Acquired Electronics360

Analog/Mixed Signal

Power Management for Energy Harvesting in WSNs

12 June 2014

Introduction

The portable power application space is both broad and diverse. Products range from wireless sensor nodes (WSNs) that consume average power measured in microwatts to cart-based medical or data acquisition systems with multi-hundred Watt-hour battery packs. However, despite this variety, a few trends have emerged; namely, designers continue to demand more power in their products to support increased functionality and they want to charge the battery from any available power source. The first trend would imply increasing battery capacities. Unfortunately, users are often impatient, and these increased capacities must be charged in a reasonable amount of time, which leads to increased charge currents. The second trend requires tremendous flexibility from the battery charging solution, as there is a need to handle a broad range of input sources and power.

Fortunately, at the low end of the power spectrum are the nanopower conversion requirements of energy harvesting systems, such as those commonly found in WSNs, that necessitate the use of power conversion ICs that deal in very low levels of power and current. These can be 10s of microwatts and nanoamps of current, respectively.

State-of-the-art and off-the-shelf energy harvesting technologies (e.g., vibration energy harvesting and indoor photovoltaic cells) yield power levels on the order of milliwatts under typical operating conditions. While such power levels may appear restrictive, the operation of harvesting elements over a number of years can mean that the technologies are broadly comparable to long-life primary batteries, both in terms of energy provision and the cost per energy unit provided. Moreover, systems incorporating energy harvesting are typically capable of recharging after depletion, something that systems powered by primary batteries cannot do. Nevertheless, most implementations will use an ambient energy source as the primary power source, but will supplement it with a primary battery that can be switched in if the ambient energy source goes away or is disrupted.

An Energy Harvesting WSN

There is plenty of ambient energy in the world around us and the conventional approach for energy harvesting has been through solar panels and wind generators. However, new harvesting tools allow us to produce electrical energy from a wider variety of ambient sources. Furthermore, it is not the energy conversion efficiency of the circuits that is important, but rather the amount of “average harvested” energy that is available to power it. For instance, thermoelectric generators convert heat (or cold) into electricity, piezo elements convert mechanical vibration, photovoltaics convert sunlight (or any photon source) and galvanics convert energy from moisture. This makes it possible to power remote sensors, or to charge a storage device such as a capacitor or thin film battery, so that a microprocessor or transmitter can be powered in a remote location without a local power source.

In general terms, the necessary IC performance characteristics needed for inclusion and use in the alternative energy market include the following:

  • Low standby quiescent currents – typically <6 µA and as low as 450 nA
  • Low start-up voltages – as low as 20 mV
  • High input voltage capability – up to 34 V continuous and 40 V transients
  • Ability to handle AC inputs
  • Multiple output capability and autonomous system power management
  • Auto-polarity operation
  • Maximum Power Point Control (MPPC) for solar inputs
  • The ability to harvest energy from as little as 1°C temperature delta
  • Compact solution footprints with minimal external components

WSNs are basically self-contained systems consisting of some kind of transducer to convert the ambient energy source into an electrical signal, usually followed by a DC/DC converter and manager to supply the downstream electronics with the right voltage level and current. The downstream electronics generally consist of a microcontroller, a sensor and a transceiver.

When trying to implement WSNs, a good question to consider is: How much power do I need to operate it? Conceptually this would seem fairly straightforward; however, in reality it is a little more difficult due to a number of factors. For instance, how frequently does a reading need to be taken? Or, more importantly, how large will the data packet be and how far does it need to be transmitted? This is important because the transceiver consumes approximately 50% of the energy used by the system for a single sensor reading. Several factors affect the power consumption characteristics of the energy harvesting system of a WSN. See Table 1.

Table 1

Of course, the energy provided by the energy harvesting source depends on how long the source is in operation. Therefore, the primary metric for comparison of scavenged sources is power density, not energy density. Energy harvesting is generally subject to low, variable and unpredictable levels of available power so a hybrid structure that interfaces to the harvester and a secondary power reservoir is often used. The harvester, because of its unlimited energy supply and deficiency in power, is the energy source of the system. The secondary power reservoir, either a battery or a capacitor, yields higher output power, but stores less energy, supplying power when required but otherwise regularly receiving charge from the harvester. Thus, in situations where there is no ambient energy from which to harvest power, the secondary power reservoir must be used to power the WSN. Of course, from system designers’ perspective, this adds a further degree of complexity since they must now take into consideration how much energy must be stored in the secondary reservoir to compensate for the lack of an ambient energy source. Just how much they will require will depend on several factors. These will include:

  1. The length of time the ambient energy source is absent
  2. The duty cycle of the WSN (that is the frequency with which a data reading and transmission has to be made)
  3. The size and type of secondary reservoir (capacitor, supercapacitor or battery)
  4. Is enough ambient energy available to act as both the primary energy source and have sufficient energy left over to charge up a secondary reservoir when it is not available for some specified period?

Ambient energy sources include light, heat differentials, vibrating beams, transmitted RF signals, or just about any other source that can produce an electrical charge through a transducer. Table 2 below illustrates the amount of energy that can be produced from different energy sources.

Table 2

There are a plethora of applications where these power levels make sense for a system deployment. Here are just a few examples:

1. Aircraft corrosion sensors

2. Auto-dimming windows

3. Bridge condition monitors

4. Building automation

5. Smart utility meters

6. Gas sensors

7. Health monitors

8. HVAC controls

9. Light switches

10. Remote pipeline monitors

11. Water meters

An excellent example of the opportunities presented by alternative energies is the market for solar-powered electronic devices. It continues to grow as companies look for ways to reduce energy consumption. Consider Smart meters for instance. These are deployed on the smart grid and it would be advantageous for them to be powered by an ambient energy source in order to reduce operating energy costs. And one viable and abundant source of energy comes from solar power. However, because solar power is variable and unreliable, nearly all solar-powered devices feature rechargeable batteries. Thus, an important goal would be to extract as much solar power as possible to charge these batteries quickly and to maintain their state of charge for use as an energy source when solar power is not available.

A Nanopower IC Solution

It is clear that WSNs have very low levels of energy available. This, in turn, means that the components used in the system must be able to deal with these low power levels. While this has already been attained with the transceivers and microcontrollers, on the power conversion side of the equation, there has been a void. However, devices have been introduced to specifically address this requirement. For example, Linear Technology's LTC3330 is a complete regulating energy harvesting solution that delivers up to 125 mA of continuous output current to extend battery life when harvestable energy is available. It requires no supply current from the battery when providing regulated power to the load from harvested energy, and only 750 nA, operating, when powered from the battery under no-load conditions.

The LTC3330’s energy harvesting power supply, consisting of a full-wave bridge rectifier accommodating AC or DC inputs and a high-efficiency synchronous buck converter, harvests energy from piezoelectric (AC), solar (DC) or magnetic (AC) sources. The primary cell input powers a synchronous buck-boost converter that operates from 1.8 V to 5.5 V at its input, and can be used when harvested energy is not available to regulate the output whether the input is above, below, or equal to the output. The device automatically transitions to the battery when the harvesting source is no longer available. This has the added benefit of allowing the battery-operated WSN to extend its operating life from 10 years to over 20 years if a suitable energy harvesting power source is available at least half of the time, and even longer if the energy harvesting source is more prevalent. This is significant since a Tadiran ‘C’ cell costs around $16 each. So the costs to swap them out with manpower are significant. Alternatively, a user could use a smaller (shorter life) battery and lower the overall system cost.

Conclusion

Even though portable applications and energy harvesting systems have a broad range of power levels for their correct operation, from microwatts to great than 1 W, there are many power conversion ICs available for selection by the system designer. However, it is at the lower end of the power range, where nanoamps of currents need to be converted, where the choice becomes limited.

Fortunately, devices like the LTC3330 are available to prolong battery life for keep-alive circuits in portable electronics, and enables a new generation of energy harvesting applications such as WSNs.

Tony Armstrong is director of product marketing for Linear Technology Corporation.



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