Connect a gas constraint to dispatch
Fuel adequacy requires commodity supply and deliverable transport at the required time. Pipeline capacity, nominations, storage and linepack constrain delivery. A generator with an electricity connection can still lack fuel; a daily gas balance can conceal an hourly shortfall.
The lesson
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01Trace the fuel before promising electrical output
Your question: can the plant deliver the evening commitment?
A generator is asked to supply 120 MW for two hours. Its fuel account shows 2,400 GJ. A colleague says that is enough, so the commitment is safe. Your job is to test the missing delivery assumption—not just the total fuel quantity. All numbers in this lesson are invented teaching inputs, not real asset capabilities.
Follow three linked boundaries
- 01
Commodity
Gas purchased or allocated: how much energy, where, for which period, and with what delivery rights?
- 02
Transport
Pipeline, compression, storage withdrawal and site delivery: what rate can actually arrive during the event?
- 03
Conversion
Plant heat rate, available MW and operating condition: how much net electricity can that fuel produce?
| Input | Assumption |
|---|---|
| Requested output | 120 MW net for 2 hours |
| Heat rate | 8 GJ per net MWh; constant for this exercise |
| Available fuel allocation | 2,400 GJ for this event |
| Confirmed delivery rate | 640 GJ/h throughout the event |
| Excluded | Starts, ramps, pressure dynamics, station-use adjustments and other operating constraints |
Check your understanding
Which input cannot be inferred from the 2,400 GJ total?
Show reasoning
The hourly delivery rate at the plant.
Enough fuel over a day or event does not establish that it can arrive quickly enough. Obtain a time-matched delivery profile and the applicable operating limits.
02Convert gas energy to electricity on a consistent basis
GJ measures energy, not flow
One TJ is 1,000 GJ; one MWh is 3.6 GJ of energy. A heat rate of 8 GJ/MWh means eight units of thermal energy input per MWh of electrical output. It does not mean 8 MW of fuel or an 8% efficiency. Use net or gross electrical output consistently and keep the same higher- or lower-heating-value convention throughout.
Fuel needed for the electrical commitment
Fuel (GJ) = MW × hours × heat rate (GJ/MWh)
Constant net output and heat rate; startup fuel and part-load changes excluded. Gas is an energy quantity, not an unconverted volume in cubic metres.
Size the fuel quantity
Electrical energy
120 × 2 = 240 MWh
Use the duration of the actual commitment.
Fuel energy
240 × 8 = 1,920 GJ = 1.92 TJ
The 2,400 GJ allocation exceeds this simplified quantity requirement by 480 GJ.
Efficiency cross-check
3.6 ÷ 8 = 0.45 = 45%
A dimensional check on matched fuel and electrical boundaries, not a guarantee of efficiency at every load.
Result: The quantity test passes. The delivery-rate test is still unresolved.
Check your understanding
At a heat rate of 10 GJ/MWh, what fuel quantity and efficiency would the same 240 MWh require?
Show reasoning
2,400 GJ and 36% on matching bases.
240 × 10 = 2,400 GJ; 3.6 ÷ 10 = 0.36. A higher heat rate means more fuel per electrical MWh, not higher efficiency.
03Find the binding limit: fuel quantity or delivery rate
Translate delivery into a power ceiling
Fuel-limited MW = gas delivery (GJ/h) ÷ heat rate (GJ/MWh)
Steady delivery and conversion, with no additional usable on-site fuel buffer. Pressure and transient studies are outside this screening model.
The original commitment fails
Required delivery
120 × 8 = 960 GJ/h
This is the instantaneous rate the simplified plant needs while producing 120 MW.
Available electrical output
640 ÷ 8 = 80 MW
The delivery ceiling is below the requested 120 MW even though total purchased gas is sufficient.
Delivered energy and gap
80 × 2 = 160 MWh; shortfall = 240 − 160 = 80 MWh
The gas account cannot remove a physical bottleneck. A dispatch commitment would need to change or another feasible resource would be needed.
Result: Base case: delivery rate binds, not the total fuel allocation.
| Case | Fuel GJ | Delivery GJ/h | Maximum constant output for 2 h |
|---|---|---|---|
| Base | 2,400 | 640 | 80 MW — delivery binds |
| More delivery | 2,400 | 960 | 120 MW — requested target met in the simplified model |
| Less fuel | 1,280 | 960 | 80 MW — total fuel now binds |
Check your understanding
In the less-fuel case, could the plant run 120 MW for part of the period instead?
Show reasoning
Yes, for 1⅓ hours in this idealised calculation, but not for the full two-hour commitment.
1,280 ÷ (120 × 8) = 1.333… hours. The same total energy can have a different delivery profile. An average does not prove compliance in every interval.
04Connect fuel price to cost without confusing it with availability
A fuel-only operating cost
Fuel A$/MWh = heat rate × gas A$/GJ
Matched fuel-energy basis. Transport, startup, variable maintenance, emissions obligations and other costs are not included unless explicitly added.
Use a price sensitivity, not a price forecast
At 8 GJ/MWh and an assumed A$12/GJ, fuel costs A$96/MWh. With A$5/MWh of variable maintenance, the simplified running cost is A$101/MWh. A A$3/GJ gas increase adds A$24/MWh, bringing that running cost to A$125/MWh. These are hypothetical prices, not current quotations.
Check your understanding
After restoring sufficient delivery, suppose the unit produces 240 MWh at an electricity price of A$130/MWh. What is contribution before startup and excluded costs at A$101/MWh running cost?
Show reasoning
A$6,960: 240 × (130 − 101).
If starting the unit costs A$8,000, the simplified run contribution becomes negative A$1,040. A positive per-MWh spread alone is not a complete start decision.
05Read gas-market evidence without overclaiming what it proves
Markets and information services answer different questions
AEMO's DWGM concerns Victoria's declared transmission system. STTM hubs operate in Sydney, Adelaide and Brisbane. GSH products support trading at designated delivery points. A market price is evidence about a specified product or hub, not proof that gas can reach your generator on the required schedule.
| Evidence | Use it for | Do not infer |
|---|---|---|
| Commodity contract / allocation | Quantity, delivery point and contractual rights | An hourly delivery profile not stated in the document |
| Transport and site confirmation | Applicable capacity, nominations and operating constraints | Unlimited withdrawal or unrestricted operation |
| Gas Bulletin Board data | Published system flows, capacity and facility context | Your own contractual entitlement or a guarantee of future delivery |
| Plant operating record | Observed availability, net output and fuel consumption | Identical capability under different conditions |
Storage and linepack are not synonyms for spare firm supply
Stored gas has inventory and withdrawal-rate limits. Linepack is gas held under pressure in pipelines; any usable flexibility depends on the network's operating conditions. Do not count it as an extra fuel source unless the relevant arrangements and physical availability establish that contribution. Avoid double-counting the same gas as both contracted delivery and storage release.
Check your understanding
A public dashboard reports high pipeline flow. Does that show your plant has spare firm transport capacity?
Show reasoning
No. Observed system flow and your transport entitlement are different things.
Check the actual route, period, product, allocation, contract and relevant operational restrictions. Use public data for context and reconciliation, not as a substitute for site-specific evidence.
06Write a fuel-adequacy briefing another analyst can reproduce
A defensible recommendation
- 01
Quantify
Target: 240 MWh; fuel need: 1,920 GJ; required flow: 960 GJ/h.
- 02
Challenge
Base delivery: 640 GJ/h. Maximum constant output: 80 MW. Two-hour shortfall: 80 MWh.
- 03
Act
Do not assume the 120 MW commitment is available. Ask the accountable parties to confirm an alternative schedule or feasible resource.
| Section | Minimum useful content |
|---|---|
| Boundary | Net output, time window, gas-energy basis and excluded items |
| Calculation | MWh, required GJ, required GJ/h and both limiting tests |
| Sensitivity | Change flow to 960 GJ/h, then separately reduce fuel to 1,280 GJ |
| Uncertainty | Startup, pressure, ramps, plant condition and gas delivery confirmation |
| Next step | Evidence owner, decision deadline and condition for revising the conclusion |
Check your understanding
A replacement battery has 80 MWh usable-delivered energy but only 20 MW output. Can it cover the base-case 40 MW gap for two hours alone?
Show reasoning
No. Energy is sufficient, but its 20 MW power limit is half the required 40 MW.
Apply Topic 5's two-limit discipline. A feasible combination might involve more battery power or verified demand response, but each component needs its own timing, boundary and availability check.
Watch the explanation
Key terms in plain English
- MW
- Megawatt: a measure of power, or how quickly energy is transferred. A plant's maximum MW is not its actual output in every interval.
- MWh
- Megawatt-hour: a quantity of energy. A steady 1 MW for one hour delivers 1 MWh.