Aharon S. Roy
Dept, of Chemical Engineering, Ben-Gurion Univ. of the Negev, Beer-Sheva, ISRAEL
ABSTRACT
When a solar power plant is assisted by fossil fuel-firing, the question is often legitimately asked as to the actual magnitude of the contribution of the solar resource vs. that of the fossil, or as to the real savings in fossil fuel brought about by the solar part of the hybrid plant. In addition, it may be of interest to assess the fossil-saving potential of the hybrid, in face of advanced-cycle power plants based on fossil fuel alone. A closely related issue is the assessment of the overall efficiency of the solar subsystem (solar to electricity) of the hybrid. By way of example these questions are raised and analyzed in a simplified manner; a solution is offered and a generalization made. It is shown that the second-law (exergy) and the power-engineering approaches are compatible and yield a justifiable yardstick for assessing power plants, whether conventional or solar-hybrid, and for giving adequate answers to the questions of the solar contribution and solar efficiency. Energy analysis and ranking of solar power plants of different genera are thereby facilitated.
KEYWORDS
Solar-thermal; power plants; hybrid; exergy; thermodynamics; second law; analysis; efficiency; fossil fuel.
INTRODUCTION
The advent of sizeable, grid-connected solai—thermal power plants which utilize both solar and fossil fuel inputs, is a most striking development in the field of solar energy utilization. The hybridization of the solar plant by fossil fuel-firing is a key element in advancing the viability and the capabilities of the plant in several ways: (1) it enables electricity generation at full capacity inde
pendent of solar insolation intensity and cloud cover, thus gaining utility capacity-credit; (2) it obviates the need for energy storage — a very expensive technological option which also entails substantial energy degradation; (3) it allows a fuller utilization of the solar collector field (start-ups become faster and parasitics smaller), and of the power block (longer operation hours throughout the year); and (4) it improves operability, by eliminating substantial problems deriving from the intermittency of solar irradiation. Consequently, improved economics has been exhibited by hybrid solar power plants.
A question, though, legitimately arises on how to quantify the solar contribution as distinct from that of the fossil, towards the total plant output, and what is the ultimate gain brought about by the specific hybrid system as compared to a strictly fossil-firing power plant. In the absence of an accepted methodology for calculating a justifiable partitioning of the energy output of a hybrid plant into distinguishable portions, each rightfully assignable to its particular energy input (solar or fossil), one cannot derive solar efficiency parameters for the system or make an adequate comparison between one type of technology and another. We should recall that it is the replaceability of fossil fuel by renewable solar energy which interests us, and the degree to which it can be accomplished by a specific technology is of definite relevance and significance. This matter relates to our efforts to assess and commercialize solar plants.
The degree of fossil replaceability is translatable into a measui— able decrease of fossil reserves depletion and quantitative reduction of pollution. Unfortunately, one cannot decipher these from published data on hybrid power plants with adequate certainty. Furthermore, the rules which one should apply for deriving the requested information have not been clarified, and need addressing.
