Sunday, June 12, 2011

Savings in CO2 emissions (ECET)

Today I shall present an estimate of CO2 emissions savings that would be achievable if my ECET concept were to be introduced.  In marketing-speak, these savings would be described as low-hanging fruit.

ECET is the acronym for the Expansion-Cycle Evaporation Turbine, the continuous-flow version of a new thermodynamic cycle I invented in 2004.  In operation, the ECET expands hot air, evaporatively cools it at sub-atmospheric pressure and then re-compresses it to ambient pressure.  Several cooling and re-compression stages are advantageous.  Power is received in expansion and expended in re-compression (as well as in water purification and injection), and there is surplus work available in the cycle provided the air is hot enough and the adiabatic efficiencies of the turbo-expanders and compressors are good enough.

For further details, please visit and follow the link to “Expansion-Cycle Evaporation Turbine”.  At that web page, I describe a case study in which the ECET is used to boost the power of an Open-Cycle Gas Turbine (OCGT) by more than 20%, without using any extra fuel, and at a specific capital cost ($/MW) expected to be no more than that for the upstream OCGT that provides the hot exhaust.  The ECET boost is employed when the OCGT is operated, namely at peak demand in the electricity grid.

Let’s proceed with the estimate …

In 2008-09, the total electricity production in Australia was 266 TWhr or 9.6 ×10^17 J [1].  I don’t have data saying how much of this was produced by OCGTs, but let me estimate 5%, giving 4.8 × 10^16 J.  In the case study referred to above, the ECET boosts the power of an OCGT by more than 20%, so it is argued that 0.2/1.2 = 0.17 of the electricity currently produced by OCGTs could be produced by ECET boost.

The specific cost ($/MW) of ECET hardware required to produce that electricity is reckoned to be no more than the specific cost for the hardware for the upstream OCGT.  If the total amount of generation capacity need not be altered, neither would be the total capital cost.

That gives a plausible Australian ECET electricity production of 0.17 × 4.8 × 10^16 = 8.2 × 10^15 J.  That represents electricity that does not have to be produced by OCGTs, say at an average thermodynamic efficiency of 0.36.  Therefore the primary energy (as natural gas) saved is 8.2 × 10^15 J / 0.36 = 2.3 × 10^16 J.

Note added 20 July 2011.  In what follows, I now think I need to use the Higher Heating Value for the energy content of natural gas, rather than the Lower Heating Value.  Appropriate changes, which have a relatively minor effect, have been made and are indicated in red.

Now natural gas has energy content (Lower Heating Value) of 50.0 × 10^6 J /kg [2], so the amount of natural gas saved is 2.3 × 10^16 J / 55.5 × 10^6 J /kg = 4.1 × 10^8 kg.

Each kg of natural gas produces 2.75 kg of CO2 [3], so the emissions savings are 2.75 × 4.1 × 10^8 kg = 1.1 × 10^9 kg CO2 = 1.1 million tonnes CO2.

If the price of gas is AUD 4 / GJ [4], then the value of the natural gas saved is AUD 4 × 2.3 × 10^7 = AUD 92 million.

Each tonne of CO2 that is avoided is therefore associated with a saving, not a cost, of AUD 92 million / 1.1 million tonnes CO2 = AUD 81 / tonne CO2.

The following table gives a summary of the estimates for Australia:

Primary energy savings per year
2.3 × 10^16 J
Savings in natural gas per year
0.41 million tonnes
Savings in CO2 emissions per year
1.1 million tonnes [5]
Value of natural gas saved per year
AUD 92 million
Benefit of CO2 emissions reduction
AUD 81 / tonne CO2 [6]
To obtain corresponding figures for the entire world, a rule of thumb is that Australia represents around 1% of the world market.  Simply multiply the above estimates (except for the benefit) by 100 to get a global estimate.

I think these estimates justify my use of the term “low-hanging fruit”.  (Even if my unsubstantiated estimate of 5% for the amount of electricity produced by OCGTs is wrong by a factor of 5, the estimates would still be impressive.)


[5] this is 0.24 % of Australia’s annual CO2 emissions, see
[6] This is a benefit, not a cost; in an important recent publication, the Australian Productivity Commission gives implicit costs for CO2 abatement for various countries and technologies; see Figure 2 on p. XXXI , Productivity Commission 2011, Carbon Emission Policies in Key Economies, Research Report, Canberra.  Abatement costs for large-scale renewable such as wind are around AUD 40-60/t CO2.  Most solar projects have abatement costs above AUD 200/t CO2.

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