Solar Energy Vol. 61, Nos. 3, pp. 219–224, 1997© 1997 Elsevier Science Ltd

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DESIGN AND DEVELOPMENT OF A WATER PISTON SOLARPOWERED STEAM PUMP

D. J. PICKEN,† K. D. R. SEARE and F. GOTODe Montfort University, The Gateway, Leicester, LEl 9BH, U.K.

Received 4 May 1994; revised version accepted 8 April 1997

Communicated by PETER FRAENKEL

Abstract—Most solar pumping systems are based on photovoltaic receivers driving electric pumps. Analternative system is to use a boiling water solar receiver to operate a direct acting steam pump. Theadvantages lie in the relative simplicity of the pump giving ease of manufacture, maintenance, reliabilityand low cost. At De Montfort University a simple direct acting steam pump has been developed overmany years, so it has been a natural development to link this with a boiling water solar receiver. Thispaper is primarily concerned with the operation of the pump and overall system employed. © 1997 ElsevierScience Ltd.

1. INTRODUCTION There are two principle elements to the system,the pump and the solar collector. The solar

It is generally accepted that concentrating solarcollector used in this design is based on a heat

collectors can generate temperatures in excesspipe that is housed within an evacuated glass

of 200°C. This is sufficient to generate steamtube. The collector is a commercially available

pressures of 15 bar (absolute) or a pumpingitem and has a typical conversion efficiency of

head of nearly 150 m. Any direct acting pump24%. The cost of such units is high, typically

will use this steam in a non-expansive manner.£350 per m2 of collector. They have been used

Thus the volume of water delivered by thebecause of their compact nature which lends

pump will be limited to the volume of steamitself to laboratory testing. The fragile nature

generated and in practice will be less becauseof these devices would make them unsuitable

of losses due to condensation. In the systemfor what must be a robust design in the field. It

developed, heat pipes encased in an evacuatedis possible that alternatives can be used if the

tube are used to provided the necessary heatrequired pumping head is reduced accordingly.

input to the water for steam generation.The pump is a simple displacement pump

where water is displaced by steam in the left-hand cylinder (cylinder A in Fig. 2), and the2. PROPOSED PUMPING SYSTEMsteam condenses when the water level reaches

Referring to Fig. 1, the main elements of thethe bottom of the ‘‘U’’ bend. There is no

proposed system are shown schematically.

Fig. 1. Schematic diagram of pumping system.

Fig. 2. Schematic diagram of the steam operated pump.†Author to whom all correspondence should be addressed.

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expansive use of the steam and thus the pump The pumping cylinder volume must beadjusted so that it enables the boiler pressureoperates on the Papin cycle.to vary only within fine limits (to avoid fatigueproblems) and yet be large enough to operate3. ANALYSIS OF PUMP’S POTENTIALat between 10 and 1 cycles per minute. A 1°C

Considering the solar collector as the start of temperature variation of the solar collectorthe system, let Q be the solar energy received would seem to be a reasonable limit toper second per unit area of the solar collector, avoid unnecessary temperature and pressurelet Ac be the effective area of the collector and fluctuations.T be the effective period of solar radiation per Then, if we take as the first approximationday. One further restriction is that the mass of that the pumping stroke is fast relative to thewater in the collector system must be sufficient reheating time of the boiler, we can assume thatto provide the steam for the whole of the all the enthalpy required for the pumping cyclepumping period. Let this mass be M per unit comes from the water in the solar collector.area of receiver. Thus, initially

When radiation starts, this mass must first beMCpw(Dh)=mhfg (6)brought to the boiling temperature that corres-

ponds to the required working pressure, i.e. where M is the mass of water in the boiler, mpumping head. This will take t seconds. is the mass evaporated per cycle and Dh is the

Thus chosen temperature drop per cycle. As pumpingproceeds, M will reduce linearly but this effectQt=MCpw(hb−h

1) (1)

will be reduced by the buffer effect of theor increased steam volume in the boiler. The value

of m can then be used to estimate the requiredQAct=MAcCpw(hb−h1) (2)

cylinder volume.where Cpw=specific heat of water, h1=initial The above calculations represent the idealtemperature of water and hb=boiling temper- conditions in which none of the steam generatedature at the working pressure. is lost in condensation during the pumping

Also during boiling QAc(T−t)=MAchfg, cycle.where hfg is the latent heat of vaporisation or

Q(T−t)=Mhfg (3)4. REDUCTION OF LOSSES DUE TO

Therefore CONDENSATION

One mechanism for the direct use of thet

T−t=

Heating Time

Boiling Time=

Cpw(hb−h1)

hfg(4) steam generated would be to contain one whole

day’s volume of water in a large cylinder andThe heating time can be calculated from this, release the steam into the cylinder as it is

and for normal conditions will be about 15% generated. This has the mechanical disadvan-of the radiation period. The mass of water tage of requiring a very large pressure cylinderevaporated per unit area of collector is found of the order of 1 m3 per m2 of solar collector.as: It also has the thermal disadvantage that the

steam would be in contact with the water andM=

Q(T−t)

hfgkg/day (5) the cylinder walls for the maximum possible

period thus ensuring that the losses would bemaximised – in fact it would not work.and the corresponding ideal volume of water

pumped calculated from the relevant specific It is therefore necessary to store up energy inthe solar collector until there is enough energyvolume vg, i.e. volume pumped per day=Mvg

if there are no condensation losses. For example, to fill the cylinder without reducing the steampressure below that which is necessary fortake the case of an 8 hour day during which

the sun is high enough for adequate radiation, pumping. This means that before releasingsteam to the pumping cylinder, pressure musti.e. T=28 800 seconds. Thus, for each kW of

radiation reaching the boiler (assuming constant build up in the boiler such that the energyrelease from the boiler during the pumpingpower) the ideal water pumped per day and the

ideal pump efficiency can be derived as shown stroke does not decrease boiler pressure belowthat necessary for pumping.on graph 1.

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Graph 1. Ideal water pumped per unit power and e ciency vs head.

This means that apart from the small amount (NRV ) displaces water from cylinder A andof radiation received during pumping, the forces it through cylinder B and the outletenergy required to do the pumping work, and NRV.fill the pump cylinder with steam must be sup- (3) When the water level reaches the junctionplied by the flash evaporation of boiler water of cylinders A and B condensation isas its pressure drops. initiated.

(4) Condensation in cylinder A creates a partialvacuum and draws water through the NRV5. PUMP PERFORMANCE ALLOWING FORfrom the well and refills cylinder A.REAL LOSSES

Unwanted condensation of the steam occursIf the pump were to operate with no energy in the following parts of the cycle:

losses other than during the condensation part (1) condensation at the steam–water interfaceof the cycle the latent heat of condensation of during pumping,the steam contained in the pumping cylinder (2) condensation on the relatively cool and wetwould be received by the water entering the exposed walls of cylinder A duringcylinder. This can again be calculated over the pumping,range of pressures considered. (3) condensation of the steam which flows into

Practical tests have shown water temperature cylinder A during the filling process.rises varying from 3 to 10°C, indicating that the Good design can reduce (i) by keeping theefficiency relative to the Papin cycle is between

interference area (cross sectional area of A)10 and 3%. This would indicate a matching

low. By making cylinder A from a low conduc-reduction in cylinder size unless the efficiencytivity and low wetting material such as glass,can be improved. There will be, of course, apolyurethane or nylon, (ii) can be significantlycorresponding decrease in the quantity of waterreduced. The elimination of steam admissiondelivered.during the condensation/refilling phase can beTo understand the losses in efficiency, thecontrolled by the addition of a pressure main-operation of the pump must be considered. Thetaining valve (PMV ) in the steam line.pump developed over many years at De

The design of the PMV is shown diagrammat-Montfort University for operation on a constantically in Fig. 3.steam supply is shown diagrammatically in

Its operation is as follows:Fig. 2 and described in Refs. 1,2.When the pressure reaches the desired levelReferring to Fig. 2, the operation is as

it will lift the shut-off valve by overcoming thefollows:mass, Mv.(1) The pump chambers, A and B are filled

with water.(2) Steam entering via a non-return valve PAv=Mvg (7)

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manner as the pump refill each cycle. In thecase of a deep well a refill reservoir is suppliedwith a small drain from the outlet to providethe source of recharge water. The final systemdesign chosen, using the above dimensions, isshown diagrammatically in Fig. 1.

This design has the advantage that the pumpunit is circular in section and in cases wherepumping from a deep well is required, can belowered into the well to a depth within 5 m ofthe water level to prevent suction being inade-

Fig. 3. Schematic diagram of the pressure maintaining valve. quate for filling the cylinder.

where P=boiler pressure and Av=area of seal7. DESIGN DATA FOR PROTOTYPE PUMPopening.

Steam pressure then lifts the diaphragm to The basic calculations enable Graph 2 to beopen fully the shut-off valve and allows steam developed from a simple spreadsheet such thatto reach the pump. the optimum pump cylinder volume can be

When the pump cylinder goes into vacuum chosen for any given head.due to condensation, the reduced pressure Condensation can be induced at the end ofacting on the diaphragm closes the shut off the pumping cycle by allowing steam to leakvalve. It cannot open again until the cylinder into the water in the manner described in Ref. 1.pressure has risen and the boiler pressure is Boiler pressure control should be achieved byrestored. The pre-load (weight) on the valve the pressure maintaining valve.must be adjusted to open at the required boilerpressure to pump the required head. The dia-

8. DESIGN OF THE PROTOTYPE PUMPINGphragm must be large enough to close the valve

SYSTEMagainst the steam pressure and is usually threeto four times the area of the shut-off valve. It was decided that the prototype would be

built on a small scale in order that solar radia-tion could be simulated by a bank of tungsten

6. RECHARGING THE SOLARhalogen light units in the laboratory. This has

BOILER/COLLECTORthe advantage of relatively low cost, but carriesthe disadvantage of low efficiency due to highThis is achieved diurnally by the collector

cooling at night and thus condensing and draw- surface to volume ratio within the pump. Thesolar collector at this stage is supplied bying in fresh water from the well in the same

Graph 2. Optimum cylinder volume.

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Thermomax and is of the evacuated tube heat tion of a water film on the inner surface ofthe cylinder.pipe design delivering heat to a single manifold

which serves as the boiler for the system and The design of the cylinder head is such thatthe pressure maintaining valve, water inlet NRVhas the following properties:and water outlet NRV (in the form of flap

Length of evacuated tube 1 mvalves) can be made from a single sheet of

Number of tubes 20rubber which is sandwiched between the two

Total collection area 2 m2sections of the cylinder head.

Boiler volume 3 lEfficiency of collection at 24%

10. INITIAL TEST FINDINGSboiler temperature of110°C The system was tested in the configuration

shown in Fig. 1. Most of the testing is concen-trated on reliability, particularly of the non-return valves each of which have to work per-9. PUMP DESIGNfectly each cycle if the system is not to fail and

The design calculations indicated that the leakage causes a further reduction in the alreadypump cylinder volume should ideally be chosen low efficiency.to suit the pumping head in the desired applica- With careful design and manufacture of thesetion. This meant that the design should prefera- items the system operated reliably at heads ofbly be modular, allowing different cylinder 1 and 2 m. However, some failures occurred atvolumes to be fitted to a cylinder head contain- a 3 m pumping head and the system would noting all the valves in the system. work at a 5 m head. The failure was attributed

The design chosen was made from a block of to the late onset of condensation at the end ofoil impregnated nylon which could be machined the pumping stroke. As a result, an automaticto incorporate the U-tube design of pumping blow-down valve was added to release steamand condensing cylinders. The cylinder length pressure at the end of the pumping stroke.can be varied by inserting extension pieces into Thereafter, no failures occurred and perfor-the centre section of the pump body. mance testing became possible. The blow-down

The material chosen for the pump was an oil valve is based on the float-operated automaticimpregnated nylon which has the following bleed valve often used on hot water systems.properties:

10.1. Test results(1) low thermal conductivity,(2) easy machining characteristics, The system has been tested in the U.K. using

two methods:(3) good resistance to thermal deformation,(4) some water repellence to reduce the reten- (a) With an electrically heated boiler to obtain

Graph 3. Heat input vs output flow rate using electric heating.

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Graph 4. Predicted and measured results for a 2 m2 solar collector and 800 W m–2 solar radiation.

results with a known and controllable energy materials using only simple techniques. Asstated earlier, the type of solar collector used ininput. The results of these tests are shown on

graph 3. The input energy values used corre- this test system is too costly and fragile for atruly low cost robust system. It is evident thatspond to solar radiation levels of 1000 and

1600 W m−2. further development and assessment of alterna-tive boiling solar collectors is needed so that(b) As a complete system with the solar

collector providing the sole energy input. The larger capacity pumping systems can be madeat a reasonable cost.results of this series of tests, carried out in June

1995, on the roof of a university building are The results obtained in the U.K. have beensufficient to justify the system being taken toshown in Graph 4. The solar radiation was

measured using a suitably calibrated pyrano- Zimbabwe where an extensive programme oftests can be carried out under true operatingmeter and found to be 800 W m−2 on average.conditions. To reduce the cost of the system, aseries of alternative solar collectors will be tested11. CONCLUSIONSto ascertain their performance when coupled to

The pump in its present form is reliable, but the pump, the results of which will be reportedof very low efficiency. Typical values are in the later.order of 0.05%, this being determined by:

Efficiency=rgQH

Input Power(8) REFERENCES

Picken D. J. and Brewin D. F. A low technology steamCorresponding values of Q (volume flow rate) pump applied to an anaerobic digester system. Int. Power

Generation Vol 4, No 4; May 1981.and H (head) can be obtained from Graphs 3Picken D. J. (1990). Design and Manufacture of thermallyand 4.

operated pumps. Proceedings of the 3rd InternationalThe pump is of a simple low cost design and Conference on Small Engines and their Fuels for Develop-

ing Countries, University of Reading.lends itself to manufacture from a variety of