The condensing engine employs steam at atmospheric pressure. It consists of a boiler, a cylinder with piston and a condenser. Initially, steam is drawn into the cylinder. Once the piston has reached the uppermost position, the boiler valve is closed, and the condenser valve opened. The condenser has a pressure close to vacuum, and is kept at a temperature of around 20C. Then the condenser valve is opened, the steam rushes into the condenser and condenses, thereby retaining the vacuum (remember that 1 litre of water gives you 1690 litres of steam). The steam valve on the other side of the piston then opens, and the new working stroke begins. The video illustrates the cycle:
Unfortunately, the efficiency of this process has a theoretical upper limit of only 6.4%, since only the atmospheric displacement work of the steam can be recovered. The picture above shows our Mk 1 engine, which delivers around 12 Watt with a stroke of 100 mm and a piston diameter of 50 mm.
What are the advantages then? There are several, most importantly: the Condensing Engine operates at 100°C (212 Fahrenheit), well below the standard operational temperatures of steam engines of 250 to 300°C (480 to 570 F). This means, that energy sources with much lower temperatures such as solar thermal systems can be used to drive the CE.
It operates under atmospheric pressure, so that it can use the process steam from industry which is often simply blown into the atmosphere. This is a waste of energy. 100 litres (25 gallons) of steam per second contain a thermal energy of 250 kW. With a conversion efficiency of 10%, this could generate 25 kW of electricity or enough to supply 50 houses.
Operating under atmospheric pressure also means that there is absolutely no danger of a boiler explosion (google “boiler explosion” and you will see the effect of such an event), that the safety requirements are therefore much less than for a pressurized engine, and that operation, maintenance and inspection costs are substantially reduced.
The Condensing Engine uses water as working fluid, which is not toxic, cheap and available everywhere, not inflammable and not poisonous.
There are of course other technologies for low temperatures such as the organic Rankine Cycle (ORC) systems which employ organic fluids with evaporation temperatures below 100°C / 212°F, and which operate with pressures of 6 to 20 bar (40 to 140 PSI). These conditions however result in fairly complex machines, which really are only suitable for power ratings of 250 kW or more.
So, and what’s new about the Condensing Engine?
Several things. Firstly, we employ steam expansion, which allows to increase the theoretical efficiency from 6.4% to around 15%, giving more power for the same volume of steam (see “Theory”.
Then, this being modern times, we of course have electronic control and electric valves to reduce the mechanical complexity. We also use a vacuum pump to evacuate the condenser, and to remove the condensate. Again this reduces the complexity of the system and employs standard components.
One big problem with steam expansion is the variable pressure on the piston, which results in a variable power output during every stroke. However, today we can simply add a flywheel (again, an off-the-shelf component) which largely balances the power fluctuation. In addition, a capacitor at the generator further smoothens the electricity output.
Modern insulation material greatly reduces heat losses in the engine, and plastic material for main components reduces costs.
We also want to use the condenser as heat exchanger, to reduce the cooling requirement.
Are there any disadvantages?
Of course, there are. Most importantly, because of the low working pressures (compared with standard steam engines or turbines), the volume of the cylinder needs to be quite large. We intend to overcome this disadvantage by using plastic as material for cylinder and piston, which should reduce costs quite considerably. The space demand however remains.
There is a requirement to keep the condenser temperature as low as possible, so some sort of cooling is required. This however is the case for most thermal engines, and the lower the operating temperature becomes the more important the problem gets.
Where do we see the main use of the Condensing Engine?
The first application that springs to mind is the direct use of waste steam, which is today often simply and literally wasted.
Solar thermal systems and industrial waste heat would be the second largest field
Where does the development go?
Aah, here it starts to get really interesting. One thing I already mentioned is the use of plastic for the main components, which should reduce costs quite dramatically.
Then, the next very exciting thing: the theory of the condensing engine suggests that the operation with temperatures below 100°C, still using water as working fluid is possible. This would mean, that thermal energy which is currently very difficult to make use of now con be converted into electricity. Of course, the efficiencies reduce and the volumes of the machines increase further. This is however absolutely new territory.