District Power-To-Heat/Cool Complemented by 2 Sewage Heat Recovery

: District heating and cooling (DHC), when combined with waste or renewable energy sources, is an environmentally sound alternative to individual heating and cooling systems in 18 buildings. In this work, the theoretical energy and economic performances of a DHC network 19 complemented by compression heat pump and sewage heat exchanger are assessed through 20 dynamic, year-round energy simulations. The proposed system comprises also a water storage and 21 a PV plant. The study stems from the operational experience on a DHC network in Budapest, in 22 which a new sewage heat recovery system is in place and provided the experimental base for 23 assessing main operational parameters of the sewage heat exchanger, like effectiveness, parasitic energy consumption and impact of cleaning. The energy and economic potential is explored for a 25 commercial district in Italy. It is found that the overall seasonal COP and EER are 3.10 and 3.64, 26 while the seasonal COP and EER of the heat pump alone achieve 3.74 and 4.03, respectively. The 27 economic feasibility is investigated by means of the levelized cost of heating and cooling (LCOHC). With an overall LCOHC between 79.1 and 89.9 €/MWh, the proposed system can be an attractive solution with respect to individual heat pumps.


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The electricity of the PV system is used to drive the system pumps and the heat pump, in 118 conjunction with the electricity purchased from the grid. If PV electricity is generated in excess, the 119 surplus is supplied to the electricity grid. The remuneration of the electricity supplied to the grid is 120 determined according to the net metering regulation currently in force [14], which allows selling at a 121 tariff constituted by the sum of the wholesale electricity price and a contribution related to the system 122 charges and general transmission and distribution costs. In the following, the mathematical models of the main system components are presented. The In conclusion, a differential equation for is obtained in the form: Since under the current simplifying assumptions ̇∝ √Δ , the following expressions for 189 ̇ can be derived: where and = 1/( √Δ ) are identified experimentally (see Figure 6) and their values are 191 provided in Table A2.

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The result can be generalized to the case of variable pressure drop, assuming constancy of , , = ̇ , where = Δ /ΔP. Expressions (7) and (8)     The district heating plant must be sized according to both heating and cooling loads, therefore 220 suitable heating and cooling hourly profiles must be generated using a building energy model. The two-node capacitive building model is shown in Figure 7, whose states are room air temperature ( )

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Based on outdoor ambient temperature and radiation on the horizontal plane, the model where , is the cumulated heat delivered by the condenser in heating mode operation and is 250 the associated electrical consumption of the compressor.

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2. Heat pump seasonal EER: where , is the cumulated cool delivered by the evaporator in heating mode operation and is 253 the associated electrical consumption of the compressor.

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Concerning sewage water, its temperature is subject to seasonal variation due to the influence

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The individual heat pumps are assumed to deliver hot water at 50 °C for heating and chilled

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The base values of the main design parameters are reported in Table 1.

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The first optimization step (see Figure 13) consists in decreasing heat pump capacity and 387 simultaneously increasing storage volume. With a reduced capacity, the heat pump will work at full

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It is observed that, although all the different configurations are cost competitive with respect to 396 the reference system, the LCOHC increases with decreasing heat pump capacity because the benefit 397 deriving from a smaller heat pump is offset by the cost of a larger storage.

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The second step concerns the optimization of the size of the SHX (see Figure 14). Increasing its 399 size while setting the same limiting flow passage area (in absolute terms) has an influence on the 400 frequency of cleaning operations, since in a larger SHX that processes an equal amount of water the 401 operating time needed to reach the critical flow passage area is prolonged. This lowers maintenance 402 cost but also increases investment cost. In fact, cleaning maintenance decreases and LCOHC 403 increases, showing that more frequent cleaning is always more economical than larger SHX size.

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However, it is also noticed that a larger SHX provides better energy performance with an increased 405 system COP. This is because the heat pump is forced to operate in partial load whenever the sewage 406 flow rate is reduced by effect of the SHX degradation. Therefore, as a best compromise between economic performance and risks associated to frequent cleaning, an optimal SHX size is selected at Lastly, the contribution of PV is considered by progressively increasing the installed peak

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The final configuration of the system is reported in Table 2.

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The behaviour of the system is investigated also when the SHX is fouled and the sewage water 438 flow rate is reduced with respect to its nominal value (see Figure 16). The heat pump is forced to 439 operate in partial load for the whole day to keep the chilled water in the heat pump evaporator 440 sufficiently far from its freezing point. In this critical condition, storage thermal capacity is essential 441 in order to cover the peak heating load.

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When the SHX is fouled, the sewage water flow rate is reduced with respect to its nominal value 457 (see Figure 18) and the heat pump is forced to operate in partial load even when the cooling load is 458 at its peak value to limit the temperature lift between evaporator and condenser. This strategy allows 459 maintaining a good energy performance while limiting the temperature increase of the sewage.

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However, the analysis has also shown that the sewage heat recovery system is a rather expensive 513 technology, and margin should exist to lower the associated investment costs and foster the storage that allows maintaining cleaning at a manageable, cost-effective frequency must be pursued