A Long Heat Pump

Renewable Energy 29 (2003) 633–639
www.elsevier.com/locate/renene
Technical note
Long-term performance of solar-assisted heat
pump water heater
B.J. Huang , C.P. Lee
Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, ROC
Received 8 September 2002; accepted 30 July 2003
Abstract
A long-term reliability test of an integral-type solar-assisted heat pump water heater

(ISAHP) was carried out. The prototype has been running continuously for more than
13,000 h with total running time >20,000 h during the past 5 yr. The measured energy con-
sumption is 0.019 kWh/l of hot water at 57 oC that is much less than the backup electric
energy consumption of the conventional solar water heater.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: Solar energy; Heat pump; Solar heat pump
1. Introduction
The direct expansion solar-assisted heat pump water heater has a better perform-
ance since the Rankine refrigeration cycle is directly coupled with a solar collector
that acts as an evaporator and the refrigerant directly expands inside the evapor-
ator to absorb the solar energy [1–3]. The research group at the New Energy
Center, Department of Mechanical Engineering, National Taiwan University, star-
ted to develop an integral-type solar-assisted heat pump water heater (ISAHP) [4]
in 1997. The ISAHP integrates all components of the machine into a single pack-
age that can be completely fabricated in the factory. No field assembly is required
except the connection of a water pipe line and electric power connector. With
proper design, the ISAHP absorbs heat simultaneously from solar radiation and
ambient air [5]. Cost reduction as well as better thermal performance can be
achieved. However, the evaporator of the ISAHP will absorb solar energy at an

Corresponding author. Tel.: +886-2-2362-4790; fax: +886-2-2364-0549.
E-mail address: bjhuang@seed.net.tw (B.J. Huang).
0960-1481/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.renene.2003.07.004
634 B.J. Huang, C.P. Lee / Renewable Energy 29 (2003) 633–639
Nomenclature
Em electric energy consumption of ISAHP per liter of hot water at 57 oC

(kWh/l)
Ht daily total solar radiation incident on horizontal surface (MJ/m2 day)
mh daily hot water load (l/day)
Ta daily-mean ambient temperature at operation of ISAHP (oC)
unsteady state due to the unsteadiness of the solar incident radiation and outdoor
weather variation. This will cause the heat pump to run under much more severe
conditions than conventional heat pumps such as air conditioners. The reliability
thus remains a big question for a direct expansion solar-assisted heat pump water
heater. In order to understand the long-term energy efficiency as well as the
reliability of the ISAHP built in 1997 (ISAHP-1), a long-term performance test was
carried out.
2. Design of ISAHP
The ISAHP consists of a Rankine cycle unit, a collector/evaporator unit, and a

heat exchanger/condenser unit that combines the condenser of the Rankine cycle
and the heater of a thermosyphon loop (Fig. 1) [4]. The ISAHP absorbs energy
from solar radiation and ambient air simultaneously [5] and transports the heat to
the storage tank through the Rankine cycle. For obtaining high reliability and
reducing cost, a heat exchanger/condenser unit is designed with a thermosyphon
loop to transfer the heat from the condenser to the water storage tank. The con-
denser releases condensing heat from the Rankine cycle to the waterside of the heat
exchanger for producing a natural-circulation flow in the thermosyphon loop. The
compressor is the only component that has moving part in the ISAHP.
In ISAHP-1, the unglazed solar collector is divided into four parts: top(50 74
cm), front(50 120 cm) and two sides(50 60 cm). The total collector area is 1.44
m2. The capacity of the water storage tank is 105 l. ISAHP-1 utilizes an R134a
compressor (250 W, 110VAC) commonly used in household refrigerators for the
Rankine cycle. A counter-flow heat exchanger is used as the condenser/heat
exchanger unit.
3. Instrumentation
This laboratory-made ISAHP (ISAHP-1) (Fig. 2) was equipped with some

instruments to monitor long-term performance. To simulate the daily hot water
load, we installed an automatic hot water drainage system in order to discharge the
B.J. Huang, C.P. Lee / Renewable Energy 29 (2003) 633–639 635
Fig. 1. Schematic diagram of an ISAHP.
Fig. 2. Prototype of ISAHP.

hot water and feed cold water into the water tank every day before sunrise. The
feed water is from a city water line that has a temperature of about 2–4 oC below
the ambient air temperature. However, the water temperature in the tank at the
startup moment of ISAHP-1 (daily initial water temperature) will increase signifi-
cantly if the hot water produced before the day (105 l) is not completely used, i.e.
mw < 105 l/day. The daily quantity of drained hot water is around 80 l, that is
about 80% of the designed capacity.
A temperature controller is used to shut down the heat pump when water tem-
perature has reached the designated value. The water temperature setting was 57 oC
in the test. A power meter was installed to record the total electric energy con-
sumption of ISAHP-1 during operation. A water meter was also installed to mea-
sure the total water consumption.
4. Long-term performance results of ISAHP
ISAHP-1 has been running continuously since January 1, 2001. No mechanical

failure has ever occurred for a continuous operation of more than 13,000 h until
the end of June 2002, with a total running time of >20,000 h during the past 5 yr
including the research period. This is due to fact that ISAHP-1 has only one mov-
ing part, i.e. Freon compressor. The machine will be highly reliable if the system
was well designed with proper sizing of all the components. ISAHP-1 has shown
itself to be a good design.
The electricity consumption per liter of hot water at 57 oC, Em, is mostly
between 0.01 and 0.03 kWh/l, with an average value 0.019 kWh/l (Fig. 3). Em is
larger at cloudy or rainy days with low solar radiation as shown in Fig. 4 that is
Fig. 3. Long-term performance test results of ISAHP-1.

Fig. 4. Variation of electricity consumption with solar irradiation.
directly converted from the results of Fig. 3. At higher solar irradiation (H t > 5
MJ/m2 day), Em approximates a constant value. Em also increases with decreasing
daily hot water load mw. The abnormal test results with higher Em as shown in
Fig. 3 are due to low mw that is caused by an adjustment problem of the monitor-
ing system at the early stage of the long-term performance test. Low daily hot
water load mw will increase the daily initial water temperature in the tank and
decrease the COP as well as Em for ISAHP [4]. This result also indicates that
ISAHP must be designed to properly meet the daily hot water load requirement.
Oversized design in hot water supply capacity will cause the energy efficiency to
drop significantly.
Fig. 5. Variation of electricity consumption with mean daily ambient temperature.

Fig. 6. Electricity consumption of various water heaters.
For an approximately constant hot water load (71–93 l/day, average 84 l/day),
Em drops linearly with increasing ambient temperature Ta as shown in Fig. 5. This
indicates that the energy consumption of ISAHP-1 changes more sensitively with
ambient temperature Ta. Em becomes higher at cold days (lower Ta).
The electricity consumption of the conventional solar hot water heating system
using electric backup heater ranges from 0.02 to 0.05 kWh/l. A long-term field test
for electric and conventional solar heaters was also carried out in the laboratory. It
shows that ISAHP-1 consumes much less electricity than the others (Fig. 6). The
present study verifies that ISAHP has a very low energy consumption and high
reliability even under long-term severe outdoors operating conditions subject to
very rough variations of solar incident radiation intensity, wind speed/direction,
ambient temperature and rain etc.
5. Conclusion
The present study measured the long-term performance of an ISAHP continu-

ously for longer than 1.5 yr (>13,000 h). The total accumulated operating time for
the prototype is over 20,000 h during the past 5 yr. No machine failure has ever
occurred. This is due to the good engineering design of ISAHP-1 with only one
moving part (compressor) and with good sizing of all the components. The life test
is still going on and the expected total lifetime for the prototype ISAHP-1 will be
longer than 6 yr. The average energy consumption of ISAHP-1 is 0.019 kWh/l of
hot water at 57 oC that is much less than the backup electric energy consumption
of the conventional solar water heater and the pure electric heater. The present
study has verified that ISAHP is very reliable if it is carefully designed. The com-
mercialization of ISAHP is now underway.
Acknowledgements
The present study was supported by the Energy Commission, Ministry of Econ-
omic Affairs, Taiwan, and by National Science Council, Taiwan, through grant no.
NSC89-2212-E-002-072 for the heat pump technology.
References
[1] Chartuvedi SK, Chiang YF, Roberts AS. Analysis of two-phase flow solar collectors with applica-
tions to heat pump. ASME Paper 80-WA/Sol-32; 1980.
[2] Chaturvedi SK, Shen JY. Thermal performance of a direct expansion solar-assisted heat pump. Solar
Energy 1984;33(2):155–62.
[3] Ito S, Miura N, Wang K. Performance of a heat pump using direct expansion solar collectors. Solar
Energy 1999;65(3):189–96.
[4] Huang BJ, Chyng JP. Integral-type solar-assisted heat pump water heater. Journal of Renewable
Energy 1998;16:731–4.
[5] Huang BJ, Chyng JP. Performance characteristics of integral type solar-assisted heat pump. Solar
Energy 2001;71(6):403–14.

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Renewable Energy 29 (2003) 633–639

A long-term reliability test of an integral-type solar-assisted heat pump water heater

Keywords: Solar energy; Heat pump; Solar heat pump

Em electric energy consumption of ISAHP per liter of hot water at 57 oC

The ISAHP consists of a Rankine cycle unit, a collector/evaporator unit, and a

This laboratory-made ISAHP (ISAHP-1) (Fig. 2) was equipped with some

Fig. 1. Schematic diagram of an ISAHP.

Fig. 2. Prototype of ISAHP.

4. Long-term performance results of ISAHP

ISAHP-1 has been running continuously since January 1, 2001. No mechanical

Fig. 3. Long-term performance test results of ISAHP-1.

Fig. 4. Variation of electricity consumption with solar irradiation.

Fig. 5. Variation of electricity consumption with mean daily ambient temperature.

Fig. 6. Electricity consumption of various water heaters.

The present study measured the long-term performance of an ISAHP continu-

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