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Potential for Free-Cooling by Ventilation

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Potential for Free-Cooling by Ventilation Solar Energy 80 (2006) 402–413 www.elsevier.com/locate/solener Potential for free-cooling by ventilation Cristian Ghiaus a,b,*,1, Francis Allard a a Laboratoire d’Etude des Phenomenes de Transfert Appliques au Batiment (LEPTAB), Universite´ de La Rochelle, Av. M. Cre´peau, 17000 La Rochelle, France b Technical University of Civil Engineering, Bd. P. Protopopescu, Bucharest, Romania Received 24 November 2004; received in revised form 31 May 2005; accepted 31 May 2005 Available online 29 September 2005 Abstract Natural ventilation is one of the most effective techniques for cooling. Its potential for cooling may be assessed by using a method based on the indoor–outdoor temperature difference of the free-running building, the adaptive comfort criteria and the outdoor temperature. It is demonstrated that the free-running temperature may be used instead of the balance temperature in energy estimation methods. The indoor–outdoor temperature difference of the free-running building becomes a characteristic of the thermal behavior of the building which is decoupled from comfort range and outdoor tem- perature. A measure related to the energy saved and the applicability of free-cooling is given by the probabilistic distribu- tion of the degree-hours as a function of the outdoor temperature and time. Weather data for this method are available in public domain from satellite investigation. The method can be applied when buildings similar to existing ones are con- structed in a new location, when existing buildings are retrofitted or when completely new buildings are designed. The method may be used to interpret the results of building simulation software or of the field measurements. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Natural ventilation; Energy; Building design; Free-running temperature 1. Introduction Augenbroe, 2002). Building simulation programs are based on first principles and require as inputs The most important decisions that affect the ther- the geometry of the building, the comfort criteria mal performance of buildings are taken in the initial and even the specification of the HVAC technology. stages of design (Holm, 1993; de Wilde et al., 2002). These prerequisites make simulation more adapted Design evaluation may be supported by the results for evaluations in the final stages of design rather of building simulation, by simplified guidelines or than a support for decisions when the building is by expert advice based on experience (deWit and sketched (Clarke, 1985; Shaviv, 1998; Al-Homoud, 2000; Hong et al., 2000; Olsen and Chen, 2003; * Corresponding author. Address: Laboratoire dÕEtude des Clarke et al., 2004). In the initial design, the archi- Phenomenes de Transfert Appliques au Batiment (LEPTAB), tects have many important issues on which the pro- Universite´ de La Rochelle, Av. M. Cre´peau, 17000 La Rochelle, ject is evaluated and investing more in thermal France. Tel.: +33 5 4645 7259; fax: +33 5 4645 8241. E-mail address: [email protected] (C. Ghiaus). design would make them less competitive. The ther- 1 ISES Member. mal analysis is done usually by engineers after the 0038-092X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.05.019 C. Ghiaus, F. Allard / Solar Energy 80 (2006) 402–413 403 Nomenclature dhc degree-hour for cooling (K h) Qc energy needed for cooling (J) dhfr degree-hour for free-cooling (K h) Tb balance temperature (K) DHc frequency distribution of degree-hour for Tbin temperature interval for a bin (K) cooling Tcl lower limit of comfort temperature (K) DHfc frequency distribution of degree-hour for Tcu upper limit of comfort temperature (K) free-cooling Tdiff temperature difference between indoor DHmc frequency distribution of degree-hour for and outdoor in free-running (K) mechanical cooling Tfr free-running temperature (K) f relative density To outdoor temperature (K) Ktot total cooling loss coefficient of the build- ing (W/K) Greek letters N total number of samples dc condition for cooling (À) Pdf probability distribution dfr condition for free-cooling (À) Pr probability density dmc condition for mechanical cooling (À) qc energy rate needed for cooling (W) qgain total heat gains from sun and internal sources (W) building is designed in order to estimate the heating the thermal mass (Zalba et al., 2003; Khudhair and cooling loads. Energy consumption can be esti- and Farid, 2004). Putting the problem in this way mated and checked if it is within accepted limits, allows us to assess the potential of architectural ele- complying thus with minimal requirements (Mihala- ments, e.g. sun-space for heating and cooling (Argi- kakou et al., 2002; CSTB, 2003). But these calcula- riou et al., 1994; Mihalakakou, 2002), green roofs tions have practically no influence on the building (Del Barrio, 1998) and pond roofs (Kishore, 1988; design (Ellis and Mathews, 2001). Since the thermal Erell and Etzion, 2000), or methods, such as passive calculations are not done in the initial stages of de- cooling (Belarbi and Allard, 2001). sign, it is essential that the architects are able to take The potential of free-cooling represents a mea- decisions that affect the thermal design. They can sure of the capability of ventilation to ensure indoor use simplified models (Ellis and Mathews, 2001), a comfort without using mechanical cooling systems. combination of simplified models and expert knowl- A way to measure this ability is by using a variant edge (Yezioro and Shaviv, 1996; Shaviv, 1999), or of the bin method in which the annual energy con- just rules of thumb based on acquired experience. sumption is evaluated for different temperature Although these results are not optimal, the heuristic intervals and time periods. The energy consumption rules indicate design solutions for building orienta- is estimated for several values of the outdoor tem- tion, geometry, and thermal mass (Shaviv et al., perature, called bins, and then the total is obtained 1996; Shaviv, 1999). by multiplying by the number of hours in the tem- Another approach would be to consider the perature interval (ASHRAE, 2001a,b). The bin building as a system that should have the potential method employs the balance temperature for calcu- to ensure indoor comfort. The indoor temperature lating the degree-hours in a bin. Balance tempera- is the result of the balance of energy fluxes and accu- ture concept supposes that the indoor temperature mulation. Then, from a thermal point of view, the is constant which implies the use of a heating or aim of the architectural design is to provide the po- air conditioning system. In buildings that use natural tential to implement the control of the energy fluxes ventilation, the indoor temperature varies making through the envelope and to allow energy storage in the concept of balance temperature unsuitable. the building structure. Arguably, the main gains are In order to estimate the relative influence of the solar and internal (Givoni, 1991; Littlefair, 1998), building design, of the comfort criteria and of the the main losses are by advection and conduction climate, it is important to have a method that (Florides et al., 2002), and the energy storage is in decouples these three characteristics. 404 C. Ghiaus, F. Allard / Solar Energy 80 (2006) 402–413 2. Thermal comfort zone Fig. 1 shows a comparison of the comfort zones for the European climatic conditions. The comfort Thermal comfort standards specify the range of range in real HVAC controlled buildings (Fountain conditions or comfort zones where 80% of sedentary et al., 1996) is compared to the ASHRAE comfort or slightly active people feel the environment ther- zone (ASHRAE, 2001a,b) and the standard for nat- mally acceptable. ASHRAE Standard 55 indicates ural ventilation (Brager and de Dear, 2000). The summer and winter comfort zones that correspond approximate parameters of the comfort zones are to typical clothing level of 0.5 and 0.9 clo, respec- given in Table 1. According to the comfort standard tively. ASHRAE comfort range is 3 °C, with a sea- for natural ventilation, the indoor mean tempera- sonal shift of 3 °C(ASHRAE, 2001a,b). Field ture in summer is considered to be 25 °C, corre- studies showed that indoor temperature in fully sponding to the mean monthly temperature of HVAC controlled buildings is maintained in nar- 22 °C(Fig. 2). This comfort standard may be used rower ranges, with the mean temperature of 23 °C, for thermal adaptive control of buildings (McCart- standard deviation of 1–1.5 °C and seasonal shift ney and Nicol, 2002). of 0.5–1 °C(Fountain et al., 1996). But because people adapt themselves to the environment, ther- 3. Balance temperature and the free-running mal comfort in naturally ventilated buildings has temperature larger seasonal ranges than assumed by ISO 7730 and ASHRAE 55 Standards (de Dear et al., 1997; A common way to compare the energy consump- Brager and de Dear, 1998; Nicol and Humphreys, tion of buildings is by applying the bin method 2002). (ASHRAE, 2001a,b). The estimation of the degree 32 30 c d 28 C) ° 26 b 24 a 22 20 (a) A/C 18 (b) ASHRAE Indoor comfort temperature ( 16 (c) 90% accept. in NV (d) 80% accept. in NV 14 0 51015 20 25 30 3540 Mean monthly temperature (°C) Fig. 1. Comfort range for air conditioning and for natural ventilation: (a) air conditioning; (b) ASHRAE comfort range; (c) natural ventilation, 90% acceptability limits; (d) natural ventilation, 80% acceptability limits (Table 1). Table 1 Comfort zones Mean Winter mean Summer mean Range Seasonal shift (°C) (°C) (°C) (°C) (°C) (a) Air conditioned buildings 23.0 22.5 23.5 1.5 1.0 (b) ASHRAE comfort zone 23.5 22.1 24.9 3.5 2.7 (c) Standard for natural ventilation, 90% acceptability limits 23.9 19.5 25.0 max 28.3 5.0 8.8 (d) Standard for natural ventilation, 80% acceptability limits 23.9 19.5 25.0 max 28.3 7.0 8.9 C.
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