Dual-objective optimization of large-scale solar heating systems integrated with water-to-water heat pumps for improved techno-economic performance

Water-to-water (WWHP) heat pump has been proved highly effective in improving the thermal performance of concentrated solar district heating system (CSDHS). The holistic design method is important to realize the optimal matching and techno-economic performance for CSDHS combined with WWHP. However, such holistic designs are still lacking and thus this study proposes a dual-objective optimization model adopting NSGA-Ⅱ to bridge the research gap. A real CSDHS in Nagarze County, China was used as a case study to validate the proposed model. Solar fraction (SF), heat collection efficiency (HCE) and levelized cost of heat (LCOH) were used as the key indicators to further evaluate the performance of the optimized system. The results showed that compared with the existing system, the performance of the optimized system is significantly improved. With the heating area of 200000m², the SF and LCOH can be improved from 53.26% and 0.2750 CNY/kWh to 74.17% and 0.2297 CNY/kWh, respectively. Meanwhile, as AHS, the boiler is found inferior to the air source heat pump (ASHP) in the economic performance, but it has larger HSU volume and WWHP capacity. Associated maximum increments of SF and HCE can reach 16.2% and 4.4% respectively. The in-depth study shows that WWHP can constantly improve the thermal performance of the system, but it only improves the system economic performance in the periods with low AHS efficiency or high energy price. The study provides a novel design method which can help improve the techno-economic performance of large scale and complex CSDHS.