Resource Evaluation, Uvita, Costa Rica

For:  [redacted]

By:  Paul Collar

Osa Water Works, S.A.

May 21, 2008

INTRODUCTION AND OBJECTIVES

Osa Water Works was contracted to undertake a resource evaluation of a property in the mountains behind Uvita.  The objective of the study was to determine alternative energy power options for the development of two well-anointed homes on the property.  This report summarizes the findings.

POWER DEMANDS

Power projections for the anticipated Uvita facility were based on the actual distribution of appliances and domestic infrastructure of Villas Manu in Hatillo.  This assumption was made at the request of [redacted] as a fair representation of what is expected for the Uvita facility.  Table 1 summarizes the electrical appliances and equipment in use at Villas Manu and provides an estimate of the amount of time each appliance is used daily at full occupancy.

WATER DEMANDS

Contemporary water supply in industrialized societies presumes a per capita daily water consumption of 100 gallons.  If we presume a peak occupancy of twelve persons divided between the two homes, this amounts to a daily water demand of 1200 gallons at peak occupancy.   This amount is adequate to include pool water needs but does not allow for summertime irrigation, as irrigation rates can easily exceed ten times the amount needed for residential water supply.  A volume of 1200 gallons delivered over a 24 hour period corresponds to a continuous flow rate of 0.83 gallons per minute.  This means that with adequate storage capacity, a sustained flow rate of less than one gpm is able to provide enough water for 12 full-time occupants. 

Table 1.  Summary of anticipated power consumption patterns based on infrastructure present at Finca Manu and projections of reasonable daily usage at the Uvita facility during peak occupancy.  (DAILY refers to watt-hours consumed in total; SAME TIME refers to the instantaneous wattage required for the number of units and the expected simultaneity of usage.

FIELD SURVEY

Figure 1 shows the plano of the property with the GPS track of the field survey superimposed.  While the superimposition is not exactly to scale, the estimate is reasonably close.  Figure 2 shows the overlay of the waypoints on the Google Earth satellite photo of the finca.  Field notes are given as the appendix to this report.  The

Figure 1.  Plano of the property overlain with GPS track of hikes made during the field survey

Figure 2.  Google Earth satellite image with GPS waypoints overlain (see Appendix 1 for waypoint descriptions.

 

location of relevant points on the property are shown in Figure 3 and include:  1)  the most likely cleared area with the capacity to deploy solar panels (PV-FARM);  2)  the streams located on the property (INTAKES);  3)  the two building sites (GUEST and MAIN); and 4) where the stream crosses the southerly property boundary (HYDRO GENERATION).

Figure 3.  Enlarged portion of the finca, showing locations key to the property power system.

 

ENGINEERING ANALYSIS AND DESIGN

The following analysis presumes:  1) the power demands as presented in Table 1, and 2)  hydroelectric and solar power as the only charging sources.

Charging Source

The hydroelectric power generation potential of the property is very low during the dry season.  A summertime flow rate of 50 gpm can be captured from the larger of two  streams and diverted across a vertical relief of 45 meters for a peak summertime hydroelectric potential of only 16 Kw per day.  Whereas another 5 gpm of summertime flow can be captured from the small stream at the same elevation for a conjoined dry season output of perhaps 18 Kw, this option was discarded for the purposes of this analysis as it is a lot of cost for little benefit.

If we add together the power demands summarized in Table 1 and apply them as an approximation of the anticipated Uvita power demands, this amounts to a total power demand of 126 Kw per day.  While Villas Manu include three houses rather than the two expected for the Uvita finca, for the moment let us assume that the two households are fully electrical and free of propane use whatsoever.  For a carbon-free household, let us assume, therefore, that the 126 Kw for Alex’s three houses approximate the propane-free power requirements of Mike’s projected two houses.

With 16 Kw of daily hydroelectric capacity, this leaves a power deficit of 110 Kw per day.  This power deficit must be satisfied by a separate power source, assumed for this report to be solar. 

Still, this assumes that 100% of the dry season flow rate of the stream is used for hydroelectric power generation.  In reality, we need 20 gpm of this flow rate to energize a hydraulic ram pump to deliver 2 gpm continuously for potable water supply for the two houses.  Coupled with an adequately-sized storage tank (5,000 gallons per house), this is enough water for 28 full time residents.

Subtracting this amount of potable water demand leaves us with only 30 gpm for power production, reducing the summertime hydro potential to a mere 10 Kw per day, meaning that the power deficit that must be made up with solar panels is actually 116 Kw.

During an 8-hour solar day, it will require 65 224-watt panels to make up the projected power deficit. 

In fact, 10 kilowatts hydro is equivalent to 6 panels, so it is arguable as to whether an investment in hydroelectric is even worthwhile, since a hydroelectric power generation system is going to cost more than six extra panels.

However, during the rainy season, it is almost certain that 8 hours of daily sunshine is an unreasonable expectation.  In fact, because of the location of the property on the mountains, it is arguable whether 6-hours of daily insolation can be reliably forecast, and during the peak rainy months, it may be even less than that.

The 30 gpm summertime hydroelectric flow rate swells to a raging river in the winter that will easily allow for diversion of 250 gpm in a four-inch pipeline, capable of providing 80 kilowatts across 24 hours.  This means that the power deficit that must be filled by solar power is reduced in the winter to only 46 kilowatts.  This amount of power is generated by 65 panels in only 3.2 hours of sunshine.

The analysis reveals that hydroelectric power generation can provide for less than 10% of the projected facility power demands during the summer months of expected peak occupancy.  However, in the absence of hydroelectric power, even a robust solar array is likely to fall short of full facility power requirements in the winter.  Since the investment in the hydroelectric component is unlikely to exceed $20,000 in capital costs, it is certain that hydroelectric power will be economically favored over dramatically expanding the solar array to account for lower hours of sunlight during the rainy season.  Of course the energy deficit could be compensated by running a generator every day, but a hydroelectric capacity means that a generator may not ever be needed at all except in the case of a catastrophic failure or emergency.

Once we assume a hybrid solar/hydro system, then we can modify the design assumptions somewhat and plan for 10 hours of daily insolation during the summer months.  By subtracting the 10 Kw of summertime hydro from the 126 Kw of anticipated daily power demands, and assuming a 10-hour solar day, this requires a total of 52 solar panels.  This implies a savings in panels of around $16,000, enough to capitalize the expected cost of the entire hydroelectric installation.  Calculating backwards during the wintertime, 52 solar panels provide the projected 46-Kw deficit in 4 hours.  It is entirely reasonable to anticipate an average of 4 hours of effective sunlight even during the month of November, meaning that our design remains a robust one in which it is not expected that a fossil-fuel generator will be needed, even at peak occupancy.

Power Transmission

Figure 4 shows the key locations on the finca and the distances between key system locations.  The most practical means of distributing the power generated is to deploy a power bodega at the location of the solar farm.  The inverter, battery bank and controllers would all be located at that point.  Due to the distance between the power center and the two houses, it will be necessary to use step up / step down transformers to transmit power without excessive line losses.  The size of the cable required to transmit power to the two houses will vary as a function of the voltage achieved with the transformers.  The higher the voltage, the lower the size of the cable, so additional analysis is required to determine the most economic combination of transformer and cable sizing.  For transmission of power from the hydroelectric generation point, an induction generator, which produces high-voltage AC output especially for transmission over longer distance than traditional turbines, can be carried to the power center through relatively inexpensive cable.  The high-voltage AC power must be stepped down at  the PV-farm in order to pass through the charge controller and into the batteries. 

Figure 4.  Conceptual system design with distances between key features shown in meters.

Inverter, Charge Controlling, and Battery Bank

The size of the inverter governs the total amount of simultaneous power usage that can be obtained.  For instance, if two 4000-watt dryers are on at the same time, a 10-Kw inverter will allow only 2000 watts of additional power use.  If we have a coffee maker on and a few lights, that means that nothing else will turn on.  In practice it is unlikely that two dryers will ever need to be run simultaneously.  However, for two well-anointed all-electric homes it is arguably sub-adequate to deploy an inverter of less than 15 kilowatts for the two households.  Also, it must be a 240-V inverter, since the stove, dryer, water heater, and pool pumps all require 240-V power. 

The rule of thumb for battery bank sizing is 200 amp-hour capacity for every kilowatt of inverter capacity.  Foregoing more rigorous design analysis for the moment, this presupposes a relatively beefy 3000 amp-hour battery bank capacity.  Further system design is an array of branching design alternatives beyond the scope of this report, including decisions regarding the input system voltage (12, 24, or 48), the type, amperage rating, and voltage of batteries to be used, and varying combinations of cable size and transformer rating as discussed below.  Despite a considerable additional engineering effort required to arrive at a final optimal system design, it is likely that the eventual system would be 48-Volt using 2-Volt cells, presuming a bank of 24 batteries, with a 3000 amp-hour capacity.  The configuration of the panels would most likely favor the deployment of two separate charge controllers, each to control half the array, and a third charge controller for the hydroelectric charging source.  The system would likely include four arrays of 14 panels each, and a secure battery, electronics, and generator enclosure with a footprint of around 24 square meters.

CONCLUSIONS AND RECOMMENDATIONS

By substituting gas-appliances, including a hot water heater, hybrid dryer, gas stoves, and deploying high-efficiency DC-refrigeration, the overall facility electrical demands can be reduced by two thirds and the capital costs reduced by likely more than half.   On the other hand, a fossil-fuel dependent alternative will necessarily depend on large amounts of propane combustion.  Propane dryers use a lot of gas, and once hot water, and cooking are factored in, monthly propane consumption in two houses is likely to be a significant expense at peak occupancy, not to mention a source of operational requirements in transporting propane and tanks back and forth.  At some finite point in time, the extra capital costs for a fully electrical household will pay for themselves through savings in propane consumption, particularly as the price of fossil fuel increases. 

A hybrid solar/hydro system as summarized in this resource evaluation, replete with a backup 25-Kw generator for emergencies, is not expected to exceed a cost of $250,000, installed.  The cost may be halved for a system that includes fossil-fuel and DC appliances, and even less if a fossil fuel generator is planned not just for emergencies but actually for operations to augment the system for peak electrical demand in order to reduce the alternative solar and hydro design to accommodate for merely normal and not peak power demands.  To arrive at exact costs will require a considerable additional effort, including:  1)  ownership decisions on operational philosophy and rigorous definition of appliances and power demand expectations;  2) a more extensive engineering and design analysis based on those decisions and best-design practices with the technologies selected and an economic analysis involving actual propane consumption expectations;  3) product sourcing, price negotiation, transport, and import mechanisms used;  and lastly, 4) an installation work plan and budget. 

Independent of whether hybrid appliances shall be used or not, this entire analysis is predicated upon the electrical power demand predicted by the infrastructure deployed at Villas Manu and estimates of the amount of time each appliance may be used on average in Uvita.  As the estimates have been made somewhat conservatively, it is of course possible to micro-manage usage beyond simple decisions about the use of compact fluorescent bulbs for lighting and strive to adhere to an operational plan that is much more conservation-minded than the Table 1 usage would suggest.  In this manner it may be possible to shave the system capacity and cost down considerably.  The use of electric clothes dryers presents a good example.  During the rainy season, when there is an abundance of hydroelectric power and clothes cannot be dried effectively any other way, electric dryers can be used with impunity to dry clothes.  However, in the summertime, when the dry season allows for drying clothes on the line, a dramatic power savings is possible by simply not using the dryer.  Likewise, the energy requirements for summertime hot water can be likely reduced by using passive solar hot water heating in combination with an electric water heater rather than to rely on electric water heating alone.  When the sun is less powerful in the wintertime, this measure is not needed because of the increased power available from the hydroelectric potential. 

PROPOSAL DEVELOPMENT

Osa Water Works is prepared to elaborate a full engineering analysis with all practical alternatives following ownership feedback on design objectives and boundaries for a fee of $1000.  The final report would include installation budgets for the practical alternatives and an engineering analysis and an itemized procurement and installation bid for the alternative favored by the ownership.  Terms include a deposit of 50% to proceed with the engineering and design work, the balance due upon delivery of a final itemized budget, work plan, and installation proposal.

APPENDIX.  Field Notes