One of the best uses of electric double-layer capacitors (EDLCs), also known as ultracapacitors or supercapacitors, is in the design of hybrid power supplies. The primary supply can be a battery, an ac power source, or a combination of both.
In all cases, ultracapcitor devices can augment the primary supply either to improve power density or to provide carry-through to maintain power quality. In all but the simplest cases, a control or interface device is required to allow the primary power source and the ultracapacitor bank to operate properly.
The amount of power delivered by the capacitor bank in most applications is high—on the order of kilowatts. Also, the capacitor bank often isn’t connected to the main power bus unless there is a sudden need for extra power. In such a case, there must be some way to sense the need and subsequently to engage the capacitor bank, and this generally requires a smart controller.
One of the first considerations is the connection to the power bus. In most instances, the best method of making the connection is through the use of fast-switching transistors, such as high-speed MOSFETs. For the capacitor bank to deliver high power, it is essential to keep the effective series resistance (ESR) of the capacitor bank as low as possible since the amount of load taken by the capacitor bank depends on the ratio of the primary source’s effective internal resistance to that of the ultracap pack. Thus, when choosing the switching transistor, its resistance in the forward conducting state should be as low as possible.
The use of contactors is also possible but is generally to be avoided, since they introduce a higher resistance to the bus connection and increase the effective ESR of the ultracapacitor bank. To engage and disengage the ultracapacitor bank, the controller must sense current and voltage from the primary source and the ultracapacitor bank. The ultracapacitor bank must be monitored to ensure that when power is demanded, that bank is fully charged and at the appropriate voltage level to provide the required power.
The Goals Of Design
The control circuits that we design for specific customer needs are programmable so various parameters can be adjusted under software control. If the ultracapacitor bank used to provide power to a motor under heavy load conditions in hybrid supplies is not engaged for a sufficiently long period of time, its usefulness is diminished. In many applications, we have found it necessary to reprogram controllers to lengthen or shorten the time of engagement.
The controller is the heart of many systems that we design, and its function clearly depends on the particular application. The preliminary considerations noted earlier are general and apply to virtually every application in which control circuits are required. One simple application that doesn’t require any sophisticated control is the use of a bank of ultracapacitors with an automotive starting battery.
Starter batteries typically suffer degradation in cold weather and frequently cannot provide sufficient current to start the car’s engine. Using an ultracapacitor bank in parallel with the battery can provide sufficient current to charge the bank. When the starter motor is engaged, the ultracapacitor pack provides the power required to start the car.
This simple example illustrates two important features. First, in cold weather, the effective internal resistance of the battery is increased, reducing its ability to provide enough power to start the engine. Second, the fact that ultracapacitor banks are fully functional to –40°C makes them more immune to the effects of cold weather.
Even though this particular application does not require a control circuit, a better and more efficient version would employ a control circuit performing such tasks as limiting the current draw on the battery when the ultracapacitor bank is totally discharged and engaging/disengaging the ultracapacitor bank for starting and disengaging once the engine is started.
As stated above, each application calls for control functions that are unique. Some requirements are common to most applications. However, each application will have its own set of unique control requirements.
In a recent control module built to customer specifications, it was necessary to disengage the ultracapacitor bank when the primary power supply, in this instance a bank of lead-acid storage batteries, was removed. The issue was one of safety since removal of the primary power source would lead maintenance personnel to believe the power bus was dead. If the ultracapacitor bank were engaged, unbeknownst to the maintenance personnel, a nasty surprise could be in store.
Energy Harvesting
With the emphasis on increased efficiency, energy that was formerly wasted can now be harvested and used. Most regenerative energy recovery systems fall into this category. Without the aid of a smart controller, such systems would be impossible to build. For example, ultracapacitor devices are widely used in automotive regenerative braking systems.
The basic concept is simple: to brake the automobile, slow it down by generating electrical power, which is stored in a bank of ultracapacitors. The ultracapacitor bank is superior to recharging the automobile’s battery because the ultracapacitor bank can absorb much more energy than would be used in recharging the battery. This is easily understood in terms of the power density of the ultracapacitor bank versus the battery, as the battery is limited by the reaction rate, whereas the ultracapacitor’s charging rate is limited only by its intrinsic ESR.
An automobile’s regenerative braking system is far more complicated than the previous examples discussed due to safety issues and the functions that the controller must execute. The “electric brake” converts mechanical energy (the kinetic energy of the car) to electrical energy using a generator that feeds a load, namely the ultracapacitor bank.
If the driver wishes to decrease speed from 60 mph to 45 mph over a 30-second interval, the braking can be accomplished entirely by the electric brake, provided the ultracapacitor bank is in a state of discharge and can absorb the electrical energy. If the ultracap bank is fully charged, then either the electrical energy is shunted to a resistor bank or the car is slowed by application of the mechanical brake.
The control circuitry is key to making this happen. To accomplish these tasks, the state of charge of the ultracap bank must be known, the rate of speed decrease must be monitored, and the appropriate action is related to the pressure the driver puts on the brake pedal. This pressure is the primary input to the control circuit and dictates the rate of speed decrease. The entire system also interfaces with the ABS system of the car, introducing a third element. The control circuitry must respond to the pressure applied to the brake pedal and implement the appropriate response.
Suppose the electric brake fails due to an open condition in which the load is not connected to the generator. In this instance, no braking will occur. But the controller that is sensing there is no decrease in speed must respond to this situation by causing the mechanical brake to engage. It should be obvious that the control circuit in this type of scenario is the heart of the system. If computer-based control circuitry were not available, regenerative braking would be little more than a fantasy.
In the example above, if a car is to decellerate from 60 to 45 mph, the work done to slow the car is 157 Joules/kg. Assuming this energy is stored in an ultracapacitor bank that is initially uncharged with a nominal voltage rating of 15 V, where the voltage has been chosen to be consistent with a 12-V automotive system, the minimum capacitance per kilogram is on the order of 1.5 F/kg. For a 2200-pound car (1000) kg, this translates to 3300 Farads of capacitance. Using just an ultracap bank for temporary storage of this amount of energy is currently not cost effective.
But with the aid of a controller that can distribute the acquired energy to provide power for other functions so the regenerative power can be used immediately, the capacitor bank can be used to rapidly acquire the energy. The controller can then be designed to redistribute this energy for other needs. Clearly, the size of the ultracapacitor bank will determine the maximum amount of energy that can be harvested. As the cost of ultracapacitor devices comes down and the sophistication of control circuitry increases, these systems will become more and more cost-effective.
Trains, Cranes, And Trees
The concept of energy harvesting is not limited to regenerative braking. Rather, there are many instances where energy can be recovered that would otherwise be wasted. Two examples that come to mind are regenerative braking systems for trains and recovery of energy in cranes and elevator systems.
The first example is essentially the same as regenerative braking in automobiles, though the amount of energy is far greater. For a commuter railroad, it is possible to store and share energy among several trains using large ultracapacitor banks located at intervals along the commuter route. This holds the energy on a short-term basis until it is required by a train for acceleration or other electrical needs.
The use of energy harvesting in a shipyard is another natural application. The loading and unloading of ships requires the lifting of transport containers. Containers must be lowered at a controlled speed. To accomplish this, a brake is employed. Rather than just being wasted, the energy could again be stored in an ultracapacitor bank and used to assist in the next lift.
The number of potential applications is unlimited. On a smaller scale, using ultracapacitors in conjunction with piezoelectric converters allows small vibrations to be converted to electrical energy, and that energy could then be stored in an ultracapacitor bank for later use.
A final and novel example of energy harvesting comes from an unexpected source. A sustained electrical potential exists between the xylem of many plants and their surrounding soil. In a paper written in 2008, researchers discussed the possibility of taking advantage of such sustained voltage differentials to generate small amounts of electrical energy (see “Tree-To-Earth pH Difference May Generate Harvestable Energy”). The generated electrical energy could be stored and used to power sensors in remote locations for the purpose of fire detection and/or other environmental monitoring purposes, eliminating the need for any external power source.
Conclusion
In addition to a smart controller, the use of power electronics allows for energy storage in an ultracapacitor bank to be converted to a higher or lower dc voltage utilizing boost and buck circuitry. It also allows for the conversion of dc voltage levels to ac voltage levels, again using switch-mode power conversion techniques. The use of switch-mode power techniques, micro-based smart controllers, and ultracapacitor devices has opened up a vast array of potential applications that will allow for better power quality and more efficient use of electrical energy.
Reference
1. “Source of Sustained Voltage Difference Between Xylem Of A Potted Ficus Benjamina Tree and Its Soil,” Chirstopher J. Love, Shuguang Zhang, Andreas Mershin, Center for Biochemical Engineering, Massachusetts Institute of Technology