Archive for november, 2012

Solar powered thermoacoustic cooling

Solar powered thermoacoustic cooling is presumably one of  the first commercial application of thermoacoustics on short term. To day onset and operating temperatures of multi-stage traveling wave thermoacoustic engines are at such a level that heat from vacuum tube solar collectors (120°C-160 °C) can be utilized effectively to power thermoacoustic (heat driven) heat pumps for cooling in domestic and rural applications. Main assets of the concept are the lack of environmental issues, absence of mechanical moving parts and the linear relation between cooling power and solar irradiation.

Market introduction is in preparation by a joint venture together with Solar collector manufacturer Watt Sp. z o.o and Thermo Acoustic Solutions Sp. z o.o both established in Poland. One of the activities within the framework of this collaboration is the build of two prototypes. (1) a representative prototype equipped with fluid-gas heat exchanger for testing in combination with vacuum tube collectors under realistic conditions and (2) a transportable device  for demonstration purposes. The demonstration prototype is completed by last week and is depicted below.


For simplicity this demonstrator operates without any fluid circuit and is externally powered by cartridge heaters keeping the hot heat exchanger at 160ºC (simulating  input heat from the vacuum tube collectors) and is cooled by forced convection keeping the cold heat exchanger around 40ºC, which both are representatives temperatures for our applications.

Ice_webTo allow visual inspection of the construction, no isolation is applied at all. Nevertheless the temperature lift of the cooler section is  more than 40ºC and ice is formed quickly at the refrigerator cold heat exchanger as is shown at the left.

The working gas in the demonstration set-up is helium at a mean pressure of 1 MPa, the pressure amplitude is 35 kPa and the oscillation frequency 138 Hz.  Net cooling power at -5ºC is about 50W at a net heat input of 380W at 160ºC. After correction for the absence of any isolation the exegetic efficiency of both the engine section and the refrigerator section is found to be in the order of 0.35 .

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.