How to use heat energy for refrigeration

Solar energy is recognized as one of the most suitable, safest, greenest and ideal alternative energy sources for human beings in the future, with the advantages of convenient access, huge energy, no pollution and good safety. According to relevant data, China is a country rich in solar energy resources, and the annual total radiation in two-thirds of the areas is more than 5020MJ/m2, so the development and utilization potential of solar energy is huge. Using solar energy to drive air conditioning system can greatly reduce the consumption of non-renewable energy and power resources, and on the other hand, it can reduce the environmental pollution caused by burning conventional fuels such as coal, which is a hot topic in the field of air conditioning and refrigeration technology.

Main ways of driving refrigeration

According to the different energy conversion modes, there are two main modes of solar-driven refrigeration: one is to realize photoelectric conversion first, and then use electricity for refrigeration; The second is to carry out photothermal conversion, and then use thermal energy for refrigeration.

Study on solar photoelectric conversion refrigeration

It is a method of converting solar energy into electric energy through photoelectric conversion devices, and then driving semiconductor refrigeration system or conventional compression refrigeration system to realize refrigeration, namely photoelectric semiconductor refrigeration and photoelectric compression refrigeration. The premise of this refrigeration mode is to convert solar energy into electric energy, and the key is photoelectric conversion technology. Photoelectric conversion receiver, namely photovoltaic cell, must be adopted, and its working principle is photovoltaic effect.

Solar semiconductor refrigeration. Solar semiconductor refrigeration is a special refrigeration method that uses the electric energy generated by solar cells to supply semiconductor refrigeration devices to realize heat energy transfer. The theoretical basis of semiconductor refrigeration is the thermoelectric effect of solid, that is, when direct current passes through a circuit composed of two different conductive materials, it will absorb heat or release heat at the junction. How to improve the performance of materials and find more ideal materials has become an important topic of solar semiconductor refrigeration. Solar semiconductor refrigeration is widely used as coolers for electronic equipment and instruments in the fields of national defense, scientific research, medical and health care, or for low-temperature instruments and meters, or for making small thermostats. At present, the efficiency of solar semiconductor refrigeration device is still relatively low, and the COP is generally around 0.2 ~ 0.3, which is much lower than compression refrigeration.

Photoelectric compression refrigeration. In the process of photoelectric compression refrigeration, solar energy is first converted into electric energy by photoelectric conversion devices, and the refrigeration process is conventional compression refrigeration. The advantage of photoelectric compression refrigeration is that it can easily obtain refrigeration capacity by using mature and efficient compression refrigeration technology. Photoelectric compression refrigeration system has been applied in some countries and regions with good sunshine and lack of power facilities, such as African countries for life and drug refrigeration. But its cost is about 3 ~ 4 times higher than that of conventional refrigeration cycle. With the improvement of photoelectric conversion device efficiency and the reduction of cost, photoelectric solar refrigeration products will have broad development prospects.

Study on solar photothermal conversion refrigeration

Solar photothermal conversion refrigeration first converts solar energy into heat energy, and then uses heat energy as external compensation to achieve the purpose of refrigeration. Photothermal conversion can mainly realize refrigeration from the following directions: solar absorption refrigeration, solar adsorption refrigeration, solar dehumidification refrigeration, solar vapor compression refrigeration and solar vapor jet refrigeration. Among them, solar absorption refrigeration has entered the application stage, while solar absorption refrigeration is still in the experimental research stage.

Study on solar absorption refrigeration. The research of solar absorption refrigeration is closest to practical application. The most common configuration is that solar collectors are used to collect solar energy and drive single-effect, double-effect or two-stage absorption refrigerators. Lithium bromide-water is mainly used as working medium, and oil or coal-fired boiler can be used for auxiliary heating when solar energy is insufficient. The main structure of the system is basically the same as that of the ordinary absorption refrigeration system, the only difference is that the heat source at the generator is solar energy, instead of high-temperature steam, hot water or high-temperature waste gas generated by ordinary boiler heating.

Solar adsorption refrigeration. The refrigeration principle of solar adsorption refrigeration system is to realize refrigeration cycle by using the periodic adsorption and desorption process of refrigerant by solid adsorbent in adsorption bed. Solar adsorption refrigeration system is mainly composed of solar adsorption collector, condenser, accumulator, evaporator and valve. Commonly used adsorbents include activated carbon-methanol, activated carbon-ammonia, calcium chloride-ammonia, silica gel-water, metal hydride-hydrogen and so on. Solar adsorption refrigeration has the advantages of simple system structure, no moving parts, low noise and no need to consider corrosion, and its construction cost and operation cost are relatively low.