Hydroelectric Resource Evaluation:  Division, Costa Rica

For:  [--redacted--]

By:  Paul Collar

Osa Water Works, S.A.

May 22, 2008

INTRODUCTION AND OBJECTIVES

Osa Water Works was hired to perform a resource evaluation on an 80-hectare finca just outside of Division.  The property includes two houses that are continually occupied and facility equipment and tools.  This report summarizes the current electrical power demand and the hydroelectric potential of the property and alternatives for exploiting those resources.

FIELD STUDY AND FINDINGS

Figure 1 shows a GPS field track of the survey undertaken on May 14, 2008 overlain on the plano of the property.  The overlay is a rough estimate and not intended to be exactly to scale.  A series of small streams all gathered at waypoint 624, suggesting a likely water intake point.  Based on the guide’s description of summertime flow rates, it is clear that as much as 300 gpm could be diverted at this point for hydroelectric power generation.

Figure 1.  Plano of the property with a GPS track of the field survey undertaken overlain approximately.

Figure 2 shows the waypoints overlain on the satellite photo on Google Earth.  Field notes for this site survey are given in the Appendix of this report.

Figure 2.  Waypoints overlain on satellite image from Google Earth; note the owner’s and watchman’s house clearly visible to the left of waypoint 618 at the top of the image.

Figure 3 shows an energy profile of the points visited on the field survey, consisting of the elevations measured plotted as a function of the waypoints.  The diagram reveals an elevation drop of 215 meters over a linear distance of 610 meters.  The elevation difference between waypoints 624 (the intake point) and 627 (as far as we hiked downstream) was 45 meters across 150 meters of distance. 

Figure 3.  Energy profile of the terrain hiked during the field survey.

We did not continue because of a dangerous waterfall (Figure 5), but it was clear that as much head is available as desired moving further downstream and across a relatively short distance due to the very steep slope of the watershed. 

POWER CONSUMPTION DEMANDS

Table 1 shows the electrical appliances in use at the two houses and on the facility grounds.  The column “Demand” summarizes the overall power estimation for a full day at peak usage, so the sum of this column represents the charging source required to satisfy the projected peak facility power demands.  The column SAME TIME refers to those appliances that may be on at the same time as an estimate needed for sizing of the required inverter.

ENGINEERING ANALYSIS AND DESIGN

Charging Source

The required charging source of 97 kilowatts across 24 hours corresponds to an hourly charging source requirement of 4.04 kw.  A four-inch pipeline will carry around 200 gpm without excessive head losses, so if we solve for the required vertical drop using the projected power need and the flow of water available, this amounts to 224 vertical feet, or 68.5 meters.  Assuming the watershed slope continues about the same as the stretch of river that we hiked, we are able to achieve an elevation drop of 70 meters across a total pipeline distance of 233 meters.

Power Transmission

The transmission distance from the point of generation in the stream up to the point of use at the site of the two homes is 610 meters in a straight line, say 700 to include sway in the lines and to allow for a bit of divergence from a straight line.  This is too great a distance for conventional hydroelectric 120 Volt power to be transmitted without dramatic line losses.  Induction hydroelectric turbines are designed for this precise application, however, and produce a 480-Volt output that will permit transmission across the 700 meter distance in as small as a number 6 cable with less than 2% losses. 

Inverter and Battery Bank

The size of the inverter governs the total amount of simultaneous power usage that can be obtained.  Since the on-demand electrical hot water heater has a 9-kilowatt draw all by itself, a minimum inverter size of 10-kw is required just to heat water.  The electrical dryer is 4000-watts, and with the humidifier, hot plates, and other electrical appliances, to have the ability to use several at the same time, a 15 kw inverter is the smallest practical inverter size. 

The property design of the battery bank presumes an allowance of three days of independent battery capacity.  Since you cannot draw down a battery completely, a factor of 80% drawdown is used in conjunction with a 90% inverter efficiency.  For a 48-Volt input, 100 kw of daily consumption will require a battery bank of 92 2-Volt, 2170 amp-hour cells in four parallel arrays of 24 batteries per series.

The last part of the equation is a charge controller, which will cost somewhere around $1000.

Financial

Ballpark costs for the system are itemized in Table 2.  These prices are in some cases mere estimates and do not include negotiated discounts nor have the labor costs been firmly determined.  The estimation is presented as strictly a ballpark figure for comparative purposes and is not to be misconstrued as a bid.

An all-electric fully robust home system completely free of fossil-fuel requirements can be expected, therefore, to run in the neighborhood of $85,000, predicated on a daily consumption of 100 kw-hours as calculated from the power usage figures shown in Table 1.

TABLE 2.  Ball-park budget estimate for all-electrical facility.

However, the electrical bill is reported by Carlos Granera to document a usage of 800-1300 kw-hours per month rather than the hypothetical 3000 kw-hours that Table 1 would imply across a full month.  If we assume that the monthly power consumption averages 1000 kw-hours, that means a daily power demand of only 33 kw-hours.  This amount of power could be satisfied with a 1.4 kw turbine, which could be operated with considerably less water.  Since the same instantaneous power demands apply, the inverter would still have to be a 15 kw 240-AC unit, no savings there.  However, the battery bank could be effectively divided in three.  Overall savings under this configuration would put the cost of the entire system somewhere around $50,000.

One final alternative is to replace the hot water heater, the electric stove, refrigeration, and the clothes dryer with appliances that are either high-efficiency 12-Volt units (in the case of refrigeration) or propane units in the case of the stove, water heater, and hybrid dryer.  This would change the power consumption demands reported in Table 1 to the ones that are reported below in Table 3.

Based on the power consumption reported by the meter, it appears that the daily estimate that I have calculated is running twice as high, so I consider it safe to assume that with hybrid appliances, the total daily charging source is unlikely to actually exceed 20 kw-hours and the instantaneous power capacity will be satisfied with an inverter of 4 kw, a 120-volt inverter this case rather than the 240-volt inverter required for an all-electric complex.

Table 3.  Summary of anticipated power demands presuming high-efficiency DC-power refrigeration, propane stove and hot water heating, and hybrid clothes dryer

Under the new power consumption patterns described above, we need a charging source now of only 1 kw.  This means that we can easily achieve this power generation capacity through a two-inch pipeline transmitting 50 gpm of water across a total vertical relief of 70 meters.  This pipeline run is identical to the first configuration but in 2” pipe rather than 4” pipe.  Also, the smaller power demands suggest that it is entirely reasonable for us to deploy a 24-Volt inverter (rather than a 48-Volt inverter).  With a 24 Volt inverter and an assumption of 20 kw-hours daily power demand, three days of battery capacity corresponds to a battery bank of only 1200 amp-hours, which is achievable with a single series of batteries (rather than multiple series in a parallel configuration) and since the voltage is 24, we need only 12 individual 2-Volt cells (rather than 96 for the capacity required for the first case discussed).  The charge controller for this reduced system will run closer to $500.  The overall system estimate is summarized below but does not include the cost of the appliances that will be required to replace the all-electric appliances currently in use.

TABLE 4.  Ball-park budget estimate for facility using propane hot water heating, stove, high-efficiency 24-Volt refrigeration, and hybrid dryer.

CONCLUSIONS AND RECOMMENDATIONS

Completion of a final system design will require additional input from the ownership.  There are many ways to tweak the system through either appliance substitution or changes in patterns of lifestyle that have the potential to dramatically effect the final system design required.  The single most problematic appliance on the facility grounds are the on-demand hot water heaters.  The second most consumptive appliance is the electric dryer.  Two widely divergent designs were presented in this analysis, an all-electric system that comes in around $90,000 and a hybrid system that comes in at around $30,000.  Beyond those two, it is possible to make a variety of changes in design assumptions to reach the optimal balance between quality of life, independence from the power grid and from inflation in the price of oil, and capital costs.

Following careful review of this report and reflection on the part of the ownership, Osa Water Works is prepared to do a second design iteration following discussions with the ownership to determine if and where adjustments are to be made to either expand or reduce capacity to arrive at either a firm bid or a cost-plus estimate for final installation.  This final round of design, engineering, and costing will carry a fee of $500.

Figure 4.  A view of the upper watershed of the target stream.

Figure 5.  This is a photo of the waterfall, which was as far as we were able to hike during the field survey.  It is hard to tell, but this waterfall is about 15 meters in height.

APPENDIX:  FIELD NOTES