Hydroelectric Resource Evaluation: Mountains of Hatillo
For: [ redacted ]
By: Paul Collar
Osa Water Works, S.A.
Puerto Jimenez Costa Rica
May 18, 2008
INTRODUCTION AND OBJECTIVES
On May 16, 2008, a site survey was undertaken by Alexander Hruby and Paul Collar of a finca in the mountains behind Hatillo. The objective of the study was to define the hydroelectric potential of the stream that transects the property.
FIELD FINDINGS
A plano of the property was not available, so the trajectory of the hike is referenced only to verbally indicated place names, including: 1) the entrance to the property; 2) the building site; 3) the waterfall base; and 4) the river base (the furthest point that can be reached by vehicle.
Figure 1 shows the field track defined by the GPS used in the field survey. Figure 2 shows the locations of the waypoints imported into Google Earth. Unfortunately, the low resolution of the satellite coverage of this particular area makes features indistinguishable, at least until the coverage is updated with new satellite imagery.
Figure 1. Trajectory of field hike: 635: entrance to property; 637: building site; 639: waterfall base; 641: river base.
Figure 2. Location of waypoints taken during field hike shown in Google Earth.
Figure 3 is a graph showing the relative elevations of the places visited in the field. A datum of 0 was assigned to the river base and corresponds approximately to the lowest river elevation on the finca.
Figure 3. Energy diagram showing the elevations of the points visited during the field survey.
Figure 3 reveals that from the waterfall base to the river base there is a vertical relief of 40 meters. Whereas we did not climb to the top of the waterfall to get an altimeter reading, we estimated the height of the waterfall at 30 meters.
A flow rate of 1500 gpm +/- 30% was estimated at the base of the waterfall. Hruby reports that at the driest time of the year, flow is approximately 1/3 this amount. During rainy season months, my experience would lead me to estimate that rainy season flows average 2-5 times the flow rate observed on our field investigation.
The hydroelectric potential of the stream varies dramatically according to the season, therefore, and also varies according to whether the intake is situated above the waterfall or below the waterfall.
ENGINEERING ANALYSIS
Dry Season Hydroelectric Potential
If we assume a flow of 500 gpm at the driest time of the year and divert one half of that amount for hydroelectric power generation, we can do so comfortably in a four-inch pipe. A flow of 250 gpm corresponds to a hydroelectric potential of 3 kw presuming an intake at the base of the waterfall. If this flow is diverted from the top of the waterfall, the hydroelectric potential is 5.2 kw.
Since this potential is too low to sustain a house with power using an AC-Direct mini-hydroelectric configuration, it is most practical to capitalize on this power with a micro-hydroelectric system that includes inverter, battery bank, and charge controller. In this manner, energy that is not consumed as it is generated is stored in batteries rather than having to be wasted as heat. Correspondingly when power demands exceed the instantaneous hydroelectric production, the excess power is withdrawn from the battery bank so that power demands in excess of generation rate can be sustained as a function of the battery bank capacity presuming that periods of lower demand (at night for instance) are adequate to recharge the batteries.
Rainy Season Hydroelectric Potential
In the rainy season, diversion of 1500 gpm through a 12” pipeline would provide a mini-hydroelectric potential of 40.3 kw from the top of the waterfall and 23 kw from the base. While this is expected to be adequate to provide even peak energy demands of a relatively well-anointed villa (exclusive of air conditioning), any energy not used during power generation must either be burned off as heat or else distributed to the national power grid.
DISCUSSION
Despite the high rainy season hydroelectric potential, it is the dry season summer months that typically are the highest occupancy periods and require both the greatest amount of power and water. For this reason, a rational approach to facility planning must be able to satisfy the demands of the months December through April. Once the demands of these summer months are accommodated, the remainder of the year will pretty much fall into place.
On an annualized basis, if there were a grid connection, and if the facility were able to negotiate the sale of excess energy to the grid, then a 40-75 kw mini-hydroelectric facility would almost certainly be the preferred alternative economically. Under this alternative, excess power would be sold to the electrical utility during the rainy months, and during the summer months when the mini-hydroelectric would either be turned off or operated at a diminished capacity, energy would be purchased from the grid. It is expected that the facility would have substantial net revenues that would be the basis for calculating return on investment and payback period for the non-trivial capital investment in pipeline, power generation equipment, grid extension, and power transmission.
However, the above configuration does not provide for energy independence from the grid and would leave the facility vulnerable to regional and national power outages and brown-outs. A micro-hydroelectric configuration, on the other hand, would enable reliable, consistent, environmentally sustainable hydroelectric power generation all year round. Since micro-hydro turbines can be added in a modular sense, the system can be operated at varying power generation capacities depending on how much water is available and how much energy is desired.
Another dramatic advantage of a battery/inverter based system is that separate charging sources can be used simultaneously. So, solar panels, wind turbines, even fossil fuel power generation sources can be connected to the system, all contributing to the power reserves available, all automated. In this manner, solar panels are able to contribute additional power during the dry months when there is less water and more sun. And in the rainy months, hydroelectric power is able to provide essentially any foreseeable power demand. As a backup, a generator can be deployed that will automatically start if sustained high demands exceed the capacity of the batteries.
RECOMMENDATIONS
The stream on the property provides considerable versatility to the energy planning for development of the Hatillo finca. In purely economic terms, it is likely to be favorable to go the route of large-scale mini-hydroelectric, presuming that terms can be negotiated for sale of excess power to the grid. However, for environmental sustainability, energy independence, and optimization of resource potential, a micro-hydroelectric configuration is without question the most efficient use of available resources.
The eventual system design must necessarily be predicated on the anticipated facility power demands. For a robust energy system year round, it is likely that a six – eight inch pipeline will enable scaling of power production from an expected low of 72 kw per day in the summer time (diverting only one half of available water from the base of the waterfall) to a high of 430 kw per day in the winter time (eight inch pipeline from top of waterfall). Summertime facility power demands would then determine the number of solar panels (or wind turbines) needed to supplement summertime hydroelectric demands in a hybrid power co-generation system.
