Night cooling control strategies. Final report.

Night cooling is an established technique allowing ventilation to take place at night with the intention of removing heat gains that have built up during the preceding day. By permitting the cool night air to flow through a building, heat is removed and a store of the cool air within the building fabric, furniture and fittings is achieved thus providing a cooling effect the following day. In order to achieve the full benefit of this technique it is essential that appropriate night cooling control strategies are used thus achieving the maximum amount of cooling whilst avoiding overcooling and subsequent re-heating. In this way energy consumption relating to mechanical cooling and mechanical ventilation is minimized and consequently significant cost savings can be made. The research carried out by BSRJA involved site monitoring of four night cooling control strategies, each of which was used in actual office buildings installation based upon a 'low energy' design. The performance of the control strategies was further verified through the use of thermal simulation software. This was used to mode! a basic representation of one of the case study buildings and the performance of a range of night cooling control strategies, including those applied in the case study buildings were tested against a selection of variables that could influence performance. The variables were designed to represent the range of conditions that could be experienced in actual buildings. The work undertaken showed that ail of the strategies are successful in helping to produce a satisfactory thermal environment as part of an overall low energy strategy. The research demonstrated that the application of complex control algorithms is not necessary in practice to achieve cost savings, although there may be benefits when trying to achieve energy savings. What is important is the careful selection of the control set points to initiate night cooling and in order to optimise the amount of night cooling taking place. The research showed that during the summer months ail of the night cooling control strategies operated for the maximum amount of time. It was du ring the marginal summer months (May, June, September and October) where the different control strategies resulted in a wide variation regarding utilisation. Over-cooling of the space was generally not beneficial since it was only achieved under marginal conditions when the requirement for night cooling is reduced. (ie over cooling of the space under peak temperature conditions would lead to improved corn fort conditions later in the day). A minimum zone setpoint should be specified to prevent overcooling. This should be related to the heating setpoint so that the situation where over-cooling followed by heating is avoided. The results showed that during peak conditions the external air temperature remained higher than the internal temperature until late into the evening. Therefore night cooling should only be permitted when the zone temperature exceeds the external temperature. Night cooling by mechanical ventilation was typically no more successful than natural ventilation and also imposed a significant energy penalty. The use of mechanical ventilation should be restricted to bad weather conditions when natural ventilation is not feasible where the ventilation systems allow this. Where mechanical ventilation is utilized it was found that starting the plant at 21:00 hours rather than 18:00 hours resulted in a similar internal temperature being achieved at the end of the night cool period due to the lower external temperatures later in the evening.