Domestic air-conditioner and integrated water heater for subtropical climate

Applied Thermal Engineering 23 (23) 581 592 www.elsevier.com/locate/apthermeng Domestic air-conditioner and integrated water heater for 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 2326, China b Division of Building Science and Technology, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Received 25 July 22; accepted 15 November 22 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 outdoor unit of a split-type air-conditioner so that space cooling and water heating can take place simultaneously, the energy performance can be raised considerably. Ó 22 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 195s for space heating and for domestic hot water heating. They were not reliable at that time and the maintenance cost was high. After the oil crises in the 197s, the family-use heat pump has undergone rapid development. Desuperheaters, which worked for heat pumps and air conditioners, were introduced in the United States market by three manufacturers. They were then claimed to offer almost free water heating in summer whenever * Corresponding author. Tel.: +852-2788-789; fax: +852-2788-9716. E-mail address: bsttchow@cityu.edu.hk (T.-T. Chow). 1359-4311/2/$ - see front matter Ó 22 Elsevier Science Ltd. All rights reserved. PII: S1359-4311(2)228-4

582 J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 Nomenclature COP coefficient of performance Q heat flow rate (W) W power input (W) s time period (s) Subscripts 1 condenser 2 evaporator c space cooling h space heating w water heating avg averaged space cooling was required, and to reduce total domestic electricity demand in houses which otherwise 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 of 2 3 years were anticipated [2]. Air-conditioning units with an integral hot-water storage tank and immersed condenser, using ambient air as a heat source, were available [3]. In early 198s, over 1, units of these air-to-water heat pumps for homes were sold every year in US. These early models 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 two surviving manufacturers were only selling about 2 residential units per year. One of them was producing a high-end integrated heat pump/storage tank. And the other was producing a compact stand-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; the additional air-conditioning and heat recovery ventilation are the by-products. As a matter of fact, a condensing temperature higher than 6 C is difficult to obtain with refrigerants like R-22 and R134a. Heat pump water heaters in their normal capacities can heat water at a rate 4 1% of the electric resistance units, and 3 5% of the gas units. To provide quick recovery, a household must have a large heat pump, an unusually large storage tank, and a control system that turns on the electric resistance backup heater whenever required. However, large water tanks are more expensive, take up more space, and use more energy to maintain a set point. A higher-capacity heat pump is undesirable as it costs more and the frequent start stop often results in lower efficiency. Some designs let the small heat pump cater for most of the hot water load, but include control 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, an option 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

J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 583 energy-saving alternative. A heat pump water heater can provide hot water two to three times more energy efficient than an electric-resistance heater. If installed properly, it can also provide useful by-products space cooling, dehumidification, or heat-recovery ventilation. The extended use 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-conditioners merely for space cooling in summer. The use of heat pump is not popular because of the relatively short and mild winter. Instead, electric heaters are used during the cold days. To obtain hot water, the families rely on various types of water heaters, such as electric water heater, gasfired 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 the year; the utilization is low. (b) A family buys both air-conditioner and water heater; this may not be economical in terms of cost and space. (c) The operating cost of the gas-fired or electric water heater can be higher than that of the heat pump. (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 combustion products. (e) Condenser heat dissipation from the air-conditioners can be a source of heat pollution; this is elaborated below. For high-rise apartment buildings in Hong Kong and in the Mainland China, the building reentrant an open space where the kitchens receive daylight and ventilating air is a popular place to accommodate the outdoor units of the split-type air-conditioners. The presence of these aircooled condensing units however, introduces a thermal problem the thermal buoyance as a result of heat dissipation leads to the development of a rising air plume; inadequate air exchange at this recessed space elevates the ambient temperature, and the insufficient cooling significantly affects the condenser performance, especially for those at the upper floor levels. The result could be an overall degradation of the capacity and the efficiency of the air-conditioners at the entire building re-entrant. As most condensing units are unlikely to function properly at elevated on-coil temperature because of the abnormal condenser working pressure, the problem is therefore not only energy wastage but also equipment malfunction [6,7]. Similar problems occur in windowtype 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 the mentioned problems and limitations. Two prototypes have been produced. The following presents their basic design concepts and the laboratory test results that illustrate the operation and economic advantages.

584 J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 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 device possesses the dual functions of space air-conditioning and water heating. It is designed as a splittype unit. A water tank with immersed condensing coils is integrated in the outdoor unit. In the warm seasons, the novel air-conditioner provides space cooling as its primary task, and produces domestic hot water through the water-bath heat-recovery condenser as its secondary task. In the process, the water temperature will rise. The performance will drop because of the elevated condenser working temperature and pressure. To overcome this limitation, an air-cooled condenser is connected in series with the immersed condenser. This is to overtake the duty of the immersed condenser when the water temperature reaches a high limit. And, because of the mixed effect of water- and air-cooling, the average performance in the cooling mode will be generally higher 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 2% more than the evaporator refrigerating effect, the recovery of all rejected heat for hot water production is not necessary. The hot water consumption depends on the needs of the individual family, say on the number of family members and their living habits. As the operation time for space cooling can be lengthy, the water 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 domestic hot water. In this mode, the indoor unit is not in use. The air-cooler at the outdoor unit becomes the evaporator, which works together with the immersed condenser. In order to improve the 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 winter in the subtropical region, the outdoor temperature will not drop below C. In Hong Kong for example, the average temperature in winter is 15 C, there is minimum defrosting problem even when the ambient air is used as the heat pump heat source. 3.1.3. Provision of space heating In 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 evaporator. The indoor unit will then provide space heating. In terms of economy in equipment operation, 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 coefficient of performance (COP), which carries different definitions depending on its specific appli-

J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 585 cations. Additional performance definitions can be used in the case of the novel air-conditioner. 3.2.1. Space cooling and water heating mode Because 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: COP c ðtþ ¼ Q 2ðtÞ ð1þ W ðtþ and the overall COP value when the water waste heat for water heating is included is then: COP cw ðtþ ¼ Q 2ðtÞþQ w ðtþ ð2þ W ðtþ Their average values over a time period s is COP c avg ¼ R s Q 2ðtÞdt R s W ðtþdt ð3þ COP cw avg ¼ R s ½Q 2ðtÞþQ w ðtþšdt R s W ðtþdt ð4þ In the above equations, Q 2 ðtþ is the heat exchange rate at the indoor unit; Q w ðtþ is heat exchange rate at the immersed condenser; W ðtþ is the electric power input to the novel air-conditioner, and s is the time duration of the measurement. 3.2.2. Water-heating-only mode The water heating efficiency can be defined as COP w ðtþ ¼ Q wðtþ W ðtþ R s COP w avg ¼ Q wðtþdt R s W ðtþdt ð5þ ð6þ 3.2.3. Space-heating-only Similarly, the COP for space heating can be defined as COP h ðtþ ¼ Q 1ðtÞ W ðtþ R s COP h avg ¼ Q 1ðtÞdt R s W ðtþdt ð7þ ð8þ

586 J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 where Q 1 is the condenser heat energy released to the indoor space for a reversed cycle. This COP h for space heating is expected the same as the conventional air-to-air heat pump, when the water bath condenser is left idle. 4. The prototypes Two prototypes of slightly different design were fabricated for performance testing. Their features are described below. 4.1. Two-function air-conditioner The two-function air-conditioner is able to provide space cooling and water heating, but not space heating. This is especially suitable for applying in the tropic and subtropical areas where space heating is not essential. The water bath condenser consists of two heat transfer coils acting side by side. The two alternative working cycles are shown in Fig. 1. The white arrows in the figure indicate the direction of refrigerant flow in the space cooling and water heating mode (Mode 1), and the black arrows indicate the flow directions in the water-heating-only mode (Mode 2). The switching between these two modes is by means of the electromagnetic valves. In Mode l, Valve 1 and Valve 3 are closed and Valve 2 is open. The refrigerant discharged from the compressor passes through Coil 1 at the water bath condenser, then directly to the air-cooled condenser without passing through the Water bath condenser Coil 1 Valve 2 Air-cooled condenser Valve1 Coil 2 Compressor Capillary tube 1 Valve 3 Indoor unit Evaporator Capillary tube 2 Fig. 1. Working cycles of the two-function air-conditioner.

J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 587 Water-bath condenser Valve 1 Air-cooled condenser Four-way valve Valve 2 Capillary tube 1 Compressor Valve3 Indoor unit Evaporator Capillary tube 2 Fig. 2. Working cycles of the three-function air-conditioner. Capillary tube 1. In Mode 2, Valve 1 and Valve 3 are open and Valve 2 is closed. Both Coil 1 and Coil 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 space heating. This is particularly suitable for applying in cold climatic regions where space heating is generally required. However, it should not be subject to extremely cold weather; otherwise the proposed product with a low-temperature heat source will suffer from low efficiency. Also defrosting 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 for water-heating-only, and Mode 3 is for space-heating-only. The directions of refrigerant flow in these three modes are indicated by the white, black and gray arrows respectively. The four-way valve 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 condensation heat transfer in different modes and different thermal environment (i.e. the temperature and relative humidity of air) were measured.

588 J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 The energy performance of the three-function air-conditioner was tested in a national-standard laboratory. In the absence of standard testing procedures for this innovative product, the measurements were making reference to the Chinese National Standard GB/T7725-1996 on space heating and cooling devices. The thermal environments of the performance tests on the cooling and heat modes respectively are listed in Table 1. All laboratory measurements were taken placed in a room-type test chamber, of which the key features and provisions are shown in Fig. 3. The entire chamber was provided with high quality thermal insulation, in that the heat exchange (including thermal radiation) across the external enclosure was not to exceed 3 W, based on a temperature difference of 11 C. The two environmental chambers were separated by a well-insulated partition. Individual air-handling unit (AHU) was provided in each chamber to keep the air 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 outdoor units of the novel air-conditioners were placed. Surrounding each of the two chambers was an equalizing temperature compartment, where the air temperature was maintained the same as that in either chamber. A pressure-balancing device was inserted to monitor the pressure difference between the two chambers, and to facilitate the measurements of air leakage and air recirculating rates. For the water-heating-only mode, the performance tests were carried out solely in the outdoor chamber at four different dry bulb air temperature conditions. These were respectively 31, 25, 15 and 4.5 C. The measurements were stopped when the water bath temperature rose to 7 C. And for the space-heating-only mode, the þ7 C testing conditions is regarded as 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 2 15 (max) 7 6 Fig. 3. Key features of air-conditioner performance test chamber.

J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 589 35 3 25 2 15 1 5 5.2. Performance test results 15 3 45 6 75 9 15 Time(minute) cooling rate(w) input po wer(w) COPcw 1 water te mperature 1(deg.C) COPc 1 Fig. 4. Performance test results of three-function air-conditioner at space cooling and water-heating mode. Fig. 4 shows the testing results of the three-function air-conditioner when it operated with the space-cooling and water-heating mode. While both the input electric power and water temperature increased with time, the COP cw and COP c drop mildly. The COP cw avg is found to be 4.2 and the COP c 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 COP w gradually decreases with time as the water bath temperature increases. During the four testing periods, the COP w avg dropped from 3.42 to 2. when the setting of the controlled ambient temperature shifted from 31 to 4.5 C. It should be noted that the listed COP w avg values are for reference only, since the initial water bath temperatures 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 COP h remained constant (equal to COP h avg ) as far as the indoor and outdoor thermal environments were fixed in accordance with the specified testing conditions stated in Table 1. The performance 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 same design, except for the complexity in the refrigeration circuit. 6. Discussions The COP c of a conventional air-conditioner is in the range of 2.2 2.4. According to our test results, the novel air-conditioner has a higher COP c (2.91 in average) than the conventional

59 J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 8 6 4 2 5 9 13 17 2 23 Time(minute) (a) Ambient temperature at 31 o C input power/1(w ) water temperature(deg.c ) COPw 1 8 6 4 2 4 8 12 16 2 24 28 31 Time(minute ) input power/1(w) water temperature(deg.c) COPw 1 (b) Ambient temperature at 25 o C 8 6 4 2 4 8 12 16 2 24 28 32 Time(minute) input power/1(w) water temperature(deg.c) COPw*1 (c) Ambient temperature at 15 o C 8 6 4 2 5 1 15 2 25 3 35 4 45 5 55 6 65 Time(minute) (d) Ambient temperature at 4.5 o C input power/1(w) water temperature(deg.c) COP w 1 Fig. 5. Performance test results of three-function novel air-conditioner at water-heating-only mode. split-type air-conditioning unit. This is because of the water-cooling effect. If the novel air-conditioner is also used as a heat pump water heater, the COP cw avg of 4.2 becomes much attractive. This value is 38% higher than the COP c avg, and can be even higher if there is a continuous hot water consumption. Since once the water in tank has been consumed substantially and refilled, the heat sink temperature will be lowered and the heat pump water heater will perform more efficiently. In order to evaluate the effect of different hot water consumption modes on the energy performance, the variation of daily COP in different hot-water usage pattern should be further investigated. Furthermore, as a novel product, the equipment should be made cost effective by controlling well the initial cost and the maintenance cost. Over-sizing should be avoided.

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 COP cw avg 4.2 COP c avg 2.91 Mode 2: water-heating-only J. Ji et al. / Applied Thermal Engineering 23 (23) 581 592 591 COP w avg at 31 C 3.42 25 C 3.25 15 C 2.52 4.5 C 2. Mode 3: space-heating-only COP h 2.72 According to an investigation of the China market in 1999, the annual sale volume of airconditioners in the country was 9,. Only 3% of the families living in cities owned airconditioners. The figure reduced to 1% for the rural area. Meanwhile, the annual output of gas water heaters was 4, and the annual output of solar water collectors was 25,. The sale volume of electric water heater shared 6% of the total water heater market. Following the economic growth of the country, it is expected that more and more families will have a demand on air-conditioners and water heaters. On the other hand, the available water heaters have problems like safety, reliability, and energy saving. At this end, the proposed air-conditioner integrated with water heater is functionally safe, flexible, economical, and environmental friendly. It is expected to have a very good market in China, and even in other countries as well. Nishimura [9] gave a precise 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 warming effect. By integrating a water-cooled condenser to the outdoor unit of a heat pump, there are a number of advantages, i.e. (a) By combining the air-conditioning unit and the water heater into one product, the novel airconditioner 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 improved. (c) It can work independently as a domestic heat pump water heater; the heating efficiency is much higher than that of the electric water heater. (d) The conventional air-conditioner only works in summer. In most time of a year, it becomes idle. 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 performance under 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-type unit.

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