Matching of hot water storage tank and auxiliary energy

Matching of hot water storage tank and auxiliary energy

The heat preservation hot water storage tank is the main energy storage component of the hot water system. Generally, the manufacturer has a wrong understanding, thinking that if Party A proposes how much hot water is needed, it is considered qualified if it is equipped with a water tank of the corresponding capacity. This is correct for an insulated hot water storage tank that only contains hot water, but it is far from that for the insulated circulating hot water storage tank of a solar hot water system that uses the “top-water method” to fetch water. We imagine that a thermal insulation circulating hot water storage tank with a capacity of 100L is filled with hot water with an average temperature of 45°C. We use cold water with an average temperature of 15°C to squeeze the hot water with cold water on top of the hot water. Because there is a temperature difference of 30°C between cold and hot water, that is to say, for every liter of cold water added, theoretically about 5L of hot water will be mixed and cooled to below 40°C. Calculating in this way, we use the “top water method” as long as 40L of cold water is mixed to replace about 40% of the standard hot water, and the average water temperature in the heat preservation circulating hot water storage tank will not reach the minimum operating temperature lower limit of 35°C. Up. Even if the water temperature stratification of the water tank is good, after 50% of the available hot water can be ejected from the water tank, the average temperature of the water in the water tank is lower than the human body temperature, and it is no longer suitable for bathing. In other words, whether you are in a natural circulation system or a forced circulation system, as long as we use the “top water method” to extract hot water, we must take into account that the hot water produced by the system cannot be fully extracted and used on that day. Cold water in winter This is especially true when the temperature is low. Therefore, when designing the system, we must not only fully consider the actual available hot water output of the system, but also consider the increased heat collection area and the excess capacity of the water tank due to different water intake and water methods.

In addition, for systems with higher requirements for the hot water guarantee rate, the problem of matching auxiliary heating devices must also be considered. Currently, gas water heaters, electric heaters or heat pumps can be used as auxiliary energy sources for solar water heaters. Given that the thermal efficiency of gas water heaters is only 75%, the efficiency of electric water heaters is 90%, and the average “energy consumption ratio” of heat pumps can reach more than 300%, from the perspective of safety and energy saving, the preferred heat pump unit as the auxiliary energy for solar water heaters is The more ideal solution, the possible problem with the use of heat pump units is that the system is more complicated and the supporting costs are relatively high. Secondly, choose an electric water heater; a simple electric heating device is an electric heating tube, a temperature probe and a temperature controller with over-current, over-voltage, and anti-leakage protection devices.

The theory of heat transfer tells us that heat can only be transferred spontaneously from high temperature to low temperature, and it is impossible for a low-temperature heat source to automatically enrich heat into a high-temperature heat source. If you want to realize the temperature transfer in the opposite direction, that is, to collect the heat energy in the low temperature area and transport it to the high temperature area, you must first select a working fluid with a lower boiling point than the low temperature heat source as the heat carrier, and first expand the working fluid through an expansion valve Evaporate and vaporize, and then use the expanded and vaporized low-temperature working fluid to fully absorb the heat of the low-temperature heat source in the evaporator; then start the compressor as a mechanical power to transport the low-temperature working fluid with latent heat vaporization to the place where the heat energy needs to be enriched; At the same time, pressure is applied to force the low-temperature working fluid to release the heat carried in the condenser, re-condensate into liquid and flow back to the liquid storage tank, waiting to enter the next heat absorption ~ heat release cycle. Only through the above-mentioned intermediate conversion of heat absorption to heat release of the low-temperature working fluid, it is possible to enrich the heat of the low-temperature heat source and realize the transfer to the high-temperature hot zone. Scientists call the physical phenomenon that can transfer heat in the low temperature region to the high temperature region as the “reverse Carnot cycle.” This kind of “reverse Carnot cycle” is like lifting the water in a low pond to a high water tower through a water pump. Therefore, people vividly refer to the mechanical device that can realize the “reverse Carnot cycle” to transport heat energy, simply referred to as the “heat pump”-that is, the “pump” that can enrich the low temperature heat energy and transfer it to the high temperature heat storage device. .

Matching of hot water storage tank and auxiliary energy
heat pump

That is to say: the so-called “heat pump” is a kind of equipment used to collect the heat energy of the warm heat source in the surrounding (air, water, geothermal) environment, and realize the enrichment of low-temperature heat energy, and effective transportation and transmission. The heat pump uses a low-boiling substance as a working fluid, uses the physical phenomenon of low-pressure expansion and vaporization of the working fluid to absorb heat, and high-pressure condensation and liquefaction to release heat to achieve effective collection of low-temperature heat. A device that effectively transfers to high-temperature energy storage components remotely. In the process of “reverse Carnot cycle”, the low boiling point working fluid used by the heat pump has only undergone morphological changes and reductions, without any irreversible fundamental changes in internal energy. If, when the heat energy is circulated in the forward direction, we can use the cycle efficiency (η) to express the evaluation of the heat conduction. Then, to measure the effectiveness of the reverse cycle of thermal energy, we usually use the energy consumption ratio (COP) to express. The so-called energy consumption ratio (COP) refers to the ratio of the thermal energy (QC) obtained by the system to the power (W0) consumed in the process.
which is

If: the heat energy obtained by the system is QC; the heat absorbed by the working fluid from the natural environment (low temperature heat source) is QA. According to the first law of thermodynamics, the actual work W0 consumed in the “reverse Carnot cycle” process is:

According to the above formula:

It can be seen from the above formula that in an ideal reverse Carnot cycle, the energy consumption ratio (COP) is inversely proportional to the temperature difference. Assumption: The temperature of the heat output of the condenser is T1; the temperature of the low-temperature heat source (air, water or geothermal temperature) input to the evaporator is T2; then

In the above formula, since the output temperature (T1) is always higher than the input temperature (T2); therefore, it can be said that the energy consumption ratio (COP) of the heat pump is always greater than 1: Therefore, we can say with certainty that if you choose a “heat pump” As an auxiliary energy source for solar water heating systems, it must have good economic benefits. Also from the above formula, we can also see that the energy consumption ratio (COP) decreases as the difference (T1-T2) decreases, and the COP value will increase more; in other words, for the air source heat pump, As the ambient temperature (T2) increases, the conversion efficiency of the heat pump and the energy consumption ratio (COP) will be higher. In other words, if we can use solar energy to heat the air temperature (T2) near the heat pump and the evaporator as much as possible, the heat production efficiency of our heat pump will be greatly improved.

Through the W0=QC-QA formula, we know that the actual energy consumption of the heat pump system is composed of two parts. If we can effectively use the (QA) part, the total economic benefits of the system will be significantly improved. As we all know, when the air passes through the heat pump evaporator and exchanges the heat contained in the air to the low-temperature working fluid, the air temperature generally decreases by 5°C~7°C on average: if we can effectively recover the cooled air at the same time, After proper treatment, it is sent to the room as fresh cold air for air conditioning to cool down, which is equivalent to saving a large amount of energy costs for air conditioning cooling (QA). According to this idea, in 2002, Kunming Nankai Energy Research Institute specially developed a new technology product-“air source heat pump cold air recycling device”; using this device will pass through the air source heat pump evaporator after being heat exchanged and cooled The cold air is collected, synchronously through electrostatic dust removal, ionized bromine and oxygen sterilization, and then input into the room as fresh cold air, which is a good realization of the economic goal of achieving two birds with one stone, energy saving and consumption reduction.

As far back as the 1950s, when scientists discovered the principle of “reverse Carnot cycle”, and successfully replaced the heat energy from the low-temperature air heat source, reversed the process of “pumping” the heat to the high-temperature heat utilization zone. Vividly named it “air source heat pump”; the same reasoning is that the device that replaces heat from the water of rivers, rivers, lakes and wells is called “water source heat pump”; and the system that collects geothermal energy from the latent heat of deep underground wells, then It is called “ground source heat pump”. Generally speaking, it is more economical and reasonable to use air source heat pumps to match solar water heaters in the south. In northern regions, considering the freezing and severe cold in winter, the use of ground source heat pumps is more reliable and applicable. Water source heat pumps are obviously more ideal for those close to non-freezing rivers and lakes in winter. At present, some manufacturers in the market have changed the name of the air source heat pump that scientists have accurately named as “air energy water heater”. If this alternative term is used, it does not include new technologies with independent intellectual property rights and independent innovation content. That would completely violate the moral bottom line of intellectual property that not discoverers or inventors have the right to rename scientific and technological achievements that have been clearly defined. The kind of commercial hype that transforms other people’s scientific and technological development results for personal gain does not respect science or history. It must be pointed out that the term “air energy” lacks scientific rigor, because it is easily confused with the meteorological energy formed by the absorption of solar light and heat radiation by the atmosphere around the earth, which has been clearly defined by the scientific and technological circles, and we don’t know new terms. How does the inventor explain this so-called “air energy” and what is the principle difference between wind power generation that really uses air kinetic energy to generate electricity and hot chimney power generation that uses convection with large temperature differences? In addition, the connotation is not exact, because when talking about the characteristics of air, people often first think of it not the low-temperature heat energy it contains, but the dust, germs, ice crystals, smog, electric charge, and condensed water vapor clouds mixed with it. And cold air and cold currents. All in all, in the face of commercial publicity advertisements for the general public, we must pay attention to science, explain the profound things in simple terms, and seek truth from facts; we cannot unscrupulously stand out, sensationalize, and mislead consumers; respect science and respect knowledge should become the most basic professional ethics of the new energy industry.