Defrosting is necessary for the normal operation of air source heat pump (ASHP) units to solve the frosting in the winter, which deteriorates the performance of ASHP. During this defrosting period, the ASHP cannot provide heat to the indoor space, leading to adverse thermal comfort, which may reduce the sleep quality. Hence, a task/ambient ASHP (T-ASHP) system was designed in this study, and the transient thermal environment was investigated through a computational fluid dynamic (CFD) method. The duration of toz (average air temperature in the occupied zone) increase period was 4.33 times as long as toz decrease period, although the duration of the phase 1 heating period was 1.9 times as that of defrosting period. During the defrosting procedure, the PMV value decreased to the lowest value of −0.83, and the period of feeling cool and slightly cool lasted for 45.4 min. The hand, foot and leg parts were much cooler than the whole body, and the non-uniformity of the PMV value on body parts was obviously enlarged during the defrosting period. Higher draught risk around 22% near the head and trunk regions was found during the defrosting period and was significantly reduced in the follow-up heating period.
Practical application: The results from this study emphasize the sleeping thermal comfort during defrosting of air source heat pumps. The present study gave a map of transient thermal comfortable levels influenced by the operating of defrosting and re-heating. These results should serve as a recommendation to air source heat pump designers to determine suitable defrosting technology to maintain normal indoor thermal comfort.
SongMDengSPanDMaoN. An experimental study on the effects of downwards flowing of melted frost over a vertical multi-circuit outdoor coil in an air source heat pump on defrosting performance during reverse cycle defrosting. Appl Therm Eng2014; 67: 258–265.
2.
SuJLiJLuoXYuCWGuZ. Experimental evaluation of a capillary heating bed driven by an air source heat pump and solar energy. Indoor Built Environ2020; 29(10): 1399–1411.
3.
ZhaoMKangWLuoXYuCWMengXZGuZ. Performance comparison of capillary mat radiant and floor radiant heating systems assisted by an air source heat pump in a residential building. Indoor Built Environ2017; 26(9): 1292–1304.
4.
YaoYJiangYDengSMaZ. A study on the performance of the airside heat exchanger under frosting in an air source heat pump water heater/chiller unit. Int J Heat Mass Tran2004; 47: 3745–3756.
5.
ByunJ-SLeeJJeonC-D. Frost retardation of an air-source heat pump by the hot gas bypass method. Int J Refrig2008; 31: 328–334.
6.
KimJChoiH-JKimKC. A combined dual hot-gas bypass defrosting method with accumulator heater for an air-to-air heat pump in cold region. Appl Energy2015; 147: 344–352.
7.
KondepudiSNO’NealDL. Frosting performance of tube fin heat exchangers with wavy and corrugated fins. Exp Therm Fluid Sci1991; 4: 613–618.
8.
O’NealDPetersonKAnandNSchliesingJ. Refrigeration system dynamics during the reverse cycle defrost. ASHRAE Trans1989; 95: 689–698.
9.
WangFWangZZhengYLinZHaoPHuanCWangT. Performance investigation of a novel frost-free air-source heat pump water heater combined with energy storage and dehumidification. Appl Energy2015; 139: 212–219.
10.
SongMDengSDangCMaoNWangZ. Review on improvement for air source heat pump units during frosting and defrosting. Appl Energy2018; 211: 1150–1170.
11.
DingYMaGChaiQ. Experiment investigation of reverse cycle defrosting methods on air source heat pump with TXV as the throttle regulator. Int J Refrig2004; 27: 617–678.
12.
ByunJ-SJeonC-DJungJ-HLeeJ. The application of photo-coupler for frost detecting in an air-source heat pump. Int J Refrig2006; 29: 191–198.
13.
QuMXiaLDengSJiangY. Improved indoor thermal comfort during defrost with a novel reverse-cycle defrosting method for air source heat pumps. Build Environ2010; 45: 2354–2361.
14.
DongJ-kJiangY-qYaoYZhangX-d. Operating performance of novel reverse-cycle defrosting method based on thermal energy storage for air source heat pump. J Cent South Univ Technol2011; 18: 2163–2169.
15.
HuWJiangYQuMNiLYaoYDengS. An experimental study on the operating performance of a novel reverse-cycle hot gas defrosting method for air source heat pumps. Appl Therm Eng2011; 31: 363–369.
16.
ZhangLDongJJiangYYaoY. A novel defrosting method using heat energy dissipated by the compressor of an air source heat pump. Appl Energy2014; 133: 101–111.
17.
TangJGongGSuHWuFHermanC. Performance evaluation of a novel method of frost prevention and retardation for air source heat pumps using the orthogonal experiment design method. Appl Energy2016; 169: 696–708.
18.
CuellarNGRatcliffeSJChienD. Effects of depression on sleep quality, fatigue, and sleepiness in persons with restless legs syndrome. J Am Psychiatr Nurses Assoc2006; 12: 262–271.
19.
LiXZhouBShenLWuZ. Exploring the effect of mattress cushion materials on human-mattress interface temperatures, pre-sleep thermal state and sleep quality. Indoor Built Environ2021; 30(5): 650–664.
20.
CaoTLianZDuHMiyazakiRBaoJ. Differences in environmental perception of gender and sleep quality in self-regulating sleep thermal environment. Indoor Built Environ2021; 30(9): 1568–1579.
21.
YuJHassanMTBaiYAnNTamVWY. A pilot study monitoring the thermal comfort of the elderly living in nursing homes in Hefei, China, using wireless sensor networks, site measurements and a survey. Indoor Built Environ2020; 29(3): 449–464.
22.
PanDDengSLinZChanM. Air-conditioning for sleeping environments in tropics and/or sub-tropics - a review. Energy2013; 51: 18–26.
23.
MaoNPanDChanMDengS. Parameter optimization for operation of a bed-based task/ambient air conditioning (TAC) system to achieve a thermally neutral environment with minimum energy use. Indoor Built Environ2017; 26(1): 132–144.
24.
TongZChenYMalkawiA. Defining the influence region in neighborhood-scale CFD simulations for natural ventilation design. Appl Energy2016; 182: 625–633.
25.
SongMChenAMaoN. An experimental study on defrosting performance of an air source heat pump unit with a multi-circuit outdoor coil at different frosting evenness values. Appl Therm Eng2016; 94: 331–340.
26.
MaoNPanDDengSChanM. Thermal, ventilation and energy saving performance evaluations of a ductless bed-based task/ambient air conditioning (TAC) system. Energy Build2013; 66: 297–305.
27.
MaoNSongMPanDDengS. Computational fluid dynamics analysis of convective heat transfer coefficients for a sleeping human body. Appl Therm Eng2017; 117: 385–396.
ANSI/ASHRAE Standard 55-2020. Thermal Environmental Conditions for Human Occupancy. Atlanta, GA: ASHRAE, 2020.
31.
SevilgenGKilicM. Numerical analysis of air flow, heat transfer, moisture transport and thermal comfort in a room heated by two-panel radiators. Energy and Buildings2011; 43: 137–146.
32.
LinZDengS. A study on the characteristics of nighttime bedroom cooling load in tropics and subtropics. Build Environ2004; 39: 1101–1114.
33.
LuYJ. Study on the Use of Phase Change Materials in Light Building Envelope-A Case in Tianjin Area. Thesis. Tianjin, China: Tianjin University, 2013.
34.
GB/T 50176-2016. Thermal Design Code for Civil Buildings. Beijing, China: Ministry of Housing and Urban–Rural Development of the People’s Republic of China, China Architecture & Building Press, 1993. (in Chinese).
35.
McCulloughEAZbikowskiPJJonesBW. Measurement and prediction of the insulation provided by bedding systems. ASHRAE Tran1987; 93: 1055–1068.
36.
LinZDengS. A study on the thermal comfort in sleeping environments in the subtropics-Measuring the total insulation values for the bedding systems commonly used in the subtropics. Build Environ2008; 43: 905–916.
ZhaiZQZhangZZhangWChenQY. Evaluation of various turbulence models in predicting airflow and turbulence in enclosed environments by CFD: Part-1: summary of prevalent turbulence models. HVAC&R Res2005; 13: 853–870.
39.
RaithbyGDChuiEH. A finite-volume method for predicting a radiant heat transfer in enclosures with participating media. J Heat Transf1990; 112: 415–423.
40.
ChenQSrebricJ. A procedure for verification, validation, and reporting of indoor environment CFD analyses. HVAC&R Res2002; 8: 201–216.
41.
LinZDengS. A study on the thermal comfort in sleeping environments in the subtropics-Developing a thermal comfort model for sleeping environments. Build Environ2008; 43: 70–81.
42.
ZhouXXiongJLianZ. Predication of skin temperature and thermal comfort under two-way transient environments. J Therm Biol2017; 70: 15–20.
43.
DahlanNDGitalYY. Thermal sensations and comfort investigations in transient conditions in tropical office. Appl Ergon2016; 54: 169–176.
