Optimizing the cost and energy performance of district cooling system with the low delta-T syndrome
Almost every chilled water system is affected by the low delta-T syndrome in which the supply and return chilled water temperatures falls short of the design level, particularly at low loads. This results in inefficient chillers and higher energy consumption of the chiller plant. This research is aimed at designing a district cooling system (DCS) that can accommodate the low delta-T problem and minimize its impact on the DCS’ energy performance. Methodologies were developed to minimize DCS energy consumption and running cost, particularly those related to the chiller plant and pumping station. A hypothetical urban district and a baseline DCS were set up for simulation of alternative designs to be evaluated and compared. Energy efficiency enhancement measures related to chiller system configuration, pumping station configuration and chilled water temperature were also evaluated. \ud \ud Moreover, mathematical models that simulate the performance of major DCS components were developed. These models were integrated to become a DCS model for identifying an optimum design. A life cycle cost (LCC) model was also adopted for identifying a cost optimal design solution that would result in the lowest LCC and an optimum energy performance when the DCS was operated under low delta-T conditions.\ud \ud The variants of DCS design evaluated include five combinations of chiller system configuration, eight chilled water temperature regimes, and 36,192 arrangements of pumping stations. A simple heuristic strategy was adopted to greatly reduce the number of design solutions to be studied. The energy, financial and environmental performances of these possible solutions were then evaluated. \ud \ud The results show that the optimum design in respect of energy performance, denoted as “Solution E”, could save 15.3% of the annual total electricity consumption of DCSO. After evaluating the LCC of each possible solution, it was found that instead of Solution E, “Solution C” was the most cost-effective. This cost-optimal design was about 7.5% lower in LCC than the baseline case. The LCC saving would amount to HK$332 million in present value. There were 15 equally-sized variable speed chillers in Solution C. Six pumping stations were located along both the main chilled water supply and return pipes, with five pumps in each station, and the chilled water supply and return temperatures were 5oC and 13oC respectively. This design could lead to a 14.6% reduction in the electricity consumption of DCSO. Although this percentage was about 1% lower than that achieved by Solution E, the LCC of Solution C was more financially favourable due to lower initial capital cost, and life-cycle replacement and maintenance cost.\ud \ud The methods devised in the presented research can help to provide a direction in the search for an integrated DCS design solution that could mitigate the impacts of degrading delta-T on the energy performance of the DCS. The results obtained from this study will enable a DCS owner to evaluate the energy benefits and the associated financial trade-offs. Moreover, the energy-optimal solution identified could lead to fewer impacts on the environment. Had we been able to account for the costs of the environmental impacts as well, the energy-optimal solution could well be the cost-optimal solution as well. This factor should be considered in a selection of the design to adopt in order to help our society achieve a more sustainable future.