Efficiency Enhancements

In parallel, another option to be considered for CO2 mitigation involves something as mundane as improving the efficiency of processes we already use. Whether the fuel is gas or coal, efficiency improvements always work to reduce CO2 emissions. Market forces are already inherently at work here, albeit for different reasons (demand for commodities such as oil and natural gas sometimes quickly outpaces supply). Fuel-efficient hybrid vehicles and diesel engines are preferred over less efficient, larger, and heavier motor vehicles, particularly where fuel price is an issue. Airlines will optimize flight schedules to reduce fuel consumption, which commensurately reduces CO2 emissions. In many cases, an aircraft may be replaced entirely with a more fuel-efficient version—saving money but also mitigating CO2 emissions.

There are significant efficiency enhancements which can yield CO2 reduction in power generation. The results noted in Fig. 10.5 are based on using the 2004 average US fossil fleet (coal) performance figures. By improving overall fleet efficiency by just 1% (from the average 33% to 34%) could reduce CO2 emissions by about 60 million tonnes per year. Making minor changes to a fleet of 1,400 fossil units is no small feat, but clearly the opportunities for quick returns on reduction of CO2 are possible, and almost irresistible not to mention the savings related to reduced fuel consumption.

Fig. 10.5 CO reduction potential from fossil coal utility fleet with efficiency enhancements

Going further, we could consider operating the entire United States coal fleet at or near supercritical conditions where thermal efficiencies of 40% HHV (Higher Heating Value) are considered the benchmark for new fossil steam plants. This could yield 400+ million tonnes of CO2 reduction, which is about half the CO2 increase in the United States economy since 1990 and what would work in the US markets could be readily deployed globally. A similar exercise with natural gas shows that efficiency improvements reduce CO2, but because the CO2 emission factor for natural gas is so much lower initially, the reductions obtained in the gas fleet are not nearly as large as with the fossil coal fleet.

Even though new fossil plants can be rated at 40% efficiency and greater, the fully integrated system (generation, transmission, and distribution, storage, etc.) does not perform at this level all the time. One reason for this is that load and demand must always be in balance, and since demand changes throughout the day (and the season), the system must respond accordingly. A plant operating at maximum efficiency at 4 PM may only be required to support a much-reduced load at night, and invariably this comes with reduced efficiency as the output drops. Virtually all peaking plants will be offline in the late hours. The net result is that the yearly average of the fleet drops from the design maximum continuous rating of the individual components that make up the fleet.

Conflicting plant operational requirements can make improvements in efficiency difficult, especially in older facilities where open loop cooling (for the condenser heat exchanger) has been the norm. A recent study by Tetratech examined the impact of switching from open loop cooling for a conventional thermal power station. Cooling water extracted from offshore can be reduced by 94+%, but for a gas-fired thermal station in California it also worsens the plant heat rate by 1.5-1.7%, and reduces total plant output by about 2.7%. The importance of what might appear to be a minor issue shouldn't be minimized: changing the facility operating characteristics to solve one problem (reducing the amount of water consumed for cooling) comes at a price of reduced efficiency and output, a tack that takes the facility away from improving the CO2 emission footprint [12].

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