Investigation of a hybrid system of nocturnal radiative cooling and direct evaporative cooling Ghassem Heidarinejad a, * , Moien Farmahini Farahani a , Shahram Delfani b a Department of Mechanical Engineering, Tarbiat Modares University, PO Box 14115-143, Tehran 14117, Iran b Building and Housing Research Center (BHRC), PO Box 13145-1696, Tehran, Iran article info Article history: Received 5 August 2009 Received in revised form 24 December 2009 Accepted 4 January 2010 Keywords: Nocturnal radiative cooling Cooling coil Direct evaporative cooling Hybrid cooling system abstract In this paper, the results of a study on a hybrid system of nocturnal radiative cooling, cooling coil, and direct evaporative cooling in Tehran have been discussed. During a night, the nocturnal radiative cooling provides required chilled water for a cooling coil unit. The cold water is stored in a storage tank. During eight working hours of the next day, hot outdoor air is pre-cooled by means of the cooling coil unit and then it enters a direct evaporative cooling unit. In this period, temperature variation of the conditioned air is investigated. This hybrid system complements direct evaporative cooling as if it consumes low energy to provide cold water and is able to fulfill the comfort condition whereas direct evaporative alone is not able to provide summer comfort condition. The results obtained demonstrate that overall effec- tiveness of hybrid system is more than 100%. Thus, this environmentally clean and energy efficient system can be considered as an alternative to the mechanical vapor compression systems. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Cooling is an essential issue in air conditioning of most buildings in warm and humid climates. In fact, due to great consumption of energy in buildings, there are increasing demands to design building heating, ventilation, and air conditioning (HVAC) equip- ments and systems energy efficiently. Among the HVAC compo- nents and systems, cooling systems consume the largest amount of electrical energy. The issues of climatic change caused by global warming, the consumption of fossil fuels, the resources depletion, and demand for reducing pollutant particles have led to a growth use of natural resources instead of conventional energy resources or partly replacement of active cooling system. The usage of passive cooling has been considered to drive cooling cycles to provide comfort cooling. In addition, evaporative cooling system can be an economical alternative, or as a pre-cooler in the conventional systems. Also, it is known due to its zero pollution, easy mainte- nance, low energy consumption, simplicity, and good indoor air quality [1–8]. Passive cooling resources are the natural heat sinks of the planet in order that understanding their parameters is worthwhile for all varieties of cooling methods. Three heat sinks of nature are the sky, atmosphere, and the earth. Energy transfer to sky is entirely done by radiation in the wave-length interval from approximately 8–14 mm. In fact, the only means which the earth can lose heat is radiative cooling [1,3]. Significant thermal comfort can be achieved during summer by passive cooling in buildings with a great reduction of cooling loads. A black object at ambient temperature interacts with all tempera- ture range of atmospheric layers and causes cool down beneath of ambient temperature in optimum situations. Heat dissipation techniques are based on the transfer of excess heat to a lower temperature natural sinks. Regarding sky, heat dissipation is carried out by long-wave radiation from a building to the sky that is called radiative cooling. The sky equivalent temperature is usually lower than the temperature of the most bodies on the earth, therefore, any ordinary surface that interact with the sky has a net long-wave radiant loss [2,3]. Direct evaporative cooling (DEC) is the oldest, and the most widespread form of air conditioning. The underlying principle of DEC is the conversion of sensible heat to latent heat. Through a direct evaporative cooling system, hot outside air passes a porous wetted medium. Heat is absorbed by the water as it evaporates from the porous wetting medium, so the air leaves the system at a lower temperature. In fact, this is an adiabatic saturation process in which dry bulb temperature of the air reduces as its humidity increase (constant enthalpy). Some of the sensible heat of the air is transferred to the water and become latent heat by evaporating some of the water. The latent heat follows the water vapor and diffuses into the air. The minimum temperature that can be obtained is the wet bulb temperature of the entering air [8–11]. * Corresponding author. Tel.: þ98 21 82883361; fax: þ98 21 88005040. E-mail address: gheidari@modares.ac.ir (G. Heidarinejad). Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv 0360-1323/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.01.003 Building and Environment 45 (2010) 1521–1528