Assessing the techno-economic impact of low-temperature subnets in conventional district heating networks

The 4th generation Low-Temperature District Heating (LTDH) is envisioned as a more efficient and environmentally friendly solution to provide heating services to the building stock. Specifically, in countries already with a large share of well-established DH systems, conventional DH and LTDH technologies will be operating simultaneously in the near future. Newly built or refurbished buildings have lower heat demands, which in combination with LTDH brings potential savings compared to conventional DH. This work explores the advantages in DH operation by connecting these loads via LTDH subnets to a conventional DH system, supplied by a Combined Heat and Power (CHP) plant. A techno-economic analysis was performed, through modelling and simulation, by estimating the annual DH operating costs and revenues achieved by the reduction in return temperatures that LTDH would bring. The savings are related to: (1) the reduction in distribution heat losses in the return pipe; and (2) lower pumping power demand. Likewise, additional revenues are assessed from: (3) improved Power-to-Heat ratio for electricity production; and (4) enhanced heat recovery through Flue Gas Condensation (FGC). The annual savings per kWh of delivered heat are estimated as a function of the penetration percentage of 'energy efficient' loads over the conventional DH network. Key outcomes show the trade-offs between the potential savings in operating costs and the reduction in heat demand: relative losses in this scenario are maintained at 13.1% compared to 15.3% expected with conventional DH; and relative pumping power demand decreased as well. In other words, the costs of supplying heat decrease, even though the total heat supplied is less.