Developing a new frosting map to guide defrosting control for air-source heat pump units

Accepted Manuscript Developing a new frosting map to guide defrosting control for air-source heat pump units J.H. Zhu, Y.Y. Sun, W. Wang, S.M. Deng, Y.J. Ge, L.T. Li PII:

S1359-4311(15)00635-3

DOI:

10.1016/j.applthermaleng.2015.06.076

Reference:

ATE 6769

To appear in:

Applied Thermal Engineering

Received Date: 14 April 2015 Revised Date:

17 June 2015

Accepted Date: 28 June 2015

Please cite this article as: J.H. Zhu, Y.Y. Sun, W. Wang, S.M. Deng, Y.J. Ge, L.T. Li, Developing a new frosting map to guide defrosting control for air-source heat pump units, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.06.076. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Developing a new frosting map to guide defrosting control

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for air-source heat pump units

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J.H. Zhua, Y.Y. Suna, W. Wanga,*, S.M. Dengb, Y.J. Gea, L.T. Lia Department of Building Environment and Facility Engineering, College of

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Architecture and Civil Engineering, Beijing University of Technology, No.100

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Pingleyuan, Chaoyang District, Beijing 100124, China b

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Department of Building Services Engineering, The Hong Kong Polytechnic

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University, Kowloon, Hong Kong SAR, China

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Abstract

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The development of a new frosting map for air-source heat pumps (ASHPs) based on

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a temperature-humidity chart is reported in this paper. Through a parametric analysis,

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two boundary lines (a condensing line and a frosting line) were obtained, dividing the

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map into three regions: a non-frosting region, a condensing region and a frosting

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region. Using the simulation results from a generalized numerical model of frost

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growth, the frosting region was further divided into three indicative zones

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representing different frosting levels: a severe frosting zone, a moderate frosting zone

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(I, II) and a mild frosting zone (I, II). Lab and field tests were conducted in order to

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verify the correctness of the zoning and to obtain indicative defrosting intervals for

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ASHPs operated at different zones. Based on the test results and using the developed

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Corresponding author. Tel.: +86 010 67391608-804; fax: +86 010 67391608-804 E-mail address: [email protected] (W. Wang) 1

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map, reference defrosting intervals were suggested for different frosting zones, and

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they can be used in conjunction with a conventional defrosting control method to

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avoid potential mal-defrosting.

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Keywords: Air-source heat pump; Frosting map; Defrosting interval; Mal-defrost;

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Field test

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Nomenclature Frost mass, g

Q

Heating capacity, kW

RH

Air relative humidity, %

Ta

Air temperature, ºC

Td

Air dew point, ºC

Tdc

Defrosting interval, min

Tdis

Discharge temperature, ºC

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Freezing point of water, ºC

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Tf

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Mf

Tsuc

Suction temperature, ºC

Tw

Coil temperature of outdoor heat exchanger, ºC

Va

Air velocity, m s−1

W

Input electrical power, kW

∆T

Heat transfer temperature difference, ºC

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1 Introduction

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Air-source heat pump (ASHP) technology is widely accepted for the merits of

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energy-saving and environmental protection. It serves as a heating and cooling source

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in many parts of the world [1,2]. However, for an ASHP unit, when its outdoor heat

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exchanger surface temperature is below both air dew point and freezing point of water,

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frost will form on the outdoor heat exchanger surface. The experimental results [3]

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indicated that a frost layer on an outdoor heat exchanger surface can reduce its heat

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flux by about 40 %. According to Sanders [4], the heating capacity of an ASHP unit

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can be deteriorated by 35 % under frosting conditions. In the previous studies by

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Wang et al. [5], the heating capacity of an ASHP unit was decreased by 29 % to 57 %

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when its outdoor heat exchanger was covered by frost. Frosting not only increased the

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heat transfer resistance and reduced coil’s efficiency [6], but could also lead to the

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shutdown of an ASHP unit [7].

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Therefore, periodic defrosting is necessary, although defrosting itself consumes

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energy and causes an uncomfortable fluctuation of the indoor air temperature in a

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heated space [8]. What is more, an improper defrosting-control scheme might result in

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mal-defrosting [9]. Although various defrosting control methods have been developed,

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such as temperature difference control [10], artificial neural network [11], fan power

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sensing [12], photoelectric technology [13], etc., temperature-time (T-T) is the most

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widely used defrosting control method. As reflected by its name, T-T control is based

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ACCEPTED MANUSCRIPT on temperature and time and is adopted in most ASHP units due to its simplicity.

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However, its reliability is questionable. Baxter and Mogers [14] found that, in a T-T

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controlled ASHP unit, approximately 27 % of defrosting was carried out when no

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frost was observed. According to the field test results [15], nearly 68 % of defrosting

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was unnecessary in a heating season in Beijing, China. For a T-T controlled ASHP

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unit, mal-defrost mainly resulted from the fact that one important parameter, air

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humidity, was not taken into account. Both air temperature and humidity affect

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frosting in an ASHP unit. Without knowing both parameters, it is hardly possible to

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make a correct judgment on whether an ASHP unit is actually operating under frosting

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conditions, nor is it possible to make a correct judgment as to the level of frosting.

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Therefore, the use of T-T control can lead to mal-defrosting. To avoid mal-defrosting,

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it is necessary to know how different combinations of ambient air temperature and

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humidity affect frosting and its level.

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Limited studies related to developing a frosting map have been previously reported.

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For example, Adachi et al. [16] proposed the ranges of outdoor air parameters within

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which frosting may occur, as shown in Fig. 1 a). In Fig. 1 a), the shaded area is the

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frosting region inside which, when an ASHP unit is operated, frosting on its outdoor

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heat exchanger may occur. Frosting conditions were confined between Ta of −5 ºC

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and 7 ºC and between RH of 60 % and 100 %. However, the level of frosting cannot

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be specified by using this map. Furthermore, field test results [15] suggested that the

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surface of the outdoor heat exchanger in an ASHP unit may also be frosted at Ta of

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−0.6 ºC to 1.6 ºC and RH of 45 % to 52 %. But this is not covered in Fig. 1 a).

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Fig. 1 Frosting maps developed in two previous studies

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Wang and Chen [17] also proposed a frosting map using simulation results, as shown

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in Fig. 1 b). The map was divided into three regions, a non-frosting region (Region I),

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a condensing region (Region II) and a frosting region (Region III). It was developed

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at a specific air velocity (Va) of 2 m s−1, a level not considered by Adachi et al. [16].

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Region III was further divided into five parts based on different frosting rates (Wm).

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Therefore, this frosting map presented more details. However, although the frosting

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rate was considered, as shown in Fig. 1 b), it cannot be directly used as a base for

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defrosting control. Furthermore, it was not verified by experiments or field tests.

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According to the field test results [15], the outdoor heat exchanger in an ASHP unit

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may also be frosted in the non-frosting region. Therefore, the frosting map should be

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further improved.

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In this paper, therefore, the development of a new frosting map for ASHPs with a

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further zoning of the frosting region on a temperature-humidity chart, which can be

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regarded as a further development to the frosting map developed by Wang and Chen

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[17] shown in Fig. 1 b), is reported. Similar to the map developed by Wang and Chen

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[17], the new map was also divided into three regions based on the

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temperature-humidity chart: a non-frosting region, a condensing region and a frosting 5

ACCEPTED MANUSCRIPT region. By using a generalized numerical model for predicting frost growth on a cold

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flat plate [18], the frosting region was further divided into three distinctive zones

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representing different frosting levels, i.e., a severe frosting zone, a moderate frosting

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zone (I, II) and a mild frosting zone (I, II). To verify the correctness of the zoning and

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to obtain indicative defrosting intervals for ASHPs operating at different zones, lab

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and field tests were conducted. Based on the tests results, reference defrosting

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intervals for ASHP units operating at different frosting zones are proposed. The

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potential uses of the new frosting map are also discussed.

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2 The developed new frosting map

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A frosting map based on a temperature-humidity chart indicates frosting situations

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and levels on the outdoor heat exchanger surface in an ASHP unit. The procedure to

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develop the new frosting map is shown in Fig. 2, and is explained in this section.

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Fig. 2 The flow chart for developing the new frosting map

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2.1 Assumptions

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Frost formation and growth is a transient process featuring dynamic heat and mass

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transfer. It is impacted by the following primary parameters: air temperature, air

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relative humidity, air velocity, air cleanliness and the temperature and wettability of 6

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an outdoor heat exchanger surface [19]. When developing the new frosting map, the

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following were assumed:

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1) As air cleanliness and the wettability of the outdoor heat exchanger surface were not easily quantified, they were neglected;

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2) In practical application, air velocity at the outdoor heat exchanger surface in an

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ASHP unit is between 1.5 m s−1 and 3.5 m s−1 [17]. In this study, Va was set as 2

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m s−1;

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3) The mass flow rate of the refrigerant was evenly distributed to each refrigerant

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circuit;

4) A constant speed compressor was used.

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2.2 Division of the frosting map through a parametric analysis

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1) The range of the new frosting map

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The range of the new frosting map needed to be determined. While air relative

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humidity varied from 0 % to 100 %, the lower limit of the air temperature was set at

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−15 ºC, a value widely accepted by manufacturers of ASHP units [20]. On the other

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hand, heating may not be required in regions where the average temperature in

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January is higher than 11.5 ºC [20], so this value was set as the upper limit of the air

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temperature. Therefore, for the new frosting map to be developed for ASHP units, its

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temperature range was set at −15 ºC to 11.5 ºC, and the relative humidity range was 0 %

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to 100 %.

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ACCEPTED MANUSCRIPT 2) Physical phenomena of condensing and frosting

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Non-frosting: If Tw > Td, both condensation and frost do not occur on the outdoor heat

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exchanger surface.

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Condensing: If Tw < Td and Tw > Tf, condensation occurs on the outdoor heat

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exchanger surface.

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Frosting: If Tw < Td and Tw < Tf, frost occurs on the outdoor heat exchanger surface.

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There are two critical conditions for the above three physical phenomena. Tw = Td is

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the critical condition for condensing. Tw < Td and Tw = Tf are the critical conditions for

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frosting. Therefore, by comparing the values of Tw, Td and Tf, the frosting map can be

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divided into three regions: a non-frosting region, a condensing region and a frosting

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region.

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3) Determination of Tw and Td

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Based on the above analysis, two crucial parameters, Tw and Td, need to be determined

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first.

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Tw can be estimated by the following correlation:

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 =
 − ∆


(1)

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where ∆T is the heat transfer temperature difference in an outdoor heat exchanger. The

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value of ∆T is usually 11 ºC to 13 ºC [20], varying with the changes of Ta. To

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determine a practical relationship between ∆T and Ta, a field test was conducted using

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the test system described in Section 3.2 in this paper. The field test results are shown

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in Fig. 3. 8

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Fig. 3 Heat transfer temperature difference with the change in air temperature

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As shown in Fig. 3, ∆T decreased as a result of a decrease in Ta. Using the method of

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linear regression, the following linear regression equation linking Ta and ∆T was

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obtained, with the correlation coefficient R2 = 0.69:

∆
=  +  ∙
 ( = 10.260;  = 0.172)

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(2)

Based on Equations (1) and (2), Tw can be calculated by using Ta.

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Td can be calculated by using two air state parameters, Ta and RH. On the

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temperature-humidity chart, the constant dewpoint curves can be obtained by

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connecting the air state points, which have same values of Td.

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4) The condensing and frosting line

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The condensing line is a boundary line used to judge whether condensation will occur

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on the outdoor heat exchanger. According to the above analysis, the values of the two

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parameters (Tw, Td) on any point of the frosting map can be determined. As shown in

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Fig. 4, by connecting these points (Tw = Td) on the frosting map, the condensing line

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was obtained. Below the condensing line, Tw is higher than Td, and there is no

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condensation. Above the condensing line, Tw is lower than Td, and condensation will

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occur on outdoor heat exchanger surface. If Tw is also lower than Tf, the outdoor heat

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exchanger surface will be frosted.

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ACCEPTED MANUSCRIPT Fig. 4 The frosting map with three regions

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The frosting line is a boundary line used to judge whether frost will occur on the

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outdoor heat exchanger. Based on Equations (1) and (2), when Ta is 12 oC, Tw is equal

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to Tf. If Ta is lower than 12 oC, Tw is lower than Tf, and frost will occur on the outdoor

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heat exchanger surface. However, due to the temperature difference between the fins

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and coils of an outdoor heat exchanger, the temperatures of most parts of the outdoor

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heat exchanger would be higher than Tw. Based on the field test data, when Ta was

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between 6 and 12 oC, frost and condensation occurred on the outdoor heat exchanger

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surface at the same time, but condensation was dominant. Only when Ta was lower

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than 6 oC, did frost occur on most parts of the outdoor heat exchanger. Therefore, the

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frosting line was determined at 6 oC.

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5) The non-frosting, condensing and frosting region

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Based on the two boundary lines, the frosting map was divided into three regions:

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Condensing region (above the condensing line and on the right side of the frosting line). When an ASHP unit works in this region, condensation occurs on its outdoor heat exchanger surface.

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Non-frosting region (under the condensing line). If an ASHP unit is operated in this region, its outdoor heat exchanger surface remains dry and no frost is formed.

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Frosting region (above the condensing line and on the left side of the frosting line). In this region, the ASHP unit works with frost on the outdoor heat 10

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exchanger surface.

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2.3 Further division for the frosting region

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To specify frosting levels, the frosting region may be further divided based on

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simulation results using a generalized numerical model for predicting frost growth on

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a cold flat plate [18]. In this way, the frosting map may be used to predict frosting

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levels of an ASHP unit operated under different temperature-humidity conditions. By

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plotting curves of the constant frosting rate, the frosting region was, therefore, further

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divided into three indicative zones representing different frosting levels, i.e., a severe

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frosting zone, a moderate frosting zone and a mild frosting zone. In order to obtain a

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more detailed frosting rate, the moderate frosting zone and the mild frosting zone

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were both divided into two parts (I, II) reflecting different frosting levels. The division

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of the frosting region is shown in Fig. 5.

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Fig. 5 Indicative divisions in the frosting region

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3 Lab and field tests

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To verify the correctness of the zoning of the frosting region and to obtain indicative

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defrosting intervals for ASHPs operated at different zones, both lab tests and field

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tests were carried out. 11

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3.1 Description of lab tests

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In the severe frosting zone, three lab-based test cases (Cases 1–3) were carried out

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using a lab-based test ASHP unit with a nominal heating capacity of 60 kW. This has

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been previously described [21]. The reason for using a lab-based test instead of using

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a field test was that, in Beijing, the outdoor weather conditions in winter were outside

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the severe frosting zone. As a result, it was not possible to carry out field tests at the

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severe frosting zone. The operating conditions of these test cases are listed in Table 1

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and shown in Fig. 6, using the averaged tested air temperature and humidity.

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Table 1 Detailed operating conditions for all test cases

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Fig. 6 Test conditions shown on the frosting region in the new frosting map for

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Cases 1–15

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3.2 Description of field tests

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A field test system was used to investigate the 12 cases (Cases 4–15) in the moderate

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frosting zone (I, II) and the mild frosting zone (I, II). The operating conditions of

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these test cases are also listed in Table 1 and shown in Fig. 6 using the averaged tested

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air temperature and humidity. The field tests were carried out during two heating

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seasons of 2012–2014 in an office building (test building) in Beijing, China. There 12

ACCEPTED MANUSCRIPT were 11 rooms with a total heating area of 185 m2. A commercial ASHP unit served as

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both the heating and cooling sources for the test building. The nominal heating

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capacity and input electric power were 28 kW and 8.66 kW respectively, based on

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nominal conditions for provision of hot water to a floor radiant heating system. The

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ASHP unit was equipped with two constant speed compressors. Reversed-cycle

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defrosting was used. Fig. 7 shows the schematics of the test ASHP unit.

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Fig. 7 Schematic of the field test ASHP unit and its associated space heating

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system

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A measuring system was built to monitor and record the outdoor testing environment

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and the operating parameters of the test ASHP unit. Table 2 presents detailed

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information of the measuring system. Furthermore, photos of frost growth on the

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outdoor heat exchanger surface of the test ASHP unit were taken.

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Table 2 Details of the measuring system for the ASHP unit used in field tests

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Using the measuring system detailed in Table 2, the following parameters were

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monitored.

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pressure;

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Air side: Outdoor heat exchanger inlet and outlet temperature, humidity and




Refrigerant side: Compressor suction and discharge temperatures and pressures, 13

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outdoor heat exchanger surface temperature (tube & fin), expansion valve inlet

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and outlet temperatures;


Water side: Condenser inlet and outlet temperatures, water flow rate;

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Others: Input electrical power to fan, compressor and water pump, frost mass, etc.

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All sensors and measuring devices were able to output a direct current signal of 4–20

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mA or 0–10 V to a data acquisition system (DAS) for logging and recording. The

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DAS collected and recorded all the measured data at an interval of 5 s.

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4 Test results

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The operating characteristics of both the lab test ASHP unit and the field test ASHP

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unit at different frostings during laboratory and field tests are presented in this section.

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Fig. 8 presents the results of lab-based test cases (Cases 1–3) in the severe frosting

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zone. As shown in Fig. 8 a) and Fig. 8 b), in all three cases, as the frosting on the

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outdoor heat exchanger proceeded, the operating performance of the test ASHP unit

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deteriorated quickly. For example, in Case 1, the discharge temperature (Tdis) was

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increased from 99.5 ºC at 10 min to 115.8 ºC at 26 min, and the suction temperature

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(Tsuc) was decreased from −3.6 ºC at 10 min to −12.6 ºC at 26 min. The output heat

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capacity (Q) of the test ASHP unit dropped by 30 %, and the COP decreased by 26 %

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at 26 min, compared with the values at 10 min. For Cases 1–3, defrosting started at 26

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ACCEPTED MANUSCRIPT min, 27 min and 27 min respectively. Furthermore, as seen from the photos shown in

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Fig. 8 c), over 90 % of the outdoor heat exchanger’s surface was covered by heavy

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frost for each case at the start of defrosting. This suggested that, when an ASHP unit is

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operated in the severe frosting zone, frost will quickly form on its outdoor heat

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exchanger surface. Therefore, it can be recommended that the defrosting interval for

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ASHP units operating in the severe frosting zone should be less than 30 min.

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Fig. 8 Test results of Cases 1–3

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The full results for all the remaining cases are shown in Table 3. As shown in Fig. 9,

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similar trends of the heating capacity and COP reductions to those in Cases 1–3 can

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be observed. Furthermore, as shown in Table 3, defrosting was initiated when capacity

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loss was at about 30 % and Mf was about 1320 g for all cases but was initiated at

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different times, depending on the frosting zone in which the test ASHP unit was

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operated. At moderate frosting zones I and II, defrosting was started at 45 min and

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60–75 min, and, at mild frosting zones I and II, defrosting was started at 90–140 min

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and 180–240 min respectively.

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Table 3 Results of four field tests (Cases 4–15)

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Fig. 9 Results of selected field tests in moderate and mild frosting zones (Case 5,

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5 Discussion

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Based on the test results of Cases 1–15 and using the new frosting map, the reference

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defrosting intervals for ASHP units operating at different frosting zones were

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proposed, as shown in Table 4.

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Table 4 Reference defrosting intervals for ASHP units operating at different

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frosting zones

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These reference defrosting intervals can be used in conjunction with a conventional

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time control or T-T control to avoid potential mal-defrosting. For example, when an

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ASHP unit is operated at moderate frosting zone II, its reference defrosting interval is

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45 to 90 minutes. Therefore, if its T-T control calls for defrosting in less than 45

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minutes, possible mal-defrosting can be expected because the defrosting operation is

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carried too early and the outdoor heat exchanger may only be covered with slight frost.

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On the contrary, if the defrosting operation is carried out for more than 90 minutes,

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possible mal-defrosting can be also expected. Because the outdoor heat exchanger

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may be covered with heavily frost, the performance of the unit may seriously decrease.

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This may even lead to the shutdown of the unit. In addition, the new frosting map can

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be used to indicate if ASHP units may be appropriately applied in a specific area. For

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example, it is not recommended to use ASHP units if most conditions for winter days

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are in the severe frosting zone, even if it is not very cold, because frequent frosting

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and defrosting will seriously decrease the performance of ASHP units.

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Due to the difference in ∆T, appropriate modifications might have to be introduced

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when the frosting map is used for other ASHP units. ∆T is an important parameter to

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estimate Tw according to Ta. Different ∆Ts will result in different frosting maps.

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Normally, ∆T is 11 ºC to 13 ºC when Ta is 7 ºC [20]. For the tested unit, ∆T was about

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11.5 ºC when Ta was 7 ºC. For another unit, ∆T may be different, and Equation (2) (a

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linear regression equation between ∆T and Ta described in Section 2.2) should first be

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modified. In Equation (2), the main parameter is k1 as its value is much higher than

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the value of  ∙
 . Therefore, to modify Equation (2), k2 should remain unchanged

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while k1 should be altered according to the change of ∆T when Ta is 7 ºC. If ∆T is 13

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ºC when Ta is 7 ºC, k1 should increase by about 1.5. Once Equation (2) is modified,

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the frosting map can be correspondingly modified. In the frosting map, the frosting

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line will move right for about 2 ºC, and the condensing line will move down for about

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5 % RH. The above analysis shows that, for different ASHP units, the bigger ∆T is,

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the lower Tw is and the bigger the range of the frosting region is. Nevertheless, if there

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is only a slight difference in ∆T, the frosting map can be directly used.

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6 Conclusions

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In this paper, the development of a new frosting map for ASHP units is reported.

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Through a parametric analysis, two boundary lines (a condensing line and frosting 17

ACCEPTED MANUSCRIPT line) were obtained, dividing the map into three regions: a non-frosting region, a

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condensing region and a frosting region. Furthermore, the frosting region was divided

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into three indicative zones representing different frosting levels: a severe frosting zone,

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a moderate frosting zone (I, II) and a mild frosting zone (I, II). The operating

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characteristics of the test ASHP unit at different frosting zones during laboratory and

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field tests were presented. Based on the test results and using the new frosting map,

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reference defrosting intervals were also suggested for ASHP units operating in

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different frosting zones, and these intervals can be used in conjunction with a

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conventional defrosting control method to avoid potential mal-defrosting.

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Acknowledgements

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This work was supported partially by the National Natural Science Foundation of

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China (Grant No. 51178015), National Twelfth Five-year Science & Technology

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Support Plan (Grant No. 2011BAJ03B10), Beijing New-star Plan of Science and

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Technology, and the science and technology plan project of Beijing Municipal

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Education Commission (Grant No. KM 201410005022).

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References [1]

W. Wang, Z.L. Ma, Y.Q. Jiang, Y. Yao, S. Xu, Z. Yang, Field test investigation of a double-stage coupled heat pumps heating system for cold regions, Int. J.

[2]

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Refrig. 28 (5) (2005) 672–679.

M. Mohanraj, S. Jayaraj, C. Muraleedharan, Applications of artificial neural networks for refrigeration, air-conditioning and heat pump systems-A review,

P.L.T. Brian, R.C. Reid, Y.T. Shah, Frost deposition on cold surfaces. Ind. Eng.

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[3]

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Renew. Sust. Energ. Rev. 16 (2) (2012) 1340–1358.

Chem. Fundam. 9 (3) (1970) 375–380. [4]

C.T. Sanders, Testing of air coolers operating under frosting conditions, in heat and mass transfer in refrigeration systems and in air-conditioning, Int. J. Refrig.

[5]

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(1972) 383–396.

W. Wang, Y.C. Feng, J.H. Zhu, L.T. Li, Q.C. Guo, W.P. Lu, Performances of

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air source heat pump system for a kind of mal-defrost phenomenon appearing in moderate climate conditions, Appl. Energ. 112 (12) (2013) 1138–1145. [6]

R. Lazzarin, D. Nardotto, Study of Frost Growth on an Air Heat Pump Outside

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1

Coil, Proceedings Congress "Sustainable Refrigeration and Heat Pump Technology", Stockholm, (2010).

[7]

S.N. Kondepudi, D.L. O’Neal, Effect of different fin configuration on the performance of finned-tube heat exchanger under frosting conditions, ASHRAE Trans. 96 (2) (1990) 439–444.

[8]

W.J. Hu, Y.Q. Jiang, M.L. Qu, L. Ni, Y. Yao, S.M. Deng, An experimental study 19

ACCEPTED MANUSCRIPT on the operating performance of a novel reverse-cycle hot gas defrosting method for air source heat pumps, Appl. Therm. Eng. 31 (2011) 363–369. [9]

W. Wang, J. Xiao, Q.C. Guo, W.P. Lu, Y.C. Feng, Field test investigation of

phenomena, Appl. Energ. 88 (12) (2011) 4470–4480.

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the characteristics for the air source heat pump under two typical mal-defrost

[10] T.R. Buick, J.T. Mcmullan, R. Morgan, R.B. Murry, Icedetection in heat pumps

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and coolers, Int. J. Energ. Res. 2 (1) (1978) 85–98.

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[11] D. Datta, S.A. Tassou, Implementation of a defrost on demand control strategy on a retail display cabinet, Proc. IIF-IIR–Commission D, 1 (2002) 218–226. [12] E.D. Muller, A new concept for defrosting refrigeration plants, Kalte, 28 (2) (1975) 52–54.

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[13] J. Xiao, W. Wang, Y.H. Zhao, F.R. Zhang, An analysis of the feasibility and characteristics of photoelectric technique applied in defrost-control, Int. J. Refrig. 32 (6) (2009) 1350–1357.

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[14] V.D. Baxter, J.C. Mogers, Field-measured cycling, frosting and defrosting (2B)

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losses for a high-efficiency air-source heat pump, ASHRAE Trans. 91 (1985) 537–554.

[15] Y.C. Feng, Mal-defrost accident research of air source heat pump. (MPhil Thesis), Beijing University of Technology, Beijing, (2013) [In Chinese].

[16] M. Adachi, S. Inoue, T. Aizawa, On the refrigeration cycle property of heat pump air conditioners operating with frost formation, Refrigeration, 50 (576) (1975) 818–820 [In Japanese]. 20

ACCEPTED MANUSCRIPT [17] J.F. Wang, G.M. Chen, Study of frosting characteristics of air source heat pumps during winter heating, Refrigeration, (1) (1997) 8–11 [In Chinese]. [18] W. Wang, Q.C. Guo, W.P. Lu, A generalized simple model for predicting frost

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growth on cold flat plate, Int. J. Refrig. 35 (2) (2012) 475–486. [19] C.M. Robinson, A.M. Jacobi, A study of frost formation on a plain fin. Air and

Refrigeration

Center,

Urbana-Champaign, IL, (2001) TR-188.

University

of

Illinois

at

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Conditioning

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[20] W. Wang, F.R. Zhang, Q.C. Guo, J. Xiao, Y.H. Zhao, Investigation of frost status for air source heat pump in different regions of China, J. Hunan Univ. 36 (12) (2009) 9–13 [In Chinese].

[21] W. Wang, J. Xiao, Y.C. Feng, Characteristics of an air source heat pump with

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novel photoelectric sensors during periodic froste-defrost cycles, Appl. Therm.

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EP

Eng. 50 (1) (2013) 177–186.

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ACCEPTED MANUSCRIPT Figure captions Frosting maps developed in two previous studies

Figure 2

The flow chart for developing the new frosting map

Figure 3

Heat transfer temperature difference with the change in air temperature

Figure 4

The frosting map with three regions

Figure 5

Indicative divisions in the frosting region

Figure 6

Test conditions shown on the frosting region in the new frosting map for

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Cases 1–15 Figure 7

Schematic of the field test ASHP unit and its associated space heating system

Figure 8

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Figure 1

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1

Test results of Cases 1–3

Figure 9

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Results of selected field tests in moderate and mild frosting zones (Case

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5, 8, 11, 14)

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ACCEPTED MANUSCRIPT 100 90

70

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RH (%)

80

60 50

Frosting region 40 0

5

10

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-5

Ta (oC)

100

III

1.

90

W 80

1.2 5

=0

.6 3

88

2. 50

x1

x1 0 -5

x1

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RH (%)

m

70

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a) The frosting map developed by Adachi et al. (1975)

0 -5

x

10 -

II

5

0 -5

kg

(m 2

s) - 1

I

Va = 2 m s-1

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60 -10

-5

0

5

10

o

Ta ( C)

1 2

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b) The frosting map developed by Wang and Chen (1997) Fig. 1 Frosting maps developed in two previous studies

23

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2

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1

Fig. 2 The flow chart for developing the new frosting map

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24

ACCEPTED MANUSCRIPT 16 Experimental data Fitting line

14

o

△T ( C)

12 10

6 4 -10

-8

-6

-4

0

Ta (oC)

1

2

4

6

8

10

Fig. 3 Heat transfer temperature difference with the change in air temperature

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2

-2

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8

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25

ACCEPTED MANUSCRIPT 100 2

A

3 B

a

70

4 C

5 D

b

RH (%)

60

7

e

region

8

F

d

9

G

10

H

f

12

J

13

K

i

50

Condensing line

11

I

g h

j

k

L l

14 M m

N n

40 30

15 O o

16 P p

17

18 19 Q 20 R 21 S q 22 T r 23 U s 24 V t 25 W u 26 X v w x Y Z 27 28 y z AA AB 29 30 aa ab ACAD 31 32 ac ad AE AF 33 34 35 ae af AGAH AI 36 37 ag ah 38 39 40 41 ai AJ aj AK 42 43 44 ak AL al AM amAN 45 46 47 an AO ao AP ap AQ 48 49 50 aq AR ar AS as AT 51 52 53 54 at AU au AV av AW awAX ax AY ay AZBA az ba BB bb

Non-frosting region

20 10 0 -15

Condensing

Frosting region

6 E

c

-12

-9

-6

-3

0 o

Ta ( C)

1 2

3

6

9

Fig. 4 The frosting map with three regions

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26

0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14

1 A a

-15 -16 -17 -18 -19 -20 -21 -22 -23 -24 -25 -26 -27 -28

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80

Constant o dewpoint ( C)

Frosting line

11.5

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90 1

ACCEPTED MANUSCRIPT 100

80

ΙΙ

RH (%)

70 60

Severe frosting zone

Moderate frosting zone Ι

Mild frosting zone

Ι ΙΙ

50 40 30

Frosting region Condensing region Non-frosting region

20 10 -12

-9

-6

-3

0 o

Ta ( C)

1

6

9

Fig. 5 Indicative divisions in the frosting region

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3

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0 -15

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90

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27

11.5

ACCEPTED MANUSCRIPT 100 Averaged test conditions

C1 C2 C3 Severe frosting zone

90 Moderate frosting zone Ι

ΙΙ

C4 C7

70

Ι

60

C9

ΙΙ

C10

50

C13

C11 C15

C14

C12

40

-9

-6

-3

Ta ( C)

1

0

3

6

SC

-12

o

Fig. 6 Test conditions shown on the frosting region in the new frosting map for

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Cases 4-15 (Field test)

C6

C5 C8

Mild frosting zone

30 -15

Cases 1-3 (Lab-based test)

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RH (%)

80

Cases 1–15

3

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4

28

1

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Fig. 7 Schematic of the field test ASHP unit and its associated space heating

3

system

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ACCEPTED MANUSCRIPT 120

C1

C2

C3

80 60

Tdis 20

Tsuc

10 0 -10 -20 0

10

20

30 0

10

20

60

C2

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C1

40

Q

30

W 20 10 0 0

6

10

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Q & W (kW)

50

30

20

30 0

10

20

30 0

C3

10

20

Time (min)

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b) Variation of Q and W over time

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5

20

a) Variation of Tdis and Tsuc over time

2

4

10

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Time (min)

1

3

30 0

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Tdis & Tsuc (oC)

100

26 min (Case 1)

27 min (Case 2)

27 min (Case 3)

7

c) Photos of the frosted outdoor coil

8

Fig. 8 Test results of Cases 1–3

9 30

30

ACCEPTED MANUSCRIPT 120 110

90 80

Tdis

C5 C8

20

C11

Tsuc

10 0 -10 -20 0

45 60

120

140

a) Variation of Tdis and Tsuc over time

2

12

Q & W (kW)

10 8

C5 C8

6

2 0 0

45 60

120

140

180

Q W

C14

240

Time (min)

3

6

C11

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4

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b) Variation of Q and W over time

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5

240

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1

4

180

Time (min)

C14

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Tdis & Tsuc (oC)

100

45 min (Case 5)

60 min (Case 8)

140 min (Case 11)

240 min (Case 14)

7

c) Photos of the frosted outdoor coil

8

Fig. 9 Results of selected field tests in moderate and mild frosting zones (Case 5,

9

8, 11, 14) 31

ACCEPTED MANUSCRIPT Table captions Table 1

Detailed operating conditions for all test cases

Table 2

Details of the measuring system for the ASHP unit used in field tests

Table 3

Results of four field tests (Cases 4–15)

Table 4

Reference defrosting intervals for ASHP units operating at different

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frosting zones

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Table 1 Detailed operating conditions for all test cases Ta (ºC)

RH (%)

Frosting

Case Avg.

Range

Avg.

zone

C1

−0.4 to 0.6

0.1

90.3 to 92.5

91.5

Severe

C2

0.5 to 0.9

0.7

91.2 to 92.4

92.0

Severe

C3

0.6 to 2.1

1.3

90.5 to 92.1

91.4

Severe

C4

−2.2 to −1.5

−2.0

73.6 to 75.7

74.5

Moderate I

C5

2.8 to 3.1

2.9

73.5 to 75.2

74.3

Moderate I

C6

4.2 to 5.3

4.8

68.1 to 69.2

68.7

Moderate I

C7

−6.0 to −5.1

−5.7

68.6 to 69.8

69.2

Moderate II

C8

3.9 to 4.5

−1.0

63.6 to 67.1

66.4

Moderate II

C9

5.0 to 5.6

5.2

58.8 to 62.7

60.7

Moderate II

C 11

−0.5 to 1.0

C 12

3.3 to 4.7

C 13

−3.0 to −1.5

C 14 C 15

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−1.9 to 0.1

Field test

−1.2

54.6 to 58.3

56.8

Mild I

0.2

50.6 to 54.4

52.8

Mild I

3.8

50.2 to 55.3

52.1

Mild I

−2.2

47.8 to 49.9

49.2

Mild II

−1.9 to −0.8

−1.3

45.1 to 48.1

46.3

Mild II

0.1 to 1.1

0.5

47.5 to 49.8

48.5

Mild II

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C 10

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Range

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2

Remark

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ACCEPTED MANUSCRIPT Table 2 Details of the measuring system for the ASHP unit used in field tests Device

Number

Accuracy of reading

Full scale

Temperature sensor (PT1000)

18

Class A

−40 to 140 ºC

Humidity sensor

2

±3.5 %

0 to 99 %

Differential pressure sensor (air)

2

±3.0 %

Pressure sensor (refrigerant)

4

±0.4 %

Power sensor

3

±1.0 %

--

Flow-meter

1

±0.5 %

0.5 to 10 m3 h−1

1

--

15 Megapixels

1

--

14 Megapixels

1

±0.1 g

0 to 6000 g

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1

0 to 100 Pa 0 to 40 bar

Digital camera

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Electronic scale 2

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Video camera

0 to 25 bar

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ACCEPTED MANUSCRIPT Table 3 Results of four field tests (Cases 4–15)

Moderate II

Mild I

Mf (g)

Loss of Q (%)

Tdc (min)

C4

1393

30

45

C5

1386

30

45

C6

1381

29

45

C7

1375

30

70

C8

1362

30

60

C9

1406

31

75

C 10

1363

C 11

1361

C 12 C 13 C 14

Mild II

C 15

30

90

31

140

1365

31

120

1162

30

200

1192

30

240

1089

29

180

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Moderate I

Case

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Frosting zone

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Table 4 Reference defrosting intervals for ASHP units operating at different

2

frosting zones Reference defrosting intervals (min)

Severe frosting zone

Tdc ≤ 30 I

30 ≤ Tdc ≤ 45

II

45 ≤ Tdc ≤ 90

I

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Frosting zones

Moderate frosting zone

90 ≤ Tdc ≤ 150

Mild frosting zone

150 ≤ Tdc ≤ 240

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ACCEPTED MANUSCRIPT Highlights
A new frosting map guiding defrosting control for ASHPs was reported.
The map was divided into three regions, non-frosting, condensing, frosting

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region.
Frosting region was divided into three zones, severe, moderate, mild frosting zone.

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Lab and field tests were carried out to verify the correctness of the zoning.

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Reference defrosting intervals were proposed for different frosting zones.