Outdoor Air Relative Humidity Research Articles

Numerical study on heat transfer characteristics of spray cooling double skin façade

Using spray cooling technology to solve the overheating issue in double skin façade (DSF) is an important attempt to reduce building energy consumption. While, the thermal performance of the spray cooling double skin façade (SC-DSF) is not very clear, especially the complicated heat transfer characteristics. In this study, a comprehensive numerical model of SC-DSF system including solar radiation and two-phase flow problem is established by using three-dimensional steady Euler-Lagrange method to analyze the heat transfer characteristics from a microscopic perspective. And the CFD model is validated by the experiment. Meanwhile, a quantitative analysis is conducted to investigate the thermodynamic and kinetic behavior of the SC-DSF in hot humid climate region in China. The influences of outdoor air relative humidity, solar radiation intensity, and outdoor wind speed on the water spray cooling effect of the DSF are analyzed. The cooling efficiency of SC-DSF system is sensitive to the outdoor relative humidity. In a sufficiently cooled condition, the minimum temperature in the cavity is close to the outdoor wet-bulb temperature. The spray cooling efficiency reaches 28.2 % with the outdoor relative humidity of 20 %, but decreases to only 9.3 % as relative humidity rises to 80 %. Outdoor wind speed also has some impact on the spray cooling system, an optimal outdoor wind speed of 1.5 m/s can enhance cooling efficiency by 6.6 %, but it has adverse effect on the cooling efficiency when wind speed exceeds 3 m/s. However, in addition to increasing surface temperature of the DSF, solar radiation has little influence on the cooling performance. Excessive humidity exceeding 80 % and wind speed surpassing 3 m/s may render the spray system ineffective. Therefore, the effects of environmental conditions should be fully considered and utilized in the design and application of spray cooling systems.

Investigation and assessment of the optimum conditions of solar heat source hybrid desiccant cooling system under different modes

In this study, the hybrid desiccant cooling system (HDCS) with solar heat source is experimentally studied, and various alternative methods are suggested for the problem of the cooling system which is of concern due to the characteristics of solar power, empirical experiments and the applicability of solar heat sources to HDCS. The results of empirical experiment are analyzed through daily data, while cooling temperature is set to Tset = 22 °C (setting temperature) under the conditions of TOA = 32 °C (outdoor air temperature), RHOA = 45 % (outdoor air relative humidity), Tst = 70–80 °C (storage top temperature), Treg = 70 °C (regeneration temperature), dehumidification = 4.5 g/kg, COPelec = 4.8, and COPth = 2–4. When Tst is higher than 85 °C, it may damage the system due to material characteristics of desiccant rotor. Therefore, flow rate of the load-side pipe is lowered by adding a bypass pipe to load-side pipe, and OA (outdoor air) of 60–70 °C suitable for regeneration of the desiccant rotor can be supplied. When the Tst is low, the Tst is increased by lowering the Treg and rather absorbing the heat of the condenser and radiating the heat in the heat storage tank. If the Tst is higher than 85 °C but no separate action can be taken on the load side, a radiator can be installed at the solar system to control the temperature of storage tank. If hot water supply is controlled according to the temperature of storage tank, the solar heat source can be stably applied to the HDCS.

Effect of airflow rate on frost formation and performance of membrane energy exchangers in sub-zero conditions

Frost formation in membrane energy exchangers under cold climates poses significant operational challenges. This study comprehensively investigates the influence of airflow rate on frost formation and the performance of the energy exchanger through experimental and theoretical analyses. It was found that higher airflow rates increase the risk of frost formation while reducing the airflow rate effectively mitigates frost formation. Frost had a greater adverse effect on sensible effectiveness than latent effectiveness, leading to decreased heat transfer performance. The sensible effectiveness decreased by 15.3 % at 400 m3/h after one hour, while the latent effectiveness only decreased by 13.0 %. The risk of frost formation was influenced by outdoor air temperature and extract air relative humidity. The impact of outdoor air relative humidity and extract air temperature was relatively small. The findings provide practical insights for optimizing airflow rates to prevent frost formation and enhance energy exchanger performance, contributing to the development of more efficient heat recovery systems.

A thermal sensation model for naturally ventilated indoor environments based on deep learning algorithms

In recent years, with the emphasis on sustainability and energy efficiency, natural ventilation has attracted increasing interest from building designers. Natural ventilation is dependent on the outdoor environments which could change rapidly, and the traditional thermal sensation models such as the predicted mean vote (PMV) are not applicable, correspondingly. The deep belief neural network can reveal nonlinear patterns in processing big data, and it can be used to predict target data with high flexibility and accuracy. This study developed a deep belief neural network model for indoor thermal sensation prediction in naturally ventilated environments with outdoor environment parameters and human factors: outdoor air temperature, average radiant temperature, outdoor air relative humidity, outdoor wind speed, clothing thermal resistance, activity level, gender, age and weight collected in 10 semi-open classrooms and 5 laboratories in April and November when natural ventilation was used. The research compared the performance of deep belief neural networks with three neural networks: BP, Elman and fuzzy neural networks. Results showed that the deep belief neural network can enhance the performance of thermal sensation prediction of natural ventilated indoor environments. The research provides a more flexible and effective solution for thermal comfort prediction of natural ventilated indoor environments.

Fibre Optic FBG Sensors for Monitoring of the Temperature of the Building Envelope.

Standard sensors for the measurement and monitoring of temperature in civil structures are liable to mechanical damage and electromagnetic interference. A system of purpose-designed fibre optic FBG sensors offers a more suitable and reliable solution—the sensors can be directly integrated with the load-bearing structure during construction, it is possible to create a network of fibre optic sensors to ensure not only temperature measurements but also measurements of strain and of the moisture content in the building envelope. The paper describes the results of temperature measurements of a building 2-layer wall using optical fibre Bragg grating (FBG) sensors and of a three-layer wall using equivalent classical temperature sensors. The testing results can be transmitted remotely. In the first stage, the sensors were tested in a climatic test chamber to determine their characteristics. The paper describes test results of temperature measurements carried out in the winter season for two multilayer external walls of a building in relation to the environmental conditions recorded at that time, i.e., outdoor temperature, relative humidity, and wind speed. Cases are considered with the biggest difference in the level of the relative humidity of air recorded in the observation period. It is found that there is greater convergence between the theoretical and the real temperature distribution in the wall for high levels (~84%) of the outdoor air relative humidity, whereas at the humidity level of ~49%, the difference between theoretical and real temperature histories is substantial and totals up to 20%. A correction factor is proposed for the theoretical temperature distribution.

UTILIZATION OF SOLAR ENERGY IN GREENHOUSE HEATING DURING WINTER SEASON IN EGYPT

The main aim of this study is to investigate the utilization of solar energy as a renewable energy source and compared to the traditional energy in greenhouses heating during winter season. To achieve that study, the effect of two different heating systems (Traditional heating (G1) and solar heating (G2)) on the cucumber growth parameters during growth period and cucumber fruit characteristics and total yield were investigated. Temperature and relative humidity during the growth season were recorded. The obtained results indicated that the indoor air temperatures ranged from 18.78 to 25.29 and 18.00 to 25.15 ᵒC for G1 and G2, respectively, whereas the outdoor air temperature ranged from 11.00 to 21.66 ᵒC. The temperatures were added by using electrical (G1) and solar energy (G2) for greenhouse heating ranged from 1.5 to 9.3 and 0.85 to 7.78 ᵒC, respectively, during experimental period. The indoor relative humidity ranged from 63.33 to 88.67 and 70.93 to 93.94 % for G1 and G2, respectively, whereas the outdoor air relative humidity ranged from 45.93 to 92.73 %. The plant length, stem diameter and total leaf area values for cucumber grown in G1 were higher than those of the plant growth in G2. The fruit length was 24.79 and 19.44 cm for G1 and G2, respectively. The fruit diameters were 3.36 and 3.19 cm for G1 and G2, respectively. The results also indicate that the fruit weight and number of fruits per plant were 115.07 and 108.73 g and 19 and 17 for G1 and G2, respectively. The total yield values of cucumber were 182.09 and 152.92 kg for G1 and G2, respectively, during growth period.

Field experiments on the effects of an earth-to-air heat exchanger on the indoor thermal environment in summer and winter for a typical hot-summer and cold-winter region

An earth-to-air heat exchanger (EAHE) is a shallow geothermal energy utilization technology, which can significantly reduce the building energy consumption. This study aims to investigate the effects of an EAHE on the indoor thermal environment in hot-summer and cold-winter regions. A full-scale experiment was conducted for a continuous 24 h period using two identical buildings (with and without an EAHE) exposed to the same outdoor conditions on typical summer and winter days. Experimental results show that the EAHE provides an average outdoor air temperature reduction of 9.12 °C under cooling conditions and increase of 5.53 °C under heating conditions. Moreover, the average outdoor air relative humidity increases by 46.89% and decreases by 27.41% in the cooling and heating conditions, respectively; the corresponding average EAHE coefficients of performance (COP) are 7.03 and 4.26, respectively. Compared to that of the building without EAHE, the average indoor air temperature of the building with EAHE decreases by 5.9 °C in summer and increases by 4.29 °C in winter, and the average walls’ internal surface temperature decreases by approximately 6 °C and increases by approximately 5.8 °C. Moreover, the EAHE system reduces the average building cooling and heating loads by 55.4 and 40.43 W/m2, respectively.

Experimental study on the performance of air source heat pump system with multiple parallel outdoor units

This paper presents a novel air source heat pump for heating of buildings named air source heat pump with multiple parallel outdoor units (ASHPMO). Multiple outdoor units were connected in parallel with the aim of realising alternate defrosting and uninterrupted heating simultaneously. An experimental apparatus of the ASHPMO system was developed. The defrosting performance was experimentally investigated under different outdoor air temperatures, outdoor air relative humidity, and condensation temperatures, among other factors. The test results showed that the novel ASHPMO system could provide continuous heating when defrosting even under an outdoor air temperature of −10°C. Variations in compressor vapour suction and discharge pressure and temperature were observed. The minimum heating capacity could still reach 60% of that without defrosting. Under the defrosting condition with outdoor air temperature −10°C, both the heating coefficient of performance (heating COP) and total energy efficiency ratio (EER) of the system can reach to 2.0 and 2.32, respectively.

Heat and mass transfer modeling in enthalpy exchangers using asymmetric composite membranes

Enthalpy exchangers using water vapor perm-selective membranes are used in building ventilation systems due to their small footprint, simplicity, reduced contaminant crossover, and relatively high efficiency. Moisture permeation properties of the membrane media that vary with operating air humidity and temperature lead to significant changes in the performance of an enthalpy exchanger and thus the whole ventilation system, impacting building energy efficiency. Evaluation of the actual energy savings potential of such energy recovery devices in building ventilation systems requires models that account for this variable membrane performance. A theoretical model is developed for current generation asymmetric composite membranes used in enthalpy exchangers. This model predicts the membrane permeability as a function of local values of air humidity and temperature, based on a limited number of kinetic water vapor sorption tests of the membrane material. The membrane model is coupled with a finite-difference model of the conjugate heat and mass transfer in full cross-flow enthalpy exchanger cores. The model predictions are validated against experimental data of a commercial-scale enthalpy exchanger. The model is used to predict the influence of outdoor air parameters (temperature, humidity) on an enthalpy exchanger and the predictions are compared against a baseline case that assumes constant membrane permeability. Such assumption can result in deviations in effectiveness predictions by up to 15%. Depending on the mode of operation, outdoor air relative humidity can increase or decrease the effectiveness of enthalpy exchangers by up to 12%. In contrast, outdoor air temperature appears to have only a minimal influence on effectiveness parameters.

The correction coefficients for the building air-conditioning load accounting for the whole building hygrothermal transfer process for major cities in China

At present, the calculation software and design specifications related to building cooling and heating load are based mainly on heat transfer theory. The effect of building moisture transfer process on the air-conditioning load is rarely researched. The aim of this paper was to investigate the effect of the whole building hygrothermal transfer (WBHT) process on the indoor air temperature and relative humidity, wall inner surface temperature and moisture flux, sensible heat load, latent heat load and total air-conditioning load in different climates. A WBHT model was developed and validated by the analytic solution and experimental data. Three humidity regions in China were classified for summer and winter according to the different outdoor air relative humidity levels. The correction coefficients of air-conditioning load of major cities were obtained accounting for the WBHT process. Our results would provide a future reference for accurately calculating heating and cooling load of buildings in different climate regions.

Potential of Solar Cooling in Latvian Conditions

The national priority axis is increase of RES use in Latvian Energy sector, so new solutions for Electricity, Heat and Cool production are searching for. In Latvian conditions cooling is required about 5000 degree hours per year (at an indoor temperature of 21°C), but the importance of outdoor air relative humidity ranging from 70-90%. Optimal relative humidity for human body is from 45 - 55%, if the moisture levels in the air are lower or higher, it would adversely affect the health, so it is important that humidity level is optimal in areas with a high human concentration. Solar energy is available at the same time when is need to cool rooms, so Solar Cooling are suitable also in Latvia. In Latvia such a system is not used, so it is important to assess the potential of this system. In the Institute of Physical Energetic Solar Energy Testing polygon the Solar Cooling system was installed with the cooling capacity 8kW in the facility, measuring equipment was connected and operating data have been monitored and technical equipment performance were improved. As in Latvian climatic conditions in the summer period (may -September) the average outside air temperature is about 15° C (max temperature of summer season average value + 26.38°C) and solar radiation - 1100 kWh/year, than the experimental equipment has operation temperature range of 55 -95° C in the driving circuit. An, evaluation of possible use, the advantages and disadvantages of the system, taking into account the reduction of fossil fuel use for cooling will be provided.

Energy performance and consumption for biogas heat pump air conditioner

Biogas engine-driven heat pump air conditioner is a new-style system which includes biogas engine-driven heat pump, primary heat exchanger, second heat exchanger, sprayed room and fans, pumps, etc. In summertime, the air can be reheated by the waste heat water from the biogas engine in the system, while the air can be reheated and humidified by the waste heat water in winter. Reducing or displacing electrical heating requirements can achieve the great opportunity for significant energy savings. This paper, therefore, aims to improve the energy performance of the AC system by using the waste heat from the biogas engine. The mathematic model was used to research the BHPAC. Explicitly, we investigated the influence of various factors including the outdoor air temperature and humidity in summer and winter. Results show that the biogas engine-driven heat pump air conditioner can save more energy than the electrical power heat pump. In summer, the minimum for percentage of primary energy saving for BHPAC is over 25%. With the outdoor air dry-bulb temperature and the relative humidity rises, the saving energy percentage rises. In winter, the minimum for percentage of primary energy saving for BHPAC is 37%. The more the outdoor air relative humidity of the outdoor air decreases, the more the BHPAC saves energy. It is proved that the system which is a highly actively fully utilizing energy technology has good partial load characteristic and good effects of energy saving.

Dynamic defrosting characteristics of air source heat pump and effects of outdoor air parameters on defrost cycle performance

The reverse-cycle defrosting characteristics of a 11.2 kW split-type air source heat pump (ASHP) were experimentally investigated under the defrosting conditions. Based on the experimental results, the effects of outdoor air parameters on defrosting cycle performance as well as the dynamic defrosting characteristics of the ASHP unit were analyzed. The experimental results indicate that with the increase of the outdoor air relative humidity at a constant air temperature and velocity, the total power consumption, defrosting time and endotherm from indoor room during defrosting period decreased linearly, and they also decreased as the outdoor air temperature increased, but the trends of the curves presented the concave-up. The duration of the defrosting period mainly depends on the wall temperature of outdoor heat exchanger, which is corresponding to the condensing pressure during the defrosting cycle. In this paper, the concept of total coefficient of performance (COP) is used to evaluate the performance of ASHP unit, and as the air temperature increased under the conditions of a fixed air relative humidity and air velocity, the total COP increased linearly, but it decreased linearly as the air relative humidity increased.