Implementation of supercapacitors in uninterruptible power supplies

Andrew Stepanov, Ilja Galkin, Lauris Bisenieks RIGA TECHNICAL UNIVERSITY

Institute of Industrial Electronics and Electrical Engineering Kronvalda boulevard 1-324

Riga, Latvia Tel.: +371– 67089918. Fax: +371– 67089941.

E-Mail: astepanov@eef.rtu.lv, gia@eef.rtu.lv, bisenieks@eef.rtu.lv

Keywords «Supercapacitor», «Uninterruptible power supply», «Switched-mode power supply»

Abstract The given paper deals with uninterruptible power supplies with supercapacitors. Parameters of the supercapacitors and common lead-acid batteries are compared. Various opportunities of supercapacitor implementation into the uninterruptible power supplies are discussed, pros and cons are analyzed. Initial numerical comparison of energy capacity for these devices is done. Experimental testing of the supercapacitors and batteries as energy sources for a boost converter has also been made.

Introduction We live in the time when electricity is integral part of our life, but energy faults are still possible. That is why important objects are connected to the supply grid via two independent transformers. Such objects have also an emergency diesel-generator. It ensures reconnection of energy sources, but does not protect against voltage drops during the switching. Besides that there are others energy problems, like harmonics, overvoltage which cannot be overcome by just double feeding. The solution is in feeding the critical equipment through an uninterruptible power supply (UPS). Its safety depends on its type: off-line, line-interactive or on-line UPS, but the best of them is “On-line” UPS. Operation time of UPS depends on the capacity of UPS energy storage, but the power of it depends on capability of the energy storage to give high current. Therefore the choice of the storage is an important part of the design procedure of UPS. This is the main topic of the paper.

Batteries In the majority of cases, lead-acid batteries (LAB) are utilized in UPS as energy storages. Amount of returned energy of LAB depends on its discharge current (Fig. 1).

Fig. 1. Discharge characteristics of a LAB

If discharge current is 0,05C the battery discharge time is 20 hours and battery gives back 100 percents of the stored energy. At 2C the battery discharge time is 15 minutes and efficiency is about 50 percents. At bigger discharge currents this value is even less. In uninterruptible power supplies discharge current is above 4-6 C and therefore batteries discharge time is often about 3-5 minutes, but the efficiency is about 30 percents. The phenomenon takes place because of chemical processes in the battery. The higher current leads to the higher crystallization of the electrodes that limits a contact surface of the electrode and electrolyte. Battery life is also affected by discharge depth (Fig. 2). The deeper discharge, the fewer charge-discharge cycles can be done.

Fig. 2. Lifetime versus depth of discharge

In order to let a battery accumulate maximum of its capacity, it is necessary to charge it correctly. One possible method is shown in Fig. 3.

Fig. 3. Charging current of a LAB There are many methods of charging the battery, but all of them prevents battery overheating at the beginning and its overcharging at the end. A fast charging method lasting 4 - 6 hours is often used in UPS, but this method is not good for battery. Taking into consideration battery characteristic it is obvious that battery life in UPS is not long.

Supercapacitor As soon as capacitors of super-value appeared on the market it became possible to use them not only as conversion elements, but also as energy storages. Brief comparison of supercapacitors (SCs) and batteries is given in the table I:

Table I: Comparison of supercapacitors and lead-acid batteries as energy sources

Supercapacitor Lead-acid battery High power density 1-14 kW/kg ++ Low power density 200-400 W/kg - Low energy density 2-10 W*h/kg (*) - High energy density 40-60 W*h/kg + High price - Low price ++ Long cycle life 10k – 1000k cycles ++ Short cycle life 300-1200 cycles - Long shelf life more then 10 years + Shelf life 3-7 years - High efficiency of cycles + Low efficiency of cycles - Low internal resistance + Environment safety + Environment dangerous - Fast self-discharge - Low self-discharge + No maintenance + No maintenance if sealed lead-acid battery Easy to control stored energy (E=0.5U2C) + Easy to charge + Voltage depends on a degree of charge -

Implementation of supercapacitors in UPS There are few topologies of UPS that are different in prices and in quality of the output voltage. They also affect utilization of SCs. High price of SCs makes unreasonable choice of low price UPS topology. That is why on-line topology for SCs based UPS was chosen. It validates advantages of the SCs. On-line UPS topology is shown in Fig. 4. It shows that a SC can be built-in in two places. Directly to the DC bus (position A in Fig. 4) and through the DC/DC converter (position C in Fig. 4). Combining these methods gives several possible UPS topologies with various properties. We will consider some of them.

Fig. 4. Topology of on-line UPS

1) Sole SC is directly connected to DC bus (position A in Fig. 4). Since DC bus has high voltage, but voltage of SC is 1.5-3V, it is necessary to connect 200-300 SCs in series. It leads to unbalanced voltage distribution and underutilization of energy density of some SCs. Also SC can source only a small part of the stored energy, because low DC-link voltages do not ensure peak value on the output. UPS with discharged SC is consumes huge starting current. 2) SC is directly connected to DC bus and battery connected through the DC/DC converter (A and B in Fig. 4). Characteristics of such connection method are similar to those of the first configuration. Advantage of the battery usage is its higher energy density. UPS with batteries delivers the same energy, but is smaller in size and weight. 3) Replace battery with SC (C in Fig. 4). Advantages of this method are: high power of UPS, faster charge and easy service. Comparing with the first circuit there is a possibility to limit the charge current and, hence, the starting current. The SC battery of high voltage is not required and almost

complete energy utilization is possible. Power of such UPS can be about ten times higher than power of battery UPS of the same weight (due to higher power density of SCs which is about 10 kW/kg, whereas the same batteries is 200-400 W/kg). Lifetime of SC can be more then 10 years and SC can perform over one million discharge-recharge cycles, whereas the batteries only 1000 cycles. Since the charge time of SC is short, UPS can be ready for mains disconnection in about 10 minutes, whereas UPS with batteries – in 4-6 hours. 4) Connect SC parallel to battery (position B and 2B in Fig. 4). This is not good connection, because it is not possible to use all the energy stored in SC. This energy depends on common voltage, but the voltage cannot fall lower than the minimal value in order keep the battery undamaged. However, large power in short-term can be supplied. 5) Connect SC and battery through different DC/DC converters (position B and C in Fig. 4). This type of connection is similar to the fourth case, due to the, that the SC is connected through the second DC/DC converter UPS disappears disadvantages that have fourth connection method. There is a possibility to charge and discharge the battery and SC independently and accordingly to use SC more efficiently.

Estimation of the energy capacity Let’s take a look at the emergency power system including UPS as well as a diesel-generator. UPS must give power as long as diesel-generator starts operate (5-15 seconds). In such system 100kW UPS is to use 250-500 kg lead-acid battery or 20-100 kg SC. Such mass allows 33 pieces of 55Ah-12V lead-acid batteries. If they were series connected we would get 396V battery. Then its discharge current:

AUPI 52.252

396100000 === (1)

Since it is 4,5C the time is about 4 -5 min. (Fig. 1). If 50 kg of SCs are used, we get 2 MJ (using energy density from [4]). Since UPS electronics cannot take energy at low voltage (30% of rated) then utilized energy is smaller:

%91100

301005.0

5.05.0%100 2

22

20

220

0

0 =−=−

=⋅−

CVCVCV

EEE ff (2)

Where Eo – stored energy, Eo-Ef – delivered energy, Vo – initial voltage, Vf – final voltage, C – capacity. It means that discharge time will be:

sPEkt 2.18

100000200000091.0 =⋅=⋅= (3)

Where E – stored energy, P – power of UPS, k – coefficient of used energy. For diesel-generator it is enough to begin operating at nominal power.

Testing the batteries and supercapacitors in UPS In order to compare their effect on the supplied energy the supercapacitors and batteries were installed in an on-line UPS. In on-line UPS, with the half-bridge topology of rectifier and inverter, voltage of DC-bus is 700-800V. In this case voltage of the energy storage must not be smaller than 70V. However, due to the limited laboratory facilities of the research institution, experiments were made with reduced system. The input voltage of the boost converter was reduced to 14V and it was ensured by a 14V supercapacitor stack and 12V lead-acid battery. Besides that, only boost part of the whole

system was investigated this time, which is, however, enough for practical estimation of the supplied energy. Stored energy of 14V and 169F supercapacitor module is:

hWJCVE ⋅==⋅=⋅= 6.416562216914

2

22

(4)

where: E - value of energy, C - capacity of supercapacitor, V - rated voltage of the supercapacitor. The task of this experiment was to compare the battery and the supercapacitor as energy storage. For reasonable comparison the battery with similar to SC’s parameters was chosen. Battery with capacity 1.2 A*h and nominal voltage 12 volts is the nearest to a capacity of supercapacitor. Stored energy in battery is:

hWJCVE ⋅==⋅=⋅= 4.14518402.112 (5) In order to compare battery and supercapacitor in similar conditions, it was decided to remove all that might influence the experiment and do conditions unequal. Only boost converter of UPS was used and load was connected directly to the DC-bus. Task of experiment was to compare battery with supercapacitor at different operating regimes, examine times that they can supply load and also calculate energy which was taken from energy storages. Output voltages of boost converter were constant (15V or 30V) and output currents were from 1A to 8A. Resistive load was used. The corresponding results are shown in a table II.

Table II: Supercapacitor and battery given energy Battery Supercapacitor Battery Supercapacitor Vout Iout

(A) E (J) t (s) E (J) t (s) Vout Iout

(A) E (J) t (s) E (J) t (s) 15 1 31379 1831 13849 797 30 1 20707 544 13398 32715 2 24248 671 13154 324 30 1.5 16710 301 12701 21215 3 17763 316 12689 210 30 2 12635 178 11991 14515 4 13809 186 12991 162 30 2.5 9001 99 10926 11015 5 9633 101 10881 101 30 3 5742 53 10541 8815 6 7017 51 10296 87 30 3.5 4996 49 9479 6815 7 4166 39 9116 58

A graphic view of table II is shown in Fig. 5. In Fig. 5.a given energy is shown depending on the boost converter output current at output voltage 15V. In Fig. 5.b the same, but output voltage is 30V.

a) b) Fig. 5. Given energy of the energy storage versus the output current at different levels of the output voltage: a) at 15 V, b) at 30V

As shown in Fig. 5 at higher currents (higher loadings) the energy given by battery was significantly decreasing. As it has been described above it causes not only internal losses in battery, but also it causes chemical processes on battery’s electrodes. The supercapacitor electrodes are not subjected to such processes and given energy is almost equal at any discharge currents. Given energy was smaller because the boost converter operates at high currents and cannot enlarge voltage from low input voltage. The higher current the higher is minimal input voltage and the lower energy is given by supercapacitor. In Fig. 6 voltage and current of supercapacitor are shown at output voltage of boost converter 15V and output current of it 8A. On the 60th second supercapacitor current stops to increase. This is the moment when the boost converter cannot adjust output voltage any more. Input voltage is about 8V. Fig. 7 shows the similar picture, but at 1A of the output current. In this case ability to regulate voltage disappears only at input voltage about 5V. Therefore given energy by supercapacitor is higher than in the case when output current is 8A. If the boost converter was optimized, or its input voltage was higher, this effect would be less distinct.

Fig. 6. Supercapacitor voltage and current at output current 8A

Fig. 7. Supercapacitor voltage and current at output current 1A This experiment shows that in cases when the discharge currents of battery are large (5C-10C), time of discharge is so short, that supercapacitor with a capacity in 3 times smaller discharge time is longer. Weight and price of this supercapacitor is higher. But as it is written above, supercapacitor has other advantages such as:

• Long life time • Large power density • Wide temperature range • No need maintenance • And etc.

It is also necessary to take into account that prices of the supercapacitors constantly falls and their parameters getting better. The cost of 1F (December, 2006) is about 0.01 $. This means that 1kJ (if use 3000F supercapacitor with voltage 2.7V) costs 2.85$ [12]. It is 10-20 times more than 1kJ of battery. But if take into account practically usable energy and long life of supercapacitor, that price of supercapacitors become almost comparable with battery price. According to newsletters of “Maxwell technologies” this price will fall down twice after a couple of years. That will make supercapacitor more attractive for use in UPS or in other spheres.

Conclusion In the given paper possibilities of utilization of supercapacitors in uninterruptible power supply were discussed. Possible topologies were analyzed and it was found that the “On-line” topology with separate converters for each energy storage is preferable. It was calculated that high power ratings are more easy achievable with supercapacitors rather than with batteries. It was experimentally found that at higher currents supercapacitors are comparable with batteries and getting better from the point of view of energy utilization. However, from the economical point of view, at the present time, direct substitution of batteries with SCs is not very effective. Nowadays it is better to combine traditional battery for higher capacity and SCs - for higher power. It must be also be mentioned, that ultracapacitors undergo intensive development and become more and more available in size and price. Therefore the overall prospects of the supercapacitors in UPS are estimated as good.

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