Solar assisted thermoelectric cooling/heating system for vehicle cabin during parking: A numerical study

Highlights

• A solar assisted thermoelectric cooling/heating system for vehicle cabin is proposed.

• •3D model is developed for evaluating temperature variations in vehicle cabin during parking.

• •The cooling/heating system performance was evaluated for both summer and winter seasons.

• •The solar simulator investigated the performance parameters for the vehicle rooftop solar panel.

Abstract

Vehicles parked under blazing sun increases the cabin temperature to a broiling point as greenhouse effect comes into play. Similar is found in winter season when vehicle is parked in open, cabin temperature falls drastically. This extreme rise and fall in the vehicle cabin temperature, results in more fossil fuel consumption to power air-conditioner for high loads, which eventually leads to increase in generation of harmful gases. Additionally, the drastic condition inside vehicle cabin makes it profusely miserable for human comfort. Thus, relevant renewable energy powered cooling/heating system is required for the vehicle cabin when parked in open areas. Therefore, in this study initially, a 3D heat and mass transfer model is developed for evaluating temperature variations in vehicle cabin for summer and winter seasons. Then, a solar powered thermoelectric cooling-heating system is proposed to resolve the extreme rise and fall of vehicle cabin temperature without running the engine or using any power from the primary source. And, the thermal performance of the proposed system is evaluated and assessed in the numerical study. The proposed system was able to reduce the average temperature of the cabin space by 17 °C in summer season and the average temperature rise of the cabin space is found by 15 °C in winter season. The solar simulator for the vehicle rooftop solar panel investigated annual averages of normalized energy per day and loss in solar energy collection due to variation in tilt angle, which was around 20%. Thus, the present study demonstrates the feasibility of proposed novel solar assisted thermoelectric cooling/heating system in both summer and winter seasons. Hence, study demonstrates that the thermoelectric technology with solar assistance is effective and viable for cooling and heating of vehicle cabin during parking.

Introduction

The vehicle cabin goes through enormous thermal environment variation under various circumstances. Thus, an air conditioning system is obligatory for the vehicle cabin for every instance. The vehicle cabin environment goes through extreme thermal variations when parked outside in open for both summer and winter seasons. However, challenges to overcome peak loads in both seasons are still open to be resolved. These cases of peak loads are generally found when vehicles are parked in open space for a long duration which are reported for summer and spring season by Grundstein et al. [1]. The most adverse effect of vehicle parking is found in the summer season under the sun. When vehicles are parked under the sun for a long period, for example outside office, shopping complex, schools, and colleges, it is unbearable for drivers as well as passengers to come and just sit inside cabin [2]. As the temperature inside the cabin will sharply rise, this would develop harmful poisonous gases for the passengers from the dashboard, seat covers, and other accessories. Children and dogs can be found dead if forgotten by parents [3]. This prolific increase in temperature increases the load on the air conditioner which demands more fossil fuels subsequently increment in chlorofluorocarbon (CFC) emission [4,5]. In case of electric vehicles, which already deal with low energy potentials, air-conditioners may consume considerable amount of total energy [6]. Further reducing the efficiency of vehicle by a substantial amount, which is one of the major parameters for electric vehicle acceptability [7,8]. For cases of heating in winter seasons, in the areas where the ambient temperature drops drastically low such as Moscow, Russia, it is found an extreme drop in vehicle cabin temperature. Thus, when vehicle is parked in an open area, it is difficult to adapt to the inside environment of vehicles by passengers and drivers [9]. However, in the case of heating the vehicle cabin recovers heat energy lost by the engine and other thermal losses while its operation for the fuel consumption vehicles [10]. Although in electric vehicles, the load on the air conditioner cannot be recovered by the same. Therefore, different techniques are being intrigued for lowering of these peak loads by cooling-heating systems that can be active while the vehicle is parked [[11], [12], [13], [14]].

Renewable energy such as solar and wind energy can be easily tapped in case of parked vehicles for providing electricity to any cooling-heating system [[15], [16], [17], [18]]. As a result, majority of the cooling/heating system reported were solar powered driven, in which solar energy was tapped in different ways. Popinski [19] patented an absorption-based cooling system driven by solar and waste heat from the motor for automotive. Similarly, Suzuki [20] also proposed an adsorption cooling system utilizing water as a working fluid for air conditioning and minimizing emissions. Since major loads are due to solar energy absorption, therefore, Levinson et al. [21] used solar reflective car shells for cabin cooling, fuel savings and emission reductions. Besides, researchers have developed solar powered DC fans-based air conditioning and ventilation systems within vehicle cabin for reducing cooling loads [[14], [15], [16], [17], [18]]. Apart from these few other distinctive techniques employed were DC air conditioning system consisting of a DC powered compressor using R134a as refrigerant, phase change material-based cooling system and portable compressed liquid air cooling system for vehicle cabin cooling during parking [11,12,17].

From the above discussed literature, it can be decisively indicated that solar energy is one of the prominent alternative opportunities for vehicle cooling input. However, the systems seem to be complicated and have daily assigned tasks for assembly and disassembly, which requires additional time and effort. Furthermore, it will be quite challenging for the passengers with less time, energy and technicality of the system. Thus, to the best of our knowledge thermoelectric cooling/heating technique is not explored for air-conditioning of vehicle cabins during parking. Therefore, employing thermoelectric technology for vehicle air conditioning can provide maintenance free single solution of both cooling and heating which can be directly operated by solar energy. The thermoelectric modules are convenient, light in weight, compact in size, noise free (no moving part), and highly reliable [22]. These modules have long lifespan and can very easily switch between cooling and heating modes. Thermoelectric devices have solid-state mechanism which works on the principle of Seebeck, Peltier and Thomson effect. Moreover, in recent years, thermoelectric cooling/heating applications have dwelled towards a significant rise [[23], [24], [25], [26], [27], [79]].

The proposed cooling/heating systems for parked vehicle cabin, found in literature are presented in Table 1. The different solution techniques reported for the problem are compared in aspect of cooling/heating operation and input power source. Thus, Table 1 suggests that solar energy has leading potential to power cooling/heating system required for vehicle cabin parking. Further, depicts the novelty of current proposed thermoelectric based system. However, techniques such as solar reflective car shells, PCM and photovoltaic powered DC fans suggest energy saving with complete green and clean operation of the system [16,17,21,28]. In addition with this the proposed system also facilitates stable structure, low maintenance and minimum size. Moreover, the tabular form for literature review clearly represents the existing techniques and outlines differences among different methods (Table 1).

Therefore, a numerical study of the proposed system is developed for both summer and winter seasons. Initially, the thermal model is assessed for evaluation of temperature variation in vehicle cabin and then the performance analysis of the proposed system is examined. In the numerical study all three modes of heat transfer are considered. However, in particular for the summer season study, the heat transfer analysis was dominated by radiation. There are numerical studies which have developed solutions for such combined natural convection-radiation problems [34,35]. In literature, different methods are found for such numerical study as; simplified backward Monte Carlo method, and finite volume method employed with SIMPLE algorithms and backward Monte Carlo method [[34], [35], [36]]. In Table 2, the numerical approaches used for parked vehicle cabin under sun are compared with the aspects of initial and boundary conditions. The major factors taken for numerical studies are supposed as vehicle cabin volume, and the solar radiation as per daytime and location. Thus, Table 2 summarises numerical studies suggested with different approaches for various vehicle cabin models under different solar radiation conditions.

However, the present article deals with time-dependent study and is solved using differential-algebraic system of equations (DAE) solver. This implicit solver uses backward differentiation formula, and is employed with free and strict time stepping. Further, specific considerations were made in the numerical study to follow on with all three heat transfer modes thermal model. Hence, in this article a solar powered thermoelectric based cooling/heating system is proposed for vehicle cabin air-conditioning when parked, for both summer and winter seasons. The thermal performance and cooling/heating distribution within vehicle cabin of the proposed system is assessed. Further, the evaluation of solar panel placed over the vehicle rooftop for both summer and winter seasons, and with variation in tilt angle is performed.