Green Dragons

Vapour Screw Engine – Professor of Energy Systems, Keith R Pullen

This Green Dragon project will show and demonstrate the working principle of a new form of low cost, low maintenance engine that takes normally wasted low temperature heat and converts it to electrical power. The engine at in an early stage of development in the laboratories of City University but operates in such a simple way that it can be understood even by non engineering students. It is called the Vapour Screw Engine and has been patented with help of City University.

 The Vapour Screw Engine takes two inventions attributed to Archimedes – liquid displacement/buoyancy and the Archimedes screw and combines them with our  modern understanding that water boils at a lower temperature when under vacuum or up a mountain.

 In its simplest form the engine is an Archimedes Screw immersed inside a sealed, water filled, cylinder. A vacuum is induced in the cylinder and the water at the bottom of the cylinder is heated. The water boils at, say, 70°C and the steam thus generated displaces water and the consequent buoyancy forces the steam up the Archimedes Screw, causing it to rotate and turn an electrical generator via a magnetic coupling. The steam condenses at the top of the cylinder and returns to the bottom to be re-heated.

 A basic form of the engine exists and several development versions have been tested on compressed air using a clear plastic cylinder. However in order to convert hot water into electrical power the engine needs to operate under vacuum pressure.  Clear acrylic plastic softens with heat and then cannot withstand the vacuum.  A steel cylinder is satisfactory, but the operation of the engine cannot then be seen from the outside. The Green Dragon funding will provide a large quartz glass cylinder, allowing the full operation of the engine to be viewed clearly for demonstration and development.

 The project has so far involved a number of students at different levels from Post doc to third year, including Resham Advani, who presented the project in a competition run by the Institution of Mechanical Engineers and won 1st prize for Communication Skills.

 A major application of this engine is to convert stored water heated by solar in developing countries into electrical power. This application will bring low capital cost electricity to households who will never be connected to the grid. 50W of power can be generated for 5 from 1 cubic m of water at 80°C which will enable LED lighting, phone charging and television in remote communities after sunset. Such households currently use paraffin lighting which is expensive and a health hazards in restricted spaces. Cheap renewable electricity after dark will transform lives, enabling children to read and do homework and parents to work after nightfall. Since the engine is very simple with one moving part, it can be easily maintained and repaired. It is essentially safe because of the low temperatures and pressure.  Batteries that are needed to use Solar Voltaic generation after dark fail after 1-2 years and can become an environmental hazard when there is no easy way to dispose of them or safely recycle in developing countries. Batteries are also expensive given the need for regular replacement.

 Another application is conversion of industrial low grade waste heat to electricity. Such low grade heat is a nuisance and costly to remove and is emitted from many factory processes and engines such as CHP systems. Considerable effort, including at City, is being devoted to development of organic Rankine cycle systems to recover energy from low grade heat. These are not expected to be viable on input temperatures below 120°C. Vapour Screw engines can operate on their waste heat, increasing the overall efficiency of the systems.

Pledges (50)=Pledge for this project and support Prof. Keith Pullen and his team to develop the Vapour Screw Engine.

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