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An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
Pergamon
PII: S0038 – 092X( 99 )00016 – X
Solar Energy Vol. 66, No. 1, pp. 1–9, 1999 1999 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0038-092X / 99 / $ - see front matter
www.elsevier.com / locate / solener
AN OPTIMUM DISPATCH STRATEGY USING SET POINTS FOR A PHOTOVOLTAIC (PV)–DIESEL–BATTERY HYBRID POWER SYSTEM M. ASHARI † and C. V. NAYAR Centre for Renewable Energy Systems Technology Australia (CRESTA), Curtin University of Technology, Perth, Western Australia 6001, Australia Received 14 February 1997; revised version accepted 9 February 1999 Communicated by ROBERT HILL
Abstract—This paper presents dispatch strategies for the operation of a solar photovoltaic (PV)–diesel– battery hybrid power system using ‘set points’. This includes determination of the optimum values of set points for the starting and stopping of the diesel generator to minimise the overall system costs. A computer program for a typical dispatch strategy has been developed to predict the long-term energy performance and the lifecycle cost of the system. 1999 Elsevier Science Ltd. All rights reserved.
delivered to the battery can be controlled either by controlling the excitation of the alternator or by incorporating a charger regulator in the renewable energy source. In this scheme, the inverter at the desired voltage and frequency can condition the load. This scheme also supplies the load without any interruption of power in the event of changing the DG or the PV generator to charge the battery bank. The design principles of this system are relatively simple to implement, but it has the following disadvantages: • low overall system efficiency due to the series configuration of system elements; a certain amount of the energy will be lost due to the battery and the inverter efficiencies; • a large inverter size should be used such that the capacity is substantially larger than the maximum peak-load demand. • when the renewable energy sources are incorporated in the system, it leads to a limited control of the diesel alternator.
1. INTRODUCTION
Hybrid power systems integrate renewable energy technologies with diesel generators (DGs), inverters (INVs) and batteries to provide grid quality electrical power. They are becoming the most commonly used systems for village electrification due to the environmental and economic advantages. Many countries in the world such as Australia, Canada, China, Indonesia, Malaysia, Mexico, Russia have currently installed hybrid systems. From the application point of view, hybrid power systems can be classified into two topologies: series and parallel (Nayar et al., 1993).
1.1. Series topology In the series hybrid system, shown schematically in Fig. 1, the diesel generator and the renewable energy source are used to charge a battery bank. The diesel generator is connected in series with the inverter to supply the load. So the diesel generator can not supply the load directly. The inverter converts the power from the battery bank to AC at mains voltage and frequency and subsequently supplies the load. The battery bank and the inverter capacity should be enough to cover the peak load demand, while the DG capacity should be adequate for meeting the peak load and charging the battery simultaneously. The power
†
Author to whom correspondence should be addressed. Tel.: 11-61-8-9266 3157; fax: 11-61-8-9266-3107.
Fig. 1. Series topology of hybrid system. 1
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M. Ashari and C. V. Nayar
Fig. 2. Parallel topology of hybrid system.
1.2. Parallel topology The block diagram of parallel hybrid system is shown in Fig. 2. This topology has superior performance over the series hybrid system. In this scheme the renewable energy sources and the DGs supply a portion of the load demand directly, resulting in higher overall system efficiency. The diesel generator and the inverter can operate in stand-alone or parallel mode. This offers some combinations of the source for meeting the load. When the load is low, either the DG or the inverter in the stand-alone mode can supply it. However, during peak load, both the sources are operated in parallel mode. Due to this parallel operation, the capacity of the inverter and the DG can be reduced. The design principles of this system are relatively complicated to implement due to the parallel operation but it has several advantages including: • system load can be met in the most optimal way; • diesel fuel efficiency can be maximised due to higher average operating power;
• diesel maintenance can be minimised due to reduction of the run time; • reduction in the capacities of diesel, battery and renewable sources while meeting the load peaks. The rectifier and the inverter shown separately in Fig. 2 can be combined in a bi-directional inverter or converter. It can either share the load with the diesel or accept power from the diesel and operate as a battery charger. A controller is needed to supervise the operation of the system, selecting the most appropriate mode operation to supply a certain load without power interruption. The actual operation of the parallel hybrid system is presented in Fig. 3. The diesel generator is connected to the converter through a coupling inductor, XL . The diesel generator output voltage, VD , is maintained constant by an automatic excitation controller while the frequency of the voltage is maintained within limit by the diesel engine governor. The converter voltage, VC , is obtained by converting the battery voltage and power electronic switches (such as Insulated Gate Bipolar Transistor, IGBT). The IGBT devices are switched according to a Pulse Width Modulated signal. The voltage VD and VC are synchronised in parallel operation. The cost of a hybrid power system depends on two factors: the size of the individual component and the dispatch strategy. In order to predict the cost and the operation of systems, researchers have developed computer programs for simulation and analysis based on different preferences and approach. Manwell et al. (1996) presented Hybrid2, a tool machine that provides general configuration of hybrid power systems but does
Fig. 3. The equivalent circuit of a parallel hybrid system.
An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
not include the optimisation. Barley (1996) has presented a simple model for the systems including the optimisation of the dispatch strategy. This paper presents dispatch strategies for the operation of a solar photovoltaic–diesel–battery hybrid power system using ‘set points’. This includes determination of the optimum values of ‘set points’ for the starting and stopping of the diesel generator to minimise the overall system costs. A computer program for a typical dispatch strategy has been developed to predict the longterm energy performance and the lifecycle cost of the system. 2. DISPATCH STRATEGY
The dispatch strategy for a hybrid power system is a control algorithm for the interaction among various system components. The system dispatch controller should determine the starting or stopping the diesel generator, battery charger operation, and cutting-in or cutting-out the renewable energy sources. These operations are usually done on the bases of certain percentage of the system load or the battery state-of-charge (SOC)—usually known as set points. Determining the best value of these set points is the key to
Fig. 4. Block diagram of a hybrid system controller.
achieve an optimum operation. The dispatch strategy set points for a parallel hybrid system are summarised in Table 1. The hybrid controller can be devised to meet all or specific objectives outline in Table 1. Fig. 4 shows the block diagram of the controller. The controller monitors the real time input data for deciding the system operation. The input data is compared to the default values resulting in an appropriate output signal. When the input is lower or greater than the appropriate reference, the output will generate ‘high’ or ‘low.’ Using digital controller logics (OR operation or AND opera-
Table 1. Dispatch strategy set points No.
Dispatch parameters of individual component
(1)
Diesel starts: (1) At specific battery level that is determined by the state of charge (SOC) or battery terminal voltage. (2) At specific site load power which is measured as a percentage of the diesel generator or inverter rated capacity. (3) At specific renewable output power as a percentage of the peak power of the PV generator. (4) After a specific time period. Diesel generator operation: (1) At fixed, at the full power rating. (2) Diesel meeting the entire load and charging the battery if required. (3) Diesel meeting the base load with battery supplying the transient load. Diesel stops: (1) At specific battery level that is determined by the SOC or battery terminal voltage. (2) At specific site load power which is measured as a percentage of the diesel generator or inverter rated capacity. (3) At specific renewable output power as a percentage of the peak power of the PV generator. (4) At specific power transferred to battery. (5) At specific diesel operating power level. (6) After a specific time period. Inverter operation: (1) To meet the transient load. (2) To meet all or part of the load. Battery charger starts: (1) At the beginning of the diesel operating. (2) At specific battery level during the running of the diesel generator. (3) At specific diesel operating power level. (4) At specific PV output power. Battery charger power level: (1) Fixed at the full power rating of the charger. (2) Varying to meet the maximum battery charging rate. Battery charger stops: (1) At specific battery level. (2) At specific diesel operating power level. (3) At specific PV output power.
(2)
(3)
(4) (5)
(6) (7)
3
4
M. Ashari and C. V. Nayar
tion), the criteria of the system dispatch are formulated. As an example, the DG starting criteria of a PV–diesel–inverter–battery hybrid system is defined as follows (refer to Table 1): • diesel starts when the battery terminal voltage has reached 1.9 V/ cell; or • when the site load has exceeded 60% of the inverter rated capacity for 60 s. After one of the above criteria is satisfied, the diesel generator will be automatically started to meet the load and to charge the battery. In this example, the time limitation of 60 s is used to ensure that the site load is going to be steady and not a transient one (for example, starting of a motor load). A computer model has been developed for the system configuration shown in Fig. 5 involving the following dispatch parameters: 1. Diesel starts at specific battery terminal voltage or site load power. 2. Diesel operates to meet the load including the transient. 3. Diesel stops when the minimum power level has been reached. 4. Inverter operates to meet all or part of the load. 5. Battery charger starts at specific battery voltage or at the beginning of DG operation. 6. Battery charger operates varying to meet the maximum charging rate. 7. Battery charger stops when the battery terminal voltage has reached the max. level. The computer program involves a part of parameters shown in Table 1. Only several of the parameters are included because some of them may be suitable for a typical system. As an
example, a system that has a battery bank should consider the battery SOC for the DG starting set point, not the PV output power. The other reason for involving a part of the parameters is the similarity of the parameter’s value. For example, the value of a specific site load for stopping the DG can be set the same as the minimum operation level of the DG. This set point can also be used as the minimum loading of the DG recommended by the manufacturer. The stored energy level of the battery is measured from the terminal voltage. This technique is easier to implement than using the SOC method. A formula that has been proposed by Copetti and Chenlo (1994) is used in this model. The formula presents the lead acid battery voltage as function of the previous charge or discharge current. Table 2 presents the various set point values for the model (Prime Power Systems, 1995). Table 3 shows the battery usage strategies that depend on the diesel operating set points. When the starting and stopping of the DG are indicated as 0%, the battery is used to meet the transient load only, while the diesel meets the actual load. For the Medium Battery Usage, the battery is discharged at a power level up to 80% of peak load. When the battery is set for Maximum Usage, it meets the entire load including the peak. In this mode, the discharging of the battery is continued until the minimum battery terminal voltage for discharge is reached. The battery voltage to represent the SOC is based on the data supplied by the manufacturer. The diesel generator operating set points can be
Fig. 5. The operation cost of components.
An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
5
Table 2. Typical range of set points No.
Description
Parameter
Set point
Relative to
(1)
Starting diesel: - Site load . - Battery voltage ,
Pi max Vmin
0–100% 0–2.7 V/ cell
Inverter rated power Batt. terminal voltage
(2)
Stopping diesel
Pd min
0–100%
Diesel rated power
(3)
Diesel max. power level for parallel mode
Pd max
0–100%
Diesel rated power
Starting batt.charger: - Battery voltage ,
Vmin
when DG start, 0–2.7 V/ cell
Batt. terminal voltage
(4) (5)
Stopping batt charger: - Battery voltage .
Vmax
0–2.7 V/ cell
Batt. terminal voltage
(6)
Charger max. power
Pc max
0–100%
Inverter rated power
Table 3. Various values of diesel set points Dispatch description
Stopping diesel (Pd min )
Starting diesel (Pi max )
Parallel mode (Pd max )
Minimum batt. usage Low battery usage Medium battery usage Maximum batt. usage
0% 30% 30% 100%
0% 30% 80% 100%
100% 100% 100% 100%
given in various values depending on the users. These should be determined properly to obtain the optimum operation. 3. SYSTEM OPTIMISATION
Determination of the stopping and starting the diesel generator at a certain power level is presented in this section. An optimum condition, which is the optimum fuel consumption cost and the battery cost applied by the hybrid system, can be obtained using these set points. One approach proposed by Barley (1995) to determine the optimum value involves comparison of the DG operating and the battery costs. The operating cost of a DG is related to the fuel consumption, while the maintenance cost depends on the diesel run time and the load that is serving while in operation. DGs have typically a maximum fuel efficiency of about 3 kWh / l when run above 80% of the rated capacity. When DG is run at loads below 30% of its rating, the fuel efficiency becomes very low. Eq. 1 shows the fuel consumption cost of a general diesel generator which is in the maximum efficiency as proposed by Skarstein and Uhlen (1989). CDf 5 (Fi 1 F0 /Popr )c f
(1)
CDf 5 (0.246 1 0.08415 3 PR /Popr )c f
(2)
The cost of battery has two components: capital cost and replacement cost. The capital cost depends on the battery size, while the replacement cost depends on the system dispatch strategy. The system dispatch strategy determines how the battery will age. The battery wear cost is defined by the cost of the delivered energy from the battery as given in Eq. 3. The energy was previously stored from the PV or diesel generator. Cbw 5 Cb /EFCave
(3)
EFC 5 No. of cyclesu DOD 3 DOD
(4)
where EFC is the Equivalent Full Cycles, DOD is Depth of Discharge and No. of cyclesu DOD is the number of charge–discharge cycles at the given DOD. An example of a typical lead acid battery is given in Table 4. Fig. 5 depicts the diesel generator fuel con-
Table 4. Battery equivalent full cycles (EFC) No. cycles
DOD
EFC
5200 2800 1000 700
310% 320% 350% 380% EFCave
5520 5560 5500 5560 535
6
M. Ashari and C. V. Nayar
sumption cost and the battery wear cost for the fuel price as A$1 per litre and the battery purchased cost as A$270 / kWh (A$1.005US$0.75). The optimum diesel stopping equals to the value of Ld , the critical DG load for discharging the battery. This is the intersection point between diesel fuel cost (CDf ) and battery wear cost (Cbw ): CDf 5 Cbw
(5)
Ld 5 Popr /PR 5 0.08415 / [(Cbw /c f ) 2 0.246]
(6)
From Fig. 5, when the diesel operating power to meet the load is lower than Ld , it is more economical to discharge the batteries rather than to operate the diesel. Therefore, below this point the diesel should be turn off and the battery, through inverter, supplies the load. When the battery price is extremely high relative to the cost of fuel, the Ld approaches zero. However, when the battery price is extremely low, Ld approaches infinity. For the example shown in Fig. 5, Ld is approximately 32% of diesel rated capacity. The determination of the diesel starting set point is more complex, because it has to consider the following: • the critical DG load for discharging the battery; • the cost of energy for charging the battery; • the battery usage dispatch strategy; • the load profile. In the hybrid system, the energy for charging can be obtained from either the diesel or the PV array. The charging cost from the diesel generator is presented in Eq. 7 involving the fuel price (c f ),
the efficiency of the battery (hb ) and the battery charger (hc ). CcD 5 0.246 3 c f /(hbhc )
(7)
The cost for charging the battery from the PV can be computed using Eq. 8. CcPV 5 (CPV 1 CR,PV ) /WPV
(8)
The cycled-energy that the energy is achieved from the source, is stored in the battery and then is delivered to the load is the sum of the charging cost and the battery wear cost. Fig. 6 shows a typical fuel consumption cost of the DG, the cycled-energy cost achieved from the DG and the PV, respectively. The PV generator in this example is rated at 2.7 kWp costing A$9 / Wp and the DG is rated at 10 kW. At this moment, charging the battery using the diesel generator is more economical than using the PV. Therefore, it is more economical if the energy from PV can be used directly by the load rather than for charging the battery. This is not an easy task for the controller, because the instantaneous demand with respect to the PV power determines where the PV energy will go. 4. RESULTS AND DISCUSSION
To demonstrate the effectiveness of the optimisation strategy, three types of dispatch strategies are considered: • Ideal Dispatch, • Maximum Battery Usage, and • Optimum Dispatch.
Fig. 6. Cycled-energy cost from the DG and the PV.
An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
The first dispatch represents the theoretical strategy that should achieved the lowest cost. The second dispatch is a strategy that is mostly applied in the hybrid system operation. Both of these strategies will be compared to the optimum dispatch strategy. The Ideal Dispatch strategy is achieved by scanning the set points based on the exact future data, resulting in the lowest system cost. This is a theoretical strategy, but it is possible to be implemented using an advanced adaptive control strategy. In the Maximum Battery Usage as defined in the previous section, the battery meets the entire load including the peak period. The discharging of the battery is continued until reaching the minimum defined battery voltage. The set point for the Optimum Dispatch Strategy is obtained as given in Eq. 6, the fuel price is given as A$1 per litre and the battery wear cost as A$0.505 per kWh. The diesel starting set point is selected as 50% of the DG rated power. This considers the average ‘low load period’ of the load profile compared to the DG rated (the load profile is given in Fig. 9). The set point values for the system example are given in Table 5. The results of the cost for three types of dispatch strategy is shown in Fig. 7, the quantity is given as follows: Cop 5 CT,Df 1 CS,Bat 1 CR,Bat
(9)
Fig. 8. Energy cost.
Figs. 7 and 8 show that Maximum Battery Usage has the highest cost, while the Optimum Dispatch can provide a lower cost. The lowest cost is presented by the Ideal Dispatch Strategy. A closer gap between the Optimum and the Ideal dispatch curves should be achieved for a more accurate calculation, e.g. using a smoother curve of battery cycles, using a more accurate battery model etc. The load profile for this simulation is given by Fig. 9. Here, the diesel is sized the same as the inverter at 10 kW, the battery bank has 57 kW h and the project lifetime is 20 years. The computer program has been validated using data from a typical Australian remote area
Table 5. Three dispatch strategies for analysis No.
Dispatch parameter description
Ideal dispatch
Max. batt usage
Optimum dispatch
1
Diesel starts: Pi max Vmin Diesel stops: Pd min Diesel max level: Pd max Charger stops: Vmax
Scanning 1.9 V/ cell Scanning 100% 2.3 V/ cell
100% 1.9 V/ cell 100% 100% 2.3 V/ cell
50% 1.9 V/ cell Ld 532% 100% 2.3 V/ cell
2 3 4
7
Fig. 7. System operation cost consisting of the fuel and batteries cost.
8
M. Ashari and C. V. Nayar
Fig. 9. Load profile with 120 kW h / day.
power system, Epenarra. This is a diesel–battery hybrid system located in Northern Territory. Fig. 10 presents the operation of the diesel and inverter compared to the simulation output. The figure shows that the operation of the system provided by the computer program agrees with the field system. The original data that is given in every 15 min has been converted into hourly data for the simulation. A sensitivity analysis has been performed for the effectiveness of the optimisation under different fuel price or load demand. The costs when the fuel price increases up to 100% and the load demand grows up to 50% are presented by Figs. 11 and 12. These show that the Optimum Dispatch still follow the Ideal Strategy although the fuel price or the load demand increased. The energy cost becomes relatively expensive when the fuel price is increased, but it rapidly decreases when a larger PV array is incorporated in the system. The energy cost for the system when
Fig. 11. Fuel price given as 200%.
Fig. 12. Load demand given as 150%.
Fig. 10. Diesel and inverter operation.
An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
using a diesel generator stand-alone is found as $1.16 per kWh. 5. CONCLUSION
Modelling of hybrid power systems including the optimisation of the dispatch strategy has been presented. The fuel consumption is one of the main component of the entire operation cost of a diesel generator over its lifetime. Therefore, determining the best time for starting and stopping the DG with respect to the load to its capacity is a crucial factor for optimisation. The inverter including the battery and the controller has also a big role in determining the system cost, because the inverter should supply the load when the DG is shut off. Operating the inverter continuously in the permissible range of the battery State of Charge without considering the load demand requires the highest cost as resumed in the Maximum Battery Usage strategy. However, operating the inverter in a strategy which considers the battery wear cost, the diesel fuel consumption cost and the load profile provides a lower system operation cost. NOMENCLATURE d u hb hc Cb Cbw CcD CcPV CDf cf Cop CPV CR, Bat CR,
PV
Phase angle between the diesel generator and the inverter voltage Phase angle between the load voltage and the load current Energy efficiency of the battery Converter efficiency Battery purchased cost Battery wear cost Charging cost from the diesel generator Charging cost from the PV Fuel consumption cost of the diesel generator Fuel price of the diesel generator Operation cost of the system over the project lifetime in the Present Value Installed cost of the PV Total replacement cost of the battery in the Present Value Replacement cost of PV over the project lifetime in the Present Value
CS, Bat CT, Df DG DOD EFCave F0 Fi INV Ld Pc max Pd max Pd min Pi max Popr PR SOC VC VD Vmax Vmin WPV XL
9
Capital cost of the battery in the Present Value Total fuel consumption cost of the DG in the Present Value Diesel generator Depth of discharge Average value of the Equivalent Full Cycles (EFC) Fuel consumption at no load of the diesel generator Incremental fuel consumption of the diesel generator Bi-directional inverter Critical load for discharging the battery Maximum operating power of the battery charger Maximum operating power of the diesel generator Minimum operating power of the diesel generator Maximum operating power of the inverter Diesel generator operating power Rated power State of charge Converter output voltage Diesel generator output voltage Maximum battery terminal voltage for charging Minimum battery terminal voltage for discharging Estimated output energy generated by the PV Coupling inductor
Acknowledgements—The authors would like to thank Advance Energy Systems Pty. Ltd. for the support for this work.
REFERENCES Barley C. D. (1996) Hybrid Power Design Handbook ( With Software), Fort Collins, CO, July 9. Barley C. D. (1995) Optimal control of remote hybrid power systems: Design guide lines. Proc. of ISES in Search of the Sun Conf., Sept 11–15, Harare, Zimbabwe. In press. Copetti J. B. and Chenlo F. (1994) Lead / acid batteries for photovoltaic applications test results and modelling. Journal of Power Sources 46, 119–127. Manwell J. F, Rogers A., Hayman G., Avelar C. T. and McGowan J. G. (1996) Draft Theory Manual for Hybrid2 the Hybrid System Simulation Model. University of Massachusetts, and National Renewable Energy Laboratory, CO. Nayar C. V., Phillips S. J., James W. L., Prior T. L. and Remmer D. (1993) Novel wind / diesel / battery hybrid energy system. Solar Energy 51(1), 65–78. Prime Power Systems Ltd. (1995) Single Phase Static Power Pack Product, Rev. 3.0, User’ s Manual Document Rev. 1.0, Prime Power System Ltd. Skarstein O. and Uhlen K. (1989) Design considerations with respect to long-term diesel saving in wind / diesel plants. Wind Engineering 13, 72–87.
PII: S0038 – 092X( 99 )00016 – X
Solar Energy Vol. 66, No. 1, pp. 1–9, 1999 1999 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0038-092X / 99 / $ - see front matter
www.elsevier.com / locate / solener
AN OPTIMUM DISPATCH STRATEGY USING SET POINTS FOR A PHOTOVOLTAIC (PV)–DIESEL–BATTERY HYBRID POWER SYSTEM M. ASHARI † and C. V. NAYAR Centre for Renewable Energy Systems Technology Australia (CRESTA), Curtin University of Technology, Perth, Western Australia 6001, Australia Received 14 February 1997; revised version accepted 9 February 1999 Communicated by ROBERT HILL
Abstract—This paper presents dispatch strategies for the operation of a solar photovoltaic (PV)–diesel– battery hybrid power system using ‘set points’. This includes determination of the optimum values of set points for the starting and stopping of the diesel generator to minimise the overall system costs. A computer program for a typical dispatch strategy has been developed to predict the long-term energy performance and the lifecycle cost of the system. 1999 Elsevier Science Ltd. All rights reserved.
delivered to the battery can be controlled either by controlling the excitation of the alternator or by incorporating a charger regulator in the renewable energy source. In this scheme, the inverter at the desired voltage and frequency can condition the load. This scheme also supplies the load without any interruption of power in the event of changing the DG or the PV generator to charge the battery bank. The design principles of this system are relatively simple to implement, but it has the following disadvantages: • low overall system efficiency due to the series configuration of system elements; a certain amount of the energy will be lost due to the battery and the inverter efficiencies; • a large inverter size should be used such that the capacity is substantially larger than the maximum peak-load demand. • when the renewable energy sources are incorporated in the system, it leads to a limited control of the diesel alternator.
1. INTRODUCTION
Hybrid power systems integrate renewable energy technologies with diesel generators (DGs), inverters (INVs) and batteries to provide grid quality electrical power. They are becoming the most commonly used systems for village electrification due to the environmental and economic advantages. Many countries in the world such as Australia, Canada, China, Indonesia, Malaysia, Mexico, Russia have currently installed hybrid systems. From the application point of view, hybrid power systems can be classified into two topologies: series and parallel (Nayar et al., 1993).
1.1. Series topology In the series hybrid system, shown schematically in Fig. 1, the diesel generator and the renewable energy source are used to charge a battery bank. The diesel generator is connected in series with the inverter to supply the load. So the diesel generator can not supply the load directly. The inverter converts the power from the battery bank to AC at mains voltage and frequency and subsequently supplies the load. The battery bank and the inverter capacity should be enough to cover the peak load demand, while the DG capacity should be adequate for meeting the peak load and charging the battery simultaneously. The power
†
Author to whom correspondence should be addressed. Tel.: 11-61-8-9266 3157; fax: 11-61-8-9266-3107.
Fig. 1. Series topology of hybrid system. 1
2
M. Ashari and C. V. Nayar
Fig. 2. Parallel topology of hybrid system.
1.2. Parallel topology The block diagram of parallel hybrid system is shown in Fig. 2. This topology has superior performance over the series hybrid system. In this scheme the renewable energy sources and the DGs supply a portion of the load demand directly, resulting in higher overall system efficiency. The diesel generator and the inverter can operate in stand-alone or parallel mode. This offers some combinations of the source for meeting the load. When the load is low, either the DG or the inverter in the stand-alone mode can supply it. However, during peak load, both the sources are operated in parallel mode. Due to this parallel operation, the capacity of the inverter and the DG can be reduced. The design principles of this system are relatively complicated to implement due to the parallel operation but it has several advantages including: • system load can be met in the most optimal way; • diesel fuel efficiency can be maximised due to higher average operating power;
• diesel maintenance can be minimised due to reduction of the run time; • reduction in the capacities of diesel, battery and renewable sources while meeting the load peaks. The rectifier and the inverter shown separately in Fig. 2 can be combined in a bi-directional inverter or converter. It can either share the load with the diesel or accept power from the diesel and operate as a battery charger. A controller is needed to supervise the operation of the system, selecting the most appropriate mode operation to supply a certain load without power interruption. The actual operation of the parallel hybrid system is presented in Fig. 3. The diesel generator is connected to the converter through a coupling inductor, XL . The diesel generator output voltage, VD , is maintained constant by an automatic excitation controller while the frequency of the voltage is maintained within limit by the diesel engine governor. The converter voltage, VC , is obtained by converting the battery voltage and power electronic switches (such as Insulated Gate Bipolar Transistor, IGBT). The IGBT devices are switched according to a Pulse Width Modulated signal. The voltage VD and VC are synchronised in parallel operation. The cost of a hybrid power system depends on two factors: the size of the individual component and the dispatch strategy. In order to predict the cost and the operation of systems, researchers have developed computer programs for simulation and analysis based on different preferences and approach. Manwell et al. (1996) presented Hybrid2, a tool machine that provides general configuration of hybrid power systems but does
Fig. 3. The equivalent circuit of a parallel hybrid system.
An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
not include the optimisation. Barley (1996) has presented a simple model for the systems including the optimisation of the dispatch strategy. This paper presents dispatch strategies for the operation of a solar photovoltaic–diesel–battery hybrid power system using ‘set points’. This includes determination of the optimum values of ‘set points’ for the starting and stopping of the diesel generator to minimise the overall system costs. A computer program for a typical dispatch strategy has been developed to predict the longterm energy performance and the lifecycle cost of the system. 2. DISPATCH STRATEGY
The dispatch strategy for a hybrid power system is a control algorithm for the interaction among various system components. The system dispatch controller should determine the starting or stopping the diesel generator, battery charger operation, and cutting-in or cutting-out the renewable energy sources. These operations are usually done on the bases of certain percentage of the system load or the battery state-of-charge (SOC)—usually known as set points. Determining the best value of these set points is the key to
Fig. 4. Block diagram of a hybrid system controller.
achieve an optimum operation. The dispatch strategy set points for a parallel hybrid system are summarised in Table 1. The hybrid controller can be devised to meet all or specific objectives outline in Table 1. Fig. 4 shows the block diagram of the controller. The controller monitors the real time input data for deciding the system operation. The input data is compared to the default values resulting in an appropriate output signal. When the input is lower or greater than the appropriate reference, the output will generate ‘high’ or ‘low.’ Using digital controller logics (OR operation or AND opera-
Table 1. Dispatch strategy set points No.
Dispatch parameters of individual component
(1)
Diesel starts: (1) At specific battery level that is determined by the state of charge (SOC) or battery terminal voltage. (2) At specific site load power which is measured as a percentage of the diesel generator or inverter rated capacity. (3) At specific renewable output power as a percentage of the peak power of the PV generator. (4) After a specific time period. Diesel generator operation: (1) At fixed, at the full power rating. (2) Diesel meeting the entire load and charging the battery if required. (3) Diesel meeting the base load with battery supplying the transient load. Diesel stops: (1) At specific battery level that is determined by the SOC or battery terminal voltage. (2) At specific site load power which is measured as a percentage of the diesel generator or inverter rated capacity. (3) At specific renewable output power as a percentage of the peak power of the PV generator. (4) At specific power transferred to battery. (5) At specific diesel operating power level. (6) After a specific time period. Inverter operation: (1) To meet the transient load. (2) To meet all or part of the load. Battery charger starts: (1) At the beginning of the diesel operating. (2) At specific battery level during the running of the diesel generator. (3) At specific diesel operating power level. (4) At specific PV output power. Battery charger power level: (1) Fixed at the full power rating of the charger. (2) Varying to meet the maximum battery charging rate. Battery charger stops: (1) At specific battery level. (2) At specific diesel operating power level. (3) At specific PV output power.
(2)
(3)
(4) (5)
(6) (7)
3
4
M. Ashari and C. V. Nayar
tion), the criteria of the system dispatch are formulated. As an example, the DG starting criteria of a PV–diesel–inverter–battery hybrid system is defined as follows (refer to Table 1): • diesel starts when the battery terminal voltage has reached 1.9 V/ cell; or • when the site load has exceeded 60% of the inverter rated capacity for 60 s. After one of the above criteria is satisfied, the diesel generator will be automatically started to meet the load and to charge the battery. In this example, the time limitation of 60 s is used to ensure that the site load is going to be steady and not a transient one (for example, starting of a motor load). A computer model has been developed for the system configuration shown in Fig. 5 involving the following dispatch parameters: 1. Diesel starts at specific battery terminal voltage or site load power. 2. Diesel operates to meet the load including the transient. 3. Diesel stops when the minimum power level has been reached. 4. Inverter operates to meet all or part of the load. 5. Battery charger starts at specific battery voltage or at the beginning of DG operation. 6. Battery charger operates varying to meet the maximum charging rate. 7. Battery charger stops when the battery terminal voltage has reached the max. level. The computer program involves a part of parameters shown in Table 1. Only several of the parameters are included because some of them may be suitable for a typical system. As an
example, a system that has a battery bank should consider the battery SOC for the DG starting set point, not the PV output power. The other reason for involving a part of the parameters is the similarity of the parameter’s value. For example, the value of a specific site load for stopping the DG can be set the same as the minimum operation level of the DG. This set point can also be used as the minimum loading of the DG recommended by the manufacturer. The stored energy level of the battery is measured from the terminal voltage. This technique is easier to implement than using the SOC method. A formula that has been proposed by Copetti and Chenlo (1994) is used in this model. The formula presents the lead acid battery voltage as function of the previous charge or discharge current. Table 2 presents the various set point values for the model (Prime Power Systems, 1995). Table 3 shows the battery usage strategies that depend on the diesel operating set points. When the starting and stopping of the DG are indicated as 0%, the battery is used to meet the transient load only, while the diesel meets the actual load. For the Medium Battery Usage, the battery is discharged at a power level up to 80% of peak load. When the battery is set for Maximum Usage, it meets the entire load including the peak. In this mode, the discharging of the battery is continued until the minimum battery terminal voltage for discharge is reached. The battery voltage to represent the SOC is based on the data supplied by the manufacturer. The diesel generator operating set points can be
Fig. 5. The operation cost of components.
An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
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Table 2. Typical range of set points No.
Description
Parameter
Set point
Relative to
(1)
Starting diesel: - Site load . - Battery voltage ,
Pi max Vmin
0–100% 0–2.7 V/ cell
Inverter rated power Batt. terminal voltage
(2)
Stopping diesel
Pd min
0–100%
Diesel rated power
(3)
Diesel max. power level for parallel mode
Pd max
0–100%
Diesel rated power
Starting batt.charger: - Battery voltage ,
Vmin
when DG start, 0–2.7 V/ cell
Batt. terminal voltage
(4) (5)
Stopping batt charger: - Battery voltage .
Vmax
0–2.7 V/ cell
Batt. terminal voltage
(6)
Charger max. power
Pc max
0–100%
Inverter rated power
Table 3. Various values of diesel set points Dispatch description
Stopping diesel (Pd min )
Starting diesel (Pi max )
Parallel mode (Pd max )
Minimum batt. usage Low battery usage Medium battery usage Maximum batt. usage
0% 30% 30% 100%
0% 30% 80% 100%
100% 100% 100% 100%
given in various values depending on the users. These should be determined properly to obtain the optimum operation. 3. SYSTEM OPTIMISATION
Determination of the stopping and starting the diesel generator at a certain power level is presented in this section. An optimum condition, which is the optimum fuel consumption cost and the battery cost applied by the hybrid system, can be obtained using these set points. One approach proposed by Barley (1995) to determine the optimum value involves comparison of the DG operating and the battery costs. The operating cost of a DG is related to the fuel consumption, while the maintenance cost depends on the diesel run time and the load that is serving while in operation. DGs have typically a maximum fuel efficiency of about 3 kWh / l when run above 80% of the rated capacity. When DG is run at loads below 30% of its rating, the fuel efficiency becomes very low. Eq. 1 shows the fuel consumption cost of a general diesel generator which is in the maximum efficiency as proposed by Skarstein and Uhlen (1989). CDf 5 (Fi 1 F0 /Popr )c f
(1)
CDf 5 (0.246 1 0.08415 3 PR /Popr )c f
(2)
The cost of battery has two components: capital cost and replacement cost. The capital cost depends on the battery size, while the replacement cost depends on the system dispatch strategy. The system dispatch strategy determines how the battery will age. The battery wear cost is defined by the cost of the delivered energy from the battery as given in Eq. 3. The energy was previously stored from the PV or diesel generator. Cbw 5 Cb /EFCave
(3)
EFC 5 No. of cyclesu DOD 3 DOD
(4)
where EFC is the Equivalent Full Cycles, DOD is Depth of Discharge and No. of cyclesu DOD is the number of charge–discharge cycles at the given DOD. An example of a typical lead acid battery is given in Table 4. Fig. 5 depicts the diesel generator fuel con-
Table 4. Battery equivalent full cycles (EFC) No. cycles
DOD
EFC
5200 2800 1000 700
310% 320% 350% 380% EFCave
5520 5560 5500 5560 535
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M. Ashari and C. V. Nayar
sumption cost and the battery wear cost for the fuel price as A$1 per litre and the battery purchased cost as A$270 / kWh (A$1.005US$0.75). The optimum diesel stopping equals to the value of Ld , the critical DG load for discharging the battery. This is the intersection point between diesel fuel cost (CDf ) and battery wear cost (Cbw ): CDf 5 Cbw
(5)
Ld 5 Popr /PR 5 0.08415 / [(Cbw /c f ) 2 0.246]
(6)
From Fig. 5, when the diesel operating power to meet the load is lower than Ld , it is more economical to discharge the batteries rather than to operate the diesel. Therefore, below this point the diesel should be turn off and the battery, through inverter, supplies the load. When the battery price is extremely high relative to the cost of fuel, the Ld approaches zero. However, when the battery price is extremely low, Ld approaches infinity. For the example shown in Fig. 5, Ld is approximately 32% of diesel rated capacity. The determination of the diesel starting set point is more complex, because it has to consider the following: • the critical DG load for discharging the battery; • the cost of energy for charging the battery; • the battery usage dispatch strategy; • the load profile. In the hybrid system, the energy for charging can be obtained from either the diesel or the PV array. The charging cost from the diesel generator is presented in Eq. 7 involving the fuel price (c f ),
the efficiency of the battery (hb ) and the battery charger (hc ). CcD 5 0.246 3 c f /(hbhc )
(7)
The cost for charging the battery from the PV can be computed using Eq. 8. CcPV 5 (CPV 1 CR,PV ) /WPV
(8)
The cycled-energy that the energy is achieved from the source, is stored in the battery and then is delivered to the load is the sum of the charging cost and the battery wear cost. Fig. 6 shows a typical fuel consumption cost of the DG, the cycled-energy cost achieved from the DG and the PV, respectively. The PV generator in this example is rated at 2.7 kWp costing A$9 / Wp and the DG is rated at 10 kW. At this moment, charging the battery using the diesel generator is more economical than using the PV. Therefore, it is more economical if the energy from PV can be used directly by the load rather than for charging the battery. This is not an easy task for the controller, because the instantaneous demand with respect to the PV power determines where the PV energy will go. 4. RESULTS AND DISCUSSION
To demonstrate the effectiveness of the optimisation strategy, three types of dispatch strategies are considered: • Ideal Dispatch, • Maximum Battery Usage, and • Optimum Dispatch.
Fig. 6. Cycled-energy cost from the DG and the PV.
An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
The first dispatch represents the theoretical strategy that should achieved the lowest cost. The second dispatch is a strategy that is mostly applied in the hybrid system operation. Both of these strategies will be compared to the optimum dispatch strategy. The Ideal Dispatch strategy is achieved by scanning the set points based on the exact future data, resulting in the lowest system cost. This is a theoretical strategy, but it is possible to be implemented using an advanced adaptive control strategy. In the Maximum Battery Usage as defined in the previous section, the battery meets the entire load including the peak period. The discharging of the battery is continued until reaching the minimum defined battery voltage. The set point for the Optimum Dispatch Strategy is obtained as given in Eq. 6, the fuel price is given as A$1 per litre and the battery wear cost as A$0.505 per kWh. The diesel starting set point is selected as 50% of the DG rated power. This considers the average ‘low load period’ of the load profile compared to the DG rated (the load profile is given in Fig. 9). The set point values for the system example are given in Table 5. The results of the cost for three types of dispatch strategy is shown in Fig. 7, the quantity is given as follows: Cop 5 CT,Df 1 CS,Bat 1 CR,Bat
(9)
Fig. 8. Energy cost.
Figs. 7 and 8 show that Maximum Battery Usage has the highest cost, while the Optimum Dispatch can provide a lower cost. The lowest cost is presented by the Ideal Dispatch Strategy. A closer gap between the Optimum and the Ideal dispatch curves should be achieved for a more accurate calculation, e.g. using a smoother curve of battery cycles, using a more accurate battery model etc. The load profile for this simulation is given by Fig. 9. Here, the diesel is sized the same as the inverter at 10 kW, the battery bank has 57 kW h and the project lifetime is 20 years. The computer program has been validated using data from a typical Australian remote area
Table 5. Three dispatch strategies for analysis No.
Dispatch parameter description
Ideal dispatch
Max. batt usage
Optimum dispatch
1
Diesel starts: Pi max Vmin Diesel stops: Pd min Diesel max level: Pd max Charger stops: Vmax
Scanning 1.9 V/ cell Scanning 100% 2.3 V/ cell
100% 1.9 V/ cell 100% 100% 2.3 V/ cell
50% 1.9 V/ cell Ld 532% 100% 2.3 V/ cell
2 3 4
7
Fig. 7. System operation cost consisting of the fuel and batteries cost.
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M. Ashari and C. V. Nayar
Fig. 9. Load profile with 120 kW h / day.
power system, Epenarra. This is a diesel–battery hybrid system located in Northern Territory. Fig. 10 presents the operation of the diesel and inverter compared to the simulation output. The figure shows that the operation of the system provided by the computer program agrees with the field system. The original data that is given in every 15 min has been converted into hourly data for the simulation. A sensitivity analysis has been performed for the effectiveness of the optimisation under different fuel price or load demand. The costs when the fuel price increases up to 100% and the load demand grows up to 50% are presented by Figs. 11 and 12. These show that the Optimum Dispatch still follow the Ideal Strategy although the fuel price or the load demand increased. The energy cost becomes relatively expensive when the fuel price is increased, but it rapidly decreases when a larger PV array is incorporated in the system. The energy cost for the system when
Fig. 11. Fuel price given as 200%.
Fig. 12. Load demand given as 150%.
Fig. 10. Diesel and inverter operation.
An optimum dispatch strategy using set points for a photovoltaic (PV)–diesel–battery hybrid power system
using a diesel generator stand-alone is found as $1.16 per kWh. 5. CONCLUSION
Modelling of hybrid power systems including the optimisation of the dispatch strategy has been presented. The fuel consumption is one of the main component of the entire operation cost of a diesel generator over its lifetime. Therefore, determining the best time for starting and stopping the DG with respect to the load to its capacity is a crucial factor for optimisation. The inverter including the battery and the controller has also a big role in determining the system cost, because the inverter should supply the load when the DG is shut off. Operating the inverter continuously in the permissible range of the battery State of Charge without considering the load demand requires the highest cost as resumed in the Maximum Battery Usage strategy. However, operating the inverter in a strategy which considers the battery wear cost, the diesel fuel consumption cost and the load profile provides a lower system operation cost. NOMENCLATURE d u hb hc Cb Cbw CcD CcPV CDf cf Cop CPV CR, Bat CR,
PV
Phase angle between the diesel generator and the inverter voltage Phase angle between the load voltage and the load current Energy efficiency of the battery Converter efficiency Battery purchased cost Battery wear cost Charging cost from the diesel generator Charging cost from the PV Fuel consumption cost of the diesel generator Fuel price of the diesel generator Operation cost of the system over the project lifetime in the Present Value Installed cost of the PV Total replacement cost of the battery in the Present Value Replacement cost of PV over the project lifetime in the Present Value
CS, Bat CT, Df DG DOD EFCave F0 Fi INV Ld Pc max Pd max Pd min Pi max Popr PR SOC VC VD Vmax Vmin WPV XL
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Capital cost of the battery in the Present Value Total fuel consumption cost of the DG in the Present Value Diesel generator Depth of discharge Average value of the Equivalent Full Cycles (EFC) Fuel consumption at no load of the diesel generator Incremental fuel consumption of the diesel generator Bi-directional inverter Critical load for discharging the battery Maximum operating power of the battery charger Maximum operating power of the diesel generator Minimum operating power of the diesel generator Maximum operating power of the inverter Diesel generator operating power Rated power State of charge Converter output voltage Diesel generator output voltage Maximum battery terminal voltage for charging Minimum battery terminal voltage for discharging Estimated output energy generated by the PV Coupling inductor
Acknowledgements—The authors would like to thank Advance Energy Systems Pty. Ltd. for the support for this work.
REFERENCES Barley C. D. (1996) Hybrid Power Design Handbook ( With Software), Fort Collins, CO, July 9. Barley C. D. (1995) Optimal control of remote hybrid power systems: Design guide lines. Proc. of ISES in Search of the Sun Conf., Sept 11–15, Harare, Zimbabwe. In press. Copetti J. B. and Chenlo F. (1994) Lead / acid batteries for photovoltaic applications test results and modelling. Journal of Power Sources 46, 119–127. Manwell J. F, Rogers A., Hayman G., Avelar C. T. and McGowan J. G. (1996) Draft Theory Manual for Hybrid2 the Hybrid System Simulation Model. University of Massachusetts, and National Renewable Energy Laboratory, CO. Nayar C. V., Phillips S. J., James W. L., Prior T. L. and Remmer D. (1993) Novel wind / diesel / battery hybrid energy system. Solar Energy 51(1), 65–78. Prime Power Systems Ltd. (1995) Single Phase Static Power Pack Product, Rev. 3.0, User’ s Manual Document Rev. 1.0, Prime Power System Ltd. Skarstein O. and Uhlen K. (1989) Design considerations with respect to long-term diesel saving in wind / diesel plants. Wind Engineering 13, 72–87.