Energy Engineering - Solar and Biomass Power Generation

Full exam

SOLAR AND BIOMASS POWER PRODUCTION AY 2019-20 26 th August 2020 Prof. Giampaolo Manzolini Time: 2 hours Instructions for the examination: 1) Clearly indicate your name on all the sheets you will deliver. 2) The score refers to exercises done in a comprehensiv e manner with exact numerical results. Numerical results correct but not accompanied by explanations will not be taken into account. The final score can be normalized according to the av erage results. 3) Answer briefly and clearly only to the asked questions. Calculations and explanations which do not respond to the questions will not be considered for ev aluation ev en if correct. 4) Talking with colleagues and / or cheating will cause the cancellation of the exam. 5) All the needed data for the resolution of exercises lies on this paper. It is NOT ALLOW ED to use material other than this (e.g. books, clipboard etc.). 6) Not all the prov ided inputs are necessary for the problem solving Exercise 1 (16 points) Calculate the LHV, HHV and the energy density of the biomass reported below on a “dry” and “as received” basis at the boiler inlet (∆h evaporation = 2.44 MJ/kg) (3 points). Determine the biomass mass flowrate assuming that the net power output of the boiler is 30 MW , thermal losses from boiler walls are equal to 1% of the fuel LHV dry thermal input, un- burned carbon is 2% (of carbon content, dry basis, LHV = 32.8 MJ/kg), stack temperature equal to 150°C (c p exhaust gases 1.1 kJ/kg K) and ashes melt at 900°C can be accounted as losses (equivalent heat capacity 2 kJ/kg K), ambient temperature is 15°C and the ( ������������������������������������������������ ������������̇������������������������������������ ) ̇ is equal to 10. (7 points) Determine the storage period of the biomass considering that the storage can be approxi- mated as a greenhouse with 1500 m 2 surface, overall heat transfer coefficient 8 W/m 2K, air temperature in the greenhouse 35°C when solar radiation above 0 W/m 2, ambient temper- ature 20°C, greenhouse absorptivity 0.7, average irradiation 3 kWh/m 2/day and irradiance 700 W/m 2, average incidence angle 20°. Final moisture content 12%. (6 points) Willow wood %, weight dry basis C 0.468 H 0.0569 O 0.4101 N 0 S 0 Cl 0.015 Ash 0.05 LHV , kJ/kg, dry basis 18540 M oisture content, % 15.0 Density (as received), kg/m 3 775 Exercise 2 (17 points) Consider a CSP plant based on solar tower system. Calculate the optical efficiency and plant performance at design conditions (Spring Equinox 925 W/m 2), knowing that the solar plant is located in Abu Dhabi (24°28′35″N 54°22′12″E STZ= GMT +4) and the optical effi- ciency as the following function (being θ z and γ z the sun Zenith and Azimuth angle respec- tively). The power block power output and thermal storage size must be assumed with rea- sonable values. The power plant consumes 1.5 MW electric for heat rejection and HTF cir- culation (8 points) ������������ ������������������������������������ =������������∙ γ ������������ 2+ ������������∙γ ������������ +������������ θz= Any a -0.0073 b 0.0 7 c 58.87 Calculate the net electric energy produced by the plant and net electric efficiency assuming the following daily profile. (7 points) Compare and comment the different conversion effi- ciencies with respect to the design conditions (2 points) 360( 284) 23.45 sin 365 nd δ +  = °⋅  360( 284) 23.45 sin 365 nd δ +  = °⋅  = STZ meridian longitude= (�): = ������������STD = ������������������������������������ ∙15 = Solar Time= (h) =:= ������������������������= ������������− (������������������������������������������������������������ − ������������������������������������������������ ) 15 + ������������������������ 60 Hour Angle (�): ������������ = (������������������������− 12 )∙15 = = cos sin sin cos cos cos sin (sin sin sin ) cos[ ( )] cos cos ZSS S S S sign θ δφ δ φ ω γ γφ δ ψφ γφ =+= − = Heliostat area 100 m2 Convective heat trans- fer coefficient 15 W/m 2K N° of heliostats 7000 T ma x 550 °C receiver height 27 m T mi n 290 °C receiver diameter 15 m T ambient 25 °C receiver emissivity 0.8 eta PB 42% Solar Hour DNI (W/m 2) Zentih/ Azimuth Optical efficiency Thermal efficiency Q HTF QTES QPB P Net 1 0 2 0 3 0 4 0 5 0 6 0 7 450 8 720 9 820 10 840 11 910 12 950 13 910 14 840 15 820 16 720 17 450 18 0 19 0 20 0 21 0 22 0 23 0 24 0