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The thermal performance of foundation piles used as heat exchangers in ground energy systems

The thermal performance of foundation piles used as heat exchangers in ground energy systems
The thermal performance of foundation piles used as heat exchangers in ground energy systems
Pile heat exchangers are expected to make a significant contribution to meeting UK and EU renewable energy and carbon dioxide reduction targets. However, design for the thermal capacity of pile heat exchangers has to date been largely based on methods developed for borehole heat exchangers. Piles have a much smaller aspect (length to diameter) ratio than boreholes and consequently their thermal behaviour is different in a number of important ways. This thesis explores these differences and makes recommendations for improved assessment of pile heat exchanger thermal capacity.

Traditionally vertical heat exchanger design assumes separation of the thermal effects in the ground and in the pile. A transient temperature response function is used to assess temperature changes in the ground and a steady state resistance is applied to the pile concrete. In this thesis existing approaches to temperature response functions are critically assessed for use with thermal piles. It is important to take into account the larger pile diameter, which causes increased temperature changes in the short term. In the long term, the shorter pile length will result in reduced temperature changes as steady state is reached more quickly.

Simple 2D numerical modelling has been carried out and the results used to derive a new method for determining pile thermal resistance. However, for large diameter piles, the time taken for the pile to reach steady state suggests that the use of a constant thermal resistance in design is not always appropriate. In these cases it is recommended that a transient temperature response function is used to assess the response of the ground and the concrete together.

The applicability of short duration thermal response testing for pile heat exchangers has been examined. Modelling and case study data has shown that the technique is only reliable for piles of 300mm diameter or less. For the special case of large diameter piles with centrally placed heat transfer pipes then it is possible to use the test to determine the thermal conductivity of the pile concrete, but not pile thermal resistance.
Loveridge, F.
fb5b7ad9-d1b8-40d3-894b-bccedf0e8a77
Loveridge, F.
fb5b7ad9-d1b8-40d3-894b-bccedf0e8a77
Powrie, William
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Loveridge, F. (2012) The thermal performance of foundation piles used as heat exchangers in ground energy systems. University of Southampton, Faculty of Engineering and the Environment, Doctoral Thesis, 206pp.

Record type: Thesis (Doctoral)

Abstract

Pile heat exchangers are expected to make a significant contribution to meeting UK and EU renewable energy and carbon dioxide reduction targets. However, design for the thermal capacity of pile heat exchangers has to date been largely based on methods developed for borehole heat exchangers. Piles have a much smaller aspect (length to diameter) ratio than boreholes and consequently their thermal behaviour is different in a number of important ways. This thesis explores these differences and makes recommendations for improved assessment of pile heat exchanger thermal capacity.

Traditionally vertical heat exchanger design assumes separation of the thermal effects in the ground and in the pile. A transient temperature response function is used to assess temperature changes in the ground and a steady state resistance is applied to the pile concrete. In this thesis existing approaches to temperature response functions are critically assessed for use with thermal piles. It is important to take into account the larger pile diameter, which causes increased temperature changes in the short term. In the long term, the shorter pile length will result in reduced temperature changes as steady state is reached more quickly.

Simple 2D numerical modelling has been carried out and the results used to derive a new method for determining pile thermal resistance. However, for large diameter piles, the time taken for the pile to reach steady state suggests that the use of a constant thermal resistance in design is not always appropriate. In these cases it is recommended that a transient temperature response function is used to assess the response of the ground and the concrete together.

The applicability of short duration thermal response testing for pile heat exchangers has been examined. Modelling and case study data has shown that the technique is only reliable for piles of 300mm diameter or less. For the special case of large diameter piles with centrally placed heat transfer pipes then it is possible to use the test to determine the thermal conductivity of the pile concrete, but not pile thermal resistance.

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More information

Published date: June 2012
Organisations: University of Southampton, Civil Maritime & Env. Eng & Sci Unit

Identifiers

Local EPrints ID: 348910
URI: http://eprints.soton.ac.uk/id/eprint/348910
PURE UUID: 1cb11f36-d719-4278-9a17-9baa08a83ac8
ORCID for F. Loveridge: ORCID iD orcid.org/0000-0002-6688-6305
ORCID for William Powrie: ORCID iD orcid.org/0000-0002-2271-0826

Catalogue record

Date deposited: 05 Mar 2013 15:06
Last modified: 13 Jun 2019 00:32

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