Crops in greenhouses located in cold climates are frequently affected by high relative humidity (RH). This study presents the design, testing, and analysis of a dehumidifier based on thermoelectric cooling. Thermoelectric dehumidifiers (TEDs) are capable of dehumidifying greenhouses in cold regions while recovering heat for indoor air heating. The design of a TED is based on the specific characteristics of thermoelectric coolers (TECs). A TED consists of a cabinet, four heat exchangers, a duct fan, a water pump, and auxiliary components. The TED performance was evaluated in a Chinese solar greenhouse (CSG) with a volume of approximately 160 m³. The input voltage of the TECs, fan airflow rate, and cold-side fin area affected the TED performance, with their influence varying in magnitude. The radar chart results show that the optimal operating parameters are as follows: a fan airflow rate of 300 m³/h, a TEC input voltage of 15 V, and a cold-side fin area of 0.15 m². With the TED running for 120 min under the optimal parameters, the RH in the CSG decreased by 25.5%, while the air temperature increased by 3.4 °C. The installation of the TED at the bottom of the CSG improved the growing environment of the crops, particularly in the vertical range between 0.2 m and 1.5 m height inside the greenhouse. These findings provide a valuable reference for applying thermoelectric cooling technology in the greenhouse field.

High-air temperature stress inhibits the growth of hydroponic lettuce. The practical application of conventional air cooling is constrained by high cost and moderate efficacy. However, root-zone cooling represents a more promising temperature regulation strategy for vegetable production, offering advantages such as ease of integration and lower cost. This study used lettuce (Spanish Green) as the plant material under four RZT treatments: T0 (control: 24.65~31.65 °C), T1 (24.5 °C), T2 (20.5 °C), and T3 (16.5 °C). Growth parameters and nutritional quality indicators under each treatment were systematically monitored, and a comprehensive evaluation was performed using the fuzzy membership function method. All cooling treatments (T1–T3) enhanced lettuce plant height, leaf area, and shoot dry weight. According to the fuzzy membership function analysis, the T1 treatment was found to exhibit the highest overall nutritional value. Although the T0 control group displayed the poorest growth performance, with a shoot dry weight 47.24% lower than that of T1, it accumulated significantly higher levels of P, Ca, and Zn. These findings demonstrate that regulating RZT to approximately 24.5 °C synergistically enhances both biomass and quality in lettuce, providing theoretical and practical support for optimizing hydroponic production in summer conditions.

In the Nutrient Film Technique (NFT) system, air-temperature stress presents a severe challenge to the cultivation of hydroponic lettuce, often limiting its yield and quality during warm seasons. Although previous studies have confirmed that regulating air temperature can alleviate the stress on crops, challenges such as uneven temperature distribution and high equipment costs hinder effective air temperature management in greenhouse environments. In contrast, independently regulating root-zone temperature (RZT) under the same air-temperature stress conditions offers a more practical approach to enhancing crop growth, which can significantly improve crop outcomes without incurring substantial additional costs. This study used 'Spanish Green' lettuce as the test material and established four RZT treatments: T0 (control: 24.65–31.65℃), T1 (24.5℃), T2 (20.5℃), and T3 (16.5℃). Over a 38-day cultivation period, we systematically monitored the effects of different RZT treatments on lettuce growth parameters (such as plant height, leaf area, and shoot dry weight) and nutritional quality indicators (including vitamin C, nitrate, and mineral element content). The fuzzy membership function method was employed for a comprehensive evaluation of lettuce quality.The results showed that all cooling treatments (T1, T2, T3) promoted increases in plant height, leaf area, and shoot dry weight. Based on calculations from the fuzzy membership function, lettuce under treatment T1 achieved the best balance between growth performance and nutritional quality. Lettuce in treatment T0 exhibited the poorest growth; compared to T0, shoot dry weight increased by 47.24%, 16.24%, and 12.21% for treatments T1, T2, and T3, respectively. However, mineral element contents such as P, Ca, and Zn were significantly higher in treatment T0 than in the other three treatments. The overall quality of lettuce in treatment T2 was superior to that in T3; although T2 promoted growth relative to T0, its growth performance was significantly lower than that of T1, and both T2 and T3 exhibited a decline in overall growth quality. Nutrient solution consumption was highest for treatment T1 and lowest for treatment T0.This study demonstrates that independently regulating RZT to approximately 24.5℃ can achieve a synergistic enhancement of biomass and quality in lettuce, providing a theoretical basis and practical guidelines for optimizing NFT system production during summer conditions.