe; (b) pulsation operation when portions of the flowing water compresses air in the specialized chamber installed on the ocean bed. It is easy to show that only the second approach is reasonable for a large scale system which must operate with great volumes of flowing water and compressed air. Therefore, in this paper we will consider the air chamber method.
246
A. M. GORLOV Oos turb*ne ~
Io
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FIG. 2. Filling cycle.
Figure 1 is a pictorial view of a tidal power plant according to the described project. For the dam structure as a possible variant a flexible plastic barrier is used.* The concept of a flexible dam for a tidal power plant was considered in detail in [1] and is out of the scope of the present paper. The compressed air which would be subsequently used for the gas turbine operation has been produced by air chamber 1. The bottom of the chamber has openings or ports which are arranged around the periphery of its base. For the flood period the water flows into the dammed bay through chamber 1, where it alternately creates compression or a partial vacuum in the upper part (Figs 2, 3). For the ebb time the same volume of water should be discharged through the chamber into the ocean, creating the same alternate compression or vacuum in the upper part of the closure. The difference between atmospheric air pressure and the chamber air pressure is used to drive an air turbine-electric generator unit. The idea of producing hydrogen for subsequent consumption to increase the capacity of the power plant comes from the air turbine's ability to increase its power by heating the compressed air supplied to it. This means that for 85--90% of the off-peak day time, the air turbine-generator unit can be used for production and storage of hydrogen on the low natural power rate. In the
FIG. 3. Emptying cycle. remaining 10-15% of the peak-load day period, the power unit works consuming compressed air heated by burning the stored hydrogen. In this case the hydrogen plays the role of an energy accumulator in the low demand consumption period. The first cycle of a turbine operation we will name (following [1]) "a single operation mode" and the second one "a dual operation mode". OFF-PEAK SINGLE O P E R A T I O N MODE In this paper we will focus on the daily off-peak tidal power operation as it is used for the production and storage of a quantity of hydrogen and its later combustion in the peak time of the same day. It does not exclude the subsequent study of other modes, when the total volume of hydrogen stored is governed on a weekly, monthly, or even seasonal basis. But the first case appears more desirable for a tidal power plant because it requires a minimum storage facility. The size of the electrolysis plant is dictated by the amount of energy available during an off-peak time, which in its turn, defines the rate of hydrogen production. As a key figure for a calculation of a capacity of electrolizers, we take (4.5-5.2) kWh required for the production of 7m 3 of hydrogen gas (de current).
* In [1] this method was named the "water sail" approach.
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT
247
The amount of available energy under the present approach, first of all, depends on the capacity and efficiency of the compressed air chamber. Figures 2 and 3 illustrate schematically the operation of the chamber with a system of vertical flap sluice gates. The chamber is a water-proof, reinforced concrete or steel structure which is installed on the ocean bed. The gates are located along two opposite sides of the chamber with the upper edge of the gates lower than minimum water level for the ebb period. When the gates from the higher water basin open, the gates from the lower water basin close. The water filling the chamber compresses air in the upper part of the closure. The compressed air flowing through the special pipe on the upper part of the chamber drives the gas turbines or reciprocating engines, until the water level in the chamber reaches the corresponding level in the high basin. After that a new cycle starts: water flowing out of the chamber into the lower basin creates a partial vacuum in the upper part of the closure, and so, the engines are driven by the air flow from the atmosphere into the chamber. A system of the air valves can be arranged so that the air flowing through the engines always has the same direction [7]. It should be noted that the volume of the water held within the housing may be orders of magnitude less than that which must pass through the system during a tidal change and which is available from, for instance, the bay side of dam. Thus, one tidal cycle corresponds to a number of filling/emptying cycles in the chamber and the air motor speed is not limited to the water velocity, as it is with hydroturbines. In operation, with water flowing from the high basin through the chamber to the low basin the pressure-actuated locks for the doors on the high side will open and water will rush in to fill the chamber. When the chamber is filled to a level corresponding to the water level on the high side of the dam, the doors on the high basin will fall back into place and lock. At this time, the doors on the low side of the dam will be unlocked because the pressure differential across these doors will have reached the predetermined pressure head. With the unlocking of the low side doors, water will rush out of the chamber until such time as the water level in the chamber drops to the level of the water at the low basin. When this happens, the pressure head at the doors on the high side reaches the predetermined pressure head and these doors are unlocked which starts the cycle over again. As can be seen, this is a simple, automatic system in which by merely locking and unlocking the doors, the chambers are made to fill and discharge water, with the doors closing when the pressure differential across them is about zero. The doors close and lock of their own weight. Referring to Figs 2 and 3 we can see two cycles of air flowing: through the gas turbine to the atmosphere expansion (compression cycle) and then suction of the air from the atmosphere through the engine into the chamber (vacuum cycle). Another mode of operation, which can be named "chamber-to-chamber operation", involves at least two adjacent air chambers. Work phases of these chambers are shifted by the time interval with respect to each other in such a manner that while the first chamber is filling (compression cycle) the second one is emptying (suction cycle). Air from the higher pressure chamber is expanding not to the atmosphere but into partial vacuum space of the adjacent chamber. After completion of the cycle (first chamber is filled, second one is emptied) the following cycle starts from the filling of the second chamber and emptying of the first one with reverse direction of the air flow. The main advantage of the "chamber-to-chamber" operation, compared to the one chamber operation mode, is a two times greater difference in the air pressure for the same water head across the dam (a more detailed description of this mode is given in [1]). As was mentioned, two different types of prime movers can effectively utilize a difference in the air pressure obtained in the chambers--gas turbine and reciprocating piston engine. A gas turbine should be more compact and more effective because of its advantage of continuous operation. But in some cases a piston engine might be favorable; for example, for the recompressing of initial low chamber pressure to higher level. Figure 4 diagrammatically represents the scheme of operation of a power plant with gas turbine units. Table 1 contains some parameters of the required turbines* and water regions. Note that the mentioned turbines are not tied to any particular ocean site but are taken as an illustration * The parameters of the turbines mentioned were calculated by Dr. M. Nackamkin per request of the author.
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of the relationship between possible capacity of a power aggregate and tidal region for this approach. Column 3 of the table contains the required air mass rate which should be produced by the air chambers to obtain the turbine's power (column 1) for the taken gauge working pressure (the difference between magnitudes of air pressure of the two chambers for the "chamber-to-chamber" operation mode, column 2). The most interesting results are represented by the figures of columns 10 and 11. The annual energy output for the gas-turbine operation (column 10) is calculated on the basis of turbine power (column 1) and about 70% of work hours during a year (t = 6132 h) for the 0.2 atm. of gauge pressure (2 m of a water head). For the pressures of 0.3, 0.4 and 0.5 atm. active time is taken correspondingly 4300 h, 3000 h, and 1840 h. The reason for the decrease in work time is that once the water head and corresponding air pressure falls below an effective level, the basin simply fills or empties. The air mass production (column 3) is directly proportional to the water volume which should be passed through a chamber. Knowing that, and taking into account as an example the mean tide h = 5.5 m for the Cobscook Bay water region, we can evaluate the areas of the ocean surface which are required for the chosen power movers (column 7) and with a rough approximation--the lengths of closing barriers for the shallow shore sites (column 8). The data in columns 10 and 11 reveal about the same magnitude of harnessed energy for the conventional hydro-turbine and gas-turbine operations. This proves the point of view that the hydro-turbine is not the unique machine for recovering energy from ocean water streams. We can conclude that the increasing velocity of the plenum (air flow for a gas-turbine in comparison with water flow for a hydro-turbine) compensates for the disadvantage of the smaller medium mass. By analogy, note that a fly-wheel's angular velocity is more important for its capacity to accumulate energy than its mass is. PEAK L O A D D U A L O P E R A T I O N MODE The conception of a dual operation mode as it regards the harnessing of tidal energy was described in detail in [1]. Below we will consider more precisely the application of this operation in the case where hydrogen is used as an activating fuel. In order to increase the energy output by heating the air flowing to the gas turbine, a hydrogen combustion chamber should be installed between the air chamber and the turbine-generator unit, as shown in Fig. 5. The hydrogen, which was stored during the single operation, is supplied into
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Gauge air pressure, Aim.
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1
Turbine power, MW
9.57 16.9 25 25 25
25
834.8
702.3 856.7 1799 1264.5 966.3
Air mass flow rate, kg/s
3
1.00 1.00 1.60 1.22 1.01 0.872
Blade length, m
4
164.2 195.9 164.2 195.9 220.1 241.1
Gas velocity, m/s
5
2.5 2.5 2.5 2.5 2.5 15
Min. blade dia./length ratio
6
1.55 1.90 4.00 2.80 2.13 1.85
Average barrier length (shallow shore location), km
Min. ocean area required, km: 2.21 2.70 5.70 4.00 3.04 2.63
8
7
TABLE 1.
136 × 166 x 351 × 246 × 187 × 162 ×
106 106 106 10~ 106
10 6
Annual potential energy of'the water region, KWh
9
x 106 x 106
× 106
× 106 x l06 × 106
35 43 91 64 49 42
x 106 x 106
× 106
x 106 × 106
× 106
Annual energy output for the case of hydroturbine operation, KWh Annual energy output for the gas-turbine operation, KWh 35 44 92 64 46 27
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A L E X A N D E R M. G O R L O V TABLE2.
No.
Input pressure, atm.
Turbine power, MW
Heat rate, kcal/kWh
Hydrogen consumption, g/kWh
Air mass flow rate, kg/s
Blade length, m
1
2
3
4
5
6
1.2 1.2 1.3 1.3 1.3 1.3 1.4 1.5
25 25 25 25 25 50 50 50
15198 16942 8558 10666 11886 11885 9350 7823
463.0 516.3 260.8 325.0 ~62.2 362.2 284.9 238.4
501.2 382.2 476.2 351.7 268.2 536.3 421.9 353
1.2 1.11 0.98 0.92 0.85 1.2 1.0 0.88
1 2 3 4 5 6 7 8
TABLE 3. GE Model 7821
GE Model 7981
Input pressure, atria.
Power, MW
Heat rate, kcal/kWh
Hydrogen consumption, g/kWh
Power, MW
Heat rate, kcal/kWh
Hydrogen consumption, g/kWh
1.0 1.2 1.3 1.5
60.0 66.5 69.1 73.8
2764 2495 2400 2247
84.2 76.0 73.1 68.5
73.2 81.4 83.9 89.3
2656 2387 2317 2178
80.9 72.7 70.6 66.4
TABLE4. Project
Operation mode Annual Production: 1. Energy, kWh
2. Hydrogen, m3 3. Oxygen, m3 4. Heavy water (D20), kg
Passamaquaddy Tribe
Single
43 x 106 (inner consumption) 8.43 × 106 4.21 × 106 Up to 460
Dual (2 peakh/day)
11 x 106 (output)
Treat Island
Single
2044 x 106 (inner consumption) 400 x 106 200 × 106 Up to 21500
Dual (2 peakh/day)
517 x 10~ (output)
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT
251
the combustion chamber at the same time as the compressed air. The flow of air heated by the combustion of hydrogen fuel then expands through the gas turbine and creates a type of "turbo-charger" which becomes a power system for the tidal power plant when combined with an electrical generator. Table 2 represents some characteristics of the gas turbines required for the different regimes of a dual operation mode. These figures were obtained by an analysis of the turbine operation under the conditions of input air pressure from 1.2-1.5 atm. pressure which corresponds to the 2-5 m of water head across the dam. The power of the turbines has been chosen within reasonable industrial limits of 25 and 50 MW. The data of Table 2 reveals the very high magnitudes of required heat consumption per one kWh energy output ("heat rate"). The figures of column 3 are several times greater than those for the industrial "state-of-the-art" gas turbines, consuming a regular No. 2 fuel oil with a heating value of 10725 kcal/kg. Usually, this rate does not exceed 3000 kcal/kg.
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TIME, hours RO. 6. Daily tidal energy balance of a square meter of ocean surface (Cobscook Bay region). The high specific fuel consumption can be explained by the low efficiency of a gas turbine which has been supplied by low pressured hot air. A better solution could be found if instead of the direct heating of low pressure air an intermediate recompression of air were done. In this case an industrial system "compressor-combustion chamber-turbine--generator" unit designed to work for the normal atmospheric pressure can serve as a power generator. If we consider this unit a "black box" for which compressed air is input and produced energy is output we can predict that increasing the input pressure will increase the efficiency of the power system and the energy output. The relationship between input and output for such a "black box" can be demonstrated by the analysis of the conventional General Electric gas turbines GE-7821 and GE-7981 (Table 3). The gas turbines mentioned are designed to use No. 2 fuel oil with a heating value 10725 kcal/kg. Hydrogen consumption figures were calculated corresponding to required heat rates. A compressed air consumption for the turbines of Table 3 is about 240 kg/s, which can be supplied from the ocean area of about 0.75 km 2 for the average tide h = 5.5 m. The first line of Table 3 is relevant to the design capacity of the power units for the normal conditions (p = 1 atm.). As it can be seen, increasing the initial pressure to 1.2atm. (line 2)
252
A.M. GORLOV
increases the power of the units about 10% and decreases the design heat rate (fuel consumption) about the same percentage. This tendency has been kept with further arising input air pressure, however, it becomes more moderate with each new higher step. We can point out some quite remarkable advantages of the power system analogous to GE-7821 or GE-7981, which uses hydrogen fuel during the dual operation cycle. 1. This system is ready to generate energy in any instant of the day regardless of the tidal conditions if there is stored hydrogen fuel. Indeed, because the power unit is supplied by its own compressor it can work even during the "dead" period between ebb and flood. 2. This is a very dynamic system which does not require time for its acceleration and deceleration. 3. The hydrogen mixed with oxygen can be an almost ideal fuel for the gas turbine operation due to its low density, viscosity and the tremendous efficiency of the combustion. For this reason we can predict that the heat rate and hydrogen consumption mentioned in Table 3 should be smaller if instead of oil we use hydrogen fuel. 4. Expenses for the storage of hydrogen is minimized because a daily portion of produced hydrogen is used for combustion during peak time the same day. The energy characteristics and efficiency of the described hydrogen combustion installation for the regular daily tidal cycle can be obtained by analysis of the hydrogen production and energy output corresponding to single and dual parts of the tidal power plant operation. Let us consider ocean conditions of the Cobscook Bay region in the state of Maine in the U.S., with average tide fluctuation of 5.5 m. Following the approach of this paper (see above), the average daily potential energy of the tide is 0.17 kWh/m E of the ocean surface. Now, assuming the time of single operation is 22 h a day and the time of dual operation is 2 h, we can construct a general pattern of the power system work. Results of the analysis are represented by Fig. 6, for the midnight flood start. The graph in Fig. 6 is relevant to the case of combustion of the whole volume of the hydrogen produced daily and for the conditions when the total energy of the tidal power plant is used for the production of the hydrogen during the time of single operation. The efficiency of this mode is taken to be 70%. Any correction of this value does not influence the general shape of the graph. If we compare both modes it can be seen that the total energy output for a dual mode operation is about four times less than for a single mode operation: Eduaj = 27.65 x 10 -3 kWh/m Eand Es~g~ = 109.2 × 10-3kWh/m 2. The ratio pointed out is about proportional to the efficiency of the "state-of-the-art" gas turbine power generator multiplied by the efficiency of hydrogen production. The data of Fig. 6, which are related to 1 m 2 of ocean surface of the Cobscook Bay, can be used for the evaluation of the capacity of any power plant for the analogous tidal conditions. Such an evaluation has been done for the Passamaquoddy Tribe and Treat Island tidal power plant projects (Table 4). The energy output of the power plants mentioned were calculated for about 5600 h of active annual time duration for the average tidal installation [1]. The heat rate, 2387 kal/kWh, and the corresponding air pressure, 1.2 atm. (2 m of the water head, (see second line of Table 3 for GE-7981 model), were used as a basis for calculation of the hydrogen production and the energy output. If a peak-time accounted for the absence of a tide between ebb and flood, which would mean the absence of the outer air supply, the plant capacity decreases about 10%. As may be seen from Table 4 a substantial volume of the heavy water can be produced annually as a by-product of electrolysis (about 460 kg by the first power plant and 21.5 t by the second plant). So, the present approach would be more attractive if the heavy water could be utilized by an industrial market. As an approximation, the general efficiency K of the described tidal power plant may be evaluated by the following product:
K= Ki KE K3 where, K1 is the mechanical efficiency of the gas turbines for single operation (K1 is taken to be 60%);/(2 is the efficiency of the electrolysis (/(2 is about 68% for the "state-of-the-art" industrial electrolysis installation); K3 is the efficiency of a gas turbine (/(3 is about 36%). Thus, the overall efficiency of the dual mode operation tidal power plant, K, becomes 15.7% for the above listed magnitudes, K~, K2 and/(3. This efficiency can be improved if we utilize hot air thrown out by the gas turbine for the energy generation by the conventional steam power
HYDROGEN AS AN ACTIVATING FUEL FOR A TIDAL POWER PLANT
253
installation ("combined cycle"). The mentioned "low efficiency" is natural for the system which involves three consecutive operations. Note that the dual system probably cannot use a chamber-to-chamber operational mode as well as a partial vacuum cycle for the air chamber operation. Because of the high temperature of the air, it has been expanded out from the turbine. In any case, presently it is not clear how to use the suction part of the cycle. For that reason we take only the compression part of the working time for the calculation of plant capacity in the case of dual operation. CONCLUSION The considered approach provides one of the possible solutions to meet peak energy demands in the case of compressed air usage for tidal power plant operation. As it was shown, the amount of a hydrogen fuel produced for the off-peak day time is enough for the power plant operation during 2 peak hours, with three times greater than average plant capacity. Definitely, this is not the only possible mode of operation. Hydrogen fuel can be collected for two or more days in order to extend the time of peaking operation. Each of these cases should be addressed to the analysis of the particular project conditions. So, the advantage of treble power plant capacity for shorter peaking operation can be brought by the worth of 75% gross energy losses. Subsequent study could reveal whether it would be reasonable or not for any particular case. On balance, even though the hydrogen fuel storage method has suffered energy losses and extra capital investment for electrolysis and hydrogen storage equipment, we see the following advantages of this approach: 1. In comparison with an air compressed storage, the described scheme does not require a gas turbine oil fuel and, definitely, it is greatly less capital-intensive because the volume of hydrogen produced daily is in several orders of magnitude less than the corresponding volume of compressed air. 2. In comparison with a hydro-pumped method, this approach does not use low-speed heavy hydro-turbines and a large and, for that reason, expensive power house. 3. The same machinery (gas turbines) can be used for production and consumption of hydrogen fuel. Note once again that the increasing peak capacity of a tidal power plant is the problem of the present paper. In general, the compressed air concept can be used for a large-scale harnessing of tidal energy (see [1]). If such a tidal power plant has been designed for production of hydrogen fuel it could work with naturally distributed power capacity, providing large quantities of hydrogen for general purposes (see second and fourth columns of Table 4). In this case the irregularity of the tides' nature does not have any significance. Acknowledgement--The present research is sponsored by the Division of Advanced Energy Projects of the Department of Energy under a 2 year contract with Northeastern University.
REFERENCES 1. A. M. GORLOV,Some new conceptions in the approach to harnessing tidal energy. Proceedings of the 2nd Miami International Conference on Alternative Energy Sources, U.S.A. (1979). 2. T. L. HAYDOCKand J. G. WARNOCK,Tidal Power and Energy Storage, Halifax, Nova Scotia, Canada (1974). 3. L. B. BERNSTEIN,A problem of harnessing tidal power and the construction of an experimental Kislaja Guba tidal power plant, Post-doctoral dissertation, Leningrad, USSR (1973). 4. W. W. WAYNE, JR., Tidal power possibilities in the United States, ASCE, Preprint 3199, Pittsburgh (1978). 5. Reassessment of Fundy Tidal Power, Reports of the Bay of Fundy Tidal Power Review Board Committee (1977). 6. F. OTro, Tensile Structures, M.I.T. Press (1973). 7. A. M. GORLOV,Apparatus for harnessing tidal power, U.S. Patent No. 4,103,490 August (1978).