CASE STUDY | BREEZE HOTEL cascade. This is a vertical shaft that drops down through the centre of the building from the roof to the ground floor where air for ventilation is cooled, heated, dried or humidified as needed, by nine water sprays fitted to the top of the shaft. Water extracted from boreholes beneath the building is sprayed into the air at a relatively constant temperature of 13C all year round. The high heat-transfer coefficient of the falling water droplets and the large active surface area of the millions of droplets in the spray ensure the climate cascade can operate at very small temperature differences between air and water, says Bronsema, who describes the climate cascade as a gravity-activated heat exchanger. In summer, the water sprays can cool the outside air from 28C to 18C. As it cools, it sinks to the base of the shaft, and the downward movement of air is reinforced by momentum transfer from the water droplets to the air. Spraying the air with water also has the effect of scrubbing particulates from the air, thereby cleaning it. This aerodynamic pressure, together with the hydraulic pressure and the downward thermal draft, makes fans superfluous, explains Bronsema. As the cooled air sinks, the pressure increases at the foot of the cascade, which Bronsema uses to move the cooled fresh air into the rooms. In winter, the water sprays are relatively warm to humidify the supply air. A heater battery is installed at the base of the cascade, which raises the temperature of the air for supply to the rooms. The injection water temperature is 13C throughout the year; in summer, this produces cooling and, in winter, it heats the air and humidifies it, says Ronald van Luijk, principal of Green Building Engineering. The spray water is collected at the base of the climate cascade and pumped through a water-treatment installation for reuse. The climate cascade and solar chimney generate only a modest pressure difference to move the ventilation air, which means the air ducts must have larger dimensions than a traditional mechanical air conditioning system to minimise the pressure drop in the system. Both the supply and exhaust systems are fitted with axial fans to assist the airflow when necessary, says Van Luijk. The system whereby air is drawn into the hotel via the climate cascade and expelled via the solar chimney enables all rooms and communal areas to be ventilated by natural means, without the need for mechanical ventilation. Fresh air is supplied to each of The plan of two floors in the hotel In the climate cascade, air for ventilation is cooled, heated, dried or humidified as needed, by nine water sprays the guest rooms through a system of large ducts to minimise the pressure drops. The ducting is designed as big as possible to reduce pressure loss, with the air introduced to each room through a constant volume valve, explains Van Luijk. A fan coil unit enables guests to control the temperature and airflow rate in their room. When guests leave their room, the conditions revert to an energy-saving condition, he adds. According to Bronsema, his system ensures that nature inhabits the building by architectural means and because the building design is fundamental to the success of the natural air conditioning concept, the engineer and architect have to work together from the outset. It is the architect who shapes the design of the building as climate machine, he says. Minimal energy is used by the natural air conditioning system, and the biggest energy consumer is the pump that delivers the borehole water to the spray heads at the top of the climate cascade. The system can be made more energy efficient by turning off the sprays when the outside temperature or relative humidity allows. We only need one small pump to pump water to the sprays, says Bronsema. With a hotel concept such as this, we had to design a very efficient and realistic building based on a good, compact floor plan, to enable maximum investment in all of the alternative energy systems, says the 28 July 2019 www.cibsejournal.com CIBSE July19 pp26-28, 30 BreezeHotel.indd 28 21/06/2019 14:46