OSA WATER WORKS

THE GREENING OF COSTA RICA

 INTRODUCTION AND OBJECTIVES

This analysis is an overview of power and fuel consumption patterns for a conventional residence located in Mountain View, Arkansas.  The objective of this analysis was to determine options for offsetting energy consumption patterns and carbon footprint through the integration of conventional green-engineering practices.

EXISTING POWER CONSUMPTION

The home is located in a temperate portion of the United States that depends on central air conditioning (with one supplemental window unit) in the summertime.  Wintertime heating and year-round cooking is powered by natural gas, but all remaining appliances and climate control is powered by electricity.  The table below summarizes monthly electrical power consumption patterns for four representative months and presents a calculation of the amount of energy used on average per day.

 Whereas details on the amount of natural gas consumed were not provided, winter heating bills average $141.52 per month for last winter’s four coldest months (Dec-Mar) and $25.78 for the remainder of the year.                                                                                            

OPTIONS FOR REDUCTION OF CARBON FOOTPRINT

 As with any existing structure, there are a great many number of changes that can be undertaken to reduce the overall power requirements of the home and to change the nature of those power demands to reduce the carbon footprint that results from consumption of fossil fuels, both directly as in the case of natural gas heating, and the consumption of conventional grid power, much of which is generated using oil and coal-burning technologies.  The least costly and most obvious ones are simple weather-proofing, which reduces utilities for the average home by as much as 10%.  Energy conservation practices can also have dramatic effects.  These include such simple practices, of which only a few are listed:  1)  turning off lights that are not in use;  2)  drying clothing on the line rather than in the dryer when possible;  3)  adjusting the temperature of central climate control systems down in the winter and up in the summer;  4)  planning refrigerator and freezer use to minimize the time the door is open;  5)  closing off rooms or portions of the house to reduce and space that is air conditioned and heated;  6)  adjusting the temperature of the hot water thermostat to a lower setting.

Unhappily, most options for boosting home efficiencies are neither simply changes in usage patterns nor relatively inexpensive like weather-proofing.  Most meaningful options also require active structural intervention and in some cases may disrupt patterns daily of existence in manners that may or may not be acceptable to home owners because of quality-of-life concerns.  The most practical way to examine the benefits and costs of home retro-fits is through an analysis of each change and what its implications are both in terms of capital and operational costs of implementing the change as well as any quality-of-life impacts that will need to be considered.  The most rational point of departure is a no-change baseline, in which the only change made is to replace the existing power supply with an alternative energy power source that does not produce any greenhouse gases.

Case 1:  No Changes

The existing patterns of power consumption reveals that the greatest electricity demands correspond to the month of August, when reliance upon air conditioning pushes the daily power demand to 86 kilowatt-hours per day.  In comparison, April power demand, when neither heating nor cooling is much required, the daily power demand is only 46 kilowatt-hours, slightly more than half as much power.  In order to determine how many solar panels are required to provide this amount of power, it is necessary to first assess seasonal insolation patterns.  In the wintertime it is not reasonable to expect on average more than five hours of daily insolation.  The production of 77 kilowatt-hours (January power demand average) during 5 hours of sun presumes a charging source of 15.4 kilowatts.  In the summertime, when 10 hours of average daily insolation can be reasonably expected, the 86 kilowatt-hours daily power demand can be satisfied with a charging source of only 8.6 kilowatts.  In order to completely meet the existing power demands, therefore, it would be necessary to go with the most conservative case.  Using 200-watt panels, a charging source of 15.4 kw amounts to a total of 77 panels.  A 48-volt system would presume, therefore a deployment of 80 panels for geometric symmetry.  This absurdly large power source, by the time that batteries, inverters, controllers, mounting racks, cable, and labor is included for installation would carry a capital costs in the vicinity of $130,000.  Using current electricity costs ($0.10 / kilowatt-hour on average), it would take 52 years to pay for this level of capital investment, and since this exceeds the life expectancy of all the components of the solar system, including the panels themselves, this simple analysis reveals that it is not economically viable to consider this alternative.  Moreover, the deployment of twice as many panels does nothing to reduce the natural gas costs during the winter, so the costs of heating and cooking and their associated greenhouse gas emissions would continue unabated.

The analysis reveals a subtle but important point.  If it is not economically viable to replace the entire power demand with green solar energy, then it is not economically viable to replace any portion of it with solar power.  In other words, the patterns of energy consumption are such that solar energy in any degree cannot be justified economically on any time frame without changes in the basic assumptions underlying the analysis.  Changes that might affect the economic analysis favorably include the following:  1)  the ownership placing irrational value (i.e. emotive rather than market-based) in reducing personal carbon emissions;  2)  dramatic increase in the cost of electricity;  3) dramatic decrease in the cost of solar power equipment;  4)  the ability to sell excess power to the power grid and factor in a revenue stream;  or 5) the ability to sell carbon credits and factor in corresponding revenue streams.

CASE 2:  Change in the use of basic appliances.

The most energy consumptive household appliances are those that involve heat exchange or heating elements.  These include the following typical residential operations:  1)  refrigeration;  2)  climate control;  3)  cooking;  and 4)  hot water heating.  Since the stove in the household is natural gas, let us set it to the side for a moment—as well as climate control—and focus on refrigeration and hot water heating.  In the household in question there is one refrigerator and one freezer, both conventional appliances.  The hot water heater for the house is a standard electrical hot water heater.  Dramatic efficiencies are possible using appliance substitution as described below.

1)     Refrigeration.  Conventional AC-powered refrigerators and freezers consume power when the compressor is on.  With typical household usage, this corresponds to approximately one third of the time on average.  The typical power consumption of a standard refrigerator is 1000 watts, so presuming average usage patterns, the freezer and refrigerator in the household under analysis are expected to consume 480 kilowatt-hours per month.  For the month of April, this represents over 30% of the total home power use.  Refrigeration that is achieved using direct-current compressors is on average 20 times more efficient than conventional AC refrigeration.  By replacing the existing appliances with DC-refrigeration, an immediate monthly savings of 456 kw-hours is anticipated.  At the ballpark cost of $0.10 per kilowatt hour, the $550 annual savings pay for the two new appliances in four years, not counting any value in resale of the existing appliances.  Since the appliances have life expectancies of 10-15 years, DC refrigeration is clearly economically favorable over conventional refrigeration on virtually any time scale.  This, as it turns out, is a universal truism for refrigeration and should arguably be folded into domestic energy policy guidelines, eventually perhaps even legislation.  The sacrifice is that DC refrigerators (except for the very expensive Sunfrost) are traditionally chest-style in order to capitalize on the greater density of cold air to provide for greater efficiency.  While a chest freezer is unlikely to represent a dramatic lifestyle change, a chest refrigerator will require considerable acclimation and may be perceived by many as a quality-of-life sacrifice.

2)     Hot Water.  Traditional hot water heaters are by their very nature inefficient.  A reservoir of water is maintained at a high temperature at all times, whether it is being used or not, including during the night and during working hours.  On-demand hot water heating circumvents this intrinsic profligacy, but it also requires smaller heaters distributed at points of use throughout the house.  Electrical hot water heaters are dramatic power consumers.  Like refrigerators, the power is used only when the heating element is on, so the actual consumption varies as a function of:  1)  amount and frequency of hot water usage;  2)  temperature gradient between feed and product water;  and 3)  the efficiency of insulation of the tank.  However, it is reasonable to assume that in a 24 hour period, a conventional hot water heater will operate an average of three hours per day.  At 7500 watts, this amounts to a whopping 675 kilowatt-hours per month.  For the month of April, it is expected that hot water heating and refrigeration alone account for 80% of the total power bill.  Whereas stand-alone passive solar hot water is not able to satisfy 100% of hot water demands in temperate and cold climates, it is capable of reducing power consumption by a factor of ten.  At a savings of $61 per month, it will take thirty months to recover the costs of a passive solar hot water addition to the house.  Unlike refrigeration, however, there are no quality-of-life issues associated with this transition, since the unit would be connected to the existing hot water heater and all changes would be opaque to the user.

3)     Solar System Sizing.  Continuing to disregard the 800-pound gorilla of climate control, the abstraction of 80% of existing power demands by employing DC refrigeration and solar hot water heating means that in April, assuming a seven hour solar day, all of the home’s power could be supplied by a 1.3 kilowatt charging source, the equivalent of seven 200-watt solar panels, eight in practical terms for the preservation of design symmetry.  However, during August, at least 26 solar panels would be required to provide the power necessary for climate control and the remaining energetic needs of the home.

Case 3:  Geothermal Heat Pump

Air conditioning and heating systems operate on the basis of transferring heat from one place to another.  Even in freezing temperatures, air possesses heat, so a home heater functions by concentrating the heat present in the cold outside air and pushing it into the house.  In the summer the exchange direction is reversed to remove heat from living quarters.  The energy required to “pump heat” in both cases varies as a direct function of the difference in temperature between outside and inside temperatures.  Therefore on very cold and very hot days, great amounts of energy are required to sustain comfortable living temperatures inside a home.  However, even during the hottest and coldest days of the year, the temperature a few feet underground is always constant and is equal to the mean annual temperature of the latitude in question, 56 degrees Fahrenheit in the case of Mountain View, Arkansas.  This means that a heat pump that uses the earth’s temperature as its basic heat source and sink requires much less energy during very hot and cold days to achieve internal climate control.  In fact, it requires less energy year round.  This reality is the theoretical underpinning of the most efficient technology for climate control available to man, the geothermal heat pump.

While results and economics vary according to site circumstances and home characteristics, a retrofitted heating and cooling system predicated around a geothermal heat pump alone can be expected to reduce air conditioning and heating utilities by as much as 80%.  If a geothermal heat pump is used in conjunction with passive solar radiant heating, even greater savings are possible during wintertime heating.  For the present case, if a full heating/cooling home retrofit were undertaken, an 80% decrease in power demands during the summer would mean that the entire household (presuming solar hot water and DC refrigeration) could be operated on as few as six 200-watt solar panels. 

A capital cost for retrofitting the home with a geothermal heat pump will naturally vary according to circumstances specific to the location.  If we assume a capital investment of $20,000 for this switchover, the annual savings of around $2500 imply a payback period of eight years, discounting energy inflation, and also would imply a reduction in the carbon footprint of the house on the order of 75% even without removal of the natural gas burner.

Case 4:  The Vegan Environmental Take-No-Prisoners Option

The preceding analysis reveals that with non-trivial capital investment and a concerted home operations model, energy consumption can be dramatically reduced.  The primary planetary driver of improvements in efficiencies is not necessarily the energy itself or its cost of production but rather the emissions produced by fossil fuel combustion to generate the electricity that is being used.  The State of Arkansas, for example produces 57.3% of its power with fossil fuels.  And while natural gas is a cleaner fuel than coal, oil, or even gasoline, it is still a fossil fuel.  Its combustion produces carbon dioxide if not particulates, soot, and other aerial pollutants and very definitely contributes to global warming.

The most environmentally conscientious solution to home retrofitting and operations, therefore, is to eliminate all fossil fuel combustion from within the household and to replace all sources of electricity that depend on fossil fuel combustion with clean sources of electricity and to cut ties with the electrical grid, at least in places like Arkansas, where it is energized mostly through the combustion of fossil fuels.  Without severing ties with the existing grid, it is not possible to achieve a carbon free footprint without buying into the debatable exercise of the emerging carbon-credit and exchange paradigm associated with reforestation, carbon fixation, and other activities not accessible to typical homeowners.

To go tree-frog viridian (as opposed to simply green) requires replacement of the natural gas heater with an electrical and/or solar radiant alternative and the replacement of the natural gas stove/oven with an electric stove.  Both of these changes will increase the electrical power demand and require a greater number of panels to compensate for the displaced natural gas.   However, the value added is in the elimination of the final remaining carbon footprint associated with typical home operations.

In the case of wintertime climate control, a solar-hot water heat exchanger is an ideal alternative for optimizing climate control at the lowest possible supplemental energy investment.  A more sophisticated analysis will be required to determine precisely how many additional solar panels will be required for carbon-free wintertime climate control for that heating component that geothermal/solar hot water heat exchange is unable to completely achieve. 

And while an electric stove is one of the most electrically consumptive of all appliances, all it takes is a few more panels and a somewhat amplified power inversion capacity to eliminate a carbon footprint from the preparation of household food.  If we assume that each meal uses on average two burners for a total of one hour for each meal, that there are three meals prepared per day, and that the oven is used for one hour per day, this daily power sink of 7 kilowatt-hours is equivalent to five solar panels if we assume an average 7-hour solar day.  While induction technology exists that is capable of reducing power consumption by 50%, with ancillary benefits, induction heating requires the procurement of a costly appliance and special cookware and may represent an undesirable quality-of-life commitment. 

It is estimated that a completely retrofitted, zero-carbon emission home, that incorporates all the different elements discussed in this report would by adequately powered at all times of the year with a 2.4-3.2 kilowatt system, that is 12-16 200-watt solar panels.  Since hot water heating, electrical clothes drying, and electrical stove, all require 220-volt service, the power inversion stack must include a minimum of four 3.5 kilowatt, 48-volt Outback inverters.  This will provide for up to 7 kilowatts of 220-volt power demand, 14 kilowatts of 110-volt power demand, or some range of both up to those capacities.  For greater instantaneous capacity, the inverters stack could be boosted to six, eight or as many as ten stacked inverters.  Equipment costs of such a system are estimated below:

SUMMARY AND CONCLUSIONS

The lesson in the analysis is that it is very expensive to retrofit an existing house to reduce its carbon footprint to zero.  While new homes can be built and outfitted at lower capital costs since no replacement is required, costs remain non-trivial even when starting from scratch.  The most expensive capital investment is with a solar power system.  The most invasive of changes to the home itself is in the replacement of the home climate control system to incorporate geothermal heat pump technology.  And perhaps the greatest quality-of-life considerations are with the use of non-familiar appliances, like DC-refrigeration, electrical or induction stoves and cookware, and perhaps the simple discipline of energy conservation an lifestyle compromises.  Nevertheless, it is entirely reasonable to completely eliminate a home’s carbon footprint and to do so within a payback period of 20-30 years without factoring in energy inflation and without considering any value to the carbon not emitted.  The estimates are for a Retrofit and New Home.   For new homes the costs shown are not the actual cost estimates of the equipment but the amount above the cost of conventional equipment. 

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