6 A PPLICATIONS OF SOLID OXIDE FUEL CELLS FUTURE ENERGY SYSTEMS
6.2 E FFECTS ON FUEL EFFICIENCY AND INTEGRATION OF RENEWABLE ENERGY
The characteristics of the energy systems to which fuel cells and electrolysers are applied are very important to the ability of the applications to improve these systems. In very in‐
efficient energy systems, the fuel cells improve the fuel efficiency to a larger extent than in energy systems which are already rather efficient. However, in renewable energy systems, fuel cells are also important, as they improve the total efficiency of the system. The fuel efficiency improvements identified in the technical energy system analyses of the different applications are illustrated in Fig. 12.
tion of power plants and improve the fuel efficiency. In the CHP system with wind power, the FC‐CHPs are more efficient than the gas turbines already installed. However, the fuel savings achieved are limited, because the heat at these plants is now produced by boilers.
The fuel savings in the 100 per cent renewable energy system are larger than in the inte‐
grated energy system, because the rather inefficient wood pellet boilers are replaced. In the 100 per cent renewable energy system, however, SOFC are already installed in all CHP plants; hence, FC‐CHP is very important to the total fuel efficiency of the system. In the in‐
tegrated energy system, the penetration of SOFCs is also rather high already at this point, and thus, an improvement in fuel efficiency can only be achieved with central FC‐CHP. For these energy systems, the flexible operation of FC‐CHP is important to the integration of fluctuating renewable energy. Renewable energy and CHP replace power plants in these systems and, hence, traditional base load plants are not important in future fuel‐efficient renewable energy systems.
Along the path towards renewable energy systems, the analyses of fuel efficiency show that improvements can be achieved by expanding the CHP areas. For Central FC‐CHP and Local FC‐CHP, the largest fuel efficiency improvements are achieved in the two electricity energy systems, because both electricity and heat demands previously covered by power plants are replaced. In the electricity energy systems with 24 TWh of wind power, the im‐
provements are almost the same as in the system without wind, because the FC‐CHPs are able to utilise heat storage and move their production to hours with no wind power, thus replacing power plant production. In these two energy systems, the electricity demand is rather high; thus, both Central and Local FC‐CHP plants have the potential for producing heat in FC‐CHP plants. For Central and Local FC‐CHP in the remaining energy systems, the savings are lower due to the fact that individual boilers are now replaced instead of electric heating. Fuel savings are generally lower in systems with wind power, because these sys‐
tems offer limited opportunities to replace production at power plants with production at FC‐CHP.
The Micro FC‐CHP applications replace a heat share similar to the amount supplied to households from district heating in the Local FC‐CHP application. The Micro FC‐CHP also replaces power plant production whenever possible; but the fuel savings achieved are in general lower than in the previous applications, mainly due to lower efficiencies. In the en‐
ergy systems with already installed CHP plants, the Micro FC‐CHPs produce at times when these would normally be operating, and thus, the heat demand in district heating areas must now be covered by boilers. This is not a big problem in energy systems in which CHP plant efficiencies are lower than those of the Micro FC‐CHP; but it does pose a problem in renewable energy systems in which the efficiencies of installed CHP plants are higher than those of Micro FC‐CHPs. Due to this situation, a small increase in fuel consumption takes place in the integrated energy system. Again, savings in the 100 per cent renewable energy
system are higher than those of the integrated energy system, because wood pellet boilers are replaced and Local and Central FC‐CHPs are more efficient.
In the cases of Central and Local FC‐CHP combined with electrolysers, marginally higher fuel efficiencies can be identified in systems with excess electricity production, because other fuels can now be replaced. Here, the electrolysers only produce at times with excess elec‐
tricity production. Most fuels are replaced in the traditional energy system with 24 TWh wind power, because this system has the largest excess electricity production. For the Mi‐
cro FC‐CHPs combined with electrolysers, the hydrogen demand must be met, and thus, excess electricity from wind power cannot cover the entire demand. This eliminates the fuel savings achieved by such applications, as identified in the case of Micro FC‐CHPs oper‐
ated on natural gas.
For comparison, 500 MWe central FC power plants have been included in the technical en‐
ergy system analyses. The technology is identical with Central FC‐CHP except from the fact that no heat is produced by FC power plants. As wind power often replaces central power plants, and thus, FC power plants have fewer possibilities of operating in these systems, the best fuel efficiency improvement is achieved in energy systems without wind power. In the integrated energy system and in the 100 per cent renewable energy systems, fuel savings are very low. This is based on the following facts: 1) A large proportion of the demand is met by fluctuating renewable energy; 2) the operation hours of power plant are reduced to a minimum with CHP plants in order to improve fuel efficiency; and 3) the efficiencies of the existing power plants are already quite high.
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Central FC‐PP Central FC‐CHP Local FC‐CHP Micro FC‐CHP Central FC‐CHP Local FC‐CHP Micro FC‐CHP
Natural gas Electrolyser hydrogen
TWh/year
Marginal fuel savings
Electric heating system Electric heating system + 24,5 TWh wind Traditional system Traditional system + 24,5 TWh wind CHP system
CHP system + 24,5 TWh wind
Integrated system 100 per cent renewable energy system
Fig. 12, Marginal fuel savings of the six applications analysed in the eight different energy systems. Central fuel cell power plants (FC‐PP) have been included for comparison.
hydrogen Central and Local FC‐CHP systems, the excess electricity can now be utilised. The natural gas Micro FC‐CHP application increases the excess electricity production marginally in energy systems with wind power. Although heat storages are used, these are less flexible than central and local FC‐CHP plants. This is due to the fact that the units are prioritised to increase the total efficiency, but are sometimes forced to produce electricity at times when the demand is already met by wind power and the production of power plants and other CHP plants has already been reduced to a minimum. This is also the case in the hydrogen Micro FC‐CHP system in the traditional and the 100 per cent renewable energy systems.
Here, electrolysers reduce excess electricity production, but this is increased again during some hours due to the electricity produced by Micro FC‐CHP.
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Central FC‐CHP Local FC‐CHP Micro FC‐CHP Central FC‐CHP Local FC‐CHP Micro FC‐CHP
Natural gas Electrolyser hydrogen
TWh/year
Marginal changes in excess electricity
Electric heating system Electric heating system + 24,5 TWh wind Traditional system Traditional system + 24,5 TWh wind CHP system
CHP system + 24,5 TWh wind
Integrated system 100 per cent renewable energy system
Fig. 13, Marginal changes in excess electricity production of the six applications analysed in the eight different energy systems.