Energy savings in tropical HVAC systems using heat pipe heat exchangers (HPHXs) (2004)
AuthorsYau, Yat Huangshow all
In hospitals, where the air must be changed at least 15 times per hour, humidity control is as important as controlling the space temperature. The high air change rate applied in hospitals instantly implies that there would be energy-saving potential in using a heat recovery system between the air being exhausted from the conditioned space and the fresh air being supplied in to replace it. In addition, the fact that in very humid climates the moisture removal requirements often necessitates overcooling air and then reheating it implies that there may be a further opportunity for heat exchange as a conservation of energy measure. Research has been undertaken on heat pipe heat exchangers (HPHXs) for coolness recovery in tropical climates to explore the potential for energy savings in HVAC systems through using HPHXs. In this work, the proprietary simulation software package, TRNSYS, has been utilised to model both the building characteristics and the HVAC system of an operating theatre suite in a Kuala Lumpur hosp ital. The model has been "driven" by the hour-by-hour climatic data for a Typical Meteorological Year (TMY) for that location, producing detailed hour-by-hour predictions of temperature and relative humidity variation within a selected week of the year, and also overall energy usage for the existing HVAC system for the complete year. To simulate accurately the influence of adding one or more HPHXs to the existing system, it was essential to know the performance characteristics of a HPHX under the situation it would experience in such a system. These operating performance characteristics in tropical climate conditions have been determined by an extensive series of laboratory measurements of the performance of an actual HPJ-I)( under the range of moist air states that it would be required to operate under in service. This experimental stage needed the specification, design and construction of a HPHX, an associated conventional chilled water coil, a fan and duct system, and a full range of properly calibrated sensors. The experimental rig also allowed the examination of the effect, if any, that tilting the HPHX would have on its effectiveness. From the experiments on inclining the HPHX, the effect of the inclination angle on the performance of the HPHX has been shown to be negligible for the range of air conditions examined and the degree of tilt investigated. Based on the results from these experiments, custom-written modules have been added to the TRNSYS package to represent the behaviour of a heat pipe heat exchanger (HPHX). The energy advantages to be gained through modifying the existing HVAC system to incorporate either one or two HPHX units have been predicted through using this extended TRNSYS model, with appropriate allowance being made for the additional fan energy penalty incurred as a result of the increased pressure drops introduced by the presence of the HPHXs. Based on this investigation, the likely energy savings as a basis for assessing economic feasibility of the HPHX in tropical cIimates have been identified. Energy savings, and the resulting pay-off period for retrofitting the HPHXs, were seen to be sensitive to the coefficient of performance (COP) of the HVAC system's chiller from the simulations. At the existing HVAC plant's claimed average COP of 4.0 the pay-off period would be 4.5 years, decreasing to 3.9 years if the average COP was 3.2. These pay-off periods could be further decreased if the application of HPHXs was incorporated in the initial HVAC system design, rather than as a retrofit. This indicates the importance of fully integrating the design process right from the outset of the system design if HPHXs are to be incorporated into a HVAC system so as to give the maximum possible energy saving benefits.