Electricity: battery or capacitor charging or discharging – Wind – solar – thermal – or fuel-cell source – With shuntless charging source control
Reexamination Certificate
2001-03-26
2002-12-03
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Wind, solar, thermal, or fuel-cell source
With shuntless charging source control
Reexamination Certificate
active
06489743
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates to electric power plants comprising at least one battery adapted to be supplied with electric power from a power source with temporally random availability.
2. Description of the Related Art
Such plants are known which comprise a battery adapted to be recharged with current by means of photovoltaic panels. The energy (power) available in a temporally random manner is then solar radiation. These plants are used in particular in developing countries, in regions having no local or national electricity supply network. Usually, to control the charging and discharging of the battery, the voltage across the terminals of the battery is monitored and a predetermined top voltage threshold and a predetermined bottom voltage threshold are used. Thus, when the voltage across the terminals reaches the bottom threshold, the users are disconnected so as to safeguard the battery from excessive discharging. Once the voltage across the terminals reaches the top threshold, the battery is disconnected from the panels and, after a timeout of a few minutes, the battery is made available for the delivery of current by reestablishing the connection with the users. An advantage of this device is its simplicity.
However, such a method of control has numerous drawbacks. Firstly, in practice there is no strict correlation between the instantaneous voltage across the terminals of the battery and its state of charge. In particular, it is possible for the voltage to be high while the charge of the battery is very low.
Moreover, the battery conventionally comprising several subassemblies each adapted to receive and to output an electromotive force, it is frequently the case that when the top threshold is reached, the charge of the battery is distributed in a very unequal manner between its various subassemblies. Hence, one of the subassemblies might thereafter reach by priority its deep discharge threshold and might overdischarge throughout the period required for the discharging of the other subassemblies. Now, a subassembly which remains deeply discharged for too long will experience a great reduction in its lifetime, so that it is a fifth or a sixth thereof for example, this correspondingly shortening the lifetime of the battery.
Furthermore, theoretically, the step of complete charging of each subassembly comprises in particular, in the case of open batteries, a phase of heterogenization of the electrolyte followed by a phase of homogenization: in the course of the first phase of the charging, concentrated electrolyte firstly gradually fills the porous electrodes of the subassembly, then seeps out of these electrodes in the form of a heavy viscous acid which runs down the electrodes and accumulates at the bottom of the bath. This therefore results in a stratification of the electrolyte: the electrolyte concentration becomes high at the bottom of the bath and low at the top of the bath, hence the expression “heterogenization”. During the second phase, the current received by the battery is apportioned into a charging current proper and into an electrolysis current producing oxygen at the positive electrodes and hydrogen at the negative electrodes. The continued charging of the battery therefore causes a release of gas in the bath, thereby causing forced convection of the electrolyte. This results in gradual homogenization of the electrolyte whose concentration ultimately becomes uniform again throughout the height of the bath. The heterogenization phase is sometimes referred to as the “charging” phase, and the homogenization phase as the “overcharging” phase. However, at the termination of the heterogenization phase, charging is incomplete and the homogenization phase is merely the continuance of charging so as to obtain complete charging. The use of a top voltage threshold to stop the charging causes the homogenization phase to be shortened and often even to be absent. In this way, a heterogenization (stratification) phase without subsequent complete rehomogenization occurs during each charging period. Consequently, the layers of acid accumulate gradually and irreversibly at the bottom of the bath during the life of the battery. Hence, only the bottom parts of the electrodes participate in the operation of the battery. This leads to their rapid destruction and considerably reduces the lifetime of the electrodes and of the battery.
One sometimes attempts to alleviate these drawbacks by overdimensioning the battery. The effective lifetime of the battery is then longer than that of a more modest battery. Nevertheless, this effective lifetime is substantially reduced relative to that normally envisaged for the battery.
SUMMARY
One object of the invention is to provide a method for controlling an electric power plant of the aforesaid type, allowing better knowledge of the effective state of charge of the battery and making it possible to increase the lifetime of the plant without overdimensioning it.
With a view to achieving this object, there is provided according to the invention a method for controlling an electric power plant associated with a power source with temporally random availability, the plant comprising at least one battery adapted to be supplied with electric current from the source, in which method, when the random power is available, the battery current supply is controlled, doing so so as to come as close as possible to a state of complete charge of the battery, and preferably to reach this state.
Thus, the search for the obtaining of the state of complete charge makes it possible to avoid any uncertainty relating to the correlation between the voltage measured across the terminals and the actual state of charge. Once complete charge has been reached, the continuous measurement of the current delivered or received by the battery makes it possible to know at any instant its effective charge with good accuracy.
Moreover, the state of complete charge implies that each of the subassemblies of the battery has also reached its state of complete charge. Subsequently, there is therefore no longer any need to fear premature total discharge on one of the subassemblies, thus making it possible to safeguard the lifetime of the battery.
Furthermore, the complete charging ensures that the phase of heterogenization of the electrolyte has been followed by a phase of sufficient rehomogenization of this electrolyte. This prolongs the lifetime of the electrodes and hence that of the battery.
Finally, the invention avoids the need to resort to an overdimensioning of the plant, which would generate excessive costs.
Advantageously, after the battery has been supplied so as to have received a quantity of electricity above a first predetermined threshold and in particular when the random power is not available, the plant is controlled in such a way that delivery of current by the battery is disabled.
Preferably, the threshold will be below or equal to the partial state of charge after which the electrolyte begins to seep out of the electrodes. Advantageously, this threshold will be as close as possible to this limit. This threshold has a particular value for each type of battery. In certain cases it corresponds to 5% of the nominal capacity of the battery.
Thus, under the aforesaid conditions, the subsequent continuance of the charging phase is favored over the immediate delivery of current. For example, if the phase of heterogenization of the bath has been interrupted for lack of random power, a bid for immediate considerable delivery of current would lead under the effect of the stratification to deep discharging and hence to the fatiguing of the lower parts of the electrodes. It is therefore preferred rather to preserve the battery state for the subsequent resumption of the charging phase causing the continuance of the heterogenization and then the complete rehomogenization of the bath. The lifetime of the battery is thus lengthened.
Advantageously, after the battery has been supplied so as to have received a quantity of electricity below the first thres
Alzieu Jean
Camps Jean-Claude
Smimite Hassan
Blakely & Sokoloff, Taylor & Zafman
Electricite de France (Service National
Tibbits Pia
Tso Edward H.
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