Heat Utilization Characteristics of Two Sensible Heat Storage Vegetable Oils for Domestic Applications

Abstract

The heat utilization characteristics of two different sensible heat storage vegetable oils for domestic applications are evaluated. Heat utilization is the heat delivered to the cooking fluid/load. The two sensible heat storage vegetable oils are sunflower oil, and Roki oil (a blend of palm oil and sunflower oil). The heat utilization characteristics of the two heat storage materials are experimentally determined in water heating experiments using 1.0, 1.5, 2.0, and 2.5 kg water loads. The stored heat in Roki oil and sunflower oil is used to heat up the water during cool down/heat utilization tests. The reason for the research is that new insights into the utilization of a locally available vegetable oil (Roki oil) in Uganda, as a sensible thermal energy storage material, is determined. Additionally, a new storage pot is designed, tested, and compared with Roki oil and sunflower oil as sensible thermal energy storage materials. The highlights of the work are that average heat utilization values are dependent on the water heating load. The average heat utilization efficiency increases with the increase in the water heating load; Roki oil shows higher heat utilization and higher average heat utilization efficiency values as compared to sunflower oil; the average heat utilization and average heat utilization efficiency values increase with the increase in the water load for both Sunflower oil and Roki oil. The results suggest that Roki oil is a potential heat storage material for domestic applications since it shows better heat utilization characteristics than sunflower oil during heat utilization.

1. Introduction

Cooking forms part of the daily activities of human beings for survival and well-being. The cooking time and heat storage time are affected by the cooking vessel, cooking fluid, and heat storage material. Thermal energy storage (TES) systems use sensible heat storage materials (SHSM) or latent heat storage materials (LHSM) for energy storage. Sensible heat storage (SHS) materials include water, sand, pebbles, rocks, steel balls, and thermal oils. For LHSM, phase change materials (PCMs) such as erythritol, eutectic solder, and adipic acid are used. Sensible heat thermal energy storage (SHTES) is cheaper than latent heat thermal energy storage (LHTES) for small storage volumes, but its energy storage density is lower [1]. Local availability, cheapness, and ease of use are three distinct advantages of SHSM over LHSM [2]

Both water and thermal oils can be used as heat transfer fluids and heat storage media. In the case of water, its advantage of cheapness at low temperatures is overshadowed at high temperatures for the cooking processes used for reaching its boiling point (100 °C), which requires pressurizing it for higher temperatures to be achieved. The usefulness of thermal oils is that they store thermal energy at temperatures greater than the boiling point of water without the need for pressurizing mechanisms, which are usually expensive. A recently used thermal oil for domestic solar applications is sunflower oil [1,3,4,5,6,7]. It is non-toxic and its fumes are generally tolerated; it is food-grade, and its flash point of around 250 °C is a temperature far above the cooking temperature of the majority of food [1,6]. Most of the characteristics shown by sunflower oil are shown by other edible oils except for the slightly different flash points. Other vegetable oils/bio-oils that have been recently used for thermal energy storage are Jatropha oil and palm olein [8,9]. Edible oils are also generally cheaper than commercial non-edible heat transfer oils [1].

Thermo-physical properties and thermal energy storage performance of two vegetable oils were experimentally studied [3]. A blend of sunflower oil and palm oil (Roki oil) and pure sunflower oil were the vegetable oils used. The results showed that during charging, Roki oil attained higher maximum temperatures compared to sunflower oil. An experimental investigation of a cooking unit integrated with a TES system was performed by [10]. Sunflower oil was used both as the heat storage material and heat transfer fluid. The results indicated that the heating rate increased with increasing flow rates. In the performance comparison of two cooking pots/vessels, sunflower oil was used as a sensible heat storage material and cooking load [5]. It was suggested that a possible future work with a cooking pot containing both sensible heat storage material and phase change material (PCM) should be carried out to study the performance comparison during TES. An experimental study of the performance of sunflower oil as a sensible heat storage medium for domestic applications was carried by [6]. The study highlighted that other edible thermal oils with reasonable low viscosities at room temperature such as Coconut oil could have been used but sunflower oil was a priority because of it price, availability, and wide usage in Sub-Saharan Africa. Vegetable oils are cheap and readily available sensible heat storage materials produced locally in most countries in Sub-Saharan Africa. Recent studies on the use of vegetable oils as sensible heat TES media in both domestic and industrial applications have been presented [11,12,13,14,15,16,17,18].

Recently, palm oil has been studied as TES material [19,20]. Their studies found that using palm oil as a bio-based phase change material as an energy storage material in building is cheaper compared to commercially available high-performance products such Rubitherm. Results obtained by Fabiani et al. [19] showed that expired palm oil from the food industry can be considered as a material for bio-based latent heat applications. The application of bio-based phase change materials for effective heat management has been recently reviewed [21]. It was noted that environmentally friendly bio-based phase-changed materials should be studied. There is the availability of a blended edible vegetable cooking oil known as Roki oil in Uganda [22], locally available on the Ugandan market. Roki oil is a mixture/blend of sunflower oil and palm oil. Roki oil is cheaper than pure sunflower oil and is widely used in Uganda in food preparation. It costs USD 3 per liter compared to USD 6 per liter for pure sunflower oil. The frying stability of different selected brands of cooking oils in the greater Metropolitan Kampala during frying cycles was reported by [23]. Roki oil exhibited the best frying stability. Roki oil is a vegetable cooking oil that is widely used in restaurants for cooking in Uganda. It is locally manufactured and relatively cheaper than sunflower oil, thus, its widespread usage in Uganda. The used-up Roki oil from restaurants can be re-used in TES for pre-heating water for cooking rice, and making hot chocolate, tea, and hot coffee, hence, reducing the demand for electrical energy needed for cooking.

In the studies of TES, electrical energy has been used in simulating solar collectors or concentrators [2,4,24,25,26]. The performance of a thermal energy storage system integrated with a cooking unit was experimentally investigated [2]. An electrical heater was used in charging the oil and oil–rock system. The result showed that the best thermal performance was by the oil–rock system. Kajumba et al. [4] used electrical energy to simulate solar energy in their experimental investigation of a cooking unit integrated with thermal energy storage system. Their results showed that the efficiencies of the cooking unit increased with the flow rates. In addition, the highest flow rate gave the highest heat transfer rate, while the lowest flow rate resulted in the lowest heat transfer rate. Mawire and McPherson [24] experimentally studied the temperature distribution of an oil–pebble bed thermal energy storage system with an electrical hot plate in thermal contact with a copper spiral coil. The hot plate was used as the source of electrical energy in simulating the solar concentrator. In the study of the thermal performance of a small oil-in-glass tube thermal energy storage system during charging, an electrical hot plate in thermal contact with a steel spiral coil was used to charge the TES system [25]. Results showed degradation of the thermal performance due to increased heat losses when the optimal charging temperature is exceeded. An electrical energy source was used to simulate solar energy. Experimental characterization of a thermal energy storage system using temperature and power-controlled charging was carried out by Mawire et al. [25]. An electrical hot plot (collector/concentrator) and a variable power controller were used. The results indicated that due to heat losses, larger deviations between the experiment and simulation were seen at later stages of the charging process. From the reviewed literature, it is clear that one can use electrical energy (hot plates) to simulate the operation of solar concentrators or solar collectors.

Most of the reviewed studies involved the use of sunflower oil as the sensible heat thermal oil for domestic TES applications. However, studies on blended vegetable cooking oils (blend of sunflower oil and other cooking oils) to enhance the thermal performance of TES has been rarely reported in the literature. Roki oil is a suitable example of a blended cooking oil to be used as an SHSM. A study would enable researchers and designers to choose the appropriate and cheaply available thermal oil for domestic TES. The aim of this paper is to perform static laboratory tests on Roki oil and sunflower oil to evaluate their performances during charging, cool-down, and heat utilization processes. Charging tests are carried out with low-power charging cycles for a period of 2 h. Cool-down tests for 1 h are carried out after low-power charging. Heat utilization tests with 1.0, 1.5, 2.0, and 2.5 kg water loads are done during the 1 h cool down periods on different days. Abedigamba et al. [3] studied the thermo-physical properties and thermal energy storage performance of the two vegetable oils. However, the effect of the load was not studied. The novelty of the study is that new insights into the utilization of a locally available vegetable oil (Roki oil) in Uganda as a sensible thermal energy storage material will be determined. In addition, a new storage cooking pot was designed and tested with a suitable new sensible heat storage material. This has rarely been investigated to the best of our knowledge. The study is justified since Roki oil is cheaper than sunflower oil. Vegetable oils are also food grade, thus, contamination of food is not an issue when compared to other commercial fossil-fuel-based blends.

2. Methodology

2.1. Charging and Discharging Experiments: Simulation with Electrical Energy

The thermophysical properties of the two thermal oils experimentally tested are shown in

Table 1. Thermophysical properties of Roki oil and sunflower oil at 25 °C [3] (Adapted with permission from [3], 2023, with permission from Elsevier).

Property	Sunflower Oil	Roki Oil
Density (g cm−3)	0.915	0.909
Specific heat capacity (J g−1 K−1)	1.863	1.888
Thermal conductivity (W m−1 K−1)	0.163	0.170
Thermal diffusivity (mm2 s−1)	0.095	0.095
Sound velocity (m s−1)	1453.100	1446.100
Viscosity (mPa s)	57.000	82.200
Refractive index	1.472	1.465
Isentropic compressibility (Pa−1)	5.2 × 107	5.3 × 107
Isobaric thermal expansion (K−1)	7.65 × 104	7.70 × 104

[3]. Most thermal properties appear similar, except for slight differences in the thermal conductivity and viscosity of the two oils. Thermophysical properties such as thermal conductivity and specific heat capacity are essential in explaining observational data and have been used in recent years [3,27,28]. The specific heat capacity (c_p), thermal conductivity ($\lambda$), and thermal diffusivity (a) were measured at NETZSCH in Germany. The density ($\rho$) at room temperature of the thermal oils was measured using the buoyancy flotation method. The thermal conductivity was computed using the values of c_p, a, and $\rho$ with the formula $\lambda$ = $\rho .$c_p. a. More details of the analysis of the physical properties are presented in a recent paper by [3]. The variation of density ($\rho )$, sound velocity (u), viscosity, and refractive index with temperature were measured at North-West University, South Africa. The isentropic compressibility, $\kappa$_s, was computed using the Newton–Laplace equation, $\kappa$_{s =} $\frac{1}{\rho u^2}$ and isobaric thermal expansion, $\alpha$_p = −$\left(\frac{1}{\rho }\right){\left(\frac{\partial \rho }{\partial T}\right)}_P.$ The details of the measurements are presented in the recent paper by [3].

2.1.1. Experimental Equipment

A photograph of the experimental setup is shown in Figure 1. The storage cooking pot containing the cooking oil was heated by an electrical heating element shown in Figure 1 as (1). The cooking pot is indicated as (2). The electrical heating element has a power control knob which is used for setting the power of the heating element. The control level 2 (600 W) on the heating element was used [3]. In Figure 1, (3) and (4) are the data logger and computer, respectively, and (6) is the power supply.

2.1.2. Temperature Monitoring

Temperature measurements are carried with thermocouples which are indicated as (5) in Figure 1. Equal volumes of oils are filled in the cavities of the pots. The duration of heating the oils was 120 min. During this period, the temperature monitoring of the oils was carried out with K-type thermocouples having an accuracy of ±2.2 °C. The data are recorded at intervals of 10 s on the data logger. Figure 2 is a schematic diagram showing the storage cooking pot. Heated oil in the wall cavities of the pot was used to heat up 1.0, 1.5, 2.0, and 2.5 kg of water as test loads during charging, heat retention, and heat utilization. Data were recorded every 10 s for 60 min. The storage cooking pots were not insulated since the experiments were performed indoors, and the aim was to find the general performance first, before optimizing it. Each experiment was repeated twice. The mean values were used in the final analysis.

2.2. Performance Analysis of Sunflower Oil and Roki Oil

The experimental thermal parameters to characterize the thermal performance of sunflower oil and Roki oil during charging and heat retention are the total energy stored $\left({\mathrm{Q}}_{\mathrm{u}\mathrm{s}}\right)$, the total heat utilization $\left({\mathrm{Q}}_{\mathrm{u}\mathrm{t}\mathrm{i}}\right)$ and heat utilization efficiency (${\mathsf{\eta }}_{\mathrm{u}\mathrm{t}\mathrm{i}})$. The different calculations are made using Equations (1)–(3), respectively.

The total energy stored ${\mathrm{Q}}_{\mathrm{u}\mathrm{s}}$ during the cooking period was calculated from Equation (1) as [5,29];

$${\mathrm{Q}}_{\mathrm{u}\mathrm{s}}=\sum \mathrm{m}\mathrm{c}∆\mathrm{T},$$

(1)

where m is the mass of the oil,$\mathrm{c}$ is the specific heat capacity of the oil, and $∆\mathrm{T}$ is the moving average temperature between the next and the previous time step at the interval of $∆\mathrm{t}.$

The total heat utilization energy during the storage cooking period was determined from Equation (2) as [5,29];

$${\mathrm{Q}}_{\mathrm{u}\mathrm{t}\mathrm{i}}=\sum {\mathrm{m}}_{\mathrm{w}}{\mathrm{c}}_{\mathrm{w}}∆\mathrm{T},$$

(2)

where ${\mathrm{m}}_{\mathrm{w}}$ is the mass of water being heated and ${\mathrm{c}}_{\mathrm{w}}$ is the specific heat capacity of water.

The heat utilization efficiency was estimated from the ratio of the total heat utilization energy to the total energy stored, and it is expressed as;

$${\mathsf{\eta }}_{\mathrm{u}\mathrm{t}\mathrm{i}}=\frac{{\mathrm{Q}}_{\mathrm{u}\mathrm{t}\mathrm{i}}}{{\mathrm{Q}}_{\mathrm{u}\mathrm{s}}},$$

(3)

where Q_uti is the total heat utilization and Q_us is the total heat stored [5,29].

3. Results

3.1. Thermal Profiles

Roki oil showed higher maximum temperatures compared to sunflower oil during charging. Figure 3 shows the temperature profile for storage during heating. In Figure 3a–d, the heat storage temperature of Roki oil is higher than that of sunflower oil. Higher maximum temperatures are achieved by Roki oil due to its slightly higher thermal conductivity. It can also be observed that the temperature difference of the two oils at the end of charging increased with the increase in the load from 1.0–2.5 kg. The temperatures were high enough to cook food. Abedigamba et al. [3] used both electrical and solar energy to study heat utilization with a 1.0 and 1.5 kg load during charging and discharging. Roki oil performed better than sunflower oil as a heat storage material during charging and discharging. A performance comparison of sunflower oil and erythritol with loads of 0.5–2.5 kg was made by Mawire et al. [30]. Storage efficiencies were calculated, and it was found that the storage efficiencies marginally increased with the increase in the load. Some works have focused on the effect of the load on thermal performance [10,31].

Roki oil showed higher maximum temperatures compared to sunflower oil during cool-down heat retention. Figure 4 shows the temperature profiles for storage materials during heat retention. In Figure 4a–d, Roki oil achieved higher final temperatures compared to sunflower oil due to its slightly higher thermal conductivity. The cooling of each thermal oil was acheived by natural convection. Due to the time lag between switching off the electric heater and adding the load for heat utilization, the initial temperatures recorded for Roki oil and sunflower oil are not the same as the final temperatures achieved during charging. Sunflower oil shows a higher rate of temperature decrease in Figure 4a–d during the 60 min for the cool-down heat retention period. At the end of the 60 min (1 h) cool-down heat retention process, the temperatures obtained are enough to warm water for bathing and to warm cold food. After about 20 min of the heat retention process for Roki oil with the lowest load, the temperature achieved is enough to cook food such as rice. This is applicable/useful during the period when electrical heating is not possible, for example, when there is scheduled load-shedding for a short duration. Sunflower oil shows a high decrease in temperature compared to Roki oil, possibly due to its lower thermal mass storage mass (mc).

Higher maximum temperatures were shown by loads utilizing Roki oil compared to sunflower oil during heat utilization. Figure 5 shows the temperature profiles of heat utilization during discharging with 1.0, 1.5, 2.0, and 2.5 kg water loads. In Figure 5a–d, the water load temperatures in Roki oil were higher than the water load temperatures in sunflower oil. This is because Roki oil has higher thermal conductivity than sunflower oil. Roki oil can quickly transfer heat to the water load. Sunflower oil has a lower thermal conductivity, thus slowing heat transfer to the load. Overall, the load temperatures in Roki oil are higher than those of sunflower oil. The water loads in Figure 5a–d are placed immediately after the cool-down heat retention commences. However, due to the time lag between starting the data logger and placing the load, the initially recorded load temperatures are higher than the room temperature one would expect. This is because the data logger was started after about 30 s, when the load was placed during the heat utilization period (time lag). It is important to also note that as the load is increased, the maximum temperatures attained by the load decreased due to the increase in the load. The maximum temperature for Roki oil decreased from 97 to 70 °C for a load increase of 1.0–2.5 kg, whereas the corresponding decrease for sunflower oil is from 93 to 55 °C.

3.2. Thermal Performance Parameters

Table 2. A summary of thermal performance parameters of Roki oil and sunflower oil * using electrical heating.

Test Case (Roki Oil, Sunflower Oil *)	Heated Load (kg)	Average Energy Stored (kJ)	Average Heat Utilization Energy (kJ)	Average Heart Utilization Efficiency (-)
Case 1	1.0	595, 591 *	150, 148 *	0.253, 0.251 *
Case 2	1.5	530, 497 *	226, 156 *	0.426, 0.314 *
Case 3	2.0	529, 457 *	289, 192 *	0.547, 0.421 *
Case 4	2.5	524, 437 *	304, 241 *	0.574, 0.485 *

* Indicates results for sunflower oil, while values without star symbols are values for Roki oil.

shows the experimental comparison of thermal performance parameters of Roki oil and sunflower oil during storage and heat utilization using four different water loads of 1.0, 1.5, 2.0, and 2.5 kg, respectively. The average energy stored for Roki oil was higher than the average energy stored in sunflower oil by a difference of 4.0 kJ for 1.0 kg of water. This implies that Roki oil can store more energy than sunflower oil with the lowest load. As the load was increased for both fluids, the energy stored decreased slightly because of lower storage temperatures attained with larger loads. The average energy stored for all loads was greater for Roki oil compared to sunflower oil, possibly due to combined effect of the slightly higher specific heat capacity and thermal conductivity of Roki oil (for detailed values of specific heat capacity and thermal conductivity, see [3] (Table 1 and Table 2)). Average heat utilization energies and average heat utilization efficiencies for Roki oil and sunflower oil increase with an increase in water load. This suggests that heating larger water loads is more efficient than heating small loads. The increase in the average heat utilization energy and heat utilization efficiency for Roki oil was higher than that of sunflower oil because of the higher load temperatures attained by Roki oil due to its slightly higher thermal conductivity. Generally, Roki oil shows a higher maximum average energy storage values compared to sunflower oil. In addition, average heat utilization efficiencies for Roki and sunflower oil increased with an increase in load.

4. Conclusions

An experimental setup to compare the performance of Roki oil and sunflower oil as sensible heat storage materials for domestic applications was presented. Roki oil and sunflower oil were experimentally compared using water heating. The thermal performance of the oils was evaluated in terms of heat stored, heat utilization energy, and heat utilization efficiency. The water loads used in the experimental tests were 1.0, 1.5, 2.0, and 2.5 kg, respectively. The main conclusions drawn were:

• The average energy stored was reduced because of slightly lower storage temperatures attained with larger loads for both pots. However, the average energy stored for Roki oil (595–524) kJ was higher compared to that of sunflower oil (591–437) kJ because of the higher storage temperatures attained by Roki oil.
• The average heat utilization energy and average heat utilization efficiency values were dependent on the water heating load. The average heat utilization efficiency increased with the increase in the water heating load.
• Roki oil showed higher average heat utilization energies (150–304) kJ, and higher average heat utilization efficiencies values (0.25–0.57) compared to sunflower oil, which showed (148–241) kJ and (0.25–0.49), respectively.

The results suggest that Roki oil, which is relatively cheaper than sunflower oil, is a potential thermal energy storage material that can be used in place of sunflower oil.

Future work will look at:

• The use of nanoparticles in Roki oil to improve its thermal conductivity.
• Experimental tests with solar energy using solar cookers for a practical solar cooking application.
• Investigating the combined use of vegetable oils and PCM to increase the thermal energy storage density, thus, improving the thermal performance.
• Comparing non-edible oils with Roki oil as sensible heat storage materials. It should be noted that non-edible oils are not food grade and will contaminate the food if spillages occur.
• Testing and comparing higher cooking loads with lower cooking loads (1.0–2.5 kg) were presented in this paper. The higher cooking loads, however, resulted in slower cooking and lower cooking temperatures, which might not be very ideal.
• Calculating heat losses.