Life cycle cost assessment of natural ventilation systems
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The background for this work is an uncertainty related to whether the use of natural ventilation concepts can compete with conventional mechanical concepts, and under which conditions it is recommended, with respect to life cycle cost (LCC). There is also a tendency to focus on the initial costs, neglecting the costs and benefits during use of the building. In addition, some methodical difficulties rise when comparing a building integrated natural ventilation system with a mechanical ventilation system. From this background, two objectives have emerged: 1. To develop and demonstrate a method for evaluation of LCC for building integrated ventilation systems. 2. To estimate whether, and under which conditions, natural ventilation concepts are recommendable with respect to LCC, compared with mechanical ventilation concepts. A case study of three ventilation systems, applied on a fictitious office building, has been performed, partly as a guide in development of the method, partly to demonstrate the method, and partly to investigate LCC of natural versus mechanical ventilation systems. The case study includes two different natural ventilation concepts, one with air inlet in the façade, the other with an embedded duct inlet. The exhaust system, a central stack with a roof cowl that utilises the wind forces, is the same on both systems. The third case is a "best practice" balanced mechanical ventilation system, which serves as a reference. The work has provided a method for the assessment of LCC of building integrated ventilation systems. An important element of the method, is guidelines for allocation of costs of items that serve other functions in addition to ventilation. Another aspect of the method is guidelines for calculation of LCC consequences of space occupied by components of a ventilation system. The results of the case study, show that the NE case has the highest LCC, about 5% above the LCC of the MB case. The NF case has the lowest LCC, about 8% lower than the LCC of the MB case. The higher LCC of the NE case in relation to the MB case is mainly caused by the initial cost of the embedded duct. The lower LCC of the NF case in relation to the MB case is mainly caused by the absence of cooling system and ductwork. The result is relative stabile against future variations of crucial input parameters such as O&M cost, replacement intervals, energy price, discount rate and calculation horizon. If the premises of the assessment are being altered, so that the client sells or let out the building, the LCC differences between the cases increase. The following general conclusions can be drawn about an LCCA of natural and mechanical ventilation systems: 1. Depending on the degree of building integration, allocation can be an important step of such an LCCA. 2. Depending on the conditions, and particularly the client-user relationship, costs caused by space occupied by the ventilation system can be important to include. 3. Natural ventilation systems, with a facade inlet can represent a significantly lower LCC than a balanced mechanical ventilation system. Natural ventilation systems with an embedded duct inlet, however, can represent a higher LCC than a balanced mechanical ventilation system. 4. A natural ventilation system with facade inlet can have a considerably lower LCC than a natural ventilation system with an embedded duct inlet. 5. Initial costs are likely to represent the largest cost group, and can also represent significant differences between alternatives. 6. OM&R costs represent a large share of the total LCC, and are important to include. There is, however, a significant uncertainty related to OM&R costs. 7. Energy costs represent a relatively small share of the LCC, but are still important to include. 8. A high replacement frequency for control components can have a particularly negative influence on the LCC of natural ventilation systems with automatic controlled facade inlet.Initial costs represent a large cost group.