--- title: "Design exceedance Q45% versus Q65%" subject: "Power Plant Design" chapter: 3 key: "semester_8/power_plant_design:q45-vs-q65-exceedance" source: https://www.bctengineer.com/q/semester_8/power_plant_design/q45-vs-q65-exceedance site: BCT Engineering --- # Design exceedance Q45% versus Q65% Power Plant Design · Chapter 3: Hydro Power Plant Design - Asked in: 2075 Bhadra, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/q45-vs-q65-exceedance - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3 ## Question The design exceedance of hydropower plant Q45%, what does that mean? Compare Q45% with Q65% while designing hydropower plant from the perspective of power producer (seller). ## Short answer Q45% means rated discharge is the flow equaled or exceeded 45% of the time. Q45% yields higher capacity and more total energy (mainly wet-season) but lower plant factor; Q65% gives smaller, cheaper plant with higher plant factor and lower spill risk. With a firm PPA, sellers usually prefer about Q40–Q45 for higher total revenue. ## Answer #### Meaning of Q45% A design exceedance of **Q45%** means the plant's design (rated) discharge is the flow that the river **equals or exceeds 45% of the time**, about $0.45 \times 365 \approx 164$ days a year. The installed capacity is based on this flow: $$ P = \eta_o \rho g\,(Q_{45} - Q_{riparian})\,H_{net} $$ For the other 55% of the time the plant runs at part load, following the river flow. In Nepal, NEA's power purchase practice for ROR projects has moved from about $Q_{65}$ (older practice) to about $Q_{40}$–$Q_{45}$. #### Comparison from the seller's (IPP) point of view | Point | Q45% design | Q65% design | |---|---|---| | Design flow | Larger | Smaller | | Installed capacity | Higher (MW) | Lower | | Annual energy | More (extra wet-season energy) | Less | | Plant factor | Lower (about 55–65%) | Higher (about 70–80%) | | Capital cost | Higher (bigger waterway, units) | Lower | | Cost per kWh | Can be higher if the extra energy is only wet-season | Lower per kWh | | Wet-season spill/curtailment risk | Higher (NEA may curtail surplus in monsoon) | Lower | | Dry-season share of energy | Smaller proportion | Larger proportion | | Revenue | Higher total, if all energy is bought | Lower total, steadier | #### Seller's view - **Q45** gives more MW and more total energy, which raises revenue if NEA buys all energy under a take-or-pay PPA. Bank financing and per-MW benefits also favour larger capacity. - But most of the extra energy comes in the monsoon, when the tariff is lower (dry-season rate is higher than wet-season rate in NEA's posted rates) and when the grid already has surplus, so spillage or curtailment can reduce income. - **Q65** gives a smaller, cheaper plant with higher plant factor and lower risk, but loses wet-season energy. - The final choice is made by comparing incremental energy and revenue against incremental cost (B/C ratio, IRR) for several exceedance values; with a firm PPA, sellers usually prefer about Q40–Q45. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a design exceedance of Q45% mean? A. The flow the river equals or exceeds 45% of the time B. The flow that is exceeded only 55% of the time in the dry season C. 45% of the average annual flow D. A flow equal to 45% of the maximum flood Answer: A. Q45 is read from the flow duration curve as the flow equalled or exceeded 45% of the time (about 164 days a year). (Key point: Meaning of Q45%) 2. Compared with a Q65 design, a Q45 design has: A. Lower installed capacity and higher plant factor B. Higher installed capacity, more annual energy and lower plant factor C. Same capacity but lower cost D. Lower capital cost and higher plant factor Answer: B. Q45 is larger than Q65, so the plant is bigger and produces more energy, but it runs at part load more often, so the plant factor is lower. (Key point: Effect of higher design flow) 3. Why may extra capacity at Q45 not give proportionally higher income to the seller? A. Extra energy comes mostly in the monsoon at a lower tariff, with spill or curtailment risk B. Extra energy comes mostly in the dry season when the tariff is low C. Q45 plants cannot sign a PPA D. Q45 reduces the head available Answer: A. The additional flow is wet-season flow, when tariff is lower and the grid has surplus, so NEA may curtail it. (Key point: Seller's risk with Q45) 4. How is the final exceedance value chosen? A. Always the lowest cost option B. By comparing incremental energy and revenue with incremental cost (B/C, IRR) for several exceedances C. By the highest plant factor only D. By the buyer without any analysis Answer: B. Several exceedances are compared on incremental benefit versus incremental cost. 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