Ecological sanitation, organic animal farm, and cogeneration: Closing the loop in achieving sustainable development—A concept study with on-site biogas fueled trigeneration retrofit in a 900-bed university hospital
摘要
Healthcare facilities mostly consume natural gas or fuel oil, utilize grid power, and are the second most energy intensive sector in the USA. Besides their high fossil fuel expenditures, hospital buildings generate large amounts of plumbing wastes and others, such that they are the largest producer of GHG emissions in the building sector. Energy costs are consuming up to 15% of their annual profits. In this paper the overall environmental and economic problems that may be associated especially with large healthcare facilities are addressed by showing ways to convert their energy and environmental disadvantages into advantages. In this respect, a concept study with ecological sanitation and formation of an energy, water, food, and education nexus by primarily employing a trigeneration system operating with biogas at an optimum fuel share with natural gas for retrofitting an existing 900-bed University hospital is presented. This case study covers two scenarios. The first scenario is the base scenario, which utilizes three trigeneration engines, with one 1,25 MWe, and two 2,2 MWe capacity each, all running on natural gas with a total capacity of 5,65 MWe. The second scenario includes three stages. The first stage mixes natural gas with biogas, which is to be produced on-site by primarily using plumbing wastes, for driving the 1,25 MWe engine, which satisfies the constant base load of the hospital for 24 h a day. The second stage produces biogas by making use of the widely available surrounding free land of the hospital in a new eco-farm development and replaces the fuel input of the first 2,2 MWe engine, which operates 16 h a day on average. In the third stage the second trigeneration unit with 2,2 MWe capacity remains on natural gas fuel input and operates approximately 8 h a day (peaking engine). Both scenarios have an absorption cooling system with the same capacity and an 8 MWc-h ice tank. This common base of identical power, heat, and cold capacities was aimed to independently focus on the environmental and economic benefits of biogas substitution covering a ten-year operational period. The next system for stage two involves a new organic 6000 livestock-animal organic farm and a dairy factory to be owned by the University, which completes the food, water, energy, education and environment nexus and serves as a full-scale hands-on farm for the Department of Agriculture students and provide an R&D platform. It has been shown that such an application closes the loop towards sustainability. The organic venture is expected to have a large economic impact and important contributions also on the dietary needs of the patients. The organic farm is envisioned to incorporate greenhouses, wind, and solar farms. Yet this study only covers the impact of the biogas supply to the trigeneration system. CO2 emissions from biogas generation is assumed to be captured and utilized for dry ice production. Analyses show that the additional cost of on-site biogas anaerobic digester and its ancillaries of the first-stage (1,25 MWe) may pay back themselves in four years. The corresponding prediction for the second stage biogas trigeneration system with biogas fuel (2,2 MWe) is also four years. Total reduction in CO2 emissions attributable to the biogas conversion of the trigeneration system is 161558,2 t CO2 over a ten-year period, taking into account the additional reductions due to improvements in rational exergy management of the energy resources. The net total savings from biogas conversion in two stages is expected to be about 4 M€ for a ten-year period.