Experimental and numerical studies for a novel bed-based air source heat pump (B-ASHP) heating system for improving indoor thermal environments in winter

Abstract

Air source heat pumps (ASHPs) have been increasingly used for space heating due to their high energy efficiency and installation flexibility. In China, with the implementation of the Chinese National strategy of clean-heating, as part of the national efforts of combating severe air pollution in winter, the use of air source heat pumps (ASHPs) has been strongly recommended to replace traditional fossil-fuel based space heating technology. However, when using a conventional ASHP system for space heating, its fan-powered convection-based indoor heating terminal is usually located at a higher level in a heated room. This results in an undesired vertical indoor air temperature gradient and thermally discomfort to occupants as a result of directly blowing warm air to their upper bodies including heads. On the other hand, in many rural areas in northern Chinese provinces, Kangs, a kind of traditional heating systems based on burning biofuels, are still being used for space heating in winter. A Kang-bed in a Chinese Kang-based heating system serves as not only a space heating terminal at daytime for daytime activities but also a heated bed for sleeping at nighttime. However, employing such a Chinese Kang-based heating system can also lead to a number of environmental and operational problems such as local air pollution and safety. Therefore, a research project on developing, based on the pros and cons of both ASHPs and Kang-based heating systems, a novel bed-based ASHP (B-ASHP) system by combining the advantages from both an ASHP and a Kang-based heating system has been carried out and the project results are presented in this Thesis. The Thesis begins with presenting an experimental study, as the first part of the research project, on the operational performances of an experimental B-ASHP system. The experimental B-ASHP system was designed and established in an experimental B­ASHP, and its bed-based heating terminal contained two sections: a horizontal section (H-Section) and a vertical section (V-Section). The dynamic and steady-state operating performances of the experimental B-ASHP system were examined in four test cases. The test results from the four test cases demonstrated that the experimental B-ASHP system can be used to achieve quicker increases in indoor air temperatures and bed-surface temperatures than Kang-based heating systems. The use of experimental B­ASHP system can lead to a more uniformed vertical indoor air temperature gradient, thus a better occupants' thermal comfort, than the use of a conventional convection-based ASHP system. Furthermore, compared to using a Kang-based heating system, a more uniformed bed surface temperature may be obtained when using a B-ASHP system. Nonetheless, although currently the experimental B-ASHP system can be satisfactorily operated, a number of issues related to the designs of a B-ASHP system such as choices of refrigerant, refrigerant tube layout optimizations, and the use of variable speed compressors for capacity control, should be carefully considered to further improve its operational safety and energy efficiency. In the experimental study, the operating characteristics of the experimental B-ASHP system were experimentally evaluated under a fix set of design/operating parameters for its bed-based heating terminal. Therefore, a fellow-up numerical study, as the second part of the research project, to evaluate the impacts of optimizing these design/operating parameters, including the location, dimension, surface temperature (tbed) and surface emissivity (ε) of the bed-based heating terminal, on the indoor thermal environment and the operating efficiency of the B-ASHP system, is reported. A CFD method to be used in the numerical study was established and experimentally validated. The numerical study results suggested that the variation in these parameters did impact both indoor thermal environment and the operating efficiency of the B-ASHP system, in terms of the uniformity of indoor thermal environment, indoor air velocity distribution and the output heating capacity, as well as the installation space requirement and cost implication. Therefore, care must be exercised in properly balancing all influencing parameters. Also, to both provide full space heating and thus maintain an acceptable indoor thermal environment that can meet the Chinese National Standard, and achieve the best possible operating efficiency of the B-ASHP system, the tbed of its bed-based heating terminal without a mattress on it may be set at around 35 °C. Furthermore, although the changes in ε did not significantly impact on indoor thermal environment, a higher ε should be preferred when a lower tbed was used, so as to further improve potentially the operating efficiency of the B-ASHP system. Furthermore, the bed-based terminal can virtually act as a bed, similar to a heated Kang­bed in a Chinese Kang-based heating system, to provide a sleeping person with a suitable micro-environment that was conducive to quality sleep. Therefore, as the third part of the research project, a further numerical study using a digital thermal manikin (DTM) on the impacts of the following four factors, i.e., operating mode of the bed-based heating terminal, tbed, thermal resistance of mattress (Rmat), and thermal resistance of quilt (Rq) on the thermal comfort of a person sleeping on top of the bed-based heating terminal is reported. In this numerical study, the CFD method developed and used in the second part of the project was modified considering the presence of the DTM and its complex geometry, and the modified CFD method was further experimentally validated. The numerical study results using the modified CFD method firstly suggested that to maintain a suitable sleeping thermal environment, the use of H-Section only was adequate. However, using H+V-Section could minimize the temperature difference between the micro-climate inside a quilt and its outer environment, but at a higher energy consumption. Secondly, the tbed value of the bed-based terminal should be carefully selected, and unlike that in the second part of the research project, a tbed at 44 °C was recommended to both meet the requirement of sleeping thermal comfort at nighttime for localized heating and achieve the highest possible operating efficiency of the B-ASHP system. Thirdly, the use of a mattress with a higher Rmat value would not only reduce the output heating capacity and lower the operating efficiency of the B­ASHP system, but also lead to a cold sleeping environment. Fourthly, although using quilts of appropriate Rq value would help prevent the heat loss from a sleeping person, a higher-than-necessary Rq would rather decrease the thermal comfort of a sleeping person.