7117

Fig. 17.12. Arc-matrix regenerator straw. (»Iter Hoffman 1976). Dimensions arc in inches.

Fig. 17.12. Arc-matrix regenerator straw. (»Iter Hoffman 1976). Dimensions arc in inches.

Fjo. 17.13, Pneumatic cfliclency of Aerojet Ihermocompressor as a function of engine cyclic rate with different regenerator sections (after Hoffman 1976: RGAP denotes the radial gap in inches; VRV denotes the regenerator void volume).

Fig. 17.14, Animal tests of Aerojet heart-assist systems arc carried out at the University of California, Davis (after Moise and Faeser l(j77).
Fig. 17.15. Block diagram of elements in the Thermo-Elcctron artificial heart system (after Watelet. Ruggles, and Hägen 1976).

THERMO-ELEiC I RON ARTIFICIAL HEART KNOINI

Work on ihe Thermo-Elcctron engine began in 197(1 and has been reported regularly in annual reports (e.g. Watelet, Ruggles, and Hagcn 1976), the annual Intersociety Energy Conversion Engineering Conference (e.g. Watelet, Ruggles. and Torti 1976). and elsewhere. In the course of development the engine sustained major changes in design. The system described here is ihe Model 4 unit reported in the above references.

The Thermo-Electron engine was originally called the annular tidal regenerator engine and was not generally reckoned to be a Stirling engine. However, the engine is. in fact, a Stirling engine and furthermore, one of unique character. The working fluid, water, changes phase Irom liquid in the cold space to vapour in the hot space and so may be described as a one-component, two-phase working fiuidt.

A schematic diagram of the Thermo-Electron system is shown in Fig. 17.15. It consists of two principal assemblies; the engine module and a pump module. In the engine module, nuclear or electric heat is converted into mechanical work to operate a hydraulic pump. The hydraulic fluid is then conveyed to the second unit to actuate the blood pump.

A simplified cross-section of the engine is shown in Fig. 17.16. It can be recognized as a Stirling engine ol Heinrici form having a piston and displacer in separate cylinders. The expansion space is the heated space tThc reader is referred to Chapter 8, on working lluids in Stirling engines, lor iurthct discussion

Expansion ^space

Expansion ^space

Fio. 17.16. Simplified cross-section of the Thermo-Electron tidal regenerator engine. This may he classified as a Stirling engine having separate cylinders for the piston and displace! and with a onc-coinponent condensing working fluid.

above the displaeer and the compression space is the cooled spaces below the displaeer and above the piston. The hydraulic pump is a metal bellows located below the displaeer with inlet and outlet check valves.

Piston motion is induced by an electric motor driving a screwed shaft on which is mounted a nut attached to the underside of the piston. Rotation of the motor in one direction causes the piston to rise; rotation in the other causes it to descend. The motor is controlled by solid-state electronic logic unit in terms of the duration and direction of rotation, and the cyclic frequency. Power for the motor and the electronic control is provided from a silicon-germanium thermoelectric unit heated by the power source.

As the piston ascends, driven by the electric motor, liquid working fluid moves into the displaeer cylinder and passes up the annular gap between the displaeer and the cylinder wall. In passage through the annular gap il boils to a vapour and is further heated to a superheated vapour.

The phase change to vapour causes an increase in pressure in the working space. When the pressure exceeds that in the hydraulic pump circuit, in the bellows below the displacer, the displacer begins to descend. This causes more working Huid to be displaced through the regenerator into the hot space, thereby increasing the pressure further, which in turn accelerates the motion of the displacer.

As the displacer descends the bellows below is compressed and hydraulic fluid is expelled through the outlet check valve at the pre-set discharge pressure.

When the piston drive motor reverses rotation the piston descends, the direction of fluid How reverses and the pressure decreases. When it becomes less than the pressure in the hydraulic circuit the bellows below the displacer expands, driving the displacer upwards. This causes the working fluid to flow back through the annulus where it is cooled and changes phase to liquid resulting in further decrease in pressure and thereby accentuating the upward displacer motion. As the bellows expands fresh hydraulic fluid is drawn through the inlet check valve into the bellows space below the displacer. Waste heat from the engine is carried by the hydraulic fluid to the blood pump which incorporates a heat exchange for dissipation of the heat in the blood.

The Thermo-lilectron artificial heart system was different to other Stirling engine systems reviewed here for the engine operated at the frequency of the natural heart. As a heart-assist system it was necessary to have a sensor which synchronized the operation of the engine/pump operation with the natural heart.

No technical details about the engine in terms of pressure range, cylinder diameter, or displacer stroke were given in any of the published material consulted. Engine efficiencies were said to be in the range of 15 to 16 per cent with maximum cycle temperatures of 480 °C (900°F) to 540 °C (1000 °F). The engine efficiency was defined as the ratio: tj = indicated work/(condenser heat + indicated work).

A cross-section of an implantable engine module is shown in Fig. 17.17. This was described by Watclet, Ruggles, and Torti (1976) as 20 cm (7.9 in) long by 6.1cm (2.4 in) diameter for a volume of 700 cm3 (42.7 in3) and a weight of 1.6 kg (3.5 lb,„). In bench tests of the complete system, a power input of 33 watts to the engine module resulted in approximately 3 watts hydraulic output from the blood pump at a frequency of 90 to 110 beats per minute.

Endurance testing of a complete inplantable engine module was carried out for 24 hour day operation at 70 beats a minute for 1200 hours. In this time two bellows failures were recorded, both being the main pump bellows beneath the displacer. A second metal bellows was used to seal the piston but apparently Ihis endured the test without incident.

Further improvement, to a value of 20 per cent, in the efficiency of the tidal regenerator engine was anticipated with the development of the

Nickel foil caps

Cylinder

Polyurethane loam insulation lingine output bellows

Electronics subassembly

Vacuum foil insulation

Fuel capsule or elcctric heater

Fro. 17.17. Cross section of implantable engine module of die Thermo-Electron tidal regenerator engine (after Wntelet. Rugbies, ami Hogen, 1976).

Fro. 17.17. Cross section of implantable engine module of die Thermo-Electron tidal regenerator engine (after Wntelet. Rugbies, ami Hogen, 1976).

binary tidal regenerator engine. This was a combination in a single machine of two tidal regenerator engines using different working fluids. The two fluids discussed by Watelet. Ruggles. and Hagcn (1976) were water and the proprietory fluid Dowtherm A. The two thermodynamic cycles, superimposed on a common temperature/entropy plane, are shown in Fig. 17.18. The engine was essentially the existing tidal regenerator engine with water as the working fluid on which was superimposed another engine using Dowtherm and operating at higher temperature. The Dowtherm displaccr was a double-wall unit with the water displaccr operating inside it. Heat was supplied to the engine at the maximum cycle temperature in the Dowtherm boiler and was transferred at some intermediate temperature from the Dowtherm condenser to the water boiler. It was rejected from the engine at the low temperature of

Cylinder

Polyurethane loam insulation

Nickel foil caps lingine output bellows

Electronics subassembly

Vacuum foil insulation

Fuel capsule or elcctric heater

Enl ropy--

FiG. 17.18. Superimposed thermodynamic cycles of the binary tidal regenerator engine with water and Dowtherm working fluids {after Watclet, Ruggles and Hagen. 1"7<>).

Enl ropy--

FiG. 17.18. Superimposed thermodynamic cycles of the binary tidal regenerator engine with water and Dowtherm working fluids {after Watclet, Ruggles and Hagen. 1"7<>).

the water condenser and hence dissipated to the blood.

Watclet, Ruggles, and Hagen (1976) described a bench model binary engine at an early stage of development: 'that started easily and generated 3.6 watts power output at a frequency of 100 beats per minute with an efficiency close to 10 per cent. The maximum engine pressure was 0.86 MN/nv (1251b per sq in) corresponding to a Dowtherm saturation vapour temperature of 3S2 °C (720 °F)\

Animal experiments associated with the Thermo-Hlectron system development have been carried out in cooperation with the Cardiovascular Surgical Research Laboratory of the Texas Heart Institute. Houston.

W E STIN G HOU SB/1 ' 111 LI PS ARTIFICIAL HI-ART ENGINE

The Westinghouse/Philips artificial heart system development was sponsored by the U.S. Department of Energy, formerly the Energy

Research and Development Administration and before thai the U.S. Atomic Energy Commission. The AEC program was reviewed bv Mott cf

An early indication o: Wcstinghouse interest in this field was given by Lance and Sclz (196K) reporting a conceptual study for a Rankine-cycle rotary steam engine with a Plutonium 238 heal source. When the present program started in the early 1970s the concept had been converted to a radioisotope-fuclled Stirling engine driving a blood pump by means of a flexible shaft. The system was a cooperative effort of the Westinghouse Electric Corporation and the North American Philips Corporation. Philips was responsible foi the Stirling engine development.

Pouchot and Daniels (1974) described the concept and early work carried out on a bench model demonstrator. A conceptual view of the complete system is shown ia Pig. 17.19 and a schematic diagram in Fig. 17.20. The engine was a low-speed (600 revolutions per minute) rhombic-drive piston-displacer unit. The shaft speed was increased by gearing to 1800 revolutions for the flywheel rotation then down to 900 revolutions for the flexible shaft drive with a further reduction in speed to 120 revolutions per minute at the blood pump. The pump was a double cup unit driven by a crank Scotch yoke mechanism as shown in Fig.

GoldOwsky and Lehrfeld (i977) have reviewed the progress in the engine development following several years' work. The prototype implantable unit has been defined to the system envelope shown in big.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

Get My Free Ebook


Post a comment