THE POWER-ENGINEERING APPROACH

The power-plant engineer recognizes that new, modern fossil-firing power plants will have conversion efficiencies in the range of 33­40/., depending on the specified fossil fuel, size of plant and local environment. For some specific conditions, it may be 33X, i. e.,

0. 33 kWh(e) net electricity will be produced per 1 kWh(t) of fuel <Lo. ht. val. ). Therefore, in a solar hybrid plant, the solar electri­city contribution should be obtainable by substraction, as follows :

Solar electricity = W„ut! – 0.33 x Win = 36.67 x 10* kWh(e)

Hence,

Solar fraction = (Wou« – 0.33 x Wi„)/Weut = 0.569

and

Solar Efficiency = (Solar electricity)/А x I = 7.8X

With this approach the value of the obtained solar fraction repre­sents the fuel displaced and pollution obviation in a much more justifiable manner than with the first-law approach. The values of such attributes of fuel displacement and pollution prevention, which are derived via the powei—engineering approach, are also justifiably relevant for comparison with those of solar photo-voltaic plants.

= 22.35 x 10* kWh(e> = 0.347 = 4. 7%

Подпись: Solar electricity Solar fraction Solar efficiency

How, one more step. A power-plant scientist versed with advanced converson systems such as combined cycles, etc. , knows that fossil fuel can yield today 50% (net) electricity out of fossil fuel (and at power-plant costs not exceeding that of the solar hybrid). Why shouldn’t this value of conversion efficiency be used as reference for the power-engineering approach? When it is, then. the fraction 0.5 will replace the 0.33 used above. This will yield :

With this approach one kind of advanced technology (solar hybrid) is compared with another (combined cycle), for its potential of saving fuel and obviating pollution. The results are striking and provide us with a first-hand grasp of the energetic rank of the specific solar technology.

In checking the particular technology treated numerically here, one notes the relatively low steam temperature obtained from the solar field, which necessitates extensive usage of fossil fuel. This in­evitably lessens the resulting, real solar contribution, and far more so when advanced high-temperature power cycles are taken for reference. In contra-distinction, other solar technologies which yield adequately high temperatures show improved fuel displacement, as expected. In fact, the analysis of the particular example data quantifies the major weakness of low-temperature solar technology.

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