Combined heating and cooling networks with waste heat recovery based on energy hub concept

Highlights: • Multi-generation heat pump systems provide savings in heating/cooling networks. • Optimal heat pump size based on cost is ∼80% of the maximal thermodynamic limit. • Putting emphasis on GHG emissions does not change the optimal use of heat pumps. • To reduce cost and GHG emissions, electricity outperforms natural gas for heating. -- Abstract: Waste heat recovery can help reducing operation costs and greenhouse gas emissions. In the present work, an "energy hub" template was employed to design combined heating and cooling networks in which heat pumps can be used to recover heat from the cooling loop and supply it to the heating loop. Heating and cooling loads of the network can be satisfied by natural gas boilers, electric boilers, chillers, and heat pumps. The design of the system and its operation over the year were optimized with respect to cost and greenhouse gas emissions under different combinations of heating and cooling loads. The introduction of 8760-h synthetic loads allowed covering several possible load profiles driving the energy hub. The contribution of each possible energy source and technology and the sizing of the heat pump system are optimized, while ensuring satisfaction of the heating and cooling demands. The optimized hub configurations for scenarios with and without waste heat recovery were compared, showing that heat pumps were beneficial in all scenarios. The optimal capacity of heat pumps to minimize total cost was found to be ∼80% of the maximal possible value from a thermodynamic analysis of the loads. The simultaneous minimization of cost and emissions revealed a relatively sharp transition from gas to electric heating as more emphasis is put on emissions than cost, but in all cases, waste heat recovery with heat pumps was heavily used to satisfy the heating and cooling loads.