Simple-cycle and combined-cycle offers cover different generating systems. A combined-cycle plant adds steam generation and a steam turbine to recover useful electrical output from gas-turbine exhaust. Compare both options at the same site delivery point, with their full equipment scope, operating profile, cooling requirements, installation work, and acceptance conditions included.
The distinction matters especially in the secondary market. An advertised capacity may describe an engine, a generating package, or the output of an entire installed plant. If the sale includes only part of that system, the headline capacity cannot describe what the buyer can install and operate without additional equipment.
Start by defining the power project and the asset boundary. Then compare a simple-cycle package with the complete combined-cycle alternative. The decision should show which system can supply the required duty and what work remains between the equipment offer and usable site power.
Key Takeaways
- Establish whether the offer covers an engine, a generating package, or an installed plant.
- Compare simple-cycle and combined-cycle ratings for the same turbine configuration and published basis.
- Keep generator output, plant net output, and campus-delivered power distinct.
- Include the steam system, heat rejection, auxiliary loads, and operating sequence in the combined-cycle assessment.
- Price the complete installation and its unresolved interfaces before comparing equipment offers.
3 offer types — an engine, a generating package, and a complete plant require different scope and capacity checks.
Establish the equipment boundary behind the capacity claim#
Ask the seller to identify every major assembly included in the offer. For a generating package, that includes the turbine, generator, controls, auxiliaries, inlet and exhaust arrangements, and any gearbox. For a combined-cycle plant, obtain the steam-system and heat-rejection scope as well.
The EPA combustion-turbine technology description explains how exhaust heat can support steam production and additional electricity generation. Use that process distinction when reading the equipment list. The document's historical cost and market figures are not current purchase benchmarks.
Equipment boundaries to distinguish in a sale offer#
| Offer description | What the buyer must establish | Capacity interpretation |
|---|---|---|
| Engine or core | Exact assembly and missing generating equipment | Engine capability does not establish a complete electrical installation |
| Simple-cycle generating package | Turbine, generator, auxiliaries, controls, and inlet/exhaust scope | Verify the published or guaranteed electrical boundary |
| Combined-cycle plant | Gas and steam generating equipment, heat recovery, cooling, auxiliaries, and integration | Verify the complete plant arrangement behind the stated output |
Combined heat and power is another distinction. Useful exhaust heat can supply a process without being converted into additional electricity. Solar's cogeneration description describes simultaneous electrical and thermal supply. A steam export should be recorded as useful thermal output, separate from electrical capacity.
Combined-cycle and CHP arrangements can also coexist where a plant supplies both electricity and useful heat. Identify the actual energy flows and delivery conditions. Adding electrical megawatts to a thermal-output figure does not establish a larger electrical generating rating.
Use the gas turbine catalogue to identify candidate families, then move to the offered unit and plant records. The family name is useful for navigation; the equipment schedule establishes what the buyer is acquiring.
Compare cycle ratings within a matched OEM configuration#
A matched OEM example can show the effect of changing the plant arrangement while retaining the identified gas-turbine variant. It is more useful than subtracting output figures for different engines or mixing a simple-cycle generator rating with an unrelated plant total.
LM6000 PC SPRINT: published simple-cycle and combined-cycle reference#
| Plant arrangement | Net output | Net heat rate, LHV |
|---|---|---|
| Simple cycle | 51.1 MW | 9,068 kJ/kWh |
| Combined cycle, 1x1 | 66.5 MW | 6,960 kJ/kWh |
GE Vernova's September 2025 LM6000 factsheet gives this 50/60 Hz offering on ISO conditions and natural gas, including inlet/exhaust losses and excluding balance-of-plant equipment. Its figures are indicative. The combined-cycle increment belongs to the expanded plant arrangement; it is not an uprate of the gas turbine alone.
Arithmetic from the matched published reference#
| Comparison | Calculation | Interpretation |
|---|---|---|
| Additional plant output | 66.5 − 51.1 = 15.4 MW | Difference between the two published plant arrangements |
| Lower heat rate | (9,068 − 6,960) ÷ 9,068 × 100 ≈ 23.2% | Lower fuel energy per unit of electricity on this reference basis |
The heat-rate difference is not a forecast of the same percentage reduction in total project cost. Fuel pricing, operating profile, maintenance, capital scope, auxiliary demand, and financing affect that comparison. Obtain a site heat balance before translating a reference rating into a project estimate.
Other OEMs use different reporting boundaries. Preserve those distinctions instead of placing every output figure in one apparently uniform ranking.
SGT-800 reference illustrating a gross-output boundary#
| Published option | Arrangement | Gross electrical output | Published basis |
|---|---|---|---|
| SGT-800, 62 MW rating option | Simple cycle | 62.5 MW | 50/60 Hz; ISO, natural gas, no inlet/exhaust losses |
| SCC-800, same rating option | Combined cycle, 1x1 | 89 MW | 50/60 Hz; ISO, natural gas; three-pressure, non-reheat bottoming cycle; no inlet/exhaust losses |
These values appear in the Siemens Energy SGT-800 technical tables. Their gross boundary differs from the GE table's stated net boundary. The figures should not be compared directly as equivalent campus-delivered capacity.
Use the LM6000 dossier and SGT-800 dossier to organize configuration enquiries. A used unit needs confirmation of the installed rating and the condition assumed in any new performance proposal.
Bring both options to the same site delivery point#
Define the electrical point at which the project needs power. Ask each bidder for output at that point and a schedule of included losses and auxiliary demand. This makes it possible to compare offers without treating different meanings of “net” as interchangeable.
Identify the gas-supply equipment, cooling system, water treatment, pumps, fans, controls, transformers, and other relevant loads. A published performance sheet may include some losses and exclude other plant equipment. Preserve the footnote and request the additional deductions needed for the site boundary.
Inputs for a comparable site performance case#
| Input | Required definition |
|---|---|
| Weather and elevation | Agreed ambient conditions, humidity where relevant, and site elevation or pressure |
| Fuel | Composition, heating-value convention, condition, and supply requirements |
| Equipment condition | New or clean reference, or an agreed degradation and maintenance basis |
| Plant arrangement | Included generating units, heat recovery, supplementary firing, and cooling configuration |
| Electrical boundary | Gross generation, included auxiliaries and losses, and required delivery point |
| Operating condition | Load, dispatch mode, active equipment, and any thermal export |
For a combined-cycle proposal, ask for the heat balance and equipment assumptions behind the steam contribution. Establish whether supplementary firing is included. Additional fuel supplied to a heat-recovery system must appear in the consumption and emissions case as well as the output figure.
Where process steam is exported, define its condition and return arrangements. Compare the electrical and thermal requirements together, while preserving their separate units and values. An offer optimized for steam export may serve a different duty from a power-only plant.
Request an agreed degradation treatment for used equipment. A performance test from before dismantling provides evidence about the prior installation under its test conditions. It does not automatically guarantee the relocated plant at a different site, with a different cooling system or exhaust arrangement.
The data-center power procurement roadmap provides the wider project context. The useful capacity figure is the power supported at the agreed boundary when the site needs it, with the supporting infrastructure available.
Evaluate dispatch and startup as a complete operating sequence#
Describe expected operating hours, starts, load range, reserve requirements, and the sequence for adding site demand. Those inputs affect how much value the project receives from heat recovery and how the generating arrangement should be designed.
The gas turbine and steam system have related but distinct operating conditions. A gas-turbine startup claim does not establish the time needed to reach the complete combined-cycle plant output. Ask for the defined initial state, required auxiliaries, sequence, and power available through the startup period.
Operating cases to obtain for both alternatives#
| Case | Question the performance response must answer |
|---|---|
| Initial startup | What conditions and external services are needed before generation begins? |
| Loading sequence | How does available electrical output develop as equipment enters service? |
| Part-load operation | What output, heat rate, emissions, and restrictions apply? |
| Equipment outage | What generation remains available and under which approved configuration? |
| Restart | How does the required sequence change with the condition of the plant? |
For a project considering simple-cycle operation before a later combined-cycle addition, request an engineered phasing plan. Establish exhaust interfaces, space, civil provisions, electrical integration, and the approvals required for each phase. The future steam-system connection needs more than an available area on a site drawing.
Ask whether the proposed plant can operate with the steam system unavailable and what equipment makes that possible. Exhaust bypass arrangements and operating permissions are configuration-specific. Do not assume every combined-cycle plant retains a usable standalone simple-cycle mode.
The backup-power procurement discussion helps connect these sequences to the site's continuity requirement. Evaluate the capacity actually available during the specified event, including startup services, fuel delivery, controls, and any reserve equipment.
A project with frequent changes in demand needs a different commercial comparison from one with sustained operation at a stable load. Use the proposed dispatch profile in the evaluation rather than treating a full-load efficiency figure as the entire operating case.
Use the gas turbine part-load performance guide to compare minimum operating limits and fuel use across the expected demand profile.
Include water, heat rejection, and site space#
A steam cycle introduces a cooling decision that can materially affect site scope. Identify the condenser arrangement, heat-rejection equipment, makeup requirements, treatment, discharge, and auxiliary power. These items belong in the plant enquiry before land or utility assumptions are fixed.
NREL's March 2011 water-use review distinguishes cooling arrangements and reports historical withdrawal and consumption observations. It supports treating cooling choice as a separate decision. Its literature factors cannot establish the water demand of the proposed plant or be transferred to a standalone simple-cycle package.
Dry heat rejection may be considered where water is constrained, but it still needs a site-specific performance and equipment assessment. Obtain the hot-day output, auxiliary load, space requirement, and operating restrictions for the proposed arrangement. Avoid assuming that removing a water demand leaves every other plant characteristic unchanged.
Include the gas turbine's inlet conditioning and combustion-related water systems separately. A dry combustor and a dry-cooled steam cycle describe different systems. The plant water balance should identify each relevant stream and its operating profile.
Review access as well as footprint. Heat-recovery equipment, steam piping, cooling equipment, lifting areas, and maintenance access can determine whether a relocated plant fits the site. Ask for an arrangement drawing showing the complete installed and service scope.
Noise and emissions equipment also need to fit the selected arrangement. Relocating a plant can change exhaust paths, stack requirements, surrounding receptors, and applicable site obligations. Have the responsible engineering and permitting teams resolve those interfaces using the actual proposed layout.
If the site is also weighing another generation technology, the fuel-cell and turbine comparison provides a separate selection discussion. Keep the load, utility, heat-management, and delivery boundaries consistent across that wider comparison.
Inspect the whole used plant and its relocation scope#
A used combined-cycle offer needs condition and documentation review beyond the gas turbine. Request equipment identities, service records, inspection findings, preservation history, and known limitations for the steam turbine, generator, heat-recovery equipment, condenser, and other included systems.
Used-plant diligence by assembly and interface#
| Scope | Evidence to request | Potential work to price |
|---|---|---|
| Gas-turbine package | Configuration, maintenance records, condition, and operating history | Service work and site adaptation |
| Heat-recovery system | Design records, inspections, condition, and included sections | Repair, dismantling, reassembly, and pressure-system acceptance |
| Steam generating train | Equipment identity, service records, preservation, and support | Inspection, overhaul, controls, and installation |
| Cooling and water equipment | Condition, capacity basis, and site suitability | Replacement, resizing, treatment, and integration |
| Plant controls and drawings | Installed versions, licenses, interfaces, and current documentation | Reconstruction, upgrades, integration, and testing |
For a GE 7F candidate, distinguish the offered gas-turbine equipment from the installed plant in which it previously operated. A familiar engine family cannot establish the inclusion or condition of that plant's steam system.
Define the dismantling and delivery condition. Ask who records match marks, preserves equipment, protects openings, packages components, and maintains the equipment register. Obtain a scope for documenting any damage or missing items before the equipment moves between parties.
Check service support and ownership of the documentation needed for reconstruction. Software access, control-system records, proprietary drawings, and service-agreement transfer can affect the practicality of relocation. Record each dependency before assigning a completion date.
Compare the existing cooling and steam equipment with the new site's requirements. Reusing every assembly is not automatically the least costly installed option when adaptation, condition work, transport, and future service are included. Price the supported alternatives on the same scope.
Compare total scope and agree the acceptance basis#
Build the commercial comparison from current, scope-defined quotations. Include equipment, immediate maintenance, dismantling, transport, civil works, fuel infrastructure, cooling and water systems, electrical integration, emissions equipment, and commissioning. State quotation dates, currency, taxes, and exclusions.
Model the proposed operating profile using consistent energy boundaries. Fuel expense depends on energy consumed at the applicable heat rate and delivered fuel price. Separate capital cost, fixed charges, maintenance, water, auxiliary electricity where externally supplied, and other costs to avoid counting the same item twice.
Show unresolved items as defined scenarios or quoted options. A lower equipment asking price does not resolve missing steam-system scope or an uncertain cooling design. The comparison should make clear what additional work the buyer must procure and which assumptions still affect the delivery date.
Agree the performance-test boundary before contracting. ASME's PTC 46 scope description addresses overall thermal performance and electrical output, including combined-cycle plants. It also distinguishes emissions, operational demonstrations, and reliability tests from that performance-test scope. Do not treat one test as proof of every contractual requirement.
Have the responsible engineers define the applicable methods, reference conditions, corrections, and acceptance criteria. Align equipment guarantees with the complete plant's obligations, including the services and operating conditions needed to achieve them.
SecondWatt provides independent equipment intelligence and connects buyers with sellers. Contact info@secondwatt.com with the intended duty, site constraints, and the scope of any candidate offer. A clear equipment boundary makes the next supplier enquiry and purchase comparison more useful.
FAQ: simple-cycle and combined-cycle gas turbine purchases#
Does a combined-cycle rating describe the gas turbine alone?#
No. It describes an expanded generating arrangement that includes steam-cycle contribution on a stated basis. Establish the equipment and auxiliaries behind the figure. An offer for the gas turbine alone does not include that entire plant output.
Is combined cycle always the better purchase?#
Evaluate the intended duty, fuel use, installation scope, cooling, service requirements, and delivery schedule. A favorable reference heat rate is one input. The appropriate choice depends on the complete project and its operating profile.
Can a simple-cycle plant be converted later?#
It may be possible with an engineered integration and phasing plan. Confirm equipment interfaces, space, cooling, electrical works, operating arrangements, and approvals. Obtain a defined future scope before attaching value to the conversion option.
Is CHP the same as combined cycle?#
CHP supplies useful heat as well as electricity. Combined cycle uses a steam cycle to generate additional power from gas-turbine exhaust. A plant can combine these functions, but thermal exports and electrical output need separate accounting.
Can a combined-cycle plant run while the steam turbine is unavailable?#
That depends on the installed exhaust and bypass arrangements, equipment limitations, controls, and operating permissions. Request the supported outage configuration and its available output. Do not assume that capability from the plant's general description.
What is the first question to ask about a used plant listing?#
Ask exactly what equipment is included and what boundary the advertised output describes. Then request configuration, condition, performance, and relocation records for that scope. This establishes whether the offer matches the project before detailed pricing begins.