KEVOS® MAKEOS · Engineering
Cogeneration (CHP) Utilisation and Primary Energy Saving Check
For a combined heat and power plant with this fuel input, electrical output and useful heat, what is its utilisation factor and power-to-heat ratio, how much fuel would separate power and heat production use, and does it save the share of primary energy you require?
Energy performance of a combined heat and power (CHP) plant: electrical and heat efficiencies, utilisation factor, power-to-heat ratio, and the fuel that separate production would need at your reference efficiencies, giving the fuel saved and the primary energy saving against your target.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Fuel energy input rate | kW | Fuel energy supplied to the CHP unit per unit time, from the gas meter and the gas heating value (state whether higher or lower heating value). |
| Net electrical output | kW | Net electrical output after the unit’s own auxiliaries, from the maker’s data or the export meter. |
| Useful heat delivered to the process or buildings | kW | Heat actually used on site from the unit: hot water, steam or absorption chilling; not heat dumped to a radiator. |
| Reference separate production | ||
| Reference efficiency of separate power generation | % | Efficiency of the electricity the CHP displaces (grid or on-site generation), on the same heating-value basis as the fuel input. |
| Reference efficiency of separate heat production (boiler) | % | Efficiency of the boiler whose heat the CHP displaces, on the same heating-value basis. |
| Target and hours | ||
| Primary energy saving required | % | Primary energy saving the project must achieve, from your business case or a funding scheme. |
| Running hours per year | h/year | Hours a year the unit runs at this load. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Fuel energy input rate
- What it is
- The rate of fuel energy supplied to the CHP unit: fuel flow multiplied by its heating value.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 1 kW to 10000000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2500 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More fuel for the same outputs lowers the utilisation factor and the primary energy saving.
- Get it right
- State the basis: natural gas on the lower heating value is about a tenth below the higher value.
What this means: Net electrical output
- What it is
- The net electrical output of the CHP unit after its own auxiliaries such as pumps, fans and controls.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0 kW to 10000000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 900 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More electricity raises the separate-production fuel strongly and so the saving.
- Get it right
- Use the output at the load point you are assessing, not the nameplate rating if the unit runs part-loaded.
What this means: Useful heat delivered to the process or buildin…
- What it is
- The heat from the CHP unit that the site actually uses in place of boiler heat: hot water, steam or heat to an absorption chiller.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0 kW to 10000000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1100 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More used heat raises the utilisation factor and the saving.
- Get it right
- Do not count heat rejected through a dump radiator or used to keep plant warm unnecessarily.
Reference separate production
What this means: Reference efficiency of separate power generati…
- What it is
- The efficiency of the power generation that the CHP electricity displaces, used to work out the fuel separate production would burn.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 5% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower reference efficiency makes the CHP saving look larger.
- Get it right
- Use the same heating-value basis as the fuel input; your energy adviser or a published reference value can give it.
What this means: Reference efficiency of separate heat productio…
- What it is
- The efficiency of the boiler that the CHP heat displaces, used to work out the fuel separate heat production would burn.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 5% to 110%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 85% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower reference efficiency makes the CHP saving look larger.
- Get it right
- Use the existing boiler’s seasonal efficiency if CHP heat would replace it.
Target and hours
What this means: Primary energy saving required
- What it is
- The primary energy saving, as a share of the fuel separate production would use, that the project must reach.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line.
- Get it right
- Set it from your business case; schemes that define high-efficiency CHP set their own thresholds.
What this means: Running hours per year
- What it is
- The number of hours a year the unit runs at this load point, used to turn power into yearly energy.
- Units and format
- Enter a number in h/year: h/year. The form accepts 0 h/year to 8784 h/year. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6000 h/year as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Yearly fuel used and saved rise in proportion; the percentages do not change.
- Get it right
- Count only hours when the heat is also used; running for electricity alone changes the saving.
Method and formulas
- Electrical efficiency η_e = W ÷ Q_fuel; heat efficiency η_h = Q_heat ÷ Q_fuel; utilisation factor = (W + Q_heat) ÷ Q_fuel.
- Fuel for separate production F_sep = W ÷ η_E,ref + Q_heat ÷ η_H,ref.
- Fuel saved = F_sep − Q_fuel; primary energy saving PES = 1 − Q_fuel ÷ F_sep.
- Example: 2,500 kW of fuel giving 900 kW of electricity and 1,100 kW of used heat has a utilisation factor of 80 % and saves 29.46 % of primary energy against 40 % power and 85 % boiler references, above a 10 % target.
Limitations
- One steady load point; real CHP savings depend on how many hours the heat is actually needed, which this point cannot show.
- All efficiencies must use the same heating-value basis (higher or lower); mixing bases moves the result by about 10 % for natural gas.
- Energy only: costs, tariffs, maintenance and emissions factors are not included, so a saving does not by itself make a business case.
- Example values are illustrations. Replace them with values for your actual site and unit.
Typical uses
- Screen a gas-engine or turbine CHP proposal for a factory, hospital or campus
- Check a supplier’s claimed savings against your own reference efficiencies
- Show how much of the saving depends on actually using the recovered heat
Who it is for
- Energy engineers and managers
- Facility and plant managers
- Engineering consultants
What you get
- Primary energy saving and fuel saved
- Utilisation factor, electrical and heat efficiencies
- Power-to-heat ratio
- Yearly fuel used and saved
Interpreting the result
- A high utilisation factor with a low primary energy saving usually means the heat is credited but cheap to make in a boiler; the electricity is where CHP earns its saving.
- If the heat is not used, CHP is just an inefficient power station: check the heat demand profile first.
Before relying on the result
- Check the site’s heat demand by hour and season against the unit’s heat output before relying on the yearly figures.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 10 (section 10-8: cogeneration and the utilisation factor). Method reference; no tables, figures or examples are reproduced.
- Directive 2012/27/EU on energy efficiency, Annex II: calculation of primary energy savings for high-efficiency cogeneration (method only; its reference values are not reproduced, enter your own).
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Explains the concept · KEVOS® original book
Entropy generated by heat transfer
What it covers: Why heat crossing a temperature difference always generates entropy, how to calculate it for heat exchangers, kettles and heaters, and how matching temperatures and staging sources cut it.
Method explained here · KEVOS® original book
Cogeneration: utilisation, primary energy saving and the business case
In On-Site Power and Heat by the Numbers
What it covers: Judge a cogeneration plant by utilisation, power-to-heat ratio and primary energy saving, size it to the heat demand, and build and stress-test the A$ business case for one site.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Cogeneration (CHP) Utilisation and Primary Energy Saving Check calculator