Cogeneration plant

Co-Generation Plant outsideLa Trobe University’s cogeneration plant was installed at the Melbourne campus in 1993 under an incentive package from the Victorian Government. At the time, the government wanted to reduce the need to invest in new power stations while also decreasing the State’s reliance on a few very large coal fired power stations.

The cogeneration plant was a chance to cut energy costs while demonstrating a commitment to the environment, in particular the reduction of greenhouse gas emissions. The plant also offers back-up power supply to reduce the effects of power outages as it can take over if the grid fails and vice versa. The plant can improve the quality of power and protects computer and laboratory equipment from interruptions to the power supply.

How it works

Cogeneration is the simultaneous production of electricity and useful heat. La Trobe University’s cogeneration plant at the Melbourne campus uses natural gas in a gas turbine to create electricity. At the same time, exhaust (useful heat) from the combustion process provides heating throughout the campus.

The cogeneration plant consists of a 6MW gas turbine coupled to a 7.7 MVA generator. In the gas turbine, air and natural gas are compressed and fed into a combustion chamber. The pressure resulting from the combustion rotates a turbine pushes a shaft in the generator and produces electricity. In 2010, the cogeneration plant produced 137,365 GJ of electrical energy. The majority of electrical energy (133,615 GJ in 2010) is used to power a large number of Melbourne campus’ buildings. Any surplus electricity energy is fed back to the grid (24,060 GJ in 2010).

The cogeneration process also uses cold water which is piped through a chamber. Exhaust gases (at 400–500°C) from the process is used to generate High Temperature Hot Water (HTHW) to 185°C. HTHW is pumped around the campus and used for heating, hot water and in absorption chillers where the heat provides the energy needed for a cooling system.

When does the cogeneration plant function?

The cogeneration plant operates at maximum output from Monday to Friday during the day (7 am to 11 pm). During the night (11 pm to 7 am) it operates at reduced capacity — just enough to cover heating requirements and with a small electricity output. On weekends, the cogeneration plant does not operate.

The electricity grid provides energy to the Melbourne campus on weekends and at times when the campus draws more energy than the output provided by the cogeneration plant. Excess electricity produced on campus is sold back into the grid, which provides revenue, making the plant economically viable. As off-peak tariffs are significantly lower than peak tariffs, operating at maximum output during off-peak times is not worth it economically. Gas boilers supplement heating requirements in winter and when the cogeneration plant is not operational.

The maximum amount of electricity that can be produced by the cogeneration plant is around 6 megavolt ampere (MVA). This is approximately the operational requirement of the Melbourne campus. Various factors affect demand for electricity and heating of the campus, and these factors determine whether electricity is imported from or exported to the grid. The main factors are air temperature, the number of students on campus, and whether it is a teaching or non-teaching period.

During the hot summer months, a significant peak in electrical power demand occurs as air conditioners are used for cooling. To supplement this electricity demand, a larger proportion of electricity is imported from the grid. During cold weather in winter, demand for heating increases on campus, but demand for electrical power decreases. Thus, there is a peak in electricity exports to the grid during the student holidays in June (winter).

How is it more efficient than other power stations?

Natural gas-fired heat and power cogeneration is a highly energy efficient approach to electricity generation. The process has a comparably lower carbon footprint in contrast to brown coal fired power which supplies the majority of grid electricity in Victoria. There are lower carbon emissions associated with the combustion of natural gas considering it is the cleanest burned fossil fuel.