Estimating global hydrogen production from wind

international journal of hydrogen energy 34 (2009) 727–736

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Estimating global hydrogen production from wind Damon Honnery*, Patrick Moriarty Department of Mechanical and Aerospace Engineering, Monash University, Building 31, 3800, Australia

article info

abstract

Article history:

It is likely that intermittent renewable sources such as wind and solar will provide the

Received 12 September 2008

greatest opportunity for future large-scale hydrogen production. Here, on-shore wind is

Received in revised form

examined. Global wind energy is estimated by placing one 2 MW turbine/km2 over the

5 November 2008

surface of the earth. Wind energy production is based on monthly mean wind speed data.

Accepted 5 November 2008

Wind turbines are grouped to form arrays that are linked to local hydrogen generation and

Available online 11 December 2008

transmission networks. Hydrogen generation is done via low-pressure electrolysis and transmission via high-pressure gas pipelines. The wind/hydrogen system is considered

Keywords:

within a global energy system that must not only provide hydrogen, but also energy for

Electricity

electricity consumption at the local generation site. The technical potential of the

Electrolysis

hydrogen produced is estimated to be 116 EJ. Uneven distribution of the hydrogen-rich sites

Transmission

results in the need to export much of the hydrogen produced to energy-poor regions. To

Wind turbine

overcome system losses, a combined wind/HVDC/hydrogen system is considered. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

In previous papers [1,2] the authors argued that intermittent renewable energy offered the most likely source for the energy needed to produce hydrogen on a large scale. Base-load renewables such as hydro, biomass and geothermal were shown unable to provide sufficient energy to supply even existing demand for electricity. Both wind and solar on the other hand, while intermittent and unevenly distributed, are abundant and so more likely to be able to provide the scale of energy generation necessary for hydrogen production on a global scale. Estimates of the global technical potential of solar energy range from 15 to 4300 EJ, and for on-shore wind 3–600 EJ [3,4]. Differences arise from assumptions made when estimating the electrical energy able to be generated from solar and wind resources. Factors such as the type of energy conversion equipment (PV cells and wind turbines), the number used and where they are placed are variable and open to speculation.

The definition of the term technical potential often used to describe the estimate is also problematic. The Intergovernmental Panel on Climate Change (IPCC) in 2001 [5], for example, estimated the annual global theoretical terrestrial potential of wind as 1728 EJ from all land with mean annual wind speeds >5.1 m/s at 10 m above the ground. This amount was then reduced to give a technical potential of only 72 EJ based on the experience of the Netherlands and the USA. By placing four 1 MW wind turbines/km2 on land considered suitable, Hoogwijk et al. [6] estimated the technical potential of wind to be 346 EJ. Archer and Jacobson [7] placed a single 2 MW turbine/km2 on land with mean wind speeds over 6.9 m/ s and above at a height of 80 m and estimated the technical potential to be 230 EJ. More recently de Vries et al. [3] tightened the land suitability constraints of Hoogwijk et al. and estimated 155 EJ. For solar energy similar problems arise, although the largest sensitivity lies in the land area utilized. As a resource, wind energy offers a number of advantages over solar energy. Peak energy intensity for wind turbines is

* Corresponding author. E-mail address: [email protected] (D. Honnery). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.11.001

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international journal of hydrogen energy 34 (2009) 727–736

Nomenclature AT Ai D c Cf di E Ec,i Ee,i Eec,i EH,i EHT EHT,i Ei Elc,i Elc,i,j ET f k Llc mi n nc

turbine rotor swept area, m2 area of grid cell i, km2 turbine rotor diameter, m scale velocity, m/s capacity factor pipe diameter, m, of grid cell i energy, EJ annual wind energy of grid cell i, EJ annual energy available for export from grid cell i, EJ annual electrolyser compressor energy consumption of grid cell i, EJ annual hydrogen energy produced from electrolyser system of grid cell i, EJ total annual hydrogen energy supplied to city networks, EJ annual hydrogen energy supplied to city networks from grid cell i, EJ annual electricity consumption of grid cell i, EJ annual total line compressor energy consumption of grid cell i, EJ annual line compressor energy consumption of compressor j of grid cell i, EJ total annual wind energy, EJ pipe friction factor Weibull shape factor line compressor separation distance, m annual mass of hydrogen available for export from grid cell i, kg number of grid cells number of line compressors

currently around 400 W/m2, more than twice that for solar PV. Energy generation costs for solar PV systems are typically 6–18 times higher than for equivalent wind turbine systems [8]. Wind and solar energy are both unevenly distributed, with the greatest resources usually being located far from populated areas. While this has the advantage of providing the large land area both need, the cost of transmission of the generated energy can become high. Wind does have the advantage that it allows complementary use of the generating site: cropping can continue on a wind farm, but is unlikely on a solar PV farm. For these reasons wind has so far proved more successful than solar energy. In the 10 years from 1997 to 2007, installed wind power capacity grew from 7.6 to 94 GW [9]; in 2005 global wind energy generation reached 0.45 EJ [10]. By comparison, installed capacity of solar PV was only 5 GW in mid-2006, with total energy production only 1/50th that of wind. For comparison, total annual primary energy consumption in 2005 was around 496 EJ [1]. Despite wind’s current small scale, in the short to medium term it is likely that it offers the most promising source of intermittent energy for hydrogen generation. A 2005 review [11] assessed the range of technologies available for wind/ hydrogen systems and concluded that use of low-pressure water electrolysis was the most probable. Once produced,

nT P Pr pe p1 p2 pr R si si,vd T ty V VH Vi Vm,R Vm,z Vo Vr z zR zo g l ha hA hac hc hn hg hpms

number of turbines/km2 power, MW turbine rated power, MW electrolyser delivery pressure, bar line compressor delivery pressure, bar line compressor inlet pressure, bar line compressor pressure ratio p2/p1 gas constant, J/kg K grid cell suitability factor grid cell suitability factor for de Vries et al. case temperature, K or C seconds per year, 31.536E6 s/yr velocity, m/s hydrogen pipe gas flow velocity, m/s turbine cut-in wind speed, m/s mean wind speed, m/s, at zR, m mean wind speed, m/s, at z, m turbine cut-out wind speed, m/s turbine rated wind speed, m/s height, m velocity reference height, m roughness scale height, m ratio of specific heats turbine spacing turbine array efficiency turbine availability factor grid cell i HVAC network efficiency compressor efficiency grid cell local transmission network efficiency gas turbine efficiency electrolyser power matching system efficiency

low-pressure hydrogen could be used locally at the site or within a local transmission network, or sent at high pressure to populated areas via a centralised pipe network. After delivery to the central network, the hydrogen could be further compressed to provide for land transport needs, expanded for low-pressure delivery to homes and industry for heating and cooking, or sent to thermal power stations for electricity production. If produced in large enough quantities, storage is also a possibility [12]. Given the various estimates of technical wind energy potential, how much hydrogen could be produced from a global scale wind/hydrogen system? Use of electrolysers to convert electrical energy and water into hydrogen can be expected to reduce the net energy produced from the wind. Use of compressors and transmission systems further reduces net energy delivery. There is also the likelihood that largescale wind power generation will need to supply electrical energy for local consumption as well as produce hydrogen. Hydrogen production therefore needs to be considered within the context of a global energy system. In this paper we seek to estimate the technical potential of a global wind/hydrogen system. Global wind energy potential is first estimated using a methodology based on the work of de Vries et al. [3] and Hoogwijk et al. [6]. This provides wind

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energy potential globally on a 0.5  0.5 grid. At each grid point, a simple model of a hydrogen production plant is placed to estimate hydrogen production. Each local system is then connected to a centralised high-pressure transmission system that delivers hydrogen to a city network, defined as the nearest grid cell with a population density threshold of 300 persons/km2. Hydrogen energy technical potential is then estimated by calculating the total energy delivered to the city networks. A transmission energy model is used to estimate losses and electricity consumed within the network is accounted for. The paper concludes with a discussion on the sensitivity of the estimate to the assumptions used.

2.

Calculating wind turbine power

(1)

where AT (m) is the area swept by the blades or rotor, V (m/s) the wind speed, and r density (kg/m3) of the air. Constants a, b and c are related to the turbine power coefficient, and its mechanical and electrical efficiency. Practical considerations impose three wind speed limits on turbine operation. A certain wind speed is required before power can be generated; this is known as the cut-in wind speed, Vi. The upper wind speed limit is known as the cut-out speed, Vo; above this the turbine will not produce power. The third speed limit is the rated power wind speed, Vr. Once Vr is reached, the turbine will produce its rated power Pr until the cut-out speed is reached. Between the Vi and Vr, power output will vary according to Eq. (1). The power output from the turbine may at any time be expressed as a fraction of the turbine rated power by P ¼ Pr fp ðVÞ:

P : Pr

Cf ¼

h i h i exp  ðVi =cÞk  exp  ðVr =cÞk k

k

ðVr =cÞ ðVi =cÞ

i h  exp  ðVo =cÞk :

where Vm is the average wind velocity of the distribution and G is the gamma function. For a typical wind distribution shape factor (k ¼ 2) this yields Vm ¼ 0.886c m/s. Thus, if the average wind velocity is known, the capacity factor for that wind speed can be determined, and for a particular rated power output, the actual power or energy of the turbine is estimated. Fig. 1 shows the variation of capacity factor with average wind speed Vm for values (Vi ¼ 4 m/s, Vr ¼ 15 m/s and Vo ¼ 25 m/s), typical of a large-size wind turbine. It is evident that Cf peaks at the rated wind speed at a value of Cf ¼ 0.56 and that performance falls at a greater rate for decreasing speed rather than for increasing speed. Rated power outputs for the largest turbines are now well over 3 MW, with 2 MW being typical. The hub height of these turbines can reach over 70 m above the earth’s surface. This is well above the typical 10 m height at which average wind measurements are undertaken. To enable calculation of wind speeds at these heights, wind speed can be modified by a log law boundary layer profile [7], which incorporates a roughness factor based on the local surface roughness scale zo (m)

(2)

lnðz=zo Þ ; lnðzR =zo Þ

E : ty Pr

(3)

If Cf is calculated in this manner, the average power output from the turbine is given by Pa ¼ CfPr, where Cf represents the average value of the function fp(V) in Eq. (2). In the absence of actual turbine power output, annual energy output E can be calculated if the hourly wind speed variation at the turbine site is known. This situation is rare and may be overcome by assuming a frequency distribution for wind speed. The 2-parameter Weibull function is commonly used for this. Expressed as a probability density function, it takes the form

(5)

where Vm,R (m/s) is the average wind speed at the measured height zR (m), and Vm,z (m/s) is the average wind speed at height z (m).

When averaged over some specified period, this expression represents the capacity factor Cf of the turbine. The capacity factor is a function of both the turbine type and the wind speed. For operational turbines, it is normally expressed as the total annual electrical energy produced E (EJ, or J) relative to energy produced at the rated power for ty ¼ 31.536E6 s/yr Cf ¼

(4)

The relationship between the scale velocity and the shape factor is given by

Vm;z ¼ Vm;R

The function fp(V) accounts for operation of the turbine from Vi to Vo which can be expressed as a rated power fraction fp ðVÞ ¼

In this equation, c (m/s) is a scale velocity and k is a shape factor. For a turbine with a particular cut-in, rated and cut-out wind speed, Abed and El-Mallah [13] have shown that use of the Weibull distribution yields the following expression for turbine capacity factor:

Vm ¼ cðGð1 þ ð1=kÞÞÞ;

The relationship between wind speed and electrical power output from a wind turbine is normally expressed by a cubic polynomial   1 P ¼ rAT aV3 þ bV2 þ cV ; 2

"   #   k1 k k V V : exp  f ðVÞ ¼ c c c

Fig. 1 – Capacity factor based on a Weibull wind distribution (k [ 2) against mean wind speed for wind turbine in Table 2. Shown are distributions for a single turbine and one in an infinite array.

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international journal of hydrogen energy 34 (2009) 727–736

For power output greater than one turbine can provide, turbines can be located adjacent to one another in an array. If the spacing between the turbines is too small, the wake from surrounding turbines can reduce performance. Performance will fall not only as spacing between the turbines reduces, but as the size of the array increases [14]. This leads to an array efficiency factor ha, which can be applied to each turbine in the array   (6) E ¼ ty ha Cf Pr : In the limit of the array being ‘infinite’ (more than 100  100 turbines) the data of Gustavson [14] may be approximated by ha ¼ 0:9 expð  74lÞ; where l is defined as the ratio of turbine rotor swept area AT to the land area occupied by each turbine. The effect of array efficiency on capacity factor is shown in Fig. 1.

3.

Global wind energy potential

The method of estimating global wind energy potential is similar to the approach of Hoodwijk et al. [6] and de Vries et al. [3], although they used a linear approximation for capacity factor in place of the Weibull distribution used here. Average wind speed is available on a global scale from the Climate Research Unit (CRU) database [15]. This data contains average wind speeds (Vm,R) at 10 m height (zR) taken at on-shore monitoring stations located around the world averaged monthly for the years 1960–1990. Discrete monitoring data is presented as spatial data on a 0.5  0.5 grid. This grid forms the basis of the calculation. To enable calculation of wind speeds at the turbine hub height, Vm,z, in Eq. (5), a roughness scale based on the local surface roughness is determined. Surface roughness is based on data obtained from the USGS Earth Resources Observation System land use/land cover system [16] which is made up of 24 separate land surface categories [17]. Within the 24 categories there are 8 specific groupings: urban, cropland, grassland, forests, wetlands (and water), barren, tundra and snow, and within these various sub-categories. Each land use category is given a value of zo based on the data from the Engineering Sciences Data Unit (ESDU) [18]. See Table 1. Once Vm,z is known, the capacity factor (Eq. (4)) at each point on the CRU global 0.5  0.5 grid can be calculated. Wind energy is then determined by assigning a number of turbines with fixed rated power per square kilometre to the grid point. Annual global wind energy potential ET is then calculated by summing the energy output from each turbine Ei determined by Eq. (7) over the land surface ET ¼

n X i¼1

Ei ¼ ty Pr nT ha hA

n X 

 Cf;i Ai si :

(7)

Table 1 – Land use factors and roughness scales. Land use category Urban Dryland, cropland and pasture Irrigated and pasture cropland Mixed dryland/irrigated cropland and pasture Cropland/grassland mosaic Cropland/woodland mosaic Grassland Shrubland Mixed shrubland/grassland Savannah Deciduous broadleaf forest Deciduous needleleaf forest Evergreen broadleaf forest Evergreen needleleaf forest Mixed forest Water bodies Herbaceous wetlands Wooded wetland Barren or sparsely vegetated Herbaceous tundra Wooded tundra Mixed tundra Bare ground tundra Snow or ice

zo (m) Unconstrained Constrained si si/si,vd 1.0 0.1

1.0 1.0

0 1.0/0.6

0.1

1.0

0

0.1

1.0

0

0.1 0.2 0.2 0.25 0.3 0.15 0.5 0.5 0.5 0.5 0.3 0.005 0.15 0.35 0.15

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.0 1.0 1.0 1.0

1.0/0.6 1.0/0.6 1.0/0.2 1.0/0.2 1.0/0.2 1.0/0.2 0 0 0 0 0 0 0 0 1.0/0.1

0.2 0.3 0.25 0.05 0.005

1.0 1.0 1.0 1.0 1.0

1.0/0.1 1.0/0.1 1.0/0.1 1.0/0.1 0

between the turbines. Data on the turbine considered in this study, representative of current 2 MW turbines, appears in Table 2.

3.1.

Results

An initial estimate of global wind potential can be made by placing an infinite array of 2 MW turbines consisting of one turbine/km2 over the land surface of the earth. Such an array has a turbine spacing of l ¼ 0.005 and array efficiency of ha ¼ 62%. In this unconstrained case all land but Antarctica is considered suitable (si ¼ 1), Table 1. Fig. 2 shows the result of the calculation for the month of January. In this figure, and other figures, global wind energy potential is presented as a summation of the energy available from locations with wind speeds equal to and greater than a given mean wind speed at a turbine hub height 70 m. Total annual global wind energy for the unconstrained case is 516 EJ, only slightly larger than present global primary

Table 2 – Turbine parameters.

i¼1

Here, nT is the number of turbines per square kilometre, Cf,i the capacity factor at grid point i, hA an availability factor for turbine operation, and Ai the area of the 0.5  0.5 grid cell in square kilometres. Following Ref. [3,6], si represents a land use suitability factor for the grid point. This is a weighting factor that enables constraints to be placed on the grid cell. ha is the efficiency of the turbine array and this depends on the spacing

Rated power Pr Turbine diameter D Hub height z Cut-in wind speed Vi Rated wind speed Vr Cut-out wind speed Vo Infinite array spacing l Availability factor hA

2 MW 80 m 70 m 4 m/s 15 m/s 25 m/s 0.005 0.95

international journal of hydrogen energy 34 (2009) 727–736

Fig. 2 – Annual global wind energy potential (A) from locations with wind speeds equal to and greater than the mean wind speed at 70 m height for the unconstrained case of Table 1. Also shown are the corresponding distributions of the population (-) and land area (:), both normalised by their respective totals. Dashed line is 2004 global electricity consumption.

energy consumption of about 496 EJ [1]. This value, however, includes all mean wind speeds >0 m/s and is therefore unlikely to represent a realistic assessment of technical potential. Archer and Jacobson [7] used wind speeds greater than class 3 (Vz,m > 6.9 m/s) at the height of the turbine hub as being representative of technical potential of the wind resource and estimated it to be 230 EJ. Limiting the present calculation to these winds yields 260 EJ. Hoogwijk et al. [6] used a limit of 4 m/s at 10 m height, equivalent to 6 m/s at 70 m height for a surface roughness figure zo ¼ 0.2. This figure was based on average wind speed data at sites of working wind farms and for this reason it is used here. Limiting turbines to these sites where the mean wind speed is 6 m/s at 70 m yields a technical potential of 341 EJ, Fig. 2. This is almost six times 2004 global electricity consumption of 57.5 EJ [19], which could be satisfied by using Vz,m > 9.7 m/s (class 7) winds only. Data presented in Fig. 2 is for the month of January; little difference in this result is found for different months of the year. Also shown in Fig. 2 are the cumulative distributions of population and land area corresponding to the regions covered by the mean wind speed. Both are normalised by the global totals. For this calculation, global population was taken from the year 2000 Gridded Population of the World database [20]. An imbalance between wind energy, population and land is evident. Around 11% of the global population, living on around 20% of the total 131 million km2 of land surface, has direct accesses to 50% of the global wind potential. Regions in which high mean wind speeds are found are North America, southern South America, northern Europe, Russia and Australia.

3.2.

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suitability of land within each land use cover type, given in the si column of Table 1. All urban areas, forests, wetlands, snow and ice-covered land, irrigated pasture and croplands are excluded, with si ¼ 0. All remaining land is weighted si ¼ 1. Fig. 3 shows the result of applying the land use constraints of Table 1 to global wind potential. Total land area utilized for the constrained case is 92 million km2 which contains around 4.1 billion people. Global wind energy potential in this area is 359 EJ. Expressed as a technical potential, wind energy falls to 229 EJ, on a land area of 24.4 million km2, with around 0.71 billion people. Also shown in Fig. 3 is the result of applying a set of constraints similar to that of Ref. [3], si,vd. For this case, land available for wind turbines drops further to 7.2 million km2, containing 0.35 billion people. As in Ref. [3], four 1 MW wind turbines/km2 are used in place of one 2 MW turbine: rated power per unit swept area is held constant at 400 W/m2. Also, their array efficiency of ha ¼ 90% is used. Global wind energy potential for this case is only 131 EJ. Despite the differences in the methods, most notably the land cover definitions, this value compares favourably to their estimate of 155 EJ.

4.

Estimating hydrogen production potential

To obtain an estimate of the technical potential of hydrogen energy from wind, a simplified hydrogen production/transmission system is located at each grid cell, Fig. 4. It is based on the conceptual models suggested by Sherif et al. [11] for wind turbines and the more detailed model for hydrogen production from solar PV systems of Mason and Zweibel [21]. In both these studies, low-pressure electrolysis is the method favoured for hydrogen production and high-pressure gas pipe for transmission. The hydrogen system is assumed to operate as part of a global energy system in which wind power is also used to supply energy for electricity consumption within the locality of the wind farm. This enables a more realistic estimate of technical potential as it is unlikely that any future global wind energy system would supply energy for hydrogen alone.

Land constraints

Not all land is suitable for wind turbines. Hoogwijk et al. [6] and de Vries et al. [3] considered the availability of land for turbine use based on a set of suitability criteria. In a similar approach, we use a weighting factor to determine the

Fig. 3 – Annual global wind energy potential available from locations with wind speeds equal to and greater than the mean wind speed at 70 m for three sets of constraints (see Table 1): (A) unconstrained case, (-) constrained case and (:) de Vries et al. [3] constraints.

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consumption is taken from Ref. [20]. As well as accounting for annual grid cell electricity consumption, an additional term Eec,i is included to account for the energy required to drive the electrolyser compressor. Provided the grid cell annual wind energy Ei is greater than the combined energy consumption in the grid cell, including losses, energy Ee,i can be exported from the grid cell via the hydrogen production system   Ec;i þ Eec;i : (8) Ee;i ¼ Ei  hac hn The electrolysers used to produce the hydrogen are connected to the grid cell HVAC system. The efficiency of the electrolysis system is set to a uniform value of he for each grid cell. As the power source is intermittent, an additional efficiency of hpms is included to account for losses in the electrolyser power matching system [11]. Annual hydrogen energy produced by the electrolysers is given by EH;i ¼ hpms he Ee;i : Annual hydrogen gas mass leaving the grid cell is calculated via its higher heating value (HHV ¼ 143 MJ/kg) mi ¼

Fig. 4 – Modelled hydrogen production system. Included in the system is an alternative method of wind energy export via HVDC transmission.

Each 0.5  0.5 grid cell is considered to be an individual wind/hydrogen system and each wind turbine within the grid cell is connected to a local HVAC network that is unique to that cell. Transmission efficiency hac for the cell HVAC network is based on the work of Barberis Negra et al. [22]. See Table 3. The HVAC network is connected to a local transmission network which operates at an efficiency of hn. Energy is taken from the local network to supply electricity for consumption in the grid cell Ec,i. Annual electricity consumption is calculated using the 2004 annual per capita electricity consumption [19] for the country the cell resides in. Grid cell population required to calculate annual energy

Table 3 – Energy export system parameters. Electrical transmission HVAC Local transmission network Hydrogen production Power matching system Electrolyser Electrolyser compressor Line compressor

Gas turbine Pipe flow

Efficiency hac Efficiency hn

98% 95%

Efficiency hpms Efficiency he Delivery pressure pe Efficiency hc Delivery pressure p1 Efficiency hc Inlet pressure p2 ¼ prp1 Delivery pressure p1 Efficiency hg Inlet velocity VH

90% 75% 1 bar 80% 69 bar 80% pr ¼ 0.8 69 bar 40% 15 m/s

EH;i : HHV

Following Ref. [20], the hydrogen is then compressed from the electrolyser exit pressure of pe to the pipe inlet pressure of p1. The energy required to drive this compressor Eec,i (Eq. (8)) is taken from the local grid transmission network and is additional to the average annual electricity consumption for that cell. Annual compressor energy Eec,i is estimated via an isentropic formulation [23] with efficiency hc #  " g1 mi RT g p1 g Eec;i ¼ 1 : pe hc g  1 Hydrogen is exported from the grid cell by a high-pressure pipe. Pipe diameter di (m) is sized by establishing a fixed pipe entrance average gas velocity VH (m/s) for the average hydrogen mass flow from the grid cell 0:5  4mi RT : di ¼ pp1 VH ty Pipe diameter and hydrogen gas temperature T ¼ 20  C are held constant for the grid cell. Recompression to the fixed entrance pressure to account for pipe friction losses is accomplished by locating line compressors along the pipes at intervals Llc (m), determined by setting an allowable pressure drop ratio of pr ¼ p2/p1.     pp1 ty 2 d5i : Llc ¼ 1  p2r 4mi RTf A constant pipe friction factor of f ¼ 0.01 is used in this expression. The line compressors are driven by gas turbine systems powered by hydrogen taken directly from the pipe system [24]. Gas turbine primary to secondary energy conversion efficiency is given by hg. Total annual energy required for the line compressors Elc,i is given by summing the individual line compressor energy Elc,i,j for the grid cell pipe system for total number of compressors nc Elc;i ¼

nc X j¼1

Elc;i;j ¼

  h nc  iX  g1 Elc;i;j1 RT g mi;j  : pr g 1 HHV hc hg g  1 j¼1

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Fig. 5 – Annual hydrogen energy EHT supplied to the city network available from locations with wind speeds equal to and greater than the mean wind speed at 70 m height: (A) unconstrained case, (-) constrained case.

Use of the hydrogen to power the compressors will mean a slight reduction in hydrogen mass flow through the pipe. This has the effect of reducing the velocity of the gas but it will remain fully turbulent. An efficiency of hc is used for all compressors. The length of the pipe leaving the grid cell is determined by calculating the distance from the grid cell to the nearest city network defined as the nearest grid cell with a population density threshold of 300 persons/km2. Each grid cell is connected to a city network via a separate pipe. Should the consumption of the hydrogen for line compressor operation be greater than hydrogen mass flow for the grid cell, export from the cell is set to zero. Annual hydrogen energy entering the city network from the grid cell pipe system is given by EHT,i, and annual global hydrogen energy EHT is given by summing all grid cells EHT ¼

n X

EHT;i ¼

i¼1

4.1.

n X 

 EH;i  Elc;i :

(9)

i¼1

Results

Fig. 5 shows the results of applying the hydrogen production and transmission model to the wind energy produced. Total potential annual energy EHT supplied to the various global city

networks as hydrogen is equal to 254 EJ for the unconstrained case and 179 EJ for the constrained case. Technical potential is 172 EJ for the unconstrained case and 116 EJ for the constrained case. As was evident from Fig. 2, hydrogen production is unequally distributed. In the constrained case, 52% of the 179 EJ of hydrogen is produced by only five countries (Table 4), and apart from the USA, production in the most populous countries (Table 5), is very limited, with almost half the world’s population producing less than 4% of the total. The distribution becomes even more uneven when technical potential is considered, with 67% of the hydrogen energy produced by the top five generating countries, and less than 0.2% total in four of the five most populous countries. When compared to their 2004 total primary energy supply (TPES), only three of these five countries (Canada, Australia and Argentina) have the capacity to export hydrogen, with total exports equalling 17.4 EJ, equivalent to around 25% of the imbalance between the USA’s hydrogen technical potential and its annual TPES. Water consumed during hydrogen production for the constrained case is also shown in Table 4 for the top five producing countries. Global annual water consumption required for the technical potential is 8740 GL with 66% used in the top five countries. Water consumption in Australia, which is expected to be one of the driest of these countries, is slightly less than half the 1688 GL used in production and supply of electricity and gas in 2001 [25]. While this suggests there is sufficient water in the fossil fuel-dominated electricity and gas sector in Australia available for hydrogen production, linking the water to the wind resources would be necessary.

4.2.

Losses

Energy losses during production and transmission are presented in Fig. 6 for the constrained case. Of the energy available for production of hydrogen 148 EJ (45.1%) is lost. The greatest loss occurs in the electrolysis process, 101 EJ (30.8%). Compressor loss is greatest for the line compressors, 30 EJ (9.1%), with the electrolyser compressor the smallest, 17 EJ (5.2%). The fraction of energy loss is generally independent of mean wind speed. As electrolysis loss makes up the greatest loss, overall loss sensitivity in the calculation of EHT to variation in transmission pipe system parameters is relatively small. Line

Table 4 – Hydrogen production, electricity supply and consumption, TPES and water consumption for the top five countries for the constrained case. Country USA Russia Canada Australia Argentina Global total

Electricity 2004 Electricity 2004 TPESb Water Potential EHT Technical potential (EJ) (%) EHT (EJ) (%) supplya EcT (EJ) (%) consumptionb (EJ) (%) (EJ) (%) consumptiona (GL) (%) 26 (14) 21 (12) 19 (11) 17 (9.5) 10 (5.6) 179 (100)

23 (20) 19 (16) 18 (15) 10 (8.6) 8 (6.9) 116 (100)

a Data for supply of technical potential. b Data from Ref. [19].

4.20 (34) 1.20 (9.6) 0.49 (3.9) 0.12 (1.0) 0.04 (0.3) 12.5 (100)

14.0 (24) 2.9 (5.0) 2.0 (3.5) 0.8 (1.4) 0.3 (0.5) 57.5 (100)

99.0 (20) 27.0 (5.4) 11.0 (2.2) 4.9 (1.0) 2.7 (0.5) 496 (100)

1590 (18) 1500 (17) 1475 (17) 750 (8.6) 535 (6.1) 8740 (100)

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Table 5 – Hydrogen production in the five most populous countries for the constrained case. Country China India USA Indonesia Brazil Global total

Potential (EJ) (%) 2.9 0.3 25.6 0.5 2.8 179.0

(1.6) (0.16) (14) (0.28) (1.6) (100)

Technical potential (EJ) (%) 0.15 0.0 23.0 0.0 0.03 116.0

(0.13) (0.0) (20) (0.0) (0.03) (100)

compressor loss, for example, is strongly related to the initial gas flow velocity VH. Reducing VH from 15 m/s to 5 m/s reduces annual line compressor loss from 30 EJ to 17.4 EJ for the constrained case, while decreasing the allowable line compressor drop ratio pr from 0.8 to 0.5 increases annual line compressor loss from 30 EJ to 43 EJ. Dropping pipe pressure p1 from 69 bar to 34.5 bar reduces both line losses (from 30 EJ to 22 EJ) and electrolyser compressor losses (17–13 EJ). Overall, these variations yield typical deviations of around 10 EJ in the calculated 179 EJ for the constrained case.

4.3.

Electricity supply

As well as supplying hydrogen to the city network, the system also supplies energy to each grid cell for local electricity consumption. Energy supplied to all grid cell local transmission networks for annual electricity consumption EcT is 23 EJ for the constrained case and 12.5 EJ when limited to technical potential. Both are well below the global 2004 total of 57.5 EJ, Table 4. These relatively low levels result from a mismatch between population centres and locations of high wind energy in each country. Highly populated areas, which could be expected to have high annual levels of electricity consumption, are located far from regions of high wind energy.

If a wind/hydrogen system were to provide the bulk of electricity within a global renewable energy system, the difference between that available for consumption in the grid cells and the global total of 57.5 EJ would need to be extracted from the hydrogen network. For the 34.5 EJ required in the constrained case, a primary to secondary energy conversion efficiency of 50% (hydrogen to electricity via gas turbine/ Rankine cycle) would see 69 EJ extracted from the hydrogen network leaving 110 EJ of hydrogen available to the city networks. When limited to technical potential, only 26 EJ would remain available to all the city networks. These estimates of hydrogen energy after supply of global annual electricity consumption do not take into account any additional losses from energy transmission to energy-poor regions. These combined with the large hydrogen production losses result in a relatively poor energy outcome. One possible modification to the system to improve overall system efficiency is to extract the energy required for global electricity consumption from a centralised high-voltage electrical network rather than from the hydrogen network, as shown in Fig. 4. Although the cost of such a system will not be examined here, such a system has a number of advantages over hydrogen transmission. Several studies (e.g. [21,26,27]) have shown that losses in renewable HVDC networks amount to around 0.4% of energy input per 100 km transmission distance. Additional losses due to voltage converter stations amount to around 1.5%. Replacing the entire hydrogen production and transmission system in each grid cell by a HVDC line commuted converter system based on the transmission loss data for wind farms in Ref. [21] results in delivery for the constrained case of 311 EJ of electrical energy globally to the city networks. Technical potential of the supply is 209 EJ, significantly more than for the hydrogen system. Combining a HVDC transmission model for global electricity consumption with the hydrogen system of Fig. 4 would enable complete supply of global annual electricity consumption and make available delivery of up to 90 EJ of hydrogen globally to the city networks, assuming an average loss factor of 45.1%. This is much more than the 26 EJ able to be supplied via a hydrogen system alone.

4.4.

Fig. 6 – Annual energy available for hydrogen production and lost during hydrogen production from locations with wind speeds equal to and greater than the mean wind speed at 70 m height for the constrained case: (-) annual global energy available, (,) total hydrogen system energy loss, (:) electrolysis loss, (B) electrolysis compressor loss Eec and (C) line compressor loss Elc.

Storage and net supply

An assumption underlying the estimation of energy production needed to supply electricity consumption is that interconnection of each grid cell via the various transmission systems overcomes the problem of intermittent wind energy supply. This implies that the entire system is able to smooth out generation variability. But supply instability is inevitable and it is likely that storage of hydrogen will become necessary. There are several options for storage, but only one will be considered here. Leighty et al. [28] have suggested that the hydrogen pipe system itself could act as a storage network for hydrogen. During periods of little or no wind, hydrogen could be drawn from the pipe for use in combined cycle power generation systems. As operated here, global storage in the pipe system amounts to 0.23 EJ for the constrained case, with a technical potential of 0.17 EJ. This can be increased by reducing VH and increasing p1. For VH ¼ 2 m/s and p1 ¼ 100 bar,

international journal of hydrogen energy 34 (2009) 727–736

storage increases to 1.3 EJ. For comparison, global average daily electricity consumption is around 0.16 EJ. One advantage of reduced velocity is that it compensates for the higher line pressure, resulting in additional 24 EJ of hydrogen at the city network due to lower pipe friction losses. The disadvantage of pipe storage is that pipe diameter, and so also material energy input costs, increases with increased storage. Although not examined here, additional energy input to the system will result from supply of the wind turbines, compressors, electrolysers and pipes, and the supply of highpressure water to the electrolysis systems. Data available from the life cycle analyses of turbines [29,30] suggests that around 700 GJ energy input per year is required to operate each 2 MW wind turbine over a 20 year life. For mean wind speeds below 3 m/s, the turbine will produce insufficient energy to supply annual input. For the 24.4 million turbines located at sites with mean wind speed equal to and greater than the 6 m/s technical potential limit, this amounts to an annual input of 17 EJ. Accounting for this input would see net hydrogen energy supply to the city networks falling to around 106 EJ, assuming energy input was via the local transmission grid. Although not a large reduction in technical potential, it does illustrate the need for a detailed energy analysis of the global wind/hydrogen energy system to enable a full assessment of the net energy available. One compensating feature of the pipe storage system may come through use of high-pressure electrolysers [31]. Although only currently available in relatively small output sizes [28], these units may be able to provide hydrogen up to 100 bar, thus reducing the need for the electrolyser compressor.

5.

Conclusions

Use of wind as an energy-generating source has been examined at the global scale in an attempt to assess the potential for future hydrogen production. By locating a 2 MW wind turbine on each square kilometre of land, wind can supply 516 EJ annually, if there are no constraints. By constraining turbine placement to locations deemed likely to be suitable, annual wind energy potential falls to 359 EJ. Introducing a wind speed limit of 6 m/s at heights of 70 m for the location of wind turbines to better estimate the technical potential of the annual supply further reduces this to 229 EJ. Conversion of the available wind energy into hydrogen via a simple but representative hydrogen generation and transmission system enables production of hydrogen with a technical potential of 116 EJ. Energy-rich regions are however poorly distributed, with the USA, Russia, Canada, Australia and Argentina providing around 66% of the technical potential, while the highly populated countries China, India, Indonesia and Brazil together account for less than 0.2%. If the wind/hydrogen system is to provide much of the world’s energy, redistribution of energy to provide for annual electricity consumption in wind-poor countries dramatically reduces the amount of hydrogen available. It is possible that energy transmission via HVDC systems would be necessary for the system to supply global electricity consumption. Combining a HVDC system with the hydrogen system could deliver the 57.5 EJ needed for present electricity consumption

735

together with 90 EJ of hydrogen, assuming the entire system is able to smooth out generation variability. Storage of hydrogen in the pipes of the transmission system is possible but limited. Losses within the hydrogen system correspond to around 45% of the energy supplied. Although not extensively analysed, additional energy input from system components further reduces overall system efficiency. A full energy analysis of the system is required to enable a complete assessment of net hydrogen technical potential within a global renewable energy system.

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