Energy and Buildings, 17 (1991) 55-62 55

A pump economizer for evaporative coolers

Arvinder Singh, Ashok Gadgil, S. Gopi and Bhaskar Natarajan Tata Energy Research Institute, 7 Jot Bagh, New Delhi 110 003 (India)

(Received June 14, 1990; accepted October 16, 1990; revised paper received June 29, 1990)

Abstract

The water flow rate in the existing evaporative coolers used in India for domestic cooling (called "desert coolers" in the local markets) is much higher than what is required to keep the fibre pads wet. Therefore, a retrofit has been designed to lower the electrical energy consumption of the coolers. We report here on the design, performance and economics of the proposed retrofit which can be very easily fitted on to existing coolers. The cost of the retrofit, including installation charges, is about Rs. 95 (1988 rupees) or about US$ 6 (US$ 1 =Rs. 15.60), and the resulting annual electricity savings are about 25 kWh. Since space cooling contributes to the summertime peak electricity demand in large North Indian urban centres such as Delhi, the device appears very attractive for conserving electrical energy and peak urban electricity demand in summertime.

1. Introduct ion

The d e m a n d for e lect r ic i ty in the Union Ter r i to ry of Delhi has b e e n g rowing s teadi ly over the years . The p e a k d e m a n d has g rown f r o m 422 MW in 1 9 7 8 - 7 9 to 1255 ~ in 1 9 8 8 - 8 9 . The p e a k d e m a n d for 1 9 8 9 - 9 0 is e s t ima ted to be 1373 ~ [1]. Dur ing the s u m m e r , the inabili ty to m e e t the d e m a n d of ten leads to load shedd ing in the ho t t e s t pa r t of the day. This is b e c a u s e the d e m a n d for s p a c e cool ing is a s ignif icant c o m p o n e n t of the de- m a n d in s u m m e r . The s a m e is e x p e c t e d to be t rue for o the r ho t and dry reg ions of India.

In the no r the rn and cent ra l u r b a n cen t re s of India, the s u m m e r s are hot and dry, wi th the o u t d o o r t e m p e r a t u r e of ten r each ing 47 °C. This has led to the ex tens ive use of air con- d i t ioners and evapora t ive ' dese r t ' coolers . Of the two, the evapora t ive coo le r s a re ve ry p o p - ular and are u sed by m o s t o f the u r b a n house - ho lds and in offices, main ly b e c a u s e of the low initial i n v e s t m e n t involved, low ope ra t i ng and m a i n t e n a n c e cos t s and ea sy instal lat ion. Man- u fac tu re of evapora t ive coo le r s be ing a small- scale industry, n u m e r o u s w o r k s h o p s manu- fac tu re evapora t ive coo le r s for local sale. Al- t hough each coo le r c o n s u m e s only a smal l

f rac t ion ( abou t one- ten th) of the e lect r ic i ty c o n s u m e d by an air condi t ioner , the i r shee r n u m b e r leads to a s ignif icant con t r ibu t ion to the s u m m e r t i m e elect r ic i ty c o n s u m p t i o n as well as the p e a k demand .

In the p r e s e n t effort, a s imple and inex- pens ive device cal led a " p u m p e c o n o m i z e r " is descr ibed , which can be re t rof i t ted to the exis t ing evapora t ive coo le r s to i m p r o v e the i r ene rgy efficiency.

2. Exis t ing evaporat ive coolers

A tradi t ional Indian evapora t ive coo le r is no rma l ly m a d e with ga lvan i sed i ron shee t s or fibre glass sheets , in the s h a p e of a cube. On th ree s ides of this coo le r are r e m o v a b l e fibre p a d ho lders and on the four th side is the exhaus t fan (Fig. 1).

The re are two elect r ical c o m p o n e n t s in the conven t iona l cooler : the fan and the p u m p . The la t ter con t inuous ly lifts w a t e r f r o m the s u m p at the b o t t o m and d is t r ibutes it into p e r f o r a t e d V-channels at the top of the fibre p a d s on th ree s ides of the coo le r to k e e p t h e m wet. The w a t e r l ifted by the p u m p en te rs the w a t e r d is t r ibut ion cup t h rough the l a t t e r ' s bot-

Elsevier Sequoia/Printed in The Netherlands

5 6

:... 3-way cup fo r

distributing w a t e r

P e d o r a t e d V-channel

Lattice for holding ............

water retaining mat

P u m p .............

.:.

S u m p for holding water

Fig. 1. Details of a conventional evaporative cooler.

...... Rubbertubinq(pvc pipe) for carr~ng waler

...... Exhaustfan

tom port. The fan pulls air through these wet pads and the cooled air is thrown into the room. The enthalpy content of the air remains constant; its drybulb temperature is lowered without any significant change in its wetbulb temperature. The magnitude of the drop in the drybulb temperature of air on passage through the cooler, depends on the airflow rate, the initial relative humidity of air, the pad char- acteristics (pad surface area, thickness, etc.) and the amount of water on the pads.

The pumps used in the conventional coolers have motors rated at 40 - 50 W. According to the manufacturer 's plate fixed on these motors, they deliver 1/70 hp (10.6 W). (Incidentally, these plates do not give any figures for the water flow rate and the head developed at 10.6 W of pump power). During our experiments, we observed that the actual mechanical power delivered by the pump under working condi- tions, i.e., when connected to the water dis- tribution system of the cooler, was about 2.4 W at the present water flow rate of 8.4 1/min. Thus, the overall efficiency of the pump and the motor, under normal working conditions, comes to about 6%.

The proposed pump economizer is based on the observation that the flow rate of water in the traditional coolers is much higher than that required for their present performance. Our experiments show that a water flow rate of about 21/min is sufficient for a cooler of medium size to deliver the cooling performance (the common flow rate is about 8 -8 .5 1/min). In

other words, the flow rates in the present coolers are about four times higher than those required for a comparable performance.

The pump economizer described here is a retrofit device which operates the pump in- termittently to reduce its flow rate from about 8 I/min to about 2 l/min. The device (including its installation costs) costs only about Rs. 95 and can be retrofitted to the existing evapo- rative coolers.

3. Descr ip t ion o f the pump e c o n o m i z e r

The pump economizer consists of a float and limit control switch in a small galvanized iron (GI) tray installed on the top of the cooler. The capacity of this tray is about one-fourth the capacity of the sump at the bot tom (which is typically about 50 litres). The pump outlet is redirected to feed water into this tray instead of directly feeding the perforated channels above the fibre pads. Water from the tray is fed to the perforated channels via a small adjustable aperture on the bot tom of the tray. This aperture is adjusted to obtain a water flow rate that is adequate for the required cooler performance.

Since the pumping rate of water into the tray (about 8 l/min) is much higher than the rate of water leaving the bot tom of the tray (about 2 1/min) for the fibre pads, the tray gets filled with water in a few minutes. At this stage, a float and limit switch switches off the

57

pump. The ent i re a r r angemen t is shown in Fig. 2.

The wa te r keeps flowing by gravi ty f rom the t ray to the fibre pads t h rough the wa te r dis- t r ibut ion cup with its b o t t o m por t sealed and wa te r enter ing it t h rough a hole at the top (as shown in Fig. 2(c)) . When the wa te r level in the t ray d rops sufficiently, the float and limit switch reac t iva te the pump until the t ray is again comple te ly filled. This cycle con t inues until the t ime when, owing to evapora t ion , the total a m oun t of wa te r left in the cooler b e c o m e s less than the a m oun t of wa te r requ i red in the t ray to switch off the pump. The pump now runs cont inuously . At this stage, the sump needs to be refilled.

With this a r rangement , the pump does not opera te cont inuously , as in the convent iona l

cooler . The flow of wa te r to the fibre pads, however , remains cont inuous . This saves a f rac t ion of the electr ical ene rgy c o n s u m e d by the pump.

4. Exper imenta l resul ts

Expe r imen t s were first co n d u c t ed to deter- mine the effect of decreas ing the wa te r flow ra te on the p e r fo rm an ce of the cooler .

Two ident ical new coolers (fan dia. 43 cm or 16 in.) were t es ted to ensure that they p e r f o r m e d identically. The tes t ing cons is ted of opera t ing the coolers ou tdoor s s imultaneously, p lacing them side by side. The inlet and out le t air t em p e ra tu r e s were m easu red with a digital t em p e ra tu r e indicator ( accu racy 0.1 °C). Rel-

~A

/

ns

(a)

, \ ........... J ® .... [,_[ ....~

(e) ! .... Fiocd tJimit switch

,~ " ....float

, L - " ~ I. Pump gels switched off Co) (d)

Terminal

~': Filament housin(~ of a bulb

~ s s tube?. Terminal "

2. Pump remains ~-~ ~. Pump switches on

Fig. 2. (a) Retrofit details. Co) Sectional elevation A-A. (c)Water distribution system: 1. bolt with a slot for water entry, 2. nut welded to the pipe, 3. welding, 4. full threaded pipe (3/4" dia.), 5. checknuts, 6. rubber washers, 7. bot tom of the tray, 8. top of the cooler body, 9. top plate of the water distribution cup, 10. water distribution cup tightened to the top plate, 11. water outlets, 12. water inlet sealed. (d) Float and limit control switch.

58

35

34

33

32

® .~ 3~

29

28

27

26

(a)

4 0

3 8

36

~ 3 4

32

3O

2 8

2 6

(c)

3 6

34

32 e =

April 22,1988

AIR TEMPERATURES 0 Cooler t (Outlet) + Cooler 2 (Outlet) o ~nbieot (Inlet)

I q I I I

II II:30 12 1~30 13

Time (Hours)

April 28, 1988

/-° o

AIR TEMPERATURES ~ ' ° - ~ _ o o ~'fOdilionol Cooler (Outlel)

(~6t~er Flow Rote 8.4 tlrs/rNn )

-~ Modified Cooler(Outlet) (Woter Flow Rote 2 8 ~s/min )

o Arnb~eot (inlet)

t ~ i i i

It 12 13 [4 15

Time (Ho~rs)

April 29,1988

AIR TEMPERATU

(Woter Row Rate 8.4 Itrs/rain )

+ Modified Cooler(l~Jllel) ('~tot e r Flow Rate 2.11trs ~ i n )

o Ambient (Inlet)

28 __~.}_~----- + ~ +

~ 0 ~ 0 ~ 0 / o ~ 0

26 = ~ I I I I . ~ I

t2 12:15 12:50 12:45 13 13:15 13~30 13:45

(e) Time (Hours)

Moy 3L, 1988 36

35

34

m

3 3 g E

32

o

o /

AIR TEMI:~R ATURES 0 Troditionol Cooler(Curler)

( W a t e r Row Rote 8.4 firs/rain }

+ Modified Cooler (C~IeH (Wuter F~w Rote 51 nrs/rain )

a Ambient {Inlet)

30 I I I I

9 I0 II i2 13

(b) Time(Hoursl

4 0

3 8

3 6

34

~ 32 E

3O

28

26

I0

(d)

May I0,1988

o j O - - - - o ° ~

AIR TEMPERATURES o Tradifionol Cooler ( C u t l e t )

(Water FlowRute 8L41ffS /rnirL)

+ Modified Cooler (OutLet) (V~er Flow Rote 2,6 ~trs/rnin )

o Ambient (~lel)

o I I I i i

12 13

Time (Hours }

Fig. 3. (a) Comparison of two coolers before modifications. Comparison of modified and tradit ional coolers: (b) mod- ified cooler with flow rate of 5.1 Vmin; (c) modified cooler with flow rate of 2.8 1/min; (d) modified cooler with flow rate of 2.6 l/min; (e) modified cooler with flow rate of 2.1 Vmin.

ative humidi ty at the inlets and the out le ts w a s a lso m e a s u r e d (accuracy 1%). The tes ts con- f irmed that both the c o o l e r s p e r f o r m e d iden- t ical ly (Fig. 3(a)) .

One o f the c o o l e r s w a s then modi f i ed ac- cording to the p r o p o s e d des ign (Fig. 2). A gate va lve w a s instal led b e t w e e n the c o o l e r and the tray so that the f low rate o f water to the fibre pads cou ld be regulated. In pract ice , a nut and s lo t ted bolt a s s e m b l y cou ld be u s e d

to contro l the f low rate o f water f rom the top tray to the fibre pads (as s h o w n in Fig. 2 (c ) ) . B o t h the coo l er s w e r e again operated s imul- t a n e o u s l y s ide by s ide in the open .

In all the exper iments , the water f low rate in the unmodi f i ed c o o l e r w a s left u n c h a n g e d . The water f low rate in the modi f ied c o o l e r w a s reduced by s tages in a ser ies o f exper iments , w h i c h w e r e des igned to de termine the m i n i m u m water f low rate at w h i c h the modi f ied c o o l e r

would pe r fo rm almost identically to the un- modified one. The compar i son of cooler per- fo rmances at different water flow rates is shown in Figs. 3 (b ) - ( e ) .

It was observed that the pe r fo rmance of the coolers remained a lmost identical*, even after the water flow rate in the modif ied cooler was r educed by a fac tor of four, i.e., f rom 8.4 1/min to 2.1 l/rain.

An exper iment was also c o n d u c t e d to de- termine the actual mechan ica l power of the p u m p in the tradit ional cooler (for the flow rate of 8.4 litres/min).

The fluid power (FP) of a p u m p is deno ted by:

FP = T Q H (watts) (1)

where T = w e i g h t densi ty of water (N/m 3) Q = flow rate of water (m3/s) H = t o t a l head deve loped by p u m p (m)

The total head deve loped by the p u m p con- sists of the static pressure head and the dynamic head. A U-tube mercu ry m a n o m e t e r was used to measure the static head (as shown in Fig. A1 in the Appendix) . The dynamic head de- ve loped was calcula ted on the basis of the average veloci ty of the flow at the exit point f rom the p u m p impeller.

5. E c o n o m i c s

The cos t benefits of such a retrofit can be calculated f rom the point of view of the elec- tricity consumer s and the e c o n o m y as a whole. A limited economic analysis of the p u m p econ- omizer has been carr ied out in this Sect ion for the u rban region of Delhi. To the consumer , the p u m p economize r would cos t Rs. 95, in- cluding the cos t of materials, fabrication, dis- t r ibut ion and installation. It is assumed, con° servatively, to have a lifetime of 10 years and no scrap value after that.

It is es t imated that residential evaporat ive coolers in and a round Delhi are opera ted for about 800 to 1000 hours in a year. The p u m p s in our exper imenta l coolers were ra ted at 42 watts. Assuming 800 hours of annual operat ion, each p u m p would c o n s u m e 33.6 k w h in a

*A difference of less than 1 °C in the outlet air tem- perature of the coolers was considered as "almost identical performance".

59

TABLE 1. Net savings (consumer)

Pump with economizer

Additional capital cost 95.0 (~.)

Savings in annual operating 15.12 costs (Rs.) (@ Rs. 0.6/kWh)

Lifetime operating cost (Rs.) 75.9 (@ 15% discount rate)

Net lifetime savings - 19.1 (Rs.) 75.9 - 95 =

TABLE 2. Net savings (society)

Pump with economizer

Additional capital cost (Rs.)

Savings in annual operating cost (Rs.) (@ Rs. 1.4/kWh)

Lifetime operating cost savings (Rs.) (@ 12% discount rate)

Net lifetime savings (Rs.) 199.3-95=

95.0

35.28

199.3

104.3

year. With the p r o p o s e d modification, 75% of this consumpt ion (i.e., 25.2 kWh) would be saved annually.

As shown in Table 1, assuming a (subsidized) electricity price of Rs. 0.6/kWh*, which the c o n s u m e r pays, it is no t cost-effective to invest in the unsubsidized p u m p economizer .

On the o ther hand, Table 2 shows that the picture is entirely different f rom the v iewpoint of social cos ts and benefits. Even if one con- servatively a s sumed the marginal cos t of elec- tricity p roduc t ion to be Rs. 1.4/kWh [2], the p u m p economize r seems to be a cost-effective investment . In case of mass product ion , the cos t of the economize r is likely to go down drastically, thus making it a m u c h more at- t ract ive proposi t ion. Thus, f rom the v iewpoint of social cos t benefit analysis, the p u m p econ- omizer appears to be an at tractive investment .

We here under take the calculat ions for the Delhi Electr ici ty Supply Under tak ing (DESU) system. A TERI s tudy [3] indicates tha t there are about 800 000 domest ic evaporat ive cool-

*Taken as the average price paid by the domestic consumer. The price paid by the commercial consumer is much higher.

60

ers used in Delhi. A conse rva t ive es t imate would be tha t the n u m b e r of evapora t ive coo le r in- s tal la t ions in c o m m e r c i a l bui ldings is at leas t 200 000 [4]. Thus, the to ta l s tock of evapo- rat ive coolers in Delhi is abou t one million. Even if 20% of the exis t ing coole rs were to be fitted with the p u m p economizer , the po- tential year ly savings at the end-use point would be 5.04 million units of electricity. This equals an annual gene ra t ion of 6.3 mill ion uni ts of electricity. In the p r e s en t s i tuat ion of shor tages , this would m e a n addi t ional p o w e r avai lable for o the r e conomic activities.

Another impor t an t advan tage of the p u m p e c o n o m i z e r could be its i m pac t on u rban peak electr ic i ty demand . In severa l u rban electr ic i ty supp ly sy s t ems in nor th India, the p e a k elec- tr icity d e m a n d in the s u m m e r is d o m i n a t e d by space cooling. The Delhi Electr ic i ty Supply Under tak ing (DESU), for example , has been forced to pu rchase expens ive gas turbine elec- tric genera t ion sets to m e e t its p e a k demand. It is quite evident tha t dur ing the s um m er , these coolers would be opera t ing , especia l ly dur ing the d e m a n d peaks . Assuming conserv- at ively tha t only half of the to ta l e s t ima ted n u m b e r of coo le rs in Delhi are in use dur ing the peak demand , i.e., half a mill ion coolers , the savings due to the use of the p u m p econ- omizer would a m o u n t to abou t 15 MW.

6. C o n c l u s i o n s

The p u m p e c o n o m i z e r retrofi t to evapora t ive coole rs has b e e n tes ted , and it a p p e a r s to be feasible and economica l ly viable. Cons ider ing the marg ina l cos t of e lectr ic i ty and the poten t ia l savings in e lectr ic i ty demand , it a p p e a r s to be an a t t rac t ive social inves tment . The p r o p o s e d p u m p e c o n o m i z e r can be re t rof i t ted to any exis t ing coo le r for r educ ing the flow ra te of wa t e r and t he r eby dec reas ing the e lectr ic i ty c o n s u m p t i o n wi thout adverse ly affect ing its pe r fo rmance .

References

1 Twelfth Power Survey Committee, Twelfth Power Survey of India, Central Electricity Authority, New Delhi, Aug. 1985.

2 Study on Costing andPric ingfor TNEB, FinalRep., Tata Energy Research Institute, New Delhi, July 1988.

3 S. Ramesh, B. Natarajan and G. Bhagat, A Study of Characteristics of DESU Load Demand, Tata Energy Research Institute, New Delhi, May 1988.

4 Public Electricity Supply, All India Statistics 1985-86, Central Electricity Authority.

A p p e n d i x 1

Figure A1 shows the fluid circuit configu- ra t ion of the expe r imen ta l se t -up to m e a s u r e the head deve loped by the pump .

We have f rom the Bernoull i equat ion, the to ta l head at any point of a fluid in a pipe, g iven by: Total head ( h ~ ) = s t a t i c p r e s su re h e a d + d y n - amic head + poten t ia l head

p V 2 hw = -- + + Z (A1)

Applying this for poin t 2 of the fluid circuit, which is the exit poin t of the pump , we get the to ta l head deve loped by the p u m p as:

hr, at po in t 2, P2 V22 = - - + + Z 2 (A2) T 2g

Pump

Woter Distn butor

5c 4 c ~::z= Z, b :. _=_--=.-=_~ 4 a 5o

A c k n o w l e d g e m e n t s

The au thor s are thankfu l to Dr R. K. Pachaur i for his suppo r t and e n c o u r a g e m e n t . This proj- ect was s u p p o r t e d by TERI.

Impeller Housing

Fig. A1.

Mercury Mo, nomeqer

where Z2 = posi t ion on the vert ical plane of point f rom da tum level (m) 172 = average veloci ty of flow at point 2 (m/s) P2 = static pressure head at point 2 (N/m e) T----weight densi ty of water (N/m3). Taking the da tum level to be at the level of point 2, we get Z2 = 0; eqn. (A2) b e c o m e s

hw, at point 2, = P2 V2 2 - - + - - ( A 3 ) y 2g

F rom the exper iments conducted , the static head at point 2 was found to be 12 cm of Hg. Thus, the static heat at point 2 = 0 . 1 2 × 1 3 . 6 (m of water).

The dynamic head at point 2 = V22/2g. From the measured flow rate and the i.d.

(de) of the t ransfer pipe at point 2; the average veloci ty 172 is c o m p u t e d as:

y~= flow rate

area of c ross-sec t ion at point 2

Flow rate = 8.4 l/rain

8.4 × 10 -3 - (m3/s)

6O

= 1 . 4 × 10 -4 (m3/s)

Area of c ross-sec t ion of pipe at point 2

= 9 . 5 × 10 -5 m 2

1 . 4 x 10 -4 g2- 9 . 5 x 1 0 - ~

= 1.473 m/s

Dynamic head at point 2 = - - V2 2

2g

(1 .473) 2

2 x 9 . 8 1

= 0 . 1 1 m

From eqn. (A3), we have the following:

Total head deve loped h i , at point 2 = 1 . 6 3 + 0 . 1 1 = 1 . 7 4 m

The fluid power developed by the p u m p is c o m p u t e d using eqn. (1):

Fluid power (FP) = T Q H watts

61

= 9 8 1 0 x 1 . 4 x 10 - 4 × 1.74 = 2.389 watts

The fluid circuit sys tem losses compr ise the following: (1) pipe friction losses be tween points 2 and 3 (2) entry loss at point 3 (3) exit loss at point 4 (4) pipe friction loss be tween points 4 and 5 (5) head loss at the end elbow at point 6.

The above-ment ioned losses could be quan- tified exactly, using the Bernoulli equat ion applied at points 2 and 6, as given below:

P2 1/'22 P6 1762 - - + + Z 2 = - - + - - + Z 6 +hlosses T 2g T 2g

(A4)

where 1/'2, 176 = average velocit ies at points 2 and 6 Z2, Z 6 = posi t ions f rom data levels of points 2 and 6 P2, P6 = static pressures at points 2 and 6.

Where Z2=O, and, P6=O (a tmospher ic) :

flow rate Y6=

total area of c ross-sec t ion at point 6 of the 3 distr ibution pipes

1.4X 1 0 - 4 X 4 = 0.928 (m/s)

3 × ~rX (0.008) 2

Equat ion (A4) b e c o m e s

P2 I122 1762 h~ . . . . . = - - + - - Z6

T 2g 2g

0.928 = 1 . 6 3 + 0 . 1 1 - 2 × 9 . 8 1 - 0 . 7 5

= 1.63 + 0.11 - 0 . 0 4 3 9 3 1 3 - 0.75

= 0 .946 m

which is the total head loss in the fluid circuit.

A p p e n d i x 2

Floa t a n d l i m i t s w i t c h There are many float and limit control

switches readily available on the market. But these switches are too expensive to be used in our p r o p o s e d design. So, we des igned a mercury switch.

As shown in Fig. 2(d), a glass tube was bent and sealed f rom one end. It was filled with about 5 gm of mercury. The filament hous ing

62

f rom a fused incandescen t l amp was fixed to the o ther side of the glass tube. The two s u p p o r t s of the f i lament ac ted as t e rmina l s t h rough which the p u m p was connec ted .

This swi tch was a t t ached to the float valve, and m o v e d up and down with it. W h e n the cooler was swi tched on, the t ray on the top was e m p t y and the float valve was at lowes t posi t ion. At this posi t ion, all the m e r c u r y in the glass tube r e s t ed at the end where the t e rmina l s were, thus m ak i ng the circuit com- ple te and ac t iva t ing the pump . W h e n wa te r

f lowed into the tray, the glass tube s t a r t ed r is ing with the r is ing float valve. W h e n the t ray was comple t e ly filled, the m e r c u r y in the glass t ube fell to the o the r end, thus switching off the pump . The wa te r kep t flowing down to the fibre p a d s by gravity. When the wa te r level in the t ray fell sufficiently, the m e r c u r y again d r o p p e d to the end with the terminals .

The float and limit swi tch desc r ibed above m a y a p p e a r crude, but it works and is rugged and ve ry inexpens ive ( abou t Rs. 15 -20 ) . Any o ther sui table switch could also be used in p lace of the one deve loped by us.