OSA WATER WORKS

THE GREENING OF COSTA RICA

Home Solar Power System Design:  Cookie Cutter Instructions

July 27, 2009

Part II of a two part series exploring solar power options for homes and businesses in Costa Rica.  See Part One here.

Introduction

Whether you are considering solar, wind, or hydroelectric, whether your existing or future home will be connected to the power grid or is so remote that is not an option, whether your home is a one-room cabin in the woods or a sprawling ocean-view mansion, the design and sizing of a power system to fit your needs is essentially the same.  In this article, I summarize the process of designing a solar system for a medium-sized home.  In practice solar design and engineering is an iterative process that seeks the best possible solution taking into account your power needs, all available equipment alternatives, and the costs associated.  For the purposes of brevity, however, I will skip the backs and forth and will instead use the design principles discussed in Section One to discuss in Section Two why certain decisions in home outfitting (AC vs. DC refrigeration, options for hot water heating, clothes drying, cooking, and air conditioning) may warrant modest lifestyle adjustments in order to provide the greatest bang for your buck.

Section One:  solar power design for a mid-sized home.

1.  Determine your Power Demand.  Make a list of ALL household electrical appliances that you will use and list the hours per day on average that you expect to use them.  Using power consumption data in watts that you may find online, make a spreadsheet to determine how many watt-hours of daily power you require for your home.  In Table 1 I have summarized a typical list of household appurtenances, their power consumption, and the average number of hours each is expected to be used daily.  Column 6 shows the watt-hours required for the daily operation of each item.  At the bottom of the column is the total watt-hours required for an average day of operation.  In this example the household power demand is projected to be 21,060 watt-hours per day.  This estimate is needed to determine how many solar panels your home will require.

2.  Instantaneous Power Requirement.  By examining your electrical appliances, determine what might be turned on at the same time during peak usage.  Column 7 in Table 1 shows the wattage for appliances that might reasonably be expected to be on simultaneously at peak usage in our example home, and at the bottom of the column we see that the peak instantaneous demand adds up to 2,970 watts.  This estimate is needed to determine the size inverter your home will require.

3.  Determine Energy Independence Required.  In order to size the battery bank required for your home, determine how many hours of energy independence you wish to have for your home, that is, the amount of time when charging sources are not active.  Clearly, for solar systems, the battery bank must be capable of carrying the household during night time hours.  Batteries are expensive, and while it may be nice to plan for three days of energy independence to account for periods of intense cloudiness and rain, this may add $10,000 in costs to a system, and it may be economically more attractive to plan for the inclusion of a $3000 fossil-fuel generator rather than to capitalize a huge battery bank.   I will use an energy independence of 10 hours in the design that follows. 

Table 1.  Household power demand calculator.  Columns:  1)  appliance;  2)  number in use;  3)  appliance wattage;  4)  hours of daily usage;  5) simultaneity of usage (for items of which there are more than one);  6)  watt-hours;  7)  Instantaneous power required (watts).

4.  Determine charging source.   Based on the number of hours of sun you can expect on average per day, determine the charging source required.  Assuming that your home is not shaded by trees and is not in a cloud forest, you can reasonably expect a 7-hour solar day on average in Costa Rica.  Dividing the daily demand of 21,060 kilowatt-hours by 7 hours, we determine that we need a charging source of 3009 watts.  If we assume the use of 224-watt solar panels (the largest presently on the market), this amounts to 3009 watts / 224 watts per panel = 13.43 panels.  Therefore this system requires 14 224-watt solar panels.

5.  Determine the input and output voltage to be used.  Rather than analyze this rigorously, a simple rule-of-thumb is normally adequate.  For small systems assume a 12-volt input, for mid-sized systems use 24-volts, and for large systems use 48-volt.  For our mid-sized application, we shall assume a 24-volt input.  I have consciously avoided the use of 220 volt appliances in the list in Table 1 so that an output voltage of 110 volts is adequate.  For our system we will use a 24/110 input/output voltage.  

6.  Determine the inverter to be used.                  Different manufacturers have inverters with different ratings and characteristics.  You will need to educate yourself on the options in order to make decisions most appropriate for your needs.  I consider Outback Power Systems to be the gold standard of the inverter industry and use this brand for all solar power applications barring unusual extenuating circumstances.  For our example the peak demand is 2970 watts (Table 1, Column 7), and therefore the 3500-watt inverter (Outback’s largest without stacking) is the most appropriate inverter.  Since we have an input voltage of 24 volts, this means the Outback 3524 model is the appropriate one.  Since the output voltage is 110 volt, we need only a single inverter.  If we had 220 volt appliances we would need two inverters stacked together to achieve 220 volt output.  Finally, for smaller applications you have the choice of sealed (FX-series) or vented (VFX-series) inverters.  Sealed inverters provide somewhat greater protection against corrosion by water vapor and salt, but they do not disperse heat quite as well as vented models and are less appropriate for hot climates.  For our case, this is not an option, since the 3524 comes only in a vented (VFX-series) housing.  Our design will therefore include the Outback VFX 3524 inverter.

7.  Determine the battery bank required.   Battery capacity is rated in amp-hours.  The power demand that we calculated in Table 1 (Column 6) is reported in watt-hours.  To convert from one to the other, you must divide the power demand in watt-hours by the input voltage.  Therefore, our daily power demand of 21,060 watt-hours / 24 volts is equal to 880 amp-hours.  However, batteries may not be drawn down beneath 80% of their capacity, and inversion of DC power to AC in the inverter is typically only 90% efficient.  Therefore the actual battery capacity equivalent to this daily power demand calculated is actually 880 amp-hours / 0.8 / 0.9 = 1222 amp-hours.  If we discard 8 hours per day as a period of system dormancy while the household is asleep, this means that the hourly battery capacity required for the demand estimated is actually 1222 / (24-8) = 76.4 amp-hours per hour of independent capacity.  So for ten hours of power independence we need a battery bank of 76.4 x 10 = 764 amp-hours.  The Rolls Surrette model 6CS-25PS 6-volt battery provides 820 amp-hours.  So, if we configure a battery bank of four of these batteries wired in series, this provides us with a 24-volt battery bank with a storage capacity of 820 amp-hours.

8.  Determine your panel array configuration and charge controller requirements.  The 24-volt, 224-watt panels proposed for this application each produces 8 amps of output.  Fourteen such panels wired in series require a charge controller capable of attenuating 14 panels x 8 amps/panel = 112 amps.  This could be achieved by using two 60 amp charge controllers.  However, it is commonly best to wire the panels at least partly in series in order to boost the transmission voltage, which has the effect of reducing line losses and increasing overall system efficiency, particularly if the distance between the panels and batteries is greater than 30 feet or so.  It also reduces the amperage and requires less charge controller capacity.  In this case, we shall wire the panels in seven parallel series of two panels each and thereby increase the transmission voltage from the 24 volt rating of the individual panels to 48 volts for each couplet of panels wired in series and reduce the amperage to 7 series x 8 amps / series = 56 amps total.  In this configuration a single 60 amp charge controller will be adequate.  We shall use an Outback FLEXmax 60-amp MPPT charge controller for this duty.

9.  Miscellaneous.  Finally, you will need to use the appropriate size and types of cable to connect the systems and need to have additional system components, including AC and DC power disconnects, power surge / lightning arrestors, a fossil fuel backup generator, a control panel / readout display that allows you to program the system and to monitor operations, and possibly additional system peripherals, depending on your budget and your affinity and taste for gadgetry and control.  You will require a frame upon which to mount the panels on the roof or a ground-mounted pedestal array.  And the entire system must be adequately grounded through the use of copper grounding rod(s).  All control equipment must be contained within an appropriate structure, batteries placed in a well-ventilated area, and the panels should be located as close as possible to the batteries to reduce line losses.

10.  Budget.   Costa Rica does not charge import duties on imported alternative energy equipment.  For equipment imported from abroad, the only surcharges that apply are the 13% sales tax to bring the equipment into the country, shipping charges, brokerage and warehousing fees, and the profit margin of the company that actually does the importing.  In Table II I have provided estimates based on current US pricing of the components proposed for our mid-sized example home, 13% sales tax, 15% profit margin, estimated shipping fees, and reasonable estimates for installation by companies operating in Costa Rica.  For do-it-yourselfers, the installation fees and profits will not apply, and on a $30,000 system, savings of as much as $6000 may be possible.

Table II.  Cost estimation of the solar power system designed for a mid-sized home.

Section II:  Analysis of high-energy routine home operations and processes

Residential solar power design is an iterative design process, as has been repeatedly stated.  Part of that process is determining what appliances to plan to use.  This section summarizes the power demands of typical residential processes and operations and provides guidance in how to determine the options that are most appropriate for your environmental intentions, lifestyle preferences, and budget.

Refrigeration.  A 16-cubic foot conventional refrigerator uses 1000 watts on average while the compressor is running.  A DC fridge of the same size uses 40 watts.  Under typical usage, the compressor operates on average 8 hours per day.  Many DC fridges are horizontal chest units, capitalizing on the higher density of cold air and their greater efficiency with respect to vertical units.  Assuming a 7-hour solar day, an AC refrigerator will require (1000 x 8 / 7 / 224 ) = 5.1 panels.   A DC refrigerator, on the other hand, requires (40 x 8 / 7 / 224 ) = 0.2 panels.    In other words, the $750 conventional vertical refrigerator will require $6200 in solar panels to operate it.  The $1800 chest-style Sundanzer 24-volt fridge, on the other hand, will require $243 in solar panels to provide the same operation.  Conventional AC refrigeration carries a capital cost, therefore—exclusive of expanded battery bank and inverter capacity—of $6900 while the equivalent capacity in solar refrigeration adds up to $1843 in capital costs.  A DC fridge is therefore 3.4 times less expensive than a conventional refrigerator.

Air Conditioning.  A two-ton central air conditioning unit capable of adequately supplying the needs of a 2500 square foot home has a power demand of 3500 watts and is expected to operate 75% of the time.  This amounts to a daily power draw of 3500 x 24 x 0.75 = 63,000 kilowatt hours.  Assuming 7 hours of average daily sunlight, this requires a total of 63,000 / 7 / 224 = 40 solar panels.  Discounting volume discounts and additional inverter and battery costs, this size central air conditioning unit requires an investment in solar panels alone of $48,000.  Air conditioning requires a lot of power and if you plan to power AC units with solar panels, be prepared to spend a lot of money to do so.

Hot Water.  All processes involving heat exchange require great investments in energy.  Like refrigeration and air conditioning, the heating of water is energy intensive.  Conventional electrical hot water heaters require 7500 watts on average and may be active as many as five hours per day on average.  This 37,500 watt-hour power demand requires 24 panels (assuming a seven hour solar day), or $29,000 in panel costs alone.  Solar hot water heating with direct solar radiation can provide the same hot water service for one tenth of this energy investment and is absolutely the preferred hot water heating technology in Costa Rica.  To complicate matters, electric hot water heaters are all 220 volt, so their inclusion in an alternative energy household absolutely requires stacked inverters.  For a 7500 watt electric hot water heater, for example, four inverters are not enough to make it operate.  Consider:  3500 watts of 220 volt power x 2 (that is four stacked inverters) is equal to only 7000 watts.  Therefore, in order to operate a single conventional hot water heater requires six stacked inverters.  So not only is there a dramatically higher solar panel cost, conventional electric hot water heating requires another $15,000 in power inversion equipment.

Clothes Dryer.  A conventional clothes dryer requires 4500 watts of 220 volt power.  If the dryer is used only once per day for one hour, that amounts to a power demand equivalent to $3600 in solar panels alone.  However, to achieve 4500 watts in 220 volts requires FOUR 3500 volt stacked inverters, or $7500 more in power inversion equipment just to operate the dryer.   To avoid this, most homeowners will find that hybrid dryers take less than one fifth the power as fully electrical models and require a single inverter to operate.  Since most households depend on natural gas for cooking and other domestic needs, this is often the best option to keep solar system costs from spiraling out of control.

Water Well.  Submersible pumps over ½ horsepower are all 220 volt.  While the charging source required may not necessarily be limiting, it means in most cases that a minimum of two stacked inverters are required to provide the voltage necessary for pump operation.  If this is the only 220 volt appliance in the house, it is possible to add an accessory (an auto-transformer) to a single inverter to provide this capacity rather than stack two inverters, but at the expense of some degree of flexibility in routine capacity.  For those households that depend on well water, this is not a variable but an absolute power demand requirement as there are no viable alternatives to electrical power when deep well water supply is the only alternative for water supply.

Electrical v. Natural Gas.  Many domestic processes present the option to use natural gas instead of electricity.  Natural gas is reasonably considered for many duties:

1)      Cooking

2)      Refrigeration

3)      Hot water heating

4)      Clothes drying

A pedestrian energy analysis will reveal that in every case electrical powering of these residential functions is more reasonable energetically when carbon emissions are taken into consideration.  Most homeowners do not yet include carbon budgets in their decision-making, and outside of those considerations, the use of natural gas remains economically favored for cooking and clothes drying at returns of investment of less than ten years at current fuel prices.  Beyond that period natural gas is not competitive.  For hot water and refrigeration, natural gas is not competitive beyond returns of investment of less than two-three years.

For Costa Rica, the best solutions to these energetically challenging routine home operations are the following:

1)      Refrigeration:  DC refrigeration

2)      Cooking:  unless you are insistent upon carbon neutrality, natural gas is your best alternative.

3)      Clothes drying:  When possible, dry clothes on the line, but for the rainy season use a hybrid natural gas / electrical clothes dryer.

4)      Hot water:  solar hot water heaters alone (passive systems) are ideal for residential applications and for homes that absolutely must have piping hot water all the time solar hot water heaters may be coupled with gas hot water heaters (active systems).

5)      Central Air conditioning:  electrical is the only option.  To reduce the power required, geothermal heat pumps may be incorporated to cut costs by 35-50 percent.

Part One of this solar mini-series focused on Costa Rica's ambitions to achieve national carbon neutrality and how that is expected to impact the normalization of grid-tie energy trading legislation. 

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