Radiant Terminals Research Articles

Experimental and numerical study on thermal performance of energy storage interior wall with phase change materials

Phase change materials (PCM) and embedded tube radiant terminals demonstrate considerable advantages with respect to heat storage, energy savings, and the provision of comfort in buildings. This paper puts forth the concept of an energy storage interior wall (ESIW) with embedded pipe radiant technology, comprising PCM, and coupled with low-grade energy sources. Compared to traditional TABS, this system uses PCM energy storage to compensate for the instability of solar energy supply, which expands the application scenarios of clean energy. At the same time, it can greatly improve the thermal mass of the building and provide cooling and heating for multiple rooms. Experimental results demonstrate that the ESIW is capable of markedly enhancing the thermal comfort and indoor temperature, with an average increase of 9.9 °C relative to the outdoor. In the numerical study based on test data, sensitivity analysis was performed on 10 characteristic parameters of the ESIW structure: wall thickness, wall density, wall thermal conductivity, wall specific heat capacity, PCM phase change temperature, PCM pipe diameter, PCM enthalpy, PCM pipe length, pipe flow diameter, and number of rows of PCM pipes. The results show that the key parameters affecting the first objective (thermal storage capacity) are: pipe flow diameter, wall density, wall thickness, and PCM pipe diameter; and the key parameter affecting the second objective (investment cost) is the diameter of the PCM pipe. After multi-objective optimization for these two objectives, the thermal storage capacity of the ESIW was improved by 96.7 %, and the investment cost was reduced by 11.49 %.

A simulation study on the heating characteristics of residential buildings using intermittent heating in Hot-Summer/Cold-Winter areas of China

Radiant air conditioning system is an important heating method with better thermal comfort, less noise and greater energy-saving potential. However, different intermittent heating modes may significantly impact both the heating efficiency and indoor thermal comfort, which has been neglected in existing studies. Therefore, to obtain optimal intermittent heating modes for achieving higher energy efficiency and better indoor thermal comfort for the residential buildings in Hot Summer and Cold Winter areas, a TRNSYS model of the radiant air conditioning system was developed and validated. The indoor thermal comfort and energy consumption characteristics during the coldest day and entire heating season were simulated and analyzed based on four different intermittent heating modes. The results demonstrated that the supply water temperature of the air source heat pump unit and the radiant terminals stabilized at the set temperature in about 1 h during the start-up stage, and the indoor air temperature stabilized at 24.0 °C after 7.5 h during the shutdown stage. The proportion of time that meets first-level thermal comfort under the four intermittent heating modes was above 90.1% and even higher at over 96.1% when considering the sleeping period. Compared to the continuous heating mode, the four intermittent heating modes saved 22.3%, 25.8%, 40.4% and 48.4% of energy. Overall, the preferred intermittent heating mode for residential buildings with the radiant air conditioning system in Hot Summer and Cold Winter areas was identified as switching on from 7:00 to 15:00 and 19:00 to 3:00, while switching off from 15:00 to 19:00 and 3:00 to 7:00. This model not only considered the immediate benefits of improved indoor thermal comfort and energy efficiency, but also showed great advantages in developing sustainable construction.

Developing a Practical Thermal Performance Index for Radiant Terminals—Structural Thermal Resistance

Radiant terminals have been widely applied in heating and cooling systems. However, few existing thermal performance evaluation indices can reflect the influence of structural forms on heat transfer performance. This study introduces the structural thermal resistance (Rs) to rapidly evaluate the structure form’s effects. First, theoretical analysis and experimental tests were introduced. Three types of terminals, including the copper conduit graphite plate (CCGP), plastic tube-embedded metal plate (PTMP), and capillary network-embedded structural plate (CNSP) were tested in the laboratory. Then, the CNSP terminals were taken as validation examples. The results show that the Rs values of the same type of radiant terminal tend to be stable and constant. The variations in Rs within the same type of radiant terminals were small both under cooling and heating conditions. Only when the terminal structure changed, the Rs would change. This suggests that the Rs can reflect the complex heat transfer processes inside the radiant terminals while distinguishing different terminal types. The validation analysis showed an average relative error of 3.4% and 2.9% for cooling and heating, respectively. Lastly, the potential application of Rs in practical applications was discussed, and a Python-based online tool was developed to help design, operate, and evaluate radiant terminals.

Developing a TRNSYS model for radiant cooling floor with a pipe-embedded PCM layer and parametric study on system thermal performance

The design of the radiant cooling floor with a pipe-embedded PCM layer (RCF-PEPCML) lacks a relevant reference basis. Simulation is the best way to determine the optimal design of the system. The simulation research of PCM radiant terminals has been developed, but there is still a need to establish a convenient transient simulation method for RCF-PEPCML, in order to conduct various optimization studies on system design. In order to meet this simulation requirement, a two-dimensional heat transfer model for a pipe-embedded PCM layer (PEPCML) was established. The numerical model was written in FORTRAN language and integrated into TRNSYS to create a new module (TYPE207) for simulating a PEPCML. The accuracy of the new module was verified by experiments conducted in a scaled experiment room with radiant floor cooling. Then, using the newly-built module and TRNSYS, a case study was conducted to explore the effect of several key parameters on the thermal performance of the RCF-PEPCML. The influencing parameters considered include the PCM's phase change temperature, phase change latent heat, and the PCM layer thickness. The results of univariate sensitivity analysis show that these three parameters all greatly affect the thermal performance of RCF-PEPCML. The recommended optimal phase change temperature should meet this condition where the lower limit of the phase change temperature interval is higher than the water supply temperature by 0–1 °C. The optimal phase change latent heat and optimal PCM layer thickness depend on the specific design conditions of the cooling terminal. It is suggested to perform optimization design analysis for specific cases through simulation. The orthogonal test analysis of multiple parameters shows that the optimal design parameters combination for the case study is the combination of a phase change temperature of 20 °C, a phase change latent heat of 160 kJ/kg, and a PCM layer thickness of 15 mm. Among the three influencing factors, the phase change temperature should be given priority.

Field studies on indoor thermal environment and start-stop characteristics of the radiant air conditioning system during intermittent heating

With the improvement of social economy, the heating demand of residents in Hot Summer and Cold Winter (HSCW) areas in China is increasing. The radiant air conditioning (A/C) system, is popular as a heating way due to its high comfort, low noise and high energy saving potential. There is still a lack of field research on the system under the intermittent heating mode in the HSCW area. To provide a guide for the operation mode of the radiant A/C system during intermittent heating in HSCW areas, this paper focused on the radiant A/C system with the fresh air system, and then studied system's practical operational parameters and dynamic characteristics in winter. The indoor and outdoor air temperatures and relative humidity, the water temperatures of inlet and outlet of the air source heat pump (ASHP) unit, the supply and return water temperatures of radiant terminals, as well as supply air temperature were analyzed in three stages, i.e., the initial start-up stage, the intermittent shutdown stage and the intermittent start-up stage. The results show that, it took 7 h and 50 min for the indoor air temperature to reach the set indoor temperature during the initial start-up stage. Compared to the initial start-up stage, the intermittent start-up stage reduced the time for the indoor air temperature to reach the set indoor temperature by 4 h and increased the indoor air temperature rise rate by 97%. The indoor air temperature rise rate was 0.46 oC/h and 0.91 oC/h, during the initial start-up and the intermittent start-up stage, respectively. The indoor air temperature decrease rate was 0.23 oC/h during the shutdown period. During the entire winter season, the ASHP unit switched on and off intermittently 10 times. The longest shutdown lasted for 111 h, and the indoor air temperature dropped to 17.2oC.

Novel Radiation-Adjustable Heating Terminal Based on Flat Heat Pipe Combined with Air Source Heat Pump

The electrification of building heating is an effective way to meet the global carbon target. As a clean and sustainable electrified heating technology, air-source heat pumps (ASHPs) are widely used in areas lacking central heating. However, as a major component of space heating, heating terminals might not fit well with ASHP in order to achieve both intermittency and comfort. Therefore, this study proposes a novel radiation-adjustable heating terminal combined with an ASHP to achieve electrification, intermittency, and better thermal comfort. Radiant terminals currently suffer from three major problems: limited maximum heating capacity, inability to freely adapt, and difficulty with combining them with ASHPs. These problems were solved by improving the structural design of the novel terminal (Improvement A–E). Results showed that the maximum heating capacity increased by 23.6% and radiation heat transfer ratio from 10.1% to 30.9% was provided for users with the novel terminal. Further, new flat heat pipe (FHP) design improved stability (compressor oil return), intermittency (refrigerant thermal inertia), and safety (refrigerant leakage risk) by reducing the length of exposed refrigerant pipes. Furthermore, a new phased operation strategy was proposed for the novel terminal, and the adjustability of the terminal was improved. The results can be used as reference information for decarbonizing buildings by electrifying heating terminals.

Experimental investigation on condensation characteristics of a novel radiant terminal based on sepiolite composite humidity-conditioning coating

The problem of condensation hinders the application and popularization of the radiant cooling system. According to the characteristics of temperature and humidity independent control of the radiant cooling system, a novel humidity-conditioning radiant terminal combined with solid humidity-conditioning material is proposed which can control temperature and humidity simultaneously, and a sepiolite composite humidity-conditioning coating (SCHCC) suitable for the radiant terminal with good humidity control ability in high relative humidity environment is prepared. The humidity-conditioning coating can adsorb water vapor in the air to reduce the risk of condensation that is different from the surface condensation of traditional radiant terminals. Then, the condensation and desorption characteristics of three humidity-conditioning boards are tested which is considered scaled-down humidity-conditioning radiant terminal. The results show that SCHCC has good humidity control ability in a high relative humidity environment, the moisture adsorption capacity of SCHCC between 85% RH and 95% RH is 0.208 g/g, which is 11.43 and 22.47 times that of gypsum and diatomite. The sepiolite based humidity-conditioning board (SHCB) with the best moisture adsorption characteristics has the longest condensation time, and the condensation time increases with the decrease of undercooling. SHCB also shows better humidity control ability even for the experimental condition without condensation within 300 min. In summary, SHCB can reduce the moisture content of indoor air through its excellent hygroscopic properties, and reduce the risk of condensation at the radiant terminal. • A novel humidity-conditioning radiant terminal is proposed. • The material has good humidity control ability under high relative humidity. • The condensation and desorption behavior of the novel radiant terminal is tested. • The novel radiant terminal can effectively prolong the surface condensation time.

Thermal performance investigation of a novel heating terminal integrated with flat heat pipe and heat transfer enhancement

Comfort heating with fast thermal response rate, which is applicable for intermittent heating, requires of small thermal inertia and a large heat transfer coefficient for heating terminals. However, existing heating terminals are generally inconsistent in terms of the response rate and thermal comfort. Therefore, a novel radiant-convective heating terminal integrated with a flat heat pipe and heat transfer enhancement is proposed in this research. The flat heat pipe, which acts as the radiant panel, provides quick thermal response and high radiation uniformity. Additionally, fins are attached to the back of the flat heat pipe to enhance the heat transfer ability via both natural and forced convection. The heating performance, including the response rate, radiation temperature, heating power was studied experimentally, and influential factors, such as heat source temperature, ambient temperature, and forced convection, were also considered. The results suggested that the heating terminal had a response rate of 920–1125 s, which is superior to other conventional radiant terminals, and the surface temperature distribution exhibited high uniformity. Furthermore, the heating power in the natural convection mode was approximately 498–724 W at the heat source temperature of 50 °C. When the forced convection was activated, the heating power and exergy efficiency increased by up to 62% and 25% respectively. Overall, the novel flat heat pipe heating terminal showed great potential for intermittent heating applications.

Inverse design of indoor radiant terminal using the particle swarm optimization method with topology concept

Radiant air conditioning system could create thermal comfortable and energy saving indoor environment. However, previous studies indicated that the design scheme, such as the area, geometry, and installation position, of the radiant terminals have significant effects on indoor thermal environment. That is because the current radiant terminal design method is a trial-and-errors process, and this time-consuming method is very difficult to achieve the optimal design. To fix that problem, this study tried to develop an efficient inverse design method, the particle swarm optimization (PSO) method combined with the topology optimization, which can inversely identify the area, geometry, and installation position of the modularity radiant terminal based on the requirements. This investigation also explored the impact of some key factors on the inverse design results and finally the proposed method was used to design modularity radiant terminals for a multi-occupant's office. The results indicated that the proposed PSO method with the topology optimization could help with the design of high energy-efficiency radiant air conditioning system and the inversely identified modularity radiant terminal could create thermally comfort indoor environment.

Performance analysis and instant/delayed characteristics of a solar heating system used in cold regions

Solar energy is widely used to meet the building heating energy demand in the cold region. Performance of the solar heating system is significantly influenced by the heating terminal. This paper firstly makes a comprehensive comparison between the air supply system and the radiant floor system in solar heating circumstance. A hybrid solar heating system combining the advantages of convective and radiant terminals is proposed to achieve 24-h diurnal heating with daytime limited insolation. Hot air heated by a solar collector is firstly delivered to the hollow cores in the radiant floor. After passing through the cores, the air subsequently enters the room, while room air returns to the collector to complete the cycle. Simulation models are built to analyze the performances of different air-based solar heating systems. It is indicated indoor operative temperature (Top) of the proposed system could be controlled between 18.0∼20.9 °C, where the average Top is 3.4 °C and 1.0 °C higher than those in the air supply system and radiant floor system respectively. It's with superiorities in both indoor temperature satisfactory and solar utilization efficiency. Influences of different parameters such as floor structure, solar collecting efficiency and air flow rate are also investigated. This research preliminarily shows the probability of this combined solar heating system for cold regions with sufficient solar radiation.

A simulation study for evaluating the performances of different types of house-hold radiant air conditioning systems

As one potential solution of independent temperature and humidity control in small-to-medium scaled residential buildings, systems of outdoor air dehumidifier with radiant terminals attracted lots of researchers’ interests. In this paper, three types of house-hold radiant air conditioning (A/C) systems, a conventional air source heat pump (ASHP) based, a high-temperature ASHP based and a novel variable refrigerant flow rate (VRF) based radiant A/C system, were proposed. Simulation models for three systems were established using Energy Plus. A prototype of the VRF based radiant A/C system was built and tested in an existed psychometric room. Testing results were compared with the simulated results showing that the average relative errors between them were less than 8%. Therefore, simulation studies were carried out to evaluate the energy performances of the three types of house-hold radiant A/C systems. Compared to the conventional ASHP based system, the high-temperature ASHP based system achieved 14.99% annual energy saving and the VRF based system 19.34%. Therefore, the VRF based system achieved the best energy efficiency, providing a promising solution for the house-hold radiant A/C system to realize the independent control of temperature and humidity in small-to-medium scaled residential buildings.

The development and experimental performance evaluation on a novel household variable refrigerant flow based temperature humidity independently controlled radiant air conditioning system

The present temperature humidity independent control (THIC) air conditioning (A/C) systems are not that applicable in small-to-medium scaled residential and office buildings for their complex configuration and additional requirement of regenerating heat. In this paper, a novel house-hold VRF based radiant A/C system adopting outdoor air dehumidifier (OAD) was developed and experimentally tested. This novel system employed the VRF system, applying one refrigerant cycle to the OAD to cooling and dehumidifying the outdoor air and another to the Refrigerant-Water plate heat exchanger (RWPHE) to providing chilled water for the adopted radiant terminals. Therefore, the supplying outdoor air from the OAD handled the indoor latent cooling load and the radiant terminals the indoor sensible load, so as to realize the independent control of indoor air temperature and humidity. Experiments were carried out at different outdoor air states for examining the performance of this novel THIC system. The experimental results showed that the supply air temperatures were controlled at proper levels with a moderate fluctuation below 2°C and the supply air humidity ratios were within 8–10g/kg, meeting the requirement of ASHRAE’s recommendation. Therefore, the results suggested promising performance of the developed system in temperature and humidity control.

Performance comparison of capillary mat radiant and floor radiant heating systems assisted by an air source heat pump in a residential building

The radiant heating system assisted by an air source heat pump has been widely applied in China for its effective energy conservation, high comfort performance and flexible utilization. Because the coefficient of performance of the system is strictly controlled by the supply water temperature heated by the air source heat pump, an efficient radiant terminal with low-temperature supply water is of significance to the coefficient of performance. In this research, the energy-saving feature of the capillary mat radiant heating system was first proved theoretically based on the influence of the heat transfer temperature difference on the coefficient of performance of the air source heat pump. In order to compare the performances of the capillary mat radiant and floor radiant heating systems, an experiment platform of two different radiant terminals assisted by an air source heat pump was established in a residential building in Xi’an, China. Experimental results showed that, to satisfy the indoor heating requirements, the supply and return water temperatures ought to be 35.0℃ and 30.9℃, respectively, and for the capillary mat radiant heating system, 43.9℃ and 38.8℃, respectively, for the floor radiant heating system. However, the electricity consumption of the capillary mat radiant heating system is 45% less than that of the floor radiant heating system. Thus, our study validated the energy-saving potential of the capillary mat radiant heating system assisted by an air source heat pump.

Simplified Building Thermal Model Used for Optimal Control of Radiant Cooling System

MPC has the ability to optimize the system operation parameters for energy conservation. Recently, it has been used in HVAC systems for saving energy, but there are very few applications in radiant cooling systems. To implement MPC in buildings with radiant terminals, the predictions of cooling load and thermal environment are indispensable. In this paper, a simplified thermal model is proposed for predicting cooling load and thermal environment in buildings with radiant floor. In this thermal model, the black-box model is introduced to derive the incident solar radiation, while the genetic algorithm is utilized to identify the parameters of the thermal model. In order to further validate this simplified thermal model, simulated results from TRNSYS are compared with those from this model and the deviation is evaluated based on coefficient of variation of root mean square (CV). The results show that the simplified model can predict the operative temperature with a CV lower than 1% and predict cooling loads with a CV lower than 10%. For the purpose of supervisory control in HVAC systems, this simplified RC thermal model has an acceptable accuracy and can be used for further MPC in buildings with radiation terminals.

Sensitivity analysis of the thermal performance of radiant and convective terminals for cooling buildings

Heating and cooling terminals can be classified in two main categories: convective terminals (e.g. active chilled beam, air conditioning) and radiant terminals. The mode of heat transfer of the two emitters is different: the first one is mainly based on convection, whereas the second one is based on both radiation and convection. In order to characterise the advantages and drawbacks of the different terminals, steady-state simulations of a typical office room have been performed using four types of terminals (active chilled beam, radiant floor, wall and ceiling). A sensitivity analysis has been conducted to determine the parameters influencing their thermal performance the most. The air change rate, the outdoor temperature and the air temperature stratification have the largest effect on the cooling need (maintaining a constant operative temperature). For air change rates higher than 0.5 ACH, differences between terminals can be observed. Due to their higher dependency on the air change rate and outdoor temperature, convective terminals are generally less energy effective than radiant terminals. The global comfort level achieved by the different systems is always within the recommended range, but differences have been observed in the uniformity of comfort.