Helium engines

Gallery opened: 29 April 2015

Updated: 3 June 2024

More on Helium turbines in Germany added
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Making an engine that uses helium as its working fluid is going to require some strong justification. Helium is expensive, and it is effectively the ultimate non-renewable substance. You can always make oil out of coal if you feel the need; the technology existed in 1932. However, helium is an element, and the only way to 'make' it is to sit around and wait for a radioactive substance to emit alpha particles, which are helium nucleii.

The main reason to make a helium engine is to liquefy helium efficently, though there is some interest in helium-cooled nuclear reactors. As for liquid air, using an expansion engine rather than a Joule-Thomson throttle valve to drop the gas pressure, as in the Linde cycle, gives more efficient cooling because work is being taken out of the system. A further complication fully explained in the link is that a gas must be below its inversion temperature to be liquefied in this way, and so simple Linde cycle liquefiers cannot normally be used to liquefy neon, (BP -246.1 degC) hydrogen, (BP -252.9 degC) or helium (BP -269 degC) and the use of an expansion engine is then essential for reasonable efficiency.

HELIUM PISTON ENGINES

Left: Helium piston engine for wet expansion

Perhaps the most remarkable thing about this engine is how conventional it looks. It is designed to work as a wet expander- in other words liquid helium forms actually inside the engine. Claude had dreadful troubles with lubrication when was liquefying air inside an engine; the fact that it can be done routinely with helium presumably demonstrates what progress has been made in dry lubrication since 1900.

Its operation is pretty much self-explanatory, but note that the valves are at the bottom, presumably so the liquefied helium is easily cleared through the exhaust valve.

Piston expanders like this one are commonly used in the temperature range 20 - 40 K. (Kelvin or degrees Absolute) Wet expanders are preferred as they are more efficient than dry expanders, in which the gas cools but does not liquefy. Typically wet piston expanders are 80% efficient compared with dry piston expanders at 70 - 75%.

The power output is tiny compared with that required to compress the helium in the first place, and is usually not worth trying to exploit it. It is usually dissipated in a small fan.

From Cryogenic Engineering Ed B A Hands. Academic Press 1986

HELIUM TURBINES

Left: Section through helium turbine

It is well known that in steam power generation turbines are more efficient and more reliable than reciprocating engines. In the helium business things do not work the same way. Turbines can be and are used as expanders, but piston expanders are in general more efficient, and maintain their efficiency over a wide range of flow rates, making plant operation easier.

This is a turbine expander for a large helium refrigerator. The helium leaves the turbine and exhausts upwards.

Note that no details are given of the bearing, except to say it is a 'journal bearing' which suggest a simple sleeve bearing. How and if it is lubricated are not stated, but the presence of an 'oil slinger ring', which would throw oil off the shsft by centrifugal force, suggests- well, oil. The power output of the turbine appears to be absorbed in an 'oil brake' which seems to be a simple cylinder immersed in oli, to absorb the power output in viscosity losses.

From Cryogenic Engineering Ed B A Hands Academic Press 1986. Image originally from Sulzer Bros.

Left: Two helium turbines of different size

Note the very small impellers compared with the size of the man's hand.

LBL is the Lawrence Berkeley Laboratory in California, which lives at 1 Cyclotron Rd, Berkeley, CA. (A well cool address) The elements Lawrencium, Seaborgium, and Dubnium were discovered there.

From Cryogenic Engineering Ed B A Hands Academic Press 1986

Left: The helium HP turbine of the HHT test system: Germany

There is currently interest in nuclear reactors cooled by helium, which is then expanded in a helium turbine. Coupling a high temperature gas-cooled nuclear reactor with a closed-cycle gas turbine using helium was first suggested by Professor Curt Keller (co-founder of the closed Brayton cycle gas turbine with Professor Ackeret) in 1945. Helium was chosen because of its radioactive stability and high thermal capacity. Helium offers other important advantages, including a lower Mach numbers and a higher Reynolds numbers than with the use of air in turbines.

The first and largest helium turbine to date (the HHT project) was constructed in Germany in 1968. It was rated at 50 MW at 750 degC. The helium was heated by a fossil-fired furnace giving 53.5 MW The operating helium pressure for tests was up to 1 MPa, equal to 145 psi.

A later project was the HHT, consisting of the Oberhausen II helium turbine cogeneration plant operated from 1974 to 1988 by the German utility EVO (Energie Versorgung Oberhausen AG), and a second facility for high-temperature testing (HHV) built in 1981.

The HHT turbomachinery was a two-shaft arrangement. The high-pressure (HP) turbine, which has a rotational speed of 5,500 rpm, drove the low-pressure (LP) compressor and high-pressure (HP) compressor on the first shaft. The low-pressure (LP) turbine on the secons shaft was directly connected to the generator with a synchronous rotational speed of 3,000 rpm. The mass flow rate of helium was 84.8 kg/second.

Source: A Review Of Helium Gas Turbine Technology For High-Temperature Gas-Cooled Reactors by Hee Cheon No et al, 2007

Helium-cooled reactors have a Wikipedia page.

Left: The turbine rotor of the HHV test system: Germany

Source: A Review Of Helium Gas Turbine Technology For High-Temperature Gas-Cooled Reactors by Hee Cheon No et al, 2007

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