Domestic Air-conditioner and Integrated Water Heater

Domestic air-conditioner and integrated water heaterfor subtropical climate

Jie Ji a, Tin-tai Chow b,*, Gang Pei a, Jun Dong a, Wei He a

a Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei,

Anhui 230026, Chinab Division of Building Science and Technology, City University of Hong Kong, Tat Chee Avenue,

Kowloon, Hong Kong

Received 25 July 2002; accepted 15 November 2002

Abstract

The technology of using a heat pump for space conditioning and domestic hot water heating in residences

has been developed for half a century. The earlier air-to-water heat pumps and water-heating heat pumps

suffered from drawbacks like high costs, unreliable operation, and inflexible applications. They were not

well positioned in the market to attract customers. This paper introduces a novel air-conditioning product

that can achieve the multi-functions with improved energy performance. The basic design principles and the

laboratory test results are presented. The results showed that by incorporating a water heater in the out-

door unit of a split-type air-conditioner so that space cooling and water heating can take place simulta-neously, the energy performance can be raised considerably.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Room air-conditioner; Heat pump; Domestic hot water heating; Refrigeration; Thermal design

1. Introduction

Heat pumps were first used in residences in 1950s for space heating and for domestic hot waterheating. They were not reliable at that time and the maintenance cost was high. After the oil crisesin the 1970s, the family-use heat pump has undergone rapid development. Desuperheaters, whichworked for heat pumps and air conditioners, were introduced in the United States market by threemanufacturers. They were then claimed to offer almost free water heating in summer whenever

*Corresponding author. Tel.: +852-2788-7089; fax: +852-2788-9716.

E-mail address: [email protected] (T.-T. Chow).

1359-4311/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S1359-4311(02)00228-4

Applied Thermal Engineering 23 (2003) 581–592www.elsevier.com/locate/apthermeng

mail to: [email protected]

space cooling was required, and to reduce total domestic electricity demand in houses whichotherwise had to rely on direct electric water heaters [1].Further feasibility studies showed that air-source water-heating heat pumps were much more

economical than solar systems, which were much more popular in application. Payback periods of2–3 years were anticipated [2]. Air-conditioning units with an integral hot-water storage tank andimmersed condenser, using ambient air as a heat source, were available [3]. In early 1980s, over10,000 units of these air-to-water heat pumps for homes were sold every year in US. These earlymodels were suffered from high purchase prices, high maintenance costs, noisy, poor longevity,and limited installation options. These drawbacks led the market to collapse. As of 1995, the twosurviving manufacturers were only selling about 2000 residential units per year. One of them wasproducing a high-end integrated heat pump/storage tank. And the other was producing a compactstand-along heat pump that retrofited onto an electric resistance storage water heater [4].Through all these years, the technology has been emphasizing on waste heat recovery and hot

water production. Services water heating is the main task of the heat pump water heater; theadditional air-conditioning and heat recovery ventilation are the by-products. As a matter of fact,a condensing temperature higher than 60 �C is difficult to obtain with refrigerants like R-22 andR134a. Heat pump water heaters in their normal capacities can heat water at a rate 40–100% ofthe electric resistance units, and 30–50% of the gas units. To provide quick recovery, a householdmust have a large heat pump, an unusually large storage tank, and a control system that turns onthe electric resistance backup heater whenever required. However, large water tanks are moreexpensive, take up more space, and use more energy to maintain a set point. A higher-capacityheat pump is undesirable as it costs more and the frequent start–stop often results in lower effi-ciency. Some designs let the small heat pump cater for most of the hot water load, but includecontrol mechanism to activate the electrical heater in the tank when the demand is high. However,this increases peak electrical demand and reduces energy efficiency. For some households, anoption is to reduce peak demand by spreading out their hot water consumption over the day.Today, technological advancements have overcome the problems mentioned above. Heat pump

water heater may turn out to be popular again. The air-to-water heat pump water heater offers an

Nomenclature

COP coefficient of performanceQ heat flow rate (W)W power input (W)s time period (s)

Subscripts1 condenser2 evaporatorc space coolingh space heatingw water heatingavg averaged

582 J. Ji et al. / Applied Thermal Engineering 23 (2003) 581–592

energy-saving alternative. A heat pump water heater can provide hot water two to three timesmore energy efficient than an electric-resistance heater. If installed properly, it can also provideuseful by-products––space cooling, dehumidification, or heat-recovery ventilation. The extendeduse of heat pump water heater in air-conditioning and heat-recovery ventilation has been reported[4]. In Europe, advanced heat pump technology has been already in place [5].

2. Applications in subtropical region

For a place in the subtropical region, like Hong Kong, most household families use air-con-ditioners merely for space cooling in summer. The use of heat pump is not popular because of therelatively short and mild winter. Instead, electric heaters are used during the cold days. To obtainhot water, the families rely on various types of water heaters, such as electric water heater, gas-fired water heater, etc. There are some limitations and problems as at present, viz.:

(a) The air-conditioner is used only in summer for space cooling and remains idle in the rest of theyear; the utilization is low.

(b) A family buys both air-conditioner and water heater; this may not be economical in terms ofcost and space.

(c) The operating cost of the gas-fired or electric water heater can be higher than that of the heatpump.

(d) The gas-fired water heater depends on the stability of water-pressure on its smooth operation;it also has the potential danger in life safety caused by the possible leakage of fuel gas or com-bustion products.

(e) Condenser heat dissipation from the air-conditioners can be a source of heat pollution; this iselaborated below.

For high-rise apartment buildings in Hong Kong and in the Mainland China, the building re-entrant––an open space where the kitchens receive daylight and ventilating air––is a popular placeto accommodate the outdoor units of the split-type air-conditioners. The presence of these air-cooled condensing units however, introduces a thermal problem––the thermal buoyance as aresult of heat dissipation leads to the development of a rising air plume; inadequate air exchangeat this recessed space elevates the ambient temperature, and the insufficient cooling significantlyaffects the condenser performance, especially for those at the upper floor levels. The result couldbe an overall degradation of the capacity and the efficiency of the air-conditioners at the entirebuilding re-entrant. As most condensing units are unlikely to function properly at elevated on-coiltemperature because of the abnormal condenser working pressure, the problem is therefore notonly energy wastage but also equipment malfunction [6,7]. Similar problems occur in window-type air-conditioners when they are placed at the building re-entrant [8].Currently under our study is a novel air-conditioner that possesses multi-functions: space

cooling, domestic hot water heating, and space heating. The design is expected to overcome thementioned problems and limitations. Two prototypes have been produced. The following presentstheir basic design concepts and the laboratory test results that illustrate the operation and eco-nomic advantages.

J. Ji et al. / Applied Thermal Engineering 23 (2003) 581–592 583

3. Design of novel air-conditioning units

3.1. General principles

The proposed air-conditioner is designed for multi-task and for year round service. The devicepossesses the dual functions of space air-conditioning and water heating. It is designed as a split-type unit. A water tank with immersed condensing coils is integrated in the outdoor unit. In thewarm seasons, the novel air-conditioner provides space cooling as its primary task, and producesdomestic hot water through the ‘‘water-bath’’ heat-recovery condenser as its secondary task. Inthe process, the water temperature will rise. The performance will drop because of the elevatedcondenser working temperature and pressure. To overcome this limitation, an air-cooled con-denser is connected in series with the immersed condenser. This is to overtake the duty of theimmersed condenser when the water temperature reaches a high limit. And, because of the mixedeffect of water- and air-cooling, the average performance in the cooling mode will be generallyhigher than the conventional split-type air-conditioner.The proposed product is able to work in one of the following three operation modes:

3.1.1. Provision of space cooling and domestic hot water

As the rejected heat energy at the condenser is about 15–20% more than the evaporator re-frigerating effect, the recovery of all rejected heat for hot water production is not necessary. Thehot water consumption depends on the needs of the individual family, say on the number offamily members and their living habits. As the operation time for space cooling can be lengthy, thewater heater is only to recover a part of the condensation heat energy.

3.1.2. Provision of domestic hot water as a heat pump water heater

Other than the cooling season, the proposed air-conditioner is primarily used to produce do-mestic hot water. In this mode, the indoor unit is not in use. The air-cooler at the outdoor unitbecomes the evaporator, which works together with the immersed condenser. In order to improvethe heat exchange at the water tank, the other side-by-side condenser coil is to be put in service.The proposed product becomes a heat pump water heater in this case. Because of the mild winterin the subtropical region, the outdoor temperature will not drop below 0 �C. In Hong Kong forexample, the average temperature in winter is 15 �C, there is minimum defrosting problem evenwhen the ambient air is used as the heat pump heat source.

3.1.3. Provision of space heatingIn the cold season, space heating may be needed. The proposed air-conditioner then behaves

like a conventional air-to-air heat pump. The air-cooler at the outdoor unit becomes the evap-orator. The indoor unit will then provide space heating. In terms of economy in equipment op-eration, simultaneous space heating and domestic water heating is not recommended.

3.2. Performance definition

The energy performance of a refrigeration device can be measured by its coefficientof performance (COP), which carries different definitions depending on its specific appli-


cations. Additional performance definitions can be used in the case of the novel air-condi-tioner.

3.2.1. Space cooling and water heating modeBecause of the gradual change in water temperature, the COP of the novel air-conditioner

varies with time, even when both indoor and outdoor thermal environments remain unchanged.At any time instance t, the COP for space cooling is given by:

COPcðtÞ ¼Q2ðtÞW ðtÞ ð1Þ

and the overall COP value when the water waste heat for water heating is included is then:

COPcwðtÞ ¼Q2ðtÞ þ QwðtÞ

W ðtÞ ð2Þ

Their average values over a time period s is

COPc avg ¼R s0Q2ðtÞdtR s

0W ðtÞdt

ð3Þ

COPcw avg ¼R s0½Q2ðtÞ þ QwðtÞ�dtR s

0W ðtÞdt

ð4Þ

In the above equations, Q2ðtÞ is the heat exchange rate at the indoor unit; QwðtÞ is heat exchangerate at the immersed condenser; W ðtÞ is the electric power input to the novel air-conditioner, and sis the time duration of the measurement.

3.2.2. Water-heating-only mode

The water heating efficiency can be defined as

COPwðtÞ ¼QwðtÞW ðtÞ ð5Þ

COPw avg ¼R s0QwðtÞdtR s

0W ðtÞdt

ð6Þ

3.2.3. Space-heating-only

Similarly, the COP for space heating can be defined as

COPhðtÞ ¼Q1ðtÞW ðtÞ ð7Þ

COPh avg ¼R s0Q1ðtÞdtR s

0W ðtÞdt

ð8Þ


where Q1 is the condenser heat energy released to the indoor space for a reversed cycle. This COPh

for space heating is expected the same as the conventional air-to-air heat pump, when the waterbath condenser is left idle.

4. The prototypes

Two prototypes of slightly different design were fabricated for performance testing. Theirfeatures are described below.

4.1. Two-function air-conditioner

The two-function air-conditioner is able to provide space cooling and water heating, but notspace heating. This is especially suitable for applying in the tropic and subtropical areas wherespace heating is not essential.The water bath condenser consists of two heat transfer coils acting side by side. The two al-

ternative working cycles are shown in Fig. 1. The white arrows in the figure indicate the directionof refrigerant flow in the ‘‘space cooling and water heating’’ mode (Mode 1), and the black arrowsindicate the flow directions in the water-heating-only mode (Mode 2). The switching betweenthese two modes is by means of the electromagnetic valves. In Mode l, Valve 1 and Valve 3 areclosed and Valve 2 is open. The refrigerant discharged from the compressor passes through Coil 1at the water bath condenser, then directly to the air-cooled condenser without passing through the

Capillary tube 1Com

pressor

Coil 1

Air-cooled

condenser

Valve 3

Evaporator

Valve 2

Capillary tube 2

Coil 2Valve1

Indoor unit

Water bath

condenser

Fig. 1. Working cycles of the two-function air-conditioner.


Capillary tube 1. In Mode 2, Valve 1 and Valve 3 are open and Valve 2 is closed. Both Coil 1 andCoil 2 of the water bath condenser are now in use to increase the heat capacity.

4.2. Three-function air-conditioner

The three-function air-conditioner is able to provide space cooling, water heating and spaceheating. This is particularly suitable for applying in cold climatic regions where space heating isgenerally required. However, it should not be subject to extremely cold weather; otherwise theproposed product with a low-temperature heat source will suffer from low efficiency. Also de-frosting can be a problem.The working cycles of the three-function air-conditioner are shown in Fig. 2. There are three

modes of operation. Mode 1 is for simultaneous space cooling and water heating, Mode 2 is forwater-heating-only, and Mode 3 is for space-heating-only. The directions of refrigerant flow inthese three modes are indicated by the white, black and gray arrows respectively. The four-wayvalve allows the correct refrigerant flow across the compressor in all situations.

5. Laboratory test

5.1. Laboratory set-up

To examine the performance of the novel air-conditioner, the evaporative and condensationheat transfer in different modes and different thermal environment (i.e. the temperature and rel-ative humidity of air) were measured.

Capillary tube 1

Com

pressor

Water-bath

condenser

Air-cooled

condenser

Valve3

Evaporator

Valve 1

Valve 2

Capillary tube 2

Four-way

valve

Indoor unit

Fig. 2. Working cycles of the three-function air-conditioner.


The energy performance of the three-function air-conditioner was tested in a national-standardlaboratory. In the absence of standard testing procedures for this innovative product, the mea-surements were making reference to the Chinese National Standard GB/T7725-1996 on spaceheating and cooling devices. The thermal environments of the performance tests on the coolingand heat modes respectively are listed in Table 1. All laboratory measurements were taken placedin a room-type test chamber, of which the key features and provisions are shown in Fig. 3. Theentire chamber was provided with high quality thermal insulation, in that the heat exchange(including thermal radiation) across the external enclosure was not to exceed 300 W, based on atemperature difference of 11 �C. The two environmental chambers were separated by a well-in-sulated partition. Individual air-handling unit (AHU) was provided in each chamber to keep theair temperature and relative humidity to the controlled conditions. These two adjoining chambers,here referred as the indoor chamber and the outdoor chamber, were where the indoor and out-door units of the novel air-conditioners were placed. Surrounding each of the two chambers wasan equalizing temperature compartment, where the air temperature was maintained the same asthat in either chamber. A pressure-balancing device was inserted to monitor the pressure differ-ence between the two chambers, and to facilitate the measurements of air leakage and air recir-culating rates. For the ‘‘water-heating-only’’ mode, the performance tests were carried out solelyin the outdoor chamber at four different dry bulb air temperature conditions. These were re-spectively 31, 25, 15 and 4.5 �C. The measurements were stopped when the water bath temper-ature rose to 70 �C. And for the space-heating-only mode, the þ7 �C testing conditions is regardedas a frost-free outside condition.

Table 1

Controlled testing environment of novel air-conditioner

Heat pump testing conditions Controlled air temperature at

indoor chamber (�C)Controlled air temperature at

outdoor chamber (�C)DB WB DB WB

Indoor unit at cooling operating mode 27 19 35 24

Indoor unit at heating operating mode 20 15 (max) 7 6

Fig. 3. Key features of air-conditioner performance test chamber.


5.2. Performance test results

Fig. 4 shows the testing results of the three-function air-conditioner when it operated with thespace-cooling and water-heating mode. While both the input electric power and water tempera-ture increased with time, the COPcw and COPc drop mildly. The COPcw avg is found to be 4.02 andthe COPc avg 2.91.Fig. 5(a)–(d) show the test results of the water-heating-only mode at four different controlled

ambient temperatures. It can be seen that in each case COPw gradually decreases with time as thewater bath temperature increases. During the four testing periods, the COPw avg dropped from3.42 to 2.0 when the setting of the controlled ambient temperature shifted from 31 to 4.5 �C. Itshould be noted that the listed COPw avg values are for reference only, since the initial water bathtemperatures were not controlled in these trial tests.Table 2 lists the averaged testing results for Modes 1, 2 and 3. It should be noted that for Mode

3, the COPh remained constant (equal to COPh avg) as far as the indoor and outdoor thermalenvironments were fixed in accordance with the specified testing conditions stated in Table 1. Theperformance was close to a conventional air-to-air heat pump.For the two-function air-conditioner, the test results are expected to be more or less the same as

the three-function air-conditioner. This is because the two prototypes are basically of the samedesign, except for the complexity in the refrigeration circuit.

6. Discussions

The COPc of a conventional air-conditioner is in the range of 2.2–2.4. According to ourtest results, the novel air-conditioner has a higher COPc (2.91 in average) than the conventional

0

500

1000

1500

2000

2500

3000

3500

0 15 30 45 60 75 90 105Time(minute)

cooling rate(W)input po wer(W)COPcw∗100water te mperature∗10(deg.C)COPc∗100

Fig. 4. Performance test results of three-function air-conditioner at ‘‘space cooling and water-heating’’ mode.


split-type air-conditioning unit. This is because of the water-cooling effect. If the novel air-con-ditioner is also used as a heat pump water heater, the COPcw avg of 4.02 becomes much attractive.This value is 38% higher than the COPc avg, and can be even higher if there is a continuous hotwater consumption. Since once the water in tank has been consumed substantially and refilled, theheat sink temperature will be lowered and the heat pump water heater will perform more effi-ciently. In order to evaluate the effect of different hot water consumption modes on the energyperformance, the variation of daily COP in different hot-water usage pattern should be furtherinvestigated. Furthermore, as a novel product, the equipment should be made cost effective bycontrolling well the initial cost and the maintenance cost. Over-sizing should be avoided.

0

20

40

60

80

0 5 9 13 17 20 23

Time(minute)

input power/100(W )

wate rtemperature(deg.C )

COPw∗1 0

0

20

40

60

80

0 4 8 12 16 20 24 28 31

Time(minute )

input power/100(W)

wate rtemperature(deg.C)

COPw∗1 0

0

20

40

60

80

0 4 8 12 16 20 24 28 32

Time(minute)

input power/100(W)

watertemperature(deg.C)

COPw*10

0

20

40

60

80

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Time(minute)

input power/100(W)

watertemperature(deg.C)

COP w∗10

(d) Ambient temperature at 4.5oC

(c) Ambient temperature at 15oC

(b) Ambient temperature at 25oC

(a) Ambient temperature at 31oC

Fig. 5. Performance test results of three-function novel air-conditioner at ‘‘water-heating-only’’ mode.


According to an investigation of the China market in 1999, the annual sale volume of air-conditioners in the country was 900,0000. Only 30% of the families living in cities owned air-conditioners. The figure reduced to 10% for the rural area. Meanwhile, the annual output of gaswater heaters was 400,000 and the annual output of solar water collectors was 250,0000. The salevolume of electric water heater shared 60% of the total water heater market. Following theeconomic growth of the country, it is expected that more and more families will have a demand onair-conditioners and water heaters. On the other hand, the available water heaters have problemslike safety, reliability, and energy saving. At this end, the proposed air-conditioner integrated withwater heater is functionally safe, flexible, economical, and environmental friendly. It is expected tohave a very good market in China, and even in other countries as well. Nishimura [9] gave aprecise prediction about the trends of using heat pump in Asia and the Pacific.

7. Conclusions

Heat pump offers a technology that reduces energy consumption, fuel cost, and global warmingeffect. By integrating a water-cooled condenser to the outdoor unit of a heat pump, there are anumber of advantages, i.e.

(a) By combining the air-conditioning unit and the water heater into one product, the novel air-conditioner turns out to be flexible, multi-functional, and economical.

(b) Through the waste heat recovery process, the overall COP of the device can be much im-proved.

(c) It can work independently as a domestic heat pump water heater; the heating efficiency ismuch higher than that of the electric water heater.

(d) The conventional air-conditioner only works in summer. In most time of a year, it becomesidle. The novel air-conditioner is suitable for use in any time of a year.

(e) The laboratory test results on the prototype have been very much promising; the performanceunder the space-cooling mode can be jacked up by 38% through simultaneous water-heating.The energy performance of the space-heating mode is close to that of a conventional split-typeunit.

Table 2

Test performance of three-function novel air-conditioner in different operating modes

Operating mode Averaged COP Test results

Mode 1: space-cooling and water-heating COPcw avg 4.02

COPc avg 2.91

Mode 2: water-heating-only COPw avg at

31 �C 3.42

25 �C 3.25

15 �C 2.52

4.5 �C 2.00

Mode 3: space-heating-only COPh 2.72


References

[1] E.F. Gorzelnik, Heat water with your air-conditioner, Electrical World 188 (11) (1977) 54–55.

[2] O�Neal, et al., Energy and economic effects of residential heat pump water heaters. CONF-790107-2, Oak Ridge

National Laboratory, USA, 1979.

[3] R.D. Heap, Heat Pumps, Second ed., E.&F.N. Spon, London, 1983.

[4] S. Bodzin, Air-to-water heat pumps for the home, Journal of Home Energy 14 (4) (1997).

[5] H.J. Laue, Regional report Europe: ‘‘heat pumps––status and trends’’, International Journal of Refrigeration 25

(2002) 414–420.

[6] T.T. Chow, Z. Lin, Prediction of on-coil temperature of condensers installed at tall-building re-entrant, Applied

Thermal Engineering 19 (1999) 117–132.

[7] T.T. Chow, Z. Lin, J.P. Liu, Effect of condensing unit layout at building re-entrant on split-type air-conditioner

performance, Energy and Buildings 34 (3) (2002) 237–244.

[8] M. Bojic, M. Lee, F. Yik, Flow and temperature outside a high-rise residential building due to heat rejection by its

air-conditioners, Energy and Buildings 33 (2001) 737–751.

[9] T. Nishimura, ‘‘Heat pumps––status and trends’’ in Asia and the Pacific, International Journal of Refrigeration 25

(2002) 405–413.


Domestic Air-conditioner and Integrated Water Heater

Documents

Transcript of Domestic Air-conditioner and Integrated Water Heater