Recycled and reutilised heat energy from sewer flows aids carbon reduction fight

Apr 05, 2011

New concepts and special solutions

For more than a decade now world governments have been pressing industry, utilities and the general public to be more aware of energy usage and to play their part in the reduction of carbon emissions which are deemed crucial in the fight against the global warming phenomenon currently afflicting the planet. To date, much of the effort towards carbon reduction has been to reduce the use of fossil fuels and to increase the application of renewable carbon-free energy sources such as wind and wave.

However, somewhat less emphasis has been placed on maximising the use of the energy already available having been created by mainly fossil fuel power production sources. Much of this energy produced is being allowed to go to waste after only one, yet not always efficient, use.
With the help of a new product, Therm-Liner, from Uhrig Kanaltechnik GmbH, this could now be changing!
A schematic of the Therm-Liner system operation. (Source: Uhrig Kanaltechnik GmbH)
A large percentage of the fossil fuel energy used in homes, office buildings, schools, factories etc goes into the heating of water and the environment in general. This in turn means that much of the wastewater, which has been used for heating or which has simply been warmed by the local environment, is flushed into the local sewer network.
This ‘wastewater’ is at a higher temperature than local ambient conditions, often running at between 12 and 15o Celsius. The fossil fuel energy used to create this heat is, at present increasing the carbon emissions and generally lost to the environment as the effluent travels through the sewer network, which is energy literally wasted.
This can be up to as much as 25% of the energy used to heat the buildings from which the wastewater and effluents carry the bulk of the energy loss into the underground sewer pipes. Under the right circumstances the recycling of the heat energy recovered from this process can lead to up to 75% reduction of ‘existing fossil fuel’ energy consumption.
Uhrig’s Therm-Liner is designed to minimise this energy loss by recovering this ‘waste’ heat for reuse in buildings.
A Therm-Liner V4A heat exchanger unit. (Source: Uhrig Kanaltechnik GmbH)
Because the wastewater sewer flows run at a higher temperature than the pipeline and surrounding ground through which the effluent runs, any heat in the effluent is generally lost through the sewer pipe wall into the local soil or dissipates into the atmosphere at the sewerage plants.
The Therm-Liner V4A heat exchanger unit is designed to minimise this loss by recovering this heat before it is lost to the environment.
Therm-Liner is a heat exchanger which is installed into the existing sewer flows, along the pipe invert, so that the effluent flows directly over the heat exchanger elements.
By connecting the Therm-Liner to a heat pump on the surface, which circulates heat extraction fluid through the Therm-Liner Elements, the Heat Energy from the wastewater or effluent is transferred to a local ‘power’ station, recycled and reutilised.
Installing the Therm-Liner system via existing manholes. (Source: Uhrig Kanaltechnik GmbH)
Here the heat from the effluent can also be used to warm water which is used as part of centralised local area heating system for local buildings such as council offices, schools or industrial sites and factories, normally within 300 m of the heat source (in this case the sewer pipes) and heat pump circulation system.
The wastewater/effluent in the sewer flows undisturbed onward on its normal path.
By utilising the ‘wastewater’ in existing sewers this way it is possible to reduce the need for the buildings on the system to use ‘new’ additional fossil fuel energy to maintain good working environments for their workforces, students etc or to heat water for production process etc, so ultimately reducing the need for carbon-loaded energy use.
It should perhaps be pointed out that the heat recovery system does not produced new electricity but simply transfers what would be ‘waste’ heat back into the system for reuse.
It can be termed ‘heat recycling’ due to maximisation of and increase in the effectiveness of the power or fossil fuel energy which was initially used to heat the water which was then introduced into the sewer system with a higher temperature.
A circular sewer pipe during installation. (Source: Uhrig Kanaltechnik GmbH)
A completed installation in an egg-shaped sewer pipe. (Source: Uhrig Kanaltechnik GmbH)
The Therm-Liner heat recovery elements are designed and shaped to fit the most commonly used pipeline formats including circular, egg-shaped and other shapes as necessary ideally starting at a nominal diameter of 800 mm, but there are solutions for smaller diameter pipes also.
The installation can be carried out through most, if not all, existing manhole structures and designs, normally eliminating any need to excavate or to remove the ‘biscuit’ for access to the sewer pipe.
The stainless steel units are normally 1 m long and can be interconnected during installation. This allows a flexible approach to the various projects and insures the possibility of an extension or removal of Therm Liner Elements. It is possible at any time to adopt and to suit changing client needs or a later dynamic increase in client energy demands.
As the Therm-Liner unit is installed in the sewer flow, heat extraction fluid is passed through pipes which form part of the Therm-Liner system. These pipes have very high heat transfer properties which allow the heat from the effluent to pass into the circulating heat recovery fluid raising its temperature.
This fluid is circulated around the heat exchanger by low-power pumps in the ‘transfer station’ on surface. In the transfer station, a heat exchanger uses this recovered heat to warm water which is passed into a local centralised heating system at temperatures of between 40 and 60oC. This now hot water is passed to buildings on the system where it is fed directly into their normal central heating systems.
Because the Therm-Liner elements are installed in the sewer pipe invert, they are always immersed in the sewer flows and recover heat even when flows are low. Specific heat recovery figures will depend on the sewer into which the system is placed and its flow characteristics but tests on the system have shown, for example, that in a 1200/1800 egg-shaped sewer with an average flow of 8.3 l/sec with just 39 m of Therm-Liner heat exchange units in place, the local area heating system saved around effective 43% of its ‘new’ energy usage which in turn lead to a 60% carbon dioxide reduction from the fuels normally used.
The pipe work arrangement for an installation in a kindergarten. (Source: Uhrig Kanaltechnik GmbH)
Commenting on the Therm-Liner system Brian Hickland, International Sales Manager for Uhrig said: “Having used this system in Europe, including Germany, France, Austria, Switzerland and other countries, for some time now the data feedback from the various installed and functional projects confirm that the heat energy recovery from sewer flows is not only possible but also very cost effective method. Within the target markets of council-run housing estates, multiple dwellings, municipal buildings (schools, hospitals offices public baths etc), industrial sites and sports facilities, the potential for energy savings in terms of both power used and cost savings are immense.”
Brian went on to say: “The system is very quick and easy to install in existing sewer pipes, once personnel have been trained correctly, which means minimal lead time and early results on reducing energy consumption from more conventional means. Taking into account we have hundreds of miles of sewers running under our Cities a future low cost energy source is literally running under our feet and is already at our door step. So, let’s not waste time but utilise this low cost source of future energy.”
The following is a list of Towns and Cities and some of the projects completed using the Therm-Liner system:
Location Project
Dresden Town hall
Furth Town Hall
Berlin University Children’s nursery
Rauenberg Children’s nursery
Bretten Primary School
Bad Cannstatt
Pinneberg Town Hall
Munich Work Sheds
Speyer Yacht harbour
Tübingen School
Dereteinfurt Fire Station House
Berlin Sport Hall
Redilsheim Private House

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