New HVAC Technology – Natural Gas-Driven Refrigeration

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We know that coal and oil are the two major conventional energy sources in the world. In comparison, natural gas is a cleaner and higher-quality fuel, and its reserves are quite abundant; as a result, it has become the world’s third-largest conventional energy source after coal and oil. Compressed refrigeration and air conditioning systems driven by internal combustion engines or gas turbines using natural gas as fuel have entered the market, replacing some electrically driven compressed refrigeration and air conditioning systems. The widespread use of this system has saved a great deal of energy, reduced investment in electricity, extended the lifespan of compressors, and improved energy efficiency. Driven by the heat energy for pumping, they can all utilize the heat energy of natural gas to achieve heat transfer. The refrigerants used for cooling are mostly chlorofluorocarbons; these refrigerants undergo only phase changes, and electric motors are primarily used as the driving mechanism. In recent years, the use of natural gas engines has been promoted as a replacement for electric motors; they allow for more flexible adjustment of cooling capacity, offer higher efficiency at partial load, and result in lower overall operating costs. Thermally driven absorption refrigeration uses a working fluid pair composed of substances with significantly different evaporation temperatures; the component with the lower evaporation temperature acts as the refrigerant, while the component with the higher evaporation temperature functions as the absorbent. During the refrigeration cycle, the working fluid pair only undergoes concentration changes without any chemical reactions. Its driving force is the heat of combustion of natural gas or various heat sources, even waste heat. The principle of an absorption refrigeration system is shown in Figure 5. The main difference between this type of refrigeration system and compression-based refrigeration systems is that a generator, an absorber, and a solution pump are used in place of a compressor ； Using thermal energy in place of electrical or mechanical energy ； Cooling is achieved when the working fluid pair is heated in the generator to produce water vapor, which then condenses into liquid water in the condenser. After passing through a throttle valve, this water enters the evaporator where it absorbs heat and evaporates, thereby providing cooling. Then, the water vapor is absorbed by the concentrated lithium bromide solution in the absorber, and the diluted lithium bromide solution is sent back to the generator using a solution pump, completing the cycle. The system operates under low pressure or vacuum conditions, does not use chlorofluorocarbons, has no large moving parts, is quiet and safe with high reliability, and requires low maintenance costs. In desiccant dehumidification systems and refrigeration and air conditioning systems, air is generally cooled to remove the moisture from it, which is known as latent heat cooling; simultaneously, the temperature of the air is reduced, which is known as sensible heat cooling. Latent heat cooling often consumes half of the refrigeration energy. The system that utilizes the adsorption (absorption) of desiccants removes moisture from air directly without cooling it. The combination of a drying system and a cooling system allows for flexible control over the humidity and temperature of the air. The combined system prevents condensation from forming in the ducts when using a single refrigeration method, reduces the humidity of items indoors, limits mold growth, and improves air quality ； Low humidity can make people feel comfortable at higher temperatures; for example, when the relative humidity is as low as 30%, it provides the same level of comfort as a temperature decrease of 2°C. Low humidity allows the cooling temperature setting in air conditioners to be increased, thereby saving energy ； Using a drying system can reduce the load on the refrigeration system, saving on investment and operating costs. Drying systems are suitable for applications that require a low dew point or where the ratio of latent heat cooling to sensible heat cooling loads is high.