At present, this technical solution is legally protected in the Czech Republic territory as a patent and an utility model. The whole heat circuit system has two basic parts: a heat pump and an exchanger situated in the sewer.

Figure 1: Schematic diagram of the exchanger
Bytový dům - Block of flats, Stoka - Sewer

The heat pump is installed in a circuit for taking off heat from the exchanger and increasing temperatures to the values used as standard in the technical systems of building services. The fundamental component of the whole system is the sewer exchanger. The exchanger has an arched shape so it can be easily positioned in the sewer. The length of the manufactured prototype is 2 m and it is designed to be located in circular sewerage with a diameter of 500 mm. The dimensions of the exchanger may differ, of course, according to the size of a particular sewer.

One important condition for the exchanger to function properly is a sufficient flow of waste water. Therefore the system is more suited for use in cities, where sewers are sufficiently long for waste water to be heated by the Earth’s mass before it arrives at the waste water treatment plant. It is very important that waste water has not been overly cooled before the cleaning process. Low temperatures would slow down the cleaning process in the waste water treatment plant.

In an experiment which was conducted after a numerical simulation, waste water was cooled by only 1 K. With such a difference in temperatures and a flow-rate of 800 l/h, a heat output of 3.1 kW was achieved. In the exchanger, water was heated from 2 °C to 5 °C and after flowing through the heat pump up to 65 °C. Such obtained heat, in combination with an appropriate reserve tank, can be used in virtually any way (heating warm water, heating, heating air in air-conditioning units etc.).

Title figure: Simulation of temperature course in a sewer exchanger