Researchers at the École Polytechnique Fédérale de Lausanne (EPFL), in Switzerland, have demonstrated that a heating and cooling distribution network using carbon dioxide (CO2) could lead to over 80% in final energy savings.

In Europe, the building sector accounts for 40% of final energy consumption and around one-third of greenhouse gas emissions. Switzerland is no exception. And in cities like Geneva and Lausanne, there are many types of buildings with diverse heating and cooling needs.

For example, to achieve a comfortable level of heat in winter, the temperature of water circulating through an old radiator must be at least 50 degrees Celsius, while floor heating requires a temperature of only 30 degrees. In the summer, supermarket air conditioners run at full blast—giving off heat at the same time—while just steps away a gas boiler heats the water that is piped to all the floors of the building. In sum: a huge waste of energy.

Samuel Henchoz, who is part of EPFL's Industrial Process and Energy Systems Engineering Group, analyzed a new concept based on a saturated liquid refrigerant—in this case pressurized CO2—circulating between buildings. As in a refrigerator, liquid CO2 evaporates in a heat exchanger to provide cooling. To produce heat, it does the opposite: CO2 gas releases heat in the heat exchanger.

In Henchoz’s system, CO2 circulates at a temperature of around 15 degrees, close to its saturation point between the liquid and gaseous states. This is also close to the temperature of soil underground, which limits the need to insulate the pipes. Because there is no risk of freezing, the pipes could even be installed under sidewalks. The pipes themselves could be much smaller in diameter than those in a water-based network, since the gas is pressurized and has a high energy density.

Another advantage to this system is that the liquid refrigerant recovers the heat given off by coolers along its path, reducing the amount of heat needed from district heating plants. While the concept is appealing on a theoretical level, Henchoz set out to determine whether using CO2 as a liquid refrigerant in urban areas would be safe, reliable, efficient and economical. Focusing on a neighborhood in the center of Geneva, he compared the energy performance and economic profitability of five variants of fluid refrigerant networks, a cold-water network and the mix of energy conversion systems that is currently in use.

The researchers simulated the network in Geneva's commercial area. Image credit: ©SITG.He found that the current system, which uses both boilers and ordinary cooling units, is the least efficient. All the variants that he examined would generate more than 80% in final energy savings in comparison. The most promising variant uses CO2 as the transfer fluid and a CO2 heat pump to regulate the temperature.

The initial cost would be somewhere between 27 and 35 million francs for the neighborhood that he analyzed and would pay for itself only four to six years after going online. The second best option, although more expensive, is a cold-water network, whose advantages include safety and the availability of components.

"Unlike synthetic liquid refrigerants, CO2 is natural, inexpensive, non-flammable and non-toxic,” says Henchoz. “The only catch is that it has to circulate under 50 bars of pressure, which is totally different and would have to be taken into account in the official standards.”

With financing from Switzerland's Commission for Technology and Innovation, Henchoz created and tested a small-scale liquid refrigerant network in collaboration with Geneva's utility company and the engineering firm Amstein + Walthert. The tests were aimed primarily at demonstrating the concept’s feasibility. The system did indeed perform as expected, at the same time validating the concept of automatic control.

“The tests boosted our confidence in the practical aspects of a CO2 network,” says Henchoz.