Tri Harder – Part 3, How does trigeneration stack up in each state?
What is the carbon benefit of trigeneration, does it pay itself back, what are the numbers please? We have heard a lot in the industry recently about the benefits and non-benefits of trigeneration but not much on the actual numbers. So as promised this post shows the numbers.
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. In Part 2 of Tri Harder I talked about the difference in the carbon intensity of the electricity grid around Australia.
The next question – if trigeneration uses natural gas (a fossil fuel) to generate electricity, how does it stack up in comparison to the carbon intensity of the grid for each state?
Lets take a typical large office building and stick a 750kWe trigeneration system into it. Using typical efficiencies of 36% electrical efficiency and 42% thermal efficiency and a two stage absorption chiller we can calculate the carbon benefit of a trigeneration unit in each state assuming a bunch of stuff about operational hours, elec profiles, cooling load etc.
The table below shows the results of the analysis for each state.
|Carbon Intensity of the Grid Electricity||Carbon Intensity of Trigen Electricity||Carbon Savings %||Carbon Payback Years||Carbon Benefit|
If you recall from Tri Harder Part 2 I suggested that the closer the carbon intensity of trigen electricity is to grid electricity the less carbon benefit you will get from trigen. There are of course variants for each building, a 750kWe isn’t necessarily the right size for each building but the above table gives a good indication of where trigeneration works and where it doesn’t.
So from the table above you can see which states trigen gives a carbon benefit (NSW, VIC, QLD and WA), the states where trigen is marginally beneficial (SA and NT) and TAS where you definitely wouldn’t do trigen for carbon reasons.
The column titled “carbon payback years” gives an indication as to how long it would take to ‘payback’ the embodied carbon of the trigeneration system through carbon savings when compared to grid electricity, assuming the grid carbon intensity doesn’t improve in the future. As can be seen a reasonable carbon payback is achieved in most states apart from TAS and longer term in SA and NT.
So that’s it in carbon terms but what about cost?
Well the cost of gas and electricity varies so much between states, cities and purchasers that it is difficult to give one answer for all situations so what I have done is calculated the equivalent cost per kWh of electricity produced from a trigen unit for different gas prices. If the equivalent electricity cost from trigen is higher than grid electricity cost then trigen will cost you money every year you operate it, if the equivalent cost is less than grid you will save money and possibly payback the initial capital investment cost.
The graph below gives an easy to use quick assessment tool for trigen systems in operating cost terms for office buildings.
How to use the graph
1. find out the gas cost per MJ including all charges and your grid electricity cost per kWh for all charges
2. draw a line vertically from your gas cost and a line horizontally across from your electricity cost
3. if the two lines intersect below the equivalent cost line trigen operational costs will be more expensive than grid, if they cross above the equivalent cost line then trigen will be cheaper to run than grid electricity
Please note that this does not include the ongoing maintenance cost of the trigen system, this will vary depending upon how much the trigen is used and cannot be easily factored in to an equivalent cost.
In summary, at the moment in most states there is an ongoing increased cost in operating a trigen system in most office buildings in Australia but this will be entirely dependent upon how much you buy your gas and electricity for. But in all states apart from TAS there is a carbon benefit of using a trigeneration system, with the most benefit in VIC. However, as mentioned in Tri Harder Sidebar these calculations are only based on scope 1&2 emissions and don’t include fugitive emissions (scope 3) from gas extraction and processing – that’s just way too hard to calculate!
Calculating the exact benefit and cost of a trigeneration system should be done for each individual building as the profile of electrical demand, cooling demand and operational profile will vary the calculations. If you are looking at a trigeneration system and need help please email me at firstname.lastname@example.org
The next Tri Harder posts will go into more detail on:
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?