Abstract-- In this study, basalt stone availability as storage material in the sensible heat storage method and the effect of heat transfer fluid flow rate were investigated. The experiments were carried out at three different flow rates, and measurements were taken at intervals of ten minutes for three days for each flow rate. Analytical results also show that the heat transfer to the water at 15 L min-1 is 660.5 W on average. On the other hand, it was determined that the average amount of heat transferred by the fluid at 30 and 45 L min-1 is 2339.9 and 2626.3 W, respectively. These results revealed that the increase in the intake temperature caused an increase in the fluid amount of heat transferred. A multiple linear regression model was created to determine the mathematical relationship between the inlet temperature and flow independent variables. Fluid heat transfer dependent variable and heat transfer values calculated by the generated model are found linear. It is seen that the amount of heat stored by the heat storage material decreases with the increase of the water flow rate. As a result of the calculations made, the water flow's heat storage efficiency adjusted at 15, 30 and 45 L min- 1 was found 43.1%, 9.5% and 6.5%, respectively. According to these results, it is concluded that the water flow rate as low as possible in such heat storage systems will increase the efficiency of the heat storage. Besides, the volumetric heat storage capacity of basalt stone was determined as 3417.3 kJ m-3 oC-1.
Keywords-- sensible heat storage, basalt stone.
I. INTRODUCTION
Energy systems play a key role in harvesting energy from various sources and converting it to the energy forms required for applications in various sectors, e.g., utility, industry, building and transportation. Energy sources like fossil fuels can be used to provide energy according to customer demand, i.e. they are readily storable when not required. But other sources such as solar and wind energy need to be harvested when available and stored until needed (Koohi-Fayegh and Roses, 2020)
Although solar energy, which is the most widely used type of renewable energy source in practice, is one of the most promising alternative energy options for fossil fuels, the most important problem in using it as an energy source is the energy source's discontinuity. The energy output acquired on sunny days is typically upwards compared to the energy needed to counterbalance the instantaneous demand, whereas, on cloudy days, the solar energy is rather inadequate (Boonsu et al., 2016) Said inefficiency of solar energy leads to the fact that it cannot be utilized at high efficiency. For these reasons, it is very important to research and develop systems that can store energy efficiently and economically to obtain energy from solar energy systems on cloudy days and night hours when the energy input is insufficient (Dinçer and Rosen, 2011). Lately, quite a few methods are utilized for storing energy. However, because of its ease of application, the storage of heat energy obtained by conversion or cycle is the most used energy storage method (Kozak and Kozak, 2012).
The storage of heat energy is referred to as the temporary storage of heat energy at high or low temperatures. The basis of heat energy storage systems; consists of providing energy to the storage system, storing this obtained energy and using stored energy at the requested time. This situation is briefly; loading, storage, and recovery as processes can be summarized. Storage of heat energy; closing the location and time difference between using and obtaining energy provides more effective and efficient use of energy in heating and cooling systems and thus achieves significant energy savings. Heat storage techniques are in two forms; sensible heat and latent heat (Yılmazoğlu, 2010). Using Phase Change Material (PCM) as storage media for latent thermal energy is an effective way for using thermal energy. PCMs give a high energy storage capacity by absorbing or releasing latent heat during phase change process during isotherm conditions. So PCMs used in many studies with different types, shapes and place of them through the system. Many types are used such as paraffin wax, stearic acid, magnesium chloride hexahydrate, vegetable-based oil and lauric acid (Malkawi and Tamimi, 2021).
In sensible heat storage systems, energy is stored by utilizing the sensible heat generated due to changing the temperature of a storage medium like water, air, oil, rock beds, bricks, sand and soil (Dinçer and Rosen, 2011). Many of the storage materials used in sensible heat storage systems are abundant and generally inexpensive. Besides, the technology developed in order to be used to store heat energy from these storage materials is suitable for use in efficient systems. For these reasons, sensible heat storage is preferred as the most used heat storage application at present. In the storage of heat energy using this method, the storage material's temperature changes during the storage and recovery of heat. An alluring property among the rest is the effectuation of a vast number of heat storage and recovery cycles (Öztürk, 2015).
There are several studies in the literature regarding the application of sensible heat storage systems. In one of the studies for this purpose, although the outdoor temperature decreased in winter, the indoor temperature was kept at 13°C using the packed bed heat storage system (Arizov and Niyazov, 1980). In another study carried out in two different greenhouses, it was determined that a temperature difference of 10°C occurred at night between the greenhouse where the storage system is located and the other greenhouse (Kürklü et al., 2003). In another study, when the sensible heat storage method was used in the greenhouse, it was determined that greenhouse agriculture would reduce the heating costs, which had a large share in the total production expenses. As a result of the calculation, 60-70% savings would be accomplished with the utilization of the so-called method from the amount of coal needed for greenhouse heating applications was found (Öztürk, 2012). In the studies carried out in the proper design of heat energy storage systems, it was determined that the sensible heat storage method is more advantageous than the latent heat storage method compared to design, construction, cost and ease of operation (Khare et al., 2013) In another study, experiments were carried out to select the best storage material among five types of rock species (quartz, basalt, granite, hornfels and marble) abundant in Morocco. As a result of the experiments, it was explained that the use of these rocks as a storage material in the sensible heat storage system increases the efficiency and reduces the cost of the system (Grirate et al., 2014)
In this study, basalt stone's usability as a heat storage material in the sensible heat storage method was investigated. The effect of the flow rate of the heat transfer fluid on the efficiency of heat storage was investigated in these systems. For this purpose, the flow rate was adjusted to 15, 30 and 45 L min-1.
II. METHODS
A. Storage Material
Approximately 2.5 tons of basalt stones were used as heat storage material in our study (Fig. 1). The reason for using basalt stone in our study is that it is one of the most common rock types on earth. On the grounds of its immense resistance to abrasion and climatic circumstances, it is commonly the favored natural stone as a base and superstructure material. Basalt used as the heat storage material was provided from the Toprakkale district of the Osmaniye province in Turkey. Toprakkale basalt is spread over an area of approximately 115 km in quaternary plateau basalts between Toprakkale and Erzin districts and the Gulf of İskenderun. Toprakkale basalts are seen as massive, black-gray colored, large-small and angular blocks. The basalt stone's physical and thermal properties are given in Table 1.
B. Heat Storage System
The already stated study was designed for day-to-day storing of
solar energy with sensible heat storage with the utilization of basalt stone
and operated in the Tayfur So-
Figure 1: Basalt stone used as heat storage material in the study.
Figure 2: Schematic representation of solar thermal storage system (P:Pump; V:Throttle Valve; F:Flowmeter; ST: Tempera-ture sensor).
Table 1. Physical and thermal properties of basalt (Eppelbaum et al., 2014)
men Campus of Hatay Mustafa Kemal University in Turkey. The schematic view of the experimental setup was given in Fig. 2.
The heat transfer fluid from the solar collectors was transferred to the heat storage system, as in Fig 2. The heat carried by heat transfer fluid circulated in the storage tank was absorbed and stored by basalt stones in the heat storage system. Subsequently to the absorption of heat carried with the means of a fluid, the cooled fluid was transferred back to solar collectors. Thus, the heat transfer fluid's continuous circulation between the heat storage unit and solar collectors was ensured.
Two standard flat plate liquid solar collectors with the same characteristics were used in the study. As a heat storage unit, the storage tank (Fig. 3) with a volume of 1 m3 in square prism shape made of corrosion-resistant galvanized, sheet material was used. Besides, with the purpose of implementing thermal insulation, glass wool was situated on the exterior surface of the storage tank.
Corrugated polyethylene (PE) pipes with a 1-inch
diameter were used to circulate the heat transfer fluid in the
Figure 3: Isometric (a), top (b) and right (c) views of the heat storage unit
heat storage system. In addition, a data logger to record the measured data, a flow meter with a measurement range of 6-60 L min-1 for flow measurement and a 3-stage circulation pump to circulate heat carrier liquid in a heat storage system was used.
The temperature was measured from points where heat transfer fluid entered the heat storage tank and exited from the heat storage tank. In addition, the temperature was measured from inside the tank in order to measure the temperature of the basalt stone. Temperature measurements carried out from 3 points were recorded via the data logger at 10-minute intervals. When evaluating the obtained data, the cases where the difference between inlet and outlet temperature of the heat transfer fluid to the storage system is negative were not considered because the discharge system was not installed in the experiment.
The changeable throttle valve ensured the flor rate needed for the circulation of the heat transfer fluid at various flow rates. Flow measurements with the flow meter were carried out by throttling the valve until the system reached the desired flow rate. The experiments were performed in three different flow rates as 15, 30 and 45 L min-1.
In the heat storage system, Eq. 1 was used to calculate the amount of heat transferred by the water, which is the heat transfer fluid and the specific heat of the water was taken as 4.18 kJ kg-1 oC-1 (Çengel, 2006)
. (1)
The amount of heat stored by basalt stones was calculated by Eq. 2 using the lowest and highest temperature differences obtained due to the temperature measurements carried out within the heat storage tank. In this calculation, the mass of basalt stone and specific heats were 2500 kg and 1.23 kJ kg-1 oC-1, respectively.
Using the ratio of the amount of heat stored by basalt stones to the amount of heat carried by the heat transfer fluid to the heat storage system, the system's efficiency was calculated as in Eq. 3.
(3)
The volumetric heat capacity of basalt stone was calculated using Eq. 4, which is the ratio of highest and lowest temperature difference measured in the heat storage tank to amount of heat stored by basalt stones, and the volume of the tank was taken as 0.9 m3.
Statistical analyzes were performed by using IBM SPSS 20 statistics program to determine the relationships between measured data and calculated stored heat quantities. The effect of water flow rate on the amount of heat stored was determined by analysis of variance. In addition, the Duncan test was used to determine whether the flow change had a statistically significant effect on the amount of heat stored. For the identification of the relationship between the inlet temperature and the amount of heat stored, an analysis was conducted among the stated two data. Finally, regression analysis was formed between water flow rate, inlet temperature and stored temperature.
III. RESULTS
A. Measurements at Different Water Flows
When the flow rate was set at 15 L min-1, the graphs of the measurements obtained at 10 minutes intervals on 29-30 April and 2 May 2017 were given in Fig. 4. As a result of the measurements carried out at the water flow rate set at 15 L min-1, the heat transferred was increased depending on the inlet temperature of water to the storage tank. Besides, the temperature difference in the storage all over the daytime was determined to be in a spectrum of almost 6 °C. The highest temperatures (38 oC) in the storage were measured after midnight. Simultaneously, it was determined that the water temperature entering the tank was lower than the leaving water temperature and the heat storage system lost heat.
Figure 4: Measurements obtained at 15 L min-1 water flow rate.
Figure 5: Measurements obtained at 30 Lmin-1 water flow rate.
When the flow rate was set at 30 L min-1, the graphs of the measurements obtained at 10 minutes intervals on 25-26 and 27 May 2017 were given in Fig. 5. As a result of the measurements carried out at the water flow rate set at 30 L min-1, the heat transferred was increased depending on the inlet temperature of water to the storage tank. Additively, the temperature difference in the storage all over the daytime was determined to be in a spectrum of almost 6 °C. The highest temperatures (40 °C) in the storage were measured after midnight.
As a result of the measurements carried out at the water flow rate set at 45 L min-1, the heat transferred was increased depending on the inlet temperature of water to the storage tank and given in Fig. 6. In addition, it was determined that the temperature change in the storage throughout the day was in a range of approximately 5 °C. The highest temperatures (42 °C) in the storage were measured after midnight. At the flow rate set at 45 L min-1, it was observed that the difference between the inlet and outlet temperatures of the water was higher than the differences in the other flow rates. At the noontime, notably, a temperature change of about 2 °C was observed, and by virtue of the aforementioned temperature contrast, the extent of heat transferred at a flow rate of 45 L min-1 was remarkably above with respect to the values acquired at a flow rate of 15 L min-1. On the other hand, it was determined that the amount of heat transferred at a flow rate of 30 L min-1 was approximately the same.
Figure 6: Measurements obtained at 45 Lmin-1 water flow rate.
Figure 7: Amount of heat stored and heat storage efficiency.
B. Heat Storage Quantity and Efficiency
The heat storage amount of the basalt stone used as heat storage material in the system and the system's storage efficiency were calculated and given in Fig. 7 and Table 2.
As a result of the experiments and calculations, it was determined that the flow rate affects the amount of heat storage. It has correspondingly realized that the development of the heat transfer fluid flow rate diminished the heat storage performance of the storage material.
C. Volumetric Heat Capacity
The values obtained using the calculated heat storage amount and the measured temperature differences are given in Table 3.
