Upgrading waste heat to process heat by a thermoacoustic heat pump

In order to upgrade waste heat to process heat, a heat pump is required that can generate a temperature lift of 50-100ºC. A thermoacoustic (TA) system is capable of doing this. A general description of the operation of a thermoacoustic system can be found elsewhere.

A very powerful acoustic wave is needed in order to drive a TA-heat pump. The required acoustic energy can be generated in several ways. ECN is concentrating on TA-engines that use a heat source to create the acoustic wave. Two situations can be distinguished:

• The waste heat temperature is high enough to drive a TA-engine. The required temperature is at least 120ºC.
• The TA-engine is driven by a high temperature source like an electrical heater or a burner

Waste heat engine

In this type of engine, the temperature difference between waste heat and the ambient atmosphere is used a strong acoustic wave. ECN, together with her partners, is developing a unique multiple regenerator system in which several regenerator units are applied within one TA-engine in order to generate sufficiency acoustic power from the relatively low waste heat temperatures. The picture below shows a TA-engine containing three regenerator units which is already running at a waste heat temperature of 100ºC. This low starting temperature enables the application of waste heat for driving TA-engines.

The acoustic energy is subsequently being used in a TA-heat pump to upgrade waste heat to usable process heat at the required temperature. The picture below visualises the whole system. The TA-engine is located at the right side and generates acoustic power from a stream of waste heat stream at a temperature of 140ºC. The acoustic power flows through the resonator to the TA-heat pump. Waste heat of 140ºC is upgraded to 180ºC in this component. The total system can be generally applied into the existing utility system at an industrial site.

The goal of the ECN activities is to develop this technology in order to upgrade industrial waste heat to usable process heat in a cost effective way. The objective is to upgrade about 20% of the waste heat that is presently being released to the ambient atmosphere. The energy saving potential using this technology within the Netherlands amounts to about 5 PJ, which is comparable with the energy use of more than 50.000 households.

The present activities are carried out on a scale of 1-5 kW. In the longer term the expected size will be about 1 MW. The picture below gives an impression of such a system.

The information presented here about the upgrading of waste heat by a thermoacoustic heat pump can also be found in this brochure.

Burner drive engine

An additional heat source is needed when waste heat temperatures are too low to drive a TA-engine. This can be a burner, an electrical heater, or another high temperature source. Applications are for example distillation columns, office buildings, and even residential dwellings. The process scheme for the application of a thermoacoustic heat pump at distillation columns is patented by ECN. In this scheme the heat that is released during condensation of vapours in the top of a column is used again to provide heat to the reboiler side of the column. Ambient heat is used as a waste heat source for the build environment.

From an energetic point of view, driving a system by a burner would be a favorable. Important part of the research in this area is how to transfer the energy from the burner to the TA-engine. Within the EC funded project THATEA (THermoAcoustic Technology for Energy Applications), ECN has built a TA-engine that is driven by heated air (up till 750°C) that simulates flue gasses. The picture below depicts this engine, connected to an acoustic resonator. More information on the THATEA project can be found on the project website www.thatea.eu.

The acoustic energy from the engine is used to pump up heat from the waste heat level to the desired temperature. The picture below visualises this for the application at a distillation column. The heat pump is being integrated with the existing column. In this example the temperature at the bottom of the column amounts 100ºC while the top temperature equals 50ºC. Energy savings are realised by reusing the heat of the condensing vapours at the top of the column.

The goal of the ECN research is to develop the most efficient conversion in both the engine and heat pump part of the system. The amount of energy that can be saved by this technology depends on the temperature level of both the waste heat and the required heat. An ECN study showed that application in the Dutch distillation market would save about 9 PJ per year, comparable to more than 90.000 households. A energy saving potential of 5 PJ per year in the Netherlands is realised by application in office buildings.

Information on the application of a thermoacoustic heat pump at a distillation column can be found in this brochure. Application of the technology in office buildings is described in this brochure.