An important yardstick for renewable energy technology evaluation is the cost of one ton of the fuel carbon (dioxide) avoidance by the specific technology. 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 55%. Perhaps 30%? Of course, any assumption will produce a different result for the avoided carbon amount. The problem requires an agreed reference efficiency or standard. In case we find that we need more than one standard, this should be well reasoned. Whatever the standard applied, it should always be transparent.

Fig. 1, drawn after the diagram pattern of Geyer [2] illustrates the Fuel Consumption Ratios (FCR) for model thermal power-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 (shown by the vertical scale on the right) when the power plant has operated for the full 8760 hours of the year (full yearly

SHAPE * MERGEFORMAT

kHrs

Fig. 1. The relative fuel consumption (hence emissions) ratio and the
green energy fraction of systems over full load hours.

9 30% fuel in SEGS solar hours.

*50% fuel in SEGS solar hours.

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 a practical, efficient power plant of available technology at present. One of the less efficient generators is represented by 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 3.333 kWht for the 30% conversion. The avoided carbon in weight (grams) for each case is derived through the FCR value and the particular fuel stoichiometry (chemical composition-based accounting) of the respective fuel. It is of significance to recognise 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. This concept is an essential key point for analyzing environmental parameters of hybrid systems. Green Energy Fraction (GREF)

For achieving the goals of climate protection, many kinds of solar energy systems have been developed and a variety of design strategies have emerged to enhance marketing prospects. The blending of fuel with solar has been invoked in order to enhance capacity, smooth power production and exhibit reduced costs for the mixed electricity output. The value of (1-fCr) defines the Green Energy Fraction (GREF) (shown on the left vertical scale) which represents the avoided-fuel fraction of the energy output of a system. It signifies the green (CO2-free) portion of the system energy output. This is the clean or green energy, which explicitly contributes to climate protection, which we aim at. We want this green energy not to be lost upon blending with fuel energy. Both the (GREF) and (1-FCR) of a system must be based on the same reference standard.