Performance analysis of the different radiant floor cooling systems in the office building in Shandong province, China

. Radiant cooling systems have good application prospects. A variety of composite radiative cooling systems have been proposed to save energy, but few studies have focused on comparing the performance of different radiant floor cooling systems. This study compares the cooling performance operating conditions of a floor radiant system with direct ground cooling and conventional radiant cooling systems. In addition to the comfort conditions required by the standard, Jinan, Shandong also studied the low risk of condensation on the floor surface. At the same time, the energy consumption of different radiative cooling systems is compared and the potential of the system is analyzed. Preliminary results found a 26% reduction in energy consumption for system operation compared to conventional radiant cooling systems. Although the radiant floor system using the ground direct cooling system does not reduce energy consumption when the water tank is turned on for energy storage at night, the electricity cost is reduced due to the peak and valley electricity price at night. The research will provide architects and engineers with more efficient and energy-efficient technologies that reduce carbon emission.


Introduction
Space cooling accounted for nearly 16% (about 1,885 TWh) of final electricity consumption in the building sector in 2020.Space cooling consumption has been on the rise and without action, global electricity demand for building cooling could increase by as much as 40% by 2030 [1].An effective solution is to optimize the indoor environmental control and energy utilization of the cooling system, which can realize the energy-saving potential under the conditions of maintaining thermal comfort in the building, thereby restraining the impact of rapidly rising cooling demand.
Among all kinds of air conditioning systems, the radiant floor cooling system has been widely concerned because of its high thermal comfort and energy savings.The radiant cooling systems employ active cooling surfaces to remove indoor sensible heat loads by radiation and convection, usually circulating cooling water to maintain the radiant surface temperature [2].Xiang et al [3] compared the cooling capacity of the radiant floor cooling system with that of the all-air system in an environment of high solar radiation and proved that the radiant floor cooling system can greatly improve the terminal cooling capacity.Radiant cooling systems are currently mostly used in public buildings.To ensure indoor comfort, the cooling water temperature of the radiation system is high, which improves the efficiency of the chiller.In addition, the hightemperature cooling water can make full use of natural * Corresponding author: jxl83@sdjzu.edu.cncold sources, which further improves the energy saving potential of the system.Shallow geothermal energy, as a kind of renewable energy, has been widely studied, because it keeps the same temperature at a certain depth, under the premise of ensuring the balance of land heat exchange, it can continuously provide free cooling for buildings in summer through the circulating water of underground heat exchangers, and has great potential in energy saving and improving indoor thermal comfort [4].There have been a lot of studies on radiant cooling with a direct-ground cooling source system with good application prospects.Romani et al. [5] Found experimentally that a radiant wall system combined with a floor heat exchanger shows high energy savings potential in the cooling mode.In addition, the radiant wall shows potential for peak load shifting and nighttime precooling.Arghand et al. [6] Simulated that for direct ground cooling systems, the temperature control method makes the indoor air temperature more stable and more likely to prevent condensation risk than the water flow control method.Although many studies have shown that radiant cooling with a direct-ground cooling source system has great potential to maintain the thermal comfort of the building under good control.But in reality, the application of the system in China is still at a low level [7].Thermal comfort and performance analysis of the system is still necessary.
Taking Jinan, Shandong Province as an example, this paper simulates and compares the indoor operating temperature (Top), PMV (Predicted Mean Vote) and https://doi.org/10.1051/e3sconf/202235601047E3S Web of Conferences 356, 01047 (2022) ROOMVENT 2022 local percentage dissatisfied caused by floor surface temperature.and condensation time of the direct supply radiant floor cooling system and the conventional floor radiant cooling system in an office building under temperate climate, and compares the energy consumption and electricity cost. of the three different system cases.It provides a reference for the engineering application of the radiant floor cooling system with ground heat exchanger.

Case introduction
TRNSYS (Transient System Simulation Program) is used to simulate three different radiant floor cooling systems.Case 1 is the direct supply radiant floor cooling system with ground heat exchanger, case 2 is the direct supply radiant floor cooling system with ground heat exchanger using water tank, and Case 3 is the conventional radiant floor system.The schematic diagram of the three cases is shown in Fig. 1.In case 1, the high temperature chilled water is supplied to the floor radiant system by the direct supply system with ground heat exchanger, and the average water supply temperature is 20℃, and the chilled water of 7℃ is supplied to the fresh air unit by the heat pump unit.In that second case, the high-temperature cold wat is supplied to the floor radiation system by the ground pipe direct supply system, the average wat supply temperature is 20℃, the fresh air unit is provided by the water tank, and the water tank store the 7℃ chilled water produced the heat pump.The cold source of case 3 is two ground source heat pumps, one of which provides 16℃ cold water for the floor radiation system, and the other provides 7℃ chilled water for the fresh air unit.The three cases' ground source heat pump units extract high-temperature cooling capacity from underground buried pipes.The case operation strategy is shown in Table 1.The floor radiation system of case 1 and case 2 operates all day.The floor radiation system of case 3 operates from 7:00 to 18:00.Due to the large thermal inertia of the building, it starts 2 hours in advance.In all three cases ventilation systems operate from 9:00 to 18:00.The supply air temperature of each case is set to 20°C, and the relative humidity is about 60%.To deal with the fluctuation of indoor load, a simple control strategy is set.Monitor the indoor air temperature.When the indoor temperature is greater than 27°C, turn on the cooling, and when the indoor temperature is less than 25°C, turn off the cooling.The building air supply parameter is 30 m 3 / (h•person).According to the operation strategy of the case shown in Table 1, a system simulation model is built in TRNSYS.

Building model
The building model used in the simulation is shown in Fig. 2. The building is an office building with a floor height of 3 m and a total of five floors.The building area is 5000 m 2 and the shape coefficient is 0.25.The building model envelope structure information is shown in Table 2 and the heat gain in the building is set to light 15 (W/m 2 ), equipment 9 (W/m 2 ) and personnel 10 (m 2 /person), and meet the "Design Standard for Energy Efficiency of Public Buildings" (GB 50189-2019) [8].
In the simulation process, the climatic conditions from July to August in Jinan were taken as an example.Cases 1 and 2 were directly supplied by the buried pipe direct supply system to the ground radiation system.. Regardless of pipe heat loss and temperature fluctuations, the simulation time step is 15 minutes.

Thermal comfort analysis
This paper selects three parameters to compare and analyze different radiant floor cooling cases, namely operating temperature, PMV and local percentage dissatisfaction caused by floor surface temperature.It is assumed that the labor intensity of the personnel in the office building is level I (light labor), the people in the room are sitting, the metabolic rate is 69.78 W/m 2 , and the clothing insulation is 0.08 m 2 •K/W.The parameters of the three cases all meet the comfort requirements.
Since case 1 and case 2 have the same control results in the room, only case 1 and case 3 are analyzed in detail.

Operate temperature analysis
Fig. 3 shows the variation of the indoor top of the building on August 1.The variation trend of the indoor operate temperature of both cases is the same, but the fluctuation of case 3 is smaller, specifically, the top is lower in the daytime and higher at night.The Top of case 1 was lower from 9:00 to 12:00, and the Top of case 3 was lower from 12:00 to 18:00, but the difference between the two cases was within 1℃.This is due to the intermittent control of case 3, and the lower temperature of the water supply of the floor radiant cooling system during working hours.

PMV analysis
Fig. 4 shows the indoor PMV changes in buildings on July 24.Since the outdoor temperature on that day was at a high level, the indoor PMV values of the two cases were greater than 0 from 9:00 to 18:00.Case 1 has an obvious difference in indoor temperature between day and night.This is also because the sensible heat load borne by the floor radiant system in case 1 is smaller than that in case 3.During the simulation, Case 3 always kept the PMV in the range of -0.4~0.4.The thermal inertia of the building causes Case 3 to have a high PMV value between 8:00 and 12:00.

Local percentage dissatisfied caused by floor surface temperature analysis
Fig. 5 shows the floor surface temperature on July 14.The average floor temperature of case 1 is 21.3℃, which is 0.3℃ lower than that of case 3, and the fluctuation of floor surface temperature is smaller.As shown in Fig. 6.
The local dissatisfaction rate caused by the floor surface temperature of the two cases is all less than 10%.

Condensation analysis
To avoid condensation on the floor surface, the temperature of the radiation surface should be 1~2°C higher than the dew point temperature of the indoor air.According to statistics, case 3 can meet the requirements, case 1 has a temperature difference of less than 1℃ in 6.8h.Fig. 7 is a graph showing the variation of the floor surface temperature concerning the air dew point temperature on July 4. On that day, the average outdoor temperature was 18.2 ~ 27.8℃ and the relative humidity was 54 ~ 94%.Since the indoor sensible heat load is small from 5:00 to 7:00 in the morning, the continuous radiant floor causes the indoor air temperature to decrease.At this time, the ventilation system does not start to operate, the dew point temperature is high, and there is a risk of condensation.When the floor radiant system stops operating at night, the risk of condensation at night is greatly improved.

Analysis of the energy saving potential of the cases
Fig. 8 shows the distribution of each case's energy consumption and electricity cost.The energy consumption of cooling system is divided into two parts: floor radiation system and ventilation system.The ventilation system plays an important role in the whole cooling system, and its energy consumption is relatively large.Because case 2 increases the water storage of water tank, it will increase the energy consumption of water pump, resulting in high energy consumption of ventilation system.Although case 3 has the least operating time, the floor radiant system of case 3 bears more room load, so case 3 increases the energy consumption of the radiant system by 47% compared to option 1.However, the cold storage system will save electricity costs, which is 32% lower than that of case 3.There are still some deficiencies in the analysis of cases.The article only compares the thermal comfort of the whole room and does not analyze the PMV change of the occupied area.This paper mainly focuses on the analysis of the simulation results, and the next step will consider increasing the experimental assistance to improve the reference value of the actual project.

Conclusions
The buried pipe direct supply floor radiant cooling system can directly use the natural cooling source to significantly reduce the energy consumption of the system, and at the same time there is still great potential maintaining indoor thermal comfort.
(1) The indoor comfort of case one is better than that of case one in the morning.
(2) Because of the large temperature difference between day and night in Jinan in summer, the floor radiation refrigeration can be intermittently adjusted according to the indoor thermal conditions, to avoid indoor supercooling, reduce the risk of indoor condensation and save energy.
(3) Under the condition of ensuring indoor thermal comfort, the energy consumption of case 1 is the lowest, which is 26% lower than that of case 3. Option 2 brings the lowest electricity bill due to the peak and valley electricity price in Jinan.

Fig. 3 .
Fig. 3. Changes in the indoor operating temperature of the building on August 1.

Fig. 6 .
Fig. 6.Distribution map of local percentage dissatisfied caused by floor surface temperature.

Fig. 7 .
Fig. 7. Changes in the difference between floor surface temperature and air dew point temperature on July 4.

Fig. 8 .
Fig. 8. Distribution of system energy consumption and electricity cost in each case.

Table 1 .
The case operation strategy.