Co-generation, an effi cient method of generating energy, involves producing electricity at the same time as creating heat and steam for industrial purposes, such as drying timber in kilns or drying milk powder.
In large factories, wood waste, gas or coal is used to generate electricity for the factory. Surplus electricity can be exported to the National Grid. Co-generation can produce both electricity and hot steam for factory use.
What types of co-generation are there?
In New Zealand, the main types of co-generation are open-cycle gas turbines and steam turbines.
How is electricity generated through co-generation?
Co-generation at a paper mill (open-cycle gas turbine)
Co-generation uses the heat from an open-cycle gas turbine in a boiler, which produces steam. This steam is used in two ways:
yrotcaf eht ni repap dellim ylhserf yrd ot •enibrut maets a nrut ot •
The steam turbine drives a generator. The generator then produces electricity. The paper mill uses the electricity to run its business and exports the rest to the National Grid for everyone to use.
Co-generation at a dairy factory (steam turbine)
Some of the heat produced by a dairy factory is used to turn water into steam. The steam turns a turbine, driving a generator. The generator then produces electricity. The dairy factory uses the electricity to run its business and exports the rest to the National Grid for everyone to use.
What are the advantages of co-generation?
Co-generation makes use of excess heat, which helps to improve overall effi ciency of the generator. In comparison to other ways of generating energy, it’s more effi cient, as it uses the heat and energy generated by fossil fuels or wood waste. Co-generation can save businesses money as they’re generating their
THE CARTER HOLT HARVEY PULP AND PAPER MILL IN KINLEITH IS A GOOD EXAMPLE OF A CO-GENERATION SCHEME. SOURCE: GENESIS ENERGY
WHAT IS CO-GENERATION?Co-generation is the production of electricity in combination with another industrial process. Co-generation plants are often thermal generators installed in industries that produce or require heat. In e�ect, the heat is used twice – once to generate electricity and once for another purpose, resulting in a very e�cient process overall.
In New Zealand, co-generation plants are found in the wood processing, dairy and steel indus-tries.
WHAT TYPES OF CO-GENERATION ARE THERE?Co-generation systems can work in one of two ways.
Bottoming cycleHeat is produced in the form of high-pressure steam and used in the industrial process. The leftover heat is then recovered and used to make steam that drives a turbine and generator.
Topping cycle This works the other way around. First, electricity is generated, usually through an open-cycle gas turbine, then the waste heat is used to produce steam for industrial purposes.
Many power stations now use the more e�cient combined-cycle gas turbines. Technically speaking, combined-cycle gas turbines are themselves examples of co-generation. First, gas is ignited. It expands as it burns in a gas turbine connected to an electrical generator, then the leftover heat from this process is used to make steam, which drives a steam turbine and a second generator. The second generator is not as e�cient as the �rst, but still produces electricity from what could have been waste heat.
WHAT ARE THE ADVANTAGES OF CO-GENERATION?Co-generation makes use of excess heat, which helps to improve the overall e�ciency of the process. Co-generation can save businesses money, as they can generate their own electricity and steam supply at the same time. They can sometimes also sell electricity to the national grid, which provides power to New Zealand.
own electricity and steam supply.
What are the disadvantages of co-generation?
Even though co-generation uses electricity generated effi ciently in the business, it can burn non-renewable sources which produce carbon dioxide emissions.
How is co-generation used in New Zealand?
Co-generation is a relatively new technology in New Zealand. It has the potential to be used throughout the country due to our primary industries, such as dairy, paper,and steel production.
A few co-generation schemes operate in New Zealand today. The Fonterra Plant in Te Awamutu and the Carter Holt Harvey Pulp and Paper Mill in Kinleith are two examples. Combined, these co-generation plants have a capacity of 67MW, enough capacity to power 60,000 homes.
ACTIVITYSmall group brainstorming activity.
taht ssenisub rehto eno tsael ta fo knihT •could use co-generation. Look at the types of waste the business is creating and how the waste could be used to generate energy.
na si noitareneg-oc kniht uoy oD •environmentally-friendly type of energy generation? Explain.
dezis muidem ro llams rehtehw ssucsiD •enterprises could pool their resources and use co-generation as a method of generating energy.
SUPPORTING RESOURCES
WHAT ARE THE DISADVANTAGES OF CO-GENERATION? Even though co-generation produces electricity e�ciently on site, it burns non-renewable sources that produce carbon dioxide emissions.
HOW CAN CO-GENERATION BE USED IN NEW ZEALAND?Although co-generation is a relatively new technology in New Zealand, several of our primary industries are being made more e�cient by installing co-generation plants. For example, a timber mill or paper mill might burn waste wood scraps to produce heat. This might be used to drive a steam turbine and generate electricity for the mill. The excess heat from the turbine might be further used, such as for drying purposes, while any electricity that is not needed could be fed into the national grid. This would be an extremely e�cient use of what may have been unwanted waste.
The steel industry uses huge amounts of energy in its iron melters. In the steel-making process, there is a lot of waste gas from the furnaces. In a co-generation plant, this waste gas would be recovered and burnt, and the heat used to produce steam to drive a steam turbine and generator.
- Energy E�ciency and Conservation Authority: www.eeca.govt.nz- Ministry of Economic Development: www.med.govt.nz- Genesis Energy: www.genesisenergy.co.nz
ACTIVITYSmall group brainstorming activity
Think of at least one other business that could use co-generation. Look at the types of waste the business is creating and how the waste could be used to generate energy.Do you think co-generation is an environmentally-friendly type of energy generation? Explain.Discuss whether small- or medium-sized enterprises could pool their resources and use co-generation as a method of generating energy.
For industries that require heat for drying, such as the dairy industry when drying milk powder, it can be economical to produce their own electricity by running a small generating plant, and then to use the waste heat from the turbine for the drying process.
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