Integrating a heat pump into a 4th generation district heating (4GDH) system – Two-mode configuration inputting operational data

Abstract

The aim is to examine integrating a heat pump (HP) into a district heating (DH) system versus potential savings in the separate scenario. It takes into account all the properties of both DH system and a HP operation, and gives the understanding of temperature, pressure and enthalpy of refrigerant at each cycle point. Contribution to the pool of knowledge is considering a novel configuration, which allows to get rid of a heat exchanger between the water-source HP and the network. Scenarios of (i) separate operation of a HP, and (ii) integrating it into a DH system and inputting design data only, or (iii) utilizing operational data have been analyzed. Multiple thermal simulations have been performed. We also allow traditional coverage of heat demand with the help of a supply line of high-temperature DH (HTDH), thus elaborated what happens when the outdoor air temperature drops below lower threshold. Hence, the model identifies two stages of operating a HP integrated into a DH system. Starting from it, the temperature of the supply water should be 50 °C and above, so it is more effective to use supply water directly until the outdoor temperature rises again. This outdoor temperature implies change of operational mode, which leads to the following results. On cold winter days, the available amount of heat is lower than the energy consumption, therefore, energy from a supply line of HTDH is utilized, which gives overall energy saving effect of about 14 %. Mass flow rates through the configuration suggested are identical during moderate-cold fall and spring days, since the control logic is established with the help of variation of supply temperature only. Single HP has the lowest efficiency because of high temperatures needed: with a nominal temperature difference of 25 °C, the coefficient of performance (COP) rises to 5.0 only. At the break-even point, HP in each configuration has COP ranging between 3.51 and 4.43, while the rest of the time COP is just above 2. In different cases, the lowest threshold of HP's performance is 2.4, 2.6, and 2.89, respectively. The DH-assisted scenario, where peak energy demand was covered by DH system directly, clearly outperformed separate installation, where a HP was the only thing responsible to provide the necessary supply temperatures. For this case, lower COP stipulates less energy is supplied from the DH system, and therefore a lower share of consumption is supplied directly from supply line, which decreases the overall seasonal COP. Scenario (iii) of inputting DH operational data has the highest seasonal COPs, reasoned to higher than design return temperatures. Given the low performance of a HP raising water temperature to 55 °C (the lowest threshold acceptable in winter) a single HP with no backup capacity accounts for 23 % more energy use. Supported by a coal-fired combined heat-and-power plant, distribution of primary energy consumption is more favorable, i.e. coal-fired generation covers the peak. The positive effect is expanded by converting a part of heat demand from HP operation to cheaper HTDH operation.