Power Management Power up
As the demand for solar photovoltaic and wind turbine generated power increases how can the energy generated be efficiently and safely stored? John Wynne considers the options
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n order to provide output power from solar photovoltaic (PV) installations and wind turbine systems over a longer time period that natural conditions allow – whether in darkness or with insufficient wind speed – it is necessary to store the generated energy by some means, so it will be available when demanded later. But how can the generated energy be stored in banks of Li-ion cells? Which products can deliver safe and efficient system operation over time and temperature?
The storage components used in a solar PV system are identical with those in a wind turbine system. The output of a small array of solar panels is boosted up to a level which facilitates the output inverter to deliver the desired ac-output level. The boost converter also supports MPPT (Maximum Power Point Tracking) functionality under the control of the local processor. Following the input boost circuit there is a V/I charger circuit which generates a suitable charging current required to charge the array of Li-ion cells. The Li-ion cells are generally grouped in modules or packs usually containing between 6 to 12 cells. These modules are then bolted together in series (creating a battery string) to produce whatever is the desired overall stack voltage. The stack voltage is generally chosen to ease the job of the dc/dc inverter so, for instance, to generate an output of 110V ac, the overall stack voltage might well be 180V dc. From a quick glance through various manufacturers’ literature a typical Li-ion cell recommended for small UPS installations has a nominal voltage of 3.7V for a 4/5 D-size cylindrical cell and a typical capacity of 4.4 Ah. This would lead to 48 cells in a series string, arranged in eight packs of 6 cells. Each string has a capacity of 780Wh. To increase the energy storage capacity of the battery bank additional strings are added in parallel. Assembling a battery stack with a capacity sufficient to supply a private household for a number of hours might well require numerous strings; for example, 26 strings in parallel will give a total capacity of 20KWh.
Assuming the useable power in the
battery stack is 80% of rated capacity this produces a useable capacity of approximately 16KWh. It cannot be stated often enough and clearly
24 July/August 2010 Components in Electronics
Figure 1: Typical energy storage system showing two strings of Li-Ion cells in parallel
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enough that such high voltage battery stacks are completely lethal and proper and correct procedures must at all times be adhered to. Your first mistake with a high voltage stack is generally your last. Please ensure that safety is always a priority.
This 20KWh capacity requirement translates into a total of 1,248 Li-ion cells in the array. A solar PV system with just two strings is shown in Figure 1. A typical system will have many more strings and with so many strings in parallel it would be prudent to have some means of isolating individual strings from the overall array. This might be some sort of high current relay or contactor in series with each string.
Each string has its own dedicated monitoring circuitry which is independent from adjoining strings. This might sound like extravagance but assigning dedicated monitoring components to each string eases individual string isolation if necessary and allows tighter monitoring over the Li-ion cells within a string. Overcharging Li-ion batteries can result in local heating leading to widespread heating, possibly leading to thermal runaway and eventually fire. Close monitoring avoids this scenario. Although the price of battery technology is dropping as
manufacturing capacity ramps up and more suppliers enter the market, such
stacks are still expensive. For instance, when the price of all the necessary components are added together (including connectors, wiring harnesses, mechanical housings, monitoring electronics and so on) a figure of between $750/kW and $1,000/kW is not unreasonable This would put the stack in this example at a minimum of $15,000 (approximately £10,300). This investment needs to be carefully monitored in order to ensure a long lifetime.
Highly specialised cell monitor products can provide full voltage and temperature monitoring capabilities for each Li-ion cell in the string, while the individual string current can be monitored by a microconverter. In addition to having current and voltage monitoring input channels each microconverter also communicates with its string of Li-ion cell monitors, downloading individual cell data and running cell balancing algorithms. The main system functionality in this example is under the control of a Blackfin (BF50X) processor. This communicates with the various system blocks via iCouplers to allow the entire front end system to float and to electrically isolate the back-end from high voltages for safety reasons. When not required to generate power and also for safety purposes during maintenance it will be necessary to disconnect the battery stack from the inverter and this is the function of the three relays or contactors. Again for safety reasons it is a good idea to switch in a discharge path across the inputs of the inverter when the contactors are opened for whatever reason. This ensures that the inverter capacitors are drained of
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