Renewable Energy 35 (2010) 2040–2052
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Renewable Energy
journal homepage: www.elsevier.com/locate/renene
Natural cooling of stand-alone houses using solar chimney and evaporative cooling cavity
M. Maerefat*, A.P. Haghighi
Mech. Eng. Department, Faculty of Engineering, Tarbiat Modares University, Tehran 14115-143, Iran article info abstract
Article history: In this study a low-energy-consumption technique to enhance passive cooling and natural ventilation in Received 24 December 2008 a solar house, using a system consisting of a Solar Chimney (SC) and an Evaporative Cooling Cavity (ECC) Accepted 4 February 2010 has been proposed. The capability of the system to meet the required thermal needs of individuals and Available online 26 February 2010 the effects of main geometric parameters on the system performance has been studied. The dependence of the system performance on outdoor air temperature has been studied to determine the operative Keywords: conditions for appropriate effectiveness, regarding thermal comfort criteria. To determine the heat and Passive cooling mass transfer characteristics of the system, a mathematical model based on conservation equations of Solar chimney Evaporative cooling cavity mass and energy has been developed and solved by an iterative method. The findings show that the system is capable of providing good indoor air condition at daytime in a living room, even with poor solar intensity of 200 W/m2. The results show that when the relative humidity is lower than 50%, the system can make good indoor air condition even at 40 C, and a higher performance is achieved using ECC with cocurrent configuration. It is found that the proposed system may be applied successfully in hot arid climates to fulfill the indoor thermal comfort expectations. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Employing natural or passive cooling system can be an alter- native way to maintain a cool house or reduce air-conditioning Providing good indoor conditions become problematic when load. A passive cooling system employs non-mechanical proce- outdoor temperature is high. The most effective method to cool dures to maintain suitable indoor temperature. Ingenuity of ancient a building is to avoid heating. The basic source of heat in a building architectures has showed how a rational use of traditional passive is the sunlight energy absorbed through the roof, walls, windows as techniques, along with a smart design, was involved in having well as the heat generated by appliances and air leakages. Although, desired summer comfort without a need to pursue mechanical there are some specific methods to prevent the accumulation of cooling systems. Recently, there is much inclination toward these heat, adopting all those strategies might not be possible or systems, especially due to economic and environmental reasons. adequate. Air-conditioning provides some relief. However, its initial This study attempts to present a new low-energy passive tech- costs as well as the related costs of electricity consumption make it nique to ventilate and provide thermal needs of occupants in the an expensive technology to afford. In addition, conventional air- buildings. Here, the solar energy and an evaporative cooling tech- conditioners use refrigerants made of chlorine compounds which nique are used simultaneously. Many researches have been con- are unfortunately involved in depletion of the ozone layer and ducted on using natural ventilation and evaporative cooling for global warming. It is noteworthy that human production of producing cool air so far. First of all, Bahadori introduced the idea of greenhouse gases, especially CO2, is changing the global climate. capturing the wind within a wind tower and then passing it through Statistics show that nearly 50% of CO2 emissions in developed wetted conduit walls [1]. Verma et al. studied numerically the countries are derived from energy consumption in the buildings. performance of a passive cooling system which uses evaporative Thus, it is obvious that the well-controlled and energy-efficient cooling technique on the roof. They concluded that this method is an ventilation systems are prerequisite for low-energy-consumption effective technique which significantly reduces air temperature of as well as a substantial reduction in CO2 emissions. the room [2]. Giabaklou and Ballinger presented another method of cooling in low-rise multi-storey buildings through a simple water cascade associated with openings and balconies of individual units * Corresponding author. Tel.: þ98 21 8288 3360; fax: þ98 21 8288 3381. to provide a passive cooling. This system applies the evaporative E-mail address: [email protected] (M. Maerefat). cooling technique to reduce ambient air temperature by passing air
0960-1481/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.02.005 M. Maerefat, A.P. Haghighi / Renewable Energy 35 (2010) 2040–2052 2041 over the water falling film [3]. Aboulnaga analytically studied the In the cooling cavity, circulating water is sprayed onto the top of roof solar chimney coupled with wind cooled cavity to obtain the the wall where it flows as a thin film along the wall surfaces of the air best performance of the system [4]. Raman et al. described two passage. The air, near the water film, is at an average temperature of passive models for the summer cooling system. The first model the water spray film. Since partial pressure of water vapor at the consisting of two solar chimneys: one placed on the roof to ventilate interface is higher than air pressure, there is a mass transfer of the room air and the other on the ground adjacent to the room to act evaporated water into the air. This is associated with a latent heat as an evaporative cooler during summer. The second model con- transfer and water vaporization. At the same time, convective heat sisting of a south wall collector and a roof duct is cooled from the top transfer takes place due to the temperature difference between by a sack of cloth evaporative cooling system. The results showed surface of the water and the air. At the end of evaporation process, that thermal performance of the second model is better than the the cooled falling water is collected in a basin that is provided at the first one [5]. Manzan and Saro studied a passive system consisting of bottom of the evaporative cavity. This water is recycled again to the a ventilated roof with a wet lower surface of the cavity which flows sprinkler by means of a pump. The water finally achieves an adiabatic the external air. They investigated thermal performance of the saturation temperature of the air naturally after several circulations. system by numerical modeling of evaporative cooling process Thus we can call it a direct evaporative passive cooling system. through the chimney [6]. Dai et al. presented a new passive cooling The system operates as follows: the solar energy heats up the system for humid climate using the solar chimney and adsorption room air flowing through the chimney, and the hot air generates cooling system. Its performance was investigated based on a math- the draft in the chimney. This draft induces air ventilation in the ematical model. The system increases the rate of ventilation and whole system: solar chimney, room, and cooling cavity. The provides the cooling without increasing humidity of the room [7]. chimney effect causes the air to be drawn through the cooling Chungloo and Limmeechokchai experimentally investigated the cavity with wetted cool surfaces and to remove heat from this air performance of a passive cooling consisting of a solar chimney and and brings cooled supply air into the room. Therefore, both cooling water spraying system that was placed on the roof under hot and and ventilation are provided during daytime by solar energy. humid climate. They reported that the system performed well in The present work intends to investigate the capability of the high ambient temperature [8]. proposed system to provide thermal comfort condition in a living The review of the related literature shows that the combination room. Therefore, the system is modeled and solved and the of both a solar chimney and an evaporative cooling which provides obtained indoor thermal condition is checked against the Adapted natural ventilation together with temperature reduction inside Comfort Standard (ACS) specified for thermal comfort in naturally a building has not been investigated yet. This ventilation system ventilated buildings. The adapted comfort standard is shown in has the merits of being environmentally friendly and energy saving Fig. 2 which is drawn based on data reported by Ref. [9]. The ACS at the same time. only gives the acceptable temperature range of indoor air when the Fig. 1 illustrates a schematic plan of the system consisting of two outdoor temperature is within the range of 0–40 C and does not parts: a solar chimney and a cooling cavity. The solar chimney recommend the suitable rate of ventilation. However, the ventila- comprises a glass surface oriented to the south where the solar tion rate is set approximately around 3 air changes per hour to radiation crosses through an absorber wall which acts as a capturing reduce possible pollution concentration and to ensure that the surface. thermal comfort condition is provided.
Fig. 1. Schematic diagram of solar chimney and cooling cavity. 2042 M. Maerefat, A.P. Haghighi / Renewable Energy 35 (2010) 2040–2052
at the outlet of cooling cavity and room air temperature. For modeling the cooling cavity, heat and mass transfer from the water film into the air flow and the overall energy balance equa- tions are taken into account. Some postulations are assumed to enable solving the mathe- matical model. The major assumptions are summarized as follows:
1. The air at the room is at a uniform temperature. 2. Air enters the chimney at room air temperature. 3. Only buoyancy force is considered and wind induced natural ventilation is not included. 4. The flows in the channels are laminar, and hydrodynamically and thermally are fully developed. 5. The glass cover is opaque for infrared radiation. 6. Thermal capacities of glass and wall are negligible. 7. The air flow in the channel is radiative non-participating medium. Fig. 2. Adaptive standard for naturally ventilated buildings. 8. All thermophysical properties are evaluated at an average temperature. 9. Thermal resistance of water film is negligible. 2. Modeling the system 10. The spray enthalpy is negligible. 11. The air enthalpy is only expressed as a linear function of wet Modeling the system includes the models of two distinctive bulb temperature. devices: the Solar Chimney (SC), and the Evaporative Cooling 12. The Lewis number relating heat and mass transfer is 1.0. Cavity (ECC) shown in Figs. 3 and 4, respectively. For estimating 13. The system is at steady-state condition. the ventilation rate of the proposed solar house as a whole, it is important to determine the air flow rate which can be handled 2.1. Mathematical modeling of solar chimney under a particular design and operating conditions. Therefore, an overall energy balance on the chimney is considered. This includes An element of the modeled SC is shown in Fig. 3. Based on the the energy balances for glass cover wall, the black absorber wall, energy conservation law, a set of differential equations are obtained and the air between. Writing the energy balance equations and along the length of the SC. The energy balance equation for the solving them for Tg, Tabs,andTf to calculate air flow rate have glass cover is: sought a mathematical solution. The chimney modeling has been done in accordance with Ong model [10]. Indoor thermal comfort SgAg þ hrabs gAabs Tabs Tg ¼ hgAg Tg Tfsc condition depends on room air temperature. A simplified steady state model is developed to determine the air mean temperature þ Ug aAg Tg Tfsc (1)
The overall top heat loss coefficient from the glass cover to ambient air (Ug a), can be written as:
Ug a ¼ hwind þ hrg sky þ hg a (2) The convective heat transfer coefficient due to the wind is given by Ref. [11] as follow:
hwind ¼ 2:8 þ 3:0 uwind (3)
Fig. 3. Schematic diagram of the heat transfers in the solar chimney. Fig. 4. Schematic diagram of heat and mass transfer in the cooling cavity. M. Maerefat, A.P. Haghighi / Renewable Energy 35 (2010) 2040–2052 2043
The solar radiation heat flux absorbed by the glass cover is where, dA is equal to bdx, and f represents wettability percentage of given by: the plate. hfcc is defined as the dry heat transfer coefficient calcu- lated using any relevant correlation for a flow between parallel Sg ¼ agI (4) plates [13]. The following relation is adopted in this study:
The radiative heat transfer coefficient from the outer glass surface to the sky considering the ambient temperature is adopted 0:7 hfcc ¼ 57 ufcc (14) from Ref. [10]: The heat transfer from the room air into the water film is 2 2 s3g Tg þ T T þ T Tg T obtained from: sky g sky sky hrg sky ¼ (5) Tg Ta dQr ¼ Ur wðTr TwÞdA (15) 1:5 where, the sky temperature is Tsky ¼ 0:0552 Ta [11]. The radiation heat transfer coefficient between the absorber The heat transfer from the ambient into the air flow is obtained plate and the glass cover is [10]: from: