Non Steady Analytical Models for Energy Pile Testing and Design

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Europe's building stock is increasing in floor area by approximately 1% per annum. This represents an additional operational energy demand of over 4.5 million tonnes of oil equivalent, year on year. Of that energy requirement up to three quarters is related to space heating (or cooling), representing about half of total energy usage in Europe and North America. Ground source heat pumps, which can reduce the net consumption of energy for space heating by approximately 75%, can therefore play a significant and timely role in tackling the energy and carbon emissions challenge. Despite the urgent need to curb the increasing energy requirements of new buildings, the market for ground source heat pump systems faces a number of barriers to expansion. While some of the barriers are related to regulation, investment cost intensity remains an important factor. Consequently, research and development must focus on increasing energy efficiency and reducing capital costs. One route to reducing investment capital costs is through the combined use of building foundations for the heat exchanger component of the system, thereby avoiding the need for construction of special purpose heat exchangers such as boreholes. This has the potential to both reduce capital expenditure and deliver increased energy per drilled metre of the heat exchanger. Piles are the most common type of deep foundation. These are typically constructed by augering a hole which is infilled with concrete and steel reinforcement. Energy pile is the term used for a foundation pile which is equipped with heat transfer pipes to act as the heat exchanger part of a ground source heat pump system. Energy piles were first trialled in Austria in 1984, but thermal analysis and design methods have lagged substantially behind the practical application. Recent breakthroughs have shown the importance of the concrete part of the pile in storing thermal energy on a short term basis. This is significant because fluctuating operational energy demand means that a thermal steady state in the piles is rarely achieved. Despite this, most routine design approaches still characterise energy pile in terms of a steady state thermal resistance parameter. This means that any storage of energy in the concrete is neglected and the energy capacity of the system is routinely underestimated. Indeed, the steady state assumption has been shown to underestimate the potential energy saving available from energy piles by around 20%. This proposal outlines planned work which will develop new non-steady models for use with the thermal analysis of energy piles. The work will also include application of these models to in situ thermal response tests which are used to determine the thermal characteristics of the soil surrounding the pile. Hence the new models will contribute to both improved soil parameter selection and less conservative design approaches. This work is novel because there are currently no analytical models that appropriately simulate the transient behaviour of energy piles. By the introduction of appropriate non steady models this work will lead to improved and less conservative assessment of energy available from energy piles and hence increase their uptake in practice. This work is pressing because the alternative of using inappropriate steady state models will result in the under-prediction of ground source heat pump system performance and thereby inhibit uptake of this key renewable heat technology.