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Project Title: Modelling the Viability of Heat Recovery from Underground Pipes

PhD Student: Mohamad Abdel-Aal

Supervisors: Dr Mostafa Mohamed, Dr Alma Schellart (University of Sheffield) and Professor Simon Tait (University of Sheffield)

Background

The increasing demand on energy consumption along with tighter EU regulations regarding carbon emissions has boosted the desire for producing alternative energy. The literature showed that domestic heating has one of the highest demands and therefore, generating heat by utilising a more sustainable technique than the conventional methods is needed. Attractive heating options for the domestic sector were found to be the wastewater and Ground Source Heat Pumps (GSHP).

Recovering heat from wastewater running in sewer pipes may result in temperature drops at the wastewater treatment plant (WwTP). Some treatment processes, such as nitrification, require certain wastewater temperatures and hence large temperature drops can result in treatment problems. Therefore, it is crucial to predict the temperature drop along the sewer pipe profile and use this to estimate the best location for a heat exchanger to be fitted in the pipe.

Installation of heat pump loops usually requires large spaces and therefore may prove challenging for implementation in small house gardens. However, minimising the required space can be achieved by enhancing the heat recovery process from the soil surrounding the buried heat pump loops. Laboratory experimental works, carried out in Bradford, have proven that increasing the moisture level in soil increases its thermal conductivity. Modelling the temperature variation along the heat pump loop is therefore necessary to examine the performance of the GSHP system

Aims

The aim of this project is to demonstrate the viability of heat recovery from underground pipes using computational techniques. Underground pipes, in this context, are sewer pipes and ground source heat pump loops.

Methodology

Five sewer pipes were utilised in collaboration with Aquafin, Belgium to measure wastewater and in-sewer air temperatures at the upstream and downstream ends of each sewer pipe. Flow rates and local soil temperatures were also monitored. Three sewers can be categorised as urban sewers, i.e. with dry weather flow (DWF) of 35-50 m3/hour while the other two are large main collectors with DWF of 350 and 1300 m3/hour.

Heat pump systems were installed in three houses in Dewsbury. Single and double loop panels were installed to compare the performance of each type. Infiltration trenches were installed above two loop types to examine the impact of moistening the soil on GSHP performance. Figure 1 shows some of the work done in Dewsbury.

igure 1: Installation of heat pump loops in Dewsbury. Photo on the left shows one panel of the double heat pump loop. Right photo shows the infiltration trench used to utilise rainfall water.

Figure 1: Installation of heat pump loops in Dewsbury. Photo on the left shows one panel of the double heat pump loop. Right photo shows the infiltration trench used to utilise rainfall water

Studying the heat transfer processes in partially and fully filled pipes has led to the development of two deterministic models. The first model is designed to estimate the temperature variation along a partially filled pipe profile. The latter model estimates how far from the WwTP a heat exchanger can be installed to recover heat without compromising on the wastewater treatment process. The second model estimates the temperature variation along the GSHP loop and therefore enables the prediction of the GSHP performance. Data collected from the Belgian sewers was used for modelling validation and calibration. Current work is being held to develop the partially filled pipe model to estimate temperature variation in sewer networks. Work is underway to validate and calibrate the fully filled pipe model using measured data in Dewsbury.

Results

Figure 2: Measured data in an urban sewer site in Belgium with a length of 175m.

Figure 2: Measured data in an urban sewer site in Belgium with a length of 175m

Heat recovery application example on a 2 km sewer pipe.

Figure 3: Heat recovery application example on a 2 km sewer pipe with similar characteristics to an urban sewer in May 2012. Source: Abdel-Aal, M., et al. 2014. Modelling the viability of heat recovery from combined sewers. Water Science & Technology, 297-306

Acknowledgement

Thanks to the EU Interreg IVB Inners, http://inners.eu/, for funding this project.

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