Research and application of energy storage dual-core

stainless steel pressurized water tank

Introduction

With the acceleration of the integration of solar energy applications and buildings across the country, various renewable energy technologies that are compulsorily suitable for the resource conditions and building utilization conditions of the region are promoted. The enthusiasm of local governments, developers, and solar energy companies has been greatly promoted, and a number of large-scale projects have been implemented one after other particularly balcony wall-mounted solar water heating systems used in high-rise residential buildings in real estate, which have been widely used as the main source of hot water. A set of efficient, safe and reliable hot water systems, however, not only requires high-efficiency solar collector technology to support, but also needs heat storage equipment with efficient heat transfer as a guarantee of quality. Because the balcony wall-mounted solar water heating system is limited by certain conditions in the middle and high-rise building applications, in order to improve the energy efficiency rating index of the balcony wall-mounted solar water heating system entirely, the heat storage equipment of the balcony wall-mounted solar water heating system should have a high-efficiency heat transfer function to convert solar energy into heat energy for storage in addition to the function of heat storage.

Through the research and analysis of the way to influence the heat transfer effect, the new energy storage dual-core stainless steel pressurized water tank made by changing the structural design of the water tank heat exchange layer has a heat exchange efficiency of 26% higher than that of the ordinary hot water storage tank currently used in the market, and the energy efficiency index of the solar water heating system of the balcony wall is improved 10% as a whole, creating a huge development space for the application of balcony wall-mounted solar water heating system in middle and high-rise buildings.

1. Heat transfer efficiency is the core idea of the design of accumulating dual-core stainless steel pressurized water tank

At present, the heat exchange process of the solar water heating system of the balcony wall-mounted solar water heating system is basically a non-phase-varying convective heat transfer process, that is, when the high-temperature fluid that absorbs solar heat flows through the surface of the heat exchange solid, due to the two different temperatures, the transfer of heat occurs. During this process the fluid does not undergo a phase change (condensation and boiling), but a single-phase fluid. According to the basic calculation formula of Newtonian cooling φ=h×A×Δt, it can be known that increasing the heat exchange area A, increasing the heat transfer temperature difference Δt between the fluid and the wall surface, and increasing the surface heat transfer coefficient h all have the effects of enhancing heat transfer. In the process of enhancing heat transfer, increasing the heat transfer area A and increasing the heat transfer temperature difference Δt between the fluid and the wall surface is relatively easy to determine, while the surface heat transfer coefficient h value reflecting the heat transfer strength is affected by various factors, which is difficult to determine.

The core idea of the energy storage dual-core stainless steel pressurized water tank is how to greatly improve the heat exchange efficiency. In the design process of energy storage dual-core stainless steel pressurized water tank, the idea of strengthening the heat transfer of the water tank is always adhered to. By increasing the heat exchange area A as much as possible, increasing the heat transfer temperature difference Δt between the fluid and the heat exchange surface, and increasing the surface heat transfer coefficient h to enhance heat transfer to achieve a significant increase in water tank heat transfer efficiency.

The energy storage dual-core stainless steel pressurized water tank is mainly composed of a heat storage inner tank (1), a heat collecting liner that is welded around the outer wall of the heat storage liner into a narrow heat exchange space (2), heat exchange circulation pipe inlet and outlet interface (5) ( 6), cold and hot water layered into the water pipe interface (8), water tank insulation layer, water tank liner, electric auxiliary heating and control accessories. The laminated jacket double core liner is used on the water tank structure. The laminated jacket double core liner is adopted on the water tank structure, and the heat exchange surface of the accumulator dual-core stainless steel pressurized water tank heat storage inner tank is rolled from a smooth surface into a spiral rising groove heat exchange surface (4), which not only expands to the 35% of the whole heat exchange area of the water tank, at the same time, due to the change of the shape, size and position of the heat exchange surface, the spiral rising groove type heat exchange surface in the heat exchange process will continuously change the direction during the forward movement, causing secondary circulation in the cross section, which can generate turbulence at a lower Reynolds number, and then enhances heat transfer.

At the same time, the accumulating dual-core stainless steel pressurized water tank collector liner is designed as a narrow heat exchange space. On the one hand, under a certain ratio of heat transfer path and small flow constant pressure condition, the fluid can maintain a good flow velocity and achieve high efficiency of heat transfer; on the other hand, the fluid layer can be thinned during the heat exchange process, and the heat transfer coefficient is further increased.

Because the liquid flow direction of the water tank in the water tank is heated from bottom to top, the water tank heat exchange inlet and outlet pipes are designed to be 180 degree countercurrent circulation heat exchange mode, and the hot fluid medium flows in from the upper part of the heat exchange wall, after 180 degree heat exchange, it flows out from the lower part of the heat exchange wall. Under the same fluid inlet and outlet temperature, the average temperature difference between the hot and cold fluid is the largest, which enhances the heat transfer.

Through the above different ways and methods for the design change of the water tank heat transfer structure, the overall heat exchange effect of the water tank can be greatly improved by 26%, highlighting the energy efficiency of the energy storage dual-core stainless steel pressurized water tank.

2. Technical innovation measures for increase the energy quality of dual-core stainless steel pressurized water tank

The energy storage dual-core stainless steel pressurized water tank not only has the performance advantages of high-efficiency heat transfer, but also has the technical guarantee of high quality quality. There are many factors affecting the quality of the heat transfer water tank including whether there is water leakage after the using of water tank for a period of time, whether the water tank strength meets the operation requirements of high temperature and high pressure, whether the water tank is insulated and energy-saving, and whether the water tank is convenient to use and so on. To analyze the factors affecting the quality of the heat transfer water tank, the first thing that needs to be solved is the choice of the material making the heat transfer water tank. At present, the heat exchange water tanks in the market are mainly divided into two types: enamel inner tank and stainless steel inner tank. The enamel inner liner is mainly composed of two kinds of materials with different expansion coefficients, such as porcelain layer and carbon steel plate. The porcelain layer is mainly used for anti-corrosion. Under the condition of  75 ° C water temperature and relatively stable pressure, the different expansion forces of porcelain layer and steel plate are generally smaller than the adhesion between porcelain layer and steel plate, and there is no phenomenon of porcelain removal and cracking. Therefore, the inner tank of the electric water heater using the temperature under 75 ° C basically adopts the enamel liner.

However, in the solar energy utilization system, the absorption of solar heat can cause the temperature of the heat exchange water tank reach above 100 °C and the temperature and pressure exhibit irregular pulse fluctuations, which may cause that the different expansion force of the porcelain layer and the steel plate would be greater than the adhesion between the porcelain layer and the steel plate. The cracks will appear on the enamel surface during the alternating heat and cold process, and then the porcelain powder will fall off, the anti-corrosion function of the porcelain layer will be lost, finally the carbon steel of the water tank substrate will corrode and leak. Moreover, the energy storage dual-nuclear heat transfer pressure tank used in the balcony wall-mounted hot water system should uniformly sprayed the porcelain powder to each part in a narrow space of 5 mm. Hence, it would be hard to image the difficulty of this process, and the quality of the enamel can not be guaranteed. Therefore, in the solar water heating system with high temperature and high pressure unsteady working conditions, the material for heat exchange pressure water tank is recommended to be made of food grade 304 stainless steel material which has certain anti-corrosion function and is easy to form and weld. In areas with high chloride ion content, 316L stainless steel is available. According to the design structure of the water tank, the thickness of the water tank plate is determined by the strength calculation of the selected materials, and the plate, end cover and pipe fittings of the water tank can be automatically welded in the structural process, and the human factors are eliminated to ensure the overall strength requirement of the water tank. The energy storage dual-core stainless steel pressurized water tank made of 304 stainless steel has no leakage and no deformation after 250,000 pressure cycles between 15% and 100% of the maximum working pressure.

Secondly, the energy storage dual-core stainless steel pressurized water tank not only uses materials that can adapt to the application of stainless steel in the solar water heating system, but also uses multiple protection measures in the tank anti-corrosion technology. In the design of the water tank welding structure, the inner tank of the water tank can be welded by argon gas to prevent the material change of the stainless steel material during the welding process. In order to prevent that welding may affect the anti-corrosion function of the material, after the welding of the inner liner assembly, the accumulator dual-core stainless steel pressurized water tank must be subjected to the passivation treatment of the water tank surface through the configured metal chemical liquid under the strict technical standards, and the surface of the water tank generates a very thin, dense, well-covered, chromium-rich passivation film that can be firmly attached to the metal surface. The potential of the chromium-rich passivation film can be as high as +1.0V, which is close to the noble metal potential, and form a multi-layer CrO3 or Cr2O3 structure, which can completely separate the metal from the corrosive medium, preventing the metal from contacting the corrosive medium directly. Therefore, the metal basically stops dissolving, and form a passive state to prevent corrosion. The salt spray test of the stainless steel water tank after metal passivation treatment was 20 times higher than that of the untreated stainless steel water tank. When the blue dot method was used, no blue dot appeared for more than 35 seconds. At the same time, the energy storage dual-core stainless steel pressurized water tank also adopts the sacrificial anode cathodic protection measure. A rod-shaped magnesium rod is arranged as the anode in the water tank, and the metal tank inner tank is used as the cathode, so that there forms a primary battery between the cathode and the anode passing through the conductive water medium. The anode magnesium rod of the battery is corroded and consumed by itself, while the cathode liner is protected, providing the functions of insulation and corrosion protection.

Thirdly, energy saving, comfort, convenience and convenience are the humanized design concepts that the energy storage dual-core stainless steel pressurized water tank always adheres to. Energy storage is the most basic function of the water tank. How to prevent the stored heat absorbed by the water tank not to lost is an important part to save energy. After the linearly calculation of water tank energy-saving insulation effect, if the 80-120 liter water tank want to achieve a good energy-saving effect, the thickness of the insulation layer should be no less than 50 mm. The energy storage dual-core stainless steel pressure tank insulation layer is designed to be 52mm, making of environmentally-friendly polyurethane foaming agent by constant temperature and constant pressure, the density reaches 36kg/m3 and the inherent loss of the water tank is 1.2KWh and the level of water tank energy efficiency reaches level A. The water consumption method of the pressurized hot water tank generally adopts the cold water while the hot water is being used. And if the treatment of the water tank structure is not good and the phenomenon of hot and cold water mixing is easy to occur, causing that the user's water temperature being unstable and the water outlet rate of the water tank is extremely low. The energy storage dual-core stainless steel pressurized water tank is equipped with a cold water buffer water separator in the cold water inlet, which would change the direction of the cold water flow route and block the mixing of cold water and hot water, then form a layer of hot and cold water in the water tank and maximize the utilization rate of the hot water tank. In order to facilitate the installation of the water tank during construction, the energy storage dual-core stainless steel pressurized water tank is equipped with multi-functional and quick-installing components, which are designed in proportion to the modulus of the building materials, and can be quickly and perfectly installed without any special tools. At the same time, the intelligent controller of the water tank can be integrated with the water tank, or it can be installed separately from the water tank according to the user's installation position. LCD display, intuitive display, one-button operation, precise control, simple and convenient operation, effective integration of various protection measures, safe and worry-free.

3. Application and test of energy storage dual-core stainless steel pressurized water tank

The energy storage dual-core stainless steel pressurized water tank is a new type of heat exchange water tank developed for the use of balcony wall-mounted solar water heating system for medium and high-rise residential buildings. It adopts laminated jacket double core liner structure, connecting dual core sandwich and solar collector directly and separating domestic water from the endothermic circulation medium completely. The heat exchange between the solar collector and the hot water storage tank does not require any additional power. According to the different temperature density of the circulating medium, the thermosiphon acts naturally, that is, the collector absorbs the solar high-temperature medium and flows into the natural circulation of the low-temperature circulating medium in the jacket heat exchange water tank. It exchanges heat through the conduction of the inner wall of the jacketed living tank. The schematic diagram of the application of the energy storage dual-core stainless steel pressurized water tank is as follows:

According to the above installation method, we will compare the heat storage dual-core stainless steel pressure water tank and the well-known brand enamel double-chamber pressurized water tank on the market with the same type and model flat collector for the hot water system energy efficiency level comparison test.

The daily useful heat q test basic conditions:

◆Daily solar radiation H≥16MJ/m2, water temperature in the hot water tank: tb=(20.0±1.0)°C, ambient temperature 8°C≤tad≤35°C, ambient air flow rate ν≤4m/s;

◆Water tank capacity 100L; configuration of a flat plate collector 1.84m2；

After a total of 8 hours from 4 hours before noon to 4 hours after noon, the cumulative solar radiation is 17.89 MJ (d.m2); the temperature rise curve of the water tank is shown in Figure (1), based on the daily useful heat calculation formula we can obtain:

       q=ρw×VS×Cpw×(te-tb)/1000×AC----------⑴

q——daily useful heat MJ/m2；

r——water density,1kg/L；

s——water tank capacity L；

pW——constant pressure specific heat capacity of water，4.186KJ/（kg.c）；

te——stop temperature of water in the water tank（℃）；

tb——initial temperature of water(℃)；

Ac——daylighting area m2；

The test data is substituted into the formula (1) and the figure (1) is calculated.：

图（1）

       The heat storage system of the heat storage dual-core stainless steel pressurized water tank hot water system has a value of 7.9 MJ/m2;

       The enamel double-chamber pressurized water tank hot water system has a daily heat value of 7.3MJ/m2;

The daily useful heat when converted into a solar radiation amount of 17 MJ (d.m2) is:

q17=17q/H----------⑵

Since the cumulative solar radiation is 17.79 MJ (d.m2) during the 8-hour test, substitute  17.79 into equation (2) is available:

The daily useful heat q17 value of the heat storage dual-core stainless steel pressurized water tank hot water system is 7.51MJ/m2;

The daily useful heat q17 value of the enamel double-chamber pressurized water tank hot water system is 6.97MJ/m2;

The above tests are in line with the thermal performance of the household split dual-circuit solar water heating system of 6.6MJ/m2. At the same time, it can be seen that the hot water system of the heat storage dual-core stainless steel pressurized water tank is obviously increased by about 10% compared with the daily heat of the enamel double-chamber pressurized water tank hot water system.

Average heat loss factor USL test basic conditions:

◆Water temperature in the hot water storage tank: take tb=(55±1.0)°C, ambient temperature 8°C≤tad≤35°C, ambient air flow rate ν≤4m/s；

◆Water tank capacity 100L; configuration of a flat plate collector 1.84m2；

After 10 hours of testing from 8:00 pm to 6:00 the next morning, the temperature drop curve of the water tank, as shown in Figure (2). Based on the USL calculation formula of the average heat loss factor we can obtain:

--------------------（3）

USL: average heat loss factor（W/m3.K)；

ρw: water density（kg/m3)；

Cpw: specific heat capacity of water 4.186KJ/（kg.c）；

Δτ: time interval (S)；

ti: initial temperature in the water tank（℃）；

tf: final temperature in the water tank（℃）；

tas(av): ambient air temperature（℃）；

Substituting test data into equation (3) to calculate the graph (2)：

图（2）

      The average heat loss factor USL of the hot water system of the heat storage dual-core stainless steel pressurized water tank is 11.8 (W/m3.K);

The average heat loss factor value of the hot water system of the enamel double-chamber pressurized water tank is USL 12.7.（W/m3.K)；

The energy efficiency coefficient of the domestic solar water heating system can be obtained by the following formula:

CTP=QS(e)/QS(m)-αUsl(e)/Usl(M)--------------------（4）

      Where: QS(e): measured daily useful heat MJ/m2 under test conditions;

      QS(m): The minimum value of the useful heat for the household split double circuit is 6.6MJ/m2;

       α: the weight coefficient is 0.9;

       Usl(e): average heat loss factor (W/m3.K) measured under test conditions;

       Usl (M): the average split heat loss factor of the household split double loop is 16 (W/m3.K);

The test value QS(e)=7.51MJ/m2 and Usl(e)=11.8(W/m3.K) of the heat storage dual-core stainless steel pressurized water tank will be assigned to the formula (4)：CTP=0.47

The test value QS(e)=6.97MJ/m2 and Usl(e)=12.7(W/m3.K) of the enamel double-chamber pressurized tank hot water system will be substituted into the formula (4)：CTP=0.34

Through the calculation of the above test values, it can be seen that the water storage double-core stainless steel pressurized water tank hot water system can reach the energy efficiency grade I standard, while the energy efficiency rating of the enamel double tank pressurized water tank hot water system is II standard. Therefore, it can be seen that the application of the heat storage dual-core stainless steel pressurized water tank to the balcony wall-mounted natural circulation hot water system can improve the overall energy efficiency index of the system.

Of course, with good thermal performance indicators, there must be a reliable operational guarantee. Although installation and use seem to be very simple of energy storage dual-core stainless steel pressurized water tank applied into solar balcony wall hanging indirect natural hot water system, in the actual installation application, the following points must be noted:

◇Before installing the water tank, it must be verified that the strength of the water tank installation position must be able to meet the total load carrying capacity of the water tank (the total weight of the water tank after filling the water).

◇A check valve must be installed before the inlet of the water tank cold water pipe. When the inlet pressure is greater than the tank's rated pressure, a pressure relief valve must be installed at the tank inlet.

◇The water tank must be fitted with a safety valve that defines the tank pressure. Taking into account the safe discharge of hot water, it is necessary to ensure that the safety valve vent is unobstructed and properly connected to the building drainage system.

◇During the user's use, the safety valve must be manually operated every other month to check the reliability of the device.

◇The tank should be installed at a height of 0.5 m above the highest point of the solar collector. The installation pipe distance is recommended to be no more than 5 meters.

◇The installation position of the pressurized water tank should not exceed 3.5 meters above the water point. Otherwise, it is strictly forbidden to use hot water or install an inhalation valve at the hot water outlet in the water stop state.

◇If the user goes out for a long time or goes out on vacation, please check whether the safety valve exhaust port is unblocked, cut off the system power supply, and close the water supply control valve.

◇The electronic magnesium rod must be replaced regularly.

◇Check the meter and the indoor power cord, and ensure that it matches the tank load. Overloading is strictly prohibited.

◇The water tank embedded controller is strictly prohibited from being used in places where it is wet or has flammable or explosive gases such as fuel to avoid danger;

◇In areas where the ambient temperature is below zero degrees Celsius, the circulating medium must be antifreeze.

◇The system circulating medium and the use of hot water must comply with national standards. It is strictly forbidden to use untreated river water, lake water and sea water.

Therefore, in the indirect natural hot water system of the solar balcony wall, the application of the energy storage dual-core stainless steel pressurized water tank can only be installed and used in strict accordance with the requirements of the specification, so as to reflect the characteristics of energy saving, comfort, convenience and convenience.

5 Conclusion

        In summary, the energy storage dual-core stainless steel pressurized water tank is developed for the mid-rise and high-rise residential buildings using balcony wall-mounted indirect natural circulation solar water heating system. It has the following unique advantages and features, and the market application prospect is very broad.

1. More ways to adopt the structural design to enhance heat transfer effect, the heat transfer efficiency of the water tank is increased by 26%, and the water tank reaches the A-level energy efficiency level; The water tank is applied to the indirect natural circulation hot water system of the balcony wall, which can improve the energy efficiency index of the system as a whole by 10%.

2. The energy storage dual-core pressurized water tank must be made of food grade stainless steel. The enamel liner can not meet the high temperature, high pressure, cold and heat alternating expansion force of the solar water heating system. There is no guarantee for the interlayer enamel process.

3. Laminated jacket double-core liner origins "Platinum Shield" all-round stainless steel anti-corrosion technology multiple protection, salt spray test is 20 times higher than ordinary stainless steel water tank, blue dot method detection, and no blue dot appears for more than 35 seconds, can be guaranteed for 15 years There is no water leakage above, and the water quality is clean and pollution-free.

4. Material strength verification calculation and strict process welding guarantee that the water tank passes through 250,000 times of fatigue life pulse test and 2 times working pressure static test intact, and the water tank is reliable and durable.

5. In the water tank insulation test at minus 15 degrees Celsius, environmentally friendly, the energy loss of non-polluting, high-density thick foam layer and mature foaming process can reach the A-class standard in the 24-hour, and the water tank is more energy-efficient.

6. Cold and heat stratified space heat storage structure can meet the needs of large-capacity instantaneous hot water, bathing would be more comfortable and more assured.

7. Multi-functional quick-installing components, designed in proportion to the modulus of building materials, can be quickly and perfectly installed with no special tools.

8. Water tank and intelligent control humanized interface design, the operation is simpler, highlighting the fashion and color, protecting for your safety and care.

9. Heat exchange expansion space design reserved and temperature pressure valve settings ensure safe and reliable system operation.