Optimal Integration Of Heat-Pump And Solar Thermal Energy In The Pre-heating Loop Of Wood And Gas Boiler Based District Heating System

TL;DR

This paper develops an MILP model to optimize the integration of solar thermal energy and heat pumps in district heating systems, balancing economic, environmental, and operational factors to improve system performance.

Contribution

It introduces a multi-criteria optimization framework with temperature discretization and dynamic modeling, reducing solution time and enhancing integration strategies.

Findings

Solar integration up to 11,932 m² increases reliance on solar energy.

Including wood boilers reduces solar dependency and lowers costs.

Higher carbon taxes promote solar adoption but face storage challenges.

Abstract

The integration of renewable sources is essential for decarbonizing heat production in district energy networks. Beyond biomass-based solutions, solar thermal energy, with or without heat pumps, presents a significant opportunity. However, system performance is highly dependent on outdoor and setpoint temperatures. This study aims to optimize system design using a multi-criteria approach that considers techno-economic and environmental (CO2) factors. A Mixed-Integer Linear Programming (MILP) model is developed, incorporating temperature discretization for problem linearization and capturing key dynamic characteristics of heat generators. The model improves convergence, reducing a 19% MIP gap in 26 hours to 10% in 12 hours by dissipating 6% excess solar heat. A multi-scenario analysis under two carbon taxation levels and different CO2 emission cases revealed solar integration up to 11,932 m${}^2$ but increased gas reliance (50%) and TES losses (49%). Wood boiler inclusion reduced solar dependency, covering 45% of heat, lowered LCOH, but limited renewable penetration. Higher carbon taxes boosted solar adoption but faced storage inefficiencies, while biomass enhanced cost efficiency and system stability.