Dynamic thermal simulation of a solar chimney with PV modules

Transcription

1 International Conerence Passive and Low Energy Cooling 89 Dynamic thermal simulation o a solar chimney with PV modules J. Martí-Herrero and M.R. Heras-Celemin Energetic Eiciency in Building, CIEMAT, Madrid, España ABSTRACT The issue o this paper is to present theoretical results or a solar chimney with thermal mass, where the glass surace is replaced by photovoltaic (PV) modules. A portion o the heat absorbed by the PV modules is dissipated to the air channel in convective orm, and it exchanges radiation heat with the concrete wall. These cooling phenomena or the PV modules improve their eiciency with a lower working temperature. Both phenomena are heating process to the air and the concrete wall, that produce natural ventilation. The solar chimney is supposed to be isolated rom any building. The results obtained are an average air mass low rate around 0.02 kg/s along the day and night, and a maximum PV temperature o 32 K.. INTRODUCTION The energy consumption or cooling and heating buldings increases every year. The construction o buildings with low thermal eiciency, without taking in consideration any solar design, is supplied with the thermal conditioning o buildings by electrical heating and cooling machines. Natural cooling and heating techniques must been employed to improve the energy eiciency o buildings. These techniques have been employed in the traditional buildings or long time. Some o them can be recovered and adapted, other ones are being developed. Natural ventilation in buildings is a cooling passive technique appropriate or warm climates. Solar chimneys are natural ventilation systems that take advantage o solar radiation to generate convective air lows. This convection pulls air out rom inside the building, replacing it with air rom outside. The irst studies on solar chimneys have been reported in 993 by Bansal et al. They present a stationary state model to describe a solar chimney, consistent by a conventional chimney linked to an air solar heater. Hirunlabh et al. report on 999 the results o an experimental solar chimney, composed by a glass surace, air channel and a metallic black wall as collector surace. Aonso and Oliveira published on 2000 the experimental results o a conventional chimney and a solar chimney to evaluate the contribution o the natural ventilation in buildings. Khedari et al. on 999 make a comparative study among dierent conigurations o solar chimneys, classiied like; roo solar collector, modiied Trombe wall, Trombe wall and metallic solar wall. On 2003 Ong and Chow report the experimental and theorical results o a solar chimney similar to the Hirunlabh one. On 2004 Martí-Herrero and Heras Celemin propose a dynamical model or a solar chimney with thermal inertia, composed by a glass surace, air channel, and a 20cm wide concrete wall. Dierent conigurations have been studied or solar chimneys. The metallic solar wall type produces an air low due to the solar radiation, indicated or tropical climates. The thermal inertia type storage heats during the day, producing ventilation, even without solar radiation during the night, indicated or Mediterranean climates. The design o the proposed PV solar chimney is shown in Figure. 2. PHYSICAL MODEL The physical model proposed to describe the

2 892 International Conerence Passive and Low Energy Cooling Figure : Design o a solar chimney with thermal mass and PV modules. Figure 2: Heat transer phenomena considered in the dynamic model which describe the thermal perormance o the solar chimney. thermal behaviour or a thermal mass solar chimney with PV modules, is based on the one reported by Martí-Herrero and Heras Celemin on 2004, or a thermal inertia solar chimney. This dynamical model characterizes the system by representative temperatures or the dierent components (Ong and Chow, 2003): T or PV modules temperature, T or the air channel temperature and 6 T wi(i,6) or the concrete wall temperature. The model meteorological inputs are: solar radiation, ambient temperature and wind velocity. The physical properties o the dierent components are provided to the model. It calculates all the heat transer phenomena considered and the representative temperatures o the system. The heat transer phenomena considered in the dynamical model are shown in Figure 2 and they are: - S : Solar radiation gain or the PV modules; - G : Power generated by de PV modules; - h rs : Radiative heat transer PV-sky; - h wind : Convective heat transer by wind; - h rw : Radiative heat transer PV-wall; - h : Convective heat transer PV-air channel; - q: Heat loss by the air that leave the chimney; - S w : Solar radiation solar gain by the wall; - h w : Convective heat transer wall-air channel; - δt/δx: Heat conduction equation; - h rws : Radiative heat transer wall-sky. The dynamic model proposed makes the energy balance or the PV modules, air channel and the concrete wall as shown in equations -4. S + h S h wind T ) + h a T ) + h rw rs T w0 T ) + ) s () q h T T ) + h T ) (2) w + h h w wind ( w w0 rw w0 Twi T k 2 x h T w0 ) Tw Tw T ) + k 2 x wi + wi Ta ) + h rws wi T s ) (3) (4) The conduction heat transer or the interior o the concrete wall is described by: 2 T T (5) 2 x α t Resolved by numerically using the next approximations: y yi yi + z z (6) 2y yi+ 2yi + yi (7) z2 z2 The convective heat transer due to the wind or the external surace o the PV modules and concrete wall is described by the experimental equation (McAdams, 994):

3 International Conerence Passive and Low Energy Cooling 893 h wind V (8) where V is the wind velocity. The sky temperature, T s, or the radiative heat transer between the external suraces and the sky is given by the experimental equation (Swinbank, 963): Ts (9) Ta The heat loss rom the solar chimney due to the heated air that leaves the channel is given by the expression (Bansal et al., 993): mc Ta) q (0) γwl where γ is 0.75 (Bansal et al., 993), W y L are the dimensions o the chimney and m is: A 2gL( T ρ o Cd + A T o / Ai a m T ) a () where C d is the discharge coeicient, A o and A i are the outlet and inlet areas and g us the gravity. The convective heat transer coeicients, h and h w, are calculated using the Nusselt number Nu i where i is the solid surace: N ui K hi (2) L The Nusselt number is calculated by a mathematical expression (Incropera and De Witt, 996) dependent to the Rayleigh number, Ra i, and Prandtl number, Pr. I Ra i < 0 9, a laminar low is considered: / Rai Nui (3) 9 /6 ( + ( / Pr) ) 4 / 9 I Ra i > 0 9, a turbulent low is considered: 0.387Ra / 6 i 2 Nui ( ) (4) 8 / 27 9 /6 ( + ( / Pr) ) Figure 3: Experimental colar chimney with thermal inertia existing at LECE (Almería, Spain). 3. CASE STUDY The solar chimney with thermal inertia and PV modules proposed in this research, is based on the dimensions o the solar chimney with thermal inertia constructed in the Energetic Research Laboratory or Construction Components (LECE, Almería, Spain) (Fig. 3). The dimensions are: - 4m high; - 20cm air channel width; - 20cm concrete wall width; m 2 inlet area; m 2 outlet area. The reinorced concrete wall is black painted on its interior surace or better absorption o the solar radiation. It is supposed that the solar radiation absorption, α w, is 0.82; emissivity, ε w, is 0.95; thermal conductivity, k w, is.63 W/mK; heat capacity, c w, is 090 J/kgK and the density, ρ w, is 2400 kg/m 3. The physical properties o the air are given by empiric correlations dependent on the temperature, and itted to the interval K (Ong, 2003). The PV modules have been supposed as generics ones, which properties are a solar radiation absorption, α, o 0.70; electrical generation eiciency o 0.5; and relection index, τ, o 0.5. The PV modules are supposed to have no thermal inertia or thermal capacity. The meteorological data is taken rom a weather station at LECE or two sunny days at summer. It is shown in Figure K ambient temperature, 450 W/m 2 solar radiation over vertical surace and 8 m/s wind velocity are reached.

4 894 International Conerence Passive and Low Energy Cooling source, and the rest o phenomena are cooling process or the PV module, except during the night, when the radiative heat transer rom the wall heats the PV module. Figure 4: Meteorological real data rom two days in summer at LECE, employed in the model proposed. 4. RESULTS The results obtained by the application o the model proposed, with the real meteorological data available are shown in Figure 5. In all graphics is represented the ambient temperature. The top graphic is the air mass low rate in two cases; a) solar chimney with thermal inertia and glass cover and b) solar chimney with thermal inertia and PV modules. The air mass low rate is higher during the two days in the irst case. The results obtained in the case o PV modules show that, even with only 0% o solar radiation incidence over the concrete wall, a natural ventilation is obtained during the night. More than 0.02 kg/s o air mass low is reached during most o the night. The second graphic shows the temperatures evolution o the system, in the case o PV modules. The estimated work temperature or the PV modules reaches, as maximum, 32 K, and a maximum dierence with ambient temperature o 0 K. The wall suraces temperature show its thermal inertia with a sot wave variation. The third graphic is the thermal luxes evolution to the channel interior air. In this model the air absorption coeicient is null, so only by convective heat transer can be heated. This graphic shows how the PV heat the air during the day, and the wall during the night. In the orth graphic appear all the heat transer phenomena considered in the model or the PV modules. The solar gain is the heating 5. DISCUSSION The air mass low rate obtained is high enough to consider this solar chimney coniguration as a valid natural ventilation system (Khedari et al., 999). The PV modules transer heat to the air during the day, and the concrete wall during the night, and both give to this solar chimney coniguration almost natural ventilation during all day. Only in the aternoon (7 h), when the PV modules temperature, inner surace wall temperature and ambient temperature tends to be equal, minor natural ventilation is produced. During two hours in the early morning, no natural ventilation is produced as well, but at these time it is not necessary. The PV module estimated temperature reach 32 K, 0 k over ambient temperature. For equal solar irradiance, experimental measurements on ventilated PV açade (Wilshaw et al., 996) are around 8 K dierence temperature. Experimental thermal data on photovoltaic modules (Alonso and Balenzategui, 2004) obtains or the same solar irradiance, 5 K dierence, but with 302 K ambient temperature, instead 33 K on this paper. The more important cooling process or the module is the wind convective heat transer according to the results obtained, due the real meteorological data, that show peaks o 8 m/s wind velocity. Equation (8) rules the convective heat transer by the wind, and this equation has been questioned beore (Martí-Herrero and Heras Celemin, 2004b) and is necessary to be revised. 6. CONCLUSIONS The solar chimney with thermal inertia and PV modules replacing the glass surace proposed, acts as natural ventilation system appropriate to Mediterranean climates due to the nocturnal ventilation produced. From the PV module point o view, the system does not overheat, and reaches work temperatures lowers to those reported on the bibliography, or similar in the ventilated cases. To validate the model is required experimental

5 International Conerence Passive and Low Energy Cooling 895 Figure 5: Theorical results obtained or a solar chimney with thermal mass and PV modules. First graphic: Evolution o air mass low rate (AMFR) or the case o solar chimney with glass surace, and PV modules case. Second graphic: Temperatures evolution or all the system (PV modules case). Third graphic: Evolution o heat transer process to inner air (PV modules case). Forth graphic: Evolution o heat transer process in the PV modules. work, considering heat capacity o the module (Knaupp, 992), and revising the wind convective heat transer equation (eq.4) (Marti- Herrero, et al., 2004b) REFERENCES Aonso, C. and A. Oliveira, Solar chimneys: Simulation and experiment. Energy and Buildings 32, 7-79.

6 896 International Conerence Passive and Low Energy Cooling Alonso García M.C. and J.L. Balenzategui, Estimation o photovoltaic module yearly temperature and perormance based on Nominal Operation Cell Temperature calculations. Renewable Energy. Vol 29 iss Bansal, N.K., R. Mathur and M.S. Bhandari, 993. Solar chimney or enhanced stack ventilation. Building and environment 28, Hirunrabh, J., W. Kongduang, P. Namprakai and J. Khendari, 999. Study o Natural ventilation o houses by a metallic solar wall under tropical climate. Renowable Energy 8, Incropera, F.P. and D.P. DeWitt, 996. Fundamentals o Heat and Mass Transer, 4 th Edition. John Wiley, New York. Khedari, J., B. Boonsri and J. Hirunlabh, 999. Ventilation impact o a solar chimney on indorr temperature luctuation and air change in a school building. Energy and Buildings 32, Knaupp, W., 992. Thermal description o photovoltaic modules. th photovoltaic solar energy conerence. Montreux, Switzerland. Martí-Herreo, J. and M.R. Heras Celemin, 2004a. Dynamical physical model or a solar chimney, submitted to Solar Energy on June Martí-Herreo, J., and M.R. Heras Celemin, 2004b. Evaluación de modelos ísicos para describir el comportamiento energético de una chimenea solar. XXVIII Semana Nacional de la Energía Solar, Oaxaca, México, 0-8 OX. McAdams, W.H., 994. Heat transmision. 3rd ed. New York: McGraw-Hill. Ong, K.S., A mathematical model o a solar chimney, Renewable energy 28, Ong, K.S. and C.C. Chow, Perormance o solar chimney. Solar energy 74, -7. Swinbank, W.C., 963. Long wave radiation rom clear skies. Q. J. R. Meteoro. Soc. 963;89;339. Wilshaw, A.R., J.R. Bates and N.M. Pearsalt, 996. Photovoltaic module operating temperature eects. Eurosun 96, pp