Abstract
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 buildings. In this work, the theoretical energy and economic performances of a DHC network complemented by compression heat pump and sewage heat exchanger are assessed through dynamic, year-round energy simulations. The proposed system comprises also a water storage and a PV plant. The study stems from the operational experience on a DHC network in Budapest, in which a new sewage heat recovery system is in place and provided the experimental base for 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 commercial district in Italy. It is found that the overall seasonal COP and EER are 3.10 and 3.64, while the seasonal COP and EER of the heat pump alone achieve 3.74 and 4.03, respectively. The 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.
Highlights
District heating and cooling (DHC) is considered more efficient than individual, distributed systems for heating and cooling, especially because DHC solutions can benefit from locally available, low-cost energy sources, like environmental heat and cool, industrial waste heat and solid waste incineration [1], but DHC, as a heat/cool demand aggregator with thermal storage capacity, can offer flexibility in managing energy demand
Annual benefits (B) include the sales at wholesale price (WP) of PV electricity generated in excess and the contribution related to avoided cost of transmission and distribution (CU), calculated according to the following expression [14]: B = min PUN · E purch ; WP· Esold + CU ·min E purch ; Esold where the valorization of the electricity sold to the grid (Esold ) is capped by the electricity purchased (E purch ) and valorized at the national average selling price (PUN)
When the sewage heat exchanger (SHX) is fouled, the sewage water flow rate is reduced with respect to its nominal value and the heat pump is forced to operate in partial load even when the cooling load is at its peak value to limit the temperature lift between evaporator and condenser
Summary
District heating and cooling (DHC) is considered more efficient than individual, distributed systems for heating and cooling, especially because DHC solutions can benefit from locally available, low-cost energy sources, like environmental heat and cool, industrial waste heat and solid waste incineration [1], but DHC, as a heat/cool demand aggregator with thermal storage capacity, can offer flexibility in managing energy demand. With respect to sewage potential and availability, it is worth noticing that Italy is characterized by a large per capita water consumption, 175 liter/day/person [12] Such a system would offer the following main advantages: (1) ability to exploit the increasing share of renewable electricity in the electricity mix, which implies lower CO2 emissions as compared to gas boilers for heating; (2) ability to purchase electricity on the wholesale market at competitive tariffs, as compared to the average electricity consumer; (3) zero local emissions of air pollutants (e.g., PM, NOx), as compared to gas boilers and DH conversion systems relying on combustion (e.g., CHP); (4) possibility to exploit PV electricity generated on-site by a large capacity PV plant that can benefit from economies of scale. A detailed mathematical model of the system is built and simulated in Trnsys [13]
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