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Power Generation from Renewable Energy Sources Fall 2013 Instructor: Xiaodong Chu Email：[email protected] Office Tel.: 81696127 Flashbacks of Last Lecture • Configurations of grid-connected PV systems – – – – – – PV array Combiner box Array disconnect switch Maximum power tracker (MPPT) Inverter Breaker • Protection design of grid-connected PV systems – Islanding – Reclosing Flashbacks of Last Lecture • Factors influencing performance of grid-connected PV systems – Mismatch – Reclosing Flashbacks of Last Lecture • Example 9.6 of the textbook: you should master it! Photovoltaic Systems–Stand-Alone PV Systems • A general stand-alone PV system includes a generator backup as well as the possibility for some loads to be served directly with more-efficient dc and others with ac • A combination charger–inverter has the capability to convert ac to dc or vice versa Photovoltaic Systems–Stand-Alone PV Systems • Off-grid stand-alone PV systems must be designed with great care to assure satisfactory performance • Users must be willing to check and maintain batteries, they must be willing to adjust their energy demands as weather and battery charge vary, they may have to fuel and fix a noisy generator, and they must take responsibility for the safe operation of the system Photovoltaic Systems–Stand-Alone PV Systems • The design process for stand-alone systems begins with an estimate of the loads that are to be provided for • The user may try to provide the capability to power everything that grid-connected living allows • Trade-offs are made between more expensive, but more efficient, appliances and devices in exchange for fewer PVs and batteries – A key decision involves whether to use all dc loads to avoid the inefficiencies associated with inverters, or whether the convenience of an all ac system is worth the extra cost, or perhaps a combination of the two is best – Another important decision is whether to include a generator back-up system and, if so, what fraction of the load it will have to supply Photovoltaic Systems–Stand-Alone PV Systems AC Micro- (Nano-) Grid Photovoltaic Systems–Stand-Alone PV Systems DC Micro- (Nano-) Grid Photovoltaic Systems–Stand-Alone PV Systems • Power needed by a load, as well as energy required over time by that load, is important for system sizing • In the simplest case, energy (watt-hours or kilowatt-hours) is just the product of some nominal power rating of the device multiplied by the hours that it is in use • The situation is often more complicated – Many appliances use different amounts of power during different portions of their operating cycle – Many devices use power even while they are turned off since some circuits remain energized awaiting the turn-on signal – Major appliances encounter the surge of power required to start their electric motors, which is important for sizing inverters, wires, fuses, and other ancillary electrical components in the system Photovoltaic Systems–Stand-Alone PV Systems • A inverter is needed for a stand-alone PV system to converter dc PV power to ac power of load side • When no load is present, a good inverter will power down to less than 1 watt of standby power while it waits for something to be turned on that needs ac • When it senses a load, the inverter powers up and while it runs on the order of 5–20 W of its own, which means those standby losses associated with so many of electronic devices may keep the inverter running continuously, even though no real energy service is being delivered Photovoltaic Systems–Stand-Alone PV Systems • Inverters are specified by their dc input voltage as well as by their ac output voltage, continuous power handling capability, and the amount of surge power they can supply for brief periods of time • The inverter’s dc input voltage, which is the same as the voltage of the battery bank and the PV array, is called the system voltage – The system voltage is usually 12 V, 24 V, or 48 V – One guideline is based on keeping the maximum steady-state current drawn below around 100 A so that readily available electrical hardware and wire sizes can be used Photovoltaic Systems–Stand-Alone PV Systems • The maximum ac power that the inverter needs to deliver can be estimated by adding the power demand of all of the loads that will ever be anticipated to be operating simultaneously • The most important specification for an inverter is the amount of ac power that it can supply on a continuous basis • It is also critically important that the inverter be able to supply surges of current that occur when electric motors are started Photovoltaic Systems–Stand-Alone PV Systems • Stand-alone systems obviously need some method to store energy gathered during good times to be able to use it during the bad • While various exotic technologies are possible including flywheels, compressed air, or even hydrogen production, it is the battery that makes the most sense today • Batteries provide several other important energy services for PV systems – Provide surges of current that are much higher than the instantaneous current available from the array – Control the output voltage of the array so that loads receive voltages that are within their own range of acceptability Photovoltaic Systems–Stand-Alone PV Systems • Energy storage in a battery is typically given in units of amphours (Ah) at some nominal voltage and at some specified discharge rate • A lead-acid battery, has a nominal voltage of 2 V per cell and manufacturers typically specify the amp-hour capacity at a discharge rate that would drain the battery down to 1.75 V over a specified period of time at a temperature of 25◦C Photovoltaic Systems–Stand-Alone PV Systems • Battery storage capacity is specified in amp-hours rather than watt-hours – A 200-Ah battery that is delivering 20 A is said to be discharging at a C/10 rate, where the C refers to Ah of capacity and the 10 is hours it would take to deplete (C/10 = 200 Ah/10 h = 20 A) – Rapid draw-down of a battery results in lower Ah capacity, while long discharge times result in higher Ah capacity – Deep-cycle batteries intended for photovoltaic systems are often specified in terms of their 20-h discharge rate (C/20), which is more or less of a standard, as well as in terms of the much longer C/100 rate that is more representative of how they are actually used Photovoltaic Systems–Stand-Alone PV Systems • The amp-hour capacity of a battery is not only ratedependent, but also depends on temperature • The combination of cold temperature effects on battery performance—decreased capacity, decreased output voltage, and increased vulnerability to freezing when discharged— mean that lead-acid batteries need to be well protected in cold climates • However, the apparent improvement in battery capacity at high temperatures does not mean that heat is good for a battery Photovoltaic Systems–Stand-Alone PV Systems Photovoltaic Systems–Stand-Alone PV Systems • Battery efficiency is more easily expressed in terms of current efficiency than in terms of energy efficiency • Assume charging a battery with a constant current IC over a period of time ΔTC during which time applied voltage is VC, inputting energy Ein VC IC TC • Suppose that the battery is discharged at current ID and voltage VD over a period of time ΔTD, delivering energy Eout VD I D TD Photovoltaic Systems–Stand-Alone PV Systems • The energy efficiency of the battery is Eout VD I D TD Energy Efficiency Ein VC I C TC • If we recognize that current (A) times time (h) is Coulombs of charge expressed as Ah, then V I T V Ah Energy Efficiency D D D D out VC IC TC VC Ahin • The ratio of discharge voltage to charge voltage is called the voltage efficiency of the battery, and the ratio of Ahout to Ahin is called the Coulomb efficiency Energy Efficiency (Voltage efficiency) (Coulomb efficiency) Photovoltaic Systems–Stand-Alone PV Systems Photovoltaic Systems–Stand-Alone PV Systems • If good weather could be counted on, battery sizing might mean simply providing enough storage to carry the load through the night and into the next day until the sun picks up the load once again • The usual case is one in which there are periods of time when little or no sunlight is available and the batteries might have to be relied on to carry the load for some number of days • During those periods, there may be some flexibility in the strategy to be taken – Some noncritical loads might be reduced or eliminated; and if a generator is part of the system, a trade-off between battery storage and generator run times will be part of the design Photovoltaic Systems–Stand-Alone PV Systems • Given the statistical nature of weather and the variability of responses to inclement conditions, there are no set rules about how best to size battery storage • The key trade-off will be cost • Sizing a storage system to meet the demand 99% of the time can easily cost triple that of one that meets demand only 95% of the time Photovoltaic Systems–Stand-Alone PV Systems Photovoltaic Systems–Stand-Alone PV Systems • A curve fitting technique could be used to approximate the relationship between storage days and peak sun hours • Would you like to have a try on some curve fitting problems? – Many tools are available, e.g., Curve Fitting Toolbox of Matlab Photovoltaic Systems–Stand-Alone PV Systems • The simplest PV–battery system consists of just a single module connected to a battery and a DC load, with no charge controller, inverter, or anything else to complicate things • Such a system might provide someone with a bit of light at night and maybe a few other simple amenities • Some systems allow the battery to leak current back through the PV module at night, which raises the question of whether it might be worthwhile to add a blocking diode to prevent that nightly discharge Photovoltaic Systems–Stand-Alone PV Systems Photovoltaic Systems–Stand-Alone PV Systems • The equivalent circuit of a single PV cell will help us analyze the potential nighttime battery loss problem • Ignoring the insignificant impact of the very small series resistance and eliminating the ideal current source ISC because the cell is in the dark at night • Current through the diode in the equivalent circuit for a cell (at 25◦C) is given I d I 0 (e38.9Vd 1) Photovoltaic Systems–Stand-Alone PV Systems • The nighttime current from the battery through each cell will be I B I d I RP I 0 (e38.9Vd 1) Vd RP where the voltage Vd across the diode will be equal to the battery voltage VB divided by the number of cells n in the PV module • With this simple nighttime equivalent circuit, we can decide how much leakage will occur from the battery through the PVs