The usage of shallow groundwater heat by heat pumps in a high- temperature district heating system

Abstract

The general tendency in district heating (DH) is the lowering of grid temperatures. To achieve this, heat pumps (HPs) are necessary technology that has an important role in very low (below 60 ⁰C) and ultra-low (below 45 ⁰C) DH supply temperatures. A significant variation in outdoor temperatures during the heating season in the climate of Serbia makes the implementation of HPs in the high-temperature DH systems possible. The goal of the paper is to analyze the implementation of HPs that extract heat from the shallow groundwater to satisfy a part of heat requirements in a boiler room of a DH system. This is a superheated water system with a nominal capacity of 37.728 MW and a nominal temperature regime 130/75⁰C that belongs to the PUC Toplana in Kraljevo. The city is placed at an alluvial plane rich with shallow groundwater with temperatures in the range of 12 to 18⁰C. The analysis is carried with two operating modes: (i) present, with an everyday stoppage between 9 PM and 5 AM, and (ii) continuous operation with limited overnight heating. Not to endanger the inflow, the maximal extraction of the heat from groundwater of 755 kW was assumed. The present operating mode would have seasonal COP 3.87 and negative economic indicators, whereas the continuous operation would lead to a sustainable project with a financial rate of return of 10.5%, benefit-cost ratio of 1.192, and the reduction of CO2 emissions of 201 t/year. The seasonal COP of the HPs would be 4.29.