30 Apr Ultracapacitors in Hybrid Systems for Military Vehicles
Much attention is presently being given to low-emission and zero-emission technologies based on distributed energy generation devices, in particular, renewable energy sources and hybrid cogeneration systems. The key elements used for the start-up of large-scale cogeneration systems based on internal combustion engines are batteries and ultracapacitors. Ultracapacitors and batteries are also used in heavy vehicles with conventional drivetrains as well as in trucks and electric vehicles. In the case of batteries, during operation in difficult conditions, i.e., low ambient temperature, their operational parameters significantly decrease because the internal resistance of the battery increases (in particular, the resistance of the electrolyte) and the electromotive force and voltage at the connection points decrease. In such conditions, the mechanical resistance in the internal combustion engine is higher too, which is caused by the higher density of engine oil. This, in conjunction with the decreased available electric power, results in a higher load on the battery, leading to a decrease in its expected lifetime. In order to increase energy and power, start-up batteries of greater capacity or a greater number of batteries connected in parallel are used, which increases costs. One of the possible solutions to this problem is the implementation of a hybrid energy storage method based on the parallel connection of a battery and an ultracapacitor.
The advancement in battery technology has made the start-up of combustion engines in standard operating conditions an easy task, with batteries being cheap, effective, and relatively durable. It is worth pointing out that operation in difficult conditions can occur as well, where the high cranking current causes an overly deep drop in battery voltage, a decrease in the power of the starter motor, and combustion engine start-up difficulties. Examples of such difficult conditions are low ambient temperature, long time gaps between engine operation phases, significant power draw from vehicle onboard electrical appliances (i.e., the dashboard), and in the case of vehicles featuring start–stop systems, high frequency of system operation (frequent combustion engine start-ups and short periods of battery charging—dynamic conditions that cause the battery to lose charge at a faster rate, decreasing the usable capacity and durability. Additional drawbacks of batteries are self-discharge and difficulty in detecting the discharge point of the battery (often, full discharge occurs suddenly, with no warning.
According to the scientific literature, the rate of self-discharge can range from 5% to even 30% SOC over one month, depending on the type of battery and storage conditions (i.e., ambient temperature). Use of a parallel connection of a battery and ultracapacitor allows for availability of stored energy even if the voltage of the hybrid system drops to a value indicating a discharged battery (equivalent to SOC = 0). The amount of energy stored at that voltage in the ultracapacitor still allows for start-up of an internal combustion engine. This situation may occur for vehicles that are used sporadically or in cold climates (i.e., service and military vehicles) as well as vehicles with an increased load present on the DC electric installation.
Hybrid Energy Storage Based on Battery and Ultracapacitor
Properties of ultracapacitors and batteries are often complementary to each other. It can be seen especially when a parallel connection of the two components into a system that combines the benefits of both energy storage solutions can be achieved. In particular, the parallel configuration of ultracapacitors and batteries will simultaneously have high energy (effect of batteries) and high power (effect of ultracapacitors), even at very low ambient temperatures (effect of ultracapacitors) and at a low SOC.
Currently available on the market are single ultracapacitor cells and modules with the serial or serial-parallel connection of several cells. Modules are equipped with electronic control systems, such as a battery management system (BMS), that ensure an equal charge level in all cells and overcharge protection. With such a module, parallel connection with batteries is relatively simple if the required “soft start” system for limiting the amount of current equalizing the voltages at the moment of connection of a discharged ultracapacitor with the battery is kept in mind. To ensure protection against deep discharge of the batteries, a DC/DC converter is required for raising the ultracapacitor output voltage to a level safe for the battery. Ultracapacitor modules featuring these kinds of electronic systems (BMS, “soft start”, and DC/DC converter) in a single case are similar in size to a starter battery and are available on the market in 24 V versions.
The main advantages of ultracapacitor plus battery systems are:
- A greater power density of the hybrid system in comparison to a battery-only system.
- The energy is available in a wider range (ultracapacitor effect). So, even if the battery is completely discharged (SOC = 0), the ultracapacitor can still deliver energy to the starter motor system, which allows for engine start-up, especially in difficult conditions (such as low ambient temperature, infrequent start-up, very high momentary current peaks, etc.).
- The reduction of costs related to the replacement of the battery (extension of the battery lifetime). In the considered system, the ultracapacitor takes over the larger part of the load, which means the battery is not loaded with high current values (maximal current values are not exceeded), which directly influences the extension of the battery lifetime.
A disadvantage of such systems is:
- Self-discharge. For the battery, the rate of self-discharge ranges from about 5% up to even 30% SOC over a one-month period depending on the type of battery and the storage conditions (e.g., ambient temperature). In the case of the ultracapacitor, the rate of self-discharge is about 6.25% per month (75% per year).
This is an excerpt of the journal article: Research on Ultracapacitors in Hybrid Systems: Case Study, by Piotr Piórkowski, Adrian Chmielewski, Krzysztof Bogdziński, Jakub Możaryn and Tomasz Mydłowski. Published: September 2018 in Energies, 11(10), 2551; DOI: https://doi.org/10.3390/en11102551 under a Creative Commons Attribution License (CC BY 4.0).