The key element of an Automatic Meter Reading (AMR) system is of course the  so-called ‘smart meter’ itself. For rapid and cost-effective deployment,  smart meters must be plug-and-play, transparently replacing existing meters. In addition to communicating detailed power-consumption data, many power meters also report a range of additional information, such as power-outage and restoration status, self-diagnostics, power service, wiring stability and health, tamper detection, and meter-clock deviations and corrections.

For these advanced features to be successfully integrated into a competitive product, the size, lifespan, and cost of the communication hardware’s power supply are key considerations for the wireless meter designer. In particular where meters report power-outage events, they require some form of energy storage to continue operating.

Wireless meters popularly use batteries as a power source to allow data transmission to continue in the event of an outage. But most secondary cell chemistries only provide a three to five year operating life. In addition, traditional lead-acid batteries are relatively large and heavy. Whatever the technology, battery replacement can be a costly process – especially in remote sites. 

Needed: an inexpensive, reliable and compact energy source 
Alternative energy sources need to be able to draw power from the meter power supply at a relatively constant rate and then handle the power peaks demanded by wireless connectivity. Power peaks dramatically reduce the life of conventional batteries.

A further consideration for AMR applications is the ability to operate over the widest operating temperature range, -40°C to +70°C. Such thermal extremes diminish operating performance and shorten the life of batteries, while also degrading their packaging.  

Ultracapacitors - also called “supercapacitors” or “electrochemical double-layer capacitors” – offer an innovative alternative technology for energy storage and power delivery. By switching to ultracapacitors, operating life can be extended to over ten years. Andy they provide many more benefits over conventional solutions.

Why ultracapacitors? 
Relatively inexpensive, the ultracapacitor requires no special charging regimes, operates over the AMR’s complete temperature range, and has a proven installed life of up to ten years.

Since the energy storage mechanism of the ultracapacitor is not a chemical reaction, charging and discharging of ultracapacitors can occur at the same rate. Therefore, the rated current for the ultracapacitor applies for both charge and discharge:  efficiency values of charge and discharge are essentially the same. Hence a variety of methods can be used for charging, either through constant current or constant power charging via a DC source, or through AC charging methods. This flexible "opportunity charging" allows a system designer to make best use of their energy sources.

Variable power requirements can benefit from the ultracapacitor's ability to "cache" power and deliver it when needed for meeting peak demands. In many applications, a conventional primary energy storage device is designated to meet on-going low power requirements, and the ultracapacitor is available to supply peak power needs.
Whether for managing power peaks or outages, the first step in ultracapacitor selection is to size the system based on the maximum system voltage, the minimum allowable application voltage, typical current or power needed, peak current or power needed and load duration. These can be input into sizing tools like the model produced by Maxwell Technologies to determine the best device for the job.

An example is available online at http://www.maxwell.com/ultracapacitors/support/maxwell_model_3.xls

The second step is to analyse the system environment, including typical and extreme ambient conditions, together with self-created conditions around the ultracapacitor.

AMR applications rely on extended temperature range components and a further advantage of ultracapacitors is the low freezing point of their organic based electrolytes. This enables them to be deployed over a wide range of temperatures with relatively unaffected performance. But not all ultracapacitors are equal. Maxwell Technologies’ BOOSTCAP ultracapacitor is a double-layer capacitor incorporating a unique metal/carbon electrode and an advanced non-aqueous electrolytic solution. When a voltage potential is applied across the terminals, ions migrate to the high surface area electrodes. The combination of available surface area and proximity to the current collector provide an ultra-high capacitance for this electrostatic process. As a result, performance of these ultracapacitors is very stable over a wide operating temperature.

Ideally, operating temperature should be kept as low as practicable. Consideration should be made for the duty cycle and resulting capacitor temperature as well as the anticipated ambient temperature the device will be operating under. The combination of the two should not exceed the operating temperature for the ultracapacitor. Vendors such as Maxwell Technologies provide tools that enable meter designers to apply known thermal profiles to their projects, for example to assess whether heat sinks are required. Cooling at the capacitor ends or terminals is the most efficient where devices have electrically insulating shrink sleeving around the capacitor body.

Operating voltage has an impact too
In addition to temperature, ultracapacitor life is affected by the operating voltage. Ultracapacitors have an unlimited shelf life when stored in a discharged state, and product data sheets reflect the change in performance, typically decreasing capacitance and an increase in resistance. The ultracapacitor does not experience a true end of life – rather, its performance continually degrades over the life of the use of the product.

Typical degradation behaviour resembles exponential decay. The majority of the performance change occurs during initial use of the ultracapacitor and this performance change then levels off over time. The most dramatic effect of the life degradation is on the internal resistance of the device.

In many applications, the ultracapacitors will be maintained at working voltage until needed. Manufacturers should be able to provide data showing degradation in rated capacitance for ultracapacitors held at typical working voltages for long periods of time, and at different temperatures. To give one example, a 15% reduction in rated capacitance and a 40% increase in rated resistance may occur for an ultracapacitor held at 2.5 V after 88,000 hours at 25°C. The plots, along with the fact that the influence of temperature has a doubling effect for every 10 centigrade degrees, can be used to predict the expected performance change for a variety of conditions.

AMR system designers can use voltage de-rating not only to offset high ambient requirements, but also to extend component life. What is more, de-rating can help protect from voltage imbalance so reducing the need to apply active or passive balancing.

A further consideration is the number of charge/discharge cycles the device has to endure. Again, manufacturers should be able to provide the data designers need, backed-up by real-life test results. For example Maxwell’s cycle testing demonstrates that, under typical conditions, the BOOSTCAP ultracapacitor family is expected to provide in excess of 1 million duty cycles with an approximate 20% reduction in rated capacitance. Details of the testing and plots of capacitance versus cycles are available to designers on request.

Laboratory results are not the only demonstration of ultracapacitors’ superiority. There is ample evidence from real-life AMR applications, too.

Real-world benefits
A recently-developed smart wireless meter incorporates Maxwell Technologies’ BOOSTCAP ultracapacitors, replacing the more traditional energy sources such as lithium-ion or lead-acid batteries. Ultracapacitors are used both to provide short burst currents to RF modems (higher than the native meter power supply can normally deliver) and to provide backup power during utility power outages
Life expectancy of the meter’s power supply is extended to over ten years:  a 100- to 300-percent improvement over lead-acid batteries.
Its energy storage sub-system contains six BOOSTCAP PC10s and utilises a state- of-the-art invention under a patent-pending application. This design also leverages the PC10’s lightweight, rugged packaging and numerous mounting options. Cost savings of over $200 per meter have been achieved by eliminating the battery-replacement task during its 10-year cycle. Furthermore, hardware costs have been reduced through the smaller footprint of the new ultracapacitor-based power supply.

Customers have provided very positive feedback, for example reporting that the smart meters are easy to install and operate in the field. They have better product integration, are working well, and are meeting or exceeding all expectations.

The utility meter can perform numerous new functions, which are possible only with the integration of new components. Conventional components are inadequate because of their cost, size, performance, and reliability.

Adopting new component technology increased the performance and reduced the cost of the AMR meter, in turn providing greater value for its customers and building new markets for its systems.  Just one more example of how companies that understand how to effectively integrate new capabilities while driving down costs will be those that succeed.