Conventional geothermal systems occur naturally, due to deep heat sources such as magma chambers, which most often occur near plate boundaries where tectonics has induced melting of the Earth’s crust. The fluid in most geothermal systems is groundwater which is heated near this deep heat source and then moves upwards closer to the surface, a heat transfer process called convection. Geothermal systems are therefore dynamic systems, the size and shape of which depends on the depth and temperature of the heat source and the permeability structure of the shallower rocks through which the convecting fluid moves.
The chemical content of geothermal fluid depends on the original composition of the groundwater, any inputs of fluids from the magma chamber (or deeper), the composition of the rocks through which the fluid travels, and the pressure and temperature (which affect the rate of fluid-rock interaction). Geothermal fluid contains CO2, methane and hydrogen sulphide, and in the natural state (pre-development) these are discharged through obvious natural surface features such as fumaroles and bubbling pools, and less obviously as a flux through the soil.
After development of a geothermal system for power generation, during operation of the power plant some of the geothermal gases (CO2 and others) can become separated from the geothermal fluid as a result of changes in temperature and pressure. The gases that have become separated are non-condensable and are released to the atmosphere as a part of the power generation process.
It is useful to consider emissions for electricity generation in terms of an “emissions factor” (also called “emissions intensity” or “carbon intensity”) of grams CO2-equivalent per kilo-watt hour gCO2(eq)/kWh, which enables comparison to electricity from other energy sources. The measure “grams CO2-equivalent” is a useful way to combine the CO2 and methane into one number – it is the mass of actual CO2 plus an equivalent mass of CO2 to represent the methane. As a greenhouse gas, methane has 25 times more impact than CO2 and so the mass of methane times 25 gives the equivalent mass of CO2.
The emissions factors for geothermal power stations in New Zealand for the calendar year 2018 are given in the table and figures below. These are the emissions of CO2(eq) released from the geothermal fluid during operation of the plant. The median emissions factor for 2018 is 62 gCO2(eq)/kWh and this is a standard measure of the central tendency of this kind of dataset with outliers. The use of the median (and other percentiles) is the same approach as used by the IPCC in the 2011 Special Report on Renewable Energy Sources and Climate Change Mitigation. Another useful statistic is the weighted average of 76 gCO2(eq)/kWh, which is weighted using the total energy generated from each plant, thus accounting for the fact that not all plants are the same size and hence their individual numbers for emission factor do not carry the same weight.
For comparison, the emissions factors from other renewable energy sources during operation are:
- Hydro: > 0 gCO2(eq)/kWh (some methane is emitted from decomposition of organic material in the reservoir, though this is hard to quantify)For comparison, the emissions factors from other renewable energy sources during operation are:
- Solar photovoltaic (PV): 0 gCO2(eq)/kWh (no emissions from sunlight)
- Wind: 0 gCO2(eq)/kWh (no emissions from wind)
And emissions factors from fossil fuel plants from fuel combustion during operation are (national average NZ, source MBIE):
- Coal: 970 gCO2(eq)/kWh
- Natural gas (open-cycle gas turbine): 530 gCO2(eq)/kWh
- Natural gas (combined-cycle gas turbine): 390 gCO2(eq)/kWh
All the emissions factors discussed so far are for operation of the plant only. For a true comparison between the different energy sources, the full life-cycle of the plant needs to be considered. A life-cycle assessment (LCA) includes carbon emissions related to: materials/construction, operation, and decommissioning. An international review of LCAs for all energy sources is available in the 2011 IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation (see figure and table below).
For all the energy types currently present in NZ, median LCA CO2(eq) emissions factors from the IPCC report are as follows:
- Coal: 1001 gCO2(eq)/kWh (high certainty – 169 data points)
- Natural gas: 469 gCO2(eq)/kWh (high certainty – 83 data points)
- Solar PV: 46 gCO2(eq)/kWh (high certainty – 124 data points)
- Geothermal: 45 gCO2(eq)/kWh (uncertain – only 8 data points)
- Wind: 12 gCO2(eq)/kWh (high certainty – 126 data points)
- Hydro: 4 gCO2(eq)/kWh (medium certainty – 28 data points)
The IPCC life-cycle (LCA) median emissions factor for geothermal is 45 gCO2(eq)/kWh, which seems low when the median NZ emissions in 2018 were 62 gCO2(eq)/kWh from operation only. This may be due to the limited number of geothermal LCAs – only 8 data points met the criteria to be included in the IPCC review. All the other energy types had many more LCAs and are likely to be more representative.
Although the true median geothermal life-cycle emissions factor will likely be higher, it is still useful to consider these life-cycle numbers, as it can be seen that:
- All energy types emit CO2(eq), there is no zero-carbon energy.
- All the renewable energy types present in NZ (hydro, wind, solar PV and geothermal) have emissions factors at least one order of magnitude lower than the fossil fuel plants (gas and coal).
Geothermal emissions intensities are relatively complex, and change over time. The intensity decreases due to field degassing, and can increase or decrease to some degree due to operational changes to the steamfield or plant, as shown schematically in the figure below. Additionally, there are two important factors which offset some of the CO2(eq) emissions from geothermal electricity generation, which are not usually accounted for:
- Geothermal plants have the benefit of producing large volumes of hot water. This is often just reinjected back into the reservoir, however the hot water can be used (and is being used) as process heat for various industries, which would otherwise burn fossil fuels to obtain that heat.
- Geothermal systems in their natural state emit CO2 and methane from natural surface features, such as fumaroles, bubbling pools, and flux through the soil. Development of geothermal power stations has often resulted in a decline in surface CO2 and methane emissions, although this is very difficult to quantify.
Geothermal Fluid Use and Emissions Trading Requirements
- Application – Electricity Generation and Industrial Heat
The New Zealand Emissions Trading Scheme applies to using geothermal fluid for generating electricity or industrial heat, where the emissions of carbon dioxide-equivalent (CO2-e) exceed 4,000 tonnes from a given installation per annum.
- Legislative Requirements
The Climate Change Response Act 2002 (update as at 8 Dec 2009) requires industries to register, to set up holding accounts, to gather data, to monitor emissions, to provide regular data returns for prescribed periods at specified times. Payment is according to default emissions factors for a given facility as specified in the Climate Change (Stationary Energy and Industrial Processes) Regulations 2009 unless an application for a unique emissions factor is made and approved under the Climate Change (Unique Emissions Factors) Regulations 2009.
Geothermal facilities supplying geothermal fluid for generating electricity or industrial heat are subject to the Climate Change (Stationary Energy and Industrial Processes) Regulations 2009. These regulations consider fluid supply as either geothermal steam (Schedule 2, Table 6, Part A) or geothermal fluid (Schedule 2, Table 6, Part B). Prescribed or default emission factors are defined in Schedule 2, Table 6 of the regulations for these two fluid types. The measured annual fluid production is multiplied by the prescribed emissions factor to derive the reportable annual emissions from a given facility.
There is an option for the prescribed emissions factor to be substituted with a unique emissions factor. The methodology to develop a unique emissions factor for a geothermal facility is covered in the Climate Change (Unique Emissions Factors) Regulations 2009, clauses 14 to 17. Aspects of determination of unique emissions factors covered are in a letter on the Climate Change Act – Geothermal Sampling Procedures dated 23 September 2010 from GNS Science to the New Zealand Geothermal Association. This letter identifies appropriate sampling methods that comply with the legislative requirements.
- Other Commentary and Information
A number of companies have analysed their processes, determined that it is cost effective to make an application for a unique emissions factor and have subsequently applied for and been granted a unique geothermal emissions factor.
The carbon emissions scheme effectively taxes industries for their emissions. For the geothermal industry, which has comparatively low carbon emissions, this increases their economic performance with respect to other higher emitting generators.
For more on the emissions trading scheme visit the NZ Government climate change web site
- Practical methods of minimizing or mitigating environmental effects from integrated geothermal developments; recent examples from New Zealand
- Houghton, B.F. 1989: Inventory of New Zealand Geothermal Fields and Features. Geological Society of NZ
- B.F. Houghton 1982. Geyserland: A Guide to the Volcanoes and Geothermal Areas of Rotorua. Geological Society of New Zealand Guidebook N. 4.
- B.F. Houghton, E.F. Llyod and R.F. Keam 1980: The Preservation of Hydrothermal System Features of Scientific and Other Interest – A Report to the Geological Society of New Zealand.
- Parliamentary Commissioner for the Environment 2003. Electricity, energy and the environment. Part A making the connections.