An assessment of indirect evaporative cooling as an energy efficient and cost effective method of air conditioning with energy recovery
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Indirect evaporative cooling (lEC) exhibits favourable potential for energy recovery when operated on its own or when it is integrated with a vapour compression system to form a hybrid system. However, very few systematic and holistic design approaches have been carried out to analyse its strengths and weaknesses relative to other available technologies. This thesis reports research on developing a novel low energy air conditioning system in which an indirect evaporative cooling unit in the form of a polymer plate cross-flow heat exchanger is integrated with a vapour compression system or a chilled water coil. Two design approaches are taken, one after the other. In the first approach the thermal aspect of this particular heat exchanger is described (Chapters 1 to 3). A model for basic effectiveness is developed from the physical principles involving energy balance, use of moist air properties and a psychrometric chart. This new development explains the sensitivity of effective operating conditions and the link between sensible heat ratio and flow ratio. In the second part of this thesis, (chapter 4 to 7) a functional design approach is employed that considers criteria which are common to air conditioning system design and product development. For the DICER system, technology assessment and the original case study for ventilation air pre-treatment are described. This part of the thesis also describes life cycle costing, materials, manufacturing and the influence of volume production on cost along with a case study. When considering manufacturing or fabrication on a larger scale a simple tool using geometrical relations of the mould size, shape and material specifications is used to estimate the material quantity for large scale production. This is illustrated with a specific model of heat exchanger housing and considering fibreglass as a preferred material for fabrication. An economic evaluation is carried out based on the material requirements for existing manufacturing and proposed manufacturing method. Cost reduction opportunity is presented using optimised batch quantity. This cost reduction is then extended to other models of the heat exchanger housing and compared with existing manufacturing methods. This total approach of combining thermal science with materials, production and engineering design activity identifies the strengths, weaknesses and suitability of this method of air conditioning for commercial exploitation. The research conducted by this approach has provided valuable insights and understanding of the technology as well as its merits and limitations when compared with existing commercial products such as vapour compression systems. A life cycle cost (LCC) analysis method is developed based on the operating cost, initial cost, performance and discount rate over future time for the economic lifetime of the product. This model compares the life cycle cost of a particular design or product when evaluating several energy recovery options. This costing tool will aid design engineers to establish a balance between performance and cost. Alternatives with different design, performance and initial costs are assessed and analysed for operating life, taking replacement within the comparison period into account. The key contributions of the work described in this thesis are: 1. A simplified effectiveness mode] based on sensible heat ratio and using a psychrometric chart which explains sensitivity of effectiveness when considering dry and wet surface heat transfer. 2. The case study involving ventilation air pre-treatment in a commercial building using the DICER method of energy recovery, where the cross-flow polymer plate heat exchanger is integrated with the chilled water coils supplied from the main plant. 3. Qualifications to the benefits of this method of ventilation air pre-treatment for peak demand reduction as well as annual energy conservation combined with site evaluation for potential application in retrofit operation. 4. Guidelines are developed based on the knowledge gained throughout the case study which will aid similar future designs. 5. Technology assessment is carried out to point out the strengths and weaknesses of the DICER system for its next stage of design optimisation. 6. A simplified quantity estimation technique is presented using the geometric relation of mould shape; size and material specification. Optimum batch quantities are presented for the existing and recommended method of manufacturing for further cost optimisation.
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