Trade-off between temperature and air-movement for reducing air-conditioning energy use in residential buildings in Hong Kong

Abstract

A study aimed to minimise the energy consumption for thermal comfort control in residential buildings in Hong Kong has been carried out. The hypothesis of the study was that the use of air-conditioners could be minimised and, when their use was inevitable, a higher temperature setting could be used, if a higher air speed was maintained inside the residential units with the assistance of ceiling or oscillating fans. Central to the application of this method is the knowledge about the thermal sensation of residents and their tendency to use air-conditioners when subject to different indoor temperature, humidity and air speed. The study has the objective to confirm this hypothesis; to find out the increase in indoor temperature that can be achieved within the limits that the air movement would remain acceptable; and to quantify the potential saving in energy use that could be realised. Participation in a survey of energy end-uses and environmental conditions in residential buildings in Hong Kong formed part of the current study. The survey provided the key data required in this study, including knowledge about the availability of air-conditioners and fans, and other appliances, in households and the pattern of use of these appliances. Measurement of the indoor temperature and humidity in 10 households had also been conducted in a supplementary survey, allowing the operation patterns to be verified and conditions maintained during the air-conditioned periods ascertained. Experimental work was conducted in an environmental chamber to investigate the thermal comfort requirement of human subjects and trade-offs among the environmental parameters, particularly between the air temperature and speed. Experimental results from tests involving 50 young adults responding to the environmental conditions maintained inside an environmental chamber allowed the trade-off to be quantified. It was found that under an indoor relative humidity of 80%, the indoor temperature could be raised to 28C without significantly affecting the thermal comfort sensation of the subjects provided the speed of air movement around the subjects was raised to about 0.8m/s. Site measurements were also carried out in an existing flat. Instead of the fan test rig in the environmental chamber, an oscillating fan was used, as it is a more realistic appliance that would be available in households, but the fluctuating airflow that would result would be different from the steady airflow maintained in the environmental chamber. 50 young adults participated in this set of experiments. The key finding of this part of the experimental study was that the preferred air speed, quantified by the maximum air speed at the position of the subject, would be just marginally higher than the preferred steady air speed. The data obtained in the survey provided a basis for the simulation studies for predicting the effects of using higher air speed to allow the air-conditioning needs to be deferred, and a higher set point temperature to be used for air-conditioning. The difference in the energy consumption of using mechanical fans in conjunction with air-conditioners from that of using air conditioners alone for maintaining the same comfort level were predicted using the computer software HTB2 and BECRES. It was found that up to 27% of the annual energy use for thermal comfort control could be saved by using electric fan to assist air-conditioners in residential buildings in Hong Kong. The proposition raised in this thesis can be applied to other types of premises such as public transport facilities and factories.