Performance and Profitability Perspectives of a Co2 Based District Energy Network in Geneva’s City Center

A new type of district energy network capable of providing simultaneously heating and cooling is being investigated. It is based on the use of CO2 as a heat transfer fluid by taking advantage of the latent heat of vaporization, to store and transfer heat across the network. The goal of the present study is to determine the performance of a CO2 network when applied to a real urban area. It focuses first on determining the requirements for the various thermal energy services for a part of Geneva’s city centre. The energy consumption is first computed for the energy conversion technologies now in place in this area namely fuel boilers and vapour compression chillers. Then the new energy consumption is computed if a CO2 network were used instead of the existing technology. Finally a profitability analysis of the CO2 network variant is done accounting for investment, energy purchasing, equipments replacement, operation, and maintenance costs. For an interest rate of 6% and a price of the delivered heating/cooling energy at 0.13 CHF per kWh, a net present value of 82.8 million CHF after 40 years is achieved, while the break-even is reached after 5 years of operation. INTRODUCTION District heating and cooling networks have been used to deliver energy in urban areas for many decades. Generally, these networks rely on centralized and efficient energy conversion technologies supplying heating and/or cooling to the users through a water loop. In most of the cases, the supply temperature of such a network is selected according to the most demanding consumer connected. Thus all the other users are supplied at a temperature beyond their need – often far beyond their need. Furthermore, when heating and cooling have to be supplied, two independent water loops are needed. Finally, most of the time, heat discharged by the cooling users in the district cooling network is not transferred to the district heating network, and thus not recovered. A new type of district energy network is being investigated. It is based on the use of CO2 as a heat transfer fluid. It uses the latent heat of vaporization, instead of sensible heat, to store and transfer heat. The pressure of the network is selected such that evaporation/condensation takes place at around 12 to 14 °C. This level of temperature allows free cooling to be used for most of the cooling applications, while decentralized heat pumps transfer heat, from the network to each heating user, at the required temperature level. Furthermore only two pipes are required by the network, thus allowing the recovery of waste heat from the cooling users to be recovered by the heating users. Obviously, the heat required by the heating users may, most of the time, not be strictly equal to the waste heat discharged by the cooling users. Hence, a central plant is needed such that the required amount of heat can be taken from/released into the environment. Fig 1 provides a schematic view of a CO2 network. A more detailed description of the concept is done by Weber and Favrat in [2]. A presentation of a list of possible energy services and associated conversion technologies that can be included in a CO2 network was done by Henchoz et al. in [4]. Fig. 1 Schematic representation of a CO2 based district energy network. DHC13, the 13 th International Symposium on District Heating and Cooling September 3 rd to September 4 th , 2012, Copenhagen, Denmark AREA STUDIED It was chosen to study the area of “Rues Basses” in Geneva. Data such as the energy reference area (ERA) and buildings affectation was made available to the authors by Geneva’s cantonal office for energy [12]. Fig 2 shows a map of the area studied. The area was divided in 32 groups of buildings for which the 3 possible affectations were considered:  Commercial buildings