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Wind power in Jamaica
So/at EnergyVol. 44, No. 6, PP. 355-365, 1990 Printed in the U.S.A.
0038-092X/90 $3.00+ .00 Copyright ~ 1990 PergamonPress I~C
WIND POWER IN JAMAICA A. A. CHEN,* A. R. DANIEL, S. T. DANIEL, and C. R. GRAYt Department of Physics, University of the West Indies, Kingston 7, *National MeteorologicalService, Kingston 10, Jamaica Abstract--Parameters to evaluate the potential for using wind energy to generate electricityin Jamaica were obtained. These include the average wind power scaled to a height of 20 m at existing weather stations and temporary anemometer sites, the variation in annual and monthly wind power, and the frequencydistribution of wind speed and wind energy available. Four small commercial turbines were assumed to be operating at some of the sites, and the estimated energy captured by them, the time they operated above their cut-in speed and their capacity factors were also determined. Diurnal variations of wind speed and prevailingwind directions are discussed and a map showing wind power at various sites was produced. Two stations with long-term averages, Manley and Morant Point, gave results which warranted further investigation. Results from some temporary stations are also encouraging. Mean wind speeds at two other sites in the Caribbean are given for comparison. A method for estimating the power exponent for scaling the wind speed from climatic data is described in Appendix 2.
I. INTRODUCTION
The island of Jamaica in the Caribbean Sea lacks the rich endowment of fossil fuel and the potential for large-scale hydroelectricity. For the past ten years the Government of Jamaica has been committed to investigating the development of all alternative energy resources, including solar and wind energy. This paper sets out the results of a resource study of wind energy undertaken jointly by the Physics Department, University of the West Indies and the National Meteorological Service on behalf of the Ministry of Mining and Energy, Government of Jamaica. The objective of the study was to reduce and analyse available wind data and thereby determine various wind parameters, which would be needed by the Ministry of Mining and Energy to evaluate the potential for using wind energy to generate electricity in Jamaica. Results of work done subsequently to this study are also listed. The parameters which were looked at included average and seasonal variation of wind power, distribution of wind speed, wind energy available and the fraction captured by hypothetical turbines.
teorological Service weather stations permanently situated at these locations, where readings were taken on the hour. However, except for the international airports, a significant number of hourly data were missing. Hourly data from Discovery Bay, Fairy Hill, Flagaman, Fullerswood, Hillside, Manchioneal, Passley Gardens, Pimento Hill, Rowlandsfield, Vinery and Yallahs were obtained from temporary stations, where in all cases readings were taken for less than one year. The equivalent number of years of data collected at each station are indicated by the second number in parenthesis in Fig. I. All readings were taken at approximately l0 m above the surface, except at Manley International Airport where readings were taken at 21.95 m from 1957 to 1961, and at Vinery (18 m). The physical characteristics of the sites are listed in Table I. After the completion of the study, the National Meteorological Service recorded wind speeds at 20 m at 5 sites, Folly Point, Galina, Hellshire, Munroe and Spur Tree, for a short period. These stations are also shown in Fig. 1.
3. M E T H O D OF ANALYSIS AND SCALING H E I G H T 2. DATA SOURCES
The data source consisted of all available wind data from the Meteorological Service at the time of analysis. The data tended to be mainly from the coastal region or elevated land near to the sea coast. The interior of the island is mountainous and there are a few exposed hilltops where one experiences strong winds at times. However, anemometer readings from these sites were not available at the time of analysis. The locations where anemometers were sited axe shown in Fig. 1. Data for Bodies, Crawford, Manley International Airport, Morant Point, Mason River and Sangster International Airport were obtained from the National Me-
* ISES member. 355
The method of analysis is fully described elsewhere[l]. The variations of the monthly and annual average wind power were obtained for each station, as well as the frequency and energy distribution of the wind speed. The expected distributions of the energy captured by assumed commercial wind energy conversion systems (WECS) were also obtained. Weibuil statistics were also used to obtain the average wind power and turbine power output[2]. The frequency distribution of hourly wind speed was obtained in the ranges commonly used by the National Meteorological Service (i.e., the metric equivalent of the following ranges in knots: 0-3, 4-6, 7-10, I 1-16, 17-21, 22-27, 28-33, 34-40, 41-47, 48-55, 56-63, >63 knots). The wind speeds were scaled to the height of 20 m, which is the turbine hub height, by the methods described
356
A.A. CHEN et ell
Discover Bay (200, 0.3) \ \ \
Sangster (145, 19.8) l
Mason River 124 1 2~ ' "'
Pimento Hill (200, 0.6)
~
~ 1
~" ~
--
(27
~ '
-
.......
_
/
..
-
Fullerswood (14,0,2~ .
.
.
I
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.
Snur T r .
I- "-'/
~
~
~,/
.
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....... /
Hillside (140, o.1)
,~
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Passley Gardens
Vinery / ~ (70, 0.3) ,G~ina, ~~, J ,~v, ~ (236, 0:1) ~
~
I
~
]
_
(70, 0.4) / Fairy Hill
/
(134,0.1) Manochioneal
/-
7
Yallahs (I00, 0.5)
Bodies (31.4.2)
/
/
Morant Point {193 1.7) . . . .
(55, 0.9)
Fig. 1. Location of wind assessment sites in Jamaica and average wind power at each site in W m-: (first number in brackets) at 20 m. The second number in brackets gives the equivalent number of years of data. Values for Folly Point, Galina, Hellshire. Munro, and Spur Tree are based on actual measurements at 20 m.
Table 1. De~fiption o f ~ a t i o n sitesand ~ a l i n g e x p o n e n t u ~ d Station
Elevation
(m)
Anemometer
p,Scaling
Height (m)
Exponent
Bodies Crawford Discovery Bay
30 30 near sea level
10 10 12
0.14 0.14 0.22
Fairy Hill
78
10
0.26
Flagaman
60
S.1
0.21
Folly Point
near sea level near sea
20
Fullerswood
10
0.14
level Galina
Hellshire
n e a r sea level near sea level
Description
Flat, tall grass Flat, tall grass Jutting coastline, s u r r o u n d e d by mangrove upwind, and b u i l d i n g downwind H i l l s i d e w i t h moderate slope, trees 5 10 a t a l l Sloping terrain. wind u p s l o p e , buildnearby mangrove and low grass
20
20
Hillside
30
9.1
0.14
Flat, cane grass
Manehoneal
6
S.1
0.14
Low g r a s s
Manley
near sea
0.I0
Flat, mowed grass
level
21.95 & 10
10
0.32
Morant
near sea
i0
0.I0
S u r r o u n d e d by t a l l trees Sand
Point Munro Passley Gardens
level 792 15
20 9.1
0.24
Pimento Hill
305
9.1
0.21
Mason R i v e r
R o w l a n d s f i e l d 305
9.1
0.14
Sangster
near sea level
10
0.I0
Spur T r e e Vinery
610 457
20 18
0.24
Yallahs
60
9.1
0.23
and s h r u b s
Elevated land near sea c o a s t , trees nearby Hill-top, trees thinly scattered
Sloping h i l l ,
short grass F l a t , mowed g r a s s Hill-top, high density of t r e e s Flat, tall r u s h e s
Wind power in Jamaica
357
Table 2. Turbine characteristics and estimated annual production and operating hours, of assumed turbines (100% availability) TURBINE A B C O
TURBINE
RATED RATED CUT-IN CUT-OUT ROTOR POWER (KW) SPEED (m/s) SPEED (m/s) SPEED (m/e) DIAMETER (m) 1.9 8.05 3.13 26,8~ 6.88 2.0 9.57 3.58 none 6.15 3.0 11.18 3.58 none 6.15 10.0 11.18 6.02 26,82 9.62
ENERGY PRODUCTION (KWHr)
(MANLEY 57 A B C D
61:)
(MANLEY 61 A B C D
83:)
CAPACITY FACTOR*
APPROX H O U R S OF LOW WIND
APPROX H O U R S OF OPERATION
APPROX H O U R S OF F U L L POWER
6090 3830 3860 13030
0.25 0.22 0.15 0.15
5000 5500 5500 5750
3760 3260 3260 3010
960 260 160 160
6730 6230 5030 16250
0.28 0.26 0.19 0.19
6600 5000 5000 5300
~160 3760 3760 3660
1060 660 360 360
(SANGSTER:) A B C D
~020 3690 6010 12910
0.2~ 0.20 0,15 0.15
5100 5500 5500 5800
3660 3260 3260 2960
760 360 160 160
(MORANT POINT:) A B C D
6220 5120 5670 18250
0.37 0,29 0.22 0.21
2600 3300 3300 6000
6160 5~60 56&0 6760
760 660 160 160
* Capacity factor = annual energy production/(8760 × rated power).
below. The wind power density for a given range was obtained from p = ½PU~ (W m-2)
(1)
where p is the air density (obtained from pressure and temperature readings) and u,, is mid-point of the wind speed range at 20 m. The energy distribution was obtained from the power and frequency distribution of the wind speed. The energy generated by the commercial turbines was calculated by using the power curves provided by the manufacturers and adjusting for differences in air density. The characteristics and power curves of the turbines are given in Table 2 and Appendix 1, respectively. In scaling wind speeds in the atmospheric boundary layer, the vertical distribution of wind speed can be represented by the power law[3] u = uh(z/h) p
(2)
where us is the average speed measured at height h, u is the average speed at any height z, and the exponent p is a constant. Hsu[4] found that the value o f p varied from 0.15 in extremely unstable conditions to 0.4 in extremely stable conditions at St. Croix in the Caribbean for a site with average roughness of 0.24. Thus the conventional value of ~[3] only gives an approximate value for p in extremely unstable conditions for a site with roughness equal to 0.24. Hsu[4] found the atmosphere in the daylight hours to be mainly ex-
tremely unstable in St. Croix. In Jamaica there is a large diurnal variation in the wind speed with the strongest winds in the daytime, and it can be assumed that most of the wind energy captured by the turbine occurs during the daytime. A detailed discussion of this assumption is given below. Since the sites at Bodles, Crawford, Fullerswood, Hillside, Manchioneal, and Rowlandsfield had roughness approximately equal to 0.24, and it was assumed that most of the energy of the wind would be captured during the daytime under unstable conditions, the conventional value of ~ was used for these sites. Sites with larger (smaller) roughness values will have larger (smaller) values ofp. For stations having larger roughness, i.e., all other stations, except Manley, Sangster, and Morant Point, the following eqn (2) was used to calculate p: p = [0.37 - 0.088 l n ( u h ) ] / [ l -- 0.088 l n ( h / l O ) ]
(3)
Because of availability of other meteorological data at Manley, the value o f p for that station was calculated by a method described in Appendix 2. Because of similarity of topography and environment, the same value (~) was used for Sangster International Airport. The value of ~ was also used for Morant Point since the site was located on sand and the value o f p would not be expected to be higher than that at Manley. No measurements were available to obtain a more accurate value. Fig. 4.10 of reference [2] suggests that an equation similar to eqn (3) can be used to calculate p for sites with very small roughness by replacing 0.37 by
358
A.A. CHENet al.
0.26 in the first term on the right ofeqn (3). The value of p for Morant Point calculated by making this adjustment was also approximately ~. The computed values o f p for all sites are listed in Table 1.
1965, 1972, 1973, 1977 to I983. The sharp increase from 1981 to 1983 is not attributable to change of anemometer location or exposure, nor to biased data, It is probably associated with the el-nifio event in 1982 to 1983, which was the most intense since 1957 to 1958 and possibly since 1925. During the 1982 to 1983 event severe droughts occurred at some locations in the tropics[5], including Jamaica. Drought conditions in Jamaica, which include low relative humidity, high temperatures, and intense solar radiation, have been shown to be associated with high wind[6]. The winds at Manley decreased in 1984 and 1985 from the abnormal levels in 1982 and 1983, as can be seen from Fig. 8. Data for 1969 to 1970 were unavailable at Manley. The fraction of time during which the hypothetical turbines generated useable energy, the annual value of that energy, the capture efficiency and the capacity factors of the turbines were also found. Table 2 lists the characteristics of the turbines, their annual energy production and capacity factor for the 3 sites based on the assumption of 100% availability and no unlisted inefficiencies. The hours of winds below the cut-in speed (low winds), the total hours of operation and the number of hours of operation above the rated speed are also listed. Turbine A, with the lowest cut-in speed, had the highest capacity factor, while turbines C and D, with the highest rated speed, had the lowest capacity factor. Turbine B, which had the same cut in speed as turbine C but a lower rated speed, had a higher capacity factor than turbine C. Figures 3 to 5 show the graphical representations of some of these results for turbine A. Figure 3 gives
4. RESULTS AND DISCUSSION
Figure 2 gives the variation of the annual average wind power at 6 stations, Manley, Sangster, Morant Point Lighthouse, Bodles, Crawford, and Mason River. The other stations with short-term averages are not included. The yearly variation in wind power is pronounced, but it should be kept in mind that the wind power varies as the cube of the wind speed and that the variation in average wind speed is much less pronounced. For example, the average wind speed at 10 m at Manley varied from 4.8 m s-t in 1981 to 5.7 m s-t in 1982 and 6.3 m s-~ in 1983, although the wind power varied from under 300 W m -2 to over 400 W m -2 in the period. The 1983 Manley data is also exaggerated because only the first eight months of the year were available at the time of analysis, so that the least windy months (October to December) were not included in the average. Clearly, Bodies, Crawford and Mason River are unsuitable sites for generating electricity by wind power. Of the three remaining sites Sangster is the least favourable. The average wind power at Morant Point Lighthouse was close to, or above, 200 W m - : between 1971 and 1977 but ends on a declining note. This could well have been a result of the degradation of the anemometer which was not well maintained, At Manley the wind power was close to, or above, 200 W m -2 in
450
400 350 I
E
300
L~ /:
250
o 0.. a z
200 150 100 50 0
I
I
58
ca
I
I
60
I
62
I
I
64.
1
I
66
I
1
68
I I I I
70
4/
I
72
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I
7
76
I
I
78
I
I
I
80
I
8
Y~R MAN
+
SAN
o
MOR
BOD
×
CRA
v
MAS
Fig. 2. Yearlyvariation of average wind power adjusted to 20 m at Manley, Sangster, Morant Point, Bodies, Crawford and Mason River. Point for Manley 1983 is based on 8 months only, not including the least windy months.
Wind power in Jamaica Monley
359
1961--83
14
13 12
I
11 t-
10v
9bJ'O
Zt.,-
hlO
87-
o2
(/.11-~v
6-
5-
0 "T
4-
d 7
3-
I
0 o
I
I
NO. H R S
I
t
4
I
8 +
I
12
WIND S P E E D WIND E N E R G Y
16
20
(re.s--l) o
TURBINE
ENERGY
Fig. 3. "Histograms" for annual number o f hours of wind in a given wind-speed range (12]), available energy passing through area swept out by turbine (+), energy generated by turbine (~) at 20 m. (The ranges are listed on p. 355.)
histograms for the number of hours of wind in a given wind speed range, the available energy passing through the area swept by turbine A and the estimated energy generated at Manley for the period 1961 to 1983. The data on which the distributions were based, are really Moront
data for histograms, i.e., values over a range of wind speeds. However for clarity the graphs are drawn as continuous distributions. The ranges are nonuniform since they were based on the nonuniform ranges used by the National Meteorological Service. It can be seen Point
1971--79
14 13
12-~
1 r
v
10
I
9 L~J'O ZC
8
7
02
6
~v 5 0 1"
d z
4 3 2
-A ~
0 0
NO. H R S
I
4
I
I
I
8
WIND S P E E D WIND E N E R G Y
1
I
1
12
[]
16
I
I
20
(re.s--l)
Fig. 4. Same as for Fig. 3.
o
TURBINE
ENERGY
360
A. A. CHEN el al. Songster
1962--82
14 1..'.3 12 11
iy
10
9
>,Z t-i,ID
~
7
~ 0 "1-
5 4
d z
3 2 1 0 0 n
NO.
4 HRS
8 +
WIND
12
WIND SPEED ENERGY
16
20
(rn.s--1) o
TURE}INE E N E R G Y
Fig. 5. Same as for Fig. 3.
that from 1961 to 1983 at Manley the wind was above the cut-in speed (3 m . s -r) approximately 50% of the time and the annual energy generated by the turbine was approximately 4730 KW hr. This represented a capture efficiency of 15% of the available energy. The corresponding histograms for Morant Point and Sangster are given in Figs. 4 and 5. At Morant Point Lighthouse the wind speed was above the cut-in speed approximately 70% of the time, and the turbine output was approximately 6220 KW hr per year, which was 20% of the energy available in the wind. At Sangster the wind blew above the cut-in speed about 40% of the time and the annual output of the turbine was approximately 4020 KW hr. This represented 17% of the available energy. From Figs. 3 to 5 and from the values quoted it can be seen that the wind power at Manley and Morant Point are comparable and about 20% higher than at Sangster. However there is more power in the wind at higher speeds at Manley than at Morant Point, and conversely at lower speeds there is more power in the wind at Morant Point. This accounts for the high capacity factor for turbine A at Morant Point. By a more efficacious choice ofturbine the power generated at Manley can be made comparable to that at Morant Point. There is a much higher percentage of calms or near calms at Manley and Sangster than at Morant Point. The corresponding results at the other sites are not listed because of the low wind power and/or the small amount of data available. However the estimated average wind power (W • m -2) at 20 m at all the sites are listed in Fig. l by the first number in brackets. Because the cube of the average is less than the average of the cube, the histogram method can lead to
an underestimation of the true average wind power, since the method effectively employs an average wind speed when it uses the mid-point of the range to determine the power in that range. In the range of wind speed which are of interest, 3.6-5.2, 5.7-8.2, 8.7-10.8 m . s -~ (7-10, 11-16, and 17-21 knots), this method can lead to a 10% maximum underestimation of the wind power. However, this effect could be counteracted by the probability that the wind speed distribution in a given range is not normal, but is skewed toward the lower end of the range. This effect would be particularly true at higher wind speed ranges, where use of the mid point as the average would overestimate the actual average of the range. Weibull statistics were also used to obtain the average wind power and turbine power[2]. If the Weibull distribution were a good fit for the wind distribution, it would have provided better estimates of average wind power. However, because of the large number of occurrences of calms in the data set (at night), the WeibuU distribution was not a good fit, except at Morant Point where the occurrences of calms were less frequent. The average wind power at 20 m at Morant Point was estimated to be 198 W m -2 by the histogram method and 177 W m -2 by the Weibull method, a difference of approximately 10%. The differences were over 20% at Manley and Sangster. Figure 6 shows the average monthly variation in wind power at Manley from 1961 to 1983, Morant Point Lighthouse from 1971 to 1979 and Sangster from 1962 to 1982. The average wind power at Manley peaked at over 300 W . m -2 in June and July, and was between 200 W - m -2 and 300 W . m -2 in February, March, May and August. For Morant Point Lighthouse the wind power peaked close to 300 W . m -z in January
Wind power in Jamaica
361
400
350
11
300 I
E v
250
tY bJ
0 r~ Z
200
i
,x ~
150
/
/
/x
/
/\
/
/xx
100
50
: I
d
I
I
A
[7-7}
MAN
F
f
1/ v V
/ / / /
~ o
/ /
V N," V /
<
M
I
I
d
d
/
/
/ / / / / /
/, / /
/
M
[
v
A
MONTH SAN
~
/ / / / I
l
I
S
0
N
MOR
Fig. 6. Average monthly variation in wind power at Manley (1961 to 1983) Morant Point (1971 to 1979) and Sangster (1962 to 1982) at 20 m.
and was between 200 W - m -~ and 300 W . m - : for January to April, N o v e m b e r and December. The average wind power did not exceed 200 W . m -2 in any month at Sangster, but it was close to that value in March, April, and July. Table 3 gives the estimated monthly energy production and the hours of operation for turbine A operating at the 3 sites on the assumption o f 100% availability and no unlisted inefficiencies. It can be seen that for the months in which the wind power exceeded or was close to 300 W m -2, the energy production was very high with a capacity factor of 0.43 or higher. For the months in which the wind power was between 200 W m -2 and 300 W m -~, the energy production was reasonable with of capacity factor of 0.28 or higher. Because the turbine was better matched
for Morant Point the turbine gave very high outputs even below a wind power of 300 W m -2.
Diurnal patterns The other parameters of interest are the diurnal variation and direction o f the wind. The average diurnal variation of the 10 m windspeed at Manley for the period 1957 to 1982 is plotted in Fig. 7. The wind speed peaks in the early afternoon due to the superposition o f t h e sea-breeze and the prevailing easterlies, and is lowest at nights. By the use of the power law it was determined that the average daytime wind speed (from 0600 to 1800) at 20 m was 5.6 m s -~ and the average nighttime windspeed (1800 to 0600) was 3.3 m s -t. There is, therefore, a large diurnal variation in
Table 3. Estimated monthly production and operating hours of turbine A (100% availability) STATN:
MONTH
3AN FEB
MAR APR MAY 3UN 3UL AUG SEP OCT NOV DEC
MANLEY61 -
83
SANGSTER
MORANT POINT
ENERGY ENERGY ENERGY PRODUCTION HOURS PRODUCTION HOURS PRODUCTION HOURS (KWHR) OPERATION (KWHR) OPERATION (KWHR) OPERATION 330
300
370
340
700
590
370 410 360
320 350 330
350 400
320 340 340
570 580 530
530 540 500
480 660 hi0 460 350 240 200 2bO
410 500 450 370 330 250 230 280
270 270 330 290 230 200 310 370
~50 540 520 380 370 330 510 660
490 570 560 460 410 370 520 590
400 310 320 410 320 230 200 310 400
Maximum production (31 days): 1414 kW hr (744 hrs).
362
A.A. CHENet al.
6 O ot-X v
_.1 n,"
5
4
O
E
v
3
ta kd !% 1/1
1
i
,
,
,
0100
i
i
,
,
,
I
i
t
,
0600
1100
+
TIME O F DAY WIND S P E E D
,
I
,
1
,
1 600 ~
,
|
,
,
2100
LOAD
Fig. 7. Diurnal variation of 10 m wind speed at Manley and of demand load on Jamaica Public Service Co. Ltd. (public utility) for the peak demand day in 1984.
the wind speed at Manley and most of the wind energy would be captured by the wind turbine in the daytime. At Sangster the daytime and nighttime averages at 20 m were 5.9 m s-t and 2.1 m s-t, respectively, for the period 1962 to 1982. A similar diurnal variation would be expected at the other stations. However, the variation would be expected to be less pronounced at Morant Point since it is at the most easterly point of Jamaica, and the local sea breeze would have less effect on the prevailing easterlies. There was no readily available data to test this hypothesis. Calm and near calm conditions occur mainly at night. Table 4 shows the percentage occurrence of calms for Manley, Sangster and Morant Point for various periods. Significant amount of calms occurred during all periods, but were
less pronounced at Manley during periods of stronger winds (May to July and 1980 to 83) and at Morant Point. Prevailing winds The superposition of the trade winds and the local sea breeze is also reflected in the directions of the wind. From data given by the Meteorological Service[7] it was estimated that the wind is primarily between E and SE at Manley (58% of the time), while it is largely from the East at Morant Point (84%) and between E and NE at Sangster (74%); however, at Manley, there is also a northerly wind between N and NW (28% of the time), which is strongest between November and January. The E and SE components at Manley are
Table 4. Percentage occurrences ofcalms or near calms (wind speed < 1.54 m-s -1) and prevailingwind direction for various periods at Manley, Morant Point, and Sangster. Anemometer heights are listed in Table I %Calm
P r e v a i l i n g Wind D i r . (~ o f time)
Manley 1957-61
35%
E t o SE (58%)
Manley 1961-83
30%
Manley 1980-83
20%
Manley O c t . - D e c . ,
1961-B3
39%
Manley May - 3 u n . ,
1961-83
21%
Morant P o i n t
1971-79
9%
Morant P o i n t D e c . - F e ~ . , 1971-79 Morant P o i n t A u g . - O c t . , Sangster
1962-82
1971-79
East
(84%)
5% 17% 41%
NE (74%)
Wind power in Jamaica strongest in June and July. The data[7] were for the period prior to 1973 but the directions given above would not be expected to change significantly for later periods. The results are also summarized in Table 4.
363
cember), the wind power for at least one ofthe stations is close to 300 W . m -2 or above. For another 6 months (February, March, April, May, August and November) the wind power is greater than 200 W • m -2 for at least one of the stations, and only in September and October are both stations below that value. Based on the above it was recommended firstly, that an analysis be carried out to determine if there is an economical advantage to operate large scale WECS or wind farms at two sites, in the vicinity of Manlcy and Morant Point Lighthouse, in a tandem fashion, with special consideration given to the fact that when the wind power is not excellent or reasonable at one site, it is excellent or reasonable at the other site. Secondly, it was recommended that a study be carried out to predict the approximate time at which the wind at Manley and/or Morant Point Lighthouse would reach the cut-in speed for a WECS, and to predict the duration and magnitude of the wind. Such a prediction would allow a load dispatcher at the public utility system to determine how much back-up could be expected from a WECS on a particular day. Further it was recommended that monitoring of wind power should continue at Manley and Morant Point Lighthouse, as well as at Discovery Bay, Flagaman and Pimento Hill, since the small amount of data at the last three sites were encouraging. From the later data provided by the National Meteorological Service, it appears that the sites at Galina, Hellshire, Munroe, and Spur Tree may be favourable for wind energy utilization. From the combined data three areas stand out as prospective sites for wind farms: (i) Pimento Hill--Galina; (ii) Manley-Hellshire; and (iii) FlagamanMunro-Spur Tree. No recommendation was made for Sangster since its annual average power did not exceed
5. CONCLUSIONS AND RECOMMENDATIONS Other than at Manley, Morant Point Lighthouse, and Sangster the wind speeds were too low and/or the data too sparse to deserve further comment. The wind speed at Manley and Morant Point Lighthouse peaked in the early afternoon. It is possible, therefore, that WECS erected at these sites could be used to backup the conventional generators during the peak demand at mid-day and early afternoon. Figure 7 gives the plot o f t h e d i u r n a l variation in load demand on the public utility for the day of maximum demand in 198418], as well as the annual diurnal variation in the wind speed at Manley. The diurnal variation ofload on other days are similar and the diagram shows that the center of the two peaks coincide during the daytime. The load demand has a broader peak than the wind speed. Nonetheless, the coincidence is such that when the peak demand is reached, the wind speed has reached a sufficient level on both the ascending and descending sides (approx. 6 m s-') to suggest that reasonable back-up power could be expected from a wind turbine during the peak hours (approx. 5 hours). The available wind power at Manley is excellent in the months of June and July, when there is an increased demand in electricity for airconditioning units. Morant Point Lighthouse on the other hand is a near excellent site in the months of December and January when conditions at Manley are not as favourable. Overall, for 4 months of the year (January, June, July and De-
18 17
16 15 14 13 12
E
11
10 IJ.I
n Ul ¢3 Z
9 8 7
6 5 4 3
2 1 0
"
"
"
•
57
I
"
'
62
'
"
I
'
'
'
67
'
I
'
72
"
'
'
I
'
'
'
'
77
!
'
'
'
*
82
YEAR +
MANL~(
&
ADAMS
x
BIRD
Fig. 8. Annual mean wind speed at Manley Intl. Airport (Jamaica), Adams Intl. Airport (Barbados) and Bird Intl. Airport (Antigua) at l0 m.
87
364
A . A . C H E N et al.
200 W m72 in any year. However, monitoring continues at Sangster since it is an international airport, and this site should always be looked at whenever there is a reassessment o f sites for utilizing wind energy. Favourable sites other than the ones listed above should exist. Munroe and Spur Tree are good examples of inland elevated sites where the effects of diurnal variation are not as pronounced as at coastal sites. However, the prospects for good elevated sites inland may be reduced by the weakened prevailing winds due to the blocking effect of the large land mass of Hispaniola, which is approximately 150 miles ENE o f Jamaica. Turbulence caused by the rugged inland terrain which exists in Jamaica may also tend to reduce the prospects for good elevated sites inland by dissipating the energy in the wind. In this respect Jamaica is unlike Barbados or other Eastern Caribbean islands which enjoy strong prevailing winds. Figure 8, which gives the annual mean wind speed at airport sites in Barbados, Antigua and Jamaica illustrates the difference.
2. 3. 4. 5. 6. 7. 8. 9. 10. I 1.
Acknowledgment--The authors thank the Ministry. of Mining. Energy and Tourism of the Government of Jamaica for supporting this project and the National Meteorological Service for its cooperation. The assistance of Mr. A. Archibald is especially acknowledged.
12.
13.
REFERENCES
14.
1. A. A. Chen, Techniques for assessment of wind power. In: Meteorological Data for Solar and HTnd Energy Applications. Commonwealth Science Council Publication
APPENDIX
Series 161, CSC(85)ENP-5, 70-84, Commonwealth Secretariat, London (1985). C. G. Justus, Winds and wind system performance, The Franklin Press, Philadelphia (1978). L. Sedefian, On the vertical extrapolation of mean wind power density, J. Appl. Meteorol. 19, 488--493 (1979). S. A. Hsu, Determination of the power law wind profile exponent on a tropical coast, J. Appl. Meteorol. 21, 11811190 (1982). World Climate Programme Newsletter, WMO Secretariat, Geneva, p. 3, No. 5 (Nov. 1983). A. R. Daniel, Simulation and forecasting of wind speed and wind power by stochastic models, M. Phil. thesis, University of the West Indies, Kingston 7, Jamaica (1987). The Climatic Branch of the Jamaican Meteorological Service, The Climate of Jamaica, Meteorological Service, Kingston (1973). K. C. Tomlinson, Jamaica Public Service, private communication (1989). The 1979 wind access catalog. In: Wind Power Digest (Winter 1978). W. Brutsaert, Evaporation into the atmosphere, D. Reidell Publishing Co., Dordrecht (1982). H. L. Penman, Natural evaporation from open water, bare soil and grass, Proc. Roy. Soc. London, A193, pp. 120-146 (1948). W. O. Pruitt, D. L. Morgan, and F. J. Lourence, Energy. momentum and mass transfers above vegetative surfaces. Tech. report ECOM-0447 (E)-F, Dept. Water Sci. Eng., Univ. Calif., Davis (1968). O.G. Sutton, Micrometeorology. In: Evaporation and its role in water resources management. McGraw-Hill, New York (1953). W. O. Pruitt, Emperical method of estimating evapotranspiration using primarily evaporation pans, in evapotranspiration and its role in water resources management, Am. Soc. Agric. Eng., St. Joseph, Mich., 57-61 (1966).
1
data. That process must be done if it is decided that the wind power potential in Jamaica should continue to be studied.
The power curves for the assumed turbines are shown in Fig. AI. The source of the data is given in ref. [9]. No attempt was made to properly match the wind turbine to the wind 11 10
D
9 8
,¢
7
t,
6
no oj c
5
4 3
C
2
B A
1
0
I
2.0
i
4.0
|
I
6.0
I
!
|
8.0 Wind S p e e d
|
10.0 (m/s)
Fig. A I. Power curves for turbines described in Table 2.
|
!
12.0
l
14.0
Wind power in Jamaica
365
APPENDIX 2
A M e t h o d used to d e t e r m i n e the p o w e r exponent at M a n l e y
In the atmospheric boundary layer, the vertical distribution o f wind speed u may be represented by the use o f boundary layer similarity theory[3] as ( k z / u * ) d u Idz = 4~( z / L )
In the original derivation[l I] G was taken to be negligible. Measurements taken over grass[12] indicate that G is small and often negligible in comparison to the other terms in eqn (7A). The Penman equation can be written as[10]
(IA) E = [A/(A + v)]R,./Le + [3'/(A + 3")]E.
(8A)
which on integration yields where u(z) = (u*/k)[ln(z]Zo) - ~b,,,,(z/L)]
(2A) A = d e * / d T , the slope of the graph of saturation va-
where 0 and ¢~,, are universal functions of height z, relative to the similarity scale L, which is the Monin Obukhov stability length, and
pour e* as a function of temperature T 3' = the psychrometric constant. Eo is a measure of the drying power o f the air and is calculated from[10]
• k = yon Karman's constant u* = friction velocity Zo = roughness length.
Eo = a~u*p(q** - q.)[ln(z./:o~) - e,~(z,,/L)] -I
Since the power law (eqn (2)), gives the variation of wind speed with height, the exponent can be expressed as p = 4~(~[L)/[In(Yr]Zo) - Cs,.(2"/L)]
where
(3A)
a~ = ratio of yon Karman's constant for water vapour and dry air q~* = saturated specific humidity at height z° qo = specific humidity at height z. z = 1.83 m, height o f temperature, vapour pressure and pressure measurement zo~ = water vapour roughness length ¢J~, = 2 In[( I + 0-2)/2], flux profile relationship for water vapour.
where ;~ = geometric mean height between the anemometer height and the height to which the measurement is scaled. The functional forms of ~ and Cs,. have been empirically determined in the atmospheric surface layer and depends on the state of stability of the atmosphere. From measurements made in the Caribbean region[4] the atmosphere in the daytime is unstable ( z / L < 0). For such conditions the values have been given[10] as 4:(z/L) = (I - 16z/L) -°'2~
(4A)
Zo. can be expressed as
¢,.,(zlL) = 2/n[(l + 0-~)/2] +/n[(l + 0-2)/2] -
(9A)
Zov = 7.4ZoeXp(-2.25Z°o ~:5)
(10A)
2 tan-l(4~-l) + 7r/2 (SA)
where The stability length L is traditionally defined[ I0] as L = - u ~ . p / [ k g { ( n / ( T q , ) ) + 0.61E}I
(6A)
where g = cp = T= H = E =
acceleration due to gravity specific heat capacity o f air at constant pressure air temperature heat flux into the air evapotranspiration.
L was determined by an energy budget method using the Penman semiempirical method[ 10,1 I] for estimating potential evapotranspiration. Thus, R. = H + E . L , + G where Rn = net incoming radiation L, = latent heat of vaporization G = heat flux to soil.
(7A)
Zo* = (U*Zo)b,, roughness Reynolds number
u = kinematic viscosity z = 2.3 cm, surface roughness length for thick grass[ 13] (environment in which measurements to evaluate E , were made).
The solution for the Obukov stability length L was obtained by the following iteration procedure. An initial guess was made o f L, and used to calculate ~s,. and ~ in eqns (5A) and (10A). ~s,. and ~ provided first values of u* and Eo by use o f e q n s (2A) and (9A), respectively. By using this value of Eo and a value o f E, estimated from Class A pan evaporation measurement, values o f R. and H were obtained from eqns (8A) and (7A), respectively. These initial values of H and u* provided a first value of L by means o f eqn (6A). This value of L allowed second estimates o f ~ and ~ ; whence the process can be repeated. The process is halted when successive values of L are sufficiently close. The value o f p can then be calculated from eqn (3A). In actual practice the potential evapotranspiration E was calculated by multiplying the average pan measurement by a pan coefficient o f 0.8[14]. The value of L was found to have an average value o f - 4 . 6 .
0038-092X/90 $3.00+ .00 Copyright ~ 1990 PergamonPress I~C
WIND POWER IN JAMAICA A. A. CHEN,* A. R. DANIEL, S. T. DANIEL, and C. R. GRAYt Department of Physics, University of the West Indies, Kingston 7, *National MeteorologicalService, Kingston 10, Jamaica Abstract--Parameters to evaluate the potential for using wind energy to generate electricityin Jamaica were obtained. These include the average wind power scaled to a height of 20 m at existing weather stations and temporary anemometer sites, the variation in annual and monthly wind power, and the frequencydistribution of wind speed and wind energy available. Four small commercial turbines were assumed to be operating at some of the sites, and the estimated energy captured by them, the time they operated above their cut-in speed and their capacity factors were also determined. Diurnal variations of wind speed and prevailingwind directions are discussed and a map showing wind power at various sites was produced. Two stations with long-term averages, Manley and Morant Point, gave results which warranted further investigation. Results from some temporary stations are also encouraging. Mean wind speeds at two other sites in the Caribbean are given for comparison. A method for estimating the power exponent for scaling the wind speed from climatic data is described in Appendix 2.
I. INTRODUCTION
The island of Jamaica in the Caribbean Sea lacks the rich endowment of fossil fuel and the potential for large-scale hydroelectricity. For the past ten years the Government of Jamaica has been committed to investigating the development of all alternative energy resources, including solar and wind energy. This paper sets out the results of a resource study of wind energy undertaken jointly by the Physics Department, University of the West Indies and the National Meteorological Service on behalf of the Ministry of Mining and Energy, Government of Jamaica. The objective of the study was to reduce and analyse available wind data and thereby determine various wind parameters, which would be needed by the Ministry of Mining and Energy to evaluate the potential for using wind energy to generate electricity in Jamaica. Results of work done subsequently to this study are also listed. The parameters which were looked at included average and seasonal variation of wind power, distribution of wind speed, wind energy available and the fraction captured by hypothetical turbines.
teorological Service weather stations permanently situated at these locations, where readings were taken on the hour. However, except for the international airports, a significant number of hourly data were missing. Hourly data from Discovery Bay, Fairy Hill, Flagaman, Fullerswood, Hillside, Manchioneal, Passley Gardens, Pimento Hill, Rowlandsfield, Vinery and Yallahs were obtained from temporary stations, where in all cases readings were taken for less than one year. The equivalent number of years of data collected at each station are indicated by the second number in parenthesis in Fig. I. All readings were taken at approximately l0 m above the surface, except at Manley International Airport where readings were taken at 21.95 m from 1957 to 1961, and at Vinery (18 m). The physical characteristics of the sites are listed in Table I. After the completion of the study, the National Meteorological Service recorded wind speeds at 20 m at 5 sites, Folly Point, Galina, Hellshire, Munroe and Spur Tree, for a short period. These stations are also shown in Fig. 1.
3. M E T H O D OF ANALYSIS AND SCALING H E I G H T 2. DATA SOURCES
The data source consisted of all available wind data from the Meteorological Service at the time of analysis. The data tended to be mainly from the coastal region or elevated land near to the sea coast. The interior of the island is mountainous and there are a few exposed hilltops where one experiences strong winds at times. However, anemometer readings from these sites were not available at the time of analysis. The locations where anemometers were sited axe shown in Fig. 1. Data for Bodies, Crawford, Manley International Airport, Morant Point, Mason River and Sangster International Airport were obtained from the National Me-
* ISES member. 355
The method of analysis is fully described elsewhere[l]. The variations of the monthly and annual average wind power were obtained for each station, as well as the frequency and energy distribution of the wind speed. The expected distributions of the energy captured by assumed commercial wind energy conversion systems (WECS) were also obtained. Weibuil statistics were also used to obtain the average wind power and turbine power output[2]. The frequency distribution of hourly wind speed was obtained in the ranges commonly used by the National Meteorological Service (i.e., the metric equivalent of the following ranges in knots: 0-3, 4-6, 7-10, I 1-16, 17-21, 22-27, 28-33, 34-40, 41-47, 48-55, 56-63, >63 knots). The wind speeds were scaled to the height of 20 m, which is the turbine hub height, by the methods described
356
A.A. CHEN et ell
Discover Bay (200, 0.3) \ \ \
Sangster (145, 19.8) l
Mason River 124 1 2~ ' "'
Pimento Hill (200, 0.6)
~
~ 1
~" ~
--
(27
~ '
-
.......
_
/
..
-
Fullerswood (14,0,2~ .
.
.
I
l
.
Snur T r .
I- "-'/
~
~
~,/
.
~
t~ y
....... /
Hillside (140, o.1)
,~
I
I
Passley Gardens
Vinery / ~ (70, 0.3) ,G~ina, ~~, J ,~v, ~ (236, 0:1) ~
~
I
~
]
_
(70, 0.4) / Fairy Hill
/
(134,0.1) Manochioneal
/-
7
Yallahs (I00, 0.5)
Bodies (31.4.2)
/
/
Morant Point {193 1.7) . . . .
(55, 0.9)
Fig. 1. Location of wind assessment sites in Jamaica and average wind power at each site in W m-: (first number in brackets) at 20 m. The second number in brackets gives the equivalent number of years of data. Values for Folly Point, Galina, Hellshire. Munro, and Spur Tree are based on actual measurements at 20 m.
Table 1. De~fiption o f ~ a t i o n sitesand ~ a l i n g e x p o n e n t u ~ d Station
Elevation
(m)
Anemometer
p,Scaling
Height (m)
Exponent
Bodies Crawford Discovery Bay
30 30 near sea level
10 10 12
0.14 0.14 0.22
Fairy Hill
78
10
0.26
Flagaman
60
S.1
0.21
Folly Point
near sea level near sea
20
Fullerswood
10
0.14
level Galina
Hellshire
n e a r sea level near sea level
Description
Flat, tall grass Flat, tall grass Jutting coastline, s u r r o u n d e d by mangrove upwind, and b u i l d i n g downwind H i l l s i d e w i t h moderate slope, trees 5 10 a t a l l Sloping terrain. wind u p s l o p e , buildnearby mangrove and low grass
20
20
Hillside
30
9.1
0.14
Flat, cane grass
Manehoneal
6
S.1
0.14
Low g r a s s
Manley
near sea
0.I0
Flat, mowed grass
level
21.95 & 10
10
0.32
Morant
near sea
i0
0.I0
S u r r o u n d e d by t a l l trees Sand
Point Munro Passley Gardens
level 792 15
20 9.1
0.24
Pimento Hill
305
9.1
0.21
Mason R i v e r
R o w l a n d s f i e l d 305
9.1
0.14
Sangster
near sea level
10
0.I0
Spur T r e e Vinery
610 457
20 18
0.24
Yallahs
60
9.1
0.23
and s h r u b s
Elevated land near sea c o a s t , trees nearby Hill-top, trees thinly scattered
Sloping h i l l ,
short grass F l a t , mowed g r a s s Hill-top, high density of t r e e s Flat, tall r u s h e s
Wind power in Jamaica
357
Table 2. Turbine characteristics and estimated annual production and operating hours, of assumed turbines (100% availability) TURBINE A B C O
TURBINE
RATED RATED CUT-IN CUT-OUT ROTOR POWER (KW) SPEED (m/s) SPEED (m/s) SPEED (m/e) DIAMETER (m) 1.9 8.05 3.13 26,8~ 6.88 2.0 9.57 3.58 none 6.15 3.0 11.18 3.58 none 6.15 10.0 11.18 6.02 26,82 9.62
ENERGY PRODUCTION (KWHr)
(MANLEY 57 A B C D
61:)
(MANLEY 61 A B C D
83:)
CAPACITY FACTOR*
APPROX H O U R S OF LOW WIND
APPROX H O U R S OF OPERATION
APPROX H O U R S OF F U L L POWER
6090 3830 3860 13030
0.25 0.22 0.15 0.15
5000 5500 5500 5750
3760 3260 3260 3010
960 260 160 160
6730 6230 5030 16250
0.28 0.26 0.19 0.19
6600 5000 5000 5300
~160 3760 3760 3660
1060 660 360 360
(SANGSTER:) A B C D
~020 3690 6010 12910
0.2~ 0.20 0,15 0.15
5100 5500 5500 5800
3660 3260 3260 2960
760 360 160 160
(MORANT POINT:) A B C D
6220 5120 5670 18250
0.37 0,29 0.22 0.21
2600 3300 3300 6000
6160 5~60 56&0 6760
760 660 160 160
* Capacity factor = annual energy production/(8760 × rated power).
below. The wind power density for a given range was obtained from p = ½PU~ (W m-2)
(1)
where p is the air density (obtained from pressure and temperature readings) and u,, is mid-point of the wind speed range at 20 m. The energy distribution was obtained from the power and frequency distribution of the wind speed. The energy generated by the commercial turbines was calculated by using the power curves provided by the manufacturers and adjusting for differences in air density. The characteristics and power curves of the turbines are given in Table 2 and Appendix 1, respectively. In scaling wind speeds in the atmospheric boundary layer, the vertical distribution of wind speed can be represented by the power law[3] u = uh(z/h) p
(2)
where us is the average speed measured at height h, u is the average speed at any height z, and the exponent p is a constant. Hsu[4] found that the value o f p varied from 0.15 in extremely unstable conditions to 0.4 in extremely stable conditions at St. Croix in the Caribbean for a site with average roughness of 0.24. Thus the conventional value of ~[3] only gives an approximate value for p in extremely unstable conditions for a site with roughness equal to 0.24. Hsu[4] found the atmosphere in the daylight hours to be mainly ex-
tremely unstable in St. Croix. In Jamaica there is a large diurnal variation in the wind speed with the strongest winds in the daytime, and it can be assumed that most of the wind energy captured by the turbine occurs during the daytime. A detailed discussion of this assumption is given below. Since the sites at Bodles, Crawford, Fullerswood, Hillside, Manchioneal, and Rowlandsfield had roughness approximately equal to 0.24, and it was assumed that most of the energy of the wind would be captured during the daytime under unstable conditions, the conventional value of ~ was used for these sites. Sites with larger (smaller) roughness values will have larger (smaller) values ofp. For stations having larger roughness, i.e., all other stations, except Manley, Sangster, and Morant Point, the following eqn (2) was used to calculate p: p = [0.37 - 0.088 l n ( u h ) ] / [ l -- 0.088 l n ( h / l O ) ]
(3)
Because of availability of other meteorological data at Manley, the value o f p for that station was calculated by a method described in Appendix 2. Because of similarity of topography and environment, the same value (~) was used for Sangster International Airport. The value of ~ was also used for Morant Point since the site was located on sand and the value o f p would not be expected to be higher than that at Manley. No measurements were available to obtain a more accurate value. Fig. 4.10 of reference [2] suggests that an equation similar to eqn (3) can be used to calculate p for sites with very small roughness by replacing 0.37 by
358
A.A. CHENet al.
0.26 in the first term on the right ofeqn (3). The value of p for Morant Point calculated by making this adjustment was also approximately ~. The computed values o f p for all sites are listed in Table 1.
1965, 1972, 1973, 1977 to I983. The sharp increase from 1981 to 1983 is not attributable to change of anemometer location or exposure, nor to biased data, It is probably associated with the el-nifio event in 1982 to 1983, which was the most intense since 1957 to 1958 and possibly since 1925. During the 1982 to 1983 event severe droughts occurred at some locations in the tropics[5], including Jamaica. Drought conditions in Jamaica, which include low relative humidity, high temperatures, and intense solar radiation, have been shown to be associated with high wind[6]. The winds at Manley decreased in 1984 and 1985 from the abnormal levels in 1982 and 1983, as can be seen from Fig. 8. Data for 1969 to 1970 were unavailable at Manley. The fraction of time during which the hypothetical turbines generated useable energy, the annual value of that energy, the capture efficiency and the capacity factors of the turbines were also found. Table 2 lists the characteristics of the turbines, their annual energy production and capacity factor for the 3 sites based on the assumption of 100% availability and no unlisted inefficiencies. The hours of winds below the cut-in speed (low winds), the total hours of operation and the number of hours of operation above the rated speed are also listed. Turbine A, with the lowest cut-in speed, had the highest capacity factor, while turbines C and D, with the highest rated speed, had the lowest capacity factor. Turbine B, which had the same cut in speed as turbine C but a lower rated speed, had a higher capacity factor than turbine C. Figures 3 to 5 show the graphical representations of some of these results for turbine A. Figure 3 gives
4. RESULTS AND DISCUSSION
Figure 2 gives the variation of the annual average wind power at 6 stations, Manley, Sangster, Morant Point Lighthouse, Bodles, Crawford, and Mason River. The other stations with short-term averages are not included. The yearly variation in wind power is pronounced, but it should be kept in mind that the wind power varies as the cube of the wind speed and that the variation in average wind speed is much less pronounced. For example, the average wind speed at 10 m at Manley varied from 4.8 m s-t in 1981 to 5.7 m s-t in 1982 and 6.3 m s-~ in 1983, although the wind power varied from under 300 W m -2 to over 400 W m -2 in the period. The 1983 Manley data is also exaggerated because only the first eight months of the year were available at the time of analysis, so that the least windy months (October to December) were not included in the average. Clearly, Bodies, Crawford and Mason River are unsuitable sites for generating electricity by wind power. Of the three remaining sites Sangster is the least favourable. The average wind power at Morant Point Lighthouse was close to, or above, 200 W m - : between 1971 and 1977 but ends on a declining note. This could well have been a result of the degradation of the anemometer which was not well maintained, At Manley the wind power was close to, or above, 200 W m -2 in
450
400 350 I
E
300
L~ /:
250
o 0.. a z
200 150 100 50 0
I
I
58
ca
I
I
60
I
62
I
I
64.
1
I
66
I
1
68
I I I I
70
4/
I
72
I
I
7
76
I
I
78
I
I
I
80
I
8
Y~R MAN
+
SAN
o
MOR
BOD
×
CRA
v
MAS
Fig. 2. Yearlyvariation of average wind power adjusted to 20 m at Manley, Sangster, Morant Point, Bodies, Crawford and Mason River. Point for Manley 1983 is based on 8 months only, not including the least windy months.
Wind power in Jamaica Monley
359
1961--83
14
13 12
I
11 t-
10v
9bJ'O
Zt.,-
hlO
87-
o2
(/.11-~v
6-
5-
0 "T
4-
d 7
3-
I
0 o
I
I
NO. H R S
I
t
4
I
8 +
I
12
WIND S P E E D WIND E N E R G Y
16
20
(re.s--l) o
TURBINE
ENERGY
Fig. 3. "Histograms" for annual number o f hours of wind in a given wind-speed range (12]), available energy passing through area swept out by turbine (+), energy generated by turbine (~) at 20 m. (The ranges are listed on p. 355.)
histograms for the number of hours of wind in a given wind speed range, the available energy passing through the area swept by turbine A and the estimated energy generated at Manley for the period 1961 to 1983. The data on which the distributions were based, are really Moront
data for histograms, i.e., values over a range of wind speeds. However for clarity the graphs are drawn as continuous distributions. The ranges are nonuniform since they were based on the nonuniform ranges used by the National Meteorological Service. It can be seen Point
1971--79
14 13
12-~
1 r
v
10
I
9 L~J'O ZC
8
7
02
6
~v 5 0 1"
d z
4 3 2
-A ~
0 0
NO. H R S
I
4
I
I
I
8
WIND S P E E D WIND E N E R G Y
1
I
1
12
[]
16
I
I
20
(re.s--l)
Fig. 4. Same as for Fig. 3.
o
TURBINE
ENERGY
360
A. A. CHEN el al. Songster
1962--82
14 1..'.3 12 11
iy
10
9
>,Z t-i,ID
~
7
~ 0 "1-
5 4
d z
3 2 1 0 0 n
NO.
4 HRS
8 +
WIND
12
WIND SPEED ENERGY
16
20
(rn.s--1) o
TURE}INE E N E R G Y
Fig. 5. Same as for Fig. 3.
that from 1961 to 1983 at Manley the wind was above the cut-in speed (3 m . s -r) approximately 50% of the time and the annual energy generated by the turbine was approximately 4730 KW hr. This represented a capture efficiency of 15% of the available energy. The corresponding histograms for Morant Point and Sangster are given in Figs. 4 and 5. At Morant Point Lighthouse the wind speed was above the cut-in speed approximately 70% of the time, and the turbine output was approximately 6220 KW hr per year, which was 20% of the energy available in the wind. At Sangster the wind blew above the cut-in speed about 40% of the time and the annual output of the turbine was approximately 4020 KW hr. This represented 17% of the available energy. From Figs. 3 to 5 and from the values quoted it can be seen that the wind power at Manley and Morant Point are comparable and about 20% higher than at Sangster. However there is more power in the wind at higher speeds at Manley than at Morant Point, and conversely at lower speeds there is more power in the wind at Morant Point. This accounts for the high capacity factor for turbine A at Morant Point. By a more efficacious choice ofturbine the power generated at Manley can be made comparable to that at Morant Point. There is a much higher percentage of calms or near calms at Manley and Sangster than at Morant Point. The corresponding results at the other sites are not listed because of the low wind power and/or the small amount of data available. However the estimated average wind power (W • m -2) at 20 m at all the sites are listed in Fig. l by the first number in brackets. Because the cube of the average is less than the average of the cube, the histogram method can lead to
an underestimation of the true average wind power, since the method effectively employs an average wind speed when it uses the mid-point of the range to determine the power in that range. In the range of wind speed which are of interest, 3.6-5.2, 5.7-8.2, 8.7-10.8 m . s -~ (7-10, 11-16, and 17-21 knots), this method can lead to a 10% maximum underestimation of the wind power. However, this effect could be counteracted by the probability that the wind speed distribution in a given range is not normal, but is skewed toward the lower end of the range. This effect would be particularly true at higher wind speed ranges, where use of the mid point as the average would overestimate the actual average of the range. Weibull statistics were also used to obtain the average wind power and turbine power[2]. If the Weibull distribution were a good fit for the wind distribution, it would have provided better estimates of average wind power. However, because of the large number of occurrences of calms in the data set (at night), the WeibuU distribution was not a good fit, except at Morant Point where the occurrences of calms were less frequent. The average wind power at 20 m at Morant Point was estimated to be 198 W m -2 by the histogram method and 177 W m -2 by the Weibull method, a difference of approximately 10%. The differences were over 20% at Manley and Sangster. Figure 6 shows the average monthly variation in wind power at Manley from 1961 to 1983, Morant Point Lighthouse from 1971 to 1979 and Sangster from 1962 to 1982. The average wind power at Manley peaked at over 300 W . m -2 in June and July, and was between 200 W - m -2 and 300 W . m -2 in February, March, May and August. For Morant Point Lighthouse the wind power peaked close to 300 W . m -z in January
Wind power in Jamaica
361
400
350
11
300 I
E v
250
tY bJ
0 r~ Z
200
i
,x ~
150
/
/
/x
/
/\
/
/xx
100
50
: I
d
I
I
A
[7-7}
MAN
F
f
1/ v V
/ / / /
~ o
/ /
V N," V /
<
M
I
I
d
d
/
/
/ / / / / /
/, / /
/
M
[
v
A
MONTH SAN
~
/ / / / I
l
I
S
0
N
MOR
Fig. 6. Average monthly variation in wind power at Manley (1961 to 1983) Morant Point (1971 to 1979) and Sangster (1962 to 1982) at 20 m.
and was between 200 W - m -~ and 300 W . m - : for January to April, N o v e m b e r and December. The average wind power did not exceed 200 W . m -2 in any month at Sangster, but it was close to that value in March, April, and July. Table 3 gives the estimated monthly energy production and the hours of operation for turbine A operating at the 3 sites on the assumption o f 100% availability and no unlisted inefficiencies. It can be seen that for the months in which the wind power exceeded or was close to 300 W m -2, the energy production was very high with a capacity factor of 0.43 or higher. For the months in which the wind power was between 200 W m -2 and 300 W m -~, the energy production was reasonable with of capacity factor of 0.28 or higher. Because the turbine was better matched
for Morant Point the turbine gave very high outputs even below a wind power of 300 W m -2.
Diurnal patterns The other parameters of interest are the diurnal variation and direction o f the wind. The average diurnal variation of the 10 m windspeed at Manley for the period 1957 to 1982 is plotted in Fig. 7. The wind speed peaks in the early afternoon due to the superposition o f t h e sea-breeze and the prevailing easterlies, and is lowest at nights. By the use of the power law it was determined that the average daytime wind speed (from 0600 to 1800) at 20 m was 5.6 m s -~ and the average nighttime windspeed (1800 to 0600) was 3.3 m s -t. There is, therefore, a large diurnal variation in
Table 3. Estimated monthly production and operating hours of turbine A (100% availability) STATN:
MONTH
3AN FEB
MAR APR MAY 3UN 3UL AUG SEP OCT NOV DEC
MANLEY61 -
83
SANGSTER
MORANT POINT
ENERGY ENERGY ENERGY PRODUCTION HOURS PRODUCTION HOURS PRODUCTION HOURS (KWHR) OPERATION (KWHR) OPERATION (KWHR) OPERATION 330
300
370
340
700
590
370 410 360
320 350 330
350 400
320 340 340
570 580 530
530 540 500
480 660 hi0 460 350 240 200 2bO
410 500 450 370 330 250 230 280
270 270 330 290 230 200 310 370
~50 540 520 380 370 330 510 660
490 570 560 460 410 370 520 590
400 310 320 410 320 230 200 310 400
Maximum production (31 days): 1414 kW hr (744 hrs).
362
A.A. CHENet al.
6 O ot-X v
_.1 n,"
5
4
O
E
v
3
ta kd !% 1/1
1
i
,
,
,
0100
i
i
,
,
,
I
i
t
,
0600
1100
+
TIME O F DAY WIND S P E E D
,
I
,
1
,
1 600 ~
,
|
,
,
2100
LOAD
Fig. 7. Diurnal variation of 10 m wind speed at Manley and of demand load on Jamaica Public Service Co. Ltd. (public utility) for the peak demand day in 1984.
the wind speed at Manley and most of the wind energy would be captured by the wind turbine in the daytime. At Sangster the daytime and nighttime averages at 20 m were 5.9 m s-t and 2.1 m s-t, respectively, for the period 1962 to 1982. A similar diurnal variation would be expected at the other stations. However, the variation would be expected to be less pronounced at Morant Point since it is at the most easterly point of Jamaica, and the local sea breeze would have less effect on the prevailing easterlies. There was no readily available data to test this hypothesis. Calm and near calm conditions occur mainly at night. Table 4 shows the percentage occurrence of calms for Manley, Sangster and Morant Point for various periods. Significant amount of calms occurred during all periods, but were
less pronounced at Manley during periods of stronger winds (May to July and 1980 to 83) and at Morant Point. Prevailing winds The superposition of the trade winds and the local sea breeze is also reflected in the directions of the wind. From data given by the Meteorological Service[7] it was estimated that the wind is primarily between E and SE at Manley (58% of the time), while it is largely from the East at Morant Point (84%) and between E and NE at Sangster (74%); however, at Manley, there is also a northerly wind between N and NW (28% of the time), which is strongest between November and January. The E and SE components at Manley are
Table 4. Percentage occurrences ofcalms or near calms (wind speed < 1.54 m-s -1) and prevailingwind direction for various periods at Manley, Morant Point, and Sangster. Anemometer heights are listed in Table I %Calm
P r e v a i l i n g Wind D i r . (~ o f time)
Manley 1957-61
35%
E t o SE (58%)
Manley 1961-83
30%
Manley 1980-83
20%
Manley O c t . - D e c . ,
1961-B3
39%
Manley May - 3 u n . ,
1961-83
21%
Morant P o i n t
1971-79
9%
Morant P o i n t D e c . - F e ~ . , 1971-79 Morant P o i n t A u g . - O c t . , Sangster
1962-82
1971-79
East
(84%)
5% 17% 41%
NE (74%)
Wind power in Jamaica strongest in June and July. The data[7] were for the period prior to 1973 but the directions given above would not be expected to change significantly for later periods. The results are also summarized in Table 4.
363
cember), the wind power for at least one ofthe stations is close to 300 W . m -2 or above. For another 6 months (February, March, April, May, August and November) the wind power is greater than 200 W • m -2 for at least one of the stations, and only in September and October are both stations below that value. Based on the above it was recommended firstly, that an analysis be carried out to determine if there is an economical advantage to operate large scale WECS or wind farms at two sites, in the vicinity of Manlcy and Morant Point Lighthouse, in a tandem fashion, with special consideration given to the fact that when the wind power is not excellent or reasonable at one site, it is excellent or reasonable at the other site. Secondly, it was recommended that a study be carried out to predict the approximate time at which the wind at Manley and/or Morant Point Lighthouse would reach the cut-in speed for a WECS, and to predict the duration and magnitude of the wind. Such a prediction would allow a load dispatcher at the public utility system to determine how much back-up could be expected from a WECS on a particular day. Further it was recommended that monitoring of wind power should continue at Manley and Morant Point Lighthouse, as well as at Discovery Bay, Flagaman and Pimento Hill, since the small amount of data at the last three sites were encouraging. From the later data provided by the National Meteorological Service, it appears that the sites at Galina, Hellshire, Munroe, and Spur Tree may be favourable for wind energy utilization. From the combined data three areas stand out as prospective sites for wind farms: (i) Pimento Hill--Galina; (ii) Manley-Hellshire; and (iii) FlagamanMunro-Spur Tree. No recommendation was made for Sangster since its annual average power did not exceed
5. CONCLUSIONS AND RECOMMENDATIONS Other than at Manley, Morant Point Lighthouse, and Sangster the wind speeds were too low and/or the data too sparse to deserve further comment. The wind speed at Manley and Morant Point Lighthouse peaked in the early afternoon. It is possible, therefore, that WECS erected at these sites could be used to backup the conventional generators during the peak demand at mid-day and early afternoon. Figure 7 gives the plot o f t h e d i u r n a l variation in load demand on the public utility for the day of maximum demand in 198418], as well as the annual diurnal variation in the wind speed at Manley. The diurnal variation ofload on other days are similar and the diagram shows that the center of the two peaks coincide during the daytime. The load demand has a broader peak than the wind speed. Nonetheless, the coincidence is such that when the peak demand is reached, the wind speed has reached a sufficient level on both the ascending and descending sides (approx. 6 m s-') to suggest that reasonable back-up power could be expected from a wind turbine during the peak hours (approx. 5 hours). The available wind power at Manley is excellent in the months of June and July, when there is an increased demand in electricity for airconditioning units. Morant Point Lighthouse on the other hand is a near excellent site in the months of December and January when conditions at Manley are not as favourable. Overall, for 4 months of the year (January, June, July and De-
18 17
16 15 14 13 12
E
11
10 IJ.I
n Ul ¢3 Z
9 8 7
6 5 4 3
2 1 0
"
"
"
•
57
I
"
'
62
'
"
I
'
'
'
67
'
I
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72
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'
'
I
'
'
'
'
77
!
'
'
'
*
82
YEAR +
MANL~(
&
ADAMS
x
BIRD
Fig. 8. Annual mean wind speed at Manley Intl. Airport (Jamaica), Adams Intl. Airport (Barbados) and Bird Intl. Airport (Antigua) at l0 m.
87
364
A . A . C H E N et al.
200 W m72 in any year. However, monitoring continues at Sangster since it is an international airport, and this site should always be looked at whenever there is a reassessment o f sites for utilizing wind energy. Favourable sites other than the ones listed above should exist. Munroe and Spur Tree are good examples of inland elevated sites where the effects of diurnal variation are not as pronounced as at coastal sites. However, the prospects for good elevated sites inland may be reduced by the weakened prevailing winds due to the blocking effect of the large land mass of Hispaniola, which is approximately 150 miles ENE o f Jamaica. Turbulence caused by the rugged inland terrain which exists in Jamaica may also tend to reduce the prospects for good elevated sites inland by dissipating the energy in the wind. In this respect Jamaica is unlike Barbados or other Eastern Caribbean islands which enjoy strong prevailing winds. Figure 8, which gives the annual mean wind speed at airport sites in Barbados, Antigua and Jamaica illustrates the difference.
2. 3. 4. 5. 6. 7. 8. 9. 10. I 1.
Acknowledgment--The authors thank the Ministry. of Mining. Energy and Tourism of the Government of Jamaica for supporting this project and the National Meteorological Service for its cooperation. The assistance of Mr. A. Archibald is especially acknowledged.
12.
13.
REFERENCES
14.
1. A. A. Chen, Techniques for assessment of wind power. In: Meteorological Data for Solar and HTnd Energy Applications. Commonwealth Science Council Publication
APPENDIX
Series 161, CSC(85)ENP-5, 70-84, Commonwealth Secretariat, London (1985). C. G. Justus, Winds and wind system performance, The Franklin Press, Philadelphia (1978). L. Sedefian, On the vertical extrapolation of mean wind power density, J. Appl. Meteorol. 19, 488--493 (1979). S. A. Hsu, Determination of the power law wind profile exponent on a tropical coast, J. Appl. Meteorol. 21, 11811190 (1982). World Climate Programme Newsletter, WMO Secretariat, Geneva, p. 3, No. 5 (Nov. 1983). A. R. Daniel, Simulation and forecasting of wind speed and wind power by stochastic models, M. Phil. thesis, University of the West Indies, Kingston 7, Jamaica (1987). The Climatic Branch of the Jamaican Meteorological Service, The Climate of Jamaica, Meteorological Service, Kingston (1973). K. C. Tomlinson, Jamaica Public Service, private communication (1989). The 1979 wind access catalog. In: Wind Power Digest (Winter 1978). W. Brutsaert, Evaporation into the atmosphere, D. Reidell Publishing Co., Dordrecht (1982). H. L. Penman, Natural evaporation from open water, bare soil and grass, Proc. Roy. Soc. London, A193, pp. 120-146 (1948). W. O. Pruitt, D. L. Morgan, and F. J. Lourence, Energy. momentum and mass transfers above vegetative surfaces. Tech. report ECOM-0447 (E)-F, Dept. Water Sci. Eng., Univ. Calif., Davis (1968). O.G. Sutton, Micrometeorology. In: Evaporation and its role in water resources management. McGraw-Hill, New York (1953). W. O. Pruitt, Emperical method of estimating evapotranspiration using primarily evaporation pans, in evapotranspiration and its role in water resources management, Am. Soc. Agric. Eng., St. Joseph, Mich., 57-61 (1966).
1
data. That process must be done if it is decided that the wind power potential in Jamaica should continue to be studied.
The power curves for the assumed turbines are shown in Fig. AI. The source of the data is given in ref. [9]. No attempt was made to properly match the wind turbine to the wind 11 10
D
9 8
,¢
7
t,
6
no oj c
5
4 3
C
2
B A
1
0
I
2.0
i
4.0
|
I
6.0
I
!
|
8.0 Wind S p e e d
|
10.0 (m/s)
Fig. A I. Power curves for turbines described in Table 2.
|
!
12.0
l
14.0
Wind power in Jamaica
365
APPENDIX 2
A M e t h o d used to d e t e r m i n e the p o w e r exponent at M a n l e y
In the atmospheric boundary layer, the vertical distribution o f wind speed u may be represented by the use o f boundary layer similarity theory[3] as ( k z / u * ) d u Idz = 4~( z / L )
In the original derivation[l I] G was taken to be negligible. Measurements taken over grass[12] indicate that G is small and often negligible in comparison to the other terms in eqn (7A). The Penman equation can be written as[10]
(IA) E = [A/(A + v)]R,./Le + [3'/(A + 3")]E.
(8A)
which on integration yields where u(z) = (u*/k)[ln(z]Zo) - ~b,,,,(z/L)]
(2A) A = d e * / d T , the slope of the graph of saturation va-
where 0 and ¢~,, are universal functions of height z, relative to the similarity scale L, which is the Monin Obukhov stability length, and
pour e* as a function of temperature T 3' = the psychrometric constant. Eo is a measure of the drying power o f the air and is calculated from[10]
• k = yon Karman's constant u* = friction velocity Zo = roughness length.
Eo = a~u*p(q** - q.)[ln(z./:o~) - e,~(z,,/L)] -I
Since the power law (eqn (2)), gives the variation of wind speed with height, the exponent can be expressed as p = 4~(~[L)/[In(Yr]Zo) - Cs,.(2"/L)]
where
(3A)
a~ = ratio of yon Karman's constant for water vapour and dry air q~* = saturated specific humidity at height z° qo = specific humidity at height z. z = 1.83 m, height o f temperature, vapour pressure and pressure measurement zo~ = water vapour roughness length ¢J~, = 2 In[( I + 0-2)/2], flux profile relationship for water vapour.
where ;~ = geometric mean height between the anemometer height and the height to which the measurement is scaled. The functional forms of ~ and Cs,. have been empirically determined in the atmospheric surface layer and depends on the state of stability of the atmosphere. From measurements made in the Caribbean region[4] the atmosphere in the daytime is unstable ( z / L < 0). For such conditions the values have been given[10] as 4:(z/L) = (I - 16z/L) -°'2~
(4A)
Zo. can be expressed as
¢,.,(zlL) = 2/n[(l + 0-~)/2] +/n[(l + 0-2)/2] -
(9A)
Zov = 7.4ZoeXp(-2.25Z°o ~:5)
(10A)
2 tan-l(4~-l) + 7r/2 (SA)
where The stability length L is traditionally defined[ I0] as L = - u ~ . p / [ k g { ( n / ( T q , ) ) + 0.61E}I
(6A)
where g = cp = T= H = E =
acceleration due to gravity specific heat capacity o f air at constant pressure air temperature heat flux into the air evapotranspiration.
L was determined by an energy budget method using the Penman semiempirical method[ 10,1 I] for estimating potential evapotranspiration. Thus, R. = H + E . L , + G where Rn = net incoming radiation L, = latent heat of vaporization G = heat flux to soil.
(7A)
Zo* = (U*Zo)b,, roughness Reynolds number
u = kinematic viscosity z = 2.3 cm, surface roughness length for thick grass[ 13] (environment in which measurements to evaluate E , were made).
The solution for the Obukov stability length L was obtained by the following iteration procedure. An initial guess was made o f L, and used to calculate ~s,. and ~ in eqns (5A) and (10A). ~s,. and ~ provided first values of u* and Eo by use o f e q n s (2A) and (9A), respectively. By using this value of Eo and a value o f E, estimated from Class A pan evaporation measurement, values o f R. and H were obtained from eqns (8A) and (7A), respectively. These initial values of H and u* provided a first value of L by means o f eqn (6A). This value of L allowed second estimates o f ~ and ~ ; whence the process can be repeated. The process is halted when successive values of L are sufficiently close. The value o f p can then be calculated from eqn (3A). In actual practice the potential evapotranspiration E was calculated by multiplying the average pan measurement by a pan coefficient o f 0.8[14]. The value of L was found to have an average value o f - 4 . 6 .