Two plants on the same industrial estate burn the same volume of gas every month. One delivers about 32 percent of that fuel energy to the busbar and sends the rest up the stack at roughly 500 degrees C. The other recovers that stack heat and delivers close to 45 percent.
On a 20 MW machine running 7,000 hours a year, that gap is worth several billion Naira in fuel over the life of the plant. That is the simple cycle vs combined cycle gas turbine argument in one paragraph, and it is also why the decision gets misread so often.
The efficiency gain is real and repeatable. What varies from site to site is whether your plant runs enough hours, has enough water, and can absorb the extra staffing and outage burden needed to collect it.
This article sets out how the two configurations differ mechanically, what each costs to install and operate in Nigeria, and the running hours threshold below which a bottoming cycle does not pay back. The figures are indicative. Any of them can move by 20 percent or more once your load profile, gas contract and site layout are properly measured.
Where a combined cycle finds its extra output
A gas turbine in simple cycle does one energy conversion. Air is compressed, fuel is burned in the combustor, and the hot gas expands through the turbine to drive both the compressor and the generator. Exhaust leaves the stack somewhere between 450 and 600 degrees C depending on machine class and load.
That exhaust still carries roughly 60 to 65 percent of the fuel energy you paid for. A combined cycle installs a heat recovery steam generator, or HRSG, in the exhaust path, raises steam, and expands that steam through a separate steam turbine driving a second generator.
Nothing extra is burned. The bottoming cycle produces power from energy that was already going up the stack, which is why combined cycle output usually lands 30 to 50 percent above the gas turbine alone on the same fuel flow.
The catch is that a steam cycle is a plant in its own right, not a bolt-on. You are adding pressure parts with a statutory inspection regime, a condenser, a cooling system, feedwater treatment and demineralisation, a steam turbine with its own lube oil and control systems, and a second generator with its own protection scheme.
If you are at the configuration stage now, the sequence matters. Get the load profile logged at one minute resolution for two normal production weeks, then price the options against that data. Request a technical proposal and we will scope the measurement before anyone specifies equipment.
Simple cycle vs combined cycle gas turbine: the efficiency and capex numbers
At the industrial scale most Nigerian plants operate, which is broadly 3 MW to 50 MW, the numbers below are what you should expect. Note that these are lower heating value figures at ISO conditions, and your site will not run at ISO conditions.
| Item | Simple cycle | Combined cycle | Cogeneration (back-pressure) |
|---|---|---|---|
| Electrical efficiency | 28 to 35% | 40 to 50% | 22 to 30% |
| Total fuel utilisation | 28 to 35% | 40 to 50% | 65 to 80% |
| Installed cost per kW | Lowest of the three | Highest, roughly 60 to 90% above simple cycle | Between simple and combined cycle |
| Cooling water makeup | Negligible | 1.5 to 2.5 m3 per MWh of steam turbine output | Low, steam returns as condensate |
| Cold start to full output | 10 to 20 minutes | 2 to 4 hours for the full block | 1 to 2 hours |
| Plot area, relative | 1.0 | 2.0 to 2.5 | 1.4 to 1.8 |
| Operating staff, relative | 1.0 | 1.8 to 2.5 | 1.4 to 1.8 |
| Minimum sensible run hours | Any | 5,500 to 6,000 per year | 4,000 per year with steam demand |
| Best fit | Peaking, standby, swinging load, single shift | Continuous baseload with no process steam demand | Continuous load plus a real steam requirement |
Two figures in that table decide most cases. The first is installed cost per kW, where combined cycle runs roughly 60 to 90 percent above simple cycle for the same total output. The second is the minimum sensible run hours, because a bottoming cycle carries most of its cost whether it runs or not.
The efficiency band for combined cycle at this scale is worth stressing. Large utility class machines reach 58 to 62 percent, and those numbers circulate widely, but they come from F class and H class turbines above 150 MW with reheat steam cycles. A 10 MW industrial unit with an unfired single pressure HRSG will land closer to 40 to 44 percent. The International Energy Agency publishes useful context on where gas fired plant sits in modern systems, but the utility scale efficiency figures do not transfer to a 10 MW captive installation.
What a bottoming cycle costs that the proposal will not show you
The capital number is the easy part. Four running costs are consistently underestimated at the feasibility stage.
Water comes first. A wet cooled condensing steam cycle consumes roughly 1.5 to 2.5 cubic metres of makeup water per MWh of steam turbine output. A 3.6 MW steam turbine running continuously will need somewhere between 110 and 180 cubic metres a day, treated to boiler feed standard. On a site already trucking water or drawing from a stressed borehole, this alone can end the discussion.
An air cooled condenser removes the water demand. It costs 15 to 25 percent more on the bottoming cycle and gives back 2 to 5 percent of steam turbine output on a 36 degree C afternoon, which is when your load peaks.
Water chemistry comes second. HRSG tube failures from poor feedwater control are the most common reason retrofits underperform against their business case. This is a daily discipline requiring competent staff, reliable dosing and consistent sampling, not a commissioning task.
Third is the outage burden. You are adding pressure vessel inspections, a steam turbine major inspection at its own interval, and a condenser and cooling system that need cleaning. The steam turbine will eventually need a bearing and blade path inspection on a schedule that has nothing to do with your gas turbine hot gas path outage, so planning the two together takes real effort. Our steam turbine overhaul service exists precisely because that second machine has its own life cycle.
Fourth is start flexibility. The gas turbine still starts in minutes, but the steam turbine rotor and casing must be warmed through to avoid differential expansion and thermal stress. A hot start needs 40 to 90 minutes. A cold start needs 2 to 4 hours. If your plant starts and stops daily, a combined cycle spends much of its life warming up, and the cycling damages steam turbine life.
Running hours decide the configuration, not nameplate efficiency
The arithmetic is straightforward. The bottoming cycle saves you fuel only in the hours it actually runs, but it costs you staff, water, chemicals and maintenance broadly regardless.
Below about 4,000 equivalent operating hours a year, a condensing combined cycle almost never pays back within a normal industrial evaluation window. Between 4,000 and 6,000 hours the answer depends on gas price, water availability and cost of capital, and needs a proper model. Above 6,000 hours with firm gas supply, it usually wins.
Derating matters here too. A gas turbine loses roughly 0.5 to 0.7 percent of output for every degree C above the ISO reference of 15 degrees C. At a 35 degree C ambient you are already down 10 to 14 percent, and the steam side follows the exhaust flow down with it. Harmattan season adds compressor fouling on top of that, which we cover in harmattan dust and turbine derating.
Also confirm your permitting position early. Captive generation above 1 MW requires a permit from the Nigerian Electricity Regulatory Commission, and adding a second generator to an existing installation changes what you declared.
To model your own case properly, book a plant assessment and we will build the hours based comparison against your metered data rather than nameplate assumptions.
Choosing simple cycle or combined cycle gas turbine on three site profiles
The following are anonymised and indicative. They show how the same question resolves three different ways.
Hypothetical example: two 5.5 MW units, 6,900 hours a year at 85 percent average load. A bottoming cycle across both exhausts would add about 3.6 MW. At an assumed landed gas price (use your own delivered cost per standard cubic metre), the extra 21,000 MWh a year in avoided fuel is worth a large sum over the plant’s life at typical Nigerian gas and fuel costs.
Against that, the retrofit itself, including cooling and water treatment, carries a substantial capital cost plus its own added annual operating cost. At these hours the avoided fuel can repay that within the plant’s life, and the payback you calculate from your own capital quote and gas price will lengthen once financing at Nigerian commercial rates is applied. The build runs about 14 months with a three week tie-in outage, so the cost of that outage window belongs in the business case, not in a footnote. This one is worth doing.
Hypothetical example: a plant needing 12 tonnes an hour of process steam at 9 bar. Here a condensing combined cycle is the wrong answer, because it would throw away exactly the steam the plant is currently buying gas to raise in package boilers. A 4 MW gas turbine with an unfired HRSG raising around 11 tonnes an hour, with supplementary firing available to reach 15 tonnes an hour, lifts total fuel utilisation to roughly 70 to 76 percent against about 40 percent for separate power and boiler plant.
Payback on that configuration is usually the quickest of the three because the steam has a direct, already funded fuel value. The choice between passing out steam and condensing the balance is a design decision covered in back-pressure versus condensing turbines.
Hypothetical example: a single shift operation, 2,600 hours a year, on an 8 MW simple cycle unit. A bottoming cycle would add about 2.9 MW, but at those hours the extra output is worth far less than the added operating cost, let alone the capital required. Payback does not arrive within any sensible planning horizon. It is not a close call.
That plant should spend a fraction of the money on inlet air filtration, evaporative inlet cooling and control system tuning, which can recover 3 to 6 percent of output for a small share of the capital.
What to measure before you commit either way
Five things should be on paper before a configuration is fixed. Get the annual load duration curve from metered data, not from connected load. Get a firm written position on gas volume, pressure and composition, including interruptibility clauses.
Establish raw water availability and quality, with a treatment cost, or price an air cooled condenser instead. Establish the plot area actually available, because a combined cycle needs two to two and a half times the footprint and the steam plant usually cannot be squeezed into a corner.
Finally, put the outage plan in writing before you sign. A retrofit tie-in on a running plant is not a maintenance job. It needs an isolation plan, a defined outage window and qualified engineers on site through the tie-in.
A structured power plant audit covers all five and produces the load and heat balance data a credible configuration decision requires. If your plant is at that stage, request a technical proposal and we will set out the scope, duration and deliverables in writing.
Frequently Asked Questions
Is a combined cycle always more efficient than simple cycle?
In electrical efficiency terms, yes, a condensing combined cycle will always beat the same gas turbine in simple cycle. In total fuel utilisation terms, no. A cogeneration plant supplying process steam typically uses 65 to 80 percent of the fuel energy, well above a condensing combined cycle, because none of the heat is rejected to a condenser.
What is the smallest gas turbine worth combining?
Below roughly 5 MW of gas turbine output, a conventional steam bottoming cycle rarely justifies its own cost and staffing. The pressure parts, water treatment and operating headcount do not scale down proportionally. For smaller exhaust streams, an organic Rankine cycle unit or a straightforward cogeneration arrangement is usually the better route if there is any heat demand on site.
How long does a combined cycle retrofit outage take?
The HRSG, steam turbine and cooling plant are largely built alongside the running unit, so most of a 12 to 18 month programme does not touch generation. The tie-in itself, where the exhaust is diverted into the HRSG, needs the gas turbine off line for typically two to four weeks depending on ducting and diverter damper design. That window should be planned against your annual shutdown, not treated as separate.
Can we build simple cycle now and add the bottoming cycle later?
Yes, and this is often the sensible route while load is still growing. It only works if the original layout reserves plot space for the steam plant, the exhaust stack is specified to accept a diverter damper, and the electrical single line is sized for a second generator. Retrofitting a bottoming cycle to a plant that was laid out without provision for one commonly adds 20 to 35 percent to the retrofit cost.