The carbon footprint caused by the oversizing of building service systems : A case study of an NHS Hospital

Sammanfattning: Energy usage in buildings is a main contributor to CO2 emissions and in order for the EU to reach the 2050 goal of carbon-neutrality, there is a great need to improve the energy efficiency in buildings, particularly commercial buildings that often are substantially overdesigned. Excess margins in the design process of building services result in an oversizing of these systems which has great environmental impacts, divided up as the operational and embodied carbon footprints. The heating and cooling system of an NHS Hospital in southern England was studied and modelled in order to identify whether the system was overdesigned and to quantify the oversizing’s carbon footprint, which was the aim of the study. The cooling system of the NHS Hospital was determined potentially oversized and the focus of the thesis was therefore on the cooling system. It included the chillers that provide cooling, and the associated adiabatic coolers that provide heat rejection, as well as the affiliated pumps. The carbon footprint of this system was quantified, based on the operational energy use, the current grid carbon factor, environmental performance evaluations of units, observations and assumptions, and its cooling capacity was compared to the demand of the hospital. An optimised alternative was developed through analysis of the current system and its capacity, and the demand at the site, as well as based on the learnings of the background research. The system was designed to consist of smaller chillers and a reduced pumping system, to more correctly match the cooling demand. The optimised system was also modelled, its capacity compared to the demand, and its carbon footprint quantified. A future estimation of the two systems’ carbon footprints was calculated for year 2035, based on a projected grid carbon factor. The systems’ setups and carbon footprints were then compared for the current and projected scenarios, and the results discussed, also in regard to mitigation strategies that could lead to a reduction of oversizing and lower the environmental impacts. The results indicate that the yearly carbon footprint difference for the current scenario was approximately 539 tonnes CO2 eq, which was 43% greater than the optimised system’s carbon footprint. Whereas the yearly difference for the projected scenario was estimated to approximately 562 tonnes CO2eq, which was 752% greater than the optimised system’s carbon footprint in a possible future. This demonstrates the great environmental impact caused by the oversizing of cooling systems. The current system’s embodied carbon footprint was estimated to 3.3% of the total carbon footprint for the current scenario, and 4.8% for the projected scenario. Whereas the optimised system’s embodied carbon footprint was estimated to 1.5% for the current scenario, and 8.6% for the projected scenario. This demonstrates the large share of the embodied carbon footprint of the current, oversized system, compared to the optimised system that is sized more correctly for the cooling demand. Furthermore, it shows the anticipated raised proportion of the embodied carbon footprint of a product or system’s total future carbon footprint, since it increases for both the systems with time. The elevated share of the embodied carbon footprint in the future raises the need to address this factor and make it a priority. The key to a correctly sized system that meets the demand was determined to be precise calculations of the requirements and the elimination of excess margins that lack quantifiable justification. This results in an improved environmental performance where the system operates at its optimum level. The stakeholders’ involvement and influence throughout a transparent design process with clear communication, and incentives that provide financial aid to appropriately sized systems, as well as environmental impact evaluations of products, among others, are essential factors with major influence on the outcome. These elements are considered crucial for the reduction of the excess carbon footprint caused by the oversizing of building service systems.