Combined heat and power (CHP) is the simultaneous generation of electricity and useful heat from a single fuel, on the site that uses them. It is the same thing as cogeneration. This guide explains how a CHP plant works, what it is made of, which fuel suits which site, and when it is worth installing.
What is combined heat and power?
A power station turns fuel into electricity and throws the heat away. A boiler turns fuel into heat and makes no electricity. A combined heat and power plant does both jobs with one engine: it generates electricity, and the heat the engine gives off is captured and put to work. Using both outputs is what lifts the overall efficiency of a 2G CHP plant to between 90% and 104% of the fuel’s energy, against roughly 35% for grid electricity once generation and transmission losses are counted.
In Australia the terms are used interchangeably: CHP, cogeneration and cogen all describe the same plant. When an absorption chiller is added to make chilled water as well, the plant is called trigeneration.
What a CHP plant is made of
- Gas engine. A reciprocating engine built for the fuel: natural gas, biogas or hydrogen. 2G engines run at low speed compared with the very high speeds of micro-turbines, which is part of why they last longer and are easier to service.
- Generator. Driven directly by the engine, producing three-phase electricity synchronised with the site supply.
- Heat recovery. Heat exchangers on the engine cooling circuit, oil circuit and exhaust gas deliver hot water, or steam from the exhaust, to the site’s heating, hot water or process systems.
- Gas treatment. On biogas sites, treatment removes moisture, hydrogen sulphide and siloxanes so the engine runs cleanly on raw gas. See biogas treatment.
- Enclosure and controls. The whole plant is built as one connection-ready module in a container or acoustic enclosure, with an integrated control system that adjusts to changing energy demand and gas quality and reports remotely.
How a CHP plant works, step by step
- Fuel is fed to the engine. Natural gas comes from the network; biogas comes from a digester, landfill or wastewater treatment process on site.
- The engine drives the generator. Electricity goes to the site’s switchboard and displaces what would otherwise be bought from the grid.
- Heat exchangers recover the engine’s heat and deliver it as hot water or steam to wherever the site needs it: process heat, space heating, hot water, digester heating or pool heating.
- The control system matches output to demand and keeps the plant within its efficiency band, and the site keeps a grid connection for top-up and export.
Natural gas, biogas or hydrogen?
| Fuel | Where it comes from | Suits | 2G range |
|---|---|---|---|
| Natural gas | The gas network | Any site with a gas connection and a steady heat and power demand: food manufacturing, aquatic centres, hospitals, cold storage, data centres | 20 kW to 4,000 kW |
| Biogas | Anaerobic digestion of sewage sludge, food waste, animal waste or landfill gas, produced on site | Wastewater treatment plants, meat and food processors, piggeries and dairies, landfills | 50 kW to 3,000 kW |
| Hydrogen | Green hydrogen, or a hydrogen blend in the gas network | Sites planning for zero-carbon operation; 2G engines can be built hydrogen-ready and converted later | Hydrogen CHP |
Natural gas has one of the lowest CO2 emission values of the fossil fuels, and running it through a CHP plant cuts emissions further because the heat is not wasted. Biogas is sustainable and carbon neutral, and on a wastewater or food site it is often being produced and flared already. Where the biogas is worth more as pipeline gas, a biomethane plant upgrades it for grid injection instead of burning it in an engine.
When combined heat and power pays off
CHP earns its keep on sites that need electricity and heat at the same time for long hours. The questions Evo’s engineers ask first are:
- How many hours a year does the site run? A plant that runs 6,000 hours or more a year is working for most of its life; a site that is busy only in daylight hours needs a smaller plant sized to its base load.
- Can the heat be used? The economics rely on the recovered heat displacing gas or electricity you would otherwise buy. Pools, process steam, hot water, digester heating and absorption cooling are all good uses.
- What are the site’s electricity and gas prices? The wider the gap between the price of grid electricity and the price of the fuel, the faster the payback.
- Is there a reliability case? Cold storage, data centres and critical infrastructure often value on-site generation and islanded operation as much as the savings.
Evo models each site from its actual interval data before recommending a plant, so the payback figure you get is yours, not an industry average.
CHP in practice
Evo Energy Technologies has installed 2G combined heat and power plants across Australia, including a 3 MW cogeneration plant at Simplot Bathurst, a 1 MW system at NewCold Truganina, a 4,000 kW multi-unit biogas project at Kilcoy Global Foods, landfill gas generation at Hyland Highway that offsets 80% of Latrobe City Council’s electricity, and biogas CHP at wastewater plants including Coombabah, Kawana, Mount Martha and Colac. Browse the project portfolio.
Frequently asked questions
Is CHP the same as cogeneration?
Yes. Combined heat and power and cogeneration are two names for the same technology. Trigeneration is cogeneration with cooling added.
How efficient is a CHP plant?
A 2G CHP plant uses between 90% and 104% of the energy in its fuel when both the electricity and the recovered heat are counted, compared with around 35% for electricity delivered from the grid.
Does a CHP plant replace the grid connection?
Usually no. The plant covers the site’s base load and the grid supplies peaks and backup, although plants can be designed to keep running in island mode during a grid outage.
Can an existing boiler stay?
Yes. Boilers are normally kept for peak heat demand and backup, with the CHP plant meeting the base heat load so it runs as many hours as possible.
Who looks after the plant?
Evo Energy Technologies designs, installs, commissions and services every plant it supplies, as the authorised Australian partner for 2G Energy.
Talk to Evo about CHP for your site
Send us a recent electricity and gas bill and a note on how your site uses heat, and we will tell you whether combined heat and power stacks up. Call 1300 012 471 or contact us.