ENGINES

Engines and power generation systems are of two types:

1-      OPEN SYSTEMS –these are usually piston engines such as the Otto or gasoline engine

2-      CLOSED SYSTEMS-these are mostly turbine type systems                                                                   

OPEN SYSTEMS

 The efficiency of an open system has several limiting factors. The fuel is usually liquid and is injected into the engine this way when the engine is at top dead center. If the fuel were more in a gas state there would be more complete combustion and less fuel would be used to get the piston to move. As the system is now all of the fuel is used and a certain percentage is wasted unconsumed along with a great deal of heat. The piston itself is the cause of inefficiency for a couple of reasons. The first is that the explosive energy in the fuels energy must overcome the forces of friction that prevent the piston from moving and by the time the gasses have expanded the fuel has not (a) been fully burned and (b) the gasses has not fully exerted all its internal energy on the piston. Lastly, the temperatures available for cooling yield low Carnot efficiencies.

Otto engines are usually about 20% efficient and are of the piston variety of engines. They are usually low compression engines. There several links to more information on these engines and why they perform the way they do. PLEASE take the time to read the information it will help understand the problem but more importantly it will help you understand our solution.

Diesel engines generally are approximately 40% efficient and are also of the piston variety of engines. These are high compression engines.

                      CLOSED SYSTEMS

STIRLING ENGINES are generally a closed system that uses lower temperatures. Originally, they used air as a working fluid. They still primarily operate with pistons and Carnot efficiencies. Attempts have been made to over come the problems of working with air as a working fluid and with pistons by using other working fluids and higher temperatures. They still have not taken off as a power generation device, although there are devices that do perform well as refrigeration machines.

STEAM SYSTEMS are usually closed systems. The reason is simple. If this were an open system at some point the working fluid would run out. The other reason is that as the working fluid was evaporated the residue would cause inefficiencies in the boiler.

THE WORKING FLUID-water- is heated----changed from a liquid to a gas or vapor. Then more heat/energy is added causing the steam to be pressurized. This pressurized steam is then led through a nozzle and into a turbine, which allows the gas to expand thereby giving the turbine motion. As the gas passes through the turbine, it cools. Since all the energy in the gas is not given up to the turbine, it must be cooled to the point of condensing the steam to water and/or being able to compress or pump the working fluid to the boiler for reheating to steam.

THE TURBINE—is the device that converts the energy in the gas to motion or power. It is constructed of sections of blades that become larger and larger due to the fact that the steam expands in the turbine. As the steam expands the water particles condense and become larger. If one lets the water condense too much the water droplets cause wear against the turbine blades. There is a limit of about 30-50% of the heat that engineers allow to be taken out of the system. The rest of the heat in the steam must be removed. This feat is accomplished by putting a cooling coil at the exhaust of the turbine to cool/condense some of the steam to water and some is compressed both are fed back to the boiler for reheating.

THE CONDENSER—is a device that is usually placed at the exhaust of the turbine. Generally, it uses water from a large source at about 52F to cool the steam and precipitate it in the system. This cooling water has its temperature raised by 75-100F and is then put back into the ‘river’ it came from.

Due to the above factors the efficiency of a steam plant is usually about 30%

ORGANIC RANKINE CYCLE SYSTEMS--- is a closed system very much like the steam turbine system. Its advantage is that it uses a refrigerant as a working fluid. This enables the system to work off of thermal sources that are much lower than water based systems. The disadvantage is that the difference between its high thermal source and its lower cooling source is much smaller than the steam system and the turbine has the same deficiencies in transforming energy into power. This translates into higher costs for the system to be constructed. For any given power output one would need two to three times as much energy put into the system and the ability to cool two to three times as much waste heat. These systems are geothermal or waste heat systems that rely on coils or heat exchangers to get heat into the working fluid. These coils would need to be two to three times as big and for cooling two three times as much water at 52F would need to be pumped. These systems usually run at about 10-20% efficiency.