Potential of polymer sewage heat exchangers with enhanced thermal properties

Abstract

Buildings contribute to 30% of global CO₂ emissions and consume 40% of the global energy supply (Yang et al., 2014). Heating and cooling requirements of the buildings form the major part of the energy consumption in buildings (Culha et al., 2015). Thus to solve this problem, one of the solutions currently being
looked at is to recover heat from the sewage. Cities have large sewage flows and in the winter the sewage is warmer than ambient and in the summer it is colder than the ambient, thus making it a good heat source and sink respectively. This work involves the integration of waste water heat exchanger with heat pump
to form a Waste Water Source Heat Pump (WWSHP). This system was further integrated with Aquifer Thermal Energy Storage (ATES) system. The WWSHP system was modeled in Matlab and the aquifer was modeled in COMSOL. COMSOL Live-link Matlab feature was used to integrate the two models.
Polymers were chosen as heat exchanger material due to their low cost, low weight (lower CO₂ emissions during transportation), flexibility, non corrosive nature and low energy requirement in manufacturing (Hussain et al., 2017). Two systems were proposed to support the heating and cooling demands of the concert venue and convention centre of the Rotterdam, the ’Doelen’. The objective of this work was to illustrate the potential of polymer sewage heat exchangers. The first system was called the WWSHP system. In this system, heat was recovered from the sewage in the winter through polymer heat exchangers and was upgraded in a heat pump for use in the heating network of the ’Doelen’. The heat pump was a reversible one, thus, in the summer, heat was extracted from the cooling network of the ’Doelen’ and rejected to the sewage through the same polymer heat exchangers. To obtain more heat in the winter, a second system was proposed. This system was called WWSHP + ATES system. In this system, heat was extracted from the sewage and an aquifer. This extracted heat was upgraded in a heat pump for supply to the ’Doelen’. In the summer, the heat extracted from the ’Doelen’ along with the heat recovered from the sewage were used to refill the warm well of the aquifer to maintain thermal balance. The scope of the work also included optimizing the dimensions, material and cost of the waste water heat exchanger. In both the systems, the summer and the winter models were different, hence they were simulated separately. The heat recovery model was built based on a sewage channel near the ’Doelen’. The sewage channel data and the sewage flow and temperature data were provided by the Gemeente of Rotterdam. The waste wa-ter heat exchanger was chosen to be a multi row tube polymer heat exchanger. Various polymer options were available, among which the option with the highest thermal conductivity, High Density Poly-ethylene (HDPE) was chosen. Among six combinations of standard HDPE tube lengths and diameters, tube length of 30 m and tube inner diameter of 29 mm were found to be the most optimum in terms of economics and heat recovery. Based on the optimized tube dimensions, heat delivered by the system to the ’Doelen’ per unit cost was compared for different materials and the results confirmed that HDPE with a cost of 0.54 €/kg was the best choice. Thus, using the optimized combination of tube dimensions and HDPE as tube material, 374 MWh of heat was recovered from the sewage in the winter and 486 MWh of heat was supplied to the heating network of the ’Doelen’ through the heat pump. In the summer, 23 MWh was removed from the ’Doelen’ by the heat pump and 26 MWh was rejected to the sewage using the same HDPE heat exchangers. Among the different polymer and filler combinations, PE (Polyethylene) with 30% graphite filler was foundto be the best choice. Using PE with 30% graphite resulted in 32% higher heat recovery from the sewage in the winter and 15% higher heat rejection to the sewage in the summer when compared to HDPE with no fillers. Thermal enhancement of polymer tubes, although increased the amount of heat exchanged with the sewage in the winter and the summer, it reduced the system economic performance (kWh/€) in the winter.

The WWSHP + ATES system supplied 1244 MWh of heat to the ’Doelen’ in the winter and removed 388 MWh of heat from the ’Doelen’ in summer. Furthermore, thermal enhancement of polymers of the waste water heat exchangers reduced the performance (kWh/€) of the WWSHP+ATES system in both the summer and the winter. WWSHP + ATES system proved to be capable of handling higher heating and cooling demand than the WWSHP system. The costs of heat exchangers and electricity were also much higher for this option, thus making it less economical. For instance, the WWSHP model supplied 66 kWh to the ’Doelen’ per € spent, as opposed to the WWSHP + ATES system which supplied only 34 kWh/€. Thus, only high heating and cooling requirements would justify the use of WWSHP + ATES system.