1. Introduction
The world’s residential electricity consumption is growing rapidly, mainly for domestic water heating [
1]. It is estimated that domestic water heating accounts for about 15–20% of the total domestic electricity consumption of an average household in Hong Kong and the United States [
2]. With such a high demand for energy, there has been a growing interest in more efficient installations, especially since the doubling of oil prices in 2005 led to the increase in electricity rates [
3].
Global warming keeps worsening and climate anomalies are occurring increasingly frequently around the world. After the Kyoto Protocol, which aims to curb the global greenhouse gas emissions, came into force on 16 February 2005, the innovation of energy technologies and the change of social behavior for energy conservation will be an effective strategy for carbon dioxide reduction [
4]. Due to the growing influence of global warming and the rapid changes in the ecological environment, many unusual ecological crises have started to occur all over the world. Therefore, the search for “Renewable Energy” is a matter of great importance to all governmental and non-governmental organizations [
5,
6]. For the U.S., the U.K., and China, the goal is for renewable energy to account for at least 20% of total electricity generation by 2020, and it is expected that renewable energy will contribute more than 50% of total electricity generation in some countries by 2050 [
7].
Economic development depends on an adequate supply of energy, and there is an urgent need for a clean, environmental-friendly and inexhaustible source of renewable energy, and the best choice is “solar energy”. The sun emits 3.845 × 10
26 J of energy per second and the earth receives 1.743 × 10
17 J of energy therefrom per second, which is equivalent to about the heat quantity generated by 6.1 million tons of coal burning and 16,700 times the global demand [
8]. In terms of the conversion and application of solar radiation, it can be divided into solar water heating systems, solar heater systems, solar air conditioning and refrigeration systems, solar power generation and so on [
9] according to the solar energy density. For low temperature applications (<100 °C), it is mainly used for water-heating and warming. For medium temperature applications (100 to 200 °C), it is for industrial heating and air conditioning. For high-temperature applications (>200 °C), it focuses on solar power generation, smelting and toxic decomposition [
10].
The solar water heating system uses solar collectors (such as flat plate or vacuum tube type) to convert solar radiation into thermal energy to heat up water. Such products were introduced as early as 1891, and by 2001 solar collectors had been installed over 70 million square meters worldwide [
11].
It is known that water heaters include gas water heater, electric water heater, heat pump water heater and solar water heater. Of these four types of water heaters, the heat pump water heater and solar water heater are the most energy efficient and environmentally friendly. The difference in energy efficiency between these two types of water heaters is not significant and must be determined by the weather and region. Generally, if the annual average sunshine hours are more than 1/2 of the year, reaching over 210 sunshine days (daily sunshine hours of more than 4 h), using a solar water heater will save more energy and money. Conversely, if the number of sunshine days is less than 180 days, the heat pump hot water system will be more cost-efficient than the solar water heater [
12].
Taiwan is a very sunny country, even in the northern part of Taiwan where the latitude is higher, people can still use solar water heaters. However, one of the most unbelievable solar-related products commercially available on the market is the solar water heater. Why is there a need to install an electric heating rod for secondary heating when claimed to be the cleanest and most environmentally friendly renewable energy? The reason for this is that the design of the water storage tank of the conventional 1st generation water heater was not ideal. The water storage tank was hollow and could not separate the hot water from the cold water added thereafter. When bathing, with the hot water being consumed, the temperature of the hot water still in the tank drops significantly because the cold water is being replenished therein, so it is necessary to use an additional electric heating device to reheat the water, thus increasing the energy consumption.
Many homes around the world use electric water heaters (EWHs) to generate and store hot water, and the energy required can be supplied by solar energy using solar panels. For a single tank or auxiliary tanks (parallel or series connection), an electric heater is the most common heating element used in water heating storage systems [
13]. The only drawback of an electric heating system is that it consumes a large amount of electrical energy to produce the required hot water. Although these systems are driven by electrical energy, a disadvantage compared to direct solar water heaters, they are still widely used due to their practicality and low installation cost [
14].
Aviv et al. [
15] investigated the results of inputting and mixing cold and hot water in a vertical water tank. In such a system, the speed of mixing of the hot water produced and the incoming cold water will affect the thermal performance of the storage tank. The researchers recommended using a horizontal dividing disk above the vertical inlet at the bottom of the tank, and they found that the dividing disk would not be needed for a very low flow rate (2–3 L/min) of incoming cold water. However, for a higher water flow rate (5–7 L/min), it was found that a single stir was sufficient to mix the hot and cold water evenly.
Hegazy [
3] stated that thermal stratification is affected by the length-to-width ratio of the inlet of the incoming cold water and the storage tank. He proposed a new design for an inlet diffuser (wedged type) in which the incoming cold water does not interfere with the hot water. It directs the cold water flow to the bottom of the tank and establishes a cold water pan partition at the bottom of the tank. This new design reduces the mixing speed of hot and cold water.
Assari et al. [
16] investigated the impact of the water inlet and outlet location of the fluid on thermal performance in a cylindrical storage tank. It was found that location of hot water inlet to the tank has a high impact on performance enhancement. With the increase in vertical height of the heating location, better performance was obtained due to less mixing of hot and cold water.
Due to the unsatisfactory design of the storage tank of a conventional solar water heater, in which while the hot water is being consumed during bathing, the cold water is replenished at the same time, resulting in a significant drop in the temperature of the hot water in the storage tank, some manufacturers adopted the above practice of using a horizontal dividing disk (2nd generation solar water heating tank) near the water inlet. This new design effectively reduces the mixing speed of hot and cold water, but unfortunately, it can only reduce the “mixing speed of hot and cold water” and eventually the temperature of the hot water in the tank will still drop.
To address this shortcoming, the 3rd generation solar water heating tank is designed to improve the situation. It adds a “movable dividing disk” inside the tank to completely isolate the hot water and cold water, so that the entire storage tank of hot water is 100% utilized. However, there is an invisible drawback to this new design. For the 3rd generation solar water heating tank, when the hot water is exhausted and the user wants to continue using hot water, an electric heating rod must be used to reheat the entire tank of water, or a separate small heating tank needs to be added. To address this issue, this article proposes the 4th generation intelligent energy-saving solar water heating tank, in which a “fixed dividing disk” is installed inside the water heating tank to separate a small space for local heating so that the hot water can be supplied more efficiently and quickly.
3. Experiment and Discussion
From
Figure 8, it is known that the 4th generation solar water heating tank can be regarded as a combination of the 3rd generation solar water heating tank and 2nd generation solar water heating tank. It retains the “movable dividing disk” of the 3rd generation solar water heating tank while adding the “fixed dividing disk” of the 2nd generation solar water heating tank. Therefore, the impacts are only studied both “movable dividing disk” and “fixed dividing disk” have on the water heating efficiency of the storage tank to know the efficiency of the 4th generation solar water heating tank, and the optimal position of the fixed dividing disk.
An experiment was conduced to verify the effectiveness of the “movable dividing disk” of the 3rd generation solar water heating tank and to test the optimal position of the “fixed dividing disk” of the 2nd generation solar water heating tank. Especially for making a storage tank model to test the changes in water temperature in the 1st generation, 2nd generation and 3rd generation water heating tanks. The test environment and hypothesis for this experiment were as follows.
Water storage tank has a capacity of 400 L; the temperature of hot water is 60 °C.
The cold water injected is 20 °C.
Bathing for each person consumes 50 L of water.
There is a 15-minute interval between the shower of each person.
The water temperature will be measured from the water outlet.
The actual experiment used a smaller 2000 mL tank to simulate a 400 L storage tank, so the water consumption of 50 L per person for bathing was proportionally reduced to 250 mL. When conducting experiments with the “fixed dividing disks” of the 2nd generation solar water heating tank, the tank was then divided into 20 equal parts and we placed the fixed dividing disks in the positions of 1/20, 2/20, 3/20, …10/20, etc. to measure the temperature change after each person takes a bath. The
Figure 10 shows the dividing disk at the 4/20 position.
In addition to the commonly used 400 L water storage tank on the market, there were other sizes of 100 L, 200 L, 300 L, 400 L, 600 L, 800 L, etc. If you want to use the same 2000 mL small bucket for simulation, the bath water consumption during simulation must be adjusted accordingly, as shown in
Table 1. Moreover, the list of instruments adopted for this experiment was also provided, as can be seen in
Table 2.
Table 3 shows the experimental data obtained by using a 2000 mL tank to simulate a 400 L tank, which mainly measured the change of water temperature after each person takes a bath using hot water.
Unless it is a particularly enlarged water storage tank, the water storage tank can only bathe about 5–6 people, so this experiment only set up to 6 people to bathe. When the hot water in the water storage tank was exhausted for shower 7, 8, 9 and 10, the general practice was to use an electric heating rod for heating.
Table 3 is the actual experimental data obtained by using a 2000 mL tank to simulate a 400 L tank but not by calculation. For example, the temperature of the 1st generation tank in the first row was 58.5 °C after 1 person taking a bath (this temperature was the actual measurement). If by calculation, the temperature would be: (350 × 60 °C + 50 × 20 °C)/400 = 55 °C.
Why was the temperature obtained from the calculation 55 °C but the actual temperature measured by thermometer was 58.5 °C? That is because after one person took a bath, cold water would fill in from the right side of the tank, and the hot water outlet is at the far left side (it is where the temperature was measured during the experiment because the temperature there was the closest to the temperature of our bath), so the hot and cold water is not completely and fully mixed. In other words, the water temperature inside the tank was unevenly distributed, and the condition presented was that the temperature near the hot water outlet on the left was higher, while that near the cold water injection outlet on the right was lower. Of course, if the hot and cold water were mixed for several hours, the measured temperature would be closer to the calculated value (but then the actual temperature might be lower than 55 °C because of the limited heat retention capacity of the water storage tank).
In
Table 3, the temperature of the water storage tank of the no dividing disk (1st generation) after the 6th shower was 37.2 °C. If the heating rod started to be heated to 60 °C at this time, it will consume energy at this time: 37,000 × (400/1000) × ((60 − 37.2)/(60 − 23)) = 9120 Kcal (it is mentioned in [
9] that 1000 L of cold water needs 37,000 Kcal to be heated from 23 degrees to 60 degrees). The temperature of the water storage tank of the movable dividing disk (3rd generation) after the 6th shower was 57.2 °C. If the heating rod is turned on at this time to be heated to 60 °C, it will only consume energy at this time: 37,000 × (400/1000) × ((60 − 57.2)/(60 − 23)) = 1120 Kcal. The energy consumption of the 3rd generation water storage tank was only 12.28% of that of the first-generation water storage tank (9120/1120 = 0.1228).
The above experiment is the average results of the two experiments. From the data, it is found that the 3rd generation water heating tank developed by us had the best temperature performance. After 6 persons taking baths, the temperature of hot water was still at 57.2 °C, with its theoretical value being 60.0 °C, the temperature still dropped from 60.0 to 57.2 °C, mainly because of the loss of temperature inside the water storage tank. This is the error caused by poor insulation of the water storage tank.
In addition, from the experimental data of
Table 3, it is known that the 2nd generation water heating tank had the best temperature performance when the “fixed dividing disk” was placed at the 2/20 position. The temperature performance was 41.0 °C when the dividing disk was at the 1/20 position, already much better than 37.2 °C without the disk. The temperature performance was best when the dividing disk was at the 2/20 position, and then slowly dropped to 37.0 °C at the 5/20 position, which was similar to the 37.2 °C without the disk. It means that if the fixed dividing disk was installed at the 5/20 position; it is about the same as having no dividing disk at all. When its position was moved back, the temperature would slowly rise again. Since the two sides of the storage tank were symmetrical, the temperature at the 18/20 position was expected to be similar to that at the 2/20 position.
Since the 2nd generation water heating tank had the best temperature performance when the “fixed dividing disk” was placed at the 2/20 position, the original temperature of the hot water was 60 °C, and the temperature of hot water was 41.5 °C after 6 persons took baths, which means the temperature performance was best when the buffer area accounted for about 10% of the entire water heating tank. As the 4th generation solar water heating tank can be seen as a combination of the 2nd generation and 3rd generation solar water heating tank, the “heating area” of the 4th generation water heating tank was also proposed to take up about 10% of the whole water heating tank, as shown in
Figure 11.
When the first tank of hot water is used up and a secondary heating with an electric heating rod is required, as the “heating area” takes up only 10% of the entire water heating tank, there is no need to reheat the entire tank of water, and about 90% of the energy can be saved in that instance.
Since the 4th generation solar water heating tank is an evolutionary version of the 3rd generation solar water heating tank, the advantages and disadvantages are summarized as
Table 4:
Finally, the 4th generation water heating tank can be regarded as a combination of the 2nd generation and 3rd generation water heating tank. It has the advantages of 100% hot water use in the 3rd generation water heating tank. When the water is depleted, the 2nd generation water heating tank can also play the role of blocking, only for local heating of small areas, so this hot water tank can be regarded as the best solution at present. Moreover, the cost of a general hot water tank is about US$125. The 4th generation water heating tank adds the movable dividing disk, fixed dividing disk and pumping motor, so the total cost will increase about US$31.25, which is about 25% of the total cost.
4. Conclusions
In the era of environmental awareness, the application of renewable energy and the development of green energy technology are gradually being emphasized. Many products around the world have been developed with the aspirations of energy saving, environmental protection and cherishing the Earth’s resources. This study successfully developed the 3rd generation solar water heating tank, and designing a “mobile dividing disk” to avoid the issue of mixing of hot and cold water that occurred in the 1st and 2nd generation solar water heating tanks. As a result, the hot and cold water could be completed separated, moreover the whole tank of hot water could be 100% utilized.
However, there is still an invisible drawback to that new design. When the hot water runs out and a secondary heating is required, the whole tank must be heated with an electric heating rod, or an additional small storage tank must be added for reheating. To address this problem, the 4th generation solar water heating tank was developed in this article. An additional “fixed dividing disk” is designed to be added inside the tank, and the separated heating area uses an electric heating rod for secondary heating, which saves 90% of energy and electricity cost in that instance compared to heating the entire tank of water, while eliminating the cost of making a small additional heating tank, thus saving energy and reducing carbon emissions. This improvement was the most important feature of this work, making this work more suitable for living needs and more convenient.
The 4th generation water heating tank proposed in this article can not only be used in solar water heaters, in fact, it can be used as long as it needs to store hot water in a water tank, such as gas water heaters, power heaters, heat pump water heaters, etc., are all applicable. Therefore, if this 4th generation water heating tank can be commercialized, thus it will greatly contribute to energy saving and carbon reduction. Furthermore, the cost of a general hot water tank is about US$125. The 4th generation water heating tank adds a movable dividing disk, fixed dividing disk and pumping motor, so the total cost will increase to about US$31.25, which is about 25% of the total cost.