GB2472604A - Free piston thermo electrical power generator - Google Patents

Abstract

A low temperature thermo-electrical power generator comprising a double acting low pressure expander which has a sealed cylinder 1, and a magnetised free piston 2, an evaporator, a condenser and a feed pump, the cylinder walls are encircled by induction coils 13 which generate electricity when the magnetised piston passes them. The inlet and outlet valve of the expander may be in the form of a perforated rotating disc and be driven by an electronic control system. Preferably the pressure in the fluid circuit is regulated by means of a variable volume. The heat source may be solar, a thermal store, geothermal or from an exothermal biological process and may have a temperature below 50°C. Preferably the mean operating pressure in the system is be sub-atmospheric causing evaporating temperatures of the fluid to be reduced to correspond with the temperatures available for collection.

Description

Low Temperature Thermo-electrical Generator This invention relates to a low temperature thermo-electrical power generator.

The principle of generating electrical power from a thermal source is well known and comprises a means of transferring the thermal energy generated by a combustion (or similar) process to a fluid creating potential energy which is then converted to mechanical energy by the process of expansion also known as the Rankine Cycle. The energy sources for this process include combustion of a number of fossil fuels, nuclear fusion or fission, geothermal and focused solar thermal energy. All of these processes require the generation of temperatures sufficient to generate superheated steam at high temperatures and pressures in order to maximise the overall efficiency. There is also well known the principle of using organic compounds in place of water to utilise temperatures in excess of 80°C and typically in excess of 150°C.

This invention provides a means of generating electricity using a Rankine Cycle generator in which the system uses a heat source below 50°C. The system comprises a sealed circuit containing the fluid, a double acting linear expander, evaporator, condenser, feed pump and variably timed inlet and exhaust valves to control the operation of the expander. The reciprocal motion of the magnetised piston passing through a plurality of coils fixed to the cylinder walls generates electrical current. The mean operating pressure in the system is sub-atmoshperic causing the evaporating temperatures of the fluid to be reduced. Means are provided to control the mean operating pressure thereby optimizing performance.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 illustrates the double acting linear expander Figure 2 shows a typical circuit diagram Figure 3 illustrates the variable timing inlet / outlet valve mechanism Referring to the drawing Figure 1 the Double acting expander comprises a closed cylinder 1 with inlet and outlet ports 3 and 4 at both ends, a free piston 2 which also incorporates a high density field magnet 14. Fitted around the cylinder wall are a number of induction coils for the generation of electrical power 13. A means of detecting the position and velocity of the free piston is provided 15. The system is controlled by means of an electronic, software driven control module. This uses a combination of hot gas pressure detected by sensor 17 and free piston velocity and position to control the opening and closing of the variable timing inlet and outlet valves 3 and 4.

Referring to the circuit diagram Figure 2 heat is absorbed by device 9 which may, by way of example, take the form of an unglazed flat plate solar collector and is transferred by means of the heat transfer medium and pump 8 to the pre-heat heat exchanger 5a and evaporator 5. Heat is transferred to the fluid in the generator circuit and converts this fluid to a superheated gas capable of doing work. Feed pump 7 replenishes the evaporator with sub-cooled fluid via the pre-heat heat exchanger 5a. When the free piston is at the end of its stroke, exhaust valve 4 is closed and inlet valve 3 opened allowing superheated gas to enter the cylinder 1, the inlet port 3 is kept open until the free piston reaches a position in the cylinder determined by the control system when the port is closed allowing the gas to continue to expand until the free piston reaches the end of its travel when the exhaust port 4 is opened. The exhaust gas is then passed through the condenser 6 which causes it to change state from a gas to a sub-cooled liquid. Heat extracted from the exhaust gas is rejected through heat exchange device 11 by means of a heat transfer fluid driven by pump 10. Overall system pressure is regulated by the Pressure Regulating Actuator 12 this may by way of example be evaporating and condensing at 0.07 bar and 0.02 bar respectively, however the actual operating pressures are directly related to the temperatures available for evaporating and condensing, the algorithm for this is contained within the control system software. The control module 16 uses input data from pressure sensors 17 and 18 and temperature sensors 19 and 20 to control the Pressure Regulating Actuator 12. This enables the system to optimise performance against evaporating and condensing conditions.

Referring to the drawing Figure 3 Variable timing InletlOutlet the valve consists of the body 26 together with a rotating disc 21 incorporating a shaped port 25. The disc 21 is provided with gear teeth on the perimeter, which mesh with the driving gear 24 causing it to rotate as directed by the control system 16. Seals 22 and 22a are provided to ensure the valve is gas tight when in the open and closed position. The gear 24 can, by way of example, be driven by a stepper type electric motor 25 ensuring accuracy in positioning of the port.