Although there is a growing market for environmentally sensitive energy production, key challenges to widespread adoption remain. Among the most significant is creating consistent, reliable stores of energy from unpredictable natural resources such as solar and wind power. In the wind turbine market in particular, variable weather conditions create a demand for an energy storage system that can respond quickly regardless of conditions. In many cases today, the storage system is a battery. This must change if the market for wind-generated power is to fulfill its potential for growth.
Challenged by wind
Most wind turbines use three rotor blades to spin generators. The pitch of these blades can be quickly adjusted to respond to current wind conditions and optimize the output power. Variable winds create challenges to maximizing energy output because they also creates waste. The energy storage frequently used is sized to meet the highest possible power demands, even when those periods occur briefly and sporadically.
Several issues emerge when wind turbines use batteries to store emergency power. First, batteries struggle under moments of high peak power and perform poorly in low temperatures. In extreme conditions, a battery’s operating life is extremely limited, creating a situation in which maintenance crews must frequently swap out components under potentially dangerous conditions. Worse yet, batteries do a poor job delivering frequent, short power boosts wind turbines need to make rapid rotor-blade adjustments under changing conditions.
Ultracapacitors provide a solution because they are more reliable over a wide temperature range. These power storage devices reduce overall system size and have a far longer lifespan than batteries, making them significantly more cost-effective. As the wind market grows, ultracapacitors will become an increasingly important ingredient in wind-generated power production.
Growth in the wind market
Wind-turbine installations worldwide have remained relatively flat over the past two years, with new installed capacity averaging 38 GW. Estimates for new installed capacity from 2010 to 2015 suggest a rebound in growth with new cumulative installations of 236 GW. Assuming an average turbine size of 2 MW, the power figure translates to 118,000 newly installed turbines through 2015. The trend in these new wind turbines will favor ultracapacitors for several reasons.
For example, pitch control on each blade ensures best position for efficient use of the wind speed regarding performance and safety. Engineers adjust blade pitch either mechanically or electrically. Mechanical control through hydraulics raises concerns for reliability, necessitating undesirable maintenance aspects. Electric control systems replace some mechanical devices with more reliable electric systems. At issue for the electric control method is the implementation of battery-based backup systems. The maintenance requirements for batteries often clouds the perceived maintenance advantage of electrical systems over hydraulic versions.
In the last few years, designs for backup have included ultracapacitors rather than batteries. With more than 14,000 turbine installations, ultracapacitors have provided maintenance-free operation along with performance advantages and a longer life relative to batteries in extreme environments. As a result, ultracapacitors let electric pitch-control systems capitalize on their inherent reliability advantages.
Present market estimates are that 60% of newly installed turbine systems use electric pitch controls. The market share should continue to expand as more new turbine developments focus on electric-control systems. With ultracapacitors as a driver for such market shifts, it’s useful to take a detailed look at ultracapacitor performance.
An ultracap primer
Ultracapicators have a higher power density than standard capacitors. What differentiates ultracapacitors from their traditional counterparts, electrolytic capacitors, is their higher energy density, allowing them to store more energy in a small package. The capacitors most design engineers are familiar with have short time constants. This means their voltage cycles quickly, whereas ultracapacitor arrays have time constants of the order of tens of seconds to minutes. The large capacitance and low frequency time constants allow using ultracapacitors in applications that have not been practical or economical for conventional capacitors.
Because ultracapacitors are still rather new to the electronics industry, few people are familiar with how to use them. The goal of this article is to familiarize people with the properties of ultracapacitors and suggest applications for which they are well suited.
Ultracapacitors have a maximum cell voltage of 2.7V, so they must be connected in series to reach a required working voltage. With any identical capacitors, the capacitance of a series array decreases as they are connected in series, but the working voltage increases by the rated voltage of each additional cell. Replacing a six-cell lead-acid battery requires six ultracapacitors, because a 12-V battery is actually charged to 14.4V. If five ultracapacitors were used, the maximum voltage across each cell would be 14.4V/ 5 = 2.88V, which would cause premature cell failure. At higher-voltage-battery configurations, it is possible to have slightly fewer ultracapacitor cells than lead-acid cells. But, in general, the number of ultracapacitor cells equals the number of lead-acid cells when directly connected in parallel with the battery. Because there is a minimum of six cells required and 250F (Farads) is the minimum capacitance, the cell capacitance has to be at least 6 x 250F, or about 1,500F. Several manufacturers, including Ioxus Inc, offer several different sizes of ultracapacitors close to this capacitance.
Consider a 2,000F prismatic cell. The ESR (Equivalent Series Resistance) specified for these cells is 0.0006 Ohm/cell, resulting in a total ESR of 0.0036 Ohms.
In general, use of an ultracapacitor in combination with a battery is an excellent way to increase overall power density of the source and decrease strain on the battery. In addition, a smaller battery could be used because the available power of the hybrid power source is more than required. In any case, when energy storage requires high-peak power, it is likely an ultracapacitor will be useful.
Benefits versus batteries
Ultracapacitors are similar to traditional film capacitors because their energy storage is based on surface area electrostatic- charge accumulation at the positive and negative plates. Highly porous electrodes in ultracapacitors allow a significant charge accumulation in comparison to traditional capacitors. The release of energy in capacitors and ultracapacitors comes at high rates due to this loose-charge accumulation attraction. Resistance to the energy release is primarily driven by the resistivity of the electrolyte. In contrast, batteries rely on current flow between positive and negative plates through chemical reactions of plating and decomposition of the positive and negative plates. As such, the energy release or power capability of the technology is significantly reduced in comparison. Because chemical reactions are involved, rate kinetics also negatively impact power delivery at lower temperatures.
Ultracapacitors also have significant life advantages for the same reasons. Ultracapacitors have no plating or chemical reactions so there is no wear mechanism in the technology. Therefore, they tolerate millions of charge and discharge cycles with limited performance degradation. Any performance fade in the devices are predictable and easily monitored so any end- of-application life is easily predicted.
Battery life is not so easily predicted. In fact, the general practice is to replace batteries at specified intervals regardless of the actual battery health.
The accompanying table compares a few a key characteristics of batteries to ultracapacitors. Basic life-cycle costs can be generated based on these characteristics with additional maintenance cost considered.
Wind turbine maintenance is costly and potentially dangerous. One way to reduce maintenance is with newer technologies that create safer operating conditions. Supporting safer operation will be paramount as wind turbines are deployed in greater numbers. Part of that effort will require reducing the maintenance demand from energy storage. This is a cost issue as well. Generally, the cost of the maintenance event outweighs the cost of a new battery or ultracapacitor. This will be especially true in offshore turbines. At present, about 3 GW of turbines are installed worldwide. Experts project that more than 50 GW of offshore installations will occur through the end of the decade. These installations require the highest level of reliability for cost-effective turbine operations.
Demand for ultracapacitors in electrical pitch-control systems is growing as the market for wind turbines expands, and it’s easy to see why. Unlike batteries, ultracapacitors deliver a simple, long-lasting, cost-effective, and reliable means of storing energy and increasing the safety of modern wind turbines.