Sun-Powered Refrigerator: Design, Testing, and Limitations

Vapor Compression Refrigeration (VCR) systems are the most commonly utilized and easily accessible options for the storage of temperature-sensitive products. The challenge of ensuring the availability of vaccines, medicines, and temperature-sensitive products in remote or off-grid regions is complex and requires addressing the issue of unreliable electricity access in a constructive manner, since the safety and functional properties of perishable products can be affected by inadequate cooling facilities. In addition, improvements in energy consumption of refrigeration systems are crucial for the refrigeration industry to reduce its environmental impact. During last decades, the scientific community has focused its attention on new emerging refrigeration technologies, such as solid-state refrigeration [1, 2], new refrigerants [3] able to reduce the so-called direct emissions (related to refrigerant leakages). In particular, to address fluorine-based environmental impact, current scientific literature focuses on solutions involving the use of natural refrigerants such as carbon dioxide [4-6] or isobutane (R600a) [7], showing promising energy and environmental performances. In addition, renewable power sources and energy recovery [8] are spreading as promising solutions to reduce indirect emissions (related to the production of electric energy to power refrigeration systems) both in static and transport refrigeration sectors. Integrating solar photovoltaic (PV) systems with refrigeration technology has become a promising solution to meet the critical need for cooling [9] thanks to its supply-demand matching characteristics (during periods of higher solar radiation, the demand for cooling increases). Moreover, solar cooling is an environmentally conscious solution, since it contributes to decreasing reliance on fossil fuels, reducing or eliminating the indirect emissions of refrigeration systems. PV solar cooling systems utilize electricity generated by the PV modules to power the cooling operations, resulting in a system where solar energy drives cooling [10]. Solar PV cooling, also called solar-assisted vapor compression refrigeration, comprises four essential components: PV modules, a battery pack, an inverter or a charge regulator, and a VCR system. The specific cost of PV panels is decreasing over time, and their efficiency is continuously improving, so vapor compression cycles combined with PV panels represent the most attractive solution in economic [11] and technical [12] terms. Several experimental studies have evaluated the effectiveness of vapor compression solar cooling systems, including both alternating current (AC) and direct current (DC) cases [13-15]. The difference between AC and DC systems is related to the device acting as the interface between the PV modules and the load, i.e. performing the DC-AC or DC-DC conversion, respectively. In particular, when considering an AC refrigeration system, a hybrid inverter with specific characteristics for photovoltaic purposes (performing the Maximum Power Point Tracking-MPPT, for example) must be used. In the case of DC-powered refrigeration units, no AC/DC conversion is required, and a charge controller should be used in order to modify the PV panel operating voltage to meet the battery needs (if present) and power the load properly. In this case, the efficiency in the DC/DC conversion is greater than that of AC/DC conversion, reducing the power losses [16] and increasing the available electric energy provided by the PV panels. In addition, several researchers have conducted theoretical investigations and feasibility studies regarding PV solar cooling systems [9, 17-19]. Feasibility is proven, both technically and economically, and its application is currently under study also in the refrigerated transport sector [20]. However, drawbacks like the necessity of batteries (thermal or electrical) and the higher investment costs with respect to standard solutions prevent the extension of these systems. The battery pack is necessary to achieve maximum emission savings by allowing the use of green energy during insufficient solar energy periods, such as at night. At the same time, a PV array output deviates from the maximum power point without an inverter or a charge regulator [21], causing an inefficient use of solar energy. Charge and discharge cycles during operation can cause a premature degradation of the battery because of an increase in temperature [22]. However, in the case of solar-powered VCR systems, batteries do not reach dangerous conditions, so a thermal management system is not required (differently from automotive cases).

Even if the current scientific literature is rich in studies regarding PV solar cooling systems, it needs to be pointed out that PV panels powering a refrigeration unit (both refrigeration and air conditioning systems) are usually dislocated with respect to the VCR system. This solution allows having no limits to the available solar surface but strongly reducing the compactness and portability of the system.

This study presents and thoroughly analyses a compact solar-assisted refrigerator, developed inside the spinoff ARES S.r.l. (SU.P.E.R. - SUn PowEred Refrigerator project). The photovoltaic panel is mounted on the rooftop of a DC refrigerator, making it suitable for transportation and deployment in remote areas. The results of initial experimental tests aim to validate the sun-powered refrigerator's feasibility and efficacy in operating without the need for a backup power source, and assess the refrigerator's performance under varying operating conditions. The experimental results are also discussed in terms of opportunities and limitations of the proposed system.

In this study, four experimental tests have been performed to prove feasibility and highlight the proposed PV-powered system's drawbacks and limitations. Four different set-point temperatures are considered in the experimental campaign to evaluate the system's potential for different applications. In particular, internal temperatures of -20℃, -10℃, 0℃ and +4℃, with a hysteresis of ±1℃, are tested to cover a wide range of medical products applications (e.g. 2–8℃ for common medical products, between -25℃ and -15℃ for Moderna COVID-19 vaccine [24]). To consider the worst operating conditions, no products are stored in the refrigerator during the tests, since they act as thermal masses increasing the thermal inertia of the whole system. The internal air temperature is brought to the desired set-point temperature before beginning each experimental test, which starts at 17:00 in full-charged battery conditions and lasts 24 hours. The chosen starting time is intended to face nighttime as soon as possible, as solar radiation decreases and the external temperature starts to decrease after 18:00. Furthermore, in this condition, the PV panel power production is almost negligible, so most of the energy required to power the VCR system is provided by the battery.

The tests are performed in a temperature-controlled laboratory room to avoid dependence on actual daily climatic conditions. A remotely controlled DC power supply simulates the PV panel's dynamic behaviour. In particular, the well-known single diode model, given by Eq. (1), is employed to calculate the V-I and V-P curves, and consequently the maximum power point, according to climatic conditions (external temperature and solar irradiance) provided by the PVGIS-SARAH database.

${{I}_{PV}}={{I}_{ph}}-{{I}_{sat}}\left( {{e}^{\frac{{{V}_{PV}}+{{I}_{PV}}\cdot {{R}_{s}}}{\eta \cdot {{V}_{t}}}}}-1 \right)-\frac{{{V}_{PV}}+{{I}_{PV}}\cdot {{R}_{s}}}{{{R}_{h}}}$             (1)

where, I_ph is the photoinduced current, I_sat is the diode’s saturation current, V_PV and I_PV are the PV panel voltage and current, respectively. V_t is the thermal voltage of the PV cell (related to the PN junction) depending on the cell's temperature, R_s and R_h are series and shunt resistances used in the equivalent circuit, respectively, and η is the ideality factor of the diode. All these parameters are evaluated based on the specific PV module datasheet. A schematic of the above-described testing device is shown in Figure 2.

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Figure 2. Schematic of the testing device

In this experimental campaign, climatic conditions of Fisciano (Salerno, Italy) on a generic day in July are considered to test the worst conditions in terms of thermal load and energy consumption. The external temperature is varied through an electric heater to maintain 30℃ during the day (with a hysteresis of ± 0.5℃), while night-related attenuation is simulated by turning the heater off.

During the tests, external and internal temperatures are measured through two four-wire RTDs Pt100 with an accuracy of ±0.1℃. Electric energy measurements are also performed to evaluate energy fluxes across the charge regulator. In particular, an energy meter is used to measure both voltage and current related to the PV panel, the battery and the VCR system’s compressor, with an accuracy of ±10mV and ±10mA, respectively.

In this study, the feasibility of a compact solar-powered refrigerator is analysed. The proposed system consists of a commercial cockpit DC-powered refrigerator, with the addition of a PV panel mounted on its top surface, a solar charge controller and a lead-acid battery to store electric energy. Four experimental tests at different set point temperatures have been conducted in order to evaluate the dynamic behaviour of the whole system for 24 hours considering July climatic conditions. The set point temperatures are chosen according to medical products storage temperature range. Results have shown that the system is able to operate independently from the electric power grid. The PV panel can produce all the required electric energy during the 24 hours, so that the battery is fully recharged after the 24 hours. These results prove the feasibility of the system and its efficacy in storing medical products in remote and undeserved areas in the worst climatic conditions in terms of thermal load. In addition, the proposed components also allow the system to be a possible solution towards sustainability in the cold chain, since 100% of the required electric energy is provided by a renewable energy source, leading to CO_2,e savings of 67-201 g_CO2,e. Anyway, the energy required by the refrigerator varies according to the climatic conditions, as well as the PV panel’s electric energy production, so further experimental tests at different environmental conditions are required. In addition, the system still requires a battery with high capacity, size and weight to face low-solar radiation conditions, reducing the actual compactness of the whole system. A possible solution to this limitation is to employ phase change materials as thermal energy storage system, allowing an increase in the PV panel’s energy production and reducing the size of the battery.