Page 86 - Piston Engine-Based Power Plants

Piston Engine Cogeneration and Combined Cycles 77

With these measures it may be possible to achieve a fuel to electricity
conversion efficiency of close to 55%. This was the target for efficiency
for a plant in Wasa, Finland, installed in 1998. The plant has two
17 MW diesel engines and a single steam turbine. Efficiency in this
case was improved by using seawater cooling for the steam turbine
condenser. The additional expense of the waste heat recovery and
steam turbine will generally only prove cost-effective if the engine is to
be used for base-load operation.

ORGANIC RANKINE CYCLE
5
An organic Rankine cycle turbine is a small turbine that is identical
in design to a steam turbine but which uses a low boiling point organic
fluid as its working fluid instead of water and steam. The turbine is
packaged into a closed cycle system with a condenser and heat
exchanger. Waste heat is supplied to the heat exchanger and this
vaporises the organic fluid. The vapour drives the small turbine with a
condenser on its outlet to ensure a high pressure drop across the tur-
bine. The re-condensed fluid is returned to the heat exchanger. The
turbine, meanwhile, is connected to a generator which provides an
electrical output.
The ORC has been used to exploit low-quality heat from a number
of different sources. They have been used with low-temperature geo-
thermal reservoirs and can be added as bottoming cycles for a range of
different heat engine generating systems. The ORC would not nor-
mally be considered a cost-effective addition to a reciprocating engine
system. Recently, however, small mass produced and packaged ORC
systems that can exploit any waste heat source have become available.
These can relatively easily be adapted for use with a reciprocating
engine. One company that has developed an ORC system adapted for
a 1 MW diesel generator package has claimed an improvement in fuel
efficiency of 10% 12% and a payback time of 2 3 years for a diesel-
based plant. 6

5 The Rankine cycle describes the operation of the steam turbine as a heat engine.
6 Efficiency Gains by Bottoming Reciprocating Engines with an ORC, Rob Emrich,
ElectraTherm, 2015.