In the power generation work for microsystems at Holst Research Centre, in Eindhoven in the Netherlands, the main focus is on 'harvesting' unused ambient energy such as body heat and stray light. In particular, it is investigating:
- Energy harvesting concepts
- Thermal
- RF
- Mechanical / vibration (dealt with in Part One)
- Small, indoor photovoltaics
In each case, the underlying concept and eventual implementation depends heavily on the end application. This ensures the best use of the resonant frequencies (vibrational), frequency spectra (RF and photovoltaic) or temperature gradients (thermal) specific to each application.
Microbatteries, supercapacitors and miniature fuel cells
Holst also analyzes energy storage systems (ESS) such as micro-sized batteries, super capacitors and fuel cells. This includes testing and benchmarking for partners, and advice for commercial battery suppliers.
Sensor node design
The ultimate goal is micro-power modules that can be easily integrated into complete sensor node designs. "So we are investigating ways to integrate the ESS with the energy scavenger and antenna into micro-power modules that may include multiple harvesting modes (e.g. thermal and photovoltaic). For the most promising concepts, we build fully integrated demonstrators including power generation, conditioning, storage and management systems."
The piezoelectric, thermal, RF and indoor photovoltaic energy harvesting are for power microgenerators, for example to support its program of work on body area networks and wireless sensor nodes in general.
Harvesting for MEMS
MEMS processing both in plane and large area topography is carried out together with a system level approach for the micropower module. Indeed, MEMS are also developed for both chemical and mechanically based gas sensors, one example using a SiN substrate supporting a printed organic material that swells in response to the input, electrodes being made of sputtered platinum.
RF energy harvesting
There is also work on RF energy harvesters ie providing local energy to small electronic devices by directing an RF beam, though some would say this is strictly not energy harvesting because it does not employ ambient energy. One application is charging a microbattery to run its microgenerators. Key aspects here are their knowhow in antenna design and in integrating antenna, rectifier and battery.
Thermal energy harvesters
Despite the failure of Seiko Thermic wristwatches in the marketplace because of the inability to provide reliable thermal gradients adequate for the Seebeck devices used, Holst has progressed similar options for microgenerators in medical use. It is modelling thermoelectric generation, getting practical knowledge of human body applications issues from using demonstrators, some with record breaking energy output.
Indoor photovoltaics
Sony is working on indoor photovoltaics because so many of its products need to work indoors. That takes it into the realms of Dye Sensitised Solar cells DSSC and organic solar cells. Holst research centre seeks indoor photovoltaics as well, in this case targeting its wireless sensor node and smart tag applications. It is characterising the infrastructure in a system level approach.
Sensor and actuator technology
Small, low power sensors and actuators are vital for systems of all kinds to interact with their environment. Holst Centre and its partners are developing ultra low power (bio)chemical sensors, actuators and signal acquisition / conditioning ICs that can be implemented in standard CMOS processes.
The organisation reports that, "Small, low power sensors and actuators are vital for systems of all kinds to interact with their environment. Holst Centre and its partners are developing ultra low power (bio)chemical sensors, actuators and signal acquisition / conditioning ICs that can be implemented in standard CMOS processes.
There is a wide range of possible sensing mechanisms and implementations. So we are evaluating transducer technologies based on thermal, optical (plasmonic), mechanical and electrochemical effects to better understand their strengths and weaknesses for various applications.
On the actuation side, we're developing miniaturized components such as micro-pumps and micro-fluidic channels made mainly from polymers. In both cases, surface modification and the creation of new thin-film materials plays a key role.
The many different sensing technologies present a big challenge for the design of the signal acquisition and conditioning circuits (read-out electronics, op amps, data converters, etc). Consequently, the program's analog and mixed signal design experts work closely with the sensor developers, as well as with DSP and power experts from other programs. The goal is to combine sensor, actuators and electronics onto a single chip by developing and integrating advanced processes.
Power consumption is a key challenge. A complete wireless sensor node (including signal processing and communication functionality) will typically have a power budget of 100 µW. So we're targeting a power level of 40 µW for the sensor / actuator / signal acquisition and conditioning module. Achieving this requires good application knowledge to enable an optimal balance between power and speed / resolution.
The program bridges the worlds of academia and industry. It features people with a wide range of expertise, from quantum physics and chemistry to IC design and manufacturing processes. Industrial partners include semiconductor, electronics and medical systems companies, and the program also has links with manufacturing equipment suppliers."
For more see www.holstresearch.com 
Top image of Holst Centre, source Holst Centre
For more attend: Energy Harvesting & Storage USA 2009
