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PV System Components ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner PV system components Key issues in components: • • • • Power loss in each component Implication of components on maintenance, life time of system Matching of components Effect on cost of adding component/s PV system components: • • • • • PV modules Batteries Power conditioning Loads Balance of systems ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Batteries • Batteries are required for the following reasons: – Load profile doesn’t match solar radiation profile – Mitigating effect of variability in solar radiation • Have a major impact on the performance of PV systems, altering the maintenance, design, reliability and safety. They are a large portion of the total cost of a system. • The key issues for batteries: – Why and how battery voltage and capacity change with system and battery parameters – Operating constraints on batteries; constraints of state of charge, charge/discharge rate, temperature – Lifetime and safety issues with batteries ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Batteries Battery basics: • Redox reactions • Components of batteries • Equilibrium voltage – Nernst equation • Ideal battery capacity – calculations • Non-equilibrium voltage and capacity: Effect of reaction rates – Polarization: Activation, Mass transport, Resistance – Effect of charging, discharging, age • Time dependent effects in batteries i.e. aging • General safety issues with batteries ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Batteries Battery characteristics • • • • • Charging/discharging parameters: State of Charge (SOC), Depth of Discharge (DOD), discharge rate Capacity Efficiency Energy, volumetric and power density Other electrical parameters Lead Acid batteries • • • • Operation of lead acid batteries Lead acid battery characteristics Issues specific to lead acid batteries Potential problems ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Batteries Lead acid batteries configurations • • • • Electrode materials: addition of small amounts of other materials affects gassing, charging, sulfation etc.. Electrolytes: Gelled or Absorptive Glass Mat (AGM) batteries Battery housing: flooded, sealed, valve-regulated Types of lead-acid batteries Other battery systems • Nickel-cadmium ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Chemistry basics ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Chemistry basics • Mole, molar mass – Mole is unit of amount of a substance i.e. one mole of Iron contains the same number of atoms as one mole of gold etc.. – Molar mass is how much one mole of an element weighs in grammes, so if the molar mass of Silver (Ag) is 107.87 it means 107.87 grammes of Ag contains one mole of Ag atoms • Molarity – Concentration of a solution expressed as moles per litre • Balancing chemical equations – Need to make sure we are losing elements or gaining them – Can also figure out which reactions should take place and which shouldn’t ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Chemistry Basics • Oxidation refers to when an element or molecule loses electrons i.e. increase in oxidation state • Reduction refers to when an element or molecule gains electrons i.e. decrease in oxidation state • The oxidation and reduction reactions are paired in what is called reduction-oxidation or redox reactions • Redox reaction represents the transfer of electrons (and hence charge) from one material to another • Also tell us if a reaction will occur spontaneously • Redox reactions are therefore the basis for batteries • Redox reactions typically take place in close proximity to each other ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery basics • Redox reaction; physically separated • Oxidation reaction generates electron at surface of anode; electron transported to cathode; ion in electrolyte transported to anode; voltage due to chemical potential difference between anode and cathode Components • Electrodes (anode, cathode) • Electrolyte • Electron transport (wire) • Ion transport (electrolyte) • Housing ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery basics • Primary battery is one that only discharges – cannot recharge • Secondary battery can be re-charged • Anode and cathode collectively referred to as electrodes • Oxidation takes place at anode • Reduction takes place at cathode • Need to make electrical contact to both electrodes • Solution surrounding electrodes is referred to as the electrolyte ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery basics • Primary battery is one that only discharges – cannot recharge • Secondary battery can be re-charged • Need to be able to transport ions to maintain charge throughout the reaction • Require a conductive medium • Conceptually use a salt bridge • Practically, it makes more sense to use a common electrolyte for both half-reactions ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Redox reactions • Redox reactions are paired reduction/oxidation reactions where electrons are transferred between the reactions • The transferred electrons come about due to changes in the valence energy of the materials in the reactions • Reduction reaction: acquires electrons in order to lower the valence state of a material • Oxidation reaction: electrons emitted to increase the valence state of a material ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Redox reactions • Overall redox reaction: • Example • Example Balance equation and identify the elements that are oxidized and reduced respectively ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Redox reactions • Often helps to separately write oxidation and reduction reactions (“half-cell reactions) • Important to figure out these half-cell reactions, some elements can have multiple oxidation states (think Fe) • Look up tables exist for these reactions – very useful, we will see why in a second • What about the other stuff like SO42- ions? They don’t change oxidation state, so don’t care about them ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Electrochemical Potential • • Electrochemical potential is a measure of the average energy of the valence states of a material – analogous to the Fermi energy in metals/semiconductors Lowest energy state is a full outer valence band, typically meaning 8 electrons – Valence changes that get you closer to the ideal state are spontaneous occurring if an electron is available or an extra electron can be taken by another material – Spontaneous changes defined as being positive ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Electrochemical Potential • A material has a certain electrochemical potential which depends on the electronic configuration of the compound • In going from one form to another, the compound changes it’s electrochemical potential, dependant on the change in valence state of compounds • We can measure the change in electrochemical potential and under standard conditions (25 C, 1 M solutions, compared to H2), the change in electrochemical potential can be measured, and is given in a table – Large positive number indicates a good acceptor of electrons: good oxidizing agent, it gets reduced in a reaction – Large negative number indicates a good source of electrons: good reducing agent, it gets oxidized in a reaction ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Table ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Table ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Electrochemical potential • Table of electrochemical potentials gives direction of spontaneous reactions and standard potential of a halfreaction • The standard potential of a full reaction is simply the sum of the potentials for the two half-reactions – An overall positive standard potential indicates the reaction will proceed spontaneously – A negative overall standard potential indicates the reaction will not proceed spontaneously – Reversing the direction of a (half-)reaction reverses the sign of the standard potential – When combining the standard potentials we do not multiply the potentials in any way even if we have done so to balance the equation: the standard potential is always the same ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Electrochemical potential • • • • Electrochemical potentials given for reduction reactions So if negative number means reducing agent (it is oxidized) Reversing reaction direction reverses sign of potential Electrochemical potential for reaction is sum of those for two half-reactions • Example: Zn and Cu; from standard potentials table Nett reaction Standard potential for reaction is 1.1 V ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Electrochemical potential • Example Will the following reaction proceed? ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Electrochemical potential • Example Will the following reaction proceed? From standard potentials table The total standard potential for the reaction is -1.43 V Since it is negative the reaction will not proceed spontaneously ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Electrochemical potential • Example Find the potential for the following reaction ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Electrochemical potential • Example Find the potential for the following reaction Zn(s) + Fe 2+ (aq) → Zn 2+ (aq) + Fe(s) From the standard potential table we have Fe 2+ + 2e − → Fe − 0.41V However we also have Zn 2+ + 2e − → Zn − 0.76V Want it the other way around Zn → Zn 2+ + 2e − 0.76V So total standard potential is 0.35 V. It is positive, so it proceeds spontaneously ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner How many electrons do we get? • We are interested in the current from a battery, as well as it’s voltage, so important to determine number of electrons required by reaction • Definition: 1 Farad(ay) = 1 mole of electrons = 6.022 x 1023 electrons = 96,485 C • So for example in the following reaction So for 1 mole of O2 gas we need 4 x 96,485 = 36,000 C of electrons Under standard conditions 1 mole of gas is 22.4 litres, so we have 5.8 x 10-5 litres per C Now, 1 A = 1 C per second. So we get 5 x 10-5 l/s of O2 for 1 A of current flow ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner How many electrons do we get? • Battery capacity Capacity(Ah) = n.F. 1 hour 3600 sec Energy Capacity = Ah × Battery Voltage ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Nernst equation • In reality the chemical potential for a set of reactions also depends on the concentration in equilibrium of the products and reactants • Given by the Nernst equation RT 0.0592 0 0 E=E − ln Q = E − ln Q nF n R is gas constant 8.314 Jmol-1K-1 T is temperature F is Faraday constant E0 is standard potential of reaction n moles of electrons transferred in reaction Q is reaction quotient ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Nernst equation • Reaction quotient defined for the following reaction aA + bB → cC + dD as [C ]c [ D]d Q= [ A]a [ B ]b This can allow us to find the voltage for a battery under a wide range of conditions. But remember it is for when the battery is in equilibrium Q depends on concentration of the products and the reactants. Pure liquids or solids are taken as 1. Liquids in solution are given by their molarity and gases by their pressure ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Nernst equation • Example A copper-zinc cell has E0 = 1.101 V. If the reaction is done in a cell with 5.0 Zn2+ and 0.40 M Cu2+ at 25 C what is the cell voltage? ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Nernst equation • Example A copper-zinc cell has E0 = 1.101 V. If the reaction is done in a cell with 5.0 Zn2+ and 0.40 M Cu2+ at 25 C what is the cell voltage? First the equation Now find Q Q = [Zn2+]/[Cu2+]= 5.0/0.4 = 16.7 Two electrons transferred in reaction so n = 2 E = 1.065 V ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Non-ideal effects • Nernst equation holds for the battery being in equilibrium, if non-equilibrium, the voltage is different due to polarization • There are three main reasons for this – Activation over-voltage – Mass transport over-voltage – Resistive effects ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Non-ideal effects • Over-voltage: need a higher voltage to re-charge the battery than what was obtained when discharging battery • Resistive Drops: voltage drops at all parts of the battery, including the surface, meaning higher voltages at terminals than at battery itself (when charging) – Non-uniform resistances can cause uneven charging in series connected systems • Mass transport effects (involving the surface): Products and reactants need to diffuse away from or towards the surface – Regions may have locally a different concentration, causing a nonuniform voltage ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Non-ideal effects • Activation effect due to reversibility of reactions: most reactions are not completely reversible due to the formation of intermediate products in the reactions – Intermediate product may involve a higher energy state – A fraction of the initial reactants will have this higher energy state – To reverse the reaction, need to apply a higher potential – Electrolysis of water has a high over potential ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery systems • Key characteristics of battery systems – – – – – – Volumetric/Gravimetric energy density Depth of discharge Charging regimes Efficiency Voltage Cost ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery Characteristics • Battery capacity – Measured in Amp-hours rather than conventional Watt-hours due to variation in battery voltage – Battery capacity depends on age of battery and the history • Depth of discharge • Battery lifetime: number of discharges • Battery voltage and variation of voltage • Effect of temperature ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Lead-acid batteries • • For secondary batteries (re-chargeable) the most common type is leadacid Reaction and standard potential: Overall • • • PbO 2 + Pb + H 2SO 4 ⎯discharge ⎯⎯ ⎯→ 2PbSO 4 + 2H 2 O ←⎯charge ⎯⎯⎯⎯ Electrodes are lead oxide and ‘spongy’ or porous lead Electrolyte is sulfuric acid Standard potential 2.041 V ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Lead-acid batteries • Some issues – Depth of discharge, lifetime and number of cycles – Reduced life due to sulfation of the battery and shedding of the plate material – Gassing of battery: high voltages or fast charging leads to electrolysis of water ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Lead-acid batteries • Issues in lead-acid batteries – Stratification of the electrolyte: electrolyte cannot be readily mixed – Physical damage to electrodes due to soft electrode material – Spillage of sulfuric acid • Safety issues particularly for transport – Temperature dependence of the battery • Freezing of battery at low temperatures • Increasing temperature increases the voltage; must be taken into account for system design particularly the charging part of the system ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Lead-acid batteries • Batteries have modified electrode materials, geometries and electrolyte composition and volume to tailor battery to each application • Electrode composition and geometry – Trace amounts of materials (Antimony, Calcium) added to lead plates to increase hardness, however, does affect gassing and depth of discharge of battery • Electrolyte – Increasing volume of electrolyte: less sensitive to losses of water, but makes the battery heavier – Captive electrolyte: gelled or absorptive glass mat: can be either shallow or deep discharge, can be sealed ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Lead-acid batteries • Flooded batteries – Can add water, check status of electrolyte etc. – Depending on configuration, can be deep or shallow cycle • Sealed and valve regulated batteries – Often are gelled batteries – Need to control charging regime carefully to prevent generation of hydrogen • Can also be classified by use: starting, lighting, ignition (SLI); Motive power or traction; Marine or RV batteries; Stationary; Deep cycle ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Power Conditioning • Power requirements and matching to PV module • Functions of power conditioning: – Keep the module operating at it’s maximum power point (maximum power point trackers) – Optimizing charging of the batteries to prevent damage to the batteries (battery charger) – Prevent discharging of batteries through the solar cell module (blocking diode) – Converting from DC to AC (inverter) for applications or feeding the grid ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Blocking diode • Must be sized to handle array current • Introduces voltage drop that can be significant for lower voltage systems e.g. for a 12 V system it can be ~5% • Higher voltage system (also power) typically incorporate the blocking diode in the charge controller for the battery ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Maximum power point tracking • Function of MPPT is to maximize the power generated by the PV module by ensuring that it operates at it maximum power point • Recall the characteristic resistance is the resistance corresponding to the maximum power point. A MPPT tries to maintain the load seen as this resistance ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Maximum power point tracking • Constant power curves give I and V for a constant power • Operating point of the solar panel is given by the intersection of load line and IV curve • From constant power curve can determine the amount of power lost when operating away from mpp ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Maximum power point tracking • MPPT allows the PV module to operate at its mpp even as light intensity changes or effective load changes • MPPT consists of two components: a DC-DC converter and a circuit which searches for the mpp of the PV module and ensures the module is biased at mpp MPPT have efficiencies > 90% ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery Charging • Batteries are very sensitive to charging parameters and the lifetime of batteries can be dramatically altered by improper battery charging or leaving the battery at low charge • Function of battery charger to provide appropriate charging conditions for the battery, particularly by preventing either over-charging or excessive discharging of the battery • The type of battery regulator used must be matched to the battery used and a battery charger designed for one type of battery will not be effective for another type of battery ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery Charging • Issues in battery charging – If charging current is too high, towards the end of the charging cycle tend to get a lot of electrolysis of the water, resulting in gassing rather than storage – Abrupt stopping of the charging, however, results in the batteries not reaching their fully charged state, if this happens regularly the battery capacity is reduced – Solution is to taper the current as battery approaches full charge – Charging too slowly also has disadvantages: • Low charging current increases likelihood that power from solar module is used inefficiently • Want to fully charge the batteries whilst sun light is available to us ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery Charging • Multi-stage battery charging – Maintenance charge during float operation: keeping the battery fully charged by continuous low levels of current or by short bursts of power. Maintenance charging also called float or trickle charging, is typically used from mid-day into late afternoon when the batteries may be fully charged and: – Boost charging: used only in open, flooded lead-acid batteries to prevent stratification of electrolyte. If the electrolyte is gelled or absorbed in a AGM, boost charging should not be used • Pulsed charging: battery is given short pulses of charge followed by a resting period. Allows charging to higher capacities whilst minimizing outgassing ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery Charging • Types of charging regimes – Self-regulation • Self-regulation does not employ a separate regulator but rather relies on the fact that as the battery voltage increases, the current available from a PV panel connected in parallel is decreased • Only really works if the load, solar insolation and module temperature don’t vary too much – For example, in hot weather when the PV panel experiences lower voltage the battery may be under charged, while under cooler ambient conditions module voltage will be higher and may over-charge the battery ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery Charging – Shunt regulator • When voltage reaches certain level shunt regulator is switched on and current diverted through it • Since shunt dissipates the power from the modules only used for small system where shunt can handle the array current – Series regulator • When a voltage is reached the series switch opens • Doesn’t allow reduced currents as voltage rises, hence not optimal charging ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Battery Charging • Pulse Width Modulation Circuits – A PWM circuit is used to provide a variable voltage, which allows greater control over types of charging methods – Requires more complex circuitry than shunt or series-switch circuits • Amp-hour meters and controllers – Battery charge measures and controls the amps flowing into and out of the batteries and continuously determines the state-ofcharge of the batteries – Allows potentially more precise information about the BSOC but requires periodic calibrations ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Inverters • Function of inverter is to convert low voltage DC power from PV array to high voltage AC power as required by many loads • Characteristics of inverters – Power quality • Waveform shape • Total harmonic distortion – Input and output voltage ratings (input determined by battery voltage, output by that required by loads, typically 120 or 240 V) – Power and surge power ratings – Inverter efficiency – Connection and interaction of inverter with other AC components • Other inverters • Grid power ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Inverters • Basic topology: DC to AC conversion and AC voltage transformer – 2 basic approaches: • High frequency transformer for AC voltage conversion followed by high frequency to grid frequency conversion • Direct transformer for AC voltage conversion to grid frequency ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Inverter • Waveform – Square wave output: simple, reliable but very poor approximation to sine wave – Modified square (quasi-sine) wave: • RMS and peak voltage are close to that for a sine wave • Still very harmonic content and DC component in waveform • Relatively simple and inexpensive – Square and modified square wave not suited to sensitive loads ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Inverter • Sine wave inverter – Reproduces sine wave of grid supply – Suitable for all loads – Can interact with other AC systems including generators and grid – Power output measured by total harmonic distortion (THD) – More complex circuitry ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Inverters • Inverter efficiency – Different types of inverters have substantially different performances away from rated power and will be matched to different load profiles – Best matched inverter depends on load characteristics; how high are the peaks; how long do they last; is there a light load for a large portion of the day; are there any seasonal variations in the daily load profile? ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Inverters • Surge ratings of inverters – Inverter power loss is caused mainly by current flow – Below the absolute current limit of the switching transistors, the current is limited by the temperature of the semiconductor junction – Junction temperature depends on ambient temperature, power loss and time ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Inverters • Other inverter characteristics – Parallelable: • Inverters can be connected in parallel, to efficiently provide a greater power range than is possible with a single inverter • Important when there is a large variation in the load which persists for substantial times – Bi-directional inverter: • Inverter can both provide power for the battery for charging and source power from the battery • Bi-directional inverters used when there are other power sources in the system, and these can be used to charge batteries ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Inverters • Two main types: stand-alone or grid-tied • Grid-tied inverters can include batteries but many don’t – If no batteries inverter can be used as MPPT but the input voltage may vary more than when batteries are present – If batteries, then grid can be considered another AC power source • Regulatory constraints for grid-tied inverters: grid-tied inverters must meet specification set by utility – Typically includes specification on power quality – Islanding: if power failure occurs when PV array is producing power, a grid-tied inverter may produce an island of live grid • Dangerous for workers on the lines • Must have provision to isolate the system from the grid when the grid power goes down ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Balance of System • Anything else needed to complete the system, including: – Module mounting: • Mounting may be fixed or tracking • Tracking: module tracks the sun to give ~20% extra power over a year – One axis tracking: tracks sun along one axis, usually eastwest – Two axis tracking: module tracks sun in both east-west and north-south directions. Usually not used in conventional PV systems. Used in concentrator systems. – Wiring, cabling – Housing for batteries, electronics – Land ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Loads • Types of loads – Heating: water heating, cooking, small appliances – Motor/magnetic components: refrigeration and many other applications – Lighting – Electronic circuits: TV, radio, clocks – Combination electronic/motor ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Loads • In determining load need to characterize: – Power used by load – AC/DC and variation possibility i.e. can we use a DC fridge – Power quality required; harmonic content, DC offset, peak voltage levels – Variation of load in seasons – Peak power used by entire load – Variation of input voltage and current with time (i.e. surge current) – Is load critical (Availability required >>99% ?) • Efficiency of loads – In general efficiency of appliances has been improving over the last 5 years, particularly for motors (20-30% improvement) – Need life cycle costing to determine if it makes sense to replace appliance with newer, more efficient version (DC maybe?) ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Loads • Phantom Loads – Phantom loads are loads which, though typically quite small and not counted in load calculations have appreciable (and often unaccounted) power drain on the system – Inverter efficiencies are typically not constant with power level (highest when closest to their rated power), so continuous small loads appear much larger − Due to low inverter efficiency a 20 W load will ‘look’ like a 200 W load − An increasing number of loads have cts power requirements, typically anything with a clock, remote control, indicator lights ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Loads • Heating loads – Generally energy intensive, and often can be accommodated in other fashions e.g. water heating, house heating, some cooking – Small heating loads usually not worth converting – Key design principle is to eliminate as many heating loads as possible primarily water heating and electric range • Electronic loads – Examples include VCR, TV circuitry etc. – May be damaged by peak voltages outside of range of utility voltage – Appliance may use power signal not only as power source but also as timing signal • Timing signal can be triggered from rising or falling voltage level • Noise, fluctuations, changes in slope of voltage waveform can alter triggering of signal disrupting operation; means additonal requirements for the quality of the output waveform ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner Summary • We have seen the major issues in batteries dealing with how they operate by reduction-oxidation reactions and what some major degradation and failure modes are • These issues help guide in deciding the best charging regimes to be used in systems that use batteries to reduce variability in output • Looked at the role of inverters for systems connected to AC appliances or connected to the grid • Finally we looked at the role of the load itself in determining how efficiently a system can operate ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner