Page 72 - Piston Engine-Based Power Plants

64 Piston Engine-Based Power Plants

requirements. The engines operate at high pressure and this imposes
severe constraints, particularly on materials for engine construction
and for piston seals. In order to resist the forces inside the engines due
to this high pressure, Stirling engines are relatively heavy compared to
similar internal combustion engines. In addition, the use of gases such
as hydrogen and helium makes sealing the gas inside the engine, while
allowing piston movement and lubrication, technically challenging.
Both of these gases have small molecules that can diffuse easily so seal-
ing the engines requires specialist materials and designs. Some engine
designers have experimented with nitrogen or air as replacements, but
these do not have the same advantageous thermal properties of either
hydrogen or helium.

Many Stirling engines operate at relatively high speed, providing a
high power density. However high-speed engines tend to have shorter
lives, with greater wear, than slower speed engines. Larger engines
tend to have lower rotational speeds by virtue of their greater mass.
Stirling engines also operate at relatively high temperatures, with the

heat source in engines typically reaching around 700 C while some
operate at temperatures as high as 1000 C.

Higher temperatures lead to higher thermodynamic efficiency.
Some commercially available engines have achieved up to 30% effi-
ciency and in principle 50% efficiency is feasible. However the high
temperature gradient in an engine can lead to thermal stress between
the hot and cold sections of the engine and high-performance materials
are required to manage this.

Another technological challenge is to achieve fast heat transfer
from the heat source to the working fluid in the hot cylinder and from
the cold cylinder into the heat sink. The choice of hydrogen or helium
as the working fluid helps with heat transfer but the heat transfer
elements of the engine body itself must also be extremely efficient at
conducting heat. The design of an effective regenerator can also be
challenging. As with the heat transfer elements, this must be able to
absorb and release heat rapidly and effectively. Most regenerators
are made from some form of fine mesh or wire construction that pre-
sents a high surface area for heat transfer as the working fluid passes
through it. However this can get blocked if there are any particles
within the working fluid.