Tri Harder – Part 2, So what is the carbon intensity of tri-gen?
In Part 1 of Tri Harder I talked about the different carbon intensities of grid electricity around the world. Australia has one of the worst performing power grids in terms of carbon, being beaten by all of the developed world and most of the developing world too.
The next question – is the whole of Australia that bad or do some of the states perform better than others? Well, according to the Department of Climate Change and Energy Efficiency we do have a significant variance across the states. Victoria is worse at 1.19 kgCO2 / kWh and Tasmania is best, mainly due to hydro electricity at 0.26 kgCO2 / kWh.
So if trigeneration is used for reducing the carbon emissions of electricity in buildings how does it compare to grid carbon intensity?
The predominant form of fuel for trigeneration systems is natural gas – natural gas has a lower carbon intensity in electrical generation than black or brown coal. When natural gas is used in a power station rather than coal the carbon intensity drops from 0.93 kgCO2/kWh to 0.55 kgCO2/kWh of electricity produced.
In a gas fired trigeneration system, gas is used to basically run a very large car engine. The engine is used to produce electricity (first generation of energy), the waste heat from the engine is captured (second generation of energy), the waste heat is then used directly in the building for heating or hot water OR is converted to chilled water (third generation of energy) using an absorption chiller. The name trigeneration (tri-gen) denotes that three sources of energy are used, if an absorption chiller wasn’t used but the waste heat is used for heating then there are only two uses of energy and it is called cogeneration (co-gen). If none of the waste heat is used then it can be called onegeneration (one-gen) – I don’t think one-gen is the right word but it makes logical sense!
Back to the maths. In Tri Harder Part 3 I will post the calculations used to compare trigeneration to grid connected but in the meantime through calculations the average carbon intensity of trigeneration in terms of electricity produced is approx. 0.55 kgCO2/kWh. There will be some subtle variances in engine efficiency which generally gives a range of 0.5 to 0.6 kgCO2/kW. A one-gen system will have a carbon intensity of approx. 0.7 – 0.8 kgCO2/kWh.
So if you are considering building a trigeneration system in Tasmania, don’t – the grid is a much better option in terms of carbon intensity. If you are considering trigeneration in South Australia be very cautious as the carbon payback of a trigeneration system in SA is likely to be greater than 15 years – potential longer than the life of the engine.
In other words as a simple test – if the carbon intensity difference between grid and trigeneration isn’t that great or if the grid is less than trigeneration then trigeneration is not a good option.
That’s it in simple terms, but as with most things its not simple. The next Tri Harder posts will go into more detail on:
Tri Harder Part 3 – How does Trigeneration stack up in each state?
Tri Harder Part 4 – Why do we consider carbon payback?
Tri Harder Part 5 – What happens if the carbon intensity of the grid improves?
Tri Harder Part 6 – What about flue emissions and maintenance?
Tri Harder The Final Chapter – Why do we have so many trigeneration systems going into office buildings and how do I know if trigeneration is right for my building?