Electricity: power supply or regulation systems – In shunt with source or load – Using a three or more terminal semiconductive device
Reexamination Certificate
2000-07-20
2001-03-20
Han, Jessica (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using a three or more terminal semiconductive device
C323S299000, C323S906000, C320S102000
Reexamination Certificate
active
06204645
ABSTRACT:
BACKGROUND OF THE INVENTION
Most electricity used in the United States comes from power grids fed by large power stations. However, for many reasons alternate energy systems are becoming economically attractive in special situations. These alternate energy systems include solar, wind, hydroelectric and thermoelectric generators. Solar electric generators (SG's) have been commercially available in the United States for about 25 years. These units generate electric power from the energy of sunlight, which is free. Attempts have been made to produce electric power from sunlight to supply utility electric grids but these efforts have been largely unsuccessful because the total cost per kilowatt-hour from the solar generators substantially exceed the cost per kilowatt hour for electric power generated at central generating stations powered by burning coal, oil, gas or by nuclear power plants.
The RV Market
However, when it is not feasible to hook up to a power grid fed by a central generating station, the solar electric generator is often the power source of choice. Competitive power sources include gasoline powered motor generating units and thermoelectric devices. A very lucrative market for solar generators is to provide electric power for recreation vehicles (RV's) when the engine of the vehicle is not being utilized for travel. In this situation, the solar unit provides electric power (considering all applicable cost including depreciation, maintenance, etc.) at a small fraction of the cost of operation the vehicle gasoline engine to charge the battery or batteries of the RV. The typical RV has one or two batteries. When there are two batteries, one is for the engine and one is for the “house” portion of the RV. A controller is needed to control the supply of electricity to the batteries.
Prior art controllers have typically been rather simple devices and not much effort has gone into utilizing controllers to maximize the efficiency of solar power generators. Perhaps, the thinking has been “why worry about efficiency when the energy (from the sun) is free?”
The typical prior art solar generating unit sold for RV units is designed to produce power at about 17 volts for charging 12-volt batteries. The typical control unit comprises control switches (either relay control switches or solid state control switches) for connecting the output of the solar generator to the battery and a control unit which monitors the battery voltage and opens the switch when the battery voltage reaches a high target voltage, such as 14 volts and closes the switch when the respective battery voltage drops to a low target voltage such as 13 volts. The prior art control units are also typically constructed with a series diode to assure that current does not flow in reverse through the solar generator discharging the battery at night.
Constant Current Generators
Most solar generating units are designed to operate in what is called constant current mode. This means that for a given level of solar radiation such as 1000 W/m
2
, a substantially constant current is produced for any battery voltage within the design range of the solar generating unit. For example,
FIG. 1
shows current vs. voltage for a typical solar unit, which is the BP275 Module available from BP Solar with offices in Fairfield, Calif. This graph shows that in the sunshine of 1000 W/m
2
at a solar generator temperature of about 25° C., the current produced by this unit is about 4.7 amps for battery voltages between 0 and 14 volts. The current drops off slightly to about 4.5 amps at 17 volts and drops to substantially zero at 21.4 volts. This is referred to as the open circuit voltage. Power is the product of current and voltage. Thus, if the battery being charged is at a low voltage level the rate of power delivery, and hence charging, can be substantially reduced.
Battery lifetime can be adversely affected if it is not maintained in accordance with instructions of the manufacturers. These instructions include recommendations on techniques for charging and maintaining the charge of the batteries.
What is needed is a better controller permitting the solar generating unit to function safely at or near its maximum power capacity and at same time to provide charging to maintain long battery life.
SUMMARY OF THE INVENTION
The present invention provides a controller for a solar electric generator that permits the generator to produce power substantially at its maximum capacity while also providing efficient charging at three charging stages; i.e., bulk charging, acceptance charging and float charging. Power is transferred from the generator to a temporary electric storage device that is periodically partially drained of power to maintain the temporary electric storage device at a voltage corresponding to the voltage needed by the generator to provide maximum generator power. The electric power drained from the temporary storage device is used to charge conventional batteries. In a preferred embodiment, the temporary storage device is a capacitor that is part of a buck regulator operating at 50 kHz with duty factor control between 0% and 100%. This buck topology switching type regulator provides the periodic draining. In the preferred embodiment control of the duty factor of the buck regulator is utilized to limit current, to prevent battery over charging, to test for the voltage corresponding to maximum power, and to operate the solar generator at is maximum power voltage. When operated at its maximum power operating point, the output to the battery is constant power, providing greater battery charge current than prior art controllers. Additional controls are provided to adjust battery charge voltage to permit maximum current flow during bulk charging, and at a first pre-selected charge voltage during acceptance charging and at a second pre-selected charge voltage during float charge. In a preferred embodiment provision is made for periodic equalization overcharging to improve battery performance and lifetime.
REFERENCES:
patent: 5270636 (1993-12-01), Lafferty
patent: 5530335 (1996-06-01), Decker et al.
patent: 6081104 (2000-06-01), Kern
patent: 6111391 (2000-08-01), Cullen
Han Jessica
Ross John R.
Ross, III John R.
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