Resource Evaluation:  Finca Rio Magnolia Home and Bungalows

La Alfombra, Tinamastes, Costa Rica

For:  Maureen and John Paterson

By:  Paul Collar

Osa Water Works, S.A.

June 22, 2007

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INTRODUCTION AND OBJECTIVES

Finca Río Magnolia is a property of about 280 acres containing both forested and pasture mountain land in the Tinamastes region of Costa Rica, near the town La Alfombra.  The ownership of the finca has extended an all-season four-wheel drive road into the portion of the property most amenable for development and has groomed building sites, access roads, and cut preliminary drainage for a residential and commercial complex comprising one main house and five pads for the construction of upscale bungalows for rentals on nightly and longer time-scales.  The overall project anticipates construction of ten bungalows in total.

Electrical power from the national grid extends to within about two and a half kilometers of the development site.  However, the property contains a perennial stream with fairly large flow and considerable relief that offers the possibility of onsite hydroelectric power generation potential.  Osa Water Works was contracted to undertake a detailed resource evaluation to determine if the hydroelectric potential of the stream is adequate to provide for the anticipated demands of the commercial/residential facility and if so to identify reasonably hydroelectric options and provide recommendations as to which alternative is technically and economically most appropriate.

On June 12, 2007 a field investigation was undertaken on the property by Paul Collar, owner of Osa Water Works, S.A., in the company of the Finca Rio Magnolia co-owner John Paterson and the former owner of the property.  This report summarizes the findings made during the field investigation as well as an engineering analysis of hydroelectric feasibility and an economic comparison of development options available to the ownership.

LOCATION AND METHODS

Figure 1 shows the location of the property on the topographic quadrangle for the region, and Figure 2 is a plano of the property in question, showing the approximate location of the area that has been groomed for building.

Figure 1.  Topographic map showing the outline of the 280-acre Finca Rio Magnolia and the location of the Quebrada Magnolia that passes through the property from the northeastern section to the southewestern corner of the property.

The building sites were selected and developed by the ownership according to existing criteria.  Firstly, the area comprised an extensive rolling pastureland slope that did not require any deforestation.  Secondly the slopes of this pastureland were significantly shallower and more buildable than the more steeply inclined slopes on other portions of the property.  Finally, the location could be accessed following a modest investment in road development.  Either by design or circumstantially, the river that transects the property, Quebrada Magnolia, is located in the valley that bounds the building site and is geographically very close, making the possibility of hydroelectric development worth investigating in detail.

This study was undertaken using field equipment to measure distances and elevations, including a Casio digital altimeter and a Garmin global positioning system.  Standard formulae for the calculation of hydroelectric potential and head loss in pipes were employed to determine the hydroelectric potential of the site.  Facility electrical demand was estimated using conventional values for the wattage of equipment the ownership anticipates for the main house and the facility bungalows.  The demands were predicated on the basis of an eventual total of ten bungalows, each with similar power requirements.  Both micro- and mini-hydroelectric alternatives were considered in the course of this study and are reported on.  Conceptual designs were based upon established engineering practices for hydroelectric system design.  Costing estimation was based on current market value of materials and estimations of installation time and effort, and an economic analysis of the three alternatives identified was based on the estimates. 

Figure 2.  In this property site map, the location of the Quebrada Margarita is shown as well as the approximate location of the access road that was put in by the ownership (shown in red).  The purple oval approximately represents the portion of the property that is presently groomed for building the main house and bungalows.

FACILITY POWER DEMANDS

Facility power demands will almost necessarily vary as a function of the type of hydroelectric system used.  For an AC-direct hydroelectric power generation system, the power generated cannot be stored as it can in the case of DC micro-hydro systems, which require deep-cycle batteries for precisely this purpose.  For AC-Direct, or mini-hydroelectric, it is use it or lose it.  Also, since there is no reserve power in storage the facility utilization can never exceed the actual amount of power being generated at any given time.  This is not the case for a micro-hydro system, so the design variables and design process are significantly different for both.  Owing to these basic differences in functionality, there is little advantage in an AC-Direct system to purchase expensive power conservation equipment (like 12-Volt or propane refrigeration, or hybrid clothes dryers).  Facility power demand has correspondingly been estimated for this report in two sections, one presuming the power supply is AC-Direct hydroelectric power generation, the second section presuming a DC-hydroelectric origin.

AC-Direct Facility Power Demand

Table 1 is a summary of likely appliances and fixtures to be used in the main house, in each of the ten anticipated bungalows, and in the guardhouse.  The totals indicate that if everything is used at the same time, the total facility power demand adds up to around 50 kw.  However, this situation is unlikely to ever happen in real life, or at most represent a rare occurence.  An approximation of routine everyday use in the main house, full utilization of the guard house, and normal electrical consumption of the bungalows when all ten are occupied corresponds to a facility demand of around 15 kw.  However, if a system is designed for 15 kw, that means that the facility can never exceed this basic threshold of usage without a supplemental source of power.  The design process is to determine how close to get to the peak capacity and still stay within reasonable economic bounds.

In societies in which power utilities are required by law to purchase excess power from individual power producers (such as the United States), there is an economic incentive for private power producers to design and install systems capable of approaching or surpassing peak demands.  However, Costa Rica does not require this, nor does the nation make this option available.  In the case of Rio Margarita, moreover, the utility power lines are two kilometers away anyway, so it would not even be theoretically possible without an extension of the grid to the project location.  Consequently, to design a system to satisfy peak demands means that power during routine usage patterns must be continuously burned off just to get rid of the excess power, typically in the form of heating water.  Since each kw of extra power carries non-trivial capital costs, it becomes more practical to design the facility capacity for more than routine operational demands and to either adapt to peak demands through operational adjustments (washing clothes during non-peak hours, for instance, or limiting the use of the pool pump, microwave ovens, coffee makers, etc.), or introducing a fossil-fuel generator to boost the capacity periods of peak demand.

Table 1.  List of anticipated appliances and wattage estimates for conventional, off-the-shelf models.  The list is divided between the expected needs of the main house, the individual cabins, and the guardhouse. BASE represents the expected normal power consumption of the facility at full occupancy.  TOTAL represents the demands when everything is turned on at once.

The other alternative is to eschew AC-Direct power generation altogether and to opt for micro-hydroelectric power generation, in which power that is generated can be stored in a battery bank and saved for use specifically when momentary demands exceed the actual hydroelectric generation potential. 

Facility Power Demand assuming  Micro-Hydroelectric Power Generation

For a micro-hydroelectric application, the energy generated can be stored in batteries and does not have to be burned off if it is not used.  Also, the entire facility electrical capacity is limited by the capacity of the inverter that is used to convert direct current to alternating current.  For these reasons, the use of energy conservation appliances and fixtures is essential to the optimal system design.  Table II presents a re-assessment of facility demands that have now been tailored for a micro-hydroelectric application.  Note that the wattage of lights has been reduced and conventional 110 V refrigeration has been surplanted with 12-V alternatives, and the 220-V conventional clothes dryer has been replaced with a hybrid dryer in which propane is used to generate the required heat.

Table 2.  List of anticipated appliances and expected wattage for conservation models.  Also presented are the expected hours of daily usage.  The list is divided between the expected needs of the main house, the individual cabins, and the guardhouse.

The calculations reveal that the total power consumption for the entire facility at peak occupancy and assuming reasonable appliance usage patterns is 84 kw-hours, which corresponds to a continuos micro-hydroelectric power generation demand across 24 hours of only 3.5 kw, i.e. less than one tenth of the hydroelectric power generation demand for an AC-Direct system.  The column labeled “INVERT” in Table 2 lists a series of appliances that might reasonably be expected to be on at the same time and represents the capacity required by the inverter(s) to be used to supply the needed alternating current.  The calculations reveal that a single inverter with a 10 kw capacity satisfies the demands.  Applying a safety margin of 20%, the design capacity for a single inverter for the entire facility is therefore 12 kw.  Alternately, it is likely to be more reasonable to divide the load between two 6 kw inverters, one to serve the bungalows and guardhouse and the other to power the main house.

FIELD SURVEY

The extremely dense forest that carpets the valley walls descending to the river channel made it impractical during the field survey to access the river along its entire length.  The steepness of the channel itself was such that the guide for the expedition did not feel comfortable hiking along the river channel owing to the dangerous footing and the many waterfalls that would have had to be circumvented through thick steep jungle.  As a consequence, the investigating party was able to actually gain the river channel in only two reaches, one near the bridge in the upper portion of the field hike and in a second location about two thirds of the way to the bottom of the hike undertaken.  In two other places the field party was able to overlook the river channel from a height of about 30 feet or so above the channel itself.  The flowrate of the river was estimated to be around 5000 gpm, and was reported by the previous owner to not drop from its observed flow rate by more than one third in even the driest time of year.

Figure 3 shows the overlay of a GPS track of the area hiked onto the topographic map of the area.  The two maps are not possible to reconcile completely, though the correspondence is adequate for the purposes of this study.  Points A, B, C, and D in Figure 3 represent the points on the stream that were reached and where elevations and cartographic locations were measured. 

The elevations of each with respect to the arbitrary datum Point D­ are given in Table 3.  The distances between each of these points was determined using GPS mapping software.  These data were used to elaborate the stream channel profile diagram given in Figure 5.

Figure 3.  Overlay of GPS track on the topographic basemap.  The alignment was done visually with the ridge house site and the stream used as reference.  Whereas the two scales are within 5% of one another, the stream location in the vicinity of the bridge cannot be reconciled completely between the two mapping mechanisms.

Table 3.  Locations along the stream for which location and altitude were measured.  Locations of points are shown in Figure 3.

Figure 5.  Stream profile of the portion of the stream surveyed during the survey.  Elevations and distances are referenced to the Datum, point D, the lowest point of the stream visited during the field survey.  Point A represents the low-water bridge.

ANALYSIS

Hydroelectric potential is estimated using the empirical formula given below

                                                             Power = Head x Flow x 0.18 x E                                             (1)

Where Power is given in watts / hour, Head is measured in feet of relief,  Flow is measured in gallons per minute, 0.18 is a units conversion factor, and E is the efficiency of the turbine, 50% for the Harris micro-hydro pelton wheel, 65% for the Canyon AC-Direct mini-hydro turbines.

The design variables of flow rate and head from the watershed are balanced against needs for the property and economic constraints in each potential application to determine the optimal system.  The stream channel profile shown in Figure 5 reveals that the reaches between A and B and between C and D are steeper than the longer reach between B and C.   Given the high costs of pipeline installation, if a system can be designed using a shorter reach of river, it is likely to be cost competitive (the least expensive design is one that approaches vertical, to keep pipeline costs as low as possible) so the first step is to determine what would be required from both micro- and mini-hydro installations to electrify Finca Rio Magnolia, to then see if it can be optimized with respect to the resources available.

Mini-Hydroelectric System Requirements

To generate the entire 50 kw required to fully sustain the facility’s anticipated peak demand, we can apply Formula (1) backward to tell us how much water is required for the drop available.  If we use the entire 105 meters of vertical relief surveyed, then only 1240 gpm of flow are required in a frictionless world to generate 50 kw of power.  This means that even in the dry season the flow needed would only be about one third of the total expected stream flow, which is environmentally sustainable.  Therefore, AC-Direct power harvesting can satisfy the full power needs for the entire facility.  Unhappily, the lowest point visited on the field survey (Point D) remains a pretty fair distance (2400 feet) from the bungalow farthest from the house.  The geographically optimal generation point would be somewhere in the vicinity of Point B in Figure 4, to minimize transmission distance.  If we assume 55 meters of head based on an intake located in the vicinity of the bridge in order to achieve a generation point around Point B, then 2400 gpm of water is required (discounting friction) to generate 50 kw.  This is very close to the total amount of water expected to be present during the dry season.  Even if the stream can physically sustain this withdrawal, it is unlikely to be a poster-child for environmentally sustainability.  Therefore, in order to design a short run, high-yield, AC-Direct application that is conscientiously palatable, it is important that dry season flow rates be measured and accommodated into any final design.

Table 4 details the total head losses for the range of possible pipeline diameter sizes, assuming a 1300 gpm flow across a head of 105 meters.  This 50 kw system design, as represented in the conceptual design shown in Figure 6 assumes a 12” pipeline 2400 feet in length, a 50 kw AC-direct turbine, step-up and step-down transformers to reduce transmission line losses, 2000 feet of aluminum cable for above ground power transmission from the river to the area of the bungalows, and 1000 feet of buried copper transmission cable to distribute power internally among the ten bungalows and main house.  Estimates for the materials and installation are presented in Table 5 for this system.

Table 5.  Estimated costs for a competed AC-Direct system.

Micro Hydro System Requirements

Setting mini-hydro aside for a moment, let’s consider micro.  As was determined earlier, only 3.5 kw of continuous power generation is required to satisfy the power demands of the entire facility.  Each Harris pelton wheel generates 2.5 kw at 48 volts, or 1.5 kw at 24 volts.  Therefore two 48-volt Peltons would generate up to 5 kw of continuous power.  If we generate this amount of power across the steepest part of the stream in order to minimize the pipeline distance between points A and B in Figure 7, we can expect to easily work with 40 meters of relief.   It takes only 425 gpm of water at 40 meters of head in a frictionless world to generate 5 kw of power.  Head loss is expressed as a function of flow across an anticipated pipeline distance of 1000 feet in Table 6, and the rational choice is eight-inch pipe.

The conceptual diagram for the micro-hydro system is shown in Figure 7.  For the sake of comparison with the mini-hydro model previously developed, we will assume for this system that the first transmission leg is run in #0 gage aluminum and the final distribution leg is buried in #2 gage copper.  Also, to reduce line losses we include a step-up / step-down generator but leave the final specification of power transformation versus modification of cable gage as a subsequent engineering task once the system has been more extensively refined.  Based on the conceptual micro-hydro system presented in Figure 7, an estimate of costs is presented in Table 7.

Table 6.  Pipe losses for micro-hydro pipeline

Table 7.  Estimated costs for a competed micro-hydro system.