Feasibility Survey for Hydroelectric Power Generation and Water Supply

Puerto Jimenez, Costa Rica

May 26, 2000

Water and energy represent two of the most basic demands by home builders and owners. These fundamental needs are particularly relevant in remote building sites, where neither basic commodity can be taken for granted. In areas that receive a large amount of rainfall and have significant topographical relief, it is possible to implement dual-use water supply / hydroelectric generation systems that provide comprehensive answers to the questions of water and energy demands. The Tinamastes region of Pacific Costa Rica is an area that has an abundance of potential hydroelectric generation sources. At the Finca Fila Vista, there are several streams that could reasonably supply all the electrical demands of a single dwelling. Given the breadth of the property, it might additionally be possible to combine the hydroelectric potential from dispersed sources to supply the power demands of entire communities. This feasibility evaluation was undertaken on the basis, however, of the energy and water demands of a single home.

The objective of this study was three-fold:

 

• To identify a native source of water capable of supplying the hydroelectric generation demands of a fully-functional, large, "American-style" home.
• To identify a native source of potable water capable of supplying the water demands from domestic, irrigation, and pool use during the driest time of the year.
• To provide a conceptual design of the proposed system for refinement upon confirmation of the assumptions made about the basic design variables (anticipated daily and continuous power usage, and anticipated daily water demand.)

   

Investigative methods included the assimilation of verbal reports, the physical inspection of promising watersheds, and the detailed surveying and characterization of a target stream.

Our field methods of investigation consisted of collecting data by a variety of means. Simple surveying methods employing a tape measure, inclinometer, compass, and trigonometric formulae were used to determine distances, slopes, and to calculate vertical drops. Corroboration was obtained with an instrument that measures positive and negative pressure over a known linear distance. Flow measurement was accomplished by measuring the time taken to fill a known volume. Water quality samples collected from the spring were preserved and transported to San Jose for analysis by Laboratorios Aqylasa. Interpretation of the water quality results will be provided in a following report.

The study area included the immediate vicinity of the planned building site shown below.

The topographic map of the area has a scale too small to work from and is shown in Figure 1 simply as a reference. A copy of the plano encompassing the study area is shown at the top of the following page for additional detail.

Interviews with personnel with lifelong familiarity with the property encompassed by the Finca Fila Vista revealed the availability of three potential hydroelectric/water supply sources near the anticipated home site. Two of these water sources were hiked, and the stream shown in Figure 2 was identified as a primary target stream. This stream was the closest to the anticipated home site and by all verbal reports had the greatest discharge of all subjacent streams during both seasons and in drought.

The target stream has a confluence of two major tributaries in a location about 1000 feet from the planned home site. Since these two tributaries are major sources of the stream's flow, the ideal location for a hydroelectric pipeline water intake is below the confluence of the tributaries. Fortuitously, the stretch of river immediately beneath the confluence of these tributaries is ideally suited for the installation of an infiltration gallery for the intake of water necessary to charge the hydroelectric pipeline (Figure 4). A resident of the area of over 30 years reports that the target stream has never dried up. He reports that in the worst drought in memory the target stream shrank to only about one third of the size it presented during the survey. Approximately 100 linear feet and 25 vertical feet beneath the proposed hydroelectric water intake lies a freshwater spring emerging from bedding planes and fractures in solid rock adjacent to a small waterfall. This spring appears to be an excellent source of domestic, irrigation, and pool water supply (Figure 5). About 930 linear feet and 205 vertical feet lower lies the location where we would place the hydroelectric generator for a system with a charging potential of 60 kW per day.

Whereas the conceptual design presented herein depends upon a vertical drop of only 270 feet, it is practical on the basis of our findings to extend the scope of the project by siting the power generation installation farther downstream. The versatility presented by the land surface characteristics of the property in question makes it possible to refine the system design if the energy and water supply of the conceptual design are deemed insufficient by the property owner.

One of the fundamental design variables of generating hydroelectric energy is the flow rate of the stream that provides the force to drive the hydroelectric turbine. Ideally, this flow measurement should be taken in the driest time of the year. Although rains had begun in the area the week before the survey, the stream water was free of turbidity, indicating that ideal base flow conditions were in effect at the time of flow measurement. Flow rates of the target stream and the spring intended for potable water supply were measured by confining the existing flow through pipes, making all efforts to minimize the losses around the piping employed. Pipe discharges were then measured by filling a known volume during a measured period of time. Results of the discharge measurement are presented in Table 1.

  The final critical design parameter for a hydroelectric generation system is the amount of available vertical drop, which determines the water pressure that drives the turbine. To determine the vertical drop available and the resulting length of the penstock, or pipeline, needed for installation of this system, the target stream was surveyed from the site of the proposed infiltration gallery downstream. The results of this survey are shown in Figure 7.

The amount of power available to any hydroelectric system depends on the following variables: 1) dynamic head (in feet), which is equal to the vertical drop from the source to the power-generation site; 2) the rate of water flow (gpm); 3) pipeline efficiency (cumulative head losses); and 4) the efficiency of the turbine/generator, or "Pelton wheel," which is typically 50% for DC turbines. We used a design flow rate of 120 gpm, which is about 1/3 of the target stream's measured flow, which corresponds to the lowest flow observed during drought conditions. We sized the system to provide 270 feet of head. Using an estimated friction coefficient of 0.18, this corresponds to a charging source equivalent to 60 kW per day.

  Power in Watts = (head)*(flow rate)(friction coefficient)*(Pelton wheel efficiency)

We propose a system which utilizes a single pipeline feeding twin turbines. The force of water is directed at the runners (Pelton wheels) through a diffusion system which focuses 4 individual nozzles on the runner of each machine. In this fashion, we are able to maximize electrical output (90 kW/day) during times of high flow rates by operating the full array of nozzles, or increase the efficiency of a single machine to operate at near maximum output (40 kW/day) in times of unseasonably low flow. This system will consist of a DC generating source which will, in turn, charge a bank of batteries sized adequately to meet an emergency demand of at least one week of autonomy, in the unlikely event a system error were to occur. The Trace inverting system will draw from the battery bank, providing 110/220 volt three-phase power to meet the demands of the household.

In contrast, a typical AC generation system of comparable output requires flow rates of water large enough to meet the maximum surges placed on the system. Oftentimes this requires flows of thousands of gallons per minute and sensitive and costly monitoring systems, which are not often designed to hold up well in these types of environments. The DC charging system, on the other hand, is easily serviceable, efficient, and cost effective.

We expect to increase the observed spring water supply discharge from its present flow rate of 3.5 gpm to at least twice that much and possibly more. We have included a 5000 gallon water storage tank in our design that will ensure an adequate water supply even during periods of high water usage, such as when filling the swimming pool.

Proposed Conceptual Design

A block diagram of the components of the proposed hydroelectric design is given in Figure 8. Physical structures that must be built for the hydroelectric system design include: 1) a small dam at the intake location for installation of the infiltration gallery in the stream channel; 2) a concrete housing for the two parallel Pelton generators at the end of the pipeline; 3) power station (site of DC-AC inversion and battery storage unit); and 4) a small utility structure at the planned home site for housing the AC system's main breaker and water pressure tanks. Figure 9 illustrates the proposed water system, which includes the following physical structures/equipment: 1) a spring collection box built in the rock face where the spring is located, 2) a water storage tank equipped with a submersible pump, and 3) main AC breaker box enclosure and pressure tank storage at the planned home site.

Power and water demands for a large, single-residence dwelling can be supplied through hydroelectric power generation and spring water supply. We have presented a conceptual design that will result in a charging source of 60 kW/day and the potential for a continuous supply of 15 kW. We will provide a detailed design and bid for the complete installation of the system upon the authorization to proceed. If greater power demand is anticipated, we will design and bid a system according to the demands that are anticipated. Osa Water Works has a window of opportunity for installation of a finalized design during a two-week period in the month of July. Sistema Solar Internacional, S.A., a member of OWW, has a container of equipment coming from the United States in end of June, which makes it possible to have all the components in place for a July installation date. Due to existing work loads, travel schedules, and the heavy rains that begin in September, if the work cannot proceed in July or August, then we may not be able to install a system until December or January. We will complete the installation in a 15-day time period. The final installation can be left indefinitely in an idle state and be ready to turn on when construction efforts begin. We recommend that the home owner provide us with an authorization to proceed with bidding the 15 kW continual system introduced herein or to provide us with an alternate desired output. We will be able to turn around a detailed bid for installation of the system within 1week of authorization.

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