Green Energy Fraction (GREF)

Climate protection requires new technologies, hence many kinds of renewable energy sources systems have been developed like wind, solar, geo-thermal and bio-mass, and a variety of strategies for marketing have emerged. The case of solar thermal hybrid systems is significant and is also illustrative for renewable energy hybrid systems of other technologies. 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 directly contributes to climate protection, and which we are targeting 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.

A fuel-assisted solar thermal hybrid system

At the lower left side of Figure 1 a solar thermal hybrid system with a Rankine-cycle generator is modelled. The use of fuel enables the solar thermal SEGS (solar electricity generation system) to accomplish increased generation capacity, e. g., extend more hours of operation throughout the year far beyond the solar hours (say, 2000 solar hours as shown), by fuel firing at the SEGS generator (33.2% net efficiency). Partial fuel is used also during the solar hours in order to augment heat input when solar radiation is low and additional purposes. Three operation modes of such systems are noted. Mode 1 (the 2 kHrs point on the horizontal scale) indicates no use of any fuel during the solar hours (GREF = 1). Mode 2 (the full circular point), the use of 30% fuel, and mode 3 (the full triangular point), of 50% fuel.

At the point of 2000 operation hours with mode 1 the FCR will be zero, as there is no use of fuel. With mode 2, the FCR will be 0.11, and with mode 3, FCR of 0.2. The Green Energy Fraction (GREF) values will show 1,0.89 and 0.8, for the 3 modes, respectively. With full fuel
firing beyond the 2000 solar hours, the steep, relatively thick, three dashed lines (marked by SEGS 33.2%) show how fast the fuel-consumption grows. At 5700-6700 hours the lines reach the horizontal baseline standard level, which stands for the fully fuel fired CC (gas turbine combined cycle) (FCR=1). In other words, by operation for that period of time the solar-hybrid has consumed as much fuel (and produced emissions) as a 60% efficient fuel fired CC will do during the full year 8760 hours. At around 8000 hours, the solar-hybrid will have produced emissions much more than the CC and nearly as much as a fuel fired Rankine cycle running at a 40% level conversion (which is a secondary standard). In terms of GREF (green energy fraction) the green energy (fuel avoidance kind of energy) which has been produced by the solar system during the solar hours, now will largely diminish or be totally wiped out because of the extended operation hours. This green energy annihilation results from the long hours of firing fuel relatively inefficiently (33.2%) as compared to the baseline standard of 60%. It may also be nearly so with respect to the secondary standard of 40%. The differences between the 3 modes are noticeable.