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An experimental study on a modified air conditioner with a domestic hot water supply (ACDHWS)
ARTICLE IN PRESS
Energy 31 (2006) 1789–1803 www.elsevier.com/locate/energy
An experimental study on a modified air conditioner with a domestic hot water supply (ACDHWS) Huimin Jiang, Yiqiang Jiang, Yang Wang, Zuiliang Ma, Yang Yao Department of Building Thermal Engineering, Harbin Institute of Technology, Box 2651, Harbin 150090, China Received 10 May 2005
Abstract The recovery of condenser heat in air conditioners is attractive because of its great economy and environmental value. This work experimentally studies a modified air conditioner with a domestic hot water supply (ACDHWS) that operates in the space-cooling and water-heating mode. The working principles and the basic features of the ACDHWS are introduced in this paper. This is followed by an experimental study on dynamic operation characteristics, hot water supply performance, energy efficiency and the temperature distribution of hot water in the storage tank of the unit. The results show that the ACDHWS can reliably be used to heat domestic hot water without losing its cooling capacity when it is controlled well in different operation conditions. Comparatively, the coefficient of comprehensive energy performance (COP2) of the ACDHWS is about 38.6% higher than that of the original unit. Furthermore, it is proved that the ACDHWS can continuously supply hot water for householders if a suitable hot water storage tank is installed. All these may be much helpful to develop a perfect ACDHWS product. r 2005 Elsevier Ltd. All rights reserved. Keywords: Experiments; Air conditioners; Domestic hot water supply; Condenser heat recovery; Energy-saving
1. Introduction A hot water supply is a necessity in developed countries, and is becoming more and more popular in developing countries. In developed countries, there is usually a central air-conditioning system with one central domestic hot water supply system in most residential buildings. In developing countries, a room air conditioner and an electrical, gas-fired or oil-fired water heater are also needed for catering for space cooling and domestic hot water supply, respectively. Usually, condenser heat from air conditioners has to directly be discharged to the outside. This not only engenders energy wasting, but yields thermal pollution to the outside environment. At the same time, most water heaters also consume much raw energy. In developed countries, energy consumption of domestic hot water is secondary after that of domestic heating and air-conditioning [1]. Therefore, it will be economical if condenser heat can be used to heat hot water instead of discharging it to the atmosphere in vain—provided the cooling capacity of an air conditioner is not reduced. Corresponding author. Tel.: +86 451 8229 1520; fax: +86 451 8628 2346.
E-mail address: [email protected] (H. Jiang). 0360-5442/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.07.004
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Nomenclature COP EER Twp Tp Pc Pe Gw Qe DT W U P r t H Tw z Qhw m c Tdb COP1 COP2 B V T
coefficient of performance energy efficiency ratio mean temperature of hot water, 1C discharge temperature, 1C condensing pressure, MPa evaporating pressure, MPa flow rate of hot water, kg h1 cooling rate, kW difference between inlet cold water and outlet hot water temperature, 1C power rate, kW uncertainty precision error density, kg m3 Time, min height of the tank, cm temperature of hot water, 1C compressive ratio heat flow rate for hot water, kW mass flow rate, kg s1 specific heat of hot water, kJ kg1 1C1 air dry-bulb temperature, 1C coefficient of performance in the space-cooling mode coefficient of performance of comprehensive utilizing energy of the ACDHWS bias error volume flow rate, m3 s1 temperature, 1C
Subscripts cw i hw o
cold water inlet hot water outlet
The recycling of condenser heat had been applied to many engineering projects to save energy [2–4], e.g. reusing condenser heat to heat office buildings and markets, utilizing it to produce hot water for crafts in beer factories, sugar refineries, papermaking and animal stock husbandries, etc. Luigi [5] put forward a modified air-conditioning system in office buildings to recover condenser heat to heat domestic hot water in Italy. The results showed that compared with the conventional system, this system could save 100% fuel cost for hot water in the hottest month and 70% in other months. However, the use of condenser heat is only limited to large installations where an amount of condenser heat is substantial. With the popularization of residential air conditioners, the recycling of condenser heat should be expected for its great economy value. In 1965, Healy and Wetherington [6] firstly demonstrated the potential for condenser heat as ‘‘free’’ energy that was used to produce hot water in residential buildings. They found that it could save 70% energy for heating domestic hot water every year. In succession, Cook [7], Ying [8], Toh and Chan [9] and Goldschmidt [10] launched experiments and simulations on using condenser heat to heat hot water and proved its feasibility. They concluded that the modified system could provide domestic hot water without losing its cooling capacity. It was also found that the actual recovery was about 20% of the total
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discharged condenser heat and COP of the system was increased. Olszeweki [11] analyzed the economic feasibility of desuperheaters in heat pumps for supplying domestic hot water. The assessment indicated that this system was especially suitable for areas where the air-conditioning load was substantial and the price of electricity was high. However, its use has not by far been fully exploited. In China, there has not been found in open literatures on it until recent years. Ying et al. [12] presented a new heat pump with hot water supply. The further research on it has not been found as yet. Shi et al. [13] put forward an assumption on an inverter aircooled heat pump with a water heater. They calculated the all-year-round operation energy consumption of the system. But this system was too complex to be realized in practice. Later, Tan and Deng [14] developed a simulation research on utilizing condenser heat from the water chiller to generate hot water. The simulation results showed that the use of the system would achieve high-energy efficiency. Ji et al. [15] carried out experiment and simulation researches on energy performance of the air conditioner integrated a water heater, and found that EER of the unit was about 30% higher than that of the conventional unit. Even now, the controllability, reliability, dynamic operation characteristics and hot water supply performance of this unit have not been discussed yet. Nor has hot water temperature distribution in the storage tank of the unit been studied. From above-mentioned, it is obvious that most studies only laid emphasis on feasible forecast and energy performance of the system, although the conception of recovering condenser heat to supply domestic hot water was put forward in 1960s. Up to now, an ideal ACDHWS product has seldom been found in sale. Factors that led to this status are various. Therefore, to develop the ACDHWS product, more in-depth studies should be done, such as the controllability, reliability and some other operation characteristics of the unit. This paper aims to solve some existing problems that will prevent from developing the ACDHWS product. Firstly, a modified air conditioner with a domestic hot water supply (ACDHWS) is developed in this paper. Compared with former systems, it is simpler, cheaper and easier to be controlled and will be easier to be adopted by consumers. Dynamic operation characteristics, hot water supply performance and energy efficiency of the unit were tested in the space-cooling and water-heating mode. And then the research on the hot water storage tank was conducted and the comparison between the modified ACDHWS and the original was also done. All these may be much helpful to develop a perfect ACDHWS product. 2. ACDHWS 2.1. The description of the ACDHWS The sketch of the ACDHWS is shown in Fig. 1. It mainly consists of a compressor, a hot water storage tank with immersed condenser coils, an outdoor heat exchanger, an indoor heat exchanger, an expansion valve and a set of control valves. In contrast to the original, the ACDHWS is added with a hot water storage tank and a set of control valves. It is designed as a multi-task and year-round service unit with five modes, i.e. a spacecooling mode, a space-heating mode, a water-heating mode, a space-cooling and water-heating mode and a space- and water-heating mode. In the former two modes, the ACDHWS behaves like a conventional airsource heat pump. It becomes a heat pump water heater in the water-heating mode. In winter, the unit has to satisfy space heating for rooms and domestic hot water supplying simultaneously. Therefore, some problems may occur in the space- and water-heating mode, e.g. the amount of supplying heat may be insufficient, the flow rate of domestic hot water may be smaller than the required, the temperature of hot water may be too low to use. To solve these problems, a variable speed compressor, an auxiliary heater or a more big heat storage device may be used in the system. Hence, the ACDHWS is especially suitable for tropic and subtropical areas where space-cooling and water-heating are necessary, while space-heating is not essential. In this paper, the space-cooling and water-heating mode is presented in detail. In Fig. 1, the black arrows indicate the direction of refrigerant flow, while the white ones indicate the direction of hot water flow. The ACDHWS can be designed as the split-type unit. Its water tank may be integrated with the outdoor unit, or laid in the bathroom or the kitchen separately. The temperature of hot water in the storage tank will rise when the unit runs for a period of time. The operation performance of the unit will drop because of the elevated working pressure, condensing temperature and discharge temperature of the compressor. To overcome these limitations, an air-cooled condenser will be connected in series with the
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Fig. 1. Schematic of the ACDHWS in space-cooling and water-heating mode. 1-compressor, 2-evaporator, 3-condenser, 4-thermal expansion valve, 5-domestic water tank, 6-four-way valve, 7-gas-liquid separator, 8-water pump, 9–15-electromagnetic valve, 16protection valve, 17-electricity heaters.
hot water storage tank. The air-cooled condenser will not operate until the temperature of hot water in the tank reaches a high limit. Thus, the average performance of the ACDHWS in this mode will be generally higher than that of the original unit because of the corporate effect of water- and air-cooling. 2.2. The design of the ACDHWS The experimental sample unit was designed as a split-type air conditioner with a water heater integrated with the outdoor unit. The unit had a nominal capacity of 4.5 kW with R-22 as the refrigerant. It consisted of the outdoor and the indoor unit with physical sizes of 900 500 960 and 440 250 300 mm, respectively. The volume of the hot water storage tank mainly depends on some factors, such as the capacity of the air conditioner, operation hours per day, hot water using patterns of occupants, etc. [7]. In this paper, the tank holding 130 l water was designed as a cylindrical tank with the diameter of 540 mm and the height of 850 mm. The amount of hot water is sufficient for a typical family [16]. The chosen size of the hot water storage tank should satisfy the minimum requirement for providing a continuous hot water supply for shower when hot water is heated to the required temperature, say about 45 1C. As shown in Fig. 2, the hot water storage tank was insulated with 50 mm-thickness rock wool wrapped with a layer of aluminum foil. A sight glass was fixed to check the level of water and a relief valve to discharge air in the tank. Its heating coils consisted of copper tubes of 12 mm in diameter and 83.5 m in length. Thirteen thermocouples were installed to measure the variation of hot water temperature along the depth of the tank. Moreover, to measure temperature and pressure, pressure gauges, thermocouples and isolation valves were also installed at certain parts of the ACDHWS (see Fig. 1). 2.3. Laboratory set-up for the ACDHWS To examine the performance of the ACDHWS, heat transfer of the evaporator and condenser in different thermal environment (i.e. the temperature and relative humidity of air) were measured. The experiment was carried out in a laboratory built on the basis of Chinese National Code (GB/T17758-1999). The main thermal
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Fig. 2. Schematic of the domestic hot water tank.
Table 1 Main testing environment Controlled air temperature at indoor chamber (1C)
Controlled air temperature at outdoor chamber (1C)
DB
WB
DB
WB
2570.1
17.570.1
2870.1
19.770.1
3570.1
2470.1
environments of the experiment and test instruments are listed in Tables 1 and 2, respectively. All tests were conducted in several room-type test chambers whose key features and provisions are shown in Fig. 3. The entire chamber was provided with high quality thermal insulation. The two adjoining chambers were separated by a well-insulated partition. Individual air-handling unit (AHU) was provided to control air temperature and relative humidity of each chamber. Indoor and outdoor units of the ACDHWS were placed in the two adjoining chambers, respectively. A pressure-balancing device was inserted to monitor the pressure difference between the two chambers and facilitate the measurements of air leakage. A computer in the control-room was used to monitor, collect and process experimental data. The error analysis of experimental data is carried out in detail in Appendix A and the results of uncertainty analysis are summarized in Table 3.
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Parameters
Instruments
Measure range
Type and precision
1 2 3 4 5
Pressure Temperature Flux Power Time
Pressure transfer device Thermocouple Rotameter flowmeter Power meter Stopwatch
0–2.1 MPa 40–400 1C 10–70 L min1 0.4–12 kW 0–60 s
Type: SEPRA2801, Precision: 0.001 MPa Precision: 0.11C Type: MBLG, Precision: 1.5 level Type: 7950A, Precision: 7(0.1% range+0.4% reading) Precision: 0.1 s
Fig. 3. Sketch of the ACDHWS performance test chamber. 1-outdoor unit, 2-water storage tank, 3-indoor unit, 4-standard fan coils, 5air-conditioning unit, 6-water-cooling unit, 7-adding moisture instrument, 8-supply water instrument, 9-controlled platform, 10-AHU, 11power ark, 12-manostat, 13-wiring ark, 14-air meter instrument.
Table 3 Summary of error analysis Items
Bias errors (%)
Precision errors (%)
Uncertainty (%)
V cm V hm T i;cm T o;cm T i;hm T o;hm Qe Qhm W COP1 COP2
0.0137 0.028 2 2 0.4 0.4 2.84 0.57 0.044 2.84 2.90
0.01 0.02 2 2 0.4 0.4 2.84 0.56 0.9 2.98 3.03
0.017 0.034 2.83 2.83 0.57 0.57 4.02 0.81 0.9 4.1 4.2
3. Results and discussion In space-cooling and water-heating mode, the ACDHWS was tested in five different operation conditions of water-heating. (A) Water-heating without the air-cooled condenser. (B) Water-heating with the air-cooled condenser in natural convection condition.
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(C) Water-heating with the air-cooled condenser in forced convection condition. (D) Simulation test I—firstly, water-heating without using hot water, secondly withdrawing hot water without water-heating to simulate continuous use of hot water, and then water-heating without using water again. (E) Simulation test II—water-heating with withdrawing hot water to simulate the amount of hot water provided continuously by the unit in the given temperature. The ACDHWS was controlled to run in condition (A) at the start, and then switched to run in condition (B) and condition (C) in turn because of increasing of condensing pressure and discharge temperature, finally steadily run in condition (C). At the same time, the unit was also controlled to run in condition (D) and condition (E) in order to test its performance, respectively. All test results and discussion are as follows. 3.1. Dynamic operation characteristics of the ACDHWS Dynamic operation characteristics were tested under the condition of the indoor air temperature 25 1C and the outdoor air temperature 35 1C. Figs. 4(a) and 5(a) show the change of mean temperature of hot water, evaporating pressure, condensing pressure and discharge temperature of the compressor with time after the start of the unit. The mean temperature is calculated by arithmetical average of the hot water temperature along the depth of the tank. The rising rate of the mean temperature of hot water is the largest during the initial stage of heating water without the air-cooled condenser, condition (A). When the mean temperature rises to about 36 1C and the discharge temperature of the compressor reaches 115 1C (exceeding the permitted discharge temperature of the compressor), the unit is quickly switched to condition (B). Here, the discharge temperature rises to about 117 1C very quickly, and then the unit is promptly turned into condition (C). In condition (C), the discharge temperature, the evaporating pressure and the condensing pressure remain some value hereabout, and the unit runs steadily. This is because the rising of hot water temperature in the tank makes the cooling capacity of the water-cooled condenser (i.e. the hot water storage tank) drop, superheat degree increase and flow rate of refrigerant decrease. When the forced air-cooled condenser starts, the above-mentioned problems are completely solved. In other words, the ACDHWS can be switched from (A) condition to (C) condition directly in order to obtain better operation performance in practice. In the experimental process, the judgment parameter of switching is the discharge temperature. In fact, the condensing pressure, the discharge temperature and COP of the system may decide switching condition. Fig. 5(b) shows the change of cooling capacity, power and COP with time after the start of the unit. In late condition (A), the cooling capacity and COP are dropping sharply, even if the phenomenon has not been ameliorated in condition (B). When the unit operates in condition (C), its performance is improved instantly.
heating 117 minutes
heating 97 minutes
heating 107 minutes
heating 77 minutes
heating 57 minutes
heating 47 minutes
50 H (cm)
35 Twp (°C)
heating 37 minutes
60
heating 25 minutes
c
a
heating 17 minutes
b
40
heating 1 minutes
70
45
heating 9 minutes
50
30
40 30
25 20
20
a-only water heating b-water heating with natural air-cooled c-water heating with forced air-cooled
15
10
10
(a)
0
20
40
60 80 100 Time t (minute)
120
140
160
(b)
15
20
25
30 35 Tw (°C)
40
45
50
Fig. 4. (a) Mean water temperature in the tank vs. time. (b) Variation of water temperature vs. time along the height of the tank.
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1.2
120
7
1.0
110
0.8
100
Pc / Pe (MPa)
a
b
c
0.6
Qe W
COP a-only water heating b-water heating with natural air-cooled c-water heating and forced air-cooled a b c
2
6 5 4
1
90 3
0.4
80 a-only water heating b-water heating with natural air-cooled c-water heating with forced air-cooled
0.2 0.0 0
(a)
Q e / W (kW)
8
Tp (°C)
130
1.4
3
9
140
Pc Pe Tp
1.6
COP
1796
20
40
60 80 Time t (minute)
100
2 70 1 120
0
20
40
(b)
60 80 Time t (minute)
100
0 120
Fig. 5. (a) Discharge temperature, evaporating pressure and condensing pressure vs. time. (b) Rate of refrigeration, power and COP vs. time.
55 50 45
Tw (°C)
40 35
c
b 30
Tw1
a
T
w12
Tw10
25
Tw7
A-using hot water B-stop using hot water a-only water heating
20 15
b-water heating with natural air-cooled c-water heating with forced air-cooled
B
A
Tw4
10 0
20
40
60 80 Time t (minute)
100
120
140
Fig. 6. Variation of water temperature in the tank vs. time in using hot water.
Therefore, to obtain high performance, the ACDHWS may be switched from condition (A) to condition (C) directly on basis of the COP value (e.g. switching about at COP 2.4), not only the discharge temperature and the condensing pressure. Thus, it is proved that judgment parameters of switching are the discharge temperature, the condensing pressure and the COP value of the ACDHWS to obtain better performance. It will be the theoretical foundation to control well the ACDHWS product in the future. Fig. 4(b) shows the variation of hot water temperature along the height of the tank with time. The change of temperature is not obvious in most parts of the water tank above 200 mm. The temperature in most parts of the water tank is high. It is very favorable for householders to use hot water. Fig. 6 shows the variation of hot water temperature in the hot water storage tank in condition (D). In Fig. 6, Tw1, Tw4, Tw7, Tw10 and Tw12 are points 750, 480, 300, 165 and 75 mm away from the bottom of the tank, respectively. Tw1 is also the temperature of supplying hot water. When hot water is heated to about 45 1C, hot
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150 Pc Pe Tp
140
110
1.0 a
b
A-using hot water B B-stop using hot water
A
0.6
a-only water heating
c
100
b-water heating with natural air-cooled c-water heating with forced air-cooled
3 b-water heating with natural air-cooled
5
120 Tp (°C)
Pc / Pe (MPa)
6
130
1.2
0.8
A-using hot water B-stop using hot water a-only water heating c-water heating with forced air-cooled
2 A
a
B
b
c
4
COP
1.4
Q e / W (kW)
1.6
1797
3
80
1
Qe W COP
90 2
0.4 70 1
0
20
40
60
(a)
80 100 120 Time t (minute)
140
160
0
0
180
20
40
(b)
60
80 100 120 Time t (minute)
140
160
180
Fig. 7. (a) Discharge temperature, evaporating pressure and condensing pressure vs. time in using hot water. (b) Rate of refrigeration, power and COP vs. time in using hot water.
5 Tp Twp
120
b
1.8 a
Pc Pe
c
1.6
100
Z
1.4 c
60 40
a-only water heating b-water heating with natural air-cooled c-water heating with forced air-cooled
1.2 1.0
4
0.8 0.6
a-only water heating b-water heating with natural air-cooled c-water heating with forced air-cooled
20
Z=Pc / Pe
80
Pc / Pe (MPa)
Tp / Twp (°C)
b a
3
0.4 0.2
0
(a)
100
200
300
400 500 600 Time t (minute)
700
800
0
900
(b)
100
200
300
400 500 600 Time t (minute)
700
800
900
Fig. 8. (a) Discharge temperature and hot water temperature vs. time without using hot water. (b) Evaporating pressure, condensing pressure and pressure ratio vs. time without using hot water
water is drawn from the tank at a rate of 6.5 L/min (quantity of shower bath per capita in a typical family) and the tank is refilled by water from the main. The temperature of hot water is too low to use after the withdrawal lasts about 20 min. Then, water in the tank will continuously be heated for another 70 min to use again. After that, hot water is drawn from the tank again and the cycle is repeated. The above withdrawal of hot water simulates the use of hot water for shower in a typical family [8]. The amount of hot water is enough to cater for one-person shower in a typical family. If time interval of using hot water is shortened, then the initial water temperature will be lifted or the size of the tank will be increased. Fig. 7(a) and (b) show the change of evaporating pressure, condensing pressure, discharge temperature, cooling capacity, power and COP with time from the start of using hot water. The results show that the performance of the unit is improved and COP is also increased in using hot water process without air-cooled condenser. It is proved that the ACDHWS could provide a continuous hot water supply for householders if it could be reasonably controlled. Fig. 8(a) and (b) show the change of evaporating pressure, condensing pressure, discharge temperature of the compressor and pressure ratio with time within 15 h after the start without using hot water. Thus, the
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reliability of the ACDHWS could be examined by this experiment. In condition (A), the ACDHWS has been running for 80 min, when evaporating pressure, condensing pressure and discharge temperature of the compressor rise sharply, especially the discharge temperature of the compressor (rising to about 120 1C). Then the system is switched to operate in condition (B). After the unit has been running for 15 min in condition (B), the discharge temperature rises to about 120 1C again. And it is switched to operate in condition (C). Then, the unit steadily runs in condition (C). It is tested that the ACDHWS is reliable after having been operated for 15 h steadily in this condition. In heating process, electromagnetic valve 11 (Fig. 1) is switched to avoid superheating hot water in the tank. It is concluded that the unit can steadily provide space cooling and hot water supply simultaneously—provided it could reasonably be switched in different operation conditions. 3.2. Hot water supply performance and energy efficiency of the ACDHWS The performance of supplying hot water and energy efficiency of the ACDHWS were tested under the conditions of the indoor air temperature 25 1C and the outdoor air temperature 35, 32 and 28 1C, respectively. Fig. 9(a) shows the results of the continuous flow rate of hot water vs. the ambient air temperature and
130 120 110
35 °C hot water supply 45 °C hot water supply 50 °C hot water supply 55 °C hot water supply
4.1
100 90 Q e (kW)
Gw (kg/h)
35 °C hot water supply 45 °C hot water supply 50 °C hot water supply 55 °C hot water supply
4.2
80 70
4.0
3.9
60 50
3.8 40 30
3.7 28
30
(a)
32 Tdb (°C)
34
28
36
2.0 1.9 1.8
35 °C hot water supply 45 °C hot water supply 50 °C hot water supply 55 °C hot water supply
1.6
COP
W (kW)
1.7
1.5 1.4 1.3 1.2 28
(c)
30
32 Tdb (°C)
30
(b)
34
36
3.4 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.9
34
36
35 °C hot water supply 45 °C hot water supply 50 °C hot water supply 55 °C hot water supply
28
(d)
32 Tdb (°C)
30
32 Tdb (°C)
34
36
Fig. 9. (a) Hot water flow-rate vs. ambient air temperature and hot water temperature with cooling and using hot water. (b) Cooling rate vs. ambient air temperature and hot water temperature with cooling and using hot water. (c) Power rate vs. ambient air temperature and hot water temperature with cooling and using hot water. (d) COP vs. ambient air temperature and hot water temperature with cooling and using hot water.
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Table 4 Comparison of experiment results between the ACDHWS and the original unit Outdoor (1C)
Types
Pe (MPa)
Pc (MPa)
z ¼ Pc/Pe
Qe (kW)
W (kW)
COP1
Gw (kg h1)
COP2
28
The Original The ACDHWS
0.418 0.411
1.415 1.297
3.385 3.156
4.0 3.9
1.750 1.636
2.3 2.4
— 66.63 (45 1C) 28.61 (55 1C)
2.3 3.8 3.2
35
The Original The ACDHWS
0.494 0.469
1.763 1.564
3.569 3.335
4.3 4.3
2.160 1.926
2.0 2.2
— 91.40 (45 1C) 52.12 (55 1C)
2.0 3.7 3.3
the hot water temperature. These mainly test the capacity of supplying hot water with space cooling by the ACDHWS in the given water temperature. Fig. 9(b)–(d) show the results of the cooling rate, the power rate and COP vs. the ambient air temperature and the hot water temperature, respectively. When the outdoor air temperature is constant, the flow rate of hot water, the cooling rate and the COP value are increasing with the drop of hot water temperature, except the power rate is decreasing. When the temperature of hot water is constant, the flow rate of hot water and the power rate are increasing with the ascending of the outdoor air temperature, yet the cooling rate and the COP valve are decreasing. E.g. at the outdoor air temperature 35 1C, the flow rate of hot water rises by 132.9%, the cooling rate increases by 2.4%, the power rate drops by 12.8% and COP ascends by 11.4% when the temperature of hot water drops from 55 to 45 1C. At the temperature of hot water 45 1C, the flow rate of hot water rises by 22.2%, the cooling rate decreases by 5.7%, the power rate rises by 16.6% and COP drops by 16.7% when the outdoor air temperature rises from 28 to 35 1C. It is obvious that the variation of the cooling rate is smaller in contrast to other parameters. It is essential that the ACDHWS could retain the cooling rate whether supplying hot water or not. 3.3. Comparison of the ACDHWS with the original air conditioner The tests were carried out under the condition of the indoor air temperature 25 1C and the outdoor air temperatures 35 and 28 1C. The results are shown in Table 4. In Table 4, P Q þ Qe Q Q P P COP2 ¼ ; COP1 ¼ P e and Qhw ¼ chw mhw DT hw . ¼ hw W W W According to the test results, COP1 of the original unit is in the range of 2.0–2.3. Comparatively, the ACDHWS has higher COP1 (2.2–2.4) by 10% than that of the original unit. If the ACDHWS is also used as a water heater, COP2 with 3.2–3.8 becomes much attractive. The average COP2 of the ACDHWS is about 38.6% higher than that of the original unit. It can be even higher if there is much more hot water consumption. Therefore, it is obvious that the ACDHWS is more economical than the original unit. 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 patterns should be further investigated. As a novel product, the equipment should be made economical by controlling the initial cost and the maintenance cost well. Furthermore, comprehensive economical evaluation on the ACDHWS should be developed further in the future. According to an investigation of the China markets in 1999, the annual sale quantity of air conditioners was about 900,000. Only about 30% of the families living in cities owned air conditioners. Meanwhile, the sale quantity of electric water heater shared about 60% of the total water heater markets. With the economy growth of the country, it is expected that more and more families will have a demand on air conditioners and water heaters. The ACDHWS is functionally safe, flexible, economical and environmental. It is expected to have a very good market not only in China, but also elsewhere in the world.
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4. Conclusions A novel ACDHWS is presented in this paper. And its dynamic operation characteristics, hot water supply performance, energy efficiency and the temperature distribution of hot water in the storage tank were tested in different operation condition by using the experimental rig developed. The results verify that the ACDHWS is a flexible, multi-functional and economical unit, and is completely reliable even under the condition of the operation without using hot water for a long time if it can reasonably be switched in different operation conditions. Furthermore, the comparison of operation performance between the original unit and the ACDHWS shows that COP1 of the ACDHWS is about 10% higher than that of the original unit, and COP2 of the ACDHWS is about 38.6% higher than that of the original unit. Therefore, the ACDHWS not only has a better operation performance than the original unit, but also can provide a continuous hot water supply for householders. Thus, the results can also provide valuable reference to the industry of water heaters and air conditioners. Acknowledgement The project sponsored by Shandong Jin Ji Yuan Electrical Ltd in China and by SRF for ROCS, SEM is gratefully acknowledged. Appendix A A.1. Error analysis The total experimental error of the measurement is also called experimental uncertainty. It is classified by bias error and precision error. The 95% confidence uncertainty is calculated as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U ¼ B2 þ P2 . (1) For example, Qe ¼ ccw mcw DT cw ¼ ccw mcw ðT o;cw T i;cw Þ,
(2)
Qhw ¼ chw mhw DT hw ¼ chw mhw ðT o;hw T i;hw Þ,
(3)
mcw ¼ V cw rcw ; mhw ¼ V hw rhw ,
(4)
COP1 ¼
Qe , W
Qhw þ Qe . W The 95% confidence uncertainty of cooling capacity is calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U Qe B Qe 2 P Qe 2 ¼ þ Qe Qe Qe COP2 ¼
bias limit: BQe 2 BT i;cw 2 BT o;cw 2 Bccw 2 Bmcw 2 ¼ þ þ þ Qe ccw mcw DT cw DT cw precision limit: PQe 2 PT i;cw 2 PT o;cw 2 Pccw 2 Pmcw 2 ¼ þ þ þ , Qe ccw mcw DT cw DT cw where Bccw ; Pccw ; Brcw ; Prcw ; Bchw ; Pchw ; Brhw ; Prhw ¼ 0 since C w ; rw is constant.
(5) (6)
(7)
(8)
(9)
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Therefore, bias limit: B Qe 2 BT i;cw 2 BT o;cw 2 BV cw 2 ¼ þ þ , Qe V cw DT cw DT cw
(10)
precision limit: P Qe 2 PT i;cw 2 PT o;cw 2 PV cw 2 ¼ þ þ . Qe V cw DT cw DT cw 1. Calculation of bias error BV cw ¼ 0:0137%, V cw BV hw ¼ 0:028%, V hw BT i;cw BT o;cw ¼ ¼ 2%, DT cw DT cw BT i;hw BT o;hw ¼ ¼ 0:4%, DT hw DT hw BW ¼ 0:044%, W
BQe ¼ Qe
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi BV cw 2 BT i;cw 2 BT o;cw 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 0:01372 þ 22 þ 22 % ¼ 2:84%, þ þ V cw DT cw DT cw
BQhw ¼ Qhw
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi BT i;hw 2 BT o;hw 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi BV hw 2 ¼ 0:0282 þ 0:42 þ 0:42 % ¼ 0:57%. þ þ V hw DT hw DT hw
2. Calculation of precision error PV cw ¼ 0:01%, V cw PV hw ¼ 0:02%, V hw PT i;cw PT o;cw ¼ ¼ 2%, DT cw DT cw
(11)
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PT i;hw PT o;hw ¼ ¼ 0:4%, DT hw DT hw PW ¼ 0:9%, W P Qe ¼ Qe
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PT i;cw 2 PT o;cw 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PV cw 2 ¼ 0:012 þ 22 þ 22 % ¼ 2:84%, þ þ V cw DT cw DT cw s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi PV hw 2 PT i;hw 2 PT o;hw 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 0:022 þ 0:42 þ 0:42 % ¼ 0:56%. þ þ V hw DT hw DT hw
PQhw ¼ Qhw
3. Calculation of uncertainty s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi U Qe BQe 2 P Qe 2 ¼ 4%, ¼ þ Qe Qe Qe U Qhw ¼ Qhw UW ¼ W
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi BQhw 2 PQhw 2 ¼ 0:8%, þ Qhw Qhw
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 BW PW ¼ 0:9%, þ W W
U COP1 ¼ COP1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U Qe 2 UW 2 ¼ 4:1%, þ Qe W
U COP2 ¼ COP2
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U Qe 2 U Qhw 2 UW 2 ¼ 4:2%. þ þ Qe Qhw W
References [1] Meyer J. Domestic hot water consumption of the developed and developing communities in South Africa. ASHRAE Trans 1999;105(Part 1):173–8. [2] Xu Bangyu, Lu Yajun, Ma Zuiliang. Heat pump ( ). Beijing: China Construction Industry Press; 1988 (in Chinese). [3] Westinghouse Electric Co. Beer pasteurize with heat pump. ASHRAE J 1980;(9):23–6. [4] Kohloss KH. Use of heat rejection equipment to conserve energy. ASHRAE Trans 1984;90(Part 1):377–84. [5] Luigi Schibuola. Experimental analysis of condenser heat recovery in an air condition plant. Energy 1999;24(4):273–83. [6] Healy CT, Wetherington Jr TI. Water heating by recovery of rejected heat from heat pump. ASHRAE J 1965;(4):68–74. [7] Cook RE. Water storage tank size requirement for residential heat pump/air-conditioner desuperheater recovery. ASHRAE Trans 1990;96(Part 2):715–9. [8] Ying WM. Performance of room air conditioner used for cooling and hot water heating. ASHRAE Trans 1989;95(Part 2):441–4. [9] Toh KC, Chan SK. Thermosiphon heat recovery from an air- conditioner for a domestic hot water system. ASHRAE Trans 1993;99(Part 1):259–64. [10] Goldschmidt VW. Desuperheater water-heater and air-to-air heat pump system: representative performance data. ASHRAE Trans 1990;96(Part 2):417–21.
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[11] Olszeweki M. Economic viability of heat pump desuperheater for supplying domestic hot water. ASHRAE Trans 1984;90(Part 1B): 169–77. [12] You Ying, Harmon Steve, Sidney Phil. A new heat pump system for year-round water heating and seasonal space cooling. In: Proceedings of the seventh International Energy Agency conference on heat pump technologies, vol. 2, 2002. p. 780–4. [13] Shi Wenxing, Shao Shuangquan, Wang Baolong, Ma Jie, Li Xianting, Yan Qisen. Thermodynamics analysis and thermoeconomics evaluation on heat recovery inverter air-cooling heat pump with domestic hot water. In: Proceedings of the seventh International Energy Agency conference on heat pump technologies, vol. 2, 2002. p. 889–94. [14] Tan Kunxiong, Deng Shiming. A simulation study on a water chiller complete with a desuperheater and a reversibly used water cooling tower (RUWCT) for service hot water generation. Build Environ 2002;37(7):741–51. [15] Jie Ji, Tintai Chow, Gang Pei, Jun Dong, Wei He. Domestic air-conditioner and integrated water heater for subtropical climate. Appl Therm Eng 2003;23(5):581–92. [16] Chen Yaozong. Architecture water supply and drainage design handbook ( ). Beijing: China Construction Industry Press; 1995 (in Chinese).
Energy 31 (2006) 1789–1803 www.elsevier.com/locate/energy
An experimental study on a modified air conditioner with a domestic hot water supply (ACDHWS) Huimin Jiang, Yiqiang Jiang, Yang Wang, Zuiliang Ma, Yang Yao Department of Building Thermal Engineering, Harbin Institute of Technology, Box 2651, Harbin 150090, China Received 10 May 2005
Abstract The recovery of condenser heat in air conditioners is attractive because of its great economy and environmental value. This work experimentally studies a modified air conditioner with a domestic hot water supply (ACDHWS) that operates in the space-cooling and water-heating mode. The working principles and the basic features of the ACDHWS are introduced in this paper. This is followed by an experimental study on dynamic operation characteristics, hot water supply performance, energy efficiency and the temperature distribution of hot water in the storage tank of the unit. The results show that the ACDHWS can reliably be used to heat domestic hot water without losing its cooling capacity when it is controlled well in different operation conditions. Comparatively, the coefficient of comprehensive energy performance (COP2) of the ACDHWS is about 38.6% higher than that of the original unit. Furthermore, it is proved that the ACDHWS can continuously supply hot water for householders if a suitable hot water storage tank is installed. All these may be much helpful to develop a perfect ACDHWS product. r 2005 Elsevier Ltd. All rights reserved. Keywords: Experiments; Air conditioners; Domestic hot water supply; Condenser heat recovery; Energy-saving
1. Introduction A hot water supply is a necessity in developed countries, and is becoming more and more popular in developing countries. In developed countries, there is usually a central air-conditioning system with one central domestic hot water supply system in most residential buildings. In developing countries, a room air conditioner and an electrical, gas-fired or oil-fired water heater are also needed for catering for space cooling and domestic hot water supply, respectively. Usually, condenser heat from air conditioners has to directly be discharged to the outside. This not only engenders energy wasting, but yields thermal pollution to the outside environment. At the same time, most water heaters also consume much raw energy. In developed countries, energy consumption of domestic hot water is secondary after that of domestic heating and air-conditioning [1]. Therefore, it will be economical if condenser heat can be used to heat hot water instead of discharging it to the atmosphere in vain—provided the cooling capacity of an air conditioner is not reduced. Corresponding author. Tel.: +86 451 8229 1520; fax: +86 451 8628 2346.
E-mail address: [email protected] (H. Jiang). 0360-5442/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.07.004
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Nomenclature COP EER Twp Tp Pc Pe Gw Qe DT W U P r t H Tw z Qhw m c Tdb COP1 COP2 B V T
coefficient of performance energy efficiency ratio mean temperature of hot water, 1C discharge temperature, 1C condensing pressure, MPa evaporating pressure, MPa flow rate of hot water, kg h1 cooling rate, kW difference between inlet cold water and outlet hot water temperature, 1C power rate, kW uncertainty precision error density, kg m3 Time, min height of the tank, cm temperature of hot water, 1C compressive ratio heat flow rate for hot water, kW mass flow rate, kg s1 specific heat of hot water, kJ kg1 1C1 air dry-bulb temperature, 1C coefficient of performance in the space-cooling mode coefficient of performance of comprehensive utilizing energy of the ACDHWS bias error volume flow rate, m3 s1 temperature, 1C
Subscripts cw i hw o
cold water inlet hot water outlet
The recycling of condenser heat had been applied to many engineering projects to save energy [2–4], e.g. reusing condenser heat to heat office buildings and markets, utilizing it to produce hot water for crafts in beer factories, sugar refineries, papermaking and animal stock husbandries, etc. Luigi [5] put forward a modified air-conditioning system in office buildings to recover condenser heat to heat domestic hot water in Italy. The results showed that compared with the conventional system, this system could save 100% fuel cost for hot water in the hottest month and 70% in other months. However, the use of condenser heat is only limited to large installations where an amount of condenser heat is substantial. With the popularization of residential air conditioners, the recycling of condenser heat should be expected for its great economy value. In 1965, Healy and Wetherington [6] firstly demonstrated the potential for condenser heat as ‘‘free’’ energy that was used to produce hot water in residential buildings. They found that it could save 70% energy for heating domestic hot water every year. In succession, Cook [7], Ying [8], Toh and Chan [9] and Goldschmidt [10] launched experiments and simulations on using condenser heat to heat hot water and proved its feasibility. They concluded that the modified system could provide domestic hot water without losing its cooling capacity. It was also found that the actual recovery was about 20% of the total
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discharged condenser heat and COP of the system was increased. Olszeweki [11] analyzed the economic feasibility of desuperheaters in heat pumps for supplying domestic hot water. The assessment indicated that this system was especially suitable for areas where the air-conditioning load was substantial and the price of electricity was high. However, its use has not by far been fully exploited. In China, there has not been found in open literatures on it until recent years. Ying et al. [12] presented a new heat pump with hot water supply. The further research on it has not been found as yet. Shi et al. [13] put forward an assumption on an inverter aircooled heat pump with a water heater. They calculated the all-year-round operation energy consumption of the system. But this system was too complex to be realized in practice. Later, Tan and Deng [14] developed a simulation research on utilizing condenser heat from the water chiller to generate hot water. The simulation results showed that the use of the system would achieve high-energy efficiency. Ji et al. [15] carried out experiment and simulation researches on energy performance of the air conditioner integrated a water heater, and found that EER of the unit was about 30% higher than that of the conventional unit. Even now, the controllability, reliability, dynamic operation characteristics and hot water supply performance of this unit have not been discussed yet. Nor has hot water temperature distribution in the storage tank of the unit been studied. From above-mentioned, it is obvious that most studies only laid emphasis on feasible forecast and energy performance of the system, although the conception of recovering condenser heat to supply domestic hot water was put forward in 1960s. Up to now, an ideal ACDHWS product has seldom been found in sale. Factors that led to this status are various. Therefore, to develop the ACDHWS product, more in-depth studies should be done, such as the controllability, reliability and some other operation characteristics of the unit. This paper aims to solve some existing problems that will prevent from developing the ACDHWS product. Firstly, a modified air conditioner with a domestic hot water supply (ACDHWS) is developed in this paper. Compared with former systems, it is simpler, cheaper and easier to be controlled and will be easier to be adopted by consumers. Dynamic operation characteristics, hot water supply performance and energy efficiency of the unit were tested in the space-cooling and water-heating mode. And then the research on the hot water storage tank was conducted and the comparison between the modified ACDHWS and the original was also done. All these may be much helpful to develop a perfect ACDHWS product. 2. ACDHWS 2.1. The description of the ACDHWS The sketch of the ACDHWS is shown in Fig. 1. It mainly consists of a compressor, a hot water storage tank with immersed condenser coils, an outdoor heat exchanger, an indoor heat exchanger, an expansion valve and a set of control valves. In contrast to the original, the ACDHWS is added with a hot water storage tank and a set of control valves. It is designed as a multi-task and year-round service unit with five modes, i.e. a spacecooling mode, a space-heating mode, a water-heating mode, a space-cooling and water-heating mode and a space- and water-heating mode. In the former two modes, the ACDHWS behaves like a conventional airsource heat pump. It becomes a heat pump water heater in the water-heating mode. In winter, the unit has to satisfy space heating for rooms and domestic hot water supplying simultaneously. Therefore, some problems may occur in the space- and water-heating mode, e.g. the amount of supplying heat may be insufficient, the flow rate of domestic hot water may be smaller than the required, the temperature of hot water may be too low to use. To solve these problems, a variable speed compressor, an auxiliary heater or a more big heat storage device may be used in the system. Hence, the ACDHWS is especially suitable for tropic and subtropical areas where space-cooling and water-heating are necessary, while space-heating is not essential. In this paper, the space-cooling and water-heating mode is presented in detail. In Fig. 1, the black arrows indicate the direction of refrigerant flow, while the white ones indicate the direction of hot water flow. The ACDHWS can be designed as the split-type unit. Its water tank may be integrated with the outdoor unit, or laid in the bathroom or the kitchen separately. The temperature of hot water in the storage tank will rise when the unit runs for a period of time. The operation performance of the unit will drop because of the elevated working pressure, condensing temperature and discharge temperature of the compressor. To overcome these limitations, an air-cooled condenser will be connected in series with the
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Fig. 1. Schematic of the ACDHWS in space-cooling and water-heating mode. 1-compressor, 2-evaporator, 3-condenser, 4-thermal expansion valve, 5-domestic water tank, 6-four-way valve, 7-gas-liquid separator, 8-water pump, 9–15-electromagnetic valve, 16protection valve, 17-electricity heaters.
hot water storage tank. The air-cooled condenser will not operate until the temperature of hot water in the tank reaches a high limit. Thus, the average performance of the ACDHWS in this mode will be generally higher than that of the original unit because of the corporate effect of water- and air-cooling. 2.2. The design of the ACDHWS The experimental sample unit was designed as a split-type air conditioner with a water heater integrated with the outdoor unit. The unit had a nominal capacity of 4.5 kW with R-22 as the refrigerant. It consisted of the outdoor and the indoor unit with physical sizes of 900 500 960 and 440 250 300 mm, respectively. The volume of the hot water storage tank mainly depends on some factors, such as the capacity of the air conditioner, operation hours per day, hot water using patterns of occupants, etc. [7]. In this paper, the tank holding 130 l water was designed as a cylindrical tank with the diameter of 540 mm and the height of 850 mm. The amount of hot water is sufficient for a typical family [16]. The chosen size of the hot water storage tank should satisfy the minimum requirement for providing a continuous hot water supply for shower when hot water is heated to the required temperature, say about 45 1C. As shown in Fig. 2, the hot water storage tank was insulated with 50 mm-thickness rock wool wrapped with a layer of aluminum foil. A sight glass was fixed to check the level of water and a relief valve to discharge air in the tank. Its heating coils consisted of copper tubes of 12 mm in diameter and 83.5 m in length. Thirteen thermocouples were installed to measure the variation of hot water temperature along the depth of the tank. Moreover, to measure temperature and pressure, pressure gauges, thermocouples and isolation valves were also installed at certain parts of the ACDHWS (see Fig. 1). 2.3. Laboratory set-up for the ACDHWS To examine the performance of the ACDHWS, heat transfer of the evaporator and condenser in different thermal environment (i.e. the temperature and relative humidity of air) were measured. The experiment was carried out in a laboratory built on the basis of Chinese National Code (GB/T17758-1999). The main thermal
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Fig. 2. Schematic of the domestic hot water tank.
Table 1 Main testing environment Controlled air temperature at indoor chamber (1C)
Controlled air temperature at outdoor chamber (1C)
DB
WB
DB
WB
2570.1
17.570.1
2870.1
19.770.1
3570.1
2470.1
environments of the experiment and test instruments are listed in Tables 1 and 2, respectively. All tests were conducted in several room-type test chambers whose key features and provisions are shown in Fig. 3. The entire chamber was provided with high quality thermal insulation. The two adjoining chambers were separated by a well-insulated partition. Individual air-handling unit (AHU) was provided to control air temperature and relative humidity of each chamber. Indoor and outdoor units of the ACDHWS were placed in the two adjoining chambers, respectively. A pressure-balancing device was inserted to monitor the pressure difference between the two chambers and facilitate the measurements of air leakage. A computer in the control-room was used to monitor, collect and process experimental data. The error analysis of experimental data is carried out in detail in Appendix A and the results of uncertainty analysis are summarized in Table 3.
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Parameters
Instruments
Measure range
Type and precision
1 2 3 4 5
Pressure Temperature Flux Power Time
Pressure transfer device Thermocouple Rotameter flowmeter Power meter Stopwatch
0–2.1 MPa 40–400 1C 10–70 L min1 0.4–12 kW 0–60 s
Type: SEPRA2801, Precision: 0.001 MPa Precision: 0.11C Type: MBLG, Precision: 1.5 level Type: 7950A, Precision: 7(0.1% range+0.4% reading) Precision: 0.1 s
Fig. 3. Sketch of the ACDHWS performance test chamber. 1-outdoor unit, 2-water storage tank, 3-indoor unit, 4-standard fan coils, 5air-conditioning unit, 6-water-cooling unit, 7-adding moisture instrument, 8-supply water instrument, 9-controlled platform, 10-AHU, 11power ark, 12-manostat, 13-wiring ark, 14-air meter instrument.
Table 3 Summary of error analysis Items
Bias errors (%)
Precision errors (%)
Uncertainty (%)
V cm V hm T i;cm T o;cm T i;hm T o;hm Qe Qhm W COP1 COP2
0.0137 0.028 2 2 0.4 0.4 2.84 0.57 0.044 2.84 2.90
0.01 0.02 2 2 0.4 0.4 2.84 0.56 0.9 2.98 3.03
0.017 0.034 2.83 2.83 0.57 0.57 4.02 0.81 0.9 4.1 4.2
3. Results and discussion In space-cooling and water-heating mode, the ACDHWS was tested in five different operation conditions of water-heating. (A) Water-heating without the air-cooled condenser. (B) Water-heating with the air-cooled condenser in natural convection condition.
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(C) Water-heating with the air-cooled condenser in forced convection condition. (D) Simulation test I—firstly, water-heating without using hot water, secondly withdrawing hot water without water-heating to simulate continuous use of hot water, and then water-heating without using water again. (E) Simulation test II—water-heating with withdrawing hot water to simulate the amount of hot water provided continuously by the unit in the given temperature. The ACDHWS was controlled to run in condition (A) at the start, and then switched to run in condition (B) and condition (C) in turn because of increasing of condensing pressure and discharge temperature, finally steadily run in condition (C). At the same time, the unit was also controlled to run in condition (D) and condition (E) in order to test its performance, respectively. All test results and discussion are as follows. 3.1. Dynamic operation characteristics of the ACDHWS Dynamic operation characteristics were tested under the condition of the indoor air temperature 25 1C and the outdoor air temperature 35 1C. Figs. 4(a) and 5(a) show the change of mean temperature of hot water, evaporating pressure, condensing pressure and discharge temperature of the compressor with time after the start of the unit. The mean temperature is calculated by arithmetical average of the hot water temperature along the depth of the tank. The rising rate of the mean temperature of hot water is the largest during the initial stage of heating water without the air-cooled condenser, condition (A). When the mean temperature rises to about 36 1C and the discharge temperature of the compressor reaches 115 1C (exceeding the permitted discharge temperature of the compressor), the unit is quickly switched to condition (B). Here, the discharge temperature rises to about 117 1C very quickly, and then the unit is promptly turned into condition (C). In condition (C), the discharge temperature, the evaporating pressure and the condensing pressure remain some value hereabout, and the unit runs steadily. This is because the rising of hot water temperature in the tank makes the cooling capacity of the water-cooled condenser (i.e. the hot water storage tank) drop, superheat degree increase and flow rate of refrigerant decrease. When the forced air-cooled condenser starts, the above-mentioned problems are completely solved. In other words, the ACDHWS can be switched from (A) condition to (C) condition directly in order to obtain better operation performance in practice. In the experimental process, the judgment parameter of switching is the discharge temperature. In fact, the condensing pressure, the discharge temperature and COP of the system may decide switching condition. Fig. 5(b) shows the change of cooling capacity, power and COP with time after the start of the unit. In late condition (A), the cooling capacity and COP are dropping sharply, even if the phenomenon has not been ameliorated in condition (B). When the unit operates in condition (C), its performance is improved instantly.
heating 117 minutes
heating 97 minutes
heating 107 minutes
heating 77 minutes
heating 57 minutes
heating 47 minutes
50 H (cm)
35 Twp (°C)
heating 37 minutes
60
heating 25 minutes
c
a
heating 17 minutes
b
40
heating 1 minutes
70
45
heating 9 minutes
50
30
40 30
25 20
20
a-only water heating b-water heating with natural air-cooled c-water heating with forced air-cooled
15
10
10
(a)
0
20
40
60 80 100 Time t (minute)
120
140
160
(b)
15
20
25
30 35 Tw (°C)
40
45
50
Fig. 4. (a) Mean water temperature in the tank vs. time. (b) Variation of water temperature vs. time along the height of the tank.
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1.2
120
7
1.0
110
0.8
100
Pc / Pe (MPa)
a
b
c
0.6
Qe W
COP a-only water heating b-water heating with natural air-cooled c-water heating and forced air-cooled a b c
2
6 5 4
1
90 3
0.4
80 a-only water heating b-water heating with natural air-cooled c-water heating with forced air-cooled
0.2 0.0 0
(a)
Q e / W (kW)
8
Tp (°C)
130
1.4
3
9
140
Pc Pe Tp
1.6
COP
1796
20
40
60 80 Time t (minute)
100
2 70 1 120
0
20
40
(b)
60 80 Time t (minute)
100
0 120
Fig. 5. (a) Discharge temperature, evaporating pressure and condensing pressure vs. time. (b) Rate of refrigeration, power and COP vs. time.
55 50 45
Tw (°C)
40 35
c
b 30
Tw1
a
T
w12
Tw10
25
Tw7
A-using hot water B-stop using hot water a-only water heating
20 15
b-water heating with natural air-cooled c-water heating with forced air-cooled
B
A
Tw4
10 0
20
40
60 80 Time t (minute)
100
120
140
Fig. 6. Variation of water temperature in the tank vs. time in using hot water.
Therefore, to obtain high performance, the ACDHWS may be switched from condition (A) to condition (C) directly on basis of the COP value (e.g. switching about at COP 2.4), not only the discharge temperature and the condensing pressure. Thus, it is proved that judgment parameters of switching are the discharge temperature, the condensing pressure and the COP value of the ACDHWS to obtain better performance. It will be the theoretical foundation to control well the ACDHWS product in the future. Fig. 4(b) shows the variation of hot water temperature along the height of the tank with time. The change of temperature is not obvious in most parts of the water tank above 200 mm. The temperature in most parts of the water tank is high. It is very favorable for householders to use hot water. Fig. 6 shows the variation of hot water temperature in the hot water storage tank in condition (D). In Fig. 6, Tw1, Tw4, Tw7, Tw10 and Tw12 are points 750, 480, 300, 165 and 75 mm away from the bottom of the tank, respectively. Tw1 is also the temperature of supplying hot water. When hot water is heated to about 45 1C, hot
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150 Pc Pe Tp
140
110
1.0 a
b
A-using hot water B B-stop using hot water
A
0.6
a-only water heating
c
100
b-water heating with natural air-cooled c-water heating with forced air-cooled
3 b-water heating with natural air-cooled
5
120 Tp (°C)
Pc / Pe (MPa)
6
130
1.2
0.8
A-using hot water B-stop using hot water a-only water heating c-water heating with forced air-cooled
2 A
a
B
b
c
4
COP
1.4
Q e / W (kW)
1.6
1797
3
80
1
Qe W COP
90 2
0.4 70 1
0
20
40
60
(a)
80 100 120 Time t (minute)
140
160
0
0
180
20
40
(b)
60
80 100 120 Time t (minute)
140
160
180
Fig. 7. (a) Discharge temperature, evaporating pressure and condensing pressure vs. time in using hot water. (b) Rate of refrigeration, power and COP vs. time in using hot water.
5 Tp Twp
120
b
1.8 a
Pc Pe
c
1.6
100
Z
1.4 c
60 40
a-only water heating b-water heating with natural air-cooled c-water heating with forced air-cooled
1.2 1.0
4
0.8 0.6
a-only water heating b-water heating with natural air-cooled c-water heating with forced air-cooled
20
Z=Pc / Pe
80
Pc / Pe (MPa)
Tp / Twp (°C)
b a
3
0.4 0.2
0
(a)
100
200
300
400 500 600 Time t (minute)
700
800
0
900
(b)
100
200
300
400 500 600 Time t (minute)
700
800
900
Fig. 8. (a) Discharge temperature and hot water temperature vs. time without using hot water. (b) Evaporating pressure, condensing pressure and pressure ratio vs. time without using hot water
water is drawn from the tank at a rate of 6.5 L/min (quantity of shower bath per capita in a typical family) and the tank is refilled by water from the main. The temperature of hot water is too low to use after the withdrawal lasts about 20 min. Then, water in the tank will continuously be heated for another 70 min to use again. After that, hot water is drawn from the tank again and the cycle is repeated. The above withdrawal of hot water simulates the use of hot water for shower in a typical family [8]. The amount of hot water is enough to cater for one-person shower in a typical family. If time interval of using hot water is shortened, then the initial water temperature will be lifted or the size of the tank will be increased. Fig. 7(a) and (b) show the change of evaporating pressure, condensing pressure, discharge temperature, cooling capacity, power and COP with time from the start of using hot water. The results show that the performance of the unit is improved and COP is also increased in using hot water process without air-cooled condenser. It is proved that the ACDHWS could provide a continuous hot water supply for householders if it could be reasonably controlled. Fig. 8(a) and (b) show the change of evaporating pressure, condensing pressure, discharge temperature of the compressor and pressure ratio with time within 15 h after the start without using hot water. Thus, the
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reliability of the ACDHWS could be examined by this experiment. In condition (A), the ACDHWS has been running for 80 min, when evaporating pressure, condensing pressure and discharge temperature of the compressor rise sharply, especially the discharge temperature of the compressor (rising to about 120 1C). Then the system is switched to operate in condition (B). After the unit has been running for 15 min in condition (B), the discharge temperature rises to about 120 1C again. And it is switched to operate in condition (C). Then, the unit steadily runs in condition (C). It is tested that the ACDHWS is reliable after having been operated for 15 h steadily in this condition. In heating process, electromagnetic valve 11 (Fig. 1) is switched to avoid superheating hot water in the tank. It is concluded that the unit can steadily provide space cooling and hot water supply simultaneously—provided it could reasonably be switched in different operation conditions. 3.2. Hot water supply performance and energy efficiency of the ACDHWS The performance of supplying hot water and energy efficiency of the ACDHWS were tested under the conditions of the indoor air temperature 25 1C and the outdoor air temperature 35, 32 and 28 1C, respectively. Fig. 9(a) shows the results of the continuous flow rate of hot water vs. the ambient air temperature and
130 120 110
35 °C hot water supply 45 °C hot water supply 50 °C hot water supply 55 °C hot water supply
4.1
100 90 Q e (kW)
Gw (kg/h)
35 °C hot water supply 45 °C hot water supply 50 °C hot water supply 55 °C hot water supply
4.2
80 70
4.0
3.9
60 50
3.8 40 30
3.7 28
30
(a)
32 Tdb (°C)
34
28
36
2.0 1.9 1.8
35 °C hot water supply 45 °C hot water supply 50 °C hot water supply 55 °C hot water supply
1.6
COP
W (kW)
1.7
1.5 1.4 1.3 1.2 28
(c)
30
32 Tdb (°C)
30
(b)
34
36
3.4 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.9
34
36
35 °C hot water supply 45 °C hot water supply 50 °C hot water supply 55 °C hot water supply
28
(d)
32 Tdb (°C)
30
32 Tdb (°C)
34
36
Fig. 9. (a) Hot water flow-rate vs. ambient air temperature and hot water temperature with cooling and using hot water. (b) Cooling rate vs. ambient air temperature and hot water temperature with cooling and using hot water. (c) Power rate vs. ambient air temperature and hot water temperature with cooling and using hot water. (d) COP vs. ambient air temperature and hot water temperature with cooling and using hot water.
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Table 4 Comparison of experiment results between the ACDHWS and the original unit Outdoor (1C)
Types
Pe (MPa)
Pc (MPa)
z ¼ Pc/Pe
Qe (kW)
W (kW)
COP1
Gw (kg h1)
COP2
28
The Original The ACDHWS
0.418 0.411
1.415 1.297
3.385 3.156
4.0 3.9
1.750 1.636
2.3 2.4
— 66.63 (45 1C) 28.61 (55 1C)
2.3 3.8 3.2
35
The Original The ACDHWS
0.494 0.469
1.763 1.564
3.569 3.335
4.3 4.3
2.160 1.926
2.0 2.2
— 91.40 (45 1C) 52.12 (55 1C)
2.0 3.7 3.3
the hot water temperature. These mainly test the capacity of supplying hot water with space cooling by the ACDHWS in the given water temperature. Fig. 9(b)–(d) show the results of the cooling rate, the power rate and COP vs. the ambient air temperature and the hot water temperature, respectively. When the outdoor air temperature is constant, the flow rate of hot water, the cooling rate and the COP value are increasing with the drop of hot water temperature, except the power rate is decreasing. When the temperature of hot water is constant, the flow rate of hot water and the power rate are increasing with the ascending of the outdoor air temperature, yet the cooling rate and the COP valve are decreasing. E.g. at the outdoor air temperature 35 1C, the flow rate of hot water rises by 132.9%, the cooling rate increases by 2.4%, the power rate drops by 12.8% and COP ascends by 11.4% when the temperature of hot water drops from 55 to 45 1C. At the temperature of hot water 45 1C, the flow rate of hot water rises by 22.2%, the cooling rate decreases by 5.7%, the power rate rises by 16.6% and COP drops by 16.7% when the outdoor air temperature rises from 28 to 35 1C. It is obvious that the variation of the cooling rate is smaller in contrast to other parameters. It is essential that the ACDHWS could retain the cooling rate whether supplying hot water or not. 3.3. Comparison of the ACDHWS with the original air conditioner The tests were carried out under the condition of the indoor air temperature 25 1C and the outdoor air temperatures 35 and 28 1C. The results are shown in Table 4. In Table 4, P Q þ Qe Q Q P P COP2 ¼ ; COP1 ¼ P e and Qhw ¼ chw mhw DT hw . ¼ hw W W W According to the test results, COP1 of the original unit is in the range of 2.0–2.3. Comparatively, the ACDHWS has higher COP1 (2.2–2.4) by 10% than that of the original unit. If the ACDHWS is also used as a water heater, COP2 with 3.2–3.8 becomes much attractive. The average COP2 of the ACDHWS is about 38.6% higher than that of the original unit. It can be even higher if there is much more hot water consumption. Therefore, it is obvious that the ACDHWS is more economical than the original unit. 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 patterns should be further investigated. As a novel product, the equipment should be made economical by controlling the initial cost and the maintenance cost well. Furthermore, comprehensive economical evaluation on the ACDHWS should be developed further in the future. According to an investigation of the China markets in 1999, the annual sale quantity of air conditioners was about 900,000. Only about 30% of the families living in cities owned air conditioners. Meanwhile, the sale quantity of electric water heater shared about 60% of the total water heater markets. With the economy growth of the country, it is expected that more and more families will have a demand on air conditioners and water heaters. The ACDHWS is functionally safe, flexible, economical and environmental. It is expected to have a very good market not only in China, but also elsewhere in the world.
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4. Conclusions A novel ACDHWS is presented in this paper. And its dynamic operation characteristics, hot water supply performance, energy efficiency and the temperature distribution of hot water in the storage tank were tested in different operation condition by using the experimental rig developed. The results verify that the ACDHWS is a flexible, multi-functional and economical unit, and is completely reliable even under the condition of the operation without using hot water for a long time if it can reasonably be switched in different operation conditions. Furthermore, the comparison of operation performance between the original unit and the ACDHWS shows that COP1 of the ACDHWS is about 10% higher than that of the original unit, and COP2 of the ACDHWS is about 38.6% higher than that of the original unit. Therefore, the ACDHWS not only has a better operation performance than the original unit, but also can provide a continuous hot water supply for householders. Thus, the results can also provide valuable reference to the industry of water heaters and air conditioners. Acknowledgement The project sponsored by Shandong Jin Ji Yuan Electrical Ltd in China and by SRF for ROCS, SEM is gratefully acknowledged. Appendix A A.1. Error analysis The total experimental error of the measurement is also called experimental uncertainty. It is classified by bias error and precision error. The 95% confidence uncertainty is calculated as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U ¼ B2 þ P2 . (1) For example, Qe ¼ ccw mcw DT cw ¼ ccw mcw ðT o;cw T i;cw Þ,
(2)
Qhw ¼ chw mhw DT hw ¼ chw mhw ðT o;hw T i;hw Þ,
(3)
mcw ¼ V cw rcw ; mhw ¼ V hw rhw ,
(4)
COP1 ¼
Qe , W
Qhw þ Qe . W The 95% confidence uncertainty of cooling capacity is calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U Qe B Qe 2 P Qe 2 ¼ þ Qe Qe Qe COP2 ¼
bias limit: BQe 2 BT i;cw 2 BT o;cw 2 Bccw 2 Bmcw 2 ¼ þ þ þ Qe ccw mcw DT cw DT cw precision limit: PQe 2 PT i;cw 2 PT o;cw 2 Pccw 2 Pmcw 2 ¼ þ þ þ , Qe ccw mcw DT cw DT cw where Bccw ; Pccw ; Brcw ; Prcw ; Bchw ; Pchw ; Brhw ; Prhw ¼ 0 since C w ; rw is constant.
(5) (6)
(7)
(8)
(9)
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Therefore, bias limit: B Qe 2 BT i;cw 2 BT o;cw 2 BV cw 2 ¼ þ þ , Qe V cw DT cw DT cw
(10)
precision limit: P Qe 2 PT i;cw 2 PT o;cw 2 PV cw 2 ¼ þ þ . Qe V cw DT cw DT cw 1. Calculation of bias error BV cw ¼ 0:0137%, V cw BV hw ¼ 0:028%, V hw BT i;cw BT o;cw ¼ ¼ 2%, DT cw DT cw BT i;hw BT o;hw ¼ ¼ 0:4%, DT hw DT hw BW ¼ 0:044%, W
BQe ¼ Qe
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi BV cw 2 BT i;cw 2 BT o;cw 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 0:01372 þ 22 þ 22 % ¼ 2:84%, þ þ V cw DT cw DT cw
BQhw ¼ Qhw
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi BT i;hw 2 BT o;hw 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi BV hw 2 ¼ 0:0282 þ 0:42 þ 0:42 % ¼ 0:57%. þ þ V hw DT hw DT hw
2. Calculation of precision error PV cw ¼ 0:01%, V cw PV hw ¼ 0:02%, V hw PT i;cw PT o;cw ¼ ¼ 2%, DT cw DT cw
(11)
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PT i;hw PT o;hw ¼ ¼ 0:4%, DT hw DT hw PW ¼ 0:9%, W P Qe ¼ Qe
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PT i;cw 2 PT o;cw 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PV cw 2 ¼ 0:012 þ 22 þ 22 % ¼ 2:84%, þ þ V cw DT cw DT cw s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi PV hw 2 PT i;hw 2 PT o;hw 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 0:022 þ 0:42 þ 0:42 % ¼ 0:56%. þ þ V hw DT hw DT hw
PQhw ¼ Qhw
3. Calculation of uncertainty s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi U Qe BQe 2 P Qe 2 ¼ 4%, ¼ þ Qe Qe Qe U Qhw ¼ Qhw UW ¼ W
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi BQhw 2 PQhw 2 ¼ 0:8%, þ Qhw Qhw
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 BW PW ¼ 0:9%, þ W W
U COP1 ¼ COP1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U Qe 2 UW 2 ¼ 4:1%, þ Qe W
U COP2 ¼ COP2
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi U Qe 2 U Qhw 2 UW 2 ¼ 4:2%. þ þ Qe Qhw W
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