Coupling night ventilative and active cooling to reduce energy use in supermarkets with high refrigeration loads

Night ventilation is used extensively as a low energy strategy to cool buildings in climates where night temperatures are suitable. It can be used for spaces utilising natural or mechanical ventilation systems as well as active refrigerant cooling. Most published work focuses on domestic and relatively simple in operation commercial buildings such as offices. This paper presents a study of the cooling benefits of night ventilation for frozen food supermarkets with high cooling demand. Supermarkets present a unique space conditioning challenge because of the interaction between the HVAC system and the refrigerated display cabinets. HVAC systems are the largest consumer of energy after refrigeration in supermarkets depending on system design, geographical location and controls. The most common HVAC system used in supermarkets is the constant air volume (CAV) system integrating heating, cooling and ventilation in one system although different types of systems to decouple cooling and ventilation such as a combination of variable refrigerant flow (VRF) for heating and cooling and mechanical balanced or extract ventilation have been tried out to improve thermal comfort and reduce energy use.
This paper presents two case-studies (CS) which differ from typical supermarkets as they belong in a frozen food supermarket chain. CS1 is served by a typical CAV system for heating, cooling and ventilation (coupled HVAC), while CS2 provides conditioned air through a VRF system and ducted extract ventilation (decoupled HVAC). First, an analysis of their measured energy use and indoor environmental conditions is presented to highlight similarities and differences of the two systems. Then, a coupling approach for dynamic simulation of the air-conditioning with the refrigeration system by EnergyPlus is presented and the resulting models are validated against the monitored data from the two case-study supermarkets. Using the validated models, a parametric study of the coupled operation of night ventilation and active cooling for the climatic conditions of south east England is carried out and optimisation strategies are modelled. The parametric analysis indicates that the air flow rate of night ventilation and climatic conditions are significantly correlated with the impact of night ventilation on the total energy consumption of the supermarket. Simulations have revealed that night ventilation results to lower cooling energy use for both HVAC systems. Night ventilation is in use in CS1 but optimised control strategy with lower air flow rate reduced the total annual energy use of the store by 3% due to reduction in fan energy use and active cooling, although refrigeration energy use was remain stable. In CS2 active cooling during the night is replaced with night ventilative cooling which leads to a reduction of energy use by 3.3%. Such a percentage reduction equates to 35 kWh/m2/annum. The paper discusses the differences of the two systems (all air or decoupling of ventilation from heating/cooling) in terms of HVAC energy use, total energy use, impact on the refrigeration system and the importance of controls for the night ventilative cooling.