Home Energy Awareness
by Chuck Wright
The evidence for climate change due to human activities is becoming stronger each year. Most scientists who study global climate agree that the effects are real.
It has been calculated that in order to stabilize the global climate by the end of the 21st century, atmospheric carbon emissions need to be reduced 60 percent from current levels. What this means is that if all citizens of Earth are allowed to emit the same amount (just to be fair), and the total amount is cut back by 60 percent, then we Americans need to reduce our emissions to about a tenth of what they are now.
Every major city has its dome of brown air, the result mainly of fossil fuel combustion. The health consequences of living in this are becoming increasingly well known.
Although we burn fossil fuels as though they are limitless, they are in fact finite. In the context of several thousand years of human history, our last 100 years of exponentially growing energy consumption is notable.
For example, most assessments of "Estimated Ultimately Recoverable" oil are around 2000 billion barrels, meaning that 2000 billion barrels is all the oil that can ever be recovered. If this sounds like a lot, consider some facts:
To use half of this major resource in less than one percent of human history is certainly an amazing achievement, but doesn't inspire confidence in a long future!
Americans are excessively wasteful in our use of energy. Per capita, we use twice as much as Europeans, and ten times as much as the Chinese (though they are trying hard to catch up!).
Energy Basics - Watts, Kilowatt Hours, Therms
Power Versus Energy
Power and energy are commonly confused, but before we start a discussion of energy in the home, we need to get straight on what they are and how they are related.
You can think of energy as a "stuff" that you can buy, transport, or use, like gallons of gasoline. Power is simply the rate at which energy is delivered, like gallons of gasoline per minute. (This is a good analogy, because a gallon of gasoline contains a particular amount of chemical energy, which, if burned, will be released as heat).
So, Kilowatts are power, Kilowatt Hours are energy. BTUs are energy, BTUs per hour (or BTUH) are power.
If someone tells you that he used 5 kilowatts (a unit of power) today, you should be confused, because he is. This is a meaningless statement. It would be meaningful if he said he used 5 kilowatt hours (a unit of energy) today, or that he used 5 kilowatts average for an entire day (which would be 5 * 24 = 120 kilowatt hours).
Laws of Thermodynamics
You may have heard of the "Laws of Thermodynamics", and probably think that they are just for engineers. In reality, these are fundamental to everyday life. The First Law says simply that energy can change form, but cannot be created or destroyed.
The Second Law says basically that any energy conversion is inefficient: some of what you start with always gets thrown off as heat.
These are not difficult concepts, but they are far reaching. They govern how a power plant operates and how your car engine works. They also tell you that the guy who wants you to invest in his "Energy from Air" scheme is either sadly off base, or a crook.
These basic principles were worked out about the time that humans were starting to harness steam. Since then, they and the body of engineering that has grown around them, thermodynamics, have proved to be fundamental. You should have your alarm bells well-tuned to hints of violations of these basic laws. Lots of people have tried to cheat on them, but nobody has ever succeeded!
Forms of Energy
Energy can appear in a variety of different forms, and one form can usually be converted to another, but always with some loss:
Note that all of the fossil fuel energy that we use arrived at Earth in the form of solar energy, a long time ago. Plants used sunlight to break apart Carbon Dioxide from the air, storing the carbon and releasing oxygen. A small amount of this carbon was covered up and eventually ended up as coal, gas or oil.
Some other interesting representations of the Kilowatt Hour can be found at http://www.wattsonschools.com.
For your home, you probably buy energy (in the form of
electricity) from the electric company. Electricity is
sold in units of the "kilowatt hour". If you run a 1000
watt (1 kilowatt) appliance for an hour, you have used a
Natural Gas Energy
You may also buy energy in the form of Natural Gas. Natural Gas is primarily Methane, which is a gas composed of Carbon and Hydrogen (CH4). It can be burned to produce heat.
Natural Gas is commonly sold by the amount of heat energy that can be obtained from burning it, specifically in units called Therms" (a Therm being 100,000 BTU). If the gas company wanted to be less confusing, they could sell it by the kilowatt hour, instead. A therm is about 29.3 kilowatt hours.
Natural Gas is sometimes sold by a unit called "CCF" which is a hundred cubic feet. It happens that a hundred cubic feet (envision a cube about 4.5 feet on a side) holds about a therm of energy.
One reason that people find concepts of energy so confusing is that there are so many different units. Some of the units are Joules, BTUs, Therms, and Kilowatt Hours. Just remember that although different units are commonly used to represent different forms of energy, this is only for historical reasons. In reality, you can express any form of energy in any unit, and freely convert between units.
If you are interested in energy and power conversions, you might want to look at http://www.infinitepower.org/calc_watts.htm.
Behind The Scenes
Think about turning on a light switch. You are familiar with what happens next. The light comes on. That's all you see, but there's more to it.
When you turn on the switch, the current that flows through your light bulb comes from a power plant somewhere. As a result of the extra load that you personally place on the electric grid, the power plant has to stoke up their fire a little bit. Granted, in the context of a billion watt power plant, your 100 watt light bulb isn't very noticeable, but it definitely causes things to happen. What?
Suppose your power plant burns coal. The heat from the coal burning boils water to produce steam. The steam runs a turbine, and the turbine turns a generator to produce electricity. When you use a kilowatt hour of electricity, you burn about 1.2 pounds of coal, on average.
Your power plant might also burn natural gas. It works just like coal-fired plants, but the heat comes from a big gas flame. If your power plant burns natural gas, that kilowatt hour of electricity requires about 10 cubic feet of gas.
Think of a 100 watt incandescent light bulb. It probably has a lifetime of about 750 hours (about a month of continuous operation). When you buy that light bulb, think of your committment to buy a 90 pound sack of coal to provide the electricity to light it (not considering the energy used to manufacture it, of course). Or, 825 cubic feet of natural gas (a cube about 9.4 feet on a side).
Of course, burning of the fossil fuel to produce the electricity
causes emission of various pollutants. The following table summarizes
typical emissions of gas and coal plants, as well as those for
two Austin area electric providers.
Something else that you may not realize. On average, the power plant converts only about a third of the heat from the fuel that it burns into electricity (recall that Second Law of Thermodynamics). When you turn on the 100 watt bulb, 200 watts get thrown off as heat at the power plant. Power plants use massive amounts of water to cool themselves. When you use a kilowatt hour of electricity, the power company uses about 0.86 gallons of water. So, your 100 watt light bulb also comes with about 64 gallons of water use.
Nuclear power plants are like coal and gas plants, except they get heat for making steam from a controlled nuclear fission reaction. Especially as concern over carbon emissions increases, advocates of nuclear energy point out that nuclear plants have no emissions of CO2 or any of the other pollutants that fossil fuel plants do (other than a heavy use of cooling water).
Compared to the waste from fossil fuel plants, the actual amount of waste from a nuclear plant is tiny. A gigawatt nuclear plant, for example, is said to produce only about a cubic meter of waste in a year. Well, it's actually bigger than that, because you have to spread it out to avoid a nuclear chain reaction.
Unfortunately, the other facts about nuclear waste are very difficult for humans to put into perspective. The small amount of it that exists is extremely dangerous and remains that way for a long time (thousands of years), needing protection and maintenance many times longer than any political entity in history has ever been stable.
Equally unfortunate is the fact that the only discussion presented to the public is heavily slanted either in favor or against.
Look Around Your Yard
Take a look at your yard and think about trimming grass and tree limbs. Think about the process that created them. Inside each leaf, there is a little factory, powered by sunlight, that removes carbon from the air, combines it with water from the ground, and spits oxygen back into the air. Think of the amount of carbon that all of your yard waste represents. Compare it to the tons of carbon that your home energy habits, driving habits, and industrial activities to support your lifestyle are throwing into the air. Ask yourself where your personal forest is that brings things into balance.
http://www.infinitepower.org/calc_carbon.htm should help you study your personal carbon balance.
Consumption Around the House
Heating and Cooling
You probably use an air conditioner in the summer. You
may or may not have paid attention to what it does. It pumps heat
out of your house and dumps it outside. The following figure illustrates
Qc represents the rate of heat removal from the house. W is the electrical power used to run the Air Conditioner. The efficiency of the unit is Qc/W. In typical home units, this is typically about 3. The industry rates efficiency using Energy Efficiency Ratio, which uses units of BTU per hour for heat flow rate, and watts for power input. Knowing that a BTU per hour is 0.293 watts, a unit with of efficiency of 3 has an EER of 3/0.293, or about 10. (in SEER, S stands for Seasonal, so SEER is an average of EER over a season).
Supposing that an air conditioner is rated at 42,000 BTUH cooling capacity, and has an EER of 10. It requires 42000/10 = 4200 watts input. If we go back to our earlier discussion of power plants, we can see that if this unit runs half time for a month, we have used 360 kilowatt hours of electricity, burning
When you heat with natural gas, the flame heats a "heat exchanger", through which air is circulated. Uncommonly realized is that a certain amount of heat goes up the stove pipe. The best modern furnaces claim efficiencies of 95 percent (though in the past, they were much lower). Assuming you have a high effiency furnace, delivering 100,000 BTUs of thermal energy to your house requires close to tht amount of natural gas energy. This would be about 105 cubic feet of gas. (but with an older, 70% efficient furnace, it would be more like 143 cubic feet).
Electric Strip Heat
Suppose you heat with electricity ("strip heat"), which has a reputation for being expensive and wasteful. Is it? In fact, yes. Electric heat in your house is close to 100 percent efficient. But remember that the power plant has to burn about three times the amount of fuel than the energy that is actually delivered. Getting 100,000 BTU of heat in your house ultimately requires 300 cubic feet of gas, two to three times as much as if you used the gas directly. So yes, electric strip heat is indeed wasteful and expensive.
A more efficient way of heating with electricity is to use a
heat pump. The Heat Pump is just an Air Conditioner running in
reverse. It pumps heat from the cool outdoors to the warm indoors.
The following diagram illustrates this.
Like the air conditioner, it is possible to pump more heat energy than electricity is used to run the cycle. When running in this mode, the efficiency of the cycle is Qc+W/W. As with Air Conditioners, efficiencies of 3 or more are possible.
Consider a heat pump powered by electricity from a gas-fired power plant. A given amount of electrcity requires three times that much energy in gas being burned. But, the heat pump delivers three times more heat than the incoming electric energy. Overall, the efficiency is close to unity, better than the average gas furnace.
Although electric heat using a heat pump is still somewhat more expensive than heat from natural gas, the overall efficiency of fuel use is comparable. Environmentally, it is similar a high efficiency gas furnace. Still, expect to pay quite a bit more. 100,000 btu of heat is about 29.3 kilowatt hours. Electrical input will be about a third of this, or 9.7, which will cost about 80 cents. The same amount of gas might cost about 50 cents, depending on where you are.
Think about why this might be: to heat with a heat pump requires a huge infrastructure of power plant and distribution network, compared to that of distributing gas, so it makes sense that, even given comparable overall energy efficiency, the gas would be cheaper.
Remember the laws of thermodynamics? They also say how efficient an air conditioner of heat pump can be. The larger the temperature difference across which it operates, the lower the efficiency. Heat pumps get less efficient when it is really cold outside, so they are best for moderate climates.
Some companies make "ground source" heat pumps. Instead of hanging their outdoor coils in the air, they couple them to the ground, which has less temperature variation than the air. By doing this, they are claimed to attain higher efficiency overall (though there cases in which this can work against them).
After heating and cooling, the next big household energy user is typically heating water. Let's take a look at the energy required.
Consider a nice, hot shower. Assuming 2 gallons per minute flow rate, 105F shower water, and 75F for unheated water, this water is carrying energy at the rate of 8,500 watts.
If you are heating with electricity, a 6 minute shower then represents the burning of about a pound of coal or 8.5 cubic feet of natural gas. If you are heating with gas, it represents about 4.3 cubic feet of gas.
Take a look at your water heater. If it holds 50 gallons, and the water inside is at 130F, and the incoming water was at 75F, and it is electric, its contents represent 6.4 kilowatt hours of electricity, amounting to 7.7 pounds of coal burned at a coal-fired plant, or 64 cubic feet of natural gas from a gas-fired plant. If the water heater is gas-fired, its contents represent about 33 cubic feet of gas.
You have certainly heard of ompact fluorescent lamps. They are very small, and can in many cases directly replace an incandescant light bulb.
You may have also read the "hype" on the package, about how much money they will save you. It probably sounded too good to be true.
Well, in reality, it wasn't hype, and they save a great deal of electricity. Here is how to calculate it.
Suppose you have a 100 watt lamp. You will certainly use it until it burns out. Over the estimated 750 hour lifetime of this bulb, and assuming the bulb costs 75 cents, with electricity costing 8 cents per kilowatt hour, this bulb will cost you
$0.75 + (0.1 kw * 750 hours * $0.08/kwh) = $6.75
A compact fluorescent bulb is rated at 10,000 hours of life. The cost of incandescant bulbs and their energy, over 10,000 hours, is $90.
The cost of a CF lamp is about $10 these days. It uses a fourth to a third the electricity of the incandescant. Assuming a third, the electricity cost of the CF light is a little less than $27 (at $.08/kwh). Adding the cost of the lamp, the total cost for 10,000 hours of light is $37, or $53 less than the incandescant. The CF lamp itself costs about the same as the 13 incandescants that it replaces. The rest is gravy. Who wouldn't pick up a $50 dollar bill lying on the ground?
Perhaps you mistrust the claim of a 10,000 hour life? How long does it take for the CF to recoup its investment? It turns out that the CF only has to last about 2.5 times as long as the incandescant to break even. The rest is gravy.
Drying the Clothes
I have an electric clothes dryer. Drying a large load, it uses about 5 kilowatt hours, according to my electric meter. As with other appliances around the house, we can look behind the scenes. If powered by a coal-fired plant, this load of clothes burns about 6 pounds of coal (and produces 110 cubic feet of CO2). If from a gas-fired plant, that's about 50 cubic feet (a cube about 44 inches on a side).
Your dryer might also get its heat from natural gas. Supposing that it is 70 percent efficient, you will have used about 24 cubic feet of gas.
Americans, and particularly Texans, travel a lot. If you add up the fuel value of all of the gasoline that you use, it probably matches the results of all of your home energy use. The solutions are simply a matter of doing it:
Modern economic theory says that consumption is good. The more the better. In an infinite world, perhaps. This thought is in conflict with the notion of a finite planet. Just as there is a power plant behind that electric switch, there is energy expenditure behind every item that you buy.
Detailed discussion of this is not the topic here, but it is something everyone should think about.
Central Texas Climate
Every climate is different. If you want to be comfortable at home while using energy efficiently, the first step is understanding the local climate. Everyone knows how hot it is in Texas. Most people spend quite a bit more running air conditioners than on heating. I have heard many times that "Texas is a Cooling Climate". But, rather than just accept this and buy a big A/C, let's try to understand it.
When calculating heating or cooling load, a basic concept is the "degree day", calculated by multiplying the difference between indoor and outdoor tmperature by the time of that temperature difference. If your indoor-outdoor temperature difference is 10 degrees for a week, for example, that is seventy degree days. Degree days indicate roughly how much heat will flow across a partition, and so are related to heating or cooling cost. Typically, both heating degree days (HDD) and cooling degree days (CDD) are specified.
For Austin, Texas, the National Renewable Energy Laboratory reports that an average year has HD=1688 and CDD=3016 confirming that we need to spend much more cooling than heating. Right? Well, not really. HDD and CDD are always specified against a particular indoor reference temperature. By convention both of these are specified against a a base of 65 degrees F. Most people keep their houses warmer than this in the winter (perhaps 68?), and a lot warmer in the summer (say, 78).
If you recalculate degree days for these more typical conditions (68F heating, 78F cooling), heating requirements are way up (HDD=2334) and cooling way down (CDD=749). This says that if you consider only air temperatures, central Texas is not such a hot place after all. In fact, considering air temperature alone, we should probably use about 3 times as much energy heating as cooling.
HDD and CDD are interesting, but our practical experience is that cooling is indeed a problem. The degree day numbers simply tell us that something other than outside temperature needs to be considered if we seek effective cooling in Central Texas. The place to look is the sun. This can be viewed as an opportunity: if we can effectively control the summer sun, a large part of our cooling problem is solved.
The following section examines windows, because they are a large part of the regular thermal load, and because they are such a huge contributor to unwanted heat gain in the summer. If we wish for winter heat gain, they can help there, too.
Take a look at your windows. What's to see? They give you a view, and maybe some ventilation, and some light. Is that all? Let's take a look at windows in terms of their operating cost or benefit, and in terms of their environmental effect.
Think about the window when it's hot outside and cool inside, or vice versa. What happens? Heat flows through the glass, in proportion to the temperature difference across it and to its area. Some windows are better than others, but all windows conduct far better than an average wall.
Let's consider the monetary cost of a couple of the basic aluminum frame, single-pane window, assuming we have a heat pump. At Home Depot, such a window, 3 x 3 feet, costs about $30.
The yearly cost of this window (see calculations) is about $6.56. If we spend an extra $24 for double pane glass, our cost drops by about $2. This amounts to a return on investment of about 8 percent per year, more if fuel cost rises.
Suppose we spend an extra $20 for a better window, with insulating vinyl frame. These perform considerably better than the aluminum variety. Our operating cost drops to about $2.64. Overall, the investment in a higher quality window of $44 more than doubles the cost of the window, but the investment pays back at a rate of 9 percent per year. And, it saves 59 pounds of coal or 540 cubic feet of gas, 103 pounds of CO2 emissions, 42 gallons of water per year, plus reducing other pollutants.
The following table summarizes these results.
That's not all there is to windows, though. We need to consider the sunlight that goes through them. This goes into the house and warms it. In the winter this is good. In the summer, it's not. Of course, there's always the benefit of having daylight, which can offset energy used for lighting, and generally make the interior more pleasant. You can see that it's a complicated situation.
Still, it is interesting to consider the cost or benefit of the sunlight coming through the window. This is a complicated matter in itself, depending on the exact shading, orientation, and sun conditions. Average values are available, though, that will allow us to bound the problem somewhat. These are published by the National Renewable Energy Laboratory, in a publication called "Solar Radaiation Data Manual for Buildings". It presents radiation data calculated for various window exposures at each of 239 sites around the US. It is available at http://rredc.nrel.gov/solar/.
A South Window
Consider a south facing, double-glazed window with insulated frame, in Austin, with shading arranged to shade the window from direct summer sunlight. This window gains energy every month of the year, more during winter months. If we total the energy gain for the window during the months of May through September, we get a total heat gain of about 49,900 BTU/sq ft. To remove this heat will require about 43 kilowatt hours of electricity for the season.
For this time period, we calculated 8 kwh to overcome the conductive losses. The radiant load is indeed dominant. A small south window shaded from direct summer sunlight costs us $3.44 for the cooling season. Addressing summer radiant gain is clearly very important.
Before deciding to shade the window permanently, let us see what value it might be to us in the winter. Picking out the data from months November through March, we see that we have a benefit from the window of 96 kwh, worth $7.68, for the heating season. Not bad. Overall, the window benefits in winter more than it costs in summer. An ideal situation might be to add shading (solar screens? awning? creative planting?) during the summer. One would certainly hate to lose the benefit of the winter solar gain.
Consider our south window, now, including conduction losses. Overall, the window costs us $2.64 in yearly conduction losses, $3.44 in summer radiant gain, and saves us $7.68 in the winter, so overall it saves us $1.60 per year. Any summer shading that we add to this south facing window adds directly to the economic benefit of this window.
This is clearly a much more complicated situation than presented here. Of special interest is the effect of vegetation on the window's gain.
It is especially important to note that simply shading a window from the direct beam radiation is not adequate. This south window is shaded to receive almost direct summer sunlight. The gain comes from diffuse and ground reflected sources.
A West Window
What about the dreaded West facing window? A similar analysis can be done for it, as shown in the calculations section. In the summer, this window is a disaster, costing over $8 to cool. West windows clearly need to have serious shading in the summer.
In the winter, it does produce some useful gain, worth about $5. Still, the difficulty of controlling summer gain make the west (and similar east) window troublesome.
The following table summarizes the performance of the windows
discussed above (3x3 foot, double pane, insulated frame, heat pump
heating, Tcool=78F, Theat=68F,
Electricity cost = $0.08/kwh, locale = Austin, TX).
Insulation as an Investment
Whenever there is a temperature difference across any surface, heat flows. If the surface is a wall or ceiling of a house, the heat usually flows in the "wrong" direction. When we run heating or air conditioning to maintain the temperature difference, we pay money to support this heat transfer.
We can reduce this heat flow by adding insulation. The insulation reduces the heating or cooling cost, but it costs something itself. When installing insulation, we are investing money in a home improvement that will have a measurable return.
http://chuck-wright.com/calculators/insulpb.html will help you calculate the monetary return on an insulation investment.
Consider an attic in an old, 1500 square foot house in Austin. Suppose the house is maintained between 68F and 78F, and that it has no insulation in the attic. For these inside conditions, Austin has 2334 Heading Degree Days and 749 Cooling Degree Days. Adding R19 to this attic (assuming initially R3), at a cost of $500, will save 42 percent of the original investment each year, with gas heat, almost 50 percent with a heat pump, and a whopping 125 percent if you use electric heat (in which case you might also consider an investment in a heat pump!).
Going back to that coal power plant, we can see that we would have saved the burning of about 11,000 pounds of coal, and the emission of almost 20,000 pounds of Carbon Dioxide.
Let's consider that old, 1500 square foot house again. Perhaps the weather stripping isn't too good, and there are some places to caulk. Home energy experts talk about house tightness in terms of Air Changes per Hour (ACH). It can be measured under standard conditions, but it depends heavily on weather conditions, so this calculation is a little flaky. Even so, we plunge ahead. Suppose our house has 2 air changes per hour, and we live in Austin, with HDD=2336 and CDD=749.
The calculation in the appendix shows that the annual energy expenditure for this is about 23.9 million BTU (7000 kilowatt hours) for heating, and 7.7 million BTU (2250 KWH) for cooling.
We know how to calculate the fuel consumption a variety of different ways. For a heat pump / AC with an efficiency of 3, this air exchange will cost us 3700 kilowatt hours of electricity, worth $296. If the electricity comes from a coal power plant it will burn 4,440 pounds of coal, and emit 7800 pounds of CO2.
A couple of hundred dollars invested could probably cut this substantially.
Earlier, we discussed Degree Days, and how they are related to energy use. Although it is only partly true in the summer, because of high radiant load, it is quite true in the winter.
We also discussed how Degree Days vary with selection of the base temperature. The following table shows heating and cooling degree days recalculated for a variety of temperatures, for Austin, Tx.
Suppose we are willing to put on a sweater in the winter and lower our thermostat from 68F to 66F. Seeing Degree Days decline by about 13 percent, we would expect our heating energy to decline similarly.
All through your house, you are likely to have very small, "phantom" loads that consume power continuously. Little transformers for small electronic devices, televisions, audio systems, all consume small amounts of power. The problem is that they consume it continuously.
Here is a table showing some phantom loads in my house.
Together, these four appliances use only 11.5 watts. But over the period of a month, they waste 8 kilowatt hours, representing consumption of 10 pounds of coal or the equivalent in other fossil fuels, plus the pollutant emissions that their burning causes.
Appendix A: Selected Calculations
Heat balance a single pane, Al frame, 3 x 3 foot window
Conductive losses. Cost: ~$30 U =~1.27 BTU/Hr Ft**2 F To simplify, combine heating and cooling, assume heat pump with same COP heeating ad cooling. Heating/cooling cost = 9 ft**2 x 1.27 x 3083 DD x 24 (H/D) x 0.000293 kwh/btu x .33 (heat pump/ac cop = 3) = 82 kwh 82 kwh x 1.2 pounds coal/kwh = 98 pounds coal 82 kwh x 11 cu ft gas/kwh = 902 cubic feet gas 82 kwh x 0.08 $/kwh = $6.56
Heat balance a double pane, Al frame, 3 x 3 foot window
Conductive losses. Cost: ~$54 U =~1.27 BTU/Hr Ft**2 F Heating/cooling cost = 9 ft**2 x .87 x 3083 DD x 24 (H/D) x 0.000293 kwh/btu x .33 (heat pump cop = 3) = 56 kwh 56 kwh x 1.2 pounds coal/kwh = 67 pounds coal 56 kwh x 11 cu ft gas/kwh = 616 cubic feet gas 56 kwh x 0.08 $/kwh = $4.48
Heat balance a double pane, vinyl frame, 3 x 3 foot window
Conductive losses. Cost: ~$74 U =~1.27 BTU/Hr Ft**2 F Heating/cooling cost = 9 ft**2 x .51 x 3083 DD x 24 (H/D) x 0.000293 kwh/btu x .33 (heat pump cop = 3) = 33 kwh 33 kwh x 1.2 pounds coal/kwh = 40 pounds coal 33 kwh x 11 cu ft gas/kwh = 360 cubic feet gas 33 kwh x 0.08 $/kwh = $2.64 Cooling cost alone = 749/3038 * 33kwh = 8 kwh
Pollution savings of window investment
Based on conductive losses. kwh savings = 82 - 33 = 49 kwh per year Coal Savings = 59 pounds Nat Gas savings = 542 cu ft CO2 Reduction (coal) = 103 pounds Water savings = 42 gallons
South Window in Austin
Detrimental and beneficial heat gains from solar radiation. NSRDB monthly average transmitted radiation for shaded window JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC btu/sqftday 830 700 410 320 320 320 320 330 340 620 840 860 Days 31 29 31 30 31 30 31 31 30 31 30 31 Heating * * * * * Cooling * * * * * Cooling gain = sum of gain * month length = 49,900 BTU/sq ft x 9 sq ft x .000293 kwh/btu x .33 = 43 kwh Heating gain = sum of gains * month length = 110,600 BTU/sq ft x 9 sq ft x .000293 x .33 = 96 kwh
West window in Austin
Detrimental and beneficial heat gains from solar radiation. NSRDB monthly average transmitted radiation for shaded window JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC btu/sqftday 430 530 650 720 760 830 830 780 690 620 480 400 Days 31 29 31 30 31 30 31 31 30 31 30 31 Heating * * * * * Cooling * * * * * Cooling gain = sum of gain * month length = 119,000 BTU/sq ft x 9 sq ft x .000293 kwh/btu x .33 = 104 kwh Heating gain = sum of gains * month length = 75,650 BTU/sq ft x 9 sq ft x .000293 x .33 = 66 kwh
Calculation of energy flow rate (power) to a shower. Assume: flow = 2 gallons per minute initial water temperature = 75F shower temperature = 105F duration = 6 minutes = 0.1 hour Heat flow rate = 2 * (105 - 75) * 1 BTU/#mF * 8 #m/gal * 60min/Hour = 28,800 BTU/hour = 8,400 watts. Multiply by 0.1 to get 0.84 kwh Coal use ~= 1 pound
Assume: volume = 50 gallons ( * 8 pounds/gallon) = 400 #m temperature = 130F initial temperature = 75F Heat content = (130 - 75) * 1 BTU/#mF * 400 = 22,000 BTU * 0.000293 = 6.4kwh Electric water heater: Fuel = 7.7 pounds coal or 64 cubic feet of natural gas Gas water heater: Fuel ~= 22,000 BTU * 1 therm/100,000BTU / 0.65 (efficiency) * 100cu ft/therm ~= 33 cubic feet.
Calculation of Energy Cost of Infiltration
Assume: HDD = 2334 CDD = 749 House Volume = 1500 sq ft * 8 ft Air Changes per Hour (ACH) = 2 Specific Heat of Air = 0.2403 BTU/#m Specific Volume of Air = 13.5 ft**3/#m Heating loss = 2 ACH * 1500ft**2 * 8 ft * 24 H/D * 1#m/13.5ft**3 * 0.24BTU/#mF * 2334HDD = 23.9E6 BTU = 7,000 KWH Cooling loss = 2 ACH * 1500ft**2 * 8 ft * 24 H/D * 1#m/13.5ft**3 * 0.24BTU/#mF * 749HDD = 7.7E6 BTU = 2.25E3 KWH Combined --> 31.6E6 BTU, 9.25E3 KWH
Appendix B: Resources
Intergovernmental Panel on Climate Change (IPCC), http://www.ipcc.ch. Instead of reading what everyone else is saying about what the IPCC says, why not read it for yourself? I find this material to be very conservatively written and to have the "ring of truth".
National Renewable Energy Laboratory - Renewable Resource Data
Texas Solar Energy Society:
BP Review of World Energy
Software to help you become more energy aware: