The cost of one ton of the fuel carbon (dioxide) avoidance by the specific technology is an important yardstick for renewable energy technology evaluation. However, there is a fundamental dilemma here: How to compute the avoided carbon amount per one kWhe that need not be generated from fossil sources. Is it by assuming, say, a 40% efficiency conversion of the fuel (heat of combustion) to electricity? Why not 30%, perhaps 55%? Any assumption will lead to a different result for the avoided carbon amount. The problem requires an agreed reference efficiency to serve as standard. In case more than one standard becomes necessary, this should be temporary and well reasoned. Full transparency of applied standard(s) (recognized or implied) is essential, with no exception.

Analysis is in place before we can answer the question. Figure 1, drawn after the diagram pattern of Geyer [2] illustrates the Fuel Consumption Ratios (FCR) for model thermal power

The relative fuel consumption (hence emissions) ratio and the green energy fraction of systems over full load hours. a 30% fuel in SEGS solar hours. a 50% fuel in SEGS solar hours.

systems of various conversion efficiencies, by the thin solid lines with varying slopes. In the figure the 60% conversion Combined Cycle (CC) is taken as a baseline standard for reference, thus having an FCR of 1 (as shown by the vertical scale on the right) when the power plant has operated for the full 8760 hours of the year (full yearly load). The 60% CC baseline seems to serve as the recommended standard reference for large power plants at sites where natural gas may be available. The CC is at present a practical, efficient power plant of available technology. Line slopes of various efficiencies are shown on the chart. For example, the 30% conversion line, which shows an FCR of 2 (the inverse efficiency-ratio with respect to the 60% standard) for the full year operation. It reflects the relative excess fuel consumption (hence emissions) of the 30% system as compared to the 60% (baseline standard). As the amount of fuel (in terms of the heat of combustion) is 1.667 kWht (per 1 kWhe electricity) for the 60% conversion, it will double to a value of 3.333 kWht for the 30% conversion. The avoided carbon in weight (ton/MWh) for each case is derived through the FCR value and the particular fuel stoichiometry (chemical compsition-based accounting). It is of significance to recognize that the avoided carbon amount per one kWhe (green electricity) output is not explicit without a clear decision on the baseline or reference standard [3]. By setting the reference standard, we can resolve the above-mentioned problem of how to compute the avoided carbon amount per one kWhe electrical energy output. Thus, environmental parameters of hybrid systems are governed by set standards.