Experimental Investigation of Thermal Performance for Selected Oils in Solar Energy Storage and Cooking

1. Introduction & Overview

This research investigates the thermal performance of locally available oils in Uganda—specifically refined sunflower oil, refined palm oil, and the industrial mineral oil Thermia B—for application in solar thermal energy storage and rural cooking systems. The core challenge addressed is identifying a cost-effective, safe, and efficient heat transfer fluid (HTF) and storage medium suitable for off-grid, rural contexts where conventional industrial HTFs are prohibitively expensive.

The study is motivated by the limitations of common media like air (low heat capacity) and water (vaporization risk at high temperatures). Vegetable oils present a promising alternative due to their higher thermal stability, safety in case of leakage, and local availability, which aligns with sustainable development goals.

2. Experimental Methodology

The experimental approach was designed to evaluate both static heat retention and dynamic heat transfer capabilities under conditions simulating solar thermal charging.

2.1. Oil Samples & Properties

Three oils were selected based on local availability and relevance:

Refined Sunflower Oil: A common vegetable oil.
Refined Palm Oil: Another widely available vegetable oil.
Thermia B: A commercial mineral-based heat transfer fluid used as a benchmark.

Key thermo-physical properties (density $\rho$, specific heat capacity $c_p$, thermal conductivity $k$) were sourced from literature (Mawire et al., 2014), showing vegetable oils generally have higher density and specific heat capacity than Thermia B.

2.2. Heat Retention Capacity Test

A primary experiment measured passive heat retention. A 4.5L cylindrical tank, insulated and fitted with a 1.5 kW electric heater, was filled with 4L of each oil. The oil was heated to a temperature near its smoke point (a safety and performance limit). Heating was then stopped, and the cooling curve was recorded over approximately 24 hours using K-type thermocouples connected to a TC-08 data logger (see Fig. 1 schematic). This test quantified the oil's ability to store and retain thermal energy without active circulation.

Chart/Figure Description (Fig. 1): The schematic shows an insulated cylindrical tank containing the oil sample. An immersion heater is present. Three thermocouples are inserted at different heights (5cm apart) to measure temperature stratification. Wires from the thermocouples connect to a data logger (TC-08), which is interfaced with a computer for real-time monitoring and data recording.

3. Results & Analysis

3.1. Thermal Performance Comparison

The experimental data revealed clear performance hierarchies:

Heat Gain Rate

Vegetable Oils > Thermia B
Sunflower and palm oil reached target temperatures faster than the mineral oil during the charging phase, indicating potentially better heat absorption in a solar collector.

Heat Retention Duration

Sunflower Oil > Palm Oil > Thermia B
Sunflower oil demonstrated the slowest cooling rate, retaining usable heat for the longest period after the heat source was removed.

Total Energy Stored

Sunflower Oil > Palm Oil > Thermia B
Calculations based on cooling curves and heat capacity showed sunflower oil stored the greatest amount of thermal energy per unit volume.

3.2. Key Findings & Data

The study conclusively identified refined sunflower oil as the most suitable candidate among the tested oils for integrated heat transfer and storage in solar cooking systems. Its superior specific heat capacity and thermal retention directly translate to higher system efficiency and longer cooking times from a single charge. Palm oil performed respectably but was outperformed by sunflower oil. Thermia B, while a dedicated industrial fluid, was less effective in this specific application context, likely due to its lower volumetric heat capacity.

Key Insight: The best performer was not the specialized industrial fluid but a locally sourced, food-grade vegetable oil, highlighting the value of context-appropriate technology.

4. Technical Deep Dive

4.1. Mathematical Models & Formulas

The energy stored in the oil during the experiment can be modeled using the fundamental calorimetry equation:

$$Q = m \int_{T_{initial}}^{T_{final}} c_p(T) \, dT$$

Where $Q$ is the thermal energy (J), $m$ is the mass of oil (kg), and $c_p(T)$ is the temperature-dependent specific heat capacity (J/kg·K). The study used empirical formulas for $c_p$ from Mawire et al. (2014), e.g., for sunflower oil: $c_p = 2115.00 + 3.13T$.

The cooling process can be analyzed using Newton's Law of Cooling, approximating the rate of heat loss:

$$\frac{dT}{dt} \approx -k (T - T_{ambient})$$

Where $k$ is a cooling constant dependent on the oil's properties and system insulation. The slower $dT/dt$ for sunflower oil indicates a more favorable $k$ for energy storage.

4.2. Experimental Setup Description

The core apparatus was a well-insulated tank to minimize parasitic heat loss to the environment, ensuring measured cooling curves primarily reflected the oil's intrinsic properties. The use of multiple thermocouples allowed observation of thermal stratification—a warmer layer atop a cooler one—which is typical in stagnant fluid storage. The data logging system provided high-resolution temporal temperature data critical for accurate energy calculations and comparative analysis.

5. Critical Analysis & Industry Perspective

Core Insight: This paper delivers a powerful, counter-intuitive punch: in the niche of low-cost, rural solar thermal storage, a commonplace kitchen staple (sunflower oil) can out-engineer a purpose-built industrial fluid (Thermia B). The real breakthrough isn't a new material, but a radical re-contextualization of an existing one. It shifts the innovation focus from high-tech synthesis to smart, appropriate technology selection.

Logical Flow: The research logic is admirably straightforward and application-driven. It starts with a clear, real-world problem (cost and safety of HTFs for rural cooking), defines relevant performance metrics (heat gain, retention, total storage), and sets up a controlled experiment that directly simulates key system operations (charging and passive cooling). The comparison between local vegetable oils and an industrial benchmark is its masterstroke, providing immediate, actionable relevance.

Strengths & Flaws:
Strengths: The study's greatest strength is its pragmatic validity. The experimental conditions (near smoke-point temperatures, 24-hour cooling) closely mirror real-use scenarios. The choice of locally available oils ensures the findings are immediately implementable, reducing technology transfer barriers. This aligns with the growing field of "frugal innovation" documented by institutions like the World Bank's Energy Sector Management Assistance Program (ESMAP).
Flaws: The analysis is primarily empirical and comparative, lacking a deep dive into the why behind the performance differences. While it cites property data, it doesn't fully explore the molecular or compositional reasons sunflower oil outperforms palm oil. Furthermore, the study omits long-term stability tests—critical for real applications. Vegetable oils can polymerize, oxidize, and degrade under repeated thermal cycling (a phenomenon well-studied in frying oil research). Will sunflower oil form sludge after 100 heating cycles? The paper is silent on this operational crux. It also doesn't address potential impacts on cooking food quality or smell transfer.

Actionable Insights: For engineers and NGOs working on solar cookers for developing regions, the mandate is clear: prototype with sunflower oil now. The performance benefit is proven. The next critical R&D phase must be durability and lifecycle testing. Partner with food chemists to understand and mitigate thermal degradation. Explore simple filtration or additive strategies to extend oil life. Furthermore, this work should catalyze a broader material search: if sunflower oil works, what about other locally abundant, high-heat-capacity fluids like certain seed oils or even sugar-based solutions? The research framework established here is a perfect template for such a systematic, locality-specific screening process.

6. Analysis Framework & Case Example

Framework for Evaluating Local Thermal Storage Fluids:
This research provides a replicable framework for assessing any potential fluid in a specific socio-technical context. The framework consists of four sequential filters:

Context Filter (Availability & Safety): Is the material locally available, affordable, and non-hazardous (e.g., non-toxic, non-flammable in a way water is)? Sunflower oil passes; synthetic oil may fail on cost/availability.
Property Filter (Thermo-physical): Does it have a high volumetric heat capacity ($\rho c_p$) and operational temperature range? Data from literature or simple lab tests applies here.
Performance Filter (Experimental): How does it actually behave in a simulated system? This involves the heat gain and retention tests described in the paper.
Durability & Lifecycle Filter (Long-term): Does it maintain performance over repeated cycles? What is its degradation profile and total lifecycle cost?

Case Example Application:
An NGO in India wants to develop a solar thermal storage unit for community cooking. Using this framework:
1. Context: They identify mustard oil and coconut oil as widely available, affordable, and safe for incidental contact with food.
2. Property: Literature search shows coconut oil has a high specific heat (~2000 J/kg·K) and a high smoke point (~177°C), making it promising.
3. Performance: They build an identical test rig to the paper's Fig. 1, comparing mustard oil, coconut oil, and a baseline of water. They find coconut oil retains heat 40% longer than water for their target temperature band.
4. Durability: They run 50 consecutive heat-cool cycles on the coconut oil, monitoring viscosity and acidity. A significant increase in viscosity after 30 cycles indicates a need for oil replacement or treatment, defining maintenance protocols for the final system design.

7. Future Applications & Research Directions

The implications of this research extend beyond simple solar cookers:

Cascading Solar Thermal Systems: Sunflower oil-based storage could provide not only cooking heat but also lower-grade heat for space heating or water pre-heating in rural clinics or schools, improving overall system economics.
Integration with Solar Parabolic Troughs: Small-scale parabolic trough collectors could use vegetable oils as the direct HTF and storage medium, simplifying system architecture for decentralized applications.
Material Science Hybrids: Future research should investigate creating "enhanced vegetable oils" with dispersed nanoparticles (e.g., alumina, graphite) to boost thermal conductivity ($k$) without sacrificing safety or cost, a concept explored in advanced nanofluids research (e.g., studies published in the International Journal of Heat and Mass Transfer).
AI-Optimized Blends: Machine learning models could be trained on thermo-physical property databases to predict optimal blends of different local oils to maximize $\rho c_p$ and minimize cost for a given climate zone.
Circular Economy Models: Research into using waste cooking oil (after proper treatment) as a thermal storage medium could create a compelling circular economy loop, further reducing costs and waste.

The critical next step is moving from laboratory performance to field-validated, durable system design, addressing the long-term stability questions this foundational study opens.

8. References

Nyeinga, K., Okello, D., Bernard, T., & Nydal, O. J. (2017). Experimental Investigation of Thermal Performance for Selected Oils for Solar Thermal Energy Storage and Rural Cooking Application. ISES Solar World Congress 2017 Proceedings. doi:10.18086/swc.2017.14.05
Mawire, A., McPherson, M., & van den Heetkamp, R. R. J. (2014). Simulated performance of storage materials for pebble bed thermal energy storage (TES) systems. Applied Energy, 113, 1106-1115. (Source for thermo-physical property data).
Okello, D., Nyeinga, K., & Nydal, O. J. (2016). Experimental investigation of a rock bed thermal energy storage system for solar cooking. International Journal of Sustainable Energy.
World Bank / ESMAP. (2020). Frugal Innovation in the Energy Sector: A Guide to Doing More with Less. [Online Report].
International Energy Agency (IEA) Solar Heating and Cooling Programme (SHC). (2021). Task 58: Material and Component Development for Thermal Energy Storage. [Research Program].
Said, Z., et al. (2021). Recent advances on nanofluids for low to medium temperature solar collectors: energy, exergy, economic analysis and environmental impact. Progress in Energy and Combustion Science, 84, 100898. (For nanofluid enhancement context).