Thermal management is extremely important for renewable energy systems such as photovoltaics (PV), thermophotovoltaics (TPV) and concentrating photovoltaics (CPV). Elevated operating temperatures not only reduce the efficiency of PV modules, but also substantially reduce their lifetimes. This is an even more acute issue for higher heat load systems, such as TPV and CPV, where low-bandgap PV cells are commonly used, making the system more sensitive to temperature increase. The encapsulated housing of CPV and TPV systems further suppresses convective cooling, leading to dramatic temperature rises.
Heat transfer methods potentially relevant to CPV and TPV systems are conduction, convection and radiation. Conventional PV cooling approaches usually only utilize convective or conductive heat transfer, such as heat sinks, convective or forced air cooling, liquid cooling, and the like. Some of these strategies require extra energy input and specially designed cooling systems, which can increase the cost, decrease system efficiency, and reduce overall system reliability. Radiative cooling, on the other hand, has been dismissed as being limited for most of the indoor and low-temperature applications, as the temperature difference between the object and ambient is not large enough to fully exploit its potential. Thus, there remains a need for more efficient heat reduction techniques that do not consume power. The present novel invention addresses these needs.
Radiative cooling can reject significantly more waste heat than convection and conduction at high temperatures by sending it directly into space. As a passive and compact cooling mechanism, radiative cooling is lightweight and does not consume energy. These qualities are promising for thermal management in outdoor systems generating low grade heat, such as concentrating photovoltaics (CPV) and thermophotovoltaics (TPV). Radiative cooling is effective over a wide range of working conditions, including heat loads from 6 to 100 W with different CPV cooling designs. A CPV system integrated with radiative coolers may achieve a 5 to 36° C. temperature drop and an 8% to 27% relative increase of open-circuit voltage for a GaSb solar cell, under a heat load of above 6 W. The temperature drops from radiative cooling may significantly improve CPV system lifetimes.
Radiative cooling can exceed convective cooling when:
σ(T4−Tr4)>h(T−Ta)\sigma(T{circumflex over ( )}4−T_a{circumflex over ( )}4)>h(T−T_a)
where σ\sigma is the Stefan-Boltzman constant, T is the system temperature, Tr T_r is the radiative cooling reservoir temperature, TaT_a is the convective coolant temperature, and h is the convective coefficient.
Radiative cooling can exceed conductive cooling when:
σ(T4−Tr4)>k∇T\sigma(T{circumflex over ( )}4−T_r{circumflex over ( )}4)>k\nabla T
where k is the conductive coefficient.
In one embodiment, a radiatively cooled solar array 10 include a downwardly-facing solar cell 15, a mirror 20 positioned below the solar cell 15 and oriented to direct sunlight onto the solar cell 15, and a heat sink 25 in thermal communication with the solar cell 15 and disposed opposite the mirror 20 (between the solar cell 15 and the sky). The heat sink 25 is thus in radiative communication through Earth's atmosphere with outer space.
The radiatively cooled solar array 10 further includes an optical concentrator 30 (such as a Fresnel lens) positioned between the mirror 20 and the solar cell 15 for focusing sunlight onto the solar cell 15. At least one glass or like composition (zinc sulfide, adhesive tape, or the like) radiative cooler 35 is operationally connected to the heat sink 25 for radiating heat away from the solar cell 15. The mirror 20 is adapted to track a solar source of sunlight and wherein the heat sink 25 radiates to an outer space location spaced from the solar source of sunlight, such as through an electronic controller 40 operationally connected to a solar sensor 45 and to a motor 50 that is likewise operationally connected to the solar array 10. A scaffold 55 may be operationally connected to the solar cell 15 and mirror 20, and a thermocouple or like temperature sensor 6o may be connected in thermal communication with the solar cell 15.
For the purposes of promoting an understanding of the principles of the invention and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
For outdoor applications, radiative cooling is a powerful tool for thermal management for buildings and PV systems as a result of direct access to the Earth's atmosphere transparency window from 8 to 13 μm. Photons with wavelengths in this range can go through the atmosphere and exchange heat directly with outer space at a temperature around 3 Kelvin. This large temperature difference enables outdoor radiative coolers to efficiently remove a great deal of waste heat. It should be noted that above-ambient cooling does not require outdoors operation.
Radiative cooling can be classified as either below-ambient cooling or above-ambient cooling, as shown in
A wide range of materials and structures have been demonstrated to provide radiative cooling. For example, nighttime below-ambient cooling does not require suppression of emission within the solar wavelengths. Bulk and composite materials with strong emissivity in the transparency window are also useful in the radiative transmission of waste heat. Daytime below-ambient cooling was not achieved until very recently, due to the challenges of simultaneously producing both strong IR emittance and low solar absorption. The emergence of nanophotonic and metamaterial coolers has now made it possible to tailor the emittance spectrum more precisely than has been achieved with traditional bulk materials. Much stronger and flatter emittance plateaus in the atmospheric transmission window have now been achieved while suppressing solar absorption, enabling net cooling even under direct sunlight. Meanwhile, above-ambient cooling with broadband IR emittance has also been proposed and studied recently, which can provide a great deal of cooling for objects at high temperatures. Moreover, given a proper design, broadband coolers may also be used for below-ambient cooling, since the thermal heat exchange with the sky outside of the transparency window can provide additional cooling power at near-ambient temperature.
Different types of radiative cooling may be elected, depending on the working temperature of a given system. For example, below-ambient cooling is widely used for thermal management of buildings, while above-ambient radiative cooling is more suitable for dissipating low grade heat from PV systems, as the elevated working temperature and high sky transmittance creates ideal conditions for maximizing the cooling power. Unlike forced air or liquid cooling for PV systems, which can consume from 2% to 5% of total output power, radiative cooling is passive with no extra energy consumption. It is also compact, lightweight, and reliable, without the bulky heat sinks or moving parts as are required for air and/or liquid cooling. This aspect may be especially beneficial with PV or CPV modules integrated with tracking systems. More importantly, the radiative power is significantly larger and grows quickly at high temperatures. The rate of heat dissipation is proportional to the fourth power of the temperature difference between the two objects, and scales much faster than conduction and convection. The efficient, compact, and passive nature of radiative cooling makes it an outstanding cooling mechanism for PV systems.
Recent research has shown the effects of radiative cooling in PV, TPV and CPV systems. For example, a GaSb based CPV system with a soda-lime radiative cooler has been experimentally demonstrated. A 10° C. drop of the solar cell was achieved under 13 suns, leading to a relative increase of 5.7% in open-circuit voltage and an estimated 40% increase in lifetime.
The instant novel technology relates to the radiative cooling performance of CPV in three different cooling structures, under a range of wind speeds and solar heat loads. Multiple outdoor experiments were conducted covering the worst and best possible working scenarios for radiative cooling to check the overall performance. The experiments show that radiative cooling, depending on the working conditions, can contribute roughly 25 to 62% of the overall cooling power of a CPV system equipped with flat-plate heat sink, while adding little weight and no extra power consumption. A high-concentration PV system integrated with radiative cooling was designed, refined, and fabricated. The average heat load on the solar cell was ˜5 to 6 W. By applying two soda-lime radiative coolers positioned on either side of the heat sink, the temperature drop of GaSb cell at steady-state for worst/best cases are 5° C. and 36° C., respectively. The maximum temperature drop even outperforms some active air-cooling methods. The temperature decrease also results in an 8% to 27% (28 to 75 mV) relative (absolute) increase in the open-circuit voltage of our GaSb PV cell, as well as a projected lifetime extension for various types of solar cells which potentially can be used in CPV systems. Using detailed simulations, a peak radiative power flux of 157 to 310 W/m2 was estimated to be present, thereby increasing the cooling performance per unit weight by 25% to 81%. This improvement is particularly beneficial to PV systems with solar trackers. To better illustrate the concept, the specific cooling power Sp is given as:
where Pr and Pc are the radiative and non-radiative cooling power, respectively; m is the total weight of the entire cooling assembly; Tcell is the solar cell temperature; and Ta is the ambient temperature.
For cooling systems working at the same temperature, a higher Sp indicates a greater cooling power per unit weight. By integrating radiative cooling into the CPV, the Sp increase can be calculated as a ratio factor f, which is given by
where Sp,r and Sp,c are the specific cooling power of an assembly with and without radiative cooling, respectively; heff is the effective coefficient for non-radiative heat transfer; σ is the Stefan-Boltzmann constant; and ε is emissivity of the cooler. The approximation is valid as long as the coolers are much lighter than the remainder of the cooling assembly, and the operating temperatures remain the same. It is straightforward to show that f˜Tcell3 when Tcell is large, which implies that radiative cooling is more resilient to high temperature systems than other cooling methods.
A radiative cooling measurement platform was built having three chambers, as illustrated in
On the other hand, the open-chamber structure (without top LDPE) is best compared to passively air-cooled CPV, which is commonly found in commercial CPV. Both structures were tested outdoors and showed considerable temperature drops. Electrode probes and type-K thermocouples were mounted to the solar cells in Chambers 1 and 2 to measure their open-circuit voltages (VOC) and temperatures (Texpt), as shown in
A daytime outdoor cooling experiment was conducted as shown in
The measured real-time solar cell temperatures Texpt, as well as a simulation of this experiment are shown in
To investigate radiative cooling in the more challenging case of high convection, a similar daytime outdoor cooling experiment without a top LDPE film installed was conducted on a windy day (wind speed 20 km/h). The results can be seen in
More experiments were conducted on different days to cover a wider range of heat loads and convective heat transfer coefficients. The heat transfer uniformity of Chamber 1 and 2, as well as the electrical characteristics of GaSb solar cells in both chambers have also been tested in separate experiments and show a very close performance.
The experiments described above were also simulated. Results are shown in Table 1.
For the sealed-chamber case, the net cooling power of the top soda-lime glass cooler and the Al reflector are found to be ˜310 W/m2 and ˜38 W/m2, respectively. The power provided by the cooler is almost one order of magnitude greater than that given by the Al reflector. Although the bottom cooler and the Al reflector do not face the sky, the cooling power from cooler is still significantly higher than that provided by the Al reflector. Thus, both the top and bottom coolers contribute a large amount of cooling power, providing ˜62% of the total. This also illustrates that radiative cooling can still provide considerable cooling power for above-ambient applications, even without direct sky access. The effective convective coefficient heff for the assembly disk (including top, bottom, and side surface areas) is 2.8 W/m2/K in Chamber 1 and 3.7 W/m2/K in Chamber 2. The higher convective coefficient in Chamber 2 is likely caused by higher buoyancy-driven convection, induced by higher operating temperatures. Moreover, due to the compactness and high cooling flux of radiative cooling, the specific cooling power Sp of the assembly disk is greatly improved, as illustrated in Equation 1. In the test assembly, the Sp in Chamber 1 is 0.49 W/kg/K, as a result of both radiative and convective cooling; while in Chamber 2, the Sp is only 0.27 W/kg/K without radiative cooling. Simply by applying two layers of soda-lime glass wafers, the Sp can increase by 81% without any extra energy input in a sealed-chamber structure. For the open-chamber case, despite a lower temperature drop, the radiative cooling power from coolers still greatly exceeds that of Al reflectors, contributing ˜25% of the total cooling power, with net values of 157.2 W/m2 and 10.9 W/m2 from the top surfaces, respectively. Direct access to ambient air increases the effective convective coefficients leff of the cooling assembly disks to 19.1 and 19.3 W/m2/K, respectively, whereas the convective coefficient hair of the top surface in direct contact with ambient air is 28.5 W/m2/K for both chambers. As a reference, for outdoor experiments with the open chamber, the typical value of heff is approximately 10 W/m2/K. The unusually high heff causes a lower temperature drop in the open-chamber experiment at only ˜4° C. Simulations show that the temperature drop should reach around 10° C. if under the same weather as the sealed-chamber experiment. Also, because of the additional cooling power from radiative coolers, Sp rises from 1.30 W/kg/K to 1.62 W/kg/K (˜25% relative improvement). Combined, these two experiments show the most extreme cases for radiative cooling, where the Neff ranges from the lowest to highest possible values for the test assembly. In most other conditions, radiative cooling provides a temperature drop between these two values.
The performance of CPV radiative cooling can vary significantly with the choice of cooling design and working environment. For example, the radiative cooling power usually increases when the heat load becomes larger, as well as becoming less obvious when the convective coefficient is higher. Therefore, choice of heat sink, wind speed, and heat load all affect performance. With a better cooling design, greater heat loads can be accepted by the solar cell to improve the overall efficiency and output power. To acquire a comprehensive understanding of radiative cooling, three groups of simulations were carried out to study the performance and upper limit of multiple CPV cooling designs at 28° C. under various heat loads and wind speeds, including: a flat-plate heat sink in sealed chamber (with top LDPE film), a flat-plate heat sink in open air (without top LDPE film), and a finned heat sink in open air (without top LDPE film). The geometries of the first two groups are the same as shown in
To summarize the radiative cooling performance under different working conditions, the temperature drop versus heat load and Neff is simulated using a flat-plate geometry.
Lower operating temperatures also dramatically improve the lifetime of most solar cells, including commercially available products. In general, solar cells can be modeled to degrade over time following the Arrhenius rate equation. Depending on the material, type and fabrication quality of the PV module, the degradation rate can vary dramatically as a result of variations in the failure mechanisms and the associated activation energy Ea. Since many different types of solar cells are used in CPV systems, including III-V, multi-junction, and high-performance silicon (Si) solar cells, the following discussion will encompass these possibilities, instead of focusing on the GaSb solar cell used in the CPV test assembly. For most Si solar cells, Ea usually falls in the range from 0.7 eV to 0.9 eV. Activation energies for other materials range from 0.49 eV to 0.85 eV. Using temperature data from the experiments and simulations, the lifetime improvements by radiative cooling for each line shown in
A GaSb CPV system integrating soda-lime glass-based radiative coolers is demonstrated and tested in outdoor experiments. The cooling performance is quantitatively modeled by an opto-thermal simulation, which shows a good match with experimental data. Three different cooling designs (flat-plate heat sink in sealed chamber; flat-plate heat sink in open chamber; finned heat sink in open chamber) have been investigated and quantitatively analyzed. Depending on the cooling design, heat load, and wind speed, radiative cooling performance can vary to a large extent. For flat-plate heat sinks in sealed chambers, a large temperature drop of 36° C. is achieved experimentally at a heat load of 6.1 W (DNI=1019 W/m2, wind speed=6 km/h), with a 75 mV increase of VOC (27% relative). A total cooling power of 310 W/m2 and 170 W/m2 from the top and bottom coolers, respectively, is estimated, representing 62% of the overall cooling power; furthermore, the cooling power per unit weight of the assembly disk is increased by 81%. This overall temperature reduction from radiative cooling is comparable to some active air-cooling systems, yet requires no extra power input. For this cooling design, the maximum heat load is limited to ˜16 W without active cooling. The second cooling structure uses the same flat-plate heat sink, but operates in open air to fully take advantage of natural convection. However, no active-air cooling system can be applied beyond the open-chamber structure. A temperature drop over 5° C. is achieved in outdoor tests, under a heat load of 6.4 W (DNI=1069 W/m2, wind speed=20 km/h), resulting in a VOC increase of 28 mV (8% relative). The radiation power from top and bottom coolers is 157 W/m2 and 45 W/m2, respectively, contributing to 25% of the total cooling power, which improves the specific cooling power by 25%. Three groups of simulations are conducted to further study radiative cooling performance under heat loads from 6 to 100 W, with different wind speeds and cooling designs. The results clearly show that radiative cooling benefits all cases, despite variations in heat sinks and weather conditions. While the temperature drop from radiative cooling becomes less obvious with better convective cooling, the absolute increase of maximum heat load improves (from flat-plate heat sink to finned heat sink). Lifetime extensions from the reduced operating temperatures for the corresponding designs are predicted for different types of solar cells which may be used in similar CPV systems; if confirmed, this would provide substantially improved reliability for the entire CPV system.
The detailed structure of Chamber 1 is shown in
Chamber 2 was almost an identical structure as Chamber 1, but the coolers were replaced with Al reflectors as a control. The Al was commonly used as a solar reflector to minimize solar heating of devices under direct sunlight, because of its light weight, reasonable cost, and widespread availability. Therefore, among non-radiative cooling materials, Al is one of the best choices for suppressing the temperature of outdoor systems.
Chamber 3 had the same enclosure as Chamber 1 and 2, while the inside assembly was replaced with a thermal power sensor. A meter console was connected to the sensor to measure the focused solar power. Chamber 2 and 3 are not shown here separately due to their similarities.
The reflectance and transmittance of the first-surface Al mirror, PMMA lens and LDPE films are shown in
Simulations are carried out with COMSOL Multiphysics to quantitatively analyze the cooling performance and verify the experimental results. Transient heat transfer process is modeled in the software and matches well with experiment, as previously shown in
The model includes heat transfer, laminar air flow and thermal radiation to reflect the real physics process, ensuring the reliability of the result. Material properties, including the density, thermal conductivity, heat capacity are extracted from manufacturer's data sheets and online database. The surface emissivity for each material is measured on the spectrophotometer and the FTIR spectrometer. The boundary conditions, including solar irradiance, wind speed, humidity, ambient temperature were defined based on the experimentally-measured solar power and the local weather reports, to faithfully reflect real-world experimental conditions.
Those skilled in the art will recognize that nigh-infinite modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described. Other implementations may be possible.
This patent application claims priority to co-pending U.S. provisional patent application Ser. No. 63/143,059, filed on Jan. 29, 2021.
This invention was made with government support under DE-EE0004946, awarded by the Department of Energy; EEC1454315, EEC-1227110, and CBET-1855882, awarded by the National Science Foundation; and No0014-15-1-2833 and No0014-19-S-B001, awarded by the Office of Naval Research. The government has certain rights in the invention.
Number | Date | Country | |
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63143059 | Jan 2021 | US |