Optimal Energy Management Of Hybrid Systems Connected To Hvac And Water Heating Systems In Healthcare Institutions

Abstract

Healthcare institutions consume large amounts of energy, ranking the second highest energy intensive building in the commercial sector. This has further been exacerbated by the COVID 19 global pandemic, as healthcare facilities worldwide demand more energy with the substantial increases in patient occupancy. Within developed countries, the energy consumption of healthcare institutions may account for up to 18% of the overall energy usage, in commercial sectors. Within developing countries, such as South Africa, the energy consumption of healthcare institutions is observed to be a close second to the food service sector. Energy consumption of healthcare institutions per bed, typically range from 43 - 92kWh per day. The high energy consumption of these buildings may be attributable to energy intensive systems, that are required to operate at full scale, 24 hours a day. In retrospect, energy intensive equipment, operating continuously or during peak energy periods, result in exceedingly high energy costs, particularly when the consumer is subjected to time-based electricity pricing and maximum demand charges. The major energy intensive processes that may have their energy efficiency significantly improved, were identified to be heating, ventilation, air conditioning (HVAC) and water heating (WH) processes. The combined energy usage of HVAC and water heating processes may account for approximately 50% of the total energy consumption in the majority of modern healthcare facilities in South Africa. These systems are critical to patient health and may be classified as non-deferrable loads. Accordingly, demand side management techniques are difficult to implement without additional equipment. This renders the scheduling of loads to the lower-cost regions of the Time-of-Use (ToU) tariff, while constantly meeting load requirements, a formidable task. Additionally, the balance between the maximum demand charge and ToU tariff has to be maintained, in order to effectively minimize energy costs. Generally, to improve the potential for demand side management and energy efficiency of these processes, various methods exist. These include the implementation of energy storage systems, equipment retro fitment/replacement and the application of effective control approaches. Within the majority of private and a few public hospitals, the energy efficiency of existing equipment leaves little room for improvement. However, the decision to commission additional equipment such, as renewables and energy storage systems, is usually made with caution. The economic feasibility of these systems, at the time of the study, particularly the payback period, appears to be just beyond the acceptable threshold for adequate justification. Consequently, introducing renewable energy systems and energy storage schemes, may reduce energy usage and associated costs, while the application of optimal control techniques may improve the feasibility of such costly implementations. Additional feasibility improvements may include waste thermal energy recovery from processes such as HVAC systems and is transferred to water heating equipment so that energy savings may further be increased. Effectively applied energy management schemes, using advanced optimization techniques, remain imperative for operational cost minimization. Therefore, in this study, various methods for improving the energy efficiency of HVAC and water heating systems are identified and applied to a large-scale hospital building as a case study. These methods include the implementation of renewable energy technologies with energy storage, equipment retro fitment and the application of optimization strategies. The objective is to minimize energy usage and associated costs, with respect to the ToU tariff and maximum demand charges. A dual axis PV tracking system and energy storage scheme, with optimal control was proposed, to supply HVAC, water heating systems and other equipment. A model was developed to represent the operation of this hybrid energy scheme. A second model of a multifarious water heating system, with a total of 57 electric storage tank water heaters (ESTWHs), connected to an HVAC energy recovery system was established. The operation of this multifarious water heating system was simulated to represent the operation of the system. In this case, the simultaneous operation of the various ESTWHs was avoided to lower the risks of incurring unnecessary maximum demand penalties. Optimal control algorithms were developed, for both models, to minimize energy costs, based on the ToU tariff and maximum demand charges. SCIP (Solving Constraint Integer Problems) in the MATLAB OPTI-Toolbox was used to solve the optimal control problems. In hindsight, the feasibility of implementing the proposed dual axis PV tracking system with energy storage, supplying mainly HVAC and water heating loads, has been evaluated and discussed. According to the study, if the proposed system were to be installed, the system would break-even within 9.3 years, with lifecycle cost savings of 24.5% over a 20 year period. Applying optimal control to this proposed system, will potentially decrease the break-even point to 7.5 years and increase cost savings by up to 34.5%. Additionally, the study with a focus on HVAC and water heating processes, with waste heat recovery, revealed that, with the implementation of a multifunctional chiller (MFC), the project should break-even in 7.4 years, with project lifetime energy cost savings of up to 22.02%. The application of the proposed optimal control approach, resulted in a potential break-even point of 5.3 years, with maximum potential savings of 68.23%, achievable over a 20 year life cycle. These results further indicated that, with the application of optimal operation control algorithms, the feasibility of high investment energy efficiency activities may be improved significantly. This, in turn, serves as a greater incentive for building energy managers to implement these “deep energy retrofit” projects as to better align with international policies to stabilize global greenhouse gas concentrations.