STUDY OF LOW TECHNOLOGY SOLAR THERMAL WATER PUMPS FOR USE IN DEVELOPING COUNTRIES

Presented byM.SYAM (06761A0324)

Department of Mechanical Engineering,LAKIREDDY BALIREDDY COLLEGE OF

ENGINEERING, Mylavaram

ABSTRACT

Solar water pumps, based on electro-mechanical pumps powered by PV

arrays, are commonly used and commercially available. However, one of the

difficulties for their wider application in developing countries .where there is high

average insolation, is their relatively excessive cost. This arises mainly due to the high

cost of the PV elements. Hence, this paper describes some developmental work and

results of experimental tests on “low-tech” solar thermal water pumps which were

built on the basis of Stirling engines with fluid pistons couple to flat-plate solar

collectors.

Temperatures and pressures in the cycle are comparatively low. Thus cheap

design materials, such as glass and plastic, and a simple technology, available in the

majority of mechanical workshops, can be used for their manufacture and

consequently reduce their cost.

Several design modifications of the above solar thermal water pumps have

been studied. The results obtained demonstrate that existing installations can be

effectively applied for water pumping with a dynamic head which varies between 2-

5m.

Furthermore, data from experimental tests shows that the pulsating motion of

water in channels of the flat-plate solar collectors increases the collector’s efficiency

by approximately 8-10%, which is a considerable advantage when a pump is used as

part of a house solar heating system.

INTRODUCTION:

Solar water pumping is mainly associated with electro-mechanical pumps

powered by PV arrays. At present numerous commercial companies supply the

market with a wide range of such pumps for different applications .The majority of

existing solar water pumps are build with piston or rotary type mechanical pump

driven by electrical motor. The cost of a complete installation, including PV elements

and control and power storage systems, can be relatively high.this the case even when

the required pumping capacity and the dynamic head is low,(0.2m3/hour and 2-

10m),hence restricting any wide application of such pumps in developing countries.

At the same time there is a considerable demand for low cost and low maintenance

installations which are capable of pumping water with a relatively low dynamic head

In order to water plantations and animals and for drainage and salt plants.

Such simple, “low-tech” solar water pumps can be created for this purpose on the

basis of solar free-piston Strirling engines.

There are two types o such engine which can be used for this purpose-free-

piston Strirling engines with conventional “solid” Pistons and machines with “fluid”

pistons. The engines with “solid” pistons very often require the use of special solar

concentrators which then makes the cost of the installation unacceptably high. The

free-piston striling engines with “liquid” pistons are called “Fluidyns.”This is widely

used terminology and is name for free piston Stirling engines with fluid pistons.

“Fluidyns”basically consist of two columns of water in a “U’ tube with the enclosed

spaces above the surfaces of water columns being the expansion and compression

spaces of the Stirling engine. The columns are connected by the channels of the heat

exchangers and oscillate at a frequency close to that of the natural frequency of the

system. The kinetic energy of the oscillating columns then can be converted into

u se fu l w ork . However, one of the main disadvantages of employing “Fluidyns” is

their unstableness under changing loads.

The aim of this paper is to describe new designs of thermal water pumps

which, although very similar to the “fluidyns” configuration have several specific

design features which enable them to operate steadily when the cycle parameters and

a mechanical or hydraulic load significantly vary. Furthermore, the developed solar

thermal water pumps are coupled to a set of flat-plate collectors which provide the

energy for the operation.

A LABORATORY PROTOTYPE OF A THERMAL CONVERTER FOR

WATER PUMPING :

Fig shows the general scheme of a laboratory prototype of such an

installation .A thermal converter consists of hot (1) and cold(2) coaxial cylinders

filled with water and there are enclosed spaces (4)and (5) above the water surfaces

that form expansion (5) and compression (5) spaces are connected by channels of a

regenerator (6) and the water columns in hot and cold cylinders are divided by a

piston actuator(7) with a mechanical spring .The water column in the cold cylinder is

connected to an internal water column(9) and diaphragm(10) separates the thermal

converter from a pumping unit(11) with puppet valves(12) .An air space(13)is used as

an additional pneumatic spring for the piston actuator(7).The water in the hot cylinder

is heated by an electrical coil heater(14) and there are relatively larger surface area

heat exchangers (15)&(16) which are installed in hot and cold cylinders respectively.

There is a tubular cooler (17) within the cold cylinder.

OPERATION :

Water in the hot cylinder (1) is heated to 75-95 0 C using the electrical coil

heater (14) and the heat exchanger (15) with large surface area intensifies a

vaporization process. Due to evaporation of hot water in the hot cylinder the pressure

of the air-steam mixture in the expansion (4) and compression (5) spaces rises. heat is

introduced into the cycle).The pressure increase in the working spaces (4)&(5) causes

the downward motion of the water column in the hot cylinder together with the piston

actuator until the latter reaches it’s position of bottom dead center. During it’s

downward motion the piston initiator (7) increases the level of water column in the

cold cylinder and increases the pressure on the diaphragm from the side of thermal

converter. These are both further enhanced by the oscillation of water in the internal

column (9) which is in counter phase with the oscillation of water in the cold cylinder.

The force which is proportional to the difference of the water levels in the hot

and cold cylinders, and the force from the mechanical spring act upon the piston

actuator while it is in the position of the bottom dead center causing it’s motion

upwards. Hence in its turn the motion of the piston actuator results in an increase in

the level of the water in the hot cylinder and in a decrease of the water level in the

cone cylinder together with the motion of diaphragm to the left. During the period

when the piston actuator is moving upwards, the pressure of the air-steam mixture in

the working spaces (4) & (5) decreases. The reduction in pressure in the stage of cycle

is further enhanced by the intensification of steam condensation process from the

additional heat exchanger (16). Heat is rejected from the cycle with the use of tubular

cooler (17).The cycle is completed when the piston is its position of top dead center.

The variation of the pressure of air-steam mixture in the working spaces (4) &

(5) provides the energy for pumping water through the pumping unit (11) of the

thermal converter

The working temperatures in the hot cylinder varies 75-95oC and the operating

pressures are 0.8-1.6 bar (less pressure) , Thus cheap materials such as glass and

plastic materials were used to manufacture the water pumping thermal converter ,so

there is no sensible friction and wear in the components, this ensures a long life

operation and low maintenance costs.

EXPERIMENTAL RESULTS :

Fig 2 shows the variation of pumping capacity with dynamic head at different

values of the power of the electrical heater. The maximum capacities of the converter

are0.5, 1.5 and 2.0 m3 /hr when the power of electrical heater is 0. 8, 2 and 3 KW

respectively.

Fig 3 shows the change in pumping capacity as a function of the average

temperature of the air-steam mixture in the hot cylinder for different values of

dynamic head .It can be seen that the pumping capacity increases with the rise of

temperature of the air mixture in the hot cylinder.

In order to evaluate the possibility of the operation of thermal converter with

solar collectors, to collectors have been manufactured and tested. Both the collectors

are of flat type, with parallel channels and headers, and end inlets and outlets.

Furthermore they are single glazed and have glass fiber back insulation which is 10

cm thickness.

The first collector has

an integral tube and fin

absorber plate which is

formed by roll-bonding

process that fuses two

aluminum plates

together by heat and

pressure. Thickness of

the aluminum plate is

1mm.The absorber has a

flat black painting. The second collector is brazed copper tube and fin assembly with

a black chrome coating .The aperture absorber is 2 m2 , the thickness of the walls of

the copper tubes and fins is 0.5mm, and the diameter and the length of nine tubes are

12mm and 2 m.

The fig 4 shows a test rig for determination of collector’s performance .The

water from the vessel (1) is circulated by electrical pump (2) through the channels of

collectors(3).During the test intensity of the solar radiation I and temperatures of the

water in the outlets and inlets from the collectors and the mass flow rate have been

measured.

Fig 5 presents sum of the results from experiments and a comparison have

been made with performance of the collectors. Fig 5(a) shows the efficiency of the

newly manufactured collectors as a function of (Tinlet+Toutlet)/2 of the water in the

collector. It has been observed that the efficiency is 30% and 39% for the first and

second collectors when the water temperature is above 80o C .Fig 5(b) shows a

comparison of efficiencies of the present collectors with those of the collectors as a

function of parameter (Taverage+Tambient)/2.These collectors can provide an adequate rise

in the water temperature which is sufficient for the operation of the thermal power

converter.

A “U”-TYPE EXPERIMENTAL

PROTOTYPE FOR A SOLAR

THERMAL WATER PUMP :

The fig 6 shows the general scheme of

a “U”-type experimental prototype of a

solar thermal water pump. In this the

hot cylinder (1) and the cold cylinders

(2) are arranged in “U” shape. The

installation uses solar collector (3) and

a cooler (4) with the hydraulic pumps

(5) to introduce and reject heat from the cycle. A radiator (6) of a cooler (4) is placed

in the water pool(7) and the main pumping unit(8) and the hydraulic pumps (5) have

puppet valves(9).There is a pneumatic camera(10) is in the pumping unit (8) which

maintains a relatively uniform water flow throughout the pump

The operation cycle of the solar thermal water pump is similar to the cycle of

laboratory prototype pf the thermal converter.

The water which flows through the channels of the solar collector (3), is

heated up to 75-95oC and is taken to the surface of the heat exchanger in the hot

cylinder (1).This heat exchanger with a large surface intensifies the vaporization

process. Due to the evaporation of the hot water in the cylinder (1), the pressure of

the air-steam mixture in the expansion and compression spaces rises. The pressure

increase in the working spaces causes the downward motion of the water column in

the hot cylinder and of the piston actuator until the latter reaches its BDC. The

remaining part of the cycle is identical to laboratory prototype

The variation of the pressure in the air-steam mixture in the working spaces

causes the flow of water through hydraulic pumps (5).when the pressure of air-steam

mixture in the working spaces rises the water flows from the converter into the

hydraulic pumps through their lower puppet valves. When the pressure of the air-

steam reduces when water into the expansion compression spaces from the hydraulic

pumps (5) through their upper puppet valves.

The fig 7 shows the pumping capacity of the solar thermal water pump as a

function of intensity of solar insolation when the converter has been completed with

the collectors. When the solar pump connected to the collectors with integral tube and

absorber, its pumping capacity

increases from 0.18-0.55 m3 /hr as the intensity of solar radiation changes from 680-

790 W/m2, respectively.

The efficiency increases due to the pulsating water flow through the channels.

The fig 8 shows the efficiency of the collectors when they are coupled with

solar thermal water pump as a function of the parameter (Taverage+Tambient) /I. It can be

seen that the pulsating flow of water increases the efficiency of the collectors by 8-

10%.

A CO-AXIAL SOLAR THERMAL WATER PUMP COUPLED TO A COLLECTOR WITH A SELECTIVE

COATING:

Fig 9 shows the general scheme of a test rig with the second design of solar water

pump. This design has a co-axial hot (1) and cold (2) cylinders and a set of brazed

copper tube and fin collectors (3) with a selective coating and aperture of 6 m2 to

supply the pump with heat. The water is pumped from a storage vessel(4) through a

section chamber (5).Water is discharged from discharging chamber (8)with a control

valve(9) and manometer(10)The main pumping unit (11) is supplied with puppet

valves(10).The water jacket(13) surrounds the cold cylinder(2) and is used to reject

heat from the cycle. The

distance H from the surface of the water to the level of discharging chamber (8) is

determined by the design of the solar thermal water pump, and is 1.6m.

Fig 11 shows the average value

of the intensity of insolation has

been registered at 850W/m2.The

pumping capacity of solar thermal

pump is about 0.7m3/hr.For both

series of tests when the value of section head and lift head are both 1.5m. The test

results shows that the pump works more efficiently when the water is pumped from

some depth in comparison with the case when the water is lifted from the surface unto

the corresponding height. Solar thermal water pumps can provide a stable operation

over a wide range of values of mechanical load.

CONCLUSIONS:

New designs of the solar thermal water pumps on the basis of a stirling

engines with fluid pistons coupled to set of flat plate solar collectors have been

developed and tested. The working fluid in the cycle is an air-steam mixture

with the maximum temperature and the pressure in the cycle at 95oC and

1.8bar, respectively, the frequency of the working cycle varies in between

1and 2 Hz and the pumping capacity changes from 0.7 to 0.2 m3/hr when the

value of dynamic head is increased from 1.5 to 5m.

The designs of the developed thermal solar water pumps provide a stable

operation over a wide variation in the intensity of solar radiation and dynamic

head.

The results of the experimental tests shows that the pulsating motion of the

water in the channels of the flat plate solar collectors coupled to a solar

thermal water pumps increases the collectors efficiency by approximately

10%.

Low cost materials and a simple technology have been used for their

manufacture and those solar thermal water pumps cam be used for watering of

the plants and animals, in the solar house heating system and for changing the

water in pools in the fish-agro industry.

REFERENCES :

1. “Photovoltaic, Wind & Diesel”-Hammad.M

2. Stirling engines – Walker.G

3. Liquid piston Stirling engine – Wiley & John.

4. An introduction to application of solar energy-Mc Veigh.

5. Principles of application of Stirling engines, Spriner-verlag.