Sustainable cooling alternatives for buildings

[摘要] ENGLISH ABSTRACT: The thesis was initiated by a Consulting Engineering Company (KV3) as a research projectto investigate various options in which the efficiency and energy utilisation of conventionalair conditioning systems may be enhanced by using alternative and renewable energy.Initially, eight options had been identified and through a process of determining the degree ofcommercialisation the alternative options were reduced to three. These options, referred to asthe sustainable cooling alternatives, are active mass cooling, night flushing and roof coolingsystem.The roof cooling system comprised a roof-pond, roof-spray, pump and storage tank. The roofcooling system was mathematically and experimentally modelled. The roof coolingexperiment was performed under a variety of weather conditions with the roof-pond andstorage tank temperatures continuously recorded. The experimentally recorded temperatureswere compared to the temperatures generated by the theoretical simulation calculations forthe same input and weather conditions. Good agreement was found between themathematical and experimental model. The largest discrepancy found between the simulatedtemperature and the experimental temperature was in the order of 1 ºC.A one-room building has been assumed to serve as a basis to which the sustainable coolingalternatives could be applied to for theoretical simulation. The one-room building had fourfaçade walls and a flat roof slab. Night flushing, active mass cooling and the roof coolingsystem were applied to the one-room building such that the room air temperature and spacecooling load could theoretically be simulated. The theoretical simulations were also repeatedfor the case where the roof-pond and roof-spray were applied as standalone systems to theone-room building. The theoretical simulation calculations were performed for typicalsummer weather conditions of Stellenbosch, South Africa.Under base case conditions and for a room thermostat setting of 22 ºC the peak cooling loadof the one-room building was 74.73 W/m². With the application of night flushing between thehours of 24:00 and 07:00, the room cooling load was reduced by 5.2% by providing3.9 W/m² of cooling and reducing the peak room temperature by 1.4 ºC. The active masscooling system was modelled by supplying water at a constant supply temperature of 15 ºC toa pipe network embedded in the roof slab of the one-room building. The sea may typically beconsidered as a cold water source for buildings situated at the coast. The active mass coolingsystem reduced the peak cooling load of the one-room building by 50% by providing37.2 W/m² of cooling and reducing the peak room temperature by 6.7 ºC.When the roof-spray and roof-pond systems were applied as standalone systems to the oneroombuilding, the peak cooling load of the one-room building could be reduced by 30% and51% respectively. This is equivalent to 22.3 W/m² of peak cooling by the roof-spray and38 W/m² of peak cooling by the roof-pond. The roof-spray reduced the peak roomtemperature by 3.71 ºC while the roof-pond reduced the peak room temperature by 5.9 ºC.Applying the roof cooling system to the one-room building produced 46 W/m² of peakcooling which resulted in a 61.1% reduction in peak cooling load. The roof cooling systemreduced the peak temperature by 8 ºC. By comparing the sustainable cooling alternatives, the roof cooling system showed to be the most effective in reducing the one-room building peakcooling load. Over a 24 hour period the roof cooling system reduced the net heat entry to theone-room building by 57.3%.In a further attempt to reduce the peak cooling load, the sustainable cooling alternatives wereapplied in combinations to the one-room building. The combination of night flushing androof-spray reduced the peak cooling load by 36% while a combination of night flushing andactive mass cooling reduced the peak cooling load by 55%. Combining night flushing withthe roof-pond also yielded a 55% peak cooling load reduction. The combination of roofpond,active mass cooling and night flushing provided 51 W/m² of cooling whichcorresponded to a 68% reduction in peak cooling load. Utilising the sustainable coolingalternatives in a combination in the one-room building gave improved results when comparedto the case where the sustainable cooling alternatives were employed as standalone systems.It is illustrated by means of a sensitivity analysis that the ability of the roof cooling system toproduce cool water is largely influenced by ambient conditions, droplet diameter and roofsprayrate. Under clear sky conditions, an ambient temperature of 15 ºC, relative humidity of80%, a roof-spray rate of 0.02 kg/sm² and a roof-pond water level of 100mm, water could becooled at a rate of 113 W/m². The roof-spray energy contributed to 28 W/m² whilst the nightsky radiation was responsible for 85 W/m² of the water cooling. It must however be notedthat the water of the roof cooling system can never be reduced to a temperature that is lowerthan the ambient dew point temperature.

[发布机构] Stellenbosch University