Energy Storage
Following the fluctuating grid demand for electricity is a problem as old as power stations. In principle, there are two solutions to cope with this problem: frequent shutting down and starting up, or storing energy in such a way that the power station can run continuously while following the demand pattern. In both cases, additional equipment is required that stands idle for part of the time. Even in the extreme case that electricity could be stored as such, additional equipment would be required.
When low capital cost power stations with a high efficiency can be built that can be started up and shut down within minutes, then in most cases no energy storage would be required. Topping units using only gas turbines and diesel generators are relatively low cost but have a low efficiency of about 40%. The Tophat cycle discussed in Section 7.3.3, with an efficiency of about 60% comprising only a gas turbine, could be a possible candidate in case fast start-up gas turbines such as aero-derivatives are used. But even in this case it is more advantageous to run the gas turbine proper continuously and opt for energy storage.
Of the various options for energy storage, such as flywheels, magneto-hydrodynamic rings, reversible chemical reactions, pressurized air in underground strata, and hydro, only the latter has become commercially successful. All the other options appear to have severe drawbacks that make them at least in the short — and medium — term unlikely candidates to solve the problem of efficient large-scale energy storage. On a smaller scale as required for IGCC power stations there are a number of options that are discussed below. These can be divided into the production of intermediate peak shaving fuels, methanol, and storage of liquid oxygen.
Peak Shaving Fuels
In the past it has been suggested to produce methanol, Fischer-Tropsch liquids, or dimethyl-ether from synthesis gas during the off-peak hours, and then to use these clean fuels as additional gas turbine fuels during the peak hours. It should be realized, though, that all these options are capital intensive, and that the “battery” efficiency of all these options is low and at best 70-80%. Moreover, these options require very pure synthesis gas and hence additional gas-treating facilities and CO shifting. All these conversion processes mean that the overall efficiency of the fuel conversion train will be negatively affected.
Methanol
Of these options methanol is still the most attractive, as this product can be made with an almost 100% conversion of the synthesis gas in a single-stage reactor and has the lowest heat of reaction and hence the highest conversion efficiency. Fuel — grade methanol can be used that will reduce the capital cost and increase slightly the process efficiency. Further, methanol may be reformed into synthesis gas with low — level heat of 300-350°C that will increase the efficiency of the overall fuel conversion train. The battery efficiency of methanol is over 80%.
Liquid Oxygen Storage
Some storage of liquid oxygen is always required in IGCC power stations in order to cope with sudden changes in demand. In order to use the oxygen for peak shaving, generally a much larger storage capacity is needed. This does increase the capital cost of the plant, but it has the advantage that the ASU does not have to be designed for the peak demand but for the average daily demand. So it is a matter of balancing these two cost items against each other. It should be mentioned that apart from the additional storage capacity required for liquid oxygen, means also have to be installed to recuperate the cold generated by evaporation of the oxygen.
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Chapter 8