Introducción a la ESF y a los sistemas autónomos Alberto Escudero-Pascual IT+46

Table of Contents ● Brief intro to stand alone photovoltaic systems ● Components – Solar panels – Batteries – Regulators – Converter and inverters – Load ● Measurements

Stand alone photovoltaic System ● Stand alone = isolated – not connected to the grid ● Photovoltaic – The use of use of solar energy for the production of electrical energy

image OF FULL sTSTEM

Solar Panel ● Composed of solar cells ● Collecting solar radiation and convert it to electrical energy ● A set of panels forms arrays ● Current supplied depend of solar energy and total amount of solar cells ● Solar cells can not store energy – batteries are needed for non-solar hours

Battery (accumulator) ● Stores produced energy that is not consumed immediately ● Stored as chemical energy ● Most common type in solar applications are the maintenance-free lead-acid batteries, AKA: – Recombinant – VRLA (valve regulated lead acid)

Regulator ● Electricity from solar panels and batteries are DC – Load normally run on AC ● DC/AC converter (inverter) is needed – Energy is lost during the conversion ● Converters can also be used to obtain DC at another voltage level that supplied by the batteries ● DC/DC converter – Implies energy losses! – Avoid by designing you system according to your load

The load ● Equipment connected to your energy system that consume the generated energy and stored energy – Wireless communications equipment, routers, workstations, lamps, TV sets, VSAT modems, etc. ● A good estimation of your load is crucial – Dimensioning of panels, batteries etc. ● The use of efficient and low power equipment should be considered

Let's start from the beginning...

Solar Panels

Physical design ● Typically 32 or 36 solar cells of crystalline silicon ● Depending on size of solar cells, the area of the module varies between 0'1 and 0'5 m² ● Two electrical contacts (positive and negative) ● Bypass diodes (optional) – protects the panel against “hot-spot” – takes places when some of the cells are in shadow and start to behave as a load (consumes energy) which leads to that other solar cells increase their temperatures (85 and 100ºC)

Electrical Performance ● IV characteristic curve – Represents all possible values of voltage/current ● Depend on two main factors – temperature – solar radiation received by the cells ● Current generated is directly proportional to solar irradiance (G) ● Voltage reduces slightly with an increase of temperature.

Electrical Performance ● The point of operation of solar panel is determined by the “load” that is present between its electrical contacts. ● A good regulator will try to maximize the amount of energy that a panel provides by tracking the point that provides maximum power (V x I). ● The maximum power corresponds to the knee of the IV curve.

Electrical performance

Solar Panel Parameters 1. Short Circuit Current (I SC ) ● The maximum current that the panel provides ● Corresponds to the current produced when connectors are short circuited 2. Open Circuit Voltage (V OC ) ● The maximum voltage that the panel is provided with ● Corresponds to the case where the terminals are not connected to any load or the circuit is open ● Normally 22 V for panels that are going to work in 12 V systems

Solar Panel Parameters 3. Maximum Power Point ● An operation point where the power supplied by the panel is maximum ● P max = I pmax. V pmax ● Measured in Watts (W) or peak Watts (Wp) ● Remember! The panel will normally not work at peak conditions as the voltage of operation is fixed by the “loads” or the regulator ● Typical values of V Pmax and I Pmax should be a bit smaller than the I sc and V oc

Solar Panel Parameters 4. Fill factor (FF) The relation between the maximum power that the panel can give and the product I SC. V OC It gives an idea of the quality of the panel The closer the FF is to 1, the greater the power a panel can provide Normal values between 0'7 and 0'8.

Solar Panel Parameters 5. Efficiency (h) ● The ratio between the maximum electrical power that the panel can give to the load and the power of the solar radiation (P L ) incident on the panel ● Normally around 10-12% ● Depending on the type of cells (monocrystalline, polycrystalline, amorphous or thin film) ●  = P M / P L = FF. I SC. V OC / P L

Solar Panel Parameters ● The values of I SC, V OC, I Pmax and V Pmax are provider by the manufacturer ● Refer to a a standard conditions of measurement of: – Irradiance (G) = 1000 W/m2 – A temperature (T) of solar cells of 25 o C – Measured at sea-level

IV Curve

IV curve vs temperature

Parameters for dimensioning ● To estimate the number of panels needed – Current and voltage at the point of maximum power (I Pmax and V pmax ) ● The panel will not always work at the point of maximum power – Assume a 5% loss of efficiency

Interconnection of panels ● Array of solar panels – a number of panels interconnected electrically – common support structure ● Important that all panels of an array are identical – Same brand and same characteristics – Any dispersion will affect the operation and performance of your system

Interconnection of panels ● By interconnecting solar panels we can – obtain a level of voltage that is near (but greater) than the level of voltage of the batteries – supply a level of current big enough to feed our equipment and to load the batteries

What is a good panel? ● Low soiling geographical areas – Panels with low affinity for soil retention ● Check the mechanical construction of the panel – Verify that the glass is hardened and the aluminum frame robust and well built. – Solar cells has a long life time (20 years) if they are treated well as they are very fragile

What is a good panel? ● Make sure that the manufacture provides you with the variation of the power with the irradiation and temperature! ● Look for the manufactures quality guarantees in terms of expected power output and mechanical construction. ● A quick measure of cost efficiency when buying panel is to compare the ratio: Watt/Price

Battery

The Battery ● Reversible chemical reaction – Makes it possible to store electrical energy which later can be retrieved when needed. ● Lead-acid batteries consist of – Contains a set of elements (cells) arranged in series – Two submerged lead electrodes in an electrolytic dissolution of water and sulfuric acid – Most common batteries (in PV systems) have a nominal tension of 12 Vor 24 V.

The Mission ● To supply electrical energy to the system when energy is not supplied by the solar panels ● Cyclical process of charging and discharging energy depending on the presence or absence of sun light ● When sun: energy that is not consumed immediately charges the battery ● No sun: all consumed energy is taken from the battery which discharges

Autonomy: Cost vs Availability ● Autonomy: how long the system can provide energy to its load without being charged ● The level of autonomy must be defined – depending on what load the system is serving – infrastructure vs client equipment ● Oversized system – Costly and inefficient ● Undersized system – Exhausted battery (no energy!)

Types of Batteries ● Most suitable type of batteries for photovoltaic applications are stationary batteries, designed for: – having a fixed location (fix station) – scenarios with irregular power consumption – no need to produce high currents in short periods of time but should accommodate deep discharge cycles ● Alkaline Electrolyte (like Nickel-Cadmium) – Reliable and resistant but expensive ● Acid Electrolyte (Lead-Acid) – cheaper, good enough

Car Battery (traction battery) ● Can be used when no stationary batteries are available, but not recommendable ● Designed to provide a great intensity during few seconds (when starting), not low intensity for a long period of time ● Shortening of life when used in photovoltaic systems ● Need very frequent maintenance ● Should not discharged more than the 70% of its total capacity – You can only use a maximum of 30% of their nominal capacity

State of Charge ● Overcharge ● Overdischarge

Overcharge ● The water of the sulfuric acid dissolution begins to break, producing oxygen and hydrogen (gasification) ● Disadvantage: Oxidation of the positive electrode ● Advantage: the presence of gas prevents the stratification of the acid. ● A compromise needs to be found – Allow light periodical and controlled overcharges – Ensured by regulator

Overdischarge ● Disadvantage: Deterioration of the battery ● The regulator prevents that more energy is extracted from the battery. – When voltage of battery reaches minimum limit, the regulator disconnects the load ● Deep and long term discharges of a battery is very results in – Formation of hard sulfation (crystallized sulfate) – Loosening of battery plate active material – Plate buckling

Battery Parameters 1. Nominal Voltage V NBat (12 V) 2. Nominal Capacity C NBat ● Maximum amount of energy that can be extracted from the battery (Ah or Wh) ● Depends on the “speed” of the extraction process ● Capacity of a battery should be specified based on different discharging times

Battery Parameters 3. Maximum Depth of Discharge DoD max ● Percentage of energy extracted from battery in a discharge ● Limited by regulators, normally calibrated to allow a max DoD of 70% ● Life expectancy depends on how deep the battery gets discharged in every cycle ● Manufacturer must provide information about the number of charge-discharge cycles in relation to the life of the battery ● Avoid to discharge a deep cycle battery more than 50% and no more than 30% for traction batteries

Battery Parameters 4. Useful Capacity C UBat ● The real (usable) capacity of a battery ● Equal to the product of the nominal capacity (C NBat ) and the DoD max ● Example: A stationary battery of nominal capacity of 120 Ah and a DoD max of 70%, has a useful capacity of 84 Ah.

Measuring the State of Charge ● Assume: – 12V sealed lead-acid battery – linear discharge of voltage during operation ● Fully loaded: 12.8 V ● Load attached (fully loaded): 12.6 V ● Discharged: 11.6 V ● 70% is reached at: 11.9 V

Battery Protection ● To protect battery and installation from short circuit and malfunctions: – Thermomagnetic circuit breakers – One time fuses ● Fuses are rated with a maximum current and a maximum usable voltage ● Maximum current should be chosen slightly over the maximum expected current – Maximum current of 4A: a fuse with a 5A rating is suitable

Battery Protection ● Never replace a fuse with a wire or a higher rating fuse. ● Always replace a broken fuse with one that has the same characteristics ● Two types of fuses: – Slow blow – Quick blow

Temperature effects ● Increase of temperature ● Decrease of temperature

Increase of temperature ● Nominal capacity (given at 25°C) increases with the temperature at the rate of 1%/°C ● In the case of too high temperature – Same type of oxidation that takes place during overcharging – Reducing the life expectancy of battery ● In hot areas: – never direct sunlight, cooling system

Decrease of temperature ● The “useful life” increases, but the risk of freezing increases ● Freezing point depends on the density of the solution – which is directly related to the state of charge of the battery. ● The bigger the density the small the risk of freezing. ● In areas of low temperatures we should avoid having batteries discharged

Life expectancy vs no of cycles ● Only component in a PV system that needs to be replaced regularly ● Reduce the number of deep discharge (>30%) cycles to increase the life of a battery ● Discharging a battery every day results in replacement after less than one year. ● By using only 1/3 of the capacity the battery can last more than 3 years. ● n be cheaper to buy a 3 times bigger capacity than to change the battery every year.

Regulator

● Also knows as – charge controller – voltage regulator – charge-discharge controller – charge-discharge and load controller ● Sits between the array of panels and the batteries and the battery and the loads.

Regulator ● Connected in series (not parallel!) ● They disconnect the array of panels from the battery to avoid overcharging ● They disconnect the battery from the load to avoid overdischarging ● The connection/disconnection is done by means of switches – Electromechanical (relays) or – Solid state (bipolar transistor, MOSFET)

Regulator ● Modern regulators can automatically: – Disconnect the panels during the night to avoid discharging of the battery – Overcharge the battery periodically (equalization) to improve their life time – Use a mechanism known as pulse width modulation (PWM) to prevent excessive gassing

Regulator Parameters ● Maximum current that the regulator can handle – Must be at least 20% bigger that the array of panels it is connected to ● Operational Voltage: 12, 24, or 48 V

Regulator Parameters ● Values of LVD, LRV and HVD ● Support for temperature compensation – Takes the temperature in consideration with calculating cut-off and reconnection points. ● Instrumentation and gauges – To measure the voltage of the panels and batteries, the state of charge (SoC) or Depth of Discharge (DoD). – Alarms that indicate that panels or loads have been disconnected, etc.

Converter

● In PV systems, we deal with: – DC/DC converter – DC/AC converter (inverter)

DC/DC converter ● Transform a continuous voltage (DC) to another continuous voltage but with a different value ● Used between the output of the battery and the load ● Two conversion methods – Linear conversion (only decrease the voltage, simple but not efficient) – Switching conversion (increase or decrease the voltage, even inverse the voltage, more complex and efficient)

Example: Linear converter ● DC/DC conversion from 12 V to 5 V ● For a 1 A output current the linear regulator would consume 1 A on the input voltage. ● P input = 12 V x 1 A = 12 W (into the converter) ● P output = 5 V x 1 A = 5 W (out of the converter) ● Efficiency = P output /P input = 5 W/ 12 W= 42% ● For a linear regulator the efficiency decrease when the difference between the input voltage and the output voltage increase.

Example: Switching converter ● Typically, a switching regulator has an efficiency of 80% or more ● For a 5W output the input power would be only 6.25 W or only 0.52 A on the 12V input. ● Switching regulators can be constructed with electronic components or can be purchased as modules

DC/AC Converter ● Used when the load needs AC power ● Chop and invert the DC current to generate a square wave, a modified sine wave or a pure sine wave ● Efficiency: Modified sine wave inverters perform better than pure sinusoidal inverters ● Not all the equipments accept a modified sine wave as voltage input – Laser printers

DC/AC Converter Features ● Reliability in the presence of surges ● Conversion Efficiency ● Battery charging – many inverters can also incorporate the inverse function: the possibility of charging batteries in the presence of an alternative source of current (grid, generator, etc). Known as charger/inverter. ● Automatic fall-over – The capability to automatically switch between different sources of power (grid, generator, solar)

DC/AC Converter ● When using telecommunication equipments avoid the use of DC/AC converters and feed them directly from a DC source. ● Most communications equipments can accept a wide range of input voltage ● Investigate carefully to ensure optimal performance and avoid burning a few capacitors

Load

Loads (equipment) ● The greater the consumption, the more expensive is the installation ● Two fundamental aspects – A realistic estimate of the maximum consumption – When the installation is in place, respect this established maximum consumption to avoid frequent failures in the provision.

Suitable Loads ● Illumination (halogen or fluorescent bulbs) ● More expensive lamps but more efficient ● LED lamps: efficient and DC fed ● Low and constant consumption house appliances ● TV, radio ● Not recommended for any application that transform energy into heat (thermal energy) ● Stove, heater ● Use solar heating or butane as alternative

Suitable Loads ● Illumination – Halogen or fluorescent bulbs (more expensive but more efficient) – LED lamps: efficient and DC fed ● Low and constant consumption house appliances – TV, radio

Non-suitable Loads ● Not recommended for any application that transform energy into heat (thermal energy) – Stove, heater – Use solar heating or butane as alternative

Loads that needs special attention ● Washing machines ● Use conventional automatic washing machines avoiding the use of any washing programs that include centrifuged water heating ● Refrigerators ● Low power consumption. ● Some refrigerators work in DC although their consumption is high (around 1000 Wh/day).

Loads that need special attention ● Washing machines – Use conventional automatic washing machines to avoid the use of any washing programs that include centrifuged water heating ● Refrigerators – Select one with low power consumption – Some refrigerators work in DC although their consumption is high (around 1000 Wh/day).

Power consumption of typical load

Wireless equipment

Consumption in Wireless equipment ● Power consumption of wireless equipment depend – the number of network interfaces, radios, types of memories/storage and traffic ● Rules of the thumb – A wireless board of low consumption consumes in the range of 2-3 W – A 200 mW radio card consumes as much as 3 W – High power cards (like Ubiquity, 400 mW) consume around 6 W – A repeating station with two radios can range between 8 and 10 W.

Consumption in Wireless equipment ● IEEE incorporates a power saving (PS) mechanism – not as good as expected ● Main mechanism for energy saving – Allow stations to put its wireless cards to sleep periodically by means of a timing circuit. – When the wireless card wakes up it verifies if a beacon exists indicating pending traffic. ● The energy saving takes place in the client side – Access point needs always to remain awake

Measurements

● Need to measure the current and voltage at different points with a multimeter ● To measure a voltage the multimeter must be connected in parallel ● To measure a current the circuit must be broken and the multimeter connected in series. ● Note! Never use the ampere meter in parallel, it would create a short circuit and either the multimeter or the installation would break