Elsevier

Energy

Volume 200, 1 June 2020, 117516
Energy

Performance analysis of the sky radiative and thermoelectric hybrid cooling system

https://doi.org/10.1016/j.energy.2020.117516Get rights and content

Highlights

  • The radiative sky and thermoelectric coolers are combined into a hybrid cooler.

  • A feasibility study involving many variables and a steady state model is conducted.

  • Including the RSC could reduce the TEC power consumption by over 10%.

  • TEC power saving can be maximized by increasing the RSC versus TEC surface area.

  • Solar absorption coefficient shall be under 0.02 to ensure reasonable performance.

Abstract

In this paper, the radiative sky cooler (RSC) and thermoelectric cooler (TEC) are integrated to form the RSC-TEC hybrid cooling system that can reduce the TEC required power consumption and increase the system’s cooling capacity over a standalone RSC. Specifically, a feasibility study is conducted to evaluate the design and working conditions that allow this system to have superior performance; For example, the TEC module type and number, RSC surface area and radiative emissivity value, solar absorption coefficient and air convective heat transfer coefficient have been parametrically swept to assess their effects on the system’s cooling capacity and the TEC power saving coefficient, a metric to define the degree of TEC power consumption reduction due to the RSC. The analyzes have been conducted through a non-dimensional steady-state mathematical model of the hybrid system that cools an enclosed space. Results demonstrate that a 0.1 m2 RSC could reduce the required power consumption of a TEC module (size 4 cm by 4 cm) by up to 10%. Moreover, increasing the RSC surface area further improved the TEC power saving coefficient, but the solar absorption coefficient had to be under 0.02 to maintain a reasonable TEC power saving coefficient.

Introduction

Cooling energy is a fundamentally important component of everyday living such as for personal comfort/health and food preservation. This is especially true in warmer regions such as near the equator where it is virtually impossible to find a natural or passive cooling source. Unfortunately, the production of cooling energy by active means is often very energy intensive, thus motivating researchers and engineers to develop an efficient, economic and compact cooling energy device. Although the vapor compression cycle (VCC) has been the most successful and widely used cooling technology for over a century [1], this technology also has some drawbacks including the involvement of environmentally harmful refrigerants and the involvement of moving parts which leads to the need for frequent maintenance. Even today, researchers are still determining ways to improve the VCC technology, as reviewed by Park et al. [2], where example aspects include implementing high-grade energy compensation ratio concept by Dehu et al. [3] or including an ejector into the cycle [4].

Therefore, alternative to the VCC, other cooling technologies have also been developed and these include the radiative sky cooler (RSC) and thermoelectric cooler (TEC). The RSC is one that utilizes the principle of heat radiation into the cold outer space, and it does so by having a high transparency for thermal radiation in mid-infrared wavelength bands (i.e. the atmospheric window, within 8–13μm) [5]. The key advantage of the RSC over the VCC is that it obtains the cooling energy passively which means it does not need a device to power its operation, and the RSC is also fully solid-state. Unfortunately, the RSC also has several major disadvantages such as having a very low cooling density (under 100W/m2 in many practical cases [6,7]) and the inability for a conventional design to operate during the daytime. Therefore, many researchers have undertaken extensive researches to improve the RSC’s cooling performance in many aspects with one being that of materials design. For example, Yang et al. [8] developed a dual-layer structure RSC which experimentally achieved a 0.99 solar reflectance and a radiative emittance of 0.9 in the mid-infrared region, thus their RSC design was highly suitable for daytime radiative cooling. In another study by Jeong et al. [9], multi-layered titanium oxide was proposed as another RSC material, which could achieve a 0.94 solar reflectance, mid-infrared radiative emittance of 0.84 and operate normally even when the air relative humidity (RH) was up to 0.7. Alternatively, the study by Wong et al. [10] proposed integrating the RSC with an asymmetric electromagnetic transmission window (AEMT), where the latter material allows outgoing radiative transmission by the RSC but reflects incoming solar irradiation.

Alternative to materials design, other researchers have also applied the RSC technology to real-world applications, such as to a “Nightcool” building in Parker and Sherwin’s study [11] to reduce the space cooling requirement by the primary cooling device. The economic feasibility of the RSC technology in a two-floor single family house in several cities in USA was also studied by Zhang et al. in Ref. [12], and it was found there that this system increased the cooling system’s average coefficient of performance (COP) by more than 39.4%. Besides these studies, other applications have also been considered such as the usage of RSCs on a building’s attic [13] and demonstrating the all-day operation of a 1 kW-scaled RSC device [14]. Moreover, another innovative concept that has been recently introduced is developing a device that integrates the RSC’s function at night time and solar energy utilization at daytime by Hu et al. [15] and Zhao et al. [5]. Specifically, the study by Hu et al. collected solar energy as heat during the daytime whereas the study by Zhao et al. involved a PV panel to produce power.

Also, the thermoelectric cooler (TEC) is another type of cooling energy device which operates under the Peltier effect principle to produce the cooling energy, and it can be seen as being another type of heat pump. The TEC can also operate in the reverse direction to convert heat to electricity via the Seebeck effect, which is known as the thermoelectric generator or TEG [16]). The TEC attracts wide research interest because it does not involve moving parts, operates without noise and does not emit any pollutants [17]. However, the TEC also has its drawbacks; For example, the COP and the specific volume power density are relatively low compared to other counterpart technologies such as the previously mentioned vapor compression cycle (VCC), as demonstrated in the comparative analysis by Hermes et al. [18]. Thus, the TEC is most often used for small scale applications, such as cooling of portable electronic devices [19], CPUs or items in small portable refrigerators [17,19]. Conversely, the indoor cooling of buildings is almost always achieved by using the VCC [2].

Therefore, to suitably apply the TEC to the building cooling application, either a separate device is required for support or the TEC should be utilized in a unique way. For example, Irshad et al. [20] proposed to integrate the photo-voltaic (PV) wall to the TEC, which has two useful purposes; The first is to provide the power required by the TEC; The second is to act as an extra thermal insulation layer against the solar irradiation. Therefore, the cooling ability to the building room by TEC operation has greatly increased over adopting the TEC via conventional means (e.g. supply the power from the grid). In another study, Zhao et al. [21] proposed that, instead of cooling the entire building space, thermal comfort could also be achieved by developing localized thermal envelopes around the individual occupants, thus lowering the cooling energy requirement for the building itself. Therefore, Zhao et al. developed a portable TEC energy conversion unit that can provide such a thermal envelope and showed that an average of only 24.6 W cooling power is required by each occupant. Most recently, Xia et al. [22] proposed to integrate the thermoelectric device (TEG) to the RSC, where the TEG utilizes the temperature difference between the ambient and the produced cooled space by the RSC to generate power. Although this system could effectively generate power silently and without any moving parts, the reported output power was only in the order of nano-Watts, which indeed is too low for any practical applications. In an alternative study, Zhao et al. [23] integrated the RSC to the TEC application for buildings, where the RSC was specifically used to dissipate heat from the TEC’s hot side. Nevertheless, such an implementation limits heat dissipation at the TEC hot side to only by RSC operation and eliminates other options such as convective air cooling, which in turn limits the total cooling capacity of the TEC itself.

In contrast to placing the RSC at the TEC hot side, this paper performs a feasibility study of setting the RSC and TEC to both provide their cooling energy to the target object. This implementation can not only enable a higher cooling capacity over a standalone RSC by TEC operation, it also reduces the required TEC power consumption for achieving a specified cooling requirement by RSC operation. Furthermore, rather than focusing on a specific design size, a feasibility study is conducted to determine the design choices and working conditions that the RSC-TEC hybrid system can suitably operate or cannot operate with; For example, design variables such as the type of TEC module and number as well as RSC surface area have been explored to evaluate their influences on the system’s TEC power saving coefficient and overall cooling capacity. Furthermore, the commonly known environmental impacts on the RSC such as solar absorption and air convective heat transfer have also been included in the study, where the corresponding coefficients have been parametrically swept to assess the working conditions in which RSC-TEC hybrid system is acceptable for normal operation. These characteristics have been calculated through a non-dimensional steady-state mathematical model for the RSC-TEC hybrid system that cools an enclosed space, and its TEC model was verified with in house obtained experimental data.

The remainder of this paper is structured as follows. Section 2 presents the mathematical model and related equations of the RSC-TED hybrid system. Section 3 then presents a validation model that proves the model of Section 2 is accurate and reliable. Section 4 presents the case study that this paper will analyze and the list of parameters that are set as constants. Section 5 analyzes the simulation results and Section 6 concludes this paper.

Section snippets

Radiative sky cooler model

Fig. 1 shows the overall system structure and a generalized heat balance model of the proposed RSC-TEC hybrid cooling system. Here, cooling is given to an internal space, where the RSC is shown on the top surface and faces the sky direction and the TEC is located on the opposite side. The RSC produces cooling via radiation to the outer space environment as the variable QR. In addition to this, other parasitic effects such as input solar radiation during daytime operation (QS) and convective

Model validation

To ensure that the adopted TEC simulation model and relevant material property values are accurate, an experimental platform is developed to obtain the TEC module’s output characteristics. The experimental results are then compared with those from the simulation model under the same working conditions, where the two results should be similar or ideally be identical. In the experiment, two TEC1-12715 modules are connected in series, where the cold side is attached to an electric heater and the

Study case

Table 1 lists the working conditions that have been imposed in this paper. The selected parameters are set to represent a portable cooling box application, where the required cooling power may be up to 30 W and the box be placed in a hot environment of up to 310 K. A limit of 30 W was chosen because higher values lead to significant difficulties in practically managing heat dissipation at the TEC hot side. The RSC surface area, type of TEC and number of TEC modules are all parametrically swept

Typical target cooling power

Initially, the target cooling power value is set at 20 W and the cooling space temperature is varied. By setting the RSC surface area at 0.1 m2 and adopting a single TEC1-12715 module, the contribution of cooling power, TEC power consumption and TEC’s COP are plotted in Fig. 4 (a), (b) and (c) respectively.

By observing Fig. 4 (a), the RSC’s contribution to the cooling power decreases from 7.5 W to 2.5 W as the cooling space temperature is further lowered from the ambient. In doing so, the TEC

Conclusion

This paper has combined the radiative sky cooler (RSC) and thermoelectric cooler (TEC) to form a hybrid cooling system that can reduce the required TEC power consumption and increase the cooling capacity over a standalone RSC. A steady-state mathematical model for cooling an enclosed space through the proposed hybrid system is introduced, and its TEC model was verified with in house obtained experimental data. The effect of the TEC module type and number, RSC surface area, solar absorption and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Trevor Hocksun Kwan: Conceptualization, Data curation, Formal analysis, Writing - original draft, Investigation, Methodology, Visualization. Bin Zhao: Writing - review & editing, Project administration, Resources, Software. Jie Liu: Writing - review & editing, Validation. Gang Pei: Funding acquisition, Supervision.

Acknowledgments

This research was sponsored by the National Natural Science Foundation of China (NSFC 5171101721, NSFC 51776193).

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