In cities, the demand for heating and cooling varies between districts, but also between buildings in the same district. Ever more buildings require cooling for their operations, and the influx of people to cities drives an increasing demand for heating. On top of this, it is not uncommon to find adjacent buildings where one is cooled and the other is heated – at the same time. The 5th generation compact district heating and cooling networks proposes a solution with heat pumps, air conditioning and a heat sink offering synergy between users and allowing the concept of a city without chimneys or cooling towers on every other building.

Using CO₂ as refrigerant and transfer fluid
One specific concept uses the latent heat of CO₂ operating as both transfer fluid and refrigerant in the heat pumps. This allows for a compact network operating at very low temperatures: 10 to 16 °C. The concept includes one pipe for saturated liquid CO₂ and one pipe for saturated vapour CO₂. Cooling is provided when CO₂ is taken from the liquid pipe and evaporated into the vapour pipe. The other way around, heating is provided when the vaporised CO₂ is condensed by a heat pump. Thus, depending on the actual need for heating and/or cooling, the flows in the pipes goes in either direction and the CO₂ is vaporised or condensed.

A system like this comes with both benefits and challenges. On the plus side is the fact that CO₂ will not freeze and thus no particular minimum depth is needed for the pipes, and insulation can also be minimized. Also, the size of the pipes is decreased to such an extent that they are easily installed in the ground or in existing utility channels. On the minus side is the high pressure required: in the order of 50 bars. Furthermore, there is a potential risk of CO₂ leakage. In the scaled-down lab facility in Geneva the tests run so far have not indicated any major operational difficulties such as strong pressure surges or two-phase flow instabilities in the main network.

Potential for decreased need for final energy
The implications for the future are very interesting. Theoretical studies of a real district in Geneva show that up to 80 % of the final energy can be saved in urban areas. And this at a lower cost than for conventional technologies. The 5th generation compact district heating and cooling networks opens for synergies between users and providers of heat. If also PV panels are installed, it allows to design districts with nearly zero emissions.

Samuel Henchoz, Daniel Favrat, Luc Girardin, Francois Marechal
Ecole Polytechnique Federale de Lausanne, Switzerland
The text is shortened by HPC

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