Archive for the ‘ FACT ’ Category

Blue print for a simple, reliable and low cost heat driven thermoacoustic generators for rural areas producing 20-50W electric power.

Thermoacoustics is a conversion technology in which the compression, expansion and displacement of the working gas is driven by acoustic wave motion rather than by pistons, valves  and displacers. Using acoustic wave motion eliminates mechanical friction and wear and therefore drastically increase lifespan and minimize maintenance. Because of the lack of moving mechanical parts in the thermodynamic process the construction tolerances and material requirements are relaxed allowing for (potential) low production and investment costs. These properties makes thermoacoustics not only a second generation energy conversion technology but also a candidate for low cost, small scale conversion technology for rural and developing areas.

These opportunities were recognized in, for example, the Score project in England (www.score.uk.com) and by initiatives supported  by the  FACT Foundation in the Netherlands (www.fact-foundation.com). The aim of these projects, started a few years ago, is to develop low cost thermoacoustic devices generating electricity combined with wood stoves for cooking or heating water.  These projects also address the social and economic aspects involving charities and local communities. This type of devices could  contribute to improving local living conditions by the use of small scale air operated multi-purpose devices for preparing hot water, cooking and generating some electricity. It could also stimulate  labour related to local production installation and maintenance of these devices.

The document below describes the design approach and underlying physics of a pre-production version of a small scale near atmospheric air operated  thermoacoustic generator and provide a blueprint for the further (local) development and production of such generators in and for rural areas and developing countries.  To make this happen, background information and construction drawings are available from Aster as input for local projects, students, scientist and construction companies who will intend to do experimenting and furthering this technology.

Design and build of a 50W thermacoustic generator(2)

Demonstration of the thermoacoustic water heater-generator

At the “Bioenergy Innovaton Program 2012 partner day”organized by the FACT foundation and held last week in the Netherlands, Aster demonstrated a functional prototype of the combined water heater and thermoacoustic generator for use in rural area’s. This thermoacoustic generator utilize the temperature difference between an arbitrary heat source (wood, gas) and the water to be heated for generating electricity. This is shown schematically below.

FACT principe

The single stage thermoacoustic engine is constructed coaxially with the high temperature heat exchanger positioned at the lower end for thermal contact with the heat source. The low temperature heat exchanger actually becomes part of the bottom of the water tank. A potential cost reduction in this concept is the use of a small bi-directional impulse turbine equipped with standard rotating generator for the conversion of acoustic power into electricity (see previous posts).

FACT demo 27 novFACT details

The left picture shows the thermoacoustic section.  This unit will be placed on the bottom of the water tank in such a way that the hot hex protrude the bottom for interacting with the heat source beneath the tank. In the final version the whole thermoacoustic device is immersed in the water (not shown on this picture) for keeping the cold hex temperature low without the need for a circulation pump. The demonstration was performed with air at atmospheric pressure which allows for visual observation of the turbine rotor. In this setting only 5 W electricity was generated for powering the led lights.

Normal operation is with compressed air at 2-3 barg at which 50W electric output is aimed. In order to make this become true we are working now on an improved bi-directional turbine which can be pressurized as well. An option for further increasing output power or reducing the dimensions of the acoustic tubing is to apply a 2-stage thermoacoustic unit.

The acoustic to electric conversion issue

Converting acoustic power from thermoacoustic engines into electricity using resonant linear alternators is a common approach but has severe limitations in terms of cost and scalability. The increase of moving mass with increasing power finally sets a practical limit caused by the extreme periodic forces in the construction and impossibility to maintain clearance seals stable over a large stroke.

An option, in particular for low cost generators for rural areas (e.g. FACT, Score) and for high power industrial applications( e.g TAP), is to convert the acoustic wave motion into rotation first. This allows for deploying standard generators. Rotational speed could be made arbitrary high. Therefore this type of alternators require much less magnetic material which significantly reduce size, weight and cost.

Linear alternators with pistons or membranes make use of the pressure variation of the acoustic wave. There is however no physical reason why not using the periodic velocity component of the acoustic wave. A way to convert such a bi-directional flow into rotation is known from shore and off-shore electricity production plants based on an oscillating water column (OWC). In this type of power stations, waves force a water column in a chamber to go up and down. This chamber is connected to the open atmosphere and the periodic in- and outflow of air drives a bi-directional turbine of which the rotation direction is independent of the flow direction.

This class of “rectifying turbines” is explored extensively and in principle they can be deployed for conversion of periodic acoustic wave motion as well. Common to uni-directional turbines they could lift or impulse based. For this application the impulse version seems to be most appropriate because of they are self-starting and could operate over a large amplitude range.

For testing and validating this option both an radial and axial bi-directional turbine designed and build using 3-D rapid prototyping. Below the 3D design and the “3D printed” result for the radial turbine. The radial turbine could be positioned at the junction between at the end of a feedback tube and the volume in front of the cold heat exchanger (e.g. TAP).

rad_turb_3Drad_turb_printed

 

 

 

 

 

 

In addition, an axial bi-directional turbine is designed  and produced because of such a type could  be placed in line with the acoustic feedback tube (FACT)

axial_turb_rotor

axial_turb_gen

 

 

 

 

 

Both turbines are provided with a brush less electro motor acting as generator. For the measurements the turbines are acoustically driven by the loudspeaker of the impedance measuring set which allows for both setting the frequency and measuring the acoustic input power of the turbines. The results are encouraging. The conclusion from these preliminary experiments is that this small turbine,

  • convert acoustic wave energy into rotation
  • can be place in-line with the thermoacoustic section
  • operate at acoustic frequencies up to > 100 Hz
  • shows an acoustic to mechanical  (rotor) efficiency in the order of 40% under small signal conditions

For such small turbines (80 mmØ) a rotor efficiency of 40% is according to theory. The same rotor scaled up to 300 mmØ will have an efficiency of 70%. Because of the periodic flow is at a fixed frequency, acoustic manipulation of the flow around the in- and outer vanes could raise the efficiency over 80%. This is for further study.

In order to validate the concept, a full scale radial bi-directional impulse turbine is under construction now for testing in the TAP (see previous posts) replacing the current linear alternators.