Theoretical Evaluation Of The NightCool Nocturnal Radiation Cooling Concept

An evaluation has been conducted of the potential of a night sky cooling system to substantially reduce space cooling needs in homes in North American climates. The report describes the innovative residential cooling system which uses nocturnal night sky radiation from a roof integrated radiator. The system uses a sealed attic covered by a highly conductive metal roof which is selectively linked by air flow to the main zone with the attic zone to provide cooling- largely during nighttime hours. Available house mass is used to store sensible cooling. Additional dehumidification is done during the evening hours as warranted by an interior control humidistat. The report describes a detailed simulation model of the relevant physical night cooling phenomenon, examining each particular parameter which was found to have an appreciable impact on performance. A 225 square meter metal roof structure is modeled in Tampa, Florida. Under a series of standard nighttime conditions approximating humid nighttime summer weather, the model predicts a cooling rate of about 2,140 Watts (7,300 Btu/hr). The model features several enhancements (such as constraining the radiator temperature to the dewpoint temperature) never before incorporated into such a model. The report finds that the major weather-related influences on achieved cooling performance are outdoor air temperature, dewpoint temperature, cloudiness and wind speed. Physical factors with a large influence are the system return air temperature (and hence radiator temperature) air flow rate and fan and motor efficiency. For Tampa, Florida, the model predicts an average summer cooling benefit of about 15 kWh per day for 1.4 kWh of fan power for a system seasonal energy efficiency ratio (SEER) of about 37 Btu/Wh. Performance in less humid climates with more diurnal temperature swing was predicted to be substantially better. A follow-up experimental plan is described to obtain empirical data on concept performance using two highly instrumented test sheds