MYP · 18 · extended response
A community energy plan that survives the demand peak
Climate and energy systems · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- discriminating
- Marks
- 6
- Topics
- 2
- Answer
- Complete
Six-hour average demand
Supplied question data. Read the labelled axes and use the observations to test the physical model.
Vertical axis (y): Average demand / kW
Horizontal axis (x): Period start / h
Read the exact data as a table
| Period start / h | Average demand / kW |
|---|---|
| 0 | 2 |
| 6 | 5 |
| 12 | 4 |
| 18 | 9 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Optional enrichment: this challenge may extend beyond the usual syllabus depth and assumes confidence with algebra and graph interpretation. A community uses 120 kWh of electricity per day, with the supplied six-hour average demand levels. A solar array generates 24 kWh per installed kW per week, and a wind installation generates 60 kWh per installed kW per week. A proposed plan uses 14 kW of solar plus 8.4 kW of wind. Treat the quoted outputs as weekly averages; neither source is dispatchable on demand. Supplied data (Period start / h; Average demand / kW): (0, 2), (6, 5), (12, 4), (18, 9).
- (a)
Check daily demand and weekly energy balance. Explain why average energy balance is insufficient to establish reliability.
3 marks - (b)
An evening backup battery must supply the full 9 kW for 6 h. Its usable depth of discharge is 80% and discharge efficiency is 90%. Find nominal energy capacity and state two further decision criteria.
3 marks
Ready to self-mark?Reveal the detailed answer guide
(a)
Check daily demand and weekly energy balance. Explain why average energy balance is insufficient to establish reliability.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
The four six-hour blocks consume 6(2+5+4+9) = 120 kWh per day, or 840 kWh per week.
- 2
Proposed weekly generation is 14(24)+8.4(60) = 336+504 = 840 kWh, matching the average weekly energy.
- 3
Matching totals does not align generation with the evening 9 kW demand or calm cloudy intervals. Storage, flexible demand or reliable backup is required, with losses and power limits included.
(b)
An evening backup battery must supply the full 9 kW for 6 h. Its usable depth of discharge is 80% and discharge efficiency is 90%. Find nominal energy capacity and state two further decision criteria.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
Required delivered energy is 9(6) = 54 kWh. Usable delivered fraction is 0.80×0.90 = 0.72.
- 2
Nominal energy capacity must be at least 54/0.72 = 75 kWh. This capacity calculation is separate from the required 9 kW output-power rating.
- 3
Compare capital and lifecycle costs, resource variability, maintenance, local noise or land impacts, material supply and recycling. Stakeholder priorities and environmental effects should be stated explicitly rather than hidden in one energy total.
Complete marking guidance
Use these worked steps alongside the part-by-part answers. They explain the method and mark allocation; they do not add extra marks.
- The four six-hour blocks consume 6(2+5+4+9) = 120 kWh per day, or 840 kWh per week.
- Proposed weekly generation is 14(24)+8.4(60) = 336+504 = 840 kWh, matching the average weekly energy.
- Matching totals does not align generation with the evening 9 kW demand or calm cloudy intervals. Storage, flexible demand or reliable backup is required, with losses and power limits included.
- Required delivered energy is 9(6) = 54 kWh. Usable delivered fraction is 0.80×0.90 = 0.72.
- Nominal energy capacity must be at least 54/0.72 = 75 kWh. This capacity calculation is separate from the required 9 kW output-power rating.
- Compare capital and lifecycle costs, resource variability, maintenance, local noise or land impacts, material supply and recycling. Stakeholder priorities and environmental effects should be stated explicitly rather than hidden in one energy total.
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