Energy and Buildings 42 (2010) 2131–2138 Contents lists available at ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild A two-stage system of nocturnal radiative and indirect evaporative cooling for conditions in Tehran Moien Farmahini Farahani a , Ghassem Heidarinejad a,∗ , Shahram Delfani b a Department of Mechanical Engineering, Tarbiat Modares University, P O Box 14115-143, Tehran, Iran b Building and Housing Research Center (BHRC), P O Box 13145-1696, Tehran, Iran article info Article history: Received 18 February 2010 Accepted 5 July 2010 Keywords: Nocturnal radiative cooling Indirect evaporative cooling Cooling coil Two-stage cooling systems abstract In this paper, the results of an investigation on a two-stage cooling system have been studied. This system consists of a nocturnal radiative unit, a cooling coil, and an indirect evaporative cooler. During the night in summer, requisite chilled water for a cooling coil unit is provided by nocturnal radiative cooling and is stored in a storage tank. During the next day, the water in the tank provides chilled water for the cooling coil unit and hot outdoor air passes through two-stages: the cooling coil unit and an indirect evaporative cooler. Three sources provide secondary air for the indirect evaporative cooler. The sources are outdoor air, the air leaving from the cooling coil, and the air leaving from the indirect stage (regenerative). The investigation has been conducted in weather conditions in the city Tehran. The results obtained demon- strate that the first stage of the system increases the effectiveness of the indirect evaporative cooler. Also, the regenerative model provides the best comfort conditions. Therefore, this environmentally friendly and energy-efficient system can be considered as an alternative to the mechanical vapor compression systems. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Due to great consumption of energy in buildings, there are increasing demands to design energy-efficient heating, ventilation, and air-conditioning (HVAC) equipments and systems for build- ings. In fact, among the HVAC components and systems, cooling systems consume the largest amount of electrical energy. Cli- matic changes due to global warming, the depletion of fossil fuel resources, and demands for reducing pollution are the motivations for replacing conventional energy resources with natural resources. Methods of passive cooling such as, radiative cooling and evapo- rative cooling can be economical alternatives, or can be used as a pre-cooler in conventional systems. Also, these methods are known to have zero pollution, low energy consumption, and produce good indoor air quality [1–5]. Passive cooling resources are the natural heat sinks of planet earth. Three heat sinks of nature are the sky, the atmosphere, and the earth. Heat dissipation techniques are based on the transfer of excess heat to a lower temperature natural sinks. Heat dissipation from a building to the sky occurs by long-wave radiation, a pro- cess called radiative cooling. In fact, the only means by which the ∗ Corresponding author. Tel.: +98 21 82883361; fax: +98 21 88005040. E-mail addresses: gheidari@modares.ac.ir, gheidari@alum.mit.edu (G. Heidarinejad). earth loses heat is radiative cooling. The sky equivalent tempera- ture is usually lower than the temperature of most bodies on earth; therefore, any ordinary surface that interacts with the sky has a net long-wave radiant loss [1–3]. Indirect evaporative cooling (IEC) provides low energy cost air- conditioning. An IEC system consists of two impervious separate air passages: a dry, primary air passage and a wet secondary air passage. In the primary passage, outdoor airflow is sensibly cooled without adding water, while the secondary air and water flow in the secondary passage. The evaporation of water in the sec- ondary passage decreases the wall temperature. As a result, the cold wall decreases the primary air temperature and obviously its wet-bulb temperature. A proportion of the air leaving, which has lower wet-bulb, can be used as secondary air. Use of the air leav- ing as secondary air is a process known as regenerative indirect evaporative cooling [6,7]. Cooling coils exchange the cooling load of hot air to chilled water loop by pushing airflow through the coil. Also, Cooling Coil Unit (CCU) can be utilized as pre-cooler systems to decrease both dry and wet-bulb temperature of hot air and to increase efficiency. Totally, taking advantage of cooling coils has a positive effect on performance of HVAC systems [8,9]. Several research papers have studied the issues of nocturnal cooling. Berdhal and Martin [10], Argiriou et al. [11], Ali et al. [12], Mihalakakou et al. [13], Al-Nimr et al. [14], Spronken-smith [15], Erell and Etzion (1999, 2000) [16,17], Meir et al. [3], Bagior- 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.07.003