Pioneering low temperature waste heat recovery system using ammonia

Although long considered unfeasible, a low temperature waste heat recovery system has been pioneered in Japan. The system, which can be used with ammonia harnesses waste heat and generates electricity using a rotary heat engine (RHE).

In May 2011 a Japanese start-up working with the University of Tokyo, finalised the development of a system capable of capturing the energy from waste heat and using this energy to rotate a rotor that generates electricity. The suggested working fluids for the system include ammonia.

The Wankel-based system currently under development builds upon a 500 watt model already available.

System specifications

The waste heat recovery system uses an external combustion type rotary heat engine (RHE) driven by the Rankin cycle. Capable of using heat of 80°C to 200°C the 30 kilowatt system consists of a rotor, power generator, evaporator, condenser and working fluid.

When external waste heat is applied to the engine the working fluid is vaporised in the evaporator. This in turn rotates the rotor, which uses a directly-coupled generator to produce electricity. The gas from the evaporated fluid turns back into fluid in the condenser, where it is fed back into the evaporator and the cycle begins again.

Using ammonia as a working fluid for waste heat temperatures around 50°C can ensure the waste heat is recovered efficiently.

Revolutionary system could help reduce energy consumption

Converting waste heat into useful energy could result in significant energy savings by reducing the consumption of fossil fuels for electricity production. In comparison to solar heat, wind power and other intermittent renewable energy sources, the electricity production from waste heat would be at a stable constant rate.

Current waste heat recovery systems can be found in steam turbines and Stirling engines. However, these technologies are only applicable to recovering high temperature wasted heat, and unlike the Da Vinci system are unsuitable for low temperatures. For example steam turbines ustilise heat at over 400°C, whilst Stirling engines require temperatures of over 300°C, and Organic Rankine Cycle of over 200°C.

The smaller rotary heat system, which targets waste heat less than 200°C could potentially be utilised to capture waste heat from factories, incinerators, geothermal facilities, or cogeneration systems and boilers.

Background

Da Vinci Co. Ltd. is a Japanese environmental equipment manufacturer specialised in the heat business.

Da Vinci Co. Ltd. has been conducting research into heat related technologies since 1991. The company began research into waste heat recovery in 2000, and in addition to the rotary heat engine the company has developed Seebeck devices, dual Peltier modules, and magnetic heat pumps.