Keywords

1 Introduction

The clean and long-lasting sources of the energies are the present need. Solar thermal energy is a promising source to meet-up the thermal energy demand of the world for low to high-temperature requirements. The solar thermal energy storage (STES) systems have special relevance in the solar thermal systems (STS). There are various types of STSs that directly collect the heat and serve the purpose of any heating applications in the range of 60–1000 °C. The variable, intermittent and unpredictable nature of solar radiation limit the operation of these systems due to the heat collection mismatch between the load and the supply. These storage systems provide heat backup for the system application in the absence of daylight or during insufficient solar radiation. Thermal energy storage (TES) systems are being widely used in solar thermal collectors for water heaters and as building heating systems [1, 2]. Among three kinds of storage systems (i) sensible heat storage (store energy while heating and release energy when cooling of a liquid or solid material), (ii) latent heat storage (store or release energy during phase change of material) and (iii) thermo-chemical storage (TCS) (uses chemical reactions to store and release thermal energy), the sensible method is the most useful. Due to the low density, the sensible heat storage requires a larger volume than that of PCM and TCS systems, respectively [2]. The properties of various storage materials are summarized in Table 1 [3,4,5,6,7,8,9,10,11,12,13,14].

Table 1 Storage materials and their properties
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Although various mediums have been tested and reported, water is the most used medium for storing sensible energy to use at low or medium temperature solar thermal systems. Storage at more than the boiling temperature of water needs pressure vessels which makes the system costly. For this, other liquids, viz. octane, isopentanol and butanol with lower density and specific heat than water are used, but, those are flammable, so requires additional safety of the system [13, 15]. The present work is emphasized for low-temperature, so water is used as storage medium. A significant number of research work has been published on the design of heat storage systems for solar thermal collectors. Parsad and Muthukumar [16] have numerically investigated a sensible heat storage unit for high-temperature application using a 3D mathematical model. The storage is configured with cast steel, iron and concrete. The predicted values are in good match with the reported values. Nkhonjera et al. [17] discussed in detail various storage units for cooking in different research work and presented a good review on STSs. Celador et al. [18] described a methodology to select the different design parameters for thermal storage systems to decrease the system cost.

It is understandable that a small-scale solar thermal system, a heat storage system with reasonable cost will increase the utility and adoption of this technology among potential users. The present work is aimed to design an effective small capacity storage tank at low cost. Two storage tanks are designed for this purpose, and a thermal performance study is conducted with the help of a lab scale experimental set-up.

2 Development of Heat Storage System

A storage system extracting heat from a heat source is depicted in Fig. 1. This heat source is a solar collector when the storage is used to provide thermal backup or diurnal storage of solar energy. In the present study, this heat source is an electric heater with a controlled energy input to a metallic heat exchanger immersed in a heat transfer fluid. This arrangement is analogues to the solar thermal energy system for testing of heat storage system under indoor laboratory conditions.

Fig. 1
An illustration of the heat storage system. The heat source on the right has the following components, power supply, thermistor, and electric heater. Electric heater contains heat transfer fluid, sensors, copper tube cylinder, insulated metallic container, ad temperature sensor. The heat storage on the left has a cylindrical vessel. It has water, a water pump, and sensors. Both sensors are connected to a temperature data logger.

Charging set-up (heat extraction) of heat storage systems from a heat source

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The energy balance equation for this set-up is written as:

$$q_{{{\text{us}}}} = q_{{{\text{af}}}} - q_{{{\text{ls}}}}$$
(1)
$$q_{{{\text{us}}}} = (m_{w} C_{w} + m_{s} C_{s} )\frac{{{\Delta }T_{s} }}{{{\text{dt}}}}$$
(2)
$$q_{{{\text{ls}}}} = U_{{{\text{ls}}}} A_{s} \left( {\overline{{T_{s} }} - \overline{T}_{a} } \right)$$
(3)
$$q_{{{\text{af}}}} = q_{{{\text{ah}}}} - q_{{{\text{lh}}}} = m_{f} C_{f} \frac{{{\Delta }T_{f} }}{{{\text{dt}}}}{ }$$
(4)
$$q_{{{\text{ah}}}} = q_{e} \eta_{e}$$
(5)
$$q_{{{\text{lh}}}} = (m_{h} C_{h} )\frac{{{\Delta }T_{f} }}{{{\text{d}}t}}.$$
(6)

where \(q_{{{\text{us}}}}\) is the rate of energy utilization by the storage system. \(q_{{{\text{af}}}}\) and \(q_{{{\text{ah}}}}\) are the rates of available heat to the heat transfer fluid and the heat source, resptively. \(q_{{{\text{ls}}}}\) and \(q_{{{\text{lh}}}}\) are the rates of energy loss from heat storage and heat source, respectively. \(q_{e}\) is the rate of electric energy supplied to electric heater and \(\eta_{e}\) is the conversion (electric energy to thermal energy) efficiency. \(m_{w}\), \(m_{s} , m_{f} {\text{and}} m_{h}\) are masses of water (sensible storage medium), storage tank, heat transfer fluid and heat exchanger, respectively. \(C_{w}\), \(C_{s} , C_{f} {\text{and}} C_{h}\) are the specific heats of water (sensible storage medium), storage tank material, heat transfer fluid and heat exchanger material, respectively. \(\Delta T_{s}\) and \(\Delta T_{f}\) are the rise of temperatures of storage system (i.e. water) and heat transfer fluid in \(dt\) time interval. \(U_{ls}\) and \(A_{s}\) are overall heat loss coefficient and total area of the storage system, respectively. \(\overline{{T_{s} }}\) and \(\overline{T}_{a}\) are average storage and ambient temperatures during the time period \(\Delta t\). In the above equation, temperature stratification is neglected due to low capacity of storage system.

Using Eqs. (1)–(6)

$$q_{{{\text{us}}}} = q_{e} \eta_{e} - m_{f} C_{f} \frac{{{\Delta }T_{f} }}{{{\text{d}}t}} - U_{{{\text{ls}}}} A_{s} \left( {\overline{{T_{s} }} - \overline{T}_{a} } \right)$$
(7)

The above equation is for charging cycle of the system. For discharging cycle energy balance equation is:

$$q_{{{\text{ls}}}} = m_{s} C_{s} \frac{{{\Delta }T_{s}^{^{\prime}} }}{{{\text{d}}t}}$$
(8)

where \({\Delta }T_{s}^{^{\prime}}\) is decrease in storage system temperature in the dt time period. Clearly, charging and discharging of a the heat storage system are govern by the various heat losses occurred in a storage system. For low charging and high discharging time, these losses should be minimized while keeping the cost of the system reasonably fair. These heat losses mainly depend on the (i) storage structure elements, (ii) operation temperature range, (iii) temperature stratification and (iv) thermal losses [19, 20].

Here, focus is on minimize the heat losses by keeping (i) low operation temperature range (50–90 °C), (ii) low capacity of storage system to avoid temperature stratification and (iii) use of locally available material for base and layered insulation material to reduce fabrication cost. Design parameters of the system are given in Table 2. All the systems are cylindrical in shape having a maximum water capacity of 8 L.

Table 2 Design parameters of the heat storage systems
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3 Experimental Method

The charging and discharging curves of storage systems are compared with a commercial storage system. The component details of these systems are given in Table 2. The laboratory set-up consists (Figs. 1 and 2) of mainly four sections (i) a heat source that is an electric heater with a controlled energy input, (ii) a metallic heat exchanger to receive direct heat from the heat source, (iii) a heat storage system that receives heat from the heat exchanger and (iv) the temperature data logger that records temperature through the sensors which are kept at various positions in the system. An electric heater (power consumption 1.5 kWh) is used to give thermal energy to HTF. The heat exchanger is an insulated metallic cylinder container (capacity 2 L) made of copper and it contains the heat transfer fluid. A copper tube cylinder is immersed in it for heat exchange from HTF. The metallic container is an insulated container with an inner diameter of 0.115 m, outer diameter of 0.18 m and height of 0.175 m.

Fig. 2
A photo of an experimental setup. A container is kept on a heater. A cylindrical container has a water pump at the center. The mouth of the container is covered with holes for the pump and the pipes. The containers are connected by thick pipes.

Lab scale testing set-up with commercial heat storage system

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In the present study, Hytherm (1 L) is used as HTF. The copper tube cylinder (CTC) is designed using a copper tube of 1 m length. The tube is rounded keeping the diameter 0.10 m to make spiral rings of height 0.08 m. This copper tube cylinder is kept in the HTF container. This heat exchanger having HTF and CTC is placed upon the electric heater to receive the heat directly from the electric heater. By this heat, the temperature of HTF increases continuously. To obtain the constant temperature of HTF, an auto cut of power supply is set using a thermistor (±5 °C). In the present study, this temperature is kept at 150 °C in all the experiments. The water is chosen as a heat storage material. The water in the heat storage is circulated through the copper tube cylinder. Both the ends of this cylinder is connected with silicon tubes, and these tubes are immersed in the heat storage system. A water pump is dipped in the storage tank to circulate the water in the copper tube cylinder and exchange the heat from HTF. The temperatures of HTF and HS are measured by using k-type thermocouples dipped in the respective containers. The sensors’ inputs are recorded through a data logger (85XX + Masibas data logger, LC 0.1 °C). For the charging experiments, the temperatures are recorded in 01 min interval while for the discharging set-up this time interval is 15 min.

4 Results and Discussion

The representative thermal profiles are shown in Figs. 3, 4, 5, 6 and 7. Figure 3 and 4 depict the charging of heat storage systems (HS, HS-1, HS,2) up to 90  °C while keeping all experimental conditions identical, i.e. HTF material and its temperature (initial and saturation), power source (heat input), initial room temperature and flow rate. The load capacity is 6 L and 8 L in the charging cycle. The discharging cycle under environmental conditions is also recorded for the different load capacities of the heat storage systems. These are depicted in Figs. 5 and 6. The temperature of heat transfer fluid (hytherm) is plotted in Fig. 7 for the different sets of experiments.

Fig. 3
A graph plots charging time against temperature. The increasing curves are labeled H S, H S 1, and H S 2. H S, (0, 30), (10, 35), (20, 50), (25, 55), (35, 65), (45, 75), (50, 90), and (55, 90). H S 1, (0, 30), (10, 34), (20, 48), (25, 50), (35, 60), (45, 70), and (50, 80). H S 2, (0, 30), (10, 33), (20, 40), (25, 45), (35, 58), and (50, 78). All values are approximated.

Charging time period vs temperature curve for heat storage systems (HS, HS-1 and HS-2) for 6 L water load capacity

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Fig. 4
A graph plots charging time against storage tank temperature. The increasing curves are labeled H S, H S 1, and H S 2. H S, (0, 30), (10, 35), (20, 45), (25, 55), (35, 65), (45, 75), (50, 80), (55, 85), and (60, 90). The other curves start from the same point (0, 30) and follow a similar trend. All values are approximated.

Charging time period vs temperature curve for heat storage systems (HS, HS-1 and HS-2) for 8 L water load capacity

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Fig. 5
A graph plots discharging time against storage tank temperature. The decreasing curves are labeled H S, H S 1, and H S 2. H S, (0, 90), (90, 85), (180, 75), (270, 72), (360, 70), (450, 65), (540, 62), (630, 60), (720, 58), (810, 57), and (900, 55). The other curves below H S follow a similar trend. All values are approximated.

Discharging time period vs temperature curve for heat storage systems (HS, HS-1 and HS-2) for 6 L water load capacity

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Fig. 6
A graph plots discharging time against storage tank temperature. The decreasing curves are labeled H S, H S 1, and H S 2. H S, (0, 90), (90, 85), (180, 80), (270, 75), (360, 70), (450, 68), (540, 65), (630, 62), (720, 61), (810, 60), and (900, 59). The other curves below H S follow a similar trend. All values are approximated.

Discharging time period vs temperature curve for heat storage systems (HS, HS-1 and HS-2) for 8 L water load capacity

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Fig. 7
A graph plots time against hytherm temperature. The curves labeled H S (8 L), H S 1 (8 L), H S 2 (6 L), and H S (6 L) follow a concave down increasing pattern. H S 8 L, (0, 30), (5, 65), (10, 90), (20, 100), (30, 120), (40, 124), (45, 125), and (50, 130). The other curves follow a similar trend. All values are approximated.

Temperature profile of Hytherm (HTF) in different set of experiments

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A comparative curve (Fig. 3) for charging of heat storage tank up to 90 °C indicates that the lab designed heat storage systems (HS-1 and HS-2) took around 6–10 min extra to reach 90 °C in comparison with the commercial HS for 6 L water, while for 8 L (Fig. 4), the time taken by systems HS and HS-1 is almost same. For all the systems, the heating of the 8 L water load took a larger time (5 to 8 min) than the 6 L load. The discharging plots shown in Fig. 5 infer that after 15 h, the temperatures of HS, HS-1 and HS-2 are 51.1 °C, 42.9 °C and 39.6  °C, respectively, for 6 L water load. The temperature difference of about 10–12 °C is found in this. In the other experiment for higher load capacity (8L), the temperature falls after 15 h are found 53.1, 47.6 and 44.5 °C for systems HS, HS-1 and HS-2, respectively. Overall, in the 15 h discharging time period, a little difference of about 6–12 °C is noted in the comparison with the commercial system. The rate of increase of the temperature for HS-1 is better than HS-2 due to good insulation material, and this rate is also equal to the commercial system (HS). The rate of decrease in temperature for both the developed system is slightly higher than HS. This is due to the marginally larger diameter of HS-1 and HS-2 about 0.03 m than the commercial one (HS). Using Eq. (8), Table 2, Figs. 5 and 6, the overall loss coefficients for the designed systems are computed and are given in Table 3. The curves between \(\Delta T_{s}\) and\(\left( {\overline{{T_{s} }} - \overline{T}_{a} } \right)\) are plotted to obtain the curve slopes for each system under 6 and 8 L water conditions. The time interval to compute \(\Delta T_{s}\) is taken 1 h. The specific heat of water is taken to be 4186 J/kg °C.

Table 3 Heat loss coefficients of heat storage systems
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Table 3 infers that the minimum heat loss coefficient is found for HS-1 system, while highest value is obtained for HS-2. This is a clear indication of better performance of the system HS-1 in comparison with the HS and HS-2. The costs of developed systems are about 60–65% less than the commercial ones due to use of different layers of cost-effective insulation materials. These initial results obtained at the lab scale infer that the lab designed systems have significant potential.

5 Conclusion

The heat storage systems are the important segment of any solar thermal collector system. Two low-cost solar thermal storages are fabricated to test low-temperature applications, and their experimental studies are performed and compared to the commercial one. The different charging and discharging tests are performed with the systems with water as heat storage material. The load capacities of all the systems are the same. The thermal performance of the fabricated systems are found to be competitive to commercial ones. The costs of the fabricated systems are reasonable.