SIMULATION OF A COMBINED WIND AND SOLAR
POWER PLANT
M. T. SAMARAKOU
Athens University, Electronics Laboratory, Solonos /04, Athens, Greece
AND
J. C. HENNET
Laboratoire d'Automatique et d'Analyse des Systemes du C.N.R.S. 7, avenue du Colonel Roche-3/077 Toulouse Cedex, France
The combined generation of electricity by wind and solar energy is a very attractive solution for isolated regions with high
levels of yearly wind energy and insolation. A computer model is developed for the simulation of the electricity system of a
Mediterranean island, including a wind power plant, a photovoltaic power plant and a storage system. In order to obtain an
overall view of the system performance and economic aspects, the model also incorporates a number of diesel generators.
Daily simulations for the Greek island Kythnos show that such a combined system of moderate size can provide a large
fraction of the electrical energy requirements. Various parameters calculated in the simulation can be used to improve the
configuration of the system and to estimate the cost of the electrical energy unit.
In many Mediterranean islands, the energy of the wind has always been considered a basic factor for economic
development. But the use of wind machinest generate electricity is a relatively new technological advance. Its
future development relies on the possibility of storing electricity in large battery units at reasonable costs.
Because of its high wind potential, the Greek island Kythnos, in the Egean sea, has been chosen for the
setting up of five 'AEROMAN' wind machines of rated power output 20 kW each. In Kythnos the wind
velocity is on average greater than 6·6 mls during fifty per cent of the time.
Considered as random processes, wind speed magnitudes are characterized by irregular distributions with
important standard deviations. On the other hand, insolation curves show that solar energy has much
smoother daily and yearly distributions. The average daily insolation is characterized by seasonal variations
with a maximal value in the summer when wind velocities are minimal. Thus the two processes present a
complementary relation which indicates the possible efficiency of the combined use of theseetwo energy
sources. Therefore, it was decided to also equip the island with 10 kW peak output 'PHOTOWATT'
photovoltaic generators for the exploitation of the solar potential.
But at the present time, the energy policy does not fully use all the possibilities. For historical and structural
reasons, it mainly relies on the diesel units, which have been over-sized. The purpose of this work is to simulate
the operation of the island electricity system which integrates the renewable energy devices, a battery storage
system for damping load and electricity production fluctuations and diesel generators only to be used when the
load is higher than the combined production and the stored energy. Computer simulation is a convenient
method of system analysis and evaluation. It requires models of environmental conditions, of system
components and of the energy policy which is to be evaluated.
0363 -907X/86 1010001 -10$0 1.00
© 1986 by John Wiley & Sons, Ltd.
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The data used by the computer code are the real meteorological data of Kythnos. Hourly load data were not
available for this island. Only the daily load curves could be obtained from a M.A.N.· study and the hourly
electricity consumptions during the days of minimal and maximal load were provided by P.P.c. (1982).t The
characteristics of the simulated system components are approximately those ofKythnos power plant. But the
simulation model is limited to the operational conditions and does not take into account the technical
particularities of the existing equipment.
The typical structure of the system is shown in Figure 1. Continuous lines represent energy flows and dotted
lines represent information flows for observations and actions on switches.
The electricity produced by each wind machine through an asynchronous generator can either be used to
satisfy the load or to charge the battery. The wind generators' specifications are given in Table I.
In a similar way, the electricity produced by the photovoltaic generators can either contribute to satisfy the
load or to charge the battery. Specifications of the photovoltaic installation are given in Table II. The tilt angle
of the panels is equal to the local latitude (37 25').
0
Table I. Specifications of the wind generators
the Kythnos wind park
installed at
Number of units
Rated power output
Total power output
5
20kW
lookW
Rated wind speed
Cut in wind speed
Furl up wind speed
Rotor diameter
Rotor speed
11·1 m/s
3·2 m/s
24·0 m/s
Type of unit
Type of generator
Aero MAN. 11/20
Asynchronous voltage: 400 V
N = 1500 RPM
Cos = 0·8
400-15000 V
Step up
Electro-hydraulic
Power and speed control
• M.A.N.: Maschinenfabrick Augsburg Niirnberg.
t P.P.c.: Power Public Corporation of Greece.
11·6 m
88/95 RPM
Photovoltaic
plant
Total area of solar park
Rated output (peak)
Rated voltage
Rated battery voltage
Rated voltage of PV plant
Number of solar modules
Rated module voltage
Rated module output
Photovoltaic
7500 m3
100kW
160 V
250 V
380 V (3 phase, 50 Hz)
800
9V
l20W
modules/cells
Monocrystalline silicon
dimensions
1009 x 1462 x 82,5 mm
Weight
27 kg
The storage system is a lead-acid battery with a minimal storage level of 120kWh, a maximal storage level
of 600 kWh and an efficiency of 0·80.
The diesel generators consist of 2 units of 530 kW each and 4 units of 80 kW.
Electricity flows are organized according to operational rules implemented by a central control unit. The
system also includes an electro-hydraulic power and speed control of the wind machines, which lies out of the
scope of this paper. The main control tasks considered in this study are represented in Figure 1. They are
decomposed into 4 basic functions which can be sequentially performed according to the diagram of Figure 2.
The roles that we assign to each of these control units can be described as follows.
1. The load control
We assume that in low demand periods, it is technically possible to meet the load without using the diesel
units. Such operating conditions with 100 per cent wind or solar/wind penetration have been shown to be
achievable with appropriate voltage and frequency regulations (Tsitsovits and Freris, 1983). However, in
turbulent wind conditions, the respect of performance and stability constraints may occasionally induce some
liiidations upon the wind energy penetration.
If the load can be totally met by the energy from the wind machines, solar energy and the excess wind energy
are directed to the battery.
If the sum of the wind energy and of the solar energy is higher than the load, any excess energy from the
renewable energy units can be directed to the battery.
If the sum of solar and wind energies can only meet a part of the load, the remaining load, called the net load,
must be met if possible, by the battery and (or) by the diesel units.
2. The charge control
The energy to be stored in the battery cannot exceed the difference between the maximal capacity and the
current charge level of the storage unit. If the battery reaches its maximal charge level, any excess energy from
the wind machines, from the photovoltaic generators or from the diesel units gets lost.
3. The discharge control
If the energy stored in th battery cannot totally meet the net load, the diesel option is taken up.
If the diesel contribution is not sufficient to cover the net load, the battery is discharged down to the
minimum permissible level. If the discharge is not sufficient to meet the residual load, there is a failure.
4. Diesel control
If the net load is higher than 300 kW, one of the two large diesel units is started up, and any residual load is
met by one or more of the small diesel units.
If the net load is less than 300 kW the large diesel units are not needed and only as many small diesel units as
required are started up.
=
=
=
W(I) =
N =
I
L(I)
S(I)
index of the hour
total load during hour I (kWh)
energy production from photovoltaic generators (kWh)
energy production from wind machines (kWh)
number of hours of the evaluation period,
The evaluation period corresponds to the periodicity of the stochastic series L(I), S (I), and W(I), that is one
year (N ~ 8760). We use the real data over one specific year, 1982, as possible sample sequences of the three
stochastic processes.
Simulation of electricity generation by the photovoltaic cells is based on real hourly values of global solar
radiation onto a horizontal surface, corrected by a factor depending of the tilt angle of the panels (37 25'). The
conversion efficiency of the cells is supposed constant with value 0·08. Wind hourly speeds V(I) have been
measured in Kythnos at a height of 150 m. The analytical expressions used to calculate the hourly energy
produced by each wind machine is classical (Joubert and Pechenx, 1981).
0
0, for
V(I)
<
V MIN
~ CpP A [V(I)J3,
P,
for VR ~
0, for
V(I)
V(I)
is the rated speed (VR =
ll'10m/s)
= 24 m/s)
is the rotor area (A = 105'7 m2)
is the wind generator efficiency (Cp =
VMAX is the cut-out speed (V MAX
A
Cp
~
> VMAX
VMIN is the cut-in speed (V MIN = 3 m/s)
VR
for VM1N
0'25)
VMAX
~
V(I)
<
V
p
P
is the air density (p = 1'3)
is the rated power (P = i C ppA V k)
The load value at any hour L (I) has been obtained by multiplying the daily load value by a typical percentage
of the daily load associated with the considered hour of the day. The average percentage curve has been drawn
from the load curves of the days of minimal and maximal electric consumption.
Computation of N L(I) leads to two different alternatives:
1. If N L(I) ~ 0 there is an excess of production over electricity demand. Let B(I -1) be the current charge
level of the battery.
BM1N
BMAX
= 120 kWh
= 600 kWh
(a) if BMAX - B(I - 1) < IN L(I)I, the battery can get charged up to the level B(I) = B(I - 1) + IN L(I)I·
(b) if B MAX - B(I - 1) < IN L(I) I, the battery can only get charged up to the level B(I) = BMAX' The excess
energy B(I - 1) + IN L(I)
BMAX gets lost.
2. If N L(I) > 0 the discharge control unit is simulated as follows:
(a) if I] (B(I - 1)- BM1N) ~ N L(I), I] being the battery discharge efficiency (I] = 0'80), then the load is met,
and B(I) = B(I -;- 1)- N L(I)/I]
(b) if I](B(I -1) - BM1N) < N L(I), then the diesel option is chosen. Two cases are possible:
(ex) if N L(I) ~ 300 kWh, one of the two large diesel units is started up. Then the residual load N L(I)
- 300 is dealt with as in case 2.
(13) if N L(I) < 300 kWh, the four small diesel units are started up one by one. Their output is
incremented from a minimal value of 20 kW to their maximal output, 80 kW, at a constant rate.
The simulation program is written in FORTRAN 77. Calculations are repeated hour by hour for the 8760
hours of a complete year. The total energy provided by the solar, wind, storage and diesel equipments are
accumulated as the calculation proceeds, and the hourly, daily and yearly totals are printed. A typical yearly
simulation requires about 20 s on a CII-Honeywell Bull Mini 6 computer. A cost subroutine has been aded to
the program in order to compute the fixed and variable costs associated with the dimensions and the operation
of the combined system.
1-
The performance of the system and the relative shares of each energy source (wind machines, solar generators,
battery and diesel units) are shown on the yearly curves in Figures 3(a), (b) and (c).In Figure 3(a), the load curve
is typical of a touristic region with a high electricity consumption in the summer and a local peak at the Easter
holiday. The production curve of the photovoltaic generators is relatively smooth with average values of
approximately 300 kWh per day in January and 700 kWh per day in June. The production curve of the wind
machines is totally different. The maximum values are about the same (around 2300 kWh per day) all the year
round. What differs is the frequency of windy days. There are many more windy days in winter than in summer.
Figure 3(b) shows that the share of the energy extracted from the battery is much lower than the share of
diesel units. However, the role of the battery is fundamental for efficiently using the two renewable energies. As
shown in Figure 3(c),the charge level of the battery is subject to a great number of strong variations which help
regulating the combined production and specially the wind energy production.
Daily curves (Figures 4(a), (b)) show various operating conditions of the system with the data of5 May, 1982.
Figure 4(a) shows that the wind energy production exceeds the load during the 2nd, the 4th and the 11th hours.
This fact causes the battery to get charged during these three hours (Figure 4(c)), and also from the 8th to the
14th hour, when the combined production of sun and wind energies is higher than the load. The rest of the time,
before the 20th hour, the battery discharge is used to meet the demand. From the 20th to the 24th hours,
the battery is not used any more since it cannot totally meet the demand. The net load is then totally met by the
diesel units.
I
i
I
I
I
~,
II
I
~
I
On a yearly basis, a comparison of the relative shares of the load met by solar energy, by wind energy and by
the diesel units yields the following results:
(a) for a total load of 1049·6MWh, 58·5 per cent is produced by the diesel units, 28·3 per cent by the wind
machines and 16·2per cent by the solar generators. Three per cent of the total energy produced is lost
because of the battery capacity and efficiency.
(b) in term of autonomy of the combined production, there are 694 hours of autonomy provided by the
wind power only, 501 hours of autonomy provided by the wind and the solar power, 1356 hours of
autonomy provided by the wind power, and solar power and the battery.
Altogether the combined system is autonomous during 2551 out of the 8760 hours.
The combination of a storage system with a wind/diesel or a wind/solar/diesel system has rarely been
investigated. But some authors (Infield et aI., 1983) have pointed out that it can considerably decrease the
number of diesel stop/start cycles. Moreover, the introduction of a storage system is specially relevant for an
isolated system with solar and wind machines output often bigger than the load. Simulation shows that for the
Kythnos system, the battery can typically provide 80 MWh per year, and such an output makes the storage
system competitive, as shown in Table III.
In order to evaluate the profitability of the whole system by a cost-benefit analysis, we have characterized
each component by its prospective yearly average cost (fixed cost + maintenance and usage).
The unit cost values of Table III have been roughly estimated from various data of a comparable real system.
The cost per kWh of diesel units (including the cost of the fuel) has been chosen relatively high to take into
account the isolated location of the island and the large size of the generators.
8
M. T. SAMARAKOU AND J. C. HENNET
Table III. Cost-benefit analysis
Unit cost per
year
Photovoltaic equipment
Wind machine
Storage
Diesel
Total
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10 $/kWh
of capacity
0'15 $/kWh
of output
Output,
MWh
Cost,
thousands
$
Unit cost
per
kWh
1200 m2
5 x 105'7
528·5 m2
160
200
60
26·4
0·375
0·132
600 kWh
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4 x 80+2 x 530
610
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Figure 4(c). Hourly
evolution
and change in the battery
level: (1) evolution;
e:
HOUR
(2) level
Photovoltaic panels have a relatively important share of the total cost and their efficiencyis low. Two ways to
economic feasibility are currently investigated: cost decrease and efficiency improvement.
In some isolated region, wind power can favourably compare with diesel units. The use of a storage system is
then economically and technically desirable.
Except for reliability, there is no interest in over-sizing the diesel units. Simulation shows that with a diesel
capacity of 500 kW, the risk of failure would still remain very low. If such an option had been chosen from the
begining, the cost-benefit analysis would probably have shown that in the present price context wind energy
has not quite reached the profitability level.
There are distinct advantages economically and ecologically to the renewable energy sources. These
advantages are: (i) it is a 'clean' type of energy; (ii) such a system could eliminate the difficulties of
transportation of conventional fuels and their cost; (iii)it could help stabilize the economies of countries which
depend on other countries for fuel resources.
As far as the combination of the two types of renewable energies is concerned, their complementarity, for
certain sites, reinforces the autonomy of the system. On the other hand the lower cost of the wind energy
production affects positively the reduction of the overall cost. However, with the present data, the cost of the
combined system is still rarely competitive with classical electrical sources.
Wind energy is now recognized as one of the most promising renewable energy sources for the future. Much
research has been devoted to the subject during the last decade and many options are still under study at the
theoretical and at the industrial levels. The weight and the cost of modern wind turbines has been considerably
reduced and the rated power increased up to 5MW and even 10 MW. Wind/diesel hybrid systems operating on
local grids should now be studied with similar rated powers for the two subsystems. The use of a battery
storage system is recommended in this context.
Infield, D. G., Slack G. W., Lipman, N. H. and Musgrove, P. J. (1983). 'Review ofwindjdiesel strategies', lEE Proc. A, 130, (9), 613-619.
Joubert, A. and Pecheux, J. (1981). 'Etude du comportement d'un systeme energetique fonctionnant a partir du couplage des energies
solaire et eolienne', Revue de Physique Appliquee, 16(7), 397--403.
Klein, S. A. (1977). 'Calculation of monthly average insolation on tilted surfaces', Solar Energy, 19(4), 325-329.
Powell, R. (1981), 'An analytical expression for the average output power of a wind machine', Solar Energy, 26(1), 77-80.
P.P.c. (Public Power Corporation). (1982). Measurementsfor
Development of Solar and Aeolic Potential of Greece for Energy Purposes.
Samarakou, M. T., Avaritsiotis, J., Grigoriadou-Kouki, M., Liolioussis, K. T. and Caroubalos, C. (1983). Theoretical study of an
autonomous system combining a photovoltaic generator and wind machines under real data', I.E.E.E. MELECON Congress, May,
Athens, Greece, May.
Tsitsovits, A. J. and Freris L. L. (1983). 'Dynamics of an isolated power system supplied from diesel and wind', lEE Proc. A, 130 (9),
587-595.