SIMULATION OF A COMBINED WIND AND SOLARPOWER PLANT

M. T. SAMARAKOUAthens University, Electronics Laboratory, Solonos /04, Athens, Greece

AND

J. C. HENNETLaboratoire 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 highlevels of yearly wind energy and insolation. A computer model is developed for the simulation of the electricity system of aMediterranean island, including a wind power plant, a photovoltaic power plant and a storage system. In order to obtain anoverall 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 largefraction of the electrical energy requirements. Various parameters calculated in the simulation can be used to improve theconfiguration 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 economicdevelopment. But the use of wind machinest generate electricity is a relatively new technological advance. Itsfuture 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 thesetting up of five 'AEROMAN' wind machines of rated power output 20 kW each. In Kythnos the windvelocity 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 withimportant standard deviations. On the other hand, insolation curves show that solar energy has muchsmoother daily and yearly distributions. The average daily insolation is characterized by seasonal variationswith a maximal value in the summer when wind velocities are minimal. Thus the two processes present acomplementary relation which indicates the possible efficiency of the combined use of theseetwo energysources. 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 structuralreasons, it mainly relies on the diesel units, which have been over-sized. The purpose of this work is to simulatethe operation of the island electricity system which integrates the renewable energy devices, a battery storagesystem for damping load and electricity production fluctuations and diesel generators only to be used when theload is higher than the combined production and the stored energy. Computer simulation is a convenientmethod of system analysis and evaluation. It requires models of environmental conditions, of systemcomponents 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 notavailable for this island. Only the daily load curves could be obtained from a M.A.N.· study and the hourlyelectricity consumptions during the days of minimal and maximal load were provided by P.P.c. (1982).t Thecharacteristics of the simulated system components are approximately those ofKythnos power plant. But thesimulation model is limited to the operational conditions and does not take into account the technicalparticularities of the existing equipment.

The typical structure of the system is shown in Figure 1.Continuous lines represent energy flows and dottedlines represent information flows for observations and actions on switches.

The electricity produced by each wind machine through an asynchronous generator can either be used tosatisfy 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 theload or to charge the battery. Specifications of the photovoltaic installation are given in Table II. The tilt angleof the panels is equal to the local latitude (37025').

Table I. Specifications of the wind generators installed atthe Kythnos wind park

Number of unitsRated power outputTotal power outputRated wind speedCut in wind speedFurl up wind speedRotor diameterRotor speedType of unitType of generator

520kWlookW

11·1 m/s3·2 m/s24·0 m/s11·6 m88/95 RPMAero MAN. 11/20Asynchronous voltage: 400 VN = 1500 RPMCos = 0·8400-15000 VStep upElectro-hydraulicPower and speed control

• M.A.N.: Maschinenfabrick Augsburg Niirnberg.t P.P.c.: Power Public Corporation of Greece.

Photovoltaic plantTotal area of solar parkRated output (peak)Rated voltageRated battery voltageRated voltage of PV plantNumber of solar modulesRated module voltageRated module output

Photovoltaic modules/cellsMonocrystalline silicondimensions 1009 x 1462 x 82,5 mmWeight 27 kg

7500 m3

100kW160 V250 V380 V (3 phase, 50 Hz)8009Vl20W

The storage system is a lead-acid battery with a minimal storage level of 120kWh, a maximal storage levelof 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 thescope of this paper. The main control tasks considered in this study are represented in Figure 1. They aredecomposed 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 dieselunits. Such operating conditions with 100 per cent wind or solar/wind penetration have been shown to beachievable with appropriate voltage and frequency regulations (Tsitsovits and Freris, 1983). However, inturbulent wind conditions, the respect of performance and stability constraints may occasionally induce someliiidations 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 energyare 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 therenewable 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 thecurrent charge level of the storage unit. If the battery reaches its maximal charge level, any excess energy fromthe 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 ismet 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 asrequired are started up.

I = index of the hourL(I) = total load during hour I (kWh)S(I) = energy production from photovoltaic generators (kWh)W(I) = energy production from wind machines (kWh)

N = 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 oneyear (N ~ 8760). We use the real data over one specific year, 1982, as possible sample sequences of the threestochastic processes.

Simulation of electricity generation by the photovoltaic cells is based on real hourly values of global solarradiation onto a horizontal surface, corrected by a factor depending of the tilt angle of the panels (37025'). Theconversion efficiency of the cells is supposed constant with value 0·08. Wind hourly speeds V(I) have beenmeasured in Kythnos at a height of 150 m. The analytical expressions used to calculate the hourly energyproduced by each wind machine is classical (Joubert and Pechenx, 1981).

0, for V(I) < VMIN

~ CpP A [V(I)J3, for VM1N ~ V(I) < V

P, for VR ~ V(I) ~ VMAX

0, for V(I) > VMAX

VMIN is the cut-in speed (V MIN = 3 m/s)VR is the rated speed (VR = ll'10m/s)VMAX is the cut-out speed (V MAX = 24 m/s)A is the rotor area (A = 105'7 m2

)

Cp is the wind generator efficiency (Cp = 0'25)

p is the air density (p = 1'3)P is the rated power (P = i CppA V k)

The load value at any hour L (I) has been obtained by multiplying the daily load value by a typical percentageof the daily load associated with the considered hour of the day. The average percentage curve has been drawnfrom the load curves of the days of minimal and maximal electric consumption.

Computation of NL(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 = 120 kWhBMAX = 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 levelB(I) = BMAX' The excess

energy B(I - 1)+ IN L(I) 1-BMAX gets lost.2. If NL(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 NL(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 areaccumulated as the calculation proceeds, and the hourly, daily and yearly totals are printed. A typical yearlysimulation requires about 20 s on a CII-Honeywell Bull Mini 6 computer. A cost subroutine has been aded tothe program in order to compute the fixed and variable costs associated with the dimensions and the operationof the combined system.

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 curveis typical of a touristic region with a high electricity consumption in the summer and a local peak at the Easterholiday. The production curve of the photovoltaic generators is relatively smooth with average values ofapproximately 300 kWh per day in January and 700 kWh per day in June. The production curve of the windmachines is totally different. The maximum values are about the same (around 2300 kWh per day) all the yearround. 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 ofdiesel units. However, the role of the battery is fundamental for efficiently using the two renewable energies. Asshown in Figure 3(c),the charge level of the battery is subject to a great number of strong variations which helpregulating 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 the14th 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 thediesel units.

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On a yearly basis, a comparison of the relative shares of the load met by solar energy, by wind energy and bythe diesel units yields the following results:

(a) for a total load of 1049·6MWh, 58·5per cent is produced by the diesel units, 28·3per cent by the windmachines and 16·2per cent by the solar generators. Three per cent of the total energy produced is lostbecause of the battery capacity and efficiency.

(b) in term of autonomy of the combined production, there are 694 hours of autonomy provided by thewind power only, 501 hours of autonomy provided by the wind and the solar power, 1356 hours ofautonomy 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 thenumber of diesel stop/start cycles. Moreover, the introduction of a storage system is specially relevant for anisolated system with solar and wind machines output often bigger than the load. Simulation shows that for theKythnos system, the battery can typically provide 80 MWh per year, and such an output makes the storagesystem competitive, as shown in Table III.

In order to evaluate the profitability of the whole system by a cost-benefit analysis, we have characterizedeach component by its prospective yearly average cost (fixed cost + maintenance and usage).

The unit cost values of Table IIIhave 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 intoaccount 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

Cost, Unit costUnit cost per Output, thousands per

year Size MWh $ kWh

Photovoltaic equipment 50 $/m2 1200m2 160 60 0·375Wind machine 50 $/m2 of 5 x 105'7 200 26·4 0·132

rotor swept 528·5m2

areaStorage 10 $/kWh 600 kWh 80 6 0·075

of capacityDiesel 0'15 $/kWh 4 x 80+2 x 530 610 91·5 0·15

of outputTotal 1050 183·9 0'175

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Photovoltaic panels have a relatively important share of the total cost and their efficiencyis low. Two ways toeconomic 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 isthen economically and technically desirable.

Except for reliability, there is no interest in over-sizing the diesel units. Simulation shows that with a dieselcapacity of 500 kW, the risk of failure would still remain very low. If such an option had been chosen from thebegining, the cost-benefit analysis would probably have shown that in the present price context wind energyhas not quite reached the profitability level.

There are distinct advantages economically and ecologically to the renewable energy sources. Theseadvantages are: (i) it is a 'clean' type of energy; (ii) such a system could eliminate the difficulties oftransportation of conventional fuels and their cost; (iii)it could help stabilize the economies of countries whichdepend on other countries for fuel resources.

As far as the combination of the two types of renewable energies is concerned, their complementarity, forcertain sites, reinforces the autonomy of the system. On the other hand the lower cost of the wind energyproduction affects positively the reduction of the overall cost. However, with the present data, the cost of thecombined 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. Muchresearch has been devoted to the subject during the last decade and many options are still under study at thetheoretical and at the industrial levels.The weight and the cost of modern wind turbines has been considerablyreduced and the rated power increased up to 5MW and even 10 MW. Wind/diesel hybrid systems operating onlocal grids should now be studied with similar rated powers for the two subsystems. The use of a batterystorage 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.