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3-012 (O)
Proceedings of the
2nd Regional Conference on Energy Technology Towards a Clean Environment
12-14 February 2003, Phuket, Thailand
Night radiative cooling and underground water
storage in a hot humid climate: a preliminary
investigation
Auttapol R. T. Golaka, and R. H. B. Exell
Energy Division, The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology
Thonburi, Thungkru, Bangkok, 10140, Thailand.
Abstract
The technical feasibility of long-term storage of water cooled by nocturnal
radiation and stored underground has been investigated for the hot humid climate
of the northern region of Thailand for application in residential air conditioning
during the hot season. The system consists of a special sky radiator and two
underground storage tanks, one for warm water and one for cold water. During the
night water from the warm tank is pumped though the sky radiator to be cooled by
long wave radiation emission. The chilled water is then stored in the cold tank.
During the day the cold water is delivered to a cooling unit in a residential room.
A computer model of the system using surface meteorological data for Chiang Rai
indicates that the underground cold water can be kept at temperatures 14.5ºC
to 22ºC from December to July. In March and April the predicted water
temperatures are 16ºC to 18ºC. This suggests that the system would be useful for
air conditioning in the hottest season.
Keywords
nocturnal cooling, sky radiator, and underground storage
Introduction
The cooling of water by nocturnal radiation to the sky and the long-term storage
of the cooled water in an underground tank for residential air conditioning in
Thailand during the hot season has the highest potential for success in the
northern region. Underground storage of thermal energy by using rock beds and
water have been studied by Givoni, B., [1], and by Arbel, A. and Sokolov, M., [2].
All theses studies have stored water at a high temperature for seasonal heating
applications in cold countries such as Sweden, Netherlands and Germany,
proposed by Lundin et al., [3], Bohoven et al., [4] and Kubler et al., [5]
respectively.
Passive cooling and cool storage has been widely exploited in the hot arid climate
of the Middle East. Night radiative cooling, applied in the natural ice maker was
reported by Sayigh [6], and cooling by flowing water has been studied by
Al-Nimr et al., [7]. In a humid region, Japan, where strong radiative cooling at
night could not be expected, the radiative cooling process enhanced by an
uncovered sky radiator surface painted black, has been studied for storing thermal
energy by Ito and Mimura [8].
In Thailand, where the atmosphere at night has strong downward atmospheric
radiation as predicted by Exell, [9], Khedari et al., [10] stated that the northern
region of Thailand has the highest potential for application of night radiative
3-012(O)
Proceedings of the
2nd Regional Conference on Energy Technology Towards a Clean Environment
12-14 February 2003, Phuket, Thailand
cooling because the downward atmospheric radiation of this region is lower than
in other regions in this country. This was confirmed in the earlier research that
employed radiative cooling to the sky at night in the northern region by
Boon-long, [11].
The objective of this investigation is to perform a computational simulation of an
integrated system with underground water storage cooled by night radiation under
varying meteorological conditions in a hot humid climate for application in air
conditioning.
Sky Radiator A room in house
TcTh
Ground Level
0.00 m
Insulation with Water
Proof
Pump
Fan coil unit
Low Temperature Tank
High Temperature
Tank
Water Flowmeter
Tr_out
Tp
Tr_in
Tc
Tl
Tg
Tug
Tc , Tg , Th , Tl , Tp , Tr_in , Tr_out and Tug are temperatures at various parts of the system.
Figure 1. The integrated underground water cool storage and night radiative
cooling system.
Description of the system
A schematic diagram of the system studied including underground water storage
with sky radiator, along with a residential cooling device is illustrated in Fig.1.
Sky radiators for cooling at night have been extensively studied [12,13]. Typically
they have been painted black and covered with polyethylene film [14,15] and
light weight [16], but the use of TiO2 white paint to boost the emissive power has
been vindicated [17,18]. In this study a novel design for the tropical climate has
been introduced (Fig. 2). This radiator consists of a flat plate coated with white
paint and thin rectangular aluminum sheets serving as windshields instead of the
polyethylene film as in other sky radiators. No insulation is need on the sides and
the bottom. The advantages of this design are lightweight, low cost, and heat
released readily to the environment at night. The basic equation of net heat flux of
this radiator is:
qr = εσ (Tp4 – Ts4) + U(Tp – Ta). (1)
Water inlet
Figure 2. The schematic of sky radiator for tropical climate
The underground storage is separated into two tanks, one for warm water at
temperature Th and one for cool water at temperature Tc, as shown in Fig.1. It is
assumed for simplicity that the water temperatures in each tank are uniform, so no
stratification occurs. The equations describing the energy balance on the storage
tanks account for heat radiated by the sky radiator in the nighttime, energy
removed by load as well as energy lost to the surroundings of the underground
tank. They are as follows.
3-012(O)
Proceedings of the
2nd Regional Conference on Energy Technology Towards a Clean Environment
12-14 February 2003, Phuket, Thailand
Energy balance in high temperature side:
(ρV)CpdT/dt = UhAh(Tg– Th) + UugAug(Tug– Th) + m'Cp(Tl – Th) – m'Cp(Th – Tr_out). (2)
Energy balance in low temperature side:
(ρV)CpdT/dt = UcAc(Tg– Tc) + UugAug(Tug– Tc) + m'Cp(Tc – Tl). (3)
As shown in Fig.1, water cooled by the sky radiator during the nighttime, will be
pumped out of the high temperature tank with temperature Th, underneath the
radiative surface, through the sky radiator. The water will be cooled by emitting
long wave radiation to the sky, before being returned to the underground cool
storage with temperature Tr. During the daytime water from the low temperature
tank at temperature Tc enters the fan coil unit where it receives heat from the room
cooling load, and is then returned to the high temperature tank at temperature Tl.
The energy transfer between the sky radiator with high temperature tank and also
the energy transfer between cool storage with residential room can be expressed
by equations (4) and (5) respectively.
Qr = m'Cp(Th–Tr_out), Ql = m'Cp(Tc–Tl). (4), (5)
The maximum allowable water mass flow rate into the heat exchange is given by:
m' max = Ql / (Tc–Tl)Cp. (6)
Heat exchange between the underground tanks and the soil continues during the
daytime as well as at night, but this is small because the tanks are insulated.
Simulation techniques and assumptions
In this simulation, it is assumed that the heat loss between the pipe and the
environment is negligible and the water temperature inside the high temperature
tank is equal to the temperature of the inlet water to sky radiator (Th = Tr_in= Tp).
For simplicity, it is also assumed that, the outlet water temperature from the fan
coil unit returning to the high temperature tank is equal to the ambient
temperature (Tl = Ta). The surface observation data, used for the simulation, were
measured at the Chiang Rai meteorological station. The climate can be divided to
three seasons: winter from November to mid March, with average ambient
temperature about 13°C to 23.5°C increasing to the maximum temperature 27°C
to 28°C in summer from April to May. In the wet season the temperature is at
about 26°C to 25°C until November. The downward atmospheric radiation was
calculated by the Idso-Jackson model corrected for cloud cover by Exell’s method
[9]. Then, the effective sky temperature Ts was computed by equation (7) and the
results are shown in Fig. 3.
T
s = (R /σ) 0.25. (7)
The monthly average underground soil temperatures, at 1 meter depth in Chiang
Rai are depicted in Fig. 4. The specifications of the underground storage tank and
the sky radiator are tabulated in Table 1. Note that, for this sky radiator, the
convective heat transfer between ambient air and radiative surface in period of the
nighttime is not considered because the nocturnal boundary layer of this region is
stable and also the radiative surface was protected by the wind guard.
3-012(O)
Proceedings of the
2nd Regional Conference on Energy Technology Towards a Clean Environment
12-14 February 2003, Phuket, Thailand
0
10
20
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mounth
Temp.C
0
10
20
30
1112131
Day
Temp C
Ta Tr_out Th Tc
-15
-10
-5
0
5
10
15
1 31 61 91 121 151 181 211 241 271 301 331 361
Da
y
Temp c
Day
Table 1 Specifications of the sky radiator and underground cool storage system
Underground storage
High temp. tank Made of concrete; thickness 0.08 m
Volume 4 m3
Low temp. tank Made of concrete; thickness 0.05 m
Styrofoam insulation; thickness 0.075 m
Volume 4 m3
Sky radiator Total area 15 m2
Surface emissivity; white cooler 0.95
Made of aluminum sheet; thickness 0.0015 m
Wind guard high 0.03 m
Water flow rate 0.02 kg / sec
Figure 3. The sky temperature at night for
one year calculated from 10-year
average surface temperatures and
cloud amounts at Chiang Rai.
Figure 4. The monthly average ground
temperature at 1 meter depth
at Chiang Rai.
Simulation results and discussion
Figure 5. The relationship between water temperature and ambient temperature in
December.
The results for December, show (Fig. 5) that the outlet water temperatures are
4°C to 12°C at the end of the month but the average outlet water temperatures
from the sky radiator over the whole month are about 8°C to 15°C, which can
reach 10°C lower than the ambient temperature because the sky temperatures
during the winter season are low. During the rainy season in August, as shown in
Fig. 6, the outlet water temperature from the sky radiator is only 2-3°C lower than
ambient temperature because of the cloudy skies. In the summer, from March to
April, the cool water temperatures from the sky radiator were approximately
3-012(O)
Proceedings of the
2nd Regional Conference on Energy Technology Towards a Clean Environment
12-14 February 2003, Phuket, Thailand
0
10
20
30
12131
Day
Ta Tr_out Th Tc
11
Te
mp C
0
40
1112131
Day
Temp C
Ta Tr_out Th Tc
20
10
30
14°C to 17°C, or approximately 8°C to 10°C lower than ambient temperature
as shown in Fig. 7.
0
10
20
30
May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr
Temp C
Figure 6. The relationship between water
temperature and ambient
temperature in August.
Figure 7. The relationship between water
temperature and ambient
temperature in April.
The water temperature in the underground cool storage from the second half of
October to March is approximately 14.5°C to 22°C, as shown in Fig. 8. At this
time, underground soil temperatures are higher (see Fig.4), but the heat flow from
the outside is not a major effect on the cooled water temperature inside the tank.
The predicted results show that the system cannot work well during the rainy
season, because the temperature of the outlet water from the sky radiator is close
to the ambient temperature.
Figure 8. The average water temperature in the underground cool storage tank.
Conclusions
The mathematical model used to investigate the thermal behavior of the system
has been successfully performed. The computational results for the northern
region of Thailand at Chiang Rai confirmed that the underground water cool
storage can be kept at approximately 14.5°C to 22°C, from December until July,
and in March to April the water temperature was 16°C to 18°C, which implies that
it can be used for residential air conditioning in the summer season. In further
work the computational model will be made more realistic and the design of the
system will be improved by allowing the water to be cooled to lower temperatures
by many passes through the sky radiator.
Acknowledgements
The authors express their deep appreciation to the Thai Meteorological
Department for providing the Chiang Rai surface observation data.
3-012(O)
Proceedings of the
2nd Regional Conference on Energy Technology Towards a Clean Environment
12-14 February 2003, Phuket, Thailand
Nomenclature
q net heat flux of sky radiator, W. m–2 t time, sec
T temperatures, οC V volume of storage tank, m3
U heat transfer coefficient, W. m–2. K –1 A area, m2
Q thermal energy transfer, W m' mass flow rate, kg. s–1
R atmospheric downward radiation, W. m–2. K4 Cp specific heat of water, kJ. kg. K–1
Greek symbols
ρ density of water, kg .m–3 ε emissivty of sky radiator 0.92
σ Stefan - Boltzmann constant 5.67× 10 –8, W. m–2. K4
Subscripts
r radiator
p radiator surface
s effective sky temperature a ambient
h hot temperature tank side c low temperature tank side
l load g tank surface on the side
ug tank surface on the bottom r_out outlet from sky radiator
r_in inlet to sky radiator max maximum
References
[1] Givoni, B., (1977), Solar Heating and Night Radiation Cooling by a
Roof Radiation Trap, Energy and Buildings, 1, 141-149.
[2] Arbel, A. and Sokolov, M., (1991), Greenhouse Heating with a Fresh
Water Floating Collector Solar Pond: A Feasibility Study, Transactions
of ASME J. Solar Energy Engineering, 113, 66-72.
[3] Lundin, S. - E., Nordell, B., and Dalenback, J., -O., et al, (1998), Solar
Heating with Seasonal Storage in Boreholes for Dwelling Area in
Anneberg, Danderyd, Sweden Council for Building Research,
Stockholm, Sweden.
[4] Bokhoven, T., P., Van Dam, J., and Kratz, P., (2001), Resent Experience
with Large Solar Thermal System in the Netherlands, Solar Energy,
71:5, 2001, 347-352.
[5] Kubler, R., Fisch, N., and Hahne, E., (1997), High Temperature Pit
Storage Project for the Seasonal Storage of Solar Energy, Solar Energy,
61, 2, 97-105.
[6] Sayigh, A.A.M., (1979), Solar Energy Application in the Buildings,
New York, Academic Press, 221-223.
[7] Al - Nimr, M., Tahat, M., Al - Rashdan, M., (1999), A Night Cool
Storage System Enhanced by Radiative Cooling - A Modified Austrian
Cooling System, Applied Thermal Engineering, 19, 1013-1026.
[8] Ito, S., and Mimura, N., (1989), Study on the Radiative Cooling Systems
for Storing Thermal Energy, Transactions of ASME J., Solar Energy
Engineering, 111, 493-496.
[9] Exell, R.H.B., (1978), Atmospheric Radiation in a Tropical Climate,
AIT. Research Report No.71, Bangkok, Asian Institute of Technology,
33 p.
[10] Khedari, J., Waewsak, J., (2000), Field Investigation of Night Radiative
Cooling under Tropical Climate, Renewable Energy, 20, 183-193.
[11] Boon-Long, P., (1986), Passive Cooling Project, Final Report,
Department of Mechanical Engineering, Chiang Mai University, 270 p.
3-012(O)
Proceedings of the
2nd Regional Conference on Energy Technology Towards a Clean Environment
12-14 February 2003, Phuket, Thailand
[12] Boldrin, B., and Scalabrin, G., (1978), Cooling of a Fluid by a Nocturnal
Radiator, Energy Conservation in Heating Cooling and Ventilating
Building, 2, 839-851.
[13] Givoni, B., (1982), Cooling by Long Wave Radiation, Passive Solar
Journal, 1: 3, 131-150.
[14] Kimball, B., A., (1985), Cooling Performance and Efficiency of Night
Sky Radiators, Solar Energy, 34:1, 19-33.
[15] Saitoh, T., and Ono, T., (1984 a), Utilization of Seasonal Sky Radiation
Energy for Space Cooling, Transactions of ASME J. Solar Energy
Engineering, 106, 493-496.
[16] Mihalakakou, G., Ferrant, A., and Lewis S.O, (1998), The Cooling
Potential of A Metallic Nocturnal, Energy and Buildings, 28, 251-256.
[17] Harrison, A.W., and Walton, M.R., (1978), Radiative Cooling of TiO2
White Paint, Solar Energy, 2, 185-187.
[18] Berdahl, P., Matin M. and Sakkal F., (1983), The Thermal Performance
of Radiative Cooling Panels, Int. J. Heat Mass Transfer, 26, 871-880.
