Abstract
This study investigated a double loop network operated with ultra-low supply/return temperatures of 45/25 °C as a novel solution for low heat-density areas in Denmark and compared the proposed concept with a typical tree network and with individual heat pumps to each end-users rather than district networks. It is a pump-driven system, where the separate circulation of supply and return flow increased the flexibility of the system to integrate and displace heating and cooling energy along the network. Despite the increased use of central and local water pumps to operate and control the system, the simulated overall pump energy consumption was 0.9% of the total energy consumption. This was also an advantage at the design stage as the larger pressure gradient, up to 570 Pa/m, allowed minimal pipe diameters. In addition, the authors proposed the installation of electrically heated vacuum-insulated micro tanks of 10 L on the primary side of each building substation as a supplementary heating solution to meet the comfort and hygiene requirements for domestic hot water (DHW). This, combined with supply water circulation in the loop network, served as a technical solution to remove the need for bypass valves during summer periods with no load in the network. The proposed double loop system reduced distribution heat losses from 19% to 12% of the total energy consumption and decreased average return temperatures from 33 °C to 23 °C compared to the tree network. While excess heat recovery can be limited due to hydraulic issues in tree networks, the study investigated the double loop concept for scenarios with heat source temperatures of 30 °C and 45 °C. The double loop network was cost-competitive when considering the required capital and operating costs. Furthermore, district networks outperformed individual heat pump solutions for low-heat density areas when waste heat was available locally. Finally, although few in Denmark envisage residential cooling as a priority, this study investigated the potential of embedding heating and cooling in the same infrastructure. It found that the return line could deliver cold water to the end-users and that the maximum cooling power was 1.4 kW to each end-user, which corresponded to 47% of the total peak heat demand used to dimension the double loop network.
Highlights
The building sector is responsible for 40% of the total energy demand in Europe [1] and as part of the Energy Efficiency Directive (EED 2018) and Energy Performance of Buildings Directive (EPBD) [2,3], the mitigation of greenhouse gas emissions and the reduction of energy consumption from buildings must be a cornerstone of every climate change strategy in the member states
The electric heaters in the micro tanks had the effect of increasing the use of electricity for domestic hot water (DHW) preparation, as this was a function of the set point temperature in the primary side, which had an impact on the operating costs and economy of the double loop network
The combination of optimized design and lower operating temperatures led to distribution heat losses of 12% of the total final energy consumption for the investigated double-loop networks compared to 19% for the tree network
Summary
The building sector is responsible for 40% of the total energy demand in Europe [1] and as part of the Energy Efficiency Directive (EED 2018) and Energy Performance of Buildings Directive (EPBD) [2,3], the mitigation of greenhouse gas emissions and the reduction of energy consumption from buildings must be a cornerstone of every climate change strategy in the member states. District heating and cooling (DHC) is recognized as one of the key technologies in the transition towards a fossil-fuel-free energy system. The competitiveness of DHC has been related to the capacity of using low-grade or renewable energy sources—e.g., heat recovery from CHP or deep-water cooling from seas. This ensures the security of supply, higher flexibility, and competitive energy prices compared to individual heating and cooling solutions [4,5]. The technological challenge, considering the future fossil-fuel-free energy system, is that the DHC infrastructure needs to be integrated with the electricity grid and transportation sector to efficiently exploit local energy sources and to further increase the flexibility and security of the energy supply due to the intermittent nature of renewable sources [6]. The Copenhagen DC network showed the possibility to reduce the energy consumption and CO2 emissions by 40% and 70% respectively compared to a scenario using air-conditioners, and it is expected that DC will be fast-growing in both temperate and warm climate countries in the coming years [17,18]
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