If you think of energy harvesting only as a means of using the sun, vibration or temperature variation to generate the few milliwatts needed to power a wireless sensor node or a wearable electronic device, think again. Despite conventional wisdom suggesting that renewable energy sources can only output power in the range of microwatts per square centimeter (µW/cm2), in the U.S. alone there are five solar thermal power stations producing 250 or more MW (yes, that’s MEGA Watts) of power. According to the Solar Energy Industries Association database, as of April 2016 there were 4,000 major solar projects of one MW and above operating, under construction or under development, representing 72 GW of capacity.
You shouldn’t be. Technology to utilize what nature provides for doing large-scale work has been around since the days of the first sailing ships. Looking specifically at solar energy, the power density of solar radiation available for conversion to electricity averages 170 W/m2 worldwide, and in sunnier locations around the globe it can reach more than 200 W/m2.
Obviously we’re talking about utility-scale plants and not the single-family photovoltaic panels installed on a roof or in a backyard. The scaled-up variety use concentrated solar power, known as CSP, which requires a lot of land (a typical CSP plant requires five to 10 acres of land per MW of capacity) but can be very cost-efficient when done on a large scale (Figure 1). A CSP plant operates most efficiently when built in sizes of 100 MW and higher. CSP plants use mirrors to concentrate the energy from the sun to drive traditional steam turbines or engines that create electricity. The thermal energy concentrated in a CSP plant can be stored and used to produce electricity when it is needed, day or night.
For example, at the Ashalim Solar Thermal Power Station being constructed in Israel’s Negev desert, approximately 35km south of the city of Be'er Sheva, the sun will be used to supply 121 MW of clean, renewable energy—enough electricity to meet the needs of more than 120,000 households. Once on line next year, it will support Israel’s commitment to use renewable sources to supply at least 10% of the country’s total electricity needs by 2020.
The project, being constructed by BrightSource Energy, Inc. in partnership with General Electric and the NOY Infrastructure and Energy Investment Fund, will employ more than 50,000 flat, sun-tracking mirrors, known as heliostats, to focus sunlight onto a receiver—essentially a boiler that sits atop a 787-foot (240-meter) tower (see Figure 2 above). When the concentrated sunlight strikes the receiver, it heats a transfer fluid to high temperatures, which is then circulated to boil water, producing superheated steam of up to 540° C to power a steam turbine that generates electricity.
Ashalim’s heliostats are positioned in a 3.15-square-kilometer solar field. Each heliostat measures 4 x 5.2 meters and is individually controlled via wireless communications (Figure 3). Motors rotate and precisely tilt each mirror by means of a dual-axis tracking system capable of 360° positioning—all to concentrate a maximum amount of energy toward the solar receiver.
Each heliostat motor is powered by a small battery pack using six Tadiran TLI AA-size industrial-grade rechargeable Li-ion batteries. Tadiran’s rechargeable energy packs not only drive the heliostat motors, but they also deliver high pulses to support wireless connectivity throughout the network. With low impedance the batteries can deliver the high pulses (5A for a AA-size cell) required for two-way wireless system control and data analysis. By wirelessly connecting each heliostat with the solar field’s integrated control system, the batteries thus help reduce expensive cabling and wiring (by as much as 85% compared to previous solutions) that would otherwise be required, reducing costs and accelerating the construction schedule. Long-term reliability is also improved.
While applications such as powering hand-held flashlights, toys and remote-control devices are well suited for inexpensive, consumer-grade Li-ion cells, these consumer batteries have limitations that cannot be overlooked in an industrial setting. These include a short life expectancy (fewer than five years), a low maximum cycle life (1,000 cycles), high annual self-discharge (up to 60% per year), and a limited temperature range (0° C to 60° C) with no possibility of charging at lower and higher temperatures.
Eschewing consumer cells in favor of industrial-grade Li-ion batteries made sense for the Ashalim solar power station in view of the extreme environmental conditions of the Negev desert, as these batteries offer an extended temperature range (-40° C to 85° C) and are ruggedly constructed, using battery cans that are precision-welded to create a hermetic seal. Inexpensive consumer-grade batteries, on the other hand, use crimped seals that can leak.
The Tadiran batteries will power the heliostats for up to 25 years and 5,000 full recharge cycles without replacement. In a CSP solar facility, extended battery life is critically important as the cost of a system-wide battery change-out to replace consumer-grade batteries every five years would be enormous, far exceeding any initial savings that could be realized by using consumer- grade cells.
For the Ashalim project, the industrial-grade batteries also won out over bulkier supercapacitors (also known as ultra-capacitors or Electric Double-Layer Capacitors). Rather than employing a dielectric between plates as in a standard capacitor, a supercapacitor has an electrolyte and a thin insulator (for example, cardboard or paper). Unlike batteries, supercapacitors store energy in an electrostatic field rather than in a chemical state. In general, batteries provide higher energy density for storage, while capacitors have more rapid charge and discharge capabilities. Supercapacitors are limited to low voltages (high voltage would break down the electrolyte) but very high capacitance, and as such they are most commonly used in lower-power consumer applications, such as providing memory backup for mobile phones, laptops and digital cameras.
Supercapacitor technology has other inherent drawbacks when considered for industrial solar- power applications, including short duration power, linear discharge characteristics that do not allow for use of all the available energy, low capacity, very high self-discharge (up to 60% per year), and the need for cell balancing when they are linked in a series.
Supercapacitors are also much bulkier than comparable industrial-grade Li-ion batteries as shown in Figure 4 above, where three large packs of supercapacitors consisting of six D-size cells each (or 18 cells total) can be replaced by a small battery pack containing six AA-size TLI Series rechargeable Li-ion batteries. Besides reducing size and weight, the Li-ion battery pack also delivers superior performance characteristics versus the supercapacitors, including:
- Higher practical capacity: 330 mAh (the equivalent pseudo-capacitance is 1200F). A supercapacitor having the same volume has about 10F maximum (3.6V).
- Lower self-discharge: 1 to 2uA of self-discharge current compared to 20 to 50uA of discharge current for a supercapacitor having about the same external volume. Self- discharge is the voltage drop on a charged battery or superconductor cell after a set period of time without a load. Unlike batteries that can discharge a fairly constant voltage, the supercapacitor cells act very similarly to traditional capacitors and will drop their voltage as they discharge their stored energy.
- A higher number of cycles: a AA-size Li-ion cell can be charged and discharged for 35,000 cycles between 2.8 V and 3.9 V (80% depth of discharge).
- Significantly higher cell voltage than that of a supercapacitor under the high-current pulse needed to power two-way communications for system control.
Beyond its use in the Ashalim Solar Thermal Power Station, Tadiran’s battery-powered solution has relevant implications for other remote wireless applications, including many types of devices that will soon be integrated into the Industrial Internet of Things.
In addition to its TLI Series batteries, Tadiran manufactures a complete line of lithium thionyl chloride batteries, including a variety of primary cylindrical batteries, coin-sized cells, battery packs and Pulses Plus™ batteries for high-current pulse applications. Tadiran products are available in a variety of terminations and assemblies. For more information visit www.tadiranbat.com.