LCA - Life Cycle Assessment
LCA is a technique used to assess the environmental impacts of a product or project. It typically includes the impacts of all project 'stages', from the extraction of raw materials, to manufacturing, distribution, use, maintenance and disposal. This is often referred to as 'cradle-to-grave'. Typically, an LCA will report on environmental impacts such as global warming potential (GHG emissions), acidification, photochemical ozone creation potential, eutrophication, resource depletion and human health impacts. The aim of the LCA conducted for this project was to compare emissions from installing and operating a deep geothermal single well (DGSW) for 50 years (the expected lifetime of the system) with a counterfactual case. In this instance the counterfactual case was 4 gas boilers (100kW each) for the same duration. It was assumed that under the DGSW case a 400kW boiler would be installed as backup and replaced every 10 years, but not used.
Emissions associated with the installation of a DGSW and associated plant equipment were calculated at 248 tonnes of carbon dioxide equivalents (tCO2e). This includes embodied emission of the materials used, their transport to site, plant equipment fuel use and maintenance (i.e. the replacement of key items such as the water pump). The main contributor to construction emissions is the drilling of the 2km well and installation of the steel case, responsible for about 90% of construction emissions. The replacement of the steel case with glass-reinforced plastic (GRP) was considered, along with steel with a high-recycled content (59%). Figure 32 summarises construction related emissions and the saving that can be achieved (30%) by simply sourcing steel pipes with a high-recycled content.
Figure 32: Deep geothermal well construction emissions - steel versus plastic pipe casing
The counterfactual case, whereby heating is provided through 4x100kW boilers, does not have any construction related emissions because the boiler system already exists today. However, the replacement of the boilers every 10 years does have a manufacturing and transport impact that was included. The impact is measured at just under 4 tCO2e over 50 years.
Heat supply from the DGSW has very low operational carbon emissions, approximately 35 tonnes per year (based on current grid carbon factors). This performance is 13 times better than gas fired boilers. This result was expected given the high efficiency of the deep geothermal system. The input electricity is used to circulate water to collect geothermal heat from the well base; as opposed to the gas boiler that is based on continuous combustion of a fossil fuel to produce heat. Both systems would require a plant room with controls that use minimal amounts of electricity. As a result, the carbon emissions for the gas boiler system are slightly lower under the decarbonising grid scenario.
Figure 33: Carbon Performance of Heating Systems, under current grid intensity and future decarbonised grid scenarios
Overall LCA Results - DGSW vs Counterfactual
Over a period of 50 years, the DGSW is projected to emit a total of 2,008 tCO2e (including construction, operation and maintenance emissions) whilst delivering an annual heat output of 1,800,000 kWh. This compares favourably to the gas boilers counterfactual whereby four 100kW boilers would deliver a similar heat output as the DGSW, but with lifecycle emissions over 50 years of 24,188 tCO2e. In this instance, the DGSW is almost 12 times more efficient than the counterfactual gas boilers, achieving a saving of over 22,170 tCO2e over 50 years. Figure 34 plots the cumulative emissions of the two scenarios over 50 years. Construction, operational and maintenance emissions are included in the graph. Although the DGSW option has a higher carbon footprint initially (due to construction) compared to the gas boiler, once the DGSW is operating, the annual savings are significant enough to offset construction emissions in less than a year.
Figure 34: Life-cycle carbon emissions of a DGSW compared to the gas boilers counterfactual scenario
Sensitivity Analysis and Uncertainties
The life-cycle emissions of the DGSW and the gas boilers were modelled using both fixed and projected grid carbon intensity. Results show how the carbon savings achieved by the DGSW over the gas boilers are not particularly sensitive to switching from a fixed UK grid, to a decarbonised projection. A 3% increase was modelled, equivalent to 730 tCO2e saved in addition by the DGSW over 50 years. This is a function of the small amount of energy the system draws off the power grid.
Figure 35: Life-cycle emissions of a DGSW compared to gas boilers, and their variance depending on UK grid projections
Figure 35 compares life-cycle emissions of the DGSW with gas boilers, and how these emissions may vary depending on fixed or decarbonised grid emissions. It is interesting to note how emissions associated with the DGSW are more sensitive to grid decarbonisation than the gas boilers. This is because the DGSW uses electricity to power the submersible pump, while the gas boilers' energy input is gas whose carbon content is fixed.
Figure 36: DGSW life-cycle emissions (fixed UK grid) by element and % contribution
Under the fixed UK grid scenario, the majority of emissions are from operating the DGSW (i.e. running the submersible pump, see Figure 36). This suggests that procuring efficient submersible water pumps is likely to have the most significant overall impact. If a decarbonised power grid projection is applied, the proportional mix of emissions changes (see Figure 37). Operational emissions are still the largest at 56%, but material and plant emissions also increase in significance. The largest contributor to material emissions is the steel casing, whilst the drilling rig dominates on-site construction emissions. Using recycled steel for the casing, or alternative materials such as GRP can reduce emissions. Currently the transport assumptions assume a 200km round trip for delivering all materials to site. Even a doubling in distance makes no significant difference to the overall life-cycle footprint. As a result, a sensitivity analysis of logistics was not undertaken.
Figure 37: DGSW life-cycle emissions (decarbonised UK grid) by element and % contribution
The lifecycle cost of the DGSW is characterised by low operating costs and high capital costs associated with drilling the well. This is the opposite of the gas boiler system for which the bulk of the lifecycle cost is associated with the fuel cost. The major advantage of the DGSW cost profile is that it is less vulnerable to price volatility in the energy market and the levelised cost of heat production is significantly below the market price. The geothermal solution could therefore offer price stability and savings to the customer and contribute to the UK's energy security. Figure 38 below provides the breakdown of lifecycle costs for both the DGSW and counterfactual. The capital costs associated with installation and upfront cost of equipment of the counterfactual were not included in the analysis because the demonstrator site currently has gas boilers installed. However, the gas boilers be replaced at least three times over 50 years and therefore their cost has been accounted for under equipment replacement costs. The annual fuel cost of the gas boiler is nearly 10 times that of the DGSW.
The analysis of the levelised cost of heat production over 50 years demonstrates the relative significance of capital, fuel and operational costs for each system. The overall result (Figure 38) demonstrates that the DGSW has a lower levelised cost of heat production than the gas boiler. This suggests that the DGSW could offer a price discount to the consumer, depending on the heat supply agreement and financing structure.
It can also be seen from Figure 40 that the levelised capital cost of the DGSW (which includes construction and equipment costs) supply is less than the fuel costs associated with the counterfactual. The operational costs of the gas boiler are therefore much more significant in driving up the lifecycle cost than that of the high capital cost of the well. It is important to note that these results are based on a 50-year heat supply, which, although consistent with the lifetime of a well, is currently beyond the norm for heat supply contracts that lie between 15 to 20 years and are most commonly specified for district heat networks.
Figure 38: Levelised cost of heat over 50-year lifetime - comparison between deep geothermal well and gas boiler (counterfactual)
Email: Johann MacDougall
There is a problem
Thanks for your feedback