Monday, July 31, 2006

My Solar Photovoltaic System. Past, Present, and Future.

My solar photovoltaic (PV) system started out small, and is periodically upgraded as funds become available. When power outages have compelled me to use the system I’ve been well pleased with its performance, but at the same time I’ve wished that it could do more. With that in mind, I’ll attempt to outline the past, present, and future capabilities of the system.

Initially, my system components included one solar panel, a charge controller, some gel-cell batteries, and a 600-watt modified sinewave inverter. With the appropriate wiring, panel mounting hardware, and miscellaneous items, the total cost of the system was about $600.00. The batteries were throw-a-ways that I got from the IT department where I work. They were not like new, but still had some life in them.

Using a couple of 75-watt light bulbs as a test load, I found that the battery bank would last for just a little over an hour. While the solar panel was capable of producing more than 400-watts of power on a sunny day, the fully-charged batteries were only able to deliver about 150-watts. Clearly, the battery-bank needed to be upgraded.

I replaced the gel-cell battery bank with two marine deep-cycle batteries. The rated capacity of these is 105ah each, for a total of 210ah. Using the 150-watt test load, the battery bank lasted eight hours. Battery capacity jumped from 150-watts to 1200-watts (150-watts times 8-hours). However, since my single solar panel can only produce about 400-watts of power on a sunny day, it will take three days to fully charge a depleted battery bank. Still, the capabilities are much greater than they were before the battery upgrade. I need not worry about running out of power when the load consists of low-power devices such as compact fluorescent (cf) lamps. The cost of the battery upgrade, including wiring, was about $150.00, and was well worth it.

A recent upgrade included another solar panel and two more marine deep cycle batteries. I now have 800-watts (per sunny day) of charging capacity, and the ability to store 2400-watts of power in the battery bank. The system is limited in that it will take three days to fully recharge a depleted battery bank, but I have an abundance of power. During a recent power outage, I ran several cf lights, fans, a radio, and a cell-phone battery charger. I even connected a TV for a short time. The drop in battery voltage by morning was hardly noticeable.

My next goal is to be able to power our small chest freezer in the event of an electrical grid failure. I’ve measured the power consumption of the freezer, and found that it uses about 0.76 Kilowatt hour per day. This is just slightly less than the total daily capacity of the two-panel array that I currently have. Without another system upgrade, I’ll have very little power left over for lights and other things. Still, running the freezer is better than watching food spoil. And, if you consider that my fully-charged battery bank will supply enough power to run the freezer for more than two days, I should be OK for the short term power outages that we typically experience.

The inverter that I’m currently using may not be able to handle the high motor-starting current of the freezer, so my next step will be to purchase a 1000-watt true sinewave inverter. I’ll also install another solar panel so that I won’t have to skimp on lights and other low-power devices in order to use the freezer. This upgrade will also allow me to use a small microwave oven, and other high-power devices for a short period of time. At this point I’ll have just over two thousand dollars invested. The performance of the system in an emergency will be well worth it.

In the event of an electrical grid power failure in the winter, I’ll want the ability to use the system to provide heat for my home. Having installed a corn-burning stove this summer, the PV system will provide electrical power to the stove’s blowers, and auger. To meet the requirements of the stove, I’ll add one or more solar panels, and two more batteries. I’ll measure the actual power requirements of the corn-burning stove on high and on low settings later this year. The only time I’ll need to conserve power is when cold weather forces me to use the stove for an extended period of time. At all other times I’ll have plenty of power for lights and small appliances.

If I had been thinking only in terms of emergency heat I would have opted for something other than a corn-burning stove. I could have installed a wood-burning stove which could have heated my entire home without any electrical power requirements. However, wood burning is more work, dirtier, and not as safe as burning corn. My previously stated goal is to live comfortably in the event of an extended power failure. All of the chores that go along with wood-burning do not meet my definition of living comfortably.

For a little over $3,000.00 I’ll have a system that will meet all of my basic household electrical needs in the event of a grid power failure. Knowing that I could have opted for a generator for about a third of that, I have no regrets. While the initial cost of a generator would have been much less, the cost of fuel to run the generator would quickly surpass the cost of the PV system. And, fuel is not always available in an emergency. My power source, the sun, is free. It recharges the battery bank each sunny day.

Having met my emergency electrical needs, I’ll next focus on upgrades that will help to reduce my grid-powered electrical load on an everyday basis. I already use the system to provide power to my grow-lights for my plants, but it would be a shame to use the system for that alone. I want to replace grid-supplied power with solar-generated power as much as possible. I’ll run the chest freezer on a continuous basis and add wiring to my computer desk. I’m also considering the replacement of our refrigerator with a highly efficient Sun Frost model. I’ll add that to the solar-powered load. With periodic upgrades, I hope to power my entire house eventually.

I like the fact that my electrical and heating systems help to reduce carbon emissions, a leading cause of global warming. Someday, everyone will have to do their part in this effort. If voluntary efforts fail to produce the desired results, conservation will become mandatory, or perhaps electricity will become so expensive that grid-connected consumers are forced to cut back. Either way, I’m glad I have a head-start.


Friday, July 21, 2006

Renewable Energy to the Rescue (Again)

On April 4th a tornado ripped through my neighborhood resulting in widespread damage, and a prolonged power outage. This event provided me with my first opportunity to put my small-scale photovoltaic (PV) system to practical use. On July 19th, my area was hit with another severe storm, and another extended power outage.

Prior to the latest storm, I prepared an emergency kit. This kit includes three light fixtures with compact fluorescent (cf) bulbs installed, extension cords, power strips, and a radio. When the power goes out, I run the cords to three areas of the house, and connect the light fixtures. Because cf bulbs use so little power, I don’t worry about the drain on the system if the lights are left on.

This most recent power outage occurred during a heat wave. Since our central air conditioning was unavailable, I connected a couple of fans to the PV system. To keep track of the storms progress, I also connected a radio.

Because our telephone service is provided by the cable company, a power outage results in a loss of service. Fortunately, our cell phones continued to work, and the solar PV system provided power to keep their batteries charged.

I tried to use a microwave oven with the system, but the inverter couldn’t handle the power that it requires. It tripped the breaker. I’ll make a note to myself to get a more powerful inverter as funds become available.
Had the power outage lasted longer, I would have connected our chest freezer to the solar PV system. The energy-efficient chest freezer would have prevented frozen food from spoiling, and I could have made ice to use in a cooler in order to protect perishables from our refrigerator. To keep things humming along in the event of an extended power failure, I'll need one or two more solar panels. I've calculated that my battery bank will keep things running for at least 24 hours, but the additional solar panels are needed to keep the batteries fully charged.

My system doesn’t do everything that I want it to do at the present time, but still it is extremely useful. While most of my neighbors have only candlelight or flashlights, we have plenty of light. We have a source of power for a radio, and I’ve even connected a small TV. We can keep somewhat cool with fans, and can keep our cell-phone batteries charged. Our power source is quiet, and inexpensive to operate. One of my neighbors has a noisy, gas-guzzling generator. I wonder how much it cost him to run that now that gas prices are so high.

Eventually, I’ll have enough power to handle the electrical requirements of our corn-burning stove. This will serve as our emergency heating system in the event of a winter weather power outage. Meanwhile, it’s good to observe that when a power outage occurs, life remains fairly normal in our home.

Shown here are my batteries and control electronics.

Extension cords and sockets bring light to other parts of my home.

Wednesday, July 19, 2006

Automating Solar PV System Performance Tracking

After installing my small solar photovoltaic (PV) system, I found myself constantly monitoring array and battery voltages, and performing tests to assess the capabilities of the system. A typical test involves monitoring a voltage over a long period of time. As an example, I might want to know how long a given battery bank will last when connected to a known load. To perform this simple test a voltage reading must be taken every few minutes over a period of several hours. A notation is made when the battery bank voltage falls below a predetermined value. This is just too time-consuming. To automate this task, I purchased a Data Logger.

The range of the Lascar Voltage USB Data Logger is 0 to 32 volts dc, making it ideal for measuring array and battery voltage. Software furnished with the data logger allows the user to set up logging rate, ranging from one reading per second to one reading every 12 hours. Typically, I use the one minute sample rate.

Another handy feature of the Lascar Data Logger is the option to set the start time. I often want voltage readings to begin just before daylight, but I don’t want to wake up early in the morning to start the data logger. I program the data logger the day before, and connect it to the voltage source to be measured. The data logger remains idle until the preset date and time, and then begins measuring voltage at the preprogrammed sample rate.

The software supplied with the data logger runs under Windows 98, 2000 and XP. Software installation is easy, and it’s hard to imagine how the software could be easier to use. After connecting the data logger to a USB port, simply run the software and answer the prompts. You only need to enter a file name for the current test, upload date and time information, start time, and sample rate. Then, remove the data logger from the computer’s USB port and connect its alligator clips to the voltage source to be sampled. A flashing green light tells you that all-is-well. As it takes each voltage reading, the data logger creates a txt file. This is a type of file that can be viewed with any word processing program, or with a text editor, such as Microsoft’s WordPad program. The software also creates a printable graph from the data.

By maintaining a log of battery bank data, I now have a baseline with which to compare performance in the future. Using this information, I’ll know when it’s time to replace my battery bank. In the same way, analyzing PV array voltage over time can alert me to solar panel problems.

Measuring voltage is helpful, but it is also important to know how much power is being supplied by the array or the battery bank. Measuring power with the data logger is not as straightforward as measuring voltage, but it is possible. The first step is to add a resistor in series with the array or battery wiring. This must be a low-resistance, high-power resistor. Next, use the data logger to measure voltage across the resistor. Then, use Ohms Law to calculate current and power. In the example below, I used a one-tenth Ohm resistor, and I measured 1.2 volts across it.

First, calculate current: Current (I) = E/R or:

Voltage (E) = 1.2
Resistance (R) = 0.1
Current (I) = 1.2/0.1
Current (I) = 12 amperes

Then, calculate power: Power (P) = E times I or:

P = 1.2 times 12
P = 14.4 watts
Note: This is the amount of power dissipated by the resistor, not the power that the array produces.

Caution: Batteries and solar panels are capable of supplying a tremendous amount of power. Choose a resistor of sufficient wattage so that it will not heat up excessively. If in doubt, don’t perform these tests unattended. A fire could result.

Shown below are samples from a logging session. The left-most column is the sample number, beginning with the first sample. Next, the date and time are recorded, followed by the PV array voltage reading for that date and time. Notice that as the sun rose, the voltage readings increased. To avoid a long blog entry, I’ve only shown the first hour of here. Notice that the voltage rose sharply at 5:21:01.

1, 05/06/2006 05:00:01, 0.05
2, 05/06/2006 05:01:01, 0.05
3, 05/06/2006 05:02:01, 0.10
4, 05/06/2006 05:03:01, 0.15
5, 05/06/2006 05:04:01, 0.20
6, 05/06/2006 05:05:01, 0.30
7, 05/06/2006 05:06:01, 0.40
8, 05/06/2006 05:07:01, 0.50
9, 05/06/2006 05:08:01, 0.65
10, 05/06/2006 05:09:01, 0.80
11, 05/06/2006 05:10:01, 1.00
12, 05/06/2006 05:11:01, 1.25
13, 05/06/2006 05:12:01, 1.55
14, 05/06/2006 05:13:01, 1.85
15, 05/06/2006 05:14:01, 2.15
16, 05/06/2006 05:15:01, 2.50
17, 05/06/2006 05:16:01, 2.85
18, 05/06/2006 05:17:01, 3.25
19, 05/06/2006 05:18:01, 3.70
20, 05/06/2006 05:19:01, 4.25
21, 05/06/2006 05:20:01, 4.80
22, 05/06/2006 05:21:01, 12.50
23, 05/06/2006 05:22:01, 12.50
24, 05/06/2006 05:23:01, 12.50
25, 05/06/2006 05:24:01, 12.50
26, 05/06/2006 05:25:01, 12.50
27, 05/06/2006 05:26:01, 12.50
28, 05/06/2006 05:27:01, 12.50
29, 05/06/2006 05:28:01, 12.50
30, 05/06/2006 05:29:01, 12.50
31, 05/06/2006 05:30:01, 12.50
32, 05/06/2006 05:31:01, 12.50
33, 05/06/2006 05:32:01, 12.50
34, 05/06/2006 05:33:01, 12.50
35, 05/06/2006 05:34:01, 12.50
36, 05/06/2006 05:35:01, 12.50
37, 05/06/2006 05:36:01, 12.50
38, 05/06/2006 05:37:01, 12.50
39, 05/06/2006 05:38:01, 12.50
40, 05/06/2006 05:39:01, 12.50
41, 05/06/2006 05:40:01, 12.50
42, 05/06/2006 05:41:01, 12.50
43, 05/06/2006 05:42:01, 12.50
44, 05/06/2006 05:43:01, 12.50
45, 05/06/2006 05:44:01, 12.50
46, 05/06/2006 05:45:01, 12.50
47, 05/06/2006 05:46:01, 12.50
48, 05/06/2006 05:47:01, 12.50
49, 05/06/2006 05:48:01, 12.50
50, 05/06/2006 05:49:01, 12.50
51, 05/06/2006 05:50:01, 12.50
52, 05/06/2006 05:51:01, 12.50
53, 05/06/2006 05:52:01, 12.50
54, 05/06/2006 05:53:01, 12.50
55, 05/06/2006 05:54:01, 12.50
56, 05/06/2006 05:55:01, 12.50
57, 05/06/2006 05:56:01, 12.50
58, 05/06/2006 05:57:01, 12.50
59, 05/06/2006 05:58:01, 12.50
60, 05/06/2006 05:59:01, 12.50

Details from this session:

5:00:01 (am) Begin logging voltage at sunrise.
5:21:01 (am) Voltage sharply rises to about 12.5 volts just after sunrise.

Additional details from this session, but not part of the raw data shown above:

5:30 to 11:00 Power from the PV array tops off the battery charge.
11:00 Charge Controller senses a fully charged battery bank and reduces
charging current. Because the load on the PV array is reduced, PV
array voltage rises.
11:00 to 16:30 Clouds and shadows cause voltage fluctuations.
16:30 As the sun sets, PV array voltage drops.
20:30 PV array voltage drops to zero at dusk.

This data not only shows me that the PV array is producing power, but also that the Charge Controller is working properly. This is good to know. A faulty Charge Controller might damage the batteries, or cause them to produce a dangerous amount of hydrogen gas. Fortunately, I have an option to set alarms when programming the data logger. If the alarm feature is used, the data logger produces an audible signal when a preset low or high voltage set point is reached. By responding to the alarm, I can prevent further damage to the equipment, and avoid a buildup of hydrogen gas.

It would be better to monitor PV array and battery bank voltage simultaneously, but that would require a more expensive data logger. I’m pleased with the performance of my single-channel data logger. Over time, I’m compiling a useful log of system performance data. Since the amount of sunlight varies from day to day and season to season, each logging period produces different results. The load on my battery bank also varies over time, and logging voltage over time is the only meaningful way to monitor battery condition and performance. All things considered, the data logger was a very worthwhile investment.

Solar John

Thursday, July 13, 2006

Peak Oil: Bring it on! The sooner the better!

Oil is a finite resource, and many believe that the production of oil has peaked or will soon peak. From that point on, higher costs and shortages are to be expected. There are those who believe that the chaos caused by the sharp decline of oil production will result in the end of the world. Books have been written, and websites created, specifically dealing with the effects of peak oil. Sadly, most fail to recognize the positive effects of the end of cheap oil.

If we are to believe what we read, the industrial age is about to end, and we’re on the verge of returning to an agrarian society. So what’s wrong with that? If you’ve ever tasted fresh produce you know that it sure beats what you buy in the grocery store. Highly processed food lacks the nutritional value of the fresh variety, and often contains harmful preservatives and additives. HFCS, an additive in soft drinks and many other food products, is a major contributor to childhood obesity. In addition, genetically modified food products that line the shelves of grocery stores present a health risk. On the other hand, produce that’s grown locally, and perhaps organically, tastes better and is better for you. Organic methods will replace chemical fertilizers and pesticides, and the result will be greater yields and healthier food. As a bonus, the soil will be replenished instead of being used up as it currently is.

If the industrial age is to end abruptly, you may no longer have to commute to work each day. Can you imagine how that will improve your life? No more rush hour traffic. No more road rage. No more breathing exhaust fumes. You’ll have more time to spend with your family.

As less fossil-fuels are burned, the ozone layer will begin to heal, and the global warming trend will start to reverse itself. Some scientists believe that this is already hapenning due to a variety of steps that have already been implemented. The planet is, or will soon be, on-the-mend. However, if we substitute coal for oil, we might just erase all of the gains we've already made. This must not be allowed to happen.

When we stop buying oil, we stop sending money to countries that export hate and terrorism. I can hardly wait for that to happen.

This, of course, is an oversimplification. The transition will be difficult. However, it is possible to live a good life without petroleum-based products. Everything we currently do that involves oil can be done another way, and often in a much better way. The transition will open up new career opportunities, and not just in agriculture. Technically savvy folks will find interesting work in energy production, construction, service, and transportation. Those who are the quickest to accept post-oil technologies will be the most successful.

Cuba made a quick transition from a budding industrial power to an agrarian society when the collapse of the Soviet Union resulted in the loss of oil imports. We can learn a lot from their experience. Perhaps the most important lesson is that every individual needs to prepare, rather than to wait for the government to take care of them. Without food on the table, government programs are useless. Cubans who had never worked in agriculture before learned to grow food crops, and other survival skills.

The sooner we start to look beyond petroleum, the better. And, since our elected leaders don't seem to grasp the urgency of the current siuation, it is up to each individual to prepare for the future. Fortunately, there are those such as Al Gore and Arnold Schwartzanegger who do understand the urgency and are doing something about it now. Hopefully, others will follow. Please remember them at election time.

Solar John

My Solar Photovoltaic (PV) System

A solar photovoltaic (PV) system can be as simple as a single module with the ability to charge the battery of a cell phone, or a huge installation designed to supply the electrical needs of an entire community. My PV system began with one panel and a small battery bank, and continues to grow as my finances will allow.

People often ask “how much money do you save on your electric bill?” They seem surprised when I tell them almost nothing. At that point, I imagine that they’re wondering why I bother with a system of this small size at all, and I feel compelled to explain why I do what I do.

I hope someday that my PV system will be able to supply all of my electrical needs, but I have already benefited in many ways from my modest system.

It’s a learning experience. I can apply what I’ve learned to a full-blown system. As I expand, I’m not overwhelmed by the technology.

I often use my system to supply power to night-lights, and to grow-lights for my plants. I’ve even used it to power the lights on a Christmas tree.

My system served as an emergency power source recently when a tornado caused a power outage. I had plenty of power for lights, cell-phone battery charging, a TV, a radio, and other small appliances. I can use my system to keep a small freezer running to prevent food from spoiling in the event of an extended power failure.

By reducing electrical usage from the grid, I’m reducing the load on a fossil-fueled power plant, and therefore reducing carbon emissions. Carbon emissions, more than anything else, are responsible for global warming.

Considering the ever-increasing cost of fuel, it’s good to have an alternative. It’s hard to predict how much money I’ll save in the future with my system.

As the price of fuel increases, the demand for PV equipment will rise. This is likely to cause supply problems and dramatic price increases. I’m glad that I already have a PV system in place.

Working with a limited power source has taught me to think about energy-efficient lights and appliances. Each time I purchase an energy-efficient device I’m reducing my utility bill and increasing the usefulness of my system.

In an effort to reduce my home heating costs, I’ve recently installed a corn-burning stove. Using this stove, I expect to be able to heat my home for significantly less than it costs to heat it with natural gas. My PV system will provide power for the stove’s fans and auger. However, I’ll need to add PV panels and batteries if I am to have enough electricity to make it through a cold winter night.


Wednesday, July 12, 2006

Burning Corn to Heat Your Home

Burning shelled corn to heat your home is a lower-cost alternative to fuels such as natural gas, propane, coal, and wood. Corn burns cleaner than those fuels, and does not release dangerous chemicals into the air. Corn-burning stoves are easy to use and maintain.

Burning corn is good for the planet. Corn absorbs carbon dioxide as it grows, and much of that remains in the stalks and roots. These are usually composted back into the soil. As it grows, corn releases oxygen and helps to rid the air of pollutants. Unlike oil and coal, corn is a renewable resource. A new crop is produced each year.

Still, there are those who do not support the idea of using corn for fuel. The effects of fertilizers and the operation of machinery in the growing, harvesting, processing, and transportation of corn may result in some damage to the environment. Hopefully, with enlightened leadership, these issues will be resolved in the not-to-distant future.

According to studies at Cornell University and the University of California-Berkley, turning corn into fuel uses much more energy than the resulting ethanol or biodiesel generates.
However, corn stoves are designed to run on shelled corn. No refinery is required.

Royal Dutch Shell considers using food crops to make biofuels "morally inappropriate", as long as there are people in the world who are starving.
I guess that's what one might expect an OIL COMPANY executive to say. I've done a little research and found that while malnutrition is a real problem in some parts of the world, it is not due to an inability to grow food crops, but rather a problem with harvesting and storing them. Using corn for fuel does not interfere with food supplies anywhere in the world. The National Corn Growers Association (NCGA) says; "American farmers have continued to easily meet all demands for US corn from foreign countries".

Burning corn kernels for home heating bypasses the negative effects of ethanol conversion, but does not address the issue of the need for an environmentally friendly way to generate electricity. Wind, hydro, and photovoltaic systems are a few of the most appropriate solutions for that task. The best current technology for heating water is direct heat from the sun. As the price of oil continues to rise, and with increasing pressure to limit carbon emissions, it will be interesting to see if the automobile industry is able to produce a product that the public will accept.

Do a Google search for corn stoves and visit manufacturers and distributors websites for more information. You'll also find stoves that burn pellets made from wood byproducts. I decided to go with a corn-burner becaus they burn cleaner and corn is less expensive than wood pellets. I'm even growing a small crop of field corn this year to burn in my stove. Now that's energy independence! I don't have enough land to grow a crop big enough to supply me with corn for the entire heating season, but at least I'm doing something to reduce carbon emissions and my dependence upon petroleum-based products for heating.

Here's a great website concerning the use of corn for heat:

Solar John

Friday, July 07, 2006

Carbon Emissions and Global Warming

For the most part, carbon emissions are due to the burning of fossil fuels. Since carbon emissions are responsible for the undesirable effects of global warming, and since the situation continues to get worse over time, it is inevitable that the government of the United States will take measures aimed at reducing carbon emissions. The automobile industry already has a mandate to produce more efficient vehicles, but that effort alone will not be enough. Only about 20 percent of carbon dioxide emissions come from cars and light trucks in the United States. About 40 percent of carbon dioxide emissions in the United States are the result of burning fossil fuels for the purpose of electricity generation.

Fortunately, there are many things that consumers can do to minimize electricity usage. Switching from incandescent light bulbs to compact fluorescent types is a relatively inexpensive option. Home improvements, such as the replacement of old windows and the addition of insulation are very helpful, as is the replacement of appliances with energy-efficient ones. Producing your own electricity via solar photovoltaic installations, hydroelectric systems, or windmills is another way to limit or eliminate the use of electricity from fossil-fueled power plants.

When legislators understand that they can wait no longer to take action aimed at reducing carbon emissions, it is likely that they will start by eliminating subsidies for fossil fuels and perhaps increasing taxes related to their use. After all, this is what has been happening with the tobacco industry. At the present time, taxes make up about 70 to 80 percent of the cost of a pack of cigarettes. This action was supposed to result in a voluntary reduction in the use of tobacco-products, but in reality has had only a modest effect. Still, taxes on tobacco-related products continue to rise. If the government attempts to encourage voluntary cutbacks of electricity usage in the same way it has attempted to discourage the use of tobacco products, the outcome may be a dismal failure. If the dumping of carbon into the atmosphere is to be reduced in a timely manner, it will be the result of a commitment by ordinary people.

Many people are already implementing alternative energy systems. Hopefully, the rest of us won’t wait until it’s too late. Scientists believe that at some point, unless we take serious steps to reverse the trend, we will reach a tripping point from which there will be no return. Still, it appears that most people choose to ignore these warnings. Perhaps this is due in part to overblown predictions of doom such as the Year 2000 (Y2K) problem.

The fact that our country has an abundance of cheap coal might also convince some people that we don’t have to change our lifestyle. While oil is getting increasingly expensive, coal may prove to be a viable substitute. Modern mining methods, such as Mountaintop Removal, greatly improve the efficiency of coal production. Unfortunately, large sectors of mountains are being turned into wastelands. But that’s another story. Burning coal contributes to global warming even more than the use of petroleum-based products does, and legislators are likely to heavily tax the use of coal. However, that’s not really a bad thing. Even though we have an abundance of coal, it’s better to preserve some of it for future generations than to use all we can for today’s population. Using renewable sources of energy not only helps to mitigate the effects of global warming, it will cost the consumer less in the long run, and help to preserve natural resources.


Follow up: Here's an interesting article related to this post.