Fuel saved in year j FLCFI iF) (12.23)
where
F = solar fraction, obtained from Eq. (12.1)
L load(GJ)
CF1= first year unit energy cost delivered from fuel(like parameters CEA and CEL, it is the product of fuel heating value and heater efficiency) ($/GJ)
if = fuel inflation rate.
It should be noted that the product FL represents the energy saved because of the use of solar energy. Summing the fuel saved in year (j) over the payback time(np) and equating to the initial system cost, (C), given by Eq. (12.2), gives