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The cooling performance of a radiator based roof component
Solar Energy 80 (2006) 1039–1047 www.elsevier.com/locate/solener
The cooling performance of a radiator based roof component A. Dimoudi
a,*,1
, A. Androutsopoulos
b
a
b
Ministry of the Environment, Phys. Planning and Public Works, Direct. of Housing Policy and Shelter, 36, Trikalon Str., Mesogeion Aven., 115 25 Athens, Greece Buildings Department, Division of Energy Efficiency, Centre for Renewable Energy Sources (CRES), 19th km Marathonos Aven., 190 09 Pikermi, Greece Received 16 September 2003; received in revised form 31 May 2005; accepted 6 June 2005 Available online 20 September 2005 Communicated by: Associate Editor Matheos Santamouris
Abstract Energy conservation in buildings is becoming an issue of great importance. Space cooling is getting important in most countries and different techniques have been developed one of which is radiative cooling. A prototype roof component, exploiting radiative cooling, was built and tested in the outdoor test facilities of the Centre of Renewable Energy Sources in Greece. The component comprises a radiator, which is utilizing water as the fluid medium and its cooling performance was investigated. This paper presents the construction of the component, the experimental set-up and the results taken during the monitoring procedure. 2005 Elsevier Ltd. All rights reserved. Keywords: Radiative cooling; Passive cooling of buildings; Experimental results
1. Introduction The design and construction of the roof of a building plays an important role at the heating and cooling needs of a building. In the Mediterranean region, the solar radiation received on a horizontal surface is about 2–3 times greater than the one received on a south oriented surface, depending on the latitude of the place and month of the year. Thus, the appropriate construction of the roof can
*
1
Corresponding author. Tel./fax: +30 210 80 87 959. E-mail address: [email protected] (A. Dimoudi). Formerly with CRES.
play a significant role at the thermal performance of a building. The roof is needed to protect the building from the outdoor climatic conditions (mainly by application of thermal insulation) and on the other hand can be used as an integrated component of the building that exploits the environmental sinks. In hot climates, roof may be used as an integrated building cooling component, for cooling with either evaporation or with radiation. In the last three decades, special attention has been given towards the energy conservation in buildings for both heating and cooling purposes. Especially for the latter, a very important issue for countries with hot climate, several techniques have been developed one of which exploits radiative
0038-092X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.06.017
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A. Dimoudi, A. Androutsopoulos / Solar Energy 80 (2006) 1039–1047
Nomenclature C1 C2 C3 Ca
=C2*Tamb/(C3 + Tamb) 17.08085 234.175 cloudiness coefficient, Ca = 1 + 0.0224n 0.0035n2 + 0.00028n3 cp specific heat of water (J kg1 K1) hconv convective heat transfer coefficient (W m2 K1) m_ water mass flow rate (kg s1) n total opaque cloud amount, 0 6 n 6 1 nrad radiatorÕs efficiency qconv convective heat transfer at the radiatorÕs surface (W m2) (Eq. (2)) Qcool,rad cooling power of the radiator (W) (Eq. (3)) qcool,rad cooling rate of the radiator (W m2 pipe) qnet net radiative heat exchange (W m2) (Eq. (1)) qtot total or effective cooling rate (W m2) RH relative air humidity, 0 6 RH 6 1 Tambient ambient temperature (C)
cooling. Different approaches have been made to design applicable systems and investigate their performance. Studies were carried out both for air and water radiators. Water radiators are either water pond systems (Hays and Yellot, 1969; Martin, 1989; Givoni, 1994; Goswami et al., 2000) or flat plate radiator systems. Different designs and pipe materials were theoretically (Argiriou et al., 1993, 1994; Michalakakou et al., 1998) or/ and experimentally investigated for flat plate radiator systems (Etzion and Erell, 1991; Erell and Etzion, 1992, 1999, 2000; Al-Nimr et al., 1998; Meir et al., 2002). The same systems were also investigated during winter for space solar heating (Matsuta et al., 1987; Erell and Etzion, 1996). The present work investigates the cooling performance of an unglazed water radiator connected to a series of water pipes embedded into the roof of the building (cooling panel). A prototype roof component, for the exploitation of radiative cooling, was built and tested at the outdoor test facilities of the Centre of Renewable Energy Sources in Greece. The motivation was to design a system that not only meets the cooling demand of a building but it should have the potential for architectural integra-
absolute ambient air temperature (K) Tamb Tcon concrete slab mean temperature (C) Tcon,in, Tcon,out inlet, outlet temperature of the cooling panel pipes (C) Tdp dew point temperature (C) Tin air temperature inside the test room (C) Trad absolute temperature of the radiator plate (K) Trad eff absolute temperature of the radiator pipes (K) Trad,in, Trad,out inlet, outlet radiator pipes water temperature (C) Tstag stagnation temperature (K) v wind speed (m s1) Greek symbols erad emissivity of the radiator plate esky emissivity of clear sky r Stefan–Boltzmann constant 108 W m2 K4)
(5.67 ·
tion. In the design of the component, the traditional and the contemporary construction practice in Greece was taken into account, as well as the application and appropriateness of the cooling techniques for covering the energy needs of a building. This paper presents the construction of the component, the experimental set-up and the results of the monitoring procedure during summer period. 2. Description of the roof component structure The tested roof component consisted of a water radiator linked to a cooling panel. The water radiator was comprised of a series of pipes exposed to the ambient environment, linked to a series of pipes embedded into the concrete roof slab (cooling panel). The dimensions of the roof component were 2.715 m wide by 4.970 m long. The water radiator was constructed with a series of heavy duty steel pipes of 3/4 in. diameter, 10 pipes per meter roof width. The pipes were fixed on a steel plate, both of them painted white, for minimisation of the solar gains during the day. The water radiator system was placed on 8 cm insulation layer. Water pipes of the same material and
A. Dimoudi, A. Androutsopoulos / Solar Energy 80 (2006) 1039–1047
diameter (3/4 in.) were embedded into the centreline of the concrete slab, with a between distance of 20 cm (5 pipes per meter). The thickness of the concrete slab was 12 cm. Two plenums across the width of the roof, at the North and South side of the test cell, connected the water radiator pipes with the water pipes inside the concrete slab and the water circulates with a circulator in a close loop. Construction details of the roof component are shown in Figs. 1 and 2. Special attention was given at the design and the construction of the unit, in order to eliminate any heat bridges at the envelope of the building and any air leakage paths as the envelope of the test cell is constructed to very high airtightness standards with negligible infiltration losses, normally less than 0.5 ACH (Wouters and Vandaele, 1994). The proposed design, with the insulation layer permanently in place beneath the radiator ensures the protection of the roof from heat gains. The operation of a water circulator during the night switches the roof to a cooling mode. The proposed roof element can be easily applied to flat roofs which are widely used in Greek buildings. With this construction, the structural mass of the building is cooled than the interior air. Its construction details can ensure thermal insulation of the roof according to the existing regulations. It can also be applied in renovation of buildings, as water pipes can be added on top of an existing concrete roof, covered with a layer of concrete, a layer of insulation and the water radiator. The same component can also be used during winter as a heating component.
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Fig. 2. View of the tested roof component.
3. Experimental procedure and instrumentation The tests took place at the upgraded PASLINK test cell of CRES (Wouters and Vandaele, 1994), which is a highly insulated box, with dimensions of 8.4 m · 3.8 m · 3.6 m, located at the outdoor environment, into which a full scale building component, wall or roof, can be tested under real weather conditions. The test cell is partitioned into two rooms: the Test Room with dimensions of 2.75 m · 2.75 m · 5.00 m and the Service Room which contains auxiliary equipment for the operation of the test procedures. Its main characteristic is the fully controlled thermal conditions of the Test Room and the high degree of the envelope thermal insulation that results to minimization of the heat losses from all surfaces of the Test Room except the one where the tested component is installed.
Fig. 1. Cross section of the prototype roof component—construction details.
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The test cell is equipped with a sophisticated heating and cooling system controlled by a data acquisition and control unit for implementation of specially developed dynamic test sequences. Both, indoor and outdoor climatic conditions are monitored using a standard set of sensors, including: solar radiation (diffuse and direct), long wave radiation, wind speed and direction, relative humidity, air and surface temperatures, heating and cooling power. All measurements are recorded at one minutely intervals by the central data acquisition system. The aim of the applied cooling technique is to cover a fraction of the cooling load of the building. This coolness is stored in the ceiling structure and the energy should be released in accordance to the load profile of the building. Continuous measurements were taken for 22 days during June–July (Julian Days 160–182). The air temperature inside the test room was kept constant at about 28 C and different water flow rates were applied at the water radiator. The standard instrumentation of the test cell was used to monitor the thermal performance of the room. Additional sensors were installed to measure the performance of the roof component. For the environmental parameters, the ambient temperature (PT 100, shielded) and the wind speed at the height of the roof (cap anemometer) were measured. For the roof component: 21 T-type thermocouples on different positions and heights at the component, 4 PT 100 sensors for the water temperature, an electromagnetic flow meter, measuring in the range of 0–2.5 l s1 and 2 heat flux sensors (HFS) were used. The position of the roof sensors is shown in Fig. 3. The data from all sensors were collected at the data acquisition system at 1 min intervals, averaged to a 10 min basis.
A water filter was also installed at the water circuit. The electric pump was controlled by a timer switch so that water circulated on a 12 h basis (from 19:00 to 07:00). The flow rate was controlled through a valve positioned next to the pump. The water was circulating during the night collecting heat from the concrete slab and extracting it to the environment. 4. Experimental results The use of the concrete slab as a cooling panel was found to be an effective way to extract heat from the roof. Fig. 4 depicts the distribution of the concrete slab mean temperature (Tcon), the air temperature inside the test room (Tin), which was controlled to be constant, at about 28 C, the ambient temperature (Tambient), the inlet/outlet temperatures of the cooling panel pipes (Tcon,in, Tcon,out) and the radiator pipes (Trad,in, Trad,out) during the night period of a typical test day. The temperature of the concrete slab was decreasing during the night and always being lower than the temperatures of the water pipes and the room air which was controlled to be constant during the whole day. The concrete slab temperature reduction ranged from 3 to 6 C during the night operation of the system. During the testing period, the ambient temperature was smoothly changing during the day while during the night fluctuation of the temperature was observed indicating cloudy nights. During the test period, the weather conditions were quite constant, with Tamb,min = 17.1 C and Tamb,max = 34.9 C. The radiator surface exchange heat with the external environment through two different mechanisms: radiative heat transfer to the sky and convec-
Radiator pipes Steel sheet
North
South
Water pipes Temperature sensor Heat flux sensor
Insulation Flow meter Wind speed
Concrete slab
Fig. 3. SensorsÕ positions on the roof component (drawing not to scale).
34
0.8
32
0.7 0.6
30
0.5
28
0.4 26
0.3
24
0.2
22
0.1
20
0
Water flow rate (m3 hr -1)
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19 :0 0 19 :3 0 19 :5 9 20 :3 0 21 :0 0 21 :2 9 22 :0 0 22 :3 0 22 :5 9 23 :3 0 00 :0 0 00 :2 9 01 :0 0 01 :3 0 01 :5 9 02 :3 0 03 :0 0 03 :2 9 04 :0 0 04 :3 0 04 :5 9 05 :3 0 06 :0 0 06 :2 9 07 :0 0 07 :3 0
Temperature (oC)
A. Dimoudi, A. Androutsopoulos / Solar Energy 80 (2006) 1039–1047
Time (hr) Trad,in
Trad,out
Tcon,in
Tcon,out
Tambient
Tin
Flow rate
Fig. 4. Distribution of test room interior, ambient air and water pipe temperatures (25th June).
tive heat transfer to the ambient environment. The net radiative exchange (W m2) is expressed from the following relation: qnet ¼ erad rðT 4rad C a esky T 4amb Þ
ð1Þ
where erad is the emissivity of the radiator plate, r is the Stefan–Boltzmann constant (5.67 · 108 W m2 K4), Trad is the absolute temperature of the radiator plate (K), Ca is the cloudiness coefficient that can be deducted from the following relation Ca = 1 + 0.0224n 0.0035n2 + 0.00028n3, n is the total opaque cloud amount [0 for clear sky, 10 for overcast sky], Tamb is the absolute ambient air temperature (K) and esky is the emissivity of the clear sky. The sky emissivity is given by the expression (Martin and Berdahl, 1984a,b): esky ¼ 0:711 þ 0:56ðT dp =100Þ þ 0:73ðT dp =100Þ2 The dew point temperature, Tdp, is evaluated by the relation: T dp ¼ C 3
lnðRHÞ þ C 1 C 2 ðlnðRHÞ þ C 1 Þ
where C1 = C2 * Tamb/(C3 + Tamb), C2 = 17.08085, C3 = 234.175 and RH is the relative air humidity ranging from 0 to 1. The convective heat transfer at the radiatorÕs surface (W m2) is given by the following relation: qconv ¼ hconv ðT rad T amb Þ
ð2Þ
where hconv is the convective heat transfer coefficient (W m2 K1), Trad and Tamb are the radiator plate and ambient air temperature accordingly.
The convective coefficient for wind speeds from 1.5 to 5 m s1 can be evaluated from the expression: hconv = 2.8 + 0.76 * v, where v is the wind speed (Clark and Berdahl, 1980). The same expression can be applied with negligible absolute error for velocities ranging from 0.5 to 1.5 m s1. For wind speeds lower than 0.5 m s1, the hconv can be taken equal to 3.5 W m2 K1). The measured wind speed close to the radiator surface in the present experiments (Fig. 3) reached values up to 4.90 m s1, with an average value of 0.36 m s1 during the night period. The instantaneous cooling power of the radiator (W) can be determined by the temperature difference of the water at the radiatorÕs inlet and outlet together with the flow rate: _ rad;in T rad;out Þ Qcool;rad ¼ cp mðT
ð3Þ
where cp is the specific heat of water (J kg1 K1), m_ is the water mass flow rate (kg s1), and Trad,in Trad,out is the difference between the radiator inlet and outlet water temperatures (C). The total or effective cooling rate (qtot) varied considerably from night to night and were calculated as the cumulative effect of the net radiative (qnet) and convective (qconv) heat exchange according to Eqs. (1) and (2) (Fig. 5). Depending on the temperature of the radiator surface, the two mechanisms may be either complementary or counteract to each other. Fig. 5 also shows the modeled cooling rate of the radiator (qcool,rad), derived from Eq. (3) by using the monitored values of the experimental _ Trad,in and Trad,out of June–July. In Fig. 6, data m, the averaged cooling rates are presented against
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80 70
Cooling rate (Wm-2)
60 50 40 30 20 10 0 -10 19
20
21
22
23
-20
0
1
2
3
4
5
Hours Qcool,rad
Qnet
Qconv
Qtotal
Fig. 5. Cooling rates on a typical night period (12th–13th June).
120
Cooling rate (Wm-2)
100 80 60 40 20 0
0
0.2
0.4
0.6 0.8 1 Water flow rate (m3 hr-1)
1.2
1.4
Fig. 6. Averaged nightly cooling rates against flow rate.
the water flow rate. During the test period, the radiator cooling rates ranged from 9.5 to 97.8 W m2, with a mean value of 55.9 W m2 for water flow rates ranging from 0.03 to 1.27 m3 h1. The calculation of the cooling rates was based on a 8 h operation (from 21:00 to 05:00). It can be seen that the cooling rate increases with the flow rate. The observed cooling rate can be enclosed by two lines, as shown in Fig. 6, and the change is represented by a curve of logarithmic form. Simulation studies (Erell and Etzion, 2000) show that for each temperature difference DT = (Trad,in Trad,out), the change of the cooling rate is represented by a separate curve. Increasing the flow rate, the temperature dif-
ference between the inlet and outlet decreases, which in turn results at an increase of the mean radiatorÕs surface temperature and thus, increased radiative cooling under all environmental conditions. An upper limit exists due to the fact that as the flow rate increases, the radiatorÕs surface temperature approaches the water inlet temperature. In practice, a limitation also exists as higher flow rates require higher pump power to circulate the water through the radiator. The water flow rate has a significant effect on the water temperature change between the inlet and exit of the radiator as shown in Fig. 7. At low flow rates, the temperature difference between the inlet and exit
A. Dimoudi, A. Androutsopoulos / Solar Energy 80 (2006) 1039–1047
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Temperature difference (oC)
7 y = -1.5598Ln(x) + 0.4609
6
2
R = 0.9505
5 4 3 2 1 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Water flow rate (m-3 h-1) Fig. 7. Averaged radiator inlet exit temperature difference against flow rate.
at the radiator pipes reaches about 6.5 C whereas, at high flow rates decreases to 0.5 C. An exponential relationship describing the mean daily change of the water temperature between inlet and exit of the radiator pipes and the water flow rate is given by the expression DT = 0.4609 1.5598 * ln(Q) which has a correlation coefficient R2 = 0.95. The efficiency of the radiator was assessed using the following relationship (Sacadura, 1993): nrad ¼
T rad;in T rad;out T arad;in T stag
where Tstag is the stagnation temperature, and it the lower temperature that the radiator can reach, and
is given by the following formula (Argiriou and Santamouris, 1995; Santamouris et al., 1996): T stag ¼ T amb
erad rðT 4amb esky T 4amb Þ hconv þ 4erad rT 3amb
where erad is the emissivity of the radiator plate, r is the Stefan–Boltzmann constant (5.67 · 108 W m2 K4), Tamb is the ambient absolute temperature (K), esky is the emissivity of the clear sky, hconv is the convective heat transfer at the radiatorÕs surface (W m2 K). Fig. 8 presents the change of the radiator efficiency in relation to the water flow rate through the radiator. It can be seen that the radiatorÕs
0.9 0.8
y= -0.1658Ln(x) + 0.1357 R2 = 0.9206
Radiator efficiency
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Flow rate (m3 hr-1) Fig. 8. RadiatorÕs efficiency change against the water flow rate through the radiator.
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A. Dimoudi, A. Androutsopoulos / Solar Energy 80 (2006) 1039–1047
120
Cooling rate (W m-2)
100 80 60 40 20 0
-3
-2
-1
0
1 2 Trad eff - Tamb (K)
3
4
5
6
Fig. 9. Cooling panel and ambient air temperature difference vs. cooling rate.
efficiency is drastically reduced with the increase of the water flow rate. In practice, a high efficiency of the radiator, results to decrease of the potential of nocturnal cooling. The effective operation of such a cooling system requires the appropriate choice of materials and sizing of the radiator and the concrete slab and also control of the water flow rate through the radiator, always in conjunction with the prevailing climatic conditions. The effect on the cooling rate of the temperature difference between the cooling panel and the ambient air—which follows the same pattern with the temperature difference between the radiator and ambient—is shown in Fig. 9. The observed cooling rate can be enclosed by two lines, where the upper line is interpreted as the cooling rate for clear night sky and the lower line for cloudy sky (Meir et al., 2002). A greater temperature difference results to higher averaged cooling rates. Other meteorological factors, like the ambient temperature, the sky temperature and the wind speed measured at the radiatorÕs surface, although they affect cooling rate, did not seem to have a strong correlation with the cooling rate for the recorded climatic conditions. It must be noted that during the test period the weather conditions were quite constant (Tamb,min = 17.1 C and Tamb,max = 34.9 C) so extreme conditions could not be investigated. 5. Conclusions This paper has presented the potential for radiative cooling of a radiative–cooling panel system from experimental tests carried out in real weather
conditions. Investigation of the potential of nocturnal radiative cooling of buildings using the roof of a building showed that integration of a water radiator in the roof can contribute to the cooling of building constructions. The regulation of the water flow rate is a key issue to the radiatorÕs effectiveness, which, together with the effort to keep the temperature of the radiator warm, can significantly affect the cooling power of the radiator. Maintaining the temperature of the radiator higher than the DBT can increase the cooling power due to reduced convection losses but it can also prevent condensation on the surface of the radiator that in turn obstructs radiation to the sky. The applicability of radiative cooling systems is restricted in areas with uniformity in the height of buildings and thus, its application is limited in urban areas without dense texture and varied building heights. City centres in several Mediterranean areas and especially in Greece are characterized by buildings of about the same height with flat roof, and thus, potentially this system can be applied. The efficiency of the system depends on the available exposed flat roof area and its cooling potential can be mainly explored in single floor buildings or by the top floor in multi floor buildings. The insulating value this system can add to a flat roof should be mentioned along with its applicability in renovation of existing, not insulated flat roofs. The extra weight of the water and the whole structure should be considered in relation to the limitation of the weight withstand of the building structure. The investigated radiator cooling system, while being at an experimental phase, can be improved
A. Dimoudi, A. Androutsopoulos / Solar Energy 80 (2006) 1039–1047
with the investigation of parameters like the radiator pipesÕ distance, the radiator and pipes material and colour, the contact of the pipes with the radiator in order to maximize the active surface of the radiator. Computer simulations of the system can also be used in future work in order to optimize the design characteristics and the performance of the system. The system has also the potential for use during the heating period in areas characterized by hot and arid summers but cold and sunny winters, like in many Mediterranean areas. Acknowledgements This work was carried out in the frame of the EC Joule project ROOFSOL (JOR3-CT96-0074), cofunded by the European Commission (DG XII) and the General Secretary of Research and Development of the Greek Ministry of Development. The contributions of Mr. G. Sutherland, for the initiation and early design of this project and of Mr. M. Vallindras for the design and construction of the roof component should be acknowledged. References Al-Nimr, M.A., Kodah, Z., Nassar, B., 1998. A theoretical and experimental investigation of a radiative cooling system. Solar Energy 63 (6), 367–373. Argiriou, A., Santamouris, M., 1995. Natural Cooling Techniques. CIENE-University of Athens, European Commission, Directorate General XVII for Energy, Athens. Argiriou, A., Santamouris, M., Balaras, C., Jeter, S., 1993. Potential of radiative cooling in southern Europe. Int. J. Solar Energy 13, 189–203. Argiriou, A., Santamouris, M., Asimakopoulos, D., 1994. Assessment of radiative cooling potential of a collector using hourly weather data. Energy 19 (8), 879–888. Clark, E., Berdahl, P., 1980. Radiative cooling: resource and applications. In: Miller, H. (Ed.), Proceedings of the Passive Cooling Workshop. Massachusetts, Amherst, pp. 177–212.
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Erell, E., Etzion, Y., 1992. A radiative cooling system using water as heat transfer medium. Architect. Sci. Rev. 35, 35–49. Erell, E., Etzion, Y., 1996. Heating experiments with a radiative cooling system. Build. Environ. 31 (6), 509–517. Erell, E., Etzion, Y., 1999. Analysis and experimental verification, of an improved cooling radiator. Renew. Energy 16, 700–703. Erell, E., Etzion, Y., 2000. Radiative cooling of buildings with flat plate solar collectors. Build. Environ. 35 (4), 297–305. Etzion, Y., Erell, E., 1991. Thermal storage mass in radiative cooling systems. Build. Environ. 26 (4), 389–394. Givoni, B., 1994. Passive and Low Energy Cooling of Buildings. Van Nostrand Reinhold, New York, ISBN 0-442-01076-1. Goswami, D.Y., Kreith, F., Kreider, J., 2000. Principles of Solar Engineering, second ed. Taylor and Francis Publ., ISBN 15600327146. Hays, H., Yellot, J., 1969. Natural air conditioning with roof ponds and natural air conditioning. ASHRAE Trans. 75 (1), 165–177. Martin, M., 1989. Radiative cooling. In: Cook, J. (Ed.), Passive Cooling. MIT Press. Martin, M., Berdahl, P., 1984a. Summary of results from the spectral and angular sky radiation measurement program. Solar Energy 33, 241–252. Martin, M., Berdahl, P., 1984b. Characteristics of infrared sky radiation in the United States. Solar Energy 33, 321– 336. Matsuta, M., Terada, S., Ito, H., 1987. Solar heating and radiative cooling using a solar collector-sky radiator with a spectrally selective surface. Solar Energy 39 (3), 183–186. Meir, M.G., Rekstad, J.B., Lovvik, O.M., 2002. A study of a polymer-based radiative cooling system. Solar Energy 73 (6), 403–417. Michalakakou, G., Ferrante, A., Lewis, J.O., 1998. The cooling potential of a metallic nocturnal radiator. Energy Build. 28 (3), 251–256. Sacadura, J.R. (Ed.), 1993. Initiation aux Transferts Thermiques, fourth ed. Technique et Documentation, Paris. Santamouris, M., Asimakopoulos, D. (Eds.), Argiriou, A., Balaras, C., Dascalaki, E., Dimoudi, A., Mantas, D., Tselepidaki, I., 1996. Passive Cooling of Buildings. James & James (Sciences Publishers) Ltd., London, ISBN 1 873936 47 8. Wouters, P., Vandaele, L. (Eds)., 1994. The PASSYS Services— Summary report of the PASSYS project. European Commission, DG XII for Science, Research & Development, EUR 15113 EN.
The cooling performance of a radiator based roof component A. Dimoudi
a,*,1
, A. Androutsopoulos
b
a
b
Ministry of the Environment, Phys. Planning and Public Works, Direct. of Housing Policy and Shelter, 36, Trikalon Str., Mesogeion Aven., 115 25 Athens, Greece Buildings Department, Division of Energy Efficiency, Centre for Renewable Energy Sources (CRES), 19th km Marathonos Aven., 190 09 Pikermi, Greece Received 16 September 2003; received in revised form 31 May 2005; accepted 6 June 2005 Available online 20 September 2005 Communicated by: Associate Editor Matheos Santamouris
Abstract Energy conservation in buildings is becoming an issue of great importance. Space cooling is getting important in most countries and different techniques have been developed one of which is radiative cooling. A prototype roof component, exploiting radiative cooling, was built and tested in the outdoor test facilities of the Centre of Renewable Energy Sources in Greece. The component comprises a radiator, which is utilizing water as the fluid medium and its cooling performance was investigated. This paper presents the construction of the component, the experimental set-up and the results taken during the monitoring procedure. 2005 Elsevier Ltd. All rights reserved. Keywords: Radiative cooling; Passive cooling of buildings; Experimental results
1. Introduction The design and construction of the roof of a building plays an important role at the heating and cooling needs of a building. In the Mediterranean region, the solar radiation received on a horizontal surface is about 2–3 times greater than the one received on a south oriented surface, depending on the latitude of the place and month of the year. Thus, the appropriate construction of the roof can
*
1
Corresponding author. Tel./fax: +30 210 80 87 959. E-mail address: [email protected] (A. Dimoudi). Formerly with CRES.
play a significant role at the thermal performance of a building. The roof is needed to protect the building from the outdoor climatic conditions (mainly by application of thermal insulation) and on the other hand can be used as an integrated component of the building that exploits the environmental sinks. In hot climates, roof may be used as an integrated building cooling component, for cooling with either evaporation or with radiation. In the last three decades, special attention has been given towards the energy conservation in buildings for both heating and cooling purposes. Especially for the latter, a very important issue for countries with hot climate, several techniques have been developed one of which exploits radiative
0038-092X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.06.017
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A. Dimoudi, A. Androutsopoulos / Solar Energy 80 (2006) 1039–1047
Nomenclature C1 C2 C3 Ca
=C2*Tamb/(C3 + Tamb) 17.08085 234.175 cloudiness coefficient, Ca = 1 + 0.0224n 0.0035n2 + 0.00028n3 cp specific heat of water (J kg1 K1) hconv convective heat transfer coefficient (W m2 K1) m_ water mass flow rate (kg s1) n total opaque cloud amount, 0 6 n 6 1 nrad radiatorÕs efficiency qconv convective heat transfer at the radiatorÕs surface (W m2) (Eq. (2)) Qcool,rad cooling power of the radiator (W) (Eq. (3)) qcool,rad cooling rate of the radiator (W m2 pipe) qnet net radiative heat exchange (W m2) (Eq. (1)) qtot total or effective cooling rate (W m2) RH relative air humidity, 0 6 RH 6 1 Tambient ambient temperature (C)
cooling. Different approaches have been made to design applicable systems and investigate their performance. Studies were carried out both for air and water radiators. Water radiators are either water pond systems (Hays and Yellot, 1969; Martin, 1989; Givoni, 1994; Goswami et al., 2000) or flat plate radiator systems. Different designs and pipe materials were theoretically (Argiriou et al., 1993, 1994; Michalakakou et al., 1998) or/ and experimentally investigated for flat plate radiator systems (Etzion and Erell, 1991; Erell and Etzion, 1992, 1999, 2000; Al-Nimr et al., 1998; Meir et al., 2002). The same systems were also investigated during winter for space solar heating (Matsuta et al., 1987; Erell and Etzion, 1996). The present work investigates the cooling performance of an unglazed water radiator connected to a series of water pipes embedded into the roof of the building (cooling panel). A prototype roof component, for the exploitation of radiative cooling, was built and tested at the outdoor test facilities of the Centre of Renewable Energy Sources in Greece. The motivation was to design a system that not only meets the cooling demand of a building but it should have the potential for architectural integra-
absolute ambient air temperature (K) Tamb Tcon concrete slab mean temperature (C) Tcon,in, Tcon,out inlet, outlet temperature of the cooling panel pipes (C) Tdp dew point temperature (C) Tin air temperature inside the test room (C) Trad absolute temperature of the radiator plate (K) Trad eff absolute temperature of the radiator pipes (K) Trad,in, Trad,out inlet, outlet radiator pipes water temperature (C) Tstag stagnation temperature (K) v wind speed (m s1) Greek symbols erad emissivity of the radiator plate esky emissivity of clear sky r Stefan–Boltzmann constant 108 W m2 K4)
(5.67 ·
tion. In the design of the component, the traditional and the contemporary construction practice in Greece was taken into account, as well as the application and appropriateness of the cooling techniques for covering the energy needs of a building. This paper presents the construction of the component, the experimental set-up and the results of the monitoring procedure during summer period. 2. Description of the roof component structure The tested roof component consisted of a water radiator linked to a cooling panel. The water radiator was comprised of a series of pipes exposed to the ambient environment, linked to a series of pipes embedded into the concrete roof slab (cooling panel). The dimensions of the roof component were 2.715 m wide by 4.970 m long. The water radiator was constructed with a series of heavy duty steel pipes of 3/4 in. diameter, 10 pipes per meter roof width. The pipes were fixed on a steel plate, both of them painted white, for minimisation of the solar gains during the day. The water radiator system was placed on 8 cm insulation layer. Water pipes of the same material and
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diameter (3/4 in.) were embedded into the centreline of the concrete slab, with a between distance of 20 cm (5 pipes per meter). The thickness of the concrete slab was 12 cm. Two plenums across the width of the roof, at the North and South side of the test cell, connected the water radiator pipes with the water pipes inside the concrete slab and the water circulates with a circulator in a close loop. Construction details of the roof component are shown in Figs. 1 and 2. Special attention was given at the design and the construction of the unit, in order to eliminate any heat bridges at the envelope of the building and any air leakage paths as the envelope of the test cell is constructed to very high airtightness standards with negligible infiltration losses, normally less than 0.5 ACH (Wouters and Vandaele, 1994). The proposed design, with the insulation layer permanently in place beneath the radiator ensures the protection of the roof from heat gains. The operation of a water circulator during the night switches the roof to a cooling mode. The proposed roof element can be easily applied to flat roofs which are widely used in Greek buildings. With this construction, the structural mass of the building is cooled than the interior air. Its construction details can ensure thermal insulation of the roof according to the existing regulations. It can also be applied in renovation of buildings, as water pipes can be added on top of an existing concrete roof, covered with a layer of concrete, a layer of insulation and the water radiator. The same component can also be used during winter as a heating component.
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Fig. 2. View of the tested roof component.
3. Experimental procedure and instrumentation The tests took place at the upgraded PASLINK test cell of CRES (Wouters and Vandaele, 1994), which is a highly insulated box, with dimensions of 8.4 m · 3.8 m · 3.6 m, located at the outdoor environment, into which a full scale building component, wall or roof, can be tested under real weather conditions. The test cell is partitioned into two rooms: the Test Room with dimensions of 2.75 m · 2.75 m · 5.00 m and the Service Room which contains auxiliary equipment for the operation of the test procedures. Its main characteristic is the fully controlled thermal conditions of the Test Room and the high degree of the envelope thermal insulation that results to minimization of the heat losses from all surfaces of the Test Room except the one where the tested component is installed.
Fig. 1. Cross section of the prototype roof component—construction details.
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The test cell is equipped with a sophisticated heating and cooling system controlled by a data acquisition and control unit for implementation of specially developed dynamic test sequences. Both, indoor and outdoor climatic conditions are monitored using a standard set of sensors, including: solar radiation (diffuse and direct), long wave radiation, wind speed and direction, relative humidity, air and surface temperatures, heating and cooling power. All measurements are recorded at one minutely intervals by the central data acquisition system. The aim of the applied cooling technique is to cover a fraction of the cooling load of the building. This coolness is stored in the ceiling structure and the energy should be released in accordance to the load profile of the building. Continuous measurements were taken for 22 days during June–July (Julian Days 160–182). The air temperature inside the test room was kept constant at about 28 C and different water flow rates were applied at the water radiator. The standard instrumentation of the test cell was used to monitor the thermal performance of the room. Additional sensors were installed to measure the performance of the roof component. For the environmental parameters, the ambient temperature (PT 100, shielded) and the wind speed at the height of the roof (cap anemometer) were measured. For the roof component: 21 T-type thermocouples on different positions and heights at the component, 4 PT 100 sensors for the water temperature, an electromagnetic flow meter, measuring in the range of 0–2.5 l s1 and 2 heat flux sensors (HFS) were used. The position of the roof sensors is shown in Fig. 3. The data from all sensors were collected at the data acquisition system at 1 min intervals, averaged to a 10 min basis.
A water filter was also installed at the water circuit. The electric pump was controlled by a timer switch so that water circulated on a 12 h basis (from 19:00 to 07:00). The flow rate was controlled through a valve positioned next to the pump. The water was circulating during the night collecting heat from the concrete slab and extracting it to the environment. 4. Experimental results The use of the concrete slab as a cooling panel was found to be an effective way to extract heat from the roof. Fig. 4 depicts the distribution of the concrete slab mean temperature (Tcon), the air temperature inside the test room (Tin), which was controlled to be constant, at about 28 C, the ambient temperature (Tambient), the inlet/outlet temperatures of the cooling panel pipes (Tcon,in, Tcon,out) and the radiator pipes (Trad,in, Trad,out) during the night period of a typical test day. The temperature of the concrete slab was decreasing during the night and always being lower than the temperatures of the water pipes and the room air which was controlled to be constant during the whole day. The concrete slab temperature reduction ranged from 3 to 6 C during the night operation of the system. During the testing period, the ambient temperature was smoothly changing during the day while during the night fluctuation of the temperature was observed indicating cloudy nights. During the test period, the weather conditions were quite constant, with Tamb,min = 17.1 C and Tamb,max = 34.9 C. The radiator surface exchange heat with the external environment through two different mechanisms: radiative heat transfer to the sky and convec-
Radiator pipes Steel sheet
North
South
Water pipes Temperature sensor Heat flux sensor
Insulation Flow meter Wind speed
Concrete slab
Fig. 3. SensorsÕ positions on the roof component (drawing not to scale).
34
0.8
32
0.7 0.6
30
0.5
28
0.4 26
0.3
24
0.2
22
0.1
20
0
Water flow rate (m3 hr -1)
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19 :0 0 19 :3 0 19 :5 9 20 :3 0 21 :0 0 21 :2 9 22 :0 0 22 :3 0 22 :5 9 23 :3 0 00 :0 0 00 :2 9 01 :0 0 01 :3 0 01 :5 9 02 :3 0 03 :0 0 03 :2 9 04 :0 0 04 :3 0 04 :5 9 05 :3 0 06 :0 0 06 :2 9 07 :0 0 07 :3 0
Temperature (oC)
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Time (hr) Trad,in
Trad,out
Tcon,in
Tcon,out
Tambient
Tin
Flow rate
Fig. 4. Distribution of test room interior, ambient air and water pipe temperatures (25th June).
tive heat transfer to the ambient environment. The net radiative exchange (W m2) is expressed from the following relation: qnet ¼ erad rðT 4rad C a esky T 4amb Þ
ð1Þ
where erad is the emissivity of the radiator plate, r is the Stefan–Boltzmann constant (5.67 · 108 W m2 K4), Trad is the absolute temperature of the radiator plate (K), Ca is the cloudiness coefficient that can be deducted from the following relation Ca = 1 + 0.0224n 0.0035n2 + 0.00028n3, n is the total opaque cloud amount [0 for clear sky, 10 for overcast sky], Tamb is the absolute ambient air temperature (K) and esky is the emissivity of the clear sky. The sky emissivity is given by the expression (Martin and Berdahl, 1984a,b): esky ¼ 0:711 þ 0:56ðT dp =100Þ þ 0:73ðT dp =100Þ2 The dew point temperature, Tdp, is evaluated by the relation: T dp ¼ C 3
lnðRHÞ þ C 1 C 2 ðlnðRHÞ þ C 1 Þ
where C1 = C2 * Tamb/(C3 + Tamb), C2 = 17.08085, C3 = 234.175 and RH is the relative air humidity ranging from 0 to 1. The convective heat transfer at the radiatorÕs surface (W m2) is given by the following relation: qconv ¼ hconv ðT rad T amb Þ
ð2Þ
where hconv is the convective heat transfer coefficient (W m2 K1), Trad and Tamb are the radiator plate and ambient air temperature accordingly.
The convective coefficient for wind speeds from 1.5 to 5 m s1 can be evaluated from the expression: hconv = 2.8 + 0.76 * v, where v is the wind speed (Clark and Berdahl, 1980). The same expression can be applied with negligible absolute error for velocities ranging from 0.5 to 1.5 m s1. For wind speeds lower than 0.5 m s1, the hconv can be taken equal to 3.5 W m2 K1). The measured wind speed close to the radiator surface in the present experiments (Fig. 3) reached values up to 4.90 m s1, with an average value of 0.36 m s1 during the night period. The instantaneous cooling power of the radiator (W) can be determined by the temperature difference of the water at the radiatorÕs inlet and outlet together with the flow rate: _ rad;in T rad;out Þ Qcool;rad ¼ cp mðT
ð3Þ
where cp is the specific heat of water (J kg1 K1), m_ is the water mass flow rate (kg s1), and Trad,in Trad,out is the difference between the radiator inlet and outlet water temperatures (C). The total or effective cooling rate (qtot) varied considerably from night to night and were calculated as the cumulative effect of the net radiative (qnet) and convective (qconv) heat exchange according to Eqs. (1) and (2) (Fig. 5). Depending on the temperature of the radiator surface, the two mechanisms may be either complementary or counteract to each other. Fig. 5 also shows the modeled cooling rate of the radiator (qcool,rad), derived from Eq. (3) by using the monitored values of the experimental _ Trad,in and Trad,out of June–July. In Fig. 6, data m, the averaged cooling rates are presented against
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80 70
Cooling rate (Wm-2)
60 50 40 30 20 10 0 -10 19
20
21
22
23
-20
0
1
2
3
4
5
Hours Qcool,rad
Qnet
Qconv
Qtotal
Fig. 5. Cooling rates on a typical night period (12th–13th June).
120
Cooling rate (Wm-2)
100 80 60 40 20 0
0
0.2
0.4
0.6 0.8 1 Water flow rate (m3 hr-1)
1.2
1.4
Fig. 6. Averaged nightly cooling rates against flow rate.
the water flow rate. During the test period, the radiator cooling rates ranged from 9.5 to 97.8 W m2, with a mean value of 55.9 W m2 for water flow rates ranging from 0.03 to 1.27 m3 h1. The calculation of the cooling rates was based on a 8 h operation (from 21:00 to 05:00). It can be seen that the cooling rate increases with the flow rate. The observed cooling rate can be enclosed by two lines, as shown in Fig. 6, and the change is represented by a curve of logarithmic form. Simulation studies (Erell and Etzion, 2000) show that for each temperature difference DT = (Trad,in Trad,out), the change of the cooling rate is represented by a separate curve. Increasing the flow rate, the temperature dif-
ference between the inlet and outlet decreases, which in turn results at an increase of the mean radiatorÕs surface temperature and thus, increased radiative cooling under all environmental conditions. An upper limit exists due to the fact that as the flow rate increases, the radiatorÕs surface temperature approaches the water inlet temperature. In practice, a limitation also exists as higher flow rates require higher pump power to circulate the water through the radiator. The water flow rate has a significant effect on the water temperature change between the inlet and exit of the radiator as shown in Fig. 7. At low flow rates, the temperature difference between the inlet and exit
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Temperature difference (oC)
7 y = -1.5598Ln(x) + 0.4609
6
2
R = 0.9505
5 4 3 2 1 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Water flow rate (m-3 h-1) Fig. 7. Averaged radiator inlet exit temperature difference against flow rate.
at the radiator pipes reaches about 6.5 C whereas, at high flow rates decreases to 0.5 C. An exponential relationship describing the mean daily change of the water temperature between inlet and exit of the radiator pipes and the water flow rate is given by the expression DT = 0.4609 1.5598 * ln(Q) which has a correlation coefficient R2 = 0.95. The efficiency of the radiator was assessed using the following relationship (Sacadura, 1993): nrad ¼
T rad;in T rad;out T arad;in T stag
where Tstag is the stagnation temperature, and it the lower temperature that the radiator can reach, and
is given by the following formula (Argiriou and Santamouris, 1995; Santamouris et al., 1996): T stag ¼ T amb
erad rðT 4amb esky T 4amb Þ hconv þ 4erad rT 3amb
where erad is the emissivity of the radiator plate, r is the Stefan–Boltzmann constant (5.67 · 108 W m2 K4), Tamb is the ambient absolute temperature (K), esky is the emissivity of the clear sky, hconv is the convective heat transfer at the radiatorÕs surface (W m2 K). Fig. 8 presents the change of the radiator efficiency in relation to the water flow rate through the radiator. It can be seen that the radiatorÕs
0.9 0.8
y= -0.1658Ln(x) + 0.1357 R2 = 0.9206
Radiator efficiency
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Flow rate (m3 hr-1) Fig. 8. RadiatorÕs efficiency change against the water flow rate through the radiator.
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120
Cooling rate (W m-2)
100 80 60 40 20 0
-3
-2
-1
0
1 2 Trad eff - Tamb (K)
3
4
5
6
Fig. 9. Cooling panel and ambient air temperature difference vs. cooling rate.
efficiency is drastically reduced with the increase of the water flow rate. In practice, a high efficiency of the radiator, results to decrease of the potential of nocturnal cooling. The effective operation of such a cooling system requires the appropriate choice of materials and sizing of the radiator and the concrete slab and also control of the water flow rate through the radiator, always in conjunction with the prevailing climatic conditions. The effect on the cooling rate of the temperature difference between the cooling panel and the ambient air—which follows the same pattern with the temperature difference between the radiator and ambient—is shown in Fig. 9. The observed cooling rate can be enclosed by two lines, where the upper line is interpreted as the cooling rate for clear night sky and the lower line for cloudy sky (Meir et al., 2002). A greater temperature difference results to higher averaged cooling rates. Other meteorological factors, like the ambient temperature, the sky temperature and the wind speed measured at the radiatorÕs surface, although they affect cooling rate, did not seem to have a strong correlation with the cooling rate for the recorded climatic conditions. It must be noted that during the test period the weather conditions were quite constant (Tamb,min = 17.1 C and Tamb,max = 34.9 C) so extreme conditions could not be investigated. 5. Conclusions This paper has presented the potential for radiative cooling of a radiative–cooling panel system from experimental tests carried out in real weather
conditions. Investigation of the potential of nocturnal radiative cooling of buildings using the roof of a building showed that integration of a water radiator in the roof can contribute to the cooling of building constructions. The regulation of the water flow rate is a key issue to the radiatorÕs effectiveness, which, together with the effort to keep the temperature of the radiator warm, can significantly affect the cooling power of the radiator. Maintaining the temperature of the radiator higher than the DBT can increase the cooling power due to reduced convection losses but it can also prevent condensation on the surface of the radiator that in turn obstructs radiation to the sky. The applicability of radiative cooling systems is restricted in areas with uniformity in the height of buildings and thus, its application is limited in urban areas without dense texture and varied building heights. City centres in several Mediterranean areas and especially in Greece are characterized by buildings of about the same height with flat roof, and thus, potentially this system can be applied. The efficiency of the system depends on the available exposed flat roof area and its cooling potential can be mainly explored in single floor buildings or by the top floor in multi floor buildings. The insulating value this system can add to a flat roof should be mentioned along with its applicability in renovation of existing, not insulated flat roofs. The extra weight of the water and the whole structure should be considered in relation to the limitation of the weight withstand of the building structure. The investigated radiator cooling system, while being at an experimental phase, can be improved
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with the investigation of parameters like the radiator pipesÕ distance, the radiator and pipes material and colour, the contact of the pipes with the radiator in order to maximize the active surface of the radiator. Computer simulations of the system can also be used in future work in order to optimize the design characteristics and the performance of the system. The system has also the potential for use during the heating period in areas characterized by hot and arid summers but cold and sunny winters, like in many Mediterranean areas. Acknowledgements This work was carried out in the frame of the EC Joule project ROOFSOL (JOR3-CT96-0074), cofunded by the European Commission (DG XII) and the General Secretary of Research and Development of the Greek Ministry of Development. The contributions of Mr. G. Sutherland, for the initiation and early design of this project and of Mr. M. Vallindras for the design and construction of the roof component should be acknowledged. References Al-Nimr, M.A., Kodah, Z., Nassar, B., 1998. A theoretical and experimental investigation of a radiative cooling system. Solar Energy 63 (6), 367–373. Argiriou, A., Santamouris, M., 1995. Natural Cooling Techniques. CIENE-University of Athens, European Commission, Directorate General XVII for Energy, Athens. Argiriou, A., Santamouris, M., Balaras, C., Jeter, S., 1993. Potential of radiative cooling in southern Europe. Int. J. Solar Energy 13, 189–203. Argiriou, A., Santamouris, M., Asimakopoulos, D., 1994. Assessment of radiative cooling potential of a collector using hourly weather data. Energy 19 (8), 879–888. Clark, E., Berdahl, P., 1980. Radiative cooling: resource and applications. In: Miller, H. (Ed.), Proceedings of the Passive Cooling Workshop. Massachusetts, Amherst, pp. 177–212.
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Erell, E., Etzion, Y., 1992. A radiative cooling system using water as heat transfer medium. Architect. Sci. Rev. 35, 35–49. Erell, E., Etzion, Y., 1996. Heating experiments with a radiative cooling system. Build. Environ. 31 (6), 509–517. Erell, E., Etzion, Y., 1999. Analysis and experimental verification, of an improved cooling radiator. Renew. Energy 16, 700–703. Erell, E., Etzion, Y., 2000. Radiative cooling of buildings with flat plate solar collectors. Build. Environ. 35 (4), 297–305. Etzion, Y., Erell, E., 1991. Thermal storage mass in radiative cooling systems. Build. Environ. 26 (4), 389–394. Givoni, B., 1994. Passive and Low Energy Cooling of Buildings. Van Nostrand Reinhold, New York, ISBN 0-442-01076-1. Goswami, D.Y., Kreith, F., Kreider, J., 2000. Principles of Solar Engineering, second ed. Taylor and Francis Publ., ISBN 15600327146. Hays, H., Yellot, J., 1969. Natural air conditioning with roof ponds and natural air conditioning. ASHRAE Trans. 75 (1), 165–177. Martin, M., 1989. Radiative cooling. In: Cook, J. (Ed.), Passive Cooling. MIT Press. Martin, M., Berdahl, P., 1984a. Summary of results from the spectral and angular sky radiation measurement program. Solar Energy 33, 241–252. Martin, M., Berdahl, P., 1984b. Characteristics of infrared sky radiation in the United States. Solar Energy 33, 321– 336. Matsuta, M., Terada, S., Ito, H., 1987. Solar heating and radiative cooling using a solar collector-sky radiator with a spectrally selective surface. Solar Energy 39 (3), 183–186. Meir, M.G., Rekstad, J.B., Lovvik, O.M., 2002. A study of a polymer-based radiative cooling system. Solar Energy 73 (6), 403–417. Michalakakou, G., Ferrante, A., Lewis, J.O., 1998. The cooling potential of a metallic nocturnal radiator. Energy Build. 28 (3), 251–256. Sacadura, J.R. (Ed.), 1993. Initiation aux Transferts Thermiques, fourth ed. Technique et Documentation, Paris. Santamouris, M., Asimakopoulos, D. (Eds.), Argiriou, A., Balaras, C., Dascalaki, E., Dimoudi, A., Mantas, D., Tselepidaki, I., 1996. Passive Cooling of Buildings. James & James (Sciences Publishers) Ltd., London, ISBN 1 873936 47 8. Wouters, P., Vandaele, L. (Eds)., 1994. The PASSYS Services— Summary report of the PASSYS project. European Commission, DG XII for Science, Research & Development, EUR 15113 EN.