2007 - Exergy and Reliability Analysis of Wind Turbine Systems. A Case Study
2007 - Exergy and Reliability Analysis of Wind Turbine Systems. A Case Study
2007 - Exergy and Reliability Analysis of Wind Turbine Systems. A Case Study
Abstract
The present study undertakes an exergy and reliability analysis of wind turbine systems and applies
to a local one in Turkey: the exergy performance and reliability of the small wind turbine generator
have been evaluated in a demonstration (1.5 kW) in Solar Energy Institute of Ege University (latitude
38.24 N, longitude 27.50 E), Izmir, Turkey. In order to extract the maximum possible power, it is
important that the blades of small wind turbines start rotating at the lowest possible wind speed. The
starting performance of a three-bladed, 3 m diameter horizontal axis wind turbine was measured in
field tests. The average technical availability, real availability, capacity factor and exergy efficiency
value have been analyzed from September 2002 to November 2003 and they are found to be 94.20%,
51.67%, 11.58%, and 0–48.72%, respectively. The reliability analysis has also been done for the
small wind turbine generator. The failure rate is high to an extent of 2.28 104 h1 and the factor of
reliability is found to be 0.37 at 4380 h. If failure rate can be decreased, not only this system but also
other wind turbine systems of real availability, capacity factor and exergy efficiency will be improved.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Efficiency; Energy; Exergy; Renewable energy; Reliability analysis; Wind energy
Corresponding author. Tel.: +90 236 24121 44/240; fax: +90 232 388 6027.
E-mail addresses: onder.ozgener@ege.edu.tr (O. Ozgener), leyla.ozgener@bayar.edu.tr (L. Ozgener).
1364-0321/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.rser.2006.03.004
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Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1812
2. Case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1815
2.1. Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1815
2.2. Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1816
2.3. System operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1818
3. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1818
3.1. Mean time between failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1819
3.2. Capacity factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1820
3.3. Performing exergy analysis of the system studied . . . . . . . . . . . . . . . . . . . . . . . 1820
4. Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1821
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825
1. Introduction
The reliability aspects of alternative sources of energy are of growing importance. This is
largely because of the fact that renewable energy sources are contributing to major power
systems more than in the past [1]. Turkey’s total theoretically available potential for wind
power is found to be about 88,000 MW/yr. Besides this, Turkey’s wave power potential is
estimated to be around 18,500 MW/yr, with an average wave energy capacity of 140
billion kWh annually. These figures indicate that Turkey has considerable potential for
generating electricity from wind and wave power [2,3]. Today, distributed small wind
electric systems can make a significant contribution to Turkey’s energy needs. To date,
four wind power plants were installed with a total capacity of 20.1 MW in Turkey, while a
wind power plant with a total capacity of 39.2 MW will be commissioned in 2006 summer,
at Cesme, Izmir. Due to recent increase in the price of fossil fuels, it is becoming ever more
costly to provide energy for our abodes, and there is also the fact pollution is being created
to provide this energy. This study aims to develop more efficient and more useful small
wind turbine system (SWTS) for rural areas increasing energy and exergy efficiencies, and
decreasing costs of stand-alone and wind systems in the Aegean Region, Turkey.
Renewable energy is abundant and its technologies are well established to provide
complete security of energy supply [4]. Among renewable energy sources, wind energy
plays an important role. From the late 1800s to the early 1900s, thousands of US farmers
and ranchers used windmills to pump water, grind grain, charge batteries, and provide
power for radios, lights, and washing machines. The use of windmills to provide electric
power died out in the early 1930s when the Rural Electrification Administration made
cheap electricity generated at centralized power stations available to farms and ranches
across the country. The cost of electricity in many areas is spiraling upwards and weak
electrical grids make power to remote farms and ranches less reliable than in the past. Even
urban homeowners are faced with unexpected jumps in power costs [5].
Researchers estimate that 50% of the US has enough wind resources for small turbine
development and 60% of US homes are located in those wind resource areas. Using small
wind turbines, farmers, ranchers, and homeowners can reduce their utility bills, stabilize
their electricity supplies, and contribute to nation energy supply to play an important role
in securing our energy future. Distributed wind electric systems represent an opportunity
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Nomenclature
Greek letters
Abbreviations
for some nation especially American households to return to the energy independence of a
past century [5].
Small wind turbines need to be affordable, reliable and almost maintenance free for the
average person to consider installing one. This often means a sacrifice of optimal
performance for simplicity in design and operation. Thus, rather than using the generator
as a motor to start and accelerate the rotor when the wind is strong enough to begin
producing power, small wind turbines rely solely on the torque produced by the wind
acting on the blades. Furthermore, small wind turbines are often located where the
generated power is required, and not necessarily where the wind resource is best. In
these low or unsteady wind conditions, slow starting reduces the total energy generated.
Also, a stationary wind turbine fuels the perception of wind energy as an unreliable energy
source [6].
Wind energy researches on its applications and effects have rapidly increased in the
world (e.g. [7–24]), so efficiency of wind energy constructions is getting importance.
Theoretically, maximum benefit is from 59.2% blowing wind according to the Betz
Criteria. Today, available wind energy ratio reaches about on average 40–45% in modern
wind turbine types. In order to extract the maximum possible power, it is important that
the blades of small wind turbines start rotating at the lowest possible wind speed [6].
The aerodynamic and structural design of rotors for horizontal axis wind turbines
(HAWTs) is a multi-disciplinary task, involving conflicting requirements on, for example,
maximum performance, minimum loads and minimum noise. The wind turbine operates in
very different conditions from normal variation in wind speed to extreme wind
occurrences. Optimum efficiency is not obtainable in the entire wind speed range, since
power regulation is needed to prevent generator burnout at high wind speeds. Optimum
efficiency is limited to a single-design wind speed for stall regulated HAWTs with fixed
speed of rotation. The development of suitable optimization methods for geometric shape
design of HAWT rotors is therefore a complex task that involves off-design performance
and multiple considerations on concept, generator size, regulation and loads [8].
NACA 63-nnn series blades can be preferred to other examples in applications for
performance improvement, because of the fact that these profiles have shown excellent
properties for wind turbine blades and their average power coefficients are higher than
other blades [11,12]. Wind turbines very often have to operate in high turbulence related,
for example, with lower layers atmospheric turbulence or wakes of other wind turbines.
Most available data on airfoil aerodynamics concerns mainly aeronautical applications,
which are characterized by a low level of turbulence (generally less than 1%) and low
angles of attack [9].
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The presented reliability analysis section was especially inspired by publication of [10], in
which for the first time the performance and reliability of the wind turbine generators have
been evaluated in a demonstration wind farm. The reliability analysis has also been done
for the small horizontal axis wind turbine generator, three NACA series bladed in Solar
Energy Institute, Izmir, Turkey. In addition, we examine exergy efficiencies and their
trends of the wind turbine system according to wind speeds and different temperatures of
blowing air and also briefly describe an easy-to-follow procedure for the exergy analysis of
wind turbine systems and how to apply this procedure to assess the system performance by
calculating exergy destruction. The case study, the small wind turbine system (SWTS)
processing, is selected for analysis assessment and evaluation purposes.
2. Case study
A test facility was constructed to study the requiring electricity needs environment lights
of Solar Energy Institute during night’s conditions. Consumed energy for environment
lights purposes depends on seasons and daily changing climatic conditions. Figs. 1 and 2
illustrate a schematic diagram of the constructed experimental system and a view of
system. The main characteristics of the elements of the experimental setup are given in
Table 1. Moreover, Table 2 shows that thermal data used the case study. The experimental
system consists of five major parts as follows: (a) Electronics; charge controller, power
conversion, inverter, charger, warmth control equipment, thermocouple (thermic), (b)
storage batteries, (c) mechanics: tower, nose cone, yaw bearing, slip rings, tail, vane, nacelle
assembly, (d) 1.5 kW non-synchrony generator (alternator) and blades, (e) environmental
Sun
· , Vr Wind; P1, m· , Vr
Wind; P2, m 2 2 1 1
32-100 Hz AC
90-185 V
12 m
AC/DC/AC
96/220V
20 W 20 W 20 W 20 W 20 W
50 Hz 220 V AC
2m
Ground level
50 Hz 220 V AC 50 Hz 220 V AC
energy saving five lamps total power 100 W, (f) five moving and light sensors for energy
saving.
The random or stochastic nature of wind is the single most unique design constraint that
differentiates wind turbines from aircraft designs. The majority of today’s wind turbines
operate within the first 100 m of the earth’s surface. This region, which occupies the lowest
portion of the planetary boundary layer (PBL), is extremely turbulent and driven by
variations, which occur with diurnal changes in atmospheric boundary conditions. The
vertical variation of temperature and wind speed with height defines the PBL behavior
characteristics.
Tower location and height are the principal factors of system efficiency. Wind average
speed depends on many parameters and can vary a lot in the same area. The wind laminar
flow over the surface is disturbed by many obstacles and topographic variations. This has
two consequences: wind speed decreasing near the earth and turbulences. Both of them
diminish as the height increases. A reasonable security margin is 10 m above any obstacle
within 100 m. Even in smooth areas, 10 m is advisable [18–26].
2.2. Measurements
The following data were regularly recorded with a time interval daily during the
experimental period from the month of September 2002 to August 2003.
Table 1
The main characteristics of the elements of the SWTS system studied [7]
Table 2
Thermal data used in the example
(b) measurement of wind velocities at the ground level by anemometer and then these
values were calculated for 12 m by using Hellmann equation,
(c) uncertainty analysis is needed to prove the accuracy of the experiments and an
uncertainty analysis was performed using the method described by Holman [27].
The total uncertainties of the measurements are estimated to be 71.30% for the wind
velocities, 71.02% for voltage and current in the system, and 73.03% for power factor.
Rotor begins to rotate (spine) when the wind speed reaches approximately 2.4 m/s
(8.64 km/h). Data sets in which the rotor accelerated from rest up to 250 rpm, which we
define as a successful ‘start’, were selected from about 70 h of field test data, yielding 160
starting sequences. Battery charging commences at a slightly higher speed, depending on
the battery state of charge. When the battery is fully charged, the charge controller
disconnects the turbine from battery. The turbine produces a three-phase alternating
current (AC) that varies in voltage and frequency as the wind speed varies. The controller
(regulator) rectifies this AC into the direct current (DC) required for battery charging and
controls the energy supplied to the batteries to avoid overcharging. SWTS has electronic
energy analyzers that show every system status data (phase voltages (VLN), phase currents
(I), total current (SI), power factor (P.F.) Cos C, apparent power, etc.).
3. Analysis
Power performance of a wind turbine can be expressed from fixed angular speed. This
parameter is defined by
Cp
CM ¼ . (2)
l
Wind turbines indicate various Cp values depending on wind velocities. Therefore, their
efficiency is best represented by a Cp–l curve. The tip speed ratio, l is given by
oR
l¼ , (3)
Vr
where l is tip speed ratio, R is maximum rotor radius (m), o is rotor speed (rad/s) and Vr is
wind velocity (m/s).
The air flowing with the wind has the same properties as the stagnant atmospheric air
except that it possess a velocity and thus some kinetic energy. This air will reach the dead
state when it is brought to a complete stop. Therefore, the availability of the blowing air is
simply the kinetic energy it possesses:
V 2r
Exergy of kinetic energy ¼ availability ¼ ke1 ¼ . (4)
2
To determine the available power, we need to know the amount of air passing through
the rotor of the windmill per unit time, the mass flow rate. Assuming standard atmospheric
conditions (25 1C, 101 kPa) in this study, the density of air is 1.18 kg/m3, and its mass flow
rate is
_ ¼ rAV r ¼ rpR2 V r .
m (5)
Thus,
_ 1 Þ.
Available power ¼ W ¼ ðmke (6)
This is the maximum power available to the windmill. Most windmills in operation
today harness about 20–40% of kinetic energy of the wind [23].
Kinetic exergy is a form of mechanical energy, and thus it can be converted to work
entirely. Therefore, the work potential or exergy of kinetic energy of a system is equal to
the kinetic energy itself, regardless of temperature and pressure of the environment [23].
Any measured wind velocity value can be estimated for different height by using the
following Hellmann equation:
H m
V r ¼ V ref , (7)
H ref
where Vr is the calculated wind velocity, and Vref the wind velocity at reference height. In
this study, a Hellmann coefficient (m) of 0.28 was assumed [24], because tower location is
near city.
When a system is often unavailable due to breakdowns and is put back into operation
after each breakdown. The mean time between breakdowns is defined as the mean time
between failures (MTBFs). During the operating period, when failure rate is fairly
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constant, the MTBF is the reciprocal of the constant failure rate [10]:
1
MTBF ¼ . (8)
l
MTBF is also referred to as the average time of satisfactory operation of the system. In this
case, the larger the MTBF, the higher is the reliability of the system. If the reliability factor of
the system is to be determined, initially the individual reliability of the subsystems or elements
have to be estimated. If each component exhibits a constant failure rate, then the reliability
factor for each component will be in the form of exponential (lt). The tth element will have
a reliabilty of exp (lt). Hence, the reliabilty for the system will be [10,28]
X
RðtÞ ¼ exp½ðl1 þ l2 þ l3 þ þ ln Þt ¼ expð li tÞ, (9)
where l is the failure rate which is the reciprocal of MTBF, t is time (h). The value of
reliability R(t) is 1 at t ¼ 0, and it decreases continuously thereafter with time. When ‘t’
becomes very large, all the components will fail, thus R(t) will reach a value of zero [10].
The capacity factor, which is called the rational efficiency or the overall rational
efficiency, is defined as the ratio of the desired actual useful energy production to the
theoretical maximum useful energy production:
W aw
Capacityfactor ¼ . (10)
W teo
or
We
¼ (11b)
Wu
Useful work:
_
m
W u ¼ ðP1 P2 Þ (12)
r
Exergy destruction:
_ dest ¼ W u W e
Ex (13)
or
Exdest ¼ ðEx1 Ex2 Þ W e , (14)
where the exergy rate
_ a.
Ex ¼ mc (15)
The total flow exergy of air is calculated from Eq. (16) or (17) [30,31]
ca ¼ ðC p;a þ oC p;v ÞT 0 ½ðT=T 0 Þ 1 lnðT=T 0 Þ þ ð1 þ 1:6078oÞRa T 0 lnðP=P0 Þ
þ Ra T 0 ð1 þ 1:6078oÞ ln½ð1 þ 1:6078o0 Þ=ð1 þ 1:6078oÞ þ 1:6078o lnðo=o0 Þ ,
ð16Þ
ca ¼ ðC p;a þ oC p;v Þ ðT T 0 Þ T 0 bðC p;a þ $C p;v Þ lnðT=T 0 Þ
ðRa þ oRv Þ lnðP=P0 Þc þ T 0 ½ðRa þ oRv Þ ln ð1 þ 1:6078o0 Þ=ð1 þ 1:6078$Þ
þ1:6078oRa ln o=o0 , ð17Þ
where the specific humidity ratio
_ w =m
o¼m _ (18)
and, T0, P0 are reference temperature and atmospheric pressure which are taken 25 1C,
101.325 kPa in this study, respectively.
Performance data from the wind turbine are stored. Output power and wind speed are
sampled over periods of time and average values of wind for each period are stored in wind
speed. Table 3 shows measured and calculated performance parameters average values of
the SWTS. According to Table 3, actual useful energy measured was 111.8 kWh, and the
main reason for this is that moving and light sensors were used on lights for saving energy,
and the SWTS was under maintenance some days. Performance test results show that the
average wind speed is 7.5 m/s, 616 W and 76 Hz. Electricity is produced by alternator, but
power consumptions value is constant always, because total power, voltage, and currents
of environment lamps are 100 W, 220 V, and about 0.5 A, respectively [7].
In the present study of wind energy system, there were repetitive machine faults which
occurred on the electric, electronic grid feed and mechanic control of the system. A
considerable number of the types of faults and problems such as electronic automation,
over and lower load electricity problems according to wind speeds and its frequencies,
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Table 3
Measured and calculated performance parameters average values of the SWTS [7]
Vr Cp _
m ke1 _ ke1
P ¼ ðWÞ ¼ m Pa (W) ¼ Cp*P Pea Wab,c,d(kWh/month)
0.6
0.4
R2 = 0.99
0.2
0
0 5000 10000 15000 20000 25000 30000
Time in hours (h)
Reliability of SWTS with existing failures
0.8
Reliability factor (-)
0.6
0.4
R2 = 0.99
0.2
0
0 5000 10000 15000 20000 25000 30000 35000 40000
Time in hours (h)
Reliability of SWTS after removing 50% of mechanic and electronic faults
Colak et al. [32]. Their results show that 0–2.5 m/s wind speed duration (Dti) is 4234 h and
wind speed frequency is 48.33%.
The capacity factor or exergy efficiency has been calculated for the wind turbine
generators, using actual generation and installed capacity. The capacity factor is less than
11.58% in a year. During the winter season from the month of November to March, the
wind velocity is high and it leads to high capacity factor. This indicates that the capacity
factor is also highly dependent on wind velocity. Theoretically useful energy is found as
964.873 kWh (corresponding to measured 1998 wind velocity speeds), and that is the
maximum electricity that will be produced in a year; however, actual seasonal performance
useful energy was measured as 111.8 kWh from September 2002 to August 2003, and the
main reason for the discrepancy is the largely low wind speed frequency distribution, the
use of moving and light sensors used on lights for saving energy, and the SWTS was under
maintenance some days.
Here, we now analyze exergy efficiencies and their trends. Exergy efficiencies are
compared in Figs. 4 and 5. Table 4 and Fig. 4 show that exergy efficiency changes between
0% and 48.7% at different wind speeds by using Eqs. (11a) and (12). Furthermore, 52.3%
of the total exergy entering the system is lost, while the remaining 48.7% is utilized at
7.5 m/s wind speed. Nonetheless, maximum energy output (1335 W) and exergy destruction
(2876.36 W) are produced at 12 m/s wind speed. Exergy efficiency value of the wind turbine
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0.6
0.5
0 2 4 6 8 10 12 14
Wind speed (m/s)
Fig. 4. Exergy efficiency of the three-bladed 3 m diameter horizontal axis wind turbine system, according to wind
speed based on pressure differences between state points shown in Fig. 1 (Dead state 25 1C and 101.325 kPa).
1
Exergy efficiency (-)
0.8
0.6
0.4
0.2
0
0 2 4 6 8 10 12 14
Wind speed (m/s)
0 ºC 5 ºC 10 ºC
15 ºC 20 ºC
Fig. 5. Exergy efficiency of the wind turbine system, three-bladed 3 m diameter horizontal axis, according to
different temperatures of blowing air (Dead state 25 1C and 101.325 kPa).
5. Conclusions
Although significant progress has been made in developing modern horizontal wind
turbines on world, it is necessary to improve research and development facilities in order to
develop more efficient, low-cost small horizontal wind turbines which are more reliable
and feasible to meet the local energy demands. An experimental system was installed for
investigating performance of an SWTS to ensure power supply to some environment lights
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Table 4
Exergy, exergy destruction rates, exergy efficiencies and other properties at various wind velocities
Pressure Air flow rate, Useful power, Wind Power at Exergy Exergy
difference at _
m(kg/s) Wu (W) velocity, Vr inverter destruction, efficiency, e
state 1 and 2, (m/s) output, We Exdest (W) ()
DP (Pa) (W)
Dead state temperature, atmospheric pressure, and density were taken as 25 1C, 101.325 kPa and 1.18 kg/m3,
respectively.
of Solar Energy Institute Building. The results obtained during the month of September
2002 till August 2003 were given and discussed. We can extract some concluding remarks
from this study as follows:
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