| The
Rankine cycle, like the Stirling cycle is an external combustion
cycle. That is the combustion process is external to cylinder
containing the working gas. The Rankine cycle is characterised by
the working gas undergoing a phase change (from liquid to gas) which can
be utilised to achieve high power densities. The most familiar
Rankine engine is the steam engine in which water is boiled by an
external heat source, expands and exerts pressure on a piston or turbine
rotor and hence does useful work. A number of the products below
make use of this concept. However, one of them (the Baxi Inergen)
is an organic Rankine engine which uses an organic fluid ( a
refrigerant) and operates at temperatures and pressures much closer to
conventional heating and refrigeration appliances. This has the
significant advantage of allowing the use of conventional, mass produced
components and eliminates many of the technical challenges of steam
engines.
For further discussion of the relative merits of these engine types see section on papers. |
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| Baxi Inergen | Climate Energy | Otag | Enginion | Cogen Micro |
| The
Baxi Inergen unit, based on the Inergia prototype developed by the
Battelle Institute in the USA, was initially a 3kWe/30kWt unit intended
for small commercial applications. However, the benefits of the
organic Rankine design mean that it is possible to use off the shelf
components for the majority of the appliance, leading to low production
costs. In effect, it is a refrigeration unit working in reverse,
using the expanding working gas to power a scroll expander
(compressor). It is now being further developed as an add-on
module suitable for connection to a gas boiler, and is lightweight and
compact making it suitable for wall-mounting.
It's relatively low electrical efficiency means that it produces less electricity than some other units, although this is compensated for to some extent by the anticipated low cost. The photograph below is of the 3kWe prototype. |
The
Climate Energy unit also uses a scroll expander, with benefits of
simplicity and potentially low manufacturing cost.
However, the use of steam as a working gas raises technical challenges regarding reliability and safety. Against this, the developers claim that the thermodynamic properties of steam permit high heat transfer rates (resulting in compact heat exchangers), tolerate leakage and have no inherent material risks. It is also non-toxic and thermally stable which, combined with the zero cost make this a competitive technology. Climate Energy LLC have recently launched the Honda ICE based unit in the USA and it is uncertain as to whether they intend to continue development of their external combustion unit. |
Otag have developed a steam powered unit,
utilising a single cylinder with two opposing pistons. An integral
linear generator converts the reciprocating motion directly into
electrical energy with variable output 3 phase output. Further details including an excellent animation of the cycle are shown on the Otag website (link below). The unit is being launched in Germany during 2006. |
The
Enginion unit was initially developed as a small commercial unit with a
power output of around 50kWe. It too is now evolving into a
unit more appropriate for small apartments and hotels, although the believed
target power output of ~5kWe still makes it unsuitable for domestic
installations.
Enginion was placed in receivership during late 2005. |
The
Cogen Micro unit, developed by an engineering consultancy (Applidyne) in
Australia is unusual in that it uses reciprocating (single piston)
technology.
Historically the very low efficiencies achievable at reasonable temperatures and pressures have not been attractive. However, novel injector design has enhanced the performance of prototype units which may lead to an ultimately viable product. |
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| For further details contact: | For further details contact: | For further details contact: | For further details contact: | For further details contact: |
| Electrical
output
1kWe |
Electrical
output
3kWe |
Electrical
output
0.2-3kWe
|
Electrical
output
5kWe |
Electrical
output
2.5kWe |
| Thermal
output
10kWt (modulating) |
Thermal
output
30kWt |
Thermal
output
2-16kWt
|
Thermal
output
|
Thermal
output
12kWt |
| Application
Individual family homes |
Application
Homes & small commercial |
Application
Homes & small commercial |
Application
Small commercial |
Application
Homes & small commercial |
| Availability
2008? |
Availability
? |
Availability
2006 |
Availability
2008? |
Availability
2009? |