Heat-pipe enhanced solar-assisted heat pump water heater
B.J. Huang
a,*, J.P. Lee
a, J.P. Chyng
ba
Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan
b
Department of Electrical Engineering, Chinmin College, Toufen, Miaoli, Taiwan Received 17 October 2003; received in revised form 20 May 2004; accepted 10 August 2004
Available online 21 September 2004 Communicated by: Associate Editor Charles Kutscher
Abstract
A heat-pipe enhanced solar-assisted heat pump water heater (HPSAHP) is studied. HPSAHP is a heat pump with dual heat sources that combines the performance of conventional heat pump and solar heat pipe collector. HPSAHP operates in heat-pump mode when solar radiation is low and in heat-pipe mode without electricity consumption when solar radiation is high. HPSAHP can thus achieve high energy efficiency. A prototype was designed and built in the present study. An outdoor test for a HPSAHP in the present study has shown that COP of the hybrid-mode operation can reach 3.32, an increase of 28.7% as compared to the heat-pump mode COP (2.58).
2004 Elsevier Ltd. All rights reserved.
Keywords: Solar heat pump; Solar thermal; Heat pump; Solar water heater
1. Introduction
The New Energy Center at National Taiwan Univer-sity has been devoted to the development of solar-assisted heat pump water heater (SAHP) since 1997. Several types of SAHP have been designed and tested. The SAHP con-sists of a Rankine refrigeration cycle coupled with a solar collector that acts as an evaporator. The refrigerant is di-rectly expanded inside the evaporator to absorb the solar energy. By a proper design of the Rankine cycle and the solar collector for a specific operating condition, heat may be absorbed from, rather rejected to, the ambient. That is, the SHAP can absorb heat from both solar radi-ation and ambient air simultaneously (Huang and Chyng,
2001).Huang and Chyng (1998, 1999)first proposed the design of an integral-type solar-assisted heat pump water heater (ISAHP) that integrates the heat pump, solar col-lector and water storage tank together to become a single package that is easy to install everywhere.
The COP of a small ISAHP (100 l, 250 W compressor input) designed byHuang and Chyng (2001) is >2.5 at ambient temperature >25C. However, it can be further improved if the compressor can be shut down during high solar radiation periods.
An ISAHP is a heat pump with dual heat sources. It absorbs solar radiation and ambient heat simultaneously through the evaporator of the Rankine cycle. This means that the compressor must be turned on in order to make hot water no matter how high the solar radia-tion is. The present study intends to design a heat-pipe enhanced SAHP (called HPSAHP) which combines the performance of a heat pipe solar collector and a SAHP. When solar radiation is low, the compressor is turned on 0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.solener.2004.08.009
*
Corresponding author. Tel.: +886 2 2363 4790/2362 4790; fax: +886 2 2364 0549.
E-mail address:[email protected](B.J. Huang).
and a solenoidal valve is closed such that HPSAHP operates in heat-pump mode. When solar radiation is high, the compressor is turnd off and HPSAHP operates in heat-pipe mode without electricity consumption. Thus, the HPSAHP can achieve higher energy efficiency.
The schematic diagram of HPSAHP is shown inFig. 1.
2. Process of HPSAHP
HPSAHP operates in two modes: heat-pipe mode and heat-pump mode.
2.1. Process of heat-pipe loop
The flowchart for the process of the heat-pipe loop is shown inFig. 1in dashed lines. In heat-pipe mode oper-ation, the compressor is turned off and the refrigerant
di-rectly enters the condenser without passing through the compressor, after absorbing the heat from sun in the evaporator. The vapor is condensed in the condenser that is immersed in a hot water tank, and releases heat to the water. After that, the refrigerant passes through a sole-noid valve and back to the evaporator. There is no
elec-tricity consumption during the heat-pipe mode
operation. For absorbing heat from ambient air in heat-pump mode operation, the evaporator is designed in two parts. The refrigerant absorbs solar radiation only in the half part of the evaporator facing toward the sun. A check-valve (Va) is installed in between to avoid the refrigerant flowing to the other half instead of flowing into the condenser. In order to avoid the heat loss by a re-verse flow from the condenser in the hot water tank to the absorber during nighttime or cloudy weather, another check valve (Vc) is installed between the outlet of the evaporator and the inlet of the condenser.
Nomenclature
Ac heating area of the evaporator, m
2
COP coefficient of performance of HPSAHP
Cp specific heat of water, J kg1C1
Ew daily total energy collection, J
Ht daily total heating capacity, MJ/m2/day
M total mass of water in the storage tank, kg
Rpw heat-pipe thermal resistance,C W1
Ta average ambient air temperature,C
Te average evaporation temperature,C
Tw water temperature,C
Twall collector wall temperature,C
Twi initial water temperature,C
Twf final water temperature,C
t time, s
(UA)e daily overall heat transfer coefficient from
ambient to collector, J/C
Qin heat input rate by electric heater, W
qin heat input density by electric heater, W/m2
a solar radiation absorb coefficient of the
evaporator
g efficiency of HPSAHP heat pipe mode
2.2. Process of heat-pump loop
In heat-pump mode operation, the solenoid valve is closed such that the refrigerant enters the compressor after absorbing the heat from solar radiation and ambi-ent air. After being compressed, the vapor ambi-enters the condenser and then flows back to the evaporator through the capillary tube.
3. Experimental setup 3.1. Prototype design
The HPSAHP prototype used a bare collector/evapo-rator. The collector is of tube-in-sheet type using copper tube (6 mm diameter) soldered on copper sheet (0.4 mm thick). The size of the each sheet is 10 cm· 180 cm. Twenty one copper tube-sheets are used in the collector. The total surface area of the collector is 3.78 m2. The collector surface is a selective black surface. The round-shape collector is surrounding the water tank. All copper-in-tube plates are connected in parallel by two header pipes (20 mm diameter).
The evaporator/collector is designed in circular shape. Hence, it always absorbs both solar radiation and ambient air by the half part facing the sun and ab-sorbs the ambient air energy only from the other half that is back the sun.
For convenience in experiment indoor, electric heat-ing films are pasted on the back surface of the 11 copper plates to simulate incident solar radiation. The Rankine refrigeration cycle unit is mounted on the top of the HPASHP. A small R134a reciprocating-type hermetic compressor with piston swept volume 12 cm3and rated input power 550 W is used.
The condenser heat exchanger is immersed in the storage tank and the design is tube-in-sheet type similar to the evaporator. Fifteen vertical copper tubes (13 mm diameter) are connected in parallel by two circular header pipes (20 mm diameter). Water absorbs the con-densation heat from the refrigerant vapor inside the copper tubes. The HPSAHP uses a 240 l tank for hot water storage.
A capillary tube (200 cm length, 1.2 mm diameter) is used for regulating the refrigerant flow in the Rankine refrigeration cycle. A filter is installed downstream of the condenser and an accumulator is installed down-stream of the collector/evaporator for protecting the compressor from wet compression. A solenoid valve is installed at the outlet of the condenser for the operation of two different modes in the HPSAHP. Three check valves are installed in the system to prevent the refriger-ant backflow. The prototype of the HPSAHP is shown inFig. 2. Design specifications of the HPSAHP are listed inTable 1.
3.2. Instrumentation
Two T-type thermocouples are installed in the suc-tion and discharge ports of the compressor for the temperature measurement, labeled as T1and T2,
respec-tively. Another two T-type thermocouples are installed at the upstream and the downstream of the capillary
tube for the temperature measurement, labeled as T3
and T4, respectively. The other two T-type
thermocou-ples are installed at the outlet of the evaporator and the inlet of the condenser for the temperature measure-ment, labeled as T5 and T6, respectively (see Fig. 1).
Two RTD sensors are used to measure the water tem-perature in the tank. One is at one-third of the tank height from the bottom, and the other is at two-thirds
Fig. 2. Prototype of the HPASHP.
Table 1
Design specifications of HPSAHP
Collector area 3.78 m2selective surface, unglazed 21
tubes-in-sheet (1.98 m2facing the sun;
1.8 m2back the sun)
Water tank 240 l (50 cm diameter, 120 cm tall) 6 cm PU
insulation layer
Rankine cycle Compressor/R134a, 550 W/110 VAC,
12 cm3/stroke
Condenser/tube bundles (15 tubes, 12.7 mm i.d., 1 m long)
of the tank height. Two pressure gages are used to meas-ure the suction and the discharge pressmeas-ures of the com-pressor, P1 and P2. The condensing temperature Tc is
converted from the discharge pressure data P1 using
thermodynamic chart of R134a (ASHRAE, 1989). Since
the pressure loss in the compressor suction line may be large, the evaporating temperature is determined from T4. The input power of the electric heating film is varied
by a transformer to simulate the solar radiation inten-sity. The heat input rate (Qin) of the electric heating films
is measured by a power-meter. The total power input to the compressor is measured by a wattmeter.
4. Test results
The HPSAHP includes two modes. The two opera-tional modes were tested separately first in order to ana-lyze the performance of each mode. Then, a test for a hybrid mode is performed.
4.1. Heat-pump mode
In heat-pump mode operation, the solenoid valve is closed and the refrigerant enters the compressor after absorbing the energy from the solar radiation and the ambient air. After being compressed, the vapor enters the condenser and then flows back to the evaporator through the capillary tube.
The heat-pump operates on the principle of Rankine cycle, the same as the ISAHP. The compressor is always running during the heat-pump mode.
The heat-pump mode experiment is tested indoor. The final temperature is set at 55C at which point the operation is terminated. The distribution of water
tem-perature is not uniform in the tank. Thus, a water pump is used to mix the water before taking water temperature measurements. We used the electric heating to simulate the solar radiation absorbed by the evaporator. The heat input rate is converted into the heat input density, qin=
Qin/Ac. The ambient temperature is between 21C and
23C during the test. The experimental result shows that the electricity consumption per liter of hot water is be-tween 0.012 and 0.018 kW h/l. COP of the HPSAHP in heat-pump mode varies from 2.5 to 3 and increases with increasing solar radiation, as shown inFig. 3. COP is de-fined as follows
COP¼ MCpðTwf TwiÞ
Total electricity consumption ð1Þ
where M is the weight of the water (240 kg); Cp is the
specific heat of water; Twfis the final water temperature;
Twiis the initial water temperature.
The heat-pump mode operation of the HPSAHP is
the same as the ISAHP.Huang and Chyng (2001)have
derived an instantaneous correlation of ISAHP for the total energy absorption at the evaporator. For long-term performance, a correlation can thus be derived accord-ing to the similar procedure:
Cday Ew ðHt AcÞ ¼ ðUAÞe Ta Te ðHt AcÞ þ a ð2Þ
where Ewis the daily total energy collection in the tank
minuses total electricity consumption of compressor Welet; (UA)e is the daily-total heat transfer coefficient
from ambient air to the refrigerant. Htis the daily total
heating capacity defined as Ht= qint, a is the solar
absorption coefficient representing energy collection from solar radiation. The correlation is satisfactory and the test results as shown inFig. 4.
Heat-pump mode test
Initial water temperature Twi =19ºC , Final water temeperature Twf =55ºC
0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0 200 400 600 800 1000
Heat input density qin (W/m²)
Electricity consumption (kWh/Liter)
4.2. Heat-pipe mode
In heat-pipe mode operation, the solenoid valve is opened. The refrigerant absorbs the heat from the evap-orator, and then flows into the condenser directly. It flows back to the evaporator through the solenoid valve after being condensed in the condenser. Many parame-ters influence the heat pipe mode performance including the refrigerant charge mass, water temperature, solar radiation etc. (Chi, 1976). We charged different refriger-ant mass to test the performance (Fig. 5). The heat input density qinis controlled at the 500 W/m2, and the heating
time is fixed at 6 h. The heat-pipe mode efficiency g is de-fined as follows.
g¼MCpðTwf TwiÞ qin t Ac
ð3Þ where M is the weight of the water (240 kg); qin is the
heat input rate by the electric heater; t is heating time. The efficiency of heat-pipe mode was measured at differ-ent refrigerant charges.Fig. 5shows that the efficiency does not change much for the charging between 3.5 and 6.5 kg. The test was carried out for total heating
time 6 h with heating power 500 W/m2. The thermal
resistance of the heat pipe from the evaporator plate
to water is also determined, by varying total heating rate Qinfrom 400 W to 1600 W. Six T-type thermocouples are
used to measure the wall temperature. Two are installed on one plate of the evaporator. The thermal resistance Rpwis defined as follows:
Rpw¼
Twall Tw
Qin
ð4Þ where Twall is the average wall temperature; Tw is the
average water temperature; Qinis the heating rate. The
thermal resistance Rpw remains between 0.01 and
0.02 K/W when the heat input rate varies from 500 W to 1500 W (Fig. 6).Fig. 7also shows the heat-pipe mode efficiency decreases with increasing initial water temper-ature. The test was carried out for total heating time 6 h with heat input density qin= 500 W/m
2
. The efficiency g is defined as Eq.(3). The results show that the lower the initial water temperature the higher efficiency since the heat transfer rate is higher temperature difference be-tween the refrigerant and water.
4.3. Hybrid mode
The hybrid mode performance includes the combined operation of heat pipe mode and heat pump mode. The 0 1 2 3 4 5 6 7 8 9 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 (Ta-Te)/(HtAc) , ºC day/MJ Cday Cday=2.3683*(Ta-Te)/(qintAc)+0.237 R²=0.9963
Fig. 4. Correlation of HPSAHP.
10 15 20 25 30 35 40 45 3 4 5 6 7 refrigerant charge (kg)
Heat-pipe mode test
Heating capacity=500W/m2; Heating time:6 hours; Initial water temperature=24˚C
relative error=5% E ff ici en cy ( % )
Fig. 5. Heat-pipe mode performance.
0 0.01 0.02 0.03 0.04 0 500 1000 1500 2000
Heat input rate Qin (W)
Heat resistance R, K/W
Heat pipe mode test
Initail water temperature=22ºC Charged refrigerant = 3.83kg relative error=5%
Fig. 6. Thermal resistance of heat-pipe mode.
Heat pipe mode test
Heat input density:500W/m2, Heating time=6 hrs;
0 10 20 30 40 50 15 20 25 30 35
Initial water temperature (Twi,ºC)
Efficiency(%)
heat-pipe mode is changed to the heat-pump mode by switching off the solenoid valve. To keep a high perform-ance for hybrid mode operation, a proper charge quan-tity of refrigerant is important. Fig. 5 shows that for heat-pipe mode operation, the weight of the refrigerant charge do not influence the efficiency very much. It was found experimentally that 3.825 kg is suitable for the heat-pump mode operation. The efficiency of the heat-pipe mode is satisfactory.
Fig. 7indicates that the efficiency of heat-pipe mode operation is high (around 40%) at low initial water tem-perature. Thus, for hybrid-mode operation, the heat pipe mode is suggested to adopt in the beginning of a day since the water temperature is low early in the morning.
The COP of the hybrid mode is tested indoor. We first start the heat-pipe mode for 4 h and then change to the heat-pump mode. The experiment is run at differ-ent heat input rates. The test results are shown inFig. 8. COP of the hybrid mode is higher than heat-pump mode operation. COP varies from 2.5 to 3.5. The improvement of hybrid-mode over the heat-pump mode only is obvi-ous at high input density. This is due to the fact that heat-pipe mode is performed without electricity con-sumption. Thus, the difference between the COP of the hybrid-mode and the COP of the heat-pump mode is lager only when the heating capacity is higher.
4.4. Outdoor test
A new prototype HPSAHP-B (Fig. 9) is fabricated
for verification by outdoor test. The principle of HPSAHP-B is the same as HPSAHP described before. The HPSAHP prototype used a bare collector/evapora-tor. The collector is tube-in-sheet type, using copper
tube (6 mm diameter). Copper tubes are soldered on the copper sheets (110 cm· 110 cm). The unglazed col-lector surface is made in wavy shape and is painted in black. The unglazed collector is set at a 23 inclined angle in order to absorb more solar radiation. The con-denser heat exchanger is immersed in the storage tank similar to HPSAHP. Ten vertical copper tubes (13 mm diameter, 50 cm long) are connected in parallel by two circular header pipes (20 mm diameter). The HPSAHP-B uses a 100 l tank for hot water storage. The specifica-tions are listed inTable 2. The heat-pump mode and the hybrid-mode are tested separately. In order to test the COP of the heat-pump mode only, the compressor is turned on at AM10:00 and turned off when the water temperature is 53C. For hybrid mode test, the heat-pipe mode is operated first in the early morning. Then, the heat-pipe mode is changed into the heat-pump mode when the solar radiation is smaller than 300 W/m2after PM12:00. The total solar radiation is measured by a
pyr-Initial water temperature Twi=21ºC , Final water temperatur Twf =55ºC
2 2.5 3 3.5 4 4.5 5 0 200 400 600 800 1000 Heat input density qin(W/m²)
COP
Heat pump mode Hybrid mode
Fig. 8. HASHP hybrid-mode performance.
w
Fig. 9. Prototype design of the HPASHP-B.
Table 2
Design specifications of HPASHP-B
Collector area 1.21 m2selective surface, unglazed Water tank 120 l (35 cm diameter, 105 cm tall) 6 cm PU
insulation layer
Rankine cycle Compressor/R134a, 250 W/110 VAC, 5.29 cm3 Condenser/tube bundles
(10 tubes, 12.7 mm i.d., 0.5 m long)
Table 3
Test results of HPASHP-B
Mode Initial water
temperature (C)
Final water temperature (C)
Daily total solar radiation (MJ/m2) Ambient temperature (C) COP Hybrid 28.5 52.5 14.72 34.9 3.32 Heat pump 27 53 15.55 35.9 2.58
anometer which is installed at a 23 inclined angle. The test result is shown inTable 3. The COP increases obvi-ously for the hybrid-mode. The COP of the hybrid-mode operation can reach 3.32, an increase of 28.7% as com-pared to the heat-pump mode only (2.58).
5. Conclusions and discussions
HPSAHP operates as the heat-pump mode and the heat-pump mode. These two modes are tested sepa-rately. In the heat-pump mode, it shows that the COP increases with increasing heat input density. In the heat-pipe mode, the efficiency is tested in different refrig-erant mass, heating capacity, and the initial water tem-perature. The thermal resistance of the heat pipe is between 0.01 and 0.02 W/K. The efficiency increases with decreasing initial water temperature.
How to decide the switching between the heat-pump mode and the heat-pipe mode is important for a HPSAHP. The present study shows that the overall per-formance can be better if the heat-pipe mode operates at lower temperature.Fig. 7shows that COP becomes lower when the water temperature is higher than 30C. Hence, it is better to switch to heat-pump mode when the water temperature is over 30C.Fig. 8also shows that the influ-ence of the heat-pipe mode is obvious when the heat input density is large than 400 W/m2. Therefore, the heat-pipe mode had better be operated at the water temperature <30C and at the heat input density >400 W/m2
. HPSAHP can operate without electricity consump-tion at high solar radiaconsump-tion using the heat-pipe mode
operation. An outdoor test for the HPSAHP in the pre-sent study has shown that COP of the hybrid-mode operation can reach 3.32, an increase of 28.7% as com-pared to the heat-pump mode COP (2.58). COP of the HPSAHP thus can be improved as compared to an ISAHP. The highest COP obtained in the present study is 3.5 which is thought to be high for a system using small compressor.
Acknowledgments
The present study was supported by Energy Commis-sion, Ministry of Economic Affairs, and National Sci-ence Council, ROC, through Grant no. NSC91-2212-E-002-077.
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