مدل سازی حرارتی از بهره برداری در مقیاس بزرگ از انرژی منبع زمین در سفره های آب شهری به عنوان یک ابزار مدیریت منابع

As part of its legal commitment to reducing CO2 emissions, the UK has outlined a roadmap for significant increases in the use of ground source heat pumps for heating and cooling buildings. The technology is particularly suitable in new buildings, and in large commercial buildings. Such development is focussed in urban areas of economic growth. This paper presents an aquifer scale model of the impact of the expansion of open loop ground source energy schemes deployed in London. The model predicts the impact for currently operating schemes, and also the potential impact of all open loop schemes that have been licensed in central London. It is concluded that there will be thermal interference between these schemes and that in areas with such a high density of ground source energy schemes, the resulting loss of efficiency will mark an effective limit to the energy available for unbalanced ground source cooling. The current unregulated approach to managing the energy resource of the Chalk aquifer beneath London will not be able to sustain the demands implied by the UK roadmap for ground source energy. A more actively managed approach is needed if these energy demands are to be met, economically, in London and other centres of economic growth.

Heating and cooling our buildings accounts for a significant proportion of the EU energy use, which has been largely supplied by fossil fuels, but which must increasingly be met by renewable energy sources. The UK government, in common with other EU partners is committed by the European Renewable Energy Directive to ambitious increases in renewable energy, and by the Climate Change Act 2008 to reducing carbon emissions. The UK has outlined a roadmap for achieving national targets of 15% renewable energy by 2020 implies that about 6% of this target will be from non-domestic ground source heat pump installations [1]. This corresponds to a 30-fold increase in installed capacity of heat pumps. Much of the early development across Europe has been for small scale residential heating systems. In contrast, the IEA [2] identify that the biggest contribution to carbon reduction by heat pumps can be made through use of natural (‘sensible’) or engineered thermal energy storage systems providing balanced heating and cooling for larger commercial buildings. The IEA roadmap envisages dramatic growth in the use of this technology worldwide. Where buildings are located above aquifers, as in London and many of Europe’s major cities, the most cost effective approach for installing heat pumps in a large commercial building is generally an open loop ground source scheme. Above a heating or cooling capacity of 100 kW, closed loop schemes require large numbers of boreholes and are increasingly uneconomic to develop. In an open loop scheme, groundwater is directly abstracted from a small number of boreholes and reinjected to the ground after passing through a plate heat exchanger which extracts the energy from the water. The thermal energy load is thus transferred directly to groundwater in the underlying aquifer. Most of the installed capacity of approximately 0.4 GWth for non-domestic heat pump systems in the UK at the end of 2010 [1], is in new buildings or fitted as part of major redevelopment works. This is because heat pump heating and cooling systems work most efficiently at relatively low temperature differentials and thus are most effective in well insulated modern buildings. It has meant that the deployment of the heat pump technology has been focussed in areas of economic growth, and in particular, in the UK, this has meant that the majority of installations are located in London and the south-east of England. If the country is to achieve its ambitions to make the transition from fossil fuels to electrical heating, and meet its renewable energy and carbon emission targets, the required dramatic growth in installed capacity is most likely to continue to be focussed in this part of the country. This paper presents a modelling assessment of the potential thermal impact on the Chalk aquifer beneath central London, and estimates physical constraints to the energy that can be derived from this resource. The model shows that large open loop schemes that are well balanced utilise the energy resource afforded by the Chalk aquifer beneath London as an energy storage system, and are a genuinely long term sustainable solution for building heating and cooling. However, many commercial buildings have unbalanced, cooling dominated demands. These schemes have the potential to develop large heat plumes in the aquifer and can be seen instead as directly ‘mining’ the energy resource of the aquifer. The magnitude of the available resource available from an aquifer beneath a city depends on the scale of the aquifer from which the energy is extracted, but also on how balanced heating and cooling demands are. If energy demands are balanced over an annual cycle, the aquifer can provide a long term sustainable resource. If however, unbalanced cooling loads are developed in a small area, there is an increasing risk of schemes interfering with each other leading to loss of efficiency, or even schemes failing to deliver the required performance. The exploitation of thermal energy from groundwater is not directly regulated in the UK, introducing heat to a groundwater body is not introducing a contaminant and the temperatures over which heat pumps are permitted to discharge water back to the aquifer does not constitute pollution. The consequential uncertainty and risk of reduced efficiency, acts as a significant barrier to greater uptake of the technology. Indeed, the investment in a ground source energy scheme has no value unless the scheme operates more efficiently than alternative energy sources with lower capital costs. Whilst heat pumps can, in principle, continue to operate when thermal interference occurs, the scheme is no longer viable when its efficiency falls to uneconomic levels. This makes it difficult to specify the absolute size of the energy resource of an aquifer, but it is clearly no longer available when interference between open loop schemes is likely. This paper concludes with a discussion of issues for the management or regulation of the thermal resource of aquifers, and a tool that can be used to identify risks to the sustainable large-scale exploitation of the resource. The model developed here successfully represents the cumulative impact of schemes implemented in London to date and shows that there is little risk of interference compromising their performance. However a scenario predicting the impact of all schemes for which abstraction licences have been agreed shows that these schemes, if implemented, would lead to thermal plumes extending over 10% of the aquifer beneath central London. Several schemes are predicted to lead to interference and it is clear that the renewable energy targets set out in the UK roadmap for renewable energy would not be able to be met by extending the implementation of ground source energy for cooling taken from the Chalk aquifer beneath central London.

The current approach to the regulation of heat discharges is only to regulate open loop GSE schemes through individual abstraction licences and only to consider the thermal impact of individual schemes in isolation. Our models show for the first time that it is now feasible, and prudent, to create and manage an energy resource model analogous to water resource management tools. This can identify physical limits to the exploitation of the thermal resource. It could be used to advise new applicants of the thermal setting in which their proposed scheme will be developed and the implications for the risk of interference and the corresponding efficiency and feasibility of their scheme. Indeed, it is the uncertainty over risks to GSE performance that is preventing a more rapid uptake of this renewable technology as envisaged by the UK renewable energy roadmap [1]. Whilst too much regulation can create a barrier to the deployment of new technologies such as GSE, the uncertainty associated with a regulatory vacuum is deterring many potential clients from investing in open loop GSE for new buildings and building refurbishment. We have used the available data on current operational and proposed schemes in central London in this case study. Here, the economic costs favour open loop for large building demands and the model represents most large impacts associated with currently installed or currently planned heat pumps. The operational open loop schemes within the central zone are estimated to correspond to amount to 0.02 TW h or 4% of the estimated total UK 2010 installed capacity, and the total model area incorporates a capacity of 6% of the total UK 2010 installed capacity. However this rises to 0.07 TW h of operational capacity within the central zone if one also includes cooling energy. At this level of exploitation, 2.9% of the area of the central zone is impacted by thermal plumes. The current proposed schemes would double the installed ground source energy capacity within the central zone to 0.14 TW h, and this is predicted to lead to thermal impacts over 9.9% of area the central zone. It has been suggested that to meet UK targets, there might need to be an increase of an order of magnitude in the installed capacity of GSE energy loads [1]. This work shows that such an expansion of GSE, with current regulation, would lead to significant problems in cities where new development is concentrated. The data available to characterise thermal impacts on aquifers is often incomplete. For example, in the UK, no information is readily available on closed loop schemes. Many of these have significant heat loads and the larger schemes are typically installed in boreholes having a similar thermal impact as open loop schemes. If the thermal resource is to be managed, more comprehensive data on the actual cumulative heating and cooling loads is required. Thermally balanced open loop schemes utilise the energy resource of the groundwater as a ‘sensible’ thermal energy storage system as identified in the IEA Technology Roadmap [2], and are a sustainable solution for building heating and cooling. However, many commercial schemes have unbalanced, cooling dominated demands. They develop large heat plumes in the aquifer and can be seen as directly ‘mining’ the energy resource of the aquifer. Whilst regulation is often seen as a barrier to development of GSE, unregulated expansion of ground source energy is likely to be increasingly led by schemes with large net cooling loads. The thermal impact of these on the aquifer is much larger than for balanced schemes and it is likely that this will lead to interference between schemes. Such concerns will be a significant barrier to further development of ground source energy which could be avoided by management of the resource using models such as that presented here. The exact area where there is now a high risk to the sustainable development of new GSE cannot be directly drawn from the model outputs such as Fig. 5. Instead, new schemes would need to be incorporated in this resource model to determine their feasibility on a case by case basis, taking into account the design loads and whether the loads are balanced or are cooling dominated. Similar limits and risks will limit the exploitation of the energy resource in aquifers wherever ground source energy schemes are concentrated to the extent seen in central London. These risks can be identified where sufficient data is available to develop thermal models of the ground source energy resource. They could be avoided if there is a more balanced exploitation of the resource as an interseasonal energy store, but this would require more direct regulation of the resource.