Using Solar Energy to Reduce the Evaporation Rate of Gasoline in Fuel Storage Tanks

1. Introduction

Gasoline is a highly volatile fuel, and exposure to its vapors poses serious risks to human health while also contributing significantly to environmental pollution. From an economic perspective, the continuous growth in gasoline consumption over recent decades, coupled with high production and import costs, makes the reduction of fuel losses a critical issue. Statistical reports indicate that approximately 2% of crude oil extracted from wells is lost due to evaporation [1]. According to domestic and international experts, nearly 75% of these losses occur during storage and transportation in tanks. Such evaporation not only leads to substantial economic losses by reducing the available volume of gasoline but can also degrade fuel quality [2], as preferential evaporation increases the concentration of additives, potentially causing damage to vehicle engines.

Beyond economic implications, gasoline vapor emissions present serious environmental and public health concerns. Although most petroleum vapors are not acutely toxic, they actively participate in photochemical reactions that lead to smog formation. These processes can result in adverse health effects such as eye, nasal, and throat irritation, dizziness, headaches, short-term memory impairment, and, in long-term exposure scenarios, cancer and genetic mutations. In the presence of nitrogen oxides and ultraviolet radiation, gasoline vapors contribute to the formation of photochemical pollutants and acidified aerosols, ultimately producing photochemical smog [3].

A critical emission pathway occurs at fuel dispensing stations, particularly during fuel transfer from tanker trucks to underground gasoline storage tanks. As gasoline is discharged into the tank, an equivalent volume of vapor must be displaced to prevent overpressure. This vapor is typically released through vent pipes installed at the top of the storage tank. While necessary for pressure regulation and operational safety, conventional venting systems can unintentionally intensify evaporative losses not only during refueling events but also under static storage conditions.

This effect is strongly influenced by solar radiation. When exposed to sunlight, the vent pipe absorbs radiant heat, which increases the temperature of the air inside the vent. As the air temperature rises, its density decreases, generating buoyancy forces that drive airflow upward. Two heat transfer mechanisms are primarily responsible for this behavior: radiative heat transfer from solar radiation to the external vent surface and free convection that heats the internal air within the vent. The resulting upward flow reduces the pressure above the gasoline surface, thereby accelerating evaporation.

This process, commonly referred to as the chimney effect (or stack effect), has historically been utilized in natural ventilation systems of traditional architecture. In gasoline storage tanks, however, it leads to an increased mass transfer coefficient between the liquid fuel and the vapor space above it, intensifying breathing losses [4]. Similar physical principles are employed in modern solar chimney technologies, though typically to enhance airflow or power generation.

The primary objective of the present research is to invert the conventional role of the chimney effect in vent paths at fuel dispensing stations. Instead of promoting airflow, this study proposes exploiting solar‑induced thermal effects through a modified vent geometry—referred to as a solar vent—to suppress gasoline evaporation. Previous numerical investigations, including computational fluid dynamics (CFD) analyses reported in an earlier ISI publication by the authors [10], showed that appropriate geometric modification of the vent outlet can significantly alter buoyancy‑driven flow patterns and stabilize pressure fluctuations within the vapor space.

Building upon these findings, the present study focuses on a process‑oriented experimental assessment of the proposed solar vent concept. By modifying the vent outlet geometry, this work aims to utilize heat absorption by the vent walls in a controlled manner that increases positive pressure above the fuel surface and reduces evaporative mass transfer. The influence of buoyancy‑driven airflow on evaporation behavior is evaluated using dimensionless parameters such as the Schmidt number [5]. As increased air velocity is known to enhance mass transfer rates, controlling airflow velocity within the vent path is therefore essential for minimizing evaporation losses.

From a broader regulatory perspective, emission control strategies for gasoline storage tanks are often assessed under frameworks such as Reasonably Available Control Technology (RACT). The U.S. Environmental Protection Agency (EPA) has established regulations to limit volatile organic compound emissions from storage tanks [6]. Honors reviewed various vapor recovery and control systems applied to oil storage tanks, including their fundamental design principles and emission mitigation mechanisms [7].

Further studies by the Air Pollution Control Association examined hydrocarbon vapor emission control in gasoline storage tanks, presenting theoretical models of evaporative losses, associated control equipment, maintenance considerations, and calculation methodologies for emission reduction [8].

Research primarily focuses on large‑scale storage systems, such as external floating‑roof tanks and refinery‑related emission inventories. In contrast, direct experimental investigations addressing evaporation mitigation in small‑scale, fixed‑roof gasoline storage tanks representative of fuel dispensing stations—particularly through passive vent geometry modification—remain scarce in the open literature. This relative scarcity reflects the underexplored nature of the topic rather than a lack of practical relevance and motivates the experimental emphasis of the present study.

Within this context, the present work introduces a passive, low‑cost venting modification specifically targeted at gasoline storage tanks used in fuel dispensing stations, providing an experimentally grounded discussion of its evaporation‑mitigation potential without reliance on active control systems or complex moving components.

Clarification on data reuse: The experimental setup, datasets, figures, and tables reported in the present manuscript are based on experiments that were previously published in the authors’ related ISI article [10]. These data are intentionally reused here to provide an independent, process‑oriented experimental discussion focusing on evaporation reduction performance. While the earlier publication presented a combined experimental and numerical (CFD) analysis of the proposed solar vent, the present study differs in scope and objectives by focusing exclusively on experimental assessment, engineering interpretation, and practical implications for fuel dispensing station storage tanks.

2. Research Method and System Description

Figure 1 illustrates the geometry of the new vent pipe proposed in this research, while Figure 2 presents the operational principle of the solar vent. Figures 3 and 4 show the conventional vent geometry and the newly designed vent geometry, respectively, both intended for gasoline storage tanks used in fuel dispensing stations.

Conventional venting systems typically employ pressure–vacuum (P&V) relief valves to protect atmospheric and low‑pressure storage tanks. These valves are designed to remain closed up to a specified internal pressure threshold; for instance, when the tank pressure reaches approximately 30 kPa, the valve is still closed, while further pressure increase causes the valve to open and release gasoline vapor to the atmosphere. P&V valves operate unidirectionally: they limit vapor discharge below a defined pressure, but allow ambient air to enter the tank under vacuum conditions. In practical fuel station operations, during vehicle refueling and fuel withdrawal from underground storage tanks, a relative vacuum may be created inside the tank, which is relieved by air ingress through the P&V valve and vent system.

Despite their protective function, conventional vent configurations can lead to significant vaporization losses, particularly during warm seasons. These losses are intensified under solar exposure when vent pipes are heated, promoting buoyancy‑driven airflow and enhancing evaporation. To address this limitation, the present study proposes an innovative passive vent configuration, referred to as the solar vent, specifically designed to reduce gasoline evaporation in fuel station storage tanks.

As shown schematically in Figure 2, in the proposed configuration, gasoline vapor flows through a central inner pipe, while ambient air circulates within an annular outer passage. Solar radiation heats the air in the outer passage, causing it to rise due to buoyancy effects. This upward airflow leads to air accumulation in the upper section of the vent assembly, thereby increasing local air pressure. The resulting convergence of airflow at the outlet of the inner vapor pipe creates a pressure barrier that suppresses vapor discharge. Consequently, vapor release is delayed until the gasoline vapor pressure inside the tank exceeds the elevated air pressure in this region, leading to a reduced evaporation rate. Under abnormal or potentially unsafe conditions, when vapor pressure becomes sufficiently high, it overcomes this pressure barrier, allowing controlled vapor release and maintaining operational safety.

For experimental evaluation, two identical laboratory‑scale gasoline storage tanks were constructed to represent the operational conditions of fuel dispensing stations. Tank No. 1 was equipped with a conventional vent system, while Tank No. 2 incorporated the proposed solar vent design. In both tanks, the gasoline free‑surface area was 0.442 m², the total tank volume was 220 L (0.22 m³), the liquid gasoline volume was 0.057 m³, and the vapor‑space volume (defined as the difference between the total tank volume and liquid volume) was 0.163 m³. The vent pipes were fabricated from black steel to ensure similar thermal absorption characteristics under solar radiation.

The devices and instruments used for constructing and monitoring the experimental tanks are listed in Table 1. Photographs documenting the fabrication, assembly, and installation stages of the laboratory‑scale experimental setup are provided in Figure 5.

3. Results and Discussion

The experiments were conducted simultaneously on two identical laboratory‑scale gasoline storage tanks, designated as Tank No. 1 (conventional vent) and Tank No. 2 (solar vent), under comparable environmental conditions from August 28, 2022, to September 15, 2022. Measurements were recorded at multiple time intervals throughout each day, and the summarized results are presented in Table 2.

Figure 6 illustrates the variations in ambient temperature and relative humidity during the experimental period. Figure 7 compares the wall temperature and gasoline temperature profiles for both tanks. As shown in this figure, the wall and gasoline temperatures remained very similar in the two tanks throughout the test period. This observation indicates that the modification of vent geometry does not have a measurable influence on either the tank wall temperature or the bulk gasoline temperature under the tested laboratory conditions.

In contrast, a clear difference is observed in the vapor‑space pressure behavior. Figure 8 shows that the relative pressure inside Tank No. 2 (solar vent) is consistently higher than that of Tank No. 1. In the conventionally vented tank, the internal pressure occasionally drops below atmospheric pressure, resulting in intermittent negative gauge pressure. The formation of such negative pressure conditions enhances mass transfer at the gasoline–vapor interface and increases evaporation. Conversely, the solar vent configuration promotes the formation of a slightly positive and more stable pressure above the gasoline surface, which suppresses vapor release and lowers the evaporation rate.

Based on the mass balance data summarized in Table 2, Tank No. 1 contained 55 L of gasoline at the end of the experimental period, corresponding to an evaporative loss of 2 L from the initial 57 L. In comparison, Tank No. 2 retained 56.5 L of gasoline, indicating a loss of only 0.5 L. Expressed as percentages, the evaporation loss in the conventional tank was approximately 3.5% of the initial gasoline volume, while the loss in the solar‑vented tank was less than 0.9%.

Under the specific experimental conditions of this study, these results correspond to an approximately 75% reduction in evaporative loss for the tank equipped with the solar vent, equivalent to a 2.6% conservation of stored gasoline over the 17‑day test period. It should be emphasized that this value is derived from a single paired experimental comparison and represents a descriptive outcome rather than a statistically generalized prediction.

From a transient and safety perspective, the solar vent did not prevent pressure relief under elevated vapor pressure conditions. When internal vapor pressure increased beyond the locally induced air pressure near the vent outlet, vapor discharge occurred, indicating that the proposed geometry preserves the fundamental safety function of conventional venting systems while moderating pressure fluctuations during normal thermal breathing cycles.

Although extrapolation to full‑scale storage tanks must be approached with caution, the observed reduction in evaporation suggests that vent geometry can play a meaningful role in mitigating gasoline losses at fuel dispensing stations. The present results are limited to laboratory‑scale conditions representative of service‑station storage tanks and do not account for long‑term operation, seasonal variability, fuel composition differences, or large‑scale industrial effects. Further experimental repetition, extended monitoring periods, and scale‑up studies are therefore required before quantitative predictions can be made for large tanks (e.g., 50,000 L).

• Tables, Figures, and Diagrams

Fig. 1. An early prototype of the modified vent geometry (solar vent) for investigation (proposed model of the design)

Fig. 2. Schematic illustrating the operational principle of the solar vent

 Table 1. List of equipment and raw materials used for constructing the samples

Fig. 5. Some views of the steps of making laboratory samples: a) the tanks, b) the vent pipes, c) the sensors’ monitor

Fig. 6. Variations in ambient air temperature and relative humidity for different experiment numbers

Fig. 7. Variations in relative pressure inside the tank

Fig. 8. Variations in wall and gasoline temperatures in the first and second tanks

Table 2. Results of experimental investigations

 3.2. Mathematical Formulas and Relationships

The governing equations presented in this section describe the fundamental conservation laws and classical turbulence concepts that provide a theoretical framework for interpreting the vent‑induced buoyancy and evaporation mechanisms investigated experimentally in this study. These relations are based on well‑established formulations in buoyancy‑driven flow analysis and are included solely to clarify the physical principles underlying the observed experimental behavior, rather than for predictive numerical simulations.

Mass Conservation Equation

For incompressible flow, the continuity equation in Cartesian coordinates is expressed as [9]:

where ujuj​ represents the velocity component in the xj​ direction.

The momentum equation for incompressible flow with constant thermophysical properties, incorporating buoyancy effects through the Boussinesq approximation, is written as [9]:

 where is fluid density, is pressure, is the gravitational acceleration component, β is the thermal expansion coefficient, ​ is the reference temperature, and   denotes the Reynolds stress tensor.

This equation accounts for buoyancy‑driven flow resulting from temperature gradients in the vent pipe.

Energy Equation

The energy equation for incompressible fluid flow with constant thermophysical properties is written as [9]:

where α is the thermal diffusivity and  represents turbulent heat flux.

This relation governs heat transfer induced by solar radiation and its effect on buoyancy within the vent.

Turbulence Modeling (k–ε Model)

The standard k–ε turbulence model is based on the eddy viscosity concept: .

with the turbulent viscosity defined as [9]:

where Cμ​ is a model constant, k is the turbulent kinetic energy, and ε is the dissipation rate of turbulent kinetic energy.

The transport equation for turbulent kinetic energy is expressed as:

, ,  &  constants.

 &  represent the effect of buoyancy forces.

  is the production of turbulence due to viscous forces.

Role of the Mathematical Framework in This Study

Although no new CFD simulations are presented in the current manuscript, the above equations are provided solely to establish a qualitative theoretical basis for understanding buoyancy‑driven flow, pressure stabilization, and evaporation suppression within the vent geometry. Accordingly, the experimental observations reported in the Results section are discussed from an interpretive perspective rather than through numerical prediction.

4. Conclusion

Given the large volumes of gasoline stored in fuel service stations, evaporation losses from storage tanks remain an important environmental and economic concern. These losses contribute to the emission of volatile organic compounds and impose direct financial penalties due to fuel loss. Conventional mitigation approaches at service stations often rely on imported pressure and vacuum (P&V) relief valves, which operate by regulating preset positive and negative pressures above the fuel surface. While effective, such systems are relatively expensive and require periodic calibration and maintenance.

In this study, a passive and low-cost vent geometry was designed and experimentally evaluated as an alternative approach for reducing gasoline evaporation without the need for specialized maintenance. A laboratory-scale experimental setup representative of fuel service station conditions was developed, consisting of two identical 220‑L gasoline tanks operated under the same environmental conditions over 17 days. One tank was equipped with a conventional vent, while the other employed the proposed vent configuration. Tank wall temperature, gasoline temperature, vapor‑space pressure, and fuel loss due to evaporation were monitored.

The experimental results indicate that the proposed vent geometry promoted a slightly higher and more stable positive vapor‑space pressure above the gasoline surface compared to the conventional vent. Under the specific laboratory conditions tested, this pressure stabilization was associated with a noticeable reduction in gasoline evaporation. In comparative tests, the conventionally vented tank lost approximately 2 L of gasoline (about 3.5% of the initial volume), whereas the tank equipped with the proposed vent lost approximately 0.5 L (about 0.9%) over the same period. These observations correspond to an approximate 75% reduction in evaporative loss relative to the reference tank.

Nevertheless, the present results suggest that passive vent geometry modification may offer a promising supplementary approach for mitigating evaporation losses in fuel service station storage tanks, warranting further controlled testing and scale‑up evaluation.

The limited number of directly comparable experimental studies highlights the need for further systematic investigations across different tank scales, climatic conditions, and operational scenarios.