Application study of radiant floor compo site air-conditioning system for heating and cooling

Radiant floor air-conditioning systems are characterized by energy efficiency and comfort. However, at higher sensible heat loads and higher humidity conditions, problems such as condensation, poor comfort, slow thermal response, and difficulty in zoning control may occur. To mitigate these limitations, this study presents a composite air-conditioning system for radiant flooring that includes an air source heat pump., an underfloor coil, a fan coil and a dehumidifier for fresh outdoor air. The composite system was experimentally tested and analyzed for indoor thermal comfort under various operation modes in summer and winter in a specific thermal zone. On the basis of the experiments, the composite system comprises multiple energy supply points, and varying combinations between these functional endpoints can fulfill the comfort needs of the human body even in year-round working conditions. The radiant floor temperature varies from the dew point air temperature by roughly 7°C. Following the test day, summer radiant systems take approximately 30 minutes to reach the set indoor temperature and humidity level.. An hour after the indoor air has reached the desired temperature and humidity, the winter radiation system is activated.


Introduction
Radiant air conditioning systems often use the building envelope as a source of cold or heat, whereby heat is exchanged with indoor air by means of radiant heat and convection heat exchanges [1].The method of heat transfer based on radiant heat transfer has been proven to markedly enhance the thermal comfort experienced by individuals [2].Compared to conventional air conditioning while preserving equivalent indoor thermal comfort, radiant air conditioning systems maintain higher interior design temperatures in summer and lower interior design temperatures in winter.This is among the factors contributing to its energy efficiency [3].To minimize the possibility of condensation on radiant panels, some studies have recommended combining dry ventilation to reduce the air humidity [4], or installing dehumidification coils [5], or using infrared transparent covers [6].

System Introduction
The composite system's operational flowchart, as examined in this paper, is illustrated in Figure 1.The control strategy of the terminal equipment is as follows: the system starts up, the fan coil operates in a high air volume mode and the indoor air is quickly processed while the radiant floor starts to operate.As the radiant floor carries the main cooling and heating load , the fan coil's air volume gradually decreases.The fresh air dehumidifier operates as per the set indoor humidity level.

Parameters of the main heating and cooling equipment
This paper studies the composite floor radiation air conditioning in the fresh air dehumidifier, radiation floor and fan coil composition and heat exchange with indoor air to provide cold heat, while the air source heat pump to produce cold heat.Outdoor fresh air dehumidifier air volume of 500 m 3 / h.Floor coil tube diameter of 1 cm with a tube spacing of 5 cm.so that small tube diameter, small spacing to achieve a greater unit area of heat transfer.The fan coil adopts the side-feeding and down-return mode.

Experimental program and experimental apparatus
Multiple monitoring points were set up to observe air temperature and humidity inside and outside the experimental room, indoor air flow rate and indoor radiant temperature.Additionally, heat flux sensors have been installed in the floor center and on each wall to measure surface temperature and heat flux.Lastly, a black ball thermometer was included to determine the radiant temperature within the space.The data logger DaqPRO 5300 was used to gather all test data every 600 seconds.The temperature and humidity recorder is situated outside, away from direct sunlight, to document external air temperature and humidity data.The information on the summer and winter models can be found in Table 1.

Experimental program
Experimental tests and data analysis were conducted on a hybrid air conditioning system with radiant flooring in both summer and winter.The experiments were conducted during standard working hours, with the system operating between 9:00 am and 5:00 pm.To achieve natural heating/cooling, the window was to be left open a day prior to the experiment.Six lights, a computer, and a person were employed as internal heat sources.The indoor temperature is regulated to 27°C/21°C during the summer and winter months, respectively, while maintaining an indoor humidity level of 60%/45%.

3.3.1.
Outdoor temperature and humidity on test days in summer and winter.During Changsha's summer experiment, the outdoor temperature ranges from 26.5-34.0℃and a need for cooling arises.
Similarly, during the winter experiment, the outdoor temperature drops to 3.7-9.7℃,requiring heating.

3.3.2.
Indoor temperature and humidity on test days in summer and winter.Figure 2 and Figure 4 illustrate alterations in indoor temperatures during the winter and summer months respectively.Contrarily, Figure 3 and Figure 5 illustrate variations in indoor humidity levels during the summer and winter seasons respectively.Indoor relative humidity in winter Winter mode 1 for the radiant floor alone operating conditions, the indoor air temperature warming is slow, in 13:40 or so the indoor temperature reached 21℃, the experimental test period, the average indoor temperature of 20.7℃, in this mode, the indoor relative humidity is low, the average relative humidity is only 19.9%, the indoor air is relatively dry, indicating that floor heating alone is difficult to meet the indoor transient heat load demand quickly.During summer mode 1 and winter mode 2, the fan coil operates under specific conditions.In summer mode 1, the indoor air temperature decreases rapidly to below 27℃ within 30 minutes of system startup and remains at around 25.5℃, but the indoor humidity is high at 69%.In winter mode 2, the temperature rises quickly to 21℃ about an hour after system startup and then stays at around 21℃.The results demonstrate the fan coil's efficient handling of sensible heat load.Summer Mode 2 and Winter Mode 3 are combined fan coil and radiant floor operation modes.In summer mode 2, the system indoor air temperature dropped rapidly to below 27°C within 30 minutes and then remained around 25.5°C.However, the indoor humidity remains high at 69.5%.During winter mode 3, the system warms up quickly after startup, reaching 21°C after 1 hour and then gradually rising to an average temperature of 22.6°C.The fan coil unit's high airflow operation initially enhances the radiant floor system's response in comparison to winter mode 1.The radiant floor and fresh air dehumidifier operate jointly under the conditions of summer mode 3 and winter mode 4.The indoor temperature in summer mode 3 is about 27.8°C, and the cooling capacity is still insufficient.Winter mode 4 is similar to winter mode 1, with slower warming after system startup, and the indoor temperature reaches 21°C in 2 hours of the system.However, the addition of fresh air dehumidifier makes the indoor relative humidity between 40% and 60% in summer and winter to meet the human comfort requirements.Summer mode 4 and winter mode 5 are joint operation conditions for the fan coil, radiant floor, and fresh air dehumidifier.When in summer mode 4, the temperature indoors drops rapidly to 26.4℃ just 30 minutes after the system is started.In winter mode 5, however, the system rapidly raises the temperature indoors to 21℃ an hour after startup.The system responds quickly in this operating condition, and the humidity meets human comfort requirements.A thorough examination of the combined operation of summer and winter modes, fan coil, radiant floor, and fresh air dehumidifier enhances the system's response speed and increases its heat load handling capacity.Additionally, it ensures that indoor humidity remains within the human comfort range.

Vertical temperature differential in indoor air.
Air temperatures were recorded at varying vertical positions during the stable stabilization stage of the indoor environment.The recording was conducted in various system modes, which can be visualised in Figure 6 and Figure7.The vertical dissatisfaction rate in the thermal comfort standard, ISSO 7730, should be applied to analyze the strength of local discomfort that may arise from vertical temperature differences.The vertical dissatisfaction rate should be computed for summer and winter months, as shown in Table 2 and Table 3. Please refer to the equation for the calculation formula.(1)shows, Vertical dissatisfaction rate., is Vertical air temperature difference between head and ankle.
Figure 6 and Table 2 demonstrate that during summer conditions, there is an uplift of hot air, resulting in higher indoor air temperatures the further away from the ground one measures.The vertical peak temperature differences for modes 1, 2, 3, and 4 are 0.39, 0.52, 1.05, and 0.75.The vertical temperature difference in modes 1, 2, and 3 is smaller than in mode 3 due to the disruptive impact of the fan coil outlet air on the indoor air.Further, the dissatisfaction rate while sitting or standing in modes 1, 2, and 4 is lower than in mode 3.This implies that in summer cooling conditions, the fan coil has a negative impact on reducing the vertical temperature difference of indoor air.As shown in Figure 7 and Table 3, for winter conditions, the vertical peak temperature differences for Modes 1, 2, 3, 4, and 5 are 0.03, 0.38, 0.11, 0.17, and 0.14.Due to the inclusion of the radiant flooring, Modes 1, 3, 4, and 5 all have lower vertical peak temperatures, seated vertical dissatisfaction rates, and standing vertical dissatisfaction rates than Mode 2, which is operated with the fan coils alone.in particular, Mode 1 vertical peak valley temperatures, sitting and standing vertical dissatisfaction rates were the lowest.In wintertime, the use of radiant floor air conditioning exclusively can result in a decreased temperature gradient indoors, demonstrating radiant floor air conditioning's heating benefits for the season.

Risk of condensation and consumption simulation analysis
In areas with warm summers and chilly winters, outdoor air with high temperatures and humidity heightens the probability of condensation on radiant slabs when utilizing radiant air conditioning cooling configurations.The changes in air dew point temperature and surface temperature of the radiant floor during summer operation mode 4 are displayed in Figure 8.The average temperature of the floor that emits radiation and the average temperature at which air reaches the dew point are 23.56°C/16.71°C,respectively.The discrepancy between the two is almost 7°C.To mitigate floor condensation, implement PID-based mixing valves to regulate water supply flow to the radiant floor for temperature control.Complementarily, employ fresh air dehumidifiers to reduce indoor air moisture.
The study analysed a composite system comprising of radiant floor cooling, fan coil, and fresh air dehumidifier, using OpenStudio software in summer mode 4. As per Figure 9, the hourly average energy consumption of heating and cooling sources, fan coils, pumps, and fresh air dehumidifiers was found to be 2.61 kWh, 0.04 kWh, 0.18 kWh, and 1.31 kWh, respectively.The total average energy consumption amounted to 4.14 kWh.The air source heat pump, which serves as the cooling and heating source, exhibited the highest power consumption of all the equipment.It was followed by the fresh air dehumidifier.This highlights the significance of accounting for the power consumption of fresh air units when integrating them into air conditioning systems.

Conclusion
Condensation, poor comfort, slow thermal response, and difficulty in zoning control were resolved through the use of radiant floor air conditioning.During summer and winter experiments of the radiant floor composite air conditioning system, fan coil units were utilized to enhance system response rates.Meanwhile, a fresh air dehumidifier regulates the latent heat in the room, while the radiant floor air conditioning ensured thermal comfort in the thermal zones.

Figure 2 . 3 .
Figure 2. Indoor air temperature in summer Figure 3. Indoor relative humidity in summer

Figure 4 .
Figure 4. Indoor air temperature in winter Figure 5.Indoor relative humidity in winter Winter mode 1 for the radiant floor alone operating conditions, the indoor air temperature warming is slow, in 13:40 or so the indoor temperature reached 21℃, the experimental test period, the average indoor temperature of 20.7℃, in this mode, the indoor relative humidity is low, the average relative humidity is only 19.9%, the indoor air is relatively dry, indicating that floor heating alone is difficult to meet the indoor transient heat load demand quickly.During summer mode 1 and winter mode 2, the fan coil operates under specific conditions.In summer mode 1, the indoor air temperature decreases rapidly to below 27℃ within 30 minutes of system startup and remains at around 25.5℃, but the indoor humidity is high at 69%.In winter mode 2, the temperature rises quickly to 21℃ about an hour after system startup and then stays at around 21℃.The results demonstrate the fan coil's efficient handling of sensible heat load.Summer Mode 2 and Winter Mode 3 are combined fan coil and radiant floor operation modes.In summer mode 2, the

Figure 6 .
Figure 6.Vertical indoor temperature distribution in summer

Figure 8 .Figure 9 .
Figure 8. Indoor air dew point temperature and floor temperature

Table 1 .
System operating mode

Table 2 .
Vertical dissatisfaction rate

Table 3 .
Winter vertical dissatisfaction rates