Applied Energy 86 (2009) 1423–1430

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Applied Energy

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A new version of a solar water heating system coupled with a solar water pump

Kittiwoot Sutthivirode, Pichai Namprakai *, Natthaphon RoonprasangDivision of Energy Technology, School of Energy Environment and Materials, King Mongkut’s University of Technology Thonburi, 126 Pracha-U-thit rd.,Bangmod, Thungkhru, Bangkok 10140, Thailand

a r t i c l e i n f o

Article history:Received 15 June 2008Received in revised form 22 November 2008Accepted 1 December 2008Available online 7 January 2009

Keywords:Efficiency improvementEnergy savingLow costNo heat exchangerSolar water pumpWeight reduction

0306-2619/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.apenergy.2008.12.002

* Corresponding author. Tel.: +662 02 470 8622; faE-mail addresses: ipicakai@gmail.com, ipicakai@

mte805@hotmail.com (N. Roonprasang).

a b s t r a c t

This research target was to improve the thermal efficiency of a solar water heating system (SWHS) cou-pled with a built-in solar water pump. The designed system consists of 1.58-m2 flat plate solar collectors,an overhead tank placed at the top level, the larger water storage tank without a heat exchanger at thelower level, and a one-way valve for water circulation control. The discharge heads of 1 and 2 m weretested. The pump could operate at the collector temperature of about 70–90 �C and vapor gage pressureof 10–18 kPa. It was found that water circulation within the SWHS ranged between 15 and 65 l/d depend-ing upon solar intensity and discharge head. Moreover, the max water temperature in the storage tank isaround 59 �C. The max daily pump efficiency is about 0.0017%. The SWHS could have max daily thermalefficiency of about 21%. It is concluded that the thermal efficiency was successfully improved, except forthe pump one. The new SWHS with 1 m discharge head or lower is suitable for residential use. It adds lessweight to a building roof and saves electrical energy for a circulation pump. It has lower cost compared toa domestic SWHS.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

A solar water heating system (SWHS) is the device that uses so-lar energy for hot water production. It depends on thermosyphonor force-circulation methods for circulating the hot water withinthe system [1–2]. The first one has a heavy and high-pressure stor-age tank (ST) integrated while the second one adds less weight to abuilding roof. However, the latter cannot save electrical energy.Forced circulation system generally comprises two types of sys-tems with and without heat exchangers [3]. Efficiency of the forcedcirculation solar water heater is around 50–60%, while that of thenatural circulation one is approximately 34–38% [4]. Hot ST waterstored at 45–50 �C is sufficient for residential use [5]. In order tosave energy for water circulation within the SWHS, a solar waterpump was introduced. Various developments of such pumps werereported in literatures [6–13].

Sumathy [6] did an experiment on a solar thermal water pump,which comprised a 1 m2 solar collector (SC), had overall efficiencyof 0.12–0.14% for 6–10 m discharge heads and performed 12–23cycles a day. The water mass of 15 kg was lifted for each cycle.Wong and Sumathy presented a solar water pump performancebased on n-pentane and ethyl ether as working fluids [7] and ther-modynamic analysis in conjunction with the optimization of thesolar thermal water pump [8]. They concluded that ethyl ether

ll rights reserved.

x: +662 02 470 8623.kmutt.ac.th (P. Namprakai),

was the best choice in terms of efficiency and economics. Pickenet al. [9] investigated the development of a water piston solar pow-ered steam pump. They used the 2 m2 SC comprising an evacuatedtube combined with a heat pipe. The boiling steam at 110 �C wasproduced to pump water from a well. Their pump efficiency wasin the order of 0.05% for the pumped water of 10–20 l/h. The testpumping heads were between 2 and 8 m. Liengjindathawornet al. [10] presented the experimental and theoretical studies of apulsating-steam water pump. The pumping system used an elec-tric heater as an energy source to produce a working water vaporat low temperature (90–120 �C). The experimental pump efficiencywas around 0.005–0.03% for the pumped water of 1–8 l/cycle andsuction heads of 1–2.5 m. However, the system was manuallyoperated. Wong and Sumathy [11] reviewed a more detail of solarwater pumps.

Recently, Roonprasang et al. [12] developed the SWHS inte-grated with a new solar water pump. Their system consists of1.58 m2 flat plate solar collector, an overhead tank placed at thetop level, and the larger water storage tank with a heat exchangerat the lower level. The pump is workable when solar energy inputis equal to or greater than 580, 600 and 630 W/m2 for dischargeheads of 1, 1.5 and 2 m. The mean water temperature in the SC isabout 75–78 �C. Moreover, the water temperature in the ST isaround 46–61 �C. Heat is transferred to the ST by means of abuilt-in heat exchanger. The mean pump efficiency is about0.0014–0.0019%. The water circulation within the SWHS is be-tween 12 and 59 l/d. Moreover, the SWHS has daily thermal effi-ciency of about 7–13%. It was concluded that more thermal

Nomenclature

Ac collector area (m2)cpw water specific heat (kJ/kg oC)t time, sg acceleration of gravity (9.806 m/s2)Htot total solar irradiation incident on the SC (kJ, MJ/m2 d)h system discharge head (m)IT solar irradiation (kW/m2)kd loss coefficient for discharge (dimensionless)mw mass of water in the ST (kg)N number of a water circulating cycle per dayPc fluid gage pressure in the SC (kPa)Qs heat stored in the ST (kJ)_Qs rate of heat stored in the ST (kW)Ta ambient temperature (�C)Tc mean water temperature in the SC (�C)Toh mean water temperature in the OT (�C)

Ts mean water temperature in the ST (�C)Vc volume of pumped water per cycle (m3)vd discharge fluid velocity (m/s)Wh required hydraulic work per cycle (kJ)c specific weight (N/m3)qw water density (kg/m3)gp daily pump efficiency (%)gt daily system thermal efficiency (%)

AcronymsCV control valveOT overhead tankSC solar collectorSET separation tankST hot water storage tankSWHS solar water heating system

1424 K. Sutthivirode et al. / Applied Energy 86 (2009) 1423–1430

energy loss occurred at the heat exchanger within the ST. Theabove system may be workable only for 1–3 m pumping headsdepending on the solar energy input. However, it can save electri-cal energy for water circulation and add less weight to a buildingroof.

In the current research the authors aimed to improve thermalefficiency of such SWHS in Ref. [12] using the same equipment, testplace and nearly similar condition but without the use of heat ex-changer so that more thermal energy losses at the ST could beeliminated. The new system designed could use a shorter dischargehead for a wider range of the SC–ST elevation differences then con-sumed less energy inputs for pumping hot water.

2. Experimental setup

An enhanced SWHS consists of various parts as shown in Fig. 1.

1. A solar collector (SC) has 1.58 m2 absorbing area (4 panels,0.638 m � 0.619 m each) and 14� inclination. It initially con-tains 4.1 l water and 1.3 l air. It was used to produce high-pres-sure vapor for pumping purpose.

Fig. 1. A schematic d

2. An overhead tank (OT) as a condenser is made of a 0.5 mmstainless steel and has 0.4 � 0.4 � 0.3 m volume with an airvent. It is above and jointed with the SC via tubes. The OT levelshould be high enough to ensure that the water head is greaterthan the head loss at the control valve during the suction stage.For each cycle, a small float valve installed inside the OT pro-vides 3.1 l water at ambient condition from the local supplytank to the SC.

3. A hot water storage tank (ST) is a cylinder tank of 42 cmheight and 50 cm diameter. It has an air vent and no heatexchanger within it compared to Roonprasang et al.’s [12]. Itis well insulated with a 2.54 cm aeroflex (thermal conductivityof 0.040 W/m K) and placed below the SC. It initially containsno water. It was used to store hot water pumped from the SCdirectly.

4. A separation tank (SET) is a 4 cm dia. thin plastic cylinder of 15cm length and well insulation with a 0.9 cm aeroflex. It is joinedbetween the SC outlet and the ST inlet tubes. The SET is flushwith the OT elevation and is open to surrounding air at its topin order to prevent siphon effect occurring between the SCand ST.

iagram of SWHS.

K. Sutthivirode et al. / Applied Energy 86 (2009) 1423–1430 1425

5. A control valve (CV) is a one-way valve placed between the SCand OT. Water can flow from the OT to SC because of a sufficientwater head when the SC pressure is equal to one atmosphere.On the contrary, water cannot flow from the SC to OT becausethe SC pressure is lower than the pressure drop across the valveand water flowing from the SC to the ST through the SET ismuch easier than that through the valve.

The hot water flows by vapor pressure from the SC to the STwhere the hot water is stored. The experiment was tested for dis-charge heads of 1 and 2 m. The discharge head is the difference be-tween the SC outlet and the SET levels. All data were collectedduring 8:00–17:00 at School of Energy Environment and Materials,King Mongkut’s University of Technology Thonburi.

The vapor gage pressure inside the SC represents the pressuredifference between the SC and the OT when the OT pressure alwaysis equal to one atmosphere. A pressure transducer (Cole Parmer)measured the pressure with accuracy of ±0.25%. Nine sets of K-typethermocouples connected with a hybrid recorder (Yokogawa) wereused to measure temperatures of surrounding air, water, and vaporat the overhead tank; the SC and the ST with accuracy of ±0.5 �C.Solarimeter (Kipp & Zonen) was used to measure solar irradiationwith accuracy of ±2 W/m2. Wind velocity was measured by a

Fig. 2. Points of measure

hot-wire anemometer with accuracy of ±0.2 m/s. Points of mea-surements were shown in Fig. 2.

3. System operation

An operation of the SWHS involves mainly three stages: heat-ing, pumping and suction as shown in Fig. 3.

3.1. Heating stage

Fig. 3a, control valve (CV) is automatically closed due to a veryhigh-pressure drop across this valve for the path from the SC to theOT compared to a lower pressure drop in the other path via theSET. During this stage, water in the SC is heated by solar energy.The heating stage continues until the pressure in the SC is high en-ough to move hot water from the SC to the SET.

3.2. Pumping stage

Fig. 3b, when the vapor pressure head in the SC is slightly morethan the discharge head of the system, the SC hot water then islifted upward through the connecting pipe to the SET by the pres-sure and to the ST by a gravitational force. After that (Fig. 3c), the

ment in the SWHS.

Fig. 3. The operation of the SWHS.

1426 K. Sutthivirode et al. / Applied Energy 86 (2009) 1423–1430

SC vapor can flow to the SET and then surrounding air at the airvent outlet due to buoyancy effect. There is less vapor flowing fromthe SET to the ST.

3.3. Suction stage

Fig. 3d, vapor from the collector continues flowing to the SETwhile some cooler air from the SET can flow into the SC until the

SC pressure balances with the surrounding air pressure. Then thewater head at the OT is greater than the head loss across the valveso that the CV is open automatically, the lower temperature waterfrom the OT moves down to the SC by a gravitational force and pro-vides suction. The water flow will stop when the SC pressure devel-oped is high enough to resist the flow. One cycle of the operation ofself-pumping is thus completed and the system is now ready forthe next step.

K. Sutthivirode et al. / Applied Energy 86 (2009) 1423–1430 1427

4. System analysis

Cumulative energy stored in the ST:

Q s ¼ mwCpwðTend � T initialÞ ð1Þ

where mw is the water mass in the ST, cpw the water specific heat,and (Tend � Tinitial) the rise in water temperature within the ST. Dailysystem thermal efficiency in percentage gt is defined as the ratio ofdaily thermal energy stored in the ST to the total solar irradiationincident on the SC:

gtð%Þ ¼R _Qsdt

AcR

IT dt� 100; ð2Þ

or gtð%Þ ¼Q S

Htot� 100; ð3Þ

where _Qs is the rate of energy stored, Ac the collector area, IT the so-lar irradiance, and Htot the total solar irradiation incident on the SC.Daily pump efficiency in percentage is given by [6]:

gpð%Þ ¼NWh

Htot� 100 ð4Þ

where N is the number of water circulating cycles per day. Wh therequired hydraulic work per cycle is expressed by:

Wh ¼ Vcqwgh ð5Þ

where Vc is the pumped water volume per cycle, qw the water den-sity, g the acceleration of gravity, and h the discharge head of thesystem.

The fluid flows to SET according to the Bernoulli’s equation:

PC

c¼ hþ ð1þ kdÞ

v2d

2gð6Þ

where PC is the SC gage pressure, c the specific weight, vd the dis-charge fluid velocity, and kd the loss coefficient for discharge. It isassumed that fluid velocity within the SC is zero and the velocityalong the discharge tube is unchanged. The SET pressure is alwaysat one atmosphere. There is less vapor flowing from the SET tothe ST. Instead, it mainly flows out at the air vent outlet due tobuoyancy force.

Furthermore, critical solar irradiance was introduced. The criti-cal value is a solar energy threshold supplied to the pump in orderthat it can start pumping water.

5. Results and discussion

5.1. Effect of discharge heads and energy input on the SWHSperformance

Table 1 shows the uncertainty of the performance of the systemduring a pumping period. As the discharge head increases the dailypump efficiency and the daily system thermal efficiency decrease,but the SC temperature increases due to more energy input con-

Table 1Uncertainty of the SC, ST, OT and ambient temperatures; SC pressure, pumped water volumheads (h) and solar irradiation for 1.58 m2 SC (present system).

Item h (m) Tc (�C) Ts (�C) Toh (�C) Ta (�C) Pc (kPa)

Mean 1 74.4 57.3 35.3 35.2 9.1Mean 2 74.8 44.8 34.5 32.7 10.4Min 1 61.8 54 31.3 27.7 �2.0Min 2 58.2 39.1 31.4 23.3 �3.2Max 1 80.8 58.6 40.5 41.2 14.4Max 2 96.3 55.6 38.5 39.2 21.6S.D. 1 2.9 1.0 1.9 2.3 2.4S.D. 2 5.8 6.6 1.2 2.8 4.7

sumed for each cycle and hence a small number of pumping cyclesgained when mean solar irradiation is between 18.9 and 21.1 MJ/m2 d.

It is found that the pump can circulate 3.1 l water for each cycle.The pump of the SWHS is only workable when solar energy input isequal to or greater than the critical values of 624 and 733 W/m2 forthe discharge heads of 1 and 2 m, respectively (Fig. 4). Moreover,this system can pump water when the SC vapor pressure head isslightly more than a discharge head. The effective vapor gage pres-sures found at the SC are between 10.1–13 kPa and 17–21.2 kPa for1 and 2 m discharge heads respectively (Fig. 4).

Fig. 4 shows the effect of an instantaneous solar energy input onthe system performance for 2 days with nearly similar solar irradi-ation (24 and 23.4 MJ/m2). It is found that as the system dischargehead increases the total number of pumping cycles as well as thestored energy decrease. For the 2 m discharge head, the peak of va-por pressure and the SC temperature for each cycle in the collectorare higher, compared to the other case because more energy inputis used.

Shown in Fig. 5 for the solar irradiation of 23 MJ/m2 d, it wasfound that the pumped water decreased with the increase in a dis-charge head. The main reason is that the system required higherenergy input to lift water at a higher discharge head.

5.2. Performance comparison between the current and other works

The current system comprises the same flat plate collector settested at the same place as Ref. [12]. According to Fig. 4 of bothworks for 1 m discharge head, the ambient temperatures arearound 35 and 30 �C; mean solar irradiations are 24 and 22 MJ/m2 d; initial SC water temperatures (first cycle) are 48 �C (withhigher solar energy and a clear day condition) and 36 �C for thecurrent work and ref. [12] respectively. For the next cycles in bothworks, the initial SC water temperature is nearly similar (70 �C or abit more). However, in terms of thermal and pump efficiencies thecomparison could be made successfully under nearly the sameconditions.

According to Table 2 describing the performance of the currentwork compared with Ref. [12] for 1 m discharge head, it was foundthat Toh was nearly equal to Ta and lower than Roonprasang et al.’s[12] leading to a lower Tc and hence Ts. The corresponding thermalefficiency is higher due to more energy losses reduction at the STwith the use of no heat exchanger. Even though lower Toh normallyleads to more air mass suctioned and hence more SC pressure pro-duced for pumping, the current pump efficiency cannot be en-hanced because the current discharge head definition is differentfrom Ref. [12]. In Ref. [12], with 30 l water initially stored at theST, the stored thermal energy (QS) highly depends on dischargeheads in terms of the pumped water for transferring heat to theST via the heat exchanger. For instance, mean pumped wateramounts are 42 and 46 l/d for the same solar energy input of20 MJ/m2 d and the discharge heads of 1.5 and 1 m, respectively.

e, daily system thermal efficiency and daily pump efficiency as a function of discharge

Pumped water (l/d) gt (%) gp (%) Solar irradiation (MJ/m2d)

50.4 17.4 0.0014 21.122.4 4.1 0.0011 18.924.8 11.9 0.0010 15.715.5 2.3 0.0009 16.165.1 20.9 0.0017 23.931.0 6.6 0.0014 23.417.8 3.9 0.0004 3.2

5.5 1.6 0.0001 2.9

y = 4.8218x - 51.646

0

10

20

30

40

50

60

70

15 17 19 21 23 25

1m

2m

linear (1m)

Pum

ped

wat

er (

l/d)

Solar irradiation ( MJ/m2d )

Fig. 5. Relationship between pumped water and solar irradiation for the systemwith discharge heads of 1 and 2 m (present system).

010

2030

4050

6070

8090

100

0

0.5

1

1.5

2

2.5

3

3.5

4Tc Ts

Toh TaIT

Tem

pera

ture

( o C

)

Sola

r ir

radi

ance

( k

W/m

2 )

daeH egrahcsiD m 2daeH egrahcsiD m 1

-25

-20

-15

-10

-5

0

5

10

15

20

25

8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0Pc V

Qs

Pres

sure

( k

Pa )

, Air

Vel

ocity

( m

/s )

Qs (

MJ

)

daeH egrahcsiD m 2daeH egrahcsiD m 1

TIME OF DAY

Fig. 4. Solar irradiance (IT), thermal energy stored in the ST (Qs), water temperature in the SC (Tc), water temperature in the ST (Ts), water temperature in the OT (Toh), ambienttemperature (Ta), vapor gage pressure in the SC (Pc), wind speed (V) as a function of time for the discharge heads of 1 and 2 m. When solar energy inputs are 24 MJ/m2 d (1 mhead), 23.4 MJ/m2 d (2 m head). (present system).

1428 K. Sutthivirode et al. / Applied Energy 86 (2009) 1423–1430

At over 3 m discharge head, the pump cannot work more effi-ciently. On the contrary, the current stored ST thermal energy de-

Table 2Mean and max performances comparison between the present work and Ref. [12] for 1 m

Item h (m) Tc (�C) Ts (�C) Toh (�C) Ta (�C) P

Present work Mean 74.4 57.3 35.3 35.2 5Ref. [12] Mean 75.0 59.1 36.2 33 4Present work Max 80.8 58.6 40.5 41.2 6Ref. [12] Max 84.6 61.0 46.3 32.5 5

pends only on solar energy input when the discharge head is fixedat 1 m.

The current SWHS uses the 1.58 m2 SC to produce hot water ofaround 54–59 �C and 65 l max volumes (sufficient for 1.5 persons)in comparison with the system of Khalifa [4] which used the2 � 1.42 m2 SC and 170 l storage tank in order to obtain roughly45–50 �C hot water when solar irradiation was around 23 MJ/m2

d. Thermal efficiency of the latter (force-circulation system) wasaround 50–60% evaluated by excluding electrical energy used bya circulation pump.

Considering Table 1, if heat losses at the SC and ST are reducedby using better insulation and more efficient SC, the ST tempera-ture may reach 80 �C or higher. Consequently, for 1 m dischargehead, the maximum thermal efficiency could increase to 36.8%(65.1 � 4.2 (80.8 � 30) � 100/(23.9 � 103 � 1.58), obtained fromEq. (3)), compared to 35–40% efficiency of the thermosyphonSWHS [4], assuming that initial water temperature is 30 �C. Addi-tionally, if the discharge head decreases to less than 1 m, the sys-tem efficiency can further increase i.e. more hot water can bepumped. The SWHS with such a solar water pump can reach 33–42% efficiency [13].

discharge head and 1.58 m2 SC.

umped water (l/d) QS (MJ/d) gt (%) gp (%) Hto (MJ/m2 dt)

0.4 5.8 17.4 0.0014 21.15.9 3.64 11.3 0.0014 20.45.1 7.9 20.9 0.0017 23.98.9 3.97 11.8 0.0017 21.3

Table 3Technology comparison between the present work and Ref [12].

Ref. [12] (original one) Present system (improved one)

1. Use of heat exchanger: Heat is transferred to the ST by means of a built-in heatexchanger, so some of thermal energy can be lost during the exchange processbetween the heat of discharged water and water stored in the ST. The other lossesare due to imperfect insulation and vapor leakage at the OT

1. No heat exchanger was used; all hot water from the SC can be stored in the ST sothat more thermal energy losses at the ST could be eliminated. This is animprovement. However, the remaining losses are due to imperfect insulation andvapor leakage at the SET

2. The OT as a condenser receives heat from the discharged water recycled, so it isless efficient

2. The OT as a condenser receives cool water from the local water supply at ambienttemperature, so it is more efficient

3. The difference between the SC and ST levels, a discharge head can not be set atover 3 m

3. The SWHS with 1-m discharge head or lower is sufficient. This will be anadvantage. The discharge head is a difference between SC and SET levels

4. Use of vapor for circulating discharged water that transfers heat to the ST via theheat exchanger

4. Use of vapor for pumping discharged water from the SC to the ST directly

5. No electrical energy is used. System water can flow automatically6. The compared systems use the same set of the SC with the ST placed below the SC. Both systems use solar water pump7. Both systems add less weight to the building roof

y = 1.0544x - 4.7149

R2 = 0.9989

-1

4

9

14

19

24

29

15 17 19 21 23 25

Dai

ly th

erm

al e

ffic

ienc

y (

% )

Solar irradiation ( MJ/m2d )

Fig. 6. Daily thermal efficiency as a function of solar irradiation for 1 m dischargehead. (outdoor experiment, present system).

y = 1E-04x - 0.0006

R2 = 0.9987

0.0005

0.0007

0.0009

0.0011

0.0013

0.0015

0.0017

0.0019

0.0021

520251

Dai

ly p

ump

effi

cien

cy (

% )

Solar irradiation ( MJ/m2d )

Fig. 7. Daily pump efficiency as a function of solar irradiation for 1 m dischargehead. (outdoor experiment, present system).

K. Sutthivirode et al. / Applied Energy 86 (2009) 1423–1430 1429

5.3. Technical analysis for the improvement of the current workcompared to Ref. [12]

The disadvantages of the original system [12] involve two majorlosses: thermal loses at the heat exchanger installed in the ST andenergy loss due to more discharge heads used and hence morehydraulic energy required for the pumping. The improvementmade in the current study is shown in Table 3 regarding technol-ogy of the current work compared with Roonprasang et al.’s [12].It is obvious that no heat exchanger is used leading to the substan-tial reduction of energy losses. In addition, the pumping systemcan be workable for every SC–ST elevation difference when the dis-charge head is fixed. Lower OT water temperature found in thenew OT (condenser) can make the relevant pump more efficient.

5.4. Long term performance analysis for the current system

The daily thermal and pump efficiencies in percentage areshown in Figs. 6 and 7. Assume a linear relation between the effi-ciencies and solar irradiation based on Figs. 6 and 7:

gt ¼ 1:0544Htot � 4:7149; ð7Þ

Table 4Mean thermal and pump efficiencies (%) of the SWHS as a function of daily mean solar ir

January February March April May June

Htot 16.7 17.8 19.1 20.4 18.2 18.2gt 12.9 14.0 15.4 16.8 14.5 14.5gP 0.0011 0.0012 0.0013 0.0014 0.0012 0.0012

where Htot is equal to 4.5 MJ/m2 d, when gt = 0.

gP ¼ 0:0001Htot � 0:0006; ð8Þ

where Htot is equal to 6 MJ/m2 d, when gP = 0.As a conclusion, the lowest solar irradiation for possible pump-

ing by the system is of the order of 6 MJ/m2 d. However, the SWHSpump is only workable during the period that solar energy input isequal to or greater than the critical value of 624 W/m2. The annualefficiencies can then be evaluated when monthly mean solar irra-diation is known as shown in Table 4.

In the case of low potential in solar energy input to the systemfound in winter, the present system like a conventional one caninstall auxiliary heating sources: electric heater and thermal en-ergy from woods, at the ST. Another alternative way to applythe developed system at a very low solar energy input is to de-crease the discharge head to not less than 16 cm (when 63.8-cmSC length � sin 14 = 15.4 cm) in order to make the pump workableat this particular condition. During that circumstance, the hotwater temperature may be as low as around 30 �C, which is stillsuitable for a bath.

radiation (MJ/m2 d) for 1 m discharge head (present system).

July August September October November December

16.3 16.1 14.7 15.1 15.4 15.312.5 12.3 10.8 11.2 11.6 11.4

0.0010 0.0010 0.0009 0.0009 0.0009 0.0009

1430 K. Sutthivirode et al. / Applied Energy 86 (2009) 1423–1430

6. Conclusions

1. For 1 m discharge head system (Table 2), it was found that thenew ST stored energy (Q s) was 1.6–2 times higher compared toRoonprasang et al.’s [12] because nearly all thermal energy ofthe discharged water could be stored at the ST. Consequently,even though the new ST water temperature (Ts) was less thanRoonprasang et al.’s [12], the corresponding thermal efficiencyfor the current system was higher.

2. Lower OT water temperature found in the new OT (condenser)makes the relevant pump more efficient.

3. The ST can be placed at any elevations below the SC panelswhen the discharge head is fixed at 1 m or lower. The pumpedwater can increase when the discharge head decreases becausethe hydraulic energy required for each cycle of pumping isreduced.

4. The present system is cheaper, compared to the product of Thai-land (thermosyphon, 100 l storage tank) [12]. The advantages ofthe present system are that it can save cost of conventionalenergy for water circulation in the system and add less weightto the building roof.

5. This system is suitable when local water supply for supplyingwater into the OT is available. The most effective solar irradia-tion of the SWHS is between 18–22 MJ/m2 d.

Acknowledgements

The authors gratefully acknowledged the financial support pro-vided by the Energy Policy and Planning Office, Ministry of Energy,

Thailand and the Energy Technology Division, School of EnergyEnvironment and Materials, King Mongkut’s University of Technol-ogy Thonburi.

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