Environmental Engineering Reference
In-Depth Information
Fig. 4.11
Traditional concept of cogeneration efficiency
instance, Fig.
4.11
is typical illustration of the advantages of cogeneration, showing
how in a large-scale thermal power station with a generation efficiency of 40 %,
60 % of the energy in the fuel is wasted. In contrast, cogeneration can achieve a ther-
mal output of 45 % and an electrical conversion efficiency of 35 %, so the overall
efficiency is said to be 80 %.
Such claims are very 'Eco' and imply that the main benefit of cogeneration is
that its overall efficiency is high. Taking at face value this implies that the power
generation efficiency is not so important even if it is low or, to take to extremes,
that an 80 % 'overall efficiency' is satisfactory even with just a boiler that does not
generate electricity! This comparison shows that it is not just the
amount
of energy
that should be considered but also its
quality
.
As mentioned above, Carnot efficiency teaches that low temperature heat is only
able to produce very limited work. We can say this is a
low-quality energy
. On the
other hand, electricity can do work with an efficiency of 100 %. In evaluation of
cogeneration systems therefore, those two types of energies must not be simply
added. The effectiveness of cogeneration also varies with the conditions of the site;
the demand ratio of electricity to heat, and how they change with time. Residential
CHP, which is sold in Japan as “Ene-farm”, is a suitable application for fuel cells,
especially for SOFC, since an average family uses electricity and hot water in simi-
lar amounts. The problem is however, that the everyday life of each family is not
always in the 'average' pattern. When CHP is operated just to meet the electricity
demand, the hot water may be surplus or inadequate.
More effective utilization of fuel cell CHP would be possible if local society
has networks of electricity and heat supply between nearby houses, offices, shops,
restaurants, etc. Fuel cells of an appropriate size for the local network would make it
