In recent years, the penetration of electric fans in urban households across China has exceeded 200%, while air conditioner ownership remains below 10%. This significant gap is primarily driven by cost and energy consumption. Electric fans are much more affordable, with a price difference of 300 to 2000 yuan compared to single-cooling window air conditioners. Moreover, there's a lack of a mid-range cooling solution that bridges this gap. Over the past decade (from April 1985 to December 1995), China’s patent system saw over 200 applications aimed at filling this void, focusing on non-mainstream cooling technologies.
From a refrigeration source perspective, water evaporation-based systems dominate, accounting for about one-third of all applications. Other types include underground cooling, semiconductor refrigeration, refrigerator cooling, low-power air conditioning, solar cooling, gas refrigeration, and heat pipe cooling. Water evaporation is particularly popular due to its low cost and high vaporization heat value. At 30°C, water has a vaporization heat of 2451 J/g, meaning 1 gram can absorb 2451 J of heat. If this process occurs within a second, it equates to a cooling power of 245 watts—comparable to a small window air conditioner.
However, the effectiveness of these evaporative coolers depends heavily on ambient humidity and airflow speed. They perform well in dry, hot regions but struggle in humid southern areas where high humidity limits evaporation. In such conditions, misting may increase indoor humidity instead of reducing it, worsening discomfort. To address this, some solutions have been proposed, like using cold plates to separate dry and wet air, or dehumidifying air using salts like calcium chloride before re-humidifying it for cooling.
Underground cooling systems fall into three categories: groundwater, tap water, and underground wind. These require large amounts of water, making them unsuitable for most urban households. The tap water type, for example, requires up to 180 liters per hour, which is impractical for city dwellers. Underground wind systems are limited to homes near basements or civil defense facilities, and often bring in stale or odorous air.
Semiconductor cooling fans use the Peltier effect, offering advantages like quiet operation, compact design, and adjustable cooling. However, they suffer from low energy efficiency and require DC power supplies, limiting their use to vehicles and aircraft.
Refrigerator-based cooling involves either modifying existing units or using cooled blocks placed in fans. These systems offer limited cooling capacity and are best suited for small spaces. Low-power air conditioners, while energy-efficient, have poor air circulation and limited coverage.
Solar cooling systems, though promising, face challenges in efficiency and cost. To achieve 2000 watts of cooling, a large solar collector area is needed, making them expensive for home use. Similarly, heat pipe and gas-based refrigeration systems remain experimental and require further validation.
Overall, while many innovative cooling technologies exist, practical adoption is still limited by cost, efficiency, and environmental factors.
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