Electric Power from Small Streams

New Zealand Journal of Agriculture, Volume 63, Issue 4, 15 October 1941, Page 277

Electric Power from Small Streams

S Because of their distance from power lines, many B B farms are unable to enjoy the benefits of electrical B ■ power supply. On the other hand, such farms are B J often endowed with streams which, if suitably har- g B nessed, could supply this power for a comparatively J B small outlay. This article gives a full description of B B how a small electric power plant can be constructed B B on the farm. B ■iiiiiiiiiiiiiiiiiiioiiiiiiiiiiiiiyihiiiiiiiiiiiiiiii JIHIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIN

NEW Zealand’s main national hydroelectric stations have now a large and increasing rural load. In thinly settled districts, however, there is little immediate likelihood of reticulation from this source, and the internal combustion engine burning imported oil fuel or petrol is likely to remain for many years the mainstay for farm power. Fortunately, our country districts are often richly endowed with streams, which, if suitably harnessed, could at the flick of a switch supply light, heat, and energy for many farm and domestic needs. How, then, might one gauge the suitability of a stream for this purpose? What power is likely to be available, and what is the likely initial cost and maintenance of the installation? Some information on these points, together with illustrations of successfully operating schemes in the Nelson district, are featured in this article.

Developing Stream Power

Most farmers . have sufficient ability to make at least the preliminary investigations so that the question may be settled“ls a small electric power installation practicable on my property or not?” Later, if the work is to be' carried out, the assistance of a competent electrical engineer will be indispensable, and a permit will be required from the Public Works Department. Permission must also be obtained from the local power board

if the farm is in a power board area, and the county council if the lines are to be taken across a public roadway. Registration and a long-term licence obtainable from the Public Works Department cost £1 Is., with an annual rental of 2s. 6d. per kilowatt on the rated capacity, with a minimum fee of ss. yearly. (1 kilowatt = 1000 watts, or approximately 1% h.p.)

Before construction work is begun the first essential is to determine the power the stream is capable of developing. This can be found by measuring the volume of water , passing a given point in a known time, together with the fall, or “head,”’ available from the intake down to the power house. Necessarily, these measurements must be made at a period

By

D. M. E. MERRY,

' Instructor in Agriculture, Nelson.

of minimum or normal flow; otherwise there will be likelihood of a power shortage in dry weather.

Measuring Water Volume

The amount of water measured in cubic feet per minute passing in the stream can be computed as follows: —

FLOAT METHOD. Select a fairly uniform section where the velocity is low, and measure accurately the stream width and average depth (Fig. 1). Along the bank measure out a known length, say 20 ft. or more, and observe the time for a float to travel the measured distance. The best form of float is a weighted stick which will float free of obstruction and with only a few inches above water. At least three readings should be taken and the times averaged. The mean velocity is approximately onefifth less than this average, due to the lag in the flow near the stream banks and bed. The discharge in cubic feet

P er minute equals four-fifths of the average velocity of the float (reckoned in feet per minute) multiplied by the average cross-sectional depth of the stream.

EXAMPLE:— Width of stream = sft. 6in. Average of depths taken every 9 inches across width: l|in., 3in., 6in., BJin., 4in., lin. = 4 in., or %ft. Rate of flow: Float takes average 60 20 time of 5 seconds for 20 ft. X 5 1 240 ft. per minute. Volume of water flowing in cubic ft. per minute —■ width X average depth 11 1 240 X rate of flow = X X 2 3 1 = 440 cubic ft. per minute. Actual volume after correction for 440 4 lag = X 352 cubic ft. per min. 1 ' 5 ~ WEIK METHOD. The weir method is a more accurate method for smaller streams. A weir or dam of planks is erected in a quiet part of the stream, the top gauge board of which should be bevelled on the downstream edge and accurately levelled. Sand bags should be used to prevent water leakage at the sides and bottom. A stake should then be

driven into the stream bed about 3ft upstream and to the level of the top of the gauge board. The depth of water is measured from the stake top (Figs. 2 and 3). The quantity of water flowing over the weir can then be found from the accompanying table.

, Colums 3 to 9 refer to depths in fractions of 1 inch between the whole numbers for. weir depths in inches as set out in column 1. EXAMPLE:— Width of flow over gauge board = 4 ft. Depth of flow over ' gauge board 5f in. Then, from the table, 5j inches depth = 56.76 cubic feet per minute, per foot depth of weir. . .' As weir is 4 ft. wide, flow is 4 X 56.76 227.04 cubic feet per minute.

TANK METHOD.

For very small quantities an accurate measurement may be made by diverting the water into a tank or drum of known capacity -and > measuring the time taken to fill the receptacle.

Measuring “Head”

The fall or “head” available is, for all practical purposes, the difference in level between the water surface at the intake and the floor level at the power house. In the case of turbines fitted with a draught or suction tube, the

“head” is the difference in level between the water surface at the intake and the discharge level in the . tailrace. Usually the machinery is set sft. to 10ft. above tail water in order to be clear of floods. Where a surveyor’s level and sighting staff are not available, fairly accurate results can be obtained in measuring the water head by sighting along an ordinary carpenter’s level on to a measuring rod held by an assistant. This is conveniently done by fitting the

level to a tripod to which a plumb-bob has been attached (Fig. 4). The first sight is taken levelled to the proposed height of the filled dam, and a reverse view from the same position taken in the downstream direction to a sighting pole notched in feet and inches. The reading is the fall between the point on which the pole rests and the proposed dam water level. Further readings down to the power house site should be taken downstream with the

sighting level and tripod erected to the same height (hence the necessity for the plumb-bob) over the point where the staff had been previously held. The sum of these readings less the height above ground of the level at each shift' after the first will give the total head. The working head will, where an open headrace is necessary, be the total head less the fall between the dam level and the pipeline intake. \

Estimating Theoretical H.P. The theoretical H.P. available can be calculated from the formula H.P. = (head measured in ft.) X flow (cubic ft. per minute) 4- 530. EXAMPLE:— Assuming head 40 ft.; flow cubic ft. per minute (previous example) 224 4530. (40 X 224) 4- 530 = approx. 17 h.p. Actual Power Available It should be realised that the theoretical energy so estimated cannot all be translated to electrical energy for a number of reasons. Unavoidable losses will be caused by such factors as seepage in the intake raceway, length and diameter of the pipeline* from intake to power house, pipeline bends, and

Fig. — Listing the total power demand.

the efficiencies of the turbine or waterwheel, together with the dynamo, in converting mechanical to electrical energy. Thus, in reckoning the actual H.P. a safe allowance is 50 per cent, of the theoretical H.P. Most farm power plants are designed so that the generator is- running continuously, and in these circumstances the dam serves only to divert the volume of water along the raceway .which is immediately required to operate the plant. Large storage dams similar to those of . the major hydro-electric schemes would, on the score of cost, be out of all reason for a small farm hydro-electric scheme. If the power demand is for short periods only and the stream is incapable of sustaining this load continuously, a small storage dam will be necessary to build up a water reserve which can be drawn on when power is required.

Reckonin'? Power Requirements The power needed to operate electric . equipment is usually expressed in 1 watts, 746 watts being equal to one H.P. (Fig. 5). Farm power schemes utilising current for domestic and shed use usually operate with a minimum of 3J h.p., the load being made up as follows: - Milking machine motor 1J to 2 H.P. (old style) or | to 1 H.P. for modern

rotary vacuum pump; water pump, separator, or shearing units, each -| H ’ P ; ; h °t-water cylinders each 750 watts or 1 HR; llghting about 4 H - P ” cooking range (heat storage type), 1 The full power load may be estimated in this manner as H.P. by totalling the wattage of all appliances needed and dividing the figures obtained \ by 746. It is necessary to point out that this will be the “total” load were

everything listed in use at the one time. As such a power demand is most unlikely, the “peak” load of the energy required to operate the appliances in use at any one time is often more important. Consequently, the total load can be built up above the generator’s capacity provided everything is not in use simultaneously. Installations having insufficient power to run the total

load all at once may be operated successfully by switching off water heaters when other equipment or machinery is in use. In regard to the domestic cooking range, the heat storage type, which is efficient and popuar, has one element burning continuously, and the range is therefore always ready for immediate use. A further element will quickly raise the oven heat as required, while

a draught vent controls minimum temperatures. Water Wheels, Pelton Wheels or Turbines? Water wheels are an age-old means of converting the energy of moving water to mechanical power. While it is not proposed to detail their construction, they can be of the overshot, breast, or undershot type. Water wheels are most suitable for operating where there is a fairly large flow of water with a low head, or fall. While cheaper than a turbine to install, if erected in the stream bed a water wheel is subject to “drowning” in periods of flooding. Again, water wheels turn relatively slowly, and require a high gearing to the dynamo; also, the flow of water to the wheel is not easily adjusted to compensate for

sudden variations in electrical load. This latter point may be the cause of overloading and the burning out of elements, motors, or lights. Metal water wheels of the type shown in Fig. 13 are superior to wheels constructed of wood, as they can be accurately balanced and will remain balanced. Wood will, in time, become waterlogged, and rebalancing is difficult following any repair. A 10ft. to 12ft. head of water is, in many cases, quite suitable for driving a water wheel of the overshot type. Pelton wheels operate best under, a fairly high head of water, 30 to 300 ft. or more, and with little trouble can usually be located out of harm’s way in the event of flooding. The generators in two of the power houses illustrated (Figs. 5 and 16) are driven by pelton wheels. In addition, pelton wheels' can be constructed in New Zealand —a decided advantage at the present —and they also operate on a low maintenance, and give little trouble. Further, they are readily fitted with an automatic oil pressure governor controlling the stream of water at the jet, which, in turn, regulates the electrical

current and prevents sudden overloading when one or other electrical appliance is switched off. The governors, worked by oil pressure, are constructed so that a stainless steel deflector is interposed between the nozzle and the pelton wheel to deflect suffiicent water away from the wheel to keep it at regular and constant speed up to the limit of full load. Turbines are made in diverse types for high or low heads, commonly 10 to 65 ft. spiral or casing types, and are more suitable where large volumes of water can be utilised and are available throughout the year. They are efficient

but initially expensive machines, and their maintenance may be greater under some conditions than the simpler pelton wheel. Turbines, like pelton wheels, may be conveniently erected beyond harm’s reach in the event of flooding, and they have the advantage that, by fitting a draught tube, the total head of water down to discharge level below the power house floor can be used. From these brief descriptive notes it may be realised that the particular scheme and finance available will

largely determine the type of power generating equipment necessary.

Dam and Intake Race Construction

As may be seen from Fig. 6, a stonewalled, earth-filled dam, constructed at the minimum of expense, is diverting stream water to the raceway outlet (Fig. 7, right) in this scheme. Concrete, while undoubtedly the most suitable building material for strength and permanence, is unfortunately expensive. Whether the dam is of logs, earth and tree-planted or stone-walled, it is wise to concrete both the spillway and the intake lead to the raceway. The illustrated raceway (Fig. 8) was dug with the water following on behind to a depth of about 3 inches while digging progressed. No grading or setting out of levels on the hillside was therefore necessary, and when the initial grade had been found the water was blocked where it entered at the dam and the raceway channel deepened on a slightly falling

grade. A fall of 1 ft. in 300 to 400 ft. is usually sufficient for this purpose (1 ft. in 396 ft. = 2 in. per chain). At the end point of this earthworks channel (Fig. 9) it was found necessary to concrete a small chamber to allow the fixing of the pipes in which the water was fed down to the power house, about 60 ft. below (Fig. 10). A metal screen to keep back sticks and other obstructing objects is necessary at the pipeline intake (Fig.’ll), and with any considerable fall in the raceway' an overflow pipe or, channel should also be provided at this point. In this scheme the pipes were 8 in. in diameter, the pelton wheel 3 ft. in diameter, and the output horse power 5. The pelton wheel was coupled by vee belt to the dynamo, with an overdrive on the same beltline to a locally patented oil pressure type governor. This ensured automatic control of water to the pelton wheel jet so that the voltage remained practically constant and without fluctuation throughout the range of the generator’s load.

An alternative method to that already described for setting out level or easy sloping grades which may be of use in constructing either watc’ raceways or laying off grades for farm roads is as follows: —An A-frame similar to Fig. 20 is made of light timbers with a span of 10 ft. or more. From the apex of this frame a weighted string of the length shown is hung. Next, two pegs are driven the correct distance apart in a shallow pond so that their tops are at water level. The frame is then rested on the pegs and the position at which the string passes across the cross-bar is marked. This is checked by turning the frame around end for end. If the string does not come against the same mark the

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pegs are not quite level and a second mark should be made. The mean of the two positions is permanently marked on the frame as level position. In setting oil a grade of, say, 1 ft. in 396 ft. with a frame of lift, span, it will be necessary to raise one end one-third of an inch. A piece of wood of this thickness is fastened to one foot of the frame. In use, the A-frame is then rested on pegs driven at, say, 11 ft. intervals, the raised end being on the peg at the lower level. When this latter peg is driven so that the string on the frame crosses the previously marked level position the required grade has been set off. Cost The cost of the completed power house, including shed, pelton wheel,

governor and dynamo of sh.p. output as in the scheme illustrated (Figs. 6 to 12), is at present-day prices approximately £9O. I am informed that maintenance on the whole installation, including motors and domestic appliances, should not exceed 5 per cent, of the capital cost per annum. All electrical apparatus, piping and wiring, was, of course, an additional charge, and the farmer himself did the work required in the erection of the dam, raceway, etc. In this district many such installations are proving highly efficient and worthy investments saving otherwise heavy fuel purchase and giving cheap, continuous and reliable service.

Acknowledgments

The writer acknowledges technical assistance in the preparation of certain material in this article to Mr. J. Ford, Public Works Department staff, Nelson, and Messrs. Geo. Higgins Ltd., electrical engineers, Nelson. ' ' . : :

NOTE. —Depths less than 2in. are not reliable.

Weir Depth. Inches. , sin. Jin. tin. jin. • L_ tin. fin. tin. fin. fin. jin. fin. fin. 2 12.90 14.16 15.42 16.74 18.06 19.44 20.82 22.26 3 23.70 25.20 26.70 28.26 29.88 31.50 33.12 34.74 4 36.42 38.16 39.90 41.70 43.50 45.30 47.16 . 49.02 5 50.24 52.86 54.78 56.76 58.80 60.78 62.88 64.80 6 66.90 69.00 71.10 73.20 75.48 77.64 79.80 82.08 7 .. 84.36 86.64 88.22 91.20 93.60 96.00 98.28 100.68 8 .. .: 103.08 105.60 108.00 110.40 112.80 115.32 117.90 120.60 9 .. .. 123.00 125.40 128.10 130.80 133.50 136.20 138.60 141.30 10 144.00 146.70 142.40 152.10 155.10 158.10 160.80 163.50 11 166.20 169.20 172.20 174.90 177.60 180.60 183.30 186.30 12 189.30 192.30 195.30 198.30 201.30 204.30 207.30 210.30

CUBIC FEET PER MINUTE PER FOOT WIDTH OF WEIR.

■ ■ ■ ■ • Watts. Watts. SHED: Milking shed motor 1 h.p. . . Separator and Water Pump 746 Motors, 2-J h.p. 374 Hot water cylinder, 750w. .. 750 Shed lights, 3-60w. . . . . 180 DOMESTIC: . . Electric range (2 elements ' burning) . . . . . . 1,500 Hot water cylinder . . 750 House lights, 7-60w., l-150w. . Toaster 550w., Radio ■ 60w., 570 Iron 750w., Kettle l.OOOw., Cleaner 44w., Washer 186w., etc. . . . . ■. . 2,590 Total 7,460 H.P. " . . 10

Power Requirements