SYSTEMS

Technical Field

[0001] The present invention relates to methods for determining a control strategy of a system, methods for operating a generator, control circuits and systems.

Background

[0002] External electrical power supply may be disrupted, and it may be desired to provide a power back up system. Furthermore, for off-grid locations, it may be desired to provide a system to generate electricity from fuel. However, running a such a system may be expensive in terms of operational cost and in terms of wear of the system. Thus, a cost efficient system may be desired.

Summary

[0003] According to the present invention, a method for determining a control strategy of a system as claimed in claim 1 is provided. A method for operating a generator according to the invention is defined in claim 5. A control circuit according to the invention is described in claim 9. A system according to various embodiments is described in claim 10. The dependent claims define some examples.

Brief Description of the Drawings

[0004] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments are described with reference to the following drawings, in which:

Fig. 1 shows an illustration of a typical diesel generator cost curve;

Fig. 2 shows an illustration of the hybrid operation strategy;

Fig. 3 shows a flow diagram illustrating the commonly used control strategy;

Fig. 4 shows a diagram illustrating a method for determining a control strategy of a system according to various embodiments;

Fig. 5 shows a flow diagram illustrating a control strategy for generator operation according to various embodiments;

Fig. 6 shows an illustration of relevant factors for the optimal power according to various embodiments;

Fig. 7 shows an illustration of an example of the nomination of power fractions;

Fig. 8 shows an illustration of optimal diesel generator power output calculation for one load demand point;

Fig. 9 shows an illustration of a diesel operating point based on the approach according to various embodiments; and

Fig. 10 shows an illustration of the system topology according to various embodiments.

Description

[0005] Various embodiments provide devices and methods which may improve the control strategy for running an off grid or power back up system where diesel is the only primary source of energy. There are two different commonly used systems: The diesel only system and the conventional diesel-battery hybrid system.

[0006] In the following, diesel only systems will be described. For the majority of off-grid and power back up sites, diesel generators are the only source of energy. [0007] In conventional diesel systems the generator is running continuously. But most of the time the generator is just powering the load, running at anywhere from 30% to 50% of maximum load. This may mean that the operational costs are high, and may include diesel fuel cost (which may be high because the diesel generator runs at low efficiency), high cost for refuelling (which is usually needed every one or two weeks) at a very remote site, costs for generator maintenance (which may be proportional to the hours of operation), and costs for replacement of the generator (which in continuous duty may only last two years).

[0008] Fig. 1 shows an illustration 100 of a typical diesel generator cost curve 106, indicating the cost per generated kWh by the diesel generator. The x-axis 102 shows the power output of the diesel generator in percentage to the maximum power output referred as 1. The y- axis 104 shows the cost in Euro per kWh. It can be noticed that the lowest energy cost arises at the maximum power output.

[0009] In Fig. 1 it can be noticed that the lowest energy cost arises at the maximum power output. By running the diesel generator on maximum power output and additionally integrating batteries into the system, diesel consumption can be reduced. This is called "hybrid operation", where the generator only runs part of the time and the battery supplies to the load while the generator is off. In hybrid operation the generator mostly runs to charge the batteries.

[0010] In the following, a commonly used hybrid diesel -battery system (which herein may be referred to as "commonly used system") will be described. By changing the way the generator is controlled, an operator can achieve savings in running costs. In hybrid operation, the generator is always running at close to full load. In hybrid operation, this may be achieved by the generator mostly operating to charge the batteries. Once the batteries are charged, the generator may stop.

[0011] Fig. 2 shows an illustration 200 of the hybrid operation strategy where the generator only runs part of the time (and battery runs the load when generator is off). Time is assumed to run from left to right in Fig. 2, and a cycle time is indicated by 202. For example the cycle time may be a 24 h cycle, or may be determined by the battery state of charge (SOC). In the upper portion of Fig. 2, the SOC 214 is shown. In the lower portion of Fig. 2, the generator run time

216 is shown. The operating sequence may be as follows:

[0012] - The system starts with a fully charged battery (204).

[0013] - The generator is turned off (206) and the battery takes the load.

[0014] - Once the battery has discharged to a pre-determined level (208), the generator starts on full power again (210) and quickly recharges the battery.

[0015] - The generator is turned off again (212) and the cycle repeats.

[0016] Fig. 3 shows a flow diagram 300 illustrating the commonly used control strategy.

Each control cycle may start in 316.

[0017] The following definitions are used in the description of the control chart:

[0018] SOC _mi „: battery dependent minimum required state of charge; e.g. end of discharging;

[0019] SOCtarget- planned maximum state of charge; e.g. end of charging;

[0020] V _EOC '- end of charge voltage as the maximum allowed voltage during charging; and

[0021] Pcm _aX : maximum allowed battery charge power.

[0022] In 302, it may be determined whether the generator is on. If the generator is on, processing may proceed in 306. If the generator is not on, processing may proceed in 304. In 304, it may be determined whether the state of charge (SOC) of the battery is equal to or larger than SOC _mi „. If SOC>= SOC _mi „, processing may proceed in 310. If SOC<SOC _m ;„, processing may proceed in 312. In 306, it may be determined whether SOC>= SOC _to¾e . If SOC>= SOCtarget, processing may proceed in 310. If SOC<SOC, _argei , processing may proceed in 308. In 308, it may be determined whether the battery voltage (V) is smaller than _E oc- I V^ _EOC , processing may proceed in 312. If V>=V _EOC , processing may proceed in 314. In 310, the generator is off; and the battery supplies load, if there is load. In 312, the generator is on full power; the generator supplies load, if there is load; and the battery is charged with the surplus power up to the maximum allowed battery charge power P max- In 314, the generator power is controlled; the generator supplies load, if there is load; the battery is charged in constant voltage mode. Each control cycle may end in 318.

[0023] The commonly used systems optimize the diesel operation based on its efficiency- curve. The losses in battery and converters are neglected.

[0024] According to various embodiments, control strategy may be provided which lead to a joint optimum of the diesel efficiency curve together with the battery and conversion losses.

[0025] Considering also these losses, according to various embodiments, a diesel operation strategy may be provided, which leads to overall costs savings.

[0026] The control strategy according to various embodiments operates on the same or similar topology as the commonly used hybrid diesel battery system. A new control strategy may be provided.

[0027] According to various embodiments, devices and methods may be provided for control of hybrid energy systems.

[0028] Fig. 4 shows a diagram 400 illustrating a method for determining a control strategy of a system according to various embodiments. The system may include an electrical energy storage (for example a chemical energy storage; for example a battery; for example a chargeable battery) and a generator configured to provide electrical energy to the electrical energy storage and to a load. In 402, a cost of operation of the system may be determined based on a model of the generator and based on a model of the electrical energy storage. In 404, an (optimal) output power of the generator may be determined based on the determined cost.

[0029] In other words, when determining an output power of the generator of the system, properties of the generator and properties of the battery may be taken into account to determine a cost of operation, and the output power may be determined based on this determined cost. [0030] According to various embodiments, the cost may be determined further based on the load.

[0031] According to various embodiments, the cost may be determined further based on a model of a converter of the system. The converter may be configured to convert electrical energy between the electrical energy storage and the load (and for example the generator) (for example from the diesel generator to the storage (from AC (alternating current) to DC (direct current)) and from the storage to the load (from DC to AC).

[0032] According to various embodiments, determining the output power of the generator may include or may be optimizing the cost of the operation of the system.

[0033] According to various embodiments, a method for operating a generator may be provided. The method for operating the generator may include: operating the generator to provide an output power which is determined based on one of the methods described above. For example, the generator may be load following, and a converter at the electrical energy storage (for example battery) may be controlled to charge the battery with a determined power.

[0034] According to various embodiments, the output power may be determined at predetermined points of time (for example continuously; for example (directly) after a previous determination is finished; for example at a pre-determined clock frequency).

[0035] According to various embodiments, the output power may be determined at predetermined points of load. The lookup table may assign a generator output power at a certain load.

[0036] According to various embodiments, the output power may be determined by optimizing the cost of the Operation of the system.

[0037] According to various embodiments, the output power may be determined based on a look-up table (for example, the look-up table may be determined based on one of the methods as described above for various load situations). [0038] According to various embodiments, a control circuit configured to control a system may be provided. The system may include an electrical energy storage and a generator configured to provide electrical energy to the electrical energy storage and to a load. The control circuit may be configured to perform any one of the methods as described above.

[0039] According to various embodiments, a system may be provided. The system may include: an electrical energy storage; a generator configured to provide electrical energy to the electrical energy storage and to a load; and a control circuit configured to perform any one of the methods as described above.

[0040] Fig. 5 shows a flow diagram 500 illustrating a control strategy for generator operation according to various embodiments. Various steps of the control strategy are identical or similar to the control strategy shown in Fig. 3, so that the same reference signs may be used and duplicate description may be omitted. The control strategy shown in Fig. 5 differs from the one shown in Fig. 3 in state 2 (shown in 502 in Fig. 5, instead of 312 of Fig. 3).

[0041] If the generator is not running ("no" after determination 302),

[0042] - and if the battery state of charge (SOC) is higher or equal to SOC _mi „ ("yes" after determination 304), the diesel generator stays turned off, and if there is load, it can be supplied by discharging the battery (State 1 ; 310);

[0043] - else if the battery state of charge is smaller than SOC _m ,-„ ("no" after determination 304), the diesel generator has to be turned on to charge the battery up to the maximum allowed charge rate and eventually supply the load (State 2; 502; the diesel generator may operate controlled on optimal power, and the battery may be charged with optimal power).

[0044] If the generator is running ("yes" after determination 302),

[0045] - and if the battery state of charge (SOC) has reached the SOQ _fl;¾ei ("yes" after determination 306), the diesel generator is turned off, and if there is load, the load is supplied by discharging the battery (State 1 ; 310); [0046] - else if the battery SOC is lower than SOC _to ^ _ei ("no" after determination 306), the diesel generator stays turned on,

[0047] - and if the battery voltage is below V _E oc ("y ^eS " ^a ft ^er determination 308), the diesel generator operates controlled on optimal power, and the battery is charges with optimal power , and the diesel generator and supplies to the load, if there is load (State 2; 502);

[0048] - else if the battery voltage is equal or higher than VEOC ("no" after determination 308), the diesel generator charges the battery at constant voltage (State 3; 314). Then the diesel output power is restricted by the battery's absorption power.

[0049] A difference between the control chart in Fig. 3 and Fig. 5 is the power level of the diesel generator in State 2.

[0050] In the commonly used control (Fig. 3), the operation is due to the diesel generator efficiency over the power output. At maximum power output the diesel consumption is lowest for each generated kWh (see Fig. 1). Hence the diesel generator power generation is set to its maximum power output. The surplus power after load supply which is charged to the battery is only limited by the maximum allowed power ?cmax-

[0051] This operation may be optimal for the diesel generator but it might not be optimal for the overall efficiency. Due to the fact that the battery usage and the necessary power conversions from AC/DC and DC/AC are related with losses it might not be beneficial to charge the battery with the surplus power generated by the diesel generator running on maximum power.

[0052] The control strategy according to various embodiments may optimize the consumable energy together with the diesel generator efficiency. In state 2, the diesel generator and the battery may be operated on system optimal power level (Fig. 5).

[0053] The commonly used diesel-battery system operates the diesel generator at full power because of its highest efficiency at that point. The losses in battery and converters and the battery aging due to usage are neglected. [0054] The control strategy according to various embodiments leads to a joint optimum of the diesel efficiency together with the battery and conversion losses and aging. The optimal diesel generator output power is identified. A difference between the commonly used strategy and the control strategy according to various embodiments may be the power output level of the diesel generator.

[0055] In the following, relevant factors for identifying the optimal power will be described.

[0056] In state 2 of the commonly used control, the diesel (engine or generator) runs on maximum power. The power charged to the battery may be the diesel generator power minus the load power. This power may be called generator to battery power (G2B). With maximum diesel generator output, the G2B power varies with a dynamic load. Additionally FG2B may be limited by Pcmax- This restriction must be followed in any system.

[0057] Fig. 6 shows an illustration 600 of relevant factors for the optimal power according to various embodiments. In the strategy according to various embodiments, the power charged to the battery and hence the diesel generator power output may be calculated (for example by a battery charge controller 614) by an optimization function taking into account the diesel generator efficiency curve f(G) 602 (which may be a model of the generator), the current (or actual) load (x,y,z) 604, 606, 608 (wherein x,y,z may be placeholder names for loads, for example mobile base station - transmitters, or loads at off grid villages), the battery efficiency and usage f(B) 612 (in other words: battery efficiency and battery aging due to usage; which may be a model of the battery), the conversion efficiency f(Q 610 (which may be a model of the converter), and the maximum battery charge power Vcmax-

[0058] In the following, three relevant factors for the calculation of the optimal power will be described: converter efficiency, battery efficiency, and battery usage cost.

[0059] With respect to converter efficiency, the conversion losses are related to the converter used and its efficiency at different input power levels. The round trip efficiency includes AC/DC conversion during charging and DC/AC conversion during discharging. Typical round trip efficiencies for converters are around 85 - 90%.

[0060] Battery efficiency depends on the technology of the battery. For example, lead acid batteries may have an efficiency of around 80 - 95% and lithium batteries of about 90% to 99%.

[0061] The battery efficiency of a specific battery may vary with the charging and discharging current also defined as C-rate. The C-Rate may be the current over the battery capacity in Ah.

[0062] Higher C-rates may lead to lower efficiency. Therefore, accurate control of charge rate may be essential.

[0063] Efficiency may relate to charge and discharge efficiency. The C-rate during charge may be controllable as it is the power from generator to battery (?G2B)- The C-rate during discharge may not be controllable as it is determined by the load. In this approach the discharge rate may be constant and may represent the expected average load.

[0064] Lifetime of a battery may be specified either by cycle lifetime or calendar lifetime. Cycle lifetimes for different lead acid batteries may be in the wide range between 200 to 1200 full cycles while calendar lifetime may be around 10 years. These numbers may vary with the battery type.

[0065] In most off-grid applications, battery replacement happens due to cyclic lifetime.

[0066] Battery usage costs may relate to every kWh of energy passing through the battery. If the accumulated energy passed through the battery reaches the expected cycles at the applied minimum SOC, the battery needs to be replaced. The higher the minimum SOC, the higher the expected cycles until end of life.

[0067] Next to the minimum SOC, there may be other influencing factors on the cycle lifetime. For example these may be the actual SOC history, the C-rate, and the ambient temperature. These factors may slow down or accelerate aging. For this approach, the main influencing factor, the Ah Throughput, may be taken into account only. A more detailed aging calculation may be integrated for refinement.

[0068] The following variables and parameters are used for factorizing the losses and the usage with costs. All costs are indicated with a "c". Capital "C" is cost/h and small "c" is cost/kWh:

[0069] ^' ■ round trip efficiency (conversion and storage);

[0070] F _GEN : power generation of the diesel generator in kW;

[0071] VG2B- power from generator to battery in kW;

[0072] F 'G2B josses ^' - losses due to charging with G2B (conversion and storage) in kW;

[0073] c _gen : price/kWh for diesel generator, e.g. diesel efficiency/cost curve;

[0074] G2B_iosses- price for the diesel power generation of the losses of ¥G2B for 1 h;

[0075] Cb _at _ _use : price/ kWh battery usage; and

[0076] Cbat _j ise ^' ■ price for lh battery usage at PG2B-

[0077] The function for the round trip efficiency (r| _r ound_eff) may include the efficiency of the battery (which may also be referred to as Conversion)- The normalized battery efficiency is based on the C-rate .

[0078] The round trip efficiency may dependent on the G2B power and the power discharged based on the average expected load:

( average discharge)■

[0079] The power lost (PG2B_iosses) may represent the Watts which are lost due to the inefficiencies of the battery and the converter during charge and discharge when the battery is charged from the diesel generator with the power PG ₂ B:

G2B losses - ( 1 - ground eff (^G2B)) * ?G2B-

[0080] ^G2B_iosses may be represented in costs by multiplying with the costs of generating energy:

CG2B_losses ^~ P G2B Josses * ^gen- [0081] It is to be noted that c _gen may be not a constant. It may vary with P _gen . [0082] The battery usage costs may be the costs for every kWh of energy passing through the storage. They may be calculated as the price of the battery divided by Ah throughput until end of life:

__ battery _ price

^bcit use ^' ·

nom _ capacity _ in _ kWh x (1 - SOC min) x cycles _ to _ failure [0083] The Ah Throughput until end of lifetime may be calculated as the nominal battery capacity times the number of full cycles until end of life. The cycles to failure may be the counted partial cycles at the applied SOC _m j _n . The cycles to failure times the fraction (1- SOCmin) of the partial cycling may be the full cycles.

[0084] The unit for the battery usage costs may be costs/kWh. Depending on the power, the energy passing through the battery in 1 h may be a multiple of Cbat_use ^' -

Cbat_use ^~ ^bat_use * ((PG S ^ average discharge)!^' )')

in units: $/h = $/kWh * kW.

[0085] Like described above, equations for V _G 2Bjosses and b _a t_use niay be set up by analyzing the behavior of the components, utilizing component datasheets, and making assumptions.

[0086] In the following, determination of the optimal power according to various embodiments will be described.

[0087] P ^' basis may denote the load (at any point in time) in kW. c _{cost 0} f energy may denote the price/kWh for the diesel power generation with losses (cost of energy). Let V _basis be the load demand at any point in time and F _gen the power generated by the diesel generator.

[0088] Fig. 7 shows an illustration 700 of an example of the nomination of power fractions, for example the composition of Pt, _as is 706, P _gen (which may be the sum of Pbasis 706 and PG ₂ B) and PQ ₂ B (which may be the sum of the usable fraction of ?G2B 704 and its losses ?G2B josses

702. [0089] The problem to be solved is to minimize the costs of energy e.g. the price/kWh for the diesel power generation inclusive the losses of charging the battery with the diesel generator. This cost of energy may be the absolute price of generating P _gen for 1 hour plus the battery usage costs divided by the energy usable after losses.

[0090] The time duration of lh may be reduced to the controller step time. Then c _{C0St 0} f _energy is. not the costs for generating each IkW for lhour but for generating each IkW for the next step time.

[0091] In other words, the problem may be: minimize c _{cost 0} f energy, wherein

_ absolute price of generating Pgen for 1 hour plus the battery usage costs

Ccost of energy ^~~ ~ ^~ ?

energy usable after losses in other words: (.Cgen ^gen ^batjxse) X 1Λ.

mm C _{cost 0} f energy ^— _ _n ^~ Th

\ ^r gen ^r G2B Josses) ^{Λ ,t}

[0092] P _G2 B may be the optimal power for charging the battery and P _gen may be the optimal diesel generator power output for state 2 in the control chart shown in Fig. 5. [0093] To solve this equation, it may for example be differentiated with respect to P _ge „ or with respect to ?G2B as they are in direct relation (PG2B - gen - P basis) ^' -

. . . d

Minimize C _cos t of energy: , Ccost of energy ^~ 0.

d P

[0094] The load may be dynamic. As such, the minimization problem according to various embodiments may be solved in each control step. Hence P _oasis may be known in each time step. P _g e„ may be the only variable. 0095] In dependency to ? _gen , the cost of energy may be:

c (P )x P + c . (((P - P„ )+ P , „ 2)

P - [(n TP )x _n (jp Tp + p f ]/2 ))x ₇₇ ' IP ))x (p - p

[0096] In the following, implementation of the minimization problem will be described. [0097] Another option besides the differential approach to solve the minimization problem may be the following: m ^{in c} cost of energy —

^gen (^gen ^G2 B osses )^ ^ ^gen ^G2 B _Iosses ^bat _ttse

^~ P gen - P G2 B _losses P gen - P GIB _losses c pen x P G2B losses + C bat iise

- C +— = = .

g ^en p _ P

gen G2 B _losses

[0098] Fig. 8 shows an illustration 800 of optimal diesel generator power output calculation for one load demand point.

[0099] According to various embodiments, the following steps may be taken:

[00100] 1. The cost curve c _gen is shown by curve 802 in Fig. 8. It is to be noted that c _gen reduces (or decreases) with increase in load due to improvement in diesel efficiency.

[00101] 2. Let be the load demand at any point in time. cJ _ge „ (808) shows the cost of diesel energy at load P _¾aiii .

[00102] 3. Cost of charging the battery with the G2B is shown by curve 806. It is to be noted that CG2B is zero at P _BASIS . With increase in cycle charging power, losses in converters and battery also increase which explains the shape of the CG2B function.

[00103] 4. The diesel load at which the overall cost of energy (illustrated by curve 804) is minimum may be located, which is shown by point c2 _ge „(810), in this example at a generator load of about 0.65 of the nominal load.

[00104] In the following, a diesel generator operating point analysis will be described.

[00105] The cost of energy minimization problem may be implemented in a simulation environment. Diesel generator output may be optimized at different loads. Results are shown in Fig. 9 with two different price scenarios. [00106] Fig. 9 shows an illustration 900 of a diesel operating point based on the approach according to various embodiments. The method according to various embodiments may be deployed to a real time controller. In the illustration of the diesel optimization cost break up (in other words: the diesel generator optimized operating point based on load demand), the solid line 902 illustrates operation at a first parameter setting (for example at a first fuel price and at a first battery usage costs), the dashed line 904 illustrates operation at a second parameter setting (for example at a second fuel price and at a second battery usage costs), and the dotted line 906 illustrates load without cycle charging. The horizontal axis 908 illustrates the load demand, and the vertical axis 910 illustrates the generator load (which may also be called the generator output power).

[00107] It can be observed from Fig. 9 that for the same load demand, the diesel generator operating point (or the optimal diesel generator output) may change depending on diesel fuel cost and battery usage cost. As an example for the first parameter setting it is not beneficial to charge battery for loads greater than 3 kW. However, for the second parameter setting, charging battery with diesel is beneficial all the way up to 7.5 kW.

[00108] Fig. 10 shows an illustration 1000 of the system topology according to various embodiments. A diesel generator 1002, a load 1004, and a storage 1006 may be provided. The approach according to various embodiments may include a strategy for involving the storage 1006, like indicated by dashed line 1008.

[00109] - From Fig. 1, it can be seen that the cost of diesel energy is high at low load operating point.

[00110] However, it should also be noted that for the network stability it may be required to have a high capacity diesel generator to provide for spinning reserve (20%) and reactive power requirements.

[00111] According to various embodiments, a storage 1206 may be provided to make the diesel operation more efficient. The excess energy produced by diesel is stored in the battery. This approach improves the overall system efficiency. In this approach, during diesel operation the optimal diesel operating point is identified at each time step as described above. It is to be noted that this operating point is a function of diesel generator parameters like cost, efficiency, fuel price, maintenance cost, start-up cost and battery parameters like usage cost, charging current and converter efficiency.

[00112] In the following, a comparison of different operating strategies will be described. Once the storage capacity is fixed, different operating strategies i.e. diesel only, commonly used strategies, and strategies according to various embodiments may be compared. In diesel only case, the generator is load following i.e. the generator is operated at the nominal load. In a commonly used approach, the diesel generator is constantly running at full power charging the battery and switching off the diesel generator after a certain SOC target has been reached. Whereas in the approach according to various embodiments, the diesel generator is always operated at optimal operating point whereby resulting in efficient battery usage.

[00113] According to various embodiments, savings may be provided compared to diesel only system. These savings may come from optimal diesel operation and efficient battery charging.

[00114] Fuel savings compared to diesel only may be provided. This may result in reduced emissions and refuelling costs. As such, the approach according to various embodiments may be referred to as a green approach.

[00115] Despite the commonly used approach having less diesel operating hours compared to the approach according to various embodiments, the approach according to various embodiments shows lower LCOE (Levelised Cost of Energy). This may be the result of efficient diesel generator operation and optimal battery charging.

[00116] Diesel only operation is highly inefficient, as diesel generator is made to operate at loads below minimum operating load for significant amount of time resulting in frequent (23 times more) engine replacements over the system lifetime. [00117] In the following, commercialization of the control strategy according to various embodiments will be described. To make this control easily available for any hybrid system, a control setup design tool may be provided. This tool may allow the system integrator to put in component specific sizes and performance parameters of the hybrid diesel -battery system to be built up. The output of this tool may be a generator output curve like the one shown in Fig. 9. This generator output curve gives the diesel generator output level P _gen for every load Pba _s is- The curve may then be transferred as a look up table to the real time controller of the hybrid system.

[00118] The system integrator may enter these component specific sizes and performance parameters:

[00119] - Diesel generator: manufacturer/model (if available in the component database) or fuel consumption at 1/4, 1/2, 3/4 and full power levels; diesel fuel price (full costs incl. transport); and minimum diesel generator power output (specified by manufacturer);

[00120] - Storage: Capacity (Ah); Battery cost (incl. installation, transport, etc.); Nominal system voltage (default: 48 V); Maximum C-rate (default: 2C); Cycles to failure at SOCmi _n from data sheet; and Battery type (for example flooded, gel);

[00121] - Converter: Efficiency curve from data sheet;

[00122] - Average expected load.

[00123] According to various embodiments, when the diesel fuel price, the converter efficiency or another input parameter is changed, the diesel generator output power changes. In contrast thereto, for commonly used systems, the diesel generator is either off, operates load following or is on full power. This operation is not supposed to change with diesel price or other system parameters.

[00124] It will be understood that although the above has been described for a diesel generator, any generator may be used, for example any generator with a combustion engine, no matter which kind of fuel it uses (for example diesel or gasoline). [00125] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.