RIDER
A Hybrid System of Radiative Cooling Integrated with Desiccant-Assisted Evapo-Radiant Cooling
See more on RIDER Hybrid System in the research paper.
(Pending Publication)
Hybrid System Design
A hybrid system integrating radiative cooling with complementary technologies is proposed to achieve enhanced energy effi ciency while addressing the indoor cooling load. At the core of the system is a radiative emitter that allows for optimal thermal exchange with the sky while minimizing solar heat gain of the emitter surface. The emitter is linked to thermal storage through a closed-loop working fluid circuit. During nighttime, when radiative cooling efficiency is maximized due to the absence of solar gain, heat is transferred from the thermal storage to the emitter and released to the sky. This process signifi cantly cools the storage medium, which can then serve as the primary cooling source during the following daytime period.
During daytime operation, the radiative emitter is less efficient due to solar gain, which elevates its surface temperature. While unsuitable for cooling the thermal storage under such conditions, the emitter remains effective as a pre-cooling component. Outdoor air, often hot and humid in the target climates, is first passed through a liquid desiccant bed to reduce humidity, yielding hot and dry air. This air is then directed to a heat exchanger directly connected to the radiative emitter for pre-cooling, where its temperature is reduced. After pre-cooling, the air is further cooled via thermal exchange with the chilled thermal storage. This sequential process improves the cooling performance of the thermal storage and ensures the delivery of cool, dry air to the indoor environment even under high ambient temperature and humidity conditions.
To address scenarios where thermal storage capacity is insufficient or ambient conditions degrade system performance, an auxiliary heat pump can be employed to further lower the air temperature before distribution to the conditioned space. The cool, dry air circulated within the space absorbs heat and returns slightly warmer and more humid. This returned air, however, retains residual evaporative potential, which is exploited through the integration of an Evapo-Radiant Panel. The panel features thin water films that evaporate as the returned air passes through, absorbing heat from the indoor environment as a radiant cooling surface and enhancing the system’s overall cooling capacity. Once the air has fully depleted its evaporative potential, it is expelled from the system.
Left: Air state as air pass through the proposed system.
Up: Hypothetical air state as air passes through the proposed system is represented on the psychrometric chart.
Radiative Cooling // Experimental Verification
The prototype consists of two main components: the radiative cooling emitter and the thermal storage system. The 24” x 24” emitting surface is housed within a wooden container elevated above the thermal storage unit. The emitting surface comprises two metal sheets: a flat upper sheet and a corrugated lower sheet, which sandwich polyethylene tubes that serve as the working fluid circuit. This circuit facilitates heat transfer from the thermal storage to the emitting surface, where it is dissipated to the sky. To maximize radiative efficiency, the emitting surface is coated with matte black paint, increasing its emissivity to over 0.9, thereby optimizing radiation loss. The wooden container housing the emitter is insulated with multiple layers of batt insulation to minimize heat gain from external sources. The system is further sealed with a polyethylene film on top, reducing convective heat transfer to the emitting surface. The thermal storage system consists of 5 gallons of water contained within an insulated ice chest designed to minimize heat exchange with the external environment. The thermal storage is integrated with the emitter via the working fl uid circuit, with water serving as the circulating fl uid. This configuration ensures effective heat transfer and stable operation of the cooling system.
Radiant-Evapo Panel // Experimental Verification
The effectiveness of the proposed hybrid system's evapo-radiant panel is validated through physical experimentation. The prototype employs a Maisotsenko (M) cycle, in which return air from an indoor conditioned environment is supplied to the middle channel. The middle channel features a top plate made of aluminum, with its upper surface coated in a continuous water film.
As air flows through the middle channel, a portion of this air is redirected following the M-cycle configuration to the upper channel. In the upper channel, the air comes into contact with the continuous water film, leveraging its evaporative cooling potential. This process reduces the air temperature in the upper channel, and the resulting thermal gradient is transferred through the highly thermally conductive aluminum plate to further cool the air passing through the middle channel.
A fraction of the pre-cooled air from the middle channel is directed into the lower channel, completing the M-cycle. The lower channel also features a bottom aluminum plate, with its upper surface coated in a continuous water film. The evaporative cooling potential of the air in the lower channel is further utilized, reducing the air temperature further. This enables the lower surface of the aluminum plate to function as a radiant cooling surface, effectively dissipating heat.
Through this multi-stage evaporative cooling process, the system enhances the cooling performance and demonstrates the feasibility of integrating evaporative and radiant cooling in a single configuration. The experimental results validate the prototype's capacity to achieve significant cooling while maximizing evaporative and radiant heat transfer efficiencies.
