Towards a water-wise world Annual report 2014

Almere case study shows possibilities  

Using heat and cold from drinking water and sewers

19 May 2014BTO

The greatest portion of the energy used in household water consumption is employed in heating water. The water companies have asked KWR to research the possibilities of recovering the thermal energy (heat and cold) contained in drinking water and wastewater. The possibilities are very dependent on local conditions; KWR has developed a model to research the possibilities and applied it to the town of Almere.

Within the water sector’s joint research programme (BTO), KWR studies the possibilities of using the thermal energy (heat and cold) contained in drinking water and wastewater. An important driving force behind the initiative is society’s need for greater sustainability and energy efficiency. Moreover, the use of thermal energy from pipes perhaps gives water companies the opportunity to offer new services for their clients.
The possibilities of using thermal energy from pipes are very dependent on local conditions. KWR has developed models to identify these conditions for each location – including the physical location of the sites where the energy can be recovered and used. This is important, because the feasibility of heat and cold recovery from drinking water or sewer pipes is very dependent on there being a short distance between the energy’s recovery and its use locations. In addition, the supply of energy must be sufficiently large and continuous.

Almere case study

These conclusions were drawn from the case study of the town of Almere, among others. Since wastewater’s temperature decreases rapidly during transport, its thermal energy is of greatest interest at its source – for example through the use of shower heat exchangers. The recoverable thermal energy in drinking water varies less than in the case of wastewater, which makes the use of drinking water for cooling and heating more interesting. In addition, energy is easier to recover from drinking water: the quality of drinking water is far superior to that of wastewater, so that heat exchangers can be used more efficiently, and, furthermore, they don’t become so quickly fouled. In this way, the available temperature difference in the case of drinking water pipes can – technically speaking – be exploited to the maximum.

Microbial risks

When heat is extracted from raw water, purified water or drinking water, the water’s temperature naturally drops. This involves no risks. But when this water is used for cooling purposes, its temperature rises. In order to prevent the regrowth of bacteria such as Legionella, the law establishes that the temperature of drinking water at the tap cannot exceed 25°C. This microbial risk has to be taken into account when heat is added to drinking water. But the soil around the drinking water pipes, between where it is used for cooling purposes and the client’s tap, also has a strong influence on the water’s temperature. In 2015, an in-depth study will be started and focus essentially on establishing the microbial risks connected to extracting cold from drinking water, by monitoring practical situations.

Financial risks

The financial risks associated with recovering thermal energy from the water cycle are related to 4 issues: uncertainty in the thermal source (the supply has to be sufficiently large and continuous); uncertainty on the part of the client; the technical possibilities; and the long-term evolution of energy prices. By using the Total Cost of Ownership method, one can effectively determine the financial risks on a case-by-case basis.

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