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Leveraging Thermal and Compressed Air Energy Storage

Oct. 1, 2004
Active Power recently revealed the principles by which it has achieved extended battery-free power backup in its CleanSource XR product. The company combines

Active Power recently revealed the principles by which it has achieved extended battery-free power backup in its CleanSource XR product. The company combines two energy storage mechanisms — thermal and compressed air — along with flywheel energy storage to create a self-contained power backup system capable of delivering 80 kW for 15 min. A prototype of this system was announced in June (see “Battery-Free Energy Storage System Doubles Output” in Power Electronics Technology, July 2004, page 10).

By applying mature energy storage techniques in a novel way, the company achieves runtimes comparable to battery-backed power systems, while improving system performance and reliability. For the customer, the technology has a higher initial cost than battery-based energy storage. But because of the reduced maintenance costs (there are no batteries to replace), this energy storage technology is projected to cost much less than batteries over the life of the product.

In using three energy storage mechanisms, Active Power balances the strengths and weaknesses of each one. Compressed air energy storage offers an environmentally benign energy source that can provide long backup times and has proven reliability. For example, the utility industry has used compressed air energy storage to achieve load balancing. During off-peak hours, power is used from the grid to compress air and store it in in wells or aquifers. Then, during peak hours of energy demand, the compressed air is heated with natural gas and used to drive expansion turbines to generate hundreds of megawatts of electrical energy. However, efforts to find other applications for compressed air storage are hampered by its poor energy density, slow response times and storage requirements.

Thermal energy storage, on the other hand, offers both long backup time and high energy density. And like compressed air, thermal energy storage is harmless to the environment. Nevertheless, generating electricity from thermal energy requires other technologies. Furthermore, it has never been used in UPS applications.

In examining these two energy storage mechanisms, Active Power discovered that a combination of the two approaches could overcome the drawbacks of each. Its system employs a small compressor to charge a bank of compressed air cylinders and an electrically heated block of steel with holes drilled in it to allow passage of air. In operation, compressed air from the cylinders is passed through the heated steel, which transfers its thermal energy efficiently to the air. The heated air is then used to drive a turbine/alternator, which generates electricity. Then, the alternator's output is converted to dc by a power supply within the system (see the figure).

The feasibility of this approach rests largely on the company's ability to assemble a working system using readily available components. The air cylinders, air compressor, air control valves, resistance heaters and turbine-alternator unit can all be obtained as off-the-shelf products. The one device that must be custom built is the thermal storage unit (TSU). However, this device is simply a machined block of steel, so its cost and availability do not present any obstacles to product design or manufacturability.

The key limitation of the compressed air and thermal energy storage approach is its sluggish dynamic response. Startup of the system requires requires 1 or 2 sec, which would not be acceptable in backup power applications. To compensate for this limitation, Active Power added a small flywheel-based power source (see the “Bridging Motor/Generator” in the figure) to the system. The flywheel provides 3 to 4 sec of bridging energy to the load, while the long-term energy storage system powers up. The system is configured with a base unit that integrates all the various subsystems except for the air storage tanks, which are housed in a separate base unit (see the figure).

The system incurs about 1 kW to 2 kW of standby power losses, which represents roughly 1% to 2% of its rated 85-kW output. Of those losses, about 750 W is required to keep the flywheel spinning and about 700 W are required to heat the TSU.

Although these losses represent a relatively small amount of waste heat in standby operation, the system itself can actually provide cooling to its surroundings during backup power operation. This is because the turbine vents cool air (approximately 55°F) outside the unit. That cool air may be sufficient to keep air temperatures relatively stable when air-conditioning units go off during power outages.

One limiting factor in system operation may be the time required to recharge the air tanks. Using the supplied compressor, a full recharge takes between 24 and 30 hr. In comparison, a battery used in a comparably sized backup power system would recharge to 80% capacity in 4 to 8 hr.

The long recharge time in the new system reflects design tradeoffs made to meet size and cost goals for the finished system. Use of a larger, more-powerful compressor would charge the tanks faster at the expense of a larger footprint and higher price tag. However, for systems that require fast recharge, customers may opt to keep a spare set of air tanks on hand. Such tanks are readily available.

Despite the longer recharge period, the new energy storage system provides a much more predictable state of charge than batteries. Typically, batteries experience significant degradation in capacity overtime, causing system designers to over-specify battery capacity. However, with thermal and compressed air energy storage, there is no decay in capacity over time.

The company will begin accepting orders for its thermal and compressed air storage system in Q2 of next year. Full volume production is slated to begin in Q3.

For a copy of a white paper explaining the technology or for more details, visit www.activepower.com.

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