The Cooling Potential of Sky Radiation With Variations in System Parameters

2019 ◽  
Vol 141 (4) ◽  
Author(s):  
M. Adrienne Parsons ◽  
M. Keith Sharp
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Pipe ◽  
Storage Capacity ◽  
Thermal Storage ◽  
Ambient Air ◽  
View Factor ◽  
Water Tank ◽  
Cooling Load ◽  
Tank Wall

This study evaluated the building cooling capacity of sky radiation, which was previously identified to have the greatest cooling potential among common ambient sources for climates across the U.S. A heat pipe augmented sky radiator system was simulated by a thermal network with nine nodes, including a thin polyethylene cover with and without condensation, white (zinc oxide) painted radiator plate, condenser and evaporator ends of the heat pipe, thermal storage fluid (water), tank wall, room, sky and ambient air. Heat transfer between nodes included solar flux and sky radiation to cover and plate, wind convection and radiation from cover to ambient, radiation from plate to ambient, natural convection and radiation from plate to cover, conduction from plate to condenser, two-phase heat transfer from evaporator to condenser, natural convection from evaporator to water and from water to tank wall, natural convection and radiation from tank wall to room, and overall heat loss from room to ambient. A thin layer of water was applied to simulate condensation on the cover. Nodal temperatures were simultaneously solved as functions of time using typical meteorological year (TMY3) weather data. Auxiliary cooling was added as needed to limit room temperature to a maximum of 23.9 °C. For this initial investigation, a moderate climate (Louisville, KY) was used to evaluate the effects of radiator orientation, thermal storage capacity, and cooling load to radiator area ratio (LRR). Results were compared to a Louisville baseline with LRR = 10 W/m2 K, horizontal radiator and one cover, which provided an annual sky fraction (fraction of cooling load provided by sky radiation) of 0.855. A decrease to 0.852 was found for an increase in radiator slope to 20 deg, and a drop to 0.832 for 53 deg slope (latitude + 15 deg, a typical slope for solar heating). These drops were associated with increases in average radiator temperature by 0.73 °C for 20 deg and 1.99 °C for 53 deg. A 30% decrease in storage capacity caused a decrease in sky fraction to 0.843. Sky fractions were 0.720 and 0.959 for LRR of 20 and 5, respectively. LRR and thermal storage capacity had strong effects on performance. Radiator slope had a surprisingly small impact, considering that the view factor to the sky at 53 deg tilt is less than 0.5.

The Potential of Sky Radiation With Change in Design Parameters

2016 ◽  
Author(s):  
Adrienne M. Parsons ◽  
M. Keith Sharp
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Pipe ◽  
New Orleans ◽  
Storage Capacity ◽  
Thermal Storage ◽  
Ambient Air ◽  
Dew Point ◽  
Cooling Load ◽  
Tank Wall

This study evaluated the building cooling capacity of sky radiation, which was previously identified to have the greatest cooling potential among common ambient sources for climates across the US. [Robinson, et al. 2013b]. A heat pipe augmented sky radiator system was simulated by a thermal network with nine nodes, representing a thin polyethylene cover, white (ZnO) painted radiator plate [Duffie & Beckman 2013], condenser and evaporator ends of the heat pipe, thermal storage fluid (water), tank wall, room, sky and ambient air. Heat transfer between nodes included solar flux and sky radiation to cover and plate, wind convection and radiation from cover to ambient, radiation from plate to ambient, natural convection and radiation from plate to cover, conduction from plate to condenser or, two-phase heat transfer from evaporator to condenser, natural convection from evaporator to water and from water to tank wall, natural convection and radiation from tank wall to room, and overall heat loss from room to ambient. Nodal temperatures were simultaneously solved as functions of time using Typical Meteorological Year (TMY3) weather data. Auxiliary cooling was applied as needed to limit room temperature to a maximum of 23.9°C. For this initial investigation, a moderate climate (Louisville, KY) was used to evaluate the effects of radiator orientation, thermal storage capacity and cooling load to radiator area ratio, LRR. Louisville and two challenging climates (Miami, FL and New Orleans, LA) were then used to evaluate five cover configurations — zero, one and two covers with unconstrained temperature, and zero and one cover with temperature limited to the dew point of ambient air to simulate condensation on the cover. Results were compared to a Louisville baseline with LRR = 10 W/m2K, horizontal radiator and one cover with constrained temperature, which provided an annual sky fraction (fraction of cooling load provided by sky radiation) of 0.861. A decrease to 0.857 was found for an increase in radiator slope to 20°, and a drop to 0.833 for 53° slope (latitude + 15°, a typical slope for solar heating). These drops were associated with increases in average radiator temperature by 0.2°C for 20° and 1.5°C for 53°. A 25% decrease in storage capacity caused a decrease in sky fraction to 0.854. Sky fractions were 0.727 and 0.963 for LRR of 20 and 5, respectively. Sky fractions for the baseline system in Miami and New Orleans were 0.505 and 0.603, respectively. In all three climates, performance was little affected by constraining the cover temperature and by adding a second cover. These results confirm the potential for passive cooling of buildings by radiation to the sky. Climate, LRR and thermal storage capacity had strong effects on performance, while the cover configuration did not. Radiator slope had a surprisingly small impact, considering that the view factor to the sky at 53° tilt is less than 0.5.

Effect of Thermal Storage on the Cooling Capacity of Ambient Sources

2014 ◽  
Author(s):  
Brian S. Robinson ◽  
M. Keith Sharp
Keyword(s):  
Storage Capacity ◽  
Thermal Storage ◽  
Ambient Air ◽  
Cooling Capacity ◽  
Ground Temperature ◽  
Cooling Load ◽  
Space Cooling ◽  
Wide Range ◽  
Sky Temperature ◽  
Night Sky

Ambient sources, including ambient air at dry-bulb and wet-bulb temperature, ground temperature and night sky temperature, were evaluated for their potential to provide space cooling in locations across the U.S. While ground temperature is constant beyond a certain depth, the other sources have fluctuating temperatures, which present intermittent potentials for cooling. Simultaneously, cooling demands also fluctuate with outdoor temperature. Thermal storage can bridge intervals of time during which cooling is needed in the building, but ambient source temperature is too high to provide cooling. The duration of these intervals and the thermal storage capacity required to meet cooling needs based on ambient source potential prior to the interval were quantified for all eleven climate zones across the continental U.S using TMY3 weather data. The thermal storage capacity required to meet the entire annual cooling load is dictated by the span of time without ambient source cooling potential that has the greatest ratio of cooling load to ambient source cooling potential prior to the interval. This maximum thermal storage capacity, normalized by building overall loss coefficient, (this ratio has units of time) was one day or less for night sky temperature for all but the three warmest climates. This ratio was one day or less for wet-bulb temperature for four locations, and for dry-bulb temperature for only two locations. Ground temperature provided continuous cooling potential in all but the three warmest climates, where ground temperature was warmer than the indoor comfort temperature. Because the maximum thermal storage capacity was determined in most climates by uncommon and infrequent coincidence of high cooling demand and low ambient source cooling potential, smaller thermal storage provided substantial cooling capacity in most cases. For instance, ten percent of the maximum supplied 99% of the cooling load for the dry-bulb ambient air source in Albuquerque, and 0.1% of the maximum served over 90% of the cooling load with night sky radiation in New Orleans and Phoenix. While considerable development of hardware and control algorithms to utilize ambient sources for space cooling has occurred, this study shows the potential of these sources to further reduce demands for conventional energy for space cooling across a wide range of climates.

Natural Convection in a Horizontal Layer of Water Cooled From Above to Near Freezing

1975 ◽  
Vol 97 (1) ◽  
pp. 47-53 ◽  
Author(s):  
R. E. Forbes ◽  
J. W. Cooper
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Natural Convection ◽  
Hydrodynamic Instability ◽  
Convective Heat Transfer Coefficient ◽  
Free Water ◽  
Ambient Air ◽  
Maximum Density ◽  
Surface Boundary ◽  
Initial Water

Natural convection in horizontal layers of water cooled from above to near freezing was studied analytically. The water was confined laterally and underneath by rigid insulators, and the upper horizontal surface was subjected to: (1) a constant 0C temperature, rigid conducting boundary, and (2) a free, water to air convection boundary condition, in which the convective heat transfer coefficient was held constant at values of 5.68 W/m2 · K and 284 W/m2 · K (1.0 and 50.0 Btu/hr ft2F) and the temperature of the ambient air was maintained at 0C. The ratios of the width to the depth of the rectangular water layers under consideration were W/D = 1, 3, and 6. Initially the water is assumed to be at a uniform temperature of either 4C or 8C, and then the upper surface boundary condition was suddenly applied. It was observed in all cases for which the initial water temperature was 4C, that the water remained stagnant and became thermally stratified. Heat transfer application of either of the surface boundary conditions to water initially at 8C produced large convective eddies extending from the bottom to the top of the layer of water. As the liquid layer cooled further, two distinct horizontal regions appeared, the 4C isothermal line separating the two. This produces a region of hydrodynamic instability in the fluid since the maximum density fluid (4C) is physically located above the less dense fluid in the lower portion of the cavity. The large eddies which appeared initially were confined to the hydrodynamically unstable region bounded by the 4C isotherm and the bottom of the cavity. The action of viscous shearing forces upon the stable water above the 4C isotherm produced a second “layer” of eddies. An alternating direction implicit finite difference method was used to solve the coupled system of partial differential equations. The paper presents transient isotherms and streamlines and a discussion of the effect of maximum density on the flow patterns.

Transient Natural Convection in Enclosures With Application to Solar Thermal Storage Tanks

1992 ◽  
Vol 114 (3) ◽  
pp. 175-181 ◽  
Author(s):  
D. T. Reindl ◽  
W. A. Beckman ◽  
J. W. Mitchell
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Exchangers ◽  
Thermal Storage ◽  
Solar Thermal ◽  
Storage Tanks ◽  
Time Duration ◽  
Source Temperature ◽  
Fluid Field ◽  
Isothermal Fluid

Many previously studied natural convection enclosure problems in the literature have the bounding walls of the enclosure responsible for driving the flow. A number of relevant applications contain sources within the enclosure which drive the fluid flow and heat transfer. The motivation for this work is found in solar thermal storage tanks with immersed coil heat exchangers. The heat exchangers provide a means to charge and discharge the thermal energy in the tank. The enclosure is cylindrical and well insulated. Initially the interior fluid is isothermal and quiescent. At time zero, a step change in the source temperature begins to influence the flow. The final condition is a quiescent isothermal fluid field at the source temperature. The governing time-dependent Navier-Stokes and energy equations for this configuration are solved by a finite element method. Solutions are obtained for 103≤RaD≤106. Scale analysis is used to obtain time duration estimates of three distinct heat transfer regimes. The transient heat transfer during these regimes are compared with limiting cases. Correlations are presented for the three regimes.

Analysis of a Latent Heat Storage Device With Radial Fins

2013 ◽  
Author(s):  
Y. Kozak ◽  
T. Rozenfeld ◽  
G. Ziskind
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Latent Heat ◽  
Close Contact ◽  
Thermal Storage ◽  
Ambient Air ◽  
Contact Melting ◽  
Storage Device ◽  
Melting Time ◽  
Cfd Model

Phase-change materials (PCMs) can store large amounts of heat without significant change of their temperature during the phase-change process. This effect may be utilized in thermal energy storage, especially for solar-thermal power plants. In order to enhance the rate of heat transfer into PCMs, one of the most common methods is the use of fins which increase the heat transfer area that is in contact with the PCM. The present work deals with a latent heat thermal storage device that uses a finned tube with an array of radial fins. A heat transfer fluid (HTF) flows through the tube and heat is conducted from the tube to the radial fins that are in contact with the bulk of the PCM inside a cylindrical shell. The thermal storage charging/discharging process is driven by a hot/cold HTF inside the tube that causes the PCM to melt/solidify. The main objective of the present work is to demonstrate that close-contact melting (CCM) can affect the storage unit performance. Accordingly, two different types of experiments are conducted: with the shell exposed to ambient air and with the shell submerged into a heated water bath. The latter is done to separate the PCM from the shell by a thin molten layer, thus enabling the solid bulk to sink. The effect of the solid sinking and close-contact melting on the fins is explored. It is found that close-contact melting shortens the melting time drastically. Accordingly, two types of models are used to predict the melting rate: numerical CFD model and analytical/numerical close-contact melting model. The CFD model takes into account convection in the melt and the PCM property dependence on temperature and phase. The analytical/numerical CCM model is developed under several simplifying assumptions. Good agreement is found between the predictions and corresponding experimental results.

Effect of Natural Convection on Thermal Energy Storage in Supercritical Fluids

2013 ◽  
Author(s):  
Reza Baghaei Lakeh ◽  
Adrienne S. Lavine ◽  
H. Pirouz Kavehpour ◽  
Gani B. Ganapathi ◽  
Richard E. Wirz
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Natural Convection ◽  
Energy Storage ◽  
Supercritical Fluid ◽  
Supercritical Fluids ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Storage System ◽  
Thermal Storage

Heat transfer to the storage fluid is a critical subject in thermal energy storage systems. The storage fluids that are proposed for supercritical thermal storage system are organic fluids that have poor thermal conductivity; therefore, pure conduction will not be an efficient heat transfer mechanism for the system. The current study concerns a supercritical thermal energy storage system consisting of horizontal tubes filled with a supercritical fluid. The results of this study show that the heat transfer to the supercritical fluid is highly dominated by natural convection. The buoyancy-driven flow inside the storage tubes dominates the flow field and enhances the heat transfer dramatically. Depending on the diameter of the storage tube, the buoyancy-driven flow may be laminar or turbulent. The natural convection has a significant effect on reducing the charge time compared to pure conduction. It was concluded that although the thermal conductivity of the organic supercritical fluids are relatively low, the effective laminar or turbulent natural convection compensates for this deficiency and enables the supercritical thermal storage to charge effectively.

The manipulation of the streamwise vortex instability in a natural convection boundary layer along a heated inclined flat plate

2002 ◽  
Vol 470 ◽  
pp. 31-61 ◽  
Author(s):  
MARK A. TRAUTMAN ◽  
ARI GLEZER
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Natural Convection ◽  
Surface Temperature ◽  
Streamwise Vortex ◽  
Surface Heat ◽  
Convection Boundary Layer ◽  
Water Tank ◽  
Surface Heat Transfer ◽  
Convection Boundary

Flow instabilities leading to the formation of streamwise vortices in a natural convection boundary layer over a heated inclined plate submerged in a water tank are manipulated using spanwise arrays of surface-mounted heating elements. The flow over the plate is driven by a two-ply surface heater comprised of a uniform, constant- heat flux heater and a mosaic of 32 × 12 individually controlled heating elements that are used as control actuators. Surface temperature distributions are measured using liquid crystal thermography and the fluid velocity in cross-stream planes is measured using particle image velocimetry (PIV). Time-invariant spanwise-periodic excitation over a range of spanwise wavelengths leads to the formation of arrays of counter-rotating streamwise vortex pairs and to substantial modification of the surface temperature and heat transfer. The increase in surface heat transfer is accompanied by increased entrainment of ambient fluid and, as a consequence, higher streamwise flowrate. Subsequent spanwise-periodic merging of groups of vortices farther downstream retards the streamwise increase of the surface heat transfer rate. Finally, the suppression of small-amplitude spanwise disturbances by linear cancellation is demonstrated.

The Potential of Night Sky Radiation for Humidity Control

2015 ◽  
Author(s):  
Zachary Springer ◽  
M. Keith Sharp
Keyword(s):  
Storage Capacity ◽  
Volume Flow Rate ◽  
Ambient Air ◽  
Dew Point ◽  
Weather Data ◽  
Cooling Load ◽  
Floor Area ◽  
The Us ◽  
Night Sky ◽  
Mountain West

Ambient energy sources, including ambient air, ground and night sky, have potential for space cooling. The night sky offers the lowest temperature and, therefore, the greatest potential across most of the US. Compared to a previous analysis that considered only the sensible cooling load, the objective of this new project was to evaluate the potential of night-sky radiation (NSR) to also serve the latent cooling load. ASHRAE standard 55 was used to establish the comfort limits (22°C for room temperature and 60% relative humidity). Condensation was evaluated as the mechanism for humidity reduction, thus the dew-point temperature, 13.9°C, corresponding to the ASHRAE limits was the maximum target temperature for night-sky cooling. Typical meteorological year (TMY3) weather data was used for eleven locations representing ASHRAE climate zones. Building heat gain, infiltration/ventilation requirements and night-sky radiator size were characterized by a load-to-radiator ratio LRR defined as the infiltration/ventilation volume flow rate times the ratio of building floor area to radiator area. Three values of LRR were evaluated: 0.35, 3.5 and 35 m/hr. Three thermal storage cases were considered: 1. Annual NSR cooling potential (seasonal storage), 2. Diurnal storage, and 3. The minimum storage capacity to serve the entire annual load, as well as the effects of capacity less than the minimum. To evaluate the effect of night-sky radiator temperature on storage capacity, six NSR temperatures Trad = 13.9 to −26.1°C were tested. Results showed that even in Miami, FL (the most challenging climate evaluated), annual NSR potential exceeded the total sensible and latent cooling load, at least for the lowest LRR and highest Trad. For diurnal storage, NSR could serve less than 20% of the load in the hot and humid southeast, but the entire load in the mountain west. The minimum storage capacity to meet the entire annual load corresponds to the capacity required to bridge the span of time without NSR availability during which the largest cooling load occurs. This capacity decreases with decreasing LRR and decreasing Trad. For the southeast, large capacity is required, but for Louisville, for instance, sufficient capacity is provided by the equivalent of as little as 0.05 m of water over the floor area of the building for LRR = 0.35 m/hr. These results demonstrate that for much of the US, night-sky radiation has the potential to serve the entire annual sensible and latent cooling load.

scholarly journals Thermal Charging Optimization of a Wavy-Shaped Nano-Enhanced Thermal Storage Unit

Molecules ◽  
2021 ◽  
Vol 26 (5) ◽  
pp. 1496
Author(s):  
Mohammad Ghalambaz ◽  
S.A.M. Mehryan ◽  
Ahmad Hajjar ◽  
Mohammad Yacoub Al Shdaifat ◽  
Obai Younis ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Phase Change ◽  
Volume Fraction ◽  
Thermal Storage ◽  
Wave Number ◽  
Storage Unit ◽  
Charging Time ◽  
Phase Change Heat Transfer ◽  
The Impact

A wavy shape was used to enhance the thermal heat transfer in a shell-tube latent heat thermal energy storage (LHTES) unit. The thermal storage unit was filled with CuO–coconut oil nano-enhanced phase change material (NePCM). The enthalpy-porosity approach was employed to model the phase change heat transfer in the presence of natural convection effects in the molten NePCM. The finite element method was applied to integrate the governing equations for fluid motion and phase change heat transfer. The impact of wave amplitude and wave number of the heated tube, as well as the volume concertation of nanoparticles on the full-charging time of the LHTES unit, was addressed. The Taguchi optimization method was used to find an optimum design of the LHTES unit. The results showed that an increase in the volume fraction of nanoparticles reduces the charging time. Moreover, the waviness of the tube resists the natural convection flow circulation in the phase change domain and could increase the charging time.

scholarly journals Commercial Building Thermal Mass Precooling for Conditioning Reduciton Under the Operative Temperature Metric

2021 ◽  
Author(s):  
Christopher Raghubar
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Convective Heat Transfer Coefficient ◽  
Ambient Air ◽  
Thermal Mass ◽  
Cooling Load ◽  
Operative Temperature ◽  
Occupant Comfort ◽  
Cooling Effects

Building thermal mass precooling is highly variable due to the uncertainty of the convective heat transfer coefficient, and current research neglects the radiative cooling effects of reduced interior surface temperatures. The research presented aims to address the shortcomings of current research by modelling night ventilation through concrete slab hollow cores, increasing confidence in the heat transfer coefficient estimate; and modelling the operative temperature experienced by an occupant in an open office with simplified geometry. The cooling load of a baseline non-ventilated slab was determined through a custom numerical model and the operative temperature of the baseline was assigned to the same model with hollow core slab ventilation to determine the ambient air setpoint temperature associated cooling load. The ventilated model was found to achieve 35% cooling energy savings compared to the baseline, with compromised occupant comfort in the early morning, and improved occupant comfort for the rest of the day.

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